I don't know enough about battery tech to know what the standard measure is, so I'll just accept that the standard measure of capacity is energy/volume as you say. Still, my point remains. The mass density (mass/volume) also needs to be known to assess the practical merit of the tech. They are independent properties. If a device has a given whr requirement, one tells you how much space the battery will require and the other tells you how heavy it will be. Of course, the "dilution" you speak of complicates things.