Flexible mica films coated by magnetron sputtered insulating layers for high‐temperature capacitive energy storage

Abstract

High‐temperature energy storage performance of dielectric capacitors is crucial for the next generation of power electronic devices. However, conduction losses rise sharply at elevated temperature, limiting the application of energy storage capacitors. Here, the mica films magnetron sputtered by different insulating layers are specifically investigated, which exhibit the excellent high‐temperature energy storage performance. The experimental results revealed that the PbZrO₃/Al₂O₃/PbZrO₃ (PZO/AO/PZO) interface insulating layers can effectively reduce the high‐temperature leakage current and conduction loss of the composite films. Consequently, the ultrahigh energy storage density (W_rec) and charge‒discharge efficiency (η) can be achieved simultaneously in the flexible mica‐based composite films. Especially, PZO/AO/PZO/mica/PZO/AO/PZO (PAPMPAP) films possess excellent W_rec of 27.5 J/cm³ and η of 87.8% at 200°C, which are significantly better than currently reported high‐temperature capacitive energy storage dielectric materials. Together with outstanding power density and electrical cycling stability, the flexible films in this work have great application potential in high‐temperature energy storage capacitors. Moreover, the magnetron sputtering technology can deposit large‐area nanoscale insulating layers on the surface of capacitor films, which can provide technical support for the industrial production of capacitors.

INTRODUCTION

Among various energy storage devices, dielectric capacitors possess the highest power density, which are important component in modern electronic and power systems. However, the relatively low energy storage density of dielectric materials limits the long-term stable operation of energy storage devices.^1–3 Therefore, the development of dielectric materials with high energy storage properties will effectively promote the progress of energy storage technology. In dielectric energy storage materials, polymer dielectrics have become the preferred materials for dielectric capacitors due to the high breakdown strength, good flexibility, and high reliability. The energy storage performance of current polymer film capacitors seriously deteriorates as the temperature increases, so they cannot meet the rapid energy storage and conversion in high-temperature operating environments.⁴ For example, commercial biaxially oriented polypropylene (BOPP) film can be only worked continuously under 85°C. Therefore, in order to ensure the stable operation of energy storage devices under high-temperature environments, it is necessary to introduce the additional cooling system, which will result in a large amount of energy consumption.⁵ In addition, low dielectric constant of BOPP films results in its highest energy storage density of only about 5 J/cm³. In order to overcome the shortcomings of BOPP films, a series of high-temperature system polymer films with high glass transition temperatures have been gradually studied in recent years. Relevant research results showed that the leakage current and conduction loss of polyimide (PI), polyether imide (PEI), polycarbonate (PC), and other high-temperature-resistant composite films increase exponentially under high temperature and high electric field conditions, which seriously deteriorates the energy storage performance.^6–9 Inorganic dielectric materials, such as lead zirconate (PbZrO₃), barium titanate (BaTiO₃), and sodium bismuth titanate (Bi_0.5Na_0.5TiO₃), have good temperature stability, but they are usually grown on rigid substrates and exhibit poor flexibility.^10–12 Therefore, it is urgent to develop dielectric materials with excellent temperature stability and flexibility for energy storage capacitors.

