Novel composite diaphragm for lithium battery and preparation method of novel composite diaphragm

Abstract

The invention relates to the field of lithium batteries, and particularly discloses a novel composite diaphragm for a lithium battery and a preparation method of the novel composite diaphragm. The novel composite membrane for the lithium battery comprises a PE base membrane and a functional membrane arranged on the PE base membrane, and the functional membrane is prepared from the following raw materials in percentage by weight: 20-25wt% of a composite flame-retardant material, 20-25wt% of an anti-oxidant, 5-10wt% of a solvent and the balance of a solvent. 7-8 wt% of an adhesive; 0.2 to 0.85 wt% of a thickening agent; 0.1 to 0.4 wt% of a wetting agent; 0.3 to 0.55 wt% of a dispersant; the balance is water; the composite flame-retardant material comprises an inorganic filler, a flame retardant and a nano-filler, the inorganic filler comprises boehmite and aluminum oxide, the flame retardant comprises melamine and derivatives thereof, and the nano-filler comprises nano-cellulose and nanowires. The product provided by the invention can effectively solve the problem of thermal runaway of the battery cell, and meets the requirements of the lithium battery on safety in application scenarios such as high energy density and fast charge.

Description

Technical Field

[0001] This application relates to the technical field of lithium batteries, and in particular to a novel composite separator for lithium batteries and its preparation method. Background Technology

[0002] In today's society, lithium batteries, with their significant advantages such as high energy density, have been widely used in numerous fields, including electronic devices and electric vehicles, powerfully driving the rapid development of these industries and bringing great convenience to people's lives and production. With continuous technological advancements, people are placing higher demands on the performance of lithium batteries in applications such as high energy density and fast charging, leading to a sustained increase in market demand. However, the safety of lithium batteries has gradually become a key bottleneck restricting their further development in these high-end applications, and their safety risks are becoming increasingly serious. Meanwhile, although solid-state batteries are considered a promising solution, they still require significant time and technological accumulation to improve in areas such as environmental adaptability, cost control, and solid-solid contact interface processing.

[0003] In addressing lithium battery safety issues, the industry routinely employs various methods. The needle penetration test is a crucial method for assessing lithium battery safety. It artificially creates a large-area, low-resistance, through-type internal short circuit by piercing the battery with a steel needle at a certain speed. Regarding the selection of positive electrode materials for lithium batteries, lithium iron phosphate is often chosen because of its relatively high decomposition temperature and the fact that it does not release oxygen during decomposition, making it relatively stable in thermal runaway during needle penetration. While the separator has closed-cell characteristics, it shrinks rapidly at high temperatures. In the event of thermal runaway in the cell, where the heat rapidly increases beyond the separator's melting point, it can easily lead to fire and explosion. To mitigate this risk of thermal runaway, existing methods include coating the PE or PP separator with heat-resistant materials, or having cell manufacturers coat the electrodes with flame-retardant materials.

[0004] However, existing technologies have certain drawbacks. Ternary and lithium cobalt oxide systems have poor thermal stability and low decomposition temperatures, making them highly susceptible to ignition and explosion during nail penetration tests. Moreover, some flame-retardant materials used in separator coatings experience a rapid temperature rise to their decomposition temperature during battery nail penetration, leading to a further increase in cell temperature. This fails to effectively address the issue of thermal runaway in cells and makes it difficult to meet the safety requirements of lithium batteries in high-energy-density and fast-charging applications. Summary of the Invention

[0005] To address the aforementioned technical problems, this application provides a novel composite separator for lithium batteries and its preparation method.

[0006] In a first aspect, this application provides a novel composite separator for lithium batteries, comprising a PE base film and a functional film disposed on the PE base film. The raw materials used to prepare the functional film, by weight percentage, include the following components: Composite flame retardant material 20-25wt%; Adhesive 7-8wt%; Thickener 0.2-0.85 wt%; Wetting agent 0.1-0.4wt%; Dispersant 0.3-0.55 wt%; Water balance; The composite flame-retardant material comprises inorganic fillers, flame retardants, and nanofillers in a weight ratio of 6:(2-3):(1-2). The inorganic fillers include boehmite and alumina, the flame retardants include melamine and its derivatives, and the nanofillers include nanocellulose and nanowires.

[0007] Optionally, the adhesive includes acrylate adhesives, polyacrylic adhesives, and styrene-acrylic emulsion adhesives.

