A multilayer gradient structure nanocomposite dielectric thin film and a preparation method thereof

Abstract

The application discloses a kind of multilayer gradient structure's nanocomposite dielectric film and preparation method thereof, including symmetrically arranged outer pressure layer and polarization layer located between two pressure layers, characterized in that buffer layer is arranged between pressure layer and polarization layer, buffer layer is made of P (VDF-HFP) and PMMA mixed in proportion of 60% of volume ratio;Pressure layer, polarization layer and buffer layer are filled with Ni (OH) 2, and its preparation method comprises the following steps: 1) using hydrothermal method to prepare Ni (OH) 2 nanosheet;2) Ni (OH) 2 nanosheet is added to pressure layer, buffer layer and polarization layer, and electrospinning is carried out according to multilayer gradient structure;3) multilayer gradient structure's nanocomposite dielectric film is obtained by hot pressing into film under preset conditions;The advantage is that the dielectric difference between different functional layers of multilayer structure can be significantly alleviated, the breakdown field strength and dielectric constant are improved, and the energy storage density is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of dielectric capacitor, in particular to a multilayer gradient structure nanocomposite dielectric film and a preparation method thereof. BACKGROUND

[0002] With the increasing demand for energy storage, dielectric capacitors have become an ideal energy storage device due to their high power density and ultrafast charge and discharge speed, and are widely used in electronic circuits, microwave communication, power systems and other fields and play a vital role. However, its energy storage density and charge and discharge efficiency are relatively low, which causes the volume and weight of the capacitor to increase sharply and undesirable energy loss to be converted into heat. This is obviously contrary to the development trend of future energy storage devices to store and release energy efficiently and stably. Therefore, achieving the double improvement of the energy storage density and efficiency of dielectric capacitors is the persistent pursuit of researchers. The discharge energy density and efficiency of dielectric materials can be defined as:

[0003] where Pm is the maximum polarization, Pr is the residual polarization, E is the applied electric field, U loss is the energy loss. For linear dielectrics, the discharge energy density can be expressed as: where ε0 is the vacuum permittivity, ε r is the relative permittivity of the dielectric material. Obviously, ε r and E b are two important parameters that determine the energy storage density of the material, and the influence of E b is more critical than that of ε r .

[0004] As an energy storage device, dielectric capacitors have the ability of fast charge and discharge and high power density, and have been widely used in many advanced power systems compared to chemical capacitors and batteries. In practical applications, the low energy storage density of dielectric capacitors will lead to an undesirable increase in the volume and weight of the device, which is contrary to the miniaturization of the device. The materials used in dielectric capacitors can be roughly divided into organic polymers and inorganic ceramics, the former has a higher breakdown field strength, but the dielectric constant is often low, and the latter has a higher dielectric constant, but the breakdown field strength is not ideal. It is difficult for the above two types of dielectric materials to achieve high energy storage density alone, and the nanocomposite film composed of organic polymers and inorganic ceramic fillers and their respective advantages can exceed the energy storage performance of both, which has great potential to solve this problem. With further research, it is found that part of the composite film has a high breakdown field strength at low filler content, but the dielectric constant does not reach a high level, and the other part of the composite film with high filler content has a high dielectric constant, but the breakdown field strength decreases sharply, which makes the research of composite film dielectric energy storage density in a dilemma.

[0005] The multilayer structure dielectric film is composed of an insulating layer of a polymer resistant to breakdown or a ceramic filler capable of hindering a breakdown path, and the problem of difficult simultaneous improvement of breakdown field strength and dielectric constant is overcome by different functional layers of a pressure-bearing layer with high breakdown field strength and a polarization layer with high dielectric constant. However, research shows that although the dielectric difference between layers can cause interface polarization and thus improve the dielectric constant, it also causes electric field distortion at the junction of adjacent layers and a large dielectric difference between different layers, and uneven electric field distribution in an external electric field can lead to early breakdown, which is not conducive to further improvement of the energy storage density of the dielectric film. SUMMARY

[0006] The technical problem to be solved by the present application is to provide a multilayer gradient structure nanocomposite dielectric film capable of significantly alleviating the dielectric difference between different functional layers, improving the breakdown field strength and dielectric constant, and improving the energy storage density.

