Understanding of Spinel Phases in Lithium-Rich Cathode for High-Energy-Density Lithium-Ion Batteries: A Review

Layered Li-rich oxides have attracted much attention because of higher capacity than that of traditional layered oxides (more than 250 mAh g⁻¹). However, the intrinsic issues of Li-rich cathode materials suffer from lattice oxygen loss, poor rate capability, voltage fade, and limited cycle life. To tackle these problems, the Li-rich cathode containing intergrown layer and spinel phases was proposed, and this heterostructure material meets the requirements of high energy and stable surface with a fast Li⁺ diffusion channel. Herein, we review the recent progress and in-depth understanding about heterostructure including microstructure and morphology, performance of advancement and degradation mechanisms, and modification strategies. Special attention is given to the high-performance energy mechanism as follows: (a) spinel phase and oxygen vacancy jointly enhance the lattice structure and prevent the irreversible oxygen release, (b) higher capacity is achieved by promotion of activation of Li2MnO3 phase and control of the activation rate to realize stable long-term cyclability, and (c) spinel phase provides the 3D interconnected Li⁺ diffusion channels and protects the surface region from side reactions. The other issue that aroused interest is the undesirable changes of phase transition and degradation mechanisms as follows: (a) the key reconstruction process is to produce a “good” spinel to maintain the surface and interior structure stability. (b) It is significant to figure out the structure degradation and phase transition mechanism in the cycled heterostructure. This review aims to provide inspiration and opportunities for the design of high-energy-density cathode materials, thereby bridging the gap between laboratory research and practical battery applications.