[0008] By adopting the above technical solutions, the inorganic fillers boehmite and alumina, the flame retardant melamine and its derivatives, and the nanofillers nanocellulose and nanowires in the composite flame retardant material of this application work together. Among them, alumina can improve the thermal stability and ion conduction efficiency of the diaphragm and prevent the diaphragm from shrinking and shortening. Boehmite, as a precursor of alumina, can form a porous structure when the diaphragm temperature rises sharply, which can enhance the heat resistance and puncture resistance of the diaphragm and improve the wettability of the electrolyte. Its surface hydroxyl properties can also help to combine with other materials. Melamine and its derivatives decompose to produce gases such as N2, which inhibits combustion. When combined with inorganic materials, it can make up for the brittleness of inorganic materials and form a dual flame retardant system of "gas phase-condensed phase". Nanocellulose, as an organic flexible matrix, can improve the flexibility of the diaphragm and the affinity of the electrolyte. When combined with inorganic materials, it can form an organic-inorganic hybrid network and enhance the mechanical strength of the diaphragm. The one-dimensional structure of nanowires can improve the tensile strength of the diaphragm and at the same time act as a heat conduction channel to accelerate heat dissipation. More importantly, in the composite flame-retardant material of this application, the gas-phase flame retardancy of melamine derivatives is combined with the condensed-phase flame retardancy of inorganic materials to form a multi-barrier structure of "heat insulation-dilution-carbonization", which significantly improves the limiting oxygen index. Nanocellulose and nanowires can enhance flexibility, while boehmite and alumina provide rigid support. The composite membrane maintains structural integrity at high temperatures. Nanowires can bridge the organic and inorganic phases in the system, reduce interface defects, and improve ion conduction efficiency. Overall, the composite flame-retardant material has an inorganic-organic hybrid design, achieving a synergistic improvement in flame retardancy, mechanical properties, and thermal stability.

[0009] The raw materials used in the functional membrane in this application also include binders, thickeners, wetting agents, and dispersants. The binder enables the components of the functional membrane to bond together better, enhancing the adhesion between the functional membrane and the PE base membrane. The thickener can adjust the viscosity of the composite membrane coating liquid, giving it good fluidity and coating performance. The wetting agent can reduce the surface tension of the liquid, allowing the coating liquid to better wet the surface of the PE base membrane and improve coating uniformity. The dispersant can ensure that the composite flame-retardant materials are uniformly dispersed in the coating liquid, avoiding agglomeration. As a result, the prepared novel composite membrane for lithium batteries has good flame-retardant performance and stability, reducing the risk of battery thermal runaway.

[0010] In summary, the novel composite separator for lithium batteries of this application can achieve the effects of endothermic decomposition, absorbing a large amount of heat, reducing the surface temperature of the material, and delaying or interrupting the polymer thermal degradation process, thereby improving the thermal stability of lithium batteries. This effectively solves the problem of thermal runaway in battery cells and meets the safety requirements of lithium batteries in high-energy-density, fast-charging applications. Preferably, the flame retardant includes melamine and melamine polyphosphate.

[0011] Preferably, the weight ratio of melamine to melamine polyphosphate is 1:(1-2).

[0012] By adopting the above technical solution, this application utilizes melamine and melamine polyphosphate as flame retardants. Melamine decomposes upon heating to produce inert gases such as ammonia and nitrogen, diluting the concentration of oxygen and combustible gases, while simultaneously promoting the formation of a carbon layer on the material surface. Melamine polyphosphate decomposes at high temperatures to generate phosphoric acid and polyphosphoric acid, catalyzing the dehydration and carbonization of the polymer to form a dense carbon layer, while releasing gases such as nitrogen, further diluting oxygen and expanding the carbon layer, enhancing the heat and oxygen insulation effect. Furthermore, through phosphorus-nitrogen synergy and the compounding with inorganic materials, melamine and its derivatives achieve highly efficient and environmentally friendly flame retardant effects. Compared to other melamine derivatives, such as melamine hydrobromide, melamine polyphosphate, and formaldehyde polymelamine hydrochloride, melamine polyphosphate exhibits a better synergistic effect with melamine, further improving the flame retardant capability of the novel composite separator for lithium batteries. Meanwhile, this application also controls the weight ratio of melamine and melamine polyphosphate to be 1:(1-2). Under this weight ratio, the two can achieve a better synergistic effect, further improve the flame retardant effect, and enhance the adaptability of lithium batteries in high energy density, fast charging and other application scenarios.

[0013] Preferably, the adhesive is an acrylate adhesive, and the specific preparation method includes the following steps: blending acrylamide, 1-vinylimidazole and tetrahydrofuran acrylate, and carrying out a polymerization reaction under inert gas protection to obtain the adhesive.

