Solid-state calcination profoundly influences the structural integrity and electrochemical performance of polycrystalline layered oxide cathode materials in lithium-ion batteries. As temperatures rise, heterogeneous phase transitions driven by solid-state diffusion can result in structural non-uniformity. In this study, we employ operando characterization techniques and high-resolution electron microscopy to scrutinize the inherent heterogeneity observed in the early-stage of the solid-state lithiation process and its subsequent influence on the formation and merging of resultant LiNi0.9Co0.05Mn0.05 (NCM90) grains on the surface of the secondary particle. We found a conformal atomic layer deposited WO3 layer on the hydroxide precursor could be in-situ lithiated to form LixWOy compounds, which are stable and none-dissolvable at the grain boundaries, further acting as segregation layer to prevent the merging of grains during the formation of layered phase on the surface of secondary particles, which preserves the route for the uniform lithiation for the inner part of the secondary particles. These investigations shed light on the effect of solid-state reaction heterogeneity and present a novel methodology for mitigating the persistent challenge by grain boundary engineering.
