Cylindrical battery

Abstract

In this cylindrical battery (10), when a point (P) is defined as the position of the center in the width direction of a positive electrode (11) that overlaps a positive electrode start end (11x) in the radial direction of an electrode body (14) and is positioned on the outside of one turn of the positive electrode start end (11x), a circle (α1) is defined as a perfect circle passing through the point (P) with a winding center (Z) as the center, and a circle (α2) is defined as a perfect circle passing through three points of a first point shifted by 3° from the point (P) toward a winding start side along the center in the width direction of the positive electrode (11), a second point shifted by 3° from the point (P) toward a winding end side, and the point (P), the ratio (R2 / R1) of a curvature R2 of the circle (α2) to a curvature R1 of the circle (α1) is 0.90 or less.

Description

Cylindrical battery

[0001] The present disclosure relates to a cylindrical battery, and more particularly to a cylindrical battery including a wound electrode body.

[0002] The cylindrical battery includes a wound electrode body in which a positive electrode and a negative electrode are wound in a spiral shape with a separator interposed therebetween (see, for example, Patent Document 1). The positive electrode and the negative electrode constituting the wound electrode body have a core body and a mixture layer formed on the core body. In order to prevent metal deposition on the surface of the negative electrode, a negative electrode mixture layer is always disposed in a range facing the positive electrode mixture layer in the radial direction of the electrode body. Therefore, the start end of the positive electrode mixture layer, which is the end of the positive electrode mixture layer on the winding start side of the electrode body, is sandwiched from both sides in the radial direction of the electrode body by the negative electrode mixture layer via the separator. Note that Patent Document 1 discloses a method for manufacturing an electrode body that forms a winding structure using a special-shaped bobbin having a chamfered shape.

[0003] Japanese Patent Application Laid-Open No. 11-111328

[0004] As a result of the study by the present inventors, it has been found that deformation of the negative electrode is likely to occur in the vicinity of the start end of the positive electrode mixture layer. The electrode body expands and contracts during charging and discharging of the battery. At this time, stress is likely to concentrate on the negative electrode in the vicinity of the start end of the positive electrode mixture layer. For example, a part of the negative electrode located on the outer side of the winding of the start end of the positive electrode mixture layer bends so as to protrude inward of the winding. When such deformation of the negative electrode occurs, it may lead to a decrease in cycle characteristics due to non-uniformity of the charge and discharge reaction, occurrence of local short circuits, etc. Therefore, suppressing such deformation of the negative electrode is an important issue. Note that in the electrode body of Patent Document 1, such deformation of the negative electrode cannot be sufficiently suppressed.

[0005] The cylindrical battery according to this disclosure comprises a positive electrode, a negative electrode, and a separator, an electrode body in which the positive electrode and the negative electrode are wound around the separator, and a bottomed cylindrical outer casing for housing the electrode body, wherein the positive electrode has a positive electrode core and a positive electrode mixture layer formed on the positive electrode core, and the negative electrode has a negative electrode core and a negative electrode mixture layer formed on the negative electrode core, and in a cross section obtained by cutting the electrode body in a direction perpendicular to the axial direction, the starting end of the positive electrode mixture layer, which is the end of the positive electrode mixture layer on the winding starting side of the electrode body, overlaps with the radial direction of the electrode body, When the position of the widthwise center of the positive electrode located one full turn outside the starting end is defined as point P, the midpoint of the longest straight line drawn from point P toward the widthwise center of the positive electrode opposite the winding core portion of the electrode body is defined as the winding center Z, the circle α1 is a perfect circle centered on the winding center Z and passing through point P, and the circle α2 is a perfect circle passing through the three points: a first point moved 3° toward the winding start side from point P along the widthwise center of the positive electrode, a second point moved 3° toward the winding end side from point P, and point P, the ratio of the curvature R2 of circle α2 to the curvature R1 of circle α1 (R2 / R1) is characterized in that it is 0.90 or less.

[0006] According to the cylindrical battery described herein, deformation of the negative electrode near the starting end of the positive electrode mixture layer can be effectively suppressed.

