Wound-type non-aqueous electrolyte secondary battery

Abstract

To provide a winding-type nonaqueous electrolyte secondary battery enabling suppression of degradation of charge-discharge cycle characteristics thereof.SOLUTION: A winding-type nonaqueous electrolyte secondary battery according to an embodiment comprises a negative electrode (12) that has a negative electrode current collector (30), an inner-circumference-side negative electrode mixture layer (32) disposed on an inner peripheral-side surface of both surfaces of the negative electrode current collector (30), and an outer-circumference side negative electrode mixture layer (34) disposed on an outer-circumference-side surface. The inner-circumference-side negative electrode mixture layer (32) and the outer-circumference-side negative electrode mixture layer (34) each include a negative electrode active material, and carbon nanotubes. The carbon nanotube of the outer-circumference-side negative electrode mixture layer (34) which is larger in average fiber length than the carbon nano-tube of the inner-circumference-side negative electrode mixture layer (32), has an average fiber length of 5-30 μm.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a technique for a wound nonaqueous electrolyte secondary battery. [Background technology]

[0002] In recent years, non-aqueous electrolyte secondary batteries have been widely used as high-power, high-energy density secondary batteries. These batteries include a positive electrode, a negative electrode, and a non-aqueous electrolyte, and are charged and discharged by transferring lithium ions or the like between the positive electrode and the negative electrode.

[0003] For example, Patent Document 1 discloses a wound nonaqueous electrolyte secondary battery that includes an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, and at least one of the positive electrode and the negative electrode is an electrode in which composite layers containing an active material and a conductive material are disposed on both sides of a current collector, and in which the content of the conductive material contained in the composite layer disposed on the outer peripheral surface of the current collector in a wound state of the electrode is made higher than the content of the conductive material contained in the composite layer disposed on the inner peripheral surface of the current collector.

[0004] Furthermore, for example, Patent Documents 2 to 4 disclose the use of carbon nanotubes as the conductive material contained in the composite layer of the negative electrode. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 4-34855 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-319186 [Patent Document 3] Japanese Patent Application Laid-Open No. 2005-4974 [Patent Document 4] International Publication No. 2012 / 147647 Summary of the Invention [Problem to be solved by the invention]

[0006] In an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, tensile stress is applied to the composite layer disposed on the outer peripheral surface of the current collector, which can cause cracks in the composite layer on the outer peripheral surface. This cracking is particularly likely to occur when the thickness of the composite layer is increased to increase capacity. When this cracking occurs, isolation of the active material progresses from the cracked area, which can lead to a deterioration in the charge-discharge cycle characteristics of the battery.

[0007] Therefore, an object of the present disclosure is to provide a wound-type nonaqueous electrolyte secondary battery that can suppress deterioration in charge-discharge cycle characteristics. [Means for solving the problem]

[0008] A wound nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure includes an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, and a nonaqueous electrolyte. At least one of the positive electrode and the negative electrode includes a current collector, an inner circumferential composite layer disposed on the inner circumferential surface of the current collector, and an outer circumferential composite layer disposed on the outer circumferential surface thereof. The inner circumferential composite layer and the outer circumferential composite layer contain an active material and carbon nanotubes. The carbon nanotubes in the outer circumferential composite layer have an average fiber length of 5 μm to 30 μm that is longer than that of the carbon nanotubes in the inner circumferential composite layer. [Effects of the Invention]

[0009] The wound nonaqueous electrolyte secondary battery of the present disclosure can suppress deterioration in charge-discharge cycle characteristics. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment; [Figure 2] 3 is a partial cross-sectional view of a negative electrode seen from the direction of the winding axis of an electrode body having a wound structure. FIG. [Figure 3] 3 is a partial cross-sectional view of a positive electrode seen from the direction of the winding axis of an electrode body having a wound structure. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an example of an embodiment of the present disclosure will be described with reference to the drawings.

