Raman Spectroelectrochemistry of a Lithium/Polymer Electrolyte Symmetric Cell

A symmetric lithium cell LiTFSI/Li has been studied at 80°C by confocal Raman microspectrometry while current densities of up to are passed through the cell. The Raman observation is performed on the edge of the cell along a line of points extending from one electrode to the other at a depth of about 20 μm within the electrolyte. Local salt concentration is measured in the electrolyte as a function of time, current density I , and electrolyte thickness L (90 and 160 μm). When the steady‐state regime is reached, linear and symmetric salt concentration gradients are observed. They are proportional to I and L as expected from the theoretical predictions for a binary electrolyte containing a fully dissociated salt with negligible ionic associations. In addition, preliminary results have been obtained concerning the establishment and relaxation of the steady state. From these data, it is shown that salt diffusion coefficient and ionic transport numbers can be determined with a reasonable precision. Confocal Raman microspectrometry can therefore be considered as a new and powerful spectroelectrochemical method to study transport properties in polymer electrolytes.