Acid-Assisted Surface Modification of O3-type Sodium Layered Oxides for Long-Life and Safe Sodium-Ion Batteries

Sodium-ion batteries have attracted significant attention for large-scale energy-storage applications owing to the natural abundance and low cost of sodium resources. However, the widely studied layered oxide cathodes are highly susceptible to forming surface impurities and phase transformation after even brief exposure to humidity in ambient air, which severely degrades their cathode performance. In this work, we demonstrate that simple phosphoric acid and boric acid treatments effectively convert the surface impurities on O3-NaNi_0.5Mn_0.5O₂ into protective phosphate and borate surface layers. These chemically robust surface layers substantially stabilize the cathode-electrolyte interface. The improvement in thermal stability is quantitatively evaluated by using a thermal stability index (TSI) derived from differential scanning calorimetry (DSC) measurements. This study highlights the critical role of surface engineering in enhancing the safety and stability of sodium-layered oxide cathodes, thereby advancing the development of reliable sodium-ion batteries.