The development and utilization of renewable, sustainable, and environmentally friendly energy sources are essential to mitigate the continuously rising global energy demand, shortage of fossil fuels, and environmental pollution caused by non-renewable sources ^[1][2][3][1,2,3]. To this end, various energy harvesting, storage, and recycling technologies based on external sources (e.g., solar power, thermal energy, and chemical energy) have been developed. Among them, mechanical energy sources are readily available in nature and daily human activities, such as human movement with fingers, hands, arms, legs, etc., speaking, respiration, airflow, vibrations, frictional forces, water precipitation, and hydraulics (waves in nature, blood flow inside organisms, etc.) ^[4][5][6][7][4,5,6,7]. Over recent years, self-powered wearable sensors and human energy harvesters based on nanogenerators (NGs) have attracted considerable attention, including piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs). These wearable sensors can be used to detect, monitor, and record real-time information on the human physiological status.

Poly(vinylidene fluoride) (PVDF) is one of the most interesting semicrystalline polymers and is often used in sensors, actuators, energy harvesters, etc., because of its high biocompatibility, film-forming ability, low cost, excellent chemical stability, and good electroactive characteristics, including piezo-, pyro-, and ferro-electric properties ^{[8][9][10][11]}[8,9,10,11]. Notably, PVDF-based NGs can effectively harvest energy from organic systems and human activities, such as body motion and even breathing ^{[12][13][14][15]}[12,13,14,15]. In addition, the excellent biocompatibility of PVDF-based polymers makes them desirable for application in flexible membranes, energy sensors, energy-harvesting electronic skins (e-skins), and even implantable devices and artificial prosthetics [16]. However, they still have some drawbacks, such as low ionic conductivity, low crystallinity, and shortage of reactive groups [17]. The low crystallinity especially can limit their piezoelectric properties, charge mobility, and dielectric constant.

Moreover, two copolymers of PVDF are popular candidates for self-powered electronics and energy harvesting. The first one, poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)), is promising due to its thermodynamic stability and high crystallinity. TrFE has a more rigid and ordered structure compared to the vinylidene fluoride (VDF) monomer in PVDF. This structural difference promotes the formation of crystalline regions within the copolymer, where it allows for efficient alignment of polymer chains, resulting in enhanced charge generation in response to mechanical stress or strain. However, the high cost, poor thermal stability, limited stacking integrity, chemical reactivity, and poor ferroelectric dipole density of P(VDF-TrFE) restrict its large-scale device fabrication. Although the copolymerization units of TrFE can improve the crystallinity of PVDF, their crystal defects often cause current leakage paths ^[18][19][18,19]. Compared with P(VDF-TrFE), PVDF homopolymers exhibit a higher dipole density and thermal stability. The other PVDF copolymer, poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)), has a relatively higher piezoelectric sensitivity and electrostrictive strain ^{[20][21][22][23]}[20,21,22,23]. Furthermore, the piezoelectric coefficient of P(VDF-HFP) (13.5 with 5% HFP) is much higher than those of PVDF (≈12.9) and P(VDF-TrFE) (≈10.4) copolymers [24]. Furthermore, the P(VDF-HFP) copolymer has a unique piezoelectric response, which makes it more suitable for fabricating self-powered wearable, stretchable electronic devices, compared with the other polymers. The PVDF- and PVDF-HFP-based materials exhibit immense potential as electrolytes in solid-state lithium-ion batteries, owing to their large dielectric constant, chemical stability, and high mechanical strength ^[25][26][25,26].

Apart from PVDF co-polymers (e.g., P(VDF-TrFE) and P(VDF-HFP)), other polymers such as polypropylene (PP), Nylon11, polylactic acid (PLLA), and poly (lactic-co-glycolic acid) (PLGA) also exhibit piezoelectric properties [27]. Moreover, their soft property is suitable for wearable electronics. Although several traditional ceramic materials such as lead zirconate titanate (PZT) can efficiently convert mechanical energy into electrical energy, they are rigid and difficult to manipulate and machine. On the other hand, owing to its flexibility, PVDF shows excellent long-term stability and does not depolarize when exposed to extremely strong alternating electric fields. Consequently, PVDF-based flexible films have gained immense popularity in recent years.

