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Electrochimica Acta 67 (2012) 87– 94

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

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ffects of aqueous electrolytes on the voltage behaviors of rechargeable Li-airatteries

ui Hea, Wei Niua, Nina Mahootcheian Aslb, Jason Salimc, Rongrong Chena,∗, Youngsik Kima,b,∗∗

Richard G. Lugar Center for Renewable Energy, Indiana University Purdue University Indianapolis, Indianapolis, United StatesDepartment of Mechanical Engineering, Indiana University Purdue University Indianapolis, Indianapolis, United StatesDepartment of Electrical and Computer Engineering, Indiana University Purdue University Indianapolis, Indianapolis, United States

r t i c l e i n f o

rticle history:eceived 29 October 2011eceived in revised form 27 January 2012ccepted 1 February 2012vailable online 14 February 2012

eywords:queous Li-air batterylkaline aqueous electrolyteischarge–charge voltage efficiency

a b s t r a c t

Aqueous Li-air batteries have attracted a great deal of attention due to their high theoretical energycapacities. However, while still in the early stages of research, the reported energy capacities of Li-airbatteries are far from what has been theoretically predicted. In this research, we have designed a Li-air battery that has a Li | organic liquid electrolyte | Li+-conducting glass ceramic plate (LiGC plate) |aqueous electrolyte | Pt air electrode structure and studied the impacts of the compositions of the aqueouselectrolyte on the battery performance. With lower concentrations of alkali aqueous electrolytes (≤0.05 MLiOH), a discharge voltage of approximately 3.5 V (at 0.05 mA cm−2) and a voltage efficiency up to 84%were observed. The addition of LiClO4 into the aqueous solution slightly lowered the discharge voltageto 3.3 V but dramatically decreased the internal resistance of the battery to 35.4 � cm−2. With a charge

−2

ir electrode voltage plateau observed at 3.90 V at a current of 0.05 mA cm , the Li | organic liquid electrolyte | LiGC| 1 M LiClO4 | Pt air battery showed an 85% voltage efficiency at room temperature. Adding LiClO4 intothe aqueous electrolytes resulted in an impedance reduction and slowed the pH increase of the alkaline-based electrolyte due to the fast or long-term discharge of the air electrode in the Li-air battery. Thedischarge and charge voltage behaviors of the battery and the changes to the pH values of the aqueous

rrent

electrolyte at different cu

. Introduction

The Li-ion rechargeable battery has been successfully used inmall portable electronic devices because it has many advantages,ncluding its high gravimetric energy density (120–150 Wh kg−1),elatively short charging time, and long cycle life. However, it is cru-ial to increase the energy density of the battery to further developortable electronic devices that can better meet today’s needs. Thebility to increase the energy density of the present Li-ion batter-es is limited by the use of Li intercalation solid compounds, which

ave been employed as both negative and positive electrodes, suchs LixC6 and Li1−xCoO2 [1]. With air (O2) as the positive electrodend Li metal as the negative electrode, a Li-air battery has beeneveloped. The specific energy density of the Li-air battery rangesetween 5789 and 11248 Wh kg−1, which is more than ten timesigher than that of Li-ion batteries [2]. Based on the nature of the

∗ Corresponding author at: Richard G. Lugar Center for Renewable Energy, Indiananiversity Purdue University Indianapolis, Indianapolis, United States.el.: +1 317 274 4280.∗∗ Corresponding author at: Department of Mechanical Engineering, Indiana Uni-ersity Purdue University Indianapolis, Indianapolis, United States.el.: +1 317 274 9711.

