Non-aqueous electrolyte secondary battery

Abstract

The present invention provides a nonaqueous electrolyte secondary battery which is suppressed in the occurrence of an internal short circuit at the outer winding end of an electrode body. A nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure comprises: an electrode body which is obtained by winding a positive electrode and a negative electrode, with a separator being interposed therebetween; a nonaqueous electrolyte; and an outer case for housing the electrode body and the nonaqueous electrolyte. The negative electrode comprises a first negative electrode active material and a second negative electrode active material that has a greater expansion coefficient than the first negative electrode active material during charging; and if the proportion of the mass of the second negative electrode active material with respect to the total mass of the first negative electrode active material and the second negative electrode active material is defined as the second negative electrode active material ratio, the second negative electrode active material ratio at the outer winding end is smaller than the second negative electrode active material ratio at the inner winding end.

Description

[Technical Field]

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

[0002] Conventionally, non-aqueous electrolyte secondary batteries have been widely used, in which a wound electrode body, consisting of a strip-shaped positive electrode and a strip-shaped negative electrode wound around a separator, is housed in an outer casing. In recent years, from the viewpoint of increasing the capacity of batteries, the use of silicon (Si)-based materials, which can absorb more lithium ions per unit mass compared to carbon-based materials such as graphite, has been considered as the negative electrode active material contained in the negative electrode mixture layer. Patent Document 1 discloses a negative electrode containing a Si-based material in a predetermined proportion in the mixture layer.

[0003] Patent Document 2 discloses a non-aqueous electrolyte secondary battery in which a wound electrode body is housed in a space below (bottom side) the grooved portion of the outer casing, sandwiched between insulating plates on the top and bottom. Patent Document 2 also discloses a negative electrode containing two types of Si-based materials, in which the mixing ratios of the two types of Si-based materials differ in the two layers separated in the thickness direction of the negative electrode mixture layer. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2010-212228 [Patent Document 2] Japanese Patent Publication No. 2020-149821 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] In the non-aqueous electrolyte secondary battery disclosed in Patent Document 2, when the negative electrode expands due to charging, the negative electrode located below the grooved portion may bend without being able to push the insulating plate above the grooved portion, potentially causing the adjacent separator to break and resulting in a micro-short circuit. In particular, since Si-based materials undergo a larger volume change due to lithium ion intercalation compared to carbon-based materials, the risk of micro-short circuits is greater in negative electrodes containing Si-based materials. Patent Documents 1~ 2 However, the suppression of internal short circuits at the outer end of the electrode body has not been considered, and there is still room for further investigation.

[0006] The object of this disclosure is to provide a non-aqueous electrolyte secondary battery that suppresses the occurrence of internal short circuits at the outer end of the winding. [Means for solving the problem]

[0007] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure comprises an electrode body in which a positive electrode and a negative electrode are wound with a separator between them, a non-aqueous electrolyte, and an outer casing that houses the electrode body and the non-aqueous electrolyte. The negative electrode includes a first negative electrode active material and a second negative electrode active material having a greater expansion rate during charging than the first negative electrode active material. The ratio of the mass of the second negative electrode active material to the total mass of the first and second negative electrode active materials is defined as the second negative electrode active material ratio, and the second negative electrode active material ratio at the outer end of the winding is smaller than the second negative electrode active material ratio at the inner end of the winding. [Effects of the Invention]

[0008] The non-aqueous electrolyte secondary battery according to this disclosure can suppress the occurrence of internal short circuits at the outer end of the electrode body and improve the reliability of the battery. [Brief explanation of the drawing]

[0009] [Figure 1] This is an axial cross-sectional view of a secondary battery in its initial state, which is an example of an embodiment. [Figure 2] This is an enlarged view of a cross-section near the grooved portion of a secondary battery, which is an example of an embodiment, after repeated charging and discharging. [Figure 3] It is a perspective view of an electrode body in an example of an embodiment. [Figure 4] It is a front view showing the positive electrode and the negative electrode constituting the electrode body in an example of the embodiment in a developed state. [Figure 5] (a) to (d) are diagrams showing changes in the second negative electrode active material ratio in the longitudinal direction of FIG. 4. [Figure 6] It is a diagram corresponding to FIG. 2 in a secondary battery according to the prior art.

