Negative electrode for nonaqueous electrolyte secondary battery, nonaqueous electrolyte secondary battery, and battery pack

Abstract

In the present invention, a negative electrode (12) comprises: a long negative electrode core body (30); and a negative electrode mixture layer (31), which contains, as a negative electrode active material, a first graphite species having a particle interior porosity of 5% or less and a second graphite species having a particle interior porosity of 8 to 20%, and which is provided on the negative electrode core body (30). The negative electrode (12) constitutes a wound electrode body (14). The negative electrode mixture layer (31) is divided into a central region (33) positioned in the widthwise central region of the negative electrode core (30), and end regions (34) positioned closer to both widthwise end sides of the negative electrode core (30) than the central region (33). The ratio (α / β) of the porosity of the central region (33) (α) to the porosity of the end region (34) (β) is greater than 1.0 and 2.0 or less.

Description

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

[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, etc., such as in-vehicle applications and power storage applications. Since the negative electrode, a major component of a battery, significantly affects these performances, many studies have been conducted on the negative electrode. For example, Patent Document 1 discloses a negative electrode in which two types of graphite are added to a negative electrode mixture layer, and when the negative electrode mixture layer is divided into two equal parts in the thickness direction, a large amount of graphite particles with a small internal porosity are added to a surface region of the mixture layer.

[0003] International Publication No. 2019 / 239652

[0004] However, as a result of investigations by the present inventors, it has been found that in a high-capacity non-aqueous electrolyte secondary battery equipped with a wound-type electrode assembly, repeated rapid charging and discharging at a large current causes the potential within the negative electrode to become non-uniform, accelerating deterioration of the battery's performance. An object of the present disclosure is to provide a negative electrode for a non-aqueous electrolyte secondary battery that can reduce the potential difference within the electrode plate during rapid charging and discharging of the battery while maintaining high capacity.

[0005] The negative electrode for a non-aqueous electrolyte secondary battery according to 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, the negative electrode mixture layer including, as negative electrode active materials, first graphite having an intra-particle porosity of 5% or less and second graphite having an intra-particle porosity of 8% or more and 20% or less, the negative electrode mixture layer being divided into a central region located in the widthwise central portion of the negative electrode core and end regions located on both end sides of the central region in the widthwise direction of the negative electrode core, and the ratio (α / β) of the porosity (α) of the central region to the porosity (β) of the end regions is greater than 1.0 and not more than 2.0.

[0006] 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.

[0007] The negative electrode for a non-aqueous electrolyte secondary battery according to the present disclosure can effectively reduce the potential difference within the electrode plate during rapid charging and discharging of the battery while ensuring high capacity. A non-aqueous electrolyte secondary battery using the negative electrode according to the present disclosure has, for example, high capacity and high durability.

[0008] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention;

[0009] As a result of intensive research into the above-mentioned problem, the inventors have found that when the ratio (α / β) of the porosity (α) of the central portion of the negative electrode to the porosity (β) of the widthwise end portion of the negative electrode is set to more than 1.0 and 2.0 or less, the potential difference within the negative electrode during rapid charge and discharge is specifically reduced. In high-capacity batteries, repeated rapid charge and discharge at large currents is thought to cause uneven potential within the negative electrode, accelerating the deterioration of battery performance. When the potential difference within the negative electrode increases, for example, Li precipitates, increasing the risk of short circuit occurrence and reducing the reliability of the battery.

[0010] Therefore, the inventors succeeded in achieving uniform potential within the negative electrode during rapid charge / discharge by increasing the porosity (α) of the widthwise center of the negative electrode relative to the porosity (β) of the widthwise end portions and controlling the porosity ratio (α / β) of the negative electrode mixture layer within a specific range. A uniform potential difference within the negative electrode means that the electrolyte is distributed throughout the entire negative electrode, resulting in a uniform battery reaction throughout the entire negative electrode. As a result, battery degradation is suppressed and battery durability is improved.

[0011] 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, a nonaqueous electrolyte secondary battery using the negative electrode, and a battery pack using the nonaqueous electrolyte secondary battery 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.

[0012] 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.

