Cross-Regime Anion Chemistry Dictating Interfacial Architecture in Aqueous and Acetonitrile-Based Metal-Free Zinc-Ion Capatteries

The gravimetric and volumetric penalties associated with metallic zinc anodes motivate the development of metal-free zinc-ion architectures. However, the interfacial chemistry governing their operation remains poorly understood, particularly across distinct solvation environments. Here, we investigate how electrolyte anion chemistry in aqueous ZnSO 4 and nonaqueous Zn(Otf) 2 /acetonitrile electrolytes influences the solvation structure, interface composition, and charge transfer kinetics in a zinc-ion capattery employing an interstitially engineered zinc vanadate cathode and a hard carbon anode. MD simulations reveal fundamentally different primary solvation structures: strongly hydrated [Zn(H 2 O) 6 ] 2+ complexes in aqueous media and mixed acetonitrile-triflate coordination in nonaqueous systems. These differences translate into distinct interfacial chemistries. In the aqueous electrolyte, interfacial hydroxylation dominates, leading to a dynamically evolving, redox-active cathode electrolyte interface. In contrast, triflate-rich solvation promotes the formation of a fluorinated organic–inorganic interface composed of ZnF 2 and cyano-derived species, generating a stable, ion-permeable barrier. State-of-charge-resolved impedance spectroscopy correlates these interface architectures with diffusion-controlled kinetics in aqueous systems and stabilized charge transfer in nonaqueous media. Together, these findings establish a mechanistic link between anion-governed solvation structure and the emergent interfacial architecture, providing a framework for electrolyte design in metal-free zinc-ion energy storage.