The Association of Zn${}^{2+}$-SO${}_4^{2-}$ and Mg${}^{2+}$-SO${}_4^{2-}$ in Aqueous MgSO${}_4$/ZnSO${}_4$ Hybrid Electrolytes: Insights from All-Atom Molecular Dynamics Simulations Molecular Dynamics Simulations

Magnesium sulfate ($\mathrm{M}\mathrm{g}\mathrm{S}{\mathrm{O}}_4$) is used as an additive to reduce capacity fading in rechargeable zinc-ion batteries. This study investigates the ion pairing and solvation structure of $\mathrm{Z}{\mathrm{n}}^{2+}-\mathrm{S}{\mathrm{O}}_4^{2-}$ and $\mathrm{M}{\mathrm{g}}^{2+}-\mathrm{S}{\mathrm{O}}_4^{2-}$ in mixtures of $[\mathrm{Z}\mathrm{n}\mathrm{S}{\mathrm{O}}_4]2\mathrm{M}+[\mathrm{M}\mathrm{g}\mathrm{S}{\mathrm{O}}_4]0\mathrm{M}$, $[\mathrm{Z}\mathrm{n}\mathrm{S}{\mathrm{O}}_4]1\mathrm{M}+[\mathrm{M}\mathrm{g}\mathrm{S}{\mathrm{O}}_4]1\mathrm{M}$, and $[\mathrm{Z}\mathrm{n}\mathrm{S}{\mathrm{O}}_4]0\mathrm{M}+[\mathrm{M}\mathrm{g}\mathrm{S}{\mathrm{O}}_4]2\mathrm{M}$. Using molecular dynamics simulations, we evaluated dipole dynamics, spatial distributions, and coordination of ions through radial distribution functions, running coordination numbers, and potentials of mean force. The results reveal that increasing $\mathrm{M}\mathrm{g}\mathrm{S}{\mathrm{O}}_4$ concentration disrupts $\mathrm{Z}{\mathrm{n}}^{2+}$ self-association, while $\mathrm{M}{\mathrm{g}}^{2+}$ shows enhanced association. The analysis indicates different solvation structures, with $\mathrm{Z}{\mathrm{n}}^{2+}$ maintaining a more ordered coordination with $\mathrm{S}{\mathrm{O}}_4^{2-}$ than $\mathrm{M}{\mathrm{g}}^{2+}$. Preferential binding analysis highlights stronger $\mathrm{Z}{\mathrm{n}}^{2+}$ affinity for $\mathrm{S}{\mathrm{O}}_4^{2-}$, shedding light on ion interaction dynamics in mixed electrolytes.