Aluminum foil with improved wettability

Abstract

Aluminum foils with a thickness of 4 μm to 100 μm made from alloys of type AA1xxx, AA3xxx and / or AA8xxx are in a cold-solidified state and contain a rolling oil layer on their surface made of polyalkylene glycol or a compound containing a polyalkylene oxide structure, which can be easily coated with an electrode suspension for producing a battery membrane.

Description

[Technical Field]

[0001] The present invention relates to aluminum foil, further processing thereof to form a battery film, and the use of aluminum foil in manufacturing a battery film. [Background technology]

[0002] Batteries, such as lithium-ion batteries, are used as storage media for electrical energy, particularly in electric mobile vehicles. The performance of such batteries depends on the speed and efficiency of the charge and material transport processes during charging and discharging. These transport processes are determined, in particular, by the structure of the electrodes within the battery.

[0003] Electrodes, particularly positive electrodes, often consist of a substrate made of metal foil and further having an electrode coating or positive electrode material on its surface. Here, the chemical-physical processes at the interface between the substrate and the electrode coating are crucial for battery performance. Therefore, to improve battery characteristics, the surface properties of the substrate must be configured to ensure optimal adhesion of the electrode coating and, as much as possible, reliable contact.

[0004] Aluminum foil used in battery electrodes, which serves as the base material for the electrodes, is typically manufactured by a rolling process. The aluminum foil is generally provided with a cooling lubricant during the rolling process to reduce friction in the roll gap, to cool the aluminum and the rolls, and to reduce roll wear and prevent the rolled product from seizing during the rolling process.

[0005] When electrode material is applied to aluminum foil while it is rolled, lubricated, and cold-solidified, surface residues from the cooling lubricant and further residues from the rolling process, such as particles, abrasion, and reaction products, can lead to wetting and adhesion problems of the electrode coating on the aluminum foil. Furthermore, the distribution of the cooling lubricant on the surface of the aluminum foil after rolling can become so uneven that areas with localized excessive cooling lubricant occupancy are formed, preventing optimal coating and contact.

[0006] Therefore, when used in aluminum foil for batteries, the mass of the cooling lubricant should be reduced or removed before coating with an electrode coating. For example, for this purpose, the aluminum foil can be annealed at a low temperature to evaporate or oxidize any residual cooling lubricant, and the aluminum foil will soften. However, especially during low-temperature annealing, residues of the cooling lubricant may remain on the aluminum foil, thus posing a risk of problems in further processing, such as coating. In particular, even after annealing, locally severely fluctuating wettability may still occur due to uneven occupancy by the cooling lubricant. Similarly, heating the aluminum foil may cause an oxide layer to grow on the surface, with the thickness of the oxide layer increasing, especially in the edge regions. Due to the unevenness in the thickness of the oxide layer, various boundary resistances may occur at the interface with the aluminum foil after calendering of the electrode coating. Furthermore, a considerable amount of time is required for annealing, especially long annealing times required for degreasing at low temperatures, resulting in increased costs.

[0007] Similarly, aluminum foil can be chemically degreased to prepare its surface for coating. For this purpose, cooling lubricants are removed in pickling grease, and the roll oxide layer is dissolved. In the subsequent rinsing process, a thicker porous hydroxide layer is formed on the surface of the aluminum foil. These correspondingly prepared aluminum foils are characterized by uniform surface properties and high wettability, making them suitable for coating with aqueous suspensions. However, this chemical degreasing also results in relatively high processing costs.

[0008] Flame degreasing of aluminum foil can also be used as an alternative surface treatment for degreasing and conditioning the surface. However, the high energy input required for flame degreasing can lead to undesirable softening of the aluminum foil and significant growth of oxide layers, which degrades the contact of the aluminum foil surface. The same results can be obtained from NIR and plasma treatments.

