Phase‐Purity Stoichiometric Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ Cathode Material via Reversible Reaction Modulation for High‐Performance Sodium‐Ion Batteries

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, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mn>2</mml:mn> <mml:mi>P</mml:mi> <mml:msubsup><mml:mi>O</mml:mi> <mml:mn>4</mml:mn> <mml:mrow><mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:mrow> </mml:msubsup> <mml:mo>+</mml:mo> <mml:mn>2</mml:mn> <mml:msup><mml:mi>H</mml:mi> <mml:mo>+</mml:mo></mml:msup> <mml:mspace></mml:mspace> <mml:mo>⇌</mml:mo> <mml:mspace></mml:mspace> <mml:msub><mml:mi>P</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:msubsup><mml:mi>O</mml:mi> <mml:mn>7</mml:mn> <mml:mrow><mml:mn>4</mml:mn> <mml:mo>-</mml:mo></mml:mrow> </mml:msubsup> <mml:mo>+</mml:mo> <mml:msub><mml:mi>H</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>O</mml:mi></mml:mrow> <mml:annotation>$2PO_{4}^{3-}+2{{H}^{+}}~\rightleftharpoons ~{{P}_{2}}O_{7}^{4-}+{{H}_{2}}O$</mml:annotation></mml:semantics> </mml:math> ) 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.