Dependence of Impedance Response of Li-S Battery on Polysulfide Solubility of Electrolyte II - Mid - End Stage of Discharge

Abstract

Sulfur is a promising cathode material for the lithium based battery because of its advantages such as eco-friendness, abundance in nature, and high theoretical capacity of 1675 mAh g-1. However, sulfur cathodes are not without their own intrinsic problems such as low ionic and electronic conductivity, large volumetric expansion upon lithiation from S8 to Li2S, and the dissolution of so-called polysulfide intermediates (Li2Sx where x represents a value between 4 and 8). In previous studies, glyme-Li+ solvent ionic liquid (SIL)1) and polypyrrole film containing ionic liquid2) were proposed to mitigate dissolution of polysulfide. Meanwhile, dependence of sulfur cathode’s reaction mechanism on polysulfide solubility of electrolyte is still unclear. In this study, electrochemical impedance spectroscopy (EIS) at various depth of discharge was performed in two types of lithium-sulfur batteries using electrolytes with different polysulfide solubility. This paper describes that polysulfide solubility in electrolyte largely influences the impedance response at mid - end stage of discharge in Li-S batteries. Figure 1 shows discharge curves of Li-S cells with polysulfide soluble electrolyte (1M LiTFSI in 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME) 1:1 (v/v)) and with polysulfide insoluble SIL electrolyte (Li(G3)TFSI/hydrofluoroether (HFE) 1:4 (v/v)). Figure 2 (a) and (b) show Nyquist plots obtained by the cells using DOL/DME and SIL, respectively. A remarkable difference was observed in the diameter of semicircles in lower frequency range of 10Hz – 0.1 Hz at potential of 2.1 - 1.5 V. The semicircle was observed with SIL at 2.1 V, and the diameter of the semicircle increased with discharge progression. No semicircle was observed at 1.9-1.5V. On the other hand, no semicircle with DOL/DME was observed at 2.1 - 1.5 V. However, impedance response at 2.1 V changed to that at 1.9 - 1.5V. The difference at 2.1V was attributed to DOD and/or state of polysulfide in electrolyte. There was no large difference of impedance response with these electrolytes at 1.9 V -1.5 V. It is thought that similar (semi-) solid-state reactions without electrolyte was occurred, because of no effect of solving polysulfide of Li2S2 and Li2S in both electrolytes.AcknowledgementThis work was partly supported by Advanced Low Carbon Technology Research, Development Program Special Priority Research Area “Next-Generation Rechargeable Battery” (ALCA-Spring) from the Japan Science and Technology Agency (JST) (Grant Number JPMJAL1301).Reference(1) K. Dokko et al., J. Electrochem. Soc., 160, A1304-A1310 (2013).(2) N. Nakamura et al., J Power Sources, 274, 1263-1266 (2015). Figure 1