Non-aqueous electrolyte secondary battery

Abstract

A non-aqueous electrolyte secondary battery is provided which enables improving charge / discharge cycle characteristics. This non-aqueous electrolyte secondary battery is provided with an electrode body which comprises a first electrode and a second electrode with mutually different polarities that face each other across from a separator, and a battery case which houses the electrode body. The first electrode has a mixed layer that contains a conductive agent, and, used in a state in which a non-aqueous electrolyte secondary battery is fixed, the mixed layer has a first region on the upper side in the vertical direction and a second region on the lower side in the vertical direction; the void ratio of the mixed layer in the first region is higher than the void ratio of the mixed layer in the second region, and the conductive agent content ratio in the first region is lower than the conductive agent content ratio in the second region; and the conductive agent contained in the first region contains fibrous carbon and the conductive agent contained in the second region contains granular carbon.

Description

[Technical Field]

[0001] This disclosure relates to a non-aqueous electrolyte secondary battery. [Background technology]

[0002] In recent years, non-aqueous electrolyte secondary batteries, which charge and discharge by moving lithium ions and other elements between the positive and negative electrodes via a non-aqueous electrolyte, have been widely used as high-power, high-energy-density secondary batteries.

[0003] The battery capacity of non-aqueous electrolyte secondary batteries may decrease due to repeated charging and discharging, and improvements to the charge-discharge cycle characteristics of non-aqueous electrolyte secondary batteries are being investigated. Patent Document 1 discloses a technique for improving the charge-discharge cycle characteristics of a non-aqueous electrolyte secondary battery having a wound electrode body, by making the void ratio on the winding center side of the electrode body higher than the void ratio on the peripheral side of the electrode body, while also making the content of conductive agent on the winding center side of the electrode body higher than the content of conductive agent on the peripheral side of the electrode body. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2011-165388 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Incidentally, when a non-aqueous electrolyte secondary battery is used in a stationary state, the non-aqueous electrolyte inside the battery may become ubiquitous at the bottom due to gravity. In this case, repeated charging and discharging will result in differences in the non-aqueous electrolyte content between the top and bottom of the battery, leading to differences in the degree of degradation and a decrease in charge-discharge cycle characteristics.

[0006] This disclosure aims to provide a non-aqueous electrolyte secondary battery that enables improved charge-discharge cycle characteristics. [Means for solving the problem]

[0007] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure comprises an electrode body in which a first electrode and a second electrode having opposite polarities face each other via a separator, and a battery case housing the electrode body, wherein the first electrode has a composite layer containing a conductive agent, and when the non-aqueous electrolyte secondary battery is used in a fixed state, the composite layer has a first region on the upper side with respect to the vertical direction and a second region on the lower side with respect to the vertical direction, the porosity of the composite layer in the first region is higher than the porosity of the composite layer in the second region, the content of the conductive agent in the first region is lower than the content of the conductive agent in the second region, and the conductive agent contained in the first region contains fibrous carbon, and the conductive agent contained in the second region contains granular carbon.

[0008] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure comprises an electrode body in which a first electrode and a second electrode having opposite polarities face each other via a separator, a bottomed cylindrical outer can containing the electrode body, and a sealing body that closes the opening of the outer can. The first electrode has a composite layer containing a conductive agent, and when the direction in which the electrode body is inserted into the outer can is the insertion direction, the composite layer has a first region on the sealing body side with respect to the insertion direction and a second region on the bottom side of the outer can with respect to the insertion direction. The porosity of the composite layer in the first region is higher than the porosity of the composite layer in the second region, the content of the conductive agent in the first region is lower than the content of the conductive agent in the second region, and the conductive agent contained in the first region contains fibrous carbon, and the conductive agent contained in the second region contains granular carbon.

[0009] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes an electrode body in which a first electrode and a second electrode having different polarities face each other via a separator, a bottomed cylindrical exterior can that houses the electrode body, and a sealing body that closes an opening of the exterior can. The first electrode has a mixture layer containing a conductive agent. When the direction in which the electrode body is inserted into the exterior can is defined as the insertion direction, the mixture layer has a first region on the sealing body side with respect to the insertion direction and a second region on the bottom side of the exterior can with respect to the insertion direction. The porosity of the mixture layer in the second region is higher than the porosity of the mixture layer in the first region, the content rate of the conductive agent in the second region is lower than the content rate of the conductive agent in the first region, the conductive agent contained in the second region includes fibrous carbon, and the conductive agent contained in the first region includes granular carbon.

