In Situ Bulk Oxygen Vacancy Manufacturing and Surface Spinel Layer Coating Enable High-Performance Na-Ion Layered Fe–Mn Based Cathodes

Abstract

Iron-manganese based layered transition metal (TM) oxides are promising cathodes for sodium-ion batteries owing to their high specific capacity and cost effectiveness, but they are confronted by severe Mn³⁺ Jahn-Teller distortion, lattice oxygen loss, and irreversible Fe migration. Herein, in situ bulk oxygen vacancy manufacturing and surface spinel (MnFe₂O₄) layer coating for the P2-Na_0.67Fe_0.3Mn_0.7O₂ (NFM(OV)+MnFe₂O₄) cathode are realized via an argon-atmosphere calcination method. The bulk oxygen vacancies improve the Mn redox capacity by reducing Mn valence, and favor the reversible Fe interlayer migration to enhance oxygen redox activity through the Na─O─(TM vacancy) configurations. The migrated Fe ions to Na sites can serve as “temporary pillars” to suppress the TMO₂ layers gliding in the deep charged state and return to their original positions upon discharge. The spinel coating layer can mitigate the lattice oxygen escape, prevent the interfacial side reactions, and alleviate the Mn³⁺ Jahn-Teller distortion. Therefore, the tailored NFM(OV)+MnFe₂O₄ cathode affords high discharge capacities (185.7 and 84 mAh g⁻¹ at 0.1 and 5 C, respectively) and desirable cycling stability (82.6% capacity retention after 300 cycles). This study paves the way for fabricating high-performance Fe─Mn based layered oxide cathodes by simultaneously tuning the bulk and surface structures.