Composite cathode current collector and method for manufacturing the same

Abstract

The application discloses a composite positive electrode current collector and a preparation method thereof. The composite positive electrode current collector comprises a polymer substrate layer and composite functional layers arranged on both sides of the thickness direction of the polymer substrate layer. The composite functional layers comprise, in sequence along the direction away from the polymer substrate layer, a conductive metal layer and a scale-like brittle functional layer. The scale-like brittle functional layer is formed by stacking of obliquely grown flaky grains. The axial direction of the flaky grains and the normal direction of the polymer substrate layer form an angle of 30-60 degrees. The relative density is 70-85%. During preparation, the incident direction of evaporated atoms forms an angle of 25-55 degrees with the normal of the substrate, and the shadow effect is used to induce the oblique growth of the flaky grains. In the application, the toughness tearing during needling is changed into cleavage fracture along the interface of the flaky layer. The average length of the needle pricking burrs is reduced compared with the traditional pure aluminum layer composite foil, the elastic modulus can be greatly improved, and the safety of battery needle pricking is effectively improved.

Description

Technical Field

[0001] This invention relates to the field of composite current collector materials technology, and more specifically, to a composite positive electrode current collector and its preparation method. Background Technology

[0002] The composite aluminum foil current collector adopts a sandwich structure of polymer substrate and double-sided metal aluminum layer. Compared with traditional pure aluminum foil, it can achieve a weight reduction of more than 40%. Moreover, the polymer insulating layer can block the current when the cell is short-circuited. It is one of the core materials for improving the energy density and safety performance of power batteries.

[0003] However, while pure aluminum layers possess good plasticity and high toughness, during needle punching or extrusion of the battery cell, the aluminum layer undergoes plastic elongation under millisecond-level dynamic shear stress, forming long and sharp metal burrs. These burrs penetrate the separator, triggering a severe internal short circuit and negating the safety gains of the polymer layer. Therefore, needle punching safety has become a key bottleneck for the industrialization of composite aluminum foil.

[0004] To address this issue, the industry has proposed several improvement solutions, but all have limitations: First, the interface oxide modification solution adds a thin oxide interface layer between the polymer substrate and the aluminum layer to improve interfacial adhesion and electrolyte resistance. However, this oxide layer is located below the conductive metal layer and only serves for interface anchoring and corrosion resistance; it cannot control the fracture behavior of the aluminum layer as a whole and has no substantial effect on reducing needle burrs. Second, the microporous structure solution constructs the surface aluminum layer into a microporous structure and supplements it with an alumina film. It relies on stress dispersion in the microporous region to improve the needle burr throughput. However, its mechanism still relies on the deformation of the metal layer itself to accommodate needle burrs. The first problem is that the micropores only shorten the surface layer rather than eliminate it, and they also damage the continuity of the surface conductivity. The second problem is that the layered toughening scheme limits the short-circuit current and deflection cracks by alternating layers of pure aluminum and oxide or by using a biomimetic soft-hard alternating structure. However, the oxide layers are all dense columnar crystals, which still form a continuous sharp fracture surface when broken. The number of layers is large and the process is complicated. Moreover, the outermost layer is still a ductile metal, which cannot solve the problem of surface burrs. The third problem is that the current process is limited. Mass production roll-to-roll evaporation uses normal incidence deposition. The film layer is a dense equiaxed crystal or columnar crystal structure. The fracture mode is mainly ductile tearing. The industry lacks the means to actively control the fracture behavior of the film layer.

[0005] It is particularly important to note that, regarding needle penetration safety, the mainstream improvement direction in the field of composite current collectors has always been toughening and reinforcement, that is, increasing the elongation of the aluminum or oxide layer through doping, toughening intermediate layers, or biomimetic structural design. The industry generally believes that the higher the material toughness, the stronger the puncture resistance. However, through extensive experiments, the inventors have discovered that the above understanding constitutes a technical bias: although toughening can improve static mechanical strength, under the millisecond-level severe dynamic shear stress generated by needle penetration, the toughened metal layer undergoes unsteady necking, forming long and straight dagger-shaped burrs, which actually exacerbates the short circuit risk.

[0006] In view of this, the present invention is specially proposed. Contents of the Invention

[0007] An object of the present invention is to provide a composite positive current collector and a preparation method thereof.

[0008] The present invention is implemented as follows: In a first aspect, the present invention provides a composite positive current collector, comprising a polymer base material layer, and composite functional layers respectively disposed on both sides of the polymer base material layer in the thickness direction; In a direction away from the polymer base material layer, the composite functional layer sequentially comprises a conductive metal layer and a scaly brittle functional layer; The scaly brittle functional layer has a scaly morphology formed by stacking obliquely grown flaky crystal grains, the included angle between the axial direction of the flaky crystal grains and the normal direction of the polymer base material layer is 30° to 60°, and the relative density of the scaly brittle functional layer is 70% to 85%.

[0009] In the present application, the relative density is defined as the densification degree of the scaly brittle functional layer along the normal direction of the polymer base material, and is the ratio of the actual density of the layer along the normal direction of the base material to the theoretical density of the corresponding bulk material constituting the flaky crystal grains.

[0010] In an optional embodiment, the total thickness of the composite functional layer on a single side is 100 to 2000 nm; and / or, the thickness of the scaly brittle functional layer accounts for 30% to 50% of the total thickness of the composite functional layer on a single side, and the lateral dimension of a single scale is 50 to 150 nm; and / or, the conductive metal layer has a dense equiaxed crystal structure.

[0011] In an optional embodiment, the composite functional layer further comprises a primer layer located between the polymer base material layer and the conductive metal layer, the primer layer is bonded to the polymer base material layer through oxygen bonds, the thickness of the primer layer is 5 to 30 nm, and the thickness accounts for 2% to 10% of the total thickness of the composite functional layer on a single side; and / or, the material of the primer layer is at least one of aluminum oxide, silicon dioxide and aluminum-silicon composite oxide; the molar ratio of aluminum to silicon in the aluminum-silicon composite oxide is 1 to 2:1; and / or, the material of the conductive metal layer comprises aluminum; An AlOx gradient transition layer is disposed between the scaly brittle functional layer and the conductive metal layer, where 0<x<1.5, and the oxygen content of the AlOx gradient transition layer continuously increases in the thickness direction.

