A Holistic Picture of the Phase Construction Process of O3-Structured NaNi1/3Mn1/3Fe1/3O2 for Sodium-Ion Batteries

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.