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Ind. Eng. Chem. Res. 2008, 47, 3876–3882
MATERIALS AND INTERFACES Improvement of Electrochemical Performance of Li[Ni0.8Co0.15Al0.05]O2 Cathode Materials by AlF3 coating 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 UniVersity, Seoul 133-791, South Korea, and Department of Chemical and Biological Engineering, Illinois Institute of Technology, 10 West 33rd Street, Chicago, Illinois 60616
A thin AlF3 layer of ∼10 nm was uniformly coated on the particle surface of Li[Ni0.8Co0.15Al0.05]O2. The AlF3 coating improved cycle performance at 55, 30, and –10 °C and improved storage characteristics at 60 °C. In studies of differential capacity (dQ/dV) versus voltage, the AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 showed little variation in redox peaks with cycling. Electrochemical impedance spectroscopy suggested that the AlF3 coating 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).1,2 Nirich Li[Ni1-xMx]O2 (M ) metal) cathode materials are alternatives to LiCoO2 and have lower cost, lower toxicity, and higher reversible capacity.3–5 However, Ni-rich Li[Ni1-xMx]O2 has been found to exhibit low thermal stability, poor cycling behavior, and increased impedance during cycling, particularly at high temperatures.6,7 One of the most promising materials is Li[Ni0.8Co0.15Al0.05]O2.8,9 The substitution of Ni with Co and Al improves its structural and thermal stability without sacrificing discharge capacity. In spite of the improved electrochemical performance of the Li[Ni0.8Co0.15Al0.05]O2, the major barriers that hinder commercial-scale application are its insufficient 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 encapsulate the Ni-based active materials with Li[Ni0.5Mn0.5]O2, a stable active material.10,11 Another problem inherent to Li[Ni0.8Co0.15Al0.05]O2 with high Ni content is its rapid reaction to moisture and ambient CO2, resulting in the formation of LiOH and Li2CO3 on the cathode particle surface.12,13 Li 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 oxide14–18 and metal fluoride19–21 to reduce the catalytic activity of the transition metal with a high oxidation state in LiMO2 (M ) Co, Ni, Mn). The electrochemical properties and thermal stability of the coated cathode materials show significant improvement over uncoated ones. Though the reason is not wellunderstood, the protection of the cathode surface provided by the coating may be responsible for the enhanced electrochemical * Corresponding author. Phone: +82-2-2220-0524. Fax: +82-22282-7329. E-mail:
[email protected]. † Hanyang University. ‡ Illinois Institute of Technology.
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.17,18,22 The 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 AlF3 coating over Li[Ni0.8Co0.15Al0.05]O2 on electrochemical performance at various temperatures. We also investigate the reason for the improvement of the electrochemical performance in the presence of the coating by comparing the differential capacity (dQ/dV) versus voltage profiles of pristine and AlF3-coated Li[Ni0.8Co0.1Mn0.1]O2. Experimental Section [Ni0.8Co0.15Al0.05](OH)2 powders were prepared by coprecipitation, as reported previously.23 An aqueous solution of NiSO4 · 6H2O, CoSO4 · 7H2O, and Al2(SO4)3 · 16H2O was pumped into a continuously stirred tank reactor (CSTR, capacity 4 L) in an N2 atmosphere. Simultaneously, an NaOH solution (aq) and an appropriate amount of NH4OH solution (aq), a chelating agent, were fed separately into the reactor. The spherical [Ni0.8Co0.15Al0.05](OH)2 powders were dried at 110 °C for 24 h to remove adsorbed water. Finally, a mixture of the dehydrated [Ni0.8Co0.15Al0.05](OH)2 and LiOH · H2O were preheated to 480 °C for 5 h and then heated at 750 °C for 20 h in an oxygen flow. The chemical compositions of the prepared powders were analyzed using atomic absorption spectroscopy (AAS, Vario 6, Analyticjena). To coat the surface of the Li[Ni0.8Co0.15Al0.05]O2 with AlF3, ammonium fluoride (Aldrich) and aluminum nitrate nonahydrate (Aldrich) were separately dissolved in distilled water. After the prepared Li[Ni0.8Co0.15Al0.05]O2 powders were immersed in the aluminum nitrate nonahydrate solution, the ammonium fluoride solution was slowly added. The molar ratio of F to Al was fixed at 6, and the amount of AlF3 in the solution corresponded to 0.25 mol % of the Li[Ni0.8Co0.15Al0.05]O2 powders. The solution containing the active material was
10.1021/ie0715308 CCC: $40.75 2008 American Chemical Society Published on Web 04/29/2008
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Figure 2. Bright-field TEM images of (a) pristine and (b) AlF3-coated Li[Ni0.8Co0.15Al0.05]O2. Figure 1. XRD patterns of (a) pristine and (b) AlF3-coated Li[Ni0.8Co0.15Al0.05]O2.
continuously stirred for 5 h at 80 °C, accompanied by a slow evaporation of solvent. The Li[Ni0.8Co0.15Al0.05]O2 powders obtained were heated at 400 °C for 5 h in an oxygen flow. Powder X-ray diffraction (XRD) (Rigaku, Rint-2000) employing Cu KR radiation was used to identify the crystalline phase of the prepared powders at each stage. The surface of the AlF3coated powder was also observed using transmission electron microscopy (TEM, JEOL 2010). Differential scanning calorimetry (DSC) experiments were carried out for the positive electrode materials by fully charging the coin cell to 4.3 V at a constant current and voltage and opening it in an Ar-filled dry room. The measurements were carried out in a differential scanning calorimeter 200 PC (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 40 and 100 mA/g at 30 °C. The cell consisted of positive and negative electrodes of lithium metal separated by a porous polypropylene film. To fabricate the cathode, the prepared powders were mixed with carbon black and polyvinylidene fluoride (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 2032 coin-type cell using Li metal for the anode. The electrolyte solution used was 1 M LiPF6 in ethylene carbonatediethyl carbonate (1:1 in volume, Cheil Industries, Inc., Korea). Results and Discussion Figure 1 shows the XRD patterns of the pristine and AlF3coated Li[Ni0.8Co0.15Al0.05]O2 materials. It was confirmed that the two powders have a well-defined layer structure based on the hexagonal a-NaFeO2 structure with space group Rm. The lattice constants, a and c, of the AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 powders were 2.865(5) and 14.194(7) Å, as calculated by a least-squares method. These values approximate those of the pristine material, a ) 2.864(1) and c ) 14.191(0) Å, indicating that the AlF3 was not incorporated into the host structure. Chemical analysis showed that the prepared powder composition was Li[Ni0.796Co0.153Al0.051]O2. Figure 2 shows a TEM image of the AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 powder. The TEM image proves the presence of the uniform AlF3 coating layer of 10 nm on the particle surface. Energy-dispersive X-ray spectroscopy confirmed the composition of the coating layer to be rich in Al. Figure 3 displays the initial charge/discharge curves of the Li/pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 cells by applying a constant current of 40 mA/g (0.5 C-rate) between 3.0 and 4.3 V vs Li+/Li. The charge voltage of the AlF3-coated
Figure 3. Initial charge–discharge curves of Li/pristine and Li/AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 cells at a current density of 40 mA/g between 3.0 and 4.3 V.
Li[Ni0.8Co0.15Al0.05]O2 electrode shows lower resistance than the pristine Li[Ni0.8Co0.15Al0.05]O2 in all operating regions, which increases the discharge capacity and the rate capability. The initial discharge capacity of the AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 is 178 mAh/g, as compared to that of the pristine material, which has a capacity of 168 mAh/g. It is speculated that a thin AlF3-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 4 show the discharge capacity of Li/ pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 cells over 50 cycles at 30 and 55 °C, respectively. The cells were first activated by applying a current density of 0.2 C-rate (20 mA/ g) for two cycles and were then cycled at 0.5 C-rate (40 mA/g) between 3.0 and 4.3 V vs Li+/Li. During the 30 °C cycling, the AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 electrode delivered a discharge capacity of 173 mAh/g, while the pristine electrode delivered only 160 mAh/g. The pristine Li[Ni0.8Co0.15Al0.05]O2 showed a gradual decrease in capacity, leading to a capacity retention of 91% after 50 cycles. The AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 displayed a slightly improved capacity retention of 97% after the same cycling period. However, the cycling behavior of the AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 at 55 °C was remarkably enhanced, showing capacity retention of 94% after 62 cycles. The pristine electrode showed a gradual decrease in capacity, leading to a capacity retention of only 73%. Recently, Abraham et al. reported that a LixNi1-xO-type layer was observed on the surface of a Li[Ni0.8Co0.2]O2 particle on which surface layers were 2-5 nm thick in samples from 0% power fade cells, and >35 nm thick in samples from 43% power fade cells.6 It is believed that the thin AlF3 coating layer lowers the catalytic activity of Ni4+ with electrolyte species in highly delithiated Li1-x[Ni0.8Co0.15Al0.05]O2 and, thus, reduces the formation of the
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Figure 6. Variation of the discharge capacities of (a) Li/pristine and (b) AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 cells in the charged state of 4.3 V at 60 °C as a function of storage time. Figure 4. Variation of the discharge capacities of Li/pristine and Li/AlF3coated Li[Ni0.8Co0.15Al0.05]O2 cells at (a) 30 and (b) 55 °C between 3.0 and 4.3 V at 0.5 C-rate (40 mA/g).
