Cylindrical lithium-ion secondary battery

Abstract

The present invention provides a cylindrical lithium-ion secondary battery having a positive electrode plate that constitutes an electrode body. The positive electrode plate has a positive electrode mixture layer formed on the surface of a positive electrode core, a positive electrode tab bonded to an exposed surface formed on only one side, in the electrode plate transverse direction, of at least a part of the positive electrode plate in the electrode plate longitudinal direction, and an insulating tape applied to the positive electrode plate to cover the exposed surface. The positive electrode mixture layer includes a raised part with a maximum thickness that is 10% or more greater than the average thickness of the positive electrode core in the vicinity of a first exposed end of the exposed surface in the electrode plate longitudinal direction. The insulating tape covers the exposed surface and a portion of the raised part beyond a maximum thickness position M in the electrode plate longitudinal direction, the portion beyond the maximum thickness position going farther beyond the position where the thickness is less than 1.05 times the average thickness of the positive electrode mixture layer.

Description

Cylindrical lithium-ion secondary battery

[0001] The present disclosure relates to cylindrical lithium-ion secondary batteries.

[0002] Conventionally, cylindrical lithium-ion secondary batteries have been known, each comprising an electrode assembly in which a positive electrode plate and a negative electrode plate are wound with a separator interposed therebetween, and an outer can containing the electrode assembly and a nonaqueous electrolyte. In recent years, there has been a demand for secondary batteries to have higher input / output capabilities for use in electric vehicles and the like. To this end, in lithium-ion secondary batteries, a method has been proposed in which multiple current collecting leads (current collecting tabs) are bonded to multiple exposed surfaces of a substrate, where the composite material is removed from the positive electrode plate, to improve battery characteristics, as described in Patent Document 1. In this case, the exposed substrate surface is formed only on one side of the positive electrode plate in the short direction of the electrode plate, thereby increasing the proportion of the composite material in the positive electrode plate.

[0003] Japanese Patent Application Laid-Open No. 2000-243376

[0004] However, when the exposed surface of the core is formed only on one side of the positive electrode plate in the short direction, the application of the composite material becomes unstable near the end of the positive electrode plate in the long direction, which tends to cause the composite material to bulge. This causes dendritic lithium deposition near the position of the negative electrode plate facing the bulge. This lithium deposition can cause short circuits and deterioration of the secondary battery.

[0005] An object of the present disclosure is to suppress Li deposition in a cylindrical lithium ion secondary battery configured such that an exposed surface of a substrate is formed on only one side of a positive electrode plate in the short side direction of the electrode plate.

[0006] The cylindrical lithium-ion secondary battery according to the present disclosure includes an electrode assembly in which a positive electrode plate and a negative electrode plate are wound with a separator interposed therebetween, and an outer can that houses the electrode assembly. The positive electrode plate includes a positive electrode core, a positive electrode mixture layer formed on the surface of the positive electrode core, a positive electrode tab formed on only one side in the short direction of the electrode plate at least in a part of the long direction of the electrode plate, the positive electrode tab being joined to an exposed surface where the positive electrode core is exposed and extending from one end of the positive electrode core in the short direction of the electrode plate, and an insulating tape attached to the positive electrode plate so as to cover the positive electrode tab superimposed on the exposed surface and the exposed surface. The positive electrode mixture layer has a raised portion on at least one exposed surface of the positive electrode core near a first exposed end in the longitudinal direction of the electrode plate, and the raised portion has a maximum thickness that is 10% or more greater than the average thickness of the portion of the positive electrode mixture layer excluding the raised portion. The insulating tape covers the exposed surface and the raised portion beyond the maximum thickness position in the longitudinal direction of the electrode plate, and the portion beyond the maximum thickness position also extends beyond a position where the thickness becomes smaller than 1.05 times the average thickness of the positive electrode mixture layer. This is a cylindrical lithium-ion secondary battery.

[0007] In the cylindrical lithium-ion secondary battery according to the present disclosure, in a configuration in which an exposed surface of the core is formed only on one side of the positive electrode plate in the short direction of the electrode plate, the protruding portion of the positive electrode mixture layer, which is a cause of Li deposition, can be covered almost entirely with insulating tape, thereby suppressing Li deposition.

