Lithium–Air Batteries: Performance Interplays withInstability FactorsLuhan Ye,[a] Weiqiang Lv,[a] Junyi Cui,[a] Yachun Liang,[a] Peng Wu,[a] Xiaoning Wang,[a]

Han He,[a] Senjun Lin,[b] Wei Wang,[c] James H. Dickerson,[d, e] and Weidong He*[a, f, g]

1. Introduction

The fossil-fuel-based economy has become increasingly unsus-tainable over the past decades. The emerging energy crisis hasaffected various aspects of human society. To meet futuresocial and industrial energy demands, scientists are facing un-precedented challenges. Much effort has been focused on thedevelopment of sustainable energy devices including solarcells, and those driven by biomass and hydrogen.[1–4] How toproduce power efficiently has been a central theme through-out the industrial age. In addition, for better energy utilization,another strategy is to seek new methods for efficient energystorage.[5, 6] Until now, Li-ion batteries have dominated the bat-

tery market. They have particularly long cycle lives and highoperating voltages, and are thus suitable for certain practicalapplications such as powering laptops and cellphones.[7] Al-though the energy density of Li-ion batteries (�160 W h kg�1)is relatively high compared to traditional batteries, they arestill far from being applied in electrical vehicles. Cost-effectiveand environmentally friendly storage devices with high energydensities are urgently needed. The lithium–air battery, due toits excellent energy density, can potentially meet this need.[8, 9]

The first rechargeable nonaqueous lithium–air battery was in-troduced by Abraham and Jiang in 1996.[10] The theoreticalenergy density has been demonstrated to be up to11 430 W h kg�1, which is comparable to fossil fuels and ismuch higher compared with other widely used batteries (suchas the Li-ion and Ni–Cd batteries), as shown in Figure 1.[9] Thehigh energy density of lithium–air batteries is promising forpowering electric vehicles and hybrid electric vehicles. Despitetheir high energy density, the applications of lithium–air bat-teries are still limited significantly by their instability.[11–16] Com-pared to the lead–acid battery widely used in industry, the per-formance of lithium–air batteries is affected directly by their in-stability. Conventional lithium–air batteries, including nonaqu-eous and aqueous types, are assembled as shown inFigure 2.[17] Much work has demonstrated their instabilityduring charge–discharge processes between the application tointernal structures; a subtle disturbance can lead to a largedegradation in capacity and poor cyclic performance.[18–20]

For instance, the main reaction product, Li2O2, is insoluble inorganic solvents, and might consequently clog electrodepores. Furthermore, Bruce et al.[21] reported that carbon elec-trodes are unstable as the charge voltage increases beyond3.5 V. Electrolytes also decompose during operation cycles.[19]

In brief, overall physical transportation problems, decomposi-tion and parasitic reactions cause a sensitive instability in lithi-

Lithium–air batteries are considered to be promising electro-chemical storage devices, due to their high specific energydensity. However, instability limits their cyclic performance andrate capacity and also leads to a high overpotential ; lithium–air batteries are typically characterized by capacity degradationand short cycle life. Such challenges prevent lithium–air batter-ies from entering and competing in the battery market. Elec-trodes, organic solvents, the interface between electrolyte and

cathode, and ambient conditions have all been demonstratedto impact substantially the stability of the lithium–air battery.In this Minireview, we focus on electrode and electrolyte de-composition, side reactions, and physical mass transport inaprotic lithium–air batteries, as well as other types of lithium–air batteries, and aim to understand comprehensively their per-formance and association with instability factors.

[a] L. Ye, W. Lv, J. Cui, Y. Liang, P. Wu, X. Wang, H. He, Prof. W. HeSchool of Energy Science and EngineeringUniversity of Electronic Science and TechnologyChengdu, Sichuan 611731 (P.R. China)

[b] S. LinDepartment of Industrial DesignZhejiang University of TechnologyHangzhou, Zhejiang, 310014 (P.R. China)

[c] W. WangDepartment of Material Science and EngineeringShenzhen Graduate School, Harbin Institute of TechnologyShenzhen 518055 (P.R. China)

[d] J. H. DickersonCenter for Functional NanomaterialsBrookhaven National LaboratoryUpton, NY 11973 (USA)

[e] J. H. DickersonDepartment of Physics, Brown UniversityProvidence, RI 02912 (USA)

[f] Prof. W. HeInterdisciplinary Program in Materials ScienceVanderbilt University, Nashville, TN 37234-0106 (USA)

[g] Prof. W. HeVanderbilt Institute of Nanoscale Science and EngineeringVanderbilt University, Nashville, TN 37234-0106 (USA)E-mail : weidong.he@uestc.edu.cn

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um–air batteries. The stability is correlated with the currentdensity, specific capacity and cycle life. Instability in a batterysystem leads to a high overpotential and consequently reducesthe energy output. To tackle the instability issues, many im-proved materials and protected structures have been proposedrecently, including the lithium anode with a protective mem-brane architecture and stable ionic liquid (IL) electrolyte.[11, 12]

Although much attention has been paid to the developmentof long-term-stable lithium–air batteries by using various ap-proaches, instability issues remain to be addressed in this field.As these unstable systems inevitably undergo rapid fading ofcapacity and high energy loss, the lithium–air battery will be oflittle practical significance unless the overall instability issue isproperly resolved.

In summary, to obtain a high-performance lithium–air bat-tery, instability is the key obstacle to be tackled. In this Mini-review, the instability associated with the different parts of lith-ium–air batteries, including the electrode, the electrolyte andthe ambient environment, is discussed, and insights into thedevelopment of advanced stable materials and structures areprovided. The aim of this Minireview is to give the readera comprehensive understanding on the stability of lithium–airbatteries.

