Composite diaphragm and lithium ion battery

Abstract

The invention relates to a composite diaphragm and a lithium ion battery. The composite separator includes: a base film; the ceramic coating is located on at least one side of the base membrane, and the ceramic coating comprises a plurality of ceramic particles; the PTFE porous membrane is positioned on one side, far away from the base membrane, of the ceramic coating; wherein the aperture of the PTFE porous membrane is larger than the particle size of the ceramic particles, and at least part of the ceramic particles are embedded into the PTFE porous membrane. According to the composite diaphragm disclosed by the invention, a part of ceramic particles are embedded into the PTFE porous membrane, and the pore diameter of PTFE is controlled to be greater than the particle diameter of the ceramic particles, so that the comprehensive performance of the diaphragm is remarkably improved: the ceramic particles and the PTFE membrane form a mechanical interlocking structure, the shrinkage behavior of the diaphragm at a high temperature is effectively inhibited, and the dimensional stability and the thermal safety are enhanced; the introduction of the ceramic particles improves the lyophilic property of the originally hydrophobic PTFE membrane surface, thereby significantly improving the wetting speed and retention ability of the electrolyte and reducing the internal resistance of the battery. And through the synergistic effect of the two components, the wettability of the electrolyte can be improved while the thermal shrinkage is improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a composite separator and a lithium-ion battery. Background Technology

[0002] In high-energy-density lithium-ion batteries, the separator, as a key component ensuring safety, not only needs to possess good ion conductivity but also needs to maintain dimensional stability at high temperatures to prevent short circuits between the positive and negative electrodes due to thermal shrinkage. To this end, existing technologies often employ composite high-temperature resistant materials on the surface of polyolefin-based films to improve overall thermal stability. However, as battery systems evolve towards high-nickel cathodes and silicon-carbon anodes, the cathode side faces challenges such as high-voltage oxidation, lattice oxygen release, and free radical attack, while the anode side presents challenges such as strong reducing properties, lithium dendrite growth, and interfacial side reactions. Traditional composite separator structures face new interfacial compatibility challenges.

[0003] Furthermore, to improve the thermal stability of the separator, existing technologies typically employ a strategy of coating the polyolefin-based membrane with a ceramic layer or compounding it with other high-temperature resistant polymer layers (such as polytetrafluoroethylene, PTFE). PTFE possesses excellent thermal stability and chemical inertness, making it a preferred material for the high-temperature layer of the base membrane. However, in the highly reducing environment of the negative electrode side (especially at lithium metal or deeply lithiated graphite / silicon-carbon interfaces), it may undergo interfacial side reactions, including electron injection-induced defluorination reactions, the generation of HF-like corrosive products, or impact on SEI membrane stability, leading to gas generation, increased impedance, and decreased cycle life.

[0004] Therefore, existing composite membranes with symmetrical structures (i.e., the positive and negative electrode sides use the same material and stacking order) cannot simultaneously meet the different interface environment requirements of the positive and negative electrode sides.

[0005] Therefore, there is an urgent need for a new type of diaphragm structure design that can overcome the limitations of symmetry and adapt to the functional differences between the positive and negative electrodes. Summary of the Invention

[0006] The technical problem to be solved by the embodiments of the present invention is to provide a composite separator and a lithium-ion battery that can overcome the symmetry limitation and adapt to the functional differences between the positive and negative electrodes.

[0007] To address the aforementioned technical problems, the present invention provides a composite separator, comprising a base membrane having a positive electrode side and a negative electrode side opposite to each other along its thickness direction; the positive electrode side having a first ceramic coating and a first PTFE porous membrane layer stacked along a first direction; the negative electrode side having a second ceramic coating and a second PTFE porous membrane layer stacked along the first direction; and the side of the second PTFE porous membrane layer away from the positive electrode side is not the outermost side of the negative electrode side; wherein, the first direction is parallel to the thickness direction of the base membrane.

[0008] In one feasible implementation, the first direction is from the negative electrode side to the positive electrode side.

[0009] In one feasible implementation, the first direction is from the positive electrode side to the negative electrode side; the second PTFE porous membrane layer is further provided with a protective layer on the side away from the positive electrode side.

[0010] In one feasible implementation, the protective layer is a single-layer structure made of ceramic material, polymer material, or a mixture of ceramic and polymer.

[0011] In one feasible embodiment, the ceramic material is selected from at least one of alumina, boehmite, silicon dioxide, zirconium oxide, boron nitride titanium oxide, magnesium oxide, lithium phosphate, and lithium aluminate.

[0012] In one feasible embodiment, the polymer material is selected from at least one of polyimide, aramid, polyacrylonitrile, polyvinylidene fluoride-hexafluoropropylene copolymer, polyetheretherketone, polyphenylene sulfide, polytetrafluoroethylene, polybenzimidazole, and polyethylene oxide.

[0013] In one feasible implementation, the first ceramic coating comprises a plurality of ceramic particles, and the second ceramic coating comprises a plurality of ceramic particles.

[0014] In one feasible implementation, the ceramic particles comprise one or a mixture of more than one of oxide ceramics, nitride ceramics, and carbide ceramics.

[0015] In one feasible implementation, the thickness of the first ceramic coating and the second ceramic coating is 0.5 micrometers to 6 micrometers.

[0016] In one feasible implementation, the thickness of the first PTFE porous membrane layer and the second PTFE porous membrane layer is 10 micrometers to 35 micrometers.

