20262 Phys. Chem. Chem. Phys., 2013, 15, 20262--20271 This journal is c the Owner Societies 2013

Cite this: Phys. Chem.Chem.Phys.,2013,15, 20262

Electrochemical properties of graphene flakes as an aircathode material for Li–O2 batteries in an ether-basedelectrolyte†

Se Young Kim,a Ho-Taek Leeb and Kwang-Bum Kim*a

We employed graphene flakes as an air-cathode material for Li–O2 batteries and investigated their

electrochemical properties in the dimethyl ether electrolyte. Graphene flakes were prepared by microwave-

assisted reduction of graphene oxide, and their electrochemical properties were compared with those

of Ketjen Black and carbon nanotubes. The catalytic effect of the prepared graphene flake-air cathode

was demonstrated using cyclic voltammetry and discharge–charge testing performed under a limited

discharge capacity. The catalytic effect of graphene flakes was also supported by morphological and

spectroscopic analysis of the discharge–charge products formed on the graphene surface. Scanning

electron microscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy revealed that Li2O2,

Li2O, and Li2CO3 were the main discharge products on all carbon-air cathode surfaces. Raman spectro-

scopy revealed that LiRCO3 was additionally formed on Ketjen Black and carbon nanotubes during the

first discharge; however, its formation was not observed on the graphene flakes. The catalytic effect

of the graphene flakes and the absence of LiRCO3 in the discharge product could explain the higher

Coulombic efficiency in the discharge–charge tests.

1. Introduction

Rechargeable Li–O2 batteries are emerging as a breakthroughtechnology for use as next-generation energy storage systemsfor electrical vehicles (EVs) owing to their high specific capacity.1–3

During the discharge of a Li–O2 battery, O2 is reduced to form asolid discharge product such as Li2O or Li2O2 on the surface of thecarbon-air cathode. This material eventually covers the reactionsites and clogs the pores of the carbon structure within the aircathode. Different forms of carbon such as Vulcan XC-72, KetjenBlack (KB), and Super P have been investigated as air cathodematerials. The discharge capacity of a Li–O2 battery depends onits physical properties such as specific surface area and porosityof the carbon material in the air cathode,4–9 and is in the rangeof around 2000–3000 mA h gcarbon

�1.10–12 In the subsequentcharging process, the discharge products are electrochemicallydecomposed to Li+ ions and O2;13 however, they are reported to be

only partially decomposed, mainly due to the low electronicconductivity (B10�4 S m�1) of the solid products14,15 and aweak catalytic effect of carbon toward oxygen evolution reac-tions (OER).11

In order to improve the Coulombic efficiency and over-potential of the carbon-air cathode during charging, carbonnanocomposites with various metal and metal-oxide catalystssuch as a-MnO2,8,16 MnCo2O4,17 and Pt–Au nanoparticles11,18

have been investigated. For example, a mesoporous NiCo2O4

nanoflake catalyst was reported to lower discharge–chargeoverpotentials and provide enhanced cycling stability comparedto that of a pure carbon material (Super P) in 1 M Li bis(trifluoro-methylsulfonyl)imide (LiTFSI) in tetraethylene glycol dimethylether (TEGDME) solution.19 Graphene flakes (GF) consist of afew layers of graphene in which the carbon atoms are tightlypacked into 2-dimensional (2D) honeycomb sp2 carbon latticesthat possess structural defects as well as a high concentration ofedge sites.20–22 Graphene has recently been investigated for useas an air-cathode material in Li–O2 batteries.23–26 Sun et al.investigated GF as an air-cathode material for non-aqueous Li–O2

batteries for the first time, and reported a discharge capacity ashigh as 8705 mA h g�1.27 Zhang et al. reported a discharge capacityof 15 000 mA h g�1 in an ether-based electrolyte from an aircathode composed of hierarchically porous graphene balls.This large discharge capacity was attributed to defects andfunctional groups within the graphene structure, which favoured

a Department of Material Science and Engineering, Yenisei University,

134 Shin chon-Dong, Seodaemun-Gu, Seoul 120-749, Republic of Korea.

E-mail: kbkim@yonsei.ac.kr; Fax: +82-2-312-5375; Tel: +82-2-365-7745b Hyundai Motor Company, 37, Cheoldobangmulgwan-ro, Uiwang-si, Gyeonggi-do,

437-815, Republic of Korea

† Electronic supplementary information (ESI) available: Potential profiles ofcarbon air cathodes for the first deep discharge–charge and FT-IR spectra ofthe carbon air cathodes after the first discharge–charge. See DOI: 10.1039/c3cp53534g

Received 20th August 2013,Accepted 14th October 2013

DOI: 10.1039/c3cp53534g

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the formation of isolated nanosized Li2O2 particles, and helpedprevent air blocking in the air electrode.6 Yoo and Zhou.evaluated GF for use as an air-cathode material in Li–O2

batteries with an aqueous–non-aqueous hybrid electrolyte,demonstrating the electrocatalytic activity of GF toward theoxygen reduction reaction (ORR) in (1 M LiNO3 + 0.5 M LiOH)aqueous solution.28 Wang et al. also reported the catalytic effectof GF, observing a smaller difference between the charge anddischarge potentials of GF (1.22 V) compared to a Vulcan XC-72carbon electrode (1.69 V) in a carbonate-based electrolyte.29

In this study, we prepared GF via solid-state microwaveirradiation treatment of graphite oxide (GO) under an Ar atmo-sphere30 and investigated its catalytic effect toward ORR andOER in a non-aqueous dimethyl ether (DME) electrolytesolution using cyclic voltammetry (CV) and discharge–chargetests. The discharge capacity for the discharge–charge tests wasintentionally limited to 1280 mA h gcarbon

�1 within thepotential window from 2.5 to 4.2 V (vs. Li/Li+), as excessaccumulation of the insulative discharge products mightobscure the catalytic effect of GF due to the limited transport ofLi+ ions, O2, and electrons in the discharge products. In addition tothe electrochemical characterization of GF, morphological andspectroscopic analysis of the discharge–charge products on GFwere performed to further investigate their formation and removalduring discharge–charge cycles.

