Concentration-Dependent Solvation Structure, Ion Association and Transport in Aqueous ZnCl₂ Electrolytes: A Self-Consistent Force- Field Study with Effective Charge Scaling

Abstract

Concentrated aqueous ZnCl₂ solutions determine the stability of mild zinc batteries, yet how rising salt concentration reshapes the Zn ²⁺ first solvation shell and ion association, and whether a classical non-polarizable force field can describe this behaviour, remain controversial. Here we ran all-atom molecular dynamics simulations of aqueous ZnCl₂ at four concentrations (1–10 mol·kg ⁻¹ ) at 298.15 K and 1 bar, with three independent replicas and sampling long enough to permit ligand exchange at every concentration. The simulations reproduce the experimental first-shell structure: with rising concentration, water is progressively displaced from the first coordination sphere by chloride, and the speciation evolves from the octahedral hexa-aqua complex [Zn(H₂O)₆] ²⁺ toward higher chloro-complexes such as [ZnCl₃] ⁻ and [ZnCl₄] ²⁻ . Self-diffusion of all species decreases with concentration, while the Nernst–Einstein conductivity first rises and then declines, reproducing the experimentally reported maximum near 6 mol·kg ⁻¹ . Finally, an effective-charge-scaling (ECC) boundary test shows that charge scaling alone erases Zn–Cl contact ion pairing, and only its combination with a dedicated Zn–Cl term retains physically meaningful association. These results quantify the minimum physics a classical model requires for a quantitative description of concentrated ZnCl₂ electrolytes.

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