High-voltage Polarization Research Articles

Enhancing the Reversibility of Lithium Cobalt Oxide Phase Transition in Thick Electrode via Low Tortuosity Design.

Lithium cobalt oxide (LCO) is a widely used cathode material for lithium-ion batteries. However, it suffers from irreversible phase transition during cycling because of high cutoff voltage or huge concentration polarization in thick electrode, resulting in deteriorated cyclability. Here, we design a low tortuous LiCoO2 (LCO-LT) electrode by ice-templating method and investigate the reversibility of LCO phase transition. LCO-LT thick electrode shows accelerated lithium-ion transport and reduced concentration polarization, achieving excellent rate capability and homogeneous actual operating voltage. Moreover, LCO-LT thick electrode exhibits a durable phase transition between O2 and H1-3, mitigated volume expansion, and suppressed microcrack formation. LCO-LT electrode (25 mg cm-2) delivers improved capacity retentions of 94.4% after 200 cycles and 93.3% after 150 cycles at cutoff voltages of 4.3 and 4.5 V, respectively. This strategy provides a new concept to improve the reversibility of LCO phase transition in thick electrode by low tortuosity design.

A High‐Performance Room‐Temperature Li||Ga–Sn Liquid Metal Battery for Grid Energy Storage

Herein, a room‐temperature liquid metal battery (LMB) with a solid lithium anode electrode and gallium–tin (Ga–Sn) alloy cathode electrode is reported. With the improved wettability of the electric collector and grain‐refined liquid metal droplets cathode, the aforementioned LMB exhibits good cyclic reversibility and negligible self‐discharge. The result shows that the assembled Li||Ga–Sn battery has a satisfactory specific capacity (409 mAh g−1) and high energy efficiency (up to 92%). The low melting point Ga–Sn alloy can construct fast kinetics for the redox reaction and improve the electrode reaction kinetics, imparting the Li||Ga–Sn system with high discharge voltage (0.77 V at 1C) and smaller polarization voltage at different current densities, which ensure the high Coulomb efficiency and good rate performance of the battery. Moreover, the battery stably cycles at 35 °C for 60 cycles at the 1C rate, with no significant capacity degradation. With good electrochemical performance, simple structure, easy maintenance, and high safety, this room‐temperature Li||Ga–Sn battery may be a promising choice for power grid energy storage applications.

A Review of Piezoelectric PVDF Film by Electrospinning and Its Applications.

With the increase of interest in the application of piezoelectric polyvinylidene fluoride (PVDF) in nanogenerators (NGs), sensors, and microdevices, the most efficient and suitable methods of their synthesis are being pursued. Electrospinning is an effective method to prepare higher content β-phase PVDF nanofiber films without additional high voltage poling or mechanical stretching, and thus, it is considered an economically viable and relatively simple method. This work discusses the parameters affecting the preparation of the desired phase of the PVDF film with a higher electrical output. The design and selection of optimum preparation conditions such as solution concentration, solvents, the molecular weight of PVDF, and others lead to electrical properties and performance enhancement in the NG, sensor, and other applications. Additionally, the effect of the nanoparticle additives that showed efficient improvements in the PVDF films was discussed as well. For instance, additives of BaTiO3, carbon nanotubes, graphene, nanoclays, and others are summarized to show their contributions to the higher piezo response in the electrospun PVDF. The recently reported applications of electrospun PVDF films are also analyzed in this review paper.

