A growing interest in electrochemical behavior of metal-organic frameworks (MOFs) has emerged in past years due to their potential candidate in electrode materials for energy storage devices.¹ In this presentation, an intercalated MOFs (iMOFs) consisting of p-conjugated dicarboxylate salts, which shows an electrochemically reversible intercalation reaction, is introduced as a novel electrode material for lithium ion batteries^{2, 3} or lithium ion based hybrid capacitors.⁴

As shown in Fig. 1a and b, the proposed iMOF electrode material, 2,6-naphthalene dicarboxylate dilithium (2,6-Naph(COOLi)₂) as a representative iMOF, has a characteristic organic-inorganic layered crystalline structure based on π-stacked naphthalene packing layers and tetrahedral LiO₄ units, respectively, and undergoes a two-electron-transfer Li intercalation reaction (220 mAh g⁻¹) with a flat plateau at a potential of 0.8 V (vs. Li/Li⁺), which is between the potentials of graphite carbon (0.05 V) and Li₄Ti₅O₁₂ (1.55 V). This intermediate operating potential in iMOF electrodes can be expect to overcome both the internal short-circuit risk due to Li dendrite in graphite and the low cell voltage in Li₄Ti₅O₁₂. In addition, the 2,6-Naph(COOLi)₂ electrode undergoes a very small change in unit-cell volume (0.33%) while maintaining the framework during Li intercalation.² We have proposed that the iMOF negative electrodes bring Li-based energy storage devices with new design concepts such as high-voltage bipolar batteries² and high-energy-density hybrid capacitors.⁴

Although these materials have been considered as having poor electronic conductivity, the essential mechanisms of electronic conduction responsible for the electrochemical reversibility remain to be elucidated. Here we will introduce the detailed electronic and ionic conduction mechanism leading to such reversible electrochemical properties of the proposed iMOF electrode materials from theoretical and experimental evidence.⁵

Band structure calculation indicates that electronic conduction occurs in the 2D π-stacking naphthalene layers when the band gap is decreased to 0.99 eV by Li intercalation into the 2,6-Naph(COOLi)₂ framework. And electron hopping conduction mechanism determined on the basis of the Marcus theory shows the formation of an anisotropic electron hopping conduction pathway at Li intercalation state. In the I-V response (Fig. 1c) and frequency characteristics of electronic resistances (Fig. 1d) for Li-intercalated 2,6-Naph(COOLi)₂ (Li₂-2,6-Naph(COOLi)₂) prepared using a chemically reductive lithiation agent exhibits current flow and low electronic resistance, respectively,with sufficiently high electronic conductivity, even though it displays insulating characteristics in the pristine state.

From temperature dependence of in situ powder XRD patterns of Li₂-2,6-Naph(COOLi)₂ for temperatures of up to 400°C, the structure exhibiting electronic conductivity remains stable up to 200°C and reverts to an insulating structure, without collapsing at 400°C, offering the potential for a shutdown switch for battery safety during thermal runaway.

References

(1) Z. Liang, C. Qu, W. Guo, R. Zou and Q. Xu, Adv. Mater., 1702891 (2017).

(2) N. Ogihara, T. Yasuda, Y. Kishida, T. Ohsuna, K. Miyamoto and N. Ohba, Angew. Chem. Int. Ed. Engl., 53, 11467 (2014).

(3) T. Yasuda and N. Ogihara, Chem. Commun., 50, 11565 (2014).

(4) N. Ogihara, Y. Ozawa and O. Hiruta, J. Mater. Chem. A, 4, 3398 (2016).

(5) N. Ogihara, N. Ohba and Y. Kishida, Sci. Adv., 3, e1603103 (2017).