Improved High-Voltage Cycling Stability of Nano-Thin TiO2 Layer Coated Ni-Rich Layered Oxides By Polydopamine Pre-Treatment

Abstract

Demand for electric vehicles or large-scale energy storage system boost the development of high-energy and stable lithium-ion batteries (LIBs) and the utilization of cathode materials having higher capacity has been a key issue to enhance the energy density of current LIBs. Accordingly, a large attention has been paid to Ni-rich layered oxides which potentially achieve a higher specific capacity than LiCoO2, and many researches have shown that Ni-rich layered oxides are promising candidate cathode materials with high capacity over 220 mAh/g. Increasing the Ni content help to obtain high capacity, however, structural instability of Ni-rich layered oxides results in serious degradation of the cycling stability. Generally, this problem is mainly related to the surface degradation which can be ascribed to side-reaction products and NiO formation from the generation of highly reactive Ni4+ ions during charging to high-voltage (≥ 4.3 V). This surface degradation is accelerated when the cell is operated at high temperature (>60 oC) because side reactions at the electrode/electrolyte interface and structural degradation are seriously worsen at elevated temperature. To solve this problem, coating strategy has been an effective way to mitigate the surface degradation, and the uniformity of the coating layer is critical to suppress the side reaction at an interface between particles and electrolyte. However, conventional coating methods such as ball-milling or wet coating have a limitation to obtain the uniform coating layer due to the lack of the driving force for even distribution of coating materials on the surface of the electrode material.In this study, a stable Ti-based oxide coating layer is introduced on LiNi0.6Co0.2Mn0.2O2 (NCM). To obtain a nano-thin uniform coating layer, we decorated the polydopamine (PDA) on the NCM particles surface to trap Ti4+ ions through forming O-H catechol groups. This helps to attract Ti(OEt)4 molecules in the coating solution to the NCM surface, thus uniform TiO2 nanolayer can be obtained after the calcination process. We confirmed that the uniformly coated TiO2 nanolayer helps to suppress side reactions and improve the surface stability. Consequently, it enhances the cycling stability in the high-voltage operation (4.5 V), and improves the high temperature (60 oC) cycling performance. The presented results are a further step toward a wise design of stable Ni-rich based cathodes for high energy density LIBs. Figure 1