The approach presented here, addresses the general problem to determine properties of binary liquid electrolytes by a simple, reliable and fast measurement. In particular, we focus on the determination of the binary salt diffusion coefficient. Alternative methods applied so far require long measurements or complex and expensive measurement equipment. For instance, Pulsed-Field-Gradient NMR measurements lead to diffusion coefficient values for ionic but also neutral species. Thus, the transferability of the parameter values determined by that method to electrochemical applications is questionable. Pulsed galvanostatic or potentiostatic polarization experiments demand chemically stable cells due to their long measurement time which is challenging due to the reactive nature of the components, e.g. metal lithium, being involved.
Our patented approach uses a simple, symmetrical cell with lithium metal electrodes at both ends of an electrolyte-filled chamber, which separates the electrodes by a few millimeter. During measurements, a linear increasing current is applied to the cell while potential difference is measured between the two electrodes.
The recorded potential-current curve enable an insight into two main electrolyte properties: The electrolytes conductivity can be determined quantitatively and the diffusion coefficient can be estimated qualitatively.
Our approach is based on the application of the Fick’s law to a semi-finite electrode-electrolyte-interface: Increasing currents lead to a depletion of lithium ions because of limited diffusion velocities. Electrolyte layers with low lithium concentration cause high resistances and, in the case of applied currents, high potential drops. Electrolytes with high diffusion coefficients allow for higher limiting currents. We also show this effect of different temperatures on the limiting current in case of a typical commercial available battery electrolyte (LiPF6 in EC:DMC (1:1 / v:v)): The limiting current significantly increases with increasing temperature.