[0012] In an optional embodiment, the composite functional layer further includes a brittle metal layer disposed on the scaly brittle functional layer in a direction away from the polymer substrate layer. And / or, the brittle metal layer is made of at least one of the elemental metals of chromium, molybdenum, and silver or their alloys; And / or, the thickness of the brittle metal layer accounts for 3% to 10% of the total thickness of the composite functional layer on one side, the thickness is 10 to 70 nm, the surface roughness Ra is 30 to 200 nm, and it has an island-like particle stacking morphology.

[0013] Secondly, the present invention provides a method for preparing the composite positive current collector according to any one of the foregoing embodiments, comprising the following steps: Step 1: The polymer substrate is sent into a vacuum chamber and activated by plasma surface cleaning to obtain a polymer substrate layer; Step 2: Evaporate along the normal direction of the polymer substrate layer to deposit a dense conductive metal layer, controlling the deposition thickness to be 10%~40% of the total thickness of the single-sided composite functional layer; Step 3: Make the incident direction of the evaporating atoms form an angle of 25°~55° with the normal direction of the polymer substrate layer, and introduce the reactive gas to form the scaly brittle functional layer by vapor deposition, controlling the deposition thickness to account for 30%~50% of the total thickness of the single-sided composite functional layer; Step 4: Repeat steps 2 and 3 on the other side of the polymer substrate layer to complete the double-sided sequential vapor deposition and obtain the composite positive electrode current collector.

[0014] In an optional embodiment, in step one, the surface of the polymer substrate has an underlayer, and the material of the underlayer is at least one of alumina, silicon dioxide and aluminum-silicon composite oxide. And / or, the cavity vacuum degree in step one is ≤5×10 -3 Pa; In step two, the evaporation temperature is 800~1400℃, and the film-moving speed is 5~25 m / min; And / or, in step three, multiple oxygen inlets are set along the film-moving direction to increase the oxygen flow rate from 0 to 20~35 sccm along the film-moving direction, the substrate temperature is controlled below 100℃, the working gas pressure of the cavity is 0.5~3.0 Pa, the oxygen partial pressure ratio is 20%~35%, the evaporation temperature is 800~1400℃, and the film-moving speed is 10~25 m / min.

[0015] In an optional implementation, a step for preparing a brittle metal layer is added between step three and step four, including: A brittle metal layer is deposited by incident vapor deposition along the normal direction of the polymer substrate layer, controlling the deposition thickness to be 3%~10% of the total thickness of the single-sided composite functional layer, so that the metal grows in an island-like manner, forming a discontinuous island-like particle stack morphology; when the material of the brittle metal layer is chromium, the evaporation raw material for depositing the brittle metal layer is high-purity chromium particles with a particle size of 1~3 mm and a purity ≥99.95%, and the background vacuum degree of the chamber is evacuated to ≤5×10 -3 Pa.

[0016] In an optional embodiment, when the material of the scaly brittle functional layer is silicon dioxide, the evaporation raw material is high-purity polycrystalline silicon, the evaporation temperature is 1400~1600℃, and the oxygen flow rate is 20~30 sccm.

[0017] Thirdly, the present invention provides a positive electrode sheet, including the composite positive current collector described in the foregoing embodiments or the composite positive current collector prepared by the preparation method described in the foregoing embodiments.

[0018] Fourthly, the present invention provides a lithium-ion battery, including the positive electrode sheet described in the foregoing embodiments.

[0019] The present invention has the following beneficial effects: In the composite positive electrode current collector of this invention, composite functional layers are symmetrically arranged on both sides of the polymer substrate layer in the thickness direction, retaining the fundamental advantages of weight reduction and internal short-circuit blocking of the composite current collector. The scaly brittle functional layer is formed by stacking inclined plate-like grains, transforming the tough tearing during needle punching into cleavage fracture along the plate interface. The fracture surface is flat and without continuous sharp tips. The average length of the needle punch burrs is effectively reduced compared to traditional pure aluminum composite foils. The elastic modulus and stiffness of the composite positive electrode current collector are improved, which is beneficial for maintaining the interface integrity of the electrode sheet during coating, rolling, and slitting processes. The bottom layer is bonded to the polymer substrate layer through oxygen bonds. The conductive metal layer constructs the basic conductive path with a dense equiaxed crystal structure and anchors the upper and lower functional layers. The AlOx gradient transition layer achieves a smooth transition of physical properties from metal to ceramic, ensuring interlayer bonding and electrolyte resistance. This resolves the inherent contradiction between surface embrittlement and interlayer bonding, achieving a synergistic effect of surface embrittlement, conductivity, interlayer adhesion, and stiffening. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0021] In a first aspect, the present invention provides a composite positive electrode current collector, comprising a polymer substrate layer and composite functional layers respectively disposed on both sides of the polymer substrate layer in the thickness direction. Along the direction away from the polymer substrate layer, the composite functional layer sequentially includes a conductive metal layer and a scaly brittle functional layer; The scaly brittle functional layer has a scaly morphology formed by stacking inclined, plate-like grains. The angle between the axial direction of the plate-like grains and the normal direction of the polymer substrate layer is 30° to 60°, and the relative density of the scaly brittle functional layer is 70% to 85%.

