I replaced the lithium-ion battery in my T3 with a lithium-polymer. The latter is moldable to some extent, so I could fit a larger-capacity battery by shaping it (a bit like very tough clay).

Electronically the li-po are the same as a lithium-ion batteries, w.r.t. charging/overcharging/undercharging etc.

Had to get an internal overcharging protector installed though, as this was an RC battery - they are sold "raw".

@spiderx: Good summary! I've been going on and on about lithium batteries for several months here, but your shortlist got it all I think. I tend to waste much more space..