Battery diaphragm and preparation method thereof, thermal composite battery cell and secondary battery

Abstract

The invention discloses a battery diaphragm and a preparation method thereof, a thermal composite battery cell and a secondary battery, and belongs to the technical field of batteries. The battery diaphragm comprises a base membrane, the base membrane is of a single-layer structure or a multi-layer composite structure, and preparation raw materials of the base membrane comprise at least one of polyethylene with the molecular weight larger than or equal to 1.5 million, polypropylene with the molecular weight larger than or equal to 1.5 million, polyethylene terephthalate and polyimide. The base membrane has high strength and high heat resistance, can greatly simplify the preparation process of the battery cell, reduces the difficulty of the preparation process of the battery cell, shortens the preparation process time of the battery cell, improves the preparation efficiency of the battery cell, and is also beneficial to improving the safety of the battery cell.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and particularly to battery separators and their preparation methods, thermal composite cells, and secondary batteries. Background Technology

[0002] In secondary batteries, the battery separator mainly serves to isolate the positive and negative electrodes while allowing ions to pass through freely.

[0003] Battery separators typically include a base film. Currently, base films are usually made of polyolefin materials, such as polypropylene (PP), polyethylene (PE), or composites of PP and PE. However, due to limitations in current film-forming processes, the molecular weight of the PP or PE materials used is generally low. This results in inferior strength and heat resistance for the aforementioned PP and PE base films, placing higher demands on the cell assembly process and making it more complex. Summary of the Invention

[0004] In view of this, the present invention provides a battery separator and its preparation method, a thermally composite battery cell, and a secondary battery, which can solve the problem that the strength and adhesion of battery separators in related technologies need to be improved. Specifically, it includes the following technical solutions:

[0005] On one hand, a battery separator is provided, the battery separator comprising: a base membrane, the base membrane being a single-layer structure or a multi-layer composite structure, the raw materials for preparing the base membrane comprising at least one of: polyethylene with a molecular weight greater than or equal to 1.5 million, polypropylene with a molecular weight greater than or equal to 1.5 million, polyethylene terephthalate, and polyimide.

[0006] In some possible implementations, the base film has a plurality of through holes, the axial direction of which is along the thickness direction of the base film.

[0007] In some possible implementations, the plurality of through holes are arranged in a matrix.

[0008] In some possible implementations, the base film has a thermal shrinkage rate of less than or equal to 1% at a temperature of 230°C-300°C.

[0009] In some possible implementations, the base film also has at least one of the following physical properties: the thickness of the base film is 1 μm-20 μm, the average pore size of the through-holes of the base film is less than or equal to 1 μm, the porosity of the base film is 25%-85%, the tensile strength of the base film is greater than or equal to 400 MPa, and the puncture strength of the base film is greater than or equal to 6.5 N.

[0010] In some possible implementations, the battery separator further includes an adhesive layer bonded to at least one surface of the base film;

[0011] The adhesive layer includes a layered body and protruding adhesive particles on the surface of the layered body facing away from the base film. The adhesive layer contains organic adhesive particles, inorganic filler particles, adhesive, and dispersant. The particle size of the organic adhesive particles is larger than that of the inorganic filler particles. Part of the surface of the organic adhesive particles is connected to the inorganic filler particles through the adhesive and cooperates to form the layered body. Another part of the organic adhesive particles protrudes to the surface of the layered body to form the protruding adhesive particles.

[0012] On the other hand, a method for preparing a battery separator is provided, wherein the battery separator is as described in any of the above descriptions, and the method for preparing the battery separator includes:

[0013] The preparation of a base film includes: preparing the base film by means of extrusion molding, stretching pore forming or laser drilling, depending on the raw materials used for the preparation of the base film.

[0014] In some possible implementations, when the raw material for preparing the base film is polyethylene with a molecular weight greater than or equal to 1.5 million or polypropylene with a molecular weight greater than or equal to 1.5 million, the base film is prepared using the extrusion molding method, including:

[0015] The raw materials for preparing the base film are mixed with solvent oil to form a base film solution, wherein the raw materials for preparing the base film account for 15%-20% of the mass percentage of the base film solution;

[0016] The base film is prepared by extruding the base film liquid using an extruder, wherein the extrusion rate is controlled to be 250 kg / h-350 kg / h.

[0017] In some possible implementations, when the raw material for preparing the base film is polyethylene with a molecular weight greater than or equal to 1.5 million, polypropylene with a molecular weight greater than or equal to 1.5 million, polyethylene terephthalate, or polyimide, the base film is prepared using the stretching pore-forming method, including:

[0018] The raw materials for preparing the base film are pretreated, and the pretreated raw materials are extruded and molded using an extruder to form a precursor film.

[0019] The precursor film is heated to a stretching temperature, wherein the stretching temperature is lower than the melting point of the raw material.

[0020] The precursor film is stretched at the specified stretching temperature to prepare the base film.

[0021] In some possible implementations, when the raw material for preparing the base film is polyethylene with a molecular weight greater than or equal to 1.5 million, polypropylene with a molecular weight greater than or equal to 1.5 million, polyethylene terephthalate, or polyimide, the laser drilling method is used to prepare the base film, including:

[0022] A dense membrane material is prepared from the raw materials for the base membrane by melt extrusion or casting, wherein the porosity of the dense membrane material is less than 5% and the pore size is less than 0.1 μm;

[0023] A laser source is used to perform laser etching on the dense film material to form multiple micropores on the dense film material, thereby preparing a base film.

[0024] In some possible implementations, the method for preparing the battery separator further includes:

[0025] The slurry corresponding to the adhesive layer is coated onto at least one side of the base film, and then dried to prepare the adhesive layer on the surface of the base film.

[0026] On another front, a thermal composite battery cell is provided, the thermal composite battery cell comprising a plurality of positive electrode plates, a plurality of negative electrode plates, and any of the battery separators described above;

[0027] The negative electrode and the positive electrode are stacked alternately, and adjacent negative electrode and positive electrode are separated by the battery separator. The battery separator is bonded to the corresponding electrode by an adhesive layer.

