Usually lithium ion cells are used in laptop batteries. They are hard to solder that is why they are welded by spot welder, which requires a transformer. But today I bought you guys a solution by which you can solder a 3.7v lithium ion cells.
Bevel Tip For Solder ( You can get it on hardware store or you can make your own by cutting the pointed tip and rubbing tip on whetstone or concrete) See Image
Wires (Better to use Copper flex Wires) See Image
Step 1: Heating Battery Tab
Heat the battery tab for 10 seconds by placing solder on it.
Step 2: Applying Solder on Tip