What triggers the thermal runaway could be one of a number of inputs, primarily:
- electrical abuse
- short circuit
- internal particle
- dendrite growth resulting in a short
- overcharging the cell
- thermal abuse
- overheating of the cell with an external heat source
- heat from another cell
- mechanical abuse
- crushing of the cell in an impact
- puncturing of the cell
Thermal Runaway is part of the many aspects of cell and pack design that have to be understood. Thermal runaway is also a huge subject in it’s own right.
For the fuller picture of how these abuse conditions develop into failure, see Battery Thermal Runaway: Causes, Stages & Prevention, and Cell Testing for how each trigger below is verified experimentally.
Electrical Abuse
Short Circuit
A short circuit is a very low-resistance path that forms between the cell’s positive and negative sides, letting current flow well beyond what the cell is designed for. It can originate inside the cell, from contamination or dendrite growth, or outside it, from a conductive path bridging the terminals directly – see Short Circuit for the full breakdown of internal versus external causes.
Internal Short Circuit
Internal Particle
The nail test was originally designed to replicate a cell failure caused by a piece of rogue metal that gets into the cell during production.
Cell Internal Short Circuit Device – developed and patented by NREL and NASA. This is a metallic particle embedded in a phase change material (wax) that is then inserted into a cell during the assembly of the cell.
This means the particle can very closely replicate the rogue metallic particle trigger method.
Beyond a single rogue particle, the same failure mode can come from weld spatter at the tab or case seam, or physical damage to an electrode during assembly – see Techniques for Battery Quality Control in Production for how manufacturers control for this.
Dendrite Growth resulting in a Short
Dendrites are needle-like deposits of metallic lithium that build up on the anode surface instead of intercalating normally into the graphite. Left to grow, they can pierce the separator and connect the anode to the cathode directly. The underlying cause is lithium plating: either the anode simply runs out of room once it’s fully charged, or, more commonly, a high charge rate or a low cell temperature pushes lithium onto the surface faster than it can be absorbed. Fast charging protocols have to manage this trade-off directly.
External Short Circuit
Unlike a particle sealed inside the cell during manufacture, an external short is caused by something outside it entirely – crash debris bridging exposed terminals, a dropped tool during assembly, or a conductive path forming between busbars at module level. It’s mitigated differently too: fusing, a current interrupt device, and current monitoring by the BMS are the usual defences, rather than manufacturing process control. See Module External Short Circuit for how this is tested at pack level.
Overcharging the Cell
Charging a cell past its designed upper voltage limit accelerates the lithium-plating pathway above, decomposes the electrolyte and generates gas as lithium keeps leaving the cathode, and, in nickel-rich chemistries, can destabilise the cathode’s crystal structure. Guarding against this is normally split between the BMS at pack level and a current interrupt device at cell level.
Over-discharge / Voltage Reversal
The mirror case of overcharge. Discharging a cell below its minimum voltage strips the graphite anode of lithium entirely, and if current keeps flowing past that point it starts dissolving the copper current collector instead. On the next charge, that dissolved copper can replate as a copper dendrite, producing an internal short by the same mechanism as the lithium dendrite above. See the Battery Thermal Runaway overview for the full over-discharge sequence.
Thermal Abuse
Overheating of the Cell with an External Heat Source
This is normally achieved with a heating element attached to the outer surface of the cell. This can also be a laser heating source.
Laser Radiation Thermal Runaway Trigger – Shironita et al look at the energy input in laser pulse and how the cell fails.
In conclusion, the laser irradiation to the battery was effective in generating thermal runaway in a highly controlled manner. Specifically, this technique is considered to be an efficient initiation method in thermal propagation tests that do not increase the temperature of an adjacent battery in a module, and it constitutes as one of the practical uses of laser irradiation. Our findings are expected to be extremely useful for thermal propagation tests of other types and sizes of battery materials.
Sayoko Shironita, Hideki Tsuruga, Keizoh Honda, Kenichiroh Koshika, Minoru Umeda, Thermal runaway characteristics of a LiFePO4-based lithium-ion secondary battery using the laser-irradiation method, Journal of Energy Storage, Volume 40, 2021
Induction Heating – A controllable method of heating a cell that concentrates the energy within the cell. However, it is bulky and heats up the hard case of the cell leading to a failure of the enclosure.
Heat from Another Cell
When designing a battery pack it is necessary to arrange cells in series and parallel to achieve the required system voltage, energy and current delivery. The overall package is often constrained and hence there will be a requirement to arrange cells next to each other.
For how this propagates beyond the initiating cell, see Thermal Propagation and Pack Thermal Runaway.
From these results, it seems that the presence of a gap can mitigate the safety crisis by slowing the propagation of thermal runaway, allowing the driver more time to escape the vehicle. However, larger gaps result in smaller energy densities within the battery pack. Therefore, it is necessary to balance the cell-to-cell gap and energy density considerations in the design process. Modelling Thermal Runaway
Yuanmao Li, Guixiong Liu, Zuyu Li, Numerical modeling of thermal runaway in high-energy lithium-ion battery packs induced by multipoint heating, Case Studies in Thermal Engineering, Volume 38, 2022
Separator Breakdown at Elevated Temperature
This can occur without any outside heating element at all. Once a cell’s internal temperature climbs past the separator’s stability point, whether from an unresolved internal short, a cooling system failure, or high ambient exposure, the polyethylene or polypropylene separator that keeps the anode and cathode apart starts to melt, typically above around 130°C, letting the two electrodes touch directly. Some designs build in a shutdown layer or ceramic coating specifically to blunt this failure mode; see the Battery Thermal Runaway overview and Battery Thermal Management for how cooling design factors into it.
Mechanical Abuse
Crushing of the Cell in an Impact
Crush loading deforms the cell enough to buckle the internal electrode stack or jelly-roll, which can tear the separator and bring the electrodes into direct contact – the same short-circuit endpoint as the electrical triggers above, just reached mechanically. It’s tested at cell, module and pack level; see Crush Testing for how the standards vary.
Puncturing of the Cell
A puncture breaches the case and separator at a single, localised point – a sharp or rigid object forcing its way through, whether that’s a nail in a standardised abuse test or debris in a real-world collision. The nail test below is the standardised version of this; BYD’s public nail-penetration testing of the Blade cell is a useful real-world case study of how much chemistry affects the outcome of the same physical trigger.
The nail test was originally designed to replicate a cell failure caused by a piece of rogue metal that gets into the cell during production. The metal nail causes a short circuit between the active layers and hence rapid electrical heating. This is one of the most severe cell tests and part of a suite of abuse tests.