Lithium-sulfur batteries: an investigation into the electrolyte and the polysulfide species within

Abstract

The lithium-sulfur (Li-S) battery is one of the most promising candidates in next generation energy storage, offering high theoretical specific capacities through the use of inexpensive and environmentally benign positive electrode materials. However, full commercialisation has been prevented by several technical challenges, most notably polysulfide shuttling. Despite significant scientific interest in recent years, the mechanism of the discharge and charge processes is still poorly understood. Whilst it is known that a variety of different processes occur between electrochemically active species during cycling, the identity of the polysulfides species remains unknown. Additionally, the polysulfide concentrations at different stages of discharge and charge, as well as their thermodynamic and kinetic properties are still poorly understood.

In this project, the significance of cell design on the cycling performance of a Li-S battery is highlighted. The reproducibility of a selected cell design and the effect of lithium nitrate as an electrolyte additive is investigated. To obtain a thorough understanding of the electrolyte system used throughout this report (LiTFSI in DOL), various electrolytes containing different concentrations of the electrolyte salt are prepared and analysed. Using a Walden plot, it is revealed that at high salt concentrations, this electrolyte system begins to exhibit properties similar to that of an ionic liquid. Additionally, employing high salt concentrations improves the cycling performance of the Li-S battery. Two methods have been developed to quantitatively determine the total ‘sulfur’ content of an electrolyte containing polysulfides, as well as its average oxidation state. These techniques have enabled production of the first experimental ternary phase diagram for the Li-S battery. Finally, the galvanostatic intermittent titration technique (GITT) method is quantitatively analysed using a model redox system to assess its ability to determine the diffusion coefficient of the redox system. This study offers a unique assessment of the ability to use GITT to study the mass transport of polysulfide intermediates within a Li-S battery.

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ORCID for Nuria Garcia-Araez: orcid.org/0000-0001-9095-2379

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