Electrolyte/Electrode Interface Reaction in Polysulfides Insoluble Lithium-Sulfur Battery Using Li-Glyme Solvate Ionic Liquid

Abstract

Lithium-Sulfur (Li-S) battery is attracting attention as a new generation secondary battery. However, one of the major problems is dissolution of discharge products of lithium polysulfides (Li2Sn: n = 1, 2, 4, 6 and 8) that cause the self-discharge. Solvate ionic liquids (SIL), which consist of a solvate ion and its counter ion, are a subclass of ionic liquids. Li-Gn SIL is yielded by equimolar mixing lithium bis-(trifluoromethanesulfonyl)amide (LiTFSA) and glyme, an oligoether such as triglyme and tetraglyme (Gn : CH3O-(CH2CH2O) n -CH3, n = 3, 4). Watanabe and Dokko et al. [1] reported that the Li-glyme SIL had low solubility of the polysulfides, and proposed polysulfides insoluble Li-S batteries using these electrolyte solutions. Recently, a sulfolane-based super-concentrated electrolyte solution (SCES) also exhibits low solubility of the polysulfides and has been applied to Li-S battery [2]. Both of Li-Gn SIL and the SCES are polysulfide insoluble electrolytes, however, the Li-S batteries with the SCES has better battery performance than that with Li-Gn SIL, implying that the different discharge reaction paths. In this contribution, in situ electrochemical impedance spectroscopy (EIS), which can acquire charge-discharge curves and impedance spectra simultaneously, is applied to the Li-S battery using the Li-Gn SIL to obtain further insight into the discharge behavior.A positive electrode slurry was obtained by mixing a sulfur S8, a Ketjen black (KB, EC600JD, Lion Corporation) and a carboxymethyl cellulose (CMC2200, Daicel) at a weight ratio of 60 : 30 : 10. The slurry was spread on an Al foil to obtain a KB-S composite electrode. The typical loading of the sulfur was 1.0 mg cm-1. A negative electrode and a separator were used a lithium metal foil (Honjo Metal) and a glass separator (GA-55, Advantec), respectively. The Li-Gn SIL electrolyte solution was prepared by mixing LiTFSA, G4 and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) as the molar ratio of 1 : 1 : 4. The Li-S batteries were assembled using the CR 2032 coin-type cell in an Ar-filled glove box. In situ EIS measurements were conducted at different C-rate under constant-current discharging/charging condition using an electrochemical measurement system (SP-150, BioLogic). The current values were 336 and 84 μA. The DC current was superimposed with a small AC current adjusted so that the response voltage amplitude did not exceed 10 mV. The measurements were carried out in the frequency range from 500 kHz to 10 mHz.The voltage drop is observed at the DOD = 20-35 %, in the discharge profile measured at the high C-rate (336 μA), whereas that measured at the low C-rate (84 μA) has no voltage drop. The compensation for the changes over time [3,4] was performed on the obtained impedance spectra to determine instantaneous impedance spectra at a specific DOD (depth of discharge). Two capacitive semicircles and a straight line are observed for all of the investigated systems. The semicircle in the high frequency range (< 10 Hz) is asymmetrical, as if two semicircles in the high frequency range from 500 kHz to 1 kHz and in the middle frequency range from 1 kHz to 10 Hz are overlapped. For the high C-rate, the diameter of the semicircle in the middle frequency range increases with increasing DOD and remarkably increases at DOD = 20-35 %. By contrast, in the low C-rate, the diameter does not change during discharging. This semicircle is assigned to the charge transfer resistance of the negative electrode. According to the previous studies [5, 6], when the charge transfer reaction of lithium is faster than the diffusion rate of G4, the solvation structure changes due to the lack of G4 coordinating to Li+. This solvation structure change causes suppression of Li+ elution to increase the charge transfer resistance of the negative electrode. References 1) K. Dokko et al., J. Electrochem. Soc., 160, A1304-A1310 (2013).2) A. Nakanishi et al., J. Phys. Chem. C, 123, 14229-14238 (2019).3) Z. B. Stoynov et al., J. Electroanal. Chem., 183, 133-144 (1985).4) M. Itagaki et al., J. Power Sources, 135, 255-261 (2004).5) N. Serizawa et al., J. Electrochem. Soc., 160(9), A1529–A1533, (2013).6) A. Miki et al., J. Mater. Chem. A, 9, 14700–14709, (2021).