Cylindrical battery core internal puncture-resistant protective film layer for electric bicycle and use method of cylindrical battery core internal puncture-resistant protective film layer

Abstract

The invention relates to the technical field of lithium battery safety, in particular to an anti-puncture protective film layer inside a cylindrical battery cell for an electric bicycle and a using method thereof.The anti-puncture protective film layer is internally attached to the inner surface of a battery cell shell and comprises a protective film body which is of a double-layer structure and comprises a resin protective layer serving as an inner surface layer and a hot melting bonding layer serving as an outer surface layer; the resin protection layer is used for being in direct contact with electrolyte in the battery cell, and the hot melting bonding layer is used for being bonded and fixed with the inner wall of the battery cell shell; the resin protective layer is a blend of POE (Polyolefin Elastomer) resin and PP (Polypropylene) resin, and the hot-melt bonding layer is a blend of a maleic anhydride modified polyolefin hot-melt adhesive and POE resin; when the cell shell is penetrated by a puncture object, the protective film body can move along with the puncture object and cover the surface of the puncture object, so that the puncture object is separated from the positive and negative electrodes of the cell to reduce the risk of internal short circuit. Compared with the prior art, the anti-puncture protective film layer in the cylindrical battery cell for the electric bicycle and the use method of the anti-puncture protective film layer can wrap the surface of a puncture object when the battery cell is punctured, so that the puncture object is separated from the positive electrode and the negative electrode of the battery cell, the short circuit of the battery cell is prevented, and the problems of fire and explosion are further prevented.

Description

Technical Field This invention relates to the field of lithium battery safety technology, and in particular to the puncture-resistant protective film layer inside cylindrical battery cells for electric bicycles and its application method. Background Technology In recent years, safety standards for electric bicycle batteries have become increasingly stringent. The "Safety Technical Specifications for Electric Bicycles" (GB 17761-2024), released in 2024, imposed stricter requirements on battery management; simultaneously, the "Safety Technical Specifications for Lithium-ion Batteries for Electric Bicycles" (GB 43854-2024) also set new safety performance requirements for lithium-ion batteries in electric bicycles. Currently, regardless of whether lithium-ion batteries on the market use stacking or winding processes, the internal structure of the cells is very soft and fragile. Once the external separator fails, short circuits and fires can easily occur. For electric bicycles, due to their weight limitations, it is impossible to indefinitely add external protective layers, and it is also not economically feasible. Therefore, how to improve the cells themselves to prevent fires and explosions when damaged has become an urgent problem to be solved. Solid-state battery solutions hold promise for solving these problems in the future, but their commercial mass production will take time; therefore, improvements must be made to the existing battery system.

[0001] Several attempts have been made in the prior art. For example, CN106025108A discloses a puncture-resistant aluminum-plastic film, which improves the puncture resistance of the battery packaging film by introducing polytetrafluoroethylene propylene material and a warp and weft textile structure. However, this solution does not fundamentally solve the problem of combustion and explosion caused by internal short circuits after the battery cell is punctured. Another solution, CN109802068A, improves the battery casing by filling the casing interlayer with a core layer having negative Poisson bit properties. This core layer uses a star-shaped structure to provide buffering force, which can prevent the casing from being punctured or severely deformed, achieving a better puncture and compression protection effect. However, due to the addition of a thicker interlayer, this solution inevitably occupies battery volume, significantly reducing the energy density in the same volume. It is only suitable for battery cell designs with extreme safety requirements and is not economically viable.

[0002] Besides the aforementioned new solutions, the conventional approach is to coat the cell surface with an insulating film or spray an insulating adhesive layer. While this provides some electrical insulation, it does not offer further short-circuit protection on the aluminum or steel casing in the event of a puncture. The strength of most polymer materials is unlikely to exceed that of a metal casing; therefore, attempting to reinforce the outer casing to resist punctures has limited effectiveness in practical applications and significantly increases weight. Summary of the Invention To overcome the above problems, this invention proposes an internal puncture-resistant protective film layer for cylindrical battery cells used in electric bicycles that can effectively solve the above problems, and its application method.

