| Lithium ion battery has been widely used in many fields, such as mobile phones, cameras, electric devices and laptop computers, due to its characteristics of high energy, high voltage, long cycle life and environmental-friendly. In the recent years, the application of lithium ion battery in hybrid electric vehicles (HEV) and renewable energy systems is further expanded with the development of strategic and emerging industries of nation. As we all know, the performance of lithium ion battery is affected by electrode material directly."Zero-strain"Li4Ti5O12may be regarded as one of the most promising candidate as a new generation of anode material for lithium ion battery, because it has good cycling stability and satisfactory security. However, the electrochemical property of Li4Ti5O12is affected at high charge-discharge rates attributed to its low electronic conductivity and ionic conductivity, limiting the practical application of L14Ti5O12in power and energy storage fields.In this thesis, considering the inherent defect of poor performance at high rates, the physicochemical and electrochemical properties of Li4Ti5O12material were investigated mainly from aspects of high temperature solid state synthesis process and modification by addition of activated carbon and Mg2+doping in combination with X-ray diffraction, Scanning electronic microscopy, Transmission electron microscopy, Energy disperse spectroscopy, Constant current charge-discharge experiment, Electrochemical impedance scope and Cyclic voltammogram. Besides, the process conditions of high temperature solid-state synthesis was optimized so as to provide a convenient and economical technical reference to industrial production.Firstly, The influences of one-step solid state synthesis processing conditions such as raw materials, calcination temperature and time, and molar ratio of Li to Ti on the property of Li4Ti5O12were investigated by designing orthogonal experiment. The results show that molar ratio of Li/Ti is the remarkable factor during the preparation process, and the optimal condition for one-step solid state synthesis process as following:a mixture of micron-sized TiO2and Li2CO3as molar ratio of Li/Ti of0.8, sintered temperature at900℃for12h. Two-step solid state synthesis process, namely, modified high temperature solid state synthesis process, was studied based on optimization of one-step solid state synthesis process. According to DSC-TG analysis, the two-step solid state process for synthesizing Li4Ti5O12is a process of high temperature reaction with mesophase Li2Ti03as a transition phase. Moreover, the technology parameters of the two-step solid state synthesis process were optimized. The sample with the best performance was pre-heated at650℃for8h and then calcined at900℃for2h, the discharge specific capacity of the sample is165.4mAh/g,117.8mAh/g and98.1mAh/g at0.2C,3C and5C rates, respectively, and the specific capacity attenuation is only5.1%after30charge-discharge cycles at0.2C. The experiments proved that two-step, a solid state synthesis process, is superior to one-step one.Secondly, aiming at improving the conductivity of Li4Ti5O12material, the material was modified by carbon in air and argon atmosphere, respectively, and the effect of the addition of carbon on the properties of the Li4Ti5O12material was investigated in argon atmosphere. The results show that additive of carbon in air constrained the growth of particles effectively and the rates performance of the material was improved in certain extent, the discharge specific capacity of the material is103.5mAh/g at5C, which is1.7times of the pristine one; The Li4Ti5Oi2material obtained in argon atmosphere with addition of6.0wt%carbon shows the best rates performance, At10C, its initial discharge specific capacity is106.5mAh/g, retaining85.2%of initial discharge specific capacity of5C, and the specific capacity retention of98.6%was obtained after10charge and discharge cycles.Lastly, Taking Mg2+as doping source to modify the anode material Li4Ti5O12, and Li4-xMgxTi5O12(x=0.0,0.12,0.2,0.3,0.5) were synthesized by high temperature solid-state reaction. The obtained Li3.8Mgo.2Ti5012material has the smallest charge transfer resistance and the optimal rates performance as doping quantity x=0.2, the first discharge specific capacity of Li3.8Mgo.2Ti5O12material is108.0mAh/g at10C, which is87.0%of initial discharge specific capacity of5C(93.3mAh/g). High specific capacity and good cycling stability at high rates were exhibited for the sample Li3.8Mg0.2Ti5O12/C, the initial discharge specific capacity as high as118.1mAh/g was obtained at10C with the capacity fade only4.0%after50charge-discharge cycles, electronic conductivity and ionic conductivity of the Li3.8Mgo.2Ti5O12/C were improved greatly. |
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