Advantages of the Invention

[0010] According to one aspect of the present disclosure, it becomes possible to improve the charge-discharge cycle characteristics.

Brief Description of the Drawings

[0011] [Figure 1] It is a cross-sectional view of a non-aqueous electrolyte secondary battery which is an example of an embodiment. [Figure 2] It is a side view showing a state in which the non-aqueous electrolyte secondary battery shown in FIG. 1 is fixed. [Figure 3] It is a perspective view of a wound electrode body used for the non-aqueous electrolyte secondary battery of FIG. 2. [Figure 4] It is a side view showing another example of a state in which the non-aqueous electrolyte secondary battery shown in FIG. 1 is fixed. [Figure 5] It is a perspective view of a wound electrode body used for the non-aqueous electrolyte secondary battery of FIG. 4. [Figure 6] It is a developed perspective view showing a part of the wound electrode body used for the non-aqueous electrolyte secondary battery of FIG. 2 and showing a positive electrode mixture layer formed on the surface of the positive electrode.

Modes for Carrying Out the Invention

[0012] An example of an embodiment will be described with reference to the drawings. Note that the non-aqueous electrolyte secondary battery of this disclosure is not limited to the embodiments described below. Also, the drawings referenced in the description of the embodiments are schematic.

[0013] Figure 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment. The non-aqueous electrolyte secondary battery 10 shown in Figure 1 comprises a wound electrode body 14 in which a positive electrode 11 and a negative electrode 12 are wound around a separator 13, a non-aqueous electrolyte, insulating plates 18 and 19 arranged above and below the electrode body 14, respectively, and a battery case 15 that houses the above components. The battery case 15 is composed of an outer casing 16 and a sealing body 17 that closes the opening of the outer casing 16. In addition, other forms of electrode bodies may be used instead of the wound electrode body 14, such as a laminated electrode body in which the positive electrode and negative electrode are alternately stacked with a separator. Examples of battery cases 15 include cylindrical, square, coin-shaped, button-shaped, and other bottomed cylindrical outer casings, and pouch outer casings formed by laminating a resin sheet and a metal sheet.

[0014] The outer casing 16 is, for example, a metal case in the shape of a bottomed cylinder. A gasket 28 is provided between the outer casing 16 and the sealing body 17 to ensure airtightness inside the battery. The outer casing 16 has, for example, a protruding portion 22 that supports the sealing body 17, which is a part of the side surface that protrudes inward. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17.

[0015] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, with the insulating member 25 interposed between their respective peripheral edges. When the internal pressure of the non-aqueous electrolyte secondary battery 10 rises due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 towards the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0016] In the non-aqueous electrolyte secondary battery 10 shown in Figure 1, the positive electrode lead 20 attached to the positive electrode 11 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 attached to the negative electrode 12 extends outside the insulating plate 19 towards the bottom of the outer casing 16. The positive electrode lead 20 is connected by welding or the like to the lower surface of the filter 23, which is the bottom plate of the sealing body 17, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected by welding or the like to the inner surface of the bottom of the outer casing 16, and the outer casing 16 becomes the negative electrode terminal.

[0017] In the non-aqueous electrolyte secondary battery 10 shown in Figure 1, the side with the sealing body 17 is considered the upper side, and the bottom side of the outer casing 16 is considered the lower side. Also, the direction from the bottom of the battery case 15 toward the sealing body 17 is considered the height direction of the non-aqueous electrolyte secondary battery 10.

[0018] Next, with reference to Figures 2 to 5, we will describe the configuration in which the non-aqueous electrolyte secondary battery 10 shown in Figure 1 is fixed.

