Recent Progress on Structure Manipulation of Poly(vinylidene fluoride)‐Based Ferroelectric Polymers for Enhanced Piezoelectricity and Applications

Poly(vinylidene fluoride) (PVDF)‐based polymers demonstrate great potential for applications in flexible and wearable electronics but show low piezoelectric coefficients (e.g., −d33 < 30 pC N−1). The effective improvement for the piezoelectricity of PVDF is achieved by manipulating its semicrystalline structures. However, there is still a debate about which component is the primary contributor to piezoelectricity. Therefore, current methods to improve the piezoelectricity of PVDF can be classified into modulations of the amorphous phase, the crystalline region, and the crystalline–amorphous interface. Here, the basic principles and measurements of piezoelectric coefficients for soft polymers are first discussed. Then, three different categories of structural modulations are reviewed. In each category, the physical understanding and strategies to improve the piezoelectric performance of PVDF are discussed. In particular, the crucial role of the oriented amorphous fraction at the crystalline–amorphous interface in determining the piezoelectricity of PVDF is emphasized. At last, the future development of high performance piezoelectric polymers is outlooked.


Introduction
3][4] When applying a mechanical stress, the charge density on the surface of piezoelectric materials changes, and this is called the "positive or direct piezoelectric effect" (Figure 1a).While the inverse piezoelectric effect is observed as the deformation of piezoelectric materials in response to an electric field (Figure 1b).Piezoelectric ceramics show high piezoelectric coefficients, especially those that have a morphotropic phase boundary (MPB). [5,6]For example, 0.55Pb(Ni 1/3 Nb 2/3 )O 3 -0.135PbZrO 3 -0.315PbTiO 3 (PNN-PZT) has a large-signal piezoelectric strain coefficient d 33 * (i.e., the ratio of the maximum strain and the maximum electric field) of ≈1753 pm V −1 near their MPB, [7] and the lead-free strontium (Sr)-doped (K, Na)NbO 3 (KNN) can reach a d 33 * as high as 2100 pm V −1 . [8]owever, piezoceramics are brittle in nature and the leadcontaining ones are toxic, which limits their applications in flexible and implantable electronic devices.Unlike ceramics, polymers have good flexibility, lightweight, processability, and biocompatibility, [9][10][11] showing great potential to fill the application gap for piezoelectric ceramics.Nonetheless, even for poly(vinylidene fluoride) (PVDF), which is believed to have the highest piezoelectricity among all existing piezoelectric polymers, its piezoelectric coefficients are quite low (i.e., absolute values < 30 pC N −1 ). [12,13]Therefore, it cannot meet the requirements for practical piezoelectric applications.
Due to the chain flexibility, PVDF is semicrystalline and has five different crystal forms, , , , , and . and  forms are nonpolar phases, and , , and  forms are ferroelectric polar phases (Figure 2a-e). [14,15]These crystal forms share three different conformations, i.e., the trans-gauche (TGTG′) conformation (Figure 2f), the all-trans (TTTT) conformation (Figure 2h), and the intermediate T 3 GT 3 G′ conformation (Figure 2g).Among these, the  form with the TTTT conformation is the most desired crystalline phase for piezoelectricity, since all fluorines and hydrogens locate on the opposite sides of the chain,   [15] Copyright 2012, American Chemical Society.
contributing to high polarity. [16,17]In the past, researchers have tried to induce  crystals to increase piezoelectric performance by incorporating different types of inorganic fillers, such as clay, [18,19] carbon nanotubes, [20,21] graphene oxide (GO), [22] and ZnO. [23]Although effective in improving the piezoelectric coefficients, the introduction of a large amount of inorganic fillers sacrifices the flexibility of PVDF, preventing the application of PVDF in flexible and wearable electronics.Therefore, it is desirable to improve the piezoelectric property of PVDF without adding inorganic fillers.
To achieve this goal, one promising way is to manipulate the bulk structures of PVDF.Nonetheless, semicrystalline PVDF typically has three components, i.e., the crystalline phase, the amorphous phase, and the crystalline-amorphous interphase (Figure 3a).Debate still exists on which component is the primary contributor to the piezoelectricity of PVDF. [24]To understand the origin of the piezoelectric behavior in PVDF-based polymers, different microscopic models have been developed.In the dimensional model, the piezoelectricity of polymers comes from the change of dipole moment density or polarization (P). [25,26]herefore, the amorphous region is believed to play an important role in the piezoelectric behavior of PVDF, because the amorphous phase is more sensitive to volumetric deformation than rigid crystals.However, the electrostriction model gives a different story.This model claims that it is the electrostriction from the crystalline phase that mainly contributes to the piezoelectricity of PVDF. [27]Afterward, a modified electrostriction model was proposed by Katsouras et al. [28] In this model, the piezoelectric effect is not only enabled by the change in lattice constants, but also affected by the coupling between the intermixed regions of the crystalline lamella and the amorphous phase.These models explain the piezoelectric behavior to certain extent, but do not reach an agreement on the physical origin of the piezoelectricity of PVDF.Therefore, research about manipulation of the bulk structure of PVDF to improve piezoelectricity is classified into three categories, i.e., the modulation of the amorphous phase, the crystalline-amorphous interface, and the crystalline phase (Figure 3b).
According to the dimensional model, the amorphous phase should play an important role in changing the volume of the sample.Therefore, decreasing the degree of crystallinity might be a good way to improve the piezoelectric property of PVDF.However, the truth is not this case.In terms of the crystalline phase, most researchers pay attention to induce the formation of the ferroelectric  phase in PVDF.][31][32][33] It is believed that the increased content and/or orientation degree of the  crystals contribute to a high piezoelectric performance. [34,35]owever, by merely inducing the  crystals, the maximum piezoelectric coefficient d 33 can only reach around −30 pC N −1 in most studies.Recently, a MPB-like behavior was discovered in poly(VDF-co-trifluoroethylene) [P(VDF-TrFE)] copolymers with the VDF content of about 50 mol%, and it was found that near the MPB, the piezoelectric coefficient d 33 increased to −63.5 pC N −1 . [36]Nonetheless, although the piezoelectric coefficient could be greatly improved for P(VDF-TrFE) via the MPB mechanism, PVDF, which is more commonly used in industry, still shows low piezoelectric coefficients and the MPB mechanism cannot be applied.To further improve the piezoelectric property of PVDF, we recently conducted a study on the modulation of the crystallineamorphous interface in a biaxially oriented PVDF (BOPVDF).It was found that a large amount of the oriented amorphous fraction (OAF) located at the crystalline and amorphous interface contributed to a largely increased piezoelectric coefficient d 33 of −62 pC N −1 . [24,37]Later, a recent study demonstrated that it was the relaxor-like secondary crystals formed in OAFs (SC OAF ) with an extended-chain crystal (ECC) structure that gave high piezoelectric performance. [30,38]n this review, the basic principles and various measurement methods for piezoelectric coefficients of ferroelectric polymers are first introduced.Then, approaches to improve piezoelectric property are discussed, regarding the structural manipulation of different components of PVDF, as shown in Figure 3.For the amorphous phase, some pioneer studies agreeing with the dimensional model are discussed.The effect of the thickness variation and Poisson's ratio on the piezoelectric response are reviewed.For the crystalline region, recent studies on inducing the  crystals by stretching, electrospinning, and chemical modifications are discussed in detail.Methods such as electric poling and thermal annealing are briefly introduced.Some new ideas to explain the piezoelectric behavior of PVDF-based materials, such as the MPB behavior, are reviewed.For the crystalline-amorphous interface, the updated understanding of the physical origin of  -g) Reproduced with permission. [24]Copyright 2021, Springer Nature.
the piezoelectricity of PVDF is discussed.Especially, the crucial role of OAF and SC OAF with an ECC structure in determining the piezoelectricity of PVDF is summarized.Finally, outlooks are given for the future development of piezoelectric properties of ferroelectric polymers.