2. Electrodes

An electrode is the intermediate bridge between the outsideatmosphere and the inner microstructure of the lithium–airbattery. Electrochemical performance correlates with Li2O2 dep-osition, air diffusion and decomposition in an electrode. Thecathodes utilized in lithium–air batteries are typically catalyst-based porous carbons. Much effort has been devoted to in-creasing the capacity retention and lowering the kinetic over-potential.[22–26] Existing literature has also demonstrated thatthe cyclical stability can be improved by controlling the cath-ode reaction and optimizing the cathode morphology.[25] Re-garding the mechanism, Xiao et al.[27] have investigated variouscarbon materials to explore the effect of pore microstructureas well as porosity and thickness. Among all the studiedcarbon sources, the highly porous Ketjen black (KB)-based elec-trode gave the best performance after assembly (Figure 3 andTable 1). Due to the high porosity of KB-based electrodes andtheir unique chainlike structure, such lithium–air batteries ex-

Figure 1. The gravimetric energy densities (W h kg�1) for various types of re-chargeable batteries compared with gasoline. Adapted from Ref. [9] .

Figure 2. Aprotic (left) and aqueous (right) lithium–air batteries. Adaptedfrom Ref. [17] .

Figure 3. Comparison of the discharge capacities of lithium–air batteriesusing different carbon sources: Ketjen black (KB), Calgon, ball-milled KB,BP2000, JMC (mesoporous carbon), and Denka. The preparative processesare presented in Ref. [27] .

Table 1. Surface area and pore volume comparison. Pore size distributionwas evaluated using the Barrett–Joyner–Halenda (BJH) method. Adaptedfrom Ref. [27] .

Surfacearea [m2 g�1]

Pore volume[cm3 g�1]

BJH poresize [nm]

Microstructurefrom XRD

KB 2672 7.6510 2.217–15.000

poorly crystallinegraphite

Ball-milled KB

342.4 0.4334 no clearpeak

amorphous

BP2000 1567 0.8350 no clearpeak

poorly crystallinegraphite

Calgon 1006 0.5460 no clearpeak

crystallinegraphite

Denkablack

102.0 0.5355 2.511 &6.000

poorly crystallinegraphite

JMC 548.7 0.2376 3.0–3.8 amorphous withorderedmesopores

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hibit high specific capacity. The results indicate that thevolume is sufficient to provide extra reaction and storage re-gions, which is favorable for achieving high battery stability.Tran et al.[28] also reported that the capacity of batteries islargely linearly correlated with electrode pore size (Figure 4).

This work indicates that the performance of the lithium–airbattery is largely affected by the porosity and pore size of elec-trodes as well as the cathode reaction. An electrode with highporosity is afforded efficient diffusion, whereas sufficientlylarge pores are preferable for Li2O2 accommodation. In this sec-tion, we will discuss first product deposition and then carbondecomposition. Following on, some advanced cathode materi-als are introduced.

2.1. Li2O2 Deposition

In an organic solvent, the main product Li2O2 is insoluble, andupon discharge Li2O2 deposits on the electrode pore surface toform a thin layer (Figure 5).[29–31] The deposition layer can1) clog a sufficient number of pores to stop the liquid frompermeating, 2) limit the air diffusion rate, and 3) lead to lowcharge–discharge efficiency. Among these problems, the firsttwo result in energy loss and the third leads to a sudden dis-charge voltage drop as well as large polarization, which makesthe practical capacity much smaller than the theoreticalvalue.[28–36] Tests on various carbon-based air electrodes haverevealed that expansion of the Li2O2 storage region can deliverhigher performance compared to smaller areas, indicating thatthe accommodation of Li2O2 requires abundant volume.[37]

Both experiment and numerical simulation show high overpo-tentials during charge–discharge cycles, induced mainly by sur-face deposition. Here, the interface passivation in performancevariation is dominant compared with solvent and O2 transportissues, and polarization is also mainly due to passivation.[35]

Deposition layers can obstruct the contacts between O2 andcatalyst/electrode. The pore surface provides active reactionsites for O2 reduction. O2 is difficult to sufficiently transport in

or adhere to reaction sites, which influences in part the batteryperformance. More importantly, the Li2O2 layer restricts theelectrical conductivity of electrodes. It has been demonstratedthat a certain amount of Li vacancies in the layer enable thesurface to be conductive.[37] Radin et al.[38] used first-principlescalculations to explore the surface properties of Li2O2 layers,and the results indicate that Li2O2 has pronounced conductivi-ty, whereas no conduction was observed in Li2O or LiCO3.Hummelshøj et al.[39] proposed a model to describe the Li2O2

deposition and its influence on conductivity. Density functionaltheory (DFT) was used to identify the origin of the overpoten-tial. In their report, the discharge overpotential was deter-mined to be 0.43 V for discharging and 0.6 V for charging re-sulting from surface deposition. Albertus et al.[36] showeda gradual decrease of discharge voltage from an initial 2.6 V,and a sudden drop to approximately 2.0 V. Furthermore, Viswa-nathan et al.[35] proposed a first-principles metal–insulator–metal (MIM) charge-transport model to probe the Li2O2 surfaceconductivity (Figure 6). They stated that the tunneling currentshould support a high operating current, and that the MIM in-terface is critical for obtaining high stability. They describeda dramatic drop—termed “sudden death”—that occurs if thedeposition layer thickness is in the range of approximately 5–10 nm. Once the operating current exceeds the tunneling cur-rent, the sudden death occurs.

