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DOI: 10.1002/cplu.201402104 Air Electrode for the Lithium–Air Batteries: Materials and Structure Designs Zhaoyin Wen,* Chen Shen, and Yan Lu [a] # 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPlusChem 0000, 00, 1 – 19 &1& These are not the final page numbers! ÞÞ CHEMPLUSCHEM REVIEWS

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Page 1: Air Electrode for the Lithium-Air Batteries: Materials and Structure Designs

DOI: 10.1002/cplu.201402104

Air Electrode for the Lithium–Air Batteries: Materials andStructure DesignsZhaoyin Wen,* Chen Shen, and Yan Lu[a]

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Page 2: Air Electrode for the Lithium-Air Batteries: Materials and Structure Designs

Introduction

With the rapid development of modern society, the demandfor energy has been increasing rapidly and continuously.[1, 2] Atpresent, mankind’s total power consumption is 14 TWh peryear and is projected to roughly triple by 2050.[1] Such a largequantity of energy consumption has brought great threat tothe sustainable development of the earth because most of theenergy supply comes from fossil fuels, such as coal, oil, andnatural gas, thus causing a dramatic buildup of greenhousegases in the atmosphere.[2]

In this way, various renewable resources, such as solar radia-tion, wind, waves, and geothermal energy, have been exploredto reduce the dependence on fossil fuels that cause a largeamount of carbon dioxide. However, they are intrinsically fluc-tuant and intermittent because of the variety of weather con-ditions, or restricted in location, which thus essentially requireslarge-scale energy storage devices to counterbalance their vari-ability.[3] Numerous energy storage solutions enlisting mechani-cal, magnetic, and chemical storage and so forth are presentlybeing investigated. Among them, electrochemical batteries areconsidered to be one of the most efficient, simple, and reliablesystems, through converting electrical energy directly intochemical energy or vice versa by reversible electrochemical ox-idation–reduction reactions.[4]

Lithium-ion technology is traditionally considered as themost promising storage method owing to its relatively long cy-cling life (>5000 cycles) and high energy efficiency (>90 %),which has captured the portable electronics market, invadedthe power tool equipment market previously kept for Ni–metalhydride technology, and is on the verge of penetrating theelectric vehicles market. However, the energy density of com-mercial lithium-ion batteries (theoretical value is 400 Wh kg�1)is limited by the “rocking mechanism”, which is far from meet-ing the high energy demands of large-scale electricity storagefor renewable energy and electric automotive vehicles, theirlong-term applications even optimizing this technology usingcurrently available materials. Therefore, the exploration of

high-energy-density storage technologies composed of com-pletely different reaction mechanisms is an urgent require-ment.

The Li–air battery has been developed under these condi-tions. It has captured worldwide attention recently because ofits super-high specific energy density of 11140 Wh kg�1 rivalingthat of gasoline. Different from the rocking mechanism in lithi-um-ion batteries, a catalytically active oxygen reduction reac-tion (ORR) and an oxygen evolution reaction (OER) are in-volved during the electrochemical process. The structure ofa Li–air battery is composed of a lithium anode, an air elec-trode, and a separator soaked in lithium-ion-conducting elec-trolyte. The reaction mechanism can be described in brief asoxygen from the air diffusing at the surface of the cathodeduring the discharge process and being reduced to form theoxide or peroxide, thereby releasing electrons to support theload in the external circuit. The charge process is in reverse.Owing to the fact that the cathode oxygen is from air, which isinexhaustible and not stored in the cell, the metal–air familyhas a notably higher theoretical energy density than other tra-ditional batteries, such as the primary Zn–MnO2 (Zn–Mn), re-chargeable lead–acid, nickel–metal hydride (Ni–MH), and lithi-um-ion batteries. Undoubtedly, Li–air batteries are consideredto be potential next-generation energy storage devices. Thiscan be clearly observed from Figure 1.[5] Among most of therechargeable batteries, Li–air batteries showed the highestpossible driving distance for electric vehicles. For this, it hasbeen chosen as the candidate for the Battery 500 Project of

The lithium–air battery, considered to be a promising candi-date for future applications such as electric vehicles andenergy storage, has captured worldwide attention recently be-cause of its superhigh specific energy density even rivalingthat of gasoline. This Review covers the most recent and signif-icant scientific progress made in the fields relevant to Li–airbatteries, with emphasis on the structure design of air electro-des. After a brief introduction to the operation principles and

cathode electrochemical reactions, a discussion of the designand optimization of the cathode structure is presented se-quentially, and the final conclusion remarks on future challeng-es and perspectives. The aim is to provide a better understand-ing of the effect of the cathode structure on the performanceof the promising metal–air battery technologies, and to consid-er their challenges and perspectives in the future.

Figure 1. Practical specific energies for some rechargeable batteries, alongwith estimated driving distances and pack prices.

[a] Prof. Z. Wen, C. Shen, Dr. Y. LuCAS Key Laboratory of Materials for Energy ConversionShanghai Institute of CeramicsChinese Academy of SciencesShanghai 200050 (P. R. China)E-mail : [email protected]

Part of a Special Issue on “Metal–Air and Redox Flow Batteries”. A link tothe table of contents will appear here once the issue is compiled.

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Page 3: Air Electrode for the Lithium-Air Batteries: Materials and Structure Designs

IBM. Therefore, a great interest in Li–air battery technology isongoing all over the world.

The use of oxygen from the atmosphere in the Li–air batteryendowed this battery with superhigh energy density as well asraising a great challenge for the testing device. Therefore, vari-ous chemical architectures have been designed worldwide toachieve a high capacity density. In general, the architecturescan be divided into three categories according to the electro-

lytes used, as demonstrated in Figure 2: a fully aprotic liquidelectrolyte, a mixed solid electrolyte-based system with anaqueous electrolyte immersing the cathode and an aproticelectrolyte immersing the anode, and an all-solid-state batterywith a solid electrolyte. The electrochemical reactions at thepositive electrode depend on the environment (electrolyte)around it and so do the electrode potentials. When there isaqueous solution present at the positive electrode of Li–airbatteries, a series of complex electrochemical reactions, whichare proposed to involve multistep electron-transfer processesand complicated oxygen-containing species such as O2�, OH� ,O2, and HO2�, would happen.[6–10] Specifically, for alkaline elec-trolytes, the related cathode reactions can be briefly given as[Eqs. (1)–(4)]:

O2 þ 2H2Oþ 4e� ! 4OH� E0 ¼ 0:40 Vð Þ ð1Þ

O2 þ H2Oþ 2e� ! HO�2 þ OH� E0 ¼ �0:07 Vð Þ ð2Þ

HO�2 þ H2Oþ 2e� ! 3OH� E0 ¼ 0:87 Vð Þ ð3Þ

2HO�2 ! 2OH� þ O2 ð4Þ

As demonstrated, oxygen reduction may proceed througha four-electron (4 e) pathway or a two-electron (2 e) pathwayon surfaces of metals.[6, 11] The ORR pathways and mechanismsare sensitive on the catalytic materials applied and their struc-tures, even for the same catalyst. They are similar to the reac-tions in alkaline fuel cells or zinc–air batteries. The cathode re-actions of an aprotic Li–air cell differ from that in aqueous sys-tems. The possible net reactions of a Li–air battery are envi-sioned as follows, with four or two electrons transferred per O2

molecule [Eqs. (5) and (6)]:[1, 12–17]

4 Liþ O2 ! 2 Li2O ðE0 ¼ 2:91 V vs: Li=LiþÞ ð5Þ

2 Liþ O2 ! Li2O2 ðE0 ¼ 2:96 or 3:10 V vs: Li=LiþÞ ð6Þ

The reported standard cell potential of Equation (6) is notuniform, and may be derived from different Gibbs free ener-gies. Because of the close E0 values of the above two reactions,either lithium oxide or lithium peroxide is highly thermody-namically feasible to be the discharge product.[6] However,both of them are hard resolve in organic electrolytes, andcause the problems of pore blocking and large polarization inair electrodes. Although the products of the all-solid-state Li–air battery have not been proved, the deposition of the prod-ucts at the surface of the air electrode is inevitable. This issueis the same with aprotic Li–air batteries, which brought thehuge polarization and poor cycling performances of Li–air bat-teries. Therefore, efforts should be made to solve these prob-lems.

