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 compositional 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 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 concentration, 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.