­­ Synthesis and electrochemical performances of

LiNi0.6Co0.2Mn_0.2O₂ cathode materials

ZHONG Sheng-kui(钟胜奎)^{1, 2}, LI Wei(李 伟)¹, LI Yan-hong(李艳红)³,

ZUO Zheng-guang(邹正光)^{1, 2}, TANG Xin(唐 鑫)^{1, 2}

1. Department of Material and Chemistry, Guilin University of Technology, Guilin 541004, China;

2. Key Laboratory of New Processing Technology for Nonferrous Metals and Materials,

Ministry of Education, Guilin University of Technology, Guilin 541004, China

Received 10 August 2009; accepted 15 September 2009

Abstract: LiNi_0.6Co_0.2Mn_0.2O₂ was prepared from LiOH·H₂O and MCO₃ (M=Ni, Co, Mn) by co-precipitation and subsequent heating. XRD, SEM and electrochemical measurements were used to examine the structure, morphology and electrochemical characteristics, respectively. LiNi_0.6Co_0.2Mn_0.2O₂ samples show excellent electrochemical performances. The optimum sintering temperature and sintering time are 850 ℃ and 20 h, respectively. The LiNi_0.6Co_0.2Mn_0.2O₂ shows the discharge capacity of 148 mA?h/g in the range of 3.0-4.3 V at the first cycle, and the discharge capacity remains 136 mA?h/g after 30 cycles. The carbonate co-precipitation method is suitable for the preparation of LiNi_0.6Co_0.2Mn_0.2O₂ cathode materials with good electrochemical performance for lithium ion batteries.

Key words: lithium ion batteries; LiNi_0.6Co_0.2Mn_0.2O₂; cyclic voltammogram (CV)

1 Introduction

Research on cathode materials of lithium-ion battery was mainly concentrated in the transition metal oxides of layered structure such as LiCoO₂, LiNiO₂, LiMnO₂ and their derivatives. LiCoO₂ is one of the first commercial lithium-ion battery cathode materials, but its application is limited because of the scarce cobalt resources, high prices and a certain degree of toxicity [1-3]. It is hard to prepare LiNiO₂ and the crystal structure will change during the charge-discharge process, resulting in much attenuation of its capacity[4-5]. LiMnO₂ with layered structure has high theoretical specific capacity of 285 mA?h/g. However, during the process of charging and discharging, the  structure easily changes to the spinel structure, resulting in fast attenuation of capacity[6]. Recently, intensive effort has been directed towards the development of LiNi_xCo_yMn_1-x-yO₂ (e.g. LiNi_1/3Co_1/3Mn_1/3O₂) as possible replacement for LiCoO₂[7-11]. It was reported that the capacity obtained for LiNi_xCo_yMn_1-x-yO₂ was dependent upon composition, synthesis technology and charge/ discharge voltage. The first discharge specific capacity reported is often more than 180 mA?h/g with good cycling stability in the first 30 or 50 cycles when being charged to 4.6 V (vs Li/Li⁺). However, the cycling stability of this kind of material in more cycles has been less discussed because of the appearance of Li-branch on the surface of Li anode after some cycles.

In this work, a two-stage technique consisting of co-precipitation and high temperature sintering was employed to prepare the layered LiNi_0.6Co_0.2Mn_0.2O₂ cathode material. Effects of the heating temperature and time on structure and electrochemical properties are tested.