[0028] In another aspect, a secondary battery is provided, the secondary battery comprising: a housing, an electrolyte contained inside the housing, and a thermally composite battery cell, the thermally composite battery cell being as described above.

[0029] The beneficial effects of the technical solutions provided in the embodiments of the present invention include at least the following:

[0030] The battery separator provided in this invention is prepared from at least one of the following raw materials: polyethylene with a molecular weight greater than or equal to 1.5 million, polypropylene with a molecular weight greater than or equal to 1.5 million, polyethylene terephthalate, and polyimide. For polyethylene and polypropylene materials, increasing their molecular weight results in extremely long molecular chains and high entanglement density, which is beneficial for improving the tensile strength and puncture strength of the base film. Furthermore, the ultra-high molecular weight polyethylene and polypropylene materials, due to their restricted molecular chain movement, exhibit significantly lower shrinkage rates at high temperatures (e.g., above 120°C) compared to low molecular weight PE / PP separators, thus improving their dimensional stability at high temperatures, i.e., heat resistance. In summary, this invention aims to significantly improve the strength and heat resistance of polyethylene and polypropylene base films by increasing their molecular weight. Polyethylene terephthalate (PET) and polyimide (PI) materials possess properties such as high melting point, high tensile strength, and high modulus, resulting in PET-based and PI-based films exhibiting high strength and high heat resistance.

[0031] The increased strength of the base film enhances its puncture resistance, effectively preventing dust and other foreign objects from puncturing the base film and causing short circuits. This reduces the cleanliness requirements for the cell assembly environment, allowing for cell assembly in environments with lower cleanliness levels. Improved heat resistance of the base film enhances its moisture control capabilities (moisture content ≤300ppm), as this allows for higher baking temperatures to quickly and thoroughly remove moisture from the base film, further reducing moisture requirements for the cell assembly environment. Therefore, this type of base film significantly simplifies the cell manufacturing process, reduces its complexity, shortens its time, and improves its efficiency. Furthermore, the increased strength and heat resistance of the base film also reduce its thermal shrinkage rate, effectively preventing internal short circuits at high temperatures and overcharging without triggering thermal runaway, thus improving battery safety. Detailed Implementation

[0032] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] In related technologies, PP and PE base films have poor strength and heat resistance, resulting in poor puncture resistance. Therefore, when using these base films for battery cell assembly, special attention must be paid to the cleanliness of the assembly environment. The cleanliness of the assembly environment must be sufficiently high to prevent dust and other foreign objects from puncturing the separator. Additionally, the moisture content in the assembly environment must be extremely low to avoid introducing unwanted moisture into the separator and battery cell. It is evident that currently known PP and PE base films place high demands on battery cell assembly operations, making the battery cell assembly process relatively complex.

[0034] To address the technical problems existing in related technologies, embodiments of the present invention provide a battery separator, which includes: a base membrane, wherein the base membrane is a single-layer structure or a multi-layer composite structure, and the raw materials for preparing the base membrane include at least one of the following: polyethylene with a molecular weight greater than or equal to 1.5 million, polypropylene with a molecular weight greater than or equal to 1.5 million, polyethylene terephthalate, and polyimide.

[0035] The battery separator provided in this embodiment of the invention has a base film that can be a single-layer structure. For example, a single-layer base film can be a polyethylene base film with a molecular weight greater than or equal to 1.5 million, a polypropylene base film with a molecular weight greater than or equal to 1.5 million, a polyethylene terephthalate base film, or a polyimide base film.

[0036] The base film can also be a multilayer composite structure. For multilayer composite base films, it can be two-layer, three-layer, four-layer or other. The multilayer composite base film is obtained by combining any two, three or four of the following: polyethylene base film with a molecular weight greater than or equal to 1.5 million, polypropylene base film with a molecular weight greater than or equal to 1.5 million, polyethylene terephthalate base film or polyimide base film. Taking a three-layer base film as an example, it can include a polypropylene base film with a molecular weight greater than or equal to 1.5 million, a polyethylene base film with a molecular weight greater than or equal to 1.5 million and a polypropylene base film with a molecular weight greater than or equal to 1.5 million arranged in sequence.

[0037] The battery separator provided in this invention is prepared from at least one of the following raw materials: polyethylene with a molecular weight greater than or equal to 1.5 million, polypropylene with a molecular weight greater than or equal to 1.5 million, polyethylene terephthalate, and polyimide. For polyethylene and polypropylene materials, increasing their molecular weight results in extremely long molecular chains and high entanglement density, which is beneficial for improving the tensile strength and puncture strength of the base film. Furthermore, the ultra-high molecular weight polyethylene and polypropylene materials, due to their restricted molecular chain movement, exhibit significantly lower shrinkage rates at high temperatures (e.g., above 120°C) compared to low molecular weight PE / PP separators, thus improving their dimensional stability at high temperatures, i.e., heat resistance. In summary, this invention aims to significantly improve the strength and heat resistance of polyethylene and polypropylene base films by increasing their molecular weight. Polyethylene terephthalate (PET) and polyimide (PI) materials possess properties such as high melting point, high tensile strength, and high modulus, resulting in PET-based and PI-based films exhibiting high strength and high heat resistance.

[0038] The increased strength of the base film enhances its puncture resistance, effectively preventing dust and other foreign objects from puncturing the base film and causing short circuits. This reduces the cleanliness requirements for the cell assembly environment, allowing for cell assembly in environments with lower cleanliness levels. Improved heat resistance of the base film enhances its moisture control capabilities (moisture content ≤300ppm), as this allows for the use of higher baking temperatures to quickly and thoroughly remove moisture from the base film, further reducing moisture requirements for the cell assembly environment. Therefore, this type of base film significantly simplifies the cell manufacturing process, reduces its complexity, shortens its manufacturing time, and improves its efficiency. Furthermore, the improved strength and heat resistance of the base film also reduce its thermal shrinkage rate, effectively preventing internal short circuits at high temperatures and overcharging without triggering thermal runaway, thus improving battery safety.