[0007] This is an axial cross-sectional view of a cylindrical battery, which is an example of an embodiment. This is a cross-sectional view of an electrode body, which is an example of an embodiment, cut in a direction perpendicular to the axial direction, showing the starting side of the winding structure. This is a diagram for explaining the structure of an electrode body, which is an example of an embodiment.

[0008] Hereinafter, an example of an embodiment of the cylindrical battery according to this disclosure will be described in detail with reference to the drawings. Note that the cylindrical battery according to this disclosure is not limited to the embodiments described below. Furthermore, forms obtained by selectively combining the various components of the multiple embodiments and modifications described below are also included in this disclosure.

[0009] Figure 1 is an axial cross-sectional view of a cylindrical battery 10, which is an example of an embodiment, cut along a plane containing its central axis. As shown in Figure 1, the cylindrical battery 10 includes a positive electrode 11, a negative electrode 12, and a separator 13, and comprises an electrode body 14 in which the positive electrode 11 and the negative electrode 12 are wound around the separator 13, and a bottomed cylindrical outer casing 16 that houses the electrode body 14. The cylindrical battery 10 also includes an electrolyte housed in the outer casing 16 and a sealing body 17 that closes the opening of the outer casing 16. The outer casing 16 has grooves 22 formed in its side wall, and the sealing body 17 is supported by the grooves 22 and closes the opening of the outer casing 16. For convenience of explanation, in the following, the side of the cylindrical battery 10 with the sealing body 17 will be considered the top, and the bottom side of the outer casing 16 will be considered the bottom.

[0010] The electrolyte may be an aqueous electrolyte, but in this embodiment, a non-aqueous electrolyte is used. The non-aqueous electrolyte has lithium-ion conductivity. The non-aqueous electrolyte may be a liquid electrolyte (electrolyte solution) or a solid electrolyte. The cylindrical battery 10 is, for example, a non-aqueous electrolyte secondary battery, and among these, a lithium-ion battery is preferred.

[0011] A non-aqueous electrolyte comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixtures thereof. The non-aqueous solvent may also contain halogen-substituted solvents (e.g., fluoroethylene carbonate) in which at least some of the hydrogen atoms of the solvent are replaced with halogen atoms such as fluorine. Examples of electrolyte salts include LiPF4. ６ Lithium salts such as these are used.

[0012] 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, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, 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. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc.

[0013] As described above, the electrode body 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound in a spiral shape via a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all elongated strip-shaped bodies that are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape. The negative electrode 12 is formed to be slightly larger in dimensions than the positive electrode 11. That is, the negative electrode 12 is formed to be longer in both the length and width directions than the positive electrode 11. The separator 13 is formed to be at least slightly larger in dimensions than the positive electrode 11, and for example, two separators are arranged so as to sandwich the positive electrode 11.

[0014] The positive electrode 11 comprises a long positive electrode core 30 and a positive electrode mixture layer 31 formed on the positive electrode core 30. The positive electrode core 30 can be made of a metal foil that is stable within the potential range of the positive electrode 11, such as aluminum, aluminum alloy, stainless steel, or titanium, or a film with the metal arranged on its surface. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent such as acetylene black, and a binder such as polyvinylidene fluoride (PVdF), and is preferably formed on both sides of the positive electrode core 30. For example, a lithium transition metal composite oxide containing Ni, Co, Mn, Al, etc., can be used as the positive electrode active material.

[0015] The thickness of the positive electrode 11 is, for example, 150 μm to 230 μm. In this embodiment, the thickness of the positive electrode 11 is substantially constant, except for the exposed core portion (not shown) to which the positive electrode lead 20 is connected. The thickness of the positive electrode core 30 is, for example, 10 μm to 30 μm. The thickness of the positive electrode mixture layer 31 is, for example, 70 μm to 100 μm on one side of the positive electrode core 30. The positive electrode 11 can be manufactured by applying a positive electrode mixture slurry onto the positive electrode core 30, drying the coating film, and then compressing it to form the positive electrode mixture layer 31 on both sides of the positive electrode core 30.

