What is the safest method of storage for Lithium-Ion Batteries?

[koifish | 555]

I've become aware recently of people finding GBA SP and PSP systems whose batteries had swollen and had become unusable. Sometimes this also involved bending the battery cover out of shape, requiring a new replacement of that as well.

Off the top of my head, I can think of about eight game devices I own with lithium-ion batteries, between those that are behind a screw cover (GBA SP and Micro, DS and DSi, 3DS) and those that are internal and hidden within (Vita, Switch, DualShock3 controllers). I also own eight backup batteries which I've purchased from Nintendo over the years as official replacements (back in the day when I first heard about PSP batteries failing and needing to be replaced, I got concerned about my favored handhelds and so I started buying up replacement batteries for them). I also own one or two sealed systems that I bought either for collecting, or as a backup unit for "someday" when my original hardware fails to the point that I decide to sell it for parts and move on.

So after all that background, I get to the meat of the question: How do I best ensure the lifespan and continued proper operation of these batteries? I have four kinds:

1. Internal for handhelds which are inside of an easily accessed battery compartment
2. Internal for newer handhelds and controllers which are only accessible through partial disassembly of the unit
3. Spares, not installed in a device, kept in either original packaging or in aftermarket storage (electrostatic-safe containers with silica gel packs)
4. Unused batteries stored inside of original sale packaging (installed in sealed consoles)

I want them to last as long as possible and to not expand or rupture or anything else nasty. What should I do? Thanks to all the battery experts who read and reply with pro tips.