Interpretation of Kelvin Probe Force Measurements in Solid-State Electrochemical Cells

Kelvin probe force microscopy (KPFM) provides an established and reliable measurement of the work function of electronic conductors under equilibrium conditions. A less used but highly versatile application of KPFM is the characterization of electrochemical devices in operation, i.e., devices under nonequilibrium conditions. We derive the KPFM signal interpretation from basic considerations of the Volta potential and its relation to the surface potential, chemical potential of electrons, Galvani potential, and work function. As a key experiment for understanding operando measurements at electrochemical cells, we investigate a Hebb-Wagner solid-state polarization cell (HWC), constructed with a mixed ionic-electronic conductor (MIEC). Using a model-type MIEC based on amorphous Li₃PO₄, we illustrate how different potentials used in electrochemistry contribute to the KPFM signal. We show that KPFM measurements correspond to the inner electric (Galvani) potential profile along the HWC, once specific assumptions are valid. Consequently, KPFM can be very valuable in the investigation of solid electrolytes in operating electrochemical cells. Such cells are suitable models for all-solid-state batteries, candidates for future high energy density batteries.