Thermal Runaway - Battery Design

Thermal runaway can be considered at a single cell and thermal propagation at a multi-cell/pack level. Testing at a cell and pack level is a fundamental part of the overall safety sign off.

Battery Thermal Runaway – in general, thermal runaway can be attributed to 3 main types of abuse conditions; mechanical, electrical and thermal. The likelihood of each of these potential risks varies depending on chemistry, design and operating conditions, with the likelihood of failure generally becoming higher with battery aging.

If you heat a battery cell to somewhere above 130°C then exothermic chemical reactions inside the cell will increase the temperature and further reactions will take place. The result is an uncontrolled runaway and increase in temperature. The cell should vent in a controlled manner with fire and molten material. In severe cases the cell may explode. The energy released from one cell failing is likely to heat neighbouring cells that again could be triggered into thermal propagation.

The EUCAR Hazard Levels define the outcome of cell level safety testing. These levels are normally used to describe the outcome of tests such as overcharge as part of the cell specification.

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

These then lead to an electro-thermal-chemical series of events.

At a multi-cell level the thermal runaway of a single cell could be contained at the cell in question. However, if this cell causes neighbouring cells to overheat then we are likely to see the thermal runaway of the pack.

Separator

Forms the primary internal cell electrical barrier between the anode and cathode. Hence needs to stay intact and functioning to perform this function. At high temperature the separator can shrink or even melt and hence then allow an internal electrical short in the cell. Thus allowing further uncontrolled discharge and internal heat generation in the cell.

Energy Release

thermal runaway energy vs electrical energy for smaller cells

Energy released versus total energy stored in the cell is an interesting plot and gives a rough starting rule of thumb for how much energy is released by a cell during thermal runaway.

This data includes different chemistries, results versus SoC and different size / formats of cell. However, as a check of the rule of thumb it shows that the energy released in Thermal Runaway is 2x the electrical energy stored in the cell.

This plot shows the bottom end of the data with cells up to around 20Ah.

Gas Composition

Baird et al [2] have reviewed the published experimental data and pulled this together into a table showing the gas composition by chemistry and SOC:

This shows:

Increasing SOC => increasing fraction of hydrogen and carbon-monoxide
Increasing SoC => decreasing fraction of the inert carbon-dioxide decreases
- as CO2 decreases the overall hazard increases as there is less inert gas
Hydrocarbons are relatively constant versus SoC:
- 10-15% for NCA and LFP
- 20-25% for LCO

This table only shows the Volume Fraction and not the total amount of gas.

The gas composition, surrounding gas composition, laminar flame speed, lower flammability limit and maximum overpressure all need to be evaluated to understand the likelihood and severity of combustion.

The combustion metrics that were evaluated show that NCA and LCO vented gases produce higher flame speeds and maximum overpressures relative to LFP vent gases. LFP cells also have a higher LFL, which likely reduces the probability of a flammable ignition.
Austin R. Baird, Erik J. Archibald, Kevin C. Marr, Ofodike A. Ezekoye, Explosion Hazards from Lithium-Ion Battery Vent Gas, SAND2019-6428J

Gas Volume

The volume of gas released is typically 1 to 2 litres per Ah of electrical capacity. This is just a rough estimate.

Sturk et al [1] measured the release of gas during thermal runaway in an inert gas chamber and got a significant difference in the volume of gas released for NMC/NCO compared to LFP.

NMC/LMO = 780 litres/kg
LFP = 42 litres/kg

Trigger Methods

The main trigger methods for experiments are:

External heater: resistance wire is wrapped around the cell or a heating pad is applied to the surface of the cell.
Overcharge: continue charging the cell beyond 100% SoC and until the cell goes into thermal runaway. If the cell has a current interrupt device that operates based on internal pressure then this is likely to operate and stop the cell charging before runaway.
Nail test: a nail is push into the cell thus causing the cell to locally short circuit. This will normally cause the cell to go into thermal runaway.

C. Essl et al [5] look at the different trigger methods and compare an NMC pouch cell to an NMC prismatic cell. They look at thermal behaviour and vent gas composition. The results show that overcharge triggered TR can be harsher than other abuse triggers such as overtemperature and nail-penetration. The reasoning is that overcharging the cell adds significantly to the energy content of the cell. Heating the cell also adds energy to the cell and is harsher than the nail test.