[0039] It should be noted that although the embodiments of the present invention have improved the raw materials for preparing the base film and achieved excellent results, the processing difficulties of these raw materials have also been overcome in the process. In other words, it is quite difficult to prepare a base film that meets the requirements from such materials. The embodiments of the present invention have overcome this difficulty and improved the corresponding base film preparation method. The base film is prepared by using one of the improved extrusion molding method, stretching pore forming method, and laser drilling method, which will be described in detail in the section on base film preparation methods below.

[0040] As mentioned above, the high-strength base film is prepared using ultra-high molecular weight polyethylene or polypropylene with a molecular weight (specifically, weight-average molecular weight, in g / mol) greater than or equal to 1.5 million. In some examples, the molecular weight range of polyethylene is 1.5 million to 2.5 million, and the molecular weight range of polypropylene is 1.5 million to 2.5 million.

[0041] Understandably, when the molecular weight of polyethylene or polypropylene is greater than or equal to 1.5 million, the molecular weight of the prepared base film will also be greater than or equal to 1.5 million.

[0042] As mentioned above, the base film can be prepared using either the stretching pore-forming method or the laser drilling method for the aforementioned polymer raw materials. The stretching pore-forming method, also known as the dry stretching process, leverages the partially crystalline nature of polymer materials by stretching them at temperatures close to their melting points. This stretching creates defects in the amorphous regions of the polymer material, which then become lithium-ion transport channels. The laser drilling method, on the other hand, creates pores through a physical process, specifically by using laser etching to form through-holes in a non-porous base film, providing ion transport channels. When using the stretching pore-forming method, the pore size in the base film can be controlled by adjusting the stretching speed ratio and temperature. When using the laser drilling method, the size and spacing of the through-holes can be freely set.

[0043] Unlike the pore morphology formed by wet film deposition, the through holes formed by stretching and laser drilling are straight through holes in the base film. That is to say, the base film has multiple straight through holes, and the axis of the straight through holes is along the thickness direction of the base film. The base film with straight through holes is more conducive to ion transport and has higher ion conductivity.

[0044] In some examples, for through holes formed by laser drilling, multiple through holes can be arranged in a matrix. For example, the matrix formed by multiple through holes can be a square matrix, a regular polygon matrix, or other forms. This is beneficial for the uniform distribution of multiple through holes in the base film and for improving the ion transport effect of the base film.

[0045] The pore size of the battery separator directly affects the lithium-ion transport efficiency, mechanical strength, and safety (such as dendrite blocking ability). For the battery separators mentioned above in the embodiments of the present invention, the average pore size is less than 1 μm, and can be further 0.1 μm-1 μm, in order to balance ion conductivity, dendrite suppression, and mechanical strength.

[0046] The porosity of a battery separator is a key parameter affecting its ion conductivity, mechanical strength, and electrolyte retention capacity. For the battery separators mentioned above in the embodiments of the present invention, the porosity can be 25%-85%, and further can be 30%-60%, thereby achieving a balance between ion conductivity, mechanical strength, and safety.

[0047] For any of the battery separators mentioned above, the thermal shrinkage rate of its base film at a temperature of 230℃-300℃ is less than or equal to 1%. For example, the thermal shrinkage rate of the base film at a temperature of 250℃ is less than or equal to 1%, which effectively prevents internal short circuits, prevents thermal runaway from being triggered by overcharging, and improves the safety performance of the battery cell.

[0048] In some examples, the base film also has at least one of the following physical properties: the thickness of the base film is 1 μm-20 μm, the average pore size of the pores of the base film is 0.1 μm-1 μm, the porosity of the base film is 25%-85%, the tensile strength of the base film is greater than or equal to 400 MPa, for example, 400 MPa-600 MPa, and the puncture strength of the base film is greater than or equal to 6.5 N, for example, 6.5 N-10 N.

[0049] As a typical example, the base film simultaneously meets all the above parameters. It is evident that, while maintaining basic performance, the base film also possesses a relatively thin thickness and high strength. Its excellent puncture resistance effectively prevents short circuits between the positive and negative electrodes, improving battery safety.

[0050] In some examples of the battery separators mentioned above, the battery separators provided in the embodiments of the present invention further include an adhesive layer bonded to at least one surface of the base film. The adhesive layer includes a layered body and protruding adhesive particles on the surface of the layered body facing away from the base film. The adhesive layer contains organic adhesive particles, inorganic filler particles, a binder, and a dispersant. The particle size of the organic adhesive particles is larger than that of the inorganic filler particles. Part of the surface of the organic adhesive particles is connected to the inorganic filler particles by the binder and cooperates to form the layered body. Another part of the organic adhesive particles protrudes to the surface of the layered body to form protruding adhesive particles.

[0051] It should be noted that organic particles are in an adhesive state when the temperature is above their glass transition temperature, while the Mohs hardness of inorganic filler particles is greater than or equal to the hardness threshold.

[0052] The aforementioned battery separator comprises an adhesive layer containing embedded organic particles. These organic particles are partially embedded in the layered matrix of the adhesive layer and partially protrude from the surface of the layered matrix away from the base film. This design of the organic particles, on the one hand, increases the number of viscous contact points on the adhesive layer surface through the protruding arrangement of the particles, and the protruding particle molecular chains are more likely to entangle with the molecular chains on the electrode surface, thereby enhancing the adhesion between the battery separator and the electrode. On the other hand, the particle size of the organic particles is larger than that of the inorganic filler particles. By increasing the particle size of the organic particles to a larger particle form, it is easier to control the uniformity of the distribution of the organic particles in the adhesive layer, which is more beneficial for enhancing the adhesion between the battery separator and the electrode. Furthermore, the organic particles are in an adhesive state at temperatures above their glass transition temperature, allowing the battery separator to be bonded to the electrode through a thermal bonding process and maintaining stable adhesion during subsequent stacking processes.

[0053] As can be seen, the battery separator provided in this embodiment of the invention possesses both high strength and high adhesion, making it suitable for assembling battery cells with electrodes using high-speed composite lamination processes. This improves cell production efficiency, reduces production costs, and simultaneously enhances the overall performance and operational reliability of the battery cells. Furthermore, the adhesive layer also contains inorganic filler particles, which, through a binder, combine with organic adhesive particles, imparting good mechanical stability, thermal stability, and air permeability to the adhesive layer, further improving the overall performance of the battery separator.

