A friend of mine has been a volunteer firefighter for 48 years. He doesn't scare easy.
He estimates that he's helped put out 500 fires, running into burning buildings some of the time. That doesn't faze him either.
But last week, after a course on lithium-ion batteries, he was a bit shaken up, as evidenced by the statement: "Those things scare the shit out of me. They give off a deadly gas (the real word he used to describe the gas began with an "F").
He's mostly correct. There are few things in life that one wants to inhale less than hydrogen fluoride, which is a potential hazard from most conventional lithium-ion batteries, large and small. [1] (Hydrogen fluoride also contains an "F," but it has a very different meaning here.) But, as with all poisons, the dose makes the poison. Perhaps more germane here: "The battery size makes the poison." It's not the rechargeable battery for your electric toothbrush that's gonna cause real problems; it's the larger ones from e-bikes and e-cars that can once they get going (more on this below).
Hydrogen fluoride: A good excuse to be elsewhere
If you need convincing, one sentence from the chemical's MSDS [2] ought to suffice.
Signs and Symptoms of Acute Hydrogen Fluoride Exposure: Acute exposure to hydrogen fluoride will result in irritation, burns, ulcerous lesions, and necrosis of the eyes, skin, and mucous membranes. Total destruction of the eyes is possible. Other effects include nausea, vomiting, diarrhea, pneumonitis (inflammation of the lungs), and circulatory collapse.
Cameo Chemicals MSDS
(No, the fact that hydrogen fluoride is dangerous does not mean that the tiny bit of sodium fluoride in drinking water is.)
WTF? (Why the fluorine?)
A reasonable question, considering we're talking about lithium-ion batteries, not fluoride-ion batteries. Without going into too much chemistry – arguably worse than breathing HF – we need to explain this a little.
LiPF₆ (lithium hexafluorophosphate) is a fluorine-containing lithium salt commonly used in a battery’s electrolyte. Its job is to provide mobile lithium ions that carry charge between the electrodes. It is the lithium that makes the battery work. In fact, the fluorine itself is not required for a lithium-ion battery to work. LiPF₆ became popular because it provides the electrolyte with good ionic conductivity and works well with other battery components, not because fluorine is fundamental to battery chemistry. Other lithium salts, including fluorine-free ones, can do the job.
Fluorine doesn't do much, but it can sure cause problems
Lithium hexafluorophosphate may help the battery perform well, but it's also a source of toxic HF. Explaining this requires a bit of chemistry. Apologies.
One of the culprits is lithium hexafluorophosphate itself, which is no picnic either.
Toxic if swallowed Causes severe skin burns and eye damage. May cause respiratory irritation Causes damage to organs through prolonged or repeated exposure...
Fisher Scientific MSDS explaining why you shouldn't eat batteries.
B-b-b-b-b-aby you ain't s-s-s-s-s-s-een nothing yet
No, eating batteries isn't a great idea, but sometimes the batteries want to eat you.
This is because in a fire a bunch of stuff happens, all bad.
- Lithium hexafluorophosphate breaks down into lithium fluoride and phosphorus pentafluoride
LiPF6→LiF+PF5
Phosphorus pentafluoride is even less healthy than seed oils:
"Phosphorus pentafluoride is a colorless, poisonous, nonflammable, compressed gas with a pungent odor. It is extremely irritating to skin, eyes, and mucous membranes. It is very toxic by inhalation and can cause pulmonary edema. Toxic and corrosive fumes are generated when this material is exposed to water or steam."
Source: PubChem
- If the cell ruptures, then the contents can be exposed to water (or even moist air) and an entirely different type of fun ensues. Phosphorus pentafluoride reacts like a madman with water, forming hydrogen fluoride and another nasty called phosphorus oxyfluoride. This reaction is exothermic (generates heat, as if any more is needed).
PF5 + H2O → POF3 + 2 HF ← uh oh
Good news/bad news
Phosphorus oxyfluoride (still plenty toxic) also reacts with water. The good news is that it does so less enthusiastically than phosphorus pentafluoride. The bad news is that when it does, it generates three more molecules of HF. Add those to the two already produced, and we're up to five. Literally a case of pick your poison.
Why use fluorine at all?
LiPF₆ is used for a reason. It conducts lithium ions well, is chemically compatible with the rest of the battery, and helps prevent corrosion. Battery manufacturers also have decades of experience making cells with the stuff. Any fluorine-free replacement has to do all of those things, last for thousands of charge-discharge cycles, be safe, and not cost a fortune. So far, nothing has been good enough to send LiPF₆ packing.
Bottom line
So, was my firefighter friend overreacting? Not really. Most lithium-ion batteries spend their entire lives doing nothing more exciting than powering your phone, car, or toothbrush. But when a large battery catches fire, you're dealing with more than flames and smoke. You're dealing with a chemical system capable of generating some exceptionally nasty stuff, including hydrogen fluoride.
After looking into the chemistry, I understand why a badass who has spent 48 years running into burning buildings would rather not run into one of these.
NOTES:
[1] Lithium-metal batteries and lithium-ion batteries are different. Lithium-metal batteries are usually non-rechargeable and are used in devices such as watches and smoke detectors. Lithium-ion batteries are rechargeable and power everything from smartphones and laptops to e-bikes and electric vehicles. This article deals only with the lithium-ion variety.
Josh Bloom
Director of Chemical and Pharmaceutical Science
Dr. Josh Bloom, the Director of Chemical and Pharmaceutical Science, comes from the world of drug discovery, where he did research for more than 20 years. He holds a Ph.D. in chemistry.