[0022] In this invention, the bottom layer is bonded to the polymer substrate layer through oxygen bonds, which improves the interfacial bonding force and corrosion resistance, and also increases the surface elastic modulus of the polymer substrate. The conductive metal layer constructs the basic conductive path with a dense equiaxed crystal structure, while simultaneously connecting the functional layers on both sides. The scaly brittle functional layer is the outer functional layer in the composite functional layer that plays a role in regulating fracture behavior: its lamellar grains grow at an angle of 30° to 60° to the normal of the polymer substrate layer and are stacked layer by layer to form a lamellar interface that extends obliquely relative to the needle-punching direction. The relative density of 70% to 85% makes the bonding strength between the lamellar interfaces lower than the strength of the lamellar grains themselves. When the dynamic stress generated by needle punching is applied to the film layer, the crack preferentially deflects at the oblique lamellar interface and rapidly splits and propagates along the interface, rather than propagating through the film thickness. In this invention, the cleavage fracture of the scaly brittle functional layer refers to a brittle fracture mode in which cracks rapidly split along the weakly bonded lamellar interface under normal stress, with almost no accompanying plastic deformation. The fracture surface is a flat step shape consistent with the lamellar orientation and has no continuous sharp tips. This contrasts with the behavior of conductive metal layers forming sharp burrs due to ductile tearing and necking under the same dynamic stress, thus eliminating long and sharp burrs from the fracture mechanism level.

[0023] In this application, the relative density is defined as the compactness of the scaly brittle functional layer along the normal direction of the polymer substrate, and is the ratio of the actual density of the layer along the normal direction of the substrate to the theoretical density of the corresponding bulk material constituting the lamellar grains. The calculation formula is: Normal relative density = (Actual density along the normal direction of the substrate / Theoretical density of the corresponding bulk material) × 100%; X-ray reflectometer was used for testing. During the test, X-rays were incident along the normal direction of the polymer substrate, with an incident angle scanning range of 0.1° to 5° and a scanning step size of 0.01°. The X-rays were reflected at the upper surface of the scaly brittle functional layer and at the interface between the scaly brittle functional layer and the underlying structure. The two reflected beams interfered with each other to form an oscillating curve of reflectivity as a function of the incident angle. The oscillating curve was fitted using thin film analysis software to obtain the normal average electron density of the scaly brittle functional layer. Based on the direct proportional relationship between electron density and mass density, the ratio of the average electron density of the film layer to the theoretical electron density of the corresponding bulk material constituting the lamellar grains was multiplied by 100% to obtain the relative density.

[0024] In an optional embodiment, the total thickness of the composite functional layer on one side is 100~2000 nm.

[0025] In an optional embodiment, the thickness of the scaly brittle functional layer accounts for 30% to 50% of the total thickness of the composite functional layer on one side, and the lateral dimension of a single scale is 50 to 150 nm.

[0026] In this invention, the scaly brittle functional layer must reach a thickness ratio of more than 30% to dominate the propagation path of surface cracks under needle-punched dynamic shear stress. When the ratio is too low, the fracture behavior is still controlled by the ductile tearing of the underlying conductive metal layer, and the burr length increases significantly. The transverse size of the single scale and the relative density of the film layer change synergistically with the incident angle: when the scale is too large, it corresponds to a smaller incident angle, the grains tend to become columnar, the relative density of the film layer increases, the lamellar interface decreases, the local toughness increases, and the cleavage fracture ability weakens. When the scale is too small, it corresponds to a larger incident angle, the lamellar gap is too large, the relative density of the film layer is too low, the integrity and barrier performance of the film layer decrease, and when the incident angle continues to increase, penetrating pinholes will appear. Therefore, controlling the transverse size of the single scale to 50~150 nm can maintain effective cleavage fracture along the lamellar interface while ensuring the integrity and barrier performance of the film layer.

[0027] In an optional embodiment, the conductive metal layer has a dense equiaxed crystal structure.

[0028] In an optional embodiment, the composite functional layer further includes a substrate layer located between the polymer substrate layer and the conductive metal layer, wherein the substrate layer is bonded to the polymer substrate layer via oxygen bonds; the substrate layer is made of at least one of alumina, silicon dioxide, and aluminum-silicon composite oxide, has a thickness of 5-30 nm, and its thickness accounts for 2%-10% of the total thickness of the composite functional layer on one side; the molar ratio of aluminum to silicon in the aluminum-silicon composite oxide is 1-2:1.

[0029] In the present invention, the primer layer is bonded to the polymer substrate layer through oxygen bonds. A thickness of 5 to 30 nm can not only form a continuous and complete anchoring interface to ensure interfacial bonding force and corrosion resistance, but also avoid interfacial cracking caused by the inherent brittleness of an excessively thick ceramic layer: when the thickness is less than 5 nm, the film layer is discontinuous and a complete oxygen bond anchorage cannot be established; when the thickness exceeds 30 nm, the internal interfacial stress rises and its proportion gets out of control. The performance of the aluminum-silicon composite oxide is adjusted by the molar ratio of aluminum to silicon, wherein the aluminum-oxygen bonds provide bonding sites with the substrate, and the silicon-oxygen network provides structural compactness and chemical stability. An optimal balance between corrosion resistance and interfacial toughness can be obtained when the molar ratio is in the range of 1:1 to 2:1.

[0030] In an optional embodiment, an AlOx gradient transition layer is provided between the scaly brittle functional layer and the conductive metal layer, where 0<x<1.5, and the oxygen content of the AlOx gradient transition layer increases continuously along the thickness direction, realizing a smooth composition transition from metallic aluminum to aluminum oxide.

[0031] In the present invention, if the oxygen content mutates between the conductive metal layer and the scaly brittle functional layer, mutation points of elastic modulus and thermal expansion coefficient will be formed at the interface, which become the initiation crack source for interlayer peeling. The AlOx gradient transition layer enables continuous change of physical properties from metal to ceramic, so that the needle prick stress can be smoothly transferred, the interlayer adhesion is increased from the order of 1.7 N / cm to more than 2.4 N / cm, and the inherent contradiction between surface layer embrittlement and interlayer bonding force is resolved structurally.

[0032] In an optional embodiment, in the direction away from the polymer substrate layer, the composite functional layer further includes a brittle metal layer disposed on the scaly brittle functional layer; the material of the brittle metal layer is at least one of simple metals of chromium, molybdenum, silver or alloys thereof, the thickness accounts for 3% to 10% of the total thickness of the composite functional layer on one side, the thickness is 10 to 70 nm, the surface roughness Ra is 30 to 200 nm, and the layer presents an island-like particle stacked morphology.

