Lithium Ceramic Batteries
Just to bring it up for everyone who's not following the latest developments:
http://www.prologium.com shipped sample batteries to greatscott (video: )
These batteries are solid state types and apparently do not suffer from the dangers of liquid electrolyte type lithium batteries.
They can be overcharged, shorted, penetrated, cut, bend etc without catastrophic failure(fire/explosion/ignition/thermal runaway). Other than that they seem to be pretty close to regular off the shelf li-battery tech.