A lithium-sulfur battery separator with low shuttle effect and preparation method thereof

Abstract

The present invention relates to the technical field of lithium-sulfur battery separators, and discloses a lithium-sulfur battery separator with a low shuttle effect and a method for preparing the same. The present invention adopts a three-layer structure. In the surface layer A, chitosan powder is used to form hydrogen bonds or coordination effects with polysulfides through the amino groups / hydroxyl groups on its molecular chain, achieving chemical adsorption, making it difficult for polysulfides to diffuse in the electrolyte, thereby reducing the impact of polysulfides on the negative electrode and suppressing the shuttle effect. In the middle layer of the lithium-sulfur battery, the ion transmission channel is effectively improved by increasing the porosity of the separator and optimizing the pore size distribution. In addition, the modification method also significantly improves the mechanical strength and toughness of the separator, enhancing its puncture resistance. Without increasing the thickness of the separator, this technology can effectively alleviate the shuttle effect problem of lithium-sulfur batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-sulfur battery separators, and in particular to a lithium-sulfur battery separator with low shuttle effect and a preparation method thereof. Background Art

[0002] Lithium-sulfur batteries are mainly composed of a positive electrode, a negative electrode, an electrolyte, a current collector, and a separator. In lithium-sulfur batteries, the separator plays a vital role, and its main functions include the following aspects:

[0003] 1. Prevent internal short circuits in the battery by separating the positive electrode (sulfur) and the negative electrode (lithium) to prevent direct contact inside the battery and avoid short circuits. This is crucial to ensure battery safety. 2. The diaphragm needs to have good ionic conductivity to allow lithium ions to migrate from the positive electrode to the negative electrode and vice versa during charging and discharging. This is the basis for normal battery operation and efficient energy conversion. 3. In lithium-sulfur batteries, polysulfides (such as Li2S x ) will dissolve in the electrolyte during charging and may diffuse to the negative electrode, resulting in the so-called shuttle effect.

[0004] However, although lithium-sulfur batteries have high theoretical energy density and good environmental friendliness, they still face several technical challenges in practical applications. Among them, polysulfides (Li2S x , x = 4-8) will dissolve in the electrolyte and migrate to the negative electrode during charging, resulting in internal short circuit, capacity decay and shortened life of the battery, which seriously hinders the large-scale application of lithium-sulfur batteries.

[0005] In lithium-sulfur batteries, the design of the diaphragm must have both high ionic conductivity and strong polysulfide barrier capabilities. The patent with publication number CN 116387757 A discloses a high-porosity sodium-ion battery stretch diaphragm and its preparation method. The invention aims to solve the problem in the prior art that the battery diaphragm has insufficient porosity when applied to sodium-ion batteries, resulting in a limited sodium ion transmission rate; at the same time, the short-circuit rate of the diaphragm will also increase after the porosity is increased, which is not conducive to battery safety. To this end, the surface layer of the diaphragm adopts a macroporous structure to accommodate the electrolyte, and the middle layer is designed to be uniformly densely pores to maintain mechanical strength, thereby improving the sodium ion transmission efficiency while ensuring safety.

[0006] However, this separator has significant drawbacks when applied to lithium-sulfur batteries: On the one hand, the size of polysulfides in lithium-sulfur batteries (0.5-2 nm) is much smaller than the pore size of the separator (typically tens to hundreds of nanometers), resulting in the physical pore structure being unable to effectively block the shuttling of dissolved Li2Sn; on the other hand, while the high porosity design is beneficial for lithium ion conduction, it will form a rapid diffusion channel for polysulfides, exacerbating the "shuttle effect." This structural defect will directly lead to problems such as continuous loss of active materials, increased electrolyte viscosity, and negative electrode passivation, ultimately manifesting as a sharp decrease in battery cycle stability and rapid capacity decay. Therefore, the separator designed in this patent is difficult to meet the special requirements of lithium-sulfur batteries for polysulfide barrier function. Summary of the Invention

[0007] This invention aims to address the existing problem of reduced battery life due to the shuttle effect when using battery separators in lithium-sulfur batteries. By adopting a three-layer structure and optimizing the raw material composition of each layer, in surface layer A, the amino and hydroxyl groups on the chitosan powder molecular chain form hydrogen bonds or coordination interactions with polysulfides, achieving chemical adsorption. This prevents diffusion in the electrolyte, thereby reducing the impact of polysulfides on the negative electrode. In intermediate layer B, the introduction of modified polyimide powder increases the total porosity of the separator, improves the porosity, and optimizes the pore size distribution, effectively improving ion transport channels and mitigating the shuttle effect in lithium-sulfur batteries, thereby extending battery life.

