In this work, dielectric ultracapacitors were designed and fabricated using a combination of phase boundary and nanograin strategies. These ultracapacitors are based on submicron-thick Ba(Zr_0.2Ti_0.8)O₃ ferroelectric films sputter-deposited on Si at 500 °C. With a composition near a polymorphic phase boundary (PPB), a compressive strain, and a high nucleation rate due to the lowered deposition temperature, these films exhibit a columnar nanograined microstructure with gradient phases along the growth direction. Such a microstructure presents three-dimensional polarization inhomogeneities on the nanoscale, thereby significantly delaying the saturation of the overall electric polarization. Consequently, a pseudolinear, ultraslim polarization (P)–electric field (E) hysteresis loop was obtained, featuring a high maximum applicable electric field (~5.7 MV/cm), low remnant polarization (~5.2 μC/cm²) and high maximum polarization (~92.1 μC/cm²). This P–E loop corresponds to a high recyclable energy density (W_rec ~208 J/cm³) and charge‒discharge efficiency (~88%). An in-depth electron microscopy study revealed that the gradient ferroelectric phases consisted of tetragonal (T) and rhombohedral (R) polymorphs along the growth direction of the film. The T-rich phase is abundant near the bottom of the film and gradually transforms into the R-rich phase near the surface. These films also exhibited a high Curie temperature of ~460 °C and stable capacitive energy storage up to 200 °C. These results suggest a feasible pathway for the design and fabrication of high-performance dielectric film capacitors.