Non-aqueous electrolyte secondary battery

Abstract

This nonaqueous electrolyte secondary battery is provided with: an electrode body in which a first electrode (12) and a second electrode are wound with a separator interposed therebetween; a nonaqueous electrolyte; and a cylindrical outer can, the nonaqueous electrolyte secondary battery disposed such that the axial direction of the outer can extends along the vertical direction. The nonaqueous electrolyte secondary battery is characterized in that: the first electrode (12) has a first electrode core body (40) and a first electrode mixture layer (41) disposed on the first electrode core body (40); on the surface of the first electrode mixture layer (41), a recess (50) depressed in the thickness direction of the first electrode mixture layer (41) is provided; and if the length from a lower end (41X) of the first electrode mixture layer (41) to an upper end (41Y) of the first electrode mixture layer (41) is set as L in an axial-direction cross-sectional view of the outer can (15), the recess (50) is provided only in a range from the lower end (41X) of the first electrode mixture layer (41) to 0.6 L.

Description

Nonaqueous electrolyte secondary battery

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

[0002] Non-aqueous electrolyte secondary batteries are widely used as high-energy-density secondary batteries. A non-aqueous electrolyte secondary battery comprises, for example, an electrode body in which a positive electrode and a negative electrode are wound with a separator in between, a non-aqueous electrolyte, and an outer casing that houses the electrode body and the non-aqueous electrolyte. The positive electrode and the negative electrode also have a core body and a composite layer disposed on the surface of the core body. Patent Document 1 discloses a technique for providing a plurality of circular holes in plan view on the surface of the composite layer in order to reduce internal resistance.

[0003] Japanese Patent Publication No. 2021-190288

[0004] In recent years, with the increasing adoption of non-aqueous electrolyte secondary batteries in automotive and energy storage applications, there has been a growing demand for non-aqueous electrolyte secondary batteries with high energy density and excellent charge-discharge cycle characteristics. In non-aqueous electrolyte secondary batteries equipped with wound electrode bodies, when rapid charge-discharge cycles are repeated, the non-aqueous electrolyte tends to be pushed out to both axial ends of the electrode body due to volume changes in the electrode plates, etc. As a result, the concentration of electrolyte salt tends to decrease at both axial ends of the electrode body, and in particular, the concentration of electrolyte salt tends to decrease in the region below where the battery is placed. Consequently, lithium deposition may occur at the negative electrode in the region where the electrolyte salt concentration has decreased, which can lead to a deterioration in charge-discharge cycle characteristics.

[0005] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure comprises an electrode body in which a strip-shaped first electrode and a strip-shaped second electrode having opposite polarities are wound around a separator, a non-aqueous electrolyte, and a cylindrical outer container for housing the electrode body and the non-aqueous electrolyte, wherein the axial direction of the outer container is aligned with the vertical direction, and the first electrode comprises a first electrode core and a first electrode mixture layer disposed on the first electrode core, wherein the surface of the first electrode mixture layer is provided with recesses that are indented in the thickness direction of the first electrode mixture layer, and in an axial cross-sectional view of the outer container, when the length from the lower end to the upper end of the first electrode mixture layer is L, the recesses are provided only in the range from the lower end to 0.6L of the first electrode mixture layer.

[0006] According to one aspect of this disclosure, a non-aqueous electrolyte secondary battery can be provided that ensures high energy density while exhibiting excellent charge-discharge cycle characteristics.

[0007] This is an axial cross-sectional view of the non-aqueous electrolyte secondary battery of the first embodiment. This is a perspective view of the electrode body of the non-aqueous electrolyte secondary battery of the first embodiment. This is a cross-sectional view of the negative electrode of the non-aqueous electrolyte secondary battery of the first embodiment. This is a diagram showing the negative electrode of the non-aqueous electrolyte secondary battery of the first embodiment in an unfolded state. This is an axial cross-sectional view of the non-aqueous electrolyte secondary battery of the second embodiment. This is a diagram showing the negative electrode of the non-aqueous electrolyte secondary battery of the second embodiment in an unfolded state.

[0008] Hereinafter, an example of an embodiment of a non-aqueous electrolyte secondary battery according to this disclosure will be described in detail with reference to the drawings. Note that configurations obtained by selectively combining the components of the multiple embodiments and modified examples described below are included within the scope of this disclosure.

[0009] In the following, a cylindrical battery in which a wound electrode body 14 is housed in a bottomed cylindrical outer casing 15 is given as an example of a non-aqueous electrolyte secondary battery, but the battery's outer casing is not limited to a cylindrical shape. The non-aqueous electrolyte secondary battery according to this disclosure may be, for example, a rectangular battery with a rectangular outer casing. The electrode body may also be a wound electrode body formed into a flat shape.

[0010] [First Embodiment] The configuration of the non-aqueous electrolyte secondary battery (cylindrical battery) 10 of the first embodiment will be described with reference to Figures 1 and 2. Figure 1 is a schematic diagram showing a cross-section of the non-aqueous electrolyte secondary battery 10, and Figure 2 is a perspective view of the electrode body 14 constituting the non-aqueous electrolyte secondary battery 10.

[0011] As shown in Figures 1 and 2, the non-aqueous electrolyte secondary battery 10 comprises an electrode body 14 in which a first electrode and a second electrode with opposite polarities are wound around a separator 13, a non-aqueous electrolyte (not shown), an outer container 15 that houses the electrode body 14 and the non-aqueous electrolyte, and a sealing body 16 that closes the opening of the outer container 15. The non-aqueous electrolyte secondary battery 10 is arranged so that the axial direction of the outer container 15 is aligned with the vertical direction. That is, the sealing body 16 side is on the upper side and the bottom side of the outer container 15 is on the lower side. In this embodiment, the case in which the first electrode is a negative electrode 12 and the second electrode is a positive electrode 11 will be described.

[0012] The electrode body 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and the positive electrode 11 and the negative electrode 12 are wound in a spiral shape via the separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 that make up the electrode body 14 are all elongated strips, and are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape. The positive electrode 11 protrudes above the negative electrode 12 and the separator 13, and the negative electrode 12 protrudes below the positive electrode 11 and the separator 13. Two separators 13 are arranged, for example, so as to sandwich the positive electrode 11. In addition, a hollow portion 14A is formed at the winding center of the electrode body 14. The hollow portion 14A is a space that extends along the vertical direction.

[0013] The positive electrode 11 comprises a positive electrode core 30 and a positive electrode mixture layer 31 formed on the positive electrode core 30. The positive electrode core 30 can be made of a metal foil that is stable within the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film with the metal arranged on its surface. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent, and a binder, and is formed on both sides of the positive electrode core 30, excluding the non-mixture layer portion 32, which will be described later. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode core 30, drying the coating, and then compressing it to form the positive electrode mixture layer 31 on both sides of the positive electrode core 30.

