The 3D computed tomography analysis combined with the visual inspection during the characterization tests indicates significant gas formation. This gas formation causes a loss of active surface area, since these gas bubbles hinder the transport of lithium ions. This results in a significant decrease of the capacity and increase of internal resistance, as observed during extensive electrical testing (see Section 3.1
). This section discusses the possible root cause of this gassing phenomenon. Side reactions of the electrolyte most likely cause this gas formation. The electrolyte consists of 0.9 M LiPF6, 0.1 M Lifetime Electrolyte Arkema (LEA), and 10% fluoroethylene carbonate (FEC).
The LEA is a high-purity salt based upon lithium 4,5-dicyano-2-(trifluoromethyl) imidazolide (LiTDI) developed by Arkema to increase the cell’s performance. The usage of this salt has three main advantages. Firstly, it was demonstrated that adding LiTDI improves the cycling performance of silicon-based electrodes [29
]. Secondly, less LiPF
is necessary, which is beneficial for the stability and overall safety of the battery [30
]. The reason is that LiPF
forms HF, which negatively affects the stability of silicon-based electrodes and causes safety risks in the case of battery failure. Thirdly, according to a study by Lindgren et al. [29
], LiTDI undergoes only minor decomposition during cycling, making this salt unlikely to be the cause of the gas formation.
Fluoroethylene carbonate (FEC) is considered a standard additive when using silicon-based electrodes [14
]. It improves the solid electrolyte interface layer (SEI) by creating a more homogeneous and flexible SEI layer. This results in an improved cycling stability combined with an overall higher coulombic efficiency [10
]. Additionally, it reduces the cracking of the SEI layer [34
] caused by the volumetric changes of the silicon particles during charging and discharging. However, it should be noted that the working principle and FEC’s exact decomposition during battery cycling are not yet fully understood. Research by Leung et al. [35
] simulated the excess electron-induced decomposition of FEC by using density functional theory (DFT), ab initio molecular dynamics (AIMD), and quantum chemistry techniques. Only one- and two-electron reactions were considered. Leung concluded that the most probable decomposition products of FEC are LiF, CO
, and CHOCH
radicals. Jung [36
] proposed a four-electron-induced decomposition of FEC, as illustrated in Figure 8
. The decomposition products are CO
, LiF, Li
, and a partially cross-linked polymer. Additionally, Jung et al. [36
] showed by online electrochemical mass spectrometry that one molecule of CO
is produced for every molecule of FEC that is reduced. Furthermore, experiments have shown a significant increase in the production of CO
gas when using FEC when compared to non-FEC-containing electrolytes.
In conclusion, the two main decomposition products from FEC, confirmed by both modeling and experiments, are LiF and CO
. LiF is identified as the component leading to an improved SEI layer, making it more homogeneous and more resistant to cracking due to volumetric changes in the silicon particles. This results in an improved cycling stability [37
The detached areas, as seen in Figure 7
, combined with the presence of gas, could be caused by two possible mechanisms, as demonstrated in Figure 9
. More specifically, by deformation-induced gas formation and by gas formation-induced deformation. In the first case, small inhomogeneities between the opposites sides of the current collector are present due to manufacturing. This creates different in-plane stresses leading to small detachments of the layers locally [38
]. Detached areas have higher local resistances and thus create locally higher overpotentials, which is a more favorable condition for gas formation. In the second case, due to homogeneities or, for example, a difference in compressive force, gas formation occurs locally, leading to the detachment of the layers.
The observed rapid decreasing capacity is clearly linked to the rapid consumption of FEC. This is also confirmed by the significant amounts of gas produced. Due to this large consumption of FEC, the negative effects of the produced gas dominate the positive effects of the produced LiF. Therefore, this reaction rate should be reduced. The reaction rate is influenced by several parameters, the most important being temperature, concentration, and pressure. The temperature effect is less relevant since batteries need to be used within a range of temperatures given by the application itself. The concentration, in this case, is already optimized to achieve the highest energy density. However, external pressure can still be applied to the cell. Applying external pressure to the cell would probably reduce the reaction rate since it will be more difficult to form CO gas.
3.4. Investigation of the Effect of External Pressure
This section presents the investigation of whether the application of external pressure to the pouch cell reduces gassing and accordingly improves its electrical performance. This was achieved by clamping a fresh pouch cell between two parallel plates, applying homogeneous pressure onto the pouch cell. A capacity test, as defined previously in Section 3.1.3
, was performed in order to evaluate if ballooning occurs and, additionally, to evaluate the cell’s performance by means of discharge capacity and rate retention. After this test, when releasing the pressure, no ballooning of the cell could be seen. The comparison between the capacity of the clamped and unclamped pouch cell is visualized in Figure 10
. Significant increases in discharge capacity ranged between 10.8% and 19% at C/5 and 1.5C, respectively. A significant improvement of the cells’ performance, in terms of available discharge capacity and rate capability, can be observed. Taking into account these two observations—no ballooning and improved electrical performance—it can be concluded that no gas or only a small amount of gas is present in the cell. Taking into account the two possible mechanisms described in Figure 9
, it is assumed that external pressure leads to a better connection between the electrodes and thus less detached areas, and thus less favorable conditions for gas formation. Therefore, it can be concluded that the application of external pressure to the pouch cell indirectly reduces the consumption of FEC, leading to a superior performance of the cell. The application of external pressure is concluded to be a critical parameter when designing battery packs containing weight quantities of silicon. More research on this topic is needed to quantify the impact of pressure on the aging of the cell.