Journal of Energy Chemistry 22(2013)

Transcription

1 Journal of Energy Chemistry 22(2013) Coating of Al 2 O 3 on layered Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 using CO 2 as green precipitant and their improved electrochemical performance for lithium ion batteries Yingqiang Wu a,b,c, Linhai Zhuo a,b, Jun Ming a,b, Yancun Yu a,b, Fengyu Zhao a,b a. State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun , Jilin, China; b. Laboratory of Green Chemistry and Process, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun , Jilin, China; c. University of Chinese Academy of Sciences, Beijing , China [ Manuscript received December 3, 2012; revised January 15, 2013 ] Abstract Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 cathode materials were fabricated by a hydroxide precursor method. Al 2 O 3 was coated on the surface of the Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 through a simple and effective one-step electrostatic self-assembly method. In the coating process, a NaHCO 3 - H 2 CO 3 buffer was formed spontaneously when CO 2 was introduced into the NaAlO 2 solution. Compared with bare Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2, the surface-modified samples exhibited better cycling performance, rate capability and rate capability retention. The Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 electrodes delivered a discharge capacity of about 115 mah g 1 at 2 A g 1, but only 84 mah g 1 for the bare one. The capacity retention of the Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 was 90.7% after 50 cycles, about 30% higher than that of the pristine one. Key words electrochemistry; alumina; coating; layered cathode; lithium-ion batteries 1. Introduction Lithium ion batteries (LIBs) are now being intensively pursued for transportation applications, as they offer higher volumetric and gravimetric energy densities compared with other rechargeable battery systems such as nickel-cadmium and nickel-metal hydride batteries [1 4]. In 1991, the Sony Corporation introduced the first commercial LIB into the market using layered Li 1 x CoO 2 as the cathode. [5] However, the high cost, toxicity and chemical instability at deep charge prevent its large scale application in transportation. In addition, the lithium ion in Li 1 x CoO 2 can be used only in the range of x 0.5, limiting its practical capacity to 140 mah g 1 (50% of theoretical capacity) [6]. To overcome these difficulties, extensive efforts have been focused on developing alternative low-cost cathodes with higher energy densities [7]. Recently, layered Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 cathodes become appealing due to their higher capacity, lower cost and better safety compared with the conventional LiCoO 2 cathodes [8 13]. Although the Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 cathodes have demonstrated appealing performance in LIB, its rate capability diminishes gradually owing to unfavorable side reactions and lower lithium ion diffusion velocity [14]. Tailoring the materials into a nanoscale to shorten the lithium ion diffusion distance and increase the electrode-electrolyte contact area can ameliorate the velocity problem of lithium ion diffusion [15,16]. Unfortunately, increasing the surface area of the electrode materials can seriously aggravate unfavorable side reactions such as dissolution of active transition metal into the electrolyte. An effective way to avoid or suppress these side reactions is to coat the nanoparticles with a stabilizing surface layer [17 19]. To date, various coatings such as TiO 2 [20], ZrO 2 [21,22], Al 2 O 3 [23,24], AlF 3 [25,26], and AlPO 4 [27] have been shown to improve the thermal stability and rate capability of the Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 cathode materials. Especially, Al 2 O 3 keeps on being the most appropriate stabilizing coating layer for its great abundance and low cost. The methods applied in the coating of Al 2 O 3 generally include the sol-gel wet-chemical methods and the scalable technique of atomic layer deposition (ALD), etc. [23,28,29]. Corresponding author. Tel: ; Fax: ; lhzhuo@ciac.jl.cn, zhaofy@ciac.jl.cn This work was financial supported by the National Natural Science Foundation of China ( ). Copyright 2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.

