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

Multivariate Metal-Organic Frameworks (MTV-MOFs) have recently garnered attention as ultrafast lithium ion (Li⁺) conductors due to their mechanical stability, cost-effective processing, and conductive properties. However, their application in full cell or cyclic cell batteries is limited by issues with unstable or conductive solid electrolyte interphases (SEI). In this study, we demonstrate that UiO-66 functionalized with ether-based linker groups can immobilize anions, enhancing the Li⁺ transference number and achieving improved conductivity in cyclic cells, reaching up to 0.23 mS/cm at room temperature. Despite these advances, the molecular mechanisms behind Li⁺ transport in MTV-MOF electrolytes remain poorly understood. Using a combination of atomistic simulations, quantum mechanics, and molecular modeling, we identify three distinct ion-hopping mechanisms—linker-linker hopping, linker-counterion hopping, and counterion-counterion hopping—that drive Li⁺ conduction within the MOF. These mechanisms are influenced by counterion distribution, linker binding strength, and entropic factors such as linker variability. To explore how different ether-based linker topologies affect Li⁺ binding, classical molecular dynamics (MD) simulations were conducted with various MOFs at a fixed ratio of LiTFSI salt, matching experimental conditions. MD simulations revealed a reduced affinity of Li⁺ for the TFSI⁻ anion, favoring interactions with linker groups, while density functional theory (DFT) calculations indicated that specific linker groups exhibit higher Li⁺ binding affinity, providing additional hopping sites. The role of ion channels in Li⁺ transport was further examined in UiO-66, which possesses octahedral and tetrahedral pores critical to conductivity. Our analysis showed that low-conductivity MOFs rely predominantly on counterion-counterion hopping, while high-conductivity MOFs exhibit all three hopping mechanisms. Enhanced sampling techniques such as metadynamics revealed that linker groups offering hopping sites create local minima on the free energy landscape. Our computational findings align with experimental data, highlighting the importance of structure-property relationships in achieving ultrafast Li⁺ conductivity.