Effect of Composition on Polarization Hysteresis and Energy Storage Ability of P(VDF-Trfe-CFE) Relaxor Terpolymers

Article Effect of Composition on Polarization Hysteresis and Energy Storage Ability of P(VDF-TrFE-CFE) Relaxor Terpolymers

Yusra Hambal 1 , Vladimir V. Shvartsman 1,* , Daniil Lewin 1, Chieng Huo Huat 1, Xin Chen 2,3, Ivo Michiels 1, Qiming Zhang 2,3 and Doru C. Lupascu 1

1 Institute for Materials Science and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, 45141 Essen, Germany; [email protected] (Y.H.); [email protected] (D.L.); [email protected] (C.H.H.); [email protected] (I.M.); [email protected] (D.C.L.) 2 Department of Electrical Engineering, Materials Research Institute, The Pennsylvania State University, University Park, State College, PA 16802, USA; [email protected] (X.C.); [email protected] (Q.Z.) 3 Department of Materials Science and Engineering, Materials Research Institute, The Pennsylvania State University, University Park, State College, PA 16802, USA * Correspondence: [email protected]

Abstract: The temperature dependence of the dielectric permittivity and polarization hysteresis loops of P(VDF-TrFE-CFE) polymer films with different compositions are studied. Among them, the three compositions, 51.3/48.7/6.2, 59.8/40.2/7.3, and 70/30/8.1, are characterized for the first time. Relaxor behavior is confirmed for all studied samples. Increasing the CFE content results in lowering the freezing temperature and stabilizes the ergodic relaxor state. The observed double hysteresis loops are related to the field-induced transition to a ferroelectric state. The critical field corresponding Citation: Hambal, Y.; Shvartsman, to this transition varies with the composition and temperature; it becomes larger for temperatures V.V.; Lewin, D.; Huat, C.H.; Chen, X.; far from the freezing temperature. The energy storage performance is evaluated from the analysis Michiels, I.; Zhang, Q.; Lupascu, D.C. of unipolar polarization hysteresis loops. P(VDF-TrFE-CFE) 59.8/40.2/7.3 shows the largest energy Effect of Composition on Polarization density of about 5 J·cm−3 (at the field of 200 MV·m−1) and a charge–discharge efficiency of 63%, Hysteresis and Energy Storage which iscomparable with the best literature data for the neat terpolymers. Ability of P(VDF-TrFE-CFE) Relaxor Terpolymers. Polymers 2021, 13, 1343. Keywords: polymers; relaxors; energy storage; P(VDF-TrFE-CFE) https://doi.org/10.3390/polym 13081343

Academic Editor: Alexey Bubnov 1. Introduction Received: 23 March 2021 Alternative technologies in the energy generation sector, miniaturization in the elec- Accepted: 18 April 2021 tronics industry, and electric mobility have opened up many doors for advancements Published: 20 April 2021 in the field of energy storage [1]. Due to the high dielectric strength, various dielectric polymers, such as polypropylene, polycarbonate, and polyethylene terephthalate, were Publisher’s Note: MDPI stays neutral commercially used for decades in thin film and thick film capacitors, all showing a linear with regard to jurisdictional claims in dielectric response and thus a plain capacitive behavior [2–4]. For a non-linear or hysteretic published maps and institutional affil- material, the stored energy density in a polarization–electric field graph is given by the iations. area between the charging branch of a dielectric displacement—electric field hysteresis loop and the dielectric displacement axis (Figure1).

Z Dmax → → → Ustored = E (D)·dD, (1) Copyright: © 2021 by the authors. 0 Licensee MDPI, Basel, Switzerland. → This article is an open access article with the dielectric displacement given by the polarization P of the material and the → → → → distributed under the terms and dielectric permittivity of free space D = ε0·E + P(E). conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Polymers 2021, 13, 1343. https://doi.org/10.3390/polym13081343 https://www.mdpi.com/journal/polymers Polymers 2021, 13, x FOR PEER REVIEW 2 of 13

storage applications [5–8]. Ideally, all this stored energy should berecoverable, which entails that a remanent contribution to polarization should be small. In ferroelectric polymers, saturation and remanent polarization often do not differ much, which is unfavorable for the recoverable energy storage density which is defined by the area between the discharging part of the D–E hysteresis loop and the dielectric displacement axis.

𝑈 = 𝐸⃗ (𝐷⃗)·𝑑𝐷⃗ (2) Therefore, materials with a high polarizability and a slim hysteresis loop are more Polymers 2021, 13, 1343 suited for energy storage applications. Such a combination of properties is typical for re- 2 of 12 laxor ferroelectrics or shortly speaking relaxors [9].

Pmax.

Udischarged 4 Polarization (µC/cm²) Polarization 2

Prem.

0 050100 Electric Field (MV/m)

Figure 1.Figure A unipolar 1. A unipolarpolarization polarization hysteresis hysteresis loop. The loop. green The area green corresponds area corresponds to the recoverable to the recoverable energy density.energy density.The hatched The hatched area yiel areads losses yields in losses the storage in the storageprocess. process.

