Imparting Electrochemical Functionality into Extended Solids for Next-Generation Batteries

Abstract

As global energy storage demands grow exponentially, exploitation of fossil fuels grows correspondingly leading to dangerous levels of anthropogenic climate change. These global trends have necessitated developing new forms of renewable energy storage, and lithium-ion batteries have been pioneers in making that an attainable reality. However, lithium-ion batteries are approaching their theoretical capacity, leading to an impending crisis in which energy storage demands may not be met. To meet that demand, we focus on lithium-sulfur batteries as a promising contender for next-generation batteries. Compared to lithium-ion batteries, lithium-sulfur batteries offer over 4 times the amount of charge by mass and almost twice the amount of charge by volume. Moreover, sulfur is naturally abundant, making it a highly attractive material for battery technology. Limiting their implementation, however, is the polysulfide shuttle, a phenomenon in which the intermediate polysulfides are lost to the electrolyte during battery cycling leading to eventual battery failure. Additionally, the elemental sulfur at the cathode is insulating, an inherent issue in a device reliant on the movement of electrons. To improve battery performance, we employ a materials chemistry approach to improve both charge transfer and limit the polysulfide shuttle. We begin by using metal-organic frameworks (MOFs) which are porous, tunable materials composed of metal nodes and organic linkers which come together form 1D, 2D, and 3D nets. MOFs can also be modified post-synthetically for greater functionality. However, MOFs are often insulating, so, to improve charge transfer inherent to MOFs, we design a Zr-based MOF with a molecular oligosilane to instill new charge donor abilities. To interrupt the polysulfide shuttle, we post-synthetically modify another Zr-based MOF with thiophosphates to tether polysulfides and, in turn, improve battery performance. We then apply these findings to carbon nanotubes as both a more commercially available material and to better elucidate the role of the thiophosphate group and porosity on battery performance. These works offer a fundamental approach to practical implementation of next-generation batteries. Through a deeper understanding of charge transfer and the design of better cathode materials, we observe widespread improvements in battery performance, making a fossil-free future one step closer to reality