Battery diaphragm, preparation method thereof and lithium ion battery

Abstract

The invention provides a battery diaphragm, a preparation method thereof and a lithium ion battery. The battery diaphragm comprises a first film layer and a second film layer which are stacked, and the first film layer comprises a polyimide film and boehmite dispersed in the polyimide film. In the application, on one hand, boehmite is dispersed in the polyimide film, so that the overall mechanical property of the first film layer can be improved; in addition, boehmite has excellent thermal stability and can keep the structure stable at a high temperature, so that the high temperature resistance of the first film layer is improved. And on the other hand, the second film layer has relatively high mechanical strength, and the battery diaphragm simultaneously comprises the first film layer and the second film layer, so that the comprehensive mechanical property and the thermal stability of the battery diaphragm can be further improved.

Description

Technical Field

[0001] This application relates to the technical field of batteries, and particularly relates to a battery separator and its preparation method and a lithium-ion battery. Background Art

[0002] With the wide application of lithium-ion batteries in portable electronic devices and electric vehicles, the performance requirements for battery separators are getting higher and higher. The separator is a key component of the battery, located between the positive and negative electrodes. While preventing the positive and negative electrodes from contacting and causing a short circuit, the separator allows lithium ions to pass through. Therefore, the separator material needs to have good mechanical properties and thermal stability.

[0003] In related technologies, the battery separator is generally a single-layer polyolefin separator. The single-layer polyolefin separator has poor mechanical properties and is prone to shrinkage at high temperatures, affecting the safety performance of the battery. Summary of the Invention

[0004] Embodiments of this application provide a battery separator and its preparation method and a lithium-ion battery, which can improve the technical problem that the poor mechanical properties and thermal stability of the battery separator affect the battery performance.

[0005] In a first aspect, embodiments of this application provide a battery separator, which includes a first film layer and a second film layer arranged in a stacked manner;

[0006] Wherein, the first film layer includes a polyimide film and boehmite dispersed in the polyimide film.

[0007] In one embodiment, the second film layer is a polypropylene film layer; and / or

[0008] The thickness of the battery separator is 15 mm - 20 mm; and / or

[0009] The thickness of the first film layer is 4 mm - 7 mm; and / or

[0010] The thickness of the second film layer is 8 mm - 16 mm; and / or

[0011] The mass percentage of boehmite in the first film layer is 5% - 15%.

[0012] In a second aspect, embodiments of this application provide a preparation method of the battery separator as described above, including the following steps:

[0013] Provide a polyimide film adsorbed with aluminum chloride, wherein the polyimide film has a fiber structure, and the aluminum chloride is adsorbed on the surface of the fiber structure;

[0014] Immerse the polyimide film adsorbed with aluminum chloride in a dilute ammonia water solution so that aluminum chloride reacts in-situ with ammonia water on the fiber structure of the polyimide film to form boehmite, wash, and dry to obtain a composite polyimide film;

[0015] Laminate the composite polyimide film and a polypropylene film, perform hot pressing treatment, and cool to obtain the battery separator.

[0016] In one embodiment, the preparation method of the polyimide film adsorbed with aluminum chloride includes the following steps:

[0017] Provide a polyimide film having a fiber structure;

[0018] Immerse the polyimide film in an ethanol solution of anhydrous aluminum chloride so that the aluminum chloride is adsorbed on the surface of the fiber structure to obtain the polyimide film adsorbed with aluminum chloride.

[0019] In one embodiment, the concentration of the ethanol solution of anhydrous aluminum chloride is 0.2 mol / L - 0.4 mol / L; and / or

[0020] The immersion time is 2 h - 4 h, and / or the immersion temperature is 20°C - 30°C.

[0021] In one embodiment, the preparation method of the polyimide film includes the following steps:

[0022] Disperse pyromellitic dianhydride and 4,4'-diphenylether diamine in a solvent and carry out a polymerization reaction to obtain a polyamic acid solution;

[0023] Place the polyamic acid solution in an electrospinning device and carry out electrospinning to obtain a polyamic acid nanofiber membrane;

[0024] Heat and dry the polyamic acid nanofiber membrane and imidize it to obtain the polyimide film.

