Optimizing the Electron Spin States of Na₄Fe₃(PO₄)₂P₂O7 Cathodes via Mn/F Dual-Doping for Enhanced Sodium Storage

A NASICON-type Mn/F dual-doping Na4Fe3(PO4)(2)P2O7 cathode material is successfully synthesized via a spray drying method. A medium-spin of Fe is measured by DFT calculation, X-ray absorption near edge structure (XANES), temperature-dependent magnetization susceptibility (M-T) measurement, and electron paramagnetic resonance (EPR) tests. It indicates that the eg orbital occupation of Fe2+ can be finely regulated, thus optimizing the bond strength between the oxidation and reduction processes. Furthermore, from UV-vis DRS and four-point probe conductivity measurements, it can be seen that, after adjusting the electron spin states, the band gap of the material has decreased from 1.01 to 0.80 eV, and the electronic conductivity has increased from 8.5 to 24.4 mu S cm(-1), thereby leading to competitive electrochemical performance. The as-optimized Na4Fe3(PO4)(2)P2O7 displays both excellent rate performance (121.0 and 104.9 mAh g(-1) at 0.1 C and 5 C, respectively) and outstanding cycling stability (88.5% capacity retention after 1000 cycles at 1 C). The results indicate that this low-cost Mn/F dual-doping Na4Fe3(PO4)(2)P(2)O(7)cathode can be a competitive candidate material for sodium-ion batteries.