Basic Principles and Measurements of Piezoelectric Coefficient for Polymers
Piezoelectric coefficients, d ij , are important parameters to quantitatively characterize the piezoelectric effect.Basically, the higher the d ij , the better the piezoelectricity.For d ij , the first subscript i refers to the direction of polarization or the applied electric field, and the second subscript j indicates the direction of the applied stress or the induced strain.The  phase of PVDF has a C 2v symmetry, and the d ij for this symmetry is shown as [39][40][41] The axis i can be 1, 2, 3, which is the polarization direction along the x, y, and z axis, respectively.The axis j can be 1, 2, 3 with 1 being the drawn direction, 2 the transverse direction, and 3 the normal direction of the film.Stresses 4, 5, 6 represent the shear stresses along the 1, 2, and 3 directions.They are illustrated in Figure 4a.If the sample is biaxially oriented with the same ori-entation degree in both directions, the crystal symmetry becomes C ∞v .Their d ij are similar to those of the uniaxially oriented sample, except that d 31 = d 32 and d 15 = d 24 . [41]any methods have been developed to measure d ij , including the frequency resonance method, the laser interferometry method, and the quasistatic method. [42]Among them, the quasistatic method (also known as the Berlincourt method) is the simplest and could be realized by simply using a commercial piezoelectric d 33 meter.The working mechanism of a d 33 meter is shown in Figure 4b,c, where a dynamic force (F dynamic ) is applied to the sample, and the charge generated on the sample (Q sample ) is measured by a signal detector.Then, the d 33 can be simply calculated using the following equation This quasistatic Berlincourt method is widely used to measure the d 33 or d 31 for ceramics.For soft polymers, this method shows certain issues because most commercial d 33 meters can only provide a fixed peak value of F dynamic (F d,peak ), while the direct d 33 of polymers usually increases with F d, peak . [24]Therefore, the d 33 at different dynamic forces cannot be accurately measured for soft polymers using the commercial d 33 meter.Meanwhile, the d 33 often decreases with increasing the static force for soft polymers, as shown in the inset of Figure 4c.This is attributed to the fact that the applied static force can decrease the amount of available dipoles for the piezoelectric effect in response to the dynamic effect. [24]A similar situation is also observed for piezoelectric ceramics, however, at a much lower level. [40]Therefore, one should be cautious when using the d 33 piezometer to measure piezoelectric coefficients for soft polymers.
To address this issue, some new methods with varying F d,peak are developed to measure the piezoelectric response of polymers.Usually, F d,peak can be provided by force generators, and the voltages or charges are recorded by an electrometer or an oscilloscope. [43,44]The d 33 could be calculated according to Equation (2).Since these methods are easy to set up, a lot of studies employ them to measure the piezoelectric coefficients of PVDF.However, a recent study by Šutka et al. indicated that a contact electrification between the piezoelectric materials and the force generator can inevitably produce some interfacial friction, which would generate triboelectric charges and influence the piezoelectric measurement. [45]Unfortunately, quite many studies paid little attention to the overlapping piezoelectric and triboelectric effects, and counted the electrical signal from triboelectric charges into the piezoelectric charge, [46] resulting in an exaggerated d 33 .
To obtain an accurate d 33 , the triboelectricity should be avoided as much as possible.
Recently, we have designed a testing device to measure the direct d 33 of polymers to avoid the triboelectricity. [24]The schematic diagram of the device is shown in Figure 4d.The test sample was first coated with Au on both sides of the film as electrodes.To minimize the friction, a Au-plated copper disk that had the same area as the overlapping Au electrodes was put on top of the sample, since the triboelectricity is the smallest between two identical metallic materials.To insulate the sample, a flat glass disk was placed on top of the Au-coated copper disk.Moreover, a metal rod guided by a homemade fixture was placed on top of the glass disk to make sure the sample was in its right position to further avoid any friction during the test.The dynamic force and the generated charge were detected by a force sensor attached at the bottom of the metal rod and a Keithley electrometer, respectively.Then, d 33 could be obtained using Equation (2).It is noted that the Keithley electrometer is more preferred than an oscilloscope because it has a much higher input resistance (>200 TΩ) than that (≈1 MΩ) of the oscilloscope.This high input resistance could prolong the decay time of the voltage spike and a more accurate value of peak voltage could be obtained.To verify the removal of triboelectricity, a fresh PVDF film without any electrical poling (i.e., nonpiezoelectric) was tested, and no charge was generated after applying a force of 0.98 N (Figure 4e).Then, a poled PVDF was tested, and different charges were detected under different dynamic forces (Figure 4f).These results demonstrated that our piezoelectric test setup could efficiently avoid the triboelectricity.Using this setup, a d 33 ≈ −18 pC N −1 was obtained for PVDF below 0.1 MPa, which is similar to ≈21 pC N −1 determined by a commercial d 33 piezometer.This indicated the accuracy of our measurement.Moreover, compared to the commercial d 33 piezometer, our test setup showed the ability to measure d 33 under different dynamic stresses, which is more desirable for practical applications.Therefore, the test setup developed in our previous work provides a simple yet reliable way for direct d 33 measurement of polymer piezoelectrics.Following a similar principle, a test setup for d 31 (and d 32 ) was also designed in our previous study (Figure 4g), and a stress-dependent d 31 /d 32 could also be obtained. [24]ry recently, Wang and co-workers also paid attention to the triboelectricity during the piezoelectric measurement.They developed a method, called the compressed balance analysis (CBA), to quantitatively identify and extract the piezoelectric charge from a mixture of frictional and piezoelectric signals. [46]First, they divided the generation of the triboelectric-piezoelectric hybrid signal into six stages, i.e., contacting, contacted, compressing, releasing, released, and separating stages (Figure 5a).In the compressing stage, there is an electrostatic equilibrium between the frictional and piezoelectric charges, and such equilibrium could be classified into positive polarization (model I) and negative polarization (model II), as indicated in Figure 5b.These two models could be expressed using the following equations From Equations ( 3) and ( 4), Q p could be expressed as where q and q′ could be obtained from results in Figure 5c,d.The d 33 could be measured by the following equation Using this CBA method, they reported a d 33 of 33.7 pC N −1 for a PVDF film, which is consistent with the results of other outsideshielding method (33.2 pC N −1 ) and directly extracting method (32.6 pC N −1 ). [46]Then, they claimed that this CBA method can evaluate the piezoelectric coefficient correctly.
The above methods are based on the direct piezoelectric effect and special attention should be paid to minimize triboelectricity.Based on the inverse piezoelectric effect, the piezoelectric coefficient can be obtained by measuring the strain (S) curves as a function of the electric field (E). [47]The working mechanism of this method is shown in Figure 5e, [48] where a voltage generator applies an electric field to the piezoelectric material, and then the induced strain is detected by a high-resolution displacement sensor, such as the photonic sensor (or called Fotonic sensor, MTI optical fiber measurement system, fiber-optic probe), [48][49][50] laser displacement sensor (or called laser Doppler vibrometers), [51,52] and linear variable differential transformer (LVDT). [29,53]Generally, two different inverse piezoelectric coefficients are obtained from the S-E loops, i.e., the d ij calculated as the slope the S-E loops at low electric fields (below the coercive field, E c ), [30,36] and the largesignal d ij * measured at high electric fields using the equation, [47] d ij * = S max /E max , where S max and E max are the maximum strain and the maximum electric field, respectively.The d ij is accurate to evaluate the intrinsic piezoelectricity.The large-signal d ij * is fundamentally incorrect, since it includes strains from nonlinear ferroelectric switching, which in most cases overestimates the piezoelectric coefficient. [30]However, d ij * is still used by some researchers for piezoelectric ceramics since it is a parameter for overall actuation. [47]In this context, there can exist large deviation  , and Q p are the total charge in the Kapton layer, the total charge in the Al plate, and the induced piezoelectric charge, respectively.q and −q′ are the charge transfer between electrodes in the positive and the negative polarizations, respectively.The transferred charges between electrodes are measured from c) the positive and d) the negative polarization sides of the PVDF-based device with an applied force of 60 N. (a-d) Reproduced with permission. [46]Copyright 2022, Springer Nature.e) Schematic diagram of the inverse piezoelectric effect measurement by Fotonic sensors.Reproduced with permission. [48]Copyright 2005, Taylor & Francis Inc. f) Schematic representation of the S-E loop measurement fixture to transverse strain responses.Reproduced with permission. [50]Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA.g) Determination of the inverse d 31 using bipolar S 1 -E loops for QSAP films at room temperature.h) Two continuous bipolar S 1 -E loops after 60 cycles of unipolar poling at 100 MV m −1 : coP-55/45 QSAP.The poling frequency is 1 Hz with a sinusoidal waveform.(g, h) Reproduced with permission. [30]opyright 2021, Elsevier Inc. i) Amplitude hysteresis loop of the PZT-doped PVDF film.Reproduced with permission. [82]Copyright 2022, Elsevier Ltd.
between the direct piezoelectric coefficients measured by using the d 33 piezometer and the d ij * calculated from S-E curves.
Note that the electric-field-induced stain in the thickness direction of polymer thin films (10-100 μm thick) is in the nanometer scale.Therefore, it is challenging to accurately measure such small strain using the displacement sensors.As a result, S-E curves are much less frequently used for piezoelectric polymers than ceramics.We attempted to measure the inverse d 33 of the 8 μm thick BOPVDF in our previous work [24] using a Fotonic sensor, but unable to obtain accurate data due to the small strain induced in the thickness direction of the BOPVDF by low electric fields.To overcome this, we later modified the device and employed a long sample stripe to test the strain in the transverse (or 1) direction (as shown in Figure 5f), [50] resulting in an inverse piezoelectric coefficient d 31 of 77 ± 5 pm V −1 for P(VDF-TrFE) under a low electrical field of 10 MV m −1 (Figure 5g). [30]We also em-ployed a high electric field (100 MV m −1 ) to measure the inverse d 31 * and fount it was as high as 360 ± 10 pm V −1 (Figure 5h), indicating that the nonlinear ferroelectric switching gave additional contribution to the strain.Very recently, Chen et al. employed the LVDT to measure the inverse piezoelectric coefficient of a relaxor ferroelectric polymer. [29]Unlike traditional piezoelectric PVDF or P(VDF-TrFE), the piezoelectric test process for relaxor polymers requires the application of a DC bias because piezoelectricity is the electrostriction under a bias polarization. [29]In short, a highprecision displacement detector is required when using the S-E curve to measure the inverse piezoelectric coefficients, and the testing electric field needs to be chosen carefully below the E c to avoid the interference of nonlinear ferroelectric switching.
Table 1 summarized the piezoelectric coefficients obtained through various methods.Most studies show that the d ij of PVDF or its co-and ter-polymers are below −30 pC N −1 (or pm V −1 ).Recent studies have reported some high d ij values, such as ≈−60 pC N −1 in BOPVDF with OAF, [24] and P(VDF-TrFE) near the MPB, [36] and even up to ≈1000 pm V −1 in the relaxor ferroelectric PVDF tetrapolymer, [29] which will be discussed later.It is worth noting that most studies on polymers only employ one method to test the piezoelectric coefficients, making it difficult to compare the direct d ij with the inverse d ij , or to compare the d ij obtained from different methods.On the other hand, some studies on piezoelectric ceramics have simultaneously measured both the direct and the inverse d ij , [8,54] and found a significant difference between them, which is also summarized in Table 1.For example, the direct d 33 of BaTiO 3 measured by the Berlincourt d 33 meter is 413 pC N −1 , while the inverse d 33 obtained from the S-E curves is 870 pm V −1 . [54]It seems that the inverse d ij * is higher than the direct d ij .However, as we mentioned above, the d ij * involves nonlinear ferroelectric switching and thus is not the genuine piezoelectric coefficient.Future studies on piezoelectric polymers should aim to measure both the direct and inverse piezoelectric coefficients using different methods for comprehensive understanding and comparison.Piezoelectric response force microscopy (PFM) can also be employed to detect the inverse piezoelectric coefficients of PVDF. [79,80]Specifically, the sample is first prepared on a conductive substrate, which is grounded to the atomic force microscope (AFM) stage.Then, PFM is operated in a contact mode with additional alternating voltage acting to the conductive tip.The deformation of the sample induced by the voltage is monitored by the conductive tip, and the resulting oscillations of the cantilever are measured by a lock-in amplifier. [81]This method can characterize the local electromechanical properties at the nanoscale, which is important with the miniaturization of piezoelectric devices in recent years.For instance, Dong and co-workers employed the PFM technique to measure a PZT-doped PVDF-polymer nanocomposite, and a classic "butterfly shape" amplitude loops were observed, as shown in Figure 5i. [82]The piezoelectric coefficient d 33 can be calculated from the following equation where A, V, A 0 , and V 0 represent the amplitude, the voltage, the amplitude, and voltage values at the intersection, respectively.Using Equation ( 7), the d 33 was calculated as 22 pm V −1 . [82]It was reported that the PFM technique has very high lateral (≈10 nm) and vertical (≈1 pm) resolutions, and its development has achieved tremendous progress. [83]However, the charged scanning probe of the scanning probe microscopy shows a long-range electroelastic interaction with the piezoelectric samples.][85] Especially, the stray capacitance is much higher for nanoscale samples and should be subtracted when measuring d 33 using PFM. [86,87]n summary, there are several methods to measure the piezoelectric d ij for polymers.For direct d ij measurements, the key to obtain accurate values is to avoid the influence of triboelectricity.For inverse d ij measurements from S-E loops, a professionalmade fixture and a high-resolution displacement sensor are necessary to detect small strains of polymers.Additionally, the applied electrical field should not be too high to avoid the nonlinear ferroelectric switching.For PFM measurements on nanosized samples, stray capacitance is often much higher than the piezoelectric response, and it has been challenging to obtain accurate data without subtracting the stray capacitance.Future development of the accurate measurement methods for d ij is still needed, which is simple and has less limitation to the nanodimension of test samples.

Structure Manipulation of PVDF-Based Polymers to Improve Piezoelectric Properties
Manipulation of the semicrystalline structures of PVDF is a promising way to improve the piezoelectric performance.Since there is still a debate on which component of the polymer primarily contributes to the piezoelectricity, research has mainly focused on modulating the following three components: the amorphous phase, the crystalline phase, and the crystalline-amorphous interphase.

The Amorphous Phase
According to the dimensional model, the amorphous phase is the main component contributing to the piezoelectricity of PVDF; [25,26] therefore, the effect of the amorphous region on the piezoelectricity was studied by many researchers.However, research in this area was only active in early years, and recently more attention was paid to the modulation of crystalline phase and the crystalline-amorphous interface or interphase, as will be discussed later in Sections 3.2 and 3.3.Here, only some pioneer works discussing the piezoelectric response of PVDF caused by the amorphous phase are reviewed.
In 1978, Broadhurst et al. proposed that the largest contribution to the piezoelectric activity of ferroelectric polymers came from the dimensional change rather than changes in the molecular dipole moment. [88]Further investigation revealed that the thickness variation of the PVDF film contributed 2/3 of the d 33 , and the remaining 1/3 came from the variation in the dipole moment change of a constant thickness film. [25]This result proved that the amorphous region was the main component that contributed to the piezoelectricity of PVDF.It is worth noting that the thickness variation in the piezoelectric response is unique for polymers because inorganic materials are much stiffer in nature. [25]he thickness variation is highly dependent on a property known as the Poisson's ratio, which is defined as the negative transverse strain divided by the axial strain in the stretching direction. [89]From this understanding, the Poisson's ratio was thought to be an important factor to influence the piezoelectricity of PVDF.In a study by Sussner, it was proposed that the temperature-dependent Poisson's ratio could be used to explain the sudden drop of piezoelectric coefficients below the glass transition temperature (T g ). [90]In his study, Poisson's ratio above T g was as high as 0.6 for the uniaxially oriented PVDF film, because the lamellar structure showed a high shear compliance above the glass transition.While below T g , the Poisson's ratio dropped to 0.2-0.3since the chain mobility was largely frozen.The smaller Poisson's ratio below T g suggests that the thickness variation is difficult, which is the main reason for the decreased piezoelectric coefficients at low temperatures.A study by Wada and Hayakawa also proved the role of the Poisson's ratio in determining the piezoelectric response of PVDF. [26]They developed a two-phase model with polar spheres embedded in a nonpolar medium to calculate the piezoelectric stress constant e 31 .It was found that the contribution of the Poisson's ratio amounted to 78% of e 31 and was the main reason for the temperature dependence of e 31 .Tanaka et al. studied the pressure dependence of e 31 , and found the e 31 decreased with increasing pressure. [91]They believed that the decrease of e 31 came from the decrease of the Poisson's ratio due to the increase in bulk modulus of the amorphous phase, which agreed well with Wade's two-phase model theory.In addition, Tasaka and Miyata investigated the effect of the Poisson's ratio and dielectric electrostriction constant on piezoelectric d 33 of PVDF films, and found that the Poisson's ratio had the greatest influence on the piezoelectric constant. [89]Therefore, in order to improve the piezoelectricity, it is important to tailor the structure of piezoelectric films with a high Poisson's ratio.Afterward, Furukawa et al. investigated the origin of the piezoelectric property of P(VDF-TrFE) and also suggested that the piezoelectric activity was mainly from the influence of the macroscopic size effect. [92]ater, a similar conclusion was proposed by Wen. [93]n summary, the above studies demonstrated that the amorphous phase played an important role in determining the piezoelectricity of PVDF.It is worth noting that in the dimensional model, the amorphous region is the main factor but not the only factor that influences the piezoelectricity of polymers, especially for d 33 in the normal direction of the stretched polymers.The contribution from the crystalline phase and the crystalline-amorphous interphase cannot be ignored, which will be discussed later, especially for d 31 along the stretching direction of the ferroelectric polymers.

