Super\nMg²⁺ Conductivity around 10^–3 S cm^–1 Observed in a Porous Metal–Organic\nFramework

Abstract

We first report a\nsolid-state crystalline “Mg²⁺ conductor”\nshowing a superionic conductivity of around 10^–3 S cm^–1 at ambient temperature,\nwhich was obtained using the pores of a metal–organic framework\n(MOF), MIL-101, as ion-conducting pathways. The MOF, MIL-101⊃{Mg­(TFSI)₂}_1.6 (TFSI^– = bis­(trifluoromethanesulfonyl)­imide),\ncontaining Mg²⁺ inside its pores, showed a superionic conductivity\nof 1.9 × 10^–3 S cm^–1 at room\ntemperature (RT) (25 °C) under the optimal guest vapor (MeCN),\nwhich is the highest value among all Mg²⁺-containing crystalline\ncompounds. The Mg²⁺ conductivity in the MOF was estimated\nto be 0.8 × 10^–3 S cm^–1 at\nRT, by determining the transport number of Mg²⁺ (<i>t</i>_Mg²⁺ = 0.41), which is the level\nas high as practical use for secondary battery. Measurements of adsorption\nisotherms, pressure dependence of ionic conductivity, and in situ\nFourier transform infrared measurements revealed that the “super\nMg²⁺ conductivity” is caused by the efficient migration\nof the Mg²⁺ carrier with the help of adsorbed guest molecules.