[0009] Corona treatment of the surface of aluminum strips or aluminum foil is also suitable for degreasing and conditioning. In particular, corona treatment also increases surface tension. [Overview of the Initiative] [Means for solving the problem]

[0010] The object of the present invention is to provide an aluminum foil for battery electrodes that does not require costly surface preparation, has good wettability and adhesive strength to various types of electrode coatings, and simultaneously enables effective electrical contact with the electrode coating. Furthermore, the manufactured surface should have good corrosion resistance to the electrolyte used in the battery cell.

[0011] This objective is achieved by aluminum foil made of type AA1xxx, AA3xxx, and / or AA8xxx alloys having a cold-solidified state and having a compound containing polyalkylene glycol or polyalkylene oxide structure on its surface.

[0012] The present invention further relates to a method for manufacturing the aforementioned battery membrane.

[0013] Finally, the present invention relates to the use of aluminum foil as a battery membrane. [Modes for carrying out the invention]

[0014] The aluminum foil according to the present invention may consist of alloys of type AA1xxx, AA3xxx, and / or AA8xxx. As a result of low proportions of alloy additives, AA1xxx type aluminum alloys cause very little undesirable chemical process between the electrode coating and the surface of the aluminum foil, and therefore have a low corrosion effect. In this case, it is possible to use alloys of type AA1050, AA1100, AA1200, or AA1085, in particular, which may have improved mechanical properties compared to pure aluminum. Mechanical properties can be further improved by using alloys of type AA3xxx and AA8xxx.

[0015] It has been found that the surface of aluminum foil produced during cold rolling using a cooling lubricant that is substantially free of fatty acids and aliphatic alcohols but contains a compound containing polyalkylene glycol or polyalkylene oxide has a high surface energy and therefore can achieve an optimal surface for use as a substrate in battery electrodes. The surface of the aluminum foil is substantially free of fatty acids and aliphatic alcohols. Generally, these substances are contained in the cooling lubricant and therefore exist as components of residual cooling lubricant on the surface immediately after cold rolling of the aluminum. Substantially free of fatty acids and aliphatic alcohols means 1 mg / m² on the surface side of the aluminum foil in each case. 2 This means that less than a certain amount of fatty acids and / or aliphatic alcohols are provided.

[0016] The surface of the aluminum foil is only adjusted by the rolling process and does not undergo heat treatment, flame degreasing, chemical degreasing, and / or corona treatment, especially after the completion of cold rolling. The aluminum foil has good initial adhesion strength to the electrode coating.

[0017] Therefore, the aluminum foil has a cold-solidified state, and it is advantageous that the aluminum foil according to the present invention is different from the aluminum foil for battery electrodes adjusted by, for example, heat treatment or flame degreasing due to the cold-solidified structure.

[0018] In the production of aluminum strips and foils, rolling emulsions and rolling oils, which have a significant impact on the economic feasibility of production and the quality of the product, are used as cooling lubricants. During rolling, the friction coefficient between the rolling roll and the material to be rolled should not be too high or too low. A low friction coefficient improves lubrication in the roll gap, thus reducing energy consumption, frictional heat, and roll wear in the rolling process.

[0019] The aluminum foil according to the present invention is obtained by cold rolling an aluminum strip in the presence of a cooling lubricant containing a mineral oil-based base oil or a synthetic base oil, a polyalkylene glycol, and / or a compound containing a polyalkylene oxide structure, and substantially free of fatty acids and aliphatic alcohols.

[0020] Therefore, the obtained aluminum foil has no visually distinguishable defect pattern caused by fatty acids and aliphatic alcohols and has surprisingly high wettability to N-methyl-2-pyrrolidone (NMP). This is an indicator of the coatability of the aluminum foil by the electrode coating. Furthermore, the aluminum foil according to the present invention does not require corona treatment when a high surface energy of the surface of the aluminum foil is desired.

[0021] The lubricant for cooling used according to the present invention is oil-soluble. It is not compatible with water. The lubricant for cooling according to the present invention does not contain linear olefins, and particularly does not contain α-olefins having 6 to 40 carbon atoms.

[0022] Substantially free of fatty acids in the sense of the lubricant for cooling used in the present invention means that the fatty acids are contained as a lubricating additive at a ratio of at most 0.2% by weight, preferably at most 0.1% by weight, based on the mass of the lubricant for cooling.

