The energy storage density of electrical capacitors utilizing antiferroelectric compositions Pb_0.99Nb_0.02[(Zr_0.57Sn_0.43)_1−yTi_y]_0.98O₃ as dielectrics is measured at a series of temperatures in a series of dielectric compositions with and without self-confinement. Under the applied electric field of 70 kV/cm, a maximum energy density of 1.3 J/cm³ is achieved. The mechanical self-confinement was introduced by partially electroding the central portion of thedielectric ceramic disk. A phase-field model was developed and it confirms the presence of compressive stresses ∼30 MPa in the electroded portion of the dielectric disk and the contribution to the increased energy density from the mechanical confinement.