Composite diaphragm, preparation method thereof and lithium ion battery

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a composite diaphragm which comprises a middle structure layer, a first composite layer and a second composite layer. The middle structure layer comprises a polyolefin base film; the first composite layer is arranged on the upper surface of the middle structure layer, and the first composite layer comprises a polyacrylonitrile fiber matrix, a polyvinylidene fluoride-hexafluoropropylene copolymer and inorganic particles; the second composite layer is arranged on the lower surface of the middle structure layer, and the second composite layer comprises a polyvinylidene fluoride-hexafluoropropylene copolymer and polyethylene oxide. Therefore, the overall thermal stability, the ion conductivity and the electrolyte wettability of the composite diaphragm can be effectively improved, so that the dynamic performance of the battery is improved, and the safety of the battery is ensured. The invention further discloses a preparation method of the composite diaphragm and a lithium ion battery.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to a composite diaphragm and a preparation method thereof, and a lithium-ion battery. Background Art

[0002] With the widespread application of lithium-ion batteries in new energy vehicles, energy storage systems, and portable electronic devices, the requirements for their energy density, rate performance, and cycle life continue to increase. The diaphragm ensures the free passage of electrolyte ions to form the charge and discharge circuit, and its performance directly affects the ion conduction efficiency and interface stability of lithium-ion batteries.

[0003] The separators used in related technologies are generally polyolefin separators, such as polyethylene and polypropylene. These separators are widely used in lithium-ion batteries due to their mature manufacturing process and low cost. However, the thermal stability, safety, and electrochemical performance of polyolefin separators do not meet the current battery performance requirements.

[0004] During the implementation of the embodiments of the present disclosure, it was found that at least the following problems exist in the related art:

[0005] Optimizing the coating structure of polyolefin separators can improve battery performance to a certain extent. However, due to the lack of polar groups on their surface, polyolefin separators have weak interfacial bonding with electrode materials or coating structures. As a result, poor adhesion still exists, affecting the wettability of the electrolyte, thereby limiting the transmission efficiency of lithium ions and increasing the interfacial impedance.

[0006] It should be noted that the information disclosed in the above background technology section is only used to enhance the understanding of the background of this application, and therefore may include information that does not constitute prior art known to ordinary technicians in this field. Summary of the Invention

[0007] In order to provide a basic understanding of some aspects of the disclosed embodiments, a brief summary is given below. The summary is not an extensive review, nor is it intended to identify key / critical elements or delineate the scope of protection of these embodiments, but rather serves as a prelude to the detailed description that follows.

[0008] The embodiments of the present disclosure provide a composite diaphragm and a preparation method thereof, and a lithium-ion battery, which can effectively improve the overall thermal stability, ion conductivity and electrolyte wettability of the composite diaphragm, thereby improving the dynamic performance of the battery and ensuring the safety of the battery.

[0009] In some embodiments, the composite diaphragm includes an intermediate structural layer, a first composite layer, and a second composite layer. The intermediate structural layer includes a polyolefin base film; the first composite layer is disposed on the upper surface of the intermediate structural layer and includes a polyacrylonitrile fiber matrix, a polyvinylidene fluoride-hexafluoropropylene copolymer, and inorganic particles; and the second composite layer is disposed on the lower surface of the intermediate structural layer and includes a polyvinylidene fluoride-hexafluoropropylene copolymer and polyethylene oxide.

[0010] Optionally, the inorganic particles include silicon dioxide, aluminum oxide, zirconium dioxide or calcium hydroxyphosphate.

[0011] Optionally, the mass fraction of the inorganic particles in the first composite layer is 5% to 25%.

[0012] Optionally, in the infrared spectrum of the composite membrane, at 2240 cm -1 ~2250cm -1 Between, 1400cm -1 ~1465cm -1 Between, 1170cm -1 ~1400cm -1 Between 1050cm -1 ~1155cm -1 Between and 500cm -1 ~800cm -1 There are characteristic peaks between them.

[0013] Optionally, the contact angle W of the composite membrane in the liquid electrolyte is ≤18°.

[0014] Optionally, the composite membrane has a rupture temperature T≥185°C.

[0015] Optionally, the liquid absorption rate A of the composite membrane is ≥130%.

[0016] Optionally, after the composite diaphragm is heat-treated at 120° C., its dimensional shrinkage S is less than 3.5%.

[0017] Optionally, the first composite layer and the second composite layer are combined with the intermediate structural layer through an electrospinning process and a hot pressing process, respectively, to form the composite diaphragm.

[0018] In some embodiments, the method for preparing the composite membrane comprises the following steps:

[0019] The first diaphragm is obtained by depositing a first spinning solution on the surface of the polyolefin base film through an electrospinning process to form a first composite layer; wherein the first spinning solution includes a mixed solution of a polyacrylonitrile fiber matrix, a polyvinylidene fluoride-hexafluoropropylene copolymer, and inorganic particles configured in a preset mass ratio;

[0020] The second spinning solution is deposited on the lower surface of the first diaphragm to form a second composite layer through an electrospinning process, thereby obtaining a second diaphragm; wherein the second spinning solution comprises a mixed solution of polyvinylidene fluoride-hexafluoropropylene copolymer and polyethylene oxide in a preset mass ratio;

[0021] The second diaphragm is subjected to a hot pressing process to obtain a composite diaphragm.

[0022] Optionally, the first spinning solution is prepared in the following manner:

[0023] After fully dissolving the polyacrylonitrile fiber matrix and the polyvinylidene fluoride-hexafluoropropylene copolymer in N,N-dimethylformamide, a prefabricated solution is obtained;

[0024] dispersing inorganic particles in the prefabricated solution by ultrasound to obtain the first spinning solution;

[0025] Among them, in the first spinning solution, the mass fraction of the polyacrylonitrile fiber matrix is ​​35% to 50%, the mass fraction of the polyvinylidene fluoride-hexafluoropropylene copolymer is 20% to 40%, and the mass fraction of the inorganic particles is 5% to 25%, forming a first spinning solution with a solid content of 10% to 20%.

[0026] Optionally, the second spinning solution is prepared in the following manner:

[0027] The polyvinylidene fluoride-hexafluoropropylene copolymer and polyethylene oxide are dissolved in tetrahydrofuran or ethanol solvent in a mass ratio of 1:1 to obtain the second spinning solution.