It has been shown that the energy storage capacity andpower capability of Li–air batteries are strongly determined bythe air electrode, which is undoubtedly considered as the mostimportant part of the Li–air battery. The conventional air diffu-sion electrode in metal–air batteries was composed of threeparts including support, binder, and catalyst. Each of them per-

Zhaoyin Wen obtained his B.S. at the

Nanjing University of Chemical Tech-

nology, China in 1984 and M.S. and

Ph.D. from the Shanghai Institute of

Ceramics, Chinese Academy of Scien-

ces (SICCAS) in 1987 and 1998, respec-

tively. He worked on solid electrolytes

for lithium-ion batteries at Mie Univer-

sity, Japan from 1999 to 2001 as a visit-

ing professor. He is now a professor of

SICCAS with research interests focused

on secondary batteries, including lithi-

um–sulfur, Li–air, sodium–sulfur, and sodium chloride batteries,

with more than 210 peer-reviewed scientific publications. He is

now taking charge of the Chinese Society for Solid State Ionics.

Chen Shen received his B.S. at the Uni-

versity of Science and Technology of

China (USTC) in 2011. Currently he is

carrying out Ph.D. studies under the

supervision of Prof. Zhaoyin Wen at

Shanghai Institute of Ceramics, Chi-

nese Academy of Sciences (SICCAS).

His research interests are novel de-

signs of cathodes for Li–oxygen batter-

ies and the reaction mechanism

during the discharge/charge process

of Li–oxygen batteries.

Yan Lu obtained her B.S. in Materials

Science and Engineering from Shan-

dong University in 2009. Then she

joined Shanghai Institute of Ceramics,

Chinese Academy of Sciences and

started her Ph.D. work in electrochem-

istry under the supervision of Prof.

Zhaoyin Wen, during which her re-

search interests were mainly focused

on the synthesis of high-performance

nanostructured materials and their ap-

plications in lithium-based batteries in-

cluding Li–air and lithium-ion batteries. She obtained her Ph.D. in

2014 and is now working at Nanyang Technological University as

a postdoctoral research fellow in the School of Materials Science

and Engineering. Her research interests at NTU are biotemplate

synthesis of nanostructured materials and their applications in lithi-

um batteries.

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Page 4: Air Electrode for the Lithium-Air Batteries: Materials and Structure Designs

forms a unique responsibility. Specifically, the support is elec-tronically conductive and provides the place for catalyst depo-sition; the catalyst is responsible for the ORR/OER and providesthe place for the discharge product deposition in the nonaqu-eous Li–air battery; and the binder is used to confirm the in-tegrity of the electrode materials. Therefore, one of the keyfeatures of an efficient air electrode is the formation of a high-surface-area boundary between the active phases involved inthe oxygen reduction: air, the electrolyte, the catalyst, and theconductor. In aqueous electrolytes, oxygen is reduced to OH� ,which is dissolved into the electrolyte. The electrocatalyticoxygen reaction occurs at a three-phase contact zone betweenair, liquid electrolyte, and solid catalyst (except to the extentthat the oxygen may dissolve in the electrolyte), as shownschematically in Figure 3. This shows that a large three-phaseinterface in the air electrode is necessary, at which air can dif-fuse throughout and electrolyte can be adsorbed in the cata-lyst surface but must not flood it to the exclusion of gas.[6] Thesituation is different in nonaqueous electrolytes. In a Li–air cellbased on organic electrolyte, only the oxygen dissolved in theelectrolyte participates in the oxygen reduction owing to the

fact that the discharge products (Li2O2 and/or Li2O) are insolu-ble in the electrolyte and will precipitate on the surface of thecatalyst, thereby preventing continuous air diffusion andaccess. In view of the above-demonstrated importance of theinterface, rational design of the cathode structure should focuson increasing the interface areas and enhancing air diffusionand transportation. A high specific surface area of the catalystand porous structure of the support are of course prerequi-sites, whereas binder is not necessary. Therefore, a variety ofnew concepts of air electrode based on the design of support,catalyst, or binder have been proposed.

Firstly, the requirements for an ideal catalyst support materi-al can be summarized as: 1) a high specific surface area, whichprovides high dispersion and high utilization of nanocatalysts ;2) high conductivity ; 3) high chemical and electrochemical sta-bility under the Li–air operating process; and 4) low reactivitywith electrolytes in Li–air battery systems.[11] Therefore, themost popular support material in the Li–air system is carbonmaterial owing to its high conductivity, high surface area, andlow cost.[18–20] Especially great efforts of researchers made inthe field of porous carbon materials, which aimed to controlthe size, shape, and uniformity of the porous space and theatoms and molecules that define it, made it extremely suitablefor a gas diffusion electrode of Li–O2 batteries.[21] Various nano-structured carbon material electrodes, including 1 D nanostruc-tures (all-carbon-nanofiber electrodes,[22, 23] single-walledcarbon nanotube (SWNT)/carbon nanofiber buckypaper elec-trodes,[24] a hierarchical-fibril carbon nanotube (CNT) elec-trode,[25] and CNT composite with inorganic solid electrolytepowder[26]), 2 D graphene materials (graphene nanosheets,[27]

hierarchically porous grapheme,[28] metal-free grapheme nano-sheet catalysts[29]), and 3 D porous carbon materials (mesocellu-lar carbon foam,[30] three-dimensional ordered mesoporous/macroporous carbon sphere arrays,[31] hollow spherical carbondeposited onto carbon paper,[32] porous carbon aerogels,[33]

and CNT sponge cathode[34]), have been developed. Relation-ships between the discharge capacity and pore volume andpore diameter have been investigated. Most recently, some

Figure 2. Schematic representations of different architectures of Li–air batteries: a) Li–O2 batteries with nonaqueous electrolytes, b) Li–O2 (air) batteries withhybrid electrolytes, c) Li–O2 (air) batteries with all-solid-state electrolytes.

Figure 3. Illustrations of reaction interfaces in aqueous and nonaqueouselectrolytes.

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Page 5: Air Electrode for the Lithium-Air Batteries: Materials and Structure Designs

novel supporting materials, including oxides, carbides, nitrides,and so forth, have been developed to further improve the bat-tery performance and lifetime. Therefore, an overview of thedevelopment of supporting materials is critical for the develop-ment of Li–air batteries.

