Effect Of Lithium Iron And Phosphorus Matching On Physical-chemical Properties Of Lithium Iron Phosphate

With the ever-increasing domestic demand for electric vehicle application, lithium battery, as one of the kernel assembly in EVs, become a heated research topic. Olivine structure lithium iron phosphate as positive material has the advantages of stable charge-discharge plateau, high specific capacity, the cyclic longevity and safety. The carbon thermal reduction method with Li2CO3, C6H12O6 and FePO₄ as raw materials is applied in the practical production. The consistency of the production process of LiFePO₄ is poor, and the physical-chemical properties, electrical performance in battery application are sensitive to the change of raw materials and tiny fluctuation of the parameter in processing. There are many key issues in the reduction reaction, such as the Li:Fe:P molar ratio in raw materials, calcination temperature and ball-milling time. In this thesis, the reaction process and mechanism are studied systematically, and the microstructure, phase constitution and electrochemical performance of these carbon-coated LiFePO₄ samples are investigated by means of X-ray diffraction（XRD）, scanning electron microscope（SEM） and LAND electrochemistry analyser. The results are as follow:Firstly, the 0.985 Fe:P molar ratio iron phosphate as raw material was investigated. At the same calcination temperature, the Li:Fe molar ratio has a huge effect on the first particle size, morphology and the electrochemical performance. The lithium excess in LiFePO₄ leads to the particle size refining and reduces sinter and the formation of the non-active Li3PO4 phase which promotes the rate performance of LiFePO₄ materials. At the same ratio, the LiFePO₄ first particle size grow with the calcination temperature increasing from 700℃ to 800℃, and at the range from 700℃ to 775℃, the size growth rate with accelerating temperature of Li:Fe molar ratio 1.01, 1.03 and 1.04 samples are 2 nm·℃-1, 1 nm·℃-1 and 0.5 nm·℃-1 respectively. The less lithium counts in raw material, the more size varies in calcination procedure. At 800℃, all sample with different molar ratios sintered and degrade in electrochemical performance. The optimum condition is the temperature controlled in the range of 700℃ to 750℃ and Li:Fe molar ratio between 1.02 to 1.04.For comparison, the 0.957 Fe:P molar ratio iron phosphate as raw material was investigated. At 700℃ calcination temperature, the first particle reduce with increasing Li:Fe molar ratio, but compared to the 0.985 samples, under the same situation, the size grows bigger. It is promotive to the particle size that the phosphorus content excess. The samples’ first particle size grow faster at the calcination temperature between 675℃ to 775℃ at 1.03 Li:Fe molar ratio. The size growth rate with accelerating temperature is 3 nm·℃-1, which is almost steady and unchangeable with the varies in Li:Fe molar ratio, but much more than those compared with 0.985 iron phosphate samples. The electrochemical performance degrade with the calcination temperature and particle size increasing, and the specific capacity is less than that synthesized with 0.985 Fe:P molar ratio iron phosphate. The optimized synthesis condition is the calcination temperature in the range of 675℃ to 725℃ and Li:Fe molar ratio between 1.02 to 1.04.