Conductive films and their applications

Abstract

A conductive film with excellent conductivity in the thickness direction and sufficiently suppressed pinhole occurrence frequency is provided, and its application is also disclosed. The conductive film comprises: a conductive filler having a spike-like or structural body shape, and a resin binder. The ratio of the BET specific surface area of ​​the conductive filler to the spherical equivalent surface area obtained based on the D50 particle size of the conductive filler is 5.0 or more and 10 or less. The conductive filler comprises more than 3.0% by mass and less than 20% by mass relative to the total mass of the resin binder and the conductive filler.

Description

Technical Field

[0001] This specification relates to conductive films and their applications. Background Technology

[0002] A conductive adhesive tape using spiked conductive fillers is known (Japanese Patent Application Laid-Open No. 2009-79127). This tape specifies the content of spherical or spiked conductive fillers, and sets a predetermined relationship between the filler particle size and the thickness of the adhesive layer. It is described that this adhesive tape exhibits excellent adhesion, conductivity, and gradient absorption. Summary of the Invention

[0003] In recent years, research has been conducted on using conductive resin layers based on resin as current collectors or parts thereof in bipolar secondary batteries. For example, if the conductive film of Japanese Patent Application Laid-Open No. 2009-79127 is applied to the conductive layer of a bipolar secondary battery, the frequency of pinhole occurrence sometimes increases. Pinholes can cause various defects in secondary batteries. Furthermore, suppressing pinholes increases manufacturing costs. Moreover, while the conductive film of Japanese Patent Application Laid-Open No. 2009-79127 is expected to have in-plane conductivity, in bipolar secondary batteries, low resistance in the thickness direction of the conductive layer is crucial.

[0004] This specification provides conductive films with excellent conductivity in the thickness direction and sufficient suppression of pinhole occurrence frequency, as well as their applications.

[0005] According to this specification, the following means are provided.

[0006] [1] A conductive film comprising:

[0007] Conductive fillers with spike-like or structural forms, and

[0008] Resin adhesives

[0009] The BET specific surface area of ​​the conductive filler relative to the D based on the conductive filler 50 The ratio of the particle size to the converted surface area of ​​the spheres is greater than 5.0 and less than 10.

[0010] The conductive filler contains more than 3.0% by mass and less than 20% by mass relative to the total mass of the resin adhesive and the conductive filler.

[0011] [2] According to the conductive film of [1], the conductive filler has the spike-like morphology.

[0012] [3] According to the conductive film described in [1] or [2], the BET specific surface area of ​​the conductive filler is relative to the D based on the conductive filler.50 The ratio of the particle size to the converted surface area of ​​the spheres is greater than 6.0.

[0013] [4] The conductive film according to any one of [1] to [3] contains 5.0% by mass and 15% by mass of the conductive filler relative to the total mass of the resin adhesive and the conductive filler.

[0014] [5] A current collector for a bipolar secondary battery, comprising:

[0015] Conductive fillers with spike-like or structural forms, and

[0016] Resin adhesives

[0017] The BET specific surface area of ​​the conductive filler relative to the D based on the conductive filler 50 The ratio of the particle size to the converted surface area of ​​the spheres is greater than 5.0 and less than 10.

[0018] The conductive filler contains more than 3.0% by mass and less than 20% by mass relative to the total mass of the resin adhesive and the conductive filler.

[0019] [6] According to [5], the current collector of the bipolar secondary battery has a metal foil stacked on the conductive resin layer.

[0020] Based on the aforementioned conductive film, a conductive filler having a specific particle shape and BET specific surface area / sphere equivalent surface area is included within a predetermined range. This allows for the provision of an excellent conductive film with low resistance in the thickness direction and suppressed pinhole occurrence frequency.

[0021] The aforementioned conductive film is useful, for example, as a conductive resin layer constituting a current collector or part thereof in a bipolar secondary battery. Attached Figure Description

[0022] The features, advantages, and technical and industrial significance of exemplary embodiments of the present invention are described in detail below with reference to the accompanying drawings, wherein the same reference numerals denote the same elements.

