Introduction Polymer-based dielectrics with ultrahigh power density, good flexibility and easy-processing have attracted much attention in powering electronics such as capacitor, power transmission, pulsed power systems and microelectronic systems. However, the low energy density affects the application in electronics industry. Recently, introducing the ferroelectric ceramic fillers with a high dielectric constant into the polymer matrix can improve the dielectric constant of polymer composite dielectric materials. In general, the microscopy of fillers has a great influence on the energy storage performance of polymer-based dielectric capacitors. One-dimensional nanomaterials are potential to fabricating the high-performance polymer-based dielectric capacitors due to their quantum size effect, surface effect and macroscopic quantum tunneling effect. When one-dimensional nano-ferroelectric materials are introduced into the composites, a high length?diameter ratio can reduce the specific surface area and enhance the dispersion in the polymer matrix. Also, the composites can obtain better energy storage properties with a lower filler content due to its large electric dipole. In this paper, KNbO3 nanofibers with a good crystalline and a high length-diameter ratio were firstly synthesized via hydrothermal reaction. Polymer-based dielectric capacitors were then fabricated with KNbO3 nanofibers and poly(vinylidene fluoride-co-hexafluoropropylene) & polymethyl methacrylate (P(VDF-HFP)&PMMA) polymers. The recoverable energy storage density and energy storage efficiency were analyzed. Methods For the synthesis of KNbO3 nanofibers, 36 g KOH and 1 g Nb2O5 were mixed in a 60 mL beaker and then were stirred for 1 h. Afterwards, the solution was transferred to a 100 mL reactor for hydrothermal reaction at 180 ℃ for 12 h. The particles were washed using deionized water and ethanol for several times and dried at 60 ℃ for 12 h. Finally, the composite films were prepared through a solution-casting method. 0.9 g P(VDF?HFP) and 0.1 g PMMA were dissolved in 10 mL N,N-Dimethylacetamide (DMAC) at 80 ℃. KNbO3 nanofibers with different mass fractions (x=0, 2.5%, 5.0%, 7.5%，and 10.0%) were dispersed in the DMAC solution under magnetically stirring to obtain the mixture, and then the mixture was added into the P(VDF-HFP)&PMMA solutions. The composite films were prepared by a tape casting and subsequently dried at 90 oC. The thickness of the prepared films was 20 μm. Au electrodes (2 mm in diameter) were sputtered on the composite films for the coming electrical measurements.Results and discussion Based on the crystal structure of the KNbO3 nanofibers detected by XRD, the nanofibers exhibit a pure orthorhombic perovskite structure in coincidence with the standard JCPDS 32-0822. A high diffraction intensity reflects a good crystallinity of KNbO3 nanofibers. In addition, the widen diffraction peaks show the small size of KNbO3 nanofibers. From the TEM and SEM images, KNbO3 nanofibers have a uniform microscopy with a diameter of 10-20 nm and the length of ~500 nm. The distinct lattice fingers and selected area electron diffraction patterns confirm a good crystallinity of KNbO3 nanofibers. Besides, all the elements uniformly distribute for KNbO3 nanofibers. The frequency-dependent dielectric constants of composite films with x of 0, 2.5%, 5.0%, 7.5%, and 10.0% are 2.8, 5.2, 6.1, 8.2, and 11.5, respectively. KNbO3 nanofibers enhance the dielectric properties of composite films because of the intense ferroelectricity and the Maxwell-Wagner-Sillars interface effect. In addition, all the films exhibit a low dielectric loss, indicating the superior breakdown strength. The polarization intensity at the same external electric field increases with the increase of KNbO3 nanofiber fillers. Besides, the breakdown electric field reduces as the KNbO3 nanofibers increases because a severe electric field deformation nearby KNbO3 nanofiber fillers generates a high local electric field distribution. Therefore, the energy storage density firstly increases and then decreases, achieving an optimal value of 14.3 J/cm3 when 7.5% (in mass fraction) KNbO3 nanofibers are doped in the composite films. Also, the energy storage efficiency of the optimized composites is 72%. Conclusions The ferroelectric nano-materials were filled into the polymers to enhance the energy storage performance of composite capacitors. KNbO3 nanofibers prepared by a hydrothermal method exhibited a high aspect ratio and a good crystallinity. KNbO3 nanofibers were mixed with P(VDF-HFP)&PMMA polymers to prepare KNbO3/P(VDF-HFP)&PMMA composite films. The polarization strength of composite films was enhanced. Meanwhile, the breakdown strength of composite films slightly degraded. Therefore, the optimum energy storage density of 14.3 J/cm3 and efficiency of 72% were achieved when 7.5% KNbO3 nanofibers were doped in the composite films. The results indicated that KNbO3 nanofibers could effectively enhance the energy storage performance of composite capacitors.