Optimizing ionic conductivity and ion selectivity in zinc-polyiodide flow batteries with composite polyamide-porous separators

Owing to their superior theoretical energy capacity, zinc-polyiodide flow batteries (ZIFBs) are well-known energy storage devices. The practicality of ZIFBs depends on the development of cost-effective separators that can demonstrate both excellent ionic conductivity and selective ion permeability. Herein, an economical polymer blend comprising poly (ether sulfone) (PES) and sulfonated poly (ether ether ketone) (sPEEK) is fabricated via a phase inversion technique. Due to the formation of a sponge-like structure, the porous substrate indicates high ionic conductivity. Furthermore, the selected porous substrate is coated with a thin-film polyamide, synthesized from m-phenylenediamine (MPD) and 1,3,5-benzene tricarbonyl trichloride (TMC) via interfacial polymerization (IP). This polyamide layer can efficiently mitigate the undesired migration of triiodide ions, while maintaining high ionic conductivity. This innovative approach yields a composite separator having a deftly tunable structure. Using such a separator, ZIFBs can operate stably with high coulombic efficiency (CE) of 91 % over 100 cycles at a current density of 10 mAh cm−2. Overall, this study explores the trade-off between ionic conductivity and ion permeability. It is seen that by adjusting their morphology, the physical characteristics of the composite separators can be improved, ensuring stable and efficient operation of ZIFBs.