In recent years, mica has a tendency to be used as energy storage dielectrics. As shown in Figure S1, compared with other thicknesses, mica with a thickness of 10 µm has the most excellent energy storage performance at high temperature. On the one hand, mica stripped to 10 µm can show good flexibility and work stably for a long time at 1100°C. On the other hand, mica has a larger dielectric constant and breakdown strength than polymer films.¹³ Compared with polymer films and inorganic ceramic films, mica exhibits better energy storage performance under high-temperature conditions. In order to further suppress the high-temperature conduction loss of mica, the effective process is growing interface functional insulating layers on the surface to suppress charge injection at the electrodes. There are three main strategies for growing inorganic interface insulating layers: (1) in situ surface functionalization, (2) plasma-enhanced chemical vapor deposition (PECVD), and (3) magnetron sputtering. The in situ surface functionalization approach has high requirements on the material system and he surface of the films often needs to react in a strong acid or alkali solution, which affects the quality of the films.^14,15 The PECVD method not only involves common thermochemical reactions, but also complex plasma chemical reactions, which makes it difficult to control the composition ratio of the inorganic functional layers.^16,17 The magnetron sputtering can achieve rapid and low-temperature growth of inorganic thin films with high purity and uniform thickness, which has obvious advantages in the film deposition process.^18–20 In recent years, Cheng et al. successfully grew the high bandgap boron nitride interface barrier layers on the surface of PI films via magnetron sputtering technology, the energy storage density of the sandwich films has been increased by 50% at 150°C.²¹ Immediately afterwards, they used the same process to deposit a series of inorganic functional layers on both sides of the PEI films. The influence of insulating layers with different bandgaps and dielectric constants on the high-temperature energy storage performance of thin films has been systematically studied.²² The results show that the design of growing the insulating layers by magnetron sputtering process can significantly improve the high-temperature energy storage performance of thin film capacitors.

Previous reports on all inorganic antiferroelectric-insulator multilayered films have revealed that by introducing insulating layers Al₂O₃ (AO) into the antiferroelectric PbZrO₃ (PZO) layers, the insulating performance of the PZO/AO/PZO sandwich structure films is greatly improved compared to pure PZO films.^23–26 The reports inspire us to deposit interfacial insulating layers onto the surface of dielectric films to optimize energy storage performance. In this work, the dielectric and energy storage properties of mica-based flexible composite films are studied systematically. First, PZO (E_g ≈ 3.52 eV) and AO (E_g ≈ 7.26 eV) are selected as the interface insulating layers. The carrier migration will occur at the interfaces between the PZO antiferroelectric layers and the AO insulating layers. Then, the composite films with different structures on both sides of mica through radiofrequency magnetron sputtering technology, including PZO/mica/PZO (PMP), PZO/AO/mica/AO/PZO (PAMAP), and PZO/AO/PZO/mica/PZO/AO/PZO (PAPMPAP) are successfully prepared. The results show that interface insulating layers with PZO/AO/PZO (PAP) sandwich structure can effectively reduce the charge injection at the electrodes, which is conducive to improving the energy storage performance of the composite films. At 200°C, PAPMPAP exhibits an excellent energy storage density of 27.5 J/cm³ and efficiency of 87.8%, as well as superior power density and cycling stability. This work provides a new idea for the research of high-temperature energy storage capacitors.

RESULTS AND DISCUSSION

Figures 1A and S4 show the surface atomic force microscopy (AFM) images of films with the size of 1 µm × 1 µm, the root-mean-square roughness (R_rms) of mica, PMP, PAMAP, and PAPMPAM is 0.24, 17.1, 20.8, and 25.8 nm, respectively. As the number of insulating layers increases, the surface of the composite films becomes more and more rough, which is due to the grain size of PAPMPAP is larger than PAMAP and PMP during the annealing process. Figure S5 exhibits the X-ray diffraction (XRD) patterns of the films, it can be only detected the (003), (004), (005), and (006) diffraction peaks of mica, indicating that the crystallization degree of PZO and AO insulation layers is much lower than that of mica at the annealing temperature of 500°C. To further characterize the microstructure of the films, Figure S6 shows the cross-sectional scanning electron microscopy (SEM) images of films, no structural defects can be found in the mica films. The structure of the interface insulating layers cannot be clearly observed in the SEM images; therefore, Figure 1B gives the transmission electron microscopy (TEM) images of PAPMPAP. The insulating layers and the mica layers are tightly combined, and there are no obvious holes at the interfaces. The line energy spectrum mapping of PAPMPAP is given in Figure S7, it can be seen that the thicknesses of PZO and AO are about 40 and 9 nm, respectively. According to the high-resolution TEM image of red area in Figure 1C, the PZO insulating layer is partially crystalline, while the AO insulation layer is amorphous. The d-spacing is 0.31, which corresponds to (110) crystal plane of PZO.²⁷ The energy-dispersive spectroscopy element mapping of Pb, Zr, Al, and O are displayed in Figure 1D‒G, the clear interfaces between AO and PZO layers can be distinguished. During the magnetron sputtering process, there is basically no diffusion phenomenon at the interface between the PZO layer and the AO layer.

