Synthesis And Modification Of High-rate Li₄Ti₅O₁₂ Anode Material For Lithium-ion Batteries

Lithium ion battery has become research and development focus of power energy storage device due to its highlights, such as high voltage, high energy and power density, long cycle life and no pollution. Electrode is the core and key material determining the performance of lithium ion battery. Spinel and "zero-strain" Li4Ti5O12 has become one of the most promising lithium ion battery anode materials substituting for the conventional carboneous materials due to its long cycle life and satisfactory safety. However, Li4Ti5O12 is difficult for commercial application because it has the instrinsic limitations of low electronic conductivity and slowly Li+ion diffusion, which will result in fast fading of capacity and severely polarizing at high current charge and discharge.Considering the poor rate performance of Li4Ti5O12 materials, and aiming at decreasing calcined temperature and reducing particle size, the ultrasonic assisted with two step calcination process was developed, the effect of ultrasonic activation, particle size of raw materials, calcined atmospheres, etc on the structure and properties of materials was studied. The Li4Ti5O12 with small particle size and good rate performance was calcined. Aiming at improving the electronic conductivity of Li4Ti5O12, a high electrical conducting layer of TiN was prepared by in-situ self-growing on the surface of Li4Ti5O12 particles with urea as nitrogen source. In addition, a ladder network conducting layer of polyacrylonitrile carbon was prepared by in-situ coating. The effects of high temperature solid-state reaction process and calcined atmospheres on growth mechanism of crystal grain were investigated. Conducting model of Lithium titanate electrode particle was built, kinetics diffusion mechanism of charge in particle is explained. The main results of the thesis are as follows.(1) The synthesis technology of high performance Li4Ti5O12 anode material was optimized, and the best method is the ultrasonic assisted with two step calcination process with a mixture of Li2CO3 and TiO2 as molar ratio Li/Ti of 0.816, ball milling mixture for 2h, ultrasonic activated for 40min, then low calcined temperature at 600℃for 8h, sequentially high calcined temperature for 800℃for 10h. Mesophase LiTi2O4+δformed after ultrasonic activation. The discharge capacity is 170.9mAh/g at 0.1C rate, and 90mAh/g at 3C. the capacity retention is 90.7%after 100 cycles at 0.1C,(2) The effects of particle size of the raw materials and calcined atmosphere on Li4Ti5O12 were revealed. With increasing of particle size of the raw TiO2 (5nm-800nm), the size of Li4Ti5012 particles firstly decreases and then increases. With O2 partial pressure decreasing, the size of Li4Ti5O12 particles decreases. The Li4Ti5O12 obtained at high vaccum using TiO2 with the size of 100nm shows smallest particle size of 0.7um. The discharge capacity and capacity retention of this Li4Ti5012 is 98.3mAh/g at 5C and 56.7%, which is 45.16%,1.5 and 1.47 times of the material calcined in air atmosphere, respectively.(3) Mesophase Li2TiO3 as transition phase of high temperature reaction mechanism in Li4Ti5O12 prepared by two step calcination process was illustrated, effect of Calcined atmosphere on the apparent activation energy of grain growth and phase transition temperature was indicated. The kinetic equation of grain growth of Li4Ti5O12 is in accordance with the equation of seventh degree. Characteristic of grain growth for Li4Ti5O12 in two temperature region is different. With 721℃for boundary, the apparent activation energy is 60.15kJ/mol in the low temperature region and 156.24kJ/mol in the high temperature region in air atmosphere. With the decrease of oxygen partial pressure, the apparent activation energy of grain growth increases, phase transition temperature decrease. The structural transformation from Li2TiO3 phase to Li4Ti5O12 phase starts at the transition point. The apparent activation energy of grain growth, and hinders grain growth under hypoxic condition improves.(4) Through urea doping and nitriding at high temperature, a high electrical conducting TiN layer (electric conductivity is 1x104～4×104 S·cm-1) was formed by in-situ self-growing on the surface of Li4Ti5O12 particles. The first discharge specific capacity of obtained Li4Ti5O12/TiN composite material was 130.2 mAh·g-1, retain 80%of discharge specific capacity of 0.2C rate, and 1.7 times than that of the pristine Li4Ti5O12 samples at 3C rate charge and discharge. Through polyacrylonitrile doping and pyrolysis at high temperature, a polyacrylonitrile-based carbon conducting polymer layer was obtained by in-situ coating on the surface of Li4Ti5O12 particles. The discharge specific capacity of obtained Li4Ti5O12/C composite material was 120 mAh·g-1, retain 69.6% of discharge specific capacity of 0.2C rate, and 1.76 times than that of the pristine Li4Ti5O12 samples at 5 C rate charge and discharge.(5) Electrode resistance given per unit mass of Li4Ti5O12 was built according to the following expression Rm=Adn. The value of exponent in mass electrode resistance of Li4Ti5O12 is 3. The value of exponent in mass electrode resistance of Li4Ti5O12/C is 2. The transport of charge on Li4Ti5O12 particles without carbon coating follows point-contacted diffusion mechanism. The transport of charge on Li4Ti5O12/C particles with carbon coating follows ambipolar diffusion mechanism.