(Invited) Analysis of Electrochemical Reaction in Polysulfide-Insoluble Lithium Sulfur Battery with in-Situ Electrochemical Impedance Spectroscopic Method
Authors: Watanabe Hikari , Sugiura Yuto , Shitanda Isao , Itagaki Masayuki 
Published: December 2024

Lithium-Sulfur battery (LiSB) has attracted attention as a new generation secondary battery. However, one of the major problems is dissolution of discharge products of lithium polysulfides (Li2S
n
) that cause the self-discharge. Ionic liquids and super-concentrated electrolyte solution (SCES) have been attracting attention as a LiSB electrolyte due to its low solubility of the polysulfides. SCESs are electrolyte solutions which consist of only two or three times as much solvent as lithium salt. Watanabe and Dokko et al. [1] proposed polysulfides insoluble LiSBs using lithium-glyme based solvate ionic liquid (Li-Gn SIL) and sulfolane (SL)-based SCES. Various battery materials have been proposed for LiSBs; on the other hand, the discharge behavior strongly depends on the battery materials. Both the Li-Gn SIL and SL-based SCES are polysulfide-insoluble electrolytes, whereas the LiSB with the SL-based SCES has better battery performance than that with the Li-Gn SIL, implying that the discharge behavior is different between the LiSB with the SCES and LiSB with the SIL [1]. In this contribution, we carried out in situ electrochemical impedance spectroscopic measurement (EIS) [2,3] to reveal influence of different electrolytes on electrode reactions.

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. A negative electrode and a separator were used a lithium metal foil (Honjo Metal) and a glass separator (GA-55, Advantec), respectively. SL-based SCES and Li-Gn SIL are used as electrolyte solutions. The cells were assembled using three-electrode typed cell in an Ar-filled glove box. In situ EIS measurements were conducted at 0.025~0.1C-rate using an electrochemical measurement system (SP-150e, BioLogic). The DC current was superimposed with a small AC current adjusted so that the response voltage amplitude did not exceed 5~10 mV. The measurements were carried out in the frequency range from 500 kHz to 10 mHz. The measured impedance spectra during discharging/charging include the contribution of changes over time. Accordingly, the instantaneous impedance spectra at an arbitrary time were determined using the method to compensate for the changes over time [2,3].

In discharge curves of the LiSB with Li-Gn SIL, a voltage drop was observed at DOD = 20−35% during discharge at a high C-rate (0.1C). This voltage drop did not appear in the discharge curve at a low C-rate (0.025C). The capacitive semicircle at 457 Hz appeared in the instantaneous impedance spectra of the LiSB during discharging. The diameter of this semicircle increased as the voltage dropped and the semicircle could be assigned to the charge transfer resistance of the lithium negative electrode. The charge transfer resistance of the negative electrode is related to the resistance of the Li+ ion dissolution/deposition reaction. During discharging at a high C-rate, the Li+ ion dissolution reaction could be suppressed.

In the instantaneous impedance spectra of positive electrode in LiSB with SL-based SCES exhibited three capacitive semicircles and an inductive loop at DOD = 5%. Upon further discharging, the inductive loop changed into a capacitive semicircle. The Warburg impedance appeared at the end of the discharge period. This low-frequency impedance behavior was attributed to the Faradaic impedance due to elementary reactions on the electrode, such as intermediate reactions and redox species adsorption/desorption processes. This suggests that multi-step reactions occur in each DOD for the LiSB battery using SL based SCES.

References

1) A. Nakanishi et al., J. Phys. Chem. C, 123, 14229-14238 (2019).

2) Z. B. Stoynov et al., J. Electroanal. Chem., 183, 133-144 (1985).

3) M. Itagaki et al., J. Power Sources, 135, 255-261 (2004).