Study On The Preparation And Supercapacitive Behavior Of Spinel Manganese Oxide

Supercapacitors are new storage devices between batteries and traditional capacitors. They have a lot of advantages, such as high power density, high energy density, longer cycle life, low toxicity. In this dissertation, spinel electrode materials (LiMn₂O₄ andλ-MnO₂) were investigated and exploited. They have spinel structure with a space group Fd3m where the lithium ions can reversibly insert/extract into/out the spinel crystal lattice. In addition, they have high energy density, steady cycle performance, charge-discharge performance at great current and low price, etc. Therefore, the performance study of electrode materials of spinel manganese series will have a wider foreground in development of electrochemical supercapacitor.LiMn₂O₄ electrode materials were mainly synthesized by high temperature solid-state reaction with simple operation and easy industrialization. X-ray Diffraction(XRD), Scanning electron micrograph(SEM) and various electrochemical testing methods were employed to investigate the effect of synthesized materials, calcining temperature, calcining time, and ratio of reactant on the structure and capacitance performance of LiMn₂O₄ The results suggest that Li₂CO₃ and EMD are optimum lithium and manganese origin when LiMn₂O₄ was prepared by solid-state synthesis reaction. The crystal structure of the product has already been integrity by calcining at 800℃for 4h, and the calcining time further prolonged is insignificant. When the mol ratio of Li/Mn is 1.08/2, the synthesized material has high discharge capacitance. Once the mol ratio deviates from the value, discharge capacitance of the synthesized material will decrease.λ-MnO₂ was obtained by reaction of LiMn₂O₄ and dilutes acid, and the pure cubic spinel structure still is preserved. The electrochemical performances of LiMn₂O₄ andλ-MnO₂ were studied in different electrolytes. In the (NH₄)₂SO₄ solution, the electrodes of LiMn₂O₄ andλ-MnO₂ exhibit good capacitance behavior in the potential window of 0-1V(vs.SCE). The capacitance performances are also affected by the concentration of electrolytes. Higher the concentration is, higher the specific capacitance is. Besides, the mechanism of LiMn₂O₄ occurring capacitance has been discussed.The composite materials of LiMn₂O₄/C and LiMn₂O₂/MnO₂ with different LiMn₂O₄ contents were prepared, and its electrochemical performances were investigated in Li₂SO₄ solution. It is summarized that the behavior of cyclic voltammogram of electrode materials with composite carbon not only exhibits the capacitance behavior of double-layer capacitance, but also has capacitance behavior of Faraday. The composite electrode of LiMn₂O₄/C with 80% active carbon can reach to the highest capacitance. The specific capacitance of electrode material of LiMn₂O₄/MnO₂ with 20% LiMn₂O₄ is 151 F·g^-1,increasing 22% than that of electrode of MnO₂.It is suitable to be used as electrode material of supercapacitor. These performances determine that composite electrode materials are relatively perfect materials of supercapacitor.Crystal lattice structure of spinel LiMn₂O₄ was analyzed, and the stability of electronic configuration of doping metallic ions in octahedrons-field and effect of doping metallic ions on crystal-field stabilization energy of LiMn₂O₄ also were analyzed and compared. All of these supply theory guide for doped metallic ions. The research results of XRD and SEM indicate LiM_0.1Mn_1.9O₄ still is pure spinel phase structure with different contents of doped Al,Co,Ni,Zn,and Ti.The researches of electrochemical performances show that the discharge specific capacitances of LiMn₂O₄ electrodes by doping appropriate amount of Al,Co,Ni, Zn, and Ti can be improved, and cyclic performance of electrode material also can be improved. The results display that LiMn₂O₄ possesses practical value.Different kinds of hybrid supercapacitors were prepared. When hybrid supercapacitor was composed of LiMn₂O₄ and AC, a better capacitance capability was attained in Li₂SO₄ solution than in (NH₄)₂SO₄. Monocase working voltage can reach to 1.8V.Its specific capacitance and energy density is 62.3F·g^-1 and 28.0Wh·kg^-1 at the current density of 2mA·cm²,respectively.When hybrid supercapacitor was composed of LiAl_0.1Mn_1.9O₄ and AC and the current density was bigger than 10mA·cm²,the specific capacitance and energy density of LiAl_0.1Mn_1.9O₄ is higher than that of LiMn₂O₄.Therefore, the electrode material with doped Al is suitable to be charged/discharged at great current, having better capacitance performance.λ-MnO₂ used as electrode material of supercapacitor has good capacitance performance. The working potential of hybrid supercapacitor ofλ-MnO₂/Li₂SO₄/AC can reach 2.2V. It possesses high power density, energy density, and excellent capacitance performance, which supply basis for the research and application ofλ-MnO₂ used electrode materials of hybrid supercapacitor. Consequently, the studies ofλ-MnO₂ used electrode materials of hybrid supercapacitor make development of electrochemical supercapacitor have a wider prospect.