Improvement of Electrochemical Performance of Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 Cathode

MATERIALS AND INTERFACES

Improvement of Electrochemical Performance of Li[Ni 0.8Co 0.15Al 0.05]O 2Cathode Materials by AlF 3coating at Various Temperatures

Byung-Chun Park,?Hyung-Bae Kim,?Hyun Joo Bang,?Jai Prakash,?and Yang-Kook Sun*,?

Department of Chemical Engineering,Center for Information and Communication Materials,Hanyang Uni V ersity,Seoul 133-791,South Korea,and Department of Chemical and Biological Engineering,Illinois Institute of Technology,10West 33rd Street,Chicago,Illinois 60616

A thin AlF 3layer of ～10nm was uniformly coated on the particle surface of Li[Ni 0.8Co 0.15Al 0.05]O 2.The AlF 3coating improved cycle performance at 55,30,and –10°C and improved storage characteristics at 60°C.In studies of differential capacity (d Q/d V )versus voltage,the AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2showed little variation in redox peaks with cycling.Electrochemical impedance spectroscopy suggested that the AlF 3coating played an important role in stabilizing the interface between the cathode and the electrolyte.

Introduction

The high energy density and power capability of Li-ion batteries have made them a popular power source for portable electronic devices and,more recently,hybrid electric vehicles (HEVs)and plug-in hybrid electric vehicles (PHEVs).1,2Ni-rich Li[Ni 1-x M x ]O 2(M )metal)cathode materials are alterna-tives to LiCoO 2and have lower cost,lower toxicity,and higher reversible capacity.3–5However,Ni-rich Li[Ni 1-x M x ]O 2has been found to exhibit low thermal stability,poor cycling behavior,and increased impedance during cycling,particularly at high temperatures.6,7One of the most promising materials is Li[Ni 0.8Co 0.15Al 0.05]O 2.8,9The substitution of Ni with Co and Al improves its structural and thermal stability without sacri?c-ing discharge capacity.In spite of the improved electrochemical performance of the Li[Ni 0.8Co 0.15Al 0.05]O 2,the major barriers that hinder commercial-scale application are its insuf?cient cycle life and safety concerns under abuse conditions,especially at temperatures above 55°C.There have been several approaches to solve these problems.One method is to completely encap-sulate the Ni-based active materials with Li[Ni 0.5Mn 0.5]O 2,a stable active material.10,11Another problem inherent to Li[Ni 0.8Co 0.15Al 0.05]O 2with high Ni content is its rapid reaction to moisture and ambient CO 2,resulting in the formation of LiOH and Li 2CO 3on the cathode particle surface.12,13Li extraction from the lattice gives rise to cation ion mixing,in turn leading to severe capacity fading and reduced reversible capacity.One way to improve the electrochemical performance is to modify the cathode particle surface with metal oxide 14–18and metal ?uoride 19–21to reduce the catalytic activity of the transition metal with a high oxidation state in LiMO 2(M )Co,Ni,Mn).The electrochemical properties and thermal stability of the coated cathode materials show signi?cant improvement over uncoated ones.Though the reason is not well-understood,the protection of the cathode surface provided by the coating may be responsible for the enhanced electrochemical

performance.One plausible reason for the improvement is that the coating layer protects against HF attack of the electrolyte,suppressing the decomposition of the active material in cases where pristine particles would be severely degraded by cycling.17,18,22The preservation of the cathode surface stabilizes the host structure of the cathode materials,allowing for stable charge-transfer resistance during successive charge/discharge cycles.19–21

In this paper,we study the effect of an AlF 3coating over Li[Ni 0.8Co 0.15Al 0.05]O 2on electrochemical performance at vari-ous temperatures.We also investigate the reason for the improvement of the electrochemical performance in the presence of the coating by comparing the differential capacity (d Q /d V)versus voltage pro?les of pristine and AlF 3-coated Li[Ni 0.8-Co 0.1Mn 0.1]O 2.Experimental Section

