I read an article today which points out a little-discussed danger with LiPo batteries: if they are allowed to drop below freezing temperature and attempted to be recharged, they are rendered permanently dangerous and could burst into flame at any time. (Think about this when you buy any device that has these batteries and is shipped to you in winter in colder climates).
The problem is that below freezing (0C) most of the lithium ions fail to intercalate into the graphite. They instead plate out as metallic lithium on the anode. This blocks access to the lattice of the anode and thus transport of the ions; the result of that is a permanent and severe capacity loss along with much higher internal resistance (inability to deliver the desired current.) If the bad news ended there it would be bad enough but it doesn't. What's much worse is that metallic plating is not even. The introduction of lead-free solder saw a new phenomena show up in electronics called "dendrite shorts"; what happens over time is that the metal actually "grows" little spikes and if they grow far enough to reach another connection point you get a short circuit. Metallic plating inherently forms these dendrites and they are sharp and uneven. Recall that normal charging causes the anode to expand. But now, instead of a nice even surface the anode has what amount to thousands of tiny little pins sticking out of it! If mechanical shock or simply a high enough charge rate causes one or more of those "pins" to puncture the separator between the anode and cathode you get a direct short in the cell, the resulting short circuit causes the cell to heat, the electrolyte boils and bursts the case and the flammable electrolyte ignites. In other words you get a battery fire.
With lithium-chemistry batteries, however, there is a second problem which is far more-serious: They cannot be recharged below freezing temperatures without being destroyed and, even worse, rendered permanently and immediately dangerous.
Even one charge in a lithium-chemistry cell that takes place below 0C not only will do severe damage to its capacity it also renders the cell permanently unsafe. There is no way to know how unsafe the event has made it; no cell of this chemistry that has been charged while below 0C is safe to use as it can catch fire at any time without warning.
To Tesla Fans: Don't Live Up North
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