[buster]

There is a big miscalculation: Graphene seems to be extremely light weight. 1 liter of graphene will definitely not weight 1kg. Wikipedia [1] only gives measures for m²: Graphene is very light, weighing only about 0.77 milligrams per square meter.

[1] https://en.wikipedia.org/wiki/Graphene

[sherjilozair]

Here[1], the density is said to be as low as 0.03 gm/cc which is the same as kg/liter. That gives a multiplicative factor of 33 over the above calculations, i.e. this could be 33% of the energy density of lithium ion batteries. Not bad.

[1]: http://www.strem.com/uploads/resources/documents/graphene_na...

[Someone]

Energy per liter also is an important factor. Your mobile would float, but it also would be the size of your lower arm. Maybe, you should wear your battery as a jacket or a pair of trousers?

[mapt]

A square meter of a single-atom-thick layer of graphene isn't very relevant. Expect the final product to have a volumetric density in the neighborhood of graphite or diamond.

[sliverstorm]

Probably closer to graphite, as to my knowledge graphite is literally just layers of graphene.

[Retric]

You need to separate each layer with an insulator to be effective, otherwise you could just use graphite.

[claudius]

Graphene is also very thin. If you created a box of graphene, it would naturally have about the same weight as diamond or coal (plus or minus 100%, maybe, but certainly not orders of magnitude less).

[homeomorphic]

Probably not minus 100% :-)