In-situ Expansion Analysis of Silicon-Carbon Cells Under Variable Pressure Conditions

1. Abstract

The expansion behavior of silicon-carbon cells is primarily attributed to the volumetric expansion of the silicon component. Excessive accumulation of irreversible expansion during cell cycling can lead to significant capacity degradation. Current industry strategies for enhancing the cycling performance of silicon-carbon composite electrodes include¹⁻⁴

• Material Structural Modification: For example, reducing the size of silicon particles or synthesizing silicon electrodes with nanostructured architectures;
• Potential Control: To avoid the formation of crystalline Li–Si alloys;
• Development of Self-Healing Binders: To improve cohesion among active material particles;
• Utilization of Silicon Oxides: Which exhibit lower specific volumetric expansion during lithium insertion/extraction compared to crystalline silicon.

In addition to these material optimization approaches, the rate of cell expansion during operation can be mitigated by controlling the externally applied pressure. It is also noted that the testing results for cell expansion are influenced by the applied pressure magnitude and the testing control mode. In this work, the expansion behavior of silicon-carbon cells under various pressure conditions is compared using two test methodologies—constant pressure and constant gap—thereby providing researchers with a viable protocol for evaluating cell expansion.

Figure 1. Schematic Representation of Silicon-Based Electrode Degradation¹

2. Experimental Equipment and Methodology

2.1 Experimental Equipment

An IEST In-Situ Cell Swelling Testing System(Model IEST SWE2110) was used, as shown in Figure 2.

Figure 2. Exterior View of the SWE2110 Device

2.2 Test Cell Information:

Details of the tested cells are provided in Table 1.

Table 1. Test Cell Specifications

2.3 Testing Procedure for Cell Expansion

The cell under test is placed into the designated channel of the SWE2110. Using the MISS software, the corresponding cell identification numbers and sampling frequencies for each channel are configured. During the charging–discharging process, the software automatically records parameters such as cell thickness, thickness variation, stress variation, testing temperature, current, voltage, and capacity for subsequent comparative analysis.

3. Results and Discussion

As illustrated in Figure 3, there are generally three modes for measuring cell expansion:

(a) Free Expansion Measurement without any external constraint;

(b) Cell Expansion Measurement under Constant Preload, wherein a constant preload (F₀) is applied;

(c) Cell Expansion Measurement under Constant Gap Conditions.

A battery can be modeled as two elements with equivalent stiffness: the internal cell (with equivalent stiffness kₐ) and the external casing (with equivalent stiffness k꜀).

Under equilibrium, the force analysis for the three conditions (see Figure 3) is as follows:

• In the first case, the casing restricts the expansion of the internal wound electrode, and the forces on the casing and the wound electrode balance each other, resulting in a net zero external force.

• In the second case, the externally applied preload (F₀) induces an initial displacement of the battery casing (denoted as s₀ and s0,c in Figure 3b). The binding plates on both sides enhance the effective stiffness (kₛ) perpendicular to the electrode. Under equilibrium, the preload F₀ (equal to the force Fs acting on both binding plates) is balanced by the sum of the forces on the wound electrode and the battery casing.

• In the third case, when a constant measurement gap is maintained, the expansion behavior of the wound electrode and the casing deviates from that under free expansion due to the fixed gap condition.

Figure 3. Three Modes of Expansion Testing for Cell and Module Units⁵

In this study, parallel cell samples were subjected to constant pressure and constant gap charge–discharge tests under various pressure conditions. In-situ measurements of the cell expansion thickness and expansion force were obtained, yielding the data curves shown in Figure 4. During charging, the cell gradually expands, and during discharging, it contracts. Under different boundary constraints, the cell expansion is released in the form of either thickness or force.

• Figure 4(a): As the applied constant pressure increases from 0.1 MPa to 2 MPa, the maximum expansion thickness during charge–discharge cycles decreases, indicating that a certain level of external force can suppress cell expansion.

• Figure 4(b): With an increase in the initial preload from 0.1 MPa to 2 MPa, the initial cell gap correspondingly decreases. During charge–discharge cycles, the maximum expansion stress initially increases; however, once the preload reaches 0.5 MPa, the expansion stress becomes essentially invariant.

• Figure 4(c): The relationship between the maximum expansion and the initial applied load shows that the expansion thickness gradually decreases with increasing load, whereas the expansion force exhibits an initial increase before stabilizing.

Thus, when characterizing cell expansion behavior, both the magnitude of the applied pressure and the testing methodology significantly influence the results. Researchers should select the appropriate testing protocol based on the specific cell conditions.

Figure 4. Cell Charging Curve and Expansion Thickness Curve

During charging–discharging, lithium insertion/extraction in the lattice causes volumetric changes in active particles, generating internal stress fields. Concurrently, nonuniform stress due to diffusion may induce microcrack formation within the electrode. These microcracks can propagate during electrochemical cycling, leading to concomitant chemical and mechanical degradation and resulting in the capacity fade of lithium-ion batteries (LIBs). Furthermore, studies have demonstrated that appropriate external pressure can enhance interfacial contact and suppress dendritic lithium growth, thereby benefiting both liquid and solid electrolyte battery lifetimes and safety. In expansion testing, the interplay of external pressure and internal stress is critical.

Within the battery, electrochemical deformation comprises two components: reversible and irreversible. The reversible component refers to deformation that recovers its original configuration after electrochemical cycling, primarily due to lithium insertion/extraction and thermal effects. The irreversible component includes permanent plastic deformation and crack formation induced during lithiation/delithiation processes, arising from various side reactions such as active material dissolution, gas evolution, formation of surface layers (e.g., lithium plating, solid electrolyte interphase (SEI), cathode electrolyte interphase (CEI)), and electrolyte decomposition.

Lithiation-induced deformation is largely dependent on operating conditions (e.g., temperature range and voltage window) and is also closely related to electrode structure. For instance, the binder can affect deformation related to lithiation or side reactions, depending on its elastic modulus and adhesion properties; additionally, porosity plays a role—deformation may be accommodated by changes in porosity with the electrode dimensions remaining constant, or the electrode may maintain a constant porosity with the volumetric expansion resulting entirely in an increase in overall electrode dimensions. In summary, the volumetric changes in an electrode are strongly influenced by factors such as material porosity, particle arrangement, and mechanical properties.

The internal electrochemical stress in lithium-ion batteries is influenced by external constraints, the mechanical properties of the cell components, and the electrochemical deformation of the active material (AM). Higher external pressure can induce greater localized stress within the AM, increasing the likelihood of crack formation within particles to relieve internal stress. Additionally, external pressure may induce density variations; it can fragment particles into smaller sizes, thereby altering the particle size distribution. Various physical and mechanical properties of the particles—such as shape, coating materials, and elastic modulus—will affect the electrode response under external pressure.

Therefore, the choice of expansion testing conditions can influence the internal structural changes within the battery. For instance, high external pressure or a fixed gap condition that limits electrode expansion may in turn induce structural modifications such as crack formation to relieve stress, or allow active material expansion to fill available porosity. Consequently, the selection of testing conditions for volumetric expansion is critical and should closely mimic the conditions encountered during actual battery operation to accurately study battery degradation processes.

4. Conclusion

In this study, an IEST In-Situ Cell Swelling Testing System(Model IEST SWE2110) was utilized to compare the expansion behavior of silicon-carbon cells under various pressure conditions using constant pressure and constant gap testing modes. The results indicate that the expansion thickness gradually decreases with increasing applied load, while the expansion force increases initially before stabilizing. Accordingly, researchers can select the appropriate testing protocol based on their specific requirements.

5. References

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Updated on Apr. 1, 2025