[0028] In some embodiments, during the electrospinning process, the voltage is 5 kV to 20 kV, and the feed flow rate is 0.2 mL / h to 1.5 mL / h.

[0029] In some embodiments, the lithium-ion battery includes a positive electrode sheet, a negative electrode sheet, and the composite separator as described in the above embodiments.

[0030] Optionally, the positive electrode sheet includes a positive electrode current collector and a positive electrode coating coated with at least one layer containing a positive electrode active material; the positive electrode active material includes lithium nickel cobalt manganese oxide (Li1Ni x Co y Mn z M b O2) and lithium iron phosphate; wherein, 0.70≤x≤0.95, 0.15≤y<0.45, 0.05≤z<0.45, 0.0≤b≤0.25, x+y+z+b=1; the element M includes one or more of Zr, W, Ti, Al, Sr, B and Nd.

[0031] Optionally, the negative electrode sheet includes a negative electrode active material, and the negative electrode active material includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon oxide, pre-lithium silicon oxide, silicon, and deposited silicon carbon.

[0032] Optionally, the negative electrode sheet further includes a negative electrode conductive agent, and the negative electrode conductive agent includes one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes and carbon black.

[0033] Optionally, the negative electrode sheet further includes a negative electrode binder, and the negative electrode binder includes one or more of sodium carboxymethyl cellulose, polyacrylic acid, polyacrylonitrile, polystyrene-acrylic acid, and styrene-butadiene rubber.

[0034] Optionally, the lithium-ion battery further comprises: an electrolyte comprising a lithium salt, a solvent and an additive; the lithium salt comprises one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium difluorophosphate, lithium difluorooxalatoborate and lithium bis(trifluoromethylsulfonyl)imide; the solvent comprises one or more of dimethyl carbonate, diethyl carbonate, ethylene carbonate and ethyl methyl carbonate; the additive comprises one or more of fluoroethylene carbonate, bis(fluoroethylene carbonate), vinyl sulfate, vinyl sulfite, vinyl carbonate and vinyl carbonate.

[0035] Optionally, the ratio of the capacity of the negative electrode sheet to the capacity of the positive electrode sheet is 1.02 to 1.1.

[0036] The composite diaphragm and its preparation method, as well as the lithium-ion battery provided by the embodiments of the present disclosure can achieve the following technical effects:

[0037] The first composite layer, comprising a polyacrylonitrile fiber matrix, polyvinylidene fluoride-hexafluoropropylene copolymer, and inorganic particles, is provided on the upper surface of the polyolefin-based membrane. This effectively increases the membrane's structural thermal responsiveness and electrolyte affinity. A second composite layer, comprising polyvinylidene fluoride-hexafluoropropylene copolymer and polyethylene oxide, is provided on the lower surface of the polyolefin-based membrane. This effectively improves lithium salt coordination and electrolyte retention.

[0038] Because the polyacrylonitrile fiber matrix and polyethylene oxide monomers contain a high number of polar functional groups, they can improve the adhesion of the upper and lower surfaces of the composite separator and improve its wettability with the electrolyte. Furthermore, the inorganic particles can further enhance its wettability with the electrolyte. Furthermore, the first and second composite layers are functional non-woven nanofiber membranes with different properties, forming a dense and stable interface with the central polyolefin-based membrane, thereby constructing a three-layer gradient wetting structure, which in turn enhances the separator's overall thermal stability, ionic conductivity, and electrolyte wettability.

[0039] Therefore, through the above structural optimization, the wettability, liquid absorption performance, and thermal stability of the separator are significantly improved. This not only reduces the separator's contact angle, but also improves the adsorption and transfer efficiency of the electrolyte, enhances the battery's dynamic performance, and strengthens the separator's structural integrity and thermal stability at high temperatures. This provides a reliable material foundation and engineering path for a gradient functional separator system with excellent dynamic performance and high safety in lithium-ion batteries.

[0040] The above general description and the following description are exemplary and explanatory only and are not intended to limit the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] One or more embodiments are exemplarily described by corresponding drawings. These exemplary descriptions and drawings do not limit the embodiments. Elements with the same reference numerals in the drawings are shown as similar elements. The drawings do not constitute a scale limitation. In addition,

[0042] Figure 1 1 is a schematic flow chart of a method for preparing a composite diaphragm provided in an embodiment of the present disclosure;

[0043] Figure 2 is a schematic structural diagram of a lithium-ion battery provided by an embodiment of the present disclosure;

[0044] Figure 3 is a schematic structural diagram of a lithium-ion battery provided by an embodiment of the present disclosure;

[0045] Figure 4 Schematic diagram of an expanded lithium-ion battery provided by an embodiment of the present disclosure.

[0046] Reference numerals:

[0047] 1-positive terminal; 10-battery cell; 11-positive column; 12-negative terminal; 2-shell; 3-negative electrode; 4-diaphragm; 5-positive electrode. DETAILED DESCRIPTION

[0048] In order to be able to understand the features and technical content of the embodiments of the present disclosure in more detail, the implementation of the embodiments of the present disclosure is described in detail below in conjunction with the accompanying drawings. The accompanying drawings are for reference only and are not used to limit the embodiments of the present disclosure. In the following technical description, for the sake of convenience of explanation, a full understanding of the disclosed embodiments is provided through multiple details. However, one or more embodiments can still be implemented without these details. In other cases, to simplify the drawings, well-known structures and devices can be simplified for display.

[0049] In the description and claims of the embodiments of the present disclosure, as well as in the accompanying drawings, the terms "first," "second," and the like are used to distinguish similar items and are not necessarily used to describe a particular order or precedence. It should be understood that the terms used in this manner are interchangeable where appropriate to describe the embodiments of the present disclosure herein. In addition, the terms "including," "having," and any variations thereof are intended to cover non-exclusive inclusions.

[0050] In the embodiments of the present disclosure, the terms "upper", "lower", "inside", "middle", "outside", "front", "back" and the like indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings. These terms are mainly intended to better describe the embodiments of the present disclosure and their embodiments, and are not intended to limit the indicated devices, elements or components to having a specific direction, or to be constructed and operated in a specific direction. Moreover, in addition to being used to indicate directions or positional relationships, some of the above terms may also be used to indicate other meanings. For example, the term "upper" may also be used to indicate a certain dependency or connection relationship in certain circumstances. For those of ordinary skill in the art, the specific meanings of these terms in the embodiments of the present disclosure can be understood according to specific circumstances.

