A Molecular Capsule to Confine Redox-Active Metal Acetylacetonate Complex for Long-Lifetime Non-Aqueous Redox Flow Battery.

Redox flow batteries are promising candidates for grid-scale energy storage due to their intrinsic safety and low cost, but their practical applications are often limited by instability of redox-active species and crossover between the two electrolytes. Here, we design a molecular capsule formed by dimeric 4-tert-butylcalix[8]arene (TBC8). Due to their similar hydrophobic properties, the redox-active molecules (metal acetylacetonates) tend to be enriched in the cavities with tert-butyl groups of the formed capsules, which enables a several-fold enhanced solubility of redox-active species, compared with the system without TBC8. By using the electrolytes containing encapsulated redox species, the constructed non-aqueous redox flow battery shows a capacity retention of 90.41% after 1000 cycles, much better than that of the device (52.2% after 400 cycles) without using TBC8 capsules. This work demonstrates a promising strategy to design novel electrolytes containing sensitive redox-active species toward high-performance flow batteries with long-term stability.