A Critical Review of Li/Air Batteries

Journal of The Electrochemical Society, 159 (2) R1-R30 (2012) R10013-4651/2012/159(2)/R1/30/$28.00 © The Electrochemical Society

A Critical Review of Li/Air BatteriesJake Christensen,a,∗,z Paul Albertus,a,∗ Roel S. Sanchez-Carrera,b,∗ Timm Lohmann,a

Boris Kozinsky,b Ralf Liedtke,c Jasim Ahmed,a and Aleksandar Kojica

aRobert Bosch LLC, Research and Technology Center, Palo Alto, California 94304, USAbRobert Bosch LLC, Research and Technology Center, Cambridge, Massachusetts 02142, USAcRobert Bosch GmbH, Gerlingen-Schillerhohe, Baden-Wuerttemberg 70839, Germany

Lithium/air batteries, based on their high theoretical specific energy, are an extremely attractive technology for electrical energystorage that could make long-range electric vehicles widely affordable. However, the impact of this technology has so far fallen shortof its potential due to several daunting challenges. In nonaqueous Li/air cells, reversible chemistry with a high current efficiencyover several cycles has not yet been established, and the deposition of an electrically resistive discharge product appears to limitthe capacity. Aqueous cells require water-stable lithium-protection membranes that tend to be thick, heavy, and highly resistive.Both types of cell suffer from poor oxygen redox kinetics at the positive electrode and deleterious volume and morphology changesat the negative electrode. Closed Li/air systems that include oxygen storage are much larger and heavier than open systems, butso far oxygen- and OH−-selective membranes are not effective in preventing contamination of cells. In this review we discuss themost critical challenges to developing robust, high-energy Li/air batteries and suggest future research directions to understand andovercome these challenges. We predict that Li/air batteries will primarily remain a research topic for the next several years. However,if the fundamental challenges can be met, the Li/air battery has the potential to significantly surpass the energy storage capability oftoday’s Li-ion batteries.© 2011 The Electrochemical Society. [DOI: 10.1149/2.086202jes] All rights reserved.

Manuscript submitted April 15, 2011; revised manuscript received October 24, 2011. Published December 29, 2011. This articlewas reviewed by Kuzhikalail Abraham ([email protected]) and Yang Shao-Horn ([email protected]).

The Li/air cell has received significant interest in the past severalyears as researchers look at couples that may achieve a specific energysignificantly higher than current lithium-ion cells with two intercala-tion electrodes (e.g., C6/LiMO2, where “M” refers to a transitionmetal such as Ni, Mn, or Co). The main application driving interestis transportation, where specific energy and energy densityd are mostimportant, although applications in portable electronics and grid en-ergy storage are also of interest. Of particular interest in the context oftransportation is the fact that, with the specific energy and energy den-sity of today’s automotive Li-ion cells, one’s driving range is limitedto about 70 miles for a 200 kg pack, as shown in Figure 1. We assumea specific energy of 150 Wh/kg at the cell level and 105 Wh/kg at thepack level (70% of a pack’s weight is the cells). This means that in or-der to enable electric vehicles with a range similar to today’s vehiclespowered by liquid fuels, a battery system with a specific energy andenergy density much higher than today’s state-of-the-art is required.Indeed, while some observers predict that Li-ion cells may eventuallyreach 400 Wh/kg through the use of high-capacity cathode materials(275 mAh/g) and alloy anode materials (2000 mAh/g), significantlyhigher values can only be obtainedwith even higher capacity cathodes,Li metal, and improved packaging of active materials. A Li/air batteryhas the potential to truly surpass the battery technology used today,as well as that under development for deployment in the mediumterm (i.e., that which may achieve 400 Wh/kg). A cell-level specificenergy value for a Li/air cell remains uncertain, but our cell energycalculations show that 1000 Wh/kg or more should be attainable if

∗ Electrochemical Society Active Member.z E-mail: [email protected] By “specific energy” we mean energy per unit mass, and by “energy density” wemean energy per unit volume. Some authors instead use the terms “gravimetric energydensity” and “volumetric energy density,” respectively.

several fundamental challenges can be overcome (Figure 3 and theassociated discussion address the issue of practical specific energy indetail). This specific energy could enable an electric driving range ofmore than 380 miles on a single charge at the beginning of a battery’slife, a value approaching that of a gasoline-powered vehicle. Addi-tionally, the cost of a system that achieves today’s driving range maybe reduced significantly with a much higher specific-energy battery.This could bring electric vehicles to the mass market, since consumerdemand is expected to be driven strongly by vehicle cost reduction,provided adequate range is available.

There are many possible reactions involving Li and air.— Fromthe outset it is important to realize that several chemical products mayresult from the reaction of Li with O2, depending on the chemical en-vironment and mode of operation. The main distinction is whether themedium in which Li is combined with O2 is aqueous or nonaqueous,and this distinction will be noted throughout this review. Some re-searchers have also explored hybrid aqueous/nonaqueous Li/air cells,a concept in which a Li-conducting ceramic is used to separate onecompartment with a nonaqueous electrolyte containing Li metal andanother compartment with an aqueous electrolyte.1

It is important to note that throughout this review we refer toaprotic solvents as “nonaqueous,” which is not particularly precise butis consistent with much of the Li/air literature. We shall not explicitlyconsider protic nonaqueous Li/air chemistry, as published work islacking in this area, although it is expected that it shares many generalcharacteristics with aqueous Li/air chemistry. Likewise, mixtures ofwater and other protic solvents are outside the scope of this review.We also note that while many researchers refer to Li/air batteries, infact most of the laboratory work has focused on Li/oxygen batteries,as components in air such as H2O and CO2 can interfere with thedesired electrochemical behavior.

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R2 Journal of The Electrochemical Society, 159 (2) R1-R30 (2012)

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Figure 1. Driving range and battery weight for different cell-level specificenergy values. It is assumed the battery cells weigh 70% of the battery pack,the Li/air cell has an 83% energy efficiency, the Li-on cells have a 93% energyefficiency, and 300 Wh/mile are required from the battery. The range is givenat the beginning of a battery’s life and assumes 100% of the capacity can beused; in practice not all the energy can be used, and the available energy fallswith increasing battery age. The US Department of Energy has a goal for anEV battery of 200 kg.176

In nonaqueous Li/air batteries there are two principal electrodereactions of interest: 2 Li+ 1/2O2 ↔ Li2O, [1]

and

2 Li+ O2 ↔ Li2O2. [2]

In the absence of practical considerations the full reduction of O2to Li2O is desired because of its higher specific energy and energydensity, but it appears that Li2O2 is a product that forms more readilythan Li2O.2–4 In addition, when Li2O2 is formed full cleavage of theO-O bond may not be necessary, which is important from a kineticpoint of view.5,6 These reactions will be discussed in more detail inlater sections.Reactions involving Li and O2 in an aqueous medium depend on

the pH. In a basic aqueous environment O2 reduction includes H2Oas a reactant and results in the formation of LiOH:

2 Li+ 1/2O2 + H2O ↔ 2 LiOH. [3]

The product of this reaction is aqueous LiOH, which has a solu-bility limit of about 5.25 M at standard temperature and pressure.7 IfLiOH exceeds its solubility limit it will precipitate out of the solutionas a monohydrate, LiOH ·H2O, rather than LiOH.7 This is a criticalpoint for calculating the specific energy and energy density of aqueousLi/air cells. We are presently unaware of any solvent system that leadsto the precipitation of LiOH rather than LiOH ·H2O, although LiOH

may form as a film on the surface of Li metal.8 An example of thereaction of Li with O2 in a mildly acidic environment is the formationof LiCl:9

2 Li+ 1/2O2 + 2NH4Cl ↔ 2LiCl+ 2NH3 + H2O. [4]

Another example of a reaction in an acidic solution is the formationof Li2SO4 from Li, O2, and H2SO4.9 Although acidic solutions havethe advantage that carbonates are not formed as in basic solutions (e.g.,the formation of K2CO3 in KOH solutions), in the present review weexclude from detailed consideration aqueous reactions involving Liand O2 in acidic media because the examples known to the authorshave a lower specific energy than in basic media (e.g., reaction 4 hasa lower specific energy than reaction 3). Neutral solutions have alsobeen discussed, and we exclude them from consideration here forthe same reason.10 There may also be other chemical considerationsfor a given reaction, such as the fact that in reaction 4 NH3 has ahigh vapor pressure, limiting the reversibility of an open cell due toevaporation. We also note that if the only criterion for a “Li/air” cell isa reaction that includes Li and O2, there are additional reactions thatfall into this broad classification. However, in this review we focusour attention on the reactions that have thus far received the mostattention, 1–3. We also exclude from consideration the somewhatrelated reactions between Li and H2O (e.g., the “seawater battery”developed by PolyPlus Inc.), because O2 is not a reactant and H2 gasis evolved, significantly limiting the possibility of creating a secondarysystem.What, then, are the specific energy and energy density values for

the Li/air cells included in our analysis? We approach this questionby first looking at the active materials alone and then including thecomponents of a typical cell sandwich.

Energy estimates for the active materials alone.— Calculationsof the specific energy and energy density based on the weight ofthe active materials alone provide a benchmark for values that canbe obtained by practical cells, although a practical cell should notbe expected to achieve more than about half of the energy per massor volume of the active materials alone. “Active materials” refers toLi, O2, and H2O in the charged state, and Li2O, Li2O2, LiOH, andLiOH ·H2O in the discharged state. As mentioned above, to the bestof our knowledge LiOH ·H2O and not LiOH will precipitate from anaqueous solution, so its inclusion here is for the sake of comparisononly. In Table I we summarize the physical properties of the Li/airactive materials in the discharged state we consider in this review, aswell as a current and “next generation” Li-ion intercalation materialfor comparison.11 Figure 2 shows specific energy and energy densitynumbers based on active materials alone (i.e., excluding the mass orvolume of all cell components besides the active materials definedabove); in the charged state the weight of O2 is excluded. For theLiMO2 material we assume a specific capacity of 275 mAh/g anda density of 4.25 g/cm3, values appropriate for an advanced oxidematerial.12 Energy calculations for an open system like Li/air aredifferent from those for other battery systems that are closed to theexternal environment because the mass of the battery increases during

Table I. Physical properties of select Li/air and Li-ion materials positive-electrode active materials in the discharged state, as well as Li metal.13

Specific Capacity Uθ vs. Li Theoretical specific Theoretical energyActive capacity Density density metal energy (vs. Li metal) density (vs. Li metal)material (mAh/g) (g/cm3) (mAh/cm3) (V) (kWh/kg) (kWh/L)

Li2O 1794 2.01 3606 2.91 5.22 10.49Li2O2 1168 2.31 2698 2.96 3.46 7.99LiOH ·H2O 639 1.51 965 3.45 2.20 3.33LiOH 1119 1.46 1634 3.45 3.86 5.60LiMO2, M =Mn, Ni, Co 275 4.25 1169 3.75 1.03 4.36LiFePO4 170 3.6 612 3.42 0.58 2.09Li metal 3861 0.534 2062 0.0



Journal of The Electrochemical Society, 159 (2) R1-R30 (2012) R3

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Figure 2. (a) specific energy and (b) energydensity values based on active materials alonefor selected Li/air active materials, and an in-sertion reaction for comparison.

the discharge process (assuming oxygen is not carried on board).For that reason, the specific energy and energy density numbers areshown in Figure 2 for both the fully charged and the fully dischargedstates. As Figure 2a shows, a cell producing Li2O or Li2O2 has aboutthe same specific energy in the charged state, but in the dischargedstate the Li2O2 cell has a lower energy per mass due to only one O2molecule being consumed per 2 Li atoms, rather than 1/2 O2 in thecase of Li2O. The specific energy calculations for the LiOH ·H2O andLiOH systems in the charged state depend on the assumption aboutwhere the H2O in the discharge product originates. In Figure 2a weshow one case in which the H2O is provided by an external source sothat only the weight of Li metal is included in the charged state, andanother case in which the H2O in the product is stored in the chargedcell along with the Li (e.g., in a water reservoir). The main assumptionfor the LiOH and LiOH ·H2O values is that of a single equilibriumpotential. In practice the cell potential will vary with the activity ofthe reactants and products during the cycling process, but we useonly the standard cell potential here. Compared with a Li/LiMO2 cell,all four Li/air discharge products have a significantly higher specificenergy.While specific energy is important, energy density can be just as

important in automotive and other applications. Figure 2b shows theenergy density based on the active materials alone. The higher energydensity of the discharged cells than charged cells is partly a result ofthe low density of Li metal (0.534 g/cm3). The figure shows that theenergy density of a discharged Li/LiMO2 cell is higher than that of anaqueous Li/air cell, and within about a factor of two of a nonaqueousLi/air cell. Thus, the advantages of Li/air cells from a specific energypoint of view are more dramatic than from an energy density point ofview because of the relatively low density of Li/air active materialscompared to metal oxide intercalation materials.

Energy estimates for practical cells.— Energy estimates basedon the weight and volume of active materials alone should be fol-lowed with energy estimates for practical cells. Such estimates forLi/air cells are uncertain due to the absence of well developed celldesigns. However, in this section we make assumptions about pos-sible cell designs in order to arrive at initial estimates for practicalcells that can later be refined. We focus on an optimistic practical celldesign that will require additional materials development rather thanonly looking at cell components that are available today. For example,we assume that an ionically conductive lithium metal protection layerwith a thickness of 50 μm and density of 3.0 g/cm3 will be developedthat can be manufactured and used in practical cells.Our assumptions are summarized in Table II. We assume the Li/air

charged cells have a significant volume fraction of gas in the positiveelectrode (70%) while in the discharged state a small amount of gasremains (5%). A gas phase provides volume into which the solidactive materials can be deposited and provides good transport of O2into and out of the cell. We assume the cell contains 20% excessLi relative to the capacity obtained by filling 65% of the positive

electrode volume with discharge product. While some authors havesuggested using a large excess of Li (100 to 300%), we considerthis impractical, as it implies the tolerance of a significant degree ofparasitic reactions, likely involving electrolyte decomposition, overthe lifetime of the battery. The products generated by such significantparasitic reactions would likely impair cell performance well beforeall of the excess Li was consumed. Assumptions about the source ofH2O for the aqueous Li/air cells are very important. If humidity from

Table II. Cell and tank properties for practical cell energycalculations. Thickness values are at full charge while volumefraction values are at the end of discharge. ε values indicatevolume fractions, LPSL = Lithium protection separator layer.GDL = gas diffusion layer. CC = current collector. The practicalcell energies shown here are nominal, that is they do not includethe practical energy efficiency.

Property Value Units

Lpositive 200 μmLLPSL 50 μmLGDL, positive 50 μmLCC, negative 5 μmLCC, positive 7.5 μmAmount of excess Li for all cells 20 %εactive material, pos at the endof discharge

0.65

εelectrolyte, pos, Li/air cells 0.20εelectrolyte, pos, Li/LiMO2 cell 0.25εgas phase, pos at the end of discharge 0.05εinerts, pos (carbon) 0.10εGDL, pos 70 %Mass packing factor basedon charged cell w/o tank

80 %

Volume packing factor basedon charged cell w/o tank

70 %

ρLiOH (aq) elyte. (saturated) 1.105 g/cm3

ρLiPF6 in PC elyte. 1.2 g/cm3

ρinert (carbon) 2.2 g/cm3

ρLPSL 3.0 g/cm3

ρGDL, positive 1.8 g/cm3

ρCC, negative 8.9 g/cm3

ρCC, positive 2.7 g/cm3

Oxygen volume in tank 75 LBattery pack energy from whichoxygen pressure is calculated

140 kWh

Tensile strength of tank(Stainless steel)

460 MPa

Tank safety factor (additional wallthickness beyond tensile strength)

50 %

Additional mass and volume abovethat of the tank for tank components

20 %




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Figure 3. Practical (a) specific energy and(b) energy density values for selected Li/airactive materials, and an insertion reactionfor comparison. The energy of soluble LiOHin the electrolyte phase is excluded for theLiOH ·H2O and LiOH cases as it would leadto an increase of less than 5% for this celldesign.

the external air can be used to supply some of the H2O required byreaction 3 and the LiOH ·H2O precipitate, that significantly reducesthe weight and volume basis in the charged state. However, addinga system to capture water may add complexity and cost, and wetherefore use a design in our calculations that would either involvefilling a water reservoir (like filling current cars with fuel) or having awater-impermeable membrane that completely prevents water ingressand egress. Therefore, in our practical cell numbers for the chargedstate we include the weight and volume of water present in LiOHand LiOH ·H2O. These assumptions can be revised as practical celldesigns are more fully developed.Energy results for “practical” cell designs are shown in

Figure 3, with results for systems with and without an oxygen tankshown. We exclude the energy content of soluble LiOH in the elec-trolyte for the LiOH and LiOH ·H2O energy calculations because, forthis cell design, including it will increase the energy values by lessthan 5%, and it is unclear whether it is better to cycle with a saturatedelectrolyte solution or have a lower concentration in the fully chargedstage. The theoretical amount of energy stored when cycling betweena 0 M and a saturated solution (5.25 M at 25◦C) of aqueous LiOH isabout 430 Wh/kg and 475 Wh/L. Although an oxygen tank is, strictlyspeaking, not part of a Li/air cell, including its mass and volume in thecalculation underscores the potentially large disparity in the energydensity of closed vs. open systems. We assume the use of a stainlesssteel oxygen tank in the shape of a 1.25 m-long cylinder with twohemispherical ends.First considering systemswithout an oxygen tank, Figure 3a shows

that a “practical” discharged Li2O2 Li/air cell may achieve a specificenergy more than twice that of a Li/LiMO2 cell, while a LiOH ·H2Ocell may have a “practical” specific energy only slightly higher thana Li/LiMO2 cell. In terms of energy density, Figure 3b shows that aLi/LiMO2 cell has a modestly lower energy density than a Li2O orLi2O2 cell, and a modestly higher value than a LiOH ·H2O cell. Themajor change that would allow the aqueous Li/air system to have asignificantly higher specific energy and energy density would be theformation of pure LiOH rather than LiOH ·H2O, which we also showfor the sake of comparison in Figure 3. For comparisonwith our “prac-tical” numbers here, PolyPlus, a company focused on the developmentof protected lithium metal electrodes, has claimed a practical specificenergy of almost 1.0 kWh/kg for their basic-electrolyte aqueous Li/aircells.14 The numbers given in Figure 3a (0.70 kWh/kg for charged,0.66 kWh/kg for discharged), are about 30% below the number givenby PolyPlus. A number of factors may contribute to this difference,including our use of a 80% packing weight factor (they may have alower-weight packaging technique) and how much of the weight ofthe water stored in the LiOH ·H2O is included in their weight basis.In particular, if they have a design that takes some water from theexternal environment, that could significantly lower their weight ba-sis for the charged cell. Another factor is our use of a relatively thin200 μm positive electrode thickness; with a positive electrode 1 mmin thickness and all else the same, the specific energy for our practical

cell design is also about 1.0 kWh/kg. Note that such a thick electrodeis more realistic for aqueous than nonaqueaous cells because of themuch higher conductivity of aqueous electrolyte solutions. Again, westress that the “practical” cell energy numbers presented here willcertainly be revised as more detailed designs are developed, and aremeant to represent optimistic estimates.Figure 3 also shows the results of calculations including an oxygen

tank. We include these numbers because it is important to see howLi/air cells compare with a Li/LiMO2 cell if the problems associatedwith making an open system cannot be solved. For these calculationsthe mass of the oxygen is included when the cell is charged, as it isstored in the tank. We assume the oxygen in the tank has a specifiedvolume (75 L) and the tank is sized for a battery system that stores140 kWh. Additional specifications are given in Table II. Figure 3shows that the use of an oxygen tank results in a significant reduc-tion in the specific energy and energy density. In terms of specificenergy, the Li2O and LiOH cells with a tank still have a higher valuethan a Li/LiMO2 cell, but the Li2O2 cell and LiOH ·H2O cells havea slightly lower value than a Li/LiMO2 cell. In terms of energy den-sity, if an oxygen tank is used the values for all the Li/air cells willbe lower than for a Li/LiMO2 cell. These calculations demonstratethe importance of creating a Li/air battery system that is able to useoxygen from the atmosphere rather than store it onboard, although thetank results may be improved if a lighter weight tank material (e.g.,carbon fiber) or a higher pressure (and thus a smaller tank volume)could be used. Table III shows the pressure of a fully charged oxygentank for each Li/air active material, as well as the isothermal energyof compression required to go from 1 bar to the final pressure. Thenon-unity compressibility of oxygen was accounted for using the vander Waals equation. Interestingly, the LiOH and LiOH ·H2O activematerials require the smallest amount of oxygen because they react4 electrons per mole of O2 (as does Li2O) and have a higher equilib-rium potential than Li2O. Li2O2 requires significantly more oxygen,and therefore a higher pressure and heavier tank, than the other ac-tive materials because only 2 electrons per mole of O2 react. Theisothermal work of compression is relatively small in each of thesecases (<3%of the practical discharge energy) but isothermal compres-sion is probably more practical if done electrochemically; however,

Table III. Details on the oxygen tank pressures and compressionenergies that would enable a closed Li/oxygen battery system.

