Synthesis Of Li₄Ti₅O₁₂ Anode Material And Characterization Of Hybrid Supercapacitors

Spinel Li4Ti5O12 is a new promising anode material for lithium-ion batteries and supercapacitors. This material has many advantageous properties compared with other anode materials. Spinel Li4Ti5O12 has high specific capacity and excellent cyclability, and importantly, it is very safe when used as electrode material in lithium-ion batteries and supercapacitors. Howerver, due to the low electronic and Li ionic conductivity in Li4Ti5O12, the rate performance of pristine Li4Ti5O12 is rather poor. Thus in recent years many research works were carried out on methods to improve its electronic conductivity.This dissertation is a result of research work in the area of electrochemical energy storage devices, specifically Li-ion batteries and supercapacitors, and it is believed that the subject under study is a frontier of current research topics.The modified Li4Ti5O12 has been synthesized by solid-state method and sol-gel method, the influence of various contents of carbon and polyaniline (PAN) were investigated in this work through a series of experiments, and following conclusions were drawn:1. Li4Ti5O12 materials were first prepared by solid-state method, and then Li4Ti5O12-polyaniline (Li4Ti5O12-PAn) composites were prepared by in situ polymerization method. The effects of various contents of polyaniline on the electrochemical behavior of Li4Ti5O12 were studied. The results indicated that the electrical conductivity is enhanced due to the introduction of PAN to Li4Ti5O12. Li4Ti5O12-PAN composites exhibited higher rate capability and longer cyclability than the raw Li4Ti5O12-The Li4Ti5O12-PAn (PAn10%) composites showed the best electrochemical performance. The composites can deliver a specific capacity of 174.4 mAh/g at 0.5C,147.9 mAh/g at 2C and 124.8 mAh/g at 5C.2. Li4Ti5O12/C composite was synthesized via solid-state reaction using Super-P-Li conductive carbon black as reaction precursor. The added Super-P-Li did not affect the crystal structure of Li4Ti5O12, but it played an important role in improving the electronic conductivity and electrochemical performance of the Li4Ti5O12/C material. The intial discharge specific capacity of the Li4Ti5O12/C composite (C content of 10%) can reach 172.1 mAh/g and 108.7 mAh/g at the 0.5C and 5C rates respectively. After 100 cycles, the specific capacity was 169.8 mAh/g and 93.6 mAh/g, respectively.3. The Li4Ti5O12/C composite was synthesized by sol-gel method using Super-P-Li conductive carbon black as carbon source. The intial discharge specific capacity of the Li4Ti5O12/C composite (with 10% carbon) was 168 mAh/g at 1C, 154 mAh/g at 5C,138 mAh/g at 10C and 104 mAh/g at 60 C. When cycled at 2C, the discharge specific capacity of the composite only reduced 4.3% after 400 cycles.4. A hybrid capacitor AC/Li4Ti5On12 was assembled by using SUPER30 (from Norit) activated carbon as positive electrode, and Li4Ti5O12/C composite synthesized by sol-gel method as negative electrode,1 mol/L LiClO4 as electrolyte, Celgard 2400 as separator. The electrochemical performance of the AC/Li4Ti5O12 hybrid capacitor was characterized by constant current charge-discharge, rate tests, electrochemical impedance spectroscopy and cycle performance tests. The results revealed that the hybrid capacitor of AC/Li4Ti5O12 exhibited large specific capacitance, good rate performance, good cycling stability and long cycle life. In the potential range of 1.5-3.0V, the AC/Li4Ti5O12 hybrid capacitor can deliver a specific capacitance of 83.2F/g at the current density of 60 mA/g, when the current density adds up to 1A/g (60C), its value was still 67.6 F/g. The specific capacitance retains 71.9% of its initial value after 10000 deep cycles at the rate of 60C rate of charge/discharge, based on the total weight of both the active electrode materials. If considering the mass of the entire system, the specific energy and power of the AC/Li4Ti5O12 hybrid capacitor were 19.56 Wh/kg and 36.66 W/kg at the current density of 60 mA/g. The hybrid capacitor exhibited high capacitors, good rate performance and excellent cyclability.