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I've been watching some videos where people show lithium ion batteries burning/exploding. What usually happens is someone punctures the battery, and a jet of gas comes out the puncture hole, followed by the gas catching fire and the entire thing going up in flames.

I often hear that the energy for this reaction comes from the chemical energy stored in the battery. Thing is; i'm pretty sure that there wouldn't be enough chemical energy in the battery to cause such an intense reaction. I'm thinking that it's probably the lithium inside the battery reacting with the air in an exothermic manner.

So what exactly causes the reaction that we see? Does it have anything to do with the electric energy stored in the battery or is it just a lithium/oxygen reaction?

Edit: It was suggested I actually do the math. So here goes; typical energy density of a lithium ion battery would be around 200 watt-hours per kg of battery mass. Therefore a 1kg battery would have about 720 kJ of chemical energy. Proceeding from here we would need to know at the very least a.) the mass of the substance we see being 'exploded', b.) the specific heat capacity of said substance.

For example; if I assume that from a 1kg battery we'd have 300 grams of material being heated up and expelled from the battery, and that said material has a specific heat capacity of 2 kJ/(kg K) , and that said material starts at 293 kelvin, then I'd know that it would take .6 kJ of energy to heat said material up by 1 degree Kelvin. 720 kJ would be enough to heat it up to 1493 Kelvin, assuming 100% of the energy went into heating up just that 300 grams of material and not the other 700 grams. With a specific heat capacity of 3kj/(kg*kelvin) we'd get a temperature of 1013 Kelvin.

I suppose it'd really come down to what the substance we see shooting out of the battery is, and how much is shooting out of the battery, and what it's heat capacity is (and whether it's approximately constant across the temperature range of interest).

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"Electrochemical" shows the correspondence between chemical and electrical energy. The energy in a cell can be released slowly over time, or rapidly when short-circuited internally or externally, or the ingredients meet under the wrong conditions. No additional chemicals, such as oxygen from the air, are needed.

You state, "there wouldn't be enough electrochemical energy in the battery to cause such an intense reaction." Calculate how much energy is stored, instead of guessing:

The common Li-ion 18650 cell stores about 2.5 ampere-hours at 4 volts (rounded). That's 10 watt-hours, and multiply by 3,600 sec/hr to get 36,000 watt-seconds or joules (J). That is a lot of energy compared to some projectile weapons. For example, the largest cartridge shown in this reference, for the .50 caliber Browning Machine Gun, packs 18,000 J, half that of the inexpensive 18650 cell.

So the question is, does the energy release come from thermal effects due to ohmic heating, or from the direct combination of chemicals. Most likely, once the cell is physically damaged, whether by overheating or mechanical stress, direct chemical combination takes place. Though it can be exacerbated by water or atmospheric oxygen, there is more than enough energy in even small Li-ion cells to cause spectacular fires or explosions.

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I can't comment yet, so to add the answer provided by @DrMoishe Pippik.

The initial internal heating of the battery is caused by the reaction of the chemical components within the battery. It is a similar reaction as to what occurs during discharging, but with a few noteworthy aspects:

1) The rate of reaction is limited by the leakage within the battery and not by the current draw of a circuit.

2) There is no external circuit to do any work, so the energy is instead converted to heat within the battery.

As the temperature rises, the barrier separating the components can degrade further, resulting in a run-away reaction. The heat produced during this reaction readily exceeds a few hundred degrees Celsius which is often enough to deteriorate the battery package and ignite the flammable electrolyte. At this point, the energy is derived from reaction with air and is a typical combustion reaction. So, the total energy output in these spectacular explosions is the combined electrochemical potential energy and energy of combustion.

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