Prospects, challenges, and latest developments in lithium-air batteries

REVIEW PAPER

Prospects, challenges, and latest developments inlithium–air batteriesNaveed Akhtar1,*,† and Waheed Akhtar2

1Material Science and Engineering School, Beihang University, Beijing, China2Department of Chemistry, University of Wah, Wah Cantt, Pakistan

SUMMARY

Metal–air batteries are being envisioned as a clean and high energy fuel for the modern automotive industry. The lithium–air battery has been found most promising among the various practically applicable metal–air systems, that is, Al–air,Li–air, Mg–air, Fe–air, and Zn–air. The theoretical specific energy of the Li–air battery is ~12 kWh/kg, excluding theoxygen mass. This is comparable with the energy density of gasoline, which is ~13 kWh/kg. It has been hypothesized thatthe Li–air battery could supply an energy ~1.7 kWh/kg after losses from over potentials to run a vehicle ~300miles on a singlecharge. During the first decade of this century, a fair amount of research has been conducted on Li–air battery system. Yet,Li–air batteries could not make an industrial breakthrough, and are still in the laboratory phase since their birth. In thisarticle, we technically evaluated the recent developments, and the inferences have been analyzed from the practical/com-mercial point of view. The study concludes that low discharge rate, lower number of cycles, oxidation of lithium anode,discharge products at the cathode, and side reactions inside the battery are the key limiting factors in the slow progressof Li–air batteries on an industrial scale. The ongoing researches to overcome these hurdles have also been discussed. Thisanalysis will help the reader to understand the current standing of the lithium–air battery technology. Copyright © 2014John Wiley & Sons, Ltd.

KEY WORDS

lithium–air battery; cell reaction; oxidation of lithium anode; improvements in air electrode; oxygen selective membrane; solid-stateelectrolyte; energy of Li–air battery

Correspondence

* Naveed Akhtar, Material Science and Engineering School, Beihang University, Beijing, China.†E-mail: [email protected]

Received 11 April 2014; Revised 1 June 2014; Accepted 2 June 2014

1. INTRODUCTION

The energy demand of nations is increasing day by daybecause of growing population and modernization ofsocieties. Recently, the gap between energy demand andsupply has been widened up in many countries. This situa-tion is worse in developing nations, those primarily relyingon natural energy resources and looking toward advancednations to develop high-tech energy solutions. The existentenergy demands, diminishing natural resources, environ-ment concerns, and economic instability have become amotivating force to search for low cost, sustainable, andenvironment-friendly energy sources [1–3]. During the lasttwo decades, the research focus has been shifted towardelectrochemical energy production and storage techniquessuch as super capacitors, fuel cells, and rechargeablebatteries [4–6]. The electric vehicles and hybrid electricvehicles are the emerging developments of these clean en-ergy sources [7–9]. In recent times, rechargeable lithium

ion batteries have been evolved as a promising powersource for today’s electronic devices and automobileindustry [10–13]. Although the specific energy and powerdensity of lithium ion battery is still far lower to be usedin electric vehicles as a power source, the deficiencies inlithium ion battery and continuous R&D on rechargeablebatteries led the foundations of Li–air battery [14–16].

In the succeeding paragraphs, we have scrutinized theperspectives, practical hurdles, possible solutions, andrecent developments toward commercialization of Li–airbattery.

2. ENERGY OF LITHIUM–AIRBATTERY

Lithium–air or simply Li–O2 battery is one of the examplesof promising metal–air systems [17,18]. The mostly stud-ied metal–air couples [19] are Na–air, Ca–air, Mg–air,

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. (2014)

Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.3230

Copyright © 2014 John Wiley & Sons, Ltd.

Al–air, Zn–air, and Fe–air. Some of these are potentiallycapable to high energy commercial applications, that is,Zn–air battery [20] and Li–air battery [21]. Over the time,metal–air batteries have acquired fame because of theirhigh specific energy and open cell architecture [20,22]. Inthis open cell architecture, one of the constituent oxygenis directly absorbed from the atmosphere (air) during thedischarge process at the cathode. Therefore, the result islarger output of energy versus small input of raw materials.The theoretical specific energies of widely studied metal–air systems [23,24,21,18,19] have been displayed in plot1. This graph shows that Al–air and Li–air are the bestcapable systems for electric vehicle applications becauseof their higher energy densities.

