A method for recycling waste ceramic-coated polyethylene lithium battery separator, recycled polyethylene heat-conducting composite paper and a preparation method thereof

Abstract

This invention relates to the field of recycling and utilization of waste lithium-ion battery separators and preparation of polymer-based thermally conductive composite materials, and particularly to a method for recycling waste ceramic-coated polyethylene lithium battery separators, a recycled polyethylene thermally conductive composite paper, and a method for preparing the same. The recycling method includes the following steps: A) placing the waste lithium battery separator in a solvent, drawing it to negative pressure and then restoring it to normal pressure, followed by stirring, soaking, and drying to obtain the separator; B) placing the separator in an organic solvent, stirring and sonicating, and finally soaking and drying to obtain an uncoated PE film; a recycled polyethylene thermally conductive composite paper comprises at least two layers of recycled polyethylene composite film stacked sequentially and bonded by hot pressing; the recycled polyethylene composite film includes a recycled polyethylene film and a thermally conductive layer formed on at least one surface of the recycled polyethylene film; the recycled polyethylene film is an uncoated PE film prepared by the recycling method; the thermally conductive layer comprises hydroxylated boron nitride and polyvinylidene fluoride.

Description

Technical Field

[0001] This invention relates to the field of thermal conductivity technology, and in particular to a method for recycling waste ceramic-coated polyethylene lithium battery separators, recycled polyethylene thermally conductive composite paper, and a method for preparing the same. Background Technology

[0003] The service life of lithium-ion power batteries is about 5 to 7 years. At present, the recovery rate of rare and precious metals such as nickel, cobalt, lithium and manganese in retired lithium-ion power batteries has reached 98%. However, the waste polymer material of lithium-ion battery separator, which is a key component, cannot be recycled, causing environmental pollution and waste of resources.

[0004] Currently, mainstream lithium-ion battery separators mainly fall into three categories: polyolefin-based separators (PE / PP), ceramic-coated separators (PE / PP+Al2O3 / SiO2), and novel composite separators. Among them, ceramic-coated PE separators (PE+Al2O3) are widely used in the power batteries of high-end electric vehicles such as Tesla, NIO, and Li Auto due to their excellent high-temperature resistance. Recycled waste lithium-ion battery separators, due to the barrier mechanism of the double-sided alumina coating, make it difficult for PE to melt and flow at high temperatures, hindering direct thermoplastic processing. Therefore, the ceramic coating on the PE surface needs to be broken or separated to expose the PE film. Furthermore, to avoid affecting subsequent processing, electrolyte residues on the separator surface and inside must be removed as much as possible, mainly including organic solvents, lithium salts, and some additives. Ultimately, high-value utilization of lithium-ion battery separators is achieved through interface engineering design and structural control.

[0005] The rapid development of artificial intelligence has driven an explosive growth in computing power demand, leading to a sharp increase in the heat flux density generated by high-power chips, posing a severe challenge to traditional heat dissipation solutions. Ordinary polymer materials, due to their low thermal conductivity, are no longer sufficient to meet heat dissipation requirements, necessitating the development of polymer-based thermally conductive composite materials that balance high thermal conductivity, mechanical stability, and long lifespan. Existing PE-based thermally conductive composite materials are mainly prepared by simply filling a large amount of filler. This method results in disordered and chaotic heat conduction paths in the filler, offering limited improvement in thermal conductivity and failing to maintain the material's mechanical properties. Therefore, how to significantly improve the thermal conductivity of PE-based composite materials while simultaneously enhancing the interfacial bonding of the filler matrix and controlling the filler arrangement to improve the composite's mechanical properties has become a key scientific problem that urgently needs to be solved in the preparation of high thermal conductivity PE composite materials. Summary of the Invention

[0006] To address the aforementioned technical problems, embodiments of the present invention provide a method for recycling waste ceramic-coated polyethylene lithium battery separators, a method for recycling recycled polyethylene thermally conductive composite paper, and a method for preparing the same.

[0007] To achieve the above objectives, the embodiments of the present invention adopt the following technical solutions: On one hand, the present invention provides a method for recycling waste ceramic-coated polyethylene lithium battery separators, comprising the following steps: A. Place the waste lithium battery separator in a solvent, pump it to negative pressure and then restore it to normal pressure. After stirring, soaking and drying, the separator is obtained. B. Place the diaphragm in an organic solvent, stir and sonicate, and finally soak and dry to obtain an uncoated PE film.

