Solubility and dissolution kinetics of sulfur and sulfides in electrolyte solvents for lithium–sulfur and sodium–sulfur batteries

In this study, we monitor the dissolution of sulfur and sulfides in electrolyte solvents for lithium-sulfur (Li-S) and sodium-sulfur (Na-S) batteries. The first aim of this research is to assemble a comprehensive set of data on solubilities and dissolution kinetics that may be used in the simulation of battery cycling. The investigation also offers important insights to address key bottlenecks in the development and commercialization of metal-sulfur batteries, including the incomplete dissolution of sulfur in discharge and insoluble low-order sulfides in charge, the probability of shuttling of soluble polysulfides, and the pausing of the redox reactions in precipitated low order sulfides depending on their degree of solid state. The tested materials include sulfur, lithium sulfides Li2Sx, x = 1, 2, 4, 6, and 8, and sodium sulfides Na2Sx, x = 1, 2, 3, 4, 6, and 8, dissolved in two alternative electrolyte solvents: DOL:DME 1:1 v/v and TEGDME. The determined properties of the solute dissolution in the solvent include saturation concentration, mass transfer coefficient, and diffusion coefficient of the solvent in the solid solute. In general, the DOL:DME system offers high solubility in Li-S batteries and TEGDME offers the highest solubility in Na-S batteries. Low solubility sulfides are Li2S2 and Li2S for the Li-S batteries, and Na2S3, Na2S2, and Na2S for the Na-S batteries. However, it is noted that Na2S3 dissolves fast in TEGDME and also TEGDME diffuses fast into Na2S3, offering the possibility of a swollen Na2S3 structure in which Na+ ions might diffuse and continue the redox reactions in a semisolid state.