Multielectron-Capable Li-Rich Polyanion Material with High Operating Voltage: Li5V2PO4F8 for Li-Ion Batteries

Abstract

Polyanion compounds have relatively low energy density compared to plain oxides as cathode materials. We for the first time report on a Li-rich fluorophosphate compound, Li5V2PO4F8, that can have high energy density originating from both the multielectron reaction of vanadium and high voltage induced by a fluorine. The developed material has a new crystal structure that has a robust three-dimensional framework of corner-sharing octahedra VO2F4 with tetrahedra PO4 and three-dimensional lithium ion diffusion pathways, which can facilitate the (de)-intercalation of lithium ions. Its theoretical capacity is 285 mAh/g when two electrons are exploited. Practically, it shows the highest redox voltage, similar to 4.4 V (vs Li/Li+), among V3+/V4+ redox reactions, with 111 mAh/g of reversible capacity. Only in the first charge does it show an active redox reaction of the V4+/V5+ at similar to 4.9 V (vs Li/Li+) with 228 mAh/g of charge capacity. Moreover, the vanadium-deficient phase shows stable capacity retention and good rate capability at both charging and discharging rates up to the 2C rate. The discovery of the lithium- and fluorine-rich phosphate compounds reported here introduces a new family of cathode materials, and further exploration and optimization can be expected to unlock the full potential of this family.