Role of a Multivalent Ion-Solvent Interaction on Restricted Mg²⁺ Diffusion in Dimethoxyethane Electrolytes

Area of Science:

• Electrochemistry
• Materials Science
• Nuclear Magnetic Resonance Spectroscopy

Background:

• Studying magnesium ion (Mg2+) diffusion in electrolytes is crucial for battery technology.
• Traditional methods like pulsed field gradient (PFG) NMR are limited for Mg2+ due to low sensitivity and signal issues with 25Mg NMR.
• Distinguishing between coordinated and bulk solvent molecules is key to understanding ion transport.

Purpose of the Study:

• To develop and apply an NMR-based method for accurately measuring Mg2+ self-diffusivity in MgTFSI2/DME electrolytes.
• To investigate the factors limiting Mg2+ diffusion, particularly the interactions with solvent molecules.
• To compare the diffusion behavior of Mg2+ with other ions like Li+ and TFSI-.

Main Methods:

• Utilized 1H NMR to differentiate between "bound" and "free" dimethoxyethane (DME) in MgTFSI2/DME solutions.
• Employed 2D 1H EXSY NMR to determine the exchange rates between bound and free DME.
• Combined PFG diffusion NMR with analytical models for two-site exchange to calculate Mg2+ self-diffusivity via bound DME diffusion.

Main Results:

• Successfully extracted Mg2+ self-diffusivities by measuring the diffusion of bound DME.
• Observed a high activation enthalpy for DME exchange (65-70 kJ/mol), linked to structural changes in bound DME.
• Demonstrated that Mg2+ diffusion is relatively restricted compared to Li+ and TFSI-, attributed to long-range Mg2+-solvent interactions.

Conclusions:

• The developed NMR method provides a viable route to quantify Mg2+ diffusion, overcoming limitations of direct 25Mg NMR.
• Mg2+ mobility is significantly influenced by strong, long-range interactions with coordinating solvent molecules, impacting electrolyte performance.
• Understanding these interactions is vital for designing advanced magnesium-ion batteries.