High-temperature-resistant and puncture-resistant lithium ion battery separator and preparation method thereof

Abstract

The application discloses a high-temperature-resistant and puncture-resistant lithium ion battery diaphragm and a preparation method thereof. The preparation method comprises the following steps: (1) performing corona treatment on the surface of a PP base film, so that the surface of the PP base film presents a concave-convex undulating shape, and the depth of the concave pits is 50-70 nm; (2) performing cleaning, static electricity removing and other surface drying treatments on the PP base film subjected to the corona treatment; (3) mixing phase change material, nano zirconium oxide particles, a polyisocyanate adhesive and sodium bicarbonate, and adding deionized water to form a functional coating slurry; and (4) uniformly coating the functional coating slurry on the surface of the treated PP base film, drying, volatilizing the sodium bicarbonate, forming a porous functional coating, and protruding the polyisocyanate adhesive from the surface of the porous functional coating, so that the diaphragm is obtained. The diaphragm prepared by the application has the advantages of excellent heat conductivity, high heat resistance, puncture resistance, high liquid absorption rate and liquid retention rate, and stability, and is not easy to be delaminated and separated from the base film.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a high-temperature-resistant and puncture-resistant lithium-ion battery separator and a preparation method thereof. Background Art

[0002] Lithium-ion batteries have been highly sought after by the market for their superior performance and high energy density. The safety, stability, and lifespan of lithium-ion batteries largely depend on the quality and performance of their internal components, with the separator, as a core component of the battery, being particularly crucial in terms of its function and performance.

[0003] Polyolefin separators are widely used in many lithium-ion battery configurations. This is mainly due to the electrochemical stability and reasonable cost of polyolefins. However, polyolefin separators also have some serious problems. First, their membrane rupture temperature is low, which makes it difficult to quickly dissipate the heat generated during the battery charging and discharging process, which may cause the internal temperature of the battery to rise sharply. More seriously, polyolefin separators have poor heat resistance. Once the temperature reaches or approaches its melting point (for example, 120°C for polyethylene separators), the separator may undergo severe deformation, shrinkage, or even break. In this case, the positive and negative poles of the battery may come into direct contact, causing a short circuit and creating safety risks, including but not limited to overheating, combustion, and explosion of the battery.

[0004] To address the heat resistance issues of polyolefin separators, researchers have attempted to coat them with a ceramic layer. However, this treatment still has drawbacks. The adhesive in the coating can form an impenetrable adhesive layer on the surface of the polyolefin separator, significantly reducing the permeability of the ceramic separator. This dense layer hinders the rapid transfer of lithium ions, thereby affecting the battery's charge and discharge performance. Worse still, as the battery uses, delamination between the adhesive layer and the base film can occur, further impairing battery performance and shortening its lifespan.

[0005] Despite numerous attempts and research efforts to improve porous polyolefin separators, most still suffer from the problem of delamination between the coating and the base membrane after electrolyte infiltration, which can lead to serious problems such as short circuits and swelling in the battery. These issues not only threaten battery performance and stability, but also directly affect user safety.

[0006] Therefore, for the field of lithium-ion batteries, developing a diaphragm material that has excellent thermal conductivity, high heat resistance, stability, and is not easy to delaminate from the base membrane has become an important research and development direction and urgent task. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for preparing a high-temperature resistant and puncture-resistant lithium-ion battery separator. The separator prepared by the present invention has excellent thermal conductivity, high heat resistance, puncture resistance, high liquid absorption rate and liquid retention rate; it can also maintain stability and is not easy to delaminate and separate from the base film.

[0008] In order to achieve the above object, the present invention provides the following technical solutions:

[0009] The present invention provides a method for preparing a high-temperature-resistant and puncture-resistant lithium-ion battery separator, comprising the following steps:

[0010] (1) performing corona treatment on the surface of the PP base film, wherein after the corona treatment, the surface of the PP base film is concave and convex, and the depth of the pits is 50 to 70 nm;

[0011] (2) cleaning and destaticizing the corona-treated PP base film, and drying the surface to obtain a surface-treated PP base film;

[0012] (3) mixing a phase change material, nano zirconium oxide particles, a polyisocyanate adhesive, and sodium bicarbonate in a weight ratio of (25-35):(50-60):(5-8):(5-8), and adding deionized water to form a functional coating slurry;

[0013] (4) The functional coating slurry is evenly coated on the treated PP base film surface, dried, and the sodium bicarbonate is volatilized to form a porous functional coating, and the polyisocyanate adhesive protrudes from the surface of the porous functional coating, thereby obtaining the lithium ion battery separator.

