Pouch Cell In-situ Analysis of the Volume and Thickness Swelling Behavior

1. Background: Why Pouch Cell Swelling Measurement Matters

For lithium-ion batteries encapsulated with aluminum-plastic film, internal volume changes during cycling impose mechanical stress on the cell casing, tabs, and seals. Unmanaged swelling degrades capacity, rate capability, and safety—and in extreme cases drives seam fatigue and electrolyte leakage. Pouch cell swelling is therefore a primary engineering constraint in battery pack design, directly influencing the specification of cell swelling management systems and the selection of swelling pads.

Cell expansion divides into two categories. Reversible swelling is defined here as the net dimensional change that fully recovers when the cell returns to its initial state of charge; it includes lattice volume changes during Li⁺ intercalation/de-intercalation and thermal swelling from electrochemical heat generation. Irreversible swelling refers to permanent dimensional growth caused by gas evolution (SEI decomposition, electrolyte oxidation) and irreversible structural phase changes in electrode materials—this component accumulates with cycle number and is a direct predictor of end-of-life.

Quantifying both components simultaneously—in real time, under controlled conditions—is the purpose of the in-situ swelling analysis described here.

Figure 1. In-situ test schematics: (a) cell volume measurement via liquid displacement (GVM2200); (b) pouch thickness and expansion pressure measurement under mechanical constraint (SWE2110)

2. Experimental Equipment and Test Methods

2.1 Key Instruments

Accurate in-situ characterization of pouch cell swelling requires instruments that synchronize electrochemical data with mechanical measurements without interrupting the cell’s natural cycling behavior.

2.1.1 In-Situ Gas & Volume Monitor (IEST GVM2200)

• Temperature range: 20 °C – 85 °C
• Dual-channel synchronous testing (two pouch cells simultaneously)
• Continuously records cell volume, gas evolution, temperature, voltage, current, and capacity

The GVM2200 measures absolute volume change via the Archimedes displacement principle, making it the reference method for tracking total cell expansion including gas-driven irreversible growth.

Figure 2. IEST GVM2200 in-situ gas & volume monitor — dual-channel logging of volume change, gas evolution, temperature, voltage, and current

2.1.2 In-situ Swelling Analyzer (SWE2110 – IEST)

The SWE2110 directly measures thickness change of a pouch cell under a defined applied pressure, enabling researchers to measure pressure in the pouch cell environment indirectly. Operating in constant-gap or controlled-compression mode, it records expansion force over a known contact area—the key method for measuring pouch thickness changes as a function of state of charge, C-rate, and cycle number. This simulates the mechanical constraints found in a real battery pack module.

Figure 3. IEST SWE2110 in-situ swelling analyzer — pressure-dependent thickness measurement for comprehensive pouch cell swelling characterization

Together, the GVM2200 and SWE2110 provide a complete picture of pouch cell swelling: volume-based total expansion from the GVM and through-thickness local deformation with force data from the SWE. This dual approach distinguishes gas-driven irreversible growth from mechanically recoverable lattice expansion.

2.2 Test Protocols

• Conditioning: 25 °C rest for 5 min.

• Charge: 1C constant current (CC) to 4.2 V, then CV to 0.025C; rest 5 min.

• Discharge: 1C constant discharge (DC) to 2.8 V.

• Swelling is monitored across two adjacent cycles to capture repeatability and irreversible drift.

2.3 Measurement Workflows

• Cell volume measurement (GVM2200): record initial cell mass (m₀), place cell in channel, set sampling interval and channel ID in software. The system logs real-time volume change and synchronizes electrochemical data automatically.

• Cell thickness measurement (SWE2110): mount cell, define sampling frequency. The system records thickness, Δthickness, voltage, current, temperature, and capacity in real time—providing the primary data stream for measuring pouch thickness evolution during cycling.

• How to measure pressure in a pouch cell: operate the SWE2110 in constant-gap mode to record the force exerted by the expanding cell against a fixed boundary. Dividing measured force by the contact area yields equivalent internal pressure. Alternatively, combine GVM2200 volume data with a mechanical stiffness model of the pouch package to infer pressure from measured dimensional change.

3. In-Situ Pouch Cell Swelling Analysis: Results

3.1 Overall Swelling Behavior During Full Charge-Discharge Cycles

In-situ swelling analysis was performed across two consecutive charge-discharge cycles on the same cell. Cell design and capacity data are summarized in Table 1.

Table 1. Cell design and capacity parameters

Cell Design	Cell Size (mm)	Cycle	CC (Ah)	CV (Ah)	DC (Ah)	Coulombic Efficiency
NCM811/Graphite	47 × 35 × 4	1st cycle	0.5615	0.7001	0.6282	89.7%
		2nd cycle	0.5231	0.6802	0.6064	89.1%

Figure 4(a) plots cell volume vs. time and voltage; Figure 4(b) shows thickness vs. time and voltage for the same cell across one full charge-discharge cycle. Key observations:

• During CC charging, both volume and thickness increase monotonically as lithium intercalates into graphite and de-intercalates from the NCM cathode.

