Battery core package and preparation method thereof

Abstract

The invention provides a battery core package and a preparation method thereof, and belongs to the technical field of secondary batteries. The preparation method comprises the following steps: placing the battery cell pole group on the mylar film, enabling the bottom surface of the battery cell pole group to be attached to the mylar film, and carrying out primary hot-pressing fixation, so that the bottom surface of the battery cell pole group is adhered to the mylar film; turning over the mylar film to enable the mylar film to wrap the bottom surface, the top surface and one side surface of the battery cell pole group, and performing secondary hot-pressing fixation to enable the top surface of the battery cell pole group to be adhered to the mylar film; and folding the mylar film to coat the other side surface of the battery core pole group with the mylar film, and fixing to complete the preparation of the battery core pack. According to the invention, the problems of malposition of the Mylar film, poor hollowing, scratch in a shell and poor welding caused by malposition of the Mylar film after the Mylar film and the battery pole group are bonded and fixed are solved.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery technology, specifically relating to a battery cell pack and its preparation method. Background Technology

[0002] With the increasing demand for lithium-ion batteries, ultra-large capacity cells have become an industry trend. Coating the cells is a crucial safety procedure in the battery manufacturing process of the new energy lithium battery industry. The Mylar film wrapped around the cells prevents them from being scratched during installation in the battery casing and also acts as insulation, preventing direct contact between the cells and the battery's metal casing, which could lead to safety accidents. Research on Mylar films is becoming increasingly in-depth.

[0003] CN120230336A discloses a porous Mylar membrane suitable for battery structures, comprising a membrane body and multiple pores disposed on the membrane body; the porosity of the multiple pores is 10%~50%; the multiple pores are uniformly distributed or have a gradient distribution. This porous Mylar membrane significantly increases the surface area of ​​the membrane, forming efficient heat dissipation channels. Simultaneously, the electrolyte filled in the pores also facilitates heat transfer and diffusion, enabling more timely dissipation of heat generated during battery operation, effectively controlling the rise in internal battery temperature, effectively maintaining battery stability, reducing battery performance degradation and safety risks caused by overheating, and extending battery life.

[0004] However, as cell capacity increases, the length, width, and thickness of the electrode assembly become larger, making it difficult to align the Mylar film (polyester film) wrapping. In current conventional designs, the Mylar film functions as an insulating material, wrapping around the outer layer of the cell electrode assembly to prevent contact between the battery casing and the cell pack, thus avoiding side reactions and preventing aluminum casing corrosion. After wrapping, the sides are secured with adhesive, and the top is fixed with a plastic support by heat fusion. During the wrapping process, defects such as misalignment, bulging, and wrinkles of the Mylar film after wrapping the electrode assembly occur, causing scratches during casing insertion, leading to poor welding. Furthermore, gaps exist where the Mylar film cannot adhere properly to the electrode assembly, resulting in low utilization of the internal space.

[0005] Therefore, how to provide a novel, high-performance battery cell pack coated with Mylar film and its preparation method to solve problems such as Mylar film misalignment, poor air bulging, scratching during casing insertion, and poor welding caused by Mylar film misalignment after bonding and fixing Mylar film to battery electrode assembly has become an urgent technical problem to be solved. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a battery cell pack and its preparation method. By designing the preparation method and the specific composition of the Mylar membrane, the present invention yields a high-performance battery cell pack coated with a Mylar membrane, solving problems such as Mylar membrane misalignment, poor air pockets, scratches during casing insertion, and poor welding caused by Mylar membrane misalignment after bonding and fixing the Mylar membrane to the battery electrode assembly.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing a battery cell pack, the method comprising the following steps:

[0009] (1) Place the battery cell electrode assembly on the Mylar film, with the bottom surface of the battery cell electrode assembly attached to the Mylar film, and perform the first hot pressing fixation to make the bottom surface of the battery cell electrode assembly adhere to the Mylar film.

[0010] (2) Fold the Mylar film to cover the bottom, top and one side of the cell electrode assembly, and perform a second hot pressing to fix it so that the top surface of the cell electrode assembly is bonded to the Mylar film.

