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+ (tMg2+ = 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.