Rational Design and Electrochemical Mechanism of High‐Capacity Quadruple Layered Oxide Cathode Materials for Rechargeable Sodium‐Ion Batteries

O3-type layered oxides are strongly considered as a promising cathode material for rechargeable sodium-ion batteries due to the high theoretical capacity and low-cost raw materials, but are challenged by poor electrochemical performance over Na extraction above 4.0 V. Herein, a novel quadruple layered oxide is rationally designed by regulating Fe doping in the representative NaNi0.5Ti0.25Mn0.25O2 composition, and influence of Fe doping on structure and electrochemistry of the NaNi0.5-x/2Ti0.25-x/2FexMn0.25O2 (0 ≤ x ≤ 0.30) material is systematically investigated with X-ray diffraction (XRD), transmission electron microscope, cyclic voltammetry, and galvanostatic measurement. It is found that the favorable quadruple structure enables the optimized NaNi0.45Ti0.2Fe0.1Mn0.25O2 material to show superior electrochemical performance with a large practical capacity of 157 mAh g-1 at 10 mA g-1 and a high-capacity retention of 81% after 100 cycles at 100 mA g-1. Furthermore, the phase transitions and redox reactions are analyzed by using ex situ XRD and X-ray photoelectron spectroscopy. In addition, the cycling degradation of the materials during cycling is understood with dQ/dV curves and XRD technique. The results in this study indicate the effectiveness of the dual-cationic substitution strategy in designing high-performance layered oxides cathode, and suggest significant impact of cationic migration on their capacity degradation and voltage hysteresis during cycling.