[Ni 0.8Co 0.15Al 0.05](OH)2powders were prepared by copre-cipitation,as reported previously.23An aqueous solution of NiSO 4·6H 2O,CoSO 4·7H 2O,and Al 2(SO 4)3·16H 2O was pumped into a continuously stirred tank reactor (CSTR,capacity 4L)in an N 2atmosphere.Simultaneously,an NaOH solution (aq)and an appropriate amount of NH 4OH solution (aq),a chelating agent,were fed separately into the reactor.The spherical [Ni 0.8Co 0.15Al 0.05](OH)2powders were dried at 110°C for 24h to remove adsorbed water.Finally,a mixture of the dehydrated [Ni 0.8Co 0.15Al 0.05](OH)2and LiOH ·H 2O were preheated to 480°C for 5h and then heated at 750°C for 20h in an oxygen ?ow.The chemical compositions of the prepared powders were analyzed using atomic absorption spectroscopy (AAS,Vario 6,Analyticjena).To coat the surface of the Li[Ni 0.8Co 0.15Al 0.05]O 2with AlF 3,ammonium ?uoride (Aldrich)and aluminum nitrate nonahydrate (Aldrich)were separately dissolved in distilled water.After the prepared Li[Ni 0.8Co 0.15Al 0.05]O 2powders were immersed in the aluminum nitrate nonahydrate solution,the ammonium ?uoride solution was slowly added.The molar ratio of F to Al was ?xed at 6,and the amount of AlF 3in the solution corresponded to 0.25mol %of the Li[Ni 0.8Co 0.15Al 0.05]O 2powders.The solution containing the active material was

*Corresponding author.Phone:+82-2-2220-0524.Fax:+82-2-2282-7329.E-mail:yksun@hanyang.ac.kr.?

Hanyang University.?

Illinois Institute of Technology.

Ind.Eng.Chem.Res.2008,47,3876–3882

387610.1021/ie0715308CCC:$40.75 2008American Chemical Society

Published on Web 04/29/2008

continuously stirred for 5h at 80°C,accompanied by a slow evaporation of solvent.The Li[Ni 0.8Co 0.15Al 0.05]O 2powders obtained were heated at 400°C for 5h in an oxygen ?ow.Powder X-ray diffraction (XRD)(Rigaku,Rint-2000)employing Cu K R radiation was used to identify the crystalline phase of the prepared powders at each stage.The surface of the AlF 3-coated powder was also observed using transmission electron microscopy (TEM,JEOL 2010).Differential scanning calorim-etry (DSC)experiments were carried out for the positive electrode materials by fully charging the coin cell to 4.3V at a constant current and voltage and opening it in an Ar-?lled dry room.The measurements were carried out in a differential scanning calorimeter 200PC (NETZSCH,Germany)using a temperature scan rate of 1°C/min.Charge–discharge tests were performed with a coin-type cell (CR2032)having a current density between 40and 100mA/g at 30°C.The cell consisted of positive and negative electrodes of lithium metal separated by a porous polypropylene ?lm.To fabricate the cathode,the prepared powders were mixed with carbon black and polyvi-nylidene ?uoride (85:7.5:7.5)in N -methylpyrrolidinon.The slurry obtained was coated onto Al foil and roll-pressed at 120°C in air.The electrodes were dried at 120°C overnight in a vacuum state prior to use.Preliminary cell tests were conducted using a 2032coin-type cell using Li metal for the anode.The electrolyte solution used was 1M LiPF 6in ethylene carbonate -diethyl carbonate (1:1in volume,Cheil Industries,Inc.,Korea).Results and Discussion

Figure 1shows the XRD patterns of the pristine and AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2materials.It was con?rmed that the two powders have a well-de?ned layer structure based on the hexagonal a -NaFeO 2structure with space group Rm .The lattice constants,a and c,of the AlF 3-coated Li[Ni 0.8Co 0.15-Al 0.05]O 2powders were 2.865(5)and 14.194(7)?,as calculated by a least-squares method.These values approximate those of the pristine material,a )2.864(1)and c )14.191(0)?,indicating that the AlF 3was not incorporated into the host structure.Chemical analysis showed that the prepared powder composition was Li[Ni 0.796Co 0.153Al 0.051]O 2.

Figure 2shows a TEM image of the AlF 3-coated Li[Ni 0.8-Co 0.15Al 0.05]O 2powder.The TEM image proves the presence of the uniform AlF 3coating layer of 10nm on the particle surface.Energy-dispersive X-ray spectroscopy con?rmed the composition of the coating layer to be rich in Al.

