Secondary battery and method for manufacturing a secondary battery

Abstract

The present invention provides a secondary battery and a method for manufacturing a secondary battery that suppresses metal deposition in the curved portion of a wound body. [Solution] The secondary battery comprises a winding body 20 having a flat shape formed by winding a laminate in which a positive electrode sheet and a negative electrode sheet are stacked with a separator in between, and an electrolyte. The Young's modulus of the separator at the end of the curved portion 20A located in the longitudinal axis direction D3 in a cross-section including the winding direction of the winding body 20 is lower than the Young's modulus of the separator at the center of the flat portion 20B located in the longitudinal axis direction D3 in the cross-section.

Description

Technical Field

[0001] The present invention relates to a secondary battery and a method for manufacturing the secondary battery.

Background Art

[0002] A method for manufacturing a wound body of a secondary battery is to wind a laminate in which a long positive electrode sheet and a negative electrode sheet are laminated via a separator, and then press the wound body removed from the winding core in one radial direction to shape the wound body into a flat shape (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, the wound body pressed into a flat shape has, in a cross section including the winding direction, a curved portion located at both ends in the long axis direction of the wound body and a flat portion connecting the two curved portions along the long axis direction. The Young's modulus of the separator in the curved portion having a curvature larger than the curvature before pressing is higher than the Young's modulus of the separator in the flat portion. Therefore, there is a risk that a gap may occur between the separator and the negative electrode sheet in the curved portion of the wound body and metal may be deposited.

Means for Solving the Problems

[0005] The secondary battery for solving the above problems includes a wound body having a flat shape in which a laminate in which a positive electrode sheet and a negative electrode sheet are laminated via a separator is wound, and an electrolytic solution. The Young's modulus of the separator in the curved portion located at the end in the long axis direction in the cross section including the winding direction of the wound body is lower than the Young's modulus of the separator in the flat portion located at the center in the long axis direction in the cross section.

[0006] According to the above configuration, the Young's modulus of the separator in the curved portion of the wound body is lower than the Young's modulus of the separator in the flat portion of the wound body. Therefore, the gap between the separator and the negative electrode sheet, which is caused by the Young's modulus of the separator being higher in the curved portion than in the flat portion, is less likely to occur in the curved portion, and metal deposition can be suppressed.

[0007] With respect to the above secondary battery, it is preferable that the Young's modulus of the separator in the curved portion is 0.9 GPa or less, and the Young's modulus of the separator in the flat portion is 1.1 GPa or more.

[0008] According to the above configuration, the Young's modulus of the separator in the curved portion of the wound body is lower than that of the separator in the flat portion of the wound body. In addition, the Young's modulus of the separator in the curved portion is 0.9 GPa or less, while the Young's modulus of the separator in the flat portion is 1.1 GPa or more. Therefore, the gap between the separator and the negative electrode sheet, which is caused by the Young's modulus of the separator being higher in the curved portion than in the flat portion, is less likely to occur in the curved portion, and metal deposition can be further suppressed.

[0009] A method for manufacturing a secondary battery that solves the above problems is a method for manufacturing a secondary battery comprising a wound body and an electrolyte, comprising: a winding step of winding a laminate in which a positive electrode sheet and a negative electrode sheet are laminated with a separator between them onto a winding core along the winding direction; and a pressing step of pressing the wound body removed from the winding core into a flattened shape, wherein in the winding step, a first heated portion is left in the winding direction of the wound body wound onto the winding core, and an unheated portion is left in the winding direction. The process involves heating a second heated portion, which is spaced apart from the first heated portion and is a part of the core opposite to the first heated portion relative to the center of the core, from the core, and pressing the winding body removed from the core to flatten it so that the first heated portion and the second heated portion are brought closer together, and pressing the winding body after pressing so that the entire end in the longitudinal direction of the cross-section including the winding direction is the unheated portion.