[0011] (3) Fold the Mylar membrane so that it covers the other side of the cell electrode assembly and fix it to complete the preparation of the battery cell pack;

[0012] The Mylar membrane comprises a substrate and an adhesive layer that are bonded together;

[0013] The adhesive layer is made of any one or a combination of at least two of polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), or acrylates.

[0014] This invention designs a method for preparing a battery cell pack and the specific composition of the Mylar membrane. Through stepwise hot pressing, the adhesive layer in the Mylar membrane is tightly bonded to the surface of the battery cell electrode assembly under hot pressing, forming an adhesive fixation. This results in a high-performance battery cell pack covered with the Mylar membrane, solving problems such as Mylar membrane misalignment, poor air bulging, scratches during casing insertion, and poor welding caused by Mylar membrane misalignment after the Mylar membrane is bonded and fixed to the battery electrode assembly.

[0015] In this invention, by designing the structure of the Mylar membrane and further designing the material of the adhesive layer, a Mylar membrane with excellent performance was prepared. When the Mylar membrane provided by this invention is used to cover the battery electrode assembly, the adhesive layer bonds the battery electrode assembly at a hot-pressing temperature. Under high pressure, the Mylar membrane and the surface of the electrode assembly are tightly bonded, which solves the problems of misalignment, poor air bubbling, scratching during the bonding and fixing of the Mylar membrane and the battery electrode assembly, as well as poor welding caused by misalignment of the Mylar membrane.

[0016] PVDF has both polar and nonpolar groups. Using PVDF as the adhesive layer material allows the Mylar membrane to undergo intermolecular diffusion and chemical bonding (such as CF bond interactions) with the adhesive layer on the surface of the battery cell electrode assembly (the adhesive layer material includes PVDF and PMMA, the same below). During hot pressing (90℃ is close to the glass transition temperature and melting point of PVDF, but there will also be some softening at 90℃), the molecular chain movement of the PVDF adhesive layer material of the Mylar membrane intensifies, forming an interpenetrating network structure with the mixed adhesive. Contact under high pressure can enhance the bonding.

[0017] PMMA has a glass transition temperature (Tg) between 85-105℃. Therefore, at 90℃, PMMA softens or even flows. When PMMA is used as the adhesive layer material for the Mylar membrane, it exhibits good compatibility with the adhesive layer on the surface of the separator in the battery cell electrode assembly because the separator contains PMMA. Thus, under these conditions, the Mylar membrane adhesive layer adheres to the separator on the electrode assembly surface, and the adhesion between the two is relatively strong.

[0018] When acrylate is used as the adhesive layer material of the Mylar membrane, it exhibits certain compatibility with both PVDF and PMMA, the membrane materials of the battery cell electrode assembly, especially PMMA (since both acrylate and PMMA are acrylic polymers). Under hot-pressing conditions, the acrylate softens and diffuses with the membrane materials of the battery cell electrode assembly, forming an adhesion.

[0019] It should be noted that the "one side" mentioned in step (2) and the "other side" mentioned in step (3) are arranged opposite to each other and are located in opposite positions.

[0020] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The purpose and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.

[0021] Preferably, the adhesive layer is made of a combination of polyvinylidene fluoride and polymethyl methacrylate.

[0022] This invention further optimizes the use of a combination of PVDF and PMMA as the adhesive layer material, improving the flexibility and chemical stability of the Mylar membrane. PVDF exhibits strong stability, resisting degradation or performance decline in strong acid or alkali environments, making it suitable for highly corrosive environments. PMMA possesses good brittleness, softening at low temperatures (40°C-50°C) and exhibiting a certain degree of viscosity at room temperature or under specific pressure. Therefore, the adhesive layer formed using the combination of PVDF and PMMA combines the advantages of high stability and good brittleness, while also exhibiting a certain degree of viscoelasticity at room temperature or under specific pressure, thus enabling excellent interfacial bonding with the PE separator and the Mylar membrane.

