My guess is your soldering iron is one which just has a mains lead on it and doesn't plug into a little box which allows you to adjust the temperature. If so the thing that is killing your tip is too much heat. You probably need to keep tinning the tip right, and if you dont the tip oxidises and goes all black and dirty.
I had a cheap one that did this, it wasn't designed right or something and just ran too hot, I found that turning the power off every few minutes or so helped. Too anoying so I ended up buying a Weller Iron, again a non adjustable one, but a much better design- they have a mechanical temperature control- a contact pulls in when the temperature drops and lets go when the temperature gets too high, I don't understand how it works but it does. They take a special tip.
By the way it isnt higher than usual temperature you need when soldering battery cells as most soldering irons run at about the same temperature. What you need is high wattage so the iron can deliver the heat faster than the battery bar can take it away. My weller is 60W, its great for Deans but I find it right on the lower limit for soldering cells.
You're right to use an 80 Watt iron for soldering cells but probably need to update to a better one.