New High Donor Electrolyte for Lithium–Sulfur Batteries

Abstract

The attainable specific energy of Li-S cells is largely affected by the electrolyteto-sulfur (E/S) ratio, with a low value [5] thereof being prerequisite to achieve a competitive energy density for a practical cell. However, the most commonly used electrolyte, a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (i.e., v/v = 1:1), exhibits low Li 2 S 8 solubility of at most 0.7 m at room temperature. [6] This limited solubility constitutes an obstacle in the way of the full-depth utilization of sulfur under so-called lean electrolyte conditions. Although it could be overcome by using a large amount of electrolyte, this would severely impair the volumetric energy density. This notwithstanding, the DOL/DME system and its analogues served as standard electrolyte solvents in the early stages of investigation, because the stable environment they offer to Li metal anodes contributed greatly to enhancing the cycle life. Nevertheless, identifying electrolyte conditions that allow effective operation at low E/S ratios (i.e., below 2 μL electrolyte mg sulfur −1 in practice) is essential for practical cells. [7] Therefore, the development of alternative electrolyte solvents with high polysulfide solubility remains highly desirable. Electrolytes with a high Gutmann donor number, such as N,N-dimethyl acetamide (DMAc), [7a,8] dimethyl sulfoxide (DMSO), [6,7,9] N,N-dimethyl formamide (DMF), [10] and N-methyl-2-pyrrolidone (NMP) [9c] are good candidates for lean electrolyte conditions in Li-S batteries. High donicity affords an environment that promotes interaction with electrophilic cations, implying that the solvation of Li ions can facilitate the solubility of polysulfides. For example, high donor electrolytes were reported [6] to readily dissolve more than 1.6 m of Li 2 S 8. In the same line, the utilization of sulfur can be enhanced for the given amount of electrolyte introduced in the cell. Apart from the solvation capability, high donor electrolytes confer 3D morphology for the final discharging product, namely Li 2 S, contrary to low donor electrolytes that give rise to a 2D film-like morphology. The 3D morphology is beneficial in that it leaves the conductive electrode surface available for repeated dischargecharge over cycling without passivation. [11] In addition, high donor electrolytes activate reaction routes that involve S 3 •− species (Figure 1a), [8a,12] the availability of which represents diverse reaction pathways for discharge to further enhance the utilization of sulfur. Despite these remarkable advantages, high donor electrolytes are known to have a short cycle life mainly because of their catastrophic reactivity with the Li metal anode. [8a,11c] This problem was subsequently pinpointed by Gupta et al., [7a] who

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