Dual Regulation of Stability and Kinetics in Iron-Based Mixed Phosphate Cathode for All-Climate Sodium-Ion Batteries

Abstract

Iron-based mixed-phosphate Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) is a representative cathode for sodium-ion batteries owing to its low cost and environmental compatibility. The behavior of sodium ions in NFPP is governed by four crystallographically distinguishable sodium sites, yet the underlying mechanism for occupation sites and diffusion kinetics remains not fully understood. Herein, a scalable synthesis strategy for NFPP materials with tunable local structures is proposed by controlling the iron valence during preparation. The nonstoichiometric Na_4.024Fe_2.921(PO₄)₂P₂O₇ strikes an optimal balance between the ordering and proportion of inert sodium ions. These ordered inert sodium ions act as structural pillars, providing markedly enhanced stability compared to disordered counterparts. The optimized sodium environment leads to a synergistic enhancement of both thermal stability and sodium ionic kinetics. The cathode delivers a remarkable rate performance and cycling stability (90.7% capacity retention after 10,000 cycles). This exceptional stability is corroborated by a small lattice volume variation of 4.18% upon desodiation. Moreover, cylindrical full cells assembled with kilogram-scale Na_4.024Fe_2.921(PO₄)₂P₂O₇ cathode and a hard carbon anode demonstrate excellent performance across a wide temperature range from −40 to 45 °C. This work establishes the critical interplay between local environment and electrochemical properties, offering valuable insights for the design of advanced cathode materials.