[0003] The present invention provides a technical solution to solve the above-mentioned technical problems by providing an internal puncture-resistant protective film layer for a cylindrical battery cell used in electric bicycles and its application method. The film is attached to the inner surface of the battery cell casing and includes a protective film body. The protective film body has a double-layer structure, including a resin protective layer as the inner surface layer and a hot-melt adhesive layer as the outer surface layer. The resin protective layer is used to directly contact the electrolyte inside the battery cell, and the hot-melt adhesive layer is used to bond and fix it to the inner wall of the battery cell casing. The resin protective layer is a blend of POE resin and PP resin, and the hot-melt adhesive layer is a blend of maleic anhydride-modified polyolefin hot-melt adhesive and POE resin. When the battery cell casing is punctured by a puncture object, the protective film body can move with the puncture object and cover the surface of the puncture object, thereby isolating the puncture object from the positive and negative electrodes of the battery cell to reduce the risk of internal short circuits. Preferably, the thickness of the resin protective layer is 50–250 μm, and the thickness of the hot melt adhesive layer is 5–50 μm.

[0004] Preferably, the POE resin accounts for 50% to 90% of the mass of the resin protective layer, has a melting point of 80 to 180°C, and a melt index of 5 to 80 g / 10 min under a load of 2.16 kg.

[0005] Preferably, the ethylene content of the POE resin is 40% to 90%, the α-olefin portion is α-octene, or further, it is one or a combination of α-octene, α-hexene, and α-butene, and the α-olefin content is 10% to 60%.

[0006] Preferably, the PP resin is BOPP or CPP cast PP material, accounting for 10% to 50% of the mass in the resin protective layer, with a macroscopic melting point of 100 to 160°C, a melt index of 1 to 15 g / 10 min under a load of 2.16 kg, and a weight-average molecular weight of 150,000 to 350,000 g / mol.

[0007] Preferably, in the hot melt adhesive layer, the maleic anhydride-modified polyolefin hot melt adhesive is MAH-g-PE, which accounts for 60% to 90% by mass, the maleic anhydride grafting rate is >1%, and the modified substrate is LLDPE or LDPE with an elongation at break of >400%.

[0008] Preferably, the POE resin in the hot melt adhesive layer accounts for 10% to 40% by mass, and is consistent with the POE resin described in claim 3.

[0009] Preferably, the protective film is formed into a sleeve film through a multi-layer co-extrusion blown film process, with the blown diameter matching the cell diameter and allowing a negative tolerance of 1mm.

[0010] Preferably, the hot melt adhesive layer is configured to maintain adhesive stability under electrolyte immersion conditions, so that the protective film layer does not delaminate or shift, thereby moving with the puncture object and completely wrapping it during puncture.

[0011] The method for applying the puncture-resistant protective film layer inside the cylindrical battery cell for electric bicycles includes the following steps: Step S1: Prepare component one and component two and mix them. Step S2: Component 1 and Component 2 are co-extruded and blown into a hollow cylindrical sleeve film and then wound up. Step S3: Cut the sleeve film into cylindrical films of a predetermined length; Step S4: Fit the cylindrical diaphragm onto the outer surface of the air shaft and position it at a certain height; Step S5: Fit the battery cell housing to be processed into the air-expanding shaft and align the top opening of the battery cell housing with the positioning bolt 8. Step S6: Inflate the air shaft to make the cylindrical film adhere to the inner surface of the battery cell housing; Step S7: Activate the electromagnetic induction device to heat the battery cell housing, causing the hot melt adhesive layer on the outer surface of the cylindrical film to melt and adhere to the inner wall of the housing; Step S8: Cooling water is introduced to cool and shape the surface of the air-expanding shaft. Step S9: Depressurize and release air to separate the pneumatic shaft from the cylindrical membrane; Step S10: Remove the cell housing to obtain the cell housing with the inner protective film attached.

