In Situ Impedance Spectra Analysis of Lithium-Sulfur Battery Using Sulfolane-Based Super-Concentrated Electrolyte Solution

Abstract

Lithium-Sulfur (Li-S) battery has attracted attention as a new generation secondary battery. However, the Li-S battery has some problems toward its realization, one of which is the dissolution of discharge products of lithium polysulfides. Recently, Watanabe and Dokko et al. [1,2] reported that a solvate ionic liquid and a sulfolane-based super-concentrated electrolyte solution had low solubility of the polysulfides, and proposed polysulfides insoluble Li-S batteries using these electrolyte solutions. The discharge reaction paths of the Li-S batteries are currently unclear due to the strong dependence on the electrolyte solution In this contribution, in situ electrochemical impedance spectroscopy (EIS) was applied to the Li-S battery using the sulfolane-based super-concentrated electrolyte solution to obtain further insight into the reaction paths during the discharge. A positive electrode slurry was obtained by mixing a sulfur S8, a Ketjen black (KB) and a carboxymethyl cellulose (CMC2200) 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 2.0 mg cm-1. A negative electrode and a separator were used a lithium metal foil and a glass filter (GA-50, Advantec), respectively. The sulfolane (SL)-based super-concentrated electrolyte solution was prepared by dissolving lithium bis-(trifluoromethanesulfonyl)amide (LiTFSA) into the SL as the molar ratio of 1 : 2. 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) was added to the SL-based electrolyte solution as a diluent and finally the molar ratio of LiTFSA : SL : HFE was prepared at 1 : 2 : 2. The Li-S battery was assembled using the CR 2032 coin-type cell in an Ar-filled glove box. The galvanostatic charge-discharge measurement was carried out at the C-rate of 1/12 C. The measurement was stopped at the SOC (state of charge) = 50 % in the second charge step. Then, after relaxing to an equilibrium state, the in situ EIS measurements were performed using an electrochemical system (HZ-7000) in the frequency range from 100 mHz to 100 kHz. The in situ EIS measurement was carried out while charging at the C-rate of 1/8 C for 30 minutes. Afterward, the discharge process was measured under the same condition after keeping open circuit potential for 5 minutes. The amplitude of the AC current was determined to make the AC potential response 10 mV. In the charge-discharge curves for the Li-S battery using the SL-based electrolyte solution, the first and the second plateaus appeared in the range of 16 to 90 mAhg-1 and 180 to 700 mAhg-1, respectively. The charging curve during the in situ EIS measurement was almost identical to the charging curves during the 1st and the 2nd cycles. On the other hand, the discharging curve during the in situ EIS measurement was quite different from one in the steady state of discharging. That is, it took time to achieve the steady state of the discharge. Instantaneous impedance spectra [3, 4] were obtained with compensating the time of the measured impedance spectra. Two capacitive semicircles were observed in the instantaneous impedance spectra. The semicircle appeared in the low frequency region was attributable to the charge transfer resistance and constant phase element related to double-layer capacitance. Curve fittings using an equivalent circuit were performed on the instantaneous impedance spectra. Although the charge transfer resistance was almost constant during the charge, it remarkably increased during the discharge. This suggests that resistances for an increase of Li ion concentration at the positive electrode/electrolyte interface and the Li ion insertion to the active material are quite large. These resistances probably cause to take much time to reach the steady state of discharge. 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).