[0014] Preferably, in step (1), the voltage of the corona treatment is 2.0-2.2 kV, the current is 8 A, the corona speed is 40-50 m / min, and the treatment time is 8-10 s.

[0015] Preferably, in step (2), the surface wetting tension A (dyn), surface roughness Ra (μm) and friction coefficient μs of the surface-treated PP base film satisfy the relationship: A=20Ra+15μs+c, wherein 30≤c≤35.

[0016] Preferably, in step (1), the PP base film is a PP film grafted with diethylene glycol dimethacrylate by ultraviolet light irradiation.

[0017] Preferably, the thickness of the PP base film is 5 to 15 μm; the thickness of the functional coating is 1 to 15 μm.

[0018] Preferably, in step (3), the average particle size R1 of the polyisocyanate adhesive is respectively greater than the average particle size R2 of the phase change material, the average particle size R2 of the nano zirconium oxide particles, and the average particle size R4 of sodium bicarbonate; and it satisfies the following relationship: R1 / R2=(2-10):1, R1 / R3=(2-10):1, R1 / R4=(2-10):1.

[0019] Preferably, the average particle size of the polyisocyanate adhesive is 0.5 to 10 μm; the average particle sizes of the phase change material, nano zirconium oxide particles and sodium bicarbonate are respectively 0.05 to 5 μm.

[0020] Preferably, in step (3), the solid content of the functional coating slurry may be 10 wt % to 90 wt %.

[0021] Preferably, in step (3), the phase change material is polyethylene glycol, and the phase change temperature is 37-41° C. Preferably, the average relative molecular mass of the polyethylene glycol is 950-1050.

[0022] Preferably, in step (3), a crown ether-lithium complex is added to the slurry in an amount of 5% by weight of the total weight of the functional coating slurry. Crown ether is an oxygen-containing heterocyclic compound in which the oxygen atoms can stably coordinate lithium ions. The crown ether-lithium complex can provide a stable and controllable lithium source, effectively replenishing the lithium ions consumed during the first cycle, thereby improving the battery's initial efficiency and cycle performance.

[0023] Preferably, in step (4), the drying temperature is 80-120°C, and sodium bicarbonate begins to thermally decompose at 50°C.

[0024] Preferably, in step (4), the coating method is not particularly limited, and any method known in the art can be used as long as the functional coating slurry can be evenly coated on the base film. For example, micro-gravure or doctor blade coating can be used.

[0025] Preferably, in step (4), the functional coating slurry includes a first coating slurry, a second coating slurry and an Nth coating slurry, N ≥ 3; wherein the sodium bicarbonate particle size D1 in the first coating slurry, the sodium bicarbonate particle size D2 in the second coating slurry and the sodium bicarbonate particle size DN in the Nth coating slurry satisfy the relationship DN>D2>D1;

[0026] The first coating slurry, the second coating slurry and the Nth coating slurry are sequentially coated on the treated PP base film surface, dried, and the sodium bicarbonate is volatilized to form a porous functional coating with pores gradually increasing from the inside to the outside.

[0027] The present invention also provides a high-temperature resistant and puncture-resistant lithium-ion battery separator, which is prepared by the above-mentioned preparation method of the high-temperature resistant and puncture-resistant lithium-ion battery separator.

[0028] The present invention also provides a lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet and a separator spaced between the positive electrode sheet and the negative electrode sheet, wherein the separator is the above-mentioned high-temperature-resistant and puncture-resistant lithium-ion battery separator.

[0029] Compared with the prior art, the present invention has at least the following beneficial effects:

[0030] (1) The present invention performs corona treatment on the PP base film. Under the action of the corona current, the electron flow exerts a strong impact on the PP base film, which can cause the surface of the base film to become fluffy and increase the surface tension of the PP base film, which helps the adhesion and bonding of the adhesive. When the adhesive contacts its surface, it can produce good infiltration, so that the base film is firmly bonded. If observed under a high-power magnifying glass, the surface of the treated base film is obviously uneven and rough compared to the untreated base film. In addition, under the action of the high-voltage electric field, a large amount of ozone is generated. Ozone is a strong oxidant that can oxidize polypropylene molecules and produce carbonyl groups and peroxides with strong polarity. With this structure, the polarity of the base film molecules increases, the surface tension increases, the affinity for the adhesive increases, and the bonding strength between the base film and the coating is improved. In addition, due to the generation of carbonyl groups, new α-carbon atoms are generated in the molecular chain, and active hydrogen appears. This active hydrogen can react chemically with the active group isocyanate (-NCO) in the polyisocyanate adhesive, forming a strong chemical bond between the bonded material and the adhesive, further increasing the bonding strength, thereby effectively avoiding the occurrence of interlayer separation, extending the service life of the electrochemical power supply composite diaphragm, and thus extending the safety performance and cycle life of the electrochemical power supply.