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The heterostructure of PZO/AO insulating layers is crucial for electrical properties. First, the X-ray photoelectron spectroscopy (XPS) spectra of PZO and AO layer is exhibited in Figures S8 and S9 and Table S1. The atomic ratios of Pb:Zr:O and Al:O are 1:1.1:3.2 and 1:1.7, indicating the PZO and AO targets are deposited on the surface of mica successfully. The reason for the higher content of O element is associated with the O₂ in the chamber during the magnetron sputtering process. Second, the energy band information is investigated by XPS O_1s spectra and ultraviolet photoelectron spectroscopy (UPS) spectra, as shown in Figure S10 and Supporting Information S1. The bandgap energies (E_g) of PZO and AO layer are 3.52 and 7.26 eV. Combined with the electron affinity (χ) of PZO (4.29 eV) and AO (1.92 eV) derived from UPS spectra, the work functions (φ) are estimated in Figure 2A. When the PZO and AO layers come into contact, the free charge carriers on the interface are redistributed until the Fermi level reaches equilibrium. Since the φ value of PZO (5.00 eV) is higher than that of AO (4.74 eV), the electrons in the AO layer will migrate to the PZO layer and the upward band-bending for hole accumulation at the AO interface, as shown in Figure 2B.^23,28 Therefore, the built-in electric field directed from AO layer to PZO layer is formed at the interface. Under the action of the built-in electric field, the PZO layer will achieve self-polarization. Figure 2C,D exhibits the self-polarization mechanism of PZO/AO/PZO and PZO/AO interfaces, the electron carrier concentration in the AO layer will affect the self-polarization effect of PZO. Compared with PZO/AO/PZO insulating layer, the self-polarization effect of PZO in PZO/AO layer is more significant.

To study the effect of insulating layers to dielectric properties of the films, dielectric constant (ε_r) and dielectric loss (tan δ) of the films are shown in Figure S11. The mica, PMP, PAMAP, and PAPMPAP films exhibit excellent frequency (10⁰‒10⁷ Hz) and temperature (25°C‒150°C) stability. The mica films exhibit the ultrahigh ε_r (8‒9), which is two to three times than common high-temperature energy storage polymer, such as PEI, PI, Polyethylene terephthalate (PET), Polyetheretherketone (PEEK), PC, etc. Since the sandwich structure of the PZO/AO/PZO insulation layers can effectively improve the dielectric constant and reduce the conductance loss, the PAPMPAP films show the highest ε_r and the lowest tan δ compared to the mica, PMP, and PAMAP films at 200°C.

Figures 3A and S12 exhibit the high-temperature leakage current of films. As the temperature increases, the leakage current of the composite films gradually increases. This is because the high temperature promotes the carrier transport behavior at the electrode/dielectric interface, resulting in a sharp increase in conduction loss. At 200°C, the leakage current of mica, PMP, PAMAP, and PAPMPAP are 1.1 × 10⁻⁶, 9.7 × 10⁻⁷, 5.2 × 10⁻⁶, and 3.2 × 10⁻⁷ A, respectively. The construction of the interface insulating layers can effectively suppress the injection of charges at the electrode, and the PAPMPAP has the lowest leakage current. For the insulation layers of the PZO/AO/PZO structure, two built-in electric fields in opposite directions are formed at the interface between AO and PZO, which can effectively hinder the transport of carriers. The formation mechanism of the built-in electric fields has been described previously.