[0016] The negative electrode 12 comprises a long negative electrode core 40 and a negative electrode mixture layer 41 formed on the negative electrode core 40. The negative electrode core 40 can be made of a metal foil that is stable within the potential range of the negative electrode 12, such as copper, copper alloy, stainless steel, nickel, or nickel alloy, or a film with the metal arranged on its surface. The negative electrode mixture layer 41 contains a negative electrode active material and a binder such as styrene-butadiene rubber (SBR), and is preferably formed on both sides of the negative electrode core 40, with some exceptions. For example, graphite or silicon-containing materials can be used as the negative electrode active material.

[0017] The thickness of the negative electrode 12 is, for example, 145 μm or more and 235 μm or less. In this embodiment, the thickness of the negative electrode 12 is substantially constant, except for the portion including the core body exposed portion described later. The thickness of the negative electrode core body 40 is, for example, 5 μm or more and 15 μm or less. The thickness of the negative electrode mixture layer 41 is, for example, 70 μm or more and 110 μm or less on one side of the negative electrode core body 40. The negative electrode 12 can be manufactured in the same way as the positive electrode 11 by applying a negative electrode mixture slurry containing a negative electrode active material and a binder onto the negative electrode core body 40, drying the coating film, and then compressing it to form the negative electrode mixture layer 41 on both sides of the negative electrode core body 40.

[0018] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a highly heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator 13, and a filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12.

[0019] The electrode body 14 has a positive electrode lead 20 connected to a positive electrode 11 and a negative electrode lead 21 connected to a negative electrode 12. In this embodiment, the positive electrode mixture layer 31 is absent in the longitudinal center of the positive electrode 11, and a core exposed portion is formed where the surface of the positive electrode core 30 is exposed. The positive electrode lead 20 is connected to this exposed portion. On the other hand, the negative electrode lead 21 is provided at one longitudinal end of the negative electrode 12, which is located on the winding start side of the electrode body 14. At one longitudinal end of the negative electrode 12, the negative electrode mixture layer 41 is absent, and a first core exposed portion 42 (see Figure 2) is formed where the surface of the negative electrode core 40 is exposed. The negative electrode lead 21 is connected to this core exposed portion.

[0020] Insulating plates 18 and 19 are positioned above and below the electrode body 14, respectively. In the example shown in Figure 1, the positive electrode lead 20 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 extends through a through-hole in the insulating plate 19 towards the bottom of the outer can 16. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 becomes the negative electrode terminal.

[0021] A negative electrode 12 is positioned on the outermost surface of the electrode body 14, and a second core exposure portion 43 is provided where the surface of the negative electrode core body 40 is exposed. The core exposure portion 43 has a length of, for example, one or more times the circumference of the electrode body 14 and is in contact with the inner surface of the outer casing 16. By the core exposure portion 43 contacting the inner surface of the outer casing 16, both ends in the longitudinal direction of the negative electrode 12 and the outer casing 16 are electrically connected, ensuring good current collection on the negative electrode side. A winding stopper tape may be attached to the outermost surface of the electrode body 14 to maintain the winding structure.

[0022] The outer casing 16 is a bottomed cylindrical metal container. A gasket 28 is provided between the outer casing 16 and the sealing body 17 to seal the inside of the battery. The outer casing 16 has a grooved portion 22 that supports the sealing body 17, which is formed, for example, by pressing the side wall from the outside. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17. The upper end of the outer casing 16 is bent inward and crimped to the periphery of the sealing body 17.

[0023] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, with the insulating member 25 interposed between their respective peripheries. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 towards the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0024] The structure of the electrode body 14, which is an example of an embodiment, will be described in detail below with reference to Figures 2 and 3. Figure 2 is a cross-sectional view of the electrode body 14 cut in a direction perpendicular to the axial direction, showing the portion located on the winding start side of the winding structure.

