Low-Cost Al-Doped Layered Cathodes with Improved Electrochemical Performance for Rechargeable Sodium-Ion Batteries

O3-NaNi_0.25Fe_0.5Mn_0.25O₂ layered oxide is considered one of the most promising cathode candidates for sodium-ion batteries because of its advantages, such as its large capacity and low cost. However, the practical application of this material is limited by its poor cyclic stability and insufficient rate capability. Here, a strategy to substitute the Fe³⁺ in NaNi_0.25Fe_0.5Mn_0.25O₂ with Al³⁺ is adopted to address these issues. The substitution of Fe³⁺ with Al³⁺ enhances the framework stability and phase transition reversibility of the parent NaNi_0.25Fe_0.5Mn_0.25O₂ material by forming a stronger TM-O bond, which improves the cycling stability. Moreover, partial Al³⁺ substitution increases the interslab distance, providing a spacious path for Na⁺ diffusion and resulting in fast diffusion kinetics, which lead to improved rate capability. Consequently, the target NaNi_0.25Fe_0.5-xAl_xMn_0.25O₂ sample with optimal x = 0.045 exhibits a remarkable electrochemical performance in a Na-ion cell with a large reversible capacity of 131.7 mA h g^-1, a stable retention of approximately 81.6% after cycling at 1C for 100 cycles, and a rate performance of 81.3 mA h g^-1 at 10C. This method might pave the way for novel means of improving the electrochemical properties of layered transitional-metal oxides and provide insightful guidance for the design of low-cost cathode materials.