Formation is one of the most important and time-consuming process step in the production of lithium-ion batteries. During the first charge of the formation process the decomposition layers on the electrodes, the solid electrolyte interphase (SEI) on the negative (N) electrode and the cathode electrolyte interphase (CEI) on the positive (P) electrode, are formed. The properties of the SEI and CEI have a significant influence on the performance and cyclic aging behaviour of the cell. The formation of SEI and CEI is often accompanied by immobilizes lithium and gassing.

The gas produced in the formation process depends on the electrolyte used and the formation conditions, such as the C-rate. After formation, the gas must be removed to prevent negative effects, including pore blockage. To prevent that gassing compromises formation quality, pressure can be applied, thereby pushing the formation gas out of the cell. Although the application of pressure during formation is sufficient in many cases, the phenomenon of gassing is not yet fully understood, particular with regard to its effects on the subsequent cyclic aging behavior of different cell chemistries.

To investigate possible influencing parameters, NMC811 based LIB pouch cells with two different N electrode compositions, graphite and graphite+10%SiOx, using electrolytes without (“BL”) or with (“BL+VC”) 2%wt of vinylene carbonate (VC) were used in this study. The influence of pressure on capacity (loss), loss of lithium inventory (LLI) as well as the amount of gas formed during formation is investigated. The study also examines, whether there is a lasting effect of pressure applied during formation on the cyclic aging at 20 and 60℃.

The results clearly show a dependence of the gassing on both the electrolyte composition and the composition of the N electrode. As expected, larger gas volumes have a strongly negative effect on the cell performance during formation. However, this negative effect also persists beyond formation and influences critical performance indicators such as the irrversible loss of lithium inventory, caused for example by lithium plating.