[0010] According to the above method, the Young's modulus of the separator in the first and second heated portions of the wound body is lower than that of the separator in the unheated portion. In the flattened wound body, the flat portion is formed from the first and second heated portions, and the curved portion connecting the two flat portions is formed from the unheated portion. As a result, the gap between the separator and the negative electrode sheet, which is caused by the Young's modulus of the separator being higher in the curved portion than in the flat portion, is less likely to occur in the curved portion, and metal deposition can be suppressed. In addition, crystallization of the separator progresses more in the first and second heated portions of the wound body due to heating than in the unheated portion. As a result, in the wound body when wound on a core, the direction in which internal stress acts is dispersed rather than concentrated toward the center of the wound body. Therefore, the ease with which the wound body can be removed from the core is improved. [Effects of the Invention]

[0011] According to the present invention, it is possible to suppress the deposition of metal in the curved portion of the wound body. [Brief explanation of the drawing]

[0012] [Figure 1] This is a perspective view showing the schematic configuration of a secondary battery according to one embodiment. [Figure 2] This is a diagram showing a portion of the wound body of the secondary battery of the same embodiment unfolded. [Figure 3] This is a flowchart showing the method for manufacturing the wound body according to the same embodiment. [Figure 4] This is an end view showing the stress acting on a conventional wound body after winding. [Figure 5] This is an end view showing a conventional rolled body after pressing. [Figure 6] This is an end view showing the winding core and the winding body wound around the winding core according to the same embodiment. [Figure 7] This is an end view showing the stress acting on the wound body after winding according to the same embodiment. [Figure 8] This is an end view showing the wound body after pressing according to the same embodiment. [Figure 9] This is a flowchart showing the method for manufacturing a secondary battery according to the same embodiment. [Figure 10] This table shows examples and comparative examples of secondary batteries. [Modes for carrying out the invention]

[0013] [This Circumstance] An embodiment of a secondary battery and a method for manufacturing a secondary battery will be described below with reference to Figures 1 to 9. A lithium-ion secondary battery will be described as an example of a secondary battery.

[0014] [Lithium-ion rechargeable battery 10] As shown in Figure 1, the lithium-ion secondary battery 10, which is a non-aqueous secondary battery, is a cell battery that forms a battery pack when multiple lithium-ion secondary batteries 10 are combined and enclosed in a resin or metal case. The battery pack is used in hybrid vehicles and electric vehicles.

[0015] The lithium-ion secondary battery 10 includes a battery case 11 and a lid 12. The battery case 11 has a rectangular parallelepiped shape with an opening on the upper side. The lid 12 seals the opening of the battery case 11. The battery case 11 and the lid 12 are made of a metal such as aluminum or an aluminum alloy. The lithium-ion secondary battery 10 forms a sealed battery cell by attaching the lid 12 to the battery case 11.

[0016] Two positive electrode external terminals 13A and a negative electrode external terminal 13B are provided on the lid 12. The positive electrode external terminal 13A and the negative electrode external terminal 13B are used for charging and discharging electric power. A plurality of wound bodies 20 are accommodated inside the battery case 11. The wound body 20 is an electrode body. The positive electrode current collector portion 28A, which is the end portion on the positive electrode side of the wound body 20, is electrically connected to the positive electrode external terminal 13A via the positive electrode current collector member 14A. The negative electrode current collector portion 28B, which is the end portion on the negative electrode side of the wound body 20, is electrically connected to the negative electrode external terminal 13B via the negative electrode current collector member 14B. Also, a non-aqueous electrolyte is injected into the battery case 11 through a liquid injection hole (not shown). Note that the shapes of the positive electrode external terminal 13A and the negative electrode external terminal 13B are not limited to the shapes shown in FIG. 1 and may be any shape.

[0017] [Wound body 20] As shown in FIG. 2, the wound body 20 is a flat electrode body formed by winding a laminate in which a long positive electrode sheet 21 and a negative electrode sheet 24 are laminated via a separator 27. The positive electrode sheet 21, the negative electrode sheet 24, and the separator 27 are laminated such that their longitudinal directions coincide with the longitudinal direction D1. Before winding, the laminate is laminated in the order of the positive electrode sheet 21, the separator 27, the negative electrode sheet 24, and the separator 27.

[0018] [Positive electrode sheet 21] The positive electrode sheet 21 comprises a positive electrode current collector 22 and a positive electrode composite layer 23. The positive electrode current collector 22 is a foil-shaped positive electrode substrate formed in an elongated shape. The positive electrode composite layer 23 is provided on each of two opposing surfaces of the positive electrode current collector 22. The positive electrode current collector 22 has an uncoated positive electrode side portion 22A at one end in the width direction D2 where the positive electrode composite layer 23 is not formed and the positive electrode current collector 22 is exposed.

[0019] The positive electrode current collector 22 is made of a metal foil composed of aluminum or an alloy mainly composed of aluminum. The positive electrode current collector 22 functions as a current collector at the positive electrode. The unpainted positive electrode side portion 22A of the positive electrode current collector 22 is pressed against each other by opposing surfaces in the state of the wound body 20, forming the positive electrode side current collector portion 28A.

