An anti-swelling PVDF-based piezoelectric separation membrane based on coaxial electrospinning technology and its preparation method

Abstract

The present invention provides an anti-swelling PVDF-based piezoelectric separation membrane based on coaxial electrospinning technology and a preparation method thereof, which comprises adding PVDF and / or PVDF copolymers to a mixed solvent, stirring evenly to form an inner spinning solution, adding TPU to the mixed solvent, stirring evenly to form a homogeneous outer spinning solution, and then subjecting the inner spinning solution and the outer spinning solution to coaxial electrospinning to form a composite fiber film having a core-shell structure, and finally subjecting the membrane to annealing and drying treatments to obtain an anti-swelling PVDF-based piezoelectric separation membrane. The anti-swelling PVDF-based piezoelectric separation membrane prepared by the present invention can be used as a battery separation membrane for lithium-ion batteries, solving the problem of swelling of piezoelectric polymer films in the electrolyte in the prior art and greatly improving its electromechanical conversion capacity / efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of energy-capturing materials and energy storage battery devices, and relates to an anti-swelling PVDF-based piezoelectric separation membrane based on coaxial electrospinning technology and a preparation method thereof, and in particular to an anti-swelling PVDF-based piezoelectric separation membrane having a core-shell structure and used for lithium-ion batteries prepared by using coaxial electrospinning technology and a preparation method thereof. Background Art

[0002] Battery separators, also known as battery separators, are key components in battery structure, located between the positive and negative electrodes. They have a direct impact on battery safety, energy density, and overall performance. With the rapid development of the new energy industry, particularly the widespread application of lithium-ion batteries in electric vehicles, energy storage systems, and portable electronic devices, the research and development of battery separator technology has become a key focus for improving battery performance, reducing costs, and enhancing safety.

[0003] Battery separator membrane technology has undergone a continuous evolution from traditional materials to high-performance materials. In current commercial applications, battery separator membranes are primarily made from polyolefins such as polyethylene and polypropylene, fabricated into porous membranes via dry or wet processes. These materials offer excellent mechanical properties and chemical stability, but they also suffer from issues such as uneven thickness, insufficient mechanical properties, and high manufacturing costs.

[0004] In the existing material-related technical fields, piezoelectric materials have become one of the current research hotspots due to their unique self-generating properties, which can effectively capture discrete mechanical energy in the environment. Among the many piezoelectric materials, piezoelectric polymers have the characteristics of light weight, flexibility, and easy processing, and are widely used in many fields such as smart sensing, catalysis, and wearable devices. Among them, polyvinylidene fluoride (PVDF) and PVDF copolymers, such as vinylidene fluoride-trifluoroethylene copolymer (P(VDF-TrFE)), are one of the polymer piezoelectric materials with high piezoelectric properties and excellent processability.

[0005] In recent years, with the growing demand for portable energy systems, integrating piezoelectric materials and energy storage devices has become one of the mainstream research directions, among which self-rechargeable lithium-ion batteries (SC-LIBs) are the rising star in this research direction. Piezoelectric polymer films prepared from piezoelectric materials are used to replace traditional battery separators to assemble SC-LIBs, which can convert mechanical energy into electrical energy and store it as chemical energy. This process does not require external circuits and additional wires, integrating energy collection, conversion, and storage, greatly improving the portability and efficiency of energy storage systems. However, current research on SC-LIBs is limited to regulating the piezoelectric properties of piezoelectric materials and battery structure. When piezoelectric polymer films are used as battery separators, the main defect is that the piezoelectric performance is greatly reduced or lost in cyclic charge and discharge tests, making it difficult to achieve a breakthrough in the self-charging efficiency of such SC-LIBs. Summary of the Invention

[0006] In order to solve the problems in the above-mentioned prior art, the present invention provides an anti-swelling PVDF-based piezoelectric separation membrane based on coaxial electrospinning technology and a preparation method thereof. The macroscopic morphology and piezoelectric output performance of the piezoelectric fiber membrane are effectively regulated by the strategy of coaxial electrospinning and in-situ coating protection. The anti-swelling PVDF-based piezoelectric separation membrane prepared by the present invention can be used as a battery separation membrane for lithium-ion batteries, which solves the problem of swelling of piezoelectric polymer films in the electrolyte in the prior art and greatly improves its electromechanical conversion capability / efficiency.

[0007] To achieve the above objectives, the present invention is implemented by adopting a technical solution consisting of the following technical measures.

[0008] The present invention provides a method for preparing an anti-swelling PVDF-based piezoelectric separation membrane based on coaxial electrospinning technology, which mainly comprises the following steps:

[0009] (1) adding PVDF and / or PVDF copolymer into a mixed solvent and stirring uniformly to form an inner layer spinning solution;

[0010] The mixed solvent is obtained by mixing a polar solvent and a benign solvent for PVDF, and the mass ratio of the polar solvent and the benign solvent is (1-5):1.

[0011] The mass ratio of the PVDF and / or PVDF copolymer to the mixed solvent is 1:(3-10);

[0012] (2) adding TPU (thermoplastic polyurethane elastomer) into the mixed solvent and stirring evenly to form a homogeneous outer layer spinning solution;

[0013] Wherein, the mass ratio of the TPU to the mixed solvent is 1:(2-10);

[0014] (3) preparing a composite fiber film having a core-shell structure by coaxially electrospinning the inner layer spinning solution obtained in step (1) and the outer layer spinning solution obtained in step (2);

[0015] Among them, the specific process parameters of the coaxial electrospinning film formation are: using an 18G / 21G coaxial needle for electrospinning, applying high DC voltages of 16-22kV and -1.0-1.2kV to the needle and receiver electrodes respectively, the injection rates of the inner layer spinning solution and the outer layer spinning solution are 0.05-0.20mm / min and 0.10-0.40mm / min respectively, the wheelbase from the coaxial needle to the roller is 5-15cm, and the collector is collected at a speed of 30-300rpm to obtain a composite fiber film;

[0016] (4) The composite fiber film obtained in step (3) is subjected to annealing treatment and drying treatment in sequence to obtain an anti-swelling PVDF-based piezoelectric separation membrane.

[0017] In this article, the PVDF copolymer described in step (1) is a copolymer formed by copolymerization of vinylidene fluoride and other monomers. Those skilled in the art should know that the PVDF copolymer can be a commercially available PVDF copolymer, or a PVDF copolymer prepared by itself based on the process / literature disclosed in the prior art, and based on the inventive principle of the present invention, it should be a copolymer material selected with piezoelectric properties.

[0018] In order to better illustrate the present invention and provide a technical solution for reference, the PVDF copolymer selected in step (1) includes at least one of P(VDF-TrFE), P(VDF-HFP), and P(VDF-CTFE).

