(603c) Electrochmical Lithiation-Delithation of Sulfur in Sub-Nano Confinement

Despite the complexity of lithium–sulfur (Li–S) electrochemical reactions, most of the challenges of Li-S batteries are originated from the polysulfide solution phase reactions. In this study, we investigate a solid-state Li-S reaction in liquid electrolytes enabled by sub-nano confinement of sulfur. We investigate two factors that play decisive roles in Li–S electrochemical processes: the size of the sulfur confinement and the type of electrolyte solvents. To precisely capture the subtle changes in Li–S electrochemical behavior due to the different sulfur confinement size, a series of porous carbon hosts with different pore sizes (1, 2, 2.5, 3 nm) are selected. In addition, two electrolytes, ether-based electrolyte (TEGDME) and carbonate-based electrolyte (EC/DEC) with very different polysulfides solubility are utilized for the study. The results demonstrate a clear correlation between the size of sulfur confinement and the resulting Li–S electrochemical mechanisms. In particular, when sulfur is confined within sub-nano pores, we observe identical lithium–sulfur electrochemical behavior, which is distinctly different from conventional Li–S reactions, in both ether and carbonate electrolytes. We propose that Li ions can only enter the sub-nano pores through desolvation, therefore, the Li-S electrochemical reaction in the sub-nano pores is in solid state. To further study the effect of Li-ion solvation on the Li-S reaction in the sub-nano confinement, electrolytes based on a series of ethereal solvents (dimethoxyethane, diglyme, triglyme, tetraglyme, and 15-crown-5) are studied. With combination of electrochemical analysis, spectroscopic characterization and DFT-based computation, our results revealed the clear effect of solvation energy of different solvent molecules to Li-ion on the Li-S electrochemical reactions in the sub-nano confinement.