You have more faith in electrical tape than I do then. I hope you never drop or bump it.
You're trying to heat up a big hunk of metal (the battery) and that takes time/power. What wattage is your iron? You may need higher power. You might also need to hold it on the battery longer. The battery has to get to the temperature to melt solder, then it will stick. If you're just melting the solder with the iron and trying to stick it to the battery you will get a cold solder joint at best. Don't overheat the battery though. As soon as it will take the solder, make your connection and be done with it. Good solder joints take practice, so you might want to find something similar to try a few times with first.