Low temperature processing and electrochemical characterization of phosphate based cathode materials and electrolytes for Li-ion batteries

The role of poly-alcohol solvent as well as the reaction with different precursors (e.g. Fe(II)-oxalate, Fe(III)-nitrate) during the polyol process has been investigated. The reaction mechanisms of a low temperature wet chemical route for the synthesis of LiFePO4 and LiTi2(PO4)3 were investigated. The influence of precursors on the reaction mechanism of the synthesized materials in diethylene glycol solvent was studied by in-situ FT-IR gas analysis during the synthesis. In-situ FT-IR gas analysis is used for the first time in this type of research work.Whether the Poly-alcohol solvent acts as a dissolving medium and/or reducing agent depends on an intermediate amino ether compound. It was assumed that cyclic ether 1,4-Dioxane, dehydration product of DEG at boiling point temperature, along with the absence of oxidation by-products of DEG supports the stability of F e2+ and T i4+ under the polyol synthesis conditions. Where as with Fe(III) nitrate salts, it was observed that DEG acts as a reducing agent. The reaction products of DEG and NO - 3 -ions support the reduction of F e3+ to F e2+ during the synthesis of LiFePO4.Experimental parameters such as reaction temperature and time were established to control the morphology of materials used as a cathode material and solid electrolyte. The effects of different types of morphology on the electrochemical performance of LiFePO4 were studied. Furthermore, the effect of the concentrations of Li +, Fe2+ and P O 3- 4 ionic moieties on the morphology of LiFePO4 were studied and supported through SEM investigations.In polyol process, dissolution of the salts in DEG solvent is not sufficient for formation of LiFePO4 but only after thermal decomposition of salts in DEG and removal of the volatile products, e.g. CO2, CO, NH3, the formation of LiFePO4 and LiTi2(PO4)3 can start. Neither reduction into metallic iron nor oxidation into F e 3+ complexes (e.g. Fe-hydroxide) was observed during the synthesis of LiFePO4.