2 Experimental

2.1 Preparation of cathode materials

The precursor (Ni_0.6Co_0.2Mn_0.2)CO₃ was prepared  by the co-precipitation method. Cobalt acetate (Co(CH₃-COO)₂), nickel acetate (Ni(CH₃COO)₂), and manganese acetate (Mn(CH₃COO)₂) were dissolved in deionized water with n(Co)?n(Ni)?n(Mn) of 2?6?2 to obtain a mixture. Then, sodium carbonate (Na₂CO₃) solution was added into the mixture with 1?1(volume ratio) of NH₃-to-H₂O to obtain a solution with the pH value of about 11. It was subsequently heated at 60 ℃ for 3 h to react, then was rested for about 2.5 h. It was calcined at 100 ℃ for 4 h to obtain the precursor (Ni_0.6Co_0.2- Mn_0.2)CO_3. Then, the dried (Ni_0.6Co_0.2Mn_0.2)- CO₃ were ground together with LiOH·H₂O in the molar ratio n(CO₃^2-)?n(Li⁺) of 1?2. The powders were subsequently annealed at a temperature from 750 ℃ to 950 ℃ in air for 20 h followed by a natural cooling to obtain LiNi_0.6Co_0.2Mn_0.2O₂ powers. To investigate the effect of annealing time, the powders were further annealed at 850 ℃ for different time of 10, 20 and 30 h, respectively.

2.2 Sample characterization

The powder X-ray diffraction (XRD, X Pert-Pro) measurement using Cu K_α radiation was employed at room temperature to identify the crystalline phase of the synthesized materials. The particle size and morphology of the LiNi_0.6Co_0.2Mn_0.2O₂ powders were observed by scanning electron microscope (JEOL, GSM-6380LV) with an accelerating voltage of 15 kV.

2.3 Electrochemical performances evaluation

The electrochemical characterizations were performed using CR2025 coin-type cell. For positive electrode fabrication, the prepared powders were mixed with 10%(mass fraction) carbon black and 10%(mass fraction) polyvinylidene fluoride in N-methyl pyrrolidinone until slurry was obtained. And then, the blended slurries were pasted onto an aluminum current collector, and the electrode was dried at 120 ℃ for 10 h in vacuum. The test cell consisted of the positive electrode and lithium foil negative electrode separated by a porous polypropylene film, with 1 mol/L LiPF₆ in EC?EMC?DMC (1?1?1 in volume ratio) as the electrolyte. The assembly of the cells was carried out in a dry Ar-filled glove box. The cells were charged and discharged over a voltage range from 3.0 V to 4.3 V versus Li/Li⁺ at room temperature. Cyclic voltammograms were tested in the three-electrode system using metallic foils as both counter and reference electrodes at a scanning rate of 0.1 mV/s in the voltage range of 3.0-4.5 V.

3 Results and discussion

3.1 XRD patterns of synthesized LiNi_0.6Co_0.2Mn_0.2O₂ samples

Fig.1 shows the XRD patterns of prepared LiNi_0.6- Co_0.2Mn_0.2O₂ powders heated at 750, 850 and 950 ℃. All peaks are indexed on the basis of the α-NaFeO₂ structure(R3m). It is clear that all samples have a typical hexagonal layered structure because both the (006)/(102) and the (018)/(110) are well resolved, especially when the heating temperature is above 850 ℃.

Fig.1 XRD patterns for LiNi_0.6Co_0.2Mn_0.2O₂ powders heated at 750 ℃, 850 ℃ and 950 ℃

The X-ray diffraction patterns of LiNi_0.6Co_0.2- Mn_0.2O₂ powders annealed for different time from 10 to 30 h at 850 ℃ are shown in Fig.2. The XRD patterns of all samples are consistent with single phase of α-NaFeO₂ structure. It is also clear that all samples have a typical hexagonal layered structure.

Fig.2 XRD patterns for LiNi_0.6Co_0.2Mn_0.2O₂ powders annealed at 850 ℃ for different time

3.2 SEM images of synthesized LiNi_0.6Co_0.2Mn_0.2O₂  samples

Fig.3 shows the scanning electron microscopy (SEM) images of the LiNi_0.6Co_0.2Mn_0.2O₂ powders annealed at 750, 850 and 950 ℃ for 20 h. With increasing calcination temperature, the primary particle size gradually increases. It could be noticed that LiNi_0.6Co_0.2Mn_0.2O₂ powders calcined at 850 ℃ exhibit relatively uniform particle size, as shown in Fig.3(b), while LiNi_0.6Co_0.2Mn_0.2O₂ powders annealed at 750 and 950 ℃ consist of some primary particles and larger secondary ones. This indicates that raising the calcined temperature not only increases particle size, but also affects the uniformity of the materials.

