Composite separator with functionally graded coating and method of making the same

Abstract

The present application relates to a kind of composite diaphragm with function gradient coating and its preparation method, and the composite diaphragm includes: porous base film, solid electrolyte coating attached to one side or both sides of porous base film, mixed function coating attached to the surface of solid electrolyte coating, and the outermost polymer composite coating;Solid electrolyte coating includes: first solid electrolyte, first binder;Mixed function coating includes: second solid electrolyte, high dielectric constant nanomaterial, second binder;Polymer composite coating includes: in-situ polymerized three-dimensional network structure polymer, and lithium salt and third solid electrolyte filled in the interstice of three-dimensional network structure;Polymer is formed by in-situ polymerization of photoinitiator-initiated polymerizable monomer.It can improve the cycle performance of lithium battery by applying the composite diaphragm of the present application to lithium battery.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and in particular to a composite separator with a functionally graded coating and its preparation method. Background Technology

[0002] In the pursuit of higher energy density and safety reliability in the development of next-generation batteries, solid-state battery technology is considered a key breakthrough. However, whether it is a traditional liquid battery or an existing solid-state battery system, the separator and its interface characteristics remain the core factors restricting battery performance. Especially in solid-state batteries, problems such as high interfacial impedance caused by solid-solid contact between the electrolyte and the electrode, lithium dendrite growth, and interfacial side reactions severely limit their practical application performance.

[0003] Existing membranes, such as polyolefin microporous membranes or ceramic-coated membranes, mainly play a basic role in physical isolation and ion transport channels, but still have significant drawbacks: First, polyolefin membranes have poor thermal stability, are prone to thermal shrinkage leading to short circuits, and have insufficient affinity for electrolytes; Second, although conventional ceramic coatings can improve thermal stability and wettability, their intrinsic insulation properties cannot improve ionic conductivity, and they still form rigid contact with the electrodes, failing to suppress the space charge layer effect, resulting in persistently high interfacial impedance. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a composite membrane with a functionally graded coating and its preparation method.

[0005] This invention constructs a multi-layered, synergistic composite membrane structure by sequentially building a high-ionic-conductivity solid electrolyte coating, a high-dielectric-constant material and a solid electrolyte hybrid functional coating on a traditional polyolefin-based membrane or ceramic separator. Finally, a polymer composite coating is used to achieve gap filling and interface encapsulation. Specifically, the solid electrolyte coating, through its unique crystal structure, constructs a continuous ion permeation network, significantly improving the bulk ionic conductivity; the high-dielectric-constant material layer homogenizes the electric field distribution on the electrode surface, enabling Li... + The lithium flows more evenly across the entire electrode surface, achieving uniform lithium deposition / stripping. This suppresses dendrite nucleation and growth at the source, reducing concentration polarization near the electrode and thus decreasing interfacial impedance growth. Meanwhile, the polymer layer on the composite separator surface forms a dense, flexible physical barrier through in-situ molding, providing adaptive soft contact. This achieves adaptive soft contact between the composite separator and the electrode, improving interfacial mechanical contact, reducing sheet resistance, and suppressing lithium dendrite penetration. This invention, through the synergistic effect of intrinsic material properties and interfacial microenvironment regulation, provides an effective and feasible technical path to solve the current interface challenges of solid-state batteries.

[0006] To achieve the above objectives, in a first aspect, the present invention provides a composite membrane with a functionally graded coating, the composite membrane comprising: a porous base membrane, a solid electrolyte coating attached to one or both sides of the porous base membrane, a mixed functional coating attached to the surface of the solid electrolyte coating, and an outermost polymer composite coating.

[0007] The solid electrolyte coating comprises: a first solid electrolyte and a first binder; the hybrid functional coating comprises: a second solid electrolyte, a high dielectric constant nanomaterial, and a second binder.

[0008] The polymer composite coating comprises: a polymer with an in-situ polymerized three-dimensional network structure, and lithium salt and a third solid electrolyte filling the voids in the three-dimensional network structure; the polymer is formed by in-situ polymerization of polymerizable monomers initiated by the photoinitiator.

[0009] Preferably, the porous base membrane is a porous base membrane formed of polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinyl alcohol (PVA), polymethyl methacrylate (PMMA), or polyethylene terephthalate (PET), or a double-layer composite base membrane or a triple-layer composite base membrane formed by a combination of the above materials, or a ceramic diaphragm or a fiber diaphragm; the pore size of the porous base membrane is between 100 nm and 1 μm.

[0010] The porosity of the porous base membrane is 20% to 50%; the thickness of the porous base membrane is 9 μm to 16 μm.

[0011] The thickness of the solid electrolyte coating is 2μm to 5μm;

[0012] The thickness of the hybrid functional coating is 3μm to 6μm;

[0013] The thickness of the polymer composite coating is 1 μm to 3 μm.

[0014] Preferably, the first solid electrolyte, the second solid electrolyte, and the third solid electrolyte each comprise one or more of the following: pyrochlore-type solid electrolyte, garnet-type solid electrolyte material, NAS ICON-type solid electrolyte material, perovskite-type solid electrolyte material, LISICON solid electrolyte material, and their derivatives; the first solid electrolyte, the second solid electrolyte, and the third solid electrolyte may be the same or different;

[0015] Both the first adhesive and the second adhesive comprise one or more of the following: polyvinylidene fluoride, polyvinyl alcohol, polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, and hydroxyethyl cellulose;

[0016] The high dielectric constant nanomaterial includes one or more of barium titanate, strontium titanate, barium aluminate, barium strontium titanate, titanium dioxide, and zirconium oxide; the relative dielectric constant ε of the high dielectric constant nanomaterial is... r ≥20;

[0017] The polymerizable monomers include one or more of pentaerythritol tetraacrylate, polyethylene glycol diacrylate, methyl methacrylate, trimethylolpropane triacrylate, and trifluoroethyl acrylate.

[0018] The lithium salt includes one or more of the following: lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium difluorooxalate borate, lithium hexafluorophosphate, and lithium tetrafluoroborate.

[0019] The photoinitiator includes one or more of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, 1-hydroxycyclohexylphenyl ketone, and 2-hydroxy-2-methyl-1-phenyl-1-propanone.

[0020] More preferably, the pyrochlore-type solid electrolyte has the general chemical formula Li. x La y M1 z M2 w M3 u O6F, M1 is a tetravalent cation, M2 is a pentavalent cation, M3 is a hexavalent cation, and 1 < x + 3y < 5, 0 < x ≤ 2, 1 / 3 < y < 5 / 3, 0 ≤ z ≤ 2, 0 ≤ w ≤ 2, 0 ≤ u ≤ 2, z + w + u = 2, and at least one of M1, M2, and M3 exists, that is, one of z, w, and u is not 0;

[0021] The chemical formula of the garnet-type solid electrolyte material is Li7M43M52O. 12 M4 is one or more of La, Ca, Sr, Ba, and K, and M5 is one or more of Zr, Ta, Nb, and Hf.

[0022] The general chemical formula of the NASICON-type solid electrolyte material is Li. 1+m M6 m M7 2-m (PO4)3, where 0.01≤m≤0.5, M6 is one or more of Al, Y, Ga, Cr, In, Fe, Se, and La, and M7 is one or more of Ti, Ge, Ta, Zr, Sn, Fe, V, and Hf;

[0023] The chemical formula of the perovskite-type solid electrolyte material is: Li 3n M8 2 / 3-nM9O3, where n is between 0.01 and 0.5, M8 is one or more of La, Al, Mg, Fe, and Ta, and M9 is one or more of Ti, Nb, Sr, and Pr;

[0024] The chemical formula of the LISICON solid electrolyte material is: Li 14 N1(N2O4)4, wherein N1 is one or more of Zr, Cr, and Sn, and N2 is one or more of Si, S, and P.

[0025] Secondly, the present invention provides a method for preparing the composite separator described in the first aspect, the method comprising:

[0026] Step S1: Disperse the first solid electrolyte and the first binder in the first solvent and ball mill them together to obtain a first mixed slurry with a solid content of 20wt% to 50wt%.

[0027] Step S2: The first mixed slurry is uniformly coated on at least one surface of the porous base membrane, and after preliminary drying and vacuum drying to remove the first solvent, a solid electrolyte coating is formed on at least one surface of the porous base membrane.

[0028] Step S3: The second solid electrolyte, high dielectric constant nanomaterials, and second binder are dispersed at high speed in the second solvent to obtain a second mixed slurry with a solid content of 10wt% to 30wt%.

[0029] Step S4: The second mixed slurry is uniformly coated on the surface of the solid electrolyte coating. After baking to remove the second solvent, a mixed functional coating is formed on the surface of the solid electrolyte coating to obtain a composite membrane precursor.

[0030] Step S5: A polymer precursor solution is prepared by mixing polymerizable monomers, lithium salts and photoinitiators. Then, a third solid electrolyte is added to the polymer precursor solution and dispersed evenly to obtain a third mixed slurry.