[0020] The positive electrode composite layer 23 is a cured form of a liquid positive electrode composite paste. The positive electrode composite paste contains a positive electrode active material, a positive electrode solvent, a positive electrode conductive material, and a positive electrode binder. The positive electrode composite layer 23 is formed when the positive electrode composite paste dries and the positive electrode solvent vaporizes. Therefore, the positive electrode composite layer 23 contains a positive electrode active material, a positive electrode conductive material, and a positive electrode binder.

[0021] The positive electrode active material is a lithium-containing composite oxide capable of intercalating and releasing lithium ions, which are charge carriers in the lithium-ion secondary battery 10. The lithium-containing composite oxide is an oxide containing lithium and other metallic elements other than lithium. The other metallic elements other than lithium are, for example, at least one selected from the group consisting of nickel, cobalt, manganese, vanadium, magnesium, molybdenum, niobium, titanium, tungsten, aluminum, and iron contained as iron phosphate in the lithium-containing composite oxide.

[0022] For example, lithium-containing composite oxides include lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), and lithium manganate (LiMn2O4). Another example is lithium-containing composite oxide, a ternary lithium-containing composite oxide containing nickel, cobalt, and manganese, which is lithium nickel-cobalt-manganate (LiNiCoMnO2). Yet another example is lithium iron phosphate (LiFePO4).

[0023] The positive electrode solvent is an NMP (N-methyl-2-pyrrolidone) solution, which is an example of an organic solvent. Examples of positive electrode conductive materials include carbon black such as acetylene black and Ketjenblack, carbon fibers such as carbon nanotubes and carbon nanofibers, and graphite. The positive electrode binder is an example of a resin component contained in the positive electrode paste. Examples of positive electrode binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), and styrene-butadiene rubber (SBR).

[0024] The positive electrode sheet 21 may have an insulating layer at the boundary between the uncoated portion 22A on the positive electrode side and the positive electrode composite layer 23. The insulating layer contains an inorganic component having insulating properties and a resin component that functions as a binder. The inorganic component is at least one selected from the group consisting of powdered boehmite, titania, and alumina. The resin component is at least one selected from the group consisting of PVDF, PVA, and acrylic.

[0025] [Negative electrode sheet 24] The negative electrode sheet 24 comprises a negative electrode current collector 25 and a negative electrode composite layer 26. The negative electrode current collector 25 is a foil-shaped negative electrode substrate formed in an elongated shape. The negative electrode composite layer 26 is provided on each of two opposing surfaces of the negative electrode current collector 25. The negative electrode current collector 25 has a negative electrode side unpainted portion 25A at one end in the width direction D2, which is located opposite the positive electrode side unpainted portion 22A, where the negative electrode composite layer 26 is not formed and the negative electrode current collector 25 is exposed.

[0026] The negative electrode current collector 25 is made of metal foil composed of copper or an alloy mainly composed of copper. The negative electrode current collector 25 functions as a current collector at the negative electrode. In the state of the wound body 20, the unpainted negative electrode portion 25A has opposing surfaces pressed against each other to form the negative electrode current collector portion 28B.

[0027] The negative electrode composite layer 26 is a cured body of a liquid negative electrode composite paste. The negative electrode composite paste contains a negative electrode active material, a lithium salt, a negative electrode solvent, a negative electrode thickener, and a negative electrode binder. The negative electrode composite layer 26 is formed when the negative electrode composite paste dries and the negative electrode solvent vaporizes. Therefore, the negative electrode composite layer 26 contains the negative electrode active material, a lithium salt, and further, as additives, a negative electrode thickener and a negative electrode binder. The negative electrode composite layer 26 may further contain additives such as a conductive material.

[0028] The negative electrode active material is a material capable of intercalating and releasing lithium ions. Examples of negative electrode active materials include carbon materials such as graphite, poorly graphitizable carbon, and easily graphitizable carbon. The negative electrode solvent is, for example, water. The lithium salt can be one or more lithium compounds (lithium salts) selected from LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC4F9SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiI, LiBOB (lithium bisoxalate borate), etc. As an example of a negative electrode thickener, CMC (carboxymethylcellulose) can be used as a thickener containing a sodium salt. The negative electrode binder can be the same as that used for the positive electrode binder. As an example of a negative electrode binder, SAR (styrene-acrylic acid copolymer) can be used as a binder containing a sodium salt.

