WO2024106163A1

WO2024106163A1 - Metallized film for secondary battery positive electrodes

Info

Publication number: WO2024106163A1
Application number: PCT/JP2023/038681
Authority: WO
Inventors: 信男藤; 輝明都地; 幸雄長尾
Original assignee: 東レＫｐフィルム株式会社
Priority date: 2022-11-14
Filing date: 2023-10-26
Publication date: 2024-05-23

Abstract

The present invention enables the achievement of a production method which is capable of uniformly applying an active material to a film collector foil by roll conveyance without the occurrence of wrinkles or folds caused by heating or dimensional shrinkage during the roll conveyance even if the film collector foil is obtained by forming a conductive thin film layer on a resin surface and has a lower stiffness in comparison to conventional aluminum foils. According to the present invention, a metallized film for secondary battery positive electrodes is obtained by forming an aluminum metal film on at least one surface of a resin film, and winding the resulting film into a roll. This metallized film for secondary battery positive electrodes has a dimensional change rate in the MD direction of -0.10% or less and a dimensional change rate in the TD direction of -0.02% or more after a heat treatment at 150°C for 30 minutes.

Description

Metallized film for secondary battery positive electrodes

The present invention relates to a metallized film for use in the positive electrodes of secondary batteries.

In recent years, due to the miniaturization of electrical and electronic devices and environmental concerns, storage batteries such as secondary batteries and capacitors for non-gasoline-powered vehicles (hybrid and electric vehicles) are required to be smaller and lighter, while at the same time having a high output density that allows them to instantly charge and discharge large currents.

Generally, in order to improve the weight energy density, storage batteries installed in vehicles have a configuration in which the positive and negative electrodes are formed in sheet form, and the sheet-like positive and negative electrodes are wound or stacked and placed in a case with a separator also formed in sheet form between them. The sheet-like electrode plate has a structure in which a mixture layer containing active material is formed on the surface of a metal foil that serves as a current collector.

Also, one of the methods to obtain high output density is to reduce the resistance of the various materials that make up the storage battery (internal resistance of the storage battery). In storage batteries, aluminum foil is often used as the current collector, but ordinary aluminum foil current collectors have an oxide film, and it is said that the oxide film formed on the surface of the aluminum increases the internal resistance. When the internal resistance increases, the voltage drops when charging and discharging with a large current, which results in a decrease in the output of the storage battery. Generally, aluminum forms a natural oxide film that is a strong insulator with a thickness of 5 to 10 nm, but the aluminum surface has the characteristic of maintaining good conductivity. The reasons for this include the theory that current flows from defective parts of the oxide film, and the theory that in the field of quantum mechanics, a tunneling phenomenon in which particles pass through a region that is usually energy insurmountable with a certain probability, and that good electronic conduction occurs when an electronic conductor approaches an electrical insulator to a distance of about 10 nm or less. Although it is not very clear, it is thought that the aluminum oxide film itself has a strong influence on the internal resistance.

As a method for reducing the contact resistance between the electrode and the active material, which suppresses the increase in internal resistance caused by the oxide film, there is a method of making the surface of the metal foil used in the electrode uneven (for example, Patent Document 1). It is not clear whether roughening the aluminum surface increases the number of defects or whether forming many protrusions makes it easier for the tunnel effect to occur, but this is an effective method for reducing contact resistance.

As a method of improving high power density while reducing size and weight, progress has been made in thinning the electrode substrate with the aim of improving volumetric energy density and weight energy density. However, simply thinning the metal foil used in the electrodes to address this issue results in a problem of insufficient strength. Furthermore, increasing the surface irregularities of the thinned metal foil in order to reduce contact resistance is undesirable as it further reduces the strength of the metal foil. As a result, a new material has been proposed to replace metals: a material consisting of a biaxially oriented polyester thin film, which has excellent mechanical properties and heat-resistant dimensional stability, on the surface of which a conductive thin film layer of metal or the like is provided, with the function of a current collector and used as an electrode substrate (for example, Patent Document 2).

JP 2008-160053 A Japanese Patent Application Laid-Open No. 10-40919

However, when a conductive thin film layer such as a metal is applied to the surface of a resin film such as a polyester thin film, the film becomes less rigid than conventional aluminum foil, and dimensional shrinkage occurs due to heat treatment, which leads to wrinkles and creases during roll transport, making it very difficult to apply the active material uniformly when transported by roll.

In addition, the total electrical resistance value increases because the metal thickness is thinner than the metal foil that was previously used. Even if the metal surface is roughened to lower the contact resistance of the aluminum metal surface, a conductive thin film layer of metal or the like on the surface of the resin film is usually a vapor-deposited metal film formed by a vacuum deposition method or the like, and because it is a thin metal film, it is very difficult to roughen it by etching or the like. If the surface of the polyester thin film itself is roughened, it becomes more susceptible to breakage, making it difficult to transport during the manufacturing process.

