Secondary batteries

Abstract

To provide a secondary battery capable of providing good battery performance by suppressing breakage of an end portion of an electrode body.SOLUTION: A secondary battery 1 includes: an electrode body 20 in which a positive electrode sheet 40, where a positive electrode mixture layer 42 is formed on a positive electrode collector foil 41, and a negative electrode sheet 50, where a negative electrode mixture layer 52 is formed on a negative electrode collector foil 51, are laminated in a lamination direction via a separator 60; and a positive electrode collector terminal 30 attached to a portion where positive electrode tab portions 21 of the electrode body 20 are bundled, the positive electrode collector foil 41 that is exposed with no positive electrode mixture layer 42 formed thereon in the positive electrode sheet 40 being laminated in the positive electrode tab portions. The negative electrode sheet 50 has a negative electrode non-facing part 53 that does not face the positive electrode mixture layer 42 on a side of the positive electrode collector terminal 30, and a negative electrode facing part 54 facing the positive electrode mixture layer 42 via the separator 60. The negative electrode mixture layer 52 of the negative electrode non-facing part 53 is formed such that a thickness in the lamination direction is reduced compared to the negative electrode mixture layer 52 of the negative electrode facing part 54.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a secondary battery, and more particularly to a secondary battery having an electrode assembly in which a positive electrode sheet, a negative electrode sheet, and a separator are stacked. [Background technology]

[0002] Secondary batteries, such as lithium-ion secondary batteries and nickel-metal hydride batteries, are power storage devices that can be repeatedly charged and discharged. Because of their light weight and high energy density, they are widely used as portable power sources for personal computers, mobile terminals, and the like, and also as power sources for driving vehicles. For example, lithium-ion secondary batteries have high current density and high battery capacity per unit volume, making them suitable for use as high-output power sources for driving vehicles such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHVs). One form of this type of secondary battery includes an electrode assembly in which a positive electrode sheet and a negative electrode sheet are stacked with a separator interposed therebetween.

[0003] Patent Document 1 discloses a separator and an energy storage element, which is a first separator that is placed between a positive electrode plate and a negative electrode plate and is stacked in multiple layers together with the positive electrode plate and the negative electrode plate, and which includes a first layer and a second layer that is placed on the first layer and is made of a different material from the first layer, and in which the peel strength at the ends of the first layer and the second layer is higher than the peel strength at the center. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-206605 Summary of the Invention [Problem to be solved by the invention]

[0005] At the ends of an electrode assembly in which a positive electrode sheet, a negative electrode sheet, and a separator are stacked, current collecting terminals are attached to tab portions of the positive and negative electrode sheets where a composite layer containing an active material is not formed and metal foil is exposed. At this time, the tab portions where the metal foil is stacked are electrically connected to the current collecting terminals while bundled. Furthermore, after the electrode assembly is formed, the negative electrode sheet has a negative electrode facing portion that faces the positive electrode composite layer via the separator, as well as a negative electrode non-facing portion that does not face the positive electrode composite layer.

[0006] When the positive electrode tabs are bundled together and the positive electrode current collector terminal is joined, the positive electrode tabs are pulled with a large force toward the joint with the positive electrode current collector terminal, particularly on the outer side of the electrode assembly in the stacking direction, which may cause the foil of the positive electrode tabs to break near the joint. Furthermore, by bundling the positive electrode tabs, the separator sandwiched between the positive electrode tabs and the non-facing portion of the negative electrode comes into contact with the adjacent non-facing portion of the negative electrode, applying pressure to the separator. This may damage the separator and cause a short circuit between the negative electrode sheet and the positive electrode sheet.

[0007] Although the technology described in Patent Document 1 describes that peeling of layers can be suppressed in a separator having multiple layers, it is not intended to suppress damage to the separator due to contact with the non-facing portion of the negative electrode or foil tearing that can occur near the joint portion. Therefore, the technology described in Patent Document 1 has the risk of causing damage to the separator due to contact with the non-facing portion of the negative electrode or foil tearing near the joint portion, resulting in the problem of not being able to achieve the desired battery performance.