[0017] In one feasible embodiment, the first ceramic coating and the second ceramic coating further include a first adhesive, the first adhesive comprising one or more of water-based adhesives and oil-based adhesives.

[0018] In one feasible embodiment, the composite membrane further includes an adhesive layer located between the first PTFE porous membrane layer and / or the second PTFE porous membrane layer and the base membrane, for bonding the first PTFE porous membrane layer and / or the second PTFE porous membrane layer to the base membrane.

[0019] In one feasible embodiment, the adhesive layer includes a second adhesive, which includes one or more of water-based adhesives and oil-based adhesives.

[0020] Accordingly, this application also provides a lithium-ion battery, including a positive electrode, a negative electrode and the aforementioned composite separator, wherein the composite separator is located between the positive electrode and the negative electrode.

[0021] The composite separator provided by this invention features a ceramic coating and a PTFE porous membrane arranged in a consistent order on both sides of a base membrane, representing a heterogeneous layout on the positive and negative electrode sides of the base membrane. On the positive electrode side, PTFE can be directly or indirectly exposed to a high-voltage environment, and its chemical inertness helps resist oxygen release and free radical attack from the positive electrode. Simultaneously, the ceramic coating further enhances thermal stability and oxygen adsorption capacity. On the negative electrode side, due to the interfacial instability between PTFE and lithium, the PTFE porous membrane layer on the negative electrode side is limited to the outermost side, meaning it does not directly contact the negative electrode. This avoids side reactions caused by contact between the negative electrode and PTFE, optimizes the separator layout, and improves battery performance. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the composite diaphragm according to the first embodiment of the present invention; Figure 2 This is a schematic diagram of the composite diaphragm according to the second embodiment of the present invention.

[0023] The reference numerals in the figure: 10-Composite membrane, 11-Base membrane, 12-First PTFE porous membrane layer, 13-First ceramic coating, 14-Second ceramic coating, 15-Second PTFE porous membrane layer; 20-Composite membrane, 21-Base membrane, 231-First ceramic coating, 221-First PTFE porous membrane layer, 232-Second ceramic coating, 222-Second PTFE porous membrane layer, 24-Adhesive layer, 25-Protective layer. Detailed Implementation

[0024] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0026] The ordinal numbers such as "first" and "second" used herein are primarily intended to distinguish multiple components with the same or similar names (e.g., "first ceramic coating" and "second ceramic coating") to facilitate the description of the technical solution of the present invention. Unless the context explicitly indicates otherwise, these ordinal numbers do not imply any order, importance, positional relationship, or material difference. For example, "first ceramic coating" and "second ceramic coating" may have exactly the same composition and thickness, or they may be different; they are used only to refer to two coating entities located on different sides of the base film (positive electrode side and negative electrode side).

[0027] Please see Figure 1 The composite separator 10 of the first embodiment of the present invention includes a base membrane 11, a first ceramic coating 13, a second ceramic coating 14, a first PTFE porous membrane layer 12, and a second PTFE porous membrane layer 15. The base membrane 11 has a positive electrode side and a negative electrode side along its thickness direction. The positive electrode side has the first ceramic coating 13 and the first PTFE porous membrane layer 12 stacked along a first direction +Z; the negative electrode side has the second ceramic coating 14 and the second PTFE porous membrane layer 15 stacked along the first direction +Z. The side of the second PTFE porous membrane layer 15 furthest from the positive electrode side is not the outermost side of the negative electrode side. The first direction +Z is parallel to the thickness direction of the base membrane 11.

[0028] Wherein, the first direction +Z is from the negative electrode side to the positive electrode side. That is, on the negative electrode side of the base film 11, the second ceramic coating 14 is the outermost layer, and the second PTFE porous film layer 15 is located between the second ceramic coating 14 and the base film 11. Through layer sequence control, the interface protection of the second PTFE porous film layer 15 is achieved, avoiding its direct exposure to the strongly reducing negative electrode environment, thereby suppressing side reactions and improving battery interface stability and long-term cycle performance. On the positive electrode side, the first PTFE porous film layer 12 is outside the first ceramic coating 13, which can give full play to the excellent oxidation resistance and high-temperature dimensional stability of PTFE, allowing it to directly cope with the high voltage and strong oxidation environment of the positive electrode side, thereby improving the safety and cycle stability of the battery under high temperature and high voltage conditions. If the first ceramic coating 13 is located on the outer layer, although it can adsorb some active oxygen, it does not have the ability to resist free radicals, and long-term exposure may lead to deactivation due to surface reactions. After placing the first PTFE porous membrane layer on the outer layer, the first ceramic coating 13 can serve as an intermediate support layer, mainly playing the roles of enhancing thermal conductivity, buffering stress, and adsorbing residual oxygen, thereby extending its functional life.

[0029] Prior to this invention, while those skilled in the art recognized the excellent thermal stability and chemical inertness of PTFE materials and attempted to apply them to composite separators, they were typically symmetrically positioned on both sides of the base film due to technical biases aimed at simplifying processes and ensuring symmetry. This symmetrical design approach overlooked the fact that PTFE exhibits different interfacial stability under drastically different positive and negative electrode electrochemical environments: on the high-voltage positive electrode side, its inherent strong oxidation resistance is fully utilized; however, on the relatively reducing negative electrode side, there is a potential risk of interfacial side reactions with active lithium species. The applicant recognized that there is a fundamental contradiction between this duality of PTFE materials and the heterogeneity of the battery's two electrode environments, and that traditional symmetrical structures cannot resolve this contradiction. Based on this, this invention creatively proposes an asymmetrical layout that precisely matches the layer sequence of PTFE with the requirements of the two electrode interfaces: on the positive electrode side, it is directly or indirectly exposed to maximize its oxidation resistance; on the negative electrode side, the layer sequence is adjusted to avoid contact between PTFE and the negative electrode, thereby extending battery life.

