Non-aqueous electrolyte secondary battery and negative electrode for non-aqueous electrolyte secondary battery

Abstract

An object of the present invention is to provide a non-aqueous electrolyte secondary battery that suppresses cracks and peeling in a negative electrode mixture layer and has good cycle characteristics. The non-aqueous electrolyte secondary battery according to one aspect of the present invention includes a wound electrode body. The negative electrode (12) has a negative electrode current collector (40) and a negative electrode mixture layer (42). The negative electrode mixture layer (42) is formed on both side surfaces of the negative electrode current collector (40) and contains at least a negative electrode active material and a binder. The negative electrode mixture layer (42) has an inner negative electrode mixture layer (42-2) located on the inner peripheral side of the negative electrode current collector (40) and an outer negative electrode mixture layer (42-1) located on the outer peripheral side. Moreover, 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 having a swelling degree of 150 to 250%.

Description

Technical Field

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

[0002] Heretofore, a wound-type electrode body in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound with a spacer interposed therebetween has been widely used in a non-aqueous electrolyte secondary battery housed in an outer packaging body. The electrodes (positive electrode and negative electrode) of the electrode body have a binder layer containing an active material and a resin binder on both sides of each metal current collector. Sometimes, cracks are generated in the binder layer due to winding of the electrode body, or the binder layer peels off from the current collector. In particular, at the time of winding, a large stress is applied to the binder layer on the inner peripheral side, and the binder layer easily peels off from the current collector.

[0003] Patent Document 1 discloses that peeling of the binder layer on the inner peripheral side of the current collector is suppressed by increasing the content ratio of the binder contained in the binder layer closer to the center of the current collector.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent Laid-Open No. 8-17472

[0007] Patent Document 2: Japanese Patent Laid-Open No. 2012-182012 Summary of the Invention

[0008] Problems to be Solved by the Invention

[0009] Here, at the time of winding the electrode body, the inner binder layer is compressed and the outer binder layer is stretched. Therefore, in the inner binder layer, the flow path of the electrolyte becomes narrow and the diffusibility of Li ions decreases. On the other hand, cracks are likely to occur on the outer side, or peeling occurs due to expansion and contraction during charge and discharge, and the cycle characteristics are likely to deteriorate.

[0010] In the present invention, there is provided a non-aqueous electrolyte secondary battery in which cracks and peeling of the binder layer are suppressed by adjusting the swelling degree of the binder contained in the binder layer, and which has good cycle characteristics.

[0011] Means for Solving the Problems

[0012] The non-aqueous electrolyte secondary battery according to an aspect of the present invention includes: an electrode body formed by winding a strip-shaped positive electrode and a strip-shaped negative electrode with a spacer therebetween, and an outer packaging body that houses the electrode body. The negative electrode has: a negative electrode current collector and a negative electrode mixture layer. The negative electrode mixture layer is formed on both side surfaces of the negative electrode current collector and contains 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 peripheral side of the negative electrode current collector, and an inner negative electrode mixture layer located on the inner peripheral side. 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. The outer negative electrode mixture layer contains a binder having a swelling degree of 150 to 250%.

[0013] Advantages of the Invention

[0014] In the non-aqueous electrolyte secondary battery according to the present invention, the electrode reaction in the inner negative electrode mixture layer becomes uniform, and peeling of the outer negative electrode mixture layer can be prevented. Therefore, the cycle characteristics can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is an axial sectional view of a cylindrical secondary battery as an example of an embodiment.

[0016] Figure 2 is Figure 1 a perspective view of the electrode body included in the secondary battery shown.

[0017] Figure 3 It is a front view showing the positive electrode and the negative electrode constituting the electrode body as an example of an embodiment in an unfolded state.

[0018] Figure 4 It is a radial sectional view of the negative electrode of the electrode body as an example of an embodiment.

[0019] Figure 5 It is a partially enlarged view of the radial section of the negative electrode of the electrode body as an example of an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, with reference to the drawings, an example of an embodiment of a cylindrical and wound non-aqueous electrolyte secondary battery according to the present invention will be described in detail. In the following description, specific shapes, materials, numerical values, directions, etc. are examples for easy 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 there are multiple embodiments and modification examples, it is initially contemplated that their characteristic parts can be appropriately combined and used.