[0054] In some examples, the particle size D50 of organic filler particles can be 4.5μm-6.5μm, including but not limited to 4.5μm, 4.7μm, 5μm, 5.1μm, 5.2μm, 5.3μm, 5.4μm, 5.5μm, 5.6μm, 5.7μm, 5.8μm, 5.9μm, 6μm, 6.2μm, 6.5μm, etc. The particle size D50 of inorganic filler particles is 0.3μm-1.5μm, including but not limited to 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1.0μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, etc.

[0055] In some examples, the organic particles satisfy at least one of the following physical properties: (1) the protrusion height of the protruding particles relative to the layered body is 2μm-3μm, including but not limited to 2μm, 2.1μm, 2.2μm, 2.3μm, 2.4μm, 2.5μm, 2.6μm, 2.7μm, 2.8μm, 2.9μm, 3μm, etc. (2) The area covered by the protruding adhesive particles on the layered body accounts for 30%-60% of the area of ​​the layered body, including but not limited to 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, etc.

[0056] The protrusion height of organic particles relative to the layered matrix of the adhesive layer affects the adhesion, permeability, ion transport efficiency, and mechanical properties of the battery separator. To ensure a good balance among these properties, the protrusion height of the organic particles relative to the layered matrix of the adhesive layer can be 2μm-3μm.

[0057] By limiting the particle size D50 of both the organic adhesive particles and the inorganic filler particles as described above, on the one hand, the protruding arrangement of the organic adhesive particles is allowed; on the other hand, based on their suitable particle size D50 range, the organic adhesive particles not only facilitate their uniform distribution on the adhesive layer, but also help the adhesive layer provide sufficient bonding area.

[0058] In some examples, the organic particles may account for 5%-20% of the mass percentage of the adhesive layer, including but not limited to 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, etc.; and the inorganic filler particles may account for 70%-85% of the mass percentage of the adhesive layer, including but not limited to 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, etc.

[0059] When the mass percentage of organic particles is within the above range, it is more beneficial to enhance the adhesion between the adhesive layer and the electrode, while ensuring good air permeability of the battery separator.

[0060] When the mass percentage of inorganic filler particles is within the above range, it is beneficial to optimize the mechanical stability, thermal stability, and air permeability of the adhesive layer, thereby improving the overall performance of the battery separator.

[0061] In some examples, the organic particles constitute 5%-20% of the adhesive layer by mass, including but not limited to 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, and 20%. The inorganic filler particles constitute 70%-85% of the adhesive layer by mass, including but not limited to 70%, 75%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, and 90%. The adhesive constitutes 5%-15% of the adhesive layer by mass, including but not limited to 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, and 15%.

[0062] For any of the aforementioned battery separators, the inorganic filler particles are selected from at least one of boehmite particles, alumina particles, magnesium hydroxide particles, magnesium oxide particles, titanium dioxide particles, silicon dioxide particles, barium titanate particles, zinc oxide particles, nickel oxide particles, magnesium fluoride particles, zirconium oxide particles, nickel oxide particles, cerium oxide particles, and barium sulfate particles.

[0063] The aforementioned inorganic filler particles not only improve the thermal stability and mechanical strength of the coating, but also enhance the air permeability of the adhesive layer, thereby improving ion conductivity and electrolyte adsorption performance. For example, the inorganic filler particles can be boehmite particles.

[0064] Adhesives are used to bond organic adhesive particles and inorganic filler particles at room temperature, and also facilitate the bonding of adhesive layers to a base film. In some examples, suitable adhesives are selected from at least one of polymethyl acrylate, polyethyl acrylate, polybutyl acrylate, silicone-modified polyacrylate, and polyurethane-modified polyacrylate.

[0065] The aforementioned adhesive not only provides excellent adhesion but also facilitates the formation of an elastic network to suppress deformation cracking of the coating and improve the operational reliability of the coating.

[0066] For any of the battery separators mentioned above, the adhesive layer may also include a dispersant, and the mass percentage of the dispersant in the adhesive layer may be 1%-5%, including but not limited to the following: 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, etc.

[0067] Dispersants prevent organic colloidal particles from settling and agglomerating in solvents, which is beneficial for improving the uniformity of particle distribution in the adhesive layer. It also helps increase the content of organic colloidal particles in the adhesive layer without affecting its permeability. By limiting the mass percentage of the dispersant in the organic colloidal particles as described above, the addition of the dispersant can be used to avoid deteriorating the adhesiveness of the organic colloidal particles. Some suitable dispersants include sodium carboxymethyl cellulose, polyethylene glycol, and polyvinyl alcohol.

[0068] On the other hand, embodiments of the present invention also provide a method for preparing a battery separator, wherein the battery separator is as described in any of the above descriptions, and the method for preparing the battery separator includes:

[0069] The preparation of the base film includes: preparing the base film by means of one of the following methods, namely extrusion molding, stretching pore forming, or laser drilling, depending on the raw materials used for the preparation of the base film.

[0070] The raw materials for preparing the base film include at least one of the following: polyethylene with a molecular weight greater than or equal to 1.5 million, polypropylene with a molecular weight greater than or equal to 1.5 million, polyethylene terephthalate, and polyimide. The base film can be prepared by one of the following methods: modified extrusion molding, stretching pore forming, or laser drilling, so as to obtain a base film that meets the requirements.

[0071] The following will illustrate the preparation methods of the base film by taking into account specific raw materials.

[0072] In some examples, when the raw material for preparing the base film is polyethylene or polypropylene with a molecular weight greater than or equal to 1.5 million, extrusion molding is used to prepare the base film, including:

[0073] The raw materials for preparing the base film are mixed with solvent oil to form a base film solution. The solvent oil can be, for example, white oil. The raw materials for preparing the base film account for 15%-20% of the mass of the base film solution, including but not limited to 15%, 16%, 17%, 18%, 19%, 20%, etc.

