A Holistic Picture of the Phase Construction Process of O3‐Structured NaNi _1/3 Mn _1/3 Fe _1/3 O ₂ for Sodium‐Ion Batteries

Abstract The synthesis process of layered oxide cathode materials is pivotal yet underexplored in sodium‐ion battery (SIB) research. This study systematically investigates the phase construction mechanisms of O3‐structured NaNi _1/3 Mn _1/3 Fe _1/3 O ₂ (NMF) through in situ heating XRD, synchrotron‐based STXM, and electrochemical analysis, focusing on decomposition, diffusion, phase transformation, and oxidation steps during the synthesis. Three precursors—co‐precipitated hydroxides, micrometer‐sized metal oxides (MO), and nanometer‐sized oxides (sand‐milled metal oxides, SMMO)—are compared, alongside sodium sources (Na ₂ CO ₃ , NaOH, NaHCO ₃ ). Hydroxide precursors enabled a direct P3‐to‐O3 solid‐solution transition via homogeneous Na‐ion diffusion, yielding uniform structures and superior electrochemical performance (131 mAh g ⁻¹ discharge capacity). In contrast, MO precursors exhibited stepwise phase evolution: Mn ₂ O ₃ initiated P3 formation at 425 °C, Fe ₂ O ₃ promoted O3 nucleation at 675 °C, and NiO finalized the transition at 800 °C, albeit with residual impurities. Reducing precursor size (SMMO) delayed phase onset temperatures but retained diffusion‐controlled pathways. Sodium source decomposition kinetics critically influenced phase transitions: NaOH accelerated O3 formation at lower temperatures, while NaHCO ₃ delayed P3‐O3 conversion. STXM revealed heterogeneous oxidation states in oxide‐derived samples, correlating with sluggish diffusion and inferior cycling stability. This work establishes that precursor uniformity and sodium source selection govern diffusion homogeneity, phase purity, and electrochemical behavior.