A UV-curable cross-linked lithium-ion battery separator and its preparation method and lithium-ion battery

Abstract

The present invention discloses a UV-curable, cross-linked lithium-ion battery separator, a preparation method thereof, and a lithium-ion battery. The UV-curable, cross-linked lithium-ion battery separator comprises a base film and a UV-curable, cross-linked coating applied to the surface of the base film. The base film surface has a hydroxyl density of ≥5 / nm² and a contact angle of ≤35°. The UV-curable, cross-linked coating comprises a polymer matrix, nanoparticles, a UV-initiator, and a multifunctional acrylate cross-linker with a functionality of ≥4. The separator coating has a peel strength of 78 N / m and an ionic conductivity of 4.28×10 ‑4 S / cm (25℃), thermal shrinkage at 150℃ <2%.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery materials, and in particular relates to an ultraviolet-cured cross-linked lithium-ion battery separator, a preparation method thereof, and a lithium-ion battery. Background Art

[0002] As a core component of the battery, lithium-ion battery separators must have excellent ionic conductivity and mechanical stability while ensuring high safety. The current industry generally uses aluminum oxide (Al2O3) or polyvinylidene fluoride (PVDF) coatings to modify polyethylene (PE) or polypropylene (PP) base membranes. However, due to the difficulty of chemical bonding between the non-polar base membrane and the polar coating, the peel strength of traditional coatings is generally lower than 50N / m, and there is a risk of interface delamination after long-term cycling; at the same time, the high-temperature cross-linking process (>120°C) can easily cause the base membrane to have a thermal shrinkage rate of more than 5%, which seriously restricts battery safety. In addition, fast ion conductor materials (such as LLZO) are prone to particle agglomeration and interface side reactions due to their poor compatibility with the polymer matrix, making it difficult for ionic conductivity to exceed 10 -4 Although existing technologies attempt to optimize through silane coupling or thermal cross-linking, they are still unable to achieve high interfacial bonding strength, high porosity, and low-temperature process compatibility in a coordinated manner. Developing new coating technologies with comprehensive performance advantages has become a key issue that the industry urgently needs to overcome. Although existing technologies such as the Chinese patent with publication number CN107180938B use plasma treatment and nano-coating modification, their water-based binder system limits the coating thickness (30-70 nm) and does not introduce a cross-linking structure, resulting in an interfacial bonding strength of less than 50 N / m. Chinese patent applications with publication number CN114243207A and CN104157810B use UV curing technology. The Chinese patent with publication number CN104157810B includes a polymer matrix, nanoparticles, a UV initiator, and a multifunctional acrylic resin. However, the Chinese patent application with publication number CN114243207A does not combine the directional dispersion of fast ion conductors (such as LLZO) and the dual-mode pore design, resulting in an ionic conductivity of only 9.3×10 - 4 S / cm, and the high temperature thermal shrinkage rate is still higher than 3%. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a UV-curable cross-linked lithium-ion battery separator and its preparation method and lithium-ion battery in response to the shortcomings of the existing technology, so as to solve the problems of insufficient interface bonding strength, poor high-temperature stability and low ion conduction efficiency of traditional separator coatings, and is suitable for high-energy-density lithium-ion battery systems.

[0004] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0005] A UV-curable cross-linked lithium-ion battery separator comprises a base film and a UV-curable cross-linked coating applied to the surface of the base film, wherein the hydroxyl density on the surface of the base film is ≥5 / nm² and the contact angle is ≤35°, and the UV-curable cross-linked coating comprises a polymer matrix, nanoparticles, a UV initiator, and a multifunctional acrylate cross-linking agent with a functionality ≥4.

[0006] The present invention makes the surface hydroxyl density of the base film ≥5 / nm², the contact angle ≤35°, and the UV-curable cross-linking coating adopts a multifunctional acrylate cross-linking agent with a functionality ≥4, thereby enhancing the interfacial bonding strength of the diaphragm coating, improving the high-temperature stability and the ion conduction efficiency, and is suitable for high-energy-density lithium-ion battery systems.

[0007] Experiments show that when the crosslinker functionality is insufficient (such as TMPTA with a functionality of 3) or the base film hydroxyl density is less than 5 / nm², the peel strength drops to 48N / m and 42N / m, respectively, and the interfacial impedance deteriorates significantly (the growth rate reaches 20% in 30 days), verifying the necessity of the parameter thresholds in this application.