[0019] In this article, the mixed solvent in step (1) is a mixture of a polar solvent and a benign solvent for PVDF, wherein the polar solvent is a conventional polar solvent selected in the art, such as at least one of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), and dimethyl sulfoxide (DMSO); the benign solvent for PVDF is a conventional benign solvent for PVDF, such as at least one of acetone and dichloromethane. It should be noted that such benign solvents for PVDF are usually also benign solvents for PVDF copolymers.

[0020] In one of the technical solutions, in order to further improve the piezoelectric properties of the prepared anti-swelling PVDF-based piezoelectric separation membrane, the step (1) of adding PVDF and / or PVDF copolymer to the mixed solvent also includes adding a piezoelectric inorganic nanofiller, wherein the piezoelectric inorganic nanofiller is a piezoelectric functional inorganic nanofiller conventionally added in the field of piezoelectric materials, such as at least one of nano-barium titanate (BTO), nano-zinc oxide (ZnO), nano-lead zirconate titanate (PZT), carbon nanotubes (CNTs), and graphene nanosheets (GP); the amount of the piezoelectric inorganic nanofiller added is not more than 30% of the mass of the PVDF and / or PVDF copolymer.

[0021] In this article, the stirring in step (1) is uniformly formed to form an inner layer spinning solution, wherein the standard for uniform stirring is generally based on the formation of a homogeneous inner layer spinning solution, or the formation of a uniform emulsion / suspension / mixed solution. In order to better illustrate the present invention and provide a technical solution for reference, under laboratory operation conditions, in order to accelerate the dissolution of PVDF and / or PVDF copolymers, the stirring is uniformly formed to form an inner layer spinning solution, which is formed by stirring at least 4 hours at a temperature of 60 to 80°C and a stirring speed of 100 to 600 rpm. Furthermore, when the addition of piezoelectric inorganic nanofillers is also included, it is preferred that the inner layer spinning solution be subjected to an ultrasonic dispersion treatment for at least 0.5 hours before the coaxial electrospinning film formation in step (3).

[0022] Herein, the mixed solvent in step (2) is the same as the mixed solvent in step (1), or is a mixed solvent prepared according to the description of the mixed solvent in step (1) above.

[0023] In this article, the TPU (thermoplastic polyurethane elastomer) described in step (2) is a conventional TPU material selected in this technical field, usually a commercially available TPU material selected, such as at least one of S95A, BASF1165A, BASF1175A, BASF1180A, BASF1185A, and BASF1195A from BASF of Germany.

[0024] In this article, the stirring method for uniformly forming a homogeneous outer spinning solution in step (2) is consistent with the stirring method for uniformly forming an inner spinning solution in step (1). In order to better illustrate the present invention and provide a technical solution for reference, under laboratory operating conditions, in order to accelerate the dissolution of TPU, the stirring method for uniformly forming a homogeneous outer spinning solution is to stir at a temperature of 60 to 80°C and a stirring speed of 100 to 600 rpm for at least 4 hours to form the outer spinning solution.

[0025] In one of the technical solutions, in order to allow the inner layer spinning solution obtained in step (1) and the outer layer spinning solution obtained in step (2) to be better coaxially electrospun into a film in step (3), before the coaxial electrospinning into a film, the inner layer spinning solution obtained in step (1) and the outer layer spinning solution obtained in step (2) are respectively defoamed for 1 to 5 minutes, and a conventional high-speed mixer can usually be used for the defoaming treatment; further, it can be selected to mix them in a high-speed mixer for at least 15 minutes before the defoaming treatment.

[0026] In this article, the composite film in step (4) is sequentially subjected to annealing treatment and drying treatment, wherein the annealing treatment and drying treatment can directly refer to the parameters of conventional annealing treatment and drying treatment recorded in the literature / process manual of PVDF-based electrospun fabrics / fiber films in the prior art.

[0027] In order to better illustrate the present invention and provide a technical solution for reference, the annealing treatment in step (4) is a vacuum annealing treatment, specifically, the annealing treatment is performed at a temperature of 40 to 80° C. under vacuum conditions for 6 to 24 hours.

[0028] In order to better illustrate the present invention and provide a technical solution for reference, the drying treatment in step (4) is a vacuum drying treatment, specifically a drying treatment at a temperature of 40 to 80° C. under vacuum conditions for 8 to 24 hours.

[0029] The anti-swelling PVDF-based piezoelectric separation membrane prepared by the above-mentioned method for preparing an anti-swelling PVDF-based piezoelectric separation membrane based on coaxial electrospinning technology can be used as a flexible energy-harvesting device and energy storage lithium battery device, especially in lithium-ion batteries.

[0030] In this article, based on the common knowledge of lithium-ion batteries in the prior art, when the anti-swelling PVDF-based piezoelectric separation membrane is applied to a lithium-ion battery, the lithium-ion battery also includes a negative electrode, a positive electrode and an electrolyte, and the raw material selection / raw material ratio / preparation process of the negative electrode, positive electrode and electrolyte all follow the conventional lithium-ion battery process. Those skilled in the art can select a suitable process based on the existing technology according to the conventional preparation process of lithium-ion batteries or the required lithium-ion battery type to prepare a lithium-ion battery based on the anti-swelling PVDF-based piezoelectric separation membrane.

[0031] In order to better illustrate the present invention and facilitate a more comprehensive interpretation of the beneficial effects of the anti-swelling PVDF-based piezoelectric separation membrane provided by the present invention, on the other hand, the present invention also provides a self-charging lithium-ion battery based on the anti-swelling PVDF-based piezoelectric separation membrane, which is mainly composed of a negative electrode, a positive electrode, an electrolyte and the anti-swelling PVDF-based piezoelectric separation membrane.

[0032] In this article, the positive electrode is one of the components of lithium-ion batteries that are commonly known. It can be directly obtained from the market or prepared by itself according to the existing technical literature / conventional processes of lithium-ion batteries in this field, such as a conventional lithium iron phosphate (LiFePO4) positive electrode.

[0033] In this article, the negative electrode is one of the commonly known components in lithium-ion batteries, which can be directly obtained from the market or prepared by itself according to the existing technical literature / conventional processes of lithium-ion batteries in this field, such as a conventional graphite negative electrode.

[0034] In this article, the electrolyte is one of the commonly known components in lithium-ion batteries, which can be directly obtained from the market or prepared by itself according to the existing technical literature / conventional processes of lithium-ion batteries in this field, such as conventional commercial electrolyte (lithium hexafluorophosphate electrolyte, 1M LiPF6 in EC:DMC=1:1vol%).

[0035] In one of the technical solutions, when the self-rechargeable lithium-ion battery is selected as a flexible soft-pack self-rechargeable lithium-ion battery, the flexible soft-pack self-rechargeable lithium-ion battery also includes a positive electrode tab, a negative electrode tab, a packaging material and other conventional structures / components / components. Those skilled in the art can select a suitable process based on the existing technology to prepare the flexible soft-pack self-rechargeable lithium-ion battery product according to the conventional preparation process of the flexible soft-pack self-rechargeable lithium-ion battery or the required battery type. For example, the anti-swelling PVDF-based piezoelectric separation membrane is sandwiched between the positive and negative electrodes and the positive and negative tabs are connected and encapsulated in an aluminum-plastic film, and the flexible soft-pack self-rechargeable lithium-ion battery is prepared according to the steps of packaging, drying, liquid injection and formation.

