Nonaqueous electrolyte secondary battery and method for manufacturing negative electrode for nonaqueous electrolyte secondary battery

Abstract

This negative electrode (12) has a negative electrode current collector (40) and a negative electrode mixture layer (41) disposed on the surface of the negative electrode current collector (40), wherein the negative electrode mixture layer (41) contains, as a negative electrode active material, graphite particles A having an internal porosity of 5% or less and graphite particles B having an internal porosity of 8-20%, and the graphite particles A are contained in a larger amount in a region on the winding inner end side than in a region on the winding outer end side when the negative electrode mixture layer 41 is divided into two equal parts in the lengthwise direction.

Description

Nonaqueous electrolyte secondary battery and method for manufacturing negative electrode for nonaqueous electrolyte secondary battery

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery and a method for manufacturing a negative electrode for a non-aqueous electrolyte secondary battery.

[0002] Non-aqueous electrolyte secondary batteries are widely used as high-energy density secondary batteries. The negative electrode of a non-aqueous electrolyte secondary battery is composed of a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. Graphite is generally used as the negative electrode active material contained in the negative electrode mixture layer. Patent Document 1 discloses a negative electrode in which, in the thickness direction of the negative electrode mixture layer, more graphite particles with low internal porosity are arranged on the outer surface side of the negative electrode than on the negative electrode current collector side, in order to improve cycle characteristics.

[0003] International Publication No. 2019 / 239652

[0004] In recent years, with the spread of electric vehicles and other factors, there has been an increasing demand for improved cycle characteristics of non-aqueous electrolyte secondary batteries. In non-aqueous electrolyte secondary batteries equipped with an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, repeated rapid charge / discharge cycles can result in a decrease in the amount of non-aqueous electrolyte retained on the inner end side of the negative electrode, resulting in a deterioration in cycle characteristics. Patent Document 1 does not consider the amount of non-aqueous electrolyte retained on the inner end side of the negative electrode, and there is still room for improvement.

[0005] A nonaqueous electrolyte secondary battery according to one aspect of the present disclosure is a nonaqueous electrolyte secondary battery including an electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound longitudinally with a separator interposed therebetween, a nonaqueous electrolyte, and an exterior housing that accommodates the electrode assembly and the nonaqueous electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer disposed on the surface of the negative electrode current collector, the negative electrode mixture layer including, as negative electrode active materials, graphite particles A having an internal porosity of 5% or less and graphite particles B having an internal porosity of 8% or more and 20% or less, and wherein, when the negative electrode mixture layer is divided into two equal parts longitudinally, the graphite particles A are contained in a larger amount in a region closer to the inner end of the winding than in a region closer to the outer end of the winding.

[0006] Further, a manufacturing method of a negative electrode for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a manufacturing method of a negative electrode for a non-aqueous electrolyte secondary battery having a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector, the manufacturing method including: a first preparation step of preparing a first negative electrode mixture slurry containing, as a negative electrode active material, graphite particles A having an internal porosity of 5% or less in greater amount than graphite particles B having an internal porosity of 8% or more and 20% or less; a second preparation step of preparing a second negative electrode mixture slurry containing, as a negative electrode active material, graphite particles B in greater amount than graphite particles A; and a coating step of coating the first negative electrode mixture slurry and the second negative electrode mixture slurry onto the surface of the negative electrode current collector, wherein, when the negative electrode mixture layer is divided into two equal parts in the longitudinal direction, the first negative electrode mixture slurry is applied more to a region closer to the inner end of winding than to a region closer to the outer end of winding in the coating step.

[0007] According to the nonaqueous electrolyte secondary battery of one aspect of the present disclosure, cycle characteristics can be improved.

[0008] Fig. 1 is an axial cross-sectional view of a nonaqueous electrolyte secondary battery that is an example of an embodiment; Fig. 2 is a perspective view of an electrode body included in the nonaqueous electrolyte secondary battery that is an example of an embodiment, showing a part of the outer end side of the winding in an unfolded state; Fig. 3 is a front view showing, in an unfolded state, a negative electrode that constitutes an electrode body included in the nonaqueous electrolyte secondary battery that is an example of an embodiment; Fig. 4 is a schematic view showing a cross section of a graphite particle contained in a negative electrode mixture layer included in the nonaqueous electrolyte secondary battery that is an example of an embodiment; Fig. 5 is a front view showing, in an unfolded state, a negative electrode that constitutes an electrode body included in the nonaqueous electrolyte secondary battery that is another example of an embodiment; Fig. 6 is a diagram showing the correlation between the position in the longitudinal direction of the negative electrode included in the nonaqueous electrolyte secondary battery that is another example of an embodiment and the content of graphite particles A;

[0009] Hereinafter, an example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. In the following description, specific shapes, materials, numerical values, directions, etc. are examples for facilitating understanding of the present invention and can be appropriately changed according to the specifications of the nonaqueous electrolyte secondary battery. Furthermore, when the following description includes multiple embodiments and modified examples, it is initially assumed that the characteristic portions thereof will be used in appropriate combination.