In light of the above, the objective of the present invention is to provide a metallized film that can be transported without breaking or wrinkling, and does not increase contact resistance even when a conductive thin film layer is formed on the resin surface.

As a result of intensive research into the above-mentioned problems, the inventors have succeeded in creating a metallized film that does not break or wrinkle during roll transport and has low contact resistance, by controlling the processing temperature and tension during or immediately after forming the evaporated metal film using the vacuum deposition method, as well as the surface shape of the evaporated film, and have also developed a method for producing the same.

That is, the present invention relates to a metallized film for a secondary battery positive electrode, which is a resin film having an aluminum metal film formed on at least one surface thereof and wound into a roll shape, the metallized film for a secondary battery positive electrode being characterized in that, after heat treatment at 150° C. for 30 minutes, the dimensional change rate in the MD direction is −0.10% or less and the dimensional change rate in the TD direction is −0.02% or more;
the metallized film for a secondary battery positive electrode, wherein the ratio I[200]/I[111] of the peak intensity I[111] of X-ray diffraction from the aluminum 111 plane to the peak intensity I[200] of X-ray diffraction from the aluminum 200 plane of the metal film is 1.0 or more, and the specular reflectance at a wavelength of 555 nm of the metal film surface not in contact with the resin film is 30% or less;
the metallized film for a secondary battery positive electrode, wherein the surface resistance of the metal film is 0.15 Ω/□ or less;
the metallized film for a secondary battery positive electrode, wherein the resin film has a surface roughness Ra of 0.6 nm or more and 2.0 nm or less;
The metal film has a surface roughness Ra of 2.3 nm or more and 10.0 nm or less.

The present invention makes it possible to obtain a metallized film that can be transported by rolls without breaking or wrinkling, even when a conductive thin film layer is formed on the resin surface, without increasing contact resistance, and a method for producing the same.

1 is a cross-sectional schematic diagram of a metallized film of the present invention. 1 is a cross-sectional schematic diagram of a metallized film of the present invention. 1 is a cross-sectional schematic diagram of a metallized film of the present invention. 1 is a scanning electron microscope (SEM) photograph of the surface of an aluminum metal film (film thickness 1.48 μm) in which large, dense crystal grains were grown using an induction heating type deposition source with a carbon crucible. 1 is a SEM photograph of the surface of an aluminum metal film (film thickness 0.53 μm) when large, dense crystal grains were grown using an induction heating type deposition source using a carbon crucible. This is an SEM photograph of the cross section of an aluminum metal film (thickness 1.48 μm) grown with large and dense crystal grains using an induction heating type deposition source with a carbon crucible (same as Figure 4, but viewed from a different angle).

The present invention is described in detail below.

<Metallized film>
The metallized film 4 of the present invention has an aluminum metal film 3 on one or both surfaces of a resin film 1 (FIGS. 1, 2 and 3).

The metallized film 4 of the present invention is a film wound in a roll shape, and when the active material is coated on the metallized film, it can be pulled out from the roll, and after passing through the surface coating and drying processes, it can be wound into a roll shape. Processing in a roll shape makes the metallized film easier to handle, allows for faster processing speed, and makes it easier to save space in the processing device. However, since the metallized film 4 has a smaller rigidity and a larger thermal shrinkage than conventional aluminum foils, when the metallized film is transported by roll processing with added heat treatment, the metallized film is likely to break or deform into streaks, which makes it difficult to process cleanly. In particular, when the thermal shrinkage of the metallized film in the film width direction (sometimes called the TD direction), which is perpendicular to the roll transport direction (sometimes called the MD direction), is large, the thermal shrinkage in the TD direction becomes the starting point, and wrinkles and folds occur during roll transport. Therefore, it is preferable that the thermal shrinkage of the metallized film in the TD direction is small, and the dimensional change rate (negative shrinkage) in the TD direction after heat treatment at 150°C for 30 minutes is preferably -0.02% or more, and more preferably 0.00% or more.

A common method for reducing thermal shrinkage in the TD direction is to accelerate the thermal shrinkage of the resin in the metallized film by heating, but if the thermal shrinkage of the metallized film in the TD direction and the MD direction is reduced by the same amount, deformation and flexibility will be lost due to uneven shrinkage in the MD and TD directions, and the film will no longer be able to maintain flatness during transport, which may cause deformation such as wrinkles. Therefore, it is preferable that the thermal shrinkage in the MD direction be a certain value or higher. Specifically, it is preferable that the dimensional change rate in the MD direction after heat treatment at 150°C for 30 minutes is -0.10% or less, and more preferably -0.20% or less.

In order to ensure that the dimensional change rate in the MD direction of a metallized film wound in a roll shape is -0.10% or less and the dimensional change rate in the TD direction is -0.02% or more after heat treatment at 150°C for 30 minutes, this can be achieved by applying tension in the MD direction of the resin film or metallized film while performing heat treatment. The tension and heating conditions vary greatly depending on the type of resin in the resin film, the thickness of the resin film, the film width, the evaporated metal film, and the transport roll diameter and transport roll pitch during heat treatment. By adjusting and optimizing the tension and heating conditions, it is possible to adjust the dimensional change rate in the MD direction of the metallized film after heat treatment at 150°C for 30 minutes to -0.10% or less and the dimensional change rate in the TD direction to -0.02% or more.

