Mechanism of LiFePO4 solid-phase synthesis using iron (II) oxalate and ammonium dihydrophosphate as precursors

An experimental study of the thermolysis mechanism of FeC2O4, NH4H2PO4, Li2CO3, and citric acid from the viewpoint of the usage of a mixture of these compounds in lithium power engineering for the solid-state synthesis of LiFePO4 and its composite with carbon LiFePO4/C as well as comparison of experimental data with thermodynamic calculations were made in the temperature range from 25 up to 1,000 °C. The oxides Fe3O4, Fe2O3, and FeO were detected as the intermediate products of thermolysis of ferrous oxalate in these conditions. Various paths of oxalate decomposition may well proceed concurrently with the predomination of this or that path under slight changes in the experimental conditions. The formation of orthorhombic lithium phosphate Li3PO4 is detected just in a blend grinded at room temperature, and Li3PO4 and NH4PO3 are the basis of triphylite synthesis at increased temperatures (up to 800 °C). A new phase of single-substituted anhydrous lithium citrate C6H7O7Li is formed at room temperature if citric acid C6H8O7∙H2O is used as an organic precursor. The thermal treatment, at which citric acid can form a carbon coating with a maximum conductivity, was estimated experimentally. To identify the products of chemical reactions, structural characterization, and comparative analysis of samples synthesized at several temperatures, a set of techniques was used, namely TG with gas release analysis, Mossbauer spectroscopy, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, surface microanalysis, laser diffraction analyses. Galvanostatic cycling was used to study the electrochemical properties of the LiFePO4/C electrode material.