Isothermal compressionFully charged work (kWh/kWh practical

Active material O2 pressure (bar) discharge energy)

Li2O 134 0.0108Li2O2 275 0.0244LiOH ·H2O 114 0.0088LiOH 114 0.0088




having 100 to 300 bar oxygen pressure in the stack presents chal-lenges of its own, including safety. In addition, while electrochemicalcompression has been explored for hydrogen it is not well establishedfor oxygen.15 A major advantage of using electrochemical compres-sion and having the oxygen electrode operate at the same pressureas the tank, besides avoiding the need for a mechanical compressor,is that the compression work can be returned on discharge, resultingin a theoretical round-trip compression energy efficiency of 100%. Ifadiabatic mechanical compression is used (which is typical for me-chanical gas compression) the theoretical compression work is higher,although again if the oxygen electrode is directly exposed to the oxy-gen tank pressure at least some of the compression energy can bereturned on discharge. Mechanical compression seems more plausi-ble if the compressor could be located and powered from outside thevehicle.The calculations presented in Figure 3 are for secondary cells. The

specific energy and energy density of primary cells may be higher,depending on the cell designs that an application allows. Becausemany primary cells are intended for low-rate applications, a thickerpiece of Li metal may be used, increasing the mass and volume ofactive materials compared to packaging and thereby increasing thespecific energy and energy density. In addition, primary cells typicallydo not require any (or as much) excess Li metal because a non-unitycurrent efficiency does not affect later cycling.Along with our calculations of the cell energy, it is important to

present the capacity per area for the cells described above. In Figure 4we show the electrode capacity (in mAh/cm2) for each active materialusing the specifications given in Table II. The figure shows that eachmaterial has an electrode capacity above 10 mAh/cm2, and in the caseof Li2O it approaches 50mAh/cm2. Typical values for a Li-ion cell arein the range of 3–7 mAh/cm2. Of course, a thicker electrode with thesame active material and active material volume fraction has a higherarea-specific capacity, but there is a limit to how thick an electrodecan be made due to mechanical issues. In later sections we discuss theimportance of presenting not only the amount of capacity per, for ex-ample, gram of carbon, but also the capacity stored per electrode area.In conclusion, the specific energy of nonaqueous Li/air batteries

that form Li2O or Li2O2 is extremely appealing, as in a “practical”cell it may be about 2 to 4 times higher than a comparable cell witha Li metal negative electrode and an advanced Li-ion intercalationpositive electrode. The specific energy of the aqueous Li/air cell issomewhat higher than a comparable Li/LiMO2 cell, but its energydensity is lower. The relatively low density of the Li/air active ma-terials means that their specific energy is more appealing than theirenergy density when comparing against an advanced Li-ion interca-lation material. Storing oxygen onboard the vehicle in a tank willsignificantly reduce the specific energy and energy density of a Li/air

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Figure 4. Positive-electrode area-specific capacity for a thickness of 200 μmand an active material volume fraction of 0.65. For comparison, most nonaque-ous Li/air cells built thus far have an electrode capacity of less than 5mAh/cm2,even though they may achieve a large amount of capacity per weight of carbon(≥1000 mAh/g carbon).

cell to the point that a Li/air cell is no longer compelling comparedwith a Li/LiMO2 cell, unless a low-weight and high-pressure tankis used. The magnitude of reductions in the practical numbers inFigure 3 when a tank is used will result from a constrained optimiza-tion for the tank pressure that results in an acceptable tank volumeand gives the lowest cost (taking into account compression losses).

Overview of the critical challenges for the Li/air system.— Nowthat we have established the promising specific energy of both non-aqueous and aqueous Li/air systems, we turn our attention to thesignificant challenges that stand between the Li/air concept and com-mercialization. In the next section, we address the critical issues per-taining to nonaqueous Li/air cells, including those that are relevant toboth nonaqueous and aqueous systems. In the subsequent section, wediscuss issues that are relevant to aqueous systems only. Finally, wesummarize the main challenges and provide an outlook for the futureof this technology.We consider issues that limit the practical achievable specific en-

ergy, specific power, and cycle life of Li/air systems, as well as theadditional complexity involved in the open nature of the cell. Becausewe estimate that the commercialization of this technology will be pos-sible only with solution of several very difficult challenges we deferquestions of cost until the design of a viable system becomes clear.However, a significant increase in the specific energy will result in acommensurate reduction in cost if the cost per mass remains constant;this relationship remains to be established. Similarly, we suspend dis-cussions of system safety. While nonaqueous systems, if achievable,contain flammable liquid electrolyte and a Li anode with the potentialto generate dendritic shorts, there is evidence to suggest that basic-electrolyte aqueous systems with protected Li metal have insufficientreactivity to create the conditions for explosion due to the formationof a film that limits the reaction rate.16,17

Not every hurdle that lies in the path of commercialization canbe discussed in this review, as it is impossible to predict all of thechallenges considering that the ultimate design of a Li/air system maybe significantly different from present concepts. Here we shall focuson a set of significant challenges that have high priority within the nextfive years. Depending upon the results of research being carried outover that time period it may be that certain widely studied concepts areshown to be impossible or unattractive, while other related systems(e.g., aqueous Li cells18) showing more promise may be discovered.The issueswe consider are listed here and correspond to the subsectionheadings.

Issues that are broadly applicable to Li/air systems, or onlynonaqueous systems:

� Establishing truly reversible electrochemical reactions.� Obtaining high capacity in the positive electrode.� Accommodating significant volume changes.� Stabilizing the Li metal negative electrode.� Achieving adequate power capability and efficiency.� Supplying contaminant-free O2 to the system.

Issues that apply only to aqueous systems:

� Managing precipitation and dissolution of the dischargeproduct.

� Catalyzing discharge and charge when O-O bonds are broken.

Nonaqueous Li/Air Systems

While the first paper on a Li/air cell came out in the 1970s,19

and the first paper on a nonaqueous Li/air cell in the late 1990s,2

interest has grown significantly in the past ten years, with much ofthe attention focused on nonaqueous cell designs.3–5,20–26 Several re-view papers have already been published;3,27–29 we avoid repeatingthat task here and instead focus on the critical issues that must beunderstood and solved for the Li/air system to move toward commer-




Figure 5. Example of a Li/air gas diffusion cathode based on Ni-foam. Thecoating consists of 50 wt% Super P carbon black, 40 wt% PVDF binder and10wt%MnO2 additive. (a) SEM-image of a coated Ni-foam. (b) Optical imageof uncoated Ni-foam. (c) Optical image of a coated Ni-foam-cathode.

cial viability. However, we first briefly discuss the components andperformance of a nonaqueous Li/air cell. One advantage of nonaque-ous Li/air cells over aqueous Li/air cells is that a solid-electrolyteinterphase (SEI) forms on Li metal in many nonaqueous electrolytes,allowing some experiments to be conducted without using a solid-electrolyte separator. Thus, a typical experimental cell is composedof a piece of Li metal, a nonaqueous solvent, a separator (e.g., glassfiber or a Celgard separator), and an air electrode. Air electrodes forLi/air batteries can be prepared following several techniques,30–33 andthe most common used thus far involve carbon black (e.g., Super P,Ketjen Black, Active coal), a polymer binder (e.g. PVDF, PTFE, cellu-lose), and an organic solvent (e.g. NMP, Acetone) or water (in case ofPTFE-suspensions and cellulose binders) being homogenized to forma viscous slurry. The slurry is typically coated onto a metal grid, metalfoam or conducting fleece by a film-casting process30 or by ultrasonictreatment. The supporting and conducting grid or sheet influencesthe positive electrode performance if a 3-phase boundary betweenoxygen, reaction product(s), and electrolyte is formed. Common pos-itive electrode supports are Al grids,4,34 Ni meshes35 or porous Nifoams.36 The resulting air cathode has to fulfill several requirements.It should have a high surface area at reasonable pore volumes, goodelectronic (>1 S/cm) and ionic (>10−2 S/cm) conductivities, and adesign that supports fast gas transport to the reaction centers duringdischarge.Figure 5 shows an SEM image of a coated Ni foam (a) together

with optical images of uncoated (b) and coated samples (c). Thecoating consists of 50 wt% Super P carbon black, 40 wt% PVDFbinder and 10 wt% MnO2 particles. From the SEM image one canvisualize the functionality of the gas diffusion air electrode. The largepores ensure gas transport to the coated branches (covered with activeslurry). The electrochemical reaction presumably takes place withinthe mesopores of the coating. It is still an open question how thereaction distributes itself over the pore volume and it likely dependson the specific electrode formulation and operating conditions. Inorder to provide good reactivity and efficiency, the wetting behavior,oxygen solubility, ionic conductivity, and stability of the electrolyteon the cathode surface are of great importance.Figure 6 shows a sample discharge and charge cycle for a non-

aqueous Li/air cell. There is typically a significant offset between thedischarge and charge curves, often more than 1 V, indicating a highcell impedance and/or a different reaction mechanism on dischargeand charge. As discussed in subsequent sections, several physical pro-cesses contribute to the high resistance. The abscissa label in Figure 6shows that a commonway to express the charge stored in a nonaqueousLi/air cell is in mAh/g of cathode material, including carbon, binder,and catalyst. Some authors define the capacity in mAh/g of carbon, the

Figure 6. Sample discharge and charge curve for a nonaqueous Li/air batterywith a hydrophobized gas diffusion cathode cycled at a current density of0.2 mA/cm2. The electrolyte contains 1 M LiPF6 in TEGDME.). The cathode(50 wt% Super P carbon ± 40 wt% PVdF ± 10 wt% MnO2) loading ineach case is 2 mg/cm2, and the electrode thickness is 50 μm. The capacityis normalized to the total mass of the cathode excluding the current collector.For comparison the top x-axis shows the capacity normalized to the electrodesurface.

idea being that the reaction happens on the carbon surface. We stressthat it is also valuable to include the capacity in mAh/cm2, as thismetric in combination with the electrode thickness can be comparedto Li-ion and other electrodes.Figure 7 shows a typical result for the fall in capacity with cycling

for a nonaqueous Li/air cell containing a carbonate-based electrolyte.Some authors have shown more stable capacity, but no publicationhas demonstrated 100 cycles with at least 90% of the initial capacityretained. Different types of carbon material typically demonstratedifferent amounts of capacity and have different rates of capacityfade, as Figure 7 shows. For example, Super P carbon delivers thebest performance although its surface area is only 60 m2/g. Activecoals like Supra 30 have more than 1700 m2/g but deliver much lowercapacity. This demonstrates that a carbon with a higher surface area

Figure 7. Variation of carbon materials for Ni-foam based cathodes in apropylene carbonate electrolyte with 1 M LiPF6. Super P carbon black (60m2/g) shows the best performance in this comparison regarding maximumcapacity. The other materials are carbon black (Ensaco 350 G, 400 m2/g),carbon fibers (CF), and two active coals, Supra 30 (1760 m2/g) and PAK1000C(1000 m2/g). The cathode (50 wt% carbon± 40 wt% PVdF± 10 wt%MnO2)loading in each case is 2 mg/cm2, and the electrode thickness is 50 μm.




Figure 8. Influence of three different cathode designs on the electrochemicalperformance of a Li-O2 cell. The error bars indicate the capacity range observedfor many different samples of the respective type. The electrolyte is 1 MLiPF6 in propylene carbonate. The Al-grid supports a solid coating and allowsgases to diffuse through the layer. The all-carbon-based electrode design is agas diffusion layer (GDL) consisting of a dense carbon fiber network with ahydrophobic coating (a wet chemical treatment using a sliane-based precursor)and a carbon black coating of about 50 μm. The cathode (50 wt% Super Pcarbon black ± 40 wt% PVdF ± 10 wt% MnO2) loading in each case is2 mg/cm2. The slurry was applied to the carbon black layer of the GDL usinga coating knife. The Al-grid was coated by dipping it into the slurry similar toNi-foam. Note that the discharge capacity increases reproducibly between thefirst and second cycle for two of the cathode compositions. This may be dueto some rearrangement or modification of the electrode surface.

does not necessarily have a higher capacity. Typical electrolytes mostlikely cannot penetrate the nanopores present in high-surface-areacarbons; hence, reactive sites are restricted to the outer surface ofthese carbons and the available surface area is reduced. In addition,the particular air electrode design (e.g., all carbon treated with asilane-based hydrophobic layer vs. carbon/Ni-foam vs. carbon/Al-grid (expanded metal) electrodes, see Figure 8) has a great influenceon the capacity achieved.The identity of the solvent also has a significant impact on the

capacity obtained and the discharge and charge potentials. In our ownstudies we have tested n-methyl pyrrolidone (NMP) and tetraethy-lene glycol dimethyl ether (TEGDME)e in comparison with propy-lene carbonate (PC). Figure 9 shows data of the first cycle for PC,TEGDME, and NMP measured at a current density of 0.2 mA/cm2.The air electrode used in these experiments consists of silane-treatedcarbon black, PVDF binder and a MnO2 catalyst. The differences inthe capacity obtained and the potentials indicate possible differencesin the reaction pathways, discharge product morphology and growthmechanism, electrode wetting, and oxygen solubility and transport. Inthis comparison NMP shows the highest discharge potential of about2.7 V and, at approximately 35 hours, the longest discharge time.In summary, while a basic Li/air cell can be assembled using

materials commonly available and often used in Li-ion cells, manyexperimental results and physical processes remain difficult to under-stand. In the coming sections we identify several of the challengesthat have been identified, and seek to delineate where clarity has beenestablished and where additional evidence is required before a clearexplanation can be provided.

Truly reversible electrochemical reactions need to bedemonstrated.— Although efforts to commercialize primary Li/airbatteries are already underway, the ultimate objective in automotiveelectrification is to produce a high-specific-energy storage battery thatcan be cycled thousands of times. At this stage, limited cyclability ofboth nonaqueous and aqueous Li/air cells has been demonstrated;

e We note that recent interest in TEGDME follows early work on poly(ethylene glycol)dimethyl ethers in Li/oxygen cells.37

Figure 9. Potential vs. time of Li/air cells in three different nonaqueous elec-trolytes. All samples have been cycled at a current density of 0.2 mA/cm2. Theair electrode consists of hydrophobized carbon, PVDF, and a MnO2 catalyst.It is important to mention that the plateau around 3.8 V for NMP is due tooxidative decomposition, and is not related to the desired charge reaction (i.e.reversible Li2O2 decomposition). The same plateau is present during chargingof NMP cells without a prior discharge. The cathode (50 wt% Super P carbon± 40 wt% PVdF ± 10 wt% MnO2) loading in each case is 2 mg/cm2 and thethickness is 50 μm.

however, in the case of nonaqueous systems, it has not been shownthat they can be cycled with a current efficiency sufficiently highfor a practical reversible cell (>99.95% is required to reach about450 cycles with 80% of the capacity remaining). The distinction be-tween cycling and cycling reversibly is crucial; a cycle is only re-versible if the chemical makeup of the system is the same at the startand end of the cycle. For the nonaqueous battery in particular, detailedinvestigation of the chemistry of discharge and charge products is acritical issue that has thus far received insufficient attention. There isnow conclusive evidence that the electrochemical reactions that oc-cur in nonaqueous cells containing carbonate solvents during chargeare not, in part or whole, the reverse of those that occur during dis-charge. Hence, the system can be cycled several times, but only at theexpense of some component, likely the electrolyte solvent, which isconsumed irreversibly on each cycle. Without a continuous supply offresh reactant and a way to remove irreversibly generated dischargeproducts, hundreds of cycles are unattainable. In addition, any num-ber of non-electrochemical reactions may also be occurring. In thissection we provide a critical description of recent experimental andtheoretical work that has examined the fundamental processes occur-ring in the nonaqueous Li/air electrochemical cell, as well as specifythe types of studies that will be helpful in overcoming the criticalissues.Experimental evidence strongly supports irreversible reactions incarbonate-based solvents.—The first paper on a nonaqueous Li/aircell (published in 1996) described the use of an electrolyte with apolymer solvent (polyacrylonitrile) as well as the plasticizers ethylenecarbonate (EC) and propylene carbonate (PC) and a cobalt pthalocya-nine catalyst.2 It identified Li2O2 as the probable discharge productbased on qualitative analysis and Raman Spectroscopy. Although theyused some but not all of the same materials as Abraham and Jiang,later authors appear to have taken the formation of Li2O2 in carbon-ate solvents as already established.2,4, 20, 33, 34 However, several recentpapers have specifically focused on the reaction chemistry and estab-lished conclusively that in carbonate solvents (without the presenceof any polymers) Li2O2 is not a principal discharge product, and oncharge CO2 rather than O2 is evolved.38–42

In the most definitive article published so far, McCloskey et al.carried out differential electrochemical mass spectroscopy (DEMS)experiments on Li/oxygen cells fed with isotopically labeled O2 as




Figure 10. Raman spectra of discharged carbon cathodes from a pure DME-based cell, a 1:1 (v:v) EC/DME-based cell, and a 1:2 (v:v) PC/DME-basedcell. Neat P50 carbon paper is included for comparison. Discharge condi-tions: 0.09 mA/cm2 under O2 to 2 V. Caption and figure reproduced fromreference 38.