[0030] The composite separator 10 provided by this invention has a first ceramic coating 13, a first PTFE porous membrane layer 12, a second ceramic coating 14, and a second PTFE porous membrane layer 15 arranged in a consistent order on both sides of the base membrane 11, which is a heterogeneous layout on the positive and negative electrode sides of the base membrane 11. On the positive electrode side, the first PTFE porous membrane layer can be directly or indirectly exposed to a high-voltage environment, and its chemical inertness helps to resist oxygen release and free radical attack from the positive electrode. At the same time, the first ceramic coating 13 can further enhance thermal stability and oxygen adsorption capacity. On the negative electrode side, due to the interfacial instability between PTFE and lithium, the second PTFE porous membrane layer 15 on the negative electrode side is limited to the outermost side of the non-negative electrode side. That is, the second PTFE porous membrane layer 15 on the negative electrode side does not directly contact the negative electrode, avoiding side reactions caused by the negative electrode contacting PTFE, and optimizing the separator layout to improve battery performance.

[0031] The base membrane 11 can be selected from polyolefin (such as PE, PP, PE / PP, PE / PP / PE, PP / PE / PP) membranes, polyester (PET) membranes, polyimide (PI) membranes, aramid membranes, non-woven base membranes, or composite membranes of the above materials. Using these commercially viable and technologically mature membranes as the base membrane 11 not only ensures that the separator possesses good mechanical strength, electrochemical stability, and inherent thermal shutdown function, but also makes the composite separator 10 easily integrated into the existing battery manufacturing industry chain, possessing excellent mass production feasibility and cost-effectiveness.

[0032] It is understood that the present invention does not impose any restrictions on the material of the base film 11 layer. Any film that can be used as a lithium battery separator substrate, including known base films 11 modified by various physical or chemical methods, falls within the protection scope of the present invention.

[0033] Based on the aforementioned embodiments, the first ceramic coating 13 comprises a plurality of ceramic particles, and the second ceramic coating 14 comprises a plurality of ceramic particles; the ceramic particles include one or a mixture of multiple types of oxide ceramics, nitride ceramics, and carbide ceramics. Introducing these highly heat-resistant ceramic particles can greatly enhance the thermal insulation and thermal dimensional stability of the coating, making the composite separator 10 less prone to melting and shrinkage at high temperatures, thereby effectively preventing battery thermal runaway; if ceramic-based solid electrolyte particles are added, the ionic conductivity can also be enhanced to a certain extent and lithium dendrite puncture can be suppressed, further improving the battery's safety and rate performance.

[0034] It is understood that a wide variety of ceramic particles can be used in this invention. For example, oxide ceramics include silicon dioxide, boehmite, alumina, zirconium oxide, titanium oxide, magnesium oxide, zinc oxide, aluminum hydroxide, etc.; nitride ceramics include boron nitride, silicon nitride, etc.; carbides include silicon carbide, titanium carbide, zirconium carbide, tungsten carbide, boron carbide, etc. These materials have excellent electrochemical stability and heat resistance, and are preferred embodiments of this invention. In addition, ceramic-based solid electrolytes (such as lithium lanthanum zirconium oxide LLZO, lithium lanthanum titanium oxide LLTO, etc.) can also be used as a special type of ceramic particle due to their inherent lithium-ion conductivity.

[0035] In summary, all ceramic particle materials that can meet the basic requirements of electrochemical stability and heat resistance and can be applied to lithium-ion battery systems fall within the protection scope of this invention.

[0036] Preferably, the first ceramic coating 13 and the second ceramic coating 14 use ceramic particles with the same content and composition. This not only simplifies slurry preparation and production processes, and reduces raw material management and quality control costs, but also ensures that the composite separator 10 has balanced thermal conductivity, mechanical strength, and interfacial characteristics on both the positive and negative electrodes. This avoids uneven internal stress or ion transport kinetic mismatch caused by differences in the performance of the coatings on both sides, thus ensuring the consistency and reliability of the overall battery performance.

[0037] Based on the aforementioned embodiments, the thickness of the first ceramic coating 13 and the second ceramic coating 14 is 0.5 micrometers to 6 micrometers. Controlling the thickness of the first ceramic coating 13 and the second ceramic coating 14 within this optimized range ensures that the composite diaphragm 10 has sufficient thermal stability and mechanical reinforcement, while avoiding problems such as decreased overall porosity, reduced ionic conductivity, or decreased flexibility due to excessive coating thickness. This achieves an optimal balance between the thermal safety and electrochemical performance of the diaphragm. Furthermore, the thickness of the first ceramic coating 13 and the second ceramic coating 14 can be, but is not limited to, 0.5 micrometers, 0.8 micrometers, 1.0 micrometers, 2 micrometers, 3 micrometers, 4 micrometers, 5 micrometers, or 6 micrometers, as well as specific values ​​between these values.