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Under the action of the external electric field, the metal electrodes will emit electrons or holes to the films due to the Schottky effect, and charges will accumulate at the interface between the metal electrodes and the films.²⁹ The description of the Schottky emission is shown in Supporting Information S2. Figure 3B gives the Schottky conduction mechanism fitting curves of films, according to the formula of Schottky emission model, it can be seen that the slope and intercept of the fitting lines are determined by the ε_r and barrier height, respectively. The slope of the fitting line for the PAPMPAM film is the lowest, indicating that the ε_r of the PAPMPAM film is the highest, which is consistent with the results of the dielectric properties. Meanwhile, the PAPMPAP film has the lowest intercept, proving the highest injection barrier.

In order to further analyze the insulation performance of the composite films, Figures 3C,D and S13 show the breakdown performance measured at 200°C and room temperature, respectively. The E_b of films is analyzed by Weibull distribution, as shown in Supporting Information S3.^30–32 At 200°C, the breakdown performance of the films does not deteriorate significantly. The E_b of mica, PMP, PAMAP, and PAPMPAP composite films are 588.3, 665.7, 697.6, and 712.1 MV/m, and the β values are 4.7, 19.5, 16.6, and 15.1 respectively. By constructing the interface insulating layers, the high-temperature breakdown performances of the composite films are substantially improved. On the one hand, the electron carriers in the AO layer will migrate to the PZO layer, and the PZO layer realizes self-polarization, which improves the insulation strength of the AO film layer. On the other hand, compared to the PZO/AO insulating layer, PZO/AO/PZO has a higher injection barrier, which better hinders the injection of carriers at the electrodes.

The polarization behaviors measured at room temperature and 200°C of films are shown in Figures S14 and S15, whether at room temperature or 200°C, the unipolar P‒E loops of the composite films are slim. The energy storage performance can be calculated by the P‒E loops, as shown in Figure S16 and Supporting Information S4. The important parameters that affect the energy storage performance of film materials are P_m, P_r, and E_b, respectively. Figures 4A, S17, and S18 exhibit the energy storage performances measured at 200°C and room temperature of composite films. At 200°C, the W_rec of mica, PMP, PAMAP, and PAPMPAP reaches 17.0, 22.0, 26.0, and 27.5 J/cm³ with η of 88.1%, 83.6%, 89.1%, and 87.8%. After the introduction of the PZO/AO/PZO heterostructure interface insulation layer, the W_rec of the composite films has been increased by 162%. It can be seen that the η of PMP, PAMAP, and PAPMPAP composite films has decreased, and the designed heterogeneous interface insulation layers can hinder the injection of charge at the electrode and inhibit the conduction loss. The polarization loss of the antiferroelectric PZO layer is large, which degrades the η of the composite films. The η decreases first and then increases, which is caused by the field-induced phase transition of PZO layer under the applied electric field. The energy storage performances at the same electrical field (400 MV/m) and maximum electrical field are shown in Figures S19 and S20, the W_rec of PAPMPAP composite films is higher than that of PAMAP, PMP, and mica. On the one hand, the ε_r of PZO and AO layers are higher than those of mica. On the other hand, the large electric dipole moment caused by the interface polarization is another factor that enhances the polarization performance.³³

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In practical applications, it is necessary to ensure the long-term working stability of film capacitors. Figures 4B and S21 give the cycling stability performance of the PAPMPAP composite films at the electric field of 200 MV/m and the temperature of 200°C. It can be seen that under the combined action of electric field and temperature, the W_rec and η of the PAPMPAP basically do not change after 50 000 switching cycles and 10 000 bending cycles, proving that the film capacitor can work stably. The actual discharge performance of film capacitors should be also considered during the application process. Figure 4C shows the discharge voltage curves of PAPMPAP composite films. The load resistance is 10 kΩ. By integrating the discharge voltage curves in Figure 4C, the discharge energy density under different electric field is obtained, as shown in Figure 4D. At the electric field of 200 MV/m and the temperature of 200°C, the power density of the PAPMPAP composite films is 2.5 MW/cm³. The comparison of energy storage performances between this work and other representative works at 200°C is exhibited in Figure 4E and Table S2, in this work, compared with polymer films and ceramics,^22,34–41 the W_rec, η, and E_b of PAPMPAP film capacitors show obvious advantages, proving that they have good application prospects in the field of high-temperature energy storage.