[0033] The brittle metal layer disposed on the outer surface of the scaly brittle functional layer has an island-like particle stacking morphology. Under extremely thin thickness and discontinuous morphology, its macroscopic continuous plastic flow is physically suppressed. During needle puncture, it breaks off synchronously with the scaly brittle functional layer without contributing additional burrs, and at the same time, it undertakes the functions of surface conductivity and interfacial bonding. Chromium, molybdenum, and silver have different degrees of intrinsic ductility at the macroscopic bulk material level, but under the extremely thin thickness and discontinuous island stacking morphology defined in this invention, their macroscopic continuous plastic flow is physically suppressed, and they cannot form continuous dimple tensile bands. Under high-speed impact, they exhibit quasi-brittle fracture characteristics and can break off synchronously with the underlying scaly brittle oxide layer without contributing additional burrs. The upper limit of the thickness is controlled within 70 nm because beyond this thickness, the island particles gradually connect laterally, restore macroscopic ductility, and the quasi-brittle mechanism fails. The surface roughness Ra is controlled at 30~200 nm to ensure that sufficient roughness peaks form mechanical anchoring with the positive electrode active material and enhance interfacial bonding, while avoiding excessive roughness that would cause tip discharge and local current concentration. This layer simultaneously improves surface conductivity and reduces contact resistance.

[0034] In an optional embodiment, the polymer substrate layer is made of any one of polyethylene terephthalate (PET), polypropylene (PP), polyimide (PI), and polyethylene (PE), and has a thickness of 4~12 μm.

[0035] The above-mentioned substrates are all conventional polymer film materials in the battery field and can withstand the temperature of vapor deposition process: PET has balanced comprehensive performance and strong oxygen bond bonding ability with oxide underlayer, PP has good cost and good electrolyte resistance, PI has the best heat resistance and is suitable for high temperature processes, and PE has good flexibility. When the thickness is less than 4μm, the substrate is easy to tear under the tension of roll-to-roll film and has insufficient barrier performance. When it is greater than 12μm, the weight reduction advantage compared with traditional pure aluminum foil is significantly lost. 4~12μm is a balance between processability and weight reduction benefits.

[0036] In an optional embodiment, the conductive metal layer is made of aluminum with a purity greater than 99.99%.

[0037] Impurities such as iron, silicon, and copper can form localized alloying micro-regions within the aluminum layer. On the one hand, this disrupts the continuous and uniform growth of dense equiaxed crystals and introduces weak points at grain boundaries. On the other hand, it reduces electrical conductivity and becomes the starting point for resistance to electrolyte corrosion. High-purity aluminum with a purity greater than 99.99% can ensure that the conductive metal layer forms a uniform and dense equiaxed crystal structure, stably undertaking the dual functions of basic conductive path and interlayer bonding.

[0038] Secondly, the present invention provides a method for preparing the composite positive current collector according to any one of the foregoing embodiments, comprising the following steps: Step 1: The polymer substrate is sent into a vacuum chamber and activated by plasma surface cleaning to obtain a polymer substrate layer; Step 2: Evaporate along the normal direction of the polymer substrate layer to deposit a dense conductive metal layer, controlling the deposition thickness to be 10%~40% of the total thickness of the single-sided composite functional layer; Step 3: Make the incident direction of the evaporating atoms form an angle of 25°~55° with the normal direction of the polymer substrate layer, and introduce the reactive gas to form the scaly brittle functional layer by vapor deposition, controlling the deposition thickness to account for 30%~50% of the total thickness of the single-sided composite functional layer; Step 4: Repeat steps 2 and 3 on the other side of the polymer substrate layer to complete the double-sided sequential vapor deposition and obtain the composite positive electrode current collector.

[0039] In the preparation method provided by this invention, step one, argon plasma cleaning, removes contaminants adsorbed on the substrate surface and activates the surface, providing a clean deposition starting point for each functional layer; step two, a dense equiaxed pure aluminum layer is deposited with normal incidence to construct a conductive path and connect the upper and lower functional layers; step three is the angled evaporation step in the two-step method, which uses the incident angle to control the shadow effect to induce the tilted growth of lamellar grains, forming a scaly brittle functional layer in one step; step four, double-sided sequential evaporation ensures the symmetry of the composite functional layer structure on both sides.

[0040] The reason for limiting the incident angle in step three to 25°~55° is as follows: If the incident angle is less than 25°, the shading effect is extremely weak, and the film tends to grow in a dense columnar shape (relative density >90%), failing to form a scaly lamellar structure, resulting in continuous ductile tearing upon fracture; if the incident angle is greater than 55°, the shading effect is too strong, causing a sharp drop in the relative density of the film (<60%), which will produce penetrating pinhole defects, leading to electrolyte penetration and corrosion of the underlying aluminum layer. Therefore, the incident angle range of 25°~55° and the corresponding relative density of 70%~85% represent a critical balance between forming effective cleavage surfaces and ensuring the corrosion resistance of the underlying layer.

[0041] In an optional embodiment, the above preparation method is completed continuously in the same vacuum roll-to-roll evaporation equipment. The equipment is provided with a first evaporation chamber for evaporating a conductive metal layer, a second evaporation chamber for evaporating a scaly brittle functional layer, and a third evaporation chamber for evaporating a brittle metal layer in sequence along the film-moving direction. A transition chamber is provided between the second evaporation chamber and the third evaporation chamber. The evaporation source assembly of the second evaporation chamber is horizontally offset along the film-moving direction and deviates from the center of the cooling drum normal to form the incident angle of 25°~55°.

[0042] For normal incidence deposition, the evaporation source only needs to be positioned directly below the normal of the coating main roller, while for oblique incidence deposition, the evaporation source assembly only needs to be horizontally offset. Neither arrangement changes the basic configuration of the evaporation equipment. Existing mass-produced roll-to-roll evaporation lines can be modified simply by adjusting the position of the evaporation source and adding chamber partitions. The modification cost is low and it is highly compatible with large-scale mass production.

