This particular Li2O battery is a little under 700 Wh/kg, with the theoretical maximum being 11k Wh/kg, compared to gasoline's 13k Wh/kg. It's an incredible accomplishment that they have managed to get such a reaction reasonably stable. Minor improvements to the battery cited in the paper would be beyond the theoretical limits of existing commercial lithium chemistries.
> The results shown in fig. S9 indicate that this solid-state Li-air battery cell can work up to a capacity of ~10.4 mAh/cm2, resulting in a specific energy of ~685 Wh/kgcell. In addition, the cell has a volumetric energy density of ~614 Wh/Lcell because it operates well in air with no deleterious effects (supplementary materials, section S6.3)
> The results shown in fig. S9 indicate that this solid-state Li-air battery cell can work up to a capacity of ~10.4 mAh/cm2, resulting in a specific energy of ~685 Wh/kgcell. In addition, the cell has a volumetric energy density of ~614 Wh/Lcell because it operates well in air with no deleterious effects (supplementary materials, section S6.3)