The Crystalline Phase
Presently, regulation of the crystalline structure of ferroelectric polymers is widely used to improve piezoelectric properties.This could be achieved by various strategies, including stretching, electrospinning, chemical modification, annealing, electrical poling, etc.

Stretching
Early in 1969, Kawai reported that a stretched and poled PVDF film could show a piezoelectric effect. [94]After that, a lot of works were carried out to study the influence of different stretching parameters on the piezoelectricity of PVDF.These parameters include stretching temperature, stretching rate, stretching ratio, stretching direction, etc. [14,95,96] From these studies, it is widely accepted that a suitable stretching condition could induce the ferroelectric  crystals in PVDF, which contribute to a high piezoelectricity after electric poling. [14]Note that the electric poling is essential to macroscopically align the dipoles/domains to induce the piezoelectricity for ferroelectric polymers.Without electric poling, the PVDF film with even 100%  phase could not show any piezoelectric behavior. [24]In this part, effects of different stretching parameters on the formation of  crystals and the piezoelectric coefficients of PVDF are discussed.
Stretching temperature is one of the most important parameters to influence the crystalline phase of PVDF.[99] For instance, Sajkiewicz et al. reported that when stretching PVDF at a temperature between 50 and 145 °C, the nonpolar  phase could transform into the polar  phase. [96]The highest content of  phase was observed at a temperature (87 °C) close to the peak temperature of the  c relaxation, which is the relaxation process of the CF 2 -wagging motion related to the  crystals. [37,100] similar optimized temperature to maximize the content of  phase was observed by Salimi and Yousefi. [101]They employed Fourier transform infrared spectroscopy (FTIR) to investigate the effect of stretching temperature on the content of the  phase in PVDF.They found that 90 °C was the optimized temperature to get the highest  phase content (75%).The preferred temperature range to induce the formation of  crystals changes with the composition (e.g., comonomer contents) and processing conditions of the original PVDF films.Recently, we prepared a hot-pressed and quenched poly(VDF-co-chlorotrifluoroethylene) [P(VDF-CTFE)] film, [102] and found that the quenched (Q) samples contained mixed  and  crystals (Figure 6a) with the corresponding conformations of TGTG′ and T 3 GT 3 G′ (Figure 6b).After low temperature stretching of the quenched film (QS) between −20 and 0 °C, a large amount of  crystals are induced (Figure 6c).It was believed that stretching at low temperatures gave a strong internal stress to transform the / phases into the  phase.Such a stress was strong enough to pull CTFE defects into the crystalline-amorphous interfaces. [102]he stretching ratio is another important parameter to influence the generation of the  phase in PVDF (Figure 6d).Some study even believed that the it was more effective than temperature in promoting the formation of  crystals in PVDF. [101]Gaur et al. found that with increasing the stretching ratio, the  phase gradually transformed into the  phase in a PVDF film. [95]This was because the high stretching ratio could help align the polymer chains, which favors the crystallization into the more compact  phase with all-trans conformation.They also found the highest content of the  phase, 75%, was achieved at a stretching ratio of 5.After proper treatment and polarization, this sample showed a high piezoelectric coefficient of −30 pC N −1 .A similar conclusion was proposed by Wang et al. [103] They prepared a 90 nm PVDF film by coating PVDF on a stretchable poly(vinyl alcohol) (PVA) substrate using the Langmuir-Blodgett deposition method.As the stretching ratio increased from 2 to 3, the  phase content increased from 69% to 75%, and the d 33 of the stretched and poled ultrathin PVDF film could reach to −37 pm V −1 .It is noted that not all studies claimed a monotonic relationship of the stretching ratio on the content of  crystals.In a study by Guo et al., the content of the  phase first increased markedly and then decreased slightly when stretching the PVDF film to a ratio of 5, and the maximum  phase content of 88.2% was obtained at a draw ratio of 3. [104] The stretching rate also affects the formation of  crystals.Mohammadi et al. studied the effect of stretching rate on the formation of  crystals in a PVDF blown film. [32]It was found that by increasing the stretching rate from 10 to 50 cm min −1 , the content of  crystals increased by about 6%, and the highest  crystal content of 83.6% was achieved at a stretching rate of 50 cm min −1 .After electric poling, the PVDF sample with the highest content of  crystals showed a d 33 of −31 pC N −1 .Similar positive correlation between the stretching rate and  phase content was also proposed by other studies. [96,105,106]From these studies, it could be concluded that a high stretching rate favors the formation of  crystals, thus improving the piezoelectric coefficient.
The stretching direction is also an important factor to influence the effectiveness of the phase transformation from the nonpolar  phase to the polar  phase.Compared to uniaxial stretching, biaxial stretching can achieve uniform thicknesses for PVDF films and obtain better isotropic piezoelectricity. [107]From a study by Mohammadi et al., the polarized BOPVDF films showed balanced piezoelectric activities in the film plane as compared to uniaxially oriented films, which then contributed to a higher piezoelectric coefficient. [32]Ting et al. found that the biaxial stretching is easier to induce  crystals than uniaxial stretching. [108]From their study, the stretching ratio was 4 × 4 for biaxial stretching and 5 for uniaxial stretching to get a similar content of  crystals.However, the d 33 and d 31 values of the biaxially stretched PVDF films are inferior to the uniaxially stretched ones, [108] which is different from Mohammadi et al.'s study.
In summary, mechanical stretching is a promising strategy to induce the phase transition from  to  for PVDF films.Usually, the higher content of the  phase, the higher the piezoelectric coefficients.To increase the content of  crystals, it is highly desired to properly manipulate the stretching parameters.Usually,  (a-c) Reproduced with permission. [102]Copyright 2018, American Chemical Society.d) Schematic illustration of the uniaxial stretching process and structure development (phase transformation, crystalline orientation, and crystalline morphology) of PVDF films with increasing the stretch ratio.Reproduced with permission. [104]Copyright 2021, The Royal Society of Chemistry.
crystals transform into  crystals at temperatures below 90 °C, and such temperature range differs with the structure of PVDF (i.e., comonomers and the degree of crystallinity).The content of  crystals usually increases with increasing the stretching ratio and rate.For the stretching ratio, attention should be paid to avoid the formation of structural defects.For the stretching direction, it is believed that biaxial stretching can bring a more uniform film thickness and is more effective to induce  crystals when using the same draw ratio as uniaxial stretching.111]

Electrospinning
Electrospinning is a common method to prepare polymer nanofibers and has both in situ stretching and electrical poling at the same time.Therefore, on one hand, it can induce the phase transition from  to  in PVDF.114][115] Therefore, electrospinning could be employed to prepare the piezoelectric PVDF. [31]To improve piezoelectric coefficients, a lot of studies have been carried out to modulate the processing parameters in electrospinning.These parameters include those for electrospinning apparatus, electrospinning solution, and environment. [116,117]Here, we will review how these parameters influence the piezoelectric properties.Since there are many existing reviews [9,118,119] on the effect of processing parameters of electrospinning on piezoelectricity, here we only review some recent studies in this field.
The electrospinning apparatus parameters include electric field, flow rate, spin distance, needle size, collecting drum speed, etc.They can influence the stretching process of PVDF fibers, then influence the content and orientation of  crystals as well as the piezoelectric properties.Singh et al. studied the effects of these parameters on the content of  crystals in electrospun PVDF fibers. [120]From their results, the spin distance had the most profound effect on the content of  crystals with a variation of 6.6%.Drum speed, electric field, and flow rate are the next important factors that affect the percentage of the  phase with variations of 6.3%, 5.9%, and 5.7% in the  content, respectively.Nugraha et al. also studied the effect of the spinning voltage on the content of  crystals, [121] and found that increasing the voltage could increase the content.
Electrospinning solution parameters include the PVDF concentration, the surface tension, polarity of the solvents, and so on.These parameters play a major role in influencing the morphology and surface structure of electrospun PVDF nanofibers.Usually, increasing the PVDF concentration results in an increased content of  crystals.For example, Zahari et al. employed PVDF with different molecular weights to obtain varied solution concentrations, and investigated the effect of PVDF concentration on the content of  crystals and the related piezoelectric coefficient. [122]It was found that the content of the  phase and piezoelectric coefficient of PVDF gradually decreased with the increased molecular weight.When the molar mass was 180 000 g mol −1 , the  phase content in PVDF films reached a maximum value of 80.25%, and the maximum piezoelectric coefficient d 33 was −21 pC N −1 .Zaarour et al. also studied the effect of molecular weights of PVDF on the content of  crystals. [123]From their study, the  phase content gradually increased with increasing molar mass of PVDF, which is different from Zahari et al.'s study. [122]They concluded that as the molecular weight increased, the viscosity of the polymer solution increased, which prolonged the time for the solution to travel from the tip to the rotating collector.Therefore, the molecular chain had enough time for stretching, resulting in an increased content of the  phase in PVDF.More studies are needed to figure out the effect of the PVDF concentration on the  phase content in the future.
In addition to the solution concentration, the effect of the solvent polarity on the  phase content and piezoelectric performance of PVDF is also investigated.It was accepted that increasing the solvent polarity could result in a higher crystallinity, a higher  phase content, and a greater piezoelectric output. [124]ntroducing ionic liquids into the electrospinning solution can increase the polarity of solvents and then promote the generation of  crystals.From Asai et al.'s study, incorporation of an ionic liquid with only 0.25 wt% can promote the generation of  crystals. [125]However, the ionic liquid had a negative effect on the piezoelectric properties of PVDF because the ionic liquid is nonvolatile and will remain in the prepared electrospinning fibers.Since ionic liquids are ionically conductive, they would bring significant dielectric loss to PVDF, which leads to dissipation of the electric energy and thus decreased output voltage.To address this issue, Asai et al. employed protic organic solvents to replace ionic liquids, [126] and found that the protic organic solvent helped to align fluorine atoms and promoted the formation of  crystals during the electrospinning process.Moreover, protic organic solvents would evaporate before fiber deposition on the collector drum, thus it would not have a negative effect on piezoelectric properties of the PVDF nanofibers.
The environmental parameters include temperature and relative humidity (RH), which usually have influences on the content of  crystals by affecting the solvent evaporation time.Zaarour et al. investigated the effect of RH on the  phase content. [127]t was found that when RH increased from 2% to 62%, the content of  crystals increased from 55% to 73.1%, and the piezoelectric properties increased accordingly.Similar results were reported by Kong et al. [128] in a humidity controlled electrospinning system (Figure 7a).As shown in Figure 7b, the  crystal content gradually increases with the increasing of RH.The piezoelectric output voltage also shows a similar increasing tendency with the increased RH.As shown in Figure 7c, the output voltage increased from 14 to 234 V when RH increased from 10% to 70%.However, it is worth noting that the RH cannot be too high because once exceeding a "critical value," the evaporation of solvents would be impeded, which may cause a failure for the electrospinning process. [129]n summary, electrospinning is effective in producing selfpolarized piezoelectric nanofibers because of the high stretching force exerted on the electrified solution jet.In order to improve the piezoelectricity of PVDF nanofibers, the electrospinning process should be optimized by modulating the electrospinning apparatus parameters, electrospinning solution parameters, environmental parameters, etc.