2 Journal of Energy Chemistry Vol. 22 No Despite the ALD method can provide an ultrathin, uniform and conformal stabilizing surface layer to improve the performance of the nanosized electrode materials, this coating process is quite complicated, including multiple-step operations and the using of the humidity/air sensitive agent trimethylaluminum [28]. Recently, the electrostatic self-assembly method provides a green and effective way of surface-modification to stabilize the nanosized electrode materials [20,30]. The coating process includes firstly the using of ammonium hydroxide as the precipitant to prepare nanosized Al(OH) 3, and then using acetic acid to adjust the ph of the solution to a desired value between the isoelectric points of Al(OH) 3 (ph = 9.2) and the spinel LiM 2 O 4 (ph 6.0, M = Mn, Ni, Co) [31]. Once the spinel LiM 2 O 4 powders are dispersed into the solution, the nanosized Al(OH) 3 can self-assemble onto the surface of the cathode materials by electrostatic attraction, as the nanosized Al(OH) 3 has an opposite surface charge to the spinel LiM 2 O 4 at the chosen ph value. However, this electrostatic and selfassembly coating process needs an online monitoring of the ph value and multiple-step operations, which will bring some difficulties in practical large-scale manufacture. Herein, we present a simple and effective method for coating the Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 nanoparticles with an amorphous Al 2 O 3 layer using NaAlO 2 as the precursor and CO 2 as a green precipitant in a spontaneously formative NaHCO 3 -H 2 CO 3 buffer solution system. The products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). The electrochemical performance of the materials has been studied by charge-discharge measurements. 2. Experimental 2.1. Preparation of cathode materials The Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 cathode materials were fabricated by a hydroxide precursor method. The procedure involved the precipitation of the hydroxide precursors first from a solution containing manganese, nickel and cobalt sulphates (mol ratio of 1 : 1 : 1) by adding LiOH, and using NH 4 OH as the chelating agent, and then firing the oven-dried hydroxide precursors with a required amount of LiOH H 2 O at 770 C in air for 20 h with a heating rate of 1 C min 1, as reported previously [32] Preparation of mixed acid-base indicator A solution was first prepared by mixing 62.5 mg of thymol blue, 750 mg of bromothymol blue, 200 mg of methyl red and 750 mg of phenolphthalein into 125 ml of 95% EtOH, and then by adding 94 ml of H 2 O. Subsequently, 0.1 mol L 1 of NaOH was added until the solution changed to green, and then the volume of the solution was diluted to 250 ml with water Process of coating Al 2 O mg of Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 powder and a certain amount of a NaAlO 2 -NaOH solution were dispersed into 10 ml of water, and the obtained solution was transferred into a 85 ml viewable autoclave, then CO 2 was introduced into the autoclave until the pressure was up to 0.5 MPa at room temperature. After the solution was continuously stirred for one hour, the products were collected and calcined at 300 Cfor 4 h. The loading of the coating material was controlled to be 1 wt%, 2 wt% and 3 wt%. The content of Al 2 O 3 in the cathode materials was measured by the inductively coupled plasma atomic emission spectrometer (ICP-AES) Characterization The phase structures were characterized with X-ray diffraction (XRD, Bruker D8 Advance diffractometer using Cu K α (λ = Å)). The morphology of the materials was analyzed by a scanning electron microscope (SEM Hitachi S- 4800). Transmission electron microscope (TEM) determination was recorded on a Tecnai G20 operating at 200 kv for detailed microstructure information of the samples. X-ray photoelectron spectra (XPS) was recorded on a PHI quantera SXM spectrometer with an Al K α = ev excitation source, where binding energies were calibrated by referencing the C 1s peak (284.5 ev) to reduce the sample charge effect Battery fabrication and electrochemical performance measurement The cathode was constructed by mixing the cathode material with acetylene black and poly(vinylidene fluoride) powder in a weight ratio of 80 : 10 : 10. After being blended in N-methylpyrrolidinone, the slurry was spread uniformly on an aluminum foil and dried at 100 C for 12 h in vacuum. Charge and discharge performances of the electrodes were evaluated using 2025 coin cells containing an electrolyte solution of 1 M LiPF 6 in a mixed solvent of ethylene carbonate and dimethyl carbonate (1 : 1 volume ratio) in a Celgard 2320 micro-porous separator membrane. Lithium sheets served as the counter electrode and reference electrode. The cells were assembled in an argon-filled glove box (O 2 and H 2 Olevels<1 ppm). The galvanostatic charge and discharge were controlled between 2.8 and 4.5 V on a LAND CT2001 (A cell test instrument, Wuhan Kingnuo Electronic Co., China). 3. Results and discussion 3.1. Characterizations of the materials Figure 1 shows the diagram of the one-step electrostatic self-assembly method using NaAlO 2 as the precursor and CO 2 as a green precipitant. Firstly, AlO 2 was prepared by the re-

