Design, Synthesis, and Characterization of a Water–Stable Li+ ion-Conducting Sulfonate-Based Neutral Metal-Organic Framework

Abstract

The desire for ion permeable electrode separators in rechargeable batteries has led to an increase in interest in lithium ion-conducting metal-organic frameworks (MOFs). A novel, water-stable, 2D sheet-like neutral Cu (I) −sulfonate MOF, could be the first example of significant lithium ion-conductivity in a neutral, practically solvent-free, not post-synthetically modified MOF. The naphthalenediimide (NDI) ligands of this MOF are capable of simultaneously binding guest lithium ions to its carbonyls and uncoordinated sulfonate oxygen atoms, while charge diffuse perchlorate anions coordinate through anion−π interaction. When made into pristine MOF pellets, the intrinsic electrical conductivity was rather poor (4.65x10−10 S/m) at room temperature. This was due to deficient electron delocalization pathways and charge carrier density. Yet, upon the insertion of LiClO4, the ionic conductivity grew by a factor of 106 to 2.3x10−4 S/m and the activation energy for the charge carrier transport to 0.167 eV. To show the impact of the Li+ binding, the MOF was treated with Bu4NClO4, and the conductivity was essentially unchanged from the original value. The lack of conductivity can be attributed to the size exclusion and facile removal of large uncoordinated Bu4N+ cations. Thus, this presents a new strategy to develop ion-conducting sulfonate MOFs for potential battery applications