尖晶石镍锰酸锂的共沉淀合成

Electrochimica Acta52(2007)1919–1924Electrochemical characteristics of LiNi0.5Mn1.5O4preparedby spray drying and post-annealingDecheng Li a,Atsushi Ito b,Koichi Kobayakawa b,Hideyuki Noguchi c,Yuichi Sato b,∗a High-Tech Research Center,Kanagawa University,1-1-40Suehiromachi,Tsurumi-ku,Yokohama230-0045,Japanb Department of Applied Chemistry,Faculty of Engineering,Kanagawa University,3-27-1Rokkakubashi,Kanagawa-ku,Yokohama221-8686,Japanc Department of Applied Chemistry,Saga University,Honjo-1,Saga840-8502,JapanReceived28May2006;received in revised form23July2006;accepted29July2006Available online20September2006AbstractLiNi0.5Mn1.5O4was prepared by a spray drying and post-annealing process.The re-annealing treatment in O2could not only decrease the Mn3+ content,but also increased the reversible capacity and significantly improve the rate capability compared to the untreated material.Moreover, the cyclic performance of the LiNi0.5Mn1.5O4depends on both the cycling rate and operating temperature,which was ascribed to the difference between the phase transition rates between cubic I↔cubic II and cubic II↔cubic III.©2006Published by Elsevier Ltd.Keywords:Li-ion battery;Cathode material;LiNi0.5Mn1.5O4;Electrochemical properties;Spray drying and post-annealing1.IntroductionLiNi0.5Mn1.5O4has been extensively studied as a promis-ing cathode material for lithium ion batteries[1–12].Although it was initially proposed as a3V cathode materials[1],much attention has been paid to the high-voltage region near5V [2–10].Because of the difficulty in the preparation by the tra-ditional solid state reaction[2],LiNi0.5Mn1.5O4was usually obtained through a solution process,such as sol–gel[2,13], co-precipitation[6,9],emulsion drying[10],and a molten salt method[14]in order to obtain a product with high purity. LiNi0.5Mn1.5O4has a cubic structure,and the valences of Ni and Mn are determined to be divalent and tetravalent,respec-tively.The Ni2+would be oxidized to Ni4+via Ni3+during lithium ion extraction from the lattice,while the valence of Mn remains at the same value[7].It was found that the cations distribution in the lattice is sensitive to the preparation condi-tions.LiNi0.5Mn1.5O4prepared at a high annealing temperature (≥850◦C)adopts a cubic spinel structure with a high symmetry (space group Fd¯3m)[15],whereas it adopts a primitive sim-∗Corresponding author.Tel.:+81454815661x3885;fax:+81454139770.E-mail addresses:Decheng.Li@(D.Li),satouy01@kanagawa-u.ac.jp(Y.Sato).ple cubic structure with a low symmetry(space group P4332) if it was annealed at a low temperature(≤700◦C).Moreover, different processes of the phase transformations were exhib-ited during the lithium extraction/insertion from/into the lattice, thereby resulting in different electrochemical behavior[14–17].The theoretical capacity of the LiNi0.5Mn1.5O4is about 148mA h g−1,the same as that of normal LiMn2O4.However, it could provide a higher power density than that of normal LiMn2O4due to its high working voltage(about4.7V).In addi-tion to these factors,it also has the same merits of low cost and toxicity as LiMn2O4;thus it is expected to be used for electric vehicles(EVs)in the future.Both the rate capability and cyclic performance at different temperatures and/or different current densities are concerns for a power supply for the EVs.Never-theless,the studies on them are not yet sufficient.In this work,LiNi0.5Mn1.5O4with a low symmetry was pre-pared by means of a spray drying and post-annealing treatment. Its electrochemical properties were also characterized.2.ExperimentalPrecursors were prepared by a spray drying method,and LiOH·H2O,Ni(NO3)2·6H2O,and Mn(CH3COO)2·4H2O were selected as the starting materials.In our initial preparation,we dissolved all starting materials,LiOH·H2O,Ni(NO3)2·6H2O,0013-4686/$–see front matter©2006Published by Elsevier Ltd. doi:10.1016/j.electacta.2006.07.0561920 D.Li et al./Electrochimica Acta52(2007)1919–1924Fig.1.XRD pattern of