Battery aluminum foil and method for manufacturing the same

Abstract

The application provides a battery aluminum foil and a preparation method thereof, and the preparation method comprises the following steps: step S1, smelting aluminum liquid by smelting furnace through aluminum ingot and required alloy elements, and then entering a casting nozzle through a front box flow tank after refining, degassing and filtering in sequence through a refining-tilting furnace-heat preservation furnace, and obtaining a melt; the obtained melt is cooled and rolled through a double roll of a casting and rolling machine to obtain a casting and rolling roll with a thickness of 2-10 mm; wherein the average grain size in the thickness direction of the casting and rolling roll is less than or equal to 50 microns, the second phase size in the thickness direction of the casting and rolling roll is less than or equal to 5 microns, and the average size of the eutectic structure is less than or equal to 50 microns; step S2, cold rolling the casting and rolling roll and carrying out intermediate annealing treatment to obtain a foil rolling blank; step S3, carrying out foil rolling on the foil rolling blank to obtain the battery aluminum foil. The battery aluminum foil can significantly improve the elongation of the battery aluminum foil for current collectors and the production yield of battery pole pieces.

Description

Technical Field

[0001] This invention relates to the field of aluminum processing technology, and in particular to a battery aluminum foil and its preparation method. Background Technology

[0002] Lithium-ion batteries are core energy storage devices in the fields of new energy storage and power batteries. Aluminum foil, as the positive electrode current collector of lithium batteries, plays a supporting role in the skeleton of the positive electrode active material. Its mechanical properties directly determine the battery electrode processing yield and the safety of the cell in service. At present, the lithium battery industry is rapidly iterating towards high energy density and high safety and reliability. Improving the compaction density of the positive electrode active material is the mainstream technical path to improve the energy density of the cell. The electrode preparation process involves multiple strong deformation processes such as high-speed cutting, high-pressure compaction, and high-speed winding. If the elongation performance of the positive electrode current collector aluminum foil is insufficient, it is easy to cause defects such as electrode strip breakage and foil tearing, which significantly reduces the qualification rate of the electrode and battery finished products. Therefore, high elongation has become a key performance indicator of battery current collector aluminum foil.

[0003] Currently, various preparation processes for improving the elongation of current collector aluminum foil have been developed in the industry. For example, related technology CN114405998B uses Al-Ti-B refining agents and controls the deformation amount in the casting and rolling zone to refine the grains, and reduces work hardening and improves the elongation of aluminum foil by performing homogenization annealing and graded annealing in the cold rolling and foil rolling stages respectively; related technology CN116179897A controls the proportion of rolling texture by controlling the intermediate annealing thickness and heating rate, and promotes high-density recrystallization nucleation to improve the elongation of aluminum foil; related technology CN115725877B controls the thickness difference between the casting and rolling coil and the cold-rolled annealed sheet and the annealing conditions to make the grain structure more uniform and retain a large amount of annealing texture, thereby improving the elongation of aluminum foil.

[0004] However, although the existing processes mentioned above can improve the elongation of the aluminum foil for battery current collectors to some extent, the overall performance improvement is limited and not significant. The prevention of center segregation is insufficient, making it difficult to meet the stringent mechanical tolerance requirements in ultra-high voltage solid density cathode and high-speed electrode processing scenarios. Summary of the Invention

[0005] To address the shortcomings of the existing technologies, this invention proposes a method for preparing battery aluminum foil, which solves the problems of limited overall performance improvement in elongation of existing current collector battery aluminum foil, insignificant improvement, and insufficient prevention of center segregation. By suppressing center segregation of aluminum foil blanks and rationally controlling the microstructure characteristics in the thickness direction of aluminum foil, the elongation of aluminum foil can be improved.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] This invention provides a method for preparing battery aluminum foil, the method comprising the following steps: Step S1: The aluminum ingot and the required alloying elements are melted in a melting furnace and then refined, poured, held in a holding furnace, degassed, and filtered. The melt is then fed into the casting nozzle through the front box trough. The melt is cooled and rolled by the twin rolls of a casting and rolling mill to obtain a cast coil with a thickness of 2-10 mm. Wherein, the average grain size in the thickness direction of the cast-rolled coil is ≤50μm, the size of the second phase in the thickness direction of the cast-rolled coil is ≤5μm, and the average size of the precipitated eutectic structure is ≤50μm; Step S2: The cast-rolled coil is cold-rolled and then subjected to intermediate annealing to obtain a foil blank; Step S3: Foil rolling is performed on the foil blank to obtain battery aluminum foil.

