I remember reading that electric motors are much lighter than gas engines for the same amount of power. This difference can at least partially offset the added battery weight.
Also, it's possible to have multiple, small electric motors with no loss of efficiency. That isn't possible for a gas engine. Multiple motors could add some redundancy, and therefore reliability.
I think we're going to see a lot of interesting designs over the next few years.
> electric motors are much lighter than gas engines for the same amount of power
That varies wildly based on what kind of engines and motors you're talking about; it's not consistent enough to be applied as a generalization, especially when it comes to aircraft (turbines are some of the most power-dense engines available). See here [1] for examples.
Something else to keep in mind: a plane gets lighter as it burns fuel. Electric batteries don't get lighter as they expend energy.
Chip Yates flew a few experimental electric airplanes a few years back. The last I heard, he had devised some sort of battery hot-swap system so he could fly from NY to England in one go. I don't think that one ever made it off the drawing board.
Also, it's possible to have multiple, small electric motors with no loss of efficiency. That isn't possible for a gas engine. Multiple motors could add some redundancy, and therefore reliability.
I think we're going to see a lot of interesting designs over the next few years.