Nonaqueous electrolyte secondary battery and method for manufacturing the same

Abstract

The purpose of the present disclosure is to provide a non-aqueous electrolyte secondary battery with suppressed cracks and peeling of a mixture layer while suppressing an increase in the internal resistance of the battery, the internal resistance being increased by a binder contained in the mixture layer. The non-aqueous electrolyte secondary battery according to an aspect of the present disclosure comprises: a wound electrode body on which a strip-like positive electrode (11) and a strip-like negative electrode (12) are each wound with a separator interposed therebetween; and an outer package accommodating the electrode body. The positive electrode (11) comprises a positive current collector (30) and a positive mixture layer (32) that is formed on the positive current collector (30) and contains at least a positive active material and a binder. The negative electrode (12) comprises a negative current collector (40) and a negative mixture layer (42) that is formed on the negative current collector (40) and contains at least a negative active material and a binder. At least one of the positive mixture layer (32) and the negative mixture layer (42) has a binder content that is higher on a leading end (42a) than on a terminal end (42b).

Description

[Technical field]

[0001] The present disclosure relates to a nonaqueous electrolyte secondary battery and a method for manufacturing a nonaqueous electrolyte secondary battery. [Background technology]

[0002] Conventionally, non-aqueous electrolyte secondary batteries have been widely used in which a wound electrode body, in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound with a separator interposed therebetween, is housed in an exterior body. The electrodes (positive electrode and negative electrode) of the electrode body each have a mixture layer containing an active material and a binder on both sides of a metal current collector, but when the electrodes are wound, cracks may occur in the mixture layer or the mixture layer may peel off from the current collector. Patent Document 1 discloses a method of suppressing peeling of the mixture layer on the inner circumference of the current collector by increasing the content of the binder contained in the mixture layer on the inner circumference of the current collector. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 8-17472 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the technology disclosed in Patent Document 1 cannot sufficiently suppress cracking and peeling of the mixture layer at the starting end located on the winding start side of the electrode body in which the positive and negative electrodes are wound with a small radius of curvature. If the binder content is increased to suppress cracking and peeling of the mixture layer, there is a problem that the internal resistance of the battery increases and the battery characteristics deteriorate.

[0005] An object of the present disclosure is to provide a nonaqueous electrolyte secondary battery in which an increase in the internal resistance of the battery caused by a binder contained in a mixture layer is suppressed, while cracking and peeling of the mixture layer are suppressed. [Means for solving the problem]

[0006] A nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure is a nonaqueous electrolyte secondary battery including a wound electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound with a separator interposed therebetween, and an exterior body that houses the electrode assembly. The positive electrode includes a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector and containing at least a positive electrode active material and a binder, and the negative electrode includes a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector and containing at least a negative electrode active material and a binder, and at least one of the positive electrode mixture layer and the negative electrode mixture layer is characterized in that the binder content at the starting end is higher than the binder content at the end end.

[0007] A method for manufacturing a nonaqueous electrolyte secondary battery according to one aspect of the present disclosure is a method for manufacturing a nonaqueous electrolyte secondary battery including a wound electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound with a separator interposed therebetween, and an exterior body that houses the electrode assembly, the method including a positive electrode mixture layer forming step of forming a positive electrode mixture layer containing at least a positive electrode active material and a binder on a positive electrode current collector of a positive electrode, and a negative electrode mixture layer forming step of forming a negative electrode mixture layer containing at least a negative electrode active material and a binder on a negative electrode current collector of a negative electrode, characterized in that in at least one of the positive electrode mixture layer forming step and the negative electrode mixture layer forming step, a plurality of mixture slurries having different binder contents are applied from one of the starting end side and the terminal end side to the other while changing the application amount ratio of the plurality of mixture slurries. Effect of the Invention