[0025] In one embodiment, the mass ratio between pyromellitic dianhydride and 4,4'-diphenylether diamine is (1 - 2) : (1 - 2); and / or

[0026] The solvent includes at least one of N,N-dimethylformamide, dimethylacetamide, and N-methyl-2-pyrrolidone; and / or

[0027] The voltage of the electrospinning is 20 kV - 25 kV, and / or the temperature of the electrospinning is 20°C - 30°C, and / or the humidity of the electrospinning is 35% - 45%, and / or the flow rate of the electrospinning is 1 ml / h - 1.5 ml / h, and / or the time of the electrospinning is 7 h - 10 h; and / or

[0028] The temperature of the heating and drying is 100°C - 130°C, and / or the time of the heating and drying is 20 min - 40 min; and / or

[0029] The temperature of the imidization is 250°C - 350°C, and / or the time of the imidization is 1.5 h - 2.5 h.

[0030] In one embodiment, in the step of immersing the polyimide film adsorbed with aluminum chloride in a dilute ammonia aqueous solution, the immersion time is 0.8 h - 1.2 h, and / or the immersion temperature is 20°C - 30°C; and / or

[0031] The concentration of the dilute ammonia aqueous solution is 0.01 mol / L - 0.2 mol / L.

[0032] In one embodiment, the temperature of the hot pressing treatment is 160°C - 200°C, and / or the time of the hot pressing treatment is 3 min - 8 min.

[0033] In a third aspect, an embodiment of the present application provides a lithium-ion battery, including the battery separator as described above or a battery separator prepared by using the preparation method as described above.

[0034] Advantageous effects of the embodiments of the present application:

[0035] In the present application, the battery separator includes a first film layer and a second film layer arranged in a stacked manner. Among them, the first film layer includes a polyimide film and boehmite dispersed in the polyimide film. In the present application, on the one hand, boehmite itself has relatively strong mechanical strength. By dispersing boehmite in the polyimide film, the hydroxyl groups on the surface of boehmite can form hydrogen bonds with the imide groups on the polyimide molecular chain, thereby enhancing the interfacial binding force between boehmite and the polyimide molecular chain and the binding force between polyimide molecular chains, thereby improving the overall mechanical properties of the first film layer; in addition, boehmite itself has excellent thermal stability and can maintain a stable structure at high temperatures, thereby improving the high-temperature resistance of the first film layer. On the other hand, the second film layer itself has a certain mechanical strength. The battery separator includes both the first film layer and the second film layer, which can further improve the comprehensive mechanical properties and thermal stability of the battery separator. Detailed implementation manners

[0036] Next, in combination with the embodiments of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present application. In addition, it should be understood that the specific implementation manners described herein are only used to illustrate and explain the present application, and are not used to limit the present application. In the present application, unless otherwise stated, the orientation terms such as "upper" and "lower" usually refer to the upper and lower in the actual use or working state of the device; while "inner" and "outer" refer to the outline of the device.

[0037] In the related art, the mechanical properties and thermal stability of the battery separator are poor, which affects the battery performance and needs to be further improved.

[0038] To solve the above problems, an embodiment of the present application provides a battery separator. The battery separator includes a first film layer and a second film layer arranged in a stacked manner; wherein, the first film layer includes a polyimide film and boehmite dispersed in the polyimide film. In this embodiment, boehmite is dispersed on the surface and inside of the polyimide film at the same time. On the one hand, boehmite itself has strong mechanical strength. When boehmite is dispersed in the polyimide film, the hydroxyl groups on the surface of boehmite can form hydrogen bonds with the imide groups on the polyimide molecular chain, thereby enhancing the interfacial binding force between boehmite and the polyimide molecular chain and the binding force between polyimide molecular chains, and thus improving the overall mechanical properties of the first film layer; in addition, boehmite itself has excellent thermal stability and can maintain a stable structure at high temperatures, thereby improving the high-temperature resistance of the first film layer. On the other hand, the second film layer itself has a certain mechanical strength. The battery separator includes both the first film layer and the second film layer, which can further improve the comprehensive mechanical properties and thermal stability of the battery separator. In addition, boehmite has a large porosity. When the electrolyte contacts the battery separator, the pores of boehmite can be quickly filled with the electrolyte, thereby increasing the infiltration speed of the electrolyte, improving the infiltration effect of the electrolyte on the battery separator, and further improving the electrical performance of the battery.

[0039] In one embodiment, the second film layer is a polypropylene film layer. The polypropylene film layer itself has relatively high mechanical strength, and the polypropylene film layer can further improve the comprehensive mechanical properties of the battery separator.

