Quantifying Species Populations in Multivalent Borohydride Electrolytes

Ca-dimers, solvent layering, and dominant electrochemically active species in Ca(BH4)2 in THF

Divalent ions (Mg, Ca, and Zn) are being considered as competitive, safe, and earth-abundant alternatives to Li-ion electrochemistry, but present challenges for stable cycling due to undesirable interfacial phenomena. We explore the formation of electroactive species in the electrolyte Ca(BH4)2∣THF using molecular dynamics coupled with a continuum model of bulk and interfacial speciation. Free-energy analysis and unsupervised learning indicate a majority population of neutral Ca dimers and monomers with diverse molecular conformations and an order of magnitude lower concentration of the primary electroactive charged species - the monocation, CaBH[Formula: see text] - produced via disproportionation of neutral complexes. Dense layering of THF molecules within ~1 nm of the electrode surface strongly modulates local electrolyte species populations. A dramatic increase in monocation population in this interfacial zone is induced at negative bias. We see no evidence for electrochemical activity of fully-solvated Ca2+. The consequences for performance are discussed in light of this molecular-scale insight.

Elucidating the Molecular Origins of the Transference Number in Battery Electrolytes Using Computer Simulations

The rate at which rechargeable batteries can be charged and discharged is governed by the selective transport of the working ions through the electrolyte. Conductivity, the parameter commonly used to characterize ion transport in electrolytes, reflects the mobility of both cations and anions. The transference number, a parameter introduced over a century ago, sheds light on the relative rates of cation and anion transport. This parameter is, not surprisingly, affected by cation-cation, anion-anion, and cation-anion correlations. In addition, it is affected by correlations between the ions and neutral solvent molecules. Computer simulations have the potential to provide insights into the nature of these correlations. We review the dominant theoretical approaches used to predict the transference number from simulations by using a model univalent lithium electrolyte. In electrolytes of low concentration, one can obtain a quantitative model by assuming that the solution is made up of discrete ion-containing clusters-neutral ion pairs, negatively and positively charged triplets, neutral quadruplets, and so on. These clusters can be identified in simulations using simple algorithms, provided their lifetimes are sufficiently long. In concentrated electrolytes, more clusters are short-lived and more rigorous approaches that account for all correlations are necessary to quantify transference. Elucidating the molecular origin of the transference number in this limit remains an unmet challenge.

Toggling Calcium Plating Activity and Reversibility through Modulation of Ca2+ Speciation in Borohydride-based Electrolytes

Learning how to tailor Ca2+ speciation and electroactivity is of central importance to engineer next-generation battery electrolytes. Using an exemplar dual-salt electrolyte, Ca(BH4)2 + Ca(TFSI)2 in THF, this work examines how to modulate a critical parameter proposed to govern electroactivity, the BH4 -/Ca2+ ratio. Introduction of a more-dissociating source of Ca2+ via Ca(TFSI)2 drives re-speciation of strongly ion-paired Ca(BH4)2, confirmed by ionic conductivity, Raman spectroscopy, and reaction microcalorimetry measurements, generating larger populations of charged species and enhancing plating currents. Ca plating is possible when [Ca(TFSI)2] < [Ca(BH4)2] and thus BH4 -/Ca2+ >1, but a dramatic shut-down of plating activity occurs when [Ca(TFSI)2] > [Ca(BH4)2] (BH4 -/Ca2+ <1), directly evidencing the significance of coordination-shell chemistry on plating activity. Ca(BH4)2 + TBABH4 in THF, which enables enrichment of BH4 - concentrations compared to Ca2+, is also examined; ionic conductivity and plating currents also increase compared to Ca(BH4)2/THF, with the latter related in part to a decrease in solution resistance. These findings delineate future directions to modulate Ca2+ coordination towards achieving both high plating activity and reversibility.

Molecular Insight into Microstructural and Dynamical Heterogeneities in Magnesium Ionic Liquid Electrolytes

Ionic liquids (ILs) are promising designer solvents for multivalent electrolytes, enabling the modulation of molecular-level interactions of solvate species. The molecular mechanism of multivalent-ion clustering and its impact on electrolytes properties is far less studied than that of ion pairs. Herein, we explore the effect of ion clusters on the transport and electrochemical behavior of IL-based electrolytes for Mg batteries. Simulation and small-angle X-ray scattering results indicate that ILs with higher denticity effectively suppress ion agglomeration and parasitic reactions of the Mg electrolytes. Although ion clustering reduces the diffusivity of Mg²⁺, the Coulombic efficiency for the reversible Mg deposition/stripping process is improved, highlighting the importance of microstructural and dynamical heterogeneities in the rational design of enhanced multivalent electrolytes.

Concentration-dependent ion correlations impact the electrochemical behavior of calcium battery electrolytes

Ion interactions strongly determine the solvation environments of multivalent electrolytes even at concentrations below that required for practical battery-based energy storage. This statement is particularly true of electrolytes utilizing ethereal solvents due to their low dielectric constants. These solvents are among the most commonly used for multivalent batteries based on reactive metals (Mg, Ca) due to their reductive stability. Recent developments in multivalent electrolyte design have produced a variety of new salts for Mg²⁺ and Ca²⁺ that test the limits of weak coordination strength and oxidative stability. Such electrolytes have great potential for enabling full-cell cycling of batteries based on these working ions. However, the ion interactions in these electrolytes exhibit significant and non-intuitive concentration relationships. In this work, we investigate a promising exemplar, calcium tetrakis(hexafluoroisopropoxy)borate (Ca(BHFIP)₂), in the ethereal solvents 1,2-dimethoxyethane (DME) and tetrahydrofuran (THF) across a concentration range of several orders of magnitude. Surprisingly, we find that effective salt dissociation is lower at relatively dilute concentrations (e.g. 0.01 M) than at higher concentrations (e.g. 0.2 M). Combined experimental and computational dielectric and X-ray spectroscopic analyses of the changes occurring in the Ca²⁺ solvation environment across these concentration regimes reveals a progressive transition from well-defined solvent-separated ion pairs to de-correlated free ions. This transition in ion correlation results in improvements in both conductivity and calcium cycling stability with increased salt concentration. Comparison with previous findings involving more strongly associating salts highlights the generality of this phenomenon, leading to important insight into controlling ion interactions in ether-based multivalent battery electrolytes.