After seeing this pics, i've begun to think in upgrade default batteries in 1st iliad:
https://wiki.mobileread.com/wiki/ILiad_Internals
They seem two standard
li-ion 3.7v batteries with two terminals. There are
li-polymer at very low prices with high capacity like this:
www.all-battery.com
A li-Polymer 3.7V battery with 5700mAh at 26$, but we need heigh and width from the original batteries.