[0040] In one embodiment, the thickness of the battery separator is 15 mm - 20 mm. Optionally, the thickness of the battery separator can be any one of 15 mm, 16 mm, 17 mm, 18 mm, 19 mm, 20 mm, etc. or the range between any two of them. In this embodiment, if the thickness of the battery separator is too large, it is easy to cause an increase in the battery weight and a decrease in the energy density. At the same time, when the thickness of the battery separator is too large, the diffusion path of ions in the active material becomes longer, increasing the internal resistance of the battery, resulting in more heat generation during the charging and discharging process of the battery and reducing the charging and discharging efficiency of the battery; if the thickness of the battery separator is too small, it is easy to cause the battery separator to have an unsatisfactory effect in withstanding high temperature or external force, and the battery separator is prone to phenomena such as melting and shrinking under high temperature or external force, reducing the voltage consistency and cycle performance of the battery.

[0041] In one embodiment, the thickness of the first film layer is 4 mm - 7 mm. Optionally, the thickness of the first film layer can be any one of 4 mm, 5 mm, 6 mm, 7 mm, etc. or the range between any two of them. In this embodiment, if the thickness of the first film layer is too small, it is easy to cause the first film layer to have an unsatisfactory effect in improving the thermal stability and mechanical properties of the battery separator; if the thickness of the first film layer is too large, it is easy to increase the resistance of ion conduction of the battery separator, resulting in an increase in the internal resistance of the battery and a decrease in the charging and discharging efficiency.

[0042] In one embodiment, the thickness of the second film layer is 8 mm - 16 mm. Optionally, the thickness of the second film layer can be any one of 8 mm, 10 mm, 12 mm, 14 mm, 16 mm, etc. or the range between any two of them. In this embodiment, if the thickness of the second film layer is too small, it is easy to cause the second film layer to have an unsatisfactory effect in improving the mechanical properties of the battery separator; if the thickness of the second film layer is too large, it is easy to cause uneven distribution of the mechanical strength of the battery separator, affecting the durability and safety of the battery.

[0043] In one embodiment, the mass percentage of boehmite in the first film layer is 5% - 15%. Optionally, the mass percentage of boehmite in the first film layer can be any one of 5%, 7%, 9%, 11%, 13%, 15%, etc. or the range between any two of them. In this embodiment, if the mass percentage of boehmite in the first film layer is too small, it is easy to cause the boehmite to have an unsatisfactory effect in improving the mechanical properties and thermal stability of the battery film; if the mass percentage of boehmite in the first film layer is too large, too many boehmite particles will interfere with the arrangement and interaction of polyimide molecular chains, thereby reducing the mechanical properties of the first film layer.

[0044] This application also provides a preparation method of the battery separator as described above, including the following steps:

[0045] S11. Provide a polyimide film adsorbed with aluminum chloride, wherein the polyimide film has a fibrous structure, and the aluminum chloride is adsorbed on the surface of the fibrous structure;

[0046] S12. Immerse the polyimide film adsorbed with aluminum chloride in a dilute ammonia water solution so that aluminum chloride reacts in situ with ammonia water on the fibrous structure of the polyimide film to generate boehmite, wash, and dry to obtain a composite polyimide film;

[0047] S13. Stack the composite polyimide film and a polypropylene film, perform hot pressing treatment, and cool to obtain a battery separator.

[0048] In this embodiment, in step S11, the polyimide molecular chains of the polyimide film have carbonyl groups, and the oxygen atoms on the carbonyl groups can form coordination bonds with aluminum ions, thereby enabling aluminum chloride to be adsorbed on the fibrous structure of the polyimide film; in addition, the polyimide film has a fibrous structure, and the fibrous structure is an elongated, linear microstructure formed on the surface and inside of the polyimide film. The fibrous structure has a large specific surface area, which can provide more adsorption positions for aluminum chloride molecules and increase the adsorption amount of aluminum chloride; at the same time, pores are formed between the fibrous structures, facilitating the diffusion and penetration of aluminum chloride molecules in the polyimide film and improving the adsorption effect of aluminum chloride.

