Exploring the Liquid Structure and Ion Formation in Magnesium Borohydride Electrolyte Using Density Functional Theory

Abstract

Rechargeable magnesium batteries have attracted much interest due their high volumetric capacity, potential for safe operation, and the natural abundance of magnesium. However, the development of magnesium batteries for practical applications has been obstructed by the lack of understanding of the liquid structure of electrolytes. Herein, we use quantum density functional theory coupled with a continuum solvation model to investigate the structure of Mg(BH 4 ) 2 in two ethereal solvents: tetrahydrofuran (THF), and monoglyme (G1). The most energetically favorable clusters of Mg(BH 4 ) 2 , MgBH 4 + , and Mg 2+ , with associated solvent molecule ligands, are determined. The free energy required to generate monovalent ions in the electrolyte is positive and the formation of divalent complexes is prohibitive. Singly and doubly charged complexes are more stable in G1 than THF, which is consistent with experimental findings. From the standpoint of free energy, clusters containing multiple magnesium atoms are not favored. Theoretical 25 Mg-NMR, 11 B-NMR spectra, and infrared vibrational modes of borohydride were calculated for each cluster. The relationships between cluster charge and the signals of each spectrum are determined. These analytical descriptors could be useful to characterize the degree of ion dissociation in the electrolyte. For rechargeable batteries to become widespread in electric vehicle and grid energy storage applications, it is imperative that they are safe and low cost.1,2 These challenges have motivated researchers in recent years to consider alternatives to lithium-ion batteries, such as sodium 3,4 and multivalent 5,6 battery chemistries. Ever since the first prototypes were developed, 7,8 magnesium batteries have attracted much interest from the research community. Compared with lithiumion batteries, magnesium batteries have several advantages, such as their high volumetric capacity (3832 mAh/cm 3 ), which is a result of the divalency of magnesium. Additionally, the cycling of plating/stripping of Mg 2+ onto the magnesium metal anode does not produce dendrites, which alleviates one of safety concerns of lithium batteries. 9,10 However, magnesium batteries face crucial challenges before they can succeed in practical applications.11 In particular, the choice of the electrolyte is critical to their performance. 12,13 There are only a handful of electrolytes known to be stable 14 during battery cycling, due to the reactivity of the Mg anode. that an electrolyte consisting of magnesium borohydride in an ethereal solvent, such as tetrahydrofuran or monoglyme, could reversibly plate/strip Mg 2+ onto a magnesium metal anode. X-ray photoelectron spectroscopy 13 has shown that a Mg anode using this electrolyte did not yield signals for boron or carbon, implying that the electrolyte is electrochemically stable during charge and discharge cycles. This marked the discovery of the first stable, halide-free electrolyte for magnesium batteries. Since then, there have been a number of experimental 13,[20][21][22...

The critical role of configurational flexibility in facilitating reversible reactive metal deposition from borohydride solutions

“…These attributes are contrasted with those of Mg(BH4)2 electrolytes, which have been the subject of several recent studies. 5,[18][19][20][21] Our integrated investigation demonstrates that the relative concentration of ionic clusters is enhanced in high concentration Ca(BH4)2/THF, and that BH4is strongly coordinated to Ca 2+ within these clusters. From this analysis, we propose that the electroactive cationic species is CaBH 4 + and that this species is responsible for delivery of Ca 2+ to the electrode.…”

Influence of Complexing Additives on the Reversible Deposition/Dissolution of Magnesium in an Ionic Liquid

“…Focusing on BH 4 − ‐containing electrolytes, both experimental studies [15,50] and calculations [61,62] for several electrolytes have concluded that the dissociation of the Mg 2+ −BH 4 − coordination is a key aspect for Mg deposition, which, in ether‐type solvents, is influenced by the number of coordinating oxygen atoms or possibly also by coordinating TFSI − ions. The Mg 2+ coordination in BH 4 − ‐ and TFSI − ‐containing diglyme was investigated by Hu et al .…”

“…À -containing electrolytes, both experimental studies [15,50] and calculations [61,62] for several electrolytes have concluded that the dissociation of the Mg 2 + À BH 4…”

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.…”