**2. Methods**

Multiple electrochemical processes with a wide range of time constants occur in LIBs. In addition to the the charge transfer processes of the individual electrodes, diffusion processes such as solid state diffusion and diffusion of lithium ions through the surface films of the electrodes also influence the dynamic characteristics of LIBs [9]. Some processes have similar time constants and are therefore not easy to distiguish from each other only considering the spectra obtained by EIS. In Figure 1a, a spectrum of the cells examined in this research is shown.

The diffusion branch at low frequencies can be clearly assigned to the solid state diffusion as the time constant of these processes is usually a few decades larger compared to other dynamic processes [9]. The semicircle in the spectra is caused by charge transfer processes and the diffusion trough the SEI [27]. Since the time constants are of a comparable order of magnitude, these processes show overlapping effects in the impedance spectra and only a single semicircle is visible. Changes of the polarization contribution and time constants of the various processes during aging can therefore not be reliably separated and assigned to the specific processes. An approach to identify the single processes and their parameters is an analysis of the DRT [14].