well as spectroscopic analysis of discharged and charged electrodes.38

In a solvent composed of either 1:1 EC:DMC or 1:2 PC:DME, sig-nificantly more than 2 e−/O2 molecule were consumed on discharge(indicating reactions besides formation of Li2O2), XRD of dischargedair electrodes showed the absence of Li2O2, and Raman Spectroscopyshowed both the absence of Li2O2 and the presence of Li2CO3 andpossibly Li alkyl carbonates (see Figure 10). Upon charging a cellthat had undergone a first discharge, the principal gas evolved wasCO2 rather than isotopically labeled oxygen. The evidence in thispaper clearly establishes the absence of reversible cycling when, atthe very least, the specific carbonate solvents used by the authorsare present. We consider the use of isotopically labeled oxygen inquantitative, on-line DEMS experiments to be a key method for estab-lishing the reversibility of the Li/air system; in a reversible system thesame isotopically labeled O2 consumed on discharge should emergeon charge. If CO2 is evolved, isotopic labeling of carbon in the solventmay help determine whether the solvent or, if carbon based, the elec-trode decomposes. We note that when evaluating new chemistries forreversibility, it is important to quantify the number of electrons andO2 molecules consumed on discharge and charge as well as the purityof the discharge product; reversible systems have charge/dischargecapacity ratios of very nearly 1 and e−/O2 ratios of very nearly 2(assuming Li2O2 is the discharge product). Other ratios and/or thepresence of discharge products other than Li2O2 indicate the presenceof electrochemical or chemical side reactions. Quantifying electronandO2 consumption and generation as functions of current density andpotential window can help identify the nature of these side reactionsif they exist.The conclusions of McCloskey et al. are supported by several

other publications. For example, Freunberger et al. also carried outDEMS analysis on a carbonate-containing Li/air cell, as well as spec-troscopic analysis that included FTIR, NMR, and Surface-EnhancedRaman Spectroscopy (SERS).39 They found that in an electrolyte ofLiPF6 in PC on discharge the products include Li2CO3 and severalalkyl carbonates, and on charge CO2 rather than O2 evolves. Theirspectroscopic and gas-analysis findings are consistent with those ofMcCloskey et al. and clearly point to irreversible reactions during thecycling of a Li/oxygen cell with a PC solvent. In addition, similar re-sults have been reported byMizuno et al. who carried out FTIR and gasanalysis;40 Zhang et al. who carried out gas analysis during charging,FTIR, and XRD;41,43 and Veith et al. who carried out XPS, FTIR, andRaman on Li/oxygen cells with an electrolyte composed of LiPF6 ina mixture of ethylene carbonate and dimethyl carbonate.42 While sev-

eral other articles state that Li2O2 forms in a Li/oxygen cell containinga carbonate solvent (e.g., the original paper by Abraham and Jiangand a paper by Thapa et al. who built carbon-free electrodes44), noneof them employed a set of analytical tools (including gas analysis andspectroscopy) that could consistently account for the exclusive elec-trochemical formation of Li2O2.We conclude from the articles that didemploy rigorous spectroelectrochemical techniques and demonstratedthe absence of significant Li2O2 formation and reversible cycling thatcarbonate solvents do not have sufficient stability for long-term Li/aircell cycling.While the spectroscopic techniques mentioned thus far have been

helpful for identifying the discharge and charge chemistry, Zhang andco-workers demonstrated that the characteristic lithium NMR chem-ical shift for Li2O2 is difficult to distinguish from that of a reactionby-product of their carbonate-electrolyte Li/air cells.43 Similar resultswere also obtained in our recent theoretical work.45 Therefore, a directassignment for the chemical shift that indicates the presence of Li2O2could not be established by means of lithium NMR spectroscopy. Thesometimes conflicting results for the carbonate-based Li/air chemistryunderscore the importance of carefully selecting appropriate analyti-cal tools to identify discharge and charge products.No solvent has yet demonstrated highly reversible cycling; furtherexploration and quantification are necessary.—Some noncarbonatesolvents have been explored for use in a Li/oxygen cell, includingethers, ionic liquids, and acetonitrile. In this section we assess criti-cally the results for each of these solvents. Considering, first, the classof ethers, McCloskey et al. also explored the use of dimethoxyethane(DME) in the same work that addressed carbonate solvents.38 Dur-ing discharge in DME, 2.05 ± 0.05 e−/O2 were consumed andthe principal product identified by spectroscopy was Li2O2 (seeFigure 10), while on charge the same isotopically labeled oxygenconsumed by the cell on discharge was released. However, during thecharging process, in which the cell was charged by a capacity equiv-alent to the discharge capacity, 3.2 e−/O2 released were consumed,indicating a current efficiency for O2 release of only 60%. Interest-ingly, even with such a low current efficiency, no Li2O2 was found atthe end of charge, which the authors suggest may be due to a chemicalor electrochemical reaction of Li2O2 during the charging process thatdoes not have O2 as a product. The authors conclude that while DMEdoes provide for the formation of Li2O2 on discharge, it is unstableduring the charging process and is therefore an unsuitable solvent forreversible cycling. However, much of the charging process was car-ried out at high potential (∼4.5 V vs. Li), at which DME is likely todecompose electrochemically,46,47 and additional experiments (e.g.,charging at lower currents; using a cell design that ensures intimateelectrical contact between Li2O2 and conductive carbon) should beperformed before eliminating DME as a candidate solvent for non-aqueous Li/oxygen cells. Peng et al.48 showed that electrodes packedwith Li2O2 could be charged in PC solvent, whose oxidation potentialis∼5.1 to 6.0V,4,46,47, 49, 50 without evidence of solvent decomposition.Freunberger et al. also report on the chemistry of Li/oxygen cells

that make use of ethereal solvents, and found that for linear ethers liketetraglyme, as well as cyclic ethers like 1,3-dioxolane, some Li2O2forms on discharge, although significant amounts of electrolyte de-composition products are also found.51 They did not report resultsfor DME as a solvent, possibly explaining the difference between thefraction of Li2O2 produced during the first discharge they found com-pared with McCloskey et al. Like McCloskey et al.,38 they concludethat ethers are unsuitable solvents for reversible Li/oxygen cells, al-though they are more stable than carbonates as some Li2O2 formson the first discharge. Other authors have also discussed the fact thatLi2O2 does form in ethereal solvents, and there is at least limitedreversibility.26,52 In our view, while additional work on ethereal sol-vents is warranted, there are already indications that they too undergodecomposition reactions.A different class of solvents that has recently received attention is

ionic liquids. For example, Mizuno and Iba discuss the use of an ionicliquid, N-methyl-N-propylpiperidinium bis-trifluoromethansulfonyl-amide (PP13TFSA), as a solvent that demonstrates better reversibility




than PC, and results in O2 rather than CO2 evolution during charge.53

Other authors have also explored the use of ionic liquids for Li/airbatteries, although to date there has been no paper that demonstratesquantitatively a sufficiently high current efficiency for a practicalcell.31,54–56

Finally, several authors have explored the use of acetonitrile as asolvent for Li/air batteries.26,48 While they have found that oxygenreduction and evolution appear to be reversible, they have not ade-quately quantified the current efficiency of Li2O2 formation, and pointout that the higher vapor pressure and inferior safety characteristicsof acetonitrile make it a poor choice for a practical Li/air cell.Laoire et al.26 have suggested that suitability of a solvent for a Li/air

cell be evaluated in terms of Pearson’s “Hard Soft Acid Base” (HSAB)Theory.57 By comparing two cations (e.g., Bu4N+, a soft Lewis acid,and Li+, a hard Lewis acid), they showed that O2− is more stablein a soft acid environment, but is reduced to O22− and ultimatelyO2− in a hard acid environment. Moreover, solvation decreases thehardness of the Li+ acidity in proportion to the donor number ofthe solvent. This in turn leads to a longer lived superoxide andreversible LiO2 formation. Such theories can help guide the solventexploration process.In conclusion, while several noncarbonate classes of solvents have

been explored for use in Li/oxygen batteries, to date there has been noreport that clearly establishes highly reversible cycling, and findinga salt and solvent combination that allow truly reversible cyclingremains perhaps the first and most important challenge for Li/airbatteries at this time.Hypothesized mechanistic routes for the formation of the desiredreaction product include reactive intermediates.—There is now suffi-cient evidence that the solvent plays a critical role in determining thenature of the Li/air reaction products and the cell’s electrochemicalcyclabilty. The search for a stable electrolyte could be greatly assistedby acquiring detailed knowledge of the prevalent reactionmechanismsof the Li/air cell, including the identification of rate determining stepsand intermediate reactive species. For example, the experimental workof Abraham and co-workers25,26 resulted in a proposed reaction mech-anism that includes the reduction of oxygen to lithium superoxide:

O2 + Li+ + e− → LiO2 [5]

followed by

2LiO2 → Li2O2 + O2 [6]

or

LiO2 + Li+ + e− → Li2O2 [7]

and possibly

Li2O2 + 2Li+ + 2e− → 2Li2O [8]

Equation 5 involves the irreversible reduction of O2 via a one-electron process to formLiO2. This product disproportionates to Li2O2and O2 (equation 6) or is further reduced to form Li2O2 (equation 7).Li2O is also probably formed as the ultimate reduction product of O2,as indicated in equation 8.25,26 Recent in-situ spectroscopic data haveprovided direct experimental evidence of the formation of LiO2 as anintermediate species for the formation of Li2O2.51 LiO2 formation in-volves reduction of O2 to superoxide, O

−2 , which has been implicated

both experimentally39 and theoretically58 as the species responsible forthe decomposition of the carbonate-based electrolytes. As discussedextensively by Sawyer and Valetine, the superoxide ion is a powerfulreducing agent, which is thought to react very rapidly with a variety oforganic substrates.59 Others have also ascribed significant reactivity tothe electrochemically formed LiO2 species, which rapidly reduces thecarbonate solvent and forms solvent-decompositions products.38,43

Although the rate of formation of LiO2 and its reactivity towardcarbonate solvents remain unmeasured, preliminary XRD results byZhang and co-workers indicate that the attack of the carbonate-basedsolvent molecule by the superoxide ion proceeds at a faster rate thanthe coordination of O−

2 and Li+ to form LiO2.60 To summarize, the

evidence for whether O2− or LiO2 is more influential in unwantedparasitic reactions is not yet definitive.Independent of whether the solvent decomposition reaction pro-

ceeds via O−2 , LiO2, or another Li/air reactive species, it is now

clear that a solvent that is resistant to attack by reduced O2 speciesmust be discovered in order to achieve a practical Li/air cell witha long cycle life. Recent advances in surface-sensitive spectroscopyand microscopic analyzes38,51 coupled with first-principles modelingsimulations58 could help unveil the fundamental elementary processesof the Li/air electrochemical cell and explain the reasons for the poorreversibility of the reactions in typical Li-ion battery solvents.When screening electrolytes that do not form a stable SEI on Li metalan appropriate counter electrode should be used.—The carbonate sol-vents of widespread use in Li-ion batteries can form solid-electrolyteinterface layers on Li metal electrodes that are sufficiently stable toallow tens or even hundreds of cycles. When noncarbonate solventsare being screened for their stability toward positive-electrode reac-tions, care must be taken that side reaction products generated at thecounter electrode do not diffuse across the cell and interfere. Ideally, acounter electrode and reference electrode that have a potential at whicha solvent is stable should be used. For example, Li4Ti5O12, with anequilibrium potential of 1.5 V vs. Li metal may be a good candidate,although it does not have cyclable Li as synthesized (active materialsare typically synthesized in the discharged state). To circumvent thisproblem Li2O or Li2O2 could be packed into the positive electrode.Another possible counter and reference electrode is LiFePO4. In anycase, we stress the importance of a careful experimental design evenfor research cells in order to ensure the electrochemical stability ofthe reactions occurring at all electrodes and the avoidance of solubleproducts that may obscure the interpretation of the electrochemicalsignals. A protected lithium electrode could likewise serve as a counterelectrode for screening nonaqueous solvents, and it may be necessaryfor the function of certain solvents.Outlook: Nonreactive nonaqueous solvents remain elusive and re-quire a carefully designed screening procedure.—It is now clear thatcarbonate solvents are poor candidates for reversible Li/air systemsbecause of their susceptibility to react with reaction intermediates.The search for noncarbonate solvents should involve screening incells with appropriate negative electrodes (e.g., protected Li or high-potential intercalation electrodes). The amount and identity of O2consumed and generated during discharge and charge should be mea-sured and compared to the discharge and charge capacity to quantifyreversibility. The identity of the discharge product and its absenceafter recharge should also be confirmed. Thus far linear and cyclicethers, acetonitrile, and some ionic liquids have been explored; whileLi2O2 is formed on the first discharge in some cases, highly reversiblecycling has not been quantified and reported for any of these non-carbonate solvents. In our view, finding a stable solvent that allowstruly reversible cycling remains perhaps the most important currentchallenge for Li/air research. Using a variety of analytical techniques,including gas analysis (preferably with isotopically labeled oxygenand possibly carbon), and spectroscopy such as Raman and XRD,is required to ensure that cycling is truly reversible. Identifying andunderstanding the reaction pathways that influence reversibility via acombination of spectroscopy, microscopy, and first-principles simula-tions is a worthy effort that the scientific community should undertake.

A high positive-electrode capacity needs to be achieved for aLi/air cell to achieve a high specific energy and energy density.—Conventional positive-electrode materials like LiMn2O4, LiFePO4,61

or LiNi1/3Co1/3Mn1/3O2 have maximum capacities of 100 to200 mAh/g. A practical sulfur-electrode (elemental sulfur dispensedin a carbon or polymer matrix) reaches 400 to 900 mAh/g62,63 referredto the mass of carbon, binder and sulfur. The experimentally observedcapacity range for Li/air gas diffusion electrodes is about 600 to5000 mAh/g (the typical range of loadings is 3 to 6 mg/cm2)depending on the electrolyte, carbon, binder, carrier material, andoxygen partial pressure.21,22,31 The capacity is referred to the total




mass of the carbon, binder, and additives in the cathode. This largecapacity range shows that many parameters may influence the specificenergy of a Li/air cell. Therefore the cathode has to be understoodin terms of maximum capacity and relevant mechanisms that limitthe capacity. Three major capacity-limiting issues, passivation, poreblockage, and O2 transport limitations, are discussed in the literatureand are assessed critically in the following sections.However, beforemoving to those capacity-limiting issueswe stress

that in order for a Li/air cell to achieve a high energy the cell, at the endof discharge, needs to have a high volume fraction of active material(whether it be Li2O, Li2O2, or LiOH ·H2O). Reporting the capacity inmAh/g carbon, mAh/g cathode, or even mAh/cm2 somewhat obscuresthis extremely important point. Unlike standard Li-ion cells where theactive material is built directly into the electrodes with a volumefraction of around 50 to 75%, in a Li/air cell the active materialis essentially being synthesized during the discharge process. Whilemany authors report the capacity in mAh/g carbon, we emphasize thatcarbon is not the true activematerial in the Li/air cell, as Table I shows.To enable a reader to calculate the volume fraction of active materialin an electrode or cell at the end of discharge it is important to includethe information necessary to make that calculation. For example, if thecapacity is reported in mAh/g carbon, an author should also providethe carbon loading (in mg-carbon/cm2) and the electrode thickness toallow the calculation of mAh/cm3, and from that the volume fractionof active material.Passivation by insulating discharge products appears to limit thecapacity.—Recent flat-electrode experiments and Li/air cell model-ing indicate that passivation of the electrode surface by electroni-cally insulating discharge products severely limits the capacity ofLi/air cells even at low rates of discharge, although the results maydepend on the electrolyte used and the discharge rate.23 In partic-ular, Figure 11 shows the rapid passivation that occurs during dis-charge on a flat glassy-carbon surface. The maximum thickness ob-tained for the discharge product is less than 100 nm. The cell inthis case used a carbonate solvent; hence, the discharge reactionpicture is complicated by the fact that an array of discharge prod-ucts are generated. Researchers carrying out rotating-disk electrodeexperiments have also commented on passivation.25 This limitationmay be general because the desired reaction product in the non-aqueous Li/air cell, Li2O2, has the electronic properties of an in-sulator (to be more specific, bulk Li2O2 is insulating,5,64 although it

3.0

2.8

2.6

2.4

2.2

2.0

Cel

l pot

entia

l (V

)

10x10-3

86420

Capacity (mAh/cm2)

706050403020100Thickness of discharge products (nm)

0.75 μA/cm2 (~12.2 hr)

1.50 uA/cm2 (~ 3.4 hr)

3.76 μA/cm2 (~1.0 hr)

Fit to 0.75 μA/cm2 data

Initial open-circuit potential

Highkineticresistance

Exponentialresistancerise

Figure 11. Demonstration of the passivation of a flat-electrode surface duringa discharge on glassy carbon. Reproduced from reference 23.

is possible that its surface states and grain boundaries may be moreconductive).We stress that besides Li2O2, other reaction products such as Li2O,

Li2CO3 and Li-organic compoundsmay appear depending on the elec-trolyte system. The mechanism of electrical passivation in noncarbon-ate electrolyte systems is at an early stage of investigation.52

The impedance rise associated with passivation leads to an in-creasing overpotential that may end the discharge before the availablepore volume is even moderately filled. It will therefore be necessaryto understand the detailed growth mechanism of the desired reactionproduct, which is not yet well understood. Defects or grain boundariesin the Li2O2 crystal could result in enhanced electronic conduction tothe Li2O2/electrolyte interface, or diffusion of Li and O through thefilm could enable growth from the electrode/Li2O2 interface. Theoreti-cal computations of electronic conductivity through the film have beenlimited to the consideration of fully dense, monocrystalline films.23,65

More detailed characterization of Li2O2 films grown on porous elec-trodes bathed in noncarbonate electrolytes, using a combination ofimaging (e.g., SEM, TEM, AFM) and electrochemical techniques(e.g., use of ferrocene couples, GITT), is required to elucidate theLi2O2 growth mechanism. Depending upon the results, models thatincorporate grain boundaries and/or defect chemistry could be devel-oped to improve our understanding of Li2O2 formation and growth.Pore blocking may also restrict the practical capacity.—Besides directpassivation of the electrochemically active surface, blockage ofmicro-pores and some mesopores by Li oxides or other products formed atthe beginning of discharge can limit the accessibility of some electrodesurface for electrochemical reaction. Figure 12 shows schematicallyhow Li2O2 growth on the surface of carbon in an electrode with re-stricted pores could result in either passivation or pore blocking. Bothphenomena result in unused pore volume and hence limit the dischargecapacity.In electrodes where passivation does not occur, pore blocking may

be the mechanism that most severely limits capacity. To overcomepore blocking the selection of a suitable carbon material with a suf-ficiently large pore diameters to allow the entire electrochemicallyactive surface to react is important.In order to generate innovative designs that prevent air electrode

passivation and pore blocking one needs to understand the chem-istry of the reaction process (discussed in Section 2.1) and how thecarbon/air electrodemicrostructure affects the discharge performance.Hence, it is necessary to characterize the porous network of the real gasdiffusion electrode and relate it to its electrochemical performance. Inthe literature N2-adsorption measurements (BET66) are applied for theselection of suitable carbonmaterials. Information about pore size dis-tribution, pore volume, and the available surface area can be related tostate-of-charge dependent impedance measurement data (EIS).67 Thishas been used to track the electrical passivation and relate the behaviorto structural properties of the carbon material determined by BET. Formesocellular carbon foam a capacity increase by 40% compared toother carbon blackmaterials has been achieved recently.68 The authors

