Diaphragm, preparation method thereof and lithium ion battery

Abstract

The invention provides a diaphragm and a preparation method thereof and a lithium ion battery, the diaphragm comprises a porous support layer and a gradient functional layer, the porous support layer comprises a base membrane with the porosity of 45-60% and active groups modified on the surface of the base membrane, and the gradient functional layer comprises a heat conduction strengthening layer and an ion channel layer, the heat conduction strengthening layer comprises a first polymer and a modified boron nitride nanosheet, and the ion channel layer comprises a second polymer and a solid electrolyte nanowire. The porous supporting layer and the gradient functional layer are overlapped to form the composite diaphragm, so that high thermal stability, high wettability and high ion conductivity are synergistically realized, and formation of lithium dendrites can be inhibited.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion battery manufacturing, in particular to a separator and a preparation method thereof and a lithium ion battery. BACKGROUND

[0002] With the wide application of high energy density lithium ion batteries in the fields of new energy vehicles, energy storage power stations and the like, the thermal safety and electrochemical stability of the battery system are facing severe challenges. As a key component of lithium ion batteries, the performance of the separator directly affects the electrolyte wettability, lithium ion transport efficiency and structural integrity under high temperature conditions.

[0003] The currently widely used commercial polyolefin separator (PP film / PE film) has non-polar characteristics of polyolefin molecular chains, which can cause insufficient wettability of the separator in the electrolyte, and the separator is prone to melt shrinkage at a temperature above 160℃, which poses a serious risk of thermal runaway when applied to lithium ion batteries with high energy density input and output.

[0004] In recent years, the academic community has proposed to improve the performance of the separator by coating inorganic ceramic particles (such as Al2O3, SiO2) or composite polymer layers (polyvinylidene fluoride-hexafluoropropylene copolymer PVDF-HFP, polyethylene oxide PEO) on the surface of the polyolefin-based film. However, such modification schemes still have the following bottlenecks: (1) insufficient interface stability: the interface between the inorganic coating and the polyolefin-based film is mainly physical adsorption, and the inorganic ceramic particles are prone to falling off during the cycle process, which can cause pore blockage and lead to an increase in interface impedance to more than 50Ω·cm 2 , loss of battery capacity, and system out-of-control; (2) single-function defect: existing composite separators are mostly focused on single performance optimization (such as thermal stability or wettability), and it is difficult to simultaneously solve the multidimensional requirements of dendrite inhibition, ion conduction and high temperature deformation control; (3) structural design limitation: symmetric coating or simple lamination process leads to a long ion transport path, and lacks gradient function-oriented interface engineering, which restricts the high rate charge and discharge performance, and the 10C capacity retention rate is less than 85%. SUMMARY

[0005] To solve the above problems, a composite separator is formed by stacking a gradient functional layer, which can realize high thermal stability, high wettability, high ion conductivity, and inhibit dendrite formation. The first aspect of the present application provides a separator, comprising a porous support layer and a gradient functional layer stacked on the outer side of the porous support layer, the porous support layer comprising a base film with a porosity of 45-60% and active groups modified on the surface of the base film, the active groups comprising at least one of hydroxyl, aldehyde, carboxyl, ester, carbonyl, vinyl, amino, epoxy and mercapto groups, and the gradient functional layer comprising a thermal conductivity strengthening layer and an ion channel layer, the thermal conductivity strengthening layer comprising a first polymer and modified boron nitride nanosheets surface-modified by a silane coupling agent, and the ion channel layer comprising a second polymer and solid electrolyte nanowires.

[0006] In some optional embodiments, the thickness of the porous support layer is 6-10 μm, the pore size distribution is in the range of 50-200 nm, and the glass transition temperature is ≥160°C. The base film is selected from one or more of polyvinylidene fluoride film, polyethylene film, polypropylene film, polyethylene terephthalate film, polyphenylene benzobisoxazole film, polybenzimidazole film, polyacrylonitrile film, polyimide film, polycarbonate film, polyurethane film and polyetherimide film.

[0007] In some optional embodiments, the active groups are one or more of carboxyl, aldehyde, carbonyl and epoxy groups. The thickness of the thermal conductivity strengthening layer is 2-5 μm. The first polymer comprises one or more of polyimide and its derivatives, polysulfone and its derivatives, and polyarylether and its derivatives. The silane coupling agent is an amino silane coupling agent. The modified boron nitride nanosheets have a lateral size of 200-500 nm and a surface amino content of ≥0.8 mmol / g. The mass ratio of the first polymer to the modified boron nitride nanosheets is (5.7-11.5):1.

[0008] In some optional embodiments, the thickness of the ion channel layer is 3-6 μm. The mass ratio of the second polymer to the solid electrolyte nanowires is (4-9):1. The solid electrolyte nanowires have a diameter of 20-50 nm and an aspect ratio of 30-100. The second polymer comprises polybenzimidazole and its derivatives and / or polyether ether ketone and its derivatives. The solid electrolyte nanowires comprise one or more of lithium lanthanum zirconium oxide nanowires, titanium aluminum lithium phosphate nanowires and lanthanum lithium titanium oxide nanowires.

[0009] In some optional embodiments, the second polymer is sulfonated polybenzimidazole with a sulfonation degree of 20-40%. The solid electrolyte nanowires are lithium lanthanum zirconium oxide nanowires.

[0010] In some optional embodiments, the heat-conducting reinforcing layer and the ion channel layer are arranged on two sides of the porous support layer, the separator is arranged on the positive electrode sheet with the side of the heat-conducting reinforcing layer as the contact side, and is arranged on the negative electrode sheet with the side of the ion channel layer as the contact side.

[0011] Alternatively, the heat-conducting reinforcing layer and the ion channel layer are arranged on the same side of the porous support layer, the ion channel layer is arranged in contact with the porous support layer, and the heat-conducting reinforcing layer is arranged outside the ion channel layer, and the separator is arranged on the positive electrode sheet with the side of the heat-conducting reinforcing layer as the contact side.

[0012] Alternatively, the heat-conducting reinforcing layer and the ion channel layer are arranged on the same side of the porous support layer, the ion channel layer is arranged in contact with the porous support layer, and the heat-conducting reinforcing layer is arranged outside the ion channel layer, and the separator is arranged on the positive electrode sheet with the side of the heat-conducting reinforcing layer as the contact side.

[0013] In some optional embodiments, the breaking temperature of the separator is ≥220°C, and the ion conductivity of the separator at 25°C is >0.65 mS / cm.

[0014] In some optional embodiments, the dendrite cycle life of the separator at a current density of 2 mA / cm 2 is greater than or equal to 1200 hours.

