Figure 1. Thermal runaway occurs when Li-ion battery cells enter an uncontrollable self-heating state during which their temperature rises rapidly to as much as 900° C.
Thermal runaway happens when a lithium-ion (Li-ion) cell, or a small region within a cell, reaches a critical temperature where the materials start to undergo decomposition reactions. These reactions generate significant additional heat.
The decomposition reactions are temperature-dependent, increasing exponentially as the temperature rises. Once decomposition starts, a chain reaction causes the battery to release its energy rapidly — with potentially explosive consequences.
The initial overheating can be caused by mechanical or thermal failures, overcharging or over-discharging, or an internal short circuit.
Here are four important steps that should be taken to mitigate thermal runaway in Li-ion batteries.
1. Prevent cell-to-cell propagation
It may not be possible to completely eliminate the possibility of thermal runaway within a cell. While excellent thermal management, structural design, and battery management can significantly reduce the risk, there’s always the possibility of defects or foreign objects causing internal short circuits.
Perhaps the most critical mitigation is ensuring that a thermal runaway event will not cause cell-to-cell propagation in a battery module or pack. Although no standardized methods currently exist to prevent cell-to-cell propagation, essential considerations include cell spacing, interstitial insulation, heatsinks, casing thickness, and venting features.
2. Thermal management
The thermal management system in a battery module or pack should maintain a healthy operating temperature for cells under normal operating conditions to prevent degradation and thermal runaway. The thermal management system plays a vital role in managing potential thermal runaway events should they occur in an individual cell, ensuring they have no serious consequences outside the battery and preventing cell-to-cell propagation.
The safest approach is to assume that thermal runaway will occur in one of the cells at some point and then ensure that the resulting heat will have somewhere to go without causing runaway in an adjacent cell. Such an approach can be considered the gold standard in thermal runaway management, an approach developed by NASA.
3. Battery management
Good battery management should prevent electrochemical abuse, such as overcharging and over-discharging, reducing the risk of a thermal runaway and preserving the battery condition.
In addition to the pack-level battery management system, individual cells may be fitted with simple bimetallic switches, which give an open circuit condition when a critical temperature is reached within the cell. These current interrupt devices (CID) may also be triggered by pressure as the cell cap deforms.
A CID is often a single-use device, rendering a cell useless once triggered. Another issue with a CID is that a trigger can cause arcing, potentially initiating thermal runaway by igniting hydrocarbon vapor from overheating electrolytes.
A positive temperature coefficient thermistor (PTC) can be used similarly, producing a sudden increase in resistance when a specified temperature is reached. Most cylindrical cells have PTC thermistors. However, one concern with these devices is they’re located in the cell cap, whereas the hot spot where thermal runaway initiates is often deeper within the cell structure. Thermal fuses are another similar device.
4. Structural design
The structural design of cells and battery packs can significantly reduce the risk of physical damage leading to internal short circuits.
Methods of mitigating thermal runaway are continuing to be developed. Still, no Li-ion battery packs are entirely immune from the risk of explosion, which is also true of fuel tanks. Fortunately, new cell materials that are not susceptible to thermal runaway are being developed.
Additionally, solid-state batteries could be fully immune to these risks without requiring heavy thermal management systems and protective cell spacing. In the future, battery packs could have much higher power density and lighter weight, eliminating the risk of thermal runaway.
Filed Under: Batteries, FAQs