Fig.3 SEM images of LiNi_0.6Co_0.2Mn_0.2O₂ powders annealed at 750 ℃(a), 850 ℃(b) and 950 ℃(c) for 20 h

Fig.4 shows the SEM images of the LiNi_0.6Co_0.2Mn_0.2O₂ powders annealed at 850 ℃ for 10, 20 and 30 h. With increasing calcination time, the primary particle size gradually increases. It could be noticed that LiNi_0.6Co_0.2 Mn_0.2O₂ powders calcined for 20 h exhibit relatively uniform particle size, as shown in Fig.4(b), while LiNi_0.6Co_0.2 Mn_0.2O₂ powders annealed for 10 and 30 h consist of some primary particles and larger secondary ones. This indicates that raising the calcined time not only increases particle size, but also affects the uniformity of the materials.

Fig.4 SEM images of LiNi_0.6Co_0.2 Mn_0.2O₂ powders annealed at 850 ℃ for 10 h (a) , 20 h (b) and 30 h (c)

3.3 Electrochemical characteristics

Fig.5 shows the first charge-discharge curves of LiNi_0.6Co_0.2Mn_0.2O₂ cells calcined at various temperatures at the rate of 0.5C in the voltage range of 3.0-4.3 V at room temperature. As seen in Fig.5, the initial charge capacities for LiNi_0.6Co_0.2Mn_0.2O₂ calcined at of 750, 850 and 950 ℃ are about 156, 163 and 160 mA?h/g, and the discharge capacities are about 137, 148 and 142 mA?h/g, respectively. The corresponding coulombic efficiency is 87.8%, 90.7% and 88.7%, respectively. It is clearly seen that the electrode reaction reversibility is enhanced considerably when the heating temperature is selected as 850 ℃. This is because the homogenous particle of LiNi_0.6Co_0.2 Mn_0.2O₂ leads to a uniform depth of charge (DOC) of each particle, which increases the utilization of the material. Accordingly, the coulombic efficiency is increased.

Fig.5 Initial charge-discharge curves of LiNi_0.6Co_0.2 Mn_0.2O₂ samples calcined at different temperatures

Fig.6 shows the specific discharge capacity with the cycle number for the LiNi_0.6Co_0.2Mn_0.2O₂ cell in the voltage range between 3.0 and 4.3 V. It can be seen that the 850 ℃-annealed LiNi_0.6Co_0.2Mn_0.2O₂ cell delivers a discharge capacity of 148 mA?h/g at 0.2C in the first cycle. For comparison, the initial specific discharge capacities are 137 and 142 mA?h/g for the samples annealed at 750 and 950 ℃, respectively. It is noticed that the initial specific discharge capacity increases with increasing the annealing temperature from 750 to 850 ℃, and decreases with further increasing the annealing temperature from 850 to 950 ℃. The following factors might cause this trend. First, the crystallinity of powders is one important factor affecting the electrochemical discharge capacity. The higher annealing temperature leads to higher crystallinity that helps to increase the electrode capacity. Second, the particle size of electrode material is also related to its electrochemical performance. During the first cycle, part of overall capacity is consumed to form a surface layer on the electrode particles. The larger particle size means a smaller specific surface area, resulting in lower capacity loss. Third, too high annealing temperature may be accompanied by the appearance of cation mixing and impurity phases. On the other hand, the irreversible capacity losses after 30 cycles are 12.5%, 8.2%, and 14.8%, respectively, as the annealing temperature increases from 750 to 950 ℃. The irreversibility comes mainly from the fact that a part of Ni³⁺ cannot be reduced to Ni²⁺ after the first cycle. Another main contributor is the formation of surface layer on the electrode material, especially for some material with small particle sizes.

