Diffusional motion of redox centers in carbonate electrolytes.

Abstract

Ferrocene (Fc) and N-(ferrocenylmethyl)-N,N-dimethyl-N-ethylammonium bistrifluoromethyl-sulfonimide (Fc1N112-TFSI) were dissolved in carbonate solvents and self-diffusion coefficients (D) of solutes and solvents were measured by (1)H and (19)F pulsed field gradient nuclear magnetic resonance (NMR) spectroscopy. The organic solvents were propylene carbonate (PC), ethyl methyl carbonate (EMC), and a ternary mixture that also includes ethylene carbonate (EC). Results from NMR studies over the temperature range of 0-50 °C and for various concentrations (0.25-1.7 M) of Fc1N112-TFSI are compared to values of D simulated with classical molecular dynamics (MD). The measured self-diffusion coefficients gradually decreased as the Fc1N112-TFSI concentration increased in all solvents. Since TFSI(-) has fluoromethyl groups (CF3), D(TFSI) could be measured separately and the values found are larger than those for D(Fc1N112) in all samples measured. The EC, PC, and EMC have the same D in the neat solvent mixture and when Fc is dissolved in EC/PC/EMC at a concentration of 0.2 M, probably due to the interactions between common carbonyl structures within EC, PC, and EMC. A difference in D (D(PC) < D(EC) < D(EMC)), and both a higher E(a) for translational motion and higher effective viscosity for PC in the mixture containing Fc1N112-TFSI reflect the interaction between PC and Fc1N112(+), which is a relatively stronger interaction than that between Fc1N112(+) and other solvent species. In the EC/PC/EMC solution that is saturated with Fc1N112-TFSI, we find that D(PC) = D(EC) = D(EMC) and Fc1N112(+) and all components of the EC/PC/EMC solution have the same E(a) for translational motion, while the ratio D(EC/PC/EMC)/D(Fc1N112) is approximately 3. These results reflect the lack of available free volume for independent diffusion in the saturated solution. The Fc1N112(+) transference numbers lie around 0.4 and increase slightly as the temperature is increased in the PC and EMC solvents. The trends observed for D from simulations are in good agreement with experimental results and provide molecular level understanding of the solvation structure of Fc1N112-TFSI dissolved in EC/PC/EMC.