Dual‐Metal Electrolytes for Hybrid‐Ion Batteries: Synergism or Antagonism?

Abstract

The construction of hybrid metal‐ion batteries faces a plethora of challenges. A critical one is to unveil the solvation/desolvation processes at the molecular level in electrolytes that ensure efficient transfer of several types of charge carriers. This study reports first results on simulations of mixed‐ion electrolytes. All combinations of homo‐ and hetero‐binuclear complexes of Li+, Na+ and Mg2+, solvated with varying number of ethylene carbonate (EC) molecules are modeled in non‐polar and polar environment by means of first principles calculations and compared to the mononuclear analogues in terms of stability, spatial organization, charge distribution and solvation/desolvation behavior. The used PF6− counterion is shown to have minor impact on the geometry of the complexes. The desolvation energy penalty of binuclear complexes can be lowered by the fluoride ions, emerging upon the PF6− decay. These model investigations could be extended to rationalize the solvation structure and ionic mobility in dual‐ion electrolytes.

Rivalry at the Interface: Ion Desolvation and Electrolyte Degradation in Model Ethylene Carbonate Complexes of Li+, Na+, and Mg2+ with PF6– on the Li4Ti5O12 (111) Surface

“…The results are compared with earlier studies on the desolvation energy of the same ions in vacuo and in highly polar (ε ∼ 90) implicit solvent. 27 As expected, the removal of the last solvent molecules is growingly endothermic in a relatively narrow range in polar medium. The presence of the surface compensates for the lost solvent molecules.…”

“…In dual-ion electrolytes, earlier studies revealed that the scarcity of solvent favors the formation of hetero-binuclear complexes of the type M 1 p+ M 2 q+ (EC) n . 27 Therefore, we have modeled all binuclear combinations of Li + , Na + , and Mg 2+ with n = 0, 1, 2, 3 adsorbed on the studied surface (Figure 5 and Figure S3). Unlike the single-cation EC clusters, in this case, the adsorption cannot be equally favorable for both ions, so we tested different starting geometries.…”

“…The optimized geometries revealed that the counterion plays the part of a solvent molecule as it does in the absence of an electrode surface. 27 The bare Li + PF 6 − pair adsorbs in a predictable manner on the LTO surface, the anion complementing the coordination of Li + . In Li + PF 6 − EC, the attraction of Li + holds the EC molecule close to the LTO surface, which results in the catalytic dehydrogenation of EC and formation of VC.…”

“…26 In contrast, in dual Li(Na)-Mg electrolytes at low and moderate degrees of solvation, the most stable species are heteronuclear complexes Li + Mg 2+ (EC) n and Na + Mg 2+ (EC) n , which is why they determine electrolyte behavior at the surface. 27 Thus, the ionic transport property of the Mg-based electrolyte is improved significantly by addition of Li + or Na + ions, which has a direct impact on the rate capability of hybrid Li/Mg-and Na/Mg-ion batteries. 17,18,28 Other concerns are closely related to the processes occurring at the electrode/electrolyte interface.…”

“…34,35 In the case of incomplete solvation, the counterion complements the coordination shell around the metal ion, thus making the consecutive release of solvent molecules easier. 27 Most of these findings rely on molecular modeling, where, for simplicity, the effect of the electrode surface is not explicitly accounted for. Experimentally, it has been well documented that for the conventional graphitic electrode, the formation of SEI film has a direct impact on the desolvation process.…”