[0021] Figure 1 It is an axial sectional view of a wound secondary battery 10 as an example of an embodiment. Figure 1In the secondary battery 10 shown, an electrode body 14 and a non-aqueous electrolyte (not shown) are accommodated in an outer packaging 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. It should be noted that Figure 1 The secondary battery 10 shown is cylindrical in shape, but as long as the electrode body 14 has a wound structure, the secondary battery 10 can also be square cylindrical or the like. As the non-aqueous solvent (organic solvent) of the non-aqueous electrolyte, carbonates, lactones, ethers, ketones, esters, etc. can be used, and two or more of these solvents can be used in combination. When two or more solvents are used in combination, a mixed solvent containing a cyclic carbonate and a chain carbonate is preferably used. For example, as the cyclic carbonate, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. can be used, and as the chain carbonate, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), etc. can be used. As the electrolyte salt of the non-aqueous electrolyte, LiPF6, LiBF4, LiCF3SO3, etc. and their mixtures can be used. The dissolution amount of the electrolyte salt with respect to the non-aqueous solvent can be set to, for example, 0.5 to 2.0 mol / L. It should be noted that hereinafter, for convenience of explanation, the side of the sealing body 16 will be referred to as "upper", and the bottom side of the outer packaging body 15 will be referred to as "lower" for explanation.

[0022] By closing the opening end of the outer packaging body 15 with the sealing body 16, 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 passes through the through-hole of the insulating plate 17 and extends upward, and is welded to the lower surface of the bottom plate, i.e., the filter member 22, of the sealing body 16. In the secondary battery 10, the top plate, i.e., the cap 26, of the sealing body 16 electrically connected to the filter member 22 becomes the positive terminal. On the other hand, the negative electrode lead 20 passes through the through-hole of the insulating plate 18 and extends to the bottom side of the outer packaging body 15, and is welded to the inner surface of the bottom of the outer packaging body 15. In the secondary battery 10, the outer packaging body 15 becomes the negative terminal. It should be noted that when the negative electrode lead 20 is provided at the terminal portion, the negative electrode lead 20 passes outside the insulating plate 18 and extends to the bottom side of the outer packaging body 15, and is welded to the inner surface of the bottom of the outer packaging body 15.

[0023] The outer packaging body 15 is, for example, a metal outer packaging can having a bottomed cylindrical shape. A sealing gasket 27 is provided between the outer packaging body 15 and the sealing body 16, and they are electrically insulated and the airtightness of the inside of the secondary battery 10 is ensured. The outer packaging body 15 has, for example, a groove portion 21 formed by pressing the side surface portion from the outside to support the sealing body 16. The groove portion 21 is preferably formed in a ring shape along the circumferential direction of the outer packaging body 15, and supports the sealing body 16 on its upper surface.

[0024] The sealing body 16 has a filter member 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26 laminated in sequence from the side of the electrode body 14. Each component constituting the sealing body 16 has, for example, a disc shape or an annular shape, and the components other than 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 central portions, and the insulating member 24 is interposed between their respective peripheral portions. If the internal pressure of the battery rises due to abnormal heating, for example, the lower valve body 23 breaks, whereby the upper valve body 25 expands toward the cap 26 side and separates from the lower valve body 23, thereby blocking the electrical connection between the two. If the internal pressure further rises, the upper valve body 25 breaks, and gas is discharged from the opening 26a of the cap 26.

[0025] Next, while referring to Figure 2 , the electrode body 14 will be described. Figure 2 is a perspective view of the electrode body 14. As described above, the electrode body 14 has a winding structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape with a spacer 13 interposed therebetween. The positive electrode 11, the negative electrode 12, and the spacer 13 are all formed in a strip shape, and are wound in a spiral shape around a bobbin disposed along the winding axis 28 to be in a state of being alternately laminated in the radial direction of the electrode body 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 body 14, the length directions of the positive electrode 11 and the negative electrode 12 are the winding directions, and the width directions of the positive electrode 11 and the negative electrode 12 are the axial directions. At the upper end of the electrode body 14, the positive electrode lead 19 extends axially from a substantially central position in the radial direction between the center and the outermost periphery. In addition, at the lower end of the electrode body 14, the negative electrode lead 20 extends axially from near the winding axis 28.

