Water Content Gradient Electrolyte for Highly Stable Aqueous Zinc Metal Batteries

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Recent interest focuses on aqueous zinc metal batteries (AZMBs) for large-scale energy storage systems due to their affordability and safety.1 However, the dendrite growth and parasitic reactions on the Zn anode and the dissolution on the cathode harm the sustainability of AZMBs.2 Several strategies have been proposed to stabilize zinc anode and protect oxide-based cathodes by decreasing the water content in the electrolytes.3-5 However, the actual performance of these low water-content electrolytes is unsatisfactory because of their high viscosity and low ionic conductivity. In contrast, high water-content electrolytes (i.e., conventional aqueous electrolytes) provide promising physicochemical properties, whereas water decomposition on the electrode surface is inevitable, further negatively impacting electrochemical performance. In this study, we design a unique gradient electrolyte where a water content gradient is achieved from electrode to separator. Specifically, a molecular crowding electrolyte with less water content and suppressed water activity is coated on the electrode surface, and a conventional aqueous electrolyte with high water content is dropped on the separator. By assembling the battery, the MCE on both electrodes and the AqE on the separator form a sandwich-like gradient electrolyte (GE). Due to the AqE in between, the ionic conductivity within GE is significantly improved than MCE. Meanwhile, the MCE on the electrode surface regulates uniform zinc deposition and reduces parasitic reactions related to water decomposition. As a result, the highly reversible zinc plating/stripping occurs in Zn/Cu asymmetric cells with an average Coulombic efficiency of 99.1% for over 1000 cycles. Moreover, the Zn/Zn symmetric cells can stably cycle with reduced overpotential than MCE for over 1000 hours at different current densities. Full cell batteries employing MnO2 exhibit excellent capacity retention after 1000 cycles at 2 A g–1 due to the suppressed cathodic dissolution. This gradient electrolyte offers a facile strategy to protect electrodes and prevent side reactions without sacrificing ionic conductivity and viscosity.