[0007] The technical solution adopted by the present application to solve the above technical problem is: a multilayer gradient structure nanocomposite dielectric film, comprising symmetrically arranged outer pressure-bearing layers and a polarization layer located between the two pressure-bearing layers, the pressure-bearing layer is a mixture of PMMA and P(VDF-HFP) in an equal volume ratio, the polarization layer is P(VDF-HFP), a buffer layer is arranged between the pressure-bearing layer and the polarization layer, the buffer layer is a mixture of P(VDF-HFP) and PMMA in a volume ratio of 3:2, and Ni(OH)2 is filled in the pressure-bearing layer, the polarization layer and the buffer layer. The buffer layer is introduced to alleviate the electric field mutation caused by the dielectric difference between adjacent layers, thereby improving the breakdown field strength and the energy storage density.

[0008] Further, the buffer layer is a mixture of P(VDF-HFP) and PMMA in a volume ratio of 60%.

[0009] Further, the sum of the thicknesses of one side of the pressure-bearing layer and the buffer layer is equal to the thickness of the polarization layer.

[0010] Further, the thickness ratio of the pressure-bearing layer to the buffer layer is (1-3):(3-1).

[0011] Preferably, the thickness ratio of the pressure-bearing layer to the buffer layer is 3:1.

[0012] Further, the filling mass percentage of Ni(OH)2 in the pressure-bearing layer, the polarization layer and the buffer layer is 30% respectively.

[0013] The present application also provides a preparation method of a multilayer gradient structure nanocomposite dielectric film, comprising the following steps:

[0014] (1) Adopting hydrothermal method to prepare Ni(OH)2 nanosheet;

[0015] (2) Adding Ni(OH)2 nanosheet into pressure-bearing layer, buffer layer and polarization layer, and electrospinning according to the above multi-layer gradient structure;

[0016] (3) Hot-pressing into film under preset conditions to obtain multi-layer gradient structure nanocomposite dielectric film.

[0017] Further, step (1) is specifically: weighing 1.2g NaOH and 3.565g NiCl2 into 25ml deionized water, respectively, and stirring at 600r / min for 30min until completely dissolved, then slowly dropping the NiCl2 solution into the NaOH solution and adding 30ml deionized water, and then stirring at 600r / min for 30min, then pouring the uniform mixed solution into a high-pressure reaction kettle, and fully reacting at 200℃ for 24h, then washing the reaction product with alcohol and deionized water at 8000r / min for several times, then transferring the product to a vacuum drying oven, drying at 60℃ for 24h, and then grinding to obtain light green Ni(OH)2 nanosheet.

[0018] Further, step (2) is specifically:

[0019] A. Dissolving Ni(OH)2 nanosheet in N,N-dimethylformamide (DMF), ultrasonic dispersing, and then stirring at 600r / min for 30min until uniform, to form a Ni(OH)2 nanosheet solution with a concentration of 0.3g / L;

[0020] B. Adding P(VDF-HFP) and PMMA into the Ni(OH)2 nanosheet solution, stirring at 600r / min and 60℃ for 2h, and then stirring at room temperature for 4h, to form a pressure-bearing layer solution with P(VDF-HFP) and PMMA in a volume ratio of 1:1;

[0021] C. Adding P(VDF-HFP) and PMMA into the Ni(OH)2 nanosheet solution, stirring at 600r / min and 60℃ for 2h, and then stirring at room temperature for 4h, to form a buffer layer solution with P(VDF-HFP) and PMMA in a volume ratio of 3:2;

[0022] D. Adding P(VDF-HFP) into the Ni(OH)2 nanosheet solution, stirring at 600r / min and 60℃ for 2h, and then stirring at room temperature for 4h, to form a polarization layer solution;

[0023] E. The pressure-bearing layer solution, buffer layer solution, and polarization layer solution were vacuum-treated to remove bubbles, and then electrospun at a rate of 1 mm / min in an environment with a temperature of 40°C and a humidity of 30% in the above-mentioned gradient structure order. The resulting products were collected on aluminum foil and then placed in an oven at 60°C for 2 hours to remove residual solvent to obtain a composite film.

[0024] Furthermore, the amount of the Ni(OH)2 nanosheets added to the pressure-bearing layer solution, the buffer layer solution, and the polarization layer solution is 30% by mass.