[0008] 3 is a cross-sectional view taken along the axial direction of a cylindrical lithium-ion secondary battery according to an embodiment; FIG. 4 is a perspective view showing an expanded view of a portion of an electrode body constituting a cylindrical lithium-ion secondary battery according to an embodiment; FIG. 5 is a schematic expanded view of a positive electrode plate according to an embodiment; FIG. 6 is an enlarged view of the A-A cross section of FIG. 2; FIG. 7 is an enlarged view corresponding to one side in the thickness direction of the electrode plate of FIG. 4, of a positive electrode plate constituting a cylindrical lithium-ion secondary battery according to another embodiment; FIG. 8 is a diagram showing the results of measuring the relationship between the distance from the end of the exposed surface of the core body toward the outside in the positive electrode plate and the thickness of the positive electrode mixture layer; FIG. 9 is a schematic view showing an enlarged view of portion B of FIG. 3, with the positive electrode tab omitted; FIG. 10 is a diagram showing the results of determining the relationship between the thickness of the positive electrode mixture layer at the end of the insulating tape and the state of Li precipitation in a plurality of test positive electrode plates; FIG. 11 is a diagram corresponding to FIG. 3, of a cylindrical lithium-ion secondary battery according to another embodiment;

[0009] The present inventors have found that in a cylindrical lithium-ion secondary battery, when the exposed surface of the substrate is formed only on one side of the positive electrode plate in the short direction, a raised portion of the mixture layer is likely to occur near the end of the exposed surface of the positive electrode plate in the long direction of the plate, and that this makes it easy for Li to precipitate near the raised portion.

[0010] As a result of extensive research, the inventors have found that in a configuration in which the exposed surface of the core is formed only on one side of the positive electrode plate in the short direction of the electrode plate, the above-mentioned precipitation can be suppressed by restricting the attachment position of the insulating tape attached to the positive electrode plate so as to cover the exposed surface and the positive electrode tab overlapping the exposed surface. Specifically, when the maximum thickness of the raised portion is 10% or more greater than the average thickness of the portion excluding the raised portion of the positive electrode mixture layer, the insulating tape can be configured to cover the exposed surface and the raised portion beyond the maximum thickness position in the longitudinal direction of the electrode plate and beyond a position where the portion beyond the maximum thickness position becomes thinner than 1.05 times the average thickness of the positive electrode mixture layer, thereby suppressing the above-mentioned precipitation.

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, specific shapes, materials, numerical values, directions, etc. are examples for facilitating understanding of the present invention and can be appropriately changed according to the specifications of the cylindrical lithium-ion secondary battery. Furthermore, the term "substantially" is used below to mean, for example, not only completely the same, but also substantially the same. Furthermore, when multiple embodiments and variations are included below, it is initially assumed that their characteristic features will be used in appropriate combination.

[0012] Fig. 1 is a cross-sectional view taken along the axial direction of a cylindrical lithium-ion secondary battery 10 according to an embodiment. Fig. 2 is a perspective view showing a portion of an electrode body 14 constituting the cylindrical lithium-ion secondary battery 10 in an expanded state. Fig. 3 is a schematic expanded view of a positive electrode plate 11. Fig. 4 is a cross-sectional view taken along the line A-A in Fig. 2.

[0013] As illustrated in FIGS. 1 to 4 , a cylindrical lithium-ion secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte (not shown), an outer can 15, and a sealing member 16. The wound electrode assembly 14 includes a positive electrode plate 11, a negative electrode plate 12, and a separator 13. As shown in FIG. 2 (described later), the positive electrode plate 11 and the negative electrode plate 12 are spirally wound with the separator 13 interposed therebetween. Hereinafter, one axial side of the electrode assembly 14 may be referred to as the "upper" side, and the other axial side as the "lower" side. The nonaqueous electrolyte includes a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent. Hereinafter, the cylindrical lithium-ion secondary battery 10 will be referred to as a secondary battery 10.

[0014] 2, four positive electrode tabs 20 are joined and electrically connected to the positive electrode plate 11 as multiple current collecting tabs. Each positive electrode tab 20 is a conductive member for electrically connecting a positive electrode core 11a (FIG. 3) (described later) constituting the positive electrode plate 11 to a positive electrode terminal, and extends from the upper end of the positive electrode core 11a of the electrode body 14 to one side (upward) in the axial direction α.

[0015] Two negative electrode tabs 21a, 21b are joined and electrically connected to the negative electrode plate 12. Each negative electrode tab 21a, 21b is a conductive member for electrically connecting the negative electrode core constituting the negative electrode plate 12 to the bottom of the outer can 15, which serves as the negative electrode terminal, and extends from the lower end of the negative electrode core of the electrode body 14 to the other side (downward) in the axial direction α. ​​One negative electrode tab 21a is joined to the winding start end of the negative electrode plate 12, and the other negative electrode tab 21b is joined to the winding end end of the negative electrode plate 12. One of the two negative electrode tabs 21a, 21b may be omitted.

[0016] The negative electrode plate 12 is formed to be slightly larger than the positive electrode plate 11, and is formed to be longer in the longitudinal direction and the lateral direction than the positive electrode plate 11. In addition, the two separators 13 are formed to be at least slightly larger than the positive electrode plate 11, and are arranged to sandwich the positive electrode plate 11.

