In-situ Analysis of Silicon-Carbon Composite Cell Expansion Performance During Formation Process

1. Intoduction

Formation is a crucial step in battery manufacturing. During the formation process, the negative electrode first develops a solid electrolyte interphase (SEI), after which lithium ions traverse the SEI to intercalate into the negative electrode particles. Both of these processes induce an increase in the electrode thickness. Moreover, SEI formation is accompanied by gas generation, which contributes to the overall cell expansion. Since jigs are used during formation to apply a certain pressure on the electrode, the generated gas is forced into the side gap of the cell’s aluminum-plastic laminate pouch.

Silicon carbon anode electrodes exhibit a significant increase in cell thickness during formation due to both film formation and lithium intercalation. Notably, the overall cell expansion increases with a higher silicon content because of the substantial volumetric expansion of silicon. In this study, cells that have been electrolyte-wetted are subjected to constant pressure thickness expansion tests to observe the expansion behavior of negative electrodes with varying silicon–carbon ratios.

Figure 1. Comparison of Expansion for Silicon-Carbon Composite cells with Different Ratios

2. Experimental Equipment and Testing Method

2.1 Experimental Equipment:

An In-Situ Cell Swelling Testing System (model IEST SWE2110) was employed. The external appearance of the equipment is shown in Figure 2.

Figure 2. External View of the SWE2110 Device

2.2 Testing Procedure:

2.2.1 Cell Information:

The details of the test cells are provided in Table 1.

Table 1. Test Cell Specifications

2.2.2 Formation Process:

The formation procedure is as follows: 5 min rest at 25 °C; 2 min constant current (CC) at 0.01 C; followed by CC at 0.1 C until reaching 3.9 V.

2.2.3 Cell Thickness Expansion Testing:

The cell under test is inserted into the designated channel of the analyzer. The MISS software is initiated, and parameters such as cell identification and sampling frequency for each channel are set. The software then automatically records data including cell thickness, thickness variation, test temperature, current, voltage, and capacity.

3. Results and Discussion

Figure 3 presents the charging curve and thickness expansion curve during the formation process. During charging, the cell thickness increases gradually, which is primarily attributed to SEI formation and lithium intercalation into the negative electrode. Notably, the overall expansion thickness of the 800 Si/C electrode (with higher silicon content) is greater than that of the 450 Si/C electrode (with lower silicon content). A comparison between the differential capacity curve and the thickness expansion curve in Figure 3(b) reveals noticeable inflection points corresponding to the peaks. The earlier lithiation potential and larger peak intensity observed for the 800 Si/C electrode indicate that higher silicon content leads to a more pronounced expansion due to the formation of LiₓSi alloys, which also influences the lithiation phase transition potential of graphite.

Figure 3. Formation Charging Curves

Figure 4. Thickness Expansion Curves

4. Conclusion

Using an In-Situ Cell Swelling Testing System (SWE2110), this study analyzed the thickness expansion behavior during the formation process for silicon-carbon composite cells with varying gravimetric capacities. It was found that an increase in the gravimetric capacity of the silicon–carbon negative electrode leads to greater cell thickness expansion. This phenomenon is mainly associated with the structural expansion of silicon upon alloying with lithium. Accordingly, researchers should appropriately adjust the silicon–carbon ratio and modify the silicon-based material structure to mitigate structural expansion.

5. References

[1] Andressa Y. R. Prado, Marco-Tulio F. Rodrigues, Stephen E. Trask, Leon Shaw, and Daniel P. Abraham, “Electrochemical Dilatometry of Si-Bearing Electrodes: Dimensional Changes and Experiment Design,” Journal of The Electrochemical Society, 167 (2020) 160551.

[2] Peng Li, Hun Kim, Seung-Taek Myung, Yang-Kook Sun, “Diverting Exploration of Silicon Anode into Practical Way: A Review Focused on Silicon-Graphite Composite for Lithium Ion Batteries,” Energy Storage Materials, 35 (2021) 550–576.

[3] Sujong Chae, Minseong Ko, Kyungho Kim, Kihong Ahn, and Jaephil Cho, “Confronting Issues of the Practical Implementation of Si Anode in High-Energy Lithium-Ion Batteries,” Joule, 1, 47–60, September 6, 2017.

Updated on Apr. 2, 2025