In-Situ Analysis of Pouch Cell Swelling: Measuring Volume, Thickness, and Pressure Changes

In-Situ Cell Gassing Volume Monitor Analyzer, In-Situ Cell Swelling Case

Updated on 2025/08/20

1. Preface

For lithium-ion batteries, particularly pouch cell designs encapsulated with lightweight aluminum-plastic film, external space is a critical constraint. The volume swelling that occurs during charging and discharging generates significant internal stress. This stress directly impacts crucial performance metrics like capacity, rate capability, and, most importantly, pouch cell safety and long-term reliability. Effectively characterizing and controlling this pouch cell swelling is therefore a primary focus for battery developers.

This swelling behavior is categorized into two types: reversible and irreversible. Reversible swelling stems from the structural expansion and contraction of electrode materials during lithium-ion intercalation/de-intercalation and from thermal effects. Irreversible swelling, a more serious concern, is caused by gas generation due to electrolyte decomposition or irreversible structural phase changes within the electrodes ^1-5.

This study provides a comprehensive, in-situ analysis of pouch cell swelling behavior from two critical physical perspectives: overall volume change and localized thickness change. And we explain measurement methods, present representative results, and discuss how to measure pressure in pouch cell environments for R&D and QC applications.

Figure 1. Test schematic: (a) Volume Test; (b) Thickness Test

2. Experimental Equipment and Test Methods

2.1 Key Instruments

To accurately measure pressure and dimensional changes in a pouch cell, advanced in-situ instruments are required.

2.1.1 In-situ Gas & Volume Monitor (GVM2200 – IEST)

• 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 appearance of the device is shown in Figure 2.

Figure 2. Appearance of GVM2200 Equipment

2.1.2 In-situ Swelling Analyzer (SWE2110 – IEST)

The SWE2110 directly measures the thickness change of a pouch cell under a defined applied pressure. This is critical because it allows researchers to measure pressure-dependent swelling behavior, simulating the mechanical constraints found in a real battery pack. The appearance of the equipment is shown in Figure 3.

Figure 3. Appearance of SWE2110 Equipment

These systems together enable comprehensive pouch cell swelling characterization and allow researchers to measure pressure in pouch cell indirectly (via force/expansion under controlled gaps or pressures) and to monitor gas-driven volume growth directly.

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 test (GVM2200): measure initial mass (m₀), place cell in channel, set sampling and channel IDs in software; system logs real-time volume (or gas) change and synchronizes electrochemical data.
Cell thickness test (SWE2110): mount cell, define sampling frequency; system records thickness, Δthickness, voltage, current, temperature and capacity in real time.
How to measure pressure in pouch cell: use SWE2110 in constant gap or controlled-compression modes to measure expansion force (converted to pressure over known contact area). Alternatively, infer internal pressure changes by combining volume change (GVM) with mechanical stiffness models of the pouch package.

3. In-situ Analysis of Pouch Cell Swelling Behavior

3.1 Overall Pouch Cell Swelling Behavior During Full Charge-Discharge Cycles

Perform in-situ swelling analysis on the same cell’s two continuous charging and discharging processes.

The cell design information and capacity information are shown in Table 1.

Table 1. Cell design information and capacity information

Cell Design System :NCM811/Graphite Cell Size :47354mm³
Capacity/Ah	CC	CV	DC	Coulombic Efficiency
1st cycle	0.5615	0.7001	0.6282	89.7%
2nd cycle	0.5231	0.6802	0.6064	89.1%

Figure 4a shows cell volume vs. time and voltage, while Figure 4b shows thickness vs. time and voltage for the same cell during charge and discharge. Observations:

During CC charging, both volume and thickness increase.
During the CV stage, both metrics decrease slightly as concentration gradients relax.
During CC discharge, volume first rises then falls; thickness remains nearly constant initially then decreases.
Percentage changes in total volume and thickness are broadly consistent, indicating that thickness deformation largely drives the macroscopic volume change.

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 and discharge, showing characteristic pouch cell swelling. (b) Voltage and thickness change during charge and discharge.

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) Differential capacity and volume change during constant current charging. (b) Differential capacity and thickness change during constant current charging.

3.3 Cell Swelling Behavior During Constant Voltage Charging

During the CV hold phase (Figure 6 a & b), the current decreases exponentially. As the driving force for lithium intercalation decreases, the lithium concentration gradient within the graphite anode relaxes and becomes more uniform ⁵. This relaxation process is directly observable as a gradual decrease and stabilization of both volume and thickness, indicating a reduction in internal stress within the pouch cell.

Figure 6. (a) Current and volume changes during constant voltage charging. (b) Current and thickness changes during constant voltage charging.

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) Differential capacity and volume change during constant current discharge. (b) Differential capacity and thickness change during constant current discharge.

4. Practical Guidance — Measure Pressure in Pouch Cell & Interpret Swelling Data

To measure pressure in pouch cell and use swelling metrics effectively:

Combine direct and indirect methods: use GVM to detect gas and overall volume expansion, SWE2110 to measure thickness and expansion force; convert measured force to contact pressure using known contact area.
Synchronize electrochemical data: always record voltage, current, capacity and temperature alongside mechanical signals to link swelling signatures to SOC and electrochemical events.
Separate reversible vs irreversible components: track cycle-to-cycle baseline shifts (irreversible swelling) versus periodic reversible changes tied to SOC.
Apply realistic constraints: test with realistic pack pre-load to see how containment alters measured swelling and internal pressure.
Use stress–strain loops: perform quasi-static compression sequences to extract modulus, yield and permanent set — useful for designing housing stiffness and pre-load strategies.
Modeling: feed measured volume/thickness and gas evolution into thermo-mechanical models to estimate internal pressure distribution and risk zones for delamination or pouch failure.

5. Summary & Implications for Pouch Cell Design

IEST In-situ GVM and SWE measurements provide complementary views of pouch cell swelling: GVM captures gas and volume growth, SWE records thickness and force. Together they enable reliable ways to measure pressure in pouch cell systems and to distinguish reversible intercalation swelling from irreversible gas-driven growth.

Key takeaways:

Swelling correlates strongly with electrochemical phase changes (visible in dQ/dV).
Reversible swelling is largely driven by lithium intercalation and thermal expansion; irreversible swelling arises from gas evolution and structural changes.
Measuring both volume and thickness is essential for accurate internal pressure inference and for designing mechanical mitigation (housing stiffness, pre-load, filling protocol).
Regular in-situ swelling characterization improves safety margins, enables better electrolyte and electrode material selection, and shortens R&D cycles.