[0010] Fig. 1 is an axial cross-sectional view of an example of a nonaqueous electrolyte secondary battery 10. As shown in Fig. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte (not shown), and an exterior body 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte.

[0011] As will be described in detail later, the electrode assembly 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0012] The non-aqueous electrolyte has lithium ion conductivity. 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. The electrolyte salt may include, for example, LiPF ６ Lithium salts such as

[0013] The exterior body 16 is a cylindrical metal container with a bottom and an opening at one axial end, and the opening of the exterior body 16 is closed by a sealing body 17. For ease of explanation, the sealing body 17 side of the nonaqueous electrolyte secondary battery 10 will be referred to as the "top" and the bottom side of the exterior body 16 will be referred to as the "bottom" below.

[0014] 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 side of the exterior body 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the exterior body 16 by welding or the like, and the exterior body 16 serves as the negative electrode terminal.

[0015] A gasket 28 is provided between the exterior body 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior body 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior body 16, and supports the sealing body 17 on its upper surface. The sealing body 17 is fixed to the top of the exterior body 16 by the grooved portion 22 and the open end of the exterior body 16 that is crimped to the sealing body 17.

[0016] 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 through a vent hole in the cap 27.

[0017] Next, the electrode assembly 14 will be described in detail with further reference to Fig. 2. Fig. 2 is a perspective view of the electrode assembly 14, showing a part of the outer end side of the wound electrode assembly 14 in a developed state.

[0018] 1 and 2 , the electrode body 14 has a structure in which a strip-shaped positive electrode 11 and a strip-shaped negative electrode 12 are wound in the longitudinal direction with a separator 13 interposed therebetween. The positive electrode 11, the negative electrode 12, and the separator 13 are spirally wound so that they are alternately stacked in the radial direction of the electrode body 14. In the electrode body 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the width direction of the positive electrode 11 and the negative electrode 12 is the axial direction.

[0019] The positive electrode 11 has a positive electrode current collector 30 and a positive electrode mixture layer 31 disposed on the positive electrode current collector 30. The positive electrode current collector 30 can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on its surface. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent, and a binder. 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, a binder, etc., onto the positive electrode current collector 30, drying the coating, and then compressing it to form the positive electrode mixture layer 31 on both sides of the positive electrode current collector 30.

[0020] The positive electrode mixture layer 31 contains a particulate lithium-containing composite oxide as a positive electrode active material. The lithium-containing composite oxide is a composite oxide containing metal elements such as Co, Mn, Ni, and Al in addition to Li. The metal element constituting the lithium-containing composite oxide is, for example, at least one selected from Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, and Bi. Among these, it is preferable to contain at least one selected from Co, Ni, Al, and Mn. Examples of suitable composite oxides include a lithium-containing composite oxide containing Ni, Co, and Mn, and a lithium-containing composite oxide containing Ni, Co, and Al.

[0021] Examples of the conductive agent contained in the positive electrode mixture layer 31 include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, and other carbon materials. Examples of the binder contained in the positive electrode mixture layer 31 include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, and the like. Furthermore, these resins may be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), and the like.

[0022] As will be described in detail later, the negative electrode 12 has a negative electrode current collector 40 and a negative electrode mixture layer 41 disposed on the negative electrode current collector 40. The negative electrode current collector 40 may be a foil of a metal such as copper or a copper alloy that is stable within the potential range of the negative electrode 12, or a film having such a metal disposed on its surface layer. The negative electrode mixture layer 41 contains a negative electrode active material, a binder, and, if necessary, a conductive agent.

[0023] 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. Furthermore, a highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

[0024] In this embodiment, the negative electrode mixture layer 41 is not formed on the outermost periphery of the electrode body 14, so that the negative electrode current collector 40 is exposed, and a first current collector exposed portion 44 is disposed in contact with the inner surface of the exterior body 16. By providing the negative electrode lead 21 and by contacting the first current collector exposed portion 44 with the inner surface of the exterior body 16, the internal resistance of the battery can be further reduced. Note that a separator 13 may be disposed on the outermost periphery of the electrode body 14.

[0025] Next, the negative electrode 12 constituting the electrode body 14 will be described in detail with further reference to Figures 3 and 4. Figure 3 is a front view showing the negative electrode 12 in a developed state, and Figure 4 is a diagram schematically showing a cross section of a graphite particle 50 contained in a negative electrode mixture layer 41.

[0026] 1 and 3 , the negative electrode 12 has a negative electrode current collector 40 and a negative electrode mixture layer 41 disposed on at least one surface of the negative electrode current collector 40. In this embodiment, the negative electrode mixture layer 41 is disposed on both surfaces of the negative electrode current collector 40.

[0027] The negative electrode 12 has a first current collector exposed portion 44 where the negative electrode mixture layer 41 is not formed and the negative electrode current collector 40 is exposed, and which abuts against the inner surface of the exterior body 16, and a second current collector exposed portion 45 where the negative electrode mixture layer 41 is not formed and the negative electrode current collector 40 is exposed, and to which the negative electrode lead 21 is joined. Note that the negative electrode 12 may have a region at an inner winding end 12A of the negative electrode 12 where the negative electrode mixture layer 41 is not formed and the negative electrode current collector 40 is exposed.

