Highly Conductive, Sulfonated, UV-Cross-Linked Separators for Li-S Batteries

Metal (based on Li, Na, Mg, or Al)-sulfur batteries are promising candidates for rechargeable electrochemical energy storage devices capable of high charge storage. However, the success of metal-sulfur battery technology calls for solutions of fundamental problems associated with the inherently complex solution chemistry and interfacial reactivity of sulfur and polysulfide species in commonly used electrolytes. It is understood that the dissolution and shuttling of polysulfides induce rapid capacity degradation, poor cycling stability, and low efficiency of these cells. Herein, we report on the synthesis and transport properties of membranes containing sulfonate groups that are able to rectify transport of polysulfide species in liquid electrolytes. Composed of a cross-linked polyethylene glycol (PEG) framework containing pendant SO32- groups, the membranes facilitate electrolyte wetting and Li+ ion transport, but are highly selective in preventing migration of negatively charged sulfur species (S-n(2-)) dissolved in liquid electrolytes. We argue that the ion rectifying properties originate from two sources, the small tortuous pores originating from cross-linking small PEG molecules and from repulsive electrostatic interactions between the pendant SO32- groups and large migrating S-n(2-) species. Here we demonstrate the effectiveness of these membranes in Li-S batteries and we find that the materials enable high-efficiency (>98%) cycling in LiNO3 additive-free electrolytes. Such membranes are also attractive in other electrochemical cell designs where they serve to decouple transport of positive and negative charged ions in the electrolyte to minimize polarization.