Composite diaphragm, preparation method thereof and battery

Abstract

The invention relates to a composite diaphragm, a preparation method thereof and a battery in the technical field of lithium battery production. The composite diaphragm comprises a base membrane, a first coating and a second coating, wherein the first coating and the second coating are positioned on two side surfaces of the base membrane; the base film comprises a polyolefin material; the first coating comprises modified lithium aluminum titanium phosphate (LATP) particles, and modification is characterized in that inert elements are introduced in the LATP synthesis process for doping and / or inert functional group surface grafting is carried out on the LATP particles; the second coating comprises an aramid fiber material; wherein the inert element is selected from at least one element of boron (B), europium (Eu), lanthanum (La) and yttrium (Y), and the inert functional group is selected from at least one functional group of siloxane group and amino functional group. The modified LATP coating and the aramid fiber coating are arranged on the two sides of the polyolefin base membrane respectively, and through the high-temperature-resistant characteristic of the aramid fiber coating and the ion conduction optimization synergistic effect of the modified LATP coating, the thermal stability of the diaphragm is remarkably improved, lithium dendrite puncture is prevented, the thermal runaway risk of a battery cell is reduced, and the ion transmission efficiency of the diaphragm at low temperature is improved.

Description

Technical Field

[0001] This invention relates to the field of lithium battery production technology, and in particular to a composite separator, its preparation method, and a battery. Background Technology

[0002] As the core energy carrier of power batteries and energy storage batteries, the safety performance and adaptability to extreme environments of lithium-ion batteries have become key bottlenecks restricting the development of the industry. The separator directly determines the thermal stability, ion transport efficiency, and cycle life of the battery cell. Although traditional polyolefin porous separators (such as PE and PP) have good mechanical properties and ion permeability, they have poor thermal stability (easily shrinking above 120°C). At high temperatures, separator failure can easily lead to short circuits between the positive and negative electrodes, thereby inducing the risk of thermal runaway. At the same time, in low-temperature environments (such as below 0°C, especially -20°C and below), the viscosity of the electrolyte increases significantly, the resistance to ion migration increases, and the interfacial compatibility of traditional separators is insufficient, resulting in intensified battery polarization, severe capacity decay, and a significant decrease in cycle stability. This makes it difficult to meet the actual needs of power batteries starting up in cold regions and energy storage batteries operating outdoors at low temperatures.

[0003] To address these issues, the industry has conducted relevant improvement research: On the one hand, coating the surface of polyolefin-based membranes with high-temperature resistant coatings such as aramid or ceramics (e.g., Al2O3, SiO2) can improve the thermal stability and resistance to lithium dendrite puncture, reducing the risk of thermal runaway. However, ceramic coatings have poor compatibility with electrolytes, increasing ion conduction resistance at low temperatures and leading to severe capacity decay at low temperatures. While aramid coatings are heat-resistant, their low ion conductivity means that using them alone can affect the battery's rate performance. LATP (Li 1.3 Al 0.3 Ti 1.7 (PO4)3) As a novel fast ion conductor, LATP possesses high ionic conductivity and chemical stability, and is often used as a membrane coating to improve ion transport efficiency. However, LATP readily undergoes a reduction reaction with the lithium metal anode, leading to increased interfacial impedance. Furthermore, at low temperatures, LATP particles agglomerate, and grain boundary impedance increases, limiting its application in low-temperature environments. Simultaneously, the thermal stability of a simple LATP coating is insufficient, failing to meet the high-temperature safety requirements of batteries.

[0004] Therefore, developing a composite separator that combines excellent thermal stability with efficient low-temperature ion transport performance has become a technical challenge that the lithium battery industry urgently needs to solve. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention discloses a composite separator, its preparation method, and a battery. This invention involves setting a modified LATP coating and an aramid coating on both sides of a polyolefin-based membrane. Through the synergistic effect of the high-temperature resistance of the aramid coating and the ion conduction optimization of the modified LATP coating, the thermal stability of the separator is significantly improved, lithium dendrite puncture is prevented, the risk of thermal runaway of the battery cell is reduced, and its ion transport efficiency at low temperatures is improved.

[0006] The technical solution of this invention is implemented as follows:

[0007] In a first aspect, this application provides a composite membrane, including a base membrane and a first coating and a second coating located on both sides of the base membrane;

[0008] The base membrane comprises a polyolefin material;

[0009] The first coating comprises modified lithium aluminum titanium phosphate (LATP) particles, wherein the modification is performed by introducing inert elements for doping and / or by grafting inert functional groups onto the LATP particles during the LATP synthesis process.

[0010] The second coating comprises an aramid material;

[0011] The inert element is selected from at least one element selected from boron (B), europium (Eu), lanthanum (La), and yttrium (Y), and the inert functional group is selected from at least one functional group selected from siloxane and amino functional groups.

[0012] In some embodiments, the polyolefin material includes at least one of polypropylene (PP), polyethylene (PE), or PP / PE composite porous membrane; and the porosity of the base membrane is 35-50% and the thickness is 7-14 μm.

[0013] In some embodiments, the aramid material includes poly(m-phenylene isophthalamide) and / or poly(p-phenylene terephthalamide).

[0014] In some embodiments, the total thickness of the composite membrane is 9–30 μm, the thickness of the first coating is 1–8 μm, and the thickness of the second coating is 1–8 μm.

[0015] Secondly, the present invention provides a method for preparing a composite separator, comprising the following steps:

[0016] S1. Provide a base film, using polyolefin as the base film;

[0017] S2. Preparation of modified lithium titanium aluminum phosphate (LATP) material: Modified lithium titanium aluminum phosphate powder is obtained by introducing inert elements for doping and / or grafting inert functional groups onto the surface of LATP particles during the LATP synthesis process.

