Unbiased Quantification of the Electrochemical Stability Limits of Electrolytes and Ionic Liquids

Abstract

The electrochemical window is the potential range in which an electrolyte/solvent system does not get reduced or oxidized. Usually, voltammograms are measured, and the potentials at which specific current densities are reached are identified as the electrochemical limits. We measured electrochemical limits of several electrolytes–including ionic liquids–and show that this approach has disadvantages that can be overcome by an alternate approach of defining electrochemical limits. The choice of the cut-off current density, Jcut-off, is arbitrary and strongly affects the determined electrochemical windows, which are strongly influenced by electrolyte mass transport. Moreover, the Jcut-off method does not provide an accurate estimate of the electrochemical window at electrodes with high surface areas, where the capacitive currents are large. We propose a method that requires no definition of Jcut-off. This method minimizes electrolyte mass transport effects, gives realistic electrochemical stability limits at high surface area electrodes, and is less affected by experimental parameters such as the scan rate. The method is based on linear fits of the current–voltage curve at potentials below and above the onset of electrolyte decomposition. The potential at which the two linear fits intersect is defined as the electrolyte electrochemical limit.