Non-aqueous electrolyte secondary battery

Abstract

Provided is a non-aqueous electrolyte secondary battery having improved charge and discharge cycle characteristics. A non-aqueous electrolyte secondary battery according to an aspect of the present disclosure comprises: an electrode body obtained by winding a positive electrode and a negative electrode with a separator therebetween; an electrolytic solution; and an exterior body that houses the electrode body and the electrolytic solution. The negative electrode has: a negative electrode current collector; and a negative electrode mixture layer formed on the surface of the negative electrode current collector, wherein the negative electrode mixture layer contains a negative electrode active material and fibrous carbon, and the average fiber length of the fibrous carbon contained in the central part of the negative electrode mixture layer in the width direction is greater than the average fiber length of the fibrous carbon contained in both end parts of the negative electrode mixture layer in the width direction.

Description

[Technical Field]

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery in which the negative electrode contains fibrous carbon. [Background technology]

[0002] The positive and negative electrodes of nonaqueous electrolyte secondary batteries have a metal current collector and a mixture layer formed on the surface of the current collector. The mixture layer can be formed by applying a mixture slurry containing an active material and a conductive agent to the surface of the current collector, drying it, and compressing it. To improve battery performance, fibrous carbon such as carbon nanotubes is sometimes used as a conductive agent. For example, Patent Document 1 discloses a technique for uniformly dispersing fibrous carbon in a mixture slurry by using two types of fibrous carbon with different diameters. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-238575 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in non-aqueous electrolyte secondary batteries having a wound electrode assembly, the expansion and contraction of the electrode assembly due to charging and discharging can cause uneven distribution of the electrolyte within the electrode assembly, resulting in a decrease in battery capacity after repeated charging and discharging. The technology disclosed in Patent Document 1 does not consider the distribution of the electrolyte within the electrode assembly, and there is still room for improvement in the charge-discharge cycle characteristics.

[0005] Therefore, an object of the present disclosure is to provide a nonaqueous electrolyte secondary battery with improved charge-discharge cycle characteristics. [Means for solving the problem]

[0006] A nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure includes an electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound with a separator interposed therebetween, an electrolyte, and an exterior housing for accommodating the electrode assembly and the electrolyte. The negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The negative electrode mixture layer contains a negative electrode active material and fibrous carbon, and is characterized in that the average fiber length of the fibrous carbon contained in a central portion in the width direction of the negative electrode mixture layer is longer than the average fiber length of the fibrous carbon contained in both end portions in the width direction of the negative electrode mixture layer. [Effects of the Invention]

[0007] According to the secondary battery of one aspect of the present disclosure, charge / discharge cycle characteristics can be improved. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is an axial cross-sectional view of a cylindrical secondary battery according to an embodiment of the present invention; [Figure 2] FIG. 2 is a front view of a negative electrode according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, an example of an embodiment of a cylindrical secondary battery according to the present disclosure will be described in detail with reference to the drawings. In the following description, specific shapes, materials, 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 modified examples, it is assumed from the beginning that the characteristic portions of those embodiments and modified examples can be appropriately combined and used.

[0010] Fig. 1 is an axial cross-sectional view of a cylindrical secondary battery 10 that is one example of an embodiment. In the secondary battery 10 shown in Fig. 1, an electrode assembly 14 and an electrolyte (not shown) are housed in an exterior body 15. The electrode assembly 14 has a wound structure in which a strip-shaped positive electrode 11 and a strip-shaped negative electrode 12 are wound with a separator 13 interposed therebetween. For ease of explanation, the following description will be given with the sealing body 16 side referred to as "top" and the bottom side of the exterior body 15 referred to as "bottom."

[0011] The upper open end of the exterior body 15 is closed 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 passes outside the insulating plate 18, extends toward the bottom side of the exterior body 15, and is welded to the inner surface of the bottom of the exterior body 15. In the secondary battery 10, the exterior body 15 serves as the negative electrode terminal.

[0012] Exterior body 15 is, for example, a cylindrical metal exterior can with a bottom. A gasket 27 is provided between exterior body 15 and sealing body 16, ensuring the airtightness of the interior of secondary battery 10. Exterior body 15 has a grooved portion 21 that supports sealing body 16, formed, for example, by pressing the side surface from the outside. Grooved portion 21 is preferably formed in an annular shape along the circumferential direction of exterior body 15, and supports sealing body 16 on its upper surface via gasket 27.