Mode for Carrying Out the Invention

[0010] Hereinafter, an example of an embodiment of a cylindrical 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 the understanding of the present invention, and can be appropriately changed according to the specifications of the cylindrical secondary battery. Further, in the following description, when a plurality of embodiments and modification examples are included, it is initially assumed that their characteristic parts are used in appropriate combination.

[0011] Figure 1 is an axial cross-sectional view of a cylindrical secondary battery 10 in its initial state, which is an example of an embodiment. Here, the initial state refers to the state before the first charge is performed. In the secondary battery 10 shown in Figure 1, an electrode body 14 and a non-aqueous electrolyte (not shown) are housed in an outer casing 15. The electrode body 14 has a wound structure in which a strip-shaped positive electrode 11 and a strip-shaped negative electrode 12 are wound around a separator 13. As the non-aqueous solvent (organic solvent) for the electrolyte, carbonates, lactones, ethers, ketones, esters, etc. can be used, and two or more of these solvents can be mixed and used. When using a mixture of two or more solvents, it is preferable to use a mixed solvent containing a cyclic carbonate and a linear carbonate. For example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. can be used as cyclic carbonates, and dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), etc. can be used as linear carbonates. As the electrolyte salt for the electrolyte solution, LiPF6, LiBF4, LiCF3SO3, etc., and mixtures thereof can be used. The solubility of the electrolyte salt in the non-aqueous solvent can be, for example, 0.5 to 2.0 mol / L. For the sake of explanation below, the side of the sealing body 16 will be referred to as "upper" and the bottom side of the outer casing 15 as "lower".

[0012] The inside of the secondary battery 10 is sealed by closing the upper end of the outer casing 15 with the sealing body 16. Insulating plates 17 and 18 are provided above and below the electrode body 14, respectively. The positive electrode lead 19 extends upward through a through hole in the insulating plate 17 and is welded to the lower surface of the filter 22, which is the bottom plate of the sealing body 16. In the secondary battery 10, the cap 26, which is the top plate of the sealing body 16 electrically connected to the filter 22, becomes the positive electrode terminal. On the other hand, the negative electrode lead 20 extends through a through hole in the insulating plate 18 to the bottom side of the outer casing 15 and is welded to the bottom inner surface of the outer casing 15. In the secondary battery 10, the outer casing 15 becomes the negative electrode terminal. If the negative electrode lead 20 is installed at the outer end of the winding, the negative electrode lead 20 extends outside the insulating plate 18 to the bottom side of the outer casing 15 and is welded to the bottom inner surface of the outer casing 15.

[0013] The outer casing 15 is, for example, a metal outer casing in the shape of a bottomed cylindrical container. A gasket 27 is provided between the outer casing 15 and the sealing body 16 to ensure airtightness inside the secondary battery 10. The outer casing 15 has grooves 21 that support the sealing body 16, which are formed, for example, by pressing the side surface from the outside. The grooves 21 are preferably formed in an annular shape along the circumferential direction of the outer casing 15, and their upper surface supports the sealing body 16.

[0014] The sealing body 16 has a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26, which are stacked in order from the electrode body 14 side. Each component of the sealing body 16 has, for example, a disc shape or a ring shape, and each component except the insulating member 24 is electrically connected to one another. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, with the insulating member 24 interposed between their respective peripheries. If the internal pressure of the battery rises due to abnormal heat generation, for example, the lower valve body 23 may rupture, causing the upper valve body 25 to bulge towards the cap 26 and separate from the lower valve body 23, thereby interrupting the electrical connection between the two. If the internal pressure rises further, the upper valve body 25 may rupture, and gas may be discharged from the opening 26a of the cap 26.