[0017] Referring to FIG. 3, the positive electrode plate 11 has a strip-shaped positive electrode core 11a and a positive electrode mixture layer 11b formed on the surface of the positive electrode core 11a. The positive electrode core 11a may be, for example, a foil of a metal such as aluminum, or a film with such a metal disposed on its surface. A suitable positive electrode core 11a is a foil of a metal primarily composed of aluminum or an aluminum alloy. The thickness of the positive electrode core 11a is, for example, 10 μm to 30 μm. In this example, the positive electrode mixture layer 11b is formed on both sides of the positive electrode core 11a, but may also be formed on only one surface in the thickness direction of the positive electrode core 11a.

[0018] The positive electrode mixture layer 11b preferably contains a positive electrode active material, a conductive agent, and a binder. The positive electrode plate 11 is produced by applying a positive electrode mixture slurry containing the positive electrode active material, the conductive agent, the binder, and a solvent such as N-methyl-2-pyrrolidone (NMP) to both sides of the positive electrode substrate 11a, followed by drying and rolling.

[0019] The positive electrode active material can be exemplified by a lithium transition metal composite oxide containing a transition metal element such as Co, Mn, or Ni. ｘ CoO ２ , Li ｘ NiO ２ , Li ｘ MnO ２, Li ｘ Co ｙ Ni １－ｙ O ２ , Li ｘ Co ｙ M １－ｙ O ｚ , Li ｘ Ni １－ｙ M ｙ O ｚ , Li ｘ Mn ２ O ４ , Li ｘ Mn ２－ｙ M ｙ O ４ , LiMPO ４ , Li ２ MPO ４ F (M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, and 0<x≦1.2, 0<y≦0.9, and 2.0≦z≦2.3). These may be used alone or in combination.

[0020] Examples of the conductive agent include carbon black (CB) such as acetylene black (AB) and Ketjen black, and carbon materials such as graphite. Examples of the binder include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide (PI), acrylic resins, and polyolefin resins. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like. These may be used alone or in combination of two or more.

[0021] As shown in FIG. 3 , exposed surfaces 11c, in which the surface of the metal constituting the positive electrode core 11a is exposed, are formed only at one end of the positive electrode plate 11 in the electrode plate width direction γ at four positions in the electrode plate length direction β. In FIG. 3 , the positive electrode mixture layer 11b is indicated by hatched areas. Four positive electrode tabs 20 are joined to the four exposed surfaces 11c by, for example, ultrasonic welding. The positive electrode tabs 20 are covered with insulating tape 24, thereby suppressing short circuits between the positive electrode plate 11 and the negative electrode plate 12. The insulating tape 24 covers the entire exposed surface 11c of the positive electrode core 11a as shown in FIG. 3 .

[0022] The insulating tape 24 has a base material made of, for example, an insulating resin and an adhesive layer formed on one side of the base material, and is substantially non-conductive.

[0023] There are no particular limitations on the material of the positive electrode tab 20. The positive electrode tab 20 is preferably made of a metal containing aluminum as a main component. The actual number of turns of the positive electrode plate 11 and the negative electrode plate 12 is significantly greater than that shown in Figures 1 and 2.

[0024] The negative electrode plate 12 has a strip-shaped negative electrode core and negative electrode mixture layers formed on both sides of the negative electrode core. The negative electrode core may be made of, for example, a foil of a metal such as copper, or a film with such a metal disposed on its surface. The thickness of the negative electrode core is, for example, 5 μm to 30 μm.

[0025] The negative electrode mixture layer preferably contains a negative electrode active material and a binder. The negative electrode plate 12 is produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, water, and the like to both sides of a negative electrode core, followed by drying and rolling.

[0026] The negative electrode active material is not particularly limited as long as it can reversibly absorb and release lithium ions. For example, carbon materials such as natural graphite and artificial graphite, metals that alloy with lithium such as Si and Sn, or alloys or composite oxides containing these, can be used. The binder contained in the negative electrode active material layer is, for example, the same resin as that used in the positive electrode plate 11. When preparing the negative electrode mixture slurry using an aqueous solvent, styrene-butadiene rubber (SBR), CMC or a salt thereof, polyacrylic acid or a salt thereof, polyvinyl alcohol, etc. can be used. These materials may be used alone or in combination of two or more.

[0027] The non-aqueous electrolyte has ion conductivity (e.g., lithium ion conductivity). The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte is not limited to a liquid electrolyte (nonaqueous electrolytic solution), and may be a solid electrolyte using a gel polymer or the like. The secondary battery 10 is preferably a lithium ion battery. The electrolyte salt may be, for example, LiBF ４ , LiPF ６ Examples of the non-aqueous solvent include esters such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and methyl propionate (MP), ethers, nitriles, amides, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted product in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0028] Examples of halogen-substituted compounds include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP). From the viewpoint of suppressing a decrease in the charge-discharge cycle characteristics of a nonaqueous electrolyte secondary battery or improving input characteristics, the nonaqueous electrolyte preferably contains 5% by mass or more of FEC, and more preferably 5% by mass to 15% by mass of FEC, relative to the mass of the nonaqueous electrolyte.

