Nonaqueous electrolyte secondary battery and negative electrode for nonaqueous electrolyte secondary battery

Abstract

The purpose of the present disclosure is to provide a nonaqueous electrolyte secondary battery which is suppressed in cracking and separation of a negative electrode mixture layer, and has good cycle characteristics. A nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure is provided with a wound-type electrode body. A negative electrode (12) has: a negative electrode collector (40); and negative electrode mixture layers (42) that are formed on both lateral surfaces of the negative electrode collector (40), while containing at least a negative electrode active material and a binder. The negative electrode mixture layers (42) include an inner negative electrode mixture layer (42-2) that is positioned on the inner circumference side of the negative electrode collector (40) and an outer negative electrode mixture layer (42-1) that is positioned on the outer circumference side. The swelling degree of the binder contained in the outer negative electrode mixture layer (42-1) is higher than the swelling degree of the binder contained in the inner negative electrode mixture layer (42-2); and the outer negative electrode mixture layer contains a binder that has a swelling degree of from 150% to 250%.

Description

[Technical Field]

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery and a negative electrode for a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Conventionally, nonaqueous electrolyte secondary batteries have been widely used, in which a wound electrode assembly, which is formed by winding a strip-shaped positive electrode and a strip-shaped negative electrode with a separator interposed therebetween, is housed in an outer casing. The electrodes (positive electrode and negative electrode) of the electrode assembly each have a mixture layer containing an active material and a resin binder on both sides of a metal current collector. When the electrode assembly is wound, cracks may occur in the mixture layer, causing the mixture layer to peel off from the current collector. In particular, during winding, a large stress is applied to the mixture layer on the inner circumferential side, making the mixture layer more likely to peel off from the current collector.

[0003] Patent Document 1 discloses that peeling of the mixture layer on the inner periphery of the current collector can be suppressed by increasing the binder content in the mixture layer closer to the center of the current collector. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 8-17472 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-182012 Summary of the Invention [Problem to be solved by the invention]

[0005] When the electrode body is wound, the inner mixture layer is compressed and the outer mixture layer is stretched. This narrows the electrolyte flow path in the inner mixture layer, reducing the diffusibility of Li ions. On the other hand, cracks and peeling due to expansion and contraction during charging and discharging are likely to occur on the outer side, leading to a decrease in cycle performance.

[0006] The present disclosure provides a nonaqueous electrolyte secondary battery that suppresses cracking and peeling of the mixture layer and has good cycle characteristics by adjusting the swelling degree of the binder contained in the mixture layer. [Means for solving the problem]

[0007] A nonaqueous electrolyte secondary battery according to one aspect of the present disclosure is a nonaqueous electrolyte secondary battery including: an 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 housing that houses the electrode assembly; the negative electrode includes a negative electrode current collector; and negative electrode mixture layers formed on both side surfaces of the negative electrode current collector and containing at least a negative electrode active material and a binder; the negative electrode mixture layers include an outer negative electrode mixture layer located on the outer periphery of the negative electrode current collector and an inner negative electrode mixture layer located on the inner periphery; the swelling degree of the binder contained in the outer negative electrode mixture layer is higher than the swelling degree of the binder contained in the inner negative electrode mixture layer; and the outer negative electrode mixture layer contains a binder with a swelling degree of 150 to 250%. [Effects of the Invention]

[0008] According to the nonaqueous electrolyte secondary battery according to the present disclosure, the electrode reaction in the inner negative electrode mixture layer is made uniform and peeling of the outer negative electrode mixture layer is prevented, thereby improving the cycle characteristics. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is an axial cross-sectional view of a cylindrical secondary battery as an example of an embodiment. [Figure 2] FIG. 2 is a perspective view of an electrode body included in the secondary battery shown in FIG. [Figure 3] FIG. 3 is a front view showing the positive electrode and negative electrode constituting the electrode assembly according to one embodiment in a developed state. [Figure 4] FIG. 4 is a radial cross-sectional view of a negative electrode of an electrode assembly according to an embodiment. [Figure 5] FIG. 5 is a partially enlarged view of a radial cross section of a negative electrode of an electrode assembly according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an example of an embodiment of a cylindrical, wound-type nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. In the following description, specific shapes, materials, numerical values, directions, etc. are examples for facilitating understanding of the present invention and can be appropriately changed according to the specifications of the cylindrical secondary battery. Furthermore, when the following description includes multiple embodiments and variations, it is initially assumed that the characteristic portions of these embodiments and variations can be appropriately combined and used.