[0015] Next, with reference to Figures 2 and 6, the differences between the secondary battery 10, which is an example of an embodiment, and the conventional secondary battery 50 will be explained. Figure 2 is an enlarged view of the cross-section near the grooved portion 21 of the secondary battery 10, which is an example of an embodiment, after repeated charging and discharging. Figure 6 is a diagram corresponding to Figure 2 in the conventional secondary battery 50.

[0016] In the conventional secondary battery 50 shown in Figure 6, the negative electrode 52 expands due to repeated charging and discharging. At the inner end of the winding, the negative electrode 52 pushes the insulating plate 57 upward. However, at the outer end of the winding, there is a groove 61 above the insulating plate 57, so the negative electrode 52 on the outer end side can only push the insulating plate 57 up to the groove 61, causing the tip of the negative electrode 52 to bend. The bent portion of the tip of the negative electrode 52 may compress the adjacent separator, potentially causing a small short circuit.

[0017] In the secondary battery 10, which is an example of the embodiment shown in Figure 2, the negative electrode 12 expands due to repeated charging and discharging, similar to the conventional secondary battery 50 shown in Figure 6. However, as will be described later, by adjusting the ratio of the negative electrode active material contained in the negative electrode 12, the expansion rate of the negative electrode 12 at the outer end of the winding is made smaller than the expansion rate of the negative electrode 12 at the inner end of the winding, thereby suppressing the bending of the tip of the negative electrode 12 at the outer end of the winding. This suppresses the occurrence of internal short circuits at the outer end of the winding and improves the reliability of the secondary battery 10.

[0018] Next, the electrode body 14 will be described with reference to Figure 3. Figure 3 is a perspective view of the electrode body 14. As described above, the electrode body 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound in a spiral shape via a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all formed in a strip shape and are wound in a spiral shape around a winding core arranged along the winding axis, resulting in them being alternately stacked in the radial direction of the electrode body 14. In the radial direction, the side facing the winding axis is called the inner circumference, and the opposite side is called the outer circumference. 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. The positive electrode lead 19 extends axially from approximately the center in the radial direction between the center and the outermost circumference at the upper end of the electrode body 14. The negative electrode lead 20 extends axially from near the winding axis at the lower end of the electrode body 14.

[0019] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. The material of the separator 13 is preferably an olefin resin such as polyethylene or polypropylene. The thickness of the separator 13 is, for example, 10 μm to 50 μm. The separator 13 is trending toward thinner films as batteries become more high-capacity and high-power. The separator 13 has a melting point of, for example, 130°C to 180°C.

[0020] Next, the positive electrode 11 and negative electrode 12 according to this embodiment will be described with reference to Figures 4 and 5. Figure 4 is a front view of the positive electrode 11 and negative electrode 12 constituting the electrode body 14. In Figure 4, the positive electrode 11 and negative electrode 12 are shown in an unfolded state. As illustrated in Figure 4, in the electrode body 14, the negative electrode 12 is formed to be larger than the positive electrode 11 in order to prevent lithium deposition on the negative electrode 12. Specifically, the length of the negative electrode 12 in the axial direction is greater than the length of the positive electrode 11 in the width direction. Also, the length of the negative electrode 12 in the longitudinal direction is greater than the length of the positive electrode 11 in the longitudinal direction. As a result, when wound as the electrode body 14, at least the positive electrode mixture layer 32 of the positive electrode 11 is positioned opposite the negative electrode mixture layer 42 of the negative electrode 12 via the separator 13.

[0021] The positive electrode 11 comprises a strip-shaped positive electrode current collector 30 and a positive electrode mixture layer 32 formed on the surface of the positive electrode current collector 30. The positive electrode mixture layer 32 is formed on at least one of the inner and outer circumferences of the positive electrode current collector 30, and preferably covers the entire surface of both sides of the positive electrode current collector 30, excluding the positive electrode exposed portion 34 described later. For example, the positive electrode current collector 30 can be made of a metal foil such as aluminum, or a film with the metal arranged on its surface. The thickness of the positive electrode current collector 30 is, for example, 10 μm to 30 μm.