[0026] In the spacer 13, a porous sheet having ion permeability and insulation can be used. Specific examples of the porous sheet include microporous films, woven fabrics, non-woven fabrics, etc. As the material of the spacer 13, olefin resins such as polyethylene and polypropylene are preferred. The thickness of the spacer 13 is, for example, 10 μm to 50 μm. With the increase in the capacity and output power of the battery, the spacer 13 tends to be thinned. The spacer 13 has a melting point of, for example, about 130 °C to 180 °C.

[0027] Next, Figure 3 is a front view of the positive electrode 11 and the negative electrode 12 constituting the electrode body 14. Figure 3 shows the positive electrode 11 and the negative electrode 12 in an unfolded state. As Figure 3Exemplification: In the electrode body 14, in order to prevent the precipitation of lithium in the negative electrode 12, the negative electrode 12 is formed larger than the positive electrode 11. Specifically, the length of the negative electrode 12 in the width direction (axial direction) is greater than the length of the positive electrode 11 in the width direction. In addition, the length of the negative electrode 12 in the length direction is greater than the length of the positive electrode 11 in the length direction. Thus, when the electrode body 14 is wound, at least the portion of the positive electrode 11 where the positive electrode mixture layer 32 is formed is disposed opposite to the portion of the negative electrode 12 where the negative electrode mixture layer 42 is formed with the spacer 13 therebetween.

[0028] 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, for example, a foil of a metal such as aluminum, a film having the metal disposed on the surface layer, etc. can be used. A suitable positive electrode current collector 30 is a foil of aluminum or a metal mainly composed of an aluminum alloy. The thickness of the positive electrode current collector 30 is, for example, 10 μm to 30 μm.

[0029] On both surfaces of the positive electrode current collector 30, it is suitable that the positive electrode mixture layer 32 is formed in all regions except for 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 coating 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) on both surfaces of the positive electrode current collector 30 and drying. Thereafter, the positive electrode mixture layer 32 is compressed.

[0030] As the positive electrode active material, a lithium-containing transition metal oxide containing transition metal elements such as Co, Mn, and Ni can be exemplified. The lithium-containing transition metal oxide is not particularly limited, and 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) is preferred.

[0031] As the conductive agent contained in the positive electrode mixture layer 32, carbon materials such as carbon black (CB), acetylene black (AB), Ketjen black, and graphite can be preferably used.

[0032] 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. In the case of preparing a positive electrode mixture slurry using an aqueous solvent, styrene-butadiene rubber (SBR), nitrile rubber (NBR), CMC or its salts, polyacrylic acid or its salts, polyvinyl alcohol, etc. can be used. These can be used alone or in combination of two or more. The content rate of the binder in the positive electrode mixture layer 32 is 0.5 mass% to 10 mass%, preferably 1 mass% to 5 mass%.

[0033] On the positive electrode 11, there is provided 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 the part that connects the positive electrode lead 19 and is the part where the surface of the positive electrode current collector 30 is not covered by the positive electrode mixture layer 32. The positive electrode exposed portion 34 is formed wider in the length direction than the positive electrode lead 19. It is appropriate to provide the positive electrode exposed portion 34 on both sides of the positive electrode 11 in a manner that overlaps 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.

[0034] Figure 3 In the example shown, at the central portion in the length direction of the positive electrode 11, the positive electrode exposed portion 34 is provided over the entire length range in the belt width direction. The positive electrode exposed portion 34 may also be formed at the start end portion or the end end portion of the positive electrode 11, but from the viewpoint of current collection property, it is preferably provided at a position approximately equidistant from the start end portion and the end end portion. By connecting the positive electrode exposed portion 34 provided at such a position to the positive electrode lead 19, when the electrode body 14 is wound, the positive electrode lead 19 is disposed so as to protrude upward from the end face in the belt width direction at an intermediate position in the radial direction of the electrode body 14. The positive electrode exposed portion 34 is provided by, for example, intermittent coating where a part of the positive electrode current collector 30 is not coated with the positive electrode mixture slurry.