[0049] In this embodiment, in step S12, the polyimide film adsorbed with aluminum chloride is immersed in a dilute ammonia water solution. After the reaction of aluminum chloride and ammonia water, boehmite is in-situ generated on the fibrous structure of the polyimide film. The hydroxyl groups on the surface of boehmite can form hydrogen bonds with the imide groups on the polyimide molecular chains, and then firmly bind to the fibrous structure, and boehmite is not easily detached from the polyimide film; in addition, by the method of in-situ generating boehmite on the fibrous structure, boehmite is dispersed in the polyimide film, and boehmite is not easily agglomerated, and the distribution of boehmite in the polyimide film is more uniform, further improving the mechanical properties and thermal stability of the battery separator.

[0050] In this embodiment, the surface and inside of the polyimide film have a fibrous structure. After the reaction of aluminum chloride and ammonia water, boehmite is in-situ generated on the fibrous structure of the polyimide film, and boehmite can be dispersed on the surface and inside of the polyimide film simultaneously.

[0051] In this embodiment, in step S13, after stacking the composite polyimide film and the polypropylene film, performing hot pressing treatment can make the composite polyimide film and the polypropylene film tightly combined.

[0052] In one embodiment, the preparation method of the polyimide film adsorbed with aluminum chloride includes the following steps:

[0053] S111. Provide a polyimide film with a fibrous structure;

[0054] S112. Immerse the polyimide film in an ethanol solution of anhydrous aluminum chloride so that aluminum chloride is adsorbed on the surface of the fibrous structure, obtaining a polyimide film adsorbed with aluminum chloride.

[0055] In this embodiment, by the immersion method, agglomeration between aluminum chloride molecules is not likely to occur, and aluminum chloride can be adsorbed more uniformly on the surface of the fibrous structure; in addition, pores are formed between the fibrous structures, facilitating the diffusion and penetration of the ethanol solution of anhydrous aluminum chloride in the polyimide film and improving the adsorption effect of aluminum chloride.

[0056] In one embodiment, the concentration of the ethanol solution of anhydrous aluminum chloride is 0.2 mol / L - 0.4 mol / L. Optionally, the concentration of the ethanol solution of anhydrous aluminum chloride can be any one of 0.2 mol / L, 0.25 mol / L, 0.3 mol / L, 0.35 mol / L, 0.4 mol / L, etc. or the range between any two of them. In this embodiment, if the concentration of the ethanol solution of anhydrous aluminum chloride is too high, it is easy to cause too much aluminum chloride to be adsorbed on the polyimide film, resulting in too much boehmite generated on the polyimide film subsequently; if the concentration of the ethanol solution of anhydrous aluminum chloride is too low, it is easy to cause too little aluminum chloride to be adsorbed on the polyimide film, resulting in too little boehmite generated on the polyimide film subsequently.

[0057] In one embodiment, in step S112, the immersion time is 2 h - 4 h, and the immersion temperature is 20°C - 30°C. Optionally, the immersion time can be any one of 2 h, 2.5 h, 3 h, 3.5 h, 4 h, etc. or the range between any two of them; the immersion temperature can be any one of 20°C, 22°C, 24°C, 26°C, 30°C, etc. or the range between any two of them.

[0058] In one embodiment, the preparation method of the polyimide film includes the following steps:

[0059] S01. Disperse pyromellitic dianhydride and 4,4'-diphenylether diamine in a solvent and carry out a polymerization reaction to obtain a polyamic acid solution;

[0060] S02. Place the polyamic acid solution in an electrospinning device and carry out electrospinning to obtain a polyamic acid nanofiber membrane;

[0061] S03. Heat and dry the polyamic acid nanofiber membrane and imidize it to obtain a polyimide film.

[0062] In this embodiment, a polyamic acid nanofiber membrane is prepared through an electrostatic spinning process, and a fiber structure can be formed within the polyimide film. By heating and drying the polyamic acid nanofiber membrane and imidizing it, the solvent can be removed, and at the same time, the strength and high-temperature resistance of the polyimide film can be improved.

[0063] In one embodiment, the mass ratio between pyromellitic dianhydride and 4,4'-diphenylether diamine is (1 - 2):(1 - 2). Optionally, the mass ratio between pyromellitic dianhydride and 4,4'-diphenylether diamine can be any one of 1:1, 1:1.5, 1:2, 2:1, etc., or the range between any two of them.

[0064] In one embodiment, the solvent includes at least one of N,N-dimethylformamide, dimethylacetamide, and N-methyl-2-pyrrolidone.

