Synthesis of LiFePO4/C cathode materials for Li secondary battery

Recently, LiFePO4 has received a lot of attention, owing to its potential use as a cathode material in the next-generation lithium-ion batteries [1]. It features several interesting properties, such high specific capacity (170 mAhg-1), high thermal stability and excellent electrochemical performance, and is low cost and not toxic. However the compound is an electric insulator at r.t. (10-9 Scm-1) and it shows low Li+ ionic mobility. In order to improve the physical properties of the material many studies have been focused on finding means to reduce these mentioned drawbacks. Several methods of synthesis of LiFePO4 are known in the literature the most used ones being the solid state reaction and carbon-thermal reduction. The latter favors the formation of LiFePO4/C composite, which shows a measurable electrical conductivity. Here we report on an original synthetic route to prepare LiFePO4/C composites [2,3]. The idea behind it was to provide a precursor source containing iron, phosphorus and carbon, and, for this purpose, an appropriate hybrid organic-inorganic iron(II)-phenyl phosphonate, [Fe(C6H5PO3)(H2O)] has been chosen. This compound is stable to the air and, when mixed with Li2CO3, under heat treatment provides sub-micrometric carbon coated LiFePO4/C composites. The thermal decomposition was carried out in a furnace by varying the experimental conditions, i.e. different heating temperatures, calcination times and inert gases in order to investigate the influence of the latter on the final product. The samples were characterized by elemental analysis, TG/DTA, X-ray powder diffraction analysis (XRPD), FT-IR spectroscopy. The morphology of the final products was investigated by means of scanning electron microscopy (SEM). The dependence of particle size and the role of impurities on the electrochemical behavior of the cathode material will be presented and discussed.