[0018] S3. Mix the modified LATP powder, the first solvent, and the binder, and stir until homogeneous to form a first slurry;

[0019] S4. Provide aramid raw material, add it to the second solvent under an inert atmosphere, and stir until completely dissolved to form a mixed solution;

[0020] S5. Add additives to the mixed solution and disperse it ultrasonically to form a second slurry;

[0021] S6. The first slurry and the second slurry are coated on the first surface and the second surface of the base film, respectively, and after drying, the first coating and the second coating are formed.

[0022] S7. A composite diaphragm is prepared.

[0023] The inert element is selected from at least one element selected from boron (B), europium (Eu), lanthanum (La), and yttrium (Y), and the inert functional group is selected from at least one functional group selected from siloxane and amino functional groups.

[0024] In some embodiments, step S2, which modifies LATP by doping with inert elements, includes the following steps:

[0025] A. Weigh lithium source, aluminum source, titanium source and phosphorus source according to stoichiometric ratio, and add the compound of the inert elements;

[0026] B. Ball milling mixing;

[0027] C. After drying and pressing, calcination is carried out;

[0028] D. The inert element-doped modified LATP powder is obtained by cooling and pulverizing.

[0029] The inert element compound includes at least one of H3BO3, Eu2O3, La2O3, and Y2O3; based on the mass of the LATP material as 100%, the inert element compound accounts for 0.5% to 4% of the mass of the LATP material.

[0030] In this invention, "mass of LATP material" refers to the theoretically generated Li calculated according to stoichiometric ratios. 1+x Al x Ti 2-x The mass of (PO4)3.

[0031] In some embodiments, step S2, which involves grafting LATP with inert functional groups, includes the following steps:

[0032] (1) Pretreatment: The LATP material is added to a third solvent and ultrasonically dispersed to form a suspension;

[0033] (2) Add a reagent containing inert functional groups to the suspension and add a catalyst to form a mixture;

[0034] (3) Stir the mixture and centrifuge to obtain the precipitate;

[0035] (4) The inert functional group grafted modified LATP was obtained by washing and drying.

[0036] Specifically, based on the mass of the LATP material as 100%, the reagent containing inert functional groups accounts for 1% to 6% of the mass of the LATP material.

[0037] In some embodiments, the additive in step S5 includes at least one of nano-SiO2, alumina, and boehmite.

[0038] In some embodiments, the catalyst includes at least one of dibutyltin dilaurate, glacial acetic acid, hydrochloric acid, and carbodiimide condensing agents.

[0039] Thirdly, the present invention provides a battery comprising the composite separator provided in the first aspect.

[0040] The advantages of this invention are as follows:

[0041] (1) This invention improves the thermal stability and interfacial bonding force between LATP particles and the base film by modifying them with inert elements or functional groups, which can effectively suppress the shrinkage of the base film at high temperatures. At the same time, the combination with an aramid coating with excellent high-temperature resistance can block the transfer of heat from the positive electrode to the negative electrode. The synergistic effect of the two significantly enhances the overall thermal stability of the separator and reduces the risk of battery thermal runaway.

[0042] (2) The inert components on the surface of the modified LATP coating can prevent LATP particles from agglomerating, forming uniform ion transport channels and guiding uniform lithium ion deposition; it can also avoid side reactions with the lithium anode, reduce lithium dendrite formation, and maintain interface stability. The aramid coating provides a physical barrier with its high mechanical strength, preventing lithium dendrite puncture. Together, they construct a dual mechanism of "formation inhibition + physical protection," significantly improving the membrane's resistance to lithium dendrite formation.

[0043] (3) The modified LATP coating improves interfacial compatibility and reduces interfacial impedance, ensuring the smooth flow of ion transport channels at low temperatures; the aramid coating helps optimize the cathode interfacial environment and reduce electrolyte interfacial impedance. The two work synergistically to improve the low-temperature ion transport efficiency of the battery and enhance low-temperature cycling performance and capacity retention. Detailed Implementation

[0044] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0045] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used in the detailed description is for the purpose of describing particular embodiments only and is not intended to limit the invention.

[0046] In the description of specific embodiments of the present invention, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present invention, "multiple" means two or more, unless otherwise explicitly defined.

[0047] In this invention, the reference to "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this invention can be combined with other embodiments.

[0048] In the description of the embodiments of this invention, the term "and / or" is merely a description of the relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, in this invention, the character " / " generally indicates that the preceding and following associated objects have an "or" relationship.

[0049] Throughout this invention, numerical values ​​represent approximate measurements or limits of a range to cover minute deviations from a given value, as well as embodiments with approximately the mentioned value and embodiments with the exact mentioned value. Except for the working examples provided at the end of the detailed description, all numerical values ​​of parameters (e.g., quantities or conditions) in this specification (including the appended claims) should be understood to be modified in all cases by the term “about,” regardless of whether “about” actually appears before the numerical value. “About” indicates that the stated numerical value allows for some minor inaccuracy (approaching the exact value in some way; approximately or reasonably approaching the value; almost). If the inaccuracy provided by “about” is not otherwise understood in this general sense in the art, then “about” as used in this invention at least indicates a variation that can be produced by common methods of measuring and using such a parameter. For example, “about” may include a variation less than or equal to 5%, optionally less than or equal to 4%, optionally less than or equal to 3%, optionally less than or equal to 2%, optionally less than or equal to 1%, optionally less than or equal to 0.5%, and in some respects, optionally less than or equal to 0.1%.

[0050] Additionally, the disclosure of the range includes the disclosure of all values ​​across the entire range and the disclosure of further subdivided ranges, including the endpoints and subranges given for these ranges.

[0051] Traditional polyolefin porous membranes (such as PE and PP) possess good mechanical properties and ion permeability, but they have poor thermal stability (easily undergoing thermal shrinkage above 120°C). At high temperatures, membrane failure can easily lead to short circuits between the positive and negative electrodes, thereby inducing the risk of thermal runaway. At the same time, in low-temperature environments (such as below 0°C, especially -20°C and below), the viscosity of the electrolyte increases significantly, the resistance to ion migration increases, and the interfacial compatibility of traditional membranes is insufficient, resulting in intensified battery polarization, severe capacity decay, and a significant decrease in cycle stability. This makes it difficult to meet the actual needs of power batteries for starting up in cold regions and energy storage batteries for outdoor low-temperature operation.

