Aqueous Zinc Ion Battery with Potassium Selective Hyper Cross-Linked Dibenzo-18-Crown-6-Ether Membrane

Abstract

Nonselective ion transport across the separator in zinc-iodine flow batteries (ZIFBs) alters concentration gradients, precipitates insoluble iodine, and generates uneven zinc nucleation on the anode surface. Here, we report the synthesis of a K+ ion-selective hyper-cross-linked dibenzo-18-crown-6-ether-containing polymer (HCP) that allows transfer of the K+ ion while avoiding the crossover of electroactive species. The sulfonated HCP was blended with SPEEK in compositions of 5, 10, and 15%, designated as SHCP1, SHCP2, and SHCP3, respectively. The blending of SHCP results in increases in ion exchange capacity (IEC), water uptake, ionic conductivity, and K+ ion selectivity. The best membrane, SHCP3, showed 7.6 times higher K+ ion selectivity than the neat SPEEK membrane. The galvanostatic charging/discharging over 200 cycles at 20 mA cm−2 current density showed 96% CE, 88% VE, and 84% EE for the SHCP3 membrane. It delivered 37.5 Ah l−1 capacity at 20 mA cm−2 current density, whereas Nafion-117 delivered 22 Ah l−1 capacity. The OCV of the cell with SHCP3 membranes remains at 1 V over 20 h. The results suggest that K+ ion selectivity is significantly helping in mitigating dendrite formation and enhancing the stability of the ZIFB battery, thus increasing its potential use.