Multi-angle tracking synthetic kinetics of phase evolution in Li-rich Mn-based cathodes

Abstract

As commercial cathodes with the highest practical capacity, the structural nature of Li-rich Mn-based (LMR) cathodes, composite or solid solution, is still under debate. Due to the extreme structural similarity of two layered-phase components, namely, the monoclinic phase (C2/m) and rhombohedral phase (Rm), no single tool can resolve this concern alone. Herein, we combined multiple advanced techniques to comprehensively study the structural changes during synthesis of LMR cathodes from different aspects such as the elemental distribution, local structure, long-range structure, and short-range structure, revealing a clear process from the formation of two phases to the gradual phase fusion and eventually to a nearly solid solution. Particularly, X-ray pair distribution function (PDF) analysis combined with theoretical simulations reveals for the first time that the transition metal (TM)–TM distance increases with the progress of the phase fusion, which makes the short-range structural change in TM–TM atomic pairs an effective parameter for judging the extent of phase fusion. Eventually, excellent electrochemical performance was achieved by balancing capacity and cycling stability through adjusting the phase fusion to a medium extent in lithium and sodium two layered-phase components. This study establishes an approach to investigate the structural evolution in the complicated multiple-phase system and provides valuable insights into the design and optimization of cathodes by tuning the phase fusion extent.