Electrochemical Impedance Spectroscopy of Li-S Batteries: Effect of Atomic Vanadium- and Cobalt-Modified Ketjen Black-Sulfur Cathode, Sulfur Loading, and Electrolyte-to-Sulfur Ratio

Abstract

The polysulfide shuttle mechanism and insulating characteristics of sulfur and discharge products are the two major drawbacks of Li−S batteries. These increase internal cell resistances, resulting in low battery performance and life. In this study, we investigate the effect of cathode material on the cell resistances by preparing two different cathodes: by encapsulating sulfur (S) with pure Ketjen black (KBS) and with atomic vanadium and cobalt-modified Ketjen black (VCKBS). In addition to the cathode material, the influence of crucial cell design parameters, namely electrolyte-to-sulfur (E/S) ratio and sulfur loading, on the cell resistances and battery performance is also compared. Electrochemical impedance spectroscopy (EIS) is applied to determine the individual cell resistances, whereas a system-level performance model is used to estimate the system-level specific energies and energy densities. The comparison of the cathodes shows that VCKBS significantly improves both cell- and system-level performances, which are attributed to a significant decrease in cell resistances. The cells with higher sulfur loadings and lower E/S ratios show poorer performance for both cathodes. On the other hand, an E/S ratio of 6 mg L−1 can result in high cell- and system-level performances for the VCKBS cathode.