Ionic association analysis of LiTDI, LiFSI and LiPF6 in EC/DMC for better Li-ion battery performances

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 Li-7 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 degrees C and the working temperature from 20 degrees C to 80 degrees 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 (alpha(D)) with the help of the Walden rule and the Nernst-Einstein equation. The lithium cation effective solute radius (r(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 alpha(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 alpha(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 alpha(D) and r(Li), it is concluded that the ion-pair formation process is exergonic and endothermic for the three salts in EC/DMC.