[0008] Preferably, in step S1, the large-angle grain boundaries in the thickness direction of the cast-rolled coil account for ≥50% of the total grain boundary length; The total grain boundary length is defined as the sum of the large-angle grain boundary length and the small-angle grain boundary length; the orientation difference angle between the two sides of the grain boundary is defined as θ, the small-angle grain boundary is 5° < θ < 15°, and the large-angle grain boundary is θ ≥ 15°.

[0009] Preferably, in step S2, the annealing temperature of the intermediate annealing satisfies the following relationship: T 退 = T 再 -T o ; Where, T is defined 退 T is the intermediate annealing temperature. 再 T is the temperature at which aluminum alloys fully recrystallize. o This represents the temperature difference between the complete recrystallization temperature and the intermediate annealing temperature.

[0010] Preferably, 20℃≤T o ≤150℃.

[0011] Preferably, in step S2, the number of nucleation points of the foil blank is ≥10 / mm².

[0012] Preferably, in step S3, the number of grains in the thickness direction of the battery aluminum foil is ≥50, and the average grain size in the thickness direction of the battery aluminum foil is ≤0.2μm.

[0013] Secondly, embodiments of the present invention provide a battery aluminum foil, which is prepared by the above-described method for preparing battery aluminum foil.

[0014] Compared with related technologies, in the embodiments of the present invention, through the above steps S1-S3, the average grain size in the thickness direction of the cast-rolled coil is ≤50μm, the second phase size in the thickness direction of the cast-rolled coil is ≤5μm, and the average size of the precipitated eutectic structure is ≤50μm. In this way, by suppressing the center segregation of the aluminum foil blank and reasonably controlling the microstructure characteristics in the thickness direction of the aluminum foil, the elongation rate of the current collector aluminum foil and the yield of battery electrode production are significantly improved, thus meeting the high plasticity requirements of high energy density lithium-ion power batteries for the current collector aluminum foil. Attached Figure Description

[0015] The present invention will now be described in detail with reference to the accompanying drawings. The above and other aspects of the present invention will become clearer and more readily understood through the detailed description following the accompanying drawings. In the drawings: Figure 1 A flowchart illustrating the method for preparing battery aluminum foil according to an embodiment of the present invention; Figure 2 A schematic diagram of the center segregation of the microstructure of the cast-rolled coil of battery aluminum foil provided in an embodiment of the present invention; Figure 3 A schematic diagram of the second phase and eutectic structure of the cast-rolled coil of battery aluminum foil provided in an embodiment of the present invention; Figure 4 EBSD electron backscattering diffraction antipolar orientation distribution diagram of the second phase and eutectic structure of the cast-rolled coil of battery aluminum foil provided in the embodiments of the present invention; Figure 5 This is a schematic diagram of the microstructure of the battery aluminum foil thickness section provided in an embodiment of the present invention; Figure 6 This is a schematic diagram of the internal grain interface and nucleation particle distribution morphology of the annealed and cold-rolled sheet of battery aluminum foil provided in an embodiment of the present invention. Figure 7 The EBSD reverse polarity orientation distribution diagram of the battery aluminum foil provided in the embodiment of the present invention; Figure 8 The point-to-point orientation difference angle scan curve of the battery aluminum foil provided in the embodiment of the present invention. Detailed Implementation

[0016] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein in the specification of the application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application; the terms "comprising" and "having," and any variations thereof, in the specification, claims, and foregoing drawings of this application, are intended to cover non-exclusive inclusion. The terms "first," "second," etc., in the specification, claims, or foregoing drawings of this application are used to distinguish different objects, not to describe a particular order.

[0017] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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.