[0025] As shown in Figure 2, at the winding start side of the electrode body 14, a portion of the negative electrode 12 is positioned inward from the positive electrode mixture layer start end, which is the end of the positive electrode mixture layer 31 at the winding start side, and extends toward the winding start side beyond the position opposite to the positive electrode mixture layer start end. In this embodiment, the positive electrode mixture layer start end coincides with the start end of the positive electrode core body 30, and the positive electrode mixture layer start end is the positive electrode start end 11x. Hereinafter, the structure of the electrode body 14 will be described using the term positive electrode start end 11x, but in this embodiment, "positive electrode start end 11x" can be read as "positive electrode mixture layer start end". On the other hand, in the negative electrode 12, the start end of the negative electrode mixture layer 41 and the start end of the negative electrode core body 40 do not coincide, and a core body exposed portion 42 is formed in which the surface of the negative electrode core body 40 is exposed over a predetermined length range from the negative electrode start end 12x.

[0026] As described above, the electrode body 14 has a core body exposed portion 42 and a negative electrode lead 21 connected to the inner surface of the bottom of the outer casing 16. The core body exposed portion 42 is formed to a length that does not reach the position opposite the positive electrode start end 11x from the negative electrode start end 12x. The negative electrode start end 12x refers to the start end of the negative electrode core body 40. In the winding core portion of the electrode body 14, in other words, the radial center portion, a hollow portion 14x extending in the axial direction is formed. The hollow portion 14x is a space formed after the winding core is removed, as will be described later, and functions as an exhaust path when a malfunction occurs in the battery and gas is generated. The winding center Z of the electrode body 14 is located in the hollow portion 14x.

[0027] The electrode body 14 has a first section 35 in which the curvature of the positive electrode 11 is locally reduced. The positive electrode 11 and the negative electrode 12 are wound in a spiral shape via a separator 13, and are curved along their entire length, but the curvature changes discontinuously in the first section 35, making the degree of curvature gentler. As will be described in more detail later, the first section 35 is formed in the area that overlaps radially with the positive electrode start end 11x and the electrode body 14, one turn outside the positive electrode start end 11x. There may be parts two or more turns outside the positive electrode start end 11x in which the degree of curvature of the positive electrode 11 is gentler compared to other parts of the same number of turns, but in this embodiment, the part with a gentler degree of curvature one turn outside the positive electrode start end 11x is defined as the first section 35.

[0028] The electrode body 14 is configured such that the negative electrode mixture layer 41 is always placed in the area facing the positive electrode mixture layer 31 in order to prevent lithium deposition on the surface of the negative electrode 12. As a result, the positive electrode start end 11x is sandwiched from both radial sides of the electrode body 14 by the negative electrode mixture layer 41 via the separator 13. The electrode body 14 expands and contracts during charging and discharging of the battery, and at this time, stress tends to concentrate near the positive electrode start end 11x. As a result, deformation of the negative electrode 12 may occur near the positive electrode start end 11x, but with the electrode body 14, the deformation of the negative electrode 12 is effectively suppressed by the function of the first section 35.

[0029] In this embodiment, the negative electrode lead 21 is positioned near the hollow portion 14x (winding core portion) of the electrode body 14, and the negative electrode 12 and the outer casing 16 are connected by the negative electrode lead 21, so the movement of the electrode body 14 is constrained at the winding start side. For this reason, deformation such as bending of the negative electrode 12 toward the winding side is likely to occur near the winding start side of the electrode body 14, especially near the positive electrode start end 11x. However, the first section 35 is thought to distribute the stress acting near the positive electrode start end 11x, and as a result, the stress that causes bending of the negative electrode 12 is reduced.

[0030] As described above, the first section 35 is formed in a region that overlaps radially with the positive electrode start end 11x and the electrode body 14. This is thought to effectively suppress stress concentration at the positive electrode start end 11x. On the other hand, even if the first section, in which the curvature of the positive electrode is locally reduced, is formed at a location other than the position that overlaps radially with the positive electrode start end 11x, the effect of suppressing negative electrode deformation will not be substantially obtained. However, a portion with locally reduced curvature can be formed at such other locations, to the extent that it does not impair the purpose of this disclosure. For example, a portion with locally reduced curvature can be formed at a location opposite the first section 35, with the hollow portion 14x in between. Also, the curvature of the first section 35 and the portion in question may be the same.

