Regulating Na/Mn Antisite Defects and Reactivating Anomalous Jahn–Teller Behavior for Na4Fe1.5Mn1.5(PO4)2(P2O7) Cathode Material with Superior Performance

Na4Fe3–xMnx(PO4)2(P2O7) is considered a promising candidate for commercial-scale applications due to its significantly improved energy density compared to Na4Fe3(PO4)2(P2O7). However, challenges such as intractable impurities, voltage hysteresis/decay, and sluggish Na+ kinetics hinder their practical application. In this study, failure mechanisms of Na4Fe1.5Mn1.5(PO4)2(P2O7) are intensively investigated and demystified. It is found that the issues of this material are mainly caused by surface element segregation, Na/Mn antisite defects, and the closure of Na+ channels. To address these problems, a nonhomogeneous Mg doping engineering strategy is proposed, which effectively eliminates inert impurity phases, decreases the concentration of Na/Mn antisite defects, reactivates the anomalous Jahn–Teller behavior, and inhibits Mn dissolution. The synthesized ternary polyanionic cathode material, Na4Fe1.5Mn1.35Mg0.15(PO4)2(P2O7)@C–N, demonstrates significant improvements, featuring an average operating voltage of approximately 3.5 V, an energy density of 430 Wh kg–1 at 0.2C, and an ultralong cycle life (>12,000 cycles). This work highlights the nonhomogeneous Mg doping engineering strategy and provides a promising approach for developing cathode materials with high energy density for commercial-scale sodium-ion batteries.