Re-evaluation of the Energy Density Properties of VDF Ferroelectric Thin-Film Capacitors.

Large dielectric constants and small remanent polarization of the relaxor-ferroelectric (RFE) polymers are favored for energy-harvesting applications. Here, the energy harvesting of RFE thin films of vinylidene fluoride (VDF)-based terpolymers were re-evaluated. VDF-based terpolymers with trifluoroethylene (TrFE) and chlorofluoroethylene (CFE), CFE terpolymer, and those with TrFE and chlorotrifluoroethylene were used. Thermally annealed CFE terpolymer exhibited an energy density of 8.3 J cm-3 and an energy efficiency of 82% at a field of 280 MV m-1. The high-energy efficiency was related to the narrow bipolar hysteresis of displacement (D)-electric field (E) of the CFE terpolymer film. This narrow D-E hysteresis was a sum of the unipolar hysteresis directed toward the positive electric field region and that toward the negative electric field region, which suggested antiferroelectric-like behavior.

Introduction

Recent energy trends are toward energy resources from fossil oils to natural resources, such as wind power, sunlight, and geothermal energy. Electrical power generation is a very useful and convenient method for energy harvesting. The question arises of how to store electrical power. Energy storage technologies face many challenges in their development that need to be resolved. Solutions to these challenges include batteries and capacitors. In recent years, materials possessing high energy density and high power density have been designed for energy harvesting.[1−4] Capacitors have a high power density because of their fast discharge rates but a low energy density.[4] Thus, the power density and energy density are conflicting relations. The development of film capacitors with both high energy density and high power density to the market would have an impact on these energy issues.

Ferroelectrics are a category of dielectrics, including normal ferroelectrics, relaxor-ferroelectrics (RFEs), and antiferroelectrics. In antiferroelectrics, antiparallel spontaneous polarization in the neighboring unit cells cancel each other out. The linear polarization response is measured at a lower electric field, but the polarization behavior changes to ferroelectric with a double hysteresis loop at a higher electric field.[4] Thus, the double hysteresis loop of the displacement (D)–electric field (E) is observed in antiferroelectrics. From the aspect of energy harvesting, antiferroelectric properties are favored for energy storage. RFE properties are also of interest in energy storage. Normal ferroelectric properties are less crucial in energy storage because of the single D–E hysteresis loop with large remanent polarization.

Vinylidene fluoride (VDF)-based polymers, copolymers, and terpolymers are well-known ferroelectric polymers.[5] Since the first investigation of piezoelectricity in poly(vinylidene fluoride) (PVDF) was reported in 1969,[6] PVDF has been a pioneer polymer in ferroelectrics. The introduction of the VDF copolymer to trifluoroethylene (TrFE) [P(VDF–TrFE)] has been extensively studied in different fields because of the preferential formation of the β crystallite phase and easy rotation of polar VDF moieties along the molecular chain when it was subjected to an electric field. The introduction of a third comonomer to P(VDF–TrFE) results in the VDF-based terpolymers. The VDF-based terpolymers as a RFE show antiferroelectric-like behaviors. The energy-harvesting properties of VDF terpolymers with TrFE and chlorotrifluoroethylene (CTFE) (denoted as CTFE terpolymer) have been investigated in the past.[2] In recent extensive studies for P(VDF–TrFE) with changing VDF contents, antiferroelectric relaxor phase was observed,[7] and the origin of which was attributed to the chirality very recently.[8]

In our previous study on the CTFE terpolymer and VDF-based terpolymer with TrFE, chlorofluoroethylene (CFE) (denoted as CFE terpolymer), we showed the antiferroelectric-like behavior and relaxor-ferroelectricity because of the nanodomains of the dipoles when they were subjected to positive and negative electric fields.[9] Based on our previous results, in this report, we re-evaluated the energy storage properties of these VDF-based terpolymers. Thermal annealing of CFE terpolymer enhanced the D–E double hysteresis loop with smaller remanent polarization, which led to higher energy storage properties.