[0012] Fig. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. As illustrated in Fig. 1, a nonaqueous electrolyte secondary battery 10 includes an electrode assembly 14, a nonaqueous electrolyte, and a battery case 15 that houses the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 includes a positive electrode 11, a negative electrode 12, and a separator 13 interposed between the positive electrode 11 and the negative electrode 12. The electrode assembly 14 has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound with the separator 13 interposed therebetween.

[0013] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixtures of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. The non-aqueous electrolyte is not limited to a liquid electrolyte, but may also be a solid electrolyte. The electrolyte salt may be, for example, a lithium salt such as LiPF6.

[0014] The battery case 15 is composed of a cylindrical outer can 16 with a bottom, and a sealing body 17 that closes the opening of the outer can 16 .

[0015] The outer can 16 is, for example, a cylindrical metal container with a bottom. A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure airtightness inside the battery. The outer can 16 has, for example, a grooved portion 22 that protrudes inward from a portion of the side surface and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its upper surface.

[0016] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in this order from the electrode body 14 side. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected to one another at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0017] The nonaqueous electrolyte secondary battery 10 includes insulating plates 18 and 19 disposed above and below the electrode assembly 14. In the example shown in Fig. 1 , a positive electrode lead 20 attached to the positive electrode 11 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and a negative electrode lead 21 attached to the negative electrode 12 passes outside the insulating plate 19 and extends toward the bottom of the outer can 16. The positive electrode lead 20 is connected to the bottom surface of a filter 23 in the sealing body 17 by welding or the like, and a cap 27 of the sealing body 17 electrically connected to the filter 23 serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0018] The positive electrode 11, the negative electrode 12, and the separator 13 that constitute the electrode assembly 14 will be described below.

[0019] <Negative electrode 12> Fig. 2 is a partial cross-sectional view of a negative electrode seen from the direction of the winding axis of an electrode body having a wound structure. As shown in Fig. 2, the negative electrode 12 has a negative electrode current collector 30, an inner circumferential-side negative electrode composite layer 32 disposed on one of both surfaces of the negative electrode current collector 30 that will become the inner circumferential side when wound, and an outer circumferential-side negative electrode composite layer 34 disposed on the outer circumferential side. Note that the inner circumferential side of the negative electrode current collector 30 refers to the surface of the negative electrode current collector 30 that is located inside in the radial direction of the wound negative electrode 12, and the outer circumferential side of the negative electrode current collector 30 refers to the surface of the negative electrode current collector 30 that is located outside in the radial direction of the wound negative electrode 12.

[0020] The negative electrode current collector 30 may be, for example, a foil of a metal such as copper that is stable in the potential range of the negative electrode, or a film having such a metal disposed on the surface thereof.

[0021] The inner circumferential side negative electrode mixture layer 32 and the outer circumferential side negative electrode mixture layer 34 contain a negative electrode active material and carbon nanotubes, and also contain an optional binder and the like.

[0022] The negative electrode 12 can be obtained, for example, by applying an inner-side negative electrode composite slurry containing a negative electrode active material, carbon nanotubes, an optional binder, etc. to one surface of the negative electrode current collector 30 and drying it to form an inner-side negative electrode composite layer 32, and by applying an outer-side negative electrode composite slurry containing a negative electrode active material, carbon nanotubes, an optional binder, etc. to the other surface of the negative electrode current collector 30 and drying it to form an outer-side negative electrode composite layer 34, and then rolling these negative electrode composite layers.

[0023] The negative electrode active material contained in the inner circumferential-side negative electrode composite layer 32 and the outer circumferential-side negative electrode composite layer 34 is not particularly limited as long as it is a material capable of absorbing and releasing lithium ions, and examples thereof include lithium alloys such as metallic lithium, lithium-aluminum alloy, lithium-lead alloy, lithium-silicon alloy, and lithium-tin alloy, carbon materials such as graphite, coke, and organic sintered bodies, and metal oxides such as SnO, SnO, and TiO. These may be used alone or in combination of two or more.

