Super Mg²⁺ Conductivity around 10^-3 S cm^-1 Observed in a Porous Metal-Organic Framework

We first report a solid-state crystalline "Mg²⁺ conductor" showing a superionic conductivity of around 10^-3 S cm^-1 at ambient temperature, which was obtained using the pores of a metal-organic framework (MOF), MIL-101, as ion-conducting pathways. The MOF, MIL-101⊃{Mg(TFSI)₂}_1.6 (TFSI^- = bis(trifluoromethanesulfonyl)imide), containing Mg²⁺ inside its pores, showed a superionic conductivity of 1.9 × 10^-3 S cm^-1 at room temperature (RT) (25 °C) under the optimal guest vapor (MeCN), which is the highest value among all Mg²⁺-containing crystalline compounds. The Mg²⁺ conductivity in the MOF was estimated to be 0.8 × 10^-3 S cm^-1 at RT, by determining the transport number of Mg²⁺ (t_Mg = 0.41), which is the level as high as practical use for secondary battery. Measurements of adsorption isotherms, pressure dependence of ionic conductivity, and in situ Fourier transform infrared measurements revealed that the "super Mg²⁺ conductivity" is caused by the efficient migration of the Mg²⁺ carrier with the help of adsorbed guest molecules.