Separator for lithium ion battery having coating layer having brievoid structure and lithium ion

Abstract

The invention provides a diaphragm for a lithium ion battery and a lithium ion battery using the diaphragm, wherein the diaphragm is provided with a coating layer with a Beliogang structure. The diaphragm for the lithium ion battery comprises a basement membrane and a coating layer coated on one or two surfaces of the basement membrane, and slurry consisting of an inorganic nano material and a binder is unidirectionally brushed on one or two surfaces of the basement membrane in a plurality of orientation directions to form a spiral stacking layer, in this way, a coating layer having a Brini-gang structure is obtained. The diaphragm for the lithium ion battery has improved mechanical properties, electrolyte wettability and heat resistance; meanwhile, the lithium ion battery using the diaphragm for the lithium ion battery provided by the invention has improved cycle performance and rate capability.

Description

Technical Field

[0001] The present invention relates to the field of batteries, and more particularly to a separator for a lithium-ion battery having a coating layer with a Brigand structure and a lithium-ion battery using the same. Background Art

[0002] Lithium-ion batteries have been widely used due to their advantages such as light weight, large capacity, and no memory effect. Many current digital devices use lithium-ion batteries as power sources. Although their price is relatively high, the energy density of lithium-ion batteries is very high. Its capacity is 1.5 to 2 times that of nickel-metal hydride batteries of the same weight, and it has a very low self-discharge rate. In addition, lithium-ion batteries have almost no memory effect and do not contain toxic substances, which is also an important reason for their widespread application.

[0003] The separator for a lithium-ion battery is a key component in the battery. Its main function is to isolate the positive and negative electrodes to prevent short circuits, while allowing lithium ions to pass through. Currently, commonly used separator materials include polyethylene (PE), polypropylene (PP), and their multi-layer composite materials. These materials have good mechanical strength, chemical stability, and electrical insulation. However, these separators also have some defects that limit their performance. These mainly manifest in insufficient thermal stability, poor wettability, limited mechanical strength, uneven porosity, etc. PP and PE are prone to shrinkage at high temperatures, which may cause battery short circuits. In addition, the wettability of the separator to the electrolyte is poor, affecting ion conduction. During the charge and discharge process of the battery, the separator may be damaged due to mechanical stress. Uneven porosity distribution will affect battery performance.

[0004] The separator for a lithium-ion battery is crucial for battery performance. Existing materials have problems such as poor thermal stability and wettability. Therefore, the technology for improving the performance of the separator has become a research hotspot. Currently, the performance of the separator for batteries has been improved by coating functional materials. For example, ceramic-coated separators can improve thermal stability and safety; separators coated with polymers such as PVDF perform well in enhancing wettability and mechanical strength; and the comprehensive performance of some composite-coated separators combining ceramics and polymers has been optimized.

[0005] Nevertheless, there is still a need in the art for separators for lithium-ion batteries with further improved performance, especially mechanical properties, electrolyte wettability, and heat resistance, in order to further improve the performance of lithium-ion batteries, especially the cycle performance and rate performance of the batteries. Summary of the Invention

[0006] Based on the Bouligand structure inspired by, for example, the appendage structure of mantis shrimp (which has amazing impact resistance, high strength, and energy absorption ability), the inventors unexpectedly found that by brushing a slurry with a specific composition on one or both surfaces of the base film to form a coating layer with a Bouligand structure, a separator for a lithium-ion battery with significantly improved mechanical properties, electrolyte wettability, heat resistance, etc. can be obtained, and a lithium-ion battery using this separator for a lithium-ion battery has significantly improved cycle performance and rate performance.

[0007] For this reason, in one aspect, the present invention provides a separator for a lithium-ion battery with a coating layer having a Bouligand structure, which includes a base film and a coating layer coated on one or both surfaces of the base film, wherein a helical stacked layer is formed by unidirectionally brushing a slurry composed of inorganic nanomaterials and a binder on one or both surfaces of the base film in a plurality of orientation directions, thereby obtaining a coating layer with a Bouligand structure.

[0008] In a preferred embodiment, the base film is selected from a polyethylene (PE) film, a polypropylene (PP) film, a double-layer PP / PE composite film, a three-layer PP / PE / PP composite film, a polyimide (PI) film, a polyetheretherketone (PEEK) film, or a meta-aramid (PMIA) film.

[0009] In a preferred embodiment, the thickness of the coating layer is 5 - 50 μm.

[0010] In a preferred embodiment, the inorganic nanomaterials are one or more selected from silicon dioxide (SiO2) nanowires, aluminum oxide (Al2O3) nanowires, titanium dioxide (TiO2) nanowires, zinc oxide (ZnO) nanowires, calcium silicate (CaSiO3) nanowires, carbon nanotubes, and boron nitride (BN) nanowires.

[0011] In a preferred embodiment, the binder is one or more selected from polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), polyvinyl alcohol (PVA), and polyethylene oxide (PEO).

[0012] In a preferred embodiment, in the slurry, the weight content of the binder is 0.5% - 10%.

[0013] In a preferred embodiment, in the slurry, the weight content of the inorganic nanomaterials is 1% - 30%.

[0014] In a preferred embodiment, the coating layer is a helical stacked layer formed by unidirectionally brushing the slurry in at least three orientation directions.

[0015] In a preferred embodiment, the angle between each layer of the spiral stacked layer is in the range of 10° to 90°.

[0016] In another aspect, the present invention provides a lithium-ion battery, which includes the separator for lithium-ion battery described above.

[0017] In a preferred embodiment, the lithium-ion battery is a liquid lithium-ion battery.

[0018] The coating layer of the separator for lithium-ion battery of the present invention has a Bouligand structure, which can dissipate energy by guiding the crack propagation path through the interface between the spiral stacked layers, thereby endowing the separator with excellent mechanical properties; meanwhile, the coating layer is a spiral stacked layer formed by brushing a slurry composed of inorganic nanomaterials and a binder unidirectionally, thereby endowing improved electrolyte wettability and heat resistance. Further, the lithium-ion battery using the separator for lithium-ion battery of the present invention has excellent cycle and rate performance.

[0019] In addition, the present invention breaks through the traditional homogeneous / laminated design of the separator for batteries, and for the first time introduces a biological spiral stacked layered structure (i.e., Bouligand structure) into the coating layer of the separator, realizing multi-stage energy dissipation.

[0020] In addition, the separator for batteries of the present invention realizes a bionic structure and function integration significantly different from the traditional homogeneous coating, taking into account both the mechanical strength (such as puncture resistance) and electrochemical performance (ion transport ability) of the separator for batteries, which are difficult to achieve by traditional coatings. Description of the Drawings

[0021] Figure 1 Shows a scanning electron microscope (SEM) image of the surface of the separator obtained according to Example 1.

[0022] Figure 2 Shows a SEM image of the cross-section of the separator obtained according to Example 1.

[0023] Figure 3 Shows a SEM image of the surface of the separator obtained according to Comparative Example 1.

[0024] Figure 4 Shows a SEM image of the surface of the separator obtained according to Comparative Example 2.

[0025] Figure 5 Shows X-ray diffraction (XRD) images of the separators obtained according to Examples 1-3 and Comparative Examples 1-2.

[0026] Figure 6 Shows the mechanical properties of the separators obtained according to Examples 1-3 and Comparative Examples 1-2

[0027] Figure 7 Shows the water contact angle results of the separators obtained according to Examples 1-3 and Comparative Examples 1-2.

[0028] Figure 8 Shows the ionic conductivity results of the separators obtained according to Examples 1-3 and Comparative Examples 1-2.

[0029] Figure 9 Shows the cycle performance results of the lithium-ion batteries obtained according to Example 2.

[0030] Figure 10 Shows the cycle performance results of the lithium-ion batteries obtained according to Comparative Example 1.

[0031] Figure 11 Shows the cycle performance results of the lithium-ion batteries obtained according to Comparative Example 2.

[0032] Figure 12 Shows the rate performance results of the lithium-ion batteries obtained according to Example 2.

[0033] Figure 13 Shows the rate performance results of the lithium-ion batteries obtained according to Comparative Example 1.

[0034] Figure 14 Shows the rate performance results of the lithium-ion batteries obtained according to Comparative Example 2. Detailed Description of the Invention

[0035] The biomimetic Bragdon structure is derived from the study of the appendage structure of the mantis shrimp, which has amazing impact resistance, high strength and energy absorption ability. The inventors have found that such excellent properties are due to the unique arrangement of its internal microstructure (helical hierarchical structure), in which nanofibers as basic units are arranged unidirectionally in different orientation directions in a plane to form multiple nanofiber helical stacked layers, and these nanofiber helical stacked layers are helically stacked at a certain angle to form a stable three-dimensional structure.

[0036] Based on in-depth research, the inventors unexpectedly found that on the surface of a substrate such as a PE film (also referred to as a base film in this article) of a separator for a conventional lithium-ion battery, a separator for a lithium-ion battery obtained by providing a coating layer with a Bragdon structure in a specific manner can be significantly improved especially in terms of mechanical properties, electrolyte wettability and heat resistance, and at the same time, a good balance can be achieved between the mechanical strength (such as puncture resistance) and electrochemical performance (ion transport ability) of the separator; moreover, this further significantly improves the cycle and rate performance of the lithium-ion battery using this separator.

[0037] Therefore, the separator for a lithium-ion battery provided by the present invention includes a base film and a coating layer coated on one or two surfaces of the base film, wherein a helical stacking layer is formed by unidirectionally brushing a slurry composed of inorganic nanomaterials and a binder on one or two surfaces of the base film in a plurality of orientation directions, thereby enabling the coating layer to have a desired Bouligand structure.

[0038] In the present invention, there is no particular limitation on the base film. For example, it can be any separator substrate commonly used in lithium-ion batteries. Preferably, the base film used in the present invention can be selected from a polyethylene (PE) film, a polypropylene (PP) film, a double-layer PP / PE composite film, a triple-layer PP / PE / PP composite film, a polyimide (PI) film, a polyether ether ketone (PEEK) film, or a meta-aramid (PMIA) film.

[0039] In the present invention, preferably, the inorganic nanomaterials used are in the form of nanofibers (also known as nanowires). For example, the inorganic nanomaterials used are nanowires with a diameter of ∼20 to 30 nm and a length of 1 to 10 μm. More preferably, the inorganic nanomaterials used can be one or more selected from calcium silicate (CaSiO3) nanowires, metal oxide nanowires, carbon nanotubes, and semiconductor nanowires. Particularly preferably, the inorganic nanomaterials used can be one or several selected from silicon dioxide (SiO2) nanowires, aluminum oxide (Al2O3) nanowires, titanium dioxide (TiO2) nanowires, zinc oxide (ZnO) nanowires, calcium silicate (CaSiO3) nanowires, carbon nanotubes, and boron nitride (BN) nanowires.

[0040] Methods for preparing inorganic nanowires are known in the art. Only as a non-limiting example, for instance, the calcium silicate nanowires used in the present invention can be obtained by dissolving the corresponding soluble metal salts (such as calcium nitrate and sodium silicate) separately in water, then mixing their solutions in a reaction kettle, and then leaving or reacting the sealed reaction kettle in an oven at an elevated temperature (such as 200 °C) for a period of time (such as 24 h), then filtering and washing the obtained product, and finally drying (such as in an oven at 120 °C), thereby obtaining the desired calcium silicate nanowires. Similarly, other desired inorganic nanomaterials can be obtained.

[0041] In the present invention, there is no particular limitation on the binder. For example, it can be any binder commonly used in the battery field. Preferably, the binder used can be one or more selected from polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), polyvinyl alcohol (PVA), and polyethylene oxide (PEO).

[0042] In the present invention, typically, a slurry required for forming a coating layer can be formed by dissolving inorganic nanomaterials and a binder either individually or as a mixture in a suitable solvent. Suitable solvents that can be used are known to those skilled in the art, such as but not limited to those commonly used in the battery field. Preferably, the solvents for dissolving inorganic nanomaterials and / or the binder in the present invention can include but are not limited to; aqueous solvents, such as water (e.g., deionized water or ultrapure water), water / ethanol mixed solvents; organic solvents, such as N-methylpyrrolidone (NMP), acetone, cyclopentanone, dimethylformamide (DMF)), alcohol solvents (e.g., isopropyl alcohol (IPA) or ethylene glycol (EG)); and other solvents, such as ionic liquids.

[0043] In the present invention, preferably, in the slurry formed by dissolving inorganic nanomaterials and a binder either individually or as a mixture in a suitable solvent, the weight content of the inorganic nanomaterials is 1% - 30%. In the present invention, preferably, in the slurry formed by dissolving inorganic nanomaterials and a binder either individually or as a mixture in a suitable solvent, the weight content of the binder is 0.5% - 10%. The inventors have found that when the weight contents of the inorganic nanomaterials and the binder in the slurry are respectively within the above corresponding ranges, a spiral stacked layer with the best shape can be obtained, so that the obtained separator for lithium-ion batteries can achieve the best balance between mechanical strength (such as puncture resistance) and electrochemical performance (ion transport ability), and exhibit the best performance.

[0044] For a method of forming a coating with a Bouligand structure on a substrate, for example, Si-Ming Chen et al. mentioned a shear-brushing strategy for preparing Bouligand structures in a review article (Si-ming Chen et al., Biological and bioinspired Bouligand structural materials: Recent advances and perspectives, Matter 7, 378–407, the entire content of which is incorporated herein by reference), where shear forces can induce the assembly of anisotropic fiber building blocks. In this strategy, a programmable fiber assembly platform is used, which includes four modules: "solution injection and transport", "sliding shear", "solvent evaporation and drying", and "substrate rotation". These four modules can work sequentially to facilitate the accurate assembly of fiber building blocks into biomimetic Bouligand structural materials. The assembly method utilizes shear forces to align anisotropic micro / nanofiber building blocks, and the shear forces have two sources: one comes from the spatially confined solution transport from the bottom to the tip of the brush bristles (the shear force generated can promote the initial orientation of micro / nanofiber building blocks), and the other comes from the sliding process of the brush on the substrate (the shear force generated can further promote the orientation on the substrate). In addition, the gelation induced by the evaporation of the solvent in the slurry can limit the random oscillation of micro / nanofiber building units and contribute to the formation of a dry and ordered composite micro-layer, from which a unidirectional thin sheet or unidirectional thin layer can be obtained. Subsequently, by further using the "substrate rotation" module and rotating the substrate by a certain angle multiple times, a biomimetic Bouligand structure with twisted and aligned thin sheets or thin layers can be prepared, and the mechanical properties can be optimized by adjusting the interlayer skew angle.

[0045] More specifically, in the present invention, the coating layer with a Bouligand structure for a lithium-ion battery separator can be obtained in the following manner: Fix the base film as the substrate on a flat glass plate, which is placed on a heatable platform or device (the heating temperature is, for example, 50 °C to 120 °C, which can be used to evaporate the solvent to dry the coating); then, the prepared required slurry (obtained by dissolving inorganic nanomaterials and a binder individually or as a mixture in a suitable solvent) is first brushed unidirectionally (i.e., moving the coating brush along a fixed direction) on the surface of the base film in one orientation direction once with a coating brush to form a first inorganic nanomaterial layer; thereafter, preferably after the first inorganic nanomaterial layer has dried, by rotating the glass plate by an angle (the rotation angle is, for example, 10 ° to 90 °), it is brushed unidirectionally again in another orientation direction once to form a second inorganic nanomaterial layer on the first inorganic nanomaterial layer. Further, by repeating the process of rotating the glass plate and brushing unidirectionally with the coating brush, third, fourth... inorganic nanomaterial layers can be successively formed, and spiral stacked layers (i.e., coating layers with a Bouligand structure) are formed on the base film at the above rotation angle between these layers, thereby obtaining a separator including a coating layer with a Bouligand structure.

[0046] In the present invention, preferably, the Bouligand structure of the coating layer is composed of at least two, at least three up to ten of the above-mentioned inorganic nanomaterial layers. More preferably, the coating layer is a spiral stacked layer formed by brushing the slurry unidirectionally in at least three orientation directions (i.e., rotating at least twice).

[0047] In the present invention, preferably, during the process of forming the Bouligand structure, the rotation angle each time can be the same or different, and preferably within the range of 10 ° to 90 °, for example, independently 15 °, 30 °, 45 °, 50 °, 60 °, 70 °, 85 ° or 90 °, etc.

[0048] In the present invention, preferably, the thickness of the coating layer with a Bouligand structure formed on the base film (i.e., the total thickness of the formed spiral stacked layer) can be within the range of 5 to 50 μm. The inventors have found that when a coating layer with a Bouligand structure having a thickness within the above range is formed on the base film, the obtained separator has more excellent performance.

[0049] The separator for lithium-ion batteries of the present invention can be applied to lithium-ion batteries, especially liquid lithium-ion batteries using electrolytes, such as button-type lithium-ion batteries. The process of applying the separator for lithium-ion batteries to lithium-ion batteries or assembling it into lithium-ion batteries is well-known in the art. For example, a button-type lithium-ion battery can be assembled as follows: using a lithium metal sheet and a commercial lithium iron phosphate electrode sheet as electrodes, and cutting the separator into the desired shape (such as a disc with a radius of 6 mm or 8 mm), assembling in the order of negative electrode case - lithium metal electrode sheet - separator - injecting electrolyte - lithium iron phosphate electrode sheet - stainless steel gasket - funnel spring - positive electrode case, and compacting and pressing it with a battery encapsulation machine after assembly. The electrolyte used can be, for example, a 1M LiPF6 solution in ethylene carbonate (EC) / dimethyl carbonate (DMC) (1:1) v / v + 2% vinylene carbonate (VC). When assembling the battery, preferably, if the coating layer with a buligang structure is formed only on one surface of the base film, the coating layer with a buligang structure faces the side of the lithium iron phosphate electrode sheet.

[0050] Hereinafter, the present invention will be further clearly and completely described through specific examples. It should be understood that the described examples are only for illustrative purposes of the present invention and not for restrictive purposes. At the same time, these examples are only partial examples specifically embodying the present invention, rather than all examples. Those skilled in the art can easily understand and implement other advantages and effects of the present invention according to the content disclosed in this specification.

[0051] Unless otherwise specified, various solvents and reagents used in the following examples or comparative examples are commercially available. Similarly, various starting materials used are commercially available, or can be further processed or prepared by methods well-known in the art or conventional reactions. In addition, without specific description, the reagents or raw materials used are directly used after purchase, or used after conventional treatment.

[0052] The calcium nitrate, sodium silicate, and polyethylene oxide used in the examples are all of analytical purity, and the manufacturer is Sinopharm Chemical Reagent Co., Ltd. The button battery components in the examples, the positive electrode case, negative electrode case, gasket (Φ15.8*1.0 mm), and funnel spring (304 spring Φ15.4*1.1 mm) used for assembling CR 2032 batteries are purchased from Kelude. The electrolyte component used for assembling button batteries is a 1M LiPF6 solution in ethylene carbonate (EC) / dimethyl carbonate (DMC) (1:1) v / v + 2% vinylene carbonate (VC), purchased from Nanjing Mojiesi Research Institute.

[0053] Unless otherwise specified, the reaction devices and the equipment or instruments used for analysis or characterization in the following examples or comparative examples are well-known or commonly used in the art.

[0054] The relevant instruments and equipment used in the examples include: scanning electron microscope (Supra 40, Zeiss), X-ray diffractometer (PANalytical X’pert PRO MRD, Malvern), contact angle tester (OCA 25, Dataphysics), universal mechanical testing machine (Instron 5565-A, Instron), EC-LAB test platform (VMP3, BioLogic), Neware battery test system (Shenzhen Neware), glove box (Mikrouna), manual slicer (MSK-T-06, Hefei Kejing), electric pressure-controlled button cell encapsulation machine (MSK-160E, Hefei Kejing), hydrothermal reactor (HTG500, Kemi Instrument), mechanical stirrer (RW 20 digital, IKA), heating stage (HP-6040, Hanbang).

[0055] Example 1

[0056] Dissolve 41 g of calcium nitrate and 71.1 g of sodium silicate in 500 ml of ultrapure water in a 500 ml glass beaker respectively to prepare solutions with a concentration of 0.5 mol / L each (i.e., the concentration ratio of calcium nitrate to calcium silicate is 1:1). Then, dropwise add the calcium nitrate solution to the sodium silicate solution under mechanical stirring to obtain a mixed solution, and then transfer the mixed solution to a reaction kettle lined with polytetrafluoroethylene. After sealing the reaction kettle, place it in an oven at 200 °C for 24 h. Then, take out the reaction kettle and filter the product therein under normal pressure using a Buchner funnel, and wash it three times with ultrapure water to obtain calcium silicate nanowires, and then dry them in an oven at 120 °C for 24 h to obtain dry calcium silicate nanowires. Through observation and calculation by a scanning electron microscope, the diameter of the obtained calcium silicate nanowires is in the range of 20 - 50 nm, and the length is in the range of 1 to 10 μm.

[0057] Prepare a calcium silicate nanowire dispersion with a calcium silicate concentration of 4% by dispersing an appropriate amount of calcium silicate nanowires into ultrapure water.

[0058] Weigh 1 g of polyethylene oxide and 1 g of sodium carboxymethylcellulose into a 20 ml sample bottle, add 18 ml of ultrapure water and a magnetic stir bar, and dissolve the polyethylene oxide and sodium carboxymethylcellulose completely by magnetic stirring for 12 h to obtain a binder solution with a mass concentration of 10%.

[0059] Then, add 5 ml of the above binder solution to a 100 ml beaker, and then add 45 ml of the above calcium silicate nanowire dispersion. After stirring well with a glass rod, the required slurry solution is obtained. In this slurry solution, the weight content of the binder is 1%, and the weight content of the calcium silicate nanowires is 3.6%.

[0060] Then, fix the PE separator (purchased from Kelude, with a thickness of 20 μm and a width of 60 mm) used as the base membrane on the glass plate with transparent tape. The glass plate is placed on an electrically heated platform with a heating temperature of 80°C. Then, brush the slurry solution with a coating brush as follows: Deliver the slurry solution to the root of the coating brush through a syringe pump, and brush the coating brush on one surface of the base membrane once along an orientation direction (i.e., move the coating brush in a fixed direction) (the brushing speed or the moving speed of the coating brush is 1 cm / s) to form a first calcium silicate nanowire layer with a thickness of 10 μm. Subsequently, rotate the glass plate clockwise by 30° (i.e., the first rotation), and continue to brush the coating brush unidirectionally above the first calcium silicate nanowire layer in this orientation direction (the brushing speed remains 1 cm / s) to form a second calcium silicate nanowire layer with a thickness of 10 μm; then, rotate the glass plate clockwise by 30° again (i.e., the second rotation), and continue to brush the coating brush unidirectionally above the second calcium silicate nanowire layer in this orientation direction (the brushing speed remains 1 cm / s) to form a third calcium silicate nanowire layer with a thickness of 10 μm, thereby forming a helically stacked layer with a total thickness of 30 μm and twisted at an angle of 30° (i.e., a coating layer with a Bouligand structure). After complete drying, remove the base membrane with the coating layer from the glass plate to obtain the required separator.

[0061] Finally, the obtained separator was used to assemble a CR 2032 coin-type lithium-ion battery, using a lithium metal sheet and a commercial lithium iron phosphate electrode sheet as the positive and negative electrodes, respectively. Before assembly, the lithium metal sheet and the commercial lithium iron phosphate electrode sheet were cut into circular pieces with a radius of 6 mm using a manual slicing machine, and the obtained separator was cut into a circular piece with a radius of 8 mm. The battery assembly was carried out in a glove box filled with argon, and all materials were passed through a transfer chamber to remove air before entering the glove box. The CR 2032 coin-type battery was assembled in the order of negative electrode case - lithium metal sheet - separator coated with a Brygon structure - electrolyte injection - lithium iron phosphate electrode sheet - stainless steel gasket - funnel spring - positive electrode case, and after assembly, it was compacted using a battery encapsulation machine. The electrolyte used had a composition of 1M LiPF6 solution in ethylene carbonate (EC) / dimethyl carbonate (DMC) (1:1) v / v + 2% vinylene carbonate (VC), and the volume of the electrolyte used was 80 μL, with the Brygon-structured coating layer facing the lithium iron phosphate electrode sheet side. The assembled battery was placed at room temperature for 12 h before electrochemical testing.

