Achieving High-Voltage Sodium–Ion Batteries with Phase-Transition-Free High-Entropy Oxide Cathodes via Altering Local Coordination Fields

The cocktail effect has brought Na-based high-entropy oxides (HEOs) into the spotlight in the realm of sodium-ion batteries. However, the irreversible phase transition of the HEOs cathode at high voltage induces serious voltage hysteresis during dynamic (de)sodiation processes. Herein, a P2-type fluorine-substituted Na_0.85Mn_0.4Li_0.1Ni_0.2Fe_0.1Cu_0.1Co_0.1O_1.9F_0.1 (PNHEOF) cathode was constructed to achieve phase-transition-free operation at a high state of charge. Experimental characterizations combined with theoretical calculations proved that the increased covalency of the transition metal (TM) and oxygen bond, which was caused by the rise in the valence state of TM cations, was alleviated, thereby effectively suppressing the migration of TMO₂ slabs at a deep desodiation state. Meanwhile, the high Na⁺ content not only enhances structural stability but also promotes the participation of low-valent TM in more redox reactions for charge compensation. As a result, the PNHEOF cathode delivers a specific capacity of 182 mAh g^-1, showing nearly zero-volume change (only a negligible volume variation of 0.43%) within the voltage range of 2.0-4.5 V at a rate of 0.1 C. When assembled into a commercial pouch cell, it shows an energy density of 154 Wh kg^-1 and maintains a stable output midpoint voltage of 3.2 V for practical application.