Martin LORENZ1, Franziska KILCHERT2, Pinchas NÜRNBERG1,3, Max SCHAMMER2, Arnulf LATZ2,4, Birger HORSTMANN2,4, Monika SCHÖNHOFF1
1Institute of Physical Chemistry, University of Münster, Münster, Germany
2German Aerospace Center, Stuttgart, Germany
3KIT, Institute of Meteorology and Climate Research - Atmospheric Environmental Research (IMK-IFU), Garmisch-Partenkirchen, Germany
4Helmholtz Institute Ulm, Ulm, Germany
Transport processes in ionic liquids (ILs) and respective lithium containing electrolytes are subject of intense research, as these systems could overcome the challenges of today’s common carbonate-based electrolytes. Coupled by the overall conductivity, however, the migration of the different ionic species in an applied electric field cannot be considered independent. Consequently, there should be an overall constraint governing the migration flux of all species in the electrolyte.
Describing transport in electrolytes, momentum conservation is a basic physical principle, hence typically a net zero mass flux is assumed. This is also a typical prerequisite in molecular dynamics (MD) simulations. However, the question arises if the assumption of momentum conservation appropriately describes the experimental situation with an electric field applied to the electrolyte sample placed between the electrodes.
By employing electrophoretic nuclear magnetic resonance (eNMR), direct information about the migration velocity and direction of different species carrying NMR-active nuclei is accessible. Respective measurements of IL-based electrolytes indeed could not validate the governing role of momentum conservation, generally yielding a momentum transfer in the direction of the anode. This observation holds true for a broad variety of systems, pure ILs as well as lithium containing electrolytes, and is generally independent of the chemical composition and lithium salt concentration.
Here, we show that the relation of the migration flux of the different species in IL-based electrolytes is governed by their incompressibility instead of momentum conservation. Consequently, there is no net volume flux in the electrolyte, implying that migration of species in one direction always requires a similar volume flux in the opposite direction. This local volume constraint is governing the charge transport. It further points out the necessity of reference frame transformations for the comparison of experimental data with MD-results, as the latter often employ momentum conservation as a presumed principle.