Journal of Energy Storage: Silicon-Carbon Pouch Cell In-Situ Swelling Testing Under Buffer Foam Mechanical Constraint

Academic Application, In-Situ Cell Swelling

Updated on 2026/09/10

Abstract

In-situ swelling testing of silicon-carbon pouch cells is an important characterization method in silicon-anode battery development. However, reproducible and engineering-relevant results depend strongly on the mechanical boundary conditions applied during testing and how closely they represent the compliant constraints of practical battery modules. A 2026 Mercedes-Benz study in Journal of Energy Storage compares rigid constraint using a pneumatic press with compliant, buffer foam constraint using a PCB eddy-current sensing platform for NMC811/Si-C composite pouch cells. Over 408 cycles at 250 kPa, the foam-buffered configuration resulted in approximately 4% higher capacity retention than the rigidly constrained configuration, reduced lithium plating and electrode delamination, and enabled the PCB-based platform to measure cell swelling with only a 2.8% deviation from post-disassembly micrometer measurement. The study further highlights the importance of pre-characterizing the mechanical behavior of the buffer foam through Compression Force Deflection (CFD) testing to establish well-defined mechanical boundary conditions and improve the reproducibility and comparability of swelling measurements.

📄 Source Paper

Comparative in-operando analysis of lithium-ion cell swelling in pouch cells: Constant force versus foam-buffered quasi-constant force

DOI: 10.1016/j.est.2026.121124
| Journal: Journal of Energy Storage, 2026
| Institutions: Mercedes-Benz AG

1. Introduction: Cell Swelling Behavior and the Role of Mechanical Constraint

Pairing an NMC811 cathode with a silicon-carbon composite anode is a widely adopted approach to increasing the energy density of lithium-ion batteries. However, silicon particles can undergo volume expansion up to 300% during lithiation. As a result, Si/C cells exhibit two distinct types of thickness change during cycling:

Reversible swelling (cell breathing): Reversible expansion and contraction associated with lithiation/delithiation of the electrode materials.
Irreversible thickness growth: A progressive increase in cell thickness associated with degradation processes such as continued SEI growth, gas generation, and particle pulverization. Tracking this growth can provide insight into the progression of cell degradation.

High-precision in-situ swelling testing of silicon-carbon pouch cells is therefore indispensable for both electrode material development and battery module mechanical design. Importantly, the mechanical boundary conditions imposed by the test fixture can influence both the measured swelling response and the electrochemical behavior of the cell. Therefore, a rigid, non-compliant fixture may provide a mechanical environment that differs substantially from that of a practical battery module incorporating compliant buffer materials.

2. Experimental Setup

2.1 Cell Configuration

Two types of laboratory-made NMC811/graphite-Si/C pouch cells were evaluated using the same electrode formulations:

Cathode: NMC811, areal capacity of 3.8 mAh·cm⁻²
Anode: Graphite-Si/C composite, areal capacity of 4.5 mAh·cm⁻²
N/P capacity ratio: 1.13–1.18

Type 1 (single-stack): 0.18 Ah nominal, 2-unit stack
Type 2 (multi-stack): 3.2 Ah nominal, 10-unit stack

After electrolyte filling and sealing, all cells underwent formation followed by long-term cycling at 25 °C, for a total of 408 cycles. Cell swelling and electrochemical performance were monitored throughout the test.

2.2 Two Swelling-Test Platforms (Variable: Constraint Type)

Table 1. Comparison of battery expansion constraint platforms and their roles in the study
Platform	Constraint Type	Role in Study
Pneumatic press	Rigid, no compliance	Reference group (conventional lab standard)
PCB eddy current sensor + foam	Compliant quasi-constant-force via buffer foam	Experimental group (module-representative)

2.3 Mechanical Pre-Characterization of Buffer Foam

Before cell swelling testing, three polyurethane foams with different densities were evaluated through compression force-deflection (CFD) measurements. The purpose was to quantify their compression behavior and establish a reproducible mechanical boundary condition for subsequent cell testing. Based on the CFD characterization, Buffer foam 1, with a density of 500 kg·m⁻³, was selected for the subsequent experiments.

