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

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 Li⁺ ion electrolytes. 

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