Bipolar current collector, method of making the same, electrode sheet, battery, and electrochemical device

Abstract

This invention provides a bipolar current collector and its preparation method, electrode sheet, battery, and electrochemical device, specifically relating to the field of current collector technology. The bipolar current collector comprises a copper layer, a first interface layer, a conductive metal layer, a second interface layer, and an aluminum layer stacked sequentially; wherein the conductive metal layer is a chromium layer; the first interface layer is a copper-chromium physical mixture layer; and the second interface layer is a chromium-aluminum solid solution layer. By sequentially placing the first copper-chromium physical mixture interface layer, the pure chromium conductive metal layer, and the second interface layer containing the chromium-aluminum solid solution between the copper layer and the aluminum layer, the first interface layer achieves atomic-level mechanical bonding, resulting in high adhesion and low resistance; the second interface layer is a tough Cr-Al solid solution, resistant to bending, buffering stress, and preventing delamination; the dense chromium layer completely prevents direct contact between Cu and Al and the formation of brittle phases. These three elements synergistically ensure high conductivity, strong interface stability, and long-cycle reliability.

Description

Technical Field

[0001] This invention relates to the field of current collector technology, and in particular to a bipolar current collector and its preparation method, electrode sheet, battery, and electrochemical device. Background Technology

[0002] In existing technologies, bipolar current collectors typically employ a structure where copper and aluminum layers are directly composited or connected via transition metal layers such as titanium (Ti). This aims to integrate the positive and negative current collection functions onto a single substrate, reducing redundant components within the battery, increasing energy density, and simplifying thermal management design. This type of structure has been observed in the prototype development of stacked lithium-ion batteries and bipolar battery stacks, demonstrating the potential for modular adaptation to voltage / capacity.

[0003] However, direct copper-aluminum composites exhibit weak interfacial bonding and are prone to delamination during cycling. Furthermore, they are susceptible to interdiffusion under electrochemical conditions, forming high-resistivity, brittle Cu-Al intermetallic compounds that significantly increase internal resistance and lead to failure. While using titanium as an interlayer can improve bonding strength, it easily forms high-resistivity, easily cracked, brittle Ti-Al compounds, resulting in decreased conductivity and insufficient mechanical reliability, making it difficult to simultaneously meet the requirements of high performance and long lifespan.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a bipolar current collector and its preparation method, electrode sheet, battery and electrochemical device, aiming to solve at least one of the above-mentioned technical problems.

[0006] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: The first aspect of the present invention provides a bipolar current collector, comprising a copper layer, a first interface layer, a conductive metal layer, a second interface layer and an aluminum layer stacked sequentially; wherein the conductive metal layer is a chromium layer; the first interface layer is a copper-chromium physical hybrid layer; and the second interface layer is a chromium-aluminum solid solution layer.

[0007] Furthermore, in the copper-chromium physical hybrid layer, the copper content decreases along the direction closer to the aluminum layer.

[0008] And / or, in the chromium-aluminum solid solution layer, the aluminum content increases along the direction close to the aluminum layer.

[0009] And / or, the gradient copper-chromium hybrid layer is a copper-chromium physical hybrid layer.

[0010] And / or, the gradient chromium-aluminum mixed layer is a chromium-aluminum solid solution layer.

[0011] Furthermore, the thickness of the copper layer is 3~15μm, preferably 5~10μm.

[0012] And / or, the thickness of the first interface layer is 1~5nm, preferably 2~4nm.

[0013] And / or, the thickness of the conductive metal layer is 20~200nm, preferably 50~150nm.

[0014] And / or, the thickness of the second interface layer is 10~50nm, preferably 20~40nm.

[0015] And / or, the thickness of the aluminum layer is 1~5μm, preferably 2~4μm.

[0016] And / or, the thickness ratio of the second interface layer to the first interface layer is 5 to 15.

[0017] Furthermore, the surface roughness Ra of the conductive metal layer on the side near the aluminum layer is 30~150nm.

[0018] The second aspect of the present invention provides a method for preparing the bipolar current collector, wherein chromium is deposited on the surface of a copper foil by magnetron sputtering in a vacuum environment to form a first interface layer and a conductive metal layer, thereby obtaining a copper-chromium substrate; then an aluminum metal layer is vacuum evaporated onto the conductive metal layer of the copper-chromium substrate to form a second interface layer and an aluminum layer, thereby obtaining a bipolar current collector.

[0019] Furthermore, the vacuum level in the vacuum environment is 10. -4 ~10 -3 Pa.

[0020] And / or, during the magnetron sputtering process, the target material is a chromium target with a purity of ≥99.5%, the sputtering power is 4~10kW, the working gas pressure is 0.5~2.0Pa, and the substrate winding speed is 0.2~1.0m / min.

[0021] And / or, the vacuum evaporation process is carried out under a protective atmosphere.

[0022] And / or, the protective atmosphere is argon.

[0023] And / or, the flow rate of the argon gas is 10~50 sccm.

[0024] Furthermore, during the vacuum evaporation process, the temperature of the copper-chromium substrate is 150~250℃, and the substrate winding speed is 3~15m / min.

[0025] And / or, during the vacuum evaporation process, the aluminum wire feeding speed is 200~400mm / min, and the evaporation source temperature is 1100~1300℃.

[0026] A third aspect of the present invention provides an electrode sheet comprising the bipolar current collector described in the first aspect or the bipolar current collector prepared by the preparation method described in the second aspect.

[0027] A fourth aspect of the present invention provides a battery including the aforementioned electrode sheet.

[0028] A fifth aspect of the present invention provides an electrochemical device including the battery described above.