[0024] The content of the negative electrode active material in the inner circumferential side negative electrode mixture layer 32 is, for example, preferably in the range of 90 mass % to 99 mass %, and more preferably in the range of 95 mass % to 98 mass %, relative to the mass of the inner circumferential side negative electrode mixture layer 32. The same applies to the negative electrode active material in the outer circumferential side negative electrode mixture layer 34.

[0025] The carbon nanotubes contained in the inner circumferential side negative electrode composite layer 32 and the outer circumferential side negative electrode composite layer 34 may be, for example, carbon nanotubes with a tubular structure in which graphene sheets made of six-membered carbon rings are wound parallel to the fiber axis, carbon nanotubes with a pullet structure in which graphene sheets made of six-membered carbon rings are arranged perpendicular to the fiber axis, or carbon nanotubes with a herringbone structure in which graphene sheets made of six-membered carbon rings are wound at an oblique angle to the fiber axis.

[0026] The carbon nanotubes contained in the outer-circumferential negative electrode mixture layer 34 have a longer average fiber length (i.e., average fiber length) than the carbon nanotubes contained in the inner-circumferential negative electrode mixture layer 32, with an average fiber length of 5 μm to 30 μm, and preferably 10 μm to 25 μm. The carbon nanotubes contained in the inner-circumferential negative electrode mixture layer 32 have a shorter average fiber length than the carbon nanotubes contained in the outer-circumferential negative electrode mixture layer 34, with an average fiber length of preferably 25 μm or less, and more preferably less than 5 μm. The lower limit is not particularly limited, but is desirably 1 μm or more from the viewpoint of ease of production of carbon nanotubes, etc. The average fiber length of the carbon nanotubes can be measured using a scanning electron microscope (SEM). Specifically, the fiber lengths of 10 carbon nanotubes within the field of view of the scanning electron microscope are measured, and the average value is taken as the average fiber length.

[0027] When the negative electrode 12 is wound during the preparation of the electrode assembly 14, due to the difference in curvature, a compressive stress as indicated by arrow X in FIG. 2 is applied to the inner-side negative electrode mixture layer 32 disposed on the inner side of the negative electrode current collector 30, and a tensile stress as indicated by arrow Y in FIG. 2 is applied to the outer-side negative electrode mixture layer 34 disposed on the outer side of the negative electrode current collector 30. The tensile stress can cause cracks in the outer-side negative electrode mixture layer 34, leading to isolation of the negative electrode active material at the cracked locations and resulting in a deterioration in charge-discharge cycle performance. However, as in the nonaqueous electrolyte secondary battery 10 of this embodiment, by incorporating carbon nanotubes having an average fiber length of 5 μm to 30 μm in the outer-side negative electrode mixture layer 34, which is longer than the carbon nanotubes contained in the inner-side negative electrode mixture layer 32, a high anchor effect can be achieved, improving the adhesive strength between the negative electrode active material particles and suppressing cracking in the outer-side negative electrode mixture layer 34. As a result, the deterioration of the charge-discharge cycle characteristics is suppressed.

[0028] From the viewpoint of charge / discharge cycle characteristics, the content of carbon nanotubes contained in the outer circumferential negative electrode mixture layer 34 is preferably 0.1 mass % or more, and more preferably 1 mass % or more, relative to the mass of the negative electrode active material contained in the outer circumferential negative electrode mixture layer 34. Note that the upper limit is not particularly limited, but if the carbon nanotube content is too high, the amount of negative electrode active material will decrease, which may lead to a decrease in the capacity of the secondary battery, so it is preferably 5 mass % or less, for example. As with the outer circumferential negative electrode mixture layer 34, the content of carbon nanotubes contained in the inner circumferential negative electrode mixture layer 32 is preferably 0.1 mass % or more and 5 mass % or less, for example.

[0029] The average diameter (i.e., average fiber diameter) of the carbon nanotubes contained in the inner circumferential negative electrode composite layer 32 and the outer circumferential negative electrode composite layer 34 is preferably in the range of about 4 to 200 nm, and more preferably about 4 to 150 nm. The average diameter of the carbon nanotubes can be measured using a scanning electron microscope (SEM). Specifically, the diameters of 10 carbon nanotubes within the field of view of the scanning electron microscope are measured, and the average value is taken as the average diameter.

