Nonaqueous electrolyte secondary battery

Abstract

The present invention provides a nonaqueous electrolyte secondary battery which has high capacity and excellent durability. A nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure is provided with: an electrode body that comprises a first electrode and a second electrode, which have different polarities from each other; an electrolyte solution; and an outer package which contains the electrode body and the electrolyte solution. The first electrode has a rectangular shape, and comprises a collector and a mixture layer that is formed on the surface of the collector; the mixture layer has a first region, a second region and a third region sequentially from one end toward the other end in the short-side direction of the first electrode; the respective widths of the first region and the third region in the short-side direction of the first electrode are 1% to 20% of the length of the first electrode in the short-side direction; the mixture layer contains an electrolyte salt; and the respective contents of the electrolyte salt in the first region and the third region are higher than the content of the electrolyte salt in the second region.

Description

Nonaqueous electrolyte secondary battery

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery.

[0002] The positive and negative electrodes of a nonaqueous electrolyte secondary battery each have a current collector and a mixture layer formed on the surface of the current collector. The mixture layer contains an active material capable of reversibly absorbing and releasing Li ions.

[0003] Patent Document 1 discloses a technique for incorporating a part of the electrolyte salt in the final electrolyte solution into the positive electrode in advance from the viewpoint of improving the injection property during the production of a secondary battery. Patent Document 2 discloses a technique for incorporating an electrolyte into the positive electrode in advance in order to suppress a decrease in the electrolyte salt concentration inside the positive electrode after repeated charge-discharge cycles.

[0004] JP 2011-192561 A International Publication No. 2018 / 025469

[0005] In recent years, further improvements in battery characteristics such as durability and battery capacity have been required for secondary batteries. As a result of extensive research, the present inventors have found that even if an electrolyte salt is contained in an electrode, the decrease in battery capacity due to charge / discharge cycles may not be sufficiently suppressed. Furthermore, increasing the content of electrolyte salt in the electrode relatively decreases the content of active material in the electrode, resulting in a decrease in battery capacity. The techniques disclosed in Patent Documents 1 and 2 do not consider achieving both battery capacity and durability in secondary batteries, and there is still room for improvement.

[0006] An object of the present disclosure is to provide a non-aqueous electrolyte secondary battery that has high capacity and excellent durability.

[0007] A nonaqueous electrolyte secondary battery according to one aspect of the present disclosure includes an electrode assembly including a first electrode and a second electrode having mutually different polarities, an electrolytic solution, and an exterior body that accommodates the electrode assembly and the electrolytic solution. The first electrode has a rectangular shape and a current collector and a mixture layer formed on the surface of the current collector. The mixture layer has a first region, a second region, and a third region in this order from one end to the other end in the lateral direction of the first electrode. The widths of the first region and the third region in the lateral direction of the first electrode are each 1% to 20% of the length of the first electrode in the lateral direction. The mixture layer contains an electrolyte salt, and the contents of the electrolyte salt in the first region and the third region are each higher than the content of the electrolyte salt in the second region.

[0008] According to a nonaqueous electrolyte secondary battery according to one aspect of the present disclosure, both high capacity and high durability can be achieved.

[0009] 1 is a cross-sectional view of a cylindrical secondary battery according to an embodiment of the present invention taken along an axial direction thereof;

[0010] An example of an embodiment of a cylindrical secondary battery according to the present disclosure will be described in detail below with reference to the drawings. In the following description, specific shapes, materials, values, directions, etc. are merely examples for ease of understanding the present invention and can be appropriately changed according to the specifications of the secondary battery. Furthermore, the following describes a cylindrical secondary battery in which a wound electrode body is housed in a cylindrical exterior body. However, the electrode body is not limited to a 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 between them. The exterior body is not limited to a cylindrical shape and may be, for example, rectangular or coin-shaped. Furthermore, the exterior body may be a pouch-type body made of a laminate sheet including a metal layer and a resin layer. Furthermore, in this specification, the expression "numerical value (A) to numerical value (B)" means greater than or equal to numerical value (A) and less than or equal to numerical value (B).

