  Dielectric and antiferroelectric materials are particularly promising for
high-power energy-storage applications. However, relatively low energy density
greatly hinders their usage in storage technologies. Here, we report
first-principles-based calculations predicting that epitaxial and initially
non-polar AlN/ScN superlattices can achieve an ultrahigh energy density of up
to 200 J/cm$^{\textrm{3}}$, accompanied by an ideal efficiency of 100%. We also
show that high energy density requires the system being neither too close nor
too far from a ferroelectric phase transition under zero electric field. A
phenomenological model is further proposed to rationalize such striking
features.
