Regulating Na/Mn Antisite Defects and Reactivating Anomalous Jahn–Teller Behavior for Na 4 Fe 1.5 Mn 1.5 (PO 4 ) 2 (P 2 O 7 ) Cathode Material with Superior Performance

Na 4 Fe 3– x Mn x (PO 4 ) 2 (P 2 O 7 ) is considered a promising candidate for commercial-scale applications due to its significantly improved energy density compared to Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ). However, challenges such as intractable impurities, voltage hysteresis/decay, and sluggish Na + kinetics hinder their practical application. In this study, failure mechanisms of Na 4 Fe 1.5 Mn 1.5 (PO 4 ) 2 (P 2 O 7 ) 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, Na 4 Fe 1.5 Mn 1.35 Mg 0.15 (PO 4 ) 2 (P 2 O 7 )@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.