Manipulating Na/TM Ratio‐Driven Structural Heterogeneity of O3‐NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ Cathode for High‐Voltage Sodium‐Ion Batteries

ABSTRACT The stability of O3‐type under high‐voltage cycling is dictated by how synthesis encodes lattice strain and redox heterogeneity. Here, the role of Na:TM stoichiometry is systematically resolved by tuning the NaOH:precursor ratio during solid‐state synthesis. The stoichiometric condition (Na:TM = 1.00) yields minimized microstrain, enabling uniform O3–P3 phase evolution and homogeneous multi‐metal redox with preserved octahedral symmetry. In contrast, Na‐excess compositions inherit disordered intermediates and heterogeneous distortion fields that trigger abrupt multiphase transitions and promote localized charge redistribution. In situ XRD captures the divergence in phase‐transition pathways, TXM resolves particle‐level redox heterogeneity, and XANES corroborates a stronger and more reversible Fe redox contribution at stoichiometry, shifting to diminished Fe participation and spatially inhomogeneous redox at higher Na content. These results establish Na:TM stoichiometry as a critical synthesis parameter controlling both structural coherence and redox stability. Electrochemically, the stoichiometric composition exhibits smooth voltage profiles with minimal polarization growth and retains nearly 80% of its initial capacity after 100 cycles even at an extended 4.2 V cutoff, whereas Na‐excess compositions show significantly reduced initial coulombic efficiency and rapid voltage fade. Precise stoichiometric tuning provides a scalable route to defect‐suppressed O3 frameworks, enabling structurally resilient, high‐voltage sodium‐layered cathodes.