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Alkali carbonate-coated graphite electrode

Alkali carbonate-coated graphite electrode for lithium-ion batteries

S.Komaba a,*,M.Watanabe b ,H.Groult c ,N.Kumagai b

Department of Applied Chemistry,Tokyo University of Science,1-3Kagurazaka,Shinjuku,Tokyo 162-8601,Japan b

Graduate School of Engineering,Iwate University,Morioka,Iwate 020-8551,Japan c

Universite

′P .&M.Curie-Paris6,CNRS UMR 7612,Laboratoire LI2C,4Place Jussieu,Paris F-75005,France A R T I C L E I N F O Article history:

Received 28January 2008Accepted 10April 2008Available online 22April 2008

A B S T R A C T

Charge and discharge behavior of a graphite electrode for rechargeable lithium-ion batter-ies was successfully improved by pretreatment of graphite powders with A 2CO 3(A =Li,Na,and K)aqueous solutions.In the process of the pretreatment,graphite powders were sim-ply dispersed in the aqueous solutions,and then ?ltered and dried to modify the surface of graphite powder with solid alkali carbonate.With the optimum concentration of each car-bonate,1wt.%Li 2CO 3,5wt.%Na 2CO 3,and 1wt.%K 2CO 3,the irreversible reaction at the ini-tial cycle was suppressed by the pretreatment which was capable of modifying the solid electrolyte interphase formed on the graphite electrode surface.Furthermore,the rate capability was improved by the surface modi?cation,that is,the reversible discharge capacities at 175mA g à1increased with adequate capacity retention in a 1mol dm à3LiClO 4ethylene carbonate:diethyl carbonate electrolyte solution because of the kinetics enhance-ment of lithium-ion transfer at the interface.

1.Introduction

Graphite electrode shows a high speci?c capacity,low work-ing potential close to that of lithium metal,and superior cy-cling behavior as the negative electrode of Li-ion cells [1–4].

When graphite powders are employed as the active material,the irreversible capacity appears at the ?rst cycle since reduc-tive decomposition of an electrolyte solution occurred at the graphite/electrolyte interface during the ?rst charge (electro-reduction)including the formation of solid electrolyte inter-phase (SEI).The existence of a SEI layer plays an important role in reversible lithium intercalation into the interspace be-tween graphene slabs.The kinetics of lithium intercalation was predominantly determined by the SEI because all lithium ions in an electrolyte solution must cross the interface accompanied with desolvation before the formation of lithium-intercalated graphite compound;therefore,the

chemical modi?cation of a graphite surface including a SEI ?lm has attracted wide attention to improve the battery performances.

With the aim of enhancing the negative electrode perfor-mance in lithium-based rechargeable batteries,various or-ganic and inorganic additives dissolved into an electrolyte solution are known to be effective:CO 2[5,6],HF [7–9],AlI 3,and MgI 2[10,11]for metallic lithium,and ethylene sul?te [12],vinylene carbonate [13–15],cobalt ion [16],lithium car-bonate [17,18]and sodium ion [19]for carbon anodes.These additives successfully suppress the initial irreversibility and improve their cycle performance.Besides,we found several particular additives for LiMn 2O 4/C batteries,vinyl-pyridine [20],LiI,LiBr,and NH 4I [21].

In previous reports it has been stated that,the electro-chemical lithiation properties of carbon anode were improved by its surface treatment with metal oxides [22,23]and lithium

*Corresponding author:Fax:+81352288749.

E-mail address:komaba@rs.kagu.tus.ac.jp (S.Komaba).

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carbonate[24].Our group has been recently emphasizing on the key role of the inorganic ingredient at the graphite surface,such as lithium,sodium,potassium[25],and?uorine [26];furthermore,we reported the impact of Na salts as an electrolyte additive[19,27]and coating[28,29]on the battery performance of graphite.In this study,we investigated the effects of alkaline carbonates,A2CO3(A=Li,Na,and K),as a coating chemical upon the battery performance of graphite.

2.Experimental

Reagent grade natural graphite(particle size:3–10l m in diameter,d002=0.3356nm,surface area:11.8m2gà1),Li2CO3, Na2CO3,and K2CO3(Kanto Chemical Co.,Inc.)were used.Bat-tery grade lithium foil,ethylene carbonate(EC),diethyl car-bonate(DEC),and LiClO4were used without any further puri?cation and treatment.

