Aqueous electrolyte solutions with anion-bridged secondary solvation sheaths for highly efficient zinc metal batteries

Aqueous zinc metal batteries are low-cost electrochemical devices suitable for safe grid energy storage. However, water decomposition and Zn dendrite formation detrimentally affect their coulombic efficiency. Conventional aqueous electrolyte solutions, with a concentration around 1 M, are cost-effective and exhibit high bulk ionic conductivity but cannot form a stable solid electrolyte interphase. Water-in-salt and aqueous-organic hybrid electrolyte solutions can form robust solid electrolyte interphases, but they are not kinetically efficient and cost-effective. Here, to circumvent these issues, we design variously concentrated aqueous electrolyte solutions using several salts with different donor numbers to extend anion coordination into the secondary solvation sheath. We show that salt-derived anions with donor number > 18 enter the Zn²⁺ first solvation sheath, and ensure a strong binding energy between the Zn²⁺(H₂O)₅-anion nanometric clusters and water molecules in the secondary solvation sheath. In particular, 2 M aqueous electrolyte solutions containing fluorinated anions exhibit bulk ionic conductivities of 26-35 mS cm^-1 at 25 °C and form a ZnF₂-rich solid electrolyte interphase. When tested in Zn||NaV₃O₈·1.5H₂O Swagelok cells, the best-performing electrolyte solution enables an average coulombic efficiency of 99.99% for 1,000 cycles at 1.5 mA cm^-2, corresponding to an initial specific energy of 130 Wh kg^-1 (based on the combined weight of the positive and negative electrodes).