Electrochemical performance of Al–Si–graphite composite as anode for lithium–ion batteries

Electrochemical performance of Al –Si –graphite composite as anode for lithium –ion batteriesWenchao Zhou,Shailesh Upreti,M.Stanley Whittingham ⁎Institute for Materials Research,State University of New York at Binghamton,Binghamton,NY 13902-6000,USAa b s t r a c ta r t i c l e i n f o Article history:Received 16November 2010Received in revised form 2December 2010Accepted 3December 2010Available online 13December 2010Keywords:Lithium battery Anode AluminumAl –Si –graphite composite with low 7.9wt.%Si was synthesized by ball-milling eutectic Al –Si powder and graphite.Its reversible capacity remained above 0.65Ah/g after 10cycles,corresponding to 1.6Ah/cm 3,almost double that of graphite alone.Ex-situ XRD indicates that in Al –graphite the large volume change during lithium insertion/removal causes the active material to become isolated and not available for electrochemical reactions,while in Al –Si –graphite all the material remains available.©2010Elsevier B.V.All rights reserved.1.IntroductionThere is a need to replace the carbon anodes used in lithium batteries to increase their energy density and safety [1,2].Metal based anodes (Si,Sn,Sb,Al,etc.)are believed to be the most promising replacement candidates due to their high capacity and desirable working potential [3–6].However,the large volume change while alloying/dealloying with lithium causes quick capacity decay during cycling.Among those metals,Sn reacts with 4.4lithium,with a volume expansion of more than 400%,while Al reacts with one lithium atom with ~100%volume increase.However the gravimetric capacity of Al (0.99Ah/g)is comparable to Sn (0.99Ah/g)[7].In the past decade Si and Sn based alloys/composites have been intensively studied,while less is understood about Al based material as anode for lithium batteries.The Li –Al alloy was used in the first lithium rechargeable batteries in the 1970s [8,9]to minimize lithium dendrite formation in ether-based electrolytes.Al powder [10],Al thin films [11–14]and Al –Fe alloy/composites [15,16]in carbonate electrolytes lose most of their initial capacity in a few cycles.The cycling can be improved if Al particles are dispersed in a carbon matrix but capacity decay still cannot be avoided [15].Here we report that a small amount of Si (b 8%by wt.)enables the full utilization of Al and also helps the Al-based composite maintain good capacity during cycling.2.ExperimentalAl –Si –graphite (ASG)and Al –graphite (AG)samples were synthe-sized via ball-milling.eutectic Al –Si (−325mesh,99%,Al:Si=88:12wt.%,VWR),Al (−325mesh,99.5%,VWR)and graphite (b 20μm,synthetic,Aldrich)powders were used as received.The weight ratio of the metal powder to graphite was 2:1,so that the ASG composite contains 58.1%Al,7.9%Si and 34%carbon.Ball-milling was performed in a planetary machine at 450rpm for 1h.A Scintag XDS2000diffractometer was used for structural studies,and Kapton tape was used to protect cycled anodes from air,and a ZeissSupra-55SEM for morphology studies.2325-type coin cells,with a cathode composite,a lithium anode,a Celgard 3501separator,and a 1M solution of LiPF 6in a 1:1volume mixture of ethylene carbonate and dimethyl carbonate (Novolyte)were tested on a VMP2multi-channel potentiostat (Biologic)at 0.5mA/cm 2.The composites of the active material,carbon black (Cities Service Company)and sodium carboxymethyl cellulose (CMC,VWR)in a weight ratio of 80:10:10were cast on copper foil,using an ethanol:water slurry (30:70volume ratio)and dried at 90°C in air for 12h.Typical active material loading was about 4.5mg.Cyclic voltammetric (CV)was scanned at a rate of 0.1mV/s between 0.01V and 1.5V.3.Results and discussionBulk Al and Si are reported to be immiscible at room temperature [17];however,when nanosized they can form a solid solution with the lattice parameter obeying Vegard's law [18].The lattice parameter of the Al –rich phase used here was found to be 4.043(1)Å,slightly less than the 4.049(1)Åof the aluminum powder,indicating around 2at.%of silicon in aluminum.X-ray diffraction showed no new phases beingElectrochemistry Communications 13(2011)158–161⁎Corresponding author.Tel./fax:+16077774623.E-mail address:stanwhit@ (M.S.Whittingham).1388-2481/$–see front matter ©2010Elsevier B.V.All rights reserved.doi:10.1016/j.elecom.2010.12.001Contents lists available at ScienceDirectElectrochemistry Communicationsj o u r n a l h o m e p a g e :w ww.e l s ev i e r.c o m /l o c a t e /e l e c o mformed during the ball-milling process.For the ASG and AG samples,the main diffraction peaks can be indexed to Al,Si or graphite;however,two small peaks at 2θ=36.8and 42.7°,also found in the initial Al and Al –Si powders are believed to be due to the surface oxidation of aluminum.The crystallite sizes in the Al –Si starting material,determined using the Scherrer equation are 46and 20nm for the aluminum and silicon respectively,and 35nm for the aluminum in the ASG composite.Fig.1shows the SEM figures of the raw metal powders and the synthesized composites.The aluminum powder had an irregular shape and the Al –Si contained spheres with diameters ranging from 1to 8μm.In Al –graphite,Al and graphite were easily blended,due to their