Thiazole‐Linked Covalent Organic Framework Promoting Fast Two‐Electron Transfer for Lithium‐Organic Batteries

Abstract

constitutes an untapped resource. [5][6][7] In addition, tailoring molecules by adding redox-active functional groups can modulate both redox potentials and electrontransfer rates. [8,9] Thus, the vast array of organic molecules can construct a new class of electrodes, [10,11] if the main problems, such as excessive solubility, poor electronic conductivity, and vulnerability to chemical side reactions, can be solved. [5][6][7][8][9] A promising answer to these problems is the integration of small molecules into a crystalline network scaffold. [12,13] Covalent organic frameworks (COF) are well suited to form stable molecular crystals [14,15] ; Integration of organic building blocks via strong covalent bonds creates 2D structures, which are layered upon one another and held in place by π-π interactions. The resulting 3D structures inhibit the dissolution of the organic layers. Moreover, periodic mesopores are formed across the entire structure, providing effective ion-transport channels. [16][17][18] Typically, COF electrodes comprise redox functionality and linkage molecules, where the latter is responsible for the shape, mesopore size, π-conjugation, and stability of the COF. [13,19] Heteroatom-containing molecular linkers have been widely utilized to improve the wettability of COFs in electrolyte solutions. [20][21][22][23] β-Ketoenamine-linked COFs have been extensively used for capacitor and battery electrodes, and solid-state electrolytes. [24][25][26] However, a β-ketoenamine unit typically forms a nonconjugated in-plane structure of the COF, [26,27] the effect of which during electrochemical conditions has not yet been studied in depth. Imine linkage comprises partial π-conjugation over the 2D COF layer. [28,29] However, chemical instability of imine has been widely known, for example, hydrolysis in an acidic solution. [30] Therefore, the role of imine linkage remains to be explored in the nonaqueous solution-based electrochemical cells. Thiazole linkage has been recently developed to satisfy both chemical stability and the π-conjugated system. [31][32][33] Sulfur-containing thiazole, thiophene, and tetrathiafulvalene are a new class of moieties embedded into COFs and show excellent charge mobility and conductivity. [34][35][36][37] Thus, they have extended the applications of COFs to molecular electronics, and energy conversion and storage electrodes. [38][39][40][41][42][43] However, there is still a lack of study that proves true chemical and electrochemical stability of thiazole-linked COF under the battery operating conditions.Covalent organic frameworks (COFs) have been considered a potentially versatile electrode structure if they are made highly conductive and flexible to stabilize the redox functionality. Although conceptually plausible, COF-based electrodes have rarely satisfied high capacity, cyclability, and rate capability thus far. Incorporating thiazole moieties into the organic scaffold, it is able to fabricate π-conjugated and crystalline organic electrodes and demonstrate the fast two-elec...

Toward azo-linked covalent organic frameworks by developing linkage chemistry via linker exchange

“…2e ), indicating achievement of linkage conversion. Moreover, quite different from that of Im-COF-1, the Raman spectrum of Azo-COF-1 displays broadening peaks due to the background interference of fluorescence 32 , further supporting the occurrence of framework transformation. For Azo-COF-2, similar phenomena such as widening peaks, disappearance of imine peak (1631 cm −1 ), and appearance of azo peak (1443 cm −1 ) were also observed (Fig.…”

“…Azo unit (–N=N–), as a vital functional group in chemistry and materials science, has aroused wide interest because of its intriguing features such as photo-isomerization 27 – 29 , narrow bandgap 30 , Lewis base property 31 , and redox activity 32 . In this context, construction of azo-linked COFs (Azo-COFs) undoubtedly will lead to a novel type of crystalline porous polymers with fascinating properties.…”

“…Indeed, recent theoretical calculations have revealed that Azo-COFs have great potentials in photocatalysis 33 . However, thus far –N=N– has not yet been reported as a linkage to construct COFs, albeit a few COFs carrying azo moieties were prepared through polycondensation of judiciously selected monomers with pre-installed –N=N– units 32 , 34 – 38 . In those examples the azo units do not function as linkages, but as part of building blocks.…”

Advances in electrochemical energy storage with covalent organic frameworks

“…The presence of pπ-pπ orbital coupling to the axial direction may further amplify this electron transport, which suggests the need for COFs with interlayered designs. 102,163 The fabrication of thin films using exfoliation and direct thinfilm growth has been extensively investigated to solve the problem of low electronic conductivity. However, thin films contain only a small amount of active material, which is not acceptable for industrial production.…”

“…Byon's group highlighted this issue by designing three linkages, thiazole-fused ring, imine, and β-ketoenamine, introduced to the azo moiety, and examined these COF electrodes in Li-ion cells. 102 The azo integrated with the thiazole-fused ring showed the concerted two-electron transfers within 1.5-2.0 V. The extended -conjugation structure formed by the thiazole-fused ring stabilized the COF structure during the discharging and 96 Copyright 2019, American Chemical Society. charging processes and delivered stable 5000 cycles at 10C.…”