[0014] The positive electrode composite layer 31 contains particulate lithium metal composite oxide as the positive electrode active material. The lithium metal composite oxide is a composite oxide containing metal elements such as Co, Mn, Ni, and Al in addition to Li. The metal elements constituting the lithium metal composite oxide are, for example, at least one selected from Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, and Bi. Among these, it is preferable to contain at least one selected from Co, Ni, and Mn. Examples of suitable composite oxides include lithium metal composite oxides containing Ni, Co, and Mn, or lithium metal composite oxides containing Ni, Co, and Al.

[0015] Examples of conductive agents included in the positive electrode mixture layer 31 include carbon black such as acetylene black and Ketjenblack, graphite, carbon nanotubes (CNTs), carbon nanofibers, and graphene. Examples of binders included in the positive electrode mixture layer 31 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resins, and polyolefins. In addition, these resins may be used in combination with carboxymethylcellulose (CMC) or its salts, polyethylene oxide (PEO), etc.

[0016] As shown in Figures 1 and 2, the positive electrode 11 has a non-compound layer portion 32 at the upper axial end of the electrode body 14, where the positive electrode compound layer 31 is not provided on the positive electrode core body 30. The non-compound layer portion 32 is provided over the range from the beginning end to the end end of the winding in the longitudinal direction of the elongated positive electrode 11. The non-compound layer portion 32 is bent radially inward and welded to the lower surface of the positive electrode current collector plate 17. By joining the non-compound layer portion 32 to the positive electrode current collector plate 17, the contact area between the non-compound layer portion 32 and the positive electrode current collector plate 17 is increased, so the internal resistance of the positive electrode 11 can be reduced compared to when the positive electrode 11 and the positive electrode current collector plate 17 are connected by a positive electrode tab or the like.

[0017] As will be described in more detail later, the negative electrode 12 has a negative electrode core 40 and a negative electrode mixture layer 41 formed on the negative electrode core 40. The negative electrode 12 can be manufactured by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the surface of the negative electrode core 40, drying the coating film, and then compressing it to form the negative electrode mixture layer 41 on both sides of the negative electrode core 40.

[0018] As shown in Figures 1 and 2, the negative electrode 12 has a non-compound layer portion 42 at the lower axial end of the electrode body 14 where the negative electrode compound layer 41 is not provided on the negative electrode core body 40. The non-compound layer portion 42 is provided over the range from the beginning end to the end end of the winding in the longitudinal direction of the elongated negative electrode 12, similar to the case of the positive electrode 11. The non-compound layer portion 42 is bent radially inward at its lower end and welded to the negative electrode current collector plate 18. In addition, the non-aqueous electrolyte secondary battery 10 may not have a negative electrode current collector plate 18, and the non-compound layer portion 42 may be directly joined to the inner surface of the bottom plate of the outer casing 15.

[0019] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. A heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator 13. 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.

[0020] A non-aqueous electrolyte (electrolyte solution) has lithium ion conductivity. The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. For the non-aqueous solvent, for example, esters, ethers, nitriles, amides, mixed solvents of two or more of these, and the like are used. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), mixed solvents of two or more of these, and the like. The non-aqueous solvent may contain a halogen-substituted product obtained by substituting at least a part of hydrogen atoms of these solvents with a halogen atom such as fluorine (e.g., fluoroethylene carbonate, etc.).

[0021] For the electrolyte salt, for example, a lithium salt is used. As the lithium salt, LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lower aliphatic carboxylic acid lithium, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of phosphates include lithium difluorophosphate (LiPO ２ F ２ ), lithium difluorobis(oxalato) phosphate (LiDFBOP), lithium tetrafluoro(oxalato) phosphate, and the like. Examples of borates include lithium bis(oxalato) borate (LiBOB), lithium difluoro(oxalato) borate (LiDFOB), and the like. As the imide salt, lithium bis(fluorosulfonyl)imide (LiN(FSO ２ ) ２ ), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonyl nonafluorobutanesulfonyl imide (LiN(CF ３ SO２ ) (C ４ F ９ SO ２ )), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ ) etc. are used. Of these, LiPF is used from the viewpoint of ionic conductivity, electrochemical stability, etc. ６ It is preferable to use the following. The concentration of the lithium salt may be, for example, 4 moles or less per liter of non-aqueous solvent, or 3 moles or less, preferably 2 moles or less, and more preferably 0.8 moles or more and 1.8 moles or less.

[0022] Furthermore, the non-aqueous electrolyte may contain additives. Examples of additives include unsaturated carbonate esters, acid anhydrides, phenol compounds, benzene compounds, nitrile compounds, isocyanate compounds, sultone compounds, sulfuric acid compounds, boric acid ester compounds, phosphate ester compounds, and phosphite ester compounds.

[0023] The outer container 15 is a bottomed cylindrical metal container with one end open in the axial direction, and the opening of the outer container 15 is sealed by a sealing body 16.

[0024] A gasket 27 is provided between the outer casing 15 and the sealing body 16 to ensure airtightness inside the battery. The outer casing 15 has a grooved portion 21 formed on its side surface, which protrudes inward to support the sealing body 16. The grooved portion 21 is preferably formed in an annular shape along the circumferential direction of the outer casing 15, and its upper surface supports the sealing body 16. The sealing body 16 is fixed to the upper part of the outer casing 15 by the grooved portion 21 and the open end of the outer casing 15 which is crimped to the sealing body 16.

[0025] The sealing body 16 has a structure in which a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26 are stacked in this order from the electrode body 14 side. Each member constituting the sealing body 16 has, for example, a disk shape or a ring shape, and each member except the insulating member 24 is electrically connected to each other. The filter 22 has at least one through hole. Further, the lower valve body 23 and the upper valve body 25 are connected at their respective central portions, and the insulating member 24 is interposed between their respective peripheral edge portions.

[0026] When the non-aqueous electrolyte secondary battery 10 abnormally generates heat and the internal pressure of the non-aqueous electrolyte secondary battery 10 rises, the lower valve body 23 deforms so as to push the upper valve body 25 toward the cap 26 side and breaks, whereby the current path between the lower valve body 23 and the upper valve body 25 is cut off. When the internal pressure further rises, the upper valve body 25 breaks, and gas is discharged from the through hole 26a of the cap 26. This gas discharge can prevent the non-aqueous electrolyte secondary battery 10 from bursting due to excessive rise of the internal pressure of the non-aqueous electrolyte secondary battery 10, thereby improving the safety of the non-aqueous electrolyte secondary battery 10. The configuration of the sealing body 16 is not limited to this as long as it can close the opening of the outer can 15.