Partial Discharge Imaging Correlated with Phase-Resolved Patterns in Non-Uniform Electric Fields with Various Dielectric Barrier Materials

This paper describes a correlation of partial discharge phase-resolved patterns with an optical imaging performed in a non-uniform electric field configuration. The influence of different dielectric barrier materials, placed on the plane electrode, on the discharge propagation and surface landing was investigated. The investigations were focused on the corona at positive polarity of AC high voltage. It was found that the initial positive corona stage is similar for all cases whereas the discharge propagation and surface landing strongly depends on the barrier material properties. The observed streamer discharge modes have been described by the geometrical measures such as stem length, stretch of a discharge profile on the dielectric barrier surface and an hemispherical envelope of discharge filaments. Since various dielectrics reveal different properties of charge accumulation and surface neutralization, the charge memory effect may be visible and can be related to the ability to create and sustain of additional electric field component. It may refer to subsequent discharges as well as to conditions faced at the voltage polarity reversal. The correspondence between different forms of phase-resolved patterns have been associated with the modes of streamer discharges observed by optical imaging. Presented methodology poses huge potential for both scientific investigations on underlying discharge phenomena as well as on the application in future diagnostic systems of HV insulation.

Piezoelectric PVDF membranes for use in anaerobic membrane bioreactor (AnMBR) and their antifouling performance

Membrane fouling is a common phenomenon in anaerobic membrane bioreactor (AnMBR), deteriorating membrane performance and increasing the operation costs of AnMBR. In this study, β-phase polyvinylidene fluoride (PVDF) flat sheet membranes were successfully prepared by first mixing PVDF powder with ionic liquid [BMIM]PF6 and subsequently casting them using thermally induced phase separation (TIPS), followed by high voltage polarization treatment to endow the PVDF membranes with piezoelectric properties. For the first time in AnMBR, we reported in this study on the piezoelectric PVDF membrane that generates in-situ vibrations to reduce membrane fouling by applying a given frequency and alternating voltage (AV). The results suggested that the steady-state water flux through the piezoelectric membrane in AnMBR operation increased along with the AV, enhancing it by up to 72.6% (at 20 V) compared to that of the membrane without applying AV. In addition, high water flux was obtained at the optimal operating frequency of 10 kHz when the cross-flow mode was taken into consideration in AnMBR operation in this study, although the resonance frequency with maximum amplitude of the membrane was 600 kHz. These results indicate that the piezoelectric β-PVDF membrane is effective at minimizing or alleviating the membrane fouling when the electric field simultaneously applied during AnMBR operation. During the 30-day operation of AnMBR with the β-PVDF membrane with poling, the COD removal rate was maintained at over 90%, even if the applied frequencies and voltages were changed during the operation.

Bio-waste orange peel and polymer hybrid for efficient energy harvesting

Bio-waste orange peel with poly(vinylidene fluoride) hybrids have been developed as an efficient energy harvester. Bio-wastes in huge amount cause landfills and environmental pollution. An effort in the direction of using bio-waste will be beneficial in many ways. Orange peel, a bio-waste, is mainly composed of cellulose and different proteins that are responsible for its piezoelectric effect. The hybrid exhibits significant piezoelectric properties arising through induced piezoelectricity in the polymer matrix by the smaller dimension orange peel powder as filler causing an electroactive phase of ∼70% in the hybrid. Integrated device has been fabricated for energy harvesting using the hybrid material, without conventional high voltage poling, which displays very high open circuit voltage of ∼90 V and power of 135 μW/cm2 using finger tapping. The hybrid nanogenerator is proficient in light up the LEDs from the movement of sliding door and from any human body movements like bending, twisting and walking etc. The underlying mechanism of enhanced piezoelectricity in the hybrid is revealed through structure, morphology and thermal studies of the hybrid. Thus, the piezoelectric energy harvester prepared from the bio-waste might be one of the solutions to utilize the bio-waste.

