Cell swelling and cell-to-cell barriers: what the compression data tell you

How cells load the barrier

Prismatic and pouch cells change thickness in two ways. Reversible “breathing” follows state of charge and temperature as the electrodes expand and contract. Irreversible growth accumulates over life from ageing processes such as interphase growth and gas generation. In most modules, end plates with straps or tie rods limit the stack’s length, so cell growth is shared between the cells themselves, the barriers and pads, the frame and fasteners, and any spring elements or intentional gaps in the design.

Two requirements pull against each other. The barrier must stay thick enough to insulate when a neighbouring cell fails. The stack pressure must stay within the window the cells need for long life, without overloading the end plates. The pressure window is specific to each cell design and should come from the cell supplier. Cell format also matters: pouch cells need the pressure spread evenly over the face, while the rigid can of a prismatic cell tends to bow outward at the centre, so the barrier is loaded unevenly and local pressure can be well above the average.

Published compression and conductivity data

Morgan’s datasheet lists both papers for pouch and prismatic cell-to-cell protection, chosen by the required compression characteristics. At 20 % deflection, EST C310 resists about ten times more strongly than EST C30, and both stiffen steeply as deflection increases: the resistance of EST C30 rises more than tenfold between 20 % and 60 % deflection as the fibre network densifies.

The Rogers board’s published curve reaches about 2,000 kPa at only 17 % strain, and its reported conductivity at test temperatures from 25 to 800 °C is 0.029–0.037 W/(m·K), roughly a quarter to a third of the ceramic-fibre papers’ values at 600–800 °C. Read together, these figures suggest a material that changes thickness little under load, so a design relying on it to take up large cell growth would need checking; the datasheet table gives the compression range for finished parts as “based on design”. Measured conductivity at temperature is not the same as retained barrier performance in a fire, which depends also on dimensional stability and integrity.

These are datasheet values obtained by different methods (Morgan lists ASTM C201 / C177 for conductivity; Rogers lists ISO 8301 and, for compression, ASTM D1056), specimen conditions and cycle histories. They are not a ranking of pack-level fire or propagation performance. For fibre papers in particular, permanent set on the first compression cycle means a first-cycle curve may not represent cyclic or aged behaviour. Conductivity is measured on uncompressed material.

What the datasheets usually do not tell you

• Stress relaxation. In a clamped stack the barrier is held at a fixed or slowly increasing strain. What matters is how much its force decays over time at temperature, because the stack pressure falls with it.
• Compression set and recovery. When cells contract at low state of charge or low temperature, a barrier with high compression set recovers little, and depending on preload and frame compliance the stack can lose contact pressure.
• Thermal resistance at the compressed thickness. Heat flow through the barrier scales with its conductance, conductivity divided by thickness. Compression reduces thickness and also changes conductivity, in a way that depends on the material: densifying a porous insulator can lower radiative transfer but raise solid conduction. Contact resistance and gas gaps also matter. Ask for conductivity at the end-of-life thickness or, better, a transient or full propagation result.
• Electrical insulation. Dielectric strength at temperature, volume and surface resistivity, and tolerance to pinholes and cut edges, measured on the compressed and aged part.
• Condition after ageing. Repeated breathing can crack rigid laminates or shed fibres from unencapsulated papers. Either damage can reduce dielectric isolation between cell cans and make the barrier easier for vent jets and particles to erode.

A method for specifying a barrier in a swelling stack

1. Build the displacement budget. For each barrier, deflection at end of life is roughly the assembly preload deflection, plus the irreversible cell growth and maximum reversible breathing it has to take up, minus what the frame, fasteners and other compliant layers absorb. Include manufacturing and thickness tolerances, cell-to-cell variation and thermal expansion, and check local peaks where prismatic cans bow, not just the average. Cell growth data come from the cell supplier.
2. Check the operating points. Locate the beginning-of-life, low-state-of-charge strain and the end-of-life, high-state-of-charge strain on the barrier’s compression curve. Then correct the forces using stress-relaxation data measured at the relevant temperature and dwell time, since relaxation cannot be read from a static curve. Barrier stress is not the same as cell-face pressure, which can be uneven; both should stay inside the window the cells require.
3. Consider a layered barrier. If one material cannot meet both the displacement budget and the thermal requirement, combine a thin, stiff insulating layer with a compliant layer that takes up growth. Define what each layer must do during a fire and for how long; a compliant layer may be designed to decompose, while the insulating layer usually has to keep its thickness and integrity.
4. Check the failure case. During thermal runaway the failing cell can swell before it vents, loading the barrier at the same time as it heats. The barrier must not shrink, crack or extrude out of the gap under combined temperature and pressure, and must stay in place after neighbouring cells vent. Vent gas, flame and hot particles, edge bypass paths and decomposing adhesives can dominate the outcome even when bulk conductivity is low.
5. Test in the aged, compressed state. Regulations do not normally specify barrier ageing. For example, the test added to China’s GB 38031-2025 after fast-charge cycling is an external short-circuit test, separate from the thermal-propagation (热扩散) test; check the official text for the exact clauses. Running propagation tests on barriers that have seen representative compression, relaxation and temperature is therefore part of the pack developer’s own validation, and a result from one pack design does not transfer automatically to another.

Related

• Aerogel vs mica vs ceramic fibre: choosing a cell-to-cell barrier
• Thermal runaway propagation explained
• Materials data with verified product values

Common questions

Why does cell swelling matter for a thermal barrier?

A barrier between prismatic or pouch cells sits in the mechanical load path of the cell stack. As the cells grow over life, the barrier is compressed. That changes the stack pressure on the cells and reduces the barrier's thickness, and therefore its thermal resistance, at the time a thermal runaway might occur.

What compression data should I ask a barrier supplier for?

The compression-force deflection curve with its test method and cycle number, compressive stress relaxation at temperature, compression set, and thermal conductivity or a propagation result at the compressed thickness expected at end of life. A single stress value at one deflection is not enough.

Is a stiffer barrier better?

Not in general. A stiff barrier keeps its thickness but turns cell growth into stack pressure and end-plate load. A compliant barrier keeps pressure low but loses thickness. Many packs combine a thin, stiff insulating layer with a compliant layer that takes up cell growth.