E-mail addresses: rochen@iupui.edu (R. Chen), yk35@iupui.edu (Y. Kim).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2012.02.001

rates were also recorded and are presented in this paper.© 2012 Elsevier Ltd. All rights reserved.

electrolyte and its reaction products, Li-air batteries can be dividedinto two groups:

(a) Li/O2 in non-aqueous electrolytes [2]

Li + O2 = Li2O2(peroxide) E = 3.10V (1)

4Li + O2 = 2Li2O E = 3.10V (2)

(b) and Li/O2 in aqueous electrolytes [2]

Basicelectrolyte : 4Li + O2 + 2H2O = 4LiOH E = 3.45V

(3)

Acidicelectrolyte : 4Li + O2 + 4H+ = 2H2O + 4Li+

E = 4.27V (4)

Seawater(pH8.2) : 4Li + O2 + 2H2O = 4LiOH E = 3.79V

(5)

In theory, Li-air batteries with non-aqueous electrolytes candeliver a specific energy density up to 11248 Wh kg−1 [3]. Thefirst non-aqueous electrolyte Li-air battery with a Li|organic liquidelectrolyte|air electrode structure was reported in 1996 [2]. With

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8 H. He et al. / Electroch

his battery, energy capacities of 1600 mAh g−1 and 1410 mAh g−1

ere achieved in air and pure oxygen atmospheres, respectively.hese energy capacities were also characterized by the weight ofhe carbon catalyst since a decrease in the discharge voltage wasften caused by the carbon electrode, which could be choked byhe deposition of the reaction product (Li2O2) in its pores. Interest-ngly, a better capacity, 2120 mAh g−1, was achieved by changinghe mass of the carbon catalyst [4]. Moreover, modification of the airatalytic electrode also improved the capacity up to 2825 mAh g−1

t 0.05 mA cm−2 [5]. The highest capacity for a Li-air battery in non-queous electrolytes was reported to be 5360 mAh g−1 (dischargedt 0.01 mA cm−2) by Kuboki et al. [6]. Nanometer-scale catalysts,ased on precious metals, have also been introduced into non-queous electrolyte Li-air batteries as catalysts for the air electrode,ncluding Au [7], Pd [8], Pt [9] and PtAu [7]. Higher discharge volt-ges were reported from the use of these nanometer-scale catalystsompared with metal oxide catalysts [7,8,10].

However, the use of a non-aqueous electrolyte Li-air battery as aechargeable battery has resulted in two critical problems. The firstroblem is that the non-aqueous electrolyte allows the moisture ofhe air to travel into the Li metal anode, contaminating the Li metalnd resulting in a poor cycle life for the battery. To minimize theffects of Li corrosion, the use of dry air and pure oxygen in placef atmospheric air was attempted [2,4–6,10–15], but this approachs not a cost-effective solution. The second problem is that the dis-harge products, Li2O2 and Li2O, are insoluble in the non-aqueousiquid electrolyte, and thus, the product particles gradually clog theorous air electrodes [2,4–6,10–15], which eventually yield poorycle lives.

To overcome the challenges faced by Li-air batteries with non-queous electrolytes, the use of aqueous electrolytes with a Li-airattery have been recently studied [7,16–19]. To protect the Linode from being exposed to the aqueous electrolytes, a Li+-ion-onducting glass ceramic (LiGC) plate was used to separate thenode, which contains the Li metal in a non-aqueous electrolytend the cathode, which, in turn, is composed of an oxygen reduc-ion catalyst in an aqueous solution. The theoretical specific energyensities of the Li-air batteries in the aqueous solutions are approx-

mately 5789 Wh kg−1 based on the weight of the battery, where theeight of oxygen (O2) is excluded because it is freely available from

he atmosphere and, therefore, does not need to be stored in theattery or in the cell. Compared with a non-aqueous Li-air battery,etter voltage efficiency and a longer cycle life should be possible

n an aqueous Li-air battery because the discharged product (LiOH)an be dissolved in the aqueous electrolyte solutions, which wouldield good reversible reactions. However, the solubility of LiOH inater is also a limitation for aqueous Li-air batteries to reach high

apacities [18].Recently, aqueous Li-air batteries that use a LiGC solid elec-

rolyte have been reported including Li | PEO18LiTFSI | LiGC | 1 M LiCl Pt [13], Li–Al | Li3−xPO4−yNy | LiGC | 1 M LiCl | Pt [17], Li | 1 M LiClO4n EC:DMC | LiGC |1 M KOH| Mn3O4 [18] and Li | 1 M LiPF6 in EC:DMC