2.2. Carbon Electrodes

Carbon is widely used as cathode material because it is cheapand easily incorporated into a porous electrode.[40–42] However,its actual effectiveness is often debated, due to its poor stabili-ty at the micro-/nanoscale.[19, 21] The electron/oxygen ratio isnear the ideal stoichiometric value of 2.000, based on quantita-tive differential electrochemical mass spectrometry (DEMS),and most side reactions occur after the first discharge.[19, 43–44]

The intermediate radical product O�2 and dominant reaction

Figure 4. Discharge time and specific capacity as a function of average porediameter. Adapted from Ref. [28].

Figure 5. Various growth modes of discharge products : a) cylindrical film,b) spherical film, and c) planar film. Adapted from Ref. [31] .

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product Li2O2 are both reactive, and can attack carbon or elec-trolyte. McCloskey et al.[44] revealed that Li2CO3 and LiRCO3 (R =

alkyl) are produced at the carbon–Li2O2 interface and Li2O2–electrolyte interface, respectively. Such “interfacial carbonateproblems” lead to an extra overpotential (Figure 7). Whencharged, carbon reacts chemically with Li2O2 in a high-oxidiz-ing environment according to 2 Li2O2 + 2 C + O2!2 Li2CO3 or2 Li2O2 + C!Li2O + Li2CO3. 13C was used to investigate the gen-eration of Li2CO3, as it could be distinguished from electrolyte.Thotiyl et al.[21] suggested that the reactions between Li2O2 andcarbon occur mostly at high charging potentials (U>3.5 V).They stated that during the first discharge, electrode decom-position is subtle, whereas the side reactions involving the de-composition of the electrolyte are dominant. Interestingly, fol-lowing cycles of charge–discharge, the amount of Li2CO3 in-creases with increasing voltage. As Li2CO3 is formed from the

decomposition of carbon, it begins to accumulate above 3.5 V,especially during charging.

The reactions of Li2O2, C, and Li2CO3 result in the formationof a thin layer at the cathode–Li2O2 interface. The formation ofCO2 indicates that Li2CO3 is formed simultaneously and is par-tially reduced. The electrochemical reduction of Li2CO3 pro-ceeds continuously during charging. However, such reductionis incomplete. In contrast to Li2O2, Li2CO3 is completely insulat-ing and precipitates on the reaction sites causing electrodepassivation. However, only a small proportion of Li2CO3 forma-tion occurs by the reaction between Li2O2 and carbon duringcycling.[21] In subsequent cycles, the electrolyte and carbonelectrode decompose continuously to form Li2CO3 and lithiumcarboxylates. Once carbon starts to form Li2CO3, the chargingvoltage rises rapidly. An experiment using a porous gold cath-ode showed less electrolyte decomposition compared with thecarbon-based cathode in the same electrolyte (DMSO), whichindicates that carbon not only decomposes itself but also pro-motes the electrolyte decomposition.[21, 45] In addition, the hy-drophobicity/hydrophilicity of the surface also impacts the sta-bility of the carbon and electrolyte.[21] Batteries based on hy-drophobic carbon are more stable compared to hydrophiliccarbons. This is possibly due to the existence of C�O, COOH,and C�OH groups. In summary, the direct reaction betweenLi2O2 and carbon is destructive when the charging potential isgreater than 3.5 V. Accumulation of Li2CO3 from carbon de-composition leads to high polarization and also stimulateselectrolyte decomposition.

2.3. Recent Stable Electrodes

As carbon is unstable in a highly oxidizing environment, andits decomposition promotes the decomposition of the electro-lyte, research has focused on new materials to replace carbon.

One example of an air electrode is the nanoporous gold(NPG) cathode, which was proposed by Bruce et al.[45] Afterseveral cycles, such cathodes show great stability (Figure 8).NPG cathodes in a DMSO electrolyte can retain 95 % of theircapacity after 100 cycles. During charge and discharge, the re-action is dominated by Li2O2 formation/decomposition. Fur-thermore, the NPG cathode promotes efficiently Li2O2 forma-tion/decomposition. The kinetics of Li2O2 oxidation was dem-onstrated to be tenfold faster than that of normal cathodes.NPG cathodes are known as “concept cathodes” because theyare currently too expensive to be industrially manufactured.Furthermore, the high mass fraction of gold compromises oneof the most important merits—the high specific energy densi-ty—rendering batteries containing these cathodes of little use.However, NPG cathodes represent significant progress towardsthe future of cathode materials by enabling fabrication ofstable Li–O2 batteries with a stable cathode.

In a recent report, TiC-based cathodes proposed by Thotiylet al.[46] outperformed a NPG cathode (Figure 9). TiC is fourtimes lighter than an NPG cathode and shows more robust sta-bility. Fewer side reactions were detected at the cathode–elec-trolyte interface. The authors showed that the TiC-based Li–O2

battery retained capacity of greater than 98 % after 100 cycles

Figure 6. Typical conductivity calculation setup for a) Au jLi2O2 jAu andb) Au jLi2O2–LiO2 jAu. Adapted from Ref. [35] .

Figure 7. Upper panels indicate the deposition events during charging thatcause the rising charging potential. An approximately single monolayer ofLi2CO3 forms at the carbon surface, some dispersed carbonate possiblyforms as a result of electrolyte decomposition in the Li2O2 deposit, and anapproximately single monolayer forms at the electrolyte interface duringcharging. e� represents an unspecified electrochemical reaction that produ-ces carbonate at the interface during charging. The dashed arrows indicatequalitatively the charging potential appropriate for the three panels. Adapt-ed from Ref. [44] .