Secondly, catalytic materials are also critical in Li–air batteriesbecause of the sluggish ORR (during discharge) and OER kinet-ics (during charging) in Li+-containing aprotic electrolytes.Therefore, numerous catalysis materials, including noblemetals and alloys, carbon-based material, metal oxides, noblemetal, and inorganic–organic composites, have been devel-oped. Many high-quality reviews have demonstrated the ad-vantages and disadvantages of the catalysis materials and ana-lyzed their development prospects.[6] Therefore, repeat workwill not be done here. Instead, we will focus on new conceptsof catalysis materials, such as bifunctional electrocatalyst,[35, 36]

in situ formation of oxides at the surface of a current collec-tor,[37] core–shell structures,[36, 38] durable 3 D bifunctional airelectrodes,[39] and so on. All of the new concepts of catalysismaterials have brought Li–air batteries excellent performances.

Finally, from the aspect of binder, it has always been a con-troversial component in air electrodes. The primary role playedby the binder is to link different types of small particles togeth-er and to ensure the active material adhesion to the currentcollector of the cathode during subsequent cycling.

However, the common binders used for the oxygen cathodeof the lithium–oxygen cells are polyvinylidene fluoride (PVDF)or polytetrafluoroethylene (PTFE), which have strong bindingstrength but low flexibility and moreover are readily swollen,gelled, or dissolved by the nonaqueous liquid electrolytes es-pecially at elevated temperatures.[40–42] Besides, for the insulat-ing nature of the discharge products in the oxygen cathode,the redox reactions and electron-transfer reactions can onlyoccur at the surface.[43] As is known, conventional PVDF andPTFE are nonconductive, and the lack of facile transfer net-works for electron/Li in the carbon cathode with the conven-tional binder would limit the capacity, rate capability, and cycleefficiency of the lithium–oxygen cells. Therefore, binder-free airelectrodes[44, 45] and functional binders for air electrodes[41] weredeveloped.

Herein, we intend to review the fundamentals and the mostrecent and significant scientific progress made in the fields rel-evant to Li–air batteries, with emphasis placed on the structuredesign of air electrodes. After a brief introduction to the opera-tion principles and cathode electrochemical reactions, a discus-sion of the design and optimization of cathode structure ispresented sequentially in the following sections, and the finalconclusion remarks on future challenges and perspectives. Theaim is to provide a better understanding of the effect of thecathode structure on the performance of the promising metal–air battery technologies, and to consider their challenges andperspectives in the future.

Nonaqueous Li–O2 battery

The rechargeable nonaqueous lithium–oxygen (Li–O2) batterywas first reported by Abraham and Jiang.[12] Research interest

in this battery has increased rapidly in recent years because ofits ultrahigh energy density[15, 35, 46–51] (3505 Wh kg�1 based onthe formation of Li2O2). A typical nonaqueous Li–O2 cell, as rep-resented schematically in Figure 2 a, consists of a lithium metalanode, an organic Li+-conducting electrolyte, and a porouscathode. The cycling of the battery is based on the reversibleformation/decomposition of the solid-state discharge product(Li2O2) on the surface of the porous cathode. Although a greatamount of effort has been made to improve the performanceof the Li–O2 battery, many obstacles, such as low cycle efficien-cy, capacity fading under high current, poor cycling per-formance, the possible decomposition of carbon substrate inthe cathode, and so on, have to be overcome. Many of theseproblems are related to the composition and structure of theoxygen cathode. Therefore, an optimized cathode is the key toestablishing a practical Li–O2 cell.

Unlike cathodes of Li-ion batteries, those of Li–O2 batteriesshould offer transport channels for oxygen, a three-phase(electrolyte/cathode/oxygen) interphase for the electrochemi-cal reactions, and the space for the deposition of solid-statedischarge product on the surface at the same time. Thus, thethree-dimensional structure of the air cathode is vital to thecell performance. Up to now, many reported cathodes werestructured by the mechanical mixture of catalyst and carbonsubstrate, and a porous current collector and organic binder tostick them together. These electrodes often lack the desiredstructure to fulfill the three characteristics mentioned above.Novel designs of oxygen cathodes are required.

Another challenge for rechargeable nonaqueous Li–O2 bat-teries is the decomposition of the organic electrolyte duringcharge and discharge. With the help of FTIR, NMR, and Ramanspectroscopy and mass spectrometry, Bruce et al. proved thatthe Li–O2 battery with a propylene carbonate (PC)-based elec-trolyte cycles by the decomposition of electrolyte on dischargeand decomposition of the discharge products on charging.[52]

The compounds C3H6(OCO2Li)2, Li2CO3, HCO2Li, CH3CO2Li, CO2,and H2O are formed during discharge, whereas C3H6(OCO2Li)2,Li2CO3, HCO2Li, and CH3CO2Li are oxidized during charge(Figure 4). Similar results of alkyl carbonate electrolyte decom-position have also been reported by other groups.[53, 54] Ether-based electrolytes, such as dimethoxyethane (DME) and tetra-glyme (TEGDME), are considered to be more stable in Li–O2

batteries.[55–58] But their stability has also been questioned re-cently.[59] The pursuit of a suitable organic electrolyte with sat-

Figure 4. Scheme for PC decomposition in rechargeable Li–O2 batteries. Re-printed with permission from Ref. [52] . Copyright 2011 American ChemicalSociety.

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Page 6: Air Electrode for the Lithium-Air Batteries: Materials and Structure Designs

isfactory stability is still in progress. We will just focus on theair cathode in this Review.

Carbon-based electrode

Carbon is the most widely used material in the oxygen cath-ode of Li–O2 batteries owing to its high electronic conductivity,high surface area, and low cost.[60] Although the catalytic effectof carbon material is still not clear,[60, 61] its porosity as a gas dif-fusion electrode (GDE) is vital for the cell performance. Asmentioned above, the GDE should offer a fast oxygen trans-port channel during the accommodation of discharge productsso that the discharge process does not terminate before thereis still room for Li2O2 deposition. Recent research shows thatthe high volume fraction of insoluble discharge products willlead to the increase of oxygen transport resistance significant-ly.[62] Other results also have demonstrated that instead of thesurface area, there is a strong correlation between the porestructure and the discharge capacity[63, 64] (Figure 5).

To develop a functional GDE for nonaqueous Li–O2 cells,Wang et al.[64] synthesized and tested carbon materials with dif-ferent pore structures. They pointed out that mesopores witha larger size could accommodate more discharge productswhereas pores with smaller size would be clogged at the earlystage of the discharge process. The results of Hall and col-leagues also showed that carbon aerogels with the highestpore volume (2.195 cm3 g�1) and a wide pore size (14.23 nm)showed the highest discharge capacity (1290 mA h g�1) amongthe prepared carbon aerogels.[33] Their further research onporous carbon aerogels showed that the shape and value ofthe resistance in the impedance spectrum of the Li–O2 cell de-pended strongly on the porosity of the carbon employed inthe oxygen electrode.[65] This result indicated that by optimiz-ing the pore structure of the electrode, the impedance of thecell could be decreased and the cell performance could be im-proved.

