Chloride electrolyte enabled practical zinc metal battery with a near-unity Coulombic efficiency

Rechargeable aqueous zinc batteries are finding their niche in stationary storage applications where safety, cost, scalability and carbon footprint matter most. However, harnessing this reversible two-electron redox chemistry is plagued by major technical issues, notably hydrogen evolution reaction (HER) at the zinc surface, whose impacts are often not revealed under typical measurement conditions. Here we report a concentrated electrolyte design that eliminates this parasitic reaction and enables a Coulombic efficiency (CE) of 99.95% for Zn plating/stripping measured at a low current density of 0.2 mA cm−2. With extra chloride salts and dimethyl carbonate in concentrated ZnCl2 electrolyte, the hybrid electrolyte with a unique chemical environment features low Hammett acidity and facilitates the in situ formation of a dual-layered solid electrolyte interphase, protecting zinc anodes from HER and dendrite growth. Benefiting from the near-unity CE, the pouch cell with a VOPO4·2H2O cathode sustains 500 deep cycles without swelling or leaking and delivers an energy density of 100 Wh kg−1 under practical conditions. Our work represents a critical step forward in accelerating the market adoption of zinc batteries as an energy storage system with higher sustainability. Rechargeable aqueous zinc batteries are heralded as a sustainable energy technology but still face technical challenges. The hybrid electrolyte here eliminates hydrogen evolution reaction, the most thorny issue, and allows for impressive battery performance even under harsh conditions.