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(compared with 95 % for NPG-cathode-based batteries). The re-sults of Fourier transform infrared (FTIR) spectroscopy demon-strated that Li2O2 is the dominant discharge product duringcharge–discharge. Only a minor amount of carboxylates pro-duced by electrolyte decomposition was detected. To investi-gate the origin of the stability, X-ray photoelectron spectrosco-py (XPS) data on the TiC cathode at every stage of the reaction

were collected. At the beginning of discharge, the amount ofTiO2 increases noticeably and it gradually becomes the mainsubstance on the surface. XPS results indicated that a thinlayer containing TiO2 and TiOC formed on the surface of thecathode, which was believed to be responsible for the stabilityof the cathode.

In addition, the investigation into the application of differentallotropic carbon nanoforms is also a promising strategy. De-spite the cathode reaction, advanced carbon electrodes withspecial bimodal pore size distributions are promising candi-dates for long-term-stable lithium–air batteries. Among variousallotropic carbons, carbon nanotubes, carbon nanoballs andgraphene have been widely investigated.[22–24] For instance,Xiao et al.[22] fabricated hierarchically porous graphene as theelectrode of the lithium–air battery. Using these electrodesthey obtained a capacity of 15 000 mA h g�1. The distributionsof pore sizes in this electrode are binomial, which is differentfrom traditional electrodes. The improved properties were alsodemonstrated in high-performance carbon nanoball electro-des.[23] A capacity larger by 30 %–40 % compared with KB wasobtained with a carbon nanoball electrode. Carbon nanotubes,similarly, due to their large pores and high porosity also gavesuperior performance.[24] In these electrodes, large pores canaccommodate the deposition products and small pores can fa-cilitate air diffusion. However, a thorough investigation intothe stability of such electrodes is required to improve the cath-ode reaction.

3. Electrolyte

3.1. Organic Solvents

Lithium metal is a reactive mate-rial that needs to be protectedin batteries. The direct contactof lithium with an aqueous solu-tion results in the reaction 2 Li +2 H2O!H2›+ 2 LiOH. Lithiummetal reacts rapidly if aqueouselectrolytes are used (discussedin following section) unlessa membrane separator is used.Current aprotic lithium–air bat-teries usually use organic saltsdissolved in organic solvents asthe electrolytes. The first-genera-tion electrolyte applied in thelithium–air battery was ethylenecarbonate associated with LiPF6,introduced by Abraham in1996.[10] For the first time, suc-cessful recharge performanceand a relatively high capacity(800 mA h g at 1 mA cm2) wereachieved. However, poor cyclicperformance and low coulombicefficiency limited the widespread

Figure 8. Charge–discharge curves (top) and cycling profile (bottom) fora Li–O2 cell with a 0.1 m LiClO4/DMSO electrolyte and an NPG cathode, ata current density of 500 mA g�1 (based on the mass of Au). Adapted fromRef. [45] .

Figure 9. Cycling curves and capacity retention of TiC cathodes. a) Galvanostatic discharge–charge cycles recordedin 0.5 m LiClO4 in DMSO at a geometric current density of 1 mA cm�2. b) Capacity retention for the same cell as in(a). c) Galvanostatic discharge–charge cycles recorded in 0.5 m LiPF6 in TEGDME at a geometric current density of0.5 mA cm�2. d) Capacity retention for the same cell as in (c). Adapted from Ref. [46] .

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application of such a carbonate solvent. Propylene carbonate(PC), ethylene carbonate (EC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), or their mixtures are substitutes pro-posed by others.[47] For example, using Super P as the air elec-trode and PC as solvent, Read[43] obtained the capacity of1934 mA h g at a current density of 0.05 mA cm�1. Sun et al.[41]

loaded CoO nanoparticles onto a mesoporous carbon (CMK-3)cathode, with PC as the solvent. The cycle performance wasbetter than previously reported results, and a 95 % capacity re-tention was observed after 15 cycles. Cyclic performance andvoltaic efficiency were increased correspondingly, but were stillfar from the standard required for practical application. Furtherresearch on organic carbonate solvents revealed the reactionmechanisms during the oxygen reduction reaction (ORR) andoxygen evolution reaction (OER), and eventually showed thedrawbacks of using such solvents. The mechanism was shownto involve attack on the electrode or electrolyte by superoxideradical anions or peroxide anions produced during dischargeattacks the electrode or electrolyte (Scheme 1). Such a mecha-

nism was then accepted widely to be the reaction principle forelectrolyte decomposition, which also demonstrates that alkycarbonate should not be used in long cycle lithium-air batter-ies. It has been widely proposed that during discharge, radicalO�2 is the intermediate product.[14, 48] Due to the reactivity of O�2and the subsequent radical product O2�

2 , the solvent decom-poses rapidly during the subsequent discharge process.[19, 49, 50]

Freunberger et al.[51] reported side reactions with the formationof C3H6(OCO2Li)2, HCO2Li, CH3CO2Li, CO2, and water during dis-charge (Figure 10). After several cycles, side products accumu-late rapidly. The same results were also observed byothers.[52, 53] This stability issue over highly active radicals is piv-otal for the electrolyte test. As the organic carbonates havebeen associated with disastrous drawbacks, much effort hasbeen focused on other solvents.

