Electroactive tetrathiafulvalene-based covalent organic framework with thiophene units as anode for high-performance hybrid lithium-ion capacitors

Abstract

Enhancing the performance of hybrid lithium-ion capacitors (HLICs) by regulating the structural characteristics of covalent organic frameworks (COFs) has been a challenge. In this study, electron-rich thiophene units combining with the electroactive tetrathiafulvalene (TTF) motif consists the designable organic linker, tetrathiafulvalene tetrathiophenal (TTFTTA). A novel 2D COF, TTFTTA-PDA (PDA, p-phenylenediamine), was assembled via a solvothermal method. TTFTTA-PDA exhibits reversible redox activity, a large Brunauer−Emmett−Teller surface area (457 m² g⁻¹) and high stability (pH 3∼14). Furthermore, compared with the reported tetrathiafulvalene-tetrabenzaldehyde (TTFTBA)-based COF, TTFTBA-PDA, the introduction of thiophene rings enhances the capability of electron transfer, characterized by a smaller band gap (1.45 eV) and a lower calculated energy gap (0.89 eV). As a result, the electrochemical performance of TTFTTA-PDA in HLICs is outstanding. In the full-cell configurations, TTFTTA-PDA||activated carbon HLICs exhibit impressive energy density (140 Wh kg⁻¹ at 233 W kg⁻¹), power density (9328 W kg⁻¹ at 91 Wh kg⁻¹), and cycling lifespan (the capacity retention of 81.3 % after 2200 cycles), demonstrating a certain level of competitiveness among the reported state-of-the-art HLICs utilizing metal organic framework-/COF-based anode materials. These results illustrate that the precise structural design of pristine COFs can be an effective strategy to enhancing the performance of HLICs.