2. Experimental2.1 Preparation of carbon-air cathodes

GO was synthesized from purified graphite powder using amodified Hummers method. GF was then synthesized from theas-prepared GO using a method reported in a previous study.30

The GF-air cathode was prepared as follows: GF was mixed witha polyvinylidene fluoride (PVDF, MW = 534 000 g mol�1, Sigma-Aldrich) binder at a weight ratio of 85 : 15 in N-methyl-2-pyrrolidone (NMP, Sigma-Aldrich, anhydrous, 99.5%), forminga slurry without using any additional conductive agent. Theslurry was subsequently cast onto Teflon-coated carbon paper(TGP-H-060 Carbon Paper, Toray) to act as a gas-diffusion layer,with a carbon-loading density of 2.0 mgcarbon cm�2. The slurry-coated carbon-paper air cathodes were then dried at 100 1C inan oven for 24 h.

2.2 Electrochemical measurements

For electrochemical characterization of the GF-air cathodematerial, coin cells (2032-type, Wellcos) were assembled in aglove box filled with purified Ar (99.999%). The coin cells wereassembled in the following order: stainless-steel casing bottom,gasket, stainless-steel spring, stainless-steel disk spacer(0.5 mm thickness), Li-metal anode (16 mm diameter), separator(18 mm diameter), air cathode (14 mm diameter), stainless-steelcap with a hole. The cap of the coin cells had one hole with adiameter of 1 mm. The electrolyte was 1 M Li bis(trifluoromethyl-sulfonyl)imide (LiTFSI, Sigma-Aldrich, 99.95%) in dimethyl ether(DME, Sigma-Aldrich, Z99%). A glass-microfiber filter paper(Whatman GF/D, 2.7 mm pore size) was used as the separator,

and Li foil (Iljin Materials, 0.14 mm thickness) was used as thecounter electrode. Finally, the assembled Li–O2 coin cell wasremoved from the Ar-filled glove box and placed in a separate glovebox filled with O2 (99.995%) at a pressure of 1 atm, where all of thesubsequent tests were carried out. For comparison purposes, airelectrodes using KB (Ilshin Chemtech) and carbon nanotube (CNT,Hanwha Chemical) materials were prepared and assembled in thesame manner. The assembled coin cells were fixed into a 2032-coincell holder (HITECH) in an O2-filled glove box for electrochemicalmeasurements. CV tests were performed with the coin cell using aBiologic potentiostat/galvanostat Model VMP2 (BioLab, Inc.) withina potential window of 2.0–4.5 V (vs. Li/Li+) at a potential scan rate of0.1 mV s�1 at room temperature. Discharge and charge testswere performed at a constant current density of 0.4 mA cm�2

(330 mA gcarbon�1) from 2.5 V to 4.2 V to avoid electrolyte

decomposition, while the discharge capacity of the air cathodewas set to 1280 mA h gcarbon

�1 for investigation of the catalyticeffects of GF, KB, and CNT on air-cathode performance.

2.3 Characterization of carbon materials and dischargeproducts

The physical properties of GF were characterized using a varietyof analytical techniques. Scanning electron microscopy (SEM,S-4300SE, Hitachi) was used to analyse structural and micro-scopic features of the carbon materials. X-ray diffraction (XRD,D8-Advance, Bruker) patterns were obtained on a diffracto-meter using a voltage of 40 kV and a current of 200 mA at ascan rate of 21 min�1; the degree of graphitization of the carbonmaterials was subsequently calculated from the value of the(002) reflection.31 Raman spectroscopy was carried out using aRaman microscope system (LabRam HR, Horiba Jobin Yvon)equipped with an Ar ion laser of wavelength 488 nm with 0.5 mWof laser power on samples, a 60 s exposure, and 600 gr per mmgratings. Peak intensities of the D and G bands (1360 cm�1 and1580 cm�1, respectively) after subtracting the background spectrumwere acquired at various points on the carbon powders, and thenthe average relative intensity (ID/IG) was calculated. N2 adsorption–desorption isotherms of the carbon materials were measured usinga fully automatic physisorption analyzer (ASAP ZOZO, MicrometricsCo.). The Brunauer–Emmett–Teller (BET) specific surface area wascalculated from the adsorption data in the relative pressure (P/P0)range of 0.05–0.2; total pore volume was determined from theamount of N2 adsorbed at P/P0 = 0.98. Pore-size distribution wascalculated based on the desorption branch of the isotherm usingthe Barrett–Joyner–Halenda (BJH) method.

To characterize the discharge products, the dischargedLi–O2 coin cells were disassembled in the Ar-filled glove box, andthe air cathodes were washed repeatedly with fresh anhydrousdimethyl carbonate.32 After washing, the cathodes were driedovernight under vacuum at room temperature. Discharge-productmorphology was observed using SEM; care was taken to protect thesamples from exposure to air during transfer to the SEM chamber,with a paraffin tape-sealed box prepared in the glove box used forthe transport process. XRD analysis was performed on thedischarged–charged carbon-air cathodes in an Ar-filled airtightholder with a 2y range of 30–601 at a scan rate of 21 min�1.

