Are Li Ion Batteries not just rechargeable Batteries?
No. Start here:
http://www.saberforum.com/index.php?topic=1727.0 And now i have 14500 Batteries in my Saber can i use some bigger Like 25000?
Not if you want it to keep working. Putting 25000 in it without some serious modifications will destroy it. I don't have the exact difference in voltage because I don't feel like looking it up, but I'm sure someone will be along at some point with the information.
And i have a second Saber a Staff with normal Battery Setup. Can i use only normal Batteries in it or Li Ion too?
Normal batteries only, or rechargeable batteries. But not Li-Ion. See link above for the differences.