Synthesis And Characterizations Of Li₂FeSiO₄as Cathode Material For Lithium Ion Batteries

The sustainable development of energy and environment has become the globalissue concerned by all the countries. Due to increasing global environmentdeterioration, crude oil prices and increasing energy, demand, the urgent task is thedevelopment and utilization of new type of green energy and then, the green poweremerges. It is a new type of renewable energy, which can well realize the energystorage, conversion and utilization. With battery design technology constantly updatedand the emergence of new materials, applications continue to expand in different areas.Nowadays, lithium ion battery has been widely used in all fields of human life. Thenew type of silicate (Li2MSiO4) polyanionic cathode material has caught the attentionof the researchers in recent years because of its outstanding performance. This kind ofmaterial has high theoretical capacity by achieving the insertion and extraction ofmore than one lithium ions. Because of the relatively low Fe2+/Fe3+redox Fermilevel, and the strong Si-O covalent bonds make the crystal structure more stable andreveal the better thermal stability. Li2FeSiO4also has good reversibility and cycleperformance. Improving its electrical conductivity and lithium ion diffusioncoefficient has become an important subject. Therefore, in this paper, we used severalmethods to modify the materials and systematically studied the effect of differentmodification methods on its structure, morphology and the electrochemical properties. First, we synthesize the pure phase of the nitrogen doped lithium iron silicate at650℃by sol-gel method. Powder X-ray diffraction (XRD) pattern shows that thematerial belongs to monoclinic system, P21/n space group. Scanning electronmicroscopy (SEM) shows that the particle has small particle size and uniformdistribution. At the same time, we prepare pure lithium iron silicate and carbon coatedlithium iron silicate with citric acid as the carbon source for comparisons. We foundthat the nitrogen doped material has the best electrochemical performance. At0.1Crate, the discharge specific capacity can reach130mAh g-1, and the capacity retentionis very good after50cycles.Then, we explore a simple and low cost way to modify the Li2FeSiO4materialsurface which can significantly improve high rate performance of Li2FeSiO4. Wechoose AlPO4as the target coating material, and study the effect of the coatedquantity of AlPO4on Li2FeSiO4.The coated quantity is2%,5%and8%, respectively.SEM showed that the surfaces of three kinds of modified materials are covered byAlPO4coating layer with flower shape. The roughness of the surface of the particlesincreases the effective surface area of contact with the electrolyte. The change of theeffective surface area may influence the physical and chemical properties of materials.The high specific surface area of materials is conductive to the insertion andextraction of lithium ion in Li2FeSiO4, thereby improving the high rate performance.Electrochemical performance test shows that2%AlPO4coated material has the bestelectrochemical performance, which can be supported by the dynamic performance.