[0031] Step S6: Immerse the composite membrane precursor in the third mixed slurry for vacuum impregnation, remove the vacuum-impregnated composite membrane precursor, and form a polymer composite coating after photocuring to finally obtain a composite membrane with a functional gradient coating.

[0032] Preferably, the first solid electrolyte, the second solid electrolyte, and the third solid electrolyte each comprise one or more of the following: pyrochlore-type solid electrolyte, garnet-type solid electrolyte material, NASICON-type solid electrolyte material, perovskite-type solid electrolyte material, LISICON solid electrolyte material, and their derivatives; the first solid electrolyte, the second solid electrolyte, and the third solid electrolyte may be the same or different;

[0033] Both the first adhesive and the second adhesive comprise one or more of the following: polyvinylidene fluoride, polyvinyl alcohol, polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, and hydroxyethyl cellulose;

[0034] The high dielectric constant nanomaterial includes one or more of barium titanate, strontium titanate, barium aluminate, barium strontium titanate, titanium dioxide, and zirconium oxide; the relative dielectric constant ε of the high dielectric constant nanomaterial is... r ≥20;

[0035] The polymerizable monomers include one or more of pentaerythritol tetraacrylate, polyethylene glycol diacrylate, methyl methacrylate, trimethylolpropane triacrylate, and trifluoroethyl acrylate.

[0036] The lithium salt includes one or more of the following: lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium difluorooxalate borate, lithium hexafluorophosphate, and lithium tetrafluoroborate.

[0037] The initiator includes one or more of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, 1-hydroxycyclohexylphenyl ketone, and 2-hydroxy-2-methyl-1-phenyl-1-propanone.

[0038] The first solvent and the second solvent include one or more of N-methylpyrrolidone (NMP), deionized water, ethanol, and propanol;

[0039] The mass ratio of the first solid electrolyte to the first binder is 95-99.5:0.5-5;

[0040] The mass ratio of the second solid electrolyte, the high dielectric constant nanomaterial, and the second binder is 50-90:5-45:1-10;

[0041] In the polymer precursor solution, the molar ratio of the lithium salt to the polymerizable monomer is 1:2 to 1:8, and the mass ratio of the initiator to the polymerizable monomer is 0.5:100 to 3:100; the molar concentration of the polymerizable monomer in the polymer precursor solution is 1 mol / L to 4 mol / L; and the mass ratio of the third solid electrolyte to the total solid mass in the polymer precursor solution is 10% to 40%.

[0042] More preferably, the pyrochlore-type solid electrolyte has the general chemical formula Li. x La y M1 z M2 w M3 u O6F, M1 is a tetravalent cation, M2 is a pentavalent cation, M3 is a hexavalent cation, and 1 < x + 3y < 5, 0 < x ≤ 2, 1 / 3 < y < 5 / 3, 0 ≤ z ≤ 2, 0 ≤ w ≤ 2, 0 ≤ u ≤ 2, z + w + u = 2, and at least one of M1, M2, and M3 exists, that is, one of z, w, and u is not 0;

[0043] The chemical formula of the garnet-type solid electrolyte material is Li7M43M52O. 12 M4 is one or more of La, Ca, Sr, Ba, and K, and M5 is one or more of Zr, Ta, Nb, and Hf.

[0044] The general chemical formula of the NASICON-type solid electrolyte material is Li. 1+m M6 m M7 2-m (PO4)3, where 0.01≤m≤0.5, M6 is one or more of Al, Y, Ga, Cr, In, Fe, Se, and La, and M7 is one or more of Ti, Ge, Ta, Zr, Sn, Fe, V, and Hf;

[0045] The chemical formula of the perovskite-type solid electrolyte material is: Li 3n M8 2 / 3-n M9O3, where n is between 0.01 and 0.5, M8 is one or more of La, Al, Mg, Fe, and Ta, and M9 is one or more of Ti, Nb, Sr, and Pr;

[0046] The chemical formula of the LISICON solid electrolyte material is: Li 14 N1(N2O4)4, wherein N1 is one or more of Zr, Cr, and Sn, and N2 is one or more of Si, S, and P.

[0047] Preferably, in step S1, the ball milling speed is 800 rpm to 2000 rpm, and the ball milling time is 0.5 hours to 5 hours;

[0048] In step S2, the preliminary drying is specifically performed at 40℃ to 60℃ for 5 to 30 minutes; the vacuum drying is specifically performed at 60℃ to 100℃ for 2 to 12 hours.

[0049] In step S3, the high-speed dispersion speed is 800 rpm to 3000 rpm, and the time is 0.5 hours to 5 hours.

[0050] Preferably, in step S4, the baking temperature is 50℃~80℃ and the baking time is 1 hour~4 hours;

[0051] Step S6, the vacuum impregnation specifically involves impregnating the composite membrane precursor under a vacuum condition of -0.05MPa to -0.1MPa for 10 to 60 minutes; the photocuring specifically involves curing the vacuum-impregnated composite membrane precursor under ultraviolet light with a wavelength of 365nm for 30 seconds to 5 minutes.

[0052] Thirdly, the present invention provides a lithium battery, the lithium battery comprising the composite separator described in the first aspect, or the composite separator prepared by the preparation method described in the second aspect.

[0053] The present invention provides a composite diaphragm with a functionally graded coating and a method for preparing the same, which has the following technical effects.

[0054] (1) The present invention provides a method for preparing a composite membrane with a functional gradient coating. By sequentially constructing a solid electrolyte coating with high ionic conductivity, a mixed functional coating of high dielectric constant material and solid electrolyte on a porous base membrane, and finally achieving gap filling and interface encapsulation through a polymer composite coating with a three-dimensional network structure and containing solid electrolyte, a multi-layer synergistic composite membrane with a functional gradient coating is formed.

[0055] (2) The composite membrane with functionally graded coating prepared by the preparation method provided by the present invention establishes a low-defect, high-density ion migration channel in the solid electrolyte coating. By increasing the content of active ion conductors, a three-dimensional ion path is formed inside the lattice of the solid electrolyte, which is an active ion conductor, providing a low-barrier diffusion channel for the rapid migration of lithium ions. This greatly reduces the transport resistance of lithium ions in the membrane body and ensures that the ion flow can be efficiently transported from one electrode to the other.

[0056] The hybrid functional coating contains a solid electrolyte and a high dielectric constant material. The solid electrolyte ensures that the ion transport path is not interrupted when the ions are transported outward from the first layer, avoiding the ion blockage that may be caused by using a pure high dielectric constant material layer. The added high dielectric constant material is dispersed in the hybrid functional coating and acts directly around the ion transport path. It can more efficiently and directly polarize and shield the space charge layer effect at the interface, reducing the energy barrier for ions to cross the interface.

[0057] The polymer composite coating provides excellent flexibility through the polymer matrix, enabling it to adapt to the microscopic undulations of the electrode surface and achieve adaptive soft contact with maximum area, significantly reducing mechanical contact resistance. Meanwhile, the dispersed solid electrolyte powder constructs localized ion transport shortcuts within the flexible polymer, compensating for the low ionic conductivity of pure polymers and ensuring efficient ion transfer from the separator to the electrode interface. Simultaneously, the polymer matrix fills the gaps in the separator and its coating, forming a robust physical barrier that effectively inhibits lithium dendrite penetration.

[0058] The composite membrane provided by this invention constructs a three-layer structure of solid electrolyte coating, hybrid functional coating, and polymer composite coating, forming a high-speed ion transport channel. Electric field polarization regulation and a flexible, uniform interface contact mode ensure that ions are in a highly efficient and low-resistance state throughout the entire process from the membrane to the electrode, avoiding performance bottlenecks caused by a single coating. Furthermore, by forming interconnected ion channels in each layer through a gradient transition of internal solid electrolyte content, interlayer impedance is effectively eliminated, greatly improving structural integrity and cycle stability. In addition, through the synergistic effect of the three layers, the intermediate hybrid functional coating protects the inner main structure and pre-enriches ions to drive outer layer transport, while the outer polymer layer not only provides dendrite suppression and stress buffering, but its ion transport efficiency also benefits from the support and energization of the inner layer. Ultimately, the composite membrane of this invention simultaneously achieves high ionic conductivity, low interfacial impedance, excellent mechanical contact, and superior safety.

[0059] (3) The composite separator with functional gradient coating provided by the present invention can be applied to lithium batteries. Since the composite separator has high ionic conductivity, low interfacial impedance, excellent mechanical contact and excellent safety performance, it can improve the cycle performance of lithium batteries. Attached Figure Description

[0060] Figure 1 This is a schematic diagram of a composite membrane with a functionally graded coating provided in an embodiment of the present invention.

[0061] Figure 2 This is another structural schematic diagram of a composite membrane with a functionally graded coating provided in an embodiment of the present invention.