Pretreatment of graphite was carried out in the atmo-sphere prior to battery assembly as described previously [28].Graphite powders(a few grams)were added into Li2CO3, Na2CO3,and K2CO3aqueous solutions(ca.100ml)and stirred for30min to ensure that the powder was completely wetted. Then,the graphite powders were?ltered out using reduced pressure without rinsing in water,and the wetted graphite was dried in vacuum at100°C.This pretreated graphite pow-der was used for electrochemical measurements.As control graphite,the natural graphite was used without the pretreatment.

For the preparation of working electrodes,the graphite powders(90wt.%)were mixed with polyvinylidene?uoride (10wt.%)in N-methylpyrrolidone.The slurry thus obtained was coated onto a copper foil and roll-pressed in air.The elec-trodes were dried at80–120°C in a vacuum state prior to use.

A coin-type electrochemical cell(diameter20mm,thickness:

3.2mm),which consisted of the graphite working electrode,a lithium foil as a quasi-reference electrode,a separator,and an electrolyte,was assembled in an Ar?lled glove box(dew point <à60°C).The electrolyte used was1mol dmà3LiClO4in EC:-DEC(1:1by volume).Galvanostatic charge and discharge(lith-ium intercalation and deintercalation,respectively)tests of the graphite electrode at175mA gà1(corresponding to C/2 when350mAh gà1)were carried out between0.00and

2.00V vs.Li at25±2°C.In case of the galvanostatic tests at

a higher current of350mA gà1,a beaker-type cell was assem-bled with graphite/nickel mesh,a lithium foil as reference and counter electrodes,and the electrolyte[27].The same slurry of graphite was pasted onto nickel mesh,and dried at 80–160°C in a vacuum state prior to use.

In order to estimate the loading of coating in the graphite powder,the pretreated graphite was thoroughly washed with water,and then alkali concentration of the resultant aqueous solution was analyzed with atomic absorption spectroscopy (AAS).To con?rm the presence of alkali coating after and be-fore immersing into the EC:DEC electrolyte solution,the graphite was examined using a time of?ight-secondary ion mass spectroscopy(TOF-SIMS,ULVAC-PHI TFS2000,PerkinEl-mer Inc.)surface analyzer operated at10à9Torr,equipped with a liquid Ga ion source and pulse electron?ooding.Dur-ing the analysis,the targets were bombarded by the10keV Ga beams with pulsed primary ion current varying from0.3 to0.5pA.The total collection time was300s and rastering was done over a de?nite area.

The surface of the electrode after electrochemical cycling was observed with scanning electron microscopy(SEM,N-2300NII,Hitachi High-Technologies Co.).X-ray photoelectron spectroscopy(XPS,PHI5600system,PerkinElmer Inc.)was employed using monochromatic AlK a as the incident X-rays, and depth pro?ling was made by argon ion-beam sputtering which was conducted using an ion-beam voltage of3kV.Ex situ X-ray diffraction of the electrode was employed using a plastic-wrapped electrode formed in the glove box in order to avoid exposing the electrochemically tested electrode to an atmosphere.

To prepare an electrochemically tested graphite for SEM, XPS,TOF-SIMS,and ex situ XRD measurements,a beaker-type electrochemical cell with the graphite working electrode on Ni net was assembled and disassembled before the analyses in an Ar atmosphere.The electrode after an electrochemical test was rinsed in a DEC or EC:DEC solvent and dried in vac-uum,and then transferred to the observation chamber using a specially built transport bag to prevent any exposure to air.

3.Results and discussion

3.1.Alkali carbonate coating

As reported previously[28],the pretreatment is completed with only aqueous solutions in the atmosphere at room tem-perature,so that the procedure of the pretreatment of graph-ite powders is remarkably simple and easy compared to those of oxide coating[22,23].In case of conventional additives for lithium-ion batteries,their effects result from the chemical linking with the electrode surface,e.g.its electroreductive decomposition,byproducts’solubility,and SEI properties,of course,we have to pay attention to the in?uence of additives on the opposite(positive)electrode.On the contrary,the coat-ing method is able to modify the graphite surface without any in?uences on the positive electrode.

Fig.1shows the XRD patterns of graphite without and with treatment by alkali carbonate aqueous solutions.The diffrac-tion patterns are magni?ed to check the existence of coating salts.The diffraction peaks of graphite are very sharp and in-tense,indicating that the natural graphite has high crystallin-ity and the value of d002was0.3356nm.By treating graphite powders with the aqueous solutions,the diffraction of the corresponding carbonates appeared in the patterns,and their intensities are intensi?ed with increasing the concentration for the treatment.Because the solubility of Li2CO3was rela-tively low,the concentration of Li2CO3was varied up to 1wt.%.The loading of carbonate increased with the increase in the concentration as was con?rmed by atomic absorption spectrometry.The peaks of graphite are identical for all sam-ples,indicating no in?uence of the treatment on the crystal structure of graphite.As well as the diffraction peaks of graphite which is highly crystalline,the peak widths of each carbonate are narrow,indicating that the high crystalline car-bonates are loaded by the treatment via recrystallization dur-ing drying,but the undeniable fact is that the low crystalline

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or amorphous thin coating of the carbonates exists on the graphite surface.

