Lithium-sulfur (Li-S) batteries are a promising technology to reach a target of 500-700 Wh kg^-1 as a replacement for current commercial Li-ion batteries in a number of sectors. They also offer lower environmental impact with reduced costs for the cathode materials. However, the level of currently achievable performance is still below expectations. This is due to a lack of understanding of the fundamental electrochemical processes and mitigating the undesired reactions during Li-S battery cycling. In particular the so-called redox shuttle effect, which arises from the solubility of polysulfides (PSs, Li₂S_x) species, and their potential to reduce capacity retention of the battery. Modifying electrolytes with additives and new solvents change the solubility of these intermediates and potentially improve the cycling performance of the battery.

Herein, the effects of using LiNO₃ as an additive as well as C₄mpyr-based ionic liquid electrolyte on the performance of Li-S cells are analysed using electrochemical, in situ X-ray powder diffraction (XRD) and ex situ soft X-ray absorption spectroscopy (sXAS) techniques. Whilst both LiNO₃ and C₄mpyr-based IL participate in forming a protective stable SEI layer on the lithium anode, our studies have provided further evidence for the suppression of Li₂S deposition on the electrodes when using LiNO₃ salt as an additive in the electrolyte, leading to higher capacity and better capacity retention compared to the additive-free electrolyte. In addition, based on XRD data, different species of Li₂S_x were detected during cycling of cells with organic and IL-based electrolytes, which indicates that different (electro)chemical reactions occur in these environments. Overall, cells with electrolytes containing ionic liquids and LiNO₃ showed more stable cycling.