[0029] The separator 13 (FIGS. 1 and 2) 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. The separator 13 is preferably made of an olefin resin such as polyethylene or polypropylene. The thickness of the separator 13 is, for example, 10 μm to 50 μm. Separators 13 tend to be thinner as batteries become higher in capacity and power output. The separator 13 has a melting point of, for example, about 130°C to 180°C.

[0030] Then, tape (not shown) is attached to the outermost surface of the separator 13 , which is the outermost surface of the electrode body 14 , so as to fix the winding end of the separator 13 to the outermost surface of the separator 13 .

[0031] In the example shown in FIG. 1 , an outer can 15 and a sealing body 16 constitute a metal battery case that houses an electrode assembly 14 and a nonaqueous electrolyte. The sealing body 16 is formed by stacking an upper metal terminal cap (sealing plate 17) and a lower metal current collector plate 18 with a through hole 18a, sandwiching a metal plate 19 between them. The sealing plate 17 is hat-shaped, with a bottomed, short cylindrical portion 17a bulging upward at its center. The outer can 15 has an annular grooved portion 15b formed by radially inwardly recessing the upper end of the cylindrical portion 15a around the entire circumference. The sealing body 16 is fitted into the opening at the upper end of the cylindrical portion 15a via a gasket 27, and the upper end of the cylindrical portion 15a is crimped radially inward while being engaged with the upper surface of the grooved portion 15b. As a result, the sealing body 16 is crimped and fixed to the inside of the open end of the outer can 15 via the gasket 27 on the outer periphery.

[0032] Insulating plates 28, 29 are provided above and below the electrode body 14. The four positive electrode tabs 20 extend toward the sealing body 16 through the through holes in the upper insulating plate 28, pass through the through holes 18a in the current collector plate 18, and are sandwiched and joined between the current collector plate 18 and the outer peripheral portion of the metal plate 19. In the secondary battery 10, the sealing plate 17 electrically connected to the current collector plate 18 and the metal plate 19 serves as the positive electrode terminal.

[0033] Of the two negative electrode tabs 21a, 21b, the negative electrode tab 21a joined to the winding start end of the negative electrode plate 12 passes through a through hole in the lower insulating plate 29, is bent at a substantially right angle toward the central axis of the battery, and extends in the opposite direction relative to the central axis. Of the two negative electrode tabs 21a, 21b, the other negative electrode tab 21b joined to the winding end end of the negative electrode plate 12 passes outside the insulating plate 29, and is then bent so as to overlap the negative electrode tab 21a. The overlapping portions of the negative electrode tabs 21a, 21b are joined to the inner surface of the bottom of the outer can 15. As a result, each negative electrode tab 21a, 21b is electrically connected to the outer can 15, which serves as a negative electrode terminal.

[0034] As described above, the positive electrode plate 11 has an exposed surface 11c, in which the metal surface constituting the positive electrode core 11a is exposed, at only one end of the positive electrode plate 11 in the transverse direction γ at a plurality of positions in the longitudinal direction β. It has been found that this configuration tends to cause a raised portion of the positive electrode mixture layer near the longitudinal end of the exposed surface 11c of the positive electrode plate 11. This has also been found to cause the disadvantage of Li deposition being likely to occur near the raised portion. In this embodiment, the application position of the insulating tape 24 is restricted to prevent this disadvantage. This will be described below with reference to FIGS. 4 to 8 .

[0035] Fig. 4 is an enlarged view of the A-A cross section in Fig. 2. Fig. 5 is an enlarged view of one side in the electrode plate thickness direction in Fig. 4. As shown in Figs. 4 and 5, the positive electrode plate 11 has exposed surfaces 11c on both sides opposite each other in the thickness direction, where the positive electrode core 11a is exposed. At this time, on each of both sides of the positive electrode plate 11, a raised portion 30 of the positive electrode mixture layer 11b is formed near one or the other end of the exposed surface 11c in the electrode plate longitudinal direction β.

[0036] It has been found that the raised portion 30 is likely to occur due to unstable application of the positive electrode mixture layer 11b. For example, referring to FIG. 3 , when the positive electrode plate 11 has an exposed surface 11c only on one end in the short-side direction, a positive electrode mixture slurry applicator applies the slurry from one side to the other in the long-side direction of the positive electrode substrate 11a. In this case, the applicator often applies the slurry by discharging it from multiple discharge ports aligned in the short-side direction of the positive electrode plate 11 while moving the positive electrode substrate 11a in the long-side direction of the electrode substrate. At this time, the discharge of the slurry from some discharge ports on the exposed surface 11c is intermittently stopped. However, the discharge of the slurry continues in the portion adjacent to the exposed surface in the short-side direction of the electrode plate. This slurry flows into the portion of the exposed surface 11c before the application was stopped, which is thought to result in the raised portion 30 occurring near one or the other end of the exposed surface 11c in the long-side direction of the electrode plate.

