Preparation And Modification Of LiMnPO₄ For High Performance Lithium-ion Battery Cathode

The capacity bottleneck of lithium-ion battery, as a new type of chemical power source, is still the capacity upper limit of the positive materials. It is worth mentioning that the LiMnP04 has been a hot research topic in the positive materials, which is cheaper, safer and more stable, and has 20% higher redox potential than that of LiFePO4. Therefore, it is considered as the cathode for a new generation of power battery, and thus has great development potential. However, it is restricted by the sluggish kinetics of electron and lithium ion migration in crystal lattice. As a general rule, major obstacles can be overcome by controlling nanoparticle granularity, applying electrically conductive coatings and ion-doping. In this thesis, we have developed a systematic study method to synthesize nanosized LiMnPO4/C and a universal method to study ion-doping, described as the following two parts:(1) An improved approach has been proposed for the synthesis of nanosized LiMnPO4 with more active a-c planes, a regular micromorphology and conductive carbon layer by using a two-step microwave solvothermal process. A systematic study has been conducted to find the optimum operation conditions. The sample synthesized at 160? for 10 min gives the best performance and shows excellent cyclic stability with a capacity of 155 mA h g"1 at 0.5 C after 100 cycles and a surprising rate performance of 118 mA h g-1 at 10C. The best sample (LMP/NC) shows an excellent low temperature performance retaining 80% of its theoretical capacity at 2?. These excellent results are ascribled to a specific crystal orientation and the high conductivity and nano-confinement of a carbon coating that contains a small amount of nitrogen. The overall process has short synthesis times and scale-up potential.(2) An impregnation method has been developed to investigate the influence of ion-doping in LiMnPO4 on its chemical structure and electrochemical performance. This method includes two parts that are the allocation of high concentrations of Li-Mn-PO4 solution and the selection of highly conductive supporter with large pore volume. The Mg, Ca and Ce are separately investigated on the basis of this method. The heteroatom with large radius is helpful to improve high rate performance of LiMnPO4, while the heteroatom with smaller radius is helpful to improve low rate performance. By using characterization means and comparing literatures, one can conclude that the heteroatom with large radius can widen transport channel while reduce the stability of crystal. Therefore, these two factors need to be considered simultaneously on the selection of dopant in the Li-Mn-PO4 system.