Ethers, used widely after the alkyl carbonates were pro-posed, were investigated by others.[54, 55] Because of their stabil-ity at a high charge potential, ethers were considered originallyas promising electrolyte solvents. However, as research contin-ued, the instability issues of solvents were demonstrated tostill exist even for ethers. Although ether moieties are generallymore stable than carboxyl groups, the cyclic performance andcharge–discharge efficiency with ethers is far from the desiredlevel. McCloskey et al. and Bruce et al. and many other stud-ies[19, 56, 57, 58] investigated the stability of the widely used etherdimethoxyethane (DME; Figure 11). Although the major dis-charge product in the ORR was Li2O2, side products such as

Li2CO3, HCO2Li, and C2H4(OCO2Li)2 formed at the initial cyclesand continued to accumulate. The use of isotope tracer meth-ods also indicated that the side products were mainly fromelectrolyte decomposition. In addition to DME, tetraethyleneglycol dimethyl ether (TEGDME) is a promising alternative or-ganic solvent and is now commonly used. Scrosati et al.[42] re-ported a high-performance lithium–air battery with the capaci-ty of 5000 mA h gcarbon

�1 at a relatively high current density of3 A gcarbon

�1. The high performance is believed to be associatedwith the chemical inertia of the optimized TEGDME–LiOTf elec-trolyte. The lifetime of O�2 is relatively short, and it is nearly in-stantly involved in the reaction that forms Li2O2.

As well as TEGDME, DMSO is another stable solvent accord-ing to several recent reports. Stable cathode-based examina-tion shows that the quantity of decomposition product withTEGDME is double that of the DMSO-based electrolyte. Al-though both electrolytes undergo little decomposition,TEGDME showed a higher polarization increase on cycling.[45]

In addition, Mozhzhukhina et al.[59] tested the stability of DMSO

Scheme 1. Molecular oxygen reduction in aprotic systems. Adapted fromRef. [48] with permission. Copyright 2011 Wiley-VCH.

Figure 10. FTIR spectrum of a composite electrode (SuperP/R-MnO2/Kynar)discharged in 1 m LiPF6/PC to 2 V in O2. The reference spectra for Li2O2 (smallimpurity peaks at 1080, 1450, and 1620 cm�1), Li2CO3, and the electrodebefore discharge are also shown. Adapted from Ref. [51] .

Figure 11. 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-based cell.The spectrum of neat P50 carbon paper is included for comparison. Dis-charge conditions: 0.09 mA cm�2 under O2 to 2 V. Adapted from Ref. [19] .

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using infrared spectroscopy and demonstrated that it wasstable during discharge. However, under a high potential, theDMSO undergoes electrochemical oxidation and the formationof dimethyl sulfone was observed.[59] The experiment was con-ducted using the porous gold cathode, which eliminates theinfluence of carbon decomposition. The formation of dimethylsulfone was detected at a charge potential above approxi-mately 4.3 V. Furthermore, DMSO has a high volatility. Thesame challenges occur with the use of diethyl ether. Metal lith-ium undergoes oxidization in DMSO, which is another limita-tion for the stability of this solvent. For further information onelectrolyte, the reader is referred to selected books and re-views.[60–62]

ILs, which have been used widely in the field of traditionalbatteries, are also considered to be promising electrolytes forlithium–air batteries. Their high specific heat capacity allowsthem to operate under special conditions. The use of ILs im-proves stability of the battery, and few side reactions andproducts are observed.[63–66] However, their high viscosity andlow conductivity limit battery performance. Xu et al.[66] revealedthe suitability of ILs as solvents, and showed that a cell con-taining N-methyl-N-butylpyrrolidinium bis(trifluoromethanesul-fonyl)imide (Pyr14TFSI) can be only discharged at low operatingcurrents. Further investigation of IL-based lithium–air batteriesis needed.

3.2. Lithium Salts

Organic salts such as lithium tetrafluoroborate (LiBF4), lithiumbis(oxalato)borate (LiBOB), lithium trifluoromethanesulfonate(LiOTf), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithi-um perchlorate (LiClO4), lithium hexafluorophosphate (LiPF6),lithium bromide (LiBr) and lithium nitrate (LiNO3) are highly as-sociated with the ionic conductivity of electrolytes, and greatlyinfluence the formation of the solid–electrolyte interphase. Thedecomposition of salts is also a problem impacting the stabili-ty. The capability of anti-O�2 (and O2�

2 ) is also necessary for thelithium salts. However, the decomposition of lithium salts isslightly less than of the solvent, which still causes a reductionin capacity and irreversibility.[67]

LiPF6 is widely used in both Li-ion and lithium–air batteries,and undergoes hydrolysis toproduce HF and LiF. In the initialstudy, Oswald et al.[67] stated thatLiPF6, LiClO4, and LiBOB all de-compose upon contact with re-active Li2O2. Other studies alsoconfirmed that LiPF6 decom-posed to form LiF.[55, 68] LiBOB de-composes to form boron oxideand lithium carbonates.[69] Thedegradation of organic salts isattributed to both chemical andelectrochemical effects. A sys-tematic investigation was con-ducted by Nasybulin et al.[70] to

explore the stability of different common lithium salts in lithi-um–air batteries, and their effects (Figures 12 and 13). X-raydiffraction (XRD), NMR, and XPS methods were used to investi-gate the decomposition products. Cyclic performance was as-sociated with salt decomposition by discharge–charge voltageprofiles and XPS results. This research indicates that salts, andnot only solvents, are also an important influence on the per-formance of lithium–air batteries. Of the seven lithium saltstested, LiClO4 is the most stable. Other salts form LiF (LiPF6,LiOTf, LiBF4, LiTFSI) or Li2C2O4 (LiBOB), as detected by XPS(Figure 13). LiBr, analogous to LiNO3-based electrolyte, due toits low conductivity in a glyme-based solvent, showed a some-what low capacity (0.2 Ah g�1). However, this is still a novelidea, because no adverse reaction was observed using LiNO3 inN,N-dimethylacetamide.[71] For traditional lithium salts, an inter-esting phenomenon is that the LiClO4-based battery does notdeliver the highest capacity during the cycle, which indicatesthat the degradation of salts is only a small factor in overallperformance. The decomposition of solvent contributes mostto the instability.

