Current collector, electrode, lithium ion secondary battery, and method for manufacturing current collector

Abstract

A current collector according to the present disclosure is for a secondary battery, and has a laminated structure comprising a pair of conductive layers and a resin layer positioned between the pair of conductive layers. The melting point and the softening point of the resin layer are 85°C or more, the temperature difference ΔT between the melting point and the softening point of the resin layer is 14° C or more, and the resin layer includes a stretched layer.

Description

Current collector, electrode, lithium ion secondary battery, and method for manufacturing current collector

[0001] The present disclosure relates to a current collector, an electrode, a lithium ion secondary battery, and a method for manufacturing a current collector.

[0002] Conventionally, aluminum foil (positive electrode) or copper foil (negative electrode) has been used as a current collector for secondary batteries. Secondary batteries are required to have improved energy density by being thinner and lighter. Under these circumstances, it has been proposed to use a current collector for secondary batteries in which a conductive layer is formed on one or both sides of a resin film (for example, Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2019-102429

[0004] However, the inventors' investigations have revealed that there is room for improvement in such current collectors in terms of resistance to non-aqueous electrolytes.

[0005] Furthermore, such a current collector is required to have sufficient adhesion between the resin layer and the conductive layer, and one possible method for ensuring this adhesion is to press the resin layer and the conductive layer together at a high temperature.

[0006] Incidentally, secondary batteries are sometimes produced by film converting techniques such as roll-to-roll. Therefore, current collectors are required to have sufficient strength to withstand breakage, particularly during the process of coating an active material. However, the current collectors have room for improvement in terms of breakage strength.

[0007] The present disclosure provides a current collector having excellent resistance to nonaqueous electrolytes, sufficient adhesion between a resin layer and a conductive layer, and excellent breaking strength, a method for manufacturing the current collector, an electrode including such a current collector, and a lithium-ion secondary battery including such an electrode.

[0008] In order to solve the above problems, the present disclosure provides the following current collector, electrode, lithium-ion secondary battery, and method for manufacturing a current collector. [1] A current collector for a secondary battery, having a laminated structure including a pair of conductive layers and a resin layer located between the pair of conductive layers, wherein the resin layer has a melting point and a softening point of 85°C or higher, a temperature difference ΔT between the melting point and the softening point of the resin layer is 14°C or higher, and the resin layer includes a stretched layer. [2] The current collector according to [1], wherein the temperature difference ΔT is 23°C or higher. [3] The current collector according to [1] or [2], wherein the resin layer contains an acid-modified polypropylene resin. [4] The current collector according to any of [1] to [3], wherein the resin layer consists solely of the stretched layer. [5] The current collector according to any one of [1] to [3], wherein the resin layer comprises a first layer, a second layer, and a third layer in this order, the first layer and the third layer contain an acid-modified polypropylene resin, the second layer is made of a homopolymer polypropylene resin, and the second layer is the stretched layer. [6] An electrode for a secondary battery, comprising the current collector according to any one of [1] to [5]. [7] A lithium ion secondary battery, comprising the electrode according to [6]. [8] A method for producing the current collector according to any one of [1] to [5], comprising the step of applying pressure while heating a laminate having a layered structure comprising a pair of conductive layers and a resin layer located between the pair of conductive layers to obtain the current collector.

[0009] The present disclosure provides a current collector having excellent resistance to non-aqueous electrolytes, sufficient adhesion between a resin layer and a conductive layer, and excellent breaking strength, a method for manufacturing the current collector, an electrode including such a current collector, and a lithium-ion secondary battery including such an electrode.

[0010] Fig. 1 is a schematic cross-sectional view of a current collector according to one embodiment. Fig. 2 is a diagram showing an example of a differential scanning calorimetric curve with the temperature of the resin layer on the horizontal axis and the heat flow of the resin layer on the vertical axis. Fig. 3 is a diagram showing an example of a curve with the temperature of the probe on the horizontal axis and the displacement of the probe on the vertical axis. Fig. 4 is a schematic cross-sectional view of a current collector according to another embodiment. Fig. 5 is a diagram showing a schematic configuration of a secondary battery according to one embodiment. Fig. 6 is a diagram showing a schematic configuration of a secondary battery according to another embodiment.