[0036] It should be noted that the inventors of the present invention discovered in their preliminary research that current research on SC-LIBs is limited to regulating the piezoelectric properties of piezoelectric materials and battery structures. In addition, when using piezoelectric polymer films as battery separators, a major defect occurs during cyclic charge-discharge tests, which significantly reduces or loses the piezoelectric performance. This makes it difficult to achieve a breakthrough in the self-charging efficiency of such SC-LIBs. In the process of experimentally resolving these major defects, it was discovered for the first time that the cause of this major defect is the neglect of the impact of the battery environment on the piezoelectric material, especially the electrolyte environment, which easily causes swelling behavior in the piezoelectric polymer film, causing the piezoelectric polymer film to significantly reduce or lose its piezoelectric performance. It should be noted that the view that the above defects are caused by swelling has not been found in relevant reports or literature.

[0037] Based on the above discovery that the defects of piezoelectric polymer materials are caused by swelling, the main inventive point of the present invention is that, by utilizing coaxial electrospinning film-forming technology, the outer spinning solution containing TPU is formed into a flexible shell structure, which is coated with a core material composed of PVDF and / or PVDF copolymer, thereby protecting it from the swelling of the electrolyte. Actual tests have confirmed that lithium-ion batteries using this anti-swelling PVDF-based piezoelectric separation membrane have excellent electrochemical properties, especially its advantages of high electrodynamic output performance and high energy storage capacity. As a result, the self-rechargeable lithium-ion battery prepared by the present invention has huge potential application prospects in energy storage devices, emergency energy supply and micro-intelligent wearable energy systems.

[0038] Based on the above-mentioned invention points, it was found in the actual experimental process that the mass ratio of the PVDF and / or PVDF copolymer to the mixed solvent in step (1) is 1: (3 to 10). When the mass ratio is higher than 1:3, the concentration of PVDF and / or PVDF copolymer is too high, resulting in abnormal spinning. When the mass ratio is lower than 1:10, the concentration of PVDF and / or PVDF copolymer is too low, and smooth filamentous fibers cannot be formed.

[0039] Based on the above-mentioned invention points, in one of the preferred technical solutions, it was found through actual experiments that the mass ratio of TPU to the mixed solvent in step (2) is 1:(2.5~3.5), that is, when the mass ratio of TPU to the mixed solvent is about 1:3, the anti-swelling PVDF-based piezoelectric separation membrane prepared therefrom has the best performance; if the mass ratio is too low, the flexible shell structure will be too thin, and it will not be possible to more effectively prevent the core material from swelling.

[0040] Based on the above-mentioned invention points, in one of the preferred technical solutions, it was found through actual experiments that in order to achieve a better core-shell coating effect, the injection rates of the inner layer spinning solution and the outer layer spinning solution in step (3) are 0.05-0.20 mm / min and 0.10-0.40 mm / min, respectively, and the injection rate of the outer layer spinning solution is preferably 1.5-3 times the injection rate of the inner layer spinning solution.

[0041] Based on the above invention, it was found in the actual experimental process that the wheelbase from the coaxial needle to the roller in step (3) was 5 to 15 cm. Under this condition, the piezoelectric performance of the anti-swelling PVDF-based piezoelectric separation membrane prepared was the best by comparison.

[0042] The present invention has the following beneficial effects:

[0043] 1. The present invention provides an anti-swelling PVDF-based piezoelectric separation membrane based on coaxial electrospinning technology and a preparation method thereof. The membrane has a distinct core-shell structure, and the flexible shell structure formed by the outer layer of TPU isolates the swelling behavior of the electrolyte on the core material, providing a barrier for the piezoelectric material to exert its high-voltage output performance.

[0044] 2. The present invention provides an anti-swelling PVDF-based piezoelectric separation membrane based on coaxial electrospinning technology and a preparation method thereof, which can effectively regulate the morphology and structure of the spun fiber by adjusting the concentration and injection rate of the inner / outer layer spinning solution, the wheelbase from the needle to the roller, and the size of the electric field voltage, thereby adjusting its piezoelectric output performance and ensuring the effective output of its piezoelectric performance.

[0045] 3. The anti-swelling PVDF-based piezoelectric separation membrane provided by the present invention can be used as a battery separation membrane for lithium-ion batteries. It has the advantages of not being swollen by the electrolyte, being flexible and high in strength, and having high self-charging capacity for lithium-ion batteries. It has huge potential application prospects in energy storage equipment, emergency energy supply, and micro-intelligent wearable energy systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 This is a photo of the anti-swelling PVDF-based piezoelectric separation membrane prepared in Example 2 of the present invention during piezoelectric testing.

[0047] Figure 2 This is a physical picture of the flexible soft-pack self-rechargeable lithium-ion battery prepared in Application Example 1 of the present invention.

[0048] Figure 3 These are SEM images of the fiber cross-sections of the anti-swelling PVDF-based piezoelectric separation membranes prepared in Examples 1 to 3 of the present invention.

[0049] Figure 4 The following are comparison diagrams of the swelling of the P(VDF-TrFE) piezoelectric separation membrane prepared in Comparative Example 2 of the present invention and the swelling-resistant PVDF-based piezoelectric separation membrane prepared in Experimental Example 2, when immersed in an electrolyte. Figures (a) and (b) are SEM electron micrographs of the fiber cross-section of the P(VDF-TrFE) piezoelectric separation membrane prepared in Comparative Example 2 before and after immersion in the electrolyte, respectively; Figures (c) and (d) are SEM electron micrographs of the fiber cross-section of the swelling-resistant PVDF-based piezoelectric separation membrane prepared in Experimental Example 2 before and after immersion in the electrolyte, respectively.

[0050] Figure 5This is a bar chart comparing the piezoelectric output performance (piezoelectric output voltage and output current) of the piezoelectric separation membrane samples prepared in Comparative Examples 1-2 and Experimental Example 2 of the present invention. PVDF corresponds to the PVDF piezoelectric separation membrane prepared in Comparative Example 1, PVT corresponds to the P(VDF-TrFE) piezoelectric separation membrane prepared in Example 2, and UVT corresponds to the anti-swelling PVDF-based piezoelectric separation membrane prepared in Example 2.