[0051] Furthermore, the terms "disposed," "connected," and "fixed" should be interpreted broadly. For example, "connected" can mean a fixed connection, a removable connection, or an integral structure; it can be a mechanical connection or an electrical connection; it can be a direct connection, an indirect connection through an intermediary, or an internal connection between two devices, elements, or components. Those skilled in the art will understand the specific meanings of these terms in the embodiments of this disclosure based on the specific circumstances.

[0052] Unless otherwise stated, the term "plurality" means two or more.

[0053] In the embodiment of the present disclosure, the character " / " indicates that the preceding and following objects are in an "or" relationship. For example, A / B means: A or B.

[0054] The term "and / or" describes an association between objects, indicating that three relationships can exist. For example, A and / or B means: A or B, or A and B.

[0055] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present disclosure can be combined with each other.

[0056] The present disclosure provides a composite diaphragm, comprising an intermediate structural layer, a first composite layer, and a second composite layer. The intermediate structural layer comprises a polyolefin-based film; the first composite layer, disposed on the upper surface of the intermediate structural layer, comprises a polyacrylonitrile fiber matrix, a polyvinylidene fluoride-hexafluoropropylene copolymer, and inorganic particles; and the second composite layer, disposed on the lower surface of the intermediate structural layer, comprises a polyvinylidene fluoride-hexafluoropropylene copolymer and polyethylene oxide.

[0057] The composite separator provided by the embodiments of the present disclosure comprises a first composite layer formed on the upper surface of a polyolefin-based membrane. The first composite layer comprises a polyacrylonitrile fiber matrix, a polyvinylidene fluoride-hexafluoropropylene copolymer, and inorganic particles. This effectively enhances the separator's structural thermal responsiveness and electrolyte affinity. A second composite layer, comprising a polyvinylidene fluoride-hexafluoropropylene copolymer and polyethylene oxide, is formed on the lower surface of the polyolefin-based membrane. This effectively improves lithium salt coordination and electrolyte retention.

[0058] Because the polyacrylonitrile fiber matrix and polyethylene oxide monomers contain a high number of polar functional groups, they can improve the adhesion of the upper and lower surfaces of the composite separator and improve its wettability with the electrolyte. Furthermore, the inorganic particles can further enhance its wettability with the electrolyte. Furthermore, the first and second composite layers are functional non-woven nanofiber membranes with different properties, forming a dense and stable interface with the central polyolefin-based membrane, thereby constructing a three-layer gradient wetting structure, which in turn enhances the separator's overall thermal stability, ionic conductivity, and electrolyte wettability.

[0059] Therefore, through the above structural optimization, the wettability, liquid absorption performance, and thermal stability of the separator are significantly improved. This not only reduces the separator's contact angle, but also improves the adsorption and transfer efficiency of the electrolyte, enhances the battery's dynamic performance, and strengthens the separator's structural integrity and thermal stability at high temperatures. This provides a reliable material foundation and engineering path for a gradient functional separator system with excellent dynamic performance and high safety in lithium-ion batteries.

[0060] Optionally, the inorganic particles include silicon dioxide SiO2, aluminum oxide Al2O3, zirconium dioxide ZrO2 or calcium hydroxyphosphate Ca5(PO4)3(OH).

[0061] Optionally, the mass fraction of the inorganic particles in the first composite layer is 5% to 25%.

[0062] Alternatively, in the infrared spectrum of the composite membrane, at 2240 cm -1 ~2250cm -1 Between, 1400cm -1 ~1465cm -1 Between, 1170cm -1~1400cm -1 Between 1050cm -1 ~1155cm -1 Between and 500cm -1 ~800cm -1 There are characteristic peaks between them.

[0063] In this embodiment, the composite membrane has a Fourier transform infrared spectrum (FT-IR) at 2240 cm -1 ~2250cm -1 There is a characteristic peak between them, which is the stretching vibration of the (-C≡N) bond in the polyacrylic fiber (PAN) matrix.

[0064] In this embodiment, at 1400 cm -1 ~1465cm -1 There are characteristic peaks between them, which are the bending vibrations of (-CH2) bonds and (-CH) bonds in PAN matrix and polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP).

[0065] In this embodiment, at 1050 cm -1 ~1155cm -1 There is a characteristic peak between them, which is the stretching vibration of the (COC) bond in polyethylene oxide (PEO).

[0066] In this embodiment, at 500 cm -1 ~800cm -1 There are characteristic peaks between them, which are the bending vibrations or lattice vibrations of the metal-oxygen bonds in the inorganic particles, such as the bending vibrations of the Si-O bonds in SiO2 and the lattice vibrations of the Al-O bonds in Al2O3.

[0067] Optionally, the contact angle W of the composite membrane in the liquid electrolyte is ≤18°.

[0068] Optionally, the composite diaphragm has a rupture temperature T≥185°C.

[0069] Optionally, the liquid absorption rate A of the composite membrane is ≥130%.

[0070] Optionally, after the composite separator is heat-treated at 120° C., its dimensional shrinkage S is less than 3.5%.

[0071] In this embodiment, the dimensional shrinkage S includes the dimensional shrinkage of the composite diaphragm in the moving direction during the production process and the dimensional shrinkage in the perpendicular moving direction. That is, the dimensional shrinkage MD of the composite diaphragm in the longitudinal direction is less than 3.5%; and the dimensional shrinkage TD of the composite diaphragm in the longitudinal direction is less than 3.5%.

[0072] Optionally, the first composite layer and the second composite layer are combined with the intermediate structural layer through an electrospinning process and a hot pressing process, respectively, to form a composite diaphragm.

[0073] Combine Figure 1 As shown, the embodiment of the present disclosure also provides a method for preparing a composite diaphragm, comprising the following steps:

[0074] S101, depositing a first spinning solution on the upper surface of a polyolefin-based membrane by an electrospinning process to form a first composite layer, thereby obtaining a first diaphragm; wherein the first spinning solution comprises a mixed solution of a polyacrylonitrile fiber matrix, a polyvinylidene fluoride-hexafluoropropylene copolymer, and inorganic particles configured in a preset mass ratio;

[0075] S102, depositing a second spinning solution on the lower surface of the first diaphragm to form a second composite layer through an electrospinning process, thereby obtaining a second diaphragm; wherein the second spinning solution comprises a mixed solution of polyvinylidene fluoride-hexafluoropropylene copolymer and polyethylene oxide in a preset mass ratio;

[0076] Here, the second spinning solution is deposited on the lower surface of the first diaphragm by an electrostatic spinning process to form the second composite layer, that is, the second spinning solution is deposited on the lower surface of the polyolefin-based membrane by an electrostatic spinning process to form the second composite layer.

