Study On Preparation Of Lithium Iron Phosphate And Carbon Coating By Supercritical Method

With the continuous development of social life,people’s demand for energy storage is also increasing.As a new type of energy storage device,lithium ion battery has been used more and more widely,especially in new energy electric vehicles.In recent years,it has attracted extensive attention.LiFePO₄,the cathode material of lithium-ion battery with olivine structure,has been favored by people because of its advantages such as stable cycle,non-toxicity and cheap raw material.However,due to the low diffusion coefficient of lithium ion and electron,its practical application is affected.Therefore,it is very important to improve its conductivity and ion diffusion rate.Although the traditional solid phase method has a simple synthesis process,it has the problem that the particle size of the synthesized material is not uniform and the nanometer material cannot be prepared.The supercritical method has unique advantages in the synthesis of nanomaterials,which has the characteristics of small particle size,high crystallinity and convenient operation,etc.,and is an ideal method for preparing electrode materials.In this paper,LiFePO₄ was prepared by supercritical ethanol thermal method,and the influence of process parameters on the properties of the product was investigated.The work and conclusions mainly included the following:In this study,carbon coated LiFePO₄ nanoparticles were prepared by ethanol solvothermal method.oleylamine was used as carbon source precursor.In the process of supercritical reaction,oleylamine uniformly adheres to the surface of LiFePO₄ to prevent the particles from growing up.After being calcined at high temperature under the protection of nitrogen,the oleylamine is pyrolyzed and transformed into an ultra-thin homogeneous carbon layer,which is deposited on the surface of LiFePO₄,improving the electrical conductivity of the material and its stability in the electrolyte.The crystal structure,microstructure and electrochemical properties of LiFePO₄ prepared at different reaction times under subcritical conditions were systematically studied.The results show that when the reaction time is 1h and 3h,the prepared products have good electrochemical properties,mainly because the short reaction time produces LiFePO₄ with small particle size and uniform morphology.Although the crystallization properties of LiFePO₄ without calcination are not as good as those under 6h and12h conditions.After the calcination treatment,the crystallization property of the material is significantly improved and the particle size is also reduced.Constant current impedance test shows that LiFePO₄ has the best performance under the conditions of reaction time of 1h.This paper studies the preparation of carbon-coated LiFePO₄ structure by supercritical ethanol solvothermal method.When the temperature and pressure reach the critical point,the fluid enters the supercritical state,and LiFePO₄ can be prepared in the supercritical state.The effects of reaction temperature,loading degree,ethanol/oleylamine volume ratio and material amount on the crystal structure,morphology and particle size of LiFePO₄ were systematically studied,and the formation mechanism of LiFePO₄ in this supercritical process was explained.The research shows that with the increase of temperature,loading degree and material quantity,the crystallinity of LiFePO₄ increases continuously,and the particle size of the material also increases.With the increase of the proportion of oleylamine in the reaction system,the crystallinity of the material decreases,and the particle size becomes more uniform.Based on the single factor experiment,the orthogonal experiment of four factors and three levels was designed with temperature,ethanol/oleylamine ratio,loading degree and material quantity.The LiFePO₄ material with good morphology and particle size was prepared by taking the area average particle size of the synthesized LiFePO₄ as the evaluation standard.The first discharge specific capacity was 138.5 m Ah/g.Through orthogonal experiment,the influence of various factors on the experiment was analyzed,and the optimized particle size parameters were obtained as follows:temperature 360℃,volume ratio of ethanol-oleylamine 3.5:1,loading degree 64%,and material quantity 0.5 mmol.Under the optimal reaction conditions,vanadium doped LiFePO₄ was prepared with the doping amount of 0%～20%.The crystal structure,morphology and electrochemical properties of LiFePO₄ with different vanadium doping amount were systematically studied.The results show that the material has good constant current impedance and cyclic voltammetry performance when the doping amount is 5%.