[0038] Based on the aforementioned embodiments, the thickness of the first PTFE porous membrane layer 12 and the second PTFE porous membrane layer 15 is 10 micrometers to 35 micrometers. Controlling the thickness of the first PTFE porous membrane layer 12 and the second PTFE porous membrane layer 15 within this range ensures that they possess sufficiently high porosity and excellent electrolyte retention, thereby providing unobstructed migration channels for lithium ions and reducing battery internal resistance. Simultaneously, this thickness of PTFE porous membrane layer also maintains good flexibility and mechanical strength, and does not affect the overall processability of the separator after being composited with the ceramic coating. Furthermore, the thickness of the first PTFE porous membrane layer 12 and the second PTFE porous membrane layer 15 can be, but is not limited to, 10 micrometers, 15 micrometers, 20 micrometers, 25 micrometers, 30 micrometers, or 35 micrometers, as well as specific values ​​between these values.

[0039] Preferably, the first ceramic coating 13 and the second ceramic coating 14 have the same thickness, and the first PTFE porous membrane layer 12 and the second PTFE porous membrane layer 15 have the same thickness. This symmetrical thickness design helps to simplify the manufacturing process, control the uniformity of the overall thickness of the separator, and ensure the balance of ion transport impedance on the positive and negative electrode sides inside the battery, further improving the overall electrochemical performance and consistency of the battery.

[0040] It should be noted that the first and second PTFE porous membrane layers are independent pre-fabricated membranes, rather than functional layers formed in situ on the surface of the base membrane 11 through coating or other methods. They are microporous films formed from PTFE material through processes such as extrusion and stretching, and microscopically possess a three-dimensional network of porous structures composed of interconnected fibrous structures. The first and second PTFE porous membrane layers mentioned in this application refer only to PTFE membranes with porous structures, and not to a specific type of product or structure simply named "PTFE porous membrane." This application does not have specific requirements regarding the pore structure of PTFE.

[0041] Based on the aforementioned embodiments, the first ceramic coating 13 and the second ceramic coating 14 further include a first adhesive, which includes one or more of water-based adhesives and oil-based adhesives.

[0042] This application does not impose any special restrictions on the specific type of the first binder. Any binder component suitable for battery systems can be used in this invention without departing from the inventive concept of this application.

[0043] For example, the waterborne adhesive includes any one or a mixture of several of the following: cellulose acetate (CA), methylcellulose (MC), sodium carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), styrene-butadiene rubber (SBR), nitrile rubber (NBR), styrene-ethylene-butene-styrene copolymer (SEBS), styrene-butadiene-styrene copolymer (SBS), polyvinyl alcohol (PVA), polyacrylamide (PAM), polyacrylic acid (PAA), sodium polyacrylate (NaPAA), lithium polyacrylate (LiPAA), polyacrylic acid copolymers (such as polyacrylic acid-polyacrylonitrile copolymer, polyacrylate-polyacrylonitrile copolymer), waterborne polyurethane (PU), polyethylene oxide (PEO), polyimide, ethylene-vinyl acetate copolymer, epoxy resin, sodium alginate, and lithium alginate.

[0044] For example, the oily adhesive includes any one or a mixture of several of polyvinylidene fluoride (PVDF) and polyvinylidene fluoride copolymers (such as PVDF-HFP copolymer, PVDF-TFE copolymer). The above examples are merely illustrative and are not intended to limit the scope of protection of the present invention.

[0045] Preferably, the first binder is an oil-based binder, which has excellent bonding properties. It not only ensures a strong bond between ceramic particles and between the first ceramic coating 13 and the base film 11, but also forms a good bond between the first ceramic coating 13 and the first PTFE porous membrane layer 12, and between the second ceramic coating 14 and the second PTFE porous membrane layer 15, giving the battery higher overall hardness and interface smoothness. Specifically, when PVDF-HFP is used as the binder, during hot-pressing assembly, the PVDF-HFP layer is activated by heat, releasing a small amount of PVDF components, promoting tighter bonding between layers at the interface. After subsequent cold pressing, the binder is re-cured, further eliminating interlayer voids and enhancing the structural integrity and interface stability of the composite separator 10.

[0046] Preferably, the first ceramic coating 13 and the second ceramic coating 14 use the same content and composition of the first binder formulation. This not only simplifies the slurry preparation and production process, and reduces raw material management and quality control costs, but also ensures that the composite separator 10 has balanced thermal conductivity, mechanical strength, and interfacial characteristics on both the positive and negative electrodes. This avoids uneven internal stress or ion transport kinetic mismatch caused by differences in the performance of the coatings on both sides, thus ensuring the consistency and reliability of the overall battery performance.

[0047] Based on the aforementioned embodiments, the composite diaphragm 10 further includes an adhesive layer 16, such as... Figure 1As shown. The adhesive layer 16 is located between the second PTFE porous membrane layer 15 and the base membrane 11, and is used to bond the second PTFE porous membrane layer 15 to the base membrane 11. Further, the adhesive layer 16 includes a second adhesive, which includes one or a mixture of water-based adhesives and oil-based adhesives. Preferably, the second adhesive in the adhesive layer 16 is an oil-based adhesive, which includes any one or a mixture of polyvinylidene fluoride (PVDF) and polyvinylidene fluoride copolymers (such as PVDF-HFP copolymer, PVDF-TFE copolymer). The adhesive layer 16 can effectively improve the interlayer bonding strength between the base membrane 11 and the second PTFE porous membrane layer 15, prevent interlayer delamination, ensure that the composite separator 10 maintains its intact structure during battery manufacturing and use, ensure the stability of the ion transport path, and extend the battery cycle life.

