Battery Safety Knowledge Hub
Technical Risk
Lithium-Ion Battery Thermal Runaway
Thermal runaway is often reduced to one dramatic moment: a temperature spike, flame or explosion. For engineering decisions, that is too late and too simple.
A useful thermal-runaway characterisation asks how the event develops, which signals appear first, how quickly the transition occurs and what the cell releases into the surrounding system.
THERMAL RUNAWAY IS NOT ONE TEMPERATURE
To understand cell behaviour during failure, it is necessary to determine when the event initiates, the rate of its development, and the resulting thermal load on the surrounding system. Consequently, thermal runaway characterization encompasses more than just the maximum cell temperature.
First Venting
This stage marks the initial release of gas, electrolyte vapour, or other effluent from the cell. At this point, the issue extends beyond the cell itself, making gas detection, venting direction, and the surrounding system increasingly relevant.
This stage marks the initial release of gas, electrolyte vapour, or other effluent from the cell. At this point, the issue extends beyond the cell itself, making gas detection, venting direction, and the surrounding system increasingly relevant.
First Detectable Self-Heating or Onset
This parameter refers to the earliest measurable point at which the cell generates abnormal heat. It defines the initial thermal warning window prior to event acceleration.
This parameter refers to the earliest measurable point at which the cell generates abnormal heat. It defines the initial thermal warning window prior to event acceleration.
Rapid Runaway Onset
This point is characterized by a sharp acceleration in self-heating, marking the onset of the main exothermic event. At this stage, the focus shifts from preventing individual cell failure to containing its consequences.
This point is characterized by a sharp acceleration in self-heating, marking the onset of the main exothermic event. At this stage, the focus shifts from preventing individual cell failure to containing its consequences.
Maximum Surface Temperature
This parameter represents the maximum temperature measured on the cell surface. It defines the thermal load imposed on neighbouring cells, barriers, cooling interfaces, and structural components.
This parameter represents the maximum temperature measured on the cell surface. It defines the thermal load imposed on neighbouring cells, barriers, cooling interfaces, and structural components.
Maximum Vent or Effluent Temperature
When measured, this parameter indicates the temperature of released gas and ejecta. It is particularly important for vent-path design, material selection, particle exposure, and ignition risk assessment.
When measured, this parameter indicates the temperature of released gas and ejecta. It is particularly important for vent-path design, material selection, particle exposure, and ignition risk assessment.
Event Duration and Release Rate
Total energy released does not fully describe event severity. A cell that releases heat and gas over several minutes presents a different challenge compared to a rapid release within seconds. The rate of energy release determines the available detection and response time, as well as the extent of loading on the surrounding system.
Total energy released does not fully describe event severity. A cell that releases heat and gas over several minutes presents a different challenge compared to a rapid release within seconds. The rate of energy release determines the available detection and response time, as well as the extent of loading on the surrounding system.
Collectively, these parameters indicate the initiation of failure, the rate of escalation, and the requirements that the surrounding battery system must be designed to address.
WHAT LEAVES THE CELL
When a cell enters thermal runaway, the event is not defined by temperature alone.
The cell can release heat, gas, vapour, particles, and secondary electrical or mechanical effects. Each of these can drive broader system consequences.
The cell can release heat, gas, vapour, particles, and secondary electrical or mechanical effects. Each of these can drive broader system consequences.
Heat
Heat is the primary driver of thermal loading on adjacent cells and surrounding materials. It determines the rate at which neighbouring parts approach failure and the available system response time.
Heat is the primary driver of thermal loading on adjacent cells and surrounding materials. It determines the rate at which neighbouring parts approach failure and the available system response time.
Vent Gas and Electrolyte Vapour
The cell can release a mixture of flammable, toxic, and hot gases along with electrolyte vapour. The amount, composition, temperature, and release rate influence gas accumulation, ignition risk, toxicity, and venting system requirements.