[0037] In Fig. 4 , the positive electrode mixture slurry is applied from left to right on one thickness-wise side (upper side in Fig. 4 ) of the positive electrode plate 11, and the positive electrode mixture slurry is applied from right to left on the other thickness-wise side (lower side in Fig. 4 ) of the positive electrode plate 11. A positive electrode tab 20 is joined to the exposed surface 11c on one thickness-wise side of the positive electrode plate 11, and insulating tape 24 is attached to the positive electrode plate 11 so as to cover the maximum thickness position M of the raised portion 30. The configuration of one thickness-wise side (upper side in Fig. 4 ) of the positive electrode plate 11 will be mainly described below.

[0038] In this example, the positive electrode mixture layer 11b has a raised portion 30 in the positive electrode core 11a near a first exposed end E1, which is an end on one side of each exposed surface 11c in the electrode plate longitudinal direction. The maximum thickness of each raised portion 30 is 10% or more greater than the average thickness Ta of the positive electrode mixture layer 11b excluding all of the raised portions 30. For example, the average thickness Ta can be determined by using computed tomography (CT) to photograph a cross section of a portion of the positive electrode plate 11 that is different in the electrode plate transverse direction from the exposed surface 11c, measuring the thickness of the positive electrode mixture layer 11b at multiple positions in the electrode plate longitudinal direction from the photograph, and then measuring the thicknesses of the positive electrode mixture layer 11b at multiple measured positions in the electrode plate longitudinal direction.

[0039] The insulating tape 24 has both ends attached to the surface of the positive electrode mixture layer 11b, spanning the exposed surface 11c in the longitudinal direction of the electrode plate. The insulating tape 24 also covers the exposed surface 11c and the raised portion 30 beyond the maximum thickness position M in the longitudinal direction of the electrode plate, and the portion beyond the maximum thickness position M also extends beyond a position where the thickness becomes smaller than 1.05 times the average thickness Ta of the positive electrode mixture layer 11b. The "position where the thickness becomes smaller than 1.05 times the average thickness Ta" refers to the surface position of the portion where the thickness becomes smaller than 1.05 times the average thickness Ta.

[0040] 4 shows a case where the thickness of the protruding portion 30 at point P is 1.1 times (1.1Ta) the average thickness Ta of the positive electrode mixture layer 11b, and the thickness at maximum thickness position M is greater than 1.1Ta, and shows a thickness position at point Q where the thickness is 1.05 times (1.05Ta) the average thickness Ta. Even in this case, by covering the protruding portion 30 with the insulating tape 24 as described above, it is possible to cover substantially the entire protruding portion 30 of the positive electrode mixture layer 11b, which is a cause of Li deposition, with the insulating tape 24. This makes it possible to suppress Li deposition.

[0041] Fig. 5 is an enlarged view of one side in the thickness direction of the positive electrode plate 11e constituting a secondary battery of another example of the embodiment, corresponding to Fig. 4. In the configuration of this example, the portion of the insulating tape 24a that extends beyond the exposed surface 11c, the maximum thickness position M, and the position where the average thickness Ta is reached in the longitudinal direction of the electrode plate extends further toward the opposite side from the exposed surface 11c than in the case of Fig. 4.

[0042] Let us consider the first exposed end E1 of the exposed surface 11c as the reference position X0, and measure the thickness of the positive electrode mixture layer 11b at predetermined distances in the longitudinal direction of the electrode plate from the maximum thickness position M of the protruding portion 30 toward the first tape end 31 of the insulating tape 24 on the protruding portion 30 side. Here, the predetermined distance is, for example, 10 μm. Among the multiple measurement positions measured sequentially toward the first tape end 31, the length from the reference position X0 to the previous measurement position at which the thickness of the positive electrode mixture layer 11b at the previous measurement position becomes the same as the thickness of the positive electrode mixture layer 11b at the current measurement position is defined as X1. Furthermore, the length from the reference position X0 to the first tape end 31 is defined as X2. In this example, the attachment position of the first tape end 31 is regulated so that X2 > X1 × 1.1.

[0043] In this configuration of the present example, the surface position of the positive electrode mixture layer 11b corresponding to the length X1 can be estimated to be the boundary position between the raised portion 30 and the portion having the average thickness Ta. This allows the insulating tape 24 to cover the surface of the positive electrode mixture layer 11b up to a position that is 1.1 times the length X1 from the reference position X0 to the boundary position. This allows the insulating tape 24 to more reliably cover the entire raised portion 30. In this example, the other configurations and functions are the same as those of the configurations of FIGS. 1 to 4.