[0028] The first current collector exposed portion 44 is provided at the outer winding end 12B of the negative electrode 12. The first current collector exposed portion 44 is formed, for example, over a range of a length of at least one turn and at most two turns from the outer winding end 12B of the negative electrode 12. The first current collector exposed portion 44 may be provided only on the surface of the negative electrode current collector 40 that faces the inner surface of the outer casing 16. In this case, the outer winding end 41B of the negative electrode mixture layer 41 coincides with the outer winding end 12B of the negative electrode 12. Note that the negative electrode 12 does not necessarily have to have the first current collector exposed portion 44. In other words, the negative electrode mixture layer 41 may be formed up to the outer winding end 12B of the negative electrode 12.

[0029] The second current collector exposed portion 45 is provided between the inner winding end 41A and the outer winding end 41B of the negative electrode mixture layer 41 in the longitudinal direction of the negative electrode mixture layer 41. In the example shown in Fig. 3, the second current collector exposed portion 45 is provided at a position that divides the negative electrode mixture layer 41 into two equal parts in the longitudinal direction. The second current collector exposed portion 45 may be formed on only one surface of the negative electrode current collector 40, or may be formed on both surfaces of the negative electrode current collector 40.

[0030] As will be described in more detail below, in this embodiment, the constituent components of the negative electrode active material contained in the negative electrode mixture layer 41 differ across the second current collector exposed portion 45. Specifically, the first negative electrode mixture layer 42 located closer to the inner winding end 41A than the second current collector exposed portion 45 contains more graphite particles with a smaller internal porosity than the second negative electrode mixture layer 43 located closer to the outer winding end 41B than the second current collector exposed portion 45.

[0031] When the length along the longitudinal direction from the inner winding end 41A to the outer winding end 41B of the negative electrode mixture layer 41 is defined as L, the second current collector exposed portion 45 is preferably provided within a length range of 0.25L or more and 0.60L or less from the inner winding end 41A of the negative electrode mixture layer 41. In this case, the effect of improving the cycle characteristics described below becomes significant.

[0032] The negative electrode mixture layer 41 contains at least graphite particles as a negative electrode active material. Examples of the graphite particles include natural graphite such as massive graphite and amorphous graphite, and artificial graphite such as massive artificial graphite and graphitized mesophase carbon microbeads. However, from the viewpoint of ease of adjusting the internal porosity, which will be described later, it is preferable that the graphite particles contain artificial graphite.

[0033] As shown in FIG. 4 , graphite particle 50 has, in a cross-sectional view of graphite particle 50, closed voids (hereinafter referred to as internal voids) 52 that do not connect from the inside of the particle to the particle surface, and voids (hereinafter referred to as external voids) 54 that connect from the inside of the particle to the particle surface.

[0034] Graphite particles 50 of the present embodiment include graphite particles A having an internal porosity of 5% or less and graphite particles B having an internal porosity of 8% or more and 20% or less. Here, the internal porosity of graphite particles 50 is a two-dimensional value calculated from the ratio of the area of ​​internal voids 52 of graphite particles 50 to the cross-sectional area of ​​graphite particles 50, and can be calculated by the following procedure.

[0035] <Method for Measuring Internal Porosity> (1) Exposing the cross section of the negative electrode 12. For example, a method for exposing the 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 12. (2) Using a scanning electron microscope, a backscattered electron image of the exposed cross section of the negative electrode 12 is captured. The backscattered electron image is captured at a magnification of 3,000 to 5,000 times. (3) The cross-sectional image obtained above 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 particle cross sections in the cross-sectional image are colored black and voids present in the particle cross sections are colored white. (4) From the binarized image, graphite particles 50 are selected, and the cross-sectional area of ​​the graphite particles 50 and the area of ​​the internal voids 52 present in the cross sections of the graphite particles 50 are calculated. Here, the cross-sectional area of ​​graphite particle 50 refers to the area of ​​the region surrounded by the outer periphery of graphite particle 50, i.e., the area of ​​the entire cross-sectional portion of graphite particle 50. Furthermore, for voids present in the cross-section of graphite particle 50 that have a width of 3 μm or less, it may be difficult to distinguish between internal voids 52 and external voids 54 in image analysis, so voids with a width of 3 μm or less may be considered internal voids 52. Then, from the calculated cross-sectional area of ​​graphite particle 50 and the area of ​​internal voids 52 in the cross-section of graphite particle 50, the internal porosity of graphite particle 50 (area of ​​internal voids in the cross-section of graphite particle × 100 / area of ​​cross-section of graphite particle) is calculated.

[0036] The graphite particles A and the graphite particles B are produced, for example, as follows.

