Quasi‐Solid Aqueous Electrolytes for Low‐Cost Sustainable Alkali‐Metal Batteries

Aqueous electrolytes are highly important for batteries due to their sustainability, greenness, and low cost. However, the free water molecules react violently with alkali metals, rendering the high-capacity of alkali-metal anodes unusable. Here, water molecules are confined in a carcerand-like network to build quasi-solid aqueous electrolytes (QAEs) with reduced water molecules' freedom and matched with the low-cost chloride salts. The formed QAEs possess substantially different properties than liquid water molecules, including stable operation with alkali-metal anodes without gas evolution. Specifically, the alkali-metal anodes can directly cycle in a water-based environment with suppressed growth of dendrites, electrode dissolution, and polysulfide shuttle. Li-metal symmetric cells achieved long-term cycling over 7000 h (and over 5000/4000 h for Na/K symmetric cells), and all the Cu-based alkali-metal cells exhibited a Coulombic efficiency of over 99%. Full metal batteries, such as Li||S batteries, attained high Coulombic efficiency, long life (over 4000 cycles), and unprecedented energy density among water-based rechargeable batteries.