[0025] Furthermore, step (3) is specifically as follows: the composite film prepared in step (2) is pasted on the conductive surface of the conductive glass, and is hot-pressed in a vacuum vulcanizer at 150°C and a pressure of 15 MPa for 30 minutes. After the hot pressing is completed, the composite film is placed in an oven at 200°C for 10 minutes, and then quickly placed in an ice-water mixture to complete the quenching operation. After cleaning and drying, a multi-layer gradient structure nano-composite dielectric film is obtained.

[0026] Compared with the prior art, the advantages of the present invention are as follows: a multi-layer gradient structure nanocomposite dielectric film and its preparation method, which uses a PMMA-P (VDF-HFP) blended substrate with a high breakdown field strength as the outer layer and P (VDF-HFP) with a high dielectric constant as the inner layer, can simultaneously improve the breakdown field strength and dielectric constant of the composite film; the gradient structure dielectric film reduces the dielectric difference between the substrate materials of adjacent layers of the composite material by introducing a buffer layer, and the change of dielectric constant and conductivity is continuous, making the electric field distribution within the material uniform. At the same time, due to the increase in interlayer interfaces, the breakdown path is hindered, further improving the breakdown field strength of the composite film. Secondly, Ni (OH) 2 nanosheets are used as fillers. The perpendicular electric field distribution of the nanosheets with a large aspect ratio can improve the breakdown field strength, while the parallel electric field distribution can also improve the dielectric constant. This composite film couples the above functions together, significantly improving the breakdown resistance of the composite film and optimizing the energy storage density of the gradient structure composite film.

[0027] In summary, the multi-layer gradient structure nanocomposite dielectric film of the present invention and its preparation method, the multi-layer gradient structure can achieve a significant improvement in the overall energy storage performance of the composite material, so that it has excellent breakdown resistance and dielectric properties, and the preparation process is green and environmentally friendly, the process is simple, and it can be widely applied. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 The flowchart for preparing a multi-layer gradient structure nanocomposite dielectric film;

[0029] Figure 2The scanning electron microscope micro-morphology diagram of the nanocomposite dielectric thin film with multi-layer gradient structure;

[0030] Figure 3 The dielectric constant curve of the nanocomposite dielectric thin film with multi-layer gradient structure with frequency change;

[0031] Figure 4 The dielectric loss curve of the nanocomposite dielectric thin film with multi-layer gradient structure with frequency change;

[0032] Figure 5 The column chart of breakdown field strength and energy storage density of the nanocomposite dielectric thin film with multi-layer gradient structure;

[0033] Figure 6 The finite element simulation diagram of the electric breakdown process of the nanocomposite dielectric thin film with multi-layer gradient structure;

[0034] Figure 7 The finite element simulation diagram of the electric field distribution of the nanocomposite dielectric thin film with multi-layer gradient structure;

[0035] Figure 8 The P-E curve of the nanocomposite thin film with multi-layer gradient structure in the different thickness of the pressure-bearing layer and the buffer layer. DETAILED DESCRIPTION

[0036] The application will be further described in detail below in combination with the embodiments of the drawings.

[0037] Polyvinylidene fluoride-hexafluoropropylene copolymer P(VDF-HFP) and polymethyl methacrylate PMMA were purchased from Shanghai Sanaifu New Materials Co., Ltd, DMF N, N-dimethylformamide (AR, 99.5%), NaOH sodium hydroxide (AR, 99.5%), nickel chloride NiCl2·6H2O (AR, 99.5%) bought from Aladdin. I. Specific embodiments

[0039] Example 1

[0040] A kind of multilayer gradient structure nanocomposite dielectric film, including symmetrically arranged outer pressure layer and polarization layer located in two pressure layers, pressure layer is PMMA (polymethyl methacrylate) and P (VDF-HFP) (polyvinylidene fluoride-hexafluoropropylene copolymer) mixed according to equal volume ratio, polarization layer is P (VDF-HFP), buffer layer is arranged between pressure layer and polarization layer, buffer layer is P (VDF-HFP) and PMMA mixed according to the proportion of 3:2 in volume ratio;Pressure layer, polarization layer and buffer layer are filled with Ni (OH) 2.The sum of the thickness of one side pressure layer and buffer layer is equal to the thickness of polarization layer.The thickness ratio of pressure layer and buffer layer is 3:1.The preparation method specific steps are as follows:

[0041] Step 1, Ni (OH) 2 nanosheet is prepared by hydrothermal method, as follows:

[0042] 1.2g NaOH and 3.565g NiCl2 are weighed and added into 25ml deionized water, stirred at 600r / min for 30min until completely dissolved, then NiCl2 solution is slowly dropped into NaOH solution and mixed with 30ml deionized water, then stirred at 600r / min for 30min, then the uniform mixed solution is poured into high-pressure reaction kettle, and fully reacted at 200℃ for 24h, then the reaction product is washed with alcohol and deionized water at 8000r / min for several times, then the product is transferred to vacuum drying oven, dried at 60℃ for 24h, then grinded to obtain light green Ni (OH) 2 nanosheet.

