Functionality of Oxide Coating for Li[Li0.05Ni0.4

Feb 14, 2007 - and Naoaki Kumagai†. Department of Frontier Materials and Functional Engineering, Graduate School of Engineering,. Iwate UniVersity, ...
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J. Phys. Chem. C 2007, 111, 4061-4067

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Functionality of Oxide Coating for Li[Li0.05Ni0.4Co0.15Mn0.4]O2 as Positive Electrode Materials for Lithium-Ion Secondary Batteries Seung-Taek Myung,*,†,‡ Kentarou Izumi,† Shinichi Komaba,§ Hitoshi Yashiro,| Hyun Joo Bang,+ Yang-Kook Sun,+ and Naoaki Kumagai† Department of Frontier Materials and Functional Engineering, Graduate School of Engineering, Iwate UniVersity, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan, Department of Applied Chemistry, Tokyo UniVersity of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan, Department of Chemical Engineering, Iwate UniVersity, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan, and Department of Chemical Engineering, Hanyang UniVersity, Seungdong-Gu, Seoul 133-791, South Korea ReceiVed: NoVember 10, 2006; In Final Form: January 9, 2007

Surface-modified Li[Li0.05Ni0.4Co0.15Mn0.4]O2 oxides were studied. The oxide particles were coated by heteroelements such as Al2O3, Nb2O5, Ta2O5, ZrO2 and ZnO. Metal oxide-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 did not show significant difference in X-ray diffraction patterns. Thickness of the formed coating layer was around 10 nm, as observed by transmission electron microscopy. Electrochemical properties of heteroelementcoated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 were investigated using coin type Li-ion cells employing graphite as an anode at 60 °C. Metal oxide-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 obviously showed higher capacity with good cyclability. Also, area-specific impedance was significantly lower for the metal oxide-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 during cycling, compared with that for bare Li[Li0.05Ni0.4Co0.15Mn0.4]O2. Among them, Al2O3coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 had the best electrochemical performances. The metal oxide coating layer transformed to metal fluoride layer during cycling, as proved by time-of-flight-secondary ion mass spectroscopy. The newly formed metal fluoride layer would be greatly effective against HF attack during cycling. Possible reasons for the effectiveness of the metal oxide coating are discussed.

Introduction Recently, layered O3 type Li[(Ni0.5Mn0.5)1-xCox]O2 (x ) 0 - 0.33, R3hm) oxides have given rise to a great deal of interest as positive electrode materials for lithium-ion secondary batteries, because they have comparable specific capacity with LiCoO21-3 and reduced amount of Co in the compound leads to significantly increased thermal stability and less safety concerns.4-6 It is quite interesting to note the material chemistry of Li[(Ni0.5Mn0.5)1-xCox]O2 (x ) 0 - 0.33); Ueda et al.7 reported that the average oxidation state of Ni, Co, and Mn are 2+, 3+, and 4+, respectively. Solid-state redox couples of the compound are Ni2+/3+/4+ and Co3+/4+, so that theoretical capacity is comparable to O3-type layered compounds, such as LiCoO2 and LiNiO2, of about 280 mAh (g-oxide)-1. Though tetravalent Mn does not join the electrochemical reaction, it attributes to significant structural and thermal stability at, especially highly delithiated states.8 Even though Li[(Ni0.5Mn0.5)1-xCox]O2 (x ) 0 - 0.33) shows good electrochemical properties, capacity fading resulting from Co dissolution is also observed when they are cycled beyond 4.2 V versus Li, similarly to LiCoO2.3 To achieve good cyclability, it is necessary to stabilize the structure by introducing stable elements into the host structure. However, substitution

