Non-aqueous electrolyte secondary battery negative electrode and non-aqueous electrolyte secondary battery

Abstract

A negative electrode (12) comprises an elongated negative electrode core body (30) and a negative electrode mixture layer (31) provided on the negative electrode core body (30), and constitutes a wound electrode body (14). The negative electrode mixture layer (31) is divided into a central region (33) that is positioned in the center of the negative electrode core body (30) in the width direction and end regions (34) that are respectively positioned further to both end sides of the negative electrode core body (30) in the width direction than the central region (33), and is configured such that, in a state in which the battery is 100% charged, the thickness (A) of the central region (33) is smaller than the thickness (B) of the end region (34) by not less than 1.5%, and the thickness difference (C) is not more than 10 μm.

Description

Negative electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

[0001] The present disclosure relates to a negative electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the negative electrode, and more particularly to a negative electrode applied to a wound electrode assembly and a non-aqueous electrolyte secondary battery including the wound electrode assembly.

[0002] In recent years, non-aqueous electrolyte secondary batteries such as lithium-ion batteries have been widely used in applications requiring high capacity, high durability, rapid charging performance, and the like, such as automotive applications and power storage applications. Because the negative electrode, a major component of a battery, significantly affects these performances, much research has been conducted on the negative electrode. For example, Patent Document 1 discloses that, in order to extend the life of a secondary battery equipped with a wound electrode assembly, the expansion and contraction rate of the negative electrode active material in the axial center of the negative electrode is made smaller than the expansion and contraction rate of the negative electrode active material in the axial end portion. However, Patent Document 1 does not disclose the extent to which the expansion and contraction rate of the negative electrode active material should differ between the axial center and end portions of the negative electrode.

[0003] Japanese Patent Application Laid-Open No. 2015-191879

[0004] However, as a result of investigations by the present inventors, it has been found that when the volume change of the negative electrode during charge and discharge increases due to an increase in the capacity of the battery, deformation of the electrode plate is likely to occur on the winding core side of the electrode assembly having a wound structure. An object of the present disclosure is to provide a negative electrode for a non-aqueous electrolyte secondary battery that can suppress deformation of the electrode assembly that may occur during charge and discharge of the battery.

[0005] A negative electrode for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a negative electrode for a non-aqueous electrolyte secondary battery used in a wound electrode body, and comprises a long negative electrode core and a negative electrode mixture layer provided on the negative electrode core, wherein the negative electrode mixture layer is divided into a central region located at the center of the negative electrode core in the width direction and end regions located on both ends of the negative electrode core relative to the central region in the width direction, and is configured such that, when the battery is at a charging rate of 100%, the thickness of the central region is smaller than the thickness of the end regions by 1.5% or more and the difference in thickness is 10 μm or less.

[0006] a negative electrode for a non-aqueous electrolyte secondary battery according to another aspect of the present disclosure, which is used in a wound electrode body, and which includes a long negative electrode core and a negative electrode mixture layer provided on the negative electrode core; the negative electrode mixture layer is divided into a central region located in a widthwise central portion of the negative electrode core and end regions located on both widthwise ends of the negative electrode core relative to the central region; the width (X) of the central region is 80% or less of the overall width (Z) of the negative electrode mixture layer; when the battery is at a 100% charge state, the thickness (A) of the central region is smaller than the thickness (B) of the end regions to form recesses, and the thickness difference (C) is 10 μm or less; and the recesses have a volume such that the ratio (X / Z) of the width (X) to the overall width (Z) multiplied by the ratio (C / B) of the thickness difference (C) to the thickness (B) is 0.3 or more.

[0007] A non-aqueous electrolyte secondary battery according to the present disclosure includes the above-described negative electrode, a positive electrode, and a non-aqueous electrolyte.

[0008] The negative electrode for a non-aqueous electrolyte secondary battery according to the present disclosure can effectively suppress deformation of the electrode body that may occur during charging and discharging of the battery.

[0009] It is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. It is a front view of a negative electrode according to an embodiment. It is a cross-sectional view taken along line AA in FIG. 2. It is a cross-sectional view showing a modified example of a negative electrode. It is a view for explaining a method of evaluating electrode plate deformation.

[0010] The inventors have conducted extensive research into the above-mentioned problem and have found that deformation of the electrode assembly is specifically suppressed when the thickness of the widthwise center of the negative electrode is smaller than the thickness of the ends when the battery is at 100% charged and certain conditions are met. As the capacity of the battery increases, the volume change of the negative electrode during charging and discharging increases, which is thought to increase the surface pressure acting on the widthwise center of the negative electrode, particularly, and as a result, deformation of the electrode plate is more likely to occur on the winding core side of the electrode assembly. Deformation of the electrode plate during charging and discharging, for example, increases the risk of short circuit occurrence and reduces the reliability of the battery. Deformation of the electrode plate is more likely to occur in the negative electrode; for example, repeated charging and discharging of the battery causes the negative electrode to bend near the winding start end of the positive electrode.

[0011] Therefore, the inventors succeeded in effectively suppressing plate deformation on the winding core side of the electrode assembly by selectively reducing the thickness of the central portion of the negative electrode relative to the widthwise ends to reduce the surface pressure acting on the electrode plate. Furthermore, it is expected that the electrolyte will be extruded from the electrode assembly during charge and discharge, resulting in a decrease in the electrolyte at the widthwise central portion of the electrode plate. However, by using the negative electrode according to the present disclosure, a recess is formed in the widthwise central portion of the negative electrode, ensuring a sufficient amount of electrolyte. As a result, the cycle characteristics of the battery are improved. Note that if the thickness of the widthwise central portion of the negative electrode becomes too small, for example, the internal resistance of the battery increases, causing problems such as Li deposition. Therefore, it is necessary to control the thickness to meet certain conditions.