[0008] In some embodiments, step A specifically includes the following steps: cutting the waste lithium battery separator and laying it flat on a sieve, drying it at 60 ℃ for 12 h; placing the dried separator in a beaker containing acetonitrile, and stirring it at 40 ℃ for 1 h; Then place the beaker in a vacuum drying oven, turn on the vacuum pump, slowly evacuate to -0.1 MPa and maintain for 0.5 h, then open the gas valve and purge to atmospheric pressure; The beaker was then placed on a magnetic stirring table and stirred at 40 °C for 1 h. The diaphragm was then removed from the acetonitrile solution, soaked in anhydrous ethanol, rinsed, and then spread flat on a sieve and dried at 60 °C for 12 h to obtain the diaphragm.

[0009] In some embodiments, step B specifically includes the following steps: The dried diaphragm from step A was placed in a beaker containing N-methylpyrrolidone and stirred at 60 °C for 0.5 h, followed by sonication at 60 °C for 2 h. Finally, the diaphragm was removed, soaked in anhydrous ethanol, rinsed, and spread flat on a sieve. It was then dried at 60 °C for 12 h to obtain a pure PE film.

[0010] On the other hand, the present invention also provides a recycled polyethylene thermally conductive composite paper, comprising at least two layers of recycled polyethylene composite film stacked sequentially and bonded by hot pressing; The recycled polyethylene composite film includes a recycled polyethylene film and a thermally conductive layer formed on at least one surface of the recycled polyethylene film; The recycled polyethylene film is an uncoated PE film prepared by the above recycling method; The thermally conductive layer comprises hydroxylated boron nitride and polyvinylidene fluoride.

[0011] In some embodiments, the hydroxylated boron nitride sheet has a diameter of 1-3 μm and a thickness of 100 nm.

[0012] In some embodiments, the mass ratio of hydroxylated boron nitride to polyvinylidene fluoride is 9:1.

[0013] In another aspect, the present invention provides a method for preparing recycled polyethylene thermally conductive composite paper, comprising the following steps: S1. Hydroxylated boron nitride, polyvinylidene fluoride and N-methylpyrrolidone are mixed and stirred to obtain a thermally conductive slurry; S2. The thermally conductive slurry is sprayed onto at least one side of the surface of the recycled polyethylene film, and the solvent is evaporated to obtain a recycled polyethylene composite film. S3. At least two of the recycled polyethylene composite films are stacked in sequence and hot-pressed to obtain recycled polyethylene thermally conductive composite paper.

[0014] In some embodiments, in S1, the mass ratio of hydroxylated boron nitride, polyvinylidene fluoride and N-methylpyrrolidone is 9:1:15, and in S2, the coating thickness of the thermally conductive slurry is 0.07 mm.

[0015] In some embodiments, in S2, the solvent evaporation process is maintained at 40 °C for 2 h, at 60 °C for 4 h, and at 40 °C for 12 h.

[0016] In some embodiments, in S3, the hot pressing process specifically involves first preheating at 140 °C for 5 min, then hot pressing at 8 MPa for 3 min, and finally cold pressing at 10 MPa for 5 min.

[0017] Compared with the prior art, the present invention has at least the following beneficial effects: Due to the barrier mechanism of the double-sided alumina coating, the recycled ceramic-coated PE separator is difficult for PE to melt and flow at high temperatures, making it difficult to be directly thermoplasticized. Therefore, the ceramic coating on the PE surface needs to be broken or separated to expose the PE film. To reduce the impact of residues on subsequent processing, this invention employs a method for recycling waste ceramic-coated polyethylene lithium battery separators. This involves using acetonitrile combined with negative pressure ultrasonication to remove electrolyte residues from the separator surface and internal pores, and then using N-methylpyrrolidone combined with ultrasonication to peel off the Al2O3 ceramic coating from the film surface, ultimately obtaining an uncoated PE film.

