Formation is an essential step during battery manufacturing. During this process, the cell is charged for the first time, resulting in the formation of the solid electrolyte interface (SEI). The SEI plays a key role for the performance and lifetime of the battery cell. By improving the understanding of the effects of formation parameters on the cell properties and their lifetime, formation protocols can be optimised and possibly adjusted based on the use-case of a battery cell. In this contribution we present an in-depth investigation on the influence of the formation temperature, a key parameter during formation on the initial performance as well as the degradation under cyclic ageing.

The SEI forms as a result of electrolyte reduction. Additives such as FEC and VC are used to improve SEI-properties. In addition to the electrolyte composition, formation conditions can influence cell properties.
Cells with three different electrolytes (no additives, 5% fluorethylene-carbonate and 5% vinylene-carbonate) undergo formation cycles at three different temperatures (45, 60 and 80°C). Subsequent performance is compared based on initial low-current cycling and a reference performance test. Furthermore, changes of the electrolyte in the cells are investigated using the non-destructive thermal electrolyte analysis after the reference performance test. This highlights the effectivity effectiveness of additives in preventing electrolyte degradation during formation but also demonstrates a new way of adjusting the formation temperature to achieve similar results.
After this initial characterisation, we disassemble exemplary cells for an analysis of the anode surface focussing on cells without additives. The remaining cells undergo cyclic ageing tests at 25°C and 45°C, respectively. Regular low-current cycles and reference performance tests provide additional information on the degradation of these cells. Thermal electrolyte analysis is also applied on some cells to identify changes of the electrolyte during ageing.
Our results show that formation temperatures have to be selected with respect to the electrolyte composition used. The benefits of additives heavily depend on the formation temperature. Vinylene-carbonate (VC) has negative effects at high temperatures, while FEC seems to require higher temperatures to unfold its benefits. Cells without additives show the best long-term stability for the highest tested formation temperature of 80°C.