Cation and Vacancy Engineering Reveals Coexistence of Liquid- and Solid-Like Ion Transport in Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are emerging as unique (pseudo-)solid state electrolytes that enable transport of technologically relevant cations such as Li+, Na+, K+, Mg2+, Zn2+ and Sr2+. As solids, they should be subject to the composition-al and structural engineering approaches that are common to traditional solid-state electrolytes. Yet, they also contain liquid within their pores. As such, cations may benefit from sampling both solid and liquid-like transport regimes. Here, we show that Li+, Na+, K+, Mg2+, Zn2+ and Sr2+ ions are mobile in the cation-exchangeable anionic MOF [Cu3(µ3-OH)(pyrazole-4-carboxylate)3]‒, with conductivity values spanning from 4.3 × 10−6 S/cm for Zn2+ to 1.7 × 10−3 S/cm for K+. Experiments aimed at probing the effects of solvated ion radii, interaction with the host framework, charge carrier con-centration, and vacancy sites on ionic conductivity and activation energy for individual ions demonstrate that, indeed, transport of ions in MOFs should be thought of as a combination of direct interaction with the framework, as well as liquid-like transport in the pores.