Nano-Thin TiO2 Layer Coating By Polydopamine Pre-Coating for Highly Stable Ni-Rich Layered Oxides with 4.5 V High-Voltage Operation in Lithium-Ion Batteries

Abstract

As the demand for electric vehicles or large-scale energy storage system increases rapidly, high-energy and stable lithium-ion batteries (LIBs) have been required. Accordingly, a large attention has been paid to Ni-rich layered oxides which has a higher energy density than LiCoO2, and the Ni-rich layered oxides have been applied to cathode materials for LIBs. Although the researchers succeeded in obtaining high reversible capacity by increasing the Ni content, the higher Ni content in layered oxides is still structurally unstable, which seriously degrades the cycling stability and lefts many questions on a reliability of the LIBs. Generally, the cycle degradation is mainly related to surface degradations which can be ascribed to side-reaction products and NiO formation from the oxidation of the electrolyte, and these are caused by the generation of highly reactive Ni4+ ions during charging to high-voltage (≥4.3 V). These surface degradations are accelerated when the cell is operated at high temperature, those side reactions at the electrode/electrolyte interface and structural degradation are seriously worsen. To solve the problem, coating strategy is effective way to mitigate the surface degradation, and the uniformly coated layer can can suppress the side reaction effectively for each particle. However, conventional coating methods such as ball-milling or wet coating have a limitation to achieve the uniformity of the coating layer due to the lack of the driving force for making the coating material concentrated on the whole surface.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 based on the driving force, concentrating the coating material, we coated the polydopamine (PDA) on the NCM first to trap Ti4+ ions on the surface in the coating solution through forming O-H catechol groups. This helps to attract Ti(OEt)4 molecules to the NCM surface, thus uniform TiO2 nanolayer can be obtained after the calcination process. The TiO2 nanolayer suppresses side reactions with the electrolyte generated during the charging process to improve the surface stability. We confirmed the growth of CEI (cathode electrolyte interphase) layer originated from side reactions during the cycling through XPS and TEM analyses Consequently, it enhances the cycling stability in the high-voltage operation (4.5 V), and it also improves the high temperature (60 oC) cycling. In summary, this talk will give a facile way to form a uniform and ultrathin TiO2 nanolayer on Ni-rich oxide surface by pre-coating a polydopamine layer. We expect that the PDA pre-coating method is effective and could be applied to other fields where high-quality coating is required. Figure 1