Enhancing discharged energy density and suppressing dielectric loss of poly(vinylidene fluoride-ter-trifluoroethylene-ter-chlorofluoroethylene) by a sandwiched structure

: Polymer dielectrics with high energy density and low dielectric loss are highly desired due to the rapid development of electric devices. Among known polymers, poly(vinylidene fluoride-ter-trifluoroethylene-ter-chlorofluoroethylene) P(VDF-TrFE-CFE) is one of the promising materials for energy storage capacitor applications because of its high dielectric constant. Nevertheless, it suffers from high dielectric loss especially at the high electric field, which suppresses its breakdown strength and energy storage density. Herein, sandwiched structure dielectric films were fabricated by employing polymethyl methacrylate (PMMA) as the outer layer and P(VDF-TrFE-CFE) as the central layer. By modulating the thickness of the central layer, an enhanced discharged energy density of 7.03 J/cm 3 is achieved at a high electric field of 480 MV/m, which is 132% more than that of P(VDF-TrFE-CFE) at its maximum electric field 300 MV/m. Meanwhile, this sandwiched structure film also retains a high discharge efficiency of 78% at 480 MV/m, which is never been seen in polyvinylidene fluoride-based polymers. Results show that PMMA acts as charge barrier and simultaneously enhance the breakdown strength and suppress the dielectric loss of P(VDF-TrFE-CFE).


Introduction
In the past several decades, ever decreasing natural energy resources urged human to find new energy resources. Therefore, lots of work have been conducted to alleviate this problem, such as Li-ion batteries [1,2], fuel cells [3,4], supercapacitors [5,6], and dielectric capacitors [7][8][9][10][11][12]. Among these energy storage devices, dielectric capacitors possess the highest power density and are extensively used in electrical weapons, transistors, and inverters. However, its energy density is too low to satisfy the tendency of miniaturisation of modern electrical devices. For example, the energy density of commercialised biaxially oriented polypropylene (BOPP) films is suppressed due to its low dielectric constant (∼2.2 at 1 kHz), which is not acceptable in one little electrical component.
The energy density (U e ) of dielectric materials can be calculated from U e = ∫ E dD [13], where E is the electric field, and D is the electrical displacement. For linear dielectrics, such as polypropylene and polyethylene, energy density can be obtained from U e = (1/2)DE = (1/2)ε γ ε 0 E b 2 [14], where ε γ and E b are relative permittivity and breakdown strength of dielectric materials, respectively, ε 0 is the vacuum permittivity (8.85 × 10 −12 F/m). Therefore, permittivity and breakdown strength determine the energy density of dielectric materials. For BOPP, though its breakdown strength as high as 700 MV/m which is ascribed to its ultralow dielectric loss (∼0.0002 at 1 kHz) [15], its energy storage is restricted to lower than 5 J/cm 3 depending on the processing conditions.
Over the past several decades, PVDF (polyvinylidene fluoride)based polymers have been studied for energy storage capacitors and were comprehensively researched by many scientists due to its high permittivity and high breakdown strength. For instance, the dielectric constant of PVDF film is about 10 at 1 kHz and it could exhibit an energy density as high as 14 J/cm 3 depending on the crystalline phase of PVDF [16]. Because of strong ferroelectric behaviour of PVDF resulting low charge-discharge efficiency, trifluoroethylene (TrFE) [17,18], chlorotrifluoroethylene (CTFE) [19][20][21], hexafluoropropylene (HFP) [22,23], and chlorfluoroethylene (CFE) [24] are employed to copolymerise with VDF to generate copolymers and terpolymers [25,26] with high energy efficiency. Among these copolymers and terpolymers, poly(vinylidene fluoride-ter-trifluoroethylene-terchlorofluoroethylene) (P(VDF-TrFE-CFE)) exhibits a relaxor ferroelectric behaviour and high permittivity (∼50 at 1 kHz) [26,27], which has drawn lots of attention for energy storage applications. However, its poor mechanical properties ended up with its low dielectric strength comparing with other PVDF-based polymers [26]. As a consequence, its potential application in energy storage capacitors is confined by its low breakdown strength.
