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Electrochimica Acta 82 (2012) 233– 242

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

hemical surface treatments for decreasing irreversible charge loss andreventing exfoliation of graphite in Li-ion batteries

allavi Vermaa, Tsuyoshi Sasakia,b, Petr Nováka,∗,1

Paul Scherrer Institut, Electrochemistry Laboratory, CH-5232 Villigen PSI, SwitzerlandToyota Central R&D Labs., Inc., Nagakute, Aichi, 480-1192, Japan

r t i c l e i n f o

rticle history:eceived 18 November 2011eceived in revised form 27 March 2012ccepted 27 March 2012vailable online 5 April 2012

a b s t r a c t

Chemical pretreatments modify surface properties of graphite and hence enhance its electrochemicalperformance in Li-ion battery. Surface modifications ameliorate the solid electrolyte interphase (SEI)layer formed on graphite during first cycle. An effective SEI formation is vital for reversible cycling ofgraphite in propylene carbonate (PC) based electrolytes, which is advantageous for low temperatureapplications of Li-ion batteries. For minimum surface layer thickness, the surface groups already exist-

eywords:raphiteurface treatmentxfoliationdditivesropylene carbonate (PC)

ing on graphite were modified by chemical pretreatments. Chemically pre-reduced graphites show lessirreversible charge loss, whereas chemically pre-oxidized graphites show more irreversible charge loss.Treatment of graphite by n-butyl lithium resulted in exclusively edge covered particles. This materialexhibits low irreversible charge loss, good reversible cycling in PC based electrolyte, as well as homoge-neous and flexible SEI. Cyclability of this material in standard electrolyte was optimized by stabilizingthe surface chemical composition by use of ethylene carbonate as additive.

. Introduction

Safety and energy efficiency are the main challenges facedy Li-ion battery technology for application in electric vehiclesEVs). An external method of safety using auxiliary componentsn the battery management systems demand additional space and

eight, which are very constrained in EVs. Therefore, researchas to be oriented to intrinsically avert hazardous reactions of thective materials with the electrolyte within a battery. This requireshe reactivity of the materials to be tuned. And since most of theeactions are triggered from the interface of the material and thelectrolyte, it is fundamentally important to tune the reactivity ofhe surface. One such substantial interfacial reaction occurring in

Li-ion battery is electrolyte reduction on the surface of graphite,hich results in the formation of a passive layer as interphasehich is called solid electrolyte interphase or SEI [1]. SEI has been

xtensively studied due to its critical impact on the irreversibleharge loss (ICL), self-discharge, cyclability, rate capability, andafety of the battery [2]. Once formed, the SEI electronically

assivates the surface of the electrode material, and ensuresyclability by virtue of Li-ion conduction. However, there are twoain limitations of SEI. Firstly the charge consumed for electrolyte

∗ Corresponding author. Tel.: +41 56 310 2457.E-mail address: petr.novak@psi.ch (P. Novák).

1 ISE member.

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.03.140

© 2012 Elsevier Ltd. All rights reserved.

reduction during the first lithiation is “lost” irreversibly and cannotbe retrieved in the consecutive cycles. This charge is called ICL isone of the major issues to be solved. It is calculated as follows:

ICL = charge of lithiation − charge of delithiationcharge of lithiation

(%)

ICL ranges from 8% (for graphite of d90 = 44 �m) to 22% (forgraphite of d90 = 6 �m). ICL will be more apparent for the biggerbatteries for applications in EVs, where the amount of the elec-trolyte reduction will be larger. Another perception is that the eventhough the capacity loss is only 10–20% in the first cycle, this per-centage of capacity is lost for every consecutive cycle, and hencethe total loss after 1000 cycles would be rather grave. In addition,there are other consequences of the consumption of the electrolyte.“Loss” of Li-ions from electrolyte and positive electrode materialcan lead to lower conductivity of electrolyte and unbalanced cellrespectively. Second critical issue is that the natural SEI is not ableto avert endless charge of crystalline graphite in propylene car-bonate (PC) electrolyte, which according to Besenhard et al. is dueto solvent co-intercalation of PC in graphite [3]. This results inexfoliation of graphite, where the graphene sheets open up andcontinuous irreversible decomposition of the electrolyte occurs onthe freshly exposed area. PC based electrolytes are very important

for low temperature performance of a battery [4] as they freezeat −49 ◦C, compared to +37 ◦C for the other commonly used sol-vent ethylene carbonate (EC). PC based electrolytes have higherconductivity than EC ones. Most methods used in the literature to

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mprove performance of graphite in PC are by using additives [5–7],r by employing composite electrodes [6,8]. However, more effec-ive methods have to be developed for applications in EVs, wheret is mandatory to have good low temperature performance of theattery.

Vast diversity of surface pretreatments of graphites has beenmployed for improving its electrochemical performance in Li-ionattery. Chemical methods of surface treatment of graphite includeoating graphite with carbonates or chlorides from aqueous solu-ions [9,10], coating with polymers [11,12], chemical fluorination13,14], oxidation [15–18], doping [19–22], etching [23], treatingith butyl lithium [24], acid treatment [25–27], and silylation [28].

hysical methods of surface treatment [29] include chemical vaporeposition [30–32], thermal pyrolysis [33], heat treatment underas flow [34,35], oxygen plasma [36], vacuum evaporation [37],nd chemical vapor infiltration [38]. However, the physical meth-ds are majorly used for synthesis of high specific charge compositeaterials like carbon-alloy [39] or carbon-metal oxide [40]. Physi-

al methods control the physical properties of carbons like surfacerea, surface structure, particle size, particle shape, etc. Whereashe chemical methods allow a more defined control in terms ofurface chemistry, composition, and reactivity. Alongside, chemi-al methods of surface treatment are expected to result in moreobust surface modification owing to the chemical bonds involved.ence, we have selected chemical methods of surface modificationver physical methods.