LiNi0.5Mn1.5O4prepared by one-step process. and Mn(CH3COO)2·4H2O,into citric solution and carried out the spray dry process.However,impurities,NiO and/or Li x Ni1−x O,were found in thefinal product as illustrated in Fig.1.These impurities commonly appear during the prepa-ration of the LiNi0.5Mn1.5O4[2,18].As well-known,NiO impurity was usually observed in LiNi0.5Mn0.5O2,which had been ascribed to the preferential formation of Li2MnO3to LiNiO2[19].We suspected that the mechanism for the appear-ance of NiO and/or Li x Ni1−x O is similar to the case in the LiNi0.5Mn0.5O2.In order to avoid this problem,wefirstly pre-pared the precursors without Li addition.The precisely weighed Ni(NO3)2·6H2O,and Mn(CH3COO)2·4H2O were initially dis-solved into a0.2M citric solution(Ni:Mn:citric acid is1:3:4in molar ratio.)The resulting solution was pumped into a spray dry instrument(B¨u chi Mini Spray Dryer B-290).The obtained precursor was pre-heated at900◦C for20h in air.The resulted precursors are Ni–Mn oxides with spinel structure,as illustrated in Fig.2.Then,LiOH·H2O was added into the obtained powder (7%in molar ratio of lithium is in excess in order to compensate for the possible loss),and the mixture was thoroughly ground,and then pressed into pellets.The pellets were sintered at700◦CFig.2.XRD profile of Ni–Mn–O precursor after sintered at900◦C.for24h in air.Half of them were ground into power and re-treated at500◦C for30h in O2.The XRD measurements were carried out using a Rigaku Rint1000diffractometer equipped with a monochromator and a Cu target tube.The specific surface area for each sample was analyzed by the Brunauer,Emmett,and Teller(BET)method using a Micromeritics Gemini2375in which N2gas adsorption was employed.Each sample was heated to120◦C for1h to remove adsorbed water before measurement.The Li,Ni and Mn contents in the samples were deter-mined by inductive coupled plasma spectroscopy(ICP)using SPS1500VR(SEIKO)spectrometer.Fourier transform infrared(FTIR)spectra were recorded by a KBr method using FT/IR-4100(JASCO)spectrometer.The charge/discharge tests were carried out using a CR2032 coin-type cell,which consists of a cathode and lithium metal anode separated by a Celgard2400porous polypropylenefilm. The cathode contains a mixture of20mg of accurately weighted active materials and12mg of teflonized acetylene black(TAB-2)as the conducting binder.The mixture was pressed onto a stainless steel mesh and dried at130◦C for4h.The cells were assembled in a glove boxfilled with dried argon gas.The elec-trolyte was1M LiPF6in ethylene carbonate/dimethyl carbonate (EC/DMC,1:2by volume).3.Results and discussionFig.3shows the FTIR spectra of the LiNi0.5Mn1.5O4with and without re-treatment in O2.Both samples show several absorp-tion bands in the range of400–700cm−1.However,the sample re-treated in O2shows well defined peaks compared to thoseof Fig.3.FTIR spectra of the LiNi0.5Mn1.5O4and the LiNi0.5Mn1.5O4re-treated in O2.D.Li et al./Electrochimica Acta52(2007)1919–19241921Fig.4.XRD patterns of LiNi0.5Mn1.5O4with and without the re-annealing treat-ment in O2.the untreated material,and several weak shoulder peaks appear. Although these peaks could not be precisely assigned yet,our results strongly suggested that both samples have a cubic struc-ture with the P4332space group,quite consistent with those reported[6,16,20].Fig.4shows the XRD patterns of the LiNi0.5Mn1.5O4with and without re-treatment in O2.Both samples have a high purity, and no impurities such as NiO were observed in their XRD pro-files.The elemental ratio of Li:Ni:Mn was determined to be 1.02:0.5:1.55and1.01:0.5:1.56for the untreated sample and re-treated sample,respectively.All peaks could be indexed on the basis of the primitive simple cubic structure with the P4332 space group.The lattice parameters were roughly calculated by the least squares method using10lines.The lattice parame-ters are8.1682and8.1697˚A for the re-treated sample and the untreated sample,respectively.These results