Herein, a sandwiched structure polymer films with P(VDF-TrFE-CFE) in the middle layer and polymethyl methacrylate (PMMA) in the outer layer were presented by employing a step-bystep solution casting method. As known, PMMA is a linear polymer with a glass transition temperature about 105°C [28], which makes its polymer chains in the glassy state resulting low dielectric constant and relatively low loss tangent at room temperature. Based on previous research results [10,29] that introducing low loss tangent materials into outer layer could notably reduce the charge injection from electrodes thus improving the charge-discharge efficiency. Remarkably suppressed dielectric loss and enhanced dielectric strength were realised by employing PMMA as the outer layer and P(VDF-TrFE-CFE) as the central layer. By designing the thickness of each layer, a maximum discharged energy density of 7.03 J/cm 3 with high efficiency of 78% were obtained, which are 113 and 120% of the corresponding value of pure P(VDF-TrFE-CFE), respectively. Firstly, PMMA (Adamas, Tansoole, China) and P(VDF-TrFE-CFE) (PiezoTech, France) were dissolved in Acetone (AR) and DMF (AR) at a mass concentration of 100 mg/ml, respectively. The multilayer films were obtained by three solution casting process as shown in Fig. 1a. Each process was conducted after the previous layer was fully dried. The thickness of each layer was controlled by the drawing knife. For comparison, the outer layers of PMMA were carefully set to about 4 μm. The central layer of P(VDF-TrFE-CFE) is varied by changing the distance of the drawing knife. The succinct code of P(VDF-TrFE-CFE)-200 and P(VDF-TrFE-CFE)-300 represent the thickness of the drawing knife while casting the central layer is 200 and 300 μm, respectively. As for P(VDF-TrFE-CFE)-300, the thickness of the drawing knife while casting the central layer is 300 μm, which indicates that the thickness of the wet film is 300 μm. After the solvent evaporates thoroughly, the thickness of the thus obtained film is about 20 μm. Therefore, the number behind P(VDF-TrFE-CFE) represents the thickness of the wet film. The thickness of the central layer for P(VDF-TrFE-CFE)-200 and P(VDF-TrFE-CFE)-300 is about 9 and 12 μm, respectively. After the third layer was casted, the films were further dried at 40°C in a vacuum oven overnight followed by heating at 200°C for 7 min and immediately quenched in ice water. Then the films were peeled from the substrate (glass plate) and dried at 40°C in vacuum for 12 h to be ready for the test. For comparison, PMMA and P(VDF-TrFE-CFE) films were conducted by the same procedure.

Characterisation
The scanning electron microscopy (SEM, Nova NanoSEM 450, FEI, USA) was introduced to detect the morphologies of the cross-section of multilayer films, which was prepared by fracturing in liquid nitrogen. The broadband dielectric spectroscope (BDS) was measured by employing a Novocontrol Alpha-A high resolution dielectric analyser (GmbH Concept 40) from 10 −1 to 10 6 Hz at room temperature. For BDS test, both sides of samples were sputter coated with copper with a diameter of 12 mm. Precision Multiferroic Materials Analyser (Radiant Inc.) was utilised to observe the D-E loops (measured at 10 Hz) and I − V curves. Copper with a diameter of 3 mm was sputtered on both sides of films for those tests.

Results and discussion
Two different sandwiched films with a varying thickness of central layer were fabricated. From Fig. 1b, it can be seen that the three layers are closely integrated without any obvious pores. According to Fig. 1b, the thickness of P(VDF-TrFE-CFE)-300 is about 20 μm. Fig. 1c shows the F element mapping of the cross-section of P(VDF-TrFE-CFE)-300 and the corresponding SEM picture is exhibited in Fig. 1d. It is clearly seen that the central layer containing abundant F element fills in the zone between two white dotted lines. This means that sandwiched structure films with P(VDF-TrFE-CFE) in the central layer are successfully fabricated. Meanwhile, it is hard for PMMA to dissolve in DMF at room temperature. Therefore, the second solution casting process would not affect the flatness of the PMMA layer. In addition, P(VDF-TrFE-CFE) could not easily dissolve in acetone at room temperature. So the second layer remains its flatness during the third solution casting process. As a result, these three layers possess a flat surface. Fig. 2a shows the frequency dependence of the dielectric constant of four dielectric films. P(VDF-TrFE-CFE) possesses the highest permittivity (∼50 at 1 kHz) due to its small domain size and low curie temperature [26,27]. The lowest permittivity of ∼3.2 at 1 kHz of PMMA is obtained which can be ascribed to the low polarity of MMA. For sandwiched films, the dielectric constant increases from 4.1 of P(VDF-TrFE-CFE)-200 to 5.2 of P(VDF-TrFE-CFE)-300 at 1 kHz. A detailed discussion about low permittivity of P(VDF-TrFE-CFE)-200 and P(VDF-TrFE-CFE)-300 will be shown later.