[0043] In an optional implementation, stainless steel water-cooled baffles are used to separate the evaporation chambers. The gap between the upper and lower water-cooled baffles is 1-3 mm. A support roller is provided on each side of the baffle. Each chamber is provided with a different number of air extraction ports to achieve a gradient vacuum.

[0044] The first evaporation chamber requires a high vacuum environment to deposit pure aluminum, ensuring the purity and density of the aluminum film. The second evaporation chamber requires a working gas pressure of 0.5~3.0Pa and an oxygen partial pressure to deposit a brittle, scaly functional layer. The pressure difference between the two chambers is several orders of magnitude. Without isolation, oxygen-containing atmosphere will seep into the first evaporation chamber, causing premature oxidation of the pure aluminum layer. The 1~3mm gap of the water-cooled baffle is a balance between the continuous film flow channel and gas barrier. If the gap is too small, the film material cannot pass through and is easily scratched. If it is too large, gas cross-flow will occur, and the isolation will be ineffective. The water-cooled structure also blocks heat radiation from the evaporation source from crossing the chambers. The support rollers on both sides of the baffle allow the film material to pass smoothly through the narrow gap, preventing scratches and wrinkles. Each chamber is independently equipped with a different number of evacuation ports to maintain the required vacuum gradient for each chamber.

[0045] In an optional embodiment, in step one, the polymer substrate surface has an underlayer, the material of which is at least one of alumina, silicon dioxide, and aluminum-silicon composite oxide; the cavity vacuum degree in step one is ≤5×10 - 3 In step two, the evaporation temperature is 800~1400℃ and the film flow rate is 5~25 m / min. In step three, multiple oxygen inlets are set along the film flow direction to increase the oxygen flow rate from 0 to 20~35 sccm. The substrate temperature is controlled below 100℃, the working pressure of the cavity is 0.5~3.0 Pa, the oxygen partial pressure ratio is 20%~35%, the evaporation temperature is 800~1400℃, and the film flow rate is 10~25 m / min.

[0046] Background vacuum degree ≤5×10 -3Pa prevents residual gas from contaminating the film layer; the evaporation temperature of 800~1400℃ covers the stable evaporation process window of aluminum raw materials (correspondingly increased to 1400~1600℃ when using silicon as the evaporation raw material to deposit a silicon dioxide scaly layer). If the temperature is too low, the evaporation rate is insufficient, resulting in a thin and non-dense film layer; if it is too high, the evaporation rate is too fast, and the thickness uniformity is difficult to control; the film flow rate and deposition rate together determine the single-pass film thickness. In continuous film flow, all evaporation chambers share the same film flow rate, within the range of 5~25 The thickness control accuracy and production capacity are balanced within the m / min range; the substrate temperature is controlled below 100℃ to prevent thermal deformation of the polymer substrate; the oxygen flow rate is gradually increased from 0 at the three oxygen inlets, and the metal vapor is deposited in a low-oxygen atmosphere in the early stage of evaporation. As the oxygen flow rate increases, its oxidation degree continuously increases, thus first forming an oxide gradient transition layer with continuously increasing oxygen content on the surface of the conductive metal layer, and then entering the full oxidation deposition stage of the scaly layer. This achieves the integrated continuous preparation of the gradient transition layer and the scaly layer in one fell swoop: when aluminum is used as the evaporation raw material, the gradient transition layer is an AlOx gradient transition layer and the scaly brittle functional layer is aluminum oxide; when silicon is used as the evaporation raw material, the gradient transition layer is a silicon oxide gradient transition layer with continuously increasing oxygen content and the scaly brittle functional layer is silicon dioxide; the oxygen partial pressure ratio of 20%~35% ensures that the metal vapor is fully oxidized into the corresponding oxide, and does not reduce the arrival rate of evaporation atoms due to excessive oxygen scattering.

[0047] In an optional implementation, a step for preparing a brittle metal layer is added between step three and step four, including: A brittle metal layer is deposited by incident vapor deposition along the normal direction of the polymer substrate layer, controlling the deposition thickness to be 3%~10% of the total thickness of the single-sided composite functional layer, so that the metal grows in an island-like manner, forming a discontinuous island-like particle stack morphology. The vapor deposition raw material for the brittle metal layer is high-purity chromium particles with a particle size of 1~3 mm and a purity ≥99.95%. The background vacuum degree of the chamber is evaporated to ≤5×10 -3 Pa.

[0048] In an optional embodiment, the preparation method further includes the step of depositing a brittle metal layer in a third evaporation chamber following the second evaporation chamber: the evaporation source is arranged normally incident along the coating main roller, and the deposition thickness is controlled to be 3% to 10% of the total thickness of the single-sided composite functional layer. Through an extremely thin deposition thickness and a rapid nucleation process with normal incident incidence, the metal grows in an island-like manner, forming a discontinuous island-like particle stack morphology. The evaporation raw material for depositing the brittle metal layer is high-purity chromium particles with a particle size of 1 to 3 mm and a purity ≥ 99.95%, and the chamber's background vacuum is evacuated to ≤ 5 × 10⁻⁶. -3 Pa.

[0049] Under normal incidence, evaporating atoms rapidly nucleate with a high arrival rate. Combined with an extremely thin deposition layer, subsequent atoms preferentially attach to existing nuclei, making it difficult for the nuclei to spread laterally and connect to form a film. This naturally results in an island-like particle stacking morphology, ensuring the quasi-brittle fracture behavior of this layer. The chromium particle size is controlled between 1 and 3 mm; too small a particle size results in a large specific surface area and fluctuating evaporation rate, while too large a particle size leads to insufficient melting and unstable evaporation. The background vacuum degree is ≤5×10⁻⁶. -3 Pa ensures the purity of the chromium layer.

[0050] In an optional embodiment, the material of the scaly brittle functional layer is silicon dioxide, the evaporation raw material is high-purity polycrystalline silicon, the evaporation temperature is 1400~1600℃, and the oxygen flow rate is 20~30 sccm.

