Synergistic anion-cation redox in a P2/O3 biphasic high-entropy layered oxide

Sodium-ion batteries (SIBs) face a fundamental trade-off between high capacity and structural stability in layered oxide cathodes, where anion redox reactions (ARR) induce oxygen loss and irreversible phase transitions (e.g., detrimental P2-O2). To overcome this, we design a single-crystal P2/O3 biphasic high-entropy oxide cathode Na 0.67 (Fe 1/4 Co 1/4 Ni 1/4 Ti 1/4 ) 0.2 Mn 0.6 Li 0.2 O 2 (FCNTML20). This cathode design strategy integrated high-entropy TM-layer configuration for lattice stabilization, precisely controlled P2/O3 biphasic structure (95:5 ration) to suppress phase transitions, and large single-crystal morphology eliminating grain boundaries. Ex-situ XPS analysis demonstrates that Li substitution effectively enhances the redox activity of oxygen while increasing the Mn 4+ /Mn 3+ ratio, thereby mitigating the Jahn-Teller distortion induced by Mn 3+ . In-situ XRD reveals that Li substitution and configurational entropy minimize lattice variations and inhibit P2-O2 transitions in favor of a reversible P2-OP4 pathway, enabling highly reversible synergistic anion-cation redox. Consequently, FCNTML20 achieves a high specific capacity of 207.7 mAh g −1 at 0.1C and near-100 % capacity retention after 100 cycles (1.5–4.5  V ). Full cells paired with hard carbon deliver 185.2 mAh g −1 and an energy density of 491.7 Wh kg −1 . This work establishes a paradigm for high-energy SIB cathodes by concurrently stabilizing cationic/anionic redox through entropy-enhanced biphasic engineering.