JP7810133B2 - Polyester film for lithium-ion battery current collector, film foil for lithium-ion battery current collector, and lithium-ion battery current collector - Google Patents

Description

The present invention relates to a polyester film for a lithium-ion battery current collector, a film foil for a lithium-ion battery current collector, and a lithium-ion battery current collector.

Traditionally, polyester films have been used in a variety of applications, including packaging, electronic components, electrical insulation, metal lamination, display components such as flexible displays, touch panels, anti-reflection, and shatterproofing of glass, due to their transparency, dimensional stability, mechanical properties, heat resistance, and chemical resistance.

In recent years, secondary batteries such as lithium-ion batteries have been widely used in various products such as vehicles and portable devices. Electrodes for such secondary batteries are formed, for example, by applying an electrode active material layer containing an electrode active material to the surface of an electrode current collector. This electrode current collector is a sheet-like conductive member that forms part of the conductive path from the electrode active material layer to the electrode terminal.
Electrode current collectors for secondary batteries are sometimes constructed with a laminated structure in which the surface of a resin substrate is covered with a thin metal film. Such laminated electrode current collectors have a current interrupting function in which the resin substrate melts and deforms, causing the thin metal film to rupture when abnormal heat generation occurs due to an internal short circuit or the like, and therefore have the advantage of contributing to preventing the progression of abnormal heat generation (see Patent Document 1).

For example, Patent Document 2 discloses an electrode for a secondary battery and a film used therefor, which has excellent heat resistance, is thinned to improve battery capacity and life, and is lighter than conventional products by using a film as a substrate.
Furthermore, Patent Document 3 discloses an electrode for a secondary battery, which has excellent heat-resistant dimensional stability, an improved battery capacity due to a thinner electrode material substrate, and is lighter than conventional products due to the use of a film, and a film used therefor.

Japanese Patent Application Laid-Open No. 2004-311146 Japanese Patent Application Publication No. 10-40919 Japanese Patent Application Publication No. 10-40920

However, the film in Patent Document 2 makes no mention of dimensional stability during high-temperature treatment, and the films in Patent Documents 2 and 3 may have insufficient current interruption functionality during a short circuit due to melting.

The present invention has been made in consideration of the above-mentioned circumstances, and its objective is to provide a polyester film for lithium-ion battery current collectors that contributes to stopping or preventing runaway reactions by improving the current interruption function during short circuits caused by melting, and that also has excellent dimensional stability during high-temperature treatment.

After extensive research, the inventors have discovered that the above-mentioned problems can be solved by providing the following configuration. The present invention has the following aspects.

[1] A polyester film for a lithium-ion battery current collector, having a crystalline melting temperature (Tm) of 258°C or less as determined by differential scanning calorimetry (DSC), and a shrinkage rate of 0.7% or less in both the machine direction (MD) and the width direction (TD) when heat-treated at 120°C for 5 minutes.
[2] The polyester film for a lithium ion battery current collector according to the above [1], wherein the shrinkage rate when heat-treated at 150°C for 5 minutes is 1.6% or less in both the machine direction (MD) and the width direction (TD).
[3] The polyester film for a lithium ion battery current collector according to [1] or [2] above, wherein the shrinkage rate when heat-treated at 180°C for 5 minutes is 3.7% or less in both the machine direction (MD) and the width direction (TD).
[4] The polyester film for a lithium ion battery current collector according to any one of [1] to [3] above, which contains a polyethylene naphthalate copolymer (A).
[5] The polyester film for a lithium ion battery current collector according to [4] above, wherein the content of the polyethylene naphthalate copolymer (A) in the polyester film is 50 mass% or more.
[6] The polyester film for a lithium-ion battery current collector according to [4] or [5] above, wherein the polyethylene naphthalate copolymer (A) contains 2,6-naphthalenedicarboxylic acid as the dicarboxylic acid component (a-1) and a bisphenol A-ethylene oxide adduct and ethylene glycol as the diol component (a-2).
[7] The polyester film for a lithium ion battery current collector according to [6] above, wherein the dicarboxylic acid component (a-1) contains 80 mol% or more of 2,6-naphthalenedicarboxylic acid.
[8] The polyester film for a lithium ion battery current collector according to the above [6] or [7], wherein the diol component (a-2) contains 1 mol% or more and 49 mol% or less of a bisphenol A-ethylene oxide adduct.
[9] The polyester film for a lithium ion battery current collector according to any one of [1] to [8] above, having a thickness of 1 μm or more and 12 μm or less.
[10] A film foil for a lithium ion battery current collector, comprising a metal layer on both sides of the polyester film for a lithium ion battery current collector according to any one of [1] to [9] above.
[11] The film foil for a lithium ion battery current collector according to [10] above, wherein the metal layer is made of copper or aluminum.
[12] The film foil for a lithium ion battery current collector according to the above [10] or [11], wherein the metal layer is provided by any one of vapor deposition, plating, or sputtering.
[13] The film foil for a lithium ion battery current collector according to any one of [10] to [12] above, wherein the metal layer has a two-layer structure.
[14] A lithium ion battery current collector comprising an electrode layer on the metal layer of the film foil for a lithium ion battery current collector according to any one of [10] to [13] above.