[0012] Hereinafter, with reference to the drawings, an example of an embodiment of a negative electrode for a nonaqueous electrolyte secondary battery according to the present disclosure and a nonaqueous electrolyte secondary battery using the negative electrode will be described in detail. Note that configurations obtained by selectively combining the respective components of the multiple embodiments and modifications described below are included within the scope of the present disclosure.

[0013] In the embodiment described below, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is exemplified as the nonaqueous electrolyte secondary battery, but the outer can of the battery is not limited to a cylindrical outer can. Other embodiments of the nonaqueous electrolyte secondary battery according to the present disclosure include a prismatic battery equipped with a prismatic outer can, a pouch-type battery equipped with an outer can made of a laminate sheet including a metal layer and a resin layer, and the like.

[0014] FIG. 1 is a schematic diagram illustrating an axial and radial cross section of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1 , the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container with a bottom and an open end in the axial direction, and the opening of the outer can 16 is closed by a sealing member 17. Hereinafter, for convenience of explanation, the sealing member 17 side of the battery is referred to as the top, and the bottom side of the outer can 16 is referred to as the bottom.

[0015] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. Examples of the electrolyte salt include LiPF ６ Lithium salts such as

[0016] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer in both the length and width directions than the positive electrode 11. The separator 13 is formed to be at least slightly larger than the positive electrode 11, and for example, two separators 13 are arranged to sandwich the positive electrode 11.

[0017] The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 and a negative electrode lead 21 connected to the negative electrode 12. In this embodiment, the positive electrode lead 20 is provided in the longitudinal center of the positive electrode 11. On the other hand, the negative electrode lead 21 is provided at one longitudinal end of the negative electrode 12 located on the winding core side of the electrode body 14. At both longitudinal ends of the negative electrode 12, core exposed portions 32 (see FIG. 2 described below) are formed where the negative electrode mixture layer 31 is not present and the surface of the negative electrode core 30 is exposed, and the negative electrode lead 21 is connected to the core exposed portion 32 located on the winding core side of the electrode body 14.

[0018] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in FIG. 1 , the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes through a through hole in the insulating plate 19 and extends toward the bottom of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal. In addition, a core exposed portion 32 located on the outer peripheral surface of the electrode body 14 abuts against the inner peripheral surface of the outer can 16.

[0019] The outer can 16 is a cylindrical metal container with a bottom. A gasket 28 is provided between the outer can 16 and the sealing body 17 to seal the interior of the battery. The outer can 16 has a groove 22 that supports the sealing body 17, formed, for example, by pressing the side surface from the outside. The groove 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The upper end of the outer can 16 is bent inward and crimped to the periphery of the sealing body 17.

[0020] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0021] The positive electrode 11, the negative electrode 12, and the separator 13 that constitute the electrode assembly 14, with the negative electrode 12 being particularly described in detail below.

[0022] [Positive Electrode] The positive electrode 11 has a long positive electrode core and a positive electrode mixture layer provided on the positive electrode core. The positive electrode core can be a foil of a metal stable within the potential range of the positive electrode 11, such as aluminum, an aluminum alloy, stainless steel, or titanium, or a film with such a metal disposed on the surface. The thickness of the positive electrode core is, for example, 10 μm to 30 μm. The positive electrode mixture layer preferably contains a positive electrode active material, a conductive agent, and a binder, and is provided on both sides of the positive electrode core. The average thickness of the positive electrode mixture layer is, for example, 60 μm to 120 μm on one side of the positive electrode core. The positive electrode 11 can be produced, 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, drying the coating, and then compressing it to form a positive electrode mixture layer on both sides of the positive electrode core.

[0023] The positive electrode active material is a lithium transition metal composite oxide containing transition metal elements such as Ni, Co, and Mn. Examples of metal elements contained in the composite oxide include Ni, Co, Mn, Al, Be, B, Na, Mg, Si, K, Ca, Sc, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, Ta, W, Pb, and Bi. Among these, it is preferable to contain at least one of Ni, Co, and Mn. The lithium transition metal composite oxide has, for example, a layered rock salt structure. One type of lithium transition metal composite oxide may be used alone, or multiple types may be used in combination. The content of the positive electrode active material is, for example, 90% by mass or more and 99.8% by mass or less relative to the mass of the positive electrode mixture layer.

[0024] Examples of the conductive agent contained in the positive electrode mixture layer include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, metal fibers, metal powder, and conductive whiskers. One type of conductive agent may be used alone, or multiple types may be used in combination. The content of the conductive agent is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less relative to the mass of the positive electrode mixture layer.

[0025] Examples of binders contained in the positive electrode mixture layer include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF); olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, and ethylene-propylene-butadiene copolymer; and acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide, and ethylene-acrylic acid copolymer. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like. One type of binder may be used alone, or multiple types may be used in combination. The content of the binder is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less, relative to the mass of the positive electrode mixture layer.

