Unveiling the Chemical Heterogeneity and Structural Evolution in Oxide Cathodes During Ion Exchange

Abstract Voltage decay is inevitable for the commercialization of Li‐rich nickel‐manganese oxides (LNMO). O2‐type LNMO with ABCBA oxygen stacking exhibits suppressed voltage decay, which can be prepared by chemical and electrochemical ion exchange from P2‐type sodium intermediate. Herein, the impact of these two methods on the structural properties, electrochemical behavior, and structural evolution of LNMO is systematically investigated. Additionally, the influence of the coexistence of Li/Na in the alkali metal layer on ion migration and phase transformation in LNMO has been revealed. In the electrochemical ion exchange process, phase transformation occurs prior to electrochemical activation due to the interplay of concentration gradients and electrochemical activation. This rapid phase transition from P2 to O2 leads to lattice spacing contraction, which hinders Na + extraction during the subsequent ion exchange process. The resulting impure O2‐LNMO, containing a significant amount of Na in the bulk, experiences pronounced lattice strain during cycling, leading to particle cracking and even fragmentation. To enhance the performance of the electrochemically prepared sample, it is crucial to balance the phase transition rates driven by concentration differences and electrochemical activation, ensuring a uniform and complete phase transition. In contrast, the high‐crystallinity C‐LNMO demonstrates superior battery performance and inhibited voltage decay.