[0029] [Separator 27] The separator 27 prevents contact between the positive electrode sheet 21 and the negative electrode sheet 24, and holds the non-aqueous electrolyte between the positive electrode sheet 21 and the negative electrode sheet 24. When the wound body 20 is immersed in the non-aqueous electrolyte, the non-aqueous electrolyte penetrates from the ends in the width direction D2 of the separator 27 toward the center.

[0030] The separator 27 can be, for example, a porous polymer membrane such as a porous polyethylene membrane, a porous polyolefin membrane, or a porous polyvinyl chloride membrane, or an ion-conductive polymer electrolyte membrane.

[0031] [Nonaqueous electrolyte] A non-aqueous electrolyte is a composition containing a supporting salt in a non-aqueous solvent. As the non-aqueous solvent, one or more materials selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, etc., can be used. As the supporting salt, one or more lithium compounds (lithium salts) selected from LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC4F9SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiI, etc., can be used.

[0032] [Manufacturing method for lithium-ion secondary battery 10] Next, the manufacturing method of the lithium-ion secondary battery 10 will be described with reference to Figures 3 to 9. As shown in Figure 3, the manufacturing method of the lithium-ion secondary battery 10 includes a winding step, a temporary pressing step, and a flattening step for manufacturing the wound body 20.

[0033] [Winding process] In the winding process of step S1 in Figure 3, a laminate in which the positive electrode sheet 21 and the negative electrode sheet 24 are stacked with a separator 27 is wound onto a winding core along the winding direction to produce a wound body 20.

[0034] [Preliminary pressing process] In the preliminary pressing step S2 in Figure 3, the winding body 20 removed from the core is pressed from both sides of one radial direction at the center in a radial direction perpendicular to that radial direction, thereby shaping the winding body 20 into one with a concave center in the radial direction perpendicular to that radial direction.

[0035] [Flat pressing process] In the flattening press process of step S3 in Figure 3, the pre-pressed coiled body 20 is compressed from both sides in one radial direction to form a flattened coiled body 20.

[0036] Furthermore, as shown by the arrows in Figure 4, in the conventional winding body 120, when wound around the winding core 130, the direction in which internal stress acts is concentrated toward the center of the winding body 120. For this reason, when removing the conventional winding body 120 from the winding core 130, the innermost separator 27 may shift.

[0037] Furthermore, as shown in Figure 5, when the conventional winding body 120 removed from the winding core 130 is pressed into a flattened shape, the cross-section including the winding direction has curved portions 120A located at both ends of the long axis direction D3 of the winding body 120, and a flat portion 120B connecting the two curved portions 120A along the long axis direction D3. For this reason, the Young's modulus of the separator 27 in the curved portion 120A, which has a greater curvature than the curvature before pressing, is higher than the Young's modulus of the separator 27 in the flat portion 120B.

[0038] [Heating during the winding process] Therefore, in this embodiment, as shown in Figure 6, the winding core 30 is equipped with a first heating element 40A and a second heating element 40B. The wound body 20 wound around the winding core 30 is heated from the winding core 30. That is, the wound body 20 is heated from the winding core 30 in a first heated portion 50A and a second heated portion 50B. The first heated portion 50A is a part of the wound body 20 wound around the winding core 30 in the winding direction. The second heated portion 50B is spaced apart from the first heated portion 50A in the winding direction, leaving an unheated portion 50C, and is a part of the winding core 30 on the opposite side from the first heated portion 50A with respect to the center.

[0039] As a result, the crystallization of the separator 27 progresses more rapidly in the first heated portion 50A and the second heated portion 50B of the wound body 20 than in the unheated portion 50C due to heating. Therefore, as shown by the arrows in Figure 7, in the heated wound body 20, the direction in which internal stress acts is dispersed rather than concentrated toward the center of the wound body 20.

[0040] Then, as shown in Figure 8, the winding body 20 removed from the core 30 is flattened by pressing so that the first heated portion 50A and the second heated portion 50B are brought closer together. Furthermore, the winding body 20 is pressed so that the entire end portion in the longitudinal axis direction D3 of the cross-section including the winding direction is an unheated portion 50C.