The present invention provides a polyester film for lithium-ion battery current collectors that contributes to stopping or preventing runaway reactions by improving current interruption functionality during short circuits caused by melting, and also has excellent dimensional stability during high-temperature treatment.

1 is a cross-sectional view schematically illustrating the structure of a film foil according to one embodiment of the present invention. FIG. 2 is a cross-sectional view schematically showing a case where a metal layer according to one embodiment of the present invention has a two-layer structure. FIG. 2 is a cross-sectional view schematically illustrating the structure of a film foil in a case where the metal layer according to one embodiment of the present invention has a two-layer structure.

Next, an example of an embodiment of the present invention will be described. However, the present invention is not limited to the embodiment described below.

<<Polyester film>>
The polyester film for a lithium-ion battery current collector of the present invention (hereinafter also referred to as "the film") has a crystalline melting temperature (Tm) of 258°C or less as determined by differential scanning calorimetry (DSC), and exhibits a shrinkage rate of 0.7% or less in both the longitudinal direction (MD) and the transverse direction (TD) when heat-treated at 120°C for 5 minutes.

The longitudinal direction (MD) of a film refers to the direction in which the film travels during the film production process, that is, the winding direction of a film roll, and is also called the machine direction or longitudinal direction.
The width direction (TD) of a film refers to the direction parallel to the film surface and perpendicular to the longitudinal direction, that is, the direction parallel to the central axis of the roll when the film is rolled, and is also called the transverse direction.

The present film is not particularly limited as long as it satisfies the above requirements, and may have either a single-layer structure or a laminated (multi-layer) structure. When the present film has a laminated structure, the present film may have a two-layer structure, a three-layer structure, or may have four or more layers as long as it does not deviate from the gist of the present invention. The number of layers to be laminated is not particularly limited, but is preferably 10 layers or less. If there are 10 layers or less, the thickness of each layer is sufficient, resulting in sufficient lamination during film formation, making flow marks and the like less likely to occur, and ensuring sufficient film quality.
When the present film has a laminated structure of two or more layers, a two-kind three-layer structure or a three-kind three-layer structure is preferred, and a two-kind three-layer structure is more preferred.

The film may be either an unstretched film (sheet) or a stretched film. Of these, a stretched film stretched uniaxially or biaxially is preferred. Of these, a biaxially stretched film is more preferred, as it has excellent balance of mechanical properties and flatness.

The present film preferably contains polyester as the main component resin, and when the present film has a laminated structure, the main component resin of each layer is preferably polyester.
The term "main component resin" refers to the resin that is contained in the largest proportion among the resins constituting each layer, and is, for example, a resin that accounts for 50% by mass or more, preferably 70% by mass or more, and more preferably 80% by mass or more (including 100% by mass) of the resins constituting each layer.

<Polyester>
The polyester used as the raw material for the present film is not particularly limited, and may be a homopolyester or a copolymer polyester. Specific examples include polyesters obtained by polycondensation of a dicarboxylic acid component and a diol component.

Examples of the dicarboxylic acid component include aromatic dicarboxylic acids such as terephthalic acid, 2,6-naphthalenedicarboxylic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, 3,4-furandicarboxylic acid, benzophenonedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 3,3'-diphenyldicarboxylic acid, and 4,4'-diphenyletherdicarboxylic acid; and aliphatic dicarboxylic acids such as cyclohexanedicarboxylic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and dimer acid.

Examples of the diol component include ethylene glycol, diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, polytetramethylene ether glycol, dimer diol, and bisphenols (bisphenol compounds such as bisphenol A, bisphenol F, or bisphenol S, or their derivatives, or ethylene oxide adducts thereof).

Typical polyesters include polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, and polybutylene naphthalate.

Examples of copolymer polyesters include copolymer polyesters that contain a third component as a copolymerization component other than the compound that is the main component of the dicarboxylic acid component that constitutes the polyester and the compound that is the main component of the diol component.

In particular, it is preferable that the present film contains a polyethylene naphthalate (hereinafter also referred to as "PEN")-based copolymer (A) as the polyester, from the viewpoint of easily adjusting the crystalline melting temperature (Tm) and the shrinkage rate to desired values.
The content of the PEN copolymer (A) in the present film is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more (including 100% by mass) of the resin constituting the present film.
When the present film has a laminated structure, it is preferable that the content of the PEN copolymer (A) contained in each layer satisfies the above range.

The PEN copolymer (A) specifically contains a dicarboxylic acid component (a-1) and a diol component (a-2). More specifically, the dicarboxylic acid component (a-1) is 2,6-naphthalenedicarboxylic acid, and the diol component (a-2) is ethylene glycol, and at least one of the dicarboxylic acid component (a-1) and the diol component (a-2) contains a copolymerization component.