[0019] Figure 2 is a side view showing an example of a fixed state of the non-aqueous electrolyte secondary battery 10 shown in Figure 1. The non-aqueous electrolyte secondary battery 10 of this embodiment is preferably used as a stationary or fixed power source installed indoors or outdoors, or as a power source installed in a mobile device such as an electric vehicle. As shown in Figure 2, the non-aqueous electrolyte secondary battery 10 used as such a power source is installed on a fixing part 38 such as a mounting base or case and used in a fixed state. Used in a fixed state means that after the non-aqueous electrolyte secondary battery 10 is installed on the fixing part 38 and use begins, the orientation of the non-aqueous electrolyte secondary battery 10 does not change significantly. For example, a non-aqueous electrolyte secondary battery 10 used as a power source for a mobile phone is placed in any orientation as the mobile phone is used, so it is not included in the case of being used in a fixed state.

[0020] In Figure 2, arrow Z points in the vertical direction (direction of gravity). That is, the non-aqueous electrolyte secondary battery 10 shown in Figure 2 is erected along the vertical direction. More specifically, in the non-aqueous electrolyte secondary battery 10 shown in Figure 2, the bottom of the battery case 15 is in contact with the fixing part 38, and the non-aqueous electrolyte secondary battery 10 is installed so that its height direction is aligned with the vertical direction.

[0021] Figure 3 is a perspective view of the wound electrode body 14 used in the non-aqueous electrolyte secondary battery 10 shown in Figure 2. Region A of the electrode body 14 shown in Figure 3 corresponds to the upper region 10a in Figure 2. Region A includes the first region of the composite layer of the first electrode, which will be described later. Region B of the electrode body 14 shown in Figure 3 corresponds to the lower region 10b in Figure 2. Region B includes the second region of the composite layer of the first electrode, which will be described later.

[0022] Figure 4 is a side view showing another example of the non-aqueous electrolyte secondary battery 10 shown in Figure 1 in a fixed state. In Figure 4, arrow Z points in the vertical direction (direction of gravity), and arrow Y points in the direction perpendicular to the vertical direction (horizontal direction). In the non-aqueous electrolyte secondary battery 10 shown in Figure 4, the side of the battery case 15 is in contact with the fixing part 38, and the height direction of the non-aqueous electrolyte secondary battery 10 is aligned with the direction perpendicular to the vertical direction (horizontal direction).

[0023] FIG. 5 is a perspective view of the wound electrode body 14 used in the non-aqueous electrolyte secondary battery 10 of FIG. 4. Region A of the electrode body 14 shown in FIG. 5 corresponds to the upper side region 10a of FIG. 4. Region A includes a first region of the mixture layer of the first electrode, which will be described later. Region B of the electrode body 14 shown in FIG. 5 corresponds to the lower side region 10b of FIG. 4. Region B includes a second region of the mixture layer of the first electrode, which will be described later.

[0024] Next, each component of the non-aqueous electrolyte secondary battery 10 will be described in detail. Hereinafter, an example in which the positive electrode 11 is the first electrode and the negative electrode 12 is the second electrode will be described. Note that the present embodiment is not limited to this example. For example, the negative electrode 12 may be the first electrode. Also, 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.

[0025] [Positive Electrode] The positive electrode 11 includes a positive electrode current collector and a positive electrode mixture layer provided on the positive electrode current collector. For the positive electrode current collector, a foil of a metal stable within the potential range of the positive electrode 11 such as aluminum, or a film having the metal disposed on the surface layer can be used. The positive electrode mixture layer contains, for example, a positive electrode active material, a binder, a conductive agent, and the like.

[0026] Examples of the positive electrode active material include lithium metal composite oxides containing transition metal elements such as Co, Mn, Ni, etc. The lithium metal composite oxide is, for example, Li x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1-y O z 、Li x Ni 1-y M y O z 、Li x Mn2O4, Li x Mn 2-y M yO4, LiMPO4, Li2MPO4F (M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B; 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3). These may be used alone or in combination of multiple kinds. In terms of achieving high capacity of the non-aqueous electrolyte secondary battery, the cathode 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, B; 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3), etc. lithium nickel composite oxides are preferably included.

[0027] The binder can be, for example, fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide (PI), acrylic resins, polyolefin resins, styrene-butadiene rubber (SBR), nitrile rubber (NBR), carboxymethyl cellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts, polyvinyl alcohol (PVA), etc. These can be used alone or in combination of two or more kinds. The content of the binder in the cathode mixture layer is preferably, for example, 0.5% to 10% by mass based on the mass of the cathode mixture layer, and more preferably 1% to 5% by mass.