[0041] As a result, the flat portion 20B of the flattened wound body 20 is formed from a first heated portion 50A and a second heated portion 50B, while the curved portion 20A connecting the two flat portions 20B, 20B is formed from an unheated portion 50C. Therefore, the Young's modulus of the separator 27 in the first heated portion 50A and the second heated portion 50B of the wound body 20 is lower than the Young's modulus of the separator 27 in the unheated portion 50C. In other words, the Young's modulus of the separator 27 in the curved portion 20A located at the end in the longitudinal axis direction D3 in the cross-section including the winding direction of the wound body 20 is lower than the Young's modulus of the separator 27 in the flat portion 20B located in the center in the longitudinal axis direction D3 in the cross-section. Furthermore, because the Young's modulus of the separator 27 of the wound body 20 is higher in the curved portion 20A than in the flat portion 20B, the gap between the separator 27 and the negative electrode sheet 24 is less likely to occur in the curved portion 20A, thereby suppressing the deposition of metallic lithium. Preferably, the Young's modulus of the separator 27 in the curved portion 20A of the wound body 20 is 0.9 GPa or less, and the Young's modulus of the separator 27 in the flat portion 20B is 1.1 GPa or more. The Young's modulus is a value obtained by a tensile test method in accordance with JIS K7161-1:2014, and is obtained as an average value by measuring multiple points in the entirety of the curved portion 20A and the flat portion 20B cut out from the wound body 20. Alternatively, the Young's modulus may be a value obtained by measuring a portion of the curved portion 20A and the flat portion 20B cut out from the wound body 20.

[0042] Furthermore, as shown in Figure 9, the manufacturing method of the lithium-ion secondary battery 10 includes, after manufacturing the wound body 20, a terminal joining step, a sealing step, a cell drying step, a liquid injection and sealing step, and an activation step.

[0043] [Terminal joining process] Next, in the terminal joining process of step S11 in Figure 9, the positive electrode external terminal 13A is joined to the positive electrode side current collector portion 28A, which is the positive electrode end of the winding body 20 in the axial direction, via the positive electrode side current collector member 14A. In addition, the negative electrode external terminal 13B is joined to the negative electrode side current collector portion 28B, which is the negative electrode end of the winding body 20 in the axial direction, via the negative electrode side current collector member 14B.

[0044] [Can sealing process] Next, in the sealing process of step S12 in Figure 9, the wound body 20, to which the positive external terminal 13A and the negative external terminal 13B are joined, is inserted into the battery case 11, and the lid 12 is attached to close the opening of the battery case 11, for example by laser welding.

[0045] [Cell drying process] Next, in the cell drying process of step S13 in Figure 9, the sealed battery case 11 is heated to dry the wound body 20.

[0046] [Liquid injection / sealing process] Next, in the liquid injection and sealing step S14 in Figure 9, a non-aqueous electrolyte is injected through an inlet (not shown) formed in the lid 12, and a cap (not shown) is attached to the inlet, for example by laser welding, to seal it.

[0047] [Activation process] Next, in the activation process of step S15 in Figure 9, the lithium-ion secondary battery 10 is initially charged and stored at a high temperature for a certain period of time to dissolve metallic foreign matter and stabilize the SEI (Solid Electrolyte Interphase) coating.

[0048] [Effects of this embodiment] Next, the effects of this embodiment will be described. (1) The Young's modulus of the separator 27 in the curved portion 20A of the wound body 20 is lower than the Young's modulus of the separator 27 in the flat portion 20B of the wound body 20. As a result, the gap between the separator 27 and the negative electrode sheet 24, which is caused by the Young's modulus of the separator 27 being higher in the curved portion 20A than in the flat portion 20B, is less likely to occur in the curved portion 20A, and the deposition of metallic lithium can be suppressed.

[0049] (2) In addition to the fact that the Young's modulus of the separator 27 in the curved portion 20A of the wound body 20 is lower than that of the separator 27 in the flat portion 20B of the wound body 20, the Young's modulus of the separator 27 in the curved portion 20A is 0.9 GPa or less, and the Young's modulus of the separator 27 in the flat portion 20B is 1.1 GPa or more. As a result, the gap between the separator 27 and the negative electrode sheet 24 caused by the Young's modulus of the separator 27 of the wound body 20 being higher in the curved portion 20A than in the flat portion 20B becomes even less likely to occur in the curved portion 20A, and the deposition of metallic lithium can be further suppressed.

