A Solid Zn-Ion Conductor from an All-Zinc Metal–Organic Framework Replete with Mobile Zn2+ Cations

We describe the synthesis and properties of Zn3[(Zn4Cl)3(BTT)8]2 (ZnZnBTT, BTT3- = 1,3,5-benzenetristetrazolate), a heretofore unknown member of a well-known, extensive family of metal-organic frameworks (MOFs) with the general formula MII3[(MII4Cl)3(BTT)8]2, which adopts an anionic, sodalite-like structure. As with previous members in this family, ZnZnBTT presents two crystallographically distinct metal cations: a skeletal Zn2+ site, fixed within Zn4Cl(tetrazole)8 secondary building units (SBUs), and a charge-balancing Zn2+ site. Self-assembly of ZnZnBTT from its building blocks has remained elusive; instead, we show that ZnZnBTT is readily accessed by quantitative postsynthetic exchange of all Mn2+ ions in MnMnBTT with zinc. We further demonstrate that ZnZnBTT is a promising Zn-ion conductor owing to the mobile charge-balancing extra-framework Zn2+ cations. The new material displays a Zn-ion conductivity of σ = 1.15 × 10-4 S/cm at room temperature and a relatively low activation energy of Ea = 0.317 eV, enabling potential applications in the emerging field of quasi-solid-state zinc-ion batteries.