Formation and Electrochemical Role of an Ultrathin Compositionally Complex Interfacial Layer on O3-Type Layered Cathodes

Abstract Sodium-ion batteries (SIBs) have emerged as promising alternatives for large-scale energy storage owing to the abundance and low cost of sodium. However, layered oxide cathodes suffer from severe surface degradation and irreversible phase transitions during high-voltage cycling, limiting their electrochemical performance. Here, we report an ultrathin sodium-ion conductive nanolayer composed of a Na–Ni–Fe–Mn–Ti–O multicomponent oxide with a cubic NaTiO2-type framework (NTO), formed on NaNi1/3Fe1/3Mn1/3O2 (NFM) via atomic layer deposition followed by thermal annealing. The conformal NTO layer (∼2–3 nm) preserves the bulk crystal structure and particle morphology. As a result, the NTO-coated NFM exhibits significantly improved electrochemical performance, delivering approximately fivefold higher capacity retention after 400 cycles at 0.2 C and sixfold higher discharge capacity at 1 C compared to pristine NFM. Structural and interfacial analyses reveal that the NTO layer suppresses surface degradation and inhibits the formation of a cation-disordered rock-salt phase. In situ X-ray diffraction further shows that the NTO layer induces a more solid-solution-like phase transition, enabling reversible structural evolution. This behavior is attributed to the Na concentration gradient between the surface layer and bulk, which enhances Na+ transport and alleviates structural strain. These results demonstrate that constructing an ultrathin Na-ion conductive interfacial layer is an effective strategy to improve the structural stability and electrochemical performance of layered oxide cathodes.