[0015] The second aspect of the present application provides a preparation method of a separator, the separator being the separator according to any one of the above, and the preparation of the separator comprising the following steps:

[0016] S1: dissolving the first polymer and the modified boron nitride nanosheet in N-methylpyrrolidone, and electrospinning to form the heat-conducting reinforcing layer under a voltage of 18 kV;

[0017] S2: mixing the second polymer and the solid-state electrolyte nanowire, and preparing the ion channel layer by using a flow casting process;

[0018] S3: using a radio frequency plasma device, setting the power of the radio frequency plasma device to 80 W, and the single operation time to 60 s, taking a base film with a porosity of 45-60%, and feeding the base film into the chamber of the radio frequency plasma device, introducing a compound reagent containing the active group, setting the flow rate of the compound reagent to 0.1 mL / min, and vaporizing the compound reagent to form a vapor, and performing plasma treatment on the base film in the vapor atmosphere to graft the compound, thereby preparing the porous support layer; and

[0019] S4: stacking the heat-conducting reinforcing layer, the porous support layer and the ion channel layer, and then performing roll pressing treatment, the heat pressing temperature being 100-120℃, the roll pressing pressure being 0.8MPa, and then vacuum drying at 60℃ for 24h to obtain the separator.

[0020] In some optional embodiments, the base film is a polyvinylidene fluoride film, and the preparation of the polyvinylidene fluoride film further comprises the following steps:

[0021] S31: dissolving polyvinylidene fluoride in N-methylpyrrolidone, and controlling the concentration of polyvinylidene fluoride to be 20wt%;

[0022] S33: coating the polyvinylidene fluoride solution on a steel belt or a roller through a flow die, and controlling the thickness to be 100-200μm to obtain a wet film;

[0023] S34: exposing the wet film to air with a humidity of 50-60% for 60-80s;

[0024] S35: transferring the wet film to a non-solvent bath with a mass ratio of ethanol: water being 1:3, soaking for 60-80s, and the temperature being 25℃;

[0025] S36: taking out the wet film, washing with water to remove residual reagents, and heat setting at 160℃ for 15min to obtain the base film.

[0026] The third aspect of the present application provides a lithium ion battery comprising a positive electrode sheet, a separator and a negative electrode sheet, the separator being the separator according to any one of the above, and the 10C rate discharge capacity retention rate of the lithium ion battery being ≥85%.

[0027] The present application has at least the following technical effects:

[0028] 1) The first aspect of the present application provides a separator, comprising a porous support layer, and a gradient functional layer superimposed on the outside of the porous support layer, the gradient functional layer comprising a thermal conductivity enhancement layer and an ion channel layer, wherein the modified boron nitride in the thermal conductivity enhancement layer is used to build a thermal conductivity network, which can quickly disperse local hot spots, the first polymer provides high temperature support, delays melting deformation, and synergistically enhances the high temperature support performance of the separator; the solid-state electrolyte nanowires in the ion channel layer are dispersed in the second polymer to build a conductive network, optimize the lithium ion transmission path, improve the electrochemical system kinetics, reduce local lithium ion deposition, and inhibit lithium dendrite formation. In addition, the thermal conductivity network and the conductive network can also achieve a synergistic effect of 1+1>2, the rapid heat dissipation of the thermal conductivity network can improve the membrane breaking temperature of the separator, and the improvement of the membrane breaking temperature can cause the improvement of the ion conductivity in the system, further synergizing the conductive network and inhibiting the formation of lithium dendrites. In addition, the surface of the porous support layer is also modified with active groups, which form chemical bond interactions between the first polymer in the thermal conductivity enhancement layer and / or the second polymer of the ion channel layer, compared with physical adsorption, the interface bonding force can be greatly improved, and the interface stability of the separator is improved.

[0029] 2) The second aspect of the present application provides a preparation method of a separator, electrospinning to prepare a thermal conductivity enhancement layer, flow casting to prepare an ion channel layer, plasma radio frequency to graft active groups on the surface of the base film to obtain a porous support layer, and then using a hot press roller to obtain the separator of any one of the above aspects. The combination and stacking of the above three-layer structure realizes gradient function guidance through interface engineering, breaks through the limitations of traditional polyolefin separators, and develops a separator with high thermal conductivity, high stability, high ion transmission performance, and high cycle performance, which helps the development of high safety and long cycle life of lithium ion batteries.

[0030] 3) The third aspect of the present application provides a lithium ion battery using the above-mentioned separator, and the rate performance and cycle performance of the lithium ion battery are significantly improved. DETAILED DESCRIPTION

[0031] The embodiments of the present application are described in detail below, and in the description of the present application, the meaning of several is one or more, the meaning of multiple is two or more, greater than, less than, more than, etc. are understood as not including the number, above, below, etc. are understood as including the number. If it is described as first, second, it is only used for the purpose of distinguishing technical features, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features or the sequence of indicated technical features.

[0032] In the description of the present embodiment, unless otherwise explicitly defined, the words such as arrangement, installation, connection, etc. should be understood in a broad sense, and those skilled in the art can determine the specific meaning of the above words in the present embodiment in combination with the specific content of the technical solution.

[0033] The common structure of the existing lithium ion battery includes an open-ended shell, a winding core assembled in the shell from the opening, an electrolyte injected into the shell, and a cap set above the opening of the shell. It can be understood that the lithium ion battery can be a cylindrical lithium ion battery or a square lithium ion battery.

[0034] The winding core is formed by winding the positive electrode sheet, the separator and the negative electrode sheet arranged in layers, the positive electrode sheet includes the positive electrode current collector and the positive electrode active material layer coated on both sides of the positive electrode current collector, the negative electrode sheet includes the negative electrode current collector and the negative electrode active material layer coated on both sides of the negative electrode current collector, and lithium ions complete the conversion between chemical energy and electrical energy by intercalation or deintercalation reaction on the surface of the positive electrode active material layer and the negative electrode active material layer.

[0035] The first aspect of the present application provides a separator, which comprises a porous support layer and a gradient functional layer superimposed on the outer side of the porous support layer, the porous support layer comprises a base film with a porosity of 45-60% and active groups modified on the surface of the base film, and the active groups include at least one of hydroxyl, aldehyde, carboxyl, ester, carbonyl, vinyl, amino, epoxy and mercapto groups;

[0036] The gradient functional layer comprises a thermal conductivity strengthening layer and an ion channel layer, the thermal conductivity strengthening layer comprises a first polymer and modified boron nitride nanosheet surface modified by a silane coupling agent, and the ion channel layer comprises a second polymer and solid electrolyte nanowire.

[0037] The porous support layer is formed by grafting active groups on the surface of the base film, and the gradient functional layer is superimposed on the surface of the porous support layer, and the interface is combined with each other by chemical bond, which has significantly higher stability than the physical adsorption of traditional composite membranes, and experiments have verified that the separator can realize a dendritic crystal cycle life of more than 1200 hours and a 10C discharge capacity retention rate of up to 95% when applied to a lithium ion battery.

