The Preparation Of Cathode Material Of Lithium-ion Batteries Li₂FeSiO₄ By Co-precipitation And Performance Research

In recent years, lithium-ion batteries have received particular attention as one of motive power due to its advantages. As a new type of Lithium-ion batteries anode material, Li₂FeSiO₄ has high specific capacity（332mAh/g） of theory because of the two lithium-ions. Li₂FeSiO₄ elements are also more common in the anode material, so the price is low. But the electronic conductivity velocity of pristine Li₂FeSiO₄ is low which inducing its poor electrochemical performance. In order to improve the electrochemical performance of Li₂FeSiO₄, considerable efforts have been made by optimizing the morphology structure, cladding of conductive materials and doping methods. The coprecipitation method is simple and feasible and the material under this preparation has the advantages of small particle size, consisitent morphology. The microstructure and electrochemical performance of materials are analyzed via the co-precipitation method in this paper, the glucose acts as carbon source for material and the method of manganese and nickel ion doped on Fe sites to synthesize composite cathode material Li₂Fe_1-2xMn_xNi_xSiO₄. The specific work is as follows:（1）Silicon dioxide is obtained by co-precipitation in alkaline oxidation environment, which solved the problem that ferrous iron ion precipitation is easy oxidation in alkaline oxidation environment, and the generation of ferrous oxalate precipitation is covered by silicon dioxide uniformly. The Li₂FeSiO₄ precursor material was prepared by addition of lithium source, and Li₂FeSiO₄ cathode material was synthesized through different sntering conditions. The results indicated that Li₂FeSiO₄ precursor material is prepared successfully by co-precipitation method and the pure phase Li₂FeSiO₄ cathode material is synthesized by heat teratment. The sample heated for 10 h at 850℃ presented the spherical morphology. The size of Li₂FeSiO₄ particles was approximately 400-600 nm and distributed consistently. The samples had high electrical conductivity under this condition（1.244×10-13cm2 s-1）. The first charge and discharge capacity of 162.2,153.1mAh/g could be obtained at 0.1C rate. The coulomb efficiency is 94.4%. The capacity was maintained at 90% after 50 cycles which illustrated the method of preparation of spherical Li₂FeSiO₄/C material had a good electrochemical performance.（2）Based on the study of co-precipitation, the performance influence of Li₂FeSiO₄ by carbon coating was researched via the addition of different levels of glucose. The influence of different carbon content on the structure and morphology of the Li₂FeSiO₄ were analyzed by XRD, SEM, TEM. The test results show that the main peak of material of all samples are consistent and all samples are pure Li₂FeSiO₄; SEM test indicates that: the particle size of samples（300nm⁵00nm） decreases after the carbon coating; EIS test shows that: The lithium ion diffusion coefficient which coated are increased compared with the uncoated which make clear that the electronic conductivity of the Li₂FeSiO₄ is improved after carbon coating. Electrochemical performance test shows that the first discharge capacity of 152.3mAh/g could be obtained at 0.1C rate when the carbon content is 6wt%. The capacity was maintained at 87.7% after 50 cycles at 0.5C which explained it has a better electrochemical performance.（3）On account of the advantages of the ternary composite cathode material and based on the study of co-precipitation, manganese and nickel ion doped on Fe site to synthesize composite cathode material Li₂Fe_1-2xMn_xNi_xSiO₄ was researched tentatively. The influence of different doping content on the structure and morphology of the Li₂FeSiO₄ were analyzed by XRD, SEM. The results indicate that: the Li₂Fe_1-2xMn_xNi_xSiO₄ samples are consistent of Li₂FeSiO₄. The impurity peak（Li₂SiO₃ and FeNi） was appeared with the increase of doping content. SEM test results show that: the size of Li₂FeSiO₄ particles has uniform morphology（the size is about 400nm） and the particle size distributed consistently when x=0.1. Electrochemical performance test explained that the phenomenon of phase transition was appeared in the process of inital charge and discharge. Li₂Fe_0.8Mn_0.1Ni_0.1SiO₄ had the capacity of 179.2mAh/g in second charge process. The discharge capacity was 101.6mAh/g which maintained at 73% after 30 cycles which illustrated the capacity of the sample droped quickly and the worse cycle performance. Li₂Fe_0.6Mn_0.2Ni_0.2SiO₄ had a better cycle performance.