Tuning the ion conductivity of Zr-based metal–organic framework ionogels by linker functionalization

Abstract

Solid-state ion conductors have improved significantly in the past decade. Envisioned applications range from metal anode batteries and fuel cells to sensors and medical technologies. The choice of solid electrolytes is, however, relatively limited and it remains challenging to find new systems that can be tuned and processed easily. Here, we investigate solid ion conductors based on the UiO-66 metal–organic framework (MOF) structure. Three synergistic approaches are used for the realization of MOF-based solid ion conductors: post-synthetic modifications by lithium alkoxide grafting on the [Zr₆O₄(OH)₄]¹²⁺ cluster site, the use of alternate linkers with additional functionality and the incorporation of an ionic liquid in the porous structure that leads to the formation of an ionogel. Depending on the exact configuration and composition used, the overall ionic conductivity can be improved by up to 6 orders of magnitude. Our results show that MOF-based ion conductors are complex systems in which several ions, such as Li⁺, protons and ionic liquids species, are the mobile charge carriers. Establishing the ionic species responsible for conduction constitutes one of the aims of this investigation. For instance, high total ionic conductivities of up to 0.5 mS cm⁻¹ at 293 K are recorded, with protons likely having a major contribution. Thus, our work demonstrates the versatility of the MOF-based solid ion conductors and opens the path towards advanced electrochemical applications.