DOI: 10.1002/cplu.201402035

Reversibility of Lithium-Ion–Air Batteries Using LithiumIntercalation Compounds as AnodesJinyoung Chun,[a] Hyojin Kim,[b] Changshin Jo,[a] Eunho Lim,[a] Jinwoo Lee,*[a] andYoungsik Kim*[b]

A Li (ion)–air cell was designed to investigate the reversible uti-lization of Li that was harvested from a cathode. In the Li(ion)–air cell, 0.1 m LiOH(aq) was used as the catholyte withoxygen in air and Li intercalation compounds such as graphite(C6) and Li4Ti5O12 (LTO) were used as the anodes. It appearedthat the Li metal formed on the stainless-steel anode in thefirst charge of the Li–air cell was poorly reversible in further cy-cling owing to irreversible loss of Li during cycling. However,when Li intercalation compounds were used as anodes in a Li-ion–air cell, high coulombic efficiency by reversible reactionsand good cycle performance were achieved. These resultsshow that Li intercalation materials have significant potentialas anode materials since they effectively utilize Li resources inLi–air battery systems and ensure reversibility in these systems.

As energy-storage technologies are of growing importance formeeting society’s increasing demands for electricity and for ef-fective utilization of energy resources,[1] many researchers haveexplored advanced secondary battery systems that are consid-ered to provide enormous energy-storage capacity with costeffectiveness.[2] In recent years, by incorporating ceramic solidelectrolytes into Li battery technologies, new types of batterysystems, such as, aqueous Li–air[3] and Li–aqueous liquid bat-teries, have been suggested,[4] and these are also promisingcandidates for energy-storage applications.[5] In these batterysystems, Li metal was used as the anode material to achievehigh energy densities because Li is the lightest and most elec-tropositive metal, which provides a high specific capacity andhigh-output voltage operation in combination with air and var-ious liquid cathode materials.[6]

Recent studies that used Li metal as an anode in Li–air andLi–liquid batteries have shown excellent coulombic efficienciesand stable cyclic performances over several tens of cycles.[7]

However, in most of the literature,[7] the Li metal in the anodewas not fully used in every discharging step. Instead, the dis-charge time was limited before full utilization of Li so thata great excess amount of Li metal was still left in the anodecompared with the available charge capacity from the catho-des. As a result, it is difficult to establish whether the Li metalthat is harvested through charging is fully and reversibly re-uti-lized during the following discharge/charge process. If the dis-charge capacity is obtained by the presence of an excessamount of Li metal at the anode, the cycle performance of theLi–air battery is not able to be truly investigated because it isnot possible to examine some portion of Li that can be lost byside reactions during the discharge/charge process.

In the study reported by Jang et al. , for instance, poor cyclicperformance was observed when anodic Li metal was fully de-pleted at every discharging step.[8] It has also been consideredthat Li-dendrite growth due to high chemical reactivity of met-allic Li causes critical problems for safety and cycling stabilityof Li–air batteries.[9] For these reasons, the reversibility of Li–airbattery systems must be confirmed in the absence of reactionscaused by the presence of an excess amount of Li metal, andimprovements in efficiency and safety must be achieved. Thisissue can be addressed by using Li–ion intercalation materials,which can facilitate charge/discharge reactions reversibly asanodes for the aforementioned battery systems. In this way,only Li ions harvested by charging reactions at the cathodecan be re-utilized during the following discharge/charge pro-cess, and reversible reactions will likely lead to higher coulom-bic efficiencies. In addition, the safety of battery systems canbe improved by preventing the formation of Li dendrites onthe anode. Therefore, although their specific capacities arelower than that of Li metal, Li-ion intercalation materials canperform well as anode materials in scalable and/or stationaryenergy-storage systems by ensuring reaction reversibility andthe efficient use of lithium.[2c]

Herein, we report on our investigation of the reversible uti-lization of Li harvested from cathodes by applying stainlesssteel, graphite (C6), and lithium titanate (Li4Ti5O12, LTO) toanodes of a Li (ion)–air battery system (Figure 1). A previouslydesigned Li–air battery system that uses oxygen dissolved inwater as a cathode[10] was adopted in this study to demon-strate the reversibility of a Li ion–air battery in which stainlesssteel, C6, or Li4Ti5O12 were used as the anode and LiOH(aq) wasused as the catholyte with oxygen in air. It was observed that

[a] J. Chun,+ C. Jo, E. Lim, Prof. J. LeeDepartment of Chemical EngineeringPohang University of Science and Technology (POSTECH)Pohang, 790-784 (Republic of Korea)Fax: (+ 82) 54-279-5528E-mail : jinwoo03@postech.ac.kr

[b] H. Kim,+ Prof. Y. KimInterdisciplinary School of Green EnergyUlsan National Institute of Science and Technology (UNIST)Ulsan, 689-798 (Republic of Korea)Fax: (+ 82)-52-217-3009E-mail : ykim@unist.ac.kr

[+] These authors contributed equally to this work.