[0038] Further, the gradient functional layer comprises a thermal conduction reinforced layer and an ion channel layer, wherein the modified boron nitride in the thermal conduction reinforced layer is used to build a thermal conduction network, which can quickly disperse local hot spots, the first polymer provides high-temperature support and delays melting deformation, thereby synergistically enhancing the high-temperature support performance of the separator; the second polymer in the ion channel layer has affinity with the electrolyte, which improves the liquid absorption rate of the separator, optimizes the electrolyte wettability, and the solid-state electrolyte nanowires are dispersed in the second polymer to build a conductive network, which optimizes the lithium ion transmission path, improves the electrochemical system kinetics, reduces the local deposition of lithium ions, and inhibits the formation of lithium dendrites.

[0039] In addition, the thermal conduction network synergistically with the conductive network can also achieve a synergistic effect of 1+1>2, the rapid heat dissipation of the thermal conduction network can improve the film breaking temperature of the separator, and the improvement of the film breaking temperature can cause the improvement of the ion conductivity in the system, which further synergistically with the conductive network, reduces the risk of local deposition of lithium ions, and inhibits the formation of lithium dendrites. Correspondingly, the efficient transmission of lithium ions by the conductive network can reduce the concentration polarization, thereby inhibiting local overheating.

[0040] Further, the first polymer comprises one or more of polyimide and its derivatives, polysulfone and its derivatives, and polyarylether and its derivatives.

[0041] It should be noted that the polyimide refers to a heterocyclic polymer with an imide ring (-CO-NR-CO-) in the main chain, and the repeating unit thereof is usually -[N(CO)2Ar1-(CO)2N-Ar2] n - wherein each nitrogen atom is connected to two carbonyl groups, the Ar1 group is connected to four carbonyl groups, and the Ar1 group and the Ar2 group can contain aromatic groups such as benzene rings, biphenyl, biphenyl ether, biphenyl sulfide, naphthalene rings, etc. The derivative of polyimide can be polyetherimide (polyimide main chain with ether group). Polysulfone and its derivatives refer to a polymer with aromatic rings and sulfone groups (-SO2-) alternately connected in the main chain, and the derivatives of polysulfone usually include three types: bisphenol A type polysulfone (PSU), polyphenyl sulfone (PPSU), and polyether sulfone (PESU). Polyarylether refers to a polymer with ether bonds (-O-) connecting aromatic rings in the main chain, and the aromatic rings include benzene rings, biphenyl, naphthalene rings, etc. The derivative of polyarylether usually introduces functional groups such as sulfone groups (-SO2-), ketone groups (-CO-), nitrile groups (-CN) in the main chain, for example: polyarylethersulfone, polyaryletherketone, etc.

[0042] In some optional embodiments, the second polymer comprises polybenzimidazole and its derivatives and / or polyether ether ketone and its derivatives.

[0043] It can be understood that the polybenzimidazole refers to a polymer with a benzimidazole ring in the main chain, and the benzimidazole ring unit is composed of a benzene ring and an imidazole five-membered heterocyclic ring fused and linked by aromatic and / or aliphatic. The derivative of polybenzimidazole usually introduces sulfonic group (—SO3H), ether bond (-O-), sulfone group (-SO2-), etc. Specifically, sulfonated polybenzimidazole (SPBI) can be used. Polyether ether ketone (PEEK) usually refers to a polymer with ether bond (-O-) and ketone group (-CO-) alternatingly connected with aromatic ring in the main chain. The derivative of polyether ether ketone usually includes ether ketone polymers with different ether / ketone ratios, or introduces active groups such as sulfonic group (—SO3H), sulfone group (-SO2-) on the benzene ring. Specifically, sulfonated polyether ether ketone (SPEEK) can be used.

[0044] In some optional embodiments, the solid-state electrolyte nanowire includes one or more of lithium lanthanum zirconium oxide nanowire, lithium aluminum titanium phosphate nanowire and lanthanum lithium titanium oxide nanowire. A three-dimensional lithium ion channel is formed, and the lithium ion migration efficiency is improved.

[0045] In the specific implementation process, the separator is applied to a lithium ion battery, the film breaking temperature of the lithium ion battery can be increased to 235℃, the conductivity can be increased to 0.82mS / cm, the 10C discharge capacity retention rate is as high as 95%, the 2mA / cm 2 No lithium dendrite penetration short circuit failure occurs after constant current density cycling for more than 1600 hours.

[0046] In some optional embodiments, the thickness of the porous support layer is 6-10μm, the pore size distribution range is 50-200nm, and the glass transition temperature is ≥160℃. The base film is selected from one or more of polyvinylidene fluoride film PVDF, polyethylene film PE, polypropylene film PP, polyethylene terephthalate film PET, polybenzobisoxazole film PBO, polybenzimidazole film PBI, polyacrylonitrile film PAN, polyimide film PI, polycarbonate film PC, polyurethane film PU and polyetherimide film PEI.

[0047] Specifically, when multiple film materials are used in the base film, such as PVDF, PE, PP, PET, PBO, PBI, PAN, PI, PU and PEI, the base film is formed by hot pressing. In the specific implementation process, the base film is a polyvinylidene fluoride film, and the active group is a carboxyl group. Specifically, the plasma radio frequency method is used to graft acrylic monomer (AAc) on both sides of the polyvinylidene fluoride film to form a chemical bonding interface PVDF-g-AAc.

[0048] The thickness of the porous support layer is 6-10 pm, the pore size distribution ranges from 50-200 nm, and the glass transition temperature is ≥160 °C. The thickness of 6-10 pm balances the mechanical strength and ion transmission efficiency of the separator. The smaller the thickness, the more conducive to reducing the ion diffusion path, improving the battery rate performance and energy density. The larger the thickness, the stronger the mechanical toughness of the separator, reducing the risk of short circuit or puncture between electrodes, ensuring the structural integrity during the battery charging and discharging process. In the pore size range of 50-200 nm, it can not only allow the free transmission of lithium ions (pore size > 50 nm reduces ion transmission resistance), but also effectively block the growth of lithium dendrites (pore size < 200 nm inhibits dendrite penetration), thereby significantly prolonging the battery cycle life. The high glass transition temperature of ≥160 °C ensures the dimensional stability and thermal safety of the separator under extreme temperature conditions (such as high temperature environment or overcharge scenario). This parameter prevents the separator from softening or shrinking above 160 °C, avoiding short circuit caused by electrode contact; at the same time, it is also beneficial to improve the membrane rupture temperature of the battery in subsequent applications. In specific experiments, the membrane rupture temperature of the battery using this separator is ≥220 °C, which improves the overall heat resistance of the battery by maintaining the integrity of the porous structure.