3 470 Yingqiang Wu et al./ Journal of Energy Chemistry Vol. 22 No action of Al(NO 3 ) 3 with NaOH (1 : 8 in mol ratio, and the superfluous NaOH acted as a stabilizing agent for AlO 2 ), then the as-obtained Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 powders were dispersed into the solution, in which the surface of the cathode material also took a negative charge. When CO 2 was introduced into the autoclave, the following reactions took place: H 2 O+CO 2 H 2 CO 3 NaAlO 2 +H 2 CO 3 +H 2 O NaHCO 3 +Al(OH) 3 NaOH+H 2 CO 3 NaHCO 3 +H 2 O the surface of the cathode materials by electrostatic attraction. The accuracy weight of Al 2 O 3 in the cathode materials was 2.9 wt% for the 3 wt% Al 2 O 3 -coated Li Mn 1/3 Ni 1/3 Co 1/3 O 2 sample, which revealed that the precursor (NaAlO 2 ) was almost totally converted into the reaction product (Al 2 O 3 ) during the synthesis. The XRD patterns of as-obtained Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 powders are shown in Figure 3. The diffraction patterns are sharp and well-defined and can be identified as a hexagonal R- NaFeO 2 structure with space group R3hm. The XRD patterns of the Al 2 O 3 -coated samples are the same as that of the bare Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 without any other impurity phases, indicating that the layered structures of the surface-modified samples are maintained. The reflections corresponding to the Al 2 O 3 layer are not observed in all the coating samples, and this may be due to its small quantity and/or the coating layer is amorphous when it is calcined at a lower temperature of 300 C. Figure 1. Diagram of the one-step electrostatic self-assembly coating method developed in this work It is interesting that a NaHCO 3 -H 2 CO 3 buffer solution could be spontaneously formed in the solution. The ph value of this NaHCO 3 -H 2 CO 3 buffer solution was measured by an artful experiment using a home-made acid-base indicator probe in a viewable autoclave. Figure 2(a) exhibits the varied colors versus different ph values of the solutions with the home-made acid-base indicator. A blank test without Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 in the solution of NaAlO 2 -NaOH with the acid-base indicator is shown in Figure 2(b). Once CO 2 was introduced into the autoclave, the color of the solution quickly turned from purple to green in five minutes, indicating that the final ph value of the solution was about 7. Consequently, in this spontaneously formative NaHCO 3 - H 2 CO 3 buffer solution, the Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 cathode material took a negative charge and the Al(OH) 3 took a positive charge so that the Al(OH) 3 could self-assemble onto Figure 2. (a) Varied colors versus different ph values of the solution with home-made acid-base indicator, (b) the variational process of the solution with 0.5 MPa of CO 2 Figure 3. XRD patterns of (1) bare Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 material, (2) 1 wt% Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2, (3) 2 wt% Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 and (4) 3 wt% Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 Figure4(a)showsthebareLi(Mn 1/3 Ni 1/3 Co 1/3 )O 2 fabricated from firing the hydroxide precursor with a required amount of LiOH H 2 O at 770 C in air for 20 h, from which a smooth and clean surface could be observed. However, the surfaces of the particles became obviously rough after coated with Al 2 O 3 (Figure 4b to 4d). In addition, more and more Al 2 O 3 floccules were coated on the surfaces with the increasing of the Al 2 O 3 in the materials. Further evidence can be observed from the HRTEM images, as shown in Figure 5. Comparison between the HRTEM images of the Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 and the bare sample, it is obvious that an ultrathin protecting layer is coated on the surface of the sample (Figure 5a), and it is amorphous and clearly different from the highly crystalline layered structure of the bulk region. Contrarily, in Figure 5(c), the surface region in the bare Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 has obviously the same lattice fringes as those in the bulk region. The EDS analysis (Figure 5b and 5d) also demonstrates that aluminum is present on the coating sample, but not found in the bare one.