[0077] S103, performing hot pressing treatment on the second diaphragm to obtain a composite diaphragm.

[0078] Optionally, the first spinning solution is prepared as follows:

[0079] After fully dissolving the polyacrylonitrile fiber matrix and the polyvinylidene fluoride-hexafluoropropylene copolymer in N,N-dimethylformamide, a prefabricated solution is obtained;

[0080] dispersing inorganic particles in a prefabricated solution by ultrasound to obtain a first spinning solution;

[0081] Among them, in the first spinning solution, the mass fraction of the polyacrylonitrile fiber matrix is ​​35% to 50%, the mass fraction of the polyvinylidene fluoride-hexafluoropropylene copolymer is 20% to 40%, and the mass fraction of the inorganic particles is 5% to 25%, forming a first spinning solution with a solid content of 10% to 20%.

[0082] Optionally, the second spinning solution is prepared as follows:

[0083] The polyvinylidene fluoride-hexafluoropropylene copolymer and polyethylene oxide are dissolved in tetrahydrofuran or ethanol solvent in a mass ratio of 1:1 to obtain a second spinning solution.

[0084] In some embodiments, during the electrospinning process, the voltage is 5 kV to 20 kV, and the feed flow rate is 0.2 mL / h to 1.5 mL / h.

[0085] The embodiments of the present disclosure further provide a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, and the composite separator as described in the aforementioned embodiments.

[0086] The specific structure of the composite diaphragm refers to the above embodiments. Since the lithium-ion battery adopts all the technical solutions of all the above embodiments, it has at least all the technical effects brought by the technical solutions of the above embodiments, which will not be described one by one here.

[0087] Optionally, the positive electrode sheet includes a positive electrode current collector and a positive electrode coating coated with at least one layer containing a positive electrode active material; the positive electrode active material includes lithium nickel cobalt manganese oxide (Li1Ni x Co y Mn z M b O2) and lithium iron phosphate; wherein, 0.70≤x≤0.95, 0.15≤y<0.45, 0.05≤z<0.45, 0.0≤b≤0.25, x+y+z+b=1; the element M includes one or more of Zr, W, Ti, Al, Sr, B and Nd.

[0088] Optionally, the negative electrode sheet includes a negative electrode active material, and the negative electrode active material includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon oxide, pre-lithium silicon oxide, silicon, and deposited silicon carbon.

[0089] Optionally, the negative electrode sheet further includes a negative electrode conductive agent, and the negative electrode conductive agent includes one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes and carbon black.

[0090] Optionally, the negative electrode sheet further includes a negative electrode binder, and the negative electrode binder includes one or more of sodium carboxymethyl cellulose, polyacrylic acid, polyacrylonitrile, polystyrene-acrylic acid, and styrene-butadiene rubber.

[0091] Optionally, the lithium-ion battery further includes: an electrolyte including a lithium salt, a solvent and an additive; the lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium difluorophosphate, lithium difluorooxalatoborate and lithium bis(trifluoromethylsulfonyl)imide; the solvent includes one or more of dimethyl carbonate, diethyl carbonate, ethylene carbonate and ethyl methyl carbonate; the additive includes one or more of fluoroethylene carbonate, bis(fluoroethylene carbonate), vinyl sulfate, vinyl sulfite, vinyl carbonate and vinyl carbonate.

[0092] Optionally, the ratio of the capacity of the negative electrode sheet to the capacity of the positive electrode sheet is 1.02 to 1.1.

[0093] In the embodiment of the present disclosure, the lithium-ion battery further includes a shell, and the positive electrode sheet and the negative electrode sheet are encapsulated in the shell. Figures 2 to 4 As shown, the lithium-ion battery specifically includes a cylindrical shell 2, the interior of which is used to accommodate the battery cell, wherein the top is the positive terminal 1, the bottom is the negative terminal 12, and the positive terminal 1 is provided with a positive column 11. Specifically, Figure 3 shows a schematic structural diagram of the lithium-ion battery in this application, Figure 4 FIG1 shows a schematic diagram of the lithium-ion battery in the present application. Figure 4 As shown in the stacking, then winding to form Figure 3 The cylindrical battery cell 10 shown has the electrode end at the cylinder axis at the beginning of winding and at the outer surface of the cylinder at the end of winding. The positive electrode sheet 5 comprises a strip of positive electrode foil, a positive electrode coating applied to the surface of the positive electrode foil strip, and a first hollow foil region. The negative electrode sheet 3 comprises a strip of negative electrode foil, a negative electrode coating applied to the surface of the negative electrode foil strip, and a second hollow foil region. The first and second hollow foil regions are perpendicular to the winding direction and are formed by flattening or cutting and stacking to form the top and bottom end faces of the lithium-ion battery.

[0094] On this basis, an embodiment of the present disclosure provides a method for preparing a lithium-ion battery, comprising:

[0095] Preparation of positive electrode sheet: Mix the positive electrode coating materials, apply them on both sides of the aluminum foil, dry and cold press to obtain the positive electrode sheet;

[0096] Preparation of negative electrode sheet: Mix the negative electrode coating material and apply it on both sides of the aluminum foil. Then, a depression of a certain depth is opened in the negative electrode coating. After drying and cold pressing, the negative electrode sheet is obtained.

[0097] Preparation of battery cells: The positive and negative electrode sheets are rolled and slit, and then wound together with the separator to obtain battery cells;

[0098] Assembling lithium-ion batteries: Welding the tabs of the lithium-ion battery to the electrical connectors, placing them into the battery case, and performing the electrolyte injection, sealing, and formation processes to obtain a lithium-ion battery.

[0099] In addition, an embodiment of the present disclosure provides an electrical device, comprising a lithium-ion battery for providing power as described in the present application.

[0100] The present invention is further explained below with reference to the following examples.