Figure 3displays the initial charge/discharge curves of the Li/pristine and AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2cells by applying a constant current of 40mA/g (0.5C-rate)between 3.0and 4.3V vs Li +/Li.The charge voltage of the AlF 3-coated

Li[Ni 0.8Co 0.15Al 0.05]O 2electrode shows lower resistance than the pristine Li[Ni 0.8Co 0.15Al 0.05]O 2in all operating regions,which increases the discharge capacity and the rate capability.The initial discharge capacity of the AlF 3-coated Li[Ni 0.8Co 0.15-Al 0.05]O 2is 178mAh/g,as compared to that of the pristine material,which has a capacity of 168mAh/g.It is speculated that a thin AlF 3-coating layer lowers the charge-transfer resistance,which facilitates Li +intercalation/deintercalation at the interface between the cathode and the electrolyte.19–21

Parts a and b of Figure 4show the discharge capacity of Li/pristine and AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2cells over 50cycles at 30and 55°C,respectively.The cells were ?rst activated by applying a current density of 0.2C-rate (20mA/g)for two cycles and were then cycled at 0.5C-rate (40mA/g)between 3.0and 4.3V vs Li +/Li.During the 30°C cycling,the AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2electrode delivered a discharge capacity of 173mAh/g,while the pristine electrode delivered only 160mAh/g.The pristine Li[Ni 0.8Co 0.15Al 0.05]O 2showed a gradual decrease in capacity,leading to a capacity retention of 91%after 50cycles.The AlF 3-coated Li[Ni 0.8-Co 0.15Al 0.05]O 2displayed a slightly improved capacity retention of 97%after the same cycling period.However,the cycling behavior of the AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2at 55°C was remarkably enhanced,showing capacity retention of 94%after 62cycles.The pristine electrode showed a gradual decrease in capacity,leading to a capacity retention of only 73%.Recently,Abraham et al.reported that a Li x Ni 1-x O-type layer was observed on the surface of a Li[Ni 0.8Co 0.2]O 2particle on which surface layers were 2-5nm thick in samples from 0%power fade cells,and >35nm thick in samples from 43%power fade cells.6It is believed that the thin AlF 3coating layer lowers the catalytic activity of Ni 4+with electrolyte species in highly delithiated Li 1-x [Ni 0.8Co 0.15Al 0.05]O 2and,thus,reduces the formation of

the

Figure 1.XRD patterns of (a)pristine and (b)AlF 3-coated Li[Ni 0.8Co 0.15-Al 0.05]O 2

.

Figure 2.Bright-?eld TEM images of (a)pristine and (b)AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2

.

Figure 3.Initial charge–discharge curves of Li/pristine and Li/AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2cells at a current density of 40mA/g between 3.0and 4.3V.

Ind.Eng.Chem.Res.,Vol.47,No.11,20083877

Li x Ni 1-x O-type layer resulting from Ni 4+decomposition,espe-cially at temperatures higher than 55°C.

Figure 5shows a plot of discharge capacity versus cycle number for the Li/pristine and AlF 3-coated Li[Ni 0.8Co 0.15-Al 0.05]O 2between 3.0and 4.3V at 0.2C-rate (20mA/g)and –10°C.The cells were activated for two cycles at 0.2C-rate and 30°C before the test.Although AlF 3-coated Li[Ni 0.8Co 0.15-Al 0.05]O 2showed a slightly larger discharge capacity (123mAh/g)than the pristine material (113mAh/g),the two electrodes showed a similar capacity retention of 67–69%at –10°C as compared to 30°C.However,the cycling stability was remark-

ably enhanced in the electrode with the AlF 3coating,which showed no capacity loss,while the pristine electrode suffered from a capacity loss of 14%after only 20cycles.It is believed that the improved performance at low temperatures can be attributed to the lower charge-transfer resistance.The data are consistent with our previous reports showing that Li/AlF 3-coated LiCoO 2,Li[Ni 1/3Co 1/3Mn 1/3]O 2,and Li[Ni 0.8Co 0.1Mn 0.1]O 2cells exhibit a signi?cantly lower charge-transfer resistance (R ct )than pristine electrodes during extended cycling.19–21

