UC San Diego

Metal-organic framework (MOF)-based membranes have received significant attention as separators for lithium-sulfur (Li-S) batteries due to their high porosities, well-defined and tailored structures, and other tunable features that are desirable for preventing the ‘shuttle effect’ of soluble polysulfides. Due to the insulating properties of most MOFs, composite membranes generally consist of a combination of MOFs and electrically conductive materials. In this study, we examined the property-performance relation between MOF-based separators by systematically adjusting the electrical conductivity, thickness and mass loading of MOF-based separators. Beyond the commonly referenced trapping or blocking ability of MOFs toward polysulfides, we find that by fixing the thickness of the MOF-based composite coating layer (~ 40 μm) on a Celgard membrane, the electrical conductivity of the MOF composite layer is of paramount importance to the cycling performance compared with the physical/chemical trapping ability of polysulfides. However, the trapping ability of MOFs becomes essential when the thickness of the composite layer is small (e.g., ~ 20 μm), indicating the synergetic effects of the adsorption and conversion capabilities of the thin composite layer. This work suggests the importance of a holistic design consideration for MOF-based membrane for long-life and high energy density Li-S batteries.