[0018] Uncoated PE films obtained from the separation of waste lithium-ion battery separators are made from biaxially stretched PE, resulting in lightweight, thin, and high-strength films. Direct melt processing, however, accelerates PE aging, damages the microstructure, and impairs film performance. By preparing a BN-OH (hydroxylated boron nitride) / PVDF (polyvinylidene fluoride) / NMP (N-methylpyrrolidone) slurry, a BN-OH thermally conductive layer was successfully constructed on the surface of the separated PE film through spraying and solvent evaporation. Furthermore, a multi-layered recycled PE thermally conductive composite paper was successfully prepared by overlapping hot pressing, demonstrating broad application potential in the thermal management of wearable electronic devices.

[0019] The lithium-ion battery separator, a key component, is currently a waste polymer material that cannot be recycled, causing environmental pollution and resource waste. This study successfully presents a solution for recycling waste ceramic-coated PE separators and processing them into high-performance thermally conductive materials, realizing the high-value utilization of waste lithium-ion battery components and completing the "last link" in the green, low-carbon, and circular development of the new energy industry chain.

[0020] Current research often focuses on separating the polymer matrix and ceramic particles from lithium-ion battery separators. However, the aging of the polymer matrix severely affects subsequent processing and product performance, making it difficult to provide an effective solution and high-value utilization method. This study flexibly utilizes the aging effect of PE, adapting the bonding effect of PVDF by changing the polarity of the PE molecular chain, turning a defect into an advantage, establishing a stronger mechanical interlock than hydrogen bonds, forming a high-strength interlayer bond, and preparing a novel high-strength recycled PE thermally conductive composite paper.

[0021] Current research on polymer-based thermal management materials struggles to achieve both superior thermal conductivity and mechanical properties at high filler contents, especially with filler loadings approaching 70%. This study, through interface optimization and structural design, prepared a recycled PE thermally conductive composite paper capable of supporting high filler contents. Even with a BN-OH content of 68.8 wt%, it still achieved a 100% increase in strength and a 6100% increase in thermal conductivity. Attached Figure Description

[0022] Figure 1 This is a flowchart illustrating the preparation process of the recycled PE thermally conductive composite paper according to the present invention. Figure 2 The images shown are of the actual products of this invention: (a) a discarded BYD 33Ah lithium iron phosphate battery cell; (b) a discarded BYD lithium battery separator; (c) recycled PE film; and (d) recycled PE thermally conductive composite paper. Figure 3 (a) Surface SEM images and EDS elemental analysis tables, (b) FT-IR spectra, (c) DSC curves and (d) TG curves for the recycled diaphragm and recycled PE film; Figure 4 (a, b) FTIR spectra of recycled PE film and commercial PE film, (c) surface water contact angle and (d) mechanism diagram of their interaction with PVDF surface; Figure 5 SEM images of the surface of (a) and (d) recycled PE film, (b) and (e) recycled PE composite film and (c) and (f) recycled PE thermally conductive composite paper, and (g), (h) and (i) cross-sectional SEM images of recycled PE composite film and (j), (k) and (l) recycled PE thermally conductive composite paper; Figure 6(a) original tensile stress-strain curves and (b) tensile strength and elongation at break values ​​of recycled PE film, commercial PE film, recycled PE thermally conductive composite paper, and commercial PE thermally conductive composite paper; (c) TG curves of thermally conductive composite paper; (d) optical photographs of commercial PE thermally conductive composite paper and recycled PE thermally conductive composite paper when stretched to break. Figure 7 (a) Raw data of in-plane thermal conductivity and (b) thermal conductivity growth rate calculated from the thermal conductivity data for recycled PE film, commercial PE film, recycled PE composite film, commercial PE composite film, recycled PE thermally conductive composite paper and commercial PE thermally conductive composite paper; Figure 8 (a) and (b) are cross-sectional SEM images of the nylon thermally conductive composite material in patent CN 118852738 B, respectively. (c) is a cross-sectional SEM image of the recycled PE thermally conductive composite paper in this invention. Figure 9 The infrared spectrum of the material used in this invention; Figure 10 The mechanical property test diagram of nylon thermally conductive composite material in patent CN 118852738 B; Figure 11 The image shows the thermal conductivity test results of the nylon thermally conductive composite material in patent CN 118852738 B. Detailed Implementation

[0023] The technical solutions in some embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this disclosure, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments provided in this disclosure are within the scope of protection of this disclosure.