The chemistry of the existing surface groups will be tunedo obtain thinner surface layers or modification; which wouldecrease the extent of electrolyte reduction and allow unhinderedi-ion transport. This method is expected to result in surface layersostly on the edge planes, as most of the surface groups, dangling

onds, unsaturated groups, and structural defects are present onhe periphery of the graphene sheets in graphite particles. Hence,he resulting surface modification is also expected to avert exfoli-tion via solvent co-intercalation.

Many of the literature reports on chemical surface modificationmploy oxidizing reagents [15–18] to enhance the specific chargef graphitic carbons by creating defects and voids [15]. However,ore oxidized groups would consume extra charge for undergo-

ng reduction during first lithiation. Hence, we have rather chosenhemical reagents for reducing the surface groups into lowest oxi-ation state. Pre-reduced graphite is expected to show low ICL at

east in the potential range open circuit potential (OCP) until 0.8 V.he reduced surface group could also serve as nucleophilic centersith which other organic molecules could be reacted.

The approach here is to modify the surface groups alreadyxisting on the surface of graphite by chemical treatment to tuneeactivity of the inherent surface groups. Surface modified carbonsould comprise of tuned reactivity and morphology and will play

predominant role in determining the properties of the SEI, andonsequently the performance of the battery in terms of safety,CL, exfoliation of graphite, and cyclability.

. Experimental

Treatment with reducing agents: SFG6 graphite powder (fromIMCAL® Switzerland) was treated with excess reagents 1 M BH3n THF, 1 M BH3 in THF followed by 1.6 M n-butyl lithium (BuLi)n hexane, 1.6 M BuLi in hexane, and 2 M LiAlH4 in THF, individ-ally under inert conditions. Post treatment these powders wereashed with THF, hexane, hexane, and THF respectively. The pow-

ers were dried at room temperature inside an Ar filled glove boxO2 < 0.6 ppm, H2O < 3 ppm).

Treatment with bases: SFG6 graphite powder was stirred indi-idually in excess reagents 25% NH3 in water, pyridine, pyrolle, and

Acta 82 (2012) 233– 242

1 M Li tert-butoxide in hexane. They were then washed in water,acetone, acetone, and anhydrous hexane respectively. NH3 treatedsample was dried overnight under vacuum at 80 ◦C, whereas otherswere dried at room temperature inside the glove box.

Treatment with oxidizing agents: SFG6 graphite powder wasstirred in excess reagents 1 M KMnO4 in water, and 35% H2O2 indi-vidually for 4 h and washed copiously with water. KMnO4 treatedgraphite was further reduced with aforementioned BuLi and LiAlH4reagents. Powders were dried overnight under vacuum at 80 ◦C.

Treatment with additive: Electrodes of SFG6 treated with BuLi(SFG6BuLi) using styrene butadiene rubber (SBR) binder on Ti cur-rent collector were prepared from slurry in hexane. Then ∼200 mLof neat additive was dropped on the prepared electrode. The addi-tives were allowed to react with the electrode for ∼10 min afterwhich excess of the additive was removed. Treated electrodes wereallowed to dry inside the glove box.

For electrochemical cycling, standard test cells using Li metalas counter cum reference electrode were used. To minimize thepossible effect of the binder and solvent on the chemical nature ofthe surface groups, chemically inert binder SBR and inert solventhexane was used in slurry formulation (10/90 SBR/graphite, w/w).The slurry was applied onto Ti current collector stubs and driedat room temperature. Electrochemical cycling was done galvanos-tatically at C/10 rate (one Li+ insertion per C6) in 1 M LiPF6 in 1:1(w/w) EC: DMC in the potential window 0.01 mV to 1.0 V vs. Li+/Li.Cyclic voltammetry (CV) of SFG6 electrode vs. Li metal was run at0.01 mV/s in the potential window 0.01 mV to 1.0 V vs. Li+/Li. Forexfoliation studies graphite electrodes were cycled in 1 M LiPF6 inEC:PC (15:85) at C/37 rate. All electrochemical experiments wereconducted at 25 ◦C (±1 ◦C). All potentials mentioned are vs. Li+/Li.

Ultra55 Field Emission scanning electron microscopy (SEM)was used to visualize the surface morphology of samples. Pictureswere taken using in lens detector at 3 keV accelerating voltage.An in-house built transfer chamber was used for transferring theair-sensitive and cycled samples from the glove box (where theelectrochemical cells were disassembled) to the SEM measurementchamber without exposure to air. This is necessary in order to avoidany alterations/reactions of the air sensitive SEI with O2, CO2 or H2Ofrom air. All the treated samples were washed with dimethyl car-bonate (DMC) after 50 electrochemical cycles and dried inside theglove box for post mortem SEM analysis.

Electrochemical impedance spectroscopy (EIS) measurementswere performed in three electrode electrochemical cell (from ToyoSystem Co., Ltd.) using VMP3 multipotentiostat–galvanostat (fromBioLogic Science Instruments). The electrochemical cell was ini-tially relaxed for 1 h, after that a potential perturbation of ±5 mVwas applied at the measuring point (OCP or at specific potentialduring cycling). The frequency range was 100 kHz–100 MHz.

Raman spectra were acquired on a Horiba-Jobin Yvon LabramHR800 system equipped with a helium–neon laser (wave-length = 632.8 nm). Raman spectra were recorded in spectral range1100–1900 cm−1, under confocal conditions, employing an X50objective, with spatial resolution of 2–3 �m3.

3. Results and discussion

3.1. Treatment with reducing agents

SFG6 graphite powders were treated with reducing agents ofvarying strengths starting from weak BH3, medium BH3 → BuLi,very strong BuLi, and aggressive LiAlH4. The CV of SFG6 graphites

reduced with various reducing agents is shown in Fig. 1a. Fig. 1bshows a zoom of the CV in the potential window of SEI forma-tion. It is evident that the electrolyte reduction peak near 0.8 V,which is typically associated with SEI formation [41] is drastically

P. Verma et al. / Electrochimica Acta 82 (2012) 233– 242 235

a.

0.0 0.2 0.6 0.8 1.0-150

-100

-50

0

50

100

150

Specific

curr

ent/m

A g

-1

Potential vs. (Li+/Li)/V

SFG6 pristine

SFG6 BH3

SFG6 BH3->Bu

SFG6 BuLi

SFG6 LiAlH4

figure b

b.

0.6 0.7 0. 8 0.9 1.0-10

-8

-6

-4

-2

0

Potential vs. (Li+/Li)/V

Specific

curr

ent/m

A g

-1

SFG6 pristine

SFG6 BH3

SFG6 BH3 ->BuLi

SFG6 BuLi

SFG6 LiAlH4

Fig. 1. (a) CV of SFG6 chemically reduced with different reagents in 1 M LiPF6 inEf

dsapofetsfatat

tltSp(

ittN(bn

100 20 30 40

0

100

200

300

400 SFG6 pristine cycling in EC:DMC

SFG6 BuLi cycling in EC:DMC

Specific

charg

e/m

Ah g

-1

0.8 V. This is in agreement with XPS results, which showed that thenumber of oxygen containing surface groups increased after thistreatment. However, H2O2 treated SFG6 did not show any drasticchanges as compared to pristine, this is attributed to only a minute

100 20 30 40 50 600.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5 SFG6 pristine

SFG6 H2O

2

SFG6 KMnO4

SFG6 KMnO4 LiAlH 4

SFG6 KMnO4 BuLi

Po

ten

tia

l vs.

(Li+

/Li)

/V

Specific charge/mAh g-1

C:DMC electrolyte; (b) enlarged part of the same CV in the potential window of SEIormation.

iminished for SFG6BuLi as compared to the pristine. For the otheramples too the peak was depressed but the effect was not as drastics that with BuLi. On the contrary for the LiAlH4 treated sample theeak current increased. This may be attributed to the ionic naturef this reagent, which might enhance the conductivity of the sur-ace layer due to presence of charged ionic aggregates and hencenhance charge transfer reactions. As a general trend, decrease inhe peak current is found to be more for graphites reduced withtronger reducing agents (with the exception of LiAlH4). The OCPsor pristine graphite, graphite treated with BH3, BH3 → BuLi, BuLi,nd LiAlH4 were ∼3.3 V, ∼2.8 V, ∼2.9 V, ∼2.6 V, and ∼2.1 V respec-ively. The OCPs of the graphite reduced with the stronger reducinggent was found to be lower vs. Li+/Li. This is in agreement withrend of SEI formation peak current from the CVs.

From Fig. 1b it is seen that the charge consumption in the poten-ial window of SEI formation of SFG6BuLi graphite is drasticallyower compared to pristine SFG6. The ICL of this material is 23% lesshan that of pristine. These results are in agreement with those ofcott et al. [24]. However, cyclability of SFG6BuLi was poor and theractical specific charge retrieved was only 80% of the theoreticalshown in Fig. 2).

Since, BuLi is known to be a strong base as well as a reduc-ng agent; it is unclear which of these roles was responsible forhe improvement in terms of ICL. Hence, graphite powders werereated with bases of varying strengths like pyridine (pKa = 5.2),H3 (pKa = 9.2), and pyrolle (pKa = −3.8), and Li tert-butoxide

pKa > 35). Neither the SEI formation part of the first lithiation ofase treated graphite electrodes in 1 M LiPF6 in EC:DMC electrolyteor the cyclability was not significantly different from that of

Cycle number

Fig. 2. Cyclability of SFG6 pristine and SFG6 BuLi in 1 M LiPF6 in EC:DMC electrolyte.

pristine. Thus, it was most probably not the basic property ofBuLi which was responsible for lower ICL and poor cyclability ofSFG6BuLi. It was rather the role of BuLi as a reducing agent whichwas into play. To confirm this further we treated graphites with oxi-dizing agents in order to get experimental proof from the oppositeextreme end.