[0052] To address these issues, the industry has conducted relevant improvement research: On the one hand, coating the surface of polyolefin-based membranes with high-temperature resistant coatings such as aramid or ceramics (e.g., Al2O3, SiO2) can improve the thermal stability and resistance to lithium dendrite puncture, reducing the risk of thermal runaway. However, ceramic coatings have poor compatibility with electrolytes, increasing ion conduction resistance at low temperatures and leading to severe capacity decay at low temperatures. While aramid coatings are heat-resistant, their low ion conductivity means that using them alone can affect the battery's rate performance. LATP (Li 1.3 Al 0.3 Ti 1.7(PO4)3) As a novel fast ion conductor, LATP possesses high ionic conductivity and chemical stability, and is often used as a membrane coating to improve ion transport efficiency. However, LATP readily undergoes a reduction reaction with the lithium metal anode, leading to increased interfacial impedance. Furthermore, at low temperatures, LATP particles agglomerate, and grain boundary impedance increases, limiting its application in low-temperature environments. Simultaneously, the thermal stability of a simple LATP coating is insufficient, failing to meet the high-temperature safety requirements of batteries.

[0053] Therefore, in order to solve the above-mentioned technical problems, this invention discloses a composite separator and its preparation method and battery. This invention coats a modified LATP coating and an aramid coating on both sides of a polyolefin base film, respectively. Through the synergistic effect of the high temperature resistance of the aramid coating and the ion conduction optimization of the modified LATP coating, the thermal stability of the separator is significantly improved, lithium dendrite puncture is prevented, the risk of thermal runaway of the battery cell is reduced, and its ion transport efficiency at low temperature is improved.

[0054] In a first aspect, this application provides a composite membrane, including a base membrane and a first coating and a second coating located on both sides of the base membrane;

[0055] The base membrane comprises a polyolefin material;

[0056] The first coating comprises modified lithium aluminum titanium phosphate (LATP) particles, wherein the modification is performed by introducing inert elements for doping and / or by grafting inert functional groups onto the LATP particles during the LATP synthesis process.

[0057] The second coating comprises an aramid material;

[0058] The inert element is selected from at least one element selected from boron (B), europium (Eu), lanthanum (La), and yttrium (Y), and the inert functional group is selected from at least one functional group selected from siloxane and amino functional groups.

[0059] In this invention, "at least one" means selecting one of the listed materials, or selecting a mixture of two or more materials. Those skilled in the art can freely choose within the scope of their understanding. This invention does not impose any limitations in this regard.

[0060] This invention achieves multifunctional synergistic enhancement by respectively depositing modified LATP coatings and aramid coatings on both sides of a polyolefin-based film. After modification with inert elements or functional groups, the LATP particles exhibit significantly improved thermal stability and interfacial compatibility. This not only effectively suppresses high-temperature shrinkage of the base film and reduces interfacial side reactions, but also forms uniform ion channels, guiding uniform lithium ion deposition, thereby inhibiting lithium dendrite formation and ensuring ion transport efficiency at low temperatures. Simultaneously, the aramid coating on the other side, with its excellent heat resistance and high mechanical strength, directly blocks heat from the positive electrode and provides a physical barrier. Together with the modified LATP layer, it constructs a triple synergistic mechanism of "thermal protection-dendrite suppression-ion conduction," significantly enhancing the battery's thermal safety, cycle stability, and wide-temperature performance.

[0061] The inert elements (B, Eu, La, Y) selected in this invention have specific functional orientations. For example, the introduction of boron (B) can enter the LATP lattice, stabilizing the structure and reducing grain boundary impedance; the doping of lanthanides (Eu, La) and yttrium (Y) can effectively suppress Ti... 4+ Reduction at low potentials enhances the chemical stability of the material against lithium metal. The selected inert functional groups (such as siloxane and amino groups) are primarily chosen from an interface engineering perspective. Siloxane groups can form a stable silicon-oxygen network on the surface of LATP particles through hydrolysis and condensation, and readily exhibit good compatibility with commonly used binders in coatings. Amino functional groups provide certain alkaline sites, neutralizing trace amounts of acidic components (such as HF) in the electrolyte, reducing corrosion of the active material, and improving particle dispersibility in the slurry. The common goal of these modifications is to improve the stability of the LATP material bulk and its interfaces, reduce side reactions with the electrolyte and electrodes, thereby ensuring a long cycle life and wide-temperature performance of the battery.

[0062] In some embodiments, the polyolefin material includes at least one of polypropylene (PP), polyethylene (PE), or a PP / PE composite porous membrane; and the porosity of the base membrane is 35-50% to ensure sufficient ion transport channels while maintaining the necessary mechanical strength and electronic insulation of the separator. Too low a porosity will lead to a decrease in ionic conductivity, especially at low temperatures; too high a porosity may weaken the mechanical properties of the separator and increase the risk of short circuits; a thickness of 7-14 μm is based on a comprehensive balance between battery energy density (pursuing thinner layers) and safety and reliability (requiring a certain level of mechanical strength).

[0063] In specific applications, the porosity of the base film can be 35%, 40%, 45%, 50%, etc.; the thickness can be 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, etc. The numerical ratios listed above are merely examples, and the present invention does not impose any limitations on them.

[0064] In some embodiments, the aramid material includes poly(m-phenylene isophthalamide) and / or poly(p-phenylene terephthalamide).

[0065] In some specific implementations, the second coating is in direct contact with the positive electrode. The excellent high-temperature resistance and mechanical strength of aramid materials enhance the thermal stability and puncture resistance of the diaphragm.