[0008] According to the nonaqueous electrolyte secondary battery according to the present disclosure, it is possible to suppress an increase in the internal resistance of the battery due to the binder contained in the mixture layer, while suppressing cracking and peeling of the mixture layer. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is an axial cross-sectional view of a cylindrical secondary battery as one example of the embodiment. [Diagram 2] FIG. 2 is a perspective view of a wound electrode body included in the secondary battery shown in FIG. [Diagram 3]FIG. 3 is a front view showing a positive electrode and a negative electrode constituting an electrode assembly according to an embodiment in a developed state. [Figure 4] 4(a) to 4(d) are diagrams showing changes in the content of the binder contained in the negative electrode mixture layer in the longitudinal direction of FIG. [Diagram 5] FIG. 5 is a radial cross-sectional view of a negative electrode in the vicinity of the winding axis of an electrode body according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] In the following, an example of an embodiment of a cylindrical secondary battery according to the present disclosure will be described in detail with reference to the drawings. In the following description, specific shapes, materials, values, directions, etc. are examples for facilitating understanding of the present invention, and can be appropriately changed according to the specifications of the cylindrical secondary battery. In addition, in the following description, when multiple embodiments and modified examples are included, it is assumed from the beginning that the characteristic parts of these embodiments and modified examples will be appropriately combined and used.

[0011] FIG. 1 is an axial cross-sectional view of a cylindrical secondary battery 10 as an example of an embodiment. In the secondary battery 10 shown in FIG. 1, an electrode body 14 and a non-aqueous electrolyte (not shown) are housed in an exterior body 15. The electrode body 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. As a non-aqueous solvent (organic solvent) for the non-aqueous electrolyte, carbonates, lactones, ethers, ketones, esters, etc. can be used, and these solvents can be used in a mixture of two or more kinds. When using a mixture of two or more kinds of solvents, it is preferable to use a mixed solvent containing a cyclic carbonate and a chain carbonate. For example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. can be used as the cyclic carbonate, and dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), etc. can be used as the chain carbonate. The electrolyte salt of the non-aqueous electrolyte may be LiPF6, LiBF4, LiCF3SO3, or a mixture thereof. The amount of electrolyte salt dissolved in the non-aqueous solvent may be, for example, 0.5 to 2.0 mol / L. For convenience of explanation, the sealing body 16 side will be referred to as "upper" and the bottom side of the exterior body 15 as "lower".

[0012] The opening end of the exterior body 15 is closed with the sealing body 16, so that the inside of the secondary battery 10 is sealed. Insulating plates 17 and 18 are provided above and below the electrode body 14, respectively. The positive electrode lead 19 extends upward through the through hole of the insulating plate 17 and is welded to the underside of the filter 22, which is the bottom plate of the sealing body 16. In the secondary battery 10, the cap 26, which is the top plate of the sealing body 16 electrically connected to the filter 22, serves as the positive electrode terminal. On the other hand, the negative electrode lead 20 extends through the through hole of the insulating plate 18 to the bottom side of the exterior body 15 and is welded to the inner bottom surface of the exterior body 15. In the secondary battery 10, the exterior body 15 serves as the negative electrode terminal. In addition, when the negative electrode lead 20 is installed at the terminal portion, the negative electrode lead 20 passes outside the insulating plate 18, extends to the bottom side of the exterior body 15, and is welded to the inner bottom surface of the exterior body 15.

[0013] The exterior body 15 is, for example, a cylindrical metal exterior can with a bottom. A gasket 27 is provided between the exterior body 15 and the sealing body 16 to ensure the sealing of the inside of the secondary battery 10. The exterior body 15 has a grooved portion 21 that supports the sealing body 16 and is formed, for example, by pressing a side portion from the outside. The grooved portion 21 is preferably formed in an annular shape along the circumferential direction of the exterior body 15, and supports the sealing body 16 on its upper surface.

[0014] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26, which are stacked in this order from the electrode body 14 side. Each member constituting the sealing body 16 has, for example, a disk shape or a ring shape, and each member except the insulating member 24 is electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, and the insulating member 24 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, for example, the lower valve body 23 breaks, and as a result, the upper valve body 25 swells toward the cap 26 and separates from the lower valve body 23, thereby cutting off the electrical connection between them. When the internal pressure further increases, the upper valve body 25 breaks, and gas is discharged from the opening 26a of the cap 26.

[0015] Next, the electrode body 14 will be described with reference to FIG. 2. FIG. 2 is a perspective view of the electrode body 14. As described above, the electrode body 14 has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape with the separator 13 interposed therebetween. The positive electrode 11, the negative electrode 12, and the separator 13 are all formed in a strip shape, and are wound in a spiral shape around a winding core arranged along the winding axis 28, so that they are alternately stacked in the radial direction of the electrode body 14. In the radial direction, the winding axis 28 side is called the inner circumferential side, and the opposite side is called the outer circumferential side. In the electrode body 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the strip width direction of the positive electrode 11 and the negative electrode 12 is the axial direction. The positive electrode lead 19 extends in the axial direction from approximately the center in the radial direction between the center and the outermost circumference at the upper end of the electrode body 14. Furthermore, the negative electrode lead 20 extends in the axial direction from the vicinity of the winding axis 28 at the lower end of the electrode body 14 .