[0051] Figure 6 The electrochemical performance, coulombic efficiency and self-charging performance of the flexible soft-pack self-charging lithium-ion battery prepared in Application Example 1 of the present invention are shown in Figure 1. Figure (a) shows the charge and discharge time-voltage curves of the flexible soft-pack self-charging lithium-ion battery in Application Example 1 at different rates (0.1C and 0.5C); Figure (2) shows the charge capacity and discharge capacity after 10 cycles at different rates (0.1C and 0.3C); Figure (3) shows the charge and discharge specific capacity-voltage curves of the flexible soft-pack self-charging lithium-ion battery at 0.1C and 0.5C; Figure (4) shows the self-charging performance curve (charge and discharge time-voltage curve) of the flexible soft-pack self-charging lithium-ion battery after continuous tapping at constant force and constant frequency (9N, 1.6Hz) for 600s.

[0052] Figure 7 Surface SEM images of the anti-swelling PVDF-based piezoelectric separation membranes prepared in Experimental Examples 4 to 7 of the present invention.

[0053] Figure 8 The figure shows a comparison of the piezoelectric properties of the piezoelectric separation membrane samples prepared in Example 2 and Examples 4 to 7 of the present invention. UVT / UVTB corresponds to the anti-swelling PVDF-based piezoelectric separation membrane prepared in Example 2. DETAILED DESCRIPTION

[0054] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples, but it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than for limiting the claims of the invention. Those skilled in the art can refer to the contents of this article and appropriately improve the process parameters for implementation. It is particularly important to point out that all similar replacements and modifications are obvious to those skilled in the art, and they are all considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention. Although it is believed that those of ordinary skill in the art fully understand the following terms, the following definitions are still stated to help illustrate the subject matter disclosed by the present invention.

[0055] The present invention provides a method for preparing an anti-swelling PVDF-based piezoelectric separation membrane based on coaxial electrospinning technology, which mainly comprises the following steps:

[0056] (1) adding PVDF and / or PVDF copolymer into a mixed solvent and stirring uniformly to form an inner layer spinning solution;

[0057] The mixed solvent is obtained by mixing a polar solvent and a benign solvent for PVDF, and the mass ratio of the polar solvent and the benign solvent is (1-5):1.

[0058] The mass ratio of the PVDF and / or PVDF copolymer to the mixed solvent is 1:(3-10);

[0059] (2) adding TPU (thermoplastic polyurethane elastomer) into the mixed solvent and stirring evenly to form a homogeneous outer layer spinning solution;

[0060] Wherein, the mass ratio of the TPU to the mixed solvent is 1:(2-10);

[0061] (3) preparing a composite fiber film having a core-shell structure by coaxially electrospinning the inner layer spinning solution obtained in step (1) and the outer layer spinning solution obtained in step (2);

[0062] Among them, the specific process parameters of the coaxial electrospinning film formation are: using an 18G / 21G coaxial needle for electrospinning, applying high DC voltages of 16-22kV and -1.0-1.2kV to the needle and receiver electrodes respectively, the injection rates of the inner layer spinning solution and the outer layer spinning solution are 0.05-0.20mm / min and 0.10-0.40mm / min respectively, the wheelbase from the coaxial needle to the roller is 5-15cm, and the collector is collected at a speed of 30-300rpm to obtain a composite fiber film;

[0063] (4) The composite fiber film obtained in step (3) is subjected to annealing treatment and drying treatment in sequence to obtain an anti-swelling PVDF-based piezoelectric separation membrane.

[0064] In this article, the PVDF copolymer described in step (1) is a copolymer formed by copolymerization of vinylidene fluoride and other monomers. Those skilled in the art should know that the PVDF copolymer can be a commercially available PVDF copolymer, or a PVDF copolymer prepared by itself based on the process / literature disclosed in the prior art, and based on the inventive principle of the present invention, it should be a copolymer material selected with piezoelectric properties.

[0065] In order to better illustrate the present invention and provide an embodiment for reference, the PVDF copolymer selected in step (1) includes at least one of P(VDF-TrFE), P(VDF-HFP), and P(VDF-CTFE).

[0066] In this article, the mixed solvent in step (1) is a mixture of a polar solvent and a benign solvent for PVDF, wherein the polar solvent is a conventional polar solvent selected in the art. In one embodiment, for example, at least one of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), and dimethyl sulfoxide (DMSO) is selected; the benign solvent for PVDF is a conventional benign solvent for PVDF, such as at least one of acetone and dichloromethane. It should be noted that such a benign solvent for PVDF is usually also a benign solvent for PVDF copolymers.

[0067] In one embodiment, in order to further improve the piezoelectric properties of the anti-swelling PVDF-based piezoelectric separation membrane prepared, the step (1) of adding PVDF and / or PVDF copolymer to the mixed solvent also includes adding a piezoelectric inorganic nanofiller, wherein the piezoelectric inorganic nanofiller is a piezoelectric functional inorganic nanofiller conventionally added in the field of piezoelectric materials, such as at least one of nano-barium titanate (BTO), nano-zinc oxide (ZnO), nano-lead zirconate titanate (PZT), carbon nanotubes (CNTs), and graphene nanosheets (GP); the amount of the piezoelectric inorganic nanofiller added is not more than 30% of the mass of the PVDF and / or PVDF copolymer.

[0068] In this article, the stirring in step (1) is uniformly formed to form an inner layer spinning solution, wherein the standard for uniform stirring is generally based on the formation of a homogeneous inner layer spinning solution, or the ability to form a uniform emulsion / suspension / mixed solution. In order to better illustrate the present invention and provide a technical solution for reference, under laboratory operation conditions, in order to accelerate the dissolution of PVDF and / or PVDF copolymers, the stirring is uniformly formed to form an inner layer spinning solution, which is formed by stirring at least 4 hours at a temperature of 60 to 80°C and a stirring speed of 100 to 600 rpm. In one embodiment, when the addition of piezoelectric inorganic nanofillers is also included, the inner layer spinning solution is preferably subjected to an ultrasonic dispersion treatment for at least 0.5 hours before the coaxial electrospinning film formation in step (3).

[0069] In one embodiment, the mixed solvent in step (1) is a mixture of a polar solvent and a benign solvent for PVDF, and the mass ratio of the polar solvent and the benign solvent is (1 to 5):1, for example, 1:1, 2:1, 3:1, 4:1, 5:1 or any range or point value therebetween.

[0070] In one embodiment, the mass ratio of PVDF and / or PVDF copolymer to the mixed solvent in step (1) is 1:(3-10), for example, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10 or any range or point value therebetween.

[0071] Herein, the mixed solvent in step (2) is the same as the mixed solvent in step (1), or is a mixed solvent prepared according to the description of the mixed solvent in step (1) above.

[0072] In this article, the TPU (thermoplastic polyurethane elastomer) described in step (2) is a conventional TPU material selected in the present technical field, usually a commercially available TPU material. In one embodiment, for example, at least one of S95A, BASF1165A, BASF1175A, BASF1180A, BASF1185A, and BASF1195A of BASF of Germany is selected.

