Pouch cells have become one of the most widely used cell formats in battery research and development. Their design versatility, flexible sizing, low equipment requirements, and straightforward scalability from single to multi-layer configurations make them the format of choice across a wide range of research areas — from lithium-metal and sodium-ion chemistries to solid-state prototypes and beyond. Unlike rigid cylindrical or prismatic cells, the pouch format allows researchers to apply and monitor controlled external conditions, making them well-suited for systematic experimental work.
This flexibility, however, introduces its own variables: how pressure is applied, how it is distributed across the cell face, and how it evolves as the cell swells and contracts during cycling. These factors can influence electrochemical performance as significantly as the cell chemistry itself.
Pressure Effects on Performance
The influence of external pressure on pouch cell behaviour is well established in the literature, and the evidence consistently points to two requirements: an appropriate magnitude of applied pressure, and, critically, its uniformity.
Mussa et al. investigated the effects of external compression on NMC/graphite single-layer pouch cells over a range of applied pressures (0.66–1.98 MPa). They identified an optimal pressure of approximately 1.3 MPa that minimised cyclable-lithium loss during cycling, while both insufficient and excessive pressure led to accelerated degradation. Notably, non-uniform pressure distribution was shown to induce non-uniform current distribution, resulting in inhomogeneous ageing across the electrode area [1].
Choi et al. extended this understanding using pressure-sensitive films to directly visualise stack pressure distributions in Li-metal pouch cells. They showed that non-uniform stack pressure leads to pouch cell failure regardless of the absolute pressure magnitude applied. A modified fixture incorporating force-redistributing pads enabled stable cycling at lower overall pressures, with the improvement correlated to a more uniform passivation layer thickness across the anode surface [2].
From an automotive engineering perspective, van Nuffel demonstrated that Poron foam compression pads can meaningfully extend battery longevity by reducing mechanical non-uniformities across the cell face [4] - the same material used by Wattcrafts in our holders, and the same approach adopted by the EV battery packs industry for managing cell-to-cell pressure variation.
The Experimental Setup Challenge
The most common laboratory solution for pouch cell fixturing is a pair of flat plates — acrylic (plexiglass) or aluminium — bolted together at the corners with threaded rods and wing nuts or hex bolts. The approach is inexpensive, fast to assemble, and widely available. And for many electrochemical measurements, it seems to work fine.
In practice, a number of factors can compromise pressure uniformity:
- Plate bending. Acrylic plates are a popular choice for their optical transparency and low cost, but they bend under load — bowing outward at the centre and pushing more force onto the cell edges and corners. Aluminium plates can behave the same way if they are not thick enough for the applied load.
- Cell edge effects. As the plate bends, the cell edges and corners experience disproportionate lateral stress, which can cause wrinkles in the pouch laminate, and localised mechanical loads that would not occur in practical real-world situations.
- Uneven bolt tension. Manually tightening four corner bolts to identical torque is difficult in practice. The result is that the cell is clamped harder in some areas than others.
- Pad material mismatch. Where foam or rubber pads are used, the selected material is often either too rigid to provide meaningful cushioning, or too soft and fully compresses under the applied load - in both cases not distributing efficiently the pressure.
In chemistries reliant on uniform metal deposition or sensitive to localised stress, the electrode morphology observed post-cycling will directly reflect these fixture-imposed pressure gradients - independently of any intrinsic chemistry behaviour.
Case Study: Anode-Free Sodium-Ion Technology
Anode-free batteries, in which the alkali metal plated onto the current collector during charging serves directly as the anode with no pre-loaded active material, represent one of the highest energy density architectures currently under development. In sodium-ion variants, metallic sodium deposits onto a current collector during charge and strips during discharge. The process is particularly sensitive to local mechanical conditions: insufficient or non-uniform pressure produces mossy or dendritic sodium morphologies with poor reversibility, while pressure hot spots create localised plating that further concentrates subsequent deposition.
Fig. 1. Example simple acrylic fixture.
The effect of a basic plexiglass fixture on anode-free sodium cells is readily apparent in post-cycling electrode analysis.
Fig. 2. (left) heavy, compact Na deposition at cell edges (bending issue). (middle) heavy deposition in one corner or more on one side of cell (uneven screwing). (right) visible plating lines (wrinkles/lateral stress)
Each observed deposition pattern corresponds to a specific issue with the fixture. Plating at the edges means the plate was bowing. Concentration in one corner means one side/corner was tightened more than the others. Lines across the electrode mean the pouch was wrinkling. In each case, the sodium is showing you where the pressure was highest.
These non-homogenous plating patterns compromise cycle life measurements, Coulombic efficiency data, and any post-mortem analysis aimed at characterising intrinsic cell behaviour.
Wattcrafts Pouch Cell Holders
When the same anode-free sodium-ion chemistry is cycled in a Wattcrafts Pouch Cell Holder at equivalent overall pressure, the electrode morphology changes substantially.