Based on simulation results, Zhang et al. claimed thata dual-pore system could improve the cell performance by fa-cilitating oxygen transport into the inner region of the air elec-trode.[66] With the help of a mesocellular foam silica template,Xia et al. prepared mesocellular carbon foam (MCF-C) withnarrow pore size distribution, centered at 4.3 and 30.4 nm. The

MCF-C delivered a discharge capacity of 2500 mAh g�1 at0.1 mA cm�2, which is about 40 % higher than that of Super Pelectrode (Figure 6).[30] More recently, Xia et al. synthesizedthree-dimensional ordered mesoporous/macroporous carbonsphere arrays and achieved enhanced cycle (up to 30 cycles)and rate (stable discharge/charge platform at 1000 mA g�1)performance.[31] The improved electrochemical performance ismainly a result of the ordered porous structure that benefitsthe O2 diffusion and three-phase interface formation.

One-dimensional carbon materials, such as nanotubes andnanofibers, have also been used to form a porous GDE fornonaqueous Li–O2 batteries. Yang and his group developeda binder-free all-carbon-fiber electrode for Li–O2 batteries andgained a high energy density of 2500 Wh kg�1. They attributedits improved cell performance to the efficient utilization of thecarbon material and void pores for discharge deposition in theabsence of organic binder.[22] A similar idea was also used todevelop a freestanding carbon nanofiber/CNT cathode for Li–O2 batteries.[24, 25] Recently, Kang’s group designed a hierarchicalcarbon–air electrode with controlled porosity constructed withwell-aligned CNT fibrils.[25] SEM images of the discharged elec-trode (Figure 7 a,b) proved that the porous structure of thecathode was kept because the discharge products packedclosely on the surface of the CNT fibrils. This close packingleads to the enhanced cycle performance of the electrode.[25]

These results[22, 24, 25] clearly prove that the absence of organicbinder is beneficial for the fabrication of a desired GDE for Li–O2 cells owing to the stable pore structure without bindercoating, which is important for the formation of the transportchannel for oxygen, three-phase interphase for the electro-chemical reactions, and the space for the deposition of solid-state discharge products.

Figure 5. Discharge time and specific capacity versus average pore diameter.Reprinted with permission from Ref. [64] . Copyright 2010 Elsevier.

Figure 6. Schematic of a) carbon black and b) MCF-C after discharge. Re-printed with permission from Ref. [30] . Copyright 2009 The Royal Society ofChemistry.

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Two-dimensional graphenematerial is also used in Li–O2

batteries because of its highelectronic conductivity and largesurface area.[5, 67] Sun et al. ach-ieved a discharge capacity of8705.9 mAh g�1 by preparinggrapheme nanosheets (GNSs)with a thin and wrinkled struc-ture.[27] The superior per-formance of this electrodecomes from the unique porousstructure provided by the GNSs,which allows the deposition ofdischarge products on bothsides of the nanosheets (Fig-ure 8 c) and also the edges ofthe GNSs. This phenomenon isin good agreement with Wangand Zhou’s suggestion that theunsaturated carbon atoms at theedge of the GNSs are very activeto oxygen, thereby improvingthe ORR catalytic activity.[68]

Zhang et al. developed function-al graphene nanosheets (FGSs)with a hierarchical arrange-ment.[28] The loosely packed,“broken egg” structure offerslarge interconnected tunnels foroxygen transportation and en-larges the three-phase electro-chemical interface. As a result,the FGS cathode delivers an ul-trahigh discharge capacity of15 000 mAh g�1 (Figure 9).

Besides the porosity and structure of the carbon materials,the carbon nature also affects the cell performance of thecarbon electrode.[11, 36, 69] Many studies show the improvementof the cell performance after doping carbon materials withcarbon.[69–73] Research results indicate that the conjugation be-tween the nitrogen lone-pair electrons and graphene p sy-stems may be beneficial for the ORR activity.[74] Sun et al. re-ported that sulfur doping in graphene nanosheets will alsolead to the formation of Li2O2 particles and the improvementof charge performance.[27]

Notably, the electrode thickness and carbon loading alsohave an influence on the electrochemical performance of thecarbon electrode. Plichta et al. found that the oxygen diffusionchannel was pinched-off first at the electrode surface of the airside (Figure 10).[24] The authors attributed the nonuniform dis-tribution of discharge products to the limited oxygen diffusiondepth compared to the thickness of the air electrode. Thisresult indicates that an optimized porous structure that prefersoxygen transport during the discharge process can improve

Figure 8. SEM and TEM images of GNS electrodes before (a, b) and after (c, d) discharge. e) Discharge/charge per-formance of lithium–oxygen batteries with different carbon cathodes at a current density of 75 mA g�1. Adaptedwith permission from Ref. [27] . Copyright 2011 The Royal Society of Chemistry.

Figure 7. SEM images of the CNT fibrils after first discharge at a) low mag-nification and b) high magnification. c) Discharge/charge profiles andd) cycle performance at a current rate of 2000 mA g�1 (inset: voltage versustime graph of the initial 10 cycles). Adapted from Ref. [25] with permission.Copyright 2013 Wiley-VCH.

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Page 8: Air Electrode for the Lithium-Air Batteries: Materials and Structure Designs

the discharge performance of the electrode. On the otherhand, Zhang et al. discovered that among the air electrodeswith the same thickness, the cathode with a carbon loading of15.1 mg cm�2 showed the highest specific capacity, and anyloading higher or lower than that would lead to the fading ofdischarge capacity.[75] The optimized carbon loading may be re-sponsible for the formation of a porous structure to enlargethe three-phase electrochemical interface.

Carbon-supported electrodes with various catalysts

Besides solid-state discharge deposition and oxygen transpor-tation, the large overpotential during cycling was caused bythe slow kinetics of the ORR/OER process.[76–79] Though the spe-cific catalytic mechanism for Li–O2 cells during charge/dis-charge is still not conclusive,[80–83] numerous studies haveshown that a wide range of materials (Au, Pt, Pd, transition-metal oxides) are capable of decreasing the overpotential ofLi–O2 batteries.[5, 35, 60, 81, 84, 85] As the developed catalysts areeither too expensive (noble metals such as Pt, Au etc.) orsuffer from low electronic conductivity (transition-metal oxidessuch as MnO2, Co3O4 etc.), carbon-based materials are oftenchosen as a cheap and highly conductive support material fora functional air cathode for Li–O2 cells.

Because the discharge/charge reactions are more favorableto take place on the surface of catalysts instead of the carbonsupport in a catalyst-loaded air electrode, the pore structure ofthe catalyst material also plays an important role in the cell per-

formance. Based on this understanding, porous catalysts havebeen synthesized and utilized in Li–O2 batteries.[81, 85,86, 87] Zhanget al. fabricated hierarchical perovskite La0.5Sr0.5CoO2.91 (LSCO)mesoporous nanowires assembled by mesoporous LSCO nano-rods.[81] The LSCO nanowire electrode delivered a capacity ofover 11 000 mAh g�1 (Figure 11) owing to the high ORR catalyticactivity of LSCO and the enhanced oxygen mobility along themesoporous nanowires. A similar porous La0.75Sr0.25MnO3 nano-tube-based electrode was also developed for Li–O2 batteries.[88]

Members of our laboratory also developed a mesoporousCo3O4 catalyst with controlled porosities with the help of silicatemplates.[89] Cell test results showed that the mesoporousCo3O4 demonstrated improvement in the round-trip efficiencyand specific capacity over the bulk one. Notably, the intercon-nected hydrophilic/hydrophobic channel structured by hydro-philic mesoporous Co3O4 loaded on hydrophobic carbon(Figure 12) improves the oxygen diffusion in the cathode andincreases the utilization of the pore volume.

