Reaction mechanism and morphology of the LiFePO4 materials synthesized by chemical solution deposition and solid-state reaction

Abstract

The mechanism of a low temperature wet chemical route for synthesis of LiFePO4 cathode material for Li-ion batteries is presented. Using inorganic salts as precursors and diethylene glycol (DEG) as reaction media, nanosize LiFePO4 powder is obtained. The controversially discussed role of the polyol on this reaction has been clarified by in-situ FT-IR analysis of the gaseous reaction products and a full analysis of solid products. It is observed that DEG does not act as reducing agent for iron(II) salts, nor is it oxidized during the synthesis of LiFePO4. Generally, polyols are used to reduce the oxides and hydroxides into their corresponding metals. The nucleation of Li+, PO 43− and Fe2+-ions in the DEG solvent starts at temperatures 110°C, 130°C and 170°C, respectively. LiFePO4 precipitation from Li-carbonate, Fe(II)-oxalate dihydrate and ammonium dihydrogenphosphate as precursors is driven by thermal decomposition of the salts, intermediate formation and dehydration of amino-ethoxy-ethane, yielding 1,4-dioxane as volatile by-product. With the help of in-situ investigations reaction time and temperature could be minimized. Furthermore, the effect of the concentrations of Li+, Fe2+ and PO 43− ionic moieties on the morphology of LiFePO4 materials obtained by chemical solution deposition and solid state reaction are studied with the help of FT-IR analysis. LiFePO4 platelets morphology exhibited better discharge capacity than LiFePO4 agglomerate morphology. The solid product is characterized by X-ray diffraction, scanning electron microscopy and differential thermogravimetry combined with mass spectrometry and FT-IR.