(348i) Solid State Lithiation and Delithiation of Sulfur in Liquid Electrolytes: A New Concept for Lithium-Sulfur Batteries

Rechargeable lithium-sulfur (Li-S) batteries have attracted tremendous research efforts in the past decade. In spite of the impressive improvements of performance, a fundamental question of both scientific and technological interests remains: is it possible to restrict the electroactive sulfur-containing species in solid state during the Li-S electrochemical reaction? To answer this question, we investigate in this study two factors that are believed to play decisive roles in the Li-S electrochemical processes: the size of the sulfur confinement and the type of electrolyte solvent. To precisely capture the possibly subtle changes of Li-S electrochemical behaviors due to the different sulfur confinement size, a series of resorcinol-formaldehyde derived porous carbon fibers with four different pore sizes were used in this study. Besides carbon hosts with different pore size, two specific electrolyte solvent systems are selected for this study: The first one is tetraglyme (TEGDME), which is a typical solvent for conventional Li-S batteries. The other solvent is a mixture of ethylene carbonate and diethyl carbonate (EC/DEC) with 1:1 volume ratio, which is a typical solvent for Li-ion batteries and does not work for Li-S batteries. A major difference between these two electrolytes is their solubility of lithium polysulfides, particularly for the high order ones. TEGDME is a much superior solvent for lithium polysulfides comparing to EC/DEC. Through systematical investigation, we proposed a new mechanism of solid-state Li-S electrochemical reaction in liquid electrolytes enabled by sub-nano confinement of sulfur. The results demonstrate unambiguous transition of electrochemical behaviors from superficial sulfur to sub-nano confined sulfur, and from sub-nano confined sulfur to sulfur in relatively larger confinements. It is clearly demonstrated that the lithiation and delithiation of sulfur in sub-nano confinement is thermodynamically different from the conventional liquid phase Li-S reactions. As the result, both ether-based electrolyte and carbonate-based electrolyte are viable for Li-S electrochemical reaction in the sub-nano confinement since the solid-state mechanism does not involve or require lithium polysulfides dissolution. Comparing to the conventional liquid phase Li-S electrochemical reactions, this proposed solid-state mechanism has the benefit of simplicity, which can provide a new paradigm for future Li-S battery materials design and synthesis.