[0008] The present invention provides a surface hydroxyl density of ≥5 per nm² and a contact angle of ≤35° on the surface of the base film, thereby increasing the surface energy of the base film to 50-60 mJ / m². Preferably, the present invention employs an argon-oxygen mixed plasma treatment to provide a surface hydroxyl density of ≥5 per nm² and a contact angle of ≤35° on the surface of the base film.

[0009] In a preferred embodiment of the present invention, the peel strength of the UV-curable cross-linked lithium-ion battery separator is 65-85 N / m, and the ionic conductivity at 25°C is 3×10 -4 ~5.5×10 -4 S / cm, thermal shrinkage at 150℃<2.5%.

[0010] The porosity of the composite coating is 40-50%, and the pore size is bimodal, including 200-1000nm macropores and 50-200nm mesopores.

[0011] In a preferred embodiment of the present invention, the mass ratio of the polymer matrix, the nanoparticles, the multifunctional acrylate crosslinking agent with a functionality ≥ 4, and the ultraviolet light initiator is 100:25-50:5-30:0.3-5.

[0012] In a preferred embodiment of the present invention, the nanoparticles include lithium lanthanum zirconium oxide nanoparticles and a coating particle layer coated on the surface of the lithium lanthanum zirconium oxide nanoparticles, and the coating particle layer includes one or two of aluminum oxide and lithium aluminate.

[0013] Preferably, the thickness of the coated particle layer is 2-5 nm; further preferably, the particle size of the nanoparticles is 50-300 nm.

[0014] The surface coating modification (Al2O3 / LiAlO2, 2-5nm) and particle size control (50-300nm) of lithium lanthanum zirconium oxide (LLZO) nanoparticles combined with continuous ion channel design increased the room temperature ionic conductivity of the composite coating to 4.28×10 -4 S / cm, and the capacity retention rate exceeds 92% after 500 cycles at 5C rate.

[0015] In a preferred embodiment of the present invention, the lithium lanthanum zirconium oxide nanoparticles are Ta or Nb doped, and the chemical formula is Li 6.4 La3Zr 1.4 Ta 0.6 O 12 or Li7La3Zr 1.5 Nb 0.5 O 12 .

[0016] In a preferred embodiment of the present invention, the polymer matrix includes polyvinylidene fluoride-hexafluoropropylene, and the multifunctional acrylate crosslinking agent includes one or more of dipentaerythritol hexaacrylate, trimethylolpropane triacrylate or pentaerythritol tetraacrylate.

[0017] The present invention constructs a diaphragm with a cross-linked polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) / lithium lanthanum zirconium oxide (LLZO) composite coating on the surface of a base film through ultraviolet (UV) curing technology and a preparation method thereof.

[0018] The molecular weight of polyvinylidene fluoride-hexafluoropropylene is 300-500 kDa, and the crystallinity is 20-30%.

[0019] The ultraviolet photoinitiator includes one or more of 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (PI-2959), 1-hydroxycyclohexylphenyl ketone (UV-184), or bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (PI-819).

[0020] The present invention also discloses a method for preparing the ultraviolet light-cured cross-linked lithium-ion battery separator, comprising the following steps:

[0021] S1. Activate the base film using argon-oxygen plasma to increase the hydroxyl density on the surface of the base film to ≥5 / nm² and the contact angle to ≤35°, thereby obtaining an activated base film;

[0022] S2, mixing a solvent, a polymer matrix, nanoparticles, a multifunctional crosslinking agent and a UV initiator, and evenly dispersing the mixture to obtain a slurry, coating the slurry on the surface of the base film activated in S1, and then curing the slurry with UV light to form a three-dimensional crosslinked network coating, so that the crosslinking density is ≥85%;

[0023] S3, using a gradient phase separation process to form a porous coating with a bimodal pore size distribution on the three-dimensional cross-linked network coating in S2, so that the porosity is 40-50%;

[0024] S4. Dry the porous coating in S3 in sections to obtain a UV-cured cross-linked lithium-ion battery separator.

[0025] In a preferred embodiment of the present invention, the argon-oxygen plasma activation of the base film in S1 uses an argon-oxygen mixed gas with an Ar / O2 volume ratio of 3:1-5:1, a power density of 0.8-1.2 W / cm², and a time of 20-60 s.

[0026] In a preferred embodiment of the present invention, the gradient phase separation process in S3 includes immersing the UV-cured membrane in a first gradient mixed solution comprising an organic solvent and water, a second gradient mixed solution and a third gradient mixed solution in sequence, wherein the organic solvent is at least one of N-methylpyrrolidone, N,N-dimethylformamide and tetrahydrofuran, and the volume ratio of the organic solvent to water in the first gradient mixed solution is 7:3-8:2, and the immersion time is 5-7min. The volume ratio of the organic solvent to water in the second gradient mixed solution is 4:6-5:5, and the immersion time is 3±0.5min. The volume ratio of the organic solvent to water in the third gradient mixed solution is 0:10, and the immersion time is 15±5s; the first gradient mixed solution also includes a porogen, and the mass percentage of the porogen is 1-3wt%.

