Ion Pairing and Redissociaton in Low-Permittivity Electrolytes for Multivalent Battery Applications

Area of Science:

• Electrochemistry
• Materials Science
• Computational Chemistry

Background:

• Detailed electrolyte speciation is crucial for understanding ion transport and decomposition.
• Multivalent electrolytes exhibit a strong link between anodic stability and solvation structure.
• Stable solvents for alkaline-earth metals often have low permittivity, complicating electrolyte species.

Purpose of the Study:

• To investigate the speciation and transport of magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2) in glyme solvents.
• To understand the ionic population in low-permittivity electrolytes using a multiscale thermodynamic model.
• To validate computational findings with experimental spectroscopic data.

Main Methods:

• Utilized a multiscale thermodynamic model combining first-principles calculations and molecular dynamics simulations.
• Calculated ion association and dielectric properties for Mg(TFSI)2 in monoglyme and diglyme.
• Compared modeling results with experimental Raman and dielectric relaxation spectroscopies.

Main Results:

• Observed a significant concentration of free ions in low-permittivity glymes (0.02–0.6 M).
• Explained this phenomenon using the low-permittivity redissociation hypothesis.
• Found that long-range electrostatics, including polar contact ion pairs, primarily dictate salt speciation.

Conclusions:

• The redissociation hypothesis effectively explains high free ion concentrations in low-permittivity multivalent electrolytes.
• Salt speciation is governed by long-range electrostatic interactions in these systems.
• Suggests potential for high conductivity in other low-permittivity multivalent electrolytes via redissociation.