[0008] The present disclosure has been made to solve such problems, and aims to provide a secondary battery that can achieve good battery performance by suppressing breakage of the ends of the electrode body. [Means for solving the problem]

[0009] A secondary battery according to one embodiment includes an electrode body in which a positive electrode sheet having a positive electrode composite layer formed on a positive electrode current collector foil and a negative electrode sheet having a negative electrode composite layer formed on a negative electrode current collector foil are stacked in a stacking direction with a separator interposed therebetween, and a positive electrode current collector terminal attached to a bundled portion of a positive electrode tab portion of the electrode body in which exposed positive electrode current collector foils on which no positive electrode composite layer is formed are stacked, and the negative electrode sheet has a negative electrode non-facing portion that does not face the positive electrode composite layer on the positive electrode current collector terminal side, and a negative electrode facing portion that faces the positive electrode composite layer via the separator, and the negative electrode composite layer in the negative electrode non-facing portion is formed so as to have a smaller thickness in the stacking direction than the negative electrode composite layer in the negative electrode facing portion. [Effects of the Invention]

[0010] According to the present disclosure, it is possible to provide a secondary battery that can achieve good battery performance by suppressing breakage of the ends of the electrode assembly. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a perspective view showing an example of a secondary battery according to a first embodiment. [Figure 2] 2 is a cross-sectional view showing the structure of the positive electrode current collector terminal side of the secondary battery shown in FIG. [Figure 3] FIG. 2 is a cross-sectional view showing the structure of the positive electrode current collector terminal side of a secondary battery in which the negative electrode non-facing portion and the separator end portion are not thinned. DETAILED DESCRIPTION OF THE INVENTION

[0012] Embodiment 1 Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. However, the present disclosure is not limited to the following embodiments. Furthermore, for clarity of explanation, the following description and drawings have been simplified as appropriate. What is shown in the drawings is only a part of the whole, and in reality, many other configurations that are not shown are included. In the following description, identical or equivalent elements are given the same reference numerals, and duplicate explanations will be omitted.

[0013] An outline of a secondary battery 1 according to this embodiment will be described with reference to Figures 1 and 2. In this embodiment, the secondary battery 1 will be specifically described as a lithium ion secondary battery. In a lithium ion secondary battery, during an electrochemical reaction, lithium ions, which are charge carriers, are conducted through an electrolyte between a positive electrode and a negative electrode, thereby realizing charging and discharging.

[0014] Fig. 1 is a perspective view showing an example of a secondary battery according to embodiment 1. As shown in Fig. 1, the secondary battery 1 includes a battery case in the shape of a flat rectangular parallelepiped, an electrode assembly 20 housed in the battery case, and an electrolyte.

[0015] The battery case is made of a metal material such as aluminum, aluminum alloy, or stainless steel. The battery case has a case body 10 and a lid 11. The case body 10 has a rectangular cylindrical shape with an open top and a closed bottom. The lid 11 has a rectangular flat plate shape and is provided to close the opening of the case body 10 and is joined to the case body 10 by welding or the like. The lid 11 seals the inside of the battery case when the electrode body 20 is housed inside the case body 10.

[0016] The lid 11 is provided with a positive electrode terminal 12 and a negative electrode terminal 13 used for inputting and outputting electric power. The positive electrode terminal 12 and the negative electrode terminal 13 are electrically connected to the electrode body 20 inside the battery case. In this embodiment, the positive electrode terminal 12 and the negative electrode terminal 13 are disposed on the top surface of the battery case (lid 11), but the positive electrode terminal 12 and the negative electrode terminal 13 may also be disposed on the side surface in the width direction of the battery case (case body 10).

[0017] The interior of the battery case will be described in detail with reference to Fig. 2. One of the features of the secondary battery 1 according to this embodiment is the structure of the end of the electrode body 20 arranged on the positive electrode collector terminal 30 side. Fig. 2 is a cross-sectional view showing the structure of the positive electrode collector terminal side of the secondary battery shown in Fig. 1.

[0018] 2 is a cross section of the secondary battery 1 cut along the stacking direction, showing a cross section including a portion of the electrode assembly 20 on the positive electrode collector terminal 30 side and the positive electrode collector terminal 30, and omits illustration of other components. Also, in FIG. 2, several positive electrode sheets 40, negative electrode sheets 50, and separators 60 from the outermost sides of the electrode assembly 20 in the stacking direction are shown, as well as the positive electrode collector terminal 30, but the other side in the stacking direction has the same configuration with the central axis C as the axis of symmetry.

