Anion Substitution Strategy toward an Advanced NASICON-Na₄Fe₃(PO₄)₂P₂O₇ Cathode for Sodium-Ion Batteries

Superior sodium-ion batteries (SIBs) greatly need cathode materials with higher capacity and better durability. Herein, the anion group substitution strategy is proposed to design a cathode material with extraordinary Na⁺ storage performance, NASICON-Na₄Fe₃(PO₄)_1.9(SiO₄)_0.1P₂O₇ (NFPP-Si0.1). The experimental and theoretical research revealed that modification in the local structure by anion substitution significantly boosts the ionic/electronic transfer kinetics via optimizing the electronic conductivity and reducing the Na⁺ diffusion energy barrier. Furthermore, the SiO₄^4- substitution generates a slight expansion of the crystal lattice to broaden the Na⁺ diffusion channel. Specifically, the custom-designed NFPP-Si0.1 could deliver a high rate capability of 77.6 mAh g^-1 at constant 50 C charge-discharge and excellent recyclability of 79.4% retention rate after 7000 cycles at 10 C. Besides, it also possesses outstanding low temperature reversible capacity of 95.5 mAh g^-1 at 0.1 C and long-term cyclability of 93.6% capacity retention after 1000 cycles at 5 C in -10 °C. This strategy of heterogeneous and isostructural anion group substitution provides a method for unlocking high-rate and long-life-span mixed polyanionic cathodes.