[0051] In this scheme, the outer layer of scaly silica can further improve the thoroughness of cleavage fracture and the stiffness of the composite positive current collector; silica needs to be generated by the evaporation and oxidation of silicon source. Since the evaporation temperature of polycrystalline silicon is higher than that of aluminum, the evaporation temperature is raised to 1400~1600℃ and the oxygen flow rate is controlled at 20~30sccm to ensure that silicon vapor is fully oxidized.

[0052] In an optional embodiment, the brittle metal layer is made of silver, and its preparation process parameters are as follows: the evaporation raw material is high-purity silver particles, the evaporation temperature is 900~1100℃, the deposition thickness is controlled at 10~30 nm, and the surface roughness Ra of the resulting silver island layer is 50~120 nm.

[0053] In this scheme, silver has the highest electrical conductivity among commonly used metals, and its conductivity is superior at the same thickness. Silver has a lower evaporation temperature than aluminum and chromium, and its deposition rate is mild and easy to control. Silver maintains a discontinuous island morphology with a deposition amount of 10~30nm. The rough surface with Ra of 50~120nm significantly enhances the mechanical anchoring of the positive electrode active material. The peel strength is more than 30% higher than that of the control sample without island layer. In addition, no additional burrs caused by the extension of silver layer were observed in the needle penetration test, which verifies the inhibitory effect of island morphology on macroscopic plastic flow.

[0054] Thirdly, the present invention provides a positive electrode sheet, comprising a positive active material layer and a composite positive current collector as described in any of the foregoing embodiments, wherein the positive active material layer is disposed on at least one side surface of the composite positive current collector.

[0055] Fourthly, the present invention provides a lithium-ion battery, including the positive electrode sheet described in the foregoing embodiments.

[0056] With the positive current collector provided by the present invention, cleavage fracture rather than ductile tearing occurs on the surface layer of the current collector during the nail penetration process, which fundamentally suppresses severe internal short circuits caused by burrs penetrating the separator; meanwhile, the brittle metal layer reduces the contact impedance between the current collector and the active material and improves the peeling strength, so that the pole piece maintains an intact interface during the rolling and slitting processes, and the nail penetration safety performance and processing yield of the battery are improved simultaneously.

[0057] The features and performance of the present invention are described in further detail below with reference to the examples.

[0058] Example 1 A PET film with a thickness of 6 μm is selected as the base material, on the surface of which an aluminum oxide primer layer with a thickness of 25 nm (accounting for 2.5% of the total thickness of the single-side composite functional layer) is pre-deposited. The PET base material with the primer layer is loaded into a vacuum roll-to-roll evaporation apparatus, which is provided with a first evaporation chamber for evaporating pure aluminum, a second evaporation chamber for evaporating the scaly brittle functional layer, and a third evaporation chamber for evaporating the brittle metal layer. The chambers are separated by stainless steel water-cooled baffles with a gap of 2 mm, a support roller is arranged on each of both sides of the baffle, and a different number of air extraction ports are arranged in each chamber to form a gradient vacuum degree.

[0059] The base material is cleaned and activated by argon plasma, and the vacuum degree of the chamber is pumped to 5×10 -3 Pa.

[0060] First evaporation chamber: using 99.999% high-purity aluminum wire as the raw material, evaporating with a resistive evaporation boat, the evaporation source is arranged in normal incidence along the normal line of the No.1 main roller, the evaporation temperature is 1350°C, the film running speed is 15 m / min, and a dense equiaxed crystal conductive metal layer of 400 nm is deposited, accounting for 40% of the total thickness (1000 nm) of the single-side composite functional layer.

[0061] Second evaporation chamber: the entire evaporation source assembly is horizontally offset along the film running direction, so that the incident direction of evaporated atoms forms an included angle of 40° with the normal line of the base material; using high-purity aluminum as the evaporation source, the evaporation temperature is 1350°C, the film running speed is 15 m / min, the substrate temperature is controlled at 80°C, and the working air pressure of the chamber is 1.5 Pa; three oxygen inlet ports are provided, the oxygen flow rate gradient increases from 0 to 30 sccm, and the oxygen partial pressure accounts for 28%. During the deposition process, an AlOx gradient transition layer with a continuously increasing oxygen content of 115 nm (0<x<1.5) is naturally formed on the surface of the conductive metal layer first, and then an aluminum oxide scaly brittle functional layer of 400 nm is continuously deposited, accounting for 40% of the total thickness; characterized by SEM, the included angle between the axial direction of the flaky crystal grains and the normal line of the base material is 45°, the transverse dimension of a single scale is 100 nm, and the relative density of the film layer is 78%.

[0062] Third evaporation chamber: using high-purity chromium particles with a particle size of 2 mm and a purity of 99.99% as the raw material, the evaporation source is arranged in normal incidence along the normal line of the No.3 main roller, and the background vacuum degree of the chamber is pumped to 5×10 -3Pa, a brittle metal layer (chromium) of 60 nm is deposited, accounting for 6% of the total thickness, in the form of island-like particle stacks with a surface roughness Ra of 80 nm. Subsequently, the cavity and roller are symmetrically arranged to complete the sequential evaporation on the other side, resulting in a composite positive current collector with a symmetrical double-sided structure.

[0063] Example 2 The only difference between this embodiment and Embodiment 1 is that the horizontal offset of the evaporation source in the second evaporation chamber is reduced, so that the incident direction of the evaporating atoms forms a 25° angle with the normal of the substrate; correspondingly, the axial angle between the lamellar grains of the resulting scaly brittle functional layer and the normal is 30°, the lateral dimension of a single scale is 148 nm, and the relative density of the film layer is 85%. The remaining parameters are the same as in Embodiment 1.

[0064] Example 3 The only difference between this embodiment and Embodiment 1 is that the horizontal offset of the evaporation source in the second evaporation chamber is increased, so that the incident direction of the evaporating atoms forms a 55° angle with the normal of the substrate; correspondingly, the axial angle between the plate-like grains and the normal is 60°, the lateral dimension of a single scale is 52 nm, and the relative density of the film layer is 71%. The remaining parameters are the same as in Embodiment 1.

