| Summary: | 博士 === 國立臺灣大學 === 化學研究所 === 103 === Lithium-ion batteries and supercapacitors are important energy-storage systems that enable more efficient energy storage and usage than other solutions. Both systems are excellent choices for electrical vehicles or hybrid electrical vehicles, as well as other portable devices that require both high power and high energy density. Li–ion batteries conduct electrical work by Li+ ion diffusion and faradaic redox reaction of electrical reagents between two electrodes driven by the differences in the electrochemical potential. Consequently, enormous works have focused on investigating the performances of these kinds of active materials with respect to the energy density, high capacity and cycling stability of Li–ion batteries. Supercapacitors can temporarily store a large amount of charges and then release it by a non-faradaic electrical energy storage process, which has concerned intensive attention for their high power density, fast charging–discharging, and long cycle life. In addition, Supercapacitors can store energy based on ion adsorption and/or fast surface redox reactions. Therefore, their performance is largely determined by the electrode material’s morphology, size, and porosity.
Rechargeable lithium–air (Li–O2) batteries using aprotic solvent as an electrolyte have recently attracted considerable attention because they offer an extremely high theoretical volumetric energy density of 3436 W h L-1 based on the sum of Li volumes at the beginning of discharge and Li2O2 at the end. This value is more than thrice that of state-of-the-art lithium-ion batteries (1015 W h L-1). Such fascinating features make Li–O2 batteries the most promising power source for next-generation electrical vehicles.
The key aspect to improving the performance of these kinds of energy devices is to improve the performance of active materials. The use of nanostructured materials supported on binder and conductive-agent-free electrodes are designed to enhance both ion transport and electron transport by shortening the diffusion lengths of ions (for instance Li+) and increasing the conductivity within electrode materials, respectively.
In this doctoral work, we demonstrate an easy, two-step hydrothermal/calcination approach for growing uniform Mn, Fe, and Zn cobaltite (MCo2O4) nanostructure (NS) on flexible binder-and conductive-agent-free Ni foam and carbon substrates, respectively. The hierarchical Mn, Fe, and Zn cobaltite nanowire- and nanoflake-based architectures onto conductive substrate allow enhanced electrolyte transport and charge transfer toward/from Mn, Fe, and Zn cobaltite NS surface with numerous electroactive sites. In addition, the direct growth and attachment of Mn, Fe, and Zn cobaltite NSs in supporting conductive substrates provide substantially reduced contact resistance and efficient charge transfer. These excellent features allow the use of Mn, Fe, and Zn cobaltite NS as lithium-ion battery and supercapacitors electrodes.
In the first research part, the application of nanostructured MCo2O4 electrodes as electrode material for Li–ion batteries and supercapacitors is evaluated in Chapters 3 to 5. FeCo2O4 nanoflakes electrodes exhibit a better performance towards Li-ion battery MnCo2O4 and ZnCo2O4 because of Fe electroactivity towards Li, Fe2+ free electrons, and the high-surface area of FeCo2O4 2D nanoflakes. MnCo2O4 nanowires electrodes exhibit better performance than the flower-like ZnCo2O4 nanowires because Mn can transport free electrons and acquires high-capacity, in addition, the high-surface area of the discrete MnCo2O4 nanowire structures. MnCo2O4 nanowires exhibit better supercapacitive performance and higher capacitance values than FeCo2O4 nanoflakes, because of the porous structure MnCo2O4 1D nanowires which have a higher surface area than 2D FeCo2O4 nanoflakes.
In the second research part, catalytic behaviors of MCo2O4 nanorods (M = Mn, Fe, Ni, and Zn) as cathode material for lithium–O2 battery are also been studied by synthesizing MnCo2O4, FeCo2O4, NiCo2O4, and ZnCo2O4 nanorods through an easy hydrothermal method. These nanorod structures are porous, which further improve capacity and cycling performance. The mesoporous structure enables oxygen and electrolyte flow during discharge reaction and provides a good two-phase interface for catalysis. FeCo2O4 nanorods display higher capacity and lower overpotential than other MCo2O4 nanorods which can be attributed to the higher Co3+/Co2+ ratio in FeCo2O4 system, resulting in high O2 absorption of the catalytic surface. As a result, more electropositive Co3+ ions act as active sites to adsorb oxygen molecules. Also ZnCo2O4 nanorods show a comparable electrochemical performance results with FeCo2O4 nanorods, which also is due to a relatively high amount of Co3+ ions.
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