Further Study Of Lithium Ion Battery

Compared with traditional secondary batteries, e.g., Pb-PbO₂ battery, Ni-Cd battery, Ni-MH battery and so on, lithium ion battery shows greater advantages at the aspect of rate capability, energy density, and charge-discharge performance. Moreover, Lithium ion battery also shows the advantages of long cycling life, low self-discharge rate, and green environmental conservation. Recently, lithium ion battery has been widely used in minitype electric instruments, and are actively developing toward the fields of space technology, national defence industry, electromotive vehicle and UPS. The improvement of lithium ion battery technology and performance is depended on the development of the electrode materials and the renovation of the technics of battery, and nowdays the main attack is further improving properties and decreasing costs. It is very important to investigate the cathode and anode materials of lithium ion battery because they occupy a large proportion in the costs of lithium ion battery.In this thesis, we have reviewed the newest development in research of electrode materials of lithium ion battery, prepared relevant electrode materials and explored their applications in this device in detail. The micro-structures and morphologies of these materials were investigated by thermogravimetry/diffrential thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electronic microscope (SEM) and transmission electron microscopic (TEM). The electrochemical performances have been evaluated by galvanostatic charge-discharge, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) detailedly. The main content is the following:1. The preparation methods of spinel LiMn₂O₄ cathode materials have been studied detailedly and made some improvement. Pure-phase and well-crystallized spinel LiMn₂O₄ powders as a cathode material for lithium ion battery were first successfully synthesized by a new simple microwave-assisted rheological phase method. The results of the experiments indicated that the microwave-assisted rheological phase method have the advantages of shorter sintering time and energysaving. The synthesized LiMn₂O₄ powders are pure, spinel-structure particles of regular shapes with distribution uniformly. Such well-crystallized particles benefit from the treatment of microwave because the microwave heated the precursor not only from the outside but from inside of the precursor and thus provided a uniform heating environment which shortened the synthesizing time and overcame the agglomeration of particles. It is apparent that this new microwave-assisted rheological phase method leads to the formation of better cubic spinel structure particles. Such kind of morphology will make the material have better electrochemical performance.2. The improvement of the performance of LiMn₂O₄ cathode materials by doping the spinel with other elements have been studied in detail. The results indicated that metal-doping could improve the cycle performance of LiMn₂O₄ due to inhibition of Jahn-Teller distortion and could enhance the atomic cohesive force in the cathode hosts to strengthen their stability. Among these Al, Co and Zn doped materials, the substituted cobalt manganese oxide spinel (LiCoo.05Mn1.95O4) had the best cycle performance, the capacity loss was only 3% of its initial capacity after 30 cycles. In addition, the LiCoo.05Mn1.95O4 spinel showed high rate discharge capability and good cycle stability, The discharge capacity ratio of (2C/3)/(C/3) is 97.5%. This outstanding performance provided the possibility for the practical application of LiCo_0.05Mn_1.95O₄ spinel material.Through cationic and anionic co-substitutions, Al-F double doped LiAl_0.05Mn_1.95O_3.95F_0.05 cathode materials were prepared. The results of experiment indicated that a little amount of LiF doping was allowed to play a role of reducing the melting point, which could accelerate the crystal formation of LiMn₂O₄, and improve the crystal structure and sample appearance. The cation and anion cooperation not only reduces the Jahn-Teller distortion, enhances the stability of LiMn₂O₄, but also improves the discharge capacity. Therefore, the cation and anion co-substitutions is a better way to improve the electrochemical performance of LiMn₂O₄ cathode materials than only substituted with cation.3. Polyaniline/multi-walled carbon nanotubes (PANI/MWNTs) composites were prepared by an in situ chemical oxidative polymerization method, and were firstly used as the cathode materials for rechargeable lithium battery to improve the specific capacity and coulombic efficiency of polyaniline. MWNTs have an obvious improvement effect, which makes the composites have more reversibility, more active sites for faradic reaction, and enhanced electric conductivity, lower the resistance, and facilitate the charge-transfer of the composites. Moreover, the addition of MWNTs can protect the well contact of polymers by retaining and strengthening the structure of the electrode automatically, and reinforcing the integrality of the electrode during cycling. The discharge capacity of PANI/MWNTs composites is high as 118.8 mAh g^-1, and it is only 97.8 mAh g^-1 for PANI. The cycle performance of PANI/MWNTs composites is also significantly improved by introducing MWNTs to pure PANI.4. TiO₂ nanotubes prepared by using a hydrothermal process were firstly coated with silver nanoparticles as the anode materials for lithium ion batteries by the traditional silver mirror reaction. The physical properties and electrochemical performance of Ag/TiO₂-NTs composites were investigated in detail. Because of the high electronic conductivity of metal Ag, Ag additive significantly increased the electronic conductivity of TiO₂ nanotubes, made the transfer rate of Li-ion in TiO₂ nanotubes higher, lower the resistance, reduced the first irreversible capacity, improved the reversible capacity and the cycling stability of the TiO₂ nanotube at high charge-discharge rate, and marvelously decreased the cell polarization.