Battery Thermal Runaway

Lithium-ion batteries power devices ranging from smartphone and laptops to electric vehicles; highlighting how essential they are to our everyday lives. However, despite their widespread use, safety concerns persist. Here, battery thermal runaway can lead to catastrophic explosions, fire and generation of toxic smoke. Therefore, mitigating these risks will be essential for the mass market uptake of battery powered devices.

In this article, we provide an introduction to the underpinning causes of battery thermal runaway. An overview of this article can be found in the video below, and notable academic works in the area, expanding on the topics highlighted, are included in the references. Note that this article does not include an exhaustive overview of all battery failure modes, but rather aims to provide an introduction to the key considerations of battery failure.

1. What causes thermal runaway?

In general, thermal runaway can be attributed to 3 main types of abuse conditions; mechanical, electrical and thermal, as shown in Figure 1. 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.

Figure 1 – Overview of different causes of battery thermal runaway. Adapted from Feng et al.1

An important note is that the “Smoke” in the diagram can include combustible gases, this can then burn or explode.

2. Mechanical abuse

One potential cause of thermal runaway stems from mechanical abuse of a battery. This can be triggered by crushing or penetration a battery with an external object, and it’s for this reason, that extra care needs to be taken if a battery electric vehicle experiences a crash.

When this mechanical abuse occurs, the battery deforms which can cause effects such as tearing of the polymer separator. This separator is essential as it prevents the positive and negative electrodes from coming into contact with each other. If these 2 electrodes do have an electrically conductive pathway, either from deformation of the battery components or from the penetration of an external object (Figure 2), this can cause an internal short circuit to occur. This internal short circuit can result in a significant current which can general enough heat to push the battery into thermal runaway.

Figure 2 – The short-circuiting process during a nail penetration test

3. Electrical abuse

Beyond mechanical abuse, thermal runaway can be cause by electrical abuse, either by over-charging/over-discharging.

3.1. Over-charge

Figure 3 shows the various stages in over-charging. During normal operation lithium-ions move between the anode and cathode, with most of the lithium in the anode when fully charged. Graphite is the most common anode material, and an important consideration here is that fully lithiated graphite is thermodynamically unstable in most battery electrolytes. This means that the two will react with each other causing degradation, however a passivation layer called the solid electrolyte interphase (SEI) is generated which protects the graphite surface from the electrolyte; limiting the rate of degradation.

As the battery is over-charged (i.e. charging an individual cell to > ~4.2V), a number of effects occur. In the first instance, the graphite becomes saturated with lithium and cannot host anymore within their structure. If a charging current continues to be applied, this can result in lithium being plated on the surface of the graphite, which can then manifest itself into lithium dendrites which can short-circuit the battery; leading to failure.

Beyond this, lithium is continually removed from the cathode which increases its potential, resulting in the decomposition of electrolyte and gas formation. Furthermore, in battery cathodes such as the commonly used NMC (Lithium nickel manganese cobalt oxide), this continued removal of lithium, can cause the atomic structure of the material to become unstable, again leading to the catastrophic failure of the battery.

Figure 3 – Stages in the over-charging process. Adapted from Ren et al.2

3.2. Over-discharge

In the case of over-discharge (i.e. discharging an individual cell to < ~2.7V), all of the lithium is removed from the graphite, however if a discharge current is continually applied, then this can cause the copper collector on the negative electrode to dissolve as shown in Figure 4. If this battery is then recharged, this can result in the dissolved copper, reforming as a copper dendrite; causing an internal short-circuit in the same way as the lithium dendrite.