[0023] Substantially free of aliphatic alcohols in the sense of the present invention means that the aliphatic alcohols are contained as a lubricating additive at a ratio of at most 0.4% by weight, preferably at most 0.3% by weight, based on the mass of the lubricant for cooling. When the fatty acid content and / or aliphatic alcohol content in the lubricant according to the present invention is higher than the maximum value specified above, the wetting properties of the aluminum product rolled using it are impaired.

[0024] The polyalkylene glycol to be used according to the present invention includes typical polyalkylene glycols and compounds having a polyalkylene glycol structure such as polyoxyalkylene aliphatic alcohol ethers (ethoxylated aliphatic alcohols). The alkylene groups in the polyalkylene glycol or polyalkylene oxide may be ethylene, propylene or butylene (polyethylene glycol, polypropylene glycol, polybutylene glycol). The aliphatic alcohol may contain 8 to 20 carbon atoms. The aliphatic alcohol group may be, for example, decanol, lauryl alcohol, myristyl alcohol, cetyl alcohol, stearyl alcohol. These compounds have lubricating and cooling properties during cold rolling of aluminum. The term polyalkylene glycol used below includes polyalkylene glycols and compounds having a polyalkylene glycol structure.

[0025] The polyalkylene glycol used in the lubricant for cooling has a kinematic viscosity of 5 mm2 / sec~250mm 2 / second, preferably 10mm 2 / sec~200mm 2 They may have a kinematic viscosity of 1 / second. The polyalkylene glycols used in this invention exist as liquids at temperatures above 5°C and are therefore easy to measure. They may be insoluble or soluble in water.

[0026] Particularly preferred are compounds containing polyalkylene glycol or polyalkylene oxide, such as ethoxylated aliphatic alcohols like tetraethylene glycol monododecyl ether. Corresponding polyalkylene glycols are commercially available.

[0027] The proportion of polyalkylene glycol in the cooling lubricant may be up to 10% by weight, particularly 0.01 to 8% by weight, and especially preferably 0.01 to 5% by weight, based on the mass of the rolling oil in each case. Thus, polyalkylene glycol replaces the fatty acid additives and aliphatic alcohol additives that are normally present in cold rolling lubricants. The cooling lubricant according to the present invention has a good lubricating effect or tribological effect without the aforementioned unfavorable effects of fatty acids and aliphatic alcohols.

[0028] The cooling lubricant used is based on a hydrocarbon base oil having a boiling point in the range of 180°C to 300°C, as measured according to DIN EN ISO 3405. The base oil contains linear and branched hydrocarbons. The base oil may contain a hydrocarbon mixture. The proportion of aromatic compounds therein is preferably less than 1% by weight, based on the mass of the base oil. The base oil may be mineral oil or synthetic oil. It may contain n-paraffins and / or isoparaffins.

[0029] The kinematic viscosity of this low-aromatic hydrocarbon mixture is 1.5–3.6 mm at 20°C. 2It may be / second. This kinematic viscosity provides good flow characteristics in the cold rolling stand, enabling uniform lubrication and cooling. The proportion of base oil in the cooling lubricant according to the present invention may be 90% by weight or more based on the mass of the cooling lubricant. The proportion of base oil may be, for example, 90% to 99% by weight of the mass of the cooling lubricant.

[0030] The cooling lubricant may contain typical additives, antioxidants, and conductivity enhancers to increase high-pressure lubrication properties.

[0031] Additives used to enhance high-pressure lubrication properties include linear saturated carbon. 10~14 It contains carboxylic acid esters, including, for example, butyl stearate and methyl dodecanoate. Methyl dodecanoate is particularly preferred. These may be included in an amount of up to 10% by weight, preferably 1 to 8% by weight, based on the mass of the cooling lubricant.