[0065] Example 4 The only difference between this embodiment and Embodiment 1 is that the thickness of each layer of the single-sided composite functional layer is adjusted to be: 25nm (2.5%) for the base layer, 250nm (25%) for the conductive metal layer, 165nm for the AlOx gradient transition layer, 500nm for the scaly brittle functional layer (accounting for 50% of the total thickness of 1000nm), and 60nm (6%) for the brittle metal layer. The other parameters are the same as in Embodiment 1.

[0066] Example 5 The only difference between this embodiment and Embodiment 1 is that the total thickness of the single-sided composite functional layer is adjusted to 800nm, and the thickness of each layer is as follows: 20nm (2.5%) for the base layer, 320nm (40%) for the conductive metal layer, 170nm for the AlOx gradient transition layer, 240nm (30% of the total thickness) for the scaly brittle functional layer, and 50nm (6.25%) for the brittle metal layer. The incident angle of the second evaporation chamber is 40°, and the other parameters are the same as those in Embodiment 1.

[0067] Example 6 The only difference between this embodiment and Embodiment 1 is that: the raw material for evaporation in the second evaporation chamber is high-purity polycrystalline silicon, the evaporation temperature is 1500℃, the oxygen flow rate is 25 sccm, a 115nm SiOx gradient transition layer with continuously increasing oxygen content is first formed on the surface of the conductive metal layer, followed by the deposition of a 400nm scaly brittle functional layer of silicon dioxide (accounting for 40% of the total thickness); the angle between the axial direction and the normal of the plate-like grains is 44°, the lateral dimension of a single scale is 95nm, and the relative density is 76%. The remaining parameters are the same as in Embodiment 1.

[0068] Example 7 The only difference between this embodiment and Embodiment 1 is that: the raw material for evaporation in the third evaporation chamber is high-purity silver particles, the evaporation temperature is 1000℃, the deposition thickness is controlled at 30nm, and the surface roughness Ra of the resulting silver island layer is 85nm. The thickness of the AlOx gradient transition layer is adjusted from 115nm to 145nm accordingly, and the other parameters are the same as in Embodiment 1.

[0069] Example 8 Changes to the underlayer: The alumina underlayer in Example 1 was replaced with an aluminum-silicon composite oxide underlayer with a thickness of 25 nm (Al to Si molar ratio 1.5:1), and the other parameters were the same as in Example 1.

[0070] Example 9 The only difference between this embodiment and Embodiment 1 is that the thickness of each layer of the single-sided composite functional layer is adjusted to be: 25nm (2.5%) for the base layer, 250nm (25%) for the conductive metal layer, 115nm for the AlOx gradient transition layer, 550nm for the scaly brittle functional layer (accounting for 55% of the total thickness of the single-sided composite functional layer), and 60nm (6%) for the brittle metal layer; the incident angle of the second evaporation chamber is 40°, the angle between the axial direction of the plate-like grains and the normal is 45°, the lateral dimension of a single scale is 100nm, and the relative density of the film layer is 78%. The remaining parameters are the same as in Embodiment 1.

[0071] Example 10 The only difference between this embodiment and embodiment 5 is that the scaly brittle functional layer is thinned from 240nm (accounting for 30% of the total thickness of 800nm) to 200nm (accounting for 25%), and the reduced thickness is included in the AlOx gradient transition layer (increased from 170nm to 210nm). The other parameters are the same as in embodiment 5. The incident angle of the second evaporation chamber is 40°, the angle between the axis of the plate-like grains and the normal is 45°, the lateral dimension of a single scale is 100nm, and the relative density of the film layer is 78%.

[0072] Example 11 The only difference between this embodiment and Embodiment 1 is that: oxygen is introduced into the second evaporation chamber at a constant flow rate of 30 sccm from the beginning of deposition (no gradient oxygen supply), and an interface with a sudden change in oxygen content is formed between the conductive metal layer and the scaly brittle functional layer, that is, there is no AlOx gradient transition layer, and the thickness of the original gradient layer is incorporated into the scaly brittle functional layer (515 nm, accounting for 51.5% of the total thickness of the single-sided composite functional layer). The other parameters are the same as in Embodiment 1.

[0073] Comparative Example 1 This comparative example uses the same PET substrate and alumina underlayer (25 nm) as in Example 1. The first evaporation chamber is used for positive incident deposition of a 975 nm pure aluminum layer (accounting for 97.5% of the total thickness of 1000 nm). The second and third evaporation chambers are not used, which is the traditional structure with a dense pure aluminum layer as the outermost layer.

[0074] Comparative Example 2 The only difference between this comparative example and Example 1 is that the incident angle of the second evaporation chamber is adjusted to 20° (less than 25°), while the other parameters are the same as in Example 1. The resulting oxide outer layer has a dense columnar crystal structure with a relative density of 92%, no scaly lamellar morphology, and the angle between the grain axis and the normal is about 8°.

[0075] Comparative Example 3 The only difference between this comparative example and Example 1 is that the incident angle of the second evaporation chamber is adjusted to 60° (greater than 55°), while the other parameters are the same as in Example 1. The relative density of the resulting film layer decreased to 55%, and through-hole pinhole defects appeared.

[0076] Comparative Example 4 The only difference between this comparative example and Example 1 is that the evaporation source in the second evaporation chamber is arranged to restore the normal incidence (without offset), the oxygen flow rate is constant at 30 sccm, a dense alumina outer layer of 400 nm is deposited (accounting for 40% of the total thickness), there is no scaly morphology, and the relative density is 95%.

[0077] Comparative Example 5 This comparative example uses the same structure as Comparative Example 1, except that the pure aluminum layer is replaced with an aluminum-nickel alloy toughening layer with 3% nickel doping (the elongation is increased by about 60% compared to pure aluminum), which is the mainstream toughening and reinforcement route in the industry.

[0078] Performance testing The performance testing methods in each embodiment and comparative example are as follows: (1) Length of needle burrs: A steel needle with a diameter of 3 mm was used to penetrate the sample at a speed of 25 mm / s, and the average length of the burrs at the edge of the needle hole was counted using a scanning electron microscope. (2) Elastic modulus: Tested by tensile method according to GB / T 1040.3, and the increase was calculated based on traditional pure aluminum composite foil of the same thickness; (3) Interlayer adhesion: The peel force between the metal layer and the substrate is tested according to GB / T 2792 using the 90° peel method, in N / cm.