are quite consistent with the reported values[13,14,16].It was found that the lattice parameters of the untreated sample are larger than those of the sample re-annealed in O2,implying that there is trivalent Mn3+ in the untreated sample.The initial charge and discharge curves of the LiNi0.5Mn1.5O4 and the LiNi0.5Mn1.5O4re-annealed in O2are illustrated in Fig.5.A small plateau near4V,which corresponds to the Mn3+/Mn4+redox,was observed in the initial charge and dis-charge curves of the sample without the re-annealing treatment. This result powerfully demonstrated our suspicion based the analysis of the lattice parameters and FTIR.On the other hand, this plateau was difficult to distinguish in the charge and dis-charge curves of the sample re-treated in O2,suggesting that the re-annealing treatment could effectively reduce the content of trivalent manganese in thefinal product,as reported by Idemoto et al.[13].Moreover,the initial charge and discharge capacities of the sample re-treated in O2are149and134mA h g−1,higher than those of the untreated material(139/123mA h g−1).The irreversible capacities of both samples are nearly the same.The initial charge capacity of the re-treated sample slightly exceeds its theoretical capacity,the origin of which is probably related to the electrolyte decomposition at a highvoltage.Fig.5.Initial charge and discharge curves of LiNi0.5Mn1.5O4with and without the re-annealing treatment.The rate performances of both samples are given in Fig.6. Both samples were initially operated for one cycle at a current density of0.2mA cm−2(20mA g−1).From the second cycle on,the charge current density was kept at a constant current density of0.2mA cm−2while the discharge capacities changed from0.2mA cm−2(about0.15C)to0.4,0.8,1.6,3.2,and 4.8mA cm−2(about3.5C)in subsequent cycles.As illustrated in Fig.4,the discharge capacity of the sample re-annealed in O2 decreased from134to128mA h g−1when the discharge rate increased from0.15to3.5C,with more than95%of the ini-tial discharge capacity being retained.The discharge capacity of the untreated sample was113mA h g−1at a rate of3.5C, about88%of its initial discharge capacity.This result suggests that the sample undergoing post-annealing in O2could effec-tively improve its rate capability.Although it was reported that ordered LiNi0.5Mn1.5O4with the P4332space group had poor rate capability[14,21],our results suggested that the rateperfor-Fig.6.Rate capabilities of LiNi0.5Mn1.5O4and LiNi0.5Mn1.5O4re-treated in O2.1922 D.Li et al./Electrochimica Acta52(2007)1919–1924Fig.7.SEM images of LiNi0.5Mn1.5O4and LiNi0.5Mn1.5O4re-treated in O2. mance of this ordered LiNi0.5Mn1.5O4is excellent.On the other hand,Wakihara[21]reported that there are two possible diffu-sion paths for lithium migration in the ordered LiNi0.5Mn1.5O4. One is8c–4a(vacancy)and the other is8c–12d(vacancy).The former has a lower coulomb potential value than that of the lat-ter,thereby resulting in a quick diffusion for lithium ions during cycling.Because of the excellent rate capabilities of our samples, we believed that lithium diffusion in the ordered LiNi0.5Mn1.5O4 is predominantly through the8c–4a(vacancy)path.Fig.7depicts the SEM images of LiNi0.5Mn1.5O4with and without the re-annealing treatment in O2.No significant change was observed before and after the re-annealing treatment.Both samples have polyhedral agglomerated morphologies,different from the typical octahedral shape as reported in the literature [6,22].A closer inspection reveals that the primary particle has a step–kink–terrace morphology and that the average size is about 1␮m for both samples.The specific surface area of the sample re-annealed treatment in O2is1.82m2g−1,while the specific surface area of the untreated one is1.48m2g−1.We believed that both the decrease in the remaining Mn3+and the increase inthe Fig.8.Cyclic performances of LiNi0.5Mn1.5O4and LiNi0.5Mn1.5O4re-treated in O2operated at different C rates at room temperature.specific