[0049] In some optional embodiments, the active group is one or more of carboxyl, aldehyde, carbonyl, and epoxy, the thickness of the thermal conductive reinforcement layer is 2-5 pm, the silane coupling agent is an amino silane coupling agent, the lateral size of the modified boron nitride nanosheet is 200-500 nm and the surface amino content is ≥0.8 mmol / g, and the mass ratio of the first polymer to the modified boron nitride nanosheet is (5.7-11.5):1. The thickness range of the thermal conductive reinforcement layer is used to balance the interface bonding tightness and thermal diffusion efficiency. The thinner the thickness, such as 2 pm, the total thickness and ion transmission resistance of the separator can be reduced, the lithium ion conductivity and the battery rate performance can be improved, and the interface bonding between the thermal conductive reinforcement layer and the porous support layer is also tighter, avoiding the risk of delamination. The thicker the thickness, such as 5 pm, the thermal diffusion capacity and mechanical stability of the thermal conductive reinforcement layer can be enhanced, effectively dispersing the local heat accumulation during the battery charging and discharging process, preventing thermal runaway, and in specific experiments, the membrane rupture temperature of the separator is ≥220 °C, the high temperature safety is guaranteed, and the dendrite cycle life is ≥1200 hours, prolonging the battery life by maintaining the interlayer integrity under thermal abuse conditions. The boron nitride nanosheet has a high specific surface area at a smaller size, such as 200 nm wide, which can enhance the thermal path density and thermal conductivity. At a larger size, such as 500 nm wide, the interface defects and nanosheet agglomeration can be reduced, ensuring uniform distribution in the thermal conductive reinforcement layer and improving overall thermal management efficiency. Further modification by the silane coupling agent grafts amino groups on the surface of the boron nitride nanosheet, and the surface amino content is ≥0.8 mmol / g, introducing the synergistic effect of interface engineering to strengthen the interlayer chemical bonding and support the high membrane rupture temperature of the separator above 220 °C.

[0050] Further, the silane coupling agent is KH550, the content of surface amino groups of the modified boron nitride nanosheet is controlled to be ≥0.8 mmol / g, the reactivity of active groups carboxyl, aldehyde and epoxy of the chemical bonding interface between the modified boron nitride nanosheet and the porous support layer is enhanced, the firmness of the interface bonding is improved, specifically, the carboxyl (such as -COOH provided by acrylic acid) can react with the amino (-NH2) to form a covalent bond (such as a peptide bond) to enhance the interlayer adhesion. It can be understood that the groups on the surface of the modified boron nitride nanosheet and the active groups modified on the surface of the porous support layer have correspondence, for example, carboxyl-amino; hydroxyl-carboxyl / isocyanate group / epoxy group, the hydroxyl group can react with the carboxyl group to form an esterification reaction, or form a carbamate bond with the isocyanate, or open ring reaction with the epoxy group to enhance the interface bonding force; aldehyde-amino, the aldehyde group reacts with the amino group to generate a Schiff base bond (imine bond), which is suitable for a high-stability interface; carbonyl-amino / hydrazine, the carbonyl group can form a Schiff base with the amino group, or react with the hydrazine group to improve the coupling efficiency; vinyl-mercapto / maleimide group, a stable covalent bond is formed through thiol-ene click chemistry reaction, which meets the high interface strength requirement of the separator of the application; epoxy-amino / carboxyl, the epoxy group is easy to react with the amino group or the carboxyl group under appropriate catalysis to form a covalent network, which meets the interface engineering design logic; amino-carboxyl / epoxy, the amino group can directly react with the carboxyl group to form an amide bond, or open ring polymerization with the epoxy group to ensure firm bonding; thiol-maleimide group / ethenyl, thiol participates in thiol-Michael addition or click chemistry reaction to provide fast and high-selectivity coupling.

[0051] The mass ratio of the first polymer to the modified BNNS nanosheet is (5.7-11.5):1, within this range, as the proportion of BNNS nanosheet gradually increases, the membrane breaking temperature, ion conductivity, 10C discharge capacity retention rate and dendrite cycle life of the separator also gradually increase, and when the mass ratio of the first polymer to the BNNS nanosheet is 5.7:1, the battery performance is optimal, and on this basis, if the mass ratio of the BNNS nanosheet is further increased, the performance of the separator will decrease. This is because an appropriate amount of BNNS nanosheet constructs a heat conduction network in the first polymer, quickly disperses local hot spots, and the first polymer provides high-temperature support to delay melting deformation; while an excessive amount of BNNS nanosheet will cause the pores of the heat conduction strengthening layer to be blocked, and the heat conduction capacity will decrease.

[0052] It should be noted that the size of the boron nitride nanosheet is significantly affected by the preparation method, raw materials and process parameters, and in the industry, the size is usually described only by two dimensions, i.e., the lateral size and the thickness, wherein the lateral size usually refers to the maximum width data or diameter data of the boron nitride nanosheet.

[0053] In some optional embodiments, the thickness of the ion channel layer is 3-6 μm, the mass ratio of the second polymer to the solid-state electrolyte nanowire is (4-9): 1, the solid-state electrolyte nanowire has a diameter of 20-50 nm and an aspect ratio of 30-100. The thickness of the ion channel layer is used to balance the ion transmission efficiency and the safety barrier function of the separator, avoiding delamination between layers or short circuit. The thinner the thickness, for example 3 μm, reduces the lithium ion diffusion path length, improves ion migration rate and overall conductivity, thereby achieving the index of ion conductivity > 0.65 mS / cm in the specific implementation process; the thicker the thickness, for example: 6 μm, then the mechanical strength and toughness of the separator are enhanced, effectively inhibiting the risk of lithium dendrite penetration, prolonging the cycle life, thereby achieving the long cycle performance of dendrite cycle life > 1200 hours in the specific implementation process. The diameter of the solid-state electrolyte nanowire is 20-50 nm, which has a high specific surface area, is beneficial to the rapid adsorption and diffusion of lithium ions, shortens the ion transmission path, thereby improving the ion conductivity (> 0.65 mS / cm) and reducing the interface impedance. The solid-state electrolyte nanowire with a high aspect ratio (30-100) ensures that the nanowire forms an interlaced network, creating a continuous ion channel and enhancing the uniformity of ion migration; at the same time, when the aspect ratio is 100, the nanowire can be prevented from bending or breaking, maintaining the structural integrity, which indirectly supports the long cycle life of more than 1200 hours in the subsequent experiment.

[0054] Further, the mass ratio of the second polymer to the solid-state electrolyte nanowire is (4-9): 1, within this range, gradually increasing the proportion of solid-state electrolyte nanowires, the membrane breaking temperature, ion conductivity, 10C charge capacity retention rate and dendrite cycle life of the separator are also gradually improved, and when the mass ratio of the second polymer to the solid-state electrolyte nanowire is 4:1, the battery performance is optimal, and on this basis, further increasing the mass ratio of the solid-state electrolyte nanowire sheet will cause the performance of the separator to decrease. This is due to the fact that an excessive amount of solid-state electrolyte nanowires will cause the nanowires to intertwine and not be uniformly dispersed, and the agglomerates will block the through holes between the nanowire fibers, reducing the pore connectivity and blocking the ion transmission path.