The storage characteristics of cathode materials at an elevated temperature,especially in the delithiated state,are of great interest in judging their practical use for lithium secondary batteries.Figure 6shows the variations in discharge capacities of Li/pristine and AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2cells in the charged state of 4.3V at 60°C as a function of storage time.The prepared cells were preliminarily charged and discharged between 3.0and 4.3V at 0.2C-rate and 30°C for two cycles to determine their initial capacities.The cells were then charged to 4.3V at 60°C and stored for 24,36,and 72h.The aged cells were cooled down to 30°C,and their discharge capacity was measured at each storage interval.While the pristine Li[Ni 0.8Co 0.15Al 0.05]O 2showed a rapid decrease in capacity,leading to a capacity retention of only 46%after 72h in storage,the storage capacity of the AlF 3-coated electrode greatly improved,showing a capacity retention of 91%after the same storage time.The data also show the impact of AlF 3coating.To investigate the effects of transition metal dissolution of the Li 1-δ[Ni 0.8Co 0.15Al 0.05]O 2on storage time,the chemical compositions with storage times were analyzed by AAS,and the results are shown in Table 1.The Ni and Co dissolution from the pristine Li 0.2[Ni 0.8Co 0.15Al 0.05]O 2was increased with storage time,while dissolution from the AlF 3-coated Li 0.2[Ni 0.8Co 0.15Al 0.05]O 2was kept nearly constant.For

example,

Figure 4.Variation of the discharge capacities of Li/pristine and Li/AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2cells at (a)30and (b)55°C between 3.0and 4.3V at 0.5C-rate (40

mA/g).

Figure 5.Plot of discharge capacity vs cycle number for the Li/pristine and AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2between 3.0and 4.3V at 0.2C-rate (20mA/g)and –10°

C.

Figure 6.Variation of the discharge capacities of (a)Li/pristine and (b)AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2cells in the charged state of 4.3V at 60°C as a function of storage time.

3878Ind.Eng.Chem.Res.,Vol.47,No.11,2008

the chemical compositions of the pristine and AlF 3-coated Li 0.2[Ni 0.8Co 0.15Al 0.05]O 2after 72h were estimated to be Li 0.42[Ni 0.70Co 0.103Al 0.04]O 2-δand Li 0.27[Ni 0.75Co 0.14Al 0.04]O 2-δ,respectively.Note that the Li content of the pristine materials was quite a bit larger than that of the AlF 3-coated materials.Recently,Ozawa et al.reported that capacity fading in the charged state originates from (i)the chemical reactions involving electron transfer between the electrolyte and the electrode and (ii)Li-intercalation from the electrolyte into the cathode host structure.24It is believed that the AlF 3coating is so resistant to HF attack on the cathode particle surface that dissolution of the transition metal is greatly reduced,which in turn enhances the storage performance at 60°C.

Figure 7shows the discharge capacities of the Li/pristine and AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2cells as a function of various C-rates (40-1000mA/g)between 3.0and 4.3V.The capacity retention percentage of each C-rate (compared with 0.2C-rate)is also shown in Figure 7.The cells were charged galvanostati-cally with a current density of 20mA/g (0.2C-rate)before each discharge test and then discharged at current densities from 20(0.2C-rate)to 1000mA/g (5C-rate).As observed in Figure 7,the capacity retention difference between the pristine and AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2electrode became larger with increasing C-rate.For example,the capacity retentions of AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2were 92%and 62%at 1and 5C-rate,respectively,while those of the pristine material were 89%and 54%.Notice that the AlF 3-coated Li[Ni 0.8Co 0.15-Al 0.05]O 2electrode delivers a larger discharge capacity than the pristine one across the range of C-rates.This result leads us to believe that the AlF 3-coating layer on Li[Ni 0.8Co 0.15Al 0.05]O 2plays an important role in expediting the Li +intercalation to the host structure by stabilizing the structure and reducing the interfacial resistance between the cathode and the electrolyte.19–21

Figure 8shows differential capacity (d Q /d V)versus voltage pro?les of Li/pristine and AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2cells at the ?rst,30th,and 50th cycles.The cells were cycled between 3.0and 4.3V at a current density of 100mA/g.During the ?rst cycle,the Li/pristine Li[Ni 0.8Co 0.15Al 0.05]O 2cell showed a sharp oxidation peak at 3.68V and a satellite peak at 3.76V,