is usually carried out for the electrochemically active elements so that it may reduce obtainable capacity because the amounts of electrochemically active ingredients are reduced, though their cyclabilities improve. The other possible way is to coat the active materials with nanoporous materials. Since the coating layer on the surface of the active materials is nanoporous, electrolyte can be penetrated into the porous coating layer deeply. Simultaneously, the nanoporous layer provides Li+ diffusion path, and hence, it is possible to obtain comparable specific capacity with the bare active materials.9 For these reasons, metal oxide coatings were often applied for lithiated transition metal oxides or carboneous materials.10-15 Overall, the coated materials exhibit better capacity and cyclability especially for high voltage applications, though the reason is not clarified yet. Recently, we suggested that the Al2O3 coating layer on Li[Li0.05Ni0.4Co0.15Mn0.4]O2 particles gradually transformed to AlF3 via an intermediate stage (copresence of mixed chemical bonds of Al-O-F and Al-F after 300 cycles at 60 °C) that exists on the surface of Al2O3 coating layer as it was proved by ToF-SIMS, which indicates that Al2O3 scavenges HF.16 In this investigation, we would like to study the effectiveness of metal oxide coating for lithiated transition metal oxides. Experimental

* Corresponding author. Tel: +82-31-230-7386. Fax: +82-31-232-0694. E-mail:[email protected]. † Graduate School of Engineering, Iwate University. ‡ Current address: 3M Korea, Innovation Center, 374-5, Byeongjeomdong, Hwaseong-Si, Kyonggido 445-360, South Korea. § Tokyo University of Science. | Department of Chemical Engineering, Iwate University. + Hanyang University.

In order to prepare heteroelement-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 (3M), Al[OCH(CH3)2]3, Nb[OC2H5]5, Ta[OC2H5]5, or Zr[OC2H5]5 was first completely dissolved in ethanol at room temperature. For ZnO coating, Zn(CH3COO)2‚2H2O was dissolved in distilled water by adjusting the solution pH to 10.5. The active material, Li[Li0.05Ni0.4Co0.15Mn0.4]O2, was slowly

10.1021/jp0674367 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/14/2007

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Myung et al. TABLE 1: Calculated Lattice Parameters of Heterometal Element-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 bare Al2O3-coated Nb2O5-coated Ta2O5-coated ZrO2-coated ZnO-coated

Figure 1. XRD patterns of heterometal element-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2; (a) bare, (b) Al2O3-coated, (c) Nb2O5-coated, (d) Ta2O5coated, (e) ZrO2-coated, and (f) ZnO-coated.

poured into the each solution. The starting ratio of Li[Li0.05Ni0.4Co0.15Mn0.4]O2 versus each coating medium was 99:1 in weight, because the starting ratio exhibited the best results in our previous report.16 Then the solution containing the active material was constantly stirred at 80 °C for 2 days, accompanying by a slow evaporation of solvent. For ZnO coating the solution evaporation was done at 120 °C. The solution-treated as-received and bare Li[Li0.05Ni0.4Co0.15Mn0.4]O2 powders were fired at 400 °C in air for 5 h in air. For comparison, the bare material was also poured in the ethanol, and it followed the same drying and heating procedures. X-ray diffractometry (XRD, Rigaku Rint 2200) and transmission electron microscopy (TEM; 200 kV, Hitachi, H-800) were employed to characterize the prepared powders. XRD data were obtained at 2θ ) 10 to 80°, with a step size of 0.03° and a count time of 5 s. For fabrication of positive electrodes, the prepared powders were mixed with carbon black and polyvinylidene fluoride (94: 3:3) in N-methyl-pyrrodinone. The slurry thus obtained was coated onto Al foil and roll-pressed at 120 °C in air. The electrodes were dried at 120 °C for overnight in vacuum state prior to use. Cell tests were done using 2032 coin-type cell adopting Li metal as the negative electrode. The long cyclelife tests were performed in coin-type full cell. MCMB (Osaka gas) was used as the negative electrode. The electrolyte solution used was 1 M LiPF6 in ethylene carbonate-dimethyl carbonate (1:2 in volume). A preliminary cell formation was performed for Li-ion cell: five cycles were performed at room temperature at 0.01-0.5 C rates in the voltage range of 3.0-4.2 V versus graphite. The C-rate is defined as the exchange of 0.5 F per formula unit in 1 h. The cells were charged and discharged between 3 and 4.2 V by applying constant current density of 30 mA g-1 at 60 °C. To confirm the presence of byproducts on the surface of the active materials after extensive cycling, the cycled active materials were examined using a time-of-flight-secondary ion mass spectroscopy (ToF-SIMS, ULVAC-PHI TFS2000, Perkin-Elmer) surface analyzer operated at 10-9 Torr, equipped with a liquid Ga ion source and pulse electron flooding. During the analysis, the targets were bombarded by the 10 keV Ga beams with pulsed primary ion current varying from 0.3 to 0.5 pA. The total collection time was 300 s and rastered over a 12 × 12 mm area. Results and Discussion Figure 1 shows XRD patterns of solution-treated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 fired at 400 °C for 5 h in air. The diffrac-

a-axis/Å

c-axis/Å

2.871(5) 2.871(3) 2.873(2) 2.872(5) 2.872(1) 2.871(2)

