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Condens. Matter 2021, 6, 22 6 of 9 mechanism similar to the one proposed by Barbiellini and Platzman [57] could exist in this case. Figure 2. Intercalation of one to three Li atoms (from left to right) into the MIL-101(Fe) model at B3LYP/def2-TZVP level. Energy values in italics indicate that the corresponding structures are not stable (giving imaginary frequencies). The reported electrochemical potential is calculated with reference to Li anode. The Li, Fe, C, O and H atoms are respectively shown as purple, blue, grey, red and white balls. Li intercalation is accompanied with some changes in the partial charge of the atoms (especially Fe) and variations in the energy of the highest occupied molecular orbital (EHOMO) and the energy of the lowest unoccupied molecular orbital (ELUMO), the Fermi level (EF = EHOMO + (ELUMO − EHOMO)/2 and the band gap (Eg) energies. The changes obtained from natural bond orbital (NBO) analysis are summarized in Table S1 in the SM. Table S1 shows that the intercalation reaction induces a redistribution of charge in the system: the decrease of charge on Fe atoms reflects reduction of Fe+3 to Fe+2. Notably, absolute partial charges are not equivalent to formal oxidation states [58]. However, as the density of states (DOS) plots in Figure S3 show, the changes in orbital energies and atomic charges are consistent with the changes seen in the DOS plots. Furthermore, the differential charge densities (Fukui functions [59]) shown in Figure S4 help to visualize the redox orbitals [14] in the present case. In addition, the NBO results related to the intercalation of one Li atom at the B3LYP/def2-TZVP level show that the intercalation of the Li atom mainly relies on three charge transfer processes from the lone pair (LP, Lewis type) orbital of the bridging O atoms to the anti-bonding LP (LP∗, non-Lewis) orbital of Li (i.e., LP(O) −→ LP∗(Li) charge transfer). This weaker charge transfer is similar to the charge transfer between the O and Fe atoms. LP(Fe) −→ RY∗(O) and LP∗(Fe)−→ RY∗(O) (RY∗: non-Lewis Rydberg type) charge transfer processes are also responsible for stabilizing the O-Fe bonds in MIL-101(Fe). Considering the charges of the atoms, both Fe-O and Li-O bonds are ionic, but the Fe-O bond has a stronger ionic nature while the Li-O bond shows some covalent character. 4. Conclusions We have presented a comprehensive theoretical study that provides insight into the mechanism of lithiation/delithiation and the potential variation in a MOF cathode. DFT simulations including structural distortions offer a quantitative understanding of the changes in total energy induced by the modification of the structure due to electron orbitals of transition metals and ligands. Our first principles approach enables us to gain a molecular-level understanding of the relationship between structural distortion, potential changes and redox orbitals in battery MOFs. Our method can thus elucidate structural- activity relationships of working MOF-based battery materials to improve their perfor- mance. Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: Geometries of the MIL-101(Fe) model at various spin states; Figure S2: Li intercalation energy at different levels of theory; Figure S3: DOS plots; Figure S4: Fukui functions; Table S1: Orbital energies and natural atomic charges of the MIL-101(Fe) model.PDF Image | Electrochemical Potential MIL-101(Fe) as Cathode Material in Li-Ion Batteries
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