Influence of high voltage polarity in multi-pin upward electrospinning system on the Fiber morphology of poly (vinyl alcohol)

This study was carried out to evaluate the influence of high voltage polarity on a newly designed multi-pin upward electrospinning spinneret to produce poly (vinyl alcohol) nanofibers. Two different high voltage polarity configurations were applied to the spinneret and collector, namely collector charging configuration (configuration-1) and spinneret charging configuration (configuration-2). The outcomes demonstrated that the collector charging configuration is better in case of upward multi-pin electrospinning setup, to their conservative framework in terms of stable Taylor cone formation, smaller fiber diameter and less bead formation. Electrical field profile in the electrospinning zone was investigated using finite element modelling for both the configurations. No difference in electric field intensity norm (|E|) was observed near the tip of the pins for both the polarities (configurations). Although having the same magnitude, the dominant electrical field component, Ez for configuration -1 is found to be of opposite sign to that of configuration -2. The electrostatic simulations also suggested that all the pins were not at the same potential and that there was a minor difference in pin potentials. Also, optimizations of pin length to reduce the potential differences between the pins were theoretically estimated through COMSOL3D simulation software.

Mechanistic understanding of the Sulfurized-Poly(acrylonitrile) cathode for lithium-sulfur batteries

Sulfurized−poly(acrylonitrile) compound (SPAN) cathode decipher polysulfide dissolution and shuttling effect and deliver promising electrochemical performance. However, the synthesis reaction mechanism, chemical structure, and lithium storage mechanism of the compound are yet not clearly understood. Herein we report a plausible synthesis reaction mechanism, chemical structure, and lithium storage mechanism of SPAN using high−resolution cross−polarization/magic angle spinning (CP−MAS) solid states nuclear magnetic resonance (ssNMR), Fourier transform infrared spectroscopy (FTIR), X−ray photoelectron spectroscopy (XPS), elemental analysis (EA) techniques, and electrochemical analysis. The analysis reveals that the SPAN structure contains N–S and NC–S bonds in addition to C–S and S–S bonds. Electrochemical cleavage of the covalent bonds results in high initial discharge capacity with high voltage polarization in the first cycle. Once the covalent bonds are cleaved in the first cycle, the electron-donating effect of the Li–C and Li–N bonds increases the electron density of the conjugated heterocyclic structure and benefits the decrease of the charge/discharge voltage hysteresis after the second cycle. As a result, it provides high reversible specific capacity, high coulombic efficiency, and extended cycle life with low voltage hysteresis after the second cycle. The results show that the covalent molecular interactions of SPAN cathode completely overcome the polysulfide dissolution, thereby prevent the loss of active materials. Furthermore, the structural elucidation is further applicable for designing an organic cathode achieving high active materials loading concomitantly with improving the electrochemical performances.

Poling and annealing of piezoelectric Poly(Vinylidene fluoride) micropillar arrays

This work reports on the effect of calcination and poling processes on the crystalline phase and piezoresponse of poly(vinylidene fluoride) (PVDF) micropillar arrays. PVDF micropillars were prepared by the imprinting method, heated and treated with high-voltage poling. The effect of the treatment conditions on the crystallization behaviour and the piezoelectric properties of the patterned PVDF films was investigated by piezoresponse force microscopy (PFM), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). DSC data show that poling of the PVDF micropillars increases the crystallinity of the polymer from 12% to 22.7%. FTIR measurements of PVDF films show that the calcination and poling processes affect the γ to β phase transformation. In the imprinted and annealed samples, the γ phase was predominant (58% and 46%, respectively). For the poled samples, up to 42% of the β phase was found. Piezoelectric measurements using PFM showed that the poled PVDF micropillars possess a much higher piezoelectric coefficient (29 pm/V) compared to the annealed sample (10 pm/V). The piezoresponse of the PVDF micropillar arrays is thus substantially enhanced by poling.