[0019] The electrolyte may be, for example, an electrolytic solution. The electrolytic solution is, for example, a non-aqueous electrolytic solution in which a lithium salt is dissolved in an organic solvent. Examples of the lithium salt include LiClO4, LiPF6, LiAsF6, LiBF4, and LiSO3CF3. Examples of the organic solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane; sulfur compounds such as ethyl methyl sulfone and butane sultone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate. These may be used alone or in combination as the electrolytic solution.

[0020] The electrode body 20 is housed in a battery case. The electrode body 20 has a positive electrode tab portion 21 at the end on the positive electrode current collector terminal 30 side, and a negative electrode tab portion at the end on the negative electrode current collector terminal side. The positive electrode tab portion 21 is electrically connected to the positive electrode terminal 12 via the positive electrode current collector terminal 30. The negative electrode tab portion is electrically connected to the negative electrode terminal 13 via the negative electrode current collector terminal.

[0021] The positive electrode current collector terminal 30 is attached to the bundled portion of the positive electrode tab portion 21. Positive current collector terminal 30 is joined to the positive electrode tab portion 21 by, for example, welding or the like. The negative electrode current collector terminal is attached to the bundled portion of the negative electrode tab portion. The negative electrode current collector terminal is also joined to the negative electrode tab portion by welding or the like.

[0022] The positive electrode current collector terminal 30 and the negative electrode current collector terminal are each made of a metal with good conductivity, and the same material can be preferably used for the positive electrode current collector foil 41 and the negative electrode current collector foil 51, corresponding to the electrodes. The arrangement of the positive electrode current collector terminal 30 and the negative electrode current collector terminal is not particularly limited and can be designed appropriately depending on the configuration of the secondary battery 1. In this embodiment, the positive electrode current collector terminal 30 is arranged on one side in the width direction within the battery case, and the negative electrode current collector terminal is arranged on the other side in the width direction.

[0023] The electrode assembly 20 has a positive electrode sheet 40, a negative electrode sheet 50, and a separator 60. Electricity stored in the positive electrode sheet 40 and the negative electrode sheet 50 is collected by the positive electrode tab portion 21 and the negative electrode tab portion, respectively, and output from the positive electrode terminal 12 and the negative electrode terminal 13. The electrode assembly 20 may be a stacked electrode assembly in which a plurality of positive electrode sheets 40 and a plurality of negative electrode sheets 50 are stacked with a separator 60 interposed therebetween, or may be a wound electrode assembly in which long positive electrode sheets 40 and a long negative electrode sheet 50 are stacked with a long separator 60 interposed therebetween and wound in the lengthwise direction.

[0024] The positive electrode sheet 40 has a positive electrode current collector foil 41 and a positive electrode composite layer 42 formed on the positive electrode current collector foil 41. The positive electrode composite layer 42 is formed on a portion of each of both sides of the positive electrode current collector foil 41. The positive electrode sheet 40 also has a positive electrode tab portion 21 where the positive electrode composite layer 42 is not formed on either side of the positive electrode current collector foil 41 and the positive electrode current collector foil 41 is exposed.

[0025] The positive electrode current collector foil 41 is made of a foil of a metal with good conductivity (e.g., aluminum, aluminum alloy, nickel, titanium, stainless steel, etc.). The positive electrode mixture layer 42 is held by the positive electrode current collector foil 41 and contains at least a positive electrode active material capable of absorbing and releasing lithium ions. The positive electrode mixture layer 42 may also contain other optional components such as a conductive material and a binder. As the positive electrode active material, for example, lithium cobalt oxide (LiCoO), lithium nickel oxide (LiNiO), lithium manganese oxide (LiMnO), lithium iron phosphate (LiFePO), nickel cobalt lithium aluminum oxide (NCA), nickel cobalt lithium manganese oxide (NCM), etc. can be used alone or in combination.

[0026] The positive electrode sheet 40 is produced, for example, as follows. A positive electrode composite composition containing at least the above-described positive electrode active material is applied to both sides of the positive electrode current collector foil 41, excluding the positive electrode tab portion 21, and then dried and pressed to obtain the positive electrode sheet 40 in which the positive electrode composite layer 42 is formed on the positive electrode current collector foil 41. The positive electrode composite composition may be, for example, a paste or slurry composition having an appropriate viscosity prepared by kneading the positive electrode active material, other optional components, and a solvent.