[0035] The negative electrode 12 has a strip-shaped negative electrode current collector 40 and negative electrode mixture layers 42 formed on both side surfaces of the negative electrode current collector 40. As the negative electrode current collector 40, for example, a foil of a metal such as copper or a film having the metal disposed on the surface layer can be used. The thickness of the negative electrode current collector 40 is, for example, 5 μm to 30 μm.

[0036] On both sides of the negative electrode current collector 40, it is appropriate to form the negative electrode mixture layer 42 in all regions except for the 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 coating a negative electrode mixture slurry containing a negative electrode active material, a binder, and a solvent such as water on both sides of the negative electrode current collector 40 and drying. Thereafter, the negative electrode mixture layer 42 is compressed.

[0037] Figure 3In the example shown, at the starting end portion in the length direction of the negative electrode 12, a negative electrode exposed portion 44 is provided over the entire length range in the strip width direction of the negative electrode current collector. The negative electrode exposed portion 44 is the portion where the negative electrode lead 20 is connected, and is the 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 wider in the length direction than the width of the negative electrode lead 20. It is preferable that the negative electrode exposed portion 44 is provided on both sides of the negative electrode 12 so as to overlap in the thickness direction of the negative electrode 12.

[0038] In the present embodiment, the negative electrode lead 20 is joined to the inner peripheral side surface of the negative electrode current collector 40 by, for example, ultrasonic welding. One end portion of the negative electrode lead 20 is disposed in the negative electrode exposed portion 44, and the other end portion extends downward from the lower end of the negative electrode exposed portion 44.

[0039] The arrangement position of the negative electrode lead 20 is not limited to Figure 3 the example shown. The negative electrode lead 20 may be provided only at the terminal end portion of the negative electrode 12. Alternatively, the negative electrode lead 20 may be provided at both the starting end portion and the terminal end portion of the negative electrode 12. In this case, the current collection property is improved. By bringing the negative electrode exposed portion 44 at the terminal end portion of the negative electrode 12 into contact with the inner peripheral surface of the outer packaging body 15 (refer to Figure 1 ), the terminal end portion of the negative electrode 12 can be electrically connected to the outer packaging body 15 without using the negative electrode lead 20. The negative electrode exposed portion 44 is provided by, for example, intermittent coating in which the negative electrode mixture paste is not applied to a part of the negative electrode current collector 40.

[0040] As the negative electrode active material, there is no particular limitation as long as it is a material capable of reversibly occluding and releasing lithium (Li) ions. For example, carbon materials such as natural graphite and artificial graphite, metals alloyed with lithium such as Si and Sn, or alloys, oxides, etc. containing these can be used.

[0041] The binder contained in the negative electrode mixture layer 42 is usually resin-based (resin binder). As examples thereof, fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide (PI), acrylic resins, polyolefin resins, etc. can be cited. When preparing the negative electrode mixture paste with an aqueous solvent, styrene-butadiene rubber (SBR), nitrile rubber (NBR), polyacrylic acid or its salts, polyvinyl alcohol, etc. can be used. As the binder, from the viewpoint of the 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 preferable. These can be used alone or in combination of two or more. The content rate of the binder in the negative electrode mixture layer 42 is 0.5 mass% to 10 mass%, preferably 1 mass% to 5 mass%.

[0042] Figure 3In [the figure], the starting end portion 42a of the negative electrode mixture layer 42 is a portion adjacent to the exposed portion 44 of the negative electrode. On the other hand, the terminal end portion 42b of the negative electrode mixture layer 42 is the same as the terminal end portion of the negative electrode 12. The negative electrode mixture layer 42 continuously exists from the starting end portion 42a to the terminal end portion 42b.

[0043] Next, while referring to Figure 4 , the winding radius of the negative electrode 12 near the starting end portion of the negative electrode mixture layer 42 will be described. Figure 4 It is a radial cross-sectional view of the negative electrode 12 in the vicinity of the winding axis 28 of the electrode body 14 as an example of an embodiment. In Figure 4 , the description of the positive electrode 11 and the spacer 13 is omitted.

