Improved High Rate Performance and Cycle Performance of Al-Doped O3-Type NaNi_0.5Mn_0.5O₂ Cathode Materials for Sodium-Ion Batteries

O3-type layered transition metal oxides have shown great promise as high-capacity cathode materials for sodium-ion batteries in large-scale energy storage, due to their low cost and the abundance of sodium resources. However, the limited interlayer spacing and the unstable structure of the O3 phase result in inferior cycling stability and poor rate capability, which could even hinder their practical application process. Hereby, we present that doping with nonelectrochemically active Al in NaAl_x(Ni_0.5Mn_0.5)_1–xO₂ (x = 0, 0.02, 0.06, 0.1) can effectively alleviate these issues. Among these materials, materials with x = 0.02 exhibited the best electrochemical behavior with improved capacity and better cycling property than other materials. Specifically, the discharge capacity of a 2 mol % Al-doped material possesses 63.2% capacity retention after 200 cycles at a current density of 240 mA g^–1, which is 21.4% higher than that in NaNi_0.5Mn_0.5O₂. In addition, a 2 mol % Al-doped electrode also shows outstanding rate capability and delivers 90 mAh g^–1 at a high rate of 480 mA g^–1, compared to only 67 mAh g^–1 for that of the pristine one. X-ray diffraction (XRD), cyclic voltammetry (CV), and galvanostatic intermittent titration technique (GITT) analysis elucidated that the introduced Al dopant can effectively improve the structural stability and promote kinetics of Na⁺ diffusion mobility. Therefore, the strategy of doping inactive aluminum elements for optimizing the electrochemical performance of NaNi_0.5Mn_0.5O₂ was verified, which could also open an avenue for the design of other O3-type sodium cathode materials.