[0062] Figure 3A flowchart illustrating the preparation method of a composite membrane with a functionally graded coating provided in an embodiment of the present invention. Detailed Implementation

[0063] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0064] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0065] This invention provides a composite separator with a functionally graded coating. This composite separator can have two structures, one of which is as follows: Figure 1 As shown, the composite membrane includes: a porous base membrane, a solid electrolyte coating attached to one side surface of the porous base membrane, a hybrid functional coating attached to the surface of the solid electrolyte coating, and a polymer composite coating attached to the surface of the hybrid functional coating and the other side surface of the porous base membrane.

[0066] Another structure of composite membranes with functionally graded coatings, such as Figure 2 As shown, the composite membrane includes: a porous base membrane, a solid electrolyte coating attached to both sides of the porous base membrane, a hybrid functional coating attached to the surface of the solid electrolyte coating, and a polymer composite coating attached to the surface of the hybrid functional coating.

[0067] Specifically, the aforementioned solid electrolyte coating includes: a first solid electrolyte and a first binder. The thickness of the solid electrolyte coating is 2 μm to 5 μm.

[0068] The hybrid functional coating comprises a second solid electrolyte, a high dielectric constant nanomaterial, and a second binder. The thickness of the hybrid functional coating is 3 μm to 6 μm.

[0069] The polymer composite coating comprises: a polymer with an in-situ polymerized three-dimensional network structure, and lithium salt and a third solid electrolyte filling the voids in the three-dimensional network structure; the polymer is formed by in-situ polymerization of polymerizable monomers initiated by a photoinitiator. The thickness of the polymer composite coating is 1 μm to 3 μm.

[0070] The porous base membrane is a porous base membrane formed of polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinyl alcohol (PVA), polymethyl methacrylate (PMMA), or polyethylene terephthalate (PET), or a double-layer composite base membrane or a triple-layer composite base membrane formed by a combination of the above materials, or a ceramic diaphragm or a fiber diaphragm; the pore size of the porous base membrane is between 100 nm and 1 μm.

[0071] The porosity of the porous base membrane is 20%–50%; the thickness of the porous base membrane is 9μm–16μm.

[0072] The first solid electrolyte, the second solid electrolyte, and the third solid electrolyte all include one or more of the following: pyrochlore-type solid electrolyte, garnet-type solid electrolyte material, NAS I CON-type solid electrolyte material, perovskite-type solid electrolyte material, LI SI CON solid electrolyte material, and their derivative materials; the first solid electrolyte, the second solid electrolyte, and the third solid electrolyte may be the same or different.

[0073] Both the first and second adhesives include one or more of the following: polyvinylidene fluoride, polyvinyl alcohol, polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, and hydroxyethyl cellulose.

[0074] High dielectric constant nanomaterials include one or more of the following: barium titanate, strontium titanate, barium aluminate, barium strontium titanate, titanium dioxide, and zirconium oxide; the relative dielectric constant ε of high dielectric constant nanomaterials is... r ≥20.

[0075] Polymerizable monomers include one or more of pentaerythritol tetraacrylate, polyethylene glycol diacrylate, methyl methacrylate, trimethylolpropane triacrylate, and trifluoroethyl acrylate.

[0076] Lithium salts include one or more of the following: lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium difluorooxalate borate, lithium hexafluorophosphate, and lithium tetrafluoroborate.

[0077] Photoinitiators include one or more of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, 1-hydroxycyclohexylphenyl ketone, and 2-hydroxy-2-methyl-1-phenyl-1-propanone.

[0078] The general chemical formula of pyrochlore-type solid electrolytes is Li x La y M1 z M2 w M3 uO6F, M1 is a tetravalent cation, M2 is a pentavalent cation, and M3 is a hexavalent cation, and 1 < x + 3y < 5, 0 < x ≤ 2, 1 / 3 < y < 5 / 3, 0 ≤ z ≤ 2, 0 ≤ w ≤ 2, 0 ≤ u ≤ 2, z + w + u = 2, and at least one of M1, M2, and M3 exists, that is, one of z, w, and u is not 0; M1 can be one or more of Zr, Ti, Hf, Si, Ge, and Sn; M2 can be one or more of Nb, Sb, Bi, V, and Ta; M3 can be one or more of W, Cr, Mo, and Mn.

[0079] The general chemical formula of garnet-type solid electrolyte materials is Li7M43M52O 12 M4 is one or more of La, Ca, Sr, Ba, and K, and M5 is one or more of Zr, Ta, Nb, and Hf.

[0080] The general chemical formula of NAS ICON type solid electrolyte material is Li 1+m M6 m M7 2-m (PO4)3, where 0.01≤m≤0.5, M6 is one or more of Al, Y, Ga, Cr, In, Fe, Se, and La, and M7 is one or more of Ti, Ge, Ta, Zr, Sn, Fe, V, and Hf.

[0081] The general chemical formula for perovskite solid electrolyte materials is: Li 3n M8 2 / 3-n M9O3, where 0.01≤n≤0.5, M8 is one or more of La, Al, Mg, Fe, and Ta, and M9 is one or more of Ti, Nb, Sr, and Pr.

[0082] The general chemical formula of LISICON solid electrolyte material is: Li 14 N1(N2O4)4, wherein N1 is one or more of Zr, Cr, and Sn, and N2 is one or more of Si, S, and P.

[0083] The pyrochlore-type solid electrolyte in this invention refers to a solid electrolyte with a "pyrochlore" crystal structure. The first, second, and third solid electrolytes in this invention are preferably pyrochlore-type solid electrolytes. This is because pyrochlore-type solid electrolytes are fluoride oxide solid electrolytes, which inherently possess high mechanical strength, high density, high purity, high volumetric energy density, low internal resistance, and excellent ion conductivity. Furthermore, pyrochlore-type solid electrolytes have a rigid structure with adjustable elemental composition. By introducing diverse coordination environments through multivalent cation doping, open channels conducive to lithium-ion transport can be formed. Simultaneously, fluorine doping can further enhance the polarity and interfacial wettability of the material, improve interfacial contact with the electrolyte and electrodes, and reduce interfacial impedance. When pyrochlore-type solid electrolytes are present in all three coating layers, the oxyfluoride-containing pyrochlore-type solid electrolyte particles form a continuous ion transport permeation network through mutual contact. Lithium ions can not only penetrate the interior of the particles but also be rapidly transported through the interfaces between the particles, resulting in an overall ionic conductivity far exceeding that of traditional porous membranes.

[0084] This invention provides a method for preparing a composite separator, such as... Figure 3 As shown, the specific steps include:

[0085] Step S1: Disperse the first solid electrolyte and the first binder in the first solvent and ball mill them together to obtain the first mixed slurry.

[0086] The first solid electrolyte includes one or more of the following: pyrochlore-type solid electrolyte, garnet-type solid electrolyte material, NASICON-type solid electrolyte material, perovskite-type solid electrolyte material, LIS ICON solid electrolyte material and its derivative materials.

[0087] Specifically, the general chemical formula of pyrochlore-type solid electrolytes is Li. x La y M1 z M2 w M3 u O6F, M1 is a tetravalent cation, M2 is a pentavalent cation, and M3 is a hexavalent cation, and 1 < x + 3y < 5, 0 < x ≤ 2, 1 / 3 < y < 5 / 3, 0 ≤ z ≤ 2, 0 ≤ w ≤ 2, 0 ≤ u ≤ 2, z + w + u = 2, and at least one of M1, M2, and M3 exists, that is, one of z, w, and u is not 0; M1 can be one or more of Zr, Ti, Hf, Si, Ge, and Sn; M2 can be one or more of Nb, Sb, Bi, V, and Ta; M3 can be one or more of W, Cr, Mo, and Mn.

[0088] The general chemical formula for garnet-type solid electrolyte materials is Li7M43M52O. 12 M4 is one or more of La, Ca, Sr, Ba, and K, and M5 is one or more of Zr, Ta, Nb, and Hf.

[0089] The general chemical formula of NASICON-type solid electrolyte materials is Li. 1+m M6 m M7 2-m (PO4)3, where 0.01≤m≤0.5, M6 is one or more of Al, Y, Ga, Cr, In, Fe, Se, and La, and M7 is one or more of Ti, Ge, Ta, Zr, Sn, Fe, V, and Hf.

[0090] The general chemical formula for perovskite solid electrolyte materials is: Li 3n M8 2 / 3-n M9O3, where 0.01≤n≤0.5, M8 is one or more of La, Al, Mg, Fe, and Ta, and M9 is one or more of Ti, Nb, Sr, and Pr.

[0091] The general chemical formula of LISICON solid electrolyte material is: Li 14 N1(N2O4)4, wherein N1 is one or more of Zr, Cr, and Sn, and N2 is one or more of Si, S, and P.

[0092] The first binder includes one or more of the following: polyvinylidene fluoride, polyvinyl alcohol, polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, and hydroxyethyl cellulose.

[0093] The mass ratio of the first solid electrolyte to the first binder is 95-99.5:0.5-5.

[0094] The first solvent includes one or more of N-methylpyrrolidone (NMP), deionized water, ethanol, and propanol.