The heat treatment can be performed before, during, or after deposition, but it is more preferable to perform it during vacuum deposition, which does not require a separate heat treatment step.

<Aluminum metal film>
The aluminum metal film 3 in the present invention is an aggregate of aluminum metal formed by laminating one or more layers whose main component is aluminum. The main component means that the aluminum content exceeds 80 atomic % when the entire layer is taken as 100 atomic %.

The thickness of the aluminum metal film 3 in the present invention is preferably 0.7 μm or more and 3.0 μm or less, and more preferably 1.0 μm or more and 2.5 μm or less.

For electrode applications, the lower the electrical resistance, the better, and in terms of surface resistance, the metal film surface resistance is preferably 0.15 Ω/□ or less, and more preferably 0.05 Ω/□ or less. On the other hand, since thinning is necessary to improve energy density, simply making the metal film thicker is not preferable. Considering the electrical resistance of the electrode, the thickness of the aluminum metal film is preferably 0.7 μm or more, and if it is 1.0 μm or more, the resistance will be lower and the rise in internal resistance can be reduced. On the other hand, in order to improve volumetric energy density, it is necessary to further thin the electrode substrate, and a thickness of 3.0 μm or less is preferable, and 2.5 μm or less is even more preferable.

The aluminum metal film 3 in the present invention has the characteristic of being able to reduce contact resistance by controlling the crystal growth of the metal film so that the surface irregularities become large when the film is formed by vacuum deposition.

　When the aluminum metal film 3 of the metallized film 4 is placed on top of a 10 mm thick NR sponge rubber, and two gold-plated copper plates measuring 25 mm x 25 mm are placed with a 1 mm gap between them and a 500 g weight is placed on each of the copper plates, and the resistance between the two copper plates is taken as the contact resistance, the contact resistance is preferably 15 mΩ or less, and more preferably 10 mΩ or less, in order to reduce the increase in internal resistance. The contact resistance here is a value that includes the contact resistance of the two electrode areas of 25 mm x 25 mm and the film resistance (surface resistance) between the two electrodes. Therefore, the ratio of the contact resistance value to the surface resistance value [contact resistance value]/[surface resistance value] can indicate a value that is less affected by the surface resistance, and the ratio of the surface resistance value [contact resistance value]/[surface resistance value] is preferably 0.35 or less, and more preferably 0.25 or less.

As a feature of the surface of the aluminum metal film 3, which controls the crystal growth of the metal film and reduces contact resistance, the specular reflectance of the surface of the aluminum metal film 3 that is not in contact with the resin film at a wavelength of 555 nm is preferably 30% or less, and more preferably 20% or less. This is because the surface of the aluminum metal film is finely roughened, which reduces the specular reflectance of visible light at 555 nm, and is effective in reducing contact resistance.

As a feature of the surface of the aluminum metal film 3 that controls the crystal growth of the metal film and reduces the contact resistance, the surface roughness Ra of the surface of the aluminum metal film 3 that is not in contact with the resin film 1 is preferably 2.3 nm or more and 10.0 nm or less, and more preferably 5.0 nm or more and 10.0 nm or less. When the aluminum surface is sufficiently roughened and a certain degree of size is ensured for the height of the convex and concave portions, the contact resistance tends to be low, and the larger the surface roughness Ra, the more preferable it tends to be. However, if the surface roughness Ra is too large, the thin aluminum metal film 3 can cause the metal film to break when transported or folded, so it is preferable that it be 10.0 nm or less.

As a feature of the surface of the aluminum metal film 3 that controls the crystal growth of the metal film and reduces the contact resistance, the ratio I[200]/I[111] of the X-ray diffraction peak intensity I[111] of the aluminum 111 plane of the aluminum metal film 3 to the X-ray diffraction peak intensity I[200] of the aluminum 200 plane is preferably 1.0 or more, and more preferably 2.0 or more.

For reference, aluminum is a cubic crystal, so when aluminum is in powder form, the crystal orientation is random, so the peak intensity of the X-ray diffraction of the 111 plane is the largest, and the intensity ratio I[200]/I[111] is less than 1.0. On the other hand, rolled aluminum foil becomes dense through the rolling process, and the crystal orientation is aligned, so the peak intensity I[111] of the X-ray diffraction of the diagonal 111 plane is weaker, and the intensity ratio I[200]/I[111] is greater than 1.0. The larger the intensity ratio I[200]/I[111], the more densely aligned the crystal orientation of the aluminum metal film 3 is, so the film resistance (surface resistance) is smaller, and the contact resistance of the contact surface is also reduced. The metal particles of the aluminum metallized film produced by the normal vacuum deposition method are columnar crystal films with large gaps, and the peak intensity of the X-ray diffraction of the 111 plane is the largest, and the intensity ratio I[200]/I[111] is less than 1.0.

