Lithium Ion Diffusion in a Metal-Organic Framework Mediated by an Ionic Liquid

Metal-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 (EMI_0.8Li_0.2)TFSA in the micropores was 2 orders of magnitude lower than that of the bulk (EMI_0.8Li_0.2)TFSA. However, the activation energy for the diffusion of lithium ions in the micropores of ZIF-8 was comparable with the bulk (EMI_0.8Li_0.2)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.