EV Thermal Runaway: Causes and Prevention Strategies

Thermal runaway in Lithium-Ion batteries is a safety issue for electric vehicles (EVs). Although the probability of thermal runaway of Lithium-Ion battery packs and cells is low, the associated risks are high.

We want to avoid the potential for fire and explosion due to rapid and uncontrollable escalations of battery temperature. We must design thermal management systems, robust Lithium-Ion batteries, and early detection systems. These measures can mitigate the risks of electric car thermal runaway. Thus, we ensure the safety of drivers, passengers, and passers-by.

An example of high-temperature values is 150°C - 200°C.

At these temperatures, the organic electrolytes used in batteries can decompose. The chemical process of decomposition releases thermal energy and flammable gases.

Read on to learn causes and prevention strategies.

You will learn the physical and chemical mechanisms behind thermal runaway events and the thermal management solutions to avoid them.

Final Consequences: Fire and Explosion

Let us get straight to the potentially disastrous conclusion of the thermal runaway process (Fig.2). When the battery's electrolyte and other materials decompose, they provoke gas release, increasing the pressure inside the battery cell. Unfortunately, released gases are flammable gases.

As the pressure builds up, it can exceed the mechanical limits of the cell casing. Suppose an ignition source, such as a spark from a short circuit or the hot cell itself, ignites the gases. In that case, battery fires can start and quickly spread from cell to cell in the battery pack, worsening the thermal runaway event. 

In severe cases, the rapid release of energy and gases can lead to an explosion, which poses safety risks to the passenger compartment, emergency responders, and anyone nearby.

Causes of EV Thermal Runaway

Let us now examine what internal or external causes such a dangerous thermal event in electric vehicles (Fig. 3).

Electric Vehicles - BEV vs. PHEV vs. ICE

Two types of electric vehicles (EVs) exist: plug-in hybrid electric vehicles (PHEVs) and battery EVs (BEVs). The energy efficiency of EVs typically ranges from 85% to 90%—this means that 85-90% of the energy stored in the battery is used to power the vehicle, with much less energy lost than in internal combustion engine (ICE) vehicles (Fig. 4).

Battery Cell Short Circuits and Chemical Reactions

One primary cause of thermal runaway in EV battery packs is internal short circuits within individual battery cells. These can result from manufacturing defects in the battery system, damage, or degradation over time. When a cell reacts internally, it generates excess heat, which can initiate a thermal runaway.

Elevated Temperature and Heat Dissipation

Another factor is high temperatures during regular cell operation.

Fig. 5 shows the range of temperatures from normal operation (25°C) to the onset of thermal runaway.

Poor thermal management can lead to inadequate thermal energy dissipation, raising the battery pack's temperature. This is especially critical in a densely packed battery module, where heat from individual cells can accumulate and spread to adjacent cells.

When the battery is overcharged (Fig. 6), the voltage exceeds the safe limit, causing electrolyte decomposition and SEI layer breakdown, leading to localized heating and thermal runaway.

Detailed Chemistry of Thermal Runaway

Exothermic reactions are central to thermal runaway. When battery cells reach a critical temperature, the electrolyte can decompose, releasing flammable gases (ethylene, methane, and hydrogen). This decomposition is highly exothermic.

EV Battery SEI Decomposition

During regular battery operation, the Solid Electrolyte Interphase (=SEI) layer forms on the anode surface.

During the initial cycles of a Lithium-Ion battery, the cell electrolyte (which contains organic carbonates such as ethylene carbonate, EC) is reduced at the cell anode, forming compounds, including lithium alkyl carbonates (ROCO₂Li), which form the passivation layer on the anode. This layer prevents further electrolyte decomposition by limiting the direct contact between the electrolyte and the anode.

However, SEI can decompose at elevated temperatures, releasing heat:

SEI → Li₂CO₃ + Li₂​O + ROCO₂​Li + heat

where "R" represents an alkyl group such as methyl (CH₃-), ethyl (C₂H₅-), propyl (C₃H₇-).