Chemical Modification
Besides stretching and electrospinning, chemical modification is also important to increase the fraction of  crystals in PVDF.One method is to employ dehydrofluorination (DHF) reaction to promote the TTTT conformation.The DHF reaction leads to the loss of hydrogen fluoride from PVDF, leading to the formation of carbon-carbon double bonds in the backbone. [130,131]These bulky and planar double bonds have significant steric effects on the polymer chains with TGTG conformation while having a negligible influence on the TTTT conformation chains. [132]As a result, the formation of planar TTTT conformation is promoted.
The DHF agents could be strong bases, such as sodium hydroxide and potassium hydroxide. [133,134]However, these agents may induce an overreaction of DHF, generating conjugated double bonds and even carbon-carbon triple bonds. [130]This would hinder the crystallization of PVDF and thus reduce the piezoelectric properties.To better control the DHF reaction, Lin et al. employed a weak organic base, ethylene diamine (EDA), as the DHF agent, and then induced double bonds and cross-linked structures subsequently. [135]From Figure 8a, the percentage of dehydrofluorinated VDF units in PVDF (%DHF) increases linearly with the reaction time.The carbon-carbon double bond fractions are very close to the %DHF values when the reaction time is within 12 h, indicating that majority of the EDA-induced DHF leads to double bond formation in the early stage of the reaction (Figure 8a).Further study revealed that the relative fraction of planar chain conformation started to increase when the %DHF increased to 5.43% (Figure 8b).When %DHF reached 10.2%, the fraction of planar zigzag chain conformation reached a saturation of 82.3 ± 2.1%.Further increase of %DHF did not show any significant effect on the fraction of planar zigzag conformation in dehydrofluorinated PVDF.From the X-ray diffraction (XRD) measurement, the untreated PVDF was dominated by the  phase, and when the %DHF is 10.3%, only  and  phases The inset is a schematic operating principle of the TENG device in the vertical contact-separation mode.Reproduced with permission. [128]Copyright 2020, American Chemical Society.
were observed.The piezoelectric properties were also measured, and the drawn and undrawn dehydrofluorinated PVDF samples showed d 31 of 35.1 ± 0.7 and 25.1 ± 1.1 pC N −1 , respectively, [135] higher than 21.9 ± 0.3 pC N −1 of a commercial uniaxially drawn PVDF film without any DHF reaction (Figure 8c).Wang et al. also proved the positive effect of the DHF reaction on the piezoelectric performance of PVDF. [136]From their study, the content of the  phase could reach to 92.1% at the DHF reaction time of 5 h.At the same time, the piezoelectric output voltage increased from ≈10 to ≈30 V.
[139] These monomers include TrFE, 1,1-chlorofluoroethylene (CFE), CTFE, and so on.Among them, TrFE is one of the most promising monomers that can greatly improve the piezoelectricity of PVDF, since it can decrease the Curie transition temperature (T c ) lower than the melting point (T m ) when its content is higher than 18 mol%. [140]It was reported that below the T c , P(VDF-TrFE) always shows a ferroelectric phase with a TTTT conformation, regardless of the processing method used. [24]Therefore, a high piezoelectric performance can be expected for P(VDF-TrFE).[143][144][145][146] Most of them consider that increasing the content of the ferroelectric phase can increase the piezoelectric coefficients.This seems consistent with the case of improving the piezoelectric coefficient for PVDF as the  content increases.In this part, we will introduce some new understanding about the piezoelectric behavior of P(VDF-TrFE).
Liu et al. discovered a MPB-like behavior in organic materials for the first time.They proposed that near the MPB, P(VDF-TrFE) would show the highest d 33 , just like the ceramic piezoelectric materials.They synthesized a batch of P(VDF-TrFE) random copolymers with the VDF content ranged from 45 to 80 mol%, and then investigated their tacticity, conformation, and crystalline structures. [36]From Figure 8d, there existed a critical VDF content of 49 mol% where the tacticity distribution of the  (a-c) Reproduced with permission. [135]Copyright 2020, American Chemical Society.(d-g) Reproduced with permission. [36]Copyright 2018, Springer Nature.(h, i) Reproduced with permission. [147]Copyright 2022, American Chemical Society.
TrFE-TrFE segment changed.Such a change caused the competition between the trans-planar and 3/1 helical phases, similar to the MPB in oxide perovskites.XRD results further confirmed the existence of MPB-like behavior in P(VDF-TrFE) (Figure 8e,f).From the piezoelectric measurement, a maximum d 33 of −63.5 pC N −1 was achieved near MPB with the VDF content of 50 mol% (Figure 8g).Except for the discovery of MPB, Katsouras et al. [28] and our group [30] also proposed some new understandings about the piezoelectric behavior of P(VDF-TrFE).However, these studies considered that the piezoelectric properties were not only contributed by the crystalline region, but also by the crystallineamorphous interactions or interfaces, so they will be reviewed in the next section.
In addition to TrFE, other monomers were also introduced to improve the piezoelectricity. [29,147]Note that these monomers are usually introduced into P(VDF-TrFE) random copolymers, rather than PVDF homopolymer, since TrFE is essential to pre-expand the interchain distance of PVDF to accommodate other termonomers in the crystalline region. [102,148]For instance, we previously tried to include CTFE monomer into PVDF crystals directly without the help of TrFE, but unfortunately, this cannot be realized even under high pressure or high mechanical stretching. [102]an et al. introduced CTFE monomers into P(VDF-TrFE) and investigated the effect of the CTFE content on the microstructure, chain conformation, and piezoelectric coefficient. [147]As shown in Figure 8h, a similar MPB-like behavior was found in  [29] Copyright 2022, The American Association for the Advancement of Science.
P(VDF-TrFE-CTFE) terpolymers with the CTFE content being 1.7-5.0mol%, where both ferroelectric and relaxor ferroelectric phase coexisted.When the CTFE content was 2.4 mol%, the d 33 reached a maximum value of −55.4 pC N −1 (Figure 8i).They also believed that it was the appearance of the MPB that contributed to high d 33 .However, since P(VDF-TrFE-CTFE) has a low degree of crystallinity, they also mentioned that the contributions from the amorphous region and the crystalline-amorphous interface cannot be ignored.
Very recently, Zhang and co-workers introduced a small amount of fluorinated alkyne (FA) into P(VDF-TrFE-CFE), and studied the effect of FA on the electrostrictive (without any bias voltage) and piezoelectric (under a bias voltage) properties of the P(VDF-TrFE-CFE-FA) tetrapolymer. [29]It was believed that the FA units were included in the crystalline regions and further enhanced the electrostriction.From the polarization-electric field (P-E) loop measurements (Figure 9a,b), FA defects in the stretched tetrapolymer (s-tetrapolymer) lowered the local polarization switching, which resulted in a higher polarization response at electric fields below 60 MV m −1 (Figure 9c) and led to large electrostriction.As a result, under a low-DC bias field of 40 MV m −1 , the s-tetrapolymer with 1.9 mol% FA showed a high d 33 of ≈1050 pm V −1 and electromechanical coupling coefficient k 33 of 88% (Figure 9d).These values are even higher than those of PZT piezoceramic, making the s-tetrapolymer attractive for practical applications such as energy harvesting, sensors and actuators in soft robots and wearable electronics, and transducers for ultrasonic imaging. [29]n summary, the chemical modification of PVDF is an effective way to induce the conformation change in the crystalline phase.The DHF reaction generates carbon-carbon double bonds, providing a steric effect to the TGTG conformation, then promoting the formation of the TTTT confirmation.It is usually believed that the more TTTT confirmation (or the  phase), the higher the piezoelectric response for PVDF-based polymers.However, the extent of DHF reaction should be suitable to avoid the formation of carbon-carbon triple bonds or other defects.The copolymerization method also intends to induce more TTTT conformation.Recently, some new understandings have been proposed to explain the piezoelectric behavior of PVDF-based polymers, such as inducing a MPB-like behavior or decreasing the energy barrier for conformational changes.These new findings advance our understanding about the piezoelectricity of PVDF and brought about significantly boosted piezoelectric performance.

