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

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.