Regulating Na/MnAntisite Defects and ReactivatingAnomalous Jahn–Teller Behavior for Na₄Fe_1.5Mn_1.5(PO₄)₂(P₂O₇) Cathode Material with Superior Performance

Na₄Fe_3–<i>x</i>Mn_<i>x</i>(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 Na₄Fe_1.5Mn_1.5(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, Na₄Fe_1.5Mn_1.35Mg_0.15(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.