[0101] Example 1

[0102] This embodiment 1 provides a method for preparing a lithium ion battery as follows:

[0103] Preparation of positive electrode sheet: Select positive electrode active material (Li1Ni 0.8 Co 0.1 Mn 0.1 O2), conductive carbon black, carbon nanotubes and polyvinylidene fluoride (PVDF) were thoroughly stirred and mixed in an N-methylpyrrolidone solvent system at a mass ratio of 96:1:1:2 to obtain a positive electrode coating material; the positive electrode coating material was then coated on a 12.0μm thick aluminum foil, and after drying, cold pressing, slitting and cutting, a positive electrode sheet was obtained. The compacted density of the positive electrode sheet is 2.45g / cm 3 .

[0104] Preparation of negative electrode sheet: Graphite, carbon nanotubes, sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) as the negative electrode active material were selected and stirred in deionized water at a mass ratio of 96:1.5:1:1.5 to form a negative electrode coating material. The negative electrode coating material was then coated on a 15 μm thick copper foil. After drying, cold pressing, slitting, and cutting, the negative electrode sheet was obtained. The compaction density of the negative electrode sheet was 1.5 g / cm 3 .

[0105] Preparation of electrolyte: lithium salt lithium hexafluorophosphate (LiPF6), organic solvent ethylene carbonate (EC), dimethyl carbonate (DMC), first type additive fluoroethylene carbonate (FEC), second additive vinyl sulfate (DTD) and third type additive vinylene carbonate (VC) are mixed in a mass ratio of 10:20:55:2:8:5 to obtain an electrolyte.

[0106] Preparation of the diaphragm: including pre-treatment of the intermediate structural layer, preparation of the first composite layer and preparation of the second composite layer.

[0107] The middle structural layer was pretreated. Specifically, a polypropylene (PP) membrane with a thickness of 4 μm was selected as the middle structural layer and dried in a dust-free drying oven at 60° C. for 4 hours to remove surface moisture.

[0108] Preparation of the first composite layer: 50% by mass of PAN matrix and 30% by mass of PVDF-HFP were weighed and dissolved in an N,N-dimethylformamide (DMF) solution. The mixture was stirred for 12 hours until completely dissolved to obtain a prefabricated solution. Subsequently, 20% by mass of Al2O3 was added to the prefabricated solution and ultrasonically dispersed for 30 minutes to achieve uniform dispersion, thereby obtaining a first spinning solution. The Al2O3 mass accounted for 15% of the total mass. The first spinning solution was then electrospun onto the top surface of a polypropylene membrane at a voltage of 15 kV and a feed rate of 0.8 mL / h to 1.2 mL / h, forming a 4 μm thick nonwoven top layer, thus obtaining the first membrane.

[0109] Preparation of the second composite layer: PVDF-HFP and PEO were weighed in a mass ratio of 1:1 and dissolved in tetrahydrofuran (THF) or ethanol solvent, stirred for 12 hours until completely dissolved to obtain a second spinning solution, and then the second spinning solution was spun and deposited on the lower surface of the first diaphragm at an electrospinning process at a voltage of 15 kV and an injection rate of 0.8 mL / h to 1.2 mL / h to form a composite lyophilic layer with a thickness of 4 μm, thereby obtaining the second diaphragm.

[0110] The second diaphragm was placed in a flat-plate hot press and hot pressed for 60 seconds at a temperature of 100°C and a pressure of 0.5 MPa to form a dense and stable interface between the first composite layer, the second composite layer and the intermediate structural layer, thereby obtaining a functional three-layer composite diaphragm.

[0111] The prepared composite membrane was then placed in a vacuum oven and vacuum dried at 60° C. for 12 hours to remove the residual solvent.

[0112] Lithium-ion battery assembly: The positive and negative electrode sheets are rolled, slit, and then wound together with the separator to form a cylindrical battery core. The core is then welded to the electrical connector and assembled into the battery housing. After completing the injection, sealing, and formation processes, the lithium-ion battery of Example 1 is obtained. The housing of this lithium-ion battery is cylindrical, with dimensions of 46.0 mm in diameter and 80.0 mm in length.

[0113] Example 2

[0114] Example 2 provides a lithium-ion battery, which differs from Example 1 in that the mass percentage of PVDF-HFP in the first composite layer of the composite separator is 20%. All other aspects are the same as Example 1.

[0115] Example 3

[0116] Example 3 provides a lithium-ion battery, which differs from Example 1 in that the mass percentage of PVDF-HFP in the first composite layer of the composite separator is 30%. Other aspects are the same as Example 1.

[0117] Example 4

[0118] Example 4 provides a lithium-ion battery, which differs from Example 1 in that the mass percentage of PVDF-HFP in the first composite layer of the composite separator is 40%. Other aspects are the same as Example 1.

[0119] Example 5

[0120] Example 5 provides a lithium-ion battery, which differs from Example 1 in that the mass percentage of Al2O3 in the first composite layer of the composite separator is 5%. Other aspects are the same as those of Example 1.

[0121] Example 6

[0122] Example 6 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass percentage of Al2O3 in the first composite layer of the composite separator is 10%, and the rest is the same as Example 1.

[0123] Example 7

[0124] Example 7 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass percentage of Al2O3 in the first composite layer of the composite separator is 20%, and the rest is the same as Example 1.

[0125] Example 8

[0126] Example 8 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass percentage of Al2O3 in the first composite layer of the composite separator is 25%, and the rest is the same as Example 1.

[0127] Example 9

[0128] Example 9 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass ratio of PVDF-HFP to PEO in the second composite layer of the composite diaphragm is 7:3, and the rest is the same as Example 1.

[0129] Example 10

[0130] Example 10 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass ratio of PVDF-HFP to PEO in the second composite layer of the composite separator is 6:4, and the rest is the same as Example 1.

[0131] Example 11

[0132] Example 11 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass ratio of PVDF-HFP to PEO in the second composite layer of the composite separator is 4:6, and the rest is the same as Example 1.

[0133] Example 12

[0134] Example 12 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass ratio of PVDF-HFP to PEO in the second composite layer of the composite diaphragm is 3:7, and the rest is the same as Example 1.

[0135] Example 13

[0136] Example 13 provides a lithium-ion battery. The difference between this example and Example 1 is that the temperature during hot pressing of the second diaphragm is 60° C., and the rest is the same as Example 1.

[0137] Example 14

[0138] Example 14 provides a lithium-ion battery. The difference between this example and Example 1 is that when the second diaphragm is hot-pressed, the temperature is 80° C., and the rest is the same as Example 1.

