Non-aqueous electrolyte secondary battery

Abstract

A first electrode of a non-aqueous electrolyte secondary battery according to the present disclosure has a current collector and a mixture layer formed on the surface of the current collector. An exposed portion in which the current collector is exposed is formed on the surface of the first electrode. The exposed portion is in contact with only one end portion of both end portions in the lateral direction of the first electrode, and an electrode tab is connected to the exposed portion and led out from one end portion. The mixture layer contains an active material, granular carbon, and fibrous carbon. When the mixture layer is divided into a first mixture layer aligned with the exposed portion in the longitudinal direction of the first electrode and a second mixture layer adjacent to the exposed portion and the first mixture layer in the lateral direction of the first electrode, the first mixture layer contains granular carbon, and the second mixture layer contains fibrous carbon. The content of fibrous carbon in the first mixture layer is 0.1 mass % or less, and the content of the fibrous carbon in the second mixture layer is higher than the content of the fibrous carbon in the first mixture layer.

Description

Nonaqueous electrolyte secondary battery

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery including an electrode on which two mixture layers having different contents of fibrous carbon are formed.

[0002] In recent years, non-aqueous electrolyte secondary batteries, which transfer lithium ions or the like between two electrodes of different polarities via a non-aqueous electrolyte to charge and discharge, have been widely used as high-power, high-energy density secondary batteries. Patent Document 1 discloses a positive electrode in which two adjacent mixture layers are formed along the longitudinal direction of a strip-shaped positive electrode current collector, one of the mixture layers containing highly conductive fibrous carbon. Patent Document 1 describes that the above-described configuration of the positive electrode can suppress the internal resistance of the battery, and it is believed that the presence of a highly conductive mixture layer in the electrode can suppress a decrease in battery capacity even with repeated charge and discharge.

[0003] International Publication No. 2019 / 193882

[0004] Generally, when forming a mixture layer on the surface of a current collector, the mixture slurry is applied from one end of the current collector to the other end in the longitudinal direction. However, intermittent application, in which the mixture slurry is not applied to a portion of the current collector, can result in a current collector exposed portion where the current collector is exposed. After extensive research, the inventors discovered that when application is stopped to form the current collector exposed portion, the mixture slurry may be applied beyond the specified application range, and this tendency is particularly pronounced when a mixture slurry containing fibrous carbon is used. Because an electrode tab is connected to the current collector exposed portion, if the mixture layer is formed beyond the specified range, the mixture layer may intervene between the electrode tab and the current collector, potentially deteriorating current collection performance. The technology described in Patent Document 1 does not address poor application of the mixture slurry, and there is still room for improvement.

[0005] An object of the present disclosure is to provide a nonaqueous electrolyte secondary battery that is excellent in current collection properties and charge / discharge cycle characteristics.

[0006] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes an electrode assembly including a strip-shaped first electrode and a second electrode having mutually different polarities, and a non-aqueous electrolyte. The first electrode has a current collector and a mixture layer formed on the surface of the current collector. An exposed portion where the current collector is exposed is formed on the surface of the first electrode. The exposed portion is in contact with only one of both ends in the short direction of the first electrode. An electrode tab is connected to the exposed portion and is led out from the one end. The mixture layer contains an active material, granular carbon, and a non-aqueous electrolyte. and fibrous carbon, and when the mixture layer is divided into a first mixture layer aligned with the exposed portion in the longitudinal direction of the first electrode and a second mixture layer adjacent to the exposed portion and the first mixture layer in the transverse direction of the first electrode, the first mixture layer contains granular carbon and the second mixture layer contains fibrous carbon, the content of fibrous carbon in the first mixture layer is 0.1 mass% or less, and the content of fibrous carbon in the second mixture layer is higher than the content of fibrous carbon in the first mixture layer.

[0007] According to the nonaqueous electrolyte secondary battery according to the present disclosure, it is possible to improve the charge / discharge cycle characteristics while ensuring current collection performance.

[0008] 1 is a longitudinal cross-sectional view of a cylindrical secondary battery according to an embodiment of the present invention; FIG. 2 is a front view showing a positive electrode and a negative electrode constituting an electrode assembly according to an embodiment of the present invention in a developed state;

[0009] An example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail below. Hereinafter, a cylindrical battery in which a wound electrode assembly is housed in a cylindrical exterior body will be exemplified. However, the electrode assembly is not limited to the wound type and may be a laminated type in which multiple positive electrodes and multiple negative electrodes are alternately stacked one by one with separators interposed therebetween. Furthermore, the exterior body is not limited to a cylindrical shape and may be, for example, prismatic or coin-shaped, or may be a battery case made of a laminate sheet including a metal layer and a resin layer.