[0029] Compared with the prior art, the present invention has at least the following beneficial effects: The bipolar current collector provided by this invention achieves a triple synergistic effect by sequentially setting a first interface layer of copper-chromium mixture, a pure chromium conductive metal layer, and a second interface layer of chromium-aluminum solid solution between the copper and aluminum layers: the first interface layer forms an atomic-level embedded mechanical interlock at the copper / chromium interface, without the formation of brittle compounds, ensuring extremely high bonding force between the copper and chromium layers and allowing electrons to pass through almost unimpeded, significantly suppressing interface resistance; the second interface layer is a tough solid solution structure formed by aluminum / chromium interdiffusion, which has strong bonding force, excellent stress buffering capacity, and low resistivity, and can effectively absorb volumetric strain and bending stress during battery cycling, preventing delamination or cracking; the intermediate chromium layer itself acts as a dense barrier layer, completely blocking the direct contact and interdiffusion between copper and aluminum, eliminating the risk of Cu-Al brittle phase formation from the source, thereby maintaining high conductivity while achieving structural stability and electrical performance consistency of the current collector under long-term service.

[0030] The preparation method provided by this invention involves a magnetron sputtering process that simultaneously forms a copper-chromium mixed first interface layer and a dense pure chromium conductive metal layer on the surface of a copper foil. Utilizing the atomic shot-peening effect generated by high-energy chromium particle bombardment, atomic-level mechanical interlocking and self-compactment of the copper / chromium interface are achieved, resulting in a copper-chromium substrate with low porosity, high bonding strength, and controllable surface roughness. Subsequently, aluminum is vacuum-deposited, allowing aluminum atoms to diffuse downwards under the drive of concentration gradient and thermal activation, reacting with the chromium layer to form an aluminum-chromium solid solution second interface layer, upon which a continuous and dense aluminum layer is formed. This preparation method eliminates the need for adhesives, welding, or high-temperature annealing, avoiding the formation of brittle compounds and abrupt changes in interlayer stress. It ensures that the first and second interface layers evolve synergistically according to the designed state, thereby guaranteeing the bipolar current collector's comprehensive performance of high bonding strength, low interfacial resistance, and excellent cycle stability from the very beginning of the preparation process. Attached Figure Description

[0031] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0032] Figure 1 This is a schematic diagram of a bipolar current collector structure.

[0033] Icons: 10 - Copper layer; 20 - First interface layer; 30 - Conductive metal layer; 40 - Second interface layer; 50 - Aluminum layer. Detailed Implementation

[0034] The embodiments and examples of the present invention will be described in detail below with reference to the implementation methods and examples. However, those skilled in the art will understand that the following implementation methods and examples are only for illustrating the present invention and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] The first aspect of this invention provides a bipolar current collector, the schematic diagram of which is shown below. Figure 1 As shown, it includes a copper layer 10, a first interface layer 20, a conductive metal layer 30, a second interface layer 40, and an aluminum layer 50 stacked sequentially; wherein, the conductive metal layer 30 is a chromium layer; the first interface layer 20 is a copper-chromium physical hybrid layer; and the second interface layer 40 is a chromium-aluminum solid solution layer.

[0036] The bipolar current collector provided by this invention achieves a triple synergistic effect by sequentially setting a first interface layer 20 of copper-chromium mixture, a pure chromium conductive metal layer 30, and a second interface layer 40 of chromium-aluminum solid solution between the copper layer 10 and the aluminum layer 50: The first interface layer 20 forms an atomic-level embedded mechanical interlock at the copper / chromium interface, without the formation of brittle compounds, which not only ensures extremely high bonding force between the copper layer 10 and the chromium layer, but also allows electrons to pass through almost unimpeded, significantly suppressing interface resistance; The second interface layer 40 is a tough solid solution structure formed by aluminum / chromium interdiffusion, which has strong bonding force, excellent stress buffering capacity and low resistivity, and can effectively absorb volumetric strain and bending stress during battery cycling, preventing delamination or cracking; The intermediate chromium layer itself acts as a dense barrier layer, completely blocking the direct contact and interdiffusion between copper and aluminum, eliminating the risk of Cu-Al brittle phase formation from the source, thereby maintaining high conductivity while achieving structural stability and electrical performance consistency of the current collector under long-term service.

[0037] Furthermore, in the copper-chromium physical hybrid layer, the copper content decreases along the direction close to the aluminum layer 50.

[0038] And / or, in the chromium-aluminum solid solution layer, the aluminum content increases along the direction close to the aluminum layer 50.

[0039] And / or, the gradient copper-chromium mixed layer is a copper-chromium physical mixed layer, and the first interface layer 20 is a gradient copper-chromium physical mixed layer with a thickness of nanometers. Its composition is distributed in a continuous gradient along the thickness direction (the copper content decreases from the copper layer 10 side to the aluminum layer 50 side). The copper-chromium physical mixed layer is a non-equilibrium interface structure with a continuous distribution of composition along the thickness direction. Combined with the characteristics of the Cu-Cr binary equilibrium phase diagram, copper atoms and chromium atoms have significant differences in lattice type, electronic configuration and atomic coordination environment. The solid-state miscibility at room temperature is extremely low (less than 0.1 wt%). There is no intrinsic driving force for spontaneous interdiffusion and lattice dissolution at the thermodynamic level. The two will not form a copper-chromium solid solution, nor will they generate copper-chromium intermetallic compounds. They belong to a thermodynamically incompatible system. This layer is not a thermodynamically stable alloy phase or intermetallic compound, but rather an atomically interlocked mechanical hybrid structure induced in situ by high-energy particle bombardment in the early stages of magnetron sputtering. Cu and Cr atoms undergo limited-scale physical blending and local interlocking in the interface region, without significant interdiffusion, and no brittle Cu-Cr intermetallic compound is formed. This layer exhibits no lattice solute dissolution behavior and no brittle intermetallic phase formation. The continuous gradient design eliminates the abrupt stress change at the interface between the copper layer 10 and the chromium layer. High-strength bonding between the copper layer 10 and the chromium layer is achieved solely through atomically mechanical interlocking and physical interlocking. This avoids lattice distortion and stress concentration caused by solid solution phase transformation, completely eliminates the problems of interface cracking and increased internal resistance caused by brittle phases, and ensures smooth electronic conduction at the interface.