[0013] 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.

[0014] The positive electrode 11, negative electrode 12, separator 13, and electrolyte that constitute the secondary battery 10, particularly the negative electrode active material contained in the negative electrode mixture layer that constitutes the negative electrode 12, will be described in detail below.

[0015] [Positive electrode] The positive electrode 11 includes, for example, a strip-shaped positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. As shown in FIG. 1 , the positive electrode 11 may include a positive electrode mixture layer 32 on both sides of a positive electrode current collector 30. The positive electrode current collector 30 may be a foil of a metal, such as aluminum, that is stable within the potential range of the positive electrode 11, or a film having such a metal disposed on its surface. The positive electrode mixture layer 32 may include, for example, a positive electrode active material, a binder, a conductive agent, and the like. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, and the like onto the positive electrode current collector 30, drying the slurry, and then compressing the slurry to form the positive electrode mixture layer 32.

[0016] Examples of the positive electrode active material include lithium transition metal composite oxides containing transition metal elements such as Co, Mn, and Ni. x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M1-y O z , Li x Ni 1-y M y O z , Li x Mn2O4, Li x Mn 2-y M y O4, LiMPO4, Li2MPO4F (M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3) can be exemplified. These may be used alone or in combination of two or more. In terms of achieving a high capacity for the non-aqueous electrolyte secondary battery, the positive electrode active material is Li x NiO2, Li x Co y Ni 1-y O2, Li x Ni 1-y M y O z (M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3), etc. lithium nickel composite oxides are preferably included. In addition, inorganic compound particles such as aluminum oxide and lanthanoid-containing compounds may be adhered to the particle surface of the lithium transition metal composite oxide.

[0017] 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. The conductive agent may also include fibrous carbon such as carbon nanotubes (CNT) described later. These may be used alone or in combination of two or more.

[0018] 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. These may be used alone or in combination of two or more. Furthermore, these resins may be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like.

[0019] [Negative electrode] The negative electrode 12 has a strip-shaped negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. As shown in Fig. 1, the negative electrode 12 may have a negative electrode mixture layer 36 on both sides of a negative electrode current collector 34. The negative electrode current collector 34 may be a foil of a metal such as copper that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface layer.

[0020] The negative electrode mixture layer 36 contains a negative electrode active material and fibrous carbon. The negative electrode active material is not particularly limited as long as it can reversibly absorb and release lithium ions, and for example, a carbon material such as graphite can be used. The graphite may be any of natural graphite such as flake graphite, lump graphite, and amorphous graphite, or artificial graphite such as lump artificial graphite and graphitized mesophase carbon microbeads.

[0021] As the negative electrode active material, metals that can be alloyed with Li, such as Si and Sn, metal compounds containing Si and Sn, and lithium-titanium composite oxides may be used. For example, SiO x Si oxide represented by (0.5≦x≦1.6) or Li 2y SiO (2+y)Si-containing compounds in which fine particles of Si are dispersed in a lithium silicate phase represented by (0 < y < 2) may be used in combination with a carbon material such as graphite. Since the volume change due to charge and discharge of the above metal-based materials containing Si, Sn, etc. is larger than that of the carbon material, when the above metal-based materials are used as the negative electrode active material, non-uniform distribution of the electrolytic solution is likely to occur in the negative electrode binder layer 36. Therefore, when the above metal-based materials are used as the negative electrode active material, by distributing fibrous carbon in the negative electrode binder layer 36 as described later, the improvement of the charge and discharge cycle characteristics becomes more remarkable.

[0022] The fibrous carbon contained in the negative electrode binder layer 36 functions as a conductive agent and improves the permeability of the electrolytic solution as described later. The content of the fibrous carbon contained in the negative electrode binder layer 36 may be 0.1% to 5% by mass, preferably 0.3% to 3% by mass, and more preferably 0.5% to 1.5% by mass with respect to the mass of the negative electrode active material. Within this range, it is possible to improve the permeability of the electrolytic solution in the negative electrode binder layer 36 while ensuring the dispersibility in the negative electrode binder slurry.

