Reversible Phase Transition of an Oxide Cathode in High-Voltage Sodium-Ion Batteries

Layered oxides (LOs) show great potential as high-capacity cathode materials for sodium-ion batteries (SIBs). However, severe phase transitions of LOs at high operation voltages induce large volume variations and anisotropic lattice strain. Herein, the irreversible phase transition of O3-NaNi 0.5 Mn 0.5 O 2 at 4.3 V is regulated through entropy modulation in Na 0.846 K 0.049 Zn 0.081 Ni 0.322 Fe 0.102 Mn 0.398 Ti 0.097 O 2 (HENM). This facilitates formation of an O/P intergrowth phase in HENM upon deep desodiation, which is induced by the enhanced electron density around oxygen in transition-metal slabs. The reduced lattice strain and layer gliding decrease local stress to maintain the structural integrity of HENM. These reward SIBs with a capacity of 160.2 mAh/g through the reversible redox of Ni 2.03+ ↔ Ni 3.83+ and Fe 3.00+ ↔ Fe 3.90+ . The pouch cell of HENM||hard carbon shows a high energy density of 155.1 Wh/kg and retains 88% of its capacity after 2000 cycles. This study offers new guidance for the development of high-voltage cathode materials.