Structure-Activity Relationships in Ether Functionalized, Solid-State Metal-Organic Framework Electrolytes

The structure-property relationships of metal-organic framework (MOF) based solid-state electrolytes are not well understood. Herein, a systematic investigation of twelve Zr(IV)-based UiO-66 MOFs with varying ether-chain functional groups was carried out to elucidate the critical microscopic interactions that facilitate improved solid-state electrolyte performance. Enhanced sampling molecular dynamics (MD) simulations were employed and revealed a three-tier ion hopping mechanism: linker-linker hopping, linker-counterion hopping, and counterion-counterion hopping. Detailed structural analysis of the MD trajectories revealed that the chemistry and morphology of the linker groups affects the relative stability and population distribution of the electrolyte components, such that crown-ether based linker groups enhances the probability of extended, low-barrier ion percolation pathways. As a result, we were able to tune the ion conductivities by means of rationally manipulating the counterion distributions, linker binding strengths, and the configurational entropy (multi-variability of the linkers). The resulting performance of these MOF-based solid-state electrolytes was significantly enhanced, with a methoxy-functionalized framework (UiO-66-L1-100) achieving high ionic conductivities of 2.32 × 10^-4 S/cm and 2.07 × 10^-3 S/cm at 30 °C and 90 °C, respectively, a magnitude greater than other all-solid-state MOF electrolyte systems. The electrolyte stability was evaluated with LiIn|LPSCl|MOF:LiTFSI|LPSCl|LiIn symmetric cells, showing excellent Li plating/stripping processes for over 2 months.