[0023] For PVDF, heating it above or near its softening point, such as to 70°C~90°C, allows it to bond with both polar and non-polar materials. PMMA, due to its inherent viscosity at room temperature, bonds with non-polar materials at lower temperatures. Considering membrane safety (excessive membrane temperature leads to thermal shrinkage, and lithium batteries are typically kept below 100°C) and energy consumption (reducing hot-pressing temperature decreases energy consumption), this invention further prefers a combination of PVDF and PMMA as the adhesive layer material. This mixing improves the compatibility of PVDF or PMMA with non-polar materials, thereby achieving bonding at lower temperatures. Furthermore, the adhesive layer is cost-effective.

[0024] Mylar membranes prepared by using a blend of PVDF and PMMA as the binder layer can further improve the flexibility and adhesion of the binder layer. During the hot pressing process of the Mylar membrane onto the battery cell electrode assembly, the surface temperature of the hot pressing heating plate in contact with the battery cell electrode assembly gradually increases. When the surface temperature of the Mylar membrane rises to 40-50℃, the PMMA softens and adheres first, playing a preliminary fixing role. At 70-90℃, the PVDF further adheres, and at this point, the Mylar membrane and the separator on the surface of the battery cell electrode assembly form a precise fit, further improving the bonding effect of the hot pressing of the battery cell electrode assembly and the Mylar membrane.

[0025] Preferably, the mass ratio of polyvinylidene fluoride to polymethyl methacrylate is 1:(1-9), for example, it can be 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8 or 1:9, etc.

[0026] This invention further improves the overall performance of Mylar membrane and the overall performance of battery cell pack by controlling the mass ratio of polyvinylidene fluoride and polymethyl methacrylate within a specific range.

[0027] Preferably, the polyvinylidene fluoride has a weight-average molecular weight of 100,000 to 1,000,000 (e.g., 100,000, 200,000, 300,000, 400,000, 500,000, 600,000, 700,000, 800,000, 900,000, or 1,000,000), a glass transition temperature ≥ -40°C (e.g., -40°C, -37°C, -35°C, -33°C, -30°C, -28°C, -25°C, -22°C, or -20°C), and a melt viscosity of 60 KP to 90 KP (e.g., 60 KP, 63 KP, 66 KP, 68 KP, 70 KP, 72 KP, 75 KP, 78 KP, 81 KP, 84 KP, 86 KP, 88 KP, or 90 KP).

[0028] In this invention, the weight-average molecular weight of polyvinylidene fluoride is tested according to standard ASTM D4001, the glass transition temperature is tested according to standard ASTM E1356, and the melt viscosity is tested according to standard ASTM D3835.

[0029] Preferably, the polymethyl methacrylate has a weight-average molecular weight of 50,000 to 1,000,000 (e.g., 50,000, 100,000, 200,000, 300,000, 400,000, 500,000, 600,000, 700,000, 800,000, 900,000, or 1,000,000), and a glass transition temperature of 85°C to 105°C (e.g., 85°C, 88°C, 90°C, 92°C, 94°C, 96°C, 98°C, 100°C, 102°C, 104°C, or 105°C).

[0030] Preferably, the weight-average molecular weight of the acrylate is 30,000 to 100,000 (e.g., it can be 30,000, 40,000, 50,000, 60,000, 70,000, 80,000, 90,000 or 100,000, etc.), and the glass transition temperature is ≥-50℃ (e.g., it can be -50℃, -40℃, -30℃, -20℃, -10℃, 0℃, 10℃, 20℃ or 30℃, etc.).

[0031] Preferably, the base layer comprises a PP film or a PET film.

[0032] The thickness of the base layer is 50-150 μm, for example, it can be 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm or 150 μm, etc.

[0033] Preferably, the thickness of the adhesive layer is 1-2 μm, for example, it can be 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm or 2 μm, etc.

