Ionic association analysis of LiTDI, LiFSI and LiPF₆in EC/DMC for better Li-ion battery performances

Abstract

New lithium salts such as lithium bis(fluorosulfonyl)imide (LiFSI) and lithium 4,5-dicyano-2-(trifluoromethyl)imidazole-1-ide (LiTDI) are now challenging lithium hexafluorophosphate (LiPF₆), the most used electrolyte salt in commercial Li-ion batteries. Thus it is now important to establish a comparison of these electrolyte components in a standard solvent mixture of ethylene carbonate and dimethyl carbonate (EC/DMC: 50/50 wt%). With this aim, transport properties, such as the ionic conductivity, viscosity and ⁷Li self-diffusion coefficient have been deeply investigated. Moreover, as these properties are directly linked to the nature of the interionic interactions and ion solvation, a better understanding of the structural properties of electrolytes can be obtained. The Li salt concentration has been varied over the range of 0.1 mol L^-1 to 2 mol L^-1 at 25 °C and the working temperature from 20 °C to 80 °C at the fixed concentration of 1 mol L^-1. Experimental results were used to investigate the temperature dependence of the salt ion-pair (IP) dissociation coefficient (<i>α</i> _D) with the help of the Walden rule and the Nernst-Einstein equation. The lithium cation effective solute radius (<i>r</i> _Li) has been determined using the Jones-Dole-Kaminsky equation coupled to the Einstein relation for the viscosity of hard spheres in solution and the Stokes-Einstein equation. From the variations of <i>α</i> _D and r_Li with the temperature, it is inferred that in EC/DMC LiFSI forms solvent-shared ion-pairs (SIP) and that, LiTDI and LiPF₆ are likely to form solvent separated ion-pairs (S₂IP) or a mixture of SIP and S₂IP. From the temperature dependence of <i>α</i> _D, thermodynamic parameters such as the standard Gibbs free energy, enthalpy and entropy for the ion-pair formation are obtained. Besides being in agreement with the information provided by the variations of <i>α</i> _D and r_Li, it is concluded that the ion-pair formation process is exergonic and endothermic for the three salts in EC/DMC.