Re: folding batteries.
A proper folding phone might sell by the boatload. It would have to be a trifold device so it unfolds to 16:9 and is MUCH larger than what you could get with a non-folding phone, weigh less than 8 ounces, and be no more than 1/2" thick when folded up.
IMHO, of course, but I don't see a device that unfolds to a roughly 4:3 like this one, and is only 7.3" at that, being much of a success. Most people would want a bigger screen for watching videos or gaming, and both need 16:9. They also want something MUCH bigger, getting something only a little bigger than what you can get now if you buy one of the larger phones isn't worth the higher price, greater likelihood of problems, heavier weight, reduced battery life, etc. etc.