Optimizing carbon coating to synthesize phase pure LiVPO4F cathode through a combined sol-gel and PTFE reaction

Abstract

Phase pure carbon-coated high voltage cathode material LiVPO4F (LVPF/C) is synthesized by a facile sol-gel method and reacting the precursor powder with PTFE. Secondary phases such as Li3V2(PO4)3, Li9V3(P2O7)3(PO4)2, V2O3 and LiVP2O7 are shown to get mitigated by the addition of lauric acid, which also acts as carbon source. Phase pure LiVPO4F with good conducting properties are produced under the optimum use of oleic acid (OA; 1.31 mg, 0.84 wt. %) and lauric acid (LA; 0.25 mg, 0.16 wt. %) which are used to control particle size and carbon coating reagent, respectively. Polytetrafluoroethylene (PTFE, 35 wt.%) is reacted with the precursor during annealing to control fluorine loss. Rietveld refinement indicates the formation of phase pure LiVPO4F. The optimized cathode, LVPF/C (0.25 g LA), exhibit flat potential during the lithiation and dilithiation process with plateaus ~ 4.21 V and ~4.17 V at 0.1C-rate. Discharge capacities of 96.3 mAh g−1 (44.6 mAh g−1) are obtained at 0.1C(10C)-rate. Diffusion coefficient ~10−15 cm2s−1 is estimated from cyclic voltammetry studies. Carbon additive decreases the internal resistance and enables easy charge transfer resistance through the electrode-electrolyte interface. The cumulative effect of synthesizing phase pure compound, carbon coating, with optimum diffusion coefficient and charge transfer resistance provide the better electrochemical performance of the LVPF/C cathode.