As　a　latent　candidate　for　energy　storage　material,　K1/2Bi1/2TiO3　(KBT)　attracts　the　interest　of　researchers　due　to　its　high　saturation　polarization　and　inherited　relaxor　feature.　But　the　low　dielectric　breakdown　strength　Eb　of　KBT-based　ceramics　limits　the　potential　for　the　dielectric　energy　storage　application.　Efficient　improving　on　the　Eb　while　keeping　the　field-induced　polarization　difference　AP　at　high　level　is　of　significance　for　designing　KBT-based　dielectric　energy　storage　ceramics.　In　this　work,　by　constructing　a　ferroelectric-relaxor　phase　boundary　in　KBT-Na1/2Bi1/2ZrO3　(NBZ)　and　introducing　the　recipient　ferroelectric　SrHfO3　(SH)　with　wide　bandgap,　the　synergistic　improvement　on　the　Eb　and　AP　were　realized.　Our　experimental　results　reveal　that,　0.9KBT-0.1NBZ　behaves　as　a　strong　ferroelectric　relaxor,　and　SH　addition　into　0.9KBT-0.1NBZ　increases　the　band　gap　and　suppresses　the　leakage　current,　thus　enhances　the　dielectric　breakdown　strength　Eb　significantly.　At　SH　addition　of　6.0　mol.%,　excellent　polarization　behavior　and　energy　storage　performance　were　achieved:　a　super-large　AP　of　47.8　mu　C/cm2,　an　ultrahigh　recoverable　energy　density　Wr　of　5.33　J/cm3,　and　a　high　efficiency　above　75%.　Our　work　proposes　a　novel　design　route　to　construct　KBT-based　ceramics　as　a　candidate　for　energy　storage　capacitors.