In this paper, in-situ swelling thickness tests were performed on two soft coated cores prepared with different proportions of mixed silicon carbon materials (specific capacities of 450mAh/g and 800mAh/g, respectively) to compare the differences in swelling properties of the two.

Improvement in the material and process aspects are needed to enhance the energy density of the battery. The theoretical ratio capacity of silicon based material forming Li xSi (x=0~4.4) alloy in the process of lithium embedding is about 4200mAh/g, much greater than the commonly used graphite material of 372mAh/g. Therefore, silicon based material becomes the most potential negative material to enhance the energy density of the battery. However, due to the serious volume swelling and contraction of silicon negative electrode in the process of removing lithium, the poor battery circulation performance has limited its wide application in the lithium industry. How to effectively inhibit the swelling of silicon negative electrode has become an urgent difficulty to overcome in the lithium industry. At present, most of the silicon-based anode materials that can be actually used are silicon carbon mixed according to a certain proportion to explore the impact of different mixing ratios on the battery swelling performance, which will help researchers to deepen the understanding of the swelling of silicon-based materials and make corresponding improvement measures, and accelerate the wide application of silicon-based materials.

Figure 1. Overview of silicon-graphite composite electrodes ¹

Experimental Equipment and Test Methods

1. Experimental Equipment:

In-situ swelling analyzer, model SWE2110 (IEST), the equipment appearance is shown in Figure 2.

Figure 2. SWE2110 Equipment Appearance

2. Test Methods

The 2.1  core information is shown in Table 1.

2.1 Table 1 Test Cell Information

2.1.1 Charging and Discharge Process: 25℃ R est 5min; 0.5C CC to 4.2V,CV to 0.025C; rest 5min; 1C DC to 2.75V.

2.1.2 Cellular Thickness Swelling Test: Put the power to be measured into the corresponding channel of the equipment, open the MISS software, and set the corresponding cell number and sampling frequency parameters of each channel. The software automatically reads the cell thickness, thickness change amount, test temperature, current, voltage, capacity, etc.

3. In-situ Analysis of Silicon Carbon System Cell Swelling Behavior

FIG. 3 shows the cell charge and discharge curve and the thickness swelling curve. In the process of charge and discharge, the thickness of the cell increases and then decreases, which is mainly related to the transition of the silicon carbon structure. When lithium constantly exits from the positive electrode and enters the negative electrode structure, lithium forms Li xSi alloy with silicon and LiCx intercalation compound with graphite will cause negative swelling, which can further analyze silicon and graphite swelling behavior combined with the differential capacity curve.

FIG. 3 Cell charge and discharge curve and thickness swelling curve

FIG. 4 shows the comparison of the differential capacity curve and the thickness swelling curve of the core. From the differential capacity curve, three obvious off-embedded lithium peaks appeared in both the charging and discharge processes, and the positive electrode of this cell is NCM811, and the negative electrode is silicon carbon materials with different grams of capacities respectively. Compared with the peak2 of the two cores, this peak is mainly the phase transition site of NCM811, while compared with peak1 and peak3, 800Si/C is significantly greater than 450Si/C, shows that when Si is more, the formation of silicon carbon alloy is also more, and then causes more thickness swelling when charging.

Fig. 4 The charge and discharge differential capacity curve and the thickness swelling curve

4. Summary

The in-situ swelling analyzer (SWE) is used to expand the charging thickness, which found that the swelling thickness of silicon carbon is related when forming silicon carbon alloy. Developers should reasonably regulate the silicon carbon structure and modify the silicon-based material structure to inhibit structural swelling, so as to ensure that the battery has good cycle stability.

5. Reference

1.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.

2.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, J.Electrochem.Soc.167 160551.

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.