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This range was chosen, as the main XRD peaks from thedischarge products were known to occur within this range, withthose from the carbon paper and electrolyte appearing outside ofthe range. Fourier-transform infrared (FTIR, Vertex 70, Bruker)spectra of samples were obtained by forming KBr pellets with thedischarged–charged carbon materials from the air cathodes andtaking measurements in the frequency range of 400–4000 cm�1

under an Ar atmosphere. Raman spectra of the electrodes weremeasured using a Raman microscope system (LabRam HR,Horiba Jobin Yvon) in the range of 100–2000 cm�1, with an Arion laser of wavelength 488 nm.

3. Results and discussion3.1 Physical properties of GF, KB, and CNT materials

Fig. 1 shows SEM images of the three different carbon materialsused for preparing the air cathode. In this study, GF wasprepared using a solid-state microwave-irradiation techniqueunder an Ar atmosphere. As shown in the inset of Fig. 1(a), theGF exhibited a wrinkled or worm-like morphology, which ischaracteristic of the thermally exfoliated reduced GO.33–35 Thegraphene sheet edges were linked as a continuous sheet net-work, leading to the formation of a highly porous GF structure.As shown in Fig. 1(b), KB was composed of spherical carbonparticles approximately 20–50 nm in diameter, and displayed arough surface. The CNT material, as shown in Fig. 1(c), wasobserved to have an outer diameter ranging from 5 to 10 nmand a length of hundreds of nanometers within an entanglednetwork.

Fig. 2 shows the physical properties of the three carbonmaterials, as analyzed using XRD, Raman spectroscopy, andN2 gas adsorption–desorption measurements. The degrees ofgraphitization were calculated using Bragg’s law from the valueof the (002) reflection in the XRD spectra (Table 1). GF gave thehighest d002 value of 3.78 Å, with a broad XRD peak visiblearound 2y = 231 (Fig. 2(a)), indicating that the graphene sheetsin GF were relatively well exfoliated.36 KB was considered to beless graphitized than the CNTs, as it provided a higher d002

value of 3.68 Å.37,38 The CNTs were found to be the mostgraphitized carbon material among those investigated in thisstudy, with a d002 value of 3.41 Å, which is quite similar to thetheoretical value for graphite (3.35 Å).39

Fig. 2(b) shows atomic bonding characteristics of the carbonmaterials, as analyzed by Raman spectroscopy. The spectracontained two prominent peaks at 1360 cm�1 and 1580 cm�1,corresponding to the D and G bands, respectively.40 The G bandrepresents the first-order scattering of the E2g mode from thesp2 carbon domains, and the D band originates from thedisorder-induced mode associated with structural defects andimperfections.41 The intensity ratio (ID/IG) is generally used as ameasure to evaluate the quality of graphitic structures.30,42 Thevalue for GF was calculated to be 0.82, while KB and CNTs gavehigher values of 1.23 and 1.07, respectively, suggesting that GFhad fewer defects than the other carbon materials.

Fig. 2(c) shows a linear N2 gas adsorption–desorption iso-therm and the BJH pore-size distribution (inset) of the GF, KB,and CNT materials. The isotherm for the GF was noted to havea typical type IV shape. The steep increment in the adsorptioncapacity of GF at relative pressures (P/P0) ranging from 0.45 to0.55 can be attributed to capillary condensation, with thecorresponding hysteresis loop being characteristic of porousmaterials with a narrow mesopore size distribution.43 Theisotherm for KB showed steep increments in the adsorptioncapacity at relative pressures (P/P0) around 0.5, with a clearhysteresis loop, indicating that KB also contains mesopores. Onthe other hand, the isotherm for the CNT material exhibited agradual increase, with an adsorption step at a P/P0 value of 0.9,indicating that the CNT air cathode is less porous than theother carbon structures. The inset in Fig. 2(c) shows the pore-size distribution of the carbon materials obtained using theBJH method. The GF exhibited mesopores of around 3.5 nm indiameter, while the majority of pores in the KB were around2 and 4 nm. In contrast, the pores of the CNT material weremuch larger, approximately 30–40 nm. The specific surface areaand pore volume of the three materials are shown in Table 1.

3.2 Electrochemical properties of GF, KB, and CNT-aircathodes

CV has been previously used to investigate the electrochemicalproperties of carbon materials as electrodes in air cathodes.16,44–46

In the present study, CV experiments were performed to evaluatethe electrochemical properties of GF, KB, and CNT materials withrespect to the ORR and OER in a potential window of 2.0–4.5 V(vs. Li/Li+) at a scan rate of 0.1 mV s�1. Fig. 3(a) shows a comparison

Fig. 1 SEM images of (a) GF, (b) KB, and (c) CNT.

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of the first-cycle voltammograms for GF, KB, and CNT-aircathodes under Ar (inset) and O2 atmospheres. As shown inthe inset of Fig. 3(a), all carbon-air cathodes examined showed aweak and monotonous current response at all potentials under anAr atmosphere, which was attributed to the adsorption–desorption

of Li+ on the carbon surface.47,48 The double-layer charging anddischarging current increased in the order CNT o GF o KB,which is expected, as the current response due to the electricdouble-layer is proportional to the specific surface area ofcarbon used to prepare the electrode. The high voltage tailextending to 4.3 V during charging indicates decomposition ofthe DME electrolyte at potentials above this value.

On the other hand, under an O2 atmosphere, each carbon-air cathode gave CVs with cathodic and anodic current peaks(Fig. 3(a)). Similarly-shaped CV peaks have been previouslyreported for a glassy carbon-air electrode in the DME electrolytein a bulk electrolysis cell.44 The cathodic current peak presentin the CVs corresponds to the ORR (2Li+ + 2e� + O2 - Li2O2),which is responsible for the discharge of the air cathode, andthe anodic current peak corresponds to the OER (Li2O2 -

2Li+ + 2e� + O2), which is responsible for the charge process.The theoretical potential for the ORR and OER is E1 = 2.96 V(vs. Li/Li+); however, the ORR potential was usually observed ataround 2.7 V during discharge, with the OER being seen ataround 4.3 V during charge.