The surface of graphite treated with the carbonates was examined by TOF-SIMS as shown in Fig.2.TOF-SIMS mea-surement con?rmed the existence of the corresponding alkali components after the treatment.We observed the alkali-free surface and the existence of hydrogen and carbon(m/z=1 and12,respectively,in Fig.2)for the control natural graphite. The alkali fragments of6,7Li+,23Na+,and39K+were much clearly detected compared with those of carbon(m/z=12), whereas the diffraction intensity of graphite was much higher than those of carbonates depicted in Fig.1.In general,

the

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whole powder is re?ected in powder XRD data,and XRD mea-surements hardly detect microcrystal and low crystalline/amorphous compounds.However,TOF-SIMS can identify chemical composition within a very thin region of <1nm depth,independent of crystallinity;furthermore,quasi-quan-titative estimation could be possible from intensity ratios of each ion mass peak.From these considerations,the most part of graphite surface should be successfully modi?ed and coated with a thin layer of the carbonates from the intensity ratios of alkali and carbon in the TOF-SIMS spectra.And more,the active part such as edge plane of graphite was pref-erably modi?ed with their deposit because the edge plane would successfully serve as adsorbing and/or nucleating seed for their coating during drying.

Furthermore,TOF-SIMS analysis was carried out for graph-ite after immersion of graphite electrode into the LiClO 4EC:-DEC solution over six days to ensure that the coating remained at the electrode/electrolyte interface.As is seen in Fig.2,the de?nite amount of the undissolved coating was de-tected for Na 2CO 3,and K 2CO 3,while the large peaks of lithium (m /z =6and 7)appeared because of the electrolyte ingredient

in spite of washing with the salt-free solvent.Although the TOF-SIMS analysis was not able to distinguish lithium that originated in the coating or electrolyte,Zhang et al.described that the Li 2CO 3coating remained on the surface due to its low solubility [24].Consequently,the carbonate coatings re-mained at the surface during immersion of the graphite elec-trode in the electrolyte;therefore,the in?uence of the carbonate coating on electrochemical lithium intercalation into modi?ed graphite was investigated in the liquid electrolyte.

3.2.Anode properties in Li-ion battery

Fig.3shows the initial galvanostatic charge and discharge (intercalation and deintercalation,respectively)curves of the modi?ed graphites as negative electrodes at a rather high current density of 175mA g à1.Their capacities,ef?ciencies,and amounts of alkali are summarized with a comparison with those of control graphite in Table 1.The control natural graphite exhibited 253mAh g à1with the initial cou-lombic ef?ciency of 83.9%,de?ned as (discharge

capacity)/

Table 1–Initial capacities and coulombic ef?ciencies of graphite electrodes treated with A 2CO 3solutions compared with those of control graphite,and loading of alkali components in graphite powder

Charge (mAh g à1)

Discharge (mAh g à1)

Ef?ciency (%)

Loading of alkali a (mg (g of graphite)à1)

Control

30125383.9–1wt.%Li 2CO 335532090.30.925wt.%Na 2CO 336731686.217.91wt.%K 2CO 3

363

324

89.2

5.40

a Determined with AAS by dispersing the treated graphite in water.

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(charge capacity)at the ?rst cycle,which is rather low to be feasible in the practical lithium-ion batteries,though the con-trol revealed the nearly theoretical capacity of graphite (372mAh g à1)at lower current.Clearly,the treatment with the alkali carbonates did expand the reversible capacity of graphite because of the reduced polarization during both intercalation and deintercalation of lithium as seen in Fig.3.The coulombic ef?ciencies at the ?rst cycle increased up to 86–90%,and the de?nite amount of alkali was loaded and modi?ed the graphite surface,as seen in Table 1.This improvement agrees with the previous literature by Zhang et al.[24]claiming that Li 2CO 3coating was effective to im-prove the electrochemical performance of a graphite anode.It is found that the effects of the other alkali carbonates are similar to that of Li 2CO 3.The chronopotentiograms of Fig.3show no indicative features of any additional reactions,such as intercalation and electroplating of sodium and potassium,compared to those of unmodi?ed graphite,and ex situ XRD measurements of fully charged graphite con?rmed that the

00l diffraction whose d spacing was 3.71A

?appeared for con-trol and carbonate-treated graphites,which is close to

d 00l =3.72A

?in LiC 6.If Na +and K +ions intercalated or cointer-calated into graphite,the d 00l would be larger than 3.72A

?be-cause of larger ionic radii of Na +and K +than that of Li +

[27].From these results,we know that the redox capacity is due to electrochemical lithium intercalation into graphite with no apparent additional redox capacity in spite of the carbon-ate modi?cation.

