High-voltage Polarization Research Articles

Optimizing the polar phase transformation in PVDF-HFP/BaTiO3-activated carbon composite films with the mixture design of experiments

ABSTRACT A salient bottleneck to driving the wider use of polyvinylidene fluoride (PVDF) and its copolymers in smart applications is the requirement for a high-voltage poling process required to activate their β -phase crystal structure before the realisation of the piezoelectricity response. In pursuit of enhancing the β-phase content of PVDF-based films, various approaches have been employed in recent years. Yet, a perfect solution to the challenge remains elusive. Here, we report a preliminary investigation into the production of unpoled poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) films reinforced with barium titanate (BaTiO3) and activated carbon (AC) to improve the piezoelectric property. Employing the mixture design of experiments (MDOE) method, various formulations of the composite’s constituents weight fractions were determined. Afterwards, series of composite films were produced via a toxic-free solution casting method and characterised by Fourier Transform Infrared Spectrum (FTIR) to determine the β-phase content. A non-linear polynomial regression model was established from the experimental FTIR data, and the adequacy of the model was confirmed using ANOVA. Subsequently, an MDOE-enabled optimisation was undertaken, generating a formulation of the composite to obtain the highest β-phase content to be 15 wt.% BaTiO3 and 0.74 wt.% AC. This combination produces a theoretically predicted β-phase content of 72.7 %. A further experimental test employing the optimal formulation produced a β-phase content of 71.90 %, achieved with neither mechanical stretching, thermal annealing, nor electrical poling.

Effect of Nanostructure Morphology and Concentration on the Piezoelectric Performance of Flexible Pressure Sensor based on PVDFTrFE/ Nano-ZnO Composite Thin Film

Background: The development of high-performance piezoelectric pressure sensors with outstanding sensitivity, good linearity, flexibility, durability, and biocompatibility is of great significance for smart robotics, human healthcare devices, smart sensors, and electronic skin. Thus, considerable progress has been achieved in enhancing the piezoelectric property of PVDF-TrFEbased composite pressure sensors by adding various ZnO nanostructures in PVDF-TrFE polymer acting as a nucleating agent and dielectric material. Aims: In this work, flexible pressure sensors with a sandwich structure based on PVDFTrFE/ nano-ZnO composite sensing film were fabricated using a simple spin-coating method and post-annealing process, while electrospinning and high-voltage polarization processes were not adopted. Methods: Poly (vinylidene fluoride-trifluoroethylene) (PVDF-TrFE)/nano-ZnO composite films were prepared via spin coating to fabricate flexible piezoelectric pressure sensors. ZnO nanoparticles (ZnO NPs), tetrapod ZnO (T-ZnO) and ZnO nanorods (ZnO NRs) were used as nano-fillers for piezoelectric PVDF-TrFE, to enhance the beta-crystal ratio as well as the crystallinity of PVDF-TrFE. The structural and surface morphologies of the composite films were investigated using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results: Among three different types of ZnO nanostructures with a concentration range (0-7.5 wt%), the sensor with 0.75 wt% ZnO NRs nanofiller exhibits a maximum output voltage of 1.73 V under an external pressure of 3 N and a maximum sensitivity of 586.3 mV/N at the range of 0-3 N. Further, the sensor can generate a clear piezoelectric voltage under bending and twisting deformation as well as compression and tensile deformation. Conclusion: To summarize, the addition of different concentrations of nano-ZnO can remarkably improve the piezoelectric performance of the composite sensor, and ZnO NRs can achieve better piezoelectric properties of the sensor as compared to ZnO NPs and T-ZnO. In addition, the sensor with 0.75 wt% ZnO NRs as nanofiller has the highest piezoelectric response, which is about 2.4 times that of the pure PVDF-TrFE sensor. It is demonstrated that the sensor has great potential applications in wearable health monitoring systems and mechanical stress measurement electronics.

