A detailed capacity fade analysis was carried out for a commercial lithium ion battery with a mixed LiMn2O4/NMC cathode, cycled at room temperature with a continuous discharge rate of 1C. Complementary electrochemical and physical–analytical investigations revealed that the most significant aging processes was loss of cycleable lithium due to SEI-layer formation on the anode. The layer formation is accelerated by transition metals coming from the cathode. Impedance spectroscopy proved a significant increase of the cathode charge transfer resistance and of the serial resistance due to electrolyte decomposition and the formation of a surface layer on the anode. The changing of the impedance spectra of the lithium ion battery with aging could be interpreted with the help of impedance spectra of symmetric cells. From DRT analysis equivalent circuits for anode and cathode were derived, which were used for fitting of the impedance spectra.