[0026] [Negative Electrode] Fig. 2 is a front view of the negative electrode 12, and Fig. 3 is a cross-sectional view taken along line AA in Fig. 2 . Fig. 3 shows a cross section of the negative electrode when the nonaqueous electrolyte secondary battery 10 is at a 100% state of charge. As shown in Figs. 2 and 3 , the negative electrode 12 has an elongated negative electrode core 30 and a negative electrode mixture layer 31 provided on the negative electrode core 30. The negative electrode mixture layer 31 is divided into a central region 33 located in the widthwise center of the negative electrode core 30 and end regions 34 located on both widthwise ends of the negative electrode core 30 relative to the central region 33. The central region 33 of the negative electrode mixture layer 31 is located in the axial center of the electrode body 14, and the end regions 34 are located at both axial ends of the electrode body 14. As will be described in more detail later, the negative electrode mixture layer 31 is configured so that, when the battery's state of charge (hereinafter referred to as "SOC") is 100%, the thickness (A) of the central region 33 is smaller than the thickness (B) of the end region 34 by 1.5% or more, and the thickness difference (C) is 10 μm or less.

[0027] The negative electrode core 30 can be made of a foil of a metal stable within the potential range of the negative electrode 12, such as copper, a copper alloy, stainless steel, nickel, or a nickel alloy, or a film having such a metal disposed on its surface. Among these, copper foil or a copper alloy foil is preferably used. The thickness of the negative electrode core 30 is, for example, 5 μm to 20 μm, or 7 μm to 15 μm. The negative electrode mixture layer 31 preferably contains a negative electrode active material and a binder and is provided on both sides of the negative electrode core 30. The negative electrode mixture layer 31 may contain the same conductive agent as the positive electrode mixture layer. The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder onto the negative electrode core 30, drying the coating, and then compressing it to form the negative electrode mixture layer 31 on both sides of the negative electrode core 30.

[0028] As in the case of the positive electrode 11, the binder contained in the negative electrode mixture layer 31 can be a fluororesin, an olefin resin, PAN, a polyimide, a polyamide, an acrylic resin, or the like. However, polyvinyl acetate, styrene-butadiene rubber (SBR), or the like may also be used. Among these, SBR is preferably used. One type of binder may be used alone, or multiple types may be used in combination. Furthermore, the negative electrode mixture layer 31 preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. The content of the binder is not particularly limited, but is, for example, 0.05% by mass or more and 3% by mass or less, or 0.1% by mass or more and 1% by mass or less, relative to the mass of the negative electrode mixture layer 31.

[0029] The negative electrode mixture layer 31 preferably contains graphite and a Si-containing material as the negative electrode active material. The combined use of graphite and a Si-containing material can more effectively suppress deformation of the electrode body 14 and facilitates achieving both high capacity and high durability of the battery. Examples of Si-containing materials that function as the negative electrode active material include silicon alloys, silicon compounds, and Si-containing composite materials. Among these, Si-containing composite materials are preferred.

[0030] When graphite and a Si-containing material are used in combination as the negative electrode active material, the graphite content is, for example, 80% by mass to 99% by mass, 85% by mass to 97% by mass, or 90% by mass to 95% by mass of the total mass of the negative electrode active material. The Si-containing material content is, for example, 1% by mass to 20% by mass, 3% by mass to 15% by mass, or 5% by mass to 10% by mass of the total mass of the negative electrode active material. When the contents of the graphite and the Si-containing material are within these ranges, it becomes easier to achieve both high capacity and high durability of the battery.

[0031] Examples of graphite that can be used include artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, and amorphous graphite, and mixtures thereof. The volume-based median diameter (D50) of the graphite is, for example, 1 μm or more and 30 μm or less, and preferably 5 μm or more and 25 μm or less. D50 refers to the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size. The particle size distribution of the carbon material can be measured using a laser diffraction particle size distribution analyzer (for example, SALD-2000A, manufactured by Shimadzu Corporation) using water as a dispersion medium.

[0032] The Si-containing material (composite material) is preferably a composite particle containing an ion-conducting phase and a Si phase dispersed in the ion-conducting phase. The ion-conducting phase is, for example, at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. The silicide phase is a phase of a compound consisting of Si and an element more electropositive than Si, such as NiSi, Mg ２ Si, TiSi ２ The Si phase is formed by dispersing Si in the form of fine particles, and the ion-conducting phase is a continuous phase formed by an aggregation of particles that are finer than the Si phase.

[0033] The D50 of the Si-containing material is generally smaller than that of graphite. The D50 of the Si-containing material is, for example, 1 μm or more and 20 μm or less, or 1 μm or more and 15 μm or less. The Si-containing material may also have a conductive layer covering the surface of the ion-conductive phase. The conductive layer is made of a material with higher conductivity than the ion-conductive layer and forms a good conductive path in the negative electrode mixture layer 31. The conductive layer is, for example, a carbon coating made of a conductive carbon material. Examples of conductive carbon materials that can be used include carbon black such as acetylene black and ketjen black, graphite, and amorphous carbon (amorphous carbon) with low crystallinity.

[0034] The Si content in the Si-containing material is preferably 40% by mass or more and 70% by mass or less. If the Si content is too low, the capacity of the Si-containing material decreases, so the Si content is more preferably 50% by mass or more. On the other hand, if the Si content is too high, for example, the volume change of the particles during charge and discharge increases, making particle cracking more likely. As a result, it is thought that side reactions between the non-aqueous electrolyte and Si are more likely to occur, resulting in a decrease in the cycle characteristics of the battery.