[0074] The base film is prepared by extruding the base film liquid through an extruder. The extrusion rate is controlled to be 250kg / h-350kg / h, including but not limited to: 250kg / h, 260kg / h, 270kg / h, 280kg / h, 290kg / h, 300kg / h, 310kg / h, 320kg / h, etc.

[0075] Related technologies typically use polyethylene with a lower molecular weight to prepare the base film. This is because the extrusion and plasticizing capabilities of the equipment are limited. If the molecular weight of polyethylene is higher, it makes the preparation of polyethylene film materials more difficult.

[0076] This invention innovatively improves the extrusion molding process to allow polyethylene or polypropylene with a molecular weight greater than or equal to 1.5 million to be easily prepared into the desired base film. Specifically, on the one hand, the mass percentage of polyethylene in the base film slurry (i.e., the powder-to-oil ratio) is reduced, and on the other hand, the extrusion amount is reduced, so that both are within the aforementioned limited range, thereby improving the plasticizing and molding capacity of the extruder, enabling ultra-high molecular weight polyethylene or polypropylene to be successfully extruded and molded to obtain the desired base film.

[0077] In other examples, when the raw materials for preparing the base film are polyethylene, polypropylene, polyethylene terephthalate, or polyimide with a molecular weight greater than or equal to 1.5 million, the base film is prepared using a stretching pore-forming method, including:

[0078] The raw materials for preparing the base film are pretreated. For example, the pretreatment can be drying to remove moisture, or it can be blending modification to enhance the cleanliness or processability of the raw materials.

[0079] The pretreated raw materials are extruded using an extruder to form a precursor film. The extrusion temperature is determined based on the melting point of the material.

[0080] The precursor film is heated to a stretching temperature, wherein the stretching temperature is lower than the melting point of the raw material, and the difference between the melting point of the raw material and the stretching temperature can be 5℃-20℃.

[0081] The precursor film is stretched at a stretching temperature to form a base film. Optionally, the stretched base film can be further heat-set at a temperature below the stretching temperature to fix its through-hole structure.

[0082] In some examples, the stretching process can be unidirectional stretching (e.g., longitudinal or transverse) or bidirectional stretching (e.g., longitudinal stretching followed by transverse stretching, or transverse stretching followed by longitudinal stretching).

[0083] During the stretching process, the pore size, porosity, and pore morphology of the through-holes formed in the base film can be controlled by controlling the stretching speed ratio and stretching temperature. The stretching speed ratio refers to the ratio of the length (or width) of the polymer film after stretching to the original length (or width) along the same stretching direction during the stretching process.

[0084] In some further examples, when the raw materials for preparing the base film are polyethylene, polypropylene, polyethylene terephthalate, or polyimide with a molecular weight greater than or equal to 1.5 million, laser drilling is used to prepare the base film, including:

[0085] A dense membrane material is prepared from the raw materials used to prepare the base membrane using melt extrusion or casting methods. It should be noted that the dense membrane material designed in the embodiments of this invention has a porosity of less than 5% and a pore size of less than 0.1 μm, or even 0 μm.

[0086] A laser source is used to perform laser etching on a dense film material to form multiple micropores on the dense film material, thereby preparing a base film.

[0087] For example, the laser source can be an ultraviolet laser (355nm), an ultrashort pulse laser (femtosecond laser), etc., to avoid damaging the film material and to facilitate the formation of through holes, i.e., the hole walls are straight.

[0088] In some examples, the process parameters for laser etching can be as follows:

[0089] Energy density: 10 mJ / cm³ 2 ~50mJ / cm 2 (Avoid excessive heat that could lead to material carbonization).

[0090] Pulse frequency: 10kHz~100kHz (the better the frequency, the higher the hole formation speed).

[0091] Spot size: 0.1μm-1μm (spot size affects the diameter of the via).

[0092] By controlling the relevant process parameters of laser etching, the diameter and distribution of vias can be precisely controlled.

[0093] In conjunction with the structure of the battery separator, when the battery separator further includes an adhesive layer, the preparation method of the battery separator also includes: coating the slurry corresponding to the adhesive layer onto at least one side surface of the base film, and drying it to prepare the adhesive layer on the surface of the base film.

[0094] For example, the method for preparing the adhesive layer includes:

[0095] The monomers and additives for synthesizing the colloidal particles are dispersed in a solvent for polymerization to obtain a colloidal system containing organic colloidal particles.

[0096] Inorganic filler particles and adhesive are added to the adhesive system and stirred evenly to obtain a slurry.

[0097] The slurry is coated onto both sides of the base film and dried to form an adhesive layer on the base film surface, thus obtaining the battery separator.

[0098] The adhesive system obtained during the preparation of organic colloids can also be called an aqueous adhesive. This invention directly uses the adhesive system obtained during the preparation of organic colloids. Inorganic filler particles and a binder are added to the adhesive system, and the mixture is stirred until homogeneous to obtain a slurry. The adhesive layer is then prepared based on this slurry.

[0099] In this process, the particle size of the organic colloids can be controlled by adjusting at least one of the following: the concentration of the synthetic monomer, the temperature and time of the polymerization reaction, the stirring speed, and the concentration of the additives. By controlling these factors, the particle size of the organic colloids can be easily controlled within the desired range, facilitating the acquisition of large-particle organic colloids.

[0100] When coating the slurry onto the base film, the applicable coating process can be scraping, spraying, brushing, etc. After the slurry is coated, a liquid film of a certain thickness is formed. By controlling the coating process, the organic particles are evenly distributed in the liquid film and the thickness of the liquid film is controlled. Then the liquid film is dried, for example, by placing it in an oven to dry, thereby removing the solvent components in the liquid film and forming a solid coating.

[0101] In another aspect, embodiments of the present invention also provide a thermal composite battery cell, which includes a plurality of positive electrode plates, a plurality of negative electrode plates and a battery separator as described above, wherein the negative electrode plates and positive electrode plates are stacked alternately, adjacent negative electrode plates and positive electrode plates are separated by the battery separator, and the battery separator is bonded to the corresponding electrode plates by an adhesive layer.