LiGC |1 M LiOH| CoMn2O4 graphene composites [20]. Wang et al.19] also proposed a new Li–air fuel cell using metallic copper ashe catalyst for the O2 electrochemical reduction. The achievedesults from testing of this Li–air fuel cell demonstrate that theycle between Cu and Cu2O can be used to catalyze the O2 elec-rochemical reduction based on the copper corrosion mechanism.owever, further study is needed to verify this system. Interest-

ngly, a large capacity of 50,000 mAh g−1 (based on total mass ofatalytic electrode) was obtained after discharging Li | 1 M LiClO4n EC:DMC | LiGC |1 M KOH| Mn3O4 for 500 h [18], but the chargeerformances, including the voltage efficiencies, were not accept-

ble for use as rechargeable batteries. The Li | PEO18 LiTFSI | LiGC

1 M LiCl | Pt air cell [16] showed a favorable rechargeable perfor-ance at the increased temperature of 60 ◦C, but it was reported to

cta 67 (2012) 87– 94

discharge and charge for only one hour. As for the LiGC solid elec-trolyte, it was found to not be stable in the aqueous Li-air batteriesfor extended use when strong alkaline solutions were used [16–18].

To the best of our knowledge, the LiGC electrolyte is the onlyceramic solid electrolyte that is currently commercially availablefor use in aqueous Li-air batteries. While there are worldwideefforts to develop Li-ion conducting ceramics that are stable instrong alkaline or acid solutions, an alternative approach is to opti-mize the aqueous electrolyte and to alleviate the high pH by tuningthe chemical compositions of the aqueous solutions.

In this study, weak alkaline aqueous solutions (≤0.05 M) werefirst used as electrolytes for a Li-air battery of the following struc-ture: Li | organic liquid electrolyte | LiGC | aqueous electrolyte | Ptcatalytic electrode. Platinum was selected as the electrocatalyst forthe air electrode in this study because Pt has been considered tobe the bench-mark active and stable catalyst for the oxygen reduc-tion reaction via a direct 4-electron pathway [21]. The use of 1 MLiClO4 was also attempted to reduce the impedance and alleviatethe problem of the chemical instability of the LiGC plate in strongalkaline solutions. The effects of the compositions of the aqueouselectrolytes on the electrochemical performance were studied indetail.

2. Experimental design

2.1. Preparation of the anode and electrolytes

A Li ribbon (99.9%) with a 0.38-mm thickness was pur-chased from Sigma Aldrich, and disks with 0.8 cm diameterswere cut for use as the anode. An organic non-aqueous liq-uid electrolyte, 1 M LiPF6 in ethylene carbonate (EC):dimethylcarbonate (DMC) (1:1 volume ratio), was purchased fromNovolyte Corp. The Li-ion-conducting glass ceramic (LiGC) plate,Li1.3Ti1.7Al0.3(PO4)3, measuring 1 in. × 1 in. with a 150-�m thick-ness and a �Li ≈ 10−4 S/cm, was purchased from OHARA Inc.

2.2. Preparation of the air catalytic electrode

The carbon-supported electrocatalyst, Pt/C (50 wt.% metal oncarbon), was purchased from Alfa Aesar and was used as received.The air catalytic electrode included a catalyst layer and a gas diffu-sion layer. Teflon treated carbon paper (Fuel Cell Store, Inc., 200 �mthickness) was used as the gas diffusion layer. Pt/C ink solutionswere prepared by mixing Pt/C (80 wt.%), ionomer (Tokuyama A4,20 wt.%) as a binder, and tetrahydrofuran (Acros Organics) as a solu-tion in an ultrasonic bath for 1 h. The ink solution was sprayed ontoone side of the Teflon-treated carbon paper. The finished air cat-alytic electrode was soaked in 1 M KOH overnight to activate theionomer [22], and then it was soaked in DI water to remove anyresidual KOH from the surface of the air catalytic electrode. The areaof the air electrode was 4 cm2, and the mass loading of the catalystlayer was 1 mg cm−2 with a thickness of approximately 10 �m.

2.3. Assembly of the Li-air battery

Fig. 1 shows a schematic diagram of the laboratory-size aqueousLi-air battery that was tested. The LiGC solid electrolyte plate wasfirst placed on the top of the anode and sealed by epoxy. Next, thesealed anode was placed in an argon filled box where the waterand oxygen concentrations were kept to less than 4 ppm. The Limetal disk and the organic liquid electrolyte were loaded into theanode inside the box. After assembling the anode, the assemblage

was moved out of the box. The aqueous electrolyte was poured ontop of the LiGC plate, and then the air catalytic and diffusion layerswere placed on the aqueous electrolyte.

H. He et al. / Electrochimica Acta 67 (2012) 87– 94 89

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.4. Testing of the Li-air battery

The assembled Li-air battery was exposed to atmospheric air andonnected to the testing station. A Solartron 1470 cell tester wassed to perform the charge and discharge tests. Electrochemical

mpedance spectroscopy (EIS) experiments were performed usingn open circuit voltage with a Solartron 1260 workstation. The ACerturbation signal was ±5 mV, and the frequency range was from

mHz to 105 Hz.

. Results and discussion

Fig. 1 shows the structure of our Li-air battery that mainly con-

ists of an anode, a solid electrolyte, and a cathode. The anodeonsists of the Li metal anode and a non-aqueous electrolyte, 1 MiPF6 in EC:DMC. The cathode is composed of an aqueous elec-rolyte and an air catalytic electrode. A LiGC solid electrolyte is used

ry-sized aqueous Li-air battery.

to separate the anode and cathode by preventing the two liquidelectrolytes from mixing and provides continuous Li-ion mobilitybetween the anode and cathode sides during the discharge/chargeprocess. The Pt-based air catalytic electrode that includes the cat-alytic and gas diffusion layers was used for this system and placedbetween the aqueous electrolyte solution and the atmospheric airto provide a continuous liquid–solid–gas three-phase interface.

In recent literature, a similar structure to the Li-air battery sys-tem was reported [18], but a strong alkaline solution, 1 M KOH, wasused as the electrolyte. In contrast, our work tested weak alkalinesolutions (≤0.05 M LiOH) as electrolytes for the aqueous Li-air bat-tery; pure DI water (0.00 M LiOH) was also tested. Because therewere no Li-ions in pure DI water at the initial state of the bat-tery (before discharging), a high resistance was observed. However,

the discharge process caused the water to turn into a weak alka-line solution by producing LiOH in the water, which eventuallydecreased the internal resistance of the cell. This subject is dis-cussed in more detail in Section 3.2. Overall, the use of a weak

90 H. He et al. / Electrochimica Acta 67 (2012) 87– 94

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lkaline electrolyte minimizes the effects of the chemical instabil-ty of the LiGC plate on the discharge/charge voltage behaviors ofhe battery.

Fig. 2 shows the schematic diagram of the operating principlesf the aqueous Li-air battery. Here, the electrode reactions withinhe aqueous Li-air battery can be summarized as follows:

athode : O2 + 2H2O + 4e− → 4OH− (6)

node : Li → Li+ + e− (7)

holereaction : 4Li + O2 + 2H2O → 4Li+ + 4OH− (8)

During the discharge of the battery, O2 from the air should dif-use into the porous catalytic electrode, where an electrocatalyticxygen reduction reaction occurs. Eq. (6) shows the oxygen reduc-ion reaction. Simultaneously, Li metal changes into Li+ ions thatiffuse into the aqueous solution through the organic liquid elec-rolyte and LiGC plate. In return, the charge of the battery allowshe Li+ ions in the aqueous solution to turn back to Li metal byccepting electrons from the external circuit. Furthermore, the OH−

ons become O2 and H2O while extruding electrons to the currentollector.