In relaxors,Therefore, the transition polymers into exhibiting a long-range high polarizabilityordered ferroelectric such as ferroelectricstate is hindered polyvinylidene by structuralfluoride or charge P(VDF) disorder.Therefore, and its copolymers polariza withtion trifluoroethylene is correlated at (TrFE), the local hexafluoropropylene scale within polar nanoregions(HFP), chlorofluoroethylene (PNRs) having asize (CFE), of a and few chlorotrifluoroethylenenanometers [9]. At high (CTFE),temperatures, as well the as blends dipole betweenmoments themof these or with PNRs other are lineardynamic dielectric and are polymers, easily rotated were studiedby an electric for energy field, storage facilitatingaapplications large polarizability [5–8]. Ideally, of relaxors. all this storedWhen energythe field should is removed, berecoverable, the PNRs whichreturn entails to a disorderedthat a remanent state, resulting contribution in a small to polarization remanent polarization. should be small. To distinguish In ferroelectric from the polymers, paraelectricsaturation state, andthis remanentstate is also polarization called the oftenergodic do notrelaxor differ state. much, which is unfavorable for the In recoverableorder to achieve energy relaxor storage behavior density which in P(VDF-TrFE) is defined by co-polymers, the area between various the dischargingtech- niques,part such of as the electron D–E hysteresis beam and loop γ-beam and theirradiation, dielectric mechanical displacement stretching, axis. and defect modification were implemented. By defect modification, another bulky monomer such as Z Dmax → → → chlorofluoroethylene (CFE), hexafluoropropyleneUdischarged = (HFP),E (D )or·d Dchlorotrifluoroethylene (2) (CTFE) is incorporated into P(VDF-TrFE) [10–13].Drem It was reported that terpolymers P(VDF-TrFE-CFE)Therefore, with materials the VDF/TrFE with amolar high polarizabilityratio below 75/25 and and a slim the hysteresis molar amount loop areof more CFE > 4suited mol% forexhibit energy relaxor storage behavior applications. with a broad Such and a combination frequency-dependent of properties peakof is typicalthe for dielectricrelaxor permittivity ferroelectrics [10,11,14]. or shortly For speakingP(VDF-TrFE-CFE), relaxors [9 ].both single hysteresis loops (SHL) [15] [16],In relaxors, which are the transitiontypical for into relaxo a long-rangers, and double ordered hysteresis ferroelectric loops state (DHL) is hindered [12] by were reported.structural The or DHL charge behavior disorder.Therefore, was attributed polarization to a field-induced is correlated phase at transition. the local scale The within double polarhysteresis nanoregions behavior (PNRs) of the polymer having asizerelaxors of ais few not nanometerswell-understood [9]. At yet high [17]. temperatures, It was shown thethat dipole both SHL moments and DHL of these behavior PNRs can are be dynamic observed and in are the easily same rotated polymer by composi- an electric field, tion dependingfacilitatinga on the large synthesis polarizability conditions of relaxors. [13]. When the field is removed, the PNRs return Smallto a remanent disordered polarization state, resulting and inslim a small hysteresis remanent make polarization. P(VDFx-TrFE To1-x-CFE distinguishy) terpol- from the ymers attractiveparaelectric for state, energy this storage state is applications also called the [18,19]. ergodic A large relaxor electrocaloric state. effect was In order to achieve relaxor behavior in P(VDF-TrFE) co-polymers, various techniques, such as electron beam and γ-beam irradiation, mechanical stretching, and defect modification were implemented. By defect modification, another bulky monomer such as chlorofluoroethylene (CFE), hexafluoropropylene (HFP), or chlorotrifluoroethylene (CTFE) is incorporated into P(VDF-TrFE) [10–13]. It was reported that terpolymers P(VDF-TrFE- CFE) with the VDF/TrFE molar ratio below 75/25 and the molar amount of CFE > 4 mol% exhibit relaxor behavior with a broad and frequency-dependent peakof the dielectric permittivity [10,11,14]. For P(VDF-TrFE-CFE), both single hysteresis loops (SHL) [15,16], which are typical for relaxors, and double hysteresis loops (DHL) [12] were reported. The DHL behavior was attributed to a field-induced phase transition. The double hysteresis behavior of the polymer relaxors is not well-understood yet [17]. It was shown that both Polymers 2021, 13, 1343 3 of 12

SHL and DHL behavior can be observed in the same polymer composition depending on the synthesis conditions [13]. Small remanent polarization and slim hysteresis make P(VDFx-TrFE1-x-CFEy) terpolymers attractive for energy storage applications [18,19]. A large electrocaloric effect was also recently reported in these materials [20,21]. However, among the many possible compositions, only a few have been studied so far. An investigation of dielectric and ferroelectric behavior in a broader concentration range should help to understand the mechanism of the formation of the relaxor state and the relationship between the property and composition in these polymers. In the present work, we have studied temperature dependence of dielectric permittivity and polarization of ﬁve different P(VDFx-TrFE1-x-CFEy) compositions (Figure S1). To our knowledge, for the three compositions: 51.3/48.7/6.2, 59.8/40.2/7.3, and 70/30/8.1, such studies are reported for the ﬁrst time. The relaxor behavior of the terpolymers is analyzed. The effect of the CFE content on the relaxor behaviour, the shape of the hysteresis loops, and the energy storage capability of the studied compositions is discussed.

2. Materials and Methods The terpolymer powders used in this study, which were synthesized via suspension polymerization [13], were purchased from Piezotech, Pierre-Bénite, France. The studied compositions are listed in Table1.

