Preparation Of Nano Lithium Iron Phosphate Cathode Material And Carbon Coating Modification Research

Lithium iron phosphate LiFePO₄ material is the most promising cathode material for lithium batteries at present,but there are still problems of low conductivity and poor lithium ion diffusion.To solve this problem,the effects of solvent preparation and carbon coating on the conductivity of nanoparticles have been discussed,but the synergistic effect of co-solvent and composite surfactants,and the effects of nano and carbon coating on their properties are rarely studied.Therefore,this thesis uses the combination of nano and carbon coating,based on the action mechanism of organic aqueous solvent on lithium iron phosphate,explores the influence of different proportions of ethylene glycol and water and auxiliary surfactant on the performance of LiFePO₄,and prepares nano-sized particles to improve the diffusion coefficient of lithium ion.Finally,on this basis,the influence of different carbon sources on the performance of LiFePO₄/C was explored to further improve its specific capacity.The results are as follows:Firstly,the precursor of Fe PO₄ was prepared by hydrothermal method.The effects of different iron-phosphorus ratio,oil bath temperature,single surfactant and compound surfactant on the shape and particles of the precursor were investigated.The results showed that when p H was 1.5,the oil bath time was 6h,the iron-phosphorus ratio was 1:1.06,and the oil bath temperature was 105℃,The addition of cationic surfactant CTAB can obtain particles with good morphology,and the addition of ascorbate-Span60 complex surfactant can obtain the precursor of iron phosphate with good morphology and small particles.Subsequently,the influence of different proportions of glycol water as co-solvent on the performance of LiFePO₄ battery and the influence of different co-solvent-assisted complex surfactants on the electrochemical performance of LiFePO₄ cathode material and battery were explored.The experiment showed that by changing the volume ratio of glycol and water（1:9,2:8,3:7,4:6）,the obtained particles were all nanosheet.When the volume ratio of ethylene glycol to water was 2:8,the particle dispersion of lithium iron phosphate was better,and the crystallinity was higher.The specific capacity of initial discharge was 136.5 m Ah/g,and the diffusion coefficient of lithium ion was 1.2×10^-13cm²/s.On the basis of ethylene glycol to water 2:8 as the co-solvent,Ca-Span60,Ac-Span60 and Tween60-Span60 were added respectively to obtain LFP/Ca-S,LFP/Ac-S and LFP/T-S.At 0.2C,the specific discharge capacities were 138.2 m Ah/g,144.3 m Ah/g and 115.7 m Ah/g,respectively.The particles of LFP/Ac-S and LFP/Ca-S were about 200-300 nm,and the discharge specific capacity of LFP/Ac-S at high rate was the best,and the lithium ionization diffusion coefficient of LFP/Ac-S was 1.57×10^-13cm²/s.Finally,the effects of different coating methods,different carbon sources and different coating amounts on the physical and electrochemical properties of LiFePO₄/C were investigated XRD,SEM,TEM,Raman,TG,blue electricity and electrochemical characterization were used to characterize Life Po₄/C.The results showed that the two-step coating was superior to the one-step coating.Coated with glucose,polyvinylidene fluoride and polyethylene glycol,LFP/G,LFP/P and LFP/PG were obtained at 0.2 C discharge capacity of 153.9 m Ah/g,149.6 m Ah/g and 147 m Ah/g,respectively.The lithium ionization diffusion coefficient of LFP/G was 8.39×10^-13cm²/s.And high cyclic stability;On this basis to further explore the influence of glucose（3%,5%,7%,10%）coating quantity on its performance,the results showed that when the coating quantity was 7%performance was the best,0.2 C discharge specific capacity of 161.7 m Ah/g cycle retention rate was 99.51%,Lithium ion diffusion coefficient was 1.53×10^-12cm²/s,1 C,2 C its discharge capacity can also reach 153.6 m Ah/g,148.5 m Ah/g.