The cell can release a mixture of flammable, toxic, and hot gases along with electrolyte vapour. The amount, composition, temperature, and release rate influence gas accumulation, ignition risk, toxicity, and venting system requirements.
Particles and Ejecta
Thermal runaway can also eject hot solid material, droplets, or fragments. These can transfer heat directly, damage nearby surfaces, erode insulation, and create additional ignition or propagation pathways.
Thermal runaway can also eject hot solid material, droplets, or fragments. These can transfer heat directly, damage nearby surfaces, erode insulation, and create additional ignition or propagation pathways.
Electrical and Mechanical Consequences
A failing cell can create pressure buildup, swelling, casing rupture, short-circuiting, and damage to surrounding electrical interfaces. These secondary effects can escalate a local cell failure into a broader system event.
A failing cell can create pressure buildup, swelling, casing rupture, short-circuiting, and damage to surrounding electrical interfaces. These secondary effects can escalate a local cell failure into a broader system event.
Together, these outputs define the thermal, chemical, electrical, and mechanical loads that the surrounding system must manage. Understanding not only what leaves the cell, but also the quantity, rate, direction, and conditions of release enables the design of effective propagation control, venting, detection, and mitigation strategies.
WHAT CHANGES THE EVENT
Thermal runaway behaviour is variable and depends on multiple factors.
A single cell may exhibit markedly different failure modes depending on its chemistry, condition, triggering method, and surrounding environment.
A single cell may exhibit markedly different failure modes depending on its chemistry, condition, triggering method, and surrounding environment.
Cell Chemistry and Materials
Cell chemistry determines fundamental failure characteristics, such as onset temperature, heat release, gas production, venting behaviour, and mass loss. Materials including the electrolyte, separator, and electrode design further influence the severity of the event.
Cell chemistry determines fundamental failure characteristics, such as onset temperature, heat release, gas production, venting behaviour, and mass loss. Materials including the electrolyte, separator, and electrode design further influence the severity of the event.
Cell Format and Capacity
Cylindrical, pouch and prismatic cells do not vent or fail in the same way. Format influences vent direction, casing behaviour, pressure build-up and ejection, while capacity affects how much energy, gas and material can be released.
Cylindrical, pouch and prismatic cells do not vent or fail in the same way. Format influences vent direction, casing behaviour, pressure build-up and ejection, while capacity affects how much energy, gas and material can be released.
State of Charge (SOC)
The state of charge significantly influences the severity of thermal runaway events. Higher SOC typically reduces thermal stability and increases heat release, gas production, and overall event intensity.
The state of charge significantly influences the severity of thermal runaway events. Higher SOC typically reduces thermal stability and increases heat release, gas production, and overall event intensity.
Ageing Path and Cell Condition
Aged cells often fail differently compared to new cells. Factors such as degradation, swelling, lithium plating, increased impedance, and prior abuse can alter onset behaviour and affect both the severity and repeatability of thermal runaway.
Aged cells often fail differently compared to new cells. Factors such as degradation, swelling, lithium plating, increased impedance, and prior abuse can alter onset behaviour and affect both the severity and repeatability of thermal runaway.
Trigger Method
Triggering mechanisms such as thermal heating, internal short circuit, nail penetration, overcharge, or mechanical abuse result in distinct failure behaviours. The chosen trigger method significantly influences event initiation, acceleration rate, and the parameters measured during testing.
Triggering mechanisms such as thermal heating, internal short circuit, nail penetration, overcharge, or mechanical abuse result in distinct failure behaviours. The chosen trigger method significantly influences event initiation, acceleration rate, and the parameters measured during testing.
Compression and Installation State
Cell restraint, compression, and mounting conditions influence swelling, venting, heat transfer, and the progression of failure. Installation methods within the system can therefore significantly alter the characteristics of the event.
Cell restraint, compression, and mounting conditions influence swelling, venting, heat transfer, and the progression of failure. Installation methods within the system can therefore significantly alter the characteristics of the event.