[0095] The solid content of the first mixed slurry is 20wt% to 50wt%.

[0096] The ball milling process is a standard operation and can be performed using a ball mill. The ball milling speed is 800 rpm to 2000 rpm, and the ball milling time is 0.5 hours to 5 hours.

[0097] Step S2: The first mixed slurry is uniformly coated on at least one surface of the porous base membrane, and after preliminary drying and vacuum drying to remove the first solvent, a solid electrolyte coating is formed on at least one surface of the porous base membrane.

[0098] Specifically, the initial drying process involves drying at 40℃ to 60℃ for 5 to 30 minutes.

[0099] Vacuum drying specifically involves vacuum drying at 60℃~100℃ for 2 to 12 hours.

[0100] In this step, the coating method is a conventional method, such as any one of the following: gravure coating, slot extrusion coating, spraying, or dip-coating.

[0101] The first mixed slurry is uniformly coated on at least one surface of the porous base membrane with a coating thickness of about 2 μm to 5 μm; the thickness of the final solid electrolyte coating is 2 μm to 5 μm.

[0102] Step S3: The second solid electrolyte, high dielectric constant nanomaterial, and second binder are dispersed at high speed in the second solvent to obtain the second mixed slurry.

[0103] The second solid electrolyte includes one or more of the following: pyrochlore-type solid electrolyte, garnet-type solid electrolyte material, NASI CON-type solid electrolyte material, perovskite-type solid electrolyte material, LISI CON-type solid electrolyte material and their derivatives.

[0104] Specifically, the general chemical formula of pyrochlore-type solid electrolytes is Li. x La y M1 z M2 w M3 u O6F, M1 is a tetravalent cation, M2 is a pentavalent cation, and M3 is a hexavalent cation, and 1 < x + 3y < 5, 0 < x ≤ 2, 1 / 3 < y < 5 / 3, 0 ≤ z ≤ 2, 0 ≤ w ≤ 2, 0 ≤ u ≤ 2, z + w + u = 2, and at least one of M1, M2, and M3 exists, that is, one of z, w, and u is not 0; M1 can be one or more of Zr, Ti, Hf, Si, Ge, and Sn; M2 can be one or more of Nb, Sb, Bi, V, and Ta; M3 can be one or more of W, Cr, Mo, and Mn.

[0105] The general chemical formula for garnet-type solid electrolyte materials is Li7M43M52O. 12 M4 is one or more of La, Ca, Sr, Ba, and K, and M5 is one or more of Zr, Ta, Nb, and Hf.

[0106] The general chemical formula of NAS ICON type solid electrolyte material is Li 1+m M6 m M7 2-m(PO4)3, where 0.01≤m≤0.5, M6 is one or more of Al, Y, Ga, Cr, In, Fe, Se, and La, and M7 is one or more of Ti, Ge, Ta, Zr, Sn, Fe, V, and Hf.

[0107] The general chemical formula for perovskite solid electrolyte materials is: Li 3n M8 2 / 3-n M9O3, where 0.01≤n≤0.5, M8 is one or more of La, Al, Mg, Fe, and Ta, and M9 is one or more of Ti, Nb, Sr, and Pr.

[0108] The general chemical formula of LISICON solid electrolyte material is: Li 14 N1(N2O4)4, wherein N1 is one or more of Zr, Cr, and Sn, and N2 is one or more of Si, S, and P.

[0109] High dielectric constant nanomaterials include one or more of the following: barium titanate, strontium titanate, barium aluminate, barium strontium titanate, titanium dioxide, and zirconium oxide; the relative dielectric constant ε of high dielectric constant nanomaterials is... r ≥20.

[0110] The second binder includes one or more of the following: polyvinylidene fluoride, polyvinyl alcohol, polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, and hydroxyethyl cellulose.

[0111] The mass ratio of the second solid electrolyte, the high dielectric constant nanomaterial, and the second binder is 50–90: 5–45: 1–10.

[0112] The second solvent includes one or more of N-methylpyrrolidone (NMP), deionized water, ethanol, and propanol.

[0113] The high-speed dispersion speed is 800 rpm to 3000 rpm, and can be any value within this range, such as 800 rpm, 1000 rpm, 1500 rpm, 2000 rpm, 2500 rpm, 3000 rpm, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable. The time is 0.5 hours to 5 hours.

[0114] The solid content of the second mixed slurry is 10wt% to 30wt%.

[0115] Step S4: The second mixed slurry is uniformly coated on the surface of the solid electrolyte coating. After baking to remove the second solvent, a mixed functional coating is formed on the surface of the solid electrolyte coating to obtain the composite membrane precursor.

[0116] The second mixed slurry is uniformly coated on the surface of the solid electrolyte coating with a coating thickness of about 3μm to 6μm; the final mixed functional coating has a thickness of 3μm to 6μm.

[0117] The baking temperature is 50℃~80℃, and the baking time is 1 hour~4 hours.

[0118] In step S4, the coating method is a conventional method, such as any one of the following: gravure coating, slot extrusion coating, spraying, or dip-coating.

[0119] Step S5: Prepare a polymer precursor solution by mixing polymerizable monomers, lithium salts and photoinitiators, and then add a third solid electrolyte into the polymer precursor solution and disperse it evenly to obtain a third mixed slurry.

[0120] The polymerizable monomers include one or more of the following: pentaerythritol tetraacrylate, polyethylene glycol diacrylate, methyl methacrylate, trimethylolpropane triacrylate, and trifluoroethyl acrylate.

[0121] Lithium salts include one or more of the following: lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium difluorooxalate borate, lithium hexafluorophosphate, and lithium tetrafluoroborate.

[0122] Initiators include one or more of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, 1-hydroxycyclohexylphenyl ketone, and 2-hydroxy-2-methyl-1-phenyl-1-propanone;

[0123] The third solid electrolyte includes one or more of the following: pyrochlore-type solid electrolyte, garnet-type solid electrolyte material, NAS ICON-type solid electrolyte material, perovskite-type solid electrolyte material, LIS ICON solid electrolyte material and their derivative materials; the first solid electrolyte, the second solid electrolyte and the third solid electrolyte are the same or different.

[0124] Specifically, the general chemical formula of pyrochlore-type solid electrolytes is Li. x La y M1 z M2 w M3 uO6F, M1 is a tetravalent cation, M2 is a pentavalent cation, and M3 is a hexavalent cation, and 1 < x + 3y < 5, 0 < x ≤ 2, 1 / 3 < y < 5 / 3, 0 ≤ z ≤ 2, 0 ≤ w ≤ 2, 0 ≤ u ≤ 2, z + w + u = 2, and at least one of M1, M2, and M3 exists, that is, one of z, w, and u is not 0; M1 can be one or more of Zr, Ti, Hf, Si, Ge, and Sn; M2 can be one or more of Nb, Sb, Bi, V, and Ta; M3 can be one or more of W, Cr, Mo, and Mn.

[0125] The general chemical formula for garnet-type solid electrolyte materials is Li7M43M52O. 12 M4 is one or more of La, Ca, Sr, Ba, and K, and M5 is one or more of Zr, Ta, Nb, and Hf.

[0126] The general chemical formula of NAS ICON type solid electrolyte material is Li 1+m M6 m M7 2-m (PO4)3, where 0.01≤m≤0.5, M6 is one or more of Al, Y, Ga, Cr, In, Fe, Se, and La, and M7 is one or more of Ti, Ge, Ta, Zr, Sn, Fe, V, and Hf.

[0127] The general chemical formula for perovskite solid electrolyte materials is: Li 3n M8 2 / 3-n M9O3, where 0.01≤n≤0.5, M8 is one or more of La, Al, Mg, Fe, and Ta, and M9 is one or more of Ti, Nb, Sr, and Pr.

[0128] The general chemical formula of LISICON solid electrolyte material is: Li 14 N1(N2O4)4, wherein N1 is one or more of Zr, Cr, and Sn, and N2 is one or more of Si, S, and P.

[0129] In the polymer precursor solution, the molar ratio of lithium salt to polymerizable monomer is 1:2 to 1:8.

[0130] In the polymer precursor solution, the mass ratio of initiator to polymerizable monomer is 0.5:100 to 3:100.

[0131] In the polymer precursor solution, the molar concentration of polymerizable monomers is 1 mol / L to 4 mol / L.

[0132] The mass ratio of the third solid electrolyte to the total solid mass in the polymer precursor solution is 10% to 40%.

[0133] Step S6: Immerse the composite membrane precursor in the third mixed slurry for vacuum impregnation, remove the vacuum-impregnated composite membrane precursor, and form a polymer composite coating after photocuring, finally obtaining a composite membrane with a functional gradient coating.

[0134] Specifically, vacuum impregnation involves immersing the sample in a vacuum condition of -0.05 MPa to -0.1 MPa for 10 to 60 minutes.