Figure 12. Schematic illustration of the pore filling during discharge. Thegrowing Li2O2 layer leads to cathode passivation by electrical isolation (topright) and pore blocking (bottom right).




attribute the enhanced performance to their large pore volumes andultra-large mesoporous structures, which allow more lithium oxidedeposition during discharge.It should be possible to explore in a quantitative way the degree

of pore blocking that develops. For example, an electrode may beharvested, its porosity checked, and a Gurley test conducted to deter-mine the resistance to flow through the porous structure for a definedpressure drop. In addition, a careful design of experiments that sep-arates the influence of total surface area, overall porosity, pore size,and pore size distribution would be helpful to truly quantify the de-gree of pore blocking that occurs. Such experiments should also takeinto account differences in electrode wetting among different poredesigns. Coupled with flat-electrode experiments that are useful forassessing passivation, such a study may help separate the effects ofpore blocking and passivation, which may have a similar overall effecton electrochemical performance.Oxygen transport limitations may arise in flooded electrodes; gaschannels will improve oxygen transport; 3-phase percolation isimportant.—Oxygen transport limitations are expected to become im-portant even at low current densities (i.e., <1 mA/cm2, although theprecise value depends on the electrode thickness and other parame-ters) in fully flooded Li/air cells because the solubility of oxygen intypical nonaqueous electrolytes is less than 5 mM, and thus at leasta factor of 20 lower than the typical concentration of the salt in anonaqueous electrolyte. The diffusion coefficient of oxygen is prob-ably somewhat higher than that of typical electrolyte salts, althoughrigorous measurements are not available.21,69

While optimizing the composition of the electrolyte involves avery large permutation of salts, solvents, and additives, the ability ofan electrolyte to dissolve oxygen can be investigated systematically.Read et al. measured the ionic conductivity σ, the viscosity η andthe Bunsen coefficient α of various electrolytes and compared thesevalues to the capacities obtained from cycling experiments of lithiumhalf-cells.20,21 Furthermore they tested the influence of the oxygenpartial pressure on the cell capacity. With respect to each parameter,the cell capacity increases and then saturates as shown in Figure 13,

although the relationship is notmonotonic. At low discharge rates (i.e.,the lines labeled f with a current density of 0.05mA/cm2), the capacityreaches its maximum at relatively low values of the inverse viscosity1/η and the Bunsen coefficient α, while at high rates (i.e., the lineslabeled a with a current density of 0.5 mA/cm2) the capacity increasessteadily over the measured range (Figure 13a and 13b). The viscosityand Bunsen coefficient influence oxygen transport in the electrolyte,which becomes more critical at higher rates. Similarly, the capac-ity plateau shifts to higher oxygen partial pressures if the dischargerate increases (Figure 13c). The plateau’s position is related to therate-dependent saturation concentration of oxygen in the respectiveelectrolyte.While the oxygen partial pressure, and the electrolyte’s oxygen

solubility and diffusion coefficient, have a strong influence on thehigh-rate capacity, the fact that the low-rate capacity saturates atless than 2000 mAh/g carbon implies that O2 and Li+ transport inthe electrolyte do not ultimately limit the discharge capacity. How-ever, if passivation or other limitations can be avoided, selecting elec-trolytes with high oxygen solubility, low viscosity, and high conduc-tivity, and using a high oxygen partial pressure may afford higherrate capability. The work of Read et al. suggests that ether-basedelectrolytes and solvent blends made of propylene carbonate andtris(2,2,2-trifluoroethyl) phosphate possess favorable properties re-lated to oxygen transport.70,71

To further enhance oxygen transport, continuous gas-diffusionpaths could be created within the porous framework of the airelectrode.72,73 Adjusting the O2 partial pressure slightly above am-bient pressure may be sufficient to force the electrolyte into wettingpores and create a desirable 3-phase percolation throughout the airelectrode. This can also be achieved by employing suitable combi-nations of binders, carbons, and current collector grids, as well asby fabricating the composite electrode with a particular pore-sizedistribution.74 We performed measurements under systematic varia-tion of the amount of electrolyte in the cell, which suggest that thecell performance goes through a maximum with the amount of elec-trolyte. While too much electrolyte impedes the supply of oxygen

Figure 13. Specific cell capacity (per g carbon) depending on four different electrolyte parameters at various discharge rates: (a) Electrolyte viscosity η; (b)Bunsen coefficient α which represents the oxygen solubility; (c) oxygen partial pressure; (d) ionic conductivity σ. The letters a-f at the individual curves indicatethe discharge rate: a) 0.5, b) 0.4, c) 0.3, d) 0.2, e) 0.1, f) 0.05 mA/cm2. Figures reproduced from reference 21.




3.0

2.8

2.6

2.4

2.2

2.0

1.8

Cel

l pot

entia

l (V

)

With O2 transport limits,with passivation

With O2 transport limits,without passivation

Without O2 transport limits,with passivation

~20,500 mAh/g Super P carbon


(a)

3.0

2.8

2.6

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)

8006004002000

Specific capacity (mAh/g Super P carbon)

With O2 transport limits,with passivation

With O2 transport limits,without passivation

Without O2 transport limits,with passivation

~7,350 mAh/g Super P carbon


(b)

Figure 14. Simulation-only results demonstrating the relative impacts of elim-inating oxygen transport limitations and eliminating the electronic resistance ofthe discharge products. (a) shows results at a current density of 0.08 mA/cm2;the two simulations with passivation superpose. (b) shows results at a currentdensity of 0.47 mA/cm2. Caption and figure reproduced from reference 23.

gas, too little electrolyte results in poor effective ionic conductivityand diffusivity. If, as described in the introduction, a Li/air systemis designed with a pressurized oxygen tank and the stack is exposeddirectly to those high pressures, the quantity of oxygen in the elec-trolyte of a flooded cell would increase significantly, which wouldhave the effect of minimizing oxygen transport limitations. In otherwords, running the stack at a high oxygen pressure could have thepractical effect of increasing the capacity at a given current density,or moving to the right in Figure 13c.Albertus et al. carried out modeling work to assess the principal

limits on the capacity of a Li/air cell. While the experimental systemon which their model was based contains carbonates, they neverthe-less concluded that electrical passivation by the discharge product isa key limitation in nonaqueous systems, as both Li2O and Li2O2 areelectrically resistive and have a minimal solubility in typical nonaque-ous electrolytes. Figure 14 shows the effect of electrical passivationby the discharge product and oxygen transport limitations for a non-aqueous Li/air cell. The figure shows that at low current densities(<0.5 mA/cm2) removing the passivation limitation leads to a moresignificant increase in capacity than removing oxygen transport limi-tations. At current densities at and above about 0.5 mA/cm2 oxygentransport limitations are important for the particular cell design andassumptions used by the authors, but again the removal of passiva-tion results in a more significant increase in capacity (i.e., removingoxygen transport limitations while keeping passivation increases thecapacity by less than a factor of two; removing passivation whilekeeping oxygen transport limitations increases the capacity by abouta factor of 30). This result demonstrates that while efforts to improvegas flow channels can make notable improvements to the capacity thatcan be achieved by nonaqueous Li/air cells, if the discharge product

is electrically insulating and remains at the reaction site that will bethe principal capacity-limiting mechanism.Enhancing Li2O2 solubility could enable non-passivating Li2O2

precipitation.—Passivation of the electrode surface occurs becausebulk Li2O2 is electrically insulating and is insoluble in the elec-trolyte. Enhancing the solubility of Li2O2 could potentially enablenucleation and precipitation of solid Li2O2 away from reactive sitesin the air electrode. PolyPlus has proposed the use of noncarbon-ate solvents with limited Li2O2 solubility such as ethylene gly-col and dimethylformamide, which could be enabled through theuse of a protected lithium anode.75 Researchers have also proposedthe use of boron-based anion receptor additives to significantly en-hance the solubility of Li2O2,76,77 some of which may assist passi-vation of the negative electrode at low potentials.77 However, thesestudies have mainly been carried out in carbonate solvents, and onemust be careful to distinguish between Li2O2 solubility in and reac-tivity with the solvent.Novel designs may address passivation and improve capacity.—Toovercome capacity limitations related to passivation, it may be neces-sary to find radically new solutions that address the specific propertiesof the Li/air system. Very recent work indicates that tailored electrodestructures,78 surface treatments,35,79 introduction of defects into thedischarge product,5 and elevating the operating temperature may leadto higher discharge capacities without passivation.Mitchell et al. recently described an electrode consisting of a hol-

low carbon-nanofiber “carpet” grown via chemical vapor depositionon porous anodized alumina coated with thin Ta and Fe layers.78

High discharge capacities (7200 mAh/g carbon at 63 mA/g carbon)were obtained, and the Li2O2 discharge product, identified by XRD,grew as nodules on the fibers and developed into toroids (up to1 μm) over the course of discharge before eventually forming amonolithic mass (see Figure 15). It should be noted, however, thatthe carbon loading was rather low (∼0.1 mg/cm2), and that the ef-fective low-rate capacity (0.7 mAh/cm2) is roughly a factor of 2–10lower than in Li-ion cells. Additional work with higher carbon load-ings would be very illuminating. Similar air electrode designs mightbe realized using carbon nanotubes grown on catalyst particles (e.g.,nickel) which are placed on the electrode surface. Alternatively onecould grow metal nanowires by chemical vapor deposition80 or elec-trochemically. We also note that a systematic study of the morphologyof Li2O2 that forms during discharge, including as a function of sub-strate type, current density, and electrolyte, is a high-impact area forresearch.Surface treatment (e.g., hydrophobization35) of the electrode sur-

face may also be an appropriate method for mitigating passiva-tion. For example, C. Tran et al. recently demonstrated that sur-face treatment of the carbon material leads to an enhanced capac-ity and a discharge curve that implies diminished passivation (seeFigure 16).79 In our own experiments on electrodes with a hydropho-bized carbon surface, we found that the oxygen reduction productsform small crystallites on the hydrophobized surface instead of adense film. Open pores between the crystallites may ensure chargeand mass transfer to the reaction site. Nevertheless, conclusive ev-idence of this coating-passivation correlation is still lacking in theliterature.Besides sophisticated air electrode geometries it may be possible

to increase the conductivity of the reaction product by doping. Hum-melshøj et al. predict by DFT calculation of the Li2O2 density of statesthat the introduction of Li-defects (vacancies) increases the densityof states around the Fermi energy.5 The calculated density of statesfor pure Li2O2 and Li2O2 with Li vacancies is shown in Figure 17.If one can find a way to create Li-vacancies in-situ during dischargethe maximum capacity could be increased and the subsequent chargeis believed to be facilitated. Similarly, introduction of dopants dur-ing discharge could facilitate continuous Li2O2 growth. A practicalsolution for achieving such defects is unavailable.Another possibility to make the discharge product more conduc-

tive is to operate the Li/air cell at elevated temperatures (e.g., 60◦C)because the conductivity of the insulating or semiconducting dis-




Figure 15. Evolution of Li2Ox discharge product morphology on carbon nanofibers (electrolyte = 0.1 M LiClO4 in DME). Insets show the correspondingdischarge voltage profile. (a–b) Galvanostatic discharge to a capacity of 350 mAh/g carbon at 68 mA/g carbon. Li2O2 particles appear to first form on the carbonnanofiber sidewalls as small spheres with ≤100 nm diameters. (c–d) Intermediate galvanostatic discharge to 1880 mAh/g carbon at 64 mA/g carbon. Particlesappear to develop a toroidal shape as the average particle size increases to 400 nm. (e–f) Full discharge to 7200 mAh/g carbon at 63 mA/g carbon. The discreteparticles merge to form a monolithic Li2O2 mass with low porosity. Caption and figure reproduced from reference 78.

charge product increases with temperature. First measurements in ourlaboratories indicated an increase of the discharge capacity by 50%compared to room temperature. For certain electrolytes a significantly(∼500 mV) lower charge potential was also observed. While O2 sol-ubility and transport of both O2 and Li+ would also be improved,operating Li/air cells at elevated temperature could make the searchfor a stable electrolyte even more challenging.Outlook: Overcoming product resistivity and transport limitations isthe key to achieving high-energy nonaqueous Li/air cells.—Under-standing the morphology and conductivity of the discharge productin Li/air cells will help pave the way to high-capacity nonaqueousLi/air batteries. Experimental measurements of film conductance, us-ing both flat and roughened substrates, as well as first-principles and

Figure 16. Discharge curve for uncoated and coated carbon in a gas diffusionelectrode. The different slopes, indicated by the black lines, suggest that thecoating mitigates electrical passivation, leading to a larger discharge capacity.Reproduced from reference 79.

continuum scale modeling of film conductance and growth are newand open areas of research that could help improve this understand-ing. In addition, there is currently no systematic understanding of thegrowth process of Li2O2 and the morphology of particles. This is akey area for exploration.The use of solubilising electrolyte solvents or additivesmay reduce

or eliminate the product resistivity problem, while the development ofnovel electrode architectures could enable high pore volume utiliza-tion by avoiding or in spite of passivation. Appropriate pore structuresare required to avoid pore blockage, and a combination of wetting

Figure 17. Calculated density of states for (a) pure Li2O2 and (b) Li2O2 witha concentration of 1/16 Li vacancies. The black curve shows the DFT single-particle spectrum and the red curve shows the GW quasiparticle spectrum. In(a), the top of the valence bands have been aligned and in (b) the Fermi levelshave been aligned. Reproduced from reference 5.




Li2O2 cell before dis charge

P os itive electrode (70 vol. % gas )

L i metal (w/ 20% exces s )

Li2O2 cell after dis charge

P os itive electrode (65 vol. % Li2O 2)

20% exces s L i metal

Li/LiMO2 cell before dis charge

P os itive electrode (65 vol % MO 2)

L i metal (w/ 20% exces s )

Li /LiMO2 cell after dis charge

P os itive electrode (65 vol. % LiMO 2)

20% exces s L i metal

LP S L

LP S L

LP S L

LP S L

G DL

G DL

C C C C C C C C

C C C C C C C C

Figure 18. Schematic showing the significant amount of volume change that occurs when Li metal cells are discharged. The cell on the left has a high-capacityLi/air active material (Li2O2) while the cell on the right has a lower-capacity intercalation active material (LiMO2). LPSL = Lithium Protection Separator Layer.CC = current collector. GDL = gas diffusion layer. The layer thicknesses are drawn in proprtion.

and nonwetting pores may enable high transport rates for both lithiumions and oxygen to the reaction sites during discharge. We reiteratethat it is important to report directly the volume fraction of productin the discharged electrode (or cell), or provide sufficient informationfor the reader to do so. Reporting only the capacity in mAh/g carbonis not enough to determine whether an experimental cell can actuallyachieve a high capacity and energy. However, reporting the mAh/cm2

and the electrode thickness is sufficient.

Significant volume changes in Li/air cells need to beaccommodated.— In a Li/air cell both the Li metal electrode andthe positive electrode have materials that undergo significant volumechanges. In earlier sections we discussed the formation of new phasesin the positive electrode, and in the section on aqueous Li/air cells wewill discuss the formation of a solid LiOH ·H2O phase. However, inthis section we focus on quantifying the magnitude of volume changesfor a cell with the parameters given in Table II, and then critically re-viewing solutions that help manage those significant volume changes.Volume changes in Li/air cells are particularly challenging becausethe cell sandwich accumulates mass (oxygen) during discharge andreleases mass during charge. Note that it is important to distinguishbetween electrodes that undergo significant volume change and a cellthat undergoes significant volume change. Balanced volume changesat each electrode are possible if the density of the products and reac-tants are matched, but this is not true for the active materials alonein the case of Li/air cells. We consider the general topic of reversiblyaccommodating major volume changes in solid systems to be a high-impact area for research.The high capacity of the cathode materials and the formation of newphases make volume changes significant.—A cell with a Li metalnegative electrode undergoes significant changes in the thickness ofthe cell sandwich during a cycle, provided a significant amount ofthe Li metal is actually cycled. Volume changes are a feature of anymetal electrode in which cycling involves plating/striping, but areparticularly significant in the case of Li metal because the density ofLi metal is so low (0.534 g/cm3 at 25◦C). Remarkably, the density ofthe Li (in mol Li/cm3, which is directly proportional to the capacitydensity in mAh/cm3) stored in Li2O and Li2O2 is significantly higherthan in pure Li metal, as shown in Table I. Indeed, the Li/air cell isan interesting case of a cell that increases in mass and decreases involume during discharge. In a Li metal cell with a positive electrodethat intercalates Li and has minor volume changes (<15% in manypositive-electrode intercalation materials), the Li metal is the only

region of the cell undergoing significant volume changes. However, ina Li/air cell the discharge process involves the creation of a new phaserather than the incorporation of Li atoms into a pre-existing phase.The creation of a new phase in Li/air cells implies that electrolytedisplacement will occur unless the new phase displaces a gas phase.Figure 18 shows a diagram of the volume changes in a Li2O2 anda Li/LiMO2 cell for the cell specifications given in Table II; for theLi2O2 cell it is assumed that the positive electrode begins with avolume fraction of gas of 0.70 that is pushed out of the cell sandwichduring the discharge process by the Li2O2 that forms.A more quantitative depiction of volume change is given in

Figure 19. The results in this figure depend strongly on assump-tions about the initial components in the positive electrode, partic-ularly on the quantity of electrolyte vs. gas phase in the chargedcell. As discussed above, for our “practical” nonaqueous Li/air cellswe assume the active materials that are produced displace a gasphase rather than electrolyte, while in the aqueous cases we in-clude an initial reservoir of H2O. The results in Figure 19 showthat the volume change is more significant for Li2O than Li2O2

1.0

0.8

0.6

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al /

initi

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olum

e


Cell sandwich alone Including H2O reservoir

Figure 19. Plot showing the volume change of fourLi/air cells and aLi/LiMO2cell. LiOHhas not been observed to precipitate and is included only for compar-ison. Volume change is calculated as discharged volume / charged volume. TheLiOH ·H2O case shows the volume change when an H2O reservoir is excludedand included (it is assumed the H2O is stored outside the cell sandwich).




because of the higher capacity density of Li2O. The volume changesfor the LiOH ·H2O, LiOH,and Li/LiMO2 cases are similar. The vol-ume change for LiOH ·H2O and LiOH are smaller than the Li2O andLi2O2 cases because their capacity density is lower than that of Li2O orLi2O2 (see Table I) and therefore less Li is required. In the LiOH ·H2Oand LiOH cases, when taking into account the volume of water in areservoir in a charged cell the overall volume change of the cell islarger. Note that we exclude the additional volume from packaging inFigures 18 and 19.Pressure should be applied to cells changing volume to maintain goodcontact.—Given the significant volume changes that occur in Li-metalcells it is important to maintain good contact between the layers of thecell sandwich in order to prevent significant contact resistances fromarising. In particular, maintaining a good solid-solid contact betweenLi metal and the lithium protection separator layer is important, asthere will be a great deal of lithium transport across that interface.Application of external pressure on the cell, depending on its design,may alleviate this variability.62 Another way of dealing with this issueis to use a block copolymer that maintains good adhesion through theability of the conducting block to flow.81

Minimizing changes in contact resistance may prove more diffi-cult for wound cells. Establishing relationships between mechanicaldesign and cell performance and durability will become an importantarea of research after fundamental chemical challenges are overcome.It is important to quantify the evolution of contact resistances withcycling and the influence of pressure on this evolution.Volume change can be accommodated through the use of a flexi-ble seal.—How can cell packaging respond to significant volumechanges? One promising idea for dealing with volume change is touse a flexible seal that allows the cell sandwich to move without pro-viding significant stress on the cell packaging. Such a sealing methodhas been outlined by the PolyPlus battery company in a patent.82 Thepatent outlines a method to maintain good ionic, mechanical, andelectrical connectivity between a Li metal electrode and both the Limetal backplane and a Li protection layer. However, this approach hasso far been applied only to planar double-sandwich cells. Wound andstacked cells may require more intricate packaging.Volume change can be accommodated through the use of an electrolytereservoir.—If a Li/air cell is discharged to the point that the activematerial being formed (e.g., Li2O2) displaces all of the gas volume, itwill begin to move electrolyte within or out of the cell sandwich. Ifthe electrolyte remains within the cell sandwich and is pushed towardthe Li metal, it may lead to poor contact between the Li metal andlithium protection separator layer or between the lithium protectionseparator layer and the positive electrode. Alternatively, it may bedrawn off in a direction perpendicular to the direction of current flow.Pollard and Newman treated an electrolyte reservoir in the context ofthe Lithium-Aluminum, Iron Sulfide cell.83

Outlook: The operating principle and durability of cells that repeat-edly undergo significant volume changes have to be evaluated.—Ac-commodating the significant volume changes in both the Li metalelectrode and within the positive electrode in a highly reversible man-ner is one of the principal challenges for the successful commercial-ization of Li/air cells. Most Li/air cells demonstrated thus far havecycled only a small amount of Li, although primary cells have dis-charged significant thicknesses.9,14, 84 While there are promising ideasfor maintaining good seals and contact between cell layers, this is anarea in which major progress will need to be made.

