Current collector for secondary battery and secondary battery

Abstract

According to the present disclosure, there is provided a current collector for a secondary battery, which is easy to block current inside the battery when an internal short circuit occurs, and which has excellent capacity retention rate and resistance reduction rate. The current collector disclosed herein has a laminated structure in which a resin layer and metal layers formed on both surfaces of the resin layer are laminated. The surface of the metal layer has a rough surface portion provided with a plurality of protrusions and a plurality of recesses. The rough surface portion is formed with a resin coating layer, and at least some of the protrusions among the plurality of protrusions have an exposed portion that protrudes from the resin coating layer.

Description

Technical Field

[0001] The present invention relates to a current collector used in a secondary battery and a secondary battery including the current collector. Background Art

[0002] In recent years, secondary batteries such as lithium-ion secondary batteries have been favorably used as portable power supplies for personal computers and mobile terminals, and as power supplies for driving vehicles such as battery-electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] Electrodes commonly used in secondary batteries include a current collector (e.g., a metal foil) and a composite material layer formed on the surface of the current collector. The composite material layer contains an active material that can occlude and release charge carriers (e.g., lithium ions), and the volume change of the active material occurs as a result of the occlusion and release of the charge carriers. Since the composite material layer is easily peeled off from the current collector due to repeated volume changes of the active material, it is expected that the load caused by the above-mentioned volume change will be alleviated. For example, Patent Document 1 discloses a laminated metal foil for a lithium ion secondary battery stacked with a plurality of metal foils as a current collector. It is described that the metal foil has a roughened surface, and the roughened surfaces of the metal foils are stacked in a manner that the roughened surfaces are in contact with each other so that there are gaps between the roughened surfaces. Thus, the load on the metal foil caused by the volume change of the active material can be alleviated.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2007-26913 Summary of the Invention

[0007] However, when foreign matter is mixed into the electrode of the secondary battery, an internal short circuit may occur. When an internal short circuit occurs, the temperature of the secondary battery will rise due to the short-circuit current. Therefore, it is desired that the secondary battery has a function of more quickly blocking the current inside the battery in order to suppress the temperature rise when the internal short circuit occurs. In addition, from the perspective of improving the battery performance such as the improvement of the capacity retention rate of the secondary battery and the reduction of the resistance, it is preferred that the composite material layer is not easily peeled off from the collector.

[0008] The present invention has been developed in view of the above circumstances and aims to provide a secondary battery current collector that easily blocks current flowing through the battery during an internal short circuit and exhibits excellent capacity retention and resistance reduction. Another object is to provide a secondary battery comprising the current collector.

[0009] To achieve the above object, the current collector disclosed herein is characterized by being a current collector used in an electrode of a secondary battery, having a laminated structure in which a resin layer and metal layers formed on both surfaces of the resin layer are laminated. The surface of the metal layer has a rough surface portion provided with a plurality of protrusions and a plurality of recesses, and the rough surface portion is formed with a resin coating layer. Also, at least some of the protrusions have an exposed portion that protrudes from the resin coating layer.

[0010] According to the above configuration, when an internal short circuit occurs, the resin layer and the resin coating layer are melted by heat generated by the short circuit current, and the volume of the resin layer and the resin coating layer increases. Thus, pressure is applied to the metal layer, and the metal layer (e.g., a thin recess) is broken. As a result, the resistance sharply increases, and the current inside the battery is blocked. In addition, according to the research by the present inventors and others, it was confirmed that the presence of the resin coating layer on the rough surface portion of the metal layer makes the composite layer difficult to peel off, and the capacity retention rate is improved and the resistance is reduced. It is presumed that this is due to the effects of the higher adhesion of the composite layer to the resin coating layer than to the metal layer, the increase in the contact area of the composite layer to the current collector by the protrusions protruding from the resin coating layer, and the like. In addition, particularly at low temperatures, the resin coating layer shrinks, and the composite layer is pulled toward the current collector, so the resistance at low temperatures (hereinafter, also referred to as "low-temperature resistance") can be reduced.

[0011] In addition, in a preferred embodiment of the current collector disclosed herein, the proportion of the protrusions having the exposed portion is 20% or more of the total number of the plurality of protrusions. Thus, the peeling of the composite layer and the reduction of the resistance generated between the composite layer and the metal layer can be suppressed.

[0012] In addition, in a preferred embodiment of the current collector disclosed herein, the average of the proportion of the exposed portion in the height direction of the protrusions having the exposed portion is 5% to 99%. In addition, in a more preferred embodiment, the average of the proportion of the exposed portion is 95% or less. Thus, the balance between the increase in the contact area of the exposed portion of the protrusions to the composite layer and the contribution of the resin coating layer to the prevention of the peeling of the composite layer becomes good, and more excellent capacity retention rate and resistance reduction can be achieved.

[0013] In addition, in a preferred embodiment of the current collector disclosed herein, when n (n is a natural number of at least 4 or more than 4) SEM images of the cross section of the current collector along the stacking direction of the resin layer and the metal layer are taken in different fields of view, the average value X (μm) of the thickness x of the thinnest portion of the metal layer observed in each SEM image and the average value Y (μm) of the thickness y of the thickest portion satisfy 0.8 μm ≤ X ≤ 3 μm and 2 μm ≤ Y ≤ 11 μm. Thus, the capacity retention rate is improved and the resistance (e.g., low-temperature resistance) is reduced at a particularly high level.

[0014] In addition, in a preferred embodiment of the current collector disclosed herein, the resin coating layer contains conductive particles. Thus, the resistance (e.g., low-temperature resistance) between the composite material layer and the current collector can be further reduced.

[0015] In addition, in a preferred embodiment of the current collector disclosed herein, the resin layer contains conductive particles. In addition, in a preferred embodiment, the resin layer contains inorganic fillers. Thus, the capacity retention rate and the low-temperature resistance are further improved.

[0016] In addition, as another aspect for achieving the above object, a secondary battery having a positive electrode and a negative electrode is provided. The secondary battery has the current collector disclosed herein in at least either of the positive electrode and the negative electrode. Thus, a secondary battery that easily blocks the current inside the battery at the time of internal short circuit and has excellent capacity retention rate and reduction rate of resistance is achieved. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a cross-sectional view schematically showing the constitution of a lithium ion secondary battery of one embodiment.

[0018] Figure 2 is an enlarged cross-sectional view schematically showing the constitution of a negative electrode current collector of one embodiment.

[0019] Figure 3 is an enlarged cross-sectional view schematically showing the constitution of a negative electrode current collector of one modification.

[0020] SYMBOL DESCRIPTION

[0021] 10 exterior body

[0022] 20 electrode body

[0023] 42 positive electrode current collector terminal

[0024] 44 negative electrode current collector terminal

[0025] 50 positive electrode

[0026] 52 positive electrode current collector

[0027] 54 positive electrode composite material layer

[0028] 56 Positive terminal junction

[0029] 60 negative electrode

[0030] 62 negative electrode collector

[0031] 62a Resin layer

[0032] 62b Metal layer

[0033] 62b1 convex part

[0034] 62b2 recess

[0035] 62b3 exposed part

[0036] 62c resin coating

[0037] 64 negative electrode composite material layer

[0038] 66 Negative terminal joint

[0039] 70 Isolators

[0040] 100 lithium-ion secondary batteries DETAILED DESCRIPTION

[0041] Hereinafter, the technology disclosed herein will be described in detail with reference to the accompanying drawings, taking as an example a current collector appropriately adopted in a lithium-ion secondary battery, which is a typical example of a secondary battery. It should be noted that matters other than those specifically mentioned in this specification and matters required for implementation can be grasped as design matters for those skilled in the art based on the existing technology in this field. The technology disclosed herein can be implemented based on the contents disclosed in this specification and the technical common sense in this field. In addition, in the following drawings, the same symbols are marked for components and parts that play the same role, and repeated descriptions are sometimes omitted or simplified. In addition, the dimensional relationships (length, width, thickness, etc.) in each figure do not reflect the actual dimensional relationships. In addition, the symbol W in each figure represents the "width direction" and the symbol T represents the "thickness direction". It should be noted that these directions are directions specified for the convenience of explanation and are not intended to limit the arrangement of the secondary battery disclosed herein.

[0042] In this specification, when a numerical range is described as A to B (where A and B are arbitrary numerical values), it means that it is greater than A and less than B, and includes a range greater than A and a range less than B.

[0043] Note that the "secondary battery" in this specification is a term for all chargeable and dischargeable power storage devices, and is a concept including so-called secondary batteries (chemical batteries) such as lithium-ion secondary batteries, sodium-ion secondary batteries, nickel-hydrogen batteries, and the like, and electric double layer capacitors (physical batteries) such as electric double layer capacitors. The "lithium-ion secondary battery" in this specification is all batteries that use lithium ions as charge carriers and can be repeatedly charged and discharged with movement of the charge carriers between the positive electrode and the negative electrode. The electrolyte in the lithium-ion secondary battery can be, for example, a nonaqueous electrolyte, a gel-like electrolyte, a solid electrolyte, or the like.

[0044] Figure 1 is a cross-sectional view schematically showing the constitution of a lithium-ion secondary battery 100 of one embodiment. As shown in Figure 1 the lithium-ion secondary battery 100 houses an electrode body 20 and a nonaqueous electrolyte (not shown) in the inside of an exterior body 10. One end of a positive electrode current collector 42 is electrically connected to a positive electrode 50 in the inside of the exterior body 10, and one end of a negative electrode current collector 44 is electrically connected to a negative electrode 60 in the inside of the exterior body 10. In addition, the other ends of the positive electrode current collector 42 and the negative electrode current collector 44 are exposed to the outside of the exterior body 10.

[0045] The exterior body 10 is formed in a bag shape by a laminate film. The exterior body 10 has a housing space in which the electrode body 20 and the nonaqueous electrolyte are housed, and can be sealed by heat sealing (heat sealing) the periphery of the housing space.

[0046] The material constituting the laminate film is not particularly limited, and typically, a laminate film constituted by bonding a foil-like metal and a resin sheet can be used. For example, in order to impart heat resistance, sealing strength, impact resistance, and the like, a laminate film of a three-layer structure in which a metal layer of aluminum or the like is provided on the surface of a non-stretched polypropylene film (CPP) for heat sealing, and further an outer resin layer constituted by a polyethylene terephthalate (PET) film, a polyamide (PA) film, or a nylon film is provided on the surface of the metal layer can be used.

[0047] The electrode body 20 is constituted by stacking a plurality of electrode sheets of the positive electrode 50 and the negative electrode 60 in a state of being insulated by the separator 70. Here, the electrode sheets of the positive electrode 50 and the negative electrode 60 have a rectangular wide surface, and are stacked in a manner that the wide surfaces face each other. Note that here, the stacking direction of the electrode body 20 is the thickness direction T.

[0048] The electrode sheet of the positive electrode 50 (positive electrode sheet) has a sheet-like positive electrode current collector 52 having a rectangular wide surface, and a positive electrode composite material layer 54 coated on the surface of the positive electrode current collector 52. The positive electrode current collector 52 has a width direction (width direction of the positive electrode sheet) parallel to the wide surface of the positive electrode sheet and a length direction (length direction of the positive electrode sheet) perpendicular to the width direction of the positive electrode sheet. Figure 1The edge portion of one side in the W direction is provided with a positive electrode current collector exposed portion where the positive electrode composite material layer 54 is not formed. The positive electrode current collector exposed portions of the stacked positive electrode sheets are joined in a bundle shape from the stacking direction (T direction) to form a positive electrode terminal joining portion 56. Figure 1

[0049] The electrode sheet (negative electrode sheet) of the negative electrode 60 is provided with a sheet-shaped negative electrode current collector 62 having a rectangular wide surface, and a negative electrode composite material layer 64 coated on the surface of the negative electrode current collector 62. The edge portion of one side in the W direction is provided with a negative electrode current collector exposed portion where the negative electrode composite material layer 64 is not formed. The negative electrode current collector exposed portions of the stacked negative electrode sheets are joined in a bundle shape from the stacking direction (T direction) to form a negative electrode terminal joining portion 66. Figure 1 Figure 1

[0050] The positive electrode current terminal 42 is a plate-shaped conductive member, one end of which is joined to the positive electrode terminal joining portion 56 inside the exterior body 10, and the other end of which is exposed outside the exterior body 10. Two laminated films are overlapped in a manner that sandwiches the positive electrode current terminal 42 from the thickness direction T at the portion of the positive electrode current terminal 42 that penetrates the exterior body 10, and the laminated films are fused to the surface of the positive electrode current terminal 42. Note that, in order to improve the strength of the above fusion, for example, a fusion film composed of a resin can be interposed between the positive electrode current terminal 42 and the laminated film.

[0051] The negative electrode current terminal 44 is joined to the negative electrode terminal joining portion 66 at one end, and can have the same configuration as the above-described positive electrode current terminal 42.

[0052] As the positive electrode current collector 52 provided to the positive electrode 50, for example, an aluminum foil can be used. The positive electrode composite material layer 54 provided to the positive electrode 50 contains a positive electrode active material. As the positive electrode active material, for example, a lithium complex metal oxide (for example, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNiO2, LiCoO2, LiFeO2, LiMn2O4, LiNi 0.5 Mn 1.5 O4, LiCrMnO4, LiFePO4, etc.) can be cited. In addition, the positive electrode composite material layer 54 can contain a conductive aid, a dispersant, a binder, etc. As the conductive aid, for example, a carbon black such as acetylene black (AB), other carbon materials (graphite, etc.) can be appropriately used. As the dispersant, for example, polyvinyl alcohol (PVA), etc. can be used. As the binder, for example, polyvinylidene fluoride (PVDF), etc. can be used.

[0053] ​​​The positive electrode composite layer 54 can be formed by dispersing a positive electrode active material and a material used as necessary (a conductive aid, a binder, etc.) in a suitable solvent (for example, N-methyl-2-pyrrolidone: NMP), preparing a paste-like (or slurry-like) composition, and coating a suitable amount of the composition on the surface of the positive electrode current collector 52, and drying.

[0054] Figure 2 is a schematic cross-sectional enlarged view showing the configuration of the negative electrode current collector 62. The detailed configuration of the negative electrode current collector 62 will be described later.

[0055] The negative electrode composite layer 64 is formed on both surfaces of the negative electrode current collector 62 (see FIG. 1). Figure 1 The negative electrode composite layer 64 contains a negative electrode active material, and for example, a carbon material such as graphite, hard carbon, soft carbon, a silicon material such as Si, SiO, or the like can be used. In addition, the negative electrode composite layer 64 can further contain a binder, a thickening agent, or the like. As the binder, for example, styrene-butadiene rubber (SBR) or the like can be used. As the thickening agent, for example, carboxymethyl cellulose (CMC) or the like can be used.

[0056] The negative electrode composite layer 64 can be formed by dispersing a negative electrode active material and a material used as necessary (a binder, etc.) in a suitable solvent (for example, ion exchange water), preparing a paste-like (or slurry-like) composition, and coating a suitable amount of the composition on the surface of the negative electrode current collector 62, and drying.

[0057] As the separator 70, various microporous sheets similar to those used in the lithium ion secondary battery in the past can be used, and for example, a microporous resin sheet composed of a resin such as polyethylene (PE), polypropylene (PP), or the like can be mentioned. The microporous resin sheet can be a single layer structure, or a multilayer structure of two or more layers (for example, a three-layer structure in which a PP layer is laminated on both surfaces of a PE layer). In addition, the surface of the separator 70 can also have a heat-resistant layer (HRL).

[0058] The non-aqueous electrolyte can be the same as that used in conventional lithium-ion secondary batteries. For example, a non-aqueous electrolyte containing an auxiliary salt in a non-aqueous solvent can be used. As the non-aqueous solvent, aprotic solvents such as carbonates, esters, and ethers can be used. Among them, carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) can be appropriately used. Alternatively, fluorinated solvents such as fluorinated carbonates such as monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC), and dimethyl trifluorocarbonate (TFDMC) can be preferably used. Such non-aqueous solvents can be used alone or in appropriate combinations of two or more. As the auxiliary salt, for example, lithium salts such as LiPF6, LiBF4, and LiClO4 can be appropriately used. The concentration of the auxiliary salt is not particularly limited, but is preferably about 0.7 mol / L to 1.3 mol / L.

[0059] In addition, the non-aqueous electrolyte may contain ingredients other than the above-mentioned non-aqueous solvent and auxiliary salt, as long as the effects of the present invention are not significantly impaired. For example, it may contain various additives such as gas generators, film formers, dispersants, and thickeners. As the above-mentioned additives, specifically, there can be mentioned film formers such as lithium bis(oxalatoborate) (LiBOB), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), and fluoroethylene carbonate (FEC); overcharge additives composed of compounds that can generate gas during overcharging, represented by aromatic compounds such as biphenyl (BP) and cyclohexylbenzene (CHB); surfactants; dispersants; thickeners; antifreeze agents, etc. The concentration of these additives relative to the entire non-aqueous electrolyte varies depending on the type of additive, and for example, can be 6% by mass or less (typically 0.5% by mass to 4% by mass).

[0060] The structure of the negative electrode current collector 62 is described in detail below. Figure 2 As shown, the negative electrode current collector 62 has a laminated structure of a resin layer 62a, a metal layer 62b, and a resin coating layer 62c. Generally speaking, the metal layer 62b is formed on both sides of the resin layer 62a, and the resin coating layer 62c is formed on the surface of the metal layer 62b.

[0061] Resin layer 62a can be any resin whose volume increases due to a temperature rise during an internal short circuit, for example, a thermoplastic resin. When a thermoplastic resin reaches its melting point, it changes state from solid to liquid, increasing its volume. The increase in volume of resin layer 62a applies pressure to metal layer 62b, causing it to rupture. The rupture of metal layer 62b causes a sharp increase in resistance, thereby blocking the current flowing within the battery. This suppresses the temperature rise of the battery during an internal short circuit.

[0062] The thermoplastic resin constituting the resin layer 62a is not particularly limited, and a thermoplastic resin having a melting point of 265°C or lower under 1 atm is preferably used. As the thermoplastic resin having the above-mentioned property, for example, polyethylene terephthalate (PET), polyamide (PA), polyvinyl alcohol (PVA), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), thermoplastic polyester, acrylonitrile-butadiene-styrene (ABS), acrylonitrile-styrene (AS), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF), polyvinylidene chloride (PVDC), or the like can be given. Further, a thermoplastic resin having a melting point of 200°C or lower under 1 atm is more preferably used, and, for example, PVA, PE, PP, PVDF, or the like is preferably used. By the resin layer 62a having a lower melting point, the current inside the battery can be blocked at a lower temperature when an internal short circuit occurs in the lithium ion secondary battery 100. On the other hand, in the case where the melting point of the resin constituting the resin layer 62a is too low, there is a possibility that the resin layer 62a also melts at the time of ordinary use of the lithium ion secondary battery 100. Therefore, the melting point of the resin constituting the resin layer 62a under 1 atm is preferably 90°C or higher, and, for example, can be 100°C or higher. Note that the thermoplastic resin can be used singly or in combination of two or more. Note that the melting point of the resin layer can be measured by general differential scanning calorimetry (DSC).

[0063] The resin layer 62a can contain electrically conductive particles. By the resin layer 62a containing electrically conductive particles, the electric conductivity of the negative electrode current collector 62 is improved, and thus the resistance is further reduced. As the electrically conductive particles, for example, carbon powder, electrically conductive metal powder, or the like can be used. As the carbon powder, various carbon blacks (for example, acetylene black, furnace black, ketjen black), graphite powder, or the like can be used. As the electrically conductive metal powder, for example, copper powder, aluminum powder, nickel powder, or the like can be used, and, of these, an electrically conductive metal powder composed of the same metal as that constituting the metal layer 62b is preferably used. For example, in the case where the metal layer 62b is a copper foil, copper powder is preferably used as the electrically conductive particles. Note that the electrically conductive particles can be used singly or in combination of two or more.

[0064] The size of the electrically conductive particles is not particularly limited, and, for example, the average particle diameter is about 10 nm to 10 μm, and is preferably 20 nm to 5 μm. Note that the "average particle diameter" in this specification refers to the cumulative 50% particle diameter (D50) in the particle size distribution on a volume basis obtained by a laser diffraction scattering method. 50

[0065] ​The resin layer 62a can contain an inorganic filler. As the inorganic filler, for example, an inorganic oxide having insulating properties, glass can be used. By the resin layer 62a containing an inorganic filler having insulating properties, the electric resistance is reduced. It is presumed that this is because the unevenness of the electron density that can occur in the metal layer 62b is improved by the inorganic filler. As the inorganic oxide, for example, alumina (AI2O3), magnesia (MgO), silica (SiO2), titania (TiO2), or the like can be used. The inorganic filler can be used singly or in combination of two or more. Note that the shape of the inorganic filler is not particularly limited, and for example, can be in the form of particles, fibers, plates, flakes, or the like.

[0066] The average particle diameter of the inorganic filler is not particularly limited, and for example, is 0.1 μm to 10 μm, or 0.5 μm to 5 μm based on the average particle diameter by laser diffraction scattering method.

[0067] The average thickness of the resin layer 62a is not particularly limited, and for example, can be 10 μm or more if it is 0.1 μm or more. If the average thickness is as described above, when the resin layer 62a is melted, the volume changes with the pressure that causes the metal layer 62b to break, and thus is preferable. Note that the average thickness of the resin layer 62a is not particularly limited, and typically is 80 μm or less, for example, can be 40 μm or less. From the viewpoint of safety at the time of internal short circuit, the greater the thickness of the resin layer 62a, the greater the volume change at the time of melting, and thus is preferable, but from the viewpoint of high capacity of the battery and light weight of the battery, it is not preferable to have a thickness that is greater than necessary. Note that the thickness of the resin layer 62a can be measured, for example, by observation with a scanning electron microscope (SEM) or the like.

[0068] The metal layer 62b is a layer having conductivity. The metal layer 62b can be composed of a metal foil used in the negative electrode current collector of a conventional lithium ion secondary battery, and for example, a metal foil of copper, an alloy in which copper is the main component, titanium, nickel, or the like can be used. Note that here, the alloy in which copper is the main component means an alloy in which 50 wt% or more of the constituent components is copper.

[0069] Figure 2 In the negative electrode current collector 62 illustrated, the surface of the metal layer 62b (the surface of the side opposite to the side on which the resin layer 62a is provided) has a rough surface portion in which a plurality of protrusions 62b1 and a plurality of recesses 62b2 are provided, and the surface of the opposite side is flat. The rough surface portion is provided with a resin coating layer 62c, and the resin is accumulated in the recesses 62b2. At least a part of the plurality of protrusions 62b1 has a protrusion 62b1 in which a protruding portion 62b3 is exposed from the resin coating layer 62c.

[0070] By having the rough surface portion in the metal layer 62b, it is possible to provide a portion in the metal layer 62b that is relatively thin in thickness, and thus the metal layer 62b is easily broken at the portion that is thin in thickness when internal short-circuiting occurs. In addition, by having the exposed portion 62b3, it is possible to increase the contact area between the negative electrode composite material layer 64 and the negative electrode current collector 62 formed on the surface of the negative electrode current collector 62, and thus the negative electrode composite material layer 64 becomes less likely to peel off. Furthermore, the metal layer 62b can be brought into direct contact with the negative electrode composite material layer 64 by the exposed portion 62b3, and thus it is possible to ensure a good conductive path and reduce the resistance.

[0071] The arithmetic average roughness Ra of the rough surface portion of the metal layer 62b is not particularly limited, and is, for example, 0.5 μm to 15 μm, and can be 0.5 μm to 5 μm. The arithmetic average roughness Ra can be measured, for example, based on JIS B0601:2001, using a stylus-type surface roughness measuring instrument.

[0072] The average value X (μm) of the thickness x (μm) of the thinnest portion of the metal layer 62b is not particularly limited, and is, for example, 0.1 μm or more, preferably 0.5 μm or more, and more preferably 0.8 μm or more, from the viewpoint of ensuring the strength of the metal layer 62b. In addition, the thinnest portion of the metal layer 62b becomes the starting point of breakage when internal short-circuiting occurs, and thus is, for example, 5 μm or less, preferably 4 μm or less, and more preferably 3 μm or less.

[0073] The average value Y (μm) of the thickness y (μm) of the thickest portion of the metal layer 62b is not particularly limited, and is, for example, 1 μm or more, and can be 2 μm or more, or 4 μm or more. In addition, the average value Y is, for example, 12 μm or less, and can be 11 μm or less.

[0074] According to the research by the present inventors and others, when the average value X (μm) of the thickness x (μm) of the thinnest portion of the metal layer 62b is 0.8 μm to 3 μm (0.8 μm ≤ X ≤ 3 μm), the average value Y (μm) of the thickness y (μm) of the thickest portion of the metal layer 62b is preferably 2 μm to 11 μm (2 μm ≤ Y ≤ 11 μm), and X < Y. Thus, the negative electrode composite material layer 64 becomes less likely to peel off, and the capacity retention rate and the reduction rate of the resistance become more favorable.

[0075] The thickness x (μm) of the thinnest portion and the thickness y (μm) of the thickest portion of the metal layer 62b can be measured based on the direction along which the resin layer 62a and the metal layer 62b are stacked (the direction of the arrow A in FIG. 1). Figure 2The thickness x (μm) of the thinnest portion and the thickness y (μm) of the thickest portion of the metal layer 62b are measured from the cross-sectional SEM image of the metal layer 62b (in the T direction). The above cross-sectional SEM image is taken n times (n is a natural number of 4 or more, for example, 10 or more) at a prescribed magnification (for example, magnification: 4000 times) so as to become different fields of view. Then, the thickness x (μm) of the thinnest portion and the thickness y (μm) of the thickest portion of the metal layer 62b can be measured in each SEM image and the average value X (μm) of the above x and the average value Y (μm) of the above y can be calculated. Note that, as shown in FIG. 8, the above thickness x of the thinnest portion is typically present at the position of the recessed portion, and the above thickness y of the thickest portion is typically present at the position of the protruding portion. Note that, in each field of view, in a case where the thickness y of the thickest portion of the metal foil does not fall within the field of view, the magnification can be appropriately adjusted so as to measure the above thickness y. Figure 2

[0076] In addition, in the metal layer 62b, the distance between adjacent protruding portions 62bl is not particularly limited, and is preferably 1 μm or less. Thus, the number of protruding portions 62bl increases, and the surface area of the metal layer 62b increases. As a result, the contact area between the metal layer 62b and the negative electrode composite material layer 64 increases, and the negative electrode composite material layer 64 becomes less likely to peel, and thus the capacity retention rate improves. Note that, the "distance between adjacent protruding portions 62bl" refers to the distance between the respective highest portions of adjacent protruding portions 62bl in the cross-sectional SEM image in the stacking direction (thickness direction T) of the resin layer 62a and the metal layer 62b.

[0077] The roughened portion of the metal layer 62b can be formed by a publicly known roughening treatment method, and for example, laser irradiation, etching, sputtering, ion plating, PLD (Pulsed Laser Deposition), or the like can be used. Of these, etching treatment using a chemical solution is preferable. If etching treatment is used, the distance between the formed protruding portions 62bl can be made shorter, and for example, can be 1 μm or less.

[0078] ​The resin coating layer 62c is formed on the rough surface of the metal layer 62b. More specifically, the resin coating layer 62c is formed mainly in the recessed portions 62b2 so as to cover at least a part of the protruding portions 62b1. In addition, among the plurality of protruding portions 62b1, there can be protruding portions whose entire portions are covered with the resin coating layer 62c (i.e., the protruding portions 62b1 that do not have the exposed portions 62b3). The resin coating layer 62c has a function as an adhesive that makes the negative electrode composite material layer 64 more firmly adhere to the negative electrode current collector 62, and can more effectively prevent peeling of the negative electrode composite material layer 64. In particular, at low temperatures, the resin coating layer 62c shrinks, and can make the negative electrode composite material layer 64 further adhere to the negative electrode current collector 62, and thus can further reduce the resistance. In addition, the resin coating layer 62c melts due to temperature rise at the time of internal short circuit, and thus can apply pressure to the recessed portions 62b2 of the metal layer 62b. Thus, at the time of internal short circuit, the metal layer 62b is pressed from both the resin layer 62a and the resin coating layer 62c, and thus can more reliably break.

[0079] The kind of resin constituting the resin coating layer 62c can be the same as the resin exemplified as the resin that can constitute the above-described resin layer 62a. Among them, as a preferable example, there can be exemplified a resin that has been conventionally used as an adhesive for a composite material layer, and for example, PVDF can be appropriately used. Thus, peeling of the negative electrode composite material layer 64 can be more appropriately prevented.

[0080] The resin coating layer 62c and the resin layer 62a can be constituted of the same resin, or can be constituted of different resins, and it is preferable that the difference between the melting points of the resin constituting the resin layer 62a and the resin constituting the resin coating layer 62c be small. For example, the difference between the melting points can be 100°C or less, and more preferably 50°C or less. Thus, the timing at which the resin layer 62a and the resin coating layer 62c melt at the time of internal short circuit is close, and thus it becomes easy to break the metal layer 62b.

[0081] From the viewpoint of improving the conductivity between the negative electrode composite material layer 64 and the metal layer 62b, it is preferable that the resin coating layer 62c contain conductive particles. As the conductive particles, the conductive particles that can be used in the above-described resin layer 62a can be used.

[0082] From the viewpoint of increasing the contact area between the metal layer 62b and the negative electrode composite material layer 64, it is preferable that the proportion of the protruding portions 62b1 having the exposed portions 62b3 be high. For example, the proportion of the protruding portions 62b1 having the exposed portions 62b3 with respect to the total number of the plurality of protruding portions 62b1 is preferably 20% or more in terms of number, more preferably 30% or more in terms of number, further preferably 50% or more in terms of number, particularly preferably 80% or more in terms of number, and for example, can be 100% in terms of number.

[0083] The convex portion 62b1 having the exposed portion 62b3 has the exposed portion 62b3 and the portion covered by the resin coating layer 62c. However, if the proportion of the exposed portion 62b3 is too low, the contact area between the metal layer 62b and the negative electrode composite material layer 64 becomes too small. As a result, the negative electrode composite material layer 64 is easily peeled off, and the resistance increases. Therefore, in the height direction ( Figure 2 In the thickness direction T), the average value of the ratio of the height T3 of the exposed portion 62b3 (i.e., the difference between the height T1 of the convex portion 62b1 and the thickness T2 of the resin coating layer 62c) to the height T1 of the convex portion 62b1 can be, for example, 5% or more, preferably 10% or more, for example, 30% or more. On the other hand, when the ratio of the exposed portion 62b3 is too high, the thickness of the resin coating layer 62c is insufficient, and the effect of the resin coating layer 62c is insufficient. Therefore, the average value of the ratio of the height T3 of the exposed portion 62b3 to the height T1 of the convex portion 62b1 can be, for example, 99% or less, more preferably 95% or less, or 90% or less. It should be noted that the ratio of the exposed portion 62b3 can be measured based on a cross-sectional SEM image along the stacking direction of the resin layer 62a and the metal layer 62b. At this time, m (m is a natural number of 5 or more) convex portions 62b1 having exposed portions 62b3 are randomly selected, and the average value of the proportions of the exposed portions 62b3 in the selected convex portions 62b1 is calculated.

[0084] The height T1 of the convex portion 62b1, the thickness T2 of the resin coating layer 62c, and the height T3 of the exposed portion 62b3 can be measured similarly to the method for measuring the thickness of the metal layer 62b described above, and can be measured based on a cross-sectional SEM image taken along the stacking direction of the resin layer 62a and the metal layer 62b. It should be noted that the height T1 of the convex portion 62b1 can be determined by finding the intersection of a line connecting the lowest points (lowest height portions) of the concave portions 62b2 located to the left and right of the convex portion 62b1 in the cross-sectional SEM image and a line drawn from the apex (highest point) of the convex portion 62b1 toward the stacking direction, and then measuring the distance between the apex of the convex portion 62b1 and this intersection.

[0085] In the negative electrode current collector 62, the roughened portion of the metal layer 62b and the resin coating layer 62c are preferably provided over the entire area where the negative electrode composite material layer 64 is formed. This makes it difficult for the entire negative electrode composite material layer 64 to be peeled off from the negative electrode current collector 62. Furthermore, even if an internal short circuit occurs at any location in the negative electrode composite material layer 64, the metal layer 62b will rupture rapidly, thereby enhancing safety.

[0086] The resin layer 62a and the resin coating layer 62c are preferably not formed in the negative electrode terminal joint 66. Thus, the negative electrode terminal joint 66 can directly join the metal layers 62b of the plurality of negative electrode sheets, thereby achieving a negative electrode terminal joint 66 with excellent conductivity and joining strength.

[0087] The above describes the structure of the current collector (negative electrode current collector 62) disclosed herein, but the structure of the current collector disclosed herein is not limited thereto. Figure 3 Provide explanation.

[0088] (Variation)

[0089] Figure 3 : is an enlarged cross-sectional view schematically showing the structure of a modified example of the negative electrode current collector 62. This modified example also has a rough surface at the interface between the metal layer 62b and the resin layer 62a. As a result, the contact area between the resin layer 62a and the metal layer 62b can be increased, and the metal layer 62b can be more firmly bonded to the resin layer 62a. It should be noted that in the modified example, the thickness y (μm) of the thickest part of the metal layer 62b is as follows: Figure 3 As shown, the thickness up to and including the protrusions present on both side surfaces of the metal layer 62b is measured.

[0090] The lithium-ion secondary battery 100 can be used for various purposes. For example, it can be suitably used as a high-output power source (driving power source) for a motor mounted on a vehicle. The type of vehicle is not particularly limited. Typically, an automobile, such as a plug-in hybrid electric vehicle (PHEV), a hybrid electric vehicle (HEV), or an electric vehicle (BEV) can be cited. The lithium-ion secondary battery 100 can also be used in the form of a battery pack in which a plurality of batteries are electrically connected.

[0091] While the configuration of the current collector disclosed herein has been described above using the negative electrode current collector 62 as an example, the current collector disclosed herein can also be employed as the positive electrode current collector 52. In this case, the metal foil constituting the metal layer 62b can be replaced with a metal foil used in the positive electrode (e.g., aluminum foil), and the remaining configuration can be the same.

[0092] In addition, as an example, a laminated non-aqueous electrolyte lithium-ion secondary battery with a stacked electrode body is described in detail, but this is only an example and does not limit the scope of protection claimed. The technology described in the scope of protection claimed includes technical solutions for various deformations and changes of the above-described embodiments. For example, a wound electrode body in which a positive electrode sheet and a negative electrode sheet are wound with a separator can be provided instead of the stacked electrode body. In addition, a battery case made of a metal material such as aluminum can be used instead of the outer body 10. In addition, it can also be a fully solid-state battery using a solid electrolyte as the electrolyte, or a polymer battery using a polymer electrolyte.

[0093] Hereinafter, test examples related to the technology disclosed herein will be described, but the technology disclosed herein is not intended to be limited to the technical aspects shown in these test examples.

[0094] [Test Example 1]

[0095] <Preparation of negative electrode current collector>

[0096] In Test Example 1, 11 types of negative electrode collectors, Examples 1 to 11, were prepared to evaluate their performance as lithium-ion secondary batteries. First, copper foil and a resin sheet (thickness: 10 μm) made of polyethylene (PE) were prepared. In Examples 1 and 2, copper foil without a rough surface was used as the negative electrode collector. In Examples 3 to 11, copper foils with rough surfaces on both sides were bonded to both sides of the resin sheet, and hot pressed at 100°C to 250°C for 1 minute to 30 minutes, thereby cladding the copper foil on the resin sheet. Next, the copper foil clad on the resin sheet prepared above was immersed in a mixed solution of conductive particles, PVDF and N-methyl-2-pyrrolidone (NMP), and the surface of the copper foil was dip-coated. Thereafter, the copper foil was dried to form a resin coating layer on the surface of the copper foil. Then, the resin coating layer was ground with a brush at a speed of 200 rpm to adjust the thickness of the resin coating layer and prepare the negative electrode collector of each example. In the examples where the resin coating layer contained conductive particles, acetylene black (AB) or copper (Cu) particles were used as the conductive particles. The types of conductive particles used in each example are shown in Table 1. Furthermore, Examples 7 to 9 used resin sheets containing at least one of AB as the conductive particles and alumina (Al2O3) powder as the inorganic filler. The types of inorganic fillers used in each example are shown in Table 1.

[0097] <Production of negative electrode sheet>

[0098] Prepare a mixture of silicon oxide powder and graphite in a weight ratio of 80:20 as the negative electrode active material. In addition, prepare styrene-butadiene rubber (SBR) as a binder and carboxymethyl cellulose (CMC) as a thickener. Then, weigh them in a weight ratio of negative electrode active material: SBR: CMC = 90:5:5, add ion exchange water as a dispersion medium to make the solid content ratio 66% by mass, and mix using a planetary mixer. Thus, prepare a paste for forming a negative electrode composite material layer.

[0099] Next, the negative electrode composite material layer-forming paste was applied in a stripe-like pattern to the copper foil surfaces on both sides of the negative electrode current collector prepared in each example using a die coater and then dried. This process was repeated on both sides of the negative electrode current collector in each example. The negative electrode current collector was then pressurized to produce a negative electrode sheet.

[0100] <Production of lithium-ion secondary batteries for evaluation>

[0101] Prepare LiNi as the positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM) powder, acetylene black (AB) as a conductive additive, polyvinylidene fluoride (PVDF) as a binder, and polyvinyl alcohol (PVA) as a dispersant were weighed to a weight ratio of NCM:AB:PVDF:PVA = 90:8:1.8:0.2. NMP was added as a dispersion medium to a solid content of 56% by mass, and mixed using a planetary mixer. This prepared a paste for forming a positive electrode composite material layer.

[0102] Next, the positive electrode material layer forming paste was applied to both sides of the aluminum foil in a stripe shape using a die coater and dried. The aluminum foil coated with the dried positive electrode material layer forming paste was then pressurized to produce a positive electrode sheet.

[0103] Separately, a porous polyolefin sheet having a three-layer structure of PP / PE / PP was prepared as a separator.

[0104] The positive electrode sheet and the negative electrode sheet produced above are stacked opposite each other with the separator above, to produce a stacked electrode body. The collector terminal is mounted on the stacked electrode body and housed in an aluminum laminate bag. Then, the non-aqueous electrolyte is impregnated into the stacked electrode body, and the opening of the aluminum laminate bag is sealed and sealed to produce a lithium-ion secondary battery for evaluation. As the non-aqueous electrolyte, a solution is used in which LiPF6 as an auxiliary salt is dissolved at a concentration of 1.0 mol / L in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 1:3 and further 2% by mass of vinylene carbonate (VC) is added.

[0105] <Analysis of the cross section of the negative electrode sheet>

[0106] The cross-sections of the resin layer and the copper foil in the stacking direction of the negative electrode current collectors of Examples 3 to 11 prepared above were observed using an SEM at a magnification of 4000 times, and cross-sectional SEM images of four different fields of view were obtained. The minimum thickness x and the maximum thickness y of the copper foil were measured in each cross-sectional SEM image obtained. It should be noted that since copper foil existed on both sides of the resin layer in each example, the minimum thickness x and the maximum thickness y of each copper foil were measured in each field of view. Then, using the values ​​of the minimum thickness x and the maximum thickness y obtained from the cross-sectional SEM images of the four different fields of view, the average value X (μm) of the minimum thickness and the average value Y (μm) of the maximum thickness of the copper foil of each example were calculated. The values ​​are shown in Table 1 as "average minimum thickness X (μm)" and "average maximum thickness Y (μm)". It should be noted that Examples 1 and 2 show the measured values ​​of the thickness of the copper foil.

[0107] Furthermore, the number of protrusions present on the surface of the copper foil on the side where the resin coating layer was formed in each field of view was counted, with the number of protrusions where 5% or more of the height of the protrusions was exposed from the resin coating layer being measured. The ratio of the number of protrusions exposed from the resin coating layer is then reported in Table 1 as "Ratio of protrusions with exposed portions (number %)."

[0108] Activation treatment and initial capacity measurement

[0109] Each of the evaluation lithium-ion secondary batteries prepared above was placed in an environment of 25°C. After each of the evaluation lithium-ion secondary batteries was charged with a constant current at a current value of 1 / 3C until it reached 4.2V, it was charged with a constant voltage until the current value reached 1 / 50C, reaching a fully charged state. Thereafter, each of the evaluation lithium-ion secondary batteries was discharged with a constant current at a current value of 1 / 3C until it reached 3.0V. The discharge capacity at this time was taken as the initial capacity. It should be noted that here, "1C" refers to the current size that can charge the SOC (state of charge) from 0% to 100% in 1 hour.

[0110] <Measurement of low-temperature resistance>

[0111] The activated lithium-ion secondary batteries for evaluation were adjusted to an open voltage of 3.70 V and placed in a temperature environment of -5°C. Under the above environment, the batteries were discharged at a current value of 3C for 25 seconds, and the voltage change ΔV at this time was calculated. In addition, the current value at which 20C was reached was calculated. Then, the calculated value of (ΔV / current value of 20C) was calculated as the low-temperature resistance. The ratio of the low-temperature resistance of the lithium-ion secondary batteries for evaluation of each example when the low-temperature resistance of the lithium-ion secondary battery for evaluation of Example 1 is 1.00 is shown in Table 1 as the standardized low-temperature resistance.

[0112] <Determination of Capacity Retention Rate>

[0113] The activated lithium-ion secondary batteries for evaluation were adjusted to an open voltage of 3.3V and placed in an environment of 25°C. The charge and discharge were performed in a constant current mode, charging at a current value of 1C until 4.2V, and then discharging at a current value of 1C to 3.3V. This charge and discharge was regarded as one cycle, and repeated 500 cycles. Thereafter, the capacity after 500 cycles was measured using the same method as the initial capacity. Then, the value of (capacity after 500 cycles / initial capacity) × 100 was calculated to determine the capacity retention rate (%). The results are shown in Table 1.

[0114] Safety Test

[0115] After the activated lithium ion secondary batteries for evaluation were charged with a constant current at a current value of 1 / 3C until 4.2V, they were charged with a constant voltage until the current value reached 1 / 10C, ​​reaching a fully charged state. Thereafter, the fully charged lithium ion secondary batteries for evaluation were placed in an environment of 25°C. Next, an iron nail with a diameter of 3mm was passed through the center of each lithium ion secondary battery for evaluation at a speed of 10mm / sec. The outer surface temperature of each lithium ion secondary battery for evaluation at this time was measured with a thermocouple to determine the maximum temperature. The case where the maximum temperature at this time was less than 150°C was evaluated as "◎", the case where it was above 150°C and less than 200°C was evaluated as "0", and the case where it was above 200°C was evaluated as "×". The results are shown in Table 1.

[0116] [Table 1]

[0117]

[0118] As shown in Table 1, compared to Examples 1 and 2, which used metal foil without a rough surface as the current collector, Examples 3 to 11 showed good safety test results and improved capacity retention. Furthermore, Examples 3 to 11 also showed a lower low-temperature resistance than 1. This demonstrates that using a current collector having a resin layer, a metal foil with a rough surface, and a resin coating layer not only provides excellent safety but also improves battery performance, such as increased capacity retention and reduced low-temperature resistance.

[0119] Comparing Examples 3 to 11, it is clear that the inclusion of conductive particles in the resin coating layer resulted in superior capacity retention and reduced low-temperature resistance. Furthermore, inclusion of conductive particles in the resin coating layer also resulted in better safety test results.

[0120] Furthermore, when comparing Examples 4, 5, 10, and 11, it is found that when the ratio of the number of protrusions exposed from the resin coating layer is 30% or more, the capacity retention rate is improved and the low-temperature resistance is reduced at a higher level.

[0121] Furthermore, when Examples 6 to 9 are compared, it can be seen that when the resin layer contains conductive particles and / or an inorganic filler, an improvement in the capacity retention rate and a reduction in the low-temperature resistance can be achieved at particularly high levels.

[0122] [Test Example 2]

[0123] In Test Example 2, the thickness of the resin coating layer was studied. Specifically, attention was paid to the convex portions having exposed portions exposed from the resin coating layer, and the proportion of the exposed portions in the convex portions was investigated. Here, 8 types of negative electrode current collectors, Examples 12 to 19, were prepared and tested. These negative electrode current collectors were prepared in the same manner as in Example 5 of Test Example 1 above, and resin coating layers of various thicknesses were prepared by adjusting the proportion of grinding the resin coating layer with a brush in each example. Then, a lithium ion secondary battery for evaluation was prepared in the same manner as in Test Example 1, and various evaluation tests were carried out in the same manner as in Test Example 1. It should be noted that in Test Example 2, in the analysis of the cross section of the negative electrode sheet, 4 convex portions with exposed portions were randomly selected in each field of view (i.e., a total of 16 in each example), and the proportion of the exposed portion in the height direction of the convex portion (the stacking direction of the resin sheet and the copper foil) in the total height of the convex portion was calculated. The results are shown in Table 2. Although not shown in the table, the number of protrusions having exposed portions in the negative electrode current collectors of Examples 12 to 19 was approximately 30%.

[0124] [Table 2]

[0125]

[0126] Table 2 shows that when the ratio of the exposed portion in the height direction of the protrusion is 99% or less, a lithium-ion secondary battery with high safety, excellent capacity retention, and low-temperature resistance can be obtained. It is particularly evident that when the ratio of the exposed portion in the height direction of the protrusion is 95% or less, a higher level of safety, as well as better capacity retention and low-temperature resistance, can be achieved.

[0127] [Test Example 3]

[0128] In Test Example 3, the ranges of the average minimum thickness X and average maximum thickness Y of the copper foil used were studied. Specifically, the copper foil used was etched to prepare multiple copper foils with varying degrees of roughness. These were used to produce negative electrode current collectors for Examples 20 to 26. The composition other than the copper foil was the same as in Example 5 above, and lithium-ion secondary batteries for evaluation of each example were produced. Various evaluation tests were then carried out in the same manner as in Test Example 1 above. The results are shown in Table 3.

[0129] [Table 3]

[0130]

[0131] Table 3 shows that all of Examples 5 and Examples 20-26 achieved excellent safety, capacity retention, and low-temperature resistance. Among them, Examples 5 and Examples 24-26 achieved particularly excellent capacity retention and low-temperature resistance. This indicates that an average minimum thickness X of the metal layer of 0.8 μm to 3 μm and an average maximum thickness Y of 2 μm to 11 μm are appropriate.

Claims

1. A current collector used in an electrode of a secondary battery, A laminated structure comprising a resin layer and metal layers formed on both sides of the resin layer. The surface of the metal layer has a rough surface portion provided with a plurality of convex portions and a plurality of concave portions, A resin coating layer is formed on the rough surface portion. At least some of the plurality of protrusions have exposed portions exposed from the resin coating layer, The ratio of the convex portions having the exposed portion is 20% or more of the total number of the plurality of convex portions.

2. The current collector according to claim 1, wherein In the convex portion having the exposed portion, an average value of a ratio of the exposed portion in the height direction of the convex portion is 5% to 99%.

3. The current collector according to claim 2, wherein The average value of the ratio of the exposed portion is 95% or less.

4. The current collector according to any one of claims 1 to 3, wherein When n SEM images of the cross section of the collector along the stacking direction of the resin layer and the metal layer are obtained in different fields of view, the average value X of the thickness x of the thinnest part of the metal layer and the average value Y of the thickness y of the thickest part observed in each SEM image have 0.8μm≤X≤3μm and 2μm≤Y≤11μm, n is a natural number of 4 or greater, and the unit of the average value X of the thickness x and the average value Y of the thickness y is μm.

5. The current collector according to any one of claims 1 to 3, wherein The resin coating layer contains conductive particles.

6. The current collector according to any one of claims 1 to 3, wherein The resin layer contains conductive particles.

7. The current collector according to any one of claims 1 to 3, wherein The resin layer contains an inorganic filler.

8. A secondary battery comprising a positive electrode and a negative electrode, At least one of the positive electrode and the negative electrode includes the current collector according to any one of claims 1 to 7.

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