[0027] The non-aqueous electrolyte secondary battery 10 has a metal positive electrode current collector plate 17 on the upper side of the electrode body 14. The positive electrode current collector plate 17 is made of, for example, aluminum, an aluminum alloy, or the like. A mixture layer non-forming portion 32 of the positive electrode 11 is connected to the lower surface of the positive electrode current collector plate 17 by welding or the like. The shape of the positive electrode current collector plate 17 is not particularly limited, and for example, it may have a disk shape, an annular shape, or a substantially cross shape. Further, a positive electrode lead 20 is connected to the upper surface of the positive electrode current collector plate 17. The positive electrode lead 20 extends to the sealing body 16 side through the through hole of the insulating plate 19, and the upper end portion of the positive electrode lead 20 is connected to the lower surface of the filter 22 of the sealing body 16 by welding or the like. Therefore, the cap 26 constituting the top plate of the sealing body 16 is electrically connected to the filter 22, and the cap 26 serves as a positive electrode terminal.

[0028] Further, the non-aqueous electrolyte secondary battery 10 includes a metal negative electrode current collector plate 18 below the electrode assembly 14. The mixture layer non-formation portion 42 of the negative electrode 12 is connected to the upper surface of the negative electrode current collector plate 18. Also, the negative electrode current collector plate 18 is joined to the inner surface of the bottom plate of the outer can 15. Therefore, the outer can 15 electrically connected to the negative electrode 12 via the negative electrode current collector plate 18 serves as the negative electrode terminal.

[0029] Next, the negative electrode 12 will be described in detail with further reference to FIG. 3 and FIG. 4. FIG. 3 is a cross-sectional view of the negative electrode 12, and FIG. 4 is a developed view of the negative electrode 12.

[0030] As shown in FIG. 3, the negative electrode 12 includes a negative electrode core 40 and a negative electrode mixture layer 41 disposed on the negative electrode core 40. As described above, in the present embodiment, the non-aqueous electrolyte secondary battery 10 is arranged such that the axial direction of the outer can 15 is along the vertical direction. Therefore, the negative electrode 12 is arranged with its width direction along the vertical direction.

[0031] In the present embodiment, the negative electrode mixture layer 41 includes a first negative electrode mixture layer 43 (first layer) disposed on the surface of the negative electrode core 40, and a second negative electrode mixture layer 44 (second layer) disposed on the surface of the negative electrode 12 and having a larger porosity than the first negative electrode mixture layer 43. That is, the negative electrode mixture layer 41 has a two-layer structure in which the first negative electrode mixture layer 43 and the second negative electrode mixture layer 44 are stacked in this order from the negative electrode core 40 side. Hereinafter, the first negative electrode mixture layer 43 and the second negative electrode mixture layer 44 may be collectively referred to as the negative electrode mixture layer 41. Note that the negative electrode mixture layer 41 may alternatively have a single-layer structure.

[0032] For the negative electrode core 40, a metal foil such as copper that is stable in the potential range of the negative electrode 12, or a film having such a metal disposed on the surface thereof, or the like can be used. The thickness of the negative electrode core 40 is not particularly limited, but from the viewpoint of balancing the strength and weight reduction of the negative electrode 12, it is, for example, 5 µm or more and 30 µm or less. The negative electrode mixture layer 41 is preferably formed on both surfaces of the negative electrode core 40 excluding the mixture layer non-formation portion 42 (see FIG. 4) provided at the lower end of the negative electrode 12.

[0033] The negative electrode mixture layer 41 contains a negative electrode active material, a binder, and optionally a conductive agent. Preferably, the negative electrode mixture layer 41 contains a carbon material and a silicon-containing material as the negative electrode active material. Including a silicon-containing material as the negative electrode active material makes it easier to achieve high capacity. In addition, the negative electrode active material may also contain, for example, an element that alloys with Li such as Sn, and at least one of a material containing said element.

[0034] From the viewpoint of increasing capacity, the silicon-containing material content is preferably 5% by mass or more, and more preferably 10% by mass or more, of the total mass of the negative electrode active material. Generally, silicon-containing materials undergo a larger volume change during charging and discharging compared to carbon materials. Therefore, when a silicon-containing material is included as the negative electrode active material, the negative electrode mixture layer 41 tends to expand when charging and discharging is repeated. This makes it easier for the non-aqueous electrolyte to be pushed out from both the upper and lower sides of the electrode body 14. Thus, when a silicon-containing material is included as the negative electrode active material, the effects of the disclosure described later are exhibited more significantly.

[0035] The carbon material that functions as the negative electrode active material is, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. In particular, it is preferable to use artificial graphite such as massive artificial graphite (MAG) or graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, or earthy graphite, or a mixture thereof as the carbon material. The volume-based D50 of the carbon material is, for example, 1 μm or more and 30 μm or less, preferably 5 μm or more and 25 μm or less.

[0036] Silicon-containing materials can be any material containing Si, and examples include silicon alloys, silicon compounds, and Si-containing composite materials. Among these, Si-containing composite materials are preferred. The D50 of composite materials is generally smaller than that of graphite. The volume-based D50 of composite materials is, for example, 1 μm or more and 15 μm or less. One type of silicon-containing material may be used alone, or two or more types may be used in combination.

[0037] A suitable silicon-containing material (composite material) is a composite particle comprising an ionic conductive phase and a Si phase dispersed within the ionic conductive phase. The ionic conductive phase is, for example, at least one selected from the group consisting of a silicate phase, an amorphous carbon phase, a silicide phase, and a silicon oxide phase. The Si phase is formed by dispersing Si in the form of fine particles. The ionic conductive phase is a continuous phase composed of an aggregate of particles finer than those of the Si phase. Furthermore, a conductive layer composed of a material with higher conductivity than the ionic conductive phase may be formed on the surface of the ionic conductive phase.

[0038] An example of a suitable composite material containing Si is one having a sea-island structure in which fine Si particles are dispersed substantially uniformly in an amorphous silicon oxide phase, and the overall general formula is SiO ｘ These are composite particles represented by (0 < x ≤ 2). The main component of silicon oxide may be silicon dioxide. The oxygen content ratio (x) to Si is, for example, 0.5 ≤ x < 2.0, and preferably 0.8 ≤ x ≤ 1.5.

[0039] The binder contained in the negative electrode mixture layer 41 may include fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc., but styrene-butadiene rubber (SBR) is preferred. Furthermore, the negative electrode mixture layer 41 preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. In particular, it is preferable to use SBR in combination with CMC or a salt thereof, PAA or a salt thereof. The negative electrode mixture layer 41 may also contain a conductive agent such as CNT.

