Bipolar current collector and preparation method and application thereof

Abstract

The invention provides a bipolar current collector and a preparation method and application thereof. The bipolar current collector sequentially comprises an aluminum layer, a buffer layer and a copper layer, the buffer layer is made of alloy, the alloy comprises at least one of Ni-W alloy, Ni-P alloy, Co-W alloy and Fe-Cr-B alloy, the thermal expansion coefficient of the alloy is 9-16 (* 10 <-6 > / K), and the microhardness of the alloy is 600-1300 HV. The bipolar current collector can effectively solve the problem that the interface bonding strength of the copper layer and the aluminum layer in the existing bipolar current collector is weak, so that the conductivity, the mechanical strength and the electrochemical stability of the bipolar current collector can be effectively improved.

Description

A bipolar current collector, its preparation method and application Technical Field

[0001] This invention belongs to the field of current collectors and lithium battery technology, specifically relating to a bipolar current collector, its preparation method and application. Background Technology

[0002] All-solid-state lithium batteries, as a next-generation energy storage technology, have attracted much attention due to their high safety and high energy density. The bipolar current collector is one of the core components of an all-solid-state battery, typically employing a copper-aluminum composite structure, where aluminum foil serves as the positive current collector and a copper layer as the negative current collector. This design reduces the use of inactive materials inside the battery, thereby improving its energy density and power density.

[0003] In existing technologies, aluminum-copper bipolar current collectors are mainly bonded by direct mechanical composite (such as pressing, rolling) or welding. However, this direct interface structure has a series of interrelated fundamental defects, as follows: (1) Thermal stress concentration and fatigue failure: Due to the significant difference in the coefficients of thermal expansion (CTE) of aluminum and copper, they will undergo uncoordinated expansion and contraction during battery charge-discharge cycles or thermal cycles caused by changes in ambient temperature. At the directly bonded interface, this deformation mismatch cannot be effectively coordinated or absorbed, resulting in a continuous concentration of huge shear thermal stress at the interface. Repeated stress cycles are very likely to induce the initiation and propagation of microcracks, eventually leading to interface delamination or fracture, causing the current collector's conductivity to decay or even fail completely.

[0004] (2) Inherent weakness of mechanical bonding and stress sensitivity: Direct composite mainly relies on physical contact and limited metallurgical bonding, and its intrinsic interface bonding strength is low. When subjected to external mechanical loads (such as battery assembly pressure, vibration, impact) or internal stress, this weak interface becomes a stress concentration point, which is prone to slippage or peeling.

[0005] (3) Electrochemical corrosion and interface degradation: In an electrolyte environment, direct contact between aluminum and copper will form a galvanic corrosion pair, with aluminum, as the anode, being more susceptible to corrosion. This not only erodes the interface itself and weakens the bonding force, but the corrosion products may also increase the interfacial resistance and even cause local overheating. The existing interface itself has weak strength and is even less able to resist this further degradation of mechanical properties caused by corrosion. Summary of the Invention

[0006] To address the problems and shortcomings of existing technologies, this invention provides a bipolar current collector, its preparation method, and its application. This bipolar current collector can effectively solve the problem of weak interfacial bonding strength between the copper and aluminum layers in existing bipolar current collectors, thus effectively improving the conductivity, mechanical strength, and electrochemical stability of the bipolar current collector.

[0007] According to a first aspect of the present invention, a bipolar current collector is provided, comprising, in sequence, an aluminum layer, a buffer layer, and a copper layer; the buffer layer is made of an alloy, including at least one selected from Ni-W alloy, Ni-P alloy, Co-W alloy, and Fe-Cr-B alloy; and the coefficient of thermal expansion of the alloy is 9~16 (×10⁻⁶). -6 / K), with a microhardness of 600~1300HV.

[0008] In current bipolar current collectors, aluminum and copper are typically bonded directly. However, this direct bonding leads to stress concentration and crack initiation at the interface. Therefore, addressing the poor interfacial strength between aluminum and copper has become a major challenge in bipolar current collectors.

[0009] This invention inserts a buffer layer between the two layers. The coefficient of thermal expansion (CTE) and microhardness of the alloy material constituting the buffer layer are designed to be within the aforementioned specific range, i.e., having a low CTE and a high microhardness. First, the CTE of the buffer layer's alloy material is lower than that of copper. This characteristic ensures that the buffer layer exhibits minimal dimensional change during repeated thermal cycling (heating-cooling), becoming a stable "anchor point" in the entire laminated structure. Therefore, this characteristic allows the buffer layer to simultaneously constrain the excessive expansion or contraction of the copper and aluminum layers with their high CTE during temperature changes, forcing their deformations to converge towards the buffer layer, thereby more effectively suppressing the generation of thermal stress and fundamentally reducing the risk of material fatigue and interface failure. Second, the high hardness of the buffer layer's alloy material makes it a super-strong "load-bearing skeleton," capable of withstanding and evenly distributing most of the mechanical stress from the copper and aluminum layers. This greatly protects the fragile copper-aluminum interface, thereby improving bonding strength and fatigue resistance, and preventing direct damage from stress concentration. This not only improves the overall interfacial bonding force under static conditions, but also significantly enhances the fatigue resistance and long-term reliability of the structure under dynamic thermal cycling or mechanical loads.