.1. Open circuit voltage in weak alkaline aqueous solutions

The Li-air battery, which has the structure of a Li | organic liquidlectrolyte | LiGC | weak alkaline solutions (≤0.05 M LiOH) | Pt airlectrode, shows that the open circuit voltages (OCV) for the elec-rolytes 0.00 M LiOH (water), 0.01 M LiOH, and 0.05 M LiOH were.75 V, 3.61 V, and 3.57 V, respectively. The higher OCV correlatingith the lower concentration of OH− can be explained by applying

he Nernst equation:

= E0 − RT

zFln

aRed

aOx(9)

ere, E is the reduction potential at the temperature of inter-st, E0 is the standard reduction potential, R is the universal gas

g principles of aqueous Li-air batteries.

constant (8.314472 J K−1 mol−1), T is the absolute temperature, zis the number of moles of electrons transferred in the reaction,F is the Faraday constant (96485.33 C mol−1), and aRed and aOxrepresent the chemical activities for the reductant and oxidant,respectively. To calculate aRed and aOx, the following equation wasused: aX = �XcX, where cX and �X are the concentration and activitycoefficient of species X, respectively. However, because the activ-ity coefficients tend to unify at low concentrations, the activitiesin the Nernst equation are frequently replaced by simple concen-trations. In this equation, the use of a stronger alkaline solutionmade the concentration of the reductant (OH−) increase, causingthe chemical activity of the reductant (aRed) to become higher andthe oxygen reduction reaction potential (E) shift to become morenegative, which resulted in a decrease in the open circuit voltage.The theoretical difference between the reduction voltage in pure DIwater and that in 0.01 M LiOH and 0.05 M LiOH can be calculated viathe Nernst equation and are 0.15 V and 0.17 V, respectively; both ofthese values are comparable to the 0.14 V and 0.18 V measured inthis experiment.

3.2. Discharge voltages in weak alkaline aqueous solutions

As shown in Fig. 3(a), the discharge voltage plateaus (vs. Li+/Li)for the batteries containing 0.00 M, 0.01 M, and 0.05 M LiOH wereobserved to be 3.53 V, 3.40 V, and 3.31 V, respectively, at the currentrate of 0.05 mA cm−2 or 100 mA g−1

carbon. The battery containingpure DI water showed a large voltage drop in the first 1 h of dis-charge compared to the discharge voltage curves in the 0.01 M or0.05 M LiOH electrolytes. This voltage drop is due to the low ionicconductivity of pure DI water, which resulted in a high resistanceinside the battery. However, discharging the battery also led to the

formation of LiOH in water, which eventually decreased the internalresistance of the battery and allowed for a voltage plateau of 3.53 Vto be reached after 2 h of discharging the battery. This result is con-sistent with the measurement of the impedance spectra, where the

H. He et al. / Electrochimica Acta 67 (2012) 87– 94 91

Fig. 3. Discharge and charge performances of the developed Li-air battery at a0.05 mA cm−2 (100 mA g−1

carbon) current density: (a) discharge curves in pure DIwater and 0.01 M LiOH and 0.05 M LiOH aqueous solutions in the initial state at0D0

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.05 mA cm−2 and (b) impedance spectra of the developed Li-air battery with pureI water as the aqueous electrolyte in the initial state before and after discharge at.05 mA cm−2 for 10 h.

arge battery impedance with the water electrolyte decreased afterischarging the cell, as shown in Fig. 3(b). An initial voltage dropas also observed for the Li-air battery containing the low alkaline

olution electrolyte, but the voltage drop was less significant com-ared with the voltage drop of the battery containing the DI waterlectrolyte.

Interestingly, a higher discharge voltage was observed for theattery containing lower concentrations of LiOH in the aqueouslectrolyte. This higher discharge voltage was mainly due to thearious degrees of solubility of the oxygen in the alkaline aqueousolution – a lower oxygen solubility occurs in higher concentra-ions of alkaline solutions [23]. Before further explaining the abovehenomenon, a challenging issue in designing the cathode for thequeous Li-air battery must be mentioned. The current designsf the aqueous Li-air battery systems, including ours and othereported designs [18], note the potential problem of water floodingn the air electrode, which is also a critical problem with alkalineuel cells [24]. The catalyst layer of the air catalytic electrode ismmersed in the aqueous electrolyte, and although the catalystayer is hydrophobic, due to the capillary phenomenon, the aqueouslectrolyte spontaneously travels into the pores of the catalyst layer

see Fig. 2), flooding the air electrode with water. Therefore, it iselieved that the oxygen required for the electrocatalytic reductioneaction is from the air that dissolves into the aqueous electrolytend not directly from the air in the atmosphere.