[0040] And / or, the gradient chromium-aluminum mixed layer is a chromium-aluminum solid solution layer, and the second interface layer 40 is also a gradient chromium-aluminum solid solution layer, with its composition also showing a gradient distribution. The aluminum content increases from the copper layer 10 side to the aluminum layer 50 side. Combined with the characteristics of the Cr-Al binary phase diagram, chromium and aluminum atoms can form a continuous substitutional solid solution thermodynamically, without the tendency to form highly brittle intermetallic compounds. Moreover, the solid solution phase has excellent electrical conductivity, toughness, and stress buffering capacity. During the vacuum evaporation process, this layer system is formed by the diffusion of gaseous Al atoms to the surface of the Cr layer under thermal activation and concentration gradient drive, and the limited mutual solubility with Cr. Its microstructure is a metastable face-centered cubic or body-centered cubic Cr-Al solid solution, rather than thermodynamically stable brittle intermetallic compounds such as Al3Cr and Cr2Al. This solid solution layer combines high toughness, excellent stress buffering capacity, and low resistivity, which can effectively absorb volume expansion / contraction stress and bending deformation during electrode cycling, suppress interlayer delamination or microcrack initiation, and maintain good longitudinal conductivity.

[0041] Furthermore, the thickness of the copper layer 10 is 3~15μm, preferably 5~10μm. A thickness of ≥3μm in the copper layer 10 can provide the necessary bending stiffness and yield strength, effectively suppressing plastic deformation, wrinkling and fracture during winding, stacking and cycling processes, and can also reasonably limit the amount of inactive copper material used; a thickness of ≤15μm in the copper layer 10 significantly reduces the current collector mass ratio, avoiding excessive sacrifice of energy density.

[0042] Typically, but not limitingly, the thickness of the copper layer 10 can be, for example, 3 μm, 5 μm, 8 μm, 10 μm, 12 μm or 15 μm, or any value in the range of 3 to 15 μm, preferably 5 to 10 μm.

[0043] And / or, the thickness of the first interface layer 20 is 1~5nm, which is sufficient to form a continuous and uniform atomic-level mixing structure on the surface of the copper layer 10, ensuring good coverage and bonding stability of the interface on a large-area substrate; at the same time, its nanoscale is significantly smaller than the mean free path of electrons, so that the scattering effect on electrons when passing through the layer is minimal, thereby maintaining a low interface resistance. If the thickness is less than 1nm, the mixing region is too thin, making it difficult to ensure the continuity of film formation and compositional uniformity across the entire area under roll-to-roll continuous sputtering process, which may easily lead to local unmixing or insufficient coverage. In subsequent processes (such as rolling, edge trimming) or long-term use during battery cycling, there is a risk of uneven bonding force, which may easily cause delamination and affect conductivity. If the thickness is greater than 5nm, the resistance contribution of the mixing layer itself increases, and it may be accompanied by broadening of the compositional gradient or local segregation, affecting the overall conductivity.

[0044] Typically, but not limitingly, the thickness of the first interface layer 20 can be, for example, 1 nm, 2 nm, 3 nm, 4 nm or 5 nm, or any value in the range of 1 to 5 nm, preferably 2 to 4 nm.

[0045] And / or, the thickness of the conductive metal layer 30 is 20~200nm. This thickness range ensures the formation of a continuous, pinhole-free, dense film on the surface of the copper layer 10, thereby completely covering the inherent microscopic unevenness of the copper foil and achieving effective physical barrier to direct contact between copper and aluminum, fundamentally preventing mutual diffusion and the formation of brittle phases. Simultaneously, during the subsequent vacuum evaporation of aluminum, this thickness provides a suitable and controllable amount of chromium atoms, allowing them to undergo limited mutual solubility with aluminum atoms under thermal activation and concentration gradient driving, forming the designed gradient chromium-aluminum mixture in situ. The second interface layer 40 ensures that the interface layer has sufficient thickness and bonding strength, while avoiding complete consumption of the chromium layer due to excessive thinness, or diffusion obstruction and insufficient reaction due to excessive thickness, which would affect the integrity and functional stability of the interface structure. In addition, the chromium layer within this thickness range exhibits good mechanical adaptability, maintaining structural continuity as the copper foil substrate bends and deforms, and is not prone to cracking or peeling. If it exceeds this range, the excessive thickness of the chromium layer will lead to increased internal resistance and microcracks due to increased internal stress during bending, which would damage the reliability and conductivity of the interface.

[0046] Typically, but not limitingly, the thickness of the conductive metal layer 30 can be, for example, 20 nm, 50 nm, 80 nm, 100 nm, 150 nm or 200 nm, or any value in the range of 20 to 200 nm, preferably 50 to 150 nm.

[0047] And / or, the thickness of the second interface layer 40 is 10~50 nm. The second interface layer 40 is located between the chromium layer and the aluminum layer 50, forming an extension of the copper-aluminum barrier structure. This helps maintain the integrity of the overall interface system and further reduces the risk of direct contact or indirect diffusion between copper and aluminum. During vacuum evaporation, this thickness supports the diffusion of aluminum atoms to the surface of the chromium layer under thermal activation and concentration gradient drive, forming a chromium-aluminum solid solution structure with a gradient composition, thereby achieving a stable bond between the chromium layer and the aluminum layer 50. Simultaneously, this solid solution structure possesses a certain degree of plastic deformation capability, which can buffer interfacial stress during battery fabrication and electrochemical cycling, suppressing the tendency for delamination caused by volume changes. A thickness greater than 50 nm in the second interface layer 40 will also lead to an increase in resistance.

[0048] Typically, but not limitingly, the thickness of the second interface layer 40 can be, for example, 10 nm, 20 nm, 30 nm, 40 nm or 50 nm, or any value in the range of 10 to 50 nm, preferably 20 to 40 nm.