[0010] Therefore, this invention effectively reduces the thermal deformation and mechanical stress of the copper-aluminum composite structure by introducing a buffer layer with both low CTE and high hardness between the aluminum and copper layers. This significantly improves the mechanical load-bearing capacity and durability of the interface, resulting in a breakthrough improvement in the interfacial bonding force, dimensional stability, and fatigue life of the bipolar current collector. Consequently, the mechanical strength, conductivity, and electrochemical stability of the aluminum-copper bipolar current collector are significantly enhanced, thereby effectively improving the cycle stability, safety performance, and other electrochemical performance of batteries prepared using it, such as solid-state batteries.

[0011] It is further important to clarify that the coefficient of thermal expansion and microhardness of the buffer layer material should not be too low or too high. For example, if the coefficient of thermal expansion of the buffer layer material is too low, the expansion of aluminum and copper during heating will be strongly constrained by this "overly stable" layer, resulting in huge tensile stresses at the aluminum / buffer layer and buffer layer / copper interfaces, causing the buffer layer to peel off from the substrate or the interface to crack. Furthermore, materials with too low a coefficient of thermal expansion are often more brittle, potentially leading to poorer processability and bonding with metals. If the coefficient of thermal expansion of the buffer layer material is too high, it cannot effectively constrain and coordinate the larger expansion of the aluminum layer. If the microhardness of the buffer layer material is too low, the buffer layer is too soft and cannot effectively disperse mechanical and thermal stresses, which is detrimental to improving the interfacial bonding strength. If the microhardness of the buffer layer material is too high, when it bonds with relatively soft copper and aluminum, new stress concentration points may arise at the interface due to modulus mismatch. Additionally, an overly hard buffer layer may cause difficulties in subsequent rolling, stamping, and other processing steps, easily leading to microcracks.

[0012] It should be noted that the parameters such as the coefficient of thermal expansion and microhardness of the alloy material of the buffer layer can be designed through composition and process. For example, when the atomic ratio of Ni to W in Ni-W alloy is 75~85:15~25, or the atomic ratio of Ni to P in Ni-P alloy is 87~90:10~13, or the atomic ratio of Co to W in Co-W alloy is 75~85:15~25, or the atomic ratio of Fe, Cr, B in Fe-Cr-B alloy is 65~80:5~10:15~25, the above-mentioned parameters of thermal expansion and microhardness of the alloy material are within a specific range.

[0013] Preferably, the alloy has an amorphous and nanocrystalline composite structure. Furthermore, the buffer layer designed in this invention is made of amorphous / nanocrystalline alloy systems such as Ni-W and Ni-P. These materials often have nanoscale crystal particles embedded in an amorphous matrix. This composite structure often outperforms pure amorphous or coarse-grained structures. The simultaneous existence of long-range disordered amorphous and extremely fine-grained nanocrystalline structures, on the one hand, can efficiently transfer and relax the stress between the copper and aluminum layers like a robust gradient spring, effectively improving the problem of copper-aluminum ductility mismatch; on the other hand, due to its dense structure and lack of fast diffusion channels, it constructs a strong atomic diffusion barrier, fundamentally inhibiting the formation of brittle intermetallic compounds, thus simultaneously achieving excellent mechanical compatibility and superior interfacial stability. This extremely low interfacial stress and stable microstructure ensure uniform electrode surface current density, effectively suppressing lithium dendrite initiation and effectively improving the cycle performance of batteries using this bipolar current collector, especially all-solid-state batteries.

[0014] Furthermore, the buffer layer designed in this invention, composed of an amorphous / nanocrystalline alloy system, has an amorphous structure that is beneficial to improving the thermal stability of the bipolar current collector. This allows the bipolar current collector to maintain interface integrity and high mechanical strength even when the battery is operating at high temperatures or under thermal runaway conditions, thus delaying or preventing adverse chain reactions caused by interface failure of the bipolar current collector and improving the safety performance of the battery.

[0015] It should be further noted that the composite structure of amorphous and nanocrystalline alloys can be obtained through relevant process control.