[0010] Fig. 1 is an axial cross-sectional view of a cylindrical secondary battery 10 according to an embodiment. In the secondary battery 10 shown in Fig. 1, an electrode assembly 14 and a non-aqueous electrolyte (not shown) are housed in an exterior body 15. The opening of the exterior body 15 is closed with a sealing body 16, thereby sealing the interior of the secondary battery 10. For ease of explanation, the sealing body 16 side will be referred to as "top" and the bottom side of the exterior body 15 as "bottom."

[0011] The non-aqueous electrolyte has ion conductivity (e.g., lithium ion conductivity). The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte. The liquid electrolyte (electrolytic solution) 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 mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. The electrolyte salt may include, for example, LiPF ６ Lithium salts such as

[0012] Examples of the solid electrolyte include solid or gel polymer electrolytes and inorganic solid electrolytes. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. Examples of the matrix polymer include a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin. Examples of the inorganic solid electrolyte include materials known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.).

[0013] The electrode assembly 14 has a wound structure in which a strip-shaped positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. Insulating plates 17 and 18 are provided on the top and bottom of the electrode assembly 14, respectively. A positive electrode tab 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. Meanwhile, the negative electrode tab 20 extends through a through-hole in the insulating plate 18 toward the bottom of the exterior body 15 and is welded to the inner bottom surface of the exterior body 15. The negative electrode 12 is disposed on the outermost peripheral surface of the electrode assembly 14, and the exposed portion of the surface of the negative electrode current collector (see FIG. 2 described below) constituting the negative electrode 12 contacts the inner peripheral surface of the exterior body 15. In the secondary battery 10, the exterior body 15 serves as the negative electrode terminal.

[0014] The exterior body 15 is, for example, a cylindrical metal exterior can with a bottom. A gasket 27 is provided between the exterior body 15 and the sealing body 16 to ensure the airtightness of the interior of the secondary battery 10. The exterior body 15 has a grooved portion 21 that supports the sealing body 16, formed, for example, by pressing the side surface from the outside. The grooved portion 21 is preferably formed in an annular shape along the circumferential direction of the exterior body 15, and supports the sealing body 16 on its upper surface.

[0015] 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 rupture, 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 rupture, allowing gas to be released from the opening 26a of the cap 26.

[0016] Next, with reference to FIG. 2 , the positive electrode 11, negative electrode 12, and separator 13 constituting the electrode assembly 14 will be described in detail, particularly the positive electrode 11. FIG. 2 is a front view showing the positive electrode 11 and negative electrode 12 in a developed state according to an example embodiment, with the separator 13 omitted. In FIG. 2 , the short-side direction of the positive electrode 11 and negative electrode 12 is the axial direction of the electrode assembly 14, and the long-side direction of the positive electrode 11 and negative electrode 12 is the winding direction of the electrode assembly 14. Below, an example will be described in which the positive electrode 11 is the first electrode and the negative electrode 12 is the second electrode. Note that this embodiment is not limited to this example. For example, the negative electrode 12 may be the first electrode. Alternatively, the second electrode may have the same characteristics as the first electrode, and both the positive electrode 11 and the negative electrode 12 may have the characteristics of the first electrode.

[0017] [Positive Electrode] The positive electrode 11 has a positive electrode current collector 30 and a positive electrode mixture layer 32 formed on the surface of the positive electrode current collector 30. As described below, the positive electrode mixture layer 32 can be divided into a first positive electrode mixture layer 32a and a second positive electrode mixture layer 32b. The positive electrode mixture layer 32 is preferably formed on both sides of the positive electrode current collector 30. A positive electrode current collector exposed portion 34 is formed on the surface of the positive electrode 11, where the positive electrode current collector 30 is exposed and not covered by the positive electrode mixture layer 32.

[0018] The positive electrode current collector 30 has a strip-like outer shape that substantially matches the outer shape of the positive electrode 11. A foil of a metal such as aluminum that is stable within the potential range of the positive electrode, or a film having such a metal disposed on the surface layer, can be used for the positive electrode current collector 30. The thickness of the positive electrode current collector 30 is, for example, 10 μm to 30 μm.

