Impact of Frictional Interactions on Conductivity, Diffusion, and Transference Number in Ether- and Perfluoroether-Based Electrolytes

Abstract

There is growing interest in fluorinated electrolytes due to their high-voltage stability. We use full electrochemical characterization based on concentrated solution theory to investigate the underpinnings of conductivity and transference number in tetraglyme/ LiTFSI mixtures (H4) and a fluorinated analog, C8-DMC, mixed with LiFSI (F4). Conductivity is significantly lower in F4 than in H4, and F4 exhibits negative cation transference numbers, while that of H4 is positive at most salt concentrations. By relating Stefan-Maxwell diffusion coefficients, which quantify ion-solvent and cation-anion frictional interactions, to conductivity and transference number, we determine that at high salt concentrations, the origin of differences in transference number is differences in anion-solvent interactions. We also define new Nernst-Einstein-like equations relating conductivity to Stefan-Maxwell diffusion coefficients. In H4 at moderate to high salt concentrations, we find that all molecular interactions must be included. However, we demonstrate another regime, in which conductivity is controlled by cation-anion interactions. The applicability of this assumption is quantified by a pre-factor, , b +-which is similar to the "ionicity" pre-factor that is often included in the Nernst-Einstein equation. In F4, b +-is unity at all salt concentrations, indicating that ionic conductivity is entirely controlled by the Stefan-Maxwell diffusion coefficient quantifying cation-anion frictional interactions.

Characterising lithium-ion electrolytes via operando Raman microspectroscopy

“…Grundy et al reported the same trend with LiTFSI in G4. 41 The absolute values appeared to agree with Grundy’s findings, although their chosen salt was LiTFSI, so would expect to have a slightly lower D a p p . Compared to other lithium-ion electrolytes, D a p p was quite low, with 1 M PC and EMC-based electrolytes exhibiting values 2–6 times higher.…”

Characterising Lithium-Ion Electrolytes via Operando Raman Microspectroscopy

“…Grundy et al reported the same trend with LiTFSI in G4. [38] The absolute values appeared to agree with Grundy's findings, although their chosen salt was LiTFSI, so would expect to have a slightly lower D app . Compared to other lithium-ion electrolytes, D app was quite low, with 1M PC and EMC-based electrolytes exhibiting values 2-6 times higher.…”

Electric-Field-Induced Spatially Dynamic Heterogeneity of Solvent Motion and Cation Transference in Electrolytes

“…x-2 field is applied across an electrolyte of uniform concentration [14][15][16][17]. ) measured by 7 Li, 1 H, and 19 F electrophoretic NMR (eNMR) at 30°C: cation (blue), solvent (black) and anion (red). Velocities are in the lab frame of reference.…”

“…, 𝑣 ' and 𝑣 " are the averaged species velocities of the cation, anion, and solvent, respectively, 𝐹 is the Faraday constant, c is salt concentration and E is the applied electric field [18]. The ionic conductivity and transference number at dilute concentrations can be determined using the Nernst-Einstein equation [19]. For concentrated electrolytes, more rigorous frameworks that capture the ion-ion and the ion-solvent correlations are required [20][21][22][23][24][25], commonly with the aid of computer simulations.…”

“…1. (a) Average species velocities in LiTFSI/tetraglyme electrolytes (r = [Li + ] / [O]) measured by7 Li, 1 H, and19 F electrophoretic NMR (eNMR) at 30°C: cation (blue), solvent (black) and anion (red). Velocities are in the lab frame of reference.…”