A detailed post-mortem analysis was carried out for commercial lithium ion batteries stored at 4.2 V and 4.0 V at 60 °C. Complementary electrochemical and physical–analytical investigations revealed that the most significant aging processes for the cells aged at 4.2 V were loss of cycleable lithium, decomposition of the electrolyte and loss of active cathode material (LiMn2O4/Li(Ni0.5Mn0.3Co0.2)O2). The cells aged at 4.0 V also exhibited loss of cycleable lithium, but at a smaller extent. In fact, the aged anodes did not show significant changes compared to the new anode. Electrochemical impedance measurements including symmetric laboratory test cells gained from new and aged cells revealed valuable information about changing charge-transfer processes. The 4.2 V-cathode and both aged anodes surprisingly exhibited a decreased charge-transfer resistance, while the 4.0 V-cathode’s charge-transfer resistance increased.