AlF3包覆LiCoO2

Signi?cant improvement of high voltage cycling behavior

AlF 3-coated LiCoO 2cathode

Y.-K.Sun

a,*

,J.-M.Han a ,S.-T.Myung b ,S.-W.Lee a ,K.Amine

a Center for Information and Communication Materials,Department of Chemical Engineering,Hanyang University,Seoul 133-791,South Korea

VK Corporation,67Jije-Dong,Pyongtaek-City,Kyonggi-Do 450-090,South Korea

Electrochemical Technology Program,Chemical Engineering Division,Argonne National Laboratory,9700S.Cass Avenue,Argonne,IL 60439,USA

Received 21February 2006;received in revised form 23March 2006;accepted 28March 2006

Abstract

To improve the capacity retention of LiCoO 2at 4.5V cut-o?cycling,the AlF 3as new coating material was introduced.The AlF 3-coated LiCoO 2showed no large di?erence in the bulk structure and a uniform AlF 3-coating layer whose thickness is of about 10nm covers LiCoO 2particles,as con?rmed by transmission electron microscopy.The AlF 3-coated LiCoO 2delivered the similar initial dis-charge capacity to pristine LiCoO 2to 4.5V versus Li.Surprisingly,the capacity retention and rate capability were greatly enhanced com-paring to the pristine LiCoO 2.Ac-impedance results showed that the AlF 3-coated LiCoO 2has stable charge transfer resistance (R ct )on cycling.Reduced Co dissolution was also seen for the AlF 3-coated LiCoO 2.ó2006Elsevier B.V.All rights reserved.

Keywords:AlF 3coating;Cathode materials;Impedance;Lithium secondary batteries

1.Introduction

Layered LiCoO 2is the most widely used positive elec-trode material in commercial lithium secondary batteries due to ease of preparation,high electronic conductivity,good rate capability,and excellent cycling performance.Li x CoO 2enables to deliver a reversible capacity of around 140mAh/g typically charged to 4.2V (x %0.5),which is much lower than its theoretical capacity of 274mAh/g.In order to increase the reversible capacity of LiCoO 2,the positive electrode has to be charged above 4.2V.How-ever,the capacity retention is dependent on the upper cut-o?voltage,and the raised cut-o?voltage derives phase transformation and Co dissolution in LiCoO 2,causing severe capacity fade [1,2].To maintain the high reversibility of LiCoO 2at high voltage,the substitution of Al,Mg,and Zn for Co to give LiCo 1àx M x O 2have been intensively studied [3–5].However,capacity fading has been still

observed even though some improvement has been achieved [5].

As an alternative,there has been an extensive research by coating cathode materials with metal oxides;Kweon et al.[6]?rst reported that the electrochemically cycling performance of LiCoO 2at high voltage (>4.2V)have been fairly enhanced by Al 2O 3coating.After that,several sur-face modi?cations of LiCoO 2by coating Al 2O 3,ZrO 2,MgO,SnO 2,TiO 2,and SiO 2have been widely studied.The electrochemical properties (capacity retention and rate capability)of the coated LiCoO 2at high voltage were sig-ni?cantly improved compared with uncoated one.Cho et al.[7]reported that the coating of Al 2O 3and ZrO 2on LiCoO 2e?ectively suppressed the lattice-constant changes and thereby resulted in zero-strain cathode material.On the other hand,though Dahn’s group also reported the enhanced capacity retention by coating of ZrO 2,they proved that the ZrO 2coating has no e?ect on the phase transition,as it was revealed by in situ XRD [8].Our group clari?ed that the excellent cyclability of the ZnO-coated LiNi 0.5Mn 1.5O 4electrode in the 5-V region has been

Corresponding author.Tel.:+82222200524;fax:+82222827329.E-mail address:yksun@hanyang.ac.kr (Y.-K.Sun).

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Electrochemistry Communications 8(2006)

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ascribed to the reduced HF content and thus suppression of Mn dissolution[9,10].Recently,Myung et al.[11]have veri?ed the role of Al2O3coating layer on Li[Li0.05Ni0.4-Co0.15Mn0.4]O2cathode surface as studied by ToF-SIMS. The Al2O3coating layer acted as a HF scavenger,which reduced HF generation resulting in less decomposition of the cathode particle by forming AlF3on the surface of Al2O3coating layer.

From the above review,all of surface treatment on the cathode particle has been done by metal oxide.In this work,we?rst report the e?ect of surface modi?cation of LiCoO2by AlF3on electrochemical performance at higher cut-o?voltage limit to4.5V.

