Metal-Organic Frameworks As Solid-State Li-Ion Electrolyte

Monday, 27 July 2015
Hall 2 (Scottish Exhibition and Conference Centre)
M. J. Mees (Eindhoven University of Technology/imec), G. Pourtois (imec), A. Stesmans (University of Leuven), S. De Gendt (imec), M. Creatore, W. M. M. Kessels (Eindhoven University of Technology), and P. M. Vereecken (imec)
The use of nanostructured materials can make a significant contribution to the development of solid-state batteries [1-3]. We here focus on a class of nanostructured materials called metal-organic frameworks (MOFs), which exhibits promising properties that potentially meet the requirements of a solid-state Li-ion electrolyte in terms of high ionic conductivity (> 10-4 S/cm) and low electronic leakage. MOFs are hybrid nanoporous solids that result from a reaction of organic ligands and metallic cations to create a three-dimensional controlled skeleton. Fig. 1 illustrates the crystal structure of the so called MIL-121 MOF [4]. The 1D pores of this structure by which the transport of ions is carried out have promising dimensions. Indeed, the large nano-crystalline pores of the MOFs are expected to lead to a high ionic conductivity, as the mobile ions are not sterically hindered inside the pores. The diffusion mechanism in a functionalized MOF is expected to occur in a similar way to the one of an adatom on top of a surface. A general used approach to create a Li+ ion conductive MOF focusses on the inclusion of a lithium salt in the MOF pores. Yanai et al. [5] reported that the incorporation of a complex of polyethylene glycol with LiBF4 into the nanochannels of a Zn-MOF leads to a liquid-like mobility of the Li-ions (activation energy of diffusion equals 0.18 eV). Wiers et al. demonstrates the uptake of lithium isopropoxide (LiOiPr) salt in a Mg-MOF. The salt is electrostatically bound to the open metal centers of this Mg-MOF. An ionic conductivity of 3.1x10-4 S/cm is reported. By a similar approach Ameloot et al. [6] introduced a lithium salt in a Zr-MOF. Here, the lithium salt is chemically bound to the organic ligands of the MOF.

Fig. 1 shows the crystal structure of the MIL-121 which we investigated with first-principles techniques based on density functional theory (DFT). The carboxyl-groups in this structure rotate themselves such that their acidic framework protons (labeled A in Fig. 1) interact with two carboxyl-groups. Consequently, an open-pore structure is obtained. To include lithium in the pores of the MIL-121 the acidic framework protons labeled A in Fig. 1 are substituted by Li+ ions. This idea is similar to the one proposed by Himsl et al. [7]. Only here, the lithium-proton exchange reaction occurs on a carboxyl-group rather than on a hydroxyl one. The higher acidity of the carboxyl-groups with respect to the hydroxyl ones potentially results in a higher lithium concentration. Furthermore, the weak bond between the –COO- anion and the Li+ cation, and the rotational freedom of the carboxyl-group are expected to further improve the ionic conductivity.

We will present the modeling methodology that enables us to simulate Li+ ion conductivity in the MOF pores, followed by a discussion on the obtained results. The results cover the evolution of the structural parameters and the electronic structure of the MIL-121 unit cell for different lithium concentrations. Furthermore, the binding energies of the different acidic framework protons A, B, and C (see Fig. 1) are compared to each other. Finally, the dynamics of the atoms in the unit cell of the MIL-121 is studied for the highest possible lithium concentration in which all acidic framework protons A are substituted by a Li+ ion. The results of this study give us atomistic insight on the behavior of the Li-ions in MOF pores. These results lead to useful suggestion to further develop MOF as solid-state Liion electrolytes. 

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