Fig. 4. Electrochemical performance for a sealed Li-air battery: (a) design ofthe sealed Li-air battery; (b) discharge curve for the sealed Li-air battery at0.05 mA cm−2; and (c) details for the sealed Li-air battery.

To prove this hypothesis, we designed another battery (seeFig. 4(a)), where the cathode was completely sealed from atmo-spheric air so that no oxygen would be supplied to the aqueouselectrolyte during discharge. Fig. 4(b) shows the discharge curve forthe battery at 0.05 mA cm−2 when no atmospheric air was supplied.Two discharge voltage plateaus were found. One was approxi-mately 3.50 V, and the other one was 2.58 V. The higher dischargevoltage plateau is similar to what was observed in the Li-air battery,and the lower discharge voltage plateau (∼2.58 V) corresponded tothe discharging of pure DI water. The theoretical discharge voltageof the pure DI water is reported to be 2.22 V [25]. In our experiment,the air cannot be removed completely from water, so the dischargevoltage is slightly higher than the theoretical discharge voltage.

Li+ + H2O + e− → LiOH + ½H2 E = 2.22V (10)

Therefore, this experiment shows that the electrocatalyticreduction reaction can occur inside aqueous solutions by usingsoluble oxygen. When the catalyst layer is soaked in the aqueous

electrolyte, even if the catalyst layer is hydrophobic, the solubleoxygen in the aqueous electrolyte is the main source of oxygenfor the electrocatalytic oxygen reduction reaction, instead of theoxygen from atmospheric air. Therefore, the solubility of oxygen in

9 imica Acta 67 (2012) 87– 94

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2 H. He et al. / Electroch

he aqueous electrolyte becomes an important factor that affectshe electrochemical performance of the aqueous Li-air battery. Its reported in the literature [23] that the solubility of oxygen inqueous solutions decreases with higher concentrations of alka-ine and, as a result, fewer oxygen reduction reactions (ORR) willccur in such solutions. It is also reported that pure DI water hashe highest oxygen solubility, ∼1 × 10−3 mol cm−3 [24] comparedith LiOH solutions, which explains why the discharge voltageecreased with increasing concentrations of LiOH in the solution,s shown in Fig. 3(a).

The discharge voltages for our Li-air battery in different aque-us electrolyte solutions were 3.53 V for pure DI water, 3.40 V forhe 0.01 M LiOH solution, and 3.31 V for the 0.05 M LiOH solutiont 0.05 mA cm−2. The number of moles of LiOH in each solutionlightly increased during discharge. In fact, the increased con-entration of LiOH in the solution after discharging for 10 h wasstimated to be approximately 9 × 10−6 mol. One can clearly seehat the discharge voltage decreased with the increasing concen-rations of LiOH. However, the discharge voltages observed in theow alkaline electrolyte Li-air battery were higher than any othereported Li-air battery, as listed in Table 1.

.3. Voltage efficiency in weak alkaline aqueous solutions

Li-air batteries with low alkaline aqueous electrolytes areharged after discharging for 10 h. For this experiment, the bat-ery was discharged for only 10 h because the pH of the aqueouslectrolyte was measured and increased to 7.16, 7.72, and 9.25 atischarge times of 10 h, 20 h, and 40 h, respectively. Because theolid LiGC electrolyte has been reported to be chemically unstablen strong alkaline solutions [17], the 10-h discharge was chosen to

inimize the effects of the solid electrolyte on the voltage behav-ors of the aqueous Li-air battery being investigated.

Fig. 5 shows the charge voltage behaviors of the batteries with.00 M (water) and 0.01 M LiOH, respectively, after dischargingor 10 h. The charge voltage of the battery with water sharplyncreased at the end of the charging time because the amount ofiOH formed during the discharge decreased as the charging timencreased, which led to a higher internal resistance. On the otherand, the charge voltage of the 0.01 M LiOH electrolyte Li-air bat-ery remained constant at 4.09 V, even at the end of the charge,hich was lower than that of the pure DI water Li-air battery

4.20 V). Therefore, it is obvious that the internal resistance wasecreased by the addition of LiOH into the water.

Surprisingly, with the use of a low concentration alkaline solu-ion as an electrolyte, we were able to obtain an 84% voltagefficiency (the ratio of charge voltage to discharge voltage) at.05 mA cm−2 in the Li-air battery system having the structure ofi | organic liquid electrolyte | LiGC | water | Pt catalytic electrode.his higher voltage efficiency of the developed Li-air battery is dueo the Li-air battery having comparable charge voltages but higherischarge voltages than those that have been observed by others.

