EV fires

EV fires

Subject: Science

Section: Msc

Context: EVs catching fire

Concept:

Drawback of Li ion batteries:

The main reason for fire in Li ion batteries is a phenomenon of thermal runaway

What is thermal runaway?

Thermal runaway is one of the primary risks related to lithium-ion batteries. It is a phenomenon in which the lithium-ion cell enters an uncontrollable, self-heating state.

Thermal runaway can result in:

• Ejection of gas, shrapnel and/or particulates (violent cell venting)
• Extremely high temperatures
• Smoke
• Fire

Fundamentals of lithium ion battery

A lithium-ion battery cell consists of the following:

• Cathode: the positive terminal of the battery – typically a form of lithium metal oxide or similar
• Anode: the negative terminal of the battery – typically graphite
• Separator: a thin permeable polymer or similar that separates the cathode and anode
• Electrolyte: a solvent (typically alkyl carbonate based) containing conductive salts that permit the flow of ionic charge

• To produce electricity, an oxidation reaction at the anode, releasing electrons. Simultaneously, a reduction reaction occurs at the cathode, allowing the cathode to receive the electrons released by the anode, forming an electrical circuit.
• The anode and the cathode are isolated from each other via the separator, but over time, separators have the potential to wear down. Breaching of the separator causes high amounts of current to flow directly between the anode and cathode, short-circuiting the cell and producing tremendous amounts of trapped thermal energy.
• At the onset of thermal runaway, the battery heats in seconds from room temperature to approximately 700°C. As a result, the electrolyte breaks down into constituents such as methane, ethane, and ethene, as well as flammable and toxic gases such as carbon dioxide, carbon monoxide, and hydrogen gas. The cathode then begins to decompose, releasing oxygen, further accelerating the thermal runaway process. When the flammable electrolyte gases react with oxygen in the presence of heat, combustion occurs. The risk for explosion increases as the pressure in the cell builds.
• Mechanical damage of lithium-ion batteries such as accidental rupture or puncturing can result in the release of the electrolyte leading to the exposure of possibly toxic, corrosive, and flammable chemicals

Why li-ion battery preferred for E vehicle /smartphones?

• light weight
• high energy density
• ability to recharge.
• longer lifespan compared to a lead acid battery
• A Li-ion battery can typically store 150 watts-hour per kg as compared to a lead-acid battery which stores only around 25 watts-hour per kg.

However,

High energy density can also be a potential reason for catching fire