Fig. 3. Current collector from anode-free Na cell cycled in Wattcrafts holder with pads - showing uniform sodium plating across the full electrode area.
Dense, homogenous deposition covering the full active area reflects the uniform pressure provided by the fixture. The mossy morphologies and deposition artefacts observed with the acrylic holder are absent.
Design Principles
The Wattcrafts pouch cell holder is designed from the ground up around the requirement for uniform, reproducible pressure. The key design elements are:
Rigid frame. The fixture is machined from aluminium and stainless steel, providing dimensional stability across a wide force range. The structural geometry eliminates plate bending without requiring impractically thick or heavy components.
Compliant interface. The contact between the fixture plates and the cell is provided by a high-quality Poron foam pad. Poron is specifically engineered to maintain consistent compressive resistance across its operating range — it does not fully compress under applied load and recovers reliably across many cycles. This is the same material class used in commercial EV battery modules to manage inter-cell pressure distribution. The pad adapts to the cell surface, compensating any irregularities, such as the slightly thicker edges where the single layer pouch is sealed.
Controlled tightening. Clear procedure on tightening screws and optional levelling tool adding, that visually aids even tightening of 4 corner screws.
To validate the pressure uniformity, we used pressure-sensitive film (Fujifilm Prescale) placed on top of the single layer pouch cell and applied 1600 N (300 kPa) compression load. The colour intensity of the film after compression directly maps the local pressure distribution.
Fig. 4. Pressure-sensitive film test. (left) Wattcrafts holder, no pads. Shows high-intensity (dark) regions along the cell seal edges. (right) Wattcrafts holder with flexible pads. Shows uniform colour distribution across the full cell face.
With the Poron pad installed, the applied load is distributed across the complete cell area. The pad compensates for the seal-edge thickness difference and smooths any remaining local pressure variations are effectively eliminated.
For maximum control: pressure mapping sensors
Pressure-sensitive film gives a useful snapshot of pressure distribution, but it is a one-time measurement taken before cycling begins. For applications where tracking pressure distribution throughout the test matters, or where cell-to-cell variability needs to be characterised systematically, thin-film pressure mapping sensors can be integrated directly into the fixture, providing continuous spatial pressure data alongside electrochemical measurements. This is particularly relevant for solid-state cells, where interfacial contact pressure evolves during cycling, or for any study where understanding how pressure distribution changes with state of charge is part of the research question.
Fig. 4. Example snapshot of pressure distribution measured with thin-film mapping sensor.
If this is something you are exploring, get in touch with us at Wattcrafts to discuss what is possible.
Relevance Beyond Anode-Free Systems
Anode-free sodium-ion cells represent an extreme case of pressure sensitivity, but the same fundamental issue applies across a broad range of pouch cell chemistries and research applications:
- Silicon-anode lithium-ion cells, where lithiation-induced volume expansion of up to ~300% generates large internal stresses that interact strongly with external mechanical constraint.
- Lithium-metal anodes, where dendrite nucleation and SEI formation are both strongly dependent on local stack pressure — the same mechanisms observed in anode-free systems apply directly.
- All-solid-state cells, where maintaining intimate electrode–electrolyte interfacial contact requires carefully controlled, homogeneous pressure. Current redistribution resulting from localised contact loss can accelerate degradation elsewhere in the electrode stack [3].
- Cycle life and ageing studies in any chemistry, where non-uniform pressure leads to uneven degradation across the electrode area, making results analysis difficult to interpret.
Getting reliable data from pouch cells starts with the fixture. Pressure magnitude and uniformity both matter, and as the examples above show, a poorly designed fixture changes the experiment. This is true across cell chemistries, not just the most sensitive ones.
Interested in setting up controlled-pressure testing for your pouch cell research? Contact us or browse our product range.
References
[1] Mussa, A. S. et al. "Effects of external pressure on the performance and ageing of single-layer lithium-ion pouch cells." Journal of Power Sources 385, 18–27 (2018). https://doi.org/10.1016/j.jpowsour.2018.03.020
[2] Choi, B., Lee, M., Woo, S.-G. et al. "Importance of uniformly redistributing external pressure on cycling of pouch-type Li-metal batteries." Korean Journal of Chemical Engineering 40, 524–531 (2023). https://doi.org/10.1007/s11814-022-1361-3
[3] Thorpe, M. A. et al. “Controlling stack pressure inhomogeneity in anode-free solid-state batteries using elastomeric interlayers”. Matter 8, 3, (2025). https://doi.org/10.1016/j.matt.2024.101955
[4] van Nuffel, K. "Optimizing Lithium-ion Battery Longevity with Foam Compression Pads." MTZ Worldwide 86, 54–59 (2025). https://doi.org/10.1007/s38313-025-2104-8
Note: The anode-free sodium-ion experiments presented in this post were carried out at Swansea University.