[0037] <Graphite Particles A> For example, graphite particles A of a desired size are obtained by pulverizing coke (precursor), which is the main raw material, to a predetermined size, agglomerating the coke with a binder, firing the agglomerated coke at a temperature of 2600°C or higher, graphitizing the coke, and then sieving the resulting coke. The internal porosity can be adjusted to 5% or less by adjusting the particle size of the pulverized precursor or the particle size of the agglomerated precursor. The average particle size (median diameter in volume terms, hereinafter sometimes referred to as D50) of the pulverized precursor is preferably in the range of 12 μm or more and 20 μm or less. Furthermore, when the internal porosity is reduced to a range of 5% or less, it is preferable to increase the particle size of the pulverized precursor.

[0038] <Graphite Particles B> For example, coke (precursor), which is the main raw material, is crushed to a predetermined size, agglomerated with a binder, and then press-molded into a block. This block is then fired at a temperature of 2600°C or higher to graphitize it. The graphitized block is crushed and sieved to obtain graphite particles B of the desired size. The internal porosity can be adjusted to 8% or more and 20% or less by adjusting the amount of volatile components added to the block. When a portion of the binder added to the coke (precursor) volatilizes during firing, the binder can be used as a volatile component. Pitch is an example of such a binder.

[0039] As a result of investigations by the present inventors, it was found that repeated rapid charge-discharge cycles over an extended period of time tend to degrade the cycle characteristics. When the battery is charged and discharged, the inner end 41A of the negative electrode mixture layer 41 is subjected to a greater pressure load due to expansion and contraction of the negative electrode mixture layer 41 than the outer end 41B of the negative electrode mixture layer 41, and the nonaqueous electrolyte tends to be extruded. As a result, it is presumed that when rapid charge-discharge cycles are repeated over an extended period of time, the amount of nonaqueous electrolyte retained at the inner end 41A of the negative electrode mixture layer 41 decreases, resulting in a degradation of the cycle characteristics.

[0040] When the negative electrode mixture layer 41 of this embodiment is divided into two equal parts in the longitudinal direction, the graphite particles A are contained in a larger amount in the region closer to the inner winding end 41A than in the region closer to the outer winding end 41B. In other words, the content of graphite particles A relative to the total mass of graphite particles A and graphite particles B is larger in the region closer to the inner winding end 41A than in the region closer to the outer winding end 41B. Therefore, by disposing more graphite particles A on the inner winding end 41A side of the negative electrode mixture layer 41, more voids are formed between the graphite particles, and the amount of nonaqueous electrolyte held in the negative electrode mixture layer 41 increases. As a result, the cycle characteristics can be improved when rapid charge-discharge cycles are repeated over a long period of time.

[0041] On the other hand, the pressure load due to expansion and contraction of the anode mixture layer 41 is smaller on the outer end 41B side of the anode mixture layer 41 than on the inner end 41A side of the anode mixture layer 41, and the nonaqueous electrolyte is less likely to be extruded. Therefore, if a large amount of graphite particles A is arranged in the longitudinal direction of the anode mixture layer 41, the amount of nonaqueous electrolyte retained on the outer end 41B side of the anode mixture layer 41 may increase excessively. This results in significant nonuniformity in the charge / discharge reaction between the inner end 41A side and the outer end 41B side of the anode mixture layer 41. As a result, it may not be possible to sufficiently improve the cycle characteristics when rapid charge / discharge cycles are repeated over a long period of time. Therefore, by arranging a large number of graphite particles A with a small internal porosity on the inner winding end 41A side of the negative electrode mixture layer 41 and a large number of graphite particles B with a large internal porosity on the outer winding end 41B side of the negative electrode mixture layer 41, it is possible to suppress the bias of the reaction inside the electrode body 14 and sufficiently improve the cycle characteristics.

[0042] When the negative electrode mixture layer 41 is divided into two equal parts in the longitudinal direction, the content of graphite particles A relative to the total mass of graphite particles A and graphite particles B in the region of the negative electrode mixture layer 41 on the inner winding end 41A side is preferably 70 mass% or more, and more preferably 75 mass% or more. By making the content of graphite particles A relative to the total mass of graphite particles A and graphite particles B 70 mass% or more, a sufficient amount of voids can be formed between the graphite particles, and the amount of nonaqueous electrolyte retained on the inner winding end 41A side of the negative electrode mixture layer 41 can be further increased. Furthermore, graphite particles B may not be contained in the region of the negative electrode mixture layer 41 on the inner winding end 41A side. That is, the content of graphite particles A relative to the total mass of graphite particles A and graphite particles B may be 100 mass%. Therefore, when the negative electrode mixture layer 41 is divided into two equal parts in the longitudinal direction, the content of the graphite particles A relative to the total mass of the graphite particles A and the graphite particles B in the region on the inner winding end 41A side of the negative electrode mixture layer 41 is preferably 70 mass% or more and 100 mass% or less, and more preferably 75 mass% or more and 100 mass% or less.