14.279(3) 14.279(5) 14.2802) 14.279(7) 14.279(5) 14.278(3)

tion patterns can be identified as a hexagonal R-NaFeO2 structure with space group R3hm for the bare and heteroelementscoated materials in Figure 1a,b-f. Though the Li[Li0.05Ni0.4Co0.15Mn0.4]O2 was coated by several elements, it is difficult to find other impurities due probably to the smaller amount of coating media in the final products of Figure 1b-f. The XRD patterns appeared to be almost identical for the all samples. The calculated lattice parameters by a least-square method for the coated powders also showed that there is no significant difference in the crystal structure after the coating, as shown in Table 1. Provided that the coating media were replaced in the parent oxide matrix, the resulting lattice should be changed somewhat, which is originated from the difference in ionic radius of the coating media. If the lattice parameters are changed, it indicates that foreign element should be incorporated into the crystal structure. In fact, the a-axis parameter corresponds directly to the interatomic distance of metal and oxygen. Again, the above results show that the heteroelements were not incorporated into the Li[Li0.05Ni0.4Co0.15Mn0.4]O2 structure. Additionally, the coating materials would be an amorphous state because the coating media hardly show crystallinity when they are fired at the temperature as low as 400 °C. The coated amount of heteroelements was approximately 0.25 wt % in the final products. Figure 2 illustrates bright-field TEM images for the bare and heteroelement-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2. The bare material showed a smooth edge line without any other layer on the surface in Figure 2a. For coated materials in Figure 2b-f, one can clearly observe the newly formed layer on the surface of Li[Li0.05Ni0.4Co0.15Mn0.4]O2 particles. The estimated thickness of the coating layers ranged around 5-10 nm. From the images, it seems that the formed coating layer would be porous relative to the active material as seen from the contrasts of the pictures in Figure 2. From the above results, it is found that the Li[Li0.05Ni0.4Co0.15Mn0.4]O2 particles are coated by heteroelements (Al2O3, Nb2O5, Ta2O5, ZrO2, and ZnO). Figure 3 shows the initial charge and discharge curves of bare and heteroelements-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 powders (∼0.25 wt %). The applied current density across the positive electrode was 30 mA g-1 between 3.3 and 4.3 V versus Li at 25 °C. As shown in Figure 3, the all materials exhibited different electrochemical behaviors. Compared to the bare material, the Al2O3-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 showed slightly higher operation voltage in the whole range during discharge with much higher capacity, 167 mAh (g-oxide)-1. A slight decrease in the operation voltage was seen for the Nb2O5, Ta2O5, and ZnO-coated materials. Only Al2O3-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 delivered slightly higher specific discharge capacity than the bare material in Figure 3. Similar results have been reported in the literature for the case of Al2O3 coatings.9,11,12,16,17 On the other hand, all others showed slightly smaller discharge capacity compared to the bare material. Among them, ZrO2-coated material showed the lowest charge and discharge capacity in Figure 3. Similar tendency was also observed in ZrO2-coated LiCoO2.15,17 In fact, the metal oxide

Oxide Coating for Li[Li0.05Ni0.4Co0.15Mn0.4]O2

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Figure 2. TEM bright-field images of heterometal element-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2; (a) bare, (b) Al2O3-coated, (c) Nb2O5-coated, (d) Ta2O5-coated, (e) ZrO2-coated, and (f) ZnO-coated. The scale bar indicates 10 nm.