[0024] Material: (1) Objects to be processed: Waste BYD lithium battery separator (PE + double-sided Al2O3 coating, purchased from Henan Zhenhua Juying Machinery Technology Co., Ltd., BYD 33Ah lithium iron phosphate cell) Commercial separator (specification 12+2+2, 12-micron PE matrix, 2-micron Al2O3 coating on both sides, purchased from ASML New Energy New Lithium Battery). (2) Solvent: ① Anhydrous ethanol (Chengdu Kelong Chemical Co., Ltd.) ② Acetonitrile (Chengdu Kelong Chemical Co., Ltd.) ③ N-Methyl pyrrolidone (NMP, Chengdu Kelong Chemical Co., Ltd.) (3) Thermally conductive filler: Hydroxylated boron nitride (BN-OH, powder, flake diameter 1~3 μm; thickness 100 nm, purchased from Beijing Deco Island Gold, which obtained hydroxylated boron nitride by treating boron nitride with a silane coupling agent) (4) Auxiliary materials: Polyvinylidene fluoride (PVDF, powder, molecular weight 1 million, Zhongxin Plastics) Characterization methods: The microstructure of the samples was analyzed using a scanning electron microscope (SEM, JSM-IT800, Jielu Science and Trade Co., Ltd., Beijing). The recycled PE composite film and recycled PE thermally conductive composite paper before and after cleaning were placed in liquid nitrogen and frozen for 10 min, and then cut in liquid nitrogen. The cross-section was selected as the object of observation.

[0025] The characteristic functional groups of the recycled PE film, recycled PE composite film, and recycled PE thermally conductive composite paper before and after treatment were analyzed using a Fourier transform infrared spectrometer (FT-IR, Nicolet iS50, Thermo Scientific, Shanghai).

[0026] The recycled PE film and recycled PE thermally conductive composite paper before and after treatment were characterized and their Al₂O₃ and BN-OH contents were analyzed using a thermogravimetric analyzer (TG, TA300, TA Instruments, USA). When determining the filler content of the recycled PE thermally conductive composite paper, the temperature rise program was set to rise from room temperature to 800 °C and hold for 2 h. During this period, both PE and PVDF decomposed and disappeared, leaving behind only non-decomposable BN-OH, the mass of which was considered the filler content of the PE thermally conductive composite paper.

[0027] The surface hydrophilicity of recycled PE film and commercial PE film was analyzed using a water contact angle analyzer (WCA, OCA 50, DataPhysics Instruments GmbH, Germany).

[0028] The mechanical properties of recycled PE film, commercial PE film, recycled PE thermally conductive composite paper, and commercial PE thermally conductive composite paper were analyzed using a universal testing machine. Samples were cut into uniform rectangular strips 10 mm wide and 60 mm long. Pneumatic clamps were used to hold both ends of the strips, with a gauge length of 40 mm and a stretching rate of 2 mm / min, until the strips broke.

[0029] The thermal conductivity of recycled PE film, commercial PE film, recycled PE composite film, commercial PE composite film, recycled PE thermally conductive composite paper, and commercial PE thermally conductive composite paper was analyzed using a thermal constant analyzer. The transient plate method was used for measurement. The in-plane thermal conductivity of the samples was tested, and the thermal conductivity growth rate was calculated. The formula for calculating the thermal conductivity growth rate is as follows: , where K f K is the in-plane thermal conductivity of the sample. i The thermal conductivity is that of pure PE.

[0030] Experimental steps: (1) Membrane cleaning and separation: ①Preparation: Cut the waste BYD lithium battery separator and lay it flat on a sieve, then dry it at 60°C for 12 hours.

[0031] ② Cleaning work: The dried diaphragm was placed in a beaker containing acetonitrile and stirred at 40 °C for 1 h. The beaker was then placed in a vacuum drying oven, the vacuum pump was turned on, and the pressure was slowly evacuated to -0.1 MPa and maintained for 0.5 h. The gas valve was then opened, and the pressure was slowly increased to atmospheric pressure. After this, the beaker was placed on a magnetic stirring table and stirred at 40 °C for 1 h. Finally, the diaphragm was removed from the acetonitrile, soaked in anhydrous ethanol, rinsed, and then spread evenly on a sieve and dried at 60 °C for 12 h.