[0031] 2) The present invention provides a functional coating, and the phase change material has good thermal conductivity and sufficiently large latent heat, which can effectively transfer or absorb heat, avoiding overheating caused by charging and discharging inside the battery, thereby effectively avoiding the existing diaphragm from shrinking or even rupturing due to heat, resulting in the possibility of direct contact between the positive and negative electrodes inside the power supply and short circuit, causing battery safety hazards. In addition, the sodium bicarbonate added to the functional coating can undergo thermal decomposition during the subsequent drying process to leave gaps, increase the pore structure of the functional coating, not only improve the transmission efficiency of lithium ions, but also improve the liquid absorption and liquid retention rate of the diaphragm, and also improve the heat dissipation effect of the diaphragm. Nano-zirconia particles can not only improve the temperature resistance of the diaphragm, but also effectively improve the mechanical strength and puncture resistance of the diaphragm.

[0032] 3) By comparing PP base films before and after treatment, the present invention found that the maximum surface depth of the uncorona-treated PP film did not exceed 28 nm. After corona treatment, the maximum depth reached 70 nm, and the surface exhibited distinct undulations, with the raised areas appearing brighter. Analysis revealed these brighter areas to be particulate matter, primarily composed of low-molecular-weight oxides. The changes in surface microroughness and the increase in microscopic real area clearly reflect the effects of corona treatment on the structure and composition of the PP film. Furthermore, the protruding polyisocyanate adhesive within the porous functional coating can bond to the electrode in lithium-ion batteries, suppressing deformation and ensuring battery safety and smoothness. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Schematic diagram of the structure of the diaphragm in one embodiment of the present invention.

[0034] Among them, 1-PP base film, 2-functional coating, 21-polyisocyanate adhesive, 22-phase change material, 23-nano zirconium oxide particles, 24-sodium bicarbonate. DETAILED DESCRIPTION

[0035] To make the technical solutions and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0036] According to the first aspect of the present application, the present application provides a method for preparing a high-temperature resistant and puncture-resistant lithium-ion battery separator, comprising the following steps:

[0037] (1) performing corona treatment on the surface of the PP base film, wherein after the corona treatment, the surface of the PP base film is concave and convex, and the depth of the pits is 50 to 70 nm;

[0038] (2) cleaning and destaticizing the corona-treated PP base film, and drying the surface to obtain a surface-treated PP base film;

[0039] (3) mixing a phase change material, nano zirconium oxide particles, a polyisocyanate adhesive, and sodium bicarbonate in a weight ratio of (25-35):(50-60):(5-8):(5-8), and adding deionized water to form a functional coating slurry;

[0040] (4) The functional coating slurry is evenly coated on the treated surface of the PP base film, dried, and the sodium bicarbonate is volatilized to form a functional coating, thereby obtaining the high temperature resistant and puncture resistant lithium ion battery separator.

[0041] In one embodiment of the present application, in step (1), the corona treatment voltage is 2.0-2.2 kV, the current is 8 A, the corona speed is 40-50 m / min, and the treatment time is 8-10 s. The inventors tested the adhesion and firmness of the functional coating and found that the adhesion and firmness of the functional coating on the PP base film after corona treatment were significantly improved. When the corona current reached 8 A, the firmness of the functional coating on the surface of the PP base film was the highest. When the corona current was greater than 8 A, the adhesion of the functional coating showed a sharp decline. It can be seen that the size of the corona current plays a decisive role in the adhesion of the PP base film.

[0042] As the corona current increases, the kinetic energy of the particles generated by the corona discharge increases, facilitating the breaking of chemical bonds within the long molecular chains on the plastic surface. This gradually increases the surface activity and, consequently, the surface tension. Therefore, the surface tension of the PP film increases with increasing corona current, facilitating adhesive adhesion and bonding. Further research revealed that the peak surface tension of the PP film occurs at a corona current of approximately 8A. As the current increases, the surface tension actually decreases. This is because the amount of air between the electrode and the corona roller has reached a relatively stable state, where the oxygen content in the air is constant. Even increasing the electrode voltage and current cannot activate more oxygen molecules, allowing more oxygen-containing functional groups to remain on the PP film surface. Furthermore, excessive corona discharge severely damages the film's surface structure. Consequently, as the corona current increases, the surface tension of the PP film decreases rapidly. Compared to untreated PP film, the porous carbon coating on the corona-treated PP film exhibits significantly improved durability. With the increase of corona current intensity, the firmness of ink film on the surface of PP base film is greatly improved. After corona treatment with 8A current, the surface tension of PP base film reaches the maximum.