The GF-air cathode gave ORR and OER peak current densitiesof around 3 A gcarbon

�1 at 2.4 V and 1 A gcarbon�1 at 3.1 V,

respectively. The KB-air cathode gave the highest ORR peakcurrent among the three carbon-air cathodes, with a value of5.3 A gcarbon

�1 at 2.3 V (Fig. 3(a)), and also exhibited an OERcurrent density of 0.8 A gcarbon

�1 at 3.3 V. The CNT-air cathodegave ORR and OER current densities of 2.5 A gcarbon

�1 at 2.15 Vand 0.4 A gcarbon

�1 at 3.45 V, respectively, with a broad currentpeak. It should be noted that the GF-air cathode exhibited theleast cathodic and anodic overpotentials for ORR and OERamong the three carbon-air cathodes under an O2 atmosphere,corresponding to the smallest peak potential separation of 0.7 Vin comparison to KB (1.0 V) and the CNT material (1.3 V).

The differences between the cathode materials discussedabove are due to their different physical properties, and hencethe catalytic effect of the carbon-air cathodes on ORR and OERkinetics.27 The cathodic peak current density responsible forthe ORR increased in the order CNT o GF o KB, whichcorresponded to the trend in the specific surface area of thethree carbon materials. In general, having a large surface areaand highly porous structure for the carbon component of thesecathodes enhances ORR kinetics by providing easy access ofelectrolytic O2

� and Li+ to the air cathode surface.44,49 More-over, the catalytic effect of the carbon material can lead tohigher anodic peak current with less anodic overpotential forthe OER in CV.29 Despite the fact that GF had a smaller surfacearea than KB, the GF-air cathode showed a higher anodiccurrent density for the OER than the KB air cathodes, suggestingthat the GF-air cathode is electrochemically more active towardthe OER reaction than the KB cathode. Furthermore, the lowanodic overpotential of the GF-air cathode supports the existenceof a catalytic effect on the electrochemical discharge-productdecomposition attributed to the carbon material.

Fig. 3 also shows the variation in the CV for the threedifferent carbon-air cathodes under an O2 atmosphere over20 cycles. For all of the cathodes, the ORR and OER current

Fig. 2 (a) XRD patterns, (b) Raman spectra, and (c) linear plot of the N2 gasadsorption–desorption isotherms of GF, KB, and CNT (inset: BJH pore-sizedistribution).

Table 1 Physical properties of the carbon materials used in this study

Morphology d002 (Å)BET surfacearea (m2 g�1)

Pore volume(cm3 g�1) ID/IG ratio

GF Sheets 3.78 455 2.38 0.82KB Particle shells 3.68 1294 2.57 1.23CNT Tubes 3.41 194 0.71 1.07

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responses decreased during the repetitive CV cycling within thepotential window of 2–4.5 V. For the GF-air cathode (Fig. 3(b)),the OER peak current centred at 3.1 V gradually decreased withcycling, disappearing around the 17th cycle. The ORR peakcurrent at 2.4 V also gradually decreased; however, it was stillvisible during the 20th cycle, with a slight shift in the ORRpotential from 2.4 V to 2.25 V. KB showed a decrease in the OERcurrent density at 3.3 V upon cycling, with the OER currentdisappearing at the 9th cycle (Fig. 3(c)). Furthermore, comparedto GF, the KB cathode showed a rapid decay in the ORR currentdensity, with an ORR peak potential shift from 2.3 V to 2.17 Vduring cycling. For the CNT-air cathode, the anodic peakcurrent at 3.45 V was barely seen after the third cycle, with adecrease in the cathodic peak current and shift in the peakpotential also evident (Fig. 3(d)).

Since CV analysis suggested that GF was electrochemicallymore active towards the OER in comparison to KB and CNT, thedischarge–charge behaviour was evaluated with respect to thedischarge–charge profile and Coulombic efficiency. Fig. 4 showsthe obtained discharge–charge profiles of the three carbon-air

cathodes over 10 cycles at 0.4 mA cm�2 (330 mA gcarbon�1). In

this study, the discharge capacity was limited to 1280 mA h g�1

within the potential window from 2.5 to 4.2 V, as excessaccumulation of the insulative discharge products mightobscure the catalytic effect of GF due to the limited transportof Li+ ions, O2, and electrons in the discharge products. Inaddition, the discharge–charge potential range was set between2.5–4.2 V to avoid decomposition of the electrolyte above 4.3 V,and the formation of irreversible by-products below 2.3 V.16,45

Fig. 4(a) shows the discharge–charge profiles of the GF-aircathode for the first 10 cycles. The GF-air cathode showed adischarge onset potential of 2.8 V for the ORR, which is close tothe thermodynamic equilibrium potential of 2.96 V (vs. Li/Li+).It should be noted that discharge proceeded at a constantpotential plateau of 2.75 V for the 10 consecutive cycles. Incontrast, during the charge cycle, sloping charge curves wereobserved, with an onset potential of 3.0 V; the onset potentialsof ORR (2.8 V) and OER (3.0 V) from the galvanostatic test are ingood agreement with those observed in the CV results shown inFig. 3(b). More importantly, the shapes of the discharge–charge

Fig. 3 (a) CVs of GF, KB, and CNT-air cathodes under Ar (inset) and O2 atmospheres for the first cycle. CVs of (b) GF, (c) KB, and (d) CNT-air cathodes under an O2

atmosphere during the first 20 cycles. The starting scan potential for carbon-air cathodes was around 2.9 V (V vs. Li/Li+); CV measurements were carried out under1 atm of gas pressure at 25 1C.