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

[0009] A lithium-sulfur battery separator with low shuttle effect, wherein the structure is an A / B / A symmetrical structure;

[0010] A is the surface layer, and the raw materials include chitosan grafted polypropylene;

[0011] B is the middle layer, and its raw materials include polypropylene and modified polyimide powder;

[0012] The raw materials of the chitosan grafted polypropylene of the surface layer include: 80-90 parts of polypropylene, 1-10 parts of β-crystal nucleating agent, 3-8 parts of chitosan powder, 0.5-1 parts of dicumyl peroxide as initiator, and 1-2 parts of maleic anhydride grafting monomer;

[0013] The intermediate layer is prepared from the following raw materials in parts by weight: 85-92 parts of polypropylene, 3-6 parts of α-crystal nucleating agent and 5-8 parts of modified polyimide powder.

[0014] This invention utilizes a chitosan-modified separator to reduce the shuttle effect as the surface layer and a polyimide powder-modified separator to enhance ion conductivity as the middle layer. This unique design allows the surface layer to accommodate more electrolyte, thereby promoting the passage of more lithium ions through the surface layer. Furthermore, the uniform pore structure of the middle layer effectively enhances the lithium ion transport efficiency and mechanical properties of the separator while ensuring its strength.

[0015] The present invention utilizes different types of polypropylene materials as the surface and middle layers. After stretching, these materials form pores of varying sizes and distributions. This three-layer structure, through its staggered pore design, further enhances the separator's overall puncture strength and other mechanical properties, significantly improving battery safety.

[0016] Preferably, the particle size of the chitosan powder is 200-600 nm, the molecular weight of the chitosan powder is 100,000-500,000 Da, the deacetylation degree of the chitosan powder is 60-80%, and the specific surface area is 20-50 m² / g; the particle size of the modified polyimide powder is 50-100 nm.

[0017] This invention precisely controls the chitosan powder particle size to ensure that the separator exhibits excellent electrolyte wettability while maintaining high porosity. If the chitosan powder particle size is too small, it may be difficult to effectively reduce the "shuttle effect" and increase the difficulty and cost of material processing. Conversely, if the particle size is too large, it may adversely affect the smoothness of the separator.

[0018] If the particle size is too large, the polyimide powder will have difficulty forming a uniform pore structure, which will lead to uneven pore distribution in the separator, thus affecting the penetration and distribution of the electrolyte. Conversely, if the particle size is too small, not only will the specific surface area of ​​the powder increase, making it more prone to agglomeration, but the contact area between powder particles will also be reduced, and the bonding strength will be weakened, making the separator more susceptible to tearing or damage.

[0019] Preferably, the preparation method of chitosan grafted polypropylene comprises the following steps:

[0020] (1) Chitosan powder pretreatment: dissolving chitosan powder in 1% acetic acid solution, freeze-drying, and then grinding to obtain porous micron-sized chitosan particles;

[0021] (2) Preparation of graft modification raw materials: dicumyl peroxide and maleic anhydride are mixed in a mass ratio of 1:4 to 1:1;

[0022] (3) Mixing materials: 80-90 parts of polypropylene, 1-10 parts of β-crystal nucleating agent, 3-8 parts of chitosan powder and the grafting modified raw material in step (2) are mixed at high speed;

[0023] (4) Melt extrusion: under nitrogen protection, melt extrusion was performed by a twin-screw extruder with segmented temperature control of 160°C / 170°C / 180°C / 170°C, screw speed of 100-200 rpm, and residence time of 2-4 minutes;

[0024] (5) Granulation and post-processing: the extruded material strips were water-cooled and pelletized, and vacuum-dried at 60°C for 12 hours to obtain chitosan-grafted polypropylene.

[0025] The present invention forms a porous structure by dissolving chitosan with acetic acid and freeze-drying it, thereby significantly improving its specific surface area and adsorption capacity; and utilizes maleic anhydride and dicumyl peroxide to initiate a grafting reaction, thereby effectively enhancing the compatibility of chitosan with polypropylene.

[0026] The β-crystal nucleating agent is selected from one or more of an amide β-nucleating agent, a rare earth β-nucleating agent, and a quinoline carboxylate β-nucleating agent. In the present invention, polypropylene to which the β-crystal nucleating agent and chitosan powder are added is used as the raw material in the surface layer A. Co-extrusion with the nucleating agent produces more β-crystal polypropylene. Through uniaxial stretching, the β-crystal polypropylene is transformed into a smaller α-crystal polypropylene, thereby forming a porous structure. Furthermore, the chitosan powder added to the surface layer tightly bonds with the polypropylene matrix through hydrogen bonds or other chemical bonds, thereby enhancing the durability of the separator and preventing deformation and cracking during charge and discharge cycles.

[0027] On the one hand, introducing chitosan into the polypropylene separator can significantly enhance the hydrophilicity of the separator, improve its wettability and impregnation, and ensure efficient conduction of lithium ions inside the battery; on the other hand, chitosan can adsorb and fix polysulfide intermediates (Li2Sx), effectively inhibiting the shuttle effect in lithium-sulfur batteries, thereby improving the battery's cycle stability and capacity retention.

