Ion Pairing and Diffusion in Magnesium Electrolytes Based on Magnesium Borohydride

One obstacle to realizing a practical, rechargeable magnesium-ion battery is the development of efficient Mg electrolytes. Electrolytes based on simple Mg(BH₄)₂ salts suffer from poor salt solubility and/or low conductivity, presumably due to strong ion pairing. Understanding the molecular-scale processes occurring in these electrolytes would aid in overcoming these performance limitations. Toward this goal, the present study examines the solvation, agglomeration, and transport properties of a family of Mg electrolytes based on the Mg(BH₄)₂ salt using classical molecular dynamics. These properties were examined across five different solvents (tetrahydrofuran and the glymes G1-G4) and at four salt concentrations ranging from the dilute limit up to 0.4 M. Significant and irreversible salt agglomeration was observed in all solvents at all nondilute Mg(BH₄)₂ concentrations. The degree of clustering observed in these divalent Mg systems is much larger than that reported for electrolytes containing monovalent cations, such as Li. The salt agglomeration rate and diffusivity of Mg²⁺ were both observed to correlate with solvent self-diffusivity: electrolytes using longer- (shorter-) chain solvents had the lowest (highest) Mg²⁺ diffusivity and agglomeration rates. Incorporation of Mg²⁺ into Mg²⁺-BH₄^- clusters significantly reduces the diffusivity of Mg²⁺ by restricting displacements to localized motion within largely immobile agglomerates. Consequently, diffusion is increasingly impeded with increasing Mg(BH₄)₂ concentration. These data are consistent with the solubility limitations observed experimentally for Mg(BH₄)₂-based electrolytes and highlight the need for strategies that minimize salt agglomeration in electrolytes containing divalent cations.