Table 1. The molar content of each component in the studied P(VDFx-TrFE1-x-CFEy) compositions. It is important to note that the molar contents are mentioned in a way that the amount of VDF and TrFE adds up to 100.

VDF (mol. %) TrFE (mol. %) CFE (mol. %) 51.3 48.7 6.2 63.8 36.2 7.2 59.8 40.2 7.3 70 30 8.1 68 32 8.5

The polymer films were prepared by the drop-casting method. The polymer powder was dissolved in Dimethylformamide (DMF; VWR Chemicals, Radnor, PA, USA) and stirred overnight at room temperature. The concentration of the solution was set to 20 g L−1. The polymer solution was filtered and drop-coated onto a glass substrate (Corning Inc. Corning, New York, NY, USA). The coating was dried at 60 ◦C for 20 h, followed by annealing under vacuum (260 mbar) at 100 ◦C for 8 h and slowly cooled down to room temperature. The film with the substrate was then immersed in distilled water for some time, peeled off, and dried using a fiberless tissue. The final thickness of the freestanding films was around 20 µm. To perform electric measurements, silver electrodes were deposited onto the films through sputtering using a sputter coater 208 HR (Cressington Scientific Instruments, Watford, UK). The approximate thickness of the sputtered electrodes was 50 nm. The dielectric permittivity was measured as a function of temperature in a frequency range 103–106 Hz using a Solartron 1260 impedance analyzer with a dielectric interface of 1296 (Solartron Analytical, Farnborough, UK). The measurements were performed upon heating, as well as upon cooling within a temperature range of 270 K to 370 K, and the data points were collected every 2 K. The polarization hysteresis curves were measured using a TF Analyzer 2000 (aixACCT, Aachen, Germany) at temperatures between 298 and 323 K. The applied voltage was a triangular wave function with a frequency of 10 Hz. The thermophysical properties were investigated through differential scanning calorimetry measurements that were performed using a DSC-204 (Netzsch, Selb, Germany) setup. Polymers 2021, 13, 1343 4 of 12

3. Results 3.1. Thermal Properties The impact of compositional variation on the thermal properties (melting and recrystallization temperatures) of the polymer powder was studied. Aluminum crucibles were used and 10 mg of polymer powder was encapsulated in it. The curves were measured with a heating/cooling rate of 10 K min−1. The melting and recrystallization temperatures of all samples were determined from the positions of the endothermic and exothermic peaks on the thermograms (Figure S2), and are enlisted in Table2. The melting temperature of the studied compositions varies from 394 K to 405 K, while the recrystallization temperature lies between 368 K and 382 K.

Table 2. The melting and recrystallization temperatures of the studied compositions estimated from DSC measurements.

P(VDF -TrFE -CFE ) x 1-x y Melting Temperature (K) Recrystallization Temperature (K) (mol. %) 51.3/48.7/6.2 404 382 63.8/36.2/7.2 394 368 59.8/40.2/7.3 405 379 70/30/8.1 399 370 68/32/8.5 398 372

3.2. Relaxor Properties Figure2 shows the temperature dependence of the relative dielectric permittivity and dielectric loss tangent of the samples under study, measured at frequencies from 1 kHz to 1 MHz upon cooling. All samples manifest broad peaks in the dielectric permittivity and loss tangent. With increasing frequency, the positions of these peaks shift towards higher temperatures, the values of the dielectric permittivity become smaller, while the dielectric loss tangent increases. Such a behavior is a distinctive feature of relaxor materials [9]. For the same frequency, the dielectric permittivity peak occurs at a lower temperature for the composition with a higher CFE content. The maximal relative dielectric permittivity lies in the range of 45 to 90, which is in good agreement with the previous reports [22]. The permittivity-temperature curves were further analyzed to characterize the relaxor behavior in detail. In order to estimate the degree of the relaxor behavior, the modiﬁed Curie-Weiss law is often used to approximate the temperature dependence of ε(T) above Tm (Equation (3)) [23]. ε T − T γ m − 1 = m (3) ε(T) 2σ

Here, εm is the maximal value of the dielectric permittivity, Tm is the temperature at the maximum permittivity, the parameter σ describes the broadening of the dielectric peak, and the exponent γ reflects the degree of the relaxor behavior. For classical ferroelectric materials, the value of γ is equal to 1; while for the canonical relaxors, its value approaches 2 [24]. An example of the fit of ε(T) by Equation (3) for the P(VDF-TrFE-CFE) 68/32/8.5 film is shown in Figure3b. The fitting curve matches the experimental data well. The best fit values for γ for the studied polymers lie between 1.5 and 1.7 (Table3), which confirms the strong relaxor behavior. PolymersPolymers 20212021, 13, 13, x, FOR 1343 PEER REVIEW 5 of5 of13 12

60 0.6 (a) (f)

1 kHz - 1000 kHz 40 0.4 δ Tan 20 0.2

Dielectric permittivity Dielectric 51.3/48.7/6.2 0 0.0 80 (b) (g) c 0.6

δ 0.4 40 Tan 0.2 20 Dielectric permittivity Dielectric 63.8/36.2/7.2 0 0.0 80 (c) (h) 0.6 60 δ 0.4 40 Tan

