Shifting-reference concentration cells to refine composition-dependent transport characterization of binary lithium-ion electrolytes

Abstract

We present a novel 'shifting-reference concentration-cell' method, altering the traditional protocol for measuring liquid-junction potentials by using a sequence of reference concentrations in regularly spaced intervals, rather than a fixed reference. The method, applied to solutions of lithium hexafluorophosphate (LiPF6) in propylene carbonate (PC) and ethyl methyl carbonate (EMC) at 25 °C, helps to determine thermodynamic factors more accurately, and is useful across a wider concentration range. For LiPF6:PC, good agreement with prior fixed-reference measurements is shown, and new data at low concentrations is consistent with Debye-Hückel theory. Original composition-dependent property correlations are produced for LiPF6:EMC up to 2 M, including the density and thermodynamic factor, as well as isothermal-transport properties such as transference number, conductivity, diffusivity, and viscosity. Polarization-relaxation simulations validate these correlations. For LiPF6:EMC, the low thermodynamic factor and cation/anion Stefan-Maxwell diffusivity, as well as Walden analysis, suggest that ion association dominates, even at high dilution.

Characterising Lithium-Ion Electrolytes via Operando Raman Microspectroscopy

“…Wang et al showed this by comparing a PC-based electrolyte with an EMC-based electrolyte. [11] G4 has a ε r that lies between PC and EMC (7.91 vs. 64.92 and 2.96 respectively), and its χ M dependence on concentration agrees with this. The concentration at which the curve crosses χ M = 1 is indicative of when ion-solvent effects out-compete ion-ion effects.…”

Characterising lithium-ion electrolytes via operando Raman microspectroscopy

“… 9 , 10 D a p p is characterised through Fick’s laws of diffusion, and is defined as the proportion of current carried by the cation. Each is conventionally measured through different electrochemical techniques, with D a p p occasionally assessed using restricted-diffusion cells, 7 , 11 , 12 and from either the Hittorf method 7 , 11 , 13 – 15 , pulsed-field gradient NMR 16 or a “steady-state current” method, like the Bruce–Vincent. 17 – 19 Each have their limitations, with Hittorf often requiring large volumes of electrolyte and not being able to measure in situ concentration changes, and steady-state methods making assumptions such as the electrolyte being infinitely dilute (perfectly ideal).…”

Characterising Lithium-Ion Electrolytes via Operando Raman Microspectroscopy

“…Compared to other lithium-ion electrolytes, D app was quite low, with 1M PC and EMC-based electrolytes exhibiting values 2-6 times higher. [7,11] Decreasing D app with concentration is a result of increasing association of ions and increasing complexation of Li + with G4; an increasing frictional , showing an increase at low concentrations, then generally plateauing, and increasing again at 2m. e) (dc/dx) x=0,L , which was quite constant at low concentrations but increased rapidly after 1m.…”

“…Illustrated in Figure 4f, the electrolyte behaved according to extended Debye-Huckel theory described in ESI †, section 3.5; χ M is plotted against the square-root of concentration. [35] Similar to Hou [7] and Wang's [11] approach, the data was fitted with an extended Debye-Huckel Guggenheim equation; a x 1/2 power series. [40] This shape is akin to those of other organic-based electrolytes; [7,11,13,14]at low concentrations, a drop in χ M suggests long-range ion-ion association dominates, but at higher concentrations it is shorter range solvation effects that dominate.…”

“…[9,10] D app is characterised through Fick's laws of diffusion, and t 0 + is defined as the proportion of current carried by the cation. Each are conventionally measured through different electrochemical techniques, with D app occasionally assessed using restricted-diffusion cells, [7,11,12] and t 0 + from either the Hittorf method [7,11,[13][14][15], confocal Raman microspectrometer, a time-series of one-dimensional line scans was performed to obtain a progression of concentration gradients; measured with the stage moving in the z-direction. The cell was placed vertically on the stage to avoid natural convection.…”