[0028] The α-crystal nucleating agent is selected from one or more of sorbitol benzylidene derivatives, lignin and its derivatives, and sodium benzoate.

[0029] The present invention uses polypropylene added with an α-crystal nucleating agent and polyimide powder as the raw material for the intermediate layer B. During the membrane preparation process, the nucleating agent can refine the grains and form a uniform and fine grain structure by inhibiting excessive grain growth. This uniform grain structure helps to build a uniform pore structure, thereby improving the overall performance of the membrane. At the same time, the polyimide powder added to the intermediate layer can, on the one hand, form a more uniform pore distribution, optimize the ion conduction channel, and improve the lithium ion conduction efficiency; on the other hand, it can enhance the mechanical strength and toughness of the membrane, improve the durability and rupture resistance of the membrane, and effectively prevent physical damage to the membrane and the risk of short circuits.

[0030] Preferably, the preparation method of the modified polyimide powder is:

[0031] (1) reacting a diamine monomer and a dianhydride monomer in a polar solvent at 180-200° C. for 3-5 hours to obtain a polyamic acid solution;

[0032] (2) Add silane coupling agent to the solution in an amount of 1-3% of the total weight of the monomer and stir to react for 1-2 hours;

[0033] (3) Spray drying the mixed solution with an inlet temperature of 200-220°C and an outlet temperature of 80-100°C;

[0034] (4) The obtained powder was heated stepwise under nitrogen protection: room temperature → 250℃ / 1 h-2 h → 300℃ / 1-2 h → 350℃ / 1 h-2 h to complete the imidization;

[0035] (5) Finally, 50-100 nm modified polyimide powder was obtained by ball milling.

[0036] The present invention modifies polyimide powder using a silane coupling agent, thereby significantly improving its dispersibility and interfacial bonding strength; and adopts a step-by-step temperature increase method (250°C → 350°C) for imidization treatment to ensure the thoroughness of the reaction, thereby greatly enhancing the thermal stability and mechanical strength of the material.

[0037] The diamine monomer is selected from one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, and 2,2'-bis(trifluoromethyl)benzidine;

[0038] Preferably, the diamine monomer is a mixture of 4,4'-diaminodiphenyl ether and p-phenylenediamine, with a molar ratio of 8:2 to 9:1;

[0039] The dianhydride monomer is selected from one or more of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and 4,4'-oxydiphthalic anhydride;

[0040] Preferably, the dianhydride monomer is a mixture of pyromellitic dianhydride and 3,3',4,4'-biphenyltetracarboxylic dianhydride, with a molar ratio of 7:3 to 9:1;

[0041] The silane coupling agent is selected from one or more of 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane and 3-mercaptopropyltrimethoxysilane.

[0042] The total thickness of the diaphragm is 16-24 μm, and the thickness of the intermediate layer is 45-55% of the total thickness of the diaphragm.

[0043] The porosity of the diaphragm is 35-45%, the tensile strength in the longitudinal direction is not less than 1300 kgf / cm², the tensile strength in the transverse direction is not less than 110 kgf / cm², and the puncture strength is not less than 450 gf.

[0044] The present invention also provides a method for preparing a lithium-sulfur battery separator with low shuttle effect, comprising the following steps:

[0045] (1) Preparation of chitosan-modified polypropylene and modified polyimide powders;

[0046] (2) Casting sheet: The raw materials of the surface layer and the raw materials of the middle layer are mixed separately, extruded through a three-layer co-extrusion process, and then pulled and cooled to finally obtain a casting sheet;

[0047] (3) stretching film formation: the obtained casting sheet is subjected to heat treatment, longitudinal cold stretching, longitudinal hot stretching, and heat setting in sequence to obtain the high-performance lithium-sulfur battery separator;

[0048] In step (2), the extrusion temperature during the three-layer co-extrusion is 230 to 260°C; the cooling temperature is 90 to 110°C, and the pulling speed is 25 to 55 m / min;

[0049] The heat treatment temperature in step (3) is 120 ~ 145 ° C, and the heat treatment time is 5 ~ 14 h; the longitudinal cold stretching temperature is 35 ~ 65 ° C, and the stretching ratio is 1.3 ~ 1.6; the longitudinal hot stretching temperature is 135 ~ 155 ° C, and the stretching ratio is 1.5 ~ 3.2; the heat setting is 135 ~ 150 ° C, and the heat setting time is 1 ~ 10 min.

[0050] Compared with the prior art, the present invention has the following beneficial effects.

