Ion Pairing and Redissociaton in Low Permittivity Electrolytes for Multivalent Battery Applications.

Detailed speciation of electrolytes, as a function of chemical system and concentration, provides the foundation for understanding bulk transport as well as possible decomposition mechanisms. In particular multivalent electrolytes have shown a strong coupling between anodic stability and solvation structure. Furthermore, solvents that are found to exhibit reasonable stability against alkaline earth metals generally exhibit low-permittivity which typically increases the complexity of the electrolyte species. To improve our understanding of ionic population and associated transport and in these important classes of electrolytes, the speciation of MgTFSI2 in monoglyme and diglyme systems are here studied via a multiscale thermodynamic model using first principles calculations for ion association and molecular dynamics simulations for dielectric properties. The results are then compared to Raman and dielectric relaxation (DRS) spectroscopies, which independently confirm the modeling insights. We find that the significant presence of free ions in the low permittivity glymes in concentration ranges from 0.02 M to 0.6 M is well explained by the low permittivity redissociation hypothesis. Here, salt speciation is largely dictated by long-range electrostatics, which include permittivity increases due to polar contact-ion pairs. The present results suggest that other low permittivity multivalent electrolytes may also reach high conductivities due to redissociation.