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1、Electrochimica Acta 54(2009)46554661Contents lists available at ScienceDirectElectrochimica Actajournal homepage: study of LiCo1zAlzO2prepared by solid-stateand co-precipitation methodsWenbin Luoa,b,J.R.Dahna,aDepartment of Physics and Atmospheric Science,Dalhousie University,Halifax B3H3J5,CanadabS
2、chool of Metallurgical Science and Engineering,Central South University,Changsha 410083,PR Chinaa r t i c l ei n f oArticle history:Received 24 December 2008Received in revised form 22 March 2009Accepted 24 March 2009Available online 5 April 2009Keywords:Aluminum substituted LiCoO2Lithium-ion batter
3、iesAluminum substituted positive electrodematerialsCo-precipitationSolid-state synthesisa b s t r a c tLiCo1zAlzO2(0z0.5)samples were prepared by co-precipitation and solid-state methods.Thelattice constants varied smoothly with z for the co-precipitated samples but deviated for the solid-state samp
4、les above z=0.2.The solid-state method may not produce materials with a uniform cationdistribution when the aluminum content is large or when the duration of heating is too brief.Non-stoichiometric LixCo0.9Al0.1O2samples were synthesized by the co-precipitation method at variousnominal compositions
5、x=Li/(Co+Al)=0.95,1.0,1.1,1.2,1.3.XRD patterns of the LixCo0.9Al0.1O2sam-ples suggest the solid solution limit is between Li/(Co+Al)=1.1 and 1.2.Electrochemical studies of theLiCo1zAlzO2samples were used to measure the rate of capacity reduction with Al content,found tobe about 25030(mAh/g)/(z=1).Li
6、terature work on LiNi1/3Mn1/3Co1/3zAlzO2,LiNi1zAlzO2andLiMn2yAlyO4demonstrates the same rate of capacity reduction with Al/(Al+M)ratio.These studiesserve as baseline characterization of samples to be used to determine the impact of Al content on thethermal stability of delithiated LiCo1zAlzO2in elec
7、trolyte.2009 Elsevier Ltd.All rights reserved.1.IntroductionThe safety of lithium-ion batteries is central to their contin-ued success in the market place.LiCoO2-based Li-ion batteriesare presently dominant in cell-phone and computer applicationswhere the thermal instability of delithiated LiCoO2in
8、electrolytecan be managed due to the small size of the cells.Larger cellsoften incorporate less expensive and less reactive materials such asLiFePO4and LiMn2O4positive electrode materials.However thesehavesmallervolumetricenergydensitythanLiCoO2andothercom-mon layered positive electrode materials.Th
9、ere has recently beenresearchfocusingonimprovingthethermalstabilityoflayeredpos-itive electrode materials so they are more benign than LiMn2O4atonly a small penalty in energy density.This has been accomplishedthrough substitutions of Al in LiNi1/3Mn1/3Co1/3zAlzO21 andLiNi1/2zMn1/2zAl2zO22.Theimpacto
10、fAlsubstitutionsonthethermalstabil-ity of layered lithium transition metal oxides is well-known.For example,Ohzukus group showed that Al-substitution inLiNi1zAlzO2reduced the reactivity of the charged electrodematerial with electrolyte 3.Al substitutions have been used toCorresponding author.Tel.:+1
11、 902 494 2991;fax:+1 902 494 5191.E-mail address:jeff.dahndal.ca(J.R.Dahn).make LiNi0.8Co0.15Al0.05O2(NCA)with improved thermal sta-bility compared to the parent material without Al.Ceder et al.have reported that Al-substituted LiCoO2has a higher averagepotential versus Li than LiCoO2according to th
12、e first principlescalculation and that it has better thermal stability 4,5.In spiteof these well-known results,the mechanism for the thermal sta-bility improvement is not understood in detail.In addition,theimpact of aluminum on the kinetics of the reaction betweendelithiated positive electrode mate
