Sandwich-Structured Aqueous Electrolyte with Water-Content Gradient for Enhanced Longevity and Reaction Kinetics in Zinc Metal Batteries.

Conventional low-concentration aqueous electrolytes (AqE) for Zn metal batteries face undesirable parasitic reactions, severely deteriorating thei.r sustainability. Although low-water-content electrolytes have shown promise in mitigating water splitting, their high viscosity and limited ion transport lead to sluggish reaction kinetics. In this work, we propose a water-content gradient electrolyte (GE) by constructing a sandwich-like structure, where two molecular crowding electrolyte (MCE) layers are applied on both electrode surfaces, while a conventional AqE occupies the space in between. The low-water-content MCE effectively suppresses electrode corrosion and dissolution, while the high-water-content AqE improves ionic conductivity. As a result, Zn/Zn symmetric cells utilizing the GE demonstrate exceptional long-term cycling for over 2000 hours at 2 mA cm^-2 to 4 mAh cm^-2 and over 300 hours at 7.5 mA cm^-2 to 15 mAh cm^-2. The Zn-vanadium and Zn-manganese full cells in GE also show remarkable longevity, with cycling lives exceeding several thousand cycles at 2 A g^-1, and excellent reaction kinetics across varying current densities. Overall, the GE successfully integrates the benefits of both AqE and MCE, leading to enhanced electrode protection without compromising ion transport, thereby offering a new avenue for developing long-lasting aqueous Zn metal batteries.