[0040] As described above, the negative electrode mixture layer 41 of this embodiment includes a first negative electrode mixture layer 43 and a second negative electrode mixture layer 44 having a larger porosity than the first negative electrode mixture layer 43. By making the porosity of the second negative electrode mixture layer 44, which is arranged on the surface of the negative electrode 12, larger than that of the first negative electrode mixture layer 43, which is arranged on the negative electrode core 40 side, the non-aqueous electrolyte can diffuse more easily to the negative electrode core 40 side, and the variation in the concentration of the non-aqueous electrolyte within the negative electrode mixture layer 41 is reduced. As a result, for example, the internal resistance is reduced, and a non-aqueous electrolyte secondary battery 10 with high energy density can be realized. Furthermore, by making the porosity of the second negative electrode mixture layer 44 larger than that of the first negative electrode mixture layer 43, the diffusivity of the non-aqueous electrolyte on the surface of the negative electrode 12 is improved, and the effect of improving the diffusivity of the non-aqueous electrolyte by providing the recess 50, which will be described later, is more pronounced.

[0041] Furthermore, if the negative electrode mixture layer 41 is composed only of the second negative electrode mixture layer 44, which has a large porosity, the amount of negative electrode active material contained in the negative electrode mixture layer 41 decreases, and the energy density tends to decrease. Therefore, by including both the first negative electrode mixture layer 43 and the second negative electrode mixture layer 44 in the negative electrode mixture layer 41, it is possible to maintain energy density while reducing the concentration variation of the non-aqueous electrolyte within the negative electrode mixture layer 41.

[0042] When the porosity of the first negative electrode mixture layer 43 is S1 and the porosity of the second negative electrode mixture layer 44 is S2, the ratio of S2 to S1 (S2 / S1) may be greater than 1 and 5 or less, preferably 1.2 or more and 4 or less, and more preferably 1.4 or more and 3 or less. When the ratio (S2 / S1) is 1.2 or more and 4 or less, the amount of negative electrode active material contained in the negative electrode mixture layer 41 is ensured, while the non-aqueous electrolyte diffuses more easily to the negative electrode core 40 side, and the concentration variation of the non-aqueous electrolyte within the negative electrode mixture layer 41 becomes smaller.

[0043] The porosity S1 of the first negative electrode mixture layer 43 is, for example, 5% or more and 30% or less, and may be 10% or more and 30% or less. The porosity S2 of the second negative electrode mixture layer 44 is preferably 20% or more and 50% or less, and more preferably 25% or more and 45% or less. The porosity of the negative electrode mixture layer 41 is a two-dimensional value obtained from the ratio of the area of ​​voids to the cross-sectional area of ​​the negative electrode mixture layer 41. More specifically, it is calculated by the following procedure.

[0044] (1) A portion of the negative electrode 12 is cut off and processed using an ion milling device (e.g., Hitachi High-Tech Corporation, IM4000) to expose the cross-section of the negative electrode mixture layer 41. (2) A backscattered electron image of the cross-section of the first negative electrode mixture layer 43 of the exposed negative electrode mixture layer 41 is taken using a scanning electron microscope. (3) The cross-sectional image obtained above is imported into a computer and binarized using image analysis software (e.g., ImageJ, National Institutes of Health, USA) to obtain a binarized image in which the particle cross-sections in the cross-sectional image are made black and the interparticle voids are made white. (4) To determine the porosity of the first negative electrode mixture layer 43, the area of ​​the interparticle voids in the measurement range (e.g., 50 μm × 50 μm) is calculated from the binarized image. Then, the measurement range is set to the cross-sectional area of ​​the first negative electrode mixture layer 43 (2500 μm). ２ The area of ​​the interparticle voids is set to 50 μm x 50 μm, and the porosity of the first negative electrode mixture layer 43 (area of ​​interparticle voids x 100 / cross-sectional area of ​​the first negative electrode mixture layer 43) is calculated from the calculated area of ​​the interparticle voids. The porosity of the second negative electrode mixture layer 44 is measured in the same manner.

[0045] One method for adjusting the porosity of the first anode mixture layer 43 and the second anode mixture layer 44 is to adjust the compressive strength of the anode active material contained in each layer. For example, the compressive strength of the graphite particles (carbon material) contained in the second anode mixture layer 44 is made greater than that of the graphite particles contained in the first anode mixture layer 43. This makes it less likely for the graphite particles in the second anode mixture layer 44 to be crushed when rolled during the production of the anode 12, and thus increases the porosity of the second anode mixture layer 44. Alternatively, the porosity of the first anode mixture layer 43 and the second anode mixture layer 44 may be adjusted by changing the mixing ratio of the carbon material and the silicon-containing material in each layer.

[0046] Another method for adjusting the porosity of the first negative electrode mixture layer 43 and the second negative electrode mixture layer 44 is to adjust the rolling force applied to the first coating film constituting the first negative electrode mixture layer 43 and the second coating film constituting the second negative electrode mixture layer 44 during the formation of the negative electrode mixture layer 41. By reducing the rolling force, the particles of the negative electrode active material become less likely to be crushed, and the porosity can be increased. In addition, the porosity can be adjusted by changing the compression characteristics by changing the particle size distribution of the negative electrode active material.

[0047] The thickness of the first negative electrode mixture layer 43 may be 20% or more and 80% or less of the thickness of the negative electrode mixture layer 41, and is preferably 30% or more and 70% or less. For example, the thickness of the first negative electrode mixture layer 43 is 20 μm or more and 200 μm or less. Note that the thickness of the negative electrode mixture layer 41 (and the same applies to the first negative electrode mixture layer 43 and the second negative electrode mixture layer 44) refers to the thickness on one side of the negative electrode core body 40.

[0048] The thickness of the second negative electrode mixture layer 44 may be 10% or more and 70% or less of the thickness of the negative electrode mixture layer 41, and preferably 20% or more and 60% or less. When the thickness of the second negative electrode mixture layer 44 is 20% or more and 60% or less of the thickness of the negative electrode mixture layer 41, it becomes easier to improve the diffusivity of the non-aqueous electrolyte while ensuring energy density. The thickness of the second negative electrode mixture layer 44 may be approximately the same as the thickness of the first negative electrode mixture layer 43, or it may be less than the thickness of the first negative electrode mixture layer 43. For example, the thickness of the second negative electrode mixture layer 44 is 10 μm or more and 150 μm or less.

[0049] As shown in Figures 3 and 4, the surface of the negative electrode mixture layer 41 (second negative electrode mixture layer 44) is provided with a plurality of recesses 50 that are indented in the thickness direction of the negative electrode mixture layer 41. The recesses 50 are formed, for example, by laser processing.

[0050] In a non-aqueous electrolyte secondary battery 10 equipped with a wound electrode body 14, when rapid charge-discharge cycles are repeated, the volume change of the positive electrode 11 and the negative electrode 12 tends to push the non-aqueous electrolyte to both ends in the vertical direction of the electrode body 14. As a result, the concentration of electrolyte salt at both ends in the vertical direction of the electrode body 14 decreases compared to the concentration of electrolyte salt at the center of the electrode body 14 in the vertical direction. In particular, when the axial direction of the outer casing 15 is arranged along the vertical direction, as in this embodiment, the decrease in electrolyte salt concentration is more pronounced in the lower region of the battery.

