A Confinement Continuum for Aqueous Zinc Batteries: A Unified Design Logic for Water-in-Salt, Polymer-in-Salt, and Hybrid Electrolytes

Abstract Aqueous zinc (Zn) battery electrolytes have advanced rapidly through the development of water-in-salt, polymer-in-salt, and hybrid architectures, yet these systems are still too often treated as separate material classes. In this perspective, we argue that these apparently distinct electrolyte families are better understood through a single confinement continuum, in which performance is dictated by how strongly water molecules, Zn2+ ions, and local coordination environments are restricted across the bulk electrolyte and the electrochemical interface. This shifts electrolyte design from composition-based classification to mechanism-based control. Using this framework, we examine how confinement reshapes the key processes that govern zinc electrochemistry, including Zn2+ solvation structure, water activity, ion association, transport, desolvation, deposition behavior, and interphase evolution. Low-confinement aqueous systems provide fast bulk transport but suffer from water-rich interfacial environments that promote hydrogen evolution, corrosion, passivation, and non-uniform zinc deposition. Strongly confined systems suppress these parasitic processes by restructuring coordination chemistry and limiting water reactivity, but often introduce new penalties in viscosity, ion correlation, and interfacial rearrangement. Polymer-rich and hybrid systems are especially important because they show that chemical and physical confinement can be combined to regulate both transport and interfacial selectivity within the same electrolyte. We further highlight how operando characterization, multiscale modeling, and descriptor-based analysis can make this framework experimentally actionable. This perspective provides a unified mechanistic language for interpreting recent progress in zinc electrolytes and offers a more rational basis for designing reversible, durable, and practically relevant aqueous zinc battery systems.