Enhanced recoverable energy storage density and breakdown strength in cation-site modified K0·5Bi0·5TiO3-based ergodic relaxor ferroelectric

Bi-containing ferroelectrics like Na0·5Bi0·5TiO3 (NBT) and K0·5Bi0·5TiO3 (KBT)-based materials are extensively investigated to achieve high recoverable energy storage density (Wrec) and efficiency (η). In this work, Bi-site modified KBT-based compositions have been studied for energy storage application at room temperature. KBT has been transformed from a non-ergodic to an ergodic relaxor state through the Eu3+ substitution. High breakdown voltage and slim polarization vs electric field loop are observed in 6 mol% Eu3+-substituted KBT, and high maximum polarization (Pmax) is retained along with low remnant polarization (Pr) in this composition. These features help to attain the Wrec and efficiency of 1.3 J/cm3 and 72%, respectively in 6 mol% Eu3+-substituted KBT at room temperature. This work demonstrates a strategy to enhance the Wrec and η by forming a stable ergodic relaxor state in the KBT-based material.

High corrosion resistance and through-plane electrical conductivity of C/Ti and C/Cr coated metal bipolar plates used in PEMFC

To improve the corrosion resistance and enhance the electrical conductivity in a simulated proton exchange membrane fuel cell, carbon bound SS316L stainless steel plates were modified with titanium (Ti) or chromium (Cr). The SS316L interfacial contact resistance of 312.38 mΩ·cm2 was reduced to 1.59 mΩ·cm2 when modified with Ti and to 1.32 mΩ·cm2 when modified with Cr. The increase in potential enhanced the shedding of C films and the oxidation and dissolution of Ti and Cr buffer layers. The corrosion current density in the C/Ti coating was lower than C/Cr coating under high voltage polarization. The oxidation of Ti and Cr buffer layers led to poor appetency between the C film and the SS316L and to degradation of the through-plane electrical conductivities in the C/Ti and C/Cr coated metal bipolar plates.

Is surface modification effective to stabilize high-voltage cycling for layered P2-Na2/3Ni1/3Mn2/3O2 cathodes?

Layered transition metal oxides (TMOs), like the P2-type Na2/3Ni1/3Mn2/3O2, are promising cathodes for sodium-ion batteries but suffer rapid capacity degradation at high voltages. Surface engineering is a popular strategy to modify the high-voltage stability of cathode materials, yet its efficacy for sodium layered TMOs remains elusive, especially given the deleterious layer-gliding phase transition during high-voltage operation. Here, we examined the effect of surface coatings on the high-voltage cycling stability of Na2/3Ni1/3Mn2/3O2, finding that they suppress high-voltage polarization but do not significantly affect capacity retention, which is mainly impacted by bulk structure degradation. Hence, surface engineering must be complemented with bulk structure modification to stabilize high-voltage cycling.

Self-poled PVDF/recycled cellulose composite fibers utilizing cellulose nanocrystals to induce PVDF β-phase formation through wet-spinning as a flexible fabric piezoelectric sensor

In general, PVDF (Polyvinylidene fluoride) needs polarization treatment to exhibit its piezoelectric properties. Introducing nanofillers or applying stretching during the processing can promote the β-phase transformation in PVDF, achieving self-polarization. Herein, by incorporation of cellulose nanocrystals (CNCs), continuous β-phase enriched PVDF/cellulose composite fibers are prepared via wet spinning and drawing, in which the cellulose is recycled from waste cotton textiles to improve the wet spinnability of PVDF and reduce the cost. The obtained composite fibers are woven into fabrics and subsequently assembled into a flexible sensor, exhibiting excellent piezoelectric performance (d33 = 26.2 pC/N) without requiring additional polarization treatment. It generates an open-circuit voltage of 12 V and a short-circuit current of 100nA, much higher than those of the previously reported piezoelectric PVDF sensors with high-voltage polarization. This fabric-based flexible sensor also exhibits notable characteristics, including stable output signals and heightened sensitivity, thereby fulfilling the demands for monitoring both minute and extensive human movements. Furthermore, it shows great potential for widespread use in sound sensing for human-machine interfaces. The self-poled PVDF composite fibers offer the advantage of being easily woven together with other fiber materials, which contributes towards the development of self-powered flexible piezoelectric fabric sensors.

