I've heard of zinc-air and aluminum-air cells. The raw data from reliable sources suggest these have a great potential for long range in a reasonably sized package. The reason no big money is pursuing how to improve them has been stated above...they are disposable, not rechargeable.
The materials are easily recyclable, and the materials to make them are non toxic, which sounds good.
As another poster already suggested, there may be military applications. The military still uses a LOT of disposable batteries for radios, weapon sights, and night vision.
There would be a core charge when buying, so you would have an incentive to return it for recycling. Even if you cut the watt-hour per volume claims in half, these are very impressive, but...who would buy them?