Revealing the chemical separated two-phase structure in lithium-manganese-rich cathode.

Lithium-manganese-rich (LMR) oxides are regarded as one of the most promising cathode materials for next-generation batteries. However, their poor rate capability and performance degradation during cycling present significant challenges for practical applications. Understanding how to optimize their microscopic structures during synthesis may provide critical insights for enhancing their performance. In this work, we investigated the structural evolution during the solid-state sintering of LiNiMnO from Li-/Mn-/Ni-carbonate precursors. Combining X-ray diffraction and transmission electron microscopy (TEM) techniques, we observed the nucleation of a nanoscaled solid-solution phase at 550°C, accompanied by secondary phases of spinel-like, layered and rock salt. At 800°C, a relatively pure solid-solution phase is formed. Notably, we uncovered, for the first time, a phase transition from a solid-solution structure to a chemically separated two-phase structure when annealing the sample from 850°C to 900°C. Atomic resolution scanning-TEM (STEM) imaging clearly distinguished the phase from the phase, separated by a coherent grain boundary, as confirmed by using STEM-energy-dispersion spectroscopy mapping. Our calculations indicate that the diffusion of Ni²⁺ induced by high-temperature activation plays a significant role in facilitating the phase separation. The relatively large chemically separated two-phase structure is expected to exhibit different performance characteristics compared with the previously reported nanosized two-phase structures, providing a new foundation for further improving high-energy-density LMR cathodes.