[George from New England]

I contend that Lithium batteries are rather safe if they are idle. No charge or discharge. And safe if no physical damage occurring.

Charging can of course leave deformed and undetected internal damages. That can at anytime afterward be a quick trigger to fire.

[minnesota_bound]

I came across on a bookshelf, a 2011 cell phone battery that was expanding. A very real fire hazard.
When it started I do not know but the phone first came out in 2010 and I have not used that phone for a decade +.

Imagine electric car batteries expanding then WHOOSH!
Cars stored away or dumped in a lot or anywhere.