Electrode and secondary battery

Abstract

In a positive electrode, which is an example of an embodiment, a positive electrode mixture layer (31A) has a tail region (33A) in which there are formed a plurality of tail portions (34A) extending in the length direction of a positive electrode core body (30) from one edge of a core body exposed portion (32A), and a positive electrode mixture layer (31B) has a tail region (33B) in which there are formed a plurality of tail portions (34B) extending in the length direction of the positive electrode core body (30) from one edge of a core body exposed portion (32B). The positive electrode mixture layers (31A, 31B) are formed so that at least tail leading end regions (37A, 37B) do not overlap in the thickness direction of the positive electrode core body (30).

Description

Electrode and secondary battery

[0001] The present disclosure relates to an electrode and a secondary battery including the electrode.

[0002] Generally, an electrode for a secondary battery has a core made of metal foil and a mixture layer formed on both sides of the core. It is manufactured by applying a mixture slurry containing active material particles to both sides of the core, drying the coating, and compressing it. In this process, the mixture slurry may be applied intermittently to provide a core exposed portion where the surface of the core is exposed. The core exposed portion is, for example, a portion to which an electrode lead is connected, and is provided in the longitudinal middle of the electrode.

[0003] When the core exposed portion is formed by intermittently applying the mixture slurry, a tail portion is formed at the end of the applied mixture slurry. The tail portion is a thin, linear portion extending long in the direction of application of the mixture slurry, and the active material particles are scattered particularly at its tip. Therefore, when pressure is applied to the electrode, the core is strongly compressed at the portion sandwiched between the tail portions, and the active material particles may bite into the core, resulting in a localized thinning of the core thickness.

[0004] A technique for forming a protective layer at a planned cutting location has been known in the past to prevent burrs from being generated during the cutting process of an electrode due to the tail portion (see Patent Document 1). Patent Document 1 describes that providing a protective layer at the portion where the tail portion is formed prevents burrs from being generated during the cutting process.

[0005] International Publication No. 2019 / 077943

[0006] Multiple tailing portions are formed to extend from one edge of the exposed core portion, and as a result of studies by the present inventors, it was found that the tailing portions cause damage to the core when the secondary battery is repeatedly charged and discharged. If cracks in the core grow large enough to cause the core to break, problems such as increased resistance and decreased capacity occur, so suppressing damage to the core caused by the tailing portions is an important issue.

[0007] An electrode according to one aspect of the present disclosure comprises a long core body having a first end and a second end in a length direction, and first and second mixture layers respectively disposed on both sides of the core body, and the electrode is provided with first and second core exposed portions that overlap in a thickness direction of the core body at a longitudinal intermediate portion of the core body, the first mixture layer having a first tail region in which a plurality of first tail portions are formed extending in the length direction of the core body from an edge on the first end side of the first core exposed portion, and the second mixture layer having a first tail region in which a plurality of first tail portions are formed extending in the length direction of the core body from an edge on the first end side of the first core exposed portion, and The core body has a second tailing region in which a plurality of second tailing portions are formed extending from the edge of the exposed portion on the first end side in the longitudinal direction of the core body, and when the ranges from the tip of the tailing portion located closest to the first end to the tip of the tailing portion located closest to the first end in each of the first and second tailing region are defined as first and second tailing tip regions, the first and second mixture layers are formed so that the first and second tailing tip regions do not overlap in the thickness direction of the core body.

[0008] Another aspect of the present disclosure is an electrode comprising a long core body having a first end and a second end in the longitudinal direction, and first and second mixture layers respectively arranged on both sides of the core body, and having first and second core exposed portions that overlap in the thickness direction of the core body at the middle part of the longitudinal direction of the core body, wherein the first mixture layer has a first tailing region in which a plurality of first tailing portions are formed extending in the longitudinal direction of the core body from the edge on the first end side of the first core exposed portion, and the second mixture layer has a second tailing region in which a plurality of second tailing portions are formed extending in the longitudinal direction of the core body from the edge on the first end side of the second core exposed portion, and the first and second tailing regions are formed so that a range of 50% or more of the length of each region along the longitudinal direction of the core body does not overlap in the thickness direction of the core body.

[0009] The secondary battery according to the present disclosure is a secondary battery equipped with a wound electrode assembly including a positive electrode, a negative electrode, and a separator, characterized in that the above-described electrode is applied to the positive electrode.

[0010] According to the electrode according to the present disclosure, damage to the core body caused by the tail portion can be effectively suppressed.

