Solvothermal Synthesis And Modification Of LiFePO₄as A Cathode Material For Lithium Ion Batteries

Due to its high theoretical discharge capacity170mAh·g-1, low toxicity, high thermal stability, environmental benignity, long cycling life and the natural abundance of raw materials, lithium iron phosphate has been extensively developed into a possible cathode material replacement for LiCoO2in lithium ion batteries use for electric vehicles (EVs) and hybrid electric vehicles (HEVs). However, the key barrier to wide applications was its intrinsically low electrical conductivity, low Li ion diffusion rate and low tap density. Considerable efforts have been made to improve the electrochemical properties and enhance the tap density of LiFePO4In order to solving these problems, some works have been done in this paper as the follows:(1) The cathode material LiFePO4of Li ion battery has been successfully synthesized via solvothermal method. The composition, morphology, microstructure and electrochemical properties of the as-prepared samples were characterized by XRD, IR, SEM, HR-TEM, SAED and charge/discharge tests. The effects of reaction time, reaction temperature and ethanediamine on the morphologies and structures are also investigated. The results show that relatively high crystallinity LiFePO4platelets with diameter of0.5～1.5μm and thickness of around50nm were obtained by solvothermal with the addition of2ml ethanediamine at200℃for24h. A reasonable formation mechanism of LiFePO4platelets is proposed based on time dependent experiments. The electrochemical testing results show that the discharge specific capacities of the sample reach to120.6,104.9and68.8mAh·g-1at0.1C,0.2C and1C, respectively.(2) The nest-like LiFePO4microstructures with tap density of ca.1.2g/cm3was synthesized by solvothermal method. These microstructured LiFePO4consisting of nanoplates or platelets exposured (100) face have a relatively high crystallinity, and the average size is about5～11μm in length,3～7μm in diameter. The hierarchically nest-like microstructures retain excellent electrochemical performance of platelets or nanoplates as well as a high tap density of spherical structure. The Brunauer-Emmett-Teller surface area calculated from the adsorption isotherm is6.86m2/g. The corresponding BJH poresize distribution reflects that the average pore size is20nm. This kind of pore structure is beneficial to lithium ion diffusion and electrolyte permeation. The contrast experiment shows that P123have a grate influences to morphology. A reasonable formation mechanism is proposed based on time dependent experiments. The electrochemical testing results show that the electrochemical performance of the sample with the addition of P123is better than without the addition of P123.(3) We used glucose as carbon source and polypyrrole to coat the pure phase LiFePO4The LiFePO4/C was characterized by XRD, IR, SEM, TEM. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic discharge testing results indicate that LiFePO4/C composite exhibits a good electrochemical performance. LiFePO4/PPy composite is synthesized via in-situ chemical oxidative polymerization method. The LiFePO4/PPy composite was characterized by IR, SEM. Galvanostatic discharge testing results show that the discharge specific capacities of the LiFePO4/PPy reach155.0,145.0,130.8and117.0mAh·g-1at0.1C,0.2C,0.5C and1C, respectively.