[0043] Step 2, Ni (OH) 2 nanosheet is added into pressure layer, buffer layer and polarization layer, and electrospun according to the above multilayer gradient structure, as follows:

[0044] A.0.03g of Ni (OH) 2 nanosheet is dissolved in 10mL N,N-dimethylformamide (DMF), ultrasonically dispersed, then stirred at 600r / min for 30min to form Ni (OH) 2 solution with concentration of 3g / L;

[0045] B.0.6033g P (VDF-HFP) and 0.3967g PMMA are added into Ni (OH) 2 nanosheet solution, stirred at 600r / min and 60℃ for 2h, then stirred at room temperature for 4h, to form pressure layer solution with P (VDF-HFP) and PMMA volume ratio of 1:1, (the final volume ratio of the two is calculated according to mass divided by density);

[0046] C. 0.6953 g P(VDF-HFP) and 0.3047 g PMMA were added to the solution of Ni(OH)2nanosheet solution, stirred at 600 r / min magnetic force, 60°C for 2 h, and stirred at room temperature for 4 h to form a buffer layer solution with a P(VDF-HFP) and PMMA volume ratio of 3:2;

[0047] D. P(VDF-HFP) was added to the solution of Ni(OH)2nanosheet solution, stirred at 600 r / min magnetic force, 60°C for 2 h, and stirred at room temperature for 4 h to form a polarization layer solution, wherein the addition amount of Ni(OH)2nanosheet in the pressure-bearing layer solution, the buffer layer solution and the polarization layer solution was 30% by mass;

[0048] E. The pressure-bearing layer solution, the buffer layer solution and the polarization layer solution were respectively subjected to vacuum treatment to remove bubbles, and electrospun in the above gradient structure order at a speed of 1 mm / min in an environment with a temperature of 40°C and a humidity of 30%, and collected on an aluminum foil paper, and then placed in an oven at 60°C for 2 h to remove residual solvent to obtain a composite film.

[0049] Step 3, hot-pressing into a film under preset conditions, as follows:

[0050] The composite film prepared in step 2 was pasted on the conductive surface of conductive glass, and hot-pressed in a vacuum vulcanizing machine at a temperature of 150°C and a pressure of 15 MPa for 30 min. After hot-pressing, the composite film was placed in an oven at 200°C for 10 min, and then quickly placed in an ice-water mixture to complete the quenching operation. After cleaning, drying and obtaining a multi-layer gradient structure nanocomposite dielectric film.

[0051] Comparative Example 1

[0052] The same as Example 1 above, except that in step 2C, 0.7802 g P(VDF-HFP) and 0.2198 g PMMA were added to the solution of Ni(OH)2nanosheet solution, stirred at 600 r / min magnetic force, 60°C for 2 h, and stirred at room temperature for 4 h to form a buffer layer solution with a P(VDF-HFP) and PMMA volume ratio of 70%.

[0053] Comparative Example 3

[0054] The same as Example 1 above, except that in step 2C, 0.8589 g P(VDF-HFP) and 0.1411 g PMMA were added to the solution of Ni(OH)2nanosheet solution, stirred at 600 r / min magnetic force, 60°C for 2 h, and stirred at room temperature for 4 h to form a buffer layer solution with a P(VDF-HFP) and PMMA volume ratio of 80%.

[0055] Comparative Example 4

[0056] The same as Example 1, except that 0.9319 g P(VDF-HFP) and 0.0681 g PMMA were added to the solution of Ni(OH)2 nanosheet solution in Step 2C, and the P(VDF-HFP) and PMMA buffer layer solution with a volume ratio of 90% was formed after stirring at 60℃ for 2 h and at room temperature for 4 h at a magnetic force of 600 r / min.

