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

Abstract

The performance of rechargeable magnesium batteries is strongly dependent on the choice of electrolyte. The desolvation of multivalent cations usually goes along with high energy barriers, which can have a crucial impact on the plating reaction. This can lead to significantly higher overpotentials for magnesium deposition compared to magnesium dissolution. In this work we combine experimental measurements with DFT calculations and continuum modelling to analyze Mg deposition in various solvents. Jointly, these methods provide a better understanding of the electrode reactions and especially the magnesium deposition mechanism. Thereby, a kinetic model for electrochemical reactions at metal electrodes is developed, which explicitly couples desolvation to electron transfer and, furthermore, qualitatively takes into account effects of the electrochemical double layer. The influence of different solvents on the battery performance is studied for the state-of-the-art magnesium tetrakis(hexafluoroisopropyloxy)borate electrolyte salt. It becomes apparent that not necessarily a whole solvent molecule must be stripped from the solvated magnesium cation before the first reduction step can take place. For Mg reduction it seems to be sufficient to have one coordination site available, so that the magnesium cation is able to get closer to the electrode surface. Thereby, the initial desolvation of the magnesium cation determines the deposition reaction for mono-, tri-and tetraglyme, whereas the influence of the desolvation on the plating reaction is minor for diglyme and tetrahydrofuran. Overall, we can give a clear recommendation for diglyme to be applied as solvent in magnesium electrolytes.

Boron–Based Electrolytes for Rechargeable Magnesium Batteries: Biography and Perspective

“…[58] Drews et al developed a kinetic model coupling the desolvation to electron transfer at metal Mg anode. [59] They demonstrated that it is not necessary to peel the entire solvent molecule from the solvated magnesium cation before the first step of reduction. As shown in Figure 5(g and h), the Mg[B-(Ohfip) 4 ] 2 /DME and/DG demonstrate huge similarities in the desolvation process.…”

“…Drews et al. developed a kinetic model coupling the de‐solvation to electron transfer at metal Mg anode [59] . They demonstrated that it is not necessary to peel the entire solvent molecule from the solvated magnesium cation before the first step of reduction.…”

Modeling of Magnesium Intercalation into Chevrel Phase Mo6S8: Report on Improved Cell Design

“…However, the high charge density of the magnesium cations also causes strong coulomb interactions with anions as well as solvent molecules and therefore the kinetic barriers for desolvation and solid-state diffusion are much higher than for lithium ions. [1,[9][10][11][12] Consequently, the choice of suitable electrolytes and cathode materials is not straightforward. Reversible magnesium insertion with reasonable kinetics was possible for the first time with a Chevrel Phase (CP) cathode (Mo 6 S 8 ).…”

“…[15,23] In general, the desolvation of the electroactive specie is an important step, which can kinetically hinder the electrochemical reaction at the anode as well as at the cathode side. [1,9,10,[38][39][40][41] Thereby, the presence of chloride anions is also beneficial for the desolvation process. [1,9,10,42,43] However, the corrosive nature of the chlorides is an issue for commercializing magnesium batteries and therefore research increasingly focuses on chloride-free electrolyte systems, whereby the stateof-the-art electrolytes are based on the non-nucleophilic magnesium tetrakis(hexafluoroisopropyloxy)borate salt Mg[B-(hfip) 4 ] 2 .…”

Continuum Modelling As Tool for Optimizing the Cell Design of Magnesium Batteries

“…[1] Moreover, energetic barriers for desolvation and solid-state diffusion of the double-charged magnesium ion are usually very high, which can have a crucial impact on the battery performance. Former can significantly hinder the electron-transfer reaction, [2] whereas latter makes the choice of suitable cathode materials very challenging. Consequently, a good understanding of the limiting processes in rechargeable magnesium batteries is key to develop novel high-capacity / high-voltage cathode materials.…”