Formation is one of the most important steps in battery cell production. This final step forms the Solid Electrolyte Interphase (SEI) on the negative active material and the Cathode Electrolyte Interphase (CEI) on the positive active material. These passivation layers are essential to suppress the continuous electrolyte degradation, thus enabling extended use of lithium-ion batteries (LIBs). In this context, passivation layers with good homogeneity, low thickness, high ionic conductivity and effective electrical insulation are particularly desirable. In this respect, the formation has a direct impact on the key properties of LIBs, such as lifetime, performance and safety.

Driven by these promising potentials and many unanswered questions, this study takes a systematic approach to investigate formation. Formation was interrupted at a range of different cut-off voltages and the battery cells were then allowed to age for approximately 3 weeks. The differently formed cells were then analysed for gassing behaviour, self-discharge, impedance and general performance. Using this matrix, different electrolytes in the 1Ah pouch cells (NMC811||Gr-5%SiOx) were studied for their formation behaviour and subsequent effect on performance.

This interrupted formation cycle allowed the different stages of formation to be studied. This approach offers the potential for a better understanding of the passivation layers of different electrolytes, paving the way for tailored formation protocols. In addition, the importance of complete gassing during formation was investigated.

The authors thank the Federal Ministry of Education and Research (BMBF) for funding this work in the project “FoFeBat TP3” (03XP0501C).