[0011] FIG. 1 is an axial cross-sectional view of a wound secondary battery 10, which is an example of an embodiment. The secondary battery 10 shown in FIG. 1 includes an electrode assembly 14 and a non-aqueous electrolyte (not shown) housed in an exterior case 15. The electrode assembly 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. Although the secondary battery 10 shown in FIG. 1 is cylindrical, the secondary battery 10 may have a rectangular tubular shape or the like as long as the electrode assembly 14 has a wound structure. Examples of non-aqueous solvents (organic solvents) for the non-aqueous electrolyte include carbonates, lactones, ethers, ketones, and esters, and two or more of these solvents can be mixed and used. When two or more solvents are mixed and used, 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 can be LiPF6, LiBF4, LiCF3SO3, etc., or a mixture thereof. The amount of electrolyte salt dissolved in the non-aqueous solvent can be, for example, 0.5 to 2.0 mol / L. For ease of explanation, the sealing body 16 side will be referred to as the "top" and the bottom side of the exterior body 15 as the "bottom."

[0012] The open end of the exterior body 15 is sealed with the sealing body 16, thereby sealing the interior of the secondary battery 10. Insulating plates 17 and 18 are provided above and below the electrode body 14. The positive electrode lead 19 extends upward through a through hole in the insulating plate 17 and is welded to the underside of a filter 22, which is the bottom plate of the sealing body 16. In the secondary battery 10, a cap 26, which is the top plate of the sealing body 16 and is electrically connected to the filter 22, serves as the positive electrode terminal. On the other hand, the negative electrode lead 20 extends through a through hole in the insulating plate 18 toward the bottom 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. Note that if the negative electrode lead 20 is installed at the terminal end, the negative electrode lead 20 passes outside the insulating plate 18, extends toward the bottom 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, which electrically insulates them and ensures the internal seal of the secondary battery 10. The exterior body 15 has a grooved portion 21 that supports the sealing body 16, formed, for example, by pressing the side surface 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 component of the sealing body 16 has, for example, a disk or ring shape, and all components except for the insulating member 24 are 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, with the insulating member 24 interposed between their respective peripheral edges. If the internal pressure of the battery increases due to abnormal heat generation, for example, the lower valve body 23 may break, causing the upper valve body 25 to bulge toward the cap 26 and separate from the lower valve body 25, thereby cutting off the electrical connection between them. If the internal pressure continues to increase, the upper valve body 25 may break, and gas may be released from the opening 26a of the cap 26.

[0015] Next, the electrode assembly 14 will be described with reference to FIG. 2. FIG. 2 is a perspective view of the electrode assembly 14. As described above, the electrode assembly 14 has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The positive electrode 11, the negative electrode 12, and the separator 13 are all formed in strip shapes and spirally wound around a winding core disposed along the winding axis 28, resulting in a state in which they are alternately stacked in the radial direction of the electrode assembly 14. In the radial direction, the side of the winding axis 28 is referred to as the inner peripheral side, and the opposite side is referred to as the outer peripheral side. In the electrode assembly 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 axially from approximately the center in the radial direction between the center and the outermost periphery at the upper end of the electrode assembly 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 for 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. Separators 13 tend to be thinner as the capacity and output of batteries increase. The separator 13 has a melting point of, for example, about 130°C to 180°C.

[0017] Next, FIG. 3 is a front view of the positive electrode 11 and the negative electrode 12 that constitute the electrode assembly 14. FIG. 3 shows the positive electrode 11 and the negative electrode 12 in a developed state. As illustrated in FIG. 3, in the electrode assembly 14, the negative electrode 12 is formed to be larger than the positive electrode 11 to prevent lithium deposition on 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. Furthermore, the length of the negative electrode 12 in the longitudinal direction is larger than the length of the positive electrode 11 in the longitudinal direction. As a result, when wound into the electrode assembly 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, with the separator 13 interposed therebetween.

