Solvent Effects on Polysulfide Redox Kinetics and Ionic Conductivity in Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have high theoretical energy density and low raw materials cost compared to present lithium-ion batteries and are thus promising for use in electric transportation and other applications. A major obstacle for Li-S batteries is low rate capability, especially at the low electrolyte/sulfur (E/S) ratios required for high energy density. Herein, we investigate several potentially rate-limiting factors for Li-S batteries. We study the ionic conductivity of lithium polysulfide solutions of varying concentration and in different ether-based solvents and their exchange current density on glassy carbon working electrodes. We believe this is the first such investigation of exchange current density for lithium polysulfide in solution. Exchange current densities are measured using both electrochemical impedance spectroscopy and steady-state galvanostatic polarization. In the range of interest (1-8 M S), the ionic conductivity monotonically decreases with increasing sulfur concentration while exchange current density shows a more complicated relationship to sulfur concentration. The electrolyte solvent dramatically affects ionic conductivity and exchange current density. The measured ionic conductivities and exchange current densities are also used to interpret the overpotential and rate capability of polysulfide-nanocarbon suspensions; this analysis demonstrates that ionic conductivity is the rate-limiting property in the solution regime (i.e. between Li2S8 and Li2S4).