Molecular Dynamics Simulation Study on Heterogeneous Structure, Rheology, and Dynamic Correlation of Concentrated...

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Understanding the liquid structure of electrolyte solutions is crucial for their application in electrochemical devices.
• Lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) is a common electrolyte salt, but its structural dynamics in solution are not fully understood.
• Relating microscopic structure to macroscopic properties like viscosity is a key challenge.

Purpose of the Study:

• To investigate the relationship between the liquid structure and frequency-dependent shear viscosity of aqueous LiTFSA solutions.
• To elucidate the origins of the observed viscoelastic relaxation modes.
• To understand the influence of salt concentration on solution structure and dynamics.

Main Methods:

• Molecular dynamics (MD) simulations were employed to model aqueous LiTFSA solutions at various concentrations.
• Analysis of the structure factor to identify domain formation.
• Measurement of frequency-dependent shear viscosity and its relation to structural dynamics.
• Cross-correlation analysis between shear stress and two-body density.

Main Results:

• A low-q peak in the structure factor indicated a heterogeneous structure with distinct water and anion domains.
• Lithium ions were predominantly found in the water-rich domains due to strong hydration.
• Frequency-dependent shear viscosity exhibited bimodal relaxation, with the slower mode increasing in contribution with salt concentration.
• The slower viscoelastic mode was attributed to the dynamics of the heterogeneous domain structure.

Conclusions:

• Aqueous LiTFSA solutions exhibit a concentration-dependent heterogeneous structure.
• The observed viscoelastic behavior is directly linked to the dynamics of these water and anion domains.
• MD simulations provide valuable insights into the structure-property relationships in electrolyte solutions.