Regulation of supercooled liquid sulfur in lithium-sulfur batteries

Abstract

Lithium–sulfur batteries offer high theoretical energy density but remain limited by complex sulfur-conversion reactions and uncontrolled sulfur redistribution. This thesis investigates how electrolyte additives regulate the formation and evolution of supercooled liquid sulfur using a customized in situ optical electrochemical cell. Two mediator systems were examined: lithium iodide (LiI) and reduced isopropylxanthic disulfide (DIP-Red). In the baseline LiTFSI electrolyte, sulfur droplets formed mainly on or near the Pt wire. Adding LiI caused earlier droplet formation, higher visible droplet density, and broader sulfur distribution, supporting an iodine-mediated polysulfide-oxidation pathway. In contrast, DIP-Red did not generate sulfur throughout the electrolyte but promoted post-nucleation droplet growth, displacement, detachment, and redistribution. Raman spectroscopy confirmed that the observed droplets contained elemental sulfur. These results demonstrate that LiI and DIP-Red regulate supercooled liquid sulfur through distinct mechanisms and highlight the value of direct optical visualization for studying dynamic sulfur-conversion processes.