Phase Transition during Sintering of Layered Transition Metal Oxide Sodium Cathodes

Layered cathodes derived from precursor materials have garnered significant attention in sodium ion battery (SIB) research. However, the structure evolution mechanisms during the sintering process remain inadequately understood. In this work, two precursors with irregular and regular morphologies were subjected to identical calcination conditions to synthesize O3-NaNi_0.4Fe_0.2Mn_0.4O₂ cathodes. Comprehensive analysis revealed that the irregular precursor underwent heterogeneous Na⁺ diffusion, resulting in an <i>R</i>3̅<i>m</i> structure shell encapsulating a substantial rock-salt phase core during the solid-state sodiation process. This leads to drastic phase transition and generated unfavorable pores in the subsequent high-temperature process. In contrast, the regular quasi-spherical precursor maintains a uniform Na⁺ accessibility throughout the sintering process, which facilitated optimal phase evolution and yielded superior electrochemical performance. This investigation elucidates the critical relationship between precursor morphology and phase transition dynamics, providing crucial insights into the rational design of precursor-derived layered cathodes in SIB applications.