Abstract
Systematic understanding of how solvent property influences Li-S redox chemistry is required to develop an effective electrolyte for Li-S batteries. In this study, we investigate the correlation between solvent property and Li-S redox chemistry in nine non-aqueous electrolyte solvents that cover a wide range of three main solvent physiochemical properties, namely dielectric constant (ɛ), Gutmann donor number (DN), and acceptor number (AN). We exploit various analytical techniques including cyclic voltammetry, rotating ring disk electrode technique, UV-Vis spectroscopy and galvanostatic measurement in a two-compartment cell. We show that the potential of S8-reduction increases with increasing AN and that the polysulfide-reduction/oxidation is strongly influenced by the DN. The common discrepancy in the literature on the role of dielectric constant and donor number is addressed by examining the redox reactions, polysulfide stability, and the effect of salt concentration in acetonitrile - a solvent with high dielectric constant and low DN. We show that the DN is the primary descriptor for polysulfide redox reactions, as it controls the effective charge density of the solvated cation (Li+), which affects the stability of polysulfides with different charge density via Pearson's Hard Soft Acid Base theory.
Highlights
Li-ion batteries, lithium-sulfur (Li-S) batteries have been intensively researched due to their high theoretical capacity (≈1675 mAh/gsulfur), high natural abundance, and non-toxicity of elemental sulfur.[1,2,3,4,5,6] The reversible conversion between elemental sulfur (S8) and lithium sulfide (Li2S) in a Li-S battery is normally associated with a series of intermediates, namely polysulfides (Li2Sn, 2 ≤ n ≤ 8), which are believed to be soluble in common organic solvents and in the state-of-the-art glyme-based electrolyte mixture, i.e. 1,2-dimethoxyethane (DME):1,3-dioxolane (DOL) (1:1, v:v).[1]
Group 1: cyclic voltammetry (CV) recorded in DMSO, DMF, and DMA largely resemble each other and are consistent with the CVs reported in literature (DMSO,[15,19] DMF,[20] DMA),[19] having three oxidation peaks and two reduction peaks with large peak separation ( E > 500 mV, see Figs. 1a–1c, CVs in blue)
We study the correlation between solvent property and Li-S redox chemistry in nine nonaqueous electrolyte solvents that cover a wide range of dielectric constant (ε), donor number (DN) and acceptor number (AN)
Summary
Li-ion batteries, lithium-sulfur (Li-S) batteries have been intensively researched due to their high theoretical capacity (≈1675 mAh/gsulfur), high natural abundance, and non-toxicity of elemental sulfur.[1,2,3,4,5,6] The reversible conversion (described in Reaction 1) between elemental sulfur (S8) and lithium sulfide (Li2S) in a Li-S battery is normally associated with a series of intermediates, namely polysulfides (Li2Sn, 2 ≤ n ≤ 8), which are believed to be soluble in common organic solvents and in the state-of-the-art glyme-based electrolyte mixture, i.e. 1,2-dimethoxyethane (DME):1,3-dioxolane (DOL) (1:1, v:v).[1]. We note that TEGDME with lower DN of 16.6 has a higher absorption at 620 nm compared to DME (DN ≈ 20 or 24), which can be explained by 1) the chelate effect, the known cage structure formed by TEGDME that can better solvate Li+ cation[33,34] and 2) that donor number, an empirical parameter derived from the solvation of SbCl5, cannot fully represent the solvation of Li+.35 The distinct speciation of polysulfides in different solvents are consistent with the literature and can be well explained by the Pearsons Hard Soft Acid Base (HSAB) theory.[36] The Li ions solvated in high-DN solvents (strongly solvated Li+, soft acid) preferentially stabilize polysulfides with lower charge density
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