[0030] Examples of binders that can be used in the inner circumferential side negative electrode composite layer 32 and the outer circumferential side negative electrode composite layer 34 include fluorine-based resins such as polyvinylidene fluoride (PVdF), PAN, polyimide-based resins, acrylic-based resins, polyolefin-based resins, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof, and polyvinyl alcohol (PVA).

[0031] The thickness of each of the inner circumferential side negative electrode composite layer 32 and the outer circumferential side negative electrode composite layer 34 is preferably in the range of 10 μm to 100 μm, for example. Generally, as the negative electrode composite layer becomes thicker, cracks in the negative electrode composite layer (particularly cracks in the outer circumferential side negative electrode composite layer 34) tend to occur more easily. However, in this embodiment, the occurrence of cracks in the negative electrode composite layer due to the thickening of the negative electrode composite layer is suppressed.

[0032] <Positive electrode 11> 3 is a partial cross-sectional view of a positive electrode seen from the direction of the winding axis of an electrode body having a wound structure. As shown in FIG. 3, positive electrode 11 has a positive electrode current collector 36, an inner periphery-side positive electrode composite layer 38 disposed on one of both surfaces of positive electrode current collector 36 that will be the inner periphery due to winding, and an outer periphery-side positive electrode composite layer 40 disposed on the outer periphery-side surface. Note that the inner periphery side of positive electrode current collector 36 refers to the surface of positive electrode current collector 36 that is located inside in the radial direction of the wound positive electrode 11, and the outer periphery side of positive electrode current collector 36 refers to the surface of positive electrode current collector 36 that is located outside in the radial direction of the wound positive electrode 11.

[0033] The positive electrode current collector 36 may be, for example, a foil of a metal such as aluminum or an aluminum alloy that is stable in the potential range of the positive electrode 11, or a film having such a metal disposed on the surface thereof.

[0034] The inner periphery-side positive electrode mixture layer 38 and the outer periphery-side positive electrode mixture layer 40 contain a positive electrode active material, carbon nanotubes, an optional binder, and the like.

[0035] Positive electrode 11 can be obtained, for example, by applying an inner circumferential side positive electrode composite slurry containing a positive electrode active material, carbon nanotubes, an optional binder, etc. to one surface of positive electrode current collector 36 and drying it to form inner circumferential side positive electrode composite layer 38, and by applying an outer circumferential side positive electrode composite slurry containing a positive electrode active material, carbon nanotubes, an optional binder, etc. to the other surface of positive electrode current collector 36 and drying it to form outer circumferential side positive electrode composite layer 40, and then rolling these negative electrode composite layers.

[0036] The positive electrode active material includes, for example, a lithium-containing transition metal oxide. The metal element constituting the lithium-containing transition metal oxide is, for example, at least one selected from magnesium (Mg), aluminum (Al), calcium (Ca), scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), gallium (Ga), germanium (Ge), yttrium (Y), zirconium (Zr), tin (Sn), antimony (Sb), tungsten (W), lead (Pb), and bismuth (Bi). Among these, it is preferable to include at least one selected from Co, Ni, Mn, and Al.

[0037] The content of the positive electrode active material in the inner periphery-side positive electrode mixture layer 38 is preferably in the range of 90 mass % to 99 mass %, and more preferably in the range of 95 mass % to 98 mass %, relative to the mass of the inner periphery-side positive electrode mixture layer 38. The same applies to the positive electrode active material in the outer periphery-side positive electrode mixture layer 40.

[0038] The carbon nanotubes contained in the inner circumferential side positive electrode composite layer 38 and the outer circumferential side positive electrode composite layer 40 may be, for example, tubular structured carbon nanotubes, pullet structured carbon nanotubes, herringbone structured carbon nanotubes, etc., as with the negative electrode side.

