Composite sulfur–carbon electrodes were prepared by encapsulating sulfur into the micropores of highly disordered microporous carbon AC1 with micrometer-sized particles and and carbonized polyacrylonitrile (PAN) cloth. We demonstrated that in bis(fluorosulfonyl)imide (FSI) based ionic liquid and in organic carbonates based electrolyte solutions the performance of the obtained electrodes is governed by the formation of solid electrolyte interphase (SEI) during the initial discharge. In both types of the electrolyte solutions the formation of SEI starts at potentials lower than 1.5V vs. Li/Li⁺ (Fig.1). We attribute the process of the formation of SEI to the reaction of lithium polysulfides with FSI anions or organic carbonates, respectively. Subsequent galvanostatic cycling is characterized by one plateau voltage profile specific to quasi-solid-state reaction of Li ions with sulfur encapsulated in the micropores in solvent deficient conditions. The stability of the SEI thus formed, is critically important for the effective desolvation of Li ions participating in quasi-solid-state reactions.

We proved that realization of the quasi-solid-state mechanism is controlled not only by the porous structure of the carbon host but also by the nature of the electrolyte solution composition and the discharge cut off voltage value. We showed that the FEC-based electrolyte solution demonstrates improved capacity retention during charge–discharge cycling of S/C composite electrodes compared to the EC-based electrolyte. The obtained results are very promising for the development of SLS batteries with FEC-based electrolyte solution which is in progress.

 The cycling behavior of these cathodes is highly dependent on sulfur loading. Despite cracking of the particles observed due to a large volume expansion of sulfur during the lithiation, the best performance and initial coulombic efficiency at room temperature in FSI-based IL electrolyte solutions were demonstrated by S–AC1 composite electrodes with a sulfur loading of 60 wt%. This sulfur loading prevents the access of electrolyte solution molecules to the micropores, and the quasi-solid-state reaction of Li ions with encapsulated sulfur occurs in a solvent free environment. This insight into the mechanism of operating of S–C composite electrodes provides a new approach to the development of new electrolyte solutions and additives for Li–S cells.