[0016] Preferably, the coefficient of thermal expansion of the Ni-W alloy is 13~15 (×10⁻⁶). -6 / K), with a microhardness of 600~800HV; the coefficient of thermal expansion of Ni-P alloy is 14~16 (×10) -6 / K), with a microhardness of 500~700HV; the coefficient of thermal expansion of the Co-W alloy is 10~14 (×10) -6 / K), with a microhardness of 700~1100HV; the coefficient of thermal expansion of the Fe-Cr-B alloy is 9~13 (×10⁻⁶K). -6 / K), with a microhardness of 800~1300HV. The above-mentioned alloy materials with different buffer layers, by controlling different coefficients of thermal expansion and microhardness, are more suitable for the performance of these alloys themselves and their bonding ability with aluminum and copper. Therefore, they are more conducive to improving interface strength, reducing the formation of IMC at the interface, and further optimizing the mechanical stability and electrochemical performance of bipolar current collectors.

[0017] Preferably, the thicknesses of the aluminum layer, buffer layer, and copper layer are L1, L2, and L3, respectively, with L1:L2:L3 = 10~20:0.5~3:4~6. The buffer layer is designed to be relatively thin, which can effectively play its core role in thermo-mechanical performance gradient transition and stress buffering, while avoiding unnecessary brittleness or resistance caused by excessive thickness. The aluminum layer is the thickest because it not only serves as the positive electrode current collector but, more importantly, acts as the mechanical support base for the entire sandwich structure. It needs to provide sufficient rigidity, flatness, and strength to support the buffer layer and copper layer above, and withstand the mechanical stress in all subsequent processes such as electrode coating, rolling, and battery assembly. Therefore, the aluminum layer needs a certain thickness to ensure structural stability. The main function of the copper layer is as the active interface for the deposition / dissolution of the negative electrode (such as lithium metal) and as a high-efficiency current collector. Its core requirements are electrochemical stability, high conductivity, and surface uniformity, rather than providing major mechanical support. Therefore, an excessively thick copper layer does not contribute much to mechanical strength and may even have a negative impact. Moreover, aluminum has a lower density. Using a thicker aluminum layer, while maintaining the same mechanical strength, can significantly reduce the overall weight of the bipolar current collector compared to a thicker copper layer, which is crucial for improving battery energy density. Aluminum is also cheaper, making cost control easier. Furthermore, in solid-state batteries, the positive electrode side (aluminum layer) typically needs to withstand higher compaction density and potential volume changes; a thicker aluminum layer provides better dimensional stability and creep resistance. The negative electrode side (copper layer) requires an ultra-flat, uniform interface to guide uniform lithium deposition. A high-quality, dense thin copper layer better meets this requirement than a thick copper layer and reduces the absolute volume change stress during lithium deposition / stripping.

[0018] Therefore, the above-mentioned "thick-thin-thin" structure of aluminum layer-buffer layer-copper layer is more conducive to taking into account multiple aspects such as structural mechanics, material cost, lightweight requirements and electrochemical function.

[0019] Preferably, L1 = 10~20 μm. Preferably, L2 = 1~2 μm. Preferably, L3 = 4~6 μm. L1, L2, and L3 falling within these ranges are beneficial for achieving the optimal balance between the bipolar current collector interface stability, mechanical properties, and electrochemical performance, thus better balancing the battery's cycle performance and energy density.

[0020] Preferably, the aluminum layer includes a nano-aluminum layer and a non-nano-aluminum layer, with the nano-aluminum layer having a thickness of 1-5 μm. On the surface region of the side where the aluminum layer and buffer layer are composite, a nano-aluminum layer with a nanocrystalline structure is provided. This nanocrystalline metal structure has several advantages: First, it has an extremely high grain boundary density, providing more channels for atomic diffusion and significantly promoting atomic-level metallurgical bonding and physical anchoring between the aluminum layer and the subsequently deposited amorphous / nanocrystalline buffer layer, resulting in bonding strength far exceeding that of conventional microcrystalline aluminum materials. Second, the nano-sizing of the aluminum layer surface greatly enhances the hardness and strength of the aluminum layer interface region, forming a strong and tough transition zone that can effectively bear the stress transmitted by the buffer layer and prevent interface failure due to local yielding of the aluminum layer during processing or cycling. Third, the uniform and fine nanocrystalline structure can absorb and homogenize local stress concentration at the interface through grain boundary sliding and atomic rearrangement, buffering thermal stress caused by differences in thermal expansion coefficients, thereby inhibiting the initiation of microcracks. Fourth, the nanocrystalline structure allows for better control of the surface roughness of the aluminum layer. Appropriate roughness enables the aluminum layer and the buffer layer to jointly construct an extremely stable and durable composite interface. Therefore, the nanostructuring of the aluminum layer surface is more conducive to further optimizing the mechanical strength and thermal stability of the bipolar current collector, and improving the overall machinability and cycle life of the bipolar current collector.

