Manipulating Ion Chemistry in Biphasic Electrolytes Toward Durable High‐Energy Zinc–Bromine Batteries

ABSTRACT Zinc–bromine batteries (ZBBs) are considered a promising candidate for long‐duration energy storage, but their practical implementation is critically hampered by the crossover of polybromides. This bottleneck can be alleviated by deploying aqueous‐organic biphasic electrolytes, which leverage the pronounced difference in polybromide solubility between two immiscible phases to achieve effective confinement. However, a profound mechanistic understanding of ion‐specific functions in such systems remains elusive, and the full‐cell performance still falls short of commercial requirements. Herein, we systematically investigate the ion‐manipulated solvation environment and biphasic equilibrium of the electrolytes that correlate with the electrochemical behavior of ZBBs. Beyond anion‐driven phase separation, cations dictate ion‐pairing interactions that govern component distribution across the two phases. Compared to monovalent and trivalent counterparts, divalent cations strike an optimal thermodynamic–kinetic balance, achieving a trade‐off between polybromide confinement and electrode reaction kinetics. Furthermore, a dual‐functional zwitterion is demonstrated to concurrently suppress polybromide shuttle and stabilize zinc deposition. The resulting biphasic ZBBs deliver an energy density of 40.6 Wh L −1 and sustain a cycling life over 1000 cycles, considerably outperforming reported biphasic systems. Coupled with a low system‐level cost of ∼$100 kWh −1 , the biphasic ZBBs represent a compelling technology for grid‐scale energy storage.