The PEN copolymer (A) preferably contains 2,6-naphthalenedicarboxylic acid in the dicarboxylic acid component (a-1) in an amount of 80 mol % or more, more preferably 90 mol % or more, and even more preferably all (100 mol %) of the dicarboxylic acid component (a-1) is 2,6-naphthalenedicarboxylic acid.
By setting the content of 2,6-naphthalenedicarboxylic acid in the dicarboxylic acid component (a-1) to 80 mol % or more, it becomes easier to adjust the shrinkage rate to within a desired range.

The PEN copolymer (A) preferably contains 51 mol% or more, more preferably 60 mol% or more, even more preferably 70 mol% or more, particularly preferably 80 mol% or more, and especially preferably 90 mol% or more of ethylene glycol in the diol component (a-2). On the other hand, the upper limit of the ethylene glycol content in the diol component (a-2) is preferably 99 mol% or less, more preferably 98 mol% or less, even more preferably 97 mol% or less, particularly preferably 96 mol% or less, and especially preferably 95 mol% or less.
By setting the content of ethylene glycol in the diol component (a-2) within this range, the crystalline melting temperature (Tm) can be easily adjusted to fall within a desired range.

The PEN copolymer (A) preferably contains 20 mol % or less, more preferably 10 mol % or less of a copolymerization component in the dicarboxylic acid component (a-1), and even more preferably, all of the dicarboxylic acid component (a-1) is 2,6-naphthalenedicarboxylic acid, i.e., the copolymerization component is 0 mol %.
The PEN copolymer (A) preferably contains 49 mol% or less of the copolymer component in the diol component (a-2), more preferably 40 mol% or less, even more preferably 30 mol% or less, particularly preferably 20 mol% or less, and especially preferably 10 mol% or less. On the other hand, the lower limit of the copolymer component in the diol component (a-2) is preferably 1 mol% or more, more preferably 2 mol% or more, even more preferably 3 mol% or more, particularly preferably 4 mol% or more, and especially preferably 5 mol% or more.
By setting the content of the copolymerization component in the dicarboxylic acid component (a-1) and/or the diol component (a-2) within this range, the crystalline melting temperature (Tm) and the shrinkage rate can be easily adjusted to the desired range.

Examples of copolymerizable components that can be added to the dicarboxylic acid component (a-1) include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, 3,4-furandicarboxylic acid, benzophenonedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 3,3'-diphenyldicarboxylic acid, and 4,4'-diphenyletherdicarboxylic acid; and aliphatic dicarboxylic acids such as cyclohexanedicarboxylic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and dimer acid. From the standpoint of moldability, isophthalic acid, 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, and 3,4-furandicarboxylic acid are preferred. These copolymerizable components can be used alone or in combination of two or more.

Furthermore, examples of copolymerization components that can be added to the diol component (a-2) include diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, polytetramethylene ether glycol, dimer diol, and bisphenols (bisphenol compounds such as bisphenol A, bisphenol F, or bisphenol S, or derivatives thereof, or ethylene oxide adducts thereof). From the viewpoint of maintaining film strength, bisphenols are more preferred, and as the bisphenol, bisphenol A-ethylene oxide adducts are preferably used. These copolymerization components can be used alone or in combination of two or more.

That is, it is most preferable that the PEN copolymer (A) contains 2,6-naphthalenedicarboxylic acid as the dicarboxylic acid component (a-1) and a bisphenol A-ethylene oxide adduct and ethylene glycol as the diol component (a-2).
In this PEN copolymer (A), the dicarboxylic acid component (a-1) contains preferably 80 mol % or more, more preferably 90 mol % or more, and even more preferably 100 mol % of 2,6-naphthalenedicarboxylic acid.
In addition, in this PEN copolymer (A), the diol component (a-2) contains a bisphenol A-ethylene oxide adduct in an amount of preferably 1 mol % to 49 mol %, more preferably 2 mol % to 40 mol %, even more preferably 3 mol % to 30 mol %, particularly preferably 4 mol % to 20 mol %, and especially preferably 5 mol % to 10 mol %; and contains ethylene glycol in an amount of preferably 51 mol % to 99 mol %, more preferably 60 mol % to 98 mol %, even more preferably 70 mol % to 97 mol %, particularly preferably 80 mol % to 96 mol %, and especially preferably 90 mol % to 95 mol %.

Usually, when polyester is produced (polycondensed) using ethylene glycol as one of the raw materials, diethylene glycol is by-produced from the ethylene glycol. In this specification, this diethylene glycol is referred to as by-product diethylene glycol. The amount of diethylene glycol by-produced from ethylene glycol varies depending on the type of polycondensation, but is approximately 5 mol% or less of the ethylene glycol. In the present invention, 5 mol% or less of diethylene glycol is defined as by-product diethylene glycol, and this by-product diethylene glycol is also considered to be included in ethylene glycol and is distinguished from a copolymerization component. On the other hand, depending on the diethylene glycol content, more specifically, when diethylene glycol is contained in excess of 5 mol%, diethylene glycol is treated as a copolymerization component rather than as a by-product diethylene glycol.