[0011] Fig. 2 is an axial cross-sectional view of a cylindrical battery that is an example of an embodiment; Fig. 3 is a front view showing a part of a positive electrode that is an example of an embodiment; Fig. 4 is a cross-sectional view taken along line AA in Fig. 2; Fig. 5 is an enlarged view of a tail portion of a positive electrode and its vicinity, showing (a) a first surface (front view) of the positive electrode and (b) a second surface (rear view) of the positive electrode, respectively; Fig. 6 is a cross-sectional view showing a conventional example of an electrode;

[0012] As described above, when the core exposed portion is provided by intermittently applying the mixture slurry, a tail portion is formed at the end of the application of the mixture slurry. The mixture slurry is applied, for example, by a die coater, but it is difficult to stop the application of the slurry so that the position of the mixture end along the edge of the core exposed portion is in a straight line, so the slurry is applied from the die head at the application end portion in a tail-like manner, and multiple tail portions are formed.

[0013] Figure 5 shows a cross section of a conventional electrode taken along the length of the tail. As shown in Figure 5, active material particles are scattered, particularly near the tip of the tail. For example, when the negative electrode expands during charging and discharging of a secondary battery, the scattered active material particles exert a large localized pressure on the core. As a result, the active material particles may bite into the core, causing the core to become locally thin.

[0014] As a result of investigations by the present inventors, it was found that when a secondary battery is repeatedly charged and discharged, damage such as cracks occurs in the portion of the core where the tailing portion is located.The inventors then discovered that damage to the core can be effectively suppressed by shifting the tailing portions on the front and back of the electrode plate so that the tip regions of the tailing portions do not overlap in the thickness direction of the core, or by shifting the tailing portions on the front and back so that a range of 50% or more of the length of the tailing regions does not overlap in the thickness direction of the core.

[0015] Hereinafter, an example of an embodiment of an electrode and a secondary battery according to the present disclosure will be described in detail with reference to the drawings. The embodiment described below is merely an example, and the present disclosure is not limited to the following embodiment. Furthermore, the present disclosure also includes embodiments obtained by selectively combining the components of the embodiments described below.

[0016] In the following, as an example of an embodiment of a secondary battery according to the present disclosure, a cylindrical battery 10 in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is illustrated, but the outer can of the battery is not limited to a cylindrical outer can. The secondary battery according to the present disclosure may be, for example, a prismatic battery with a prismatic outer can or a pouch-type battery with an outer can made of a laminate sheet including a metal layer and a resin layer.

[0017] FIG. 1 is a schematic diagram illustrating an axial cross section of a cylindrical battery 10 according to an embodiment. As shown in FIG. 1 , the cylindrical battery 10 includes an electrode assembly 14, an electrolyte, and a bottomed, cylindrical outer can 16 that accommodates the electrode assembly 14 and the electrolyte. The electrode assembly 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and is configured such that the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a bottomed, cylindrical metal container that is open on one axial side, and the opening of the outer can 16 is closed by a sealing member 17. Hereinafter, for convenience of explanation, the sealing member 17 side of the cylindrical battery 10 is referred to as the top, and the bottom side of the outer can 16 is referred to as the bottom.

[0018] The electrolyte may be an aqueous electrolyte, but in this embodiment, a non-aqueous electrolyte is used. The non-aqueous electrolyte has lithium ion conductivity. The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte. The cylindrical battery 10 is, for example, a non-aqueous electrolyte secondary battery, and preferably a lithium ion battery.

[0019] The liquid electrolyte (electrolytic solution) contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. Examples of the electrolyte salt include LiPF ６ Lithium salts such as

[0020] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc. can be used. As the inorganic solid electrolyte, a material known in all-solid-state lithium ion secondary batteries, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halogen-based solid electrolyte, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. As the polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. can be used.

[0021] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer in the length direction and width direction (short direction) than the positive electrode 11. The separator 13 is formed to be at least slightly larger than the positive electrode 11, and two separators 13 are arranged to sandwich the positive electrode 11. The cylindrical battery 10 includes insulating plates 18 and 19 arranged above and below the electrode assembly 14, respectively.

[0022] The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like. In this embodiment, the positive electrode lead 20 is connected to the longitudinal center of the positive electrode 11, and the negative electrode lead 21 is connected to the longitudinal end of the negative electrode 12 located on the outer side of the wound electrode body 14. The positive electrode lead 20 is connected to the lower surface of an internal terminal plate 23, which is the bottom plate of the sealing body 17, and the negative electrode lead 21 is connected to the inner surface of the can bottom of the outer can 16. Therefore, the sealing body 17 serves as a positive electrode external terminal, and the outer can 16 serves as a negative electrode external terminal.