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profiles were well retained over the 10 cycles, and the initialdischarge capacity of 1280 mA h gcarbon

�1 could be achieved ineach discharge. Coulombic efficiency was calculated to be 99%for the first cycle and 87% for the 10th cycle for the GF-aircathode.

The discharge–charge profiles of the KB-air cathode inFig. 4(b) showed an onset potential of approximately 2.7 V atthe first discharge, decreasing gradually to 2.55 V upon cycling.

The discharge potential plateau at the first discharge changedto a sloping discharge curve with repetitive cycling. Initially, adischarge capacity of 1280 mA h gcarbon

�1 was observed, how-ever, this gradually decreased, reaching 92 mA h gcarbon

�1 at the10th discharge. During charging of the KB-air cathode, slopingcharge curves were observed with an onset potential of 3.1 V. Acharge capacity of 874 mA h gcarbon

�1 was observed; however,this sharply decreased, reaching 81 mA h gcarbon

�1 at the 10thdischarge.

Fig. 4(c) displays discharge–charge profiles for the CNT-aircathode over 10 cycles. The discharge onset potential for theORR was noted at around 2.65 V, which decreased rapidly to2.55 V upon cycling. The potential plateau at the first dischargechanged to a sloping discharge curve after the third cycle. Thefirst discharge capacity of 1280 mA h gcarbon

�1 rapidlydecreased, reaching 180 mA h gcarbon

�1 at the 10th discharge.During the initial charging of the CNT-air cathode, steepsloping curves were observed, with an onset potential of around3.15 V. The deep discharge–charge profiles of the GF, KB andCNT-air cathodes in the potential range of 2.5–4.2 V (vs. Li/Li+)also showed the highest Coulombic efficiency for GF (see ESI,†Fig. S1). Analysis of the CV and discharge–charge profilesclearly shows that GF is the more electrochemically active ofthe three carbon-air cathodes (including KB, CNT) toward ORRand OER.

The significant differences observed in the electrochemicalproperties of the three carbon-air cathodes employed in Li–O2

batteries using DME electrolyte can be ascribed to the differencesin their physical properties and catalytic activities. GF, producedfrom the reduction of GO, contains a mixture of sp2 andsp3-hybridized carbon atoms, along with defects and oxygenfunctionalities such as carboxyl, carbonyl, hydroxyl, epoxy, andperoxy groups, with carboxylic acids the most likely to belocated along the sheet edges.50,51 Zhang et al. suggested thatthe defects in the GF structure might facilitate the ORR andhinder the migration of Li2O2 on the carbon surface during theOER.6 In addition, it was previously reported that the edge sitesof graphene sheets could have specific chemical reactivitiescompared with non-edge carbon atoms because the carbonatoms at the edge of graphene sheets have partial radicalcharacter.29,52 Therefore, the enhanced catalytic effect of GFon the ORR and OER might come from the relatively largenumber of edge sites introduced into the GF during its synth-esis. The detailed mechanisms of ORR and OER on GF are notyet completely clear and are currently under investigation.

The morphological features of the air electrodes duringdischarge–charge were examined using ex situ SEM at differentstates of the cycling process. Fig. 5 shows SEM images of theGF-air cathode after the first discharge (a), the first charge (b),and the 10th charge (c) at 0.4 mA cm�2 (330 mA gcarbon

�1) with acontrolled discharge capacity of 1280 mA h gcarbon

�1. After thefirst discharge, the GF surface was seen to be covered withparticles of discharge material several nanometers in size,while the macroporous structure was retained. After the firstcharge, most of the discharge products disappeared from theGF surface, mainly due to their electrochemical decomposition

Fig. 4 Potential profiles of (a) GF, (b) KB, and (c) CNT-air cathodes with adischarge capacity limit of 1280 mA h gcarbon

�1 over 10 discharge–charge cyclesat 0.4 mA cm�2.

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during charging. After the 10th charge, the GF surface appearedpartially covered with the discharge products; however, themacroporous nature of the GF in the air cathode was retained(Fig. 5(c)).

Fig. 5 also shows SEM images of the KB-air cathode. Afterthe first discharge (Fig. 5(d)), discharge products coveredthe KB-air cathode, while the morphology of the as-preparedelectrode was retained. After the first charge (Fig. 5(e)) however,the solid product formed during the first discharge processcovering the KB surface was still evident. After the 10th charge(Fig. 5(f)), the KB-air cathode appeared heavily covered, withthe pores between the carbon nanoparticles filled with solidproducts; further, the initial structural features of the KB aircathode were no longer evident. The SEM image of the CNT-aircathode after the first discharge (Fig. 5(g)) showed disk-shapeddischarge products covering the surface, with the morphologyof the as-prepared CNT material not visible. After the firstcharge (Fig. 5(h)), the discharge products appeared to beremoved, with the initial morphology of the as-prepared CNT

electrode being retained. After the 10th charge (Fig. 5(i)), theCNT material appeared covered with solid products, with themacroporous nature lost.

Analysis of SEM images of the GF, KB, and CNT materialsacquired at different states of the cycling process indicates thatthe GF was able to electrochemically decompose the dischargeproduct in the subsequent charge cycle, retaining the initialmacroporous nature of the carbon-air electrode, in contrast tothe KB and CNT materials. The morphological features of theair electrodes during discharge–charge examined by ex situSEM at different states of the cycling process are consistentwith the CV analysis and discharge–charge profiles shown inFig. 3 and 4, respectively.