3.2. Treatment with oxidizing agents

Model glassy carbon (GC) discs (GC-G type from HTW) and SFG6graphite powders were treated with a mild oxidizing agent H2O2and a strong one 1 M KMnO4 in water. From X-ray photoelectronspectroscopy (XPS) analysis it was found that O/C ratio of pris-tine GC was 1.8%, that of H2O2 treated GC was 4.6%, and that ofKMnO4 treated sample was 12.3% (with C 1s peaks range from285.5 eV to 287 eV). Thus treatment with oxidizing agents’ resultsin enhanced oxidized surface groups and the extent of surface oxi-dation depends on the strength of the oxidizing agent.

Initial part of the first lithiation curve of pristine graphite andgraphite treated with different oxidizing agents in 1 M LiPF6 inEC:DMC electrolyte is shown in Fig. 3. It is observed that after treat-ment with KMnO4 there was an increase in the charge consumptionfor reduction of the surface groups in the potential range OCP to

Fig. 3. Initial part of the first lithiation curve of pristine graphite and graphite treatedwith different oxidizing agents in 1 M LiPF6 in EC:DMC electrolyte; cyclability ofthese graphite electrodes in the same electrolyte.

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36 P. Verma et al. / Electroch

ncrease in the surface groups (as per XPS). After the reduction ofMnO4 treated graphite by LiAlH4 and BuLi individually, it wasbserved that the charge consumption from OCP to 0.8 V as wells from 0.8 V until 0 V was drastically reduced. This result implieshat charge consumption for reduction of the surface groups as wells that for electrolyte reduction during first lithiation of the mate-ial in EC:DMC electrolyte is lower for pre-reduced samples andigher for pre-oxidized samples.

The surface group oxidation and reduction are also reflected inhe OCPs of the treated samples. OCP for pristine, H2O2 treated,MnO4 treated, KMnO4 treated sample reduced by LiAlH4, andMnO4 treated sample reduced by BuLi were ∼3.3 V, ∼3.4 V, ∼3.5 V,1.7 V, and ∼1.2 V respectively. As expected, oxidized surfaceroups have higher OCP than pristine and the reduced ones haveower OCP than pristine.

.3. SFG6 treated with BuLi

To understand the poor cyclability of SFG6BuLi treated sampleshown in Fig. 2) analysis of this material by SEM was done. SEMmages of the SFG6 treated with BuLi are shown in Fig. 4a and b. It isisible that graphite particles are edge decorated with a ruffle cov-ring the prismatic planes. The basal planes on the other hand areare, and no surface film is detectable. This surface morphology wasot reported by Scott et al. [24], who also treated graphite with BuLi.his ruffle could be a result of physical interactions/effects since its tens of nm thick. However, the exact nature and chemical com-osition of this film is unknown. This kind of surface modificationill be of utmost importance for averting exfoliation of graphite in

C based electrolyte, where one wishes to have a layer preventingolvent co-intercalation exclusively on edge planes. These resultsill be reported in Section 3.5.

To understand the poor cyclability of this sample post mortemEM was performed after 10 electrochemical cycles (when practicalpecific charge becomes negligible) (shown in Fig. 4c). It is evidenthat the particles after 10 electrochemical cycles are completelyovered with a thick dense layer, which may result in particle iso-ation and hence in failure of the cell. It is clear that the ruffles onhe edge do not prevent SEI formation completely, even though itppears so from CV curves in Fig. 1. There is continued ICL (between% and 1.5% until 9 cycles) occurring during consecutive cycles,hich results in extra SEI formation. One of the main possible

auses for instable cycling of this material shown in Fig. 2 coulde that the treatment with BuLi might have resulted in highly reac-ivity surface groups. These may be reacting with the electrolytend undergoing continued reactions during cycling.

One of the most probable reasons for poor cyclability of SFG6reated with BuLi is increase in polarization due to continuousncrease in resistance of the film due to continued reactionsf the reactive surface film. Hence evolution with cycling ofharge/discharge curves of SFG6 pristine and SFG6 treated withuLi was studied (shown in Fig. 5a and b). Shrinking of the curveso lower specific charge in Fig. 5a showing pristine SFG6 electrodeould be due to loss of active material from the electrode dur-ng cycling. This could be attributed to poor electrode engineering,

hich had to be with SBR binder inside the glove box so as to beomparable with other materials which are air sensitive. There iso obvious increase in polarization during cycling in this case ashe shrinkage is only along horizontal and not vertical axis. On theontrary the curves shown in Fig. 5b show a shift in both specificharge (horizontal) as well as potential (vertical) axes. This implies

hat BuLi treated SFG6 graphite material exhibits strong polariza-ion. Continued increase in polarization with cycling is attributedo continued reactions of the highly reactive surface layer with thelectrolyte and consequently thickening of resistive passive layer.

Acta 82 (2012) 233– 242

To assess the assumed resistance of the surface layer, EIS ofSFG6BuLi was performed. The evolution of the interface resistanceof the sample with cycling was studied by taking electrochemicalimpedance spectra at different stages during cycling. EIS results forpristine and BuLi treated SFG6 graphite are shown in Fig. 6.