[0031] The first section 35 may have a smaller curvature and a shape that is nearly flat compared to other parts of the positive electrode 11 (hereinafter sometimes referred to as the "second section") that are at a similar distance from the winding center Z. As will be described in detail later, the curvature of the first section 35 is smaller than the curvature R1 of circle α1 (see Figure 3) which is at the same distance from the winding center Z. The curvature of the second section is, for example, a curvature that approximates the curvature R1 of circle α1. Because the positive electrode 11 is wound in a spiral shape, the curvature of the second section changes continuously so as to gradually decrease towards the end of the winding, but the degree of change is gradual. A bent section 36 with a larger curvature is formed at the boundary between the first section 35 and the second section.

[0032] The positive electrode 11 may have a portion with a smaller curvature than other portions of the same number of turns, two or more turns outside the positive electrode start end 11x. In the example shown in Figure 2, the gentle curve of the first section 35 is reflected up to two or more turns outside the positive electrode start end 11x. Note that deformation of the negative electrode 12 near the positive electrode start end 11x is likely to occur in the portion located one turn outside the positive electrode start end 11x. For this reason, the positive electrode 11 only needs to have the first section 35 one turn outside the positive electrode start end 11x, but the shape of the first section 35 will affect the shape of at least the portion that overlaps radially with the electrode body 14.

[0033] The gentle curved shape resulting from the first section 35 may be reflected, for example, up to the outermost circumference of the positive electrode 11 in the area that radially overlaps with the first section 35. Alternatively, the shape of the first section 35 may be reflected only in the range of turns that is less than or equal to half the total number of turns of the positive electrode 11. In this case, there is no area with locally reduced curvature at the end of the winding of the positive electrode 11, and the second section is formed around the entire circumference. Furthermore, the shape of the positive electrode 11 including the first section 35 is also reflected in the shape of the negative electrode 12 facing it via the separator 13.

[0034] As described above, the positive electrode 11 has a bent portion 36 formed at the boundary between the first section 35 and the second section. The bent portion 36 has a greater degree of curvature compared to other parts, and its curvature is maximized. The bent portion 36 is formed at the winding start end and winding end end of the first section 35, but both are located away from the position where the positive electrode start end 11x and the electrode body 14 overlap radially. In other words, the first section 35 is formed within a predetermined length range that includes the position where the positive electrode start end 11x overlaps radially. As will be described in detail later, the length of the first section 35 along the winding direction of the electrode body 14 is, for example, 5% to 20% of the circumference of the positive electrode 11 in the portion including the first section 35.

[0035] Figure 3 is a cross-sectional view of the electrode body 14 cut in a direction perpendicular to the axial direction, showing a circle α1 passing through point P with the winding center Z as the center, and a circle α2 passing through the three points P, P1, and P2. In Figure 3, the positive electrode core 30 and the negative electrode core 40 are omitted from the illustration for clarity.

[0036] As shown in Figure 3, the curvature R2 of circle α2, which is formed passing through point P and along the first section 35, is smaller than the curvature R1 of circle α1, which is centered at the winding center Z and passes through point P. The curvature of the first section 35 approximates the curvature R2 of circle α2, and the curvature of the second section approximates the curvature R1 of circle α1. When the condition R1 of circle α1 > R2 of circle α2 is met, a negative electrode deformation suppression effect is achieved near the positive electrode start end 11x, but it is preferable that the ratio of curvature R2 to curvature R1 (R2 / R1) is 0.90 or less. In this case, the negative electrode deformation suppression effect becomes more pronounced.

