Operando Analysis of the Gassing and Swelling Behavior of Lithium-ion Pouch Cells during Formation

Improving the energy density of lithium-ion batteries advances the use of novel electrode materials having a high specific capacity, such as nickel-rich cathodes and silicon-containing anodes. These materials exhibit a high level of gas evolution during formation, which poses a safety hazard during operation. Analyzing the gas volume and the gassing duration is thus crucial to assess material properties and determining suitable formation procedures. This paper presents a novel method for evaluating both gassing and swelling simultaneously to determine the operando gas evolution of pouch cells with volume resolutions below 1 μl. Dual 1D dilatometry is performed using a cell expansion bracket which applies a quasi-constant force on the cell, thus providing reproducible formation conditions. The method was validated using the immersion bath measurement method and NCM/graphite pouch cells were compared to high-energy NCA/silicon-graphite pouch cells. Silicon-containing cells exhibited gas evolution higher by a factor of seven over ten successive cycles, thus demonstrating the challenges of high-silicon anodes. The concurrent dilation analysis further revealed a constant thickness increase over the formation, indicating continuous SEI growth and lithium loss. Consequently, the method can be used to select an ideal degassing time and to adjust the formation protocols with respect to gas evolution.