0.2 20 Dielectric permittivity Dielectric 59.8/40.2/7.3 0 0.0 80 (d) (i) 0.8

60 0.6 δ 40 Tan 0.4

20 0.2 Dielectric permittivity Dielectric 70/30/8.1 0 0.0 60 (e) 0.6 (j) i

40 δ 0.4 Tan 20 0.2

Dielectric permittivity Dielectric 68/32/8.5 0 0.0 280 300 320 340 360 280 300 320 340 360 Temperature [K] Temperature [K]

Figure 2. Temperature dependence of the dielectric permittivity (a–e) and dielectric loss tangent

Figure(f–j) of2. P(VDFTemperaturex-TrFE 1-xdependence-CFEy) ﬁlms of measuredthe dielectric at frequencies permittivity from (a–e 1) and kHz dielectric to 1 MHz loss upon tangent cooling. (f–j) of P(VDFx-TrFE1-x-CFEy) films measured at frequencies from 1 kHz to 1 MHz upon cooling. Table 3. Parameters describing the relaxor behavior of the studied the P(VDFx-TrFE1-x-CFEy) ﬁlms. Table 3. Parameters describing the relaxor behavior of the studied the P(VDFx-TrFE1-x-CFEy) films. Degree of Fitting Parameters for the P(VDFx-TrFE -CFEy) Freezing 1-x Relaxor FittingVogel-Fulcher Parameters Equation for the P(VDFx-TrFE(mol.1-x %)-CFEy) Degree of Relaxor FreezingTemperature, Tempera-Tf (K) Behavior, γ Vogel-Fulcher Equation (mol. %) Behavior, γ ture, Tf (K) ln(f 0) Ea (meV) ln(f0) Ea (meV) 51.3/48.7/6.2 1.61 306 ± 2 15.8 ± 0.9 6 ± 2 51.3/48.7/6.2 1.61 306 ± 2 15.8 ± 0.9 6 ± 2 63.8/36.2/7.2 1.52 285 ± 4 16.3 ± 0.8 14 ± 4 63.8/36.2/7.259.8/40.2/7.3 1.52 1.64 285286 ± 43 16.317.3 ± 0.80.7 1614 ± 44 59.8/40.2/7.370/30/8.1 1.64 1.50 286280 ± 32 17.317.5 ± 0.70.5 2116 ± 43 70/30/8.168/32/8.5 1.50 1.55 280278 ± 22 17.517.6 ± 0.50.5 2021 ± 33 68/32/8.5 1.55 278 ± 2 17.6 ± 0.5 20 ± 3

Unlike ferroelectric materials, the relaxors do not undergo a phase transition into the ferroelectric state at the Curie temperature. Instead, the slowing-down of the PNRs dynamics results in a transition into a glassy-like state with short-range correlated polarization (non-ergodic relaxor state) at the freezing temperature, Tf [9]. The freezing temperature can be determined by using the Vogel-Fulcher equation (Equation (4)) [25].

Polymers 2021, 13, x FOR PEER REVIEW 6 of 13

𝐸 𝑓 = 𝑓 ×𝑒𝑥𝑝 (4) 𝑘𝑇(𝑓) −𝑇

Here, f is the frequency of the applied electric field, Tm(f) is the corresponding temperature of the maximum of the dielectric permittivity, Tf is the freezing temperature, Ea is the activation energy, f0 is the attempt frequency, and k is the Boltzmann constant. Figure 3a shows an example of the Vogel-Fulcher fit of Tm(f) for the P(VDF-TrFE-CFE) 68/32/8.5 film. The best-fitting parameters for the studied samples are given in Table 3. Figure 3c illustrates the variation in the freezing temperature for the different compositions. One can see that with an increase in the CFE content, the freezing temperature decreases, while the activation energy increases. In general, it can be concluded that the increase in the CFE content ex- Polymers 2021 13 , , 1343 pands the range of the ergodic relaxor state towards lower temperatures. 6 of 12

16 310 a c 14 300 12 (K) f T 10 290 ln(f) (Hz)ln(f)

8 280 6 300 310 320 330 340 .3 .5 /7 /8 2 2 Tm (K) 0. /3 70/30/8.1 68 1.3/48.7/6.2 9.8/4 0 5 63.8/36.2/7.2 5 b -2 ε )/ ε - -4 Slope = 1.55 max ε

ln( -6

-8 024

ln(T-Tmax) (K)