[0032] Suitable antioxidants include monovalent, divalent, and trivalent phenols and polynuclear phenols with steric barriers, particularly tert-butylphenols. A typical representative of this group is methylene-4,4'-bis-(2,6-di-tert-butylphenol). Further suitable antioxidants include amines such as diphenylamine, phenyl-α-naphthylamine, p,p'-tetramethyldiaminodiphenylmethane, and N,N'-diphenyl-p-phenyldiamine. The aforementioned antioxidants may be used in combination with further antioxidants such as sulfides and polydisulfides in conventional concentrations.

[0033] The cooling lubricant used allows for further processing of the aluminum products obtained after cold rolling for a range of applications without the need for corona treatment. Despite the absence of corona treatment, sufficient surface energy for many applications is achieved on the surface of the aluminum foil. Furthermore, the surface of the aluminum foil exhibits high wettability to water and N-methyl-2-pyrrolidone (NMP).

[0034] This aluminum foil contains residues of polyalkylene glycols used in cooling lubricants on its surface. These residues are embedded in the base oil. The amount of polyalkylene glycol or a compound containing a polyalkylene oxide structure on the surface of the aluminum foil according to the present invention after cold rolling is at most 5 mg / m 2 , for example 0.01 mg / m 2 ~5 mg / m 2 and can be set as such.

[0035] It has been found that by using the cooling lubricant described in this specification, a significant reduction in the number of visually distinguishable defect patterns on the produced aluminum foil is also achieved. This is probably due to the fact that the rolling oil component does not form deposits that are difficult to remove on the material being rolled. The absence of aliphatic alcohols is thought to increase this reduction.

[0036] The surface tension of the aluminum foil can be specifically adjusted by the type of polyalkylene glycol or a compound containing a polyalkylene oxide structure used in the cooling lubricant according to the type of various electrode coatings. In particular, the surface tension of the aluminum foil is adjusted by the type of polyalkylene glycol or a compound containing a polyalkylene oxide structure used in the cooling lubricant so that the surface tension of the aluminum foil is substantially the same as the surface tension of the suspension of the electrode coating. It is advantageous that the surface tension of the aluminum foil is "substantially the same" as the surface tension of the suspension for electrode coating when it is in the range of ±20% of the surface tension of the suspension for electrode coating.

[0037] The surface tension of the corresponding suspension is determined, in particular, by the solvent, and in many cases, a nonpolar NMP is used with an optional mixture for the electrode material. Surprisingly, it has been found that relatively high surface tension can be obtained in the type of aluminum foil production according to the present invention using a selected cooling lubricant, as well as in suspensions with higher polar solvents, and even in purely aqueous suspensions, so that the aluminum foil according to the present invention can be used as well.

[0038] In one embodiment of the present invention, the contact angle in a droplet test on the surface of aluminum foil is 75° to 50°. In the droplet test, a droplet of deionized water with a droplet volume of 5 μl is applied to the surface of the aluminum foil. Under indoor climate conditions (22°C ± 2°C, 30% ± 10% relative humidity), the contact angle of the droplet is determined using a Drop Shape Analyzer DSA10 contact angle measuring device, commercially available from Kruess GmbH. The contact angle in the droplet test is a further indicator of the wettability, and therefore suitability, of the aluminum foil for use as a substrate in battery electrodes, with a smaller contact angle indicating better wettability. Several suspensions for aluminum foil can be reliably used with a contact angle of 85° to 45° in the droplet test. Particularly good results are achieved in the ranges of 55° to 85° and 65° to 80°.

[0039] Typically, to manufacture electrode foils, aluminum foil may be coated as a substrate in a slot die coating process. Good wettability of the aluminum foil surface to the electrode coating suspension (slurry) is required for the most precise adjustment of the wet film thickness and coating width, and to achieve a uniform coating. Here, wettability depends on the surface tension between the suspension and the aluminum foil, and it is advantageous if the surface tension of the aluminum foil is approximately the same as or higher than that of the suspension. By using the type and amount of polyalkylene glycol or compounds containing a polyalkylene oxide structure used in the cooling lubricants described herein, the surface tension of the aluminum foil can be set without any growth of oxide layers, and it is also possible that the surface of the aluminum foil can be influenced to suit various compositions of the electrode coating suspension by a specific selection of energy input.