[0022] The positive electrode mixture layer 32 preferably contains a positive electrode active material, a conductive agent, and a binder. The positive electrode mixture layer 32 can be manufactured, for example, by coating both sides of the positive electrode current collector 30 with a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, and a solvent such as N-methyl-2-pyrrolidone (NMP), drying it, and then rolling it.

[0023] The positive electrode 11 is provided with a positive electrode exposed portion 34 where the surface of the positive electrode current collector 30 is exposed. The positive electrode exposed portion 34 is the portion to which the positive electrode lead 19 is connected, and is the portion where the surface of the positive electrode current collector 30 is not covered by the positive electrode mixture layer 32. The positive electrode exposed portion 34 is formed to be wider in the longitudinal direction than the positive electrode lead 19. Preferably, the positive electrode exposed portion 34 is provided on both sides of the positive electrode 11 so as to overlap in the thickness direction of the positive electrode 11. The positive electrode lead 19 is joined to the positive electrode exposed portion 34 by, for example, ultrasonic welding.

[0024] In the example shown in FIG. 4, a positive electrode exposed portion 34 is provided over the entire length in the width direction at the central portion in the longitudinal direction of the positive electrode 11. The positive electrode exposed portion 34 may be formed at the inner winding end portion or the outer winding end portion of the positive electrode 11, but from the viewpoint of current collecting property, it is preferably provided at a position substantially equidistant from the inner winding end portion and the outer winding end portion. By connecting the positive electrode lead 19 to the positive electrode exposed portion 34 provided at such a position, when wound as the electrode body 14, the positive electrode lead 19 is disposed so as to project upward from the end face in the width direction at substantially the center in the radial direction of the electrode body 14. The positive electrode exposed portion 34 is provided, for example, by intermittent coating in which a positive electrode active material slurry is not applied to a part of the positive electrode current collector 30.

[0025] Examples of the positive electrode active material contained in the positive electrode active material layer 32 include lithium transition metal oxides containing transition metal elements such as Co, Mn, and Ni. The lithium transition metal oxide is, for example, Li x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1-y O z 、Li x Ni 1-y M y O z 、Li x Mn2O4, Li x Mn 2-y M y O4, LiMPO4, Li2MPO4F (M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3). These may be used alone or in combination of a plurality of kinds. In terms of achieving a high capacity of the non-aqueous electrolyte secondary battery, the positive electrode active material is Li x NiO2, Li x Co y Ni 1-y O2, Li x Ni 1-y M y Oz It is preferable to contain a lithium nickel composite oxide such as (M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≦ 1.2, 0 < y ≦ 0.9, 2.0 ≦ z ≦ 2.3).

[0026] Examples of the conductive agent contained in the positive electrode mixture layer 32 include carbon-based particles such as carbon black (CB), acetylene black (AB), ketjen black, carbon nanotube (CNT), graphene, and graphite. These may be used alone or in combination of two or more.

[0027] Examples of the binder contained in the positive electrode mixture layer 32 include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic resins, and polyolefin-based resins. These may be used alone or in combination of two or more. When preparing the positive electrode mixture slurry with an aqueous solvent, styrene-butadiene rubber (SBR), nitrile rubber (NBR), CMC or its salt, polyacrylic acid or its salt, polyvinyl alcohol, etc. may be used.

[0028] The negative electrode 12 has a strip-shaped negative electrode current collector 40 and a negative electrode mixture layer 42 formed on the surface of the negative electrode current collector 40. The negative electrode mixture layer 42 is formed on at least one of the inner peripheral side and the outer peripheral side of the negative electrode current collector 40, and is preferably formed over the entire area excluding the negative electrode exposed portions 44 described below on both surfaces of the negative electrode current collector 40. For the negative electrode current collector 40, for example, a metal foil such as copper or a film having the metal disposed on the surface layer is used. The thickness of the negative electrode current collector 40 is, for example, 5 μm to 30 μm.

