Characterization of the effect of critical cathode design parameters on the lithium sulfur cell resistance using electrochemical impedance spectroscopy

Lithium-sulfur batteries have gained significant attention as an alternative for Li-ion batteries for electric vehicle applications in the recent years. This is due to their high theoretical specific energy (2600 Wh/kg) coming from one of the most abundant and non-toxic element, sulfur and the lightest metal lithium. On the other hand, Li-S batteries have some drawbacks, which are Li anode degradation, polysulfide shuttle mechanism and low electronic conductivity; these all cause low specific capacity and poor cyclability. Hence, Li-S cells suffer from some transport and kinetic resistances. In this study, the effect of the electrolyte to sulfur ratio (E/S) and the carbon to sulfur ratio (C/S), which are two critical cathode design parameters, on the transport and kinetic resistances in a Li-S cell are investigated as a function of degree of discharge in the first cycle by using electrochemical impedance spectroscopy. First, the main resistances in the cell are identified as the electrolyte, charge transfer, film and Warburg resistances. The effect of E/S ratio on the cell resistance is investigated for Li-S cells having E/S ratios of 34 µl/mg, 19 µl/mg, 12 µl/mg, 6 µl/mg and 3 µl/mg at a C/S ratio of 1. For these cells, the highest resistances are obtained with 3 µl/mg and 6 µl/mg E/S ratios for all of the resistances. Furthermore, the lowest resistances are obtained for the cell with E/S=19 µl/mg and further increase of the E/S ratio do not reduce the resistances. Secondly, the effect of C/S ratio on the cell resistance is investigated for Li-S cells with the C/S ratios of 3.5, 2.0, 1.0, 0.5 and 0.3 having 19 µl/mg E/S ratio. The cell with a C/S ratio of 3.5 has considerably higher resistances than the other C/S ratios for all the three resistances whereas the lowest resistances are obtained with C/S=1; lower C/S ratios result in higher resistances. These results showed that, there is an optimum point for both E/S and C/S ratios, where the cells deliver minimum transport and kinetic resistances.