Al doping in NFAPP enhances sodium-ion diffusion and structural stability for high-rate and long-cycle life sodium-ion batteries

Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) has attracted growing interest as a cathode material for sodium-ion batteries (SIBs) because of its non-toxic nature, economic cost, and robust structure. However, its practical application is limited by relatively poor electronic conductivity, which affects its electrochemical performance. The introduction of Al³⁺ ions, characterized by their smaller ionic radius, leads to lattice contraction. This not only minimizes lattice distortion during the sodium ion transport process but also significantly enhances the structural stability of the material. Furthermore, coating the material with a carbon layer significantly boosts its electrical conductivity. Consequently, the optimized NFPP exhibits superior sodium-ion diffusion, enhanced electronic conductivity, and improved structural integrity. Research findings show that the formulated non-stoichiometric, carbon-coated Na₄.₄Fe₃.₄Al₀.₁(PO₄)₂.₅P₂O₇/C cathode delivers exceptional electrochemical performance.It achieves a high specific capacity of 133 mAh g⁻¹ at 0.2 C, maintains good rate capability with 91 mAh g⁻¹ at 20 C, and shows excellent cycling stability, preserving 88 % of its initial capacity after 2500 cycles at 50 C.