[0051] 1. The synergistic effect of polypropylene and a β-crystal nucleating agent on the surface layer creates a highly uniform porous structure. This creates a more uniform pore structure, resulting in a more consistent pore distribution across the separator. This dense and uniform pore structure helps hinder the diffusion of polysulfides, reducing their shuttle effect, thereby extending the battery's lifespan. Furthermore, the introduction of the nucleating agent reduces the thermal shrinkage of the polypropylene separator, ensuring battery safety during operation. Furthermore, by modifying the surface layer with chitosan, the separator's mechanical strength, hydrophilicity, and electrochemical properties are enhanced, effectively suppressing the shuttle effect of polysulfides and further enhancing battery safety.

[0052] 2. In the middle layer, a composite system of polypropylene and α-crystal nucleating agent is used. Due to the increase in nucleation points, the polypropylene grains formed are finer and more uniform. This change gives the material higher strength and toughness, making the pore distribution formed during the stretching process more uniform and improving the consistency of pore size. In addition, the polyimide powder added to the middle layer, due to its high melting point and high strength, effectively enhances the temperature resistance and strength of the diaphragm.

[0053] 3. This invention utilizes a three-layer gradient pore structure design, using carefully selected polypropylene materials with different modifications as the surface layer and the middle layer. Through a stretching process, these materials are formed into a structure with pores of varying sizes and uneven distribution. The three-layer structure, through the staggered arrangement of pores, creates a three-dimensional interlaced pore network, significantly improving the puncture strength and other mechanical properties of the separator, thereby enhancing battery safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 Schematic diagram of the low shuttle effect of the lithium-sulfur battery of the present invention.

[0055] 1. Lithium foil, 2. Separator, 3. Shuttle effect, 4. Sulfide.

[0056] Figure 2 SEM schematic diagram of the diaphragm with low shuttling effect of the present invention. DETAILED DESCRIPTION

[0057] In the present invention, unless otherwise specified, all equipment and raw materials can be purchased from the market or are commonly used in the industry. The methods in the following embodiments, unless otherwise specified, are all conventional methods in the art.

[0058] Overall embodiment

[0059] First, the present invention relates to a lithium-sulfur battery separator with low shuttle effect, which has an A / B / A symmetrical structure and a total thickness of 16-24 μm; the thickness of the intermediate layer is 45-55% of the total thickness of the separator.

[0060] A is a surface layer, and the raw materials of the chitosan grafted polypropylene of the surface layer include: 80-90 parts of polypropylene, 1-10 parts of β-crystal nucleating agent, 3-8 parts of chitosan powder, 0.5-1 parts of dicumyl peroxide as an initiator, and 1-2 parts of maleic anhydride grafting monomer;

[0061] The preparation method of chitosan grafted polypropylene comprises the following steps:

[0062] (1) Chitosan powder pretreatment: dissolving chitosan powder in 1% acetic acid solution, freeze-drying, and then grinding to obtain porous micron-sized chitosan particles;

[0063] (2) Preparation of graft modification raw materials: dicumyl peroxide and maleic anhydride are mixed in a mass ratio of 1:4 to 1:1;

[0064] (3) Mixing materials: 80-90 parts of polypropylene, 1-10 parts of β-crystal nucleating agent, 3-8 parts of chitosan powder and the grafting modified raw material in step (2) are mixed at high speed;

[0065] (4) Melt extrusion: under nitrogen protection, melt extrusion was performed by a twin-screw extruder with segmented temperature control of 160°C / 170°C / 180°C / 170°C, screw speed of 100-200 rpm, and residence time of 2-4 minutes;

[0066] (5) Granulation and post-processing: the extruded material strips were water-cooled and pelletized, and vacuum-dried at 60°C for 12 hours to obtain chitosan-grafted polypropylene.

[0067] B is the middle layer, and its raw materials include polypropylene and modified polyimide powder; its preparation raw materials are 85-92 parts of polypropylene, 3-6 parts of α-crystal nucleating agent and 5-8 parts of modified polyimide powder;

[0068] The preparation method of modified polyimide powder is:

[0069] (1) reacting a diamine monomer and a dianhydride monomer in a polar solvent at 180-200° C. for 3-5 hours to obtain a polyamic acid solution;

[0070] (2) Add silane coupling agent to the solution in an amount of 1-3% of the total weight of the monomer and stir to react for 1-2 hours;

[0071] (3) Spray drying the mixed solution with an inlet temperature of 200-220°C and an outlet temperature of 80-100°C;

[0072] (4) The obtained powder was heated stepwise under nitrogen protection: room temperature → 250℃ / 1 h-2 h → 300℃ / 1-2 h → 350℃ / 1 h-2 h to complete the imidization;

[0073] Finally, 50-100 nm modified polyimide powder was obtained by ball milling.