In Fig. 6a the Nyquist plot of SFG6 pristine and SFG6BuLi areshown after 1st cycle. A very simple model is assumed where thediameter of the semicircle at high frequency (which lies on the lefthand side of the Nyquist plot) is assumed to be proportional to theinterface resistance. It is evident by comparing the radius of the dis-torted semicircles in this case that BuLi treated sample has higherinterfacial resistance. The diameter of the semicircle in the Nyquistplot indicates two phenomena namely, SEI resistance and chargetransfer resistance at the interface [42,43]. Since this EIS measure-ment was done at 1 V, it is expected that there is quite less Li+

intercalation and hence negligible charge transfer reaction in thesemicircle in the Nyquist plot; thus the source of resistance is pri-marily SEI/surface layer. From Fig. 6b the total resistance includingcharge transfer as well as that of SEI can be compared. The Nyquistplot shows that resistance at 10 mV during the second cycle ofSFG6BuLi is higher than that of SFG6 pristine. This indicates thatapart from the resistance of the SEI/surface layer, that of chargetransfer may also be higher in the BuLi treated sample, owing tothe presence of the heavy surface film on prismatic planes. How-ever, from Fig. 6c it is evident that the resistance of charge transfer isdecreasing as lithiation progresses during the 1st cycle. This impliesthat the charge transfer reaction is not blocked, but only impededby the presence of the surface layer. And in Fig. 6d, the Nyquist plotshows the resistance of SEI after the 1st and 2nd cycle of SFG6BuLi.It is observed that the radius of the semicircle is bigger after thesecond cycle, than after the first cycle. This evidence supports thehypothesis that the surface film present on SFG6BuLi is reactive andis continuously reacting with the electrolyte to form a thicker andmore resistive SEI.

3.4. Treatment of SFG6 BuLi with additives

In order to stabilize the chemical composition of the surface film,some cyclic film forming additives were used on SFG6BuLi elec-trodes. Use of additives is one of the most commonly employedmethod for improving electrochemical performance of batterymaterials in terms of improving SEI formation, positive electrodeprotection, salt stabilization, overcharge protection, fire retarda-tion, ion solvation enhancement, Al corrosion inhibition, wetting,or viscosity dilution [44]. For the purpose of stabilizing the reac-tive nucleophilic surface layer, cyclic additives with electrophiliccenters were chosen. These additives are expected to undergonucleophilic attack from the surface groups on SFG6BuLi and openup upon reacting with SFG6BuLi to give stable chemical compo-sition. Four classes of additives were chosen based on differentchemistry. This study is also expected to elucidate at which poten-tial various additives reduce (since there is no electrolyte reductionplateau at 0.8 V for SFG6BuLi, no overlap between “standard” SEIformation and additive reduction is expected). Additives used forthis set of study along with their structures and abbreviations areillustrated in Table 1.

The first class of additives comprises of cyclic carbonates, withvarying inductive effect. This includes EC (no inductive effect), viny-lene carbonate VC (−I effect, and also polymerizable olefin unit),and PC (+I effect from methyl group). These additives are expectedto open up upon nucleophilic attack to form stable alkyl carbonates.VC has been extensively studied as an additive and proved to form

effective SEI [45–48]. The second class of additives comprises of5-membered cyclic compounds with varying oxygen content. Thestrain of the pentacyclic rings is expected to propel ring opening.Varying oxygen content would compare the stability of resulting

P. Verma et al. / Electrochimica Acta 82 (2012) 233– 242 237

F ted wi

crraetcp

ig. 4. (a and b) SEM images of SFG6 treated with BuLi; (c) SEM image of SFG6 trea

arbonate, carboxylic acid, and carbonyl functional group froming opening of EC, butyryl lactone (BL), and cyclopentanone (CP)espectively. Lactone has been studied as an electrolyte solvent [49]nd as additives [50] in the literature. The third class of additives is

poxides with varying length of side chains. Epoxides are expectedo easily open up upon nucleophilic attack due to ring strain to formorresponding ethers. The varying length of side chain allows com-arison in terms of inductive effect as well as degree of disorder

a. 0.5

0.4

0.3

0.2

0.1

0.0

Po

ten

tia

l vs.

(Li+

/Li)

/V

5004003002001000

Specific charge/mAh g-1

SFG6 pristine

Fig. 5. Cycling curves of first 30 electrochemical cycles in 1 M LiPF6 in EC

th BuLi after 10 electrochemical cycles at C/10 in 1 M LiPF6 in EC:DMC electrolyte.

in the surface layer. Ethers used are propylene oxide (PO) andbutylene oxide (BO); they are compared with a linear halogenatedalcohol chloropropanol, followed by H exchange with BuLi. Thiscombination of reagents is abbreviated as chloropropanol BuLi

(CPB). The nucleophilic attack is expected on Cl, which would allowcovalent attachment of the alkyl chain with alcohol end along withloss of LiCl. The last family of additives comprises of molecules con-taining heteroatoms. This includes ethylene sulfite (ES), which is

b. 0.5

0.4

0.3

0.2

0.1

0.0

Po

ten

tia

l vs.

(Li+

/Li)

/V

3002001000

Specific charge/mAh g-1

SFG6 BuLi

:DMC electrolyte of (a) SFG6 pristine; (b) SFG6 treated with BuLi.