[0028] The conductive agent can use fibrous carbon or granular carbon. Examples of fibrous carbon include carbon nanotubes (CNT), carbon nanofibers (CNF), vapor-grown carbon fibers (VGCF), electrospun carbon fibers, polyacrylonitrile (PAN)-based carbon fibers, pitch-based carbon fibers, etc. These can be used alone or in combination of two or more kinds.

[0029] Examples of granular carbon include carbon black (CB), acetylene black (AB), Ketjenblack, and graphite. These can be used individually or in combination of two or more.

[0030] In the following, an example of a positive electrode mixture layer will be described with reference to Figure 6. Figure 6 is an unfolded perspective view showing a portion of the wound electrode body 14 used in the non-aqueous electrolyte secondary battery 10 of Figure 2, with the positive electrode mixture layer formed on the surface of the positive electrode 11. Note that the positive electrode 11 is not limited to the following example; for example, in the positive electrode 11 shown in Figure 5, the first and second regions are configured in a different manner than in the following example, and the effects of this disclosure are realized.

[0031] In Figure 6, the positive electrode mixture layer has a first region 40 on the upper side with respect to the vertical direction and a second region 42 on the lower side with respect to the vertical direction. In other words, when the direction in which the electrode body 14 is inserted into the outer can 16 is considered the insertion direction, the positive electrode mixture layer has a first region 40 on the side of the sealing body 17 with respect to the insertion direction and a second region 42 on the bottom side of the outer can 16 with respect to the insertion direction.

[0032] The porosity of the positive electrode mixture layer in the first region 40 is higher than that of the positive electrode mixture layer in the second region 42. This suppresses the uneven distribution of the non-aqueous electrolyte in the vertical direction downwards, thereby improving the charge-discharge cycle characteristics. In a non-aqueous electrolyte secondary battery 10 used in a fixed state, the non-aqueous electrolyte in the battery case 15 is unevenly distributed in the vertical direction downwards due to gravity, and the non-aqueous electrolyte tends to be depleted in the vertical direction upwards. This uneven distribution of the non-aqueous electrolyte leads to a decrease in charge-discharge cycle characteristics. However, by making the porosity of the positive electrode mixture layer in the first region 40 higher than that of the positive electrode mixture layer in the second region 42, the retention of the non-aqueous electrolyte in the vertical direction upwards is improved. In this embodiment, a non-aqueous electrolyte secondary battery 10 having a cylindrical bottomed battery case and a wound electrode body was described as an example, but similar effects can be obtained in the case of a rectangular bottomed cylindrical battery case or a non-aqueous electrolyte secondary battery having a stacked electrode body, etc.

[0033] The porosity of the positive electrode mixture layer in the first region 40 and the second region 42 is, for example, 10% to 40% by volume. The porosity of the positive electrode mixture layer is calculated from the bulk density of the positive electrode mixture layer and the true density and mass of each component contained in the positive electrode mixture layer, such as the positive electrode active material, conductive agent, and binder, according to the following formula. By adjusting the compressibility of the positive electrode mixture layer, the bulk density of the positive electrode mixture layer can be changed, and therefore the porosity of the positive electrode mixture layer can be changed. Porosity of the positive electrode mixture layer = 1 - (Sum of (mass content / true density) of each component × bulk density of the positive electrode mixture layer)

[0034] The content of the conductive agent in the first region 40 is, for example, 0.01% to 1% by mass relative to the mass of the positive electrode mixture layer in the first region 40. The content of the conductive agent in the second region 42 is, for example, 0.1% to 5% by mass relative to the mass of the positive electrode mixture layer in the second region 42.

[0035] The conductive agent contained in the first region 40 includes fibrous carbon, and the conductive agent contained in the second region 42 includes granular carbon. Fibrous carbon can achieve equivalent or better conductivity with a smaller content than granular carbon. Therefore, by including fibrous carbon in the first region 40 and granular carbon in the second region 42, good conductivity can be obtained in both the first region 40 and the second region even if the content of the conductive agent in the first region 40 is lower than the content of the conductive agent in the second region 42.

