We report a low-cost water-in-salt electrolyte (WiSE), of 30 m ZnCl2, which enables a dendrite-free Zn metal anode to possess a high coulombic efficiency.

A great challenge for all aqueous batteries, including Zn-metal batteries, is the parasitic hydrogen evolution reaction on the low-potential anode. Herein, we report the formula of a highly concentrated aqueous electrolyte that mitigates hydrogen evolution by transforming water molecules more inert. The electrolyte comprises primarily ZnCl 2 and LiCl as an additive, both of which are inexpensive salts. The O-H covalent bonds in water get strengthened in a chemical environment that has fewer hydrogen bonding interactions and a greater number of Zn-Cl superhalides, as suggested by integrated characterization and simulation. As a result, the average Coulombic efficiency of zincmetal anode is raised to an unprecedented >99.7% at 1 mA cm −2 . In the new electrolyte, the plating/stripping processes leave the zinc-metal anode dendrite-free, and the zinc-metal anode delivers stable plating/stripping cycles for 4000 hours with an areal capacity of 4 mAh cm −2 at 2 mA cm −2 . Furthermore, the high Coulombic efficiency of zinc-metal anode in the ZnCl 2 -LiCl mixture electrolyte is demonstrated in full cells with a limited anode. The V 2 O 5 •H 2 O||Zn full cell with an N/P mass ratio of 1.2 delivers a stable life of more than 2500 cycles, and the LiMn 2 O 4 ||Zn hybrid cell with an N/P mass ratio of 0.6 exhibits 1500 cycles in its stable life.

The redox-activity of Cl−-hosting polypyrene cathode in dual-ion batteries can be tuned by tailoring the linking patterns.