Preparation And Doping Modification Of Lithium Iron Phosphate By Solvent Method

In this thesis,olivine LiFePO₄,a popular positive material for power lithium batteries,is taken as the research object.In view of its low conductivity and low ionic diffusivity caused by its own structure,the material is modified by solvent method and doping,and the phase and morphology of the prepared material are analyzed.And electrochemical testing to explore material properties innovatively explored the lithium iron potential co-doping and solvent doping to prepare lithium iron phosphate materials,and carried out the following work:（1）The two-step method is used to prepare LiFePO₄ materials.By exploring different hydrothermal time,hydrothermal temperature,p H value and other conditions to prepare materials with better conditions,doping different metal elements to prepare materials with superior performance,Li Fe_0.98Mn_0.02PO₄ materials are successfully prepared.The specific initial discharge capacity of Li Fe_0.98Mn_0.02PO₄ is 133 m Ah·g^-1,and the performance of the material is improved by 13%.After Mn doping,the material showed higher rate performance.After the impedance test analysis,the charge transfer resistance of LiFePO₄material doped with Mn is the smallest,the diffusion coefficient of lithium ion is the largest,and the difference of the REDOX peak of the material is a small 0.52V.Appropriate Mn doping modified LiFePO₄ can improve the performance of the positive electrode material.（2）Lithium site doping is introduced.Firstly,the influence of lithium site doping and iron site doping on material properties is discussed.Secondly,the influence of excessive lithium in lithium site doping and different doping modes on material properties is further discussed.The doped vanadium element material does not show regular morphology,the particle size is 200-400nm,0.2C rate charge and discharge,the charge and discharge capacity of the first circle is 134.05m Ah·g^-1,discharge capacity is 117.2m Ah·g^-1.Excessive lithium in solid phase doping does improve the electrochemistry of the material,solid phase preparation of lithium excessive doping material cycle 0.2C initial discharge capacity of127.1 m Ah·g^-1,after 100 cycles of the discharge capacity of 123.4 m Ah·g^-1,The material prepared by liquid phase method shows good cyclic stability in different rate charging and discharging tests,and the specific discharge capacity attenuation of the material is small at high rate charging and discharging.（3）The solvothermal method is used to prepare LiFePO₄materials.Different solvents,different adding methods,and the influence of olvothermal doping on material properties are studied.The pure phase LiFePO₄ materials are prepared by solvent method.The lithium iron phosphate material prepared by solvent method has 140m Ah·g^-1 specific capacity and137.9m Ah·g^-1 initial discharge capacity at 0.2C rate,and the first coulomb efficiency is 98%.The LiFePO⁴ prepared with surfactant CTAB has 161.8m Ah·g^-1 initial charge-discharge capacity.Discharge specific capacity is 150.3m Ah·g^-1,the first coulomb efficiency is 92%,the two use solvent method to prepare LiFePO₄ material,than did not use solvent method to prepare LiFePO₄ material of the first charge-discharge capacity increased by 16%and 34%,discharge specific capacity increased by 16%and 26%.The lithium ion diffusion coefficient and electrochemical properties of the materials doped by solvent method are improved.