Dependence of Impedance Response of Li-S Battery on Polysulfide Solubility of Electrolyte I - Initial Stage of Discharge

Abstract

Lithium-sulfur (Li-S) battery has higher theoretical specific capacity than commercial lithium-ion batteries. However, one of serious problems in sulfur cathode to be solved to realize a commercial Li-S battery is polysulfide dissolution, causing lowering charge-discharge capacity and round-trip efficiency. 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 initial stage of discharge in Li-S batteries. For cathode and electrolyte, X-ray absorption near edge structure (XANES) was also performed in order to analyze the valence of sulfur species at various depth of discharge.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 a frequency range of 1 kHz – 10 Hz; in both cases, semicircle decrease as discharge progression in the same manner, while decrease of the semicircles with DOL/DME was more notable. In addition, another semicircle in a frequency range of 10 Hz - 1 Hz was observed only in the Nyquist plots with SIL electrolyte. These results indicate that the semicircles in higher frequency region (1 kHz – 10 Hz) are attributed to dissolved polysulfide and the semicircle in lower frequency region (10 Hz – 1 Hz) is attributed to undissolved polysulfide. XANES measurement confirms that signal by sulfur (2472 eV) decreased until potential reached 2.1 V with DOL/DME and until 1.5 V with SIL. These results indicate that reduction of sulfur proceeds until 2.1 V with DOL/DME and until 1.5 V with SIL which is in good agreement with the result from impedance response by EIS. Acknowledgement This 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) Kaoru Dokko et al., Journal of the Electrochemical Society, Volume 160, Issue 8, A1304-A1310 (2013).(2) Natsuki Nakamura et al., J Power Sources, 274, 1263-1266 (2015). Figure 1