3.3 Discharge product analysis

The discharge products found on the different carbon-aircathodes at different cycles of the discharge–charge processeswere characterized using XRD, FT-IR, and Raman spectroscopy.Fig. 6 shows the XRD patterns of the GF-air cathode with

Fig. 5 SEM images of the air cathode after the first discharge–charge, and after 10 cycles at 0.4 mA cm�2 with a discharge capacity limit of 1280 mA h gcarbon�1. The

top row images are of the GF-air cathode (a) after the first discharge, (b) after the first charge, and (c) after the 10th charge. The middle-row images are of the KB-aircathode (d) after the first discharge, (e) after the first charge, and (f) after the 10th charge. The bottom-row images are of the CNT-air cathode (g) after the firstdischarge, (h) after the first charge, and (i) after the 10th charge.

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discharge capacity controlled at 1280 mA h gcarbon�1. The

intense peak at around 2y = 541 was attributed to the carbonpaper. The XRD analysis indicated that Li2CO3 was formedduring the first discharge in the DME electrolyte, and was notremoved in the subsequent charge cycle for all carbon materialstested in this study.44 Formation of Li2O2 and Li2O could not beconfirmed by XRD analysis because of their poor crystallinity.53–55

FT-IR spectroscopic analysis was performed to further investigatethe reaction products formed (Li2O2, Li2O, and Li2CO3) on the GFsurface by assessing the atomic vibration bands corresponding to

C–O and Li–O.56 In the spectrum of the as-prepared GF (Fig. 7),broad absorption peaks were observed at around 1247 cm�1

and 1075 cm�1, corresponding to the C–O vibration band fromremaining oxygen-containing functional groups in the GF. Afterthe first discharge, peaks attributed to Li2O2 and Li2O wereobserved at around 505 cm�1 and 564 cm�1 (Li–O), respectively.Upon subsequent charging, the peak intensities from thedischarge products decreased; however, they did not completelydisappear, and their intensities tended to increase uponrepeated discharge. This suggests that the Li2O2 and Li2Odischarge products could not be completely removed duringthe charge cycle. Formation of Li2CO3 was also confirmed fromthe presence of peaks at 1640 cm�1, 1360 cm�1 (–LiCO3)1436 cm�1 (O–CO2

�), and 867 cm�1 (C–O–C).5,45 For Li2CO3,the peak intensities increased gradually upon repeated discharging,and decreased slightly upon subsequent charging, indicating thatLi2CO3 was hardly decomposed during charging.44 Analysis of theFT-IR spectra of KB and CNT-air cathodes indicated the formationof the same products after the first discharge. However, a decreasein LiO2 and Li2O2 peak intensities of around 505 cm�1 and564 cm�1 (Li–O) after the first charge was clearly less than that forGF, suggesting that discharge product removal was more effectivefor GF (see ESI,† Fig. S2).

Fig. 8 shows the Raman spectra of GF, KB, and CNT-aircathodes. The ID/IG ratio of the three carbon materials showedlittle change, regardless of the discharge–charge state. It shouldbe noted that new peaks appeared during discharge at 1130 cm�1

and 1525 cm�1 for the KB and CNT-air cathodes, respectively,corresponding to alkyl C–C stretching modes.57–59 These Ramanpeaks did not disappear during the subsequent charge. Luntz et al.reported the formation of alkyl groups of LiRCO3 on a glassycarbon-air cathode in the DME electrolyte. LiRCO3 was found tobe an irreversible by-product of the electrochemical reactionbetween Li2O2 and DME during discharge–charge.44 LiRCO3 hasbeen previously reported as being one of the remaining solidproducts covering the air cathode reaction sites, increasing

Fig. 6 XRD patters of air cathodes after the first discharge, the first charge and asa prepared electrode under the discharge capacity limit of 1280 mA h gcarbon

�1:(a) pristine CNT, (b) pristine KB, (c) pristine GF, (d) carbon paper, (e) CNT after thefirst discharge, (f) CNT after the first charge, (g) KB after the first discharge, (h) KBafter the first charge, (i) GF after the first discharge, and (j) GF after the first chargeof air cathodes. Charge–discharge current density is 0.4 mA cm�2.

Fig. 7 FT-IR spectra of the GF-air cathode.

Fig. 8 Raman spectra of the GF, KB, and CNT-air cathodes with a dischargecapacity limit of 1280 mA h–gcarbon

�1. Raman bands at 1130 cm�1 and1525 cm�1 correspond to the C–C stretching modes of alkyl groups.

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20270 Phys. Chem. Chem. Phys., 2013, 15, 20262--20271 This journal is c the Owner Societies 2013

the overpotential for the ORR and OER upon cycling.45,60

In contrast, in the present study, the GF-air cathode did notshow any Raman peaks due to LiRCO3 during repeated discharge.This might explain the markedly different discharge–chargebehaviours of GF in comparison to the KB and CNT-air cathodesshown in Fig. 4. The possible formation of LiRCO3 on KB andCNT-air cathodes is currently under investigation.

4. Conclusions

In this study, we employed GF as an air-cathode material forLi–O2 batteries, and investigated its electrochemical propertiesin the DME electrolyte. GF was prepared by microwave-assistedreduction of GO, and its electrochemical properties were com-pared with those of KB and CNT-based air cathodes. The GF-aircathode showed peak-shaped CVs under an O2 atmosphere,with lower ORR and OER overpotentials observed in compar-ison to KB and CNT. The catalytic effect of the GF-air cathodewas confirmed using discharge–charge testing, and was corro-borated by morphological and spectroscopic analyses of dis-charge–charge products on the surface of the GF. Ramanspectroscopy revealed that LiRCO3 was formed on the KB andCNT during the first discharge, but not on the GF. In combi-nation with the catalytic effect of GF, this might explain thehigher Coulombic efficiency of GF observed in the discharge–charge tests. These results demonstrate that GF has greatpotential for use as a cathode material for improved Li–O2

batteries.

