Shear rheology senses the electrical room-temperature conductivity optimum in highly Li doped dinitrile electrolytes

Abstract

Glutaronitrile (GN) doped with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at concentrations below and above the room-temperature conductivity optimum near 1M of Li salt is investigated using dielectric spectroscopy and shear rheology. The experiments are carried out from ambient down to the glass transition temperature Tg, which increases considerably as LiTFSI is admixed to GN. As the temperature is lowered, the conductivity optimum shifts to lower salt concentrations, while the power-law exponents connecting resistivity and molecular reorientation time remain smallest for the 1M composition. By contrast, the rheologically detected time constants, as well as those obtained using dielectric spectroscopy, increase monotonically with increasing Li salt concentration for all temperatures. It is demonstrated that the shear mechanical measurements are, nevertheless, sensitive to the 1M conductivity optimum, thus elucidating the interplay of the dinitrile matrix with the mobile species. The data for the Li doped GN and other nitrile solvents all follow about the same Walden line, in harmony with their highly conductive character. The composition dependent relation between the ionic and the reorientational dynamics is also elucidated.

Translational and reorientational dynamics in carboxylic acid-based deep eutectic solvents

“…We utilize dielectric spectroscopy to simultaneously detect translational ionic and rotational dipolar motions, shear rheological spectroscopy to analyze structural dynamics, and differential scanning calorimetry (DSC) to access the glass-transition temperature. As previously shown for other DESs, [53][54][55][56][57] this combined approach is useful to clarify the relation between ionic, reorientational, and structural dynamics, as well as the role played by glass formation.…”

“…% of maline consists of dipolar molecules, the presence of a dipolar relaxation feature is expected, as reported in various other DESs containing dipolar constituents. [53][54][55][56][57][68][69][70] Upon cooling, the rotational dynamics of the dipoles gradually slow down, leading to the observed shift of the α relaxation step toward lower frequencies [Fig. 1(a)].…”

“…The rheological observable that can best be compared to the dielectric permittivity ε * is the complex shear compliance J * = J ′ − iJ ′′ . The correspondence of these two quantities has turned out useful for a range of different ion-conducting glassformers 55,57,[82][83][84] and therefore will also be exploited. In the linear response regime, J * is the inverse of the complex shear modulus, G * = G ′ + iG ′′ = 1/J * .…”

“…Such super-Arrhenius behavior is a distinctive characteristic of glass-forming liquids 75,87 and has been observed in various other DESs. [53][54][55][56][57]68,70 To characterize this non-Arrhenius freezing of the…”