[0023] The negative electrode 12 may be disposed on the outer peripheral surface of the electrode body 14, and the exposed portion of the negative electrode core may contact the inner peripheral surface of the outer can 16, thereby electrically connecting the negative electrode 12 and the outer can 16. Alternatively, a negative electrode lead may be connected to the center of the negative electrode 12 in the longitudinal direction.

[0024] The positive electrode 11 has a positive electrode core and a positive electrode mixture layer disposed on the positive electrode core. The positive electrode core can be a foil of a metal stable within the potential range of the positive electrode 11, such as aluminum, an aluminum alloy, stainless steel, or titanium, or a film having a surface layer made of such a metal. The thickness of the positive electrode core is preferably 5 μm to 25 μm, more preferably 10 μm to 20 μm. The positive electrode mixture layer contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core. The thickness of the positive electrode mixture layer is greater than the thickness of the positive electrode core, and is, for example, 60 μm to 120 μm on one side of the positive electrode core.

[0025] The positive electrode active material uses a lithium transition metal composite oxide containing transition metal elements such as Ni, Co, and Mn. Examples of metal elements contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, Be, B, Na, Mg, Si, K, Ca, Sc, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, Ta, W, Pb, and Bi. Among these, it is preferable to contain at least one of Ni, Co, and Mn. One type of lithium transition metal composite oxide may be used alone, or multiple types may be used in combination.

[0026] The positive electrode active material is a particle having a volume-based median diameter (D50) of, for example, 10 μm or more and 30 μm or less, preferably 15 μm or more and 25 μm or less. D50 means the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size. The particle size distribution of the lithium transition metal composite oxide can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrac-Bell Co., Ltd.) with water as the dispersion medium. The particle size distribution can also be measured using an image particle size distribution analyzer (e.g., CAMSIZER X2, manufactured by Microtrac-Bell Co., Ltd.) with water as the dispersion medium.

[0027] The thickness of the positive electrode core may be greater than the D50 of the positive electrode active material particles, but is preferably equal to or less than the D50 of the active material particles from the viewpoint of achieving high capacity. However, if the particle diameter of the active material is large relative to the core thickness, damage to the core due to the tailing portion is more likely to occur. As will be described in detail later, by applying a configuration in which the tailing regions are offset on the front and back of the electrode plate to the positive electrode 11, damage to the positive electrode core can be effectively suppressed even in such cases.

[0028] Examples of conductive agents contained in the positive electrode mixture layer include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, metal fibers, metal powder, and conductive whiskers. Examples of binders include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, and ethylene-propylene-butadiene copolymer, and acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide, and ethylene-acrylic acid copolymer. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like.

[0029] The positive electrode 11 can be produced by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto a positive electrode core, drying the coating, and then compressing it to form a positive electrode mixture layer on both sides of the positive electrode core. The positive electrode 11, which has a core exposed portion to which the positive electrode lead 20 is connected at the middle portion in the longitudinal direction, is produced by intermittently applying the positive electrode mixture slurry and pausing the application of the slurry midway, and the portion to which the slurry is not applied becomes the core exposed portion. For example, N-methyl-2-pyrrolidone (NMP) is used as the dispersion medium for the positive electrode mixture slurry.

[0030] The negative electrode 12 has a negative electrode core and a negative electrode mixture layer disposed on the negative electrode core. The negative electrode core can be a foil of a metal stable within the potential range of the negative electrode 12, such as copper, copper alloy, stainless steel, nickel, or nickel alloy, or a film having a surface layer made of such a metal. The negative electrode mixture layer contains a negative electrode active material and a binder and is preferably provided on both sides of the negative electrode core. As with the positive electrode 11, the negative electrode 12 can be fabricated by applying a negative electrode mixture slurry to the negative electrode core, drying the coating, and then compressing it to form a negative electrode mixture layer on both sides of the negative electrode core. Metallic lithium foil can also be used as the negative electrode 12. Alternatively, the negative electrode 12 may be composed only of a negative electrode core, with metallic lithium being precipitated on the core surface during battery charging.

[0031] The negative electrode active material is not particularly limited as long as it reversibly absorbs and releases lithium ions, and generally, carbon materials such as graphite are used. Furthermore, elements that alloy with Li, such as Si and Sn, or materials containing such elements may also be used as the negative electrode active material. Among these, silicon-containing materials containing Si are preferred. Furthermore, lithium titanate, which has a higher charge / discharge potential relative to metallic lithium than carbon materials, may also be used as the negative electrode active material. One type of negative electrode active material may be used alone, or multiple types may be used in combination.

[0032] As in the case of the positive electrode 11, the binder contained in the negative electrode mixture layer can be a fluororesin, an olefin resin, PAN, a polyimide, a polyamide, an acrylic resin, or the like. However, polyvinyl acetate, styrene-butadiene rubber (SBR), or the like may also be used. Among these, SBR is preferably used. One type of binder may be used alone, or multiple types may be used in combination. Furthermore, the negative electrode mixture layer preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. The negative electrode mixture layer may also contain a conductive agent such as CNT.

[0033] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. A highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13. A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12.

[0034] As described above, the outer can 16 is a cylindrical metal container with a bottom and an opening on one axial side. A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure sealing of the battery interior and insulation between the outer can 16 and the sealing body 17. The outer can 16 has a groove 22 formed by a portion of the side surface protruding inward. The groove 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the outer can 16 by the groove 22 and the open end of the outer can 16, which is crimped to the sealing body 17.

[0035] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. If an abnormality occurs in the battery and the internal pressure increases, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0036] The configuration of the positive electrode 11 will be described in further detail below with reference to Figures 2 to 4. Figure 2 is a front view of the positive electrode 11, showing the core exposed portion 32A and its vicinity. Note that the protective tape 29 is shown by an imaginary line in Figure 2. Figure 3 is a cross-sectional view taken along line AA in Figure 2, Figure 4(a) is an enlarged view of the tailing region 33A and its vicinity, and Figure 4(b) is an enlarged view of the tailing region 33B and its vicinity.

[0037] As shown in FIGS. 2 to 4 , the positive electrode 11 includes a long positive electrode core 30, a first positive electrode mixture layer 31A disposed on a first surface of the positive electrode core 30, and a second positive electrode mixture layer 31B disposed on a second surface of the positive electrode core 30. The positive electrode core 30 has a structure in which core exposed portions 32A and 32B are provided in the longitudinal middle portion of the positive electrode core 30. The first surface of the positive electrode core 30 may face either the outer or inner surface of the electrode assembly 14. In this embodiment, the first surface is the outer surface (front surface) facing the outer surface of the electrode assembly 14, and the second surface is the inner surface (back surface) facing the inner surface of the electrode assembly 14. The positive electrode lead 20 is bonded to the core exposed portion 32A on the first surface. The positive electrode lead 20 is positioned, for example, so as not to overlap with the positive electrode mixture layer 31A and is ultrasonically welded to the positive electrode core 30.

[0038] The positive electrode core 30 is preferably made of an elongated metal foil including a first end and a second end in the length direction. In Figures 2 and 4, the length direction of the positive electrode core 30 is indicated by X and the width direction by Y, with one side in the X direction being the first end side of the positive electrode core 30 in the length direction. The first end of the positive electrode core 30 is located, for example, at the winding start side of the positive electrode 11. The core exposed portions 32A, 32B may be formed at positions substantially equidistant from the first end and the second end of the positive electrode core 30. Furthermore, the core exposed portions 32A, 32B are arranged to overlap in the thickness direction of the positive electrode core 30.

[0039] In this embodiment, one core exposed portion is formed on each of the front and back surfaces of the positive electrode 11, but multiple core exposed portions may be formed at intervals in the length direction (X direction) of the positive electrode 11. When multiple core exposed portions are formed in the length direction of the positive electrode 11, it is preferable to apply the configuration of tailing regions 33A and 33B described below to all of the core exposed portions.

[0040] The core exposed portion 32A has dimensions that allow the positive electrode lead 20 to be disposed without overlapping with the positive electrode mixture layer 31A. The core exposed portion 32A is longer in the Y direction than in the X direction and is formed across the entire width of the positive electrode core 30. The core exposed portion may be formed with a length that does not extend from one widthwise end of the positive electrode core 30 to the other. The width (length in the X direction) of the core exposed portion 32A is preferably 1.1 to 2.0 times, and more preferably 1.2 to 1.5 times, the width of the positive electrode lead 20. The width of the core exposed portion 32A refers to the length along the X direction from the coating start end 35A to the coating end 36A (see FIG. 4 ). Multiple tail portions 34A are formed in the coating end 36A, and the position corresponding to the coating end 36A is the edge on the winding start side of the core exposed portion 32A (the first end side of the positive electrode core 30). Similarly, the width of the core exposed portion 32B refers to the length along the X direction from the application starting end 35B to the application terminal end 36B, and the position corresponding to the application terminal end 36B is defined as the edge on the winding start side of the core exposed portion 32B. As will be described in detail later, the core exposed portion 32B is formed wider than the core exposed portion 32A.

[0041] Protective tape 29 is attached to the front and back of the positive electrode 11, covering the positive electrode lead 20 and each of the core exposed portions 32A, 32B. The protective tape 29 is, for example, an insulating tape including a tape substrate and an adhesive layer formed on one side of the substrate. Examples of resins constituting the tape substrate include polyester, polypropylene, polyimide, polyphenylene sulfide, polyetherimide, and polyamide. The thickness of the tape substrate is, for example, 10 μm or more and 30 μm or less. The width of the protective tape 29 is greater than the width of the core exposed portions 32A, 32B, and the protective tape 29 covers the entire core exposed portions 32A, 32B. In addition, the protective tape 29 is preferably attached to the front and back of the positive electrode 11 so as to cover the entire tail regions 33A, 33B.

[0042] On the front and back surfaces of the positive electrode 11, tailing regions 33A, 33B including a plurality of tailing portions 34A, 34B are formed, respectively. In this embodiment, the X direction is the application direction of the positive electrode mixture slurry, and both of the tailing portions 34A, 34B extend in the X direction. The tailing portion 34A is a portion of the positive electrode mixture layer 31A formed in a thin line shape, and the thickness decreases sharply toward the tip of the tailing portion 34A. The positive electrode mixture layer 31A is configured by stacking a plurality of positive electrode active material particles in the thickness direction of the layer in the region other than the tailing portion 34A, but the tip of the tailing portion 34A has a single-layer structure in which active material particles are scattered (see FIG. 5 ).

[0043] The first tailing portions 34A have similar lengths and widths, but their dimensions are not uniform. For example, the length of the tailing portion 34A is between 1 mm and 5 mm, and the width of the tailing portion 34A is 30% or less of the length. The tailing portion 34A has a tapered shape, with the thickness decreasing from the base to the tip and the width gradually decreasing. The second tailing portion 34B has the same shape and dimensions as the first tailing portion 34A.

[0044] The first tailing region 33A is formed along the edge of the first core exposed portion 32A on the winding start side (the edge on the first end side of the positive electrode core 30). The second tailing region 33B is similarly formed along the edge of the second core exposed portion 32B on the winding start side. Because the tailing regions 33A, 33B are formed at the end portions of the application of the positive electrode mixture slurry, in this embodiment, the positive electrode mixture slurry is applied from the first end side of the positive electrode core 30. Furthermore, because the positive electrode mixture layers 31A, 31B are formed across the entire width of the positive electrode core 30, the tailing regions 33A, 33B are also formed across the entire width of the positive electrode core 30.

[0045] The tailing regions 33A, 33B formed on the edges on the winding start side of the core exposed portions 32A, 32B are arranged so that at least the tip portions do not overlap in the thickness direction of the positive electrode core 30. On the other hand, the edges on the winding end side of the core exposed portions 32A, 32B (the edges on the second end side of the positive electrode core 30) overlap in the thickness direction of the positive electrode core 30. The application start ends 35A, 35B located in positions corresponding to the edges on the winding end side of the core exposed portions 32A, 32B extend linearly in a front view and overlap each other in the thickness direction of the positive electrode core 30. No tailing portion is formed at the application start end of the positive electrode mixture slurry.

[0046] The application start ends 35A, 35B only need to substantially overlap. The positions of the application start ends 35A, 35B may be shifted within a range that is considered to be substantially overlapping (for example, about 1 mm in the X direction). It is also possible to move the position of the application start end 35B toward the start of winding relative to the position of the application start end 35A, but in that case, it becomes difficult to align the core exposed portions 32A, 32B, and as a result, it is necessary to increase the width of the core exposed portions 32A, 32B. By aligning the positions of the application start ends 35A, 35B on the front and back of the positive electrode 11, the core exposed portions 32A, 32B can be formed with the minimum necessary width, contributing to increasing the capacity of the battery.

[0047] In this embodiment, the range from tip P1, which is located closest to the second end of the positive electrode core 30, to tip P2, which is located closest to the first end of the positive electrode core 30, among the tips of the multiple tailing portions 34A, is defined as a first tailing tip region 37A. Furthermore, the range from tip P1, which is located closest to the second end of the positive electrode core 30, to tip P2, which is located closest to the first end of the positive electrode core 30, among the tips of the multiple tailing portions 34B, is defined as a second tailing tip region 37B. The positive electrode mixture layers 31A, 31B are formed so that the tailing tip regions 37A, 37B do not overlap in the thickness direction of the positive electrode core 30.

[0048] The tail tip region 37A is a region of a certain width along the Y direction that extends from the tip P1 of the tail portion 34A to the tip P2 of the tail portion 34A in the X direction (the same applies to the tail tip region 37B). The X direction lengths of the tail tip regions 37A and 37B may be different from each other, but are, for example, substantially the same. The X direction length of the tip region 37A is less than 50% of the average length of the tail portion 34A, for example, between 3% and 30%.

[0049] As described above, the thickness and width of the tailing portions 34A, 34B decrease from the application end portions 36A, 36B at the base toward the tip, and the active material particles are particularly likely to become scattered in the tailing tip regions 37A, 37B. As a result of studies by the present inventors, it was found that when the negative electrode 12 expands due to repeated charge-discharge cycles, if the tip portions of the tailing portions overlap on the front and back of the core body, the core body is locally compressed and thinned, whereas if the tailing portions exist on only one side of the core body, thinning of the core body that leads to damage such as cracks (foil tears) does not occur.

[0050] The tailing regions 33A, 33B may be arranged so that a range of 50% or more of the length of each region along the X direction does not overlap in the thickness direction of the positive electrode core 30. The positive electrode mixture layers 31A, 31B are formed so that a range of the length of the tailing region 33A corresponding to 50% or more of the length of the longest tailing portion 34A does not overlap with the tailing region 33B, and so that a range of the length of the tailing region 33B corresponding to 50% or more of the length of the longest tailing portion 34B does not overlap with the tailing region 33A.

[0051] The tailing regions 33A, 33B may be arranged so that the entire regions do not overlap in the thickness direction of the positive electrode substrate 30. In this embodiment, the tailing regions 33A, 33B are arranged completely offset on the front and back of the positive electrode 11, and the width of the substrate exposed portion 32B is larger than the width of the substrate exposed portion 32A by the width of the tailing region 33A.

[0052] The positive electrode 11 having the substrate exposed portions 32A, 32B in the longitudinally intermediate portion of the electrode plate is fabricated by intermittent application of the cathode mixture slurry, in which application of the cathode mixture slurry is temporarily stopped. At this time, the portions not coated with the cathode mixture slurry become the substrate exposed portions 32A, 32B, and the tailing regions 33A, 33B are formed at the coating end portions 36A, 36B where application of the slurry is stopped. In this embodiment, from the viewpoints of coating thickness stability and improved productivity, the coating film of the first cathode mixture layer 31A is formed before the coating film of the second cathode mixture layer 31B. The coating film of the positive electrode mixture layer 31B is formed by shifting the positions of the coating end portions so that the tailing tip regions 37A, 37B do not overlap on the front and back of the positive electrode 11 and so that the tailing regions 33A, 33B do not overlap within a length range exceeding 50% of the X-direction length.

[0053] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0054] Example 1 [Preparation of Positive Electrode] A lithium nickel composite oxide with a D50 of 10 μm was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a solids mass ratio of 97:2:1, and a positive electrode mixture slurry was prepared using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. The slurry was applied to both sides of a positive electrode core made of aluminum foil with a thickness of 15 μm using a die coater. After drying, the coating was compressed using a rolling roller. The positive electrode was then cut to a predetermined electrode size to obtain a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode core. The core exposure portion, which was the connection portion for the positive electrode lead, was formed in the middle portion in the longitudinal direction of the positive electrode.

[0055] The die coater was controlled to apply the positive electrode mixture slurry to the first surface (surface) of the positive electrode core so that the first core exposed portion was approximately 10 mm wide (length in the X direction). At this time, the first tailing region formed at the end of the application of the slurry was approximately 3 mm wide (length in the X direction). The first core exposed portion was the area from the tip of the first tailing region (tip of the longest tailing portion) to the application start end where the application of the slurry resumed. The amount of slurry applied could be controlled by changing the distance between the positive electrode core and the die head, and this distance was set so that the thickness of the positive electrode mixture layer was 50 μm.

[0056] After drying the first coating film formed on the first surface of the positive electrode substrate, the positive electrode mixture slurry was intermittently applied to the second surface (back surface) to form a second coating film, forming a second substrate exposed portion wider than the first substrate exposed portion. The position of the application start end of the second coating film was aligned with the application start end of the first coating film. Meanwhile, the position of the application end end of the slurry was shifted so that the second tailing region formed at the application end end of the slurry did not overlap with the first tailing region by more than 50% of its length. Specifically, the die coater was controlled to apply the positive electrode mixture slurry so that the second substrate exposed portion had a width of approximately 11.5 mm. At this time, the second tailing region formed at the application end end of the slurry was approximately 3 mm wide (X-direction length).

[0057] In the positive electrode of Example 1, the X-direction length of the first tail tip region is shorter than 50% of the length of the tail portion located closest to the second end of the positive electrode substrate (the same applies to the X-direction length of the second tail tip region), and the first and second tail tip regions do not overlap in the thickness direction of the substrate. An aluminum positive electrode lead was ultrasonically welded to the first substrate exposed portion, and protective tape was attached to the front and back of the positive electrode so as to entirely cover the positive electrode lead and each substrate exposed portion.

[0058] [Preparation of Negative Electrode] Graphite was used as the negative electrode active material. The negative electrode active material, a dispersion of styrene butadiene rubber, and sodium carboxymethyl cellulose were mixed in a solid mass ratio of 98:1:1, and a negative electrode mixture slurry was prepared using water as a dispersion medium. The slurry was applied to both sides of a negative electrode core made of a long copper foil with a thickness of 8 μm using a die coater, and the coating film was dried and then compressed using a rolling roller. The resulting negative electrode was then cut to a predetermined electrode size to obtain a negative electrode having a negative electrode mixture layer formed on both sides of the negative electrode core.

[0059] A negative electrode mixture layer was formed on the entire negative electrode core except for the exposed portion, leaving a core exposed portion at the end in the longitudinal direction of the negative electrode core. A nickel negative electrode lead was ultrasonically welded to the exposed portion of the core, and a protective tape was attached to cover the negative electrode lead.

[0060] [Fabrication of Electrode Assembly] The positive electrode, the negative electrode, and a polyethylene separator were spirally wound around a cylindrical core member, and a stop tape was attached to the outermost surface to obtain a wound electrode assembly. At this time, the negative electrode was positioned so that the negative electrode lead was located on the outside of the wound electrode assembly. After forming the wound structure of the electrode assembly, the core member was removed to obtain a wound electrode assembly with a cavity formed in the core portion.

[0061] [Preparation of non-aqueous electrolyte] 5 parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of a mixed solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1:3 (25°C), and LiPF ６ was dissolved in a concentration of 1.5 mol / L to prepare a non-aqueous electrolyte solution.

[0062] [Fabrication of Cylindrical Battery] After placing insulating plates above and below the electrode assembly, the negative electrode lead was welded to the inner bottom surface of a cylindrical outer can with a bottom, and the positive electrode lead was welded to the internal terminal plate of a sealing member, and the electrode assembly was housed in the outer can. Thereafter, a nonaqueous electrolyte solution was injected into the outer can under reduced pressure, and the opening of the outer can was sealed with a sealing member via a gasket, thereby obtaining a cylindrical battery.

[0063] Example 2 A positive electrode and a cylindrical battery were fabricated in the same manner as in Example 1, except that the width of the second substrate exposed portion was set to about 15 mm, and the positive electrode mixture slurry was intermittently applied to the second surface of the positive electrode substrate so that the entire second tailing region did not overlap with the first tailing region.

[0064] <Comparative Example 1> A positive electrode and a cylindrical battery were fabricated in the same manner as in Example 1, except that the positions of the application end points of the slurry were aligned on the front and back so that the second tailing region overlapped the first tailing region.

[0065] [Cycle Test] Each battery of the Examples and Comparative Examples was charged at a constant current of 1 C in a temperature environment of 25°C until the battery voltage reached 4.4 V, and then discharged at a constant current of 1 C until the battery voltage reached 2.5 V. This charge / discharge cycle was repeated 250 times. After that, the batteries were disassembled, the positive electrodes were removed, and the exposed portions of the core were visually inspected for cracks (foil breaks). The evaluation results are shown in Table 1. The denominator of the evaluation results indicates the number of batteries evaluated, and the numerator indicates the number of batteries in which foil breaks were confirmed.

[0066]

[0067] As shown in Table 1, in the comparative example battery, foil tearing was observed in the exposed core portion of both of the two batteries subjected to the cycle test (2 / 2), but no foil tearing was observed in the example battery (0 / 4). In other words, by forming the positive electrode mixture layer so that more than 50% of the length of the tailing portion does not overlap on the front and back of the positive electrode, damage to the core caused by the tailing portion is specifically suppressed. Note that, since the active material particles are scattered at the tip of the tailing portion, when the negative electrode expands due to charge / discharge cycles, the positive electrode core is locally compressed and thinned in the portion sandwiched between the active material particles. Therefore, it is believed that foil tearing can be sufficiently suppressed by preventing the tip regions of the tailing portion from overlapping on the front and back of the positive electrode.

[0068] In the above embodiment, the case where the configuration in which the tail portions on the front and back of the electrode plate are shifted in the X direction is applied to the positive electrode has been exemplified, but this configuration can also be applied to the negative electrode.

[0069] The present disclosure is further illustrated by the following embodiments. Configuration 1: An electrode comprising a long core body having a first end and a second end in the longitudinal direction, and first and second mixture layers respectively arranged on both sides of the core body, and having first and second core exposed portions that overlap in the thickness direction of the core body at a longitudinal middle portion of the core body, wherein the first mixture layer has a first tailing region in which a plurality of first tailing portions are formed extending in the longitudinal direction of the core body from an edge on the first end side of the first core exposed portion, and the second mixture layer has a second tailing region in which a plurality of second tailing portions are formed extending in the longitudinal direction of the core body from an edge on the first end side of the second core exposed portion, and wherein, when the ranges from the tip of the tailing portion located closest to the second end to the tip of the tailing portion located closest to the first end are defined as first and second tip regions in each of the first and second tailing regions, the first and second mixture layers are formed such that the first and second tip regions do not overlap in the thickness direction of the core body.

[0013] A configuration 2: An electrode comprising an elongated core body having a first end and a second end in the longitudinal direction, and first and second mixture layers respectively disposed on both sides of the core body, wherein first and second core exposed portions are provided at a longitudinal intermediate portion of the core body that overlap in the thickness direction of the core body, wherein the first mixture layer has a first tailing region in which a plurality of first tailing portions are formed extending in the longitudinal direction of the core body from an edge on the first end side of the first core exposed portion, and the second mixture layer has a second tailing region in which a plurality of second tailing portions are formed extending in the longitudinal direction of the core body from an edge on the first end side of the second core exposed portion, wherein the first and second tailing regions are formed so that a range of lengths of 50% or more of each region along the longitudinal direction of the core body does not overlap in the thickness direction of the core body.

[0014] A configuration 3: The electrode according to configuration 1 or 2, wherein the edges on the second end sides of the first and second core exposed portions overlap in the thickness direction of the core body. The electrode according to any one of the preceding configurations, wherein the first and second tailing regions are arranged so that the entire regions do not overlap in the thickness direction of the core body. The electrode according to any one of the preceding configurations, wherein the thickness of the core body is equal to or less than the volume-based median diameter (D50) of the active material particles contained in the first and second mixture layers.Configuration 6: A secondary battery having a wound electrode assembly including a positive electrode, a negative electrode, and a separator, wherein the electrode according to any one of configurations 1 to 5 is applied to the positive electrode.

[0070] REFERENCE SIGNS LIST 10 Cylindrical battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 29 Protective tape, 30 Positive electrode core, 31A, 31B Positive electrode mixture layer, 32A, 32B Core exposed portion, 33A, 33B Tail region, 34A, 34B Tail region, 35A, 35B Coating start end, 36A, 36B Coating end portion, 37A, 37B Tail tip region, P1, P2 Tip

Claims

1. An electrode comprising a long core body having a first end and a second end in the longitudinal direction, and first and second mixture layers respectively arranged on both sides of the core body, and having first and second core exposed portions overlapping in the thickness direction of the core body at a longitudinal middle portion of the core body, wherein the first mixture layer has a first tail region in which a plurality of first tail portions are formed extending in the longitudinal direction of the core body from an edge on the first end side of the first core exposed portion, and the second mixture layer has a second tail region in which a plurality of second tail portions are formed extending in the longitudinal direction of the core body from an edge on the first end side of the second core exposed portion, An electrode in which, when the ranges from the tip of the tail portion located closest to the second end to the tip of the tail portion located closest to the first end in each of the first and second tailing regions are defined as first and second tailing tip regions, the first and second mixture layers are formed such that the first and second tailing tip regions do not overlap in the thickness direction of the core body.

2. An electrode comprising a long core body including a first end and a second end in the longitudinal direction, and first and second mixture layers respectively arranged on both sides of the core body, and having first and second core exposed portions overlapping in the thickness direction of the core body at the longitudinal middle portion of the core body, wherein the first mixture layer has a first tail region in which a plurality of first tail portions are formed extending in the longitudinal direction of the core body from the edge on the first end side of the first core exposed portion, and the second mixture layer has a second tail region in which a plurality of second tail portions are formed extending in the longitudinal direction of the core body from the edge on the first end side of the second core exposed portion, and the first and second tail regions are formed so that a length range of 50% or more of each region along the longitudinal direction of the core body does not overlap in the thickness direction of the core body.

3. An electrode as described in claim 1 or 2, wherein the edges of the first and second core exposed portions on the second end side overlap in the thickness direction of the core.

4. An electrode as described in claim 1 or 2, wherein the first and second tail regions are arranged so that the entirety of each region does not overlap in the thickness direction of the core body.

5. An electrode as described in claim 1 or 2, wherein the thickness of the core body is equal to or less than the volume-based median diameter (D50) of the active material particles contained in the first and second mixture layers.

6. A secondary battery comprising a wound electrode assembly including a positive electrode, a negative electrode, and a separator, wherein the electrode according to claim 1 or 2 is applied to the positive electrode.