Truxenone-Based Covalent Organic Framework/Carbon Nanotube Composite for High-Performance Low-Temperature Sodium-Ion Batteries.

Low-temperature rechargeable batteries face great challenges due to the sluggish reaction kinetics. Redox covalent organic frameworks (COFs) with porous structures provide a viable solution to accelerate the ionic diffusion and reaction kinetics at low temperatures. However, the applications of COFs in low-temperature batteries are still at their infancy stage. Here, a composite electrode containing truxenone-based COF and a small amount of CNTs is reported for low-temperature sodium-ion batteries. The truxenone-based COF possesses high ratio of active centers and enables high theoretical capacity; while the CNTs guarantee the accessibility of the active sites, the charge transfer efficiency, and the fast reaction kinetics. As a result, the composite electrode shows a high reversible capacity of 365 mAh g-1 at 0.05 A g-1 (295 mAh g-1 after subtracting carbon contribution). Notably, it offers excellent low-temperature performance that maintains a high discharge capacity of 263.8 mAh g-1 (203 mAh g-1 after subtracting carbon contribution) at 0 °C, which represents 74% of the capacity at room temperature. Even under extreme weather of -30 °C, the capacity maintains 156.8 mAh g-1 (119 mAh g-1 after subtracting carbon contribution) after 100 cycles, with a capacity retention rate of 97%. This work illustrates the synergistic effect of the fast ionic diffusion of truxenone-based COF and the excellent charge transfer of the CNTs, offering a feasible way to realize low-temperature sodium-ion batteries.