Modeling of sulfur based polymer cathodes for Li-S batteries : structural analysis and Raman characterization

Abstract

The optimization of lithium-sulfur batteries highly depends on the exploration of novel cathode materials. This thesis focuses on the development of sulfur/carbon co-polymers as a promising class of cathodes to replace crystalline sulfur. These co-polymers offer a flexible atomic structure and a potential for high reversible capacity. In particular, we dive into the investigation of poly(sulfur-n-1,3-diisopropenylbenzene) (S/DIB). Our exploration begins with a comprehensive analysis of the atomic structure of sulfur-n-1,3-diisopropenylbenzene co-polymers, using density-functional theory calculations. The primary goal was understanding the local structural properties, with a focus on identifying the optimal sulfur chain length (Sn with n=1...8) bridging two DIB units. Our findings reveal a preference for shorter sulfur chains n=4 in DIB-Sn-DIB co-polymers. Subsequently, we complement our findings with ab initio Raman spectroscopy simulations and experimental Raman measurements. This combined approach facilitates the identification and characterization of various sulfur/carbon co-polymers with distinct sulfur contents. We demonstrate that S/DIB co-polymers featuring short and long sulfur chains exhibit distinguishable Raman activity in the 400-500 cm-1 range, providing crucial insights into their structural composition. Significantly, the results presented herein apply to the fully charged state of the cathode. Furthermore, we extend our investigation to explain the discharge state of the battery, focusing on the transformation of sulfur co-polymer cathode materials upon lithiation. Specifically, we explore how sulfur chains evolve during lithiation and perform the same ab initio Raman spectroscopy methodology for their characterization.