The higher energy density of Li–air system has resultedfrom two significant reasons: (i) lithium is the lightestmetal and possesses the highest specific capacity(3.86 × 103mAh/g), corresponding to a specific energy of1.14 × 104Wh/kg for a theoretical potential of 3.0V, whichis much higher than any other metal–air battery or pres-ently best lithium ion batteries (4.2 × 102Wh/kg); and (ii)the second reason is the cathode active material ‘oxygen’,which is absorbed during the discharge process from externalair without occupying the battery volume [25,26]. The Li–airbattery possesses the highest theoretical specific energy[27,28], and it also provides high power density, which is anequally important criterion in automobile industry.

3. PROSPECTS AND CHALLENGES

A large number of scientists believe that Li–air batterytechnology has an inherent potential to replace the gasolinefuel [5,28,25]. But, at the same time, many researchersare convinced that it might take a considerable time before

entering into the industrial phase. The prospect depicted inFigure 1 has not yet been achieved practically in the realworld. In plot 2, theoretically predicted specific energies versuspractically achieved specific energies of various rechargeablebatteries have been compared. It is equitably obvious from thisplot that until the present date, researchers could not reach thehalf-mark of the projected capacity (Figure 2).

In these days, a fair amount of research [29,7] is going ontoward the commercialization of this technology. A modelLi–air battery capable of replacing traditional energy fuels musthave the characteristics summarized in Table I. The chiefdifficulties in the sluggish industrial progress of the Li–airbattery cited in the literature have also been listed in Table I.

4. CHEMISTRY OF LITHIUM–AIRBATTERY

A typical Li–air cell consists of a Li–metal anode, a porouscarbon cathode and an electrolyte. During the discharge,the Li–metal anode oxidizes to LiO2 and electrons flowthrough an external circuit, while LiO2 diffuses towardthe cathode via the electrochemical potential gradient.The atmospheric oxygen (O2) is reduced at the porouscathode, forming Li2O2 and other products such as Li2Oin less extent. A typical Li–air cell construction is shownin Figure 3, where the oxygen is being directly absorbedfrom the air. The Li–air batteries could be classified intofour types on the basis of electrolytes, and these are asfollows:

4.1 Aprotic Li–air battery: consists of a Li–metalanode, a liquid organic electrolyte, and a porouscarbon cathode.

Figure 1. Theoretical specific energies of various metal–air couples.

Latest developments in lithium–air batteriesN. Akhtar and W. Akhtar

Int. J. Energy Res. (2014) © 2014 John Wiley & Sons, Ltd.DOI: 10.1002/er

4.2 Aqueous Li–air battery: consists of a Li–metalanode, an aqueous electrolyte, and porous carboncathode.

4.3 Hybrid Li–air battery: consists of a Li–metal anodedipped in organic electrolyte and a porous carboncathode dipped in aqueous electrolyte, in-betweenthe two there is a lithium conducting membranecalled as ‘separator’.

4.4 Solid-state Li–air battery: consists of a Li–metalanode, a polymeric electrolyte or glass-ceramic,and a porous carbon cathode.

The cell design of an aprotic Li–air battery is given inFigure 4. In aprotic battery, the fundamental chemistry dur-ing the discharge process is thought to be the electrochem-ical oxidation of lithium–metal at the anode and reductionof oxygen at the cathode [26].

Reaction at anode : Li→ Liþ þ e-

Reaction at cathode :2Li þ O2 þ 2e� → Li2O2

4Li þ O2 þ 4e� → 2Li2O

The cathode’s discharge products (Li2O2 or Li2O) areinsoluble in the organic electrolyte, and these could blockthe pores of the cathode and also harm the catalysts ofthe cathode. Accumulating discharge products on cathodecan reduce the penetration of O2 gas, essentially neededfor the discharge process. This might affect the

Table I. Eminent features of an industrially ideal Li–air battery and shortcomings in today’s Li–air battery.

Eminent features of an ideal Li–air battery Shortcomings in today’s Li–air battery

Low manufacturing cost Low discharge ratesHigh specific energy Poor cycle abilityHigher power density Oxidation of the Li–metal anodeLarge cycle life Insoluble discharge productsEasy to operate Intermediate chemical reactions and productsLightweight Side reactions between electrolyte and atmospheric gassesConvenient working temperature Low oxygen supply to air electrodeUser safety and environment-friendly Rigorous working conditions

Figure 2. Theoretical versus practical energy densities of rechargeable batteries.

