The high power density and fast charging-discharging rate inherent in dielectric energy storage capacitors render them promising in various electric and electronic equipment. However, their low energy densities limit their applications. Here, a composite capacitor consisting of two dielectric layers sandwiched by three electrodes is proposed to further improve the energy density. The work function difference between electrodes generates opposite electric fields inside the two dielectric layers and reshape the energy landscape, causing the whole capacitor in the antiferroelectric nature. Phase-field simulations demonstrate that a giant energy storage density, up to 4 times that of conventional capacitors, can be achieved by carefully choosing the work function difference, misfit strain, and film thickness. This proposal opens a new door for the design of novel dielectric energy storage facilities.