[0050] (3) The Young's modulus of the separator 27 in the first heated portion 50A and the second heated portion 50B of the wound body 20 is lower than that of the separator 27 in the unheated portion 50C. The flat portion 20B of the flattened wound body 20 is formed from the first heated portion 50A and the second heated portion 50B, and the curved portion 20A connecting the two flat portions 20B is formed from the unheated portion 50C. As a result, the gap between the separator 27 and the negative electrode sheet 24 caused by the Young's modulus of the separator 27 being higher in the curved portion 20A than in the flat portion 20B is less likely to occur in the curved portion 20A, and the deposition of metallic lithium can be suppressed. In addition, the crystallization of the separator 27 progresses more in the first heated portion 50A and the second heated portion 50B of the wound body 20 due to heating than in the unheated portion 50C. Therefore, in the wound body 20 when wound around the core 30, the direction in which internal stress acts is dispersed rather than concentrated toward the center of the wound body 20. As a result, the ease with which the wound body 20 can be removed from the core 30 is improved.

[0051] (Other embodiments) The above embodiment can be implemented with the following modifications. The above embodiment and the following modifications can be combined with each other to the extent that they do not contradict each other technically.

[0052] In the above embodiment, the winding body 20 was pressed in the pressing process such that the entire end portion in the longitudinal axis direction D3 of the cross-section including the winding direction was an unheated portion 50C. Alternatively, the winding body 20 may be pressed in the pressing process such that a portion of the end portion in the longitudinal axis direction D3 of the cross-section including the winding direction was an unheated portion 50C.

[0053] In the above embodiment, the Young's modulus of the separator 27 in the curved portion 20A of the wound body 20 is 0.9 GPa or less, and the Young's modulus of the separator 27 in the flat portion 20B is 1.1 GPa or more. However, it is sufficient if the Young's modulus of the separator 27 in the curved portion 20A of the wound body 20 is lower than the Young's modulus of the separator 27 in the flat portion 20B.

[0054] In the above embodiment, the present invention was applied to a lithium-ion secondary battery 10. However, the present invention is not limited to a lithium-ion secondary battery 10; it may also be applied to other secondary batteries, such as nickel-metal hydride secondary batteries, as long as they have a flattened wound body.

[0055] Secondary batteries may be installed in automated transport machines, special vehicles for cargo handling, electric vehicles, hybrid vehicles, etc., as well as in computers and other electronic devices, or they may constitute other systems. For example, they may be installed in mobile bodies such as ships and aircraft, or they may be part of a power supply system that supplies electricity from a power plant to buildings and homes where secondary batteries are installed via substations, etc.

[0056] [Examples] Next, with reference to Figure 10, examples and comparative examples of lithium-ion secondary batteries 10 will be described. Note that these examples and comparative examples do not limit the method of manufacturing secondary batteries. The separator 27 of the lithium-ion secondary batteries 10 in the examples and comparative examples consists of three layers: porous polypropylene (PP) resin, porous polyethylene (PE) resin, and porous polypropylene (PP) resin.

[0057] In the following, as shown in Figure 10, lithium-ion secondary batteries 10 of examples and comparative examples were prepared by changing the combination of heating conditions and the Young's modulus of the curved portion 20A and the flat portion 20B of the separator 27 of the wound body 20. The heating conditions refer to the surface temperature of the heating element. The Young's modulus of the curved portion 20A of the separator 27 is the Young's modulus of the entire curved portion 20A cut out from the wound body 20. The Young's modulus of the flat portion 20B of the separator 27 is the Young's modulus of the entire flat portion 20B cut out from the wound body 20. Note that the Young's modulus can be changed by changing the tension of the separator 27 during winding. For each example and comparative example, the precipitation resistance of the curved portion 20A of the separator 27 and the tightness of the wound body 20 were evaluated.

[0058] [Comparative Example 1] Without a heating element on the winding core 30, and without heating the winding body 20, the Young's modulus of the separator 27 in the curved portion 20A of the winding body 20 is 0.8 [GPa], and the Young's modulus of the separator 27 in the flat portion 20B is 0.8 [GPa].

[0059] [Comparative Example 2] Without a heating element on the winding core 30, and without heating the winding body 20, the Young's modulus of the separator 27 in the curved portion 20A of the winding body 20 is 1.1 [GPa], and the Young's modulus of the separator 27 in the flat portion 20B is also 1.1 [GPa].

[0060] [Comparative Example 3] Without a heating element on the winding core 30, and without heating the winding body 20, the Young's modulus of the separator 27 in the curved portion 20A of the winding body 20 is 1.5 [GPa], and the Young's modulus of the separator 27 in the flat portion 20B is 1.5 [GPa].

