Xuefan Zhou

Ultra-high discharged energy density capacitor using high aspect ratio Na0.5Bi0.5TiO3 nanofibers

Ceramic/polymer nanocomposites are attractive for energy storage applications due to their ability to exploit the high permittivity of ceramic fillers and high breakdown strength of the polymer matrix. One challenge for the development of high performance nanocomposites based on ceramic particulates or fibers in a polymer matrix is that they often require a high volume fraction (>50%) to achieve a high permittivity, which is often at the expense of a reduction in dielectric strength and mechanical flexibility. In this paper we demonstrate by both experiment and finite element simulation that high aspect ratio nanofiber fillers offer an effective approach to achieve high energy density and dielectric strength. Lead-free ferroelectric Na0.5Bi0.5TiO3 (BNT) nanofibers with a high aspect ratio (>200) are synthesized by a hydrothermal method and dispersed in a poly(vinylidene difluoride-co-hexafluoropropylene) (P(VDF-HFP)) matrix. The increased fraction of β-phase and the alignment of BNT nanofibers perpendicular to the direction of the applied electric field lead to an enhanced dielectric strength, compared to spherical BNT/P(VDF-FHP) nanoparticles and pure P(VDF-HFP), and experimental measurements are compared with numerical simulations. The results demonstrate that the nanofiber nanocomposites exhibited an ultra-high discharged energy density (12.7 J cm−3) and provide an innovative approach to produce high-energy storage density materials.

Significantly enhanced energy storage density of sandwich-structured (Na0.5Bi0.5)0.93Ba0.07TiO3/P(VDF–HFP) composites induced by PVP-modified two-dimensional platelets

Two-dimensional (Na0.5Bi0.5)0.93Ba0.07TiO3 (NBBT) platelets with a size of up to ca. 5 μm and thickness of 0.2–0.5 μm were introduced as fillers into a polymer matrix to prepare energy storage composites for the first time. The NBBT platelets were treated with an aqueous solution of H2O2 and coated with polyvinylpyrrolidone (PVP) before mixing with poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF–HFP)). The final composite was denoted as NBBT@PVP/P(VDF–HFP). Composites were prepared with NBBT@PVP loadings from 1 to 30 vol%. The relative permittivity of the composites increased significantly with increasing NBBT@PVP loading, while the breakdown strength decreased. To improve the breakdown strength of the composites, a sandwich-structure of multilayer films was developed, which used NBBT@PVP/P(VDF–HFP) composites with 1 vol% NBBT loadings as central hard layers and the composites with 30 vol% NBBT loadings as neighboring soft layers. The five-layered film, which contained three central hard layers and neighboring soft layers, showed excellent energy storage properties. The breakdown strength and the maximum energy storage density of the film reached 258 kV mm−1 and 14.95 J cm−3, respectively. The energy efficiency remained 0.9 at an electric field of 200 kV mm−1. The findings provide a new approach to produce energy storage materials with high performance.

Enhancement of dielectric properties and energy storage density in poly(vinylidene fluoride-co-hexafluoropropylene) by relaxor ferroelectric ceramics

In this study, a relaxor ferroelectric ceramic, 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 (PMN–PT), was synthesized by a molten-salt growth method with lower remnant polarization and slimmer hysteresis loops than traditional ferroelectric ceramics. The PMN–PT particles remained homogeneously dispersed in the composite and adhered tightly to a poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) matrix due to the modification of the particles with dopamine. The composites had a maximum dielectric constant of 65.1 and a low dielectric loss of less than 0.037 at 1 kHz. Due to the low remnant polarization of the relaxor ferroelectric ceramic of PMN–PT, the energy density of the composites significantly increased. The discharged energy density of the sample with 50 vol% PMN–PT was 4 times that of P(VDF-HFP) at 80 kV mm−1. It was demonstrated that the dopamine functionalized PMN–PT/P(VDF-HFP) composite was a potential dielectric material with potential future applications in energy storage.

Significantly enhanced energy storage density by modulating the aspect ratio of BaTiO3 nanofibers

There is a growing need for high energy density capacitors in modern electric power supplies. The creation of nanocomposite systems based on one-dimensional nanofibers has shown great potential in achieving a high energy density since they can optimize the energy density by exploiting both the high permittivity of ceramic fillers and the high breakdown strength of the polymer matrix. In this paper, BaTiO3 nanofibers (NFs) with different aspect ratio were synthesized by a two-step hydrothermal method and the permittivity and energy storage of the P(VDF-HFP) nanocomposites were investigated. It is found that as the BaTiO3 NF aspect ratio and volume fraction increased the permittivity and maximum electric displacement of the nanocomposites increased, while the breakdown strength decreased. The nanocomposites with the highest aspect ratio BaTiO3 NFs exhibited the highest energy storage density at the same electric field. However, the nanocomposites with the lowest aspect ratio BaTiO3 NFs achieved the maximal energy storage density of 15.48 J/cm3 due to its higher breakdown strength. This contribution provides a potential route to prepare and tailor the properties of high energy density capacitor nanocomposites.

Morphology control and piezoelectric response of Na0.5Bi0.5TiO3 synthesized via a hydrothermal method

In this study, lead-free sodium bismuth titanate (Na0.5Bi0.5TiO3, NBT) was synthesized using a hydrothermal method with processing temperatures of 100–180 °C and NaOH concentrations of 2–14 M. NBT spherical agglomerates of primary nanocubes, NBT nanowires and NBT microcubes were obtained and their morphologies exhibited a strong correlation with the synthesis conditions. NBT nanowires with a high aspect ratio were proven to be single-crystalline with a [110] growth direction by high-resolution TEM analysis. The in situ transformation process and dissolution–recrystallization mechanism were successfully used to explain the formation of different NBT morphologies. Domain structures and piezoelectric characteristics were systematically studied for NBT by piezoresponse force microscopy (PFM). Clear ferroelectric domain structures and obvious polarization switching behaviors were observed in all types of NBT. The NBT microcubes possessed a larger piezoresponse compared with the NBT spherical agglomerates and nanowires.