Local Electronic Structure Regulation Enabling Fluorophosphates Cathode with Improved Redox Potential and Reversible Capacity for Sodium-Ion Batteries

Abstract

Mn-based fluorophosphates have attracted much attention as cathodes for sodium-ion batteries owing to their high cost effectiveness, considerable capacity, and stable framework. However, the fascinating Mn^3+/2+ redox couple suffers from inadequate activation due to the Mn-O covalent character and poor electronic conductivity, impeding its further applications. Herein, a local electronic structure regulation strategy is proposed to improve the Mn^3+/2+ redox potential and reversible capacity simultaneously through introducing elements with low-energy 3d orbitals to expand the energy gap between the e_g orbitals and Fermi energy of Na. Moreover, the 3d element substitution serves to narrow the band gap toward the improved intrinsic electronic conductivity. In comparison with pristine Na₂Fe_0.45Mn_0.55PO₄F, the as-prepared Na₂Fe_0.45Mn_0.4V_0.1PO₄F cathode achieves an increase from 3.5 to 3.6 V in the high-voltage platform and an improvement in energy density from 330 to 361 Wh kg^-1. This work inspires new ideas in adjusting the redox potential of polyanionic cathodes through deliberate regulation of the local electronic structure.