Comparative study on transport properties for LiFAP and LiPF_6 in alkyl-carbonates as electrolytes through conductivity, viscosity and NMR self-diffusion measurements

摘要

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, LiPF_6; 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℃ and 20 to 70℃ 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 PF_6~- or FAP~- anions species, in each studied electrolyte, were then independently determined by observing ~3Li, ~(19)F and ~(31)p nuclei with the pulsed-gradient spin-echo (PGSE) NMR technique over the same temperature range from 20 to 80℃. 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 LiPF_6-based electrolyte, for example at T ≈ 75℃ (high temperature), the self-diffusion coefficients of Li~+ cations in solution (D (Li~+) ≈ 5 × 10~(-10) m~2 s~(-1)) is 1.6 times smaller than that of PF_6~- anions (D (PF_6~-) = 8.5 × 10~(-10) m~2 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) m~2 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 t_(Li)~+, effective radius of solvated Li~+ and of PF_6~- 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℃. 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.