Data-driven approaches for the fast formation of lithium-ion pouch cells using operando gassing analysis

Formation is among the most time-intensive process steps during lithium-ion battery production due to complex electrochemical reactions. To reduce production costs, fast formation strategies are required that do not diminish the solid electrolyte interphase (SEI) and thus the cell quality. In this work, a novel approach to optimize formation based on inline gas analysis is presented. Critical potential ranges for SEI formation are revealed by coupling impedance analysis with gas volume measurement using a pouch cell with a gold wire reference electrode and a cell expansion bracket. Based on the gas evolution profile of 4Ah NMC/graphite pouch cells, two formation strategies were derived that reduce the processing time from approx. 53.3h to 5.3h and 2.4h. A comprehensive end-of-line (EOL) test, which included a 72h calendar aging, C/3 capacity determination as well as EIS and DCIR measurements at 80%, 50%, and 20% SOC, showed no significant decrease in cell quality. The subsequent cycle life test revealed an 11.5% reduced mean cycle life for the 5.3h formation protocol, and a 6.6% increased cycle life for the 2.4h formation protocol, in relation to a common three-cycle reference protocol. Finally, early cycle life prediction was performed to enable a rapid evaluation of the cell quality using the first 50 cycles from cycle life testing only. A mean test accuracy of 7.4% was achieved when using multiple input features from the EOL test and early cycling. The results highlight the great potential of data-driven approaches to optimize formation strategies and accelerate process development in battery production.