A hybrid-aqueous biphasic electrolyte for suppressed shuttle effects and self-discharge of zinc bromide batteries

Aqueous zinc bromine batteries (ZBBs) attract extensive research interest owing to their high theoretical energy density, high operating voltage, and low cost. However, they suffer from severe self-discharge and poor cycle life caused by the uncontrolled shuttle of polybromides. Herein, we design a hybrid-aqueous biphasic electrolyte to prevent the shuttle effects of ZBBs, where the hybrid phase consists of polybromides and bromine ions (Br-) dissolved in a co-solvent of tetraethylene glycol dimethyl ether and water (TEGDME/H2O), and the aqueous phase mainly contains zinc ions (Zn2+) and sulfate ions (SO42-) in H2O. The polybromides can be confined successfully in the hybrid phase, which is attributed to the stronger interactions of polybromides with TEGDME than H2O and the unique molecular structure of the biphasic electrolyte. The superiority of the biphasic electrolyte is also confirmed by its high ionic conductivity, low volatility and non-flammability. The ZBBs using the biphasic electrolyte show significantly suppressed self-discharge, and a long 400-cycles life at 5 mA h capacity with 99.6% coulombic efficiency, which considerably exceed those of conventional ZBBs (270-cycles life, and 47.4% coulombic efficiency). A strategy of using an ion-conductive biphasic electrolyte was adopted to suppress the shuttle effects of polybromides in Zn-Br2 batteries.