[0019] Please see Figures 1-8 As shown in the figure, this invention provides a method for preparing battery aluminum foil, the method comprising the following steps: Step S1: The aluminum ingot and the required alloying elements are melted in a smelting furnace, and then the molten aluminum is successively refined, poured into a furnace, held in a holding furnace, degassed, and filtered before entering the casting nozzle through the front box trough to obtain the melt. The melt is then cooled and rolled by the twin rolls of a casting and rolling mill to obtain a cast coil with a thickness of 2~10mm.

[0020] Specifically, the aluminum ingot alloy melt undergoes smelting, two refining processes, degassing, and filtration to thoroughly remove impurities such as hydrogen bubbles and solid inclusions, preventing these impurities from becoming nucleation sites for subsequent rolling cracks and local segregation. The melt is then stably transported through the front box, with precise control of superheat and stable depth of the liquid cavity in the casting and rolling zone, thus reducing the tendency of solute element core enrichment from the process source.

[0021] Specifically, the average grain size in the thickness direction of the cast-rolled coil is ≤50μm, the second phase size in the thickness direction of the cast-rolled coil is ≤5μm, and the average size of the precipitated eutectic structure is ≤50μm. In particular, the 2~10mm thin blank cools and solidifies faster between the two rolls, and the longitudinal length of the liquid cavity is smaller, preventing solute elements from accumulating in large quantities in the core of the cast-rolled coil, thus significantly reducing the probability of center segregation.

[0022] Specifically, an average grain size of ≤50μm can rapidly solidify and homogenize the composition to form fine equiaxed grains, replacing the coarse columnar grains that are prone to inducing central segregation; fine grains can uniformly disperse the alloy solute, eliminating the compositional gradient difference in the core; at the same time, the deformation coordination between fine grains is stronger, and the stress is uniformly dispersed during subsequent multi-pass rolling, making it less likely to generate internal microcracks.

[0023] The second phase size is ≤5μm. The second phase retains its heritability in the subsequent rolling process. If the second phase particle size is too large, it will cause stress concentration inside the aluminum foil, which will easily cause problems such as strip breakage and pinholes during rolling or coating. The second phase particle size is also too large, which will cause uneven surface roughness of the aluminum foil and affect the uniform coating of the positive electrode paste.

[0024] The average size of the eutectic structure is ≤50μm, which makes the coarse eutectic a brittle and hard phase, and the main origin of rolling cracks. The fine and uniform eutectic phase weakens the brittleness of the matrix and can follow the aluminum matrix in plastic deformation during deformation without destroying the continuity of the matrix.

[0025] Step S2: The cast-rolled coil is cold-rolled and then subjected to intermediate annealing to obtain a foil blank.

[0026] Specifically, the cast-rolled coil is first cold-rolled to create a uniformly distributed work-hardened dislocation cell structure in the matrix. Subsequent intermediate annealing strictly adheres to a temperature below the full recrystallization temperature of the aluminum alloy, constituting a recovery-type incomplete recrystallization annealing. This low-temperature annealing fully releases the internal stress generated by cold rolling, relaxes high-density tangled dislocations, significantly weakens the work-hardening effect, and directly improves the billet's plasticity. Since the temperature does not reach the full recrystallization threshold, coarse equiaxed recrystallized grains are not generated, preserving a uniform fine-grained matrix and moderately deformed texture. After annealing, the billet achieves a significant increase in plasticity, enabling it to withstand large reductions in foil rolling and mitigating the risk of strip breakage during ultra-thin foil forming.

[0027] Step S3: Foil rolling is performed on the foil blank to obtain battery aluminum foil.

[0028] Specifically, after pre-casting and rolling to control segregation, fine grain quantification and regulation, and cold rolling annealing and tempering, the foil billet has no severe central segregation, no large-size brittle phases, uniform distribution of grains and second phases in the thickness direction, and sufficient plasticity reserves. The billet can stably withstand the ultra-large total reduction rate of foil rolling, and finally form ultra-thin lithium battery cathode current collector aluminum foil. The finished product has high consistency of microstructure in the thickness direction and stable and excellent elongation performance.