Li metal electrodes are chemically and morphologically un-stable in most electrolytes and must be protected.— Reversible chargestorage in the Li metal electrode is accomplished by plating (deposit-ing) and stripping (dissolving) Li on/from the surface of the electrode.This deposition-dissolution reaction can be simply modeled by anappropriate kinetic expression, such as the Butler-Volmer equation.However, two factors complicate the process. First, the nonaqueousliquid electrolyte commonly used in Li metal cells is unstable at thepotential of Li, and a passivating film (the solid electrolyte interphase,

or SEI) must form at the surface in order to stabilize the interface. TheSEI imparts additional ohmic and mass-transfer resistance to dissolu-tion and deposition of the metal. Second, the plating and stripping ofLi inherently involves change in volume of the active material. Thisvolume change often involves roughening of the Li surface, evolu-tion of the metal morphology (i.e., formation of grain boundaries),and macroscopic shape change. In addition, the contact resistance be-tween the Li and the electrolyte or separator may change dependingon the direction of current, electrode state of charge, age of the cell,and pressure applied to the cell sandwich. Although SEI formationallows for research on nonaqueous Li/air cells without ex-situ appli-cation of protection layers, SEIs are not robust against volume andsurface changes of the Li anode.85 Hence, formation of an SEI shouldbe inhibited by using a solid electrolyte separator that is stable againstLi. This solid electrolyte should also provide mechanical resistance toLi morphology development and roughening.We note that despite decades of interest and development, thus far

there has been no successful andwidespread introduction of secondaryLi-metal cells into the market. Research on methods to enable the useof Li metal in secondary cells remains a high priority.It is challenging to form stable SEIs on Li metal.—Li is an alkalinemetal and hence quite electropositive, chemically reactive, and rathersusceptible to oxidation. This property, which makes it very attractivefor use as a negative electrode, also makes it prone to react with othercomponents in the cell. Although passivation layers on Li were pre-viously examined in aqueous16,17,86, 87 and nonaqueous88 electrolytes,Peled coined the now popular term “Solid Electrolyte Interphase”(SEI) for the layer that forms on the surface of Li metal via decom-position of the electrolyte,89 which is unstable at the Li potential.Comprehensive reviews of the SEI, which forms on lithium, graphite,and other Li-insertion anodes, include those of Aurbach,90 Balbuenaand Wang,91 Winter et al.,85 and Ogumi and Inaba.92

The SEI on Li differs significantly from that on graphite in that itforms immediately upon contact between the electrode and electrolyte.Hence, it is much more challenging to control the chemistry andmorphology of the SEI during formation on Li. Rather, the electrolytesalt, solvents, and additives must be selected carefully to achieve ahighly stable SEI.Evolution of the SEI on Li is also very different from that on

graphite due to the dramatic volume change of the electrode. De-pending on the chemistry, temperature, mechanical design of the cell,and cycling conditions, the Li-metal SEI may evolve more or lessdramatically along with morphology changes in the underlying metal(see Figure 20). If films are not elastic enough to accommodate thevolume change of the active material, fracture and reformation orthickening of the film may occur as the cell is cycled.93 Spotnitz mod-eled SEI growth on graphite using an empirical relationship that takesinto account the rate-dependence of film fracture.94 However, detailedmodeling of the SEI during evolution of the Li metal morphology is achallenging endeavor that has apparently not yet been undertaken.Dendrites and morphology changes limit cyclability.—The most com-mon failure mode for cells with Li metal anodes is that of dendritegrowth and increase in electrode surface area. Needle-like dendritescan grow through the separator during charging of the cell, resultingin an internal short. “Soft shorts” that burn out rapidly result in a tem-porary self discharge of the cell, while “strong shorts” consisting of ahigher, more stable contact area can lead to complete discharge of thecell, cell failure, and possibly thermal runaway. While dendrites growthrough the separator during charge, shorts can sometimes developduring discharge depending on the external pressure placed on thecell and/or internal volume changes that occur in both the negativeand positive electrodes.Because Li metal is highly electronically conductive, surfaces tend

to roughen as the metal is plated and stripped. Peaks in the surfacegrow as dendrites during charge; while the surface is smoothed duringdischarge, some roughness typically remains at the end of discharge,and, depending on the depth of discharge, the overall roughness can beamplified from one cycle to the next. Because the metal is essentially




Figure 20. Illustration of differences in SEI formation and evolution on the surfaces of (a) graphite and (b) metal (Li or Li-alloys). Reproduced from reference 85.

at the same electrochemical potential throughout, potential and, to alesser extent, concentration gradients in the electrolyte phase drive thechange in morphology. Previous Li dendrite growth modeling workhas shown that themoving front of a dendrite tends to accelerate duringcell charge due to the higher current density localized at the dendritetip relative to its base.95 Application of thermodynamic models hasshown that dendrite initiation (i.e., initial roughening of an almostperfectly smooth surface) can be suppressed by applying mechanicalstress and selecting solid electrolytes with shear moduli on the orderof 10 GPa at room temperature.96,97 The same models indicate thatsurface tension at metal-fluid interfaces is insufficient to suppressdendrite initiation.96,97

Related to dendrite initiation and growth is development of theLi morphology, which tends to increase the electrode surface areawith cycling and consumes solvent to generate fresh passivationlayers. Formation of high-surface-area mossy Li tends to occur dur-ing low-rate deposition from a liquid electrolyte, especially if the saltconcentration is high.98 The high surface area combined with highreactivity of Li and flammability of the organic solvent makes for avery reactive and dangerous cell.Sion Power has reported a diminishing grain size in its Li/S cells

without application of external pressure to the cell. This results ina gradual increase in the volume of the anode as the cell is cycled.Moderate pressures (∼10 bar) tend tomitigate this type ofmorphologydevelopment (see Figure 21). Sion Power also proposed a strategy tominimize the surface morphology changes in the Li anode, whichconsists of ensuring complete stripping and re-plating of all lithiumon the rigid current collector in an anode-capacity-limited cell.62

Modeling needs in the area of morphology change include modelsof grain nucleation and growth during Li deposition, which includethe influence of pressure; cell-level aging models that account forsolvent consumption at the anode; and cell-level electrochemome-

Figure 21. SEM images of Li foil anodes, cycled 50 times, (a) with and(b) without application of 10 kg/m2 nonisotropic pressure. Reproduced fromreference 62.

chanical models that account for gradual swelling of the anode withcycling.Alloying Li with other elements such as Mg may help elevate

the surface energy and reduce morphology changes,99,100 and morestudies are needed in this direction.Shape change can occur due to nonuniform current densities.—Pro-vided that Li dendrites and other forms of microscopic morphologydevelopment can be suppressed, macroscopic changes in the shape ofthe anode may occur due to nonuniform current density distributionthroughout the cell. When there is high resistance to electronic currentflow from the tab to the outermost edges of the current collector (e.g.,if the current collector thickness is too small relative to its area), thecurrent density may be very nonuniform, with a higher current den-sity near the tabs. Hence, Li will deposit and dissolve preferentiallynear the tabs, as depicted conceptually in Figure 22. Application ofpressure may alleviate this problem.Li-conducting solid electrolytes can provide chemical and mechanicalprotection of Li metal.—Because of the enormous challenge involvedin stabilizing the Li surface chemically and mechanically throughthe use of electrolyte additives, such that passivation remains in ef-fect over hundreds to thousands of cycles, the preferred treatment forrechargeable Li-based cells is the use of a solid-electrolyte membranethat is mechanically robust and chemically stable against both elec-trodes. Such a barrier removes several simultaneous constraints thatthe liquid electrolyte otherwise must satisfy, but the requirements forits properties are nonethelessmultifaceted and challenging to obtain ina single material. The barrier must be chemically stable with respect tosome or all of the following: the liquid electrolyte in the positive elec-trode, electronic conductors and catalysts in the positive electrode,the metallic Li negative electrode, reactive species such as oxygenmolecules and reaction intermediates, and (in aqueous cells) water.Solid electrolytes must also have sufficient Li+ conductivity over theoperating temperature range of the cell, negligible electronic con-ductivity, and high elastic modulus to prevent Li dendrite initiation.In order to provide cheap, robust, lightweight protection, a methodmust be developed to produce relatively thin (< 50 μm), pinhole-freesolid-electrolyte layers at a reasonable cost.

Li metal

Separator

CathodeCathode

Cu tab

Al tab

Li metal

CathodeCathode

Cu tab

Al tab

1000 cyclesSeparatorSepSepSepSepSepSepepaaaaaaaarararaararararaaaaaaattttttorororororrror

Li metal

Separator

CathodeCathode

Cu tab

Al tab

Li metal

CathodeCathode

Cu tab

Al tab

1000 cyclesSepepSepSepepSepepeSSSSS aaaaaaaararararararar tttttttororororororooSeparator

Figure 22. Depiction of macroscopic Li redistribution in a cycled Li/air cellat the end of charge.




Figure 23. Ionic conductivities of several classesof solid electrolytes (LLTO = (La,Li)TiO3; LAGP= Li1+xAlxGe2−x(PO4)3; PVdF-HFP = poly(vinylidene fluoride)-hexafluoropropylene; PEO= polyethylene oxide). Reproduced from reference103. Figure numbers refer to those in the originalpaper.

Mechanical stability is another important aspect of Li-metal elec-trode protection that influences the age and safety of the cell. Thetwo primary classes of Li solid electrolytes, inorganic ceramics andsolid organic polymers, have different mechanical properties. Ceram-ics have high elastic moduli and are thus more suitable for rigid flatcell designs. Polymers are softer and more flexible, but at the sametime less chemically and thermally stable, and also susceptible toabsorption and transport of liquids. Flexibility is an advantage dueto the high anode volume change, and in wound designs in whichchanges in the separator curvature could cause fracturing of ceramicmembranes. At the same time, the lower shear moduli of polymersare likely insufficient to prevent Li metal dendrite initiation,97 leadingto safety concerns due to possible electrical shorts. Elasticity may beimportant in order to reduce fracture and tearing after multiple cyclesand to achieve wound cell designs.A number of candidates that satisfy some of the requisite properties

for Li protection have been proposed.10,82, 101–128 For extensive reviewssee references101–103 and references therein.A variety of inorganic compounds (sulfides, oxides, phosphates)

in crystalline, polycrystalline or amorphous morphologies, as wellas solid dense polymer-based materials, have been investigated withconductivities at room temperature ranging from 10−8 to 10−2 S/cm(see Figure 23).10,82, 101–128 Most inorganic crystalline and glass mate-rials have lower conductivities than liquid electrolytes, by at least 1-2orders of magnitude. It is worth mentioning that because Li motionin solid state systems is a thermally activated Arrhenius-type process,conductivity increases with temperature, sometimes by two orders ofmagnitude or more over the range 0 to 200◦C.While operating a Li/aircell at elevated temperature (>80◦C) may increase the rate capabilityand capacity, this presents numerous engineering challenges at thesystem level.Li3N has high conductivity (∼10−3 S/cm at room temperature),

but is unstable at high potentials (>0.445 V vs. Li).101,104 Li3Phas an order of magnitude lower conductivity, but is stable up to2.2 V.101,105 The Li analog to sodium β-alumina, Li2O·11Al2O3, has ahigh room-temperature single-crystal conductivity of 3×10−3 S/cm,but is extremely hygroscopic and challenging to prepare dry.101,106

The so-called Li Super-Ionic CONductors (LiSICONs) are γII-Li3PO4 type oxysalts101,102 (e.g., Li14Zn(GeO4)4107,108) that containinterstitial Li ions, but show a drop in conductivity over time at lowtemperature because of Li trapping by the immobile sublattice via de-fect complex formation.101,109 Room temperature conductivities aregenerally less than 10−4 S/cm.101 Bates and coworkers found that ra-

dio frequency magnetron sputtering of lithium silicates, phosphates,or phosphosilicates resulted in N2 incorporation to form LiPON, anamorphous analog to LiSICON.110 Thin-film batteries with Li anodesand Lithium Phosphate OxyNitride (LiPON) separators have demon-strated thousands of cycles.111,112 However, mechanical stability suf-ficient for long cycle life has not been established in a thick-electrodecell design, and the low conductivity of LiPON (2×10−6 S/cm at25◦C110) precludes the development of cells with thick LiPON mem-branes.Thio-LiSICON, the S analog to LiSICON (e.g., Li4GeS4113), can

achieve high room-temperature conductivity and low activation en-ergy (e.g., 2.2×10−3 S/cm and 20 kJ/mol for Li3.25Ge0.25P0.75S4102).Glass ceramics with structures related to thio-LiSICON exhibit evenbetter performance (3.2×10−3 S/cm, Ea = 12 kJ/mol for 70Li2S-30P2S5102,114). For these systems glass ceramics have roughly an or-der of magnitude higher conductivity at room temperature than theiramorphous counterparts (glasses), although there are several excep-tions to this trend.103 Despite high conductivities (10−3 S/cm at roomtemperature), ceramic films are not easy to fabricate and often havepoor chemical durability.115

Early work on the Li analog to NaSICON, LiM2(PO4)3, led todiscoveries of some materials with high conductivity, but poor chem-ical stability against Li (M = Ti116,117), and others with good sta-bility, but poor conductivity (M = Ge118,119). It was found that Alsubstitution via solid state reactions resulted in NaSICON structures(Li1+xAlxGe2−x(PO4)3, or LAGP) with 4 orders of magnitude higherconductivity.120

Workers at Ohara found that by heat treating glasses theyformed glass ceramics including a NaSICON crystalline phase(Li1+xAlxTi2−x(PO4)3, or LTAP), with a high conductivity up to1.3×10−3 S/cm, depending on heat-treatment temperature.115,121 Notonly is the material stable against Li metal, it is also stable in aque-ous solutions, making it a suitable candidate for aqueous Li/air cells,and was recently shown to be stable in acidic solutions.122 However,this material tends to be difficult to manufacture with large area andlow thickness. The general composition of water-stable glass ceram-ics produced by Ohara and used by PolyPlus in its aqueous Li/airand Li/water cells is Li1+x(M,Al,Ga)x(Ge1-yTiy)2−x(PO4)3, where x<= 0.8, 0< = y< = 1, and M is a member of the Lanthanide series;and/or Li1+x+yQxTi2−xSiyP3−yO12, where 0 < x < = 0.4 and 0 < y <= 0.6, Q = Al or Ga.10,82,121,123The garnet family of ceramics have lower conductivity than

NaSICON-type phosphates and perovskite-type oxides (8×10−4 S/cm




at 300 K124), but they have a lower grain boundary resistance and arestable against Li metal.102 The garnet chemical space is still beingwidely explored.Grain boundaries often limit the conductivity of solid electrolytes;

for instance, perovskites (ABO3) such as Li0.5−3xLa0.5+xTiO3125–128

have bulk conductivities as high as 1×10−3 S/cm, but with grainboundaries the composite conductivity is only 2 × 10−5 S/cm.125,126

The grain boundary contribution to total resistance is commonly sen-sitive to the ceramic composition and depends on foreign phases thatprecipitate at the boundaries. Thus making progress in ionic conduc-tivity of a ceramic polycrystalline material requires understandingoptimization of each contribution separately. To get a clear picture onthe limitations of bulk transport, one needs to synthesize and performtransport measurements on single crystal samples, which is often verychallenging. First-principles computations of hopping barriers andmolecular dynamics offer an alternative route which is proving usefulto understand atomic-level mechanisms.Once the bottlenecks in a given material are identified, it is critical

to engineer the composition and fine-tune the synthesis process tooptimize the microstructure and density. Even more difficult is toachieve these optima for a film that is only 50 μm thick. Thin-filmmanufacturing processes, which vary from sputtering to chemicalvapor deposition, tend to be expensive or result in inhomogeneouscoatings.Multi-layer encapsulation of Li is needed to meet stability andimpedance requirements.—It is unlikely that a single material will bediscovered that satisfies all the requirements for protecting Li metalin Li/air cells. Most Li/air cell designs to date involve the use of inter-layers between Li and the solid-electrolyte membrane that are stableat the Li potential and reduce considerably the contact resistance.PolyPlus has patented a method for producing a thin layer (∼0.2 to 1μm) of Li3N between Li and the water stable NaSICON-type separa-tor (LTAP) supplied by Ohara.123 This interlayer reduces the contactresistance that results from decomposition of the LTAP surface ex-posed to Li. Electricite de France (EDF) has deposited a thin layer(0.5 to 2μm) of LiPON onto LTAPf to prevent degradation of the sep-arator; however, imperfections in the coating allowed reaction of Liwith the LTAP during cycling, ultimately resulting in the formation ofcracks in the separator, and Li morphology development and detach-ment from the separator were observed.129 Nonaqueous electrolyteshave also been used to eliminate contact issues between Li and theseparator.1,27, 130

In its Li/S cells, Sion Power usesmulti-layermembrane assembliesincluding ceramic and polymer components to prevent reaction of theelectrolyte with Li,62 which may contribute to morphology develop-ment and capacity fade. Polymer components may provide sufficientflexibility to achieve wound cell designs, while ceramic componentsmay provide improved stiffness to prevent dendrite initiation.Outlook: A low-resistance multi-layer solid-electrolyte encapsulationof Li metal is required for stable operation and good performance.—We identify three critical issues in Li metal protection: good ionicconductance, chemical stability, and mechanical stability. There isoften a tradeoff between the three properties, depending on the celldesign and positive-electrode electrolyte chemistry. In-situ protectionvia an SEI is unlikely to prevent morphology development duringcycling. Solid-electrolyte membranes that are flexible, strong, thin,pinhole free, easy tomanufacture, and have high conductivities are stillmissing. More experimental and computational work to understandand design such materials is needed in the direction of mechanicalstrength and ionic conductivity. Interlayers between Li metal andthe solid-electrolyte membrane are required to improve the stabilityand decrease the contact resistance of the interface. Application ofpressure may be required to prevent Li metal morphology changesduring cycling.

f While the authors of reference 129 refer to the material as “Lisicon,” their referenceto an article from PolyPlus and the fact that no other Li-protection material besidesOhara’s LTAP is known to be stable in water suggest that this is a misnomer. We shallassume that LTAP was intended.

