Evaluating the Gassing and Swelling Behavior of Lithium-Ion Pouch Cells Using in-Situ Pressure Analysis

Abstract

The goal to reduce cost-intensive and rare elements in Lithium-ion batteries promotes the use of high-energy, nickel-rich cathodes. These materials are characterized by a high specific capacity but show an increased gas evolution during initial cycling. The main sources for the gas generation are the decomposition of electrolyte solvents at both electrodes when forming the SEI and CEI, as well as the structural release of oxygen at the cathode. A pronounced gas evolution imposes a high hazard potential during operation due to high pressures within the cell. Hence, analyzing the gas volume and the gassing period is crucial to determine optimal formation protocols and degassing steps. Existing optical or mechanical gas detection methods lack the in-situ applicability in a high-throughput production and are error-prone due to the overlaying effects of gassing and swelling. This work presents a novel method to evaluate both gassing and swelling independently to determine the gassing behavior of pouch cells with volume resolutions below 100 µl. In-situ pressure analysis is performed using an expansion cell bracket that provides a constant force on the cell, thus providing realistic and reproducible formation conditions. The method is validated using the established immersion bath measurement and three different cell chemistries are quantified: NMC/Graphite, LNMO/Graphite, and NCA/Si-Graphite. Pouch cells containing LNMO and NCA exhibited a 30 and 20 times higher gas evolution volume than conventional NMC/graphite cells, respectively. As a consequence, multi-cycle formation protocols are required as gas evolution continues within the first five charging and discharging cycles. Conclusively, a procedure to adapt the formation protocol to the gassing behavior is proposed, thus reducing processing time and ensuring the ideal degassing point.