Li⁺ Ion-Conducting Sulfonate-Based Neutral Metal–Organic Framework

Abstract

Lithium ion-conducting metal–organic frameworks (MOFs) are rapidly gaining interest because of their potential application as ion-permeable, robust electrode separators in rechargeable batteries, arguably the most ubiquitous portable clean energy storage devices developed to date. A novel, water-stable 2D sheet-like neutral Cu(I)–sulfonate MOF featuring π-acidic naphthalenediimide (NDI) ligands that can simultaneously bind guest lithium ions with its carbonyl and uncoordinated sulfonate oxygen atoms and charge diffuse perchlorate anions through anion−π interaction has been constructed. While the pristine MOF pellets displayed poor intrinsic electrical conductivity (4.65 × 10–10 S/m) at room temperature due to inadequate charge carrier density and electron delocalization pathway, upon infiltration of LiClO4, its ionic conductivity surged almost million times to 2.3 × 10–4 S/m, and the activation energy for charge carrier transport dropped to a mere 0.167 eV. In contrast, the conductivity of Bu4NClO4-treated MOF remained practically unchanged from its original value possibly due to size exclusion and/or facile removal of large uncoordinated Bu4N+ cations, revealing the positive impact of Li+ ion infiltration and binding. Thus, this report presents a rare, if not the first, example of significant lithium ion conductivity of a neutral, practically solvent-free, not post-synthetically modified MOF and offers a new strategy to develop ion-conducting sulfonate MOFs for potential battery applications.