Comprehensive Crystal Structural Insights into Phase Evolution of Spinel Co-Free Lithium Nickel Manganese Oxide

High-voltage Co-free LiNi0.5Mn1.5O4 (LNMO) is considered a promising candidate for next-generation high-performance lithium-ion batteries. However, there is a significant absence of full understanding of elaborate evolution of crystal structures. This review article aims to thoroughly examine the latest advancements in state-of-the-art LNMO and establish a systematic framework that outlines the varied phase evolution mechanisms under different synthesis conditions and during cycling from the perspective of the crystal structure. First, an analysis of the intrinsic structural properties of LNMO is conducted for optimizing the synthesis process, including crystal orientation, oxygen deficiency, disorder–order transition, and surface phase reconstruction under various synthesis conditions. Then, the phase transformation mechanisms during cycling have been systematically elucidated to develop strategies to mitigate any detrimental effects and improve the electrochemical performance. Finally, the insights into further development directions in LNMO are offered at relevant length scales from the crystal structure level to the electrode level.