Preparation and Electrochemical Studies of Metal Fluorides for Lithium Batteries

摘要

Conventional positive (cathodes) electrode materials for lithium batteries, use mixed-conducting lithium containing transition metal oxides, metal phosphates etc. which are able to store both lithium and electrons by changing their oxidation state. In this presentation, we discuss the facile synthesis of materials suitable for alternative electrode concepts including Transition metal fluorides MF_2 (M=Fe, Mn, Zn) were obtained by reacting Fe, Mn, Zn-Metal salts with fumaric acid as a one-dimensional metal organic framework, and a polymer (PVDF) as Fluorine source, final products are obtained by heating at 600°C, 6h in Ar gas. The MOF were initially prepared by a simple chimie douce method. Incorporation of the MOF with a fluorinated polymer and the eventual decomposition lead to carbon coated metal fluoride nanoparticles of high surface area of >200 m~2/g for FeF_2. The obtained materials will be characterized in detail by X-ray diffraction, Scanning and Transmission electron microscope (SEM/TEM) are used to evaluate the structure and morphology, X-ray photoelectron spectroscopy are used understand structure, vibrational bands and oxidation state of the materials and BET surface area method. Electrochemical studies were carried out in the voltage, range 4 to 1.0 vs. Li, at current rate of 50 mA/g (0.1 C) using 1MLiPF_6 (EC;DEC) as liquid electrolyte and tested with Li-metal as a counter and reference electrodes. The cyclic voltammetry at scan rate of 0.075 mV/sec at room temperature (24°C). Galvanostatic cycling of FeF_2 demonstrates that the material exhibit stable and good reversible capacity of 580 mAh/g during the first cycle and slight capacity fading has been observed after 20 cycles. Further studies on rate performance is being carried up to 2.5 C rate. Whereas MnF_2 and ZnF_2 showed reversible capacity of 220 and 200 mAh/g and retained a capacity around 100 mAh/g after 20 cycles and showed lower capacity than FeF_2. Discuss the structural, reaction mechanism of FeF_2 during charge-discharge cycling by in situ/operando X-ray diffraction and electrochemical impedance spectroscopy studies discuss in detail.