[0016] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. The material of the separator 13 is preferably an olefin resin such as polyethylene or polypropylene. The thickness of the separator 13 is, for example, 10 μm to 50 μm. The separator 13 tends to be thinner as the capacity and output of the battery increase. The separator 13 has a melting point of, for example, about 130° C. to 180° C.

[0017] Next, with reference to Figs. 3 to 5, an embodiment in which the binder content of the negative electrode mixture layer 42 in the longitudinal direction is not uniform will be described. Fig. 3 is a front view of the positive electrode 11 and the negative electrode 12 constituting the electrode body 14. In Fig. 3, the positive electrode 11 and the negative electrode 12 are shown in a developed state. As illustrated in Fig. 3, in the electrode body 14, the negative electrode 12 is formed larger than the positive electrode 11 in order to prevent lithium deposition in the negative electrode 12. Specifically, the length of the negative electrode 12 in the strip width direction (axial direction) is larger than the length of the positive electrode 11 in the strip width direction. In addition, the length of the negative electrode 12 in the longitudinal direction is larger than the length of the positive electrode 11. As a result, when wound into the electrode body 14, at least the portion of the positive electrode 11 where the positive electrode mixture layer 32 is formed is disposed opposite the portion of the negative electrode 12 where the negative electrode mixture layer 42 is formed via the separator 13.

[0018] The positive electrode 11 has a strip-shaped positive electrode current collector 30 and a positive electrode mixture layer 32 formed on the positive electrode current collector 30. The positive electrode mixture layer 32 is formed on at least one of the inner and outer circumferential sides of the positive electrode current collector 30. For the positive electrode current collector 30, for example, a foil of a metal such as aluminum, or a film having such a metal disposed on its surface layer, is used. A suitable positive electrode current collector 30 is an aluminum foil or an alloy foil mainly composed of aluminum. The thickness of the positive electrode current collector 30 is, for example, 10 μm to 30 μm.

[0019] The positive electrode mixture layer 32 is preferably formed over the entire area of both surfaces of the positive electrode current collector 30, excluding the positive electrode exposed portion 34 described later. The positive electrode mixture layer 32 preferably contains a positive electrode active material, a conductive agent, and a binder. The positive electrode mixture layer 32 is formed by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, and a solvent such as N-methyl-2-pyrrolidone (NMP) to both surfaces of the positive electrode current collector 30 and drying it (positive electrode mixture layer formation step). Thereafter, the positive electrode mixture layer 32 is compressed.

[0020] Examples of the positive electrode active material include lithium-containing transition metal oxides containing transition metal elements such as Co, Mn, and Ni. The lithium-containing transition metal oxide is not particularly limited, but is preferably a composite oxide represented by the general formula Li 1+x MO2 (wherein -0.2 < x ≦ 0.2, and M contains at least one of Ni, Co, Mn, and Al).

[0021] Examples of the conductive agent contained in the positive electrode mixture layer 32 include carbon materials such as carbon black (CB), acetylene black (AB), ketjen black, and graphite.

[0022] Examples of the binder contained in the positive electrode mixture layer 32 include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide (PI), acrylic resins, and polyolefin resins. When preparing the positive electrode mixture slurry with an aqueous solvent, styrene-butadiene rubber (SBR), nitrile rubber (NBR), CMC or its salt, polyacrylic acid or its salt, polyvinyl alcohol, etc. can be used. From the viewpoint of the flexibility of the positive electrode 11, rubber-based resins having a repeating molecular structure of double bonds and single bonds such as SBR and NBR are preferable as the binder. These may be used alone or in combination of two or more. The content of the binder in the positive electrode mixture layer 32 is 0.5% by mass to 10% by mass, preferably 1% by mass to 5% by mass.

[0023] The positive electrode 11 is provided with a positive electrode exposed portion 34 where the surface of the positive electrode current collector 30 is exposed. The positive electrode exposed portion 34 is a portion to which the positive electrode lead 19 is connected, and is a portion of the surface of the positive electrode current collector 30 that is not covered by the positive electrode mixture layer 32. The positive electrode exposed portion 34 is formed wider in the longitudinal direction than the positive electrode lead 19. The positive electrode exposed portion 34 is preferably provided on both sides of the positive electrode 11 so as to overlap in the thickness direction of the positive electrode 11. The positive electrode lead 19 is joined to the positive electrode exposed portion 34 by, for example, ultrasonic welding.