Figure 3. A typical cell design of Li–air battery.

Latest developments in lithium–air batteries N. Akhtar and W. Akhtar


performance of the cathode, and ultimately, the dischargecapacity of the battery would be reduced. This phenome-non is well explained in Figure 4.

Aqueous Li–air battery has similar cell architecture asshown in Figure 3, but the cell reaction is different as itutilizes aqueous electrolytes and that could be acidic oralkaline in nature. The reaction mechanisms are given inthe succeeding text [30]:

Cell reaction in basic electrolyte :

4Li þ O2 þ 2H2O → 4LiOH

Cell reaction in acidic electrolyte :

4Li þ O2 þ 4Hþ → 4LiOH þ 2H2O þ Liþ

Aqueous electrolytes offer many advantages over non-aqueous systems. They are inexpensive and offer highionic conductivities. The discharge products are solublein aqueous electrolytes, and also, there is a minimum issueof moist air. However, the use of Li–metal anode inaqueous electrolyte requires some extra safety because ofits high reactivity with water.

A hybrid Li–air battery comprises a Li–metal anode im-mersed in organic electrolyte and an air-electrodeinterfaced with aqueous electrolyte while there is a thinfilm of solid-state electrolyte in-between them (Figure 5).This makes a two compartment cell of aqueous and non-aqueous electrolytes, sharing a common interface ofsolid-state Li+ conducting membrane (i.e., LISICON).The electrode reactions of this type of system can be writ-ten as follows [29]:

Reaction at anode: Li→Li+ + e�

Reaction at cathode: O2 + 2H2O+4e�→ 4OH�

Cell reaction: 4Li +O2 + 2H2O→ 4Li+

+ 4OH�

The discharge process of a hybrid battery has beenshown in Figure 5. During operation, O2 molecules from

the air continuously diffuse into the porous cathode, wherethe reduction reaction takes place. At the same time, thelithium anode produces Li+ ions, and then, Li+ ions diffusefrom the nonaqueous solution to the aqueous solution viasolid-state conduction film (separator).

Hybrid type battery has been developed to accommo-date the discharge products and moisture effects in aproticsystem. There are no such issues of clogging of porouscathode because of discharge products in hybrid battery.However, hybrid type batteries provide a lesser powerdensity, which is a drawback over long-term operations,and one of the identified reasons behind is the lithium ionconducting membrane [31].

The solid-state Li–air battery contains all its compo-nents (Figure 6) in the solid-state: (i) Li–metal anodesurrounded by a conductive glass or coatings; (ii) electro-lyte composed of some inorganic Li+ conducting materialssuch as LISICON [32], glass-ceramic, polymer-ceramic, orsolid organic electrolytes; and (iii) composite cathodeprepared from carbon and glass-ceramic powders. Theoxidation reduction reaction taking place in this type ofcell is given in the succeeding text:

2Liþ þ O2 þ 2e� → Li2O2

Or

4Liþ þ O2 þ 4e� → 2Li2O

A solid-state Li–air battery provides many advantages,that is, Li anode protection, fewer effects of moistair, and good stability and safety. In a recent study,solid-state Li–air battery showed excellent thermal sta-bility and rechargeability in a wide temperature range(30–105 °C) [33].

Figure 4. Aprotic Li–air battery, showing discharge process anddischarge products.

Figure 5. Hybrid Li–air battery, cell design and dischargeprocess.



5. RECENT DEVELOPMENTS INLITHIUM–AIR BATTERY

The Li–air battery technology is still in the developmentalphase, and there are many serious issues yet to be resolvedbefore entering into the commercial phase. Researchers areworking on every area that could enhance the performanceof the battery, that is, cathode design, electrolytes,catalysts, discharge products, moisture effects, corrosionof the lithium–metal anode, Li+ ion conducting mem-branes, oxygen selective membranes, and overall celldesign [34,14,7,35].