Figure 3. Initial charge-discharge curves of heterometal elementcoated Li[Li0.05Ni0.4Co0.15Mn0.4]O2/Li cells. The cycles were done by applying a current of 30 mA g-1 at 25 °C.

coating layers have electric insulating layer so that it may hinder electron and ion transfer. From the TEM images in Figure 2, it is likely that the formed oxide coating layers are thin and porous because the contrasts are lighter relative to Li[Li0.05Ni0.4Co0.15Mn0.4]O2. Groner et al.18 reported that electron migrations can

be somewhat faster by the tunneling effect when the pore size is less than several nanometers. Therefore, the thin and porous oxide coating layers would attribute to the high capacity. Accelerated cycling tests of the bare and coated materials were carried out by employing graphite negative electrode with applying a current density of 30 mA g-1 at 60 °C. It was thought that the elevated test temperature is quite severe condition for the Li-ion cells so that degradation of the positive electrode material would occur faster. Hence, one can easily discriminate the effect of each coating layer during degradation by electrochemical cycling. The same electrodes used for the half cell tests were applied for the full cell test in Figures 4 and 5. For the bare material, we can observe relatively higher initial charge and discharge capacities. However, the capacity faded fast with cycling in Figures 4a and 5. In particular, the operation voltage decay by cycling seems to be quite severe in Figure 4a, meaning a great increase in the internal resistance during cycling. For the case of coated materials, capacity fading was not so drastic relative to the bare material in Figure 5. The trend in the capacity in Figure 4 was similar to that in found for half cell tests shown in Figure 3. Comparing to the bare material, starting discharge voltages were also higher during extensive cycling for the coated

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Figure 4. Continuous charge-discharge curves of heterometal elementcoated Li[Li0.05Ni0.4Co0.15Mn0.4]O2; (a) bare, (b) Al2O3-coated, (c) Nb2O5-coated, (d) Ta2O5-coated, (e) ZrO2-coated, and (f) ZnO-coated. Graphite electrode was used as the negative electrode. The cyclings were done by applying a current of 30 mA g-1 at 60 °C.

Figure 5. (a) Cyclability and (b) capacity retention of heterometal element-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2. Graphite electrode was used as the negative electrode. The cyclings were done by applying a current of 30 mA g-1 at 60 °C.

materials in Figure 4, which means that the internal resistance of the cells was quite reduced by the metal oxide coating. Area-specific impedances (ASI) were also monitored to examine the resistance for the coating medium during cycling at 60 °C. The ASI was determined according to the equation (A∆V)/I, where A is the cross-sectional area, ∆V is the voltage variation during current interruption for 30 s at 100% SOC, and I is the current applied during galvanostatic cycling.19 As seen in Figure 6a, the initial ASI values for the bare material

Myung et al.

Figure 6. (a) Area-specific impedance (ASI) variation measured at each 100% SOC with cycling and (b) increased ASI after 100 cycles. Graphite electrode was used as the negative electrode. The cyclings were done by applying a current of 30 mA g-1 at 60 °C.

was about 100 Ω cm2, which is much greater than that of Al2O3, ZrO2, and ZnO-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2. With cycling the R value increased drastically, and it reached around 280 Ω-cm2, showing that a value is 4 times greater than that of the ZnO-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 in Figure 6a. The ASI values for the heteroelement-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 have relatively good stability during cycling. However, the initial R values depend on the coating media. Among the coatings, Al2O3, ZrO2, and ZnO-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 showed significantly lower R values and the values increased relatively slow with cycling. When the ASIs were compared after 100 cycles with the initial state, one can see that the Al2O3-coated material exhibited the smallest variation in ASI during cycling in Figure 6b. On the other hand, the bare material showed the greatest increase in the ASI after 100 cycles in Figure 6b. From these results, it is suggested that the metal oxide coatings greatly improve the cathode kinetics and capacity retention during cycling. To find out the possible reason why the coating of the active material is substantially effective, the cycled positive electrode materials were examined by ToF-SIMS, which is one of the sensitive surface analytic tools, though quantitative analysis is difficult. The extensively cycled cells at 60 °C were disassembled and the positive electrodes were carefully washed with salt-free dimethyl carbonate in a glove box for one week to remove the remained lithium salt completely. As we previously reported, positive electrode oxide material was decomposed by HF attack from the electrolyte, which is generated by the decomposition of the electrolytic salt with following reaction as suggested by Aurbach et al.20 and Edstro¨m et al.:21