[0013] A battery pack as an example of an embodiment is an assembled battery configured using a plurality of nonaqueous electrolyte secondary batteries, and is also called a battery module. The plurality of nonaqueous electrolyte secondary batteries that make up the battery pack are electrically connected to each other and housed in a predetermined case. The plurality of nonaqueous electrolyte secondary batteries may be connected in series or in parallel. The battery pack as an example of an embodiment is configured, for example, by connecting a plurality of battery groups connected in series in parallel.

[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. As shown in FIG. 2, the negative electrode 12 has a long 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. 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 the surface of the negative electrode core 30. The central region 33 and the end regions 34 of the negative electrode mixture layer 31 are preferably formed using different negative electrode mixture slurries.

[0027] As will be described in detail later, the negative electrode mixture layer 31 includes, as negative electrode active materials, a first graphite having an intraparticle porosity of 5% or less and a second graphite having an intraparticle porosity of 8% to 20% or less, and the ratio (α / β) of the porosity (α) of the central region 33 to the porosity (β) of the end region 34 is greater than 1.0 and less than 2.0. This allows the electrolyte to be distributed throughout the negative electrode 12 even during rapid charging and discharging, effectively suppressing the occurrence of uneven liquid. As a result, a uniform battery reaction can be achieved throughout the negative electrode 12, and the potential difference within the negative electrode 12 is reduced. The use of the negative electrode 12 suppresses battery degradation and improves battery durability.

[0028] The negative electrode core 30 can be made of a foil of a metal that is 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 or more and 20 μm or less, or 7 μm or more and 15 μm or less. The negative electrode mixture layer 31 contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core 30.

[0029] The thickness of the negative electrode mixture layer 31 is substantially constant throughout the entire region, including the central region 33 and the end region 34. The thickness of the negative electrode mixture layer 31 is preferably 50 μm or more and 100 μm or less, or 60 μm or more and 90 μm or less, on one side of the negative electrode core 30. The configuration of the negative electrode mixture layer 31 is substantially the same on both sides of the negative electrode core 30. The thicknesses of the negative electrode 12 and the negative electrode mixture layer 31 are measured by observing a cross section of the negative electrode using a scanning electron microscope (SEM).

[0030] The binder contained in the negative electrode mixture layer 31 can be, as in the case of the positive electrode 11, fluororesin, olefin resin, PAN, polyimide, polyamide, acrylic resin, etc., but polyvinyl acetate, styrene-butadiene rubber (SBR), etc. 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. The negative electrode mixture layer 31 preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. 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. The negative electrode mixture layer 31 may also contain the same conductive agent as the positive electrode mixture layer.

[0031] The negative electrode mixture layer 31 preferably contains a Si-containing material in addition to graphite as the negative electrode active material. The combined use of graphite and a Si-containing material facilitates achieving both high capacity and high durability of the battery. Furthermore, by appropriately using the Si-containing material, it becomes easier to adjust the capacity per unit area to the same level when the difference in porosity between the central region 33 and the edge regions 34 is increased. The content of the Si-containing material may be the same in the central region 33 and the edge regions 34, but is preferably higher in the central region 33 than in the edge regions 34. Examples of Si-containing materials that function as negative electrode active materials include Si alloys, Si compounds, and Si-containing composite materials. Among these, Si-containing composite materials are preferred.

[0032] 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.

[0033] 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.

[0034] As described above, the negative electrode 12 is manufactured through a first step of forming the negative electrode mixture layer 31 on the long negative electrode core 30, and a second step of compressing the negative electrode mixture layer 31. In the first step, the negative electrode mixture layer 31 is divided into a central region 33 and an end region 34, and the negative electrode mixture layer 31 is formed using, as the negative electrode active material, graphite A having a low intraparticle porosity (for example, an average intraparticle porosity of 3% or more and 5% or less) and graphite B having a high intraparticle porosity (for example, an average intraparticle porosity of 10% or more and 30% or less) such that the ratio (α / β) of the porosity (α) of the central region 33 to the porosity (β) of the end region 34 is more than 1.0 and 2.0 or less.