Results and Discussion

Crystal Structures

The crystal structures of the sample films were investigated using wide-angle X-ray diffraction (WAXD) measurements. WAXD patterns are shown for all the sample films in Figure . The peak was separated to evaluate the crystallite quantities of the crystallite size and lattice spacing. Gaussian or Lorentzian models were used for the evaluation. Table summarizes the measured diffraction peak 2θ angle, crystallite size, lattice spacing, Curie temperature (Tc), and melting point (Tm) for each polymer. Tc and Tm were taken from our previous report.[9] The P(VDF–TrFE) copolymer film possessed a diffraction peak at 19.9° because of the (110) and (200) planes of the β-crystal with a lattice spacing of 0.445 nm.

Figure 1

WAXD patterns. (a) CFE terpolymer film thermally annealed for 4 h. (b) Nonannealed CFE terpolymer film. (c) Annealed CTFE terpolymer film. (d) P(VDF–TrFE) film. Black curve: measured WAXD pattern. Green curve: crystallite part. Blue curve: amorphous halo. Red: fitted curve, summation of green and blue curves.

Table 1

Diffraction Peak 2θ Angle, Lattice Spacing Measured from the WAXD Patterns, Curie Temperature (Tc), and Melting Point (Tm) for VDF-Based Copolymer and Terpolymers

sample	peak 2θ (deg)	crystallite size (nm)	lattice spacing (nm)	crystallinity (%)	phasea	Tcb (°C)	Tmb (°C)
annealed P(VDF–TrFE–CFE)	18.2	40.3	0.486	61.7	RFE	19.4	127.7
	19.4	11.8	0.456	3.5	FE
nonannealed P(VDF–TrFE–CFE)	18.3	23.6	0.485	15.5	RFE
	19.4	6.3	0.458	9.3	FE
annealed P(VDF–TrFE–CTFE)	18.3	13.1	0.484	19.6	PE	23.3	120.6
	18.8	8.9	0.472	13.5	FE
annealed P(VDF–TrFE)	19.9	14.7	0.445	43.5	FE	119.5	149.8

Phase, RFE: relaxor-ferroelectric, FE: β-crystallite ferroelectric, PE: α-crystallite paraelectric.

Taken from our previous report.[9]

The diffraction peak at 18.2–18.3° in the spectrum of the CFE terpolymer film is ascribed to the diffraction from the (110) and (200) planes of the β-crystallite RFE phase, and the diffraction peak at 19.4° is attributed to the ferroelectric (FE) β-crystallite phase, as discussed in reference.[10] Annealing increases the crystallite size and the crystallinity of the RFE phase and decreases those of the FE phase. These results are consistent with the temperature dependence of the WAXD results for CFE terpolymer in a previous report.[10] The diffraction peak at 18.8° in the spectrum of CTFE terpolymer is ascribed to the diffraction from the (110) and (200) planes of the β-crystallite FE phase, and the peak at 18.3° is related the diffraction from the (110) plane of the α-crystallite phase.[11] In the FE phase, adding bulky CFE into the P(VDF–TrFE) crystal deceases the packing density, which leads to the smaller angle of 2θ. Adding more bulky CTFE further decreases the packing density and leads to smaller angle of 2θ.

Current–Electric Field (J–E) and D–E Hysteresis Loop

When sinusoidal electric field is applied to the sample film, the total current flow on an external circuit (J) is the summation of the displacement current (JD) and the conduction current (JC). In ferroelectrics, JD is the summation of dielectric term and polarization current term because of the ferroelectric reversal of the ferroelectric large domain with many dipoles and RFEs of the nanodomain with a few dipoles.[10,12]JC is the resistance term. Thus, the total current flow (J) is expressed as followswhere Pi is the polarization term for ferroelectrics, ε0 is the permittivity in vacuum, ε is a dielectric constant, E is the sinusoidal electric field, and ρ is a resistivity.

D is calculated by the integration of JD

The bipolar current and displacement responses are investigated for the annealed CFE terpolymer and nonannealed CFE terpolymer, annealed CTFE terpolymer, and annealed P(VDF–TrFE) when each sample is exposed to an applied sinusoidal electric field at frequencies ranging from 10 to 100 kHz. VDF-based terpolymers exhibit a broad bipolar JD–E loop and the resulting narrow D–E hysteresis loop, both of which are shown in Figures and 4 for CFE terpolymer and Figure for CTFE terpolymer. On the other hand, a typical bipolar JD–E loop and the resulting hysteresis loop of D–E were measured for the P(VDF–TrFE) copolymer, both of which are summarized in Figure S1 in the Supporting Information. The thicknesses of annealed P(VDF–TrFE), annealed CTFE terpolymer, nonannealed CFE terpolymer, and annealed CFE terpolymer were 202, 115, 125, and 178 nm, respectively.