[0037] The definitions of circles α1, α2, and the winding center Z are as follows. In this embodiment, in a cross section obtained by cutting the electrode body 14 in a direction perpendicular to the axial direction, (1) point P is the position of the widthwise center of the positive electrode 11 which overlaps the positive electrode start end 11x in the radial direction of the electrode body 14 and is located one full turn outside the positive electrode start end 11x, (2) the winding center Z is the midpoint of the longest straight line Y drawn from point P toward the widthwise center of the positive electrode 11 opposite each other across the hollow portion 14x which is the winding core portion of the electrode body 14, (3) circle α1 is a perfect circle centered on the winding center Z and passing through point P, and (4) circle α2 is a perfect circle passing through the three points: a first point P1 moved 3° toward the winding start side from point P along the widthwise center of the positive electrode 11, a second point P2 moved 3° toward the winding end side from point P, and point P. Although not shown in Figure 3, the positive electrode core 30 is positioned at the center of the positive electrode 11 in the width direction, so points P, P1, and P2 can be said to be positions on the positive electrode core 30.

[0038] Preferably, the first section 35 is formed with substantially the same length on both the winding start side and the winding end side, centered on point P. That is, the pair of bent portions 36 located at the boundary between the first section 35 and the second section are formed at positions equidistant from point P. Preferably, the positive electrode 11 has bent portions 36 in a range of 5° to 20° on both the winding start side and the winding end side of the electrode body 14, along the widthwise center of the positive electrode 11 from point P.

[0039] The angle θ from point P to the bent portion 36 with respect to the winding center Z is preferably 5° to 20°, and more preferably 10° to 40°, as described above. In this case, the effect of the first section 35 becomes more pronounced. Furthermore, it is preferable that the angle θ is substantially the same in a pair of bent portions 36. The curvature of the bent portion 36 is, for example, 0.15 to 0.70 times the curvature of the second section. The curvature of the bent portion 36 is measured in the same way as the curvature R2 of circle α2.

[0040] The ratio (R2 / R1) of the curvature R2 of the circle α2 to the curvature R1 of the circle α1 is preferably 0.90 or less, more preferably 0.80 or less, as described above. The lower limit of the ratio (R2 / R1) is not particularly limited from the viewpoint of suppressing the negative electrode deformation. However, considering the improvement of the productivity of the electrode body 14, the increase in the capacity of the battery, etc., it is preferably 0.10 or more. An example of a preferable ratio (R2 / R1) is 0.20 or more and 0.80 or less. In this case, the effect of suppressing the negative electrode deformation becomes more remarkable.

[0041] The electrode body 14 can be manufactured, for example, using a winding core having a shape in which a part of the outer peripheral surface of a cylindrical shape is flatly cut along the axial direction. By winding the positive electrode 11 in a state where the positive electrode start end 11x is positioned at the center of the flatly cut portion, the first section 35 centered on the point P is formed. The curvature (curvature R2) of the first section 35 can be controlled by adjusting the tension during winding using the winding core. Generally, even when the same winding core is used, the higher the tension during winding, the more easily the shape of the winding core is reflected in the positive electrode 11, and the curvature of the first section 35 tends to be smaller.

[0042] As described above, according to the cylindrical battery 10 having the above configuration, the deformation of the negative electrode 12 in the vicinity of the positive electrode start end 11x can be effectively suppressed. In the vicinity of the positive electrode start end 11x, the negative electrode 12 is likely to be deformed such that it bends to the inner side of the winding. However, by providing the first section 35 where the curvature of the positive electrode 11 is locally small at a position radially overlapping the positive electrode start end 11x, it is considered that the stress acting in the vicinity of the positive electrode start end 11x is effectively reduced. As a result, the deformation of the negative electrode 12 is highly suppressed.

[0043] Note that the above embodiment can be appropriately modified in design without impairing the object of the present disclosure. For example, in the above embodiment, the negative electrode lead 21 is disposed in the hollow portion 14x of the electrode body 14, but the negative electrode lead may be disposed outside the hollow portion 14x, such as at the end portion on the winding end side of the electrode body 14.

