Study On The Synthesis And Properties Of Carbon Coated LiFePO₄ By Organic Compound As Carbon Source

Lithium-ion battery cathode materials of lithium iron phosphate?LiFePO₄? attracted much attention for its good thermal stability and environmental friendliness, etc. But by its own structural limits, LiFePO₄ intrinsic electronic conductivity and lithium ion mobility is very low, resulting in poor conductivity, high current charge and discharge capacity fading fast and other problems, which hindered their commercial development. Where in using the carbon-coated of a modified method to improve the electrochemical properties has become a hot topic.Based on high residue carbon and reaction activity with metal ions of isocyanate groups,in this paper, we selected of 4,4'-diphenyl methane diisocyanate?MDI? trimer as carbon source, lithium carbonate, diammonium phosphate and iron oxalate as raw materials by high temperature solid state synthesis of phase-pure carbon coating LiFePO₄/C materials.The structures and performance of prepared LiFePO₄/C are characterized by XRD, SEM and other analysis methods, and the materials of electrochemical properties analyze by cyclic voltammetry and AC impedance measurements. The main contents are as following:a) Synthesis and performance of MDI trimer: MDI trimer is synthesis of solvent method and its thermal stability study, results showed that the trimer of main two weightlessness at 350? and 400?, which the final residual carbon content up to 50%.While the synthetic material of lithium iron phosphate thermal stability studies, in order to determine the high temperature solid state synthesis of carbon-coated lithium iron phosphate temperature program.b) Synthesis and performance of carbon source coated LiFePO₄: MDI trimer as carbon source coated lithium iron phosphate anode material was prepared by high temperature solid state method.The characterized by XRD and FT-IR Indicates that iron phosphate is a standard olivine type structure.The factors influencing conclude the MDI trimer doping amount ? carburizing temperature and soaking time change the crystal structure of lithium iron phosphate ? micro topography and electrochemical properties through orthogonal experiment. Electrochemical studies were carried out on the anode materials of lithium iron phosphate with different carbon source coating and different coating quality.c) Study on electrochemical performance of carbon coated LiFePO₄:Electrochemical studies were carried out on the anode materials of lithium iron phosphate with different carbon source coating and different coating quality. The best conditions forthe MDI trimer doped amount of 9%, 750?under thermal insulation 8h. Through analysis of the SEM and particle size distribution, the added amount of MDI trimer is to low not effectively coated lithium iron phosphate particles, cause cannot afford to block the excessive growth of particles; however, the added of MDI trimer is to too much effect the serious aggregation of the particles because of complex nature of the isocyanate groups and the metal ions,which lead to transporting sudden decreased the electrochemical properties of lithium ion. The best conditions for the MDI trimer doped amount of 9%,750 ? under thermal insulation 8h. which lithium ion migration rate of up to 3.87 x10-11cm2·s-1. The results show that with less doping amount the lithium iron phosphate to MDI trimer was able to tightly cover. When the addition of a higher amount of lithium ion mobility drop, indicating that the binding capacity of MDI trimer and lithium iron phosphate is very strong, play a certain role in the battery charging and discharging process to prevent particles from breaking. Generally speaking, MDI trimer can in-situ purolysis reverse coat LiFePO₄ and block the grain excessive growth of LiFePO₄ to improve the the electrochemical performance of carbon coated LiFePO₄ as cathode materials.