When the present film has a multilayer structure, the type and content of the copolymer component constituting the PEN copolymer (A) contained in any layer, preferably in each layer, may be the same as described above, and the resin constituting each layer and the PEN copolymer (A) in each layer may be the same or different.

<Polymerization catalyst>
The polycondensation catalyst used in polycondensing polyester is not particularly limited, and any conventionally known compound can be used, such as a titanium compound, a germanium compound, an antimony compound, a manganese compound, an aluminum compound, a magnesium compound, and a calcium compound.

<Particle>
The present film may contain particles, which impart slipperiness to the polyester film and prevent scratches during each process, improving handling.
The type of particles contained in the present film is not particularly limited as long as they are particles that can impart lubricity, and specific examples include inorganic particles such as silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, aluminum oxide, and titanium oxide, as well as crosslinked polymers such as crosslinked silicone resin particles, crosslinked acrylic resin particles, crosslinked styrene-acrylic resin particles, and crosslinked polyester particles, and organic particles such as calcium oxalate and ion exchange resins.
Furthermore, precipitated particles obtained by precipitating and finely dispersing a part of a metal compound such as a catalyst during the polyester production process can also be used.

There is no particular limitation on the shape of the particles used, and any of spherical, blocky, rod-like, flat, etc. may be used.
There are also no particular limitations on the hardness, specific gravity, color, etc. Two or more types of particles of this series may be used in combination as needed.

The average particle size of the particles used is usually 0.01 to 5 μm, preferably 0.03 to 4 μm, and more preferably 0.05 to 3 μm. If the average particle size is within this range, the film can be easily handled and has good transparency.
In addition, when the particles are in the form of a powder, the average particle size of the particles can be determined by measuring the powder using a centrifugal sedimentation particle size distribution analyzer (e.g., the "SA-CP3" model manufactured by Shimadzu Corporation) and determining the particle size at an integrated volume fraction of 50% (d50) in the equivalent spherical distribution. The average particle size of particles in a film, layer, or resin can be determined by measuring the diameters of 10 or more particles using a scanning electron microscope (SEM) and calculating the average value. In this case, in the case of non-spherical particles, the average value of the longest and shortest diameters can be measured as the diameter of each particle.

When particles are incorporated into the present film, it is preferable to provide a surface layer and an intermediate layer and incorporate particles into the surface layer. Also, when the film has a three-type, three-layer structure with different front and back layers, it is possible to incorporate particles into at least one of the surface layers.
The particle content in the particle-containing layer is preferably 0.0003 to 5% by mass, more preferably 0.01 to 3% by mass, and even more preferably 0.01 to 0.5% by mass, although this depends on the average particle size. Within this range, the film can have good slip properties and transparency.

The method for adding particles to the film is not particularly limited, and any conventionally known method can be used. For example, particles can be added at any stage during polyester production, but it is preferable to add them after the esterification or transesterification reaction is complete.

<Others>
In order to suppress the amount of precipitation of oligomer components, the film may be produced using a polyester having a low content of oligomer components as the raw material. As a method for producing a polyester having a low content of oligomer components, various known methods can be used, such as a method in which solid-state polymerization is carried out after the polyester is produced.
The amount of oligomer component precipitation may be suppressed by forming the present film into a three or more layer structure and using a polyester raw material with a low content of oligomer components as the surface layer of the present film.
The polyester may also be obtained by carrying out the esterification or transesterification reaction, followed by melt polycondensation at a higher reaction temperature under reduced pressure.

In addition to the particles described above, conventionally known UV absorbers, antioxidants, antistatic agents, heat stabilizers, lubricants, dyes, pigments, etc. may also be added to this film as needed.

The present film may contain resins other than polyester as long as the effects of the present invention are not impaired.
Other resins include polystyrene resins, polyvinyl chloride resins, polyvinylidene chloride resins, chlorinated polyethylene resins, polycarbonate resins, polyamide resins, polyacetal resins, acrylic resins, ethylene vinyl acetate copolymers, polymethylpentene resins, polyvinyl alcohol resins, cyclic olefin resins, polylactic acid resins, polybutylene succinate resins, polyacrylonitrile resins, polyethylene oxide resins, cellulose resins, polyimide resins, polyurethane resins, polyphenylene sulfide resins, polyphenylene ether resins, polyvinyl acetal resins, polybutadiene resins, polybutene resins, polyamideimide resins, polyamide bismaleimide resins, polyetherimide resins, polyether ether ketone resins, polyether ketone resins, polyethersulfone resins, polyketone resins, polysulfone resins, aramid resins, and fluorine-based resins.

The thickness of the present film is preferably from 1 μm to 12 μm, more preferably from 1 μm to 10 μm, even more preferably from 2 μm to 8 μm, and particularly preferably from 2 μm to 6 μm.
By setting the thickness to 1 μm or more, the film strength can be maintained within a practical range. On the other hand, by setting the thickness to 12 μm or less, when the film foil is used as a film foil for a lithium-ion battery current collector, it can contribute to reducing the thickness and weight of the film foil, and ultimately the lithium-ion battery current collector. The thickness can be adjusted by the film formation conditions.
The thickness of the film was measured at five random locations within the film surface using a 1/1000 mm dial gauge, and the average was taken as the thickness.

