The Synthesis And Study Of Lithium Manganese Oxide For Lithium Ion Battery Cathode Material

The research focused on the two following questions: cation and cathode ion multi-doping (two cations and fluorine ion) method,Rare earth element doped and surface coating.Spinel lithium manganese oxide and multi-doping (Cobalt,Chromium,Fluorine) spinel Li1.01CoxCr0.2-xMn1.8O3.95F0.05 were synthesized with the method of sol-gel technology at the first. The research emphasized on anion substitute for Mn3+ in order to improve the structure stability of cathode material and emphasized on F-doping and the results that the initial capacity of F-doping spinel was elevated and the cycle ability in 3V region was improved, also. Their structure and electrochemical properties were characterized. The capacity of lithium manganese oxide Li1.01Mn2O4 is 120 mAh/g, which is 90 mAh/g after 20-cycle nmber at room temperature. The ratio of capacity retention is 75%. The capacity of lithium manganese oxide Li1.01Mn2O4 is only 30 mAh/g after 20-cycle number at high temperature (55℃). The ratio of capacity retention is only 25%. While, The multi-doping materials have the high initial capacity (Li1.01Co0.15Cr0.05Mn1.8O3.95F0.05: 118mAh/g) and good cycle ability (the ratio of Li1.01Co0.15Cr0.05Mn1.8O3.95F0.05 is 96.6% after 20-cycle number), and its cycle ability at elevated temperature is good than that of spinel (the ratio of Li1.01Co0.15Cr0.05Mn1.8O3.95F0.05 is 70% after 20-cycle number at elevated temperature while that of spinel is only 25%). It is found that the CV curves of spinel and multi-doping spinel showed two current peaks, which indicating that the oxidation and reduction involves two steps. The redox couple peaks in low voltage are weak than those of high voltage in the CV curves of spinel, while, it is inversing in the CV curves of multi-doping spinel. The result is that the doping anion did not play a role during lithium ion insertion and extraction in the spinel electrode.LiMn2O4 and rare earth element Dy doping spinel LiDyxMn2-xO4 were synthesized with the method of sol-gel technology. The research emphasized on anion substitute for Mn3+ in order to improve the structure stability of cathode material. It is found that the LiDy0.025Mn1.975O4 has the best electrochemical characteristic and the high initial capacity (108.5mAh/g) and good cycle ability (the retained capacity amount of LiDy0.025Mn1.975O4 is 99.5mAh/g after 20 cycle number). The results displayed that the electrochemical oxidation and reduction for the spinels could be divided into two steps,Mn dissolution mainly occurred at the second step (reactionⅱ-high voltage region). The second part of the article was the studying of TiO2-coated spinel LiMn2O4.The XRD analysis,SEM analysis showed that they had the appearance of full crystal and their structure was still the cubic spinel and that their grain distribution was uniform. The electrochemical analysis of these samples also showed that the TiO2-coated spinel LiMn2O4 cathode materials could decrease the amount of Mn dissolving in the electrolyte and had better performances. The capacity of 2-wt% coated lithium manganese oxide is 114 mAh/g. The ratio of retained capacity amount of TiO2-coated spinel LiMn2O4 is 100% after 20-cycle number and it is 95% after 100-cycle number at room temperature. But the TiO2-coated materials did not exhibit good stability at high temperature of 55℃ (the ratio of retained capacity amount of TiO2-coated (2 wt%) spinel LiMn2O4 is 38.6% after 20 cycle number). It is found that the charge transfer resistance of LiMn2O4 is littler than that of TiO2 coated LiMn2O4, which is due to the lithium ion insertion and extraction from the TiO2 coating film; and it is also found that the cyclic voltammogram curves of coated samples changed bigly compared with un-coated sample; the couple of oxidation and reduction relayed, which is due to the TiO2 coating film. Doping and coating affect the course of lithium ion insertion/extraction into/from spinel LiMn2O4 lattice, which tend to take place at the more stable one-phase structure (reactionⅰ-low voltage region).In...