Silicon Anode Materials Swelling Percentage: Rapid Coin-Cell Screening of Three Silicon-Carbon Materials

1. Preface: Why Silicon Anode Swelling Percentage Matters

Silicon anode materials, with their unique advantages of high theoretical capacity (4,200 mAh/g) and abundant resources, are expected to replace widely used graphite negative electrodes as the dominant anode material for next-generation lithium-ion batteries. The most commercially promising silicon-based anodes are silicon-carbon (Si/C) anodes and silicon-oxygen anodes, both offering high specific capacity. However, the alloying/de-alloying mechanism of silicon causes significant structural swelling that damages the pre-existing solid electrolyte interface (SEI) on the silicon surface.^[2] This drives continuous SEI destruction and regeneration during cycling, consuming electrolyte and ultimately causing rapid battery capacity decay. Evaluating a candidate silicon anode material’s swelling percentage—alongside specific capacity, initial efficiency, and cycle efficiency—is therefore essential, not optional, in materials screening.

Conventional swelling evaluation methods require silicon anode material to first be built into a pouch cell or stacked cell, then monitored for in-situ swelling using force structures and high-precision sensors (such as the IEST SWE swelling series). This powder-to-finished-cell pipeline requires a mature cell production line and a long evaluation cycle—a significant bottleneck for materials researchers who need to screen many candidate silicon-carbon formulations quickly.

IEST has developed a four-channel Silicon-Based Anode Swelling In-Situ Screening System (Figure 1) that, modeled on coin-cell assembly, directly measures silicon anode electrode swelling at the electrode level. This eliminates the labor, material, and time costs of building finished cells, enabling fast, low-consumption evaluation of the most important silicon anode performance indicators. The system also supports conventional swelling testing of small pouch cells and stacked cells (100×100 mm), giving it dual-purpose utility across both rapid screening and standard cell-level validation.

Figure 1. IEST Silicon-Based Anode Swelling In-Situ Rapid Screening System (four channels) — measures silicon anode swelling percentage directly at the coin-cell electrode level

2. Swelling Test of Three Different Silicon-Carbon Materials

2.1 Test Sample Information

• Cathode electrode: NCM811, cut into 14 mm diameter discs.
• Anode electrodes: Materials B, C, and D have matched capacity (~5.9 mAh) but different modification methods, cut into 16 mm diameter discs. Material B is a specially modified, low-expansion silicon-carbon anode material from a battery materials company in Ningbo; materials C and D are two common commercial silicon-carbon materials.
• Electrolyte: Commercial electrolyte.
• Separator: PP separator, cut into 18 mm diameter discs.

2.2 Test Information and Process

2.3 Analysis of Swelling Results

In a glove box, the three silicon-carbon materials were each assembled into a coin-cell full battery (using the same NCM811 cathode to preserve a single-variable comparison), then rapidly tested using the IEST silicon anode swelling in-situ screening system (Figure 2). All three silicon-carbon materials expand during charging and contract during discharge—consistent with anode swelling during lithium-ion intercalation on charge and anode contraction during de-intercalation on discharge. Although a full cell was assembled, overall cell swelling behavior is dominated by the negative (silicon-carbon) electrode, since cathode expansion and contraction is much smaller than anode swelling.^[3] The inflection points of each material’s swelling curve align closely with the inflection points of its charge/discharge voltage curve, confirming that the swelling curve faithfully tracks lithium-ion intercalation/de-intercalation behavior.

The coin-cell swelling evaluation method also clearly resolves swelling differences between the three silicon-carbon materials. Over the same operating voltage range, material B’s overall swelling is much smaller than that of materials C and D, demonstrating that B’s specialized modification treatment substantially suppresses silicon-carbon anode swelling—reducing the side reactions swelling causes and ultimately improving cycling performance.

Figure 2. Coin-cell in-situ swelling thickness curves for silicon-carbon materials B, C, and D over 3 charge-discharge cycles — dotted line: voltage vs time; solid line: swelling thickness vs time. Material B (modified, low-expansion) shows substantially lower swelling than common commercial materials C and D.

Tables 2 and 3 quantify swelling thickness and swelling rate per cycle for all three materials. Two key patterns emerge:

• First-cycle swelling is highest: All three materials show greater swelling on the first charge than on the first discharge or on subsequent cycles, with measurable irreversible swelling in the first charge-discharge cycle. This occurs because the negative electrode forms an SEI film during the first charge—producing irreversible swelling on the active particle surface in addition to reversible lithium-intercalation swelling.
• Stabilized silicon anode swelling percentage: Comparing the last two charge-discharge cycles, modified material B shows an average swelling thickness of only ~4.2 µm and a swelling rate of ~8.9%. Material C’s average swelling is approximately 3.7× that of material B, and material D’s is approximately 5× that of material B—demonstrating that B’s modification strategy delivers a clear, quantifiable swelling-suppression effect.

3. Electron Microscope Verification of Silicon-Carbon Electrode Swelling

To validate the coin-cell swelling measurements against an independent method, the fully charged silicon-carbon electrodes were disassembled and their cross-sections measured by scanning electron microscope (Figure 3). After subtracting copper foil thickness, the type B silicon-carbon electrode coating expanded from ~50.81 µm to ~55.45 µm when fully charged—a total swelling of ~4.64 µm, closely matching the average swelling thickness measured by the coin-cell screening method. Materials C and D showed coating thickness expansion of approximately 11.98 µm and 14.65 µm respectively after full charge, consistent with the swelling data from the last two cycles in Table 2.

Whether measured by in-situ coin-cell screening or by disassembly and SEM cross-section measurement, the swelling trend across the three silicon-carbon materials is consistent: D > C > B.

Figure 3. SEM cross-section images of three silicon-carbon electrodes before (Fresh) and after full charge (Full Charged): (a–b) material B; (c–d) material C; (e–f) material D — confirms the coin-cell swelling trend D > C > B

4. Summary

This study rapidly evaluated the swelling of three silicon-carbon materials with different modification conditions using the IEST Silicon-Based Anode Swelling In-Situ Screening System (RSS1400). None of the three silicon-carbon anodes required preparation as a pouch cell or stacked cell—each was assembled directly into a coin-cell format for in-situ swelling thickness measurement, eliminating the cumbersome finished-cell preparation step and substantially improving silicon material swelling-evaluation throughput.

The coin-cell in-situ test results show that specially modified material B’s silicon anode swelling percentage is far lower than common commercial materials C and D. Independent SEM cross-section measurement after full charge confirmed the same trend, validating that the IEST screening system enables direct, accurate evaluation of silicon anode swelling performance at the electrode level—with minimal material consumption and maximum throughput for R&D screening.

5. References

[1] M. Ashuri, Q.R. He and L.L. Shaw, Silicon as a potential anode material for Li-ion batteries: where size, geometry, and structure matter. Nanoscale 8 (2016) 74–103.

[2] X.H. Shen, R.J. Rui, Z.Y. Tian, D.P. Zhang, G.L. Cao and L. Shao, Development on silicon/carbon composite anode materials for lithium-ion battery. J. Chin. Cream. Soc. 45 (2017) 1530-1538.

[3] R. Koerver, W.B. Zhang, L. Biasi, S. Schweidler, A. Kondrakov, S. Kolling, T. Brezesinski, P. Hartmann, W. Zeier and J. Janek, Chemo-mechanical expansion of lithium electrode materials – on the route to mechanically optimized all-solid-state batteries. Energ. Environ. Sci. 11 (2018) 2142-2158.

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