13、rials and electrolyte at hightemperature is not known.These reaction kinetics are requiredfor models of the response of full Li-ion cells to abuse scenarios68.MacNeil et al.have made substantial studies of the kinetics ofthe reactions between delithiated LiCoO2and electrolyte at ele-vated temperatur
14、e 9,10.It is our goal to build on this earlierresearch by studying the impact of Al on the kinetics of reactionbetweendelithiatedLiCo1zAlzO2andelectrolyte.Insuchstudies,LiCo1zAlzO2wouldserveasamodelsystemandhopefullyresultslearned could be applied to other Al-substituted layered transitionmetal oxid
15、es.As the first step,a careful characterization of synthe-sis,structure and electrochemical properties of LiCo1zAlzO2isrequired.Scattered results can be found in the literature already,and will be referred to as appropriate below,but none is com-plete to the level we would like.In this paper,the co-
16、precipitationmethodandthetraditionalsolid-statemethodwereusedtoprepareLiCo1zAlzO2.We document the structure and electrochemical0013-4686/$see front matter 2009 Elsevier Ltd.All rights reserved.doi:10.1016/j.electacta.2009.03.0684656W.Luo,J.R.Dahn/Electrochimica Acta 54(2009)46554661performance of Li
17、Co1zAlzO2prepared by two methods and com-pare to literature results.2.Experimental2.1.Material preparation2.1.1.LiCo1zAlzO2samples prepared by the co-precipitationmethodA LiOHH2O(Sigma Aldrich,98%)solution and a mixed solutionof Co(NO3)26H2O(Sigma Aldrich,98%)and Al(NO3)39H2O(SigmaAldrich,98%)were s
18、imultaneously added over the course of about30mintoastirredflaskusingatwo-channelperistalticpump(Mas-terflex C/L pump,Barnant Co.).The concentrations of the solutionswere adjusted to set the Al:(Al+Co)ratio,z.The precipitate was fil-tered out and washed with distilled water several times to removean
19、y dissolved salts and dried at 80C overnight.The variation instructural properties and chemical composition of the precipitatewith Al content,z,have been reported in another publication 11.The dried precipitate was mixed with an appropriate amountof Li2CO3(Alfa Aesar,99%)and ground.The precursors we
20、reheated in air.Samples were heated at selected temperatures forselected times to be described below.Samples with 0z0.5in LiCo1zAlzO2were prepared.In addition,non-stoichiometricLixCo0.9Al0.1O2samples were synthesized at various nominalcompositions x=Li/(Co+Al)=0.95,1.0,1.1,1.2,1.3.2.1.2.LiCo1zAlzO2s
21、amples prepared by the solid-state methodThe starting materials for synthesis were Li2CO3(Alfa Aesar,99%),Co3O4(Alfa Aesar,99.7%)and Al(OH)3(Sigma Aldrich).Sto-ichiometric amounts of Co-and Al-containing starting materialsalong with an appropriate amount Li2CO3were mixed and groundtogetherusinganaut
22、omaticgrinder(RetschRM-0).Finallythesam-ples were heated at a selected temperature for a selected time inair.2.2.Material characterization2.2.1.X-ray diffractionXRD patterns were collected with a Siemens D5000 diffrac-tometer equipped with a Cu target x-ray tube and a diffractedbeammonochromator.LiC
23、o1zAlzO2samplesweremeasuredovera scattering angle range between 10and 90using 0.05steps anda 10s counting time.2.2.2.Electrochemical testingCoin cells(23mm diameter and 2.5mm thick)were used fortesting the electrochemical performance of the samples.Positiveelectrodes were prepared by mixing the acti
24、ve material,Super SCarbon Black,and PVDF in a weight ratio of 90:5:5.An appropriateamount of NMP was added to the mixture to form a slurry,whichwas then thoroughly mixed.The slurry was coated on Al foil anddried overnight in an oven.The electrochemical cells used a singlelithium metal foil as the co
25、unter electrode,Celgard 2320 microp-orousfilmastheseparatorand1MLiPF6inethylenecarbonate(EC)and diethyl carbonate(DEC)(1:2,v/v)as the electrolyte.Cells wereassembled in an argon-filled glove box.The cells were removedfrom the glove box and connected to a computer-controlled charg-ing system(E-One/Mo
