A high-performance pseudocapacitive negatrode for lithium-ion capacitor based on a tetrathiafulvalene-cobalt metal–organic framework

Lithium-ion capacitors (LICs) with both high specific energy and high specific power are greatly desired to meet the ever-growing needs for energy storage. Pseudocapacitive materials with fast and reversible redox features can be acted as effective negatrodes of LICs. Herein, two pristine tetrathiafulvalene (TTF)-based cobalt(II) metal–organic frameworks, [Co2(py-TTF-py)2(BDC)2]·2DMF·H2O (TTF-Co-MOF 1) and [Co2(py-TTF-py)2(BPDC)2]·3DMF·3H2O (TTF-Co-MOF 2) (py-TTF-py = 2,6-bis(4′-pyridyl)tetrathiafulvalene, H2BDC = terephthalic acid, H2BPDC = biphenyl-4,4′-dicarboxylic acid) are suggested as the pseudocapacitive negatrodes of LICs with the electrolyte of 1.0 mol L−1 LiPF6 in ethylene carbonate and diethyl carbonate (1:2 v/v ratio). The optimized TTF-Co-MOF 1 negatrode with appropriate proportion of conductive agent of lithium-ion batteries (LIBs) presents greatly enhanced discharge specific capacity of 1509 mAh g−1 at 200 mA g−1 at 307th cycle and superb rate capability with the specific capacity of 1382 and 1138 mAh g−1 at 5 and 10 A g–1 after running 171 and 175 cycles, respectively. The excellent performance has never been reported for the MOFs-based negatrode materials of LIBs. The pseudocapacitive behaviors of the TTF-Co-MOFs negatrodes are judged by cyclic voltammetry and the results show that the capacitive contribution ratios can be finely tuned by the porosity of MOFs and fabrication composites. The LICs with TTF-Co-MOF 1||AC (AC = activated carbon) configuration presents competitive performance with high specific energy of 132.4 Wh kg−1 at a specific power of 0.25 kW kg−1, high specific power of 12.5 kW kg−1 at a specific energy of 86.8 Wh kg−1, and stable cycling performance with 88% capacity retention after 8000 cycles at 2 A g−1 within the operating voltage range of 1.0–4.0 V. Furthermore, the full cell with TTF-Co-MOF 1||NMC 622 (NMC 622 = LiNi0.6Mn0.2Co0.2O2) configuration delivers the capacity of 96 mAh g−1 at 100 mA g−1 with the CE of 99.40% at the 600th cycle and 85 mAh g−1 at 1 A g−1 with the CE of 98.52% at the 300th cycle. The results help to extend the application of pristine insulating MOFs as remarkable negatrodes for the performance complementation between LIBs and electrochemical capacitors.