[0012] Compared with the prior art, the puncture-resistant protective film layer inside the cylindrical battery cell for electric bicycles and its application method of the present invention adopt the principle of "prevention is worse than cure" and design a protective film inside the hard-shell battery cell. This protective film can cover the surface of the punctured object when the battery cell is punctured, thereby isolating the punctured object (metal) from the positive and negative electrodes of the battery cell, preventing short circuits, and thus preventing fire and explosion problems. Attached Figure Description Figure 1 A schematic diagram of a device for applying an anti-puncture protective film layer inside the cylindrical battery cell for electric bicycles according to the present invention; Figure 2 This is a cross-sectional view of the air-expanding shaft in a device for internally attaching the puncture-resistant protective film layer inside the cylindrical battery cell for electric bicycles of the present invention; Figure 3 This is a schematic diagram showing the puncture-resistant protective film layer inside the cylindrical battery cell for electric bicycles of the present invention being punctured. Figure 4 This is a flowchart illustrating the method of using the puncture-resistant protective film layer inside the cylindrical battery cell for electric bicycles according to the present invention. Detailed Implementation To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0013] It should be noted that in the embodiments of the present invention, all directional indications (such as up, down, left, right, front, back, etc.) are limited to relative positions on the specified view, rather than absolute positions.

[0014] Furthermore, in this invention, descriptions involving "first," "second," etc., are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0015] Please see Figures 1 to 4 The present invention provides an internal puncture-resistant protective film layer for a cylindrical battery cell used in electric bicycles, which is attached to the inner surface of the battery cell housing 10. The protective film body 9 has a double-layer structure, including a resin protective layer (component one) as the inner surface layer and a hot-melt adhesive layer (component two) as the outer surface layer. The resin protective layer is used to directly contact the electrolyte inside the battery cell, and the hot-melt adhesive layer is used to bond and fix it to the inner wall of the battery cell housing 10.

[0016] The resin protective layer is a blend of POE (metallocene polymeric polyolefin elastomer) resin and PP (polypropylene) resin, and the hot melt adhesive layer is a blend of maleic anhydride modified polyolefin hot melt adhesive and POE resin.

[0017] When the battery cell housing 10 is penetrated by the puncture object 99, the protective film body 9 can move with the puncture object 99 and cover the surface of the puncture object 99, thereby isolating the puncture object 99 from the positive and negative electrodes of the battery cell to reduce the risk of internal short circuit.

[0018] The thickness of the resin protective layer is 50–250 μm, and the thickness of the hot melt adhesive layer is 5–50 μm.

[0019] The POE resin accounts for 50% to 90% of the mass of the resin protective layer, has a melting point of 80 to 180°C, and a melt index of 5 to 80 g / 10 min under a load of 2.16 kg.

[0020] The POE resin has an ethylene content of 40% to 90%, and the α-olefin portion is α-octene, or further, one or a combination of α-octene and α-hexene / α-butene, with an α-olefin content of 10% to 60%.

[0021] The PP resin is BOPP or CPP cast PP material, accounting for 10% to 50% of the mass in the resin protective layer. Its macroscopic melting point is 100 to 160°C, its melt index under a load of 2.16 kg is 1 to 15 g / 10 min, and its weight-average molecular weight is 150,000 to 350,000 g / mol.

[0022] Component 1 uses a special resin material that is resistant to carbonate electrolytes. It comes into direct contact with the electrolyte inside the battery cell, which can isolate the electrolyte from the corrosion of MAH-g-PE (maleic anhydride grafted polyethylene) and avoid performance degradation due to electrolyte immersion.

[0023] In the hot melt adhesive layer, the maleic anhydride-modified polyolefin hot melt adhesive is MAH-g-PE (maleic anhydride-grafted polyethylene), which accounts for 60% to 90% by mass, with a maleic anhydride grafting rate of >1%. The modified substrate is LLDPE or LDPE, with a melting point in the range of 80-180℃ and an elongation at break of >400%.