[0034] This invention prepares a high-performance Mylar film by controlling the thickness of the adhesive layer within a specific range. If the adhesive layer is too thin, the resulting Mylar film will have poor adhesion to the battery cell electrode assembly, making it difficult to solve problems such as misalignment, poor air pockets, scratches during casing installation, and poor welding caused by misalignment of the Mylar film during bonding and fixing to the battery electrode assembly. If the adhesive layer is too thick, it will affect the thickness of the Mylar film after assembly with the electrode assembly, affecting casing installation. Controlling the thickness of the adhesive layer to 1-2 μm ensures the required adhesion without affecting the overall thickness of the electrode assembly.

[0035] This invention does not impose any special limitations on the preparation method of the Mylar membrane; commonly used preparation methods in the art are applicable. Exemplary methods include, but are not limited to, the following steps in the preparation method of the Mylar membrane:

[0036] A solution containing an adhesive layer material is applied to the substrate and dried to obtain the Mylar film.

[0037] Preferably, the coating method includes spraying.

[0038] Preferably, the solvent in the solution containing the adhesive layer material includes N-methylpyrrolidone (NMP) and / or dimethylformamide (DMF).

[0039] It should be noted that the present invention does not impose any special restrictions on the mass concentration of the binder material in the solution containing the binder material, and it can be adjusted according to the actual production situation. The mass concentration range is exemplarily including, but not limited to, 10-20%, such as 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 20%, etc.

[0040] It should also be noted that the present invention does not impose any special restrictions on the drying temperature and time, which can be adjusted according to the actual production situation.

[0041] Preferably, the temperature for the first hot pressing is 70℃ to 90℃, for example, it can be 70℃, 72℃, 74℃, 76℃, 78℃, 80℃, 82℃, 84℃, 86℃, 88℃ or 90℃.

[0042] Preferably, the pressure of the first hot pressing is 10 to 14 tons, for example, it can be 10 tons, 10.5 tons, 11 tons, 11.5 tons, 12 tons, 12.5 tons, 13 tons, 13.5 tons or 14 tons.

[0043] Preferably, the time for the first hot pressing is 5 s to 15 s, for example, 5 s, 6 s, 7 s, 8 s, 9 s, 10 s, 11 s, 12 s, 13 s, 14 s or 15 s.

[0044] Preferably, the temperature for the second hot pressing is 70°C to 90°C, for example, it can be 70°C, 72°C, 74°C, 76°C, 78°C, 80°C, 82°C, 84°C, 86°C, 88°C or 90°C.

[0045] Preferably, the pressure of the second hot pressing is 10 to 14 tons, for example, it can be 10 tons, 10.5 tons, 11 tons, 11.5 tons, 12 tons, 12.5 tons, 13 tons, 13.5 tons or 14 tons.

[0046] Preferably, the second hot pressing time is 5 s to 15 s, for example, it can be 5 s, 6 s, 7 s, 8 s, 9 s, 10 s, 11 s, 12 s, 13 s, 14 s or 15 s.

[0047] Preferably, the fixing method described in step (3) includes, but is not limited to, fixing with tape.

[0048] In a second aspect, the present invention provides a battery pack, the battery pack comprising a battery electrode assembly and a Mylar film covering the bottom surface, top surface and two sides of the battery electrode assembly.

[0049] It should be noted that the two sides are in relative positions.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] (1) By designing the preparation method of the battery cell pack and the Mylar film, the present invention obtains a high-performance battery cell pack covered with Mylar film, which solves the problems of Mylar film misalignment, poor air bulging, scratching during casing and poor welding caused by Mylar film misalignment after the Mylar film is bonded and fixed to the battery electrode assembly.

[0052] (2) By designing the structure of the Mylar membrane and further designing the adhesive layer, the present invention has prepared a Mylar membrane with excellent performance, and then prepared a battery pack with excellent performance. The probability of problems such as Mylar membrane misalignment, poor air bulging, scratching during casing insertion, and poor welding caused by Mylar membrane misalignment in the battery pack is ≤0.1%. Moreover, the Mylar membrane provided by the present invention has a moderate peel force of 0.8-2.0 N / 25mm.