[0019] The positive electrode current collector exposed portion 34 is in contact with only one end portion 11a of both end portions 11a, 11b in the lateral direction of the positive electrode 11. The positive electrode current collector exposed portion 34 has, for example, a rectangular shape, and has an end portion in the lateral direction of the positive electrode 11. The positive electrode current collector exposed portion 34 is preferably provided on both surfaces of the positive electrode 11 so as to overlap with the positive electrode 11 in the thickness direction.

[0020] The positive electrode tab 19 is connected to the positive electrode current collector exposed portion 34 and extends from one end 11a. The positive electrode tab 19 is joined to the positive electrode current collector exposed portion 34 by, for example, ultrasonic welding. In the example shown in Fig. 2, the positive electrode current collector exposed portion 34 is provided at approximately the center in the longitudinal direction of the positive electrode 11. Note that a plurality of positive electrode current collector exposed portions 34 may be present in the longitudinal direction of the positive electrode 11.

[0021] The positive electrode mixture layer 32 contains, for example, a positive electrode active material, a conductive agent, and a binder. The positive electrode mixture layer 32 contains, for example, 0.1 mass % to 3 mass % of the conductive agent and 0.1 mass % to 5 mass % of the binder relative to the content of the positive electrode active material. The conductive agent includes granular carbon and fibrous carbon.

[0022] The positive electrode active material contained in the positive electrode mixture layer 32 may be a lithium-containing transition metal oxide containing a transition metal element such as Co, Mn, or Ni. The lithium-containing transition metal oxide is not particularly limited, but may be any of the following oxides represented by the general formula: Li １＋ｘ MO ２ (wherein, −0.2<x≦0.2, and M contains at least one of Ni, Co, Mn, and Al) is preferred.

[0023] 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-based resins, acrylic-based resins, polyolefin-based resins, etc. These may be used alone or in combination of two or more.

[0024] The positive electrode mixture layer 32 can be divided into a first positive electrode mixture layer 32a aligned with the positive electrode current collector exposed portion 34 in the longitudinal direction of the positive electrode 11, and a second positive electrode mixture layer 32b adjacent to the positive electrode current collector exposed portion 34 and the first positive electrode mixture layer 32a in the lateral direction of the positive electrode 11.

[0025] The first positive electrode mixture layer 32a contains granular carbon, and the second positive electrode mixture layer 32b contains fibrous carbon. The fibrous carbon content of the first positive electrode mixture layer 32a is 0.1 mass% or less. The fibrous carbon content of the second positive electrode mixture layer 32b is higher than the fibrous carbon content of the first positive electrode mixture layer 32a. This allows the positive electrode mixture layer 32 to contain fibrous carbon while preventing the first positive electrode mixture layer 32a from protruding into the positive electrode current collector exposed portion 34, thereby improving current collection performance and charge / discharge cycle characteristics. Both the first positive electrode mixture layer 32a and the second positive electrode mixture layer 32b may contain granular carbon and fibrous carbon. Furthermore, the granular carbon and fibrous carbon contents of the second positive electrode mixture layer 32a may be lower than the granular carbon and fibrous carbon contents of the first positive electrode mixture layer 32a.

[0026] Examples of fibrous carbon include carbon nanotubes (CNTs), carbon nanofibers (CNFs), vapor-grown carbon fibers (VGCFs), electrospun carbon fibers, polyacrylonitrile (PAN)-based carbon fibers, and pitch-based carbon fibers. These may be used alone or in combination of two or more. The fibrous carbon is preferably CNT. Examples of CNTs include single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), with MWCNTs being particularly preferred. SWCNTs are carbon nanostructures in which one layer of graphene sheets forms a cylindrical shape, and multi-walled carbon nanotubes are carbon nanostructures in which two or more layers of graphene sheets are concentrically stacked to form a cylindrical shape.

[0027] Examples of granular carbon include carbon materials such as carbon black (CB) such as acetylene black (AB) and Ketjen black, and graphite. These may be used alone or in combination of two or more. The granular carbon is preferably CB, and more preferably AB.

[0028] The thickness of each of the first positive electrode mixture layer 32a and the second positive electrode mixture layer 32b is, for example, 10 μm to 150 μm on one side of the positive electrode current collector 30. In this embodiment, the thickness of the first positive electrode mixture layer 32a and the thickness of the second positive electrode mixture layer 32b are the same, but they may be different from each other.