Table 1.Chemical Compositions of Pristine and AlF 3-coated Li 1-δ[Ni 0.8Co 0.15Al 0.05]O 2Charged to 4.3V with Storage Time

samples

36

72

pristine Li 0.24[Ni 0.8Co 0.15Al 0.05]O 2

Li 0.34[Ni 0.72Co 0.13Al 0.05]O 2-δLi 0.42[Ni 0.70Co 0.10Al 0.04]O 2-δAlF 3-coated Li 0.24[Ni

0.8Co 0.15Al 0.05]O 2

Li 0.25[Ni 0.76Co 0.15Al 0.05]O 2-δ

Li 0.27[Ni 0.75Co 0.14

Al 0.04]O 2-δ

Figure 7.Rate capability test of Li/pristine and Li/AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2cells at various C-rates.

Figure 8.Differential capacity vs voltage

of (a)Li/pristine and (b)Li/AlF 3-coated Li[Ni 0.8Co 0.1Mn 0.1]O 2cells in the voltage range of 3.0–4.3V and 30°C.

Figure 9.Differential capacity vs voltage of (a)Li/pristine and (b)Li/AlF 3-coated Li[Ni 0.8Co 0.1Mn 0.1]O 2cells in the voltage range of 3.0–4.3V and 60°C.

Ind.Eng.Chem.Res.,Vol.47,No.11,20083879

as well as several other peaks during charging due to multiphase transitions.25,26Corresponding redox peaks at 3.67,3.93,and 4.13V were also observed.The redox peaks became more polarized and shifted farther apart.For example,the oxidation peak height at 3.68V and the corresponding reduction peak at 3.67V decreased after 50cycles,indicating surface degradation of the host structure and/or increase of the interfacial impedance between the cathode and the electrolyte.20,21No peak variation was observed for the AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2elec-trode during cycling.

Figure 9shows the discharge differential capacity (d Q/d V )versus voltage pro?les of Li/pristine and AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2cells charged to 4.3V at 60°C as a function of storage time.Both the pristine and AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2electrodes showed three distinct reduction peaks at 3.68,3.94,and 4.16V at 30°C,which corresponded to a phase transition between two O 3structures.25However,the two electrodes showed different reduction peak changes after storage.For the pristine Li[Ni 0.8Co 0.15Al 0.05]O 2electrode,storage at 60°C caused increased polarization of the reduction peaks and shifted them farther apart.For example,the peaks at 3.68and 3.94V shifted to 3.31and 3.3V,respectively,after a 72h storage time.Notice that the reduction peak at 4.16V corre-sponding to the H2-H3phase change disappeared after 36h of storage.Surprisingly,the reduction peaks for the AlF 3-coated

Li[Ni 0.8Co 0.15Al 0.05]O 2remained almost unchanged even after a 72h storage time,though the peak positions moved to a lower angle.

To study the reason for the improved electrochemical performances of the AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2,elec-trochemical impedance spectroscopy (EIS)was carried out for the pristine and AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2in a charged state of 4.3V as a function of cycle number at 55°C and storage time at 60°C.Figure 10shows Nyquist plots of the pristine and coated materials.Expanded views of the two overlapped semicircles in the high-to-medium frequency region are also shown in the inset of Figure 10.The equivalent circuit used in this study was reported in a previous study.The scattering symbol represents experimental data,and the continuous lines represent ?tted results using the equivalent circuit.These simulated lines correspond well to the plots of observed data.The semicircle occurring at a high frequency could be attributed to the resistance of the surface ?lm (R sf ),and the second circle appearing at high-to-medium frequency is associated with the charge-transfer resistance (R ct ).Notice that the charge-transfer resistance (Li[Ni 0.8Co 0.15Al 0.05]O 2/electrolyte interface)for both electrodes rapidly increased with cycling,while the surface-?lm resistance was kept almost stable at values of 30–40?(Table 1).Similar phenomena were observed for AlF 3-coated LiCoO 2,Li[Ni 1/3Co 1/3Mn 1/3]O 2,and Li[Ni 0.8Co 0.1Mn 0.1]O 2,as we previously reported.19–21Variations with cycle number in surface-?lm resistance (R sf )and charge-transfer resistance (R ct )for both the pristine and AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2electrodes are shown in Table 2.The R ct value of the pristine Li[Ni 0.8Co 0.1Mn 0.1]O 2electrode after the ?rst cycle was 15.2?and increased rapidly to 167.0?after the 50th cycle.However,the R ct of the AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2electrode held nearly constant,growing from 14.9?after the ?rst cycle to 89.0?after the 50th cycle.