[0029] Specifically, this invention utilizes a triple-restructuring mechanism of fine-grained stress dispersion during casting and rolling, synergistic deformation of the dispersed second phase, and incomplete recovery annealing to significantly increase the elongation of aluminum foil compared to the comparative patent process. This fully meets the stringent plasticity requirements of ultra-high pressure compaction cathodes and high-speed slitting / winding for high-energy-density batteries.

[0030] Specifically, through step S1 above, the average grain size in the thickness direction of the cast-rolled coil is ≤50μm, the second phase size in the thickness direction of the cast-rolled coil is ≤5μm, and the average size of the precipitated eutectic structure is ≤50μm. By rationally controlling the microstructure characteristics in the thickness direction during the aluminum foil production process, the elongation rate of the current collector aluminum foil and the yield of battery electrode production are significantly improved, meeting the high plasticity requirements of high-energy-density lithium-ion power batteries for current collector aluminum foil. Furthermore, the obtained cast-rolled coil has no obvious central segregation within its cross-section; a eutectic structure length ≥50μm is defined as central segregation.

[0031] In this embodiment, in step S1, the proportion of large-angle grain boundaries in the thickness direction of the cast-rolled coil to the total grain boundary length is ≥50%. Preferably, the proportion of large-angle grain boundaries in the thickness direction of the cast-rolled coil to the total grain boundary length is 55%.

[0032] The total grain boundary length is defined as the sum of the lengths of large-angle grain boundaries and small-angle grain boundaries. The misorientation angle between the two sides of a grain boundary is defined as θ. The small-angle grain boundary (LAGB) is defined as 5° < θ < 15°, and the large-angle grain boundary (HAGB) is defined as θ ≥ 15°. θ < 5° represents a subgrain boundary, indicating the angle between two adjacent grains / subgrains in terms of crystal spatial orientation; this is a core parameter for quantitatively describing differences in grain orientation.

[0033] Specifically, the high-proportion, large-angle grain boundary structure is naturally formed during the casting and rolling solidification process, and works in synergy with fine grains, dispersed second phase, and fine eutectic structure to build a stable micro-grain boundary network from the source of casting and rolling. This network then acts sequentially on the entire process of cold rolling, intermediate annealing, and foil rolling, ultimately comprehensively optimizing the plasticity, crack resistance, and microstructure uniformity of the aluminum foil matrix.

[0034] In this embodiment, in step S2, the annealing temperature of the intermediate annealing satisfies the following relationship: T 退 =T 再 -T o ; Where, T is defined 退 T is the intermediate annealing temperature. 再 T is the temperature at which aluminum alloys fully recrystallize. o This represents the temperature difference (temperature drop) between the complete recrystallization temperature and the intermediate annealing temperature.

[0035] In this embodiment, 20℃≤T o ≤150℃. Where, T o The larger the size, the lower the annealing temperature, the less softening, the more work hardening is retained, the higher the strength of the substrate, and the limited increase in elongation.

[0036] T o The smaller the value, the closer the annealing temperature is to the complete recrystallization temperature, the more complete the softening, and the more significant the increase in aluminum foil elongation, but the strength will be reduced. T o The temperature difference range of 20~150℃ is a balance window specifically adapted to the aluminum foil of the battery current collector: while significantly improving the elongation and subsequent rolling processing performance, it avoids complete recrystallization, which would cause coarse grains and insufficient tensile strength, thus meeting the mechanical matching requirements of the lithium battery current collector.

[0037] Specifically, by appropriately annealing to fully release the residual internal stress of cold rolling, the plasticity of the billet is improved in a targeted manner. In subsequent processes such as foil rolling, electrode coating, high-pressure rolling, high-speed slitting and cell winding, defects such as foil tearing and strip breakage are significantly reduced, effectively improving the finished product qualification rate of aluminum foil and lithium electrode sheets and reducing industrial production losses.

[0038] In this embodiment, as Figure 6 As shown. In step S2, the number of nucleation points in the foil blank is ≥10 / mm².