[0035] An example of a suitable Si-containing material includes an amorphous silicon oxide phase, a Si phase dispersed in the silicon oxide phase, and has the general formula SiO ｘ The silicon oxide may be mainly composed of silicon dioxide. The silicon oxide phase may be doped with Li. The oxygen to Si content (x) is, for example, 0.5≦x<2.0, and preferably 0.8≦x≦1.5.

[0036] Another example of a suitable Si-containing material is a composite particle containing an amorphous silicate phase and a Si phase dispersed in the silicate phase. A suitable silicate phase is a lithium silicate phase containing Li. The lithium silicate phase can be, for example, a compound represented by the general formula Li ２ｚ SiO （２＋ｚ） (0<z<2). The lithium silicate phase is a composite oxide phase represented by the formula: Li ２ SiO ３ (Z=1) or Li ２ Si ２ O５ It is preferable that (Z=1 / 2) is used as the main component.

[0037] Another example of a suitable Si-containing material is composite particles (hereinafter referred to as "SiC") containing an amorphous carbon phase and a Si phase dispersed in the amorphous carbon phase. SiC has a sea-island structure in which fine Si phases are dispersed in a continuous phase composed of amorphous carbon. The Si phase changes in volume with charge and discharge, but the stress caused by the volume change of the Si phase is alleviated by the amorphous carbon phase. Therefore, by using SiC, the volume change of the negative electrode 12 with charge and discharge can be kept small.

[0038] The amorphous carbon phase is made of, for example, a carbon material having an average interplanar spacing of (002) planes greater than 0.34 nm as determined by XRD measurement, and is obtained by heat treating a pitch having a softening point of 200° C. or higher at a temperature of 700° C. to 900° C. The amorphous carbon phase may contain a metal oxide that does not react with Li, such as zirconium oxide, aluminum oxide, titanium oxide, nickel oxide, or yttrium oxide.

[0039] As described above, the negative electrode mixture layer 31 is configured such that, when the nonaqueous electrolyte secondary battery 10 has a 100% SOC, the thickness (A) of the central region 33 is at least 1.5% smaller than the thickness (B) of the edge region 34, and the thickness difference (C) between the central region 33 and the edge region 34 is 10 μm or less. When this condition is met, the thickness of the central region is selectively reduced relative to the widthwise edge of the negative electrode 12, effectively reducing the surface pressure acting on the electrode plate. As a result, electrode plate deformation on the winding core side of the electrode assembly 14 is suppressed. The upper charge voltage limit of the nonaqueous electrolyte secondary battery 10 is not particularly limited, but is set, for example, in the range of 4.2 V to 4.8 V. The SOC is 100% when the battery voltage reaches a predetermined upper charge voltage limit.

[0040] The thickness of the anode mixture layer 31 may be, for example, 50 μm to 100 μm, or 60 μm to 90 μm, on one side of the anode core 30 immediately after the manufacture of the anode 12 and when the SOC is low. The thicknesses of the central region 33 and the end regions 34 may be substantially the same. The configuration of the anode mixture layer 31 is preferably substantially the same on both sides of the anode core 30. As the SOC of the battery increases, the thickness of the anode mixture layer 31 increases, and the thickness difference (C) between the central region 33 and the end regions 34 gradually increases. The volume expansion of the anode mixture layer 31 is controlled so that the thickness (A) of the central region 33 is 1.5% or more smaller than the thickness (B) of the end regions 34 when the SOC is 100%. A recess recessed in the thickness direction of the anode 12 is formed in the widthwise center of the anode 12 when the SOC is high.

[0041] In the example shown in Fig. 3, the thicknesses of the central region 33 and the edge regions 34 are constant across the entire width of each region, but the thickness of each region may vary in the width direction of the negative electrode mixture layer 31. Unless otherwise specified, in this specification, the thickness (B) of the edge regions 34 is measured at a portion located inward in the width direction from both ends of the negative electrode mixture layer 31 in the width direction by a length equivalent to 5% of the entire width (Z) of the negative electrode mixture layer 31. Furthermore, the thickness (A) of the central region 33 is measured at a portion located in the center of the central region 33 in the width direction. The thicknesses of the negative electrode 12 and the negative electrode mixture layer 31 are measured by observing the cross section of the negative electrode using a scanning electron microscope (SEM).

[0042] The width (X) of the central region 33 is preferably 20% to 80% of the overall width (Z) of the negative electrode mixture layer 31, and more preferably 50% or less. In this embodiment, the negative electrode mixture layer 31 is formed across the entire width of the negative electrode core 30, and therefore the widths of the negative electrode mixture layer 31 and the negative electrode core 30 are equal. The width (X) of the central region 33 is preferably 20% to 45% of the overall width (Z) of the negative electrode mixture layer 31, and more preferably 25% to 40%. If the width (X) of the central region 33 is within this range, deformation of the electrode body 14 can be efficiently suppressed while avoiding problems such as a decrease in capacity and an increase in resistance.

[0043] The central region 33 and the end regions 34 are formed, for example, using different negative electrode mixture slurries. The first negative electrode mixture slurry forming the central region 33 and the second negative electrode mixture slurry forming the end regions 34 differ in at least one of the type of negative electrode active material contained therein and the content ratio of the multiple types of negative electrode active materials. The coating films formed by applying the first and second negative electrode mixture slurries to the surface of the negative electrode core 30 are compressed, for example, with the same amount of force. In this case, the thicknesses of the central region 33 and the end regions 34 are substantially the same immediately after the production of the negative electrode 12.

