Effects of sulfur loading, cathode porosity, and electrolyte amount on Li-S battery performance with solvate ionic liquid electrolyte

Abstract

Recently, electrolytes in which lithium polysulfides (PS) is sparingly soluble, such as solvate ionic liquids (SILs), were proposed and used as electrolytes for lithium-sulfur (Li-S) batteries. Li-S batteries with the SIL electrolyte showed good cycle stability and Coulombic efficiency; however, the actual energy density of preliminary tested cells was much lower than the theoretical value due to the low sulfur loading and excess electrolyte. In this study, we employed Al foam sheets as the cathode current collector in order to three-dimensionally collect the current, which resulted in increased sulfur loading and enabled us to control the cathode porosity by mechanically pressing the Al foam sheets. High sulfur loading on the cathodes (up to 6 mg-sulfur cm−2) was achieved with an initial discharge capacity of 1000 mAh g−1. Simultaneously, in order to reduce the amount of electrolyte, the cathode porosity was controlled; the discharge capacity in the SIL electrolyte was maintained at ~1000 mAh g−1 when the cathode porosity was no less than 30%. At sufficiently high porosity, the discharge capacity was independent of the electrolyte amount and was 1000 mAh g−1 for electrolyte volume/total void volume ratios higher than 3 in a coin cell configuration.