[0073] In this article, the stirring method for uniformly forming a homogeneous outer spinning solution in step (2) is consistent with the stirring method for uniformly forming an inner spinning solution in step (1). In order to better illustrate the present invention and provide an embodiment for reference, under laboratory operating conditions, in order to accelerate the dissolution of TPU, the stirring method for uniformly forming a homogeneous outer spinning solution is to stir at a temperature of 60 to 80°C and a stirring speed of 100 to 600 rpm for at least 4 hours to form the outer spinning solution.

[0074] In one embodiment, the mass ratio of TPU to mixed solvent in step (2) is 1:(2-10), for example, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10 or any range or point value therebetween.

[0075] In one embodiment, in order to allow the inner layer spinning solution obtained in step (1) and the outer layer spinning solution obtained in step (2) to be better coaxially electrospun into a film in step (3), before the coaxial electrospinning into a film, the inner layer spinning solution obtained in step (1) and the outer layer spinning solution obtained in step (2) are respectively defoamed for 1 to 5 minutes, and a conventional high-speed mixer can usually be used for the defoaming treatment; further, it can be selected to mix them in a high-speed mixer for at least 15 minutes before the defoaming treatment.

[0076] In this article, the composite film in step (4) is sequentially subjected to annealing treatment and drying treatment, wherein the annealing treatment and drying treatment can directly refer to the parameters of conventional annealing treatment and drying treatment recorded in the literature / process manual of PVDF-based electrospun fabrics / fiber films in the prior art.

[0077] In order to better illustrate the present invention and provide an embodiment for reference, the annealing treatment in step (4) is a vacuum annealing treatment, specifically, the annealing treatment is performed under vacuum conditions at a temperature of 40 to 80° C. for 6 to 24 hours.

[0078] In order to better illustrate the present invention and provide an embodiment for reference, the drying treatment in step (4) is a vacuum drying treatment, specifically a drying treatment at a temperature of 40 to 80° C. under vacuum conditions for 8 to 24 hours.

[0079] The anti-swelling PVDF-based piezoelectric separation membrane prepared by the above-mentioned method for preparing an anti-swelling PVDF-based piezoelectric separation membrane based on coaxial electrospinning technology can be used as a flexible energy-harvesting device and energy storage lithium battery device, especially in lithium-ion batteries.

[0080] In this article, based on the common knowledge of lithium-ion batteries in the prior art, when the anti-swelling PVDF-based piezoelectric separation membrane is applied to a lithium-ion battery, the lithium-ion battery also includes a negative electrode, a positive electrode and an electrolyte, and the raw material selection / raw material ratio / preparation process of the negative electrode, positive electrode and electrolyte all follow the conventional lithium-ion battery process. Those skilled in the art can select a suitable process based on the existing technology according to the conventional preparation process of lithium-ion batteries or the required lithium-ion battery type to prepare a lithium-ion battery based on the anti-swelling PVDF-based piezoelectric separation membrane.

[0081] In order to better illustrate the present invention and facilitate a more comprehensive interpretation of the beneficial effects of the anti-swelling PVDF-based piezoelectric separation membrane provided by the present invention, on the other hand, the present invention also provides a self-charging lithium-ion battery based on the anti-swelling PVDF-based piezoelectric separation membrane, which is mainly composed of a negative electrode, a positive electrode, an electrolyte and the anti-swelling PVDF-based piezoelectric separation membrane.

[0082] In this article, the positive electrode is one of the components of lithium-ion batteries that are commonly known. It can be directly obtained from the market or prepared by itself according to the existing technical literature / conventional processes of lithium-ion batteries in this field, such as a conventional lithium iron phosphate (LiFePO4) positive electrode.

[0083] In this article, the negative electrode is one of the commonly known components in lithium-ion batteries, which can be directly obtained from the market or prepared by itself according to the existing technical literature / conventional processes of lithium-ion batteries in this field, such as a conventional graphite negative electrode.

[0084] In this article, the electrolyte is one of the commonly known components in lithium-ion batteries, which can be directly obtained from the market or prepared by itself according to the existing technical literature / conventional processes of lithium-ion batteries in this field, such as conventional commercial electrolyte (lithium hexafluorophosphate electrolyte, 1M LiPF6 in EC:DMC=1:1vol%).

[0085] In one embodiment, when the self-rechargeable lithium-ion battery is selected as a flexible soft-pack self-rechargeable lithium-ion battery, the flexible soft-pack self-rechargeable lithium-ion battery also includes a positive electrode tab, a negative electrode tab, a packaging material and other conventional structures / components / components. Those skilled in the art can select a suitable process based on the existing technology to prepare the flexible soft-pack self-rechargeable lithium-ion battery product according to the conventional preparation process of the flexible soft-pack self-rechargeable lithium-ion battery or the required battery type. For example, the anti-swelling PVDF-based piezoelectric separation membrane is sandwiched between the positive and negative electrodes and the positive and negative tabs are connected and encapsulated in an aluminum-plastic film, and the flexible soft-pack self-rechargeable lithium-ion battery is prepared according to the steps of packaging, drying, liquid injection, and formation.

[0086] It should be noted that the inventors of the present invention discovered in their preliminary research that current research on SC-LIBs is limited to regulating the piezoelectric properties of piezoelectric materials and battery structures. In addition, when using piezoelectric polymer films as battery separators, a major defect occurs during cyclic charge-discharge tests, which significantly reduces or loses the piezoelectric performance. This makes it difficult to achieve a breakthrough in the self-charging efficiency of such SC-LIBs. In the process of experimentally resolving these major defects, it was discovered for the first time that the cause of this major defect is the neglect of the impact of the battery environment on the piezoelectric material, especially the electrolyte environment, which easily causes swelling behavior in the piezoelectric polymer film, causing the piezoelectric polymer film to significantly reduce or lose its piezoelectric performance. It should be noted that the view that the above defects are caused by swelling has not been found in relevant reports or literature.

[0087] Based on the above discovery that the defects of piezoelectric polymer materials are caused by swelling, the main inventive point of the present invention is that, by utilizing coaxial electrospinning film-forming technology, the outer spinning solution containing TPU is formed into a flexible shell structure, which is coated with a core material composed of PVDF and / or PVDF copolymer, thereby protecting it from the swelling of the electrolyte. Actual tests have confirmed that lithium-ion batteries using this anti-swelling PVDF-based piezoelectric separation membrane have excellent electrochemical properties, especially its advantages of high electrodynamic output performance and high energy storage capacity. As a result, the self-rechargeable lithium-ion battery prepared by the present invention has huge potential application prospects in energy storage devices, emergency energy supply and micro-intelligent wearable energy systems.

[0088] Based on the above-mentioned invention points, it was found in the actual experimental process that the mass ratio of the PVDF and / or PVDF copolymer to the mixed solvent in step (1) is 1: (3 to 10). When the mass ratio is higher than 1:3, the concentration of PVDF and / or PVDF copolymer is too high, resulting in abnormal spinning. When the mass ratio is lower than 1:10, the concentration of PVDF and / or PVDF copolymer is too low, and smooth filamentous fibers cannot be formed.