[0044] The end region 34 includes a first region 35 located on one widthwise end side of the negative electrode mixture layer 31 and a second region 36 located on the other widthwise end side of the negative electrode mixture layer 31. The width (Y) of the end region 34 may be different between the first region 35 and the second region 36, but in this embodiment, the widths of the first region 35 and the second region 36 are substantially the same. The width (Y) of the first region 35 and the second region 36 is preferably 10% or more and 40% or less, and more preferably 25% or more and 35% or less. The first region 35 and the second region 36 are formed using the same type of negative electrode mixture slurry.

[0045] The central region 33 and the end regions 34 are formed, for example, with a substantially constant width and a substantially constant thickness over the entire length of the negative electrode mixture layer 31. The negative electrode 12 has core exposed portions 32 at both longitudinal ends, and the central region 33 and the end regions 34 are continuously formed in a stripe shape between the core exposed portions 32 at both ends. The stripe shape of the central region 33 and the end regions 34 can be formed only in part of the length of the negative electrode mixture layer 31, but is preferably formed over the entire length of the negative electrode mixture layer 31 from the viewpoints of productivity of the negative electrode 12, uniformity of the battery reaction, etc. The negative electrode lead 21 is connected to one of the core exposed portions 32, and an insulating tape 29 is attached so as to cover the positive electrode lead 21.

[0046] The central region 33 is preferably formed in a range including the widthwise center α of the negative electrode mixture layer 31. In this embodiment, the width of the central region 33 on both sides of the widthwise center α of the negative electrode mixture layer 31 is substantially the same. That is, the widthwise center of the central region 33 is located at the widthwise center α of the negative electrode mixture layer 31. In this case, the surface pressure acting on the widthwise center portion of the negative electrode 12 can be effectively reduced, and electrode plate deformation due to charge and discharge can be more reliably suppressed.

[0047] The ratio (100 × C / B) of the difference (C) between the thickness (B) of the end region 34 and the thickness (A) of the central region 33 to the thickness (B) of the end region 34 is 1.5% or more when the SOC is 100%. In this case, deformation of the electrode body 14 is effectively suppressed, and the cycle characteristics of the battery are improved. On the other hand, if this ratio is too large, the internal resistance of the battery increases and problems such as Li deposition occur. Therefore, the upper limit of this ratio is preferably 5%. An example of a suitable range for the ratio (100 × C / B) is 1.5% or more and 5% or less, or 1.5% or more and 3.5% or less. Furthermore, the thickness difference (C) between the central region 33 and the end region 34 is preferably 10 μm or less, and more preferably 5 μm or less, when the SOC is 100%.

[0048] The central region 33 and the edge regions 34 have at least different mixing ratios of graphite and Si-containing material, or different expansion rates of the Si-containing material during charging and discharging of the battery. For example, when the same types of graphite and Si-containing material are used in the central region 33 and the edge regions 34, the proportion of the Si-containing material in the negative electrode active material is made lower in the central region 33 than in the edge regions 34. Because the Si-containing material expands in volume more than graphite during charging, the thickness of the central region 33 in the charged state can be reduced by reducing the content of the Si-containing material in the central region 33. However, in this case, the difference in capacity per unit area between the regions may become large.

[0049] Alternatively, a Si-containing material with a small expansion rate during charging may be added to the central region 33, and a Si-containing material with a large expansion rate during charging may be added to the edge regions 34. Specifically, SiC may be used in the central region 33, and a Si-containing material other than SiC (e.g., SiO ｘ ) is used. SiC is ｘSince SiC has a smaller expansion rate during charging than other Si-containing materials such as SiO, the thickness (A) of the central region 33 in the charged state can be reduced by using SiC. ｘ Since the capacity of the central region 33 is equal to or greater than that of other Si-containing materials such as those mentioned above, the thickness (A) of the central region 33 can be reduced without causing a decrease in the capacity of the negative electrode 12 .

[0050] The density of the central region 33 may be lower than the density of the edge regions 34. By lowering the density of the central region 33, the thickness (A) of the central region 33 can be made smaller than the thickness (B) of the edge regions 34 when the SOC is high. As described above, the density of each region can be changed somewhat by changing the type of negative electrode active material, but it can also be easily adjusted by changing the content of negative electrode active material per unit volume. The density of the central region 33 is, for example, 95% to 105% of the density of the edge regions 34. If the density of the central region 33 is too low, the battery capacity will decrease significantly. Therefore, it is preferable that the density difference with the edge regions 34 be within this range.

[0051] Specifically, by making the application amount of the first anode mixture slurry that forms the central region 33 less than the application amount of the second anode mixture slurry that forms the edge region 34, a configuration in which the density of the central region 33 is less than the density of the edge region 34 can be achieved. In this case, the anode mixture slurries may be the same. Alternatively, by making the application amounts of the anode mixture slurries the same and making the content of the anode active material in the first anode mixture slurry less than the content of the anode active material in the second anode mixture slurry, a configuration in which the density of the central region 33 is less than the density of the edge region 34 can also be achieved.

[0052] The ratio of the capacity per unit area of ​​the central region 33 to the capacity per unit area of ​​the end region 34 is, for example, 0.90 to 1.10, preferably 0.95 to 1.05. If the capacities per unit area of ​​each region are different, the charge and discharge of the battery must be controlled according to the region with the lower capacity to prevent problems such as Li deposition. Therefore, if the difference in capacity per unit area becomes too large, the energy density of the battery may be significantly reduced. For this reason, it is preferable to keep the difference in capacity per unit area between each region to 10% or less.

