Novel Stress‐Evolution Model for NaNi 1/3 Fe 1/3 Mn 1/3 O 2 Cathodes: Determining Localized Accumulation and Diffusive Transfer Based on Tension‐Compression Asymmetry

Abstract

ABSTRACT O3‐NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NFM) is a promising high‐energy‐density cathode for sodium‐ion batteries. However, the long‐term cycling performance is hindered by particle fracture caused by non‐uniform stress distribution. The mechanisms of non‐mechanical stress accumulation in intergranular and intragranular regions, as well as the long‐range transfer of stress interactions, remain unclear. Herein, we used DFT calculations and mechanical parameter analyses at the unit‐cell scale to identify SOC‐dependent “tension‐compression asymmetry” (Δ E SOC ) as a key descriptor. Correlation analysis reveals that Δ E SOC governs the continuity of intragranular strain and the magnitude of intergranular strain, enabling precise mapping of local stress accumulation. We developed a stress‐transfer model to describe directional stress propagation, demonstrating that cumulative stress is the product‐sum of Δ E SOC values along the transfer path. This model elucidates how intragranular stress drives microcrack formation. Guided by these insights, we employed Ca 2+ doping to regulate Δ E SOC , effectively reducing stress accumulation and enhancing cycling stability. After 1000 cycles at 1C, the Ca‐doped NFM retained 62.1% of the initial capacity, significantly outperforming the pristine NFM (18.3%). This work unifies localized stress accumulation and diffusive transfer mechanisms, providing a deeper understanding of stress‐induced failure in layered cathodes.