[0044] Hereinafter, the present disclosure will be further described by way of examples, but the present disclosure is not limited to these examples.

[0045] <Example 1>[Fabrication of the positive electrode] Lithium cobaltate was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed at a solid content mass ratio of 98:1:1, and a positive electrode mixture slurry was prepared using N-methylpyrrolidone (NMP) as the dispersion medium. The slurry was applied to both sides of a positive electrode core made of a long aluminum foil with a thickness of 15 μm, and the coating film was dried and compressed to obtain a positive electrode in which positive electrode mixture layers were formed on both sides of the positive electrode core. Note that a core exposed portion where no positive electrode mixture layer exists was provided at the central portion in the length direction of the positive electrode, and a positive electrode lead made of aluminum was ultrasonically welded to the exposed portion.

[0046] [Fabrication of the negative electrode] A mixture of graphite powder and a Si-containing material mixed at a mass ratio of 95:5 was used as the negative electrode active material. The negative electrode active material, a dispersion of styrene-butadiene rubber, and sodium carboxymethyl cellulose were mixed at a solid content mass ratio of 98:1:1, and a negative electrode mixture slurry was prepared using water as the dispersion medium. The slurry was applied to both sides of a negative electrode core made of a long copper foil with a thickness of 8 μm, and the coating film was dried and compressed to obtain a negative electrode in which negative electrode mixture layers were formed on both sides of the negative electrode core. Note that first and second core exposed portions where no negative electrode mixture layer exists were provided in a predetermined length range from both ends in the length direction of the negative electrode, and a negative electrode lead made of nickel was ultrasonically welded to the first core exposed portion.

[0047] [Fabrication of the electrode body] The above positive electrode, the above negative electrode, and a separator made of polyethylene were wound in a spiral shape using a winding core described later, and winding tapes were attached to both ends in the axial direction of the outermost peripheral surface to obtain a wound-type electrode body. At this time, the negative electrode was arranged so that the first core exposed portion of the negative electrode to which the negative electrode lead was joined was located on the winding start side of the electrode body. Also, on the winding start side of the electrode body, the negative electrode was extended from the positive electrode start end, and a non-opposing portion of the negative electrode that does not face the positive electrode was provided. Note that the non-opposing portion of the negative electrode is also provided on the winding end side of the electrode body. After forming the winding structure of the electrode body, the winding core was removed to obtain a wound-type electrode body in which a hollow portion was formed in the winding core.

[0048] As the winding core used, a winding core with a cylindrical outer surface was used, in which a portion of the outer surface was cut flat along the axial direction. That is, the contour of the cross-sectional shape of the winding core was composed of an arc and a straight line connecting the two ends of the arc. The positive electrode start end was placed at the center of the straight section of the winding core, and the positive electrode, negative electrode, and separator were wound in a spiral. As a result, a first section was formed one full turn outside the positive electrode start end, centered at a position that radially overlapped with the positive electrode start end. The definitions of the winding center Z, circle α1, circle α2, and angle θ are as described above. The curvature (curvature R2) of the formed first section was 0.45, and the ratio of curvatures (R2 / R1) was 0.90. The angle θ indicating the position of the bent part at the end of the first section was 35°.

[0049] [Preparation of Non-Aqueous Electrolyte] Mix ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:3 (at 25°C) to form a mixed solvent, and add LiPF4 to achieve a concentration of 1.5 mol / L. ６ A non-aqueous electrolyte was prepared by dissolving the substance.

[0050] [Fabrication of Cylindrical Battery] After placing insulating plates above and below the electrode body, the negative electrode lead was welded to the inner surface of the bottom of a bottomed cylindrical outer can, and the positive electrode lead was welded to the internal terminal plate of the sealing body, thereby housing the electrode body inside the outer can. Subsequently, a non-aqueous electrolyte was injected into the outer can using a reduced pressure method, and the opening of the outer can was sealed with the sealing body via a gasket to obtain a cylindrical battery. The exposed portion of the second core of the negative electrode forms the outermost surface of the electrode body and is in contact with the inner surface of the outer can.

