Effects of Polysulfide Solubility and Li Ion Transport on Li–S Batteries Using Sparingly Solvating Electrolytes

Abstract

Sparingly solvating electrolytes are an emerging class of electrolytes used in Li–S batteries. In this type of electrolytes, polysulfide dissolution and shuttling can be suppressed, leading to high Coulombic efficiency and cycle life. To optimize the electrolytes for high energy density Li–S cells, effects of polysulfide solubility and Li ion transport properties on Li–S battery performance were investigated for tetraglyme (G4)- and sulfolane (SL)-based electrolytes diluted with a hydrofluoroether (HFE). The Li2S8 solubility was found to be very low (1 mM in atomic S concentration) in [Li(G4)0.8][TFSA]-4.3HFE and [Li(SL)2][TFSA]-4.0HFE. Cells with [Li(SL)2][TFSA]-4.0HFE exhibited better rate capability compared with cells with [Li(G4)0.8][TFSA]-4.3HFE despite lower ionic conductivity of [Li(SL)2][TFSA]-4.0HFE. With regard to the Li ion coordination structure in [Li(SL)2][TFSA]-4.0HFE, the two oxygen atoms of the SL sulfonyl group coordinates to two different neighboring Li ions and TFSA anions form ionic clusters with Li ions. The unique SL- and anion-bridged, chain-like Li ion coordination structure was considered to be involved in the unusual Li ion self-diffusion behavior and Li ion tranport for [Li(SL)2][TFSA]-4.0HFE. The higher Li ion transference number measured under anion-blocking condition (t Li +) of [Li(SL)2][TFSA]-4.0HFE was suggested to contribute to the better rate performance, rather than the polysulfide solubility and ionic conductivity. Furthermore, [Li(SL)2][TFSA]-4.0HFE demonstrated an initial discharge capacity of 1130 mAh g−1 at a low electrolyte volume to sulfur weight ratio of 4, whereas a typical organic electrolyte failed to achieve such a high capacity owing to limitations of the redox mechanism mediated by dissolved polysulfides. In addition to the low solubility of polysulfides, the high t Li + is crucial for achieving high energy density Li–S batteries by reducing the electrolyte amount. The SL-based electrolytes were found to manifest a significant improvement in Li ion mass transfer as a sparingly solvating electrolyte, enabling the solid-state sulfur redox reactions in high-performance Li-S batteries. Acknowledgement This study was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST). References 1) A. Nakanishi. et al, J. Phys. Chem. C, 2019, 123, 14229.2) M. Yanagi et al, J. Electrochem. Soc., 2020, 167, 070531. Figure 1