[0027] The porogen was added in an amount of 1-3 wt% to induce a bimodal pore size distribution.

[0028] The porogen is polyethylene glycol or polyvinyl pyrrolidone, and the added amount accounts for 1-3wt% of the total mass of the mixed solution.

[0029] In a preferred embodiment of the present invention, the segmented drying in S4 includes three stages of heat treatment: the temperature of the first stage heat treatment is 60-80°C, and the time is 30-60 min; the temperature of the second stage heat treatment is 80-100°C, and the time is 1-3 h, and the vacuum degree is 5±1 kPa; the temperature of the third stage heat treatment is 100-120°C, and the time is 5-10 min.

[0030] The first stage heat treatment removes free solvent on the surface. The second stage heat treatment removes residual solvent inside. The second stage heat treatment eliminates internal stress and reduces the residual solvent content to <200ppm.

[0031] The three-stage heat treatment process (vacuum stage pressure 5±1kPa) can control the solvent residue to below 200ppm, avoiding battery cycle attenuation or flatulence problems caused by excessive solvent residue (≥500ppm).

[0032] The present invention also discloses a lithium-ion battery, comprising the UV-curable cross-linked lithium-ion battery separator, or a battery separator prepared by the method for preparing the UV-curable cross-linked lithium-ion battery separator. The lithium-ion battery has a capacity retention rate of ≥85% after 500 cycles at a 5C rate, a thermal shrinkage rate of ≤2.5% at 150°C, and an interface impedance growth rate of ≤5% / month in a 1M LiPF6 EC / DMC electrolyte.

[0033] Preferably, the base film in S1 is a polyethylene (PE) or polypropylene (PP) base film with a thickness of 4-20 μm and a porosity of 40-60%.

[0034] Preferably, the solvent in S2 includes other solvents and acetone in a volume ratio of 5:5 to 7:3, and the other solvent includes at least one of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and tetrahydrofuran (THF).

[0035] Preferably, the dispersion in S2 includes centrifugal stirring and ultrasonic treatment, the centrifugal stirring speed is 2000-4000 rpm, the time is 30-60 min, and the ultrasonic treatment is 400-600 W for 1-3 hours.

[0036] Preferably, the ultrasonic treatment adopts a pulse mode with a working cycle of 2s on / 1s off, a power of 200-500W, and a total time of 30min-2h.

[0037] Preferably, the slurry prepared in S2 has a viscosity of 3000-5000 mPa·s at 25° C., and a light transmittance of >75% at a wavelength of 365 nm.

[0038] Preferably, S2 is coated using micro-gravure coating with a coating thickness of 1-2 μm.

[0039] Preferably, the UV curing in S2 has a wavelength of 350-380 nm, a temperature of ≤60°C, an intensity of 40-60 mW / cm², and a duration of 20-40 seconds. This ensures that the double bond conversion rate during the UV curing process is ≥95%. The surface temperature of the coating after UV curing is ≤60°C.

[0040] The low-temperature characteristics of the UV curing process (≤60°C) are highly compatible with roll-to-roll production lines, which can greatly improve coating speed and product yield, and has significant industrial application value.

[0041] Preferably, the residual solvent content after the S3 gradient phase separation process is <200 ppm.

[0042] Preferably, S4 also includes interface optimization treatment: a hexagonal boron nitride buffer layer is constructed on the coating surface using aerosol deposition technology, wherein the 002 crystal orientation of the hexagonal boron nitride buffer layer is ≥80%, the XRD half-peak width is ≤0.5°, the buffer layer thickness is 2-10nm, and the interface impedance is ≤50Ω·cm².

[0043] Preferably, the surface density and thickness of the h-BN buffer layer satisfy the relationship: ρ=0.1+0.08t (ρ: mg / cm², t: nm), and the correlation coefficient R²≥0.95.

[0044] The following are the abbreviations of chemical formulas:

[0045] Polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP);

[0046] Alumina (Al2O3), lithium aluminate (LiAlO2), lithium lanthanum zirconium oxide (LLZO);

[0047] dipentaerythritol hexaacrylate (DPHA), trimethylolpropane triacrylate (TMPTA) or pentaerythritol tetraacrylate (PET4A).

