Supramolecular and covalent polymer networks for improved stability of lithium-ion batteries [electronic resource]
Keywords: Chemical Engineering.

Lithium ion batteries have become the dominant form of energy storage used in consumer electronics and, recently, electric vehicles. However, high costs have prevented widespread deployment of lithium ion batteries for applications other than portable electronics, and the safety issues associated with traditional materials remain to be addressed. In order to enable the greater utilization of electric vehicles, allow for grid scale energy storage, and meet the demands of new electronic applications, new materials for high energy density batteries must be developed. High capacity electrode materials like silicon and lithium metal have the potential to facilitate these technologies, but significant hurdles must first be overcome. Silicon, due to its brittle nature and large volume expansion during lithiation, has poor cycling stability, and lithium metal suffers from significant side reactions, poor quality deposition, and the potential to form hazardous dendrites. Therefore, strategies to enhance the mechanical and chemical stability of these electrode materials is key to their successful application in commercial batteries. In this thesis, the use of polymeric materials to address issues of stability in lithium ion batteries is explored. The first portion of my research describes how varied crosslinking density in a supramolecular, hydrogen-bonding polymer affects the cycling stability of silicon anode materials. We identified that a balance of viscoelastic stress relaxation and stiffness is required to enable our self-healing electrode concept and maintain the mechanical integrity of the electrode. Following this study, a modified version of the polymer was carefully developed to be used as a coating that enabled the dendrite free deposition of lithium metal anodes. With this new concept, improved lithium deposition was obtained at high current densities, and we observed a cycling Coulombic efficiency of over 97% for more than 180 cycles. Further investigation was carried out to understand how polymer coatings generally affected lithium metal deposition. Specific polymer chemistries were found to influence the shape of Li nuclei, while coating thickness and mechanical properties affected the global deposition properties. The hydrogen bonding, self-healing polymer used in the first portions of my research inspired the design of a new self-healing elastomer that enabled the development of the first high capacity stretchable electrode based on a graphitic carbon/silicon composite material. The composite electrode was stretchable up to 88% strain and delivered a high specific capacity of 719 mAh/g. Further development of this class of tough elastomers led to the creation of a highly resilient lithium ion conducting material. This new electrolyte is able to elastically absorb as much energy as Kevlar based electrolyte materials while conducting lithium ions at the same order of magnitude as commercial separators. A battery made with this material maintains operation after multiple impacts as high as 30 N. Overall, this thesis contributes new materials to be used in electric vehicles, grid scale storage, and new electronic devices, and uses these materials to develop fundamental understanding about how materials properties affect the stability of lithium ion batteries in each application.