Application of Porous Coordination Polymer Containing Aromatic Azo Linkers as Cathode-Active Materials in Sodium-Ion Batteries

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 (N═N) 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 N═N/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.