[0065] In one embodiment, the voltage for electrospinning is 20 kV - 25 kV, the temperature for electrospinning is 20°C - 30°C, the humidity for electrospinning is 35% - 45%, the flow rate for electrospinning is 1 ml / h - 1.5 ml / h, and the time for electrospinning is 7 h - 10 h. Optionally, the voltage for electrospinning can be any one of 20 kV, 21 kV, 22 kV, 23 kV, 25 kV, etc., or the range between any two of them; the temperature for electrospinning can be any one of 20°C, 23°C, 25°C, 28°C, 30°C, etc., or the range between any two of them; the humidity for electrospinning can be any one of 35%, 37%, 39%, 41%, 45%, etc., or the range between any two of them; the flow rate for electrospinning can be any one of 1 ml / h, 1.1 ml / h, 1.2 ml / h, 1.3 ml / h, 1.5 ml / h, etc., or the range between any two of them; the time for electrospinning can be any one of 7 h, 8 h, 9 h, 10 h, etc., or the range between any two of them.

[0066] In one embodiment, heating and drying can remove the residual solvent. The temperature for heating and drying is 100°C - 130°C, and the time for heating and drying is 20 min - 40 min. Optionally, the temperature for heating and drying can be any one of 100°C, 105°C, 110°C, 115°C, 120°C, 130°C, etc., or the range between any two of them; the time for heating and drying can be any one of 20 min, 24 min, 28 min, 32 min, 36 min, 40 min, etc., or the range between any two of them.

[0067] In one embodiment, the imidization temperature is 250°C - 350°C, and the imidization time is 1.5 h - 2.5 h. Optionally, the imidization temperature can be any one of 250°C, 280°C, 300°C, 330°C, 350°C, etc. or the range between any two of them; the imidization time can be any one of 1.5 h, 1.8 h, 2.0 h, 2.2 h, 2.5 h, etc. or the range between any two of them.

[0068] In one embodiment, in step S12, the impregnation time is 0.8 h - 1.2 h, and the impregnation temperature is 20°C - 30°C. Optionally, the impregnation time can be any one of 0.8 h, 0.9 h, 1.0 h, 1.1 h, 1.2 h, etc. or the range between any two of them; the impregnation temperature can be any one of 20°C, 22°C, 26°C, 28°C, 30°C, etc. or the range between any two of them.

[0069] In one embodiment, in step S12, the concentration of the dilute ammonia water solution is 0.01 mol / L - 0.2 mol / L. Optionally, the concentration of the dilute ammonia water solution can be any one of 0.01 mol / L, 0.05 mol / L, 0.1 mol / L, 0.15 mol / L, 0.2 mol / L, etc. or the range between any two of them. In this embodiment, if the concentration of the dilute ammonia water solution is too high, it is easy to cause the reaction of aluminum chloride in ammonia water to be too violent, resulting in incomplete reaction or generation of by-products, reducing the purity of boehmite; if the concentration of the dilute ammonia water solution is too low, it is easy to cause a decrease in the yield of boehmite, and the effect of boehmite in improving the mechanical properties and thermal stability of the battery separator is not obvious.

[0070] In one embodiment, the hot pressing temperature is 160°C - 200°C, and the hot pressing time is 3 min - 8 min. Optionally, the hot pressing temperature can be any one of 160°C, 170°C, 180°C, 190°C, 200°C, etc. or the range between any two of them; the hot pressing time can be any one of 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, etc. or the range between any two of them. In this embodiment, if the hot pressing temperature is too high, it is easy to cause deformation and shrinkage of the composite polyimide film and the polypropylene film, affecting the mechanical properties and thermal stability of the battery separator; if the hot pressing temperature is too low, it is easy to cause weak connection between the composite polyimide film and the polypropylene film.

[0071] This application also provides a lithium-ion battery, including the battery separator as described above or the battery separator prepared by the preparation method as described above.

[0072] In this embodiment, the lithium-ion battery includes a battery separator, which includes a first film layer and a second film layer arranged in a stacked manner. The first film layer includes a polyimide film and boehmite dispersed in the polyimide film. The battery separator has high mechanical properties and thermal stability, and can further improve the cycle performance and safety performance of the lithium-ion battery.

[0073] The above solution will be further described below in conjunction with specific implementation examples. The preferred embodiments of the present application are described in detail as follows:

[0074] Example 1

[0075] This embodiment provides a method for preparing a battery separator, which includes the following steps:

[0076] 1. Dissolve pyromellitic dianhydride and 4,4'-diphenyl ether diamine in N,N-dimethylformamide to form a polyamic acid solution, wherein the mass ratio between pyromellitic dianhydride and 4,4'-diphenyl ether diamine is 1:1.