[0074] Second, the present invention relates to a method for preparing the above-mentioned lithium-sulfur battery separator with low shuttle effect, comprising the following steps:

[0075] (1) Preparation of chitosan-modified polypropylene and modified polyimide powders;

[0076] (2) Casting: The raw materials of the surface layer and the raw materials of the middle layer are mixed separately, and the surface layer and the middle layer diaphragms are pulled and cooled through a three-layer co-extrusion process to finally obtain a casting sheet; the extrusion temperature during the three-layer co-extrusion is 230~260℃; the cooling temperature is 90~110℃, and the pulling speed is 25~55m / min;

[0077] (3) Stretching film formation: The obtained casting sheet is subjected to heat treatment, longitudinal cold stretching, longitudinal hot stretching, and heat setting in sequence to obtain the high-performance lithium-sulfur battery separator; the heat treatment temperature is 120~145℃, and the heat treatment time is 5~14 h; the longitudinal cold stretching temperature is 35~65℃, and the stretching ratio is 1.3~1.6; the longitudinal hot stretching temperature is 135~155℃, and the stretching ratio is 1.5~3.2; the heat setting temperature is 135~150℃, and the heat setting time is 1~10 min.

[0078] The present invention is described below by way of specific examples. It should be understood that these examples are intended only to illustrate the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit and scope of the inventive concept, any changes and advantages that can be imagined by those skilled in the art are included in the present invention, and the scope of protection of the present invention is defined by the appended claims and any equivalents thereof.

[0079] Example 1:

[0080] A lithium-sulfur battery separator with low shuttle effect, an A / B / A symmetrical structure, and a total thickness of 18 μm;

[0081] A is the surface layer, and the thickness of each layer A is 4.4 μm. The raw materials include, by weight: 80 parts of polypropylene, 3 parts of β-crystal nucleating agent TM B-5, 5 parts of chitosan powder, 0.75 parts of dicumyl peroxide, and 1.5 parts of maleic anhydride.

[0082] The preparation method of chitosan grafted polypropylene comprises the following steps:

[0083] (1) Chitosan powder pretreatment: dissolving chitosan powder in 1% acetic acid solution, freeze-drying, and then grinding to obtain porous micron-sized chitosan particles;

[0084] (2) Preparation of graft modification raw materials: dicumyl peroxide and maleic anhydride are mixed in a mass ratio of 1:2;

[0085] (3) Mixing materials: 90 parts of polypropylene, 3 parts of β-crystal nucleating agent TM B-5, 5 parts of chitosan powder, 0.75 parts of dicumyl peroxide and 1.5 parts of maleic anhydride are mixed at high speed;

[0086] (4) Melt extrusion: under nitrogen protection, melt extrusion was performed by a twin-screw extruder with segmented temperature control of 160°C / 170°C / 180°C / 170°C, a screw speed of 100 rpm, and a residence time of 2 minutes;

[0087] (5) Granulation and post-processing: the extruded material strips were water-cooled and pelletized, and vacuum-dried at 60°C for 12 hours to obtain chitosan-grafted polypropylene.

[0088] B is the middle layer, and each layer B has a thickness of 9.2 μm. The raw materials include, by weight: 86 parts of polypropylene, 6 parts of α-crystal nucleating agent sodium benzoate (commercially available, particle size 600 nm), and 8 parts of modified polyimide powder.

[0089] The preparation method of modified polyimide powder is:

[0090] (1) reacting a mixture of 4,4'-diaminodiphenyl ether and p-phenylenediamine in a molar ratio of 8:3 and a mixture of pyromellitic dianhydride and 3,3',4,4'-biphenyltetracarboxylic dianhydride in a molar ratio of 7:3 in a polar solvent at 180°C for 3 hours to obtain a polyamic acid solution;

[0091] (2) Add 3-aminopropyltriethoxysilane to the solution in an amount of 1-3% of the total weight of the monomers and stir to react for 1 hour;

[0092] (3) Spray drying the mixed solution with an inlet temperature of 200°C and an outlet temperature of 80°C;

[0093] (4) The obtained powder was heated stepwise under nitrogen protection: room temperature → 250℃ / 1 h → 300℃ / 2 h → 350℃ / 1 h to complete the imidization;

[0094] (5) Finally, 50 nm modified polyimide powder was obtained by ball milling.

[0095] The method for preparing the lithium-sulfur battery separator with low shuttle effect comprises the following steps:

[0096] (1) Preparation of chitosan-modified polypropylene and modified polyimide powders;

[0097] (2) Casting: The raw materials of the surface layer and the raw materials of the middle layer are mixed separately, and the surface layer and the middle layer diaphragms are pulled and cooled through a three-layer co-extrusion process to finally obtain a casting sheet; the extrusion temperature during the three-layer co-extrusion is 230°C; the cooling temperature is 85°C, and the pulling speed is 45 m / min;

[0098] (3) Stretching film: The cast sheet obtained in step (2) is heat treated at a temperature of 135 ° C for 8 h; then the heat-treated cast sheet is first longitudinally cold stretched at a temperature of 75 ° C with a stretching ratio of 1.3, and then longitudinally hot stretched at a temperature of 140 ° C with a stretching ratio of 1.8, and then heat-set at a temperature of 140 ° C for 5 min to obtain the lithium-sulfur battery separator with low shuttle effect.

