Explained | Why are electric vehicles catching fire? 

The story so far: The Union Government has constituted an expert panel to probe the recent series of battery explosions in electric vehicles (EVs). Manufacturers like 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 Telangana’s Nizamabad district after the battery of an electric scooter exploded while being charged. After the enquiry, the Ministry of Road Transport intends to issue guidelines for EVs, including tests for compliance with safety norms.

Why is the world poised to transition to electric vehicles?

The growing concern over climate change has led to global efforts to electrify the transportation sector. In parallel, cost of Li-ion (Lithium-ion) battery technology has decreased by a staggering order of magnitude in the last decade. The convergence of these two factors has resulted in a unique time in our history where we are at the cusp of a dramatic transition in the transportation sector, with electric vehicles poised to replace petrol vehicles.

The world has taken note of this moment with governments providing incentives to usher in the transition and private industry ramping up plans for capturing the market. There is a worldwide race emerging, with vehicle companies, battery manufacturers, and material suppliers vying with each other for market share. However, Li-ion batteries are complex devices requiring a level of sophistication that can takes years to perfect. Hurrying the development of this complex technology without careful safeguards can lead to increasing safety incidents, as evidenced recently on Indian roads.

What goes into a Li-ion battery?

Every Li-ion battery consists of three active components: the anode, typically graphite; the cathode, typically based on a nickel, cobalt, and manganese-based oxide; and an electrolyte, typically a salt of lithium in an inorganic solvent. Battery manufacturing is a complex operation involving forming sheets of the anode and cathode and assembling them into a sandwich structure held apart by a thin separator.

Separators, about 15 microns in thickness — about a fifth of the thickness of the human hair — perform the critical function of preventing the anode and cathode from shorting. Accidental shorting of the electrodes is a known cause of fires in Li-ion cells. It is important that the various layers are assembled with high precision with tight tolerances maintained throughout the manufacturing process. Safety features, such as thermal switches that turn off if the battery overheats, are added as the sandwich is packaged into a battery cell.

Battery cells are assembled into modules and then further assembled into packs. Li-ion batteries require tight control on the state of charge and the temperature of operation to enhance safety and increase usable life, achieved by adding multiple sensors. Packs are designed to ensure uniform temperature profile with minimal thermal variation during operation. Ensuring robust detection, coupled with battery management systems that interpret the data and change operation based on changes to the batteries state, remain critically important in enhancing battery performance.

Battery packs are integrated into the vehicle in unique formfactors depending on the design of the vehicle. The location of the battery should protect it from external penetration, ensure passenger safety while talking into consideration the overall weight distribution. Close interaction between vehicle manufacturers and battery manufacturers is essential so that the whole is greater than the sum of the parts.

There are multiple tradeoffs in this complex ecosystem: engineering higher safety often results in higher costs and lower driving range. In this competitive landscape where companies are vying for market share, a race to the bottom can compromise safety.

What causes battery fires?

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

Battery fires, like other fires, occur due to the convergence of three parts of the “fire triangle”: heat, oxygen, and fuel. If an adverse event such as a short circuit occurs in the battery, the internal temperature can raise as the anode and cathode release their energy through the short. This, in turn, can lead to a series of reactions from the battery materials, especially the cathode, that release heat in an uncontrolled manner, along with oxygen.

Such events also rupture the sealed battery further exposing 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 flammable and serves as a fuel. The combination leads to catastrophic failure of the battery resulting in smoke, heat, and fire, released instantaneously and explosively.

The trigger for such events can be a result of internal shorts (like a manufacturing defect that results in sharp objects penetrating the separator), external events (an accident leading to puncture of the cell and shorting of the electrodes), overcharging the battery which leads to heat releasing reactions on the cathode ( by a faulty battery management system that does not shut down charging despite the battery achieving its designed charge state), or bad thermal design at the module and pack level (by not allowing the battery internal heat to be released). Any of these triggers may cascade into a significant safety incident.(see graphic).

Are battery fires inevitable?

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

Preventing fires requires breaking the fire triangle. Battery cathodes are a leading cause of the heat release. Some cathodes, such as ones with lower nickel content or moving to iron phosphate, can increase safety. Tightly controlled manufacturing will prevent accidental shorts in the cells, eliminating a leading cause of fires. Many companies now add a ceramic layer on the separator to mechanically prevent shorts. Sensing the state of the battery and integrating this data into sophisticated battery management systems is an important aspect of design. Protecting the cell with robust thermal management is critical, especially in India where ambient temperatures are high. Finally, battery packs need to be protected from external penetration. Any large-scale manufacturing process inevitably has a certain percentage of defects; therefore, such steps are needed to minimise the number of adverse events.

Long term changes are also underway. Safety remains a concern for Li-ion manufacturers worldwide especially as cell sizes become larger for applications like solar-connected storage. Companies are developing internal “switches” that turn off parts of the battery that undergo thermal events to stop them at their inception. Research is now underway to replace the flammable liquid electrolyte with a solid electrolyte to eliminate one part of the fire triangle. A similar thread of research is the development of nonflammable liquid electrolytes. All these changes promise to remove the threat of battery fires as the roll out of mass electrification takes place.

Engineering safety requires commitment from all parts of the battery supply chain and tight integration between vehicle companies and battery companies. Further, regulators play an important role, providing the testing and certification needed to ensure that technology innovations perform at the level that is promised. Li-ion batteries are not forgiving of shoddy engineering and approaches that rely on cutting corners. Companies with tightly controlled manufacturing with years of experience can maintain the number of adverse safety incidents to a minimum. Such batteries maybe more expensive, but safety should not be “just another” metric. Rather, ensuring safety should be the priority for manufacturers.

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