Impedance spectra of a high energy lithium-ion battery (graphite vs. NMC) are investigated in detail, using the distribution of relaxation times (DRT) and complementary methods. Two predominant processes are found, one at lower frequencies and one at moderate frequencies. By investigating impedance spectra of corresponding half cells (graphite vs. Li and NMC vs. Li), it is found that the process at lower frequencies can clearly be assigned to the NMC's charge transfer. Further, it is assumed that the process at moderate frequencies is correlated to the graphite's solid electrolyte interphase (SEI). However, the relevant frequency-range of the anode half-cell is seemingly overshadowed by the lithium counter-electrode and thus cannot be investigated properly. To investigate this issue, symmetrical lithium cells are build and their impedance spectra are investigated as well. Indeed, the lithium's main-process occurs at the presumed frequencies. Eventually, a combination of several measurements and analyses show that the process at moderate frequencies is correlated to the graphite's SEI. Among these are i) an investigation of a calendric aged commercial cell and its corresponding half-cells and ii) a differential regression analysis (DRA), which is established in this work and proved to be a powerful analysis. tool.