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cplu.201402035.

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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high coulombic efficiency by reversible reactions and goodcycle performance were achieved when C6 or Li4Ti5O12 wereused as the anode in the Li ion–air battery system. However,when stainless steel was used as the negative electrode inwhich Li metal was formed during charging of the Li–air bat-tery, poor coulombic efficiency was observed owing to an irre-versible loss of Li.

These results show that Li-ion intercalation materials can beused not only to promote reversibility in battery systems, butas efficient, long-cycle-life, and safe anode materials.

The overall configuration and charge/discharge reactions ofthe Li–air battery with oxygen dissolved in water as the cath-ode are shown in Figure 1a. A Li–air battery with the followingstructure was assembled: Li metal/nonaqueous liquid electro-lyte (1 m LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC))/solid electrolyte (Li1 + x + yTi2�xAlxP3�ySiyO12)/0.1 m LiOHaqueous solution.[10] The first discharge voltage observed fromthe prepared battery was approximately 3.0 V versus Li+/Li0, atthe rate of 0.1 mA cm�2 (curve B of Figure 2a). During dis-charge, Li ions, generated at the Li-metal anode, migrate intothe water catholyte through the solid electrolyte. At the cath-ode, two possible chemical reactions can occur because bothoxygen dissolved in water and water itself can be reducedduring the charging process. The following Equations (1) and(2) show the possible overall reactions that can occur duringdischarge:

LiðsÞ þ 1=2 H2OðlÞ þ 1=4 O2ðgÞ ! LiOHðaqÞ E ¼ 3:44 V ð1Þ

LiðsÞ þ H2OðlÞ ! LiOHðaqÞ þ 1=2 H2ðgÞ E ¼ 2:21 V ð2Þ

There is a large voltage difference between the oxygen andwater involved in these two overall reactions. In our experi-ment, the observed discharge voltage of curve B (Figure 2a)was approximately 3.0 V, which is higher than the approxi-mately 2.0 V associated with the water reaction but similar tothe 3.44 V of the oxygen reaction. The discharge voltage of thepreviously reported Li–air battery exposure to air was also ap-proximately 3.0 V (curve A),[7b, 10] which is comparable to thatobserved in this experiment (curve B of Figure 2a). It is clearthat oxygen dissolved in water must be involved in the dis-charge reaction, although its content is reported to be small.[11]

To confirm the effect of dissolved oxygen on the dischargevoltage of the water, removal of oxygen was realized by bub-bling a Ar/H2 (95:5 wt %) gas mixture through the water for20 h prior to discharging the cell. A sharp voltage drop, fromapproximately 3.0 V to approximately 2.0 V, was observed inthe Ar/H2 bubbled water, compared to the non-bubbled water,under the experimental conditions described in Figure 2a(curve C). Similar voltage drops for oxygen-free water havebeen reported in the literature.[3a, 12] This experiment showsthat an electrocatalytic reduction reaction can occur insideaqueous solutions using soluble oxygen (�1 � 10�3 mol cm�3).This means that instead of using atmospheric oxygen, thesoluble oxygen in an aqueous electrolyte can be used as themain oxygen source for an electrocatalytic oxygen-reductionreaction in aqueous Li–air batteries.

Figure 1. Schematic of the (a) Li–air and (b) Li-ion–air battery system withoxygen dissolved in water as cathodes.

Figure 2. (a) Discharge voltage of the oxygen dissolved in water comparedwith that of air- and Ar/H2-bubbled water. (b) Charge/discharge voltage pro-files of a Li–air cell with an oxygen dissolved in water cathode and a Li-metal anode.

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To investigate the re-chargeability of the Li–air cell withoxygen dissolved in water as the cathode and a Li-metalanode, the cell was cycled 10 times between 4.5 and 2.0 V ata rate of 0.1 mA cm�2 (Figure 2b). By focusing on confirmingthe facile re-chargeability of the cell, charging and dischargingtimes were limited to 10 h. Figure 2b presents charge–dis-charge curves of the prepared cell. It shows that the chargingand discharging voltages of the cell were maintained at almostconstant values during the 10 cycles and stable cycling per-formance, with 100 % coulombic efficiency, was observed. Thisresult demonstrates that a Li–air cell that uses oxygen dis-solved in water as its cathode can be operated stably overrepeat charge and discharge cycles. Moreover, it appears thatfully reversible charge/discharge reactions can progress withinthe limited usage of active materials (Li metal).

