Thermal runaway propagation in battery modules and packs represents a critical safety challenge in energy storage systems and electric vehicles. When one cell enters thermal runaway, the intense heat and ejected materials can trigger adjacent cells to fail in a cascading manner, potentially leading to complete pack destruction. Understanding the mechanisms behind this propagation is essential for designing safer battery systems.

The heat transfer pathways between cells occur through three primary mechanisms: conduction through metallic connections like busbars, convection through cooling systems or air gaps, and radiation from hot surfaces. Busbars designed for electrical current carrying also serve as efficient thermal bridges. Aluminum busbars with typical thermal conductivity of 200 W/m·K can transfer substantial heat to neighboring cells. Cooling plates intended to regulate temperature may paradoxically aid propagation if not properly segmented, as heat spreads through the coolant channels and metal surfaces.

Module housings made of aluminum or steel conduct heat laterally, with aluminum's 160 W/m·K conductivity creating effective thermal bridges. The spacing between cells directly affects propagation speed, with industry data showing that gaps below 2 mm significantly increase cascade risks. Thermal barriers such as ceramic inserts or aerogel sheets with thermal conductivities below 0.02 W/m·K can disrupt these pathways when properly implemented.

The domino effect follows a characteristic sequence beginning with cell venting, followed by jetting of hot electrolytes and particulates. Time delays between successive cell failures vary based on design, with experimental data showing intervals ranging from 15 seconds in tightly packed modules to several minutes in properly mitigated systems. The propagation front moves at speeds between 0.5 to 5 cm/s depending on the thermal management system effectiveness.

Venting design critically influences propagation dynamics. Poorly oriented vent channels direct hot gases and particles toward adjacent cells, while upward-venting designs reduce cross-cell heating. Large-scale fire tests demonstrate that packs without adequate venting paths experience 40% faster propagation rates compared to designs with managed venting. The vented materials include combustible gases like hydrogen, methane, and ethylene that contribute to secondary combustion events.

Industry standards provide specific testing protocols for evaluating propagation. ISO 6469-1 specifies thermal propagation tests requiring no fire spread to adjacent cells within 5 minutes after initial failure. China's GB 38031 mandates monitoring for at least one hour after thermal runaway initiation to verify containment. These standards drive design improvements in several key areas.

Cell spacing remains one of the most effective propagation controls, with 5 mm gaps demonstrating 60% slower propagation compared to 1 mm gaps in experimental comparisons. However, this comes at the expense of energy density, creating an engineering tradeoff. Some designs employ phase change materials between cells that absorb heat during melting, with materials like paraffin wax showing 30% propagation delay in controlled tests.

Thermal barrier materials must withstand temperatures exceeding 800°C while maintaining structural integrity. Mica-based barriers demonstrate effectiveness due to their layered structure and high temperature stability. Some advanced systems use intumescent materials that expand when heated, creating insulating char layers that block heat transfer.

Cooling system design plays a dual role in propagation. Active cooling can suppress propagation if sufficiently powerful, with liquid cooling systems demonstrating better performance than air cooling in arresting cascades. However, failed cooling systems or low coolant flow rates may inadvertently aid propagation by slowly removing heat while allowing sufficient energy transfer to adjacent cells.

Electrical disconnection systems attempt to isolate failed cells by melting fusible links or triggering pyro switches. These systems must activate within milliseconds to be effective, as experimental data shows busbars can transfer enough heat to trigger adjacent cells within 10-15 seconds after initial failure.

Pack-level containment strategies include fire-resistant coatings on module housings and compartmentalization designs that create physical barriers between cell groups. Large-scale tests show that steel enclosures with ceramic liners can contain thermal runaway events for over 30 minutes, providing critical time for emergency response.

Material selection for components affects propagation resistance. Stainless steel busbars with lower thermal conductivity than aluminum reduce heat transfer but increase electrical resistance. Some designs use composite busbars with thermal breaks that maintain electrical conductivity while blocking heat flow.

Gas management systems address the combustible vapors released during thermal runaway. Some pack designs incorporate catalytic converters or flame arrestors that prevent gas ignition. Test data indicates proper gas management can reduce peak temperatures during propagation events by up to 200°C.

Early detection systems using multiple sensor types can trigger countermeasures before propagation begins. Temperature rate-of-change sensors combined with gas composition sensors provide reliable early warnings, with response times under 500 milliseconds in validated systems.

The progression of thermal runaway follows distinct phases beginning with internal short circuits, followed by separator collapse and cathode decomposition. Each phase releases different amounts of heat, with lithium nickel manganese cobalt oxide cathodes generating approximately 800 kJ/kg during decomposition. This energy release drives the propagation process.

Industry testing reveals that propagation patterns differ between cylindrical, prismatic, and pouch cells due to their structural differences. Pouch cells show more rapid propagation due to larger surface area contact, while cylindrical cells demonstrate more directional propagation along their length. Prismatic cells fall between these extremes, with propagation heavily influenced by their internal construction.

Mitigation strategies must account for the entire failure sequence from initial trigger to final containment. Multi-layer approaches combining thermal barriers, advanced cooling, early detection, and physical separation prove most effective. System-level validation through standardized testing ensures these measures work cohesively during actual failure events.

Future developments focus on materials that intrinsically resist propagation, such as solid-state electrolytes that eliminate flammable liquid components. Current research indicates these materials could increase the thermal runaway onset temperature by over 100°C, fundamentally changing propagation dynamics. However, new failure modes may emerge that require continued study and mitigation.

The comprehensive understanding of thermal runaway propagation informs safer battery system design across transportation and energy storage applications. Through careful engineering of thermal pathways, containment systems, and early warning mechanisms, the risks associated with cascading cell failures can be systematically reduced while maintaining performance requirements.