Fluorine anion modification of high-voltage Mn-rich phosphate cathodes for high-power and long-lasting sodium-ion batteries

Abstract

High-voltage cathodes with high power and stable cyclability are needed for future high-performance sodium-ion batteries. However, the low electronic conductivity and poor capacity retention impede the development of Mn-rich phosphate cathodes. Here, a light-weight F doping strategy is reported to modify high-voltage Na4MnCr(PO4)3 cathodes. The light-weight F dopant could decrease the energy gap to 0.22 eV from 1.52 eV and strengthen the adjacent chemical bonding around Mn as confirmed by density functional theory calculations, which ensure the boosted electronic conductivity and suppress Mn dissolution. The combination of in-situ and ex-situ techniques determine that the F dopant has no influence on the Na+ storage mechanisms. As a result, an outstanding rate performance up to 40 C and an improved cycling stability (1000 cycles at 20 C) are thus achieved (best reported performances until now). This work presents an unusual and widely available light-weight anion doping strategy for high-performance polyanionic cathodes.