LiAlxCo1xO2 as 4 V cathodes for lithium ion batteries

Ž.Journal of Power Sources81–821999690–695 r locate r jpowsourLiAl Co O as4V cathodes for lithium ion batteriesx1y x2Haitao Huang a,),G.V.Subba Rao a,B.V.R.Chowdari a,ba Institute of Materials Research and Engineering,Blk.S7,LeÕel3,National UniÕersity of Singapore,Singapore119260,Singaporeb Department of Physics,National UniÕersity of Singapore,Singapore119260,SingaporeAbstractŽ.Nominal LiAl Co O with x ranging from0.1to0.3was prepared by heating mixture of Al OH,Co O and LiOH at7508C in x1y x2334air.The effect of substitution of non-transition metal,Al,in LiCoO is investigated as a4V cathode for lithium ion.X-ray diffraction2q Ž.Ž.XRD indicates formation of a single phase R3m within this range of substitution.When cycled between4.5and2.5V vs.Li r Li at acurrent density of1mA cm y2,the LiAl Co O cathode exhibits reversible capacity of160mA h g y1initially.XRD of the cathode0.150.852made at the end of10cycles reveals no significant change on host structure.q1999Elsevier Science S.A.All rights reserved.Keywords:Lithium ion batteries;Cathodes;Aluminium;Cobalt1.IntroductionLiCoO is a well established4V cathode material with 2respect to synthesis and performance,and has been used inw x commercial lithium ion batteries since19901,2.How-ever,due to the high costs of cobalt,it is now facing serious competition from other materials,such as LiMn O24 w x w xŽ3–7,LiNiO8–13and LiCo Ni O or more gener-20.50.52.w xally LiCo Ni O,0F x F113–17.These alterna-x1y x2tives are considered to be attractive because of the fact thatŽthey are cheap and r or they provide higher capacity Ni .oxides.Obviously,partial replacement of Co in LiCoO2 by other less expensive metals is of interest to yield a material which is cheaper than LiCoO,but with many of2its advantages.In addition,substituted LiCoO may pos-2sess high reversible capacity.LiNi Co O is an exam-0.50.52y1w x ple;its reversible capacity is over150mA h g17,in excess of the value of136mA h g y1which corresponds to 0.5Li per formula being involved on intercalation r dein-tercalation.This is of great significance because reversible lithium insertion r extraction in Li CoO is limited to0.5y2w xF y F113,18,19.Aluminium is a non-transition metal, and has been used as dopant to modify intercalation com-w xpounds such as LiNiO20–22.However,large amount2of aluminium substitution in LiCoO,to our knowledge,2w xhas been less investigated.Recently Ceder et al.23have Ž.explored Li AlCo O as cathode material based on first-2)Corresponding author:Tel.:q65-874-2607;Fax:q65-872-0785; E-mail:ht-huang@.sg principles calculation.In this paper,we investigate Alsubstituted LiCoO by considering the facts that aluminum2is a lighter element and its compounds are relatively cheap and less toxic.Ž.Since a-LiAlO is isostructural with LiCoO R3m22w x3q3q20and the ionic radii of Al and Co are very close˚Ž0.54and0.55A,respectively,with coordination number .of six,it is likely that a wide range of LiAl Co Ox1y x2 solid solutions exist.Sufficiently high Al content is essen-tial so as to cut down the cost of the cobalt material.As an inactive element,however,the content of Al should be not too high to compromise capacity.Therefore,we focused on substitution of Co with Al from10to30at.%.Theelectrochemical performance of LiAl Co O was evalu-x1y x2ated using galvanostatic charge and discharge.X-ray Ž.diffraction XRD studies were carried out to monitor structural change during lithium extraction and on cycling so as to explore long cycling use in the view of structural stability.2.Experimental2.1.SynthesisStarting materials for preparing LiAl Co O werex1y x2Ž.Ž. LiOHH O Fluka,)99.5%,or Li CO Merck,)99%, 223Ž.Ž.Ž.Co O Fluka,G71%Co and Al OH Merck,or 343Ž.