Synthesis And Study Of Electrochemical Performance Of Li_0.5La_0.5TiO₃ And Carbon Co-coated LiFePO₄

In the 1990 s,LiFePO₄ with olivine structure was first discovered as Li-ion battery cathode material.The study found that LiFePO₄ not only had the advantages of wide range of sources and low cost,but also LiFePO₄ as the cathode of Li-ion battery had the advantages of environment-friendliness,excellent cycle performance and low self-discharge rate.Since then LiFePO₄ became the research hotspot in the field of lithium-ion batteries.In the past several years,due to its own structural reasons,a variety of modification methods had been proposed and experimented on account of the shortcomings of electrochemical performance caused by low conductivity and low lithium ion diffusion rate.In this paper,the electrochemical performance of LiFePO₄ cathode material was improved by surface coating technique.The electrochemical properties of LiFePO₄ and LiFePO₄/C cathode materials coated with different amount of Li_0.5La_0.5TiO₃?LLTO?were investigated.The purpose was to determine the optimal amount of LLTO.Two kinds of ethylene glycol?ethylene glycol and N,N-dimethyl acetamide?solvents were selected for the synthesis of Li FePO₄ and Li FePO₄/C cathode materials using a solvothermal method.The glucose was used as carbon source.LLTO was synthesized by sol-gel method,and LLTO was coated on the cathode material by alcohol suspension method.Phase of the samples was characterized by X-ray diffraction.The morphologies of the samples were X-ray diffraction by scanning electronic microscopy.Cycle performance and rate performance were tested by constant current charge/discharge test system.The AC impedance and cyclic voltammetry were tested by electrochemical workstation.The synthesized LiFePO₄ and LiFePO₄/C cathode materials were coated by LLTO with the proportion of 1wt.%⁴wt.%,respectively.The charge/discharge test results showed that when LLTO coating proportion was 3wt.%,LiFePO₄/LLTO cathode material exhibited the best electrochemical performance with discharge capacities of 147.5mAh/g,144.7mAh/g,141.1mAh/g,129.0 mAh/g and 119.8mAh/g at 0.1C,0.5C,1C,2C and 5C,respectively.However,the material without LLTO merely showed 113.7mAh/g,110.2mAh/g,108.5 mAh/g,99.4mAh/g and 91.0mAh/g at 0.1C,0.5C,1C,2C and 5C,respectively.When LLTO coating proportion is 3wt.%,the loss rate of the capacity of the samples was 23.9% after 30 cycles from low rate to high rate.But the loss rate of the capacity of the samples without LLTO was as high as 28.0%.It was indicated that the specific capacity and cycling performance of LiFePO₄ material coated with LLTO was improved.The curve of cyclic voltammetry indicated that the peak of the LiFePO₄ sample coated with 3wt.% LLTO was the largest and the symmetry was the best.The voltage between redox peaks was only 0.117 V,so this kind of sample had the best reversibility.The results of AC impedance tests showed that LiFePO₄ coated LLTO could reduce the charge transfer resistance,LiFePO₄ sample coated with 3wt.% LLTO had a minimum impedance value of 22?.The results of the analysis of LiFePO₄/C cathode material coated by LLTO showed that the sample had the best performance when the coating amount of LLTO was 2wt.%.The initial discharge capacities were 163.7mAh/g,159.8 mAh/g,156.1mAh/g,144.9mAh/g and 125.3mAh/g at 0.1C,0.5C,1C,2C and 5C,respectively.After 30 cycles from low rate to high rate,the loss rate of the capacity of the samples was 24.7%,but the LiFePO₄/C material without LLTO merely showed 138.6mAh/g,134.2mAh/g,27.3mAh/g,109.8mAh/g and 96.4 mAh/g.After 30 cycles from low rate to high rate,the loss rate of the capacity of the samples was 35.6%.It was indicated that the co-coating carbon and LLTO could improve the material's specific capacity and cycling performance.The results of cyclic voltammetry tests and AC impedance tests showed that LiFePO₄/C prepared by solvothermal method coated with LLTO of 2wt.% had the best electrochemical performance.