[0135] The photocuring process involves curing the vacuum-impregnated composite membrane precursor under ultraviolet light with a wavelength of 365 nm for 30 seconds to 5 minutes, resulting in a polymer composite coating with a thickness of 1 μm to 3 μm formed on both sides of the composite membrane precursor.

[0136] The composite separator with a functionally graded coating prepared by the method provided in this invention can be assembled with negative and positive electrodes to form a lithium battery. Lithium batteries include any one of the following: liquid lithium-ion batteries, semi-solid-state lithium batteries, all-solid-state lithium batteries, lithium-sulfur batteries, or lithium-air batteries.

[0137] The positive electrode includes any one of the following: lithium cobalt oxide positive electrode, ternary material positive electrode, lithium manganese oxide positive electrode, and lithium iron phosphate positive electrode; wherein the ternary material can be NCM523, NCM622, NCM811, etc. The positive electrode is prepared using conventional methods.

[0138] The negative electrode sheet includes any one of the following: lithium metal sheet, negative electrode sheet containing silicon-carbon negative electrode material, negative electrode sheet containing graphite, negative electrode sheet containing graphene, and negative electrode sheet containing transition metal. The negative electrode sheet is prepared using conventional methods.

[0139] To better understand the technical solution provided by the present invention, the following uses several specific examples to illustrate the preparation method and characteristics of the composite membrane with functionally graded coating of the present invention.

[0140] Example 1

[0141] This invention provides a method for preparing a composite membrane with a functionally graded coating, specifically including the following steps.

[0142] (1) Weigh out solid electrolyte Li with a particle size Dv50 of 0.8 μm at a mass ratio of 98:2. 1.25 La 0.58 Nb2O6F powder and polyvinylidene fluoride were dispersed in NMP and placed in a ball mill. The mixture was ball-milled at 800 rpm for 4 hours to obtain a first mixed slurry with a solid content of 35 wt%.

[0143] (2) Using a slit extrusion coating method, the first mixed slurry is uniformly coated on one side of a PE porous base film with a pore size of 200nm and a porosity of 40%, with a coating thickness of 2μm. The coated base film is initially dried at 60℃ for 15 minutes, and then transferred to an 80℃ vacuum drying oven for 6 hours to completely evaporate the solvent and form a solid electrolyte coating on one side of the porous base film.

[0144] (3) Weigh out solid electrolyte Li with a particle size Dv50 of 0.8 μm at a mass ratio of 65:30:5. 1.25 La 0.58 Nb2O6F powder, barium titanate nanoparticles and PVDF binder were placed together with solvent NMP in a high-speed disperser and subjected to high-speed dispersion and shearing stirring at 2000 rpm for 1 hour to obtain a second mixed slurry with a solid content of 15 wt%.

[0145] (4) Using a micro-gravure coating method, the second mixed slurry is uniformly coated on the surface of the solid electrolyte coating with a coating thickness of 3μm. After baking at 60℃ for 2 hours to remove NMP, a mixed functional coating is formed on the surface of the solid electrolyte coating to obtain the composite membrane precursor.

[0146] (5) Using pentaerythritol tetraacrylate as the polymerizable monomer and lithium bis(trifluoromethanesulfonyl)imide as the lithium salt, wherein the molar ratio of polymerizable monomer to lithium salt is 4:1, and 1 wt% of photoinitiator 2-hydroxy-2-methyl-1-phenyl-1-propanone is added, the mixture is thoroughly mixed to prepare a polymer precursor solution, wherein the molar concentration of polymerizable monomer in the polymer precursor solution is 2.5 mol / L; subsequently, solid electrolyte Li with a particle size Dv50 of 0.8 μm is added. 1.25 La 0.58 Nb₂O₆F powder was evenly dispersed in a polymer precursor solution to obtain a third mixed slurry, in which Li 1.25 La 0.58 The mass of Nb2O6F powder accounts for 20% of the total solid mass in the polymer precursor solution.

[0147] (6) The composite membrane precursor is immersed in the third mixed slurry and vacuum impregnated for 30 minutes under a vacuum of -0.08MPa. The vacuum impregnated composite membrane precursor is taken out and photocured under ultraviolet light of 365nm wavelength for 2 minutes to form a polymer composite coating. The thickness of the polymer composite coating is 1μm, and finally a composite membrane with a functional gradient coating is obtained.

[0148] Example 2

[0149] This invention provides a method for preparing a composite membrane with a functionally graded coating, specifically including the following steps.

[0150] (1) Weigh out solid electrolyte Li with a particle size Dv50 of 0.8 μm at a mass ratio of 95:5. 1.25 La 0.58 Ti 0.5 SbW 0.5 O6F powder and polyacrylonitrile were dispersed in NMP and placed in a ball mill. The mixture was ball-milled at 800 rpm for 4 hours to obtain a first mixed slurry with a solid content of 40 wt%.

[0151] (2) Using a slit extrusion coating method, the first mixed slurry is uniformly coated on one side of a PE porous base film with a pore size of 200nm and a porosity of 40%, with a coating thickness of 5μm. The coated base film is initially dried at 60℃ for 15 minutes, and then transferred to an 80℃ vacuum drying oven for 6 hours to completely evaporate the solvent and form a solid electrolyte coating on one side of the porous base film.

[0152] (3) Weigh out solid electrolyte Li with a particle size Dv50 of 0.8 μm at a mass ratio of 55:40:5. 1.25 La 0.58 Ti 0.5 SbW 0.5 O6F powder, barium titanate nanoparticles and PVDF binder were placed together with solvent NMP in a high-speed disperser and dispersed and sheared at 2000 rpm for 1 hour to obtain a second mixed slurry with a solid content of 10 wt%.

[0153] (4) Using a micro-gravure coating method, the second mixed slurry is uniformly coated on the surface of the solid electrolyte coating with a coating thickness of 5μm. After baking at 60℃ for 3 hours to remove NMP, a mixed functional coating is formed on the surface of the solid electrolyte coating to obtain the composite membrane precursor.

[0154] (5) Using polyethylene glycol diacrylate as the polymerizable monomer and lithium bis(fluorosulfonyl)imide as the lithium salt, wherein the molar ratio of polymerizable monomer to lithium salt is 4:1, and 1 wt% of photoinitiator 1-hydroxycyclohexylphenyl ketone is added, the mixture is thoroughly mixed to prepare a polymer precursor solution, wherein the molar concentration of polymerizable monomer in the polymer precursor solution is 3 mol / L; subsequently, solid electrolyte Li with a particle size Dv50 of 0.8 μm is added. 1.25 La 0.58 Ti 0.5 SbW 0.5 O6F powder was uniformly dispersed in a polymer precursor solution to obtain a third mixed slurry; wherein, Li 1.25 La 0.58 Ti 0.5 SbW 0.5 The O6F powder accounts for 40% of the total solid mass in the polymer precursor solution.

[0155] (6) The composite membrane precursor is immersed in the third mixed slurry and vacuum impregnated for 30 minutes under a vacuum of -0.08MPa. The vacuum impregnated composite membrane precursor is taken out and irradiated under ultraviolet light of 365nm wavelength for 2 minutes to form a polymer composite coating with a thickness of 1.5μm. Finally, a composite membrane with a functional gradient coating is obtained.

[0156] Example 3

[0157] This invention provides a method for preparing a composite membrane with a functionally graded coating, specifically including the following steps.

[0158] (1) Weigh out solid electrolyte Li with a particle size Dv50 of 0.8 μm at a mass ratio of 98:2. 1.25 La 1.25 Ti2O6F powder and polyvinylidene fluoride were dispersed in NMP and placed in a ball mill. The mixture was ball-milled at 800 rpm for 4 hours to obtain a first mixed slurry with a solid content of 35 wt%.

[0159] (2) Using a slit extrusion coating method, the first mixed slurry is uniformly coated on one side of a PE porous base film with a pore size of 200nm and a porosity of 40%, with a coating thickness of 2μm. The coated base film is initially dried at 60℃ for 15 minutes, and then transferred to an 80℃ vacuum drying oven for 6 hours to completely evaporate the solvent and form a solid electrolyte coating on one side of the porous base film.

[0160] (3) Weigh out solid electrolyte Li with a particle size Dv50 of 0.8 μm at a mass ratio of 70:25:5. 1.25 La 1.25 Ti2O6F powder, barium titanate nanoparticles and PVDF binder were placed together with solvent NMP in a high-speed disperser and subjected to high-speed dispersion and shearing stirring at 2000 rpm for 1 hour to obtain a second mixed slurry with a solid content of 15 wt%.

[0161] (4) Using a micro-gravure coating method, the second mixed slurry is uniformly coated on the surface of the solid electrolyte coating with a coating thickness of 3μm. After baking at 60℃ for 2 hours to remove NMP, a mixed functional coating is formed on the surface of the solid electrolyte coating to obtain the composite membrane precursor.