[0044] The innermost winding radius of the negative electrode 12 in the electrode body 14 is, for example, 1 mm to 5 mm. The innermost circumference of the negative electrode 12 is a portion that makes one full turn starting 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. For increasing 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 invention, since cracks and peeling of the negative electrode mixture layer 42 are suppressed, R is preferably 1 mm to 5 mm. Thereby, it is possible to cope with the high capacity of the secondary battery 10. The innermost winding radius of the negative electrode 12 can be adjusted by adjusting the radius of the bobbin used when winding the positive electrode 11, the negative electrode 12, and the spacer 13.

[0045] Figure 5 It is a partial enlarged view of the radial cross-section of the negative electrode 12. Thus, the outer negative electrode mixture layer 42-1 is located on the outer peripheral side of the negative electrode current collector 40, and the inner negative electrode mixture layer 42-2 is located on the inner peripheral side. If 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 curvature radius of the electrode near the bobbin is small, the outer negative electrode mixture layer 42-1 is stretched, and due to repeated expansion and contraction during charge and discharge, cracks are likely to occur or peeling from the negative electrode current collector 40 occurs, resulting in a decrease in the capacity retention rate. On the other hand, in the inner negative electrode mixture layer 42-2, the gap for electrolyte movement becomes narrow, the electrode reaction becomes uneven, and the internal resistance is likely to increase.

[0046] In the negative electrode 12 of the non-aqueous electrolyte secondary battery of the present invention, the inner negative electrode mixture layer 42-2 contains an adhesive with a relatively low degree of swelling, and the outer negative electrode mixture layer 42-1 contains an adhesive with a relatively high degree of swelling. For example, the degree of swelling of the adhesive contained in the inner negative electrode mixture layer 42-2 is 100 to 150%, and the degree of swelling of the adhesive contained in the outer negative electrode mixture layer is 150 to 250%.

[0047] For example, in styrene-butadiene rubber (SBR), if acrylonitrile is added to its constituent monomers, the degree of swelling becomes higher. Therefore, when styrene-butadiene rubber (SBR) is used as an adhesive, the degree of swelling of the adhesive can be adjusted by adjusting the content of acrylonitrile. In addition, as shown in Patent Document 2, the degree of swelling varies depending on the type of adhesive. Therefore, adhesives with different degrees of swelling can be used.

[0048] Here, the adhesive with a high degree of swelling expands due to electrolyte absorption, and its particle size is large. Therefore, when it adheres to the active material, the flow path of the electrolyte between the active materials becomes narrow, and the diffusivity of lithium ions decreases. On the other hand, the adhesive with a low degree of swelling has less expansion due to electrolyte absorption and is difficult to elongate, and its particle size is small. Therefore, even when it adheres to the active material, it is not easy to block the flow path of the electrolyte, and the diffusivity of lithium ions does not decrease.

[0049] Therefore, by making the inner negative electrode binder layer 42-2 contain an adhesive with a low degree of swelling, the inner negative electrode binder layer 42-2 can ensure the diffusivity of Li ions when being wound. In addition, by making the outer negative electrode binder layer 42-1 contain an adhesive with a high degree of swelling, when being wound, the adhesive can follow the unfolding between the active materials, suppress the generation of cracks, and maintain the adhesion to the negative electrode current collector of the outer negative electrode binder layer 42-1.

[0050] Thus, according to the present invention, the electrode reaction in the inner negative electrode binder layer 42-2 can be made uniform, and the peeling of the outer negative electrode binder layer 42-1 can be suppressed, improving the cycle characteristics.

[0051] Examples

[0052] Hereinafter, the present invention will be further described by way of examples, but the present invention is not limited to these examples.

[0053] <Example 1>

[0054] [Fabrication of Positive Electrode]

[0055] 95 parts by mass of 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 paste with a solid content of 70% by mass. Then, this positive electrode mixture paste was coated on both sides of a strip-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 for drying. After compressing the dried coating film using a roller, it was cut into a specified electrode plate size to fabricate a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode current collector. At approximately the central part in the length direction of the positive electrode, a positive electrode exposed part where the positive electrode current collector surface was exposed without a positive electrode mixture layer was provided, and an aluminum positive electrode lead was welded to the positive electrode exposed part.

