Analysis of electrolyte degradation products in cylindrical automotive lithium-ion cells during thermal aging

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 are becoming more important. The electrolyte composition can be a limiting factor for the lifetime of the cell and highly effects the performance of LICs. In this research the aging process of LIC electrolytes at increased temperatures (55°C) is investigated. First, the electrolyte is analyzed separately from the cell materials and afterwards the electrolyte is extracted from thermally aged LICs to investigate the aging effect on the electrolyte and the cell. By performing specific conductivity measurements of thermally aged electrolytes with different conductive salts and various concentrations, representative information for the entire electrolyte was obtained. High performance liquid chromatography-mass spectrometry measurements revealed that the selected conductive salt has a significant impact on the quantity of formed oligocarbonates. Furthermore, a correlation between the LiPF ₆ concentration and the amount of formed oligocarbonates could be identified. Finally, large format cylindrical LICs containing LiPF ₆ or LiFSI electrolytes were investigated for their electrolyte degradation products after the thermal aging process. Understanding the thermal aging process of different electrolyte compositions and transferring the knowledge to large format LICs is a key factor for the automotive industry aiming for the development of long-lasting, high-performance cells.