3. Key Results and Analysis

3.1 Effect of Buffer Foam and Selection of the Test Pressure

As the cell thickens during cycling, the buffer foam accommodates changes in cell thickness through compression, providing a more compliant mechanical boundary than rigid platens. Pressure-film measurements further showed that the foam promoted a more uniform pressure distribution across the cell surface, reducing localized pressure concentrations at the edges and corners.

The applied pressure also influenced both mechanical stability and cell performance. At 170 kPa, excessive foam deformation resulted in unstable pressure conditions, whereas 500 kPa excessively constrained cell breathing and was associated with greater capacity degradation over 408 cycles.

At 250 kPa, the foam operated within a relatively stable compression range with limited pressure fluctuation. This condition was therefore selected for the subsequent comparison between the PCB-based and pneumatic-press configurations.

Figure 1. Mechanical characterization and cell-level evaluation of the buffer foam: (a) compression force-deflection (CFD) curves for the three foam types; (b) calibration methods for Foam 1; (c) long-term relaxation behavior; and (d) capacity retention (filled markers) and cell breathing amplitude (open markers) under different applied pressures.

3.2 Swelling Measurement: PCB Platform vs Pneumatic Press

At 250 kPa, the PCB-based and pneumatic-press configurations produced comparable swelling profiles over 408 cycles. The measured maximum thickness changes and cell-breathing amplitudes during formation and long-term cycling were within the experimental variability.

For offline validation, the single-stack swelling measured by the PCB platform deviated by only 2.8% from post-disassembly micrometer measurements and showed a trend consistent with electrode thickness changes observed by cross-sectional SEM. These results support the measurement reliability of the calibrated PCB-based platform under the conditions evaluated in this study.

Figure 2. Swelling behavior measured using the PCB-based and pneumatic-press configurations at 250 kPa: (a) minimum and maximum thickness change per unit stack and cell-breathing amplitude over 408 cycles, with an enlarged view of the formation stage; normalized thickness change per unit stack at (b) cycle 6, (c) cycle 208, and (d) cycle 408.

Key Performance Comparison: Buffer Foam vs Rigid Constraint

Table 2. Summary of buffer foam vs rigid constraint effects on silicon-carbon pouch cell expansion measurement and electrochemical performance over 408 cycles.
Parameter	Rigid Clamping (Pneumatic Press)	Buffer Foam at 250 kPa (PCB Platform)
Capacity retention (408 cycles)	Lower	+4% vs rigid
DCIR internal resistance increase	Larger increase	Smaller increase
Negative electrode lithium plating (post-cycle SEM)	Large-area Li plating observed	None observed
Electrode coating delamination	Present (SEM)	Intact electrodes, uniform SEI
Pressure distribution (pressure film)	Local high-pressure at corners	Uniform across full electrode area
Module relevance	Diverges from module mechanical boundary	Replicates real module foam constraint
Expansion measurement accuracy vs micrometer	—	2.8% deviation (PCB+foam)

3.3 Effect of Buffer Foam Constraint on Electrochemical Performance

After 408 cycles, the buffer-foam-constrained cells show 4% higher capacity retention relative to the rigid-press group, and lower DCIR internal resistance growth. Post-cycle SEM reveals large-area lithium plating on the negative electrode and coating delamination in rigid-press cells, while buffer-foam cells show intact electrodes, uniform SEI film, and significantly reduced localized aging risk.

Figure 3. Electrochemical performance under different mechanical constraint conditions at 250 kPa: (a) capacity retention of Type 1 and Type 2 cells; (b–d) changes in DCIR at 50%, 25%, and 75% SOC, respectively. The Type 2 Press data include an outlier attributed to an incorrect four-wire connection.