The dielectric loss and conductivity dependency of frequency are illustrated in Figs. 2b and c. The dielectric loss of P(VDF-TrFE-CFE) at both high frequencies and low frequencies shows an increasing tendency although its value is higher than other samples. After introducing PMMA into the outer layer of sandwiched structure films, the dielectric loss is remarkably suppressed and remains at the same level with PMMA at frequencies ranging from 10 −1 to 10 4 Hz. Normally, for PVDF-based polymer films, the dielectric loss at high frequencies originates from the dielectric relaxation of molecule chain in the amorphous phase and that at low frequencies derive from interfacial or ionic polarisation [14,30]. Therefore, PMMA could reduce the ionic conduction at low frequencies as pure P(VDF-TrFE-CFE) barely exists interfacial polarisation. In addition, PMMA could restrict the relaxation loss at high frequencies. At frequencies between 10 4 and 10 6 Hz, the dielectric loss of two sandwiched structure films is slightly higher  than PMMA, which may be attributed to the high dielectric loss of P(VDF-TrFE-CFE) from the central layer. A modest dielectric loss increase can be seen in P(VDF-TrFE-CFE)-300 comparing with P(VDF-TrFE-CFE)-200, which verified previously suppose. The same trend can be seen in Fig. 2c, P(VDF-TrFE-CFE)-200 and P(VDF-TrFE-CFE)-300 share the same conductivity from 10 −1 to 10 6 Hz, which is approximately one order of magnitude smaller than that of P(VDF-TrFE-CFE). At low frequencies, nearly two orders of magnitude decrease of conductivity can be seen in sandwiched structure films comparing with P(VDF-TrFE-CFE). The temperature dependence of the real part of the dielectric constant of these four samples could be seen in Fig. 3. For pure P(VDF-TrFE-CFE) (as shown in Fig. 3d), ε r ′ difference under varied frequency becomes obvious around T g . Then its ε r ′ starts to decrease as temperature beyond its T c , which is around the room temperature. As for pure PMMA, its ε r ′ changes under a relatively slim range. For sandwiched structure samples, a similar tendency but higher ε r ′ could be seen in Fig. 3. Meanwhile, no marked change in ε r ′ could be seen in Figs. 3a and b. In addition, their highest ε r ′ at 100°C is only nearly 12 at 1 Hz, which is only a tenth of pure P(VDF-TrFE-CFE). These results are inconsistent with dielectric properties under room temperature.
D-E loops of PMMA, P(VDF-TrFE-CFE), P(VDF-TrFE-CFE)-200, and P(VDF-TrFE-CFE)-300 under monopolar electric field are shown in Fig. 4. PMMA exhibits the slimmest curves and highest maximum dielectric strength with 520 MV/m, but the electrical displacement was suppressed by its low permittivity. For P(VDF-TrFE-CFE), it shows an electrical displacement of about 0.08 C/m 2 at 300 MV/m. However, it could not endure an electric field >300 MV/m. Interestingly, both P(VDF-TrFE-CFE)-200 and P(VDF-TrFE-CFE)-300 could survive an electric field exceeds 450 MV/m, which is ∼150% that of P(VDF-TrFE-CFE). In addition, their remnant polarisation both are lower than 0.005 C/m 2 even at an electric field exceeds 400 MV/m, which indicates that both sandwiched structure films possess higher charge-discharge efficiency than pure P(VDF-TrFE-CFE).
