A water-in-lactone electrolyte with controllable water activity for highly reversible zinc anodes

Rechargeable aqueous zinc batteries, offering the advantages of intrinsic safety, affordability, and superior recyclability, represent an emerging technology for stationary energy storage. However, the practical application of the technology is fundamentally blocked by the instability of the zinc anode/electrolyte interface caused by the water-induced hydrogen evolution reaction and Zn dendrite growth. Here, we formulate a water-in-lactone electrolyte to effectively regulate the water activity by adjusting the molar ratio of H2O to green gamma-valerolactone. The gamma-valerolactone not only enters the solvation shell of Zn2+ ions, excluding nearly all bound water molecules, but also interacts with free water via forming intermolecular hydrogen bonds, thus completely confining water molecules within gamma-valerolactone network. As a result, the water-in-lactone electrolyte minimizes hydrogen evolution side reactions and promotes uniform zinc electrodeposition. The coulombic efficiency of Zn & Vert; Ti asymmetric cells is improved to 99.6 % over 500 cycles at a current density of 1 mA cm(-2) and an areal capacity of 1 mAh cm(-2). The cycling lifespan of Zn & Vert; Zn symmetric cells is extended to more than 1100 h with the water-in-lactone electrolyte, much higher than that with the pure water electrolyte (less than 250 h). When coupled with polyaniline cathodes, Zn full cells utilizing the newly formulated electrolyte demonstrate enhanced cycling stability with a capacity retention of 73 % after similar to 500 cycles at 200 mA g(-1). Moreover, the water-in-lactone electrolyte enhances the temperature tolerance of Zn ion capacitors with activated carbon cathodes, allowing stable operation from -20 degree celsius to 80 degree celsius. This work further sheds light on the significant correlation between water activity and the performance of zinc anodes, and hopefully contributes to the advancement of stable aqueous zinc batteries.