[0011] Various exemplary embodiments will be described in detail below with reference to the drawings. Note that the same or equivalent parts in each drawing are designated by the same reference numerals, and redundant explanations will be omitted.

[0012] [Current Collector] <First Embodiment> Fig. 1 is a schematic cross-sectional view of a current collector according to this embodiment. The current collector 10 shown in Fig. 1 is a current collector for a secondary battery. The current collector 10 has a laminated structure including a pair of conductive layers 1a, 1b and a resin layer 2 located between the pair of conductive layers 1a, 1b. The melting point and softening point of the resin layer 2 are 85°C or higher. The temperature difference ΔT between the melting point and softening point of the resin layer 2 is 14°C or higher. The resin layer 2 consists of only a stretched layer.

[0013] (Conductive Layer) When the current collector is used as a positive electrode, the material of the conductive layers 1a and 1b may be, for example, aluminum (Al). When the current collector is used as a negative electrode, the material of the conductive layers 1a and 1b may be, for example, copper (Cu). The material of the conductive layers 1a and 1b may be any material that does not easily form an alloy with metal ions contained in the electrolyte.

[0014] The thickness of the conductive layers 1a and 1b may be, for example, 1 μm or more and 4 μm or less.

[0015] (Resin Layer) The melting point of the resin layer 2 may be, for example, 150° C. or higher. The melting point of the resin layer 2 may be, for example, 170° C. or lower.

[0016] The melting point of the resin layer 2 is measured under the following conditions. Specifically, the differential scanning calorimetry of the resin layer 2 is measured using a differential scanning calorimeter. This results in a differential scanning calorimetric curve with the temperature of the resin layer on the horizontal axis and the heat flow of the resin layer on the vertical axis. The temperature showing the endothermic peak on the highest temperature side of the differential scanning calorimetric curve is adopted as the melting point of the resin layer.

[0017] 2 is a diagram showing an example of a differential scanning calorimetric curve with the temperature of the resin layer on the horizontal axis and the heat flow of the resin layer on the vertical axis. Endothermic peaks P1, P2, and P3 are observed in FIG. 2. In this example, the temperature at which endothermic peak P3, which is on the higher temperature side, indicates the melting point of the resin layer.

[0018] As will be described later, when the resin layer is made up of a plurality of layers, the melting point of each of the plurality of layers is measured, and the highest melting point value among the melting points of the layers is adopted as the melting point of the resin layer.

[0019] The softening point of the resin layer 2 may be, for example, 110° C. or higher. The softening point of the resin layer 2 may be, for example, 160° C. or lower.

[0020] The softening point of the resin layer 2 is measured under the following conditions. The current collector is processed to expose the cross section of the resin layer. The softening point of the resin layer in this cross section is measured by nano-TA analysis. Specifically, a scanning probe microscope is used to bring the tip of a probe into contact with the resin layer. The probe displacement is measured while the probe is heated. This results in a curve with the probe temperature on the horizontal axis and the probe displacement on the vertical axis. For the peak at the lowest temperature among the maximum peaks on the curve, a tangent line when the probe displacement increases and a tangent line when the probe displacement decreases are drawn. The temperature at the intersection of these tangent lines is taken as the softening point.

[0021] 3 is a diagram showing an example of a curve with the probe temperature on the horizontal axis and the probe displacement on the vertical axis. Maximum peaks P4 and P5 are observed in FIG. 3. For peak P4, which has the lowest temperature, a tangent line L1 is drawn when the probe displacement increases, and a tangent line L2 is drawn when the probe displacement decreases. In this example, the temperature at the intersection C1 of these tangent lines is the softening point of the resin layer.

[0022] As will be described later, when the resin layer is made up of a plurality of layers, the softening point of each of the plurality of layers is measured, and the lowest softening point among the softening points of the layers is adopted as the softening point of the resin layer.

[0023] The melting point and softening point of the resin layer can be adjusted by changing the material and content of the resin layer.

[0024] The temperature difference ΔT between the melting point and softening point of the resin layer 2 is preferably 23° C. or more, more preferably 30° C. or more, and even more preferably 35° C. or more. This tends to provide the current collector with sufficient adhesion between the resin layer and the conductive layer, while also providing better resistance to non-aqueous electrolytes and breaking strength. The temperature difference ΔT between the melting point and softening point of the resin layer 2 may be, for example, 40° C. or less.