4. Research Conclusions

Reversible cell breathing and irreversible thickness growth are important indicators of the mechanical evolution of silicon-carbon pouch cells during cycling. This study highlights four key findings:

Rigid, non-compliant fixtures may not fully represent practical module conditions. Pneumatic presses or other rigid-clamping fixtures do not replicate the compliant mechanical boundary of battery modules with buffer foam, which may limit the engineering relevance of the resulting swelling and degradation data.
Buffer foam is a key component for reproducing module compliant mechanical constraints. By promoting a more uniform pressure distribution across the cell surface, it helps mitigate lithium plating and electrode delamination during cycling.
Mechanical characterization of buffer foam is a prerequisite for standardized swelling testing. Variations in foam density and compression behavior can introduce differences in the applied mechanical constraint and compromise cross-batch data comparability. Therefore, pre-test foam characterization helps ensure reproducible and comparable swelling measurements.
Foam-buffered PCB eddy-current platforms can achieve swelling-measurement accuracy comparable to that of pneumatic presses. With appropriate stability calibration, such systems provide a viable approach for standardized swelling characterization of silicon-carbon cells.

5. Industry Perspective: Related Research and IEST Solutions

💡 Research-to-Industry Note: The mechanical boundary conditions described in this Mercedes-Benz study — buffer foam constraint, expansion measurement precision, foam CFD pre-characterization — are not unique to European automotive R&D programs. Domestic battery research institutions and manufacturers, particularly in silicon-anode and solid-state systems, have engaged with the same challenges using comparable or complementary approaches.

5.1 In-Situ Cell Expansion Testing: IEST SWE System

In China, in-situ swelling testing has become an increasingly established characterization method for silicon-based and solid-state battery research. The IEST In-Situ Cell Swelling Testing System (SWE Series) provides thickness-measurement accuracy of up to ±1 μm, enabling high-resolution monitoring of cell breathing and irreversible thickness growth throughout cycling.

Comparing with the PCB eddy current sensing system described in the Mercedes-Benz study, high-precision displacement-based measurement systems can offer advantages in long-term stability. As noted in this study, PCB-based sensing may be affected by signal drift and calibration offsets during extended testing, potentially affecting measurement reliability over long cycling periods.

Figure 4. IEST SWE Series In-Situ Cell Swelling Testing System — applicable to silicon-carbon and silicon-anode pouch cell expansion characterization under controlled mechanical constraint.

5.2 Buffer Foam Mechanical Characterization: IEST PRCD Series

Mechanical characterization of buffer foam is also becoming increasingly standardized among foam manufacturers. The IEST PRCD series can implement programmatic control of foam compression-decompression cycle evaluation at rates from 0.05 to 5 mm·min⁻¹, enabling quantitative evaluation of foam compression force-deflection behavior and cyclic mechanical response. Such measurements can support foam selection and mechanical calibration before the material is incorporated into cell swelling tests or battery module designs.

Figure 5. IEST PRCD Series Foam Compression Force-Deformation Evaluation System & PORON EVExtend 4701-43 foam performance data.

6. FAQs

6.1 What is the difference between reversible and irreversible cell swelling in silicon-carbon anodes?

In silicon-carbon anode cells, reversible swelling (also called cell breathing) refers to the cyclic expansion during lithiation and contraction during delithiation — this is the elastic volumetric response of electrode particles to lithium intercalation and follows each charge-discharge cycle. Irreversible permanent thickening is a distinct, cumulative increase in cell thickness that does not recover during discharge; it originates from continuous SEI film growth, internal gas generation from electrolyte decomposition, and silicon particle pulverization over repeated cycling. The irreversible component directly tracks battery state-of-health degradation and is why high-precision in-situ expansion monitoring is indispensable in silicon-anode battery development: reversible and irreversible contributions must be separated to understand which degradation mechanisms are active at a given point in cycle life.

6.2 Why is buffer foam compression force-deformation characterization necessary before expansion testing?

Buffer foam compression force-deformation (CFD) characterization is necessary before expansion testing because the mechanical properties of foams — stiffness, compression set, long-term relaxation — vary significantly between manufacturers, grades, and even production batches. If a foam is used for cell expansion testing without prior CFD characterization, its actual compression force at a given displacement is unknown, so the actual preload pressure applied to the cell is uncertain and likely variable over the duration of the test. This means expansion data from tests using uncharacterized foam cannot be reliably reproduced in another lab, or even in the same lab with a different foam batch. The Mercedes-Benz study demonstrated this concretely by testing three polyurethane foams at 70, 250, and 500 kPa and finding that only the 250 kPa stable-plateau condition reliably maintained the target pressure throughout the test. Without upfront CFD characterization to identify the stable plateau region, selecting the correct foam and operating condition is guesswork.