[0035] Graphite A, which has a low intra-particle porosity, is harder than graphite B, which has a high particle porosity, and is less likely to be crushed during the compression process of the anode mixture layer 31. Therefore, by adding a large amount of graphite A to the anode mixture slurry that forms the central region 33, voids between the graphite particles in the central region 33 are more easily secured, and the porosity (α) of the central region 33 can be increased. On the other hand, graphite B is more likely to be crushed during the compression process of the anode mixture layer 31, resulting in smaller voids between the graphite particles. Therefore, by adding a large amount of graphite B to the anode mixture slurry that forms the edge region 34, the porosity (β) of the edge region 34 decreases.

[0036] The negative electrode mixture layer 31 contains, as graphite, a first graphite having an intraparticle porosity of 5% or less and a second graphite having an intraparticle porosity of 8% to 20%. For example, the first graphite corresponds to the above-mentioned graphite A, and the second graphite corresponds to the above-mentioned graphite B. Although the central region 33 may contain only the first graphite and the edge region 34 may contain only the second graphite, it is preferable that both the first graphite and the second graphite are contained in each region. In this case, the porosity (α) of the central region 33 is higher than the porosity (β) of the edge region 34, making it easier to control the ratio (α / β) to a desired value. In one preferred embodiment, the content of the first graphite in the central region 33 is higher than the content of the first graphite in the edge region 34. Furthermore, the content of the second graphite in the central region 33 is lower than the content of the second graphite in the edge region 34. The negative electrode mixture layer 31 may contain graphite with an intraparticle porosity of more than 5% and less than 8%, or graphite with an intraparticle porosity of more than 20%, as long as the object of the present disclosure is not impaired.

[0037] The intraparticle porosity of graphite is a two-dimensional value calculated from the ratio of the area of ​​voids inside the graphite particle to the total cross-sectional area of ​​the graphite particle. The intraparticle porosity of graphite can be calculated by the following procedure.

[0038] <Method for Measuring Intra-Particle Porosity> (1) A graphite particle cross-section is prepared. For example, a method for preparing a particle cross-section includes cutting out a portion of the negative electrode 12 and processing it with an ion milling device (e.g., IM4000PLUS, manufactured by Hitachi High-Technologies Corporation) to expose the cross-section of the negative electrode mixture layer 31. (2) A backscattered electron image of the particle cross-section is captured using an SEM. The backscattered electron image is captured at a magnification of 3000x to 5000x. (3) The obtained cross-sectional image is imported into a computer and binarized using image analysis software (e.g., ImageJ, manufactured by the National Institutes of Health, USA). A binarized image is obtained in which the particle cross-sections in the cross-sectional image are colored black and the voids present in the particle cross-section are colored white. (4) From the binarized image, graphite particles having a particle diameter of 5 μm to 50 μm are selected, and the area of ​​the graphite particle cross-section and the area of ​​the internal voids present in the graphite particle cross-section are calculated. Here, the cross-sectional area of ​​a graphite particle refers to the area of ​​the region surrounded by the outer periphery of the graphite particle, i.e., the area of ​​the entire cross-sectional portion of the graphite particle. From the calculated cross-sectional area of ​​the graphite particle and the area of ​​the internal voids, the intra-particle porosity of the graphite particle (area of ​​the internal voids in the cross-section of the graphite particle × 100 / area of ​​the cross-section of the graphite particle) is calculated.

[0039] In one embodiment, artificial graphite can be used as the first graphite (graphite A), and natural graphite can be used as the second graphite (graphite B). The first graphite and the second graphite can be produced, for example, by crushing coke (precursor), which is the main raw material, to a predetermined size, agglomerating the crushed precursor with a binder, and then calcining the agglomerated precursor at a temperature of 2400°C or higher to graphitize it. The intraparticle porosity can be adjusted by adjusting the particle size of the crushed precursor, the particle size of the agglomerated precursor, the type and amount of binder, and the like. When a portion of the binder added to the precursor volatilizes during calcination, the binder can be used as a volatile component to form voids. For example, pitch can be used as the binder.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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 ｘThe 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.

[0044] 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 O ５ It is preferable that (Z=1 / 2) is used as the main component.

[0045] 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.

[0046] 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.