<Method of manufacturing polyester film>
Next, a specific example of the production of the present film will be described, but the production method is not limited to the following example.
For example, when producing a biaxially stretched film, a preferred method is to extrude dried polyester pellets as described above from a die using a melt extrusion device such as an extruder as a molten sheet, and then cool and solidify them on a cooling roll such as a rotating cooling drum to obtain an unstretched sheet. Here, cooling is preferably carried out, for example, to a temperature below the glass transition point of the polymer to obtain a substantially amorphous unoriented sheet (unstretched sheet). Furthermore, in order to improve the flatness of the sheet, it is preferable to increase the adhesion between the sheet and the cooling roll, and an electrostatic application adhesion method and/or a liquid application adhesion method are preferably used.

Next, the resulting unstretched sheet is stretched biaxially. In this case, the unstretched sheet is first stretched in one direction using a roll or tenter-type stretching machine. The stretching temperature is usually 70 to 120°C, preferably 80 to 110°C, and the stretching ratio is usually 2.5 to 7.0 times, preferably 3.0 to 6.0 times.

Then, the film is stretched in a direction perpendicular to the first-stage stretching direction. In this case, the stretching temperature is usually 70 to 170°C, and the stretch ratio is usually 3.0 to 7.0 times, preferably 3.5 to 6.0 times.

Subsequently, the film is heat-treated under tension or relaxation of 30% or less at a temperature of 180 to 270°C to obtain a biaxially stretched film. This heat treatment is also called a heat setting step. The heat treatment may be performed in two or more steps at different temperatures.
After the heat treatment, the film may be cooled in a cooling zone. The cooling temperature is preferably higher than the glass transition temperature (Tg) of the polyester constituting the film, more specifically, preferably in the range of 100 to 160° C. This cooling may be performed in two or more steps at different temperatures.
In the above stretching, a method of stretching in one direction in two or more stages can be adopted, in which case it is preferable to perform the stretching so that the final stretching ratios in both directions are within the above ranges.

The present film can also be produced by a simultaneous biaxial stretching method, in which the unstretched sheet is simultaneously stretched and oriented in the machine direction (longitudinal direction) and width direction (transverse direction) under temperature control, usually at 70 to 120°C, preferably 80 to 110°C, with the area stretching ratio being preferably 4 to 50 times, more preferably 7 to 35 times, and even more preferably 10 to 25 times.
Subsequently, the film is subjected to a heat treatment under tension or relaxation of 30% or less at a temperature of typically 170 to 250°C to obtain a stretched and oriented film. Regarding the simultaneous biaxial stretching device employing the above-mentioned stretching method, any conventionally known stretching method such as a screw method, a pantograph method, or a linear drive method can be employed.

<Physical properties of polyester film>
The crystalline melting temperature (Tm) of this film measured by differential scanning calorimetry (DSC) is 258°C or lower. If the crystalline melting temperature (Tm) exceeds 258°C, the temperature at which insulation melts during a short circuit when used in a battery (described later) increases, which may delay the onset of stopping or preventing a runaway reaction due to a short circuit. In other words, by lowering the temperature at which insulation melts during a short circuit, the current interruption function during a short circuit due to melting can be improved.
From this viewpoint, the crystalline melting temperature (Tm) is preferably 256°C or lower, more preferably 254°C or lower, and even more preferably 252°C or lower.
On the other hand, from the viewpoint of formability and strength retention during high-temperature treatment, the crystalline melting temperature (Tm) is preferably 200°C or higher, more preferably 220°C or higher, and even more preferably 240°C or higher.

The crystalline melting temperature (Tm) can be adjusted by the type and content of the polyester constituting the present film.
The crystalline melting temperature (Tm) can be measured by the method described in the Examples.

Furthermore, the shrinkage of this film when heat-treated at 120°C for 5 minutes is 0.7% or less in both the machine direction (MD) and the transverse direction (TD). If the shrinkage (120°C, 5 minutes) exceeds 0.7%, the dimensional stability during high-temperature treatment may be insufficient. An example of high-temperature treatment is the treatment of providing a metal layer, which will be described later. If the shrinkage during the provision of the metal layer is small, it is possible to suppress a decrease in adhesion with the metal layer and deformation of the film.
From this viewpoint, the shrinkage rate (120°C, 5 minutes) is preferably 0.6% or less, more preferably 0.5% or less, in both the machine direction (MD) and the transverse direction (TD). The lower limit of the shrinkage rate (120°C, 5 minutes) is not particularly limited, but is usually about -0.5%.