[0048] The present invention constructs a polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) / lithium lanthanum zirconium oxide (LLZO) composite coating on the surface of the diaphragm through the following steps: 1) using argon oxygen plasma to activate the base film to increase the surface hydroxyl density to ≥5 / nm²; 2) preparing a slurry containing LLZO nanoparticles, a multifunctional crosslinking agent and a UV initiator, and then micro-gravure coating and UV curing to form a three-dimensional cross-linked network (cross-linking density ≥85%); 3) a gradient phase separation process combined with a porogen to form a dual-mode pore structure (porosity 40-50%); 4) staged drying and interface optimization treatment.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] The present invention significantly improves the interfacial bonding strength and high-temperature stability of the diaphragm coating through the synergistic optimization of plasma activation and ultraviolet light curing technology. Specifically, the surface hydroxyl density of the base film treated with argon-oxygen plasma reaches ≥5 / nm², and the three-dimensional cross-linked network (cross-linking density ≥85%) constructed by combining a multi-functional cross-linking agent (functionality ≥4) increases the coating peel strength to 78N / m, which is more than 50% higher than the traditional wet coating process, effectively suppressing the interfacial delamination phenomenon during the cycle. By coupling the gradient dual-mode pore structure design with the ultraviolet light curing process, the thermal shrinkage rate of the diaphragm at 150°C can be controlled within 2.5%, which is more than 60% lower than the uncross-linked system, significantly enhancing the thermal safety of the battery.

[0051] The present invention exhibits significant advantages in terms of interface bonding strength, ion conduction efficiency and adaptability to industrial production, and is suitable for high-performance lithium-ion batteries with a high-nickel ternary positive electrode and silicon-carbon negative electrode system. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 This is a schematic diagram of the structure of a composite coating diaphragm prepared according to one embodiment of the present invention.

[0053] Figure 2 This is a flow chart of the slurry preparation process in one embodiment of the present invention.

[0054] Figure 3 Schematic diagram of the UV curing and gradient phase separation process in one embodiment of the present invention.

[0055] Figure 4 This is an SEM image of a battery separator prepared in one embodiment of the present invention, forming a cross-linked network and a pore structure.

[0056] Figure 5 1 is a comparison diagram of the interfacial impedance of the battery separators prepared in the examples of the present invention and the comparative examples.

[0057] Figure 6 Graph showing the thermal shrinkage test results of the battery separators prepared in Examples 1-3 of the present invention and Comparative Example 1.

[0058] Figure 7 1 is a comparison chart of the cycle performance of the battery separators prepared in Examples 1-3 of the present invention and Comparative Example 1.

[0059] Figure 8 This is a test chart of the ionic conductivity of the battery separator prepared in Example 1 (25° C.) of the present invention.

[0060] Figure 9 This is a test chart of the ionic conductivity of the battery separator prepared in Example 2 of the present invention (25°C).

[0061] Figure 10This is a test chart of the ionic conductivity of the battery separator prepared in Example 3 (25° C.) of the present invention.

[0062] Figure 11 This is a test chart of the ionic conductivity of the battery separator prepared in Example 3 (-40°C) of the present invention.

[0063] Figure 12 This is a comparison chart of the ionic conductivity test of the battery separator prepared in Comparative Example 1 (25° C.) of the present invention.

[0064] Figure 13 This is a comparison diagram of the appearance of the soft-pack battery of the battery separator prepared in Example 1 of the present invention and Comparative Example 4; Figure 13 a is Example 1, Figure 13 b is comparative example 4 (battery swelling). DETAILED DESCRIPTION

[0065] The present invention addresses the problems of low coating interface bonding strength (peel strength < 50N / m), large high temperature thermal shrinkage (> 5%) and insufficient ionic conductivity (< 1×10 -4 S / cm) and other technical defects, a method for preparing a composite coating diaphragm based on ultraviolet curing and cross-linking technology is provided, comprising the following steps:

[0066] Step 1. Base film pretreatment and slurry preparation.