Ž.Al NO6H O Fluka,)98%.Stoichiometric amounts 332of cobalt and aluminium compounds along with3at.%of0378-7753r99r$-see front matter q1999Elsevier Science S.A.All rights reserved.Ž.PII:S0378-77539900249-9()H.Huang et al.r Journal of Power Sources81–821999690–695691lithium compound in excess,to compensate for any loss of lithium that may occur during the firing,were mixed using an agate mortar and pestle.The mixtures were heated in air,first at5508C for20h and then at7508C for24h,with two intermittent grindings.2.2.Cells and electrodesŽ2.Composite cathodes area:0.6–0.5cm,containing76Žwt.%of active material,12wt.%of carbon Super P,.Ž. MMM,Sedema,Belgium and12wt.%of PTFE Aldrich binder,were fabricated using dry mixing method and pressed on Al gauze under2tons.This type of cathodes was applied in three electrode cells for cycling test.Counter electrode and reference one were lithium metal.Another type of composite cathodes with an area of2cm2was prepared using wet coating method.Active material,car-Ž.Žbon Super P,MMM and KYNAR polymer binder ELF-.Atochem America were,in80:10:10by weight,wellŽ. mixed in1-methyl-2-pyrrolidinone solvent Aldrich,and the resulting slurry was coated on Al foil.After drying under heating,half a ton was applied to press these composite cathodes.Two electrode cells consisting of the large size composite electrode and a lithium metal elec-trode were constructed.After electrochemical experiments, the cathodes were taken off for XRD measurements.Bat-Ž.tery-grade electrolyte Merck Germany,1molar LiPF6Ž. dissolved in mixture of ethylene carbonate EC and dieth-Ž.Ž.ylene carbonate DEC1:1by volume,was used,andŽ.Celgard2400polypropylene was soaked with the elec-trolyte and used as separator between electrodes.2.3.MeasurementsXRD measurements were undertaken by Philips X-raydiffractometer with Cu K radiation.The cathodes afteraelectrochemical tests were placed on a piece of glass and covered with Mylar film by tape sealing to exclude expo-sure to air during X-ray measurements.Solartron1286Ž. Electrochemical Interface voltage resolution:0.1mV was used to carry out electrochemical experiments.Specific capacities were based on the weight of LiAl Co O.x1y x2 3.Results and discussion3.1.Synthesis and structural characterisation ofLiAl Co Ox1y x2Ž.a-LiAlO,which has a-NaFeO structure R3m,was 22Žoriginally prepared by solid state reaction of a-Al O or23Ž..w xAlO OH and Li CO at6008C24.Recently Ohzuku et23w xal.20reported to prepare this phase by heating mixture Ž.Ž.of Al OH and LiNO or Li CO at7508C in air.We 3323Ž. used LiOH instead of LiNO and combined Al OH with33Ž.Al NO,and also obtained a single phase of a-LiAlO.332Ž.Thus,LiOH,Co O and Al OH plus a small amount of343Ž.Al NO were mixed and heated in air first at5508C for 3320h and then at7508C for24h.Fig.1presents the XRDpatterns of LiAl Co O with x ranging from0.1to0.3.x1y x2Miller indexes are given assuming a hexagonal lattice.It isevident that a single phase of LiAl Co O can bex1y x2 prepared by this method and the replacement of cobalt with aluminium up to30at.%does not change LiCoO2 phase except for somewhat broadening in the peaks at high Ž.angles)508in2u.The phenomenon of peak broaden-ing is not so significant compared with the case of solidw xsolution of LiAl Ni O20.By a least squares method,x1y x2lattice parameters,a and c,are calculated and plotted against aluminium contents in Fig.2,in which thevaluesŽ.Ž.Ž. Fig.1.XRD patterns for LiAl Co O:a x s0,b x s0.1,cx1y x2Ž.Ž.