Understanding the impedance spectrum of 18650 LiFePO4-cells

The complex nature of lithium ion batteries makes the tracking of individual physical processes difficult. However, having a good knowledge thereof is indispensable for a continuous improvement of cell performance and lifetime. In this study, the impedance response of a commercial 18650 cell was investigated within a wide frequency range (100 kHz–2 μHz) by combining electrochemical impedance spectroscopy and time domain measurements. In parallel, the original 18650 cell was opened. Thus, the electrode materials could be tested in an experimental test set-up using a reference electrode, which allowed the separation of anode and cathode processes. The different kinds of impedance data sets were de-convoluted in the space of relaxation times, enabling a refined separation of physical processes like charge transfer or solid state diffusion. This multistep approach, which is not tied to a particular electrochemistry, allowed the identification of the dominating physical processes being hidden in the impedance spectrum of the original 18650 cell.