[0024] In the example shown in FIG. 3, a positive electrode exposed portion 34 is provided in the center of the longitudinal direction of the positive electrode 11 over the entire length in the strip width direction. The positive electrode exposed portion 34 may be formed at the beginning or end of the positive electrode 11, but from the viewpoint of current collection, it is preferable to provide it at a position approximately equidistant from the beginning and end. By connecting the positive electrode lead 19 to the positive electrode exposed portion 34 provided at such a position, when the electrode body 14 is wound, the positive electrode lead 19 is arranged to protrude upward from the end face in the strip width direction at the radial middle position of the electrode body 14. The positive electrode exposed portion 34 is provided, for example, by intermittent application in which the positive electrode mixture slurry is not applied to a part of the positive electrode current collector 30.

[0025] The negative electrode 12 has a strip-shaped negative electrode current collector 40 and a negative electrode mixture layer 42 formed on both sides of the negative electrode current collector 40. For example, a metal foil such as copper or a film having a metal such as copper disposed on its surface is used for the negative electrode current collector 40. The thickness of the negative electrode current collector 40 is, for example, 5 μm to 30 μm.

[0026] The negative electrode mixture layer 42 is preferably formed on both sides of the negative electrode current collector 40 over the entire area except for a negative electrode exposed portion 44 described later. The negative electrode mixture layer 42 preferably contains a negative electrode active material and a binder. The negative electrode mixture layer 42 is formed by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, and a solvent such as water to both sides of the negative electrode current collector 40 and drying the slurry (negative electrode mixture layer forming step). Thereafter, the negative electrode mixture layer 42 is compressed.

[0027] 3, a negative electrode exposed portion 44 is provided at the beginning end in the longitudinal direction of the negative electrode 12, over the entire length of the current collector in the strip width direction. The negative electrode exposed portion 44 is a portion to which the negative electrode lead 20 is connected, and is a portion of the surface of the negative electrode current collector 40 that is not covered by the negative electrode mixture layer 42. The negative electrode exposed portion 44 is formed to be wider in the longitudinal direction than the width of the negative electrode lead 20. The negative electrode exposed portions 44 are preferably provided on both sides of the negative electrode 12 so as to overlap in the thickness direction of the negative electrode 12.

[0028] In this embodiment, the negative electrode lead 20 is joined to the surface on the inner circumferential side of the negative electrode current collector 40 by, for example, ultrasonic welding. One end of the negative electrode lead 20 is disposed in the negative electrode exposed portion 44, and the other end extends downward from the lower end of the negative electrode exposed portion 44.

[0029] The position of the negative electrode lead 20 is not limited to the example shown in FIG. 3, and the negative electrode lead 20 may be provided only at the terminal end of the negative electrode 12. The negative electrode lead 20 may also be provided at the start end and terminal end of the negative electrode 12. In this case, the current collection performance is improved. The terminal end of the negative electrode 12 may be electrically connected to the exterior body 15 without using the negative electrode lead 20 by contacting the negative electrode exposed portion 44 at the terminal end of the negative electrode 12 with the inner circumferential surface of the exterior body 15 (see FIG. 1). The negative electrode exposed portion 44 is provided, for example, by intermittent application in which the negative electrode mixture slurry is not applied to a part of the negative electrode current collector 40.

[0030] The negative electrode active material is not particularly limited as long as it can reversibly absorb and release lithium ions. For example, carbon materials such as natural graphite and artificial graphite, metals that can be alloyed with lithium such as Si and Sn, or alloys and oxides containing these can be used.

[0031] Examples of the binder contained in the negative electrode mixture layer 42 include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide (PI), acrylic resins, and polyolefin resins. When preparing the negative electrode mixture slurry using an aqueous solvent, styrene butadiene rubber (SBR), nitrile rubber (NBR), CMC or a salt thereof, polyacrylic acid or a salt thereof, polyvinyl alcohol, and the like can be used. As the binder, from the viewpoint of flexibility of the negative electrode 12, rubber-based resins having a molecular structure in which double bonds and single bonds are repeated, such as SBR and NBR, are preferred. These may be used alone or in combination of two or more. The content of the binder in the negative electrode mixture layer 42 is 0.5% by mass to 10% by mass, and preferably 1% by mass to 5% by mass.