[0039] Specifically, by limiting the number of nucleation particles in cold-rolled sheets to ≥10 per mm², these nucleation particles are mainly formed by the transformation of subgrain boundaries generated by cold rolling deformation into small-angle grain boundaries. By combining the cold rolling deformation law, dislocation evolution mechanism, and incomplete recrystallization annealing regime, the number of nucleation particles can be stably controlled. At the same time, a multi-dimensional synergistic system is formed with the original second-phase particles and large-angle grain boundaries of the cast-rolled billet, continuously optimizing the microstructure and plastic potential of the billet.

[0040] The principle of nucleation point generation and evolution is as follows: When aluminum alloy sheet undergoes plastic deformation during cold rolling, as the amount of rolling deformation increases, dislocations inside the crystal continue to multiply. As the amount of deformation increases, the dislocation density inside the alloy increases, resulting in intertwining and the formation of subgrain boundaries. Subgrain boundaries can then rotate to form small-angle grain boundaries, which serve as nucleation points.

[0041] This invention employs a reasonable cold rolling reduction to ensure sufficient plastic deformation of the sheet material, promoting the full proliferation and extensive entanglement of dislocations within the matrix, forming a sufficient number of subgrain boundaries. This provides the material basis for subsequent transformation into small-angle grain boundary nucleation particles, ensuring from the source that the number of nucleation particles reaches 10 / mm. 2 The above potential. Continuing the fine-grained advantage of the casting and rolling process, the sheet material maintains a fine-grained structure throughout the entire process from casting and rolling billets to cold-rolled billets and finished aluminum foil. Fine grains are the core foundation for improving the elongation rate and deformation coordination of aluminum foil.

[0042] In this embodiment, in step S3, the number of grains in the thickness direction of the battery aluminum foil is ≥50, and the average grain size in the thickness direction of the battery aluminum foil is ≤0.2μm. Specifically, arranging 50 or more independent grains along the thickness direction of the battery aluminum foil is equivalent to dividing the aluminum foil thickness into layers of dense grains, and there are no coarse grains, continuous grain boundaries, or defect channels that penetrate the entire thickness of the foil.

[0043] During deformation, external forces and stresses are transmitted and dispersed layer by layer along the multi-layered grains, preventing stress concentration in localized areas. Plastic deformation is completed step-by-step and collaboratively by the multi-layered grains, significantly improving the overall deformation adaptability. The multi-layered grains, combined with dense grain boundaries, form a three-dimensional barrier network that restricts long-range dislocation slip and blocks defect propagation paths. This structure can significantly reduce the gradient difference between the core and surface microstructure left by segregation in the center of the cast billet, improving the homogeneity of the aluminum foil's microstructure and properties from the surface to the core. This results in a breakthrough improvement in elongation, meeting the stringent requirements of high-energy-density batteries. In addition, the ultra-fine grains with an average grain size ≤0.2μm in the thickness direction of the battery aluminum foil bring a significant grain-refining effect, allowing the aluminum foil to maintain excellent tensile strength while achieving high elongation. As a current collector for the positive electrode of lithium batteries, it can stably serve as a skeleton carrier, firmly supporting the positive electrode active material and withstanding external forces during cell assembly and use. Furthermore, its high plasticity allows it to withstand multiple strong deformation processes, achieving an optimal balance between strength, plasticity, and load-bearing capacity. This can significantly improve the elongation of the aluminum foil used for current collectors, greatly reduce the problem of strip breakage during high-speed slitting, compaction, and winding of battery electrodes, and improve the yield of battery electrode production.

[0044] In this embodiment, the cross-sectional structure of the battery aluminum foil is as follows: Figure 5As shown, a is the surface of the aluminum foil (upper and lower outer surfaces); b is the grain boundary, which is used to separate adjacent grains and has a wavy layered shape due to the deformation of multiple rolling passes; c is the grain size in the thickness direction, which is the cross-sectional size of a single cold-deformed grain in the thickness direction of the aluminum foil; d is the cold-deformed grain, which is a thin sheet-like cold-deformed ultrathin grain that is greatly extended and stretched along the rolling direction and is stacked layer by layer along the thickness direction.