[0014] By employing the above technical solution, this application utilizes acrylamide, 1-vinylimidazole, and tetrahydrofuran acrylate to prepare an adhesive containing imidazole groups and acrylic acid. The abundant amino groups in this adhesive can self-crosslink to form a stable three-dimensional network structure and form dynamic reversible hydrogen bonds with the electrode components, thereby effectively mitigating the volume expansion effect of silicon particles and improving the cycle stability of the battery. Cations can preferentially adsorb onto the electrode surface, regulating the double-layer structure through electrostatic interaction and promoting the enrichment of anions at the interface, thus forming an interface layer conducive to rapid ion transport. The acrylate monomers form a polymer network with good flexibility and chemical stability, thereby enhancing the mechanical strength of the separator and its wettability to the electrolyte. Most importantly, the adhesive of this application has a high glass transition temperature. When blended with boehmite, it can form a rigid coating fixed to the boehmite surface, preventing thermal shrinkage of the separator, thereby simultaneously improving the electrical performance and flame retardant properties of the novel composite separator for lithium batteries.

[0015] Preferably, the weight ratio of acrylamide, 1-vinylimidazole and tetrahydrofuran acrylate is (5.5-6.5):(0.5-1.5):2.

[0016] Preferably, the weight ratio of acrylamide, 1-vinylimidazole and tetrahydrofuran acrylate is 6:1:2.

[0017] By adopting the above technical solution, this application controls the weight ratio of acrylamide, 1-vinylimidazole and tetrahydrofuran acrylate, adjusts the proportion of imidazole groups in the adhesive, and further improves the optimization effect of the adhesive on the comprehensive performance of the novel composite separator for lithium batteries.

[0018] Preferably, the nanocellulose is further modified with porous materials, and the specific modification steps are as follows: Chromium chloride and 2-aminoterephthalic acid were blended and acid was added to adjust the pore size to obtain a porous material. Nanocellulose was treated with alkali, and then the alkali-treated nanocellulose was blended with the porous material at a weight ratio of 110:(3-3.5). The mixture was stirred at 150-200 rpm for 10-15 h to obtain the porous material modified nanocellulose.

[0019] Preferably, the alkali-treated nanocellulose is blended with the porous material at a weight ratio of 110:3.2.

[0020] Since nanocellulose may lead to excessively large pores on the surface of novel composite separators for lithium batteries, this application employs the aforementioned technical solution to prepare a three-dimensional porous material using chromium chloride and 2-aminoterephthalic acid. This material possesses a large specific surface area and excellent chemical stability, as well as excellent structural stability in water. Subsequently, this application uses the porous material to modify the nanocellulose, reducing the possibility of excessively large pores on the surface of the novel composite separator for lithium batteries, optimizing the pore structure and porosity of the novel composite separator, thereby improving its liquid absorption rate and ultimately optimizing the electrical performance of the lithium battery.

[0021] Secondly, this application also provides a method for preparing the above-mentioned novel composite separator for lithium batteries, comprising the following steps: S1. Mix the dispersant with 40-50 wt% water until homogeneous to obtain solution A; S2. Mix the thickener with the remaining water until homogeneous to obtain solution B; S3. Then, mix liquid A, liquid B and composite flame retardant material together, stir evenly, add adhesive, stir evenly, and finally add wetting agent, stir evenly to obtain composite membrane coating liquid. S4. The composite separator coating liquid is coated on the surface of the PE base film and vacuum dried to obtain a novel composite separator for lithium batteries.

[0022] By adopting the above technical solution, this application mixes dispersant and thickener with water in different proportions to form liquid A and liquid B, respectively, and then mixes them with composite flame retardant material, adhesive and wetting agent in sequence to obtain coating liquid. The coating liquid is then coated on the surface of PE base film and vacuum dried to form composite separator. This process ensures that the raw materials are mixed evenly, and that the composite separator has the flame retardant effects of the composite flame retardant material, such as endothermic decomposition to reduce surface temperature, delaying or interrupting the polymer thermal degradation process, catalyzing the dehydration and carbonization of the polymer matrix to form a dense expanded carbon layer to isolate oxygen and heat, decomposing to generate gas to inhibit combustion, and being halogen-free. It also has the effects of nanocellulose, such as considering the thermal abuse of battery cells, short circuits of positive and negative electrodes and separator wettability.

[0023] Preferably, in step S4, the vacuum drying temperature is 105-110°C.

[0024] By adopting the above technical solution, this application controls the vacuum drying temperature to 105-110℃, promotes better water evaporation, gradually stabilizes the pore formation rate inside the separator, achieves a stable porosity, and can improve the liquid absorption rate of the separator, promote lithium ion transport, and thus optimize the electrical performance of the lithium battery.

