Epitaxial Entropy-Assisted Surface Engineering toward Durable High-Voltage O3-Type Layered Oxide Cathodes for Sodium-Ion Batteries

Surface structural instability and related side reactions are widely acknowledged as the primary factors leading to the failure of layered oxide cathodes during extended high-voltage cycling of sodium-ion batteries (SIBs). In this study, we propose an epitaxial entropy-assisted surface engineering strategy that integrates dual-site W/B surface doping with an in situ formed Na₂WO₄ protective coating layer to enhance the high-voltage cycling performance of the O3-type Na_0.9Ni_0.33Mn_0.40Cu_0.12Ti_0.15O₂ (NMCT). Tungsten and boron, selected for their high electronegativity, occupy octahedral and tetrahedral surface sites, respectively, which increases local configurational entropy. This codoping effectively anchors lattice oxygen at the surface, improves phase transition reversibility, and mitigates structural degradation that propagates from the surface into the bulk during high-voltage cycling. The Na₂WO₄ layer suppresses electrolyte decomposition while promoting efficient Na⁺ transport. As a result, WB-modified NMCT exhibits significantly enhanced cycling stability (79.7% capacity retention after 200 cycles at 100 mA g^-1) and rate capability (95.1 mAh g^-1 at 2000 mA g^-1) at an upper cutoff voltage of 4.3 V, outperforming pristine NMCT (60% retention and 77.1 mAh g^-1). This work offers an effective approach to rationally reconstruct the surface of layered oxide cathodes, thereby overcoming the upper cutoff voltage constraint for high-performance SIBs.