[0032] ③ Separate work: The dried diaphragm was placed in a beaker containing N-methylpyrrolidone and stirred at 60°C for 0.5 h, followed by sonication at 60°C for 2 h. Finally, the diaphragm was removed from the N-methylpyrrolidone, soaked in anhydrous ethanol, rinsed, and then spread flat on a sieve and dried at 60°C for 12 h. An uncoated PE film was obtained.

[0033] (2) Thermal conductivity enhancement modification of the diaphragm: ①Preparation of thermally conductive paste: BN-OH powder, PVDF powder and NMP were mixed in a beaker at a mass ratio of 9:1:15 and stirred continuously at 50 °C until the colloidal mixture turned a uniform milky white color.

[0034] ②Preparation of composite thin films: Uncoated PE film is laid flat on a smooth surface and sprayed with a spray gun loaded with thermally conductive paste until the film surface is uniformly covered with a 0.07 mm thick milky white coating. The sprayed film is then placed in a drying oven for solvent evaporation: kept at 40 ℃ for 2 h, 60 ℃ for 4 h, and 40 ℃ for 12 h to obtain recycled PE composite film.

[0035] ③Preparation of thermally conductive composite paper: The dried recycled PE composite film was removed from the oven, and three coated recycled PE composite films were overlapped and hot-pressed using a flat vulcanizing apparatus. The specific steps were: first, preheating at 140℃ for 5 minutes, followed by hot pressing at 8MPa for 3 minutes, and then cold pressing at 10MPa for 5 minutes, finally yielding the recycled PE thermally conductive composite paper. The detailed process is as follows: Figure 1 As shown, the actual product image is as follows. Figure 2 As shown.

[0036] Sample naming and specific designation in this invention:

[0037] Results and Discussion: ① Cleaning and separation effect: like Figure 3 In (a), the surface of the recovered diaphragm is covered with a dense layer of Al2O3, and the surface of this area shows an aluminum content of 25.56%. After cleaning and separation, no Al2O3 was observed on the diaphragm surface, and the aluminum content also showed to be 0%, indicating that the Al2O3 coating was fully removed.

[0038] In addition, the contents of other elements related to residual electrolyte and salt, including F, B, P and N, were also significantly reduced after treatment, indicating that diaphragm cleaning has a significant effect. Figure 3 (b) shows the FT-IR spectra before and after membrane cleaning and separation. The spectrum before treatment shows the characteristics of Al2O3, while the spectrum after treatment is basically consistent with pure PE, indicating that the cleaning and analysis effect is good.

[0039] The DSC curves show that the residual electrolyte in the recycled membrane caused an exothermic peak in the 80℃~100℃ range, while the endothermic curve of the recycled PE film was basically consistent with that of pure PE. Figure 3 (c) This indicates that the cleaning step effectively removes residual electrolyte.

[0040] Figure 3 Figure (d) shows the TG curves before and after membrane cleaning and separation. The weight at the end of the curve represents the residual Al2O3 content. The Al2O3 content before treatment was 51.31%, while the Al2O3 content after treatment was 0.78%, which also indicates that the cleaning and separation scheme is effective in removing the Al2O3 coating.

[0041] ② Oxidation of PE In fact, during the service life of lithium-ion batteries, factors such as the attack of active oxides generated by the decomposition of electrolytes like LiPF6 on the PE molecular chains, and localized overheating during charging and discharging can lead to PE oxidation and the breakage of some molecular chains. For example... Figure 4In (a) and (b), commercial PE films show no obvious peak around wavenumber 1740, but recycled PE films show a carbonyl peak near 1740, indicating that the PE molecular chains in the recycled PE films undergo oxidation to form ester bonds or oxidative breakage to form carboxylic acids. Although the PE molecular chains are inert, this oxidation increases their polarity, forming a rougher surface and enhancing the hydrophilicity of the membrane. Figure 4 In (c), recycled PE film has a lower water contact angle value than commercial PE film.

