Phase-Purity Stoichiometric Na₄Fe₃(PO₄)₂P₂O₇ Cathode Material via Reversible Reaction Modulation for High-Performance...

Na₄Fe₃(PO₄)₂P₂O₇ (NFPP), a prominent polyanionic material, has garnered significant attention as a promising cathode for practical sodium-ion batteries. However, the electrochemical performance of stoichiometric NFPP is often hindered by the persistent formation of the phase-impurity maricite NaFePO₄ (NFP), which is difficult to eliminate during synthesis. In this study, it is tackled this challenge by reversible reaction modulation (RRM, $2P{O}_4^{3-}+2H^{+}\rightleftharpoons P_2{O}_7^{4-}+H_{2}O$ ) to suppress NFP impurity. Through precise regulation, it is successfully synthesized phase-purity stoichiometric NFPP-0.25 which exhibits a significantly reduced NFP content and a more complete crystal structure with expanded Na⁺ diffusion channels compared to pristine NFPP. Consequently, NFPP-0.25 delivers exceptional electrochemical performance, including a high reversible discharge capacity of 108.2 mAh g^-1 at 0.2 C, excellent rate capability (∼87 mAh g^-1 at 50 C), and remarkable long-term cycling stability, retaining 95.8% capacity after 10,000 cycles at 10 C and 81.7% after 10,000 cycles at 50 C. Furthermore, NFPP-0.25 demonstrates practical viability in an 800 mAh pouch cell, while DFT calculations provide deeper insights into its superior ion transport properties. Therefore, NFPP prepared by RRM strategy could emerge as a highly competitive candidate for stationary sodium-ion batteries.