Significantly Improved Breakdown Strength and Energy

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Cite This: ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

Significantly Improved Breakdown Strength and Energy Density in Polymer Composites with High Filler Loading by Constructing Isoelectronic Traps Shanjun Ding†,‡ and Yingming Kong*,†,‡,§ †

School of Mechatronical Engineering, Beijing Institute of Technology, Beijing 100081, China Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China § State Key Laboratory of Supramolecular Structures and Materials, Jilin University, Changchun 130012, China Downloaded via NOTTINGHAM TRENT UNIV on August 31, 2019 at 06:01:03 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: High-performance dielectric polymer materials consisting of ZnS:O nanoparticles (ZSO NPs) with isoelectronic traps and polyvinylidene fluoride (PVDF) were prepared by a solution-casting method. Compared with pure PVDF and PVDF/ ZnS composites, the energy density and breakdown strength for polymer nanocomposites with high filler loading are obviously enhanced. By analyzing the physical model we built, we propose that the density and movement of charges are regulated by isoelectronic traps, resulting in increased insulation and a changed mircoelectric field distribution. In addition, this filling strategy also solves the problem of charge accumulation in polymer nanocomposites. KEYWORDS: polyvinylidene fluoride, zinc sulfide, isoelectronic traps, energy density, breakdown strength

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dielectric constant of dielectric polymer materials. Therefore, it is still difficult to increase the dielectric constant (εr) and breakdown strength (Eb) simultaneously.10−15 In order to increase εr and Eb simultaneously, polymer-based sandwich-structured nanocomposites were reported. For instance, Hu et al. reported on a sandwich-structured nanocomposite consisting of a middle layer with low BSBTnf loading and two outer layers with high TO-np loading. Finally, the breakdown strength (Eb ≈ 385 kV/cm) and dielectric constant (εr ≈ 14.5) were increased simultaneously.16 Wang et al. showed a sandwich-structured nanocomposite including a middle layer with low BT-np loading and two outer layers with high nanofiller loading. As a results, a high Eb (∼472.1 kV/mm) and εr (∼35) for polymer composites were achieved.17 However, although the above works overcome the challenge of a simultaneously increased breakdown strength and dielectric constant, these ways have

lectrostatic capacitors have been widely used in energy storage and conversion applications1−3 because of their excellent power density, breakdown strength, and fast charge− discharge as well as having the longest lifetime in comparison with other energy storage devices including supercapacitors, batteries, and so forth.4,5 However, the energy storage density for electrostatic capacitors has been limited, resulting in a restriction in their applications. For instance, as the best commercially available material, the energy density of biaxially oriented polypropylene (BOPP) is still lower than 2 J/cm3 at 640 kV/mm.6 In order to increase packing density, developing high-performance dielectric polymer materials has been desirable. According to the calculation formula of energy density,7 Ue = ∫ EdD, where E is the applied electric field, and D is the electric displacement, which is related to the applied electric field and dielectric constant (εr), improving breakdown strength (Eb) will obviously help to enhance energy density in comparison with an increasing dielectric constant.8 As a result, a larger amount of research has focused on inorganic/polymer composites with low filler loading to enhance breakdown strength.9 Yet, this filling strategy is seriously harmful for the © XXXX American Chemical Society

Received: July 7, 2019 Accepted: August 28, 2019

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DOI: 10.1021/acsaelm.9b00423 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Electronic Materials

Figure 1. (a) Weibull distribution; (b) resistivity values of PVDF/ZSO films with different oxygen contents.

Figure 2. (a) Energy density; (b) efficiency of PVDF/ZSO films with various oxygen contents.

been always limited and not used for “0−3 type” polymer composites. Therefore, a novel strategy has been extremely desired to increase Eb and εr simultaneously and solve the problem of charge accumulation in the polymer nanocomposites. Isoelectronic traps have been widely used in the optoelectronic devices field because of their distinctive electrical nature.18−20 In general, they are prepared by the substitution of the same family elements in the semiconductor matrix, forming a charged center that will capture charges nearby through the Coulomb force. As a result, the electrical nature of the semiconductor can be regulated. Inspired by this kind of semiconductor physics nature, we chose zinc sulfide (ZS) as our work object, because it not only possesses an excellent insulting property but also easily constructs isoelectronic traps because of the obvious discrepancy in electronegativity between O and S. On the basis of the above statement, we firmly think O-doped zinc sulfide (ZSO) has a huge potentiality in improving breakdown strength for polymer composites with high filler loading. Also, to the best of our knowledge, dielectric polymer composites with isoelectronic traps have not been reported so far in energy storage applications. In this Letter, the PVDF/ZSO nanocomposites with 22.8 vol % we prepared illustrated sufficiently high breakdown strength, energy density, and dielectric constant. The breakdown strength (Eb) values of PVDF/ZSO films are fitted, based on the Weibull distribution function. The fitted results are shown in Figure 1a. Eb increases with an increasing amount of oxygen, from ∼750.2 kV/cm for the nanocomposite films with 22.8 vol % of ZS to ∼2754.7 kV/cm