[0029] The negative electrode mixture layer 42 contains, for example, a negative electrode active material such as a first negative electrode active material and a second negative electrode active material, and a binder. The negative electrode mixture layer 42 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, and a solvent such as water to both surfaces of the negative electrode current collector 40, drying, and then rolling.

[0030] In the example shown in Figure 4, a negative electrode exposed portion 44 is provided at the longitudinal winding end of the negative electrode 12, extending along the entire width of the current collector. The negative electrode exposed portion 44 is the portion to which the negative electrode lead 20 is connected, and is the portion where the surface of the negative electrode current collector 40 is not covered by the negative electrode mixture layer 42. The negative electrode exposed portion 44 is formed to be wider in the longitudinal direction than the width of the negative electrode lead 20. Preferably, the negative electrode exposed portion 44 is provided on both sides of the negative electrode 12 so as to overlap in the thickness direction of the negative electrode 12.

[0031] In Figure 4, the inner end 42a of the negative electrode mixture layer 42 is adjacent to the exposed negative electrode portion 44. On the other hand, the outer end 42b of the negative electrode mixture layer 42 is the same as the outer end of the negative electrode 12. The negative electrode mixture layer 42 exists continuously from the inner end 42a to the outer end 42b.

[0032] In this embodiment, the negative electrode lead 20 is joined to the inner surface of the negative electrode current collector 40, for example, by ultrasonic welding. One end of the negative electrode lead 20 is positioned in the negative electrode exposed portion 44, and the other end extends downward from the lower end of the negative electrode exposed portion 44.

[0033] The placement of the negative electrode lead 20 is not limited to the example shown in Figure 4; the negative electrode lead 20 may be provided only at the outer end of the negative electrode 12. Alternatively, the negative electrode lead 20 may be provided at both the inner and outer ends of the negative electrode 12. In this case, current collection performance is improved. The outer end of the negative electrode 12 can also be electrically connected to the outer casing 15 without using a negative electrode lead 20 by bringing the negative electrode exposed portion 44 at the outer end of the negative electrode 12 into contact with the inner circumferential surface of the outer casing 15 (see Figure 1). The negative electrode exposed portion 44 is provided, for example, by intermittent coating of the negative electrode slurry without applying it to a part of the negative electrode current collector 40.

[0034] The negative electrode mixture layer 42 includes a first negative electrode active material and a second negative electrode active material having a greater expansion rate during charging than the first negative electrode active material. The first negative electrode active material is, for example, a carbon-based material such as natural graphite or artificial graphite. The second negative electrode active material is, for example, a metal that alloys with lithium such as Si or Sn, or an alloy or oxide containing these. The second negative electrode active material is preferably a silicon-based material. Examples of silicon-based materials include Si, alloys containing Si, and SiO2. x Examples include silicon oxides represented by (x is 0.8 to 1.6). Since the second negative electrode active material can absorb more lithium ions than the first negative electrode active material, using the second negative electrode active material as the negative electrode active material can increase the capacity of the battery.

[0035] In the electrode body 14, when the ratio of the mass of the second negative electrode active material to the total mass of the first negative electrode active material and the second negative electrode active material is defined as the second negative electrode active material ratio, the second negative electrode active material ratio at the outer end 42b is smaller than the second negative electrode active material ratio at the inner end 42a. This makes the expansion rate of the negative electrode 12 at the outer end 42b smaller than that of the negative electrode 12 at the inner end 42a, thereby suppressing the occurrence of internal short circuits at the outer end 42b. The distribution of the second negative electrode active material ratio along the longitudinal direction of the negative electrode 12 is not particularly limited as long as the second negative electrode active material ratio at the outer end 42b is smaller than the second negative electrode active material ratio at the inner end 42a, but for example, the second negative electrode active material ratio is Inner end 42a From the side Outer end 42b It may decrease at a constant rate along the side. Also, Inner end 42a From the side Outer end 42b The rate of decrease may vary between the sides. When forming the negative electrode mixture layer 42 using a multilayer die coater as described later, for ease of manufacture, the ratio of the second negative electrode active material is Inner end 42a From the side Outer end 42b It is preferable that the decrease is at a constant rate along the side.