5.1.1. Improvements in air electrodeIn these days, most of the new studies are dedicated in

search of appropriate and effective materials for cathodestructure (air electrode), superior electrode design, andsearch of suitable inexpensive catalysts. Researchers areworking on various types of materials that include modi-fied carbon, porous graphene, nanofibers, nanoparticles,and composite materials [36–42]. However, the commer-cial industry may accept only those materials, which wouldqualify the basic criteria of availability, price, performance,stability, toxicity, and safety.

Bing Sun and coworkers prepared an air electrode byusing graphene nanosheets for aprotic type battery contain-ing alkyl carbonate electrolyte. They compared theirresults with a commercially available carbon cathode(XC-72) and found that the graphene air electrode exhib-ited a much better cycling stability and lower over potentialthan the XC-72 carbon electrode [43]. Further research in

this direction has also investigated the catalyticperformance of graphene nanosheets supported with metalnanoparticles of Pt, Pd, PtAu alloy [44–46], and also metalfree graphene nanosheets catalysts [47]. These studies haveconfirmed that the cathodes comprising of graphenenanosheets showed higher catalytic activity than carbonblack in both types of electrolytes, that is, aqueous andnonaqueous electrolytes [43].

Yue Shen in 2013 reported the synthesis of Pd-modifiedcarbon nanotubes and its use to fabricate a cathode forLi–air battery. The resultant cathode was spongy, contain-ing lots of pores and Pd nanoparticles in its structure,which improved its catalytic activity. The battery utilizingthis new electrode remained stable under humid conditionsand delivered a capacity as high as 9092mAh/g [48].

In 2011, Robert Mitchell and companions weresucceeded to prepare hollow carbon fibers with an averagediameter of 30 nm. The hollow carbon fibers were grownon a ceramic porous substrate, and the fabrication of air-electrode was accomplished without any binder. Thiselectrode was found to yield high gravimetric energies(up to 2500Wh/kgdischarged) in Li–O2 battery, approxi-mately four times greater energy than the well-establishedlithium intercalation compounds such as LiCoO2

(~600Wh/kgelectrode). The greater energy was attributedto low carbon packing and efficient utilization of the avail-able carbon mass [49].

Similarly, glass-ceramics such as Li–Al–Ge–PO4 andtheir composites with carbon were studied to optimizethe air electrode for solid-state battery [50,51]. In one ofthe studies, a composite of nitrogen-doped carbon wasblended with Li–Al–Ge–PO4 to fabricate the air electrode.The composite electrode showed higher electrochemicalactivity. The discharge rate was six times higher in the caseof composite electrode than the simple nitrogen-dopedelectrode [52].

Research has also been focused on the structural designof air electrode, that is, multilayer electrode, three-dimen-sional electrode, and nanoporous electrode, which was pre-pared to provide maximum surface area/sites for oxygenreduction reaction and enhance collection of dischargeproducts [53–55]. Gao and coworkers synthesized an elec-trode having a double-layer structure of the gas diffusionlayer and catalyst layer. They achieved a high capacity dis-charge of 6587mAh/g at the rate 0.15mA/cm2 with thecarbon loading of 2.077mg/cm2 [56]. Scientists have alsoapplied different types of manufacturing techniques, forexample, rolling, coating, and spraying to study the effectsof various parameters such as particle size, surface area,and porosity [57].

The role and advantages of a catalyst in the oxygen re-duction process have been studied by many researchers[58–60]. The precious metals such as Au, Pt, Pd, Ru[61,59,58], and metal oxides such as Fe3O4, CoFe2O4,MnO2, and Co3O4 [62,63] have been used to enhance theperformance of the air electrode. The metal oxides catalystbonded with lithium–metal has also shown improved cata-lytic activity, for example, Li5FeO4, Li2MnO3, and LiFeO2

Figure 6. Solid-state Li–air battery, cell design and dischargeprocess.



[64]. The catalysts such as Mn3O4 [29], MnO2 [65], andother MnO2-based materials are cheap, easily available,and demonstrated significantly higher catalytic activity.

5.1.2. Improvements in oxygen selectivemembranes

As we have stated earlier, the Li–air battery is an opencell structure because O2 is being directly absorbed fromexternal atmosphere during the discharge process. At pres-ent, Li–air batteries in research are mostly operated in a dryand pure oxygen environment [66–69]. However, in realsituation, these batteries would also take-up H2O [66],CO2 [70], and other gasses from external air. And theseimpurities may deteriorate the performance and structureof the battery [71,66,70].