[0025] The resin layer 2 contains an acid-modified polyolefin resin. This tends to provide the current collector with sufficient adhesion between the resin layer and the conductive layer, while also providing better resistance to non-aqueous electrolytes and greater breaking strength. Examples of acid-modified polyolefin resins include acid-modified polypropylene resins and acid-modified polyethylene resins. From the same perspective, the acid-modified polyolefin resin is preferably an acid-modified polypropylene resin, and more preferably a maleic anhydride-modified polypropylene resin.

[0026] Acid-modified polyolefin resins are resins in which functional groups (polar acid groups) with biased charge distribution are introduced into polyolefin resins to increase adhesiveness. Polypropylene, a suitable example of polyolefin, has methyl groups (CH ３ However, by introducing functional groups (carboxyl groups) in place of methyl groups through a graft reaction, adhesion to metals increases. In particular, maleic anhydride-modified polypropylene resins containing maleic anhydride as a dicarboxylic acid are materials with high adhesive strength to metals.

[0027] The content of the acid-modified polyolefin resin is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, based on the total amount of the resin layer, which tends to improve the electrolyte resistance and the adhesion between the resin layer and the conductive layer.

[0028] The resin layer may or may not contain a polypropylene resin copolymer obtained by copolymerizing ethylene and propylene. The content of the polypropylene resin copolymer is preferably 45 mass% or less based on the total amount of the resin layer. This tends to further improve the current collector's resistance to the electrolyte solution and the adhesion between the resin layer and the conductive layer.

[0029] The resin layer may contain an elastomer. Examples of such elastomers include polyvinyl chloride, ethylene vinyl acetate, ethylene propylene rubber, and chlorinated polyethylene. Among these, ethylene propylene rubber is preferred because the softening point of the polyolefin can be adjusted.

[0030] The content of the elastomer may be 1% by mass or more, or 5% by mass or more, based on the total amount of the resin layer. The content of the elastomer may be 20% by mass or less, or 10% by mass or less, based on the total amount of the resin layer.

[0031] The resin layer may or may not contain a homopolymer polypropylene resin.

[0032] The resin layer is a stretched layer. Therefore, it has high crystalline orientation. The stretching treatment may be uniaxial stretching or biaxial stretching. The stretching treatment is preferably biaxial stretching. This allows the current collector to be made thinner, and tends to provide sufficient adhesion between the resin layer and the conductive layer while providing better resistance to non-aqueous electrolytes and greater breaking strength. The stretching ratio may be, for example, 15 to 30 times.

[0033] The thickness of the resin layer may be, for example, 3 μm or more and 10 μm or less.

[0034] The thickness of the current collector may be, for example, 5 μm or more and 16 μm or less.

[0035] [Second embodiment] Fig. 4 is a schematic cross-sectional view of a current collector according to this embodiment. The current collector 11 shown in Fig. 4 differs from the current collector according to the first embodiment in the resin layer 2. In the following description, only the differences from the first embodiment will be described, and a description of the same matters as in the first embodiment will be omitted.

[0036] The resin layer 2 includes a first layer 2a, a second layer 2b, and a third layer 2c in this order.

[0037] (First Layer) The first layer 2a contains an acid-modified polyolefin resin. This allows the current collector to have sufficient adhesion between the resin layer and the conductive layer, while also tending to have better resistance to non-aqueous electrolytes and breaking strength. Examples of the acid-modified polyolefin resin include acid-modified polypropylene resin and acid-modified polyethylene resin. From the same viewpoint, the acid-modified polyolefin resin is preferably an acid-modified polypropylene resin, and more preferably a maleic anhydride-modified polypropylene resin.

[0038] The first layer 2a contains a homopolymer polypropylene resin as a matrix resin, which is a main component, and may contain an acid-modified polyolefin resin as a secondary component.