[0024] The POE resin in the hot melt adhesive layer accounts for 10% to 40% of the total mass, and is consistent with the POE resin in the resin protective layer.

[0025] Component 2 uses a high-cost special adhesive material, the core function of which is to tightly bond the protective film layer to the cell housing 10, ensuring that there is no delamination or displacement after the cell is assembled and during cyclic use, and providing a stable adhesion base for the inner protective layer.

[0026] When the MAH-g-PE content is less than 60%, the interfacial force between the two phases weakens, and the initial peel force is ≤3N / 25mm. After immersion in electrolyte at 85℃ for 24 hours, the peel force decreases to ≤0.5N / 25mm, and the protective film layer peels off over a large area, losing its protective function.

[0027] With a MAH-g-PE content of 60%-90%, the grafting rate is in the optimal range, and the peel force is stable at 4.0-7.5N / 25mm. After immersion in electrolyte, the peel force retention rate is ≥85%, and it can move with the steel nail (puncture object) and completely wrap around it during puncture, effectively blocking internal short circuits.

[0028] With a MAH-g-PE content exceeding 90%, the material exhibits increased rigidity but decreased flexibility. During puncture, the protective film layer is prone to breakage and cannot be lifted by the steel nail to cover the surface. The steel nail leaves a conductive channel, causing direct contact between the positive and negative electrodes inside the battery cell, triggering an internal short circuit risk.

[0029] The protective film body 9 is formed into a sleeve film through a multi-layer co-extrusion blown film process, with the blown diameter matching the cell diameter and allowing a negative tolerance of 1mm.

[0030] The protective film body 9 is a hollow cylindrical shape, which is made by blow molding. It is then rolled up and cut. After cutting, it can be put onto the matching equipment and supported by the air expansion shaft with Teflon coating on the surface to be attached to the battery cell that needs to be internally coated. The aluminum / steel battery cell shell 10 is heated by electromagnetic induction to melt the surface hot melt adhesive. Then, the Teflon surface is internally filled with water for cooling so that it can be peeled off.

[0031] The hot-melt adhesive layer maintains its bonding stability under electrolyte immersion conditions, preventing the protective film from delamination and displacement, thus allowing it to move with the puncture object and completely wrap around it during puncture.

[0032] The present invention also provides a device for attaching a protective film body 9 to the inside of a battery cell housing 10, comprising a base 14, a support rod 13 connected to the base 14, and an air-expanding shaft 12 connected to the support rod 13.

[0033] The device also includes a clamp and an electromagnetic induction device 11, which is used to fix and heat the battery cell housing 10.

[0034] The base 14 includes a ventilation button, a water supply button, or related automated design equipment.

[0035] The support rod 13 is equipped with air and water pipes. Its height is determined by the electromagnetic induction device. The purpose is to raise the shaft and keep the electromagnetic induction device as far away from the metal table as possible to avoid interference.

[0036] The air-expanding shaft 12 includes an air-expanding top block 1, a cooling water channel 2 inside the air-expanding top block 1, a shaft sealing block 3 inside the air-expanding top block 1, a shaft body 4 inside the shaft sealing block 3, an air channel 5 inside the shaft body 4, a positioning bolt 8 concentrically arranged at the middle of one end of the shaft body 4, an expansion joint 6 between the air-expanding top block 1 and the shaft sealing block 3, and a deformation block 7 on the outer side of the air-expanding top block 1.

[0037] After the positive electrode opening at the top of the battery cell housing 10 is aligned with the positioning bolt 8, the air-expanding shaft 12 is fitted onto it. The surface of the air-expanding shaft 12 is covered with a protective film body 9, and the production process is completed through a coating step. The positioning bolt 8 is a small protrusion at the top of the shaft body 4, which is used for limiting and positioning the battery cell housing 10.

[0038] The air-expanding top block 1 can be an existing type of expansion block. The main material is rubber, and the surface is coated with a Teflon coating with a thickness of 1-10 μm. MAH-g-PE and PP materials do not need to adhere to its surface.