[0053] (3) The present invention further preferably includes a combination of polyvinylidene fluoride and polymethyl methacrylate as the adhesive layer material, which improves the flexibility and chemical stability of the Mylar membrane and further improves the overall performance of the battery pack. Attached Figure Description

[0054] Figure 1 This is a schematic diagram of the battery cell pack manufacturing process provided in Embodiment 1 of the present invention;

[0055] Figure 2 This is a schematic diagram of the structure of the Mylar membrane provided in Embodiment 1 of the present invention;

[0056] Among them, 1-Mylar film, 11-base layer, 12-adhesive layer, 2-cell electrode assembly, 21-cell electrode assembly tab, 3-adhesive tape. Detailed Implementation

[0057] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be considered as specific limitations thereof.

[0058] The sources of some components in the following examples and comparative examples are described in Table 1 below:

[0059] Table 1

[0060]

[0061] The Mylar membrane used in the following examples and comparative examples was prepared by the following method: the material of the adhesive layer was dissolved in N-methylpyrrolidone to obtain a solution with a mass concentration of 15%, the solution was coated on the substrate, and dried to obtain the Mylar membrane.

[0062] Example 1

[0063] This embodiment provides a battery cell pack and its preparation method. A schematic diagram of the battery cell pack preparation process is shown below. Figure 1 As shown, the details are as follows:

[0064] (1) Place the cell electrode assembly 2 (including the tab 21) on the Mylar membrane, with the bottom surface of the cell electrode assembly 2 in contact with the Mylar membrane 1. Under the conditions of 80°C and 12 tons of pressure, perform the first hot pressing fixation for 10 seconds to make the bottom surface of the cell electrode assembly 2 adhere to the Mylar membrane 1.

[0065] (2) Fold the Mylar film 1 so that the Mylar film 1 covers the bottom, top and one side of the battery cell electrode assembly 2. Under the conditions of 80°C and 12 tons of pressure, perform a second hot pressing for 10 seconds to make the top surface of the battery cell electrode assembly 2 adhere to the Mylar film 1.

[0066] (3) Fold the Mylar film 1 so that the Mylar film 1 covers the other side of the cell electrode assembly 2, and fix it with tape to complete the preparation of the battery cell pack;

[0067] The structural schematic diagram of the Mylar membrane 1 is shown below. Figure 2 As shown, it includes a base layer 11 and an adhesive layer 12 that are bonded together;

[0068] The base layer 11 is a PP film;

[0069] The adhesive layer 12 is composed of polyvinylidene fluoride and polymethyl methacrylate in a mass ratio of 1:4, and has a thickness of 1.5 μm.

[0070] Example 2

[0071] This embodiment provides a battery cell pack and its preparation method. The specific preparation method of the battery cell pack is as follows:

[0072] (1) Place the battery cell electrode assembly (including the tabs) on the Mylar membrane, with the bottom surface of the battery cell electrode assembly in contact with the Mylar membrane. Under the conditions of 88°C and 14 tons of pressure, perform the first hot pressing fixation for 15 seconds to make the bottom surface of the battery cell electrode assembly adhere to the Mylar membrane.

[0073] (2) Fold the Mylar film so that the Mylar film covers the bottom, top and one side of the battery cell electrode assembly. Under the conditions of 88°C and 14 tons of pressure, perform a second hot pressing for 15 seconds to make the top surface of the battery cell electrode assembly adhere to the Mylar film.

[0074] (3) Fold the Mylar membrane so that it covers the other side of the cell electrode assembly, and fix it with tape to complete the preparation of the battery cell pack;

[0075] The Mylar membrane includes a substrate and an adhesive layer that are bonded together.

[0076] The base layer is a PET film;

[0077] The adhesive layer is composed of polyvinylidene fluoride and polymethyl methacrylate in a mass ratio of 1:7, and has a thickness of 1 μm.

[0078] Example 3

[0079] This embodiment provides a battery cell pack and its preparation method. The specific preparation method of the battery cell pack is as follows:

[0080] (1) Place the battery cell electrode assembly (including the tabs) on the Mylar membrane, with the bottom surface of the battery cell electrode assembly in contact with the Mylar membrane. Under the conditions of 75°C and 10 tons of pressure, perform the first hot pressing fixation for 6 seconds to make the bottom surface of the battery cell electrode assembly adhere to the Mylar membrane.