[0023] Examples of the fibrous carbon include carbon nanotubes (CNT), carbon nanohorns (CNH), carbon nanofibers (CNF), vapor-grown carbon fibers (VGCF), electrospun carbon fibers, polyacrylonitrile (PAN)-based carbon fibers, pitch-based carbon fibers, and the like. These may be used alone or in combination of two or more.

[0024] The fibrous carbon preferably contains CNTs. The structure of the CNTs is not particularly limited and may be any of a tubular structure in which graphene sheets made of six-membered carbon rings are wound parallel to the fiber axis, a pullet structure in which graphene sheets made of six-membered carbon rings are aligned perpendicular to the fiber axis, and a herringbone structure in which graphene sheets made of six-membered carbon rings are wound at an oblique angle to the fiber axis. The layer structure of the CNTs is also not limited and may be either single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs). Since SWCNTs can form a conductive path in the negative electrode mixture layer 36 in smaller amounts than MWCNTs, the CNTs preferably contain SWCNTs. The negative electrode mixture layer 36 may contain not only SWCNTs but also MWCNTs.

[0025] The average fiber length of the fibrous carbon is preferably 0.1 μm to 40 μm, more preferably 0.3 μm to 20 μm, and particularly preferably 0.5 μm to 5 μm. In the negative electrode mixture layer 36, the electrolyte solution permeates through the fibrous carbon, and fibrous carbon having a long average fiber length has higher permeability than fibrous carbon having a short average fiber length. In other words, fibrous carbon having a short average fiber length has higher electrolyte solution retention than fibrous carbon having a long average fiber length. Here, the average fiber length of the fibrous carbon is calculated by measuring the lengths of 10 fibrous carbon fibers using a scanning electron microscope (hereinafter sometimes referred to as SEM) and averaging the measured lengths. For example, the length of the fibrous carbon can be determined from an SEM image (pixel count: 1024 × 1280) at a magnification of 50,000 times observed under an accelerating voltage of 5 kV. The diameter of the fibrous carbon may be, for example, 1 nm to 100 nm. The diameter of the fibrous carbon is calculated by measuring the thickness of 10 fibrous carbon fibers using an SEM and averaging the measured values.

[0026] Next, the distribution of fibrous carbon in the negative electrode mixture layer 36 formed on the surface of the negative electrode current collector 34 will be described with reference to Fig. 2. Fig. 2 is a front view of the negative electrode 12 as an example of an embodiment, in which both end portions 36a are regions at both ends in the width direction of the negative electrode mixture layer 36 formed on both sides of the strip-shaped negative electrode current collector 34, and the central portion 36b is a region in the center in the width direction of the negative electrode mixture layer 36.

[0027] The average fiber length of the fibrous carbon contained in the central portion 36b in the width direction of the negative electrode mixture layer 36 is both ends Part 36 a The average fiber length of the fibrous carbon contained in the electrode body is longer than the average fiber length of the fibrous carbon contained in the electrode body. This improves the charge-discharge cycle characteristics. It is presumed that this is because, during charging, the electrolyte suppresses the outflow of the electrolyte from the central portion 36b, and during discharging, the electrolyte is more likely to return to the central portion 36b from the outside of the electrode body via the both end portions 36a.

[0028] In the width direction of the negative electrode mixture layer, the area ratio of both end portions 36a to the central portion 36b is preferably within a range of 30:70 to 70:30, and more preferably within a range of 45:55 to 55:45. By setting the area ratio of both end portions 36a to the central portion 36b within this range, the balance of the electrolyte in the electrode body during charge and discharge is improved, and the charge and discharge cycle characteristics are more significantly improved.

[0029] From the viewpoint of uniformity of distribution of the electrolyte in the electrode assembly, it is preferable that the widthwise lengths of both end portions 36a are approximately the same. The ratio of the widthwise lengths of one end portion 36a, the central portion 36b, and the other end portion 36a can be, for example, 15:70:15 to 35:30:35.

