Tuning TM-O Bond Covalency to Boost Cationic Activity and Reversibility of Na4Fe1.5Mn1.5(PO4)2P2O7

The pursuit of cost-effectiveness stimulates great interest in the Na₄Fe_1.5Mn_1.5(PO₄)₂P₂O₇ (NMFPP) cathode. However, its cationic redox activity and reversibility are hardly up to expectation, accompanied by poor conductivity and rapid structural degradation. These issues can be attributed to the high ionization degree of TM-O bonds in the polyanion crystal field, which intensifies electronic localization and degrades the stability of TMO₆ octahedra under the Jahn-Teller effect. Herein, a strategy is proposed to enhance the covalency of TM-O bonds. Specifically, Ti⁴⁺ with strong electrophilicity is introduced to alter the local electronic structure of TM-O bonds, including band structure and bonding strength. Ultimately, both intrinsic conductivity and lattice stability of Ti modified Na₄Mn_1.3Fe_1.5Ti_0.1(PO₄)₂P₂O₇ (NMFTPP) are well optimized, upgrading the activity and reversibility of cationic redox. This work reveals the potential mechanism between TM-O bond covalency and the intrinsic conductivity/structural stability of polyanion materials, opening up a feasible path for the high-performance development of sodium ion batteries.