FABRICATION OF SILICON-CARBON NANOCOMPOSITES VIA ELECTROSPINNING AND ELECTROSPRAYING PROCESSES FOR THE NEXT GENERATION OF HIGH-RATE LITHIUM-ION BATTERY ANODES
In the recent years, batteries, especially lithium-ion batteries have garnered a lot of attention for the large-scale energy storage solutions and consumer electronics application. Other applications like transportation also have become a fast-growing market for deployment of different type of batteries. Lithium-ion batteries were introduced to the market in the 90s as a safer alternative to the lithium metal batteries and ever since have dominate multiple markets with even more growth projected. In spite of their ubiquity, the state-of-the-art lithium ion batteries have been using the same carbon anode since their conception. A significant improvement in the capacity of these batteries is only realizable by a change in the chemistry of the electrodes. For this reason, a lot of research has been dedicated to identifying the most viable candidates for electrode material. Silicon is a promising choice, due to the large theoretical capacity, abundance and non-toxicity. However, the large volume expansion of silicon that occurs during the lithiation process, fatigues the material and results in pulverization and loss of electric contact of the active material. In addition, the 300% volume change makes passivation of the surface difficult, therefore lithium ions get consumed over each cycle. These two mechanisms cause fast capacity fading of the electrode with cycling. Reducing the size of the active material to nano scale is a powerful remedy for pulverization. At the same time introducing material like carbon that can make a stable passivation layer, and provide electric conductivity that silicon lacks, improves the electrochemical performance of silicon-based anodes. The work that is presented at this dissertation is aiming to develop robust, environmentally friendly and economical methods for electrode fabrication for the next generation of the lithium ion batteries. Electrospinning was studied as a facile technique for electrode fabrication. A novel process was developed to directly deposit nanofibers of PVA-silicon-carbon nanotubes on the surface of the current collector, which resulted in bypassing multiple steps in the conventional electrode fabrication process. The non-woven fiber mat provided ample space for silicon expansion and ease of access for the electrolyte to the active sites, which resulted in great electrochemical performance of the mat especially at higher currents. The directly deposited method allowed for an in-situ comparison between the different conductive agents added in the nanocomposite. It was found that graphene nanoribbons are superior to their precursor carbon nanotubes, due to their edge groups and flexibility even at a lithiated state. In addition, the dimensions of the nanoribbons were significant in their efficacy as conductive pathways, with larger ribbons surpassing small nanoribbons since they can electronically connect individual nanofibers together. An effective use of the void spaces in between the can greatly enhance the electrochemical performance of the nanofiber mat. For this reason, aluminum oxide precursor was spin casted onto the fibers and the solution was cured to get a composite of the metal oxide coating and the fiber mat. The resulting electrode exhibited three-fold enhancement in longevity and significant improvement in high-rate performance compared to the uncoated fibers as well as improving the volumetric energy density. In order to reduce the void spaces, electrospraying was employed to directly spray a porous film of silicon and graphene sheets on the surface of the current collector. To ensure a uniform dispersion of the binder and nanoinclusions a controlled airflow was applied to the sheath layer of the nozzle. The electrochemical performance of these electrodes could further be improved by addition of carbon nanoribbons that can connect the silicon particles that are not directly in contact with graphene sheets. The use of bimodal conductive agent resulted in excellent performance at high currents. The current work investigated modified electrospinning and electrospraying as facile and economic electrode fabrication techniques for the next generation of lithium ion batteries. Methods developed here are versatile and can be applied to a wide array of chemistries and particle geometry and allows for precise control of the nanoinclusions, which is crucial for an enhancement of electrochemical performance.
Graphene Nanoribbons; Graphene Sheet; Silicon Nanocomposite; Energy; Lithium-ion battery; Electrospraying; Chemical engineering; Electrospinning
Joo, Yong L.
Archer, Lynden A.; Suntivich, Jin
Ph.D., Chemical Engineering
Doctor of Philosophy
dissertation or thesis