[0030] The ratio of the average fiber length of the fibrous carbon contained in the central portion 36b to the average fiber length of the fibrous carbon contained in both end portions 36a is preferably 1.2 or more. This increases the difference in liquid permeability between both end portions 36a and the central portion 36b, thereby ensuring the electrolyte in the central portion 36b, where the electrolyte that has leaked out from the electrode assembly during charging is less likely to return than in both end portions 36a. The ratio of the average fiber length of the fibrous carbon contained in the central portion 36b to the average fiber length of the fibrous carbon contained in both end portions 36a may be, for example, 5 or less, or 3 or less.

[0031] The average fiber length of the fibrous carbon contained in both end portions 36a is preferably 1 μm or more, and more preferably 2 μm or more, in which case the permeability of the electrolyte to both end portions 36a can be improved.

[0032] The content of fibrous carbon in both end portions 36 a may be greater than, the same as, or less than the content of fibrous carbon in the central portion 36 b. The content of fibrous carbon in both end portions 36 a is preferably approximately the same as the content of fibrous carbon in the central portion 36 b. Here, the content of fibrous carbon in both end portions 36 a and the content of fibrous carbon in the central portion 36 b refer to the percentage of the mass of fibrous carbon relative to the mass of the negative electrode active material in both end portions 36 a and the central portion 36 b, respectively.

[0033] The negative electrode mixture layer 36 may contain non-fibrous carbon in addition to fibrous carbon. Examples of non-fibrous carbon include carbon materials such as carbon black (CB), acetylene black (AB), ketjen black, and graphite. These may be used alone or in combination of two or more. The content of the non-fibrous carbon conductive agent in the negative electrode mixture layer 36 may be, for example, 5 mass % or less with respect to the total mass of the negative electrode mixture layer 36.

[0034] The negative electrode mixture layer 36 may further include a binder, etc. As in the case of the positive electrode 11, the binder included in the negative electrode mixture layer 36 may be a fluororesin such as PTFE or PVdF, PAN, PI, an acrylic resin, a polyolefin resin, etc., but is preferably styrene-butadiene rubber (SBR). The negative electrode mixture layer may also include CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc.

[0035] The negative electrode 12 can be produced, for example, through the following steps 1 to 3. (1) A negative electrode active material, a binder, a thickener, etc. are mixed with the first fibrous carbon in an appropriate ratio to prepare a negative electrode mixture slurry A using water as a dispersion medium. (2) A negative electrode active material, a binder, a thickener, etc. are mixed in an appropriate ratio with second fibrous carbon having an average fiber length shorter than that of the first fibrous carbon to prepare negative electrode mixture slurry B using water as a dispersion medium. (3) On the surface of the negative electrode current collector 34, the negative electrode mixture slurry A is applied to the center portion and dried, and then the negative electrode mixture slurry B is applied to the end portions, dried, and compressed to form the negative electrode mixture layer 36 having end portions 36a and a center portion 36b. In this manner, the negative electrode 12 can be manufactured. Note that after the application of the negative electrode mixture slurry A, the negative electrode mixture slurry B may be applied without drying, and then dried and compressed to form the negative electrode mixture layer 36. Alternatively, the negative electrode mixture slurry B may be applied to the end portions, and then the negative electrode mixture slurry A may be applied to the center portion.

[0036] [Separator] A porous sheet having ion permeability and insulating properties can be used as the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator 13.

[0037] [Electrolyte] The electrolytic solution (nonaqueous electrolyte) may contain a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent. Examples of the nonaqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The nonaqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents have been substituted with halogen atoms such as fluorine. Examples of the halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0038] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate.

[0039] Examples of the above ethers include cyclic ethers such as 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ether, etc.; chain ethers such as 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, etc.

[0040] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), LiPF 6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, borate salts such as Li2B4O7, Li(B(C2O4)F2), etc.; LiN(SO2CF3)2, LiN(CF 2l+1 SO2)(C m F 2m+1Examples of the lithium salt include imide salts such as LiPF6 (LiPF2) (where l and m are integers of 0 or more). The lithium salt may be used alone or in combination. Among these, LiPF6 is preferred from the viewpoints of ionic conductivity, electrochemical stability, etc. The concentration of the lithium salt may be, for example, 0.8 mol to 1.8 mol per 1 L of the non-aqueous solvent. [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> [Preparation of positive electrode] As the positive electrode active material, LiNi 0.8 Co 0.15 Al 0.05 Cobalt-aluminum-containing lithium nickel oxide represented by O2 was used. This positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVdF) were mixed in a mass ratio of 94:5:1 to prepare a positive electrode mixture slurry using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. This positive electrode mixture slurry was applied to both sides of a strip-shaped positive electrode current collector made of aluminum foil using a doctor blade method, and after drying the coating, the coating was compressed with a roller to form a positive electrode mixture layer on both sides of the positive electrode current collector. The positive electrode current collector with the formed positive electrode mixture layer was cut to the specified electrode size to prepare a positive electrode.