[0047] As described above, the negative electrode mixture layer 31 is configured so that the ratio (α / β) of the porosity (α) of the central region 33 to the porosity (β) of the edge region 34 is greater than 1.0 and not greater than 2.0. Note that as the ratio (α / β) increases, the capacity per unit area of ​​each region differs. In this case, it is necessary to control the charge / discharge of the battery according to the region with the lower capacity in order to prevent defects such as Li deposition, which is expected to significantly reduce the energy density of the battery. For this reason, as will be described in detail later, it is preferable to reduce the difference in capacity per unit area between each region. When the ratio (α / β) is large, for example, it is preferable to change the content of the Si-containing material between the central region 33 and the edge region 34, making it higher in the central region 33 than in the edge region 34.

[0048] The voids in the negative electrode mixture layer 31 mainly include voids within the graphite particles and voids between particles of the negative electrode active material. The porosity of each region of the negative electrode mixture layer 31 is a two-dimensional value calculated from the ratio of the void area to the total cross-sectional area of ​​each region including the voids, and can be measured in the same manner as the intra-particle porosity described above. The specific method for calculating the porosity of each region is as follows. The porosity of each region can also be calculated from the true density of the material constituting each region, the mass per unit volume of each region, and the thickness of each region.

[0049] <Method for Measuring the Porosity of the Central Region 33 and the Edge Regions 34> (1) A cross section of the negative electrode mixture layer 31 is exposed using an ion milling device, and a backscattered electron image of the exposed cross section of the negative electrode mixture layer 31 is taken at a magnification of 800x. (2) The obtained cross-sectional image is imported into a computer, and the central region 33 and the edge region 34 are cut out. (3) The cut-out cross-sectional image is subjected to a binarization process using image analysis software, and a binarized image is obtained in which the cross sections of the negative electrode active material particles in the cross-sectional image are converted to white, and the inter-particle voids and intra-particle voids are converted to black. (4) The porosity is calculated from the ratio of the white and black areas.

[0050] The porosity ratio (α / β) of the central region 33 to the edge region 34 is preferably 1.2 or more and 2.0 or less, more preferably 1.3 or more and 2.0 or less, and particularly preferably 1.4 or more and 2.0 or less. When the ratio (α / β) is within this range, the above-mentioned effect becomes more pronounced. That is, a high capacity can be maintained, and even when rapid charging and discharging at a large current is repeated, the electrolyte is distributed throughout the negative electrode 12, and a uniform battery reaction occurs throughout the negative electrode 12. Even if the ratio (α / β) is increased beyond 2.0, the effect of reducing the potential difference is not significantly increased and a decrease in capacity occurs. Therefore, the upper limit of the ratio (α / β) is set to 2.0.

[0051] As long as the ratio (α / β) of the porosities of the central region 33 and the edge regions 34 is within the above range, the porosity of each region is not particularly limited. However, the porosity (β) of the edge regions 34 is preferably 10% to 50%, more preferably 15% to 45%, and particularly preferably 20% to 40%. The porosity (α) of the central region 33 is greater than the porosity (β) of the edge regions 34, and is preferably 20% to 60%, more preferably 25% to 55%, and particularly preferably 30% to 50%. The density of the central region 33 is lower than that of the edge regions 34. The ratio of the density of the central region 33 to the density of the edge regions 34 is, for example, 0.5 to less than 1.0.

[0052] The width of the central region 33 is preferably 20% to 80% of the overall width 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 of the central region 33 is preferably 20% to 45% of the overall width of the negative electrode mixture layer 31, and more preferably 25% to 40%. If the width of the central region 33 is within this range, the potential difference between the central region 33 and the end regions 34 can be more effectively reduced while avoiding problems such as a decrease in capacity.

[0053] 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 central region 33 and the end regions 34 have substantially the same thickness.

[0054] 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 widths of the first region 35 and the second region 36 of the end region 34 may be different, but in this embodiment, the widths of the first region 35 and the second region 36 are substantially the same. The widths of the first region 35 and the second region 36 are 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.

[0055] 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, it becomes easier to uniformize the potential within the negative electrode 12 during rapid charge and discharge.

[0056] The central region 33 and the end regions 34 are formed, for example, with a substantially constant width and a 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 longitudinal direction 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.

