Reducing Residual Alkali by Moderated Acetic Acid Treatment for Stable Sodium‐Ion Layered Oxides

Sodium‐ion layered oxide cathode materials are prone to generating surface residual alkali (Na ₂ CO ₃ ) during synthesis, which severely deteriorates their electrochemical performance and hinders their commercial development. Currently reported strategies for residual alkali removal (such as water washing, secondary sintering, or surface conversion) are often complicated, costly, or destructive for material preparation, making them difficult for large‐scale application. In this work, taking NaNi _0.4 Fe _0.2 Mn _0.4 O ₂ as an example, we propose a simple and moderated acetic acid surface treatment approach. Results demonstrate that this method can effectively remove residual alkali residues, enhance Na ⁺ diffusion kinetics, and promote the formation of a stable cathode‐electrolyte interface (CEI) during cycling. After acetic acid modification, the cathode material exhibits an improved capacity retention of 86% after 200 cycles at 1C (1C = 140 mA h g ⁻¹ ), compared to the low capacity retention of 69% for the untreated sample. This study provides valuable insights into the post‐processing technique for sodium‐ion layered oxide cathode materials.