[0011] FIG. 1 is an axial cross-sectional view of a cylindrical secondary battery 10 according to an embodiment. As shown in FIG. 1 , the secondary battery 10 includes a wound electrode assembly 14, an electrolyte, and an exterior body 16 that accommodates the electrode assembly 14 and the electrolyte. The electrode assembly 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The exterior body 16 is a cylindrical metal container with a bottom and an opening on one axial side, and the opening of the exterior body 16 is closed by a sealing body 17. Hereinafter, for convenience of explanation, the sealing body 17 side of the battery will be referred to as the top, and the bottom side of the exterior body 16 will be referred to as the bottom.

[0012] The positive electrode 11, negative electrode 12, and separator 13 constituting the electrode assembly 14 are all rectangular, elongated bodies that are spirally wound in the longitudinal direction and alternately stacked in the radial direction of the electrode assembly 14. The separator 13 isolates the positive electrode 11 and the negative electrode 12 from each other. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 to prevent lithium precipitation. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the longitudinal and lateral directions. The two separators 13 are formed to be at least slightly larger than the positive electrode 11 and are arranged, for example, to sandwich the positive electrode 11. The electrode assembly 14 includes a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like. In the electrode assembly 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the lateral direction of the positive electrode 11 and the negative electrode 12 is the axial direction. That is, the end faces in the lateral direction of the positive electrode 11 and the negative electrode 12 form the end faces in the axial direction of the electrode body 14 .

[0013] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the exterior body 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the exterior body 16 by welding or the like, and the exterior body 16 serves as the negative electrode terminal.

[0014] A gasket 28 is provided between the exterior body 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior body 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior body 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior body 16 by the grooved portion 22 and the open end of the exterior body 16 that is crimped to the sealing body 17.

[0015] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. 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 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.

[0016] Next, the positive electrode 11, the negative electrode 12, the separator 13, and the electrolyte solution will be described in detail. An example in which the positive electrode 11 is the first electrode and the negative electrode 12 is the second electrode will be described below. Note that the present 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 will be described with reference to FIG. 2. FIG. 2 is a front view showing the positive electrode 11 according to an example embodiment in an expanded state. 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. The positive electrode mixture layer 32 is preferably formed on both sides of the positive electrode current collector 30. The positive electrode current collector 30 can be a foil of a metal that is stable within the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film having such a metal disposed on its surface.

[0018] The positive electrode mixture layer 32 has a first region 32 a, a second region 32 b, and a third region 32 c, arranged in this order from one end to the other end in the short-side direction of the positive electrode 11. In this embodiment, one end is located above the secondary battery 10, and the other end is located below the secondary battery 10.

[0019] The width t1 of the first region 32a and the width t3 of the third region 32c in the short-side direction of the positive electrode 11 are each 1% to 20% of the short-side length of the positive electrode 11. The width t1 of the first region 32a and the width t3 of the third region 32c may be different from each other, but are preferably the same. The width t2 of the second region 32b is 60% to 98% of the short-side length of the positive electrode 11.

[0020] The positive electrode mixture layer 32 contains an electrolyte salt. The positive electrode mixture layer 32 further contains, for example, a positive electrode active material, a binder, and a conductive agent. The electrolyte salt contents in the first region 32a and the third region 32c are each higher than the electrolyte salt contents in the second region 32b. This allows for both high capacity and high durability. Here, the electrolyte salt content in the first region 32a refers to the ratio of the mass of the electrolyte salt to the total mass of the first region 32a, and the electrolyte salt contents in the second region 32b and the third region 32c are defined similarly.

[0021] The content of the electrolyte salt in the first region 32a and the third region 32c is preferably 0.1% by mass to 10% by mass, more preferably 0.1% by mass to 5% by mass, and even more preferably 0.1% by mass to 1% by mass.

