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

Abstract

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.

Azo Compounds as Active Materials of Energy Storage Systems

“…These findings pave the way to the development of new cathode active materials for sodium-ion batteries. [51] As described above, azo polymers and frameworks show high battery performance because of their insolubility in organic solvents and the tunable HOMO and LUMO energies of π-conjugated structures. These findings facilitate the design of azo compounds as the active materials of rechargeable batteries with both high capacity and high rate capability and suggest that these goals can be achieved using the soft crystal features of COFs and MOFs (Table 4).…”

Azo Compounds as Active Materials of Energy Storage Systems

“…In addition, 4,4′‐azopyridine showed a higher voltage (2.2 V vs. Na + /Na, 2.5 V vs. Li + /Li) than azobenzene with metal carboxylate moieties (1.2 V vs. Na + /Na, 1.5 V vs. Li + /Li), [50] which indicated that CPL‐4 is a promising azo‐based cathode active material. These findings pave the way to the development of new cathode active materials for sodium‐ion batteries [51] …”

Electron Storage Performance of Metal–Organic Frameworks Based on Tetrathiafulvalene–Tetrabenzoate as Cathode Active Materials in Lithium- and Sodium-Ion Batteries

“…In particular, MOFs with redoxactive organic ligands have attracted significant attention as potential electroactive materials for rechargeable batteries. 16−26 So far, we have applied such redox-active MOFs to cathodes of LIBs and SIBs, where anthraquinone dicarboxylate, 27,28 disulfide organic linkages, 29 and aromatic azo compounds 30 have been used as redox-active ligands. In these studies, we identified dual-ion exchange, electrochemically dynamic disulfide covalent bonds, and sodium-ion-selective insertions as novel phenomena.…”