Prepare LifePO₄in Two Steps And Investagation Its Electrochemical Performance

As one of the lithium ion battery cathode materials and LiFePO4have showed a bright application prospect, because of its low thermal effects, good safety performance and high cycle life. However, there are still some shortcomings:firstly, the cost of a divalent iron raw material is expensive; secondly, the electronic conductivity and ion conductivity the materials are lower. Therefore, it has significant and practical value to realize the commercial development of phosphate by developing cheaper as well as high-purity raw materials and facilities preparation process. In this paper, the inexpensive ferric compounds were used as an iron source, firstly, the high purity FePO4materials with controllable morphology were synthesized by controlled crystallization method, then co we synthesized LiFePO4/C composites by carbothermal reduction method by using the above FePO4materials, lithium carbonate and sucrose as raw materials. By XRD, SEM, TEM, IR, phase structure, particle morphology and size, tap density and electrochemical properties have been studied systematically. The main results are listed as follows:Using Fe(NO3)3·9H2O and NH4H2PO4as raw materials, citric acid and ammonium oxalate as ligands, the influences on FePO4materials of reaction temperature, reaction time, concentration and the kinds of ligands have been studied systematically through controlling crystallization method. The results showed that ligands are the main factor to control the morphology of the materials. The morphologies of FePO4materials are very different with different ligands.The sample without ligand showed no regular shapes, and the synthetic FePO4with ammonium oxalate as its ligand possessed the spherical structure, and then the synthetic FePO4with ammonium oxalate as its ligand were the rod-shaped particles.The particle took on dandelion pattern structure composed by pieces, spreading outward from center when added citric acid and ammonium oxalate together as ligands. When taking ammonium oxalate as the ligand as an example to synthesize FePO4materials, and we found we could get well crystallized particles adjusting the reaction conditions. When the reaction concentration was0.1mol/L, reaction temperature was80℃, and the reaction time was3h, pure FePO4materials could be obtained by calcination treatment, whose particles was about2～4um. By carbothermal reduction method, LiFePO4/C materials were synthesized using FePO4materials as its iron source, L12CO3as its lithium source, and sucrose as its carbon source. The influence on the structure and properties of LiFePO4/C of some process parameters such as the lithium content, the added amount of carbon agent, the heat treatment temperature and the treatment time have been studied. When the molar ratio of FePO4to Li+was1:1.5, the amount of carbon-doped was10wt%, the reaction temperature was750℃and the reaction time was2h, LiFePO4/C materials possessed a single olivine structure, and had the excellent electrochemical performances. The initial discharge capacity was148mAh/g under the0.1C, and the capacity retention ratio could reach84.2%after twenty times recycling. It indicated that the good electrochemical performance of the materials is mainly related to the surface of the carbon material coated particles, the presence of carbon-coated layer has effectively improved the conductivity of the materials.