[0032] 3, a starting end 42a of the negative electrode mixture layer 42 is a portion adjacent to the negative electrode exposed portion 44. On the other hand, a terminal end 42b of the negative electrode mixture layer 42 is the same as the terminal end of the negative electrode 12. The negative electrode mixture layer 42 exists continuously from the starting end 42a to the terminal end 42b.

[0033] Next, with reference to Fig. 4(a) to Fig. 4(d), the change in the binder content in the negative electrode mixture layer 42 in the longitudinal direction of Fig. 3 will be described. In Fig. 4(a), the binder content decreases at a constant rate from the starting end to the terminal end. That is, the binder content is high at the starting end 42a where the negative electrode mixture layer 42 is prone to cracking and peeling, and the binder content is low at the terminal end 42b where the winding radius is larger than that of the starting end 42a and where cracking and peeling are less likely to occur. This makes it possible to suppress the cracking and peeling of the negative electrode mixture layer 42 while suppressing an increase in the internal resistance of the battery due to the binder contained in the negative electrode mixture layer 42.

[0034] Also, as shown in FIG. 4(b), the gradient indicating the rate of decrease in the binder content from the starting end 42a to the terminal end 42b may not be constant, and the gradient may change along the way. In FIG. 4(c), the binder content decreases from the starting end 42a to the terminal end 42b, and the binder content is constant between the starting end 42a and the terminal end 42b. In FIG. 4(d), the binder content decreases from the starting end 42a to the terminal end 42b, and the binder content is constant near the starting end 42a. Similarly, as long as the binder content decreases from the starting end 42a to the terminal end 42b, the binder content may be constant near the terminal end 42b. As shown in FIG. 4(c) and FIG. 4(d), it is sufficient that a region in which the binder content continuously decreases from the starting end 42a side to the terminal end 42b side is provided in at least a part of the negative electrode mixture layer 42. In this region, the binder content preferably decreases linearly, but may decrease non-linearly. This allows the binder content at the starting end 42a of the negative electrode mixture layer 42 to be higher than the binder content at the terminal end 42b. The binder whose content is changed in the negative electrode mixture layer 42 is preferably a rubber-based resin.

[0035] Here, a method for forming the negative electrode mixture layer 42 in which the binder content changes from one side to the other side of the starting end 42a and the ending end 42b will be described. In order to form such a negative electrode mixture layer 42, it is preferable to use a multi-layer die coater. By using the multi-layer die coater, a plurality of negative electrode mixture slurries having different binder contents can be simultaneously applied to the negative electrode current collector 40 while adjusting the application amount ratio thereof. When the negative electrode mixture slurry is applied to the negative electrode current collector 40, the negative electrode current collector 40 moves relative to the multi-layer die coater. Therefore, by applying a plurality of negative electrode mixture slurries having different binder contents to the negative electrode current collector 40 while changing the application amount ratio thereof at a predetermined timing, a region in which the binder content changes from the starting end 42a to the ending end 42b can be formed at any position in the negative electrode mixture layer 42. For example, a first negative electrode mixture slurry and a second negative electrode mixture slurry having a higher binder content than the first negative electrode mixture slurry are prepared. Next, the first and second negative electrode mixture slurries are applied from the starting end 42a to the ending end 42b of the negative electrode current collector 40 using a multi-layer die coater while increasing the application amount ratio of the first negative electrode mixture slurry to the second negative electrode mixture slurry, thereby obtaining a negative electrode mixture layer 42 having the profile shown in FIG. 4(a).

[0036] In the above, the case where the binder content in the starting end 42a of the negative electrode mixture layer 42 is higher than the binder content in the terminal end 42b has been described. However, in the positive electrode mixture layer 32, the binder content in the starting end may be higher than the binder content in the terminal end. Naturally, in both the positive electrode mixture layer 32 and the negative electrode mixture layer 42, the binder content in the starting end may be higher than the binder content in the terminal end. Note that, even in the case where the mixture layer is divided into two or more parts by the exposed part as in the positive electrode 11 shown in FIG. 3, it is sufficient that the binder content in the starting end is higher than the binder content in the terminal end, and it is preferable that a region in which the binder content decreases from the starting end side toward the terminal end side is formed in at least a part of the mixture layer continuing from the starting end.