[0040] Because pickling does not occur, a uniform and relatively thin roll oxide layer is obtained. This structure of the oxide layer allows for reliable penetration of the oxide layer through the active components of the electrode material, thereby ensuring good electrical connection of the aluminum foil in the battery. The relatively dense oxide layer present enables good passivation of the aluminum foil and, consequently, high corrosion protection. The aluminum foil according to the present invention is also highly suitable for ultrasonic welding due to the unique topography derived from the rolling process and the low oxide layer thickness after surface treatment.

[0041] In a further embodiment, the aluminum foil has a passivation layer, thereby preventing corrosion at the interface between the aluminum and the electrolyte. Surface treatment achieves good passivation properties of the aluminum foil, so that a passivation layer (e.g., containing AlF3) can be formed with the electrolyte components of the battery cell, particularly to improve corrosion protection.

[0042] In a further embodiment of the present invention, an electrode coating is arranged on at least a portion of the surface of an aluminum foil. Such an electrode coating particularly comprises at least one metal oxide, for example, lithium-cobalt(III) oxide. The electrode coating forms the aluminum foil, in particular, as a positive electrode material.

[0043] The following embodiments serve to further illustrate the present invention. [Examples]

[0044] <Example 1 - Determination of the coefficient of friction of various lubricants> The lubrication properties of the cooling lubricant according to the present invention were determined using the MTM2 Mini-Traction machine from PCS Instruments Ltd., in a standard configuration with a load-applying steel ball (19.05 mm in diameter) and an aluminum test disc that can rotate at various speeds. A load of 40N (0.5GPa contact pressure) was applied to a steel disc using a ball (3 / 4” rolling bearing steel AISI 52100 (100Cr6, 1.3505)) and the coefficient of friction (CF) was measured at various rolling speeds. Two average values ​​(MV) of the coefficient of friction measured at rolling speeds from 0.2 to 200 m / min are shown in Table 1 below. The disc was formed from aluminum alloy AA1XXX. The slip ratio (SRR) during the test was 50%. After the tribological test, the wettability of the aluminum test disc to water was tested. For this purpose, a droplet test was performed on a disc adjacent to a running track (German: Laufspur, English: running track) using demineralized water with a droplet volume of 5 μl. The standardized test procedure corresponds to the internal work instruction "Hydro CO 0620". Kinematic viscosity was measured according to DIN 51562.

[0045] [Table 1]

[0046] * PAG=40℃ 20mm2 EO / PO copolymer with kinematic viscosity / second ** 20mm at 40℃ 2 Polyethylene glycol monododecyl ether with kinematic viscosity of / second *** 33, 57, and 77 mm at 40°C in each case. 2 Poly(propylene glycol) monobutyl ether with kinematic viscosity of / second **** 75 and 225 mm at 40°C 2 A mixture of polypropylene glycol having a kinematic viscosity of 175 mm² / second, the viscosity of the mixture at 40°C. 2 / second

[0047] The formation of a lubricating film is not optimal with base oil alone, and metal soaps are formed. Lubricant sample 2 provides a good lubricating film, and although wear increases, the disk remains clean. Lubricant sample 3 according to the present invention provides better lubricating film formation. The same is true for sample 4, which shows even less running track on the sphere. This is also true for sample 5, which shows almost no wear. Samples 6-12 show good lubricating film formation. Samples 6 and 7 show some wear, and sample 8 shows almost no wear. Sample 6 shows acceptable wetting with water, sample 7 shows good wetting, and sample 8 shows very good wetting with water. Sample 10 shows almost no wear and almost no running track on the sphere. Samples 11 and 12 result in minimal running track on the sphere.