[0042] Furthermore, oxidation is also evident in the fact that recycled PE membranes turn golden yellow when heated above their melting point, while commercial PE films remain white without significant yellowing even at high temperatures. Oxidized PE is more polar than PVDF, and free PVDF more easily wets the PE surface, leading to physical entanglement and mechanical interlocking, such as... Figure 4 (d) Furthermore, the polar groups introduced by oxidation may also undergo some degree of chemical interaction with PVDF. Therefore, this polar recycled PE film can use PVDF as a bridge, utilizing its adhesiveness and film-forming properties to carry higher contents of thermally conductive fillers, thus preparing thermally conductive composite paper with a unique lamination structure, as a highly promising thermal management material.

[0043] ③ Subsequent modification of thin films Recycled PE film exhibits a porous surface structure at the microscopic level. Figure 5 In the middle (a, d), but the pore size does not exceed 350 nm. These pores are obtained by biaxial stretching of PE, serving as channels for ion exchange during the service life of the battery separator. After spraying, a layer of BN-OH coating is applied to the surface of the recycled PE composite film, but the BN-OH stacking is loose and disordered, as... Figure 5 (b, e). After hot pressing, the BN on the surface of the recycled PE thermally conductive composite paper becomes tight and dense, such as... Figure 5 (c, f). Cross-sectional morphology of the recycled PE composite film ( Figure 5 In the (g, h, i) samples, the BN-OH layer is found to be thick and loose, and randomly arranged along the film plane. In the recycled PE thermally conductive composite paper, the BN-OH layer becomes relatively thinner and denser, with a tight bond between the filler layer and the matrix layer, almost without gaps. Furthermore, the BN-OH layers are oriented along the film plane, forming a continuous thermal conductive path within the recycled PE thermally conductive composite paper. Figure 5 (j, k, l)).

[0044] ④ Mechanical properties like Figure 6In Figure (a), tensile tests were performed on recycled PE film, commercial PE film, recycled PE thermally conductive composite paper, and commercial PE thermally conductive composite paper, respectively, and their original stress-strain curves were obtained. Due to oxidation and aging during battery service, the strength of PE film (15.5 MPa) was 22.5% lower than that of commercial PE film (20 MPa). After design and processing (BN-OH content 68.8%, such as...) Figure 6 In the middle (c) section, the strength of the recycled PE thermally conductive composite paper reached 31 MPa, which is 100% higher than that of the recycled PE film. Figure 6 (b) In contrast, the strength of commercial PE thermally conductive composite paper is only 15.1 MPa, which is 25% lower than that of commercial PE film.

[0045] In fact, the stronger mechanical interlocking between the molecular chains of recycled PE film and PVDF results in a tighter bond between the PE film matrix layer and the BN / PVDF filler layer. Even when stretched to break, no interlayer delamination occurs. Figure 6 (d) However, commercial PE films are not used in battery cycles and show almost no oxidation. The PE molecular chains are non-polar and difficult to interlock with PVDF, resulting in poor adhesion between the film matrix layer and the BN / PVDF filler layer. This leads to a decrease in the overall mechanical properties of the composite paper and delamination before breakage.

[0046] ⑤ Thermal conductivity: like Figure 7 In Figure (a), the in-plane thermal conductivity of recycled PE film, commercial PE film, recycled PE composite film, commercial PE composite film, recycled PE thermally conductive composite paper, and commercial PE thermally conductive composite paper were tested respectively. Recycled PE film and commercial PE film, being essentially pure polymers, had the lowest thermal conductivity, at only 0.21 W / m². -1 K -1 For recycled and commercial PE films, the spray coating process coats the film with a uniform layer of thermally conductive filler, increasing the thermal conductivity to 4 W / m². -1 K -1 Nearby (thermal conductivity growth rate approximately 1810%) Figure 7 (b) At this point, the BN-OH sheets are relatively loosely and chaotically stacked, and no complete thermal conductivity pathway is formed in the planar direction. However, planar hot pressing further induces the BN-OH sheets to align and arrange themselves densely along the film planar direction, forming overlapping and interconnected BN-OH thermal conductivity pathways, which increases the thermal conductivity of the recycled PE thermally conductive composite paper to 13.02 W / m². -1 K -1 It is more than 6100% better than PE film.

