The Preparation Of Lithium Iron Phosphate And Its Surface Polymer Modified

Lithium iron phosphate (LiFePO4), as a new generation of cathode materials for lithium ion battery, have attracted great attention because of some good performance, such as high safety, environment protection, cyclic stability and low price. It is considered as a potential lithium ion battery anode material after spinel LiMn2O4due to the structure stability, moderate work potential, high reversible capacity, nontoxicity and low cost.However, it does have the disadvantage of low conductivity and slow diffusion of lithium ion, which leads to the loss of reversible capacity in the charge-discharge process at large current. And in terms of security, the traditional inorganic materials still have potential safety hazard as a result of severe exothermic reaction between the high-valence-state metal oxide and electrolyte with the oxygen releasing.This study aims to improve the specific capacity and cyclic stability through reducing the particle size of LiFePO4. This is because that the reduction of LiFePO4particle size can shorten the transmission distance of the electronic and lithium ion, and the conductivity will be enhanced. Moreover, polyaniline and other organic matter is used as coating layer for the further improvement of conductivity as well as the isolation from the reaction with electrolyte.We have synthetized LiFePO4with excellent performance by hydrothermal method and solvothermal method, respectively. The XRD result shows that the synthetic samples by both process have orthorhombic structure and space group of Pnmb. The morphology of the samples by the hydrothermal method is quite regular, with no occurrence of reunion and uniform particle size of200nm with uniform distribution. The nanoscale sample has the first discharge capacity of151.73mAh-g-1, First discharge efficiency of78.79%and discharge capacity of the80.39%of First discharge capacity after30times recharge cyclesAfter coated with polyaniline at the organic content of16.81%, the samples’ crystal structure do not change. After doping, the conductivity increases by an order of magnitude,from10-6to10-5. At the charge rate of0.1C, the samples reach to discharge capacity of162.39mAh·g-1and good cyclic stability of85.07%.Solvent-free LiFePO4is prepared by the pH adjustment method. In the method, LiFePO4nanoparticles, with sufficient hydroxyl on their surface, graft with organic silane quaternary ammonium salt, generating organic cation salt. Afterwards, the solvent-free LiFePO4is achieved by the electrostatic combination between the organic cation salt and sulfonic acid anion with polyoxyethylene chain. Rheological property testing proves the liquid-like behavior under the solvent-free condition. TEM analysis reveals the mechanism of the liquid-like behavoior: every nanoparticle is coated with an organic layer of about6run, as a result of which the core-shell structure guarantees the relative silp between adjacent nanoparticles.