[0099] Example 2:

[0100] The difference between Example 2 and Example 1 is that 10 parts of chitosan powder are selected for the surface layer, and the rest are the same as in Example 1.

[0101] Example 3:

[0102] The difference between Example 3 and Example 1 is that 1 part of β-nucleating agent is selected for the surface layer, and the rest is the same as in Example 1.

[0103] Example 4:

[0104] The difference between Example 4 and Example 1 is that the raw materials of the middle layer include 90 parts of polypropylene, 10 parts of sodium benzoate powder and 5 parts of modified polyimide powder, and the rest are the same as in Example 1.

[0105] Example 5:

[0106] The difference between Example 5 and Example 1 is that the raw materials of the middle layer include 90 parts of polypropylene, 10 parts of sodium benzoate powder and 10 parts of modified polyimide powder, and the rest are the same as in Example 1.

[0107] Comparative Example 1:

[0108] The difference between Comparative Example 1 and Example 1 is that chitosan powder is not added to the surface layer, and the rest is the same as in Example 1.

[0109] Comparative Example 2:

[0110] The difference between Comparative Example 2 and Example 1 is that no β-nucleating agent is added to the surface layer, and the rest is the same as in Example 1.

[0111] Comparative Example 3:

[0112] The difference between Comparative Example 3 and Example 1 is that the polyimide powder in the surface layer has not been surface modified, and the rest is the same as in Example 1.

[0113] Comparative Example 4:

[0114] The difference between Comparative Example 4 and Example 1 is that no modified polyimide powder is added to the intermediate layer, and the rest is the same as in Example 1.

[0115] Comparative Example 5:

[0116] The difference between Comparative Example 5 and Example 1 is that the middle layer uses the same chitosan powder as the surface layer, and the other components and preparation methods are the same as those in Example 1.

[0117] Comparative Example 6:

[0118] The difference between Comparative Example 6 and Example 1 is that the diaphragm structure is a symmetrical structure of B / A / B; the thickness of each layer B is 9.2 μm, and the thickness of A is 4.4 μm; the raw materials of layers A and B and the diaphragm preparation method are the same as those in Example 1.

[0119] Comparative Example 7:

[0120] The difference between Comparative Example 7 and Example 1 is that the middle layer uses the same content of diamine powder, and the other components and preparation methods are the same as those in Example 1.

[0121] Comparative Example 8:

[0122] The difference between Comparative Example 8 and Example 1 is that the middle layer uses the same content of dianhydride powder, and the other components and preparation methods are the same as those in Example 1.

[0123] The microporous membranes prepared in the above examples and comparative examples were cut into A4 size and subjected to various mechanical and thermal property tests. The results are shown in Table 1. The test items and methods are as follows: (1) Average thickness: The thickness of the membrane at different locations was measured using a micrometer screw and the average value was calculated.

[0124] (2) Tensile strength: The longitudinal and transverse tensile strength of the diaphragm were tested using a Xieqiang CTM universal testing machine. Five specimens were tested in each direction and the average value was calculated.

[0125] (3) Puncture strength: The puncture strength of the diaphragm was tested using a Xieqiang CTM universal testing machine. Five specimens were tested and the average value was calculated.

[0126] (4) Porosity: Use an analytical balance to measure the mass of a 10 cm × 10 cm sample and calculate the porosity of the diaphragm. Test five samples and calculate the average value.

[0127] (5) Air permeability: Use an air permeability tester to test the air permeability of the diaphragm. Test 5 samples and calculate the average value.

[0128] (6) Liquid absorption rate: The liquid absorption rate of the diaphragm is tested by weighing. First, the diaphragm is completely dried and its weight is recorded. Then, the completely dried diaphragm is immersed in the electrolyte for 24 hours. After the surface electrolyte is wiped dry, the weight of the diaphragm is recorded again. The difference between the two recorded values ​​is the liquid absorption rate of the diaphragm. Test 5 samples and calculate the average value.

[0129] (7) Charge and discharge test: The positive electrode of the lithium-sulfur battery is made by uniformly mixing sulfur and conductive carbon black and coating it on aluminum foil, drying it and pressing it into a sheet; the negative electrode is made of lithium foil. The electrolyte uses Li-TFSI organic solvent. The positive electrode, separator, negative electrode and electrolyte are assembled into a soft-pack battery, and then the charge and discharge test is carried out. The capacity (mAh), voltage (V), current (mA) and time (h) of each charge and discharge cycle are recorded. The discharge capacity of each cycle is calculated and the capacity change curve with the number of cycles is plotted. The coulombic efficiency (discharge capacity / charge capacity) is calculated to evaluate the charge and discharge efficiency of the battery. The cycle stability of the battery is analyzed, and the long-term performance of the battery is evaluated by the capacity retention rate (percentage of the initial capacity).