Other Methods
In addition to above methods, thermal annealing is also a common method to improve the piezoelectricity of the PVDF-based polymers.The temperature and time are the two most important parameters for annealing to influence the piezoelectricity. [149,150]sually, the annealing temperature is set between the T c and the melting temperature (T m ), where the polymers are in their paraelectric phase and chains show high sliding mobility in the crystals. [151]It is noted that T c for PVDF is higher than T m under the ambient pressure, while incorporation of more than 18 mol% TrFE can lower the T c below T m . [140]Therefore, annealing is more often to be employed for P(VDF-TrFE) to modulate the crystalline structure, rather than PVDF.For instance, Omote et al. annealed a stretched P(VDF-TrFE) film in the hexagonal phase (i.e., between T c and T m ) and obtained an extended-chain "single crystalline" (SC) sample with no amorphous phase.From their study, the poled SC film showed larger electrochemical coupling factors than those of conventional folded-chain crystal films. [152,153]For the annealing time, usually increasing the time can increase the lamellar thickness and degree of crystallinity. [154,155]However, it is worth noting that not all thermal treatments can induce phase transitions, and some only results in an increased degree of crystallinity (e.g., PVDF). [154,156]Furthermore, the rate at which the materials cools after annealing plays a significant role in the  phase formation. [69]When the annealed PVDF is cooled rapidly, the amount of  phase increases. [157]For instance, Bormashenko et al. found a significant enhancement of the peak intensity of the  phase in PVDF films with rapid cooling compared to samples prepared with slow cooling. [158]n recent reports, annealing is often combined with other processes (e.g., spin coating, stretching, electrospinning, etc.) and used as a postprocessing treatment.For instance, Shaik et al. annealed spin-coated PVDF films at a low rotation speed of 1000 rpm at 100 °C, and found that the  phase content was significantly enhanced. [159]When the rotation speed reached 9000 rpm where a large amount of  phase had already formed, the subsequent annealing did not enhance the  phase content.Mahdi et al. annealed the spin-coated P(VDF-TrFE) films and found that an annealing temperature of 100 °C (slightly above T c ) could induce a highly crystalline  phase with a rod-like crystal structure. [160]Bae and Chang annealed a torsion-stretched P(VDF-TrFE) fiber and found that annealing-cooling could increase the relative ferroelectric phase fraction (i.e., the ratio of the crystalline part over the sum of the amorphous and crystalline parts) from 48.29% to 60.78%. [157]Annealing is also employed to treat electrospun PVDF nanofibers, and numerous studies have found that both the -phase content and piezoelectricity of the nanofibers are enhanced.For instance, Satthiyaraju and Ramesh found that annealing PVDF nanofibers at 100 °C for 4 h increased the crystallinity and  phase content from 43.17% and 58.29% to 49.61% and 87.37%, respectively.The d 33 increased from −10.5 to −16.2 pC N −1 . [57]For P(VDF-TrFE) (>18 mol% TrFE) with 100% ferroelectric phase, annealing increases the crystallinity.For example, Baniasadi et al. annealed P(VDF-TrFE) nanofibers at 130 °C (i.e., between T c and T m ) in the paraelectric phase for 2 h to increase the ferroelectric phase content through increasing the crystallinity from 25% to 52%, which increased the piezoelectric coefficient of a single fiber from 22 ± 1.6 to 35.5 ± 3.4 pm V −1 . [69]t should be noted that in some studies, annealing can decrease the piezoelectric coefficients because during annealing, the mobility of molecular chain segments increases, causing the oriented dipoles in the ferroelectric crystals to depolarize and then resulting in a decrease in the macroscopic dipole moment. [57]owever, the situation is different for electrospun fibers.To explain this, some studies have been carried out.In a study by Baniasadi et al., it was demonstrated that annealing electrospun P(VDF-TrFE) nanofibers above T c not only increased the crystallinity, but also gave chances for the crystalline phase to reorient and align, leading to an overall enhancement of the total dipole moment and a larger piezoelectric coefficient. [150]In another study by Ramasundaram et al., they demonstrated that annealing the electrospun PVDF nanofiber removed the residual solvent rapidly and thus increased the degree of crystallinity. [161]oreover, they also believed the oriented polymer chains formed during electrospinning would not change its orientation state during annealing, and they enable an easy switching of dipoles that contributed to a good piezoelectric response.In general, as shown in Table 2, annealing is a simple and effective method to enhance the ferroelectric phase content and is applicable to enhance the piezoelectricity of polymers.
Poling is another commonly used approach to induce the formation of  crystals.Actually, except for electrospinning, all methods reviewed above need to combine with a poling process to align the ferroelectric domains to induce the macroscopic polarization and thus piezoelectricity for ferroelectric polymers. [24,169]Corona poling and contact poling are two most common poling methods.Corona poling is a technique used to create a uniform electric field within a dielectric material by exposing it between two electrodes with asymmetric geometry (such as a point electrode and a plate electrode) under high voltage. [167,170]Through corona poling, the content of the  crystals would be increased according to literatures. [171]For instance, Li et al. performed corona poling on P(VDF-TrFE) formed by solution spraying at a voltage of 35 kV. [163]The strong electric field induced the alignment of dipoles, which improved the ferroelectric phase by reorienting the molecular chains and reducing the number of gauche defects.This resulted in an increased amount of the TTTT conformation, i.e., the ferroelectric crystals.Recently, a modified corona poling device has been developed by replacing the single needle electrode with multiple needles. [167]his modification increases the poling area over the sample, resulting in P(VDF-TrFE) nanofibrous mats with a high d 33 value of −20.8 pC N −1 .It has been observed that corona poling is more effective than electrical poling in transforming the  phase into the  phase for PVDF. [171,172]For example, we treated a BOPVDF film with corona poling using a needle voltage of 20 kV and a grid voltage of 5 kV for 5 min, and compared it to contact poling at 200 MV m −1 for 3 h.Our results showed that the contact poling was about 56% effective as the corona poling in aligning the  dipoles. [171]By performing contact poling at elevated temperature and corona poling on PVDF films, Jiang et al. also         The F() represents the ferroelectric phase content.
found that the  phase content was higher after corona poling (67%) than after contact poling (60%). [172]However, compared to contact poling, the corona poling is easy to introduce conductive charge carriers, such as electrons and plasma ions, into the films, as we discussed before. [171]Moreover, some studies found that corona poling would decrease the crystallinity of PVDF. [60]herefore, contact poling is more often used to prepare piezoelectric PVDF films.Parameters that can affect the poling results include electric field, time, temperature, etc. [37,173] Among these, the electric field is particularly important.Usually, the higher the field, the higher  phase content will be induced. [24]However, the electric field cannot exceed the breakdown strength of PVDF.Otherwise, an early failure of the film will take place.It was reported that the DC breakdown strength of PVDF is much higher than the AC breakdown strength since the AC poling involves bidirectional dipole rotation that generates significant heat, leading to thermal breakdown. [174]Therefore, to achieve a high enough electrical field to maximize the polarization, the DC voltage or unidirectional electric poling is more attractive. [24,37]n addition, to achieve a high poling electrical field, the film thickness (usually in 10-100 μm) should be controlled as thin as possible since the voltage amplifier can only output a limited voltage (e.g., 10 000 V). Hartono et al. employed a dip-coating and annealing method to prepare a PVDF film with a thickness of 1 μm and found that the electric poling field can reach to 2 GV m −1 . [168]After the electrical poling, the  phase content increased from 58% to 83%.Poling time is also important.Usually, the piezoelectric coefficient first increases rapidly with the increased poling time and then reaches a saturated value. [175]Too long poling time should be avoided.Otherwise, structural defects would be generated. [176]For the poling temperature, 20-120 °C is often chosen for PVDF-based polymers, [177] and usually the structure changes more efficiently when poled between T c and T m .The  phase content and the piezoelectric coefficients obtained from corona and contact poling are also summarized in Table 2.
Except for pure PVDF or P(VDF-TrFE) films, poling is also a crucial step to make PVDF-based nanocomposites exhibit piezoelectricity.However, PVDF-based polymers have negative piezoelectric coefficients, which may interfere with the overall piezoelectricity of the composite system when combined with piezoelectric ceramics that typically have positive coefficients. [178]Additionally, the poling process can cause a reduction in crystallinity. [179]Thus, careful manipulation of the bulk structure of PVDF-based polymers and poling parameters is essential to achieve optimal piezoelectric properties.It is noted that not all PVDF-based nanocomposites require a poling process to exhibit piezoelectricity.Some PVDF nanocomposites can exhibit piezoelectricity directly due to the self-polarization of nanofillers. [82,180,181]For instance, Guo et al. employed flow fields during microinjection to induce the gauche-trans transition and orientation of PVDF chains, and discovered that the dopaminefunctionalized carbon nanotubes could attract and immobilize neighboring short chains of PVDF and then suppress the molecular relaxation. [182]This resulted in the in-situ formation of rich electroactive  phase with preferential alignment characteristics and intrinsic self-oriented dipoles.While other examples exist, [183][184][185] this review concentrates mainly on the correlation between structure and piezoelectricity within various polymeric systems.Therefore, the discussion of piezoelectric PVDF-based nanocomposites will be limited here.
In addition to annealing and poling, some other methods are also developed to induce the transition from  to  phase, such as nanoconfinement, [186,187] Langmuir-Blodgett (LB) method, [188,189] and integrated 3D printing. [190,191]These methods also can improve the piezoelectricity of PVDF, but are mostly suitable for laboratory studies.Therefore, here we will only give a brief review of them.Nanoconfinement refers to confining PVDF molecules within a space at nanoscale, which is more likely to induce PVDF to form  crystals, rather than  crystals.This is because under limited spaces, PVDF chains are prone to arrange in a more ordered manner, which favors the formation of the more order  crystals over the disordered  crystals. [186,192,193]As for P(VDF-TrFE) with no phase transformation, Meereboer et al. reported that the nanoconfinement had an effect on its Curie transition, [194] which has a severe impact on its practical application (i.e., a high T c for ferroelectric applications and a low T c for capacitive energy storage and solid-state cooling devices).From their study, the polarity of the surrounding matrix shows a strong influence on the Curie transition temperature. [194]Therefore, when P(VDF-TrFE) is confined to finite nanospace, the polarity of the surrounding matrix should be considered carefully.The electrospinning mentioned earlier can be regarded as one method to achieve nanoconfinement where the confined space is provided by the nanofibers.Other methods for nanoconfinement includes, but not limited to, hot-pressing, [195] spin-coating techniques, [196] nanoporous-template-assisted crystallization, [187] and incorporating nanofillers. [197,198]For instance, Hussain et al. fabricated ultrathin PVDF nanoflakes with thickness down to 7 nm by using a hot-pressing method and found that the nanoconfinement provided by the sub-10 nm flakes induced the  phase fraction as high as 92.8% and excellent piezoelectric strain of 0.7%. [195]The 50 nm thick nanoflakes showed the highest d 33 of −68 pm V −1 , which is 13% higher than the piezoresponse from 50 nm thick PZT nanofilms.Bhavanasi et al. immersed a P(VDF-TrFE) solution in anodic aluminum oxide (AAO) membrane with a pore size of 200 nm, and then dissolved AAO by NaOH to obtain the P(VDF-TrFE) nanotube. [187]This nanotube could show an improved d 33 of −44 pm V −1 .There is a good review discussing the nanoconfinement of polymers on the piezoelectric response, [199] so we will not review in detail here anymore.
The LB technique is a method used to deposit thin films of molecules and polymers onto a substrate in a controlled manner. [200]It involves the transfer of monolayers of amphiphilic molecules from the surface of a liquid onto a solid substrate. [201,202]This is achieved by first spreading a thin layer of the amphiphilic molecules onto the surface of a liquid (usually water) in a trough, and then compressing the layer using a movable barrier to increase the surface pressure.When the surface pressure reaches a critical value, the molecules start to pack together to form a monolayer. [203]The solid substrate, usually a glass or silicon wafer, is then slowly dipped into the liquid, perpendicular to the surface of the monolayer, at a constant speed.As the substrate is withdrawn, the monolayer of molecules on the surface of the liquid adheres to the substrate, forming a single layer of closely packed molecules.This process can be repeated multiple times, resulting in the deposition of multiple layers of molecules onto the substrate, with precise control over the thickness of each layer. [204,205]For example, Zhu et al. employed amphiphilic poly(N-dodecylacrylamide) nanosheets as the confined matrix to prepare highly oriented PVDF LB nanofilms with adjustable film thicknesses ranging from several to hundreds of nanometers. [206]The resulting mixed LB nanofilms exhibited a dominant ferroelectric  phase of ≈95% and negligible paraelectric  phase.By applying a DC bias of 5 V through a conductive cantilever, Kelvin probe force microscopy observations revealed a local polarized pattern on the surface of a nine-layer mixed LB nanofilm, indicating that it is possible to induce all dipoles in one direction in the mixed LB nanofilm.They believed that mixed LB nanofilm was promising for application in low voltage nanoelectronics.However, due to the rigid deposition conditions and crystallization during annealing, PVDF-based LB films encounter significant challenges, such as excessive roughness and nonuniform characteristics. [207]These issues arise even in areas smaller than 100 μm 2 , which impedes the LB technique's potential to create good quality ferroelectric films and, in turn, limits the advancement of organic ferroelectric nanoelectronics.To address this issue, He et al. introduced a simple hot-pressing approach for producing PVDF LB films on a relatively large scale with an exceptionally smooth surface. [207]The root mean square roughness of the film is 0.3 nm over a 30 × 30 μm 2 area, which is comparable to that of a metal substrate.Their method maximized the potential of LB technique for controlling thickness distribution.
3D printing techniques (also called additive manufacturing) are another important method for preparing PVDF-based piezoelectric materials.They are designed to produce a series of array structures in a template-free manner by programming layerby-layer deposition procedures to achieve high-precision arbitrary shapes. [191]Among various additive manufacturing methods, the material extrusion additive manufacturing process, also known as fused filament fabrication or fused deposition modeling (FDM), is the most widely used due to its cost-effectiveness, ease of implementation, and time-saving properties. [208]When setting the nozzle temperature higher than the melting point of PVDF, FDM 3D printing can be used to prepare PVDF samples with various shapes.As discussed earlier, the formation of the  phase in pure PVDF requires the implementation of various parameters such as stretching, annealing, and poling.The FDM 3D printing can provide some stress similar to mechanical stretching and helps to induce the formation of  crystals; however, the stretching is not strong enough to induce large amounts of  crystals (>50%). [209,210]In this regard, researchers have focused on improving the -phase content of the printed PVDF.Some commonly used methods include adjusting the 3D printing parameters, using P(VDF-TrFE) with high ferroelectric phase content to replace PVDF, adding additives, etc.After achieving high -phase content, a postprocessing poling is needed to align the dipoles to exhibit macroscopic piezoelectricity.Therefore, the development of 3D printing equipment with integrated poling function has aroused widespread interest.Based on this concept, Kim et al. developed an "integrated 3D printing and corona poling process" (IPC) that utilizes the 3D printer's nozzle and heating bed as anode and cathode, respectively, to create controlled poling electric fields within a heated environment. [190]The nozzle follows a programmed path with a fixed distance between its tip and the sample's top surface.Simultaneously, the electric field between the nozzle and the bottom heating pad promotes the alignment of PVDF dipoles.The obtained IPC PVDF contains higher -phase content of 56.8% than 46.1% without the corona poling, resulting in an increased d 31 from 1.0 × 10 −3 to 4.8 × 10 −2 pC N −1 . [190]Another example about the poling-integrated 3D printing is carried out by Lee and Tarbutton, who developed an electric-poling-assisted additive manufacturing (EPAM) process to print piezoelectric PVDF devices. [211]In the EPAM process, molten PVDF chain undergoes simultaneous in situ mechanical stretching by the leading nozzle and electrical poling by the application of a high electric field under high temperature.This also leads to the transformation of  phase to  phase of PVDF.However, despite the formation of PVDF  crystals being promoted by using the poling-integrated 3D printing, the phase content and piezoelectric properties of the printed sample remain relatively low. [209,212]Replacing PVDF with P(VDF-TrFE) is considered as an effective approach.For instance, Ikei et al. used an in situ polarization process similar to EPAM to print and pole P(VDF-TrFE) (70/30) simultaneously. [213]The printed P(VDF-TrFE) samples had a crystallinity of 85.8% and a d 33 value of −18 pC N −1 , which is much higher than those of in situ polarized PVDF or PVDF/BaTiO 3 composites.Furthermore, constructing multilayer or special shape can also enhance the piezoelectricity of 3D printed PVDF-based devices. [214]Yuan et al. discovered that the six-layer and rugby-ball-shaped P(VDF-TrFE) 3D printed device showed a high d 33 of −130 pC N −1 , which is 6 times higher than the single-layer counterpart.The output power density is 22 times higher than that the flat-shaped counterpart. [215]n summary, 3D printing techniques, especially the FDM process, have great potential for preparing PVDF-based piezoelectric materials with high precision and cost-effectiveness.Improving the -phase content of printed PVDF is a key challenge, and various methods have been explored to address this issue.