[0025] In summary, this application has the following beneficial technical effects: 1. The novel composite separator for lithium batteries in this application can achieve the effects of endothermic decomposition, absorbing a large amount of heat, reducing the surface temperature of materials, delaying or interrupting the thermal degradation process of polymers, thereby improving the thermal stability of lithium batteries. It can effectively solve the problem of thermal runaway of battery cells and meet the safety requirements of lithium batteries in high energy density, fast charging and other application scenarios. 2. In this application, dispersants and thickeners are mixed with water in different proportions to form liquid A and liquid B, which are then mixed sequentially with composite flame retardant materials, adhesives, and wetting agents to obtain a coating liquid. This coating liquid is then applied to the surface of a PE base film and vacuum dried to form a composite separator. This process ensures that the raw materials are mixed evenly, guaranteeing that the composite separator possesses the flame retardant effects of the composite flame retardant materials, such as endothermic decomposition to reduce surface temperature, delaying or interrupting the polymer thermal degradation process, catalyzing the dehydration and carbonization of the polymer matrix to form a dense, expanded carbon layer to isolate oxygen and heat, generating gases to inhibit combustion, and being halogen-free. It also incorporates the effects of nanocellulose, such as consideration of cell thermal abuse, positive and negative electrode short circuits, and separator wettability. Detailed Implementation

[0026] Material Sources: Unless otherwise specified, all raw materials used in this application are commercially available products, specifically: Dispersant 5027 was purchased from Nantong Aochen Chemical Co., Ltd., and its brand name is AC-5027. Sodium carboxymethyl cellulose was purchased from Zhenjiang Lianfeng Environmental Protection Chemical Co., Ltd., with a Fisher average particle size of 10-30 μm and a loose specific gravity of 0.6-1.0 g / cm³. 3 ; The polyether-modified silicone wetting agent was purchased from Guangzhou Rongda Chemical Co., Ltd., and its brand name is OFX-0309.

[0027] The present application will be further described in detail below with reference to embodiments and comparative examples.

[0028] Preparation Example 1.1 The method for preparing the adhesive includes the following steps: 55g of acrylamide, 20g of tetrahydrofuran acrylate, and 0.4g of initiator azobisisobutyramidine hydrochloride were dispersed in 400mL of water to obtain a pre-dispersion. 15g of 1-vinylimidazole and 0.4g of initiator azobisisobutyramidine hydrochloride were dispersed in 500mL of water, nitrogen gas was introduced, and the mixture was stirred at 200rpm for 30min. The temperature was then raised to 65℃, and all the pre-dispersion was added dropwise over a total time of 200min. The mixture was then stirred and reacted under a nitrogen atmosphere for 4h to obtain the adhesive.

[0029] Preparation Example 1.2 The method for preparing the adhesive includes the following steps: 65g of acrylamide, 20g of tetrahydrofuran acrylate, and 0.4g of initiator azobisisobutyramidine hydrochloride were co-dispersed in 600mL of water to obtain a pre-dispersion. 5g of 1-vinylimidazole and 0.4g of initiator azobisisobutyramidine hydrochloride were co-dispersed in 300mL of water, nitrogen gas was introduced, and the mixture was stirred at 200rpm for 30min. The temperature was then raised to 65℃, and all the pre-dispersion was added dropwise over a total time of 200min. The mixture was then stirred and reacted under a nitrogen atmosphere for 4h to obtain the adhesive.

[0030] Preparation Example 1.3 The method for preparing the adhesive includes the following steps: 50g of acrylamide, 20g of tetrahydrofuran acrylate, and 0.4g of initiator azobisisobutyramidine hydrochloride were dispersed in 500mL of water to obtain a pre-dispersion. 20g of 1-vinylimidazole and 0.4g of initiator azobisisobutyramidine hydrochloride were dispersed in 400mL of water, nitrogen gas was introduced, and the mixture was stirred at 200rpm for 30min. The temperature was then raised to 65℃, and all the pre-dispersion was added dropwise over a total time of 200min. The mixture was then stirred and reacted under a nitrogen atmosphere for 4h to obtain the adhesive.

[0031] Preparation Example 1.4 The method for preparing the adhesive includes the following steps: 68g of acrylamide, 20g of tetrahydrofuran acrylate, and 0.4g of initiator azobisisobutyramidine hydrochloride were co-dispersed in 700mL of water to obtain a pre-dispersion. 2g of 1-vinylimidazole and 0.4g of initiator azobisisobutyramidine hydrochloride were co-dispersed in 200mL of water, nitrogen gas was introduced, and the mixture was stirred at 200rpm for 30min. The temperature was then raised to 65℃, and all the pre-dispersion was added dropwise over a total time of 200min. The mixture was then stirred and reacted under a nitrogen atmosphere for 4h to obtain the adhesive.

[0032] Preparation Example 1.5 The method for preparing the adhesive includes the following steps: 60g of acrylamide, 20g of tetrahydrofuran acrylate, and 0.4g of initiator azobisisobutyramidine hydrochloride were dispersed in 500mL of water to obtain a pre-dispersion. 10g of 1-vinylimidazole and 0.4g of initiator azobisisobutyramidine hydrochloride were dispersed in 400mL of water, nitrogen gas was introduced, and the mixture was stirred at 200rpm for 30min. The temperature was then raised to 65℃, and all the pre-dispersion was added dropwise over a total time of 200min. The mixture was then stirred and reacted under a nitrogen atmosphere for 4h to obtain the adhesive.