Cell impedance must be significantly reduced to achieve adequatepower density and efficiency.— In addition to providing high specificenergy and cyclability, the specific power (or power density) andenergy efficiency of the Li/air cell must be acceptable for the intendedapplication. We shall assume that this potentially high-energy systemis mainly used in high-energy applications, such as electric vehicles,portable electronics, stationary storage, and potentially hybridizedapplications, in which the Li/air battery provides the base load whilea second battery or a capacitor provides load leveling.A long-range EV with advanced high-energy batteries can be ex-

pected to travel roughly 387 miles on a single charge. Assuming300 Wh of energy are required to propel the vehicle one mile, andthat only 83% of the battery’s energy is utilized, this range can besatisfied by a 140-kWh battery. If the maximum pulse discharge (i.e.,acceleration) and charge (i.e., regeneration) power is 140 kW, thenthe maximum pulsing rate of the battery should be 1C, while the av-erage discharge rate, assuming a maximum sustained freeway speedof 77 mph, could be as high as C/5. Average charging rates couldbe even higher, but we assume that overnight or partial charging canbe tolerated for drivers demanding long range. A hybrid storage sys-tem, consisting of a high-energy, low-rate battery and a low-energy,high-rate battery or capacitor could limit the high-energy battery raterequirement to the average discharge C rate. It is clear that the requiredpower capability for a long-range battery strongly depends upon thevehicle range, charging strategy, and level of hybridization. However,we can conclude that EV storage systems incapable of achieving∼C/5continuous discharge are uninteresting, and that systems that achieve> 2C pulsing are not required. Hence, the target battery-level specificpower is 140 to 1400W/kg for a 700-Wh/kg battery, depending on thedegree of hybridization. Our cell energy calculations indicate that, toachieve this high specific energy, an area-specific capacity of approx-imately 40 mAh/cm2 (similar to the capacities for Li2O and Li2O2cells showin in Figure 4, and corresponding to 194 μm of cycled Li)is required. Hence, the target current density for Li/air cells should bein the range 8 to 80 mA/cm2.The power density of a cell can be evaluated at the cell-sandwich

level as the product of the voltage (the open-circuit potential,U, minusthe cell overpotential,ηcell) and the current density, i (inA/m2), dividedby the cell-sandwich thickness, Lcs:

P = i (U − ηcell)

Lcs

The specific power is obtained from the power density and theaverage material density, ρ:

P = P

ρ

For a typical Li/air cell, the average material density isapproximately 1 g/mL (0.53 g/mL for Li, ∼3 g/mL for asolid-electrolyte separator,120 8.9 g/mL for the Cu current col-lector, 2.7 g/mL for the Al current collector, ∼1 g/mL forpolymers, water, and nonaqueous electrolytes); by comparison,Li-ion cells (without Li metal) have an average density of∼2 g/mL.The cell overpotential is a function of the current density, cell

design, state of charge, and materials used. As a function of cur-rent density, it is approximately linear at low current density and athigh current density increases exponentially due to increasing mass-transfer limitations. Hence, the optimum power is a balance betweenoverpotential and current density. Requirements of acceptable energyefficiency make this a constrained optimization (see Figure 24). Evenif the cost of energy is not of concern, a lower system efficiency im-plies a higher rate of heat generation during charge and discharge, andtherefore a higher cost, weight, and volume of the thermal manage-ment system.Several phenomena limit the rate capability of Li/air cells.—Whilethe major rate-limiting processes include poor oxygen reduction andevolution reaction (ORR and OER) kinetics and large ohmic drops




0.0

0.2

0.4

0.6

0.8

1.0

P~

cellη

effic

ienc

y

current density0.0

0.2

0.4

0.6

0.8

1.0

P~

cellη

effic

ienc

y

current densityFigure 24. Representative qualitative dependence of overpotential (ηcell),power density (P), and discharge voltage efficiency upon current density. Thevertical dashed line indicates the current density below which the efficiency isat least 80%.

through protection layers on top of Li metal, there are a number ofprocesses that contribute to the poor rate capability and efficiency ofLi/air cells. Impedance contributions in a battery can be classified askinetic, ohmic, or mass-transport limitations. The kinetic impedancefor elementary electron-transfer reactions tends to obey the Butler-Volmer expression, with approximately linear behavior at low surfaceoverpotentials and logarithmic (or Tafel) behavior at high overpoten-tials.However,ORRandOER in theLi/air systemconsist ofmulti-stepmechanisms that exhibit more complex behavior.At the negative electrode, the kinetics of Li deposition and dis-

solution are typically described by a Butler-Volmer expression, withan exchange current density that can depend on the electrolyte. De-pending on the electrolyte, electrode preparation, and measurementtechnique, reported values of the Li exchange current density are gen-erally on the order of 1 to 10mA/cm2.131–133 Assuming charge transferis described by Butler-Volmer kinetics with a symmetry factor of 0.5,these current exchange densities imply that 100 to 250 mV of over-potential will drive a current density of 100 mA/cm2, which is muchhigher than typical Li/air current densities.In aqueousLi/air cells, the kinetics of LiOH ·H2Oprecipitation and

dissolution could limit the achievable specific power, as is discussedin a later section.Ohmic limitations include the high-frequency impedance of the

electrolyte, the ionic resistance of separators or membranes that aresingle-ion conductors, and contact resistances. Nonaqueous elec-trolytes exhibit a conductivity of about 0.01 S/cm (1M LiPF6 inPC/EC/DMC) at 25◦C,134 while aqueous systems exhibit a conductiv-ity of 0.35 S/cm (saturated LiOH in H2O) at 25◦C.7 Li-conductingceramic separators have conductivities of 10−6 to 10−2 S/cm at25◦C.110,115,120,121 Some Li-conducting layers can be made thin(< 10 um)110,111 so as to minimize the overpotential associated withthe separator; however, such thin separators have not been successfullyemployed in Li/air cells. Conduction of electrons through the carbonmatrix in the positive electrode, Li metal, current collectors, and tabscan also contribute to the cell impedance, but these contributions areminor for a well-designed cell.Contact resistances also tend to be ohmic in nature (i.e., with

overpotential proportional to current density). These resistances canappear at a number of different interfaces within the cell, includingweld points, the Li/separator interface, interparticle contacts in thecarbon matrix of the positive electrode, and between the insulatingdischarge product and carbon matrix. Because of the large volume

changes that occur during charge and discharge, some contact re-sistances, especially at the Li/separator interface, may evolve duringcharge/discharge and as a function of cycle number. Likewise, gradualphysical separation of the discharge product from the carbon matrixmay result in significant changes in contact resistance as the cell iscycled.Mass transfer limitations are those that involve concentration gra-

dients resulting from finite diffusivities in multicomponent solutions.For instance, when oxygen is consumed at positive electrode reactionsites during discharge, it must be replenished by oxygen diffusingthrough the liquid electrolyte. Because the solubility of oxygen innonaqueous electrolytes is limited,20,21 even the relatively high oxy-gen diffusivity is insufficient to maintain the reaction site oxygenconcentration near its nominal bulk value in flooded cells dischargedat high rates. Hence, the ORR kinetics may suffer from a diminishedlocal O2 concentration. Limitations on the gas-phase oxygen transportin diffusion media and Li-ion transport in the electrolyte can also im-pact the cell impedance, but these contributions are typically smallerthan that of oxygen transport limitations in the electrolyte in floodedcells.Passivation of the positive electrode by an electronically insulating

discharge product is a complex phenomenon that may involve kinetic,ohmic, and mass-transfer processes. At this stage, the phenomenon isnot well understood, although it has been identified as a main con-tributor to the limited capacity and power capability of nonaqueousLi/air cells. Empirically, the impedance of the discharge product ap-pears to increase exponentially with film thickness, at least in somesolvent-salt combinations.23

In nonaqueous Li/air cells, current densities as high as 1 mA/cm2

have been demonstrated.135 However, the capacity begins to fall offat current densities above about 0.1 mA/cm2, depending on the celldesign and other parameters.23,135 Moreover, most measurements inthe literature have been carried out in carbonate-based solvent sys-tems, which show no evidence of reversible chemistry. There arelimited data available for noncarbonate systems. Passivation of theelectrode surface has the strongest influence on rate capability incarbonate systems.23 There is some evidence that it also plays arole in noncarbonate systems.136 If passivation can be overcomethen the introduction of oxygen gas channels should allow suffi-cient quantities of oxygen into the cell to support any reasonablecurrent density for a Li/air cell, as hydrogen-oxygen fuel cells canoperate at over 1 A/cm2 while much lower current densities are ex-pected for Li/air cells given their significantly higher area-specificimpedance.Aqueous systems, without O2 transport limitations or passivation,

have achieved current densities of 15 mA/cm2 in primary applica-tions with protected lithium electrodes137 and 2 mA/cm2 (at 30◦C)in secondary applications.129 However, these high current densitieshave not been demonstrated over large capacity windows. In pri-mary applications, where cells are designed to discharge over weeksor months, typical average discharge currents and specific powerare 0.5 mA/cm2 and 2 W/kg, respectively.14,84 By way of compar-ison, more than 100 mA/cm2 has been demonstrated for rechargeableZn/air systems using a bifunctional air electrode.138 Likewise, cur-rent densities well over 100 mA/cm2 have been achieved in primaryLi/air cells without protection layers.87 This underscores the lim-itation associated with the water-stable Li protection layer, whichhas a Li conductivity of up to 3.5 × 10−4 S/cm at 25◦C, butdue to its fragility is difficult to manufacture at thicknesses below∼150 μm.139 At 50 mA/cm2, this represents a voltage drop of 2.15 Vthrough the separator.Oxygen reduction and evolution are kinetically slow; the nature ofelectrocatalysis is unclear given that a solid product is generated.—In order to improve air electrode performance and selectively as-sist the formation of the desired reaction products, recent researchhas concentrated on the use of various metal and metal oxide par-ticles in the carbon matrix.22,24,33, 34, 44, 141 Despite these efforts, themolecular-level mechanistic processes that could explain the experi-mental observations (e.g., reduction of overpotentials for the charge




0 0.2 0.4 0.6 0.8 110-5

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10-2

Con

duct

ivity

(S

/cm

)

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150 m100 m

50 m

Separator too heavy

Separator too heavy

0 0.2 0.4 0.6 0.8 110-5

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duct

ivity

(S

/cm

)

C rate

150 m100 m

50 m

Separator too heavy

Separator too heavy Electrolyte

Anode ccCathode cc

Anode

Cathode

Separator

Binder, etc.

Mass breakdown: Total = 1.5963 kg/m2

Figure 25. (Left) The ionic conductivity of the separator required to attain a cell-sandwich specific energy of 1250 Wh/kg at the given C rate and separatorthickness. The conductivity of Ohara’s LTAP is indicated as a dashed line at 3.5e-4 S/cm. For a 150-μm separator, there is a C rate above which no cell design,even if the separator had infinite conductivity, will achieve 1250 Wh/kg because the separator is too heavy. (Right) The mass breakdown by component in thedischarged state is shown for this limit. Assumptions are described in the text. cc = Current Collector.

and discharge reactions and/or extended capacity upon discharge ofthe Li/air cells) are not yet clearly understood. In particular, if asolid, insoluble product forms and remains at the reaction site thecatalyst may be quickly covered, such that the catalytic activity is re-duced. Nevertheless, several reports on catalysis have been published.Debart et al.34 have shown that a cell containingα-MnO2 as the catalystsignificantly increases the discharge capacity of the Li/air cell. The au-thors suggested that the improvement in the capacity of the cell is dueto the presence of a tunneling structure that has the ability to accom-modate in close proximity the Li+ and O2− ions, which in turn leads toa subsequent incorporation of Li+ and O2−2 into a compact film. Suchproximity and incorporation is not possible in other manganese ox-ide materials. More recently, Lu et al.33,142 incorporated bifunctionalcatalysts into Li/air cells. By implementing a nano-structured PtAu/Cbifunctional catalyst into their cells, the authors were able to lower thecharge voltage and raise the discharge voltage, in order to obtain oneof the highest round-trip efficiencies of rechargeable Li/air batteriesreported to date. While there have been reports on catalysis, a recentpaper by McCloskey et al gives compelling evidence that catalysisobserved in carbonate systems only aids electrolyte decomposition,while in DME no catalysis is observed for Au, Pt, or MnO2.177

Thus far, the ORR and OER kinetics have not been well char-acterized for this system, although Xu and Shelton143 have recentlyapplied DFT methods to investigate the Li-based oxygen reductionreaction (Li-ORR) on two different metallic surfaces, Au (111) and Pt(111), and found that Au(111) is the most active surface for Li-ORR.Their results also indicated that on both metallic surfaces, lithiationsignificantly weakens the O–O bond and most likely will lead to theformation of monoxides (LixO), which will tend to aggregate to formcluster-like oxides. To the best of our knowledge, this is so far the onlystudy that has used theoretical approaches to understand the mech-anistic details of the electrochemical reactivity of the Li/air cell onmetallic surfaces, suggesting the need of further reports on this topic.Reducing the thickness of lithium-protection layers can significantlyimprove rate capability.—Solid Li-ion conductors generally have Li-ion conductivities at least an order of magnitude lower than liquid Lielectrolytes. One of the most attractive Li-protection layers, due toits relatively high conductivity (>10−4 S/cm) and stability against avariety of solvents, including water, is the class of glass ceramics fromOhara.115,121,139,140 Unfortunately, this material is brittle and difficultto manufacture at thicknesses below 150 μm.We used a 0D Matlab model to provide a rough estimate of the

separator materials design required to attain cell-level performancetargets. An optimization routine in Matlab (fmincon) was used tocompute the minimum ionic conductivity in the separator requiredto obtain a cell-sandwich specific energy of 1250 Wh/kg at speci-

fied values of C rate and separator thickness.g The positive electrodethickness and carbon loading were free to change in order to obtainthis optimum. Losses in the cell, which increased with C rate, wereincluded in the calculation of delivered specific energy (i.e., the dis-charge energy was 1250 Wh/kg, while the nominal specific energywas higher). To underscore the critical influence of the separator, onlyohmic losses were considered in the optimization. In a real cell, ki-netic losses, and potentially mass-transfer limitations, would furtherlower the delivered specific energy of the cell. The resulting dischargeefficiency for all optimized cell-sandwich designs was between 75and 85%.Figure 25 shows the conductivity of the separator required to

attain a cell-sandwich specific energy of 1250 Wh/kg at various Crates in a Li/air cell with a solid-state separator. We assume that thedensity and conductivity of the electrolyte in the positive electrodeare 1 g/mL and 0.01 S/cm, respectively, that the density of theseparator is 3 g/mL, and that the current collectors are Cu and Ni. Ananode/cathode capacity ratio of 1.2 was fixed for the optimization.The results imply that decreasing the thickness of a low-

conductivity separator may be a more effective route to attaininghigh practical specific energy at moderate C rates than increasing theconductivity of a thick separator. For a 150-μm (100-μm) separator,it is impossible to achieve the target practical specific energy at adischarge rate above 0.4 C (0.7 C) with any cell design (i.e., the max-imum falls below 1250 Wh/kg). Put simply, a thick separator, evenif highly conductive, requires correspondingly thick electrodes in or-der to achieve high energy density, but thick electrodes imply a poorpower density. On the other hand cells with thin separators and thinelectrodes could achieve both high energy density and power density.Outlook: Power density improvements require eliminating passiva-tion in nonaqueous systems and reducing the thickness of lithium-protection layers.—In nonaqueous systems, passivation phenomenaappear to be the most restrictive with regard to achieving high cur-rent density. If passivation can be eliminated, poor solubility of O2 innonaqueous solvents may require new cell designs with continuousgas, electrolyte, and electronically-conductive solid networks in orderto achieve even higher current density. Catalysts that promote facileORR and OER in nonaqueous systems may be required to improvethe voltage efficiency of nonaqueous Li/air cells.In aqueous systems, the high thickness and low room-temperature

ionic conductivity of available water-stable Li-protection layers arewhat limit the current density far below that of other metal-air sys-

g For a cell-sandwich specific energy of 1250 Wh/kg, an 80% cell packaging factorimplies a cell level specifc energy of 1000 Wh/kg. A further 43% increase in mass atthe pack level implies a system energy of 700 Wh/kg.




tems. Preliminary calculations indicate that reducing the protectionlayer thickness would have the greatest impact with regard to thepractically achievable specific energy. However, the mechanicalstability of thinner layers must be adequate for the hundreds tothousands of large-volume-change cycles expected for automotiveapplications. Similar requirements may also apply to solid separatorsfor nonaqueous systems.