[0043] In addition, if the corona speed is too fast or the corona treatment time is too short, insufficient corona treatment will occur, which will lead to a decrease in the adhesion of the base film. If the corona time is too slow or the corona treatment time is too long, the corona will easily penetrate the base film, which will lead to the adhesive's back-sticking. Therefore, it is necessary to control the corona speed and corona time within the appropriate range.

[0044] Comparing PP base films before and after treatment, the inventors found that the maximum surface depth of the untreated PP film was no more than 28 nm. After treatment, the maximum depth reached 70 nm, and the surface exhibited distinct unevenness, with the raised areas appearing brighter. Analysis revealed these brighter areas to be granular, primarily composed of low-molecular-weight oxides. This change in surface microroughness and the increase in microscopic true area clearly reflect the effects of corona treatment on the structure and composition of the PP film.

[0045] In one embodiment of the present application, in step (2), the surface wetting tension A (dyn), surface roughness Ra (μm), and friction coefficient μs of the surface-treated PP base film satisfy the relationship: A = 20Ra + 15μs + c, where 30≤c≤35. This relationship shows that there is a specific linear relationship between the surface wetting tension A of the PP base film and the surface roughness Ra and friction coefficient μs. When the friction coefficient μs or surface roughness Ra increases, the surface wetting tension A will also increase accordingly, indicating that the greater the friction between the object and the slurry, the better the surface wetting effect. This relationship can be used to determine the required surface wetting tension by controlling the surface roughness and friction coefficient during the actual preparation process, so as to better apply the functional coating slurry, thereby improving the high-resistance adhesion and performance stability.

[0046] In one embodiment of the present application, the value range of A is 38 to 42 dyn, the value range of Ra is 0.08 to 0.16 μm, and the value range of μs is 0.4 to 0.7. Among them, as the surface roughness increases, the surface contact area increases, and the tension of the slurry also increases accordingly. However, when the surface is too rough, the slurry is unevenly distributed, resulting in a smaller contact angle and a decrease in the surface wetting tension. Therefore, it is necessary to control the above parameters within an appropriate range. Therefore, the above relationship and parameter range can be used to control the properties of the PP base film surface during the actual preparation process. By controlling the relationship between the surface wetting tension, surface roughness and friction coefficient of the PP base film after surface treatment, it can be ensured that the base film surface has good wettability and roughness, which is beneficial to the coating of the functional coating and the bonding with the base film.

[0047] In one embodiment of the present application, in step (3), the average particle size R1 of the polyisocyanate adhesive is larger than the average particle size R2 of the phase change material, the average particle size R3 of the nano-zirconia particles, and the average particle size R4 of the sodium bicarbonate, and the average particle size R1 / R2=(2-10):1, R1 / R3=(2-10):1, and R1 / R4=(2-10):1. By controlling the particle sizes of the polyisocyanate adhesive, the phase change material, the nano-zirconia particles, and the sodium bicarbonate particles, the polyisocyanate adhesive is protruded from the surface of the porous functional coating, making the preparation method of the diaphragm of the present invention simple and easy to operate, and ensuring that the diaphragm has high consistency.

[0048] In one embodiment of the present application, the average particle size of the polyisocyanate adhesive is 0.5-10 μm; the average particle sizes of the phase change material, nano zirconium oxide particles, and sodium bicarbonate are respectively 0.05-5 μm.

[0049] In one embodiment of the present application, the PP base film has a thickness of 5 to 15 μm; the functional coating has a thickness of 1 to 15 μm. By controlling the thickness of the PP base film and the functional coating, the heat dissipation and ionic conductivity of the separator can be further optimized. This design ensures a better thickness match between the PP base film and the functional coating, avoiding performance instability caused by uneven thickness.

[0050] In one embodiment of the present application, in step (3), the phase change material is polyethylene glycol, and the phase change temperature is 37-41°C. Polyethylene glycol is an organic solid-solid phase change material (the ordered molecular connection structure in the solid state becomes a disordered molecular connection structure in the solid state), and its phase change temperature increases with the increase of the degree of polymerization. Therefore, its average relative molecular weight cannot be too high or too low. When the phase change temperature (37-41°C) is reached, a solid-solid phase change occurs, absorbing heat, further improving the heat dissipation effect of the functional coating. Although polyolefin separators can provide sufficient mechanical strength and chemical stability at room temperature, they exhibit large thermal shrinkage under high temperature conditions, resulting in contact between the positive and negative electrodes and rapid accumulation of a large amount of heat. For example, PP / PE composite separators can first melt the PE at a lower temperature (120°C) to block the micropores in the cold-proof polymer, blocking ion conduction while PP still plays a supporting role to prevent further electrode reactions. However, since the melting temperature of PP is only 150°C, when the temperature rises rapidly and exceeds the melting temperature of PP, the melting of the separator will cause a large-area short circuit and trigger thermal runaway, aggravating heat accumulation, generating high pressure inside the battery, and causing the battery to burn or explode. The phase change material in the functional coating of this application has sufficiently large latent heat to suppress the trend of rapid temperature rise inside the battery, prevent battery short circuit or thermal runaway, provide protection for battery safety, and extend battery life.

