Synthesis And Performance Of LiFePO₄Cathode Materials And FePO₄Precursors

Olivine-structured LiFePO₄with high theoretical specific capacity, abundant sources,low cost, environmental compatibility and good thermal stability has become the mostpromising material of the new generation of lithium-ion batteries for commercialization.FePO₄is one of excellent precursor materials for preparing LiFePO₄, which is low in costand can decrease impurities. In this paper we developed several methods to synthesizeFePO₄. And then these FePO₄were used as precursors to prepare LiFePO₄. Themorphology and microstructure of productswere characterized by X-ray diffraction （XRD）,scanning electron microscopy （SEM）, transmission electron microscopy （TEM）,thermogravimetry/differential scanning calorimetry （TG/DSC） andBrunauer-Emmett-Teller （BET）. The electrochemical performances were evaluated bygalvanostatic charge-discharge cycling, cyclic voltammetry （CV） and electrochemicalimpedance spectra （EIS）.In Chapter3, nano-sized FePO₄was synthesized via an electrochemistry method withelectrolyte of phosphoric acid. And the obtained FePO₄nanoparticles were used asprecursors to prepare LiFePO₄/C nanocrystals by one-step solid-phase carbon thermalreduction method. The thermal stability of the synthesized nano-sized FePO₄, the effects ofdifferent current density on the morphology of the FePO₄and the effects of differentmorphologies FePO₄precursors for LiFePO₄/C were studied. It was found that thesynthesized nano-sized FePO₄was amorphous; the crystallization temperature was around616oC. During the heating process, the nano-sized FePO₄would lose crystallization water.At the same time, the nanoparticles merged larger. It was found that the current densitycould affect the morphology of the nano-sized FePO₄.The FePO₄prepared with the currentdensity of11.4mA·cm^-2had the smallest particle size ranging from30to80nm. TheLiFePO₄/C derived from these nano-sized precursors showed excellent electrochemicalperformance. Its initial discharge capacity was146.4mAh·g^-1at0.1C rate, and thecapacity retention rate was above98%after50cycles at0.5C. Further microstructual study showed that this kind of FePO₄possessed a mesoporous structure, which could beused to prepare porous LiFePO₄/C with improved electrochemical performance.In Chapter4, trisodium phosphate instead of phosphoric acid was selected to optimizethe electrolysis reaction. The obtained FePO₄nanoparticles were used as precursors toprepare LiFePO₄/C nanocrystals by one-step solid-phase carbon thermal reduction method.It was found that the reaction cell voltage was decreased and the interval time of adjustingpH got longer when trisodium phosphate instead of phosphoric acid was used. But thesynthesized FePO₄absorbed a few of Na⁺from the trisodium phosphate electrolyte. Theevaluation of prepared LiFePO₄/C indicated that the trace of Na⁺did not affect the purityand electrochemical performances of LiFePO₄/C.In Chapter5, FePO₄was synthesized via a solid-phase method with industrial rawmaterials Fe₂O₃and P₂O₅. The obtained FePO₄was used as precursor to prepareLiFePO₄/C by one-step solid-phase carbon thermal reduction method. The effects ofdifferent preheating temperature on the purity and morphology of synthetic FePO₄werestudied. Impurities can be formed if the preheating temperature was too low. The optimalpreheating temperature was found to be500oC. The FePO₄precursor obtained in theoptimized condition was used to synthesize LiFePO₄/C. Its initial discharge capacity is152mAh·g^-1. However, it showed poor cycling performance with the capacity retention rateless than95%after50cycles at0.1C. The synthesis techniques need to be optimizedfurther to improve the corresponding electrochemical performance.