[0055] In some optional embodiments, the second polymer is sulfonated polybenzimidazole with a sulfonation degree of 20-40%, and the solid-state electrolyte nanowire is lithium lanthanum zirconium oxide nanowire. The sulfonic acid groups of sulfonated polybenzimidazole sPBI and lithium lanthanum zirconium oxide LLZO nanowires work together, and the —SO3 -The electrostatic repulsion and the size screening effect of the LLZO nanowires form an anion migration double-blocking effect, the anion transference number is greatly reduced, the lithium ion single conductor property of the LLZO can increase the number of lithium ion migration, reduce the concentration polarization, and inhibit local overheating; the LLZO can also form a composite heat conduction network with the sPBI to realize rapid dispersion of heat and further inhibit the generation of local hot spots. In addition, the sulfonated polybenzimidazole sPBI also has natural affinity with the electrolyte, which enhances the wettability of the electrolyte to the core.

[0056] In some optional embodiments, the heat-conducting reinforcing layer and the ion channel layer are arranged on both sides of the porous support layer, the separator is arranged to adhere to the positive electrode sheet on the side where the heat-conducting reinforcing layer is located, and is arranged to adhere to the negative electrode sheet on the side where the ion channel layer is located. The positive electrode sheet generates more heat than the negative electrode sheet, and the heat-conducting reinforcing layer is arranged to adhere to the positive electrode sheet to achieve higher heat dissipation efficiency; the ion channel layer is arranged to adhere to the negative electrode sheet to reduce the concentration polarization, avoid local lithium deposition, and inhibit the formation of lithium dendrites.

[0057] In some optional embodiments, the heat-conducting reinforcing layer and the ion channel layer are arranged on the same side of the porous support layer, the ion channel layer is arranged to adhere to the porous support layer, and the heat-conducting reinforcing layer is arranged outside the ion channel layer. The ion channel layer adheres to the porous support layer to facilitate the connection of lithium ion channels and accelerate the lithium ion transmission rate.

[0058] In some optional embodiments, the heat-conducting reinforcing layer and the ion channel layer are arranged on both sides of the porous support layer, the ion channel layer is arranged to adhere to the porous support layer, and the heat-conducting reinforcing layer is arranged outside the ion channel layer. Both sides are enhanced in terms of electrical conductivity and thermal conductivity, and the performance of the separator is optimized to the greatest extent.

[0059] In some optional embodiments, the breaking temperature of the separator is ≥220℃, and the ionic conductivity of the separator at 25℃ is >0.65mS / cm. The development trend of high-rate input and output of lithium ion batteries is adapted.

[0060] In some optional embodiments, the dendrite cycle life of the separator at a current density of 2mA / cm 2 is greater than or equal to 1200 hours. The development trend of high safety of lithium ion batteries is adapted.

[0061] The second aspect of the present application provides a preparation method of a separator, the separator being the separator according to any one of the above, and the preparation of the separator comprising the following steps:

[0062] S1: dissolving the first polymer and the modified boron nitride nanosheet in N-methyl pyrrolidone, and electrospinning to form the heat-conducting reinforcing layer under a voltage of 18kV;

[0063] S2: mixing the second polymer with the solid-state electrolyte nanowire, and preparing the ion channel layer by using a flow casting film forming process;

[0064] S3: using a radio frequency plasma device, setting the power of the radio frequency plasma device to 80 W, and setting the single operation time to 60 s, taking a base film with a porosity of 45-60%, and sending the base film into the chamber of the radio frequency plasma device, introducing a compound reagent containing the active group into the chamber, setting the flow rate of the compound reagent to 0.1 mL / min, and vaporizing the compound reagent to form a vapor, so that the base film is subjected to plasma treatment in the vapor atmosphere to graft the compound, thereby obtaining the porous support layer; and

[0065] S4: stacking the heat conduction strengthening layer, the porous support layer, and the ion channel layer, and performing roll pressing treatment, the hot pressing temperature is 100-120°C, the roll pressing pressure is 0.8 MPa, and then vacuum drying at 60°C for 24 h, thereby obtaining the separator.

[0066] The heat conduction strengthening layer is prepared by electrospinning, the ion channel layer is prepared by flow casting film forming process, the porous support layer is obtained by grafting the active group on the surface of the base film by plasma radio frequency, the vapor atmosphere ensures uniform distribution and activation of the monomer of the compound reagent in the plasma environment, and the separator is obtained by hot pressing and roll pressing. The combination and stacking of the above three layers of structure realize gradient function orientation combined with interface engineering, break through the limitation of traditional polyolefin separators, and develop a separator with high thermal conductivity, high stability, high ion transmission performance, and high cycle performance, thereby helping the development of high safety and long cycle life of lithium ion batteries.

[0067] Further, the base film is a polyvinylidene fluoride film, and the preparation of the polyvinylidene fluoride film further includes the following steps:

[0068] S31: dissolving polyvinylidene fluoride in N-methylpyrrolidone, and controlling the concentration of polyvinylidene fluoride to be 20 wt%; S32: adding a porosity adjusting agent with a mass of 5-8% of the mass of polyvinylidene fluoride;

[0069] S33: coating the polyvinylidene fluoride solution on a steel belt or a roller through a flow casting die, and controlling the thickness to be 100-200 μm, thereby obtaining a wet film;

[0070] S34: exposing the wet film to air with a humidity of 50-60% for 60-80 s;

[0071] S35: transferring the wet film to a non-solvent bath with an ethanol: water mass ratio of 1:3, soaking for 60-80 s, and the temperature is 25°C;

[0072] S36: The wet film is taken out, washed with water to remove residual reagents, and heat set at 160℃ for 15 min to obtain the base film.

[0073] The base film is prepared by a non-solvent induced phase separation method, which is simple and mature in process and has better control over porosity and pore size. In the specific implementation process, the porosity regulator can be polyethylene glycol.

[0074] The third aspect of the application provides a lithium ion battery, which comprises a positive electrode sheet, a separator and a negative electrode sheet, the separator is the separator according to any one of the above, and the 10C rate discharge capacity retention rate of the lithium ion battery is ≥85%. The rate performance and cycle performance of the lithium ion battery are significantly improved.

[0075] Further, the positive electrode sheet comprises a positive electrode active material, and the positive electrode active material comprises lithium nickel cobalt manganese oxide and lithium iron phosphate, wherein the chemical formula of the lithium nickel cobalt manganese oxide is Li1Ni x Co y Mn z M b O2, 0.70≤x≤0.95, 0.15≤y<0.45, 0.05≤z<0.45, 0.0≤b≤0.25, x+y+z+b=1, the element M comprises one or more of zirconium, tungsten, titanium, aluminum, strontium, boron and neodymium; the negative electrode sheet comprises a negative electrode active material, and the negative electrode active material comprises one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon oxide, pre-lithium silicon oxide, silicon and deposited silicon carbon; and the capacity N / P ratio of the negative electrode sheet to the positive electrode sheet is 1.04-1.09.

