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, BTT^3- = 1,3,5-benzenetristetrazolate), a heretofore unknown member of a well-known, extensive family of metal-organic frameworks (MOFs) with the general formula M^II₃[(M^II₄Cl)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 E_a = 0.317 eV, enabling potential applications in the emerging field of quasi-solid-state zinc-ion batteries.