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

Abstract

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, posing a safety hazard during operation. Analyzing the gas volume and the gassing duration is thus crucial to assess material properties and determine suitable formation procedures. We present 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 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 solid electrolyte interface 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.

Methods for Quantifying Expansion in Lithium-Ion Battery Cells Resulting from Cycling: A Review

“…The electrical contact (1) is directly connected to a battery cell testing device. [20] Apparatuses similar to that in Figure 3 (b) were also used to investigate expansion in pouch cells in several references [20,48,52,69,[87][88][89][90][91][92][93][94][95]. Some apparatuses exclude the springs (2) and pressure plate (5).…”

“…Stock et al [95] used a setup to measure the gas production and the swelling of the cell with a 1D-dilatometer. An adjustable pressure plate was fixed by four springs onto the surface of the cell with a pressure of 200 kPa.…”

“…In Figure 7, the right side displays the active forces acting on the cell. In stationary conditions, the balance of forces is, according to Ref [95], as follows:…”

“…Stock et al [95] used both the Archimedes principle, and 1D-dilatometry to compare the gas production of two different cells in the formation cycle. One cell had a NCA cathode with a SiC anode and the other had a NMC cathode and a graphite anode.…”

Methods for Quantifying Expansion in Lithium-Ion Battery Cells Resulting from Cycling: A Review

“…This could lead to failure modes with performance and safety consequences such as detachment of the electrical contact, poor heat dissipation among the cells, or, in a worst case scenario, cell bursting [9][10][11]. Expansion measurements can also be useful to assess the results of aging inside the battery cell, such as gas and metallic lithium generation [12][13][14]. For these reasons, the measurement of volume expansion is essential for the purpose of ensuring cell safety and performance.…”