The Synthesis Of Lithium Iron Phosphate Cathode Material And Its Polymer Coated Modification

Lithium ion batteries are favorable because of the properties of high voltage, high energy density, long cycling life, little local action, non-memory effect and pollution-free. Olivine LiFePO4 is acknowledged to be the most promising cathode material in HEV for its high theoretical capacity, brillianit cyele stability, cost effective raw materials and non-toxicity. But the low eleetrical conduetivity and low tap-density obstructs its'proeess of commereialization. To cope with this problem, researchers try to improve the eleetrochemical properties and enhance the tap-density of LiFePO4 powders. Aiming at solving the problem mentioned above, we started works as the follows:(1) The olivine lithium ion phosphate (LiFePO4) was prepared via solvothermal reaction using ethanol, ethylene glycol(EG), and glycerol-water as solvents respectively. The crystalline structure, particle morphology, and surface microstructure were characterized by high-energy synchrotron XRD, SEM and FTIR spectroscopy. The effects of different solvents on the morphologies and structurtes were investigated in detail. The electrochemical properties were also investigated by charge/discharge test, cyclic voltammetry (CV). The results showed that LiFePO4 nanosheet obtained by using EG as solvent has smaller size, thin features, such a structure reduced the lithium ion diffusion distance and favors the improvement of electrochemical performance. It can deliver a initial discharge capacities of 161.5mAh·g-1 at 0.1 C,132.6mAh·g-1 at 1C and the cycling capacity retention rate reaches 98.02% over 50 cycles at 0.1C.(2) LiFePO4 sphere with hierarchical microstructure self-assembled by nanoplates has been successfully synthesized via a low-temperature solvothermal reaction followed by high-temperature treatment. These resulting sphere show a uniform size distribution of～8um and are hierarchically constructed with two-dimensional nanoplates with～20nm thicknesses, while these tiny plates are densely aggregated in an ordered fashion. The hierarchical structure gives a relatively high tap density of 1.6g.cm-3, and simultaneously, the primary nanoplates could provide a huge electrochemically available surface for enhancing the rate capability of the lithium insertion/deinsertion reaction.The presence of urea and citric play an important role in the formation of nanoplates and construction of hierarchically self-assembled microsphere. A reasonable formation mechanism is proposed on the basis of the result of time-dependent experiments. The materials' physical properties were further characterized by SEM, TEM, XRD, BET and Raman spectroscopy. The charge-discharge test showed that the hierarchical microspheres as a cathode-active material demonstrated high reversible capacity (93.6mAh·g-1 at 10C) and excellent cycle stability.(3) A simple chemical oxidative polymerization of pyrrole directly onto the surface of LiFePO4 particles was applied to the synthesis of polypyrrole-LiFePO4 (PPy-LiFePO4) powder. The LiFePO4 sample without carbon coating was synthesized by a solvothermal method. The dodecyl benzenesulfonic acid, sodium salt (SDBS) was used as additive during PPy polymerization for improving the homogeneous distribution of the coating and conductivity of PPy-LiFePO4. Physical properties of resulting LiFePO4, PPy-LiFePO4 and PPy/SDBS-LiFePO4 were characterized by XRD, SEM, TG. The electrochemical behavior of the samples was examined against lithium counter electrode by galvanostatic charge/discharge measurements. Carbon-free LiFePO4 coated with PPy/SDBS hybrid film exhibited good electrode kinetics and a stable discharge capacity of 142.7mAh·g-1 at 0.1C. Impedance measurements showed that the PPy/SDBS coating decreased the charge-transfer resistance of the corresponding LiFePO4 cathode material very effectively, which was attributed to a homogeneous coating and doped effction.