Battery swelling in my build.

[CastleTiles]

Hey everyone,
I have a couple circuit sword builds and had them
unused in a drawer for a few months and pulled them out and discovered both batteries had gassed considerably. Storage conditions were optimal. I have no idea what kind of level they were charged to when I placed them there.
They were 3.7v 3500 mAh lithium cells off eBay. Part ID# 104755.

Anyone had issues with these batteries?
Any ideas why they gassed so much?
Is there some LiPo use 101 that I have completely missed?
I removed them and took them to a hazardous waste recycler down the street today. I want to get some new batteries but I also don’t want to put grenades in my drawer again.
Suggestions?

One spicy balloon!

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[MrErickson]

I have used several of those batteries across varying builds. They have not been in storage for any significant time and see semi-regular use. I'll double check them when I get home and see if any are showing signs you describe.