[0066] In some preferred embodiments of the present invention, the first coating (i.e., the modified LATP coating) is positioned facing the negative electrode of the battery, while the second coating (i.e., the aramid coating) is positioned facing the positive electrode. This design is based on the following synergistic considerations: the modified LATP coating facing the negative electrode, with its inert components or functional groups on its surface, can directly contact the lithium negative electrode, effectively suppressing the reduction side reaction between LATP and active lithium, reducing interfacial impedance, and guiding uniform lithium ion deposition, thereby suppressing the formation of lithium dendrites from the source. Simultaneously, its optimized ionic conductivity ensures efficient ion transport at low temperatures. The aramid coating facing the positive electrode, utilizing its extremely high thermal stability and mechanical strength, directly blocks the thermal shock that may occur on the positive electrode side during charging and discharging, preventing local overheating and shrinkage of the separator, and providing a physical barrier for possible expansion of positive electrode particles or whisker growth, preventing them from piercing the separator. The functions of the two coatings are clear and complementary, jointly constructing a dual guarantee of "interface stability and ion conduction on the negative electrode side" and "thermal protection and physical barrier on the positive electrode side."

[0067] In some embodiments, the total thickness of the composite separator is 9–30 μm, the thickness of the first coating is 1–8 μm, and the thickness of the second coating is 1–8 μm. This is to impart new functions to the separator (optimized ion conduction, thermal stability) while minimizing the increase in the overall thickness and internal resistance of the separator. A coating that is too thin may result in discontinuous functional layers and poor performance; a coating that is too thick will significantly increase ion migration distance and battery internal resistance, affecting rate performance and energy density.

[0068] In specific applications, the total thickness of the composite diaphragm can be 9μm, 15μm, 20μm, 25μm, 30μm, etc.; the thickness of the first coating can be 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, etc.; and the thickness of the second coating can be 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, etc. The numerical ratios listed above are merely examples, and the present invention does not impose any limitations on them.

[0069] Secondly, the present invention provides a method for preparing a composite separator, comprising the following steps:

[0070] S1. Provide a base film, using polyolefin as the base film;

[0071] S2. Preparation of modified lithium titanium aluminum phosphate (LATP) material: Modified lithium titanium aluminum phosphate powder is obtained by introducing inert elements for doping and / or grafting inert functional groups onto the surface of LATP particles during the LATP synthesis process.

[0072] S3. Mix the modified LATP powder, the first solvent, and the binder, and stir until homogeneous to form a first slurry;

[0073] S4. Provide aramid raw material, add it to the second solvent under an inert atmosphere, and stir until completely dissolved to form a mixed solution;

[0074] S5. Add additives to the mixed solution and disperse it ultrasonically to form a second slurry;

[0075] S6. The first slurry and the second slurry are coated on the first surface and the second surface of the base film, respectively, and after drying, the first coating and the second coating are formed.

[0076] S7. A composite diaphragm is prepared.

[0077] The inert element is selected from at least one element selected from boron (B), europium (Eu), lanthanum (La), and yttrium (Y), and the inert functional group is selected from at least one functional group selected from siloxane and amino functional groups.

[0078] In the modification method described in this invention, "LATP material" refers to the mixture of various raw materials required for the synthesis of LATP in inert element doping modification; and refers to the synthesized LATP particles in inert functional group surface grafting modification.

[0079] In some embodiments, step S2, which modifies LATP by doping with inert elements, includes the following steps:

[0080] A. Weigh lithium source, aluminum source, titanium source and phosphorus source according to stoichiometric ratio, and add the compound of the inert elements;

[0081] B. Ball milling mixing;

[0082] C. After drying and pressing, calcination is carried out;

[0083] D. The inert element-doped modified LATP powder is obtained by cooling and pulverizing.

[0084] The inert element compound includes at least one of H3BO3, Eu2O3, La2O3, and Y2O3; based on the mass of the LATP material as 100%, the inert element compound accounts for 0.5% to 4% of the mass of the LATP material.

[0085] In specific applications, the inert element may constitute 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, or 4% of the mass of the LATP material. The numerical ratios listed above are merely examples, and the present invention does not impose any limitations on them.

[0086] In some specific embodiments, the lithium source in this application is Li2CO3, the aluminum source is Al2O3, the titanium source is TiO2, and the phosphorus source is (NH4)3PO4, obtained by weighing according to stoichiometric ratios. 1.3 Al 0.3 Ti 1.7 (PO4)3(LATP).

[0087] In some embodiments, the compounds of lithium source, aluminum source, titanium source, phosphorus source and inert element in step A are mixed to form a first mixture.

[0088] In some embodiments, in step B, the first mixture from step A is ball-milled and mixed, with the mass ratio of the added agate balls to the first mixture being 8 to 12:1.

[0089] In practical applications, the mass ratio of the added agate balls to the first mixture can be 8:1, 9:1, 10:1, 11:1, 12:1, etc. The numerical ratios listed above are merely examples, and the present invention does not impose any limitations on them.

[0090] In some implementations, the equipment used for ball milling in step B is a planetary ball mill.

[0091] In some embodiments, a dispersing agent is also added to the first mixture, with the volume ratio of the dispersing agent to the total solids (agate balls and the first mixture) being 1:(1.5 to 2).

[0092] In practical applications, the volume ratio of the dispersing agent to the total solids (agate balls and the first mixture) can be 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, or 1:2. The numerical ratios listed above are merely examples, and this invention does not impose any limitations on them.

[0093] In some specific embodiments, anhydrous ethanol is used as the dispersing agent in this application.

[0094] In some embodiments, the ball milling mixing in step B is performed at a speed of 300–500 r / min for 4–8 h to obtain a uniform slurry.

[0095] In practical applications, the ball milling speed can be 300 r / min, 350 r / min, 400 r / min, 450 r / min, 500 r / min, etc., and the time can be 4 h, 5 h, 6 h, 7 h, 8 h, etc. The numerical ratios listed above are merely examples, and this invention does not impose any limitations on them.

[0096] In some embodiments, the temperature for drying the uniform slurry in step C is 80–100°C, and the drying time is 6–10 hours.

