Electrochemical Impedance Spectroscopy Study of Lithium–Sulfur Batteries : Effects of Electrolyte/Sulfur Ratios

Lithium–sulfur (Li–S) batteries are promising beyond lithium-ion battery technology. However, in order to compete with lithium-ion cells on a specific energy (Wh/kg) basis, it is critical to decrease the E/S ratio to a smaller value (≤ 11 μl/mg) [1]. The typical ether-based liquid electrolytes are known to have high polysulfides solubility which promote the electrochemical reaction. A low E/S ratio, which have polysulfide concentration close to or exceeding the maximum polysulfides solubility of the electrolyte, will however cause limited capacity and slow kinetics [2]. Electrochemical impedance spectroscopy (EIS) is a powerful tool for exploring properties of electrode/electrolyte interfaces [3]. Here, we carried out the EIS measurements on Li–S cell with a specially designed three-electrode configuration (Figure 1). This new configuration enables the separation of cathode and anode impedance contributions in a complete Li–S cell. In this study, impedance information of the electrolyte, electrode and their interfaces were collected at different depths of discharge (DOD) in the first cycle. Due to the electrodeposition of polysulfides, the electrolyte resistance decreases after reaching a maximum value and, simultaneously, interfacial resistance become more noticeable. In order to understand the effect of E/S ratio on the properties of electrolyte and interfaces, electrodes of three ratio values are being tested and compared. It is found that both interfacial and electrolyte resistances increase with decreasing E/S ratio. The rate-determining step during discharge process, which has the highest resistance, has been proposed. Reducing the E/S ratio significantly enlarges the electrode polarization, leading to insufficient utilization of sulfur and low output potential. The results measured from our newly developed three-electrode configuration can provide valuable information about the electrochemistry between electrolyte and interfaces, which are important for high energy density Li–S batteries design.