[0044] Fig. 6 is a diagram showing the results of measuring the relationship between the distance from the end of the exposed surface 11c of the positive electrode core 11a toward the outside and the thickness of the positive electrode mixture layer 11b in the positive electrode plate 11. Fig. 7 is a schematic diagram showing an enlarged view of part B in Fig. 3 with the positive electrode tab omitted.

[0045] 6 and 7, the inventors measured the relationship between the distance from the end E of the exposed surface 11c of the positive electrode substrate 11a of the positive electrode plate 11 toward the outside (left side in FIG. 7) and the thickness of the positive electrode mixture layer 11b. In FIG. 7, the exposed surface 11c is shown as a sandy area. The insulating tape 24 covers the exposed surface 11c and actually protrudes beyond the edge of the positive electrode plate 11 in the short direction of the plate, but the protruding portion is not shown in FIG.

[0046] The thickness of the positive electrode mixture layer 11b was measured on the positive electrode plate 11 at portions facing outward at both ends of the exposed surface 11c in the longitudinal direction of the electrode plate, and also on the other side of the positive electrode plate 11. The dashed line c1, solid line c2, two-dot chain line c3, and one-dot chain line c4 in Fig. 6 correspond to the thicknesses of the positive electrode mixture layer 11b at positions D1, D2, D3, and D4 of the positive electrode plate in Fig. 4, respectively. In Fig. 6, "streaks present" indicates that streaks due to Li deposition were observed on the negative electrode plate 12 when the exposed surface 11c was not covered with insulating tape 24, and "no streaks" indicates that no streaks were observed on the negative electrode plate 12. 6 indicates the case where the thickness of the positive electrode mixture layer 11b is the average thickness Ta as 1.0 T, 1.1 T as 1.1 times the average thickness Ta, and 1.5 T as 1.5 times the average thickness Ta. From the measurement results in FIG. 6, it was confirmed that Li deposition occurs near the portion where the thickness of the positive electrode mixture layer 11b becomes larger than the average thickness Ta and the maximum thickness when the thickness further increases becomes 1.1 times or more the average thickness Ta.

[0047] Fig. 8 is a diagram showing the results of determining the relationship between the thickness of the positive electrode mixture layer 11b at the end of the insulating tape 24 and the state of Li precipitation for multiple test samples of the positive electrode plates 11. In Fig. 8, the horizontal axis represents the test sample number of the positive electrode plate 11, and the vertical axis represents the thickness of the positive electrode mixture layer 11b at the end of the insulating tape 24. In Fig. 8, triangles indicate that Li precipitation was observed on the negative electrode plate 12 in the secondary battery 10 including the positive electrode plate 11, and black circles indicate that Li precipitation was not observed on the negative electrode plate 12.

[0048] In the measurement results of Fig. 8 , when the thickness of the positive electrode mixture layer 11b at the end of the insulating tape 24 is 1.05 times the average thickness Ta or more (when the mixture thickness on the vertical axis of Fig. 8 is 1.05 T or more), this means that a part of the portion of the positive electrode mixture layer 11b having a thickness of 1.05 times the average thickness Ta or more protrudes from the end of the insulating tape 24 and is not covered by the insulating tape 24. On the other hand, when the thickness of the positive electrode mixture layer 11b at the end of the insulating tape 24 is less than 1.05 times the average thickness Ta (when the mixture thickness on the vertical axis of Fig. 8 is less than 1.05 T), this means that the entire portion of the positive electrode mixture layer 11b having a thickness of 1.05 times the average thickness Ta or more is covered by the insulating tape 24.

[0049] From the measurement results in FIG. 8 , it was confirmed that when a portion of the positive electrode mixture layer 11b having a thickness of 1.05 times or more the average thickness Ta is not covered with the insulating tape 24, the inconvenience of Li deposition occurs, but when the entire portion of the positive electrode mixture layer 11b having a thickness of 1.05 times or more the average thickness Ta is covered with the insulating tape 24, Li deposition hardly occurs.

[0050] FIG. 9 is a view corresponding to FIG. 3 of a cylindrical lithium-ion secondary battery according to another embodiment. In this configuration, six exposed surfaces 11c are formed on only one end of the positive electrode plate 11f in the plate width direction γ at six positions in the plate length direction β. Six positive electrode tabs 20 at positions A1, A2, A3, ..., A6 in FIG. 9 are joined to the six exposed surfaces 11c, and are covered with insulating tape 24 along with the corresponding exposed surfaces 11c. In this example, the other configurations and functions are similar to those of the embodiment shown in FIGS. 1 to 4 or 5.

[0051] The number of exposed surfaces 11c of the positive electrode plate and the number of positive electrode tabs 20 are not limited to four or six as described above, but may be one, two, three, five, or seven or more.