[0021] Furthermore, setting the thickness of the nano-aluminum layer to 1~5μm serves two purposes. First, a depth of 1~5μm is sufficient to form a continuous and stable nanostructure transition layer, capable of fully bearing the interfacial shear stress and thermal stress transmitted from the buffer layer. This ensures that the strengthening effect of the interfacial bond has sufficient "thickness" support at the microscale, preventing the modified layer from being sheared through under stress due to excessive thinness. Second, limiting the depth of the surface nano-modification modified layer to within 5μm ensures that the aluminum foil substrate retains its original excellent ductility, conductivity, and low density characteristics. This avoids the overall brittleness of the aluminum layer, significant increase in resistance, or unnecessary weight increase caused by excessively deep surface treatment (such as severe work hardening), achieving the optimal balance between "interfacial strengthening" and "preservation of substrate properties."

[0022] It's important to further clarify that the distinction between nano-aluminum layers and non-nano-aluminum layers is primarily based on "grain size" and "microstructure." Conventional aluminum foil typically has a grain size in the micrometer range, classifying it as a "non-nano layer," while nano-aluminum layers have grain sizes refined to the nanoscale (typically <100nm). The thickness of nano-aluminum layers and non-nano-aluminum layers can be measured using methods such as TEM, scanning electron microscopy (SEM) combined with ion beam cutting / polishing.

[0023] According to a second aspect of the present invention, a method for preparing any of the above-mentioned bipolar current collectors is provided, comprising the following steps: S1. performing surface nano-sizing treatment on an aluminum foil; S2. depositing a buffer layer on one side of the aluminum foil after surface nano-sizing treatment in S1; S3. depositing a copper layer on the surface of the buffer layer, and annealing to obtain a bipolar current collector. In the method for preparing the bipolar current collector provided by the present invention, firstly, a strong and tough nanocrystalline activated surface layer is formed on an aluminum substrate in S1, providing an ideal substrate for subsequent deposition; then, in S2, a core amorphous / nanocrystalline buffer layer is deposited, directly forming a strong bond with the activated aluminum layer and establishing a thermo-mechanical performance gradient; finally, a functional copper layer is deposited in S3. This process ensures that each interface from the aluminum substrate to the copper surface is established on an optimized underlying layer, thereby facilitating tight bonding between layers, optimizing the interfacial strength of the material, and thus optimizing the overall mechanical strength, thermal stability, and electrochemical performance of the bipolar current collector.

[0024] Preferably, in S1, the surface roughness of one side of the aluminum foil after surface nano-sizing treatment is Ra, where Ra = 0.5~1.5 μm. This specific range of surface roughness (Ra = 0.5~1.5 μm) provides a substrate with a moderate micro-nano texture for the subsequently deposited buffer layer. This textured structure significantly increases the actual contact area between the aluminum layer and the buffer layer and creates a mechanical interlocking ("anchoring") effect, which can most effectively "lock" the buffer layer onto the aluminum layer, significantly enhancing interlayer bonding and shear resistance. This effectively optimizes the stability of the bipolar current collector interface and improves the subsequent bending, rolling, and other processing performance of the bipolar current collector, thus contributing to further improvement in the mechanical stability of the bipolar current collector.

[0025] Preferably, in S1, the surface roughness of one side of the aluminum foil after surface nano-sizing treatment is Ra, where Ra = 0.8~1.2 μm.

[0026] Preferably, in step S1, the specific operation for surface nano-sizing of the aluminum foil is as follows: Under an inert atmosphere, the surface of the aluminum foil is bombarded with bearing steel balls of 1-3 mm in diameter. The ultrasonic frequency of the bombardment process is 20 kHz ± 2 kHz, the amplitude is 30-50 μm, and the processing time is 3-5 min. Under these process conditions, the surface layer of the aluminum foil can be nano-sized without generating obvious structural defects on the surface of the aluminum foil, ensuring the formation of a clean and highly active aluminum foil surface, providing a chemical basis for subsequent atomic-level tight bonding with the buffer layer.

[0027] Preferably, in S1, the bearing steel ball is a GCr15 bearing steel ball.

[0028] Preferably, in step S1, the inert atmosphere includes argon. Preferably, before surface nano-sizing of the aluminum foil, the aluminum foil needs to be cleaned first. The specific cleaning procedure is as follows: the aluminum foil is sequentially immersed in acetone and anhydrous ethanol solutions for ultrasonic cleaning for 10-20 minutes each, followed by rinsing with deionized water and drying. Cleaning is to remove oil and organic matter from the aluminum foil surface, preventing a deterioration in the subsequent surface nano-sizing effect, which would affect the deposition effect of the buffer layer and the composite effect between the aluminum foil and the buffer layer.

[0029] Preferably, in S2, the buffer layer is deposited by magnetron sputtering, with the following specific process parameters: background vacuum ≤ 5.0 × 10⁻⁶. -4 Pa; inert gas working pressure is 0.5~0.8 Pa; sputtering power density is 2.0~3.5 W / cm²; deposition time is 30~45 min.