By increasing the surface temperature of the substrate (resin film; hereafter, the resin film will be referred to as the substrate) during deposition, the columnar crystals become larger and denser, the crystals are aligned in the 200-plane direction, and the intensity ratio I[200]/I[111] becomes greater than 1.0.

However, if the substrate is a resin film, increasing the substrate temperature will cause it to melt and break, so if the substrate is produced without proper care, the substrate temperature cannot be increased, and the metal particles in the aluminum metallized film produced by vacuum deposition will become a columnar crystal film with large gaps, resulting in an intensity ratio I[200]/I[111] of less than 1.0.

In the present invention, even if the substrate is a resin film, the columnar crystals of the aluminum metal film are grown large and dense, and the crystal grains are enlarged, which successfully forms appropriate irregularities on the surface of the aluminum metal film. By using a vacuum deposition method in which the resin film is forcibly cooled from the back side while increasing the heat generation amount of the deposition source and controlling the degree of vacuum to 9.0×10 ^-3 Pa or more and 1×10 ^-2 Pa or less, the temperature is raised only near the surface of the resin film where the deposition source is exposed, making it possible to make the columnar crystals large and dense.

The crystal grains are grown large and dense by using a deposition source of an induction heating method using a carbon crucible or a method of heating by an electron beam, which generates a large amount of heat, or by a vacuum deposition method in which the distance between the deposition source and the resin film is shortened to increase the amount of heat transferred to the resin film surface even if a heating boat method is used, and an inert gas such as argon is introduced and the degree of vacuum is controlled to 9.0×10 ⁻³ Pa or more and 1×10 ⁻² Pa or less. However, since the resin film will melt due to the heat if left as is, it is preferable to forcibly cool the resin film from the back side to a temperature just before it melts, thereby growing the columnar crystals of the aluminum metal film large and dense, and forming appropriate unevenness on the surface of the aluminum metal film.

Figures 4 and 5 are SEM photographs of the surface of an aluminum metal film when crystal grains were grown large and densely using an induction heating deposition source with a carbon crucible that generates a large amount of heat on a resin film (polyethylene terephthalate (PET) film) with a surface roughness Ra of 1.6 nm. Figure 4 is an SEM photograph of the surface of an aluminum metal film with a thickness of 1.48 μm, and Figure 5 is an SEM photograph of the surface of an aluminum metal film with a thickness of 0.53 μm. It can be seen from Figures 4 and 5 that by enlarging the crystal grains, appropriate irregularities can be formed on the surface of the aluminum metal film. The surface roughness Ra at this time was 8.3 nm in Figure 4 and 2.7 nm in Figure 5.

Figure 6 is a cross-sectional SEM photograph of the aluminum metal film in Figure 4, from which it can be seen that the aluminum metal film is columnar crystals, and that each convex part of the unevenness on the aluminum metal film surface corresponds to a single columnar crystal. From this, it can be inferred that the larger the convex part of the unevenness on the aluminum metal film surface, the larger and denser the columnar crystals are. Comparing Figures 4 and 5, it can be inferred that the growth of columnar crystals is greater in Figure 4, where the deposition time is longer and the amount of heat is greater in order to thicken the film.

Furthermore, to make the crystal grains large and the metal film dense, it is necessary to expose the film to the heat source, which is the deposition source, for a certain amount of time, and as a result, it is necessary to extend the deposition time. The thickness of the aluminum metal film is preferably 0.7 μm or more, and more preferably 1.0 μm or more.

<Method of Producing Aluminum Metal Film>
The method for forming the aluminum metal film 3 is preferably a vacuum deposition method that can form a metal film on a thin resin film without using an adhesive for the purpose of producing a thin electrode. Vacuum deposition methods include induction heating deposition, resistance heating deposition, laser beam deposition, and electron beam deposition, and among them, electron beam deposition, laser beam deposition, and induction heating deposition, which have a large amount of heat generated by the deposition source, are preferably used. The amount of heat generated by the deposition source needs to be large enough to form large and dense crystal grains of the aluminum metal film 3, and the substrate surface temperature needs to be sufficiently high, but since it is difficult to actually measure, it is judged whether the amount of heat is sufficient by confirming that the aluminum metal film 3 after deposition has the required physical properties.

The aluminum metal film 3 preferably has a specular reflectance of 30% or less at a wavelength of 555 nm on the surface of the metal film that is not in contact with the resin film, a ratio I[200]/I[111] of the peak intensity I[111] of the X-ray diffraction of the 111 plane of aluminum to the peak intensity I[200] of the X-ray diffraction of the 200 plane of aluminum is preferably 1.0 or more and 10 or less, and the surface roughness Ra of the surface of the metal film is preferably 2.3 nm or more and 10.0 nm or less. However, if the heat generation amount of the deposition source is increased to the required heat amount, the temperature of the resin film may rise and melt if the cooling function of the normal vacuum deposition method is controlled, so that the deposition is performed while controlling the cooling function so that the temperature does not rise too much during deposition and the film can be cooled uniformly. Specifically, it is necessary to uniformly cool the back side of the deposition surface with a cooling mechanism consisting of a metal plate or metal roll sufficiently cooled by a refrigerant. In order to cool uniformly, it is essential to make close contact between the resin film and the cooling mechanism without creating any gaps.

