Hydrothermal And Sol-gel Synthesis Of Lithium Manganese Oxides As Electrode Materials For Hybrid Supercapacitors

Recently the energy shortage and environmental pollution have became serious due to the economic development and the increasing population. Supercapacitors coupled with batteries and fuel cells are considered promising mid-term and long-term solutions for low-and zero-emission transport vehicles. Nanostructured MnO₂ and LiMn₂O₄ have been proven to be an effec tive strategy to improve supercacitor’s electrochemical capacitance due to their high power density, high energy density, longer cycle life and low toxicity. In this thesis, the electrochemical performances of hybrid supercapacitors of AC//MnO₂ and AC//LiMn₂O₄ system are studied.MnO₂ samples were synthesized by KMnO4 and C6H12O6 as raw materials at different temperatures（30 oC, 60 oC and 90 oC）. Their structures and morphologies were characterized by XRD and SEM. They and activated carbon（AC） electrode were used as the positive and negative electrode in hybrid supercapacitors, whose capacitive properties were investigated by cyclic voltammetry and galvanostatic charge/discharge test. The results showed that the MnO₂ sample prepared at 30 oC possessed the best electrochemical property. The maximum specific capacitance was 33.8 F·g-1 at the current density of 200 mA·g-1 in the wide potential range of 0¹.8 V. After 1000 cycles, the specific capacitance of AC//MnO₂ declined 13.4%. When the current density was increased to 600 mA·g-1, the specific capacitance remained 30.27 F·g-1.LiMn₂O₄ samples were prepared by a hydrothermal me thod at different reaction time（12 h, 24 h and 48 h）using above amorphous MnO₂ sample and LiO H when the molar ratio of LiO H/MnO₂ was 2 : 1. Their structures and morphologies of LiMn₂O₄ samples were characterized by XRD and SEM. Capacitive properties were investigated by cyclic voltammetry and galvanostatic charge/discharge tes t. The results showed that the LiMn₂O₄ sample which was prepared at 48 h exhibited the best electrochemical property. The maximum specific capacitance of AC//LiMn₂O₄ was 42.89 F·g-1 at the current density of 200 mA·g-1 in the wide potential range of 0¹.8 V. After 1000 cycles, the specific capacitance of AC//LiMn₂O₄ declined 5.9%. Furthermore, Li1.13Mn₂O₄ was achieved by the same hydrothermal route when the molar ratio of LiO H/MnO₂ was 4 : 5 and the reaction time was kept at 48 h. The maximum specific capacitance of AC//Li1.13Mn₂O₄ was 40.94 F·g-1 at the current density of 200 mA·g-1 in the potential range of 0¹.8 V. After 1000 cycles, the specific capacitance only declined 0.8%. Obviously, AC//Li1.13Mn₂O₄ exhibited better cyclic performance than AC//LiMn₂O₄.LiMn₂O₄ with octahedral microstructure was synthesized by a glucose-assisted sol-gel at different calcination temperatures（650 oC, 750 oC and 850 oC）and molar ratioes of C6H12O6 to Mn（Ac）2（6 : 1, 5 : 1, 4 : 1）. The structures and morphologies of LiMn₂O₄ samples were characterized by XRD and SEM. When the molar ration of C6H12O6 to Mn（Ac）2 was kept at 6:1, the LiMn₂O₄ sample prepared at 750 oC possessed the best electrochemical property. The maximum specific capacitance of AC//LiMn₂O₄ was 44.03 F·g-1 at the current density of 200 mA·g-1 in the wide potential range of 0¹.8 V. After 1000 cycles, the specific capacitance of AC//LiMn₂O₄ declined 4.7%. Therefore, the optimal calcination temperature was 750 oC. When the molar ratio of C6H12O6 to Mn（Ac）2 was adjusted to 5 : 1 or 4 : 1, the capacitive property became worse. The LiMn₂O₄ sample prepared with molar ratio of 4 : 1 at 750 oC showed the worst capacitive property. The maximum specific capacitance was 40.85F·g-1 at the current density of 200 mA·g-1 in the wide potential range of 0¹.8 V. After 1000 cycles, the specific capacitance was declined 7.7%. Thus, the optimal molar ratio of C6H12O6 to Mn（Ac）2 was 6 : 1.