[0033] Preparation Example 1.6 The method for preparing the adhesive includes the following steps: 57g of acrylamide, 20g of tetrahydrofuran acrylate, and 0.4g of initiator azobisisobutyramidine hydrochloride were dispersed in 500mL of water to obtain a pre-dispersion. 13g of 1-vinylimidazole and 0.4g of initiator azobisisobutyramidine hydrochloride were dispersed in 400mL of water, nitrogen gas was introduced, and the mixture was stirred at 200rpm for 30min. The temperature was then raised to 65℃, and all the pre-dispersion was added dropwise over a total time of 200min. The mixture was then stirred and reacted under a nitrogen atmosphere for 4h to obtain the adhesive.

[0034] Preparation Example 1.7 The method for preparing the adhesive includes the following steps: 63g of acrylamide, 20g of tetrahydrofuran acrylate, and 0.4g of initiator azobisisobutyramidine hydrochloride were dispersed in 500mL of water to obtain a pre-dispersion. 7g of 1-vinylimidazole and 0.4g of initiator azobisisobutyramidine hydrochloride were dispersed in 400mL of water, nitrogen gas was introduced, and the mixture was stirred at 200rpm for 30min. The temperature was then raised to 65℃, and all the pre-dispersion was added dropwise over a total time of 200min. The mixture was then stirred and reacted under a nitrogen atmosphere for 4h to obtain the adhesive.

[0035] Preparation Example 2.1 A method for preparing porous material-modified nanocellulose includes the following steps: 120g of nanocellulose was placed in 500mL of 1.6wt% sodium hydroxide aqueous solution and stirred at 90℃ for 4h for alkali treatment. After washing and drying, it was dispersed in 500mL of DMF to obtain an alkali-treated nanocellulose dispersion for later use. 2.4 g of chromium chloride and 1.8 g of 2-aminoterephthalic acid were placed in 1 L of DMF and ultrasonically dispersed for 10 min. Then 225 mL of glacial acetic acid was added and reacted at 120 °C for 12 h to adjust the pore size. After filtration, washing and drying, the porous material was obtained. All the alkali-treated nanocellulose dispersion was mixed with 3g of porous material and stirred at 200rpm for 10h. Then it was washed with a centrifuge, first with DMF 3 times and then with anhydrous ethanol 3 times. After filtration and drying, porous material modified nanocellulose was obtained.

[0036] Preparation Example 2.2 A method for preparing porous material-modified nanocellulose includes the following steps: 120g of nanocellulose was placed in 500mL of 1.6wt% sodium hydroxide aqueous solution and stirred at 90℃ for 4h for alkali treatment. After washing and drying, it was dispersed in 500mL of DMF to obtain an alkali-treated nanocellulose dispersion for later use. 2.4 g of chromium chloride and 1.8 g of 2-aminoterephthalic acid were placed in 1 L of DMF and ultrasonically dispersed for 10 min. Then 225 mL of glacial acetic acid was added and reacted at 120 °C for 12 h to adjust the pore size. After filtration, washing and drying, the porous material was obtained. All the alkali-treated nanocellulose dispersion was mixed with 3.5g of porous material and stirred at 150rpm for 15h. Then it was washed with a centrifuge, first with DMF 3 times and then with anhydrous ethanol 3 times. After filtration and drying, porous material modified nanocellulose was obtained.

[0037] Preparation Example 2.3 The method for preparing porous material modified nanocellulose differs from that in Preparation Example 2.1 in that all the alkali-treated nanocellulose dispersion is mixed with 3.1g of porous material, while the rest is the same as in Preparation Example 2.1.

[0038] Preparation Example 2.4 The method for preparing porous material modified nanocellulose differs from that in Preparation Example 2.1 in that all the alkali-treated nanocellulose dispersion is mixed with 3.2g of porous material, while the rest is the same as in Preparation Example 2.1.

[0039] Preparation Example 2.5 The method for preparing porous material modified nanocellulose differs from that in Preparation Example 2.1 in that all the alkali-treated nanocellulose dispersion is mixed with 3.3g of porous material, while the rest is the same as in Preparation Example 2.1.

[0040] Preparation Example 2.6 The method for preparing porous material modified nanocellulose differs from that in Preparation Example 2.1 in that all the alkali-treated nanocellulose dispersion is mixed with 3.4 g of porous material, while the rest is the same as in Preparation Example 2.1.