[0077] 2. Place the polyamic acid solution in an electrospinning device and perform electrospinning to obtain a polyamic acid nanofiber membrane; wherein, the voltage of electrospinning is 20 kV, the temperature of electrospinning is 20 °C, the humidity of electrospinning is 40%, the flow rate of electrospinning is 1 ml / h, and the time of electrospinning is 7 h.

[0078] 3. Heat and dry the polyamic acid nanofiber membrane and imidize it to obtain a polyimide film; wherein, the time of heat drying is 30 min, the temperature of heat drying is 120 °C, the temperature of imidization is 300 °C, and the time of imidization is 2.

[0079] 4. Immerse the polyimide film in an ethanol solution of anhydrous aluminum chloride with a concentration of 0.2 mol / L and soak it at 25 °C for 3 h. After aluminum chloride is adsorbed on the fiber surface, take out the membrane and wash it to obtain a polyimide film adsorbed with aluminum chloride.

[0080] 5. Immerse the polyimide film adsorbed with aluminum chloride in a dilute ammonia water solution with a concentration of 0.1 mol / L and soak it at 25 °C for 1 h. Take out the membrane and wash it, and dry it at 60 °C for 24 h to obtain a composite polyimide film.

[0081] 6. Stack the composite polyimide film and the polypropylene film, perform hot pressing treatment, and cool to obtain a battery separator; wherein, the thickness of the composite polyimide film is 5 μm, the area ratio of the composite polyimide film to the polypropylene film is 1:1, the thickness of the battery separator is 17 μm, the temperature of hot pressing treatment is 180 °C, and the time of hot pressing treatment is 5 min.

[0082] Example 2

[0083] Example 2 is different from Example 1 in that:

[0084] In step 4, the concentration of the ethanol solution of anhydrous aluminum chloride is 0.1 mol / L, and the rest is the same as in Example 1.

[0085] Example 3

[0086] Example 3 is different from Example 1 in that:

[0087] In step 4, the concentration of the ethanol solution of anhydrous aluminum chloride is 0.3 mol / L, and the rest is the same as in Example 1.

[0088] Example 4

[0089] Example 4 is different from Example 1 in that:

[0090] In step 4, the concentration of the ethanol solution of anhydrous aluminum chloride is 0.4 mol / L, and the rest is the same as in Example 1.

[0091] Example 5

[0092] Example 5 is different from Example 1 in that:

[0093] In step 5, the concentration of the dilute ammonia water solution is 0.01 mol / L, and the rest is the same as in Example 1.

[0094] Example 6

[0095] Example 6 is different from Example 1 in that:

[0096] In step 5, the concentration of the dilute ammonia water solution is 0.005 mol / L, and the rest is the same as in Example 1.

[0097] Example 7

[0098] Example 7 is different from Example 1 in that:

[0099] In step 5, the concentration of the dilute ammonia water solution is 0.15 mol / L, and the rest is the same as in Example 1.

[0100] Example 8

[0101] Example 8 is different from Example 1 in that:

[0102] In step 5, the concentration of the dilute ammonia water solution is 0.2 mol / L, and the rest is the same as in Example 1.

[0103] Example 9

[0104] Example 9 is different from Example 1 in that:

[0105] In step 6, the hot pressing temperature is 160 °C, and the rest is the same as in Example 1.

[0106] Example 10

[0107] The difference between Example 10 and Example 1 is that:

[0108] In step 6, the hot pressing temperature is 100 °C, and the rest is the same as in Example 1.

[0109] Example 11

[0110] The difference between Example 11 and Example 1 is that:

[0111] In step 6, the hot pressing temperature is 190 °C, and the rest is the same as in Example 1.

[0112] Example 12

[0113] The difference between Example 12 and Example 1 is that:

[0114] In step 6, the hot pressing temperature is 200 °C, and the rest is the same as in Example 1.

[0115] Comparative Example 1

[0116] The difference between Comparative Example 1 and Example 1 is that:

[0117] The operation of step 4 was not carried out,

[0118] Step 5 was correspondingly changed to: Immerse the polyimide film in a dilute ammonia water solution with a concentration of 0.1 mol / L, soak it at 25 °C for 1 h, take out the film and wash it, and dry it at 60 °C for 24 h to obtain a composite polyimide film; the rest is the same as in Example 1.