However, it should be noted that an excess amount of Li al-ready existed at the anode of the aforementioned Li–air cell.Therefore, it is possible to obtain a discharge capacity equal tothe charging capacity, even though some portion of the Li har-vested from the charging process may be lost to undesirableside reactions. Because the electrochemical potential of Limetal (mLi) lies above the lowest unoccupied molecular orbital(LUMO) of the nonaqueous liquid electrolyte, irreversible con-sumption of Li and the formation of a solid/electrolyte inter-face (SEI) layer, caused by decomposition of the electrolyte, areinevitable.[13] More importantly, it is well known that growth ofLi dendrites leads to the further consumption of Li, as well asthe isolation of Li (dead volume) from the anode.[8, 9, 14] Suchdendrite formation and inhomogeneous Li deposition on thesurface of the Li metal anode were also observed followingelectrochemical tests of this Li–air cell (scanning electron mi-croscopy (SEM) images in Figure 3b). Therefore, it is hard toassert that fully reversible reactions occurred during repeatedelectrochemical cycling of this Li–air cell.

To confirm the actual reversibility and coulombic efficiencyof the Li–air cell, we examined the electrochemical behaviorsof a Li–air cell constructed with only a current collector (stain-less steel) as the anode, in the absence of an active material (Limetal). In this configuration, only Li metal harvested from thecathode through charging can be reused for subsequent dis-charging, and thus the efficiency of the Li usage and the rever-sibility of the reactions of the Li–air battery system can be eval-uated. Figure 4a presents the charge–discharge voltage pro-files of the Li–air cell constructed with a stainless-steel anode.This cell was also cycled 10 times between 4.5 and 2.0 V ata rate of 0.1 mA cm�2. In this experiment, only the chargingtime was limited to 10 h. The results show that only 50 % ofthe Li was utilized during the discharge step of the first cycle,after harvesting Li metal from the LiOH solutions during thefirst charging step. In all subsequent cycles, the coulombic effi-ciency of the cell was less than 80 % (Figure 4b). Inferior cou-lombic efficiency of the cell using the stainless-steel anode wasalso observed in a study with a similar battery system, whenthe charging time of the first cycle was limited to 20 h.[4d]

These mean that loss of harvested Li continuously occurredthroughout every cycle and that undesirable side reactions sig-nificantly hindered the efficient use of Li.

Figure 3. SEM images of the surface of the Li metal (a) before electrochemi-cal tests, and (b) after 5 cycles (inset: after first charging).

Figure 4. (a) Charge/discharge voltage profiles of a Li–air cell with a stain-less-steel anode. (b) Comparison of charging/discharging times of a Li–aircell with Li metal to those of a cell with a stainless-steel (SUS) anode. Charg-ing times of both cells were limited to 10 h.

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One way to address the issues observed in the aforemen-tioned Li–air cell system is to employ Li-ion intercalation mate-rials as anodes, in which Li ions are stored, instead of where Limetal forms (Figure 1b). In an energy-storage system used forstationary applications, materials based on intercalation reac-tions are good anode-material candidates in terms of efficien-cy, safety, cycle life, and cost,[2c] although their specific capaci-ties are lower than that of Li metal. Graphite intercalates Liions reversibly into its layered structure at low voltages(�0.1 V versus Li+/Li0) and the SEI layers that form on its sur-face are relatively stable,[15] thus, we first used graphite for theanode of the “Li-ion”–air cell. The charge–discharge voltageprofiles of the prepared Li-ion–air cell are presented in Fig-ure 5a. The different shape and position of the voltage curvescompared to the Li-metal–air cell originates from a stepwise in-tercalation of Li ions into graphite at slightly higher potentialthan the Li+/Li0 equilibrium potential.[16] The first chargingtime was limited to 30 h (corresponding to 384 mA h g�1) toprevent the deposition of Li metal on the surface of graphitethat could be caused by overcharging. The subsequent dis-

charging time was observed to be 20 h, which indicates that67 % of the Li metal harvested from the cathode was recov-ered during the discharge process in the first cycle. The lowcoulombic efficiency of the Li-ion–air cell with a graphiteanode in the first cycle could be owing to the loss of the Liions that were consumed to form the SEI layers on the graph-ite surface. However, when the Li-ion (graphite)–air cell wascharged for 12.5 h after the first cycle, high coulombic efficien-cies of more than 90 % and stable cycle performance (Fig-ure 5c) were observed in the subsequent cycles. This means,not only that harvested Li ions were reused very efficientlyduring continuous discharge, but that the structure of theanode and the SEI layer formed at its surface are well main-tained during repeated cycling.

