A New Paradigm for Flowless Zn–Br2 Aqueous Cathodes: Fast Kinetics and Stability via Entrapped Phosphonium Bromides

Zinc–bromine batteries (ZBBs) are promising candidates for large-scale energy storage due to their low cost, inherent safety, and high theoretical energy density. However, conventional flow-based ZBBs suffer from low system-level energy density and operational complexity. Recent interest has shifted toward flowless ZBBs (FL-ZBBs) configuration, yet these face critical challenges associated with uncontrolled diffusion and self-discharge (SD) of bromine species at the cathode. One of the holy grails in addressing these challenges is the use of quaternary ammonium salts (NR₄⁺), which interact with bromine species to inhibit their diffusion. In this work, we demonstrate the first ever reported utilization of phosphonium-based bromine-complexing agents (BCAs) in FL-ZBBs. We do so by impregnation of the BCAs into porous carbon cathodes, particularly tetrabutylphosphonium bromide (TBP), outperform traditional ammonium-based analogs in FL-ZBBs. TBP enables high Coulombic efficiency (>98.5%, 1 A g⁻¹) for 145 mAh g⁻¹, reduces the SD phenomenon, and achieves long-term cycling stability of over 500 cycles with >97% CE (1.5 A g⁻¹, 145 mAh g⁻¹), even under low electrolyte concentrations (0.5M ZnBr2). The enhanced performance is attributed to faster 2Br⁻/Br₂ redox kinetics, as demonstrated by cyclic voltammetry, and improved molecular polybromide∙∙∙PR₄⁺ interactions, highlighting phosphonium salts as more attractive BCA compounds for high-performance FL-ZBBs.