Compared to inorganic and metal- organic frameworks (IFs, MOFs) based on ions of transition metals such as Mn, Fe, Co, and Ni, Cu ion-based ones have been much less studied as cathode materials for Li-ion batteries (LIBs) and gained much less success. The main reason is that for almost all the studied materials, Cu²⁺ and Cu⁺ tend to be reduced to form metallic Cu and thus the frameworks tend to decompose during the discharge steps. We notice that in the vast majority of these compounds, be simple binary compounds, polyanionic (e.g. SO₄^2- and PO₄^3-) salts, or MOFs, Cu ions are coordinated by four to six O atoms. A couple of exceptions to the above statements include the recently developed Cu(2,7-AQDC) (AQDC = Anthraquinone- dicarboxylate)¹ and the K_0.1Cu[Fe(CN)₆]_0.7·3.8H₂O/K_0.1Ni[Fe(CN)₆]_0.7·4.1H₂O core-shell structure.² The Cu²⁺/Cu⁺ are coordinated by four O atoms and six N atoms, respectively and their Cu²⁺/Cu⁺ reversible redox potentials are 3.1 and 3.2 V vs. Li/Li⁺ respectively.

We believe that in Cu(2,7-AQDC) three fused benzene rings in the anthra -quinone group play an important role in stabilizing the Cu²⁺/Cu⁺ redox pair. Therefore, we choose to investigate the electrochemical performances of Cu(im)₂ (imH = imidazole) for three reasons. First, im can form stable compounds with both Cu²⁺ and Cu⁺.^3–4. Second, unlike K_0.1Cu[Fe(CN)₆]_0.7·3.8H₂O, Cu(im)₂ is non-toxic and the Cu-im bond plays a well-established role in biological systems. Third, the CuN₄ coordination settings in all known Cu(im)₂ phases and the aromatic heterocycle of im can help understand and develop N-containing ligands that can stabilize the Cu²⁺/Cu⁺ redox pair.

The pink-phase Cu(im)₂ (p-Cu(im)₂) was prepared according to a published procedure.³ The experimental powder X-ray diffraction (PXRD) pattern match the reported (Fig. 1), indicating the formation of the desired product. Though p-Cu(im)₂ afforded highly reproducible PXRD patterns, together with the broad peak features their complex patterns could not be indexed for exact structural determination.

Despite its unknown structures so far, p-Cu(im)₂ demonstrates interesting electro- chemical performances toward Li⁺ insertion and removal. In the first discharge, p-Cu(im)₂ has a plateau at 2.17 V which should correspond to the reduction of Cu²⁺ into Cu⁺; it delivers a total of 54 mAh/g (Fig. 2a), about 40% of its theoretical capacity (136 mAh/g). In the subsequent first charge, there are a long plateau at ~2.67 V which should correspond to the oxidation of Cu⁺ into Cu²⁺; the potential gap between the discharge and charge plateaus is large (2.17 V vs. 2.67 V). There is also a short semi- plateau at ~3.40 V and the total charge capacity is 79 mAh/g.

To understand the mechanism, ex-situ PXRD analysis (Fig. 2b) was used to track the electrode during cycling. When the electrode was soaked in the cell for 21 hrs. to reach equilibrium, discharged till the end of the discharge plateau, and discharged to 1.8 V, the peak positions and intensities from the framework remained almost unchanged, suggesting the framework remains intact; further there are no apparent formation of new phases. When it was charged back to 3.6 V, the peak intensities from the framework decreased significantly, suggesting the partial decomposition of the framework. It is likely some sort of phase changes and conversion occurs in the semi- plateau at 3.4 V during the first charge. In the subsequent cycling, both the charge and discharge capacities keep decreasing while the potentials of the charge and discharge plateaus are very close. The discharge capacity reaches 19 mAh/g in the fifth cycle.

Though the first discharge capacity is only 40% of its theoretical capacity, p-Cu(im)₂ appears not to decompose in the first discharge, agreeing with the conventional wisdom that im can stabilize both Cu²⁺ and Cu⁺. Ongoing investigations aim at (1) understanding the 40% fulfillment of the theoretical discharge capacity and the large potential gap between the charge and discharge plateaus in the first cycle and (2) finding out the optimal potential window for cycling.

References

1. Zhang, Z. Y.; Yoshikawa, H.; Awaga, K. J. Am. Chem. Soc. 2014, 136, 16112–16115.
2. Asakura, D.; Li, C. H.; Mizuno, Y.; Okubo, M.; Zhou, H. S.; Talham, D. R. J. Am. Chem. Soc. 2013, 135, 2793–2799.
3. Masciocchi, N.; Bruni, S.; Cariati, E.; Cariati, F.; Galli, S.; Sironi, A. Inorg. Chem. 2001, 40, 5897–5905.
4. Huang, X. C.; Zhang, J. P.; Chen, X. M. Cryst. Growth & Des. 2006, 6, 1194–1198.