[0076] The technical solutions of the application are described below in combination with Examples 1-8 and Comparative Examples 1-3, wherein the separators in Examples 1-8 and Comparative Examples 1-3 need to be prepared into 21700 cylindrical lithium ion batteries and symmetric batteries capable of performing corresponding electrical tests. The size parameters of the cylindrical lithium ion batteries are as follows: diameter: 21.0 mm, length: 70.0 mm.

[0077] The cylindrical lithium ion battery is prepared, taking Example 1 as an example:

[0078] 1. Positive electrode sheet manufacturing method:

[0079] The positive electrode active material high-nickel ternary material LiNi 0.9 Co 0.05 Mn 0.05O2, conductive agent conductive carbon black, binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 96:1:3, then N-methyl pyrrolidone (NMP) is added, stirred and mixed uniformly, a stable positive electrode slurry is formed, the solid content of the positive electrode slurry is 70%; the positive electrode slurry is uniformly coated on a 12μm thick aluminum foil (positive electrode current collector), then after drying and cold pressing, a positive electrode sheet is obtained, the compaction density is 3.5g / cm 3 .

[0080] 2. The method for manufacturing a negative electrode sheet:

[0081] The nitrogen-doped carbon-coated graphite, silicon-carbon material, conductive agent carbon nanotube, thickening agent carboxymethyl cellulose sodium (CMC) and binder polyacrylic acid (PAA) are mixed in a mass ratio of 86:10:1.5:1:1.5, then deionized water is added and stirred to form a stable negative electrode slurry, the solid content of the negative electrode slurry is 40%; the negative electrode slurry is uniformly coated on a 8μm thick copper foil (negative electrode current collector), then after drying and cold pressing, a negative electrode sheet is obtained, the compaction density is 1.6g / cm 3 .

[0082] 3. Preparation of electrolyte:

[0083] Lithium salt lithium hexafluorophosphate (LiPF6), organic solvent ethylene carbonate (EC), dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), vinyl sulfate (DTD) and vinylene carbonate (VC) are mixed in a mass ratio of 10.0:20.0:55.0:2.0:8.0:5.0 to obtain an electrolyte.

[0084] 4. Preparation of separator:

[0085] Thermal conductive reinforcement layer: BNNS nanosheets (lateral size 200-500nm) and polyetherimide PEI (Mw=50,000) are dissolved in NMP at a mass ratio of 1:7.3, electrospun (voltage 18kV) to form a thermal conductive reinforcement layer, the thickness is controlled to be about 3μm.

[0086] Porous support layer: PVDF (Mw=600,000) is prepared into the base film by non-solvent induced phase separation method, the porosity is 55%, the thickness is controlled to be about 8μm.

[0087] Ion channel layer: sPBI with a sulfonation degree of 30% and LLZO nanowires (diameter 30nm, length 1-3μm) are compounded at a ratio of 5.67:1, and are cast into a film as an ion channel layer, the thickness is controlled to be about 4μm.

[0088] Interface treatment: RF plasma equipment was used to treat both sides of the PVDF base film with an acrylic monomer flow rate of 0.1 mL / min (power 80 W, time 60 s) to graft the acrylic monomer.

[0089] Lamination molding: sequentially stack the heat-conducting reinforcing layer, porous support layer, and ion channel layer, and perform roll pressing treatment, set the hot pressing temperature to 100°C, the roll pressing pressure to 0.8 MPa, and then remove the residual reagents by vacuum drying at 60°C for 24 h to obtain the separator of this embodiment 1.

[0090] 5. Assembly of lithium ion battery:

[0091] After the positive and negative electrode sheets are respectively subjected to roll pressing and slitting, they are wound together with the separator of this embodiment 1 obtained above, wherein the heat-conducting reinforcing layer and the ion channel layer are respectively arranged on the two sides of the porous support layer, the separator is arranged to adhere to the positive electrode sheet on the side where the heat-conducting reinforcing layer is located, and the negative electrode sheet on the side where the ion channel layer is located, and the wound core of the 21700 cylindrical lithium ion battery is obtained after winding, then the battery core and the connecting sheet are welded, and finally the battery shell is filled, after the liquid injection, sealing, and formation processes, the cylindrical lithium ion battery corresponding to this embodiment 1 is obtained, with the size parameters being diameter: 21.0 mm, length: 70.0 mm.

[0092] Embodiment 2

[0093] The difference between this embodiment 2 and embodiment 1 is that the mass ratio of BNNS nanosheet to polyetherimide PEI in the heat-conducting reinforcing layer is 1:11.5, and the others are the same as in embodiment 1.

[0094] Embodiment 3

[0095] The difference between this embodiment 3 and embodiment 1 is that the mass ratio of BNNS nanosheet to polyetherimide PEI in the heat-conducting reinforcing layer is 1:9, and the others are the same as in embodiment 1.

[0096] Embodiment 4

[0097] The difference between this embodiment 4 and embodiment 1 is that the mass ratio of BNNS nanosheet to polyetherimide PEI in the heat-conducting reinforcing layer is 1:5.67, and the others are the same as in embodiment 1.

[0098] Embodiment 5

[0099] The difference between this embodiment 5 and embodiment 1 is that the mass ratio of LLZO nanowire to sulfonated polybenzimidazole sPBI in the ion channel layer is 1:9, and the others are the same as in embodiment 1.

[0100] Embodiment 6

[0101] The difference between this embodiment 6 and embodiment 1 is that the mass ratio of LLZO nanowires to sulfonated polybenzimidazole sPBI in the ion channel layer is 1:5, and the others are the same as embodiment 1.

[0102] Embodiment 7

[0103] The difference between this embodiment 7 and embodiment 1 is that the porosity of the base film used in the porous support layer is 45%, and the others are the same as embodiment 1.

[0104] Embodiment 8

[0105] The difference between this embodiment 8 and embodiment 1 is that the porosity of the base film used in the porous support layer is 60%, and the others are the same as embodiment 1.

[0106] Comparative Example 1

[0107] The difference between this comparative example 1 and embodiment 1 is that no BNNS nanosheet is added in the thermal conduction strengthening layer, and the others are the same as embodiment 1.

[0108] Comparative Example 2

[0109] The difference between this comparative example 2 and embodiment 1 is that no LLZO nanowire is added in the ion channel layer, and the others are the same as embodiment 1.

[0110] Comparative Example 3

[0111] The difference between this comparative example 3 and embodiment 1 is that the porosity of the base film used in the porous support layer is 40%, and the others are the same as embodiment 1.

[0112] The lithium ion batteries in the above embodiments 1-8 and comparative examples 1-3 are subjected to the following treatments in parallel:

[0113] First, the lithium ion battery is discharged at a constant current to 2.5V to ensure that it is in a safe state, reducing the risk of short circuit or thermal runaway during disassembly. In the glove box (protected by argon or other inert atmosphere), the battery is carefully disassembled, and the separator in the cylindrical cell is taken out. The removed separator is soaked in anhydrous dimethyl carbonate (DMC) solution for 30 minutes to dissolve and remove residual electrolyte and possible by-products. Subsequently, a dust-free wiping paper is used to gently wipe the surface, then fresh DMC solution is replaced, and the soaking-wiping process is repeated three times to ensure that there is no residual contaminant on the surface of the separator. Finally, the separator is rinsed with anhydrous ethanol, wiped again, and placed in the glove box for 48 hours to ensure that the separator is completely dry to prevent subsequent tests from being disturbed by solvent residues.

