Tetrathiafulvalene-Based Metal-Organic Framework as a High-Performance Anode for Lithium-Ion Batteries

Metal-organic frameworks (MOFs) have aroused great interest as lithium-ion battery (LIB) electrode materials. In this work, we first report that a pristine three-dimensional tetrathiafulvalene derivatives (TTFs)-based zinc MOF, formulated [Zn₂(py-TTF-py)₂(BDC)₂]·2DMF·H₂O (1) (py-TTF-py = 2,6-bis(4'-pyridyl)tetrathiafulvalene and H₂BDC = terephthalic acid), can work as a high-performance electrode material for rechargeable LIBs. The TTFs-Zn-MOF 1 electrode displayed a high discharge specific capacity of 1117.4 mA h g^-1 at a current density of 200 mA g^-1 after 150 cycles along with good reversibility. After undergoing elevated discharging/charging rates, the electrode showed superior lithium storage performance in the extreme case of 20 A g^-1 and could finally recover the capability when the current rate was back to 200 mA g^-1. Particularly, specific capacities of 884.2, 513.8, and 327.8 mA h g^-1 were reached at high current densities of 5, 10, and 20 A g^-1 after 180, 175, and 300 cycles along with good reversibility, respectively. Such an excellent performance is first reported for the LIB anode materials. TTFs-Zn-MOF 2, namely, [Zn₂(py-TTF-py) (BDC)₂]·DMF·2H₂O (2), was prepared as a contrast to explore the relationship between the structures of the electrode materials and the electrochemical properties. Based on the structural analysis of 1 and 2 and ex situ X-ray photoelectron spectroscopy, the TTF moiety and the twofold TTF pillar play a key role in the excellent electrochemical performance. The full cell of MOF 1 with NMC 622 delivered the capacity of 131.9 mA h g^-1 at 100 mA g^-1 with the Coulombic efficiency of 99.45% after 70 cycles and exhibited the tolerance to high-current operation.