Recently, membrane separation technology has beenintroduced to encounter the problems associated withmoist air. An oxygen selective membrane also termed asoxygen permeable membrane mainly serves the followingpurposes in a Li–air battery:

• Provision of pure and dry oxygen for cathodereaction;

• Protection of electrolyte solvents from evaporation;• Shielding of Li–metal anode to avoid oxidation andcorrosion;

• Overall battery safety.

The aforementioned listed tasks are very challengingbecause H2O molecules have higher diffusivity andsmaller kinetic diameter than the O2 molecules [72], sotheir separation is not an easy job.

Zhang and coworkers successfully fabricated oxygenselective membranes comprising of silicon oil immobilizedin porous substrates. They claimed that membranes are lowcost and applicable to any primary or rechargeable metal–air battery, which is sensitive to moisture. Batteriesequipped with O2 selective membranes showed a muchhigher discharge capacity and specific energy under theambient air conditions at RH level of 20–30% [73]. Inanother study, a heat-sealable polymer membrane was usedto serve dual function, as oxygen permeable membraneand as a moisture barrier to protect the battery [71].

A number of polymeric materials have been researchedand declared as a suitable material for oxygen selectivemembranes and that include perfluorofluorocarbons such asperfluorodecalin, polysiloxanes such as polyfluorosiloxane,perfluorinated polyethers, and alkyl methacrylates such asmethyl methaylacrylate [74,75,40,76,73]. These materialsare very flexible and could be absorbed or coated on thesurface of the cathode, thus a variety of designs in electrodearchitecture is possible.

The silicone rubber membranes, that is, polysiloxaneand methacrylate-polysiloxane copolymers have alsoshown promising results. These membranes exhibited highpermeability for oxygen and blocked water vapors fromentering into the cell [77].

5.1.3. Improvements in lithium–metal anode andinterface

As we have mentioned in section 5.1.2, most of theLi–air batteries cited in the literature have been testedunder dry and pure oxygen with a partial pressure >1 atm.So far, the operation of the Li–air battery under normalenvironmental conditions is a great challenge, because ofabundant moisture and low oxygen pressure. The moisturein the air would corrode the Li–metal anode, and thebattery might be collapsed in worse conditions, if theLi–metal anode gets exposed to water. And this is one ofthe major issues that have hindered the development ofLi–air batteries for commercial applications [71,78]. Inaqueous or hybrid Li–air battery, it is not possible to useLi–metal anode directly in aqueous electrolyte unless it iswell protected. Lithium–metal electrode is prone todendrites formation during charging process, which is acommon drawback of Li anode [79]. The dendritic lithiumon the electrode surface may come in contact with thecarbon cathode, which causes an internal short-circuit andmay result in a cell explosion [34,80]. Because of thesehazards, researchers are more interested in aprotic andsolid-state batteries, to avoid undesirable reactionshappening in aqueous electrolytes [81].

Research has shown that dendritic growth in Li–metalanode could be controlled by (i) better morphology and de-sign of anode, (ii) using compatible electrolytes [82], and(iii) protection of anode surface with some suitable con-ductive material. Further, it has been revealed that solvents[83–89], additives [90–92,84], and plasticizers [93,94] inthe composition of an electrolyte also play a key role inthis phenomenon. Salts such as LiClO4, LiPF6, LiBF4,and LiCF3SO3 have been used to enhance the electrolytecomposition and conduction [95,96].

Polymer-based Li+ ion conductive films for Li anodeprotection were synthesized, utilizing polymeric materialssuch as polyethylene oxide, polypropylene oxide,polyacrylonitrile, polymethyl methacrylate, polyvinylidenefluoride, and polysiloxanes [97,98].

Surface coating of the Li anode is another effective toolto minimize the corrosion and dendrite issues. Ideally, acoating material should be thermally and chemicallystable, capable of forming an interface with electrolyteand mechanically durable. Coatings should be applied inthe form of homogeneous, uniform thin layer on the anodefor good results. Many materials have been evaluatedfor Li-anode coatings, that is, silane-based coatings[99–102], polymeric coatings [88,103], and inorganic Li+

ion conductive coatings [104,105].Some researchers have used oxygen selective mem-

branes [75] along with polymer glass electrolyte to protectthe Li anode [71]. Crowther and coworkers used a Tefloncoated fiberglass cloth as an oxygen selective membraneto prevent corrosion of the Li–metal anode during dis-charge in ambient air [76].

