Ion Pairing and Diffusion in Magnesium Electrolytes Based on Magnesium Borohydride

Area of Science:

• Materials Science
• Electrochemistry
• Computational Chemistry

Background:

• Developing efficient electrolytes is crucial for practical rechargeable magnesium-ion batteries.
• Magnesium borohydride (Mg(BH4)2) electrolytes exhibit poor salt solubility and low conductivity due to ion pairing.

Purpose of the Study:

• To investigate the molecular-scale solvation, agglomeration, and transport properties of Mg(BH4)2-based electrolytes.
• To understand how solvent choice and salt concentration impact electrolyte performance.

Main Methods:

• Classical molecular dynamics simulations were employed.
• Simulations covered five solvents (THF, G1-G4 glymes) and concentrations from dilute to 0.4 M Mg(BH4)2.

Main Results:

• Significant and irreversible salt agglomeration of Mg(BH4)2 was observed in all solvents at non-dilute concentrations.
• Mg2+ diffusivity correlated inversely with solvent chain length and was significantly reduced by cluster formation.
• Observed agglomeration is more pronounced than in monovalent cation electrolytes.

Conclusions:

• Salt agglomeration in Mg(BH4)2 electrolytes severely restricts Mg2+ ion transport.
• Strategies to minimize agglomeration are needed to improve Mg-ion battery electrolytes.
• Findings align with experimental observations of solubility limitations.