Synthesis And Electrochemical Behaviors Of Li₄Ti₅O₁₂ And Carbon Coating Li₄Ti₅O₁₂ Composite

Li-ion batteries （LIBs） have been widely used for portable electric devices, suchas mobile phones and notebooks. However, the performance of today’s commercialLIBs still cannot meet the requirements of some industrial applications, such as inelectric vehicles and sustainable energy storage, in terms of high power density, longcycle life and high safety. The spinel lithium titanate (Li₄Ti₅O₁₂, LTO) has attractedgreat interest as anode material of rechargeable lithium-ion batteries because of itsunique characteristics: high safety, excellent cycle reversibility and fast Li+insertionand de-insertion ability. However, the high rate performance of Li₄Ti₅O₁₂is still needto be improved. Therefore, in this paper, using low priced TiCl₄water solution astitanium source, TiO₂precursor and Li₄Ti₅O₁₂products in various morphologies areprepared in the moderate condition, and the conductive ability and the high rateperformance of these samples were improved by carbon coating.Nano-sized TiO₂particles with different morphologies were prepared by thehydrothermal method using aqueous TiCl₄solution of different concentrations. TheTiO₂precursor is firstly coated with a carbon layer.With the presence of carbon layer,the final product Li₄Ti₅O₁₂is effectively prevented from agglomeration even under ahigh temperature solid state reaction. Compared with samples without pre-coatingtreatment, carbon coated Li₄Ti₅O₁₂composite have shown a superior performance interms of rate capability, cycling life and capacity retention. Smaller particles promoteshorter pathways for solid state diffusion of Li-ions and result in better rate capability.And also the carbon layer can reduce electrode polarization by improving theelectronic conductivity of Li₄Ti₅O₁₂.Spinel-type Li₄Ti₅O₁₂ultrafine powders were successfully synthesized usingnon-organic hydrothermal and hydrolysis methods. Hydrothermal conditions promotethe adsorption and insertion of Li+, its precursor has formed the hydrated lithiumtitanate of C-base-centered orthorhombic lattice structure. The results show that thegrain size of Li₄Ti₅O₁₂prepared by hydrolysis method were200-700nm, while thehydrothermal method which effectively suppressed the crystalline growth andagglomeration of Li₄Ti₅O₁₂, which exhibited a particle size of100-400nm with high disperse degree. The obtained Li4Ti5Ol2electrode presents excellent performance interms of rate capability, cycling life and capacity retention. The initial dischargecapacity of Li₄Ti₅O₁₂prepared by hydrothermal method is172.8mAhg^-1at0.1C and142.4mAhg^-1at10C.Carbon-coated Li4Ti5Ol2was prepared by using ACM as a carbon precursorthrough an organic solvent-free and facile pathway. A reasonable amount of ACMcould form a thin uniform coating layer on the Li₄Ti₅O₁₂particles in the aqueoussolution, and after heat treatment, the carbon film obviously improved electricalconductivity, effectively reduced the resistance and polarization of the electrode. Theparticles prepared are comprised of highly-crystalline spinel-type Li₄Ti₅O₁₂with thesize in the range of100-400nm without any agglomeration, of which surface isuniformly covered by a thin carbon layer. The initial discharge capacity ofcarbon-coated Li₄Ti₅O₁₂at20C rate is137mAh·g^-1and remains as high as125mAh·g^-1after100cycles （91%retention）, exhibiting good rate and cyclicperformance.We studied the electrochemical performances of the Li₄Ti₅O₁₂electrode underlow potential. Under1V, Li+could intercalate into the tetrahedral sites of Li7Ti5O12to form Li9Ti5O12. The theoretical capacity of Li₄Ti₅O₁₂was limited by the number oftetravalent titanium, corresponding to291.8mAh·g^-1. At low potential, SEI film canalso be generated on the Li₄Ti₅O₁₂surface, thus reducing the initial charge/dischargeefficiency.