2.Experimental

To prepare AlF3-modi?ed LiCoO2,ammonium?uoride (Aldrich)and aluminum nitrate nonahydrate(Aldrich)was separately dissolved in distilled water.After LiCoO2(Nip-pon Chemical Co.)powders were immersed into the alumi-num nitrate nonahydrate solution,the ammonium?uoride solution was slowly added to the solution.The molar ratio of Al to F is?xed to3and the amount of AlF3in the solu-tion corresponded to2wt%of the LiCoO2powders.After the solution containing the cathode powders was con-stantly stirred at80°C for5h and then?ltered by distilled water.The obtained LiCoO2powders were heated at 400°C for5h in the nitrogen?owing to avoid the forma-tion of Al2O3.

Transmission electron microscopy(TEM,JEOL2010) and Powder X-ray di?raction(XRD,Rigaku Rint-2000) employing Cu K a radiation was used to characterize the prepared powders.Charge-discharge tests were performed with a coin type cell(CR2032).The cell consisted of the cathode and the lithium metal anode separated by the por-ous polypropylene?lm.For the fabrication of the positive electrode,a mixture containing20mg of positive electrode materials and5mg of conducting binder(3.3mg of te?on-ized acetylene black(TAB)and1.7mg of graphite)was pressed on a2.0-cm2stainless screen at500kg/cm2.The electrolyte was a1:1mixture of ethylene carbonate(EC) and diethyl carbonate(DEC)containing1M LiPF6by vol-ume.The long cycle-life tests were performed in a lami-nated-type full cell wrapped with an Al pouch. Mesocarbon microbeads(MCMB,Osaka gas)were used as the negative electrode.The fabrication of the cell was done in a dry room.A cell formation was performed for the Li-ion cell:three cycles were performed at room tem-perature at the0.1,0.2,and0.5C rates in the voltage range of3.0–4.4V.The cells,then,were charged and discharged between3.0and4.4V by applying a constant1C current at30°C.Ac-impedance measurements were performed using a Zahner Elektrik IM6impedance analyzer over the frequency range from1MHz to1mHz with the ampli-tude of10mV rms.To measure the amount of Co dissolu-tion,the cells charged to 4.5V were carefully disassembled and then active materials were stored in elec-trolyte at60°C.The amount of Co dissolution was mea-sured by atomic absorption spectroscopy(AAS,Analytik Jena AG,Vario6).

3.Results and discussion

XRD patterns of the pristine and AlF3-coated LCoO2 powders are shown in Fig.1.It was con?rmed that the two materials were well-de?ned layer structure based on hexagonal a-NaFeO2structure with space group R 3m in Fig.1a.The pristine LiCoO2particle show very smooth and clean surface in TEM image of Fig.1a inset.No impu-rity peaks related with Al containing compounds are observed from the XRD pattern in Fig.1b.The lattice con-stants of the coated LiCoO2powders were a=2.819(2)and c=14.071(7)A?,respectively,as calculated by a least square method.The values are close to those of pristine, a=2.819(8)A?,c=14.070(9)A?,indicating that the AlF3 was not incorporated into the host structure since no changes were seen in the structure.As shown in the inset of Fig.1b,a nanoscale-AlF3layer was coated homoge-neously on the surface of LiCoO2.It also exhibited a thin coating layer of about10nm in thickness and the coating layer could be a porous structure comparing to the LiCoO2 bulk.After precipitation of aluminum nitrate nonahydrate solution and the ammonium?uoride solution,the precipi-tates were dried at120°C.As one can see in Fig.1c,the product shows well ordered crystalline b-AlF3phase,which coincides with JCPDS card(31-0011).Due to the tiny amount of AlF3coating,it is likely that the b-AlF3phase was not observed in the Fig.1

Fig.1.Powder XRD patterns of(a)pristine LiCoO2and its bright-?eld TEM image and(b)AlF3-coated LiCoO2and its bright-?eld TEM image. The scale bar indicates20nm.