Figure 11represents the Nyquist plots of the pristine and AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2with storage time at

60

Figure 10.Nyquist plots of (a)the Li/pristine and (b)the Li/AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2cell at the 1st,25th,and 50th cycles.

Table 2.Surface-Film Resistance (R sf )and Charge-Transfer

Resistance (R ct )for Pristine and AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2with Cycle Number (Units Are ?)

pristine

Li[Ni 0.8Co 0.15Al 0.05]O 2

AlF 3-coated

Li[Ni 0.8Co 0.15Al 0.05]O 2

cycle number

R sf R ct R sf R ct 129.115.238.914.92531.7106.128.931.350

33.5

167.0

31.3

89.0

Figure 11.Nyquist plots of (a)the Li/pristine and (b)the Li/AlF 3-coated Li[Ni 0.8Co 0.15Al 0.05]O 2cell at fresh,36h,and 72h of storage time.

3880Ind.Eng.Chem.Res.,Vol.47,No.11,2008

°C in the charged state of4.3V.Similar impedance growth was observed for the coated electrode after storage time. However,the R sf values of both electrodes slowly increased with storage time,unlike the electrodes with cycling shown in Figure10.These results indicate that the Li[Ni0.8Co0.15Al0.05]O2has poor storage characteristics at elevated temperatures.Variations in R sf and R ct with storage time for both the pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2electrodes are given in Table3.The R ct value of the pristine Li[Ni0.8Co0.15Al0.05]O2electrode after the?rst cycle was38.6?and increased rapidly to95.3?after the50th cycle.However,the R ct of AlF3-coated Li[Ni0.8-Co0.15Al0.05]O2held nearly constant,growing from11.0?after the?rst cycle to23.0?after the50th cycle.Also,the R sf of AlF3-coated Li[Ni0.8Co0.15Al0.05]O2had a lower value than that of pristine Li[Ni0.8Co0.15Al0.05]O2.The enhanced storage performance of AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 at60°C could be attributed to reduced charge-transfer and surface-?lm resistances.It is clear from these results that AlF3coating gives rise to a signi?cant lowering of charge-transfer resistance during cycling and storage time at elevated temperatures,which in turn enhances the cycling behavior and the rate capability.

Conclusions

Li[Ni0.8Co0.15Al0.05]O2particles were uniformly coated with a thin AlF3layer.The AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 electrode showed improved electrochemical properties after storage at60°C,improved cycling performance at25,55, and–10°C,and improved rate capability.The AlF3-coated Li[Ni0.8Co0.15Al0.05]O2showed a capacity retention of91% after72h of storage,even at60°C,while the capacity retention of the pristine material decreased to only46%.From the differential capacity(d Q/d V)versus voltage pro?les,we see that an AlF3coating on cathode particles does not suppress structural changes induced by phase transition but stabilizes the host structure.Such excellent electrochemical properties could originate from the low and stable charge-transfer resistance between cathode and electrolyte and the suppression of surface degradation of the Li[Ni0.8Co0.15Al0.05]O2host structure by the AlF3coating. Acknowledgment

This work was supported by KOSEF through the Research Center for Energy Conversion and Storage.

Supporting Information Available:Included in the Sup-porting Information is a graph of pulse power ASI as a function of DID for a Li/pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 cell.This material is available free of charge via the Internet at https://www.360docs.net/doc/af118675.html,.

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Table3.Surface-Film Resistance(R sf)and Charge-Transfer Resistance(R ct)for Pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 with Storage Time(Units Are?)

pristine

Li[Ni0.8Co0.15Al0.05]O2

AlF3-coated

Li[Ni0.8Co0.15Al0.05]O2

storage time R sf R ct R sf R ct

fresh12.438.613.511.0

36h16.768.115.013.6

72h25.095.318.623.0

Ind.Eng.Chem.Res.,Vol.47,No.11,20083881

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Recei V ed for re V iew November9,2007

Re V ised manuscript recei V ed February21,2008

Accepted March1,2008

IE0715308

3882Ind.Eng.Chem.Res.,Vol.47,No.11,2008