For example, if the metal roll of the cooling mechanism has a scratch, the scratched portion will become a gap, and the resin film will not be able to cool at the scratched portion, resulting in melting. For example, if a foreign object gets into the resin film and the metal roll of the cooling mechanism, the foreign object will not be able to cool the resin film, resulting in melting. If the heat generation amount of the deposition source is increased to the required heat amount, scratches on the metal roll and inclusion of foreign objects, which are acceptable in normal vacuum deposition methods, will become a problem, so that it is necessary to further strictly manage scratches on the metal roll and inclusion of foreign objects. By increasing the heat generation amount of these deposition sources and strengthening the management of the cooling function, it is possible to grow the crystal grains of the aluminum metal film 3 large and densely, leading to a reduction in internal resistance including contact resistance. In particular, in the case of electron beam deposition and laser beam deposition, it is even more preferable to use an alumina crucible, which has better heat retention than a carbon crucible, as the deposition crucible, since it is possible to further increase the heat generation amount of the deposition source.

<Resin film>
The resin film 1 used in the present invention is preferably a thin film of a polymer such as a synthetic resin. Examples of resin films suitable for use in the present invention include polyester films, and among polyester films, polyethylene terephthalate films and polyethylene naphthalate films, or polyimide films, polyphenylene sulfide films, and polypropylene films. Of these, polyethylene terephthalate films are more preferably used. These resin films may be used alone or in combination. Furthermore, resin films coated with resin, adhesive, or the like on the surface thereof may be used.

The thickness of the resin film 1 is preferably 1 μm or more and 20 μm or less, and more preferably 3 μm or more and 10 μm or less. In order to make the electrode substrate thinner, it is more preferable that the resin film is thin, and it is preferably 20 μm or less, and more preferably 10 μm or less. However, if the resin film is too thin, there is a possibility that the yield will decrease due to breakage during the manufacturing process, so it is preferably 1 μm or more, and more preferably 3 μm or more.

The surface roughness Ra of the resin film is preferably 0.6 nm or more and 2.0 nm or less. If the surface roughness of the resin film is 0.6 nm or less, when the resin film is wound into a roll, it may stick and become difficult to transport. The surface roughness of the resin film is preferably 0.6 nm or more, and more preferably 1.0 nm or more. On the other hand, since it is preferable for the aluminum metal film surface to have large irregularities, it is not preferable to increase the surface roughness Ra of the resin film, as this makes the resin film more likely to break during transport. It is preferable for the resin film to have the minimum amount of irregularities necessary for transport and to be as smooth as possible, so the surface roughness Ra is preferably 2.0 nm or less, and more preferably 1.5 nm or less.

<Anchor layer>
An anchor layer 2 may be provided between the resin film and the aluminum metal film 3 of the metallized film 4 of the present invention. By providing the anchor layer 2, it is expected that the adhesion between the resin film and the aluminum metal film can be improved. As the anchor layer 2, it is preferable to form a metal layer on the resin film by a sputtering method. The sputtering method makes it possible to reduce the thickness of the anchor layer, which is optimal for storage battery applications that require thinner layers.

The anchor layer 2 is preferably a metal layer containing one or more selected from the group consisting of aluminum, nickel, titanium, nichrome, and chromium. At this time, it is important to keep the metal surface of aluminum, nickel, titanium, chromium, nichrome, etc. selected as the anchor layer 2 in a state where it is not oxidized while forming the aluminum metal film on it. Specifically, after forming a metal layer as the anchor layer 2 by sputtering, it is important to form the aluminum metal film 3 while maintaining a vacuum without exposing it to the atmosphere. If the metal surface of aluminum, nickel, titanium, chromium, nichrome, etc. selected as the anchor layer 2 is oxidized, a stable metal oxide film is formed, which makes it difficult to form a metal bond with the interface with the aluminum metal film 3 formed thereon, and the adhesion cannot be secured, and the aluminum metal film 3 may peel off from the anchor layer 2. Therefore, it is important not to oxidize the aluminum, nickel, titanium, chromium, nichrome, etc. selected as the anchor layer 2.

The thickness of the anchor layer 2 is preferably 3 nm to 40 nm, and more preferably 5 nm to 20 nm. If the thickness is less than 3 nm, sufficient adhesion may not be obtained. On the other hand, even if the anchor layer is made thicker than 40 nm, the effect of improving adhesion will not be great, so it is preferably 40 nm or less. If the anchor layer is produced by a sputtering method with a slow film formation speed, it is even more preferable to make the anchor layer 20 nm or less to improve productivity.