[0047] In fact, this process method has a similar effect on improving the thermal conductivity of the two types of PE films because the thermal conductivity of this thermally conductive composite paper material is only related to the filler layer, and whether the PE matrix is ​​oxidized or not has almost no effect on the thermal conductivity of the composite material.

[0048] ⑥ Compare with patent CN 118852738 B (hereinafter referred to as the comparative case) 1. Structural aspects: The recycled PE thermally conductive composite paper prepared in this invention differs from the nylon thermally conductive composite material in that it uses different raw materials (polymer matrix, thermally conductive filler, and intermediate materials). Although both are laminated structures (sandwich structures), the polymer matrix structures and filler loadings are different. Figure 8 (a, b)); the interlayer interface bonding mechanisms are different ( Figure 8 (c)

[0049] In contrast, the nylon thermally conductive composite material uses a polymer matrix of nylon mesh with a fibrous structure, a thermally conductive filler of BN, and an intermediate material of cellulose nanofibers (CNFs).

[0050] In this invention, the polymer matrix of the recycled PE thermally conductive composite paper is a recycled PE film, the thermally conductive filler is BN-OH, and the intermediate material is PVDF. Nylon is the most abundant component in the nylon thermally conductive composite material, with BN accounting for no more than 24%. Increasing the BN percentage further will affect the interlayer adhesion, continuously reduce mechanical properties, and lead to material failure. Figure 10 In this invention, the proportion of thermally conductive filler BN-OH in the recycled PE thermally conductive composite paper reaches nearly 70%, while still maintaining stable interlayer adhesion. Figure 6 (a, b, c, d)), therefore this material has greater application potential.

[0051] Figure 8 As shown, the comparative case of nylon thermally conductive composite material achieves interlayer surface bonding through hydrogen bonds between CNFs and nylon, while the recycled PE thermally conductive composite paper in the study achieves interlayer bonding through strong mechanical interlocking with PVDF due to the enhanced oxidation polarity of PE, rather than hydrogen bonds. Figure 9 The bonding effect is better, and it can load a higher content of thermally conductive filler (close to 70%). Nylon mesh is made of nylon fibers and has macroscopically visible pores, allowing water to pass through. Therefore, the nylon thermally conductive composite material described in the comparative case was prepared by depositing BN onto the surface of nylon fibers through vacuum-assisted filtration. The recycled PE film in this study is not a fibrous structure, and water molecules cannot pass through it. It was prepared by slurry spraying combined with solvent evaporation to complete the composite film preparation. The front-end preparation methods of the two are different.

[0052] 2. Performance aspects: Consistent with the previous section, in the comparative case, the strength of the nylon thermally conductive composite material decreased to approximately the same level as the pure sample when the filler content reached 24.5%. Further increasing the filler content would severely impact the mechanical properties, leading to mechanical failure of the composite material. Figure 10 (a, b, c). Furthermore, even at the filler content with the greatest strength improvement (7.5 wt%), representing a 44% increase, the composite material's thermal conductivity at this content is 1.6 W / m². -1 K -1 In this study, the content of PE thermally conductive composite paper filler recovered was 68.8% ( Figure 6 (c) The tensile strength is 100% higher than that of recycled PE film (pure sample). Figure 6 (b) The high mechanical property improvement rate maintained at high content is attributed to the strong mechanical interlocking between polar PE and PVDF, which provides stable interlayer adhesion for the recycled PE thermally conductive composite paper and prevents interlayer slippage. This interlayer adhesion mechanism is superior to the hydrogen bonding effect described in the comparative case. The PE thermally conductive composite paper obtained in this study has greater potential.

[0053] In the comparative case, the nylon thermally conductive composite material achieved a maximum BN content of 24.5% and a thermal conductivity of 4.5 W / m² while maintaining mechanical properties. -1 K -1 This content represents the maximum filler content for the nylon thermally conductive composite material in the comparative case, with a thermal conductivity improvement (relative to the pure sample) of approximately 1800%. In this study, the recovered PE thermally conductive composite paper had a BN-OH loading of 68.8 wt% and a thermal conductivity of 13.02 W / m². -1 K -1 Thermal conductivity increased by 6100% (relative to the pure sample), and strength increased by 100% (relative to the pure sample).