[0089] Based on the above-mentioned invention, in one of the preferred embodiments, it was found through actual experiments that the mass ratio of TPU to the mixed solvent in step (2) is 1:(2.5~3.5), that is, when the mass ratio of TPU to the mixed solvent is about 1:3, the anti-swelling PVDF-based piezoelectric separation membrane prepared therefrom has the best performance; if the mass ratio is too low, the flexible shell structure will be too thin, and it will not be possible to more effectively prevent the core material from swelling.

[0090] Based on the above-mentioned invention points, in one of the preferred embodiments, it was found through actual experiments that in order to achieve a better core-shell coating effect, the injection rates of the inner layer spinning solution and the outer layer spinning solution in step (3) are 0.05-0.20 mm / min and 0.10-0.40 mm / min, respectively, and the injection rate of the outer layer spinning solution is preferably 1.5-3 times the injection rate of the inner layer spinning solution.

[0091] Based on the above invention, it was found in the actual experimental process that the wheelbase from the coaxial needle to the roller in step (3) was 5 to 15 cm. Under this condition, the piezoelectric performance of the anti-swelling PVDF-based piezoelectric separation membrane prepared was the best by comparison.

[0092] The present invention will be further explained in detail below with reference to the examples. However, it will be appreciated by those skilled in the art that these examples are provided for illustrative purposes only and are not intended to limit the present invention.

[0093] Example

[0094] The embodiments of the present application will be described in detail below in conjunction with the examples, but it will be appreciated by those skilled in the art that the following examples are merely illustrative of the present application and should not be considered as limiting the scope of the present application. In the examples, if no specific conditions are indicated, the conditions are carried out according to normal conditions or manufacturer recommendations. The reagents used or the instruments that are not indicated by the manufacturer are conventional products that can be obtained commercially. The application should not be construed as being limited to the specific examples described.

[0095] 1. Raw materials

[0096]

[0097] 2. Preparation Method

[0098] (1) Add 2.5 g of P(VDF-TrFE) to 10 g of mixed solvent and stir evenly to form the inner layer spinning solution;

[0099] The mixed solvent is prepared by mixing DMF and acetone, and the mass ratio of the two is 1:1;

[0100] (2) Add 2.5 g of TPU (thermoplastic polyurethane elastomer) to 10 g of mixed solvent and stir evenly to form a homogeneous outer layer spinning solution;

[0101] The mixed solvent is prepared by mixing DMF and acetone, and the mass ratio of the two is 1:1;

[0102] (3) preparing a composite fiber film having a core-shell structure by coaxially electrospinning the inner layer spinning solution obtained in step (1) and the outer layer spinning solution obtained in step (2);

[0103] The specific process parameters of the coaxial electrospinning film formation are as follows: using an 18G / 21G coaxial needle for electrospinning, applying high DC voltages of 18kV and -1.0kV to the needle and receiver electrodes, respectively, the injection rates of the inner layer spinning solution and the outer layer spinning solution are 0.05-0.20mm / min and 0.10-0.40mm / min, respectively, the wheelbase from the coaxial needle to the roller is 10cm, and the collector is collected at a speed of 60rpm to obtain a composite fiber film;

[0104] (4) subjecting the composite fiber film obtained in step (3) to annealing treatment and drying treatment in sequence to obtain an anti-swelling PVDF-based piezoelectric separation membrane;

[0105] The annealing treatment is carried out under vacuum conditions at a temperature of 60° C. for 12 hours.

[0106] 3. Test Method

[0107] Scanning electron microscope test (SEM):

[0108] The spun fiber to be observed is broken with liquid nitrogen to obtain a longitudinal section of the fiber, which is then placed on a test bench and, after gold spraying, observed and photographed using a scanning electron microscope to obtain an electron microscope microscopic image of the longitudinal section.

[0109] X-ray diffraction (XRD) characterization

[0110] Because PVDF and its copolymers are polycrystalline polymers with multiple crystal phases existing on crystal planes, XRD is a common method for characterizing crystalline polymer phases. Spun fibers with flat surfaces were cut into 1.5 cm diameter discs. The sample surface was irradiated with a Cu-Kα X-ray source, scanning at a scan rate of 5° / min over a double angle range of 2-40°.

[0111] Fourier transform infrared spectroscopy (FT-IR) characterization

[0112] The spun fibers with smooth surfaces were cut into discs with a diameter of 1.5 cm. The film samples were tested using the ATR mode with an infrared wavenumber scanning range of 400 to 4000 cm -1 , a total of 32 scans, with a resolution of 4cm -1 .

[0113] Piezoelectric performance test

[0114] The present invention uses the open circuit voltage and short circuit current generated by the pressure on the material to test the piezoelectric performance. After the sample is fixed on the fixed plate, the material is impacted by a linear motor (LinMot model of Suzhou Kerui Limo Motor Co., Ltd.), and the open circuit voltage and short circuit current generated by the material are measured using an electrometer (6514 model of Keithley Instruments). In order to avoid the triboelectric effect during the impact process, Kapton tape is used to tightly encapsulate the surface of the material and the surface of the impact plate. Conductive copper tape is tightly attached to both sides of the film, and the charge generated by the material is guided by the conductive copper tape. The piezoelectric film in the piezoelectric test sample is cut into 30mm×30mm square pieces with a thickness of 200~220μm, the width of the conductive copper tape is 25mm, and the area directly opposite the piezoelectric film is 25mm×27.5mm.

[0115] Linear Sweep Voltammetry (LSV)

[0116] The present invention uses a CR2032 battery shell packaging film and a stainless steel sheet / lithium sheet counter electrode, and uses an Autolab electrochemical workstation from Metrohm, Switzerland, to perform an LSV test on the sample. The test voltage range is 2-6V and the scan rate is 0.01V / s.

[0117] Cyclic charge and discharge test

[0118] The present invention used the CT2001A battery testing system from Wuhan Blue Electric Electronics Co., Ltd. to conduct cyclic charge and discharge tests on the battery. Cyclic charge and discharge tests can provide relevant information such as the battery's charge and discharge curve, capacity, and cycle characteristics. The positive electrode of the present invention battery is lithium iron phosphate, and the negative electrode is graphite.

[0119] Example 1

[0120] Example 1: An anti-swelling PVDF-based piezoelectric separation membrane was prepared according to the above "2. Preparation method", wherein the injection rates of the inner layer spinning solution and the outer layer spinning solution were 0.15 mm / min and 0.225 mm / min, respectively; the prepared anti-swelling PVDF-based piezoelectric separation membrane was tested as a sample and recorded as UVT1.