[0053] In the second embodiment, the recess formed in the widthwise center of the anode mixture layer 31 has a volume such that the ratio (X / Z) of the width (X) of the central region 33 to the overall width (Z) of the anode mixture layer 31 multiplied by the ratio (C / B) of the thickness difference (C) between the central region 33 and the edge region 34 to the thickness (B) of the edge region 34 is 0.3 or greater. The recess is formed when the thickness (A) of the central region 33 is smaller than the thickness (B) of the edge region 34 when the battery's SOC is 100%, thereby reducing the surface pressure acting on the widthwise center of the anode 12. In this case, too, it is preferable that the width (X) of the central region is 20% to 80% of the overall width (Z) of the anode mixture layer 31, and the thickness difference (C) between the central region 33 and the edge region 34 is 10 μm or less.

[0054] In the second embodiment, the ratio (100 × C / B) may be less than 1.5% as long as the volume of the recess is such that the value of (X / Z) × (C / B) is 0.3 or greater. For example, if the width (X) of the central region 33 is greater than 20% of the overall width (Z) of the negative electrode mixture layer 31, the ratio (100 × C / B) may be less than 1.5%. By forming a recess of a predetermined volume in the widthwise center of the negative electrode mixture layer 31, a sufficient amount of electrolyte can be secured in the widthwise center of the electrode plate, improving the cycle characteristics of the battery. However, even in this case, it is preferable to set the lower limit of the ratio (100 × C / B) to 1.5% in order to effectively suppress deformation of the electrode body 14.

[0055] FIG. 4 is a cross-sectional view showing a modified example of the negative electrode 12. As shown in FIG. 4, the thickness of the edge region 34 may gradually decrease toward the central region 33. In this case, the step formed at the boundary between the central region 33 and the edge region 34 is reduced or eliminated, thereby dispersing the surface pressure acting at the boundary. The thickness of the central region 33 is substantially constant throughout the entire central region 33, similar to the embodiment shown in FIG. 3. The thickness of the edge region 34 may be constant in a region of a predetermined width adjacent to the central region 33, or may vary continuously up to the boundary position with the central region 33. Furthermore, the surface of the edge region 34 may be gently curved so as to be convex toward the negative electrode substrate 30.

[0056] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. The separator 13 may have, for example, a multi-layer structure including a thermoplastic resin layer such as polyolefin and a cellulose fiber layer, a two-layer structure of polyethylene (PE) / polypropylene (PP), or a three-layer structure of PE / PP / PE.

[0057] A filler layer containing an inorganic filler may be disposed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphate compounds. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13. In addition, a highly heat-resistant resin layer (heat-resistant layer) such as an aramid resin may be disposed on the surface of the separator 13. The separator 13 may have, for example, a substrate made of a porous sheet and a filler layer or heat-resistant layer disposed on the substrate.

[0058] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0059] Example 1 [Fabrication of Positive Electrode] A positive electrode active material having the composition formula LiNi ０．９１ Co ０．０６ Al ０．０３ O ２ A lithium transition metal composite oxide represented by the formula (I) was used. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a solids mass ratio of 98:1:1, and a positive electrode mixture slurry was prepared using N-methylpyrrolidone (NMP) as a dispersion medium. The slurry was applied to both sides of a positive electrode core made of a long aluminum foil with a thickness of 15 μm, and the coating was dried and compressed to obtain a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode core. An exposed core portion where no positive electrode mixture layer was present was provided in the center of the positive electrode in the longitudinal direction, and an aluminum positive electrode lead was ultrasonically welded to the exposed portion.

[0060] [Preparation of first negative electrode mixture slurry] A negative electrode active material was prepared by mixing graphite powder and SiC in a mass ratio of 90:10. The negative electrode active material, a dispersion of styrene butadiene rubber, and sodium carboxymethyl cellulose in a solid content mass ratio of 98:1:1 were mixed, and water was used as a dispersion medium to prepare a first negative electrode mixture slurry.

[0061] [Preparation of Second Negative Electrode Mixture Slurry] A negative electrode mixture containing graphite powder and SiO ｘ A second negative electrode mixture slurry was prepared in the same manner as the first negative electrode mixture slurry, except that a mixture of a Si-containing material represented by (X=1) and a Si-containing material represented by (X=1) in a mass ratio of 90:10 was used.

[0062] [Fabrication of Negative Electrode] The first and second negative electrode mixture slurries were applied to both sides of a negative electrode core made of a long copper foil having a thickness of 8 μm and a width of 64 mm, leaving first and second core exposed portions at both ends in the length direction, and the coating was dried and compressed to obtain a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode core. At this time, the first negative electrode mixture slurry was applied to the center in the width direction of the negative electrode core, and the second negative electrode mixture slurry was applied to both ends in the width direction of the negative electrode core. This resulted in a central region containing SiC and a SiO ｘA negative electrode mixture layer having an end region including the first core material and an end region including the first core material was formed. The width of the central region of the negative electrode mixture layer was 30% (20 mm) of the total width of the negative electrode mixture layer. The two end regions were formed to have the same width (35% of the total width of the negative electrode mixture layer). A nickel negative electrode lead was ultrasonically welded to the first core material exposed portion.

[0063] The thickness of the negative electrode, including the negative electrode core and the negative electrode mixture layers on both sides, is 150 μm in both the central region and the edge region. The thickness of the negative electrode immediately after manufacture can be measured in the same manner as the thickness of the negative electrode after charging, as described below. The ratio of the capacity per unit area of ​​the central region A to the capacity per unit area of ​​the edge region B is 100%.