238 P. Verma et al. / Electrochimica Acta 82 (2012) 233– 242

a. -100

-80

-60

-40

-20

0100806040200

ZR/

SFG6 pristine (1 V) SFG6 BuLi (1 V)

b. -50

-40

-30

-20

-10

050403020100

ZR/

SFG6 pristine (10 mV) SFG6 BuLi (10 mV)

c. -50

-40

-30

-20

-10

0

Z / I Z

/ IZ

/ I

Z / I

50403020100

ZR/

SFG6 BuLi OCP SFG6 BuLi 500 mV SFG6 BuLi 300 mV SFG6 BuLi 100 mV SFG6 BuLi 10 mV

d.

-40

-30

-20

-10

0403020100

ZR/

SFG6 BuLi after 1st cycle

SFG6 BuLi after 2nd

cycle

Fig. 6. (a) Nyquist plot at a potential of 1 V of SFG6 pristine and SFG6BuLi after 1st cycle; (b) Nyquist plot at a potential of 10 mV of SFG6 pristine and SFG6BuLi during2nd cycle; (c) Nyquist plot at different potential of SFG6BuLi during 1st cycle; (d) Nyquist plot at a potential of 10 mV of SFG6BuLi after 1st and 2nd cycle. The cycling andmeasurements are done at C/10 in 1 M LiPF6 in EC:DMC electrolyte.

Table 1Classification of additives used for further treatment of SFG6 treated with BuLi along with their structures and abbreviations.

# Family description Member 1 Member 2 Member 3

1 Cyclic carbonates OO

O

EC

OO

O

VC

OO

O

PC

2 Pentacyclic rings OO

O

EC

O

O

BL

O

CP

3 Epoxides

O

PO

O

BO

Cl OLi

CPB

4 Compounds containing heteroatom OS

O

O

ES

Si

OMeOMe

MeO VMSN

O

MP

P. Verma et al. / Electrochimica Acta 82 (2012) 233– 242 239

100 20 30 40

0

100

200

300

400

SFG6 pristine

SFG6BuLi

SFG6BuLi EC

SFG6BuLi VC

SFG6BuLi MP

Sp

ecific

ch

arg

e/m

Ah

g-1

Cycle number

FM

waoi

dStptc

Fta

0

100

200

300

1200

1400

0.0

0.5

1.0

1.5

2.0

2.5 SFG6 pristine

SFG6BuLi

SFG6BuLi EC

Pote

ntial vs. (L

i+/L

i)/V

Specific charge/mAh g-1

ig. 7. Cyclability of SFG6 pristine, SFG6BuLi, and SFG6BuLi treated with EC, VC, andP additives in 1 M LiPF6 in EC:DMC electrolyte.

idely studied electrolyte additive [51–54]; vinyl methoxy silox-ne (VMS), which is expected to undergo nucleophilic attack on thelefin unit; and N-methyl pyrillidinone (MP), which would resultn amide formation upon ring opening.

To ensure that the second step of treating with additivesoes not lead to layer thickening already prepared electrodes ofFG6BuLi material were treated with the additives, dried, and elec-rochemically cycled. By treating prepared electrodes rather thanowder material we ensure that only the surface area of the elec-

rodes which would be in direct contact with the electrolyte uponycling is modified. This way the additive only reacts with the

a. 0.5

0.4

0.3

0.2

0.1

0.0

Pote

ntial vs. (L

i+/L

i)/V

3002001000

Specific charge/mAh g-1

SFG6 BuLi EC

b.

50 10 15 20 25 300.05

0.10

0.15

0.20

0.25

SFG6 pristine

SFG6 BuLi

SFG6 BuLi EC

SFG6 BuLi VC

SFG6 BuLi MP

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l vs. (L

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ig. 8. (a) SFG6 treated with BuLi after using EC additive; (b) polarization evolu-ion during cycling of SFG6 pristine, SFG6 treated with BuLi, SFG6BuLi treated withdditives.

Fig. 9. First cycle of SFG6 pristine and SFG6BuLi, and SFG6BuLi EC in 1 M LiPF6 inEC:PC (15:85) electrolyte.

surface modified negative material, without having an adverseeffect on the positive electrode material.

Electrochemical performance of SFG6BuLi electrodes treatedwith additives is summarized in Table 2. It is observed that whencyclic carbonate additives are added to SFG6BuLi ICL is lower thanthat of pristine. Concomitantly, addition of these additives espe-cially EC, improves the cyclability better than pristine (shown inFig. 7). Good cyclability of this sample is attributed to the stabilityof the resulting chemical composition of the surface film, whichis expected to be alkyl carbonate. The potential of SEI formationmoves to more positive potentials vs. Li+/Li as the oxygen con-tent in the additive molecule decreases from EC to BL to CP. SEIformation at more positive potential would be advantageous, as itwould occur well in advance of the onset of Li+ intercalation. Butthis would not be the method of choice due to poor cyclability ofSFG6BuLi CP. Depending on these results a hypothesis can be statedthat more O containing moiety is more chemically stable, due tolowered nucleophilicity, and hence assures better electrochemi-cal cycling (because it does not undergo further chemical reactions

with electrolyte during cycling). For the treatment with ether basedadditives ICL is much higher and cyclability of these materialsis also worse than that of SFG6BuLi. Thus, it can be concluded

100 20 30

0

100

200

300

400

SFG6 pristine cycling in EC:DMC

SFG6 BuLi cycling in EC:DMC

SFG6 BuLi EC cycling in EC:DMC

SFG6 BuLi cycling in PC

SFG6 BuLi EC cycling in PC

Specific

charg

e/m

Ah g

-1

Cycle number

Fig. 10. Cyclability of SFG6 pristine, SFG6BuLi, SFG6BuLi treated with EC, and in 1 MLiPF6 in EC:DMC electrolyte; and that of SFG6BuLi and SFG6BuLi treated with EC in1 M LiPF6 in EC:PC (15:85) electrolyte.

