Characterization of the Effect of Electrolyte-to-Sulfur Ratio on the Lithium-Sulfur Cell Resistance Using Electrochemical Impedance Spectroscopy Method

Abstract

Lithium-Sulfur (Li-S) batteries have gained significant attention in the last decade due to their high theoretical specific energy and non-toxic, naturally abundant and low-cost active material [1, 2]. In Li-S batteries, as a consequence of the complexity of the cathode kinetics, electrochemical performance is highly dependent on the cell design [1, 2]. Such as, electrolyte-to-sulfur (E/S) ratio in the cell is a critical design parameter determining the Li-S cell performance through the reaction and shuttle mechanisms in the cathode [1, 2]. Reducing the electrolyte amount in the cell is crucial for achieving high energy density Li-S batteries [1]. However, sulfur utilization and thus the discharge capacity of the Li-S cell are limited considerably at low E/S ratios. Consequently, identifying the impact of E/S ratio on the kinetic and transport resistances in a Li-S cell will be of significant interest. In this study, we present a characterization of the Li-S battery as a function of E/S ratio in the cell using the Electrochemical Impedance Spectroscopy (EIS) method. EIS, which is a powerful electroanalytical technique [3], is used here to determine the charge transfer and transport resistances in a Li-S cell. The impedance of Li-S cells with varying E/S ratios is measured at different depths of discharge (DOD) for the initial discharge. An equivalent electrical circuit is proposed accordingly and the physical and electrochemical processes occurring in the cell are related to the elements of the circuit. As a result, the impact of E/S ratio on the kinetic and transport mechanisms taking place in the cell during discharge is identified and discussed. Nyquist plots for the Li-S cells present two semicircles, one in the high-frequency and the other in the middle-frequency region, and a straight line in the low-frequency region (Figure 1). It is apparent in Figure 1a that the semicircle in the high-frequency region shrinks with increasing discharge depth while the one in the middle-frequency grows substantially. Furthermore, the impact of E/S ratio on the cell impedance is clear in Figure 1b; both semicircles get smaller with increasing E/S ratio in the cell. In order to link the E/S ratio to the kinetic and transport resistances in the cell, an equivalent electrical circuit is proposed. In the circuit, the resistances for the electrolyte, charge transfer (high-frequency semicircle), Li2S film (medium-frequency semicircle) and Warburg diffusion (low-frequency line) are all considered. It is concluded that all resistances within the cell increase significantly for an E/S ratio of 6 mL/g S especially during the initial and final stages of the discharge. This may be explained such that at low E/S ratios increasing polysulfide concentration increases the electrolyte resistance through the electrolyte viscosity, worsens the cathode kinetics through changing the reaction mechanisms [4] and enhances the Li2S film resistance by controlling the precipitation reactions. Figure 1. Nyquist plots in the frequency range of 10 mHz -100 kHz for the initial discharge of a Li-S cell (a) for different DODs at an E/S ratio of 19 mL/g S and (b) for different E/S ratios at ~50% DOD. Carbon-to-sulfur ratio is 1, S loading is ~1 mg/cm2 and discharge rate is C/10 for all results. Acknowledgment This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK), Grant No: 116M574.