Fluorination of vanadium oxy-phosphates for high-energy cathode materials of li-ion batteries

Official URL: https://risc01.sabanciuniv.edu/record=b2486291 _(Table of Contents)

The requirement for sustainable high energy density materials for next generations of Li-ion batteries is driving the research to develop new materials with enhanced properties. The thesis work was focused on fluorination of vanadium oxy-phosphate cathode materials with an aim of increasing their energy density. The synthesis, structural and chemical characteristics including electrochemical properties of the vanadium oxy-phosphates and their fluorinated counterparts were investigated. β-VPO4O, ε-LiVPO4O and β-LiVPO4O phases were synthesized by solid state synthesis method. The fluorination process was carried out under argon atmosphere at high temperatures (<600 °C) using a sealed stainless-steel reactor. Lithium fluoride (LiF) and Polytetrafluoroethylene (PTFE) compounds were used as the fluorine resources. The heat-treatment of the powder mixes of vanadium oxy-phosphate and F-containing compounds resulted in incorporation of F into the structure of materials. The ε-LiVPO4O phase preserved the main framework structure after the fluorination by LiF, but it changed to LiVPO4F-type framework by the use of PTFE as the F source. The β-VPO4O phase formed a LiVPO4F-type structure after the incorporation of LiF. All of the fluorinated materials had a Tavorite-type crystal structure, composed of VO6 octahedra interconnected through corners to PO4 tetrahedra. The operating potential of all the precursor vanadium oxy-phosphates increased after the fluorination, due to the higher ionicity of the V-O/F ligands brought by the inductive effect of F in the structure. Those conclusions were based on a systematic characterization at both micro- and nano-scale using XRD, NMR, SEM, STEM-EDS and STEM-EELS, in addition to the electrochemical characterization