Explained | Why do electric vehicles catch fire?
Is the battery a problem? What safety measures should be taken to avoid explosions?
Is the battery a problem? What safety measures should be taken to avoid explosions?
The story so far: The Union government has set up a group of experts to investigate the recent spate of battery explosions in electric vehicles (EVs). Manufacturers like Okinawa and Pure EV have recalled certain batches of electric scooters following fires involving the vehicles. Last Wednesday, an 80-year-old man died at his home in Nizamabad district of Telangana after the battery of an electric scooter exploded while charging. After the investigation, the Ministry of Road Transport intends to issue guidelines for electric vehicles, including testing for compliance with safety standards.
Why is the world about to switch to electric vehicles?
Growing concern over climate change has led to global efforts to electrify the transport sector. At the same time, the cost of Li-ion (Lithium-ion) battery technology has fallen by a staggering order of magnitude over the past decade. The convergence of these two factors has resulted in a unique period in our history where we are on the cusp of a dramatic transition in the transport sector, with electric vehicles poised to replace gasoline-powered vehicles.
The world took note of this moment, with governments offering incentives to usher in the transition and private industry stepping up its plans to conquer the market. A global race is emerging, with automakers, battery makers and materials suppliers jostling for market share. However, Li-ion batteries are complex devices requiring a level of sophistication that can take years to perfect. Accelerating the development of this complex technology without careful safeguards can lead to an increase in security incidents, as recently witnessed by Indian roads.
What does a Li-ion battery contain?
Each Li-ion battery consists of three active components: the anode, usually made of graphite; the cathode, typically based on an oxide based on nickel, cobalt and manganese; and an electrolyte, typically a lithium salt in an inorganic solvent. Battery manufacturing is a complex operation of forming anode and cathode foils and assembling them into a sandwich structure held apart by a thin separator.
The separators, about 15 microns thick – about one-fifth the thickness of human hair – perform the essential function of preventing the anode and cathode from shorting out. Accidental shorting of the electrodes is a known cause of fire 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 devices, such as thermal switches that turn off if the battery overheats, are added when the sandwich is packed in a battery cell.
Battery cells are assembled into modules and then assembled into packs. Li-ion batteries require strict monitoring of the state of charge and operating temperature to improve safety and increase useful life, thanks to the addition of several sensors. The packs are designed to provide an even temperature profile with minimal thermal variation during operation. Ensuring robust sensing, coupled with battery management systems that interpret data and modify operation based on changes in battery condition, remains critically important to improving battery performance.
Batteries are integrated into the vehicle in unique formats depending on the design of the vehicle. The location of the battery should protect it from outside penetrations, ensure the safety of passengers while taking into account the overall weight distribution. Close interaction between car manufacturers and battery manufacturers is essential to make the whole greater than the sum of the parts.
There are multiple trade-offs in this complex ecosystem: designing for increased security often results in higher costs and reduced battery life. In this competitive landscape where companies compete for market share, a race to the bottom can compromise security.
What causes battery fires?
Although Li-ion batteries are complex, over the past three decades many companies have perfected the art of manufacturing high-quality cells and integrating them into vehicles with minimal safety issues. The energy density of gasoline is five hundred times that of a typical Li-ion battery. Therefore, security should be manageable if robust controls are in place. However, batteries store energy in a small package and if the energy is released in an uncontrolled manner, 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 may increase as the anode and cathode release their energy through the short circuit. This, in turn, can lead to a series of reactions of the battery materials, especially the cathode, which release heat in an uncontrolled manner, as well as oxygen.
Such events also rupture the sealed battery exposing the components further to outside air and to the second part of the fire triangle, namely oxygen. The last component of the triangle is the liquid electrolyte, which is flammable and serves as fuel. The combination leads to catastrophic battery failure resulting in smoke, heat, and fire, released instantly and explosively.
The trigger for such events can be the result of internal shorts (such as a manufacturing defect that causes sharp objects to penetrate the separator), external events (an accident resulting in a cell puncture and a short- circuit of the electrodes), an overcharging of the battery which leads to reactions of heat release on the cathode (by a faulty battery management system which does not stop the charge although the battery reaches its expected state of charge), or poor thermal design at the module and pack level (by not allowing internal battery heat to be released). Any of these triggers can result in a significant security incident (see graphic).
Are battery fires inevitable?
Over the past three decades, Li-ion batteries have proven to be extremely safe, with the industry increasing controls as security incidents have surfaced. Safety is a necessity and an important consideration for which battery and vehicle manufacturers can design on many levels, from battery material selection to cell, pack and vehicle level designs.
Fire prevention requires breaking the fire triangle. Battery cathodes are one of the main causes of heat generation. Some cathodes, such as those low in nickel or switching to iron phosphate, can increase safety. Tightly controlled manufacturing will prevent accidental short circuits in cells, eliminating one of the main causes of fire. Many companies now add a ceramic layer on the separator to mechanically prevent short circuits. Sensing battery condition and integrating this data into sophisticated battery management systems is an important aspect of design. Protecting the cell with robust thermal management is essential, especially in India where ambient temperatures are high. Finally, battery packs must be protected from external penetration. Any large-scale manufacturing process inevitably has a certain percentage of defects; therefore, such measures are necessary to minimize the number of adverse events.
Long-term changes are also underway. Safety remains a concern for Li-ion manufacturers around the world, especially as cell sizes increase for applications such as solar-connected storage. Companies are developing internal “switches” that turn off parts of the battery that experience thermal events to shut them down as soon as they are created. Research is currently 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 threat of battery fires as mass electrification rolls out.
Technical safety requires commitment from all parties in the battery supply chain and tight integration between vehicle manufacturers and battery manufacturers. Additionally, regulators play an important role in providing the necessary testing and certification to ensure that technological innovations perform at the level promised. Li-ion batteries do not forgive shoddy engineering and approaches that rely on shortcuts. Companies with tightly controlled manufacturing and years of experience can keep the number of security incidents to a minimum. These batteries may be more expensive, but safety shouldn’t be “just another” measure. On the contrary, ensuring safety should be the priority of manufacturers.
The author is director of the Argonne Collaborative Center for Energy Storage Science at Argonne National Laboratory, Illinois