Cathode materials in currently commercialized rechargeable Li-ion batteries (LIBs) use transition metal ions such as Mn, Fe, Co, and Ni as the redox centres. Such electrode materials include LiMn₂O₄, LiFePO₄, LiNi_0.8Co_0.15O₂, and LiCoO₂.¹ As the market for LIBs has expanded exponentially, the needs for such species have increased significantly despite their continuously dwindling natural reserves, leading to the cost increases for the electrode materials. Therefore, the search for alternative cathode materials using earth-abundant and nontoxic Cu ions has been going on for decades albeit with limited success possibly mainly due to two reasons. First, Cu²⁺/Cu⁺ tend to get reduced to metallic Cu, resulting in the framework collapses and cycling instability. Second, many Cu²⁺/Cu⁺-based inorganic frameworks tend to have their electrochemical potentials in the anode ranges. Few successful exceptions include Cu(2,7-AQDC) (AQDC = Anthraquinone -dicarboxylate)² and a core-shell structure with K_0.1Cu[Fe(CN)₆]_0.7·3.8H₂O as the core.³ However, the previous case uses a large ligand with a large molecular mass, leading to a lower theoretical capacity and the inclusion of solvent molecules into the framework and slow Li-ion diffusion in cycling. The latter uses toxic CN^- ligand and the core needs to be stabilized by the K_0.1Ni[Fe(CN)₆]_0.7·4.1H₂O shell. The Cu²⁺/ Cu⁺ in these two cases are coordinated by four O atoms and five N atoms, respectively.
Here we report the evaluation of a metal-organic framework, namely HNa[Cu₂ (Ac)₆] (HAc = Acetic acid) as a nontoxic cathode material for LIBs. A hydrothermal reaction between Cu(Ac)₂ and NaAc at 105 ºC for 24 h gave deep blue single crystals of HNa[Cu₂(Ac)₆] that were selected for single crystal structural analysis.
HNa[Cu₂(Ac)₆] contains discrete binuclear paddle-wheel [Cu₂(Ac)₆]^2- anions (Fig. 1a). In each [Cu₂(Ac)₆]^2-, four acetate (i.e. Ac^-) groups connect two Cu²⁺ as paddles with two additional Ac^- groups to occupy the two axial sites of Cu²⁺. Such paddlewheel exists in Cu(2,7-AQDC) as [Cu₂(COO)₄] that uses the COO^- groups to connect to each other via organic groups. Here each Cu²⁺ is five- coordinated with a distorted square pyramidal {O₅} donor set in which each of the five O atoms is from the five Ac^- groups. Packings of the [Cu₂(Ac)₆]^2- anions (Fig. 1b–c) show they are separated and charge-balanced by Na⁺ and H⁺ ions. Each Na⁺ is weakly coordinated by six O atoms from six Ac^- groups with a distorted octahedral coordination polyhedron. Powder X-ray powder diffraction (PXRD) analysis confirms the product is pure (Fig. 1d).
In the first discharge, HNa[Cu₂(Ac)₆] delivers more than 90 mAh/g at the 1.85 V plateau and a total of 150 mAh/g (Fig. 2a). Its discharge capacity being higher than its theoretical capacity (106 mAh/g; Mw = 505.34) based on reversible Cu^2+/+ redox pair suggests at least partial reduction of Cu²⁺ to metallic Cu. To understand the mechanism, ex-situ PXRD analysis (Fig. 2b) was used to track the electrode during cycling. When it was discharged to 1.85 V, all the main PXRD peaks from HNa[Cu₂(Ac)₆] remained with the relative intensities of some peaks changed, possibly mainly due to the insertion of Li⁺ into the framework and the accompanied structural rearrangement. There were no apparent new phases coming from the decomposition, indicating the metallic copper may be amorphous or leave the electrodes. Similar partial reduction to form metallic Cu during the initial insertion of Li⁺ into the framework is also observed in Cu₃(PO₄)₂.⁴
When HNa[Cu₂(Ac)₆] was further discharged to 1.5 V, all the PXRD peaks related to the framework disappeared and the electrode material was amorphous. Interestingly, when HNa[Cu₂(Ac)₆] was charged to 3.5 V, ca. 60% of the framework was recrystallized back judging from the relative intensity of the (110) peak at 8.34°. Thereafter, the discharge and charge capacities during the second cycle were 78 and 91 mAh/g. During the fifth cycling, the capacities were 35 mAh/g and its PXRD pattern indicates the absence of crystalline phase and the complete decomposition of the framework.
Investigations on correlating the framework structures and the coordination environments of Cu²⁺ with their electrochemical performances toward Li⁺ insertion-removal are ongoing.

References

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