[0043] [Preparation of negative electrode] A mixture of graphite powder and silicon oxide in a mass ratio of 95:5 was used as the negative electrode active material. The negative electrode active material, carboxymethyl cellulose (CMC), carbon nanotubes (CNTs) with an average fiber length of 2.4 μm, and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 100:0.8:1:1.2 to prepare negative electrode mixture slurry A using water as a dispersion medium.

[0044] The above negative electrode active material, CMC, CNTs with an average fiber length of 2 μm, and SBR were mixed in a mass ratio of 100:0.8:1:1.2 to prepare negative electrode mixture slurry B using water as a dispersion medium.

[0045] A negative electrode current collector made of copper foil was prepared, and the end, center, and end portions were separated in a widthwise ratio of 25:50:25. That is, the area ratio between the end portions and the center was 50:50. Negative electrode mixture slurry A was applied to the center of both sides of the negative electrode current collector using a doctor blade method, and the coating was dried. After that, negative electrode mixture slurry B was applied to both ends using a doctor blade method, and the coating was dried and compressed with a roller to form a negative electrode mixture layer on both sides of the negative electrode current collector. The negative electrode current collector with the formed negative electrode mixture layer was cut to a predetermined electrode size to produce a negative electrode.

[0046] [Preparation of electrolyte] An electrolyte solution was prepared by dissolving LiPF6 at a concentration of 1.2 mol / L in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4 (25°C, 1 atmosphere).

[0047] [Secondary battery production] An aluminum lead was attached to the positive electrode, and a nickel lead was attached to the negative electrode. The positive and negative electrodes were then spirally wound with a polyethylene microporous membrane separator interposed therebetween to produce a wound electrode assembly. This electrode assembly was then housed in an outer casing, and a nickel lead was welded to the bottom of the outer casing. Next, the aluminum lead was welded to a seal, and the electrolyte solution was poured into the outer casing. The opening of the seal was then sealed to produce a nonaqueous electrolyte secondary battery with a design capacity of 2500 mAh.

[0048] <Example 2> A secondary battery was fabricated in the same manner as in Example 1, except that in the fabrication of the negative electrode, the area ratio between both end portions and the central portion was changed to 70:30 (the length ratio in the width direction was 35:30:35 between the end portions, the central portion, and the end portions).

[0049] Example 3 A secondary battery was produced in the same manner as in Example 1, except that in the production of the negative electrode, the area ratio of both end portions to the center portion was changed to 55:45 (the end portions, center portion, and end portions were in a length ratio of 27.5:45:27.5 in the width direction).

[0050] Example 4 A secondary battery was fabricated in the same manner as in Example 1, except that in the preparation of the negative electrode, the area ratio between both end portions and the central portion was changed to 90:10 (the end portions, central portion, and end portions were in a length ratio of 45:10:45 in the width direction).

[0051] <Example 5> A secondary battery was fabricated in the same manner as in Example 1, except that in the fabrication of the negative electrode, the area ratio between both end portions and the central portion was changed to 30:70 (the end portions, central portion, and end portions were in a length ratio of 15:70:15 in the width direction).

[0052] Example 6 A secondary battery was produced in the same manner as in Example 1, except that in the production of the negative electrode, the area ratio between both end portions and the central portion was changed to 45:55 (the end portions, central portion, and end portions were in a length ratio of 22.5:55:22.5 in the width direction).

[0053] Example 7 A secondary battery was fabricated in the same manner as in Example 1, except that in the preparation of the negative electrode, the area ratio between both end portions and the central portion was changed to 10:90 (the length ratio in the width direction was 5:90:5 between the end portions, the central portion, and the end portions).