4. Ambient Factors

The high energy density of lithium–air batteries is partially theresult of using ambient air. The environment provides a sustain-able cathode-active material, O2, allowing higher energy stor-age compared with other modern batteries. However, ambientair is a double-edged sword, and also introduces undesiredgases into the battery interior. Among the destructive gas spe-cies, CO2 and the humidity in ambient air are the primary con-taminations.[72, 73, 76] First, water and CO2 attack the dischargeproduct Li2O2 to form Li2CO3 on its surface, according to the re-actions 2 H2O + 2 Li2O2!4 LiOH + O2›, 2 Li2O2 + 2 CO2!2 Li2CO3 + O2 and 2 LiOH + CO2!Li2CO3 + H2O. After a fewcycles, Li2CO3, instead of Li2O2, is predominant even withoutelectrolyte or electrode decomposition. The potential forcharging Li2CO3 to evolve CO2 (>4 V) is much higher thancharging Li2O2 (�3.0–3.5 V), which enhances the potential gapbetween charge and discharge.[74] Second, the anode is con-

Figure 12. a) First-cycle discharge–charge voltage profiles for Li–O2 batteries with various electrolytes at0.05 mA cm�2 current density. b) Cycling stability of Li–O2 batteries with various electrolytes at 0.05 mA cm�2

current density. Adapted from Ref. [70] .

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sumed by dissolved water, according to the reactions 2 Li +2 CO2 + O2!2 Li2CO3 and 4 Li + 2 H2O!2 Li2OH + H2›. Third,salts are also hydrolyzed with permeated water. The overall

effect of water was investigatedby Meini et al.[75] As shown inFigure 14, in the LiClO4/DMEelectrolyte, with a small amountof water present in the air, thecapacity was increased largely atthe first discharge. As it is pro-posed that the capacity of thelithium–air battery is limited bythe resistivity of Li2O2, the con-sumption of Li2O2 by water (toform soluble LiOH or H2O2)might be the reason for the in-crease of capacity in the initialcycles. However, assuming thatthe permeated water corrodesthe lithium anode and salts, thelithium–air battery should bekept free of water in the longterm. On the other hand, waterand CO2 react to form LiCO3 onthe surface, and the largecharge–discharge potential gapand the origin of CO2 are shownin Figure 15.A high charge potential is de-structive to the electrolyte andelectrode. However, most cur-rent lithium–air batteries havebeen examined under ideal con-ditions, and many excellent re-sults were obtained by operationunder pure O2. If ambient factorsare not taken into account, thehigh performance obtainedunder pure O2 conditions isviable for further applications. Asresearch continues, both unde-sired polluters and means ofprotection should be carefullyconsidered.[75]

To improve the stability of lith-ium–air batteries operated underthe ambient conditions, early ap-proaches reduced the electrodediffusivity to decrease the diffu-sion of both undesired anduseful species by other porousmembranes.[77] However, ineffi-cient air diffusion in lithium–airbatteries is a key challenge, asthe primary diffusivity is relative-ly low due to the nanostructuredpores.[33, 78] More importantly, al-

though the electrode diffusivity is decreased, a molecule ofwater is smaller than O2, and water therefore permeates intothe electrolyte. An optimized selective membrane was then

Figure 13. XPS results for a) O 1s scan of the discharge products, b) Li 1s scan of the discharge products, c) F 1sscan of the discharge products and the pure salts, d) S 2p scan of the discharge products and the pure salts,e) P 2p scan of the discharge products and the pure salts, f) C 1s scan of the discharge products, g) B 1s scan ofthe discharge products and the pure salts, and h) Cl 2p scan of the discharge products and the pure salt. The yaxis stands for the intensity (counts) of photon electrons. Adapted from Ref. [70] .

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proposed by Zhang et al.[77] O2-selective silicone oils wereloaded on porous membranes or polytetrafluoroethylene films;due to synergetic effects on moisture blocking, O2 must becarefully selected. Another air-diffusion layer inevitably de-creases the diffusivity of electrodes. With increasing cathodethickness, the maximum current density decreases. As a result,one more selective layer, due to its harmful obstruction is alsounrealizable unless an advanced and high-performance materi-al is synthesized for this purpose in the future. In addition, N2

can permeate easily into the selective membranes, which de-mands further investigation.

Furthermore, as presented above, O2 is of low solubility inthe organic electrolytes currently used. Read et al.[32] reportedthe critical role of improving the solubility of O2 for increasingthe capacity. The diffusion of O2 depends partly on the attrac-tive force generated from the solubility. The O2 transport ratecan further decrease once the reactive product (Li2O2) depositson the porous electrode surface. The pores of electrodes canalso be blocked by organic electrolyte,s as the attractive forcebetween O2 and the electrolyte is rather small.[79] Early work in-dicated that organic solvents such as diethyl ether and DMSOvolatilize in the exposed environment.[45] The organic solvents

in Li-ion batteries, which are closed systems, are relativelystable. In contrast, lithium–air batteries are semi-open systemsas the cathode exchanges gas species directly with the exter-nal environment. The high volatility of electrolytes apparentlyreduces the solvent in these systems, and as the cycle numberincreases the loss of electrolytes is substantial. Selective mem-branes or low-diffusivity electrodes can reduce solvent evapo-ration, but can cause other negative issues. Zhang et al.[80] in-troduced a blend of tris(2,2,2-trifluoroethyl) phosphate (TFP)/PC organic solvents as a low-volatility electrolyte. The evapora-tion rate of liquid blend decreases after the addition of TFP,making such batteries safe to operate under ambient condi-tions. However, the ion conductivity is also reduced, leading toa high overpotential. Interestingly, most current solutions tostability issues sacrifice the energy utilization efficiency.

