Compared with antiferroelectric (AFE) orthorhombic R phases, AFE orthorhombic P phases in NaNbO3 (NN) ceramics have been rarely investigated, particularly in the field of energy-storage capacitors. The main bottleneck is closely related to the contradiction between difficultly-achieved stable relaxor AFE P phase and easily induced P-R phase transition during modifying chemical compositions. Herein, we report a novel lead-free AFE ceramic of (1-x)NN-x(Bi0.5K0.5)ZrO3 ((1-x)NN-xBKZ) with a pure AFE P phase structure, which exhibits excellent energy-storage characteristics, such as an ultrahigh recoverable energy density (Wrec) ∼4.4 J/cm3 at x = 0.11, a large powder density PD ∼104 MW/cm3 and a fast discharge rate t0.9 ∼45 ns. The analysis of polarization-field response, Raman spectrum and transmission electron microscopy demonstrates that the giant amplification of Wrec by ≥177% should be mainly ascribed to the simultaneously and effectively enhanced AFE P-phase stability and its relaxor characteristics, resulting in a diffused reversible electric field-induced AFE P-ferroelectric phase transition with concurrently increased driving electric fields. Different from most (1-x)NN-xABO3 systems, it was found that the reduced polarizability of B-site cations dominates the enhanced AFE P-phase stability in (1-x)NN-xBKZ ceramics, but the almost unchanged tolerance factor tends to cause the AFE R phase to be induced at a relatively high x value.