[0049] And / or, the thickness of the aluminum layer 50 is 1~5μm, preferably 2~4μm. A thickness of ≥1μm can ensure sufficient density and continuity, so that the aluminum layer 50 can effectively resist the erosion of trace moisture and oxygen in the air / electrolyte during battery manufacturing and storage, and suppress the increase in contact resistance and interface passivation caused by non-uniform thickening or local perforation of the surface oxide film. A thickness of 1~5μm controls the mass ratio of the aluminum layer 50 at a reasonable level, avoiding a significant increase in the mass of inactive materials due to excessive thickness, which is conducive to improving the overall mass and volumetric energy density of the battery. Although the density of aluminum is lower than that of copper, its lightweight marginal benefit decreases sharply after >5μm.

[0050] Typically, but not limitingly, the thickness of the aluminum layer 50 can be, for example, 1 μm, 2 μm, 3 μm, 4 μm or 5 μm, or any value in the range of 1 to 5 μm, preferably 2 to 4 μm.

[0051] And / or, the thickness ratio of the second interface layer 40 to the first interface layer 20 is 5~15. The first interface layer 20 and the second interface layer 40 respectively undertake the functions of low resistance and high bonding on the copper / chromium side and tough and strong bonding on the chromium / aluminum side. The two interface layers can achieve stress matching and functional synergy. Specifically, this ratio ensures that the chromium-aluminum layer 50 has sufficient toughness to buffer cyclic stress, while avoiding excessive stress on its own which would damage the conductive integrity of the aluminum layer 50. The thicknesses of the two layers need to be matched to achieve a balance of stress and performance in the overall interface system.

[0052] If this ratio is less than 5, it means that the first interface layer 20 is relatively too thick, or the second interface layer 40 is relatively too thin. Both of these will cause stress to concentrate more easily at the relatively "brittle" second interface under the repeated stress of battery cycling, leading to fatigue damage or microcracks at this interface. Once cracks are formed, they become points of resistance surge and corrosion initiation points, causing the conductivity to decline more rapidly in the later stages of cycling.

[0053] If this ratio is greater than 15, it means that the second interface layer 40 is relatively too thick, or the first interface layer 20 is relatively too thin. Due to the mismatch in the coefficient of thermal expansion and the accumulation of deposition stress, the second interface layer 40 may experience microscopic warping, grain boundary cracking, or decreased adhesion between the aluminum layer 50 and the active material coating during subsequent processes (such as rolling and edge trimming) or long-term use. These microscopic defects directly increase the local impedance of electron transport and become corrosion channels, manifested as high initial resistance and poor stability with cycling.

[0054] Typically, but not limitingly, the thickness ratio of the second interface layer 40 to the first interface layer 20 can be, for example, 5, 8, 10, 12 or 15, or any value in the range of 5 to 15.

[0055] Furthermore, the surface roughness Ra of the conductive metal layer 30 near the aluminum layer 50 is 30~150nm. This certain surface roughness provides a large number of mechanical engagement points for the subsequent preparation of the aluminum layer 50. During the deposition process, aluminum atoms can embed themselves into the microscopic peaks and valleys of the chromium layer, forming an effective "hook" or "anchor" effect. This mechanical interlocking greatly enhances the bonding force between chromium and aluminum, providing physical protection against shear stress during battery manufacturing and use. If Ra is small, it indicates that the separator surface is too smooth, which reduces the nucleation points of aluminum atoms. This may lead to island-like growth of the aluminum film in the early stage, making it difficult to quickly form a continuous and dense film layer, and posing a risk of pores or weak areas. If Ra is large, the surface undulation is too large, which may cause aluminum atoms to be deposited too much at the "valley" and insufficient at the "peak," easily resulting in pores or uneven coverage. During subsequent charging and discharging, these microscopic protrusions are prone to current concentration and stress concentration, becoming crack initiation points.

[0056] Typically, but not limitingly, the surface roughness Ra of the conductive metal layer 30 near the aluminum layer 50 can be, for example, 30 nm, 50 nm, 80 nm, 100 nm, 120 nm or 150 nm, or any value in the range of 30 to 150 nm.

[0057] The second aspect of the present invention provides a method for preparing the bipolar current collector, wherein chromium is deposited on the surface of a copper foil by magnetron sputtering in a vacuum environment to form a first interface layer 20 and a conductive metal layer 30, thereby obtaining a copper-chromium substrate; then an aluminum target is vacuum-deposited on the copper-chromium substrate to form a second interface layer 40 and an aluminum layer 50, thereby obtaining a bipolar current collector.

[0058] The preparation method provided by this invention involves a magnetron sputtering process that simultaneously forms a first interface layer 20 and a dense pure chromium conductive metal layer 30 on the surface of a copper foil. Utilizing the atomic shot-penetrating effect generated by high-energy chromium particle bombardment, atomic-level mechanical interlocking and self-compactment of the copper / chromium interface are achieved, resulting in a copper-chromium substrate with low porosity, high bonding strength, and controllable surface roughness. Subsequently, aluminum is vacuum-deposited, allowing aluminum atoms to diffuse downwards under the drive of concentration gradient and thermal activation, reacting with the chromium layer to generate a second aluminum-chromium solid solution interface layer 40 with an increasing compositional gradient, upon which a continuous and dense aluminum layer 50 is formed. This preparation method eliminates the need for adhesives, welding, or high-temperature annealing, avoiding the formation of brittle compounds and abrupt changes in interlayer stress. It ensures that the first interface layer 20 and the second interface layer 40 evolve synergistically according to the designed state, thereby guaranteeing the bipolar current collector's comprehensive performance of high bonding strength, low interfacial resistance, and excellent cycle stability from the very beginning of the preparation process.

[0059] Furthermore, the vacuum level in the vacuum environment is 10. -4 ~10 -3 Pa.

[0060] Typically, but not limitingly, the vacuum level in the vacuum environment can be, for example, 10. -4 Pa, 8×10 -4 Pa or 10 - 3 Pa, or 10 -4 ~10 -3 Any value within the range of Pa.

[0061] And / or, during the magnetron sputtering process, the target material is a chromium target with a purity of ≥99.5%, the sputtering power is 4~10kW, the working gas pressure is 0.5~2.0Pa, and the substrate winding speed is 0.2~1.0m / min.

