Mobility-viscosity decoupling and cation transport in water-in-salt lithium electrolytes

STEINBERG, PAULA Y.; HORWITZ, GABRIELA; BURTON, GERARDO; RODRÍGUEZ, CRISTIAN R.; CORTI, HORACIO R.

ELECTROCHIMICA ACTA

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2020 vol. 359

0013-4686

Two super-concentrated aqueous electrolyes, or water-in-salt electrolytes, comprising the lithium trifluoromethanesulfonate (LiTf-H2O) binary system, and the ternary system with bis(trifluoromethanesulfonyl)-imide (LiTFSI-LiTf-H2O), were analyzed in relation to their conductivity, viscosity, diffusion of all the species and cationic transport number. The conductivity?viscosity and the ionic diffusion-viscosity decoupling were analyzed by means of the Walden law and the Stokes?Einstein relationship, respectively. The results, including those already reported for LiTFSI-H2O, reveal the existence of a significant decoupling of the Li+ mobility from the solution viscosity, while the corresponding anions follow the classical hydrodynamic behavior. The Li+ apparent transport number, derived from the self-diffusion coefficients measured by NMR, increases with concentration, due to the decoupling. These facts strongly support the formation of two types of nano-domains in the superconcentrated salt solutions with different mobility characteristics: a region formed by a net of anions with restricted mobility, percolated by clusters of reduced local viscosity that boost the mobility of Li+ ions. Finally, the difference between the experimental conductivity and that calculated using the Nernst?Einstein equation was rationalized in terms of the velocity correlation and resistance coefficients calculated using the measured transport coefficients.