[0056] [Fabrication of negative electrode]

[0057] In 95 parts by mass of graphite, 5 parts by mass of Si oxide (SiO), and 1 part by mass of carboxymethyl cellulose (CMC) as a thickener, an appropriate amount of water was mixed. In this mixture, 1.5 parts by mass of styrene-butadiene rubber (SBR) with a swelling degree of 250% relative to a non-aqueous solvent was mixed to prepare a first negative electrode mixture paste. Additionally, in 95 parts by mass of graphite, 5 parts by mass of Si oxide (SiO), and 1 part by mass of carboxymethyl cellulose (CMC) as a thickener, an appropriate amount of water was mixed to obtain the same mixture as above. In this mixture, 1.5 parts by mass of styrene-butadiene rubber (SBR) with a swelling degree of 100% relative to a non-aqueous solvent was mixed to prepare a second negative electrode mixture paste. Then, the first negative electrode mixture paste and the second negative electrode mixture paste were set in a die coater, the first negative electrode mixture paste was coated on one side of a strip-shaped negative electrode current collector made of copper foil, and the second negative electrode mixture paste was coated on the other side, and then the coating film was dried. After compressing the dried coating film using a roller, it was cut into a specified electrode plate size to fabricate 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. At the starting end, a negative electrode exposed part where the negative electrode current collector surface was exposed without a negative electrode mixture layer was provided, and a nickel / copper negative electrode lead was welded to the negative electrode exposed part.

[0058] [Preparation of electrolyte]

[0059] LiPF6 as a Li salt was dissolved in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) to prepare an electrolyte.

[0060] [Adjustment of swelling degree of binder]

[0061] As described above, when acrylonitrile is added to the constituent monomers of styrene butadiene rubber (SBR), the swelling degree increases. Therefore, the swelling degree of the adhesive is adjusted by adjusting the amount of acrylonitrile added.

[0062] [Evaluation method of swelling degree of adhesive]

[0063] The binder dispersed in the solvent was dried to prepare a film, and the film was immersed in an electrolyte solution (EC / DMC / DEC+Li salt) for 24 hours. The swelling degree was evaluated based on the mass before and after the immersion.

[0064] Swelling degree (%) = (membrane mass after immersion / membrane mass before immersion) × 100

[0065] [Production of electrode body]

[0066] The positive electrode and the negative electrode were wound around a winding core with a curvature radius of 1.5 mm via a separator made of a polyethylene microporous film with a thickness of 20 μm, and a tape was attached to the outermost circumference to produce a wound electrode body. At this time, the winding was performed in such a way that the first negative electrode mixture layer coated with the first negative electrode mixture slurry became the outer side and the second negative electrode mixture layer coated with the second negative electrode mixture slurry became the inner side.

[0067] [Manufacturing of cylindrical secondary batteries]

[0068] An insulating plate is arranged above and below an electrode body, and the electrode body is contained in an outer packaging body with a bottom cylindrical shape. Next, the negative electrode lead is welded to the inner bottom of the outer packaging body, and the positive electrode lead is welded to the sealing body. Then, after the electrolyte is injected into the interior of the outer packaging body by decompression, the opening end of the outer packaging body is riveted to the sealing body through a sealing gasket to seal the cylindrical secondary battery. The cylindrical secondary battery produced has a height of 65mm, a diameter of 18mm, and a designed battery capacity of 3000mAh.

[0069] <Example 2>

[0070] The same procedures as in Example 1 were followed except that the degree of swelling of the binder in the first negative electrode mixture layer was changed to 150%.

[0071] <Comparative Example 1>

[0072] The same procedures as in Example 1 were performed 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%.

[0073] <Comparative Example 2>

[0074] The same procedures as in Example 1 were followed except that the swelling degree of the binder in the second negative electrode mixture layer was changed to 250%.

[0075] <Comparative Example 3>

[0076] The swelling degree of the binder in the first negative electrode mixture layer was changed to 100%, and other than that, it was the same as in Example 1.

[0077] <Comparative Example 4>

[0078] The swelling degree of the binder in the first negative electrode mixture layer was changed to 300%, and other than that, it was the same as in Example 1.

