The Lithium Ion Secondary Battery Cathode Material Of Lifepo <sub> 4 </ Sub> Synthesis And Modification Of Research

Olivine-type LiFePO₄ cathode material is receiving more and more attention and considered the most application prospect for lithium-ion battery cathode material due to its low-cost, high specific capacity, non-toxic, environmental benign, excellent recycling performance and outstanding safety performance, especially in high-power or large-capacity battery, it has excellent high-temperature thermal stability. However, its low electron conductivity and slow Li-ion diffusion velocity affect its electrochemical properties, which hinder its commercialization process as a cathode material of lithium-ion batteries. Therefore, how to enhance its electron conductivity and lithium-ion diffusion coefficient were studied in this thesis.In this thesis,how to enhance the electronic conductivity and lithium-ion diffusion coefficient of LiFePO₄ were investigated, in order to synthesize high-performance LiFePO₄ material. Experiments have adopted solid-phase and thermal reduction methods to optimize the raw material and synthesis conditions. LiFePO₄ material was modified by using coated carbon and Fe-site doping. Crystal structure, surface morphology and electrochemical properties were investigated by XRD, SEM, chemical testing methods and other analytical methods. The main research work of this paper is as follows:1. LiFePO₄ and LiFePO₄/C composite materials were prepared by solid-phase method. The effect of lithium source (Li₂CO₃, LiOH, LiH2PO₄), carbon source (Hexamethylene tetramine, Citric acid, Sucrose) and the amount of carbon-coated to crystal structure, surface morphology and electrochemical properties were investigated to determine the optimal synthesis process.2. LiFe1-xCoxPO₄ was prepared by using solid-phase method and carbothermal reduction. XRD, SEM, CV, constant-current charge-discharge test methods were used to investigate the influence of preparation methods, the amount of doping on the crystal structure, microstructure and electrochemical performances. It was found that LiFe1-xCoxPO₄ has the same olivine structure as LiFePO₄ except for some tiny changes of the crystal lattice constant, LiFe0.98Co0.02PO₄ was prepared by solid-phase method has the best performance.3. LiFePO₄/C was synthesized through carbothermal reduction method using FePO₄ as iron source. The influence of synthesis temperature and the amount of carbon-coated on the material properties were investigated, the results showed that the material with the synthesize temperature of 720℃, coated with 4% carbon shows the highest discharge specific capacity and good cycle performance in the 0.2C, its specific initial discharge capacity is 135.3mAh/g and the capacity increase to 146.3mAh/g after 20 cycles.4. LiFePO₄/C was synthesized by using cheap Fe₂O₃ instead of ferrous iron, LiOH, LiH2PO₄ and Li₂CO₃ as lithium source, acetylene black, citric acid and sucrose as carbon source respectively. The influence of lithium source, carbon source, the sintering temperature and the amount of carbon-coated on the crystal structure and micro-morphology as well as electrochemical properties were investigated. Studies showed that the material has the best performance in the event of Li₂CO₃ as lithium source, sucrose as carbon source, the coating amount of 5%, at the sintering temperature of 720℃. The initial discharge capacity was 154.3mAh/g, and it still reached up to 151.8mAh/g after 20 cycles.5. Pure phase LiFePO₄ was synthesized at 720℃using Fe3+ iron salts and reduced iron powder as mixed iron sources. Feasibility of this program was proved by the results: Fe3+ was completely reduced to Fe2+, and it didn't contain any impurities. At the same time LiFePO₄/C composite materials were synthesized adopting sucrose as carbon source. Relative to the pure phase LiFePO₄, surface morphology and electrochemical performance of LiFePO₄/C have been significantly improved.