Having been an RC airplane nut in the early 00s, I was an early adopter of lithium tech, back when we built our own battery packs and learned to "balance" them manually.
Li-ion and Li-poly batteries must have all the cells at exactly the same state of charge at all times, and individual cells must never exceed the maximum voltage that's defined by the chemistry. Balance failure leads to overcharging of one or more cells, and that's when they catch fire. So my strong belief is that these battery fires we read about are caused by defective charge control, not the cells themselves. These Chinese battery assemblers undoubtedly cheap out when buying the electronics, and the result is sometimes even fatal.
We RC nuts built "charging safes". Fireproof containers in which batteries were imprisoned while charging. Military ammo cans were a common sight at flying fields. This is an idea whose time has not passed.
Since extreme temps are bad for these batteries, I can’t store them in my shed here in NH. An option I’d considered is renting a small storage unit (I have over a dozen of them in different sizes for yard equipment), but that’s expensive.
The only other option is stick them inside a wood burning stove which I’m not currently using, but that wouldn’t hold more than 5 or 6.
Any other suggestions? I don’t want to burn down my house, and I have these things sitting around the place. Thanks.