Ultrahigh energy harvesting properties in Ag decorated potassium-sodium niobite particle-polymer composite

Composite-type piezoelectric nanogenerator (PENG) can potentially provide power to the flexible electronics devices by harvesting the mechanical energy. The electricity output of the PENG is not entirely excavated until now because the polarization dipoles are not sufficiently aligned during the high-voltage poling process. In this study, some Ag particles are attached on the (K0.4425Na0.52Li0.0375) (Nb0.86Ta0.06Sb0.08)O3 (KNN) piezoelectric particles and then they are mixed with multi-walled carbon nanotubes and polydimethylsiloxane to fabricate the PENG device. The Ag particles can reduce the optimal poling electric field from 10 kV/mm down to 5 kV/mm. The PENG device with Ag particles poled at 5 kV/mm can generate the highest open-circuit voltage of 282 V, short-circuit voltage of 32.2 μA, and maximum instantaneous power of 3.5 mW under the external mechanical stress of 10 kPa without time-dependent degradation (only 27.9 V and 2.6 μA for the pure KNN-based PENG poled at 10 kV/mm). These are much better than previously reported composite-type PENG. The electrical energy generated from the PENG (20 mm × 40 mm) can light up 40 white light emitting diodes instantaneously without any storage unit during the stomping stage.

The possibility of using glycerin as the dielectric in pulse forming lines

In this paper, an approximately scaled coaxial test device is designed and constructed to study the breakdown characteristics of glycerin dielectric in pulse forming lines (PFLs). In the test device, factors affecting the breakdown strength of the glycerin are investigated, including polarity of high voltage, applied gas pressure, temperature, magnetic field, and the electrode surface characteristics. The time and temperature characteristics of the glycerin resistivity are investigated as well. The results show that glycerin has extremely high and stable resistivity, and its breakdown field strength is independent of the polarity of the applied electric field. As the nitrogen gas pressure is applied up to 2 MPa on the glycerin, the breakdown field strength is increased by 40%. When the temperature is changed from 10 to 55 °C, the breakdown field strength reduces from 310 to 165 kV/cm. It is also found that there is no effect on the breakdown field strength of glycerin on magnetic field under 1T. Moreover, the filling process of the glycerin PFL is investigated for exhausting the gas as far as possible. The glycerin PFL has been employed in pulsed power generators, and some breakdown problems in the practical application have been solved.

Structural, optical, and electrical properties of ferroelectric copolymer of vinylidenefluoride doped with Rhodamine 6G dye

An effect of Rhodamine 6G dye introduced into vinylidenefluoride and tetrafluoroethylene copolymer on a number of its structural and electrical characteristics has been detected. It was shown that at film crystallization, the inserted dopant shifts the equilibrium distribution of isomers to the side of increasing concentrations of chains with the conformation of a planar zigzag. The dye introduced strongly increases ac conductivity, especially at high electric fields. The investigation of high voltage polarization under bipolar external field conditions shows slow switching of gigantic current which is observed at fields lower than coercive ones. The estimation of the charge density indicates the non-ferroelectric nature of the phenomenon observed. The analysis of the data shows that in the system, Maxwell–Wagner relaxation processes take place, which lead to the space charge formation in the polymer matrix. It is established that current switching observed must be attributed to the relaxation of the space charge field.

Monolithic nonlinear optical chromophores with extended conjugate bridge: Large refractive index, high thermal and electro-optic stability

Monolithic electro-optic (EO) molecular glasses are promising materials for optical waveguide device owing to their high chromophore loading density, high refractive indices and large EO coefficients (r33 values) for the optimization of in-device figure of merit (n3r33). However, the stability of EO molecular glasses including the poling induced r33 stability, chemical stability under both high temperature and high voltage in poling is still an obstruction for their real application. Herein, we used the chemical structural optimization, by means of introducing a large conjugated electron-bridge indacenodithiophene (IDT), to synthesize a monolithic EO molecular glass IDTC. Comprehensive properties of excellent glass-forming ability, high thermal stability (Td = 315 °C), and high glass transition temperature (Tg = 137 °C) was achieved in monolithic IDTC thin films. More significantly, refractive index of monolithic film was exceptionally high (n = 1.8294 @ 1310 nm and n = 1.7645 @1550 nm). The poled IDTC film exhibited the r33 value of 87.6 pm/V with large figure of merit (n3r33: 536 pm/V @1310 nm), and the EO activity can be retained over 85% of the initial value after 300 h annealing at 85 °C. Meanwhile, it was found that there is no decomposition of IDTC in poled films after high temperature and high voltage poling as well as long term annealing. The poled chromophores are recyclable EO materials for the sustainable application in low-cost EO devices.