From the viewpoint of improving dimensional stability during high-temperature treatment, the shrinkage of the present film when heat-treated at 150°C for 5 minutes is preferably 1.7% or less, and more preferably 1.6% or less, in both the machine direction (MD) and the transverse direction (TD). The lower limit of the shrinkage (150°C, 5 minutes) is not particularly limited, but is usually about -0.5%.
From the same viewpoint, the shrinkage of the present film when heat-treated at 180°C for 5 minutes is preferably 4.0% or less in both the machine direction (MD) and the transverse direction (TD), more preferably 3.5% or less, even more preferably 3.0% or less, and particularly preferably 2.5% or less. The lower limit of the shrinkage (180°C, 5 minutes) is not particularly limited, but is usually about -0.5%.

The shrinkage rates (120°C, 5 minutes), (150°C, 5 minutes) and (180°C, 5 minutes) can be adjusted by the type and content of polyester constituting the present film, the film-forming conditions of the present film, etc.
These shrinkage rates can be measured by the method described in the examples.

<<Film foil>>
The film foil for a lithium ion battery current collector of the present invention (hereinafter also referred to as "the present film foil") preferably has a metal layer on both sides of the present film.
That is, as shown in FIG. 1, the present film foil 1 preferably has a metal layer 12 on both sides of a polyester film 11 .

<Metal layer>
The metal forming the metal layer is not particularly limited as long as it is a conductive metal, and examples thereof include aluminum, nickel, gold, silver, copper, cadmium, and titanium. Among these, the metal layer is preferably made of copper or aluminum, from the viewpoint of being widely used as an electrode current collector (positive electrode current collector) used in the positive electrode of a lithium ion battery or an electrode current collector (negative electrode current collector) used in the negative electrode. Here, "made of" means that the metal layer contains copper or aluminum as a main component.
The metal layer may contain an element other than a metal having electrical conductivity.

The metal layer is preferably formed by vapor deposition, plating, or sputtering. More specifically, conventional methods such as vacuum vapor deposition, electroplating, and sputtering can be used.

In particular, the metal layer preferably has a two-layer structure.
In one example of such an embodiment, the metal layer 12 has a two-layer structure consisting of a metal layer 21 formed by vapor deposition or sputtering and a metal layer 22 formed by plating (see FIG. 2). In this case, the film foil 1 has the metal layer 21 formed by vapor deposition or sputtering and the metal layer 22 formed by plating, in this order, on both sides of the polyester film 11 (see FIG. 3).
By using such a two-layer structure, it is possible to make the foil thinner and lighter than conventional metal foils while maintaining conventional performance, and it is also possible to reduce costs compared to conventional metal foils.

<<Lithium-ion battery current collector>>
The lithium ion battery current collector of the present invention (hereinafter also referred to as "the current collector") preferably includes an electrode layer on the metal layer.
The electrode layer is formed by laminating a conventionally known electrode agent on the surface of the metal layer, and can be used as an electrode for a lithium ion battery.
Furthermore, a lithium ion battery can be produced using the current collector by a conventionally known method.

<<Applications>>
The present film and film foil can be used for lithium ion battery current collectors.
Therefore, the present current collector preferably has a configuration exemplified by electrode layer (positive electrode)/metal layer/polyester film/metal layer/electrode layer (positive electrode) as a positive electrode current collector, and electrode layer (negative electrode)/metal layer/polyester film/metal layer/electrode layer (negative electrode) as a negative electrode current collector.
The present current collector has such a configuration, which makes it possible to stop or prevent a runaway reaction caused by a short circuit, compared to conventional lithium-ion battery current collectors that use metal foil, more specifically, electrode layer (positive electrode)/metal layer/electrode layer (positive electrode) or electrode layer (negative electrode)/metal layer/electrode layer (negative electrode), and yet has the advantage of contributing to thinner, lighter, and more cost-effective current collectors.

The reason why the present film, present film foil, and present current collector can contribute to stopping or preventing a runaway reaction due to a short circuit is because they have a current interruption function at the short-circuited area. The current interruption function is achieved when the film melts and deforms during abnormal heat generation, causing the thin metal film (metal layer) to rupture, or when the film substrate (polyester film) melts and insulates the short-circuited area. In particular, in the present invention, the crystalline melting temperature (Tm) of the polyester film falls within a specific range, which can lower the temperature at which melting insulation begins during a short circuit, thereby contributing to stopping or preventing a runaway reaction due to melting insulation at an earlier stage.
Furthermore, the reason why the present film, present film foil, and present current collector can contribute to thinner, lighter, and cost reductions is that, as mentioned above, by replacing part of the metal layer with polyester film, the use of metal can be reduced.

<<Explanation of terms>>
In the present invention, the term "film" includes the term "sheet", and the term "sheet" includes the term "film".
In the present invention, when it is stated that "X to Y" (X and Y are any numbers), unless otherwise specified, it means "X or more and Y or less", and also means "preferably larger than X" or "preferably smaller than Y".
Furthermore, when it is stated that the amount is "X or more" (X is any number), it also means that the amount is "preferably greater than X" unless otherwise specified, and when it is stated that the amount is "Y or less" (Y is any number), it also means that the amount is "preferably smaller than Y" unless otherwise specified.

The present invention will be described in more detail below with reference to examples.
However, the present invention is not limited to the following examples as long as it does not deviate from the gist of the present invention.