[0121] Example 2

[0122] Example 2: An anti-swelling PVDF-based piezoelectric separation membrane was prepared according to the above "2. Preparation method", wherein the injection rates of the inner layer spinning solution and the outer layer spinning solution were 0.15 mm / min and 0.30 mm / min, respectively; the prepared anti-swelling PVDF-based piezoelectric separation membrane was tested as a sample and recorded as UVT2.

[0123] Example 3

[0124] Example 3: An anti-swelling PVDF-based piezoelectric separation membrane was prepared according to the above "2. Preparation method", wherein the injection rates of the inner layer spinning solution and the outer layer spinning solution were 0.15 mm / min and 0.375 mm / min, respectively; the prepared anti-swelling PVDF-based piezoelectric separation membrane was tested as a sample and recorded as UVT3.

[0125] Example 4

[0126] Example 4 was prepared with reference to the above-mentioned "2. Preparation method", except that 0.125 g of dried barium titanate powder was added in step (1) and stirred evenly to form an inner spinning solution, and finally an anti-swelling PVDF-based piezoelectric separation membrane was prepared, wherein the injection rates of the inner spinning solution and the outer spinning solution were 0.15 mm / min and 0.30 mm / min, respectively; the prepared anti-swelling PVDF-based piezoelectric separation membrane was tested as a sample and recorded as UVTB-5.

[0127] Example 5

[0128] Example 5 was prepared with reference to the above-mentioned "2. Preparation method", and 0.25 g of dry barium titanate powder was also added in step (1), and the mixture was stirred evenly to form an inner spinning solution, and finally an anti-swelling PVDF-based piezoelectric separation membrane was prepared, wherein the injection rates of the inner spinning solution and the outer spinning solution were 0.15 mm / min and 0.30 mm / min, respectively; the prepared anti-swelling PVDF-based piezoelectric separation membrane was tested as a sample and recorded as UVTB-10.

[0129] Example 6

[0130] Example 6 was prepared with reference to the above-mentioned "2. Preparation method", and 0.5 g of dry barium titanate powder was also added in step (1), and the mixture was stirred evenly to form an inner spinning solution, and finally an anti-swelling PVDF-based piezoelectric separation membrane was prepared, wherein the injection rates of the inner spinning solution and the outer spinning solution were 0.15 mm / min and 0.30 mm / min, respectively; the prepared anti-swelling PVDF-based piezoelectric separation membrane was tested as a sample and recorded as UVTB-20.

[0131] Example 7

[0132] Example 7 was prepared with reference to the above-mentioned "2. Preparation method", and 0.75 g of dry barium titanate powder was also added in step (1), and the mixture was stirred evenly to form an inner spinning solution, and finally an anti-swelling PVDF-based piezoelectric separation membrane was prepared, wherein the injection rates of the inner spinning solution and the outer spinning solution were 0.15 mm / min and 0.30 mm / min, respectively; the prepared anti-swelling PVDF-based piezoelectric separation membrane was tested as a sample and recorded as UVTB-30.

[0133] Comparative Example 1

[0134] Comparative Example 1 is a PVDF piezoelectric separation membrane not coated with TPU, and the specific preparation method is as follows:

[0135] (1) Add 2.5 g of PVDF to 10 g of mixed solvent and stir evenly to form a spinning solution;

[0136] The mixed solvent is prepared by mixing DMF and acetone, and the mass ratio of the two is 1:1;

[0137] (2) preparing a PVDF fiber film by uniaxial electrospinning the spinning solution obtained in step (1);

[0138] The specific process parameters of the uniaxial electrospinning film formation are as follows: using a 21G needle for electrospinning, applying high DC voltages of 8 kV and -1.0 kV to the needle and receiver electrodes, respectively, the injection rate of the spinning solution is 0.15 mm / min, the wheelbase from the needle to the roller is 10 cm, and the collector is collected at a speed of 60 rpm to obtain the PVDF fiber film;

[0139] (3) subjecting the PVDF fiber film obtained in step (2) to annealing treatment and drying treatment in sequence to obtain a PVDF piezoelectric separation membrane not coated with TPU, which is used as a comparative sample;

[0140] The annealing treatment is carried out under vacuum conditions at a temperature of 60° C. for 12 hours.

[0141] Comparative Example 2

[0142] Comparative Example 2 is a P(VDF-TrFE) piezoelectric separation membrane that is not coated with TPU. The specific preparation method is as follows:

[0143] (1) Add 2.5 g of P(VDF-TrFE) to 10 g of mixed solvent and stir evenly to form a spinning solution;

[0144] The mixed solvent is prepared by mixing DMF and acetone, and the mass ratio of the two is 1:1;

[0145] (2) preparing a P(VDF-TrFE) fiber film by uniaxial electrospinning the spinning solution obtained in step (1);

[0146] The specific process parameters of the uniaxial electrospinning film formation are as follows: using a 21G needle for electrospinning, applying high DC voltages of 8 kV and -1.0 kV to the needle and receiver electrodes, respectively, the injection rate of the spinning solution is 0.15 mm / min, the wheelbase from the needle to the roller is 10 cm, and the collector is collected at a speed of 60 rpm to obtain the P(VDF-TrFE) fiber film;

[0147] (3) subjecting the P(VDF-TrFE) fiber film obtained in step (2) to annealing treatment and drying treatment in sequence to obtain a P(VDF-TrFE) piezoelectric separation membrane not coated with TPU, which is used as a comparative sample;

[0148] The annealing treatment is carried out under vacuum conditions at a temperature of 60° C. for 12 hours.

[0149] Application Example 1

[0150] A flexible soft-package self-rechargeable lithium-ion battery is prepared using the anti-swelling PVDF-based piezoelectric separation membrane prepared in Example 2, wherein the anti-swelling PVDF-based piezoelectric separation membrane is sandwiched between the positive and negative electrodes and the positive and negative electrode ears are connected and encapsulated in an aluminum-plastic film, and the flexible soft-package self-rechargeable lithium-ion battery is prepared according to the steps of packaging, drying, liquid injection, and formation.

[0151] The positive electrode is lithium iron phosphate (LiFePO4); the negative electrode is graphite; the electrolyte is lithium hexafluorophosphate electrolyte, 1M LiPF6 in EC:DMC=1:1vol%.

[0152] Application Example 2

[0153] A flexible soft-package self-rechargeable lithium-ion battery is prepared using the anti-swelling PVDF-based piezoelectric separation membrane prepared in Example 6, wherein the anti-swelling PVDF-based piezoelectric separation membrane is sandwiched between the positive and negative electrodes and the positive and negative electrode ears are connected and encapsulated in an aluminum-plastic film, and the flexible soft-package self-rechargeable lithium-ion battery is prepared according to the steps of packaging, drying, liquid injection, and formation.

[0154] The positive electrode is lithium iron phosphate (LiFePO4); the negative electrode is graphite; the electrolyte is lithium hexafluorophosphate electrolyte, 1M LiPF6 in EC:DMC=1:1vol%.