[0051] In one embodiment of the present application, in step (3), a crown ether-lithium complex is added to the slurry in an amount of 5% by weight of the total weight of the functional coating slurry. The crown ether is an oxygen-containing heterocyclic compound in which the oxygen atoms can stably coordinate lithium ions. The crown ether-lithium complex can provide a stable and controllable lithium source, effectively ensuring the release of active lithium during battery charging to compensate for the initial irreversible lithium loss, thereby forming a SEI film on the negative electrode surface and further improving the initial efficiency and cycle performance of the lithium-ion battery.

[0052] In one embodiment of the present application, in step (1), the PP base film is a PP film grafted with diethylene glycol dimethacrylate by ultraviolet light irradiation. Conventional PP base films lack polar groups, resulting in low surface properties. Although they can be well infiltrated in polar carbonate electrolytes, they have poor liquid absorption performance and low ionic conductivity. The liquid absorption rate affects the injection time and battery resistance during assembly of the diaphragm and battery, so having good liquid absorption performance is extremely important for battery performance. The present invention can improve the liquid absorption rate and liquid retention rate of the diaphragm by grafting polar groups onto the PP base film, thereby improving the cycle performance of the battery.

[0053] In one embodiment of the present application, in step (4), the drying temperature is 80-120° C. The functional coating is solidified by drying, and the sodium bicarbonate is volatilized to form a pore structure.

[0054] In one embodiment of the present application, in step (4), the functional coating slurry includes a first coating slurry, a second coating slurry, and an Nth coating slurry, where N≥3; wherein the sodium bicarbonate particle size D1 in the first coating slurry, the sodium bicarbonate particle size D2 in the second coating slurry, and the sodium bicarbonate particle size DN in the Nth coating slurry satisfy the relationship: DN>D2>D1;

[0055] The first coating slurry, the second coating slurry and the Nth coating slurry are sequentially coated on the treated PP base film surface, dried, and the sodium bicarbonate is volatilized to form a porous functional coating with pores gradually increasing from the inside to the outside.

[0056] The gradually increasing pores formed from the inside out in the porous functional coating improve the ion transfer ability of the lithium-ion battery, have good permeability to lithium ions, avoid the formation of lithium dendrites caused by lithium ion enrichment during high-rate charge and discharge, and improve the cycle performance and safety performance of the lithium battery; and increase the contact area with the electrolyte, improve the wettability to the electrolyte, and have sufficient liquid absorption and moisturizing capabilities.

[0057] According to the second aspect of the present application, the present application also provides a high-temperature resistant and puncture-resistant lithium-ion battery separator, which is prepared by the above-mentioned preparation method of the high-temperature resistant and puncture-resistant lithium-ion battery separator.

[0058] According to a third aspect of the present application, a lithium-ion battery is provided, comprising a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive and negative electrode sheets, wherein the separator is the separator described above. The composition and content of the positive electrode sheet, the negative electrode sheet, and the electrolyte are conventionally known in the art and are not further described here.

[0059] The present application will be further described below with reference to specific embodiments.

[0060] Example 1

[0061] like Figure 1 As shown, a lithium battery separator provided in this embodiment includes a PP base film 1 and a functional coating 2 arranged on both sides of the PP base film; the slurry of the functional coating 2 includes a polyisocyanate adhesive 21, a phase change material 22, nano zirconium oxide particles 23 and sodium bicarbonate 24.

[0062] Preparation of diaphragm:

[0063] This embodiment provides a method for preparing a high-temperature-resistant and puncture-resistant lithium-ion battery separator, comprising the following steps:

[0064] (1) A PP base film 1 having a thickness of 6 μm was subjected to corona treatment, wherein the corona treatment voltage was 2.0 kV, the current was 8 A, the corona speed was 40 m / min, and the treatment time was 8 s. After the corona treatment, the surface of the PP base film 1 showed an undulating shape, and the pit depth was 55 to 70 nm.