To enlarge the electrolyte/catalyst/oxygen three-phase elec-trochemical interface and achieve a high utilization of catalystsurface, the uniform distribution of catalyst material on the sur-face of the carbon support and a close combination betweenthe catalyst and support are required. However, it is a goal hardto achieve through simply ball milling the mixture of catalystand carbon support. Different methods have been developedto improve the distribution uniformity of the cathode materials.Among these methods, in situ growth of catalysts on carbonsupport provides a facile process to achieve a novel electrodenanostructure.[38, 45, 90, 91] Dong’s group developed a a-MnO2/gra-phene nanosheet (GN) catalysts with a-MnO2 nanorods growndirectly on GNs.[90] The superior catalytic ORR and OER proper-ties compared with an a-MnO2/GNs mixture prove the advant-age of the direct combination between the catalyst particlesand the conductive support. With a suitable synthesis method,high-density, atomically dispersed iron–nitrogen–carbon com-posite embedded in a carbon matrix was developed.[92] The en-hanced electrochemical performance of the as-prepared elec-trode demonstrates the influence of catalyst distribution on cellperformance. To further improve the oxygen diffusion in thecathode, Zhang’s group designed a freestanding, hierarchicallyporous carbon electrode embedded with Ni particles.[93]

Through a facile sol–gel method, Ni-loaded porous carbon wasloaded directly on nickel foam current collector without anybinder. This unique structure facilitated a continuous oxygenflow in the cathode and endowed the electrode with improvedcapacity under relatively large current densities (2020 mAh g�1

at 2 mA cm�2). What is more interesting, their further study ona freestanding palladium-modified hollow sphere carbon (P-HSC) deposited onto a carbon paper (CP) cathode delivered animpressive cycle performance (100 cycles at 300 mAh g�1 anda specific capacity limit of 1000 mAh g�1) and high-rate capacity(5900 mAh g�1 at 1.5 A g�1).[32] Field-emission scanning electronmicroscopy (FESEM) images of the discharged electrodeshowed that, unlike the toroid-like Li2O2 deposit on the surfaceof the air electrode reported before,[22,94] unique Li2O2 nano-sheets with a thickness of less than 10 nm grew on the wall ofthe hollow carbon spheres of the P-HSC deposited on the CP

Figure 9. Schematic structure of functionalized grapheme sheets (FGSs) withan ideal bimodal porous structure (left image) and the discharge curve ofthe FGS electrode at 0.1 mA cm�2. Adapted with permission from Ref. [28] .Copyright 2011 American Chemical Society.

Figure 10. SEM images of the air electrode surfaces at a) separator and b) airsides after discharge at 0.1 mA cm�2. Reprinted with permission fromRef. [24] . Copyright 2010 The Electrochemical Society.

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electrode (Figure 13). The authors suggested that the highlyactive Pd nanoparticles deposited in the walls of carbonspheres favored the formation of loosely packed Li2O2 nano-sheets (Figure 13), and this unique morphology was responsiblefor the enhanced rechargeability of the cell by offering a largerLi2O2/electrolyte interface. This discovery indicates the advan-tages offered by the combination of an optimized porouscarbon structure and homogeneous distribution of highlyactive catalyst particles in electrochemical performances.

Owing to its advantage in atomic-scale control over the sizeand composition of the deposit, atomic layer deposition hasbeen used to prepare carbon-supported air electrodes withuniformly deposited catalyst nanoparticles.[95, 96] Amine et al.synthesized a Pd and Al2O3 deposited carbon cathode witha low charge overpotential of approximately 0.2 V.[96] They be-lieved that the alumina coating for passivation of carbondefect sites was responsible for the decreased charge potential

(Figure 14). This result shows the influence of the carbon sup-port surface on the cell performance. Physical methods such asdc sputtering and pulsed laser ablation are also used to ach-ieve a uniform distribution of ultrafine catalyst particles on thesurface of the carbon support for Li–O2 cells.[84, 97]

Non-carbon electrode

Although carbon materials are widely used in air cathodes,recent research results on carbon electrodes in Li–O2 batteriesshow the instability of the carbon material in both charge anddischarge processes.[98, 99] Itkis’s group demonstrated the epoxygroups on carbon in the presence of superoxide radicals duringdischarge with the help of in situ ambient-pressure X-ray pho-toelectron spectroscopy.[98] Bruce et al. pointed out that carbonmaterial in air cathodes undergoes an oxidation process toform Li2CO3 when the charge potential is up to 3.5 V (versus Li/

Li+).[99] Their further study indi-cates that the carbon decompo-sition will even promote the de-composition of organic electro-lyte. As shown in Figure 15, thecarbon in the cathode would notonly be directly oxidized byoxygen under a certain potential,but also react with Li2O2 to formLi2CO3 according to the Gibbsfree energy calculation.[100] Whatis more, the low polarity andhigh hydrophobic character ofcarbon material will lead to an

Figure 11. The discharge curve of Li–air batteries using LSCO nanoparticle + activated carbon (AC) (a) and hierarchical mesoporous LSCO nanowires + AC (b).Bottom: schematic model diagram of constructing hierarchical LSCO nanowires. Adapted with permission from Ref. [81] . Copyright 2012 United States Na-tional Academy of Sciences.

Figure 12. Wetting angles of the DME-based electrolyte on a) acetylene carbon black (AB) and b) Co3O4. Reprintedwith permission from Ref. [89] . Copyright 2012 Elsevier.

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organic-electrolyte-flooded cath-ode. Also, the limited oxygentransmission inside the electro-lyte will seriously restrict the ki-netics of the electrode reactions,and result in hetero depositionof the discharge products, pitch-off of the electrode surface, andlow utilization of the porevolume.[14,101, 102] These discover-ies prove the limited applicationof carbon materials in Li–O2 cells,and the development of non-carbon electrodes is required.

Inorganic compounds withhigh electronic conductivity suchas TiOx, Sn, and WO3 are candi-dates for more stable supportmaterials. Some of them have al-ready been used in fuel cell sys-tems.[103, 104] More recently,Bruce’s group developeda stable air electrode with highlyconductive TiC.[48] Though thecatalytic activity of TiC is incon-clusive, the air electrode con-tained nothing but TiC and PTFEbinder and was capable of cy-cling 100 times at 1 mA cm�1

with Li2O2 as the main dischargeproduct (Figure 16). The authorsattributed this outstanding cyclestability at high current densityto the stability and electronicconductivity of the TiC material.This discovery indicates thegreat advantage of stable con-ductive inorganic materials asthe support in air cathodes.