[0051] <Example 2> A cylindrical battery was manufactured in the same manner as in Example 1, except that the tension during winding was increased to set the ratio (R2 / R1) to 0.75 in the preparation of the electrode body.

[0052] <Example 3> A cylindrical battery was manufactured in the same manner as in Example 1, except that the tension during winding was increased to set the ratio (R2 / R1) to 0.23 in the preparation of the electrode body.

[0053] <Comparative Example 1> A cylindrical battery was manufactured in the same manner as in Example 1, except that the electrode body was made using a cylindrical core with a perfectly circular cross-section. The ratio (R2 / R1) was 1.10.

[0054] [Evaluation of Negative Electrode Deformation] Each battery in the examples and comparative examples was charged at a constant current of 0.5C in a temperature environment of 45°C until the battery voltage reached 4.2V. Then, it was discharged at a constant current of 0.7C until the battery voltage reached 2.5V. After 500 cycles of this charge-discharge process, the batteries were put into a charged state, and the vicinity of the positive electrode start end of the electrode body was observed using an X-ray CT scanner (Shimadzu Corporation, SMX-225CT FPD HR) to check for the presence or absence of negative electrode deformation. The deformation of the negative electrode observed near the positive electrode start end is generally a deformation in which a part of the negative electrode bends so that it protrudes inward. Furthermore, after 1000 cycles of this charge-discharge process, the presence or absence of negative electrode deformation was checked in the same manner.

[0055]

[0056] As shown in Table 1, all of the batteries in the examples exhibit less deformation of the negative electrode near the positive electrode start end compared to the battery in Comparative Example 1. In particular, in the batteries of Examples 2 and 3, where the ratio (R2 / R1) is 0.80 or less, no deformation of the negative electrode was observed even after 1000 charge-discharge cycles.

[0057] 10 Cylindrical battery, 11 Positive electrode, 11x Positive electrode start end, 12 Negative electrode, 12x Negative electrode start end, 13 Separator, 14 Electrode body, 14x Hollow section, 16 Outer can, 17 Sealing body, 18 Upper insulating plate, 19 Lower insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode core body, 31 Positive electrode mixture layer, 35 First section, 36 Bent section, 40 Negative electrode core body, 41 Negative electrode mixture layer, 42, 43 Core body exposed section, L Straight line, P, P1, P2 Points, α1, α2 Circle

Claims

1. A cylindrical battery comprising an electrode body including a positive electrode, a negative electrode, and a separator, wherein the positive electrode and the negative electrode are wound around the separator, and a bottomed cylindrical outer casing for housing the electrode body, wherein the positive electrode has a positive electrode core and a positive electrode mixture layer formed on the positive electrode core, and the negative electrode has a negative electrode core and a negative electrode mixture layer formed on the negative electrode core, and in a cross section obtained by cutting the electrode body in a direction perpendicular to the axial direction, point P is the position of the widthwise center of the positive electrode which overlaps the starting end of the positive electrode mixture layer on the winding starting side of the electrode body and is located one full turn outside the starting end, the midpoint of the longest straight line drawn from point P toward the widthwise center of the positive electrode opposite to the winding core portion of the electrode body is the winding center Z, and a circle α1 is a perfect circle passing through point P with the winding center Z as the center. A cylindrical battery in which, when a circle α2 is defined as a perfect circle passing through three points: a first point moved 3° from point P towards the winding start side along the center in the width direction of the positive electrode, a second point moved 3° from point P towards the winding end side, and point P, the ratio of the curvature R2 of circle α2 to the curvature R1 of circle α1 (R2 / R1) is 0.90 or less.

2. The cylindrical battery according to claim 1, wherein the ratio (R2 / R1) is 0.10 or more and 0.80 or less.

3. The cylindrical battery according to claim 1 or 2, wherein the positive electrode has a bent portion with maximum curvature in the range of 5° to 20° along the center of the width direction of the positive electrode from point P, on both the winding start side and the winding end side.

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