Charting Water-in-Natural-Salt (WiNS) Electrolytes. Part I: Effect of Anion's Alkyl Tail Length

Water-in-natural-salt electrolytes are fluorine-free candidates for safer high voltage aqueous energy storage. We study 20 m solutions of potassium formate, acetate, propionate, and butyrate to test how anion amphiphilicity controls nanostructure, ion transport, and electrochemical stability. Small-angle X-ray scattering reveals a low-q pre-peak that shifts to lower q with increasing tail length, consistent with a growing modulation length and a distorted sponge-like bicontinuous morphology. Molecular dynamics assigns this pre-peak to concentration–concentration fluctuations that co-localize water and K+ in polar channels, anticorrelated with anion-centered alkyl domains. Water cluster analysis shows that average cluster size drops from formate to acetate in parallel with a pronounced widening of the electrochemical stability window, while conductivity decreases along the series. These trends identify potassium acetate as an optimal compromise between stability and mobility and suggest structural targets for fluorine-free aqueous electrolytes.