"Water-in-Water" Biphasic Electrolyte for Aqueous Zinc-ion Batteries

Aqueous zinc-ion batteries (AZIBs) are constrained by unstable Zn metal interfaces, where nonuniform Zn²⁺ transport induces concentration polarization and accelerates parasitic reactions. Here we show that Zn²⁺ 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 Zn²⁺ 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||NaV₃O₈ pouch cells sustain long-term operation. This work demonstrates an aqueous electrolyte architecture in which interfacial electrostatics, rather than solvent chemistry, governs Zn²⁺ transport.