Inner/Interface Engineered Iron/Manganese‐Based Mixed Phosphate Cathode with High Energy Density and Ultra‐Long Cycle Life for Sodium‐Ion Batteries

Abstract The iron/manganese‐based mixed phosphates (Na₄Fe_3‐zMn_z(PO₄)₂(P₂O₇)) have become focal points in the realm of cathode materials for sodium‐ion batteries due to their high voltage plateau and sturdy open framework architecture. However, their electrochemical performance is hindered by surface manganese enrichment, inert phases, Jahn‐Teller distortion, etc. In this work, a Ni(inner)‐B(interface) synergistic modification strategy is proposed to overcome these issues. By facilitating charge redistribution around Mn atoms via Ni doping, the structural damage by Mn Jahn‐Teller distortion is significantly mitigated. Furthermore, the redistribution can enhance the redox activity of Mn²⁺/Mn³⁺, alleviating the voltage hysteresis. Meanwhile, the interface engineering induced by B‐doping activates sodium ion active sites and eliminates surface manganese enrichment, thus impeding the dissolution of Mn and fortifying the structural steadiness. Besides, the synergistic effect of Ni‐B doping promotes the homogenization of internal elements, effectively inhibiting the spontaneous generation of inert phases. As anticipated, the optimal Na₄Fe₂Mn_0.9Ni_0.1(PO₄)₂(P₂O₇)/C‐B_0.1 demonstrates an impressive initial specific capacity (147.2 mAh g⁻¹ at 0.1C), outstanding rate performance (72.5 mAh g⁻¹ at 20C) and superior cycling stability (80.1% retention after 11000 cycles). This work successfully addresses the challenges through the implementation of multifunctional modification, providing valuable insights into the structure design for iron/manganese‐based cathode materials.