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

A single-phase, well-crystallized Li 5 V(PO 4 ) 2 F 2 /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 VPO 4 /C with Li 3 PO 4 and LiF. Synthesis of the VPO 4 /C precursor utilized carbon to reduce V 2 O 5 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 Li 5 V(PO 4 ) 2 F 2 composite. Quench techniques were used to isolate the metastable two-dimensional Li 5 V(PO 4 ) 2 F 2 as a pure single phase. 6 Li 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 Li 5 V(PO 4 ) 2 F 2 /C composite exhibited a reversible plateau at 4.15 V followed by a second plateau at 4.65 V, which correspond to the V 3+ /V 4+ and V 4+ /V 5+ 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 V 3+ /V 4+ couple in Li 3 V 2 (PO 4 ) 3 or LiVPO 4 F and the V 4+ /V 5+ couple in Li 3 V 2 (PO 4 ) 3 .