Dent on the flat side of battery?

[Eskie]

I think there's agreement to recycle that battery. I know I would not use it. Check your mod over carefully. That had to have been one hell of a fall to dent it like that. Check that the contacts are OK and keep an eye on it even with new batteries.

 

[Coastal Cowboy]

I wouldn't use it. in fact, I don't think it would even make complete contact in my devices.

It's a $5 cell. Not worth keeping.

 

[Baditude]

What do you mean by venting? Sorry, I just don't much about batteries at all.

Mooch gives a nice technical definition of venting and thermal runaway here:

Li-Ion Battery Chemistries - What are the differences in their safety?​

In a nutshell, any misuse/abuse/damage to a battery can result in an irreversable chemical reaction which can result in the release of very hot gases or flames from the battery.

This can occur while inside the mod, or if you carry the batteries unprotected in a pocket/purse/bag. In the later scenario, if the battery comes into contact with coins, keys, or other metalic items it can cause the battery to short circuit. This is why batteries should be transported in plastic battery cases.

plastic battery case

Regulated mods should be turned off before transporting in a pocket/purse to avoid unintentional pressing of the fire button, which can result in over-discharge of the battery. If you ever carry your unregulated mech mod in your pocket or purse you should either lock the fire button, or remove either the battery or atomizer from the mod, to prevent accidental firing of the button.

 

[AzPlumber]

You may want to google how a Li-ion battery is made.
Layers of *stuff* is wrapped like a jam roly poly with an insulator between the *stuff*. If the insulation layer has been compromised then you could potentially get a short circuit which could potentially induce a spontanious release of the stored energy causing a potential fire or explosion. (sorry for all the potentials)...
TLDR;
Buy a new one.

This ^^ post has a lot of potential.