Figure 2

Bipolar JD and the resulting D for the annealed CFE terpolymer film at various applied voltages between 10 and 50 V and different switching frequencies ranging from 10 to 100 kHz. (a) JD–E plot, (b) D–E plot at 10 Hz, (c) JD–E plot, (d) D–E plot at 100 Hz, (e) JD–E plot, (f) D–E plot at 1 kHz, (g) JD–E plot, (h) D–E plot at 10 kHz, (i) JD–E plot, and (j) D–E plot at 100 kHz. The sample thickness was 178 nm.

Figure 4

Bipolar JD and the resulting D for a nonannealed CFE terpolymer film for various applied voltages between 10 and 35 V at different switching frequencies ranging from 10 to 100 kHz. (a) JD–E plot, (b) D–E plot at 10 Hz, (c) JD–E plot, (d) D–E plot at 100 Hz, (e) JD–E plot, (f) D–E plot at 1 kHz, (g) JD–E plot, (h) D–E plot at 10 kHz, (i) JD–E plot, (j) D–E plot at 100 kHz. The sample thickness was 125 nm.

Figure 5

Bipolar JD and the resulting D for the annealed CTFE terpolymer film for various applied voltages between 10 and 30 V at different switching frequencies ranging from 10 to 100 kHz. (a) JD–E plot, (b) D–E plot at 10 Hz, (c) JD–E plot, (d) D–E plot at 100 Hz, (e) JD–E plot, (f) D–E plot at 1 kHz, (g) JD–E plot, (h) D–E plot at 10 kHz, (i) JD–E plot, (j) D–E plot at 100 kHz. The sample thickness was 115 nm.

Figure shows the bipolar JD and the resulting D versus the sinusoidal electric field at various switching frequencies between 10 and 100 kHz for the annealed CFE terpolymer film. At each switching frequency, the amplitude of the applied voltage ranged from 10 to 50 V with an increment of 5 V. The thickness of the CFE terpolymer film was 178 nm. As shown in Figures a,c, broad and complexed displacement currents were measured at lower switching frequencies of 10 and 100 Hz, and their corresponding electric displacements, as shown in Figure b,d, were narrow, with a constricted displacement near zero electric field. When the switching frequency exceeded 1 kHz, the complexed displacement current gradually disappeared, and the corresponding electric displacement also showed a broad single hysteresis loop.

In our previous report,[9] we showed that the bipolar broad and complexed polarization current of the CFE terpolymer film was a product of the sum of the unipolar polarization currents toward the positive and negative electric fields and a relaxor component using a Gaussian peak separation technique. Thus, we measured unipolar displacement currents in both positive and negative electric field regions, and the corresponding displacements were evaluated.

We selected the bipolar displacement current by applying a sinusoidal voltage with an amplitude of 30 V at 100 Hz, which is shown in Figure a. Integration of the bipolar broad polarization current resulted in the typical RFE response (electric displacement), as shown in Figure b. In the present case, a clear unipolar polarization current was measured in both positive and negative electric field regions of CFE terpolymer. The unipolar polarization currents in the positive and negative electric fields are both shown in Figure c. Integration of each unipolar polarization current resulted in the corresponding electric displacement, as shown in Figure d. Thus, the bipolar electric displacement is a summation of positive and negative unipolar electric displacements. These positive and negative unipolar electric displacements seem to be the origin of the antiferroelectric-like behavior.

Figure 3

Comparison of bipolar and unipolar displacement currents and the corresponding electric displacement of the annealed CFE terpolymer sample. (a) Bipolar displacement current, (b) bipolar displacement, and (c) unipolar displacement currents in positive and negative electric fields. The red curve is the positive unipolar displacement current, and the green curve is the negative unipolar displacement current. (d) Unipolar displacement. The red curve is the positive unipolar displacement, and the green curve is the negative unipolar displacement. The sinusoidal frequency is 100 Hz for either case. The sample thickness was 178 nm.

