Radical or Not Radical: Revisiting Lithium–Sulfur Electrochemistry in Nonaqueous Electrolytes

Abstract

present in DMSO solutions of lithium polysulfi des (neither S 4 ·− or S 2 ·− were detected). This was also the only radical species observed by Barchasz et al. [ 12 ] upon discharge of a sulfur electrode in tetraethylene glycol dimethyl ether (TEGDME). None of these studies, however, gives a quantitative estimate of the free radical content in the electrolyte, as compared with the various S n 2− dianions. Is the S 3 ·− radical prominent upon cycling in Li-S cells using dimethoxyethane (DME) and 1,3-dioxolane (DOL) solvents? In this event, could its reactivity be held responsible-through electrolyte decomposition-for capacity fading over extended cycling? If not, could the practical capacity of a Li-S cell be augmented by favoring free radicals by tuning the dielectric characteristics of the electrolyte?Herein, we assess the effect of sulfur radical species formed upon cycling of Li-S cells; in particular S 3 ·− . Based on our unequivocal observation of sulfur radicals in a Li-S cell by X-ray absorption near edge structure (XANES)-for the fi rst time under operating conditions-using an EPD solvent (DMA), we show that radicals are not stabilized in glyme-based electrolytes. However, we do show that S 3 ·− reacts with DOL at elevated temperatures, while DME remains intact. In contrast, the much greater dissociation of the anion precursor, S 6 2− , to the trisulfur radical in donor solvents such as DMA and DMSO-where trisulfur is in high concentration but nonreactive-surprisingly and importantly allows the full utilization of both sulfur and Li 2 S. The effective solvation of the latter results in the complete absence of an overpotential on charge. Chemical incompatibility between the lithium metal negative electrode and EPDsolvents can be overcome with anode protection, demonstrating their applicability as electrolytes for the Li-S or Li 2 S batteries in hybrid cells.

Polysulfides Formation in Different Electrolytes from the Perspective of X-ray Absorption Spectroscopy

“…The XANES spectrum of crystalline Li 2 S exhibits the first maximum at sulfur absorption edge at 2473 eV. An additional spectral signature was described in the literature 21 with pre-edge resonances at 2468 eV, corresponding to S 3…”

“…Using a combination of spectral components shown in Figure 1 one can describe mechanism of sulfur conversion in different electrolytes by using a linear combination analysis (LCA). Some other contributions can be identified like formation of radicals, 21 formation of H 2 S or C-S bond due to nucleophilic reaction of polysulfides with solvent molecules.…”

“…sulfonyl functional groups) composing the electrolyte. Nevertheless, different sulfur compounds (sulfur, polysulfides, sulfur radicals, Li 2 S, molecules with C-S bonds, sulfite, and sulfate) formed in the electrochemical cell can be identified by characteristic energy position of the sulfur edge and pre-edge resonances in the range between 2468 eV for sulfur radicals 21 to 2478 eV for sulfate 22 (corresponding to sulfur with the highest oxidation state). As a reference database for polysulfides, different stoichiometric mixtures between sulfur and lithium were synthesized and measured by XAS and 7 Li NMR.…”

Understanding the Charging Mechanism of Lithium-Sulfur Batteries Using Spatially Resolved Operando X-Ray Absorption Spectroscopy

“…9,10 Furthermore, several studies have shown that the exact value of the required overpotential during the first charge of Li 2 S can be manipulated and reduced by varying the charging rate, 8,9,11 cathode morphology and structure, 9,[11][12][13][14][15] and electrolyte composition. 9,[16][17][18][19][20] The results of these recent studies, however, have not yet been incorporated into a general understanding of the charging process of Li-S batteries, and the intermediates of the initial charge of Li 2 S have * Electrochemical Society Student Member. * * Electrochemical Society Member.…”

Operando Characterization of Intermediates Produced in a Lithium-Sulfur Battery

“…Recently, several Li-S reaction intermediates, including polysulfides, 27-29 S 3 •− , 30,31 and Li 2 S solid product, 27,32 have been shown to present unique features in the sulfur K-edge X-ray absorption near edge structure (XANES) spectrum. As a result, sulfur K-edge XANES has the potential to contribute to the understanding of the discharge mechanism in the 4-16 e -/S 8 discharge region, which is known as the low voltage plateau, and facilitate the rational design of improved Li-S batteries.…”