[0039] The cooling water channel 2 is a circular or flat passage used to fill cooling water to promote the cooling and shaping of hot melt polyolefin.

[0040] Since the air expansion block 1 is made of rubber, the shaft sealing block 3 should be made of soft material, so that it can directly contact the air expansion block 1 to reduce compression friction and provide a seal.

[0041] The shaft body 4 is made of rigid plastics such as epoxy resin, with a yield strength of 30MPa or higher, and serves to support the shaft.

[0042] The air passage 5 is a gas-filled chamber. When in use, it is filled with gas so that the gas expansion block 1 is pushed outward and stretched to fit the inner surface of the battery cell housing 10. The filling gas pressure is above 0.2 MPa.

[0043] The expansion joint 6 is the gap between the air expansion block 1 and the shaft sealing block 3, and is tightly connected to the shaft sealing block 3 to achieve an airtight effect.

[0044] The outer side of the pneumatic top block 1 is provided with a groove, and the deformation block 7 is disposed in the groove but not buried in the groove. The surface of the deformation block 7 is toughened so that it has the ability to stretch under air pressure, while reducing the air pressure required for stretching.

[0045] The method of using the puncture-resistant protective film layer inside the cylindrical battery cell for electric bicycles according to the present invention includes the following steps: Step S1: Prepare component one and component two and mix them. Step S2: Component 1 and Component 2 are co-extruded and blown into a hollow cylindrical sleeve film and then wound up. Step S3: Cut the sleeve film into cylindrical films of a predetermined length; Step S4: Fit the cylindrical film onto the outer surface of the air-expanding shaft 12 and position it at a certain height; Step S5: Fit the battery cell housing 10 to be processed into the air shaft 12, and align the top opening of the battery cell housing 10 with the positioning bolt 8 for positioning. Step S6: Inflate the air shaft 12 to make the cylindrical film adhere to the inner surface of the cell housing 10; Step S7: Activate the electromagnetic induction device 11 to heat the battery cell housing 10, so that the hot melt adhesive layer on the outer surface of the cylindrical film melts and adheres to the inner wall of the housing. Step S8: Cooling water is introduced to cool and shape the surface of the air-expanding shaft 12; Step S9: Depressurize and release air to separate the pneumatic shaft 12 from the cylindrical membrane; Step S10: Remove the cell housing 10 to obtain the cell housing 10 with the inner protective film body 9 fully attached.

[0046] The mixing time in step S1 is 30 minutes.

[0047] In step S6, the inflation pressure is above 0.2 MPa.

[0048] In step S7, the power of the electromagnetic induction device 11 is greater than 100W.

[0049] In step S8, cooling water is introduced through cooling water channel 2 to promote the cooling and shaping of hot melt polyolefin.

[0050] The outer surface of the air-expanding shaft 12 is coated with Teflon, which prevents MAH-g-PE and PP materials from sticking to its surface, thus facilitating peeling after deflation.

[0051] The present invention will be further described below with reference to specific embodiments.

[0052] Design requirements: Attach a puncture-resistant protective film inside the 48100 battery cell, leaving a 10mm blank space at the top and bottom. That is, a barrel-shaped protective film with a diameter of 48mm and a length of 80mm needs to be attached, which must be able to pass the puncture test.

[0053] Step 1: After mixing component 1 and component 2 according to the specified ratio, put them into the mixer and mix for 30 minutes.

[0054] Step 2: Component 1 and Component 2 are fed into a multi-layer co-extrusion blown film machine for blown film production to obtain a cylindrical blown film, which is then wound up.

[0055] Step 3: Cut the blown film roll into 80mm tubular films.

[0056] Step 4: Fit the cylindrical diaphragm onto the air shaft and adjust its height for positioning. (This step can be done using mechanical positioning and mechanical fitting.) Step 5: Use plastic grippers to transfer and position the battery cell casing, positioning the battery cell in the appropriate location.