<Evaluation method>
(1) Intrinsic viscosity (IV)
1 g of polyester from which components incompatible with the polyester had been removed was precisely weighed, and dissolved in 100 mL of a mixed solvent of phenol/tetrachloroethane = 50/50 (mass ratio), and the viscosity was measured at 30°C using a viscosity measuring device "VMS-022UPC-F10" (manufactured by Rigo Co., Ltd.).

(2) Average particle diameter of particles The particle diameter at an integrated volume fraction of 50% in the equivalent spherical distribution measured using a centrifugal sedimentation particle size distribution analyzer (SA-CP3 type) manufactured by Shimadzu Corporation was taken as the average particle diameter d50.

(3) Crystalline melting temperature (Tm)
The crystalline melting temperature (Tm) was measured using a differential scanning calorimeter (DSC 8500) manufactured by PerkinElmer Co., Ltd. in accordance with JIS K7121 (2012) when the temperature was raised from 25°C to 300°C at a rate of 10°C per minute. The extreme value of the maximum endothermic peak was taken as the crystalline melting temperature (Tm). The analysis was performed by selecting the corresponding maximum endothermic peak range from the "Peak Area" in the "Analysis" menu of the built-in software.

(4) Shrinkage Rate A 1.5 cm × 15 cm sample film was heat-treated for 5 minutes in a hot air oven maintained at a predetermined temperature (120°C, 150°C, or 180°C) in an untensioned state, and the lengths of the sample film were measured before and after the treatment, and the shrinkage rate was calculated using the following formula. The measurements were taken in both the machine direction (MD) and the width direction (TD) of the film.
Shrinkage rate (%) = {(sample length before heat treatment) - (sample length after heat treatment)} ÷ (sample length before heat treatment) × 100

<Materials used>
Raw material A: Dicarboxylic acid component (a-1): 2,6-naphthalenedicarboxylic acid = 100 mol %, diol component (a-2): ethylene glycol = 95 mol %, bisphenol A-ethylene oxide adduct = 5 mol % polyethylene naphthalate copolymer (A) (intrinsic viscosity = 0.62 dL/g)
Raw material B: homopolyethylene naphthalate (intrinsic viscosity = 0.62 dL/g)
Raw material C: homopolyethylene terephthalate (intrinsic viscosity = 0.65 dL/g)
Raw material D: homopolyethylene terephthalate (intrinsic viscosity = 0.85 dL/g)
Raw material E: homopolyethylene terephthalate (intrinsic viscosity = 0.59 dL/g)
Raw material F: Masterbatch prepared by blending homopolyethylene terephthalate with 1.0% by mass of silica particles having an average particle size of 3.5 μm (intrinsic viscosity = 0.70 dL/g)

Example 1
Raw material A was fed into a twin-screw extruder, extruded at 300°C, and cooled and solidified on a cooling roll set at 50°C using an electrostatic adhesion method, to obtain an unstretched sheet.
The unstretched sheet was then stretched 3.8 times in the machine direction (MD) at 130°C using a roll stretching machine. After preheating at 120°C in a tenter, the sheet was stretched 4.7 times in the transverse direction (TD) at 130°C. Finally, the sheet was heat-set at 230°C to obtain a 4 μm-thick biaxially stretched polyester film. The evaluation results are shown in Table 1.

Example 2
Raw material A was fed into a twin-screw extruder, extruded at 300°C, and cooled and solidified on a cooling roll set at 50°C using an electrostatic adhesion method, to obtain an unstretched sheet.
The unstretched sheet was then stretched 3.7 times in the machine direction (MD) at 125°C using a roll stretching machine. After preheating at 120°C in a tenter, the sheet was stretched 4.7 times in the transverse direction (TD) at 130°C. Finally, the sheet was heat-set at 230°C to obtain a 4 μm-thick biaxially stretched polyester film. The evaluation results are shown in Table 1.

(Comparative Example 1)
Raw material B was fed into a twin-screw extruder, extruded at 300°C, and cooled and solidified on a cooling roll set at 50°C using an electrostatic adhesion method, to obtain an unstretched sheet.
The unstretched sheet was then stretched 3.8 times in the machine direction (MD) at 130°C using a roll stretching machine. After preheating at 120°C in a tenter, the sheet was stretched 4.7 times in the transverse direction (TD) at 130°C. Finally, the sheet was heat-set at 230°C to obtain a 4 μm-thick biaxially stretched polyester film. The evaluation results are shown in Table 1.

(Comparative Example 2)
A mixed raw material obtained by mixing raw materials C, D, and F in proportions of 65 mass%, 15 mass%, and 20 mass%, respectively, was fed into a twin-screw extruder and extruded at 280°C. The mixture was then cooled and solidified on a cooling roll set at 30°C using an electrostatic adhesion method, thereby obtaining an unstretched sheet.
The unstretched sheet was then stretched 3.9 times in the machine direction (MD) at 83°C using a roll stretching machine. After preheating at 85°C in a tenter, the sheet was stretched 4.8 times in the transverse direction (TD) at 100°C. Finally, the sheet was heat-set at 230°C to obtain a biaxially stretched polyester film with a thickness of 5 μm. The evaluation results are shown in Table 1.