[0022] The content of electrolyte salt in the second region 32b is, for example, less than 0.1% by mass. Preferably, the second region 32b does not contain electrolyte salt. This increases the content of positive electrode active material in the second region 32b, thereby improving battery capacity. The content of positive electrode active material in each of the first region 32a, the second region 32b, and the third region 32c is, for example, 80% to 99% by mass with respect to the total mass of each region.

[0023] The electrolyte salt contained in the positive electrode mixture layer 32 is preferably a lithium salt. An example of the lithium salt is LiBF. ４, LiClO ４ , LiPF ６ , LiAsF ６ , LiSbF ６ , LiAlCl ４ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , Li(P(C ２ O ４ ) F ４ ), 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, Li ２ B ４ O ７ , Li(B(C ２ O ４ ) F ２ ), borates such as LiN(SO ２ CF ３ ) ２ , LiN(C １ F ２ｌ＋１ SO ２ ) (C ｍ F ２ｍ＋１ SO ２ ) {l and m are integers of 1 or more}. The lithium salt may be used alone or in combination of two or more. Among these, LiPF ６ The electrolyte salt contained in the positive electrode mixture layer 32 may be the same as the electrolyte salt contained in the electrolytic solution described below.

[0024] An example of the positive electrode active material contained in the positive electrode mixture layer 32 is a lithium transition metal composite oxide containing a transition metal element such as Ni. The lithium transition metal composite oxide preferably contains Ni and at least one element selected from Mn, Co, and Al. The content of Ni in the lithium transition metal composite oxide is, for example, 60 mol % to 95 mol % with respect to the total number of moles of metal elements excluding Li. The lithium transition metal composite oxide is, for example, a lithium transition metal composite oxide represented by the general formula Li ａ Ni ｘ M1 ｙ M2 ｚ O ２－ｂ (wherein 0.8≦a≦1.2, 0.6≦x≦0.95, 0.05≦y≦0.4, 0≦z≦0.15, 0≦b<0.05, x+y+z=1, M1 is at least one element selected from Mn, Co, and Al, and M2 is at least one element selected from Fe, Ti, Si, Nb, Zr, Mo, and Zn).

[0025] Examples of the conductive agent contained in the positive electrode mixture layer 32 include carbon-based particles such as carbon black (CB), acetylene black (AB), ketjen black, carbon nanotubes (CNT), graphene, and graphite. These may be used alone or in combination of two or more.

[0026] Examples of the binder contained in the positive electrode mixture layer 32 include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyimide-based resins, acrylic-based resins, polyolefin-based resins, polyacrylonitrile (PAN), etc. These may be used alone or in combination of two or more.

[0027] The positive electrode active material, conductive agent, and binder contained in the first region 32a, the second region 32b, and the third region 32c may be different from one another, but are preferably the same from one another.

[0028] 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 region 32a and the third region 32c and a second positive electrode mixture slurry for the second region 32b are each produced. For example, only the first positive electrode mixture slurry contains an electrolyte salt. Both the first positive electrode mixture slurry and the second positive electrode mixture slurry 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 11 and adjacent to each other in the lateral direction. At this time, the positive electrode 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.

[0029] [Negative Electrode] The negative electrode 12 has, for example, 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 is preferably formed on both sides of the negative electrode current collector. The negative electrode current collector 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.

[0030] The negative electrode mixture layer contains, for example, a negative electrode active material and a binder. The content of the negative electrode active material in the negative electrode mixture layer is, for example, 80% by mass to 99% by mass with respect to the total mass of the negative electrode mixture layer. The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing the negative electrode active material, the binder, and the like to the surface of the negative electrode current collector, drying the coating, and then rolling the coating using a roller or the like.

[0031] The negative electrode active material contained in the negative electrode mixture layer is not particularly limited as long as it can reversibly absorb and release Li 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 and Sn, 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.

[0032] Examples of the binder contained in the negative electrode mixture layer 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.

[0033] [Separator] 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 the surface of the separator 13 may be coated with a material such as an aramid-based resin or ceramic.

[0034] [Electrolyte] The electrolyte is a liquid electrolyte containing a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous 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 non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0035] 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 and γ-valerolactone; and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate.