5. Aqueous and Nonaqueous systems

Currently available lithium–air batteries consist of four electro-lytic types—aqueous, nonaqueous, hybrid, and solid state(Figure 16).[8] In general, an organic liquid is used in the non-aqueous electrolyte type and the reaction product is Li2O2.However, the application of Li+-conducting membranes makesit possible to replace unstable nonaqueous electrolytes withaqueous electrolytes. Wang et al.[82] have proposed a new typeof lithium–air rechargeable battery, by applying a special aque-ous electrolyte. The all-organic electrolyte was replaced with

Figure 14. a) Discharge capacity (1st cycle) comparison of Li–O2 cells with0.1 m LiClO4 in DME with water-free, or water-contaminated, oxygen(100 kPa(abs)), and Li–O2 cells with water introduced by means of a small leakbetween the cell and ambient air (a) in the “leaker cell”, or by connectinga water reservoir to produce H2O-saturated O2 inside the cell (g) in the“water vapor cell”. b) Discharge capacity (1st cycle) comparison of sealed Li–O2 cells using water-free, or deliberately water-contaminated electrolyte(0.1 m LiClO4 in DME) with 250 (a), 500 (g), and 1000 (d) ppm ofwater. The cells were discharged galvanostatically at 120 mA gcarbon

�1

(0.05 mA cm�2 electrode) after a 30 min rest period at open-circuit voltage(OCV) in pure 100 kPa(abs) O2. c) Nyquist impedance plots of Li–O2 cells usingwater-free (*) and water-contaminated (1000 ppm, *) electrolytes regis-tered after the 30 min OCV rest period (100 kHz to 0.1 Hz at an AC perturba-tion of 5.0 mV). The water vapor and leaker cells were discussed in Ref. [75] .Adapted from Ref. [75] .

Figure 15. Galvanostatic discharge and charge (0.47 mA cm�2) of cells dis-charged under a pure 18O2 headspace (a1–c1) and a 10:90 C18O2/16O2 mixture(a2–c2). The (a) panels present the discharge profiles. The (b) panels presentthe O2 evolution rate (n’O2) during the cell charge. Only single isotopes, thatis, 18O2 (b1) and 16O2 (b2), were evolved from each cell and corresponded tothe isotope used in the discharge headspace. The (c) panels present the iso-topic CO2 evolution rate (n’) during the cell charge. Cells were chargedunder an Ar headspace. Adapted from Ref. [74] .

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an organic liquid jLi+-conducting membrane jaq. electrolytesystem. The membrane cannot contact the metal lithium di-rectly, and thus an organic layer or IL is typically used to pro-tect the anode. O2 is consumed continuously during dischargeto form LiOH. Aqueous lithium–air batteries are of higher sta-bility compared with their nonaqueous counterparts in manyaspects. This type of lithium–air battery can be operated safelyin the ambient air as the dissolved water and CO2 affects metallithium only slightly. Only Li+ is capable of being transportedthrough conducting membranes, and CO2 and water are con-fined to the cathode side. Due to the solubility of LiOH, elec-trode passivation is alleviated compared with the nonaqueouslithium–air battery. Side reactions in the aqueous electrolyteare much fewer. Compared with the organic liquid, the aque-ous electrolyte is rather stable, even though radical ions attackit. The boiling point of water is higher than most organic sol-vents, therefore solvent evaporation occurs much less thanwith an organic liquid. O2 dissolves in aqueous electrolytesmore easily, which produces a more stable energy output.However, an aqueous system has its own problems of instabili-ty. Separated by the Li+-conducting membranes, the aqueouselectrolyte containing LiCl or LiNO3 is usually applied in theoxygen half-cell. A Li+-conducting glass or ceramic (e.g.Li1.35Ti1.75Al0.25Si0.1P0.9O12, LTAP) is typically used as the mem-brane. The solubility of LiOH is only approximately 13 g in100 g of room-temperature water, and most of the material ad-heres to the Li+-conducting membrane and positive electrodepores, thereby clogging them. Capacity reduces rapidly afterapproximately five cycles (in 10 m LiCl with only a protectedcathode).[81] For precipitating LiOH in the electrolyte instead of

at the cathode or ceramic separator, a special design is nowwidely used (Figure 17).[81, 91]

Current lithium-conducting membranes are unstable instrongly acidic or basic solutions.[83–85] After ORR, the productLiOH increases the alkalinity of the electrolyte. The develop-ment of Li+-conducting membranes is considered key to in-creasing the stability of the aqueous lithium–air battery.Hasegawa et al.[87] investigated the stability of NASICON-type glass ceramics(Li1+ z + yAlxTi2�xSiyP3�yO12), and LATP in aqueous 1 m LiOH and0.1 m HCl. XRD and scanning electron microscopy results indi-cated that after three weeks, either LiOH- or HCl-saturatedLTAP membranes were destroyed to different extents. Follow-

ing this work, Shimonishi et al.[83]

investigated the stability of anNASICON-type glass ceramic(LTAP) in an aqueous solution ofa weak acid. After immersion inan acetic acid/lithium acetate so-lution at 50 8C for three weeks,no apparent change in conduc-tivity was observed. The test re-sults suggest that LTAP is stablein both neutral and weaklyacidic solutions, but not instrongly acidic or alkaline elec-trolytes. To protect the Li+-con-ducting membrane, a lithiumacetate/acetic acid solution isproposed as the cathodic elec-trolyte. However, the catalystchoice is limited by the acidicsolution and only acid-resistantmaterials such as noble metals(e.g. Pt, Au) can be used. Utiliz-ing such catalysts notably im-proves cost effectiveness. Anoth-er approach is to utilize a con-

