Paddle-wheel mechanism in doped succinonitrile–glutaronitrile plastic electrolyte: a joint magnetic resonance, dielectric, and viscosimetry study of Li ion translational and molecular reorientational dynamics

Abstract

Mixtures of 60% SN (succinonitrile) and 40% GN (glutaronitrile) doped with LiTFSI or LiPF₆ at different concentrations are investigated using dielectric spectroscopy. Room temperature conductivities up to 10⁻³ S cm⁻¹ are measured along with an overall conductivity enhancement of almost five decades compared to pure SN. Additionally, the dynamics of the methylene (CD₂) groups of SN and that of the Li⁺ ions within the mixture are studied in a wide temperature range using ²H and ⁷Li NMR relaxometry, respectively. Static-field-gradient proton NMR combined with viscosity measurements probe the molecular diffusion. GN addition and Li doping both enhance the electrical conductivity significantly, while leaving the reorientational motion within the matrix essentially unchanged. The times scales and thus the effective energy barriers characterizing the Li ion motion as well as the molecular reorientations are very similar in the liquid and in the plastic phases, findings that argue in favor of the presence of a paddle-wheel mechanism.