[0039] The carbon nanotubes contained in the outer-periphery-side positive electrode mixture layer 40 have a longer average fiber length (i.e., average fiber length) than the carbon nanotubes contained in the inner-periphery-side positive electrode mixture layer 38, with an average fiber length of 5 μm to 30 μm, and preferably an average fiber length of 10 μm to 25 μm. The carbon nanotubes contained in the inner-periphery-side positive electrode mixture layer 38 have a shorter average fiber length than the carbon nanotubes contained in the outer-periphery-side positive electrode mixture layer 40, with an average fiber length of preferably 25 μm or less, and more preferably less than 5 μm. There is no particular restriction on the lower limit, but it is desirably 1 μm or more from the viewpoint of ease of manufacturing carbon nanotubes, etc.

[0040] In this way, by including carbon nanotubes having an average fiber length of 5 μm to 30 μm in the outer-side positive electrode mixture layer 40, which are longer than the carbon nanotubes included in the inner-side positive electrode mixture layer 38, it is thought that, for example, a high anchor effect is exhibited, the adhesive strength between the positive electrode active material particles is improved, and cracking of the outer-side positive electrode mixture layer 40 is suppressed. As a result, deterioration of the charge-discharge cycle characteristics is suppressed.

[0041] From the viewpoint of charge / discharge cycle characteristics, the content of carbon nanotubes contained in the outer circumferential positive electrode mixture layer 40 is preferably 0.1 mass% or more, and more preferably 1 mass% or more, relative to the mass of the positive electrode active material contained in the outer circumferential positive electrode mixture layer 40. While the upper limit is not particularly limited, if the carbon nanotube content is too high, the amount of positive electrode active material decreases, which may lead to a decrease in the capacity of the secondary battery. Therefore, for example, the upper limit is preferably 5 mass% or less. As with the outer circumferential positive electrode mixture layer 40, the content of carbon nanotubes contained in the inner circumferential positive electrode mixture layer 38 is preferably 0.1 mass% or more and 5 mass% or less.

[0042] The average diameter of the carbon nanotubes contained in the inner circumferential positive electrode composite layer 38 and the outer circumferential positive electrode composite layer 40 is, for example, preferably in the range of about 4 to 200 nm, and more preferably about 4 to 150 nm.

[0043] The binder contained in the inner periphery side positive electrode composite material layer 38 and the outer periphery side positive electrode composite material layer 40 can be the same as that on the negative electrode side.

[0044] The thickness of each of the inner circumferential side positive electrode composite layer 38 and the outer circumferential side positive electrode composite layer 40 is preferably in the range of 10 μm to 100 μm, for example. Generally, as the thickness of the positive electrode composite layer increases, cracks in the positive electrode composite layer (particularly cracks in the outer circumferential side positive electrode composite layer 40) tend to occur more easily. However, in this embodiment, cracks in the positive electrode composite layer caused by the increase in thickness are suppressed.

[0045] In the nonaqueous electrolyte secondary battery 10 of this embodiment, the carbon nanotubes contained in the outer peripheral mix layer of each of the positive electrode 11 and the negative electrode 12 are carbon nanotubes having an average fiber length of 5 μm to 30 μm, which is longer than that of the carbon nanotubes contained in the inner peripheral mix layer, but are not limited thereto. Since cracking of the outer peripheral mix layer due to tensile stress in either the positive electrode 11 or the negative electrode 12 can be suppressed, this leads to suppression of deterioration in charge-discharge cycle characteristics, and therefore, it is sufficient to suppress cracking of either the positive electrode 11 or the negative electrode 12. However, because cracking of the outer peripheral mix layer due to tensile stress is more likely to occur in the outer peripheral negative electrode mix layer 34 than in the outer peripheral positive electrode mix layer 40, it is preferable to apply this to at least the negative electrode 12.

[0046] <Separator 13> The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator include olefin-based resins such as polyethylene and polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator 13 may be a multilayer separator including a polyethylene layer and a polypropylene layer, and a separator whose surface is coated with a material such as an aramid-based resin or ceramic may be used. [Example]

[0047] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to the following examples.