[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 peripheral side and the outer peripheral side of the positive electrode current collector 30. For the positive electrode current collector 30, a metal foil such as aluminum or a film having the metal disposed on the surface layer is used. A preferred positive electrode current collector 30 is a metal foil mainly composed of aluminum or an aluminum alloy. 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) onto both surfaces of the positive electrode current collector 30 and drying it. 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 (where -0.2 < x ≦ 0.2 and M includes 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 binders 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 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. These may be used alone or in combination of two or more. The binder content in the positive electrode mixture layer 32 is 0.5% by mass to 10% by mass, and 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 with the positive electrode mixture layer 32. The positive electrode exposed portion 34 is formed to be wider in the longitudinal direction than the positive electrode lead 19. The positive electrode exposed portions 34 are 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 longitudinal center of the positive electrode 11, spanning the entire length in the strip width direction. The positive electrode exposed portion 34 may be formed at the starting end or the terminal end of the positive electrode 11, but from the viewpoint of current collection, it is preferably provided at a position approximately equidistant from the starting end and the terminal end. By connecting the positive electrode lead 19 to the positive electrode exposed portion 34 provided in such a position, when the electrode assembly 14 is wound, the positive electrode lead 19 is positioned to protrude upward from the end face in the strip width direction at the radially intermediate position of the electrode assembly 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 portion 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 side surfaces of the negative electrode current collector 40. The negative electrode current collector 40 may be, for example, a foil of a metal such as copper, or a film having such a metal disposed on its surface. 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 the entire surface of each of the negative electrode current collectors 40, excluding a negative electrode exposed portion 44, which will be 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 each of the two surfaces of the negative electrode current collectors 40 and drying the slurry. Thereafter, the negative electrode mixture layer 42 is compressed.

[0027] 3, a negative electrode exposed portion 44 is provided at the starting end in the longitudinal direction of the negative electrode 12, over the entire length of the negative electrode 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 with 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 surfaces 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 inner peripheral surface 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. Alternatively, the negative electrode lead 20 may be provided at both the starting end and the terminal end of the negative electrode 12. In this case, 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 bringing the negative electrode exposed portion 44 at the terminal end of the negative electrode 12 into contact 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 portion 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 (Li) 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 or oxides containing these, can be used.

[0031] The binder contained in the negative electrode mixture layer 42 is typically made of resin (resin binder), and examples thereof 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), polyacrylic acid or a salt thereof, polyvinyl alcohol, and the like can be used. From the viewpoint of flexibility of the negative electrode 12, rubber-based resins having a molecular structure of repeated double bonds and single bonds, such as SBR and NBR, are preferred as the binder. These may be used alone or in combination of two or more. The binder content 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, the 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, the ending end 42b of the negative electrode mixture layer 42 is the same as the ending end of the negative electrode 12. The negative electrode mixture layer 42 exists continuously from the starting end 42a to the ending end 42b.

[0033] 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. 4. Fig. 4 is a radial cross-sectional view of the negative electrode 12 near the winding axis 28 of the electrode body 14, which is one example of an embodiment. In Fig. 4, the positive electrode 11 and the separator 13 are omitted.

[0034] The winding radius of the innermost periphery of the negative electrode 12 in the electrode assembly 14 is, for example, 1 mm to 5 mm. The innermost periphery of the negative electrode 12 is the portion that goes around the negative electrode 12 from the starting end. The winding radius of the innermost periphery of the negative electrode 12 is determined by the distance R between the winding axis 28 and the negative electrode 12. A smaller R is preferable to increase the capacity of the secondary battery 10, but this makes the negative electrode mixture layer 42 more susceptible to cracking and peeling. However, according to the present disclosure, R is preferably 1 mm to 5 mm to suppress cracking and peeling of the negative electrode mixture layer 42. This allows for increased capacity of the secondary battery 10. The winding radius of the innermost periphery of the negative electrode 12 can be adjusted by the radius of the winding core used to wind the positive electrode 11, the negative electrode 12, and the separator 13.

[0035] FIG. 5 is a partially enlarged view of a radial cross section of the negative electrode 12. As shown, the outer negative electrode mixture layer 42-1 is located on the outer periphery of the negative electrode current collector 40, and the inner negative electrode mixture layer 42-2 is located on the inner periphery. When the electrode body 14 is wound, the outer negative electrode mixture layer 42-1 is stretched and the inner negative electrode mixture layer 42-2 is compressed. In particular, the electrode close to the winding core has a small radius of curvature, and the outer negative electrode mixture layer 42-1 is stretched and repeatedly expands and contracts due to charge and discharge, which makes it prone to cracking and peeling from the negative electrode current collector 40, resulting in a decrease in capacity retention. Meanwhile, in the inner negative electrode mixture layer 42-2, the gap through which the electrolyte moves is narrowed, making the electrode reaction non-uniform and increasing internal resistance.