Up to now, PVDF and PTFE areamong the most used organicbinders in Li–O2 batteries. How-ever, these traditional binders

Figure 13. a) Discharge/charge curves of Li–O2 cells with Pd-modified hollow spherical carbon (P-HSC) depositedonto a carbon paper (CP) cathode at different cycles. b) Variation of terminal voltage on the discharge of the Li–O2 cells with the P-HSC deposited on the CP cathode. c) FESEM images of the discharged hollow-sphere carbon(HSC) deposited onto a CP cathode. White scale bars: 1 mm. Green scale bars: 400 nm. d) Images of the dis-charged P-HSC deposited onto a CP cathode. White scale bars : 1 mm. Green scale bars : 400 nm. e) Mechanism forthe electrochemical production of Li2O2 nanosheet-like aggregates on P-HSC deposited onto a CP cathode.f) Mechanism for the electrochemical production of Li2O2 toroidal aggregates on Super-P carbon (SP) and HSC de-posited onto CP cathodes. Panels i in (e) and (f) illustrate the nucleation processes of Li2O2. Panels ii in (e) and (f)illustrate that the small elongated hexagonal nanocrystallites come into being after the nucleation of Li2O2. TheLi2O2 preferentially nucleates and grows on the prismatic crystal faces. Panels iii in (e) and (f) illustrate the forma-tion of the observed toroidal and nanosheet-like aggregates. Adapted with permission from Ref. [32]. Copyright2013 Macmillan Publishers Ltd.

Figure 14. Schematic of the nanostructured cathode architecture. The insetshows a hypothetical charge/discharge voltage profile versus capacity. Re-printed with permission from Ref. [96] . Copyright 2013 Macmillan PublishersLtd.

Figure 15. Schematic of possible reactions in the interfaces between thecathode and electrolyte in Li–O2 batteries with carbon-based and carbon-free cathodes. Reprinted with permission from Ref. [100]. Copyright 2013The Royal Society of Chemistry.

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are readily swollen, gelled, or dissolved by the nonaqueousliquid electrolytes.[52, 105] Also, the insulating binders showa lack of facile transfer networks for both electronics and Li+ .These disadvantages of conventional binders result in increas-ing impedance of the cell and capacity fading.[63] Besides, thestability of these binders in Li–O2 batteries has also been ques-tioned recently.[106] Although conductive polymers such aspolypyrrole and polyaniline have been used in Li-ion and Li–O2

batteries,[13, 41, 101] the existence of binders still limits the utiliza-tion of the catalyst surface.

Direct growth of catalyst material on the surface of the cur-rent collector is one of the solutions to avoid the use of anybinders. Such electrodes with a so-called freestanding structurehave been utilized in supercapacitors and Li-ion batter-ies.[17, 28, 101, 107] Our group has developed a freestanding designfor rechargeable oxygen cathodes (Figure 17).[51] Highly catalyt-ically active Co3O4 nanorods were grown on the surface ofa nickel foam current collector through a simple chemical dep-osition process. The resulting electrodes showed extremelylow potential gap (0.5 V) at a small current density(0.02 mA cm�2) and improved cycle stability relative to Co3O3/AB and pure Co3O4 electrodes. We believe that the uniquefreestanding structure, which offers high utilization of the cata-lyst surface, intimate electronic contact between the dischargeproducts, the catalyst, and the current collector, and the open-pore system for Li2O2 deposition and oxygen transportation, isresponsible for the superior electrochemical performance. Simi-lar ideas have been used in other metal–air battery cathodesand enhanced cell performance has been achieved.[37, 39]

Although the metal oxide-based freestanding cathodeshave many advantages, the lowelectronic conductivity of themetal oxide catalysts limits theirelectrochemical performanceunder high current densities.Bruce et al. developed a nanopo-rous gold (NPG) electrode, whichwas capable of cycling stably100 times (retaining 95 % of itscapacity after 100 cycles), withLi2O2 as the main dischargeproduct in a battery using 0.1 m

LiClO4/DMSO electrolyte(Figure 18).[106] In their design,the NPG electrode served asboth catalyst and current collec-tor. This breakthrough highlightsthe importance of the electronicconductivity in a freestanding aircathode. Their further studyshowed that the Li–O2 batterieswith NPG electrodes could evencycle 100 times at a large currentdensity of 1 mA cm�2 with thehelp of tetrathiafulvalene (TTF)redox mediator.[108] The TTF

added in the nonaqueous electrolyte acts as an electron–holetransfer agent that facilitates the oxidation of solid Li2O2

during the charge process.To achieve an air electrode with a relatively high electronic

conductivity in the absence of carbon material, conductivepolymer is used in Li–O2 batteries as cathode material.[109, 110]

Members of our laboratory have also prepared a nano-polypyr-role (PPy) tube-based electrode for Li–O2 batteries.[111] The tub-ular PPy electrode showed superior reversible capacity, round-trip efficiency, and cycle stability to both granular PPy and con-ventional acetylene carbon black (AB) electrodes. Further anal-ysis and calculation indicate that the hydrophilic property andtubular structure are responsible for the improved per-formance. The hydrophilic PPy surface leads to a high wettingangle against the organic electrolyte, which results in en-hanced oxygen diffusion and larger three-phase interface. ThePPy nanotube offers fast oxygen transmission paths and leadsto a more uniform oxygen concentration distribution alongthe cathode thickness direction (Figure 19).

Solid-state electrolyte-based lithium–O2 bat-teries (hybrid and all-solid-state Li–O2 batter-ies)

Although the use of oxygen from the atmosphere endows thelithium–O2 battery with a superhigh theoretical capacity densi-ty, it brings great challenge for the testing device at the sametime. For the mostly reported aprotic Li–air battery, a gas stor-age device storing dry air or pure oxygen is indispensable

Figure 16. Cycling curves and capacity retention of TiC cathodes. a) Galvanostatic discharge/charge cycles record-ed in 0.5 m LiClO4 in dimethyl sulfoxide (DMSO) at a geometric current density of 1 mA cm�2. b) Capacity retentionfor the same cell as in (a). c) Galvanostatic discharge/charge cycles recorded in 0.5 m LiPF6 in TEGDME at a geomet-ric current density of 0.5 mA cm�2. d) Capacity retention for the same cell as in (c). Reprinted with permission fromRef. [48] . Copyright 2013 Macmillan Publishers Ltd.

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owing to the instability of the aprotic electrolyte and the lithi-um anode, which limited its practical applications to a greatextent. Thus, a solid electrolyte protecting them from the con-taminants in the atmosphere is essential for Li–air batteriesthat can be worked directly in the air atmosphere. However,there are several problems in the all-solid-state configurationas indicated in Figure 20, including the instability of lithium

with solid electrolyte (SE), largeinterface impedance betweenlithium anode/solid electrolyteand solid electrolyte/air elec-trode (AE), and side reactions ofthe deposited discharge prod-ucts owing to contaminantssuch as H2O and CO2 in the at-mosphere.[112] To solve theseproblems, researchers have beendevoted to the solid-electrolyte-based Li–air batteries.[26, 68, 113–119]

Wang and co-workers developedhybrid Li–air batteries witha structure of Li anode/organicelectrolyte/solid electrolyte/aqueous electrolyte/air elec-trode, in which the organic elec-trolyte is just a thin liquid layer(or electrolyte adsorbed bya porous membrane) used toseparate and wet the Li anodeand solid electrolyte. The use ofaqueous electrolytes on the onehand decreased the interface im-pedance of the solid electrolyte/air electrodes, but on the otherhand increased the solubility ofdischarge products in them. Thehybrid configuration showeda special capacity of50 000 mAh g�1 based on totalmass of catalytic electrode(carbon + binder + catalyst) andobtained the extensive attentionof researchers.[26, 29, 115, 117, 118] How-ever, the evaporation of aqueous

solution is an issue for this configuration. Therefore, anotherconfiguration of Li–air batteries using ionic liquid electrolyteinstead of aqueous electrolyte was established.[34] The uniqueproperties of ionic liquid, such as negligible vapor pressure,low viscosity, high thermal stability, and wide electrochemicalwindow, make them highly suitable as the electrolyte of Li–airbatteries and efforts are being made with them.