Ž.x s0.15,d x s0.2,e x s0.25and f x s0.3.LiAl Co O werex1y x2Ž. prepared by heating the mixture of LiOH P H O,Co O and Al OH at2343 7508C except for x s0,which is commercial LiCoO from Merck.2()H.Huang et al.r Journal of Power Sources 81–821999690–695692Fig.2.Hexagonal unit cell parameters,a and c ,as a function of x in LiAl Co O .The parameters were determined by a least squares x 1y x 2method using the peaks in Fig.1.of lattice parameters for LiCoO and LiAlO are included 22for comparison.It can be seen from Fig.2that the valuesof a -axis and c -axis of LiAl Co O with 0.1F x F 0.3x 1y x 2situate between those of LiCoO and LiAlO .This is 22characteristic feature of solid solution.The a -axis varies slightly with x ,but the c -axis starts to increase more significantly at x )0.2.It is worth pointing out that the 003peak remains higher than the 104peak for LiAl Co O with 0.1F x F 0.3.The ratio of 003r 104,x 1y x 2as has been acknowledged to be an indication of cation mixing,is around 1.25,except for the case at x s 0.3.This may suggest that the degree of cation mixing is negligible at least at x -0.3.3.2.Re Õersible capacity of LiAl Co O x 1y x 2Fig.3compares the first charge and discharge curves of LiAl Co O cathode with those of LiCoO one,the 0.250.7522former cycled between 4.3and 2.5V vs.Li r Li q and the latter between 4.2and 2.5V vs.Li r Li q .It is evident that Al substitution does not change the potential profiles of intercalation r deintercalation in LiCoO significantly,but 2intercalation potential rises with the presence of Al in host,w x being consistent with the report by Ceder et al.23.As a result,capacity relatively decreases at a given charge voltage.To measure capacities,LiAl Co O cathodes x 1y x 2Ž.were first cycled between 4.3or 4.2V and 2.5V vs.Li r Li q for two times and then charge voltages raised to 4.5V vs.Li r Li q in the subsequent cycles.The charge and discharge currents were 1mA cm y 2.Fig.4presents the reversible capacity of LiAl Co O cathodes.For com-x 1y x 2parison,results from a cathode prepared from commer-Ž.cially available LiCoO Merck are also included in Fig.24.This LiCoO cathode was first cycled between 4.2and 22.5V for two times and then between 4.4and 2.5V for further two times.By charging to 4.2V,LiCoO cathode 2gives nearly 120mA h g y 1of discharge capacity,whileŽ.Ž.Fig.3.The first charge and discharge curves of LiAl Co O solid line and LiCoO dash line cathodes.Charge and discharge currents were 1mA 0.250.7522cm y 2.()H.Huang et al.r Journal of Power Sources81–821999690–695693Fig.4.Specific discharge capacities as a function of x in LiAl Co O:v the first cycle,I the second cycle,`the third cycle and'the fourthx1y x2y2ŽU.cycle.Charge and discharge currents were1mA cm.Cut-off voltages were4.3V or4.2V with marks and2.5V in the first two cycles,and4.5V ŽUU.or4.4V,with mark and2.5V in the third and fourth cycles.The curve with=marks refers to charge efficiency in the first cycle.the capacity of LiAl Co O is115mA h g y1.How-0.10.92ever,other LiAl Co O compositions have capacity lessx1y x2than110mA h g y1even under charge voltages of4.3V.As a contrast,more capacities were obtained by charging to4.5V vs.Li r Li q in the third and fourth cycles.Eventhough the capacity of LiAl Co O is still lower than0.250.752100mA h g y1,we noted that cathodes with x s0.1,0.15and0.2in LiAl Co O,exhibit153,139and120mA hx1y x2g y1of capacities,respectively,in comparison to151mA h y1Žg of capacity obtained from LiCoO charge limit is4.42.V.Charge efficiencies in the first cycles for LiAl Co Ox1y x2Žand LiCoO cathodes are also give in Fig.4marked with 2.crosses.LiCoO exhibits charge efficiency as high as292%in the first cycle,while all LiAl Co O cathodesx1y x2give values less than90%.The charge capacities lose onŽ.Žthe first discharge from15%at x s0.1to34%at .x s0.25.However,charge efficiencies do rise over96%in subsequent cycles for all Al contents under test.The nature of the capacity loss in the first discharge is not clear at the present stage.It is worth pointing out that the potential profiles of charge and discharge curves vary less in the first cycle and subsequent ones.Hence,we expectthat LiAl Co O should not have any major change in x1y x2chemical composition and crystal structure after the firstcycle.The X-ray data measured for LiAl Co O in-0.150.852Fig.5.Variation of specific capacity as a function of cycle number for LiAl Co O cathodes cycled between4.5and2.5V,and4.4and2.5V.0.150.852Charge and discharge currents were1mA cm y2.