Spatially Dependent Phase Evolution and Ni-Induced Heterogeneity in O3-Type NaNi1/3Fe1/3Mn1/3O2 during Solid-State Synthesis

Solid-state synthesis of layered oxide cathodes involves complex phase evolution and diffusion-governed crystal growth, yet the formation mechanism remains elusive. Here, O3-type NaNi1/3Fe1/3Mn1/3O2 is used to unravel phase evolution and structural reconstruction during solid-state synthesis via high-temperature in situ X-ray diffraction, scanning transmission electron microscopy (STEM), and energy-dispersive spectroscopy. The results reveal a stepwise phase evolution from the rock-salt intermediate through the P3 phase to the O3 phase. The residual P3 phase, Na2CO3, and NiO in nonsoaked samples indicate that the phase transformation is governed by diffusion kinetics. Focused-ion-beam STEM reveals local Ni heterogeneity and retained Ni-rich rock salt. Crystal growth proceeds through a rock-salt intermediate, followed by Na+-intercalation-induced reconstruction into a layered framework, ultimately yielding anisotropic hexagonal platelets. Electrochemical measurements reveal that grain-boundary rock-salt intermediates and intragranular NiO domains impede Na+ diffusion, increase polarization, and accelerate cycling degradation. This work elucidates the diffusion-limited phase evolution and crystal growth mechanisms governing O3-type layered oxide synthesis.