Solvothermal Preparation And Modification Of LiFePO₄ As Cathode Materials For Lithium Ion Batteries

Olivine-type LiFePO₄ is a cathode material,which has a bright prospect for lithium-ion batteries.It not only provides a discharge voltage of 3.4V,a considerable theoretical discharge capacity?170mAh/g?,but also has a stable structure,high safety,and sufficient resources.However,the lower electronic conductivity of the material itself and the poorer ion diffusion rate limit its large-scale application.Therefore,it is the focus of LiFePO₄ materials to find more simple synthetic methods and improve the electronic conductivity and dynamic performance of materials.Based on the above objectives,my thesis selected LiFePO₄ as research object,this paper takes material LiFePO₄ as the research object,the nano-LiFePO₄ for Li-ion battery cathode was prepared via solvothermal method,then studied the effect of different molecular weight of PEG as a mixed solvent on the structure and electrochemical properties of LiFePO₄,and we systematically investigated the volume ratio of PEG to water and the effect of pH values?in the range of 4-10?on the formation of LiFePO₄ particles with specific morphology and size under solvothermal conditions.Then,cathode material was prepared by solvothermal method and improved using rare earth elements La and Yb doped.The main contents in this article as follows:1.The Lithium ion anode material.LiFePO₄ powders were prepared by solvothermal method.In order to obtain the best solvent conditions,we investigated the effects of different molecular weights of PEG and H₂O as mixed solvents on the structure,morphology,and electrochemical performance of the positive electrode material.That is,when the molecular weight of PEG is 600,the sample has better electrochemical performance.The size of the synthesized sample was the smallest particle size and good dispersibility between 200-500 nm.The first charge-discharge ratio was 148.5 mAhg/g and 145.1 mAh/g,and the coulomb efficiency was 97.5%,which are obtained under the conditions of charge and discharge with current density of0.1 C and voltage of 2.5-4.2 V.The capacity retention rate was 92.5%after 50 cycles.EIS test results also show that the reformulated sample has a smaller charge transfer impedance.LiFePO₄ material prepared by solvothermal method has better electrochemical performance.2.Based on the solvent heat method,The Lithium ion anode material LiFePO₄powders were prepared by changing the varying volume ratio of PEG600 to water,controlling the pH values under the best solvent conditions.The test results showed that when the volume ratio of PEG600 to water is 1:1,the grain dimensions of about200-500 nm,the microparticles were observed to be uniform and mono-dispersed in diameter.The first charge-discharge ratio was 149.1 mAh/g and 145.8 mAh/g and the coulomb efficiency was 97%,and the capacity retention rate was 93%after 50 cycles.When the pH of synthesizing solution was adjusted to 6,the LiFePO₄ samples display nanoparticles with diameters ranging from 200 to 500 nm.The initial discharge capacities of this material was 1145.8 mAh/g.After 50 cycles,the capacity retention rate was 92.6%.3.The LiFe_1-xM_xPO₄?M=La,Yb?cathode materials were synthesized by solvothermal method under the best solvent conditions.The test results showed that doping rare earth elements La and Yb did not change the olivine structure of cathode materials.The sample of x=0.03 in all samples,had the highest initial charge and discharge capacity-139.2 m Ah/g and 130.5 mAh/g,which are obtained under the conditions of charge and discharge with current density of 0.1 C and voltage of 2.5-4.2V.the coulomb efficiency was 93.7%,and the capacity retention rate was 90.7%after 50cycles.The sample had the highest first discharge capacity?135.7 mAh/g?by doping Yb with x=0.03,whose the initial coulomb efficiency was 93.4%,and the capacity retention rate was 95.9%after 50 cycles.