Fig.6 Cycling performance of LiNi_0.6Co_0.2 Mn_0.2O₂ samples calcined at different temperatures

Fig.7 shows the first charge-discharge curves of LiNi_0.6Co_0.2Mn_0.2O₂ cells calcined at 850 ℃ for different time at the rate of 0.2C in the voltage range of 3.0-4.3 V at room temperature. As seen in Fig.7, it is clear that the initial charge capacities for LiNi_0.6Co_0.2 Mn_0.2O₂ calcined for 10, 20, 30 h are about 158, 163, 164 mA?h/g, and the discharge capacities are about 139, 148, 145 mA?h/g, respectively. The corresponding coulombic efficiencies are 87.9%, 90.7% and 88.4%, respectively. It is clearly seen that the electrode reaction reversibility is enhanced considerably when the heating time is 20 h.

Fig.7 Initial charge-discharge curves of LiNi_0.6Co_0.2 Mn_0.2O₂ samples calcined at 850 ℃ for different time

The electrochemical cycling performance of LiNi_0.6Co_0.2Mn_0.2O₂ samples is shown in Fig.8. It can be seen that the initial discharge capacities of LiNi_0.6Co_0.2Mn_0.2O₂ calcined for 10, 20 and 30 h are 139, 148, 145 mA?h/g, with the capacity losses of 10.8%, 8.2%, and 9.0% after 30 cycles, respectively. It is noticed that the initial specific discharge capacity increases with increasing the annealing time from 10 to 20 h and the capacity loss decreases with further increasing the annealing time from 10 to 20 h, then increases with increasing the annealing time from 20 to 30 h.

Fig.8 Cycling performance of LiNi_0.6Co_0.2 Mn_0.2O₂ samples calcined at 850 ℃ for different time

The initial cyclic voltammogram (CV) curves for LiNi_0.6Co_0.2 Mn_0.2O₂ electrode at a scanning rate of 0.1 mV/s is shown in Fig.9. It can be seen that the oxidation peak of LiNi_0.6Co_0.2Mn_0.2O₂ is observed at around 4.05 V, coupled with the reduction peak at 3.67 V. The appearance of only one couple of peak in the LiNi_0.6Co_0.2Mn_0.2O₂/Li cell between 3.0 and 4.6 V means that no structural transitions exist from hexagonal to monoclinic.

Fig.9 CV curves of LiNi_0.6Co_0.2 Mn_0.2O₂ electrode in first cycle

4 Conclusions

1) LiNi_0.6Co_0.2Mn_0.2O₂ cathode materials are prepared by co-precipitation and subsequent heating. XRD and SEM analyses show that the structures and morphology of the LiNi_0.6Co_0.2Mn_0.2O₂ samples heated at different temperatures and time are different, which results in different electrochemical performances.

2) The LiNi_0.6Co_0.2Mn_0.2O₂ sample annealed at 850 ℃ for 20 h shows the best electrochemical properties with the first specific discharge capacity of 148 mA?h/g in the range of 3.0-4.3 V with the capacity losses of 8.2% after 30 cycles.

3) LiNi_0.6Co_0.2Mn_0.2O₂ is tested to be a promising cathode material for lithium ion batteries.

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Foundation item: Projects(0991025, 0842003-5 and 0832259) supported by Natural Science Foundation of Guangxi Province, China; Project supported by the Joint Graduate Innovation Talent Cultivation Base of Guangxi Province, China; Project(GuiJiaoRen [2007]71) supported by the Research Funds of the Guangxi Key Laboratory of Environmental Engineering, Protection and Assessment Program to Sponsor Teams for Innovation in the Construction of Talent Highlands in Guangxi Institutions of Higher Learning, China

Corresponding author: ZHONG Sheng-kui; Tel: +86-773-5896446; E-mail: zhongshk@glite.edu.cn

DOI: 10.1016/S1003-6326(09)60059-5

(Edited by YANG Bing)