[0041] Example 1.1 A method for preparing a novel composite separator for lithium batteries includes the following steps: S1. Mix 0.3g of dispersant 5027 with 28.44g of water until homogeneous to obtain solution A; S2. Mix 0.2g of thickener sodium carboxymethyl cellulose with 42.66g of water until homogeneous to obtain solution B; S3. Then, all of liquid A, liquid B and 20g of composite flame retardant material (8g boehmite, 4g alumina, 2g melamine, 4g melamine polyphosphate, 1.5g nanocellulose and 0.5g alumina nanowires) are mixed and stirred evenly at 500 rpm. Then, 8g of the adhesive prepared in Preparation Example 1.1 is added and stirred evenly at 500 rpm. Finally, 0.4g of polyether modified organosilicon wetting agent is added and stirred evenly at 500 rpm to obtain the composite membrane coating liquid. S4. Use an automatic coating machine to coat the composite separator coating liquid onto the surface of the PE base film, and vacuum dry at 80°C for 30 minutes to obtain a new type of composite separator for lithium batteries.

[0042] Example 1.2 A method for preparing a novel composite separator for lithium batteries includes the following steps: S1. Mix 0.55g of dispersant 5027 with 33.25g of water until homogeneous to obtain solution A; S2. Mix 0.85g of thickener sodium carboxymethyl cellulose with 33.25g of water until homogeneous to obtain solution B; S3. Then, all of liquid A, liquid B and 20g of composite flame retardant material (10g boehmite, 5g alumina, 2.5g melamine, 2.5g melamine polyphosphate, 3g nanocellulose and 2g alumina nanowires) are mixed and stirred evenly at 500 rpm. Then, 7g of the adhesive prepared in Preparation Example 1.2 is added and stirred evenly at 500 rpm. Finally, 0.1g of polyether modified organosilicon wetting agent is added and stirred evenly at 500 rpm to obtain the composite membrane coating liquid. S4. Use an automatic coating machine to coat the composite separator coating liquid onto the surface of the PE base film, and vacuum dry at 80°C for 30 minutes to obtain a new type of composite separator for lithium batteries.

[0043] Example 2.1 A method for preparing a novel composite separator for lithium batteries differs from Example 1.1 in that, in S3, the amount of melamine used is 1g and the amount of melamine polyphosphate used is 5g, while the rest are the same as in Example 1.1.

[0044] Example 2.2 A method for preparing a novel composite separator for lithium batteries differs from Example 1.1 in that, in S3, the amount of melamine used is 5g and the amount of melamine polyphosphate used is 1g, while the rest are the same as in Example 1.1.

[0045] Example 3.1 A method for preparing a novel composite separator for lithium batteries differs from Example 1.1 in that, in S3, melamine polyphosphate is replaced with melamine hydrobromide, while the rest is the same as in Example 1.1.

[0046] Example 3.2 A method for preparing a novel composite separator for lithium batteries differs from Example 1.1 in that, in S3, melamine polyphosphate is replaced with melamine polyphosphate, while the rest is the same as in Example 1.1.

[0047] Example 3.3 A method for preparing a novel composite separator for lithium batteries differs from Example 1.1 in that, in S3, melamine polyphosphate is replaced with formaldehyde polymelamine hydrochloride, while the rest is the same as in Example 1.1.

[0048] Examples 4.1-4.5 A method for preparing a novel composite separator for lithium batteries differs from Example 1.1 in that, in step S3, the adhesive obtained in Preparation Example 1.1 is replaced with the adhesive obtained in Preparation Examples 1.3-1.7, while the rest is the same as in Example 1.1.

[0049] Examples 5.1-5.2 A method for preparing a novel composite separator for lithium batteries differs from Example 1.1 in that, in S3, all nanocellulose is replaced with porous material modified nanocellulose obtained in Preparation Examples 2.1-2.2, while the rest is the same as in Example 1.1.

[0050] Examples 5.3-5.6 A method for preparing a novel composite separator for lithium batteries differs from Example 5.1 in that, in S3, all the porous material modified nanocellulose obtained in Preparation Example 2.1 is replaced with the porous material modified nanocellulose obtained in Preparation Examples 2.3-2.6, while the rest is the same as in Example 5.1.

[0051] Example 6.1 The difference from Example 1.1 is that in S3, the adhesive prepared in Preparation Example 1.1 is replaced with a styrene-acrylic emulsion adhesive, and the rest is the same as in Example 1.1.

[0052] Example 6.2 The difference from Example 1.1 is that in S3, the adhesive prepared in Preparation Example 1.1 is replaced with a conventional acrylate adhesive, and all other aspects are the same as in Example 1.1.

[0053] Example 6.3 The difference from Example 1.1 is that in S3, the adhesive prepared in Preparation Example 1.1 is replaced with polyacrylic acid adhesive, and all other aspects are the same as in Example 1.1.

[0054] Comparative Example 1.1 The difference from Example 1.1 is that in S3, melamine polyphosphate is removed, and the amount of melamine used is 6g, while the rest is the same as in Example 1.1.

