The data acquisition time for a single impedance point is inversely proportional to frequency, while the total capacity for each O-GEIS measurement is directly proportional to both the acquisition time and current density. As a result, at both high current densities and low frequencies, very significant capacity changes (e.g., > 10% SOC) can occur during the collection of a single impedance point, which leads to severe non-stationarity (Fig. ). The effectiveness of the Fourier transform method of drift correction is also hindered by the lower number of excitation periods which can be collected at low frequencies. Non-stationarity for O-GEIS measurements manifests as a severe distortion of the impedance spectrum at low frequencies (Fig. ) . Stationarity can be quantitatively assessed with a Kramers-Kronig (KK) transformation of the impedance data, but unfortunately the KK transform is not suitable for truncated spectra, as are used in this work . Alternatively, stationarity can be assessed based on the value of the non-stationary distortion (NSD), which is calculated from a discrete Fourier transform based on the magnitude of the impedance response (|Y|) at frequencies adjacent to the fundamental frequency (f), according to: