Composite separator, method for manufacturing the same, and battery

Abstract

The application provides a composite diaphragm and a preparation method thereof. The composite diaphragm comprises a diaphragm base and magnetic carbon nanotubes arranged on at least one side of the diaphragm base. One end of the magnetic carbon nanotubes is connected to the diaphragm base, and the other end extends away from the diaphragm base. The magnetic carbon nanotubes comprise first magnetic carbon nanotubes, and the angle between the extending direction of the first magnetic carbon nanotubes and the diaphragm base is 30-90°. The use of the composite diaphragm for assembling a negative electrode-free battery can make lithium metal uniformly deposit on the negative electrode side and improve the cycle performance of the battery.

Description

Technical Field

[0001] This invention relates to a composite separator, and more particularly to a composite separator, its preparation method, and a battery thereof, belonging to the field of lithium-ion battery technology. Background Technology

[0002] As the market demand for battery energy density increases, lithium-ion batteries have received widespread attention due to their excellent mass energy density and volumetric energy density.

[0003] The separator is a crucial component of a battery, primarily used to separate the positive and negative electrodes, preventing direct contact and short circuits. It also allows ions in the electrolyte to pass freely, ensuring charge balance and smooth chemical reactions during charging and discharging. The separator not only needs good mechanical strength and chemical stability to withstand internal pressure changes and electrolyte corrosion, but also requires appropriate porosity and ionic conductivity to ensure efficient ion transport and battery performance. In lithium-ion batteries, the separator's role is particularly critical because it must not only maintain the above functions but also support the uniform deposition and extraction of lithium metal on the negative electrode surface, preventing the formation of lithium dendrites, which can lead to short circuits and even safety issues.

[0004] Therefore, there is an urgent need for a separator that can ensure that lithium ions are evenly distributed when they reach the negative electrode side of the battery, thereby achieving uniform deposition, avoiding the formation of lithium dendrites, and improving subsequent battery cycles. Summary of the Invention

[0005] To address the aforementioned shortcomings, this invention provides a composite separator with uniform pores, enabling effective and uniform distribution of lithium ions to the negative electrode side of the battery, thus preventing the formation of lithium dendrites. Using this composite separator, the battery exhibits good cycle efficiency.

[0006] This invention provides a preparation method for preparing the composite separator as described above. By using this preparation method to prepare the composite separator, a composite separator with a uniform pore distribution can be obtained, which is beneficial for lithium ions to be uniformly distributed to the negative electrode side after passing through the composite separator, thereby improving the cycle efficiency of the battery.

[0007] This invention provides a battery comprising a composite separator as described above or a composite separator prepared by the method described above. In this battery, lithium ions are uniformly distributed at the negative electrode and reduced to lithium metal without the formation of lithium dendrites, resulting in excellent cycle performance.

[0008] The present invention provides a battery pack comprising the battery as described above, which provides a stable power supply and has a long service life.

[0009] The present invention provides an electrical device that includes a battery or battery pack as described above. By using the battery as described above, the electrical device can have a stable power supply and a longer service life.

[0010] One aspect of the present invention provides a composite membrane, the composite membrane comprising a membrane substrate and magnetic carbon nanotubes disposed on at least one side of the membrane substrate, one end of the magnetic carbon nanotubes being connected to the membrane substrate and the other end extending away from the membrane substrate; the magnetic carbon nanotubes include a first magnetic carbon nanotube, the angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate being 30~90°.

[0011] In the composite membrane described above, the angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate 1 is 50~90°.

[0012] As described above, the composite membrane further includes a second magnetic carbon nanotube, wherein the angle between the extension direction of the second magnetic carbon nanotube and the membrane substrate is less than 30°.

[0013] In the composite membrane described above, the first magnetic carbon nanotube has a content of not less than 80% in the magnetic carbon nanotubes.

[0014] In the composite membrane described above, the ratio of the number of first magnetic carbon nanotubes with the same included angle to the total number of first magnetic carbon nanotubes is not less than 70%.

[0015] In the composite membrane described above, the mass percentage of the first magnetic carbon nanotube in the composite membrane is 5-20%.

[0016] The composite membrane described above, wherein the magnetic carbon nanotubes comprise at least one magnetic material selected from iron or iron oxide, cobalt or cobalt oxide, and nickel or nickel oxide;

[0017] The magnetic carbon nanotubes include at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0018] The magnetic carbon nanotubes include at least one of aminated magnetic carbon nanotubes, carboxylated magnetic carbon nanotubes, and pyrrole-functionalized magnetic carbon nanotubes.

[0019] In the composite membrane described above, the magnetic carbon nanotubes have a length of 5~50µm and a diameter of 2~30nm.

[0020] In the composite membrane described above, the aspect ratio of the magnetic carbon nanotubes is (200~25000):1.

[0021] The present invention also provides a preparation method for preparing the composite membrane as described above, the method comprising the following steps:

[0022] A slurry including magnetic carbon nanotubes is coated onto the surface of at least one side of a membrane substrate to obtain an intermediate membrane substrate. The magnetic carbon nanotubes in the intermediate membrane substrate are induced by a magnetic field, and the composite membrane is obtained after drying.

[0023] The strength of the magnetic field is 100~500mT.

[0024] In the preparation method described above, the magnetic field induction time is 10~24h, and the angle between the surface of the intermediate septum substrate coated with the slurry and the direction of the magnetic field is 40°~90°.

[0025] In the preparation method described above, the magnetic carbon nanotubes comprise 15-25% of the total mass of the slurry; the slurry also includes a binder comprising 1-5% of the total mass of the binder.

[0026] The present invention also provides a battery comprising the composite separator as described above or the composite separator prepared by the preparation method described above.

[0027] As described above, the negative electrode of the battery is the negative current collector.

[0028] The present invention also provides an electrical device comprising the battery described above.

[0029] The composite separator provided by this invention utilizes a special arrangement of magnetic carbon nanotubes induced by a magnetic field, specifically, the angle between the extension direction of the first magnetic carbon nanotube and the separator substrate is 30-90°, resulting in relatively uniform pores on the composite separator. Therefore, lithium ions can be effectively distributed on the negative electrode side of a negative electrodeless battery, achieving uniform deposition, improving subsequent battery cycles, and thus enhancing battery performance. Attached Figure Description

[0030] Figure 1 A schematic diagram of the cross-sectional structure of the composite diaphragm provided by the present invention;

[0031] Figure 2 This is a top view schematic diagram of the composite diaphragm provided by the present invention.

[0032] Explanation of reference numerals in the attached figures:

[0033] 1: Membrane matrix; 2: Magnetic carbon nanotubes. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0035] Unlike traditional lithium-ion batteries, electrodeless batteries allow lithium ions to directly pass through the separator and deposit onto the negative electrode current collector during charging, forming a lithium metal negative electrode layer. Currently, polymer separators and ceramic-coated separators are commonly used as separators for electrodeless batteries. However, the inconsistent pore size arrangement of these separators can lead to uneven longitudinal diffusion and distribution of lithium ions during deposition, resulting in uneven deposition and affecting the battery's cycle performance in later stages.

[0036] Based on this, the first aspect of the present invention provides a composite separator, such as Figure 1 and Figure 2 As shown, the composite membrane includes a membrane substrate 1 and magnetic carbon nanotubes 2 disposed on at least one side of the membrane substrate 1. One end of the magnetic carbon nanotube 2 is connected to the membrane substrate 1, and the other end extends away from the membrane substrate 1. The magnetic carbon nanotube 2 includes a first magnetic carbon nanotube, and the angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate 1 is 30~90°.

[0037] This invention does not impose any special limitations on the membrane substrate 1, as long as it can isolate the positive and negative electrodes and allow electrolyte ions to pass through. For example, the membrane substrate 1 can be at least one of polyethylene (PE), polypropylene (PP), non-woven fabric, and carbon cloth.

[0038] In this invention, the angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate 1 is 30° to 90°. Specifically, the extension direction refers to the direction from one end of the first magnetic carbon nanotube to the other, and the angle refers to the angle between the extension direction of the first magnetic carbon nanotube and the plane containing the length and width directions of the membrane substrate 1. For example, the angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate 1 includes, but is not limited to, 30°, 40°, 50°, 60°, 70°, 80°, 90°, or any combination thereof. In this invention, the first magnetic carbon nanotube only needs to have an angle between its extension direction and the membrane substrate 1 within the range of 30° to 90°; it is not required that each first magnetic carbon nanotube have the same fixed value, and the angles of each first magnetic carbon nanotube can be different.

[0039] Specifically, the magnetic carbon nanotubes 2 disposed on at least one side of the separator substrate 1 as described in this invention can be disposed on only one side of the separator substrate 1, or magnetic carbon nanotubes 2 can be disposed on both sides of the separator substrate 1 simultaneously. By disposing of magnetic carbon nanotubes 2 on the separator substrate 1, relatively uniform pores can be formed on the surface of the separator substrate 1. Lithium ions can enter the pores formed by the magnetic carbon nanotubes 2 after passing through the separator substrate 1. Since the magnetic carbon nanotubes 2 are arranged in an orderly manner, lithium ions can achieve a relatively uniform longitudinal diffusion distribution after passing through the pores provided by the magnetic carbon nanotubes 2. This is beneficial for the uniform distribution of lithium ions to the negative electrode side of the negative electrodeless battery after passing through the composite separator, thereby promoting the uniform deposition of lithium metal and improving the cycle efficiency of the battery.

[0040] In one specific implementation, the angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate 1 is 50° to 90°. For example, the angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate 1 includes, but is not limited to, 50°, 55°, 60°, 65°, 70°, 75°, 80°, 85°, 90°, or any combination thereof. By setting the angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate 1 within the above range, it is beneficial to improve the ion transport efficiency of the composite membrane. Lithium ions can be uniformly deposited on the negative electrode side after passing through the first magnetic carbon nanotube with the specific angle, which is beneficial to the subsequent cycling of the battery. When the angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate 1 is 90°, the magnetic carbon nanotube is more conducive to providing a better ion conduction channel, thereby accelerating the ion transport rate.

[0041] In addition, the magnetic carbon nanotubes also include a second magnetic carbon nanotube, the angle between the extension direction of the second magnetic carbon nanotube and the membrane substrate 1 being less than 30°.

[0042] In this invention, the angles between the first magnetic carbon nanotube and the second magnetic carbon nanotube are different. Specifically, the angle between the extension direction of the second magnetic carbon nanotube and the membrane substrate 1 is less than 30°. For example, the angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate 1 includes, but is not limited to, 5°, 10°, 15°, 20°, 25°, 29.9°, or any combination thereof. By setting the angle of the second magnetic carbon nanotube within the above range, the conductivity of the battery can be balanced, thereby improving the battery rate.

[0043] Furthermore, the first magnetic carbon nanotube constitutes at least 80% of the total number of magnetic carbon nanotubes 2. For example, the percentage of the first magnetic carbon nanotube in the magnetic carbon nanotube 2 may include, but is not limited to, 80%, 85%, 90%, 95%, 100%, or any combination thereof. Preferably, the percentage of the first magnetic carbon nanotube in the magnetic carbon nanotube 2 is 80-90%.

[0044] By limiting the percentage of the first magnetic carbon nanotube in the magnetic carbon nanotube 2, this invention can provide more channels for lithium ions to pass through, allowing them to be uniformly deposited on the negative electrode side of the battery, which is beneficial to improving the cycle efficiency of the battery.

[0045] Considering the uniformity of magnetic carbon nanotubes on the separator substrate, the ratio of the number of first magnetic carbon nanotubes with the same included angle to the total number of first magnetic carbon nanotubes is not less than 70%. Here, "same included angle" means that there are multiple first magnetic carbon nanotubes with the same included angle value, which is the average included angle of these multiple first magnetic carbon nanotubes. Same included angle means that the difference between the included angle of each first magnetic carbon nanotube and the average included angle of all first magnetic carbon nanotubes with the same included angle does not exceed 20°. These first magnetic carbon nanotubes with the same included angle ensure a more uniform arrangement and distribution of the overall magnetic carbon nanotubes, which is beneficial to the uniform longitudinal distribution of lithium ions. When the first magnetic carbon nanotubes are arranged on the separator substrate 1 as described above, the uniform arrangement of the first magnetic carbon nanotubes facilitates the uniform deposition of lithium ions on the negative electrode of the battery at a similar speed, which is beneficial to improving the cycle efficiency of the battery.

[0046] In one specific implementation, the mass percentage of the first magnetic carbon nanotube in the composite separator is 5-20%. For example, the mass percentage of the first magnetic carbon nanotube in the composite separator includes, but is not limited to, 5%, 8%, 10%, 12%, 15%, 18%, 20%, or any combination thereof. By limiting the mass percentage of the first magnetic carbon nanotube in the composite separator to the above range, it can be ensured that composite separators of different masses can all have good lithium-ion throughput efficiency, which is beneficial for controlling batch effects.

[0047] Further, the magnetic carbon nanotubes 2 include at least one magnetic material selected from iron or iron oxide, cobalt or cobalt oxide, and nickel or nickel oxide. The magnetic material is uniformly coated on the surface of the carbon nanotubes. This invention does not specifically limit the method of coating the magnetic material onto the surface of the carbon nanotubes; for example, vapor deposition, liquid deposition, and electrodeposition can be used to coat the magnetic material onto the surface of the carbon nanotubes. The magnetic material can be one or more of iron, cobalt, nickel, iron oxide, cobalt oxide, and nickel oxide. By setting the magnetic material on the carbon nanotubes, the magnetic material provides magnetic force for the alignment of the carbon nanotubes in a magnetic field, which is beneficial for controlling the alignment of the carbon nanotubes in a magnetic field, resulting in a composite separator with a uniformly distributed magnetic carbon nanotube arrangement, thereby improving the cycle efficiency of the battery.

[0048] In another embodiment, the magnetic carbon nanotube 2 includes at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0049] In another embodiment, the magnetic carbon nanotube 2 includes at least one of aminated magnetic carbon nanotubes, carboxylated magnetic carbon nanotubes, and pyrrolic modified magnetic carbon nanotubes.

[0050] By using the aforementioned magnetic carbon nanotubes, it is beneficial to adjust the battery's conductivity and cycle stability, thereby enhancing the battery's efficiency.

[0051] Based on considerations of lithium-ion transport efficiency, the magnetic carbon nanotubes 2 in this invention are configured with a length of 5-50 µm and a diameter of 2-30 nm. For example, the length of the magnetic carbon nanotubes 2 includes, but is not limited to, 5 µm, 15 µm, 25 µm, 35 µm, 50 µm, or any combination thereof; the diameter of the magnetic carbon nanotubes 2 includes, but is not limited to, 2 nm, 15 nm, 20 nm, 25 nm, 30 nm, or any combination thereof. By controlling the diameter of the magnetic carbon nanotubes 2 within the above-mentioned range, this invention helps to increase the specific surface area of ​​the magnetic carbon nanotubes, enhance their magnetism, and thus facilitate the uniform distribution of the magnetic carbon nanotubes on the composite separator, thereby improving battery performance. Controlling the length within the above-mentioned range also facilitates rapid lithium-ion transport, improving the battery's conductivity efficiency.

[0052] In one specific implementation, the aspect ratio of the magnetic carbon nanotubes 2 is (200-25000):1. For example, the aspect ratio of the magnetic carbon nanotubes 2 includes, but is not limited to, 200:1, 500:1, 900:1, 1000:1, 5000:1, 10000:1, 15000:1, 20000:1, 25000:1, or any combination thereof. By controlling the aspect ratio of the magnetic carbon nanotubes 2 within the above range, this invention facilitates the rapid transport of lithium ions and improves the conductivity efficiency of the battery.

[0053] A second aspect of the present invention provides a method for preparing the composite separator as described above, the method comprising the following steps:

[0054] A slurry including magnetic carbon nanotubes is coated onto the surface of at least one side of the membrane substrate, the magnetic carbon nanotubes 2 are induced by a magnetic field, and the composite membrane is obtained after drying.

[0055] The strength of the magnetic field is 100~500mT.

[0056] The magnetic field strength is 100~500mT. For example, the magnetic field strength includes, but is not limited to, 100mT, 200mT, 300mT, 400mT, 500mT, or any combination thereof. By limiting the magnetic field strength, the angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate can be 30~90°. This facilitates the uniform distribution of lithium ions to the negative electrode side of the electrodeless battery after passing through the composite membrane, thereby improving the battery's cycle efficiency.

[0057] This invention does not impose any particular limitation on the method by which magnetic carbon nanotubes 2 are dispersed in the slurry, as long as the magnetic carbon nanotubes 2 can be dispersed relatively uniformly in the slurry. For example, the magnetic carbon nanotubes 2 can be added to the slurry for dispersion, or the slurry can be added to the magnetic carbon nanotubes 2 for dispersion. Methods such as ultrasonic dispersion, mechanical dispersion, chemical dispersion, and electrophoretic dispersion can be used to uniformly disperse the magnetic carbon nanotubes 2 in the slurry.

[0058] The present invention is not limited to the coating method, as long as the slurry including magnetic carbon nanotubes 2 can be coated onto the membrane substrate. For example, spraying, dip coating, spin coating, and brush coating methods can be used to coat the slurry including magnetic carbon nanotubes 2.

[0059] This invention does not limit the drying method, drying temperature, or drying time, as long as the solvent in the composite membrane can be completely evaporated. For example, hot air drying, vacuum drying, microwave drying, etc., can be used, and the appropriate method can be selected according to the actual situation.

[0060] By using the above preparation method to prepare composite membranes, composite membranes with good performance can be obtained, and the preparation process is simple and easy to mass-produce.

[0061] Furthermore, the magnetic field induction time includes, but is not limited to, 10h, 12h, 14h, 18h, 24h, or any combination thereof. The intermediate separator substrate is positioned in the magnetic field such that the angle between the surface of the intermediate separator substrate coated with the slurry and the direction of the magnetic field is 40° to 90°. Preferably, the intermediate separator substrate is positioned in the magnetic field such that the surface of the intermediate separator substrate coated with the slurry is perpendicular to the direction of the magnetic field, that is, the angle between the surface coated with the slurry and the direction of the magnetic field is 90°. This invention, by limiting the magnetic field induction time and the placement of the intermediate separator substrate, facilitates the preparation of a composite separator with uniformly distributed magnetic carbon nanotubes 2, which is beneficial for improving the cycle stability of the battery.

[0062] In one specific implementation, based on the total mass of the slurry, the mass percentage of magnetic carbon nanotubes 2 is 15-25%; the slurry also includes a binder, the mass percentage of which is 1-5%. For example, the mass percentage of magnetic carbon nanotubes 2 includes, but is not limited to, 20%, 21%, 22%, 23%, 24%, 25%, or any combination thereof; the mass percentage of the binder includes, but is not limited to, 1%, 2%, 3%, 4%, 5%, or any combination thereof. By limiting the components and their mass percentages in the slurry containing magnetic carbon nanotubes 2 to the above ranges, this invention facilitates the preparation of a uniform slurry containing magnetic carbon nanotubes 2, which is beneficial for the subsequent uniform coating of magnetic carbon nanotubes 2 onto the surface of the membrane substrate 1.

[0063] It should be clarified that the above-mentioned slurry also includes a solvent. This invention does not specifically limit the type of solvent, as long as the solvent does not react with the solute in this invention and is easily removed later. For example, the solvent includes at least one of NMP (N-methylpyrrolidone), DMAC (dimethylacetamide), and H2O; the binder includes at least one of PVDF (polyvinylidene fluoride), PEO (polyoxyethylene), and SBR (styrene-butadiene rubber).

[0064] A third aspect of the present invention provides a battery comprising a composite separator as described above or a composite separator prepared by the preparation method described above. By using the above-described composite separator in the battery, the battery can exhibit better cycle stability.

[0065] In one specific implementation, the negative electrode of the aforementioned battery is the negative electrode current collector. When this battery is assembled using a conventional separator, the uneven pores of the separator can cause uneven longitudinal distribution of lithium ions, leading to the easy formation of lithium dendrites on the negative electrode current collector and causing a short circuit in the battery. This invention creates a composite separator by coating a conventional separator with a layer of magnetic carbon nanotubes. When this composite separator is applied to the battery, lithium ions can pass through the composite separator uniformly, significantly improving the uniformity of the longitudinal distribution of lithium ions. This facilitates the formation of a uniform lithium metal layer on the negative electrode current collector, thereby improving the cycle stability of the battery.

[0066] A fourth aspect of the present invention provides a battery pack comprising the battery as described above. By using the battery assembly described above, the present invention enables the battery pack to stably output power and has a long service life.

[0067] A fifth aspect of this invention provides an electrical device including the battery provided in the third aspect of this invention. This invention does not particularly limit the type of electrical device, including but not limited to mobile phones, portable devices, laptops, electric bicycles, electric cars, electric toys, energy storage devices, etc. By using the battery described above in the electrical device, a continuous and stable output of electrical energy can be provided to supply the device, resulting in a longer service life. Simultaneously, it reduces the maintenance of the battery in the device, thus saving costs.

[0068] The present invention will be further described in detail below through specific embodiments.

[0069] Example 1

[0070] The method for preparing the composite diaphragm in this embodiment includes the following steps:

[0071] 1) Mix and disperse magnetic carbon nanotubes and binder evenly in NMP to obtain a slurry containing magnetic carbon nanotubes;

[0072] Among them, the magnetic material in the magnetic carbon nanotubes is cobalt oxide, and the length of the magnetic carbon nanotubes is 8µm and the diameter is 10nm.

[0073] The mass percentage of magnetic carbon nanotubes is 20%.

[0074] The adhesive is PVDF, and the adhesive content is 2% by mass.

[0075] 2) The above-mentioned slurry including magnetic carbon nanotubes is coated onto one surface of the diaphragm substrate to obtain an intermediate diaphragm substrate; wherein the diaphragm substrate is polyethylene (PE).

[0076] 3) Place the above-mentioned intermediate septum membrane substrate in a magnetic field for magnetic field induction;

[0077] The magnetic field strength was 350 mT, the magnetic field induction time was 24 h, and the intermediate septum substrate was placed in the magnetic field such that the surface of the intermediate septum substrate coated with slurry was perpendicular to the direction of the magnetic field, that is, the angle between the surface coated with slurry and the direction of the magnetic field was 90°.

[0078] 4) The above-mentioned magnetic field-induced intermediate septum substrate is dried at 90°C for 120 min to obtain a composite septum.

[0079] In the composite membrane, the angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate is 70°~90°; the first magnetic carbon nanotubes, which account for 90% of the total number, have an angle of 80° between their extension direction and the membrane substrate; the number percentage of the first magnetic carbon nanotubes in the composite membrane is 90%; and the mass percentage of the first magnetic carbon nanotubes in the composite membrane is 10%.

[0080] Example 2

[0081] The preparation method of the composite diaphragm in this embodiment is basically the same as that in Example 1, except that...

[0082] In this embodiment, the magnetic material in the magnetic carbon nanotubes used in step 1) is iron oxide, the length of the magnetic carbon nanotubes is 10µm, and the diameter is 20nm.

[0083] The mass percentage of magnetic carbon nanotubes is 15%.

[0084] The adhesive is PVDF, and the adhesive content is 1% by mass.

[0085] In this embodiment, the magnetic field strength in step 3) is 350mT, the magnetic field induction time is 24h, and the intermediate septum substrate is placed in the magnetic field such that the surface of the intermediate septum substrate coated with slurry is perpendicular to the direction of the magnetic field.

[0086] The angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate in the obtained composite membrane is 70°~90°; the first magnetic carbon nanotubes, which account for 90% of the total number, have an angle of 80° between their extension direction and the membrane substrate; the number percentage of the first magnetic carbon nanotubes in the magnetic carbon nanotubes is 90%; and the mass percentage of the first magnetic carbon nanotubes in the composite membrane is 5%.

[0087] Example 3

[0088] The preparation method of the composite diaphragm in this embodiment is basically the same as that in Example 1, except that...

[0089] In this embodiment, the magnetic material in the magnetic carbon nanotubes used in step 1) is cobalt oxide, and the length of the magnetic carbon nanotubes is 15µm and the diameter is 25nm.

[0090] The mass percentage of magnetic carbon nanotubes is 23%;

[0091] The adhesive is PVDF, and the adhesive content is 2% by mass.

[0092] In this embodiment, the magnetic field strength in step 3) is 400mT, the magnetic field induction time is 24h, and the intermediate septum substrate is placed in the magnetic field such that the surface of the intermediate septum substrate coated with slurry is perpendicular to the direction of the magnetic field.

[0093] In the resulting composite membrane, the angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate is 70-90°; among the first magnetic carbon nanotubes, 90% of the first magnetic carbon nanotubes have an angle of 80° between their extension direction and the membrane substrate; the number percentage of the first magnetic carbon nanotubes in the magnetic carbon nanotubes is 90%; and the mass percentage of the first magnetic carbon nanotubes in the composite membrane is 20%.

[0094] Example 4

[0095] The preparation method of the composite membrane in this embodiment is basically the same as that in Example 1. The difference is that in step 1) of this embodiment, the length of the magnetic carbon nanotube is 5µm and the diameter is 35nm.

[0096] The angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate in the obtained composite membrane is 70°~90°; the first magnetic carbon nanotubes, which account for 90% of the total number, have an angle of 80° between their extension direction and the membrane substrate; the number percentage of the first magnetic carbon nanotubes in the above magnetic carbon nanotubes is 90%; and the mass percentage of the first magnetic carbon nanotubes in the above composite membrane is 10%.

[0097] Example 5

[0098] The preparation method of the composite diaphragm in this embodiment is basically the same as that in Example 1. The difference is that in this embodiment, the magnetic field strength in step 3) is 350mT, the magnetic field induction time is 24h, and the intermediate diaphragm substrate is placed in the magnetic field such that the angle between the surface of the intermediate diaphragm substrate coated with slurry and the direction of the magnetic field is 40°.

[0099] The angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate in the obtained composite membrane is 30°~50°; the first magnetic carbon nanotubes, which account for 90% of the total number, have an angle of 80° between their extension direction and the membrane substrate; the number percentage of the first magnetic carbon nanotubes in the above magnetic carbon nanotubes is 90%; and the mass percentage of the first magnetic carbon nanotubes in the above composite membrane is 10%.

[0100] Example 6

[0101] The preparation method of the composite diaphragm in this embodiment is basically the same as that in Example 1. The difference is that in this embodiment, the magnetic field strength in step 3) is 350mT, the magnetic field induction time is 24h, and the intermediate diaphragm substrate is placed in the magnetic field such that the angle between the surface of the intermediate diaphragm substrate coated with slurry and the direction of the magnetic field is 60°.

[0102] The angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate in the obtained composite membrane is 50°~70°; the first magnetic carbon nanotubes, which account for 90% of the total number, have an angle of 80° between their extension direction and the membrane substrate; the number percentage of the first magnetic carbon nanotubes in the above magnetic carbon nanotubes is 90%; and the mass percentage of the first magnetic carbon nanotubes in the above composite membrane is 10%.

[0103] Example 7

[0104] The preparation method of the composite membrane in this embodiment is basically the same as that in Example 1. The difference is that the magnetic carbon nanotubes used in step 1) in this embodiment have a mass percentage content of 12%; the binder is PVDF and the mass percentage content of the binder is 0.8%.

[0105] The angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate in the obtained composite membrane is 70°~90°; the first magnetic carbon nanotubes, which account for 90% of the total number, have an angle of 80° between their extension direction and the membrane substrate; the number percentage of the first magnetic carbon nanotubes in the above magnetic carbon nanotubes is 90%; and the mass percentage of the first magnetic carbon nanotubes in the above composite membrane is 4%.