• During the CV stage, both metrics decrease slightly as lithium concentration gradients within the anode relax.

• During CC discharge, volume first rises briefly then falls; thickness remains nearly constant initially before decreasing.

• Percentage changes in volume and thickness are broadly consistent, confirming that through-thickness deformation is the primary driver of macroscopic volume change—which validates measuring pouch thickness as a proxy for total cell expansion.

These synchronized signals demonstrate how pouch cell swelling is tied to electrochemical state: intercalation steps, phase transitions and thermal effects all produce measurable mechanical responses.

Figure 4(a). Voltage and volume change during charge-discharge — NCM811/Graphite pouch cell at 1C (GVM2200)

Figure 4(b). Voltage and pouch thickness change during charge-discharge — in-situ measurement (SWE2110)

3.2 Swelling During Constant Current Charging

Figure 5 (a) and (b) delve deeper by plotting the differential capacity (dQ/dV)—which identifies electrochemical phase transitions—against the swelling curves. The clear correlation is vital for understanding the root cause of pouch cell swelling. Each peak in the dQ/dV curve, signifying a phase change in the anode or cathode material, corresponds to a distinct change in the slope of the volume and thickness curves. This directly links mechanical expansion to electrochemical activity.

Figure 5(a). dQ/dV and volume change during CC charging — phase transitions drive incremental swelling

Figure 5(b). dQ/dV and thickness change during CC charging — correlation between electrode phase transitions and pouch cell swelling

3.3 Swelling Behavior During Constant Voltage Charging

During the CV hold phase (Figure 6a & b), current decreases exponentially. The lithium concentration gradient within the graphite anode relaxes, directly observable as a gradual decrease and stabilization of both volume and thickness.

Figure 6(a). Current and volume changes during CV charging — swelling relaxation as concentration gradient dissipates

Figure 6(b). Current and pouch thickness changes during CV hold — thickness stabilization confirms gradient equalization

3.4 Asymmetric Swelling Behavior During Constant Current Discharge

The discharge process (Figure 7 a & b) reveals the non-equilibrium nature of pouch cell swelling. An initial volume expansion of ~0.3% is observed, potentially linked to a phase transition (Hexagonal 3→Hexagonal 2) in the NCM cathode material as lithium ions are re-inserted. Notably, this initial expansion is not clearly seen in the thickness data, highlighting the value of multi-dimensional analysis. Furthermore, the pouch cell swelling during charge and discharge is not perfectly symmetrical. This hysteresis is a key indicator of irreversible swelling, caused by incomplete structural recovery of the electrode materials ⁴, which accumulates over cycles and leads to performance degradation.

Figure 7(a). dQ/dV and volume change during CC discharge — ~0.3% initial expansion (H3→H2); swelling asymmetry indicates irreversible growth

Figure 7(b). dQ/dV and pouch thickness change during CC discharge — hysteresis quantifies irreversible swelling per cycle

4. Summary

This study used the IEST GVM2200 and SWE2110 to perform comprehensive in-situ pouch cell swelling analysis on an NCM811/graphite cell across two consecutive 1C cycles. Key findings:

• Volume and thickness changes track closely throughout cycling.
• dQ/dV peaks correlate directly with inflection points in the swelling curves.
• The CV hold stage produces measurable swelling recovery via concentration gradient relaxation.
• An initial ~0.3% volume expansion at discharge onset is linked to an NCM cathode phase transition.
• Charge-discharge hysteresis quantifies irreversible swelling.

5. References

[1] Ruihe Li, Minggao Ouyang et al. Volume Deformation of Large-Format Lithium Ion Batteries under Different Degradation Paths. Journal of The Electrochemical Society, 2019, 166 (16) A4106-A4114

[2] Shiyao Zheng, Yong Yang et al. Correlation between long range and local structural changes in Ni-rich layered materials during charge and discharge process. J. Power Sources. 2019,412,336–343;

[3] Y. Reynier, R. Yazami, B. Fultz. The entropy and enthalpy of lithium intercalation into graphite. Journal of Power Sources .2003, 119–121 850–855

[4] Jan N. Reimers and J. R. Dahn. Electrochemical and In Situ X-Ray Diffraction Studies of Lithium Intercalation in LixCoO2. Journal of Electrochemical Society, 1992, 139, 8

[5] Haifeng Dai, Chenchen Yu, Xuezhe Wei, Zechang Sun. State of charge estimation for lithium-ion pouch batteries based on stress measurement. Energy, 2017,129, 16.

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