LiPF6 f LiFV + PF5

(1)

PF5 + H2O f POF3 + 2HF

(2)

POF3 + 3Li2O- f 6LiFV + P2O5V (or LixPOFy)

(3)

Previously, we showed that even though the active material is

Oxide Coating for Li[Li0.05Ni0.4Co0.15Mn0.4]O2

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Figure 7. ToF-SIMS spectra of the electrochemically cycled heterometal element-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 electrodes; (a) Ni-F, (b) Co-F, and (c) Mn-F fragments. Graphite electrode was used as the negative electrode. The cyclings were done by applying a current of 30 mA g-1 at 60 °C.

coated by Al2O3, decomposition of the active material by HF is inevitable.16 The same results are also seen in other coating system in Figure 7a-c, by the following reactions:

Bare and/or heteroelement-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 + 2HF + Li+ + e- f Li1-δ[Li0.05Ni0.4Co0.15Mn0.4]O2 + LiF + H2O

(4)

NiO + 2HF f NiF2 + H2O

(5)

CoO + 2HF f CoF2 + H2O

(6)

MnO + 2HF f MnF2 + H2O

(7)

Whatever bare is coated by other heteroelements, we can clearly see the formation of byproducts, such as NiF2, CoF2, and MnF2 in Figure 7. Therefore, it is found that the degradation of the active material occurs during cycling even though it is encapsulated by the thin nanoporous coating layers. However, decomposition of the cathode material occurs much rapidly on bare Li[Li0.05Ni0.4Co0.15Mn0.4]O2, because it is directly exposed to the strong acidic species, HF, generated from decomposition of the electrolyte. Simultaneously, such byproducts derived from the electrochemical reactions would appear on the surfaces of cathode, separator, and anode and into electrolyte so that they would attribute to increase in the internal resistance of the cells because they are electrical insulator, as shown in ASIs results of Figure 6. For the metal oxide-coated Li[Li0.05Ni0.4Co0.15-

Mn0.4]O2, on the other hand, such decomposition reactions would be delayed because the coating layers avoids the direct contact of the active materials from the HF attack, meaning that the coating layers work as protecting layers against HF, leading to less increase in the ASIs during cycling. Figure 8 shows magnified metal-fluoride ToF-SIMS spectra of the electrochemically cycled heterometal element-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 electrodes. All oxide coating layers were transformed to metal fluoride layers in Figure 8. As we previously reported in Al2O3-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2, the Al2O3 coating layer was changed to AlF3 layer. Though the reaction occurs rather slower, it is obvious that the surfaces of metal oxide layers scavenge F- from the acidic HF species into the electrolyte, leading to metal-fluoride layers from the ToFSIMS results in Figure 8. A schematic drawing of the interface between metal oxidecoated positive electrode and electrolyte is shown in Scheme 1. As confirmed by ToF-SIMS in Figure 8, we found that when the coated fresh metal oxides are attacked by the HF from the electrolyte, the metal oxide layer gradually transforms to a metal fluoride layers by scavenging F- from the HF. Further reaction by HF would lead to much thicker metal fluoride layers. By getting F- from HF on the surface of oxide coating layer, the concentration of acidic species into the electrolyte becomes significantly lower,16 leading to less degradation of active material by the acidic species during cycling. Moreover, the formed metal fluoride layers are resistant to HF and further protect the active material from the HF attack. For these reasons,

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Figure 8. ToF-SIMS spectra of the electrochemically cycled heterometal element-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 electrodes; (a) Al-F, (b) Ta-F, (c) Nb-F, (d) Zr-F, and (e) Zn-F fragments. Graphite electrode was used as the negative electrode. The cyclings were done by applying a current of 30 mA g-1 at 60 °C.