[0036] The fibrous carbon is preferably CNT, and the granular carbon is preferably AB. The CNT may be either single-walled carbon nanotubes (SWCNT) or multi-walled carbon nanotubes (MWCNT), and it is preferable that MWCNTs are included. The diameter of the CNT is, for example, 0.5 nm to 100 nm, and the length of the CNT is, for example, 0.1 μm to 40 μm.

[0037] The ratio of the width W1 of the first region 40 to the width W2 of the second region 42 is preferably in the range of 1:9 to 7:3, more preferably in the range of 2:8 to 6:4, and particularly preferably in the range of 3:7 to 5:5. In order to suppress the non-aqueous electrolyte from being concentrated in the vertically downward direction, it is preferable that the width W1 of the first region 40 is not smaller than the width W2 of the second region 42.

[0038] The difference between the surface resistance in the first region 40 and the surface resistance in the second region 42 is preferably 20% or less, more preferably 10% or less, and particularly preferably 5% or less, compared to the surface resistance in the second region 42. This makes the degree of degradation in the first region 40 and the second region 42 more uniform, thereby significantly improving the charge-discharge cycle characteristics. Here, the surface resistance can be measured using a commercially available device with a positive electrode mixture layer coated on the surface of a PET film or the like.

[0039] Furthermore, if the non-aqueous electrolyte secondary battery 10 is fixed upside down from the example shown in Figure 2, with the upper part of the battery case 15 in contact with the fixing part 38, and the height direction of the non-aqueous electrolyte secondary battery 10 aligned with the vertical direction, then the porosity of the positive electrode mixture layer in the second region 42 is greater than the porosity of the positive electrode mixture layer in the first region 40, the content of the conductive agent in the second region 42 is lower than the content of the conductive agent in the first region 40, and the conductive agent contained in the second region 42 contains fibrous carbon, while the conductive agent contained in the first region 40 contains granular carbon.

[0040] Next, an example of a method for manufacturing the positive electrode 11 will be described. For example, a positive electrode slurry A for the first region 40 is prepared by mixing the positive electrode active material, a binder, and fibrous carbon as a conductive agent with a solvent. Also, a positive electrode slurry B for the second region 42 is prepared by mixing the positive electrode active material, a binder, and granular carbon as a conductive agent with a solvent. The binder content in positive electrode slurry A and positive electrode slurry B is preferably, for example, 0.1% to 10% by mass, and more preferably 0.5% to 5% by mass, based on the total mass of solids. For example, in the case of a non-aqueous electrolyte secondary battery 10 used in the state shown in Figure 2, positive electrode slurry A and positive electrode slurry B are applied in a stripe pattern along the longitudinal direction of the positive electrode current collector and adjacent to each other in the width direction perpendicular to the longitudinal direction. Furthermore, in the case of a non-aqueous electrolyte secondary battery 10 used in the state shown in Figure 4, positive electrode slurry A and positive electrode slurry B are alternately applied along the longitudinal direction of the positive electrode current collector to a predetermined length. Then, the applied slurry is dried and the coating film (positive electrode slurry layer) is rolled to form the positive electrode 11.

[0041] [Negative electrode] The negative electrode 12 comprises a negative electrode current collector and a negative electrode mixture layer provided on the negative electrode current collector. The negative electrode current collector can be, for example, a foil of a metal that is stable in the negative electrode potential range, such as copper, or a film with the metal arranged on its surface. The negative electrode mixture layer contains, for example, a negative electrode active material, a binder, a conductive agent, etc. The negative electrode 12 can be manufactured, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, a conductive agent, etc., to both sides of the negative electrode current collector, drying the coating, and then rolling the coating (negative electrode mixture layer).

[0042] The negative electrode active material is, for example, a material that can reversibly intercept and release lithium ions, and includes carbon materials such as natural graphite and artificial graphite, metals that alloy with lithium such as silicon (Si) and tin (Sn), or alloys and composite oxides containing metallic elements such as Si and Sn.

[0043] Examples of binders include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide (PI), acrylic resins, polyolefin resins, styrene-butadiene rubber (SBR), nitrile rubber (NBR), carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts, and polyvinyl alcohol (PVA). These may be used individually or in combination of two or more. The binder content in the negative electrode mixture layer is preferably 0.5% to 10% by mass, and more preferably 1% to 5% by mass, relative to the mass of the negative electrode mixture layer.