[0064] [Preparation of non-aqueous electrolyte] 5 parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of a mixed solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1:3 (25°C), and LiPF ６ was dissolved in a concentration of 1.5 mol / L to prepare a non-aqueous electrolyte solution.

[0065] [Cylindrical Battery Fabrication] The positive electrode, the negative electrode, and a polyethylene separator were spirally wound, and a stop tape was applied to the outermost surface to obtain a wound-type electrode assembly. The negative electrode was positioned so that the first core exposed portion to which the negative electrode lead was attached was located on the winding core side of the electrode assembly. After placing insulating plates above and below the electrode assembly, the negative electrode lead was welded to the inner bottom of a cylindrical outer can with a bottom, and the positive electrode lead was welded to the internal terminal plate of the sealing member, and the electrode assembly was housed in the outer can. A nonaqueous electrolyte was then injected into the outer can under reduced pressure, and the opening of the outer can was sealed with the sealing member via a gasket to obtain a cylindrical battery. The second core exposed portion of the negative electrode formed the outermost surface of the electrode assembly and contacted the inner surface of the outer can.

[0066] Example 2 In the preparation of the first negative electrode mixture slurry, SiO ｘ A negative electrode and a cylindrical battery were produced in the same manner as in Example 1, except that a Si-containing material represented by (X=1) was used and the mass ratio of the graphite powder to the Si-containing material was changed to 92:8.

[0067] Example 3 A negative electrode and a cylindrical battery were produced in the same manner as in Example 2, except that in the preparation of the first negative electrode mixture slurry, the mass ratio of the graphite powder to the Si-containing material was changed to 94:6.

[0068] Comparative Example 1 A negative electrode and a cylindrical battery were fabricated in the same manner as in Example 1, except that the negative electrode mixture layer was formed using only the second negative electrode mixture slurry.

[0069] Comparative Example 2 A negative electrode and a cylindrical battery were fabricated in the same manner as in Example 2, except that in the preparation of the first negative electrode mixture slurry, the mass ratio of the graphite powder to the Si-containing material was changed to 91:9.

[0070] The performance of each battery in the Examples and Comparative Examples was evaluated by the following method, and the evaluation results are shown in Table 1. The surface pressure shown in Table 1 is a relative value when the surface pressure of the cylindrical battery in Comparative Example 1 is set to 100%.

[0071] [Measurement of Thickness of Central Region A and End Region B] Each battery of the Examples and Comparative Examples was charged at a constant current of 0.7 C in a temperature environment of 25 ° C until the battery voltage reached 4.2 V (maximum charging voltage). After the SOC reached 100%, the battery was disassembled and the negative electrode was removed. The thickness of the central region A and the end region B was measured by observing the cross section of the negative electrode using an SEM, and the thickness difference C between each region was calculated. The thickness of each region shown in Table 1 is the thickness of the entire negative electrode including the negative electrode core and the negative electrode mixture layers on both sides. For example, the thickness of the central region A of the negative electrode mixture layer (one side) in Example 1 is approximately 89.3 μm. The thickness of the central region A was measured at the center of the width of the negative electrode (negative electrode mixture layer). The thickness of the end region B was measured at a position inward in the width direction by a length equivalent to 5% of the total width of the negative electrode mixture layer from both ends of the width of the negative electrode mixture layer.

[0072] [Evaluation of Surface Pressure] Each battery of the Examples and Comparative Examples was charged at a constant current of 0.5 C in a temperature environment of 45°C until the battery voltage reached 4.2 V. Then, the battery was discharged at a constant current of 1.0 C until the battery voltage reached 2.5 V. After 500 cycles of this charge / discharge, the battery was disassembled, and the surface pressure applied to the separator was measured in the central region and edge regions where the positive electrode and negative electrode faced each other via the separator. Measurements were performed in the central region at a distance of 32 mm from the edge, and in the edge regions at a distance of 10 mm from the edge.

[0073] [Evaluation of electrode plate deformation (presence or absence of buckling)] Each of the batteries of the examples and comparative examples was charged at a constant current of 0.5 C in a temperature environment of 45° C. until the battery voltage reached 4.2 V. Thereafter, the battery was discharged at a constant current of 1.0 C until the battery voltage reached 2.5 V. After 500 cycles of this charge / discharge, the battery was placed in a charged state, and the vicinity of the winding core of the electrode body was observed using an X-ray CT device (Shimadzu Corporation, SMX-225CT FPD HR).

[0074] As shown in Figure 5, when deformation (buckling) of the electrode plate (at least one of the positive electrode 11 and the negative electrode 12) was confirmed at the location where the positive electrode and the negative electrode faced each other such that the angle θ was 150° or less, it was determined that buckling had occurred, and the presence or absence of buckling was evaluated. Note that buckling of the electrode plate is likely to occur in the negative electrode near the winding start end of the positive electrode.