Figure 5. Plot of discharge capacity vs cycle number for the Li/pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 between 3.0 and 4.3 V at 0.2 C-rate (20 mA/g) and –10 °C.
LixNi1-xO-type layer resulting from Ni4+ decomposition, especially at temperatures higher than 55 °C. Figure 5 shows a plot of discharge capacity versus cycle number for the Li/pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 between 3.0 and 4.3 V at 0.2 C-rate (20 mA/g) and –10 °C. The cells were activated for two cycles at 0.2 C-rate and 30 °C before the test. Although AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 showed a slightly larger discharge capacity (123 mAh/ g) than the pristine material (113 mAh/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 AlF3 coating, which showed no capacity loss, while the pristine electrode suffered from a capacity loss of 14% after only 20 cycles. 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/AlF3-coated LiCoO2, Li[Ni1/3Co1/3Mn1/3]O2, and Li[Ni0.8Co0.1Mn0.1]O2 cells exhibit a significantly lower charge-transfer resistance (Rct) 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 6 shows the variations in discharge capacities of Li/pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 cells in the charged state of 4.3 V at 60 °C as a function of storage time. The prepared cells were preliminarily charged and discharged between 3.0 and 4.3 V at 0.2 C-rate and 30 °C for two cycles to determine their initial capacities. The cells were then charged to 4.3 V at 60 °C and stored for 24, 36, and 72 h. The aged cells were cooled down to 30 °C, and their discharge capacity was measured at each storage interval. While the pristine Li[Ni0.8Co0.15Al0.05]O2 showed a rapid decrease in capacity, leading to a capacity retention of only 46% after 72 h in storage, the storage capacity of the AlF3-coated electrode greatly improved, showing a capacity retention of 91% after the same storage time. The data also show the impact of AlF3 coating. To investigate the effects of transition metal dissolution of the Li1-δ[Ni0.8Co0.15Al0.05]O2 on 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 Li0.2[Ni0.8Co0.15Al0.05]O2 was increased with storage time, while dissolution from the AlF3-coated Li0.2[Ni0.8Co0.15Al0.05]O2 was kept nearly constant. For example,
Ind. Eng. Chem. Res., Vol. 47, No. 11, 2008 3879
Figure 7. Rate capability test of Li/pristine and Li/AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 cells at various C-rates.
Figure 9. Differential capacity vs voltage of (a) Li/pristine and (b) Li/ AlF3-coated Li[Ni0.8Co0.1Mn0.1]O2 cells in the voltage range of 3.0–4.3 V and 60 °C.
Figure 8. Differential capacity vs voltage of (a) Li/pristine and (b) Li/ AlF3-coated Li[Ni0.8Co0.1Mn0.1]O2 cells in the voltage range of 3.0–4.3 V and 30 °C.
