Aromatic azo compounds
have been reported as organic active materials
of sodium-ion and lithium-ion batteries owing to the redox reaction
of azo groups (NN) and the introduction of insoluble groups.
In this study, we investigated a method to realize both high capacity
and good cycle performance of sodium-ion batteries by combining aromatic
azo compounds with redox-active atoms insoluble in the electrolyte.
Moreover, the metal–organic framework (MOF) CPL-4 ([Cu2(pzdc)2(azpy)], pzdc = pyrazine-2,3-dicarboxylate,
azpy = 4,4-azopyridine, pore size: 10 × 6 Å2)
was investigated as a cathode-active material for sodium-ion and lithium-ion
batteries based on the redox reactions of Cu2+/Cu+ and NN/N–N. In addition to evaluating the battery
performance, the reaction mechanism of CPL-4 was elucidated
through ex situ X-ray diffraction and operando X-ray absorption fine structure. First, Na+ ions were
inserted deeply into the CPL-4 particles, although Li+ ions were stored only on the surface of the CPL-4 particles. Second, during charging/discharging processes, CPL-4 exhibited reversible crystal structural changes and
redox reactions of Cu2+/Cu+. The MOFs containing
aromatic azo linkers showed interesting redox behavior as cathode-active
materials for sodium-ion batteries. These findings will contribute
to the design of cathode-active materials for high-performance sodium-ion
batteries.
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