Ambient and Boundary Conditions
Initial temperature, ambient pressure, oxygen availability, enclosure conditions, and proximity to other materials all affect the development of thermal runaway events. Results observed in open laboratory setups may differ substantially from those within operational battery systems.
Initial temperature, ambient pressure, oxygen availability, enclosure conditions, and proximity to other materials all affect the development of thermal runaway events. Results observed in open laboratory setups may differ substantially from those within operational battery systems.
Collectively, these factors determine the initiation, severity, and representativeness of thermal runaway events and their corresponding test results. A comprehensive understanding of battery thermal runaway requires detailed knowledge of the tested cell, the specific conditions, the cell state, and the trigger method used to initiate the event.
WHY THIS MATTERS AT SYSTEM LEVEL
Thermal runaway characterisation extends beyond understanding individual cell failure mechanisms. The primary value lies in utilizing cell-level data as critical design input for the overall battery system.
Detection Thresholds
Early self-heating, initial venting, and gas release behaviours inform the selection of detectable signals, optimal sensor placement, and the required sensitivity of alarm logic.
Early self-heating, initial venting, and gas release behaviours inform the selection of detectable signals, optimal sensor placement, and the required sensitivity of alarm logic.
Cooling and Mitigation
Heat release, peak temperatures, and event duration determine the effectiveness of cooling in delaying propagation and inform the selection of appropriate active or passive mitigation measures surrounding the cell.
Heat release, peak temperatures, and event duration determine the effectiveness of cooling in delaying propagation and inform the selection of appropriate active or passive mitigation measures surrounding the cell.
Barrier and Spacing Decisions
Surface temperature, vent-gas temperature, particle emission, and release intensity determine the necessary degree of thermal separation between cells, modules, and adjacent components.
Surface temperature, vent-gas temperature, particle emission, and release intensity determine the necessary degree of thermal separation between cells, modules, and adjacent components.
Venting and Pressure Management
Gas volume, release rate, and venting behaviour serve as direct inputs for vent-path design, pressure relief sizing, and the management of flammable or high-temperature effluent within the system.
Gas volume, release rate, and venting behaviour serve as direct inputs for vent-path design, pressure relief sizing, and the management of flammable or high-temperature effluent within the system.
Gas Detection
The quantity, composition, and timing of gas release determine the most relevant gas species for monitoring and assess the potential of gas detection as an early warning mechanism.
The quantity, composition, and timing of gas release determine the most relevant gas species for monitoring and assess the potential of gas detection as an early warning mechanism.
Electrical Isolation
Thermal events may damage insulation, busbars, connectors, and nearby high-voltage components. Consequently, cell failure behaviour directly influences the design of electrical separation and emergency disconnect strategies.
Thermal events may damage insulation, busbars, connectors, and nearby high-voltage components. Consequently, cell failure behaviour directly influences the design of electrical separation and emergency disconnect strategies.
Propagation Modelling
Thermal runaway data supplies essential source terms for modelling heat transfer, gas release, and failure escalation. In the absence of realistic cell-level input, system-level propagation models lack accuracy.
Thermal runaway data supplies essential source terms for modelling heat transfer, gas release, and failure escalation. In the absence of realistic cell-level input, system-level propagation models lack accuracy.
Validation Strategy
Cell-level data defines the necessary test parameters, conditions, and hierarchical levels—from cell to module, pack, or container—to ensure that the safety case accurately represents the actual system.
Cell-level data defines the necessary test parameters, conditions, and hierarchical levels—from cell to module, pack, or container—to ensure that the safety case accurately represents the actual system.
Collectively, these inputs determine whether a single-cell failure remains localized or escalates into a broader system issue. Therefore, thermal runaway characterisation serves not only as a cell-level analysis but also as a foundational element for detection, design, validation, and overall system safety.
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