[0055] Comparative Example 1.2 The difference from Example 1.1 is that in S3, melamine is removed, and the amount of melamine polyphosphate is 6g, while the rest is the same as in Example 1.1.

[0056] Comparative Example 2 The difference from Example 1.1 is that in S3, the nanofiller is removed, the amount of boehmite is 8.5g, the amount of alumina is 4.5g, the amount of melamine is 2.5g, and the amount of melamine polyphosphate is 4.5g, while the rest are the same as in Example 1.1.

[0057] Comparative Example 3.1 The difference from Example 1.1 is that in S3, nanocellulose is removed, the amount of nanowires is 2g, and everything else is the same as in Example 1.1.

[0058] Comparative Example 3.2 The difference from Example 1.1 is that in S3, the nanowires are removed, the amount of nanocellulose is 2g, and the rest is the same as in Example 1.1.

[0059] Performance testing 1. Flame retardancy test: The composite membranes obtained in the examples and comparative examples were cut into 100mm×100mm samples using a cutter, and vertically fixed to an oxygen index meter to test the limiting oxygen index % (LOI%). 2. Battery Cycle Discharge Testing: The cycle performance of lithium-ion / separator / lithium-ion half-cells was tested using the Blue Battery Testing System under the condition of 0.1 mA / cm². -2 The charging and discharging time was 1 hour each, and 100 charge-discharge cycles were performed. The capacity retention rate (%) was recorded. 3. Strength testing: The composite membranes obtained in the examples and comparative examples were cut into dumbbell-shaped samples with a total length of 35 mm, a test length of 20 mm, and a test width of 2 mm using a cutter. The samples were tested using a membrane material electronic universal testing machine at a testing speed of 5 mm / min, and their tensile strength (MPa) was recorded.

[0060] Table 1 Data Records Data Analysis: As shown in Table 1, the novel composite separators obtained in Examples 1.1-1.2 have a limiting oxygen index of 31.6-31.9%, a capacity retention rate of 92.3-92.5% after 100 cycles in battery cycle discharge testing, and a tensile strength of 131.9-132.5 MPa. It is evident that the novel composite separator for lithium batteries of this application can achieve the effects of endothermic decomposition, absorption of a large amount of heat, reduction of material surface temperature, and delay or interruption of polymer thermal degradation process, thereby improving the thermal stability of lithium batteries. It can effectively solve the problem of cell thermal runaway and meet the safety requirements of lithium batteries in high energy density, fast charging and other application scenarios.

[0061] In Examples 2.1-2.2, the amount of melamine and melamine polyphosphate was adjusted in this application. The results showed that the limiting oxygen index was significantly reduced. It can be seen that the ratio of melamine and melamine polyphosphate in this application can achieve a better synergistic effect and further improve the flame retardant ability of the novel composite separator for lithium batteries.

[0062] In Examples 3.1-3.3, this application adjusted the types of melamine derivatives. The results showed that the limiting oxygen index was significantly reduced. It can be seen that compared with other melamine derivatives, such as melamine hydrobromide, melamine polyphosphate and formaldehyde polymelamine hydrochloride, melamine polyphosphate can play a better synergistic role with melamine, further improving the flame retardant ability of the new composite separator for lithium batteries.

[0063] In Examples 4.1-4.5, this application adjusted the amount of imidazole used in the preparation of the adhesive. The results showed that the overall performance of the battery separator in Example 4.3 was significantly improved. It can be seen that this application adjusted the proportion of imidazole groups in the adhesive by controlling the weight ratio of acrylamide, 1-vinylimidazolium and tetrahydrofuran acrylate, thereby further improving the optimization effect of the adhesive on the overall performance of the novel composite separator for lithium batteries.

[0064] In Examples 5.1-5.6, the present application modified the nanocellulose, and the results showed a significant improvement in capacity retention, especially in Example 5.4 where the capacity retention reached 94.3%. This demonstrates that the present application prepared a three-dimensional porous material using chromium chloride and 2-aminoterephthalic acid, which possesses a large specific surface area and excellent chemical stability, as well as excellent structural stability in water. Subsequently, the present application modified the nanocellulose using the porous material, reducing the possibility of excessively large pores on the surface of the novel composite separator for lithium batteries, optimizing the pore structure and porosity of the novel composite separator for lithium batteries, thereby improving its liquid absorption rate and ultimately optimizing the electrical performance of the lithium battery.

[0065] In Examples 6.1-6.3, the type of adhesive was adjusted in this application, and the results showed that the overall performance was worse than that of Example 1.1. It can be seen that the adhesive of this application has good bonding ability and thermal stability, and at the same time improves the electrical performance and flame retardant performance of the novel composite separator for lithium batteries.

