Layered Lithium Vanadium Fluorophosphate, Li₅V(PO₄)₂F₂: A 4 V Class Positive Electrode Material for Lithium-Ion Batteries

Abstract

A single-phase, well-crystallized Li5V(PO4)2F2/carbon nanocomposite has been prepared by an optimized solid-state route, and its electrochemical behavior was examined as a positive electrode active material in lithium-ion batteries. The microcrystalline powder was synthesized by the reaction of amorphous VPO4/C with Li3PO4 and LiF. Synthesis of the VPO4/C precursor utilized carbon to reduce V2O5 and simultaneously create a nanoparticle reactive material, with the amount being tailored to result in a small excess at the end of the reaction to increase the bulk electronic conductivity of the final Li5V(PO4)2F2 composite. Quench techniques were used to isolate the metastable two-dimensional Li5V(PO4)2F2 as a pure single phase. 6Li MAS NMR studies identified the crystallographic lithium sites and provided information on lithium-ion mobility and exchange in the two-dimensional layered structure. Lithium cells of the Li5V(PO4)2F2/C composite exhibited a reversible plateau at 4.15 V followed by a second plateau at 4.65 V, which correspond to the V3+/V4+ and V4+/V5+ redox couples, respectively, and a charge capacity of 165 mA h g−1, which is close to theoretical (170 mA h g−1). These operating voltages are very similar to those of the V3+/V4+ couple in Li3V2(PO4)3 or LiVPO4F and the V4+/V5+ couple in Li3V2(PO4)3.