Synthesis And Electrochemical Properties Of Li₂FeSiO₄and Li₃V₂（PO₄）₃Cathode Materials

Developing cheap, sustainable and safe cathode materials is a prime target forlarge scale lithium batteries. With iron and silicon being among the most abundantand lowest-cost elements, Li₂FeSiO₄cathode materials have been attracted significantattention. Another polyoxyanion Li₃V₂（PO₄）₃cathode materials not only have goodsecurity, but also have higher Li⁺ion diffusion coefficient than LiFePO4. Therefore,Li₂FeSiO₄and Li₃V₂（PO₄）₃positive materials have a good application prospect, butthe two materials both have low electronic conductivity. In this paper, sol-gel methodand carbon reduction method are combined for preparing Li₂FeSiO₄/C andLi₃V₂（PO₄）₃/C lithium-ion battery cathode materials. The morphology, crystalstructures and the group situation are investigated by the scanning electronmicroscope （SEM）, X-ray diffraction （XRD） and infrared spectroscopy （FI-IR）,respectively. The electrochemical performances of them are characterized by thecyclic voltammograms （CV）, galvanostatic measurements and exchange impedanceanalysis （EIS）.Li₂FeSiO₄/C cathode materials were prepared by sol-gel method and carbonreduction method. The effect of the different way for adding carbon as a conductivephase, the complexing agent, the pH value, and calcining temperature on morphologyand electrochemical properties of Li₂FeSiO₄/C were investigated. The results indicatethat the right condition（pH=6） for the solution with citric acid owned three of thecarboxyl as complexing agent would complex the Fe2+ion better. Adding sugar in theball mill process, and calcining at700℃for10h for preparing the Li₂FeSiO₄/Cmaterial, the surface evenly coated a layer of carbon as conductive agent, which canreduce the charge transfer impedance, improve the electrochemical performance witha specific capacity of154.4mAh g^-1at0.1C, and142.5mAh g^-1has been maintainedafter20cycles.The Li₂Fe_1-xMn_xSiO₄/C samples were suscessfully prepared by the same methodabove-mentioned. The mild doping of Mn atoms in material would reduce the size ofparticles and crystal grains to about100-200nm. With the increasing content of Mn inthe material, the initial capacities also improved, but meantime the capactiy retentionis also decreased gradually. And the Li₂Fe_0.9Mn_0.1SiO₄shows the best results. It candeliver the initial capacites of164.8mAh·g^-1at the0.1C, and150.5mAh g^-1has beenmaintained after20cycles.Li₃V₂（PO₄）₃/C cathode materials were prepared by sol-gel method and carbon reduction method, The effect of the different way for adding carbon as a conductivephase, the complexing agent, and calcining temperature on morphology andelectrochemical properties of Li₃V₂（PO₄）₃/C were investigated. The results show that,using glycine as chelating agent can better complex of trivalent vanadium ion. TheLi₃V₂（PO₄）₃/C sample prepared by milling precursor with sucrose and annealing at700℃for10h has minimum charge transfer impedance, maximum lithium iondiffusion coefficient and the best electrochemical properties. The electrochemicalperformance with an initial discharge capacity of129.2mAh g^-1at0.5C between3.0-4.3V （vs. Li⁺/Li）, and123.3mAh g^-1retained after20charge-discharge cycling.Moreover the electrochemical performance with an initial discharge capacity of150.2mAh g^-1at0.5C between3.0-4.8V （vs. Li⁺/Li）, and80%of the initialdischarge capacity was retained after40charge-discharge cycling.