Ion Agglomeration and Transport in MgCl₂-Based Electrolytes for Rechargeable Magnesium Batteries

Abstract

Magnesium halide salts are an exciting prospect as stable and high-performance electrolytes for rechargeable Mg batteries (RMBs). By nature of their complex equilibria, these salts exist in solution as a variety of electroactive species (EAS) in equilibrium with counterions such as AlCl 4− . Here we investigated ion agglomeration and transport of several such EAS in MgCl 2 salts dissolved in ethereal solvents under both equilibrium and operating conditions using largescale atomistic simulations. We found that the solute morphology is strongly characterized by the presence of clusters and is governed by the solvation structures of EAS. Specifically, the isotropic solvation of Mg 2+ results in the slow formation of a bulky cluster, compared with chainlike analogues observed in the Cl-containing EAS such as Mg 2 Cl 3 + , MgCl + , and Mg 2 Cl 2 2+ . We further illustrate these clusters can reduce the diffusivity of charge-carrying species in the MgCl 2 -based electrolyte by at least an order of magnitude. Our findings for cluster formation, morphology, and kinetics can provide useful insight into the electrochemical reactions at the anode− electrolyte interface in RMBs.

Understanding the nature of the passivation layer enabling reversible calcium plating

“…Previous reports have shown that the desolvation process at the electrode surface is no-trivial for Mg electrolytes and can be the rate limiting step for Li + insertion into graphite anode. [29][30][31][32] The physicochemical properties of 0.45 M Ca(BF 4 ) 2 or Ca(TFSI) 2 in EC:PC, were previously investigated and significant differences in terms of Ca 2+ solvation structure were demonstrated with notable degree of ion-pair formation observed in BF 4…”

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

“…Due to ion pairing or clustering, the electrolyte might contain more than one electroactive specie ( j >1) especially in the case of chloride containing electrolytes. [ 10 , 11 , 12 , 13 , 19 , 31 , 32 , 46 , 47 ] However, the B(hfip)4 − anion is very bulky and weakly coordinating, which facilitates the dissociation of the chloride‐free magnesium salt. [26] Moreover, the redissociation phenomenon favors the formation of free charge carriers at non‐dilute concentrations.…”

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

“…Due to ion pairing or clustering, the electrolyte might contain more than one electroactive specie (j > 1) especially in the case of chloride containing electrolytes. [10][11][12][13]19,31,32,46,47] However, the B(hfip)4 À anion is very bulky and weakly coordinating, which facilitates the dissociation of the chloride-free magnesium salt. [26] Moreover, the redissociation phenomenon favors the formation of free charge carriers at non-dilute concentrations.…”