[0027] Negative electrode sheet 50 has negative electrode current collector foil 51 and negative electrode composite layer 52 formed on negative electrode current collector foil 51. Negative electrode composite layer 52 is formed on a portion of each of both sides of negative electrode current collector foil 51. Negative electrode sheet 50 also has a negative electrode tab portion where negative electrode composite layer 52 is not formed on either side of negative electrode current collector foil 51 and negative electrode current collector foil 51 is exposed.

[0028] The negative electrode current collector foil 51 is made of a foil of a metal with good conductivity (for example, copper, a copper alloy, nickel, titanium, stainless steel, etc.). The negative electrode mixture layer 52 is held by the negative electrode current collector foil 51 and contains at least a negative electrode active material capable of absorbing and releasing lithium ions. The negative electrode mixture layer 52 may also contain other optional components such as a conductive material and a binder. As the negative electrode active material, for example, carbon materials such as natural graphite, artificial graphite, hard carbon, and soft carbon can be used alone or in combination.

[0029] Furthermore, in this embodiment, the width direction length of the negative electrode composite material layer 52 included in the negative electrode sheet 50 is longer than the width direction length of the positive electrode composite material layer 42 included in the positive electrode sheet 40. As a result, the negative electrode sheet 50 after the electrode body 20 has been formed has a negative electrode non-facing portion 53 that does not face the positive electrode composite material layer 42 on the positive electrode current collector terminal 30 side, and a negative electrode facing portion 54 that faces the positive electrode composite material layer 42 via the separator 60. Note that the negative electrode non-facing portion 53 is included in the end of the electrode body 20 on the positive electrode current collector terminal 30 side.

[0030] In secondary battery 1 according to this embodiment, negative electrode composite material layer 52 in negative electrode non-facing portion 53 is formed so as to have a smaller thickness in the stacking direction than negative electrode composite material layer 52 in negative electrode facing portion 54. In other words, thickness T10 of negative electrode non-facing portion 53 in the stacking direction is smaller than thickness T1 of negative electrode facing portion 54 in the stacking direction.

[0031] Negative electrode sheet 50 is produced, for example, as follows. A negative electrode composite composition containing at least the above-described negative electrode active material is applied to both surfaces of negative electrode current collector foil 51 excluding the negative electrode tab portion, and the resulting coating is dried and pressed to obtain a laminate in which negative electrode composite layer 52 before thinning is formed on negative electrode current collector foil 51. Note that the negative electrode composite composition may be, for example, a paste or slurry composition having an appropriate viscosity prepared by kneading the negative electrode active material, other optional components, and a solvent.

[0032] Of the laminate obtained in this manner, at least the end portion that becomes negative electrode non-facing portion 53 (the end portion that is arranged on the positive electrode current collector terminal 30 side of the laminate) is pressed, whereby negative electrode composite layer 52 is formed on negative electrode current collector foil 51, and negative electrode sheet 50 having negative electrode non-facing portion 53 and negative electrode facing portion 54 can be obtained. When the end portion that becomes negative electrode non-facing portion 53 is pressed, negative electrode non-facing portion 53 is formed in which the thickness of negative electrode composite layer 52 is reduced compared to before pressing, and accordingly negative electrode facing portion 54 is formed.

[0033] Furthermore, by pressing the end portion that will become the negative electrode non-facing portion 53, it is possible to form the negative electrode non-facing portion 53 that includes the negative electrode composite layer 52 with reduced internal voids and increased density, thereby improving the electrolyte retention ability when the electrode body 20 expands and contracts due to charging and discharging of the secondary battery 1. From the viewpoint of improving the electrolyte retention ability, when performing pressing to form the negative electrode non-facing portion 53, it is preferable to press both end portions of the laminate (the end portion that will become the negative electrode non-facing portion 53 and the end portion on the negative electrode current collector side).

[0034] Separator 60 is made of an insulating porous sheet made of polyethylene (PE), polypropylene (PP), etc. Such a porous sheet may have a single layer structure or a laminated structure of two or more layers.