surface area contribute to the significant improvement in the rate capability of the re-treated sample.Fig.8shows the cyclic performances of the LiNi0.5Mn1.5O4 and the LiNi0.5Mn1.5O4re-treated in O2operated at differ-ent C rates at room temperature.Both samples have excellent cyclic performances when the cells were cycled at0.15C.The reversible capacities are135and126mA h g−1after50cycles for the re-treated sample in O2and the untreated one,respec-tively.When the cells were operated at3.5C,a slowly fading capacity was observed for both samples.The reversible capaci-ties are115and108mA h g−1after50cycles for the re-treated one and the untreated one,respectively.Fig.9shows the cyclic performances of the LiNi0.5Mn1.5O4 and the LiNi0.5Mn1.5O4re-treated in O2operated at different C rates at50◦C.Both samples exhibit excellent cyclic per-formances at0.15C.The reversible capacities are130and Fig.9.Cyclic performances of LiNi0.5Mn1.5O4and LiNi0.5Mn1.5O4re-treated in O2operated at different C rates at50◦C.D.Li et al./Electrochimica Acta52(2007)1919–19241923Fig.10.Variation in the charge and discharge curves of re-treated LiNi0.5Mn1.5O4operated at3.5C at50◦C.121mA h g−1after50cycles for the re-treated sample in O2 and the untreated one,respectively.These results are nearly the same as those values after cycling at room temperature,implying that the cyclic performances of both samples are almost unaf-fected by the operating temperature at a low rate.However,when the cells were cycled at a rate of3.5C,an obvious capacity fad-ing was observed for both samples compared to those at room temperature.The reversible capacities are102and80mA h g−1 for the re-treated one and the untreated one,respectively.These results suggest that the cycleablity of the LiNi0.5Mn1.5O4at high temperature is dependent on the charge–discharge rate.In order to elucidate the mechanism of this phenomenon,we did com-parative studies on the change in the charge–discharge curves after cycling,which are shown in Fig.10.As demonstrated in Fig.10,two plateaus are observed in the charge curves.One is centered at about4.73V and the other at about4.76V.As the cycle number increases,the length of the plateau at4.73V shortens and its voltage is also slightly depressed,while no significant change is observed in the plateau at4.76V.Regarding the discharge curves,the discharge volt-age of the cell increases slightly compared to that in thefirst discharge.These results suggest that the cell polarization is slightly reduced after being cycled.On the other hand,these variations also imply that structural change probably occur after cycling.Fig.11shows the ex-situ XRD patterns of the LiNi0.5Mn1.5O4re-treated in O2at different charge–discharge states at50◦C.Cells were operated at a rate of3.5C.In the initial charge state,the structure gradually transforms from cubic I to cubic III via cubic II as lithium ions continuously de-intercalate,quite consistent with those reported[15–17]. However,when the cell was initially discharged to3V,the diffraction peaks of an additional phase,whose pattern is quite similar to that of the cubic II phase,were observed in its XRD pattern,and the intensities of these peaks were enhanced after 50cycles,suggesting that the content of the additional phase has increased.These results imply that the capacity fadingof Fig.11.Ex situ XRD patterns of re-treated LiNi0.5Mn1.5O4operated at different charge–discharge states at50◦C.The cells were operated at a rate of3.5C. the LiNi0.5Mn1.5O4at high rate and high temperature should be closely related to this additional phase.Sun and co-workers [14]also observed the two phase coexistence in the ex-situ XRD patterns of LiNi0.5Mn1.5O4after50cycles at a rate of 3C at30and55◦C and attributed the newly formed phase to the unreacted cubic II phase.They also noted that the content of the new phase is higher than the initial phase when cell was operated at high temperature.Our results as well as Sun’s report bring out a new