Rational Design of Highly Stable Layered Oxide Cathodes for Na‐ion Batteries

ABSTRACT The O3 phase layered oxides are the most desirable cathode for commercial sodium‐ion batteries, but they suffer from two serious issues. First, the breakage and collapse of layers during the phase transitions are difficult to overcome. Second, the interlayered sodium ions are highly hygroscopic, causing sodium‐ion precipitation. Herein, a rational strategy is proposed to address the above‐mentioned challenges, integrating high‐entropy transition metal site design to mitigate structural breakage and interlayered calcium substitution to suppress interlayer sliding and sodium‐ion precipitation. Guided by the Zaanen‐Sawatzky‐Allen model, the high‐valence d 0 metal Ti 4+ and the s ‐block metal Li + dilute d ‐electron density in the transition metal layer, lowering correlation strength ( U ↓, W ↑, Δ↑), which synergistically suppresses charge ordering and stabilizes lattice oxygen. The optimal cathode composition with suitable calcium content is Na 0.9 Ca 0.01 Li 0.06 Ni 0.25 Cu 0.05 Fe 0.1 Co 0.05 Mn 0.41 Ti 0.08 O 2 . Such a high‐entropy configuration suppresses the unfavorable O3′ phase under high voltage and displays a minimal volume change of less than 1.7%. Exceptional electrochemical performance is achieved, with long‐term stability of 81.0% capacity retention after 600 cycles at 2 C. An ultra‐high‐capacity 20 Ah pouch cell exhibits capacity retentions of 95.1% after 700 cycles at 25°C and 94.7% after 400 cycles at 45°C, and delivers 86.1% of its 25°C capacity at −20°C.