Similarities and Differences between Potential-Step and Impedance Methods for Determining Diffusion Coefficients of Lithium in Active Electrode Materials

Abstract

Three common methods are used to characterize diffusion coefficients of lithium in solid phase intercalate materials: electrochemical impedance spectroscopy (EIS), the potentiostatic intermittent titration technique (PITT), and the galvanostatic intermittent titration technique (GITT). All three rely on small signal excitation to linearize the system behavior. In this work, we seek to clarify the relationship between EIS and PITT. Model calculations for a graphite half-cell with lithium reference illustrate that the frequency content of PITT data is usually much lower than what is easily measurable using EIS. Use of the Fast Fourier Transform to predict periodic PITT data from EIS data showed remarkable accuracy, demonstrating the consistency of these two different measurements. Predictions of EIS data based on periodic PITT data are limited by the sampling rate for current in the PITT measurements, which introduces noise in the higher frequency impedance calculations. In addition, a simple model is used to fit lithium diffusion coefficients to both EIS data and periodic PITT data. It was seen that inadequate fits to impedance data can sometimes result in quite good fits to periodic PITT data, depending on the time scales over which the PITT data is fit to the model.