The energy density and efficiency of four samples derived from Fig. 4 are exhibited in Figs. 5a-c. The maximum charge energy density of P(VDF-TrFE-CFE) is 9.56 J/cm 3 , while P(VDF-TrFE-CFE)-300 exhibits a slightly less energy density of 9.03 J/cm 3 and P(VDF-TrFE-CFE)-200 owns an even lower energy density of 5.77 J/cm 3 because of the low permittivity and modest dielectric strength. Due to the permittivity variation between P(VDF-TrFE-CFE) and sandwiched structure films, high dielectric strength does not give the advantages of P(VDF-TrFE-CFE)-300 in maximum energy density compared with P(VDF-TrFE-CFE). However, as shown in Fig. 5b, the maximum discharged energy density of P(VDF-TrFE-CFE)-300 reaches 7.03 J/cm 3 , which is much higher than that of P(VDF-TrFE-CFE) (6.21 J/cm 3 ). This enhancement could be attributed to the efficiency improve as shown in Fig. 5c. The charge-discharge efficiency of P(VDF-TrFE-CFE) rapidly decreases from 77 to 65% as the electric field increasing from 120 to 300 MV/m and failed to withstand higher electric field. As for P(VDF-TrFE-CFE)-200, its efficiency remains above 84% even the electric field reaches as high as 450 MV/m. Thus high efficiency at that high electric field has never been seen in any PVDF-based polymers before, which makes it promising to improve the chargedischarge efficiency of PVDF-based polymers. With increasing the thickness of P(VDF-TrFE-CFE), the discharge efficiency of P(VDF-TrFE-CFE)-300 slightly decreases to 78% at an electric field of 480 MV/m. This might be ascribed to high electric loss of P(VDF-TrFE-CFE) at high electric field [31].
Previous works [29,[32][33][34] have revealed that field distribution in polymer nanocomposites of multilayer dielectric films are not evenly. Because of the permittivity difference between two polymers or nanomaterials and polymer matrix, the high permittivity materials undertake lower electric field than the applied electric field. On the contrary, low permittivity materials suffer higher electric field than the external field. In this sandwiched structure films, the dielectric constant of P(VDF-TrFE-CFE) is nearly 16 times of PMMA, which makes PMMA suffer higher electric field than applied electric field. In the meantime, P(VDF-TrFE-CFE) undertakes a lower electric field thus exhibits lower permittivity as shown in Fig. 2a. The maximum electric field that PMMA could suffer is 520 MV/m, which makes the maximal electric field of P(VDF-TrFE-CFE)-300 at 480 MV/m sound. Fig. 5d shows the leakage current densities of four samples under an electric field of 100 MV/m, which is in relation to the conductivity of samples. P(VDF-TrFE-CFE) exhibits a leakage current density over 10 −6 A/cm 2 while the electric field excess 25 MV/m. Nevertheless, P(VDF-TrFE-CFE)-200 and P(VDF-TrFE-CFE)-300 possess one order of magnitude lower leakage current densities comparing to that of P(VDF-TrFE-CFE). These results are in accordance with discharge efficiency shown in Fig. 4c and maximum electric field of these four dielectric films.

Conclusions
In summary, a series of sandwiched structure dielectric films were successfully fabricated by applying P(VDF-TrFE-CFE) as the central layer and PMMA as the outer layer. Comparing to P(VDF-TrFE-CFE), P(VDF-TrFE-CFE)-200 and P(VDF-TrFE-CFE)-300 exhibit remarkably suppressed dielectric loss, enhanced dielectric strength and charge-discharge efficiency. In addition, P(VDF-TrFE-CFE)-300 shows enhanced discharged energy density of 7.03 J/cm 3 at 480 MV/m while maintaining a high discharge efficiency of 78%. Meanwhile, the maximum discharge energy density of P(VDF-TrFE-CFE) is only 6.21 J/cm 3 with an efficiency of 65%. This improvement could be ascribed to suppress dielectric loss and restricted leakage current density brought by outer layer PMMA. These results indicate that PMMA could act as a charge barrier preventing charge injection and enhancing the discharge efficiency of P(VDF-TrFE-CFE). Furthermore, by modulating the layer thickness of sandwiched structure films, a much more enhanced discharged energy density might be received due to the improvement of dielectric constant. These results pave a new way to fabricate high energy density and low dielectric loss dielectric films for energy storage capacitors application.