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Lithium Ion Diffusion in a Metal–Organic Framework Mediated by an Ionic Liquid
journal contribution
posted on 2015-11-10, 00:00 authored by Kazuyuki Fujie, Ryuichi Ikeda, Kazuya Otsubo, Teppei Yamada, Hiroshi KitagawaMetal–organic
frameworks (MOFs) are desirable host materials to study and control
the dynamics of molecules and ions such as lithium ions. We show the
first study of a lithium ion-doped ionic liquid (IL) incorporated
into a MOF and investigate its phase behavior and ionic conductivity.
Moreover, for the first time, we have studied the dynamics of lithium
ions in the micropores of the MOF in terms of the self-diffusion coefficient
of the lithium ions. The IL was a mixture of EMI-TFSA (1-ethyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)amide) with LiTFSA (lithium bis(trifluoromethylsulfonyl)amide),
and the MOF was ZIF-8 (Zn(MeIM)2; H(MeIM) = 2-methylimidazole).
The TFSA– anions showed a gradual decrease of mobility
in the micropores at low temperatures, which indicates the absence
of the apparent freezing transition. The mobility of the Li+ cations showed a slightly steeper decrease than that of the TFSA– anions at low temperature. The ionic conductivity
of the (EMI0.8Li0.2)TFSA in the micropores was
2 orders of magnitude lower than that of the bulk (EMI0.8Li0.2)TFSA. However, the activation energy for the diffusion
of lithium ions in the micropores of ZIF-8 was comparable with the
bulk (EMI0.8Li0.2)TFSA. These results suggest
that the Li+ cations diffuse through the micropores via
the exchange of the solvating TFSA– anions, similar
to the Grotthuss mechanism in proton conductivity.