[0119] Comparative Example 2

[0120] The difference between Comparative Example 1 and Example 1 is that:

[0121] The operation of step 6 was not carried out;

[0122] Step 5 was correspondingly changed to: Immerse the polyimide film adsorbed with aluminum chloride in a dilute ammonia water solution with a concentration of 0.1 mol / L, soak it at 25 °C for 1 h, take out the film and wash it, and dry it at 60 °C for 24 h to obtain a battery separator; the rest is the same as in Example 1.

[0123] Testing method

[0124] The battery separators obtained in Examples 1 to 12 and Comparative Examples 1 and 2 were subjected to tensile property testing, separator shrinkage rate testing, and electrochemical property testing. The specific testing methods are as follows:

[0125] (I) Tensile property testing:

[0126] The mechanical properties of different diaphragms were characterized by an intelligent electronic tensile testing machine with the equipment model PARAM XLW. The samples were cut into standard dumbbell-shaped test strips with the length and width of the middle parallel part being 20 mm and 4 mm respectively using a cutter. The average thickness was measured using a thickness gauge, and then the tensile properties of the samples were tested at a tensile speed of 50 mm / min.

[0127] (II) Testing of the thermal shrinkage rate of the diaphragm:

[0128] The diaphragm was cut into circular pieces with a diameter of 16 mm and heat-treated at 150 °C for 1 h, and the area change of the diaphragm was measured. The calculation formula for the thermal shrinkage rate (S, unit: %) is as follows: S = (S1 - S0) / S0 × 100%, where S0 and S1 are the areas of the diaphragm before and after heat treatment respectively, and the unit is m 2 .

[0129] (III) Electrochemical performance testing:

[0130] The battery performance of the half-cell was tested. Lithium iron phosphate and conductive carbon black were mixed to prepare the battery positive electrode; a lithium sheet was used as the negative electrode; an electrolyte was prepared with lithium hexafluorophosphate as the solute and a mixed solution of ethylene carbonate, dimethyl carbonate, and diethyl carbonate as the solvent; the diaphragms of the examples and comparative examples were used as the battery diaphragms, and the lithium-ion half-cell was assembled in an argon glove box. After assembly, it was left standing for 12 h, and then the charge-discharge performance and rate performance of the battery were tested using a Blue Power test system.

[0131] The test results are shown in Table 1 below:

[0132] Table 1

[0133] Sample Thermal shrinkage rate (%) Capacity retention rate (%) Tensile strength (MPa) Example 1 8.84 91.80 152.36 Example 2 9.47 86.83 100.42 Example 3 9.10 89.35 112.23 Example 4 9.00 89.21 137.21 Example 5 11.12 88.15 109.47 Example 6 12.13 86.72 101.37 Example 7 10.60 89.52 123.51 Example 8 10.99 88.67 110.94 Example 9 11.12 86.53 128.34 Example 10 12.22 85.31 100.89 Example 11 10.01 89.22 130.44 Example 12 11.34 86.38 120.04 Comparative Example 1 13.83 75.19 89.69 Comparative Example 2 13.90 71.92 80.35

[0134] From the test results of Examples 1 to 12 and Comparative Examples 1 and 2 above, it can be seen that the battery diaphragms prepared in Examples 1 to 19 have good mechanical properties, thermal shrinkage resistance, and electrochemical properties, indicating that the boehmite is dispersed in the polyimide film in the battery diaphragm prepared in this application, improving the overall mechanical properties, thermal stability, and electrochemical properties of the battery diaphragm; in addition, the polypropylene film layer itself has relatively high mechanical strength, and the battery diaphragm includes both the first film layer and the polypropylene film layer, which can further improve the comprehensive mechanical properties and thermal stability of the battery diaphragm.

[0135] From the comparison of Examples 1 - 4, it can be seen that when preparing the battery diaphragm, controlling the concentration of the ethanol solution of anhydrous aluminum chloride within an appropriate range can further ensure the mechanical properties, thermal stability, and electrochemical properties of the battery diaphragm.

[0136] From the comparison between Example 1 and Examples 5-8, it can be seen that when preparing the battery separator, controlling the concentration of the dilute ammonia water solution within an appropriate range can further ensure the mechanical properties, thermal stability and electrochemical properties of the battery separator.

