Perez Beltran, Saul (2021-07). New Insights on Lithum/Sulfur Battery Materials: A Theoretical Approach From Atomistic Modeling. Doctoral Dissertation.
Thesis
The Lithium-Sulfur (Li-S) battery is a high-energy-density rechargeable battery that promises to be a game-changer for the widespread use of renewable and even nuclear energy sources. The introduction of this electrochemical device is critical in an economy with depleting oil reserves and energy storage devices with limited capacity for high-demanding energy applications. The success of the Li-S battery depends on enhancing the sulfur's low electronic conductivity, eliminating the irreversible loss of active sulfur, lowering the 82% volume expansion after complete sulfur reduction to Li2S, and finding electrolytes to stabilize both electrodes. This task requires a holistic approach to eliminate sulfur loss and stabilize the electrolyte/electrode interfaces. This work focuses on sulfurized carbon (SC) composites derived from facile sulfur-assisted polymer pyrolyzation processes. Li-S batteries based on this promising composite show improved sulfur utilization and longer lifetimes with the potential to fill the gap unfulfilled by current energy storage technologies. However, there are still some issues associated with relatively low S loading, irreversible Li loss between the first and successive cycles, and others that require a detailed atomistic-level understanding of the mechanisms of the electrochemical behavior of this material. The study is carried out using reactive molecular dynamics, density functional theory (DFT), and ab initio molecular dynamics (AIMD) methods. The first part of this work uses ReaxFF and AIMD to address the SC's carbonized skeleton's structural features, the impact of the S-C bonding interactions on the sulfur's aggregation state, and the interfacial SC behavior with a cyclic ether electrolyte common in Li-S batteries. The sulfur's aggregation changes into a distribution of linear sulfur chains that induces an all-in-solid lithiation mechanism with an active electrochemical role of the carbonized backbone. The interfacial SC/solvent interaction study provides further information on the SC's lithiation mechanism and the solvent's deleterious capacity to solubilize the reduction products. The second part of this work focuses on studying via DFT and AIMD the electrochemical behavior specific to the sulfurized polyacrylonitrile (SPAN) composite. This material is of great interest due to the PAN's easiness to dehydrogenate and cyclize into a conjugated backbone with excellent conduction pathways, structural stability, and strong sulfur trapping ability. These calculations provide a complete mechanistic description of the lithiation process with the corresponding mechanical response, testing multiple solvents and lithium salts potential to form a passivation layer protecting the composite's surface in the long term.
