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

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 (αD) with the help of the Walden rule and the Nernst–Einstein equation. The lithium cation effective solute radius (rLi) 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 αD and rLi 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 αD, 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 αD and rLi, it is concluded that the ion-pair formation process is exergonic and endothermic for the three salts in EC/DMC.