[0044] The conductive agent can be fibrous carbon or granular carbon. Examples of fibrous carbon include carbon nanotubes (CNTs), carbon nanofibers (CNFs), vapor-grown carbon fibers (VGCFs), electrospun carbon fibers, polyacrylonitrile (PAN) carbon fibers, and pitch carbon fibers. These may be used individually or in combination of two or more types. The fibrous carbon may contain CNTs. The CNTs may be single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs), and it is preferable that they contain MWCNTs. The diameter of the CNTs is, for example, 0.5 nm to 100 nm, and the length of the CNTs is, for example, 0.1 μm to 40 μm.

[0045] Examples of granular carbon include carbon black (CB), acetylene black (AB), Ketjenblack, and graphite. These can be used individually or in combination of two or more.

[0046] In this embodiment, the configuration of the negative electrode mixture layer of the negative electrode 12 is not particularly specified, but the negative electrode 12 may have a first region and a second region, similar to the positive electrode 11.

[0047] [Separator] For the separator 13, for example, a porous sheet having ion permeability and insulating properties can be used. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator include polyethylene, olefin resins such as 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 resin. Alternatively, it may be a multilayer separator containing a polyethylene layer and a polypropylene layer, or a separator with a material such as aramid resin or ceramic coated on its surface may be used.

[0048] [Nonaqueous electrolyte] A non-aqueous electrolyte comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixtures of two or more of these. The non-aqueous solvent may also contain halogen-substituted solvents in which at least some of the hydrogen atoms of the solvent are replaced with halogen atoms such as fluorine.

[0049] Examples of the above esters include cyclic carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; linear carbonate esters 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 linear carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate.

[0050] 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., and 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.

[0051] As the above halogenated compounds, it is preferable to use fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP), etc.

[0052] The electrolyte salt is preferably a lithium salt. Examples of lithium salts 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, borates such as Li2B4O7, Li(B(C2O4)F2), LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 Examples include imide salts such as SO2){l,m are integers of 1 or more}. Lithium salts may be used individually or in mixtures of multiple types. Of these, LiPF6 is preferred from the viewpoint of ionic conductivity and electrochemical stability. The concentration of the lithium salt is preferably 0.8 to 1.8 mol per liter of solvent. [Examples]

[0053] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited to these examples.

[0054] <Example 1> [Fabrication of the positive electrode] (Preparation of positive electrode mixture slurry A) 98 parts by mass of LiNi 0.8 Co 0.15 Al 0.05 O2, 0.25 parts by mass of multi-walled carbon nanotubes (MWCNTs), and 1 part by mass of polyvinylidene fluoride (PVDF) with an average molecular weight of 1.1 million were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare cathode mixture slurry A with a solid content of 70% by mass.

[0055] (Preparation of positive electrode mixture slurry B) 98 parts by mass of LiNi 0.8 Co 0.15 Al 0.05 O2, 1 part by mass of acetylene black (AB), and 1 part by mass of polyvinylidene fluoride (PVDF) with an average molecular weight of 1.1 million were mixed, and an appropriate amount of NMP was added to prepare cathode mixture slurry B with a solid content of 70% by mass.

[0056] Next, the positive electrode mixture slurry was applied in a strip-shaped positive electrode current collector made of aluminum foil, with positive electrode mixture slurry A and positive electrode mixture slurry B applied in a strip pattern along the longitudinal direction, and then dried. The reverse side was similarly coated and dried. After compressing the dried coating using a roller, it was cut to a predetermined electrode plate size, and a positive electrode was fabricated with positive electrode mixture layers formed on both sides of the positive electrode current collector. The positive electrode mixture layer had the same form as in Figure 6, with positive electrode mixture slurry A applied to the first region and positive electrode mixture slurry B applied to the second region. The ratio of the width W1 of the first region to the width W2 of the second region was 4:6. The void ratio of the first region was 28.8 volume%, and the void ratio of the second region was 26.4%. In addition, a positive electrode exposed portion was provided approximately in the center of the longitudinal direction of the positive electrode, where there was no mixture layer and the surface of the current collector was exposed, and an aluminum positive electrode lead was welded to the positive electrode exposed portion.

