In Situ Visualizing the Electric-Field-Driven Assembly of Gradient Carbon Dot Hydrogel Electrolytes for Stable Zinc...

Aqueous zinc-ion batteries (AZIBs) have seen increasing use of carbon dots (CDs) as functional additives; however, their interfacial regulation mechanisms remain unclear due to the lack of direct in situ visualization under realistic conditions. In this study, we develop an operando electrochemical-confocal coupled platform that enables real-time tracking of fluorescent CDs under an applied electric field. Using this system, we directly visualize the electrophoretic migration and interfacial enrichment of CDs, providing clear evidence of their roles in electric-field modulation and Zn²⁺ flux regulation. Guided by this insight, a gradient-structured poly(vinyl alcohol) (PVA) hydrogel electrolyte (2PVA@CDs) is in situ constructed via electric-field-driven assembly. Zn||Zn symmetric cells with 2PVA@CDs exhibit ultralong cycling stability over 6500 h at 1 mA cm^-2 and 1 mAh cm^-2, and stable operation for 900 h at 85% depth of discharge. Combined experiments, finite element simulations, and density functional theory calculations reveal that CDs homogenize the interfacial electric field, regulate Zn²⁺ flux, reduce the nucleation energy barrier, and suppress hydrogen evolution, thereby enhancing interfacial stability and overall electrochemical performance. This work clarifies the mechanistic role of CDs and establishes a general operando visualization strategy for functional additives in metal batteries.