Supplying oxygen to the cells requires new membranes or on-board storage.— As a required reactant for discharge, oxygen mustbe drawn in from the environment or stored on board the vehicle. Themost attractive scenario is an open system in which the cell “breathes”oxygen from the ambient air. In this case, specific energies higher than1000 Wh/kg could be achieved for a Li/air cell, as shown in Figure 3.Conversely, storing oxygen inside a closed system (directly in thecells or in a tank) leads to practical specific energies between 500 and1000 Wh/kg. While a truly “air-based” system would afford higherspecific energy, the systemmost widely investigated in the literature sofar is the lithium-oxygen cell. In this section we discuss the challengesrelated to ambient air operation of Li/air batteries and present someconcepts from recent literature, including membrane solutions as wellas closed systems using pure-oxygen tanks. Note that for an aqueousLi/air cell it may be desirable for H2O to enter the cell from ambientair.Open systems are susceptible to contaminants and electrolyte evapo-ration, which result in severe performance degradation.—The designof an open lithium/air system presents many challenges associatedwith preventing contaminants from entering the cell and keeping theelectrolyte in the cell. Any molecules that can conceivably be foundin the atmosphere may enter the system and affect the cell chemistry,through catalyst poisoning, corrosion, or side reactions. Several un-desired side reactions may occur if carbon dioxide, nitrogen, or waterenter the cell. The most important side reactions include:

CO2:

4Li+ O2 + 2CO2 → 2Li2CO3 [9]

Li2O+ CO2 → Li2CO3 [10]

Li2O2 + CO2 → 1/2O2 + Li2CO3 [11]

LiOH+ CO2 → Li2CO3 + H2O [12]

H2O:

2Li+ 2H2O → 2LiOH+ H2 [13]

LiOH+ H2O → LiOH • H2O [14]

N2:

6Li+ N2 → 2Li3N [15]

Li3N+ 3H2O → 3 LiOH+ NH3 [16]

If the Li metal anode is protected from the electrolyte in the pos-itive electrode (e.g., with a solid-electrolyte separator), some of thereactions involving contaminants (9, 13, 15, and 16) can be prevented.In this case, introduction of CO2 can still result in the formation ofelectronically insulating bulk lithium carbonate and, in nonaqueouselectrolytes, H2Omay result in solvent degradation or the formation ofcorrosive acids.144–146 Li2CO3 formation may be partially irreversible,resulting in capacity fade.While the reactions with CO2 and N2 only degrade the cell perfor-

mance, introduction of water is more critical if the negative electrodeis not protected. In this case, additional gas evolution (e.g., H2 and

Table IV. Gibbs free energies of formation for important lithiumcompounds, and the reactants from which they are formed thatwas used to calculate the Gibbs formation energy. Data takenfrom.11

Compound and reactants �Gr (kJ/mol)

Li2CO3 (Li, CO2, O2) −809Li2O2 (Li, O2) −571Li2O (Li, O2) −562LiOH (Li, O2, H2O) −320LiOH·H2O (Li, O2, H2O) −329Li3N (Li, N2) −129

NH3) may lead to a hazardous overpressure and flammable gases mayreact with oxygen in a chain reaction.From a comparison of the Gibbs free energies of formation of the

relevant compounds (see Table IV) one can see that lithium carbonateis more stable than the lithium oxides while lithium nitride has arelatively small formation enthalpy. This implies that nitrogen enteringan open Li/air cell may be less critical than CO2, although the reactionkinetics must also be considered.Ideally the cell would include a gas selective membrane that allows

only oxygen to enter and exit the positive electrode. Optimization ofthe system implies a balance between effective separation of enter-ing gases and low weight and cost achieved by minimizing systemcomplexity.A second critical issue in open Li/air systems is electrolyte evap-

oration. Most recent studies of Li/air batteries use propylene car-bonate as the electrolyte solvent because of its high boiling point(BP = 240◦C) and good polarity, although it is chemically unstable,while some apparently more stable electrolytes26,48 have lower boilingpoints (e.g., acetonitrile, BP = 82◦C). In order to use the electrolytewith the best properties it is preferable to avoid the evaporation issueentirely. A gas selective membrane may serve this function as well.Besides the fundamental aspect of gas separation and elec-

trolyte enclosure a realistic solution must be able to handle theexpected gas flows at realistic current densities. A 90-kWh elec-tric vehicle Li/air system discharged at 1 C has an oxygen con-sumption of 3.85 L/s at 25◦C and 1 atm. Such practical constraintsshould be considered together with the gas selectivity of potentialmembranes.Membranes may be an effective means of keeping contaminants outof the cell and keeping the electrolyte in the cell.—In an open system,contaminants must be separated from the air that enters the positiveelectrode. We preclude membraneless gas separation processes suchas adsorption, absorption, and distillation, the components of whichwould likely be too complex, costly, large, and inefficient for automo-tive applications. Most researchers are instead exploring membranesimplemented at the cell level that prevent ingress of contaminantspresent in the air. A further advantage of membranes over externalair filters is that they often prevent evaporation of electrolyte fromthe cell. In primary aqueous Li/air cells, desiccants inside the cellhave been used to absorb water vapor from the atmosphere and retainwater,9 but this concept does not appear suitable for rechargeable sys-tems. If membranes are used to purify the air their weight and volumewill need to be included in energy calculations.Membranes with O2-selective silicone oils.—Zhang et al. demon-strated an O2-selective membrane for operation in ambient air of20–30% relative humidity, as shown in Figure 26.147 The cell de-sign contained a nonaqueous electrolyte and the membrane allowedpermeation of O2 while blocking moisture. Such membranes can beprepared by loading O2-selective silicone oils into porous supportssuch as porous metal sheets and Teflon (PTFE) films. The O2 gas wascontinuously supplied through a membrane barrier layer at the inter-face of the cathode and ambient air. The authors reported that siliconeoil of high viscosity showed reasonable performance. The immobi-lized silicone oil membrane (see Figure 26) in the porous PTFE film




Figure 26. Schematic of two kinds of O2 membrane structures. (a) Liquidimmobilized in homogeneous porous substrate, and (b) liquid immobilized insilicalitemembrane coated porousmetal substrate (asymmetrical). Reproducedfrom reference 147.

enabled Li/air batteries with carbon black air electrodes to operatein ambient air (at 20% RH) for 16.3 days with a specific capacityof 789 mAh/g carbon and a specific energy of 2182 Wh/kg carbon.Its performance was much better than a reference battery assembledwith a commercial, porous PTFE diffusion membrane as the mois-ture barrier layer on the cathode, which only had a discharge time of5.5 days, corresponding to a specific capacity of 267 mAh/g carbonand a specific energy of 704 Wh/kg carbon.The silicone based membrane prepared by Zhang et al. obtained

an oxygen permeance of 1.6 · 10−6 mol m−2 s−1 Pa−1 at an O2/H2Oselectivity between 1.5 and 3.6. In a different work Reynolds et al.used a polyperfluorocarbon liquid in a porous Celgard 2500 polymersubstrate but they obtained a four orders of magnitude lower oxygenpermeance of 1.7 · 10−10 mol m−2 s−1 Pa−1 at an O2/H2O selectivityof 3.9.148

Polymer protection membranes.—In another study, Zhang et al. used aheat-sealable polymer membrane (Melinex 301H) as both an oxygen-diffusion membrane and a moisture barrier in the operation of non-aqueous Li/air cells under ambient conditions with an oxygen partialpressure of 0.21 atm and relative humidity of ∼20%.149 A schematicis shown in Figure 27. The membrane also minimized the evaporationof the electrolyte solvent. These batteries operated in ambient condi-tions for more than one month with a specific energy of 362 Wh/kg,based on the total weight of the battery including packaging. The

thickness of the polymer membrane was 25 μm, representing ∼1%of the cell volume (see Figure 27). Current densities between 0.05and 0.1 mA/cm2 were demonstrated, but at higher current densitiesoxygen permeation through the membrane was insufficient.A promising step toward more suitable membranes has been very

recently reported by MaxPower Inc.150 The authors describe the de-velopment of oxygen-selective membranes based on polysiloxane andmethacrylate–polysiloxane copolymers which allow current densi-ties of up to 2 mA/cm2 and protect the cell against humidity andmoisture. Thinner membranes with high O2 selectivity must be de-veloped in order to meet the high power requirements of practicalapplications.Anion exchange membranes for aqueous systems.—For aqueous Li/aircells, EDF has developed an anion exchange membrane (AEM) be-tween the ORR electrode and discharge product reservoir that allowstransport of OH−, but mitigates influx of O2 or contaminants such asCO2.129 TheirAEMis composed of interpenetrating polymer networks(IPN): hydroxyl-conducting polycationic crosslinked polyepichlorhy-drine (PECH) provides ionic conduction and poly(hydroxyl ethylmethacrylate) provides mechanical strength and reduces swelling.Current densities up to 6 mA/cm2 (∼C/12) were obtained at 60◦C.However, some air can permeate the polymer, resulting in slowdegradation. Improvement of the membrane’s ability to screen CO2could enable the use of more alkaline electrolytes, which are par-ticularly susceptible to Li2CO3 formation but have a lower OERoverpotential.29

Use of tanks and compressors results in a closed system, protectedfrom contaminants.—As an alternative to membrane-enabled opensystems, Li/air batteries can be designed as fully closed or partiallyclosed systems that use oxygen from tanks rather than from the at-mosphere. This idea was already discussed in the introduction in thecontext of energy calculations, and it was concluded that the addedmass and volume of a tank significantly dampen the appeal of a Li/airsystem. The major advantage of using an oxygen tank is that all of thecomplexities associated with air purification can be avoided. Whilethe ultimate commercial viability of a Li/oxygen, rather than a Li/air,system is perhaps limited, the use of oxygen tanks will be critical dur-ing research and development phases to ensure well-defined chemistryfree of possible contaminants from air.Outlook: Until effective membranes are developed, a bulky closed sys-tem may be required.—Thus far no membrane has been completelyeffective in preventing the introduction of contaminants into the cellwhen ambient air is used as an oxygen source. O2-selective mem-branes are preferred for nonaqueous cells, whereas anion-exchangemembranes with good OH− selectivity are an attractive option foraqueous cells. In addition to achieving good selectivity and prevent-ing solvent evaporation, candidate membranes must enable high ratesof O2 or OH− transport.

Figure 27. Schematic of a sealed test cell used by Zhang et al. for ambient operation of Li/air. Reproduced from reference 149.




Onboard storage ofO2 provides amore reliable but bulkier solutionto the contaminant problem. In this case, the O2 tank and compressorsadd weight and complexity to the system, and the power required torun the compressors impacts the overall system efficiency. However,at least theoretically the efficiency loss may be small, as shown inTable III, and in open systems efficiency can also be diminished byresistive separation membranes. The added weight and volume area significant barrier to the development of closed systems, as theachievable specific energy falls close to the threshold that can beobtained by a Li/LiMO2 cell (∼500 Wh/kg), while the energy densitycould be much worse (see Figure 3).

Aqueous Li/Air Systems

One of themajor advantages of the aqueous Li/air system is that thedischarge reaction product is soluble in water, eliminating an appar-ent shortcoming of the nonaqueous Li/air batteries, namely formationof electrically resistive products that may passivate the air electrode.Moreover, selection of an electrolyte system that achieves reversiblechemistry does not appear to be a bottleneck for the development ofaqueous Li/air cells. However, effective storage of the discharge prod-uct, stability of the separator against both the Li anode and the aqueousair electrode, and catalysis of both the ORR andOER reactions remainconsiderable challenges for rechargeable aqueous Li/air cells.The operating principle of the aqueous Li/air cell is shown

schematically in Figure 28. Under the proposed scheme, the metalliclithium anode is protected by a Li-ion conducting ceramic film, whichprevents the vigorous reaction of metallic lithium with water. Anotherimportant characteristic of these cells is the need for catalysts in thepositive electrode that reduce the activation barriers for both ORRand OER.The idea of using a protected lithium electrode (PLE) was intro-

duced in 2004 by the PolyPlus Battery Company,10,82,151 who demon-strated long-term stability, high-discharge capacity, and some cyclabil-ity of lithiummetal in aqueous solutionswhen the anodewas protectedby a water-stable glass-ceramic electrolyte (LTAP) from Ohara.115,121

Imanishi et al. have also looked at the concept of using protectedLi anodes to fabricate rechargeable Li/air batteries using an aque-ous electrolyte.152,153 More recently, Zhou and co-workers introducedthe concept of a Li/air system including an organic/aqueous hybridelectrolyte. In this case, the catalytic reduction of O2 occurs in anaqueous electrolyte, while the metallic lithium remains in a nonaque-ous (organic) electrolyte; the two electrolyte systems are separatedby a LiSICON separator.1,27 Several other groups have adopted ap-proaches similar to PolyPlus’ PLE,many of which rely onNASICON-type lithium conductors.129,130,140

Figure 28. Schematic of an aqueous Li/air cell with electrically isolated cath-ode and anode in the positive electrode. Reproduced from reference 27.

The reaction mechanisms for aqueous Li/air batteries are differentfrom those of the nonaqueous Li/air batteries and can be summarizedas follows:

Positive electrode : O2 + 2H2O+ 4e− ↔ 4OH− [17]

Negative electrode : Li ↔ Li+ + e− [18]

The overall cell reaction is shown in equation 3.During the discharge process, O2 is electrocatalytically reduced to

produce hydroxyl ions (OH−) at the positive electrode (Equation 17),while Li+ ions are generated (Equation 18) at the negative electrode.LiOH is soluble in water, but precipitates to form LiOH·H2O abovethe solubility limit (5.25 M at 25◦C).7 During the charge process, O2gas is generated at the positive electrode and Li is plated at the negativeelectrode. Catalytic materials are needed to accelerate the kinetics ofboth ORR and OER. The electrocatalytic reduction of O2 in aqueoussolution often requires the use of expensive catalysts such as Pt,152,153

particularly in acidic electrolytes.29

Like the nonaqueous Li/air cell, the rechargeable aqueous system isalso at a development stage, and new concepts continue to be explored.Here we discuss two challenges specific to the aqueous Li/air system:controlling the location and morphology of the precipitated dischargeproduct, and durably catalyzing both the ORR and OER.

Precipitation and dissolution of the discharge product must bemanaged to enable high energy and acceptable power.— The dis-charge process in the aqueous Li/air cell differs from that of the non-aqueous cell in that the discharge product, LiOH, has a relatively highsolubility. Rather than forming a film directly on the surface of thepositive electrode, the dissolved product can reach a concentration ofapproximately 5.25 M (at room temperature) before it precipitates asLiOH ·H2O.154 A phase diagram is given in Figure 29. When cycled

Figure 29. Phase diagram for the LiOH ± H2O system, reproduced fromreference 154. The solid lines are fits to the data, which comes from varioussources given in reference 154. The eutectic is at a temperature of 255.15 Kand a composition of 5.27 mol/kg, while the peritectic is at a temperature of382.05 K and a composition of 7.70 mol/kg. The phase diagram shows that attemperatures below the peritectic the monohydrate LiOH ·H2O is the favoredform of LiOH, while above that temperature anhydrous LiOH is favored. Theuse of supporting salts (such as LiCl orNaOH) could change the phase diagram.




to the solubility limit, the theoretical specific energy of the system is430 Wh/kg; hence significant formation of LiOH ·H2O is key to at-taining an attractive specific energy. Because precipitation and dis-solution of LiOH ·H2O does not involve electron transfer, the solidproduct need not be in contact with the positive electrode. This poses achallenge, as impedance rise and restriction of accessible porosity canresult from an uneven product distribution, but also provides manyopportunities for novel cell and system designs as well as batterymanagement system algorithms155 that control the discharge process.In order to achieve high cell specific energy, it is desirable to fill

a significant fraction of the available reservoir or positive-electrodepore volume with the solid discharge product. Figure 3 shows thatwhen 65% of the volume of a 200 μm positive electrode is filledwith LiOH ·H2O a specific energy of about 700 Wh/kg results at thecell level for a practical design. If only smaller volume fractions areachieved the cell-level specific energy will be even lower. The ob-jective of maximizing the discharge capacity of the cell by going toa high volume fraction of LiOH ·H2O must be balanced against theundesired impedance rise resulting from restriction of the pores. Ide-ally, the pores should fill uniformly with discharge product in order tomaintain a homogeneous current distribution through the cell separa-tor. Preferential precipitation of the product in certain locations dueto gravitational fields, concentration and thermal gradients, currentheterogeneity, or electrolyte flow would tend to amplify nonunifor-mity of the current distribution and result in more rapid impedancerise, and at worst could reduce the accessibility of large regions ofthe reservoir or electrode. Current nonuniformity would also resultin uneven Li plating/stripping during charge/discharge. Coverage ofthe anode protection layer or positive electrode with a dense layer ofmonohydrate can likewise result in impedance rise and a prematureend of discharge.As with nonaqueous systems, aqueous Li/air cells involve sub-

stantial volume change, and the precipitation of large amounts ofsolid LiOH ·H2O within the pores of a reservoir or electrode couldimpart deleterious mechanical stress on the system. So far high cellreversibility involving large volumes of LiOH ·H2O (more than a fewpercent of the porosity) has not been demonstrated.Some existing prototype cells demonstrate high discharge capacitybut have limited reversible capacity.—While much of the recent Li/airliterature has focused on nonaqueous systems, several companies havepublished the results of their aqueous Li/air research and development.Although each company describes a unique cell design, a commonfeature is the concept of a discharge product reservoir that is distinctfrom the positive electrode. PolyPlus has demonstrated the dischargeof primary aqueous Li/air cells in which discharge product is storedin a porous reservoir of the cathode (see Figure 30).84 One of theirpatents describes a porous zirconia felt reservoir that expands with thedischarge product as it precipitates and is filled with a catholyte thatcontains hygroscopic salts used to draw inwater from the atmosphere.9

Figure 30. Cross section of PolyPlus’ aqueous Li/air cell. Reproduced fromreference 84.

While this decreases the mass of the cell (in the charged state) andprevents evaporation of catholyte, the use of such salts may not beappropriate for rechargeable cells.Toyota has demonstrated reversibility of aqueous Li/air system in-

cluding precipitation/dissolution of LiOH.130 A symmetric aqueouscell with Pt electrodes and Ohara separator was used to show the re-versible exchange of 200 mAh capacity in 5M saturated solution. The200 mAh of charge passed corresponded to precipitation/dissolutionof solid LiOH in/from one of the two electrodes. Raman spectra of thesolids confirmed LiOH as the product, and formation of O2 bubbleswas observed during dissolution of the LiOH. In a second experiment,two nonsaturated solutions were used, and the electrodes were cycledbetween 4 and 5 M without precipitation. In a second cell type, theair electrode consisted of LaSrCoO3, carbon, and PTFE on carbonpaper, and the anode consisted of Li metal in a LiTFSA/PC elec-trolyte, with an Ohara separator between the electrodes. In this case,34 mAh were passed, corresponding to 30 mg of LiOH (compositionconfirmed by XRD). During charge (from a second cell constructed inthe discharged state, with solid LiOH in the cathode), the solid LiOHwas consumed.EDF has proposed solutions to several critical LiOH storage issues

that enable up to 100 reversible charge/discharge cycles (100 cyclesat 0.1 mAh/cm2, 40 cycles at 2 mAh/cm2).129 Improvements includeapplication of an anionic polymer layer on the air electrode side of theirLTAP separator, which prevents nucleation and preferential depositionof a resistive LiOH layer at the LTAP/reservoir interface. Without thislayer an exponential rise in overpotential with time results. Humidifiedair is supplied to the air electrode, where oxygen is reduced andreacted with water to form hydroxyl ions (Equation 17) which in turnare conducted through the IPN to the reservoir. The IPN mitigatesinflux of CO2, which reacts with LiOH to form Li2CO3, resulting incell degradation. An oxygen evolution electrode is embedded in thereservoir to enable recharge of the cell (see Figure 31).By using a transparent cell housing EDF observed the formation of

a solid discharge product at the bottom of the reservoir (see Figure 31).Thus far, only a small amount of solid LiOH ·H2O, relative to thetotal pore volume of the reservoir, has been cycled. In cells that wereinitially charged to 73 mAh/cm2 (for an available cell specific energyof 500Wh/kg), 40 cycles were achieved at a depth of discharge (DOD)

Figure 31. Schematic of EDF’s rechargeable aqueous Li/air cell. Reproducedfrom reference 129.