Theoretical Calculations and Numerical Analysis of Piezoelectricity
The above sections have presented various studies on the crystalline structure of PVDF-based polymers and their influence on piezoelectricity.[221] For instance, Wang et al. utilized DFT to calculate the phase transformation barrier of PVDF under various electrical fields and found that the energy barrier for the  to  chain transition was ≈16.16 kJ mol −1 , while that from  to  was about 6.24 kJ mol −1 [222] Ranjan et al. employed DFT to investigate the phase diagram of PVDF and P(VDF-CTFE) as a function of the applied electric field and discovered that the nonpolar  phase was preferred by crystalline PVDF and ordered P(VDF-CTFE) up to 17% CTFE, and the polar  phase became thermodynamically favored under sufficiently high electric fields. [223]n a subsequent study, Ranjan et al. probed the nonpolar to polar phase transition as a sequence of operations: i) rotating one of the polymer chains by 180°in the rotational manifold of -PVDF yields -PVDF, and ii) -PVDF is a torsional manifold of -PVDF. [224]Kim et al. employed first-principle simulations to investigate the transition pathways from the  phase to the  phase in PVDF. [225]Among various possible pathways, they proposed two prototypical routes using the generalized solid-state nudged elastic band method.Route I, named the electric-field-induced transition path, involved transition from the  phase to the  phase and then allowing the  phase to transition to the parallel (polar) orientation  phase with a strong external electric field.Route II involved a direct transition from the  phase to the antiferroelectric  phase, followed by a rotation of the chain to transform to the parallel (polar)  phase.Except for PVDF, the conformational transformation of P(VDF-TrFE) was also studied. [226]or instance, Liu et al. employed DFT to calculate the energy differences between the 3/1 helix and the all-trans structures of P(VDF-TrFE) (C VDF = 40-60 mol%) and discovered that below the critical VDF content of 49 mol%, the 3/1 helix phase is more stable than the syndiotactic trans-planar phase. [36]heoretical simulations based on the structure and model of the  phase for predicting d 33 have also been conducted and received significant attention.First-principle quantum chemistry (QC) calculations, ab initio calculations, DFT calculations, and other methods have been applied and obtained simulation results that are close to the experimental values. [227,228]Early in 1990s, Karasawa and Goddard III developed a force field including the covalent shell model based on a combination of first principle QC calculations and experimental phonon frequencies of PVDF  crystals. [227]From their results, the d 33 was calculated as −18.8 pC N −1 , which is quite close to the experimental value (−20 ± 5 pC N −1 ).In 2013, Bystrov et al. simulated the dipole rotation and flipping under an electric field (U) and their resulted change in height (Δh) of the PVDF chain skeleton, then calculating the <d 33 > as −38.5 pm V −1 according to the equation, d 33 = Δh/U. [228]In addition, they simulated the variation of baxis of hexagonal symmetry unit cell, Δb, and obtained an average piezoelectric coefficient, <d 33 >, of −37.7 pm V −1 through another equation, d 33 = Δb/U.The "negative" value of d 33 in both calculation models is related to the redistribution of the electron molecular orbitals (wave functions) under an applied electrical field, which leads to the shifting of atomic nuclei and reorganization of all total charges to new, energetically optimal positions. [228]ystrov et al. also simulated the total polarization of PVDF (P) and obtained the value of the electrostriction coefficient (Q), and then d 33 of ≈−33.5 pC N −1 was obtained through the equation d 33 = 2Q 33  r  0 P. [228] These simulations provide valuable insights into the underlying mechanisms of the piezoelectricity in PVDF.In addition to d 33 , the piezoelectric stress constant, e 33 , can also be obtained using theoretical calculations.For example, Nakhmanson et al. calculated the e 33 using ab initio calculations based on a simple bond-dipole model. [229]The calculations for -PVDF were performed using an orthorhombic periodically repeated cell, containing four VDF monomers arranged into two chains in all-trans conformations.The e 33 for -PVDF was obtained as −0.332C m −2 .Moreover, by substituting a H atom by a F atom in one of the four VDF monomers in the cell, a 75/25 mol% P(VDF-TrFE) was constructed, and the e 33 for P(VDF-TrFE) was calculated as −0.211C m −2 .In addition to the above studies, theoretical calculations can also be obtained for shear piezoelectricity, [230] the effect of large elastic strain on piezoelectric properties, [231] the piezoelectric properties of PVDF-based composite piezoelectric materials, [232,233] and the effect of processing conditions on piezoelectric properties. [234]

The Crystalline-Amorphous Interphase
In addition to the amorphous and the crystalline phases, the crystalline-amorphous interface (or interphase) is also regarded as a component that can influence the piezoelectric activity of PVDF. [235,236]Early in 1980, Tashiro et al. utilized a theoretical method to calculate piezoelectric constants of PVDF based on a point charge model. [237]From their study, the contribution of the crystals to the macroscopic piezoelectricity was significant for d 33 M (the superscript M means macroscopic) but negligible for d 31 M .The d 31 M was found to be controlled by the electrical and mechanical coupling between the amorphous and crystalline phases. [236,237]In a study by Harnischfeger and Jungnickel, it was also found the dynamic and nonlinear piezoelectric properties were mainly contributed by the variation of the local polarization at the crystalline-amorphous interface. [235]These studies reached an agreement about the important role of the crystallineamorphous interface in determining the piezoelectricity.However, unlike the abundant studies about the crystalline phase, few studies have been conducted to explain the piezoelectricity in terms of the crystalline-amorphous interface after 1990s.Recently, more attention was paid to this area.
Katsouras et al. reported an interesting finding in 2016 to explain the origin of the negative piezoelectric coefficients of P(VDF-TrFE) by in situ dynamic X-ray diffraction measurement, and demonstrated the important role of crystalline-amorphous interfaces in influencing the piezoelectricity. [28]As shown in Figure 10a, the electroactuation strain as a function of the electric displacement (D) should be a hysteresis-free parabola (the dashed line) if the electrostrictive contribution is only from the crystalline part.Similarly, the strain as a function of D 2 ought to be a straight line, as indicated in Figure 10b.However, hysteresis loops were observed in Figure 10a,b, suggesting that an additional contribution from other structure components should exist for the piezoelectric effect.Such a contribution was proved to come from the crystalline-amorphous interfaces.Considering this fact, they proposed a modified electrostriction model and the d 33 was calculated by the following equation where d coupling is the additional contribution arising from the crystalline-amorphous interfacial coupling.Figure 10c,d shows the experimental strain and the contributions of the electrostrictive and coupling terms, as functions of the electric field E and electric displacement D. It is seen that the strain was dominated by electrostriction (the solid line), and the additional term, ′ p (the dashed line), became increasingly important with the increased electric field.As a conclusion, they proposed that the piezoelectric effect of P(VDF-TrFE) was mainly contributed by the change of the lattice constant, and additional contribution originated from the electromechanical coupling between the intermixed regions of the crystalline lamella and the amorphous phase of the semicrystalline polymers. [28]profiles for the fresh and poled BOPVDF films at room temperature.The inset shows the 2D WAXD pattern (in a logarithmic scale) for the poled BOPVDF film.The X-ray beam is along the transverse direction (TD) and the machine direction (MD) is vertical.f) FTIR spectra for the fresh and poled BOPVDF films in the transmission mode.Absorption bands for  and  crystals are labeled.g) Comparison of the bipolar D-E loops for the fresh and poled BOPVDF films at 300 MV m −1 .Nonlinear P-E loops are obtained for h) the two-phase and i) the three-phase models.The inset two-phase model in (h) contains the  lamellar crystals and the isotropic IAF.The inset three-phase model in (i) contains the  crystals, the IAF, and the OAF connecting the lamellar crystal and the IAF.(a-d) Reproduced with permission. [28]Copyright 2015, Springer Nature.(e-i) Reproduced with permission. [24]Copyright 2021, Springer Nature.
Very recently, we also found that the crystalline-amorphous interface had a profound influence on the piezoelectricity of PVDF. [24]First, a BOPVDF film was poled by an extremely high electric field (650 MV m −1 ) to induce the - phase transformation.From wide-angle X-ray diffraction (WAXD) and FTIR measurements, the fresh BOPVDF showed a mixed  and  phase with the  phase content being ≈30% (Figure 10e,f).After electrical poling, a pure  phase was obtained (Figure 10f).From the electric displacement-electric field (D-E) loop tests (Figure 10g), the spontaneous polarization (P s ) of the poled BOPVDF greatly increased from 67 to 140 mC m −2 when the poling field was 300 MV m −1 at 10 Hz.Based on the typical crystalline-amorphous model with a crystallinity of 0.52, the P s for pure  crystals (P s, ) was estimated to be 270 mC m −2 at 300 MV m −1 (Figure 10h).This high P s, was way beyond the maximum P s, of 188 mC m −2 calculated by the DFT. [229]To explain the unexpectedly high P s , we proposed a three-phase model (inset of Figure 10i) which contained the crystalline phase, the isotropic amorphous fraction (IAF), and the OAF connecting the crystalline and the IAF phases.Assuming the maximum P s for the OAF (P s,OAF ) to be P s, , the content of OAF (f OAF ) could be calculated from the following equation where f  was measured as 0.52, and f OAF is calculated to be 0.25.
Based on this fact, we concluded that the OAF must exist and participate in the ferroelectric domain formation and subsequent polarization switching under high-field electric poling.The d 33 was measured by a homemade test setup (Figure 4d), and the highly ) Reproduced with permission. [24]Copyright 2021, Springer Nature.(c-e) Reproduced with permission. [30]Copyright 2021, Elsevier, Inc.
poled BOPVDF showed a high d 33 of −62.5 pC N −1 (Figure 11a), remarkably higher than those of conventional PVDF (≈−30 pC N −1 ). [13,36]To explain the high piezoelectricity of the highly poled BOPVDF, we proposed a mechanism by taking into account the OAF, as shown in Figure 11b.In  crystals of the poled BOPVDF film, dipoles and domains were highly aligned along the poling direction.Since OAFs are located at the crystalline-amorphous interface, dipoles in the OAF must also orient more or less along the poling direction, and then gradually randomize when approaching the IAF.After applying a dynamic stress, an immediate in-plane strain () is induced, making more dipoles in the OAF align along the poling direction.This results in an increased polarization, ΔP.In turn, the aligned dipoles in the OAF would cause electrostatic repulsion in the film plane, further pushing the crystals apart.As a result, a high negative d 33 for BOPVDF is obtained.This mechanism is further confirmed by a molecular dynamics simulation. [24]fterward, a further study was performed and we demonstrated that the ECC and relaxor-like SC OAF could significantly enhance the electrostriction, contributing to a high inverse piezoelectric coefficient d 31 of 77 ± 5 pm V −1 in a poled P(VDF-TrFE) 55/45 copolymer at 55 °C (Figure 11c-e). [30]In this study, the ECCs were first induced by annealing the quenched and stretched P(VDF-TrFE) films at 130 °C, and a highly mobile SC OAF was induced by poling at 100 MV m −1 .Note that the annealing treatment was essential to obtain a high d 31 .It was reported that the d 31 of an annealed and poled QS sample is 57.6 ± 2.4 pm V −1 , while it was only 12.0 ± 0.9 pm V −1 for the poled QS sample.This could be explained by the fact that annealing above T c facilitated the formation of ECCs, which made it easier for SC OAF to form upon electric poling.Later, how the dipole mobility in the SC OAF affected the piezoelectricity of ferroelectric polymers was studied. [238]First, hot-pressed, quenched, stretched, annealed, and poled P(VDF-TrFE) 52/48 mol% sample (denoted as coP-52/48QSAP) was prepared.Then, additional uniaxial stretching and poling were performed for coP-52/48QSAP to obtain the coP-52/48QSAPSP film.Compared to the coP-52/48QSAP film, the coP-52/48QSAPSP film showed a much lower piezoelectric performance.Based on a broadband dielectric spectroscopy study, it was concluded that the decreased piezoelectric performance of the coP-52/48QSAPSP film was caused by the lower dipole mobility in SC OAF after additional stretching and poling of the coP-52/48QSAP film.
For above studies, the annealing temperature to induce ECCs in P(VDF-TrFE) must be higher than T c , and the formation of ECC made it easier for inducing SC OAF .However, the T c of PVDF is higher than T m at ambient pressure. [239]This makes it difficult for PVDF to obtain the ECC structure by crystallization under normal processing conditions, [140] which increases the difficulty for obtaining SC OAF in PVDF.To address this issue, it was found that SC OAF could be induced in a uniaxially stretched PVDF film with the help of high-power ultrasound. [38]After highpower ultrasonication for 20 min, a uniaxially stretched and poled PVDF film presented a high d 31 of 50.2 ± 1.7 pm V −1 at room temperature.By contrast, the stretched and poled PVDF without ultrasonication only showed a low d 31 of 28.5 ± 0.7 pm V −1 .From differential scanning calorimetry and broadband dielectric spectroscopy studies, the improved piezoelectricity originated from the SC OAF , which broke off from the primary crystals by ultrasonication.This work proved again the positive effect of relaxor-like SC OAF on improving the piezoelectric performance of PVDF.
In summary, the crystalline-amorphous interface also has a strong effect on the piezoelectric response of PVDF-based materials.Especially, the OAF located at the crystalline-amorphous interfaces of poled PVDF-based films plays an important role in enhancing the piezoelectric performance in both PVDF and P(VDF-TrFE) samples.Inducing the formation of SC OAF may bring further increased piezoelectric performance.

