Modeling the effect of key cathode design parameters on the electrochemical performance of a lithium-sulfur battery

A 1D model is developed for the Li-S cell to predict the effect of critical cathode design parameterscarbon-to-sulfur (C/S) and electrolyte-to-sulfur (E/S) ratios in the cathodeon the electrochemical performance. Cell voltage at 60% depth of discharge corresponding to the lower voltage plateau is used as a metric for calculating the cell performance. The cathode kinetics in the lower voltage plateau is defined with a single electrochemical reaction; thus, the model has a single apparent kinetic model parameter, the cathode exchange current density (i(0,pe)). The model predicts that cell voltage increases considerably with increasing carbon content until a C/S ratio of 1 is attained, whereas the enhancement in the cell voltage at higher ratios is less obvious. The model can capture the effect of the C/S ratio on the cathode kinetics by expressing the electrochemically active area in the cathode in carbon volume fraction; the C/S ratio in the cathode does not affect i(0,pe) in the model. On the other hand, the electrolyte amount has a significant impact on the kinetic model parameter such that increasing electrolyte amount improves the cell voltage as a result of increasing i(0,pe). Therefore, in the model, i(0,pe) needs to be defined as a function of the electrolyte volume fraction, which is known to have a crucial effect on reaction kinetics.