The nature of nanoclusters in aqueous zinc sulfate electrolytes

Abstract

The presence of ion nanoclusters or aggregates in electrolytes strongly impacts the electrolyte physical and chemical properties. Herein, we investigate the nature of ion nanoclusters in aqueous zinc sulfate electrolytes for zinc-ion batteries, specifically their size distribution, composition and lifetime, and subsequent influence on transport properties. Using a combination of X-ray scattering, molecular dynamics (MD) simulations and forward scattering calculations, we reveal the presence of free ions and a distribution of nanoclusters composed of solvent-separated ion pairs, whose size distributions are concentration-independent. Transient interactions between sulfate ions and long-lived hydrated zinc ions produce nanoclusters with varying sizes, compositions and geometries. Pair distribution functions calculated from MD simulations show strong agreement with experimental X-ray total scattering measurements. Nanoclusters were identified from MD simulations and used to forward simulate small-angle X-ray scattering (SAXS) of these nanoclusters. By fitting our measured SAXS using these forward simulations, we determine the distribution of nanocluster sizes in the electrolyte. Transport calculations from MD simulations and experimental measurements show that while nanoclusters hinder ion transport, their short-lived and dynamic nature enables efficient ion diffusion. Determining the nature of these nanoclusters is essential for understanding their impact on transport, solvation, and interfacial chemistry, and guiding the rational design of electrolytes for energy storage, chemical separations and environmental science.