Nano-li₄Ti₅O₁₂ Synthesized By Sol-Gel Method And Study On Its Modification By Carbon-Coating

In various cathode materials for lithium batteries, spinel-type lithium titanate (Li₄Ti₅O₁₂) is provided to have excellent characteristic, such as"zero strain"of the framework structure. Thus, it could be discharged fast, with better cycling capacity and higher security. So Li₄Ti₅O₁₂ get much attention all over the world. However, there are some disadvantages of it, such as lower ion and electron conductance, which limited its application. Aim at the disadvantages of lithium titanate material, Li₄Ti₅O₁₂ and Li₄Ti₅O₁₂/C materials were synthesized by sol-gel method using lithium acetate and terbutyl titanate as primary starting materials. Factors affecting sol-gel forming process and effect of sintering technics upon the structure and electrochemical capability of the production were primarily studied. Effect of different carbon sources and the adding methods of them, sintering techniques and coating-carbon amount upon structure and performance of materials were discussed.Through mechanism study on sol-gel forming reaction by terbutyl titanate hydrolyzing and systemic experiments, optimum sol-gel forming conditions were confirmed. According to TGA-DTA analysis of the gel precursor and systemic experiments, two-step sintering technique was determined. The white pure Li₄Ti₅O₁₂ gained has regular appearance and geometrical outline. It is crystallized nicely and has symmetrical diameter around 500 nm. Between 1.0～2.5 V versus Li, the Li₄Ti₅O₁₂ delivers 168.6, 155.1, 139.4 and 121.1 mAh.g^-1 at 0.2 C, 1 C, 5 C and 10 C respectively on first cycling. It put up better cycling stability at low rates, but bad at high rates.On the base of synthesizing study of pure Li₄Ti₅O₁₂, Li₄Ti₅O₁₂/C materials synthesized by adding sucrose into sol-gel system method and aniline polymerizing method was studied. According to testing results, electron conductance of the Li₄Ti₅O₁₂/C material is improved along with carbon adding. However, because gel precursor was sintered in N2 during the whole sintering process, oxygen atoms lacking in gel precursor structure could not be reinforced. Therefore, much TiO2 impurity is included in the productions of the two methods and the raw materials could not react to be Li₄Ti₅O₁₂ crystal completely. Though cycling performance of the Li₄Ti₅O₁₂/C material is improved, the discharging capacity is lower, especially at high rates.Aim at improving discharging capacity of Li₄Ti₅O₁₂/C material at high rates, Li₄Ti₅O₁₂/C cathode materials were synthesized by adding carbon source into advanced production. Two methods of sucrose mixed with production from advance sintering of gel precursor by dry-skiving and wet-skiving were contrastively studied. According to testing results, there is no TiO2 impurity included in the Li₄Ti₅O₁₂/C materials gained by the two methods. Thereinto, formless carbon in production by the dry-skiving method was loose, with a honeycomb-looking net structure. The formless carbon composes an effective electric network, which limites growing up of Li₄Ti₅O₁₂ crystals. The Li₄Ti₅O₁₂/C cathode material delivers 172.9, 144.9, 114.7 and 71.0 mAh.g^-1 at 0.2 C, 1 C, 5 C and 10 C respectively on first cycling. At 10 C rate, the material retains 92.7% of the first discharging capacity after 20 cycles, which is much improved than pure Li₄Ti₅O₁₂.Formless carbon in production by the wet-skiving method coated equably on the face of Li₄Ti₅O₁₂ crystals. Crystal diameter of Li₄Ti₅O₁₂/C material by the wet-skiving method distributes equably between 50 and 100 nm. The Li₄Ti₅O₁₂/C cathode material delivers 175.8, 146.6, 116.9 and 74.7 mAh.g^-1 at 0.2 C, 1 C, 5 C and 10 C respectively on first cycling. At 10 C rate, the material retains 97.2% of the first discharging capacity after 20 cycles, which is improved than production by the dry-skiving method.