[0043] Furthermore, when the negative electrode mixture layer 41 is divided into two equal parts in the longitudinal direction, the content of graphite particles A relative to the total mass of graphite particles A and graphite particles B in the region of the negative electrode mixture layer 41 on the outer winding end 41B side is preferably 30 mass% or less, and more preferably 25 mass% or less. In other words, the content of graphite particles B relative to the total mass of graphite particles A and graphite particles B in the region of the negative electrode mixture layer 41 on the outer winding end 41B side is preferably 70 mass% or more, and more preferably 75 mass% or more. By setting the content of graphite particles A relative to the total mass of graphite particles A and graphite particles B to 30 mass% or less, non-uniformity of charge / discharge reactions within the electrode body 14 can be further suppressed. Furthermore, graphite particles A may not be contained in the region of the negative electrode mixture layer 41 on the outer winding end 41B side. That is, the content of graphite particles A relative to the total mass of graphite particles A and graphite particles B may be 0 mass%. Therefore, when the negative electrode mixture layer 41 is divided into two equal parts in the longitudinal direction, the content of the graphite particles A relative to the total mass of the graphite particles A and the graphite particles B in the region on the outer winding end 41B side of the negative electrode mixture layer 41 is preferably 0 mass% or more and 30 mass% or less, and more preferably 0 mass% or more and 25 mass% or less.

[0044] 3 , the second current collector exposed portion 45 is provided at a position that divides the negative electrode mixture layer 41 into two equal parts in the longitudinal direction. Therefore, the content of graphite particles A relative to the total mass of graphite particles A and graphite particles B in the first negative electrode mixture layer 42 located closer to the inner winding end 41A than the second current collector exposed portion 45 is preferably 70% by mass or more and 100% by mass or less, and more preferably 75% by mass or more and 100% by mass or less. Furthermore, the content of graphite particles A relative to the total mass of graphite particles A and graphite particles B in the second negative electrode mixture layer 43 located closer to the outer winding end 41B than the second current collector exposed portion 45 is preferably 0% by mass or more and 30% by mass or less, and more preferably 0% by mass or more and 25% by mass or less.

[0045] The negative electrode mixture layer 41 may contain a material other than the graphite particles A and the graphite particles B as the negative electrode active material. Examples of materials other than the graphite particles A and the graphite particles B include graphite particles with an internal porosity of more than 5% and less than 8%, graphite particles with an internal porosity of more than 20%, and silicon-containing materials described below. The ratio of the sum of the masses of the graphite particles A and the graphite particles B to the total mass of the negative electrode active material in the negative electrode mixture layer 41 is, for example, 50% by mass or more, preferably 60% by mass or more, and more preferably 70% by mass or more. In this case, the effect of improving the cycle characteristics of the present disclosure becomes significant.

[0046] The negative electrode mixture layer 41 preferably contains a silicon-containing material in addition to graphite particles as the negative electrode active material. The silicon-containing material can occlude more lithium ions per unit mass than carbon materials such as graphite particles. Therefore, when the negative electrode mixture layer 41 contains a silicon-containing material, it becomes easier to achieve a high capacity battery. The content of the silicon-containing material relative to the total mass of the negative electrode active material in the negative electrode mixture layer 41 is, for example, 3% by mass or more and 30% by mass or less, and may be 5% by mass or more and 25% by mass or less.

[0047] Furthermore, silicon-containing materials undergo a larger volume change during charge and discharge than carbon materials such as graphite particles. Therefore, when the negative electrode active material contains a silicon-containing material, when the battery is charged and discharged, the pressure load due to expansion and contraction of the negative electrode mixture layer 41 increases on the inner end 41A side of the negative electrode mixture layer 41, and the nonaqueous electrolyte tends to be pushed out more. In other words, when the negative electrode active material contains a silicon-containing material, the effect of improving cycle characteristics of the present disclosure becomes more pronounced.

[0048] The silicon-containing material may be any material containing silicon, and examples thereof include silicon alloys, silicon compounds, and silicon-containing composite materials. Among these, silicon-containing composite materials are preferred. The D50 of the composite material is generally smaller than the D50 of graphite particles. The D50 of the composite material is, for example, 1 μm or more and 15 μm or less. Note that one type of silicon-containing material may be used alone, or two or more types may be used in combination.

[0049] A suitable composite material is a composite particle containing an ion-conducting phase and a Si phase dispersed in the ion-conducting phase. The surface of the composite particle is preferably covered with a conductive layer such as carbon. The ion-conducting phase is, for example, at least one selected from the group consisting of a silicate phase, an amorphous carbon phase, a silicide phase, and a silicon oxide phase. The Si phase is formed by dispersing Si in the form of fine particles. The ion-conducting phase is a continuous phase composed of a collection of particles finer than the Si phase. The conductive layer is made of a material with higher conductivity than the ion-conducting phase and forms a good conductive path in the negative electrode mixture layer 41.

[0050] An example of a suitable silicon-containing composite material has a sea-island structure in which fine Si is dispersed substantially uniformly in an amorphous silicon oxide phase, and the overall structure is represented by the general formula SiO ｘ The silicon oxide may be mainly composed of silicon dioxide. The oxygen to silicon content (x) is, for example, 0.5≦x<2.0, preferably 0.8≦x≦1.5.