[0062] The surface and cross-section of the obtained separator were analyzed by scanning electron microscopy (SEM) (instrument model: Supra 40, manufacturer: Zeiss), and the results are shown respectively in Figure 1 and Figure 2 respectively. Figure 1 Fig. shows the SEM image of the surface of the separator obtained according to Example 1, including a coating layer with a Brygon structure, and it can be seen from Figure 1 that there is a helically stacked coating layer on the surface of the PE substrate film, that is, a separator including a coating layer with a Brygon structure is obtained. Figure 2 Fig. shows the SEM image of the cross-section of the separator obtained according to Example 1, and it can be seen from Figure 2 that there is a certain rotation angle between the helically stacked layers of the coating layer with a Brygon structure.

[0063] Example 2

[0064] The procedure was the same as in Example 1, except that the glass plate was rotated clockwise by 45° each time and the thickness of the calcium silicate nanowire layer obtained each time was 11 μm (so that the total thickness of the coating layer with a Brygon structure was 33 μm).

[0065] As confirmed by the results of SEM analysis (images not shown), a coating layer with a Brygon structure was formed on the surface of the PE substrate film, and there was a certain rotation angle between the helically stacked layers.

[0066] Example 3

[0067] Performed in the same procedure as Example 1, except that the glass plate was rotated clockwise by 60° each time, and the thickness of the calcium silicate nanowire layer obtained after the second rotation was 9 μm (so that the total thickness of the coating layer with a Bouligand structure was 29 μm).

[0068] As confirmed by the results of SEM analysis (images not shown), a coating layer with a Bouligand structure was formed on the surface of the PE substrate film, and there was a certain rotation angle between the helical stacking layers.

[0069] Example 4

[0070] Performed in the same procedure as Example 1, except that silicon dioxide nanowires were used instead of the calcium silicate nanowire layer.

[0071] The silicon dioxide nanowires used in this example were prepared by a one-pot template method as follows. First, in a glass beaker, 5 g of polyvinylpyrrolidone (PVP) was dissolved in 50 mL of n-pentanol and sonicated for 2 hours to completely dissolve it. Next, 5 mL of absolute ethanol, 1.4 mL of deionized water, and 0.5 mL of sodium citrate solution were added in sequence, and the solution was stirred for 20 min to emulsify. Then 0.5 mL of ammonia water was added and stirred for 10 min. In addition, 0.5 mL of tetraethyl orthosilicate (TEOS) was added, stirred for 10 min, and then sonicated for 10 min. The mixture was transferred to a round-bottom flask and reacted in an oil bath at 80 °C for 18 h. After the reaction, the precipitate was separated by centrifugation. Subsequently, absolute ethanol and deionized water were added in sequence, and centrifugal washing was performed 3 times to remove impurities such as n-pentanol. The washed white solid material was collected and dried in an oven at 50 °C. Finally, the obtained white powder, namely silicon dioxide nanowires, was obtained.

[0072] As confirmed by the results of SEM analysis (images not shown), a coating layer with a Bouligand structure was formed on the surface of the PE substrate film, and there was a certain rotation angle between the helical stacking layers.

[0073] Example 5

[0074] Performed in the same procedure as Example 1, except that carbon nanotubes (carbon nanotube model is TNM2, used in the form of a carbon nanotube aqueous dispersion in TNWDIS dispersant, and the carbon nanotube content in this dispersion is 2.10 wt%) were used instead of the calcium silicate nanowire layer.

[0075] As confirmed by the results of SEM analysis (images not shown), a coating layer with a Bouligand structure was formed on the surface of the PE substrate film, and there was a certain rotation angle between the helical stacking layers.

[0076] Example 6

[0077] This was carried out in the same procedure as Example 1, except that a three-layer polypropylene / polyethylene / polypropylene composite film (PP / PE / PP composite film) (Celgard 2325, purchased from Kuraray, width 60 mm, thickness 25 μm) was used instead of the PE film as the base film.

[0078] As confirmed by the results of SEM analysis (images not shown), a coating layer with a Bouligand structure was formed on the surface of the PE base film, and there was a certain rotation angle between the helical stacking layers.

[0079] Example 7

[0080] This was carried out in the same procedure as Example 1, except that polymethyl methacrylate (analytical grade, Sinopharm Chemical Reagent Co., Ltd.) was used instead of polyethylene oxide and sodium carboxymethyl cellulose as the binder.

