From molecules to interfaces: a multiscale study of conjugated polymers in lithium-sulphur battery cathodes

Lithium-sulphur (Li/S) batteries are a promising next-generation energy storage technology because sulphur is inexpensive and widely available. However, they still face three major challenges that limit their practical use. First, sulphur and its lithium-based reaction products are poor electrical conductors. Second, some of the sulphur reaction byproducts dissolve into the electrolyte and diffuse away, causing a loss of active material. Finally, the interface between the sulphur cathode and the liquid electrolyte often becomes unstable during battery operation. To address these issues, researchers have developed sulphur-based polymer cathodes that hold sulphur within a conductive framework to improve conductivity and prevent material loss. Yet, it remains poorly understood how such polymer cathodes interact with the liquid electrolyte at the atomistic level, even though these interactions are crucial for long-term stability and efficiency. This thesis aims to fill that gap by examining the molecular interactions between a polymer cathode, PTBT (poly(4-(thiophen-3-yl)benzenethiol)), and a typical Li/S battery electrolyte. The electrolyte consists of a mixture of DME (1, 2-dimethoxyethane) and DOL (1, 3-dioxolane) solvents with the lithium salt LiTFSI (lithium bis(trifluoromethanesulfonyl)imide). We examine how DME/DOL solvent molecules and LiTFSI ions organise and bind around PTBT, and how these interactions alter the polymer conformation and entanglement in various solvent mixtures and salt concentrations. To address these questions, we combine density-functional theory (DFT) calculations with large-scale molecular dynamics (MD) simulations. DFT is used to characterise the polymer’s electronic structure and to develop an accurate simulation model (force field) for the polymer. Notably, this model incorporates a cation-πinteraction term, implemented through ion-induced dipole corrections, to capture Li+ binding to the polymer’s aromatic rings (an effect often missing in standard fixed-charge force fields). Our simulations reveal several key findings. First, in a mixed DME/DOL solvent, the polymer chain swells (adopts a more expanded conformation) more than in either solvent alone. Further, we found that cyclic DOL solvent has a stronger affinity for the polymer than the linear DME. Second, when LiTFSI salt is present, Li+ ions tend to bind with the polymer’s aromatic rings. This cation-πinteraction stabilises Li+ near the cathode and prevents the polymer chains from separating in the solvent, thereby keeping the polymer cathode more compact. Finally, we found that the polarity and size of the solvent molecules determine how deeply the electrolyte components penetrate the polymer. These solvent properties thus influence the thickness of the interface layer at the cathode surface. In conclusion, the stability and structure of the polymer cathode-electrolyte interface are significantly influenced by the solvent’s characteristics and specific ion-polymer interactions at the atomistic level. By elucidating how a sulphur-based polymer cathode behaves in different electrolyte environments, this work provides guidelines for designing more stable and efficient Li/S battery cathodes. The insights and methodology from this study can also be applied to other sulphur- rich cathode materials, such as organosulphur polymers and covalent organic frameworks.