question:why does this phenomenon occur easily at high temperature and high rate?In order to give an answer to this question,we initially tested if the lost capacity at the high rate could be recovered when the cell was recycled at the low rate,and the results were illustrated in Fig.12.Fig.12shows the cyclic performance of the LiNi0.5Mn1.5O4 re-treated in O2operated at50◦C.The cell was initially cycled at a rate of3.5C for15cycles,and then the charge and dis-charge rate was down to0.15C for the following15cycles.The discharge capacity decreases from131to117mA h g−1after 15cycles.When the charge and discharge rate shifts to0.15C, the discharge capacities at16th cycle increases to138mA h g−1 and the discharge capacity is136mA h g−1at30th cycle,same with the values given in Fig.7.All these results suggest that all lost capacity is recovered.This cycled cell wasdisassembled Fig.12.Cyclic performance of the LiNi0.5Mn1.5O4re-treated in O2operated at50◦C.Cell was initially cycled at a rate of3.5C for15cycles,and then the charge and discharge rate was down to0.15C for the next15cycles.1924 D.Li et al./Electrochimica Acta52(2007)1919–1924Fig.13.Ex situ XRD pattern of the electrode operated at varied charge and discharge rates at50◦C.and the electrode was characterized by XRD,whose pattern was illustrated in Fig.13.Fig.13shows the ex situ XRD pattern of the electrode oper-ated at varied charge and discharge rates at50◦pared with the XRD patterns that have been provided in Fig.9,the diffraction peaks of the remained cubic II after cycled at3.5C completely disappear after the cell was recycled at0.15C.These results strongly suggest that the capacity fading observed at the high rate and at high temperature is not originated from the structural exfoliation.In other words,the capacity loss should be resulted from the kinetic factors such as the phase transfor-mation rate.Furthermore,we believed that the phase transition rates between cubic I↔cubic II and cubic II↔cubic III are different.Given that the remained cubic II phase was observed at the end of the initial charge and at50th cycle,we suspect that the transformation rate from cubic II to cubic III is slow.As to why this phenomenon occurs easily at high temperature and high rate,we believed that since these two-phase transition processes have different energy barriers,they have different temperature correlations.When cell was operated at a high temperature,the discrepancy between the phase transition rates becomes large, thereby resulting in the incomplete transformation from cubic II to cubic III,and further in capacity fading.More studies are necessary to verify the kinetic characterizations during the phase transition among cubics I–III.4.ConclusionsLiNi0.5Mn1.5O4was prepared by a spray drying and post-annealing process.Samples obtained by this new route have polyhedral agglomerated morphologies with the presence of many step–kink–terraces on the surface of the primary parti-cles.The re-annealing treatment in O2could not only decrease the Mn3+content,but also increased the reversible capacity and significantly improve the rate capability compared to the untreated material.Moreover,the cyclic performance of the LiNi0.5Mn1.5O4depends on both the cycling rate and operat-ing temperature,which have been ascribed to the difference of the phase transition rates between cubic I↔cubic II and cubic II↔cubic III.AcknowledgementThis work wasfinancially supported by the High-Tech Research Center Project for Private University:matching fund subsidy from the Ministry of Education,Culture,Sports,Science and Technology(MEXT)from2001to2005.References[1]K.Amine,H.Tukamoto,H.Yasuda,Y.Fujita,J.Electrochem.Soc.143(1996)1607.[2]Q.Zhong,A.Bonakdarpour,M.Zhang,Y.Gao,J.Dahn,J.Electrochem.Soc.144(1997)205.[3]Y.Gao,K.Myrtle,M.Zhang,J.N.Reimer,J.R.Dahn,Phys.Rev.B54(1996)16671.[4]T.Zheng,J.R.Dahn,Phys.Rev.B56(1997)3801.[5]T.Ohzuku,S.Takeda,M.Iwanaga,J.Power 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