240 P. Verma et al. / Electrochimica Acta 82 (2012) 233– 242

Table 2ICL and cyclability of SFG6 pristine, SFG6BuLi, and SFG6BuLi after treating with various additives.

Sample name Irreversible chargeloss (%)

Specific chargea

in 1st cycle% Specific chargeat 10th cycle

% Specific chargeat 20th cycle

% Specific chargeat 45th cycle

SFG6 pristine 22 425 70 64 51SFG6 BuLi 17 230 54 34 21SFG6 BuLi EC 18 300 83 81 68SFG6 BuLi VC 18 300 76 63 53SFG6 BuLi PC 18 300 23 10 6SFG6 BuLi BL 21 300 10 6 3SFG6 BuLi CP 28 175 2 2 1SFG6 BuLi PO 19 300 8 3 1SFG6 BuLi BO 53 75 6 2 0SFG6 BuLi CPB 42 300 6 3 3SFG6 BuLi ES 37 300 26 11 3SFG6 BuLi VMS 19 300 46 23 10SFG6 BuLi MP 20 300 66 60 46

a Specific charge in mAh g−1.

Fig. 11. (a–d) Different magnification SEM images of SFG6BuLi cycled in 1 M LiPF6 in EC:PC (15:85) electrolyte; (e) SEM images of SFG6 pristine cycled in 1 M LiPF6 in EC:PC(15:85) electrolyte.

imica Acta 82 (2012) 233– 242 241

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1100 120 0 1300 140 0 1500 1600 1700 1800 1900

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SFG6 pristine cycled in PC

Inte

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SFG6 BuLi as synthesized

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P. Verma et al. / Electroch

hat the ether based SEI would not be very effective in maintain-ng the cyclability of graphite in carbonate based electrolyte. Thiss explainable because the resulting chemical composition of theayer would be an alkoxide, which is itself nucleophilic and isusceptible to further chemical reactions with electrolyte duringycling. Electrodes modified with MP shows cyclability compara-le as compared with to SFG6BuLi, which may be due to stabilityf the resulting amide functional group. Improved cyclability onddition of MP can be attributed to stable resulting amide grouppon ring opening. Thus, based on this study it can be said thathe additives resulting in stable chemical composition after ringpening like carbonate and amide are better SEI forming additives.he best additives found from this set of study are EC, VC, and MP.hese show low ICL and good specific charge retention upon cyclingshown in Fig. 7).

After treating SFG6BuLi electrodes with EC, the increase in polar-zation (along potential axis) is strongly suppressed. This is shownn Fig. 8a. The curves are shifted to a much smaller extent alonghe potential axis as compared to those in Fig. 5b. This supportsur argument of decreased reactivity due to stabilized chemicalomposition (carbonate) of the surface layer after treatment withC, which is less reactive with the electrolyte, and hence showsowered polarization upon cycling. Calculations of polarizationaverage potential during entire one lithiation step/delithiationtep vs. cycle number) for EC, VC, and MP treated SFG6BuLi arehown in the graph in Fig. 8b. This result also supports that theolarization of SFG6BuLi electrode is rather high and increasespon cycling as compared to pristine, and is lowered after it isreated with EC, VC, and MP additives.

The practical specific charge of all electrodes using SFG6BuLi asase material was lower than that of the pristine graphite electrode.he reason for this is that for the calculation of specific charge theass of the redox inactive surface layer has not been taken into

ccount. The specific charge has been calculated per gram of totalctive material (graphite + surface layer). The weight percentagef the layer was estimated to be ca. 30% (±10%) by washing andeighing experiment and calculation from galvanostatic cyclingith potential step at 50 mV vs Li+/Li. Due to the sensitivity of

he layer, better methods of analyzing the mass percentage of theurface layer could not be employed. However, this value justifieshe recorded lower capacity per unit mass of the material of theFG6BuLi material (Fig. 2).

.5. Prevention of exfoliation of SFG6 by treatment with BuLi

Since, treatment with BuLi results in exclusively edge decoratedarticles (shown in Fig. 4a and b), it is expected to avert solvent co-

ntercalation through the prismatic planes and hence exfoliationf graphite in PC based electrolyte. The first cycle of SFG6 pristinend SFG6 treated with BuLi in 1 M LiPF6 in EC:PC (15:85) electrolytes shown in Fig. 9. Pristine SFG6 undergoes endless charge due toontinuous electrolyte degradation along with exfoliation, whereashe graphite which was treated with BuLi shows normal reversibleehavior, without noticeable SEI formation plateau at 0.8 V vs.i+/Li. This means that surface coating on the edges of the graphitearticles acts as an artificial SEI, averts exfoliation, and makes itossible to cycle SFG6BuLi efficiently in PC based electrolyte. Thisesult is highly promising as it opens the way toward modificationf highly crystalline graphites for their usage in batteries employingC based electrolytes, for low temperature applications.