[0036] The ratio of the mass of the second negative electrode active material to the total mass of the first and second negative electrode active materials in the entire negative electrode mixture layer 42 is preferably 2% to 20% by mass, and more preferably 5% to 15% by mass. Within this range, it is possible to increase the battery capacity while further improving the reliability of the battery.

[0037] Examples of binders included in the negative electrode mixture layer 42 include styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts (PAA-Na, PAA-K, etc., or partially neutralized salts), and polyvinyl alcohol (PVA). The binder may also include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. These may be used individually or in combination of two or more types.

[0038] Next, referring to Figures 5(a) to 5(d), the change in the ratio of the second negative electrode active material in the winding direction of the negative electrode 12 (corresponding to the longitudinal direction in Figure 4) will be explained. In Figure 5(a), the ratio of the second negative electrode active material at the inner end 42a is higher than that at the outer end 42b, and the ratio of the second negative electrode active material decreases at a constant rate from the inner end 42a to the outer end 42b. By making the ratio of the second negative electrode active material at the outer end 42b smaller than that at the inner end 42a, the occurrence of internal short circuits at the outer end 42b can be suppressed, thereby improving the reliability of the battery. The ratio of the second negative electrode active material at the outer end 42b is preferably 1% to 15% by mass, and more preferably 5% to 10% by mass.

[0039] Furthermore, as shown in Figure 5(b), the slope indicating the rate of decrease in the ratio of the second negative electrode active material from the inner end 42a to the outer end 42b does not have to be constant, and the slope may change along the way. In Figure 5(c), the ratio of the second negative electrode active material decreases from the inner end 42a to the outer end 42b, and the ratio of the second negative electrode active material is constant between the inner end 42a and the outer end 42b. In Figure 5(d), the ratio of the second negative electrode active material decreases from the inner end 42a to the outer end 42b, Inner end 42a The ratio of the second negative electrode active material is constant in the vicinity. Similarly, if the ratio of the second negative electrode active material decreases from the inner end 42a to the outer end 42b of the winding, Outer end 42b The ratio of the second negative electrode active material may be constant in the vicinity. As shown in Figures 5(c) and (d), it is preferable that a region is provided in at least a part of the negative electrode mixture layer 42 in which the ratio of the second negative electrode active material decreases continuously from the inward end 42a to the outward end 42b. In this region, it is preferable that the ratio of the second negative electrode active material decreases linearly, but it may decrease nonlinearly. This makes it possible to make the ratio of the second negative electrode active material at the outward end 42b of the negative electrode mixture layer 42 smaller than the ratio of the second negative electrode active material at the inward end 42a.

[0040] Next, a method for forming a negative electrode mixture layer 42 in which the ratio of the second negative electrode active material changes from one end 42a on the inside winding side to the other end 42b on the outside winding side will be described. It is preferable to use a multilayer die coater to form such a negative electrode mixture layer 42. By using a multilayer die coater, multiple negative electrode mixture slurries with different ratios of the second negative electrode active material can be simultaneously applied to the negative electrode current collector 40 while adjusting their mixing ratios. When applying the negative electrode mixture slurry to the negative electrode current collector 40, the negative electrode current collector 40 moves relative to the multilayer die coater. Therefore, by applying multiple negative electrode mixture slurries with different ratios of the second negative electrode active material to the negative electrode current collector 40 while changing their mixing ratios at predetermined timings, a region in the negative electrode mixture layer 42 in which the ratio of the second negative electrode active material changes from the inside winding end 42a side to the outside winding end 42b side can be formed at any position. For example, a first negative electrode mixture slurry containing a first negative electrode active material and a second negative electrode active material is prepared, and a second negative electrode mixture slurry having a lower ratio of the second negative electrode active material than the first negative electrode mixture slurry is prepared. Next, using a multilayer die coater, the first and second negative electrode mixture slurries are applied from the inner end 42a to the outer end 42b of the negative electrode current collector 40 while increasing the mixing ratio of the second negative electrode mixture slurry to the first negative electrode mixture slurry, thereby obtaining a negative electrode mixture layer 42 having the profile shown in Figure 5(a).

