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

We describe the synthesis and properties of Zn-3[(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 M-3(II)[((M4Cl)-Cl-II)(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 sigma = 1.15 x 10(-4) S/cm at room temperature and a relatively low activation energy of E-a = 0.317 eV, enabling potential applications in the emerging field of quasi-solid-state zinc-ion batteries.