| In this paper, Li4Ti5O12and LiTi2(PO4)3as the anode materials for lithium ion battery, as well as Li2FeP2O7being a novel cathode material, were synthesized and modified, among which, the Li4Ti5O12were systematically studied by carbon-coating, Tin-doping and MoS2-composite. The crystal structure, morphology and the electrochemical properties of the expected compounds were investigated. It was found that the modifications could improve electrochemical performances of the original materials effectively, which could be applied in lithium ion battery industry. The main results are listed as follows:(1) Li4Ti5O12/C composite materials were synthesized by rheological phase method, using lithium acetate and titanium dioxide as raw materials, oxalic acid as a reductant, citric acid as coordination agent and carbon source. The crystal structure and microstructure were characterized by XRD and SEM. And the electrochemical properties of the samples were investigated by galvanostatic charge-discharge test and electrochemical impedance spectroscopy. The results showed that the performances of Li4Ti5O12/C composite materials synthesized by this method were better in particle size distribution, high-rate charge/discharge ability and cycling stability than those of pure Li4Ti5O12synthesized by same method.Li4Ti5O12was also modified by tin-doping. A non-stoichiometric Spinel Li4-xTi5SnyO12was synthesized by rheological phase method, and the effects of different Tin-doping ratios on Li4-xTi5SnyO12were studied. The crystal structure and microstructure were characterized by XRD and SEM. And the electrochemical properties of the samples were investigated by galvanostatic charge-discharge test and electrochemical impedance spectroscopy. The results showed that suitable tin-doping in Li4Ti5O12had better high-rate performance and cycling stability than that of pure Li4Ti5O12.Li4-xTi5Sn0.3O12(LTO/Sn(2))(Ti:Sn=1:0.06)exhibited the best electrochemical performance. The first discharge specific capacity was236mAh/g at50mA/g. When rate of charge and discharge was increased to4C, the discharge specific capacity remained109,8mAh/g after105cycles. Therefore the Li4-xTi5Sn0.3O12is an ideal candidate of anode material for Lithium ion battery application. In addition, Li4Ti5O12was composited with MoS2to modify its electrochemical performance. Li4Ti5O12was synthesized by rheological phase method, and MoS2was synthesized by hydrothermal synthesis method, using (NH4)6Mo7O24·4H2O, CS(NH2)2and H2C2O4·2H2O as starting materials. The two synthesized materials were combined in different molal ratios and Sample(9-1), Sample(7-3) and Sample(5-5) were obtained. The morphology of chosen samples was characterized by SEM techniques, and their electrochemical performances were also studied. The results showed that the three compounds had larger capacity than that of Li4Ti5O12, and their cycling stabilities were better than those of MoS2. The sample (7-3) exhibited the best general electrochemical performance among different Li4Ti5O12/MoS2composites. It had a relatively high initial capacity of343.5mAh/g. low damping rate and AC impedance, and after100cycles in different current densities, its discharge capacity still remained192.7mAh/g. In conclusion, the composites of Li4Ti5O12and MoS2in a suitable molal rate can make use of the advantages of both materials, thus it can be applied in the Li-ion battery industry.(2) Polyanionic compounds LiTi2(PO4)3were synthesized by rheological phase method, using lithium carbonate, titanium dioxide and ammonium dihydrogen phosphate as raw materials, and effects of different sintered temperatures on the properties and its reasons were studied. The temperature range of reactions were investigated by the thermal analysis, the crystal structure and microstructure of samples were characterized by XRD and SEM. And the electrochemical properties of the samples were investigated by galvanostatic charge-discharge test and electrochemical impedance spectroscopy. The results showed that although there were little differences in electrochemical properties between the samples synthesized at600℃and800℃at small current density, the former sample exhibited better high-rate performance than that of latter. LiTi2(PO4)3was regarded as a promising material for lithium ion batteries for its various advantages such as lower costs, environmental friendliness and high thermal stability. The exploration of the suitable synthesis condition of LiTi2(PO4)3is a significant work for developing uses of LiTi2(PO4)3in lithium ion battery industry.(3) Li2FeP2O7/C, the3.5V class cathode material for Li ion batteries, was synthesized by rheological phase reaction method in which lithium carbonate, iron protoxalate and diammonium phosphate were used as starting materials. The existence of reticular formation of carbon benefited the transformation of Li+and electron conduction, and also helped to alleviate the polarization during the charge-discharge process. Therefore the Li2FeP2O7/C had higher initial capacity and better cycling stability than that of Li2FeP2O7. After25cycles, the discharge capacity of Li2FeP2O7/C remained82.03mAh/g, and the efficiency was79.66%. The new pyrophosphate material possesses the advantages of known Fe-based phosphate cathodes, and exhibits the highest voltage platform among the other series of Fe(Ⅱ) salt cathode, thereby Li2FeP2O7can be promising candidate to be applied to the commercial Li ion batteries. |
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