[0075]

[0076] As shown in Table 1, the batteries of Examples 1 to 3 had a lower surface pressure acting on the widthwise center of the electrode plate compared to the batteries of Comparative Examples 1 and 2, making the electrode plate less likely to buckle. These results demonstrate that reducing the surface pressure on the widthwise center of the electrode plate effectively suppresses electrode plate deformation. However, in the battery of Comparative Example 2, the thickness of the central region of the negative electrode mixture layer after charging was 1.0% smaller than the thickness of the edge regions. Although the surface pressure on the widthwise center of the electrode plate was lower compared to the battery of Comparative Example 1, electrode plate deformation was confirmed. In other words, a thickness difference of 1.0% between the central and edge regions of the negative electrode mixture layer is insufficient to suppress electrode plate deformation; a thickness difference of 1.5% or more is required to prevent electrode plate deformation. Note that if the thickness difference is too large, the internal resistance of the battery increases, leading to problems such as lithium deposition. Therefore, it is preferable to limit the thickness difference to 5%.

[0077] The present disclosure is further described by the following embodiments. Aspect 1: A negative electrode for a non-aqueous electrolyte secondary battery used in a wound-type electrode assembly, the negative electrode comprising: a long negative electrode core; and a negative electrode mixture layer provided on the negative electrode core, the negative electrode mixture layer being divided into a central region located in the widthwise center of the negative electrode core; and end regions located on both widthwise ends of the negative electrode core relative to the central region, the thickness of the central region being smaller than the thickness of the end regions by 1.5% or more, and the difference in thickness being 10 μm or less. Aspect 2: The negative electrode for a non-aqueous electrolyte secondary battery according to Aspect 1, the negative electrode mixture layer containing graphite and a Si-containing material as negative electrode active materials, the central region and the end regions having different mixing ratios of the graphite and the Si-containing material, or different swelling rates of the Si-containing material during charging and discharging of the battery. Configuration 3: The negative electrode for a non-aqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the density of the central region is lower than the density of the edge regions, and the ratio of the capacity per unit area of ​​the central region to the capacity per unit area of ​​the edge regions is 0.90 to 1.10. Configuration 4: The negative electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the width of the central region is 20% to 80% of the overall width of the negative electrode mixture layer. Configuration 5: The negative electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the thickness of the edge regions gradually decreases toward the central region.Configuration 6: A negative electrode for a non-aqueous electrolyte secondary battery used in a wound-type electrode body, comprising: a long negative electrode core; and a negative electrode mixture layer provided on the negative electrode core, wherein the negative electrode mixture layer is divided into a central region located at the center in the width direction of the negative electrode core; and end regions located on both ends of the negative electrode core relative to the central region in the width direction, and when the battery is in a state where the charging rate is 100%, the thickness (A) of the central region is smaller than the thickness (B) of the end regions, forming recesses; a negative electrode for a non-aqueous electrolyte secondary battery, wherein the negative electrode for a non-aqueous electrolyte secondary battery is configured so that a thickness difference (C) between the central region and the edge regions is 10 μm or less, and the recess has a volume such that a value obtained by multiplying a ratio (X / Z) of a width (X) of the central region to an overall width (Z) of the negative electrode mixture layer by a ratio (C / B) of the thickness difference (C) between the central region and the edge regions to a thickness (B) of the edge region is 0.3 or more, and the width (X) of the central region is 20% to 80% of the overall width (Z) of the negative electrode mixture layer.

[0078] REFERENCE SIGNS LIST 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 29 Insulating tape, 30 Negative electrode core, 31 Negative electrode mixture layer, 32 Core exposed portion, 33 Central region, 34 End region, 35 First region, 36 Second region

Claims

1. A negative electrode for a non-aqueous electrolyte secondary battery used in a wound-type electrode body, comprising: a long negative electrode core; and a negative electrode mixture layer provided on the negative electrode core, wherein the negative electrode mixture layer is divided into a central region located in the center of the negative electrode core in the width direction, and end regions located on both ends of the negative electrode core relative to the central region in the width direction, and wherein the negative electrode for a non-aqueous electrolyte secondary battery is configured so that, when the battery is at a 100% charge state, the thickness of the central region is smaller than the thickness of the end regions by 1.5% or more, and the difference in thickness is 10 μm or less.

2. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode mixture layer contains graphite and a Si-containing material as negative electrode active materials, and at least a mixing ratio of the graphite to the Si-containing material differs between the central region and the edge regions, or a swelling rate of the Si-containing material due to charging and discharging of the battery differs between the central region and the edge regions.

3. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the density of the central region is lower than the density of the edge regions, and the ratio of the capacity per unit area of ​​the central region to the capacity per unit area of ​​the edge regions is 0.90 or more and 1.10 or less.

4. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the width of said central region is 20% to 80% of the overall width of said negative electrode mixture layer.

5. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the thickness of said edge regions gradually decreases toward said central region.

6. A negative electrode for a non-aqueous electrolyte secondary battery used in a wound-type electrode body, comprising: a long negative electrode core; and a negative electrode mixture layer provided on the negative electrode core, wherein the negative electrode mixture layer is divided into a central region located at the center of the negative electrode core in the width direction, and end regions located on both ends of the negative electrode core relative to the central region in the width direction, wherein, when the battery is at a 100% charging rate, the thickness (A) of the central region is smaller than the thickness (B) of the end regions to form recesses, and the thickness difference (C) between the central region and the end regions is 10 μm or less, and the recesses have a volume such that the ratio (X / Z) of the width (X) of the central region to the overall width (Z) of the negative electrode mixture layer multiplied by the ratio (C / B) of the thickness difference (C) between the central region and the end regions to the thickness (B) of the end regions is 0.3 or more, a width (X) of the central region that is 20% to 80% of the overall width (Z) of the negative electrode mixture layer; 7. A non-aqueous electrolyte secondary battery comprising: the negative electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6; a positive electrode; and a non-aqueous electrolyte.

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