EV Battery Cathode Decomposition in LiIon Batteries

In the EV battery, Cathode materials (LiCoO2, LiNiMnCoO2, and others) can undergo thermal decomposition, releasing oxygen and generating more heat again. The release of oxygen can further react with the electrolyte, exacerbating the thermal runaway:

LiCoO₂ → 0.5 Li₂O + 0.5 CoO + 0.25 O₂ + heat

Electrolyte Decomposition

The organic electrolyte in Lithium-Ion batteries is usually a lithium salt such as LiPF₆ dissolved in organic solvents. This electrolyte can break down at high temperatures, releasing heat and flammable gases.

Decomposition of Ethylene Carbonate (EC):

C₃H₄O₃ → C₂H₄ + CO₂ + heat

Decomposition of LiPF₆:

LiPF₆ → LiF + PF₅ + heat

Internal Short Circuit - Causes

Internal short circuits are a significant cause of thermal runaway. Various factors, including the following, can trigger them.

Manufacturing defects, such as impurities or misalignments during production, can create weak points.

Mechanical damage from physical impacts or punctures leads to internal shortcircuits.

Lithium dendrites can grow through the separator over time, causing a short circuit between the anode and cathode.

Prevention Strategies: Thermal Aspects of Thermal Runaway

The heat generated by reactions and internal short circuits can rapidly increase the temperature of the battery cell. This heat can propagate to adjacent battery cells, especially in densely packed modules, leading to a cascading effect. We will examine the primary mechanism behind heat transfer and the dissipation of heat.

Heat Conduction

Heat is conducted from the affected cell to neighboring cells within the battery pack. Good thermal conductivity can help spread the heat more evenly, but it also means that more cells can be affected quickly.

Heat Convection and Liquid Cooling

In an EV battery cooling system, heat is transferred away from the battery through the coolant fluid. Efficient energy dissipation through convection can prevent localized overheating.

Thermal Radiation

At higher temperatures, thermal radiation can also contribute to heat transfer, although it is typically less significant than conduction and convection.

Prevention Strategies in Practice

What measures prevent thermal runaway? We will focus on thermal management, battery cells, and battery pack design.

Effective Thermal Management Systems

Active thermal management is crucial for preventing thermal runaway. Battery manufacturers invest in sophisticated, active thermal management and solutions and various types of heat exchangers, such as cooling plates, to maintain optimal temperatures within the battery pack. These systems enhance thermal conductivity and ensure efficient thermal energy dissipation.

Insulation and Phase Change Materials

Incorporating insulation and phase change materials into battery modules can significantly improve safety. These materials help batteries absorb and dissipate heat, preventing it from spreading to other cells. Compression pads also maintain structural integrity and reduce the risk of internal short circuits.

Battery Design and Configuration

Designing battery packs with prismatic cells, pouch cells, or cylindrical configurations can also influence thermal stability. Each type of cell has unique properties that affect heat generation and dissipation. Proper design and configuration can mitigate the risks associated with thermal runaway events.

Early Detection Systems

Implementing early detection systems is essential for identifying potential thermal runaway scenarios before they escalate. Sensors and monitoring systems can detect exothermic reactions and temperature spikes, allowing timely intervention to prevent these thermal runaway situations.

Emergency Response and Safety Measures

Protocols in place to deal with EV battery incidents are critical for emergency responders. This includes understanding the hazards of lithium-ion batteries in an electric vehicle and having the appropriate equipment to manage battery fires safely.

Conclusions

The article has shown the causes and consequences of thermal runaway.

We saw why it can concern the electric vehicle industry, which wishes to avoid such a hazardous situation.

Fortunately, the risks can be mitigated.  Thermal management, better battery designs, and robust safety measures can prevent thermal runaway.

Battery manufacturers continue to innovate, ensuring efficient and safe EVs for consumers.

Further Readings

"Gas vs. Electric Car Fires" - AutoensuranceEZ, 19/12/2023

"Do electric cars pose a greater fire risk than petrol or diesel vehicles?" - The Guardian, 20/11/2023

"A Look Back: How Boeing Overcame The 787's Battery Problems" - Simple Flying, 20/08/2020