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This dissertation is mainly focused on the investigation of cathode in Li-air batteries using organic electrolyte and the development of high-rate rechargeable Li-air flow batteries. A Li-air battery using organic electrolyte with an air electrode made with a mixture of carbon nanotube (CNT) and carbon nanofiber (CNF) is utilized to investigate the capacity limitation effects of cathode using a multiple-discharge method. Scanning electron microscopy (SEM) images show that the discharge product mainly forms at the air side of cathode due to low oxygen solubility and diffusivity in the organic electrolyte. This inhomogeneous distribution of discharge product indicates that the Li-air cell falls short of the maximum capacity of air electrode. Electrochemical impedance spectra (EIS) demonstrated that during discharge at high current density (1 mA/cm2) pore blocking is the major factor that limits capacity; however, during discharge at low current density (0.2 mA/cm2) both pore blocking and impedance rise contribute to the capacity limitation. It's been confirmed that cathode is the dominant limitation to the discharge capacity. Also, the gradient porosity structure of cathode is able to increase the capacity based on the weight of carbon, but the electrolyte loading needs to be optimized to achieve high energy density of cell. A novel rechargeable Li-air flow battery is demonstrated. It consists of a lithium-ion conducting glass-ceramic membrane sandwiched by a Li-metal anode in organic electrolyte and a carbon nanofoam cathode through which oxygen-saturated aqueous electrolyte flows. It features a flow cell design in which aqueous electrolyte is bubbled with compressed air, and is continuously circulated between the cell and a storage reservoir to supply sufficient oxygen for high power output. It shows high rate capability (5 mA/cm²) and renders a power density of 7.64 mW/cm² at a constant discharge current density of 4 mA/cm². Adding RuO² as a catalyst in the cathode, the battery showed a high round-trip efficiency (ca. 83%), with the overpotentials of 0.67 V between charge and discharge at a current of 1 mA/cm². A Li-air flow battery using graphite as anode is also demonstrated for several cycles.
A Dissertation submitted to the Department of Electrical and Computer Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Bibliography Note
Includes bibliographical references.
Advisory Committee
Jim P. Zheng, Professor Directing Dissertation; Tao Liu, University Representative; Pedro Moss, Committee Member; Petru Andrei, Committee Member.
Publisher
Florida State University
Identifier
FSU_migr_etd-9156
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