[0162] (5) Using pentaerythritol tetraacrylate as the polymerizable monomer and lithium bis(trifluoromethanesulfonyl)imide as the lithium salt, wherein the molar ratio of polymerizable monomer to lithium salt is 4:1, and 0.5 wt% of photoinitiator 2-hydroxy-2-methyl-1-phenyl-1-propanone is added, the mixture is thoroughly mixed to prepare a polymer precursor solution, wherein the molar concentration of polymerizable monomer in the polymer precursor solution is 2.5 mol / L; subsequently, solid electrolyte Li with a particle size Dv50 of 0.8 μm is added. 1.25 La 1.25 Ti2O6F powder was evenly dispersed in a polymer precursor solution to obtain a third mixed slurry, in which Li 1.25 La 1.25 The Ti2O6F powder accounts for 30% of the total solid mass in the polymer precursor solution.

[0163] (6) The composite membrane precursor is immersed in the third mixed slurry and vacuum impregnated for 30 minutes under a vacuum of -0.08MPa. The vacuum impregnated composite membrane precursor is taken out and photocured under ultraviolet light of 365nm wavelength for 2 minutes to form a polymer composite coating. The thickness of the polymer composite coating is 1μm, and finally a composite membrane with a functional gradient coating is obtained.

[0164] Example 4

[0165] This invention provides a method for preparing a composite membrane with a functionally graded coating, specifically including the following steps.

[0166] (1) Weigh out solid electrolyte Li with a particle size Dv50 of 0.8 μm at a mass ratio of 98:2. 1.2 La 0.6 TiWO6F powder and polyvinylidene fluoride were dispersed in NMP and placed in a ball mill. The mixture was ball-milled at 800 rpm for 4 hours to obtain a first mixed slurry with a solid content of 35 wt%.

[0167] (2) Using a slit extrusion coating method, the first mixed slurry is uniformly coated on both sides of a PE porous base film with a pore size of 200nm and a porosity of 40%, with a coating thickness of 2μm. The coated base film is initially dried at 60℃ for 15 minutes, and then transferred to an 80℃ vacuum drying oven for 6 hours to completely evaporate the solvent and form a solid electrolyte coating on one side of the porous base film.

[0168] (3) Weigh out solid electrolyte Li with a particle size Dv50 of 0.8 μm at a mass ratio of 65:30:5. 1.2 La 0.6TiWO6F powder, barium titanate nanoparticles and PVDF binder were placed together with solvent NMP in a high-speed disperser and subjected to high-speed dispersion and shearing stirring at 2000 rpm for 1 hour to obtain a second mixed slurry with a solid content of 15 wt%.

[0169] (4) Using a micro-gravure coating method, the second mixed slurry is uniformly coated on the surface of the solid electrolyte coating with a coating thickness of 3μm. After baking at 60℃ for 2 hours to remove NMP, a mixed functional coating is formed on the surface of the solid electrolyte coating to obtain the composite membrane precursor.

[0170] (5) Using pentaerythritol tetraacrylate as the polymerizable monomer and lithium bis(trifluoromethanesulfonyl)imide as the lithium salt, wherein the molar ratio of polymerizable monomer to lithium salt is 4:1, and 1 wt% of photoinitiator 2-hydroxy-2-methyl-1-phenyl-1-propanone is added, the mixture is thoroughly mixed to prepare a polymer precursor solution, wherein the molar concentration of polymerizable monomer in the polymer precursor solution is 2.5 mol / L; subsequently, solid electrolyte Li with a particle size Dv50 of 0.8 μm is added. 1.2 La 0.6 TiWO6F powder was evenly dispersed in a polymer precursor solution to obtain a third mixed slurry, in which Li 1.2 La 0.6 The TiWO6F powder accounts for 35% of the total solid mass in the polymer precursor solution.

[0171] (6) The composite membrane precursor is immersed in the third mixed slurry and vacuum impregnated for 30 minutes under a vacuum of -0.08MPa. The vacuum impregnated composite membrane precursor is taken out and photocured under ultraviolet light of 365nm wavelength for 2 minutes to form a polymer composite coating. The thickness of the polymer composite coating is 1μm, and finally a composite membrane with a functional gradient coating is obtained.

[0172] Example 5

[0173] This invention provides a method for preparing a composite membrane with a functionally graded coating, specifically including the following steps.

[0174] (1) Weigh out solid electrolyte Li with a particle size Dv50 of 2.3 μm at a mass ratio of 98:2. 1.3 AI 0.3 Ti 1.7 (PO4)3 powder and polyvinylidene fluoride were dispersed in NMP and placed in a ball mill. The mixture was ball-milled at 800 rpm for 4 hours to obtain a first mixed slurry with a solid content of 35 wt%.

[0175] (2) Using a slit extrusion coating method, the first mixed slurry is uniformly coated on one side of a PE porous base film with a pore size of 500nm and a porosity of 40%, with a coating thickness of 2μm. The coated base film is initially dried at 60℃ for 15 minutes, and then transferred to an 80℃ vacuum drying oven for 6 hours to completely evaporate the solvent and form a solid electrolyte coating on one side of the porous base film.

[0176] (3) Weigh out solid electrolyte Li with a particle size Dv50 of 0.8 μm at a mass ratio of 65:30:5. 1.3 AI 0.3 Ti 1.7 (PO4)3 powder, barium titanate nanoparticles and PVDF binder were placed together with solvent NMP in a high-speed disperser and subjected to high-speed dispersion and shearing stirring at 2000 rpm for 1 hour to obtain a second mixed slurry with a solid content of 15 wt%.

[0177] (4) Using a micro-gravure coating method, the second mixed slurry is uniformly coated on the surface of the solid electrolyte coating with a coating thickness of 3μm. After baking at 60℃ for 2 hours to remove NMP, a mixed functional coating is formed on the surface of the solid electrolyte coating to obtain the composite membrane precursor.

[0178] (5) Using pentaerythritol tetraacrylate as the polymerizable monomer and lithium bis(trifluoromethanesulfonyl)imide as the lithium salt, wherein the molar ratio of polymerizable monomer to lithium salt is 4:1, and 0.5 wt% of photoinitiator 2-hydroxy-2-methyl-1-phenyl-1-propanone is added, the mixture is thoroughly mixed to prepare a polymer precursor solution, wherein the molar concentration of polymerizable monomer in the polymer precursor solution is 2.5 mol / L; subsequently, solid electrolyte Li with a particle size Dv50 of 0.8 μm is added. 1.3 AI 0.3 Ti 1.7 (PO4)3 powder was evenly dispersed in a polymer precursor solution to obtain a third mixed slurry, in which Li 1.3 AI 0.3 Ti 1.7 The (PO4)3 powder accounts for 20% of the total solid mass in the polymer precursor solution.

[0179] (6) The composite membrane precursor is immersed in the third mixed slurry and vacuum impregnated for 30 minutes under a vacuum of -0.08MPa. The vacuum impregnated composite membrane precursor is taken out and photocured under ultraviolet light of 365nm wavelength for 2 minutes to form a polymer composite coating. The thickness of the polymer composite coating is 1μm, and finally a composite membrane with a functional gradient coating is obtained.

[0180] To better illustrate the effects of the embodiments of the present invention, a comparative example is provided to be made with the embodiments described above.

[0181] Comparative Example 1

[0182] This comparative example provides a conventional ceramic membrane preparation process, which differs from Example 1 in that the membrane does not have an oxyfluoride solid electrolyte coating, a hybrid functional coating, or a polymer composite coating added. The specific preparation process is as follows.

[0183] (1) Weigh alumina powder and PVDF binder at a mass ratio of 95:5, disperse them in NMP solvent, and obtain a ceramic slurry with a solid content of 40wt%.

[0184] (2) Using a micro-gravure coating method, ceramic slurry is uniformly coated on one side of the same multi-film PE base film as in Example 1, and dried at 80°C for 4 hours to obtain a traditional ceramic diaphragm.

[0185] Comparative Example 2

[0186] This comparative example provides a method for preparing a composite membrane. Unlike Example 1, no solid electrolyte is added to the hybrid functional coating, and there is no polymer composite coating. The specific preparation process is as follows.

[0187] (1) Weigh the solid electrolyte Li according to a mass ratio of 98:2. 1.25 La 0.58 Nb2O6F powder and polyvinylidene fluoride were dispersed in NMP and placed in a ball mill. The mixture was ball-milled at 800 rpm for 4 hours to obtain a first mixed slurry with a solid content of 35 wt%.

[0188] (2) Using a slit extrusion coating method, the first mixed slurry is uniformly coated on one side of a PE porous base film with a pore size of 200nm and a porosity of 40%, with a coating thickness of 2μm. The coated base film is initially dried at 60℃ for 15 minutes, and then transferred to an 80℃ vacuum drying oven for 6 hours to completely evaporate the solvent and form a solid electrolyte coating on one side of the porous base film.

[0189] (3) Weigh barium titanate nanoparticles with a particle size of 100 nm and PVDF binder at a mass ratio of 95:5, then disperse them in NMP solvent, and stir at a high speed of 2000 rpm for 1 hour to obtain a second mixed slurry with a solid content of 15 wt%.