[0079] Table 1 Performance test results of the examples and comparative examples

[0080] The above results show that all embodiments can achieve the synergistic effect of reducing the average length of needle-punched burrs, increasing the elastic modulus, and improving interlayer adhesion; Comparative Examples 2 and 3 respectively demonstrate the effect of the 25°~55° incident angle and the corresponding 70%~85% relative density window; Comparative Example 5 proves that the toughening route will exacerbate the sharp burrs; Comparative Examples 1 and 4 respectively demonstrate the advantages of the scaly brittle functional layer in terms of fracture mechanism compared with the pure aluminum outer layer and the dense alumina outer layer.

[0081] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. 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 positive electrode current collector, characterized in that, comprising a polymer substrate layer, and composite functional layers respectively disposed on both sides of the polymer substrate layer in the thickness direction; in the direction away from the polymer substrate layer, the composite functional layer sequentially comprises a conductive metal layer and a scaly brittle functional layer; The scaly brittle functional layer has a scaly morphology formed by stacking obliquely grown flaky crystal grains, the angle between the axial direction of the flaky crystal grains and the normal direction of the polymer substrate layer is 30° to 60°, and the relative density of the scaly brittle functional layer is 70% to 85%.

2. The composite positive electrode current collector according to claim 1, characterized in that, The total thickness of the composite functional layer on a single side is 100 to 2000 nm; and / or, the thickness of the scaly brittle functional layer accounts for 30% to 50% of the total thickness of the composite functional layer on a single side, and the lateral dimension of a single scale is 50 to 150 nm; and / or, the conductive metal layer has a dense equiaxed grain structure.

3. The composite positive electrode current collector according to claim 1, characterized in that, The composite functional layer further comprises a primer layer located between the polymer substrate layer and the conductive metal layer, the primer layer is bonded to the polymer substrate layer through oxygen bonds, the thickness of the primer layer is 5 to 30 nm, and the thickness accounts for 2% to 10% of the total thickness of the composite functional layer on a single side; and / or, the material of the primer layer is at least one of aluminum oxide, silicon dioxide and aluminum-silicon composite oxide; the molar ratio of aluminum to silicon in the aluminum-silicon composite oxide is 1 to 2:1; and / or, the material of the conductive metal layer includes aluminum; An AlOx gradient transition layer is provided between the scaly brittle functional layer and the conductive metal layer, where 0<x<1.5, and the oxygen content of the AlOx gradient transition layer increases continuously along the thickness direction.

4. The composite positive current collector according to claim 3, characterized in that, in the direction away from the polymer substrate layer, the composite functional layer further comprises a brittle metal layer disposed on the scaly brittle functional layer; and / or, the material of the brittle metal layer is at least one of simple metals of chromium, molybdenum, silver or alloys thereof; and / or, the thickness of the brittle metal layer accounts for 3% to 10% of the total thickness of the composite functional layer on a single side, the thickness is 10 to 70 nm, the surface roughness Ra is 30 to 200 nm, and it presents an island-like particle stacking morphology.

5. A method for preparing a composite positive electrode current collector, characterized in that, comprising the following steps: Step 1: feeding the polymer substrate into a vacuum chamber, and performing plasma surface cleaning and activation to obtain a polymer substrate layer; Step 2: performing normal incidence evaporation along the normal direction of the polymer substrate layer to deposit a dense conductive metal layer, and controlling the deposition thickness to account for 10% to 40% of the total thickness of the composite functional layer on a single side; Step 3: making the incident direction of evaporated atoms form an angle of 25° to 55° with the normal direction of the polymer substrate layer, introducing a reactive gas, and performing evaporation to form the scaly brittle functional layer, controlling the deposition thickness to account for 30% to 50% of the total thickness of the composite functional layer on a single side; Step 4: repeating Step 2 and Step 3 on the other side of the polymer substrate layer to complete sequential evaporation on both sides, so as to obtain the composite positive current collector.

6. The preparation method according to claim 5, characterized in that, In step one, the surface of the polymer substrate has an underlayer, and the material of the underlayer is at least one of alumina, silicon dioxide and aluminum-silicon composite oxide; And / or, the cavity vacuum degree in step one is ≤5×10 -3 Pa; In step two, the evaporation temperature is 800~1400℃, and the film-moving speed is 5~25 m / min; And / or, in step three, multiple oxygen inlets are set along the film-moving direction to increase the oxygen flow rate from 0 to 20~35 sccm along the film-moving direction, the substrate temperature is controlled below 100℃, the working gas pressure of the cavity is 0.5~3.0 Pa, the oxygen partial pressure ratio is 20%~35%, the evaporation temperature is 800~1400℃, and the film-moving speed is 10~25 m / min.

7. The preparation method according to claim 5, characterized in that, The step of adding a brittle metal layer between step three and step four includes: A brittle metal layer is deposited by incident vapor deposition along the normal direction of the polymer substrate layer, controlling the deposition thickness to be 3%~10% of the total thickness of the single-sided composite functional layer, so that the metal grows in an island-like manner, forming a discontinuous island-like particle stack morphology; when the material of the brittle metal layer is chromium, the evaporation raw material for depositing the brittle metal layer is high-purity chromium particles with a particle size of 1~3 mm and a purity ≥99.95%, and the background vacuum degree of the chamber is evacuated to ≤5×10 -3 Pa.

8. The preparation method according to claim 5, characterized in that, When the material of the scaly brittle functional layer is silicon dioxide, the evaporation raw material is high-purity polycrystalline silicon, the evaporation temperature is 1400~1600℃, and the oxygen flow rate is 20~30sccm.

9. A positive electrode sheet, characterized in that, Includes the composite positive current collector as described in any one of claims 1 to 4 or the composite positive current collector prepared by the preparation method described in any one of claims 5 to 8.

10. A lithium-ion battery, characterized in that, Includes the positive electrode sheet as described in claim 9.

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