[0057] Step Six: Inflate the air tube to ensure the film on the sleeve adheres tightly to the inside of the battery cell.

[0058] Step 7: Start the electromagnetic induction heating equipment to heat up the battery cell casing and activate the adhesion performance of the outer surface of the protective film.

[0059] Step 8: Run cooling water through the water pipes to rapidly cool the surface of the air-expanded block.

[0060] Step 9: Release the air and pressure to separate the sleeve from the inner surface of the protective film.

[0061] Step 10: Remove the battery cell to obtain the battery cell casing with an internal puncture-resistant protective film.

[0062] After 10 steps, the following test scheme was obtained. Some verification data could not be obtained within the cell casing, so an aluminum casing of the same material was selected for testing.

[0063]

[0064] Adhesion test Prepare an aluminum sheet, ensuring it is clean and free of dirt. Apply a protective film of the corresponding size to the aluminum sheet. Separately, prepare an aluminum foil strip 60mm long and 10mm wide, and attach it to the back of the aluminum strip adhesive. After the heat and pressure treatment described above, cool it and peel it off using a tensile testing machine at a speed of 300mm / min.

[0065] Simultaneously prepare the sample in parallel, then immerse it in the electrolyte (lithium hexafluorophosphate + mixed esters, including but not limited to dimethyl carbonate, methyl ethyl ester, etc.), place it in an oven at 85°C for 4 hours, and then take it out to see if the protective film peels off, or dissociates into the electrolyte, or if it turns black, swells, or exhibits other phenomena.

[0066]

[0067] Protective film elongation at break and tensile strength test The protective film made by blown film is cut into strips and blocks, and tensile strength and elongation at break are tested.

[0068]

[0069] Protective membrane puncture test Cut the blown film protective film into 15mm squares. After acclimatizing in a chamber at (23±2)℃ and (50±10)% relative humidity for 8 hours, remove the protective film and lay it flat in the puncture strength testing machine fixture. Puncture the film at a rate of (100±10)mm / min. After completion, remove the sample and measure the thickness at the four corners, taking the average value. Calculate the puncture strength as follows: Fp = F0 / d Fp represents the puncture intensity, expressed in Newtons per micrometer. F0 is the force measured during membrane puncture, measured in Newtons (N). d represents the average thickness of the film, in micrometers.

[0070] (Note: For some test items, a POE resin layer will be applied after the modified polyolefin adhesive is applied.)

[0071] Testing the protective film coating on the battery casing Appearance Visually inspect the battery / battery after coating to ensure it is free of dirt, bumps, bubbles, and missed coatings; use a thickness gauge based on beta rays or X-rays / laser photothermal principles to determine if the coating is evenly applied.

[0072] High temperature 200℃ for 7 days After applying the protective film, place the battery in an environment of 200℃ for 7 days. Remove it, cool it, and observe for any signs of delamination or bubbles. (Note: For some test items, a POE resin layer may be applied after the modified polyolefin adhesive is applied.)

[0073] High temperature 380℃ for 10 minutes (instantaneous high temperature test) Place the battery casing with the protective film applied in a 380℃ environment for 10 minutes, then remove it and observe for any signs of delamination or bubbles after cooling. (Note: For some test items, a POE resin layer may be applied after the modified polyolefin adhesive is applied.)

[0074] Following the above process, this invention conducted sample preparation experiments with multiple sets of material formulations and thickness parameters, producing several sets of battery cell shells with protective films, and evaluating their performance. In the 180° peel strength test, the initial peel strength of the protective film layer with the preferred formulation (e.g., MAH-g-PE content of 60%–90% in the outer layer) reached 4–7 N / 25 mm. After immersion in electrolyte at 85 °C for 24 hours, the peel strength remained above 85% of the initial value, and no film detachment was observed. When the MAH-g-PE content in the outer adhesive layer was below 60%, the interfacial adhesion between the two phases was significantly weakened, and the peel strength dropped sharply to below 0.5 N / 25 mm after immersion, resulting in large-area detachment of the protective film layer. When the content was above 90%, the rigidity of the adhesive layer increased and the flexibility decreased, causing the protective film layer to easily break after being punctured by a steel needle in the puncture test, failing to move with the needle and leaving a conductive path. This result verifies the significant impact of the MAH-g-PE content range on adhesive performance. The present invention selects a MAH-g-PE ratio of 60%–90% to ensure optimal adhesive performance and puncture protection.