[0062] Within the aforementioned sputtering power range, the chromium atoms generated by the bombardment of the target surface by argon ions possess sufficient kinetic energy to produce an atomic shot peening effect upon reaching the copper substrate. This effect helps to remove gas molecules and weakly bound impurities adsorbed on the copper foil surface. On the other hand, it enables in-situ self-sputtering cleaning and densification of the formed chromium layer during the deposition process, thereby obtaining a dense film with low porosity and density close to that of the chromium bulk. Simultaneously, the bombardment of the copper surface by high-energy chromium particles induces limited reverse migration of surface copper atoms, which then form localized mixing with the incident chromium atoms in the interface region, constituting a copper-chromium physical mixed first interface layer 20 with a decreasing composition gradient.

[0063] Furthermore, the chromium layer formed at this power exhibits a gently undulating and uniformly distributed micro-hilly morphology. This morphology is beneficial for the surface migration and spreading of aluminum atoms during subsequent aluminum evaporation, enhancing the mechanical anchoring effect between the aluminum layer 50 and the chromium layer. It also disperses the externally applied bending stress onto a large number of micro-protrusions, avoiding stress concentration. At the same time, this morphology does not form deep grooves or sharp protrusions, ensuring that aluminum atoms can continuously cover the entire contour during migration, thereby forming a dense, pinhole-free aluminum layer 50.

[0064] If the sputtering power is less than 4kW, the kinetic energy of the chromium particles will be insufficient, resulting in a decrease in film formation rate, loose film, fine grains and obvious columnar growth, increased grain boundaries and porosity, which will affect the adhesion and density of the chromium layer. If the power is greater than 10kW, it will cause overheating and excessive ion bombardment, resulting in abnormal coarsening of chromium layer grains, unstable surface roughness, and the introduction of excessive high pressure stress, causing the chromium layer to wrinkle or even peel off.

[0065] Typically, but not limitingly, in the magnetron sputtering process, the target material is a chromium target with a purity ≥ 99.5%; the sputtering power can be, for example, 4kW, 6kW, 8kW or 10kW, or any value in the range of 4 to 10kW; the working pressure can be, for example, 0.5Pa, 1.0Pa, 1.5Pa or 2.0Pa, or any value in the range of 0.5 to 2.0Pa; the substrate winding speed can be, for example, 0.2m / min, 0.4m / min, 0.6m / min, 0.8m / min or 1.0m / min, or any value in the range of 0.2 to 1.0m / min.

[0066] And / or, the vacuum evaporation process is carried out under a protective atmosphere to reduce the mixing of impurities such as oxygen and water vapor, avoid oxidation of the aluminum layer 50 or the formation of oxide inclusions, and ensure the purity and bonding performance of the aluminum layer 50.

[0067] And / or, the protective atmosphere is argon.

[0068] And / or, the flow rate of the argon gas is 10~50 sccm, preferably 20~40 sccm. The presence of argon gas can isolate the air and prevent oxidation of the aluminum layer during the vapor deposition process; a flow rate ≥10 sccm can ensure the protective effect, and a flow rate ≤50 sccm can avoid uneven deposition thickness caused by excessive gas flow; preferably 20~40 sccm balances the protective effect and deposition uniformity.

[0069] Typically, but not limitingly, the flow rate of the argon gas can be, for example, 10 sccm, 20 sccm, 30 sccm, 40 sccm or 50 sccm, or any value in the range of 10 to 50 sccm.

[0070] Furthermore, during the vacuum evaporation process, the temperature of the copper-chromium substrate is 150~250℃, and the substrate winding speed is 3~15m / min.

[0071] During the vacuum evaporation process, the temperature of the copper-chromium substrate is controlled at 150~250℃. At this temperature, aluminum atoms obtain sufficient thermal activation energy, which allows them to migrate effectively and diffuse downward on the surface of the chromium layer, resulting in limited mutual solubility with the surface chromium atoms and forming a gradient-distributed chromium-aluminum solid solution second interface layer 40. At the same time, this temperature does not reach the thermodynamic critical condition for the large-scale precipitation of brittle intermetallic compounds in the Al-Cr system, thus which is conducive to maintaining the metastable state of the solid solution structure.

[0072] Furthermore, within this temperature range, the interdiffusion rate between copper and chromium is extremely low, which will not cause significant changes in the composition or structure of the first interface layer 20, thereby ensuring the original high bonding strength and low electron scattering characteristics of the copper / chromium interface and maintaining the stability of the pathway for efficient current transmission from the copper layer 10 to the chromium layer.

[0073] This temperature is slightly higher than the recrystallization temperature of aluminum, which is about 100~150℃. This temperature is conducive to the deposition of aluminum atoms on the substrate surface, enabling them to migrate sufficiently and spread evenly to form a continuous, dense, and low-porosity aluminum layer 50. At the same time, the moderate substrate temperature helps to alleviate the residual stress caused by the difference in thermal expansion coefficients during the cooling process after vapor deposition, reducing the risk of cracking or warping of the aluminum layer 50, thereby ensuring the overall structural integrity and conductivity reliability of the current collector.

[0074] Typically, but not limitingly, during the vacuum evaporation process, the temperature of the copper-chromium substrate can be, for example, 150°C, 180°C, 200°C, 220°C, or 250°C, or any value within the range of 150°C to 250°C; the substrate winding speed can be, for example, 3 m / min, 6 m / min, 9 m / min, 12 m / min, or 15 m / min, or any value within the range of 3 to 15 m / min.

[0075] And / or, during the vacuum evaporation process, the aluminum wire feeding speed is 200~400mm / min, and the evaporation source temperature is 1100~1300℃.

[0076] Typically, but not limitingly, during the vacuum evaporation process, the wire feeding speed of the aluminum wire can be, for example, 200 mm / min, 250 mm / min, 300 mm / min, 350 mm / min, or 400 mm / min, or any value within the range of 200 to 400 mm / min; the evaporation source temperature can be, for example, 1100℃, 1150℃, 1200℃, 1250℃, or 1300℃, or any value within the range of 1100 to 1300℃.