4 Journal of Energy Chemistry Vol. 22 No Figure 4. SEM images of (a) bare Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 material, (b) 1 wt% Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2, (c) 2 wt% Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 and (d) 3 wt% Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 Figure 5. HR-TEM image of (a) the 2 wt% Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 and (c) bare Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 particle; The EDS elemental analysis spectra of (b) the 2 wt% Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 and (d) bare Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 particle

5 472 Yingqiang Wu et al./ Journal of Energy Chemistry Vol. 22 No Surface chemical states of the cathode materials were studied by X-ray photoelectron spectroscopy analysis (XPS). The binding energies of the Mn (2p 3/2 ), Ni (2p 3/2 ), and Co (2p 3/2 ) in the spectra are in accordance with the reported values for Mn 4+ (642.2 ev), Ni 2+ (854.0 ev) and Co 3+ (779.5 ev) in Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2, [27,33,34], demonstrating that the oxidation states of Mn, Ni and Co in the samples are also 4+, 2+ and 3+ (Figure 6a to 6c). As Al 2 O 3 is coated on the particles (Figure 6d), all the intensities of the Mn 2p,Ni2p and Co 2p peaks decreased, due to the decreasing amount of the layered oxide within the XPS probing depth in the samples. The above results exhibit that the surface chemical states of the active transition metals in the samples did not show any changes after modification with Al 2 O 3, but their surface environment was changed due to the fact that the surface coating layer was protecting the cathode surface from direct contacting with the electrolyte and suppressed electrolyte decomposition at high operating voltages. It can be easy to forecast that the bare and surface-modified samples will exhibit different electrochemical performances such as cycle and rate stability. Figure 6. Comparison of the XPS spectra of Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 before and after modification with 2 wt% Al 2 O Electrochemical performance of the materials Figure 7 shows the first charge-discharge curves of the bare and the Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 cathodes at a rate current density of 20 ma g 1. The bare Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 shows a discharge capacity of 180 mah g 1, while the Al 2 O 3 -coated samples exhibit lower discharge capacities which are all about 160 mah g 1. On the other hand, the first charge capacity also decreases: the capacity of the bare sample is 218 mah g 1,whichis about 190 mah g 1 for the Al 2 O 3 -coated sample. However, the Irreversible Capacity Loss (IRC) value, which is the difference between the first charge capacity and the first discharge capacity, decreases when coating with Al 2 O 3. Choi et al. have found that the irreversible capacity loss of the LiNi 1/3 Mn 1/3 Co 1/3 O 2 cathodes is positively correlated with the BET surface area [14]. It can be understood that it is due to the enhanced side reactions between the cathode surface and the electrolyte with increasing BET surface. In this regard, coating the cathode with a stabilizing surface layer could protect the cathode surface from direct contact with the electrolyte and suppress the side reactions, which might be the reason for the lower IRC value of the Al 2 O 3 -coated samples, when compared with the bare one.

6 Journal of Energy Chemistry Vol. 22 No Figure 7. Comparison of the first cycle of charge-discharge voltage profiles for the bare and surface-modified cathodes: (a) bare Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2,(b) 1wt%Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2,(c)2wt%Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2,(d)3wt%Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 Although the Al 2 O 3 -coated samples delivered lower discharge capacity than the bare one for the first cycle, they delivered almost the same charge-discharge capacities when the current density was increased to 100 ma g 1 (0.5C), as shown in Figure 8. It is clear that the bare Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 sample exhibits more capacity fading than those of the Al 2 O 3 - coated samples at high current densities. And this difference in capacity fading trend is more remarkable during long-life cycling. It has been reported that the decrease in rate capability is mainly attributed to the strain and defect generated by fast lithium ion extraction and intercalation in the cathode material [35,36], and the coating layer can play an important role in reducing the strain and defect in the particles by stabilizing the layered structure of the Li Mn 1/3 Ni 1/3 Co 1/3 O 2 [24]. On the other hand, a thin layer of the Li-Al-O solid solution phase is reported to form on the surface of the Li Mn 1/3 Ni 1/3 Co 1/3 O 2 by coating with Al 2 O 3 during the lithium ion extraction and intercalation process, and this Li- Al-O solid solution phase has reasonably high Li ion conductivity [37,38]. At a low current density (0.1 C to 1 C), the 3 wt% sample delivered a lower capacity than the 1 wt% sample (148 mah g 1 vs.153mah g 1 at 0.5 C), which could be ascribed to the less active materials of the 3 wt% sample. However, once the current density was increased to 2A g 1 (10 C), the discharge capacity for the 3 wt% sample (115 mah g 1 ) was higher than that of the 1 wt% sample (108 mah g 1 ). This result can be probably attributed to the perfect coating on the surface of the 3 wt% sample which could be more effective in reducing the strain and protect the defect in the particles when lithium ions were fast extracted and intercalated in the cathode material at a high current density. In addition, the rate-performances of the electrodes were tested with different current densities. Figure 9 shows the variations in discharge capacities for different Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 electrodes cycled at various rates between 2.8 and 4.5 V (vs. Li/Li + ) at room temperature. All the samples exhibit a similar capacity (about 150 mah g 1 )ata low current rate. The bare sample delivered a discharge capacity of about 125 mah g 1 at the rate of 1 C (200 ma g 1 ). However, the Al 2 O 3 -coated samples delivered a higher capacity (143 mah g 1 ) than that of the bare one and this trend was more evident as the current density was further increased. At arateof10c(2a g 1 ), the Al 2 O 3 -coated samples delivered a discharge capacity of about 115 mah g 1, but only about 84 mah g 1 for the bare one.

