Ion Pairing and Diffusion in Magnesium Electrolytes Based on Magnesium Borohydride

Abstract

One obstacle to realizing a practical, rechargeable magnesium-ion battery is the development of efficient Mg electrolytes. Electrolytes based on simple Mg(BH) 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(BH) 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(BH) 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 Mg were both observed to correlate with solvent self-diffusivity: electrolytes using longer- (shorter-) chain solvents had the lowest (highest) Mg diffusivity and agglomeration rates. Incorporation of Mg into Mg-BH clusters significantly reduces the diffusivity of Mg by restricting displacements to localized motion within largely immobile agglomerates. Consequently, diffusion is increasingly impeded with increasing Mg(BH) concentration. These data are consistent with the solubility limitations observed experimentally for Mg(BH)-based electrolytes and highlight the need for strategies that minimize salt agglomeration in electrolytes containing divalent cations.

Cation Solvation and Physicochemical Properties of Ca Battery Electrolytes

“…The THF-and glyme-based electrolytes show decreased ion conductivities as a function of temperature, an unusual behavior attributed to the presence and augmentation of strong ion pairing, as previously reported for Mg-glyme systems. 33 Overall, the DMF-based electrolytes preliminarily seem to offer the best performance, and together with more standard PC-and EC-based electrolytes, maximum ionic conductivities are obtained for salt concentrations of 0.42−0.57 M ( Figure 2), after which a steep decrease is recorded, especially at lower temperatures. The maxima correspond to the lowest activation energies (E a ) in the VTF fits ( Figure S6 and Table S3).…”

Electrolytes for Energy Dense Batteries

“…The In addition, computations were used to understand the role of coordination and effects of the solvent on the solubility and dissociation in these electrolytes. [80][81] Significant and irreversible salt agglomeration in all glymes ranging from THF to tetraglyme was found in all non-dilute borohydride salt solutions. 80 The agglomeration rate and diffusivity of Mg 2+ in longer chain ethers such as tetraglyme were at their lowest and tracked with the solvent's self-diffusivity.…”

Beyond Typical Electrolytes for Energy Dense Batteries

“…Follow up studies examined other chelating solvents such diglyme (DGM) and tetraglyme, which further demonstrated the direct relationship between the number of electrodonating oxygen sites and the coulombic efficiency of Mg plating/stripping [80,81]. In addition, computations were used to understand the role of coordination and the effects of the solvent on the solubility and dissociation in these electrolytes [82,83]. Significant and irreversible salt agglomeration in all glymes ranging from THF to tetraglyme was found in all non-dilute borohydride salt solutions [82].…”

“…In addition, computations were used to understand the role of coordination and the effects of the solvent on the solubility and dissociation in these electrolytes [82,83]. Significant and irreversible salt agglomeration in all glymes ranging from THF to tetraglyme was found in all non-dilute borohydride salt solutions [82].…”