“Water‐in‐Water” Biphasic Electrolyte for Aqueous Zinc‐ion Batteries

Aqueous zinc‐ion batteries (AZIBs) are constrained by unstable Zn metal interfaces, where nonuniform Zn 2+ transport induces concentration polarization and accelerates parasitic reactions. Here we show that Zn 2+ flux can instead be regulated by aqueous–aqueous interfaces. A fully aqueous “water‐in‐water” (W/W) biphasic electrolyte, comprising dextran (Dex)‐rich phase droplets (∼10 µm) suspended in a polyethylene glycol (PEG)‐rich aqueous phase, creates phase boundaries that continuously buffer local Zn 2+ accumulation and redistribute ions during plating and stripping. As a result, Zn||Zn cells cycle stably for 1100 h at 10 mA cm −2 and 10 mAh cm −2 , and Ah‐level Zn||NaV 3 O 8 pouch cells sustain long‐term operation. This work demonstrates an aqueous electrolyte architecture in which interfacial electrostatics, rather than solvent chemistry, governs Zn 2+ transport.