[0081] The binder used in this example was prepared as follows: 2 g of polymethyl methacrylate was weighed into a 20 ml sample bottle, 18 ml of N-methylpyrrolidone (NMP) and a magnetic stir bar were added, and the polymethyl methacrylate was completely dissolved by magnetic stirring for 12 h to obtain a binder solution with a mass concentration of 10%.

[0082] As confirmed by the results of SEM analysis (images not shown), a coating layer with a Bouligand structure was formed on the surface of the PE base film, and there was a certain rotation angle between the helical stacking layers.

[0083] Example 8

[0084] This was carried out in the same procedure as Example 2, except that the glass plate was rotated clockwise by 45° four times (i.e., five single-direction brushing operations were performed), and the thickness of the calcium silicate nanowire layer obtained each time was 9 μm (so that the total thickness of the coating layer with a Bouligand structure was 45 μm).

[0085] As confirmed by the results of SEM analysis (images not shown), a coating layer with a Bouligand structure was formed on the surface of the PE base film, and there was a certain rotation angle between the helical stacking layers.

[0086] Comparative Example 1

[0087] A PE film with a porous structure formed by a wet process known in the art, that is, by high-temperature co-blending polyethylene with liquid paraffin and phthalate as diluents to form a homogeneous solution, followed by casting, biaxial stretching, solvent extraction, and heat treatment, was used as the battery separator. Its thickness was 20 μm, width was 60 mm, and areal density was 11 g / m 2, with a porosity of 41%, an air permeability of 224 S, a transverse thermal shrinkage TD of 0.1%, a longitudinal thermal shrinkage MD of 1.4%, and a longitudinal tensile strength MD of 196 kg / cm 3 , and a transverse tensile strength TD of 165 kg / cm 3 , a puncture strength of 722 g, and a melting temperature of 135 °C. Then, a coin cell was assembled according to the same procedure as in Example 1 and its performance was tested.

[0088] Figure 3 shows the SEM image of the separator obtained according to Comparative Example 1. From Figure 3 it can be clearly seen that there is no coating layer on the surface of the PE base film.

[0089] Comparative Example 2

[0090] Except for not rotating the glass plate, a coating layer with a total thickness of 29 μm was obtained by brushing the slurry on the base film 3 times directly. Then, a coin cell was assembled according to the same procedure as in Example 1 and its performance was tested.

[0091] Figure 4 shows the SEM image of the separator obtained according to Comparative Example 2. From Figure 4 it can be clearly seen that there is a messy coating layer on the surface of the PE base film, which does not have a Blijgan structure.

[0092] The separators obtained according to Examples 1-3 and Comparative Examples 1-2 were analyzed by X-ray diffraction (XRD) respectively, and the results are shown in Figure 5 . From Figure 5 the results shown in it can be clearly seen that compared with Comparative Example 1, a coating layer of the slurry composed of inorganic nanomaterials and binder was formed on the separators obtained according to Examples 1-3 and Comparative Example 2.

[0093] The separators obtained according to Examples 1-3 and Comparative Examples 1-2 were respectively subjected to mechanical property measurement by a universal mechanical testing machine (Instron5565-A, Instron). The film specimens were cut into film strips of 3 mm * 30 mm, and the test was carried out at room temperature. The displacement rate was between 0.5 - 5 mm / min, and the displacement rate was adjusted so that the film fracture time was between 5 - 8 min respectively. The results are shown in Figure 6 . From Figure 6As can be clearly seen from the results shown, compared with the separator of Comparative Example 1 without a coating layer, the separators obtained in Examples 1-3 of the present invention have significantly improved mechanical strength, indicating the improvement effect of the coating layer on the mechanical properties of the separator. Moreover, compared with the separator of Comparative Example 2 obtained with a coating layer without a Brigand structure, the separators obtained in Examples 1-3 of the present invention with a coating layer having a Brigand structure also have improved mechanical strength.

[0094] The separators obtained according to Examples 1-3 and Comparative Examples 1-2 were respectively tested for hydrophilicity (determined by water contact angle) using a contact angle tester (OCA 25, Dataphysics). The size of the test droplet was controlled at 2 μL. Subsequently, after the droplet was dropped onto the sample surface, the initial state of the droplet (i.e., 0 s) and the state after staying for 15 s were recorded. The results are shown in Figure 7 Among them, the contact angle of the initial state of the droplet reflects the hydrophilicity of the separator itself, and the change in the contact angle of the droplet after staying for 15 s further reflects the infiltration speed of the droplet. From Figure 7 As can be clearly seen from the results shown, compared with the separators of Comparative Example 1 and Comparative Example 2, the hydrophilicity of the separators obtained in Examples 1-3 of the present invention with a coating layer having a Brigand structure is significantly improved both initially (0 s) and after staying for a period of time (15 s), which is beneficial for the separator to be more easily wetted by the electrolyte of the battery during use.