[0048] Example 1 [Negative electrode] 95 parts by mass of graphite powder as a negative electrode active material, 5 parts by mass of silicon oxide as a negative electrode active material, 0.8 parts by mass of carboxymethyl cellulose (CMC), 1 part by mass of carbon nanotubes (CNTs) with an average fiber length of 30 μm, and water were mixed. 1.2 parts by mass of styrene-butadiene rubber (SBR) and water were mixed with this mixture to prepare a negative electrode composite slurry for the outer periphery. 95 parts by mass of graphite powder, 5 parts by mass of silicon oxide, 1.2 parts by mass of carboxymethyl cellulose (CMC), and water were mixed with this mixture. 0.8 parts by mass of styrene-butadiene rubber (SBR), 1 part by mass of carbon nanotubes (CNTs) with an average fiber length of 4 μm, and water were mixed with this mixture to prepare a negative electrode composite slurry for the inner periphery.

[0049] Next, the inner-side negative electrode composite slurry was applied to the surface of the negative electrode current collector made of copper foil that would become the inner periphery after winding, followed by drying to form an inner-side negative electrode composite layer. The outer-side negative electrode composite slurry was also applied to the surface of the negative electrode current collector that would become the outer periphery after winding, followed by drying to form an outer-side negative electrode composite layer. The negative electrode composite layer was then rolled using a rolling roller. This was used as a negative electrode.

[0050] [Preparation of positive electrode] LiNi as a positive electrode active material 0.8 Co 0.15 Al 0.05 O2, acetylene black (AB), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 94:5:1, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode composite slurry. Next, this positive electrode composite slurry was applied to both surfaces of a positive electrode current collector made of aluminum foil and dried to form a positive electrode composite layer. The positive electrode composite layer was then rolled using a rolling roller. This was used as a positive electrode.

[0051] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4. LiPF6 was dissolved in the mixed solvent to a concentration of 1.2 mol / L to prepare a non-aqueous electrolyte.

[0052] [Fabrication of non-aqueous electrolyte secondary battery] 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 polyethylene microporous film, and tape was applied to the outermost surface to produce a cylindrical electrode body. When wound, the negative electrode composite layer formed by coating the negative electrode composite slurry on the outer periphery was positioned on the outer periphery, and the negative electrode composite layer formed by coating the negative electrode composite slurry on the inner periphery was positioned on the inner periphery. An aluminum positive electrode lead was welded to the positive electrode, and a nickel negative electrode lead was welded to the negative electrode.

[0053] The electrode assembly was placed in a cylindrical outer can with a bottom, and the positive electrode lead was welded to a sealing member and the negative electrode lead was welded to the inner bottom surface of the outer can. After the nonaqueous electrolyte was poured into the outer can, the opening of the outer can was sealed with the sealing member to prepare a nonaqueous electrolyte secondary battery (height 65 mm, diameter 18 mm, design capacity 3000 mAh).

[0054] <Example 2> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that carbon nanotubes with an average fiber length of 5 μm were used in preparing the negative electrode composite slurry for the outer peripheral side.

[0055] Example 3 A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that carbon nanotubes with an average fiber length of 25 μm were used in preparing the negative electrode composite slurry for the inner periphery side.

[0056] <Comparative Example 1> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that carbon nanotubes with an average fiber length of 30 μm (the same material as the carbon nanotubes contained in the negative electrode composite slurry for the outer periphery) were used in preparing the negative electrode composite slurry for the inner periphery.

[0057] <Comparative Example 2> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that carbon nanotubes with an average fiber length of 5 μm were used in preparing the negative electrode composite slurry for the outer circumferential side, and carbon nanotubes with an average fiber length of 5 μm were used in preparing the negative electrode composite slurry for the inner circumferential side.

[0058] < Example 4 > A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that carbon nanotubes with an average fiber length of 4 μm were used in preparing the negative electrode composite slurry for the outer circumferential side, and carbon nanotubes with an average fiber length of 3 μm were used in preparing the negative electrode composite slurry for the inner circumferential side.

