Cation-Disordered O3-Na_0.8Ni_0.6Sb_0.4O₂ Cathode for High-Voltage Sodium-Ion Batteries

O3-type sodium-layered oxides (such as antimony-based O3 structures) have been suggested as one of the most fascinating cathode materials for sodium-ion batteries (SIBs). Honeycomb-ordered antimony-based O3 structures, however, unsatisfactorily exhibit complex phase transitions and sluggish Na⁺ kinetics during cycling. Herein, we prepared a completely cationic-disordered O3-type Na_0.8Ni_0.6Sb_0.4O₂ compound by composition regulation for SIBs. Surprisingly, the measured redox potentials for typical O3-P3 phase transition are located at 3.4 V. Operando X-ray diffraction confirms a reversible phase transition process from the O3 to P3 structure accompanied with a very small volume change (1.0%) upon sodium extraction and insertion. The low activation barrier energy of 400 meV and the fast Na⁺ migration of 10^-11 cm²·s^-1 are further obtained by first-principles calculations and galvanostatic intermittent titration technique, respectively. As a result, the O3-Na_0.8Ni_0.6Sb_0.4O₂ cathode displays a reversible capacity of 106 mA h g^-1 at 0.1C (12 mA g^-1), smooth charge-discharge curves, and a high average working voltage of 3.5 V during battery cycling. The results highlight the importance of searching for a new O3-type structure with cation-disordering and high working voltage for realizing high energy SIBs.