Cations Mediate Lithium Polysulfide Adsorption in Metal-Organic Frameworks for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are one promising alternative to Li-ion batteries due to their higher theoretical specific capacity and energy density. However, several technical challenges such as polysulfide shuttling remain. As liquid polysulfide diffusion into the electrolyte causes a loss of capacity, different material classes have been explored to anchor lithium polysulfides and reduce the active material loss. The metal-organic framework (MOF) UiO-66 has been identified as one candidate material due to its porosity, high surface area, and zirconium oxide nodes that could anchor liquid polysulfides. MOFs also allow for postsynthetic modifications that can increase their adsorption specificity toward liquid polysulfides and reduce shuttling. In this work, we combined atomistic simulations and experimental characterization to probe the molecular interactions between lithium polysulfides and functionalized UiO-66 nodes. We explored how lithium polysulfides adsorb to open sites caused by missing linker defects as well as sites functionalized with alkali cations. Our results demonstrate that lithium polysulfides adsorb favorably to UiO-66 through Li-O electrostatic interactions. In addition, we found that nodes functionalized with alkali metals demonstrated stronger adsorption of long-chain lithium polysulfides (Li2S4-8) by facilitating charge transfer to the nodes. Experimental ultraviolet-visible and Li-7 NMR measurements on Zr polyoxometalates and UiO-66 provided further evidence that lithiation favors adsorption of long-chain polysulfides. Our findings indicate that UiO-66 functionalization plays an important role in polysulfide adsorption, which may have implications in controlling the shuttle effect. The fundamental insights into polysulfide adsorption shown here provide quantitative principles to design functionalized moieties and further inhibit polysulfide shuttling.