Acknowledgements

This work was supported in part by the energy efficiency andresources of the Korea Institute of Energy Technology Evaluationand Planning (KETEP) grant funded by the Ministry of KnowledgeEconomy, Korean government (No.: 20122010100140).

Notes and references

1 K. M. Abraham and Z. Jiang, J. Electrochem. Soc., 1996, 143,1–5.

2 J. Read, J. Electrochem. Soc., 2002, 149, A1190–A1195.3 B. D. McCloskey, D. S. Bethune, R. M. Shelby, G. Girishkumar

and A. C. Luntz, J. Phys. Chem. Lett., 2011, 2, 1161–1166.4 T. Ogasawara, A. Debart, M. Holzapfel, P. Novak and

P. G. Bruce, J. Am. Chem. Soc., 2006, 128, 1390–1393.5 M. Hayashi, H. Minowa, M. Takahashi and T. Shodai,

Electrochemistry, 2010, 78, 325–328.6 J. Xiao, D. H. Mei, X. L. Li, W. Xu, D. Y. Wang, G. L. Graff,

W. D. Bennett, Z. M. Nie, L. V. Saraf, I. A. Aksay, J. Liu andJ. G. Zhang, Nano Lett., 2011, 11, 5071–5078.

7 A. Debart, J. Bao, G. Armstrong and P. G. Bruce, J. PowerSources, 2007, 174, 1177–1182.

8 A. Debart, A. J. Paterson, J. Bao and P. G. Bruce, Angew.Chem., Int. Ed., 2008, 47, 4521–4524.

9 A. Kraytsberg and Y. Ein-Eli, J. Power Sources, 2011, 196,886–893.

10 H. Cheng and K. Scott, J. Power Sources, 2013, 235, 226–233.11 Y. C. Lu, H. A. Gasteiger, M. C. Parent, V. Chiloyan and

Y. Shao-Horn, Electrochem. Solid-State Lett., 2010, 13,A69–A72.

12 J. Li, H. M. Zhang, Y. N. Zhang, M. R. Wang, F. X. Zhang andH. J. Nie, Nanoscale, 2013, 5, 4647–4651.

13 S. Nakanishi, F. Mizuno, K. Nobuhara, T. Abe and H. Iba,Carbon, 2012, 50, 4794–4803.

14 V. Viswanathan, K. S. Thygesen, J. S. Hummelshoj,J. K. Norskov, G. Girishkumar, B. D. McCloskey andA. C. Luntz, J. Chem. Phys., 2011, 135, 214704.

15 O. Gerbig, R. Merkle and J. Maier, Adv. Mater., 2013, 25,3129–3133.

16 G. Q. Zhang, J. P. Zheng, R. Liang, C. Zhang, B. Wang,M. Au, M. Hendrickson and E. J. Plichta, J. Electrochem. Soc.,2011, 158, A822–A827.

17 H. L. Wang, Y. Yang, Y. Y. Liang, G. Y. Zheng, Y. G. Li, Y. Cuiand H. J. Dai, Energy Environ. Sci., 2012, 5, 7931–7935.

18 Y. C. Lu, Z. C. Xu, H. A. Gasteiger, S. Chen, K. Hamad-Schifferli and Y. Shao-Horn, J. Am. Chem. Soc., 2010, 132,12170–12171.

19 L. X. Zhang, S. L. Zhang, K. J. Zhang, G. J. Xu, X. He,S. M. Dong, Z. H. Liu, C. S. Huang, L. Gu and G. L. Cui,Chem. Commun., 2013, 49, 3540–3542.

20 Y. B. Zhang, Y. W. Tan, H. L. Stormer and P. Kim, Nature,2005, 438, 201–204.

21 S. Gilje, S. Han, M. Wang, K. L. Wang and R. B. Kaner, NanoLett., 2007, 7, 3394–3398.

22 E. Yoo, J. Kim, E. Hosono, H. Zhou, T. Kudo and I. Honma,Nano Lett., 2008, 8, 2277–2282.

23 Y. Y. Liang, D. Q. Wu, X. L. Feng and K. Mullen, Adv. Mater.,2009, 21, 1679–1683.

24 C. Gomez-Navarro, J. C. Meyer, R. S. Sundaram, A. Chuvilin,S. Kurasch, M. Burghard, K. Kern and U. Kaiser, Nano Lett.,2010, 10, 1144–1148.

25 Y. W. Zhu, S. Murali, W. W. Cai, X. S. Li, J. W. Suk, J. R. Pottsand R. S. Ruoff, Adv. Mater., 2010, 22, 3906–3924.

26 D. K. Huang, B. Y. Zhang, Y. B. Zhang, F. Zhan, X. B.Xu, Y. Shen and M. K. Wang, J. Mater. Chem. A, 2013, 1,1415–1420.

27 Y. L. Li, J. J. Wang, X. F. Li, D. S. Geng, R. Y. Li and X. L. Sun,Chem. Commun., 2011, 47, 9438–9440.

28 E. Yoo and H. S. Zhou, ACS Nano, 2011, 5, 3020–3026.29 B. Sun, B. Wang, D. W. Su, L. D. Xiao, H. Ahn and

G. X. Wang, Carbon, 2012, 50, 727–733.30 S. H. Park, S. M. Bak, K. H. Kim, J. P. Jegal, S. I. Lee, J. Lee

and K. B. Kim, J. Mater. Chem., 2011, 21, 680–686.31 I. K. Moon, J. Lee, R. S. Ruoff and H. Lee, Nat. Commun.,

2010, 1, 73.32 G. M. Veith, J. Nanda, L. H. Delmau and N. J. Dudney,

J. Phys. Chem. Lett., 2012, 3, 1242–1247.33 H. C. Schniepp, J. L. Li, M. J. McAllister, H. Sai, M. Herrera-

Alonso, D. H. Adamson, R. K. Prud’homme, R. Car, D. A. Savilleand I. A. Aksay, J. Phys. Chem. B, 2006, 110, 8535–8539.

