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 7Li 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.