Recently, yet another technique has been evolved toprotect the Li anode from dendrites formation. Ding andcoworker in their experiments used cesium and rubidium



ions at low concentrations, these ions exhibit an effectivereduction potential below the standard reduction potentialof lithium ions. During lithium deposition cycle, theseadditive cations form a positively charged electrostaticshield around the initially formed tips of the lithium protu-berances. This forces further deposition of lithium toadjacent regions of the anode and eliminates the dendriteformation [106].

5.1.4. Improvements in electrolyte compositionsJung and coworkers in 2012 demonstrated that, by

choosing a stable electrolyte and appropriate cell design,the Li–air battery would be capable of operating over manycycles with a capacity and rate values as high as5000mAh/gcarbon and 3A/gcarbon, respectively [107]. Theideal electrolyte must have the following characteristicsfor long-term operations of Li–air batteries under normalatmospheric conditions:

• low flammability,• low vapor pressure,• wide electrochemical range,• high Li+ ion transfer rate,• salt solubility (high dielectric constant and lowviscosity),

• moisture tolerance,• high stability (oxidation/auto-oxidation resistance),and

• high compatibility with Li anode.

Most of the liquid electrolytes (ionic liquids) wouldqualify the aforementioned criteria but encounter severaldrawbacks, that is, low Li+ ion transfer rate, low Li–saltsolubility, and moisture susceptibility, which would pre-vent their use in commercial rechargeable Li–air batteries.On the other hand, previously used nonaqueous electrolytesuch as organic carbonates is now being declared as unsuit-able material because these are susceptible to nucleophilicattack by oxygen reduction species [108–111].

A Li–air battery operating under ambient conditionsmay breathe-in water vapors, carbon dioxide, and nitrogenalong with oxygen through the air-electrode. And thesegasses may reach toward the anode by crossing the electro-lyte. The lithium–metal anode may react with these impu-rities and produce many intermediates and final productsO2 +H2O [112]. Commonly used solvents in nonaqueouselectrolytes may also produce superoxide anion (O2

�) as aresult of decomposition reaction, and oxygen reduction re-actions at the cathode.

O2 þ e─ → O2─

The superoxide ions act as a strong nucleophile inaprotic solvents and render them nonsuitable electrolytes(cyclic and linear carbonates) for Li–air battery [113].

A number of studies have shown that a stable nonaque-ous electrolyte must include in their recipe, one or morekinds of alkyl carbonate, antioxidants, solvents

(dimethoxyethane and methoxybenzene), and co-solvents[108,114,115] to form a suitable electrolyte composition,which can withstand the operational challenges [35].

Crowther and colleagues studied the use of lithiumtetrafluoroborate (LiBF4) with different solvents, that is,propylene carbonate, dimethoxyethane, dimethyl carbon-ate, tetra ethylene glycol dimethyl ether, and methoxybenzene to get a better understanding about solvent effectson battery performance [76]. The solvents such as crownesters if used with alkyl carbonate electrolytes can lowerthe electrolyte viscosity, improve the ion transport, andalso increase the discharge performance [116].

Esters having low vapor pressure such as tetraethyleneglycol dimethyl ether and few others have been studiedfor aprotic Li–air battery [117]. McCloskey disclosed thatdischarge products such as Li2O2/LiO2 further reacted withelectrolyte and carbon cathode and produced Li2CO3

layers, which decreased the performance of Li–air battery[118].

Nitriles such as trimethylacetonitrile, amides such as N,N-dialkyl amides, were found more stable against oxygenreduction species than organic carbonates and ethers[119–121,85]. Sulfones-based electrolytes were also foundresistant to superoxide anion attack [122].

Nowadays, additives are being added in the composi-tion of the electrolyte to develop a superior interface withanode, protect the cathode, act as LiPF6 salt stabilizer,enhance salvation process, and also act as wetting agentand antioxidants. Various additives have been developed,which lessen the dendrites formation and prevent theelectrolyte reactions with Li anode by improving the solidelectrolyte interface layer [123,124,41].

