Cooperative Proton and Li-ion Conduction in a 2D-Layered MOF via Mechanical Insertion of Lithium Halides

Ionic conduction in highly designable and porous metal-organic frameworks has been explored through the introduction of various ionic species (H⁺ , OH^- , Li⁺ , etc.) using post-synthetic modification such as acid, salt, or ionic liquid incorporation. Here, we report on high ionic conductivity (σ>10^-2  S cm^-1 ) in a two-dimensionally (2D)-layered Ti-dobdc (Ti₂ (Hdobdc)₂ (H₂ dobdc), H₄ dobdc: 2,5-dihydroxyterephthalic acid) via LiX (X=Cl, Br, I) intercalation using mechanical mixing. The anionic species in lithium halide strongly affect the ionic conductivity and durability of conductivity. Solid-state pulsed-field gradient nuclear magnetic resonance (PFG NMR ) verified the high mobility of H⁺ and Li⁺ ions in the temperature range of 300-400 K. In particular, the insertion of Li salts improved the H⁺ mobility above 373 K owing to strong binding with H₂ O. Furthermore, the continuous increase in Li⁺ mobility with temperature contributed to the retention of the overall high ionic conductivity at high temperatures.