[0057] 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 preferably 0.90 to 1.10, more preferably 0.95 to 1.05. As described above, when the capacities per unit area of ​​each region are different, it is necessary to control the charge and discharge of the battery according to the region with the lower capacity in order 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. Therefore, it is preferable to keep the difference in capacity per unit area between each region to 10% or less.

[0058] When the ratio (α / β) of the porosity of the central region 33 to the edge region 34 is 1.2 or more, it is preferable to change the content of the Si-containing material between the central region 33 and the edge region 34, making it higher in the central region 33 than in the edge region 34. The content of the Si-containing material is adjusted, for example, so that the difference in capacity per unit area between the regions is 10% or less. When the ratio (α / β) is 1.2 or more, it is preferable that the content of the Si-containing material in the central region 33 be 1.1 to 2.0 times, or 1.2 to 1.7 times, the content of the Si-containing material in the edge region 34.

[0059] For example, the negative electrode 12 has a potential of 0 V or more and 0.5 V or less relative to the oxidation-reduction potential of lithium when the state of charge (SOC) of the nonaqueous electrolyte secondary battery 10 is 100%.

[0060] [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.

[0061] 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.

[0062] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0063] 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.

[0064] [Preparation of First Negative Electrode Mixture Slurry] As the negative electrode active material, artificial graphite A (first graphite) having an intraparticle porosity of 6% and a D50 of 23 μm, natural graphite B (second graphite) having an intraparticle porosity of 30% and a D50 of 23 μm, and SiO ｘThe negative electrode active material, a dispersion of styrene butadiene rubber, and sodium carboxymethyl cellulose were mixed in a solid content mass ratio of 98:1:1 using water as a dispersion medium to prepare a first negative electrode mixture slurry.

[0065] [Preparation of Second Negative Electrode Mixture Slurry] A second negative electrode mixture slurry was prepared in the same manner as the first negative electrode mixture slurry, except that a mixture of graphite A, graphite B, and a Si-containing material in a mass ratio of 54.8:39.2:5 was used as the negative electrode active material.

[0066] [Negative Electrode Fabrication] A negative electrode core made of a long copper foil with a thickness of 8 μm and a width of 64 mm was coated with first and second negative electrode mixture slurries on both sides of the core, leaving first and second core exposed portions at both ends in the longitudinal direction. The coating was then dried and compressed to obtain a negative electrode with a negative electrode mixture layer formed on both sides of the negative electrode core. The first negative electrode mixture slurry was applied to the widthwise center of the negative electrode core, and the second negative electrode mixture slurry was applied to both widthwise ends of the negative electrode core. This resulted in a negative electrode mixture layer including a central region and an end region with different porosities. The width of the central region was 47% (30 mm) of the total width of the negative electrode mixture layer. The two end regions were formed to have the same width (26.5% of the total width of the negative electrode mixture layer). A nickel negative electrode lead was ultrasonically welded to the first exposed portion.

[0067] The porosity α of the central region of the negative electrode mixture layer was 50%, and the porosity β of the edge region was 25%. The average intraparticle porosity of artificial graphite A was 5%, and the average intraparticle porosity of natural graphite B was 15% (the same applies to the following Examples and Comparative Examples). The negative electrode mixture layer contains a first graphite (artificial graphite A) with an intraparticle porosity of 5% or less and a second graphite (natural graphite B) with an intraparticle porosity of 8% or more and 20% or less, with the content of the first graphite being higher in the central region than in the edge region. The porosity is measured by the above-mentioned method. The porosity ratio (α / β) is 2.0.

[0068] [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.

[0069] [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.

[0070] Examples 2 to 15 Negative electrodes and cylindrical batteries were produced in the same manner as in Example 1, except that in preparing the first negative electrode mixture slurry for forming the central region of the negative electrode mixture layer and the second negative electrode mixture slurry for forming the end regions, the mass ratios of graphite A, graphite B, and the Si-containing material were set to the values ​​shown in Table 1, and the porosities of the central region and the end regions were adjusted to the values ​​shown in Table 1.

