(30k) Threshold and Excess Electrolyte-to-Sulfur (E/S) Ratios in Lean-Electrolyte Li-S Batteries

Lithium-sulfur (Li-S) batteries have been recognized as one of the most promising next-generation alternatives to Li-ion batteries (LIB). Specifically, lean-electrolyte Li-S batteries are receiving more attention for lower dead weights, and thus higher overall energy densities. It was calculated that the electrolyte-to-sulfur (E/S) ratio should be < 5uL/mg for Li-S batteries to be comparable with the state-of-art commercial LIB. Many efforts have been put into the development of lean-electrolyte Li-S batteries from all aspects (cathode, anode, separator, current collector...) and have achieved great improvements. However, most articles focused on one fixed condition of E/S ratio and sulfur loading, and few have performed a predictive analysis of Li-S battery performance in a wide range of both E/S ratios and sulfur loadings.

In this study, we analyzed the initial discharge capacity and cycling stability of Li-S batteries at E/S ratios of 2~30 uL/mg, from which we proposed a quantitative correlation. Interestingly, at lean electrolyte conditions < 10uL/mg, we found discharge capacity not proportional to E/S ratio, as expected from the polysulfide solubility. At each mass loading, there exists a threshold E/S ratio for the battery to function properly, possibly due to wetting of separator and electrode. We also determined the “excessive” E/S ratio signifying the “flooding” conditions, above which the capacity is no longer limited by amount of electrolyte. Both threshold and excess E/S ratios were found to be dependent on sulfur mass loadings, which were determined experimentally at 1~5 mg/cm². Our study was also extended to pouch cells to further approach the industrial standard, during which we found the same trend. We demonstrate our results can provide some insights into the design of lean-electrolyte high-sulfur-loading Li-S batteries which can push Li-S batteries further into practical commercialization.