Carbon Nanofiber Based Electrodes for Li-Air Batteries.

摘要

Driven by the pressure of greenhouse gas emissions, same as the tremendous business prospects of portable electronic devices and electric vehicles, developing new materials for electricity storage systems have been experiencing the non-stop exponential growth1-3. Although having essential chemical challenges and obstacle, Li-air batteries, the so-called "breathing" battery, start to draw significant attention with the attractively-high capacity, which is 5-10 times greater than that of Li-ion batteries4. The essential chemical difficulty lies in the sluggish multi-electron-transfer oxygen reduction and evolution reactions, coupling with complexity of multi-phase systems and inert chemical nature of the reaction products. Consequently, severe polarization, poor reversibility, and low cycling stability become the performance-limiting problems that must be addressed before Li-air batteries succeeding Li-ion batteries. High-surface-area and highly-conductive carbons become the desired material choice for electrodes used in Li-air batteries. Among various carbon materials, carbon nanofiber (CNF) electrodes are particularly promising for use in Li-air cathodes considering their characteristics such as low cost, high surface area, and high conductivity. Thus, this research focused on CNF-based electrodes for Li-air batteries.;In Chapter 3, non-woven porous carbon nanofiber (PCNF) electrodes were fabricated through electrospinning and carbonization of ZnCl2/polyacrylonitrile (PAN) precursors to evaluate the paradox effect of broadening surface area on improving cell reversibility and decreasing electrode conductivity for Li-air batteries. SEM, EDS, and nitrogen adsorption-desorption were used to evaluate the morphology and porosity of the obtained macro/micro-porous structure. The specific surface area of the PCNF electrode made from 30% ZnCl2/PAN precursor was found to be 20 times greater than that of carbon nanofibers (CNFs) made directly from the PAN precursor. Electrochemical performance tests showed the PCNF electrodes had lower capacity but better reversibility compared with CNF electrodes. In pursuing better electrode structure and higher performance of Li-air batteries, it is not always the larger surface area the better. Adjusting the balance between conductivity and surface area to the specific applications is of significant importance to increase the battery performance efficiently.;In Chapter 4, MnOx/CNF composite electrodes were prepared through electrospinning and heat treatment. SEM and STEM coupled with EDS revealed the morphology and chemical structure of the composite electrodes. An X-ray photoelctron spectroscope was deployed to analyze the oxidation state of Mn in MnOx/CNF composites. Compared with CNF electrode, the synthesized MnOx-CNF electrodes showed larger charge capacity and higher columbic efficiency in eight cycles, indicates the catalytic role of MnOx in oxygen evolution reactions. It was hence demonstrated that as a binder-free and free-standing catalyst/supporting carbon mat electrode, the MnOx -CNF nanocomposites offered a promising material electrode candidate for Li-air cathodes.;In Chapter 5, different MnO2/CNF electrodes with MnO 2 nanofibers anchored on the CNF surface or MnO2 nanoparticles loaded on the CNF surface were obtained through low-current and high-current electrodeposition (LCD and HCD), respectively. The morphology of CNF and deposited MnO 2/CNF was evaluated by SEM. Chemical composition and oxidation state of Mn in MnO2/CNF was analyzed by STEM/EDS and XPS. Through electrochemical analysis, both MnO2/CNF electrodes possessed much higher discharge capacity, better reversibility, and more cycling stability than regular CNF electrode. Compared with the nanoparticle/nanofiber structured HCD-MnO 2/CNF composite, the fiber-anchored-on-fiber structured LCD-MnO 2/CNF composite showed even better electrochemical performance. This illustrated the importance of catalyst morphology for Li-air batteries. This binder-free fiber-anchored-on-fiber structured MnO2/CNF electrode is a promising candidate for catalyst/carbon-matrix Li-air cathode.