Piezoelectric Devices and Applications
Piezoelectric PVDF-based polymers have been widely applied in various fields due to its high piezoelectric coefficient, lightweight, and high mechanical flexibility. [240]These applications include energy harvesters, [135] piezoelectric sensors, [241] actuators, [242,243] flexible self-charging batteries, [244,245] etc.There have been numerous studies and reviews conducted in these fields; [4,196,246,247] however, most of them have focused on the composites of PVDF or its copolymers and ceramics.Given that the present review is specifically concerned with the structural and property modulation of pure polymers, our focus will be mainly on exploring the applications of piezoelectric properties within pure PVDF-based polymers.
Energy harvesters prepared by PVDF or its copolymers are devices that convert mechanical energy into electrical energy using the piezoelectric effect. [135]Their basic structure consists of a thin film of PVDF-based polymer sandwiched between two electrodes.When the film is subjected to mechanical stress or pressure, it generates an electric charge across the electrodes.This charge can be collected and used to power electronic devices or stored in a battery for later use. [248]The harvested energy could come from environmental sources (such as wind, water, or solar energy), [215] and human motion (such as walk and run). [248]Here, we will review some representative devices for these two energy sources.First is about energy harvester for environmental energy.Dong and co-workers utilized 3D printing to fabricate ferroelectric P(VDF-TrFE) multilayers, which were then wrapped around a polydimethylsiloxane rugby ball to create a piezoelectric energy harvester (PEH) to harvest the mechanical energy from the environment. [215]The resulting PEH exhibited excellent electromechanical coupling properties and a piezoelectric coefficient of up to 130 pC N −1 .A comparison of the performance of the rugby-ball-structured PEH with that of a planar PEH revealed that the former had superior energy harvesting capabilities.Specifically, the rugby ball PEH generated ≈88.6 Vpp (peak to peak) voltage and 353 mA peak current at 10 Hz and 0.046 MPa, which were 2 and 10 times higher than those of the flat PEH, respectively.Furthermore, when subjected to a load resistance of 568 kΩ, the rugby ball PEH demonstrated a higher power density (5.81 mW cm −2 ) compared to the planar PEH (0.74 mW cm −2 ).Bae et al. have developed an ultrathin piezoelectric nanogenerator (U-PENG) with a thickness of ≈4 μm using P(VDF-TrFE). [249]In contrast to traditional, thicker PENG that lacks the necessary flexibility to respond to minor movements, the U-PENG conforms to soft human skin and generates energy from subtle movements, such as breathing or blinking, as shown in Figure 12a.These innovative devices boast an energy conversion efficiency of roughly 18.8%, which is 971% higher than thicker samples with an identical structure.Furthermore, after 10 000 cycles of 30% compressive strain, the output voltage did not decrease, demonstrating the ability of the U-PENG to produce sustainable energy from continuous and repetitive body movements.As a result, they believed their devices can be integrated with biodevices and used as power sources for wearable devices in remote medical systems.
Piezoelectric sensors are among the most widely used applications for PVDF-based piezoelectric polymers.These sensors can be classified into various types, such as body motion sensors, [250] mechanical wave sensors, [251] damage monitoring sensors, [252] and others. [241]In recent years, many studies have focused on body motion sensors, which are utilized for detecting human pulse, analyzing and recording human movement, and related applications. [250]For example, Laurila et al. have developed a tattoo-type, ultrathin (≈4.2 μm) piezoelectric sensor using P(VDF-TrFE) as the piezoelectric layer to detect arterial pulse waves, using a facile printing-based fabrication method. [250]s shown in Figure 12b, the ultrathin form factor allows access to the high bending mode sensitivity of the P(VDF-TrFE) layer, with the maximum sensitivity achieved in uniaxial and multiaxial bending being ≈1700 pC N −1 , which is around 50 times higher than the sensitivity measured in the normal mode.They mounted the fabricated sensor on the left distal radial artery of seven people and compared the test results with signals obtained from a reference device (Finapres NOVA).The results demonstrated that the arterial pulse sensor had good accuracy in measuring the radial artery pulse waves.In addition to pulse sensing, PVDF-based sensors can be used for hand motion (finger flexion, wrist movement) sensing, for detecting strain information and various hand gestures.For instance, Yuan et al. used 3D printing technology to prepare flexible P(VDF-TrFE) piezoelectric films for monitoring signals from finger presses, as well as finger and wrist joint flexion movements (Figure 12c,d). [253][256] For instance, Kong et al. developed a dual-mode pressure sensor that combines piezoelectric and piezoresistive properties by tightly wrapping a PVDF piezoelectric film and a strain-sensitive gold-coated polyurethane film around a cylindrical elastomer. [257]The resulting sensor demonstrated an ultrahigh open-circuit voltage of over 250 V and excellent sensitivity of 530 mV kPa −1 .When integrated into a shoe insole, the sensor can identify human walking and running states based on different pressures and step frequencies, as shown in Figure 12e.Additionally, the piezoelectric signal can be used to analyze the athletes' vertical jump, squat, and start movement process (e.g., reaction speed), allowing for evaluation of their movement status and improvement of their performance.
In addition to human motion, piezoelectric sensors can be used to detect mechanical waves in the environment, such as sound waves. [251]By modulating the fiber orientation and Figure 12. a) Photographs of ultrathin piezoelectric nanogenerator devices mounted beside the eye and on the wrist, together with the corresponding images under deformation.Reproduced with permission. [249]Copyright 2022, Elsevier Ltd. b) Photographs of ultrathin (≈4.2 μm) piezoelectric sensor attached on top of the radial artery at distal antebrachium without/with wrinkling.Reproduced with permission. [250]Copyright 2022, Elsevier Ltd.The action diagram and the corresponding voltage response of the 3D printed flexible P(VDF-TrFE) piezoelectric sensor installed in the human c) finger joint and d) wrist joint under different bending conditions.(c, d) Reproduced with permission. [253]Copyright 2021, Elsevier Ltd. e) Schematic illustration of piezoelectric-piezoresistive dual-mode pressure sensor integrated into the insole for human motion analysis.Reproduced with permission. [257]opyright 2022, Elsevier Ltd. f) Schematic diagram of the textile acoustic sensors for human speech detection; the output of topologically optimized structure film vibrating under acoustic waves is fed to the computer through a programmable electrometer.Reproduced with permission. [258]Copyright 2022, Wiley-VCH GmbH.g) Structure diagram of self-charging battery made of LiFeO 4 (cathode) and graphite (anode) supported by electrospinning P(VDF-TrFE) piezoelectric film; schematic of recharge of the lithium ion driven by mechanical deformation.Reproduced with permission. [244]Copyright 2021, Elsevier Ltd. h) Schematic illustration of patterned piezo-electrochromic tactile-sensation display integrating flexible piezoelectric strain sensor and electrochromic array modulus as a tactile sensation system.Reproduced with permission. [265]Copyright 2022, Elsevier Ltd. i) Schematic illustration of the solar-steam generator based on a piezoelectric composite film evaporator consisting of an anisotropic P(VDF-TrFE) fiber membrane and hydratable light-absorbing PVA hydrogel, in which the highly hydratable light-absorbing PVA hydrogel is customized with an oriented pore structure to enhance water transport.Reproduced with permission. [266]Copyright 2022, Wiley-VCH GmbH.mechanical vibration of electrospun PVDF piezoelectric films through topology optimization design, Lan et al. prepared piezoelectric films with similar dimensions but different topological patterns as textile acoustic sensors (TAS). [258]With the optimized piezoelectric membrane design, the developed TAS can capture the human voice for speech recognition with a classification accuracy of up to 100% with the help of deep learning (Figure 12f).PVDF-based sensors are also suitable for real-time online damage monitoring. [252]This is because various damage conditions can generate different piezoelectric signals during mechanical stress testing, allowing for identification of the degree of damage to composite materials.Such sensors could probably be used in health monitoring of aircraft wings to improve safety and efficiency.
Piezoelectric actuators are also an important application for piezoelectric PVDF-based materials. [242,243]Its basic structure is the same as the energy harvesters, and the difference is that piezoelectric actuators use the inverse piezoelectric effect to produce mechanical energy in response to electrical energy. [20]261][262] For example, Pabst et al. reported a inkjet-printed micropump actuator based on P(VDF-TrFE). [263]This actuator was manufactured by successive inkjet printing of a P(VDF-TrFE) film sandwiched between two silver electrodes on a poly (ethylene terephthalate substrate.Under an applied electric field, the P(VDF-TrFE) piezoelectric layer generates bending deflection due to its inverse piezoelectric effect (d 31 of about 10 pm V −1 ), which in turn promotes fluid movement.The actuator can reach a pump speed of 130 μL min −1 at a driving frequency of 30 Hz and voltage of 900 V, indicating its potential applications in microfluidic lab-on-a-chip systems.Reis et al. developed a piezoelectric PVDF actuator to explore its applications in biomedical applications. [264]hey implanted the PVDF actuator in osteotomy cuts of sheep femur and tibia, and found that the total bone area and new bone area were significantly increased around the actuators compared to the static controls.The results showed that PVDF piezoelectric actuators could effectively stimulate bone growth at the interface of bone implants, which would facilitate its application in tissue engineering and bone regeneration.
Apart from their well-known use in energy harvester, sensors, and actuators, PVDF-based piezoelectric materials have also found applications in other areas.For instance, Yu et al. developed a novel flexible self-charging battery (SCPB) by obtaining porous P(VDF-TrFE) films through an electrostatic spinning process (Figure 12g). [244]The SCPB can convert the small amounts of mechanical energy generated by human motion into electrical energy, and has the potential to charge wearable electronic devices.Bi et al. developed a PVDF-based piezo-electrochromic tactile-sensation display (PETSD) that integrates piezomodulus with an electrochromic system for real-time human-computer interaction (Figure 12h). [265]They believed that the PETSD device had the potential to revolutionize the development of sufficient identification characters for sensing, anti-counterfeiting, and optoelectronic applications.Meng et al. prepared highly anisotropic P(VDF-TrFE) fiber membranes by electrospinning and embedded them into a highly hydratable light-absorbing PVA hydrogel to create a solar-steam generator. [266]This generator could collect the kinetic energy of ocean waves and coactivate water to accelerate the rate of water evaporation (Figure 12i), providing a more practical solution for efficient and low-energy solar-powered water purification.In recent years, new applications for PVDF-based piezoelectric materials have emerged.These include their use in micro-light emitting diodes (LEDs), [267] for medical applications such as bone repair, [268][269][270] piezoelectric catalysis, [271] for environmental remediation through piezoelectric catalysis, [272] etc.
In this subsection, we provide a brief overview of the applications of devices based on PVDF and its copolymers in various fields, such as energy harvesting, haptic and acoustic sensing, damage monitoring, and self-charging batteries.Despite challenges in their application, PVDF-based piezoelectric materials offer numerous opportunities for development.Future research directions include improving energy conversion efficiency, elec-tromechanical coupling coefficient, power management and integration, and device miniaturization and integration.With advancements in technology, flexible PVDF piezoelectric materials are expected to find wider applications in the future.