Figure 3. Examples of (a) the Vogel-Fulcher and (b) the modified Curie-Weiss law fitting for the Figure 3. Examples of (a) the Vogel-Fulcher and (b) the modified Curie-Weiss law fitting for the P(VDF-TrFE-CFE) 68/32/8.5 film. (c) The freezing temperatures, Tf, of the studied samples. P(VDF-TrFE-CFE) 68/32/8.5 film. (c) The freezing temperatures, Tf, of the studied samples. Unlike ferroelectric materials, the relaxors do not undergo a phase transition into the 3.3.ferroelectric Polarization state Hysteresis at the Curie Behavior temperature. Instead, the slowing-down of the PNRs dynamics resultsThe polarization in a transition hysteresis into a glassy-like loops were state measured with short-range upon heating correlated as well polarization as cooling. The(non-ergodic results obtained relaxor during state) atthe the cooling freezing cycle temperature, are shown inTf Figure[9]. The 4. freezingThe 59.8/40.2/7.3 temperature ter- polymercan be determined exhibits slim by polarization using the Vogel-Fulcher hysteresis loops equation typical (Equation for relaxors (4)) [(Figure25]. 4b). A particular feature of other compositions is a jump of the polarization above a certain electric ! field value, which results in the DHL shape (FigureEa 4). Such DHL-shape was already re- f = f0 × exp (4) ported for some P(VDF-TrFE-CFE) polymers,k(T e.g.,m( f ) for− T 59.2/33.6/7.2f ) (63.8/36.2/7.8 in our notation). The DHL was explained in terms of a reversible, electric field-induced relaxor to ferroelectricHere, f is phase the frequency transition of [12]. the applied electric field, Tm(f) is the corresponding tem- peratureWe have of the taken maximum a detailed of look the dielectric at the polari permittivity,zation curvesTf is shown the freezing in Figure temperature, 4 to evalu- ateEa isthe the critical activation fields energy,correspondingf 0 is the to attempt the transition frequency, to the and ferroelectric k is the Boltzmann state, E1, and constant. back toFigure the relaxor3a shows state, an E2 example. These fields of the were Vogel-Fulcher estimated from fitof theT maximam(f) for theof the P(VDF-TrFE-CFE) field derivative of68/32/8.5 polarization. film. FigureThe best-fitting 5 shows the parameters temperature for thedependences studied samples of E1 and are E given2 for the in studied Table3. compositions.Figure3c illustrates It can the be variationobserved in that the the freezing P(VDF-TrFE-CFE) temperature for 63.8/36.2/7.2 the different (Figure compositions. 5b) and One can see that with an increase in the CFE content, the freezing temperature decreases, while the activation energy increases. In general, it can be concluded that the increase in the CFE content expands the range of the ergodic relaxor state towards lower temperatures.

3.3. Polarization Hysteresis Behavior The polarization hysteresis loops were measured upon heating as well as cooling. The results obtained during the cooling cycle are shown in Figure4. The 59.8/40.2/7.3 terpolymer exhibits slim polarization hysteresis loops typical for relaxors (Figure4b). A particular feature of other compositions is a jump of the polarization above a certain electric ﬁeld value, which results in the DHL shape (Figure4). Such DHL-shape was already reported for some P(VDF-TrFE-CFE) polymers, e.g., for 59.2/33.6/7.2 (63.8/36.2/7.8 in our notation). The DHL was explained in terms of a reversible, electric ﬁeld-induced relaxor to ferroelectric phase transition [12]. Polymers 2021, 13, x FOR PEER REVIEW 7 of 13

P(VDF-TrFE-CFE) 68/32/8.5 (Figure 5d) samples have the largest critical field values, (E1 = 80–90 MV·m−1) at room temperature; while for the P(VDF-TrFE-CFE) 51.3/48.7/6.2 (Figure 5a) the transition to the ferroelectric state occurs already at 40 MV·m−1. For all the studied Polymers 2021, 13, 1343 7 of 12 polymers, both E1 and E2 increase with temperature except for P(VDF-TrFE-CFE) 51.3/48.7/6.2, where E1 goes slightly down upon heating. ) ) 2 (a) 2 (d) 4 4 298 K C/cm 313 K C/cm μ 323 K μ 0 0

-4 -4 Polarization ( 51.3/48.7/6.2 Polarization ( 63.8/36.2/7.2 ) ) 2 (b) 2 (e) 4 4 C/cm C/cm μ μ

0 0

-4 -4 Polarization ( 59.8/40.2/7.3 Polarization ( 70/30/8.1 ) 2 (c) -100 -50 0 50 100 4 Electric Field (MV/m) C/cm μ

Polarization ( -4 68/32/8.5 -150 -100 -50 0 50 100 150 Electric Field (MV/m) FigureFigure 4.4. Polarization hysteresis loops ofof thethe P(VDF-TrFE-CFE)P(VDF-TrFE-CFE) 51.3/48.7/6.251.3/48.7/6.2 (a (a),), 59.8/40.2/7.3 59.8/40.2/7.3 (b (),b ), 68/32/8.568/32/8.5 (c (),c), 63.8/36.2/7.2 63.8/36.2/7.2 (d), (d and), and 70/30/8.1 70/30/8.1 (e) films (e) films measured measured as a asfunction a function of temperature of temperature at 10 at 10Hz Hz (triangular (triangular wave) wave) on on cooling. cooling.

WeWhen have we taken refer a to detailed the field-induced look at the polarizationrelaxor–ferroelectric curves shown transition in Figure in such4 to inorganic evaluate therelaxors critical as fields(Pb,La)(Zr,Ti)O corresponding3 [26] or to Na the0. transition5Bi0.5TiO3-BaTiO to the ferroelectric3, [27] it can be state, seenE that1, and the back critical to thefield relaxor value state,decreasesE2. These with temperature fields were estimated in the non-ergodic from the maxima relaxor ofstate, the but field increases derivative with of polarization.temperature in Figure the ergodic5 shows relaxor the temperature state. The non-ergodic dependences relaxor of E1 andstateE is2 forthe thestate studied below compositions.the freezing temperature, It can be observed where the that interactio the P(VDF-TrFE-CFE)n between PNRs 63.8/36.2/7.2blocks their reorientation (Figure5b) andand P(VDF-TrFE-CFE)related dynamics. 68/32/8.5According (Figure to the 5estimatid) samplesons from have the dielectric largest critical data, fieldthe freezing values, −1 (temperatureE1 = 80–90 MV of· mthe P(VDF-TrFE-CFE)) at room temperature; 51.3/48.7/6.2 while for sample the P(VDF-TrFE-CFE) lies at 306 K, i.e., 51.3/48.7/6.2 the range −1 (Figurewhere 5DHLa) the is transition observed tois thearound ferroelectric the transiti stateon occurs from alreadythe non-ergodic at 40 MV to·m the. ergodic For all there- studiedlaxor state. polymers, On the bothotherE hand,1 and otherE2 increase compositio withns temperature are in the ergodic except forrelaxor P(VDF-TrFE-CFE) state far from 51.3/48.7/6.2,the freezing temperature where E1 goes and slightly we observe down a upon growing heating. critical field within a temperature rangeWhen of 298 we K referto 313 to K, the while field-induced above 313 relaxor–ferroelectricK, a single hysteresis transition loop (SHL) in suchis observed. inorganic relaxorsAnother as (Pb,La)(Zr,Ti)O remarkable feature3 [26] or of Na the0.5 Bipolarizati0.5TiO3-BaTiOon hysteresis3,[27] it loops can be in seen all studied that the samples critical fieldis the value broadening decreases of the with hysteresis temperature loops in with the temperature, non-ergodic relaxorwhich leads state, to but an increasesincrease in with the temperaturemeasured values in the of ergodicthe remnant relaxor polarization state. The as non-ergodic well as the relaxorcoercive state field. is This the statebroadening below theis related freezing to temperature,the contribution where of leakage the interaction current [28], between which PNRs increases blocks with their temperature, reorientation as andcan also related be seen dynamics. in the dielectric According loss to curves the estimations at low-frequency from the (1 dielectrickHz) in Figure data, the2. In freezing the case temperatureof polymers, ofthis the leakage P(VDF-TrFE-CFE) current can be 51.3/48.7/6.2 related to the samplefield-induced lies at transport 306 K, i.e., of theelectronic range whereand ionic DHL charges. is observed In some is around publications, the transition this increase from the in the non-ergodic extrinsic polarization to the ergodic was relaxor con- state. On the other hand, other compositions are in the ergodic relaxor state far from the sidered as a signature of a negative electrocaloric effect (ECE) [29–31]. freezing temperature and we observe a growing critical field within a temperature range of 298 K to 313 K, while above 313 K, a single hysteresis loop (SHL) is observed.

PolymersPolymers 20212021, 13, 13, x, 1343FOR PEER REVIEW 8 of8 of 13 12

45 (a) 90 (b) E1 40 E2 80 35 30 70 (MV/m) (MV/m)

cr 25 cr E E 60 20 51.3/48.7/6.2 63.8/36.2/7.2 15 50 300 305 310 315 320 300 305 310 315 320 T (K) T (K)

50 (c)120 (d) 45 110 100 40 90 (MV/m) (MV/m)

cr 35 cr

E E 80 30 70/30/8.170 68/32/8.5 25 60 300 305 310 315 320 300 305 310 315 320 T (K) T (K)

FigureFigure 5. 5.TemperatureTemperature dependences dependences of the of critical the critical fields, fields, E1 andE1 Eand2, correspondingE2, corresponding to the tofield- the field- inducedinduced transitions transitions between between relaxor relaxor and and ferroelect ferroelectricric statesstates forfor thethe P(VDF-TrFE-CFE) P(VDF-TrFE-CFE) 51.3/48.7/6.2 51.3/48.7/6.2 ( a), (a63.8/36.2/7.2), 63.8/36.2/7.2 ( b(b),), 70/30/8.1 70/30/8.1 ((cc),), andand 68/32/8.5 68/32/8.5 ( (d) polymer films.films.

TheAnother ECE, which remarkable is the featurechange ofin thethe polarization temperature/entropy hysteresis of loops a dielectric in all studied material samples un- deris thean adiabatically/isothermally broadening of the hysteresis applied loops with electr temperature,ic field, has whichgained leads popularity to an increase as an envi- in the ronmentallymeasured values friendly of thetechnology remnant for polarization the development as well asof thecompact coercive solid-state field. This refrigeration broadening andis relatedair-conditioning to the contribution systems [32–35]. of leakage Since current direct electrocaloric [28], which increases measurements with temperature, need par- ticularas can devices also be and seen modifications, in the dielectric the indirect loss curves method at low-frequency is more widely (1 used kHz) [32,36,37]. in Figure 2The. In indirectthe case method of polymers, is based this on leakage the Maxwell current relation can be ( related∂S/∂E)T to= ( the∂P/∂ field-inducedT)E. In this case, transport the iso- of fieldelectronic temperature and ionic dependences charges. In of some the publications,polarization obtained this increase from in the the hysteresis extrinsic polarization loops are usedwas to considered evaluate the as aelectrocal signatureoric of achange negative of the electrocaloric temperature effect (ECE) [29–31]. The ECE, which is the change in the temperature/entropy of a dielectric material under an adiabatically/isothermally applied electric field, has gained popularity as an ∆T =− (𝑇/𝐶)(∂P/ ∂T)𝑑𝐸 (5) environmentally friendly technology for the development of compact solid-state refrigeration and air-conditioning systems [32–35]. Since direct electrocaloric measurements need From this equation, it is obvious that a positive temperature derivative of polariza- particular devices and modifications, the indirect method is more widely used [32,36,37]. tion implies a negative electrocaloric effect, as was reported by some groups [29–31]. The indirect method is based on the Maxwell relation (∂S/∂E) = (∂P/∂T) . In this case, the While the direct electrocaloric measurements of similar compositionsT onlyE show a positive isofield temperature dependences of the polarization obtained from the hysteresis loops electrocaloric effect [38,39]. Thus, the leakage-related extrinsic contribution to the polari- are used to evaluate the electrocaloric change of the temperature zation may lead to erroneous evaluation of the electrocaloric effect in ferroelectric and

relaxor polymers [40]. Z E2 As we mentioned already,∆T ECthe= relaxor− beha(T/CviorE)( ∂inP/ P(VDF-TrFE-CFE)∂T)EdE is promoted by(5) E1 the incorporation of bulky CFE monomers, which results in an increasing distance between Fromthe P(VDF-TrFE-CFE) this equation, it is obviouschains. This that aweak positiveens temperaturethe cooperative derivative polarization of polarization of the P(VDF-TrFE)implies a negative dipoles electrocaloric and reduces effect,the size as of was the reportedferroelectric by somedomains groups to the [29 –nanoscale.31]. While Thethe structure direct electrocaloric of relaxor P(VDF-TrFE-CFE) measurements of similarcontains compositions more or less only random show TmG a positive (m ≤ elec- 4) sequences,trocaloric effectand the [38 FE,39 ].structure Thus, the contains leakage-related relatively extrinsic long Tn contribution (n > 4) sequences to the polarization (T and G representmay lead the to trans erroneous and gauche evaluation conformations, of the electrocaloric respectively) effect [5]. in ferroelectric and relaxor polymersThe CFE [40 monomers]. possess their own dipole moment and can serve as dipole defect- pinningAs centers, we mentioned similar to already, dipole defects the relaxor built behaviorby oxygen in vacancies P(VDF-TrFE-CFE) and acceptor is promoted impuri- tiesby in the perovskite incorporation ferroelectrics of bulky [41]. CFE When monomers, the magnitude which results of the in applied an increasing electric distancefield is largebetween enough, the the P(VDF-TrFE-CFE) dipoles of CFE may chains. rotate, This promoting weakensthe the cooperativeswitching of polarization the surrounding of the dipolesP(VDF-TrFE) and the dipoles formation and of reduces a ferroelectric the size st ofate the with ferroelectric large domains domains [5]. to As the the nanoscale. field is

Polymers 2021, 13, 1343 9 of 12

The structure of relaxor P(VDF-TrFE-CFE) contains more or less random TmG (m ≤ 4) sequences, and the FE structure contains relatively long Tn (n > 4) sequences (T and G represent the trans and gauche conformations, respectively) [5]. The CFE monomers possess their own dipole moment and can serve as dipole defect- pinning centers, similar to dipole defects built by oxygen vacancies and acceptor impurities in perovskite ferroelectrics [41]. When the magnitude of the applied electric field is large enough, the dipoles of CFE may rotate, promoting the switching of the surrounding dipoles and the formation of a ferroelectric state with large domains [5]. As the field is reduced and eventually removed, these FE domains transform back to the relaxor phase. In the ergodic relaxor state, the PNRs are relatively free to rotate and can be easily aligned. Meanwhile, the thermal agitation breaks this alignment and therefore the critical field, E1, increases upon heating. When the sample is in the non-ergodic relaxor state, the frozen PNRs are relatively difficult to be reoriented, but they become less blocked on approaching the freezing temperature; therefore, we observe that the critical field decreases upon heating for P(VDF-TrFE-CFE) 51.3/48.7/6.2. The disappearance of the DHL with temperature can be attributed to the fact that the polymer chains become mobile with temperature and the pinning effect due to the presence of CFE in the polymer chain becomes less pronounced. However, this process is also obscured by the broadening of the hysteresis loops due to the increased conductivity.

3.4. Energy Storage Properties The stored and recoverable electrical energy densities are given by Equations (1) and (2). The stored energy cannot be fully recovered due to the leakage current and the hysteresis losses (Uloss). The energy storage efficiency, η, is defined as the ratio between the discharged and stored energy of the capacitor (Equation (6)) [42,43].

η = Udischarged/Ustored = Udischarged/(Udischarged + Uloss) (6)

All these characteristics can be deduced from the unipolar polarization hysteresis curves recorded for a different maximum electric field (Figure S3). Figure6 shows the maximum field dependences of the discharged energy density and energy storage efficiency calculated at room temperature. All compositions show a linear increase in the discharged energy density with the applied electric field. However, their energy storage efficiency decreases with the increasing field. There is a tradeoff between the discharged energy density and the energy storage efficiency due to the hysteresis losses. As is clear from Figure6, the P(VDF-TrFE-CFE) = 59.8/40.2/7.3 sample outperforms the other compositions in terms of the discharged energy density as well as the energy storage efficiency. It shows the highest energy density that reaches η = 5 J·cm−3 at a field of 200 MV·m−1, while retaining an efficiency of around 63%. The P(VDF-TrFE- CFE) 63.8/36.2/7.2 and 68/32/8.5 films show the comparable discharged energy density at a field of 100 MV·m−1, but have a lower energy storage efficiency due to the DHL behavior. In Table4, the discharged energy density of different compositions at a field of 100 MV·m−1 are compared with data reported in the literature. One can see that the discharged energy density of the polymers studied in this work is consistent with the previously reported values. Polymers 2021 13 Polymers 2021, 13, 1343x FOR PEER REVIEW 10 of 13 12

