LiVPO4F : A New Age High Voltage Cathode Material for Lithium Ion-Hybrid Electrochemical Capacitors

Abstract

Lithium ion hybrid capacitors are cells that combine the best of both worlds in terms of energy density and power density, where one of the electrodes stores charge via a faradaic mechanism (battery) and the other electrode stores charge using an electrochemical double layer (supercapacitor). This configuration allows it to have a higher power density when compared to batteries and a higher energy density when compared to supercapacitors. Thus making them a potential solution in powering Plug-in Hybrid Electric Vehicles in the future. LiVPO4F (LVPF) belongs to a class of materials known as tavorites and because of the presence of the poly-anionic phosphate group and fluorine it has a high redox potential of ~ 4.2 V. But because of the high degree of covalency in the molecule it also has very low electronic conductivity, making it unsuitable for use at high current rates in its pristine form, hence we have synthesized LVPF using a carbothermal route in order to ensure good conductivity of the sample , the material was further characterized using X-ray diffraction, scanning electron microscopy and high resolution tunneling electron microscope and was found to have single phase purity with no apparent morphology and a thin layer of conductive carbon which helped form facile electron migration paths in the material. Comprehensive electrochemical studies were performed on both LVPF and activated carbon (anode) to determine the precise mass loading in a full cell. Because of the relatively high redox potential of LVPF the working potential of the cell is raised allowing for a much higher energy density when compared to other systems in literature. Also because of the synthesis route undertaken the full cell (LVPF/AC) is able to cycle successfully at high current rates with minimal loss in cycleability