[0048] The method for preparing the composite diaphragm 10 provided in this embodiment may include the following steps: The ceramic particles, solvent, and first binder are mixed evenly to obtain a ceramic slurry; The ceramic slurry is coated onto the positive electrode side of the base film 11 and dried to form a first ceramic coating 13; and While the solvent in the first ceramic coating 13 has not completely evaporated, the base film 11 with the first ceramic coating 13 is hot-pressed together with the first PTFE porous film layer 12 to complete the preparation of the positive electrode side film layer. An adhesive is applied to the negative electrode side of the base membrane 11, and the second PTFE porous membrane layer 15 is attached to the base membrane 11 by the adhesive. A ceramic slurry is coated on the outside of the second PTFE porous membrane layer 15 and completely dried to form a second ceramic coating 14, thus completing the preparation of the negative electrode side membrane layer and obtaining the composite separator 10.

[0049] In preparing the ceramic slurry, the mass ratio of ceramic particles to the first binder ranges from (65~95):(5~35), and the amount of solvent used is 1~5 times the total dry mass of the ceramic particles and the first binder. This ratio ensures that the slurry has suitable coating viscosity and flowability, and guarantees that the ceramic coating formed after drying has good structural strength, adhesion to the base film, and an ideal porous structure.

[0050] The adhesive is a solution containing a second binder, with a solid content of 2% to 20%. By controlling this solid content range, it is possible to ensure that the adhesive has suitable flowability, wettability, and bonding strength, which can both achieve the bonding and composite of the second PTFE porous membrane layer 15 and the base membrane 11, and avoid the blockage of the base membrane pores due to excessive penetration of the adhesive, thereby ensuring the overall ionic conductivity of the membrane.

[0051] It should be noted that the order of preparation of the negative and positive electrode layers is not limited; that is, the positive electrode layer of the base film 11 can be prepared first, or the negative electrode layer of the base film 11 can be prepared first. This application does not impose any restrictions on this.

[0052] Please see Figure 2 The composite separator 20 of the second embodiment of the present invention includes a base membrane 21, a first ceramic coating 231, a first PTFE porous membrane layer 221, a second ceramic coating 232, and a second PTFE porous membrane layer 222. The base membrane 21 has a positive electrode side and a negative electrode side along its thickness direction. The positive electrode side has the first ceramic coating 231 and the first PTFE porous membrane layer 221 stacked along a first direction +Z; the negative electrode side has the second ceramic coating 232 and the second PTFE porous membrane layer 222 stacked along the first direction +Z, and the protective layer 25 is further provided on the side of the second PTFE porous membrane layer 222 away from the positive electrode side. The first direction +Z is parallel to the thickness direction of the base membrane 21. The first direction +Z points from the positive electrode side to the negative electrode side. The protective layer 25 constitutes the outermost layer of the composite separator 20 on the negative electrode side.

[0053] In this embodiment, on the negative electrode side: a protective layer 25 is added outside the second PTFE porous film layer 222, which can form a dense physical isolation barrier, blocking the direct contact path between the negative electrode and PTFE, and fundamentally preventing the occurrence of side reactions. When the protective layer is a ceramic material, it has strong chemical inertness, is stable in a reducing environment, and does not participate in side reactions; its high surface energy is conducive to electrolyte wetting, promotes uniform lithium ion transport, reduces local current concentration, and inhibits lithium dendrite growth. In addition, although PTFE is embedded, it is still close to the base film 21, and can exert its excellent dimensional stability at high temperatures—when the temperature rises to the melting point of the base film 21 (such as PE about 135°C), PTFE hardly shrinks, effectively supporting the overall structure and preventing short circuits between the positive and negative electrodes caused by softening or shrinking of the base film 21. On the positive electrode side: the first ceramic coating is set outside the first PTFE porous film layer, making it the functional layer closest to the positive electrode, which can give full play to the adsorption and catalytic decomposition ability of the surface hydroxyl or defect sites of the ceramic material for active oxygen, reduce the concentration of free radicals, and slow down oxidation side reactions. Meanwhile, the ceramic layer has high thermal conductivity, which helps to quickly conduct and disperse heat from local hot spots, alleviate heat accumulation, and improve the battery's thermal management capabilities. The inner PTFE porous membrane still provides mechanical support and thermal stability, maintaining the integrity of the pore structure at high temperatures and preventing excessive shrinkage of the base film 21. In addition, the ceramic layer can improve the interfacial adhesion between the separator and the positive electrode, reduce contact resistance, and to some extent inhibit the shedding of positive electrode particles or puncture by burrs.

[0054] Based on the aforementioned embodiments, the protective layer 25 is a single-layer structure prepared from ceramic materials, polymer materials, or a mixture of ceramics and polymers.

[0055] Based on the aforementioned embodiments, the ceramic material is selected from at least one of alumina, boehmite, silicon dioxide, zirconium oxide, boron nitride titanium oxide, magnesium oxide, lithium phosphate, and lithium aluminate.

[0056] Based on the foregoing embodiments, the polymer material is selected from at least one of polyimide, aramid, polyacrylonitrile, polyvinylidene fluoride-hexafluoropropylene copolymer, polyetheretherketone, polyphenylene sulfide, polytetrafluoroethylene, polybenzimidazole, and polyethylene oxide.