[0065] (2) Cleaning and destaticizing the corona-treated PP base film 1, and drying the surface to obtain a surface-treated PP base film 1;

[0066] (3) mixing a phase change material, nano zirconium oxide particles, a polyisocyanate adhesive, and sodium bicarbonate in a weight ratio of 30:60:5:5, and adding deionized water to form a functional coating slurry; wherein the average particle size R1 of the polyisocyanate adhesive is larger than the average particle size R2 of the phase change material, the average particle size R3 of the nano zirconium oxide particles, and the average particle size R4 of the sodium bicarbonate; and the relationship thereof satisfies the following formula: R1 / R2=4:1, R1 / R3=4:1, and R1 / R4=4:1;

[0067] (4) The porous carbon coating slurry is evenly coated on the surface of the treated PP base film 1, and dried at 90°C to volatilize the sodium bicarbonate to form a functional coating 2 with a thickness of 2 μm and a polyisocyanate adhesive protruding from the surface, thereby obtaining the above-mentioned diaphragm.

[0068] Preparation of lithium-ion batteries:

[0069] The negative electrode sheet, positive electrode sheet, separator and electrolyte are made into a lithium-ion battery according to conventional processes; the lithium salt of the electrolyte used is 1M LiPF6, and the solvent is EC:DEC:DMC 3:4:3; the active material used in the negative electrode sheet is artificial graphite, and the active material used in the positive electrode sheet is lithium nickel cobalt manganese oxide.

[0070] Example 2

[0071] Different from Example 1, in this embodiment, the thickness of the base film 1 is 10 μm, and the thickness of the functional coating 2 is 6 μm.

[0072] The rest is the same as in Example 1 and will not be described again here.

[0073] Example 3

[0074] Different from Example 1, in step (3) of preparing the diaphragm, the phase change material, nano zirconium oxide particles, polyisocyanate adhesive and sodium bicarbonate are mixed in a weight ratio of 28:58:7:7.

[0075] The rest is the same as in Example 1 and will not be described again here.

[0076] Example 4

[0077] Different from Example 1, in the preparation step (2) of the diaphragm, the surface wetting tension A of the surface treated PP base film 1 is 40 dyn, the surface roughness Ra is 0.1 μm and the friction coefficient is 0.4 μs, wherein the three satisfy the relationship: A=20Ra+15μs+c, and 30≤c≤35.

[0078] The rest is the same as in Example 1 and will not be described again here.

[0079] Example 5

[0080] The difference from Example 1 is that in the preparation step (1) of the diaphragm, the PP base film is a PP film grafted with diethylene glycol dimethacrylate by ultraviolet light irradiation.

[0081] The rest is the same as in Example 1 and will not be described again here.

[0082] Example 6

[0083] The difference from Example 1 is that in the preparation step (3) of the diaphragm, a crown ether-lithium complex accounting for 5% of the total mass of the slurry is added to the functional coating slurry.

[0084] The rest is the same as in Example 1 and will not be described again here.

[0085] Example 7

[0086] Different from Example 1, in the preparation step (4) of the diaphragm, the functional coating slurry includes a first coating slurry, a second coating slurry and a third coating slurry; wherein, the sodium bicarbonate particle size D1 in the first coating slurry, the sodium bicarbonate particle size D2 in the second coating slurry and the sodium bicarbonate particle size D3 in the third coating slurry satisfy the relationship, D3>D2>D1; the first coating slurry, the second coating slurry and the third coating slurry are sequentially coated on the treated PP base film surface, dried, and the sodium bicarbonate volatilizes to form a porous functional coating with pores gradually increasing from the inside to the outside.

[0087] The rest is the same as in Example 1 and will not be described again here.

[0088] Comparative Example 1

[0089] Different from Example 1, the preparation method of the diaphragm of this comparative example is: directly using PP / PE composite material as the diaphragm.

[0090] The rest is the same as in Example 1 and will not be described again here.

[0091] Comparative Example 2

[0092] Different from Example 1, the preparation method of the diaphragm of this comparative example is as follows: in step (1), the PP base film 1 is subjected to corona treatment, wherein the corona treatment voltage is 2.0 kV, the current is 10 A, the corona speed is 40 m / min, and the treatment time is 8 s.

[0093] The rest is the same as in Example 1 and will not be described again here.

[0094] Comparative Example 3

[0095] Different from Example 1, the preparation method of the diaphragm of this comparative example is as follows: in step (1), the PP base film 1 is subjected to corona treatment, wherein the corona treatment voltage is 2.0 kV, the current is 6 A, the corona speed is 40 m / min, and the treatment time is 8 s.

[0096] The rest is the same as in Example 1 and will not be described again here.

[0097] Comparative Example 4

[0098] Different from Example 1, the preparation method of the diaphragm of this comparative example is as follows: in step (3), the phase change material, nano zirconium oxide particles, polyacrylate adhesive and sodium bicarbonate are mixed in a weight ratio of 30:60:5:5.