[0037] Next, the winding radius of the negative electrode 12 near the starting end of the negative electrode mixture layer 42 will be described with reference to Fig. 5. Fig. 5 is a radial cross-sectional view of the negative electrode 12 near the winding axis 28 of the electrode body 14 as one example of the embodiment. In Fig. 5, the positive electrode 11 and the separator 13 are omitted.

[0038] The innermost winding radius of the negative electrode 12 in the electrode body 14 is, for example, 1 mm to 5 mm. The innermost winding radius of the negative electrode 12 is a portion that goes around from the starting end of the negative electrode 12. The innermost winding radius of the negative electrode 12 is determined by the distance R between the winding axis 28 and the negative electrode 12. In order to increase the capacity of the secondary battery 10, it is preferable that R is small, but cracks and peeling are likely to occur in the negative electrode mixture layer 42. However, according to the present disclosure, cracks and peeling of the negative electrode mixture layer 42 are suppressed, so R is preferably 1 mm to 5 mm. This makes it possible to respond to the increase in capacity of the secondary battery 10. The innermost winding radius of the negative electrode 12 can be adjusted by the radius of the winding core used when winding the positive electrode 11, the negative electrode 12, and the separator 13. EXAMPLES

[0039] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0040] [Preparation of positive electrode] 95 parts by mass LiNi 0.8 Co 0.15 Al 0.05O2, 2.5 parts by mass of acetylene black (AB), and 2.5 parts by mass of polyvinylidene fluoride (PVdF) with an average molecular weight of 1.1 million were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry with a solid content of 70% by mass. Next, the positive electrode mixture slurry was applied to both sides of a belt-shaped positive electrode current collector made of aluminum foil with a thickness of 15 μm, and the coating film was heated to 100 ° C to 150 ° C and dried. The dried coating film was compressed using a roller and then cut to a predetermined electrode plate size to prepare a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode current collector. A positive electrode exposed portion where no mixture layer was present and the current collector surface was exposed was provided at approximately the center of the positive electrode in the longitudinal direction, and an aluminum positive electrode lead was welded to the positive electrode exposed portion.

[0041] [Preparation of negative electrode] 95 parts by mass of graphite, 5 parts by mass of SiO, 1 part by mass of carboxymethylcellulose (CMC), and 0.8 parts by mass of styrene butadiene rubber (SBR) were mixed, and an appropriate amount of water was added to prepare a first negative electrode mixture slurry. Also, 95 parts by mass of graphite, 5 parts by mass of SiO, 1 part by mass of carboxymethylcellulose (CMC), and 1.2 parts by mass of styrene butadiene rubber (SBR) were mixed, and an appropriate amount of water was added to prepare a second negative electrode mixture slurry. Next, the first negative electrode mixture slurry and the second negative electrode mixture slurry were set in a multi-layer die coater, and the first negative electrode mixture slurry and the second negative electrode mixture slurry were applied to both sides of a strip-shaped negative electrode current collector made of copper foil with a thickness of 8 μm from the starting end to the terminal end in the same manner, while continuously changing the application amount ratio of the first negative electrode mixture slurry to the second negative electrode mixture slurry from 0:1 to 1:0, and then the coating film was dried. The dried coating was compressed with a roller and then cut to the specified plate size to produce a positive electrode with a negative electrode mixture layer formed on both sides of the negative electrode current collector. A negative electrode exposed part was provided at the starting end where there was no mixture layer and the current collector surface was exposed, and a nickel / copper negative electrode lead was welded to the negative electrode exposed part.

[0042] [Preparation of electrolyte] Five parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of a mixed solvent (EC:DMC=1:3 by volume ratio) consisting of ethylene carbonate (EC) and dimethyl methyl carbonate (DMC). LiPF6 was dissolved in the mixed solvent to a concentration of 1 mol / L to prepare an electrolyte.

[0043] [Preparation of electrode body] The above positive and negative electrodes were wound around a core having a radius of 1 mm with a polyethylene separator interposed therebetween to prepare an electrode assembly.

[0044] [Preparation of cylindrical secondary battery] Insulating plates were placed on the top and bottom of the electrode body, and the electrode body was housed in an exterior body. Next, the negative electrode lead was welded to the bottom of the exterior body, and the positive electrode lead was welded to the sealing body. After that, electrolyte was injected into the interior of the exterior body by a reduced pressure method, and the open end of the exterior body was sealed by crimping it to the sealing body via a gasket, to produce a cylindrical secondary battery. The capacity of the produced battery was 2500mAh.

