I suspect the only chemistries that will support that kind of improvement are air-breathing cells (vanadium boride-air has a specific energy that narrowly beats

I suspect the only chemistries that will support that kind of improvement are air-breathing cells (vanadium boride-air has a specific energy that narrowly beats gasoline). Unfortunately they can't be recharged in-situ, so they probably will be limited to applications where that kind of energy density is really necessary (e.g. airliners).