Cyclability of SFG6BuLi, and SFG6BuLi EC in PC based electrolytes compared with that of SFG6 pristine in EC:DMC electrolyte in

ig. 10. It is seen that fading of the specific charge SFG6BuLi in PCased electrolyte is comparable to normal cycling of SFG6 pristine

n EC:DMC electrolyte. After treating SFG6BuLi electrode with ECdditive, it shows reversible cycling in PC, and its cyclability in PC

Fig. 12. Raman spectra of (a) uncycled and cycled SFG6 pristine; (b) uncycled andcycled SFG6BuLi. Cycling done in 1 M LiPF6 in EC:PC (15:85) electrolyte at C/10 rate.

electrolyte is further improved (even better than that of SFG6BuLiEC in EC:DMC electrolyte). This implies that SFG6BuLi and SFG6BuLiEC show low ICL, cycles reversibly in PC based electrolyte, andshows excellent specific charge retention in PC based electrolyte.

For further evidence that SFG6BuLi does not exfoliate, SEM anal-ysis of SFG6BuLi cycled in PC electrolyte was carried out. Theelectrode after 50 electrochemical cycles is shown in Fig. 11. Itis evident from Fig. 11a that the SEI formed in this case is ratherhomogenous and uniform. There are no obvious cracks and frac-tures visible in the SEI. Picture in Fig. 11b shows that the edgesof the particles are covered with a slightly thicker SEI than thebasal planes. This might be the reason for prevented solvent co-intercalation. From picture in Fig. 11c, it is seen that the natureof the SEI is polymeric/flexible and obviously not brittle. The layerappears to be elastic and looks flexible, as it stretches across theprismatic planes. This image also supports effective SEI forma-tion along the edge planes, without visible exfoliation of graphite.Fig. 11d shows the edge planes of a few graphite particles. UnlikeSFG6 pristine cycled in the same electrolyte (shown in Fig. 11e),there are no signs of exfoliation and the stacking of graphene sheetsis retained after electrochemical cycling of SFG6BuLi electrodes.

To further prove that SFG6BuLi does not exfoliate on cycling,

post mortem Raman spectroscopy measurements were performedon the SFG6BuLi electrodes after 50 cycles in 1 M LiPF6 in EC:PC(15:85) electrolyte. The Raman spectra of pristine SFG6 before andafter cycling are shown in Fig. 12a, and as synthesized SFG6BuLi,

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nd cycled SFG6BuLi is shown in Fig. 12b. The spectra shown inhe Fig. 12a are not normalized, and the ones in Fig. 12b are nor-

alized with respect to the G band, which is the highest peak inoth spectra. The signal to background ratio of the cycled samples

s poorer due to additional SEI film present on the surface. The addi-ional film results in decreased intensity of G band, absorption ofhe incident photons by the SEI, and poses practical limitations ofocusing on such samples. The pristine and as synthesized SFG6BuLiample shows clearly an ordered structure due to presence of atronger G band as compared to D band. The cycled SFG6BuLi sam-le also shows that graphite particles have not been exfoliated ashe G band is still much stronger than the D band. Had there beenxfoliation, the D band (which is attributed to the disorder in thetructure) would have been larger than the G band, as is the case inhe spectrum of pristine SFG6 after cycling in PC shown in Fig. 12a.

. Conclusions

In order to obtain thinner surface layer, the existing surfaceroups of graphite were treated with various reducing agents. Itas found that pre-reduced graphite electrodes give rise to lesser

harge consumption in the potential window of SEI formation.re-oxidation on the other hand showed increase in charge con-umption for reduction of surface groups and also higher ICL. It canhus be inferred that treatment with reducing agent decreased theCL whereas that with oxidizing agent increased it. Amongst theifferent reducing reagents used, the best results were found withreatment using BuLi. BuLi treated sample showed 23% decreasedCL, owing to its reducing properties. SFG6 graphite treated withuLi was found to have exclusively edge decorated particles with auffle on the prismatic planes. The exact nature of this surface ruffles not clear but is assumed to be highly reactive and nucleophilicn nature, which could have been the reason behind its poor cycla-ility in EC:DMC electrolyte. The surface chemical composition ofFG6BuLi was stabilized by treating it with cyclic additives. Theest additives which improved the cyclability of this material wereC, VC, and MP. The reason for improved cyclability was decreasedeactions for the surface layer with the electrolyte. Graphite treatedith BuLi was also able to cycle reversibly in PC based electrolyteithout undergoing exfoliation. The cyclability of this material in

C based electrolyte was comparable to that of pristine graphite inC:DMC electrolyte. It cycles reversibly in PC and shows homoge-eous, uniform, and flexible SEI. SEM and Raman analysis provedhat there were no signs of exfoliation of graphite particles through-ut the electrode after cycling in PC.

Since it is demonstrated that edge covered graphite particles doot undergo exfoliation. This proves that solvent co-intercalation

s the main reason for exfoliation and that it can be averted by cov-ring the prismatic planes of graphite particles with an artificialEI. Surface modification methods have proved effective for ame-iorating SEI properties, and can be further extended for improvingerformance of materials for Li-ion batteries.

cknowledgments

We thank Swiss National Science Foundation for the funding.e thank Mr. Hermann Kaiser for developing the transfer chamber

or SEM. We are very grateful to Mr. Patrick Lanz for conducting theaman measurements.

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