SCHEME 1: Schematic Drawing of Interface between Positive Electrode and Electrolyte

it is possible to understand why the metal oxide coating resulted in better cyclability (Figures 4 and 5) and lower increase in ASIs during cycling in Figure 6. Though the oxide layers were transformed to metal fluoride layers, they are stable enough to resist HF attack. From these reasons, the Al2O3, ZrO2, and ZnO-coated material showed reasonable capacity and cyclability as well. Among them, the Al2O3-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 exhibits the best performance. Hence, it has the highest capacity with best cyclability, and lowest increase in the ASIs during cycling at 60 °C. On contrast, Nb2O5 and Ta2O5-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 had relatively inferior electrochemical properties to Al2O3 and ZnO-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2. Furthermore, the layers would melt at around 100 °C, so that the TaF5 (mp 80 °C22) and NbF5 (mp 96.8 °C22) fluoride layers would not be rigid as much as AlF3 (mp 1090 °C22) and ZnF2 (mp 872 °C22) layer, when the battery is operated at 60 °C. Even though the TaF5 and NbF5 layers have inferior mechanical properties among the heteroelement-coated materials, their battery performances are still better than those of bare Li[Li0.05Ni0.4Co0.15Mn0.4]O2 in Figures 4-6.

Figure 9. TEM bright-field images of extensively cycled Li[Li0.05Ni0.4Co0.15Mn0.4]O2 electrodes; (a) bare, (b) Al2O3-coated, (c) Ta2O5coated, (d) Nb2O5-coated, (e) ZrO2-coated, and (f) ZnO-coated. Graphite electrode was used as the negative electrode. The cyclings were done by applying a current of 30 mA g-1 at 60 °C.

Figure 9 shows TEM bright-field images of the extensively cycled bare and heteroelement-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 at 60 °C. For the bare material which had around 30% of capacity loss of its initial capacity, the surface of the particle was severely damaged (marked as arrows), and even the particle

Oxide Coating for Li[Li0.05Ni0.4Co0.15Mn0.4]O2 was cracked by cycling at elevated temperature in Figure 9a, compared to the fresh bare material in Figure 2a. Such disruption of the particle would mainly result from direct contact of the surface of the active material with the HF into the electrolyte. Consequently, the produced insulating byproducts, such as NiF2, CoF2, and MnF2, may not only block the direct electrical connection of active particle to particle, but disjoined active particles from their parents would lose the electrical contact between active material and conducting carbon. This leads to capacity fade and increment of impedance as seen in Figures 4-6. For coated materials, the smooth edges were well maintained even after elevated temperature cycling in Figure 9b-f. Compared with the fresh active particles (Figure 2b-f), the only morphological difference of the coating layers was thickness of the layer probably due to the penetration of the electrolyte into the porous coating layer and/or conversion of the coating layer from a metal oxide layer to a metal fluoride one on the surface of the coating layer. As mentioned in Scheme 1, the fluoride layers were formed on the oxide coating layer by scavenging of F- from HF into the electrolyte. It simultaneously dilutes the concentration of the acidic species in the electrolyte. Reduced amount of the acidic species into the electrolyte gives rise to the less degradation of the active material. Furthermore, the fluoride layers would work as the first protecting layer against HF attack. The metal fluoride layer, therefore, made it possible to effectively keep their original smooth edge lines of active particles, resulting to good capacity retention with lower increase in the impedance with cycling. Conclusion We investigated functionality of oxide coating for Li[Li0.05Ni0.4Co0.15Mn0.4]O2 as positive electrode materials for lithiumion secondary batteries. Metal oxide coatings which do not join the electrochemical reaction exhibited significantly improved cycling performance at 60 °C. The surface modification brought about quite stable variation of ASIs during cycling and the increment of the ASIs were quite smaller for, especially, Al2O3and ZnO-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2. The reasons for the effectiveness of the metal oxide coating for Li[Li0.05Ni0.4Co0.15Mn0.4]O2 would be possibly explained, as follows. From ToF-SIMS observation, it was found that the amphoteric oxide coating layers gradually transformed to metal fluoride films due to the scavenging of F- from HF, which is the one of the byproducts resulting from the electrolytic salt decomposition into electrolyte. Thus, the metal fluoride layer on the metal oxide coating layer can be formed. Metal fluorides are usually strongly resistive against acidic species so that newly formed metal fluoride layers on the surface of the active material would