[0057] It should be noted that the aforementioned ceramic and polymer materials are conventional choices for preparing functional layers of lithium-ion battery separators in this field. They not only possess good electrochemical and thermal stability, but also have mature film-forming processes and are easily combined with PTFE porous membranes. When ceramic materials are used, the protective layer 25 mainly exhibits excellent heat resistance, high hardness, and affinity for the electrolyte; when polymer materials are used, the protective layer 25 demonstrates better flexibility and a dense interfacial isolation effect; and when a mixture of ceramic and polymer is used, the advantages of both can be combined to achieve synergistic protection. Those skilled in the art can rationally select from the above materials to construct the protective layer 25 according to the specific requirements of the interface characteristics of the actual battery system. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope of this invention.

[0058] Based on the aforementioned embodiments, the composite diaphragm 20 further includes an adhesive layer 24, such as... Figure 2 As shown. The adhesive layer 24 is located between the first PTFE porous membrane layer 221 and the base membrane 21, and is used to bond the first PTFE porous membrane layer 221 to the base membrane 21.

[0059] The composite separator 20 provided by this invention has a first ceramic coating 231, a first PTFE porous membrane layer 221, a second ceramic coating 232, and a second PTFE porous membrane layer 222 arranged in a consistent order on both sides of the base membrane 21, that is, a heterogeneous layout on the positive and negative electrode sides of the base membrane 21. On the positive electrode side, PTFE can be directly or indirectly exposed to a high-voltage environment, and its chemical inertness helps to resist oxygen release from the positive electrode and free radical attack. At the same time, the first ceramic coating 231 can further enhance thermal stability and oxygen adsorption capacity. On the negative electrode side, due to the instability of the PTFE-negative electrode interface, the second PTFE porous membrane layer 222 on the negative electrode side is limited to the outermost side of the non-negative electrode side, that is, the second PTFE porous membrane layer 222 on the negative electrode side does not directly contact the negative electrode. The protective layer 25 avoids the negative electrode from contacting PTFE and causing side reactions, and also optimizes the layout of the separator and improves the performance of the battery.

[0060] It should be noted that, unless otherwise explicitly specified, the specific materials (such as the type of ceramic particles, the type of binder, etc.), component ratios, physical parameters (such as thickness, pore size, etc.) and preferred embodiments of the base film 21, the first ceramic coating 231, the second ceramic coating 232, the first PTFE porous film layer 221, the second PTFE porous film layer 222, the adhesive layer 24, and the protective layer 25 described in this second embodiment can all refer to the relevant descriptions of the base film 11, the first ceramic coating 13, the second ceramic coating 14, the first PTFE porous film layer 12, the second PTFE porous film layer 15, the adhesive layer 16, and their components in the first embodiment of this invention, and will not be repeated here. One of the core concepts of this invention is that when the first direction is from the positive electrode side to the negative electrode side, and the second PTFE porous film layer 222 on the negative electrode side may therefore directly face the negative electrode, by adding a protective layer 25 to isolate its contact with the negative electrode, the invention's purpose of protecting PTFE, improving battery interface stability, and cycle life can be achieved under another layered structure.

[0061] The method for preparing the composite diaphragm 20 provided in this embodiment may include the following steps: The ceramic particles, solvent, and first binder are mixed evenly to obtain a ceramic slurry; The ceramic slurry is coated onto the negative electrode side of the base film 21 and dried to form a second ceramic coating 232; and While the solvent in the second ceramic coating 232 has not completely evaporated, the base film 21 with the second ceramic coating 232 is hot-pressed together with the second PTFE porous film layer 222. Ceramic materials, polymer materials, or a mixture of both are mixed with a binder and solvent for the protective layer to form a protective layer slurry, which is then coated on the outside of the second PTFE porous membrane layer 222 and dried to form a protective layer 25, thus completing the preparation of the negative electrode side membrane layer. An adhesive is applied to the positive electrode side of the base membrane 21, and the first PTFE porous membrane layer 221 is attached to the base membrane 21 by the adhesive. A ceramic slurry is coated on the outside of the first PTFE porous membrane layer 221 and dried to form the first ceramic coating 231, thus completing the preparation of the positive electrode side membrane layer and obtaining the composite separator 20.

[0062] In the above preparation method, the binder used for the protective layer can be of the same type as the binder (including the first binder) described in the foregoing embodiments of the present invention. Any water-based or oil-based binder that is suitable for lithium-ion battery systems and can meet the film formation requirements of the corresponding functional layers can be used in the preparation of the protective layer slurry.

[0063] The ceramic slurry is prepared according to the preferred formulation described in the foregoing embodiments of this specification, and will not be repeated here. The solid content of the protective layer slurry is controlled within the range of 10% to 50%, and can be adjusted according to the characteristics of the selected ceramic and polymer materials and the coating process requirements to ensure that the protective layer has good film-forming properties, density, and interfacial bonding strength.

[0064] It is understood that the above-mentioned proportioning parameters for each slurry are merely illustrative examples provided to ensure process feasibility and basic performance, and are intended only to aid in understanding the technical solution of this invention, and do not limit the scope of protection of this invention in any way. The scope of protection of this invention shall be determined by the claims.

[0065] It should be noted that the order of preparation of the negative and positive electrode layers is not limited; that is, the positive electrode layer of the base film can be prepared first, or the negative electrode layer of the base film can be prepared first. This application does not impose any restrictions in this regard.

[0066] One embodiment of the lithium-ion battery includes the composite separator of any of the above-described embodiments. The lithium-ion battery further includes a positive electrode and a negative electrode, with the composite separator located between the positive and negative electrodes.

[0067] For example, the positive electrode includes a positive current collector and a positive active material layer coated thereon, the active material layer including a positive active substance, and may also add conductive agents and / or binders as needed; any known positive active substances may be used in this application without departing from the inventive concept of this application.