[0029] In the short-side direction of the positive electrode, the length L1 of the first positive electrode mixture layer 32a and the length L2 of the second positive electrode mixture layer 32b may satisfy 1 / 6≦L1 / (L1+L2)<1 / 2. This allows the second positive electrode mixture layer 32b containing fibrous carbon to be enlarged while ensuring the positive electrode current collector exposed portion 34 is large enough to stably connect the positive electrode tab 19, thereby making it possible to more significantly improve the current collection performance and the charge / discharge cycle characteristics.

[0030] The method for producing the positive electrode 11 is not particularly limited, but the positive electrode 11 can be produced, for example, as follows. (1) A first positive electrode mixture slurry for the first positive electrode mixture layer 32a and a second positive electrode mixture slurry for the second positive electrode mixture layer 32b are each produced. The first positive electrode mixture slurry and the second positive electrode mixture slurry each contain, for example, a positive electrode active material, a conductive agent, and a binder. (2) The first positive electrode mixture slurry and the second positive electrode mixture slurry are applied in stripes along the longitudinal direction of the positive electrode current collector 30 and adjacent to each other in the lateral direction. At this time, the positive electrode current collector exposed portion 34 is formed, for example, by intermittent application in which the first positive electrode mixture slurry is not applied to a portion of the positive electrode current collector 30. (3) After the applied slurry is dried, the coating is rolled using a rolling roller to produce the positive electrode 11.

[0031] [Negative Electrode] The negative electrode 12 has, for example, a negative electrode current collector 40 and a negative electrode mixture layer 42 formed on the surface of the negative electrode current collector 40. The negative electrode current collector 40 has a strip-like outer shape that substantially matches the outer shape of the negative electrode 12. The negative electrode current collector 40 can be a foil of a metal such as copper that is stable in the potential range of the negative electrode, or a film with such a metal disposed on the surface layer. The thickness of the negative electrode current collector is, for example, 5 μm to 30 μm.

[0032] The anode mixture layer 42 is preferably formed on both sides of the anode current collector 40. The thickness of the anode mixture layer 42 is, for example, 10 μm to 150 μm on one side of the anode current collector 40. The anode mixture layer 42 includes, for example, an anode active material and a binder. The content of the anode active material in the anode mixture layer 42 is, for example, 80 mass % to 99 mass % with respect to the total mass of the anode mixture layer 42. The anode 12 can be produced, for example, by applying an anode mixture slurry containing the anode active material, the binder, and the like to both sides of the anode current collector 40, drying the coating, and then rolling the coating using a roller or the like.

[0033] The negative electrode active material contained in the negative electrode mixture layer 42 is not particularly limited as long as it can reversibly absorb and release lithium ions, and generally, a carbon material such as graphite is 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. Furthermore, metals that alloy with Li, such as Si and Sn, metal compounds containing Si, Sn, and the like, and lithium-titanium composite oxides may also be used as the negative electrode active material. For example, SiO ｘ (x is 0.5 to 1.6), silicon oxide represented by the formula Li ２ｙ SiO （２＋ｙ） A silicon-containing material in which fine particles of Si are dispersed in a lithium silicate phase represented by (0<y<2), a silicon-containing material in which fine particles of Si are dispersed in a carbon phase, or the like may be used in combination with graphite.

[0034] Examples of the binder contained in the negative electrode mixture layer 42 include styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), carboxymethyl cellulose (CMC) or its salts (CMC-Na, CMC-K, CMC-NH ４ and the like, which may be a partially neutralized salt), polyacrylic acid (PAA) or a salt thereof (PAA-Na, PAA-K, and the like, which may be a partially neutralized salt), polyvinyl alcohol (PVA), etc. These may be used alone or in combination of two or more.

[0035] In the example shown in FIG. 2 , negative electrode current collector exposed portions 44, where the negative electrode current collector 40 is exposed, are formed near the winding start end and winding end end of the negative electrode 12. The negative electrode current collector exposed portions 44 are portions of the negative electrode current collector 40 that are not covered with the negative electrode mixture layer 42, and are formed, for example, by intermittent application of the negative electrode mixture slurry to a portion of the negative electrode current collector 40. A negative electrode tab 20 is connected to the negative electrode current collector exposed portion 44 near the winding start end. The negative electrode current collector exposed portion 44 near the winding end is in direct contact with the outer casing 15. The negative electrode current collector exposed portions 44 are preferably provided on both sides of the negative electrode 12 so as to overlap in the thickness direction of the negative electrode 12. The negative electrode tab 20 is joined to the negative electrode current collector exposed portion 44 by, for example, ultrasonic welding.