Despite the various configurations of solid-electrolyte-basedLi–air batteries, a unique advantage resulting from the use ofsolid electrolyte is that an integral cathode/electrolyte is devel-oped with an air electrode formed in situ on the surface of thesolid electrolyte (Figure 21).[68, 112] Undoubtedly, the design, op-timization, and electrochemical performance improvement ofcathodes are also important for solid-electrolyte-based Li–airbatteries. Although the strength of the research of cathodes isnot as much as that of aqueous Li–air batteries, the conceptsincluding carbon-based, bifunctional, freestanding, and solid-electrolyte-supported have also been involved in solid-electro-lyte-based Li–air batteries.

Figure 17. a) SEM images of Co3O4@Ni. b) TEM image and selected-area electron diffraction pattern of the Co3O4

nanorods. c, d) First discharge/charge profiles of the Co3O4@Ni-based Li–O2 cell at current densities of c) 0.02 andd) 0.1, 0.2, and 0.3 mA cm�2. e) Schematic diagram of the freestanding-catalyst-based electrode during cycling inthe Li–O2 battery. Adapted with permission from Ref. [51] . Copyright 2011 The Royal Society of Chemistry.

Figure 18. Charge/discharge curves (left) and cycling profile (right) for a Li–O2 cell with a 0.1 m LiClO4-DMSO electrolyte and a NPG cathode, at a currentdensity of 500 mA g�1 (based on the mass of Au). Reprinted with permissionfrom Ref. [106]. Copyright 2012 American Association for the Advancementof Science.

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Carbon-based air electrode

Similar to the nonaqueous Li–air batteries, carbon-based mate-rials have been mostly used in solid-electrolyte-based batteries,although the necessity of an OER catalyst in a cathode is stillunclear.[55, 94, 120] Almost all of the cathodes in Li–air batteries re-ported in the literature are carbon and carbon-supported ma-terials. Therefore, we will review carbon-based air electrodesby firstly distinguishing the carbon-supported air electrodeand carbon–air electrode.

Carbon has been used widely as catalyst support in Li–airbatteries. At the very early stage of solid-state Li–air batteries,Kumar and co-workers reported a conventional air electrodeconsisting of carbon black, LAGP (a lithium/aluminum/germa-nium/phosphorus mixed oxide), and Teflon, in which carbon isused as both electron conductor and catalyst support.[116]

Then, various air electrodes with metals or metal oxides sup-ported on various kinds of carbon materials were developed.Wang and co-workers reported a Li–air battery with a Mn3O4/acetylene black air electrode demonstrating a potential to con-

tinuously reduce O2 from air,which delivered a special capaci-ty of 50 000 mAh g�1 based ontotal mass of catalytic electrode(carbon + binder + catalyst).[119]

Yoo and co-workers used gra-phene nanosheets (GNs) directlyas catalysts in air electrodes andobtained excellent electrochemi-cal performances.[29] As Figure 22indicates, the GNs prepared bya chemical method displayedmany edge sites and defect siteslocated on the surfaces. Suchedges and defects are consideredto serve as active sites for chemi-cal reactions. Furthermore, the2 D structure of GNs providesa pathway for access by oxygengas from both sides of the nano-sheet. Not disappointing, itshowed a good electrochemicalperformance with a high dis-charge voltage near that of the20 wt % Pt/carbon black at0.5 mA cm�1, an improved cycleperformance, and a good reversi-bility.

Carbon-supported air electro-des with various catalysts

It is generally recognized thatthe electrocatalysts for the airelectrode play critical roles todetermine the cyclability, rate ca-pability, and energy efficiency ofthe Li–O2 batteries. Various kinds

of catalysts have been investigated, including carbon, transi-tion-metal oxides, and precious and nonprecious metal-basedmaterials.[122] Critical challenges that limit the practical use ofthis technology include the sluggish ORR (during discharge)and OER kinetics (during charging) in Li+-containing aproticelectrolytes. Therefore, it is vital to develop an effective elec-trocatalyst for both the ORR and OER, namely a bifunctionalelectrocatalyst.[35] Although a variety of bifunctional electroca-talysts have been introduced in nonaqueous lithium–O2 batter-ies, here we focus on the ones used in solid-electrolyte-basedLi–air batteries.[35, 122, 123] Our group designed a kind of bifunc-tional electrocatalyst with mesoporous carbon nitride (MCN)loaded with Pt nanoparticles. Their performances were testedin an all-solid-state Li–air battery with a structure of Li anode/gel membrane/lithium aluminum titanium phosphate/gelmembrane/air electrode. The results showed that the Pt@MCNelectrode exhibited the highest round-trip efficiency of up to87 % with the highest discharge voltage at around 2.87 VLi andthe lowest charge voltage at about 3.30 VLi, below 50 % capaci-

Figure 19. Schematic representation of the organic electrolyte and oxygen distributions on hydrophilic PPy nano-tubes (a), oxygen gradient across the flooded porous cathode (b), and the schematic discharge/charge process (c).First discharge/charge curve of AB, granular PPy (GPPy), and tubular PPy (TPPy)-supported Li–O2 cells at0.5 mA cm�2 in oxygen (d). Discharge/charge capacities versus cycle numbers of AB, GPPy, and TPPy at 0.1 and0.5 mA cm�2. Adapted with permission from Ref. [111] . Copyright 2012 The Royal Society of Chemistry.

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ty among these three kinds of electrodes (Pt@MCN, Pt@AB,and MCN) as indicated in Figure 23 b. This round-trip efficiencyis even higher than that in a previous report,[35] thus indicatingthat the Pt@MCN-based air electrode has excellent bifunctionalcatalytic performance for both OER and ORR kinetics. A newclass of core–corona-structured bifunctional catalyst (CCBC)consisting of lanthanum nickelate centers supporting nitrogen-doped carbon nanotubes (NCNTs) has been developed for re-chargeable metal–air battery application.[124] The nanostruc-tured design of the catalyst allows the core and corona to cat-alyze the OER and ORR, respectively (Figure 24). Cell tests ina zinc–air battery with aqueous alkaline electrolyte showed im-proved performance owing to this novel catalyst. This resultmade it a promising air electrode in hybrid Li–air batterieswith aqueous alkaline electrolyte. Besides, a wide range of bi-functional catalysts tested in metal–air batteries with an aque-ous alkaline electrolyte are promising to be used in Li–air bat-teries.[123]