[0035] Furthermore, in this embodiment, the length in the width direction of the separator 60 is longer than the length in the width direction of the positive electrode mixture layer 42 included in the positive electrode sheet 40. As a result, the separator 60 after the electrode body 20 is formed has a separator end portion 61 that does not face the positive electrode mixture layer 42 on the positive electrode current collector terminal 30 side, and a separator central portion 62 that is interposed between the positive electrode mixture layer 42 and the negative electrode mixture layer 52. Note that the separator end portion 61 is included in the end portion of the electrode body 20 on the positive electrode current collector terminal 30 side.

[0036] In the secondary battery 1 according to this embodiment, the separator end portion 61 may be formed so that its thickness in the stacking direction is smaller than that of the separator central portion 62. That is, the thickness z10 of the separator end portion 61 in the stacking direction is smaller than the thickness z of the separator central portion 62 in the stacking direction.

[0037] Separator 60 is produced, for example, as follows: Of a separator material having thickness z, at least the end portion that will become separator end portion 61 (the end portion of the separator material that is disposed on the positive electrode current collector terminal 30 side) is heated from above and below with a heat source to obtain separator 60. When the end portion that will become separator end portion 61 is heated, separator end portion 61 that is thinner than before heating is formed, and separator center portion 62 is also formed.

[0038] Furthermore, by heating the end portion that will become the separator end portion 61, the internal voids are reduced, and a highly densified separator end portion 61 can be formed, thereby improving the piercing strength of this portion. From the viewpoint of improving the piercing strength, when heating the separator material, it is preferable to heat both ends of the separator material (the end portion that will become the separator end portion 61 and the end portion on the negative electrode current collector side). When heating the separator material, the heated end portion may be pressurized as necessary.

[0039] The electrode assembly 20, in which the positive electrode sheet 40, the negative electrode sheet 50, and the separator 60 are stacked, has a region where the positive electrode composite layer 42, the negative electrode composite layer 52, and the separator 60 are stacked. The positive electrode tab portion 21 is formed by stacking the positive electrode sheet 40 with the exposed positive electrode current collector foil 41 where the positive electrode composite layer 42 is not formed, protruding outward from this region. The positive electrode tab portion 21 is bundled from both sides in the stacking direction and electrically connected to the positive electrode current collector terminal 30 attached to this bundled portion. The negative electrode tab portion is formed by stacking the negative electrode sheet 50 with the exposed negative electrode current collector foil 51 where the negative electrode composite layer 52 is not formed, protruding outward from this region. The negative electrode tab portion is also bundled from both sides in the stacking direction and electrically connected to the negative electrode current collector terminal attached to this bundled portion.

[0040] The directions in which the positive electrode tab portion 21 and the negative electrode tab portion protrude are not particularly limited and are designed appropriately depending on the structure of the secondary battery 1. In this embodiment, the positive electrode tab portion 21 and the negative electrode tab portion each protrude outward in the width direction.

[0041] Here, problems that arise when the negative electrode non-facing portion 53 and the separator end portion 61 are not thinned will be described using the secondary battery 100 shown in Fig. 3 as an example. Fig. 3 is a cross-sectional view showing the structure of the positive electrode current collector terminal side of a secondary battery in which the negative electrode non-facing portion and the separator end portion are not thinned.

[0042] 3, the secondary battery 100 has the same configuration as the secondary battery 1, except that the thickness T10 of the negative electrode non-facing portion 53 and the thickness z10 of the separator end portion 61 are not reduced. That is, the negative electrode non-facing portion 53 and the negative electrode facing portion 54 each have a thickness T1, and the separator end portion 61 and the separator central portion 62 each have a thickness z.

[0043] When joining the positive electrode tab portion 21 to the positive electrode current collector terminal 30, the periphery of the joint where the positive electrode tab portion 21 and the positive electrode current collector terminal 30 are joined is pressed from the outside in the stacking direction, and the positive electrode tab portions 21 are joined to the positive electrode current collector terminal 30 in a bundled state. Therefore, the positive electrode tab portion 21 located on the outside in the stacking direction is pulled toward the joint with a greater force. This pulling causes local stress in the buckled portion of the positive electrode tab portion 21 at edge position P3 of the positive electrode current collector terminal 30. This may cause foil tearing of the positive electrode tab portion 21 near the joint.

