Ultralean Electrolyte Li-S Battery by Avoiding Gelation Catastrophe

Due to the poor electronic conductivity of solid sulfur and sulfides, the dissolution of S^α- (α = 0, 2/8, 2/6, 2/4) into a liquid electrolyte and the vehicular diffusion of S^α- to carbon black are necessary for the electrochemical activity of a sulfur cathode in lithium-sulfur (Li-S) batteries. However, exactly how much dissolution and diffusion are required for high sulfur utilization and how this may control the minimum electrolyte/sulfur ratio, (E/S)_min, have not been quantitatively settled. In this work, we show experimentally that a dissolved polysulfide concentration which is too high (>10-20 MS) may gel the liquid electrolyte, leading to catastrophic loss of S^α- mobility, a failure mode that is especially susceptible in a high-donor-number (DN) electrolyte under a lean condition (low E/S), similar to a traffic jam, resulting in high electrochemical polarization and low sulfur utilization. In contrast, we show that a low-DN electrolyte, even with a low polysulfide solubility of 0.1-0.5 MS, will never encounter a gelation catastrophe even at extremely low E/S, leading to unprecedentedly high energy density. Specifically, high sulfur utilizations of 96% (1600 mAh g^-1) and 78% (1300 mAh g^-1) are reached in an electrolyte as lean as E/S = 2 and 1 μL mg^-1 Li-S coin cells when DME_1.6LiFSI-HFE of low solvation capability (DN = 13.9) is adopted, even paired against a high-sulfur-loading cathode (5 mg cm^-2).