<Storage battery>
The storage battery according to the present invention includes an electrode assembly and a battery case that houses the electrode assembly. The electrode assembly includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

Such storage batteries include, for example, primary batteries, secondary batteries, electric double-layer capacitors, and aluminum electrolytic capacitors, but in this invention it refers to secondary batteries.

Secondary batteries include, for example, lithium secondary batteries, lead acid batteries, nickel cadmium batteries, nickel hydrogen batteries, nickel iron batteries, silver oxide zinc batteries, manganese dioxide lithium secondary batteries, lithium cobalt oxide carbonate secondary batteries, vanadium lithium secondary batteries, etc.

Among these, secondary batteries are preferred because they can be used for a long period of time, and lithium secondary batteries are even more preferred because they achieve high energy density by using organic solvents.

The positive electrode is a current collector on which a positive electrode material consisting of an active material, a binder resin, and a conductive additive is laminated. It is preferable to use the metallized film 4 of the present invention as the current collector.

Examples of active materials include layered lithium-containing transition metal oxides such as _LiCoO2 , _LiNiO2 , and Li( _NiCoMn ) _O2 , spinel-type manganese oxides such as _LiMn2O4 , and iron-based compounds such as _LiFePO4 .

As the binder resin, a resin with high oxidation resistance can be used. Specific examples include fluorine-containing resin, acrylic resin, and styrene-butadiene resin.

Conductive additives include carbon materials such as carbon black and graphite.

Examples of the electrolyte include LiPF ₆ , LiBF ₄ , and LiClO ₄ , with LiPF6 being preferred from the standpoint of solubility in organic solvents and ionic conductivity.

Examples of organic solvents include ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, etc., and two or more of these organic solvents may be mixed and used.

Below, we will explain how to make lithium secondary batteries, which are the most commonly used storage batteries.

To manufacture a lithium secondary battery, first, an active material and a conductive assistant are dispersed in a binder resin solution to prepare a coating solution for the electrodes, and then this coating solution is applied to a current collector and the solvent is dried to obtain a positive electrode and a negative electrode. It is preferable that the thickness of the coating film after drying is 50 μm to 500 μm. Furthermore, it is preferable to apply pressure to the active material layer formed on the current collector, preferably by a method such as roll pressing, to densify it and make the current collector into a thin film.

A separator for lithium secondary batteries is placed between the obtained positive and negative electrodes so that it is in contact with the active material layer of each electrode, and then it is enclosed in an exterior material such as an aluminum laminate film. After injecting the electrolyte, the negative electrode lead and safety valve are installed, and the exterior material is sealed.

The lithium secondary battery obtained in this way has high adhesion to the electrodes, has excellent battery characteristics, and can be manufactured at low cost.

The present invention will be described below based on examples. Note that the present invention is not limited to these examples, and these examples can be modified or changed based on the spirit of the present invention, and are not excluded from the scope of the invention.

(Magnetron sputtering)
A resin film was placed in a roll-type vacuum deposition apparatus (EWC-060 manufactured by ULVAC), and a metal layer was formed using a target of 70 mm×550 mm in an argon gas atmosphere adjusted to a vacuum of 1×10 ⁻² Pa or less by applying a pulse power source.

Unless otherwise specified, sputtering and vacuum deposition were performed consecutively to prevent contact with the air between the anchor layer and the aluminum metal film.

(Vacuum deposition)
The resin film was placed in a roll-type vacuum deposition apparatus (EWC-060 manufactured by ULVAC), and a vacuum was drawn until the vacuum reached 9.0 × 10 ⁻³ Pa or less. After that, the deposition boat was heated under the conveying speed, output conditions, and feed speed for feeding the aluminum wire into the deposition boat that achieved a predetermined aluminum film thickness, and the fed aluminum wire was heated to perform vacuum deposition, thereby forming an aluminum metal film.

Alternatively, a resin film was placed inside a roll-type vacuum deposition device (ULVAC EWC-060), and vacuum deposition was performed by heating an aluminum ingot using induction heating deposition with a carbon crucible under the conveying speed and output conditions that resulted in the aluminum film thickness being the specified value, forming an aluminum metal film.

(XRD (X-ray diffraction) measurement method)
Measurements were performed using an X-ray diffraction (RIGAKU SmartLab 9kW). Measurement conditions were: X-ray tube voltage and current: 45 kV-200 mA, scanning speed: 2°/min, entrance slit: 1.0 mm, receiving slit: 1.0 mm. The peak intensity I[111] of the X-ray diffraction of the 111 plane and the peak intensity I[200] of the X-ray diffraction of the 200 plane were obtained from the measurement results, and the ratio I[200]/I[111] was calculated and compared.