[0081] (2) Fold the Mylar film so that the Mylar film covers the bottom, top and one side of the battery cell electrode assembly. Under the conditions of 75°C and 10 tons of pressure, perform a second hot pressing fix for 6 seconds to make the top surface of the battery cell electrode assembly adhere to the Mylar film.

[0082] (3) Fold the Mylar membrane so that it covers the other side of the cell electrode assembly, and fix it with tape to complete the preparation of the battery cell pack;

[0083] The Mylar membrane includes a substrate and an adhesive layer that are bonded together.

[0084] The base layer is a PP film or a PET film;

[0085] The adhesive layer is made of acrylate and has a thickness of 2 μm.

[0086] Example 4

[0087] This embodiment provides a battery cell pack and its preparation method. The only difference from Embodiment 1 is that the adhesive layer is composed of polyvinylidene fluoride and polymethyl methacrylate in a mass ratio of 1:1. Other conditions are the same as in Embodiment 1.

[0088] Example 5

[0089] This embodiment provides a battery pack and its preparation method. The only difference from Embodiment 1 is that the adhesive layer is composed of polyvinylidene fluoride and polymethyl methacrylate in a mass ratio of 1:9. Other conditions are the same as in Embodiment 1.

[0090] Example 6

[0091] This embodiment provides a battery pack and its preparation method. The only difference from Embodiment 1 is that the adhesive layer is composed of polyvinylidene fluoride and polymethyl methacrylate in a mass ratio of 1:0.5. Other conditions are the same as in Embodiment 1.

[0092] Example 7

[0093] This embodiment provides a battery pack and its preparation method. The only difference from Embodiment 1 is that the adhesive layer is composed of polyvinylidene fluoride and polymethyl methacrylate in a mass ratio of 1:12. Other conditions are the same as in Embodiment 1.

[0094] Example 8

[0095] This embodiment provides a battery cell pack and its preparation method. The only difference from Embodiment 1 is that the material of the adhesive layer is polyvinylidene fluoride, and the other conditions are the same as in Embodiment 1.

[0096] Example 9

[0097] This embodiment provides a battery pack and its preparation method. The only difference from Embodiment 1 is that the material of the adhesive layer is polymethyl methacrylate, and the other conditions are the same as in Embodiment 1.

[0098] Example 10

[0099] This embodiment provides a battery cell pack and its preparation method. The only difference from Embodiment 1 is that the adhesive layer is composed of polyvinylidene fluoride and acrylate in a mass ratio of 1:4. Other conditions are the same as in Embodiment 1.

[0100] Example 11

[0101] This embodiment provides a battery pack and its preparation method. The only difference from Embodiment 1 is that the adhesive layer is composed of polymethyl methacrylate and acrylate in a mass ratio of 1:4. Other conditions are the same as in Embodiment 1.

[0102] Comparative Example 1

[0103] This comparative example provides a battery cell pack and its preparation method, which differs from Example 1 only in that the preparation method of the battery cell pack is as follows:

[0104] (1) Place the battery cell electrode assembly (including the tabs) on the Mylar membrane, with the bottom surface of the battery cell electrode assembly in contact with the Mylar membrane. Fold the Mylar membrane so that it covers the bottom surface, top surface and one side of the battery cell electrode assembly. Under the conditions of 70℃ ~ 90℃ and pressure of 10 tons ~ 14 tons, perform hot pressing for 5 s ~ 15 s to make the top surface of the battery cell electrode assembly adhere to the Mylar membrane.

[0105] (2) Fold the Mylar membrane so that it covers the other side of the cell electrode assembly, and fix it with tape to complete the preparation of the battery cell pack;

[0106] The specific composition of the Mylar membrane is the same as that in Example 1.

[0107] Comparative Example 2

[0108] This comparative example provides a battery cell pack and its preparation method. The only difference from Example 1 is that the Mylar film is a PP film, and the other conditions are the same as in Example 1.