()H.Huang et al.r Journal of Power Sources 81–821999690–695694deed do not show major structure change after one cycle,as will be discussed later.3.3.LiAl Co O 0.150.852As indicated in Fig.4,the composition LiAl Co O 0.150.852is best from the point of a reasonable capacity associated with an essentially high Al content.Therefore,this compo-sition was investigated further in the following.Under the Žy 2.same current density as the above 1mA cm ,two cathodes of LiAl Co O were charged and discharged 0.150.852between 4.4and 2.5V,or 4.5and 2.5V,respectively,and the results are presented in Fig.5.Again,it is evident that high reversible capacity is obtainable with highchargeFig.6.XRD patterns of LiAl Co O after being charged by remov-0.150.852Ž.Ž.Ž.Ž.Ž.y 1ing i 0,ii 35,iii 135,iv 175and v 195mA h g of charge.Charge currents were 70m A cm y 2.The bars in the figure mark the Žpositions of the peaks from Al substrate of which the peak at 65.238in .2u is the most intensive .The very broad background peak at low angle Ž.-308in 2u arises from the tape used to seal the samples fromatmosphere.Fig.7.XRD patterns of LiAl Co O measured at the end of0.150.852˚˚Ž.Ž.discharge after i 10cycles hexagonal;a s 2.81A,c s 14.08A ,and ˚˚Ž.Ž.Ž.ii one cycle hexagonal;a s 2.82A,c s 14.11A .The pattern in iii ˚˚Ž.Žwas from a fresh cathode hexagonal;a s 2.82A,c s 14.10A .See .comments in caption to Fig.6.voltage.And,no significant influence of charge voltage on capacity retention is observed within the first 10cycles.Corresponding to charge voltages of 4.3,4.4and 4.5V vs.Li r Li q ,the first charge capacities were 130,162and 194mA h g y 1,while the charge efficiencies in the first cycle varies little,falling in the range of 82–83%.To monitor structure during deintercalation,several cathodes Ž2of LiAl Co O with an area of 2cm ,see Section 20.150.852.for more details were charged at a current density of 70m A cm y 2by removing 35,135,175and 195mA h g y 1of charge,respectively.These charges respectively corre-spond to the values of y in Li Al Co O to be 1y y 0.150.8520.12,0.47,0.61and 0.68.After standing on open circuit for 10h,these cathodes were taken out of cells and left for further equilibrium for at least three days before being subjected to XRD measurements.The resulting XRD pat-terns are presented in Fig.6.It has been reported that LiCoO structure changes from hexagonal to monoclinic 2Žphase over the range 0.44F y F 0.49in Li CoO corre-1y y 2y 1.sponding to the capacities from 120to 135mA h g()H.Huang et al.r Journal of Power Sources81–821999690–695695w x25–27.In addition,there exist two hexagonal phases atŽy1 the beginning of charging y-0.25,i.e.,-70mA h g.w xof charge capacity14,25.By contrast,the patterns inFig.6remain unchanged during lithium extraction al-though the peaks from Al substrate exist,which overlapthe006,104and108peaks of LiAl Co O.There are0.150.852no peaks splitting which are a sign of monoclinic deforma-tion or two phase coexistence.For example,the104peak of hexagonal phase splits when monoclinic deformation w xappears25.Such a situation is very similar to the cases ofŽ.w xNi substituted in LiCoO LiNi Co O14and Al20.50.52Ž.w x substituted in LiNiO LiAl Ni O20,which are20.250.752reported to have no observation of monoclinic phase.In this point of no phase transformation,15%of Al substitu-tion may enhance LiCoO structure stability.2To examine structure on cycling,two LiAl Co O0.150.852Ž2.cathodes with