[0051] By forming recesses 50 on the surface of the negative electrode mixture layer 41, the non-aqueous electrolyte can more easily diffuse to the negative electrode core 40 through the recesses 50. As a result, the concentration variation of the non-aqueous electrolyte within the negative electrode mixture layer 41 is reduced, enabling the realization of a non-aqueous electrolyte secondary battery 10 with high energy density and excellent charge-discharge cycle characteristics. Furthermore, by forming recesses 50 on the surface of the negative electrode mixture layer 41, the non-aqueous electrolyte that has been pushed out to both ends of the electrode body 14 in the vertical direction can more easily diffuse to the center of the electrode body 14 in the vertical direction through the recesses 50. In other words, the gaps formed by the recesses 50 function as diffusion pathways for the non-aqueous electrolyte. This suppresses the decrease in electrolyte salt concentration at both ends of the electrode body 14 in the vertical direction. As a result, lithium deposition at the negative electrode 12 is suppressed, and the charge-discharge cycle characteristics are improved.

[0052] On the other hand, if the recess 50 is formed over the entire negative electrode mixture layer 41, the volume of the negative electrode mixture layer 41 may decrease, potentially lowering the energy density. Therefore, in this embodiment, the recess 50 is formed only in the lower region where the decrease in electrolyte salt concentration is more pronounced. Specifically, when the length from the lower end 41X to the upper end 41Y of the negative electrode mixture layer 41 is L, the recess 50 is provided only in the range from the lower end 41X to 0.6L of the negative electrode mixture layer 41. This ensures the volume of the negative electrode mixture layer 41 while suppressing lithium deposition in the lower region of the negative electrode 12. As a result, a non-aqueous electrolyte secondary battery 10 with high energy density and excellent charge-discharge cycle characteristics can be realized.

[0053] The recess 50 may be provided only in the range from the lower end 41X of the negative electrode mixture layer 41 to 0.5L, or only in the range up to 0.3L. By reducing the area in which the recess 50 is formed, the volume of the negative electrode mixture layer 41 increases, making it easier to realize a high-energy-density non-aqueous electrolyte secondary battery 10.

[0054] As described above, a hollow portion 14A (see Figure 1) is formed at the winding center of the electrode body 14. During charging, excess non-aqueous electrolyte impregnated into the electrode body 14 is stored in the hollow portion 14A. Here, during the initial charge, it is preferable that the liquid level of the excess liquid inside the hollow portion 14A is located below the region where the recess 50 is provided. Because the electrolyte salt concentration is low in the excess liquid, lithium deposition is more pronounced in the region below the liquid level of the excess liquid. Therefore, by positioning the liquid level of the excess liquid below the region where the recess 50 is provided, lithium deposition can be further suppressed, and the charge-discharge cycle characteristics are further improved. The conditions for the initial charge are those described in the embodiment described later.

[0055] As shown in Figure 4, the recesses 50 are arranged to extend along the longitudinal direction of the negative electrode 12 over substantially the entire negative electrode mixture layer 41. In this embodiment, a plurality of recesses 50, which are substantially circular in plan view, are arranged along the longitudinal direction with small gaps between them. That is, the gaps between adjacent recesses 50 in the longitudinal direction are smaller than the gaps between adjacent recesses 50 in the width direction. Note that adjacent recesses 50 in the longitudinal direction may be connected. In other words, the recesses 50 are elongated holes extending along the longitudinal direction, and the spaces formed by the recesses 50 may be continuously formed along the longitudinal direction of the negative electrode 12.

[0056] The spacing between adjacent recesses 50 in the longitudinal direction is, for example, 50 μm or less, and may be between 0.05 μm and 45 μm. The spacing between adjacent recesses 50 in the width direction is, for example, between 50 μm and 500 μm, and may be between 100 μm and 1000 μm. When the spacing between adjacent recesses 50 in the width direction is between 50 μm and 300 μm, an appropriate amount of recesses 50 can be formed on the surface of the negative electrode mixture layer 41 while ensuring the area of ​​the negative electrode mixture layer 41. As a result, a non-aqueous electrolyte secondary battery 10 with high energy density and excellent charge-discharge cycle characteristics can be realized.

[0057] The diameter of the circumscribed circle of each recess 50 is, for example, 5 μm or more and 200 μm or less, and may be 10 μm or more and 150 μm or less. The shape of the recess 50 is not limited to a roughly circular shape in plan view, and may, for example, have a roughly rectangular shape in plan view.

[0058] In this embodiment, in a cross-sectional view of the negative electrode 12, the recess 50 has a substantially constant width along the thickness direction of the negative electrode 12. That is, the side walls of the recess 50 extend along the thickness direction of the negative electrode 12. The shape of the recess 50 is not limited to this, and for example, the width of the recess 50 may decrease towards the negative electrode core 40 side.

[0059] The depth of the recess 50 can be appropriately set according to the thickness of the first negative electrode mixture layer 43 and the second negative electrode mixture layer 44, but is preferably 10 μm or more and 50 μm or less, and preferably 15 μm or more and 50 μm or less. Note that unintended irregularities may be formed on the surface of the negative electrode mixture layer 41. In this specification, the recess 50 refers to a depression with a depth of 10 μm or more, and does not include depressions with a depth of less than 10 μm. In other words, depressions unintentionally formed on the surface of the negative electrode mixture layer 41 are not included in the recess 50.

[0060] As shown in Figure 3, the region of the negative electrode mixture layer 41 directly below the recess 50 is defined as the direct region 45, and the region from the periphery of the direct region 45 to 50 μm in the in-plane direction of the negative electrode mixture layer 41 (excluding the region overlapping with the direct region 45) is defined as the peripheral region 46. In this case, it is preferable that the porosity of the negative electrode mixture layer 41 in the direct region 45 is approximately the same as the porosity of the negative electrode mixture layer 41 in the peripheral region 46. In this case, the non-aqueous electrolyte can easily diffuse to the negative electrode core 40 side, and the concentration variation of the non-aqueous electrolyte within the negative electrode mixture layer 41 is reduced. Note that approximately the same porosity means that the difference in porosity between each region is 3% or less.

[0061] By forming the recess 50 by laser processing, the porosity of the region directly beneath 45 and the porosity of the peripheral region 46 can be made approximately the same. In other words, when the negative electrode mixture layer 41 is locally compressed from the surface side by embossing such as roll pressing to form the recess 50, the porosity in the region directly beneath 45 becomes smaller than the porosity in the peripheral region 46. As a result, the non-aqueous electrolyte becomes less likely to diffuse to the negative electrode core 40 side, and lithium deposition is more likely to occur near the region directly beneath 45, for example.