[0045] Specifically, the foil rolling process applies an extremely high total reduction rate, which strongly stretches the original grains in the intermediate billet along the rolling direction and compresses them violently along the thickness direction. The originally nearly equiaxed grains are rolled into ultra-thin sheet-like grains d; the interface between the grains evolves into wavy layered grain boundaries b, and finally forms a multi-layered stacked layered microstructure on the thickness section of the aluminum foil.

[0046] In this embodiment, two sets of embodiments and four sets of comparative examples of the present invention are compared. The comparison results are shown in Table 1. Table 1 - Comparison results of the grain parameters and elongation of aluminum foil in the thickness direction of the embodiments of the present invention with those of the comparative examples.

[0047]

[0048] According to the table above, comparing the comparative and exemplary cases, after EBSD (electron backscattering diffraction) detection, the number of grains in the thickness direction of the comparative cases is smaller than that of the exemplary cases, while the average grain size in the thickness direction of the comparative cases is larger than that of the exemplary cases. The elongation of the current collector aluminum foil prepared according to the microstructure control method of the present invention is significantly better than that of the comparative cases.

[0049] This invention also provides a battery aluminum foil, which is prepared by the above-described method. This battery aluminum foil significantly improves the elongation of current collector aluminum foil and increases the yield of battery electrode production, meeting the high plasticity requirements of high-energy-density lithium-ion power batteries for current collector aluminum foil.

[0050] It should be noted that the various embodiments described above with reference to the accompanying drawings are merely illustrative of the present invention and not intended to limit its scope. Those skilled in the art should understand that any modifications or equivalent substitutions made to the present invention without departing from its spirit and scope should be included within the scope of the present invention. Furthermore, unless the context otherwise requires, words appearing in the singular include those in the plural, and vice versa. Additionally, unless specifically stated otherwise, all or part of any embodiment may be used in conjunction with all or part of any other embodiment.

Claims

1. A method for preparing battery aluminum foil, characterized in that, The preparation method includes the following steps: Step S1: The aluminum ingot and the required alloying elements are melted in a melting furnace and then refined, poured, held in a holding furnace, degassed, and filtered. The melt is then fed into the casting nozzle through the front box trough. The melt is cooled and rolled by the twin rolls of a casting and rolling mill to obtain a cast coil with a thickness of 2-10 mm. Wherein, the average grain size in the thickness direction of the cast-rolled coil is ≤50μm, the size of the second phase in the thickness direction of the cast-rolled coil is ≤5μm, and the average size of the precipitated eutectic structure is ≤50μm; Step S2: The cast-rolled coil is cold-rolled and then subjected to intermediate annealing to obtain a foil blank; Step S3: Foil rolling is performed on the foil blank to obtain battery aluminum foil.

2. The method for preparing battery aluminum foil as described in claim 1, characterized in that, In step S1, the proportion of large-angle grain boundaries in the thickness direction of the cast-rolled coil to the total grain boundary length is ≥50%; The total grain boundary length is defined as the sum of the large-angle grain boundary length and the small-angle grain boundary length; the orientation difference angle between the two sides of the grain boundary is defined as θ, the small-angle grain boundary is 5° < θ < 15°, and the large-angle grain boundary is θ ≥ 15°.

3. The method for preparing battery aluminum foil as described in claim 1, characterized in that, In step S2, the annealing temperature of the intermediate annealing satisfies the following relationship: T 退 = T 再 -T o ; Where, T is defined 退 T is the intermediate annealing temperature. 再 T is the temperature at which aluminum alloys fully recrystallize. o This represents the temperature difference between the complete recrystallization temperature and the intermediate annealing temperature.

4. The method for preparing battery aluminum foil as described in claim 3, characterized in that, 20℃≤T o ≤150℃。 5. The method for preparing battery aluminum foil as described in claim 1, characterized in that, In step S2, the number of nucleation particles in the foil blank is ≥10 / mm².

6. The method for preparing battery aluminum foil as described in claim 1, characterized in that, In step S3, the number of grains in the thickness direction of the battery aluminum foil is ≥50, and the average size of the grains in the thickness direction of the battery aluminum foil is ≤0.2μm.

7. A battery aluminum foil, characterized in that, The battery aluminum foil is prepared by the method for preparing battery aluminum foil according to any one of claims 1-7.

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