Vanadium phosphide–phosphorus composite as a high-capacity negative electrode for sodium secondary batteries using an ionic liquid electrolyte

Abstract

A vanadium phosphide–phosphorus composite, V4P7/5P, is investigated as a negative electrode for sodium-ion batteries using the ionic liquid, Na[FSA]–[C₃C₁pyrr][FSA] (FSA = bis(fluorosulfonyl)amide anion and C₃C₁pyrr = N-methyl-N-propylpyrrolidinium cation), as the electrolyte. Analyses of the X-ray diffraction pattern, X-ray photoelectron spectra, and transmission electron microscopy images revealed that the V₄P₇/5P composite prepared by high-energy ball-milling is composed of a V₄P₇ crystalline phase and an amorphous phosphorus phase. This composite exhibits a high reversible discharge capacity of 738 mAh g⁻¹, with a reasonably high initial coulombic efficiency of 85.9% at 363 K, even in the absence of carbon materials. The emergence of Na3P peaks in the ex situ XRD pattern after full charging indicated sodiation of the excess amorphous phosphorus present in the composite. The peaks corresponding to V₄P₇ remained intact and desodiation of Na₃P after full discharge was also evident from the XRD pattern. The cyclability was higher in the ionic liquid electrolyte than in an organic electrolyte owing to the formation of a more efficient interfacial layer in the former.