The plots of the bipolar JD and the resulting D against the sinusoidal electric field with various switching frequencies between 10 and 100 kHz are shown for nonannealed CFE terpolymer in Figure and for annealed CTFE terpolymer in Figure . The thickness of the nonannealed CFE terpolymer film was 125 nm, and the thickness of CTFE terpolymer was 115 nm. The broad displacement current and narrow hysteresis loop were measured for all frequencies. Antiferroelectric-like hysteresis was not observed in nonannealed CFE terpolymer, as shown in Figure , of in CTFE terpolymer, as shown in Figure . The JD–E hysteresis was broad, and the resulting D–E hysteresis loop was narrow. The broad hysteresis and narrow hysteresis loop can be explained by the small crystallinity because of the RFE component in the nonannealed CFE terpolymer thin film. In the CTFE terpolymer, CTFE restricts the rotation of the neighboring VDF and TrFE components at a higher electric field, and thus, the broad hysteresis and narrow hysteresis loop due to RFE component were observed. Therefore, antiferroelectric-like rotation is restricted.

Energy Storage Properties

In linear dielectrics, the maximum energy density is given by equationwhere U is the energy density and Eb is the breakdown electric field.[13]

The schematic picture for evaluating energy density in nonlinear dielectrics is shown in Figure . For nonlinear dielectrics such as ferroelectric and antiferroelectrics, U is defined bywhere D is the displacement, Pr is the remanent polarization, and Dmax is the maximum displacement.[13]

Figure 6

Schematic picture for energy storing in the nonlinear dielectrics.

Energy efficiency η is defined bywhere Ul is the energy reduction.

We evaluated the energy density calculated using eq and the energy efficiency calculated using eq . In Figure , the electric field dependence of the energy density and energy efficiency are shown with various sinusoidal frequencies ranging from 10 to 100 kHz for all the sample films. In either case, the energy density increased with increasing electric field. The maximum energy density was measured at the highest electric field. Energy efficiency slightly decreased with increasing electric field and reached a constant at a higher electric field. At frequencies between 10 and 100 kHz, the energy density for annealed CFE terpolymer was higher than that of nonannealed CFE terpolymer and that of annealed CTFE terpolymer. At a frequency of 100 kHz, the energy density difference between annealed and nonannealed CFE terpolymer decreased. At higher frequencies, the pinned ferroelectric nanodomain of dipoles, which cause the antiferroelectric-like behavior, cannot follow the alternating applied electric field in the annealed CFE terpolymer thin-film sample.

Figure 7

Plots of energy density and energy efficiency vs electric field for each sample film at various sinusoidal frequencies ranging from 10 Hz to 100 kHz. (a) Energy density at 10 Hz and (b) corresponding energy efficiency at 10 Hz. (c) Energy density at 100 Hz and (d) corresponding energy efficiency at 100 Hz. (e) Energy density at 1 kHz and (f) corresponding energy efficiency at 1 kHz. (g) Energy density at 10 kHz and (h) corresponding energy efficiency at 10 kHz. (i) Energy density at 100 kHz and (j) corresponding energy efficiency at 100 kHz.

Figure a,b gives the plots of the measured maximum energy density and energy efficiency versus the sinusoidal frequency of applied field, respectively. Energy density and efficiency are higher for annealed P(VDF–TrFE–CFE) than for nonannealed one for either frequency. Higher crystallinity and larger crystallite size are related to the larger energy density and efficiency. The increase of the sinusoidal frequency slightly decreases the energy density and energy efficiency. An energy density of 8.3 J cm–3 and an energy efficiency of 82% were measured for the annealed CFE terpolymer thin film at an electric field of 280 MV m–1.

Figure 8

Plots of energy density and energy efficiency vs sinusoidal frequency ranging from 10 to 100 kHz. (a) Energy density plot. (b) Energy efficiency plot.

