Elsevier

Journal of Power Sources

Volume 256, 15 June 2014, Pages 20-27
Journal of Power Sources

An approach to application for LiNi0.6Co0.2Mn0.2O2 cathode material at high cutoff voltage by TiO2 coating

https://doi.org/10.1016/j.jpowsour.2014.01.061Get rights and content

Highlights

  • Anatase nano-TiO2 is successfully coated on the surface of LiNi0.6Co0.2Mn0.2O2.

  • Appropriate amount of TiO2 is beneficial to reduce cation disorder.

  • The 1.0 wt.% TiO2-coated LiNi0.6Co0.2Mn0.2O2 exhibits excellent electrochemistry properties.

  • The TiO2-coated LiNi0.6Co0.2Mn0.2O2 presents excellent thermal stability.

Abstract

Nickel-rich LiNi0.6Co0.2Mn0.2O2 cathode material is coated with nano-sized anatase TiO2 synthesized via hydrolyzation method to improve its electrochemical performance at high cutoff voltage of 4.5 V. Scanning electron microscopy (SEM), transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) results show that the anatase TiO2 is successfully coated on the surface of LiNi0.6Co0.2Mn0.2O2 with nanoscale and the coating layer thickness is about 25–35 nm. X-ray diffraction (XRD) test results indicate that appropriate amount of TiO2 coating is beneficial to form a good layered structure with less cation disorder. Charge–discharge test results demonstrate that the TiO2-coated LiNi0.6Co0.2Mn0.2O2 presents excellent cycling capability, rate capability and thermal stability at cutoff voltage of 4.5 V. The 1.0 wt.% TiO2-coated LiNi0.6Co0.2Mn0.2O2 exhibits a capacity retention of 88.7% after 50 cycles at 1 C and a discharge capacity of 135.8 mAh g−1 after 10 cycles at 5 C, comparing to those of the pristine LiNi0.6Co0.2Mn0.2O2 of only 78.1% and 85.4 mAh g−1. Electrochemical impedance spectroscopy (EIS) and differential scanning calorimeter (DSC) tests results provide evidence that the improved electrochemical properties are mainly attributed to the suppression of the interface reaction between the cathode and electrolyte and the improvement of structural stability of the material by coating.

Introduction

Layered LiNi1−xyCoxMnyO2 (0 < x, y < 1) cathode materials for lithium-ion batteries have been widely studied due to their higher capacity, excellent safety performance and lower cost compared with LiCoO2 cathode materials [1], [2], [3], [4]. Currently, LiNi1/3Co1/3Mn1/3O2 is considered to be one of the most promising cathode materials to replace the commonly used LiCoO2 [5], [6], [7]. However, its capacity of 155 mAh g−1 is still too low to meet the ever-growing capacity needs, especially for electric vehicles (EVs) [8], [9]. One approach to improve the discharge capacity of LiNi1−xyCoxMnyO2 is to increase the content of Ni. So nickel-rich layered cathode materials, LiNi1−xyCoxMnyO2 (1 − x − y ≥ 0.5), have been investigated extensively [9], [10], [11], [12], [13]. Among the nickel-rich layered cathode materials, LiNi0.6Co0.2Mn0.2O2 has been expected to be a promising cathode material due to its comparatively better comprehensive electrochemical properties [2], [10], [14], [15], [16], [17]. Unfortunately, the nickel-rich layered oxides, considered as a substitute material of LiNiO2, still inherit many intrinsic disadvantages of LiNiO2. The major problem associated with nickel-rich layered cathode materials includes the structural instability, the thermal instability at the fully charged state and the cycle instability. It is well known that the most critical factors for evaluating the performance of lithium-ion batteries are cycle life, rate capability, and thermal stability, which are mainly depended on the characteristics of the cathode materials. Therefore, there is no doubt that the commercialization of nickel-rich cathode materials is severely limited by their own deficiencies.

Furthermore, in the nickel-rich layered LiNi1−xyCoxMnyO2 system, another approach to increase the reversible capacity is raising the upper cutoff voltage. This is also a main potential advantage for nickel-rich layered LiNi1−xyCoxMnyO2 system comparing with LiFePO4, LiCoO2 or other system. However, the structural stability, cycle stability and thermal stability all will decrease at the same time [18], [19]. The main reason is that the host structural degradation due to the reaction between the cathode material and the electrolyte, leading to the increase of the interfacial impedance [19], [20].

In order to solve these problems, many efforts have been made to seek a feasible solution. Surface coating has been proved to be an effective method to improve the electrochemical performances and thermal stability of the cathode materials. Metal oxides or other materials such as Al2O3, ZrO2, V2O5, ZnO, AlF3, Al(OH)3 and AlPO4 [18], [19], [20], [21], [22], [23] have been reported to be very effective coating materials. In addition, Wu et al. [7] reported the enhanced electrochemical performance of TiO2-coated LiNi1/3Co1/3Mn1/3O2. Liu et al. [24] reported that the enhanced cycling stability was due to the fact that TiO2 coating. To the best of our knowledge, there are no reports about the effect of anatase nano-TiO2 coating on the nickel-rich layered LiNi0.6Co0.2Mn0.2O2 cathode materials, especially, at a high cutoff voltage of 4.5 V.

In this paper, anatase TiO2 nanoparticles are coated on the surface of LiNi0.6Co0.2Mn0.2O2 via a hydrolyzation method. The effects of anatase TiO2 coating on the structural and electrochemical performances of the LiNi0.6Co0.2Mn0.2O2 cathode materials in the high cutoff voltage (4.5 V) are investigated in detail.

Section snippets

Experimental

Firstly, Ni0.6Co0.2Mn0.2(OH)2 precursor was prepared by co-precipitation method. A stoichiometric amount of NiSO4·6H2O, CoSO4·6H2O, and MnSO4·H2O (cationic ratio of Ni:Co:Mn = 6:2:2) solutions with a concentration of 2.0 mol L−1 were slowly dripped into a reactor under nitrogen atmosphere. At the same time, NaOH solution (4.0 mol L−1) and NH4OH solution (1.0 mol L−1) as precipitation agent chelating agent (NaOH and NH4OH with the mole ratio of 2:1) were separately added. The reaction

Results and discussion

The XRD patterns of the pristine and TiO2-coated LiNi0.6Co0.2Mn0.2O2 are presented in Fig. 1. All diffraction peaks from the XRD patterns are indexed on the basis of a hexagonal α-NaFeO2 layered structure with space group R-3m without obvious impurities and secondary phase. The distinct splitting of (006)/(102) and (108)/(110) peaks for all samples demonstrates that these materials have a well-developed layered structure [5]. This suggests that the crystal structure of LiNi0.6Co0.2Mn0.2O2 is

Conclusion

Nickel-rich layered LiNi0.6Co0.2Mn0.2O2 particles are coated with anatase TiO2 nanoparticles. Appropriate amount of TiO2 coated on the surface of LiNi0.6Co0.2Mn0.2O2 can significantly improve its discharge capacity, cycling stability and rate capability, even at a high cutoff voltage of 4.5 V. Especially, the 1.0 wt.% TiO2-coated LiNi0.6Co0.2Mn0.2O2 electrode exhibits a capacity retention of 88.7% after 50 cycles at 1 C and shows a discharge capacity of 135.8 mAh g−1 after 10 cycles at 5 C,

Acknowledgment

Financial support by the National Basic Research Program of China (973 Program No. 2013CB934700) and The Innovation Fund for Technology Based Firm from Ministry of Science and Technology (No. 11C26215103354) are gratefully acknowledged.

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