[0036] Examples of the ethers include 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, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as ethyl 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, and tetraethylene glycol dimethyl ether.

[0037] As the halogen-substituted compound, it is preferable to use fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, fluorinated chain carboxylates such as methyl fluoropropionate (FMP), and the like.

[0038] The electrolyte salt contained in the electrolytic solution may be different from the electrolyte salt contained in the positive electrode 11, but is preferably the same as the electrolyte salt contained in the positive electrode 11. The electrolyte salt contained in the electrolytic solution preferably contains a lithium salt, and LiPF ６ The concentration of the lithium salt is preferably 0.5 mol to 2.0 mol per liter of the solvent.

[0039] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0040] [Preparation of Positive Electrode] As the first positive electrode active material, LiNi ０．８０ Co ０．１５ Al ０．０５ O ２ The first positive electrode active material, acetylene black (AB), polyvinylidene fluoride (PVDF), and lithium hexafluorophosphate (LiPF ６ The first positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 100:1:0.9:0.5 and kneaded while adding N-methylpyrrolidone (NMP) to prepare a first positive electrode mixture slurry. The first positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 100:1:0.9 and kneaded while adding N-methylpyrrolidone (NMP) to prepare a second positive electrode mixture slurry.

[0041] The first and second positive electrode mixture slurries were applied in stripes in the longitudinal direction to the surface of a rectangular positive electrode current collector made of aluminum foil, and then dried. The back surface was similarly coated and dried. The dried coating was rolled 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, and was formed by applying the first positive electrode mixture slurry to the first and third regions and applying the second positive electrode mixture slurry to the second region. In the transverse direction of the positive electrode, the ratio of the width of the first region to the width of the second region to the width of the third region was 5:90:5. In addition, a positive electrode exposed portion was formed in approximately the center of the positive electrode in the longitudinal direction by intermittently applying the first and second positive electrode mixture slurries, and an aluminum positive electrode lead was welded to the positive electrode exposed portion.

[0042] [Preparation of negative electrode] 98 parts by mass of graphite, 1 part by mass of carboxymethyl cellulose (CMC), 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 rectangular negative electrode current collector made of copper foil so that a negative electrode exposed portion was formed at the end of the winding end where the negative electrode current collector was exposed. After drying, the coating was rolled and cut to a predetermined electrode plate size to prepare a negative electrode having a negative electrode mixture layer formed on both sides of the negative electrode current collector, and a nickel negative electrode lead was welded to the negative electrode exposed portion.

[0043] [Preparation of Electrolyte Solution] Lithium hexafluorophosphate (LiPF ) was dissolved in a mixed solvent (volume ratio of EC:EMC=3:7) consisting of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) to a concentration of 1.5 mol / L. ６ ) was dissolved to prepare an electrolyte solution.

[0044] [Preparation of Secondary Battery] The positive and negative electrodes were wound with a polyolefin resin separator between them 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 negative electrode lead was then welded to the bottom of the outer casing, and the positive electrode lead was welded to a sealing member. After that, electrolyte was injected into the inner casing under reduced pressure, and the open end of the outer casing was crimped to the sealing member via a gasket to prepare a secondary battery.

[0045] Example 2 A secondary battery was fabricated in the same manner as in Example 1, except that in the fabrication of the positive electrode, the ratio of the width of the first region to the width of the second region to the width of the third region in the short-side direction of the positive electrode was changed to 2.5:95:2.5.

[0046] Example 3 A secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the positive electrode, the ratio of the width of the first region to the width of the second region to the width of the third region in the short direction of the positive electrode was changed to 10:80:10.

[0047] Comparative Example 1 A secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the positive electrode, only the second positive electrode mixture slurry was applied to the entire surface of the positive electrode current collector excluding the exposed positive electrode portion.

[0048] Comparative Example 2 A secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the positive electrode, only the first positive electrode mixture slurry was applied to the entire surface of the positive electrode current collector excluding the exposed positive electrode portion.