2. PVDF Phase Structure and Identification

2.1. PVDF Phase Structure

It has been widely established that the semicrystalline PVDF polymer shows five distinct crystalline phases: the α-, β-, γ-, δ-, and ε-phases ^{[28][29][30][31][32][33][34][35][36][37]}[28,29,30,31,32,33,34,35,36,37], which have different stereochemical macromolecular conformations. Firstly, the α-phase, the most thermodynamically stable polymorph, is a non-electroactive, nonpolar, and paraelectric phase with no piezoelectricity, and has a centrosymmetric (P2₁/c) monoclinic unit cell with alternating trans and gauge linkage (TGTG’) conformation ^[38][39][38,39]. On the other hand, the β-phase, the most electroactive phase with excellent piezoelectricity, has an orthorhombic crystal structure with all trans (TTT) planar zigzag conformation [40]. In PVDF, the electroactive β-phase is the most preferred, due to its superior piezo-, pyro-, and ferro-electric performance [41]. Usually, it is important to transform the α-phase into β- phase, because the α-phase is the major component of PVDF films [42]. High isothermal crystallization temperatures often result in the formation of the γ-phase, which also possesses an orthorhombic crystal structure with a T₃GT₃G’ conformation ^[33][34][35][33,34,35]. The δ- and ε-phases are the polar and antipolar analogues of the α- and γ-phases, respectively ^{[31][32][36][43]}[31,32,36,43]. Compared with the α-phase, the δ-phase has a non-centrosymmetric (P2₁cn) unit cell, rendering it piezoelectric, pyroelectric, and ferroelectric [44]. Similar to the β-phase, the δ-phase has superior memory functionality ^[45][46][45,46]. Therefore, the δ-phase is a promising alternative to the β-phase in PENGs. Among the five phases, the α-, β,- and γ-phases are the most widely investigated. Furthermore, the electromechanical coupling factor, k, is one of the dominate parameters for the preparation and application of PVDF-based materials. It presents the efficiency in the mechanical to electrical transformation. High crystallinity and preferred orientation in PVDF crystallites can lead to high remnant polarization, which increases the electromechanical coupling factor. PVDF with different phases have distinct electromechanical coupling factors, and they are also influenced by temperature, poling condition, etc. [47]. Another significant parameter, d₃₃, is used to represent the piezoelectric constant in PVDF-based materials, which often has a negative sign conversion resulting from the crystal structure and molecular alignment. It signifies that the resulting electric field is in the opposite direction to the applied stress or strain.

