Synthesis, Microstructure And Performance Of LiFePO₄ As Cathode Materials For Lithium Ion Batteries

In this dissertation, refining grains, depositing conductibility carbon film on the surface of the particles and doping Mg²⁺ into the lattice of LiFePO4 were adopted to improve the electro-chemical performance of the cathode material. The cathode material LiFePO4 mainly has two flaws, the low conductibility and the slow Li+ ion diffusion, which have a bad influence on the performance of the cathode material. Many methods were adopted to research the cathode material LiFePO4 from different facets, which are concluded as the following:Firstly, the cathode material LiFePO4 was synthesized successfully by microwave caicination using acetic acid lithium, oxalate ferrous iron and ammonium dihydrogen phosphate. Under the microwave calcination, the optimum time of the microwave sintering was ten minutes. The test of the electro-chemical performance showed the initial discharge capability could reach 118mAh.g-1, which was 20% more than that synthesized by conventional sold-phase synthesis.Secondly, better performance LiFePO4 /C composite was synthesized by adding proper content of sucrose to the reactants, test results exhibited the initial discharge capability could get 130mAh.g-1 and the capability didn't fade down after more than ten cycles'test. The improvement of the performance resulted from the amendment of electro-conductance & the refinement of grains & particles. Due to the high temperature, sucrose occurred to decompose and produce a film of electric conduction matter to deposit on the surface of the particles or crystal grains which could increase the conduction of the cathode material. What's more, some reducibility gases were produced to inhibit Fe²⁺ from oxidizing to Fe3+. From the test, we can see that the capability began to increase with the increase of the content of sucrose. But the content of additive sucrose shouldn't be too much, or it will affect the energy density of the electrode materials seriously.Thirdly, Mg²⁺was doped to the lattice to improve the electric conduction of the cathode material LiFePO4. Due to Mg²⁺ doped enter into the position of Lithium, the Lattice parameter occurred to change but the crystallinity of the cathode material LiFePO4 is excellent and the initial discharge capability could get 130mAh.g-1.