[0102] The thermal composite battery cell provided in this invention, based on the aforementioned high-strength, heat-resistant battery separator, not only simplifies the manufacturing process of the thermal composite battery cell but also enhances its safety. For thermal composite battery cells employing battery separators with through-hole designs, the ion transport effect is superior, which helps improve the cell's rate performance, energy efficiency, and cycle life.

[0103] When the battery separator has the aforementioned adhesive layer, it can stably bond with the positive and negative electrode sheets. In other words, the adhesive force between the battery separator and the positive and negative electrode sheets is large enough to allow thermally composite cells to be prepared through high-speed stacking processes, thereby improving cell production efficiency and reducing production costs.

[0104] In some examples, the number of negative electrodes is one more than the number of positive electrodes. The thermal composite cell includes multiple first electrode units a and a second electrode unit b arranged in sequence. The first electrode unit a includes a battery separator, a negative electrode, a battery separator, and a positive electrode arranged in sequence. The second electrode unit b includes a battery separator, a negative electrode, and a battery separator arranged in sequence. The second electrode unit b serves as a termination unit.

[0105] In this configuration, multiple first electrode units a share a continuous battery separator. The battery separator includes multiple alternating and continuous electrode bonding sections and multiple bending sections. The multiple electrode bonding sections are stacked along the thickness direction of the thermally composite cell, and any two adjacent electrode bonding sections are connected by the bending sections. Similarly, second electrode units b can also share a battery separator, which is bent once to form a bending section and two electrode bonding sections for bonding the negative electrode.

[0106] Taking the number of first electrode units a as an example of two, the structure of the resulting thermal composite cell is as follows: battery separator / negative electrode / battery separator / positive electrode / battery separator / negative electrode / battery separator / positive electrode / battery separator / negative electrode / battery separator.

[0107] The aforementioned thermal composite battery cell has one more negative electrode than positive electrode, and they are stacked in a specific manner. The extra negative electrode can provide more storage space and insertion sites for lithium ions, which is beneficial to improving battery performance. Moreover, the relatively large number of negative electrodes is also beneficial to enhancing battery safety, improving battery cycle stability, and optimizing the internal electric field distribution of the battery.

[0108] In another aspect, embodiments of the present invention also provide a method for preparing a thermally composite battery cell, wherein the thermally composite battery cell is as described in any of the above descriptions, and the method for preparing the thermally composite battery cell includes...

[0109] A stacked assembly is provided, which includes a battery separator and positive and negative electrode sheets respectively stacked on both sides of the battery separator.

[0110] The laminated assembly is subjected to thermal bonding treatment to form a hot-pressed laminated assembly.

[0111] The hot-pressed lamination assembly is laminated to obtain a thermal composite cell.

[0112] Before forming the stacked module, the separator needs to be baked. After forming the stacked module, the stacked module needs to be baked again. And before the thermal composite cell is assembled and injected with electrolyte, it needs to be baked again. It can be seen that multiple baking processes are required in the preparation of cells and batteries.

[0113] The improved heat resistance of the base film enhances its ability to control moisture (moisture content ≤300ppm), as this allows for the use of higher baking temperatures to quickly and thoroughly remove moisture from the base film. This reduces the moisture requirements of the cell assembly environment. Therefore, this type of base film can significantly simplify the cell manufacturing process, reduce its complexity, shorten its time, and improve its efficiency.

[0114] Furthermore, the increased strength of the base film enhances its puncture resistance, effectively preventing dust and other foreign objects from puncturing the base film and causing short circuits. This reduces the cleanliness requirements for the cell assembly environment, allowing cell assembly to be performed in environments with lower cleanliness levels. Therefore, the fabrication process of the thermally composite cell provided in this embodiment of the invention is simplified, significantly reducing its operational complexity.

[0115] Among them, the stacking process involves the high-speed stacking of hot-pressed stacking assemblies. Based on the high adhesion between the battery separator and the positive and negative electrode sheets, the hot-pressed stacking assemblies can be stacked according to the predetermined stacking route and stacking speed, thereby producing a hot-pressed composite battery cell with high alignment.

[0116] In another aspect, embodiments of the present invention provide a secondary battery, which includes: a casing, an electrolyte contained inside the casing, and a thermally composite battery cell, wherein the thermally composite battery cell is as described above.

[0117] The secondary battery provided in this embodiment of the invention has all the advantages of the battery separator mentioned above, which will not be repeated here. For example, the secondary battery can be a lithium-ion battery, a sodium-ion battery, etc.

[0118] This invention relates to an electrical device, which includes the aforementioned secondary battery. For example, the electrical device may be a portable electronic device (mobile phone, laptop, smart wearable device, etc.), a new energy transportation device (new energy vehicle, etc.), an energy storage system, etc.

[0119] Exemplary embodiments of the present invention will now be described in more detail. While exemplary embodiments of the present invention are described below, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Where specific techniques or conditions are not specified in the embodiments, they are performed in accordance with techniques or conditions described in the literature in the art or according to product instructions. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0120] Synthesis example 1

[0121] Synthesis Example 1 provides a base film prepared from polyethylene with a molecular weight of 1.5 million. The base film has a thickness of 10 μm, an average pore size of 0.05 μm, and a porosity of 40%.

[0122] The base film was prepared by extrusion molding, as shown below:

[0123] Polyethylene with a molecular weight of 1.5 million was mixed with solvent oil to form a base film solution, wherein polyethylene accounted for 18% of the mass percentage of the base film solution. The base film solution was extruded using an extruder at a controlled extrusion rate of 300 kg / h to prepare an ultra-high molecular weight polyethylene base film.

[0124] Tests showed that the ultra-high molecular weight polyethylene film provided in Synthesis Example 1 had a tensile strength of 405 MPa, a puncture strength of 6.8 N, a heat shrinkage rate of 0.6% after being kept at 250°C for 1 hour, and a moisture content of 30 ppm after being baked at 100°C for 5 minutes.