Although an 84% high voltage efficiency can be obtained withhe use of a low concentration alkaline solution as an electrolyte,he pH level measurements indicate that the aqueous electrolyteecame a strong basic solution through the production of moreH− at a higher current rate, as shown in Fig. 5(b). Furthermore,hen a current density higher than 2.00 mA cm−2 was applied, theure DI water became a strong basic solution with a pH higher than2, even with 1 h of discharging the battery.

Recently, Wang et al. reported a similar structure for an aqueousi-air battery, Li | 1 M LiPF6 in EC:DMC | LiGC |1 M LiOH| CoMn2O4-

raphene composite [20] that showed a high voltage efficiency ofpproximately 90% when it was only discharged and charged for00 s at 0.025 mA cm−2. However, their long discharge time dra-atically decreased the recharge efficiency of the cell and cycle life.

0.05 mA cm (100 mA g carbon) current density: (a) charge and discharge curvesat 0.05 mA cm−2 (100 mA g−1

carbon) in pure DI water and 0.01 M LiOH in the initialstate and (b) changes in the pH as a function of the current densities.

This result also indicates that a fast or long discharge of the aque-ous Li-air battery could raise the pH of the aqueous solution, whichcan damage the LiGC solid electrolyte. Therefore, a new approachwas proposed by modifying the composition of the aqueous elec-trolyte with the addition of LiClO4, which was designed to reducethe impedance and slow the pH increase of the aqueous electrolyteduring long-term use or during fast discharging of a Li-air battery.

3.4. The voltage and pH effects of LiClO4 in the aqueous electrolyte

In this work, LiClO4 was used to alleviate the pH increase ofthe alkaline aqueous electrolyte. Additionally, its influence on theelectrochemical performance was also studied. Fig. 6 shows thedischarge and charge voltage behaviors of the battery with 1.00 MLiClO4 in an aqueous solution, which was compared with thosemeasured in the low alkaline aqueous solutions and pure DI water.It was reported [26] that oxygen solubility decreases when LiClO4is added into pure DI water. Therefore, the observed decrease inoxygen solubility may correspond to the lower discharge voltageobserved in the 1 M LiClO4 aqueous electrolyte compared with thatof the pure DI water electrolyte.

The addition of 1 M LiClO4 into the aqueous electrolyte alsodecreased the internal resistance of the battery from 3082 �(1541 � cm−2) in pure DI water to 70.8 � (35.4 � cm−2) in the1.00 M LiClO4 solution (Fig. 6(b)). As a result, the 2.41 V discharge

H. He et al. / Electrochimica Acta 67 (2012) 87– 94 93

Table 1Summary of the discharge voltages depending on the catalysts and electrolytes.

Catalysts for air electrode Electrolyte Current density fordischarge (mA cm−2)

Discharge voltage (V) Ref

Pt/C Pure DI water 0.05 3.53Pt/C Pure DI water 0.50 3.27Pt/C 0.01 M LiOH 0.05 3.40Pt/C 0.05 M LiOH 0.05 3.31Pt/C 1 M LiCl 0.03 3.20 [17]CoMn2O4-graphene composite 1 M LiOH 0.025 3.20 [20]CoMn2O4-graphene composite 1 M LiOH 0.40 2.95 [20]Mn3O4/C 1 M KOH 0.50 2.80 [18]C 1 M LiBETI DOL:DME (1:1) 0.50 2.65 [5]C 1 M LiImide DOL:DME(1:1) 0.50 2.65 [5]C 1 M LiTriflate DOL:DME (1:1) 0.50 2.62 [5]C 1 M LiBr DOL:DME (1:1) 0.50 2.60 [5]C 1 M LiBr DOL:DME (1:1) 0.50 2.60 [5]MnO2 nanotube 1 M LiPF6 in PC 0.05 2.80 [10]

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2.00 mA cm−2. Comparatively, the pH of the 1.00 M LiClO4 solutionwas less than 11 and remained stable at the same current rates.