Propagation of a pulsed nanosecond discharge on a water surface in non-symmetrical configuration, and comparison with the symmetrical configuration

Non-thermal plasmas produced by pulsed nanosecond discharges at atmospheric pressure are of great interest for fundamental as well as technological and environmental applications due to their high reactivity. When generated in air in contact with water, these discharges induce many physical and chemical phenomena at the interface, including pattern formation. Although the patterns generated in symmetrical configuration have been extensively studied, those produced by asymmetrical discharges are not well characterized. In this study, we report the propagation dynamics of a nanosecond discharge produced in air in contact with water using electrodes mounted in parallel direction relative to the water surface (i.e. asymmetric configuration). The influence of the high voltage polarity and water electrical conductivity on the discharge pattern is investigated using fast imaging and electrical diagnostics. The obtained results demonstrate that under positive voltage polarity, plasma dots are produced along the ionization front. These dots have been previously observed in symmetrical configuration; however, their propagation velocity is greater in asymmetrical configuration, particularly in front of the anode. Under negative polarity conditions, a homogeneous emission pattern is observed, except in the area in front of the cathode, where dots are detected in the ionization front. Based on this data, the E-field threshold beyond which plasma dots are formed is estimated to be ∼5 × 108 V m−1. Overall, the results reported herein provide a fundamental understanding of plasma-water interactions.

Flexible regenerated cellulose/ZnO based piezoelectric composites fabricated via an efficient one-pot method to load high-volume ZnO with assistance of crosslinking

Flexible piezoelectric film has broad application in the energy devices and sensor field. Herein, a highly effective “one-pot method” was proposed to fabricate regenerated cellulose/ZnO piezoelectric composites, in which the preparation of regenerated cellulose (RC), in-suit polymerization of ZnO in RC and its cross linking can be carried out simultaneously. ZnO were in-situ synthesized from Zn2+ reduced by alkali from the dissolve system of cellulose. Simultaneously, the added zinc salt accompanied with acid degradation has promoted the dissolution of cellulose, existing a synergistic effect. In this synthetic process, ECH was used to crosslink cellulose molecules to obtain cross-linked regenerated cellulose (CRC), which enhanced regenerated cellulose framework for more uniform hydroxyl groups as growth sites, thereby further elevate growth content of piezoelectric ZnO distributed uniformly in the composite film. Consequently, the piezoelectric constant (d33) of the developed piezoelectric composite film with excellent tensile strength and flexibility reaches 26.8 ± 6.4 pC N−1. Without external high-voltage polarization, its piezoelectric output open-circuit voltage (VOC) and short-circuit current (ISC) was increased to 6.98 V and 610.27 nA. After polydimethylsiloxane (PDMS) was immersed in the CRC/ZnO aerogel to be CRC/ZnO/PDMS PENG, the piezoelectric output performance was further enlarged to be 10.02 V and 1032.57 nA, respectively, even with a high output stability. It can drive a LED and a small electronic screen, as well as has high sensitivity to pressure excitation, displaying high VOC and ISC up to 17 V and 1300 nA under the trampled state, respectively. The strategy has brought highly efficient assembly and high performance for the piezoelectric nanogenerator (PENG), directly providing a reference to other nanocellulose based PENG.

