Elucidating Transport Mechanisms of a Lithium Salt in the MOF UiO-66

The confinement of Li-based salts in metal−organic frameworks (MOFs) has attracted attention as a pathway for developing Li-ion conducting solid electrolytes. Nevertheless, optimization of these systems remains a challenge due to the complex environment that Li-ions encounter as they migrate through the salt-filled pores of the MOF. The present study aims to clarify the atomistic mechanisms governing the migration of a Li-based salt, LiTFSI, within the benchmark MOF UiO-66. The impact of LiTFSI loading, electric-field direction, and temperature on the transport behavior of Li-ions and TFSI anions is investigated by calculating the ions’ drift velocities and transference numbers. The Li+ drift velocity increases with increasing LiTFSI loading and varies by up to three orders of magnitude across low to high-loading scenarios. Enhanced mobility at high loadings arises in part from the higher density of TFSI anions, which provide shorter Li+ hopping distances between favorable anion/MOF sites. Compared to Li+, the drift velocities of the TFSI anions are slower overall and less sensitive to salt loading. At low salt loadings, TFSI migration is responsible for the majority of the (limited) ionic current. This trend is reversed at medium-to-high salt loadings, where Li+ migration dominates. Differences in drift velocities between Li+ and TFSI− are rationalized in terms of ion size effects. A Helmholtz free energy analysis reveals that at low loadings, Li+ is trapped at the MOF nodes. At higher loadings, favorable Li+ sites are distributed uniformly throughout the MOF pore space, mimicking the distribution of TFSI− anions. The migration of Li+ and TFSI− occurs via repeated trapping and escape events, where occasional long-range hops are interrupted by quiescent periods where the ions are immobile. At high loadings, migration occurs via the simultaneous (correlated) motion of Li+ and TFSI−. In contrast, at low loadings, the limited migration activity that exists does not involve correlated motion.