[0079] [Measurement of Capacity Retention Rate during Charge and Discharge Cycles]

[0080] At an ambient temperature of 25°C, after constant current charging (current 0.3It = 900 mA, termination voltage 4.2 V) of the non-aqueous electrolyte secondary batteries of each example and each comparative example, constant voltage charging (voltage 4.2 V, termination current 150 mA) was performed. Then, constant current discharge was performed at a current value of 900 mA until the termination voltage reached 2.75 V. This charge and discharge was set as one cycle, and 300 cycles were performed. Then, the capacity retention rate during the charge and discharge cycles of the non-aqueous electrolyte secondary batteries of each example and each comparative example was obtained by the following formula, and the cycle characteristics were evaluated. It should be noted that It (A) = rated capacity (Ah) / 1 (h).

[0081] Capacity retention rate = (discharge capacity of the 300th cycle / discharge capacity of the 1st cycle) × 100 The evaluation results of Examples 1 and 2 and Comparative Examples 1 to 4 are shown in Table 1.

[0082] [Table 1]

[0083]

[0084] It is considered that in Example 1, the capacity retention rate is as high as 90%, which can make the diffusion of ions in the inner negative electrode mixture layer sufficient, and the peeling of the outer negative electrode mixture layer is suppressed. In addition, in Example 2, the swelling degree of the adhesive in the outer negative electrode mixture layer is slightly lower, and the peeling suppression effect is weaker than that in Example 1. It is considered that in Comparative Example 1, the ion diffusivity of the inner negative electrode mixture layer decreases, and the peeling of the outer negative electrode mixture layer increases. It is considered that in Comparative Example 2, compared with Comparative Example 1, the peeling of the outer negative electrode mixture layer should be suppressed, but compared with Example 1, the ion diffusivity of the inner negative electrode mixture layer decreases. It is considered that in Comparative Example 3, compared with Example 1, the peeling of the outer negative electrode mixture layer is not suppressed. It is considered that in Comparative Example 4, the swelling degree of the outer negative electrode mixture layer is too high at 300%, and the outer negative electrode mixture layer peels off. Therefore, the outer negative electrode mixture layer preferably contains an adhesive having a swelling degree of 150 to 250%. The swelling degree of the adhesive in the inner negative electrode mixture layer is not particularly limited as long as it is lower than that of the adhesive in the outer negative electrode mixture layer, and the inner negative electrode mixture layer preferably contains an adhesive having a swelling degree of 100 to 150%.

[0085] Based on the above evaluation results, it was confirmed that by setting the swelling degree of the adhesive in the inner negative electrode mixture layer and the outer negative electrode mixture layer within an appropriate range as in Examples 1 and 2, the ion diffusivity of the inner negative electrode mixture layer can be improved, and cracks and peeling of the outer negative electrode mixture layer can be suppressed, thereby improving the cycle characteristics.

[0086] Description of Reference Numerals

[0087] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Spacer, 14 Electrode body, 15 Outer packaging body, 16 Sealing body, 17, 18 Insulating plate, 19 Positive electrode lead, 20 Negative electrode lead, 21 Groove portion, 22 Filter, 23 Lower valve body, 24 Insulating member, 25 Upper valve body, 26 Cap, 26a Opening portion, 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 non-aqueous electrolyte secondary battery, comprising: an electrode body formed by winding a strip-shaped positive electrode and a strip-shaped negative electrode with a separator therebetween, and an outer package body for accommodating the electrode body, wherein, The negative electrode has: a negative electrode current collector and a negative electrode mixture layer, the negative electrode mixture layer being 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 peripheral side of the negative electrode current collector and an inner negative electrode mixture layer located on the inner peripheral side. 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. 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.

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

3. A negative electrode for a non-aqueous electrolyte secondary battery, which is used for a non-aqueous electrolyte secondary battery, the non-aqueous electrolyte secondary battery comprising: an electrode body formed by winding a strip-shaped positive electrode and a strip-shaped negative electrode with a separator therebetween, and an outer package body for accommodating the electrode body, wherein, The negative electrode for a non-aqueous electrolyte secondary battery has: a negative electrode current collector and a negative electrode mixture layer, the negative electrode mixture layer being 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 peripheral side of the negative electrode current collector and an inner negative electrode mixture layer located on the inner peripheral side. 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. 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.

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