Molecular Modeling of Water-in-Salt Electrolytes: A Comprehensive Analysis of Polarization Effects and Force Field Parameters in Molecular Dynamics Simulations

Abstract

Accurate modeling of highly concentrated aqueous solutions, such as water-in-salt (WiS) electrolytes in battery applications, requires proper consideration of polarization contributions to atomic interactions. Within the force field molecular dynamics (MD) simulations, the atomic polarization can be accounted for at various levels. Nonpolarizable force fields implicitly account for polarization effects by incorporating them into their van der Waals interaction parameters. They can additionally mimic electron polarization within a mean-field approximation through ionic charge scaling. Alternatively, explicit polarization description methods, such as the Drude oscillator model, can be selectively applied to either a subset of polarizable atoms or all polarizable atoms to enhance simulation accuracy. The trade-off between simulation accuracy and computational efficiency highlights the importance of determining an optimal level of accounting for atomic polarization. In this study, we analyze different approaches to include polarization effects in MD simulations of WiS electrolytes, with an example of a Na-OTF solution. These approaches range from a nonpolarizable to a fully polarizable force field. After careful examination of computational costs, simulation stability, and feasibility of controlling the electrolyte properties, we identify an efficient combination of force fields: the Drude polarizable force field for salt ions and non-polarizable models for water. This cost-effective combination is sufficiently flexible to reproduce a broad range of electrolyte properties, while ensuring simulation stability over a relatively wide range of force field parameters. Furthermore, we conduct a thorough evaluation of the influence of various force field parameters on both the simulation results and technical requirements, with the aim of establishing a general framework for force field optimization and facilitating parametrization of similar systems.

Sodium Triflate Water-in-Salt Electrolytes in Advanced Battery Applications: A First-Principles-Based Molecular Dynamics Study

“…In the presence of Drude oscillators, all ion particles are encompassed within DCs and DPs, meaning that the first summation in eq exclusively represents interactions between water molecules, where k q = 1. In this polarizable form, q D represents the Drude charge, which characterizes the induced dipoles on polarizable atoms (see ref ) and is defined as q D = α k D with α being the atomic polarizability. The factor k LJ in eq scales down the LJ interactions between DCs in the presence of Drude oscillators to avoid double counting of polarization effects (see refs and ).…”

“…In this polarizable form, q D represents the Drude charge, which characterizes the induced dipoles on polarizable atoms (see ref ) and is defined as q D = α k D with α being the atomic polarizability. The factor k LJ in eq scales down the LJ interactions between DCs in the presence of Drude oscillators to avoid double counting of polarization effects (see refs and ). The necessity of incorporating this factor is discussed in Section .…”

“…To enhance the accuracy of dynamic properties predictions, the ionic charge scaling method has been proposed and widely applied in MD simulations of WiS solutions and ionic liquids. This approach, however, does not guarantee a reliable representation of the electrolyte structure, , a critical consideration in WiS electrolytes that impacts both the charge transport mechanism and electrochemical stability. It underscores the importance of utilizing polarizable force fields to explicitly model polarization effects within the electrolyte.…”

Water-in-Salt: Fast Dynamics, Structure, Thermodynamics, and Bulk Properties

“…For low ion concentrations, less than ∼0.1 M, accurate predictions of ionic solution properties can often be obtained from models based on the Debye–Huckel theory. , At higher concentrations (greater than ∼1 M), however, analytical theories that describe the complex nature of ionic solutions are lacking. , This is especially true for the very high concentration regime, often referred to as “water-in-salt” electrolytes, which can have as few as three water molecules per ion pair. ,− Recent theoretical investigations of the structure and dynamics of water-in-salt solutions include molecular dynamics (MD) simulations based on optimized force fields, ,, ab initio neural network-based deep potential MD, and density functional theory simulations. , These methods predict that most salinity effects arise from the clustering of salt ions, which disrupt the water hydrogen bonding network at high salt concentrations.…”

“…22,24 This is especially true for the very high concentration regime, often referred to as "water-in-salt" electrolytes, which can have as few as three water molecules per ion pair. 4,25−27 Recent theoretical investigations of the structure and dynamics of water-in-salt solutions include molecular dynamics (MD) simulations based on optimized force fields, 11,28,29 ab initio neural network-based deep potential MD, and density functional theory simulations. 30,31 These methods predict that most salinity effects arise from the clustering of salt ions, which disrupt the water hydrogen bonding network at high salt concentrations.…”