Comparative study on transport properties for LiFAP and LiPF6 in alkyl-carbonates as electrolytes through conductivity, viscosity and NMR self-diffusion measurements

• Transport properties of LiPF6 and LiFAP in EC/DMC were studied.
• The self-diffusion coefficients of the Li+ and PF6− or FAP− were measured.
• Transport number tLi+ and effective radius of Li+, PF6− and FAP− were calculated.
• The degree of dissociation of LiFAP is higher than LiPF6 in EC/DMC.

We present in this work a comparative study on density and transport properties, such as the conductivity (σ), viscosity (η) and self-diffusion coefficients (D), for electrolytes based on the lithium hexafluorophosphate, LiPF6; or on the lithium tris(pentafluoroethane)-trifluorophosphate, LiFAP dissolved in a binary mixture of ethylene carbonate (EC) and dimethylcarbonate (DMC) (50:50 wt%). For each electrolyte, the temperature dependence on transport properties over a temperature range from 10 to 80 °C and 20 to 70 °C for viscosity and conductivity, respectively, exhibits a non-Arrhenius behavior. However, this dependence is correctly correlated by using the Vogel-Tamman-Fulcher (VTF) type fitting equation. In each case, the best-fit parameters, such as the pseudo activation energy and ideal glass transition temperature were then extracted. The self-diffusion coefficients (D) of the Li+ cation and PF6− or FAP− anions species, in each studied electrolyte, were then independently determined by observing 3Li, 19F and 31P nuclei with the pulsed-gradient spin-echo (PGSE) NMR technique over the same temperature range from 20 to 80 °C. Results show that even if the diffusion of the lithium cation is quite similar in both electrolytes, the anions diffusion differs notably. In the case of the LiPF6-based electrolyte, for example at T ≈ 75 °C (high temperature), the self-diffusion coefficients of Li+ cations in solution (D (Li+) ≈ 5 × 10−10 m2 s−1) is 1.6 times smaller than that of PF6− anions (D (PF6−) = 8.5 × 10−10 m2 s−1), whereas in the case of the LiFAP-based electrolyte, FAP− anions diffuse at same rate as the Li+ cations (D (FAP−) = 5 × 10−10 m2 s−1). Based on these experimental results, the transport mobility of ions were then investigated through Stokes–Einstein and Nernst–Einstein equations to determine the transport number of lithium tLi+, effective radius of solvated Li+ and of PF6− and FAP− anions, and the degree of dissociation of these lithium salts in the selected EC/DMC (50:50 wt%) mixture over a the temperature range from 20 to 80 °C. This study demonstrates the conflicting nature of the requirements and the advantage of the well-balanced properties as ionic mobility and dissociation constant of the selected electrolytes.

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