6 100 (a) P(VDF-TrFE-CFE) (b) 51.3/48.7/6.2 63.8/36.2/7.2 )

3 59.8/40.2/7.3 90 70/30/8.1 68/32/8.5 4 80

70 2

60 Charge-Discharge Efficiency (%) Efficiency Charge-Discharge Discharged Energy Density (J/cm Density Energy Discharged

0 50 0 50 100 150 200 0 50 100 150 200

Electric Field (MV/m) Electric Field (MV/m) Figure 6. The discharged energy densities ( a) and charge–dischargecharge–discharge efficiencies efﬁciencies ( b) plotted against maximum applied electric field ﬁeld (estimated from the unipolar hysteresis loopsloops shownshown inin FigureFigure S3).S3).

Table 4. Comparison of the discharged energyenergy densitydensity forfor differentdifferent P(VDF-TrFE-CFE) P(VDF-TrFE-CFE) compositions. compositions. Electric Field Discharged Energy Density P(VDFx-TrFE1-x-CFEy) Electric Field−1 Discharged Energy Density−3 Reference P(VDFx-TrFE1-x-CFEy) (MV·m ) (Udischarged) (J·cm ) Reference (MV·m−1) (Udischarged) (J·cm−3) 51.3/48.6/6.2 100 1.2 This work 51.3/48.6/6.263.8/36.2/7.2 100 100 1.2 2 This This work 63.8/36.2/7.264/36/7.2 100 100 1.8 2 This [44 work] 64/36/7.270/30/8.1 100 100 1.381.8 This [44] work 70/30/8.170/30/8.1 100 100 1.38 2 This [44 work] 63/37/8.1 100 1.7 [45] 70/30/8.1 100 2 [44] 68/32/8.5 100 2 This work 59.8/40.2/7.363/37/8.1 100 100 1.7 2 This [45] work 68/32/8.5 100 2 This work 4. Conclusions59.8/40.2/7.3 100 2 This work Dielectric and polarization properties of P(VDF-TrFE-CFE) polymers with different 4. Conclusions compositions are compared. All samples show a similar degree of relaxor behavior. In- creasingDielectric the CFE and content polarization shifts theproperties freezing of temperature P(VDF-TrFE-CFE) down and polymers stabilizes with the different ergodic compositionsrelaxor state at are room compared. temperature. All samples The application show a ofsimilar a strong degree enough of relaxor electric behavior. field induces In- creasinga transition the intoCFE acontent ferroelectric shifts state,the freezing as manifested temperature in double down hysteresis and stabilizes loops. the The ergodic value relaxorof the critical state at electric room fieldtemperature. depends onThe the application proximity of to a the strong freezing enough temperature. electric field Correct in- ducesindirect a transition measurements into a of ferroelectric the electrocaloric state, as effect manifested are impossible in double in thehysteresis studied loops. materials The valuedue to of the the charge critical leakage electric extrinsic field depends contribution on the to proximity polarization to enhancedthe freezing at higher temperature. temper- Correctatures. Directindirect electrocaloric measurements measurements of the electroc are necessary.aloric effect In particular,are impossible it will in be the interesting studied materialsto study adue relation to the between charge theleakage field-induced extrinsic transitioncontribution to theto ferroelectricpolarization stateenhanced and the at higherelectrocaloric temperatures. effect at Direct the corresponding electrocaloric field measurements values. are necessary. In particular, it will beThe interesting characteristics to study of thea relation polymer betw filmseen for the energy field-induced storage applicationtransition to were the ferroe- tested. lectricA maximum state and energy the electrocaloric density of 5 effe J·cmct− at3 atthe 63% corresponding charge–discharge field values. efficiency and a field of 200The MV characteristics·m−1 is observed of the for polymer the new films composition for energy P(VDF-TrFE-CFE) storage application 59.8/40.2/7.3. were tested. To A maximum energy density of 5 J cm−3 at 63% charge–discharge efficiency and a field of

Polymers 2021, 13, 1343 11 of 12

further improve these characteristics, composites with inorganic ﬁllers can be developed based on these terpolymers. Such studies are now in progress.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/polym13081343/s1, Figure S1: The ternary compositional diagram of P(VDF-TrFE-CFE) indicating the studied compositions, Figure S2: Heat flow curves measured by DSC upon heating (above) and cooling (below), where the peaks correspond to melting and recrystallization, respectively, Figure S3: Unipolar polarization hysteresis curves measured as a function of voltage at 10 Hz and room temperature. Author Contributions: Conceptualization, Q.Z., X.C. and V.V.S.; methodology, Y.H. and X.C.; formal analysis, Y.H.; investigation, Y.H., D.L., C.H.H. and I.M.; resources, D.C.L.; data curation, Y.H. and V.V.S.; writing—original draft preparation, Y.H.; writing—review and editing, V.V.S., D.C.L. and Q.Z.; visualization, Y.H. and V.V.S.; supervision, V.V.S.; project administration, D.C.L.; funding acquisition, D.C.L. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: Qiming Zhang is grateful to the U. S. Office of Naval Research (award number N00014-19-1-2028) and to Alexander von Humboldt Foundation for the support. Conflicts of Interest: The authors declare no conflict of interest.

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