[0130] (8) Electrolyte analysis: The total sulfur content of polysulfides in the electrolyte after 500 cycles was analyzed by inductively coupled plasma mass spectrometry (ICP-MS) to understand the dissolution and precipitation of sulfides.

[0131] (9) Determination of Li2S4 concentration by UV spectroscopy: First, extract the electrolyte sample and prepare a standard solution. Use a UV-visible spectrometer to measure the absorbance of Li2S4 solutions of different concentrations and establish a standard curve. Then, compare the electrolyte sample with the standard curve and calculate the Li2S4 concentration based on the absorbance value. This method can monitor the changes in polysulfide concentration in the electrolyte in real time and evaluate battery performance.

[0132] Table 1: A lithium-sulfur battery separator with low “shuttle effect” and battery performance test results

[0133]

[0134] It can be seen from the data in Table 1 that the lithium-sulfur battery separators prepared using the raw materials and methods of the present invention in Examples 1 to 5 can effectively improve the porosity of the separator and the discharge capacity of the battery, while also having high tensile and puncture properties.

[0135] In Comparative Example 1, chitosan powder was not added to the surface layer, resulting in a decrease in discharge capacity and a significantly higher sulfide concentration, which proved that chitosan anchored polysulfides through polar groups (-NH2 / -OH) and inhibited the shuttle effect.

[0136] In Comparative Example 2, no β-nucleating agent was added to the surface layer, and a dense pore structure could not be obtained in the surface layer. The porosity of the diaphragm was significantly reduced compared with that in Example 1, indicating that the microporous structure induced by the β-crystal is crucial for ion transport.

[0137] In Comparative Example 3, the polyimide powder in the surface layer was not surface-modified, and the physical strength and discharge specific capacity of the separator were significantly reduced compared with those in Example 1.

[0138] Comparative Example 4 did not add modified polyimide powder to the intermediate layer, resulting in the separator having significantly lower tensile strength and puncture strength than Example 1. In addition, the discharge specific capacity of the lithium-sulfur battery decreased and the sulfide concentration in the electrolyte increased, confirming that the silane-modified polyimide not only enhanced the mechanical strength but also assisted in the adsorption of polysulfides.

[0139] In Comparative Example 5, after the middle layer was replaced with chitosan, although the sulfide concentration in the electrolyte was extremely low, the poor capacity was due to the blockage of ion channels caused by excessive adsorption of chitosan, which not only led to the collapse of mechanical strength but also caused a sharp drop in discharge capacity, thus confirming that modified polyimide plays an irreplaceable role in maintaining the integrity of the multilayer structure.

[0140] Comparative Example 6 improves the mechanical properties of the membrane compared to Example 1 by exchanging the surface layer and the intermediate layer and adopting a symmetrical B / A / B structure. However, its porosity and liquid absorption rate values ​​are reduced, and the sulfide concentration is significantly increased, indicating that Example 1 has a more excellent sulfide barrier effect. This is because the chitosan-enriched surface layer directly contacts the sulfur positive electrode, exerting a significant chemical adsorption effect. At the same time, although the porosity difference is not much compared with Example 1, the permeability is significantly reduced. The main reason is that the B / A / B structure leads to an uneven pore size distribution, which hinders gas diffusion.

[0141] In Comparative Examples 7 and 8, the modified polyimide powder was replaced with dianhydride and diamine powders of the same composition, primarily serving as precursors for the chemical reaction used to synthesize the polyimide. This substitution resulted in decreased porosity and liquid absorption, while significantly increasing the sulfide concentration, demonstrating that the α-crystal modification of sodium benzoate positively impacted the thermal stability of the separator.

[0142] The above description is only a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. All equivalent changes and modifications made based on the content of the present invention are included in the patent scope of the present invention.

Claims

1. A lithium-sulfur battery separator with low shuttle effect, characterized in that: The structure is A / B / A symmetrical; A is the surface layer, and the raw materials include chitosan grafted polypropylene; B is the middle layer, and its raw materials include polypropylene and modified polyimide powder; The raw materials of the chitosan grafted polypropylene of the surface layer include, by weight, 80 to 90 parts of polypropylene, 1 to 10 parts of a β-crystal nucleating agent, 3 to 8 parts of chitosan powder, 0.5 to 1 part of dicumyl peroxide as an initiator, and 1 to 2 parts of a maleic anhydride grafting monomer; The raw materials of the intermediate layer include: 85-92 parts of polypropylene, 3-6 parts of α-crystal nucleating agent and 5-8 parts of modified polyimide powder; The particle size of the chitosan powder is 200-600 nm, and the molecular weight of the chitosan powder is 100,000- 500000Da, the deacetylation degree of chitosan powder is 60-80%, and the specific surface area is 20-50m 2 / g; the particle size of the modified polyimide powder is 50 to 100 nm; The preparation method of the modified polyimide powder is: (1) reacting a diamine monomer and a dianhydride monomer in a polar solvent at 180-200° C. for 3-5 hours to obtain a polyamic acid solution; (2) adding a silane coupling agent to the solution in an amount of 1-3% of the total weight of the monomers and stirring the reaction for 1-2 hours; (3) spray drying the mixed solution with an inlet temperature of 200-220°C and an outlet temperature of 80-100°C; (4) The obtained powder was heated stepwise under nitrogen protection: room temperature → 250°C / 1h-2h → 300°C / 1-2h → 350°C / 1h-2h to complete imidization; Finally, 50-100 nm modified polyimide powder was obtained by ball milling.