[0030] Preferably, in S2, during the process of magnetron sputtering to deposit the buffer layer, the specific process parameters also meet at least one of the following conditions: (1) the inert gas used includes argon; (2) the power supply used is a DC power supply; and (3) the substrate rotation speed is 10~20 rpm.

[0031] Preferably, in step S3, the annealing includes a first-stage procedure and a second-stage procedure. The specific process parameters for the first-stage procedure are: heating to 130-170℃ at a rate of 4.5-6.5℃ / min and holding for 20-40 min; the specific process parameters for the second-stage procedure are: heating to 200-250℃ at a rate of 1.5-3.5℃ / min and holding for 8-15 min. First, a lower temperature of 130-170℃ is reached at a heating rate of 4.5-6.5℃ / min, and this temperature is held for a relatively long time. This aims to gently release most of the intrinsic stress (such as atomic vacancies and lattice distortion) accumulated in the buffer layer and copper layer during room temperature deposition. Simultaneously, this temperature promotes slight structural relaxation in the amorphous / nanocrystalline buffer layer, stabilizing its amorphous / nanocrystalline composite structure and initially enhancing the physical bonding between layers. This provides a stable foundation for the next stage of higher-temperature annealing, preventing interface mismatch or abnormal grain growth in the copper layer due to sudden high temperatures. Next, the temperature is increased to a higher temperature range at a slower rate. The core objective is to promote limited atomic interdiffusion at the interfaces of the aluminum / buffer layer and the buffer layer / copper layer. This controlled diffusion can form a trace amount of metallurgical bonding at the interface, significantly enhancing interlayer bonding and peel strength. At the same time, this temperature is sufficient to further eliminate residual stress and make the grain structure of the copper layer more stable, ultimately fusing the three layers into a whole with extremely low internal stress, strong interfaces, and a stable structure.

[0032] Therefore, the annealing process adopts a gradient procedure of "low first, then high, slow first, then even slower" rather than a one-time high-temperature annealing, which is more conducive to reducing the overall internal stress of the bipolar current collector and further improving the problem of interface delamination, thus obtaining a bipolar current collector with higher mechanical strength and better electrochemical performance.

[0033] Preferably, in step S3, during the annealing process, an inert gas is introduced. Preferably, the inert gas includes argon.

[0034] Preferably, in step S3, after annealing, the bipolar current collector is removed after cooling to 60°C.

[0035] According to a fourth aspect of the present invention, a battery is provided, the battery comprising any of the bipolar current collectors described above, or comprising a bipolar current collector prepared by any of the above preparation methods.

[0036] Preferably, the battery is a solid-state battery. The solid electrolyte used in solid-state batteries is a rigid solid, unlike liquid electrolytes which can flow to buffer volume changes. During charging and discharging, the volume expansion / contraction of electrode materials (especially the lithium metal anode) generates significant localized mechanical stress. Simultaneously, battery assembly requires extremely high pressure to ensure solid-solid interface contact. These two stresses easily lead to delamination at the interface of traditional bipolar current collectors (such as simple copper-aluminum composites), causing battery failure. However, when the bipolar current collector provided by this invention is applied to solid-state batteries, a specific amorphous / nanocrystalline buffer layer is designed between the copper and aluminum layers, resulting in low internal stress, high interface strength, and high-temperature stability. Therefore, this bipolar current collector can effectively resist the stress brought about by battery assembly and maintain high structural stability during solid-state battery assembly and subsequent cycling, avoiding degradation of battery cycle performance and safety performance due to delamination leading to electrode active material shedding or current path interruption. Therefore, the bipolar current collector provided by this invention is more suitable for improving the problems of solid-state batteries, i.e., it can better solve some of the current pain points of solid-state batteries. Detailed Implementation