A soft piezoelectric elastomer with enhanced piezoelastic response

This work aims to study, develop, and validate a soft piezo-polymer with enhanced piezo-elastic response and easy castable in a free shape through a single and easy process. The work identified a novel formulation for soft piezopolymers based on ambient temperature polymerizable silicone rubber, easily fabricable in 3D printed plastic moulds. Combining polymerizable silicone with a barium titanate (BaTiO3) ceramic powder and defining a detailed fabrication procedure of casting, curing and high voltage poling, we defined how to obtain a promising soft piezoelectric elastomer for countless sensing applications. This study includes information about the mould design used to realize, cure and polarize cylindric elastomeric specimens. This piezopolymer stands out for its flexibility, softness, easy fabrication at ambient temperature and obtainability in multiple shapes and bulky 3D geometries. Finally, we investigated different configurations of the piezopolymer formulation analysing the powder concentration and voltage polarization effects over the mechanical, piezoelectric and morphological characteristics. The specimens exhibit a high induced polarization with values up to 22.5 pC N−1, comparable with poled -phase polyvinylidene difluoride. We finally underlined limits encountered in the most extreme configurations.

Enhanced Piezo-Photocatalytic Performance of Na0.5Bi4.5Ti4O15 by High-Voltage Poling.

The internal electric field within a piezoelectric material can effectively inhibit the recombination of photogenerated electron-hole pairs, thus serving as a means to enhance photocatalytic efficiency. Herein, we synthesized a Na0.5Bi4.5Ti4O15 (NBT) catalyst by the hydrothermal method and optimized its catalytic performance by simple high-voltage poling. When applying light and mechanical stirring on a 2 kV mm-1 poled NBT sample, almost 100% of Rhodamine B solution could be degraded in 120 min, and the reaction rate constant reached as high as 28.36 × 10-3 min-1, which was 4.2 times higher than that of the unpoled NBT sample. The enhanced piezo-photocatalytic activity is attributed to the poling-enhanced internal electric field, which facilitates the efficient separation and transfer of photogenerated carriers. Our work provides a new option and idea for the development of piezo-photocatalysts for environmental remediation and pollutant treatment.

Reducing the Charging Voltage of a Zn–Air Battery to 1.6 V Enabled by Redox Radical-Mediated Biomass Oxidation

Zn–air batteries (ZABs) are one of the promising candidates of future energy storage technology owing to their advantages of high theoretical energy density, high safety, and low cost. However, high voltage polarization and low energy efficiency hinder their practical applications. Herein, we show that the charging voltage of a ZAB can be reduced to ∼1.6 V with a high energy efficiency of ∼70% by adding a redox radical, 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO), and a biomass, glucose, into the electrolyte. Upon charging, the oxidation of TEMPO forming the oxoammonium cation at the cathode catalyzes the oxidation of glucose to generate value-added derivatives. Operando differential electrochemical mass spectrometry, first principle calculations, and ex situ spectroscopic characterizations demonstrate the significant effects of TEMPO and glucose on inhibiting side reactions and dendrite growth of the Zn anode, which endow the TEMPO-mediated ZABs with long-term charging/discharging cycles over 400 h.

Conductive Polymer Coated Layered Double Hydroxide as a Novel Sulfur Reservoir for Flexible Lithium-Sulfur Batteries.

Lithium-sulfur battery (LSB) is widely regarded as the most promising next-generation energy storage system owing to its high theoretical capacity and low cost. However, the practical application of LSBs is mainly hampered by the low electronic conductivity of the sulfur cathode and the notorious "shuttle effect", which lead to high voltage polarization, severe over-charge behavior, and rapid capacity decay. To address these issues, a novel sulfur reservoir is synthesized by coating polypyrrole (PPy) thin film on hollow layered double hydroxide (LDH) (PPy@LDH). After compositing with sulfur, such PPy@LDH-S cathode shows a multi-functional effect to reserve lithium polysulfides (LiPSs). In addition, the unique architecture provides sufficient inner space to encapsulate the volume expansion and enhances the reaction kinetics of sulfur-based redox chemistry. Theoretical calculations have illustrated that the PPy@LDH has shown stronger chemical adsorption capability for LiPSs than those of porous carbon and LDH, preventing the shuttling of LiPSs and enhancing the nucleation affinity of liquid-solid conversion. As a result, the PPy@LDH-S electrode delivers a stable cycling performance and a superior rate capability. Flexible battery has demonstrated this PPy@LDH-S electrode can work properly with treatments of bending, folding, and even twisting, paving the way for wearable devices and flexible electronics.

Novel manufacturing method for highly flexible poly(lactic acid) foams and ferroelectrets

Poly (lactic acid) (PLA) foams have demonstrated a high variety of functional characteristics, still, the rigidity of this cellular material remains a major limiting factor when it comes to implementation options. In this contribution, PLA foams with outstanding flexibility were created for the first time by a new approach of uniaxial stretching and immediate relaxation following supercritical CO2-assisted extrusion foaming. Instead of improving the resilience of the PLA raw material, structural elasticity of the foam was achieved via altering the deformation mechanism from cell wall collapse or rupture towards reversible and extensive flexural strain. In addition, PLA foams with excellent piezoelectric properties were also achieved via high-voltage corona poling, giving additional function to the lens-like anisotropic foam cells. This foaming technology creates the opportunity to produce PLA piezoelectrets in a way entirely different from the state-of-the-art methods. Correlation between the tensile as well as compression elongations and moduli, cell morphology and longitudinal piezoelectric coefficients (d33) of electretized foam samples were studied. Unprecedented reversible tensile elongations of up to 16% and total elongations of up to 35% were reached, as well as considerable d33 values in the range of 50–320 pC/N were obtained for PLA ferroelectrets.

Design and fabrication of self-powered flexible P(VDF-TrFE)/ZnO/TiO2 fiber mats as nanogenerator for wearable applications

Polymer-based nanocomposite fibrous membranes are the basis of wearable flexible devices, but the application of high voltage poling, brittle material as dopants, or toxic compounds is the main challenge in the development of such devices. P(VDF-TrFE)/ZnO/TiO2 electrospun nanofiber mats are used in here to fabricate self-poled, lead-free, and flexible piezoelectric nanogenerators. Herein, self-poled, lead free, and flexible high performance piezoelectric nanogenerators (PENGs) are designed using the P(VDF-TrFE)/ZnO/TiO2 electrospun fiber mats. Three phase nanocomposite mats with varying TiO2 concentrations are fabricated using far-field electrospinning method. In this study, we reported the effect of ZnO and TiO2 nanoparticles on the viscoelastic property and piezoelectric output. The electroactive beta (β) phase fraction of P(VDF-TrFE) mats are observed to be enhanced as a result of the dispersion of TiO2 nanoparticles into the three phase nanocomposite films. The dielectric and viscoelastic properties are improved with the addition of TiO2 nanoparticles. The designed piezoelectric device produced short circuit current, open circuit voltage, and piezoelectric peak power of 4.16 μA, 23 V, and 95.68 μW, respectively. These piezoelectric outputs are obtained with the optimization of TiO2 concentration and the PENG devices are subjected to elbow bending, wrist bending, and finger tapping. The piezoelectric output of three phase composite mats is almost 3.6 times higher than PENG made from P(VDF-TrFE) fiber, and the increment in the performance is due to the synergistic effect of ZnO and TiO2 in the P(VDF-TrFE) polymer matrix. PENG is used to display real-time demonstrations of body movement detection and biomechanical energy harvesting to power an LED bulb. The enhanced performance and robustness of the nanogenerator make the electrospun P(VDF-TrFE)/ZnO/TiO2 mat an excellent candidates for sensing and energy harvesting applications.

Optical and Electrophysical Properties of Vinylidene Fluoride/Hexafluoropropylene Ferroelectric Copolymer Films: Effect of Doping with Porphyrin Derivatives.

Polymer films doped by different porphyrins, obtained by crystallization from the acetone solutions, differ in absorption and fluorescence spectra, which we attribute to the differences in the structuring and composition of the rotational isomers in the polymer chains. According to the infrared spectroscopy data, the crystallization of the films doped with tetraphenylporphyrin (TPP) proceeds in a mixture of α- and γ-phases with TGTG- and T3GT3G- conformations, respectively. Three bonds in the planar zigzag conformation ensures the contact of such segments with the active groups of the porphyrin macrocycle, significantly changing its electronic state. Structuring of the films in the presence of TPP leads to an increase in the low-voltage AC-conductivity and the registration of an intense Maxwell-Wagner polarization. An increased conductivity by an order of magnitude in TPP-doped films was also observed at high-voltage polarization. The introduction of TPP during the film formation promotes the displacement of the chemical attachment defects of "head-to-head" type in the monomeric units into the surface. This process is accompanied by a significant increase in the film surface roughness, which was registered by piezo-force microscopy. The latter method also revealed the appearance of hysteresis phenomena during the local piezoelectric coefficient d33 measurements.

3D ordered amorphous and porous TiO2 framework anode with low insertion barrier and fast kinetics for K-ion hybrid capacitors

TiO2 is considered as a low cost, long-term stable, and safe anode for high power K-ion hybrid capacitors (KICs) due to its abundant reserve, small volume expansion rate, and sloping voltage plateau that avoids K-ion plating at high voltage polarization. However, the enhancement of its low capacity and sluggish kinetics caused by poor electroconductivity and high insertion barrier is still challenging to further develop high-performance KICs. Herein, the reduced graphene oxide (rGO) is embedded in the walls of 3D ordered macro-/mesoporous TiO2 (termed as TiO2@rGO framework) to create intimate TiO2/rGO interfaces, ensuring the effectively electron transportation during potassiation/depotassiation of TiO2 while maintaining rapid ions/electrolyte diffusion. Furthermore, the controlled amorphous TiO2 framework can further lower the lattice insertion energies, contributing to a fast accommodation of K-ion. As expected, the amorphous TiO2@rGO framework (TiO2@rGO-1) exhibits a superior rate capability (148.8 mAh g−1 at 5 A g−1) and cycling stability (171.2 mAh g−1 at 1 A g−1 after 800 cycles). The assembled KICs can reach a high energy/power density of 125.2 Wh kg−1/4267.4 W kg−1 as well as a long-term lifespan. This tactic provides a reliable and general way to design a TiO2-based anode with fast kinetics toward high-performance KICs.

Tuning discharge voltage by Schottky electron barrier in P2-Na2/3Mg0.205Ni0.1Fe0.05Mn0.645O2

Recently, Mg doped Na metal oxide layered cathode compounds have attracted strong interest for Na-ion battery applications. Here a new type of asymmetric phase evolution between charge and discharge is found to show much-enhanced discharge voltage in P2-Na2/3Mg0.205Ni0.1Fe0.05Mn0.645O2 over the parent cathode compound. P2 solid solution is found to show an abnormal coexistence with O2-P2 two-phase reaction during discharge with simultaneous reduction of O, Ni, and Fe redoxes, distinct from the conventional P2-O2 two-phase reaction in the charge. Our analysis suggests that the P2 and O2 two-phase boundary forms a novel unidirectional Schottky barrier to impede electron and Na diffusions in discharge only, thus making the kinetically preferred P2 solid solution phase abnormally coexist with the two-phase region for high discharge voltage and low polarization. Our work demonstrates tuning dynamic evolution of electronic Schottky barrier as a new dimension for advanced kinetic design of high-performance battery cathode materials.

Fluorine-containing ferroelectric polymers: applications in engineering and biomedicine

Highly polar fluorine-containing ferroelectric polymers based on vinylidene fluoride refer to the class of electroactive polymers suitable for various engineering applications. Under usual crystallization conditions, polycrystalline texture is formed, while after high-voltage polarization, it is transformed into a non-centrosymmetric structure, which gives rise to high piezoelectric and pyroelectric responses. Such materials can be used for pyroelectric energy conversion. Owing to high impact strength, ferroelectric polymers can serve as materials for alternative energy sources that convert dynamic impact energy into electricity. The non-linearity of electric response (owing to ferroelectricity) and high breakdown fields offer opportunity of using them in capacitive charge storage devices. The intense dynamics of the amorphous phase provides a rapid discharge of the stored energy into an external circuit. In view of the considerable fraction of the amorphous phase ( 50%) in fluorine-containing ferroelectric polymers and high dielectric permittivity of the amorphous phase, it is possible to change the system entropy by applying an electric field. In the case of pulsed field, this provides a temperature drop; therefore, these materials (especially as composites) are promising for the design of solid-state refrigerators. The high thermoplasticity of these polymers is useful for the design of sensors <i>e.g.</i>, microphones) with a patterned active membrane surface and enhanced characteristics. Biocompatibility and low acoustic impedance (close to the impedance of biological tissues) accounts for the possibility of using these polymers as biosensors. When domains are present in a ferroelectric film, strong local electric fields appear, which can affect the activity of living cells. Using the direct and reverse piezoelectric effect, it is possible to stimulate and control the functions of various tissues, including the nervous tissue. <br> The bibliography includes 463 references.

Quantum Dot Hybridization of Piezoelectric Polymer Films for Non-Transfer Integration of Flexible Biomechanical Energy Harvesters.

This work presents a low-temperature engineering strategy, from quantum dot (QD) synthesis to fabrication of a hybrid from a homogeneous dispersion to thermal annealing with elaborate use of a small organic molecule dopamine, for achieving a kind of ZnO QD-hybridized piezoelectric polymer film directly integrated into a flexible electrode and a plastic substrate. This strategy is the key for non-transfer assembly of flexible piezoelectric nanogenerators (FPENGs) with both mechanical robustness and high electrical performance via direct lamination. The rational addition of dopamine plays multiple roles of (1) significantly decreasing the size of ZnO particles to a QD level (3.77 nm), (2) formation of a stable and homogeneous dispersion of a ZnO QDs/piezoelectric polyvinylidene fluoride-co-hexafluoropropylene copolymer for uniform hybridization of a piezoelectric film, and (3) increment of the piezoelectric phase via induced crystallization at a low annealing temperature. This dopamine-assisted low-temperature annealing strategy for a hybrid piezoelectric film with a high d33 value (∼31.56 pC/N, 30.56% larger than that of a pure piezoelectric polymer film) required no additional high-voltage polarization treatment and effectively avoided the delamination, distortion, or melt phenomenon between the piezoelectric layer, flexible electrode, and plastic protective layer caused by the high temperature and thermal stress. The obtained FPENGs showed significantly enhanced output performance and mechanical robustness under repeated impact and large amounts of strain conditions. Their specific output voltage and charge density were stably maintained at 7.16 V and 2.40 nC/cm2, which were 30.7 and 50.0% higher than those of FPENGs based on a pure piezoelectric polymer film, respectively. They were further used as biomechanical energy harvesters for generating electricity to charge capacitor energy storage devices for power electronics and self-powered sensors for visual motion-detecting systems, indicating their promising applications in both wearable technology and smart homes.

The Puzzles in Fast Charging of Li‐Ion Batteries

Fast charging of Li‐ion cells faces two aspects of challenges, 1) accelerated capacity fade and 2) inferior charging capability. It is commonly believed that the former is due to Li plating and its resultant reactions with electrolyte at the graphite anode, which results in a loss in the inventory of Li+ ions and an increase in the cell’s impedance. While the latter is ascribed to the high voltage polarization in relation to the slow transport of Li+ ions between two electrodes. However, there are many other hidden facts that essentially affect the fast charging performances of Li‐ion cells. This commentary intends, from the view of materials, to uncover these hidden factors, including failure of the solid electrolyte interphase and exfoliation of the graphite structure at the anode, structural degradation of the Ni‐rich layered cathode materials, as well as the high solvation and desolvation activation energies of Li+ ions in the electrolyte. Meanwhile, some solutions to the fast‐charging problems of Li‐ion cells are proposed based on the understanding of these hidden factors.