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

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 AlCl₄^-. Here we investigated ion agglomeration and transport of several such EAS in MgCl₂ salts dissolved in ethereal solvents under both equilibrium and operating conditions using large-scale 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 Mg²⁺ results in the slow formation of a bulky cluster, compared with chainlike analogues observed in the Cl-containing EAS such as Mg₂Cl₃⁺, MgCl⁺, and Mg₂Cl₂²⁺. We further illustrate these clusters can reduce the diffusivity of charge-carrying species in the MgCl₂-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.