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

[0037] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides and phosphate compounds containing metal elements such as Ti, Al, Si, and Mg. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0038] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0039] <Example> [Preparation of Positive Electrode] As a positive electrode active material, a compound having the composition formula LiNi ０．９１ Co ０．０４ Al ０．０５ O ２An aluminum-containing lithium nickel cobalt oxide represented by the formula (I) was used. The positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 100:0.8:1, and the mixture was kneaded while adding N-methylpyrrolidone (NMP) to prepare a first positive electrode mixture slurry. The positive electrode active material, multi-walled carbon nanotubes (CNT), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 100:0.5:1, and the mixture was kneaded while adding N-methylpyrrolidone (NMP) to prepare a second positive electrode mixture slurry.

[0040] The first and second positive electrode mixture slurries were applied in stripes along the length of a strip-shaped positive electrode current collector made of aluminum foil, and then dried. The back side was similarly coated and dried. The dried coating was compressed using a roller and then cut to a predetermined electrode plate size, producing a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode current collector. The positive electrode mixture layer had the same configuration as that shown in FIG. 2 , with the first positive electrode mixture slurry applied to the region corresponding to the first positive electrode mixture layer, and the second positive electrode mixture slurry applied to the region corresponding to the second positive electrode mixture layer. Furthermore, an exposed positive electrode current collector portion was formed at approximately the center of the positive electrode in the length direction by intermittently applying the first positive electrode mixture slurry, and an aluminum positive electrode tab was welded to it.

[0041] The first positive electrode mixture slurry had good coatability, and no protrusion of the first positive electrode mixture layer onto the exposed portion of the positive electrode current collector was observed. In the short direction of the positive electrode, the length L1 of the first positive electrode mixture layer and the length L2 of the second positive electrode mixture layer satisfied the relationship L1 / (L1+L2)=1 / 3.

[0042] [Preparation of Negative Electrode] 98 parts by mass of graphite, 1 part by mass of sodium salt of carboxymethyl cellulose (CMC-Na), and 1 part by mass of styrene butadiene rubber (SBR) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to both sides of a strip-shaped negative electrode current collector made of copper foil with a thickness of 8 μm, so that a negative electrode current collector exposed portion was formed at the end of the winding. After drying, this coating film was rolled and cut to a predetermined electrode plate size to prepare a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode current collector, and a nickel negative electrode tab was welded to the negative electrode current collector exposed portion.

[0043] [Preparation of Electrolyte Solution] Lithium hexafluorophosphate (LiPF ) was dissolved in a mixed solvent (volume ratio of EC:EMC=1:3) consisting of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) to a concentration of 1.0 mol / L. ６ ) was dissolved to prepare an electrolyte solution.

[0044] [Preparation of Secondary Battery] The positive and negative electrodes were wound around a 20 μm-thick microporous separator (a composite film of polyethylene and polypropylene) to prepare an electrode assembly. Insulating plates were placed on the top and bottom of the electrode assembly, and the electrode assembly was housed in a cylindrical outer casing. The outer casing had a diameter of 18 mm and a height of 65 mm. The negative electrode tab was then welded to the bottom of the outer casing, and the positive electrode tab was welded to a sealing member. After that, 5.2 mL of electrolyte was injected into the inner casing by vacuum injection, and the open end of the outer casing was crimped to the sealing member via a gasket to prepare a secondary battery.

[0045] [Evaluation of Capacity Retention Rate] At an ambient temperature of 25°C, the secondary battery was charged at a constant current of 0.3 C to 4.2 V, and then charged at a constant voltage of 4.2 V to 0.02 C. Thereafter, the battery was discharged at a constant current of 0.5 C to 2.5 V. This charge / discharge cycle constituted one cycle, and 100 cycles were repeated, and the capacity retention rate was calculated using the following formula: Capacity retention rate = (discharge capacity at 100th cycle / discharge capacity at 1st cycle) x 100

[0046] Comparative Example A secondary battery was fabricated and evaluated in the same manner as in the example, except that the first positive electrode mixture slurry was used instead of the second positive electrode mixture slurry, and the first positive electrode mixture slurry was applied to the entire surface of the positive electrode current collector except for the exposed portion of the positive electrode current collector.

[0047] The evaluation results of the secondary batteries of the examples and comparative examples are shown in Table 1. Table 1 also shows the conductive agents contained in the first positive electrode mixture layer and the second positive electrode mixture layer.