2.2. PVDF Phase Identification

Although the β- and γ-phase have a similar conformation, the piezoelectric effect of the β-phase is stronger than that of the γ-phase. Therefore, effective strategies for obtaining the electroactive phase of PVDF have garnered considerable research attention, and the identification of the α-, β-, and γ-phases is a crucial step in realizing this goal. Among the identification approaches, Fourier-transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) are considered to be the most reliable ones. Usually, both these techniques are simultaneously used to better discern the β- and γ-phases. It has been reported that the α-, β-, γ-, and δ-phases have distinct characteristic bands in the FTIR spectrum (Table 1). The intensity of these bands indicates the orientation of the CF₂ dipole moment. For example, the broad spectral band at 840 cm⁻¹ results from the overlap of the β- and γ-phases, so other identification approaches must be applied to discriminate between the two phases. The relative proportion of electroactive phases (F_EA) can be utilized to distinguish some phases. For example, taking the band at 840 cm⁻¹ as an example, S. Maji et al. [61] deconvoluted the FTIR spectrum (900–750 cm⁻¹ bands) and quantified the relative fraction of electroactive phases (F_EA), including both β- and γ-phases, using the following equation: $$F_{EA{}_{}=\frac{I_{EA{}_{}\frac{\left(\frac{K_{840{}_{}\frac{K_{763{}_{}\frac{\left(\left(\right)I_{763{}_{}+I_{EA{}_{}\frac{}{}\times 100}}\right)}{}}}{}}}{}\right)}{}}}{}}$$ where F_EA represents the proportion of the electroactive phase; I₇₆₃ and I_EA are the absorption intensities at 763 and 840 cm⁻¹, respectively; and K₇₆₃ and K₈₄₀ are the absorption coefficients at the respective wavenumbers [62]. The individual β- and γ-phases of PVDF films can also be defined by curve deconvolution of the band at 840 cm⁻¹. The ratio of the electroactive β- and γ-phases can be obtained as follows [63]: $$F\left(\beta \right)=F_{EA{}_{}\times \left(\frac{A_{\beta }\frac{A_{\beta }+A_{\gamma }\frac{}{}\left(\right)\times 100\%}{}}{}\right)}$$ and $$F\left(\mathsf{\gamma }\right)=F_{EA{}_{}\times \left(\frac{A_{\gamma }\frac{A_{\beta }+A_{\gamma }\frac{}{}\left(\right)\times 100\%}{}}{}\right)}$$ where A_β and A_γ are the total regions under the deconvoluted curves of the β and γ-phases centered at the 840 cm⁻¹ band. It has been widely accepted that the absorption band at 840 cm⁻¹ is common to both β and γ-phases, but it exists as a strong band only for the β-phase, while it appears as a shoulder of the 833 cm⁻¹ band for the γ-phase. It can be seen in Table 1 that some peaks of the α-, β- and γ-phases overlap with each other, so it is difficult to distinguish them just by FTIR spectroscopy. XRD characterization is another auxiliary approach to discriminate the structures. The representative crystal diffraction planes and diffraction angles of the various phases of PVDF are listed in Table 2. The peak at 20.6° is attributed to the (110) and (200) crystal planes of the β-phase, while the peaks at 18.5°, 19.2°, and 20.4° correspond to the (020), (002), and (110) crystal planes of the γ-phase. Although both the α- and δ-phases have similar chain conformations, the intensities of peaks at 2θ = 17.6° and 25.6° corresponding to (100) and (120) planes are different for the two phases. After heat treatment at 170 °C, the lattice shape and size as well as the symmetry of the unit cell lattice are changed [64]. However, some peaks cannot be easily distinguished. For example, the characteristic peak of the α-phase at 18.3° (020) is often overlapped with that of the γ-phase at 18.5° (020); the broad peak at 20.5° often results from the overlap of the β-phase signal at 20.6° and the γ-phase signal at 20.4°. Sometimes a broad double peak appears around 20.4°, indicating the coexistence of β- and γ-phases [37]. The overall crystallinity (X_c) is calculated according to the crystalline and amorphous regions isolated from the XRD patterns by the Gaussian function, as follows: $$X_c=\frac{\sum A_{cr{}_{}\frac{\sum A_{cr{}_{}+\sum A_{amr{}_{}\frac{}{}\times 100\%}}}{}}}{}$$ where ∑A_cr and ∑A_amr are the sums of integrated areas from crystal diffraction peaks and the amorphous halo. The crystallite size can be determined using the Debye–Scherrer formula, as follows: $$\mathrm{t}=\frac{\mathsf{\lambda }}{B\cos \theta \frac{}{}}$$ where t is the crystallite size, B is the FWHM of the diffraction peak in radians, and λ is the X-ray wavelength. Other auxiliary approaches can be implemented, based on the physical properties of PVDF. For example, the melting temperature of the α-phase is lower than that of the polar β- and γ-phases in PVDF, so differential scanning calorimetry (DSC) is suitable for identifying the α-phase in relation to the β- and γ-phases ^[69][70][69,70].