[0037] To enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0038] Example 1: The bipolar current collector in this example was prepared according to the following steps: S1. A 15μm aluminum foil was sequentially immersed in acetone and anhydrous ethanol solutions for ultrasonic cleaning for 15 min each, followed by rinsing with deionized water and drying. The cleaned and dried aluminum foil underwent surface nano-sizing treatment. The specific equipment and main process parameters are as follows: Ultrasonic Shot Peening (USP) equipment; the shot material was GCr15 bearing steel balls with a diameter of 1-3 mm; the ultrasonic frequency was 20 kHz ± 2 kHz; the amplitude was 30-50 μm; and the processing time was 3-5 minutes. min; the ambient atmosphere is an inert gas (Ar); by adjusting the parameters in the above processing, the surface roughness of the aluminum foil is made to be 1.0 μm; it should be noted that the aluminum foil used in this invention has a surface roughness of 0.2 μm before undergoing specific surface nano-sizing treatment; S2. On one side of the aluminum foil after surface nano-sizing treatment in S1, a buffer layer is deposited, the material of the buffer layer is Ni-W alloy (amorphous / nanocrystalline composite material); the specific equipment and main process parameters are as follows: high vacuum multi-target magnetron sputtering coating system, the target material is Ni-W alloy target (W content is 20 at.%, Ni content is 80 at.%); background vacuum ≤5.0×10 - 4 Pa; Ar working pressure: 0.5-0.8 Pa; substrate temperature: room temperature; power supply: DC power supply; sputtering power density: 2.5-3.5 W / cm³ 2 The substrate rotation speed is 10-20 rpm; the deposition time is 30-45 min; in this step, the above magnetron sputtering process parameters are controlled to obtain a buffer layer with a film thickness of 1 μm; S3. A copper layer is deposited on the surface of the buffer layer, and after annealing, a bipolar current collector is obtained; the equipment and main process parameters for depositing the copper layer are as follows: a high-vacuum multi-target magnetron sputtering coating system, the target material is a high-purity copper target (99.99%); the base vacuum is ≤5.0×10 -4 Pa; Ar working pressure is 0.3-0.5 Pa; substrate temperature is strictly controlled at <60°C; power supply is DC, sputtering power density is 2.0-3.0 W / cm³. 2 The deposition time is 40-60 min. In this step, the above magnetron sputtering process parameters are adjusted to obtain a copper layer with a film thickness of 5 μm. The annealing adopts gradient annealing treatment. The equipment and main process parameters are as follows: vacuum tube furnace, the atmosphere during the annealing process is high-purity argon (99.99%). The annealing is divided into two-stage gradient annealing program. The first stage: the temperature is increased to 150°C at a rate of 5°C / min and held for 30 minutes. The second stage: the temperature is increased to 220°C at a rate of 3°C / min and held for 10 minutes. After the annealing is completed, the bipolar current collector is removed after cooling to 60°C in the furnace.

[0039] Example 2: The preparation of the bipolar current collector in this example differs from that in Example 1 in that the target material in the S2 deposition buffer layer is adjusted to a Ni-P alloy (P content of 10 at.%, Ni content of 90 at.%), and the specific deposition process parameters are adjusted adaptively according to actual needs; the rest is the same as in Example 1.

[0040] Example 3: The preparation of the bipolar current collector in this example differs from that in Example 1 in that the target material in the S2 deposition buffer layer is adjusted to a Co-W alloy (W content of 20 at.%, Co content of 20 at.%), and the specific deposition process parameters are adjusted adaptively according to actual needs; the rest is the same as in Example 1.

[0041] Example 4: The preparation of the bipolar current collector in this example differs from that in Example 1 in that the target material in the S2 deposition buffer layer is adjusted to an Fe-Cr-B alloy (Fe content is 70 at.%, Cr content is 10 at.%, and B content is 20 at.%). The specific deposition process parameters are adjusted adaptively according to actual needs; the rest is the same as in Example 1.

[0042] Example 5: The preparation of the bipolar current collector in this example differs from that in Example 1 in that, during the surface nano-sizing of the aluminum foil in S1, the specific process parameters during the surface nano-sizing process are adjusted (these process parameters can be adjusted adaptively according to the specific situation) to obtain an aluminum foil with a surface roughness of 1.5 μm; the rest is the same as in Example 1.

[0043] Example 6: The preparation of the bipolar current collector in this example differs from that in Example 1 in that, during the surface nano-sizing of the aluminum foil in S1, the specific process parameters during the surface nano-sizing process are adjusted (these process parameters can be adjusted adaptively according to the specific situation) to obtain an aluminum foil with a surface roughness of 1.2 μm; the rest is the same as in Example 1.

[0044] Example 7 The preparation of the bipolar current collector in this example differs from that in Example 1 in that, in the surface nano-sizing of the aluminum foil in S1, the specific process parameters during the surface nano-sizing process are adjusted (these process parameters can be adjusted adaptively according to the specific situation) to obtain an aluminum foil with a surface roughness of 0.8 μm; the rest is the same as in Example 1.

[0045] Example 8 The preparation of the bipolar current collector in this example differs from that in Example 1 in that, in the surface nano-sizing of the aluminum foil in S1, the specific process parameters during the surface nano-sizing process are adjusted (these process parameters can be adjusted adaptively according to the specific situation) to obtain an aluminum foil with a surface roughness of 0.5 μm; the rest is the same as in Example 1.

[0046] Example 9 The preparation of the bipolar current collector in this example differs from that in Example 1 in that the deposition thickness of the buffer layer in S2 is adjusted to 0.5 μm, which can be obtained by adjusting the specific deposition time; the rest is the same as in Example 1.

[0047] Example 10: The preparation of the bipolar current collector in this example differs from that in Example 1 in that the deposition thickness of the buffer layer in S2 is adjusted to 2 μm, which can be obtained by adjusting the specific deposition time; the rest is the same as in Example 1.