[0137] From the comparison between Example 1 and Examples 9-12, it can be seen that when preparing the battery separator, controlling the hot pressing temperature within an appropriate range can further ensure the mechanical properties, thermal stability and electrochemical properties of the battery separator.

[0138] The embodiments of the present application have been introduced in detail above. Specific examples are used in this article to elaborate on the principle and implementation manner of the present application. The description of the above embodiments is only used to help understand the method and its core idea of the present application; at the same time, for those skilled in the art, according to the idea of the present application, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present application.

Claims

1. A battery separator, characterized in that: The battery separator comprises a first film layer and a second film layer which are stacked; Wherein, the first film layer includes a polyimide film and boehmite dispersed in the polyimide film.

2. The battery separator according to claim 1, characterized in that: The second film layer is a polypropylene film layer; and / or The thickness of the battery separator is 15 mm to 20 mm; and / or The thickness of the first film layer is 4 mm to 7 mm; and / or The thickness of the second film layer is 8mm-16mm; and / or The mass percentage of the boehmite in the first film layer is 5%-15%.

3. A method for preparing a battery separator according to claim 1 or 2, characterized in that: The following steps are involved: Providing a polyimide film adsorbed with aluminum chloride, wherein the polyimide film has a fiber structure, and the aluminum chloride is adsorbed on the surface of the fiber structure; The polyimide film adsorbed with aluminum chloride is immersed in a dilute ammonia solution so that the aluminum chloride and the ammonia solution react in situ on the fiber structure of the polyimide film to form boehmite, and then washed and dried to obtain a composite polyimide film; The composite polyimide film and the polypropylene film are stacked, heat-pressed, and cooled to obtain the battery separator.

4. The preparation method according to claim 3, characterized in that: The method for preparing the polyimide film adsorbed with aluminum chloride comprises the following steps: Providing a polyimide film having a fiber structure; The polyimide film is immersed in an ethanol solution of anhydrous aluminum chloride, so that the aluminum chloride is adsorbed on the surface of the fiber structure to obtain the polyimide film adsorbed with aluminum chloride.

5. The preparation method according to claim 4, characterized in that: The concentration of the anhydrous aluminum chloride ethanol solution is 0.2 mol / L-0.4 mol / L; and / or The immersion time is 2h-4h, and / or the immersion temperature is 20°C-30°C.

6. The preparation method according to claim 4, characterized in that: The method for preparing the polyimide film comprises the following steps: Dispersing pyromellitic acid dianhydride and 4,4'-diphenyl ether diamine in a solvent, and performing a polymerization reaction to obtain a polyamic acid solution; Placing the polyamic acid solution in an electrospinning device for electrospinning to obtain a polyamic acid nanofiber membrane; The polyamic acid nanofiber membrane is heated and dried, and imidized to obtain the polyimide film.

7. The preparation method according to claim 6, characterized in that: The mass ratio between the pyromellitic dianhydride and the 4,4'-diphenyl ether diamine is (1-2): (1-2); and / or The solvent comprises at least one of N,N-dimethylformamide, dimethylacetamide and N-methyl-2-pyrrolidone; and / or The voltage of the electrospinning is 20 kV-25 kV, and / or the temperature of the electrospinning is 20° C.-30° C., and / or the humidity of the electrospinning is 35%-45%, and / or the flow rate of the electrospinning is 1 ml / h-1.5 ml / h, and / or the time of the electrospinning is 7h-10h; and / or The heating and drying temperature is 100°C-130°C, and / or the heating and drying time is 20min-40min; and / or The temperature of the imidization is 250° C.-350° C., and / or the time of the imidization is 1.5 h-2.5 h.

8. The preparation method according to any one of claims 3 to 7, characterized in that: In the step of immersing the polyimide film adsorbed with aluminum chloride in a dilute ammonia solution, the immersion time is 0.8h-1.2h, and / or the immersion temperature is 20°C-30°C; and / or The concentration of the dilute ammonia solution is 0.01 mol / L-0.2 mol / L.

9. The preparation method according to any one of claims 3 to 7, characterized in that: The temperature of the hot pressing treatment is 160° C.-200° C., and / or the time of the hot pressing treatment is 3 min-8 min.

10. A lithium ion battery, characterized in that: It comprises the battery separator as claimed in claim 1 or 2, or the battery separator prepared by the preparation method as claimed in any one of claims 3 to 9.