[0054] 68.8wt% is not the limit of filler content for recycled PE thermally conductive composite paper in this system. Compared with nylon thermally conductive composite material, it has a higher upper limit of thermally conductive filler load, higher strength improvement, higher thermal conductivity and higher thermal conductivity improvement rate. Therefore, recycled PE thermally conductive composite paper has greater application potential than nylon thermally conductive composite material in the comparison case.

[0055] In the description of this specification, specific features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

[0056] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for recycling waste ceramic-coated polyethylene lithium battery separators, characterized in that, Includes the following steps: A. Place the waste lithium battery separator in a solvent, pump it to negative pressure and then restore it to normal pressure. After stirring, soaking and drying, the separator is obtained. B. Place the diaphragm in an organic solvent, stir and sonicate, and finally soak and dry to obtain an uncoated PE film.

2. The recycling method according to claim 1, characterized in that, Step A specifically includes the following steps: cutting the waste lithium battery separator and laying it flat on a sieve, drying it at 60 ℃ for 12 h; placing the dried separator in a beaker containing acetonitrile, and stirring it at 40 ℃ for 1 h; Then place the beaker in a vacuum drying oven, turn on the vacuum pump, slowly evacuate to -0.1 MPa and maintain for 0.5 h, then open the gas valve and purge to atmospheric pressure; The beaker was then placed on a magnetic stirring table and stirred at 40 °C for 1 h. The diaphragm was then removed from the acetonitrile solution, soaked in anhydrous ethanol, rinsed, and then spread flat on a sieve and dried at 60 °C for 12 h to obtain the diaphragm.

3. The recycling method according to claim 1, characterized in that, Step B specifically includes the following steps: The dried diaphragm from step A was placed in a beaker containing N-methylpyrrolidone and stirred at 60 °C for 0.5 h, followed by sonication at 60 °C for 2 h. Finally, the diaphragm was removed, soaked in anhydrous ethanol, rinsed, and spread flat on a sieve. It was then dried at 60 °C for 12 h to obtain a pure PE film.

4. A recycled polyethylene thermally conductive composite paper, characterized in that, It includes at least two layers of recycled polyethylene composite film stacked sequentially and bonded together by hot pressing; The recycled polyethylene composite film includes a recycled polyethylene film and a thermally conductive layer formed on at least one surface of the recycled polyethylene film; The recycled polyethylene film is an uncoated PE film prepared by the recycling method according to any one of claims 1 to 3; The thermally conductive layer comprises hydroxylated boron nitride and polyvinylidene fluoride.

5. The recycled polyethylene thermally conductive composite paper according to claim 4, characterized in that, The hydroxylated boron nitride sheet has a diameter of 1~3 μm and a thickness of 100 nm.

6. A recycled polyethylene thermally conductive composite paper according to claim 4 or 5, characterized in that, The mass ratio of hydroxylated boron nitride to polyvinylidene fluoride is 9:

1.

7. A method for preparing a recycled polyethylene thermally conductive composite paper as described in any one of claims 4 to 6, characterized in that, Includes the following steps: S1. Hydroxylated boron nitride, polyvinylidene fluoride and N-methylpyrrolidone are mixed and stirred to obtain a thermally conductive slurry; S2. The thermally conductive slurry is sprayed onto at least one side of the surface of the recycled polyethylene film, and the solvent is evaporated to obtain a recycled polyethylene composite film. S3. At least two of the recycled polyethylene composite films are stacked in sequence and hot-pressed to obtain recycled polyethylene thermally conductive composite paper.

8. The preparation method according to claim 7, characterized in that, In S1, the mass ratio of hydroxylated boron nitride, polyvinylidene fluoride and N-methylpyrrolidone is 9:1:

15. In S2, the coating thickness of the thermally conductive paste is 0.07 mm.

9. The preparation method according to claim 7, characterized in that, In S2, the solvent evaporation process is carried out at 40 °C for 2 h, at 60 °C for 4 h, and at 40 °C for 12 h.

10. The preparation method according to claim 7, characterized in that, In S3, the specific hot pressing process is as follows: first, preheating at 140 ℃ for 5 min, then hot pressing at 8 MPa for 3 min, and then cold pressing at 10 MPa for 5 min.

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