[0051] The negative electrode mixture layer 41 contains a binder in addition to the negative electrode active material. The content of the binder is preferably 0.5 mass % or more and 10 mass % or less, and more preferably 1 mass % or more and 8 mass % or less, relative to the mass of the negative electrode mixture layer 41. Note that the negative electrode mixture layer 41 may contain materials other than the negative electrode active material and the binder, such as a conductive agent and a thickener.

[0052] As in the case of the positive electrode 11, the binder contained in the negative electrode mixture layer 41 can be a fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but it is preferable to use styrene butadiene rubber (SBR). The negative electrode mixture layer 41 may further contain CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. It is preferable to use a combination of SBR and CMC or a salt thereof as the binder of the negative electrode mixture layer 41.

[0053] Next, an example of a method for manufacturing the negative electrode 12 having the negative electrode mixture layer 41 of this embodiment will be described. Note that the method for manufacturing the negative electrode 12 is not limited to the following method.

[0054] The method for manufacturing the negative electrode 12 includes a first preparation step of preparing a first negative electrode mixture slurry, a second preparation step of preparing a second negative electrode mixture slurry, and an application step of applying the first negative electrode mixture slurry and the second negative electrode mixture slurry to the surface of the negative electrode current collector 40.

[0055] In the first preparation step, a first negative electrode mixture slurry is prepared that contains more graphite particles A with an internal porosity of 5% or less than graphite particles B with an internal porosity of 8% or more and 20% or less. The content of graphite particles A relative to the total mass of graphite particles A and graphite particles B in the first negative electrode mixture slurry is preferably 70% by mass or more and 100% by mass or less.

[0056] In the second preparation step, a second negative electrode mixture slurry is prepared that contains more graphite particles B with an internal porosity of 8% or more and 20% or less than graphite particles A with an internal porosity of 5% or less. In the second negative electrode mixture slurry, the content of graphite particles A relative to the total mass of graphite particles A and graphite particles B is preferably 0% by mass or more and 30% by mass or less. In other words, the content of graphite particles B relative to the total mass of graphite particles A and graphite particles B in the second negative electrode mixture slurry is preferably 70% by mass or more and 100% by mass or less.

[0057] In the coating step, the first negative electrode mixture slurry prepared in the first preparation step and the second negative electrode mixture slurry prepared in the second preparation step are coated on the surface of the negative electrode current collector 40. There are no particular limitations on the method for coating the first negative electrode mixture slurry and the second negative electrode mixture slurry on the surface of the negative electrode current collector 40, and they can be coated using a die coater, for example. The coating film produced in the coating step is dried and rolled, thereby producing the negative electrode 12.

[0058] In the coating step, when the anode mixture layer 41 is divided into two equal parts in the longitudinal direction, the first anode mixture slurry is coated more on the region of the anode mixture layer 41 closer to the inner winding end 41A than on the region of the anode mixture layer 41 closer to the outer winding end 41B. In other words, the content of the first anode mixture slurry relative to the total mass of the first anode mixture slurry and the second anode mixture slurry is greater on the region of the inner winding end 41A than on the region of the outer winding end 41B. This allows the graphite particles A, which have a small internal porosity, to be disposed in greater amounts on the region of the anode mixture layer 41 closer to the inner winding end 41A than on the region of the anode mixture layer 41 closer to the outer winding end 41B.

[0059] 3 , a second current collector exposed portion 45 is provided in the longitudinal center of the negative electrode mixture layer 41. The second current collector exposed portion 45 can be formed by intermittent application in which the first negative electrode mixture slurry and the second negative electrode mixture slurry are not applied to a part of the negative electrode current collector 40.

[0060] The negative electrode 12 of this embodiment can be produced by applying only the first negative electrode mixture slurry to a region closer to the inner winding end 41A than the second current collector exposed portion 45, and applying only the second negative electrode mixture slurry to a region closer to the outer winding end 41B than the second current collector exposed portion 45. In other words, the first negative electrode mixture layer 42 located closer to the inner winding end 41A than the second current collector exposed portion 45 is made up of only the first negative electrode mixture slurry, and the second negative electrode mixture layer 43 located closer to the outer winding end 41B than the second current collector exposed portion 45 is made up of only the second negative electrode mixture slurry.

[0061] The negative electrode 12 may be produced by applying a mixture of the first negative electrode mixture slurry and the second negative electrode mixture slurry in different mass ratios to a region closer to the inner winding end 41A than the second current collector exposed portion 45 and a region closer to the outer winding end 41B than the second current collector exposed portion 45.

[0062] Next, modified examples of the negative electrode 12 will be described with reference to Fig. 5 and Fig. 6. Fig. 5 is a front view showing a modified negative electrode 12X in a developed state, and Fig. 6 is a diagram showing the correlation between the position in the longitudinal direction of the negative electrode 12X and the content of graphite particles A.