[0057] Comparative Example 5

[0058] The same as Example 1, except that the two parts of the pressure-bearing layer and the buffer layer were completely replaced by the pressure-bearing layer, specifically: including the outer pressure-bearing layer and the polarization layer located in the middle of the two pressure-bearing layers arranged symmetrically, the pressure-bearing layer was made of PMMA (polymethyl methacrylate) and P(VDF-HFP) (polyvinylidene fluoride-hexafluoropropylene copolymer) mixed in equal volume ratio, the polarization layer was P(VDF-HFP), and both the pressure-bearing layer and the polarization layer were filled with Ni(OH)2, and the thickness of one side of the pressure-bearing layer was equal to the thickness of the polarization layer.

[0059] II. Analysis of experimental results

[0060] Figure 1 A flow chart for the preparation of the multi-layer gradient structure composite film. The electrospinning preparation method of the present application is different from the previous solution casting method, which can reduce the pores and defects of the composite film, make the filler in the composite film uniform in the vertical electric field, and greatly improve the quality of the film. The preparation method basically realizes mechanized operation, can reduce the influence of human operation errors on the finished film, and is convenient for practical production and application.

[0061] Figure 2 A scanning electron microscope micrograph of the cross section of the multi-layer gradient structure composite film. As shown in Figure 2 the cross-sectional morphology of the film prepared in Example 1 has no obvious defects, and there is no obvious difference between the layers, which proves that the preparation method of the gradient composite film of the present application is very mature. In addition, the introduction of the buffer layer makes the polymer matrix PMMA and P(VDF-HFP) compatible well, and there is almost no change in the microstructure. The composite film of the present application introduces linear polymer PMMA on the basis of ferroelectric polymer P(VDF-HFP), which not only helps to improve the breakdown field strength, but also correspondingly improves the charge and discharge efficiency of the composite material. In addition, the nanosheets filled in the composite film prepared in this example are parallelly distributed, and this vertical electric field orientation distribution has a significant hindering effect on the formation of the electric breakdown path. The microstructure of the multi-layer gradient structure composite film exhibited under the scanning electron microscope fully meets the requirements of the quality of the film required by the dielectric capacitor in practical application, and can improve the energy storage performance.

[0062] Figure 3 The figure is the curve of the dielectric constant of the multi-layer gradient structure composite film with the change of frequency, and x is the value of the volume percentage of P(VDF-HFP) in the buffer layer. The relative dielectric constant of different composite films prepared in Example 1 and Comparative Examples 1-5 was characterized, and the differences in dielectric properties were compared. In Figure 3 The dielectric properties of these composite films with the change of frequency from 1 kHz to 1 MHz are shown in the figure, which shows good frequency stability. The dielectric constant of Example 1 and Comparative Examples 1-5 increases with the increase of x, which means that the gradient structure of the present application produces dielectric difference. This is due to the increase of P(VDF-HFP) in the composite film, which causes the dielectric constant to increase with the increase of gradient difference. Compared with Comparative Example 5, the dielectric difference also comes from the change of the interface, and the interface polarization caused by the increase of the interface will also cause the increase of the dielectric constant.

[0063] Figure 4 The figure is the curve of the dielectric loss of the multi-layer gradient structure composite film with the change of frequency. The dielectric loss of the gradient structure composite film studied in the present application changes in the range of 1 kHz to 1 MHz. In Figure 4 It can be found in the figure that the dielectric loss continues to increase with the increase of frequency, which is due to the relaxation behavior of the dipole orientation polarization, which causes the rapid increase of the loss. Overall, the dielectric loss of the gradient structure composite film prepared in Example 1 and Comparative Examples 1-5 is only about 0.03 at 1 kHz; at 1 MHz, the dielectric loss of Example 1 can still be the lowest (less than 0.1) compared with the comparative examples, so the gradient composite film of the present application can have lower dielectric loss and higher charge and discharge efficiency in a wider frequency range.