[0077] A third aspect of the present invention provides an electrode sheet comprising the bipolar current collector described in the first aspect or the bipolar current collector prepared by the preparation method described in the second aspect.

[0078] A fourth aspect of the present invention provides a battery including the aforementioned electrode sheet.

[0079] A fifth aspect of the present invention provides an electrochemical device including the battery described above.

[0080] The present invention is further illustrated below with specific embodiments and comparative examples. However, it should be understood that these embodiments are merely for illustrative purposes and should not be construed as limiting the invention in any way. Unless otherwise specified, the raw materials used in the embodiments and comparative examples of the present invention were carried out under conventional conditions or conditions recommended by the manufacturer. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0081] Example 1 This embodiment provides a bipolar current collector, and the specific preparation method is as follows: 1. Mount a 9μm thick electrolytic copper foil (purity ≥99.95%) onto the unwinding shaft of a roll-to-roll magnetron sputtering device, and evacuate to a vacuum of 3×10⁻⁶. -4 Pa; A chromium target with a purity of 99.5% was used as the sputtering target material. Under the process conditions of sputtering power of 15kW, working gas pressure of 1.2Pa (argon atmosphere) and substrate winding speed of 0.5m / min, continuous sputtering deposition was carried out on the surface of copper foil.

[0082] In the initial stage of sputtering (within the first 3 seconds), by controlling the bias voltage and sputtering power gradient, high-energy Cr ions bombard the copper foil surface, inducing Cu atoms to migrate upward and physically mix with Cr atoms, forming an in-situ gradient copper-chromium mixed first interface layer 20 with a thickness of 3 nm (along the thickness direction, the Cu atom content decreases linearly from 100 at.% near the copper layer 10 side to <5 at.% near the Cr layer side); then, maintaining stable sputtering parameters, a dense pure chromium conductive metal layer 30 with a thickness of 100 nm is deposited and formed. The surface roughness Ra of this chromium layer facing the aluminum layer 50 is measured to be 85 nm by a profilometer.

[0083] 2. Transfer the copper-chromium substrate to a roll-to-roll vacuum evaporation equipment and evacuate to a vacuum level of 3×10⁻⁶. -4 Pa, high-purity argon gas with a flow rate of 30 sccm and a purity of ≥99.99% is introduced as a protective atmosphere; the evaporation source temperature is set to 1200℃ (tungsten boat heating), aluminum wire (purity ≥99.999%) is continuously supplied at a constant wire feeding speed of 300 mm / min, the substrate winding speed is controlled at 8 m / min, and an infrared thermometer is used to monitor the copper-chromium substrate temperature in real time. The substrate surface temperature is stably maintained at 200±5℃ through thermal radiation compensation.

[0084] Driven by the combined effects of temperature and concentration gradients, gaseous Al atoms condense on the surface of the Cr layer and diffuse downwards, undergoing a solid solution reaction with the surface Cr to generate in situ a gradient chromium-aluminum mixed second interface layer 40 with a thickness of 30 nm (along the thickness direction, the Al atom content increases linearly from <5 at.% near the Cr layer to >90 at.% near the aluminum layer 50); vapor deposition continues until a continuous, dense, and pinhole-free aluminum layer 50 is formed above the second interface layer 40, with a final total thickness of 3 μm for the aluminum layer 50.

[0085] 3. After the vapor deposition is completed, the material is allowed to cool naturally to room temperature and then wound up to obtain the finished bipolar current collector.

[0086] Example 2 This embodiment provides a bipolar current collector. Unlike embodiment 1, the thickness of the first interface layer 20 is 0.5 nm. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0087] Example 3 This embodiment provides a bipolar current collector. Unlike embodiment 1, the thickness of the first interface layer 20 is 1 nm. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0088] Example 4 This embodiment provides a bipolar current collector. Unlike embodiment 1, the thickness of the first interface layer 20 is 2nm. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0089] Example 5 This embodiment provides a bipolar current collector. Unlike embodiment 1, the thickness of the first interface layer 20 is 5 nm. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0090] Example 6 This embodiment provides a bipolar current collector. Unlike embodiment 1, the thickness of the first interface layer 20 is 6 nm. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0091] Comparative Example 1 This comparative example provides a bipolar current collector. Unlike Example 1, it does not have a first interface layer 20. It is obtained by directly pressing copper foil and chromium foil together. The remaining steps are the same as in Example 1, and will not be repeated here.

[0092] Test Example 1 The bipolar current collectors obtained in Examples 1-6 and Comparative Example 1 were subjected to bonding force performance tests. The bipolar current collectors obtained in Examples 1-6 and Comparative Example 1 were used to prepare soft-pack batteries (the positive electrode active material was NCM811, the negative electrode active material was graphite, the electrolyte was 1M lithium hexafluorophosphate solution, and the solvent was ethylene carbonate) for electrical performance tests.

[0093] Adhesion test: The adhesion between the composite current collector membrane layers was evaluated using the 180° peel method (refer to ASTM D3330 standard).

[0094] 1. Sample preparation: Cut the prepared composite current collector sample into strips with a width of 15 mm and a length of not less than 150 mm.

[0095] 2. Testing process: a. Use high-strength double-sided tape to fix the sample (test side up) onto a flat steel plate.

[0096] b. Firmly adhere the standard test tape (3M #610) to the surface of the film layer (aluminum layer 50) to be tested, and roll it back and forth 3 times with a standard pressure roller at a constant speed and pressure to ensure no air bubbles.

[0097] c. On a universal testing machine, peel the tape at a peel angle of 180° and a constant speed of 100 mm / min, and continuously record the force values ​​during the peeling process.

[0098] 3. Data processing: Take the average force value (unit: N) during the stable phase of the peeling process, divide it by the sample width (unit: m) to obtain the peel strength, which is N / m.

[0099] Battery stability and durability testing Internal resistance test: The initial internal resistance of the battery and the internal resistance after 500 charge-discharge cycles were tested using an AC impedance meter.

[0100] The test data is recorded in Table 1.