[0109] Comparative Example 3

[0110] This comparative example provides a battery cell pack and its preparation method. The only difference from Example 1 is that the material of the adhesive layer is polyolefin, and the other conditions are the same as in Example 1.

[0111] The battery electrode assembly in the battery cell pack used in the above embodiments and comparative examples is composed as follows:

[0112] The cathode material was selected according to the following mass fractions: lithium iron phosphate 95.5%, CNT (carbon nanotubes, commercially available) 0.4%, PVDF (purchased from Shanghai Huiping Chemical, grade HSV1810) 1.5%, lithium iron phosphate 2%, and SP (conductive carbon black, commercially available) 0.6%.

[0113] The negative electrode material is selected according to the following mass fractions: C (graphite) 96.5%, SP (conductive carbon black, commercially available) 0.6%, CMC (carboxymethyl cellulose) 1.1%, and SBR (styrene-butadiene rubber binder, commercially available) 1.8%.

[0114] The membrane is 13 μm thick and is a commercially available product.

[0115] Aluminum foil with a thickness of 13 μm, and copper foil with a thickness of 8 μm;

[0116] The preparation method of the battery electrode assembly is as follows:

[0117] The positive electrode material and the negative electrode material are mixed evenly according to the formula. The positive electrode material is coated on aluminum foil and the negative electrode material is coated on copper foil. After drying, cold pressing and sheet forming, positive electrode sheets and negative electrode sheets are obtained respectively. The positive electrode sheets, negative electrode sheets and separator are then stacked into bare cells by stacking. After the negative electrode tabs are welded, the battery electrode assembly to be coated with Mylar film is obtained.

[0118] The performance of the battery cell packs provided in the above embodiments and comparative examples was tested, and the specific test methods are as follows:

[0119] (1) Mylar membrane misalignment defect rate: Record the number (X) of battery cells in 1000 cells where Mylar membrane misalignment occurs between the Mylar membrane and the battery electrode assembly. Mylar membrane misalignment defect rate = X / 1000×100%;

[0120] (2) Mylar membrane void defect rate: Record the number (Y) of battery cell packs in 1000 cell packs where Mylar membrane void occurs between the Mylar membrane and the battery electrode assembly. Mylar membrane void defect rate = Y / 1000×100%;

[0121] (3) Defect rate caused by edge welding of Mylar membrane: Record the number (Z) of battery cell packs in 1000 cell packs where edge welding of Mylar membrane caused a hole between the Mylar membrane and the battery electrode group. Defect rate caused by edge welding of Mylar membrane = Z / 1000×100%;

[0122] (4) Peel force between the Mylar film and the battery electrode assembly:

[0123] ① Sample preparation

[0124] Cutting dimensions: Typically, a sample (Mylar film) with a width of 25mm and a length of 150mm is used.

[0125] Bonding location: The Mylar film is bonded to the battery electrode assembly according to the actual process, and the cured sample is used as a test sample.

[0126] ② Testing equipment

[0127] Tensile testing machine; Fixture: 180° peel fixture;

[0128] ③ Test steps

[0129] The unbonded end of the sample is fixed to the fixture of the testing machine, while the bonded part is suspended.

[0130] Set the peeling angle (usually 180°) and peeling speed (e.g., 300 mm / min).

[0131] Start the equipment and record the maximum force value during the peeling process (unit: N / 25mm).

[0132] The performance test results are shown in Table 2 below:

[0133] Table 2

[0134]

[0135] This invention, through the preparation method of the battery cell pack and the design of the Mylar membrane, yields a high-performance battery cell pack coated with a Mylar membrane. It solves problems such as Mylar membrane misalignment, poor air pockets, scratches during casing insertion, and welding defects caused by Mylar membrane misalignment that occur after the Mylar membrane is bonded and fixed to the battery electrode assembly. The probability of these problems occurring in the battery cell pack is ≤0.1%. Furthermore, the Mylar membrane provided by this invention has a moderate peel force of 0.8-2.0 N / 25mm.