[0125] Synthesis example 2

[0126] Synthesis Example 2 provides a base film prepared from polypropylene with a molecular weight of 1.6 million. The base film has a thickness of 10 μm, an average pore size of 0.08 μm, and a porosity of 45%.

[0127] The base film was prepared by a stretching-pore-forming method, as follows: Polypropylene was pretreated to remove moisture. The pretreated polypropylene was then extruded using an extruder to form a precursor film. The precursor film was heated to a stretching temperature, wherein the stretching temperature was lower than the melting point of the raw material, and the difference between the melting point and the stretching temperature was 5°C. At the stretching temperature, the precursor film was stretched to form the base film.

[0128] Tests showed that the ultra-high molecular weight polyethylene film provided in Synthesis Example 2 had a tensile strength of 400 MPa, a puncture strength of 6.5 N, a heat shrinkage rate of 0.65% after being kept at 250°C for 1 hour, and a moisture content of 50 ppm after being baked at 100°C for 5 minutes.

[0129] Synthesis example 3

[0130] Synthesis Example 3 provides a base film prepared from polyethylene terephthalate. The base film has a thickness of 10 μm, an average pore size of 0.06 μm, and a porosity of 35%.

[0131] The base film was prepared by a stretching-pore-forming method, as follows: Polyethylene terephthalate (PET) was pretreated to remove moisture. The pretreated PET was then extruded using an extruder to form a precursor film. The precursor film was heated to a stretching temperature, wherein the stretching temperature was lower than the melting point of the raw material, and the difference between the melting point and the stretching temperature was 10°C. At the stretching temperature, the precursor film was stretched to form the base film.

[0132] Tests showed that the ultra-high molecular weight polyethylene film provided in Synthesis Example 3 had a tensile strength of 402 MPa, a puncture strength of 6.7 N, a heat shrinkage rate of 0.6% after being kept at 250°C for 1 hour, and a moisture content of 30 ppm after being baked at 100°C for 5 minutes.

[0133] Synthesis example 4

[0134] Synthesis Example 4 provides a base film prepared from polyimide. The base film has a thickness of 10 μm, and its through-holes are straight through-holes with an average pore diameter of 0.8 μm. The multiple through-holes are arranged in a square matrix, and the porosity is 30%.

[0135] The base film was prepared by laser drilling, as follows: The raw materials for the base film were processed by melt extrusion to obtain a dense film material with a porosity of less than 5% and a pore size of less than 0.1 μm. Then, the dense film material was laser-etched using an ultrashort pulse laser to form multiple micropores, thus obtaining the base film.

[0136] Tests showed that the ultra-high molecular weight polyethylene film provided in Synthesis Example 4 had a tensile strength of 410 MPa, a puncture strength of 6.9 N, a heat shrinkage rate of 0.3% after being kept at 250°C for 1 hour, and a moisture content of 40 ppm after being baked at 100°C for 5 minutes.

[0137] Example 1

[0138] Example 1 provides a battery separator comprising: a base film provided in Synthesis Example 1 and an adhesive layer stacked on both sides of the base film. The adhesive layer comprises 9 wt% polymethyl methacrylate particles, 83 wt% boehmite particles, 5 wt% polymethyl methacrylate, and the balance sodium carboxymethyl cellulose. The polymethyl methacrylate particles have a particle size D50 of 5 μm, are arranged protruding relative to the adhesive layer, and have a protrusion height of 2 μm.

[0139] The preparation method of the battery separator is as follows:

[0140] Methyl methacrylate (MMA) is dispersed in a solvent and polymerized under the action of an initiator to obtain a liquid system containing organic colloidal particles. Boehmite particles, polymethyl methacrylate, and sodium carboxymethyl cellulose are added to the liquid system and stirred until homogeneous to obtain a slurry. The slurry is coated onto both sides of a base membrane and dried to form an adhesive layer on the base membrane surface, thus obtaining a battery separator.

[0141] Example 2

[0142] Example 2 provides a battery separator, the composition and preparation method of which can be found in Example 1.

[0143] The difference between Example 2 and Example 1 is that the base film used is the base film provided in Synthesis Example 2.

[0144] Example 3

[0145] Example 3 provides a battery separator, the composition and preparation method of which can be found in Example 1.

[0146] The difference between Example 3 and Example 1 is that the base film used is the base film provided in Synthesis Example 3.

[0147] Example 4

[0148] Example 4 provides a battery separator, the composition and preparation method of which can be found in Example 1.

[0149] The difference between Example 4 and Example 1 is that the base film used is the base film provided in Synthesis Example 4.

[0150] Comparative Example 1

[0151] Comparative Example 1 provides a battery separator, the composition and preparation method of which can be found in Example 1.

[0152] The difference between Comparative Example 1 and Example 1 is that the base film used is a polyethylene base film with a molecular weight of 500,000.

[0153] Comparative Example 2

[0154] Comparative Example 2 provides a battery separator. The difference between Comparative Example 2 and Example 1 is that the particle size D50 of both polymethyl methacrylate particles and boehmite particles is 1.5 μm, so that the adhesive layer is planar and the polymethyl methacrylate particles do not protrude from the surface of the adhesive layer.

[0155] Test case

[0156] The following performance tests were performed on the battery separators provided in Examples 1-4 and Comparative Examples 1-2. The test results are shown in Table 1.

[0157] (1) Bond strength

[0158] Multiple positive electrode sheets are spaced apart along the length of the battery separator on one side of the separator, and multiple negative electrode sheets are spaced apart along the length of the battery separator on the other side of the separator. The positive and negative electrode sheets are arranged alternately to form a stacked assembly. The stacked assembly is heated and rolled at 40°C, and the positive and negative electrode sheets are bonded to the battery separator to form a hot-pressed stacked assembly. The positive electrode sheet is a commercially available lithium manganese oxide positive electrode sheet, and the negative electrode sheet is a commercially available graphite negative electrode sheet.

[0159] First, the bonding strength between the negative electrode and the battery separator in the hot-pressed laminate assembly, as well as the bonding strength between the positive electrode and the battery separator, are measured.

