Molecular Insights Shaping the Design of Metal-Organic Framework-Based Electrolytes

Understanding the molecular determinants of Li⁺ transport in quasi-solid-state electrolytes (QSSEs) is critical for the design of next-generation energy storage materials. Here, we investigate Li⁺ conduction in an isoreticular series of anionic metal-organic frameworks (MOFs), Li₃[(Cu₄Cl)₃L₈] (L^3- = linker), to disentangle the effects of pore size and linker functionality on ion conductivity. Contrary to expectation based on previous studies, despite their structural differences, the MOFs show pore size-independent Li⁺ conductivity, whereas the linker functional groups can impose an effect. Using ⁷Li magic-angle-spinning solid-state nuclear magnetic resonance, we identify the major Li⁺ conducting species residing in the open pore channels, which, upon the addition of propylene carbonate, display liquid-like transport with low activation energies. The similar local environments and liquid-like dynamics of these species give rise to conductivities on the order of 10^-5 S/cm across the isoreticular series, though the interactions between tetrazolate nitrogens in linkers and Li⁺ restrict ion mobility, leading to a modest decrease in conductivity. This study provides molecular-level insights into Li⁺ transport in anionic MOF-based electrolytes and thus establishes the design principles for developing efficient QSSEs.