[0139] Example 15

[0140] Example 15 provides a lithium-ion battery. The difference between this example and Example 1 is that the temperature during hot pressing of the second diaphragm is 120° C., and the rest is the same as Example 1.

[0141] Comparative Example 1

[0142] Comparative Example 1 provides a lithium-ion battery. The difference between this comparative example and Example 1 is that Al2O3 is not added to the first composite layer of the composite diaphragm, and the rest is the same as Example 1.

[0143] Comparative Example 2

[0144] Comparative Example 2 provides a lithium-ion battery. The difference between this comparative example and Example 1 is that PEO is not added to the second composite layer of the composite separator. Other aspects are the same as Example 1.

[0145] The lithium ion batteries of Examples 1 to 15, and Comparative Examples 1 and 2 were subjected to corresponding tests.

[0146] Discharge the lithium-ion battery to 2.5V at a constant current. Ensure the battery is in a safe state to reduce the risk of short circuits or thermal runaway during disassembly. Disassemble the battery and remove the separator from the cylindrical cell in a glove box filled with pure argon or other inert gas.

[0147] Soak the removed diaphragm in an anhydrous dimethyl carbonate (DMC) solution for 30 minutes to dissolve and remove residual electrolyte and possible byproducts. Remove the diaphragm and wipe the electrode surface with a dust-free wipe. Then replace the anhydrous dimethyl carbonate (DMC) solution and repeat the soaking and wiping process three times to ensure that there are no residual contaminants on the electrode surface.

[0148] Rinse the diaphragm with anhydrous ethanol and wipe it again to ensure that there is no residual contaminant on the diaphragm surface. Place the cleaned diaphragm in the glove box for 48 hours to ensure that the electrode is completely dry to prevent subsequent tests from being interfered with by solvent residue.

[0149] This embodiment provides an infrared spectrum measurement method for a diaphragm, which uses a Fourier transform infrared spectrometer to directly perform non-destructive testing on a diaphragm sample in an attenuated total reflection (ATR) mode.

[0150] The specific steps include: placing the diaphragm sample to be tested on the diamond crystal ATR sampling head; applying constant pressure to ensure good contact; -1 ~400cm -1 The characteristic absorption peaks were obtained by scanning within the wavenumber range.

[0151] This embodiment also provides a method for testing aluminum oxide content, comprising the following steps:

[0152] The dried membrane was gently peeled off from its upper, middle, and lower layers using a scraper. The upper membrane was placed in a thermogravimetric analyzer (TGA). The membrane was heated to 600°C in a nitrogen atmosphere at a constant heating rate of 5°C / min to completely decompose the organic matter, leaving aluminum oxide as the residue.

[0153] The alumina content is calculated by the ratio of the initial mass m to the residual mass m1 after high temperature constant weight. The alumina content is calculated by the following formula:

[0154] m1 / m×100%;

[0155] Here, the method for testing the aluminum oxide content can also be a solvent selective stripping method, which will not be described in detail here.

[0156] This embodiment also provides a method for testing liquid absorption rate, comprising the following steps:

[0157] The membrane was dried in a vacuum drying oven to a constant weight, and the mass of the dried membrane was weighed as M1;

[0158] Immerse the diaphragm completely in the electrolyte and keep it for 30 minutes to fully absorb the liquid. The electrolyte system here is EC / DMC+1M LiPF6 system;

[0159] Remove the separator and use filter paper to absorb excess electrolyte from the separator surface. Immediately weigh the wet separator, denoting its mass as M2. Calculate the liquid absorption rate A by the ratio of the initial mass M1 to the mass M2 of the wet separator.

[0160] This embodiment also provides a method for testing the contact angle of a diaphragm, comprising the following steps:

[0161] Use a microsyringe to drop 2μL to 5μL of deionized water on the treated clean diaphragm surface, take a side view image of the droplet, fit the droplet contour using the Young-Laplace equation or ellipse fitting method, and calculate the contact angle after automatic baseline detection.

[0162] This embodiment also provides a method for testing the thermal shrinkage rate of a diaphragm, comprising the following steps:

[0163] Cut the diaphragm into 100mm×100mm square specimens according to GB / T 13519-1992, with the MD and TD directions clearly marked;

[0164] Lay the sample flat on white paper to prevent it from curling due to high temperature;

[0165] Place the laid sample in an oven with a temperature control accuracy of ±1°C; set the temperature to 120°C, heat for 1 hour without external force, and then cool to room temperature;

[0166] Use a vernier caliper or laser distance meter with an accuracy of 0.01 mm to measure the initial length L0 in the MD direction and the length L1 after heating in the TD direction respectively;

[0167] The shrinkage rate is calculated using the following formula:

[0168] (L0-L1) / L0×100%

[0169] Three samples were tested for each direction and the average was taken. If the deviation exceeded 5%, retesting was required.

[0170] This embodiment also provides a method for testing the rate performance of a lithium-ion battery. The lithium-ion battery is discharged to 2.5V and placed in a constant temperature box at 25°C for 6 hours. The test is performed according to the following steps:

[0171] Under the condition of 1C charging rate, constant current and constant voltage charging to 4.2V, cut-off current is 0.1C, and it is left to stand for 30 minutes. The capacity of constant current charging to 4.2V is Q1;

[0172] At a discharge rate of 1C, discharge at a constant current until the voltage reaches 2.5V, with a cutoff current of 0.1C, and then let it stand for 30 minutes.

[0173] Under the condition of 10C charging rate, constant current and constant voltage charging to 4.2V, cut-off current is 0.1C, and it is left to stand for 30min. The capacity of constant current charging to 4.2V is Q 10 ;

[0174] At a discharge rate of 1C, discharge at a constant current until the voltage reaches 2.5V, with a cutoff current of 0.1C, and then let it stand for 30 minutes.

[0175] Here, through Q1 and Q 10 Calculate the capacity retention rate Q of lithium-ion batteries R , where Q R =Q1 / Q 10 ×100%.

[0176] This embodiment also provides a method for testing the hot box failure temperature. The lithium-ion battery is placed in a constant temperature box at 25°C for 4 hours and tested according to the following steps:

[0177] Under the condition of 1C charging rate, constant current and constant voltage charging is carried out to 4.2V, the cut-off current is 0.01C, and it is left to stand for 10 minutes; then the temperature is increased at a rate of 5℃ / min; where, the temperature is kept warm for 10 minutes every time the temperature is increased by 5℃, and the surface temperature change of the lithium-ion battery is monitored during the heating process; when the temperature starts to rise sharply, the corresponding oven temperature is the hot box failure temperature of the secondary battery.

