Advancing the CL&Pol Polarizable Force Field for Accurate Modeling of Mg²⁺, Ca²⁺, and Zn²⁺ Electrolytes

Divalent cations such as Mg²⁺, Ca²⁺, or Zn²⁺ are promising candidates for powerful and affordable post-Li batteries due to their high natural abundance and an expected two-electron transfer per charge carrier. But an accurate representation of divalent cations in electrolytes by molecular dynamics simulations remains challenging due to the effects of their high charge density. For ionic systems such as the ones studied here, a molecular interaction model with explicit polarization terms, using Drude induced dipoles, largely improves the representation of equilibrium and transport properties when compared to fixed-charge models. Some issues regarding the overpolarization of the induced dipoles require attention. Thole and Tang-Toennies screening (or damping) functions are parametrized to avoid the overpolarization effects, and were validated by simulating MgCl₂, CaCl₂, and ZnCl₂ salts in aqueous solutions. Following this, simulations with model electrolytes based on two commonly used solvents─dimethoxyethane (G1) and ethylene carbonate (EC)─were performed. The simulations of the G1 and EC electrolytes highlight significant differences in structural and dynamic behaviors between Ca²⁺, Mg²⁺, and Zn²⁺, depending on the scaling of the Lennard-Jones well-depth, which is performed when adding explicit induced dipoles to a nonpolarizable force field. The study of the impact of this scaling, investigated for both systems G1 and EC, provides a framework for future simulations involving divalent cations with the polarizable force field developed by Canongia Lopes and Padua (CL&Pol). These results, coupled with the transferability of the CL&Pol model, offer an efficient tool to simulate a broad range of systems involving divalent ions, from biological media to electrolytes for novel energy storage devices.