Figure 4 – Stages in the over-discharge process. Adapted from Guo et al.3

4. Thermal abuse

Finally, both mechanical and electrical abuse can result in internal short circuits which lead to heat generation. If this heat is not properly removed and the temperature increases, this can lead to catastrophic failure of the battery. The exact stages and temperatures at which these effects occur vary with different batteries, but generally follows the stages shown in Figure 5. If the temperature goes over ~70 °C, the protective SEI layer can decompose leading to electrolyte having access to fresh graphite surfaces. The reformation of this SEI is generally exothermic, leading to further heat generation as shown in Figure 6. If the battery continues to heat up much beyond ~130°C, the polymer separator which is often made of either polyethylene or polypropylene, can melt causing the anode and cathode to touch, resulting in a short-circuit. Note that in some battery designs, a thermal shutdown separator, which is often a multi-layer porous polymer, or a ceramic coating can be included to improve safety.   

At this stage, if a short-circuit does occur, a considerable amount of heat can be generated (depending on the state-of-charge). Above 200 °C the electrolyte and cathode can decompose, leading to the formation of oxygen (highly flammable) and components such hydrofluoric acid, which is extremely toxic.

Figure 5 – Stages in the thermal runaway of a lithium-ion battery. Adapted from Wang et al.4

The potential sequence of events is shown in Figure 6 along with the energy released from different components of the battery. Here, readers should note both the absolute amount of energy released from the decomposition of the material (vertical position of material) and also the heat release rate (size of peak), which is how fast that energy is released. In both cases, a high value causes more safety concerns. Notably, the cathode chemistry can have a significant impact on the safety of the battery. Here lithium iron phosphate (LFP) generally has good safety characteristics compared to other cathodes such as the commonly used NMC cathodes. Beyond this, there is significant interest in the development of solid-state batteries which strives to further increase the safety of a battery through the replacement of the flammable liquid electrolyte with a safer solid-state electrolyte. Here various approaches ranging from oxides, sulfide and polymer electrolytes have been proposed, but these are largely still in development.

Figure 6 – Stages in battery thermal runaway and comparison of energy release from different battery components. Adapted from Feng et al.1

5. Safe operating regions

In terms of safely operating a battery, Figure 7 strives to provide a broad operational map, however it should be noted that batteries are complex devices and this only provides a rough guide and the development of safe and consistent batteries is needed, alongside advanced monitoring systems.

Figure 7 – Regions of safe and unsafe battery operation. Adapted from Electropedia5

6. Closing remarks

Lithium-ion batteries will undoubtedly be an essential technology enabler for our low-carbon future, and technological developments continue at pace with lower cost and higher energy density systems. The safety of these batteries have improved over the last few years, however risks remain, with the potential impacts of a battery thermal runaway event being potentially catastrophic. In the first instance, understanding the root causes of these failure events is essential, with considerations spanning mechanical, electrical and thermal abuse conditions. With this understanding, manufacturers and operators can then design systems to mitigate the risk of failure through routes such as prevention of cell-to-cell thermal propagation and advanced control systems.

Future battery designs such as solid-state batteries have the potential to improve safety, however readers should also be careful that these systems are likely to not be 100% safe as flammable components such as the transition metal oxide cathode and lithium-metal anode are still used (depending on design).

Thus, many battery chemistries are unlikely to ever be 100% safe, yet with a holistic approach to designing safer batteries, monitoring/controlling them appropriately and having good processes in place for if they do fail, then the chances of the failure can start to tend to the improbable with mitigated impact.

References

  1. Thermal runaway mechanism of lithium-ion battery for electric vehicles. A review. Feng et al. Energy Storage Materials. 10. (2018) 246-267. 
  2. An electrochemical-thermal coupled overcharge-to-thermal-runaway model for lithium-ion battery. Ren et al. Journal of Power Sources. 2017, 364, 328-340
  3. Mechanism of the entire overdischarge process and overdischarge-induced internal short circuit in lithium-ion batteries. Guo et al. Scientific Reports. 2016, 6, 30248
  4. Progress of enhancing the safety of lithium ion battery from the electrolyte aspect. Wang et al. Nano Energy 2018; 55 :93–114 .
  5. Lithium Battery Failures, electropaedia

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