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Fig.2shows the initial charge/discharge curves and the discharge capacity versus number of cycle of Li/pristine and AlF3-coated LiCoO2cells by applying a constant cur-rent of20mA gà1between3.0and4.5V versus Li.Both cells had stable and smooth charge/discharge curves even on the higher cut-o?voltage range in Fig.2a.No di?erence in the operation voltage on charge and discharge also indi-cates that the coating medium of AlF3was not introduced into the LiCoO2structure,as it was previously reported by Myung et al.[11,12].The AlF3-coated LiCoO2delivered the similar discharge capacity around180mAh gà1to the pristine LiCoO2in Fig.2a.Although the pristine LiCoO2 electrode delivered a high initial discharge capacity of 180mAh gà1,it su?ered from a severe capacity fading, showing only30mAh gà1capacity after50cycles in Fig.2b.Remarkably,the cycling stability of the AlF3-coated LiCoO2was quite signi?cantly enhanced and the electrode showed98%capacity retention even up to4.5V cut-o?voltage limit in Fig.2b.

Rate capability properties also demonstrate the advan-tages of AlF3-coating.Fig.2c shows the discharge capaci-ties for the Li/LiCoO2and AlF3-coated LiCoO2cells as a function of various currents(20–320mA gà1)between3.0 and4.5V versus Li.The cells were charged galvanostati-cally with a current density of20mA gà1before each dis-charge testing,and then discharged at di?erent current densities from20to320mA gà1.It was clearly observed that the AlF3-coated electrode has better capacity retention than the pristine especially at higher currents.Although the pristine electrode showed abrupt capacity deterioration at higher currents above160mA gà1,the AlF3-coated LiCoO2electrode still retained its higher discharge capaci-ties.At320mA gà1current,the obtained discharge capac-ity of the AlF3-coated electrode is about85%,compared to that of20mA gà1.Meanwhile,the pristine showed only 35%at the current in Fig.2c.

In order to investigate the long-term cycling perfor-mance,a carbon electrode using MCMB was employed as the https://www.360docs.net/doc/6c13902319.html,minated-type lithium-ion batteries using an Al pouch with a capacity of45mAh were assembled. The fabricated batteries were charged and discharged for 500cycles at the rate of1C between3.0and4.4V.The capacity of the C/LiCoO2cell rapidly decreased with cycling and it reached to almost0mAh after500cycles in Fig.3a and c.It also exhibited a dramatic voltage drop with cycling shown in Fig.3a Such an inferior cycling behavior of the LiCoO2could be ascribed to the structural instability and dissolutions of Co resulting from the active material by HF attack,deteriorating the long-term cycling stability[13].On the other hand,the AlF3-coated LiCoO2 cell shows an excellent cycling performance and has capac-ity retention of91%after500cycles in Fig.3b and c,even at higher cut-o?voltage limit to4.4V.The voltage di?er-ence during cycling is quite lower compared to C/LiCoO2 in Fig.3b.

To investigate the possible reason of improved cycling performance of the AlF3-coated LiCoO2,Electrochemical impedance spectroscopy(EIS)for the pristine LiCoO2 and AlF3-coated LiCoO2were measured with cycle number at the charged state to4.5V versus Li,as shown in Fig.4a and b,respectively.Measured impedance is a collective response of kinetic processes.To interpret impedance data, hence,an appropriate model consistent with real system is required.The used equivalent circuit was proposed in our previous report[14].Expanded views of two overlapped

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semicircles in the high-to-medium frequency region are also shown in inset of Fig.4a and b.The scattering symbol and continuous lines represent experimental data and?tted results with equivalent circuit,respectively.The semicircle observed at the high frequency should be attributed to resistance of surface?lm,and the second one observed at high to medium frequency re?ects the bulk properties as previously reported by Shaju et al.[15].Two overlapped semicircles showed little change on cycling except for the 1st cycle.It is noticeable that the charge transfer resistance originated from pristine LiCoO2/electrolyte interface became much larger than that of AlF3-coated LiCoO2/elec-trolyte interface with cycling in Fig.4a.The similar phenomenon was observed for Al2O3-coated Li[Li0.05-(Ni0.5Mn0.5)0.8Co0.15]O2,as we previously reported[11]. For the AlF3-coated LiCoO2,the diameter of two over-lapped arcs has higher value than that of LiCoO2through all cycles,which could be attributed to increase of resis-tance by insulating AlF3coating layer in Fig.4b.Also, the third semicircle of the pristine LiCoO2arising from charge transfer resistance(R ct)drastically increases with cycling while the two overlapped arcs remain practically unchanged.