[0062] Next, an example of a method for manufacturing the negative electrode 12 having the negative electrode mixture layer 41 of this embodiment will be described. However, the method for manufacturing the negative electrode 12 is not limited to the method described below.

[0063] A method for manufacturing the negative electrode 12 includes, for example, a preparation step of preparing a negative electrode mixture slurry, a coating step of applying the negative electrode mixture slurry to the surface of the negative electrode core 40, and a recess formation step of forming a recess 50 on the surface of the negative electrode mixture layer 41.

[0064] In the preparation process, the negative electrode active material, a binder, and a dispersion medium such as water are mixed to prepare a negative electrode mixture slurry. At this time, a first negative electrode mixture slurry constituting the first negative electrode mixture layer 43 and a second negative electrode mixture slurry constituting the second negative electrode mixture layer 44 may also be prepared.

[0065] In the coating process, the first negative electrode slurry and the second negative electrode slurry are applied to both sides of the negative electrode core 40 in that order. The method of applying the first negative electrode slurry and the second negative electrode slurry to the surface of the negative electrode core 40 is not particularly limited and can be done, for example, by die coating. The negative electrode 12 can be manufactured by drying and rolling the coating film produced in the coating process. Note that drying and rolling may be performed together after applying the first negative electrode slurry and the second negative electrode slurry, or they may be performed after applying the first negative electrode slurry and after applying the second negative electrode slurry, respectively.

[0066] In the recess formation step, recesses 50 are formed on the surface of the negative electrode mixture layer 41. The recesses 50 may be formed by electrolytic treatment, chemical etching, etc., but it is preferable that they be formed by laser processing. Known methods can be used for laser processing, and the size and depth of the recesses 50 can be adjusted by adjusting the wavelength, output, frequency, etc., of the laser light.

[0067] [Second Embodiment] Next, the non-aqueous electrolyte secondary battery 10 of the second embodiment will be described with reference to Figures 5 and 6. Figure 5 is an axial cross-sectional view of the non-aqueous electrolyte secondary battery 10 of the second embodiment, and Figure 6 is a diagram showing the negative electrode 12 of the second embodiment in an unfolded state.

[0068] The non-aqueous electrolyte secondary battery 10 of this embodiment is identical to the non-aqueous electrolyte secondary battery 10 of the first embodiment, except for the recess 50 of the negative electrode 12. As shown in Figure 5, the non-aqueous electrolyte secondary battery 10 of this embodiment is arranged so that the axial direction of the outer casing 15 is aligned with the horizontal direction. That is, the width direction of the negative electrode 12 is aligned with the horizontal direction. As a result, the concentration of electrolyte salt tends to decrease in the vertically lower region inside the battery.

[0069] As shown in Figures 5 and 6, in an axial cross-sectional view of the outer casing 15, when the length from the lower end 41X to the upper end 41Y of the negative electrode mixture layer 41 is L, the recess 50 is provided only in the range from the lower end 41X to 0.6L of the negative electrode mixture layer 41. This ensures the volume of the negative electrode mixture layer 41 while suppressing lithium deposition in the lower region of the negative electrode 12. As a result, a non-aqueous electrolyte secondary battery 10 with high energy density and excellent charge-discharge cycle characteristics can be realized.

[0070] The recess 50 may be provided only in the range from the lower end 41X of the negative electrode mixture layer 41 to 0.5L, or only in the range up to 0.3L. By reducing the area in which the recess 50 is formed, the volume of the negative electrode mixture layer 41 increases, making it easier to realize a high-energy-density non-aqueous electrolyte secondary battery 10.

[0071] During charging, excess non-aqueous electrolyte is stored outside the electrode body 14. Here, during the initial charge, it is preferable that the liquid level of the excess liquid is below the area where the recess 50 is provided. Because the electrolyte salt concentration is low in the excess liquid, lithium deposition is more pronounced in the area below the liquid level of the excess liquid. Therefore, by positioning the liquid level of the excess liquid below the area where the recess 50 is provided, lithium deposition can be further suppressed, and the charge-discharge cycle characteristics are further improved. The conditions for the initial charge are those described in the embodiment below.

[0072] The above embodiments can be modified as appropriate within the scope of the purpose of this disclosure. For example, in the above embodiments, the negative electrode 12 has a non-compound layer forming portion 42, and the non-compound layer forming portion 42 is electrically connected to the outer casing 15 by being joined to the negative electrode current collector plate 18, but the current collection method of the negative electrode 12 is not limited to this. For example, the negative electrode 12 may be electrically connected to the outer casing 15 via a negative electrode tab or the like connected to the negative electrode core 40. In this case, there may be one negative electrode tab or multiple negative electrode tabs. In addition, when a non-compound layer forming portion 42 is provided as in the above embodiments, the non-compound layer forming portion 42 obstructs the flow path of the non-aqueous electrolyte, and the non-aqueous electrolyte pushed out to both ends in the vertical direction of the electrode body 14 tends not to return to the center in the vertical direction of the electrode body 14. Therefore, the effects of this disclosure are more pronounced when a non-compound layer forming portion 42 is provided.

[0073] Furthermore, in the above embodiment, the negative electrode mixture layer 41 has a two-layer structure in which a first negative electrode mixture layer 43 and a second negative electrode mixture layer 44 are stacked, but it may also have a single-layer structure. In this case, the depth of the recess is preferably 5% or more and 50% or less of the thickness of the negative electrode mixture layer 41, and more preferably 10% or more and 40% or less.

[0074] Furthermore, in the above embodiment, the negative electrode mixture layer 41 has a two-layer structure in which a first negative electrode mixture layer 43 and a second negative electrode mixture layer 44 are stacked, but the negative electrode mixture layer 41 may have other layers between the first negative electrode mixture layer 43 and the second negative electrode mixture layer 44. That is, the negative electrode mixture layer 41 may have a multilayer structure in which three or more layers are stacked. In this case, it is preferable that the recess 50 does not reach the first negative electrode mixture layer 43, but it may reach the layer between the first negative electrode mixture layer 43 and the second negative electrode mixture layer 44. That is, it is preferable that the depth of the recess 50 is less than or equal to the thickness of the portion of the negative electrode mixture layer 41 other than the first negative electrode mixture layer 43.

[0075] Furthermore, in the above embodiment, the recesses 50 are arranged to extend along the longitudinal direction of the negative electrode 12, but they may also be arranged to extend along the width direction of the negative electrode 12. That is, the spacing between adjacent recesses 50 in the width direction may be smaller than the spacing between adjacent recesses 50 in the longitudinal direction. By arranging the recesses 50 in this way, the non-aqueous electrolyte that has been pushed out to both ends in the vertical direction of the electrode body 14 can easily diffuse towards the vertical center of the electrode body 14 through the recesses 50. Note that adjacent recesses 50 in the width direction may be connected. That is, the recesses 50 are elongated holes extending along the width direction, and the space formed by the recesses 50 may be continuously formed along the width direction of the negative electrode 12.