[0048]

[0049] In the secondary batteries of the examples, as described above, the phenomenon of the first positive electrode mixture layer protruding into the exposed portion of the positive electrode current collector of the positive electrode mixture layer was not observed, and the capacity retention rate was higher than that of the comparative examples, as shown in Table 1. Therefore, it can be seen that in the electrode, by setting the content of fibrous carbon in the first mixture layer to 0.1 mass % or less and making the content of fibrous carbon in the second mixture layer higher than the content of fibrous carbon in the first mixture layer, a nonaqueous electrolyte secondary battery excellent in current collection performance and charge / discharge cycle characteristics can be obtained.

[0050] The present disclosure is further illustrated by the following embodiments. Configuration 1: A non-aqueous electrolyte secondary battery comprising: an electrode assembly including a strip-shaped first electrode and a second electrode having mutually different polarities; and a non-aqueous electrolyte; wherein the first electrode has a current collector and a mixture layer formed on the surface of the current collector; an exposed portion where the current collector is exposed is formed on the surface of the first electrode; the exposed portion is in contact with only one of both ends in the short direction of the first electrode; an electrode tab is connected to the exposed portion and is led out from the one end; the mixture layer contains an active material, granular carbon, and fibrous carbon; and when the mixture layer is divided into a first mixture layer aligned with the exposed portion in the long direction of the first electrode and a second mixture layer adjacent to the exposed portion and the first mixture layer in the short direction of the first electrode, the first mixture layer contains the granular carbon, and the second mixture layer contains the fibrous carbon; the content of the fibrous carbon in the first mixture layer is 0.1 mass % or less; A nonaqueous electrolyte secondary battery, wherein the content of the fibrous carbon in the second mixture layer is higher than the content of the fibrous carbon in the first mixture layer.Configuration 2: The nonaqueous electrolyte secondary battery according to Configuration 1, wherein the contents of the granular carbon and the fibrous carbon in the mixture layer are lower than the contents of the granular carbon and the fibrous carbon in the first mixture layer.Configuration 3: The nonaqueous electrolyte secondary battery according to Configuration 1 or 2, wherein, in the short direction of the first electrode, a length L1 of the first mixture layer and a length L2 of the second mixture layer satisfy 1 / 6≦L1 / (L1+L2)<1 / 2.Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the fibrous carbon is carbon nanotubes and the granular carbon is carbon black.Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the first electrode is a positive electrode and the second electrode is a negative electrode.

[0051] REFERENCE SIGNS LIST 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 tab, 20 Negative electrode tab, 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, 32a First positive electrode mixture layer, 32b Second positive electrode mixture layer, 34 Positive electrode current collector exposed portion, 40 Negative electrode current collector, 42 Negative electrode mixture layer, 44 Negative electrode current collector exposed portion

Claims

1. A non-aqueous electrolyte secondary battery comprising an electrode body including strip-shaped first and second electrodes having different polarities from each other, and a non-aqueous electrolyte, wherein the first electrode has a current collector and a binder layer formed on the surface of the current collector, and an exposed portion where the current collector is exposed is formed on the surface of the first electrode. The exposed portion is in contact with only one of the both end portions in the short side direction of the first electrode, and an electrode tab is connected to the exposed portion and led out from the one end portion. The binder layer contains an active material, granular carbon, and fibrous carbon. When the binder layer is divided into a first binder layer arranged in the longitudinal direction of the first electrode side by side with the exposed portion and a second binder layer adjacent to the exposed portion and the first binder layer in the short side direction of the first electrode, the first binder layer contains the granular carbon, the second binder layer contains the fibrous carbon, the content rate of the fibrous carbon in the first binder layer is 0.1 mass% or less, and the content rate of the fibrous carbon in the second binder layer is higher than the content rate of the fibrous carbon in the first binder layer.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the content rates of the granular carbon and the fibrous carbon in the second binder layer are lower than the content rates of the granular carbon and the fibrous carbon in the first binder layer.

3. The non-aqueous electrolyte secondary battery according to claim 1, wherein in the short side direction of the first electrode, the length L1 of the first binder layer and the length L2 of the second binder layer satisfy 1 / 6 ≦ L1 / (L1 + L2) < 1 / 2.

4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the fibrous carbon is a carbon nanotube and the granular carbon is carbon black.

5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the first electrode is a positive electrode and the second electrode is a negative electrode.

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