[0048] Example 11 The preparation of the bipolar current collector in this example differs from that in Example 1 in that the deposition thickness of the buffer layer in S2 is adjusted to 3 μm, which can be obtained by adjusting the specific deposition time; the rest is the same as in Example 1.

[0049] Example 12 The preparation of the bipolar current collector in this example differs from that in Example 1 in that the annealing process of S3 is carried out by a one-step heat preservation method, that is, the temperature is directly raised to 220°C at a rate of 3°C / min and held for 40min; the rest is the same as in Example 1.

[0050] Comparative Example 1: This comparative example uses a conventional rolling composite process to directly press aluminum foil and copper foil together to prepare a bipolar current collector without any intermediate buffer layer, followed by annealing at 250℃ for 1 hour.

[0051] Comparative Example 2 differs from Example 1 in that the target material in the S2 deposition buffer layer is pure nickel, i.e., a 1.0 μm pure nickel layer is deposited; the rest is the same as in Example 1.

[0052] Comparative Example 3 differs from Example 1 in that it does not perform the surface nano-sizing treatment in S1, nor does it deposit the buffer layer in S2; that is, it directly deposits a copper layer on the aluminum foil. The rest is the same as Example 1.

[0053] The bipolar current collector prepared in Comparative Example 4 differs from that in Example 1 in that, in the S2 deposition buffer layer, the W content in the Ni-W alloy target material is adjusted to 5 at.%, and the Ni content is adjusted to 95 at.%. The specific deposition process parameters are adjusted adaptively according to actual needs; the rest is the same as in Example 1.

[0054] Test Example 1. Experimental Construction Method The bipolar current collectors prepared in all the above examples and comparative examples were subjected to relevant tests as follows: (1) Tensile strength and elongation: The bipolar current collectors were tested for tensile strength and elongation according to the standard GB / T 1040.3-2006.

[0055] (2) Heat shrinkage rate: After baking the bipolar current collector at 150°C for 30 minutes, the heat shrinkage rate was tested.

[0056] (3) Interfacial bonding strength: According to the ASTM D3330 test standard, the bipolar current collector sample was cut into strips 20 mm wide, and its copper layer was fixed to the rigid substrate using 3M VHB double-sided tape. A universal testing machine was used to peel the sample at a rate of 50 mm / min at 90°. The average force value of the stable peeled section was taken, and the interfacial bonding strength per unit width was calculated, with the unit being N / m.

[0057] (4) Height of edge warping: Cut the bipolar current collector into 10*10cm size, place it naturally on a horizontal table, and measure the maximum height of edge warping of the bipolar current collector.

[0058] 2. The experimental results are shown in Table 1, which presents the relevant parameters and performance test results of the bipolar current collectors prepared in all the above embodiments and comparative examples. In Table 1, Ra represents the surface roughness of the aluminum foil after surface nano-sizing treatment with S1.

[0059] Table 1. Relevant parameters of the bipolar current collectors prepared in all the above embodiments and comparative examples.

[0060] As shown in Table 1, the bipolar current collector provided by the present invention has good mechanical strength, thermal stability and interfacial strength. Specifically, it has high tensile strength, elongation, thermal shrinkage rate and interfacial bonding force. Moreover, due to the above-mentioned excellent properties, the surface of the bipolar current collector is smoother, that is, the warping height is significantly reduced. For details, please refer to the data of the embodiments in Table 1 above.

[0061] In Comparative Example 1, a bipolar current collector was prepared by directly pressing aluminum foil and copper foil together using a rolling composite process without any intermediate buffer layer. This resulted in the bipolar current collector exhibiting lower tensile strength, elongation, interfacial bonding force, and higher thermal shrinkage, as well as a significant increase in edge warping height.

[0062] In Comparative Example 2, the target material for the buffer layer is pure nickel. Compared with alloys, pure nickel has a lower compatibility with aluminum and copper layers, which leads to a decrease in the tensile strength, elongation, and interfacial bonding of the bipolar current collector, an increase in thermal shrinkage, and an increase in the warping height.

[0063] In Comparative Example 3, the surface nano-sizing treatment in S1 was not performed, nor was the buffer layer in S2 deposited. Instead, a copper layer was deposited directly on the aluminum foil. Without the buffer layer and the nano-aluminum layer, the bonding strength between the aluminum layer and the copper layer was significantly reduced. As a result, the various properties of the bipolar current collector were significantly reduced.

[0064] In Comparative Example 4, although the material of the buffer layer is a Ni-W alloy, the coefficient of thermal expansion and / or microhardness are not within the specific preferred range, which also causes a decrease in the bonding ability between the aluminum layer and the copper layer, and consequently a decrease in the various properties of the bipolar current collector.

