Study On Synthesis And Properties Of Lithium Manganese Phosphate As Cathode Material For Lithium Ion Batteries

LiFeP04has been considered to be one of the most promising cathode materials for lithium-ion batteries because of its inherent low cost, nontoxicity, extremely high capacity and stability. However, the redox voltage for LiFePO4is3.4V versus Li/Li+, which is much lower than the conventional cathode materials of LiCoO2and LiMn2O4. This drawback has limited its development and application in large-scale applications, such as plug-in hybrid vehicles and stationary powers. As one of the olivine structured compound, LiMnPO4shows higher redox voltage of4.1V versus Li/Li+compared to LiFePO4, besides the advantages of high specific capacity and good cycling stability. Hence, it has attracted lots of attention in recent years. However, it showed inherent low electric and ionic conductivity. Many researchers have focused on improving electrochemical behavior of LiMnPO4through particle size reduction, carbon coating and cation doping. Based on these aspects, we tried to improve its specific capacity and rate capability. The main contents of this thesis are followed:1. Morphology controllable synthesis of LiMnP04through precipitating processAmmonium magnesium phosphate monohydrate (NH4MnPO4·H2O) precursor was prepared by a novel precipitating process with manganese citrate complexes as intermediate. The morphology of the precursor observed by Scanning Electron Microscope (SEM) was flower-like which was self-assembled by plate-like particles. Further analysis by X-Ray Diffraction (XRD) revealed that the lattice of the plate crystal was orientated along (010) plane. By solid-state reaction of the precursor, with lithium acetate and glucose as carbon source, pure olivine structured LiMnPO4/C composite was obtained and meanwhile, the original flower-like morphology could be retained.2. Synthesis of nano-sized LiMnPO4with in-situ carbon coating through solvothermal processNano-sized lithium manganese phosphate (LiMnPO4) is successfully prepared using a mild solvothermal method in ethylene glycol solvent. The particle size, observed by Scanning Electron Microscopy (SEM), is approximately100nm using i as the starting material. Further TEM characterization reveals that the decomposition of benzoyloxy in this lithium salt leads to a favorable configuration, with LiMnPO4particles embedded in carbon and a distinct in situ carbon layer formed even on partial particles. Using subsequent heat treatment with a certain amount of sucrose, LiMnPO4/C composites that display a good two-phase plateau during the discharge process can be formed, with a specific capacity over130mAhg"1at0.1C.3. Enhanced electrochemical performance of LiMnPO4by Cr dopingCr doped lithium manganese phosphate (LiMnPO4) with small particle size of50nm in diameter is successfully prepared by a two step sol-gel method. TEM shows that LiMn1-xCrxPO4(x=0,0.03,0.06,0.1) particles coated with4nm carbon layer are embedded in a carbon matrix. XRD measurement result and d-spacing calculated from HRTEM characterization both reveal that Cr is successfully doped into the lattice. Among the samples, LiMn0.94Cr0.06PO4/C shows the best electrochemical behavior in terms of specific capacities and reversibility. Furthermore, the composites that Cr doped could stabilize the electrolyte/carbon interface which related to the electronic structure of LiMn1-xCrxPO4shell, and the result shows that the6%Cr doped sample gives the best combination.4. Kinetics and electrochemical studies of Fe-substituted LiMnPO4One step sol-gel method was applied to synthesize LiMn(1-x)FexPO4(x=0-0.5). The kinetics of LiMn(1-x)FexPO4was studied by chemical delithiation experiment and EIS measurements. The results showed that the LiMn0.6Fe0.4PO4has the fastest conversion rate from LiMn(1-x)FexPO4to Mn(1-x)FexPO4which might be related to the best rate capability among all these samples. The electrochemical tests showed that the discharge capacity for it is about130mAhg-1,100mAhg-1,80mAhg-1and60mAhg-1at0.1C,0.5C,1C, and2C respectively. In order to to obtain nano-LiMn0.6Fe0.4PO4, two step sol-gel method would be explored here to enhance its electrochemical behavior.