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.

Atomistic Mechanisms of Mg Insertion Reactions in Group XIV Anodes for Mg-Ion Batteries.

Magnesium (Mg) metal has been widely explored as an anode material for Mg-ion batteries (MIBs) owing to its large specific capacity and dendrite-free operation. However, critical challenges, such as the formation of passivation layers during battery operation and anode-electrolyte-cathode incompatibilities, limit the practical application of Mg-metal anodes for MIBs. Motivated by the promise of group XIV elements (namely, Si, Ge, and Sn) as anodes for lithium- and sodium-ion batteries, here, we conduct systematic first-principles calculations to explore the thermodynamics and kinetics of group XIV anodes for MIBs and to identify the atomistic mechanisms of the electrochemical insertion reactions of Mg ions. We confirm the formation of amorphous Mg xX phases (where X = Si, Ge, and Sn) in anodes via the breaking of the stronger X-X bonding network replaced by weaker Mg-X bonding. Mg ions have higher diffusivities in Ge and Sn anodes than in Si, resulting from weaker Ge-Ge and Sn-Sn bonding networks. In addition, we identify thermodynamic instabilities of Mg xX that require a small overpotential to avoid aggregation (plating) of Mg at anode/electrolyte interfaces. Such comprehensive first-principles calculations demonstrate that amorphous Ge and crystalline Sn can be potentially effective anodes for practical applications in MIBs.