The Preparation Of Cathode Material LiFePO₄ For Lithium Ion Battery By The Hydrogen Thermal Reduction Method

The development of newer and cheaper cathode material in lithium-ion battery, was urged by the market needs for it with high power, large capacity and long cyclic number, such as the power source for dynamoelectric help bicycle and for mineral light using LED.In this thesis, LiFePO₄ was synthesized for lithium ion battery cathode material, using the hydrogen thermal reduction method which contains high temperature solid reactions in two different temperature ranges. In this thesis, Fe₂O₃ Fe₃O₄ and FePO₄ were used as iron sources compounds, sucrose citric acid and starch were used as carbon sources compounds, MgO as Mg²⁺ additive sources compound, pure hydrogen gas were used as reducing and protective gas. Three different cathode materials for lithium ion battery were synthesized undergone high temperature solid reactions occurred in two temperature ranges of 300℃-350℃and 600℃-700℃, they are, LiFePO₄ pure phase of olivine structure without any additive, LiFePO₄/C pure phase of olivine structure only with carbon coat, LiFe_xMg_1-xPO₄/C pure phase of olivine structure with carbon coating and Mg additive. Different methods for preparing LiFePO₄ and the performance of the target products were compared, from the views of crystal structure, particle morphology and electrochemical performance, by means of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammegram (CV), electrochemical impedance spectra (EIS) and charge-discharge performance of simulated cell. The results are showed as follow,1. The olivine structure lithium iron(II) phosphate (LiFePO₄) could be synthesized by hydrogenthermal reduction. Its crystal structure is belong to the space group Pmnb. The LiFePO₄ synthesized at 650℃had a certain discharging capability.2. A few carbon additive didn't influence the structured of LiFePO₄, also they reduced the particle size of LiFePO₄ and uniformly dispersed among the particle of the crystal, which strengthened the electricity of the particle. The results of electrochemical capability testing showed that the initial discharging time of carbon-coated LiFePO₄ was longer than the initial discharging time of the uncoated material.3.Doping Mg²⁺ was propitious to synthesizing small and uniform LiFe_xMg_1-xPO₄; From SEM we can see that coating carbon reduced the particle size and didn't influence the crystal structure , LiFe_xMg_1-xPO₄/C material sintered at 750°C aggregated in a different extent, the particle size was above 2μm, most of them were large column. While LiFe_xMg_1-xPO₄/C crystals sintered at 650℃were uniform, the average particle size was about 1μm, the samples had small particle size and coarse surface, and they have abundant microstructure, specific area had a considerable increasing. Compared with the initial discharging time at 0.01C and 0.2C of LiFePO₄ and LiFePO₄/C, the discharging time of LiFe_xMg_1-xPO₄/C are longer. The experiment result shown that the LiFe_xMg_1-xPO₄/C had the best electrochemical performance4. Compared with the LiFe_xMg_1-xPO₄/C synthesized by carbon thermal reaction, the LiFe_xMg_1-xPO₄/C synthesized by hydrogen thermal reduction method has smaller crystal lattices; the cathode peaks and anode peaks of the CV are very sharp ,also the potential difference of between two peaks is very small. But the value of cathode peaks and anode peaks of the CV of LiFe_xMg_1-xPO₄/C material synthesized by hydrogenous reduction are more close to theoretical value.5. LiFe_xMg_1-xPO₄/C cathode materials synthesized with various organic carbon are olivine-type structure, the result of CV indicate that cell system formed by LiFe_xMg_1-xPO₄/C material synthesized with citric acid as reducing agent and Lithium metal is commendably reversible.6. As can be seen from the CV graphs, compared with the peak area of the CV Li⁺ -adulterating material scanning, the peak area of the CV Mg²⁺-adulterating sample scanning are larger. The cathode peaks and anode peaks of the CV Mg²⁺-adulterating sample scanning are sharper. But the value of cathode peaks and anode peaks of the CV of Li⁺-adulterating material scanning are more close to theoretical value.