Fig.4shows the cyclic voltammograms of the graphite electrodes.The electroreduction in the potential region be-tween 0.3and 0V is due to reversible lithium intercalation into control and modi?ed graphite after the electrolyte decomposition at higher potential around 0.4–0.8V in catho-dic scan.Obviously,larger redox currents due to lithium inter-calation ?ow upon cathodic and anodic scan in case of the carbonate coatings.From Figs.2–4,it can be seen that the car-bonate coating contributes to the modi?cation of SEI layer to improve the kinetic rate of lithium intercalation,in particular,the lithium diffusion across the electrolyte/electrode inter-face.As con?rmed by TOF-SIMS measurement,the fairly thin coating should exist on the almost whole graphite surface.It is general that solid-state carbonate powder is an electrical insulator;however,such thin layer should participate in the SEI formation so that the carbonate coating would not disturb the electron transfer at the surface,resulting in the kinetic improvement.

The successive galvanostatic cycling at 175mA g à1was carried out for the graphite electrodes,and the variation of discharge (delithiation)capacities is represented in Fig.5.The discharge capacities depended on the carbonate concen-tration for treatment,that is,the highest capacities were achieved by treatment with 1wt.%Li 2CO 3,5wt.%Na 2CO 3,and 1wt.%K 2CO 3,and the capacity retention during the ini-tial 20cycles seems to be independent of the concentration.The optimal carbonate coating brought about the higher capacity than 300mAh g à1over 20cycles,whereas the control showed about 250mAh g à1.It is reasonable to think that the carbonate modi?ed SEI layer,which was basically formed dur-ing the initial cycles,in?uenced the successive cycling,so that the capacity at the initial cycle was maintained over the initial 20cycles as is seen in Fig.5.There appears the ten-dency that the lower and higher concentrations brought about less improvement of the capacity and that much higher concentration results in deterioration compared to that of the control.Among three carbonates,almost no difference of the reversible capacities between 240and 350mAg g à1

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observed in Fig.5,though the capacities at higher rate de-pended on the alkalis as described in Fig.9.Thus,we obtained the superior cyclability for the optimum carbonate coating to the pristine graphite.

The initial coulombic ef?ciency depended on the carbon-ate concentration as shown in Fig.6.It can be seen not only in Fig.5,but also in Fig.6that the adequate concentrations of treatment solution are 1wt.%Li 2CO 3,5wt.%Na 2CO 3,and 1wt.%K 2CO 3,to accomplish higher ef?ciency with higher capacity.The lower concentration brought about the less ex-tent of the effect on capacity and ef?ciency,otherwise,the higher concentration led to the deterioration in capacity and ef?ciency as is seen in Figs.5and 6.AAS analyses con?rmed that the loading of alkali ingredient in graphite samples in-creased almost linearly with increasing the concentration of the carbonate solution,and the diffraction of crystalline car-bonate was intensi?ed as is shown in Fig.1;therefore,the higher concentration led to the deposition of recrystallized carbonate which deteriorates the lithiation capacity.The modi?ed surface condition on nanometer scale must depend on the concentration,and when the optimum concentration of carbonate was used for treatment,the appropriate thin layer for the SEI formation successfully enhanced the charge and discharge performances,due to the improved kinetics of lithium transfer at the SEI.

3.3.Interface structure and kinetics of lithium

intercalation

The electrode morphologies after galvanostatic cycling were observed by microscopy as shown in Fig.7.SEM measurement

con?rmed a larger number of white deposits on the graphite surface for the control,which might consist of decomposition products such as lithium carbonate [30].It is clear that the amount of deposits decreased by the optimum coating with the alkali carbonate solutions.In the case of lithium and potassium carbonates,the smaller number of deposits ap-peared on the surface to compare with the result of control.In the case of Na 2CO 3,there appear few deposits in the image of Fig.7c,showing the suppression of electrolyte decomposi-tion and/or helping the formation of more compact,protec-tive,and highly ion-conductive SEI.