[0076] Furthermore, in the above embodiment, the first electrode is the negative electrode 12 and the second electrode is the positive electrode 11, but the first electrode may be the positive electrode 11 and the second electrode may be the negative electrode 12. That is, a recess 50 may be formed on the surface of the positive electrode mixture layer 31. Note that during charging and discharging, the volume change of the negative electrode 12 is larger than that of the positive electrode 11, so the effects of this disclosure are more pronounced when a recess 50 is formed on the surface of the negative electrode mixture layer 41.

[0077] The present disclosure will be further illustrated below with reference to examples, but the present disclosure is not limited to these examples.

[0078] <Example 1> [Fabrication of the positive electrode] As the positive electrode active material, particulate LiNi ０．９０ Co ０．０５ Mn ０．０５ O ２ A lithium transition metal oxide represented by [formula] was used. The above positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride powder as a binder were mixed in a mass ratio of 95:2.5:2.5, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was then applied to both sides of a positive electrode core made of long aluminum foil, and after the coating film was dried, the coating film was rolled with a rolling roller to produce a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode core. At this time, a 6 mm wide non-composite portion was provided at one end in the width direction of the positive electrode, where the positive electrode mixture layer was not formed.

[0079] [Preparation of the negative electrode] As the first negative electrode active material, a mixture of first graphite particles and Si oxide (SiO) in a mass ratio of 90:10 was used. As the second negative electrode active material, a mixture of second graphite particles, which have a higher compressive strength than the first graphite particles, and Si oxide (SiO) in a mass ratio of 90:10 was used. The first negative electrode active material, styrene-butadiene rubber dispersion, and carboxymethylcellulose sodium were mixed in a solid content mass ratio of 98:1:1, and the first negative electrode mixture slurry was prepared using water as the dispersion medium. Similarly, the second negative electrode active material, styrene-butadiene rubber dispersion, and carboxymethylcellulose sodium were mixed in a solid content mass ratio of 98:1:1, and the second negative electrode mixture slurry was prepared using water as the dispersion medium.

[0080] Then, a first negative electrode mixture slurry and a second negative electrode mixture slurry were applied in that order to the surface of a long copper foil negative electrode core body with a thickness of 8 μm. The coating film was dried and compressed to obtain a negative electrode in which a negative electrode mixture layer was formed on the negative electrode core body. At this time, a 6 mm wide non-composite portion was provided at one end in the width direction of the negative electrode where the negative electrode mixture layer was not formed.

[0081] Subsequently, by laser processing, recesses were formed only in a region extending 20% ​​of the widthwise length of the negative electrode mixture layer from the lower end in the widthwise direction, and extending along the longitudinal direction of the negative electrode. Each recess had a roughly circular shape in plan view, and the average diameter of the recesses was 90 μm. The distance between adjacent recesses in the widthwise direction was 200 μm, and the distance between adjacent recesses in the longitudinal direction was 40 μm. The depth of the recesses was 30 μm.

[0082] Cross-sectional analysis of the fabricated negative electrode revealed that the thickness of the first negative electrode mixture layer was 50 μm, and the thickness of the second negative electrode mixture layer was 40 μm. Furthermore, the porosity of the first and second negative electrode mixture layers 44 was calculated using the method described above and was found to be 19% and 36%, respectively. In addition, the porosity of the region directly below the recess and the peripheral region around the periphery of the region directly below were the same.

[0083] [Preparation of non-aqueous electrolyte] A mixed solvent is prepared by mixing ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 3:7, to which LiPF is added.６ A non-aqueous electrolyte was prepared by dissolving 1.5 mol / liter of [the substance].

[0084] [Preparation of Test Cell] The positive electrode, the negative electrode, and a polyethylene separator described above were wound in a spiral shape using a cylindrical winding core to obtain an electrode body. A positive electrode current collector plate and a negative electrode current collector plate were then placed on the upper and lower ends of the prepared electrode body. The portion of the positive electrode at the upper end of the electrode body where the composite layer was not formed, and the portion of the negative electrode at the lower end of the electrode body where the composite layer was not formed, were bent radially inward and welded to the positive electrode current collector plate and the negative electrode current collector plate, respectively.

[0085] Subsequently, the electrode assembly was housed in a bottomed cylindrical outer casing, the negative electrode current collector plate was welded to the bottom of the bottomed cylindrical outer casing, and the positive electrode current collector plate and the sealing body were connected with a positive electrode lead. After pouring in the non-aqueous electrolyte, the opening of the outer casing was sealed with the sealing body via a gasket to create a test cell (non-aqueous electrolyte secondary battery). The test cell was positioned with the axial direction of the outer casing aligned with the vertical direction.

[0086] [Evaluation of Initial Discharge Capacity and Capacity Retention Rate] Under an ambient temperature of 25°C, the battery was charged at 1C with a constant current to 4.2V, and then charged at 4.2V with a constant voltage to 0.02C. Afterward, it was discharged at 0.5C with a constant current to 2.5V, and the initial discharge capacity was measured. This charge-discharge cycle was considered one cycle, and 100 cycles were performed. The capacity retention rate in the charge-discharge cycle was then calculated using the following formula: Capacity Retention Rate [%] = (Discharge Capacity at Cycle 100 / Discharge Capacity at Cycle 1)

[0087] <Comparative Example 1> A test cell was prepared and evaluated in the same manner as in Example 1, except that no recesses were formed on the surface of the negative electrode mixture layer during the preparation of the negative electrode.

[0088] <Comparative Example 2> In the preparation of the negative electrode, recesses were formed extending in the width direction of the negative electrode over substantially the entire negative electrode mixture layer. Each recess had a substantially circular shape in plan view, and the average diameter of the recesses was 90 μm. The distance between adjacent recesses in the width direction was 40 μm, and the distance between adjacent recesses in the longitudinal direction was 200 μm. The depth of the recesses was 30 μm.

[0089] <Comparative Example 3> In the preparation of the negative electrode, recesses were formed extending in the longitudinal direction of the negative electrode over substantially the entire negative electrode mixture layer. Each recess had a substantially circular shape in plan view, and the average diameter of the recesses was 90 μm. The distance between adjacent recesses in the width direction was 200 μm, and the distance between adjacent recesses in the longitudinal direction was 40 μm. The depth of the recesses was 30 μm.

[0090] Table 1 shows the initial discharge capacity and capacity retention rate results for each test cell. The initial discharge capacity and capacity retention rate values ​​shown in Table 1 are relative to the initial discharge capacity and capacity retention rate values ​​of the test cell in Comparative Example 1, which are set to 100. A larger initial discharge capacity value indicates higher energy density, and a larger capacity retention rate indicates superior charge-discharge cycle characteristics.

