Intergrowth of Prismatic and Octahedral Layers to Inhibit Fe Migration in Oxide Cathodes for Sodium‐Ion Batteries

Abstract Fe‐based layered oxides have emerged as promising sustainable cathode materials for sodium‐ion batteries (SIBs) due to their earth abundance. However, the oxidation of Fe 3+ to Fe 4+ is typically accompanied by its migration to Na layers for capacity and voltage decay. Herein, we demonstrated that Fe migration can be suppressed by modulating the prismatic (P) and octahedral (O) intergrowth at deeply charged states in O3‐Na 0.8 Fe 0.4 Ni 0.1 Mn 0.4 M 0.15 O 2 (M = Ca, Mg). The strong bonding of O 2− ─Ca 2+ ─O 2− in Na layers prevents the gliding of transition‐metal (TM) layers and mitigates the P‐ to O‐type stacking transition at ≥ 3.7 V, which alleviates formation of vacant tetrahedra in O‐type stacking structure to restrict Fe migration. At the same time, Mg incorporation in TM layers promotes an upward shift of Fe 3d states toward the Fermi level and facilitates the sluggish redox of Fe 3+ /Fe 4+ . These enable its high specific capacity of 136 mAh g −1 and outstanding cycling stability with capacity retention of 72% after 1000 cycles. This work paves the pathway for design of sustainable cathode materials for SIBs and beyond.