[0064] Figure 5 The figure is the column chart of the breakdown field strength and energy storage density of the multi-layer gradient structure composite film. The breakdown resistance and energy storage performance of the multi-layer gradient structure composite film prepared in Example 1 and Comparative Examples 1-5 were studied. In Figure 5 It can be seen in the figure that the breakdown field strength of the multi-layer gradient structure composite film increases first and then decreases with the increase of the gradient difference, and the highest field strength is 663.6 MV / m at x=60, which is a powerful guarantee for realizing high energy storage density. Through calculation and processing, the energy storage density of the multi-layer gradient structure composite film prepared in Example 1 and Comparative Examples 1-5 was obtained, and the energy storage density of the gradient structure composite film increased first and then decreased with the increase of x, and the composite film had the maximum energy storage density of 25.9 J / cm 3 at 663.6 MV / m at x=60, which is equivalent to that of pure P(VDF-HFP) film (9.5 J / cm 3) and 2023% of commercial BOPP film (1.28 J / cm3). This shows that the gradient structure composite film rationally designed by the present invention has a huge improvement in energy storage density.

[0065] Figure 6 The finite element simulation diagram of the electrical breakdown process of the multi-layer gradient structure composite film. In order to better explain the mechanism of the enhanced energy storage performance caused by the breakdown path of the gradient structure designed in the present invention, Figure 7 The breakdown path of the gradient structure composite film prepared in Example 1 was simulated by finite element method. Figure 7 It can be observed that the adjacent interfaces of the gradient structure composite film can act as energy barriers to hinder carrier transport, causing the electric tree to develop laterally between the gap layers, thereby delaying the formation of the breakdown path and having a significant impact on E b There is a significant improvement. It can also be observed that the Ni(OH)2 nanosheets in the composite film are distributed laterally, perpendicular to the electric field direction, and act as electron scattering centers, also hindering the development of the breakdown path. Taking these factors into consideration, it can be demonstrated that the multilayer gradient structure composite film of the present invention can achieve excellent breakdown resistance and energy storage performance, as shown in the experimental results.

[0066] Figure 7 The finite element simulation diagram of the electric field distribution of the multi-layer gradient structure composite film. In order to better explain the mechanism of the electric field distribution of the gradient structure designed in the present invention to enhance the energy storage performance, Figure 8 Finite element simulation was performed on the electric field distribution of the gradient structure composite film prepared in Example 1. For multi-layer composite films, the distribution of the electric field depends on the dielectric constant of the adjacent layers. The smaller the dielectric difference, the more uniform the electric field distribution. Figure 8 As can be observed in the example, the buffer layer of the gradient-structured composite film makes the electric field distribution in the insulating layer and polarization layer more uniform, preventing sudden changes in the electric field that could lead to premature breakdown. This also confirms the original design concept of the present invention: by reducing the dielectric differences between adjacent layers of the composite film through the gradient structure design, the uniformity of the electric field distribution is effectively modulated, thus preventing premature breakdown of the composite film.

[0067] Figure 8 The PE curves for different thicknesses of the pressure-bearing and buffer layers in a multilayer gradient structure composite film are compared. The dielectric constant changes for thicknesses of 2μm:2μm (2-2), 3μm:1μm (3-1), and 1μm:3μm (1-3) are compared. As the pressure-bearing and buffer layer ratios of the composite film change from 1:3 to 2:2 and then to 3:1, the dielectric constant decreases (the slope reflects the dielectric constant) due to the decrease in the overall P(VDF-HFP) content. Ultimately, the polarization intensity and breakdown field strength of the composite film reach their highest values ​​at a pressure-bearing and buffer layer ratio of 3:1, indicating that the energy storage density is highest when the pressure-bearing and buffer layer ratio is 3:1.

[0068] The above description is not intended to limit the present application, and the present application is not limited to the above examples. Changes, modifications, additions, or substitutions made by those skilled in the art within the spirit and scope of the present application should be covered by the scope of the present application.

Claims

1. A multi-layer gradient nanocomposite dielectric film comprising symmetrically arranged outer pressure-bearing layers and a polarization layer located between the two pressure-bearing layers, wherein the pressure-bearing layers are formed by mixing PMMA and P(VDF-HFP) in equal volume ratios, and the polarization layer is P(VDF-HFP), characterized in that: A buffer layer is provided between the pressure-bearing layer and the polarization layer, and the buffer layer is formed by mixing P(VDF-HFP) and PMMA in a volume ratio of 3:2; the pressure-bearing layer, the polarization layer and the buffer layer are all filled with Ni(OH)2 nanosheets.

2. The multi-layer gradient structure nanocomposite dielectric film according to claim 1, characterized in that: The sum of the thicknesses of the pressure-bearing layer and the buffer layer on one side is equal to the thickness of the polarization layer.

