Bifunctional cationic polyelectrolyte additive enables dendrite-free and shuttle-suppressed zinc-bromine batteries

Zinc-bromine (Zn-Br₂) batteries are attractive candidates for large-scale energy storage owing to their low cost and high energy density. However, their practical deployment remains impeded by the zinc dendrites growth and polybromide shuttle diffusion effects, which compromise long-term cycling and energy efficiency. Herein, we introduce a low-concentration, bifunctional polyelectrolyte additive, polyquaternium-6 (P-6), that simultaneously addresses both challenges. Electrochemical analyses, in-situ Raman spectroscopy, and theoretical computation reveal that P-6 selectively adsorbs on Zn (002)/(100) facets, facilitating uniform Zn deposition along the kinetically favourable (101) orientation while suppressing dendrite formation via electrostatic shielding. Simultaneously, P-6 forms stable complexes with Br₃^- and Br₅^-, effectively inhibiting their shuttle diffusion. Consequently, Zn-Br₂ battery incorporating only 0.65 wt. % P-6 (∼44 mM) delivers a high capacity of ∼1300 μAh cm^-2 and exhibit outstanding stability over 1500 cycles with >98 % Coulombic efficiency. This bifunctional electrolyte strategy effectively addresses interfacial challenge in Zn-Br₂ batteries and may serve as a guiding principle for the design of additives in broader aqueous halogen-based system.