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 [Li 0.22 Mn 0.78 ]O 2 (LLMO) from the corresponding counterpart P3‐type Na 0.67 [Li 0.22 Mn 0.78 ]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 ≈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.