Explained | Why do electric cars catch fire?

Is the battery causing the problem? What safety measures must be observed to avoid explosions?

Is the battery causing the problem? What safety measures must be observed to avoid explosions?

The story so far: The Union government has set up a panel of experts to investigate the recent series of battery explosions in electric vehicles (EVs). Manufacturers such as Okinawa and Pure EV have recalled some batches of electric scooters following fire incidents involving the vehicles. Last Wednesday, an 80-year-old man died at his home in the Nizamabad district of Telangana after the battery of an electric scooter exploded while it was being charged. After the investigation, the Ministry of Transport intends to issue guidelines for electric vehicles, including tests to ensure compliance with safety standards.

Why is the world ready to switch to electric vehicles?

Growing concern about climate change has prompted a global effort to electrify the transportation sector. In parallel, the cost of Li-ion (lithium-ion) battery technology has fallen by a staggering magnitude over the past decade. The confluence of these two factors has resulted in a unique time in our history where we stand on the cusp of a dramatic shift in the transportation sector, with electric vehicles poised to replace petrol vehicles.

The world has taken note of this moment as governments offer incentives to initiate the transition and private industry ramps up plans to conquer the market. A global race is emerging in which vehicle manufacturers, battery manufacturers and material suppliers compete for market share. However, Li-ion batteries are complex devices that require a level of sophistication that can take years to perfect. The accelerated development of this complex technology without careful security precautions can lead to increasing security incidents, as recently demonstrated on Indian roads.

What goes in a Li-ion battery?

Every lithium-ion battery consists of three active components: the anode, typically graphite; the cathode, which is typically based on a nickel, cobalt and manganese based oxide; and an electrolyte, typically a lithium salt in an inorganic solvent. Battery manufacture is a complex process in which anode and cathode sheets are formed and assembled into a sandwich structure held apart by a thin separator.

Separators about 15 micrometers thick – about a fifth the thickness of a human hair – perform the crucial function of preventing a short circuit between the anode and cathode. Accidental shorting of the electrodes is a known cause of fires in lithium-ion cells. It is important that the various layers are assembled with great precision, maintaining close tolerances throughout the manufacturing process. Safety features such as thermal switches that turn off if the battery overheats are added when the sandwich is packaged in a battery cell.

Battery cells are assembled into modules and then further assembled into packs. Li-ion batteries require strict control of state of charge and operating temperature to increase safety and extend usage time, which is achieved by adding multiple sensors. The packs are designed to ensure an even temperature profile with minimal thermal fluctuations during operation. Ensuring robust detection coupled with battery management systems that interpret the data and alter operations based on changes in battery health remains critical to improving battery performance.

Battery packs are integrated into the vehicle in unique form factors depending on the design of the vehicle. The location of the battery should protect it from external intrusion, ensure passenger safety while considering overall weight distribution. To ensure that the whole is more than the sum of the parts, close cooperation between the vehicle manufacturer and the battery manufacturer is essential.

There are several trade-offs in this complex ecosystem: Greater security often translates into higher costs and reduced range. In this competitive landscape where companies compete for market share, a race to the bottom can jeopardize security.

What causes battery fires?

Although Li-ion batteries are complex, numerous companies have perfected the art of manufacturing high-quality cells and integrating them into vehicles with minimal safety concerns over the last three decades. Gasoline’s energy density is five hundred times that of a typical lithium-ion battery, so safety should be manageable if robust controls are in place. However, batteries store energy in a small package, and when the energy is released in an uncontrolled manner, the thermal event can be significant.

Battery fires, like other fires, are caused by the meeting of three parts of the “fire triangle”: heat, oxygen and fuel. If an undesirable event like a short circuit occurs in the battery, the internal temperature may rise because the anode and cathode release their energy through the short circuit. This in turn can lead to a series of reactions in the battery materials, particularly the cathode, which, in addition to oxygen, release heat in an uncontrolled manner.

Such events also destroy the sealed battery and further expose the components to outside air and the second part of the fire triangle, namely oxygen. The final component of the triangle is the liquid electrolyte, which is combustible and serves as a fuel. The combination results in catastrophic battery failure, resulting in smoke, heat, and fire that are instantaneously and explosively released.

The trigger for such events can be an internal short circuit (such as a manufacturing defect leading to sharp objects penetrating the separator), external events (an accident leading to cell breakdown and shorting of the electrodes), overcharging of the Battery leading to heat-generating reactions at the cathode (due to a faulty battery management system that does not turn off charging even though the battery is reaching its intended state of charge) or poor thermal design at the module and pack level (by not allowing internal battery heat will). released). Each of these triggers can lead to a significant security incident (see graphic).

Are battery fires inevitable?

Over the past three decades, Li-ion batteries have proven extremely safe, with the industry stepping up controls as safety incidents surfaced. Safety is a must and an important consideration that battery and vehicle manufacturers can design for at multiple levels, from battery material choice to cell, pack and vehicle level designs.

Preventing fires requires breaking the fire triangle. Battery cathodes are a major cause of heat release. Some cathodes, e.g. B. those with lower nickel content or switching to iron phosphate, can increase safety. Tightly controlled manufacturing prevents accidental short circuits in cells and eliminates a major cause of fire. Many companies are now adding a ceramic layer to the separator to mechanically prevent short circuits. Capturing battery health and integrating this data into sophisticated battery management systems is an important aspect of the design. Protecting the cell through robust thermal management is crucial, especially in India where ambient temperatures are high. Finally, battery packs must be protected from external intrusion. Any large-scale manufacturing process inevitably has a certain percentage of defects; Therefore, such steps are necessary to minimize the number of adverse events.

Long-term changes are also underway. Safety remains a concern for Li-ion manufacturers worldwide, especially as cell sizes for applications such as solar-connected storage continue to increase. Companies are developing internal “switches” that turn off parts of the battery that are subject to thermal events in order to stop them from occurring. Research is underway to replace the flammable liquid electrolyte with a solid electrolyte to eliminate part of the fire triangle. A similar line of research is the development of non-flammable liquid electrolytes. All of these changes promise to eliminate the risk of battery fires when mass electrification takes place.

Technical safety requires commitment from all parts of the battery supply chain and close integration between vehicle manufacturers and battery manufacturers. In addition, regulators play an important role by providing the tests and certifications needed to ensure technological innovations reach the levels promised. Li-ion batteries do not forgive shabby technology and approaches that rely on compromises. Companies with tightly controlled manufacturing and years of experience can reduce the number of unwanted security incidents to a minimum. Such batteries may be more expensive, but safety shouldn’t be “just another” metric. Rather, ensuring safety should be a priority for manufacturers.

The author is director of the Argonne Collaborative Center for Energy Storage Science at Argonne National Laboratory, Illinois

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