Quantum-mechanical study on structural evolution of polymeric sulfur cathodes during discharge cycles of Li-S batteries

• Applicant: Dr. Pouya Partovi-Azar
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 420536636

Lithium-sulfur batteries are among the most promising systems for next generation energy-storage devices, thanks to their high theoretical capacity and energy density. However, low sulfur utilization and a poor cycle life are among the issues limiting the commercialization of these batteries. These shortcomings are usually attributed to formation of Li-polysulfides at the sulfur cathode, and their dissolution into electrolyte during discharge (so-called shuttle effect). In recent years, sulfur/carbon copolymers have attracted much attention as cathode materials for Li-S batteries. It has been shown that structural flexibility, functional versatility, and great chemical stability of sulfur/carbon copolymers can overcome the known issues in Li-S batteries corresponding to poor cycling performance. In this project, we aim at elucidating lithiation reactions on polymeric sulfur cathodes and explore the possibility of Li-polysulfide formation. We will focus on two polymeric sulfur cathode materials, namely sulfur/carbon polymer composites based on polyacrylonitrile and sulfur-rich copolymers based on poly(sulfur-random-1,3-diisopropenylbenzene). The central goal is to decipher the lithiation reactions into individual steps in order to have a mechanistic picture of the structural changes of the cathode materials during the discharge cycles and to identify the spatial lithiation patterns. The knowledge on the time-evolution of lithiation patterns enables us to identify local structural features in the proposed polymers which might limit the utilization of sulfur. This ultimately allows to enhance the efficiency of the polymer cathodes by optimizing the cathode morphology. These studies will be performed theoretically using quantum-chemical calculations combined with multiscale modeling and complemented by spectroscopy simulations. By the latter, we aim to form a direct bridge to experiments and, as such, validate the reaction mechanisms predicted by theory. By direct comparison between simulated and measured spectra, we will also be able to associate spectroscopic features with specific lithiated structures.

• DFG Programme: Research Grants