It is evident from the aforementioned discussion thatmost of the studied organic electrolytes could not meetthe complete criteria of a suitable electrolyte. In these days,researchers are engaged to manufacture compound/blendelectrolyte having necessary characteristics to support theoperation of Li–air battery under normal environmentalconditions [125,126]. In one of the studies, a mixture ofpropylene carbonate and ethylene carbonate was found tobe the most practical solvent system and lithium bis(trifluoromethanesulfonyl)imide the best salt for ambientoperations of aprotic Li-air battery [127].

The solid-state electrolytes made of glass-ceramics[128] or polymers [97,129] have appeared to be an alterna-tive to liquid electrolytes, but their low ionic conductivityis a major concern for researchers. The solid-state electro-lytes have been considered a competitive alternativebecause they offer flexible, safe, low cost, and thin profilebatteries. A battery comprising of a solid electrolyte couldbe operated over a wide temperature range and has betterability to prevent dendrite formation phenomenon[130–133,129,134,135]. Also, solid electrolytes provide asubstantial barrier against diffusion of atmospheric gassesand moisture toward the Li–metal anode.

There are two general classes of materials used as solid-state electrolytes in Li–air batteries: (i) Li+ ion conductinginorganic ceramics and (ii) organic polymers. Polymeric-



based electrolytes are electrochemically stable and possessgood adhesion properties, which make them suitable elec-trolyte for Li anode [136]. Among the various solid poly-mers-based electrolyte, the polyethylene oxide containinglithium salts, for example, lithium trifluoromethanesulfonatehas been studied extensively [137–139].

The second class of solid-state electrolytes consists ofLi+ ion conducting glass-ceramic materials. The importantcharacteristics of glass-ceramic-based electrolytes are goodLi+ conductivity, high thermal and electrochemical stabil-ity for Li anode, and atmospheric gasses and safe batteryoperations at higher temperatures. Researchers haveexplored various kinds of materials such as sulfides,oxides, and phosphate compounds [140,31,141,142] tofind suitable solid-state electrolytes. Compounds such asLi–Al–Ge–PO4 and Li–Al–Ti–PO4 have been identified asfast Li+ ion conducting electrolytes for solid-state Li–airbatteries [143,144,11,145].

6. CONCLUSIONS

A huge interest expressed by the scientific community inthe development of Li–air battery is the demand of modernautomotive industry. We have identified four major areas.If properly addressed, this technology may enter the com-mercial phase in the near future.

1. Numerous studies have reported that the cell capacitydropped significantly with increasing discharge rates.This was attributed to limitations in kinetic chargetransfer. The suitable electrolyte chemistry andoptimization of charge transfer at the cathode canovercome this problem.

2. The charge overpotential is a primary concern andlimiting it to one time charge battery. It has been rec-ognized that research should also focus on intermedi-ate products and secondary reactions inside thebattery. The catalysts such as Pt, Pd, Co, Au, andMnO2 were found as an effective material to reducethe overpotential issues. But the high cost of preciousmetals is a snag in their use on a commercial scale.

3. The air-electrode that is the most crucial componentof the Li–air battery encounters some challenges,that is, incomplete discharge, removal of contami-nants from incoming air, and removal of wasteproducts. Researchers have suggested the use ofoxygen selective membranes to purify the incomingair, but no real breakthrough has happened yet. We sug-gest that researching should also be focused on the re-moval of discharge products from the air-electrode.

4. The Li–metal anode has remained to be a major con-cern in many studies because of its higher reactivitywith water. The water contents in the cell may arisefrom electrolyte, atmospheric air, side reactions,and their byproducts. Therefore, it is suggested thatattention should also be given to intermediate reac-tants and their byproducts.

NOMENCLATURE

Al = aluminumCa = calciumCd = cadmiumAu = goldFe = ironLi = lithiumPb = leadMg =magnesiumMn =manganeseNi = nickelNa = sodiumPt = platinumPd = palladiumS = sulfurZn = zincR&D = research and developmentLISICON = lithium super ionic conductorRH = relative humidity

ACKNOWLEDGEMENTS

Authors hereby acknowledge the cooperation of BeihangUniversity Beijing, China, and support of SEI Pakistanfor granting a scholarship to N. Akhtar.

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Prospects, challenges, and latest developments in lithium-air batteries