[0097] In practical applications, the drying temperature can be 80℃, 90℃, 100℃, etc., and the drying time can be 6h, 7h, 8h, 9h, 10h, etc. The numerical ratios listed above are merely examples, and the present invention does not impose any limitations on them.

[0098] In some embodiments, after drying in step C, the powder needs to be ground and sieved to obtain powder with a mesh size of 200-300.

[0099] In some embodiments, in step C, the disc is pressed into a diameter of 10-15 mm under a pressure of 5-10 MPa.

[0100] In practical applications, the pressure can be 5MPa, 6MPa, 7MPa, 8MPa, 9MPa, 10MPa, etc., and the diameter of the pressed disc can be 10mm, 11mm, 12mm, 13mm, 14mm, 15mm, etc. The numerical ratios listed above are merely examples, and the present invention does not impose any limitations on them.

[0101] In some embodiments, after pressing in step C, calcination is carried out at a heating rate of 5-10°C / min, a calcination temperature of 700-850°C, and a calcination time of 6-10 hours.

[0102] In practical applications, the calcination heating rate can be 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, etc., the calcination temperature can be 700℃, 750℃, 800℃, 850℃, etc., and the calcination time can be 6h, 7h, 8h, 9h, 10h, etc. The numerical ratios listed above are merely examples, and this invention does not impose any limitations on them.

[0103] In some embodiments, step D involves pulverizing the calcined product after it has been naturally cooled to room temperature and then ball-milling it until the particle size is 0.5–2 μm to finally obtain inert element-doped modified LATP powder. The ball-milling steps are the same as described above and will not be repeated here.

[0104] In some embodiments, step S2, which involves grafting LATP with inert functional groups, includes the following steps:

[0105] (1) Pretreatment: The LATP material is added to a third solvent and ultrasonically dispersed to form a suspension;

[0106] (2) Add a reagent containing inert functional groups to the suspension and add a catalyst to form a mixture;

[0107] (3) Stir the mixture and centrifuge to obtain the precipitate;

[0108] (4) The inert functional group grafted modified LATP was obtained by washing and drying.

[0109] Specifically, based on the mass of the LATP material as 100%, the reagent containing inert functional groups accounts for 1% to 6% of the mass of the LATP material.

[0110] In specific applications, the reagent containing inert functional groups can constitute 1%, 2%, 3%, 4%, 5%, 6% of the mass of the LATP material, etc. The numerical ratios listed above are merely examples, and the present invention does not impose any limitations thereon.

[0111] In some specific embodiments, the third solvent of this application is anhydrous toluene solution.

[0112] In some embodiments, the solid-liquid ratio of the LATP material to the third solvent in step (1) is 1:(10-15), the ultrasonic dispersion power is 200-300W, and the time is 30-60min.

[0113] In practical applications, the solid-liquid ratio of the LATP material to the third solvent can be 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, etc., the ultrasonic dispersion power can be 200W, 210W, 220W, 230W, 240W, 250W, 260W, 270W, 280W, 290W, 300W, etc., and the time can be 30min, 40min, 50min, 60min, etc. The numerical ratios listed above are merely examples, and this invention does not impose any limitations on them.

[0114] In specific applications, reagents containing inert functional groups include at least one of γ-aminopropyltriethoxysilane (KH550), γ-glycidoxypropyltrimethoxysilane (KH560), aminobenzoic acid, and hexamethylenediamine.

[0115] In some embodiments, the catalyst in step (2) includes at least one of dibutyltin dilaurate, glacial acetic acid, hydrochloric acid, and carbodiimide condensing agents.

[0116] In some embodiments, the catalyst in step (2) accounts for 0.1% to 0.5% of the mass of the LATP material.

[0117] In specific applications, the catalyst can account for 0.1%, 0.2%, 0.3%, 0.4%, 0.5% of the mass of the LATP material, etc.

[0118] In some embodiments, the stirring temperature in step (3) is 70-90°C, the stirring time is 4-8h, and the stirring rate is 200-300r / min; after the reaction is completed, the solid and liquid are separated by centrifugation, with the centrifuge set to a speed of 5000-8000r / min and a time of 10-15min.

[0119] In specific applications, the stirring temperature in step (3) can be 70℃, 80℃, or 90℃, the stirring time can be 4h, 5h, 6h, 7h, or 8h, and the stirring rate can be 200r / min, 220r / min, 240r / min, 260r / min, 280r / min, or 300r / min. This invention does not impose any limitations on these aspects.

[0120] In some embodiments, in step (4), the precipitate is washed 3 to 5 times with anhydrous ethanol and deionized water respectively to remove unreacted coupling agents and catalysts containing inert groups.

[0121] In some embodiments, the drying step in step (4) is performed under vacuum at a temperature of 60–80°C for 4–6 hours. This invention does not impose any limitations on this process.

[0122] In some embodiments, the mass ratio of the modified LATP powder, the first solvent, and the binder in step S3 is (20-40):(50-79):(1-10).

[0123] In practical applications, the mass ratio of modified LATP powder, the first solvent, and the binder can be 2:7:1, 10.3:20:0.8, 3:6:1, etc. The numerical ratios listed above are merely examples, and this invention does not impose any limitations on them.

[0124] In some embodiments, the first solvent in step S3 is selected from at least one of water, alcohol solvents, or polar aprotic solvents.

[0125] In some embodiments, the adhesive in step S3 is selected from at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, and nylon.

[0126] In some embodiments, before stirring in step S3, an ultrasonic step is performed with an ultrasonic power of 300-400W and an ultrasonic time of 30-45min; then stirring is performed for 1-2h to finally form the first slurry.

[0127] In some embodiments, the viscosity of the first slurry is 500 to 1500 mPa·s.

[0128] In some embodiments, the inert gas in step S4 includes one of nitrogen, argon, and helium.

[0129] In some embodiments, the second solvent in step S4 is selected from at least one of N,N-dimethylacetamide, N-methylpyrrolidone, and N,N-dimethylformamide.