[0114] Method for measuring the membrane breaking temperature:

[0115] The lithium ion battery was placed in a 25°C thermostat for 4h, and charged at 1C constant current and constant voltage to 4.2V, with a cutoff current of 0.01C, and rested for 10min, and then the thermostat was heated at a heating rate of 5°C / min, and every 5°C, the temperature was kept for 10min. The surface temperature change of the lithium ion battery was monitored during the heating process, and when the surface temperature of the lithium ion battery began to rise sharply, the corresponding temperature of the thermostat was the film rupture temperature of the lithium ion battery.

[0116] Method for measuring ionic conductivity:

[0117] (1) Sample preparation: Take the separators in Examples 1-8 and Comparative Examples 1-3, and immerse the separators in an electrolyte (such as a mixture of EC and DMC containing 1M LiPF6 in a volume ratio of 1:1) for at least 12 hours to ensure sufficient wetting.

[0118] (2) Assemble a symmetric battery: In an inert atmosphere (such as a glove box), two stainless steel electrodes (SS) and the separator soaked in electrolyte are clamped into a SS|separator|SS symmetric structure, i.e. an impedance testing device without active electrode.

[0119] (3) Test equipment and parameters: Use an electrochemical workstation to perform AC impedance testing (EIS), and the test conditions are as follows: frequency range: 1MHz-0.1Hz, AC perturbation voltage: 5-10mV, test temperature: 25°C.

[0120] Conductivity calculation formula:

[0121] Where: σ is the ionic conductivity (S / cm), L is the thickness of the separator (cm), R is the high-frequency semicircle intercept in the Nyquist plot (Ω), and A is the effective area of the peripheral electrode (cm 2 ).

[0122] 10C discharge capacity retention rate test method:

[0123] The batteries prepared from Examples 1-8 and Comparative Examples 1-3 were placed in a 25°C thermostat for 4h and tested according to the following steps:

[0124] (1) Constant current and constant voltage charging to 4.2V at 0.2C, with a cutoff current of 0.1C, and resting for 30min;

[0125] (2) Constant current discharge to 2.5V cutoff at 1C, and the capacity is Q1, and resting for 30min;

[0126] (3) Constant current and constant voltage charging to 4.2V at 0.2C, with a cutoff current of 0.1C, and resting for 30min;

[0127] (4) Discharge at constant current to 2.5 V cut-off at 10 C, the capacity is Q2, and rest for 30 min;

[0128] The calculation method of 10 C capacity retention rate is: Q2 / Q1 x 100%.

[0129] Li||Li symmetric cell cycle test:

[0130] The dendrite puncture resistance of the lithium ion battery separator can be tested by Li||Li symmetric cell at 2 mA / cm 2 Long-term cycle test at constant current density was carried out for evaluation. In the glove box, the separators in examples 1-8 and comparative examples 1-3 were sandwiched between two pieces of lithium metal to assemble a 2032 type button cell, and the electrolyte was a mixed solution containing 1 M LiPF6 in EC and DMC with a volume ratio of 1:1. Then, repeated lithium deposition / stripping cycles were carried out in a 25°C thermostat, until the voltage jumped or rapidly dropped to zero, indicating that lithium dendrites pierced the separator and caused short circuit, at which time the cycle time was recorded as the dendrite cycle life.

[0131] The test results of examples 1-8 and comparative examples 1-3 are shown in the following table:

[0132]

[0133]

[0134] As shown in Table 1, comparative examples 1-4 and comparative example 1, when the mass ratio of BNNS nanosheet to polyetherimide PEI increases from 1:11.5 to 1:7.3, the membrane breaking temperature, ion conductivity, 10C discharge capacity retention rate and dendrite cycle life of the separator all increase significantly, and when the mass ratio of BNNS nanosheet to polyetherimide PEI is 1:7.3, the performance is optimal, and on this basis, increasing the proportion of BNNS nanosheet, the performance of the separator decreases. This is because the appropriate amount of BNNS builds a heat conduction network in the PEI matrix, quickly disperses the local hot spot, and PEI provides high temperature support to delay melting deformation, and the synergistic effect is good, while excessive BNNS will cause the pores of the heat conduction network to be blocked, and the heat conduction capacity will decrease. If no BNNS is added to the heat conduction strengthening layer, the heat conduction strengthening layer cannot form a heat conduction network, and the separator lacks a suitable heat conduction network, and heat accumulation easily leads to local melting. The test results of comparative example 1 show that the membrane breaking temperature of the separator decreases significantly, the ion conductivity also decreases significantly, reflecting that the battery dynamics performance is seriously insufficient, and at the same time, the lithium dendrite cycle life is also significantly reduced.

[0135] From Example 1 and Examples 5-6, and Comparative Example 2, it can be seen that when the mass ratio of the LLZO nanowires to the sulfonated polybenzimidazole sPBI increases from 1:9 to 1:5.67, the membrane breaking temperature, ion conductivity, 10C charge capacity retention, and dendrite cycle life of the separator all increase significantly, and when the mass ratio of the LLZO nanowires to the sulfonated polybenzimidazole sPBI is 1:5.67, the performance is optimal, and on this basis, increasing the mass ratio of the LLZO nanowires further reduces the performance of the separator. This is because the appropriate amount of LLZO nanowires synergizes with the sulfonic acid groups of the sPBI with a sulfonation degree of 30%, and the -SO3 - The electrostatic repulsion and the size screening effect of the LLZO nanowires form an anion migration double-blocking effect, and the anion transference number is greatly reduced. The lithium ion single conductor property of the LLZO can increase the number of lithium ions transferred, reduce the concentration polarization, and inhibit local overheating. In addition, the LLZO can also form a composite heat conduction network with the sPBI to achieve rapid dispersion of heat and further inhibit the generation of local hot spots. However, an excessive amount of LLZO nanowires can cause the nanowires to intertwine and not be uniformly dispersed, and the agglomerates can block the through pores of the ion channel layer, reduce the pore connectivity, and block the ion transport path. In addition, the sulfonated polybenzimidazole sPBI also has natural affinity with the electrolyte, which enhances the wettability of the electrolyte to the core. If no LLZO nanowires are added to the heat conduction reinforcement layer, the lithium ion single conductor property of the LLZO nanowires is missing, Li + The number of lithium ions transferred tends to be that of a conventional separator, the electrolyte interface concentration polarization is intensified, local lithium deposition is induced, and lithium dendrites eventually penetrate the separator, and the dendrite cycle life is significantly reduced.