(Spectrophotometer absolute reflectance)
An absolute reflectance measuring device ASR-3105 (incident angle 5°) was attached to a spectrophotometer UV-3600i Plus with a large sample chamber unit MPC-603A manufactured by Shimadzu Corporation, and the reflectance of the measurement sample was measured using the aluminum mirror attached to the absolute reflectance measuring device as a reference. The reflectance data for visible light of 555 nm was used as a representative value.

(Contact resistance measurement)
A metallized film was placed on a 10 mm thick NR sponge rubber (NRS-06 manufactured by Wake Sangyo Co., Ltd.) with the metal film facing upwards, and two gold-plated copper plates measuring 25 mm x 25 mm were placed 1 mm apart, with a 500 g weight placed on each of the copper plates. The resistance between the two copper plates was measured with a resistance meter RM3544 manufactured by Hioki E.E. Co., Ltd., and was taken as the contact resistance.

(Surface resistance measurement)
The metallized film was cut to a size of approximately 300 mm x approximately 80 mm, and the surface resistance was measured at three points using a simple low resistivity meter (Loresta (registered trademark) EP MCP-T360 manufactured by Mitsubishi Chemical Analytech Co., Ltd.) by the four-terminal method, and the average value was used as the surface resistance value.

(Aluminum metal film thickness)
The metallized film was cut to a size of approximately 30 mm x approximately 30 mm, and 10 sheets were stacked and the thickness was measured with a micrometer to calculate the thickness of each metallized film. The thickness of the aluminum metal film was calculated from the difference between this thickness and the metallized film thickness calculated from the thickness of an undeposited resin film, which was similarly measured with a micrometer by stacking 10 sheets.

(Surface roughness)
Surface roughness Ra was measured using a scanning white light interference microscope manufactured by Hitachi High-Tech Science Co., Ltd. Measurement conditions were measurement mode "wave", light source 530 White, objective lens 50x, and values calculated using the attached analysis software under the following conditions: surface correction 4th order, complement "full", Gaussing Gaussian filter "cutoff 2 μm".

(Dimensional change rate measurement)
A metallized film of approximately 300 mm x approximately 300 mm is prepared, a hole of φ1 mm is drilled in approximately the center, and three holes of φ1 mm are drilled at positions 75 mm apart in front and behind the hole in the center in the roll conveyance direction (MD direction) of the metallized film, for a total of nine holes. Then, one hole is drilled at positions 50 mm in front and behind each of the three holes in the width direction (TD direction) of the film, for a total of nine holes.

First, the distance between adjacent holes for nine holes was measured using a manual two-dimensional image measuring machine "EXLON-Y" manufactured by Nakamura Mfg. Co., Ltd., and then the metallized film was heated in an oven at 150°C for 30 minutes, and the distance between adjacent holes for the same nine holes was measured using a manual two-dimensional image measuring machine "EXLON-Y" manufactured by Nakamura Mfg. Co., Ltd.

The average dimensional change rate in the MD and TD directions was calculated for nine adjacent holes, with each distance taken as 100%. Shrinkage was shown as a negative value, and expansion was shown as a positive value.

(Evaluation of Transportability)
The positive electrode active material was continuously applied to the aluminum metal film surface not in contact with the resin film on both sides or one side of the metallized film by the method described in the Examples, and after heating and drying, the positive electrode current collector was processed by calendar press processing. During this processing, it was carefully observed whether wrinkles or folds occurred during the transportation or winding process, and if these defects did not occur, the transportation suitability was judged to be "○", and if these defects occurred, the transportation suitability was judged to be "×".

Example 1
A biaxially oriented polyethylene terephthalate film with a thickness of 5.7 μm (Lumirror (registered trademark), type: F53, manufactured by Toray Industries, Inc.) was used as the resin film. The surface roughness of this resin film was 1.6 nm. A roll of this resin film was placed in a roll-type vacuum deposition apparatus (EWC-060, manufactured by ULVAC), and aluminum was deposited by sputtering to a thickness of 5 nm by applying a pulsed power supply. The sputtering output condition was 2.0 kW using a pulsed power supply. An aluminum metal film was then vacuum-deposited to a thickness of 1.06 μm by a vacuum deposition method in which the deposition boat was heated immediately after sputtering to heat the aluminum wire being sent out.

At this time, argon was introduced and the degree of vacuum during deposition was controlled to 9.0×10 ⁻³ Pa or more and 1×10 ⁻² Pa or less. At this time, the output of the deposition source, the conveying speed, and the tension during deposition were adjusted so that the dimensional change rate around 150° C. for 30 minutes was −0.10% or less in the MD direction and −0.02% or more in the TD direction.

The metallized film produced in this manner had a dimensional change rate of -0.28% in the MD direction and 0.06% in the TD direction at 150°C for 30 minutes, the ratio I[200]/I[111] of the X-ray diffraction peak intensity I[111] of the aluminum 111 face to the X-ray diffraction peak intensity I[200] of the aluminum 200 face was 3.1, the specular reflectance of the aluminum metal film surface not in contact with the resin film was 7.1% at a wavelength of 555 nm, and the surface roughness was 6.1 nm.