[0067] A polyethylene (PE) or polypropylene (PP) membrane was surface-modified using an argon-oxygen mixed plasma (Ar / O₂ volume ratio 3:1-5:1, power density 0.8-1.2 W / cm²) for 20-60 seconds to achieve a surface hydroxyl density of ≥5 / nm² (XPS analysis) and a contact angle ≤35°. Subsequently, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP, weight-average molecular weight 400 kDa ± 10%) was dissolved in a mixed solvent (volume ratio 5:5 to 7:3) consisting of at least one of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), tetrahydrofuran (THF), and acetone. Lithium lanthanum zirconium oxide (LLZO) nanoparticles (50-300 nm in diameter) coated with 2-5 nm aluminum oxide (Al₂O₃) or lithium metaaluminate (LiAlO₂) were added, along with a multifunctional acrylate crosslinker and a UV photoinitiator. After high-speed centrifugal dispersion (2000-4000 rpm, 30-60 min) and pulse ultrasonic treatment (200-500 W, 2 s on / 1 s off, 30 min-2 h), a stable slurry with a viscosity of 4000 ± 500 mPa·s (25°C) was obtained.

[0068] Step 2. UV curing and gradient phase separation

[0069] A 1-2μm coating is formed on the surface of the activated base film through a micro-gravure coating process (anilox roller line count 200±10 lines / inch). Subsequently, a 350-380nm UV light source (irradiation intensity 40-60mW / cm²) is used for 20-40 seconds in a nitrogen atmosphere (oxygen content <200ppm) to initiate a photopolymerization reaction of the crosslinker (double bond conversion rate >95%), forming a three-dimensional network structure with a crosslinking density ≥85%.

[0070] Subsequently, a gradient solvent replacement was performed: the film was immersed in a mixed solution of at least one of NMP, DMF, and THF and water, with the volume ratios of the gradient mixed solutions being 7:3-8:2, 4:6-5:5, and 0:10, respectively. The treatment time for each stage was 5-7 minutes, 3±0.5 minutes, and 15±5 seconds, respectively. A porogen (1-3 wt%) was introduced simultaneously, and finally a porous coating with a bimodal pore size distribution (macroporous: 200-1000 nm, mesopore: 50-200 nm) and a porosity of 40-50% was obtained.

[0071] Step 3. Thermal densification and interface modification

[0072] A three-stage heat treatment process was employed: the first stage (60-80°C, 30-60 minutes) removed free surface solvent; the second stage (80-100°C, 1-3 hours, vacuum 5±1 kPa) removed residual internal solvent; and the third stage (100-120°C, 5-10 minutes) eliminated internal stress, reducing the residual solvent content to <200 ppm. Finally, a 2-10 nm thick hexagonal boron nitride (h-BN) nanobuffer layer was deposited on the coating surface via aerosol deposition. The surface density and thickness of the h-BN buffer layer satisfied the relationship: ρ = 0.1 + 0.08t (ρ: mg / cm², t: nm), with a correlation coefficient R² ≥ 0.95. The (002) crystal orientation was ≥ 80% (XRD half-peak width ≤ 0.5°), reducing the interfacial impedance to below 50 Ω·cm².

[0073] Example 1:

[0074] This embodiment provides a method for preparing a UV-curable cross-linked lithium-ion battery separator, and the specific implementation steps are as follows:

[0075] A 12μm-thick, 45%-porosity PE-based membrane was plasma-treated for 30 seconds using an argon-oxygen mixture (Ar / O₂ volume ratio 4:1) at a power density of 1.0W / cm². X-ray photoelectron spectroscopy (XPS) analysis revealed a surface hydroxyl density of 5.2 per nm², a decrease in the contact angle from an initial 105° to 32°, and an increase in surface energy to 52.3mJ / m² (as measured by the Owens-Wendt method).

[0076] The sol-gel method was used to prepare lithium lanthanum zirconium oxide (Li 6.4 La3Zr 1.4 Ta 0.6 O 12 The surface of LLZO) nanoparticles is coated with an aluminum oxide (Al2O3) layer, which specifically includes the following steps: first, aluminum isopropoxide is dissolved in anhydrous ethanol to prepare a precursor solution with a concentration of 0.1 mol / L, and 0.5 wt% of acetylacetone is added as a chelating agent, and stirred until the solution becomes transparent; then, Li 6.4 La3Zr 1.4 Ta 0.6 O 12 LLZO nanoparticles were dispersed in ethanol at a 10 wt% solids content and ultrasonically treated at 300 W for 30 minutes to obtain a uniformly dispersed LLZO suspension. This suspension was then added dropwise to a precursor solution with a controlled aluminum to LLZO molar ratio of 1:50. The mixture was stirred at 60°C for 6 hours, during which the pH of the reaction system was adjusted to 4.0 with acetic acid. After completion of the reaction, the mixture was aged at room temperature for 12 hours, centrifuged, dried in a vacuum at 60°C, and calcined at 400°C under an argon atmosphere at a heating rate of 5°C / min for 2 hours to obtain LLZO@Al2O3 composite nanoparticles coated with a 2.0±0.3 nm aluminum oxide layer. High-resolution transmission electron microscopy (HR-TEM) revealed a uniform aluminum oxide coating with a thickness of 2.0±0.3 nm. Energy-dispersive X-ray spectroscopy (EDS) analysis confirmed a continuous distribution of aluminum on the surface of the LLZO particles.

