(Invited) Electrostriction Driven Preferential Solvation Processes for Divalent Cations in Non-Aqueous Electrolytes

Abstract

The strong electric field from divalent cations (i.e. Mg2+, Zn2+ and Ca2+) can influence conformal structural evolution of organic solvent molecules and subsequently influence emergent properties of battery electrolyte solutions. This electrostriction process typically depends on the charge and size of the ion and on certain properties of the solvent such as molecular flexibility and permittivity. In this work, we focused on the effect of cationic size and structural flexibility of organic solvent molecules commonly used in multivalent batteries. In particular, we analyzed electrostriction driven preferential interactions that lead to contact-ion pair (CIP) and solvent separated ion pair (SSIP) formation in cyclic ether solvents such as tetrahydrofuran (THF) and 2-methyl-tetrahydrofuran (2-MeTHF) and linear structured ether solvents such as glymes (G1 and G2). The concentration and temperature dependent preferential interactions of these electrolyte systems were analyzed using multinuclear (1H,19F, 17O, 25Mg, 43Ca, and 67Zn) NMR spectroscopy and PFG-NMR diffusion measurement combined with classical MD and DFT based computational modeling. Our results show that the solvent-cation interaction is more favorable in the linear glyme-based electrolytes than in the cyclic THF-based electrolytes. In particular, the glyme-based solvents (G1 and G2) drive preferential dissociation of cation-anion pairs, resulting in a higher concentration of SSIPs in these multivalent electrolyte solutions. The correlation between such preferential solvation phenomena and emergent properties including conductivity and chemical stability will be discussed..