The surface resistance of the aluminum metal film surface not in contact with the resin film of this metal film was 0.051 Ω/□, the contact resistance was 10.12 mΩ, and the ratio of the contact resistance to the surface resistance [contact resistance/surface resistance] was 0.20.

The contact resistance was small enough that the contact resistance was judged to be acceptable, and the transportability was rated as acceptable.

Example 2
A metallized film was prepared and evaluated in the same manner as in Example 1, except that argon was not introduced during deposition, the degree of vacuum during deposition was 9.0×10 ⁻³ Pa or higher, and the thickness of the aluminum metal film was as shown in Table 1. The results are shown in Table 1.

(Comparative Example 1)
A metallized film was produced and evaluated in the same manner as in Example 1, except that the output of the deposition source, the conveying speed, and the tension during deposition were not adjusted so that the dimensional change rate at about 150°C for 30 minutes was -0.10% or less in the MD direction and -0.02% or more in the TD direction, and the output of the induction heating deposition method using a carbon crucible as the deposition source was reduced to prevent the resin film from being damaged by heat as much as possible.

(Comparative Example 2)
A metallized film was prepared and evaluated in the same manner as in Example 1, except that vacuum deposition was performed under conditions in which argon was not introduced during deposition, the degree of vacuum during deposition was 9.0×10 ⁻³ Pa or higher, the output of the deposition source, the conveying speed, and the tension during deposition were not adjusted so that the dimensional change rate at about 150° C. for 30 minutes was −0.10% or lower in the MD direction and −0.02% or higher in the TD direction, and the tension on the resin film was kept as low as possible. The results are shown in Table 1.

(Comparative Example 3)
A metallized film was produced and evaluated in the same manner as in Example 1, except that the aluminum wire was vacuum-deposited under conditions in which argon was not introduced during deposition, the degree of vacuum during deposition was 9.0×10 ⁻³ Pa or more, the output, conveying speed, and tension of the deposition source during deposition were not adjusted so that the dimensional change rate at around 150° C. for 30 minutes was −0.10% or less in the MD direction and −0.02% or more in the TD direction, the tension on the resin film was kept as low as possible, and the output of the deposition boat was not increased. The results are shown in Table 1.

1 resin film 2 anchor layer 3 aluminum metal film 4 metallized film

Claims

A metallized film for a secondary battery positive electrode, which is a resin film having an aluminum metal film formed on at least one surface thereof and wound into a roll shape, characterized in that after heat treatment at 150°C for 30 minutes, the dimensional change rate in the MD direction is -0.10% or less and the dimensional change rate in the TD direction is -0.02% or more.
The metallized film for secondary battery positive electrodes according to claim 1, wherein the ratio I[200]/I[111] of the X-ray diffraction peak intensity I[111] of the aluminum 111 plane to the X-ray diffraction peak intensity I[200] of the 200 plane of the metal film is 1.0 or more, and the specular reflectance at a wavelength of 555 nm of the metal film surface not in contact with the resin film is 30% or less.
The metallized film for a secondary battery positive electrode according to claim 1, wherein the surface resistance of the metal film is 0.15 Ω/□ or less.
The metallized film for secondary battery positive electrodes according to claim 1, wherein the surface roughness Ra of the resin film is 0.6 nm or more and 2.0 nm or less.
The metallized film for secondary battery positive electrodes according to claim 1, wherein the surface roughness Ra of the metal film is 2.3 nm or more and 10.0 nm or less.
The metallized film for secondary battery positive electrodes according to claim 1, wherein the surface resistance of the metal film is 0.15 Ω/□ or less, the ratio I[200]/I[111] of the peak intensity I[111] of the X-ray diffraction of the aluminum 111 plane of the metal film to the peak intensity I[200] of the X-ray diffraction of the aluminum 200 plane of the metal film is 1.0 or more, and the specular reflectance at a wavelength of 555 nm of the metal film surface not in contact with the resin film is 30% or less.
The metallized film for secondary battery positive electrodes according to claim 1, wherein the surface roughness Ra of the resin film is 0.6 nm or more and 2.0 nm or less, the ratio I[200]/I[111] of the X-ray diffraction peak intensity I[111] of the aluminum 111 plane of the metal film to the X-ray diffraction peak intensity I[200] of the 200 plane of the metal film is 1.0 or more, and the specular reflectance at a wavelength of 555 nm of the metal film surface not in contact with the resin film is 30% or less.
The metallized film for secondary battery positive electrodes according to claim 1, wherein the surface roughness Ra of the metal film is 2.3 nm or more and 10.0 nm or less, the ratio I[200]/I[111] of the X-ray diffraction peak intensity I[111] of the aluminum 111 plane of the metal film to the X-ray diffraction peak intensity I[200] of the 200 plane of the metal film is 1.0 or more, and the specular reflectance at a wavelength of 555 nm of the metal film surface not in contact with the resin film is 30% or less.