Aging of lithium-ion cells is an inevitable phenomenon limiting the lifetime. Undesirable side reactions during cycle or calendar aging may affect the performance of all components of the lithium-ion cell. This results in a decreased capacity and an increase in the overall cell impedance. Based on electrochemical and physical characterization methods, the aging behavior during calendar aging of a 18650-cell, containing a blend of LiMn2O4 and Li(Ni1/3Mn1/3Co1/3)O2 (NMC) as cathode material and graphite as anode material was systematically investigated. To understand how the safety behavior of a lithium-ion cell changes with aging, accelerating rate calorimetry (ARC) and differential scanning calorimetry (DSC) were applied. With these methods the thermal stability behavior of the complete lithium-ion cell and its respective cathode and anode material were investigated. The focus of this work was it to generate first cause–effect relations between the aging under one exemplary aging condition and the thermal stability of a lithium-ion battery both on cell and material level.