[0068] The negative electrode includes a negative electrode current collector and a negative electrode active material layer coated thereon. The active material layer includes a negative electrode active substance and may also include conductive agents and / or binders as needed.

[0069] Meanwhile, the negative electrode can also be metallic lithium or a lithium-based composite.

[0070] It is understood that the description of the electrode structure in this application is a typical example, but the scope of protection is not limited thereto and also covers other structural forms known in the art. The active material includes known single materials, coating materials, doped materials, nanocomposites, and derivatives obtained through other optimization methods. Meanwhile, the conductive agent, binder, current collector, and other components include conventional and functionalized forms. Any known combination of materials and structural design that does not depart from the core concept of this invention is applicable to this application and will not be elaborated upon here.

[0071] The lithium-ion battery of this invention includes the aforementioned composite separator. This composite separator has a ceramic coating and a PTFE porous membrane arranged in a consistent order on both sides of a base membrane, representing a heterogeneous layout on the positive and negative electrode sides of the base membrane. On the positive electrode side, PTFE can be directly or indirectly exposed to a high-voltage environment. Its chemical inertness helps resist oxygen release from the positive electrode and free radical attack, while the ceramic coating further enhances thermal stability and oxygen adsorption capacity. On the negative electrode side, due to the interfacial instability between PTFE and lithium, the PTFE porous membrane layer on the negative electrode side is limited to the outermost side, meaning it does not directly contact the negative electrode. This avoids side reactions caused by contact between the negative electrode and PTFE, optimizes the separator layout, and improves battery performance.

[0072] Referring to the above embodiments, in order to make the technical solution of the present invention more specific, clear and easy to understand, examples of the technical solution of the present invention are given below. However, it should be noted that the content to be protected by the present invention is not limited to the following embodiments.

[0073] Example 1 Preparation of composite membrane: 10-micron PE membrane as base membrane.

[0074] Positive electrode side: Along the direction from the negative electrode side to the positive electrode side, a 3-micrometer-thick first ceramic coating (composed of alumina + 12% PVDF-HFP) and a 12-micrometer-thick first PTFE porous membrane layer are stacked sequentially.

[0075] Negative electrode side: Along the direction from the negative electrode side to the positive electrode side, a 3-micrometer-thick first ceramic coating (with the same composition as the first ceramic coating) and a 12-micrometer-thick second PTFE porous membrane layer (with the same composition as the first PTFE porous membrane layer) are sequentially stacked, so that the negative electrode and the second ceramic coating are in direct contact.

[0076] Preparation of the positive electrode: NCM, super-p, and PVDF were added to the solvent NMP in a ratio of 96:2:2. After mixing, the mixture was coated onto aluminum foil using a coating machine and dried to obtain the positive electrode.

[0077] Preparation of negative electrode: Graphite, super-P, CMC+SBR are added to deionized water in a ratio of 95:2:3. After mixing, the mixture is coated onto copper foil using a coating machine and dried to obtain the negative electrode.

[0078] The positive electrode, negative electrode, and composite separator prepared in the above steps are stacked according to the positional relationship and then liquid-filled to form a lithium-ion battery.

[0079] Example 2 Similar to Example 1, except that an adhesive layer (composed of a PVDF-HFP solution with a solid content of 10%) is added between the second PTFE porous membrane layer and the base membrane on the negative electrode side.

[0080] Example 3 Similar to Example 1, except that the thickness of the PTFE porous membrane layer on both the positive and negative electrode sides is 20 micrometers.

[0081] Example 4 Similar to Example 1, except that the thickness of the ceramic coating on both the positive and negative electrode sides is 6 micrometers.

[0082] Example 5 Preparation of composite membrane: 10-micron PP membrane as base membrane.

[0083] Positive electrode side: Along the direction from the positive electrode side to the negative electrode side, a first ceramic coating with a thickness of 2 micrometers and a first PTFE porous membrane layer with a thickness of 12 micrometers are stacked in sequence.

[0084] Negative electrode side: Along the direction from the positive electrode side to the negative electrode side, a 2-micrometer-thick second ceramic coating, a 12-micrometer-thick second PTFE porous membrane layer, and a 2-micrometer-thick protective layer (composed of polyimide) are sequentially stacked to make the negative electrode contact the protective layer.

[0085] Preparation of the positive electrode: NCM, super-p, and PVDF were added to the solvent NMP in a ratio of 96:2:2. After mixing, the mixture was coated onto aluminum foil using a coating machine and dried to obtain the positive electrode.

[0086] Preparation of negative electrode: Graphite, super-P, CMC+SBR are added to deionized water in a ratio of 95:2:3. After mixing, the mixture is coated onto copper foil using a coating machine and dried to obtain the negative electrode.

[0087] The positive electrode, negative electrode, and composite separator prepared in the above steps are stacked according to the positional relationship and then liquid-filled to form a lithium-ion battery.

[0088] Comparative Example 1 Preparation of composite membrane: 10-micron PE membrane as base membrane.

[0089] A porous membrane layer and a ceramic layer are symmetrically arranged on both sides of the base membrane.

[0090] On the positive electrode side of the base membrane: along the direction from the base membrane to the positive electrode, a first ceramic coating with a thickness of 3 micrometers (composed of alumina + 12% PVDF-HFP) and a first PTFE porous membrane layer with a thickness of 12 micrometers are sequentially stacked.