[0063] As shown in Fig. 5 , the negative electrode 12X differs from the negative electrode 12 shown in Fig. 3 in that it does not have a second current collector exposed portion 45 between the inner winding end 41A and the outer winding end 41B of the negative electrode mixture layer 41. In the negative electrode 12X shown in Fig. 5 , when the negative electrode mixture layer 41 is divided into two equal parts in the longitudinal direction, more graphite particles A are contained in the region of the negative electrode mixture layer 41 closer to the inner winding end 41A than in the region of the negative electrode mixture layer 41 closer to the outer winding end 41B. This increases the amount of nonaqueous electrolyte retained on the inner winding end 41A side of the negative electrode mixture layer 41. As a result, the cycle characteristics can be improved when rapid charge / discharge cycles are repeated over a long period of time.

[0064] As shown in FIG. 6 , in the present embodiment, the content of graphite particles A relative to the total mass of graphite particles A and graphite particles B is substantially constant on the inner winding end 41A side, gradually decreases from the middle toward the outer winding end 41B side, and becomes substantially constant again on the outer winding end 41B side.

[0065] When the length along the longitudinal direction from the inner winding end 41A to the outer winding end 41B of the negative electrode mixture layer 41 is defined as L, the region where the content of the graphite particles A gradually decreases is preferably provided in a range of 0.25L or more and 0.60L or less from the inner winding end 41A of the negative electrode mixture layer 41. In this case, the effect of improving the cycle characteristics can be further exerted. Note that the negative electrode mixture layer 41 does not necessarily have to have a region where the content of the graphite particles A gradually decreases. In other words, the content of the graphite particles A may change stepwise in the longitudinal direction of the negative electrode mixture layer 41.

[0066] The negative electrode 12X can be produced, for example, by changing the mixing ratio of the first negative electrode mixture slurry and the second negative electrode mixture slurry midway through the above-mentioned coating process. Specifically, using a die coater, only the first negative electrode mixture slurry is applied to the inner winding end 41A side of the negative electrode mixture layer 41, the mass ratio of the second negative electrode mixture slurry to the first negative electrode mixture slurry is gradually increased over a range from the middle to a predetermined length, and then only the second negative electrode mixture slurry is applied to the outer winding end 41B of the negative electrode mixture layer 41.

[0067] The present disclosure will be further described by the following embodiments. Configuration 1: A non-aqueous electrolyte secondary battery including an electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound in the longitudinal direction with a separator interposed therebetween, a non-aqueous electrolyte, and an exterior housing that accommodates the electrode assembly and the non-aqueous electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer disposed on a surface of the negative electrode current collector, the negative electrode mixture layer includes, as negative electrode active materials, graphite particles A having an internal porosity of 5% or less and graphite particles B having an internal porosity of 8% or more and 20% or less, and when the negative electrode mixture layer is divided into two equal parts in the longitudinal direction, the graphite particles A are contained in a larger amount in a region closer to the inner end of the winding than in a region closer to the outer end of the winding. Configuration 2: The nonaqueous electrolyte secondary battery according to Configuration 1, wherein the content of the graphite particles A relative to the total mass of the graphite particles A and the graphite particles B in the region of the negative electrode mixture layer on the inner winding end side is 70% by mass or more and 100% by mass or less, and the content of the graphite particles A relative to the total mass of the graphite particles A and the graphite particles B in the region of the negative electrode mixture layer on the outer winding end side is 0% by mass or more and 30% by mass or less. a first negative electrode mixture layer formed on the inner end side of the negative electrode mixture layer and a second negative electrode mixture layer formed on the outer end side of the negative electrode mixture layer, the first negative electrode mixture layer containing more graphite particles A than the second negative electrode mixture layer, and a second negative electrode mixture layer containing more graphite particles A than the second negative electrode mixture layer. Configuration 5: The nonaqueous electrolyte secondary battery according to Configuration 3 or 4, wherein the content of the graphite particles A relative to the total mass of the graphite particles A and the graphite particles B in the first negative electrode mixture layer is 70% by mass or more and 100% by mass or less, and the content of the graphite particles A relative to the total mass of the graphite particles A and the graphite particles B in the second negative electrode mixture layer is 0% by mass or more and 30% by mass or less.Configuration 6: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 5, wherein the negative electrode mixture layer contains a silicon-containing material as a negative electrode active material. Configuration 7: A method for manufacturing a negative electrode for a non-aqueous electrolyte secondary battery, the method having a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector, the method comprising: a first preparation step of preparing a first negative electrode mixture slurry, the first negative electrode mixture slurry containing, as a negative electrode active material, graphite particles A having an internal porosity of 5% or less in greater amount than graphite particles B having an internal porosity of 8% or more and 20% or less; a second preparation step of preparing a second negative electrode mixture slurry containing, as a negative electrode active material, the graphite particles B in greater amount than the graphite particles A; and a coating step of coating the first negative electrode mixture slurry and the second negative electrode mixture slurry onto the surface of the negative electrode current collector, wherein, when the negative electrode mixture layer is divided into two equal parts in the longitudinal direction, the first negative electrode mixture slurry is applied more to a region closer to an inner end of winding than to a region closer to an outer end of winding in the coating step. Aspect 8: The method for producing a negative electrode for a nonaqueous electrolyte secondary battery according to Aspect 7, wherein the content of the graphite particles A relative to the total mass of the graphite particles A and the graphite particles B in the first negative electrode mixture slurry is 70% by mass or more and 100% by mass or less, and the content of the graphite particles A relative to the total mass of the graphite particles A and the graphite particles B in the second negative electrode mixture slurry is 0% by mass or more and 30% by mass or less.