Because of the high operating voltage (�1.55 V versus Li+

/Li0) of Li4Ti5O12(LTO) and its zero-strain property, it was chosenas another attractive intercalation material for the anode in theLi-ion–air cell.[17] LTO prevents the undesirable decompositionof nonaqueous liquid electrolyte that occurs below approxi-mately 1 V (versus Li+/Li0) and, therefore, reversibility of the Li-ion–air cell system can be further improved with good cycleperformance. Figure 5b shows the charge/discharge curves for15 cycles of the cell between 2.6 and 0.5 V at a current densityof 0.1 mA cm�2. The charge voltage curves observed at approx-imately 2.5 V are related to the Li intercalation into LTO, whichwas extracted from LiOH(aq) with evolution of oxygen. The dis-charge voltage curves at approximately 1.5 V correspond tothe formation of LiOH(aq) by reaction with oxygen in the cath-ode. These are matched to the difference between the poten-tial of the oxygen evolution (�4.0 V versus Li+/Li0) or reduc-tion (�3.0 V versus Li+/Li0) reaction of the previously dis-cussed experiments (Figure 4a) and that of Li insertion intoLTO (�1.55 V versus Li+/Li0). Most significantly, these resultsshow high coulombic efficiencies of above more than 80 %during the first cycle and approximately 95 % for subsequentcycles (Figure 5c). Moreover, the specific capacity calculatedfrom the voltage profiles (�169 mA h g�1) nearly matches thetheoretical capacity of LTO (175 mA h g�1) and stable cyclabilitywas observed for up to 15 cycles. Such high reversibility withgood cycling performance could be a result of the high operat-ing voltage and structural integrity of the LTO anode duringthe intercalation reactions. SEM images (Figure S1 in the Sup-porting Information) show that the overall morphologies ofgraphite and LTO, unlike that of the Li metal surface, werealmost unchanged following the electrochemical reactions.These results demonstrate that Li-ion intercalation materials,such as graphite and LTO, have significant potential as anodesfor Li-based advanced energy-storage systems because of theirefficient utilization of Li, their cycle life, and their ability toensure reversibility in these systems.

In summary, we investigated the reversibility of Li (ion)–airbatteries with oxygen dissolved in water cathodes and severalkinds of anode materials. Although it appeared that reversiblereactions progressed when Li metal was used as an anode, byusing a stainless-steel anode it was demonstrated that theactual coulombic efficiency was low. However, we confirmedthat high coulombic efficiency, as a result of reversible reac-

Figure 5. Charge/discharge voltage profiles of Li-ion–air cells made with(a) graphite and (b) LTO anodes. (c) Comparison of charging/dischargingtimes of the Li–air cells made with graphite and LTO anodes.

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tions, and good cyclability were achieved when Li-ion interca-lation materials, such as graphite or LTO, were used as anodes.These results show that Li-ion intercalation materials have sig-nificant potential as anode materials since they effectively uti-lize Li resources in Li-based advanced energy-storage systemsand ensure reversibility in these systems.

Experimental Section

The Li–air battery with oxygen dissolved in water as a cathode wasassembled as illustrated elsewhere.[10] To observe the sole effect ofthe anode on its electrochemical performance, other cell compo-nents such as the solid and liquid electrolytes, cell body, and cur-rent collector materials were fixed. The assembled Li–air batterywas connected to a Solartron 1470 testing station to perform thecharge and discharge at a current rate of 0.1 mA cm�2. The currentrate was recorded with respect to the anode. See the SupportingInformation for details.

Acknowledgements

This study was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF) fundedby the Ministry of Science, ICT, and Future Planning (NRF-2013R1AA2074550). This study was also supported by the 2013Research Fund (1,130064,01) of UNIST.

Keywords: electrochemistry · energy storage · intercalations ·lithium · redox chemistry

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Received: February 26, 2014

Revised: April 2, 2014

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COMMUNICATIONS

J. Chun, H. Kim, C. Jo, E. Lim, J. Lee,*Y. Kim*

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Reversibility of Lithium-Ion–AirBatteries Using Lithium IntercalationCompounds as Anodes

Out of thin air : A lithium (ion)–air bat-tery was designed, in which 0.1 m

LiOH(aq) was used as the catholyte withoxygen from the air and lithium interca-lation compounds such as graphite (C6)and Li4Ti5O12 (LTO) were used as theanodes (see figure). In this system, re-versible utilization of Li harvested fromthe cathode was investigated.

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