26、li Energy).The cells were initially chargedand discharged at a C/20 rate between 3.3 and 4.3V versus Li metalfor two cycles.After the first two cycles,the chargedischarge ratewas increased to C/5.2.2.3.SEM testingA Hitachi S4700 field-emission scanning electron microscope(SEM)was used to image the m
27、aterials.Fig.1.XRD patterns of LiCo1zAlzO2(0z0.5)synthesized by both the solid-state method and the co-precipitation method at 900C for 3h.The patterns of the solid-statesamples are in the left column and the patterns of the co-precipitation samples are in the right column.Three Bragg peaks are inde
28、xed based on the LiCoO2structure in thetop panels.W.Luo,J.R.Dahn/Electrochimica Acta 54(2009)465546614657Fig.2.SEM images of LiCo0.7Al0.3O2synthesized by the solid-state method A,B and the co-precipitation method C,D.Fig.3.XRD patterns of LiCo0.8Al0.2O2synthesized by both the solid-state methodand t
29、he co-precipitation method.The XRD pattern shown in the bottom panel isfor a sample synthesized by the co-precipitation method and then heated at 750Cfor 3h.The remaining XRD patterns are for samples synthesized by the solid-statemethod at 750C with different heating times.3.Results and discussionFi
30、g.1 shows the XRD patterns(between 60and 75)ofLiCo1zAlzO2(0z0.5)synthesizedat900Cfor3hbybothsolid-stateandco-precipitationmethods.Allsamplesappearatfirsttobesingle phase,however the solid-state samples show substantiallybroader Bragg peaks for z0.2.The right panels of Fig.1 clearlyshow the smooth sh
31、ift of the Bragg peak positions versus z,sug-gesting the broad Bragg peaks for the solid-state samples in the leftpanels may be caused by non-uniform Al distribution.IncreasedBragg peak width can also be caused by smaller grain size,so SEMimageswereusedtoensurethattheparticlesizeswerecomparable.Fig.
32、2 shows scanning electron micrographs of two samples ofLiCo0.7Al0.3O2.Fig.2A and B show the morphology of the solid-state sample.Fig.2C and D show the morphology of the sampleFig.4.Lattice constants,a panel A and c panel B of LiCo1zAlzO2versus z.Datafor samples prepared by the co-precipitation metho
33、d(750C,20h and 900C,3h)and by the solid-state method(900C,3h)are included.4658W.Luo,J.R.Dahn/Electrochimica Acta 54(2009)46554661Fig.5.Lattice constants versus z for LiCo1zAlzO2samples prepared by the co-precipitation method(900C,3h)compared to previous literature reports.co-precipitation method(900
34、C;3h)are compared to the results from?ref.12,?ref.13,?ref.14,?ref.15,+ref.16,ref.17.made by the co-precipitation route.The sample synthesized bythe solid-state method shows crystallites with flat facets,havingprimary particle sizes around 2?m.The sample synthesized by co-precipitation route shows a
35、similar morphology,but has primaryparticles that are around 1?m.The solid-state sample has largerparticlesize,butbroaderBraggpeaks,suggestingthatnon-uniformcation distribution is the cause as was suggested above.Fig.3 shows the XRD patterns of LiCo0.8Al0.2O2synthesized byboth the solid-state method
36、and the co-precipitation method.Thesample in bottom panel was synthesized by the co-precipitationmethod at 750C for 3h.The sample is a single phase.The remain-ing panels in Fig.3 show samples synthesized by the solid-statemethod at 750C for different times(328h).Some Li2CO3peaksare observed in the X
37、RD patterns of LiCo0.8Al0.2O2synthesized bythe solid-state method and heated at 750C for 3h,6h,12h andFig.7.Lattice constants of LixCo0.9Al0.1O2(x=0.95,1.0,1.1,1.2 and 1.3)synthesizedby the co-precipitation method(900C,3h).16h.A pure phase of LiCo0.8Al0.2O2is only obtained by heating20h at 750C for