7 474 Yingqiang Wu et al./ Journal of Energy Chemistry Vol. 22 No Figure 8. Discharge voltage profiles of the Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 cycled at different current densities: (a) bare Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2,(b)1wt%Al 2 O 3 - coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2,(c)2wt%Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2,(d)3wt%Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 Figure 9. Variations in discharge capacities versus charge-discharge cycle number for different Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 electrodes cycled at different rates between 2.8 and 4.5 V (vs. Li/Li + ) at room temperature. 1C = 200 mah g 1 Figure 10 shows the cycling performances of the Al 2 O 3 - coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 and the bare sample. It was cycled between 2.8 and 4.5 V (vs. Li/Li + ) at a current rate of 20 ma g 1 (0.1 C) for the first charge-discharge cycle (not show here), then 100 ma g 1 (0.5 C) for the subsequent cycles at room temperature. The capacity retention of the Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 was about 90% after 50 cycles, but only about 60% for the pristine Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2. In addition, when electrochemical testing was continued to 100 cycles, despite the capacity retention (Figure 11) further decreased to about 80% for the Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2, it was still about 30% higher than that of the bare one. The reasons can be attributed to the undesirable surface reactions and active transition metal dissolution due to the corrodibility of the HF originating from the reaction of LiPF 6 with a small amount of water, which always existed in the electrolyte [39,40]. And if the voltage is enhanced to 4.5 V to achieve higher initial specific capacity, these undesirable reactions will obviously aggravate. That means, when the bare cathode materials are exposed to the electrolyte, the active transition metal can dissolve due to the MF 2 (M = Mn, Ni and Co) and water [17]. The resulted water can react with the LiPF 6 continuously and more HF is produced. Consequently, the capacity of the bare cathode materials will fade quickly during cycling. However, the situation is quite different if Al 2 O 3 is coated on the surface of the cathode materials. The Al 2 O 3 coating layer would gradually react with the HF, then change to AlF 3 and simultaneously adhere on the surface of the cathodes, which would protect the cathode surface from direct contacting with the electrolyte and thus decrease the concentration of the acidic species (HF) in the

8 Journal of Energy Chemistry Vol. 22 No electrolyte, leading to less degradation of the active materials during cycling [18]. Figure 10 and Figure 11 exhibit that the surface modification can improve the electrochemical performanceof the Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 cathode materials to a certain extent, but capacity fading is still present during long time cycling. This may be attributed to the following factors. First, the metal oxide protective coating layer is amorphous and the loading is quite low, which limits its ability to continuously protect the cathode surface from the etching by HF and suppress electrolyte decomposition at high operating voltages. Second, the phase of the cathode materials change gradually during charges despite the presence of a surface protective layer [41], which may