34 M. J. McAllister, J. L. Li, D. H. Adamson, H. C. Schniepp,A. A. Abdala, J. Liu, M. Herrera-Alonso, D. L. Milius, R. Car,

PCCP Paper

Publ

ishe

d on

14

Oct

ober

201

3. D

ownl

oade

d by

Yon

sei U

nive

rsity

on

03/0

3/20

14 0

6:47

:08.

View Article Online

This journal is c the Owner Societies 2013 Phys. Chem. Chem. Phys., 2013, 15, 20262--20271 20271

R. K. Prud’homme and I. A. Aksay, Chem. Mater., 2007, 19,4396–4404.

35 Z. S. Wu, W. C. Ren, L. B. Gao, J. P. Zhao, Z. P. Chen,B. L. Liu, D. M. Tang, B. Yu, C. B. Jiang and H. M. Cheng,ACS Nano, 2009, 3, 411–417.

36 Z. J. Fan, W. Kai, J. Yan, T. Wei, L. J. Zhi, J. Feng, Y. M. Ren,L. P. Song and F. Wei, ACS Nano, 2011, 5, 191–198.

37 O. P. Krivoruchko, A. N. Shmakov and V. I. Zaikovskii, Nucl.Instrum. Methods Phys. Res., Sect. A, 2001, 470, 198–201.

38 H. Yoshikawa, K. Fukuyama, Y. Nakahara, T. Konishi,N. Ichikuni, Y. Yoshikawa, N. Akuzawa, Y. Takahashi andK. Nishikawa, Carbon, 2003, 41, 2931–2938.

39 J. Donohue, Nature, 1975, 255, 172.40 M. A. Pimenta, G. Dresselhaus, M. S. Dresselhaus,

L. G. Cancado, A. Jorio and R. Saito, Phys. Chem. Chem.Phys., 2007, 9, 1276–1291.

41 A. C. Ferrari and J. Robertson, Phys. Rev. B: Condens. MatterMater. Phys., 2000, 61, 14095–14107.

42 K. Sato, R. Saito, Y. Oyama, J. Jiang, L. G. Cancado,M. A. Pimenta, A. Jorio, G. G. Samsonidze, G. Dresselhausand M. S. Dresselhaus, Chem. Phys. Lett., 2006, 427, 117–121.

43 S. Jun, S. H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsunaand O. Terasaki, J. Am. Chem. Soc., 2000, 122, 10712–10713.

44 B. D. McCloskey, A. Speidel, R. Scheffler, D. C. Miller,V. Viswanathan, J. S. Hummelshoj, J. K. Norskov andA. C. Luntz, J. Phys. Chem. Lett., 2012, 3, 997–1001.

45 J. Xiao, J. Z. Hu, D. Y. Wang, D. H. Hu, W. Xu, G. L. Graff,Z. M. Nie, J. Liu and J. G. Zhang, J. Power Sources, 2011, 196,5674–5678.

46 A. K. Thapa, K. Saimen and T. Ishihara, Electrochem. Solid-State Lett., 2010, 13, A165–A167.

47 H. S. Zhou, S. M. Zhu, M. Hibino and I. Honma, J. PowerSources, 2003, 122, 219–223.

48 S. R. S. Prabaharan, R. Vimala and Z. Zainal, J. PowerSources, 2006, 161, 730–736.

49 Z. Q. Peng, S. A. Freunberger, L. J. Hardwick, Y. H. Chen,V. Giordani, F. Barde, P. Novak, D. Graham, J. M. Tarasconand P. G. Bruce, Angew. Chem., Int. Ed., 2011, 50,6351–6355.

50 S. F. Pei and H. M. Cheng, Carbon, 2012, 50, 3210–3228.51 A. Kumar, S. Patil, A. Joshi, V. Bhoraskar, S. Datar and

P. Alegaonkar, Appl. Surf. Sci., 2013, 271, 86–92.52 D. E. Jiang, B. G. Sumpter and S. Dai, J. Chem. Phys., 2007,

126, 134701.53 F. Mizuno, S. Nakanishi, Y. Kotani, S. Yokoishi and H. Iba,

Electrochemistry, 2010, 78, 403–405.54 G. M. Veith, N. J. Dudney, J. Howe and J. Nanda, J. Phys.

Chem. C, 2011, 115, 14325–14333.55 Z. Q. Peng, S. A. Freunberger, Y. H. Chen and P. G. Bruce,

Science, 2012, 337, 563–566.56 W. M. Davis, C. L. Erickson, C. T. Johnston, J. J. Delfino and

J. E. Porter, Chemosphere, 1999, 38, 2913–2928.57 B. W. Zhang, L. F. Li, Z. Q. Wang, S. Y. Xie, Y. J. Zhang,

Y. Shen, M. Yu, B. Deng, Q. Huang, C. H. Fan and J. Y. Li,J. Mater. Chem., 2012, 22, 7775–7781.

58 V. S. Gorelik, A. V. Chervyakov, L. V. Kol’tsova andS. S. Veryaskin, J. Russ. Laser Res., 2000, 21, 323–334.

59 G. Socrates and G. Socrates, Infrared and Raman Character-istic Group Frequencies: Tables and Charts, Wiley, Chichester,New York, 2001.

60 J. B. Hou, M. Yang, M. W. Ellis, R. B. Moore and B. L. Yi,Phys. Chem. Chem. Phys., 2012, 14, 13487–13501.

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