[0061] [Example 1] A first heating element 40A and a second heating element 40B are provided on the winding core 30, and the wound body 20 is heated at 110°C for 10 seconds. The Young's modulus of the separator 27 in the curved portion 20A of the wound body 20 is 0.9 GPa, and the Young's modulus of the separator 27 in the flat portion 20B is 1.1 GPa.

[0062] [Example 2] A first heating element 40A and a second heating element 40B are provided on the winding core 30, and the wound body 20 is heated at 110°C for 180 seconds, resulting in a Young's modulus of 0.8 GPa for the separator 27 in the curved portion 20A of the wound body 20 and a Young's modulus of 1.2 GPa for the separator 27 in the flat portion 20B of the wound body 20.

[0063] [evaluation] For each of the above examples and comparative examples, the precipitation resistance of the curved portion 20A of the separator 27 and the tightness of the winding body 20 were evaluated. The precipitation resistance of the curved portion 20A of the separator 27 was evaluated by the ratio of the precipitation limit current of the curved portion 20A of the separator 27 to the precipitation limit current of the flat portion 20B. The tightness of the winding body 20 was evaluated by the amount of displacement of the innermost separator 27 when the winding body 20 was removed from the winding core 30.

[0064] In comparative examples 1 to 3, where the wound body 20 is not heated, increasing the Young's modulus of the separator 27 of the wound body 20 reduces or eliminates winding tightness. However, this also reduces the deposition limit current in the curved portion 20A of the wound body 20, resulting in lower deposition resistance. Therefore, there is a trade-off between deposition resistance and winding tightness, and it is difficult to achieve both simultaneously.

[0065] In Examples 1 and 2, where the portion of the winding body 20 corresponding to the flat portion 20B is heated, the Young's modulus of the separator 27 in the flat portion 20B of the winding body 20 becomes higher than that of the curved portion 20A, thereby suppressing winding tightness while maintaining precipitation resistance. Furthermore, by heating from the inner circumference side of the winding body 20, the ease with which the winding body 20 can be removed from the winding core 30 can be improved. [Explanation of symbols]

[0066] 10…Lithium-ion rechargeable battery 11…Battery case 12... Lid 13A... Positive external terminal 13B…Negative external terminal 14A... Positive electrode current collector 14B... Negative electrode current collector 20...Wound body 20A…Curved section 20B…Flat area 21…Positive electrode sheet 22...Positive electrode current collector 22A...Unpainted area on the positive electrode side 23…Positive electrode composite layer 24... Negative electrode sheet 25...Negative electrode current collector 25A...Unpainted area on the negative electrode side 26…Negative electrode composite material layer 27... Separator 28A…Positive side current collector 28B... Negative electrode current collector 30... core 40A...First heating element 40B...Second heating element 50A…1st heated part 50B…Second heated part 50C...Non-heated part

Claims

1. A secondary battery comprising a flattened wound body formed by winding a laminate in which a positive electrode sheet and a negative electrode sheet are stacked with a separator in between, and an electrolyte, The Young's modulus of the separator in the curved portion located at the end in the longitudinal direction of the cross-section of the winding body, including the winding direction, is lower than the Young's modulus of the separator in the flat portion located in the center in the longitudinal direction of the cross-section. A secondary battery characterized by the following features.

2. The Young's modulus of the separator in the curved portion is 0.9 GPa or less. The Young's modulus of the separator in the flat portion is 1.1 GPa or greater. The secondary battery according to claim 1.

3. A method for manufacturing a secondary battery comprising a wound body and an electrolyte, A winding process in which a laminate in which a positive electrode sheet and a negative electrode sheet are stacked with a separator in between is wound onto a winding core along the winding direction, The process includes a pressing step of flattening the winding body removed from the aforementioned core, In the winding process, a first heated portion, which is a part of the winding body wound on the core in the winding direction, and a second heated portion, which is spaced apart from the first heated portion with an unheated portion remaining in the winding direction and is a part of the core opposite to the first heated portion with respect to the center of the core, are heated from the core. The pressing process involves pressing the winding body removed from the core so that the first heated portion and the second heated portion are brought closer together, and pressing the winding body after pressing so that the entire end portion in the longitudinal direction of the cross-section including the winding direction is the unheated portion. A method for manufacturing secondary batteries.

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