2. The lithium-sulfur battery separator with low shuttle effect according to claim 1, characterized in that: The preparation method of the chitosan grafted polypropylene comprises the following steps: (1) pretreatment of chitosan powder: dissolving the chitosan powder in 1% acetic acid solution, freeze-drying, and then grinding to obtain porous micron-sized chitosan particles; (2) Preparation of graft modification raw materials: dicumyl peroxide and maleic anhydride are mixed in a mass ratio of 1:4 to 1:1; (3) Mixing materials: 80-90 parts of polypropylene, 1-10 parts of β-crystal nucleating agent, 3-8 parts of chitosan powder and the grafting modified raw material in step (2) are mixed at high speed; (4) Melt extrusion, under nitrogen protection, through a twin-screw extruder with segmented temperature control Melt extrusion at 160℃ / 170℃ / 180℃ / 170℃, screw speed 100-200rpm, residence time 2-4 minutes; (5) Granulation and post-processing: the extruded material strips were water-cooled and pelletized, and vacuum-dried at 60°C for 12 hours to obtain chitosan-grafted polypropylene.

3. The lithium-sulfur battery separator with low shuttle effect according to claim 1, characterized in that: The β-crystal nucleating agent is selected from one or more of amide β-nucleating agents, rare earth β-nucleating agents, and quinoline carboxylate β-nucleating agents; And / or, the α-crystal nucleating agent is selected from one or more of sorbitol benzyl derivatives, lignin and its derivatives, and sodium benzoate.

4. The lithium-sulfur battery separator with low shuttle effect according to claim 1, characterized in that: The diamine monomer is selected from one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, and 2,2'-bis(trifluoromethyl)benzidine; And / or, the dianhydride monomer is selected from one or more of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and 4,4'-oxydiphthalic anhydride; And / or, the silane coupling agent is selected from one or more of 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane and 3-mercaptopropyltrimethoxysilane.

5. The lithium-sulfur battery separator with low shuttle effect according to claim 4, characterized in that: The diamine monomer is a mixture of 4,4'-diaminodiphenyl ether and p-phenylenediamine, with a molar ratio of 8:2 to 9:1; And / or, the dianhydride monomer is a mixture of pyromellitic dianhydride and 3,3',4,4'-biphenyltetracarboxylic dianhydride, with a molar ratio of 7:3 to 9:

1.

6. The lithium-sulfur battery separator with low shuttle effect according to claim 1, characterized in that: The total thickness of the diaphragm is 16 to 24 μm, and the thickness of the intermediate layer is 45 to 55% of the total thickness of the diaphragm.

7. The lithium-sulfur battery separator with low shuttle effect according to claim 1, characterized in that: The porosity of the diaphragm is 35-45%, and the tensile strength in the longitudinal direction is not less than 1300kgf / cm 2 , transverse tensile strength not less than 110kgf / cm 2 , puncture strength is not less than 450gf.

8. A method for preparing a lithium-sulfur battery separator with low shuttle effect according to any one of claims 1 to 7, characterized in that: The following steps are involved: (1) preparing chitosan-modified polypropylene and modified polyimide powders; (2) Casting: The raw materials of the surface layer and the raw materials of the middle layer are mixed separately, extruded through a three-layer co-extrusion process, and then pulled and cooled to finally obtain a casting sheet; (3) Stretching to form a film: The obtained casting sheet is subjected to heat treatment, longitudinal cold stretching, longitudinal hot stretching, and heat setting in sequence to obtain the lithium-sulfur battery separator.

9. The method for preparing a lithium-sulfur battery separator with low shuttle effect according to claim 8, characterized in that: In step (2), the extrusion temperature during the three-layer co-extrusion is 230-260° C.; the cooling temperature is 90-110° C., and the pulling speed is 25-55 m / min; The heat treatment temperature in step (3) is 120-145°C, and the heat treatment time is 5-14h; the longitudinal cold stretching temperature is 35-65°C, and the stretching ratio is 1.3-1.6; the longitudinal hot stretching temperature is 135-155°C, and the stretching ratio is 1.5-3.2; the heat setting temperature is 135-150°C, and the heat setting time is 1-10min.

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