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 porous carbon hosts with narrow range of pore sizes is selected: resorcinol-formaldehyde derived porous carbon fibers with four different pore sizes (denoted as CF10, CF15, CF20, and CF25) were purchased from Kuraray Chemical Co., Ltd. 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.
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