FLUORINATION OF VANADIUM OXY-PHOSPHATES AS HIGH-ENERGY CATHODE MATERIALS FOR LI-ION BATTERIES
Sorour Semsari Parapari
MAT, PhD Dissertation, 2020
Prof. Dr. Mehmet Ali Gülgün (Thesis Advisor), Assoc. Prof. Dr. Robert Dominko, Prof. Dr. Cleva Ow-Yang, Prof. Dr. Melih Papila. Assoc. Prof. Dr. Sašo Šturm
Date & Time: February 28th, 2020 – 11 AM
Place: National Institute of Chemistry, Slovenia
Keywords : Lithium Ion Batteries, Cathode Materials, Vanadium Oxy-Phosphates, Fluorination
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 using a special stainless steel reactor under argon atmosphere at high temperatures. 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 fluorination. This was due to the higher ionicity of the V-O/F ligands brought by the inductive effect of F in the structure. The synthesized materials were subjected to a systematic characterization at both micro- and nano-scale using various methods such as XRD, NMR, SEM, STEM, EDS and EELS, in addition to the electrochemical characterization.