Constructing Internal Coupling Reactions Through Iron Substitution to Facilitate Manganese‐Based Mixed‐Phosphate Cathode for Sodium‐Ion Storage

Abstract Manganese‐based mixed‐phosphate (Na₄Mn₃(PO₄)₂P₂O₇, NMPP) is a promising high‐voltage cathode material for sodium ion batteries (SIBs). However, the sluggish kinetics of Mn³⁺/Mn²⁺ redox and Mn dissolution problems result in poor rate capability and cycle stability. Herein, manganese is substituted with iron to synthesize a series of Na₄Mn_3‐xFe_x(PO₄)₂P₂O₇ (0≤x≤2, NMFPP) materials. Among these, the optimized Na₄Mn_1.5Fe_1.5(PO₄)₂P₂O₇ (NMFPP‐1.5) sample exhibits the highest thermodynamic stability and electronic conductivity via theoretical calculations. Practically, NMFPP‐1.5 exhibits not only the largest gravimetric energy density of 378.5 Wh kg⁻¹ (14.4% higher than NMPP) in SIBs, but also the least Mn dissolution and the fastest Na‐ion diffusion kinetics. In situ investigations illustrate the Na‐ion extraction/insertion of NMFPP‐1.5 as an imperfect solid‐solution reaction with lattice distortions. Notably, a kinetic‐controlled electrochemical‐chemical‐coupling discharge mechanism is proposed to understand the voltage hysteresis and additional voltage plateau phenomenon caused by the great kinetics difference between Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺ redox couples. This coupling process facilitates the fast discharge capability of NMFPP‐1.5, guaranteeing for grid‐scale energy storage system application. Furthermore, the practical usability is validated by fabricating NMFPP‐1.5 with hard carbon. The full cell reaches 204.6 Wh kg⁻¹ based on the total anode and cathode mass, exhibiting excellent rate capability and high cycle stability.