[0036] In the negative electrode 12 of the nonaqueous electrolyte secondary battery of the present disclosure, the inner negative electrode mixture layer 42-2 contains a binder with a relatively low swelling degree, and the outer negative electrode mixture layer 42-1 contains a binder with a relatively high swelling degree. For example, the swelling degree of the binder contained in the inner negative electrode mixture layer 42-2 is 100 to 150%, and the swelling degree of the binder contained in the outer negative electrode mixture layer is 150 to 250%.

[0037] For example, in the case of styrene-butadiene rubber (SBR), the swelling degree increases when acrylonitrile is added to its constituent monomers. Therefore, when styrene-butadiene rubber (SBR) is used as a binder, the swelling degree of the binder can be adjusted by adjusting the content of acrylonitrile. Furthermore, as shown in Patent Document 2, the swelling degree differs depending on the type of binder, so binders with different swelling degrees can be used.

[0038] Here, a binder with a high degree of swelling expands and spreads when it absorbs the electrolyte, but because its particle diameter is large, when it adheres to the active material, the flow path of the electrolyte between the active material becomes narrow, and the diffusibility of lithium ions decreases.On the other hand, a binder with a low degree of swelling expands little when it absorbs the electrolyte and is difficult to expand, but because its particle diameter is small, it is difficult to block the flow path of the electrolyte even when it adheres to the active material, and the diffusibility of lithium ions does not decrease.

[0039] Therefore, when the inner negative electrode mixture layer 42-2 contains a binder with a low swelling degree, the inner negative electrode mixture layer 42-2 can ensure the diffusibility of Li ions when wound. Also, when the outer negative electrode mixture layer 42-1 contains a binder with a high swelling degree, the binder can follow the expansion of the active material when wound, thereby suppressing the occurrence of cracks and maintaining the adhesion of the outer negative electrode mixture layer 42-1 to the negative electrode current collector.

[0040] As described above, according to the present disclosure, the electrode reaction in the inner negative electrode mixture layer 42-2 can be made uniform, and peeling of the outer negative electrode mixture layer 42-1 can be suppressed, thereby improving cycle characteristics. [Example]

[0041] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0042] Example 1 [Preparation of positive electrode] LiNi 0.8 Co 0.15 Al 0.05A positive electrode mixture slurry with a solids content of 70% by mass was prepared by mixing 95 parts by mass of O2, 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, and adding an appropriate amount of N-methyl-2-pyrrolidone (NMP). The positive electrode mixture slurry was then applied to both sides of a strip-shaped positive electrode current collector made of aluminum foil with a thickness of 15 μm, and the coating was heated to 100°C to 150°C and dried. The dried coating was compressed using a roller and then cut to a predetermined electrode plate size to produce a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode current collector. A positive electrode exposed portion, where the positive electrode mixture layer was not present and the positive electrode current collector surface was exposed, was provided in the approximate center of the positive electrode in the longitudinal direction, and an aluminum positive electrode lead was welded to the positive electrode exposed portion.

[0043] [Preparation of negative electrode] 95 parts by mass of graphite, 5 parts by mass of silicon oxide (SiO), 1 part by mass of carboxymethyl cellulose (CMC) as a thickener, and an appropriate amount of water were mixed. 1.5 parts by mass of styrene-butadiene rubber (SBR) with a swelling rate of 250% in non-aqueous solvents was mixed with this mixture to prepare a first negative electrode mixture slurry. 95 parts by mass of graphite, 5 parts by mass of silicon oxide (SiO), 1 part by mass of carboxymethyl cellulose (CMC) as a thickener, and an appropriate amount of water were mixed to obtain the same mixture as above. 1.5 parts by mass of styrene-butadiene rubber (SBR) with a swelling rate of 100% in non-aqueous solvents was mixed with this mixture to prepare a second negative electrode mixture slurry. Next, the first and second negative electrode mixture slurries were placed in a die coater, and the first and second negative electrode mixture slurries were applied to one side and the other side of a strip-shaped negative electrode current collector made of copper foil, respectively. The resulting coating was then dried. The dried coating was compressed using a roller and cut to the specified electrode plate size, resulting in a negative electrode with an outer negative electrode mixture layer formed on one side of the negative electrode current collector and an inner negative electrode mixture layer formed on the other side. A negative electrode exposed portion was created at the starting end, where the negative electrode mixture layer was not present and the negative electrode current collector surface was exposed. A nickel / copper negative electrode lead was welded to the negative electrode exposed portion.

