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Synthesis And Performance Of LiFePO4Cathode Materials And FePO4Precursors

Posted on:2013-06-22Degree:MasterType:Thesis
Country:ChinaCandidate:L C QianFull Text:PDF
GTID:2231330377956818Subject:Materials Physics and Chemistry
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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.
Keywords/Search Tags:electrolysis, FePO4nanoparticle, cathode material, LiFePO4/C composite, electrochemical performance
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