Figures 5 and S22 demonstrate the comparison for W_rec, E_b, ε_r, P_m, η, and tan δ of six parameters to evaluate the comprehensive performance measured at 200°C and room temperature. The results reveal that the films with the excellent energy storage performance appears in the films with a large area of the enclosed pattern.^42–44 The energy storage performance of PAPMPAP is better than that of PAMAP, PMP, and mica, owing to the higher E_b, ε_r, P_m, η, and W_rec. and lower tan δ. Finally, the PZO/AO/PZO insulation layers have the most significant effect on improving the energy storage performance of composite films at 200°C and room temperature.

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CONCLUSION

In summary, the method to efficiently enhance the energy storage performance of dielectric films under high temperatures and high electric fields has been proposed. It has been demonstrated that the PAP interface insulating layers can suppress Schottky emission at the interfaces of electrodes and films at 200°C. The interface insulating layers have a positive effect on reducing high temperature conduction loss and improving breakdown strength of the composite films. As a result, the optimal mica-based films designed in this work (PAPMPAP) exhibit the most outstanding insulating performance and lowest dielectric loss at elevated temperature, which is beneficial to improve both W_rec and η. At 200°C, PAPMPAP possesses an improved W_rec of 27.5 J/cm³ and η of 87.8% under 710 MV/m, which exceeds all reported high-temperature polymer-based composite films and ceramics. In addition, the high-temperature capacitor films also possess excellent discharge energy density, power density, and electrical fatigue stability. This work innovatively reports high-temperature energy storage dielectric materials and their preparation process, which is beneficial to the design and development of high-temperature energy storage capacitors.

EXPERIMENTAL

Films preparation

The mica films are purchased from Changchun Taiyuan Fluorophlogopite Co., Ltd. Insulating layers (PZO and AO) are grown on both sides of the mica films through the magnetron sputtering process. The PZO and AO targets are obtained from Hefei Kejing Material Technology Co., Ltd. First, mica films are stripped to 10 µm through hand-exfoliated and ultrasonically treated in alcohol for 30 min to remove surface stains. Then, mica films are fixed in the chamber, PZO and AO targets are sputtered on the surface of mica films in turn. By adjusting the gas ratio of O₂ and Ar to 4:12, ensure that the air pressure in the chamber is about 1.0 Pa. The sputtering time of each insulation layer is 60 min, and the sputtering power is 60 W. During the magnetron sputtering process, the sample substrate is not heated. Finally, the films (PMP, PAMAP, and PAPMPAP) obtained after sputtering are rapidly annealed at the temperature of 500°C and annealing time of 3 min. The schematic diagram of magnetron sputtering and actual photograph of PAPMPAP films are shown in Figures S2 and S3.

Characterization

The surface morphologies of the films are analyzed by AFM. The cross-sectional morphologies of the films are characterized by TEM and SEM. The energy band structure is calculated through XPS and UPS. The crystal structure of the films is analyzed by XRD. The dielectric properties of the films are tested by the broadband dielectric spectroscopy. The electrical performance and energy storage performance are analyzed through the dielectric breakdown and ferroelectric polarization loop test system. The discharge characteristics of the film are determined by capacitor charge‒discharge test system.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (nos. U20A20308, 92366204, and 52277024) and the Fundamental Research Foundation for Universities of Heilongjiang Province (2023-KYYWF-0113).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

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