Co-Substitution Enhances the Rate Capability and Stabilizes the Cyclic Performance of O3-Type Cathode NaNi 0.45- x Mn 0.25 Ti 0.3 Co x O 2 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 mAh g-1 in the initial cycle and maintains 91.4 mAhg-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.