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 NaNi_0.5Ti_0.25Mn_0.25O₂ composition, and influence of Fe doping on structure and electrochemistry of the NaNi_0.5-x/2Ti_0.25-x/2Fe_xMn_0.25O₂ (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 NaNi_0.45Ti_0.2Fe_0.1Mn_0.25O₂ 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.