[0023] Figure 1 This is a diagram showing the conductive filler and resin binder in a conductive film.

[0024] Figure 2 This is a diagram illustrating a bipolar secondary battery.

[0025] Figure 3 This is a diagram illustrating an example of a collector.

[0026] Figure 4AThis is a microscopic photograph showing the spiked Ni powder used in the embodiments.

[0027] Figure 4B This is a microscopic photograph showing the structural morphology of Ni powder used in the embodiments.

[0028] Figure 5A This is a diagram showing the evaluation units for the samples in the embodiments.

[0029] Figure 5B This is a diagram showing the evaluation units of the comparative examples in the embodiments. Detailed Implementation

[0030] This specification discloses conductive films and their use in bipolar secondary batteries. The conductive films disclosed herein, in addition to being used in various applications requiring conductivity, can also be used as current collectors or part of bipolar secondary batteries. Bipolar secondary batteries can be, for example, lithium-ion batteries or solid-state batteries.

[0031] For bipolar rechargeable batteries, the cell shape is not particularly limited. A bipolar rechargeable battery is a battery constructed by stacking multiple bipolar electrodes—one with a positive electrode active material layer and the other with a negative electrode active material layer—with a separator containing an electrolyte sandwiched between them. Bipolar rechargeable batteries are not particularly limited in design and can be used, for example, in vehicles.

[0032] In this specification, the terms "above" and "below" to refer to numerical values ​​include the concepts of "above" and "greater than," and "below" and "less than." When a predetermined numerical value is defined as a threshold value, a useful numerical range can be formed even if it is above that value, exceeding that value. Similarly, a useful numerical range can be formed even if it is below that value, being less than that value.

[0033] The conductive film disclosed in this specification will be described below, followed by an explanation of its use as a current collector in a bipolar secondary battery. Figure 1 This represents the cross-section of the conductive film 10.

[0034] conductive film

[0035] like Figure 1 As shown, the conductive film 10 contains a conductive filler 22 and a resin binder 20. The conductive filler 22 is dispersed and held in a resin matrix (hereinafter referred to as matrix) 24 containing the resin binder 20.

[0036] In addition to the resin binder 20, the matrix 24 may also contain other components such as dispersants, crosslinking accelerators, and plasticizers. From the viewpoint of electrical stability, the total content of other components relative to the total mass of the conductive film 10 is preferably 0.001% by mass or more and 5% by mass or less, more preferably 0.001% by mass or more and 3% by mass or less.

[0037] The thickness of the conductive film 10 is not particularly limited and can be appropriately set according to the application. For example, in the case of battery applications such as current collectors, it may be 1 μm or more and 100 μm or less. Other examples include 3 μm or more, 5 μm or more, 6 μm or more, 7 μm or more, 8 μm or more, 10 μm or more, 15 μm or more, and 20 μm or more. Still other examples include 80 μm or less, 60 μm or less, 40 μm or less, 30 μm or less, 25 μm or less, 20 μm or less, and 15 μm or less. The thickness range of the conductive film can be further appropriately combined with a lower limit and an upper limit within the range of 1 μm or more and 100 μm or less, for example, 8 μm or more and 25 μm or less, 8 μm or more and 20 μm or less, and 8 μm or more and 15 μm or less.

[0038] Furthermore, the thickness of the conductive film 10 is the thickness T of the substrate 24 itself, excluding the protrusions based on the conductive filler 22. The thickness T is obtained as follows: A cross-section cut in the thickness direction of the conductive film 10 is observed using a scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDX). The average thickness measured at 10 points at any location where the conductive filler 22 is not observed, consisting only of the substrate 24 that disperses and holds the conductive filler 22, is the thickness T.

[0039] Resin adhesives

[0040] There are no particular limitations on the resin binder 20, and various resins can be used. Examples of resin binders 20 include olefin-based resins such as polyethylene, polypropylene, and polymethylpentene. Examples of resin binders 20 include acrylic-based resins such as poly(meth)acrylic acid and poly(meth)methyl acrylate. Examples of resin binders 20 include known thermoplastic resins such as polytetrafluoroethylene fluororesins and halogenated vinyl resins. Examples of resin binders 20 include thermosetting resins such as epoxy resins, vinyl ester resins, unsaturated polyester resins, phenolic resins, and melamine resins. Examples of resin binders 20 include known conductive polymers such as polyaniline, polypyrrole, polythiophene, polyacetylene, poly(p-phenylene) polymers, polyphenylene vinylidene polymers, polyacrylonitrile, and polyoxadiazole. One type of resin can be used alone, or two or more types can be used in combination. Although there are no particular limitations, a non-conductive resin is preferred in forming conductive pathways in the thickness direction formed by the conductive filler. From the viewpoint of electrical stability, olefin-based resins such as polyethylene, polypropylene, and polymethylpentene are preferred. Acrylic resins are also sometimes preferred.

[0041] conductive filler

[0042] Conductive filler 22 may include metallic materials or alloys such as nickel, aluminum, stainless steel, platinum, gold, silver, copper, and titanium. Conductive filler 22 may also include graphite and carbon black (acetylene black, furnace black, Ketjen black (registered trademark)). Conductive filler 22 may include carbon materials such as carbon nanotubes, carbon nanofibers, carbon nanohorns, diamond-like carbon, and glassy carbon. Conductive filler 22 may also include metallic compound materials such as metal carbides, metal nitrides, and metal oxides. One type or two or more types may be used. For example, in the current collector 4 of a bipolar secondary battery 100, from a conductivity point of view, metallic materials such as nickel or nickel alloys are effective.

[0043] Particle morphology of conductive fillers

[0044] The conductive filler 22 has one or both of the following morphologies: spike-like and structural. In these particle morphologies, it exhibits low resistance in the thickness direction and suppresses pinhole formation. However, in the cases of spherical, flake-like, or fibrous morphologies, it is impossible to simultaneously achieve both low resistance in the thickness direction and a low pinhole occurrence frequency.

[0045] Spiked morphology refers to particles with multiple continuous or aggregated spike-like protrusions on their surface. Furthermore, spike-shaped particles, as a whole, generally exhibit an amorphous to spherical shape. Spiked particles with nickel as the main component are available, for example, as Vale's Type 123.

[0046] A structural morphology refers to a structure formed by the fusion and connection of multiple primary particles. Structures formed by the connection of primary particles can be categorized into filamentous (chain-like) and grape-like forms, among others. Particles with nickel as the primary component in structural morphologies (filamentous, etc.) can be purchased from companies such as Vale.

[0047] Surface area characteristics of conductive fillers

[0048] The BET specific surface area (m²) of conductive filler 22 2 / g) relative to D-based 50 Surface area of ​​a sphere converted to m² 2 The ratio Z (ratio of 22 particles to 10) can be greater than 5.0 and less than 10. Ratio Z indicates how much the actual surface area (BET specific surface area) increases when the particles of conductive filler 22 are assumed to be spheres.

[0049] Here, based on D 50 The converted surface area of ​​a sphere is used in the determination of particle size distribution using D. 50 (Based on volume), the calculation assumes that the particles of conductive filler 22 are spherical. That is, based on the assumption that the particles are spherical, the weight of one particle is calculated according to the specific gravity of conductive filler 22, and then the number of conductive filler 22 particles in 1g is calculated. The surface area (m²) per 1g is then calculated based on this number. 2 ), as the converted surface area of ​​a sphere (m²) 2 / g). BET specific surface area (m²) 2 / g) Calculated based on the BET theory from the gas adsorption isotherm using nitrogen. Furthermore, D 50 In a volume-based particle size distribution, the particle size is D 50 The following particle proportions constitute 50% of the particle size. D 50 The particle size distribution can be determined using a laser diffraction / scattering particle size distribution measuring device.

[0050] If the Z-value is less than 5.0, it is difficult to form low resistance; if it exceeds 10, it is easy to produce defects such as residual voids between protrusions on the surface of the conductive filler 22, and protrusions breaking off and becoming foreign objects. This exhibits low resistance and low pinhole occurrence frequency in the thickness direction of the conductive film 10. Furthermore, a Z-value of, for example, 6.0 or higher, 7.0 or higher, or 8.0 or higher is preferred. A Z-value of, for example, 9.0 or lower, 8.0 or lower, 7.0 or lower, or 6.5 or lower is also preferred. Additionally, the range of the Z-value is, for example, 6.0 or higher and 9.0 or lower, 6.0 or higher and 8.0 or lower, or 6.0 or higher and 7.0 or lower.

[0051] Content of conductive filler

[0052] The conductive filler 22 can be present in a concentration greater than 3.0% by mass and less than 20% by mass relative to the total mass of the resin binder 20 and the conductive filler 22. This is because if it is less than 3.0% by mass, it is difficult to obtain a resistance value in the thickness direction. Furthermore, if it is 20% by mass or more, when forming the conductive film 10 by, for example, extrusion molding (extrusion lamination), the frequency of pinholes increases due to decreased film fluidity. The content of the conductive filler 22 can be, for example, 5% by mass or more, 6% by mass or more, 7% by mass or more, or 8% by mass or more. Alternatively, it can be, for example, 15% by mass or less, 14% by mass or less, 13% by mass or less, or 12% by mass or less. The range of the conductive filler 22 content is, for example, 5% by mass or more and 15% by mass or less, 6% by mass or more and 14% by mass or less, 8% by mass or more and 12% by mass or less. Furthermore, the total mass of the conductive filler 22 and the resin binder 20 relative to the total mass of the conductive film 10 is not particularly limited, but is 95% by mass or more and 100% by mass or less. The total mass of the conductive filler 22 and the resin binder 20 relative to the total mass of the conductive film 10 is not particularly limited, but is 97% by mass or more and 100% by mass or less. The total mass of the conductive filler 22 and the resin binder 20 relative to the total mass of the conductive film 10 is not particularly limited, but is 98% by mass or more and 100% by mass or less. The total mass of the conductive filler 22 and the resin binder 20 relative to the total mass of the conductive film 10 is not particularly limited, but is 99% by mass or more and 100% by mass or less.

[0053] Furthermore, the particle size of the conductive filler 22 is not particularly limited, but a particle size D is preferred. 50 The ratio of the thickness T (μm) of the substrate 24 to the thickness T (μm) is more than 1.0 and less than 2.0. Within this range, it is easy to achieve low resistance in the thickness direction and suppress the formation of pinholes.

[0054] Applications of conductive films

[0055] The conductive film 10 itself can be used for applications where conductivity is expected and as a current collector in bipolar secondary batteries. Figure 2 This represents an example of a bipolar secondary battery 100. For example... Figure 2 As shown, multiple bipolar electrodes 2 are stacked with a separator 12 between them. The bipolar electrodes 2 are formed by fixing the negative electrode active material layer 6 and the positive electrode active material layer 8 relative to the current collector 4. End current collectors 16a and 16b are formed at the ends of the secondary battery 100 and are connected to the outside. In addition, the outer periphery of the secondary battery 100 is insulated by an insulating sealing material 14.

[0056] The current collector 4 may have a conductive resin layer 10a composed of a conductive film 10. The current collector 4 may also be composed solely of the conductive resin layer 10a. Additionally, as... Figure 3 As shown, the current collector 4 may also have a conductive resin layer 10a and a metal layer 30 on at least one side of the conductive resin layer 10a. Figure 3 The example shown uses a conductive resin layer 10a as the current collector layer on the negative electrode side. In this case, a metal layer 30 can be provided on the surface of the conductive resin layer 10a opposite to the positive electrode active material layer 8. The metal layer 30 can use a known metal as the current collector on the positive electrode side. The metal layer 30 includes, for example, aluminum, typically aluminum or aluminum alloy foil. Although not shown, when the conductive resin layer 10a is used as the current collector layer on the positive electrode side, a metal layer 30 can be provided on the surface of the conductive resin layer 10a opposite to the negative electrode active material layer 6. In this case, the metal layer 30 can use a known metal as the current collector on the negative electrode side. For example, it includes copper or nickel, typically copper foil or nickel foil. The thickness of the metal layer 30 as a metal foil is not particularly limited, for example, it is 5 μm or more and 100 μm or less, or 20 μm or more and 40 μm or less.

[0057] Furthermore, the metal layer 30 can also be obtained by film formation methods such as physical evaporation (vacuum evaporation, ion plating, and sputtering), chemical evaporation, and deposition. When the metal layer 30 is formed by physical evaporation or the like, the thickness of the metal layer 30 is not particularly limited, for example, it can be set to be 5 nm or more and 200 nm or less.

[0058] Depending on the requirements, in addition to the conductive resin layer 10a, the current collector 4 may also have an electronically conductive carbon coating 32 between itself and the negative electrode active material layer 6 and / or the positive electrode active material layer 8. The carbon coating 32 comprises carbon materials such as acetylene black, carbon black, and graphite. The carbon coating 32 improves the adhesion between the current collector 4 and the active material layers 6 and 8. The carbon coating 32 is configured to contact the active material layers 6 and 8. When the current collector 4 has a metal layer 30, the metal layer 30 is provided on the surface opposite to the active material layers 6 and 8. The carbon coating 32 is primarily composed of carbon materials, and the aforementioned resin adhesive or similar resin bonds the carbon materials together. The carbon coating 32 is, for example, 0.1 μm or more and 5 μm or less.

[0059] Method for manufacturing current collectors

[0060] The method for manufacturing a current collector 4 having a conductive resin layer 10a disclosed in this specification includes a step of preparing a conductive resin composition containing a conductive filler 22 and a resin binder 20. The method for manufacturing the current collector 4 includes forming a conductive resin layer 10a as a conductive film 10 using the conductive resin composition.

[0061] According to this method, a current collector 4 for a bipolar secondary battery can be obtained, which maintains a low resistance in the thickness direction of the conductive resin layer 10a and the current collector 4. Regarding the resin binder 20, conductive filler 22 and its ratio Z, matrix 24 and its thickness T, and particle size D in the conductive resin layer 10a... 50 Various methods described regarding the conductive film 10 can be appropriately applied.

[0062] In addition to the conductive filler 22 and the resin binder 20 (or their raw materials), the conductive resin composition can also be prepared by mixing dispersants, solvents, etc., as needed, using a known mixer.

[0063] The conductive resin layer 10a is shaped to form a film of a predetermined thickness T for the substrate 24. When manufacturing the conductive resin layer 10a, those skilled in the art can adjust the thickness of the substrate 24 of the conductive resin layer 10a to the expected thickness T by means of appropriate experiments or calculations, based on the composition of the conductive resin layer 10a.

[0064] There are no particular limitations on the forming method, and any known film forming method for manufacturing the conductive film 10 can be used. From the viewpoint of efficiently manufacturing the conductive resin layer 10a, forming methods such as casting with coating or extrusion forming can be used.

[0065] When using an extrusion molding method, a thermal load is applied to the conductive filler 22. Therefore, metallic materials such as metals or alloys, or carbon materials, are preferably used as the conductive filler 22. Furthermore, in order to resist the shear force during extrusion and allow the conductive filler 22 to penetrate in the thickness direction, spike-like or structural body-like shapes are suitable. From the viewpoint of ensuring low resistance in the thickness direction and suppressing pinholes, a spike-like shape is more preferred.

[0066] When the current collector 4 is composed of a single conductive resin layer 10a, it can be directly obtained by obtaining the conductive resin layer 10a to manufacture the current collector 4. When the current collector 4 is composed of a conductive resin layer 10a and a metal layer 30, it can be manufactured by an extrusion lamination method in which the conductive resin layer 10a is laminated onto a metal foil serving as the metal layer 30 during extrusion molding. According to this method, a current collector 4 having a conductive resin layer 10a and a metal layer 30 can be obtained efficiently.

[0067] When the current collector 4 has a carbon coating 32, in addition to the carbon material and resin binder, a composition containing a dispersion medium such as NMP is applied to the metal layer 30 and / or the conductive resin layer 10a as needed, and then allowed to dry appropriately. In this way, a composition containing a dispersion medium such as NMP can be applied to the metal layer 30 and / or the conductive resin layer 10a as needed.

[0068] Furthermore, the manufacturing method of the current collector 4 has been described above, and the conductive resin layer 10a of the current collector 4 is an example of the conductive film 10. According to this specification, the manufacturing method of the conductive film 10 may also include: preparing a conductive resin composition containing a conductive filler 22 and a resin binder 20, and using the conductive resin composition to form the conductive film 10.

[0069] Secondary batteries and their manufacturing methods

[0070] The secondary battery disclosed in this specification is a bipolar secondary battery 100, which may include a bipolar electrode 2. The bipolar electrode 2 has a current collector 4, and the current collector 4 has a conductive resin layer 10a disclosed in this specification. According to this secondary battery 100, by using the current collector 4 having a conductive resin layer 10a, the resistance in the thickness direction can be maintained at a low level, suppressing the frequency of pinhole occurrence. Therefore, a secondary battery 100 that is advantageous from the viewpoint of cost and battery characteristics is provided. Furthermore, according to this specification, a bipolar electrode 2 having a current collector 4 is also provided.

[0071] The secondary battery 100 includes a bipolar electrode 2 on one side of the current collector 4, where a negative electrode active material layer 6 is formed and a positive electrode active material layer 8 is formed on the other side. Furthermore, the secondary battery 100 may, as needed, have a carbon coating 32 on one and / or the other side of the current collector 4 in a predetermined stacked configuration. Additionally, the secondary battery 100 has a structure in which multiple such bipolar electrodes 2 are stacked in series with a separator 12 in between.

[0072] This secondary battery 100 can be appropriately manufactured by anyone skilled in the art based on known methods. There are no particular limitations on the negative electrode active material layer 6, the positive electrode active material layer 8, the separator 12, the sealing material 14, and the end current collectors 16a and 16b, and known materials can be used for them.

[0073] Hereinafter, embodiments are described to illustrate the disclosure of this specification in more detail. Therefore, the following embodiments are provided to illustrate the disclosure of this specification and do not limit the scope of the invention.

[0074] Example 1

[0075] In this embodiment, various conductive resin compositions were prepared, and then current collectors (samples 1-5, comparative sample 1-5) were fabricated by integrating the obtained conductive resin layer with an Al foil. The resistance value in the thickness direction was evaluated for these current collectors. Furthermore, Ni powder with the morphology and particle size distribution shown in Table 1 was used as the conductive filler in each conductive resin composition. Additionally, adhesive polypropylene (for extrusion lamination) was used as the resin binder. The resin binder and conductive filler were added to a biaxial extrusion mixer at a mass ratio of 90:10. The mixture was kneaded at 240°C to obtain a conductive resin composition. Then, the conductive resin composition was heated to 280°C using an extrusion lamination apparatus and directly laminated onto an Al foil (40 μm thick). Furthermore, for reference, Figure 4A and Figure 4B Microscopic photographs showing Ni powder with a spike-like morphology in sample 1 and Ni powder with a structural body-like morphology in sample 4.

[0076] Furthermore, the crushed material 1 (0.30 MPa) was obtained by using a pulverizer to prepare a type of Ni powder under this pressure.

[0077] D 50 Particle size distribution (volume standard) was determined using a laser diffraction / scattering particle size distribution measuring device to obtain D. 50 Surface area of ​​a sphere converted to m² 2 / g) is calculated as follows. Using this D 50 Assuming the conductive filler particles used are spherical, the weight of one particle is calculated based on the specific gravity of conductive filler 22. Then, the number of conductive filler particles per 1g is calculated, and the surface area (m²) per 1g is calculated based on this number. 2 ), as the converted surface area of ​​a sphere (m²) 2 / g). BET specific surface area (m²) 2 / g) is calculated based on the BET theory from the gas adsorption isotherm using nitrogen.

[0078] The resistance value (mΩ) was measured by clamping the current collector with electrodes of 20mm diameter on both sides along its thickness direction. The volume resistivity was calculated by multiplying the resistance by the electrode area and dividing by the film thickness. Furthermore, for the thickness T of the conductive resin layer substrate, an EDX-enabled SEM was used to observe the cross-section cut along the thickness direction of the conductive resin layer. Ten locations where Ni powder was not observed were measured, and the average thickness was taken. The measurement results are shown in Table 1. Additionally, the appearance of various current collectors was observed, and the frequency of pinhole occurrence was evaluated (A: Excellent, B: Good, C: Average, D: Poor). The results are shown in Table 1 (NT: Not tested).

[0079] Table 1

[0080]

[0081] Based on samples 1-5 and comparative samples 1-5 in Table 1, the following can be observed: When the filler morphology is either spike-like or structural, and the BET specific surface area / spherical equivalent surface area is 5.0 or higher, the volume resistivity in the thickness direction is excellent, which helps to reduce the pinhole occurrence frequency. Furthermore, based on samples 1-5, the following can be observed: When the filler morphology is either spike-like or structural, and the BET specific surface area / spherical equivalent surface area is 6.0 or higher, it further contributes to low resistivity in the thickness direction and a low pinhole occurrence frequency. It is evident that a spike-like morphology contributes to an even lower pinhole occurrence frequency.

[0082] If the ratio of BET specific surface area to spherical equivalent surface area exceeds 10, voids will generally form between the protrusions of the filler and / or particle breakage will occur, resulting in undesirable adverse conditions. Therefore, it is believed that if the ratio of BET specific surface area to spherical equivalent surface area is below 10, low resistivity and low pinhole occurrence frequency can be maintained.

[0083] Furthermore, according to Comparative Example 1, if the filler content is 3.0% by mass, low resistivity in the thickness direction cannot be maintained. According to Comparative Example 2, if the filler content is 20% by mass, although low resistivity in the thickness direction can be maintained, the pinhole occurrence frequency increases. It is known that the filler contents of 5.0% by mass, 10% by mass, and 15% by mass in Examples 1-3, respectively, contribute to low resistivity and low pinhole occurrence frequency in the thickness direction. From the above, it can be seen that when the filler content is more than 3.0% by mass and less than 20% by mass, more preferably 5% by mass and less than 15% by mass, it contributes to low resistivity and low pinhole occurrence frequency in the thickness direction.

[0084] In addition, it can be known that D 50 A ratio of 1.0 to 2.0 relative to the thickness T of the substrate contributes to low resistivity in the thickness direction.

[0085] Example 2

[0086] In this embodiment, a coin-shaped unit with a unipolar structure was fabricated using the current collectors modulated in Example 1 (samples 1-5, comparative sample 1, 3-5), and the IV resistance was measured. The ratio of the IV resistance to the bonding foil (a conventional bipolar current collector) used as a control example is shown in Table 1.

[0087] Furthermore, Figure 5A and Figure 5B The structure of the evaluation unit for the sample and control examples is shown. For the current collector with Al foil prepared in Example 1, a carbon coating, a negative electrode active material layer, and a positive electrode active material layer are formed sequentially and integrated with the separator layer to prepare the evaluation unit. The preparation and formation of each layer are shown below.

[0088] carbon coating

[0089] Acetylene black and PVDF (mass ratio 90:10) were mixed with N-methylpyrrolidone (NMP) to form a coating solution with a solid content of 30%. The solution was then applied to the conductive resin layer side of the current collector using a coater at a concentration of 1 mg / cm². 2 Apply the coating at a unit area weight and allow it to dry.

[0090] Negative active material layer

[0091] Amorphous coated graphite (as the active material), carboxymethyl cellulose (CMC) (as a thickener), and styrene-butadiene rubber (SBR) (as a binder) were mixed at a mass ratio of 97:0.7:2.3 to prepare a negative electrode slurry. This slurry was then applied to a carbon coating using a coater, resulting in a weight per unit area of ​​22.6 mg / cm³. 2 The material is dried and rolled to form a predetermined negative electrode density (1.2 g / cm³). 2 The negative electrode layer of ).

[0092] Positive active material layer

[0093] Lithium nickel cobalt manganese oxide (NCM) as the active material, acetylene black as the conductive additive, and PVDF, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) as binders were mixed at a mass ratio of 95:2.5:2.5 to prepare a positive electrode paste. An Al foil (12 μm thick) was then coated onto a carbon coating using a coater, resulting in a weight per unit area of ​​38 mg / cm³. 2 The material is dried and rolled to form a predetermined positive electrode density (3.0 g / cm³). 2 The positive electrode layer.

[0094] Furthermore, in the comparative example, the Cu foil (10 μm) was integrated onto the Al foil (40 μm thick) via an adhesive layer (3 μm) to form the aforementioned carbon coating on the surface of the Cu foil, and the process was modulated in the same manner as described above.

[0095] As shown in Table 1, in the units using the current collectors of samples 1-5, the IV resistivity ratio was the same as that when using the laminated foil. On the other hand, in the units using the current collectors of comparative examples 1, 3-5, the IV resistivity ratio deteriorated significantly. This is believed to be due to the increase in resistance in the thickness direction and / or the increase in the frequency of pinhole occurrence.

[0096] Conductive fillers with spike-like or structural morphologies and a BET specific surface area / sphere equivalent surface area of ​​5.0 or more and less than 10 are used in amounts exceeding 3.0% by mass and less than 20% by mass. This enables the achievement of low resistivity and low pinhole occurrence frequency in the thickness direction, resulting in a current collector with excellent performance.

Claims

1. A conductive film comprising: Conductive fillers with spike-like or structural forms, and Resin adhesives The BET specific surface area of ​​the conductive filler relative to the D based on the conductive filler 50 The ratio of the particle size to the converted surface area of ​​the spheres is greater than 5.0 and less than 10. The conductive filler contains more than 3.0% by mass and less than 20% by mass relative to the total mass of the resin adhesive and the conductive filler.

2. The conductive film according to claim 1, The conductive filler has the spike-like morphology.

3. The conductive film according to claim 2, The BET specific surface area of ​​the conductive filler relative to the D based on the conductive filler 50 The ratio of the particle size to the converted surface area of ​​the spheres is greater than 6.

0.

4. The conductive film according to claim 3, The conductive filler contains 5% to 15% by mass relative to the total mass of the resin adhesive and the conductive filler.

5. A current collector for a bipolar secondary battery, comprising: Conductive fillers with spike-like or structural forms, and Resin adhesives The BET specific surface area of ​​the conductive filler relative to the D based on the conductive filler 50 The ratio of the particle size to the converted surface area of ​​the spheres is greater than 5.0 and less than 10. The conductive filler contains more than 3.0% by mass and less than 20% by mass relative to the total mass of the resin adhesive and the conductive filler.

6. The current collector of the bipolar secondary battery according to claim 5, A metal foil is laminated on the conductive resin layer.

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