Elucidating Solvation Structures for Rational Design of Multivalent Electrolytes—A Review

Abstract

Fundamental molecular-level understanding of functional properties of liquid solutions provides an important basis for designing optimized electrolytes for numerous applications. In particular, exhaustive knowledge of solvation structure, stability, and transport properties is critical for developing stable electrolytes for fast-charging and high-energy-density next-generation energy storage systems. Accordingly, there is growing interest in the rational design of electrolytes for beyond lithium-ion systems by tuning the molecular-level interactions of solvate species present in the electrolytes. Here we present a review of the solvation structure of multivalent electrolytes and its impact on the electrochemical performance of these batteries. A direct correlation between solvate species present in the solution and macroscopic properties of electrolytes is sparse for multivalent electrolytes and contradictory results have been reported in the literature. This review aims to illustrate the current understanding, compare results, and highlight future needs and directions to enable the deep understanding needed for the rational design of improved multivalent electrolytes.

Modeling of Electron‐Transfer Kinetics in Magnesium Electrolytes: Influence of the Solvent on the Battery Performance

“…[ 2 , 10 , 11 , 12 , 13 , 14 ] At the same time the desolvation of magnesium ions close to the electrode surface plays an important role during the deposition or intercalation process. [ 1 , 9 , 15 , 16 , 17 , 18 ] Since a sluggish desolvation can kinetically hinder the charge transfer reaction, the solvation of the magnesium cation should only be as good as necessary. Consequently, the choice of both – solvent and anion – is crucial for the performance of magnesium batteries.…”

Undesired Reactions in Aqueous Rechargeable Zinc Ion Batteries

“…4 Third, water can enhance the kinetics at the cathode−electrolyte interface in RZIBs due to a reduced charge transfer resistance, as demonstrated by Kundu et al 9 Finally, water, being a polar protic medium with a high dielectric constant, enables much improved zinc salt solubilities compared to conventional organic solvents. 8,10 For instance, while the solubility of Zn(CF 3 SO 3 ) 2 in acetonitrile is <0.5 M, 8,11 its solubility in water is ∼3 M. 12 This has been ascribed to the fact that the organic solvents cannot dissociate the salt cation−anion pair as effectively due to the high chargeto-size ratio of the divalent Zn 2+ cation. 13 The undissociated salt cation−anion pairing can impede the mobility of the ions and hence limit the electrolyte's bulk ionic conductivity.…”

“…13 The undissociated salt cation−anion pairing can impede the mobility of the ions and hence limit the electrolyte's bulk ionic conductivity. 11 Even though water can benefit the RZIB system in the ways mentioned above, it comes at a cost. In aqueous electrolytes, the water solvent molecules are one of the root causes for most of the undesired reactions in the RZIB system.…”

“…First, compared to conventional organic solvents, water is cheaper, less flammable, and safer. , Second, the ionic conductivities manifested by the aqueous electrolytes (∼10–60 mS/cm) , are much higher than those of the organic solvent-based electrolytes (∼1–10 mS/cm), which lead to improved RZIB rate performance . Third, water can enhance the kinetics at the cathode–electrolyte interface in RZIBs due to a reduced charge transfer resistance, as demonstrated by Kundu et al Finally, water, being a polar protic medium with a high dielectric constant, enables much improved zinc salt solubilities compared to conventional organic solvents. , For instance, while the solubility of Zn­(CF 3 SO 3 ) 2 in acetonitrile is <0.5 M, , its solubility in water is ∼3 M . This has been ascribed to the fact that the organic solvents cannot dissociate the salt cation–anion pair as effectively due to the high charge-to-size ratio of the divalent Zn 2+ cation .…”

“…This has been ascribed to the fact that the organic solvents cannot dissociate the salt cation–anion pair as effectively due to the high charge-to-size ratio of the divalent Zn 2+ cation . The undissociated salt cation–anion pairing can impede the mobility of the ions and hence limit the electrolyte’s bulk ionic conductivity …”