Three-dimension ivy-structured MoS2 nanoflakes-embedded nitrogen doped carbon nanofibers composite membrane as free-standing electrodes for Li/polysulfides batteries

Abstracts Three-dimension ivy-structured MoS2 nanoflakes @ nitrogen doped carbon nanofibers (MoS2@N-CNFs) composite membrane was designed by a combined electrospinning and hydrothermal technique. The MoS2@N-CNFs membrane is employed as a free-standing cathode for Li/polysulfides batteries. N-CNFs membrane with high electrical conductivity is used as a current collector to effectively reduce the internal resistance in the electrode and immobilize the dissolved lithium polysulfides with mesoporous N-CNFs with pyridine nitrogen. The MoS2 nanoflakes introduced into N-CNFs membrane offer a hierarchical composite structure, in which the MoS2 nanoflakes not only have chemical binding with the lithium polysulfides, but also promote fast redox reaction kinetics with high capacity and low voltage polarization. Meanwhile, the MoS2 nanoflakes have been demonstrated to show strong binding energy and be capable of anchoring polysulfides based on density functional theory calculations. As a result, with a high sulfur loading of 7.11 mg, the assembled cell with MoS2@N-CNFs + Li2S6 cathode demonstrates excellent electrochemical performances with a capacity decay rate of 0.08% per cycle over 250 cycles at 0.2C. Moreover, when the sulfur loading is further increased to 11.84 mg, a capacity decay rate of 0.25% per cycle over 100 cycles is achieved. Consequently, the method used in here provides a new insight to produce N-doped carbon nanofibers with metal sulfides/metal oxides nanoflakes composites as free-standing materials with the high sulfur loading for lithium-sulfur batteries.

Online desalting and sequential formation of analyte ions for mass spectrometry characterization of untreated biological samples.

The metal salts ubiquitously present in biological samples cause serious ion suppression, capillary clogging and signal fluctuation in ESI/nESI. Herein, a current-limited high voltage polarity reversing approach was applied for the online separation of intrinsic metal ions in biological samples, resulting in the generation of protonated analytes at the nESI tip for mass analysis without interference from salt cations. Stable and durable signals (∼30-60 s) were observed for protonated proteins, peptides and metabolites in complex biological samples, including liquids, solids and viscous samples, even with very high salt concentration (100 mM NaCl), allowing comprehensive tandem MS analysis with on average ca. 5-times higher analyte signal intensities compared to the conventional nESI analysis. Therefore this approach offers improved performance of nESI/ESI for the sensitive molecular analysis of untreated biological samples, opening new possibilities in various disciplines, including biology, medicine, chemistry, life sciences, etc.

Effective polarization of ferroelectric materials by using a triboelectric nanogenerator to scavenge wind energy

Effective polarization of ferroelectric materials can be achieved by utilizing a conventional high-voltage polarization equipment with a high voltage but a large volume. Here, we report that the polarization of a ferroelectric BaTiO3 material can be realized by using a triboelectric nanogenerator (TENG) to scavenge ambient wind energy, where the TENG is ideal for providing a higher voltage, a smaller current, and especially a smaller volume. Both the output voltage and current signals of TENG can be adjustable by controlling different wind speeds, where the TENG can deliver an output voltage pulse (<10 ms in period) of about 1000 V and an output current pulse of smaller than 100 µA under a wind speed of about 14 m/s. A piezoelectric constant of larger than 150 pC/N and a pyroelectric constant of larger than 14 nC/cm2 K were achieved by using a TENG to polarize the ferroelectric BaTiO3 material. A handheld TENG can be also utilized to polarize the ferroelectric material, demonstrating the possibility of obtaining small-scale high-voltage polarization equipments.