[0099] The rest is the same as in Example 1 and will not be described again here.

[0100] Comparative Example 5

[0101] Different from Example 1, the preparation method of the diaphragm of this comparative example is as follows: in step (3), the phase change material, nano zirconium oxide particles and polyisocyanate adhesive are mixed in a weight ratio of 30:60:10.

[0102] The rest is the same as in Example 1 and will not be described again here.

[0103] Comparative Example 6

[0104] Different from Example 1, the preparation method of the diaphragm of this comparative example is as follows: in step (3), the average particle size R1 of the polyisocyanate adhesive, the average particle size R2 of the phase change material, the average particle size R3 of the nano-zirconium oxide particles and the average particle size R4 of the sodium bicarbonate satisfy the following relationship: R1=R2=R3=R4.

[0105] The rest is the same as in Example 1 and will not be described again here.

[0106] The performance tests of the diaphragms and lithium-ion batteries prepared in Examples 1 to 7 and Comparative Examples 1 to 6 were carried out according to national standards. The test results are shown in Table 1:

[0107] Table 1

[0108]

[0109] From the test results of Example 1 and Comparative Example 1, it can be seen that the peel strength and heat resistance of the separator in Example 1 are significantly improved, and the high-temperature cycle performance of the lithium-ion battery is effectively improved.

[0110] The test results of Example 1 and Comparative Example 2 show that the interlayer adhesion of the separator in Example 1 is significantly better than that in Comparative Example 2, further indicating that the optimal current for the corona treatment in step (1) is 8A. When the corona current is greater than 8A, the surface tension of the PP base film 1 decreases, resulting in a sharp decrease in the adhesion of the functional coating. This shows that the magnitude of the corona current plays a decisive role in the adhesion of the PP base film 1.

[0111] From the test results of Example 1 and Comparative Example 3, it can be seen that the interlayer adhesion of the diaphragm in Example 1 is significantly better than that in Comparative Example 3, which further illustrates that the optimal current for corona treatment in step (1) is 8A. As the corona current increases, the kinetic energy of the particles generated during corona discharge increases, which is conducive to opening the chemical bonds of the long molecular chains on the surface of the PP base film 1. The surface activity energy gradually increases, and the surface tension also increases accordingly.

[0112] The test results of Example 1 and Comparative Example 4 show that the interlayer adhesion of the separator in Example 1 is significantly superior to that of Comparative Example 4. This further demonstrates that the adhesion of Example 1 is due to a chemical reaction between the reactive isocyanate (-NCO) group in the polyisocyanate adhesive and the active hydrogen generated during corona treatment of the PP base film 1, forming a strong chemical bond. This strengthens the bond between the PP base film 1 and the functional coating 2, effectively preventing interlayer separation. The high-voltage electric field generates highly polar groups such as carbonyl groups in the PP base film 1, which generate new α-carbon atoms in the molecular chain and produce active hydrogen.

[0113] It can be seen from the test results of Example 1 and Comparative Example 5 that the high-temperature cycle performance of the lithium-ion battery in Example 1 is better than that in Comparative Example 5, indicating that the pore structure formed by adding sodium bicarbonate to the functional coating 2 and in the subsequent drying process helps to further improve the heat dissipation effect and temperature resistance of the diaphragm.

[0114] The test results of Example 1 and Comparative Example 6 show that the high-temperature cycling performance of the lithium-ion battery in Example 1 is significantly better than that of Comparative Example 6. This demonstrates that it is possible to achieve protrusion of the polyisocyanate adhesive from the surface of the porous functional coating by controlling the size of the polyisocyanate adhesive, phase change material, nano-zirconium oxide particles, and sodium bicarbonate particles. The protruding polyisocyanate adhesive from the porous functional coating can bond with the electrode in the lithium-ion battery, thereby suppressing battery deformation. This improves the cycling performance of the lithium-ion battery and further ensures battery safety and flatness. Furthermore, the protrusion of the polyisocyanate adhesive from the porous functional coating and its bond with the electrode also has a certain effect on suppressing thermal shrinkage of the separator.

[0115] From the test results of Example 1 and Example 5, it can be seen that the high-temperature cycle performance of the lithium-ion battery in Example 5 is better, indicating that by grafting polar groups on the PP base film, the battery's liquid absorption performance, ionic conductivity, etc. can be improved. It further illustrates that increasing the liquid absorption rate and liquid retention rate of the diaphragm can improve the high-temperature cycle performance of the battery.

[0116] It can be seen from the test results of Example 1 and Example 6 that the cycle performance of the lithium-ion battery of the diaphragm of Example 6 is better, indicating that adding 5% crown ether-lithium complex in the functional coating 2 can provide a stable and controllable lithium source, which can effectively replenish the lithium ions consumed during the first cycle and improve the battery's first efficiency and cycle performance.