(Comparative Example 3)
A mixed raw material obtained by mixing raw materials C, D, E, and F in proportions of 20 mass%, 23 mass%, 48 mass%, and 9 mass%, respectively, was fed into a twin-screw extruder and extruded at 280°C. An unstretched sheet was obtained by cooling and solidifying the raw material on a cooling roll set at 28°C using an electrostatic application adhesion method.
The unstretched sheet was then stretched 2.7 times in the machine direction (MD) at 83°C using a roll stretching machine, and then stretched 1.7 times in the machine direction (MD) at 83°C. The sheet was then preheated at 78°C in a tenter and stretched 4.0 times in the transverse direction (TD) at 95°C. Finally, the sheet was heat-set at 225°C to obtain a 5 μm-thick biaxially stretched polyester film. The evaluation results are shown in Table 1.

As can be seen from the results in Table 1, the polyester films of the examples have a crystalline melting temperature (Tm) of 258°C or less as measured by differential scanning calorimetry (DSC), and a shrinkage rate of 0.7% or less in both the machine direction (MD) and the transverse direction (TD) when heat-treated at 120°C for 5 minutes. Therefore, by improving the current interruption function during a short circuit due to melting, the films can contribute to the termination or prevention of runaway reactions, and also have excellent dimensional stability during high-temperature treatment. Therefore, the polyester films of the present invention can be suitably used as current collectors for lithium-ion batteries.

On the other hand, the polyester film of Comparative Example 1 had a crystalline melting temperature (Tm) that exceeded the specified range, and the polyester films of Comparative Examples 2 and 3 had shrinkage rates that exceeded the specified range when heat-treated at 120°C for 5 minutes.

The polyester film for a lithium ion battery current collector of the present invention can contribute to stopping or preventing a runaway reaction by improving the current interruption function during a short circuit caused by melting, and also has excellent dimensional stability during high-temperature treatment.
Furthermore, the lithium ion battery current collector of the present invention is equipped with the film foil for lithium ion battery current collector of the present invention, which makes it possible to stop or prevent a runaway reaction due to a short circuit, compared to lithium ion battery current collectors that use conventional metal foils, and yet has the advantages of contributing to thinner, lighter, and cost reductions, making it of great industrial value.

1 Film foil 11 Polyester film 12 Metal layer 21 Metal layer (vapor deposition or sputtering)
22 Metal layer (plating)

Claims (13)

The crystalline melting temperature (Tm) measured by differential scanning calorimetry (DSC) is 258°C or lower;
The shrinkage rate when heat-treated at 120°C for 5 minutes is 0.7% or less in both the machine direction (MD) and the transverse direction (TD),
The thickness is 1 μm or more,
Contains a polyethylene naphthalate copolymer (A),
Polyester film for lithium-ion battery current collectors. The polyester film for lithium-ion battery current collectors according to claim 1, which has a shrinkage rate of 1.7% or less in both the machine direction (MD) and the transverse direction (TD) when heat-treated at 150°C for 5 minutes. The polyester film for lithium-ion battery current collectors according to claim 1, which has a shrinkage rate of 4.0% or less in both the machine direction (MD) and the transverse direction (TD) when heat-treated at 180°C for 5 minutes. The polyester film for a lithium-ion battery current collector according to claim 1, wherein the content of the polyethylene naphthalate copolymer (A) in the polyester film is 50 mass% or more. The polyester film for a lithium-ion battery current collector according to claim 1, wherein the polyethylene naphthalate copolymer (A) contains 2,6-naphthalenedicarboxylic acid as the dicarboxylic acid component (a-1) and a bisphenol A-ethylene oxide adduct and ethylene glycol as the diol component (a-2). The polyester film for a lithium-ion battery current collector according to claim 5, wherein the dicarboxylic acid component (a-1) contains 80 mol% or more of 2,6-naphthalenedicarboxylic acid. The polyester film for a lithium-ion battery current collector according to claim 5, wherein the diol component (a-2) contains 1 mol % or more and 49 mol % or less of a bisphenol A-ethylene oxide adduct. The polyester film for lithium-ion battery current collectors according to claim 1, having a thickness of 1 μm or more and 12 μm or less. A film foil for a lithium ion battery current collector, comprising a metal layer on both sides of the polyester film for a lithium ion battery current collector according to any one of claims 1 to 8. The film foil for a lithium-ion battery current collector according to claim 9, wherein the metal layer is made of copper or aluminum. The film foil for a lithium-ion battery current collector according to claim 9, wherein the metal layer is either a metal vapor deposition layer, a metal plating layer, or a metal sputtering layer. The film foil for a lithium-ion battery current collector according to claim 9, wherein the metal layer has a two-layer structure. A lithium ion battery current collector comprising an electrode layer on the metal layer of the film foil for a lithium ion battery current collector according to claim 9.