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