[0044] Furthermore, by bundling the positive electrode tab portions 21, the separator end portion 61 sandwiched between the positive electrode tab portions 21 and the negative electrode non-facing portion 53 comes into contact with the adjacent negative electrode non-facing portion 53, applying pressure to the separator end portion 61. When pressure is applied to the separator end portion 61 in this manner, local stress is generated in the buckled portion of the separator end portion 61 at the edge position P2 of the negative electrode non-facing portion 53. This phenomenon is more pronounced in separators 60 arranged on the outer side in the stacking direction. If the separator 60 is damaged in this manner, a short circuit may occur between the negative electrode sheet 50 and the positive electrode sheet 40.

[0045] Therefore, it is necessary to reduce these stresses and prevent breakage of the ends of the electrode body 20. According to the secondary battery 1 of this embodiment, good battery performance can be obtained by reducing these stresses and preventing breakage of the ends of the electrode body 20.

[0046] In the secondary battery 1 according to this embodiment, the negative electrode composite material layer 52 in the negative electrode non-facing portion 53 is formed to have a thickness smaller than that of the negative electrode composite material layer 52 in the negative electrode facing portion 54. With this configuration, it is possible to suppress breakage of the separator 60 caused by the separator end 61 sandwiched between the positive electrode tab portions 21 and the negative electrode non-facing portions 53 bundled in the stacking direction coming into contact with the adjacent negative electrode non-facing portion 53 and applying pressure to the separator end 61. In addition, it is possible to reduce the force with which the positive electrode tab portions 21 bundled in the stacking direction are pulled toward the joint portion with the positive electrode current collector terminal 30, thereby suppressing foil tearing of the positive electrode tab portions 21 near the joint portion.

[0047] Furthermore, as described above, since negative electrode non-facing portion 53 has densified negative electrode composite layer 52, the electrolyte retention is improved when electrode body 20 expands and contracts due to charging and discharging of secondary battery 1. As a result, an increase in the internal resistance of secondary battery 1 can be suppressed.

[0048] More preferably, the secondary battery 1 according to this embodiment is formed so that the separator end portion 61 is thinner than the separator center portion 62. This not only effectively prevents breakage of the separator 60 and foil tearing, but also improves the strength of the separator 60 as described above.

[0049] More preferably, in the secondary battery 1 according to this embodiment, when compared to a total thickness T1+z obtained by adding up the thickness T1 of the negative electrode facing portion 54 along the stacking direction and the thickness z of the separator central portion 62 adjacent to the negative electrode facing portion 54 along the stacking direction, the thickness reduction rate k (%) of the total thickness T10+z10 obtained by adding up the thickness T10 of the negative electrode non-facing portion 53 along the stacking direction and the thickness z10 of the separator end portion 61 adjacent to the negative electrode non-facing portion 53 along the stacking direction satisfies the following formula (1): k≦[{(a+b)y-ac} / {(a+b}(x+z)}]×100...Equation (1)

[0050] Here, in formula (1), a is the length along the width direction of the negative electrode non-facing portion 53. b is the length along the width direction between the negative electrode non-facing portion 53 and the positive electrode current collector terminal 30. c is the length along the stacking direction between the outer edge position P4 in the stacking direction of the positive electrode sheet 40 arranged at the outermost position in the stacking direction and the center position P5 of the electrode assembly 20 in the stacking direction. Note that the center position P5 exists on the central axis C of the electrode assembly 20. x is the thickness along the stacking direction of the negative electrode composite layer 52 of the negative electrode facing portion 54. y is the thickness along the stacking direction of the positive electrode composite layer 42. z is the thickness along the stacking direction of the separator center portion 62.

[0051] By satisfying the above formula (1), the effects of the present disclosure can be obtained more effectively. Therefore, a method for calculating the above formula (1) will be described with reference to Figures 2 and 3. In the following description, the positive electrode sheet 40, the negative electrode sheet 50, and the separator 60 each refer to the sheet arranged at the outermost side in the stacking direction.

[0052] 2 and 3, in the secondary battery 1, the angle θ1 is made smaller so as to approach the angle θ2 compared to the case of the secondary battery 100, while the angle θ3 is made larger compared to the case of the secondary battery 100. This provides the effect of suppressing breakage of the separator 60 and the effect of suppressing foil tearing.