[0057] [Fabrication of the negative electrode] A negative electrode slurry was prepared by mixing 100 parts by mass of graphite, 1 part by mass of carboxymethylcellulose (CMC), and 1 part by mass of styrene-butadiene rubber (SBR), and adding an appropriate amount of water. This negative electrode slurry was applied to a strip-shaped negative electrode current collector made of copper foil and dried. The reverse side was also coated and dried in the same manner. After compressing the dried coating using a roller, it was cut to a predetermined electrode plate size to create a negative electrode with negative electrode slurry layers formed on both sides of the negative electrode current collector. A negative electrode exposed portion was provided at the inner end of the winding where the negative electrode slurry layer was absent and the surface of the negative electrode current collector was exposed, and a nickel negative electrode lead was welded to the negative electrode exposed portion.

[0058] [Preparation of non-aqueous electrolyte solution] A non-aqueous electrolyte was prepared by dissolving LiPF6 in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC).

[0059] [Fabrication of cylindrical rechargeable batteries] (1) A wound electrode body was fabricated by winding a 20 μm thick polyethylene separator between the positive and negative electrodes. (2) The electrode body was inserted into the outer can, the negative electrode lead was welded to the bottom of the outer can, and the positive electrode lead was welded to the sealing body. When the direction in which the electrode body was inserted into the outer can was considered the insertion direction, the first region of the positive electrode mixture layer was on the sealing body side with respect to the insertion direction, and the second region of the positive electrode mixture layer was on the bottom side of the outer can with respect to the insertion direction. (3) After injecting a non-aqueous electrolyte into the outer casing, the open end of the outer casing was crimped to a sealing body via a gasket. This formed a cylindrical secondary battery.

[0060] <Example 2> A cylindrical secondary battery was fabricated in the same manner as in Example 1, except that the amount of MWCNTs added to the positive electrode mixture slurry A was changed to 0.2 parts by mass. The porosity of the first region was 29.0% by volume, and the porosity of the second region was 26.4%.

[0061] <Comparative Example 1> A cylindrical secondary battery was fabricated in the same manner as in Example 1, except that in the preparation of the positive electrode, positive electrode slurry B was applied to the first region and positive electrode slurry A was applied to the second region to form a positive electrode slurry layer. The porosity of the first region was 26.4 volume%, and the porosity of the second region was 28.8%.

[0062] <Comparative Example 2> A cylindrical secondary battery was fabricated in the same manner as in the example, except that only positive electrode slurry B was used and the positive electrode slurry B was applied to the entire positive electrode current collector. The porosity of the positive electrode slurry layer was 26.4% by volume.

[0063] <Comparative Example 3> A cylindrical secondary battery was fabricated in the same manner as in the example, except that only positive electrode slurry A was used and the positive electrode slurry A was applied to the entire positive electrode current collector. The porosity of the positive electrode slurry layer was 28.8% by volume.

[0064] [Evaluation of charge-discharge cycle characteristics] The cylindrical secondary batteries of the examples and comparative examples were placed on the mounting platform with the bottom of the non-aqueous electrolyte secondary battery in contact with the platform, so that the height of the battery was aligned with the vertical direction. Then, each cylindrical secondary battery was charged with a constant current of 0.5 It at a temperature of 25°C until the voltage reached 4.2V, and then charged with a constant voltage of 4.2V until the current reached 0.05 It. Finally, a constant current discharge was performed with a current of 0.5 It until the voltage reached 2.5V. This charge-discharge cycle was considered as one cycle, and 1000 cycles were performed. The capacity retention rate was then calculated using the following formula. Capacity retention rate (%) = (Discharge capacity at 1000th cycle / Discharge capacity at 1st cycle) × 100

[0065] [Measurement of surface resistance of the positive electrode mixture layer] In preparing the positive electrode, a PET film was used instead of a positive electrode current collector, and a positive electrode mixture layer was formed on one side of the PET film. Except for these differences, the measurement samples were prepared in the same manner as in each example and comparative example. For each measurement sample in the examples and comparative examples, the surface resistance was measured using a two-terminal method (25°C) with an AP probe using a Loresta-GP manufactured by Mitsubishi Chemical Corporation. For Examples 1 and 2 and Comparative Example 1, measurements were performed for both the first and second regions.

