| LiFePO4 with the ordered olivine structure is emerging as one of the most promisingcathode materials for high gain lithium ion batteries for its various advantages such as hightheoretical specific capacity (170 mAh/g), good cycle stability, lower cost, environmentalfriendliness, high thermal and chemical stability. However LiFePO4 has the intrinsic drawbackof poor electronic conductivity and slow Li-ion diffusivity, which limit its use in secondarybatteries, especially at higher current densities. LiTi2(PO4)3 with the NASICON structure hasgenerated interest as anode material for secondary lithium batteries, due to its non toxic, highionic conductivity, high chemical stability, and improved safety and reliability. However thekey limitation of LiTi2(PO4)3 is its low electronic conductivity, which worsen itselectrochemical properties and limit its practical use.In the aim of improving the performance of LiFePO4 and LiTi2(PO4)3 materials,LiFePO4/C and LiTi2(PO4)3/C were synthesized. The microstructure, morphology of thesamples and the structure of carbon were characterized and investigated by X-ray diffraction(XRD), scanning electron microscopy (SEM) and Raman microprobe spectra. Cyclicvoltammetry (CV), Electrochemical impedance spectroscopy (EIS) and the charge-dischargetest were used to study their electrochemical performance.The effect of synthesis temperatureand FePO4·2H2O on electrochemical performances of LiFePCVC was studied. At the sametime the synthesis mechanism and surface modification of LiTi2(PO4)3 anode material werestudied.The effect of synthesis temperature on electrochemical performances of LiFePO4/C wasstudied. LiFePO4/C composite cathode materials were synthesized by one-step solid-statereaction using FePO4 and Li2CO3 as raw materials and polypropylene as the reductive agentand carbon source. The results show that LiFePO4 /C are synthesized in the temperature rangeof 500 to 750℃. When the synthesis temperature is less than 750℃, the coating of carbonfrom the pyrolysis of polypropylene on LiFePO4 surface impedes the particle growthefficiently, which is beneficial to reduce the path length of Li+ ions movement. The degree ofgraphite carbon is increased with the increasing synthesis temperature, which improves theelectronic conductivity. The size of LiFePO4/C synthesized at 550℃for 8h is about 50～300nm, and it has discharge capacities of 151, 139 and 119 mAh/g at 1, 2, 5 C-rates (1 Ccorresponding to 170 mAh/g) respectively.The effect of FePO4·2H2O on electrochemical performances of LiFePO4/C was investigated. LiFePO4/C were synsthesised at 550℃using different FePO4·2H2O withdifferent sizes. The results show that the structure and purity of materials and the particalsizes are impacted by the size of FePO4·2H2O. And FePO4·2H2O with small sizes reacts withLi2CO3 to form Li3PO4 during ball-milling. It has higher nucleation rate, which makeLiFePCVC sizes more smaller, less than 300 nm. The apparent diffusion contant DLicalculated using EIS date reaches 3.11×10-14, and the conductivity of Li+ ions is 1.97×10-9S/cm. It has a discharge capacity 151 mAh/g at 1 C-rate. Reversely, FePO4·2H2O with bigsizes makes the reaction incompletely, which makes LiFePO4/C sizes more bigger, morethan 1μm, and the product contains Li2CO3. The discharge capacity of material is less than116 mAh/g at 1 C-rate.The research of synthesis mechanism and electrochemical performance of LiTi2(PO4)3 asanode material shows that the intermediate products TiP2O7 and LiPO3 are first synthesizedduring the solid-state reaction using LiH2PO4,TiO2 and NH4H2PO4, then they react with TiO2to form LiTi2(PO4)3. The increasing synthesis temperature is benifical to forming LiTi2(PO4)3.LiTi2(PO4)3 is synthesized at 950℃for 12 h. It has a initial discharge capacity of 42.2 mAh/gat 0.1 C-rate (1 C corresponding to 138 mA/g), and the initial coulombic efficitncy is 44.5%.The electronic conductivity of LiTi2(PO4)3 is improved by carbon coating using glucose ascarbon resource, and the high temperature is helpful to enhancing the electronic conductivityof LiTi2(PO4)3 material. LiTi2(PO4)3/C composite materials synthesized at 700℃give a initialdischarge capacity of 131 mAh/g (95% of therotical capacity) at 0.1 C-rate, and the initialcoulombic efficitncy is 96.3%. It has a discharge capacity of 103.2 mAh/g at 0.5 C-rate.
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