Measuring the Nucleation Overpotential in Lithium Metal Batteries: Never Forget the Counter Electrode!

The nucleation overpotential has been used by many researchers as an indicator of the energy required to form the Li nuclei during plating. Typically, a two-electrode system is used to measure the nucleation overpotential; this method, however, fails to show the contribution of working and counter electrodes separately. In this study, we have used a three-electrode configuration (three-dimensional nickel foam as working electrode, lithium foil as both reference and counter electrode) to deconvolute the potential associated with each electrode during the galvanostatic Li electrodeposition to obtain a clear picture of nucleation overpotential. The results indicate that, in such a system, the main source of overpotential is the sudden drop in the potential of the counter electrode, which can be attributed to the extraction of Li from the surface of lithium metal. Moreover, unlike the first half-cycle, the nuclear overpotential is dominated by the working electrode in the second half-discharge cycle, which should account for a true nucleation overpotential of the system. This finding may aid in clarifying the origins of the experimental polarization and preventing researchers from misinterpreting it in terms of nucleation overpotential.