Characterization of the effect of critical design parameters on the electrochemical performance of a lithium-sulfur battery

Lithium-sulfur (Li-S) batteries provides high theoretical specific energy and energy density and their performance is greatly sensitive to cell design as a result of the highly complex reaction and polysulfide shuttle mechanisms within the cathode. Electrolyte-to-sulfur (E/S) ratio, carbon-to-sulfur (C/S) ratio, sulfur loading and carbon type are vital design parameters with a critical influence on the battery performance. Here, an integrated research methodology coupling experimental characterization and electrochemical modeling was applied to forecast the relation between the key design parameters and the discharge capacity, cycling performance and cell- and system-level specific energy and energy density of the Li-S battery. Firstly, the effect of the E/S ratio was examined; the highest initial discharge capacity was achieved with an E/S ratio of 20 μl mg−1, whereas, the best capacity retention was observed for 13 μl mg−1. Consequently, an E/S ratio of 13 μl mg−1 presented the best performance as the impact of the E/S ratio not only on the peak discharge capacity and capacity retention but also on cell- and system-level performance were considered. Secondly, the influence of the C/S ratio was investigated; the Li-S cell having a C/S ratio of 2 and an E/S ratio of 13 μl mg-1 has provided the highest initial capacity in addition to the best capacity retention. Model predictions suggested that increasing the C/S ratio worsens the battery metrics at the pack level at low E/S ratios. Finally, Li-S cells with different carbon type and sulfur loading were studied. The capacity retention of Li-S cells with AB (Acetylene Black) was unaffected by the S loading, but Li-S cells with Super C65 retain capacity at higher S loadings. Li-S cells with KB (Ketjen Black) were unable to attain good performance at higher S loadings, which was surprising given their significantly larger surface area. Super C65 was projected to have the best pack performance. At medium S loadings, when discharge capacities are maximized, Li-S cells with both AB and Super C65 cathodes attain the greatest system-level metrics.