[0130] In some embodiments, the solid-liquid ratio of the aramid raw material to the second solvent in step S4 is 1:(6-20). This invention does not impose any limitations on this.

[0131] In some embodiments, the stirring temperature in step S4 is 40–60°C, the stirring rate is 150–250 r / min, and the stirring time is 2–4 h.

[0132] In some embodiments, the additive in step S5 includes at least one of nano-SiO2, alumina, and boehmite, which improves the mechanical properties of the coating.

[0133] In some embodiments, the additive described in step S5 accounts for 5-10% of the mass of the aramid raw material.

[0134] In some embodiments, the additive described in step S5 may constitute 5%, 6%, 7%, 8%, 9%, 10% of the aramid raw material by mass, etc. The numerical ratios listed above are merely examples, and the present invention does not impose any limitations thereon.

[0135] In some embodiments, the power of ultrasonic dispersion in step S5 is 200-300W and the time is 20-30min.

[0136] In some embodiments, step S5, after ultrasonic dispersion, also includes a filtration step, in which the particles are filtered through a 100-200 mesh filter to remove agglomerated particles.

[0137] In some embodiments, this application employs a distribution coating process to ensure coating uniformity and adhesion to the base film.

[0138] In some embodiments, a microgravure coating machine or an extrusion spray coating machine is used in step S6, and this application does not impose any restrictions on this.

[0139] In some embodiments, the coating speed during step S6 is 1-3 m / min, the drying temperature after coating is 40-60°C, the drying time is 3-5 min, the air velocity in the hot air drying channel is 1-2 m / s, and some solvent is removed.

[0140] Thirdly, the present invention provides a battery comprising the composite separator provided in the first aspect.

[0141] The embodiments of the present invention will be described in more detail below through examples and comparative examples. All examples and comparative examples are lithium-ion battery sample groups prepared using the same process.

[0142] To demonstrate the advantages of the present invention in a clear and comprehensive manner, all embodiments and comparative examples have been recorded or subjected to the following tests: thermal shrinkage rate test, ionic conductivity test, cyclic stability test, and thermal runaway critical temperature test.

[0143] It should be noted that the embodiments of the present invention are not limited to these examples.

[0144] Example 1

[0145] I. Preparation of composite membranes:

[0146] Base membrane selection: A 12μm thick polyethylene (PE) porous membrane with a porosity of 42% was selected to ensure basic mechanical support and lithium-ion transport space.

[0147] Preparation of modified LATP coating (first coating): Lithium source (Li2CO3), aluminum source (Al2O3), titanium source (TiO2), phosphorus source ((NH4)3PO4), and inert element source (boric acid, accounting for 3% of the theoretical LATP production mass) were weighed according to stoichiometric ratio. The total mass and the agate balls were placed together in a planetary ball mill at a mass ratio of 1:10. Anhydrous ethanol was added as the dispersion medium, and the solid-liquid ratio was set to 1:1.8. The mixture was ball-milled at 400 r / min for 6 h to obtain a uniform slurry. The slurry was then dried in an oven at 90℃ for 8 h and then ground and sieved (200 mesh).

[0148] The discs were pressed into 12 mm diameter discs under 8 MPa pressure; then the discs were placed in a muffle furnace and calcined at a heating rate of 7 °C / min to 800 °C, held for 8 h, and after natural cooling to room temperature, the calcined product was crushed and ball-milled to a particle size of 1 μm to obtain inert element doped modified LATP powder.

[0149] The modified LATP powder, polyvinylpyrrolidone and N-methylpyrrolidone were mixed at a mass ratio of 10.3:0.8:20 and ultrasonically dispersed at 350W for 38 min, and then stirred for 2 h to form an LATP coating slurry (first slurry) with a viscosity of 1000 mPa·s.

[0150] The above slurry was coated onto the first surface of the base film using a spray coating process, and then dried through a 40°C drying channel to finally form a modified LATP coating (first coating) with a thickness of 3 μm.

[0151] Preparation of aramid coating (second coating): Poly(p-phenylene terephthalamide) (aramid) and N,N-dimethylformamide were mixed at a mass ratio of 1:10 and stirred for 3 hours at a temperature of 50℃ and a stirring speed of 200 r / min. Then, nano-SiO2 accounting for 8% of the mass of aramid was added, and the mixture was ultrasonically dispersed at a power of 250W for 25 minutes. The mixture was then filtered through a 200-mesh filter to obtain the aramid coating slurry (second slurry).

[0152] The slurry is coated onto the second surface of the base film using a spray coating process, and then dried through a 45°C drying channel to finally form an aramid coating (second coating) with a thickness of 4μm.

[0153] Finished composite diaphragm: The total thickness of the composite diaphragm is 19μm.

[0154] II. Preparation of the positive electrode:

[0155] The positive electrode active material NCM, conductive agent Super-P, and binder PVDF are weighed and mixed evenly in a mass ratio of 95:3:2. The mixture is then mixed with solvent NMP to obtain a positive electrode slurry, which is then evenly coated onto aluminum foil to form a positive electrode sheet.

[0156] III. Preparation of the negative electrode:

[0157] The negative electrode active material graphite, conductive agent Super-P, binder CMC and SBR are weighed and mixed evenly in a mass ratio of 95:3:1:1, and then mixed with deionized water as solvent to obtain a negative electrode slurry. This slurry is then evenly coated onto copper foil to form a negative electrode sheet.

[0158] IV. Battery fabrication:

[0159] The positive electrode sheet prepared in step two, the negative electrode sheet prepared in step three, and the above-mentioned composite separator are assembled and injected with electrolyte to form a battery.

[0160] Example 2

[0161] This application provides a composite separator and its preparation method, as well as a battery. The raw materials and preparation methods of Example 2 are the same as those of Example 1. The difference from Example 1 is that the modified LATP powder, polyvinylpyrrolidone and N-methylpyrrolidone are mixed in a mass ratio of 6:1:13 to obtain a first coating with a thickness of 2μm; and poly(p-phenylene terephthalamide) (aramid) and N,N-dimethylformamide are mixed in a mass ratio of 1:15 to obtain a second coating with a thickness of 3μm.