[0077] A 6:4 volume ratio of NMP and acetone was used as the solvent system. PVDF-HFP with a weight-average molecular weight of 400 kDa was added and stirred at 60°C until completely dissolved. LLZO@Al2O3 composite nanoparticles (D50 = 200 nm) coated with 2.0 nm Al2O3, DPHA (20% of the polymer mass), and PI-2959 (5%) were then added sequentially. The mixture was dispersed by centrifugation at 3000 rpm for 30 minutes and treated with 500 W pulsed ultrasound for 2 hours to produce a homogeneous slurry with a viscosity of 4000 mPa·s (25°C) and a transmittance of >75% (at 365 nm).

[0078] A 2μm-thick coating was formed on the surface of the activated base film using a micro-gravure coating process. The film was then irradiated with ultraviolet light at a wavelength of 365nm and an intensity of 50mW / cm² for 30 seconds in a nitrogen atmosphere (oxygen concentration <200ppm). Fourier transform infrared spectroscopy revealed a 96.5% conversion of C=C double bonds, forming a three-dimensional network structure with a cross-link density of 86.2%. The coating surface temperature was kept below 58°C during the curing process.

[0079] The cured membrane was immersed in a 7:3 NMP / water mixture (containing 1% PEG400) for 5 minutes, followed by a 4:6 NMP / water mixture (containing 4:6 NMP / water) for 3 minutes. Finally, it was rinsed with pure water for 10 seconds. Mercury intrusion porosimetry revealed a porosity of 48% and a bimodal pore size distribution: macropores ranging from 500 to 1000 nm, mesopores ranging from 100 to 200 nm, and a pore tortuosity of 1.75.

[0080] A three-stage drying procedure was implemented: hot air drying at 60°C for 30 minutes to remove surface solvent, vacuum drying at 80°C for 2 hours (vacuum degree 5kPa) to remove internal residual solvent, and annealing at 100°C for 10 minutes to eliminate internal stress. The final solvent residue was <180ppm (gas chromatography-mass spectrometry analysis). An h-BN buffer layer was constructed on the coating surface using aerosol deposition technology, with a controlled thickness of 5.0nm and a surface density of 0.5mg / cm². X-ray diffraction analysis showed that the (002) crystal orientation was 83%, the half-height width was 0.47°, and the electrode / diaphragm interface impedance was reduced to 48Ω·cm². After immersion in 1M LiPF6 EC / DMC electrolyte for 30 days, the interface impedance was 50Ω·cm², with an increase rate of 4.2%.

[0081] The positive and negative active materials used are commercially available SiC (600 mAh / g) and NCM622. The electrolyte is a 1M LiPF6 solution in EC / DMC (1:1 volume ratio), supplemented with 2wt% FEC and 1wt% VC. The battery is assembled into a button cell or pouch cell.

[0082] The raw materials and reagents used in Example 1 are shown in Table 1.

[0083] The implementation results are shown in Table 2.

[0084] Example 2

[0085] This embodiment makes the following optimization adjustments based on embodiment 1 to reduce production costs while maintaining core performance:

[0086] Parameter adjustments: TMPTA and DPHA were mixed in a mass ratio of 3:1, and the total addition amount was reduced to 15% (TMPTA 11.25%, DPHA 3.75%). UV-184 was used, and the addition amount was adjusted to 3%. The Al₂O₃ (2.0 nm) coating on the surface of the LLZO nanoparticles was retained, but the LLZO mass percentage was reduced to 25%. PVP K30 was used as a porogen, and the addition amount was increased to 2%. The UV curing time was shortened to 25 seconds, and the irradiation intensity was reduced to 45 mW / cm². Other parameters were the same as in Example 1.

[0087] The implementation results are shown in Table 3.

[0088] Example 3

[0089] This embodiment makes the following optimization adjustments based on the first embodiment to target high-rate charge and discharge scenarios. The specific adjustment scheme is as follows:

[0090] Parameter adjustment: Ta-doped LLZO (Li 6.4 La3Zr 1.4 Ta 0.6 O 12 ), the surface was coated with a 3.0nm LiAlO2 layer, and the mass ratio was increased to 45%; PET4A was selected as the main cross-linking agent, and the addition amount was 25%, and 0.3% hindered amine light stabilizer (Tinuvin 770) was added simultaneously; PI-819 was used, and the addition amount was 1.5%, matched with a 395nm high-power LED light source; the water extraction gradient was adjusted to DMF / water: 8:2, 5:5, 0:10, and the first stage treatment time was extended to 7 minutes; the h-BN buffer layer thickness was controlled to 3.0±0.5nm, and the surface density was 0.3-0.38mg / cm²; the remaining parameters were the same as in Example 1.