[0136] As can be seen from the comparison of Examples 1 and Examples 4-11, the present invention improves the flexibility and chemical stability of the Mylar membrane by using a combination of polyvinylidene fluoride and polymethyl methacrylate as the adhesive layer material, and further designs the mass ratio of polyvinylidene fluoride and polymethyl methacrylate within a specific range. The peel force is moderate, at 1.5-2.0 N / 25mm, which further improves the overall performance of the battery cell pack. This results in a 0% probability of problems such as Mylar membrane misalignment, poor air pockets, scratches during casing insertion, and poor welding caused by Mylar membrane misalignment in the battery cell pack.

[0137] As can be seen from the comparison of Examples 1-11 and Comparative Examples 1-3, the present invention, through the design of the preparation method and the structure and specific selection of the Mylar membrane, and further through the design of the adhesive layer, has prepared a Mylar membrane and battery core pack with excellent performance, and solved the problems of Mylar membrane misalignment, poor bulging, scratching during the bonding and fixing of Mylar membrane and battery electrode assembly, as well as poor welding caused by Mylar membrane misalignment.

[0138] The applicant declares that the detailed process flow of this invention is illustrated by the above embodiments, but this invention is not limited to the above detailed process flow, that is, it does not mean that this invention must rely on the above detailed process flow to be implemented. Those skilled in the art should understand that any improvements to this invention, equivalent substitutions of raw materials for the product of this invention, addition of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of this invention.

Claims

1. A method for preparing a battery cell pack, characterized in that, The preparation method includes the following steps: (1) Place the battery cell electrode assembly on the Mylar film, with the bottom surface of the battery cell electrode assembly attached to the Mylar film, and perform the first hot pressing fixation to make the bottom surface of the battery cell electrode assembly adhere to the Mylar film. (2) Fold the Mylar film to cover the bottom, top and one side of the cell electrode assembly, and perform a second hot pressing to fix it so that the top surface of the cell electrode assembly is bonded to the Mylar film. (3) Fold the Mylar membrane so that it covers the other side of the cell electrode assembly and fix it to complete the preparation of the battery cell pack; The Mylar membrane comprises a substrate and an adhesive layer that are bonded together; The adhesive layer is made of any one or a combination of at least two of polyvinylidene fluoride, polymethyl methacrylate, or acrylate.

2. The preparation method according to claim 1, characterized in that, The adhesive layer is made of a combination of polyvinylidene fluoride and polymethyl methacrylate.

3. The preparation method according to claim 2, characterized in that, The mass ratio of polyvinylidene fluoride to polymethyl methacrylate is 1:(1-9).

4. The preparation method according to claim 1, characterized in that, The polyvinylidene fluoride has a weight-average molecular weight of 100,000 to 1,000,000, a glass transition temperature of ≥-40℃, and a melt viscosity of 60 KP to 90 KP.

5. The preparation method according to claim 1, characterized in that, The polymethyl methacrylate has a weight-average molecular weight of 50,000 to 1,000,000 and a glass transition temperature of 85°C to 105°C.

6. The preparation method according to claim 1, characterized in that, The acrylate has a weight-average molecular weight of 30,000 to 100,000 and a glass transition temperature of ≥-50℃.

7. The preparation method according to claim 1, characterized in that, The base layer includes PP film or PET film; And / or, the thickness of the base layer is 50-150 μm; And / or, the thickness of the adhesive layer is 1-2 μm.

8. The preparation method according to claim 1, characterized in that, The temperature for the first hot pressing is 70℃ ~ 90℃; And / or, the pressure of the first hot pressing is 10 to 14 tons; And / or, the time for the first hot pressing is 5 s to 15 s.

9. The preparation method according to claim 1, characterized in that, The temperature for the second hot pressing is 70℃ ~ 90℃; And / or, the pressure of the second hot pressing is 10 to 14 tons; And / or, the second hot pressing fixation time is 5 s to 15 s.

10. A battery cell pack, characterized in that, The battery pack includes a battery cell electrode assembly and a Mylar membrane covering the bottom, top, and two sides of the battery cell electrode assembly.

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