Probing Water Structure and Ion Correlations Across Concentrated Salt Solutions

Ion solvation and water structuring in aqueous salt solutions are fundamental to a wide range of applications, from desalination to energy storage. While many studies address battery salts at low to intermediate concentrations, research at high salt concentrations, where water behavior changes substantially, is limited. To fill this gap, we probe the local structure of water and ion correlations in salt solutions at concentrations ranging from semi-dilute to near saturation using high-energy X-ray diffraction with pair distribution function (PDF) analysis. We find that the addition of alkali and alkali-earth metal halides to water results in a slight increase in the nearest-neighbor distance of water, while decreasing the intensity and broadening the O-O peak in PDF, which is attributed to the disruption of the water tetrahedral network. We observe that bromides have a greater impact on disrupting water structural correlations than their corresponding chlorides. M⁺-O peak shifts to a longer distance with an increase in cation radius. Interestingly, the second neighbor correlation peak in Li⁺ salts is weakened, indicating a loss of medium-range order in the liquid structure compared to the other studied salts.