Co-Substitution Enhances the Rate Capability and Stabilizes the Cyclic Performance of O3-Type Cathode NaNi_0.45–xMn_0.25Ti_0.3Co_xO₂ for Sodium-Ion Storage at High Voltage

O3-type NaNiO2-based cathode materials suffer irreversible phase transition when they are charged to above 4.0 V in sodium-ion batteries. To solve this problem, we partially substitute Ni2+ in O3-type NaNi0.45Mn0.25Ti0.3O2 by Co3+. NaNi0.45Mn0.25Ti0.3O2 with co-substitution possesses an expanded interlayer and exhibits higher rate capability, as well as cyclic stability, compared with the pristine cathode in 2.0–4.4 V. The optimal NaNi0.4Mn0.25Ti0.3Co0.05O2 delivers discharge capacities of 180 and 80 mA h g–1 at 10 and 1000 mA g–1. At 100 mA g–1, NaNi0.4Mn0.25Ti0.3Co0.05O2 exhibits 152 mA h g–1 in the initial cycle and maintains 91.4 mA h g–1 after 180 cycles. Through ex situ X-ray diffraction, co-substitution is demonstrated to be effective in enhancing the reversibility of P3–P3″ phase transition from 4.0 to 4.4 V. Electrochemical impedance spectroscopy indicates that higher electronic conductivity is achieved by co-substitution. Moreover, cyclic voltammetry and the galvanostatic intermittent titration technique demonstrate faster kinetics for Na+ diffusion due to the co-substitution. This study provides a reference for further improvement of electrochemical performance of cathode materials for high-voltage sodium-ion batteries.