Decoding Li+/Na+ Exchange Route Toward High-Performance Mn-Based Layered Cathodes for Li-Ion Batteries

Li+/Na+ exchange has been extensively explored as an effective method to prepare high-performance Mn-based layered cathodes for Li-ion batteries, since the first report in 1996 by P. G. Bruce (Nature, 1996. 381, 499-500). Understanding the detailed structural changes during the ion-exchange process is crucial to implement the synthetic control of high-performance layered Mn-based cathodes, but less studied. Herein, in situ synchrotron X-ray diffraction, density functional theory calculations, and electrochemical tests are combined to conduct the systemic studies into the structural changes during the ion-exchange process of an Mn-only layered cathode O3-type Li-0.67[Li0.22Mn0.78]O-2 (LLMO) from the corresponding counterpart P3-type Na-0.67[Li0.22Mn0.78]O-2 (NLMO). The temperature-resolved observations combined with theoretical calculations reveal that the Li+/Na+ exchange is favorable thermodynamically and composited with two tandem topotactic phase transitions: 1) from NLMO to a layered intermediate through approximate to 60% of Li+/Na+ exchange. 2) then to the final layered product LLMO through further Li insertion. Moreover, the intermediate-dominate composite is obtained by slowing down the exchange kinetics below room temperature, showing better electrochemical performance than LLMO obtained by the traditional molten-salt method. The findings provide guides for the synthetic control of high-performance Mn-based cathodes under mild conditions.