softness and ductility.However,the eutectic Al –Si has a totally different hardness and mechanical property from Al.Eutectic Al –Si is used to make engine blocks due to the high fatigue strength and good wear resistance [17,19].In the ASG some of the spheres of the AS could still be found,and the local microstructure of the Al –Si is maintained and simply buried in the carbon matrix after ball-milling.The electrochemical activity of the ASG composite was clearly evidenced by CV scan (Fig.2).The peak below 0.25V is associated with the lithiation of the active material [14].In the reversed scan another peak at around 0.6V indicates the delithiation process.Both peaks become sharp in the 5th scan,suggesting that the reaction of the ASG material toward lithium is highly reversible.The redox peaks are in good agreement with the galvanostatic plot (Fig.4(A)),where the discharge and charge plateaus can be clearly seen.The cycling of the ASG electrode is shown in Fig.3.In the full cycling window (0.01V –1.5V)the lithium removal capacity (charge)attains 0.70Ah/g.After 10cycles it is still 0.65Ah/g.When the cut-off voltage was set to 0.1V and 0.15V,restricting the material utilization the initial charge capacity drops to 0.67Ah/g and 0.52Ah/g.To our knowledge these values are the best for Al based bulk anode material reported so far.The cycling performances of ASG and AG electrode are compared in Fig.3.The cut-off voltages were 0.01V –pared with the AG electrode,ASG delivers 50%more capacity.Moreover,the capacity retention and coulombic ef ficiency are improved.The electrochemicalperformances of both electrodes are summarized in Table 1.After 10cycles,ASG retains 98.0%of its initial capacity (0.65Ah/g)while AG keeps only 73.2%(0.31Ah/g).In order to investigate the impact of Si in the ASG electrode,ex-situ XRD was carried out on both the ASG and the AG electrodes (Fig.4).During the first lithium reaction with the ASG electrode (Fig.4Ba –c),the Si peak is not seen either because it has broadened or because the silicon became amorphous as reported earlier [20];the graphite peak shifted to a lower angle indicative of lithium intercalation,and the aluminum formed the LiAl alloy [6].During the charging process lithium removal began before the voltage plateau (0.5V)was reached (Fig.4Bd).After 10cycles no LiAl alloy was found when the electrode was fully charged (Fig.4Bf).However,for the AG electrode where even after being fully charged in the 10th cycle,LiAl was still present (Fig.4D).This suggests that part of the reacted anode material was no longer available for electrochemical reaction.These studies show that a small amount of Si not only contributes to the reversible capacity but also maintains good contact between the Al particles,possibly enabling a more stable SEIlayer.Fig.1.SEM images of (A)Al powder,(B)Al –Si powder,(C)AG composite,and (D)ASGcomposite.Fig.2.CV scan of the ASG electrode.Scan rate:0.1mV/s.159W.Zhou et al./Electrochemistry Communications 13(2011)158–1614.ConclusionsAddition of 7.9%silicon to an aluminum anode not only contributes to the total capacity but also improves the capacity retention,as the result of a partial solubility of silicon in aluminum.In this composite Si appears to form a superior surface layer in carbonate electrolytes that reduces the degradation of the bulk material.AcknowledgementThis work was supported by the U.S.DOE,EERE,under Contract No.DE-AC02-05CH11231,BATT Program subcontract #6807148.References[1]M.S.Whittingham,Mater.Res.Bull.33(4)(2008)411.[2]J.B.Goodenough,Y.Kim,Chem.Mater.22(3)(2009)587.[3]H.Li,X.Huang,L.Chen,Z.Wu,Y.Liang,Electrochem.Solid State Lett.2(11)(1999)547.[4]Q.Fan,P.J.Chupas,M.S.Whittingham,Electrochem.Solid-State 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508(2009)43–49.[19]S.P.Nikanorov,M.P.Volkov,V.N.Gurin,Y.A.Burenkov,L.I.Derkachenko,B.K.Kardashev,L.L.Regel,W.R.Wilcox,Mater.Sci.Eng.A 390(1–2)(2005)63.[20]L.Pimpa,J.Young-Il,J.D.Nancy,Y.M.Chiang,Acta Mater.51(4)(2003)1103.Table 1The composition and electrochemical performance of the AG and ASG electrodes.Percentage by weight Theoretical capacity a (Ah/g/Ah/cm 3)First discharge/charge capacity (Ah/g)First cycle ef ficiency Discharge capacity in the 10th cycle (Ah/g)AlSi Graphite AG 66%–34%0.78/1.940.560.4274.6%0.31ASG58.1%7.9%a34%0.98/2.410.820.6780.8%0.65aThe theoretical capacity was calculated based on the capacity of the individual elements:Al 0.99Ah/g,Si 3.5Ah/g,and graphite 0.37Ah/g.Fig.3.(A)Capacity cycling of the ASG electrode with different discharge cut-off voltages,and (B)electrochemical comparison of AG and ASG composites cycled between 0.01and 1.5V.In all cases the current density is 0.5mA/cm 2.160W.Zhou et al./Electrochemistry Communications 13(2011)158–161Fig.4.(A)First discharge –charge pro file of the ASG electrode.(B)Ex-situ XRD of the ASG electrode.(a)Pristine electrode,(b)discharge to 0.1V,(c)discharge to 0.01V,(d)charge to 0.45V,(e)charge to 0.15V,and (f)charge to 1.5V in the 10th cycle.(C)The first discharge –charge pro file of the AG electrode,and (D)the ex-situ XRD of the AG electrode in various discharge states:(a)Pristine electrode,(b)discharge to 0.01V,and (c)charge to 1.5V in the 10th cycle.161W.Zhou et al./Electrochemistry Communications 13(2011)158–161。