the chemical compositions of the pristine and AlF3-coated Li0.2[Ni0.8Co0.15Al0.05]O2 after 72 h were estimated to be Li0.42[Ni0.70Co0.103Al0.04]O2-δ and Li0.27[Ni0.75Co0.14Al0.04]O2-δ, respectively. Note that the Li content of the pristine materials was quite a bit larger than that of the AlF3-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.24 It is believed that the AlF3 coating 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 7 shows the discharge capacities of the Li/pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 cells as a function of various C-rates (40-1000 mA/g) between 3.0 and 4.3 V. The capacity retention percentage of each C-rate (compared with 0.2 C-rate) is also shown in Figure 7. The cells were charged galvanostatically with a current density of 20 mA/g (0.2 C-rate) before each discharge test and then discharged at current densities from 20 (0.2 C-rate) to 1000 mA/g (5 C-rate). As observed in Figure 7, the capacity retention difference between the pristine and AlF3coated Li[Ni0.8Co0.15Al0.05]O2 electrode became larger with increasing C-rate. For example, the capacity retentions of AlF3coated Li[Ni0.8Co0.15Al0.05]O2 were 92% and 62% at 1 and 5 C-rate, respectively, while those of the pristine material were 89% and 54%. Notice that the AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 electrode delivers a larger discharge capacity than the pristine one across the range of C-rates. This result leads us to believe that the AlF3-coating layer on Li[Ni0.8Co0.15Al0.05]O2 plays 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 8 shows differential capacity (dQ/dV) versus voltage profiles of Li/pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 cells at the first, 30th, and 50th cycles. The cells were cycled between 3.0 and 4.3 V at a current density of 100 mA/g. During the first cycle, the Li/pristine Li[Ni0.8Co0.15Al0.05]O2 cell showed a sharp oxidation peak at 3.68 V and a satellite peak at 3.76 V,
Table 1. Chemical Compositions of Pristine and AlF3-coated Li1-δ[Ni0.8Co0.15Al0.05]O2 Charged to 4.3 V with Storage Time samples
36
72
pristine Li0.24[Ni0.8Co0.15Al0.05]O2 AlF3-coated Li0.24[Ni0.8Co0.15Al0.05]O2
Li0.34[Ni0.72Co0.13Al0.05]O2-δ Li0.25[Ni0.76Co0.15Al0.05]O2-δ
Li0.42[Ni0.70Co0.10Al0.04]O2-δ Li0.27[Ni0.75Co0.14Al0.04]O2-δ
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Figure 10. Nyquist plots of (a) the Li/pristine and (b) the Li/AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 cell at the 1st, 25th, and 50th cycles. Table 2. Surface-Film Resistance (Rsf) and Charge-Transfer Resistance (Rct) for Pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 with Cycle Number (Units Are Ω) pristine Li[Ni0.8Co0.15Al0.05]O2
AlF3-coated Li[Ni0.8Co0.15Al0.05]O2
cycle number
Rsf
Rct
Rsf
Rct
1 25 50
29.1 31.7 33.5
15.2 106.1 167.0
38.9 28.9 31.3
14.9 31.3 89.0
as well as several other peaks during charging due to multiphase transitions.25,26 Corresponding redox peaks at 3.67, 3.93, and 4.13 V were also observed. The redox peaks became more polarized and shifted farther apart. For example, the oxidation peak height at 3.68 V and the corresponding reduction peak at 3.67 V decreased after 50 cycles, indicating surface degradation of the host structure and/or increase of the interfacial impedance between the cathode and the electrolyte.20,21 No peak variation was observed for the AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 electrode during cycling. Figure 9 shows the discharge differential capacity (dQ/dV) versus voltage profiles of Li/pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 cells charged to 4.3 V at 60 °C as a function of storage time. Both the pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 electrodes showed three distinct reduction peaks at 3.68, 3.94, and 4.16 V at 30 °C, which corresponded to a phase transition between two O3 structures.25 However, the two electrodes showed different reduction peak changes after storage. For the pristine Li[Ni0.8Co0.15Al0.05]O2 electrode, storage at 60 °C caused increased polarization of the reduction peaks and shifted them farther apart. For example, the peaks at 3.68 and 3.94 V shifted to 3.31 and 3.3 V, respectively, after a 72 h storage time. Notice that the reduction peak at 4.16 V corresponding to the H2-H3 phase change disappeared after 36 h of storage. Surprisingly, the reduction peaks for the AlF3-coated
Figure 11. Nyquist plots of (a) the Li/pristine and (b) the Li/AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 cell at fresh, 36 h, and 72 h of storage time.