Co3O4 1 M LiPF6 in PC

Carbon 1 M LiClO4 in PC:DME (1:2, v/v)

Pt/Au 1 M LiClO4 in PC:DME (1:2, v/v)

oltage was still able to be obtained even at the high current densityf 10 mA cm−2, as shown in Fig. 7(a). Moreover, the use of LiClO4n the aqueous electrolyte was found to slow the pH increase at

igher current rates. Fig. 7(b) shows the changes in the pH as a

unction of the current density for the DI water and the 1 M LiClO4queous solution. The pure DI water became a strong basic solution

ig. 6. Discharge and charge performances of the developed Li-air battery at a.05 mA cm−2 (100 mA g−1

carbon) current density: (a) charge and discharge curvest 0.05 mA cm−2 (100 mA g−1

carbon) in 0.01 M LiOH and 1.00 M LiClO4 in the initialtate and (b) impedance spectra of the developed Li-air battery with pure DI water,.01 M LiOH and 1.00 M LiClO4 as the aqueous electrolytes in the initial state.

0.035 2.60 [13]0.04 2.50 [7]0.04 2.70 [7]

with pH 12 after 1 h of discharging at a current density higher than

This result indicates that the addition of LiClO4 alleviates the pH

Fig. 7. Discharge performances at different current densities for the developed Li-air battery with 1.00 M LiClO4 as the aqueous electrolyte in the initial state: (a)discharge curves at different current densities as a function of time and (b) changesin the pH as a function of current densities.

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[23] C. Zhang, F.-R.F. Fan, A.J. Bard, Journal of the American Chemical Society 131

4 H. He et al. / Electroch

ncrease formed during fast or long discharging processes. Further-ore, it is believed that the presence of LiClO4 in alkaline solutions

educes OH− activity by decreasing the dissociation degree of LiOH.According to Debye–Hückel Equation below, when LiClO4 and

iOH are present in the same solution, the ionic strength (I) isarger than that of the LiOH solution, which eventually decreaseshe activity coefficients (�OH− ) of OH− ions:

lg �i = Az2i

√I

1 + Ba√

I(I = 1

2

n∑

i=1

ciz2i ) (11)

here � i is the activity coefficient of the ion species i, I is the ionictrength of the solution, zi is the charge number of the ion species, a is the size or effective diameter of the ion in angstroms, A and

are constants with values of 0.5085 and 0.3281, respectively, at5 ◦C in water, and ci is the molar concentration of ion i (mol dm−3).

t is noted that the ionic strength of a solution is a function of theoncentration of all of the ions present in the solution.

. Conclusion

The performance of a well-designed Li-air battery with a Li |rganic liquid electrolyte | LIGC | aqueous electrolytes | Pt catalyticlectrode structure was studied. It was found that the dischargeoltage increased with lower concentrations of LiOH in the aqueouslectrolyte due to the higher oxygen solubility in lower alkalineoncentrations. Hence, by using weak (≤0.05 M LiOH) instead oftrong alkaline solutions, the Li-air battery displayed discharge andharge voltages of 3.53 V and 4.19 V, respectively, at 0.05 mA cm−2,esulting in an 84% voltage efficiency. The addition of LiClO4 intohe aqueous solution further improved the voltage efficiency to 85%ith 3.32 V at the discharge and 3.90 V at the charge by reducing

he internal resistance of the cell. The use of LiClO4 also alleviated

he pH increase caused by a fast or long-term discharge of the cell.

This experiment shows that the modification of the aqueouslectrolyte could be the cause of the high voltage efficiency of thequeous Li-air battery. Therefore, this approach could be useful for

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cta 67 (2012) 87– 94

the control of the pH of the aqueous electrolyte. However, the dif-ferent types of chemistry in the aqueous electrolytes will need tobe further explored in the Li-air battery system.

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