[0101] Table 1

[0102] As shown in Table 1, in Examples 1, 4, and 5, when the thickness of the first interface layer 20 is in the range of 1-5 nm and the ratio of the second interface layer 40 to the first interface layer 20 is in the range of 5-15, the bonding force and conductivity of the bipolar current collector remain stable. Data from Examples 1, 2, and Comparative Example 1 show that when the thickness of the first interface layer 20 is small, the bonding force decreases, indicating that an excessively small thickness of the first interface layer 20 is prone to cracking during subsequent rolling processes or use, leading to increased internal resistance and affecting battery performance. When the thickness of the first interface layer 20 is large, although the bonding force is improved to some extent, the internal resistance also increases sharply, affecting battery performance. Data from Examples 1, 3, 4, and 5 show that when the ratio is outside the range, stress is easily concentrated in the second interface layer 40. During subsequent rolling processes, the stress will be transmitted upwards, acting on the uppermost aluminum conductive layer, causing microscopic warping and grain boundary cracking in the aluminum layer 50, increasing internal resistance and affecting conductivity.

[0103] Example 7 This embodiment provides a bipolar current collector. Unlike embodiment 1, the thickness of the second interface layer 40 is 8 nm. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0104] Example 8 This embodiment provides a bipolar current collector. Unlike embodiment 1, the thickness of the second interface layer 40 is 10 nm. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0105] Example 9 This embodiment provides a bipolar current collector. Unlike embodiment 1, the thickness of the second interface layer 40 is 15 nm. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0106] Example 10 This embodiment provides a bipolar current collector. The difference from Embodiment 1 is that the thickness of the second interface layer 40 is 50 nm. The remaining steps are the same as in Embodiment 1 and will not be repeated here.

[0107] Example 11 This embodiment provides a bipolar current collector. Unlike embodiment 1, the thickness of the second interface layer 40 is 55 nm. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0108] Comparative Example 2 This comparative example provides a bipolar current collector. Unlike Example 1, it does not have a second interface layer 40. It is obtained by directly pressing a copper-chromium substrate and an aluminum foil together. The remaining steps are the same as in Example 1 and will not be repeated here.

[0109] Comparative Example 3 This comparative example provides a bipolar current collector. The difference from Example 1 is that all chromium is replaced with titanium. The remaining steps are the same as in Example 1 and will not be repeated here.

[0110] Test Example 2 The performance of the bipolar current collectors obtained in Examples 7-11 and Comparative Examples 2 and 3 was tested.

[0111] The testing process is the same as in Test Example 1, and the data obtained from the test is recorded in Table 2.

[0112] Table 2

[0113] As shown in Table 2, in Examples 1 and 9, when the thickness of the second interface layer 40 is in the range of 10-50 nm and the ratio of the second interface layer 40 to the first interface layer 20 is in the range of 5-15, the bonding force and conductivity of the bipolar current collector remain stable. Data from Examples 1, 7, and 11 show that when the thickness of the second interface layer 40 is small, although the initial resistance decreases, the bonding force also decreases, and it cannot provide excellent stress buffering capacity. This leads to cracking during subsequent rolling processes or use, resulting in an increase in internal resistance during battery cycling, thus affecting battery performance. When the thickness of the second interface layer 40 is large, although the bonding force is improved to some extent, the initial internal resistance increases, affecting battery performance. The data from Examples 1, 8, 9, and 10 show that when the ratio is too small, the ability of the second interface layer 40 to absorb and release stress is not matched with the strength of the underlying first interface. Under repeated stress during battery cycling, stress is more likely to concentrate at the relatively "brittle" chromium-aluminum interface, leading to fatigue damage or microcracks and a surge in internal resistance. When the ratio is too large, greater stress accumulates at the second interface, causing the aluminum layer 50 to crack during battery cycling, resulting in a surge in internal resistance. The data from Examples 1 and Comparative Examples 2 and 3 show that without the second interface layer 40, the bonding force between the chromium layer and the aluminum layer 50 is greatly reduced, making it easy for the aluminum-chromium layer to separate during subsequent battery fabrication, leading to increased internal resistance. During battery cycling, the internal resistance further increases sharply. Replacing the metal conductive layer with a titanium layer improves the bonding force, but the resistance surges during battery cycling. The surface titanium forms intermetallic compounds with copper and aluminum, which, while improving the bonding force, are prone to cracking due to the brittle nature of the metal compounds, leading to increased resistance.

[0114] Example 12 This embodiment provides a bipolar current collector. Unlike embodiment 1, the thickness of the conductive metal layer 30 is 10 nm. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0115] Example 13 This embodiment provides a bipolar current collector. Unlike embodiment 1, the thickness of the conductive metal layer 30 is 20 nm. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0116] Example 14 This embodiment provides a bipolar current collector. Unlike embodiment 1, the thickness of the conductive metal layer 30 is 200 nm. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0117] Example 15 This embodiment provides a bipolar current collector. Unlike embodiment 1, the thickness of the conductive metal layer 30 is 250 nm. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0118] Comparative Example 4 This comparative example provides a bipolar current collector. Unlike Example 1, it does not have a conductive metal layer 30. The remaining steps are the same as in Example 1 and will not be repeated here.

[0119] Test Example 3 The performance of the bipolar current collectors obtained in Examples 12-15 and Comparative Example 4 was tested.

[0120] The testing process is the same as in Test Example 1, and the data obtained from the test is recorded in Table 3.

