Unraveling the morphological evolution mechanism of solid sulfur species in lithium-sulfur batteries with operando light microscopy

Solid-liquid phase conversion between various sulfur species in lithium-sulfur (Li-S) batteries is a fundamental reaction of the sulfur cathode. Disclosing the morphological evolution of solid sulfur species upon cycling is of great significance to achieving high energy densities. However, an in-depth investigation of the internal reaction is still lacking. In this work, the evolution process of solid sulfur species on carbon substrates is systematically studied by using an operando light microscope combined with in situ electrochemical impedance spectra technology. The observation of phenomena such as bulk solid sulfur species can form and dissolve independently of the conductive substrates and the transformation of supercooled liquid sulfur to crystalline sulfur. Based on the phenomena mentioned above, a possible mechanism was proposed in which the dissolution reaction of solid sulfur species is a spatially free reaction that involves isotropic physical dissolution, diffusion of molecules, and finally the electrochemical reaction. Correspondingly, the formation of solid sulfur species tends to be a form of crystallization in a saturated solution rather than electrodeposition, as is commonly believed. Our findings offer new insights into the reaction of sulfur cathodes and provide new opportunities to design advanced sulfur cathodes for Li-S batteries.