3. The multi-layer gradient structure nanocomposite dielectric film according to claim 1, characterized in that: The thickness ratio of the pressure-bearing layer to the buffer layer is (1-3): (3-1).

4. The multi-layer gradient structure nanocomposite dielectric film according to claim 1, characterized in that: The thickness ratio of the pressure-bearing layer to the buffer layer is 3:

1.

5. The multi-layer gradient structure nanocomposite dielectric film according to claim 1, characterized in that: The filling mass percentage of Ni(OH)2 in the pressure-bearing layer, the polarization layer and the buffer layer is 30% respectively.

6. A method for preparing a multi-layer gradient structure nanocomposite dielectric film, characterized in that The following steps are involved: (1) Preparation of Ni(OH)2 nanosheets by hydrothermal method; (2) adding Ni(OH)2 nanosheets to the pressure-bearing layer, the buffer layer, and the polarization layer, and electrospinning the multilayer gradient structure nanocomposite dielectric film according to claim 1; (3) A multilayer gradient structured nanocomposite dielectric film is obtained by hot pressing under preset conditions.

7. The method for preparing a multi-layer gradient nanocomposite dielectric film according to claim 6, characterized in that: Step (1) is specifically as follows: 1.2 g of NaOH and 3.565 g of NiCl2 are weighed and added to 25 ml of deionized water respectively, and magnetically stirred at 600 r / min for 30 minutes until completely dissolved. Then, the NiCl2 solution is slowly dripped into the NaOH solution and 30 ml of deionized water is added and mixed, and magnetically stirred at 600 r / min for 30 minutes. Then, the uniform mixed solution is poured into a high-pressure reactor and fully reacted at 200°C for 24 hours. The reaction product is alternately centrifuged and washed several times with alcohol and deionized water at a speed of 8000 r / min. After washing, the product is transferred to a vacuum drying oven, dried at 60°C for 24 hours, and ground to obtain light green Ni(OH)2 nanosheets.

8. The method for preparing a multi-layer gradient structure nanocomposite dielectric film according to claim 6, characterized in that: Step (2) is as follows: A. Dissolve Ni(OH)2 nanosheets in N,N-dimethylformamide, disperse by ultrasonication, and then magnetically stir at 600 rpm for 30 minutes until uniformly mixed, forming a Ni(OH)2 nanosheet solution with a concentration of 0.3 g / L. B. P(VDF-HFP) and PMMA were added to the Ni(OH)2 nanosheet solution, stirred at 600 rpm and 60°C for 2 h, and then stirred at room temperature for 4 h to form a pressure-bearing layer solution with a volume ratio of P(VDF-HFP) to PMMA of 1:

1. C. P(VDF-HFP) and PMMA were added to the Ni(OH)2 nanosheet solution and stirred at 600 rpm and 60°C for 2 h, followed by stirring at room temperature for 4 h to form a buffer layer solution with a volume ratio of P(VDF-HFP) to PMMA of 3:

2. D. Add P(VDF-HFP) to the Ni(OH)2 nanosheet solution and stir at 600 rpm and 60°C for 2 h, then stir at room temperature for 4 h to form a polarization layer solution. E. The pressure-bearing layer solution, the buffer layer solution, and the polarization layer solution are each subjected to vacuum treatment to remove bubbles, and the gradient-structured nanocomposite dielectric film according to claim 1 is sequentially electrospun at a rate of 1 mm / min in an environment of a temperature of 40°C and a humidity of 30%, collected on aluminum foil, and then placed in an oven at 60°C for 2 hours to remove residual solvent to obtain a composite film.

9. The method for preparing a multi-layer gradient nanocomposite dielectric film according to claim 8, characterized in that: The amount of the Ni(OH)2 nanosheets added to the pressure-bearing layer solution, the buffer layer solution, and the polarization layer solution is 30% by mass.

10. The method for preparing a multi-layer gradient structure nanocomposite dielectric film according to claim 6, characterized in that: Step (3) is specifically as follows: the composite film prepared in step (2) is pasted on the conductive surface of the conductive glass, and is hot-pressed in a vacuum vulcanizer at 150°C and a pressure of 15 MPa for 30 minutes. After the hot pressing is completed, the composite film is placed in an oven at 200°C for 10 minutes, and then quickly placed in an ice-water mixture to complete the quenching operation. After cleaning and drying, a multi-layer gradient structure nanocomposite dielectric film is obtained.

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