[0053] Here, angle θ1 is the angle between an imaginary line L1 along the stacking direction that passes through edge position P1 and the positive electrode tab portion 21 between edge positions P1 and P2. Furthermore, angle θ2 is the angle between imaginary line L1 and an imaginary line L2 that connects edge position P1 of the positive electrode tab portion 21 arranged outwardly and edge position P3 of the positive electrode current collector terminal 30. Furthermore, angle θ3 is the angle between the positive electrode tab portion 21 that is outward of edge position P3 in the width direction and the positive electrode tab portion 21 between edge positions P2 and P3.

[0054] In a right triangle having an acute angle θ1 and defined by imaginary line L1, separator end 61 extending between edge position P1 and edge position P2, and positive electrode tab portion 21, the following formula (2) holds: tanθ1=a / {y+(x+z)k / 100}...Equation (2)

[0055] In a right triangle having an acute angle θ2 and defined by imaginary lines L1, L2, and the central axis C, the following formula (3) holds. tanθ2=(a+b} / c···Eq. (3)

[0056] Then, when tan θ1=tan θ2, the angle θ3 becomes the largest, and therefore the following equation (4) is derived based on the above equations (2) and (3). k=[{(a+b)y-ac} / {(a+b ) (x+z)}]×100...Equation (4)

[0057] Therefore, the thickness reduction rate k (%) can be calculated using the above formula (4). In this way, the thickness T10 of the negative electrode non-facing portion 53 and the thickness z10 of the separator end 61 in the secondary battery 1 can be appropriately set. [Explanation of symbols]

[0058] 1, 100 secondary battery 10 Case body 11 Lid 12 Positive terminal 13 Negative terminal 20 Electrode body 21 Positive electrode tab 30 Positive current collecting terminal 40 Positive electrode sheet 41 Positive electrode current collector foil 42 Positive electrode mixture layer 50 negative electrode sheet 51 Negative electrode current collecting foil 52 Negative electrode composite layer 53 Negative electrode unopposed part 54 negative electrode facing part 60 Separator 61 Separator end 62 Center of separator C center axis L1, L2 imaginary line P1, P2, P3, P4 edge position P5 center position

Claims

1. an electrode assembly in which a positive electrode sheet having a positive electrode composite layer formed on a positive electrode current collector foil and a negative electrode sheet having a negative electrode composite layer formed on a negative electrode current collector foil are stacked in a stacking direction with a separator interposed therebetween; a positive electrode current collecting terminal attached to a bundled portion of the positive electrode tab portion of the electrode body in which the exposed positive electrode current collecting foil on which the positive electrode composite layer is not formed is laminated, the negative electrode sheet has a negative electrode non-facing portion that does not face the positive electrode mixture layer on the positive electrode current collector terminal side, and a negative electrode facing portion that faces the positive electrode mixture layer via the separator, the negative electrode composite material layer in the negative electrode non-facing portion is formed to have a thickness in the stacking direction that is smaller than that of the negative electrode composite material layer in the negative electrode facing portion, the separator has a separator end portion that does not face the positive electrode mixture layer on the positive electrode current collector terminal side, and a separator central portion that is sandwiched between the positive electrode mixture layer and the negative electrode mixture layer, When compared with a total thickness obtained by adding together the thickness of the negative electrode facing portion along the stacking direction and the thickness of a central portion of the separator adjacent to the negative electrode facing portion along the stacking direction, a thickness reduction rate k (%) of the total thickness obtained by adding together the thickness of the negative electrode non-facing portion along the stacking direction and the thickness of an end portion of the separator adjacent to the negative electrode non-facing portion along the stacking direction satisfies the following formula (1): Secondary battery. k≦[{(a+b)y-ac} / {(a+b)(x+z)}]×100...Formula (1) Here, in formula (1), a is the length of the negative electrode non-facing portion along the width direction perpendicular to the stacking direction, b is the length along the width direction between the negative electrode non-facing portion and the positive electrode current collecting terminal, c is the length along the stacking direction between the outer edge position in the stacking direction of the positive electrode sheet arranged at the outermost side in the stacking direction and the center position of the electrode body in the stacking direction, x is the thickness of the negative electrode composite layer of the negative electrode facing portion along the stacking direction, y is the thickness of the positive electrode composite layer along the stacking direction, and z is the thickness of the separator center portion along the stacking direction.

2. A secondary battery as described in Claim 1, wherein the separator end portion is formed so that the thickness in the stacking direction is smaller than that of the separator center portion.

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