Balancing polysulfide containment and energy loss in lithium-sulfur batteries

Integration of microporous membranes in lithium-sulfur (Li-S) batteries is a promising strategy for preventing capacity losses induced by the shuttling of soluble polysulfide species. However, microporous membranes also hinder the transport of lithium ions decreasing the available cell energy density. Here, a detailed experimental and numerical investigation of the energy losses induced by representative membrane candidates for Li-S batteries is reported as a function of their composition and pore size. Using Li|Li symmetric cells, we determine that widely used membrane candidates, such as MoS2 and graphene oxide, exhibit substantially higher overpotentials (>350 mV at 0.5C) than commercial PP separators. These overpotentials result in a significant loss (similar to 80%) of the energy density of Li-S cells. Insight on the mechanism underlying the buildup of the overpotential is revealed by comparative modeling and characterization of pristine and holey(/mesoporous) graphene membranes, quantifying the key role played by the membrane on lithium-ion permeability. These findings demonstrate the need and provide guidance for balancing non-metallic species confinement and metal-ion transport for a broad subset of emerging rechargeable battery systems, including metal-sulfur and metal-halogen systems.