Li⁺ Ion-Conducting Sulfonate-Based Neutral\nMetal–Organic Framework

Abstract

Lithium ion-conducting\nmetal–organic frameworks (MOFs) are\nrapidly gaining interest because of their potential application as\nion-permeable, robust electrode separators in rechargeable batteries,\narguably the most ubiquitous portable clean energy storage devices\ndeveloped to date. A novel, water-stable 2D sheet-like neutral Cu­(I)–sulfonate\nMOF featuring π-acidic naphthalenediimide (NDI) ligands that\ncan simultaneously bind guest lithium ions with its carbonyl and uncoordinated\nsulfonate oxygen atoms and charge diffuse perchlorate anions through\nanion−π interaction has been constructed. While the pristine\nMOF pellets displayed poor intrinsic electrical conductivity (4.65\n× 10^–10 S/m) at room temperature due to inadequate\ncharge carrier density and electron delocalization pathway, upon infiltration\nof LiClO₄, its ionic conductivity surged almost million\ntimes to 2.3 × 10^–4 S/m, and the activation\nenergy for charge carrier transport dropped to a mere 0.167 eV. In\ncontrast, the conductivity of Bu₄NClO₄-treated\nMOF remained practically unchanged from its original value possibly\ndue to size exclusion and/or facile removal of large uncoordinated\nBu₄N⁺ cations, revealing the positive impact\nof Li⁺ ion infiltration and binding. Thus, this report\npresents a rare, if not the first, example of significant lithium\nion conductivity of a neutral, practically solvent-free, not post-synthetically\nmodified MOF and offers a new strategy to develop ion-conducting sulfonate\nMOFs for potential battery applications.