an area of2cm were charged and discharged at a current of70m A cm y2in two electrode cells.One was charged by removing175mA h g y1ofqŽcharge and then discharged to2.5V vs.Li r Li giving y1.153mA h g of capacity.The other was subjected to10 times of charge and discharge.In the first cycle,charge capacity was also limited to175mA h g y1followed by discharging to2.5V.But in the subsequent cycles,the charge capacity was controlled to be same as that in the previous discharge process and discharge was always stopped at2.5V vs.Li r Li q.The discharge capacity was 153mA h g y1initially and135mA h g y1at the end of10 cycles.These two cycled-cathodes were subjected to XRD measurements,and the results are shown in Fig.7in which the result from an uncycled one is also included for comparison.It is clear that structure is highly recoverable. This is very encouraging for this Al-substituted material to be investigated for long term cycling use in future.4.ConclusionsSolid solution LiAl Co O with0.1F x F0.3can bex1y x2Ž.ŽŽ.. prepared by heating the mixture of Al OH or Al NO,333 Co O and LiOH in air at7508C.Intercalation potential 34increases as a result of the substitution of Al in LiCoO,2 and so higher charge voltage,e.g.,4.5V vs.Li r Li q,is of necessity to be applied.Initial capacity of160mA h g y1 can be obtained from the composition of LiAl Co O0.150.752 when cycled at a current of1mA cm y2and between4.5 and2.5V vs.Li r Li q.XRD data do not show monoclinic phase as well as two phase co-existence which occur in LiCoO.Structure of LiAl Co O retains hexagonal20.150.852symmetry after10cycles.These structural features are very encouraging for this material to be investigated fur-ther for possible practical use.AcknowledgementsThe authors thank Dr.A.Yu for useful discussions and Dr.H.Lindner for a critical reading of the manuscript. Thanks are also due to MMM and ELF-Atochem America for kindly supplying Super P carbon and KYNAR polymer samples.Referencesw x1T.Nagaura,Proc.of the5th Intl.Seminar on Lithium Battery Technol.and Applid.,Deerfield Beach,FL,March5–7,1990.w xŽ.2P.G.Bruce,mun.1919971817.w xŽ.3 D.Guyomard,J.M.Tarascon,Solid State Ionics691994222.w xŽ.4M.M.Thackeray,Prog.Solid State Chem.251997 1.w x5V.Manev, B.Banov, A.Momchilov, A.Nassalevska,J.Power Ž.Sources57199599.w xŽ.6H.Huang,P.G.Bruce,J.Power Sources54199552.w xŽ.7Y.Xia,M.Yoshio,J.Electrochem.Soc.1431996825.w x8 C.Delmas,J.P.Peres, A.Rougier, A.Demourgues, F.Weill, A.´`Chadwick,M.Broussely,F.Perton,Ph.Biensan,P.Willmann,J.Ž.Power Sources681997120.w x9 A.Rougier,P.Gravereau,C.Delmas,J.Electrochem.Soc.143Ž.19961168.w x10J.R.Dahn, E.W.Fuller,M.Obrovac,U.Von Sacken,Solid State Ž.Ionics691994265.w xŽ. 11W.Li,J.C.Currie,J.Wolstenholme,J.Power Sources681997 565.w x12T.Ohzuku, A.Ueda,M.Nagayama,J.Electrochem.Soc.140Ž.19931862.w x13T.Ohzuku, A.Ueda,M.Nagayama,Y.Iwakoshi,H.Komori,Ž.Electrochim.Acta3819931159.w xŽ.14 A.Ueda,T.Ohzuku,J.Electrochem.Soc.14119942010.w xŽ.15I.Saadoune,C.Delmas,J.Mater.Chem.61996193.w xŽ.16W.Li,J.C.Currie,J.Electrochem.Soc.14419972773.w x17U.Heider,R.Oesten,L.Heider,M.Niemann,A.Amann,N.Lotz, 15th Intl.Seminar and Exhibition on Prim.and Secondary Batteries, FL,USA,March2–5,1998.w xŽ.18K.Ozawa,Solid State Ionics691994212.w xŽ.19T.Ohzuku,A.Ueda,Solid State Ionics691994201.w xŽ. 20T.Ohzuku,A.Ueda,M.Kouguchi,J.Electrochem.Soc.1421995 4033.w x21T.Ohzuku,T.Yanagawa,M.Kouguchi,A.Ueda,J.Power Sources Ž.681997131.w xŽ.22Q.Zhong,U.von Sacken,J.Power Sources541995221.w x23G.Ceder,Y.-M.Chiang,D.R.Sadoway,M.K.Aydinol,Y.-I.Jang,Ž.B.Huang,Nature3921998694.w xŽ.24K.R.Poeppelmeier,D.O.Kipp,Inorg.Chem.271988766.w xŽ.25T.Ohzuku,A.Ueda,J.Electrochem.Soc.14119942972.w xŽ.26J.N.Reimers,J.R.Dahn,J.Electrochem.Soc.139********.w x27G.G.Amatucci,J.M.Tarascon,L.C.Klein,J.Electrochem.Soc.143Ž.19961114.。