[0160] Secondly, the hot-pressing stacking assembly is stacked using a stacking device. During this process, the stacking speed of the hot-pressing stacking assembly is controlled at 50m / min, and the alignment of the obtained hot-composite cells is observed.

[0161] (2) Heat shrinkage rate at 250℃@1h: The test method was performed according to GB / T36363-2018. Five battery separator samples were taken, and their dimensions before heating were measured. The samples were sandwiched between 6mm glass plates and placed in a 250℃ oven for 1 hour. The dimensions of the separator samples after heating were measured, and their heat shrinkage rate was calculated. The machine direction (MD) and transverse direction (TD) shrinkage rates of the separator samples were measured separately, and the units were %.

[0162] Table 1

[0163]

[0164]

[0165] As shown in Table 1, the battery separators provided in Examples 1-4, based on the use of improved base film and adhesive layer, exhibit excellent adhesive strength and high-temperature heat shrinkage performance. Furthermore, they enable the hot-pressed stacking assembly to be stacked according to a predetermined stacking route and speed, making the battery separators suitable for hot-pressing processes. In particular, Example 4 uses a polyimide base film, resulting in even better heat shrinkage performance.

[0166] Comparative Example 1 used a low molecular weight base film, which significantly deteriorated the high-temperature thermal shrinkage performance of the battery separator, thus affecting its heat resistance.

[0167] Despite using a high molecular weight base film, Comparative Example 2 suffered from significantly worse adhesive strength and hot-pressed stacking of the laminated assembly compared to Example 1 due to the use of a full-coat adhesive layer.

[0168] The above description is merely for the purpose of enabling those skilled in the art to understand the technical solutions of the present invention, and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A battery separator, characterized in that, The battery separator includes a base membrane, which is a single-layer structure or a multi-layer composite structure. The raw materials for preparing the base membrane include at least one of the following: polyethylene with a molecular weight greater than or equal to 1.5 million, polypropylene with a molecular weight greater than or equal to 1.5 million, polyethylene terephthalate, and polyimide.

2. The battery separator according to claim 1, characterized in that, The base film has multiple through holes, the axial direction of which is along the thickness direction of the base film.

3. The battery separator according to claim 2, characterized in that, The multiple through holes are arranged in a matrix.

4. The battery separator according to any one of claims 1-3, characterized in that, The base film has a thermal shrinkage rate of less than or equal to 1% at a temperature of 230℃-300℃.

5. The battery separator according to claim 4, characterized in that, The base film also has at least one of the following physical properties: the thickness of the base film is 1μm-20μm, the average pore size of the through-holes of the base film is less than or equal to 1μm, the porosity of the base film is 25%-85%, the tensile strength of the base film is greater than or equal to 400MPa, and the puncture strength of the base film is greater than or equal to 6.5N.

6. The battery separator according to any one of claims 1-5, characterized in that, The battery separator also includes an adhesive layer bonded to at least one surface of the base film; The adhesive layer includes a layered body and protruding adhesive particles on the surface of the layered body facing away from the base film. The adhesive layer contains organic adhesive particles, inorganic filler particles, adhesive and dispersant. The particle size of the organic adhesive particles is larger than that of the inorganic filler particles. Part of the surface of the organic adhesive particles is connected to the inorganic filler particles by the adhesive and cooperates to form the layered body. Another part of the organic adhesive particles protrudes to the surface of the layered body to form the protruding adhesive particles.

7. A method for preparing a battery separator, characterized in that, The battery separator is as described in any one of claims 1-6, and the method for preparing the battery separator includes: The preparation of a base film includes: preparing the base film by means of extrusion molding, stretching pore forming or laser drilling, depending on the raw materials used for the preparation of the base film.

8. The method for preparing the battery separator according to claim 7, characterized in that, When the raw material for preparing the base film is polyethylene with a molecular weight greater than or equal to 1.5 million or polypropylene with a molecular weight greater than or equal to 1.5 million, the base film is prepared using the extrusion molding method, including: The raw materials for preparing the base film are mixed with solvent oil to form a base film solution, wherein the raw materials for preparing the base film account for 15%-20% of the mass percentage of the base film solution; The base film is prepared by extruding the base film liquid using an extruder, wherein the extrusion rate is controlled to be 250 kg / h-350 kg / h.

9. The method for preparing the battery separator according to claim 7, characterized in that, When the raw material for preparing the base film is polyethylene, polypropylene, polyethylene terephthalate, or polyimide with a molecular weight greater than or equal to 1.5 million, the base film is prepared using the stretching pore-forming method, including: The raw materials for preparing the base film are pretreated, and the pretreated raw materials are extruded and molded using an extruder to form a precursor film. The precursor film is heated to a stretching temperature, wherein the stretching temperature is lower than the melting point of the raw material. The precursor film is stretched at the specified stretching temperature to prepare the base film.

10. The method for preparing the battery separator according to claim 7, characterized in that, When the raw material for preparing the base film is polyethylene, polypropylene, polyethylene terephthalate, or polyimide with a molecular weight greater than or equal to 1.5 million, the preparation of the base film using the laser drilling method includes: A dense membrane material is prepared from the raw materials for the base membrane by melt extrusion or casting, wherein the porosity of the dense membrane material is less than 5% and the pore size is less than 0.1 μm; A laser source is used to perform laser etching on the dense film material to form multiple micropores on the dense film material, thereby preparing a base film.

11. The method for preparing the battery separator according to any one of claims 7-10, characterized in that, The method for preparing the battery separator further includes: The slurry corresponding to the adhesive layer is coated onto at least one side of the base film, and then dried to prepare the adhesive layer on the surface of the base film.

12. A thermally composite battery cell, characterized in that, The thermal composite cell includes multiple positive electrode plates, multiple negative electrode plates, and a battery separator as described in any one of claims 1-6; The negative electrode and the positive electrode are stacked alternately, and adjacent negative electrode and positive electrode are separated by the battery separator. The battery separator is bonded to the corresponding electrode by an adhesive layer.

13. A secondary battery, characterized in that, The secondary battery includes: a casing, an electrolyte contained inside the casing, and a thermally composite battery cell, wherein the thermally composite battery cell is as described in claim 12.

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