Coupled Evolution of Charged-State Stacking and Redox Partitioning in O3-Type Sodium Cathodes

Abstract Elevated-voltage operation in layered Na-ion cathodes often increases capacity but accelerates degradation, yet the structural origin of this trade-off remains unclear. Here, we investigate the coupled evolution of charged-state structure and redox partitioning in O3-type NaNi1/3Fe1/3Mn1/3O2. Charging to 4.2 V delivers greater capacity but promotes O/P intergrowth with deeply desodiated O-type slabs and accelerates capacity decay, whereas a 4.0 V cutoff stabilizes a reversible P3 framework. Synchrotron X-ray diffraction, transition-metal X-ray absorption spectroscopy, Na K-edge spectroscopy, and electrochemical impedance spectroscopy combined with distribution of relaxation times analysis (EIS-DRT) reveal distinct charge-compensation pathways associated with these structures. The P3 framework is associated with a progressive decrease in the Ni contribution and a comparatively more reversible local Fe environment. In contrast, stacking evolution at 4.2 V is associated with sustained Ni-dominant redox, greater Fe local-environment reconstruction, and persistent kinetic limitations. These results identify the charged-state stacking as a key structural descriptor linking redox partitioning and Na-ion transport kinetics.