[0054] Example 8 A secondary battery was produced in the same manner as in Example 1, except that in producing the negative electrode, the average fiber length of the CNTs contained in the negative electrode mixture slurry A was changed to 1.2 μm and the average fiber length of the CNTs contained in the negative electrode mixture slurry B was changed to 1.0 μm.

[0055] <Comparative Example 1> A secondary battery was fabricated in the same manner as in Example 1, except that in the fabrication of the negative electrode, the average fiber length of the CNTs contained in the negative electrode mixture slurry A was changed to 2.0 μm.

[0056] <Comparative Example 2> A secondary battery was produced in the same manner as in Example 1, except that in the production of the negative electrode, the average fiber length of the CNTs contained in the negative electrode mixture slurry A was changed to 2.0 μm and the average fiber length of the CNTs contained in the negative electrode mixture slurry B was changed to 2.4 μm.

[0057] [Cycle test] Each secondary battery was charged at a constant current of 0.7 C in a temperature environment of 25°C until the battery voltage reached 4.2 V, and then charged at a constant voltage until the current value reached 1 / 20 C at 4.2 V. Subsequently, the battery was discharged at a constant current of 0.7 C until the battery voltage reached 2.75 V. This charge / discharge cycle was repeated 400 times. The discharge capacity at the first cycle and the discharge capacity at the 400th cycle were determined, and the capacity retention rate was calculated using the following formula. Capacity retention rate (%) = (400th cycle discharge capacity ÷ 1st cycle discharge capacity) × 100

[0058] The evaluation results for each secondary battery are shown in Table 1. Table 1 also shows the average fiber length of the CNTs contained in the end and center of the negative electrode, and the area ratio between the end and center of the negative electrode.

[0059] [Table 1]

[0060] As shown in Table 1, the batteries of the Examples have superior charge-discharge cycle characteristics compared to the batteries of the Comparative Examples. The battery of Comparative Example 1 contains CNTs with the same average fiber length at both end and center portions in the width direction of the negative electrode mixture layer. Therefore, during both charge and discharge, the electrolyte tends to be less at the center portion than at both end portions, resulting in non-uniform electrolyte distribution and reduced charge-discharge cycle characteristics. On the other hand, the batteries of Examples 1 to 8 all contain CNTs in the center portion with a longer average fiber length than the CNTs contained at both end portions. As a result, it is presumed that the batteries of Examples 1 to 8 all have improved charge-discharge cycle characteristics because the electrolyte is retained in the center portion of the negative electrode mixture layer, suppressing non-uniformity of the electrolyte in the electrode body. In particular, Example 1, in which the area ratio between both end portions and the center portion is 50:50, exhibits the greatest improvement in charge-discharge cycle characteristics. In addition, since Comparative Example 2 contains CNTs at both ends with a longer average fiber length than the CNTs contained in the center, unevenness in the electrolyte solution in the electrode body is more likely to occur, and the charge-discharge cycle characteristics are lower than those of Comparative Example 1. [Explanation of symbols]

[0061] 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, 30 positive electrode current collector, 32 positive electrode mixture layer, 34 negative electrode current collector, 36 negative electrode mixture layer, 36a end, 36b center

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; an electrolytic solution; and an exterior body that accommodates the electrode assembly and the electrolytic solution, the negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on a surface of the negative electrode current collector, the negative electrode mixture layer contains a negative electrode active material and fibrous carbon, an average fiber length of the fibrous carbon contained in a central portion of the negative electrode mixture layer is greater than an average fiber length of the fibrous carbon contained in both end portions in the width direction of the negative electrode mixture layer, an area ratio of the both end portions to the central portion in the width direction of the negative electrode mixture layer is within a range of 30:70 to 70:30; a ratio of an average fiber length of the fibrous carbon contained in the central portion to an average fiber length of the fibrous carbon contained in the both end portions is 1.2 or more.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the area ratio of the both end portions to the central portion in the width direction of the negative electrode mixture layer is within a range of 45:55 to 55:

45.

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the average fiber length of the fibrous carbon contained in the both end portions is 1 [mu]m or more.

4. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the fibrous carbon includes carbon nanotubes.

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