Li[Ni0.8Co0.15Al0.05]O2 remained almost unchanged even after a 72 h storage time, though the peak positions moved to a lower angle. To study the reason for the improved electrochemical performances of the AlF3-coated Li[Ni0.8Co0.15Al0.05]O2, electrochemical impedance spectroscopy (EIS) was carried out for the pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 in a charged state of 4.3 V as a function of cycle number at 55 °C and storage time at 60 °C. Figure 10 shows 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 fitted 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 film (Rsf), and the second circle appearing at high-to-medium frequency is associated with the charge-transfer resistance (Rct). Notice that the charge-transfer resistance (Li[Ni0.8Co0.15Al0.05]O2/electrolyte interface) for both electrodes rapidly increased with cycling, while the surfacefilm resistance was kept almost stable at values of 30–40 Ω (Table 1). Similar phenomena were observed for AlF3-coated LiCoO2, Li[Ni1/3Co1/3Mn1/3]O2, and Li[Ni0.8Co0.1Mn0.1]O2, as we previously reported.19–21 Variations with cycle number in surface-film resistance (Rsf) and charge-transfer resistance (Rct) for both the pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 electrodes are shown in Table 2. The Rct value of the pristine Li[Ni0.8Co0.1Mn0.1]O2 electrode after the first cycle was 15.2 Ω and increased rapidly to 167.0 Ω after the 50th cycle. However, the Rct of the AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 electrode held nearly constant, growing from 14.9 Ω after the first cycle to 89.0 Ω after the 50th cycle. Figure 11 represents the Nyquist plots of the pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 with storage time at 60
Ind. Eng. Chem. Res., Vol. 47, No. 11, 2008 3881 Table 3. Surface-Film Resistance (Rsf) and Charge-Transfer Resistance (Rct) for Pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 with Storage Time (Units Are Ω) pristine Li[Ni0.8Co0.15Al0.05]O2
AlF3-coated Li[Ni0.8Co0.15Al0.05]O2
storage time
Rsf
Rct
Rsf
Rct
fresh 36 h 72 h
12.4 16.7 25.0
38.6 68.1 95.3
13.5 15.0 18.6
11.0 13.6 23.0
°C in the charged state of 4.3 V. Similar impedance growth was observed for the coated electrode after storage time. However, the Rsf values of both electrodes slowly increased with storage time, unlike the electrodes with cycling shown in Figure 10. These results indicate that the Li[Ni0.8Co0.15Al0.05]O2 has poor storage characteristics at elevated temperatures. Variations in Rsf and Rct with storage time for both the pristine and AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 electrodes are given in Table 3. The Rct value of the pristine Li[Ni0.8Co0.15Al0.05]O2 electrode after the first cycle was 38.6 Ω and increased rapidly to 95.3 Ω after the 50th cycle. However, the Rct of AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 held nearly constant, growing from 11.0 Ω after the first cycle to 23.0 Ω after the 50th cycle. Also, the Rsf of AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 had 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 at 60 °C could be attributed to reduced charge-transfer and surface-film resistances. It is clear from these results that AlF3 coating gives rise to a significant lowering of chargetransfer 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]O2 particles were uniformly coated with a thin AlF3 layer. The AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 electrode showed improved electrochemical properties after storage at 60 °C, improved cycling performance at 25, 55, and –10 °C, and improved rate capability. The AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 showed a capacity retention of 91% after 72 h of storage, even at 60 °C, while the capacity retention of the pristine material decreased to only 46%. From the differential capacity (dQ/dV) versus voltage profiles, we see that an AlF3 coating 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 chargetransfer resistance between cathode and electrolyte and the suppression of surface degradation of the Li[Ni0.8Co0.15Al0.05]O2 host structure by the AlF3 coating. Acknowledgment This work was supported by KOSEF through the Research Center for Energy Conversion and Storage. Supporting Information Available: Included in the Supporting 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 http://pubs.acs.org. Literature Cited (1) Amine, K.; Liu, J.; Kang, S; Belharouak, I.; Hyung, Y.; Vissers, D.; Henriksen, G. Improved lithium manganese oxide spinel/graphite Liion cells for high-power applications. J. Power Sources. 2004, 129, 14–19.
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ReceiVed for reView November 9, 2007 ReVised manuscript receiVed February 21, 2008 Accepted March 1, 2008 IE0715308
