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You probably need to add more flux (and perhaps a higher temperature). What temperature is your iron set to? Get some extra flux which is widely available - either liquid or paste.
Scratch the plating. The batteries are not typically solderable, they are designed for spot welding. Aggressive active flux might help as well.
18650 cells are usually solderable, you just have to use a good amount of heat. Just be careful and don't overheat the cell.Lipo cell tabs are generally NOT solderable. They seem to be either aluminum or some sort of stainless steel alloy that doesn't take to solder, no matter how much flux or heat is used. That's why they're typically spot welded onto something that IS solderable like a Nickle strip or something similar. Another different alternative is to use a bolt/nut combo to clamp down on the tabs and then solder onto that instead.
Soldering to batteries is not recommended. If you absolutely must do it, make sure you use a big soldering iron tip, set hot, so that you can get the joint made fast. The last thing you want is for it to take 20 seconds to make the joint.
The best results I have gotten were by using an emery wheel or tip to clean the surface.