[0071] Comparative Examples 1 to 8 Negative electrodes and cylindrical batteries were produced in the same manner as in Example 1, except that in preparing the first negative electrode mixture slurry for forming the central region of the negative electrode mixture layer and the second negative electrode mixture slurry for forming the end regions, the mass ratios of graphite A, graphite B, and the Si-containing material were set to the values ​​shown in Table 1, and the porosities of the central region and the end regions were adjusted to the values ​​shown in Table 1.

[0072] 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, along with the mass ratios and porosity of graphite A, graphite B, and the Si-containing material in the central region and edge region.

[0073] [Measurement of Battery Capacity] Each battery of the Examples and Comparative Examples was charged at a constant current of 0.2 C relative to the design capacity up to 4.2 V in a temperature environment of 25°C, and then constant voltage charged to a current of 0.02 C. After that, the battery was rested for 20 minutes. After the rest, the capacity when the battery was discharged at a current of 0.2 C to 2.5 V was taken as the battery capacity. The battery capacities shown in Table 1 are relative values ​​when the capacity of the battery of Example 1 is taken as 100.

[0074] [Measurement of Potential Difference] 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. Thereafter, without a rest period, the battery was discharged at a constant current of 0.5 C until the battery voltage reached 2.5 V. After 300 cycles of this charge / discharge, the battery was disassembled at an SOC of 10%, and the negative electrode was removed. The potential values ​​relative to the redox potential of metallic lithium were obtained at positions divided into three widthwise sections, each of which was located at a length equivalent to 10% of the total width of the negative electrode from both ends of the widthwise section. The potential difference was calculated by subtracting the potential value at the center from the higher potential value at both ends.

[0075]

[0076] As shown in Table 1, the batteries of the Examples have a smaller potential difference between both ends and the center of the negative electrode in the width direction than the batteries of Comparative Examples 3 to 8, and it can be seen that this makes it possible to achieve uniform potential within the negative electrode during rapid charge and discharge. The batteries of Comparative Examples 1 and 2 have a small potential difference, but their capacity is significantly reduced. The batteries of the Examples can effectively reduce the potential difference (liquid unevenness) within the negative electrode during rapid charge and discharge while maintaining a high capacity.

[0077] The present disclosure is further described by the following embodiments. Configuration 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 including, as negative electrode active materials, a first graphite having an intraparticle porosity of 5% or less and a second graphite having an intraparticle porosity of 8% to 20%, the negative electrode mixture layer being divided into a central region located at 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 ratio (α / β) of the porosity (α) of the central region to the porosity (β) of the end regions being greater than 1.0 and not greater than 2.0. Configuration 2: The negative electrode for a non-aqueous electrolyte secondary battery according to Configuration 1, wherein the content of the second graphite is higher in the central region than in the end regions. Configuration 3: The negative electrode for a non-aqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the negative electrode mixture layer contains a Si-containing material as the negative electrode active material, and the content of the Si-containing material is higher in the central region than in the edge regions.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 ratio (α / β) of the porosity (α) of the central region to the porosity (β) of the edge regions is 1.2 to 2.0.Configuration 6: A non-aqueous electrolyte secondary battery comprising the negative electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 5, a positive electrode, and a non-aqueous electrolyte.Configuration 7: A battery pack comprising a plurality of the non-aqueous electrolyte secondary batteries according to Configuration 6.

[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, the negative electrode mixture layer including, as negative electrode active materials, first graphite having an intra-particle porosity of 5% or less and second graphite having an intra-particle porosity of 8% or more and 20% or less, wherein the negative electrode mixture layer is divided into a central region located in the widthwise center of the negative electrode core and end regions located on both ends of the negative electrode core relative to the central region in the widthwise direction, and wherein the ratio (α / β) of the porosity (α) of the central region to the porosity (β) of the end regions is greater than 1.0 and less than 2.

0.

2. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the content of said second graphite is higher in said central region than in said edge regions.

3. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode mixture layer contains a Si-containing material as the negative electrode active material, and the content of the Si-containing material is higher in the central region than in the edge regions.

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 ratio (α / β) of the porosity (α) of the central region to the porosity (β) of the edge regions is 1.2 or more and 2.0 or less.

6. 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 5; a positive electrode; and a non-aqueous electrolyte.

7. A battery pack comprising a plurality of non-aqueous electrolyte secondary batteries according to claim 6.

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