Did you ever just wanted to solder wires on a Li-po battery and realized it won't stick to the tab? The reason is that the tab is made of aluminum. Some batteries might even have nickel tab on one lead and aluminum on the other. Not sure why but it usually means you can only solder to the one tab.
The truth is that these tabs shouldn't be soldered to. Instead, nickel strips should be spot welded in place. But that's not what you're here for. To solder on these, all you need is a correct flux. Aluminum soldering flux to be specific. Here are a couple people tried and worked for them:
Any aluminum flux should work really. So don't feel limited by these two. Please check out the video as well as it goes quite a bit into details.
Step 1: Tips & Tricks
Keep the Battery cool. Heat will cause damage to the battery. The easiest way to do that is to solder as quickly as possible. Crank up the iron a bit( I'm using 380C). Get fat tip for the iron. The chunkier it is the better. Its mass will make sure that it won't cool down as you're soldering.
Clean up the surface before soldering. This is a no-brainer but using alcohol to prep the surface really makes things easier. Other than that it's pretty much like regular soldering.