[0052] Furthermore, in accordance with the configuration of the present disclosure, the inventors conducted experiments to determine the relationship between the length X1a from the reference position X0 to the surface position of a portion 1.05 times the average thickness of the positive electrode mixture layer 11b, the length X2 to the first tape end 31 of the insulating tape 24, and the state of Li deposition. The experiments were conducted using Examples 1 to 5 and Comparative Examples 1 to 5. The configurations of the Examples and Comparative Examples will be described below.

[0053] [Example 1] [Preparation of Positive Electrode Plate] LiNi was used as the positive electrode active material. ０．８８ Co ０．０９ Al ０．０３ O ２ Then, 100 parts by mass of LiNi ０．８８ Co ０．０９ Al ０．０３ O ２1 part by mass of acetylene black and 0.9 parts by mass of polyvinylidene fluoride (PVDF) (binder) were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. Next, the paste-like positive electrode mixture slurry was applied to both sides of a long positive electrode core 11a made of aluminum foil with a thickness of 15 μm, so that six exposed surfaces 11c for welding the positive electrode tabs 20 were formed, and the mixture was dried in a dryer. Thereafter, the positive electrode mixture slurry was applied to the positive electrode core 11a, dried, and cut to a predetermined electrode size. The cathode mixture slurry was then rolled using a roller to produce a positive electrode plate 11 in which a positive electrode mixture layer 11b was formed on both sides of the positive electrode core 11a. Aluminum positive electrode tabs 20 were fixed by welding to all of the exposed surfaces 11c of the core at six positions in the longitudinal direction of the positive electrode plate 11. In addition, in the positive electrode plate 11, X1a is 3 mm and X2 is 3.5 mm, so that X2>X1a×1.1 is satisfied.

[0054] [Preparation of Negative Electrode Plate] A mixture of 95 parts by mass of graphite powder and 5 parts by mass of silicon oxide was used as the negative electrode active material. Then, 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR) as a binder, and 1 part by mass of carboxymethyl cellulose (CMC) as a thickener were mixed. This mixture was then dispersed in water to prepare a negative electrode mixture slurry. This negative electrode mixture slurry was applied to both sides of a negative electrode core made of copper foil with a thickness of 8 μm, dried in a dryer, and compressed to a predetermined thickness with the rollers of a roll press. Then, the long negative electrode core on which the negative electrode mixture layer was formed was cut to a predetermined electrode size, and a negative electrode plate 12 in which a negative electrode mixture layer was formed on both sides of the negative electrode core was prepared. An exposed portion was provided at one longitudinal end of the negative electrode plate 12, which was located on the end side of the winding of the electrode body 14, where there was no mixture layer and the surface of the current collector was exposed, and a nickel negative electrode tab 21b was attached to the exposed portion by welding.

[0055] [Preparation of Electrode Body] The prepared positive electrode plate 11 and negative electrode plate 12 were spirally wound with a separator 13 made of a polyethylene microporous film interposed therebetween to prepare a wound electrode body 14, and the end of the winding was fixed with tape.

[0056] [Preparation of non-aqueous electrolyte] 5 parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of a mixed solvent prepared by mixing ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of EC:DMC = 1:3, and LiPF ６ The non-aqueous electrolyte solution was prepared by dissolving the compound in an amount of 1.5 mol / L.

[0057] [Fabrication of Secondary Battery] The electrode body 14 was housed in a cylindrical outer can 15 with a bottom, insulating plates 28 and 29 were placed above and below the electrode body 14, respectively, and a non-aqueous electrolyte solution was injected into the inside of the outer can 15 by a reduced pressure method. Thereafter, the sealing body 16 was crimped to the open end of the outer can 15 via a gasket 27, thereby fabricating a cylindrical secondary battery 10.

[0058] [Examples 2 to 5] In Example 2, X1a is 4 mm and X2 is 4.5 mm in the positive electrode plate. In Example 3, X1a is 5 mm and X2 is 6 mm in the positive electrode plate. In Example 4, X1a is 6 mm and X2 is 7 mm in the positive electrode plate. In Example 5, X1a is 5 mm and X2 is 5.6 mm in the positive electrode plate. Therefore, Examples 2 to 5 satisfy X2 > X1a × 1.1. Other configurations in Examples 2 to 5 are the same as those in Example 1.

[0059] [Comparative Example 1] In Comparative Example 1, X1a is 4 mm and X2 is 4 mm in the positive electrode plate. Therefore, X2 = X1a × 1.1 in Comparative Example 1. Other configurations of Comparative Example 1 are the same as those of Example 1.

[0060] [Comparative Examples 2 and 3] In Comparative Example 2, X1a is 5 mm and X2 is 4.5 mm in the positive electrode plate. In Comparative Example 3, X1a is 4 mm and X2 is 2 mm in the positive electrode plate. Therefore, in Comparative Examples 2 and 3, X2<X1a×1.1. In Comparative Examples 2 and 3, the other configurations are the same as in Example 1.