[0065] Further observation of Examples 1-4 reveals that when the alloy composition of the buffer layer material is adjusted, the coefficient of thermal expansion and microhardness of the material also change, resulting in changes in the various properties of the bipolar current collector. This indicates that the selection of alloy material has a certain impact on the various properties of the bipolar current collector.

[0066] Observing Examples 1, 5-8, it can be found that as the surface roughness of the aluminum layer undergoing surface nano-sizing changes from 1.5μm to 0.5μm, the various properties of the bipolar current collector change to different degrees. It can also be found that the bipolar current collectors of Examples 1 and 6, 7 (with roughness in the range of 0.8-1.2μm) have better performance.

[0067] Observing Examples 1, 9-11, it can be found that as the thickness of the buffer layer increases, the performance of the bipolar current collector generally shows a trend of first improving and then deteriorating. Moreover, when the thickness of the buffer layer is in the range of 1-2 μm, the performance of the bipolar current collector is better.

[0068] Observing Examples 1 and 12, the direct one-step heating method used in Example 12 resulted in a decrease in the various properties of the final bipolar current collector. This indicates that the two-step heating method of the present invention is more conducive to improving the various properties of the bipolar current collector.

[0069] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention, but such modifications or substitutions are all within the scope of protection of the present invention.

Claims

1. A bipolar current collector, characterized in that: The bipolar current collector sequentially comprises an aluminum layer, a buffer layer, and a copper layer; the buffer layer is made of an alloy, which includes at least one of Ni-W alloy, Ni-P alloy, Co-W alloy, and Fe-Cr-B alloy; and the coefficient of thermal expansion of the alloy is 9~16 (×10⁻⁶). -6 / K), with a microhardness of 600~1300HV.

2. The bipolar current collector as described in claim 1, characterized in that: The alloy has an amorphous and nanocrystalline composite structure.

3. The bipolar current collector as described in claim 1, characterized in that: The coefficient of thermal expansion of the Ni-W alloy is 13~15 (×10⁻⁶). -6 / K), with a microhardness of 600~800HV; the coefficient of thermal expansion of the Ni-P alloy is 14~16 (×10) -6 / K), with a microhardness of 500~700HV; the coefficient of thermal expansion of the Co-W alloy is 10~14 (×10) -6 / K), with a microhardness of 700~1100HV; the coefficient of thermal expansion of the Fe-Cr-B alloy is 9.0~13.0 (×10) -6 / K), with a microhardness of 800~1300HV.

4. The bipolar current collector as described in claim 1, characterized in that: The thicknesses of the aluminum layer, the buffer layer, and the copper layer are L1, L2, and L3, respectively, with L1:L2:L3 = 10~20:0.5~3:4~6.

5. The bipolar current collector as described in claim 1, characterized in that: The aluminum layer includes a nano-aluminum layer and a non-nano-aluminum layer, and the thickness of the nano-aluminum layer is 1~5μm.

6. A method for preparing a bipolar current collector as described in any one of claims 1 to 5, characterized in that, The process includes the following steps: S1. Performing a surface nano-sizing treatment on the aluminum foil; S2. Depositing the buffer layer on one side of the aluminum foil after the surface nano-sizing treatment in S1; S3. Depositing a copper layer on the surface of the buffer layer, and after annealing, obtaining the bipolar current collector.

7. The method for preparing the bipolar current collector as described in claim 6, characterized in that, In step S1, the surface roughness of one side of the aluminum foil after surface nano-treatment is Ra, where Ra = 0.5~1.5μm; and / or, the specific operation of surface nano-treatment of the aluminum foil is as follows: under an inert atmosphere, the surface of the aluminum foil is bombarded with bearing steel balls with a diameter of 1~3mm, the ultrasonic frequency of the bombardment process is 20 kHz±2 kHz, the amplitude is 30~50μm, and the treatment time is 3~5 min.

8. The method for preparing the bipolar current collector as described in claim 6, characterized in that, In step S2, the buffer layer is deposited using magnetron sputtering, with the following specific process parameters: background vacuum ≤ 5.0 × 10⁻⁶. -4 Pa; inert gas working pressure is 0.5~0.8 Pa; sputtering power density is 2.0~3.5 W / cm²; deposition time is 30~45 min.

9. The method for preparing the bipolar current collector as described in claim 6, characterized in that, In step S3, the annealing process includes a first-stage procedure and a second-stage procedure. The specific process parameters for the first-stage procedure are: heating to 130-170℃ at a rate of 4.5-6.5℃ / min and holding for 20-40min. The specific process parameters for the second-stage procedure are: heating to 200-250℃ at a rate of 1.5-3.5℃ / min and holding for 8-15min.

10. A battery, characterized in that: The battery includes the bipolar current collector as described in any one of claims 1 to 5, or includes the bipolar current collector prepared by the preparation method described in any one of claims 6 to 9.

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