EV Fires

EV Fires

Subject: Science and Tech

Section: Msc

Context- The Union government has constituted an expert panel to probe the recent series of battery explosions in electric vehicles (EVs).

Concept-

• Manufacturers such as Okinawa and Pure EV have recalled some batches of electric scooters after their vehicles caught fire.

What goes into a Li-ion battery?

• lithium-ion battery cell consists of the following:
  • Cathode: the positive terminal of the battery – typically based on a nickel, cobalt, and manganese-based oxide;
  • Anode: the negative terminal of the battery – typically graphite
  • Separator: a thin permeable polymer or similar that separates the cathode and anode
  • 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.

What causes battery fires?

• Li-ion batteries are complex.
• The energy density of petrol is five hundred times that of a typical Li-ion battery, 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 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.
• 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 a 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).
• 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.