for nanocomposites with 22.8 vol % of ZnS0.4O0.6 NPs, which is about 3.7 times of that of PVDF/ZS films. In addition, it is found that the Eb values of PVDF/ZS0.8O0.2 and PVDF/ ZS0.6O0.4 are also improved in comparison with those of PVDF/ZS composites. More importantly, the maximum Eb of PVDF/ZS0.4O0.6 films is close to that of pure PVDF (∼3037.2 kV/cm), which indicates that isoelectronic traps can restrict space charges from the interface area and increase breakdown strength. In order to further verify this result, the resistivity values of PVDF/ZSO films are shown in Figure 1b. The result shows that isoelectronic traps can tune the resistivity value by controlling the content of oxygen ions in the polymer matrix. Table S1 shows the comparison of breakdown strength values between this work and previously reported literature. Compared to the various previously reported results, the PVDF/ZnS0.4O0.6 film shows superior improvement of the Eb/ Ematrix value. More importantly, in the case that the filler loading is far higher than that of other papers, the Eb value of this work is close to that of refs 1 and 5 in the Supporting Information. For dielectric materials, the energy density (Ue) and the charge−discharge efficiency (η) for dielectric polymer composites are two key parameters. They all are calculated by the above integral formula, based on the loops of Figure S3. The calculation results are demonstrated, as shown in Figure 2. The maximum Ue of 5.6 J/cm3 is achieved at 2800 kV/cm in the PVDF/ZnS0.4O0.6 films with 22.8 vol %, which is almost 20 times of that of the nanocomposite filled with 22.8 vol % ZS NPs (∼0.28 J/cm3 at 800 kV/cm) and 1.9 times of that of pure PVDF. More interestingly, as the ratios of oxygen are 0.2 and B

DOI: 10.1021/acsaelm.9b00423 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Electronic Materials

Figure 3. Schematic diagram of the mechanism of improved breakdown strength for PVDF-based composites with high filler loading.

0.4, the maximum Eb values for PVDF-based composites with 22.8 vol % are weakened, but their maximum energy densities are enhanced compared with the PVDF matrix. To further evaluate the performances for dielectric materials, the η is further considered. Figure 2b shows that the η values of the films with 22.8 vol % of nanofillers with different oxygen contents gradually increase with an increasing amount of oxygen. For instance, as the ratios of oxygen are 0.2, 0.4, and 0.6, the η values of the PVDF-based nanocomposites with isoelectronic traps are 64.3, 68.4, and 81.8% at 2000 kV/cm, respectively, whereas that of pure PVDF is only 38.5% at 2000 kV/cm. In order to illustrate the significance of this work, the breakdown strength (Eb), charge−discharge efficiency (η), and energy density (Ue) of the composite film is compared with those of previously reported dielectric polymer nanocomposites. As shown in Table S2, the result shows that the Ue, η, and Eb of PVDF/ZnS0.4O0.6 film all are higher than those of the reported literature. To further explain the enhanced Eb for PVDF/ZSO nanocomposites with high filler loading, we propose a physical model according to the multicore model,15 as shown in Figure 3. The oxygen ions can be incorporated into the zinc sulfide lattice to replace sulfide ions, because the radius of oxygen is smaller than that of sulfur. As a result, a lattice distortion (electric neutral isoelectronic trap) will be formed around the oxygen ion site. The isoelectronic traps can capture charges and then form a charged center (red area). Subsequently, the charged centers will continue to attract other opposite charges through the Coulomb force, leading to formed electroneutral excitons consisting of negative and positive charges. The nanofillers with isoelectronic traps not only reduce the density of interface charges but also suppress the movement of charges in the polymer composites, which leads to the decrease of the number of free electrons, and the resulting high resistivity and high Eb in comparison with those of PVDF/ZS composites and other composites without isoelectronic traps.8,21,22 In addition, the distance of the electric dipole moment is decreased by isoelectronic traps through the Coulomb force, resulting in a reduced ability of polarization. The captured charges are gathered around the fillers. The distribution of the inner microelectric field around the fillers and the polarization will be tuned by the gathered charges. The dielectric properties of PVDF/ZSO films were measured, as shown in Figure S4. The

result shows that the permittivity decreases with an increasing amount of oxygen, indicating that the polarization is suppressed in the polymer composites. In summary, PVDF/ZSO composites with isoelectronic traps were fabricated by a solution-casting method. Both the breakdown strength (∼2754.7 kV/cm) and energy density (∼5.6 J/cm3) were significantly enhanced. The maximum energy density of 5.6 J/cm3 of PVDF/ZnS0.4O0.6 composites is about 20 and 1.9 times greater in comparison with those of PVDF/ZS composites and pure PVDF, respectively. The phenomenon that isoelectronic traps lead to improved breakdown strength and energy density in the polymer composites with high filler loading was investigated. We proposed that the semiconductor nanofillers with isoelectronic traps could capture charges and form neutral excitons, resulting in a decreased density of charge and confined movement of charges. Meanwhile, this filling strategy solves the problem of charge accumulation in polymer composites. The above mechanism explained the reason for the increase in breakdown strength and the energy density of PVDF/ZSO films with high filler loading.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaelm.9b00423.

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Materials synthesis and characterization; X-ray diffraction; SEM; dielectric properties; D−E loops (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shanjun Ding: 0000-0001-7538-4713 Yingming Kong: 0000-0002-9811-4641 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 51377157) and the C

DOI: 10.1021/acsaelm.9b00423 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Electronic Materials

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Shenzhen Key Fundamental Research Program (JCYJ20160608160307181).

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DOI: 10.1021/acsaelm.9b00423 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX