Secondary battery

Abstract

The purpose of the present invention is to reduce a gap between electrode plates in the vicinity of a winding start end part of a positive electrode generated by charging and discharging, and to suppress deterioration of charge / discharge cycle characteristics.　This secondary battery is provided with: an electrode body in which a positive electrode (11) and a negative electrode are wound with a separator interposed therebetween; a positive electrode lead; and a protective tape. The positive electrode (11) has a long positive electrode core body, and a positive electrode mixture layer disposed on the positive electrode core body. At one end in the width direction of the positive electrode (11), at least one exposed part in which the surface of the positive electrode core body is exposed is provided. The exposed part includes an innermost exposed part positioned radially at the most inward location within the electrode body, the positive electrode lead includes an innermost positive electrode lead connected to the innermost exposed part, and the protective tape includes an innermost protective tape (16a) covering the innermost exposed part and the innermost positive electrode lead. The winding start end part of the positive electrode (11) is disposed at a position not overlapping the innermost protective tape (16a) in the radial direction of the electrode body.

Description

secondary battery

[0001] The present disclosure relates to a secondary battery, and more particularly to a secondary battery including an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween.

[0002] In recent years, secondary batteries such as lithium-ion batteries have been widely used in applications requiring high capacity, high durability, rapid charging performance, etc., such as in-vehicle applications and power storage applications. The positive electrode, a key component of secondary batteries, has a significant impact on these performances, and therefore much research has been done on the positive electrode.

[0003] For example, Patent Document 1 discloses a positive electrode that constitutes a wound electrode body, and that includes a strip-shaped positive electrode core and a positive electrode mixture layer provided on at least one surface of the positive electrode core, and that has an exposed portion at one end in the width direction of the positive electrode where the surface of the positive electrode core is exposed in order to connect a positive electrode lead.

[0004] International Publication No. 2017 / 077698

[0005] However, when a wound electrode body is produced using a positive electrode having an exposed portion at one widthwise end of the positive electrode where the surface of the positive electrode core is exposed, a large difference in surface pressure occurs during charge and discharge between the widthwise end of the positive electrode and the other portion, i.e., the portion where the positive electrode mixture layer is continuously provided in the lengthwise direction, and gaps may form between the electrode plates near the widthwise end of the positive electrode.

[0006] Conventionally, a protective tape is provided to cover the exposed portion and the positive electrode lead connected to the exposed portion, but when a wound electrode assembly is fabricated, if the winding start end of the positive electrode is positioned so that the protective tape overlaps with the electrode assembly in the radial direction, the gap between the electrode plates formed by charging and discharging the battery described above may appear as a large gap near the winding start end of the positive electrode. If a large gap is formed between the electrode plates near the winding start end of the positive electrode due to charging and discharging, it may lead to a deterioration in charge and discharge cycle characteristics.

[0007] Therefore, an object of the present disclosure is to reduce the gap between the electrode plates near the winding start end of the positive electrode that occurs during charge and discharge, and to suppress a decrease in charge and discharge cycle characteristics.

[0008] The secondary battery according to the present disclosure comprises an electrode body in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, a positive electrode lead, and a protective tape, and the positive electrode has a long positive electrode core and a positive electrode mixture layer disposed on the positive electrode core, one end of the positive electrode in the width direction is provided with one or more exposed portions where the surface of the positive electrode core is exposed, and the exposed portions include an innermost exposed portion located innermost in the radial direction of the electrode body, the positive electrode lead includes an innermost positive electrode lead connected to the innermost exposed portion, the protective tape includes an innermost protective tape that covers the innermost exposed portion and the innermost positive electrode lead, and the winding start end of the positive electrode is positioned so as not to overlap the innermost protective tape in the radial direction of the electrode body.

[0009] The secondary battery according to the present disclosure can reduce the gap between the electrode plates near the winding start end of the positive electrode that occurs during charging and discharging, and can also suppress deterioration of the charge-discharge cycle characteristics.

[0010] 1 is a cross-sectional view of a secondary battery according to an embodiment of the present invention; FIG. 2 is a schematic plan view showing an example of a positive electrode before winding; and FIG. 3 is a schematic plan view showing a positive electrode after winding, as viewed from the direction of the winding axis.

[0011] Hereinafter, an example of an embodiment of a secondary battery according to the present disclosure will be described in detail with reference to the drawings. Note that configurations obtained by selectively combining the components of the multiple embodiments and modifications described below are included within the scope of the present disclosure.

[0012] In the embodiment described below, a cylindrical battery in which a wound electrode body is housed in a cylindrical outer can with a bottom is exemplified as the secondary battery, but the battery outer can is not limited to a cylindrical outer can, and the battery shape is not limited to the example shown in Figure 1. Other embodiments of the secondary battery according to the present disclosure include a prismatic battery with a prismatic outer can, and a pouch-type battery with an outer can made of a laminate sheet including a metal layer and a resin layer. Furthermore, although the electrolyte may be an aqueous electrolyte, a nonaqueous electrolyte is used in this embodiment.

[0013] Fig. 1 is a cross-sectional view of a secondary battery according to an embodiment. The secondary battery 1 shown in Fig. 1 includes an electrode assembly 10, a non-aqueous electrolyte, a cylindrical outer can 20 with a bottom that houses the electrode assembly 10 and the non-aqueous electrolyte, and a sealing member 30 that closes the opening of the outer can 20. The electrode assembly 10 has a positive electrode, a negative electrode, and a separator, and is structured such that the positive electrode and the negative electrode are wound with the separator interposed therebetween. Hereinafter, for convenience of explanation, the sealing member 30 side of the secondary battery 1 will be referred to as the top, and the can bottom 22 side of the outer can 20 will be referred to as the bottom.

[0014] The outer can 20 is a cylindrical metal container with a bottom and an open axial end (top end), and has a cylindrical sidewall 21 and a can bottom 22 that is circular in bottom view. The outer can 20 is generally made of a metal primarily containing iron, but may also be made of a metal primarily containing aluminum or the like. The outer can 20 also has a groove 23 formed along the circumferential direction of the sidewall 21. The groove 23 is formed near the opening of the outer can 20, at a position a predetermined distance from the opening edge (the top end of the outer can 20). The can bottom 22 of the outer can 20 may be formed with a safety valve for releasing gas in the event of an abnormality due to an increase in internal pressure.

[0015] The sealing body 30 has a disk shape as a whole and includes a cap 31, a current collector plate 32, and a gasket 33. The sealing body 30 is disposed on the grooved portion 23 of the outer can 20 and is fixed to the upper end of the outer can 20, thereby closing the opening of the outer can 20.

[0016] The cap 31 is a disk-shaped metal member that is exposed to the outside of the exterior can 20 and forms the top surface of the secondary battery 1. The cap 31 has a shape in which the radial center portion protrudes outward from the secondary battery 1. Wiring may be connected to the cap 31 when the secondary batteries 1 are modularized to form a battery pack.

[0017] The current collecting plate 32 is a metal member having a diameter similar to that of the cap 31, and is disposed closer to the electrode body 10 than the cap 31. The current collecting plate 32 has an opening 32a in its radial center and is formed in a ring shape. The cap 31 and the current collecting plate 32 are welded together, and the current collecting plate 32 is welded, for example, to a position closer to the outer periphery of the cap 31 than to its radial center. A protrusion 32b is formed on the current collecting plate 32, and the protrusion 32b forms the welded portion with the cap 31.

[0018] Gasket 33 is provided on the outer periphery of the laminate of cap 31 and current collector plate 32. Gasket 33 is a resin member that seals the inside of the battery and prevents contact between cap 31 and current collector plate 32 and outer can 20, thereby ensuring insulation between outer can 20 and sealing body 30. Gasket 33 is a ring-shaped resin member, and has an opening 33a formed in its radial center that overlaps with opening 32a of current collector plate 32 in the vertical direction.

[0019] The secondary battery 1 further includes a positive electrode lead 12 that connects the positive electrode to the current collector plate 32 of the sealing body 30, and an upper insulating plate 40 that is disposed between the electrode body 10 and the sealing body 30. In this embodiment, the positive electrode lead 12 electrically connects the positive electrode to the sealing body 30, and the sealing body 30 functions as a positive electrode external terminal. On the negative electrode side, the negative electrode core that constitutes the negative electrode is directly connected to the outer can 20, or the negative electrode lead electrically connects the negative electrode to the outer can 20, so that the outer can 20 functions as a negative electrode external terminal. The secondary battery 1 may also include a lower insulating plate that is disposed between the electrode body 10 and the can bottom 22.

[0020] In this embodiment, the upper insulating plate 40 includes a first insulating plate 41 and a second insulating plate 42 that is disposed closer to the sealing body 30 than the first insulating plate 41. The first insulating plate 41 is interposed between the electrode body 10 and the positive electrode lead 12 to prevent contact between the negative electrode and the positive electrode lead 12. The second insulating plate 42 is interposed between the grooved portion 23 of the outer can 20 and the positive electrode lead 12 to prevent contact between the outer can 20, which serves as the negative electrode external terminal, and the positive electrode lead 12.

[0021] The non-aqueous electrolyte has lithium ion conductivity and may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0022] 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

[0023] 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.

[0024] The positive electrode, negative electrode, and separator that make up the electrode assembly 10 are all strip-shaped, long bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 10. The negative electrode is formed, for example, to be slightly larger than the positive electrode in order to prevent lithium deposition. That is, the negative electrode is formed to be longer in both the length and width directions than the positive electrode. The separator is, for example, formed to be at least slightly larger than the positive electrode, and two separators are arranged to sandwich the positive electrode.

[0025] The positive electrode, negative electrode, and separator that constitute the electrode assembly 10 will be described in detail below.

[0026] FIG. 2 is a schematic plan view showing an example of a positive electrode before winding. In FIG. 2 , to clarify the configuration of the positive electrode, the insulating tape described below is shown in a perspective view and indicated by a dashed line. The positive electrode 11 shown in FIG. 2 is a strip-shaped, long body that includes a long positive electrode core 13 and a positive electrode mixture layer 14 disposed on the positive electrode core 13. The positive electrode core 13 can be, for example, a foil of a metal that is stable within the potential range of the positive electrode, such as aluminum, an aluminum alloy, stainless steel, or titanium, or a film with such a metal disposed on the surface layer. The positive electrode mixture layer 14 includes, for example, a positive electrode active material, a conductive agent, and a binder, and is provided on one or both sides of the positive electrode core 13.

[0027] As shown in Fig. 2 , a plurality of exposed portions 15, in which the surface of the positive electrode core 13 is exposed, are provided at predetermined intervals in the length direction of the positive electrode 11 at one width direction end portion of the positive electrode 11 as portions to which the positive electrode lead 12 is connected. The plurality of exposed portions 15 are arranged in a row in the length direction of the positive electrode 11 at one width direction end portion of the positive electrode 11 located on the upper end side of the electrode body 10. Although the positive electrode 11 shown in Fig. 2 has a plurality of exposed portions 15, the present invention is not limited thereto, and the number of exposed portions 15 may be one.

[0028] The length direction of the positive electrode 11 described above is a first direction along the left-right direction in FIG. 2 , and corresponds to the winding direction of the electrode body 10. The width direction of the positive electrode 11 described above is a second direction along the up-down direction in FIG. 2 , which is perpendicular to the first direction, and corresponds to the winding axis direction of the electrode body 10. The length (first direction length) of the positive electrode 11 is, for example, 10 to 100 times the width (second direction length) of the positive electrode 11, and is, for example, 600 mm to 6000 mm. The width of the positive electrode 11 is, for example, 50 mm to 100 mm.

[0029] The exposed portion 15 preferably has a rectangular shape when viewed from the front. The length of each exposed portion 15 along the width direction of the positive electrode 11 (hereinafter referred to as "width") is preferably 25% or less of the width of the positive electrode 11, and more preferably 10% to 20% of the width of the positive electrode 11. The width of each exposed portion 15 is, for example, 7 mm to 15 mm. The length of each exposed portion 15 along the length direction of the positive electrode 11 needs to be greater than the width of the positive electrode lead 12. For example, it is 5 mm to 70 mm. The spacing between each exposed portion 15 may be constant or may vary. In this specification, unless otherwise specified, the length of the exposed portion 15 means the length of the exposed portion 15 along the length direction of the positive electrode 11.

[0030] In the example shown in FIG. 2 , four exposed portions 15 are provided at one widthwise end of the positive electrode 11. Of these exposed portions 15, the exposed portion 15 located radially innermost of the electrode body 10 is referred to as the "innermost exposed portion 15a." This innermost exposed portion 15a is also the exposed portion closest to the winding start end 11a of the positive electrode (one longitudinal end of the positive electrode 11). When only one exposed portion 15 is provided on the positive electrode 11, that exposed portion 15 is the innermost exposed portion 15a. The innermost exposed portion 15a is desirably disposed at a predetermined distance from the winding start end 11a of the positive electrode 11, and is desirably disposed, for example, within a range of 5% to 20% of the length of the positive electrode 11 from the winding start end 11a of the positive electrode 11. With regard to the radius of the electrode body 10, it is desirable that the innermost exposed portion 15a be located within a circle having a radius of 5% to 10% of the radius of the electrode body 10 from the winding center of the electrode body 10 when viewed in a planar view of the electrode body 10 from the winding axis direction of the electrode body 10.

[0031] As shown in FIG. 2 , a plurality of positive electrode leads 12 are connected to the plurality of exposed portions 15. The positive electrode lead 12 connected to the innermost exposed portion 15 a among the plurality of positive electrode leads 12 is referred to as the "innermost positive electrode lead 12 a." The positive electrode lead 12 is, for example, a rectangular conductive member made of a metal containing aluminum as a main component. In this embodiment, one longitudinal end of each positive electrode lead 12 is connected to the exposed portion 15 of the positive electrode 11, and the other longitudinal end is connected to the current collector plate 32 of the sealing body 30. The connection position of the positive electrode lead 12 in the exposed portion 15 is not particularly limited, and the positive electrode lead 12 may be disposed within the range of the exposed portion 15 so as not to overlap with the positive electrode mixture layer 14. Examples of the connection method include ultrasonic welding and resistance welding.

[0032] 2 , a plurality of protective tapes 16 are provided on the positive electrode 11 so as to cover the exposed portions 15 and the positive electrode leads 12. Among the plurality of protective tapes 16, the protective tape 16 covering the innermost exposed portion 15 a and the innermost positive electrode lead 12 a connected to the innermost exposed portion 15 a is referred to as the “innermost protective tape 16 a.” By providing the protective tape 16 covering the exposed portions 15 and the positive electrode lead 12, it is possible to prevent the positive electrode lead 12 or the exposed portion 15 from coming into contact with the negative electrode and causing an internal short circuit, for example, when the separator breaks.

[0033] The protective tape 16 is made of, for example, an insulating material, such as a polyolefin resin such as polyethylene, polypropylene, or polybutylene, a polyester resin such as polyvinyl chloride or polyethylene terephthalate, or a resin tape such as polyimide, polyamide, or polyphenylene sulfide.

[0034] Fig. 3 is a schematic plan view showing the positive electrode after winding, as seen from the direction of the winding axis. In order to explain the positional relationship between the winding start end 11a of the positive electrode 11 and the innermost protective tape 16a, Fig. 3 shows only the positive electrode 11 wound several turns from the winding start end 11a of the positive electrode 11 and the innermost protective tape 16a, and omits other components.

[0035] As shown in FIG. 3 , the winding start end 11a of the positive electrode 11 is positioned so as not to overlap the innermost protective tape 16a in the radial direction r of the electrode assembly 10. That is, in a plan view of the positive electrode 11 (or electrode assembly 10) as viewed from the direction of the winding axis, when a straight line α connecting the winding center (point C) of the electrode assembly 10 and the winding start end 11a of the positive electrode 11 is extended radially outward from the winding start end 11a of the positive electrode 11, the extended line L should not intersect with the innermost protective tape 16a. This configuration reduces the gap near the winding start end of the positive electrode that occurs during charge and discharge and suppresses deterioration of the charge and discharge cycle characteristics. The mechanism behind this effect is not fully understood, but the following is presumed.

[0036] By positioning the winding start end 11a of the positive electrode 11 at a position where it does not overlap with the innermost protective tape 16a in the radial direction r of the electrode assembly 10, the radial overlap between the step caused by the winding start end 11a of the positive electrode 11 and the step caused by the thickness of the innermost protective tape 16a (including the thickness of the innermost positive electrode lead 12a) in the radial direction r of the electrode assembly 10 is eliminated, and it is presumed that this alleviates local stress applied near the winding start end 11a of the positive electrode 11 due to charge and discharge. This suppresses distortion between the electrode plates near the winding start end 11a of the positive electrode 11 due to charge and discharge, so even if a gap occurs between the electrode plates, its size is kept small. As a result, it is possible to suppress deterioration of the charge and discharge cycle characteristics.

[0037] The innermost protective tape 16a preferably has a length of 0.5 to 0.8 circumferences along the winding direction of the electrode body 10. In other words, the angle θ formed by a line β1 connecting the winding center (point C) of the electrode body 10 and the winding direction starting end E1 of the innermost protective tape 16a and a line β2 connecting the winding center (point C) of the electrode body 10 and the winding direction ending end E2 of the innermost protective tape 16a is １is preferably 72° or more and 180° or less. By ensuring that the innermost protective tape 16a satisfies the above length, when the separator breaks, it is possible to more effectively prevent the occurrence of an internal short circuit due to contact between the innermost exposed portion 15a and the innermost positive electrode lead 12a connected to the innermost exposed portion 15a and the negative electrode. Note that, typically, unless there is a design intention regarding the position of the winding start end 11a of the positive electrode 11, as the length of the innermost protective tape 16a in the winding direction increases, the innermost protective tape 16a and the winding start end 11a of the positive electrode 11 are likely to overlap in the radial direction r of the electrode body 10. In this embodiment, even if the innermost protective tape 16a has the above length, the electrode body 10 is designed so that the winding start end 11a of the positive electrode 11 does not overlap with the innermost protective tape 16a in the radial direction r of the electrode body 10.

[0038] The angle θ formed by a line α connecting the winding center (point C) of the electrode body 10 and the winding start end 11a of the positive electrode 11 and a line β2 connecting the winding center (point C) of the electrode body 10 and the winding direction end E2 of the innermost protective tape 16a ２ is preferably greater than 0° and equal to or less than 60°. When the battery is repeatedly charged and discharged, the electrode body 10 expands and contracts, and the positive electrode 11 ２ Therefore, the angle θ ２ By positioning the winding start end 11a of the positive electrode 11 so that the angle is greater than 0° and less than or equal to 60°, a sufficient distance is ensured between the winding start end 11a of the positive electrode 11 and the winding direction starting end E1 of the innermost protective tape 16a, and therefore, even if the positive electrode 11 moves along the winding direction due to repeated charge and discharge of the battery, the risk of the winding start end 11a of the positive electrode 11 overlapping the innermost protective tape 16a in the radial direction r of the electrode body 10 is reduced. As a result, even if the positive electrode 11 moves due to charge and discharge, the occurrence of a large inter-electrode gap near the winding start end 11a of the positive electrode 11 after movement is reduced, making it possible to further suppress deterioration of the charge and discharge cycle characteristics.

[0039] The average diameter of the electrode assembly 10 is, for example, 15 mm or more and 100 mm or less. It is preferable to apply the above-described technique based on the positional relationship between the winding start end 11a of the positive electrode 11 and the innermost protective tape 16a to an electrode assembly 10 whose average diameter falls within the above range. This can further suppress deterioration of charge / discharge cycle characteristics. A suitable example of the diameter of the electrode assembly 10 is 20 mm or more and 80 mm or less, or 40 mm or more and 70 mm or less. The average diameter of the electrode assembly 10 is calculated by measuring the diameter of the electrode assembly 10 at any 10 locations and calculating the measured values.

[0040] In this embodiment, it is preferable to apply the above-described technique based on the positional relationship between the winding start end 11a of the positive electrode 11 and the innermost protective tape 16a to four or more positive electrodes 11 provided with exposed portions 15 at one end in the width direction of the positive electrode 11 at predetermined intervals in the length direction of the positive electrode 11. This makes it possible to further suppress deterioration of the charge-discharge cycle characteristics.

[0041] The positive electrode mixture layer 14 is formed by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder to both sides of the positive electrode core 13, drying the coating, and compressing the coating. At this time, the positive electrode mixture slurry is applied intermittently to a portion corresponding to one end of the positive electrode 11 in the width direction, leaving a portion where the positive electrode mixture layer 14 is not present and the surface of the positive electrode core 13 is exposed. This portion becomes the exposed portion 15. The positive electrode mixture slurry is applied intermittently so as to satisfy the above-mentioned condition L1<L2.

[0042] Examples of the positive electrode active material contained in the positive electrode mixture layer 14 include lithium metal composite oxides containing transition metal elements such as Ni, Co, and Mn. Examples of metal elements contained in the lithium metal composite oxides include Li, 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. The crystalline structure of the lithium metal composite oxide is, for example, a layered rock salt structure belonging to the space group R-3m.

[0043] Examples of the conductive agent contained in the positive electrode mixture layer 14 include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, metal fibers, metal powder, conductive whiskers, etc. One type of conductive agent may be used alone, or multiple types may be used in combination.

[0044] Examples of the binder contained in the positive electrode mixture layer 14 include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide-based resins, acrylic-based resins, polyolefin-based resins, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), polyethylene oxide (PEO), etc. One type of binder may be used alone, or multiple types may be used in combination.

[0045] The negative electrode has a negative electrode core and a negative electrode mixture layer disposed on the negative electrode core. The negative electrode core can be made of a foil of a metal stable within the potential range of the negative electrode, such as copper, copper alloy, stainless steel, nickel, or nickel alloy, or a film with such a metal disposed on its surface. The negative electrode mixture layer contains, for example, a negative electrode active material and a binder, and is provided on both sides or one side of the negative electrode core. The negative electrode active material generally uses a carbon material that reversibly absorbs and releases lithium ions. The negative electrode active material may also use an element that alloys with Li, such as Si or Sn, or a material containing such an element. The binder can be the same as that used on the positive electrode side.

[0046] The separator is a porous sheet having ion permeability and insulating properties. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator include polyolefins such as polyethylene and polypropylene, and cellulose. The separator may have a single-layer structure or a multi-layer structure. The separator may have, for example, a multi-layer structure including a thermoplastic resin layer such as polyolefin and a cellulose fiber layer, a two-layer structure of polyethylene (PE) / polypropylene (PP), or a three-layer structure of PE / PP / PE.

[0047] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0048] Example 1 Preparation of Positive Electrode A lithium metal composite oxide as a positive electrode active material, acetylene black, and polyvinylidene fluoride (PVdF) were mixed in a solids mass ratio of 98:1:1, and a positive electrode mixture slurry was prepared using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. The positive electrode mixture slurry was applied to both sides of a positive electrode core made of aluminum foil to form a coating film, and the coating film was dried and compressed. The positive electrode core was then cut to a predetermined electrode size to obtain a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode core.

[0049] In Example 1, the positive electrode mixture slurry was intermittently applied to one end of the positive electrode in the width direction, forming eight exposed portions aligned in a row in the length direction of the positive electrode. The innermost exposed portion of these exposed portions was located 180 mm away from the winding start end of the positive electrode in the length direction of the positive electrode. The spacing between each exposed portion was set to 350 mm. The positive electrode had a length of 3000 mm and a width of 65 mm, and each exposed portion had a width of 10 mm and a length of 20 mm.

[0050] [Fabrication of Negative Electrode] A mixture of natural graphite and a silicon-containing material (a composite material in which a fine Si phase is dispersed in a silicon oxide phase) at a mass ratio of 98:2 was used as the negative electrode active material. The negative electrode active material, a dispersion of carboxymethylcellulose sodium (CMC-Na), and styrene-butadiene rubber (SBR) were mixed at a solids mass ratio of 100:1:1, and water was used as the dispersion medium to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was applied to both sides of a negative electrode core made of copper foil, and the coating was dried. The coating was then rolled using a roller and cut to a predetermined electrode size to obtain a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode core. An exposed portion in which the surface of the negative electrode core was exposed was provided in a portion of the negative electrode.

[0051] [Preparation of Non-Aqueous Electrolyte] LiPF 6 was dissolved in a mixed solvent of ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4 (25° C.).６ was dissolved in the solution at a concentration of 1.2 mol / L to prepare a non-aqueous electrolyte solution.

[0052] [Preparation of Test Cell] An aluminum lead serving as a positive electrode lead was welded to the exposed portion of the positive electrode, and a nickel lead serving as a negative electrode lead was welded to the exposed portion of the negative electrode. Protective tape was then placed to cover the exposed portion of the positive electrode and the aluminum lead, and protective tape was then placed to cover the exposed portion of the negative electrode and the nickel lead. The positive and negative electrodes were then spirally wound with a polyolefin separator interposed therebetween to prepare a wound electrode assembly with an average diameter of 40 mm. In the prepared electrode assembly, the winding start end of the positive electrode was positioned so as not to overlap in the radial direction of the electrode assembly with the innermost protective tape covering the innermost exposed portion of the positive electrode and the innermost positive electrode lead connected to the innermost exposed portion. The angle θ between the line connecting the winding center of the electrode assembly and the winding start end of the innermost protective tape and the line connecting the winding center of the electrode assembly and the winding end of the innermost protective tape was １ is 120°. That is, the innermost protective tape has a length of about 0.77 circumference along the winding direction of the electrode body. Also, the angle θ between the line connecting the winding center of the electrode body and the winding start end of the positive electrode and the line connecting the winding center of the electrode body and the winding end end of the innermost protective tape is ２ is 5°.

[0053] This electrode assembly was housed in a cylindrical outer can with a bottom, and the non-aqueous electrolyte solution was poured into it. The opening of the outer can was then sealed with a sealing member to obtain a cylindrical test cell.

[0054] Example 2 In the production of the electrode body, the angle θ formed by the line connecting the winding center of the electrode body and the winding start end of the positive electrode and the line connecting the winding center of the electrode body and the winding direction end end of the innermost protective tape was ２ A test cell was prepared in the same manner as in the example except that the angle was set to 15°.

[0055] Example 3 In the production of the electrode body, the angle θ formed by the line connecting the winding center of the electrode body and the winding start end of the positive electrode and the line connecting the winding center of the electrode body and the winding direction end end of the innermost protective tape was ２ A test cell was prepared in the same manner as in the example except that the angle was set to 90°.

[0056] <Comparative Example 1> In the production of the electrode body, the angle θ formed by the line connecting the winding center of the electrode body and the winding start end of the positive electrode and the line connecting the winding center of the electrode body and the winding direction end end of the innermost protective tape was ２ A test cell was prepared in the same manner as in Example 1, except that the winding angle was set to 0° and the winding start end of the positive electrode was positioned so as to overlap the innermost protective tape (specifically, the end of the innermost protective tape in the winding direction) in the radial direction of the electrode body.

[0057] <Comparative Example 2> In the production of the electrode body, the angle θ formed by the line connecting the winding center of the electrode body and the winding start end of the positive electrode and the line connecting the winding center of the electrode body and the winding direction end end of the innermost protective tape was ２ A test cell was produced in the same manner as in Example 1, except that the winding angle was set to −20° and the winding start end of the positive electrode was positioned so as to overlap the innermost protective tape in the radial direction of the electrode body.

[0058] [Charge-Discharge Cycle Test] Each test cell was charged at a constant current of 0.3 It at a temperature of 25°C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.02 It. Subsequently, the test cell was discharged at a constant current of 0.5 It until the battery voltage reached 2.5 V. This cycle of charge and discharge was counted as one cycle, and 500 cycles were performed, and the capacity retention rate was calculated using the following formula. The results are shown in Table 1. Note that a higher capacity retention rate indicates that the deterioration of the charge-discharge cycle characteristics was more suppressed. Capacity retention rate (%) = (discharge capacity at 500th cycle / discharge capacity at 1st cycle) x 100

[0059] [Evaluation of the gap between the plates near the winding start end of the positive electrode] Using the test cell after 500 cycles of charge and discharge, a CT image of the upper part of the electrode body as viewed from the winding axis direction of the electrode body was obtained. From the CT image, the presence or absence of a gap between the plates near the winding start end of the positive electrode was confirmed, and if a gap existed, the size of the gap (maximum radial length of the gap) was measured. The results are shown in Table 1.

[0060]

[0061] As shown in Table 1, all of Examples 1 to 3 exhibited smaller inter-plate gaps near the winding start end of the positive electrode and higher capacity retention rates during charge-discharge cycles than Comparative Examples 1 and 2. These results suggest that in an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed between them, the winding start end of the positive electrode is positioned so as not to overlap in the radial direction of the electrode assembly with the innermost protective tape covering the innermost exposed portion of the positive electrode and the innermost positive electrode lead connected to the innermost exposed portion. This reduces the inter-plate gap near the winding start end of the positive electrode that occurs during charge-discharge and suppresses deterioration of the charge-discharge cycle characteristics. Furthermore, among Examples 1 to 3, Examples 1 and 2 exhibited smaller inter-plate gaps near the winding start end of the positive electrode and higher capacity retention rates during charge-discharge cycles than Example 3. These results suggest that the angle θ between the line connecting the winding center of the electrode assembly and the winding start end of the positive electrode and the line connecting the winding center of the electrode assembly and the end of the winding direction of the innermost protective tape of the positive electrode is ２ By setting the angle within the range of more than 0° and not more than 60°, it is possible to further reduce the gap between the electrode plates near the winding start end of the positive electrode that occurs during charge and discharge, and to further suppress deterioration of the charge and discharge cycle characteristics.

[0062] [Notes] (1) A secondary battery comprising an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, a positive electrode lead, and a protective tape, wherein the positive electrode has a long positive electrode core and a positive electrode mixture layer disposed on the positive electrode core, one end of the positive electrode in the width direction is provided with one or more exposed portions where the surface of the positive electrode core is exposed, the exposed portions include an innermost exposed portion located innermost in the radial direction of the electrode assembly, the positive electrode lead includes an innermost positive electrode lead connected to the innermost exposed portion, the protective tape includes an innermost protective tape covering the innermost exposed portion and the innermost positive electrode lead, and a winding start end of the positive electrode is positioned so as not to overlap the innermost protective tape in the radial direction of the electrode assembly. (2) The secondary battery according to (1), wherein the innermost protective tape has a length of 0.5 to 0.8 revolutions along the winding direction of the electrode assembly. (3) The secondary battery according to (1) or (2) above, wherein an angle formed between a line connecting the winding center of the electrode body and the winding start end of the positive electrode and a line connecting the winding center of the electrode body and the winding direction end end of the innermost protective tape is greater than 0° and not greater than 60°. (4) The secondary battery according to any one of (1) to (3) above, wherein an average diameter of the electrode body is 15 mm or more and 100 mm or less. (5) The secondary battery according to any one of (1) to (4) above, wherein four or more exposed portions are provided at a predetermined interval in the length direction of the positive electrode at one end in the width direction of the positive electrode core.

[0063] REFERENCE SIGNS LIST 1 secondary battery, 10 electrode body, 11 positive electrode, 11a winding start end of positive electrode, 12 positive electrode lead, 13 positive electrode core, 14 positive electrode mixture layer, 15 exposed portion, 15a innermost exposed portion, 16 protective tape, 16a innermost protective tape, 20 outer can, 21 side wall, 22 can bottom, 23 grooved portion, 30 sealing body, 31 cap, 32 current collecting plate, 32a, 33a opening, 32b convex portion, 33 gasket, 40 upper insulating plate, 41 first insulating plate, 42 second insulating plate.

Claims

1. A secondary battery comprising an electrode body in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, a positive electrode lead, and a protective tape, wherein the positive electrode has a long positive electrode core and a positive electrode mixture layer arranged on the positive electrode core, one end of the positive electrode in the width direction is provided with one or more exposed portions at which the surface of the positive electrode core is exposed, the exposed portions include an innermost exposed portion located at the innermost radial direction of the electrode body, the positive electrode lead includes an innermost positive electrode lead connected to the innermost exposed portion, the protective tape includes an innermost protective tape covering the innermost exposed portion and the innermost positive electrode lead, and the winding start end of the positive electrode is arranged in a position not overlapping with the innermost protective tape in the radial direction of the electrode body.

2. The secondary battery according to claim 1, wherein the innermost protective tape has a length of 0.5 to 0.8 revolutions along the winding direction of the electrode assembly.

3. A secondary battery as described in claim 1 or 2, wherein the angle between a straight line connecting the winding center of the electrode body and the winding start end of the positive electrode and a straight line connecting the winding center of the electrode body and the winding direction end end of the innermost protective tape is greater than 0° and less than 60°.

4. The secondary battery according to claim 1 or 2, wherein the average diameter of the electrode body is 15 mm or more and 100 mm or less.

5. The secondary battery according to claim 1 or 2, wherein four or more of the exposed portions are provided at one end in the width direction of the positive electrode core body at predetermined intervals in the length direction of the positive electrode.

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