Development of high energy density Lithium-sulfur batteries

Abstract

Electrification is rapidly progressing across various industrial sectors, including portable electronic devices, electric vehicles, and grid storage. However, challenges, such as supply chain issues of critical materials (cobalt and nickel) combined with performance demands from emerging applications that exceeds the capabilities of traditional Li-ion battery (LIB) technology, are prompting an urgent need for next-generation energy storage systems. In this regard, Lithium-sulfur (Li-S) batteries are considered as one of the most promising technologies owing to the high theoretical energy density, cost-effectiveness, and earth-abundancy of sulfur. The lightweight nature of sulfur significantly enhances the gravimetric energy density of Li-S batteries, making them particularly suited for applications where battery mass is critical, such as aviation and heavy electric vehicles. In this dissertation, various strategies have been explored to maximize the energy density of the Li-S battery system, with an evaluation of their advantages and limitations. First, replacement of excess Li to establish an anode-free cell configuration with negative-to-positive capacity (N/P) ratio of 1 is investigated. The employment of trithiocarbonates in the system to both stabilize the anode and improve the cathode conductivity, greatly enhanced cell performances are demonstrated. This strategy has proven effective in Na-S systems as well, demonstrating its versatile application. Second, the integration of bi-metallic phosphide (Ni [subscript x] Mo [subscript y] P [subscript z]) catalysts into the cathode is explored to address the challenging delithiation process of Li₂S. The bi-metallic coupling of Ni and Mo optimizes the electronic state, showcasing superior catalytic activity compared to mono-metallic phosphides. The one-step synthesis via carbothermal reduction to produce the Li₂S @ Ni [subscript x] Mo [subscript y] P [subscript z] @ C composite facilitates efficient intermolecular charge transfer between particles, thereby significantly enhancing the cycling performance and capacity of the anode-free cells. Third, strategies to fabricate high sulfur loading cathodes are explored. The use of solvent-free dry-electrode processing with fibrillated polytetrafluoroethylene (PTFE) binder allows for the creation of high sulfur loading cathodes that exhibit exceptional mechanical stability with only 1 wt.% binder content. Systematic comparisons between slurry-processed and dry-processed electrodes demonstrate that dry-electrode maintains mechanical integrity throughout cycling, effectively resisting volume expansion without electrode pulverization. Finally, the investigation into all-solid-state Li-S batteries aims to achieve high gravimetric and volumetric energy density. The study demonstrates that application of Li-argyrodite solid electrolyte could impact cell performance due to the hydroscopic nature and severe decomposition behavior of the material. Additionally, the research confirms that optimized air exposure can misleadingly enhance cell performance, evidenced by multiple spectroscopic techniques. This highlights the complexities and potential pitfalls in developing solid-state battery technologies, emphasizing the need for careful material selection and handling.