[0066] In Comparative Examples 1.1-1.2, this application removed melamine polyphosphate and melamine, respectively. The results showed that the limiting oxygen index was significantly reduced. It can be seen that melamine decomposes when heated to produce inert gases such as ammonia and nitrogen, which dilute the concentration of oxygen and combustible gases, and at the same time promote the formation of a carbon layer on the material surface. Melamine polyphosphate decomposes at high temperature to generate phosphoric acid, polyphosphoric acid, etc., which catalyze the dehydration and carbonization of the polymer to form a dense carbon layer, while releasing gases such as nitrogen, which further dilutes oxygen and expands the carbon layer, enhancing the heat insulation and oxygen barrier effect. Furthermore, through the synergistic effect of phosphorus-nitrogen and the compounding with inorganic materials, melamine and its derivatives achieve a highly efficient and environmentally friendly flame retardant effect.

[0067] In Comparative Example 2, the nanofiller was removed in this application, and the results showed a significant decrease in the limiting oxygen index. In Comparative Examples 3.1-3.2, nanocellulose and nanowires were removed in this application, respectively, and the results showed a significant decrease in the overall performance of the diaphragm. It can be seen that the nanocellulose in this application, as an organic flexible matrix, can improve the flexibility and electrolyte affinity of the diaphragm. When combined with inorganic materials, it can form an organic-inorganic hybrid network, which can enhance the mechanical strength of the diaphragm. The one-dimensional structure of the nanowires can improve the tensile strength of the diaphragm and at the same time, it can act as a heat conduction channel to accelerate heat dissipation. The nanowires can bridge the organic-inorganic phases in the system, reduce interface defects, and improve ion conduction efficiency.

[0068] The embodiments described in this specific implementation are preferred embodiments of this application and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A novel composite separator for lithium batteries, comprising a PE base film and a functional film disposed on the PE base film, characterized in that, The raw materials used to prepare the functional membrane, by weight percentage, include the following components: Composite flame retardant material 20-25 wt%; Adhesive 7-8wt%; Thickener 0.2-0.85 wt%; Wetting agent 0.1-0.4wt%; Dispersant 0.3-0.55wt%; Water balance; The composite flame-retardant material comprises inorganic fillers, flame retardants, and nanofillers in a weight ratio of 6:(2-3):(1-2). The inorganic fillers include boehmite and alumina, the flame retardants include melamine and its derivatives, and the nanofillers include nanocellulose and nanowires.

2. The novel composite separator for lithium batteries according to claim 1, characterized in that, The flame retardant includes melamine and melamine polyphosphate.

3. The novel composite separator for lithium batteries according to claim 2, characterized in that, The weight ratio of melamine to melamine polyphosphate is 1:(1-2).

4. The novel composite separator for lithium batteries according to claim 1, characterized in that, The adhesive is an acrylate adhesive, and the specific preparation method includes the following steps: blending acrylamide, 1-vinylimidazole and tetrahydrofuran acrylate, and carrying out a polymerization reaction under inert gas protection to obtain the adhesive.

5. A novel composite separator for lithium batteries according to claim 4, characterized in that, The weight ratio of acrylamide, 1-vinylimidazole and tetrahydrofuran acrylate is (5.5-6.5):(0.5-1.5):

2.

6. A novel composite separator for lithium batteries according to claim 5, characterized in that, The weight ratio of acrylamide, 1-vinylimidazole, and tetrahydrofuran acrylate is 6:1:

2.

7. A novel composite separator for lithium batteries according to claim 1, characterized in that, The nanocellulose is further modified with porous materials, and the specific modification steps are as follows: Chromium chloride and 2-aminoterephthalic acid were blended and acid was added to adjust the pore size to obtain a porous material. Nanocellulose was treated with alkali, and then the alkali-treated nanocellulose was blended with the porous material at a weight ratio of 110:(3-3.5). The mixture was stirred at 150-200 rpm for 10-15 h to obtain the porous material modified nanocellulose.

8. A novel composite separator for lithium batteries according to claim 7, characterized in that, Alkali-treated nanocellulose was blended with porous materials at a weight ratio of 110:3.

2.

9. A method for preparing a novel composite separator for lithium batteries according to any one of claims 1-8, characterized in that, Includes the following steps: S1. Mix the dispersant with 40-50 wt% water until homogeneous to obtain solution A; S2. Mix the thickener with the remaining water until homogeneous to obtain solution B; S3. Then, mix liquid A, liquid B and composite flame retardant material together, stir evenly, add adhesive, stir evenly, and finally add wetting agent, stir evenly to obtain composite membrane coating liquid. S4. The composite separator coating liquid is coated on the surface of the PE base film and vacuum dried to obtain a novel composite separator for lithium batteries.

10. The method for preparing the novel composite separator for lithium batteries according to claim 9, characterized in that, In step S4, the vacuum drying temperature is 105-110℃.

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