[0121] Table 3

[0122] As shown in Table 3, in Examples 1, 13, and 14, the thickness of the conductive metal layer 30 is in the range of 20-200 nm, and the bonding force and conductivity of the bipolar current collector remain stable. However, data from Examples 1, 12, and 15 show that when the thickness of the conductive metal layer 30 is smaller, the bonding force decreases. This indicates that an excessively thin chromium layer cannot form a sufficiently strong interfacial bond with the copper substrate and aluminum layer 50, failing to form a continuous and dense barrier. Copper atoms can rapidly diffuse to the aluminum layer 50 through pinholes and other defects, forming a high-resistivity copper-aluminum compound at the interface. This leads to a sharp increase in internal resistance during battery cycling, thus affecting the battery's performance. Performance; When the conductive metal layer 30 is thicker, the initial internal resistance increases, and the internal resistance rises sharply during battery cycling. This indicates that the thicker the conductive metal layer 30, the greater the initial internal resistance and the greater the internal stress. During battery cycling, this can lead to cracks or even detachment of the copper layer 10 or aluminum layer 50, resulting in increased internal resistance. As can be seen in Example 1 and Comparative Example 4, the bonding force of the non-metallic conductive layer decreases, the initial internal resistance is higher, and the internal resistance surges significantly after battery cycling. This indicates that without a metallic conductive layer as an isolation layer, high-resistivity copper-aluminum compounds are easily formed between copper and aluminum, leading to increased internal resistance during battery cycling and affecting battery performance.

[0123] Example 16 This embodiment provides a bipolar current collector. Unlike embodiment 1, the temperature of the copper-chromium substrate is controlled at 130°C. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0124] Example 17 This embodiment provides a bipolar current collector. Unlike embodiment 1, the temperature of the copper-chromium substrate is controlled at 150°C. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0125] Example 18 This embodiment provides a bipolar current collector. Unlike embodiment 1, the temperature of the copper-chromium substrate is controlled at 250°C. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0126] Example 19 This embodiment provides a bipolar current collector. Unlike embodiment 1, the temperature of the copper-chromium substrate is controlled at 280°C. The remaining steps are the same as in embodiment 1 and will not be repeated here.

[0127] Test Example 4 The performance of the bipolar current collectors obtained in Examples 16-19 was tested.

[0128] The testing process is the same as in Test Example 1, and the data obtained from the test is recorded in Table 4.

[0129] Table 4

[0130] As shown in Table 4, in Examples 1, 16, 17, 18, and 19, when the substrate temperature is controlled between 150-250°C, the bonding force and conductivity of the bipolar current collector remain stable. This indicates that if the substrate temperature is too low, it is not conducive to the interdiffusion of aluminum and chromium to form a solid solution layer that meets the thickness requirements. The bonding force between aluminum and chromium decreases, and during battery cycling, it cannot provide beneficial bending and fatigue strength, leading to easy separation of the aluminum-chromium layer and increased internal resistance, which affects battery performance. If the substrate temperature is too high, the initial internal resistance increases, and the internal resistance surges during battery cycling. This indicates that at the same linear velocity, the higher the substrate temperature, the faster the interdiffusion of aluminum and chromium, resulting in an excessively thick second interface layer 40, which increases the internal resistance. Furthermore, the excessively thick second interface layer 40 has huge thermal residual stress, which causes the aluminum layer 50 to crack or peel off from the substrate during battery cycling, leading to a surge in battery internal resistance. At high temperatures, it promotes the formation of chromium-aluminum intermetallic compounds. The generation of brittle intermetallic compounds leads to interface embrittlement and cracking during cycling, resulting in increased battery internal resistance.

[0131] First interface layer 20, second interface layer 40. Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or make equivalent substitutions for some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A bipolar current collector, characterized in that, It includes a copper layer, a first interface layer, a conductive metal layer, a second interface layer, and an aluminum layer stacked in sequence; The conductive metal layer is a chromium layer; the first interface layer is a copper-chromium physical hybrid layer; and the second interface layer is a chromium-aluminum solid solution layer.

2. The bipolar current collector according to claim 1, characterized in that, In the copper-chromium physical hybrid layer, the copper content decreases along the direction closer to the aluminum layer; And / or, in the chromium-aluminum solid solution layer, the aluminum content increases along the direction close to the aluminum layer.

3. The bipolar current collector according to claim 1, characterized in that, The thickness of the copper layer is 3~15μm, preferably 5~10μm; And / or, the thickness of the first interface layer is 1~5nm, preferably 2~4nm; And / or, the thickness of the conductive metal layer is 20~200nm, preferably 50~150nm; And / or, the thickness of the second interface layer is 10~50nm, preferably 20~40nm; And / or, the thickness of the aluminum layer is 1~5μm, preferably 2~4μm; And / or, the thickness ratio of the second interface layer to the first interface layer is 5 to 15.

4. The bipolar current collector according to claim 1, characterized in that, The surface roughness Ra of the conductive metal layer on the side near the aluminum layer is 30~150nm.

5. A method for preparing a bipolar current collector according to any one of claims 1 to 4, characterized in that, In a vacuum environment, chromium is deposited on the surface of copper foil using magnetron sputtering to form a first interface layer and a conductive metal layer, resulting in a copper-chromium substrate. Then, an aluminum metal layer is vacuum-deposited on the conductive metal layer of the copper-chromium substrate to form a second interface layer and an aluminum layer, resulting in a bipolar current collector.

6. The preparation method according to claim 5, characterized in that, The vacuum level in the vacuum environment is 10. -4 ~10 - 3 Pa; And / or, during the magnetron sputtering process, the target material is a chromium target with a purity of ≥99.5%, the sputtering power is 4~10kW, the working gas pressure is 0.5~2.0Pa, and the substrate winding speed is 0.2~1.0m / min; And / or, the vacuum evaporation process is performed under a protective atmosphere; And / or, the protective atmosphere is argon; And / or, the flow rate of the argon gas is 10~50 sccm.

7. The preparation method according to claim 5, characterized in that, During the vacuum evaporation process, the temperature of the copper-chromium substrate is 150~250℃, and the substrate winding speed is 3~15m / min; And / or, during the vacuum evaporation process, the aluminum wire feeding speed is 200~400mm / min, and the evaporation source temperature is 1100~1300℃.

8. An electrode sheet, characterized in that, Includes the bipolar current collector according to any one of claims 1 to 4 or the bipolar current collector prepared by the preparation method according to any one of claims 5 to 7.

9. A battery, characterized in that, Includes the electrode sheet as described in claim 8.

10. An electrochemical device, characterized in that, Includes the battery as described in claim 9.

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