[0044] [Preparation of electrolyte] The electrolyte was prepared by dissolving LiPF6 as a Li salt in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC).

[0045] [Adjusting the swelling degree of the binder] As mentioned above, the swelling degree of styrene-butadiene rubber (SBR) increases when acrylonitrile is added to its constituent monomers. Therefore, the swelling degree of the binder was adjusted by adjusting the amount of acrylonitrile added.

[0046] [Method for evaluating the swelling degree of binder] The binder dispersed in the solvent was dried to prepare a film, which was then immersed in an electrolyte (EC / DMC / DEC+Li salt) for 24 hours, and the degree of swelling was evaluated by the mass before and after immersion. Swelling degree (%) = (film mass after immersion / film mass before immersion) x 100

[0047] [Preparation of electrode body] The positive and negative electrodes were wound around a core with a curvature radius of 1.5 mm, sandwiching a 20 μm-thick separator made of a microporous polyethylene film, and tape was applied to the outermost surface to produce a wound electrode body. The electrodes were wound so that the first negative electrode mixture layer coated with the first negative electrode mixture slurry was on the outside and the second negative electrode mixture layer coated with the second negative electrode mixture slurry was on the inside.

[0048] [Fabrication of cylindrical secondary batteries] Insulating plates were placed above and below one electrode assembly, and the electrode assembly was housed in a cylindrical exterior housing with a bottom. Next, the negative electrode lead was welded to the inner bottom of the exterior housing, and the positive electrode lead was welded to a sealing member. After that, electrolyte was injected into the interior of the exterior housing using a vacuum method, and the open end of the exterior housing was crimped to the sealing member via a gasket to produce a cylindrical secondary battery. The produced cylindrical secondary battery had a height of 65 mm, a diameter of 18 mm, and a designed battery capacity of 3000 mAh.

[0049] <Example 2> The procedure was the same as in Example 1, except that the swelling degree of the binder in the first negative electrode mixture layer was changed to 150%.

[0050] <Comparative Example 1> The procedure is the same as in Example 1, except that the swelling degree of the binder in the first negative electrode mixture layer was changed to 100% and the swelling degree of the binder in the second negative electrode mixture layer was changed to 250%.

[0051] <Comparative Example 2> The procedure was the same as in Example 1, except that the swelling degree of the binder in the second negative electrode mixture layer was changed to 250%.

[0052] <Comparative Example 3> The procedure was the same as in Example 1, except that the swelling degree of the binder in the first negative electrode mixture layer was changed to 100%.

[0053] <Comparative Example 4> The procedure was the same as in Example 1, except that the swelling degree of the binder in the first negative electrode mixture layer was changed to 300%.

[0054] [Measurement of capacity retention rate during charge / discharge cycles] At an ambient temperature of 25°C, the nonaqueous electrolyte secondary batteries of each Example and Comparative Example were subjected to constant current charging (current 0.3 It = 900 mA, cut-off voltage 4.2 V) and then constant voltage charging (voltage 4.2 V, cut-off current 150 mA). Subsequently, they were discharged at a constant current of 900 mA to a cut-off voltage of 2.75 V. This charge / discharge cycle was repeated 300 times. The capacity retention rate of each Example and Comparative Example during the charge / discharge cycle was calculated using the following formula to evaluate the cycle characteristics. It (A) = rated capacity (Ah) / 1 (h). Capacity retention rate = (discharge capacity at 300th cycle / discharge capacity at 1st cycle) x 100 The evaluation results of Examples 1 and 2 and Comparative Examples 1 to 4 are shown in Table 1.

