Regulating cation mixing for enhanced structural stability of layered oxide cathodes by ion-exchange strategy

Abstract

Typical LiNixCoyMn1-x-yO2 (NCM) layered oxides are a promising research area for high-energy–density cathode materials. However, they still suffer from structural disorder due to severe Li/Ni mixing caused by solid-state reactions at high temperatures (700–900 °C). To address these problems, most strategies focus on increasing the energy barrier of Ni2+ migration towards Li+ sites, such as doping or annealing in an O2-rich atmosphere. Herein, we report a new strategy of lowering processing temperature for effectively regulating Li/Ni mixing, achieved by hydrothermal ion exchange. The ion exchange can achieve a rapid substitution of Li+ ions for Na+ ions at relatively low temperatures (160–220 °C), allowing the NCM to maintain low Li/Ni mixing (<1%). Our ab initio calculations further demonstrate that the Ni2+ diffusion rate in ion-exchanged NCM523 at 200 °C is significantly slower in the order of 10−12 than that in NCM523 calcined at 900 °C, indicating that processing temperature is crucial to regulating Li/Ni mixing. That is, Li/Ni mixing of NCM523 materials is effectively regulated during ion exchange, further enhancing the structural stability and electrochemical performance. In addition, the universality of hydrothermal ion exchange was further proved by synthesizing NCM materials with good structural stability for different nickel contents (x = 0.5, 0.65, 0.7, 0.85). We anticipate that our ion-exchange strategy opens a new door towards regulating Li/Ni mixing by lowering processing temperature, enabling layered oxide materials with enhanced structural stability.