Synergizing phase engineering and site-selective substitution through the rational design of a high-entropy layered oxide cathode enables air-stable and long-life sodium-ion batteries

Abstract

P2-Na_0.67Ni_0.33Mn_0.67O₂ cathodes face commercialization barriers since irreversible P2–O2 phase transitions occur when charged above 4.0 V and Mn³⁺ degradation occurs if discharged to 2.0 V; this, along with moisture sensitivity, results in high costs for production and storage. To address these challenges, we realized the synergy of phase engineering and site-selective substitution through the rational design of a layered oxide, Na_0.80Mn_0.60Mg_0.07Ni_0.23Cu_0.07Zn_0.03O₂ (P2/O3-Na8M). The site-selective substitution of Zn, as a pillar, at the Na sites prevents irreversible phase transitions and resists H₂O insertion via the stronger attraction of Zn²⁺–O–Zn²⁺ than that of Na⁺–O–Na⁺. The accommodation of Cu/Mg in TM sites further enhances structural stability by suppressing oxygen loss through stronger Cu–O covalency than Ni–O, reducing Na⁺/H⁺ exchange, and stabilizing Na–O bonds by lowering the overall cationic potential of the structure, in addition to mitigating the Jahn–Teller distortions and enabling ultra-wide potential operation. Theoretical and experimental investigations reveal that the P2/O3 phase heterostructure lowers Na⁺ diffusion barrier, enabling P2/O3-Na8M to deliver 150 mAh g⁻¹ at 0.1C with 80.8% capacity retention after 300 cycles at 5C within 1.8–4.4 V while preserving the structure and performance after 35 days of air/water exposure. The full cell can achieve a high energy density of 281.2 Wh kg⁻¹ and offer 82.5% retention after 600 cycles at 1C.