Cation-doping modulated phase-transition mechanism and structural stability of layered nickel/manganese oxide cathode for enhanced electrochemical capability

Layered nickel/manganese oxides have garnered considerable attention as a high-capacity cathode material for rechargeable sodium-ion batteries, but being challenged by complex phase transitions and poor cycling performance. A novel layered oxide with suppression of phase transitions is designed by doping Ti and Al atoms into NaNi_0.5Mn_0.5O₂ lattice as a highly stable cathode material. Structural and electrochemical evolutions of the material are investigated with X-ray diffraction, X-ray photoelectron spectroscopy, cyclic voltammetry, and galvanostatic measurements. As revealed, it mainly undergoes a single-phase evolution with lattice distortion of O3 phase at a low-potential plateau region and a two-phase reaction process with coexistence of O3 and P3 phases at a long potential slope region. The reversible O3/P3 coexistence-dominated phase-transition mechanism enables the material having fast sodium-diffusion ability and good structural stability, thus leading to superior high-rate capability (135.2 mA h g⁻¹ at 10 mA g⁻¹ and 73.4 mA h g⁻¹ at 1000 mA g⁻¹) and enhanced cycling performance (92.3 % retention after 100 cycles at 100 mA g⁻¹). The origin of the improved performance is understood based on the analysis of in-situ dQ/dV curves and ex-situ X-ray diffraction. The finding suggests the feasibility of cation doping in modulating structural evolution and electrochemical reversibility of layered oxides.