6.3 How does mechanical constraint optimization for silicon-anode batteries affect cycle life?

Mechanical constraint optimization for silicon-anode batteries directly affects cycle life through the pressure distribution applied to the electrode stack during cycling. A rigid, non-compliant constraint cannot accommodate the cell’s irreversible thickness increase over hundreds of cycles, causing contact pressure to rise continuously and concentrate at cell corners and edges — creating the localized high-pressure zones that trigger lithium plating and electrode coating delamination, both of which accelerate capacity fade. A compliant buffer foam constraint at the optimal preload pressure (250 kPa in this study) deforms to absorb irreversible thickness increase while maintaining a stable, spatially uniform pressure across the full electrode area — eliminating the localized stress that drives plating and delamination. The practical result in this 408-cycle study was a 4% improvement in capacity retention and complete elimination of the lithium plating and delamination observed under rigid constraint.

6.4 Can a PCB eddy current sensor for battery expansion measurement match the accuracy of a pneumatic press?

Yes, provided a proper stability calibration is performed and the foam pre-characterization protocol is followed. In the Mercedes-Benz study, the PCB eddy current sensor platform combined with the optimized buffer foam showed measurement deviation of only 2.8% compared to destructive post-test micrometer measurements, and full-cycle expansion curves from the PCB platform and pneumatic press were indistinguishable within experimental standard deviation over 408 cycles. However, the paper explicitly notes that PCB sensors are subject to offset drift and long-term stability limitations that must be addressed through calibration — this is one of the acknowledged practical limitations of PCB-based approaches. High-precision mechanical displacement-based expansion systems, such as the IEST SWE series, address these stability constraints more directly, making them a complementary or alternative approach for research groups that require long-term stability without periodic recalibration overhead.

6.5 What is a standardized expansion testing protocol for silicon-anode batteries?

A standardized expansion testing protocol for silicon-anode batteries needs to specify and control at minimum: (1) the mechanical boundary condition — whether rigid or compliant, and the preload pressure and how it is maintained over the test duration; (2) for compliant constraint, the foam material, grade, and thickness, backed by CFD pre-characterization to confirm the foam’s stable compression plateau and calibration curve; (3) the cell type (single-stack vs multi-stack), electrode formulation, and N/P ratio, since silicon content and electrode balancing substantially affect both reversible breathing amplitude and irreversible thickness accumulation rate; (4) temperature and electrolyte conditions, as both affect SEI growth rate and therefore irreversible swelling; and (5) measurement system calibration and offset correction, particularly for PCB-sensor-based systems. Without specifying all of these parameters, expansion data from different labs or different test setups cannot be directly compared — which is the core motivation for the Mercedes-Benz study’s emphasis on pre-characterization and standardization.

6.6 Why does lithium plating occur under rigid constraint in silicon-carbon pouch cells?

Lithium plating under rigid constraint in silicon-carbon pouch cells occurs because the rigid platen cannot distribute pressure uniformly as the cell’s thickness increases irreversibly over cycles — instead, pressure concentrates at the stiffer regions of the cell (typically the electrode stack edges and corners). These high-pressure zones compress the separator and interfere with ion transport, increasing local overpotential and making metallic lithium deposition on the graphite surface thermodynamically and kinetically more favorable than intercalation. Once lithium plating initiates, it creates a self-reinforcing degradation pathway: deposited lithium consumes active lithium inventory, reduces capacity, increases resistance, and dendrite growth creates separator penetration risk. A compliant buffer foam distributes pressure uniformly across the full electrode area, eliminating the localized overpotential that initiates plating — which is why post-cycle SEM showed large-area lithium plating in rigid-constrained cells but intact electrodes in foam-constrained cells at identical test conditions.

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