The Preparation And Electrochemical Properties Of Modified Anode Material Li₄Ti₅O₁₂ For Lithium Ion Batteries

Compared with the typical commercial carbon anodes, spinel Li₄Ti₅O₁₂has beendemonstrated as the most promising anode material for large-scale lithium-ion batteries, Li₄Ti₅O₁₂exhibits many advantages, such as the flat and relatively high operating voltage （1.55V vs. Li+/Li）makes the battery much safer, because it is above the reduction potential of common electrolytesolvents, thus the generation of lithium dendrites is prevented. Moreover, the spinel Li₄Ti₅O₁₂shows excellent cycle life due to the negligible volume change, therefore it has been considered asa zero-strain material. Besides, the Li₄Ti₅O₁₂has the high thermal stability, especially at elevatedtemperature. Unfortunately, Li34Ti5O12also suffers from poor electronic conductivity （10-1S cm-1）which is the main problem that the electrochemical properties of Li₄Ti₅O₁₂cannot commendablymeet the requirements of the practical application. In this thesis, many approaches have been madein order to improve their electrochemical performance including doping Nb⁵⁺, coating with thehighly conductive N-doped carbons and designed hierarchically nano-mircro structure, concreteresearch content is as follows:（1） The1D hierarchical Li₄Ti₅O₁₂/C and Nb doped Li₄Ti₅O₁₂/C composite nanofibers wereprepared through electrospun method and the electrochemical performance of them were furtherstudied. The results show that the Nb doped Li₄Ti₅O₁₂/C samples display good rate capability andcycle performance and the Li₄Ti_4.95Nb_0.05O₁₂/C was the best among them. The discharge capacityof Li₄Ti_4.95Nb_0.05O₁₂/C nanofibers in the first cycle was at140.1mAh g1at the rate of10C, it stillmaintained139.7mAh g1after100cycles with only0.286%capacity loss. The1D nanostucturesof Li₄Ti_4.95Nb_0.05O₁₂/C nanofibers composed of nano-sized particles improved the intercalationkinetics for both electron and Li+transport as well as proved a high electrode-electrolyte contactarea, A certain amount of Ti4+transform to Ti3+due to niobium doping and uniform carbon coatingon the surface and throughout the interior of each secondary nanoparticles effectively improvedthe electronic conductivity of the materials, thus decreasing polarization especially at highcharge-discharge rates.（2） The pure Li₄Ti₅O₁₂, Li₄Ti₅O₁₂with carbon coating and N-doped carbon coating wereprepared by a carbon pre-coating process combined with ball milling which could make thesamples more homogeneous over primary particles. The effects of carbon-free, carbon andN-doped carbon coatings on the electrochemical performance of Li₄Ti₅O₁₂were investigated indetail. Both the N-doped carbon which could create the defects of the graphite and the uniformcarbon coatings which form the continuous conductive network give rise to the high conductivityof the N-doped carbon coated Li₄Ti₅O₁₂. Furthermore, the adding of carbon could restrain the growth of the Li₄Ti₅O₁₂nanoparticles which could enable the fast ion and electron transport. Atrate of20C, the N-doped carbon coated Li₄Ti₅O₁₂samples shows the discharge capacities of128.2mAh g-1with capacity retention as high as76.6%. Besides, after1000cycles at1C, thecapacity retention of the samples was95.9%with nearly ignored capacity fading.（3） We have demonstrated an efficient approach to prepare hierarchical Li₄Ti₅O₁₂microspheres and test it as anode material for lithium storage. The hierarchical nano-microstructures could be beneficial to the transport of the Li+and electron and further shortening thediffusion distance of Li+in the active materials. The effect of the ratio of Li/Ti on the purity of theproduct was also studied. The result shows that the Li₄Ti₅O₁₂microspheres show excellentperformances, at rate of1C, it shows the discharge capacities of163.5mAh g-1and after100cycles it can still maintained at161.3mAh g-1with capacity retention as high as98.7%.