J. Phys. Chem. C, Vol. 111, No. 10, 2007 4067 attribute to protect the active material from the direct contact of HF. Such behaviors would enable to preserve the original particle morphology, and it, in turn, would lead to good cyclability even at 60 °C. On the other hand, direct exposition of the active material against HF resulted in poor cyclability and significant drastic increase in ASI, which would be originated from the decomposition of the active materials. Therefore, it is conclude that surface modifications of electrode materials with amphoteric metal oxide coating are necessary to maintain high capacity with good cyclability of lithium-ion secondary batteries. Acknowledgment. The authors would like to thank Ms. Y. Mitobe from Iwate University for her helpful assistance in the experimental works. References and Notes (1) Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. Mater. Res. Bull. 1980, 15, 783. (2) Ohzuku, T.; Makimura, Y. Chem. Lett. 2001, 30, 744. (3) Lu, Z.; MacNeil, D. D.; Dahn, J. R. Electrochem. Solid-State Lett. 4, 2001, A200. (4) Lu, Z.; Beaulieu, L. Y.; Donaberger, R. A.; Thomas, C. L.; Dahn, J. R. J. Electrochem. Soc. 2002, 149, A778. (5) Kang S.-H.; Amine, K. J. Power Sources 2003, 119-121, 150. (6) Shizuka, K.; Kobayashi, T.; Okahara, K.; Okamoto, K.; Kanzaki, S.; Kanno, R. J. Power Sources 2005, 146, 589. (7) Ueda A.; Aoyama, S. Proceeding of the 42th Battery Symposium in Japan; Electrochemical Society of Japan: Yokohama, Japan, November 21-23, 2001; pp 130 and 131. (8) Yoon, W.-S.; Grey, C. P.; Balasubramanian, M.; Yang, X.-Q.; McBreen, J. Chem. Mater. 2003, 15, 3161. (9) Kim, S.-S.; Kadoma, Y.; Ikuta, H.; Uchimoto, Y.; Wakihara, M. Electrochem. Solid State Lett. 2001, 4, A109. (10) Kottegoda, I. R. M.; Kadoma, Y.; Ikuta, H.; Uchimoto, Y.; Wakihara, M. Electrochem. Solid State Lett. 2002, 5, A275. (11) Eftekhari, A. J. Electrochem. Soc. 2004, 151, A1456. (12) Van Landschoot, N.; Kelder, E. M.; Kooyman, P. J.; Kwakernaak, C.; Schoonman, J. J. Power Sources 2004, 138, 262. (13) Sun, Y.-K.; Yoon, C. S.; Oh, I.-H. Electrochim. Acta 2002, 48, 503. (14) Miyashiro, H.; Yamanaka, A.; Tabuchi, M.; Seki, S.; Nakayama, M.; Ohno, Y.; Kobayashi, Y.; Mita, Y.; Usami, A.; Wakihara, M. J. Electrochem. Soc. 2006, 153, A348. (15) Lin, Y.-M.; Wu, H.-C.; Yen, Y.-C.; Guo, Z.-Z.; Yang, M.-H.; Chen, H.-M.; Sheu, H.-S.; Wu, N.-L. J. Electrochem. Soc. 2005, 152, A1526. (16) Myung, S.-T.; Izumi, K.; Komaba, S.; Sun, Y.-K.; Yashiro, H.; Kumagai, N. Chem. Mater. 2005, 17, 3695. (17) Chen, Z.; Dahn, J. R. Electrochem. Solid-State Lett. 2003, 6, A221. (18) Groner, M. D.; Elam, J. W.; Fabreguette, F. H.; George, S. M. Thin Solid Films 2002, 413, 186. (19) Kang, S.-H.; Amine, K. J. Power Sources 2005, 146, 654. (20) Markovsky, B.; Rodkin, A.; Salitra, G.; Talyossef, Y.; Aurbach, D.; Kim, H.-J. J. Electrochem. Soc. 2004, 151, A1068. (21) Edstro¨m, K.; Gustafsson, T.; Thomas, J. O. Electrochim. Acta 2004, 50, 379. (22) Dean, J. A. Lange’s Handbook of Chemistry, 4th ed.; McGrawHill Inc.: New York, 1992; p 4.13.