π-Bond Electronic Structure Engineering for Autogenous Mn³⁺/Mn⁴⁺ Redox in Li/Co/Ni-Free Fe-Mn-Based Cathodes

Fe-Mn-based layered oxides with high specific capacity and natural abundance are among the most promising cathode materials for sodium-ion batteries. However, their low discharge plateau and the issue of irreversible transition metal migration and dissolution significantly impair their electrochemical performance, hindering further commercial application. To address the inherent constraints of Fe-Mn-based cathodes, this work developed a π-bond engineering enables Mn-valency self-regulation strategy and designed a low-cost and environmentally friendly P2-type cathode material Na_0.67Fe_0.2Mn_0.5Cu_0.15Zn_0.1Ti_0.05O₂ (CZT321) without expensive metal elements such as Co, Li, and Ni. In situ testing and DFT calculations reveal that the enhanced TM─O bonds suppress the unfavorable P2-O2/OP4 phase transitions. The unique "π-type" Cu─O─Mn bond, where the 3d orbitals of Cu and Mn are bridged via the O 2p orbital, endows the system with a self-regulating mechanism (SRM) for the Mn³⁺/Mn⁴⁺ redox potential. This mechanism inhibits the Jahn-Teller effect of Mn and significantly enhances the long-term cycling stability of the electrode material. The half-cell delivers a specific capacity of 144.0 mAh g^-1, with a capacity retention rate of 86.1% after 150 cycles at 200 mA g^-1. This work provides new insights for the further design and application of Fe-Mn-based cathode materials in sodium-ion batteries.