[0162] Example 3

[0163] This application provides a composite separator and its preparation method, as well as a battery. The raw materials and preparation methods of Example 3 are the same as those of Example 1. The difference from Example 1 is that the LATP powder is modified by grafting inert functional groups.

[0164] Take LATP powder and add it to anhydrous toluene solution (solid-liquid mass ratio 1:12). Disperse it by sonication for 45 min (power 250W) to form a suspension. Add γ-aminopropyltriethoxysilane KH550, a reagent containing inert functional groups, to the suspension at 5% of the mass of LATP, and add 0.3% dibutyltin dilaurate (catalyst) to form a mixture.

[0165] The mixture was refluxed and stirred at 80°C for 6 hours (stirring rate 250 r / min). After the reaction was completed, it was centrifuged (speed 6500 r / min, time 12 min) and the precipitate was collected.

[0166] The precipitate was washed four times each with anhydrous ethanol and deionized water to remove unreacted reagents and catalysts containing inert functional groups. Then, it was dried in a vacuum drying oven at 70°C for 5 hours to obtain inert functional group grafted modified LATP powder. The modified LATP powder, polyvinylpyrrolidone and N-methylpyrrolidone were mixed at a mass ratio of 10.5:0.8:20.

[0167] Example 4

[0168] This application provides a composite separator and its preparation method, as well as a battery. Example 4 uses the same raw materials and preparation method as Example 1. The difference from Example 1 is that the inert element is replaced by europium (introduced in the form of europium nitrate, accounting for 4% of the LATP mass). Modified LATP powder: polyvinylpyrrolidone: N-methylpyrrolidone = 10.4:0.8:20.

[0169] Example 5

[0170] This application provides a composite separator and its preparation method, as well as a battery. Example 5 uses the same raw materials and preparation method as Example 3. The difference from Example 3 is that the inert functional group is replaced by an amino group (introduced in the form of hexamethylenediamine, accounting for 6% of the LATP mass). Modified LATP powder: polyvinylpyrrolidone: N-methylpyrrolidone = 10.6:0.8:20.

[0171] Comparative Example 1

[0172] This application provides a composite separator and its preparation method, as well as a battery, which are the same as those in Example 1 in terms of raw materials and preparation method. The difference from Example 1 is that only the base film is used as the battery separator.

[0173] Comparative Example 2

[0174] This application provides a composite separator and its preparation method, as well as a battery. The raw materials and preparation method are the same as those in Example 1. The difference from Example 1 is that LATP (without introducing inert components) in the base film and the first coating is used as the composite separator.

[0175] Comparative Example 3

[0176] This application provides a composite separator and its preparation method, as well as a battery, which are the same as those in Example 1 in terms of raw materials and preparation method. The difference from Example 1 is that a base film and a second coating are used as the composite separator.

[0177] The batteries prepared in Examples 1-5 and Comparative Examples 1-3 were tested as follows.

[0178] (1) Heat shrinkage rate test

[0179] Sampling: Cut 10cm × 10cm samples from the composite diaphragm (or base membrane) of each embodiment and comparative example, and mark the longitudinal (machine direction, MD) and transverse (perpendicular to the machine direction, TD) of the samples.

[0180] Pretreatment: Place the sample in an oven and let it stand for 1 hour at 120℃, 150℃ and 180℃ respectively (simulating the high temperature operation or abnormal temperature rise environment of the battery).

[0181] Measurement: After removing the sample and cooling it to room temperature, use a vernier caliper (accuracy 0.01mm) to measure the longitudinal and transverse lengths of the sample. Measure 3 times in each direction and take the average value.

[0182] Calculation: Heat shrinkage rate = (initial length - length after treatment) / initial length × 100%, record the longitudinal and transverse heat shrinkage rates at each temperature.

[0183] (2) Ionic conductivity test

[0184] Sample preparation: The separators of each example and comparative example were cut into circular pieces with a diameter of 16 mm and immersed in 1 mol / L electrolyte (the same electrolyte used in the battery) in a glove box for 24 h to allow the separators to fully absorb the liquid.

[0185] Battery assembly: Place the absorbent membrane between two 16mm diameter stainless steel electrodes to assemble a CR2032 button cell (simulating an ion conduction environment).

[0186] Test conditions: An electrochemical workstation was used, and the AC impedance method was employed. The test frequency range was 10 Hz. 6 ~10-2Hz, with an applied AC voltage amplitude of 5mV; tested at -20℃, 0℃, 25℃, and 45℃ respectively (covering the low-temperature to normal-temperature operating range).

[0187] Calculation: Based on the impedance spectrum, the intersection of the high-frequency region and the real axis is taken as the bulk impedance (R) of the diaphragm. The ionic conductivity σ = L / (R×S), where L is the diaphragm thickness (cm) and S is the electrode area (cm2). Calculate and record the ionic conductivity at each temperature.

[0188] (3) Cyclic stability test

[0189] Battery selection: Three parallel samples were selected from each group of lithium-ion batteries (all with a capacity of 1Ah) prepared in each embodiment and comparative example.

[0190] Test conditions: Cyclic tests were conducted using a battery testing system at 25℃ (normal temperature) and -20℃ (low temperature). The charging regime was 0.5C constant current full charge, followed by constant voltage charging until the current dropped to 0.05C. The discharging regime was 0.5C constant current discharge. The cycle was 100 times, and the discharge capacity of each cycle was recorded.

[0191] Calculate: Capacity retention rate = (Discharge capacity of the nth cycle / Discharge capacity of the 1st cycle) × 100%. Record the capacity retention rate of the 100th cycle and take the average value of 3 parallel samples.

[0192] (4) Thermal runaway critical temperature test

[0193] Battery pretreatment: Charge the batteries of each embodiment and comparative example to full charge at 0.5C and let stand for 2 hours.

