Transferring Lithium Ions in the Nanochannels of Flexible Metal-Organic Frameworks Presenting Superchaotropic Metallacarborane Guests: Mechanism of Ionic Conductivity at Atomic Resolution.

Metal-organic frameworks (MOFs), owing to their unique architecture, attract consistent attention in the design of high-performance Li-battery materials. Here, we report a new category of ion-conducting crystalline materials for all-solid-state electrolytes based on the MIL53(Al) framework featuring superchaotropic metallacarborane (Li+CoD-) salt and present the first quantitative data on Li+ ion sites, local dynamics, chemical exchange, and the formation of charge-transfer pathways. We used multinuclear ss-NMR spectroscopy to examine the mechanism of ionic conductivity at atomic resolution and to elucidate order-disorder processes, framework-ion interactions and framework breathing during the loading of Li+CoD- species and transfer of Li+ ions. In this way, the MIL53(Al)@LiCoD framework was found to adopt an open-pore conformation accompanied by a minor fraction of narrow-pore channels. The inserted Li+ ions have two states (free and bound), which both exhibit extensive motion. Both types of Li+ ions form mutually communicating chains, which are large enough to enable efficient long-range charge transfer and macroscopic conductivity. The superchaotropic anions undergo high-amplitude uniaxial rotation motions supporting the transfer of Li+ cations along them, while the fluctuations of MOF aromatic linkers support the penetration of Li+ through the channel walls. Our findings provide detailed atomic-resolution insight into the mechanism of ionic conductivity and thus have significant implications for the design of the next generation of energy-related materials.