[0039] The content of the acid-modified polyolefin resin in the first layer 2a is preferably 5% by mass or more and 20% by mass or less, based on the total amount of the first layer 2a, and the content of the homopolymer polypropylene resin is preferably 50% by mass or more and 90% by mass or less. This tends to improve the electrolyte resistance and the adhesion between the resin layer and the conductive layer. From the same viewpoint, the content of the acid-modified polyolefin resin is more preferably 5% by mass or more and 10% by mass or less, based on the total amount of the first layer 2a, and the content of the homopolymer polypropylene resin is more preferably 50% by mass or more and 90% by mass or less.

[0040] The first layer 2a may or may not contain a polypropylene resin copolymer obtained by copolymerizing ethylene and propylene. The content of the polypropylene resin copolymer is preferably 45 mass% or less based on the total amount of the first layer 2a. This tends to further improve the current collector's resistance to the electrolyte solution and the adhesion between the resin layer and the conductive layer.

[0041] The first layer 2a may contain an elastomer. Examples of such elastomers include polyvinyl chloride, ethylene vinyl acetate, ethylene propylene rubber, and chlorinated polyethylene. Among these, ethylene propylene rubber is preferred because the softening point of the polyolefin can be adjusted.

[0042] The content of the elastomer may be 1% by mass or more, or 5% by mass or more, based on the total amount of the first layer 2a. The content of the elastomer may be 6% by mass or less, or 10% by mass or less, based on the total amount of the first layer 2a.

[0043] The thickness of the first layer 2a is preferably 0.5 μm or more, more preferably 1 μm or more, in order to provide sufficient adhesion between the resin layer and the conductive layer while thinning the entire current collector, and is preferably 2 μm or less, more preferably 1 μm or less, in order to provide adhesiveness and to tend to improve film strength.

[0044] The first layer 2a is a stretched layer. The stretching treatment may be uniaxial stretching or biaxial stretching. The stretching treatment is preferably biaxial stretching. This tends to provide the current collector with sufficient adhesion between the resin layer and the conductive layer, while also providing better resistance to non-aqueous electrolytes and greater breaking strength. The stretching ratio may be, for example, 15 to 30 times.

[0045] (Second Layer) The second layer 2b serves to impart strength to the resin layer 2. The material of the second layer 2b may be a biaxially stretchable thermoplastic resin having a higher melting point than the first layer 2a and the third layer 2c. This allows a roll-to-roll process to be applied when applying an active material to form a current collector.

[0046] The second layer 2b may contain a polypropylene resin as a matrix resin, which is a main component. The polypropylene resin, which is a main component, is preferably a homopolymer from the viewpoint of improving strength. The second layer 2b may or may not contain a secondary component. From the viewpoint of improving strength, the second layer 2b preferably does not contain a secondary component. In other words, the second layer 2b is preferably a layer made of a homopolymer polypropylene resin. Examples of the secondary component include an acid-modified polyolefin resin, a copolymer polypropylene resin obtained by copolymerizing ethylene and propylene, and an elastomer.

[0047] The content of the homopolymer polypropylene resin, based on the total amount of the second layer 2b, is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 100% by mass.

[0048] The thickness of the second layer 2b is preferably 4 μm or more, more preferably 3 μm or more, and even more preferably 2.5 μm or more, from the viewpoint of reducing the thickness of the current collector. The thickness of the resin layer is preferably 6 μm or less, more preferably 5 μm or less, and even more preferably 4 μm or less, from the viewpoint of reducing the thickness of the current collector while maintaining strength sufficient to withstand the coating of an active material.

[0049] The second layer 2b is a stretched layer. This tends to result in a thin, high-strength resin layer. The stretching treatment may be uniaxial stretching or biaxial stretching. The stretching treatment is preferably biaxial stretching. Biaxial stretching tends to be excellent in productivity because it is possible to reduce the thickness of the second layer 2b while maintaining a certain degree of width. The stretching ratio may be, for example, 15 to 30 times.

[0050] (Third Layer) The material and content of the third layer 2c may be the same as those of the first layer 2a. The thickness of the third layer 2c may be the same as those of the first layer 2a. The conditions for the stretching treatment of the third layer 2c may be the same as those of the first layer 2a.

[0051] [Third Embodiment] The current collector according to this embodiment differs from the current collector according to the second embodiment in the first layer 2 a, the second layer 2 b, and the third layer 2 c. In the following description, only the differences from the second embodiment will be described, and a description of the same matters as in the second embodiment will be omitted.

[0052] (First Layer) The first layer 2a is an unstretched layer, which ensures adhesion to the metal layer.

[0053] (Second Layer) Examples of the resin for the second layer 2b include polyethylene terephthalate resin, nylon resin, and polybutylene terephthalate. The content of these resins may be 70% by mass or more, 80% by mass or more, 90% by mass or more, or 100% by mass.

[0054] The second layer 2b is a stretched layer. This tends to result in a thin, high-strength resin layer. The stretching treatment may be uniaxial stretching or biaxial stretching. The stretching treatment is preferably biaxial stretching. Biaxial stretching tends to be excellent in productivity because it is possible to reduce the thickness of the second layer 2b while maintaining a certain degree of width. The stretching ratio may be, for example, 5 to 15 times.

[0055] (Third Layer) The material and content of the third layer 2c may be the same as those of the first layer 2a. The thickness of the third layer 2c may be the same as those of the first layer 2a. The third layer 2c is an unstretched layer for the same reasons as those of the first layer 2a.

[0056] The melting point of the resin layer 2 may be, for example, 224°C or higher and 260°C or lower.

[0057] The temperature difference ΔT between the melting point and softening point of the resin layer 2 is preferably 90° C. or more. This tends to provide the current collector with sufficient adhesion between the resin layer and the conductive layer, while also providing better resistance to non-aqueous electrolytes and breaking strength. The temperature difference ΔT between the melting point and softening point of the resin layer 2 may be, for example, 130° C. or less.

[0058] The current collectors according to the first to third embodiments have been described above, but the current collectors of the present disclosure are not limited to these embodiments. For example, the resin layer 2 may contain another resin instead of the acid-modified polyolefin resin. Examples of such resins include non-acid-modified polyolefin resins. In addition, in the current collector according to the second embodiment, the first to third layers are all stretched, but one of the first to third layers may be unstretched, or two of the first to third layers may be unstretched. In addition, in the current collector according to the third embodiment, the first and third layers are unstretched, but at least one of these layers may be stretched.

[0059] [Method for Manufacturing Current Collector] The method for manufacturing a current collector according to this embodiment includes a step of applying pressure to a laminate having a laminate structure including a pair of conductive layers and a resin layer located between the pair of conductive layers while heating to obtain a current collector.

[0060] The melting point and softening point of the resin layer are 85°C or higher. The temperature difference ΔT between the melting point and softening point of the resin layer is 14°C or higher. The resin layer includes a stretched layer. This allows sufficient adhesion between the resin layer and the conductive layer even when the laminate is heated at a low temperature. Furthermore, the low heating temperature suppresses deterioration of the resin layer. Therefore, the resulting current collector has excellent breaking strength while maintaining sufficient adhesion between the resin layer and the conductive layer.

[0061] The conductive layer may be the same as the conductive layer according to the above embodiment. The resin layer may be the same as the resin layer according to the above embodiment.

[0062] The temperature to which the laminate is heated is preferably 5°C higher than the softening point of the resin layer. This tends to result in a current collector having even more sufficient adhesion. The temperature to which the laminate is heated is preferably 10°C lower than the melting point of the resin layer. This tends to result in a current collector having even more excellent electrolyte resistance and breaking strength.

[0063] The pressure applied to the laminate may be, for example, 0.4 to 1.7 MPa.

[0064] [Secondary Battery] <First Embodiment> FIG. 5 is a diagram schematically illustrating the configuration of a secondary battery according to this embodiment. The secondary battery 100 illustrated in FIG. 5 includes a positive electrode p, a separator 30, and a negative electrode n inside a container 60. The positive electrode p includes a current collector 10 and a positive electrode active material 20. The negative electrode n includes a negative electrode active material 40 and a current collector 10. The container 60 is filled with an electrolyte 50. The positive electrode active material 20 is provided on the upper surface of the current collector 10 and faces the negative electrode active material 40 via the separator 30. The negative electrode active material 40 is provided on the lower surface of the current collector 10. The current collector 10 is electrically connected to a positive electrode T1. The current collector 10 is electrically connected to a negative electrode T2. Secondary batteries are available in various shapes, and the layer structure inside the container can be rolled up to increase the surface area.

[0065] The secondary battery may be a metal ion battery in which metal ions are involved in charging and discharging. The metal ions may be lithium ions. That is, the secondary battery may be a lithium ion secondary battery. The active material is a substance that undergoes an oxidation-reduction reaction in the secondary battery to convert chemical energy and electrical energy.

[0066] Examples of the positive electrode active material 20 include lithium cobalt oxide, lithium manganese oxide, and lithium iron phosphate.

[0067] Examples of the negative electrode active material 40 include graphite and silicon-based materials.

[0068] The separator 30 is made of a material containing an electrolytic solution. For the separator 30, for example, a resin film made of a polyolefin resin such as polypropylene or polyethylene and having a plurality of pores formed therein can be used.

[0069] The electrolytic solution 50 is a conductive solution containing metal ions. In this example, the electrolytic solution 50 is a solution containing lithium ions, and is a 1 molar (1M) solution of lithium hexafluorophosphate (LiPF ６ The solvent is a solution of ethylene carbonate (ethylene carbonate) in a solvent containing ethylene carbonate. Dimethyl carbonate (DMC) or the like can also be used as the solvent.

[0070] The material of the container 60 is not particularly limited as long as it is resistant to the electrolyte, but an aluminum alloy or the like can be used.

[0071] The positive electrode T1 and the negative electrode T2 may be made of a common electrode material, such as aluminum (Al), copper (Cu), or gold (Au).

[0072] Second Embodiment Fig. 6 is a diagram schematically illustrating the configuration of a secondary battery according to this embodiment. The secondary battery 101 shown in Fig. 6 is composed of two secondary batteries connected in series. The current collector 10 between adjacent secondary batteries is a bipolar current collector, thereby reducing the number of components. The other configurations are the same as those of the secondary battery according to the first embodiment.

[0073] The present disclosure will be described below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0074] [Preparation of Resin Layer] The following substrates A1 to A5 and substrates B1 to B5 were prepared as resin layers. (Substrate A1) Layer structure: Single layer (thickness: 4.5 μm) Material: Acid-modified polypropylene resin (density: 0.89 g / cm ３ ) and elastomer (product name "FX-TPO70N", ethylene propylene rubber). Acid-modified polypropylene resin: elastomer (mass ratio) = 90:10 Stretching treatment: The substrate was biaxially stretched. The stretching ratio was 30 times.

[0075] (Base material A2) Layer structure: first layer (thickness: 1 μm) / second layer (thickness: 3 μm) / third layer (thickness: 1 μm) Materials of the first layer and the third layer: homopolymer polypropylene resin (density: 0.90 g / cm ３ ), a copolymer polypropylene resin obtained by block copolymerization of ethylene and propylene, and a maleic anhydride modified polypropylene resin (density: 0.89 g / cm ３ ) Polypropylene resin: copolymer polypropylene resin: maleic anhydride modified polypropylene resin (mass ratio) = 70:25:5 Second layer material: homopolymer polypropylene resin (density: 0.90 g / cm ３Stretching treatment: The first layer, the second layer, and the third layer are integrated by coextrusion and then biaxially stretched to a stretching ratio of 30 times.

[0076] (Base material A3) Layer structure: single layer (thickness: 4.5 μm) Material: acid-modified polypropylene resin (density: 0.89 g / cm ３ ) and elastomer (FX-TPO70N). Acid-modified polypropylene resin: elastomer (mass ratio) = 95:5. Stretching treatment: The substrate was biaxially stretched. The stretching ratio was 30 times.

[0077] (Base material A4) Layer structure: first layer (thickness: 10 μm) / second layer (thickness: 4.5 μm) / third layer (thickness: 10 μm) Materials of the first and third layers: acid-modified polypropylene resin (density: 0.89 g / cm ３ Second layer material: polyethylene terephthalate resin. Stretching treatment: the second layer was biaxially stretched, while the first and third layers were unstretched. The stretching ratio of the second layer was 30 times.

[0078] (Base material A5) Layer structure: first layer (thickness: 10 μm) / second layer (thickness: 4.5 μm) / third layer (thickness: 10 μm) Materials of the first layer and third layer: acid-modified polypropylene resin (density: 0.89 g / cm ３ Second layer material: nylon resin. Stretching treatment: the second layer was biaxially stretched, while the first and third layers were unstretched. The stretching ratio of the second layer was 15 times.

[0079] (Base material B1) Layer structure: single layer (thickness: 4.5 μm) Material: random polypropylene (r-PP) resin (density: 0.89 g / cm ３ Stretching treatment: The substrate was biaxially stretched to a stretching ratio of 30 times.

[0080] (Base material B2) Layer structure: single layer (thickness: 4.5 μm) Material: homopolypropylene (h-pp) resin (density: 0.89 g / cm ３ Stretching treatment: The substrate was biaxially stretched to a stretching ratio of 30 times.

[0081] (Base material B3) Layer structure: single layer (thickness: 4.5 μm) Material: polypropylene resin (density: 0.90 g / cm３ Stretching treatment: The substrate was biaxially stretched to a stretching ratio of 30 times.

[0082] (Base material B4) Layer structure: single layer (thickness: 10 μm) Material: acid-modified polypropylene resin (density: 0.89 g / cm ３ ) and elastomer (FX-TPO70N). Acid-modified polypropylene resin: elastomer (mass ratio) = 90:10 Stretching treatment: The substrate was unstretched.

[0083] (Base material B5) Layer structure: first layer (thickness: 5 μm) / second layer (thickness: 10 μm) / third layer (thickness: 5 μm) Polypropylene resin (density: 0.90 g / cm ３ ), a polypropylene resin block copolymerized with ethylene and propylene, and a maleic anhydride modified polypropylene resin (density: 0.89 g / cm ３ The mass ratio of polypropylene resin: block copolymerized polypropylene resin: maleic anhydride modified polypropylene resin was 70:25:5. The material of the second layer was polypropylene resin (density: 0.89 g / cm ３ ) Stretching treatment: The first layer, the second layer and the third layer are all unstretched.

[0084] [Measurement of Minimum Adhesion Temperature] The minimum adhesion temperature was measured for each substrate. Specifically, each substrate was thermally laminated with a pair of conductive layers (material: aluminum foil) using a heated roll. The thermal lamination temperature was 130°C, the pressure was 0.6 MPa, and the conveying speed was 2 m / s. This resulted in a laminate. A peel test was conducted on the laminate using a Shimadzu Autograph (AGS-1kN) at a test speed of 50 mm / min, in a T-shape. Next, a laminate was prepared in the same manner as above, and a peel test was conducted in the same manner as above, except that the thermal lamination temperature was increased by 5°C. Thereafter, the above procedure was repeated. The measurement was terminated when the peel strength of the laminate reached 1 N / 15 mm or more during the peel test. The thermal lamination temperature of the laminate obtained immediately before the laminate with a peel strength of 1 N / 15 mm or more was used as the minimum adhesion temperature.

[0085] [Preparation of current collector] (Examples 1 to 5, Comparative Examples 1 to 5) The substrates shown in Table 1 were used as the resin layers in each of the Examples and Comparative Examples. Each substrate was thermally laminated with a pair of conductive layers (material: aluminum foil) using a heated roll. The thermal lamination temperature was the lowest temperature at which adhesion was possible as measured above, the pressure was 0.6 MPa, and the conveying speed was 2 m / sec. In this way, a current collector was obtained.

[0086] [Softening Point Measurement] (Examples 1 and 3, Comparative Examples 1 to 4) The current collectors of each Example and Comparative Example were embedded in resin. The resin-embedded current collectors were processed under freezing conditions using an ultramicrotome (manufactured by Leica Microsystems, product name: FC7) to expose the cross section of the substrate (resin layer). The softening point of the resin layer in the cross section was measured using nano-TA analysis. Specifically, a scanning probe microscope was used to contact the probe tip with the resin layer. The probe displacement was measured while heating the probe. This resulted in a curve with the probe temperature on the horizontal axis and the probe displacement on the vertical axis. Tangents to the increasing and decreasing probe displacement were drawn for the lowest-temperature peak among the maximum peaks on the curve. The temperature at the intersection of these tangents was determined as the softening point. The softening point was measured at three locations in the center of the thickness direction of the cross section of the resin layer and three locations near the main surface of the cross section of the resin layer. The average of the softening points measured at these six locations was used as the softening point Ts of the resin layer. The results are shown in Table 1. The conditions for the nano-TA analysis are shown below.

[0087] Apparatus: Bruker AXS Dimension Icon Mode: nano-TA Heating rate: 4°C / sec Probe: RR-DM-AN2-200-5 Temperature range: room temperature (25°C) to 200°C

[0088] (Examples 2, 4, 5, Comparative Example 5) A cross section of the substrate (resin layer) of the current collector was exposed in the same manner as in Examples 1 and 3 and Comparative Examples 1 to 4. The softening point of the first layer was measured in the same manner as in the resin layers of Examples 1 and 3 and Comparative Examples 1 to 4. The softening points of the second and third layers were also measured in the same manner as in the first layer. The lowest softening point of the first to third layers was adopted as the softening Ts point of the resin layer. The results are shown in Table 1.

[0089] [Melt Point Measurement] (Examples 1 and 3, Comparative Examples 1 to 4) Differential scanning calorimetry was performed on the substrate (resin layer) of the current collector of each Example and Comparative Example using a differential scanning calorimeter. The temperature rise rate was 5°C / min, and the temperature range was room temperature (30°C) to 300°C. This resulted in a differential scanning calorimetric curve with the temperature of the resin layer on the horizontal axis and the heat flow of the resin layer on the vertical axis. The temperature showing the endothermic peak on the highest temperature side of the differential scanning calorimetric curve was taken as the melting point Tm of the resin layer. The results are shown in Table 1.

[0090] (Examples 2, 4, and 5, Comparative Example 5) The melting point of the first layer was measured in the same manner as the resin layers of Examples 1 and 3 and Comparative Examples 1 to 4. The melting points of the second and third layers were also measured in the same manner as the first layer. The highest value among the melting points of the first to third layers was adopted as the melting point Tm of the resin layer. The results are shown in Table 1.

[0091] [Measurement of Breaking Strength] A test specimen having the shape of JIS K 7127 test specimen type 5 was obtained from the current collector. The test specimen was pulled using an autograph (AGS-1kN) manufactured by Shimadzu Corporation (test speed: 100 mm / min). The strength at the point where the test specimen broke in the obtained stress-strain curve was measured as the breaking strength. The results are shown in Table 1.

[0092] [Evaluation of Electrolyte Resistance] The electrolyte resistance of the resin layer was evaluated. Specifically, the current collector was immersed in an electrolyte at 85°C for 72 hours, and the state after 72 hours was observed. The electrolyte was 1 molar (1M) lithium hexafluorophosphate (LiPF ６ ) was used. The state after 72 hours was evaluated according to the following criteria. The results are shown in Table 1. (Criteria) A: No delamination occurs. B: No delamination occurs, but the current collector peels off when rubbed with a cotton swab. C: Delamination occurs. N / D: Current collector cannot be produced and evaluation is not possible.

[0093]

[0094] 1a, 1b...conductive layers, 2...resin layer, 10, 11...current collectors, 100, 101...secondary batteries.

Claims

1. A current collector for a secondary battery, having a laminated structure including a pair of conductive layers and a resin layer located between the pair of conductive layers, wherein the melting point and softening point of the resin layer are 85°C or higher, the temperature difference ΔT between the melting point and softening point of the resin layer is 14°C or higher, and the resin layer includes a stretched layer.

2. The current collector according to claim 1, wherein the temperature difference ΔT is 23° C. or more.

3. The current collector according to claim 1, wherein the resin layer contains an acid-modified polypropylene resin.

4. The current collector according to claim 1, wherein the resin layer consists solely of the stretched layer.

5. The current collector according to claim 1, wherein the resin layer comprises a first layer, a second layer, and a third layer in this order, the first layer and the third layer contain acid-modified polypropylene resin, the second layer is made of a homopolymer polypropylene resin, and the second layer is the stretched layer.

6. An electrode for a secondary battery, comprising the current collector according to any one of claims 1 to 5.

7. A lithium ion secondary battery comprising the electrode according to claim 6.

8. A method for producing a current collector according to any one of claims 1 to 5, comprising the step of heating and pressurizing a laminate having a laminate structure comprising a pair of conductive layers and a resin layer located between the pair of conductive layers to obtain the current collector.

Images