Nanostructuring and Underscreening of Bisalt Electrolytes with Dual-Anion Effects: Insights from Small-Angle Scattering Prepeak Analysis.

Using small-angle neutron scattering, we studied the nanostructure of a prototypical D(H)-bonded network electrolyte, alkaline sodium aluminate bisalt, at concentrations up to solute volume fraction of ∼0.5. Analysis of the structure factor prepeak at 0.1 < Q < 1.3 Å-1 showed that its evolution is associated with nanoscopic species distribution in water-anion network, with differences in characteristic distance d between OD- and Al(OD)4- anions related to their distinct sizes and interactions. When the solute volume fraction approached 0.5, a common/maximum value of ∼4.5 Å was found for the correlation length ξ that characterizes the electrostatic force in concentrated electrolyte solution before precipitation occurred. This implies that the solutions' morphology and behavior at high concentrations may be governed by geometric factors, rather than the chemistry of the specific anion. Furthermore, the scaling of ξ with concentration yielded an exponent of 1.25(1), suggesting that ξ is limited to a few Å. By combining the current observations of structural heterogeneity at the nanoscale, with dynamic heterogeneity at the microscopic scale from our previous quasi-elastic neutron scattering study, we have established a structural origin of local "caging" and restricted structural relaxation processes. These local solvent-solute interactions not only control dynamics heterogeneity in concentrated electrolytes but are also responsible for crystallization processes in industrial setting, such as aluminum production and radioactive waste treatment.