[0049] [Evaluation of Initial Battery Capacity and Capacity Retention Rate] At an ambient temperature of 25°C, the secondary batteries of the Examples and Comparative Examples were charged at a constant current of 0.3 C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value reached 0.01 C. Subsequently, the secondary batteries were discharged at a constant current of 0.5 C until the battery voltage reached 2.5 V. This cycle of charge and discharge constituted one cycle, and 100 cycles were performed. The capacity retention rate of the secondary batteries of each Example and Comparative Example in the charge and discharge cycles was calculated using the following formula. The discharge capacity at the first cycle was taken as the initial battery capacity. Capacity retention rate = (discharge capacity at the 200th cycle / discharge capacity at the first cycle) x 100

[0050] Table 1 shows the evaluation results of the secondary batteries of the Examples and Comparative Examples. In Table 1, the initial battery capacities of Examples 1 to 3 and Comparative Example 1 are shown as relative values, with the value of the initial battery capacity of Comparative Example 2 taken as 100. In Table 1, the capacity retention rates of Examples 1 to 3 and Comparative Example 2 are shown as relative values, with the value of the capacity retention rate of Comparative Example 1 taken as 100.

[0051]

[0052] The secondary batteries of Examples 1 to 3 had high initial battery capacity and capacity retention rate, and were able to achieve both high capacity and high durability. On the other hand, the secondary battery of Comparative Example 1 had a low capacity retention rate, and the secondary battery of Comparative Example 2 had a low initial battery capacity. Therefore, it can be seen that a secondary battery with high capacity and excellent durability can be obtained by making the content of electrolyte salt in a predetermined range near both ends in the short side direction of the mixture layer higher than the content of electrolyte salt in the center part in the short side direction.

[0053] The present disclosure is further described by the following embodiments. Aspect 1: A nonaqueous electrolyte secondary battery including an electrode assembly including a first electrode and a second electrode having opposite polarities, an electrolytic solution, and an exterior housing that accommodates the electrode assembly and the electrolytic solution, wherein the first electrode is rectangular and has a current collector and a mixture layer formed on a surface of the current collector, the mixture layer having a first region, a second region, and a third region in this order from one end to the other end in the lateral direction of the first electrode, the widths of the first region and the third region in the lateral direction of the first electrode each being 1% to 20% of the length of the first electrode in the lateral direction, the mixture layer containing an electrolyte salt, and the content of the electrolyte salt in the first region and the third region each being higher than the content of the electrolyte salt in the second region. Aspect 2: The nonaqueous electrolyte secondary battery according to aspect 1, wherein the first electrode and the second electrode in the electrode assembly are wound with a separator interposed therebetween. Configuration 3: The nonaqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the first electrode is a positive electrode. Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the electrolyte salt includes lithium hexafluorophosphate. Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the second region does not contain the electrolyte salt.

[0054] REFERENCE SIGNS LIST 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Exterior body, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode current collector, 32 Positive electrode mixture layer, 32a First region, 32b Second region, 32c Third region, 34 Positive electrode exposed portion

Claims

1. A non-aqueous electrolyte secondary battery comprising an electrode assembly including a first electrode and a second electrode of mutually opposite polarities, an electrolyte solution, and an exterior housing that accommodates the electrode assembly and the electrolyte solution, wherein the first electrode is rectangular and has a current collector and a mixture layer formed on the surface of the current collector, the mixture layer having a first region, a second region, and a third region in that order from one end to the other end in the short-side direction of the first electrode, the widths of the first region and the third region in the short-side direction of the first electrode each being 1% to 20% of the length of the first electrode in the short-side direction, the mixture layer containing an electrolyte salt, and the content of the electrolyte salt in the first region and the third region each being higher than the content of the electrolyte salt in the second region.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein in the electrode assembly, the first electrode and the second electrode are wound with a separator interposed therebetween.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the first electrode is a positive electrode.

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the electrolyte salt includes lithium hexafluorophosphate.

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the second region does not contain the electrolyte salt.

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