[0066] Table 1 shows the capacity retention rates for cylindrical secondary batteries in the examples and comparative examples. Table 1 also shows the type and content of conductive agents, porosity, and surface resistance in the first and second regions.

[0067] [Table 1]

[0068] The cylindrical secondary batteries of Examples 1 and 2 exhibit higher capacity retention and superior charge-discharge cycle characteristics compared to the cylindrical secondary batteries of Comparative Examples 1 to 3. Furthermore, the ratio of the difference between the surface resistance in the first region and the surface resistance in the second region to the surface resistance in the second region was 1.7% for the cylindrical secondary battery of Example 1 and 19.5% for the cylindrical secondary battery of Example 2. The capacity retention of Example 1, which had a smaller difference ratio, was higher than that of Example 2. [Explanation of symbols]

[0069] 10 Non-aqueous electrolyte secondary battery, 10a Upper region, 10b Lower region, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Battery case, 16 Outer can, 17 Sealing body, 18,19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Protruding part, 23 Filter, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 38 Fixing part, 40 First region, 42 Second region.

Claims

1. A non-aqueous electrolyte secondary battery comprising an electrode body in which a first electrode and a second electrode having opposite polarities face each other via a separator, and a battery case housing the electrode body, The first electrode has a composite layer containing a conductive agent, When the non-aqueous electrolyte secondary battery is used in a fixed state, the mixture layer has a first region on the upper side with respect to the vertical direction and a second region on the lower side with respect to the vertical direction. The porosity of the mixture layer in the first region is higher than the porosity of the mixture layer in the second region. A non-aqueous electrolyte secondary battery wherein the content of the conductive agent in the first region is lower than the content of the conductive agent in the second region, and the conductive agent contained in the first region contains only fibrous carbon, and the conductive agent contained in the second region contains only granular carbon.

2. A non-aqueous electrolyte secondary battery comprising an electrode body in which a first electrode and a second electrode having opposite polarities face each other via a separator, a bottomed cylindrical outer container housing the electrode body, and a sealing body that closes the opening of the outer container, The first electrode has a composite layer containing a conductive agent, When the direction in which the electrode body is inserted into the outer can is considered the insertion direction, the mixture layer has a first region on the sealing body side with respect to the insertion direction, and a second region on the bottom side of the outer can with respect to the insertion direction. The porosity of the mixture layer in the first region is higher than the porosity of the mixture layer in the second region. A non-aqueous electrolyte secondary battery wherein the content of the conductive agent in the first region is lower than the content of the conductive agent in the second region, and the conductive agent contained in the first region contains only fibrous carbon, and the conductive agent contained in the second region contains only granular carbon.

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the difference between the surface resistance in the first region and the surface resistance in the second region is 20% or less of the surface resistance in the second region.

4. A non-aqueous electrolyte secondary battery comprising an electrode body in which a first electrode and a second electrode having opposite polarities face each other via a separator, a bottomed cylindrical outer container housing the electrode body, and a sealing body that closes the opening of the outer container, The first electrode has a composite layer containing a conductive agent, When the direction in which the electrode body is inserted into the outer can is considered the insertion direction, the mixture layer has a first region on the sealing body side with respect to the insertion direction, and a second region on the bottom side of the outer can with respect to the insertion direction. The porosity of the mixture layer in the second region is higher than the porosity of the mixture layer in the first region. A non-aqueous electrolyte secondary battery wherein the content of the conductive agent in the second region is lower than the content of the conductive agent in the first region, and the conductive agent contained in the second region contains only fibrous carbon, and the conductive agent contained in the first region contains only granular carbon.

5. The non-aqueous electrolyte secondary battery according to claim 4, wherein the difference between the surface resistance in the second region and the surface resistance in the first region is 20% or less of the surface resistance in the first region.

6. The non-aqueous electrolyte secondary battery according to any one of claims 1, 2, 4, and 5, wherein the fibrous carbon is a multi-walled carbon nanotube and the granular carbon is acetylene black.

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