[0041] Furthermore, even if the negative electrode mixture layer 42 of the negative electrode 12 is divided into two or more parts by the exposed portion, as shown in Figure 4 for the positive electrode 11, it is sufficient that the ratio of the second negative electrode active material at the outer end 42b is smaller than the ratio of the second negative electrode active material at the inner end 42a. It is preferable that a region is formed in at least a part of the negative electrode mixture layer 42 continuous from the inner end 42a in which the ratio of the second negative electrode active material decreases from the inner end 42a side to the outer end 42b side. [Examples]

[0042] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited to these examples.

[0043] [Fabrication of the positive electrode] 100 parts by mass of LiNi 0.88 Co 0.09 Al 0.03 O2, 1 part by mass of acetylene black (AB), and 0.9 parts by mass of polyvinylidene fluoride (PVdF) were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to both sides of a strip-shaped positive electrode current collector made of 15 μm thick aluminum foil, dried, rolled, and cut to a predetermined electrode plate size to produce a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode current collector. A positive electrode exposed portion was provided approximately in the center of the longitudinal direction of the positive electrode, where there was no mixture layer and the surface of the current collector was exposed, and an aluminum positive electrode lead was welded to the positive electrode exposed portion.

[0044] [Fabrication of the negative electrode] 89 parts by mass of graphite, 11 parts by mass of SiO, 1 part by mass of carboxymethylcellulose (CMC), and 1 part by mass of styrene-butadiene rubber (SBR) were mixed, and an appropriate amount of water was added to prepare the first negative electrode mixture slurry. Similarly, 93 parts by mass of graphite, 7 parts by mass of SiO, 1 part by mass of CMC, and 1 part by mass of SBR were mixed, and an appropriate amount of water was added to prepare the second negative electrode mixture slurry. Next, the first and second negative electrode mixture slurries were set in a multilayer die coater and applied to both sides of a strip-shaped negative electrode current collector made of 8 μm thick copper foil, from the inner end to the outer end of the winding, while continuously changing the mixing ratio of the first and second negative electrode mixture slurries from 1:0 to 0:1, so that the charging capacity per unit area in the longitudinal direction remained constant. The coating was then dried. After the dried coating film was rolled using a roller, it was cut to a predetermined electrode plate size, and a negative electrode mixture layer was formed on both sides of the negative electrode current collector. negative electrode A negative electrode was fabricated. A negative electrode exposed section was provided at the inner end of the winding where the composite layer was absent and the surface of the current collector was exposed, and a nickel-copper negative electrode lead was welded to the negative electrode exposed section.

[0045] [Preparation of non-aqueous electrolytes] A non-aqueous electrolyte was prepared by dissolving LiPF6 in a mixed solvent consisting of fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC) (FEC:DMC = 1:3 by volume) to a concentration of 1.5 mol / L.

[0046] [Manufacturing of secondary batteries] An electrode body was fabricated by winding the positive and negative electrodes via a polyethylene separator. Insulating plates were placed above and below the electrode body, and the electrode body was housed in a cylindrical casing. Next, the negative electrode lead was welded to the bottom of the casing, and the positive electrode lead was welded to the sealing body. After that, the electrolyte was injected into the casing using a reduced pressure method, and the upper end of the casing was sealed by crimping it to the sealing body via a gasket to fabricate a secondary battery. The capacity of the fabricated secondary battery is 4600mAh.

[0047] <Example 2> A secondary battery was fabricated in the same manner as in Example 1, except that the first negative electrode slurry contained 90 parts by mass of graphite and 10 parts by mass of Si oxide, and the second negative electrode slurry contained 92 parts by mass of graphite and 8 parts by mass of Si oxide.