[0194] Test conditions: Using an accelerated calorimeter (ARC), a fully charged battery was placed in the test chamber and a stepped heating mode was adopted. The initial temperature was 25°C and the heating rate was 5°C / min. When the battery temperature rise rate exceeded 2°C / min, the mode was switched to adiabatic mode, and the battery temperature and pressure changes were continuously monitored.

[0195] Judgment: Record the initial temperature at which the battery temperature rises sharply (exceeding 100℃ / min), which is the critical temperature for thermal runaway; test 3 batteries in each group and take the average value.

[0196] The performance test results of each embodiment and comparative example are shown in Table 1.

[0197] Table 1 Comparison of Composite Membrane Performance Test Results

[0198]

[0199] Table 1 shows that the composite membrane described in this invention has significantly better overall performance than membranes with a single coating or those without modification. Specifically, the thermal runaway critical temperature (180°C) of Comparative Example 2 (containing only the unmodified LATP coating) is much lower than that of the other examples (above 210°C), highlighting the key role of the aramid coating in improving the overall thermal stability of the membrane and blocking heat transfer. Meanwhile, the -20°C ionic conductivity (1.5 × 10⁻³ S / cm) and -20°C capacity retention (62.5%) of Comparative Example 3 (containing only the aramid coating) are significantly lower than those of the other examples, demonstrating the decisive contribution of the modified LATP coating in optimizing the interface and improving low-temperature ion transport efficiency. Comparative Example 1 (pure base membrane) has the worst performance in all aspects, further illustrating the necessity of the coating. Examples 1-5, by combining the modified LATP coating with the aramid coating, do not simply add up the functions but produce a synergistic effect, achieving comprehensive and significant improvements in thermal safety, dendrite suppression ability, and wide-temperature performance.

[0200] This invention provides a modified LATP coating and an aramid coating on both sides of a polyolefin-based membrane. Through the synergistic effect of the high-temperature resistance of the aramid coating and the ion conduction optimization of the modified LATP coating, the thermal stability of the membrane is significantly improved, lithium dendrite puncture is prevented, the risk of thermal runaway of the battery cell is reduced, and its ion transport efficiency at low temperatures is improved.

[0201] It should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A composite diaphragm, characterized in that, Includes a base film and a first coating and a second coating located on both sides of the base film; The base membrane comprises a polyolefin material; The first coating comprises modified lithium aluminum titanium phosphate (LATP) particles, wherein the modification is performed by introducing inert elements for doping and / or by grafting inert functional groups onto the LATP particles during the LATP synthesis process. The second coating comprises an aramid material; The inert element is selected from at least one element selected from boron (B), europium (Eu), lanthanum (La), and yttrium (Y), and the inert functional group is selected from at least one functional group selected from siloxane and amino functional groups.

2. The composite diaphragm according to claim 1, characterized in that, The polyolefin material includes at least one of polypropylene (PP), polyethylene (PE), or PP / PE composite porous membrane; and the porosity of the base membrane is 35-50% and the thickness is 7-14 μm.

3. The composite diaphragm according to claim 1, characterized in that, The aramid material includes poly(m-phenylene isophthalamide) and / or poly(p-phenylene terephthalamide).

4. The composite diaphragm according to claim 1, characterized in that, The total thickness of the composite diaphragm is 9–30 μm, the thickness of the first coating is 1–8 μm, and the thickness of the second coating is 1–8 μm.

5. A method for preparing a composite diaphragm, characterized in that, Includes the following steps: S1. Provide a base film, using polyolefin as the base film; S2. Preparation of modified lithium titanium aluminum phosphate (LATP) material: Modified lithium titanium aluminum phosphate powder is obtained by introducing inert elements for doping and / or grafting inert functional groups onto the surface of LATP particles during the LATP synthesis process. S3. Mix the modified LATP powder, the first solvent, and the binder, and stir until homogeneous to form a first slurry; S4. Provide aramid raw material, add it to the second solvent under an inert atmosphere, and stir until completely dissolved to form a mixed solution; S5. Add additives to the mixed solution and disperse it ultrasonically to form a second slurry; S6. The first slurry and the second slurry are coated on the first surface and the second surface of the base film, respectively, and after drying, the first coating and the second coating are formed. S7. A composite diaphragm is prepared. The inert element is selected from at least one element selected from boron (B), europium (Eu), lanthanum (La), and yttrium (Y), and the inert functional group is selected from at least one functional group selected from siloxane and amino functional groups.

6. The method according to claim 5, characterized in that, Step S2, which describes the modification of LATP by inert element doping, includes the following steps: A. Weigh lithium source, aluminum source, titanium source and phosphorus source according to stoichiometric ratio, and add the compound of the inert elements; B. Ball milling mixing; C. After drying and pressing, calcination is carried out; D. The inert element-doped modified LATP powder is obtained by cooling and pulverizing. The inert element compound includes at least one of H3BO3, Eu2O3, La2O3, and Y2O3; based on the mass of the LATP material as 100%, the inert element compound accounts for 0.5% to 4% of the mass of the LATP material.

7. The method according to claim 5, characterized in that, Step S2, which describes the modification of LATP by grafting inert functional groups, includes the following steps: (1) Pretreatment: The LATP material is added to a third solvent and ultrasonically dispersed to form a suspension; (2) Add a reagent containing inert functional groups to the suspension and add a catalyst to form a mixture; (3) Stir the mixture and centrifuge to obtain the precipitate; (4) The inert functional group grafted modified LATP was obtained by washing and drying. Specifically, based on the mass of the LATP material as 100%, the reagent containing inert functional groups accounts for 1% to 6% of the mass of the LATP material.

8. The method according to claim 5, characterized in that, The additives mentioned in step S5 include at least one of nano-SiO2, alumina, and boehmite.

9. The method according to claim 7, characterized in that, The catalyst includes at least one of dibutyltin dilaurate, glacial acetic acid, hydrochloric acid, and carbodiimide condensing agents.

10. A battery comprising the composite separator as described in any one of claims 1 to 4.

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