[0068] REFERENCE SIGNS LIST 10 nonaqueous electrolyte secondary battery, 11 positive electrode, 12, 12X negative electrode, 12A inner winding end, 12B outer winding end, 13 separator, 14 electrode body, 16 outer casing, 17 sealing body, 18, 19 insulating plate, 20 positive electrode lead, 21 negative electrode lead, 22 grooved portion, 23 inner terminal plate, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket, 30 positive electrode current collector, 31 positive electrode mixture layer, 40 negative electrode current collector, 41 negative electrode mixture layer, 41A inner winding end, 41B outer winding end, 42 first negative electrode mixture layer, 43 second negative electrode mixture layer, 44 first current collector exposed portion, 45 second current collector exposed portion, 50 graphite particles, 52 internal void, 54 external void

Claims

1. A non-aqueous electrolyte secondary battery comprising: an electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound in the longitudinal direction with a separator interposed therebetween; a non-aqueous electrolyte; and an exterior housing that accommodates the electrode assembly and the non-aqueous electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer disposed on the surface of the negative electrode current collector, the negative electrode mixture layer containing, as negative electrode active materials, graphite particles A having an internal porosity of 5% or less and graphite particles B having an internal porosity of 8% or more and 20% or less, and wherein, when the negative electrode mixture layer is divided into two equal parts in the longitudinal direction, the graphite particles A are contained in a larger amount in a region closer to the inner end of the winding than in a region closer to the outer end of the winding.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the graphite particles A relative to the total mass of the graphite particles A and the graphite particles B in the region of the negative electrode mixture layer on the inner end side of the winding is 70 mass % or more and 100 mass % or less, and the content of the graphite particles A relative to the total mass of the graphite particles A and the graphite particles B in the region of the negative electrode mixture layer on the outer end side of the winding is 0 mass % or more and 30 mass % or less.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode has a current collector exposed portion where the negative electrode mixture layer is not disposed and the negative electrode current collector is exposed, between an inner end and an outer end of the negative electrode mixture layer in the longitudinal direction of the negative electrode, and when the portion of the negative electrode mixture layer closer to the inner end of the winding than the current collector exposed portion is defined as a first negative electrode mixture layer and the portion closer to the outer end of the winding than the current collector exposed portion is defined as a second negative electrode mixture layer, the graphite particles A are contained in a larger amount in the first negative electrode mixture layer than in the second negative electrode mixture layer.

4. The nonaqueous electrolyte secondary battery according to claim 3, wherein the length along the longitudinal direction from the inner end of the winding to the outer end of the winding of the negative electrode mixture layer is defined as L, and the current collector exposed portion is provided within a length range of 0.25 L or more and 0.60 L or less from the inner end of the winding of the negative electrode mixture layer.

5. The nonaqueous electrolyte secondary battery according to claim 3, wherein the content of the graphite particles A relative to the total mass of the graphite particles A and the graphite particles B in the first negative electrode mixture layer is 70 mass% or more and 100 mass% or less, and the content of the graphite particles A relative to the total mass of the graphite particles A and the graphite particles B in the second negative electrode mixture layer is 0 mass% or more and 30 mass% or less.

6. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode mixture layer contains a silicon-containing material as a negative electrode active material.

7. A method for manufacturing a negative electrode for a non-aqueous electrolyte secondary battery having a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector, the method comprising: a first preparation step of preparing a first negative electrode mixture slurry, as a negative electrode active material, containing graphite particles A having an internal porosity of 5% or less in greater amounts than graphite particles B having an internal porosity of 8% or more and 20% or less; a second preparation step of preparing a second negative electrode mixture slurry, as a negative electrode active material, containing the graphite particles B in greater amounts than the graphite particles A; and an application step of applying the first negative electrode mixture slurry and the second negative electrode mixture slurry to the surface of the negative electrode current collector, wherein, when the negative electrode mixture layer is divided into two equal parts in the longitudinal direction, the first negative electrode mixture slurry is applied more to an area closer to the inner end of the winding than to an area closer to the outer end of the winding.

8. The method for manufacturing a negative electrode for a non-aqueous electrolyte secondary battery according to claim 7, wherein the content of the graphite particles A relative to the total mass of the graphite particles A and the graphite particles B in the first negative electrode mixture slurry is 70 mass% or more and 100 mass% or less, and the content of the graphite particles A relative to the total mass of the graphite particles A and the graphite particles B in the second negative electrode mixture slurry is 0 mass% or more and 30 mass% or less.

Images