Figure 16. Types of lithium–air batteries: a) aqueous electrolyte, b) aprotic/nonaqueous electrolyte, c) mixed(aprotic-aqueous), and d) solid state. Adapted from Ref. [8] .

Figure 17. The bi-electrode design. The composite air electrode contains ananionic polymer electrolyte and the ceramic separator has been developedwith a cationic polymer. Adapted from Ref. [91] .

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centrated lithium salt as the electrolyte, such as 10 m LiCl solu-tion. Due to the common-ion effect, the reversible reaction,LiOHQLi+ + OH� , occurs and, therefore, the pH of decreases to8.14.[92] In addition, the ion conductivity of the lithium-con-ducting membrane is low, and we can expect advances in theconducting membrane field.

In addition to the separators, many other issues still exist.During the charging process, the OER forms LiOH instead ofLi2O2 by the reaction 4 Li+ + O2 + 2 H2O + e�!4 LiOH. The LiOHin the electrolyte also reacts with CO2 from the air. Water hasa narrow electrochemical window, leading to dramatic solventconsumption after several cycles. More importantly, an inextri-cable problem in all Li-based batteries is the formation of lithi-um dendrites.[86] Huge capacity degradation is predicted asa result of the formation of lithium dendrites. The dendrite for-mation typically occurs in an aqueous lithium–air battery dueto the special Li-separator design. Lithium dendrites enhancethe possibility of attaching the separator to bring about degra-dation of the fragile membrane. Like the aqueous systems, thesolid-state lithium–air battery structure is also attracting in-creased attention due to its evident stability.

Kumar et al.[88] presented a solid-state rechargeable lithium–air battery. The liquid electrolyte was replaced with a water-proof glass ceramic and a polymer ceramic to deliver stable re-charge performance. The solid membrane in the lithium airbattery is non-volatile and stable. In nonaqueous batteries, theelectrolyte saturates the air electrode and reacts with theactive radicals O�2 or O2�

2 in an undesired side reaction. Where-as the solid electrolyte facilitates Li+ transport, less decomposi-tion means it is stable even when being operated under ambi-ent air conditions. However, solid electrolytes have a large re-sistance. Studies also show that battery conductivity increasesonly if the operating temperature increases.[88] Based on thosefactors, current solid-state lithium–air batteries are typically op-erated at high temperatures. Interface stability is also a bigissue, as the system is assembled in a solid-solid fashion in thecathodic cell. In summary, although solid-state lithium–air bat-teries are stable, the overall performance is limited by the ma-terials. Ceramics or polymers are of great stability but are lesseffective; the high internal resistance of such a material is thecritical drawback and remains to be addressed.

6. Prospects

For further advances in the field of lithium–air batteries, stabili-ty issues must be addressed. First, stable electrodes and elec-trolyte materials are in urgent demand. Advanced electrolytesand electrodes are expected to undergo less decompositionduring cycles.[90, 93] The stability of electrodes of allotropiccarbon nanoforms, such as graphene, deserves a more detailedinvestigation. Second, a more systematic examination of ambi-ent operation should be conducted in the future and devel-oped. The O2-selective membranes represent significant prog-ress towards this, and in this respect, high-selectivity mem-branes are worth investigating. On the inside of the system,protective measures, including the use of LTAP membranes inthe aprotic lithium–air battery, also have the possibility to be

developed further.[11, 89] Third, suitable cathode materials andcatalysts have the promise to reduce the overpotential andkeep the system stable.[94–99] Future solutions to these issueswill facilitate the development of the lithium–air battery, andbe source of inspiration across the entire field of lithiumbatteries.[100]

7. Summary

The recently uncovered instability issues largely limit the appli-cation of the lithium–air battery. In this Minireview, the originof these instabilities and the effectiveness of a few recentlyproposed solutions have been discussed. Different types of sta-bility have been reviewed at the end of this article. The latestreports have shown promising prospects for the application ofthe lithium–air battery for solving the major issues that includeelectrolyte decomposition, ambient operation, and anodeprotection.

Keywords: electrochemistry · electrode · electrolyte ·instability factors · lithium–air batteries

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Received: September 17, 2014

Published online on && &&, 2014

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Minireviews

MINIREVIEWS

L. Ye, W. Lv, J. Cui, Y. Liang, P. Wu,X. Wang, H. He, S. Lin, W. Wang,J. H. Dickerson, W. He*

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Lithium–Air Batteries: PerformanceInterplays with Instability Factors

Air power: The stability of the lithium–air battery, with respect to electrodes,electrolytes, and ambient conditions, isreviewed. This Minireview is also fo-cused on the effects of overpotential,cyclic performance, capacity degrada-tion, and recent major solutions, andaims to illustrate the correlation be-tween instability and these aspects ofbattery performance.

ChemElectroChem 0000, 00, 0 – 0 www.chemelectrochem.org � 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim13 &

These are not the final page numbers! ��These are not the final page numbers! ��