From Rotten to Magical: Transition Metal Migration in Layered Sodium-Ion Battery Cathodes

Layered oxides, the most commercially promising cathode materials for sodium-ion batteries (SIBs), are extensively investigated due to their low cost, abundant raw materials, flexible structure, and high capacity. However, although the "rotten" transition metal (TM) migration phenomenon in layered oxide cathode materials leads to unfavorable structure reconstruction, sluggish Na+ diffusion and subsequent performance degradation, TM migration has not been systematically summarized and intensively discussed. Herein, a comprehensive insight into the recent advances of TM migration in layered SIB cathodes is provided, aiming to realize the "magical" utilization of TM migration. First, the negative effects of TM migration on crystal structure and electrochemical properties are discussed. Then, the origin of TM migration is intensively analyzed, and three main mechanisms of the migration are presented, including Cr4+-Cr4+-Cr4+ triplets, Jahn-Teller Fe4+ and Na-free layers. Additionally, the latest research findings on exploring the origin are fully discussed. Finally, the suppression and utilization of TM migration are discussed, and the further exploration of TM migration and the future directions of suppressing/utilizing TM migration are outlooked. It is believed that this perspective will provide guidelines for the design of high-performance layered oxide cathodes involving TM migration by turning the "rotten" into the "magical." A comprehensive, systematic, and in-depth insight into the recent advances of transition metal (TM) migration in layered oxide cathodes of sodium-ion batteries (SIBs) is presented, including the fundamental characteristics, migration mechanisms, suppression, utilization, and further development directions, and the relationship between electrochemical performance and TM migration in layered oxide cathodes of SIBs is plotted.image