[0155] 4. Test Results

[0156] like Figure 1 and Figure 2 As shown, the prepared anti-swelling PVDF-based piezoelectric separation membrane and flexible soft-pack self-rechargeable lithium-ion battery have good flexibility.

[0157] like Figure 3 As shown in FIG, with the increase of the injection rate ratio, the thickness of the outer shell of the fiber in the anti-swelling PVDF-based piezoelectric separation membrane gradually increases, which helps to protect the inner core fiber from swelling by the electrolyte.

[0158] Depend on Figure 4 It can be seen that after immersion in lithium hexafluorophosphate electrolyte (1M LiPF6 in EC:DMC=1:1vol%) for 5 days, the diameter of the P(VDF-TrFE) piezoelectric separation membrane fiber became more than twice the initial diameter, indicating that the piezoelectric separation membrane was fully swollen in the electrolyte, while the fiber diameter of the anti-swelling PVDF-based piezoelectric separation membrane coated with TPU did not increase significantly after 5 days, indicating that TPU as the outer shell formed an effective protection for the inner core P(VDF-TrFE), preventing the electrolyte from swelling it.

[0159] Depend on Figure 5 It can be seen that the piezoelectric output capacity of P(VDF-TrFE) is more than twice that of PVDF, and the piezoelectric output performance of the anti-swelling PVDF-based piezoelectric separation membrane coated with TPU is comparable to that of the uncoated P(VDF-TrFE), indicating that TPU as the outer shell effectively protects the P(VDF-TrFE) in the inner core, preventing the electrolyte from destroying the piezoelectric output performance of the inner layer.

[0160] Depend on Figure 6 As shown in (a), (b), and (c), the flexible soft-pack self-rechargeable lithium-ion battery prepared in Application Example 1 has good electrochemical properties, with a storage capacity of up to 120 mAh / g. Furthermore, the lithium-ion battery has excellent self-charging performance. At a stress frequency of 1.6 Hz, the voltage of the SC-LIB of the flexible soft-pack self-rechargeable lithium-ion battery increased from 0.80 V to 1.15 V, an increase of 350 mV. After a continuous discharge of 6952 s at a current of 5 μA, the battery recovered to its initial voltage, with a storage capacity of 9.655 μAh.

[0161] Depend on Figure 7It can be seen that with the increase of barium titanate content, the fiber diameter tends to become larger. This is because the increase of barium titanate content during the spinning process leads to changes in factors such as solution viscosity, surface tension, and solvent volatilization rate. The stable surface morphology indicates the stable output performance of the piezoelectric separation membrane and the electrochemical stability when used as a battery separator. When the barium titanate content is increased to 30%, it can be seen that the morphology of the fiber membrane has undergone tremendous changes - the fibers are stuck together into lumps, and the fiber diameter is very unstable. In order to achieve the highest possible dielectric constant, the barium titanate content should be as high as possible while ensuring the stability of the fiber membrane morphology. Therefore, Example 6 is the preferred choice of the present invention.

[0162] Depend on Figure 8 It can be seen that the UVTB-20 in Example 6 has the best open-circuit voltage (84.78V) and short-circuit current (1.63μA) output, which is a huge improvement compared to the UVT (52.35V, 1.08μA) in Example 2 without doping. This helps it drive more and faster lithium ion migration during the self-charging process, thereby improving the self-charging efficiency.

[0163] The above embodiments are preferred implementations of the present invention, but the implementation of the present invention is not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for preparing an anti-swelling PVDF-based piezoelectric separation membrane based on coaxial electrospinning technology, characterized in that The main steps include: (1) adding PVDF and / or PVDF copolymer into a mixed solvent and stirring uniformly to form an inner layer spinning solution; The mixed solvent is obtained by mixing a polar solvent and a benign solvent for PVDF, and the mass ratio of the polar solvent and the benign solvent is (1-5):

1. The mass ratio of the PVDF and / or PVDF copolymer to the mixed solvent is 1:(3-10); (2) adding TPU to the mixed solvent and stirring evenly to form a homogeneous outer layer spinning solution; Wherein, the mass ratio of the TPU to the mixed solvent is 1:(2-10); (3) preparing a composite fiber film having a core-shell structure by coaxially electrospinning the inner layer spinning solution obtained in step (1) and the outer layer spinning solution obtained in step (2); Among them, the specific process parameters of the coaxial electrospinning film formation are: using an 18G / 21G coaxial needle for electrospinning, applying high DC voltages of 16-22kV and -1.0-1.2kV to the needle and receiver electrodes respectively, the injection rates of the inner layer spinning solution and the outer layer spinning solution are 0.05-0.20mm / min and 0.10-0.40mm / min respectively, the wheelbase from the coaxial needle to the roller is 5-15cm, and the collector is collected at a speed of 30-300rpm to obtain a composite fiber film; (4) The composite fiber film obtained in step (3) is subjected to annealing treatment and drying treatment in sequence to obtain an anti-swelling PVDF-based piezoelectric separation membrane.

2. The preparation method according to claim 1, wherein: The PVDF copolymer in step (1) includes at least one of P(VDF-TrFE), P(VDF-HFP), and P(VDF-CTFE).

3. The preparation method according to claim 1, wherein: The polar solvent in step (1) includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide.

4. The preparation method according to claim 1, wherein: The benign solvent for PVDF in step (1) includes at least one of acetone and dichloromethane.

5. The preparation method according to claim 1, wherein: The step (1) of adding PVDF and / or a copolymer of PVDF to a mixed solvent further includes adding a piezoelectric inorganic nanofiller, wherein the piezoelectric inorganic nanofiller includes at least one of nano-barium titanate, nano-zinc oxide, nano-lead zirconate titanate, carbon nanotubes, and graphene nanosheets; the amount of the piezoelectric inorganic nanofiller added is not more than 30% of the mass of the PVDF and / or the copolymer of PVDF.

6. The preparation method according to claim 1, characterized in that: The annealing treatment in step (4) is a vacuum annealing treatment, specifically an annealing treatment at a temperature of 40 to 80° C. under vacuum conditions for 6 to 24 hours.

7. The preparation method according to claim 1, characterized in that: The drying process in step (4) is a vacuum drying process, specifically a drying process at a temperature of 40 to 80° C. under vacuum conditions for 8 to 24 hours.

8. The anti-swelling PVDF-based piezoelectric separation membrane prepared by the method for preparing an anti-swelling PVDF-based piezoelectric separation membrane based on coaxial electrospinning technology as claimed in claim 1.

9. The anti-swelling PVDF-based piezoelectric separation membrane as claimed in claim 8 is applied to flexible energy-harvesting devices and energy-storage lithium battery devices.

10. A self-charging lithium-ion battery using the anti-swelling PVDF-based piezoelectric separation membrane according to claim 8, characterized in that The device is mainly composed of a negative electrode, a positive electrode, an electrolyte and the anti-swelling PVDF-based piezoelectric separation membrane.

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