A mechanistic approach to developing two phase flow pattern transition maps for two-phase dielectric fluids subject to high voltage polarization

High voltage electric fields can be used as an active heat transfer enhancement technique for convective boiling or condensing dielectric fluids, with the major component of this enhancement being attributed to phase redistribution and mixing in the system due to electric body forces. Many performance prediction tools for convective boiling and condensation rely on flow pattern maps to identify the flow regime for use with flow pattern specific heat transfer and pressure drop correlations [1,2]. In this paper, a mechanistic approach to developing two phase flow pattern maps for two-phase dielectric fluids subject to high voltage fields using a fully contained, equation-based electric Froude number approach is presented and compared against the experimental flow pattern data for EHD convective boiling & condensation available in the literature and from the current experimental study. Despite the development of newer diabatic, semi-mechanistic flow pattern models for free-field heat exchanger performance analysis, it is recommended that the Steiner equations for flow pattern transition be used as a basis for the mapping when additional physics such as EHD is coupled, as the empirical fits in newer semi-mechanistic maps are unreliable when additional physics is present.

Simultaneous 3D Printing and Poling of PVDF and Its Nanocomposites

Given the ever-increasing demand for customization and miniaturization, in situ three-dimensional (3D) printing of piezoelectric polymers comes as an efficient means to cater to smart structures via multimaterial printing. Applying our hybrid printing technique to polyvinylidene-fluoride–barium-titanate (PVDF–BaTiO3) nanocomposites, to combine the fabrication and high-voltage poling steps, shrinks the manufacturing time, overcomes the disadvantages of adherence in nonconformal piezoelectric films or fabrics, and increases the sensitivity and scope for on-demand applications. A remarkable 300% improvement in piezoelectric charge output is achieved upon the addition of 10 wt % BaTiO3 nanoparticles and application of an electric field of 1 MV m–1, compared with printed unpoled neat PVDF. We demonstrate the application of the in situ poling process in the form of sensors printed directly on a shoe insole for gait analysis. The sensors fabricated in this work effectively distinguish between walking and stampin...

An effect of the film texture on high-voltage polarization and local piezoelectric properties of the ferroelectric copolymer of vinylidene fluoride

Vinylidene fluoride and tetrafluoroethylene copolymer 94/6 texturing has been carried out by one-axis drawing at Td = 75 °C up to the ratio of 6 with subsequent isometric tempering at 130 °C. Data of X-ray diffraction and IR-spectroscopy indicate partial polymorph transition from the metastable γ-phase to the polar β-modification with long segments in the conformation of the planar zigzag. The surface topography data show that the oriented film roughness turns out to be almost three times lower than that of the isotropic one. High-resolution vector piezoresponse force microscopy (PFM) has been used to investigate topography and ferroelectric domains in polymer films for visualization out-of-plane and in-plane polarization components. It is shown that, in the oriented sample, both crystallinity and the degree of polar β-phase crystal perfection are higher. It was found that conductivity “abnormal” dropping takes place at the stage of intensive Pr rising with the field growing. This fact is explained by quick trapping of charge carriers by polar planes of crystals with increasing the effective trap area.

Electrostrictive energy conversion property of cellular electrets after corona discharge

In this paper, the authors present the electrostrictive energy conversion ability of cellular electrets after the high-voltage corona polarization. Moreover, the electrostrictive effect of such foamed polymer before and after corona polarization has also been compared and discussed. The enhancement of electrostrictive effect of cellular electrets after corona polarization was observed. In particular, the impact on the electrostrictive effect of the macroscopic electric dipoles inside of cellular polymer which are generated by high-voltage corona poling procedure has been investigated. The present research has promoted the development of the application of electret in the field of energy conversion, actuator, transducers, etc.