[0095] The batteries assembled using the separators obtained according to Examples 1-3 and Comparative Examples 1-2 were respectively tested for ion conductivity performance using an EC-LAB test platform (VMP3, BioLogic). The process of assembling the battery was the same as that in Example 1 except that the lithium sheet was replaced with a stainless steel gasket and the lithium iron phosphate electrode sheet was removed. The ion conductivity results were reflected by electrochemical impedance spectroscopy, and the scanning frequency range was 100 MHz - 1 Hz. The results are shown in Figure 8 Among them. From Figure 8 As can be clearly seen from the results shown, compared with the separators of Comparative Example 1 and Comparative Example 2, the resistance of the separators obtained in Examples 1-3 of the present invention with a coating layer having a Brigand structure is lower, which improves the ion conductivity performance.

[0096] The lithium-ion batteries assembled according to Example 2 and Comparative Examples 1-2 were tested for cycle performance. Among them, the test was carried out using a Neware battery test system. The battery was placed in a 25 °C constant temperature box. The charge-discharge voltage range of the battery was 2.5 - 3.8 V. The battery was first charged and discharged at a rate of 0.1 C for 3 cycles to activate the battery, and then the charge-discharge rate was changed to 1 C for cycle testing. The results are shown respectively in Figures 9 - 11 Among them. From Figures 9 - 11As can be clearly seen from the results shown, compared with the lithium-ion batteries of Comparative Examples 1-2 (without using a separator including a coating layer having a buligang structure), the battery assembled according to Example 2 of the present invention (using a separator including a coating layer having a buligang structure) maintains a stable discharge specific capacity during 100 cycles, and the Coulombic efficiency also remains at a level close to 100%, showing significantly more stable and excellent cycle performance.

[0097] The rate performance tests were carried out on the lithium-ion batteries assembled according to Example 2 and Comparative Examples 1-2. The tests were carried out using a Neware battery test system. The batteries were placed in a constant temperature oven at 25 °C. The test voltage range was 2.5-3.8 V. The rate tests were carried out by charging and discharging 4 or 5 cycles each using rates of 0.1 C, 0.2 C, 0.3 C, 0.5 C, 1 C, 2 C, 3 C, 5 C and 0.1 C in sequence. The results are respectively shown in Figures 12 - 14 . From Figures 12 - 14 As can be clearly seen from the results shown, compared with the lithium-ion batteries of Comparative Examples 1-2 (without using a separator including a coating layer having a buligang structure), the battery assembled according to Example 2 of the present invention (using a separator including a coating layer having a buligang structure) still has a good discharge specific capacity at high rates, and as the test rate decreases, the discharge specific capacity can return to a relatively high level, indicating reversibility and excellent rate performance.

[0098] Although the specific embodiments of the present invention have been described in detail, this does not limit the present invention in any way. Any modifications, substitutions and improvements made to those details within the spirit and principle of the present invention shall be included within the protection scope of the present invention. The full scope of the present invention is defined by the appended claims and any equivalents thereof.

Claims

1. A separator for a lithium-ion battery having a coating layer with a Brigand structure, characterized in that, The separator for lithium-ion battery includes a base film and a coating layer coated on one or both surfaces of the base film, wherein a helical stacked layer is formed by unidirectionally brushing a slurry composed of inorganic nanomaterials and a binder on one or both surfaces of the base film in a plurality of orientation directions, thereby obtaining a coating layer with a Brigand structure.

2. The separator for a lithium-ion battery according to claim 1, wherein The base film is selected from a polyethylene film, a polypropylene film, a double-layer polypropylene / polyethylene composite film, a triple-layer polypropylene / polyethylene / polypropylene composite film, a polyimide film, a polyetheretherketone film or a meta-aramid film.

3. The separator for a lithium-ion battery according to claim 1, characterized in that, The thickness of the coating layer is 5 to 50 μm.

4. The separator for a lithium-ion battery according to claim 1, wherein The inorganic nanomaterials are one or more selected from silicon dioxide nanowires, aluminum oxide nanowires, titanium dioxide nanowires, zinc oxide nanowires, calcium silicate nanowires, carbon nanotubes and boron nitride nanowires.

5. The separator for a lithium ion battery according to claim 1, wherein The binder is one or more selected from polyvinylidene fluoride, polymethyl methacrylate, sodium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, polyvinyl alcohol and polyethylene oxide.

6. The separator for a lithium-ion battery according to claim 1, wherein In the slurry, the weight content of the binder is 0.5% to 10%, and the weight content of the inorganic nanomaterials is 1% to 30%.

7. The separator for a lithium-ion battery according to claim 1, characterized in that, The coating layer is a helical stacked layer formed by unidirectionally brushing the slurry in at least three orientation directions.

8. The separator for a lithium ion battery according to claim 1 or 7, characterized in that, The angle between the layers of the helical stacked layer is in the range of 10° to 90°.

9. A lithium-ion battery, characterized in that, The lithium-ion battery includes the separator for lithium-ion battery according to any one of claims 1 to 8.

10. The lithium-ion battery according to claim 9, wherein, The lithium-ion battery is a liquid lithium-ion battery.

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