The Importance of Nanoscale Phenomena in Electrolyte Behavior: Acetonitrile-Aqueous Zinc Sulfate Electrolytes

Abstract

Abstract Electrolytes composed of mixed solvents are of significant scientific and technological importance in lithium-ion and beyond-lithium-ion batteries, and more broadly across physical chemistry. Their emergence has highlighted the importance of multiscale phenomena, particularly the development of nanoscale solution heterogeneities. It is therefore essential to assess how these inhomogeneities and nanoscale aggregates influence solvation, transport, and electronic properties. Here we investigate the solution structure and transport properties of aqueous zinc sulfate electrolytes with acetonitrile (ACN) additives, where the addition of ACN leads to improved cyclability. Using complementary anomalous small-angle X-ray scattering, X-ray pair distribution function, electrochemistry, and NMR, we identify the presence of salt-rich-water-rich and salt-poor-ACN-rich nanoheterogeneities and, at higher ACN concentrations, liquid–liquid phase separation. We find that the size of zinc sulfate ion clusters in the water domains increases slightly upon ACN addition, while the ACN-rich domains grow much larger as phase separation is approached. The inverse Haven ratio, determined using the ionic conductivity and diffusivity, is much less than unity and decreases slightly upon addition of ACN, which shows ion–ion interactions. We speculate that the increased ion–ion interactions with increased ACN are a result of a reduction in free water, the increase in zinc sulfate cluster size, and/or an increase in solution tortuosity. This work highlights key phenomena to consider when engineering new water-organic-salt electrolyte systems and establishes a phase diagram for water–ACN–ZnSO4 that can guide advances, such as self-adaptive electrolytes.