Quantitative Analysis of Expansion of Different Proportions of Silicon Carbon Anode Plates

In 2020, DanielP. Abraham’s team used an in-situ electrochemical cell to characterize the thickness expansion of graphite and silicon anodes, quantitatively analyzed the difference in electrode expansion when different proportions of silicon-carbon were mixed, and analyzed several factors that affect the in-situ expansion test. Reference for subsequent researchers.

Sample preparation and testing equipment

Different test electrode sheets were prepared according to the proportions in Table 1 for the assembly of the buckling and in situ electrolysis cells. The device shown in Figure 1 was used to measure the expanded thickness of different pole pieces.

Table 1. Composition and performance parameters of test electrodes

Figure 1. Schematic diagram of the electrochemical expansion test structure

Result analysis：

It can be seen from the expansion test curve of the negative pole piece with 15% silicon mass percentage in Figure 2 that with the progress of the cycle, the maximum expansion thickness of the fully intercalated lithium state increases significantly, while the irreversible thickness of the fully delithiated state in each cycle. The expanded thickness is also gradually increasing, which may originate from the SEI film formation and the degree of lithium deintercalation in the anode.

Figure 2. Expansion curve of silicon-carbon composite anode

It can be seen from the expansion and voltage loops of silicon carbon anodes with different ratios in Fig. 3 that for pure graphite anodes, the thickness expansion mainly occurs in the first and third platforms, while the expansion ratio of the second platform is only 1 %, and during high-capacity delithiation, the thickness will shrink to a certain extent. For the pure silicon negative electrode, the maximum expansion thickness is almost 300%, and the irreversible expansion thickness is also close to 50%. After one cycle of charge and discharge, the capacity of the battery is also the largest.

Figure 3. Potential and expansion curves of silicon-carbon composite anodes with different ratios

From the differential capacity curve in Figure 4, it can be seen that the pure graphite electrode has three obvious reaction peaks when intercalating lithium, while the peak positions of the two lithium-silicon alloys of the pure silicon electrode are larger than those of graphite. A mixture of lithium intercalation peaks for the two materials is exhibited, but the reaction peak for the lithium-silicon alloy is weaker.

Figure 4. Differential capacity curves of silicon-carbon composite anodes with different ratios

Figure 5 shows that when different proportions of silicon carbon are combined, the specific capacity of the electrode increases gradually, and the maximum expansion thickness corresponding to the fully intercalated lithium state of the electrode also increases gradually, but not linearly. When the content is less than 30%, the expansion thickness of the electrode increases slightly, but when the content is greater than 30%, the slope of the thickness expansion increases significantly.

Figure 5. Specific capacity and maximum expansion ratio of silicon-carbon composite anodes with different ratios

Figure 6 shows the three influencing factors of the electrochemical expansion test device: electrolyte, test pressure, and electrode porosity. The authors found that because the sealing performance of the electrochemical in-situ cell is not as good as the charge-deduction, the potential capacity of the two structures There will be differences in the curves, but the influence can be weakened by selecting LiFSi electrolyte and reducing the test magnification. Meanwhile, the porosity of the electrode will also affect the change trend of the expanded thickness during the lithium deintercalation process of the electrode. In addition to the above three, the authors also consider the influence of displacement sensor drift, test applied pressure, and gas production on the measurement of expanded thickne.

Figure 6. Three influencing factors of electrochemical expansion test device: electrolyte, test pressure, electrode porosity

Summarize

In this paper, an in-situ electrochemical cell was used to characterize the thickness expansion of graphite and silicon anodes, and quantitatively analyze the difference in electrode expansion when different proportions of silicon and carbon were mixed. The main conclusions are as follows:

1. The maximum expansion of pure graphite pole piece is ~19%, while the maximum expansion of pure silicon pole piece is ~300%;
2. The expansion ratio of silicon-containing pole pieces will be affected by the porosity of the pole piece design and may be higher than expected;
3. For different proportions of silicon carbon anodes, the expansion is nonlinear;
4. Limiting the capacity of the silicon carbon anode can adjust the maximum expansion ratio;
5. Limiting the lithium intercalation depth of the pole piece is better than limiting the delithiation depth in inhibiting expansion;
6. The expansion ratio of the pole piece with low porosity is larger than that of the pole piece with high porosity;
7. Researchers should pay attention to the influence of several factors mentioned in the text when using electrochemical in-situ expansion devices;

References

Andressa Y. R. Prado, Marco-Tulio F. Rodrigues,Stephen E. Trask, Leon Shaw and Daniel P. Abraham, Electrochemical Dilatometryof Si-Bearing Electrodes: Dimensional Changes and Experiment Design, Journal ofThe Electrochemical Society, 167(2020) 160551.