High-Entropy-Stabilized Air- and Moisture-Resilient Sodium-Ion Cathode: An Experimental and Theoretical Approach

Abstract Sodium-layered transition-metal oxides gain attention as promising cathode materials for sodium-ion batteries (SIBs) owing to their high capacity, cost-effectiveness, and facile synthesis. However, the practical implementation is limited due to their high sensitivity toward ambient moisture and air. In this regard, we report a high-entropy Na0.98Ca0.02Ni1/7Cu1/7Al1/7Co1/7Fe1/7Mn1/7Ti1/7O2 (TM-7 HEO) cathode, which substitutes multivalent elements that induce cation disorder in the transition-metal layer and enhances TM-O bond covalency through electronic delocalization. This leads to the disruption of Na+/vacancy ordering in the Na layer, which suppresses the formation of intermediate phases. Furthermore, the reduced sodium layer spacing, strengthened Na–O bonds, and the presence of Ca2+ in the Na layer provide enhanced air and moisture stability, preserving the structure even after 6 days of storage in water and air exposure. Electrochemical studies demonstrated that TM-7 HEO delivers a reversible capacity of 132 mAh g–1 within the 2.0–4.2 V range, exhibiting O3–P3 reversible phase transitions and enhanced Na+ diffusion kinetics. In addition, first-principles calculations validate the experimental observations. The density of states (DOS) analysis shows that Ni, Cu, and Fe contribute to a delocalized charge compensation, enabling high redox activity, which is also supported by ex situ XPS and crystal field stabilization energy (CFSE) calculations. The experimental and theoretical results together highlight the improved reversible phase transition, as well as the “pillar effect” created by Ca doping, which further enhances moisture and air stability. This work provides a fundamental insight and a robust design strategy for developing the stable O3 type cathode for next-generation sodium-ion batteries.