Comparison of Energy Storage Properties

Table compares the present results obtained for VDF polymer-based nanocomposites with piezoelectric nanofillers and nanowires previously reported.[14−19] The energy density and efficiency for the VDF-terpolymer of the present P(VDF–TrFE–CFE) are higher than those for pristine VDF copolymer neat films of P(VDF–CTFE). The energy density of the present VDF-terpolymer is higher or comparable to those for VDF polymer-based nanocomposites. A higher energy efficiency of 82% is reported in the present study compared with previous studies. In a previous report, an energy density of 31.2 J cm–3 was obtained at an extremely high electric field of 800 MV m–1 for the BT@TiO2 nanofillers in P(VDF–HFP).[19]Figure shows the plots of energy efficiency versus energy density.

Table 2

U and η for Various Ferroelectric Polymers and Their Nanocomposites with Nanofillers and Nanowires

nanofiller	matrix	E (MV m–1)	U (J cm–3)	η (%)	no. in Figure 9	refs
none	P(VDF–TrFE–CFE)	281	8.3	82	1	a
(3 vol %)BTnws∥E	P(VDF–CTFE)	240	10.8	61.4	2	(12)
none	P(VDF–CTFE)	320	5.9	47.1	3	(12)
(2.5 vol %)BT@SiO2 nfs	PVDF	330	6.28	64.3	4	(13)
(3 vol %)BTnws⊥E	P(VDF–CTFE)	340	10.8	56.8	5	(12)
(2.5 vol %)BST60 nfs	PVDF	380	6.4	60	6	(14)
(2.5vol %)ST@PVP nfs	PVDF	380	6.8	60	7	(15)
MgO	PVDF	500	10.52	27.4	8	(16)
(3.0vol %)BT@TiO2 nfs	P(VDF–HFP)	800	31.2	78	9	(17)

This study.

Figure 9

Plot of energy efficiency vs energy density for various pristine VDF-based copolymers, terpolymers, and their nanocomposites. Data are taken from Table .

Conclusions

The energy storage properties of the VDF-based terpolymers were re-evaluated. CFE terpolymer of thin films exhibited a crystallite size of 40.3 nm and a crystallinity of 61.7% after thermal annealing at 110 °C for 4 h. The thermally annealed CFE terpolymer crystallite capacitor had a narrow D–E hysteresis loop, which suggested an antiferroelectric relaxor response. The obtained energy density was 8.3 J cm–3 at 281 MV m–1 with a high energy efficiency of 82%. These quantities were compared with those for nanocomposites of nanofiller- and nanowire-dispersed VDF-based polymers.

Experimental Section

Materials

P(VDF–TrFE) (75/25), the CFE terpolymer of P(VDF–TrFE–CFE) (59/33/8), and CTFE terpolymer of P(VDF–TrFE–CTFE) (64.2/27.1/8.7) were used as received. P(VDF–TrFE) was supplied from Kureha, Japan. Terpolymers were provided by Piezotech, France, through Arkema, Japan. The solvent was a methyl ethyl ketone (MEK). MEK was purchased from Nakalai Tesque, Japan.

Sample Preparation

The same spin-coating procedure formerly reported was used to prepare sample films:[9] a 3% MEK polymer solution was used; spin-coating was done at 2000 rpm for 30 s; a silicon substrate was evaporated by 5 mm diameter gold electrode; it was dried under ambient conditions for 24 h; and thermal annealing conditions in vacuum were at 135 °C for 2 h for the P(VDF–TrFE) film, at 110 °C for 4 h for the CTFE terpolymer, and at 110 °C for 8 h for the CFE terpolymer. Nonannealed CFE terpolymer was also prepared. Gold electrodes were evaporated onto a sample film through a 117 μm × 117 μm mesh mask. The thickness of the gold electrodes was 50 nm.

Characterization

A Pacific Technology Nano-R atomic force microscope (AFM) was used to determine the thicknesses of the films in close contact mode. A Rigaku RINT2500 or MiniFlex 600 X-ray diffractometer was used to record WAXD patterns of the films. The scanning range of 2θ is from 16 to 25°. The scanning step is 0.02°. The accumulation time is 20 s/step for RINT2500.

Polarization Hysteresis and Energy Density Measurements

A Toyo Corporation FCE-1/1A ferroelectric measurement tool combined with an AFM equipped with a conductive diamond probe in contact mode was used to evaluate the ferroelectric switching and energy density of the samples.