Solid-electrolyte-supported air electrode

The use of solid-state electrolyte requires a new catalyst load-ing method to decrease the interface impedance between theelectrolyte and catalysts. An imaginative design of the free-standing air electrode in solid-electrolyte-based Li–air batterieswas made by Wang and Zhou as indicated in Figure 25. Theydeveloped a novel thin-film electrode prepared by pencil-drawing on a ceramic-state electrolyte, which was applied ina Li–air battery as a natural catalytic electrode of oxygen. Itprovided a novel, interesting, and enlightening method forelectrode preparation. The resulting electrode delivered a ca-

pacity close to 1000 mAh g�1 at a current density of 0.1 A g�1,and could be cycled 15 times at 0.25 A g�1. After that, severalsolid-electrolyte-supported air electrodes were proposed.[26, 112]

Kitaura and Zhou fabricated an air electrode by using CNTs asthe catalyst at the surface of a solid electrolyte.[26, 29] The fabri-cation process is very simple by the following procedure: CNTand solid electrolyte powders were mixed and dispersed intoethanol solution. Then, the solution was dropped onto thesolid electrolyte sheet and dried at room temperature. Finally,a high-temperature heat treatment was conducted to decreasethe interface impedance between them. The as-prepared solid-state Li/photoelectrode/counter electrode/CNT cell could becycled three times with a capacity of about 400 mAh g�1 at thecurrent density of 10 mA g�1.[26] Recently, Zhou and Zhang alsodeveloped a LTAP-based Li–air battery with a SWNT/ionicliquid (IL) cross-linked network gel (CNG) cathode.[125] With thehelp of the novel cross-linked network of the gel cathode andthe Li bis(trifluoromethane)sulfonimide salt dissolved in theionic liquid, the battery could be cycled 100 times in ambientair with a discharge capacity of 2000 mAh g�2 (Figure 26). Thiswork showed the possibility of long cycle performance of Li–air batteries based on solid-state electrolyte in ambient air.

Summary and perspective

The lithium–air battery is considered to be a promising candi-date for future applications such as electric vehicles andenergy storage. Although this battery system shows potentialto offer an energy density much higher than current chemicalbatteries, numerous problems in cathodes, electrolytes, andanodes have to be addressed to realize a practical cell

Figure 20. Advantages and disadvantages of solid-electrolyte-based Li–air batteries.

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(Table 1). The reversible formation/decomposition of Li2O2 orLiOH in the cathode defines the Li–air battery and represents

one of the greatest challenges in the development of the cell.That is why we focus on the air cathode side in this Review. Todevelop a desired air cathode, systematic studies have to bedone in many aspects.

Fundamental understanding of cathode reactions (the for-mation/decomposition of Li2O2 or LiOH) and the catalyticmechanism are needed to guide us in developing novel elec-trode materials and structures.

Catalysts that specifically facilitate the formation/decomposi-tion of Li2O2 or LiOH and are stable to both oxygen and elec-trolyte are required to improve the round-trip efficiency andcycle stability of the battery. Some catalysts are believed to beresponsible for promoting the electrolyte decomposition.Therefore, their catalytic activities have been overestimated.The specific products after the discharge and charge processhave to be identified to establish the real catalytic activity ofthe catalysts in further studies.

Novel designs in cathode structure are also urgently needed.An optimized porous structure for air cathodes should offer

Figure 21. Schematic diagram of an all-solid-state Li–air battery using a lithi-um anode, an inorganic solid electrolyte, and an air electrode composed ofcarbon nanotubes and solid electrolyte particles. b) Cross-sectional FESEMimages of the air electrode on the solid electrolyte layer. Adapted with per-mission from Ref. [112] . Copyright 2012 The Royal Society of Chemistry.

Figure 22. a) Structure of the rechargeable Li–air battery based on GNs as an air electrode. b) Electrochemical performances of GNs. Adapted with permissionfrom Ref. [29] . Copyright 2011 American Chemical Society.

Figure 23. a) TEM image of Pt nanoparticles dispersed on MCN. b) Chargeand discharge curves of the first cycle of the prepared air electrodes at a cur-rent density of 0.02 mA cm�2. Adapted with permission from Ref. [121].Copyright 2012 Springer.

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oxygen diffusion paths and volume, especially for Li2O2 deposi-tion in the aprotic Li–air battery at the same time. Designswith hierarchical and several levels of pore structure may bea possible solution. The loading method of the catalyst on thesurface of the conductive support also plays an important rolein the cell performance. The uniform distribution of the cata-lyst and the close contact between catalyst and support mate-rial are vital to enlarge the oxygen/electrolyte/electrode three-phase interface and decrease the voltage drop caused by elec-tronic resistance of the cathode.

Besides, the stability and electronic conductivity of the cath-ode materials should also be taken into consideration. Carbonand some organic binders have been found to suffer fromoxygen corrosion during cell operation, especially during thecharge process. The low electronic conductivity of convention-al metal-oxide catalysts limits the cell performance at large cur-rent densities. Electronic conductive metal oxides with opti-mized structures (nanowires, nanotubes, and so on) and free-standing designs of air cathodes are possible solutions tothese problems.

In spite of all the obstacles lying in the way of developingcommercially available Li–air batteries, the ever-increasingneeds of modern society for energy storage devices withhigher energy densities make it worthwhile to devote more ef-forts into the research on Li–air cells.

Acknowledgements

This work was financially supported by NSFC ProjectNos. 51373195, 51272267, and 51432010.

Keywords: batteries · electrochemistry · energy conversion ·lithium · oxygen

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Figure 26. Schematic diagram of the SWNT/IL CNG cathode (a) and the cycle performance of the battery in ambi-ent air (b). Adapted with permission from Ref. [125]. Copyright 2013 Macmillan Publishers Ltd.

Table 1. Status, challenges, and possible solutions of materials in cathodes of nonaqueous Li–O2 batteries.

Materials Status Challenges Possible solutions

supports various carbonmaterials

corrosion in the presence of oxygen;flooded by organic electrolyte nonwet-ting behavior in aqueous electrolyte;loose contact with catalysts throughmechanical mixing

conductive metal oxides; in situ growthof catalysts; freestanding design ofcathodes; surface modification

binders organic bindersuch as PVDF,PTFE

instability under oxygen; nonconduc-tive for both electron and Li+ ; readilyswollen, gelled, or dissolved in electro-lytes

conductive polymers; binder-freedesign of cathodes

catalysts precious metals ;metal oxides;carbon-based ma-terials

low electronic conductivity (metaloxides) ; low OER/ORR catalytic activity;catalytic decomposition of componentsin electrolytes

novel catalysts with high catalytic activ-ity; combination of multiple catalysts ;uniform catalyst distribution on con-ductive support; identification of dis-charge/charge products

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Received: April 11, 2014Revised: September 10, 2014Published online on && &&, 0000

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REVIEWS

Z. Wen,* C. Shen, Y. Lu

&& –&&

Air Electrode for the Lithium–AirBatteries: Materials and StructureDesigns

Air power : The energy storage capacityand power capability of Li–air batteriesare determined by the air electrode. Theelectrocatalytic oxygen reaction occursat a three-phase contact zone betweenair, liquid electrolyte, and solid catalyst.

In aqueous electrolytes, oxygen is re-duced to OH� , which is dissolved intothe electrolyte (see figure). In cellsbased on organic electrolyte, only theoxygen dissolved in the electrolyte par-ticipates in the oxygen reduction.

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