[0048] <Example 3> A secondary battery was fabricated in the same manner as in Example 1, except that the first negative electrode slurry contained 88 parts by mass of graphite and 12 parts by mass of Si oxide, and the second negative electrode slurry contained 92 parts by mass of graphite and 8 parts by mass of Si oxide.

[0049] <Comparative Example> In preparing the negative electrode, 91 parts by mass of graphite, 9 parts by mass of SiO, 1 part by mass of CMC, and 1 part by mass of SBR were mixed, and an appropriate amount of water was added to prepare a third negative electrode mixture slurry. A secondary battery was then prepared in the same manner as in Example 1, except that only the third negative electrode mixture slurry was applied to both sides of the negative electrode current collector.

[0050] [Evaluation of the bending of the negative electrode at the outer end of the winding] The batteries of the examples and comparative examples were charged with a constant current of 0.3C at a temperature of 45°C until the battery voltage reached 4.2V, and then charged with a constant voltage of 4.2V until the current value reached 0.02C. Next, they were discharged with a constant current of 0.5C until the battery voltage reached 2.5V, which constituted one cycle. After repeating this charge-discharge cycle 1000 times, further charging was performed. Using an X-ray CT (Computed Tomography) device, cross-sectional observations were performed near the inner end of the electrode body of the charged batteries to confirm the degree of bending of the negative electrode.

[0051] The evaluation results for the examples and comparative examples are shown in Table 1. Table 1 also shows the proportion of SiO in the negative electrode active material contained in the first and second negative electrode mixture slurries (ratio of the second negative electrode active material).

[0052] [Table 1]

[0053] In Examples 1-3, even after 1000 cycles, the elongation of the negative electrode at the outer end of the winding was suppressed and no bending occurred. On the other hand, in the comparative example, it was observed that the negative electrode at the outer end of the winding had elongated and collided with the upper insulating plate, causing the negative electrode to bend. By suppressing the bending of the negative electrode in this way, the occurrence of internal short circuits can be suppressed, and the reliability of the battery can be improved. [Explanation of symbols]

[0054] 10 Rechargeable battery, 11 Positive electrode 、1 2 Negative electrode, 13 Separator, 14 Electrode body, 15 Outer casing, 16 Sealing body, 17,18 Insulating plate, 19 Positive electrode lead, 20 Negative electrode lead, 21 Grooved section, 22 Filter, 23 Lower valve body, 24 Insulating member, 25 Upper valve body, 26 Cap, 26a Opening, 27 Gasket, 30 Positive electrode current collector, 32 Positive electrode mixture layer, 34 Positive electrode exposed section, 40 Negative electrode current collector, 42 Negative electrode mixture layer, 42a Inner end of winding, 42b Outer end of winding, 44 Negative electrode exposed section

Claims

1. A non-aqueous electrolyte secondary battery comprising an electrode body in which a positive electrode and a negative electrode are wound with a separator between them, a non-aqueous electrolyte, and an outer casing that houses the electrode body and the non-aqueous electrolyte, The negative electrode comprises a first negative electrode active material and a second negative electrode active material having a greater expansion rate during charging than the first negative electrode active material. When the ratio of the mass of the second negative electrode active material to the total mass of the first negative electrode active material and the second negative electrode active material is defined as the second negative electrode active material ratio, the second negative electrode active material ratio at the outer end of the winding is smaller than the second negative electrode active material ratio at the inner end of the winding. A non-aqueous electrolyte secondary battery in which the ratio of the second negative electrode active material decreases at a constant rate from the inner end of the winding to the outer end of the winding, or decreases while the rate of decrease changes.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the first negative electrode active material is a carbon-based material and the second negative electrode active material is a silicon-based material.

3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the ratio of the second negative electrode active material at the outer end of the winding is 1% by mass or more and 15% by mass or less.

4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the ratio of the mass of the second negative electrode active material to the total mass of the first negative electrode active material and the second negative electrode active material in the entire negative electrode is 2% by mass to 20% by mass.

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