[0091] The implementation results are shown in Table 4.

[0092] Comparative Example 1

[0093] This comparative example uses the same substrate and basic process as Example 1, but eliminates the photocuring and crosslinking step to simulate a traditional coating system. The specific implementation differences are as follows:

[0094] No DPHA and UV initiator are added to the slurry; the UV curing step is eliminated, and the wet film is directly dried; the coating has a physical mixed structure and no three-dimensional cross-linked network; the surface hydroxyl density maintains the initial value of the base film.

[0095] Performance comparison is shown in Table 5.

[0096] Comparative Example 2

[0097] The difference between this comparative example and Example 1 is that the crosslinking agent is replaced by trimethylolpropane triacrylate (TMPTA) with a functionality of 3, and the addition amount is 20%. The other parameters are the same as those in Example 1. The test results show that the double bond conversion rate is 89.2%, the crosslinking density is 78%, the peel strength is 48N / m, the thermal shrinkage at 150℃ is 4.1%, and the ionic conductivity is 3.1×10 -4 S / cm, the interface impedance increased from 62Ω·cm² (initial) to 87Ω·cm² (30 days), and the capacity retention rate after 500 cycles at 5C was 85%.

[0098] Comparative Example 3

[0099] The difference from Example 1 is that the plasma treatment time is shortened to 10 seconds, the hydroxyl density of the base film surface is 2.8 / nm², and the other parameters are the same as Example 1. The test results show that the cross-linking density is 85%, the coating peel strength is 42N / m, the thermal shrinkage at 150℃ is 2.0%, and the ionic conductivity is 3.8×10 -4 S / cm, the interface impedance increased from 75Ω·cm² (initial) to 90Ω·cm² (30 days), and the capacity retention rate after 500 cycles at 5C was only 71%. Due to insufficient hydroxyl density leading to interface degradation, the interface impedance increased to 127Ω·cm² after 500 cycles at 5C, a growth rate of 56%.

[0100] Comparative Example 4

[0101] The only difference from Example 1 is that the argon-oxygen plasma treatment in step S1 is omitted, and the hydroxyl density on the surface of the base film is maintained at the original value (1.2 / nm², contact angle 105°). The other parameters are exactly the same as in Example 1. The hydroxyl density on the surface of the base film without plasma activation is low (1.2 / nm²), resulting in the inability to form an effective chemical bond between the coating and the base film, resulting in a sharp decrease in interfacial bonding strength (peel strength 18 N / m). At the same time, due to the increase in interfacial defects, ion transmission is hindered (the electrical conductivity drops to 6.5×10 -5 S / cm), and the difference in thermal expansion coefficient between the base film and the coating at high temperature causes serious delamination (thermal shrinkage rate 8.5%).

[0102] The performance comparison of Comparative Example 2, Comparative Example 3 and Comparative Example 4 with Example 1 is shown in Table 6.

[0103] Comparative Example 5

[0104] The difference from Example 1 is that the vacuum degree in the three-stage heat treatment is 3kPa, and the other parameters are the same as Example 1. The test results show that the solvent removal is not complete, the residual amount is ≥500ppm, and the battery capacity retention rate after 1C100 cycles is only 67%. Excessive solvent residue leads to increased side reactions inside the battery, resulting in battery bloating ( Figure 13 ).

[0105] Technical effect verification

[0106] 1. Peel strength test: According to ASTM D903, using an Instron 5967 universal testing machine at a tensile speed of 50 mm / min.

[0107] 2. High temperature stability verification: The diaphragm was placed in an oven for 1 hour and the dimensional change rate was measured using a laser rangefinder.

[0108] 3. Ionic conductivity test method: The composite coating was peeled from the base film surface, and a 16 mm diameter freestanding membrane sample was prepared using a precision microtome. Using a Chenhua CH760e electrochemical workstation, ionic conductivity was calculated using the formula σ = L / (R·A), where L is the coating thickness, R is the measured impedance, and A is the electrode area. The test frequency range was 0.1 Hz to 1 MHz.