[0075] The composite protective film layer of this invention also possesses excellent mechanical properties and a balanced strength and toughness. Tensile tests show that the elongation at break of the protective film layer can reach over 700%–1000%, and the tensile strength is approximately in the range of 10–20 MPa (slightly different depending on the formulation), enabling it to remain unbroken under tensile deformation. Puncture strength tests indicate that the puncture strength per unit thickness of the composite film of this invention is approximately 1.4–3.5 N / μm, significantly higher than the puncture strength of single-material film layers. For example, at the same thickness, the puncture strength of a composite film containing a higher proportion of PP (such as group B1) can reach 3.45 N / μm, while that of a pure POE / PP film is approximately 3.01 N / μm, and that of a pure MAH-g-PE film is approximately 1.87 N / μm.

[0076] Battery cell samples equipped with the protective film layer of this invention underwent a series of safety tests according to the electric bicycle battery safety specifications. The results showed that the battery cells with the protective film layer did not experience any dangerous situations such as short circuits, fires, or explosions in tests including external short circuits, free falls, water immersion, salt spray, and damp heat cycling. Their performance was comparable to or better than the unprotected control group. In the needle penetration test, the control cell without the protective film showed obvious smoke and fire, while the battery cells with the protective film layer of this invention did not catch fire (only a few samples showed slight smoke at the moment of needle penetration). Furthermore, after extreme tests such as storage at 200 ℃ for 7 days and at 380 ℃ for 10 minutes, the protective film layer showed no delamination or blistering, demonstrating good adhesion stability and heat resistance. The protective film layer material also passed the flame retardant performance test, meeting the UL94 V-0 flame retardant requirements. The above results fully demonstrate the effectiveness and reliability of the puncture-resistant protective film layer inside the cylindrical battery cell provided by this invention in improving the safety of lithium batteries for electric bicycles.

[0077] Compared with the prior art, the puncture-resistant protective film layer inside the cylindrical battery cell for electric bicycles and its application method of the present invention have the following beneficial effects: (1) The safety protection is changed from "external thickening protection" to "internal isolation protection". After puncture, the membrane layer covers and isolates the punctured object, reducing the risk of internal short circuit and explosion.

[0078] (2) Synergistic combination of double-layer materials and processes: The inner layer takes into account the resistance to electrolyte and flexibility, while the outer layer achieves stable adhesion and follow-up wrapping ability, so that the film can still move with the steel nail and completely wrap under puncture conditions.

[0079] (3) The internal bonding process of air expansion shaft + induction heating + water cooling shaping is easy to achieve automation, repeatability and consistency, and is suitable for aluminum shell / steel shell cylindrical cells.

[0080] The above description is only a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any modifications, equivalent substitutions and improvements made within the concept of the present invention should be included within the patent protection scope of the present invention.

Claims

1. A puncture-resistant protective film layer inside a cylindrical battery cell for electric bicycles, which is attached to the inner surface of the battery cell casing, characterized in that... The device includes a protective film body, which has a double-layer structure, comprising a resin protective layer as the inner surface layer and a hot-melt adhesive layer as the outer surface layer. The resin protective layer is used to directly contact the electrolyte inside the battery cell, and the hot-melt adhesive layer is used to bond and fix it to the inner wall of the battery cell housing. The resin protective layer is a blend of POE resin and PP resin, and the hot melt adhesive layer is a blend of maleic anhydride-modified polyolefin hot melt adhesive and POE resin. When the battery cell casing is pierced by a puncture object, the protective film body can move with the puncture object and cover the surface of the puncture object, thereby isolating the puncture object from the positive and negative electrodes of the battery cell to reduce the risk of internal short circuit.