[0091]

[0092] As shown in Table 1, the test cell of the example maintains the initial discharge capacity while improving the capacity retention rate compared to the test cell of Comparative Example 1. This is presumed to be because the formation of recesses facilitates the diffusion of the non-aqueous electrolyte in the thickness direction of the negative electrode mixture layer, thereby suppressing lithium deposition. In fact, when the test cell was disassembled after the cycle test and the surface of the negative electrode was observed, the amount of lithium deposition on the negative electrode surface of the test cell of the example was significantly reduced compared to the test cell of Comparative Example 1. Furthermore, it is presumed that the formation of recesses facilitated the diffusion of the non-aqueous electrolyte, which was pushed out to both ends in the vertical direction of the electrode body during repeated charging and discharging, towards the center in the vertical direction of the electrode body.

[0093] Furthermore, the test cell of the embodiment in which the recess is provided only on the lower end side of the negative electrode shows improved initial discharge capacity compared to the test cells of Comparative Examples 2 and 3 in which the recess is provided over the entire negative electrode. This is presumed to be because the volume of the negative electrode mixture layer increased by limiting the region in which the recess is formed. Therefore, it can be said that the technology of this disclosure makes it possible to provide a non-aqueous electrolyte secondary battery with excellent charge-discharge cycle characteristics while ensuring high energy density.

[0094] The present disclosure will be further described by the following embodiments. Configuration 1: A non-aqueous electrolyte secondary battery comprising an electrode body in which a strip-shaped first electrode and a strip-shaped second electrode having opposite polarities are wound with a separator between them, a non-aqueous electrolyte, and a cylindrical outer container for housing the electrode body and the non-aqueous electrolyte, wherein the axial direction of the outer container is aligned with the vertical direction, the first electrode having a first electrode core and a first electrode mixture layer disposed on the first electrode core, the surface of the first electrode mixture layer having a recess in the thickness direction of the first electrode mixture layer, and in an axial cross-sectional view of the outer container, when the length from the lower end of the first electrode mixture layer to the upper end of the first electrode mixture layer is L, the recess is provided only in the range from the lower end of the first electrode mixture layer to 0.6 L, the non-aqueous electrolyte secondary battery. Configuration 2: The non-aqueous electrolyte secondary battery according to Configuration 1, wherein the axial direction of the outer casing is arranged along the vertical direction, the electrode body has a hollow portion near the winding axis, the excess liquid of the non-aqueous electrolyte is stored in the hollow portion, and at the time of initial charging, the liquid level of the excess liquid inside the gap is located below the area where the recess is provided. Configuration 3: The non-aqueous electrolyte secondary battery according to Configuration 1, wherein the axial direction of the outer casing is arranged along the horizontal direction, the excess liquid of the non-aqueous electrolyte is stored inside the outer casing on the outside of the electrode body, and at the time of initial charging, the liquid level of the excess liquid is located below the area where the recess is provided. Configuration 4: The non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the depth of the recess is 5% or more and 50% or less of the thickness of the first electrode mixture layer. Configuration 5: The non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the recess is arranged to extend along the longitudinal or widthwise direction of the negative electrode. Configuration 6: The non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 5, wherein the porosity of the first electrode mixture layer in the region directly below the recess is substantially the same as the porosity of the first electrode mixture layer in the peripheral region, which is the region from the periphery of the region directly below to 50 μm in the in-plane direction of the first electrode mixture layer.Configuration 7: The non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 6, wherein the first electrode mixture layer comprises a first layer disposed on the surface of the first electrode core and a second layer disposed on the surface of the first electrode, and the porosity of the second layer is greater than that of the first layer. Configuration 8: The non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 7, wherein the recess is formed by laser processing. Configuration 9: The non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 8, wherein the first electrode is the negative electrode and the second electrode is the positive electrode.

[0095] 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 14A Hollow section, 15 Outer can, 16 Sealing body, 17 Positive electrode current collector plate, 18 Negative electrode current collector plate, 19 Insulating plate, 20 Positive electrode lead, 21 Grooved section, 22 Filter, 23 Lower valve body, 24 Insulating member, 25 Upper valve body, 26 Cap, 26a Through hole, 27 Gasket, 30 Positive electrode core body, 31 Positive electrode mixture layer, 32 Non-mixing layer section, 40 Negative electrode core body, 41 Negative electrode mixture layer, 42 Non-mixing layer section, 43 First negative electrode mixture layer, 44 Second negative electrode mixture layer, 45 Directly below region, 46 Peripheral region, 50 Recess

Claims

1. A non-aqueous electrolyte secondary battery comprising: an electrode body in which a strip-shaped first electrode and a strip-shaped second electrode having opposite polarities are wound with a separator between them; a non-aqueous electrolyte; and a cylindrical outer container for housing the electrode body and the non-aqueous electrolyte, wherein the outer container is arranged such that its axial direction is along the vertical or horizontal direction, the first electrode having a first electrode core and a first electrode mixture layer disposed on the first electrode core, the surface of the first electrode mixture layer having a recess in the thickness direction of the first electrode mixture layer, and in an axial cross-sectional view of the outer container, when the length from the lower end of the first electrode mixture layer to the upper end of the first electrode mixture layer is L, the recess is provided only in the range from the lower end of the first electrode mixture layer to 0.6 L.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the axial direction of the outer casing is arranged to be aligned with the vertical direction, the electrode body has a hollow portion near the winding axis, excess liquid of the non-aqueous electrolyte is stored in the hollow portion, and at the time of initial charging, the liquid level of the excess liquid inside the gap is located below the region in which the recess is provided.

3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the axial direction of the outer casing is arranged to be aligned with the horizontal direction, and inside the outer casing, excess liquid of the non-aqueous electrolyte is stored outside the electrode body, and at the time of initial charging, the liquid level of the excess liquid is located below the region in which the recess is provided.

4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the depth of the recess is 5% or more and 50% or less of the thickness of the first electrode mixture layer.

5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the recess is arranged to extend along the longitudinal or widthwise direction of the first electrode.

6. The non-aqueous electrolyte secondary battery according to claim 1, wherein the porosity of the first electrode mixture layer in the region directly below the recess is substantially the same as the porosity of the first electrode mixture layer in the peripheral region, which is the region from the periphery of the region directly below to 50 μm in the in-plane direction of the first electrode mixture layer.

7. The non-aqueous electrolyte secondary battery according to claim 1, wherein the first electrode mixture layer comprises a first layer disposed on the surface of the first electrode core and a second layer disposed on the surface of the first electrode, the porosity of the second layer being greater than the porosity of the first layer.

8. The non-aqueous electrolyte secondary battery according to claim 1, wherein the recess is formed by laser processing.

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

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