[0061] [Comparative Examples 4 to 5] In Comparative Example 4, X1a is 0 mm and X2 is 1 mm in the positive electrode plate. Therefore, in Comparative Example 3, there was no raised portion in the positive electrode plate. In Comparative Example 4, X1a is 0 mm and X2 is 1.5 mm. Therefore, in Comparative Example 5, there was also no raised portion in the positive electrode plate. In Comparative Examples 4 and 5, the other configurations were the same as in Example 1.

[0062] [Test Method] Using the cylindrical lithium-ion secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 5 described above, a constant current charge of 0.3 C was performed at 25°C until the battery voltage reached 4.2 V, followed by a constant voltage charge of 0.01 C at 4.2 V. The batteries were then discharged at a constant current of 0.5 C until the battery voltage reached 2.5 V. This cycle constituted one charge-discharge cycle. After the charge-discharge cycle, the batteries were disassembled to check for the presence or absence of Li deposition. Furthermore, the thickness of the mixture was measured by observing the cross section of the positive electrode mixture layer 11b near the exposed surface 11c of the positive electrode core 11a of the electrode assembly 14 using an X-ray CT scanner.

[0063] [Test Results] Figure 10 shows the experimental results of X1a and X2 in Examples 1 to 5 and Comparative Examples 1 to 5, and the presence or absence of Li precipitation. From the experimental results in Figure 10, Li precipitation occurred in Comparative Examples 1 to 3, where X2 ≦ X1a × 1.1. On the other hand, Li precipitation did not occur in Examples 1 to 5, where X2 > X1a × 1.1, and Comparative Examples 4 and 5, where no raised portion 30 was observed. This confirmed the effects of the present disclosure.

[0064] REFERENCE SIGNS LIST 10 Cylindrical lithium ion secondary battery (secondary battery), 11 Positive electrode plate, 11a Positive electrode core, 11b Positive electrode mixture layer, 11c Exposed surface, 11e, 11f Positive electrode plate, 12 Negative electrode plate, 13 Separator, 14 Electrode body, 15 Outer can, 16 Sealing body, 17 Sealing plate, 17a Short cylinder portion, 18 Current collector plate, 19 Metal plate, 20 Positive electrode tab, 21a, 21b Negative electrode tab, 24, 24a Insulating tape, 27 Gasket, 28, 29 Insulating plate 30 Protruding portion, 31 First tape end.

Claims

1. An electrode assembly comprising a positive electrode plate and a negative electrode plate wound with a separator interposed therebetween, and an exterior can housing the electrode assembly, wherein the positive electrode plate comprises: a positive electrode core; a positive electrode mixture layer formed on a surface of the positive electrode core; a positive electrode tab formed only on one side in the short direction of the electrode plate at least in a part of the long direction of the electrode plate, the positive electrode tab being joined to an exposed surface where the positive electrode core is exposed and extending from one end of the positive electrode core in the short direction of the electrode plate; and an insulating tape affixed to the positive electrode plate so as to cover the positive electrode tab superimposed on the exposed surface and the exposed surface, wherein the positive electrode mixture layer has a raised portion in the positive electrode core near a first exposed end in the long direction of the electrode plate of at least one of the exposed surfaces, and the raised portion has a maximum thickness that is 10% or more greater than an average thickness of a portion of the positive electrode mixture layer excluding the raised portion, the insulating tape covers the exposed surface and the raised portion in a longitudinal direction of the electrode plate, the insulating tape extending beyond a maximum thickness position of the exposed surface and the raised portion, and extending beyond a position where the portion beyond the maximum thickness position becomes thinner than 1.05 times the average thickness of the positive electrode mixture layer.

2. The cylindrical lithium-ion secondary battery according to claim 1, wherein when the thickness of the positive electrode mixture layer is measured at predetermined distances in the longitudinal direction of the electrode plate from the maximum thickness position toward the first tape end on the raised portion side of the insulating tape with the first exposed end of the exposed surface as a reference position, the attachment position of the first tape end is regulated so that X2 > X1 × 1.1, where X1 is the length from the reference position to the previous measurement position when the thickness at the previous measurement position becomes the same as the thickness at the current measurement position, and X2 is the length from the reference position to the first tape end.

3. The cylindrical lithium ion secondary battery according to claim 1 or 2, wherein the exposed surface is a plurality of exposed surfaces formed at a plurality of positions in a longitudinal direction of the positive electrode plate, and the positive electrode tabs are a plurality of positive electrode tabs joined to the plurality of exposed surfaces.

4. The cylindrical lithium ion secondary battery according to claim 3, wherein the plurality of exposed surfaces is six or more exposed surfaces, and the plurality of positive electrode tabs is six or more positive electrode tabs joined to the six or more exposed surfaces.

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