Thermal Runaway

Thermal runaway in batteries occurs when a battery cell receives a critical amount of energy which consequently triggers an exothermic reaction. Typical failure modes that can create this amount of energy are overheating due to thermal or mechanical failure, mechanical destruction that leads to an internal short circuit, overcharging or an externally, over the tabs induced short-circuit. At elevated temperatures, the battery cell materials can start to decompose in exothermic reactions, leading to self-heating behavior. When the self-heating of the battery cell exceeds the rate at which heat can be dissipated to the surroundings, the cell temperature rises exponentially, the battery structure can rupture, and all remaining thermal and electrochemical energy is released to the surroundings. The decomposition exothermic reactions produce large quantities of heat and gas at a high rate, which causes the temperature and the pressure inside the battery cell to increase nearly instantaneously.

Simcenter STAR-CCM+ provides the Thermal Runaway Heat Release and Thermal Runaway Battery Vent models to predict the heat that is released by the battery cell and venting gases during thermal runaway:

q_{g e n} = H_{r x n} R_{d}

(4391)

where:

$q_{g e n}$ is the heat generated by the solid cell parts.
$H_{r x n}$ is the heat of reaction.
$R_{d}$ is the thermal decomposition reaction rate.

The Heat Release model requires experimental data input, namely the heat rate of the battery cell in $[W]$ as a function of the cell temperature. This data is typically obtained by performing an accelerating rate calorimeter (ARC) test. The output data from the ARC test is in the form of temperature change per minute – the self heating rate ( $S H R$ ) – which is then used to calculate the thermal decomposition reaction rate ( $R_{d}$ ) using Eqn. (4392).

S H R = R_{d} (T_{\max} - T_{\min})

(4392)

where:

$S H R$ is the Self Heating Rate.
$T_{\max}$ is the maximum temperature reached during the ARC test.
$T_{\min}$ is the minimum temperature reached during the ARC test.

The ARC test also provides the mass flow rate of the venting gas as a function of temperature of the venting gas:

The heat of reaction ( $H_{r x n}$ ) for this calculation is found using Eqn. (4393), where the mass of the cell is continuously changing due to the exothermic reaction, causing the specific heat capacity of the battery cell to also change.

H_{r x n} = m_{c e l l} C_{p, c e l l} (T_{\max} - T_{\min})

(4393)

where:

$m_{c e l l}$ is the mass of the battery cell.
$C_{p, c e l l}$ is the specific heat capacity of the battery cell.