Butterfly batteries

At least you're not stuck with NiCads

Older Butterfly's came equipped with Super NiCad batteries. In October, 1995 the DX4 models started shipping with nickel metal hydride (NiMH) batteries. The two kinds are interchangeable. It looks like IBM has discontinued the Super NiCad batteries so the NiMH (part number 25H4862) is now the only kind you can buy.

Super NiCad batteries are rated at 1500 mAh and NiMH at 1900 mAh.

It's unlikely any newer technology will show up. Certainly Lithium-Ion is out of the question without a redesign of the internal charger.

There are lots of companies that may third party laptop and notebook batteries. A good starting place is a Yahoo search for something like notebook batteries or notebook batteries.

Xtend Micro Products has a nice Notebook battery FAQ that explains the differences between the various battery technologies.