[0045] <Comparative Example 1> An electrode assembly and a battery were produced in the same manner as in the Example, except that the first negative electrode mixture slurry and the second negative electrode mixture slurry were not mixed, and only the first negative electrode mixture slurry was applied to both sides of the negative electrode current collector.

[0046] <Comparative Example 2> An electrode body and a battery were produced in the same manner as in the Example, except that the first negative electrode mixture slurry and the second negative electrode mixture slurry were not mixed, and only the second negative electrode mixture slurry was applied to both sides of the negative electrode current collector.

[0047] [Evaluation of the presence or absence of cracks in the negative electrode mixture layer] The electrode assemblies of Example and Comparative Examples 1 and 2 were each disassembled to spread out the negative electrodes, and the starting end of the negative electrode mixture layer was visually inspected for the presence or absence of cracks and peeling.

[0048] [Internal resistance measurement] The cylindrical secondary batteries of Example and Comparative Examples 1 and 2 were charged at a constant current of 120 mA at a temperature of 25° C. until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value reached 8 mA. Next, the internal resistance of each battery was measured at 1 kHz using an AC four-terminal method.

[0049] The evaluation results of the examples and comparative examples 1 and 2 are shown in Table 1.

[0050] [Table 1]

[0051] In Comparative Example 1, in which only the first negative electrode mixture slurry with a low binder content was used, the internal resistance of the battery was low, but cracks occurred in the electrode body. On the other hand, in Comparative Example 2, in which only the second negative electrode mixture slurry with a high binder content was used, cracks did not occur in the electrode body, but the internal resistance of the battery was significantly increased compared to Comparative Example 1. However, in the examples in which the first and second negative electrode mixture slurries were used, cracks did not occur in the electrode body, and the increase in the internal resistance of the battery was suppressed compared to Comparative Example 1. As a result, it was confirmed that by making the binder content at the starting end higher than the binder content at the end end, the increase in the internal resistance of the battery due to the binder used in the electrode was suppressed, while cracks and peeling of the mixture layer were suppressed. [Explanation of symbols]

[0052] 10 secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 15 exterior body, 16 sealing body, 17, 18 insulating plate, 19 positive electrode lead, 20 negative electrode lead, 21 grooved portion, 22 filter, 23 lower valve body, 24 insulating member, 25 upper valve body, 26 cap, 26a opening, 27 gasket, 28 winding shaft, 30 positive electrode current collector, 32 positive electrode mixture layer, 34 positive electrode exposed portion, 40 negative electrode current collector, 42 negative electrode mixture layer, 44 negative electrode exposed portion

Claims

1. A nonaqueous electrolyte secondary battery comprising a wound electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound with a separator interposed therebetween, and an exterior body that houses the electrode assembly, The positive electrode has a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector and including at least a positive electrode active material and a binder, the negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector and including at least a negative electrode active material and a binder; at least one of the positive electrode mixture layer and the negative electrode mixture layer has a region in which the content of the binder at a starting end is higher than the content of the binder at a terminal end, and the content of the binder continuously decreases from the starting end side to the terminal end side.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the innermost winding radius of the negative electrode in the electrode assembly is 1 mm to 5 mm.

3. A method for manufacturing a nonaqueous electrolyte secondary battery comprising a wound electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound with a separator interposed therebetween, and an exterior body that houses the electrode assembly, comprising: a positive electrode mixture layer forming step of forming a positive electrode mixture layer including at least a positive electrode active material and a binder on a positive electrode current collector of the positive electrode; a negative electrode mixture layer forming step of forming a negative electrode mixture layer including at least a negative electrode active material and a binder on a negative electrode current collector of the negative electrode, a positive electrode mixture layer forming step and a negative electrode mixture layer forming step, the positive electrode mixture layer forming step and the negative electrode mixture layer forming step are performed by applying a plurality of mixture slurries having different binder contents from one of a starting end side and a terminal end side to the other while changing a mixing ratio of the plurality of mixture slurries.

4. 4. The method for manufacturing a nonaqueous electrolyte secondary battery according to claim 3, wherein a mixing ratio of the plurality of mix slurries is changed so that a region in which a content of the binder continuously decreases from the starting end side to the terminal end side is formed in at least one of the positive electrode mix layer and the negative electrode mix layer.

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