Azo-Integrated Covalent Organic Frameworks As Electrodes for Lithium-Ion Batteries

Abstract

Covalent Organic Frameworks (COFs) are two-dimensional, porous, and crystalline organic structures. The well-established frameworks provide ordered mesopores, structural stability, high density, and negligible solubility, which are particularly attractive for battery electrodes. By integrating a redox-active azo group to COFs, we designed positive organic electrodes and applied these electrodes for nonaqueous Li-ion batteries. The azo group can undergo two electrons transfer, whereas its reversibility and stability are highly dependent on adjacent functional groups [1,2]. We developed three azo-integrated COFs with three different linkages, -ketoenamine, imine, and thiazole-fused ring. These azo-COFs had identical hexagonal P6/m space groups, ~3 nm of pore size, and 600~1000 m2 g–1 of surface area. However, the redox chemistries of azo groups were remarkably distinct from the linkages. The-ketoenamine and imine based azo-COFs showed irreversible azo reactions, resulting in poor capacity retention. In contrast, the fully conjugated COF containing the thiazole-fused ring rendered excellent cycling stability showing 5000 cycles at 10 C. The reversible azo reaction was also demonstrated by using operando Raman spectroscopy. This result corresponded to the single and sharp redox wave at a formal potential of ~1.7 V vs. Li/Li+, suggesting the one-step two electrons transfer. The solid electrolyte interphase (SEI) was formed at the initial cycles only, which could protect the COF surface and inhibit the continuous decomposition of the electrolyte solution. Electrochemical impedance spectroscopy (EIS) revealed the low charge transfer resistance (~18 Ω), which decreased further after 20 cycles (~6 Ω). The mass transport region in the EIS curve was retained, indicating the access of Li+ to the bulk COF structure through mesopores for cycling. In the presentation, I will discuss the details of COF designs and the role of linkages enhancing the reversible and stable redox reactions in Li-ion cells.