38、the solid-state method.Based on the results inFig.3,it can be concluded that shorter heating times are requiredfor samples prepared by the co-precipitation method,as expected.Fig.4 shows the lattice constants(a and c)as a function of z inLiCo1zAlzO2for samples prepared by the solid-state(900C,3h)and
39、 co-precipitation methods(both 900C,3h and 750C,20h).Thelatticeconstantsweredeterminedbyleastsquaresrefinementsto the measured positions of at least 10 Bragg peaks for each sam-ple.The lattice constant a decreases and c increases as z increases.The lattice constants vary smoothly with z for the co-p
40、recipitatedsamples but deviate for the solid-state samples above z=0.15.Thisagainsuggeststhatthesolid-statemethodmaynotproducemateri-als with a uniform cation distribution when the aluminum contentis large,consistent with the results in Fig.1.Fig.5showsthevariationofthelatticeconstantsofLiCo1zAlzO2(
41、co-precipitation method,900C,3h)compared toFig.6.XRD patterns of LixCo0.9Al0.1O2(x=0.95,1.0,1.1,1.2 and 1.3)synthesized by the co-precipitation method(900C,3h)Two regions of the diffraction patterns areshown.The left panels show the presence of Bragg peaks from Li2CO3in the x=1.2 and 1.3 patterns.Th
42、e right panels show that the 018,110 and 111 Bragg peaks are crystallinefor all samples.W.Luo,J.R.Dahn/Electrochimica Acta 54(2009)465546614659Fig.8.The potential(V)versus specific capacity(mAh/g)of Li/LiCo1zAlzO2cells cycled between 3.3 and 4.3V at a rate of C/20.Samples prepared by the solid-state
43、 method(900C,3h)are shown in the left columns and samples prepared by the co-precipitation method(900C,3h)are shown in the right panels.literature results 1217.The literature results are for samplesprepared by a variety of methods,including the solgel method,the emulsion drying method,the co-precipi
44、tation method,thecitrate precursor method and the solid-state method.In general,methods(the solgel method,the emulsion drying method,the co-precipitationmethodandthecitrateprecursormethod)thatensuregood Co-Al cation mixing in the precursor yield results in goodagreement with our work.By contrast,the
45、 data set()in Fig.5that is in poor agreement is for samples prepared by solid-statemethods,which may not lead to good cation mixing.Fig.9.The differential capacity(dQ/dV)versus potential(V)of Li/LiCo1zAlzO2cells cycled between 3.3 and 4.3V at a rate of C/20.Samples prepared by the solid-statemethod(
46、900C,3h)are shown in the left columns and samples prepared by the co-precipitation method(900C,3h)are shown in the right panels.4660W.Luo,J.R.Dahn/Electrochimica Acta 54(2009)46554661Fig.6 shows the XRD patterns of LixCo0.9Al0.1O2(x=0.95,1.0,1.1,1.2,1.3)synthesizedat900Cfor3hbyco-precipitationmethod
47、.The value x in the figure shows the nominal Li/(Co+Al)molar ratio.The left panels in Fig.6 show the portion of the XRD pattern wherethe Li2CO3impurity appears for the x=1.2 and x=1.3 samples.Theright panels show that the XRD peaks remain sharp with well-resolved K?doublets in all cases.The results
48、in Fig.6 suggest thatsingle phase samples can be prepared under these conditions for0.95x1.1.Delmas et al.18 showed that a similar single phaseregion was possible for LixCoO2samples synthesized at 900C.Thus,the addition of Al does not strongly impact the single phasenon-stoichiometric range.Fig.7 sh
49、ows the lattice constants,a and c,plotted versus x,themolar ratio Li/(Co+Al).The lattice constants basically do not varywith x.This is in agreement with the findings of Delmas et al.forLixCoO2samples See Table 1 in reference 18.Fig.8 shows the first charge/discharge cycling curves for the cellshavin
50、g LiCo1zAlzO2electrodes.The results from electrodes madefrom solid-state samples(900C,3h)are in the left column whilethose from co-precipitated samples(900C,3h)are in the rightcolumn.The reversible capacity of LiCo1zAlzO2decreases withthe increasing Al content.As is well-known 5 this is because Al3+