[0117] It can be seen from the test results of Example 1 and Example 7 that the cycle performance of the lithium-ion battery in Example 7 is better than that in Example 1, indicating that the pores formed by gradually increasing from the inside to the outside can improve the ion transfer ability and permeability of the lithium-ion battery to lithium ions, avoid the formation of lithium dendrites caused by lithium ion enrichment during high-rate charge and discharge, and improve the cycle performance and safety performance of the lithium battery; it can also increase the contact area with the electrolyte, improve the wettability to the electrolyte, and make the battery have sufficient liquid absorption and moisturizing ability, thereby improving the cycle performance of the battery.

[0118] Based on the disclosure and teachings of the above description, those skilled in the art will be able to make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments. Any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention fall within the scope of protection of the present invention. In addition, although certain specific terms are used in this description, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A method for preparing a high-temperature-resistant and puncture-resistant lithium-ion battery separator, characterized in that: The following steps are involved: (1) performing corona treatment on the surface of the PP base film. After the corona treatment, the surface of the PP base film is concave and convex, and the depth of the pits is 50-70 nm; (2) Cleaning and destaticizing the corona-treated PP base film, and drying the surface to obtain a surface-treated PP base film; (3) Phase change material, nano zirconium oxide particles, polyisocyanate adhesive and sodium bicarbonate are mixed in a weight ratio of (25-35):(50-60):(5-8):(5-8), and deionized water is added to form a functional coating slurry; (4) The functional coating slurry is evenly coated on the treated PP base film surface, dried, and the sodium bicarbonate is volatilized to form a porous functional coating, and the polyisocyanate adhesive protrudes from the surface of the porous functional coating, thereby obtaining the lithium ion battery separator; In step (1), the voltage of the corona treatment is 2.0-2.2 kV, the current is 8 A, the corona speed is 40-50 m / min, and the treatment time is 8-10 s; In step (3), the average particle size R1 of the polyisocyanate adhesive is respectively larger than the average particle size R2 of the phase change material, the average particle size R3 of the nano zirconium oxide particles, and the average particle size R4 of the sodium bicarbonate; and they satisfy the following relationship: R1 / R2=(2~10):1, R1 / R3=(2~10):1, R1 / R4=(2~10):

1.

2. The method for preparing a high-temperature-resistant and puncture-resistant lithium-ion battery separator according to claim 1, characterized in that: In step (2), the surface wetting tension A, surface roughness Ra and friction coefficient μs of the surface-treated PP base film satisfy the relationship: A=20Ra+15μs+c, wherein 30≤c≤35, the unit of the surface wetting tension A is dyn, and the unit of the surface roughness Ra is μm.

3. The method for preparing a high-temperature-resistant and puncture-resistant lithium-ion battery separator according to any one of claims 1 to 2, characterized in that: In step (1), the PP base film is a PP film grafted with diethylene glycol dimethacrylate by ultraviolet light irradiation.

4. The method for preparing a high-temperature-resistant and puncture-resistant lithium-ion battery separator according to claim 1, characterized in that: In step (3), the phase change material is polyethylene glycol, and the phase change temperature is 37-41°C.

5. The method for preparing a high-temperature-resistant and puncture-resistant lithium-ion battery separator according to claim 1, characterized in that: In step (3), a crown ether-lithium complex accounting for 5% of the total mass of the functional coating slurry is added to the functional coating slurry.

6. The method for preparing a high-temperature-resistant and puncture-resistant lithium-ion battery separator according to claim 1, characterized in that: The thickness of the PP base film is 5-15 μm; the thickness of the functional coating is 1-15 μm.

7. The method for preparing a high-temperature-resistant and puncture-resistant lithium-ion battery separator according to claim 1, characterized in that: In step (4), the functional coating slurry includes a first coating slurry, a second coating slurry and an Nth coating slurry, N≥3; wherein the sodium bicarbonate particle size D1 in the first coating slurry, the sodium bicarbonate particle size D2 in the second coating slurry and the sodium bicarbonate particle size DN in the Nth coating slurry satisfy the relationship DN>D2>D1; The first coating slurry, the second coating slurry and the Nth coating slurry are sequentially coated on the treated PP base film surface, dried, and the sodium bicarbonate is volatilized to form a porous functional coating with pores gradually increasing from the inside to the outside.

8. A high-temperature-resistant and puncture-resistant lithium-ion battery separator, characterized by: It is prepared by the preparation method of the high-temperature-resistant and puncture-resistant lithium-ion battery separator according to any one of claims 1 to 7.

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