Investigation Of Carbon-coated Li-doped Li_4+xTi₅O₁₂/C

As anode materials for lithium-ion batteries（LIBs）, Li₄Ti₅O₁₂has unique longcyclic lifetime and high safety. Therefore, Li₄Ti₅O₁₂has multiple applications inelectrical vehicle and energy-storage power sources. However, the very low electronicconductivity (10^-13S cm^-1) of Li₄Ti₅O₁₂results in low capacity. Besides, Li₄Ti₅O₁₂hasfirst-cycle irreversible capacity lose, i.e., first-cycle coulombic efficiency <100%,which results in energy waste and low capacity of LIBs. In this dissertation,micro-sized Li₄Ti₅O₁₂was fabricated through solid-state reaction using micro-sizedH₂TiO₃and nano-sized TiO₂as Ti-source, respectively. Because of the large particlesize and unsatisfied electrochemical properties of the micro-sized Li₄Ti₅O₁₂,mesophase pitch was further exploited as carbon sources. For the first time,carbon-coated Li-doped Li_4+xTi₅O₁₂/C composite materials with first-cycle coulombicefficiency>100%, high cyclic capacities and structural stability were successfullyobtained by combined carbon thermal reduction and carbon coating techniques. Themicrostructure and cyclic properties of Li₄Ti₅O₁₂、Li_4+xTi₅O₁₂/C with different carboncontents and Li_4+xTi₅O₁₂/C with different Li-doping were systematically studied usingstructural and electrochemical methods. The conclusions include:（1） Micro-sized Li₄Ti₅O₁₂was fabricated using micro-sized H₂TiO₃and Li₂CO₃as source materials and reacted at850℃for12-20h. Pure Li₄Ti₅O₁₂could be obtainedonly when the excess of Li₂CO₃was24%（weight percentage）. The first-cycledischarge/charge capacities of Li₄Ti₅O₁₂are114.1mAhg^-1and108.9mAhg^-1,respectively. Nano-sized TiO₂was synthesized using hydrothermal process, whichwas exploited as Ti-source to fabricate micro-sized Li₄Ti₅O₁₂with the first-cycledischarge/charge capacities of122.7mAhg^-1and117.7mAhg^-1respectively.（2） For the first time, carbon-coated Li-doped Li_4+xTi₅O₁₂/C composite materialswith the first-cycle coulombic efficiency>100%were successfully obtained usingmesophase pitch as carbon source. The carbon addition could effectively decrease theparticle size and enhance the electrochemical properties. The optimum cyclicproperties were obtained when the carbon content was4%. The first-cycledischarge/charge capacities of Li_4+xTi₅O₁₂/C are131.6mAhg^-1and159.3mAhg^-1at0.1C rate, respectively. And the first-cycle coulombic efficiency is as high as121.1%.The charge capacity retention is96.8%over100cycles. Meanwhile, the Li_4+xTi₅O₁₂/C has high rate capabilities.（3） When the carbon content was4%, Li_4+xTi₅O₁₂/C composite materials withLi:Ti ratios of4:5、4.2:5、4.5:5and5:5at source materials were fabricated,respectively. Their first delithiation capacities were7.4mAhg^-1，17.2mAhg^-1，33mAhg^-1and48.1mAhg^-1, respectively, which correspond to Li₄.12Ti₅O₁₂/C,Li₄.29Ti₅O₁₂/C, Li₄.57Ti₅O₁₂/C and Li₄.82Ti₅O₁₂/C, respectively. The Li-doping contentin the Li_4+xTi₅O₁₂/C could be tuned by adjusting Li:Ti ratio in the source materials.Compared with general carbon-coated Li₄Ti₅O₁₂, the Li_4+xTi₅O₁₂/C compositematerials with first-cycle coulombic efficiency>100%and obtained for the first timein this study have unique advantages in saving energy and enhancing capacity andefficiency of LIBs, which are important both in theoretical studies and practicalapplications.