[0178] After Examples 1 to 15, and Comparative Examples 1 and 2 were subjected to the above tests, corresponding data were obtained.

[0179] The contact angle, liquid absorption rate, shrinkage rate MD, shrinkage rate TD, hot box failure temperature and capacity retention rate measured according to Examples 1 to 4 are shown in the following table:

[0180]

[0181] Table 1

[0182] As shown in Table 1, by comparing Examples 1 to 4, as the PVDF-HFP content in the first composite layer gradually increases, the contact angle decreases, the liquid absorption rate increases, the shrinkage rate decreases, the hot box failure temperature (membrane rupture temperature) increases, and the capacity retention rate also improves. The overall performance reaches its peak at 30%.

[0183] As shown in Table 1, increasing the PVDF-HFP content leads to a downward trend in all performance indicators. This phenomenon is attributed to the high polarity and excellent electrolyte affinity of the fluorinated groups in PVDF-HFP. Adding an appropriate amount to the composite fiber can improve electrolyte wettability, increase liquid absorption, and enhance battery dynamics. It also synergizes with PAN to form a flexible porous structure, improving thermal stability and membrane density.

[0184] In this implementation, the PVDF-HFP content was optimized to achieve an optimal balance between pore structure and polarity regulation, thereby improving ion channel continuity and interface stability. However, as the PVDF-HFP content continues to increase, the high viscosity of PVDF-HFP leads to excessive adhesion between fibers, causing partial collapse of the pores, reducing liquid absorption, leading to a decrease in battery kinetic performance, and possibly forming localized thermally fragile areas, resulting in a decrease in membrane rupture temperature.

[0185] The contact angle, liquid absorption rate, shrinkage rate MD, shrinkage rate TD, hot box failure temperature and capacity retention rate measured according to Example 1 and Example 5 to Example 8 are shown in the following table:

[0186]

[0187] Table 2

[0188] Table 2 shows that, by comparing Example 1 with Examples 5 to 8, as the Al2O3 content in the first composite layer gradually increases, the contact angle decreases, the liquid absorption rate significantly improves, and the thermal stability and battery rate performance are significantly improved. The optimal performance is achieved at an Al2O3 content of 15%.

[0189] As shown in Table 2, as the Al2O3 content continues to increase, its various performances decline. The main reason is that the surface of Al2O3 particles contains abundant hydroxyl groups, which can effectively enhance the wettability of the separator to the electrolyte, thereby improving the rate performance of the battery; at the same time, it provides thermal insulation, which is beneficial to improving the overall thermal stability.

[0190] In this implementation, when the Al2O3 content is within the 5% to 15% range, the Al2O3 is uniformly dispersed on the surface or within the nanofibers, establishing a stable lyophilic interfacial network and enhancing the skeleton strength. However, when the content exceeds 20%, the particles tend to agglomerate, causing pore blockage or localized aggregation, weakening the electrolyte adsorption capacity and pore connectivity, resulting in a decrease in lithium-ion conduction kinetics. Simultaneously, the particle-polymer interface bond is weakened, resulting in reduced thermal stability and overall performance.

[0191] The contact angle, liquid absorption rate, shrinkage rate MD, shrinkage rate TD, hot box failure temperature and capacity retention rate measured according to Example 1 and Example 9 to Example 12 are shown in the following table:

[0192]

[0193] Table 3

[0194] As the PEO content in the second composite layer gradually increases, its contact angle continues to decrease and the liquid absorption rate continues to increase, thereby improving the battery capacity retention rate. Among them, when the PEO content is 50%, its dynamic performance and thermal stability reach the best.

[0195] As shown in Table 3, increasing the PEO content further increases the liquid absorption rate and slightly enhances the kinetic performance; however, the thermal shrinkage and membrane rupture temperature begin to deteriorate. This is primarily due to the excellent lithium salt coordination and lyophilicity of the ether groups in PEO, which help enhance the second composite layer's ability to retain electrolyte and promote ion transport, thereby improving the kinetic performance of the lithium battery.

[0196] In this example, a 5:5 mass ratio of PVDF-HFP to PEO achieved a good balance between mechanical strength and lyophilic properties in the composite membrane. However, as the PEO content continued to increase, the membrane system's dominant structure, characterized by its low melting point and high rheological properties, weakened thermal stability, reduced interfiber bonding, and increased susceptibility to deformation or fracture at high temperatures, resulting in a decrease in overall membrane thermal shrinkage and membrane rupture temperature.

[0197] The contact angle, liquid absorption rate, shrinkage rate MD, shrinkage rate TD, hot box failure temperature and capacity retention rate measured according to Example 1 and Example 13 to Example 15 are shown in the following table:

[0198]

[0199] Table 4

[0200] In Table 4, by comparing Example 1 with Examples 13 to 15, it can be seen that increasing the temperature of hot pressing can promote the adhesion of the first composite layer, the second composite layer, and the interface of the intermediate structural layer, thereby improving the overall stability, liquid absorption performance, and capacity retention rate. Among them, when the hot pressing temperature is 100°C, the performance reaches the best.

[0201] As shown in Table 4, increasing the hot-pressing temperature slightly decreases the overall kinetic and thermodynamic properties. This is primarily due to the fact that the appropriate hot-pressing temperature promotes adhesion between the nonwoven nanofiber layer and the polyolefin-based membrane, forming a tight interface that improves the overall structural stability of the separator and the integrity of its ion channels without significantly damaging the nanopore structure.

[0202] In this embodiment, when the temperature is too high, some fibers may adhere or fuse, resulting in pore collapse, restricted electrolyte access, and reduced liquid absorption. Simultaneously, the pore structure becomes smaller, impairing electrolyte wetting and slightly increasing the contact angle. Furthermore, excessively high temperatures can cause uneven surface stress, increasing thermal shrinkage and reducing structural stability.

[0203] The contact angle, liquid absorption rate, shrinkage rate MD, shrinkage rate TD, hot box failure temperature and capacity retention rate measured according to Example 1, Comparative Example 1 and Comparative Example 2 are shown in the following table:

[0204]

[0205] Table 5

[0206] Table 5 shows that, by comparing Example 1 with Comparative Examples 1 and 2, the absence of Al2O3 in the first composite layer and PEO in the second composite layer of the composite separator significantly increases the contact angle and significantly decreases the liquid absorption rate, leading to a significant decrease in battery kinetic performance. Furthermore, the thermal stability of the separator is also somewhat reduced, primarily due to the loss of functional components, which reduces interfacial wettability and structural stability.

