The impact of battery electric vehicles (BEVs) using lithium-ion cells (LICs) as energy storage systems is increasing and topics as cell performance and lifetime of the cell are becoming more important. Understanding the thermal aging process of LICs is a key factor for the automotive industry aiming for the development of long-lasting, high-performance cells. The electrolyte composition can be a limiting factor for the lifetime of the cell and highly affects the performance of LICs. In this research the thermal aging process of LIC electrolytes at increased temperatures (55 °C) is investigated. For the first time the effect of thermal aging on LIC electrolytes is investigated by means of specific conductivity measurements combined with the analysis of formed degradation products. It is found that the amount of conductive salt significantly affects the conductivity decrease during thermal aging of the electrolyte. It is revealed that the selected conductive salt has a significant impact on the quantity of formed oligocarbonates. Understanding the degradation reaction of LiPF6 and lithium bis(fluorosulfonyl)imide (LiFSI) as two of the most used conductive salts is a key factor for the development of high performance electrolytes. Finally, large format cylindrical LICs containing LiPF6 or LiFSI electrolytes were investigated for their electrolyte degradation products after the thermal aging process. Transferring the knowledge toward large format LICs can support industrial project to develop high performance cells.