[0190] (4) Using a micro-gravure coating method, the second mixed slurry is uniformly coated on the surface of the solid electrolyte coating prepared in step S2, and dried at 60°C for 2 hours to obtain a composite membrane.

[0191] Comparative Example 3

[0192] This comparative example provides a method for preparing a composite membrane. Unlike Example 1, there is no mixed functional coating and no solid electrolyte is added to the polymer composite coating. The specific preparation process is as follows.

[0193] (1) Weigh out solid electrolyte Li with a particle size Dv50 of 0.8 μm at a mass ratio of 98:2. 1.25 La 0.58 Nb2O6F powder and polyvinylidene fluoride were dispersed in NMP and placed in a ball mill. The mixture was ball-milled at 800 rpm for 4 hours to obtain a first mixed slurry with a solid content of 35 wt%.

[0194] (2) Using a slit extrusion coating method, the first mixed slurry is uniformly coated on one side of a PE porous base membrane with a pore size of 200 nm and a porosity of 40%, with a coating thickness of 2 μm. The coated base membrane is initially dried at 60°C for 15 minutes, and then transferred to an 80°C vacuum drying oven for 6 hours to completely evaporate the solvent, forming a solid electrolyte coating on one side of the porous base membrane, thus obtaining the precursor membrane.

[0195] (3) Using pentaerythritol tetraacrylate as the polymerizable monomer and lithium bis(trifluoromethanesulfonylimide) as the lithium salt, the molar ratio of the polymerizable monomer to the lithium salt is 4:1, and 1 wt% of the photoinitiator 2-hydroxy-2-methyl-1-phenyl-1-propanone is added, and the mixture is mixed evenly to prepare a polymer precursor solution, wherein the molar concentration of the polymerizable monomer in the polymer precursor solution is 2.5 mol / L.

[0196] (4) The composite membrane precursor is immersed in the polymer precursor solution and vacuum impregnated for 30 minutes under a vacuum of -0.08MPa. The vacuum impregnated composite membrane precursor is taken out and photocured under ultraviolet light of 365nm wavelength for 2 minutes to form a polymer composite coating with a thickness of 1μm, thus obtaining the composite membrane.

[0197] Comparative Example 4

[0198] This comparative example provides a method for preparing a composite diaphragm, which involves coating the surface of a conventional ceramic diaphragm in Comparative Example 1 with a polymer coating. The specific preparation process is as follows.

[0199] (1) Using pentaerythritol tetraacrylate as the polymerizable monomer and lithium bis(trifluoromethanesulfonylimide) as the lithium salt, the molar ratio of the polymerizable monomer to the lithium salt is 4:1, and 1 wt% of the photoinitiator 2-hydroxy-2-methyl-1-phenyl-1-propanone is added, and the mixture is mixed evenly to prepare a polymer precursor solution, wherein the molar concentration of the polymerizable monomer in the polymer precursor solution is 2.5 mol / L.

[0200] (2) The conventional ceramic membrane prepared in Comparative Example 1 was immersed in a polymer precursor solution on the surface of the conventional ceramic membrane prepared in Comparative Example 1. It was vacuum impregnated for 30 minutes under a vacuum of -0.08MPa. The composite membrane precursor after vacuum impregnation was taken out and photocured under ultraviolet light of 365nm wavelength for 2 minutes to form a polymer composite coating with a thickness of 1μm, thus obtaining a composite membrane.

[0201] The composite membranes of Examples 1-5 and the membranes of Comparative Examples 1-4 were subjected to performance tests. The specific test items and test methods are as follows.

[0202] Test Item 1: Ionic conductivity was tested using electrochemical impedance spectroscopy (EIS) on an electrochemical workstation. Specifically, the composite membranes from Examples 1-5 and Comparative Examples 1-4 were cut into circular pieces and sandwiched between two stainless steel (SS) inert electrodes, then assembled into a battery for testing. To ensure the accuracy of the test, the test battery was placed in a temperature-controlled chamber.

[0203] In EIS testing, the frequency range is set from 0.01Hz to 1MHz, and the amplitude voltage is set to 10mV in order to accurately measure the resistance of the electrolyte.

[0204] Next, by analyzing the Nyquist impedance spectrum, the ionic conductivity of the electrolyte can be calculated using the following formula: In the formula, d represents the thickness of the electrolyte (≈20 μm ± 2), R is the volume resistance (impedance value) of the electrolyte read from the Nyquist impedance plot of EIS, and S represents the effective contact area between the electrolyte and the stainless steel inert electrode. To ensure the accuracy of the measurement, when testing the ionic conductivity at different temperatures, the constant temperature chamber needs to be set to the target temperature and maintained for half an hour to allow the test cell to reach thermal equilibrium, ensuring the stability of the test environment.

[0205] Test item 2: Surface resistance = impedance value R × area S.

[0206] Test Method 3: Thermal stability test. The electrolyte was placed in an oven with an increasing temperature gradient. Each temperature range was maintained for 10 minutes, and the critical temperature for diaphragm volume shrinkage was recorded. The test temperature ranges included: 50℃, 80℃, 100℃, 120℃, 140℃, 160℃, 180℃, 200℃, 220℃, 240℃, 260℃, 280℃, and 300℃.

[0207] Test Method 4: Test capacity retention rate after 100 cycles:

[0208] Preparation of positive electrode sheet for button cell: The positive electrode active material NCM811, oxyfluoride solid electrolyte and conductive agent (such as SP) are mixed evenly in a mass ratio of 70:25:5 using a ball mill. Then, an appropriate amount of binder (such as PVDF) is added to make a slurry, which is coated on an aluminum foil current collector and dried to obtain a composite positive electrode sheet.

[0209] Assembling coin cells: The composite separators of Examples 1-5 and Comparative Examples 1-4 are assembled into coin cells with composite positive electrode sheets and lithium negative electrodes. The assembly sequence is as follows: negative electrode shell, gasket, lithium negative electrode, composite separator or separator, composite positive electrode sheet, spring sheet, positive electrode shell. Finally, a sealing machine is used to seal the cells under a pressure of 0.75 tons to assemble them into coin cells.

[0210] The assembled button cells were tested: charged to 4.25V at a rate of 0.5C, and then discharged to 2.75V at a rate of 0.5C after charging. The charging and discharging process constitutes one cycle. The capacity retention rate was tested after 100 cycles of 0.5C, 2.75 to 4.25V. The cycle performance data of the battery are shown in Table 1.

[0211] Table 1 summarizes the test data for Examples 1-5 and Comparative Examples 1-4.

[0212]

[0213] Table 1

[0214] The comparison of test data in Table 1 shows that the ionic conductivity of Examples 1-5 is higher than that of Comparative Examples 1-4, the sheet resistance of Examples 1-5 is lower than that of Comparative Examples 1-4, the thermal shrinkage critical temperature of Examples 1-5 is higher than that of Comparative Examples 1-4, and the cycle capacity retention rate of Examples 1-5 after 100 cycles is higher than that of Comparative Examples 1-4. This indicates that the composite separators with functionally graded coatings prepared in Examples 1-5 of the present invention can significantly improve the thermal shrinkage temperature of the separator, increase the ionic conductivity, and reduce the sheet resistance of the separator interface through the synergistic effect of different coatings, thereby improving the cycle performance of the battery.

[0215] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A composite membrane with a functionally graded coating, characterized in that, The composite membrane includes: a porous base membrane, a solid electrolyte coating attached to one or both sides of the porous base membrane, a hybrid functional coating attached to the surface of the solid electrolyte coating, and an outermost polymer composite coating. The solid electrolyte coating comprises: a first solid electrolyte and a first binder; the mass ratio of the first solid electrolyte to the first binder is 95–99.5:0.5–5; the hybrid functional coating comprises: a second solid electrolyte, a high dielectric constant nanomaterial, and a second binder; the mass ratio of the second solid electrolyte, the high dielectric constant nanomaterial, and the second binder is 50–90:5–45:1–10; the high dielectric constant nanomaterial comprises one or more of barium titanate, strontium titanate, barium aluminate, barium strontium titanate, titanium dioxide, and zirconium oxide; the relative dielectric constant ε of the high dielectric constant nanomaterial is... ≥20; The polymer composite coating comprises: a polymer with an in-situ polymerized three-dimensional network structure, and a lithium salt and a third solid electrolyte filling the voids in the three-dimensional network structure; the polymer is formed by in-situ polymerization of polymerizable monomers initiated by a photoinitiator; the polymerizable monomers, the lithium salt, and the photoinitiator are formulated to obtain a polymer precursor solution; the mass ratio of the third solid electrolyte to the total solid mass in the polymer precursor solution is 10% to 40%. The first solid electrolyte, the second solid electrolyte, and the third solid electrolyte are the same.