[0091] On the negative electrode side of the base membrane: along the direction from the base membrane to the negative electrode, a second ceramic coating with a thickness of 3 micrometers (with the same composition as the first ceramic coating) and a second PTFE porous membrane layer with a thickness of 12 micrometers (with the same composition as the first PTFE porous membrane layer) are sequentially stacked.

[0092] Preparation of the positive electrode: NCM, super-p, and PVDF were added to the solvent NMP in a ratio of 96:2:2. After mixing, the mixture was coated onto aluminum foil using a coating machine and dried to obtain the positive electrode.

[0093] Preparation of negative electrode: Graphite, super-P, CMC+SBR are added to deionized water in a ratio of 95:2:3. After mixing, the mixture is coated onto copper foil using a coating machine and dried to obtain the negative electrode.

[0094] The positive electrode, negative electrode, and composite separator prepared in the above steps are stacked according to the positional relationship and then liquid-filled to form a lithium-ion battery.

[0095] Comparative Example 2 Similar to Example 5, except that no protective layer is provided on the negative electrode side.

[0096] Battery performance testing Temperature: 25℃±2℃ ① Charge to the termination voltage at 1C or the specified current, cut off current 0.05C, and let stand for 30 minutes; ② Discharge at 1C until the final discharge voltage (2.75V), record the discharge capacity, and let stand for 30 minutes; Repeat steps ① to ②, and record the discharge capacity in the 500th cycle. Capacity retention rate after 500 cycles = discharge capacity of the 500th cycle / initial discharge capacity * 100%.

[0097] The test results are as follows: Table 1 Test results of Examples 1-5 and Comparative Examples 1-2 This invention creatively employs a heterogeneous layout on both the positive and negative electrode sides of the base film, matching the layer sequence of PTFE with the actual requirements of the electrode interface. Specifically, on the positive electrode side, the porous PTFE film layer is positioned directly or indirectly facing the positive electrode, utilizing its chemical inertness to resist oxidation side reactions under high voltage; on the negative electrode side, by limiting the structure of the porous PTFE film layer to the outermost layer, direct contact between the porous PTFE film layer and the strongly reducing negative electrode is avoided, thereby eliminating the risk of interfacial side reactions caused by this.

[0098] The test results of the embodiments and comparative examples fully demonstrate that the embodiment using the asymmetric structure of the present invention exhibits a significantly higher capacity retention rate (85.0%~88.0%) after 500 cycles compared to Comparative Example 1, which uses a traditional symmetric structure. This proves that the asymmetric structure design described in the present invention can effectively synergistically improve the oxidation resistance of the battery on the positive electrode side and the interface stability on the negative electrode side, ultimately achieving a long-life lithium-ion battery.

[0099] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0100] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A composite diaphragm, characterized in that, The composite diaphragm includes A base film having opposing positive and negative electrode sides along its thickness direction; The positive electrode side is provided with a first ceramic coating and a first PTFE porous membrane layer stacked along a first direction; The negative electrode side is provided with a second ceramic coating and a second PTFE porous membrane layer stacked along the first direction; Furthermore, the side of the second PTFE porous membrane layer furthest from the positive electrode side is not the outermost side of the negative electrode side; Wherein, the first direction is parallel to the thickness direction of the base film.

2. The composite diaphragm according to claim 1, characterized in that, The first direction is from the negative electrode side to the positive electrode side.

3. The composite diaphragm according to claim 1, characterized in that, The first direction is from the positive electrode side to the negative electrode side; the second PTFE porous membrane layer is further provided with a protective layer on the side away from the positive electrode side.

4. The composite diaphragm according to claim 3, characterized in that, The protective layer is a single-layer structure made of ceramic material, polymer material, or a mixture of ceramic and polymer. Preferably, the ceramic material is selected from at least one of alumina, boehmite, silicon dioxide, zirconium oxide, boron nitride titanium oxide, magnesium oxide, lithium phosphate, and lithium aluminate; Preferably, the polymer material is selected from at least one of polyimide, aramid, polyacrylonitrile, polyvinylidene fluoride-hexafluoropropylene copolymer, polyetheretherketone, polyphenylene sulfide, polytetrafluoroethylene, polybenzimidazole, and polyethylene oxide.

5. The composite diaphragm according to claim 1, characterized in that, The first ceramic coating comprises a plurality of ceramic particles, and the second ceramic coating comprises a plurality of ceramic particles; The ceramic particles include one or a mixture of oxide ceramics, nitride ceramics and carbide ceramics.

6. The composite diaphragm according to claim 1, characterized in that, The thickness of the first ceramic coating and the second ceramic coating is 0.5 micrometers to 6 micrometers; The thickness of the first PTFE porous membrane layer and the second PTFE porous membrane layer is 10 micrometers to 35 micrometers.

7. The composite diaphragm according to claim 1, characterized in that, The first ceramic coating and the second ceramic coating further include a first adhesive, which includes one or a mixture of water-based adhesives and oil-based adhesives.

8. The composite diaphragm according to claim 1, characterized in that, The composite membrane further includes an adhesive layer located between the first PTFE porous membrane layer and / or the second PTFE porous membrane layer and the base membrane, for bonding the first PTFE porous membrane layer and / or the second PTFE porous membrane layer to the base membrane.

9. The composite diaphragm according to claim 8, characterized in that, The adhesive layer includes a second adhesive, which includes one or a mixture of water-based adhesives and oil-based adhesives.

10. A lithium-ion battery, characterized in that, The composite membrane included in any one of claims 1 to 9.

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