of 2 mAh/cm2 (13 Wh/kg) and 100 cycles were achieved at a DOD of0.1 mAh/cm2.129

While the current EDF cell has a vertical orientation, such thatLiOH ·H2O settles to the bottom of the cell and evolved gases exit thetop of the cell, this has the disadvantage that a non-uniform currentdistribution may result, with more current flowing in the top regionof the cell that does not contain LiOH ·H2O. This would lead to anon-uniform thickness of Li metal as a function of height in the cell.An alternative orientation would be to place the cell in a horizontalconfiguration such that the LiOH ·H2O precipitate would settle eitheron the AEM or the LTAP. This should result in a more uniform currentdistribution but may make it more difficult for oxygen produced at theOER electrode to exit the cell. The incorporation of the OER electrodein the cell sandwich may also be more difficult.High reversible capacities must be demonstrated.—While Toyota andEDF have shown good reversibility of the Li/air system involving asmall amount of solid discharge product, PolyPlus has demonstratedlarge discharge capacities in a primary cell. Further research on thereversibility of LiOH ·H2O reservoirs may pave the way to a cy-clable high-energy Li/air cell. There is as yet limited understandingof where the solid product precipitates and how this affects currentdistribution in the cell. The mechanical stability of candidate porousreservoirs, such as expandable zirconia felt, over many cycles shouldbe investigated. The influence of current density, reservoir geometry(porosity and thickness), and cell orientation with respect to gravita-tional field on the product distribution requires a more comprehensiveanalysis.Discharge product morphology impacts recharge rate capability.—During charge the solid particles of LiOH ·H2Omust dissolve and theLiOH must diffuse to the OER electrode. The rate of particle disso-lution will depend on their size and shape, as well as the porosity ofthe precipitate layer that forms. Providing forced convection in someway could significantly improve the dissolution rate. One importantquestion is whether, if the LiOH ·H2O particles that form are verysmall, they will undergo Ostwald ripening during a rest step. If thiswere to occur the larger particles would take longer to dissolve. Therecould also be changes to the porosity of the layer that would impactthe dissolution rate. We consider the issue of the physical form ofLiOH ·H2O deposits to be a critical area for research for aqueousLi/air cells.Novel system concepts may involve external LiOH · H2O reservoirs.—While reservoirs internal to the cell sandwich have been the focusof recent aqueous Li/air cell developments, earlier work on metal-airsystems included electrolyte circulation,29,87 which may allow for ex-ternal reservoirs. Storing the product outside the path of ionic currentflow would ensure that the pores of the separator and positive elec-trode remain open for ionic transport. Sufficiently high flow rates,relative to the applied current density, should minimize LiOH concen-tration gradients orthogonal to the applied electric field and maintaina uniform current density.External storage of the product could be enabled by a flow-through

cell connected to a storage reservoir, in which the aqueous LiOHsolution is continually circulated between the two. Several generalconcepts could be explored for separating the monohydrate productfrom the circulating stream, including gravitational (e.g., the use of asettling tank as proposed for Al/air cells156), mechanical, evaporative,and thermal separation, as well as filtration. The energy requirementsfor the separation process and for maintaining flow should be includedin overall energy efficiency calculations.Controls strategies involving thermalmanagement and current pro-

files could be explored as ameans to improve the uniformity of productdistribution in the reservoir.Outlook: LiOH · H2O formation appears to be reversible; high-capacity and long-term cycling need to be demonstrated.—There hasrecently been rapid progress in demonstrating reversibility and iden-tifying challenges to long-term cyclability of aqueous Li/air systems.Several companies have proposed promising engineering approachesto improve the energy density (adding desiccants to the discharge

product reservoir) and durability (polymer protection of the reservoirto separate it from the anode protection layer and the air electrode) ofaqueous Li/air cells.Success of companies like Sion and PolyPlus in protecting Limetal

could be leveraged to increase cyclability of the negative electrode,which appears to limit cell life. Other cycle-life limitations involvingstorage of LiOH ·H2Omay become apparent as the anode reversibilityis improved.The solutions proposed here should be attempted in rechargeable

Li/air cells in which a much higher fraction of the theoretical capacityis cycled. Volume changes associated with high-capacity charge anddischarge may require significantly different cell designs from thoseproposed today.

Charge and discharge need to be catalyzed to reduce kineticlosses.— In the aqueous Li/air system, where OH− is the cathodicreaction product, the O-O bond must be broken during oxygen reduc-tion and reformed during oxygen evolution. These bond-breaking andbond-forming reactions may result in a considerable overpotential in-crease relative to the ORR and OER in nonaqueous systems, in whichLi2O2 may be formed and one of the O-O bonds possibly remainsintact.In order to lower the overpotential of the ORR, there are few al-

ternatives to Pt with comparable performance, particularly in acidicenvironments.29 In alkaline electrolytes, most materials are unstableat the anodic potential required for the OER, and even Pt dissolves.157

Hence, to avoid expensive catalysts, it may be necessary to use analkaline electrolyte and electrically isolate the ORR and OER elec-trodes.Alternatively, a bifunctional electrode consisting of two different

catalysts for ORR and OER may be feasible, provided the activecomponents for the reduction and evolution of the O2 molecule arestable over a wide range of potentials, from 0.6–0.7 V (RHE) dur-ing discharge (ORR), to over 2.1 V (RHE) during charge (OER).158

Such bifunctional electrodes are already employed in metal-airsystems.138,159,160

Work on PEM and other fuel cells has significantly reduced the Ptloading required for ORR.—Investigation of ORRmechanisms in fuelcell cathodes is much more advanced than what has been recently pro-posed for metal-air batteries. ORR has been extensively studied in thefield of fuel cells because of its fundamental complexity, sensitivityto the electrode surface properties, and slow reaction kinetics. It hasalso been demonstrated that the ORR is a significant component of thecell overpotential and limits the efficiency of electrochemical energydevices that use air as the oxidant.161 Despite the complexity of this re-action, twomechanisms have been proposed for oxygen reduction: theso-called direct 4-electron pathway, in which peroxide is not formed,except as a possible adsorbate; and the peroxide pathway, involvinga 2-electron reduction of O2 to form peroxide, which subsequentlyreacts electrochemically to form H2O or chemically to form H2Oand O2.162 The 2-electron pathway is more common in alkaline solu-tions and on mercury, gold, carbon, and most transition metal oxides,whereas the 4-electron pathway proceeds via dissociative adsorptionon noble metallic catalysts and some transition metal catalysts andmacrocycles.162,163 To obtain the maximum redox efficiency and toavoid the corrosion of the electrode material by the peroxide interme-diate of the 2-electron reaction mechanism, it is preferable to achievea 4-electron ORR. Therefore, finding suitable inexpensive catalyststhat promote direct 4-electron reduction of the O2 molecule is still anactive area of research.Bifunctional electrodes are used in Zn/air systems.—Metal-air sys-tems involve a significant leap in complexity compared to fuel-cellsystems because OER must be catalyzed in addition to ORR. Sincethe ORR reaction mechanisms have been discussed in the previoussection, here we describe the reaction pathways involved in oxygenevolution. According to Jorissen,162 the OER mechanisms are rathercomplex and susceptible to change depending on the electrode poten-tial. On metallic surfaces, the rate-determining step for the OER is the




electron transfer from H2O in acidic media or OH− in basic mediato form adsorbed OH radicals at the electrode surface. OER has alsobeen investigated on metallic-oxide surfaces;164 in these systems theintermediate formation of H2O2 from metal-desorbed OH species hasbeen suggested to be the rate-determining step. Upon formation of thephysisorbed H2O2 species, O2 evolves in the final step of the proposedOER mechanism.164

Without a third electrode in the cell, the reduction and evolutionof oxygen in rechargeable (secondary) metal-air batteries requiresbifunctional air electrodes, which contain separate ORR and OERcatalysts connected electronically. Dating back several decades,159,160

bifunctional electrodes have emerged as a practical solution to thelack of highly active and stable bifunctional catalysts for both ORRand OER. For example, it is well-known that the most effective ORRcatalysts are those based on platinum (Pt),165 but Pt has only moderateactivity for the OER. On the other hand, ruthenium (Ru) and iridium(Ir) oxides are among the best OER catalysts,166 but they are notas active as Pt for ORR. Although, alloys of these compounds haveshown a better bifunctional-catalytic performance,167 the developmentof bifunctional catalysts for practical applications still represents achallenge as the best catalyticmaterials still consist of preciousmetals,which are in turn scarce and expensive.Since the composition and design principles used in bifunctional

electrocatalysts for metal-air batteries have been recently reviewed byNeburchilov et al. and Jorissen et al.162,168,169 here we make only afew remarks relevant to the Zn/air cell.While several metal-air chemistries have been proposed, Zn/air

batteries are the most mature technology and have contributed sig-nificantly toward the development of metal-air batteries.158,169 Theoverall discharge reaction can be summarized as follows:170

Positive electrode : O2 + 2H2O+ 4e− → 4OH−

Negative electrode : Zn → Zn2+ + 2e− [19]

Zn2+ + 4OH− → Zn(OH)2−4 [20]

Zn(OH)2−4 → ZnO+ H2O+ 2OH− [21]

Net reaction : 2Zn+ O2 ↔ 2ZnO [22]

In contrast to the Li/air cell, hydroxyl ions generated at the airelectrode migrate to the Zn anode compartment to complete the cellreaction, with a cell equilibrium potential of 1.65 V.170

In spite of the challenges involved in catalysis for secondary Zn/airbatteries, some success (150 cycles) with cells that contain bifunc-tional air electrodes has been reported by the ReVolt Company.138 Intheir work, a bifunctional electrode is constructed with a catalyst thatpromotes ORR (MnSO4, Fe, Pt, and Pd among other materials) anda bifunctional catalyst that shows high reaction rates for both ORRand OER (for example, La2O3, Ag2O, Ag, spinels and perovskites).According to Neburchilov et al., although several companies havepatented bifunctional electrodes (ReVolt among others), all of themhave insufficient ORR catalytic lifetimes at the high OER potential(∼2.1 eV).169 It is therefore reasonable to expect some delay in thecommercialization of the Zn/air batterieswhile inexpensive, abundant,active, and stable combinations of catalytic materials are sought.3-electrode cells remove restrictions on ORR and OER catalysts, butcomplicate the system design.—Perhaps the most promising solutionto the catalysis of ORR and OER in aqueous Li/air cells is the useof two separate and electrically isolated electrodes for charge anddischarge. This sidesteps the catalyst stability issue, as the catalystsappropriate for ORR are never polarized to the high anodic potentialsrequired for OER. Several groups have designed aqueous Li/air cellswith this configuration.27,129 An additional advantage of this configu-ration in the EDF cell is that the OER electrode can be located betweenthe IPN and product reservoir, thereby avoiding the overpotential as-sociated with OH− transport through the IPN during charge.129

Figure 32. Activity versus the experimentallymeasured d-band center relativeto platinum. The theoretical results are shown in black while the experimentalresults are shown in red. Reproduced from reference 173.

However, the use of two oxygen electrodes adds mass to the cellandmay significantly lower the system specific energy. It also requiresincreased complexity and cost of the electronics used to control thesystem.Computational screening could enable the discovery of suitable ORR,OER, and bifunctional catalysts.—First-principles modeling of elec-trocatalytic reactions is fast becoming an indispensable tool for achiev-ing insight into the reaction mechanisms of complex electrochemicalenvironments.171,172 Significant efforts in this area have led to de-tailed interpretations of the experimental data and to the computa-tional design of new catalytically active materials for the ORR infuel cells.173,174 In this work, many authors have extensively usedthe concept of “volcano” plots to predict the activity and selectivityof catalysts for ORR. Volcano plots (shown in Figure 32) illustratevery well Sabatier’s principle for catalytic materials, which states thatan effective catalytic surface should reach a compromise betweenhaving enough strength to break the bonds of the molecular adsor-bates and yet generate weakly-bonded intermediate species that arenot overly stabilized by the surface.175 As an example, Figure 32(reproduced from 173 and 174) presents the volcano plot for the elec-trocatalytic reduction of O2 on various Pt-complexes. The peaks inFigure 32 represent those catalytic surfaces with the largest electro-chemical activity for ORR in fuel cells.The values used to construct such plots are now readily attainable

from first-principles modeling techniques;174 it is therefore conceiv-able that such computational-based approaches could be used to ex-plore the catalytic activity of various metallic surfaces for both theORR and OER. In turn, this possibility provides some hope for amore versatile design strategy for bifunctional electrodes or bifunc-tional catalysts for the aqueous Li/air system.Outlook: Bifunctional electrodes or two-positive-electrode architec-tures are required.—Unless bifunctional catalysts are discoveredthat promote facile kinetics for both the ORR and OER in aque-ous air electrodes and are stable over the potential window 0.6 to2.1 V vs. RHE, the only viable approaches to making aqueous Li/airsystems rechargeable are bifunctional-electrode and two-positive-electrode architectures. The former design has already been used withsome success in rechargeable Zn/air cells, although it remains chal-lenging to find a combination of catalysts with fast ORR kinetics, sta-bility at theOERpotential, and lowcost. Two-electrode designs,whichavoid the kinetics-stability compromise, have been implemented inLi/air cells at the expense of higher complexity and weight.

Summary and Outlook

Lithium/air batteries are an attractive technology, particularly forlong-range electric vehicles, because of their high theoretical specific




Table V. Requirements for durable, high-energy automotive Li/air batteries (N = nonaqueous only, A = aqueous only).

Requirements for durable, high-energy automotive Li/air batteries

Li anodeRobust and flexible containment, including flexible seals

Li protection layer(s)transport properties

Sufficient conductivity over T range of interestNegligible electronic conductivity

manufacturabilitySufficiently thin (<50 μm preferable)Pinhole freeLow cost

mechanical propertiesHigh elastic modulus (to prevent dendrite initiation)Flexibility required for wound concepts

stabilityStable against O2, contaminants (CO2, N2, trace H2O)Stable against LiWater stable (A)

Air electrodeContinuous gas and electrolyte networks (N)High surface area (N)High pore volume (N)Bifunctional or 2-electrode design

MembraneHighly selective for OH- (A)High OH- transport rates (A)Good CO2 screeningGood H2O screening (N)

Product reservoirPromotes uniform LiOH·H2O precipitation (A)Flexible (A)High pore volume (A)Maintains electrolyte transport pathways (A)

CatalystsHigh activity/mass ratioHigh activity/cost ratioAbundant materials

ORR catalystBreaks O-O bond (A, and for Li2O)Robust to poisoning (A)

OER catalystPromotes O-O bond formation (A, and for Li2O)

ElectrolyteAdequate Li+ conductivity at all temperaturesGood stability at high temperaturesLow viscosityChemically inert in presence of O2 over operating voltage and temperature (N)

SystemClosed system with tanks and compressors or open system with highly selective gas or anion-exchange membranesIn flow configuration, highly efficient separation of LiOH ·H2O from electrolyte (A)

energy. While researchers have been aware of and worked sparinglyon Li/air batteries for decades, no one has yet demonstrated a Li/aircell that is reversible and can be cycled over a significant fractionof its theoretical capacity. The only short-term commercially viableproducts are primary Li/air cells that are designed for high specificenergy but not rechargeability.Recent investigations of both aqueous and nonaqueous Li/air sys-

tems have resulted in a significantly improved understanding of themain challenges for this technology. Despite several research groupsdemonstrating limited cyclability of nonaqueous cells using carbonatesolvents, it is now clear that these solvents participate in the reactionsthat consume lithium and oxygen during discharge and are not re-versibly generated during charge. Hence, the cycle life of carbonate-based Li/air cells is limited by the quantity of solvent available forreaction and the buildup of side reaction products. A search for ap-

propriate noncarbonate solvents is underway, but so far adequate re-versibility remains elusive.Once a reversible chemistry is established, the next major chal-

lenge that should be addressed for nonaqueous systems is the de-position of electrically resistive products in the air electrode dur-ing discharge. Bulk Li2O2 is electronically insulating and difficultto solvate; hence it may form an electrically resistive film on theelectrode surface, limiting the achievable capacity far below whatcould be obtained by completely filling the pore volume. Appro-priate nanostructuring of the air electrode may enable higher ca-pacities in spite of resistive products, while the use of new sol-vents or additives may enable Li2O2 precipitation away from activesites.Aqueous Li/air chemistry appears to be much more reversible than

nonaqueous Li/air chemistry. LiOH is very soluble (up to 5.25 M at




25◦C) and precipitates as a monohydrate above its solubility limitwhen using pure H2O as a solvent. The monohydrate can be dis-solved reversibly. However, the molar mass of the discharge product,LiOH ·H2O, is much higher than the discharge product in nonaque-ous systems, Li2O2, resulting in a comparatively lower theoreticalspecific energy. In fact, practical aqueous Li/air cell designs may notachieve higher energy density than Li/metal-oxide systems. More-over, breaking and forming of the O-O bond on discharge and charge,respectively, suggests the need for rate-enhancing ORR and OER cat-alysts, the stability of which may preclude the use of a bifunctionalelectrode.Controlling the distribution of reaction product is critical for

achieving high capacity and rate capability in nonaqueous and aque-ous systems. Provided passivation does not restrict accumulation ofLi2O2 to the electrode surface in nonaqueous cells, pore blockingmay restrict the accessibility of smaller pores. Poor oxygen transportcould result in higher current density near the gas diffusion layer andsubsequent accumulation of Li2O2 to block the ingress of oxygen.Appropriate pore structure and use of both wetting and nonwettingpores may be necessary to circumvent these issues. LiOH ·H2O tendsto precipitate on the surface of the LTAP separator preferred for aque-ous Li/air cells. Polymer coatings on the water side of the separatorcan prevent this from occurring. Gravitational effects (i.e., sedimen-tation) may require special orientation of nonaqueous cells to controlthe current density distribution and minimize ohmic and transportlosses.Massive volume changes at the electrode and cell-sandwich level

may require special cell-design features, including electrolyte or sol-vent reservoirs, flexible seals, application of pressure, and the useof components with particular mechanical properties. Recirculatingflow of the catholyte in aqueous cells may alleviate volume changein the positive electrode. Some components, such as flexible seals forcells with protected Li metal, have already been developed for pri-mary cells, but reversible cycling must also be demonstrated. Woundjellyroll designs may be precluded by high volume changes in Li/aircells.Another critical system-level issue involves the open nature of

tankless metal-air cells. Air contains contaminants, particularly H2Oand CO2, that are very reactive against Li and Li2O2. CO2 also re-acts with LiOH to from Li2CO3. Moreover, evaporation of the solventfrom the positive electrode compartment can occur in an open sys-tem. Several membranes have been proposed to avoid contaminationof the cell and evaporation, but there are no reports yet on total ef-fectiveness or the long-term stability of these membranes. In case amembrane solution is not adequate, a tank and compressor solutionseems feasible, at the expense of some specific energy and energydensity.Li metal is itself one of the most challenging components of the

Li/air cell, as it tends to roughen and develop dendrites with cycling.Application of pressure and the use of stiff solid-electrolyte separa-tors can diminish this morphology development, but this results inan additional challenge for the Li/air system. The separator must bechemically stable against both electrodes and provide sufficient ionicconductance over all operating conditions. It should not significantlyreduce the specific energy and energy density of the system (i.e., itshould preferably be thin), but it should also be mechanically ro-bust to thousands of cycles with significant electrode volume change.It seems lilkely that a multi-layer composite will be the ultimatesolution for Li/air cells, particularly aqueous cells. Several compos-ites meet some of these requirements, but manufacturability remainsuncertain.Table V summarizes the requirements for the various components

in Li/air cells. Those requirements that apply only to nonaqueous oraqueous systems are labeled N and A, respectively.Our prognosis for the rechargeable Li/air system is that it will

primarily remain a research topic for at least the next five years.Even if the challenges discussed in this review are successfully ad-dressed in that time period, the stringent requirements of durable andsafe operation in an automotive environment will further delay com-

mercialization of Li/air EVs. However, as a long-term solution tothe daunting challenge of low-cost, high-range electromobility, Li/airbatteries remain one of the few and most promising, and funding ofresearch to accelerate their introduction into the marketplace shouldbe a top priority for government, academic, and industrial researchinstitutions.

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