Propagation Mitigation - Battery Design

Once one cell in a pack goes into thermal runaway, the design problem changes: the goal is no longer preventing that single failure, it is stopping the failure from spreading to neighbouring cells (thermal propagation). Pack and module designers combine several of the approaches below, since no single measure is normally sufficient on its own.

Physical Separation and Air Gaps

Simply increasing the distance between cells reduces conductive and radiative heat transfer into the neighbouring cell, and gives any barrier material more volume to absorb energy into before it saturates. The trade-off is packaging density — every millimetre of air gap is a millimetre not used for active material, so this is usually combined with a thinner, higher-performance barrier rather than used as the only line of defence.

Thermal Barrier Materials

A layer placed between adjacent cells can absorb or block heat long enough for the rest of the pack to stay below its own runaway threshold. Materials used for this purpose include mica and ceramic-fibre sheets, intumescent coatings that expand and char under heat, and composite layers that combine a phase-change material (PCM) with a rigid ceramic or mica skeleton so the barrier keeps its shape once the PCM has melted [1][2]. Foams and aerogels are also used, both between cells and over sensitive components such as busbars, where they add thermal resistance without the weight of a solid ceramic layer.

Cell Clustering and Compartmentalisation

Rather than treating the module as one continuous block of cells, packs can be split into smaller clusters separated by structural firewalls. If propagation does occur, it is contained within one cluster instead of running through the whole pack, and each cluster can have its own vent path and cooling connection so a failure in one compartment does not compromise the thermal control of the rest of the pack.

Vent Gas Exhaust Management

A cell in runaway ejects hot, flammable gas well before neighbouring cells are affected by conduction alone — left to accumulate inside a sealed enclosure, that gas builds pressure and can re-ignite. Directing it away from other cells matters as much as blocking heat: vent-inducing frames and channels route the gas along a predetermined path out of the module, and quenching features inside that path (metal meshes, water mist, or engineered baffles) cool the gas and strip out particulates before it reaches an exhaust point [3][4]. Getting the vent path geometry right is now treated as being as important to propagation resistance as the barrier material itself.

Electrical Fusing and Isolation

A failing cell can keep feeding electrical energy into the fault from the rest of the string, which adds to the heat the barrier and vent system have to deal with. Cell or group-level fuses (see Fuses and Fuse Coordination) clear that electrical contribution quickly, so the remaining thermal and mechanical measures only have to deal with the energy already stored in the failed cell itself, not a continued current feed from its neighbours.

Active Cooling as a Propagation Tool

Cooling systems are normally sized for continuous operating loads, but some designs also call on them, or a dedicated auxiliary circuit, during a propagation event — for example running coolant at higher pressure or flow through the plates adjacent to an affected cluster to pull heat away faster than the barrier alone would manage [5]. This only works if the cooling loop and its plumbing survive the initial event, which is itself a design constraint on where coolant lines are routed relative to the cells most likely to fail first.

Regulatory Testing Context

Propagation resistance is now directly tested rather than just designed for. China’s GB 38031 standard (2020 edition) requires that a pack show no fire or explosion for at least five minutes after a single cell is driven into thermal runaway, and the 2025 revision tightens the requirements further. In North America, UL 9540A (5th edition, 2025) extended propagation testing up through cell, module, unit and full installation level, rather than stopping at the cell or module scale used in earlier editions [6][7]. These test regimes are a useful sanity check for any of the design measures above — a barrier or vent design that looks adequate in a single-cell bench test does not always hold up once tested at module or pack scale. At the cell and module scale specifically, NASA’s crewed-spaceflight programme uses a related but more tightly controlled test known as Passive Propagation Resistance (PPR) testing.