[0055] [Table 1]

[0056] In Example 1, the capacity retention rate was as high as 90%, which is considered to have ensured sufficient ion diffusibility in the inner negative electrode mixture layer and suppressed peeling of the outer negative electrode mixture layer. Furthermore, in Example 2, the binder swelling degree of the outer negative electrode mixture layer was slightly lower, and the peeling-suppression effect was weakened compared to Example 1. In Comparative Example 1, it is considered that the ion diffusibility of the inner negative electrode mixture layer decreased and peeling of the outer negative electrode mixture layer increased. In Comparative Example 2, peeling of the outer negative electrode mixture layer should have been suppressed compared to Comparative Example 1, but the ion diffusibility of the inner negative electrode mixture layer is considered to have decreased compared to Example 1. In Comparative Example 3, it is considered that peeling of the outer negative electrode mixture layer was not suppressed compared to Example 1. In Comparative Example 4, the swelling degree of the outer negative electrode mixture layer was too high at 300%, which is considered to have caused the outer negative electrode mixture layer to peel. Therefore, it is preferable that the outer negative electrode mixture layer contains a binder with a swelling degree of 150 to 250%. The swelling degree of the binder in the inner negative electrode material mixture layer is not particularly limited as long as it is lower than the swelling degree of the binder in the outer negative electrode material mixture layer, but the inner negative electrode material mixture layer preferably contains a binder with a swelling degree of 100 to 150%.

[0057] From the above evaluation results, it was confirmed that by setting the swelling degrees of the binders in the inner negative electrode mixture layer and the outer negative electrode mixture layer within appropriate ranges as in Examples 1 and 2, it is possible to improve the ion diffusibility in the inner negative electrode mixture layer while suppressing cracking and peeling of the outer negative electrode mixture layer, thereby improving the cycle characteristics. [Explanation of symbols]

[0058] 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, 42-1 outer negative electrode mixture layer, 42-2 inner negative electrode mixture layer, 44 negative electrode exposed portion.

Claims

1. A nonaqueous electrolyte secondary battery comprising an 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 negative electrode includes a negative electrode current collector and a negative electrode mixture layer formed on both side surfaces of the negative electrode current collector and containing at least a negative electrode active material and a binder; the negative electrode mixture layer has an outer negative electrode mixture layer located on the outer circumferential side of the negative electrode current collector and an inner negative electrode mixture layer located on the inner circumferential side, a degree of swelling of the binder contained in the outer negative electrode mixture layer in an electrolytic solution that is a nonaqueous electrolyte is higher than a degree of swelling of the binder contained in the inner negative electrode mixture layer in the electrolytic solution, the outer negative electrode mixture layer contains a binder having a swelling degree of 150 to 250%, the binder contains styrene-butadiene rubber having acrylonitrile as a constituent monomer, and the content of acrylonitrile in the styrene-butadiene rubber contained in the outer negative electrode mixture layer is higher than the content of acrylonitrile in the styrene-butadiene rubber contained in the inner negative electrode mixture layer; A non-aqueous electrolyte secondary battery characterized by:

2. the inner negative electrode mixture layer contains a binder having a swelling degree of 100 to 150%; 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte secondary battery is a non-aqueous electrolyte secondary battery.

3. A negative electrode for a non-aqueous electrolyte secondary battery, which is used in a non-aqueous electrolyte secondary battery, includes an 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, a negative electrode current collector; and a negative electrode mixture layer formed on both side surfaces of the negative electrode current collector and containing at least a negative electrode active material and a binder, the negative electrode mixture layer has an outer negative electrode mixture layer located on the outer circumferential side of the negative electrode current collector and an inner negative electrode mixture layer located on the inner circumferential side, a degree of swelling of the binder contained in the outer negative electrode mixture layer in an electrolytic solution that is a nonaqueous electrolyte is higher than a degree of swelling of the binder contained in the inner negative electrode mixture layer in the electrolytic solution, the outer negative electrode mixture layer contains a binder having a swelling degree of 150 to 250%, the binder contains styrene-butadiene rubber having acrylonitrile as a constituent monomer, and the content of acrylonitrile in the styrene-butadiene rubber contained in the outer negative electrode mixture layer is higher than the content of acrylonitrile in the styrene-butadiene rubber contained in the inner negative electrode mixture layer; A negative electrode for a non-aqueous electrolyte secondary battery.

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