lead to capacity fading during long time cycle. layer of Al 2 O 3 on Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 particles. The nontoxic, low-cost and abundant CO 2 is innovatively used as a green precipitant instead of the irritative and toxic ammonia for the fabrication of the Al 2 O 3 coating layer. This method can be applied in large-scale practical coating of Al 2 O 3 for cathode or anode materials. Compared with the bare material, the Al 2 O 3 -coated Li(Mn 1/3 Ni 1/3 Co 1/3 )O 2 electrode exhibits better cycling performance, rate capability and rate capability retention. It delivers a discharge capacity of about 115 mah g 1 at 2 A g 1, which is about 30% higher than that of the bare one. However, capacity fading is still present during cycling for the Al 2 O 3 -coated samples. More work is needed to investigate the function of the coating layers and the corresponding products resulting from the reaction of the electrolyte with the coated oxides, which is important in designing new cathode materials with better cycling performance, rate capability and rate capability retention. References Figure 10. Cycling performance of the bare and 1 wt%, 2 wt%, 3wt% Al 2 O 3 - coated Li(Mn 1/3 Ni 1/3 Co 1/3 O 2 ) at room temperature. The electrodes were charged-discharged between 2.8 and 4.5 V (vs Li/Li + ) at 100 ma g 1 Figure 11. Capacity retention as a function of cycle numbers measured during electrochemical testing at 100 ma g 1 4. Conclusions In summary, a green and effective one-step electrostatic self-assembly method is developed for coating a stabilizing [1] Goodenough J B, Kim Y. Chem Mater, 2009, 22(3): 587 [2] Whittingham M S. Chem Rev, 2004, 104(10): 4271 [3] Tarascon J M, Armand M. Nature, 2001, 414(6861): 359 [4] Cheng F, Liang J, Tao Z, Chen J. Adv Mater, 2011, 23(15): 1695 [5] Ohzuku T, Ueda A, Nagayama M, Iwakoshi Y, Komori H. Electrochim Acta, 1993, 38(9): 1159 [6] Reimers J N, Dahn J R. J Electrochem Soc, 1992, 139(8): 2091 [7] Lee M H, Kang Y J, Myung S T, Sun Y K. Electrochim Acta, 2004, 50(4): 939 [8] Choi J, Manthiram A. J Electrochem Soc, 2005, 152(9): A1714 [9] Choi J, Manthiram A. J Power Sources, 2006, 162(1): 667 [10] Manthiram A, Choi J. J Power Sources, 2006, 159(1): 249 [11] Gao P, Li Y, Liu H, Pinto J, Jiang X, Yang G. J Electrochem Soc, 2012, 159(4): A506 [12] Hashem A M A, Abdel-Ghany A E, Eid A E, Trottier J, Zaghib K, Mauger A, Julien C M. J Power Sources, 2011, 196(20): 8632 [13] Li D, Sasaki Y, Kobayakawa K, Noguchi H, Sato Y. Electrochim Acta, 2006, 52(2): 643 [14] Choi J, Manthiram A. Electrochem Solid State Lett, 2005, 8(8): C102 [15] Arico A S, Bruce P, Scrosati B, Tarascon J M, Van Schalkwijk W. Nat Mater, 2005, 4(5): 366 [16] Chan C K, Peng H L, Twesten R D, Jarausch K, Zhang X F, Cui Y. Nano Lett, 2007, 7(2): 490 [17] Myung S T, Izumi K, Komaba S, Sun Y K, Yashiro H, Kumagai N. Chem Mater, 2005, 17(14): 3695 [18] Myung S T, Amine K, Sun Y K. JMaterChem, 2010, 20(34): 7074 [19] Liu J, Manthiram A. J Mater Chem, 2010, 20(19): 3961 [20] ChenYD,ZhaoY,LaiQY,WeiNN,LuJZ,HuXG,JiXY. Ionics, 2008, 14(1): 53 [21] Li D, Kato Y, Kobayakawa K, Noguchi H, Sato Y. JPower Sources, 2006, 160(2): 1342 [22] Hu S K, Cheng G H, Cheng M Y, Hwang B J, Santhanam R. J Power Sources, 2009, 188(2): 564 [23] Riley L A, Van Atta S, Cavanagh A S, Yan Y, George S M, Liu P, Dillon A C, Lee S H. J Power Sources, 2011, 196(6): 3317 [24] Kim Y, Kim H S, Martin S W. Electrochim Acta, 2006, 52(3): 1316

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