[0207] Here, the Al2O3 surface has a large number of hydroxyl groups, which can significantly improve the affinity of the diaphragm to the electrolyte and enhance the thermal insulation structure between fibers; its absence leads to weakened hydrophilicity of the first composite layer and insufficient interface contact, limiting electrolyte penetration and ion migration.

[0208] On the other hand, the abundant ether oxygen groups in PEO can form effective coordination with lithium salts, which is beneficial to enhancing the electrolyte retention and lithium ion conductivity; if this flexible lyophilic component is lacking, the ion transfer impedance of the second composite layer will increase, the electrolyte adsorption capacity will decrease, and the electrochemical wettability and kinetic matching of the overall diaphragm will be weakened.

[0209] In addition, the composite of PEO and PVDF-HFP also has the effect of synergistically enhancing the thermal stability of the fiber, and its absence also weakens the structural retention ability and safety of the diaphragm under high temperature conditions, which is manifested in an increase in thermal shrinkage and a decrease in membrane rupture temperature.

[0210] In summary, this application significantly improves the wettability, liquid absorption, and thermal stability of the separator through the above-mentioned structural optimization. This not only reduces the contact angle of the separator, but also improves the adsorption and transfer efficiency of the electrolyte, enhances the dynamic performance of the battery, and strengthens the structural integrity and thermal stability of the separator at high temperatures. This provides a reliable material foundation and engineering path for a gradient functional separator system with excellent dynamic performance and high safety lithium-ion batteries.

[0211] The above description and the accompanying drawings sufficiently illustrate the embodiments of the present disclosure to enable those skilled in the art to practice them. Other embodiments may include structural and other changes. The embodiments represent only possible variations. Unless expressly required, individual components and functions are optional, and the order of operations may vary. Portions and features of some embodiments may be included in or replace portions and features of other embodiments. The embodiments of the present disclosure are not limited to the structures described above and shown in the accompanying drawings, and various modifications and changes may be made without departing from the scope thereof. The scope of the present disclosure is limited only by the appended claims.

Claims

1. A composite diaphragm, characterized in that: include: an intermediate structural layer comprising a polyolefin base film; a first composite layer, disposed on the upper surface of the intermediate structural layer, the first composite layer comprising a polyacrylonitrile fiber matrix, a polyvinylidene fluoride-hexafluoropropylene copolymer, and inorganic particles; The second composite layer is arranged on the lower surface of the intermediate structural layer, and the second composite layer comprises polyvinylidene fluoride-hexafluoropropylene copolymer and polyethylene oxide.

2. The composite diaphragm according to claim 1, characterized in that The inorganic particles include silicon dioxide, aluminum oxide, zirconium dioxide or calcium hydroxyphosphate, and the mass fraction of the inorganic particles in the first composite layer is 5% to 25%.

3. The composite diaphragm according to claim 1, characterized in that In the infrared spectrum of the composite membrane, at 2240 cm -1 ~2250cm -1 Between 1400cm -1 ~1465cm -1 Between, 1170cm -1 ~1400cm -1 Between 1050cm -1 ~1155cm -1 Between and 500cm -1 ~800cm -1 There are characteristic peaks between them.

4. The composite diaphragm according to claim 1, characterized in that The contact angle W of the composite diaphragm in the liquid electrolyte is less than or equal to 18°.

5. The composite diaphragm according to claim 1, characterized in that The composite diaphragm has a rupture temperature T of ≥185°C.

6. The composite diaphragm according to claim 1, characterized in that The liquid absorption rate A of the composite diaphragm is ≥130%.

7. The composite diaphragm according to claim 1, characterized in that After the composite diaphragm is heat-treated at 120° C., its dimensional shrinkage rate S is less than 3.5%.

8. The composite diaphragm according to claim 1, characterized in that The first composite layer and the second composite layer are combined with the intermediate structural layer through an electrostatic spinning process and a hot pressing process respectively to form the composite diaphragm.

9. A method for preparing a composite diaphragm according to any one of claims 1 to 8, characterized in that: The following steps are involved: The first diaphragm is obtained by depositing a first spinning solution on the surface of the polyolefin base film through an electrospinning process to form a first composite layer; wherein the first spinning solution includes a mixed solution of a polyacrylonitrile fiber matrix, a polyvinylidene fluoride-hexafluoropropylene copolymer, and inorganic particles configured in a preset mass ratio; The second spinning solution is deposited on the lower surface of the first diaphragm to form a second composite layer through an electrospinning process, thereby obtaining a second diaphragm; wherein the second spinning solution comprises a mixed solution of polyvinylidene fluoride-hexafluoropropylene copolymer and polyethylene oxide in a preset mass ratio; The second diaphragm is subjected to a hot pressing process to obtain a composite diaphragm.

10. The method for preparing a composite diaphragm according to claim 9, characterized in that: The first spinning solution is prepared as follows: The polyacrylonitrile fiber matrix and the polyvinylidene fluoride-hexafluoropropylene copolymer are fully dissolved in N,N-dimethylformamide to obtain a prefabricated solution; dispersing inorganic particles in the prefabricated solution by ultrasound to obtain the first spinning solution; Among them, in the first spinning solution, the mass fraction of the polyacrylonitrile fiber matrix is ​​35% to 50%, the mass fraction of polyvinylidene fluoride-hexafluoropropylene copolymer is 20% to 40%, and the mass fraction of inorganic particles is 5% to 25%, forming a first spinning solution with a solid content of 10% to 20%.

11. The method for preparing a composite diaphragm according to claim 9, characterized in that: The second spinning solution is prepared as follows: The polyvinylidene fluoride-hexafluoropropylene copolymer and polyethylene oxide are dissolved in tetrahydrofuran or ethanol solvent in a mass ratio of 1:1 to obtain the second spinning solution.

12. The method for preparing a composite diaphragm according to claim 9, characterized in that: In the electrospinning process, the voltage is 5 kV to 20 kV, and the injection flow rate is 0.2 mL / h to 1.5 mL / h.

13. A lithium ion battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet and a composite separator as claimed in any one of claims 1 to 8.

Images