Conclusions and Outlook
In this review, we have discussed the recent progress to improve the piezoelectric performance of PVDF-based polymers through the manipulation of the bulk semicrystalline structures.First, some basic principles and measurement methods for piezoelectricity of PVDF are reviewed.To measure direct piezoelectric coefficients, special attention should be paid to avoid the interference of electrical signal from triboelectricity.For the measurement of inverse piezoelectric coefficients, the triboelectricity is largely avoided.However, owing to the nanoscale dimensional change of polymers under electric fields, a high precision device (accurate to 1 nm) is required.Three categories of methods are discussed to improve the piezoelectricity of PVDF: 1) structural manipulation of the amorphous phase, 2) structural manipulation of the crystalline phase, and 3) structural manipulation of the crystalline-amorphous interfaces.
There are not many studies on manipulation of amorphous regions to improve the piezoelectricity of PVDF in recent years.According to the dimensional model, increasing the amorphous fraction may be a good strategy to induce a large dimensional change and thus a high piezoelectric response.On the contrary, a great deal of research was conducted on manipulating the crystalline regions of PVDF to improve the piezoelectricity.It was concluded that increasing the content of polar  crystals as well as its degree of orientation is effective for piezoelectricity enhancement.This can be realized by various methods, such as stretching, electrostatic spinning, chemical modification, annealing, poling, nanoconfinement, etc. Parameters such as stretching rate and direction, electrostatic voltage and spin distance, annealing time, and poling temperature have profound effects on the phase transformation from  to  as well as the piezoelectric coefficients.By tuning these parameters, most studies improved the d 33 to ≈−30 pC N −1 or pm V −1 .Some new theories are developed to further increasing the piezoelectric coefficients.It was found that inducing a MPB in the crystalline regions of P(VDF-TrFE) with 49-55 mol% VDF greatly improved the d 33 to −63.5 pC N −1 .Furthermore, an even higher d 33 of 1050 pm V −1 was achieved by incorporation of 1.9 mol% FA into P(VDF-TrFE-CFE) to decrease the energy barrier for conformational changes in the crystalline phase.
We have also reviewed the structural manipulation of the crystalline-amorphous interface to improve the piezoelectric performance of PVDF.Especially, the crucial role of the OAF in enhancing the d 33 of PVDF was emphasized.From our previous study, electrical poling at a high field (650 MV m −1 ) could align a large amount of OAF in the crystalline-amorphous interface of BOPVDF, which contributed to a greatly enhanced d 33 of PVDF to −62.5 pC N −1 .Some further studies proved that the ECCs and relaxor-like SC OAF significantly enhanced the electrostriction, contributing to an inverse d 31 as high as 77 ± 5 pm V −1 in a poled P(VDF-TrFE) 55/45 copolymer film at 55 °C.These studies advanced our understanding of the physical origin of the piezoelectricity, and meanwhile greatly increased the piezoelectric coefficients of PVDF, which holds great potential for application of PVDF in flexible and wearable electronic devices.In general, the piezoelectric coefficients of PVDF can be improved by structural manipulation of its semicrystalline structure.In terms of the crystalline phase, strategies that can induce more  crystals or MPB can lead to high piezoelectricity.In terms of the crystallineamorphous interface, OAF plays an important role in improving the piezoelectricity.
Although there have been numerous research efforts on the piezoelectric properties and applications of PVDF-based polymers, there is still a need for further exploration in addressing the specific issues and the following research aspects should be considered in the future.
1) The origin of piezoelectricity of polymer-while considerable research has been conducted on the mechanism of the piezoelectricity of PVDF and some new understanding of OAF in improving piezoelectric coefficients has emerged, there remain some controversies among different theories.To provide theoretical guidance for further enhancing the piezoelectricity of PVDF and its copolymers, more work is needed in the future to investigate the physical origin of the piezoelectric behavior of PVDF.This will require combining experimental and computer simulations to gain a deeper understanding of the underlying mechanisms.Machine learning and data analysis may be also used to predict the piezoelectric properties of polymers based on their structure and composition.2) Measurement of piezoelectric coefficients-the current methods for testing piezoelectric coefficients have some limitations, such as triboelectric signal in direct piezoelectric coefficient measurements and the requirement for high-precision equipment for detecting the inverse piezoelectric coefficient.
To address these issues, it is necessary to develop simpler and more accurate methods for detecting piezoelectric coefficients in the future.In particular, more research should be focus on developing simple methods to separate piezoelectric signals from triboelectric signals.Furthermore, more studies are needed to investigate the relationship between the direct and the inverse piezoelectric coefficients of PVDF-based polymers to improve our understanding of the true piezoelectric property of polymers.3) Scalable manufacturing technology-the modification of piezoelectric properties of PVDF-based polymers by stretching, annealing, electric field polarization, and chemical modification has been investigated.In order to realize the industrial application of piezoelectric PVDF-based polymers, novel methods that are easy, inexpensive, and scalable are needed to prepare piezoelectric PVDF in the future.Additionally, other processing external fields, such as high pressure, shear field, and flow field can be explored to influence the structure and piezoelectric properties of PVDF or its copolymers.Furthermore, the molecular structure of PVDF can also be modulated by introducing new monomers and exploring whether the new structure can lead to new piezoelectricity enhancement mechanisms.This could provide greater possibilities for enhancing the piezoelectricity of PVDF-based polymers.4) Application of PVDF-based piezoelectric devices-PVDFbased piezoelectric devices have already shown great potential in various applications, such as sensors and transducers.
Due to its excellent biocompatibility and stability, there is a growing interest in exploring its use in biomedical engineering.Further studies are needed to expand its application in this field and to develop new devices with improved performance and functionality.Moreover, the power management and integration in PVDF-based devices require further investigation.Typically, the output power of the device exhibits AC characteristics and transient pulses, which need to be rectified to unipolar and DC voltage to charge the energy storage units.However, this can result in voltage losses during transmission, which is a significant challenge.Therefore, more efficient power management strategies, such as energy storage and voltage regulation, need to be developed for PVDF-based devices to increase their energy conversion efficiency and overall performance.Additionally, the integration of multiple PVDF-based devices or other energy-harvesting technologies can be explored to enhance the overall system's power output and reliability.

Figure 1 .
Figure 1.Schematic diagrams of a) the direct and b) the inverse piezoelectric effects.

Figure 2 .
Figure 2. a-e)Different crystal forms and f-h) conformations of PVDF.The green arrows indicate the dipole moments.Reproduced with permission.[15]Copyright 2012, American Chemical Society.

Figure 3 .
Figure 3. a) The complex three-phase structure of semicrystalline PVDF.b) Schematic diagram of different methods to improve the piezoelectric performance of PVDF-based materials by manipulation of the bulk structures of PVDF.

Figure 4 .
Figure 4. a) Definition of different stress axes in piezoelectric materials with a C 2v symmetry.b) Schematic of the d 33 piezometer using the quasistatic Berlincourt method.c) The static (F static ) and dynamic force (F dynamic ) profiles applied to the sample.d) Scheme of the out-of-plane piezoelectric coefficient, d 33 , measurement setup.The sample geometry with gold coatings on both sides is also shown.e) Direct d 33 measurements for the unpoled BOPVDF film with a force of 0.98 N. Since the permanent remanent polarization is zero, all piezoelectric coefficients are zero.f) Direct piezoelectric charge measurements of d 33 for the poled BOPVDF.g) Scheme of the in-plane piezoelectric coefficient, d 31 /d 32 , measurement setup.(b-g) Reproduced with permission.[24]Copyright 2021, Springer Nature.

Figure 5 .
Figure 5. a) The signal generation process of the hybrid output contains six stages, including contacting, contacted, compressing, releasing, released, and separating stages.b) The electrostatic balance is achieved when the Al plate and the PVDF-based device are contacted.(I) represents the positive polarization and (II) represents the negative polarization.Q 1 , Q 2 ,and Q p are the total charge in the Kapton layer, the total charge in the Al plate, and the induced piezoelectric charge, respectively.q and −q′ are the charge transfer between electrodes in the positive and the negative polarizations, respectively.The transferred charges between electrodes are measured from c) the positive and d) the negative polarization sides of the PVDF-based device with an applied force of 60 N. (a-d) Reproduced with permission.[46]Copyright 2022, Springer Nature.e) Schematic diagram of the inverse piezoelectric effect measurement by Fotonic sensors.Reproduced with permission.[48]Copyright 2005, Taylor & Francis Inc. f) Schematic representation of the S-E loop measurement fixture to transverse strain responses.Reproduced with permission.[50]Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA.g) Determination of the inverse d 31 using bipolar S 1 -E loops for QSAP films at room temperature.h) Two continuous bipolar S 1 -E loops after 60 cycles of unipolar poling at 100 MV m −1 : coP-55/45 QSAP.The poling frequency is 1 Hz with a sinusoidal waveform.(g, h) Reproduced with permission.[30]Copyright 2021, Elsevier Inc. i) Amplitude hysteresis loop of the PZT-doped PVDF film.Reproduced with permission.[82]Copyright 2022, Elsevier Ltd.

Figure 7 .
Figure 7. a) Schematic fabrication process of PVDF nanofibers (NFs) by the electrospinning method.b) Relative amount of the  phase in PVDF NFs with varying RHs.c) Output voltage of the TENGs with the electrospun PVDF NFs under various RHs.The inset is a schematic operating principle of the TENG device in the vertical contact-separation mode.Reproduced with permission.[128]Copyright 2020, American Chemical Society.

Figure 8 .
Figure 8. a) %DHF versus reaction time for EDA-treated PVDF, obtained from combustion elemental analysis.b) The calculated planar (zigzag) chain conformation in untreated PVDF films and in dehydrofluorinated PVDF films with different %DHF values.c) Previously reported d 31 coefficients of PVDF and its copolymers compared to dehydrofluorinated PVDF.d) Tacticity of the TrFE-TrFE segment as a function of the VDF content, C VDF .The TrFE-TrFE sequence evolves to be atactic-like, supported by a considerable increase in heterotactic (mr+rm) triads and a rapid development of isotactic (mm) triads with decreasing C VDF .The MPB-like transition occurs near the critical VDF content of 49 mol%, as indicated by the gray arrow.e) XRD profiles of copolymers with C VDF = 45-65 mol%.f) Intermolecular lattice spacing versus C VDF .The light-green-shaded area indicates the transition region, across which the structure changes abruptly.g) Magnitude of d 33 as a function of C VDF .The light-green-shaded areas in (f) indicate the transition region, which shows substantially enhanced piezoelectric responses.h) d-spacing of ferroelectric and relaxor ferroelectric phases in P(VDF-TrFE).The shadowed area indicates the compositions in which both phases coexist.i) Longitudinal piezoelectric coefficient d 33 as a function of C VDF .(a-c)Reproduced with permission.[135]Copyright 2020, American Chemical Society.(d-g) Reproduced with permission.[36]Copyright 2018, Springer Nature.(h, i) Reproduced with permission.[147]Copyright 2022, American Chemical Society.

Figure 9 .
Figure 9. Polarization data and electrostriction coefficients of P(VDF-TrFE-CFE-FA) relaxor ferroelectric polymers: P-E loops at 1 Hz for a) terpolymer and b) s-tetrapolymer.The inset shows the temperature-scan dielectric spectra for s-tetrapolymer.c) Effective dielectric constant versus electric field for terpolymer and s-tetrapolymer.d) Comparison of d 33 and k 33 of commercial P(VDF-TrFE) copolymer, copolymer single crystal, and copolymer at MPB, PZT piezoceramic, and the relaxor ferroelectric tetrapolymer under a 40 MV m −1 DC bias.Reproduced with permission.[29]Copyright 2022, The American Association for the Advancement of Science.

d
33 of recently reported PVDF-based polymers prepared by different annealing and poling conditions.

Figure 10 .
Figure 10.Electroactuation strain (S) as a function of a) electric displacement (D) and b) D 2 .The experimental strain and the contributions of the electrostriction term, Q 33 D 2 (solid lines), and the coupling term, d coupling E ′ p (dotted lines), as a function of c) electric field and d) D. e) 1D WAXD profiles for the fresh and poled BOPVDF films at room temperature.The inset shows the 2D WAXD pattern (in a logarithmic scale) for the poled BOPVDF film.The X-ray beam is along the transverse direction (TD) and the machine direction (MD) is vertical.f) FTIR spectra for the fresh and poled BOPVDF films in the transmission mode.Absorption bands for  and  crystals are labeled.g) Comparison of the bipolar D-E loops for the fresh and poled BOPVDF films at 300 MV m −1 .Nonlinear P-E loops are obtained for h) the two-phase and i) the three-phase models.The inset two-phase model in (h) contains the  lamellar crystals and the isotropic IAF.The inset three-phase model in (i) contains the  crystals, the IAF, and the OAF connecting the lamellar crystal and the IAF.(a-d) Reproduced with permission.[28]Copyright 2015, Springer Nature.(e-i) Reproduced with permission.[24]Copyright 2021, Springer Nature.

Figure 11 .
Figure 11.a) Piezoelectric d 33 as a function of the dynamic stress for the highly poled BOPVDF film using the direct piezoelectric measurement in Figure 4d.The red star indicates the d 33 value measured by the d 33 piezometer with a static force of 2.5 N. b) Schematic representation of the stressinduced direct piezoelectric effect.c) 2D Small-angle X-ray Scattering (SAXS)pattern of the coP-52/48QSAP film.d) Proposed semicrystalline structures for the coP-52/48QSAP film.e) Temperature-dependent d 31 and k 31 .Data are represented as mean ± standard deviation.(a, b) Reproduced with permission.[24]Copyright 2021, Springer Nature.(c-e) Reproduced with permission.[30]Copyright 2021, Elsevier, Inc.

Liwei
Zhang is currently a Master student at the College of Materials Science and Engineering at the Shenzhen University.He received his Bachelor's degree in Textile Engineering from the Wuyi University in 2018.His current research is mainly focused on the processing of poly(vinylidene fluoride)based polymers for piezoelectric applications.Shuangfeng Li obtained his Bachelor's degree in the College of Materials Science and Engineering from the Shenzhen University in 2021.He is now a Master student at the College of Materials Science and Engineering from the Shenzhen University.His research interests focus on piezoelectric polymers and solid-state polymer electrolytes for lithium metal batteries.Yan-Fei Huang is currently an Assistant Professor of the College of Materials Science and Engineering at the Shenzhen University.She received her Ph.D. degree in Materials Processing from the Sichuan University in 2018.From 2016 to 2018, she was a Visiting Student in the Case School of Engineering at the Case Western Reserve University.Before joining the faculty at the Shenzhen University in 2020, she was a Postdoctoral Researcher at the Tsinghua University.Her research interests include piezoelectric, ferroelectric, and dielectric polymers, and solid-state polymer electrolytes for lithium-ion batteries.Lei Zhu received his Ph.D. degree in Polymer Science from the University of Akron in 2000.He joined the Institute of Materials Science and the Department of Chemical, Materials and Biomolecular Engineering at the University of Connecticut in 2002, as an Assistant Professor.In 2009, he moved to the Department of Macromolecular Science and Engineering at the Case Western Reserve University as an Associate Professor.In 2013, he was promoted to Full Professor.His research interests focus on structure-property-processing relationships in high- dielectric/ferroelectric polymers and polymer nanocomposites for advanced electrical and electronic applications. 2

Table 1 .
Summary of direct and inverse piezoelectric coefficients and their measurement methods.

Table 2 .
Summary of the degree of crystallinity, the content of ferroelectric phase, and the