[0048] <Example 2 - Determination of Wetting Angle After Cold Rolling Using Various Lubricants> In the following tests to determine the wetting angle on the foil surface, aluminum foil of type AA1XXX alloy was used. The contact angle (CA) was measured when wetted with water and NMP. The wetting angle or contact angle was determined with a 5 μl droplet volume in a droplet test using a CLUSSE DSA 10 drop shape analyzer from Krüss GmbH in Hamburg, Germany, using completely demineralized water or NMP. The measured values ​​are the average of individual measurements at four different locations on the surface of the foil sample. The measurement results are shown in Table 2 below. Furthermore, the surface energy (SFE) was determined by determining the contact angle. The corresponding values ​​are shown in Table 2.

[0049] [Table 2]

[0050] * Polyethylene glycol monodecyl ether ** Poly(propylene glycol) monobutyl ether

[0051] The results shown in Table 2 demonstrate that lubricants containing compounds with a polyalkylene oxide structure result in aluminum foils with significantly smaller contact angles with respect to, for example, NMP. This could be useful for specific applications, particularly as battery membranes.

Claims

1. Aluminum foil for battery electrodes, having a thickness of 4 μm to 100 μm and formed from type AA1xxx, AA3xxx, and / or AA8xxx alloys, having a cold-solidified state and a cooling lubricant layer on its surface containing polyalkylene glycol and / or a compound containing a polyalkylene oxide structure, wherein the alkylene groups in the polyalkylene glycol or polyalkylene oxide are ethylene, propylene, and / or butylene, and the concentration of the polyalkylene glycol or the compound containing a polyalkylene oxide structure is 0.01 mg / m² on the surface side of the aluminum foil. ２ ~5 mg / m² ２ Aluminum foil containing the above in an amount, wherein the proportion of the polyalkylene glycol or the compound containing a polyalkylene oxide structure in the cooling lubricant is a maximum of 10% by weight based on the mass of the cooling lubricant, the cooling lubricant comprises at most 0.2% by weight of a fatty acid as a lubricating additive and at most 0.4% by weight of an aliphatic alcohol as a lubricating additive based on the mass of the cooling lubricant, and the cooling lubricant does not contain linear olefins.

2. The aluminum foil according to claim 1, characterized in that the surface of the aluminum foil was not subjected to corona treatment after cold rolling.

3. The aluminum foil according to claim 1 or 2, wherein the contact angle in a droplet test using NMP on the surface of the aluminum foil is less than 32°, and the droplet test is performed by dropping a droplet of NMP having a droplet volume of 5 μl onto the surface of the aluminum foil instead of desalinated water, and measuring the contact angle of the droplet using a contact angle measuring device under indoor climate conditions of 22°C ± 2°C and relative humidity of 30% ± 10%.

4. The aluminum foil according to claim 1 or 2, wherein the contact angle in a droplet test using NMP on the surface of the aluminum foil is 24° to 15°, and the droplet test is performed by dropping a droplet of NMP having a droplet volume of 5 μl onto the surface of the aluminum foil instead of desalinated water, and measuring the contact angle of the droplet using a contact angle measuring device under indoor climate conditions of 22°C ± 2°C and relative humidity of 30% ± 10%.

5. The aluminum foil according to any one or more of claims 1 to 4, characterized in that the aluminum foil has a thickness of 6 μm to 50 μm.

6. A method for producing a battery film including an aluminum foil for battery electrodes with an electrode coating, characterized in that an aluminum strip of an alloy of type AA1xxx, AA3xxx and / or AA8xxx is cold-rolled in the presence of a cooling lubricant to form an aluminum foil with a thickness of 4 μm to 100 μm, wherein the cooling lubricant is based on mineral oil or synthetic oil, polyalkylene glycol and / or a compound containing a polyalkylene oxide structure, the alkylene group in the polyalkylene glycol or polyalkylene oxide is ethylene, propylene or butylene, and comprises at most 0.2% by weight of a fatty acid as a lubricating additive and at most 0.4% by weight of an aliphatic alcohol as a lubricating additive based on the mass of the cooling lubricant, the cooling lubricant does not contain linear olefins, and the surface of the aluminum foil is coated with an electrode coating material.

7. Use of aluminum foil according to any one of claims 1 to 5 for manufacturing a battery film including aluminum foil for battery electrodes with an electrode coating.

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