[0106] Example 8

[0107] The preparation method of the composite membrane in this embodiment is basically the same as that in Example 1. The difference is that the magnetic carbon nanotubes used in step 1) in this embodiment have a mass percentage of 30%; the binder is PVDF and the mass percentage of the binder is 6%.

[0108] The angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate in the obtained composite membrane is 70°~90°; the first magnetic carbon nanotubes, which account for 90% of the total number, have an angle of 80° between their extension direction and the membrane substrate; the number percentage of the first magnetic carbon nanotubes in the above magnetic carbon nanotubes is 90%; and the mass percentage of the first magnetic carbon nanotubes in the above composite membrane is 22%.

[0109] Example 9

[0110] The preparation method of the composite diaphragm in this embodiment is basically the same as that in Example 1. The difference is that in step 3) of this embodiment, the magnetic field strength is 200mT, the magnetic field induction time is 18h, and the intermediate diaphragm substrate is placed in the magnetic field such that the surface of the intermediate diaphragm substrate coated with slurry is perpendicular to the direction of the magnetic field.

[0111] In the resulting composite membrane, the angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate is 70°~90°; the first magnetic carbon nanotubes, which account for 90% of the total number, have an angle of 80° between their extension direction and the membrane substrate; the number percentage of the first magnetic carbon nanotubes in the above-mentioned magnetic carbon nanotubes is 75%; and the mass percentage of the first magnetic carbon nanotubes in the above-mentioned composite membrane is 22%.

[0112] Comparative Example 1

[0113] The diaphragm in this comparative example is a general diaphragm, without magnetic carbon nanotubes on it.

[0114] Comparative Example 2

[0115] The preparation method of the composite diaphragm in this embodiment is basically the same as that in Example 1. The difference is that in step 3) of this embodiment, the magnetic field strength is 50mT, the magnetic field induction time is 24h, and the intermediate diaphragm substrate is placed in the magnetic field at a 90° angle with the magnetic field direction of the surface of the intermediate diaphragm substrate coated with slurry.

[0116] The angle between the extension direction of the magnetic carbon nanotubes in the obtained composite membrane and the membrane substrate is 10~20°; the first magnetic carbon nanotubes, which account for 90% of the total number, have an angle of 80° between their extension direction and the membrane substrate; the number percentage of the first magnetic carbon nanotubes in the magnetic carbon nanotubes is 90%; and the mass percentage of the first magnetic carbon nanotubes in the composite membrane is 10%.

[0117] Comparative Example 3

[0118] The preparation method of the composite diaphragm in this embodiment is basically the same as that in Example 1. The difference is that in step 3) of this embodiment, the magnetic field strength is 90mT, the magnetic field induction time is 24h, and the intermediate diaphragm substrate is placed in the magnetic field at a 30° angle with the magnetic field direction to the surface of the intermediate diaphragm substrate coated with slurry.

[0119] In the resulting composite membrane, the angle between the extension direction of the magnetic carbon nanotubes and the membrane substrate is 20-30°; the first magnetic carbon nanotubes, which account for 90% of the total number, have an angle of 80° between their extension direction and the membrane substrate; the first magnetic carbon nanotubes account for 90% of the total number of magnetic carbon nanotubes; and the first magnetic carbon nanotubes account for 10% of the total mass of the composite membrane.

[0120] Test case

[0121] A negative electrode-free battery was prepared using the following method:

[0122] 1) 900g of positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2 material, 50g acetylene black, and 50g PVDF binder were added to 2000g NMP (nitrogen-methylpyrrolidone) solvent and stirred in a vacuum mixer to form a stable and homogeneous positive electrode slurry. The positive electrode slurry was then uniformly and intermittently coated onto both sides of an aluminum foil (160mm width, 16μm thickness) using a slot coating machine. The foil was dried at 393K and then pressed into a sheet using a roller press to obtain the positive electrode sheet. The positive electrode sheet was cut into rectangular sheets of 48mm x 56mm, and tabs were spot-welded along the width to obtain the positive electrode of a battery without a negative electrode.

[0123] 2) The separator obtained in the examples and comparative examples is alternately stacked with the positive electrode sheet obtained in step 1) and the pure copper foil as the negative electrode. The side of the separator coated with magnetic carbon nanotubes faces the negative electrode copper foil. The battery is prepared by stacking, wherein the positive and negative electrodes are alternately separated by the separator to obtain a dry cell.

[0124] 3) Place the dry cell in an aluminum-plastic film outer packaging, inject electrolyte, then vacuum seal, place at 60℃ for 48 hours, pressurize at 60℃, perform secondary encapsulation, vent, and capacity testing to obtain the battery.