[0059] < Example 5 > A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that carbon nanotubes with an average fiber length of 4 μm were used in preparing the negative electrode composite slurry for the outer circumferential side, and carbon nanotubes with an average fiber length of 30 μm were used in preparing the negative electrode composite slurry for the inner circumferential side.

[0060] < Example 6 > A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that carbon nanotubes with an average fiber length of 4 μm were used in preparing the negative electrode composite slurry for the outer circumferential side, and carbon nanotubes with an average fiber length of 5 μm were used in preparing the negative electrode composite slurry for the inner circumferential side.

[0061] [Evaluation of capacity retention rate during charge / discharge cycles] A charge-discharge cycle test was conducted on the nonaqueous electrolyte secondary batteries of each Example and Comparative Example under the following conditions: At a temperature of 25°C, the batteries were charged at a constant current (current 0.3 It = 900 mA, cut-off voltage 4.2 V) and then at a constant voltage (voltage 4.2 V, cut-off current 150 mA), followed by discharge at a current of 900 mA to a cut-off voltage of 2.75 V. This charge-discharge cycle was repeated 1000 times, and the capacity retention rate during the charge-discharge cycle was calculated using the following formula. The results are shown in Table 1. Capacity maintenance rate=(X2 / X1)×100 X1: Discharge capacity at the first cycle X2: Discharge capacity at 1000 cycles

[0062] [Table 1]

[0063] In Examples 1 to 3, the carbon nanotubes in the outer negative electrode mixture layer have a longer average fiber length than the carbon nanotubes in the inner negative electrode mixture layer, with an average fiber length of 5 μm to 30 μm. In Comparative Examples 1 and 2, the carbon nanotubes in the outer negative electrode mixture layer and the carbon nanotubes in the inner negative electrode mixture layer have the same average fiber length. Example 4 the carbon nanotubes in the outer negative electrode composite layer have an average fiber length longer than that of the carbon nanotubes in the inner negative electrode composite layer, but the average fiber length is less than 5 μm; Examples 5 and 6 In Examples 1 to 3 and Comparative Examples 1 to 4, the carbon nanotubes in the outer negative electrode mixture layer have a shorter average fiber length than the carbon nanotubes in the inner negative electrode mixture layer. 2 When comparing the results, Examples 1 to 3 are all the same as Comparative Examples 1 to 3. 2 As a result, the capacity retention rate during charge-discharge cycles was high. That is, in Examples 1 to 3, the deterioration of the charge-discharge cycle characteristics was suppressed. [Explanation of symbols]

[0064] 10 nonaqueous electrolyte secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 15 battery case, 16 outer can, 17 sealing body, 18, 19 insulating plate, 20 positive electrode lead, 21 negative electrode lead, 22 grooved portion, 23 filter, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket, 30 negative electrode current collector, 32 inner circumferential side negative electrode composite layer, 34 outer circumferential side negative electrode composite layer, 36 positive electrode current collector, 38 inner circumferential side positive electrode composite layer, 40 outer circumferential side positive electrode composite layer.

Claims

1. A wound non-aqueous electrolyte secondary battery including an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, and a non-aqueous electrolyte, At least one of the positive electrode and the negative electrode is a current collector; an inner circumferential side composite layer disposed on the inner circumferential side of both surfaces of the current collector; and an outer circumferential side composite layer disposed on the outer circumferential side of both surfaces of the current collector, the inner circumferential side composite layer and the outer circumferential side composite layer contain an active material and carbon nanotubes, The carbon nanotubes in the outer peripheral composite layer have an average fiber length longer than that of the carbon nanotubes in the inner peripheral composite layer.

2. 2. The wound nonaqueous electrolyte secondary battery according to claim 1, wherein the carbon nanotubes in the inner circumferential composite layer have an average fiber length of 25 [mu]m or less.

3. 2. The wound nonaqueous electrolyte secondary battery according to claim 1, wherein the carbon nanotubes in the inner circumferential composite layer have an average fiber length of less than 5 [mu]m.

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