2. The puncture-resistant protective film layer inside the cylindrical battery cell for electric bicycles as described in claim 1, characterized in that, The thickness of the resin protective layer is 50–250 μm, and the thickness of the hot melt adhesive layer is 5–50 μm.

3. The puncture-resistant protective film layer inside the cylindrical battery cell for electric bicycles as described in claim 1, characterized in that, The POE resin accounts for 50% to 90% of the mass of the resin protective layer, has a melting point of 80 to 180°C, and a melt index of 5 to 80 g / 10 min under a load of 2.16 kg.

4. The puncture-resistant protective film layer inside the cylindrical battery cell for electric bicycles as described in claim 3, characterized in that, The POE resin has an ethylene content of 40% to 90%, and the α-olefin portion is α-octene, or further, one or a combination of α-octene, α-hexene, and α-butene, with an α-olefin content of 10% to 60%.

5. The puncture-resistant protective film layer inside the cylindrical battery cell for electric bicycles as described in claim 1, characterized in that, The PP resin is BOPP or CPP cast PP material, accounting for 10% to 50% of the mass in the resin protective layer. Its macroscopic melting point is 100 to 160°C, its melt index under a load of 2.16 kg is 1 to 15 g / 10 min, and its weight-average molecular weight is 150,000 to 350,000 g / mol.

6. The puncture-resistant protective film layer inside the cylindrical battery cell for electric bicycles as described in claim 1, characterized in that, In the hot melt adhesive layer, the maleic anhydride-modified polyolefin hot melt adhesive is MAH-g-PE, which accounts for 60% to 90% by mass, with a maleic anhydride grafting rate of >1%, and the modified substrate is LLDPE or LDPE with an elongation at break of >400%.

7. The puncture-resistant protective film layer inside the cylindrical battery cell for electric bicycles as described in claim 6, characterized in that, The POE resin in the hot melt adhesive layer accounts for 10% to 40% by mass, and is consistent with the POE resin described in claim 3.

8. The puncture-resistant protective film layer inside the cylindrical battery cell for electric bicycles as described in claim 1, characterized in that, The protective film is formed into a sleeve film through a multi-layer co-extrusion blown film process, with the blown diameter matching the cell diameter and allowing a negative tolerance of 1mm.

9. The puncture-resistant protective film layer inside the cylindrical battery cell for electric bicycles as described in claim 1, characterized in that, The hot melt adhesive layer is configured to maintain adhesive stability under electrolyte immersion conditions, preventing the protective film layer from delamination and displacement, thereby allowing it to move with the puncture object and completely wrap around it during puncture.

10. A method for using the puncture-resistant protective film layer inside a cylindrical battery cell for electric bicycles, characterized in that... Includes the following steps: Step S1: Prepare component one and component two and mix them. Step S2: Component 1 and Component 2 are co-extruded and blown into a hollow cylindrical sleeve film and then wound up. Step S3: Cut the sleeve film into cylindrical films of a predetermined length; Step S4: Fit the cylindrical diaphragm onto the outer surface of the air shaft and position it at a certain height; Step S5: Fit the battery cell housing to be processed into the air-expanding shaft and align the top opening of the battery cell housing with the positioning bolt 8. Step S6: Inflate the air shaft to make the cylindrical film adhere to the inner surface of the battery cell housing; Step S7: Activate the electromagnetic induction device to heat the battery cell housing, causing the hot melt adhesive layer on the outer surface of the cylindrical film to melt and adhere to the inner wall of the housing; Step S8: Cooling water is introduced to cool and shape the surface of the air-expanding shaft. Step S9: Depressurize and release air to separate the pneumatic shaft from the cylindrical membrane; Step S10: Remove the cell housing to obtain the cell housing with the inner protective film attached.

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