Interplay among Dielectric, Transport, and Structural Properties of Electrolytes Containing Low-Permittivity Carbonates Studied by Dielectric Relaxation Spectroscopy and Molecular Dynamics Simulations.

Dielectric relaxation spectroscopy (DRS) provides valuable insights into the rotational dynamics of solvent molecules and ion pairs in lithium-ion battery electrolytes. However, interpretation of experimentally obtained spectra is often challenging due to the superposition of multiple relaxation modes, particularly for low-permittivity solvent solutions. In this study, we investigate LiPF6-based electrolytes with diethyl carbonate (DEC) and propylene carbonate (PC) by combining DRS measurements with molecular dynamics (MD) simulations. The analysis reveals that the dielectric response of DEC-based electrolytes is dominated by ion-pair dynamics, leading to a concentration-dependent increase in the static permittivity, in contrast to the mixed PC + DEC systems, where PC relaxation and ion-pair contributions compete. MD simulations also suggest that the increase in the dielectric constant of DEC-based electrolytes as a function of salt concentration promotes the formation of ion pairs with odd numbers of ions such as free ions and triple ions, likely leading to an increased molar ionic conductivity of the electrolyte. These results suggest that microscopic ion pairings and macroscopic properties such as static permittivity and conductivity are interconnected through mutual feedback.