Molecular polarity regulation of polybromide complexes for high-performance low-temperature zinc–bromine flow batteries

Abstract

Frigid environments notably impair the electrochemical performance of zinc–bromine flow batteries (ZBFBs) due to polybromide solidification, restricting their widespread deployment in cold regions. Here, two independently used complexing agent cations, n-propyl-(2-hydroxyethyl)-dimethylammonium (N[1,1,3,2OH]⁺) and diethyl-(2-hydroxyethyl)-methylammonium (N[1,2,2,2OH]⁺), are proposed to enable ZBFBs to exhibit excellent performance at both low and room temperatures, through the precise regulation of the molecular polarity of polybromide complexes. Benefiting from the optimized design of the carbon number and position on the skeleton, the molecular polarity of the N[1,1,3,2OH]⁺- and N[1,2,2,2OH]⁺-polybromide complexes is appropriately reduced compared with that of choline, which is conductive to the enhancement of bromine capture capability. Interestingly, the intermolecular hydrogen bonding effect of their hydroxyl groups is not significantly enhanced, ensuring that the formed complexes maintain good fluidity at low temperatures. Thus, ZBFBs with a single complexing agent not only demonstrate an impressive average Coulombic efficiency of >95% across 1600 cycles at room temperature, but also can sustain operation with a high current density of 40 mA cm⁻² for 250 cycles at −20 °C. This research significantly advances the comprehensive understanding of the mechanisms involved, thereby substantially contributing to the development of enhanced low-temperature complexing agents.