2. The composite diaphragm according to claim 1, characterized in that, The porous base membrane is a porous base membrane formed from polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinyl alcohol (PVA), polymethyl methacrylate (PMMA), or polyethylene terephthalate (PET), or a bilayer or trilayer composite base membrane formed from a combination of the above materials, or a ceramic diaphragm or a fiber diaphragm; the pore size of the porous base membrane is between 100 nm and 1 μm. The porosity of the porous base membrane is 20% to 50%; the thickness of the porous base membrane is 9 μm to 16 μm. The thickness of the solid electrolyte coating is 2μm to 5μm; The thickness of the hybrid functional coating is 3μm to 6μm; The thickness of the polymer composite coating is 1 μm to 3 μm.

3. The composite diaphragm according to claim 1, characterized in that, The first solid electrolyte, the second solid electrolyte, and the third solid electrolyte each include one or more of the following: pyrochlore-type solid electrolyte, garnet-type solid electrolyte material, NASICON-type solid electrolyte material, perovskite-type solid electrolyte material, LISICON solid electrolyte material, and their derivatives; Both the first adhesive and the second adhesive comprise one or more of the following: polyvinylidene fluoride, polyvinyl alcohol, polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, and hydroxyethyl cellulose; The polymerizable monomers include one or more of pentaerythritol tetraacrylate, polyethylene glycol diacrylate, methyl methacrylate, trimethylolpropane triacrylate, and trifluoroethyl acrylate. The lithium salt includes one or more of the following: lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium difluorooxalate borate, lithium hexafluorophosphate, and lithium tetrafluoroborate. The photoinitiator includes one or more of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, 1-hydroxycyclohexylphenyl ketone, and 2-hydroxy-2-methyl-1-phenyl-1-propanone.

4. The composite diaphragm according to claim 3, characterized in that, The pyrochlore-type solid electrolyte has the general chemical formula Li. x La y M1 z M2 w M3 u O6F, M1 is a tetravalent cation, M2 is a pentavalent cation, M3 is a hexavalent cation, and 1 < x + 3y < 5, 0 < x ≤ 2, 1 / 3 < y < 5 / 3, 0 ≤ z ≤ 2, 0 ≤ w ≤ 2, 0 ≤ u ≤ 2, z + w + u = 2, and at least one of M1, M2, and M3 exists, that is, one of z, w, and u is not 0; The chemical formula of the garnet-type solid electrolyte material is Li7M43M52O. 12 M4 is one or more of La, Ca, Sr, Ba, and K, and M5 is one or more of Zr, Ta, Nb, and Hf. The general chemical formula of the NASICON-type solid electrolyte material is Li. 1+m M6 m M7 2-m (PO4)3, where 0.01≤m≤0.5, M6 is one or more of Al, Y, Ga, Cr, In, Fe, Se, and La, and M7 is one or more of Ti, Ge, Ta, Zr, Sn, Fe, V, and Hf; The general chemical formula of the perovskite-type solid electrolyte material is: Li 3n M8 2 / 3-n M9O3, where n is between 0.01 and 0.5, M8 is one or more of La, Al, Mg, Fe, and Ta, and M9 is one or more of Ti, Nb, Sr, and Pr; The chemical formula of the LISICON solid electrolyte material is: Li 14 N1(N2O4)4, wherein N1 is one or more of Zr, Cr, and Sn, and N2 is one or more of Si, S, and P.

5. A method for preparing a composite separator according to any one of claims 1-4, characterized in that, The preparation method includes: Step S1: Disperse the first solid electrolyte and the first binder in the first solvent and ball mill them together to obtain the first mixed slurry; Step S2: The first mixed slurry is uniformly coated on at least one surface of the porous base membrane, and after preliminary drying and vacuum drying to remove the first solvent, a solid electrolyte coating is formed on at least one surface of the porous base membrane. Step S3: The second solid electrolyte, high dielectric constant nanomaterial, and second binder are dispersed at high speed in the second solvent to obtain the second mixed slurry; Step S4: The second mixed slurry is uniformly coated on the surface of the solid electrolyte coating. After baking to remove the second solvent, a mixed functional coating is formed on the surface of the solid electrolyte coating to obtain a composite membrane precursor. Step S5: A polymer precursor solution is prepared by mixing polymerizable monomers, lithium salts and photoinitiators. Then, a third solid electrolyte is added to the polymer precursor solution and dispersed evenly to obtain a third mixed slurry. Step S6: Immerse the composite membrane precursor in the third mixed slurry for vacuum impregnation, remove the vacuum-impregnated composite membrane precursor, and form a polymer composite coating after photocuring to finally obtain a composite membrane with a functional gradient coating.

6. The preparation method according to claim 5, characterized in that, The first solid electrolyte, the second solid electrolyte, and the third solid electrolyte each comprise one or more of the following: pyrochlore-type solid electrolyte, garnet-type solid electrolyte material, NASICON-type solid electrolyte material, perovskite-type solid electrolyte material, LISICON solid electrolyte material, and their derivatives; the first binder and the second binder each comprise one or more of the following: polyvinylidene fluoride, polyvinyl alcohol, polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, and hydroxyethyl cellulose; The high dielectric constant nanomaterial includes one or more of barium titanate, strontium titanate, barium aluminate, barium strontium titanate, titanium dioxide, and zirconium oxide; the relative dielectric constant ε of the high dielectric constant nanomaterial is... ≥20; The polymerizable monomers include one or more of pentaerythritol tetraacrylate, polyethylene glycol diacrylate, methyl methacrylate, trimethylolpropane triacrylate, and trifluoroethyl acrylate. The lithium salt includes one or more of the following: lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium difluorooxalate borate, lithium hexafluorophosphate, and lithium tetrafluoroborate. The initiator includes one or more of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, 1-hydroxycyclohexylphenyl ketone, and 2-hydroxy-2-methyl-1-phenyl-1-propanone. The first solvent and the second solvent include one or more of N-methylpyrrolidone (NMP), deionized water, ethanol, and propanol; In the polymer precursor solution, the molar ratio of the lithium salt to the polymerizable monomer is 1:2 to 1:8, and the mass ratio of the initiator to the polymerizable monomer is 0.5:100 to 3:100; the molar concentration of the polymerizable monomer in the polymer precursor solution is 1 mol / L to 4 mol / L.

7. The preparation method according to claim 6, characterized in that, The pyrochlore-type solid electrolyte has the general chemical formula Li. x La y M1 z M2 w M3 u O6F, M1 is a tetravalent cation, M2 is a pentavalent cation, M3 is a hexavalent cation, and 1 < x + 3y < 5, 0 < x ≤ 2, 1 / 3 < y < 5 / 3, 0 ≤ z ≤ 2, 0 ≤ w ≤ 2, 0 ≤ u ≤ 2, z + w + u = 2, and at least one of M1, M2, and M3 exists, that is, one of z, w, and u is not 0; The chemical formula of the garnet-type solid electrolyte material is Li7M43M52O. 12 M4 is one or more of La, Ca, Sr, Ba, and K, and M5 is one or more of Zr, Ta, Nb, and Hf. The general chemical formula of the NASICON-type solid electrolyte material is Li. 1+m M6 m M7 2-m (PO4)3, where 0.01≤m≤0.5, M6 is one or more of Al, Y, Ga, Cr, In, Fe, Se, and La, and M7 is one or more of Ti, Ge, Ta, Zr, Sn, Fe, V, and Hf; The general chemical formula of the perovskite-type solid electrolyte material is: Li 3n M8 2 / 3-n M9O3, where n is between 0.01 and 0.5, M8 is one or more of La, Al, Mg, Fe, and Ta, and M9 is one or more of Ti, Nb, Sr, and Pr; The chemical formula of the LISICON solid electrolyte material is: Li 14 N1(N2O4)4, wherein N1 is one or more of Zr, Cr, and Sn, and N2 is one or more of Si, S, and P.

8. The preparation method according to claim 5, characterized in that, In step S1, the ball milling speed is 800 rpm to 2000 rpm, and the ball milling time is 0.5 hours to 5 hours; the solid content of the first mixed slurry is 20 wt% to 50 wt%. In step S2, the preliminary drying is specifically performed at 40℃ to 60℃ for 5 to 30 minutes; the vacuum drying is specifically performed at 60℃ to 100℃ for 2 to 12 hours. In step S3, the high-speed dispersion speed is 800 rpm to 3000 rpm, and the time is 0.5 hours to 5 hours; the solid content of the second mixed slurry is 10 wt% to 30 wt%.

9. The preparation method according to claim 5, characterized in that, In step S4, the baking temperature is 50℃~80℃, and the baking time is 1 hour~4 hours. Step S6, the vacuum impregnation specifically involves impregnating the composite membrane precursor under a vacuum condition of -0.05MPa to -0.1MPa for 10 to 60 minutes; the photocuring specifically involves curing the vacuum-impregnated composite membrane precursor under ultraviolet light with a wavelength of 365nm for 30 seconds to 5 minutes.

10. A lithium battery, characterized in that, The lithium battery includes the composite separator according to any one of claims 1-4, or the composite separator prepared by any one of claims 5-9.

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