Variations in charge transfer resistance(R ct)as a func-tion of number of cycle for the pristine and AlF3-coated LiCoO2electrodes are described in Table1.The R ct value of the pristine LiCoO2electrode rapidly increased with cycling.For example,the R ct value at the1st cycle was 74X but it reached to2044X at the50th cycle.On the other hand,R ct value of AlF3-coated LiCoO2electrode kept about20–27X during cycling,as shown in Fig.4b and Table1.Aurbach et al.[16]reported that capacity fading of LiCoO2on cycling and storage at elevated tem-perature are attributed to increase of surface?lm resis-tance rather than to bulk degradation.Recently, Miyashiro et al.[17]also reported that the improved elec-

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trochemical performance of ZrO 2-coated LiCoO 2in the higher voltage region of 4.6V was not due to structural changes of host structure but rather the stabilization of interface between cathode and electrolyte.The abrupt var-iation of R ct could be due to surface ?lm composition which mainly composed of ethylene carbonate,and LiF,Li x PF y -type and Li x PF y O z -type compounds [16,18].Among these compounds,it is well known that LiF is highly resistive to Li ion transport (probably produced by reactions of the Li x MO y active mass with trace HF).Therefore,the pristine LiCoO 2cannot avoid being cov-ered by the passivated ?lms on the cathode surface.How-ever,the AlF 3layer may reduce the formation of LiF ?lms,and would thus suppress the increase of cathode/electrolyte interfacial impedance.

Considering our result,the contribution of two resis-tive elements (resistance of surface ?lm and bulk)observed at high and medium frequency may be excluded due to little variation with cycling.Therefore,other addi-tional reason could be speculated.It has been well known that Co dissolution by HF attack into electrolyte easily taken place during cycling and/or prolonged storage especially in the high voltage region [11,13,16].The half cells using Li metal as the anode were charged to 4.5V and were carefully disassembled in a glove box.The recovered cathode materials were stored in electrolyte at 60°C,and the used electrolytes,then,were examined to measure dissolved amount of Co by AAS.Table 2shows the amount of Co dissolved from the pristine and AlF 3-coated LiCoO 2electrode into the electrolyte solution with increasing storing time.The dissolved amount of Co from pristine LiCoO 2was nearly proportional to the storing time and the amount corresponded to approxi-mately 87ppm after 340h.On the other hand,the AlF 3-coated LiCoO 2showed greatly reduced Co dissolu-tion into the electrolyte and the amount reached to 23ppm,showing only about one fourth compared to the pristine LiCoO 2.Obviously,the AlF 3-coated LiCoO 2electrode gave signi?cant suppression of Co dissolution during storing at 60°C.Further work is now in progress to reveal the reaction mechanism and the origin of improved electrochemical performance for the AlF 3-coated LiCoO 2powders.4.Conclusions

The e?ects of AlF 3coating on LiCoO 2at high cut-o?voltage (4.5V)cycling were investigated.The AlF 3-coated LiCoO 2has much enhanced cycling performance and rate capability when cycled in 3.0–4.5V range com-pared to pristine LiCoO 2.The AlF 3-coated LiCoO 2cell has capacity retention of 91%after 500cycles in the full cell using carbon as the anode even at higher voltage limit to 4.4V,while the pristine LiCoO 2exhibited disap-pointing cycling behaviors.Even at higher currents,the capacity retention of Li/AlF 3-coated LiCoO 2cell was 85%at 320mA g à1and the pristine LiCoO 2cell retained only 35%at the current.The improved electrochemical performance could be explained by the lower charge transfer resistance (R ct ),and reduced Co dissolution of the AlF 3-coated sample.That is,the AlF 3coating layer could reduce the formation of LiF ?lms which increase of cathode/electrolyte interfacial impedance and also sup-press Co dissolution by covering LiCoO 2surface from HF attack.Acknowledgement

This work was supported by the Ministry of Informa-tion &Communications,Korea,under the Information Technology Research Center (ITRC)Support Program.References

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Table 2

The variation of Co dissolution for pristine-LiCoO 2and AlF 3-coated LiCoO 2with storage time in electrolyte at 60°C Storage (h)Dissolved Co amount (ppm)Pristine LiCoO 2AlF 3-coated LiCoO 248 5.088 4.289615.79 5.2714435.519.11336

86.96

23.28

Table 1

Charge transfer resistance (R ct )variation for pristine-LiCoO 2and AlF 3-coated LiCoO 2cells with cycle number Cycle number Charge transfer resistance,R ct (X )Pristine LiCoO 2AlF 3-coated LiCoO 2173.9295125.821.81022020.82047023.130850.821.8401337.525.750

2044.4

26.8

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