Elucidating the Suppression of High‐Voltage Degradation in O3‐type NaNi 1/3 Fe 1/3 Mn 1/3 O 2 : A Voltage‐Switching Strategy for Na‐ion Batteries

Elevating depth‐of‐charge (DoC) enhances the energy density of layered oxide cathodes for sodium‐ion batteries; however, such high‐DoC cycling inevitably triggers severe structural and interfacial degradation. In practice, battery management systems commonly adapt voltage‐derating strategies dynamically according to battery state of health, achieving a favorable trade‐off between energy density and cycle life. Nevertheless, a critical yet overlooked question remains: how high‐DoC‐induced damaged regions evolve when reverted to moderate‐DoC cycling. Here, using NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NFM111) as a prototype, a voltage‐switching strategy is employed to elucidate the dynamic and long‐term behavior of these damaged regions. During high‐DoC cycling (charging to 4.3 V), the rapid capacity fading is driven by pronounced formation of bulk defects and interfacial disordered spinel phases. However, upon switching these cells to moderate‐DoC cycling (charging to 4.0 V), these pre‐existing damaged regions become stabilized: their rapid propagation is suppressed, and they no longer accelerate degradation in surrounding intact regions. This enables degraded NFM111 to deliver capacity retention comparable to that of cells cycled solely under moderate‐DoC conditions. This work presents a dynamic and evolving picture of high‐DoC degradation and subsequent moderate‐DoC stabilization in NFM111 cathodes, providing mechanistic insights into voltage‐derating process and the lifetime prolongation of compromised cells.