[0125] On a LAND CT 2001C secondary battery performance testing device, at 25±1℃, the batteries with composite separators from the above examples and comparative examples were subjected to charge-discharge cycle tests at 0.2C. The steps are as follows:

[0126] 1) Let stand for 10 minutes; charge at a constant current of 0.2C to 4.3V, then charge at a constant voltage of 0.05C to cut off;

[0127] 2) Let it rest for 10 minutes; discharge at constant current to 3.0V, which is one cycle.

[0128] Repeat this step. During the cycle, the cycle ends when the battery capacity is lower than 80% of the initial discharge capacity. The number of cycles is the battery's cycle life. The capacity retention rate and energy retention rate of the battery at the corresponding number of cycles are then calculated. The results are shown in Table 1.

[0129] Table 1

[0130]

[0131] As shown in Table 1, the composite separators prepared using the methods of Examples 1-9 enable the batteries to achieve a higher cycle life, reaching 150 cycles, while the batteries in Comparative Examples 1-3 only achieved a maximum of 108 cycles. Therefore, the present invention improves the cycle stability of batteries by assembling batteries using composite separators comprising magnetic carbon nanotubes and reshaped by a magnetic field.

[0132] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite diaphragm, characterized in that, The device includes a membrane substrate and magnetic carbon nanotubes disposed on at least one side of the membrane substrate. One end of the magnetic carbon nanotube is connected to the membrane substrate, and the other end extends away from the membrane substrate. The magnetic carbon nanotube includes a first magnetic carbon nanotube, and the angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate is 30° to 90°.

2. The composite diaphragm according to claim 1, characterized in that, The angle between the extension direction of the first magnetic carbon nanotube and the membrane substrate is 50~90°.

3. The composite diaphragm according to claim 1 or 2, characterized in that, The magnetic carbon nanotubes also include a second magnetic carbon nanotube, wherein the angle between the extension direction of the second magnetic carbon nanotube and the membrane substrate is less than 30°.

4. The composite diaphragm according to any one of claims 1-3, characterized in that, The first magnetic carbon nanotube has a quantity percentage of not less than 80% in the magnetic carbon nanotubes.

5. The composite diaphragm according to any one of claims 1-4, characterized in that, The ratio of the number of first magnetic carbon nanotubes with the same included angle to the total number of first magnetic carbon nanotubes is not less than 70%.

6. The composite diaphragm according to any one of claims 1-5, characterized in that, In the composite membrane, the mass percentage of the first magnetic carbon nanotube in the composite membrane is 5-20%.

7. The composite diaphragm according to any one of claims 1-6, characterized in that, The magnetic carbon nanotubes comprise at least one magnetic material selected from iron or iron oxide, cobalt or cobalt oxide, and nickel or nickel oxide, wherein the magnetic material coats the surface of the carbon nanotubes to form the magnetic carbon nanotubes; and / or, The magnetic carbon nanotubes include at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes; and / or, The magnetic carbon nanotubes include at least one of aminated magnetic carbon nanotubes, carboxylated magnetic carbon nanotubes, and pyrrolic modified magnetic carbon nanotubes.

8. The composite diaphragm according to any one of claims 1-7, characterized in that, The magnetic carbon nanotubes have a length of 5~50µm and a diameter of 2~30nm.

9. The composite diaphragm according to any one of claims 1-8, characterized in that, The aspect ratio of the magnetic carbon nanotubes is (200-25000):

1.

10. A method for preparing a composite diaphragm according to any one of claims 1-9, characterized in that, Includes the following steps: A slurry including magnetic carbon nanotubes is coated onto the surface of at least one side of a membrane substrate to obtain an intermediate membrane substrate. The magnetic carbon nanotubes in the intermediate membrane substrate are induced by a magnetic field, and the composite membrane is obtained after drying. The strength of the magnetic field is 100~500mT.

11. The method for preparing the composite diaphragm according to claim 10, characterized in that, The magnetic field induction time is 10~24h, and the angle between the surface of the intermediate septum substrate coated with the slurry and the direction of the magnetic field is 40°~90°.

12. The method for preparing the composite diaphragm according to claim 10 or 11, characterized in that, Based on the total mass of the slurry, the magnetic carbon nanotubes comprise 15-25% by mass; the slurry also includes a binder, which comprises 1-5% by mass.

13. A battery, characterized in that, The composite membrane includes any one of claims 1-9 or a composite membrane prepared by the method for preparing the composite membrane according to any one of claims 10-12.

14. The battery according to claim 13, characterized in that, The negative electrode of the battery is the negative current collector.

15. A battery pack, characterized in that, Includes the battery as described in claim 13 or 14.

16. An electrical appliance, characterized in that, Includes the battery as described in claim 13 or 14 or the battery pack as described in claim 15.

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