Lithium-sulfur (Li-S) batteries are a very promising technology to achieve high-energy rechargeable batteries thanks to its high specific capacity (1672 mAh g^-1). Sulfur is an affordable material because of its abundance on the planet and its availability as a waste product of petroleum refinement^[1]. However, the discharge mechanism of a conventional Li-S battery with liquid electrolyte based on the dissolution of lithium polysulfides in the electrolyte leads to numerous issues that impede the performance of the cells: the polysulfide shuttle effect and the corrosion of the lithium negative electrode are two major hurdles faced by Li-S technology. Most importantly, this conventional mechanism requires large volumes of electrolyte to dissolve the active material of the cathode. This results in a high electrolyte to sulfur (E/S) ratio, in which limits the energy density. To achieve a lower E/S ratio, one solution consists in decoupling the electrochemical mechanism from the amount of electrolyte needed.^[2] To do so, some research teams have tuned the electrolyte or the cathode structure to prevent polysulfide dissolution and invoke a so-called “quasi-solid” (QS) reaction mechanism, which limits the dissolution of long polysulfide chains. ^[3] In order to obtain this QS mechanism, one can modify the nature of the electrolyte (e.g. sparingly solvating electrolytes)  or the structure of the cathode (e.g. confinement of the sulfur in carbon nanotubes).^[4]

Initially, we started by fabricating state-of-the-art Li-S coin cells using a standard electrolyte (1.0 M LiTFSI and 0.25 M LiNO₃ in DOL:DME (v:v/1:1)). Although they were unoptimized, our batteries showed promising results (800 mAh cm^-1 after 50 cycles, 2.80 ± 0.16 mg_s cm^-1). These batteries will serve as a foundational benchmark throughout the remainder of the thesis, providing a basis for comparison with future results obtained using QS mechanism batteries. The choice of electrolyte is crucial as it significantly influences the discharge mechanism. Therefore, we also initiated a screening of various electrolytes with limited polysulfide solubility, by measuring their ionic conductivity. Additional experiments will be conducted, such as polysulfide solubility and cycling performance.  

To reduce the environmental impact of our battery cathode, we are planning to use waste rubber from used tires as a starting material.^[5] Tire rubber already contains the main components of a sulfur cathode: sulfur, carbon and a polymeric binder, but in proportions that are not suitable for a battery. One objective of this work will be to increase the sulfur content in the composition of the waste rubber to make it a viable cathode for lithium-sulfur cells. The initial phase of this thesis involved a comprehensive analysis of tire samples (elemental analysis, XRD, FTIR-ATR, ICP-MS). The strategy involves employing inverse vulcanization of sulfur with rubber, aiming to impede sulfur dissolution in the process. Our most advanced results will be shown.