Claims

1. A method for preparing a UV-curable cross-linked lithium-ion battery separator, characterized in that: The following steps are involved: S1. Activate the base film using argon-oxygen plasma to increase the hydroxyl density on the surface of the base film to ≥5 / nm² and the contact angle to ≤35°, thereby obtaining an activated base film; S2, mixing a solvent, a polymer matrix, nanoparticles, a multifunctional crosslinking agent and a UV initiator, and evenly dispersing the mixture to obtain a slurry, coating the slurry on the surface of the base film activated in S1, and then curing the slurry with UV light to form a three-dimensional crosslinked network coating, so that the crosslinking density is ≥85%; S3, using a gradient phase separation process to form a porous coating with a bimodal pore size distribution on the three-dimensional cross-linked network coating in S2, so that the porosity is 40-50%; S4, drying the porous coating in S3 in sections to obtain a UV-curable cross-linked lithium-ion battery separator; The gradient phase separation process in S3 includes sequentially immersing the UV-cured membrane in a first gradient mixed solution comprising an organic solvent and water, a second gradient mixed solution, and a third gradient mixed solution, wherein the organic solvent is at least one of N-methylpyrrolidone, N,N-dimethylformamide, and tetrahydrofuran; the volume ratio of the organic solvent to water in the first gradient mixed solution is 7:3-8:2, and the immersion time is 5-7 minutes; the volume ratio of the organic solvent to water in the second gradient mixed solution is 4:6-5:5, and the immersion time is 3±0.5 minutes; the volume ratio of the organic solvent to water in the third gradient mixed solution is 0:10, and the immersion time is 15±5 seconds; the first gradient mixed solution further includes a porogen, and the mass percentage of the porogen is 1-3wt%; The UV-curable cross-linked lithium-ion battery separator includes a base film and a UV-curable cross-linked coating coated on the surface of the base film. The hydroxyl density on the surface of the base film is ≥5 / nm² and the contact angle is ≤35°. The UV-curable cross-linked coating includes a polymer matrix, nanoparticles, a UV initiator and a multi-functional acrylate cross-linker with a functionality ≥4.

2. The method for preparing a UV-curable cross-linked lithium-ion battery separator according to claim 1, wherein: In S1, argon-oxygen plasma activation of the base film uses an argon-oxygen mixed gas with an Ar / O2 volume ratio of 3:1-5:1, a power density of 0.8-1.2 W / cm², and a time of 20-60 s.

3. The method for preparing a UV-curable cross-linked lithium-ion battery separator according to claim 1, wherein: The segmented drying in S4 includes three stages of heat treatment: the first stage heat treatment temperature is 60-80℃, the time is 30-60min; the second stage heat treatment temperature is 80-100℃, the time is 1-3h, and the vacuum degree is 5±1kPa; the third stage heat treatment temperature is 100-120℃, the time is 5-10min.

4. The method for preparing a UV-curable cross-linked lithium-ion battery separator according to claim 1, wherein: The peel strength of the UV-curable cross-linked lithium-ion battery separator is 65-85 N / m, and the ionic conductivity at 25°C is 3×10⁻ 4 ~5.5×10⁻ 4 S / cm, thermal shrinkage at 150℃<2.5%.

5. The method for preparing a UV-curable cross-linked lithium-ion battery separator according to claim 1, wherein: The mass ratio of the polymer matrix, the nanoparticles, the multifunctional acrylate crosslinking agent with a functionality of ≥4 and the ultraviolet light initiator is 100:25-50:5-30:0.3-5.

6. The method for preparing a UV-curable cross-linked lithium-ion battery separator according to claim 1, wherein: The nanoparticles include lithium lanthanum zirconium oxide nanoparticles and a coating particle layer coated on the surface of the lithium lanthanum zirconium oxide nanoparticles, and the coating particle layer includes one or two of aluminum oxide and lithium metaaluminate.

7. The method for preparing a UV-curable cross-linked lithium-ion battery separator according to claim 1, wherein: The polymer matrix includes polyvinylidene fluoride-hexafluoropropylene, and the multifunctional acrylate crosslinking agent includes one or both of dipentaerythritol hexaacrylate and pentaerythritol tetraacrylate.

8. A lithium-ion battery, characterized in that: The battery separator prepared by the preparation method of the UV-curable cross-linked lithium-ion battery separator according to any one of claims 1 to 7, wherein the lithium-ion battery has a capacity retention rate of ≥85% after 500 cycles at a 5C rate, a thermal shrinkage rate of ≤2.5% at 150°C, and an interface impedance growth rate of ≤5% / month in a 1M LiPF6 EC / DMC electrolyte.

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