Entropy-driven disordered surface formation with durable anti-water stability for ultra-stable layered oxide cathodes in sodium-ion batteries

Conventional layered oxide cathodes for sodium-ion batteries (SIBs) suffer from severe capacity degradation due to crystalline surface reactivity, which triggers parasitic reactions with ambient H₂O/O₂, leading to surface corrosion and bulk structural collapse. Herein, we introduce a high-entropy engineering strategy that designs a self-protective cathode, Na_0.8Mg_0.1Zn_0.1Cu_0.1Fe_0.1Mn_0.6O₂ (HEO). This material spontaneously forms an entropy-stabilized amorphous surface coating with an ultralow formation energy of 0.16 eV. The coating acts as a kinetic barrier, raising the activation energy for detrimental H₂O/O₂ reactions by 160 % compared to a low-entropy counterpart (Na_0.8Mg_0.2Mn_0.8O₂). The synergy between entropy stabilization and surface amorphization delivers exceptional environmental robustness. After 90 days of water exposure, HEO retains 98 % of its initial capacity, and 99 % capacity retention over 100 cycles, surpassing state-of-the-art layered cathodes in cycling stability. This work establishes a universal framework for designing air/water-resilient cathodes, with immediate implications for scalable manufacturing and long-term storage stability of SIB systems.