Water–Salt Oligomers Enable Supersoluble Electrolytes for High‐Performance Aqueous Batteries

Aqueous rechargeable batteries are highly safe, low‐cost, and environmentally friendly, but restricted by low energy density. One of the most efficient solutions is to improve the concentration of the aqueous electrolytes. However, each salt is limited by its physical solubility, generally below 21–32 mol kg −1 (m). Here, a ZnCl 2 /ZnBr 2 /Zn(OAc) 2 aqueous electrolyte with a record super‐solubility up to 75 m is reported, which breaks through the physical solubility limit. This is attributed to the formation of acetate‐capped water–salt oligomers bridged by Br − /Cl − ‐H and Br − /Cl − /O‐Zn 2+ interactions. Mass spectrometry indicates that acetate anions containing nonpolarized protons prohibit the overgrowth and precipitation of ionic oligomers. The polymer‐like glass transition temperature of such inorganic electrolytes is found at ≈− 70 to − 60  ° C, without the observation of peaks for salt‐crystallization and water‐freezing from 40 to − 80  ° C. This supersoluble electrolyte enables high‐performance aqueous dual‐ion batteries that exhibit a reversible capacity of 605.7 mAh g −1 , corresponding to an energy density of 908.5 Wh kg −1 , with a coulombic efficiency of 98.07%. In situ X‐ray diffraction and Raman technologies reveal that such high ionic concentrations of the supersoluble electrolyte enable a stage‐1 intercalation of bromine into macroscopically assembled graphene cathode.