[0136] From Example 1 and Examples 7-8, and Comparative Example 3, it can be seen that the lower the porosity of the porous support layer, the higher the membrane breaking temperature, and the battery safety performance is correspondingly improved. However, the reduction in porosity can significantly reduce the ion conductivity, and the lithium ion transport is not timely, which can easily induce local lithium deposition and form lithium dendrites penetrating the separator, and the dendrite cycle life is significantly reduced. A porosity of 45-60% can better balance the high-temperature resistance and electrical conductivity of the battery.

[0137] Although the embodiments of the present embodiment have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, replacements, and variations can be made to these embodiments without departing from the principles and purposes of the present embodiment, and the scope of the present embodiment is defined by the claims and their equivalents.

Claims

1. A diaphragm, characterized in that: It includes a porous support layer and a gradient functional layer superimposed on the outside of the porous support layer. The porous support layer includes a base film with a porosity of 45-60% and active groups modified on the surface of the base film, wherein the active groups include at least one of hydroxyl, aldehyde, carboxyl, ester, carbonyl, vinyl, amino, epoxy and thiol groups. The gradient functional layer includes a thermal conductivity strengthening layer and an ion channel layer. The thermal conductivity strengthening layer includes a first polymer and modified boron nitride nanosheets surface-modified with a silane coupling agent. The ion channel layer includes a second polymer and solid electrolyte nanowires.

2. The diaphragm according to claim 1, characterized in that The thickness of the porous support layer is 6-10 μm, the pore size distribution range is 50-200 nm, the glass transition temperature is ≥160°C, and the base membrane is selected from one or more of polyvinylidene fluoride membrane, polyethylene membrane, polypropylene membrane, polyethylene terephthalate membrane, polyphenylene benzobisoxazole membrane, polybenzimidazole membrane, polyacrylonitrile membrane, polyimide membrane, polycarbonate membrane, polyurethane membrane and polyetherimide membrane.

3. The diaphragm according to claim 1, characterized in that The active group is one or more of a carboxyl group, an aldehyde group, a carbonyl group and an epoxy group; the thickness of the thermal conductivity strengthening layer is 2-5 μm; the first polymer includes one or more of polyimide and its derivatives, polysulfone and its derivatives, and polyarylene ether and its derivatives; the silane coupling agent is an aminosilane coupling agent; the lateral size of the modified boron nitride nanosheet is 200-500 nm and the surface amino content thereof is ≥0.8 mmol / g; the mass ratio of the first polymer to the modified boron nitride nanosheet is (5.7-11.5):

1.

4. The diaphragm according to claim 1, characterized in that The thickness of the ion channel layer is 3-6 μm, the mass ratio of the second polymer to the solid electrolyte nanowire is (4-9):1, the diameter of the solid electrolyte nanowire is 20-50 nm, and the aspect ratio is 30-100. The second polymer includes polybenzimidazole and its derivatives and / or polyetheretherketone and its derivatives, and the solid electrolyte nanowire includes one or more of lithium lanthanum zirconium oxide nanowires, lithium aluminum titanium phosphate nanowires and lanthanum lithium titanium oxide nanowires.

5. The diaphragm according to claim 4, characterized in that The second polymer is sulfonated polybenzimidazole with a sulfonation degree of 20-40%, and the solid electrolyte nanowire is lithium lanthanum zirconium oxide nanowire.

6. The diaphragm according to claim 1, characterized in that The thermal conductivity strengthening layer and the ion channel layer are respectively arranged on both sides of the porous support layer, and the diaphragm is arranged with the side where the thermal conductivity strengthening layer is located attached to the positive electrode sheet, and with the side where the ion channel layer is located attached to the negative electrode sheet; Alternatively, the thermally conductive strengthening layer and the ion channel layer are stacked on the same side of the porous support layer, the ion channel layer is attached to the porous support layer, the thermally conductive strengthening layer is attached to the outside of the ion channel layer, and the diaphragm is attached to the positive electrode sheet on the side where the thermally conductive strengthening layer is located; Alternatively, the thermal conductivity strengthening layer and the ion channel layer are provided on both sides of the porous support layer, the ion channel layer is arranged in contact with the porous support layer, and the thermal conductivity strengthening layer is arranged outside the ion channel layer.

7. The diaphragm according to claim 1, characterized in that The membrane rupture temperature of the membrane is ≥220° C., and the ionic conductivity of the membrane at 25° C. is greater than 0.65 mS / cm.

8. The diaphragm according to claim 1, wherein The membrane is at 2 mA / cm 2 The dendrite cycle life at the current density is greater than or equal to 1200 hours.

9. A method for preparing a diaphragm, characterized in that: The diaphragm is the diaphragm according to any one of claims 1 to 8, and the preparation of the diaphragm comprises the following steps: S1: dissolving the first polymer and the modified boron nitride nanosheets in N-methylpyrrolidone, and electrospinning at a voltage of 18 kV to form the thermal conductivity strengthening layer; S2: mixing the second polymer with the solid electrolyte nanowires, and preparing the ion channel layer by a tape casting film forming process; S3: using a radio frequency plasma device, setting the power of the radio frequency plasma device to 80W and the single operation time to 60s, taking a base film with a porosity of 45-60%, and placing it into the chamber of the radio frequency plasma device, introducing a compound reagent containing the active group, setting the flow rate of the compound reagent to 0.1mL / min, the compound reagent is vaporized to form steam, and the base film is plasma treated in the steam atmosphere to graft the compound to obtain the porous support layer; and S4: stacking the thermally conductive strengthening layer, the porous support layer, and the ion channel layer, and performing a roll pressing process at a hot pressing temperature of 100-120° C. and a roll pressing pressure of 0.8 MPa, followed by vacuum drying at 60° C. for 24 h to obtain the diaphragm.

10. The method for preparing a diaphragm according to claim 9, characterized in that: The base film is a polyvinylidene fluoride film, and the preparation of the polyvinylidene fluoride film further comprises the following steps: S31: dissolving polyvinylidene fluoride in N-methylpyrrolidone to control the concentration of polyvinylidene fluoride to be 20 wt %; S32: adding a pore regulator in an amount of 5-8% by mass of the polyvinylidene fluoride; S33: coating the polyvinylidene fluoride solution onto a steel belt or a drum through a casting die, controlling the thickness to be 100-200 μm, to obtain a wet film; S34: exposing the wet film to air with a humidity of 50-60% for 60-80 seconds; S35: Transfer the wet film to a non-solvent bath with a mass ratio of ethanol to water = 1:3, soak for 60-80 seconds, and the temperature is 25°C; S36: The wet film is taken out, washed with water to remove residual reagents, and heat-set at 160° C. for 15 minutes to obtain the base film.

11. A lithium-ion battery comprising a positive electrode, a separator and a negative electrode, characterized in that: The separator is the separator according to any one of claims 1 to 8, and the 10C rate discharge capacity retention rate of the lithium ion battery is ≥85%.

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