Why Dimethyl Carbonate Is Better Co-Solvent Than Other Linear Carbonates: Important Role of Polar Conformer Formation

Abstract

The underlying nature of the ion conduction in concentrated battery electrolytes still remains elusive. An archetypal example is a long-standing puzzle why a dimethyl carbonate (DMC) solution exhibit higher ionic conductivity than other linear carbonate (LC) solutions despite the similar physicochemical properties of the LC solvents. To address this, we investigate the correlation between the conformation isomer formation and salt dissociation behavior in 0.1 M–3.0 M LiPF6 DMC, ethyl methyl carbonate, and diethyl carbonate solutions using Raman spectroscopy, dielectric relaxation spectroscopy, and pfg-NMR. Importantly, we corroborate that compared to the other two LCs, DMC more readily dissociates Li-salt into free, conducting ions. The higher salt-dissociation ability of DMC was attributed to the extensive formation of polar cis-trans conformer participating in Li-ion solvation compared to non-polar cis-cis conformer. This study unveils the critical role of the solvent isomerism in the ion-solvation chemistry, suggesting an avenue to achieve highly conductive electrolytes for rechargeable batteries.