XPS measurements were carried out to investigate the chemical composition of the thin surface layer on tested graphite electrode including SEI [20,27].Fig.8shows the vari-ation in atomic concentration estimated from the relative areas of the carbon,oxygen,and alkali peaks,taking their photoionization cross sections into account,when the elec-trode was etched by argon ion-beam sputtering;therefore,the variation corresponded to the depth pro?ling.These con-?rmed the different chemical compositions and depth pro-?les expected from the different electrode morphologies shown in Fig.7.As the surface was covered with many depos-its in the case of control,the almost constant ratios of carbon,oxygen,and lithium are detected during etching.This sup-ported the deposition of thick decomposition products on graphite,such as carbonate,alkoxide,alkyl carbonate,and lithium salt by electroreduction.However,the carbon ingredi-ents increased,and the lithium and oxygen decreased during the etching because the carbonate treatment led to less deposits,that is,the thinner surface layer as shown in Fig.7.Peak shape and position of carbon 1s (not shown here)

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revealed that the observed increase in carbon was due to the appearance of graphite by etching with a decrease of carbon-ate and so on.Among the carbonate-treated graphites,the carbon of graphite signi?cantly increased by etching in the Na2CO3case,and a similar variation was observed for Li2CO3 and K2CO3,that is in agreement with the SEM pictures.These results also prove that the carbonate-coated graphite elec-trode exhibited the enhanced kinetics of lithium intercalaca-tion at the SEI of electrode/electrolyte interface.

Charge and discharge tests were carried out at a larger cur-rent of350mA gà1to compare the kinetic in?uence.Fig.9shows the variation in discharge capacities examined at 350mA gà1.Because of the different con?gurations of the electrochemical cell and graphite electrode,the control graphite on Cu foil and Ni mesh showed different capacities in coin-and beaker-type cells as seen in Figs.5and9,respec-tively,but we compared the control and carbonate-coated graphite electrodes under the same galvanostatic condition in the beaker-type cell here.Clearly,the capacities also in-creased by all carbonate coatings.It can be seen in Fig.5that the observed capacities at175mA gà1hardly depended on the alkali species of carbonates;however,there appeared the

Fig.7–SEM pictures of graphite electrodes after20cycles:(a)control and treated with(b)Li2CO3,(c)Na2CO3,and(d)K2CO3. 1190C A R B O N46(2008)1184–1193

dependence on the alkali when the cycle tests were done at 350mA g à1as shown in Fig.9.Among the carbonate-treated graphites,the Na 2CO 3-coated one demonstrated the highest capacities during 50cycles.Since the surface morphology be-came most uniform with thin SEI layer for Na 2CO 3coating as shown in Figs.7and 8,the most enhanced kinetics of electro-chemical lithiation should be achieved by the Na 2CO 3coating.We believe that it resulted from the cooperative outcome of the positive effects of both sodium [19,27,28]and carbonate

ions [6,31].Potassium ion additive by dissolving KPF 6in LiPF 6EC:DEC deteriorated the electrochemical performance of a graphite electrode in Li cell [32];on the other hand,the posi-tive effects of K 2CO 3as an electrolyte additive were reported by Zheng et al.[33],which is similar to the effect of the car-bonate coating.If the same ions are used for interface modi-?cation,their effects depend on the counter ions and on the adding procedure such as dissolution and coating.

This pretreatment method makes the application of insol-uble inorganics in electrolyte solutions possible to selectively modify the electrode/electrolyte interface with a minimal amount;furthermore,note that there is no need to take into account the in?uence of the coatings on the opposite elec-trode,and there are advantages of the cheap,environmen-tally benign,and easy process.Therefore,we believe that this method has high potential for application in practical lithium-ion batteries.Further investigation and establish-ment of this coating technique are in progress with various inorganic salts and polymers.

Conclusion

We investigated the effects of carbonate coatings on graphite anode performance.From the electrochemical tests,the lith-ium,sodium,and potassium carbonate coatings are capable of improving the electrochemical performance,due to the ki-netic enhancement.Among the tested coatings,the Na 2CO 3coating demonstrated the excellent effect on the battery per-formance:higher reversible capacities with the high coulom-bic ef?ciency of graphite anode for lithium-ion batteries.

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Considering our previous study on the Na+effect on the SEI, carbonate was included in the SEI layer resulting in the mod-i?cation of the interface structure,so that the polarization of the graphite anode was reduced by faster kinetics of lithium ions across the interface.

Acknowledgements

The authors thank Dr.H.Yashiro and N.Kumagai for TOF-SIMS measurement and helpful assistance.This study was ?nancially supported by the Iwatani Naoji Foundation’s Re-search Grant and the Yazaki Memorial Foundation for Science and Technology.

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