Non-aqueous electrolyte secondary battery

Abstract

The present invention is characterized in that a positive electrode (11) has a positive electrode current collector (30) and a positive electrode mixture layer (32) disposed on the surface of the positive electrode current collector (30), and the positive electrode mixture layer (32) has a region in which the capacity density of the positive electrode mixture layer (32) per unit mass continuously increases and the thickness of the positive electrode mixture layer (32) continuously decreases from the winding inner end (32X) side toward the winding outer end (32Y) side.

Description

Nonaqueous electrolyte secondary battery

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

[0002] Non-aqueous electrolyte secondary batteries are widely used as high-energy-density secondary batteries. A non-aqueous electrolyte secondary battery comprises an electrode body in which a positive electrode and a negative electrode are wound with a separator in between, a non-aqueous electrolyte, and an outer casing that houses the electrode body and the non-aqueous electrolyte. The positive electrode of a non-aqueous electrolyte secondary battery is composed of a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. Patent Document 1 discloses a positive electrode in which the content of the conductive agent differs in the thickness direction of the positive electrode.

[0003] International Publication No. 2015 / 049775

[0004] In wound electrode bodies, a step is formed at the outer end of the positive electrode. During charging and discharging, the volume of the electrode body changes (expansion and contraction), and when the electrode body is pressed radially inward from the outer casing, the pressure applied to the electrode body may change in plane due to this step. This can cause unevenness in the charge-discharge reaction inside the electrode body, potentially degrading the cycle characteristics. From the perspective of improving the reliability of non-aqueous electrolyte secondary batteries, it is necessary to reduce this step while ensuring battery capacity.

[0005] One aspect of the present disclosure is a non-aqueous electrolyte secondary battery comprising an electrode body in which a positive electrode and a negative electrode are wound longitudinally with a separator between them, wherein the positive electrode comprises a positive electrode current collector and a positive electrode mixture layer disposed on the surface of the positive electrode current collector, and the positive electrode mixture layer has a region in which the capacity density of the positive electrode mixture layer per unit mass continuously increases and the thickness of the positive electrode mixture layer continuously decreases from the inner end to the outer end of the winding.

[0006] According to a non-aqueous electrolyte secondary battery, one aspect of this disclosure, the step formed at the outer end of the positive electrode winding can be reduced while maintaining battery capacity. As a result, cycle characteristics can be improved while maintaining battery capacity.

[0007] This is an axial cross-sectional view of a non-aqueous electrolyte secondary battery, which is one example of an embodiment. This is a perspective view of the electrode body of a non-aqueous electrolyte secondary battery, which is one example of an embodiment. This is a cross-sectional view of a positive electrode, which is one example of an embodiment. This is a cross-sectional view of a positive electrode, which is another example of an embodiment. This is a cross-sectional view of a positive electrode, which is another example of an embodiment. This is a cross-sectional view of a positive electrode, which is another example of an embodiment. This is a cross-sectional view of the positive electrode of Comparative Example 1.

[0008] In the following, an example of an embodiment of the non-aqueous electrolyte secondary battery according to this disclosure will be described in detail with reference to the drawings. In the following description, specific shapes, materials, numerical values, directions, etc., are examples to facilitate understanding of the present invention and can be appropriately modified according to the specifications of the non-aqueous electrolyte secondary battery. Furthermore, if the following description includes multiple embodiments and modifications, it is intended from the outset that their characteristic parts may be used in appropriate combinations.

[0009] Figure 1 is an axial cross-sectional view of a non-aqueous electrolyte secondary battery 10, which is an example of an embodiment. As shown in Figure 1, the non-aqueous electrolyte secondary battery 10 comprises a wound electrode body 14, a non-aqueous electrolyte (not shown), and an outer casing 16 that houses the electrode body 14 and the non-aqueous electrolyte.

[0010] As will be described in more detail later, the electrode body 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 wound in a spiral shape via the separator 13. The electrode body 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.

[0011] Non-aqueous electrolytes are lithium ion conductive. Non-aqueous electrolytes may be liquid electrolytes (electrolytes) or solid electrolytes.

[0012] A liquid electrolyte (electrolyte solution) comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixtures thereof. The non-aqueous solvent may also contain halogen-substituted solvents (e.g., fluoroethylene carbonate) in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of electrolyte salts include LiPF4. ６ Lithium salts such as these are used.

[0013] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc., can be used. As the inorganic solid electrolyte, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc.

[0014] The outer casing 16 is a bottomed cylindrical metal container with one end open in the axial direction, and the opening of the outer casing 16 is sealed by a sealing body 17. For the sake of explanation, the side of the non-aqueous electrolyte secondary battery 10 with the sealing body 17 will be referred to as "upper," and the bottom side of the outer casing 16 will be referred to as "lower."

[0015] Insulating plates 18 and 19 are positioned above and below the electrode body 14, respectively. In the example shown in Figure 1, the positive electrode lead 20 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 extends through a through-hole in the insulating plate 19 towards the bottom of the outer casing 16. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected to the bottom inner surface of the outer casing 16 by welding or the like, and the outer casing 16 becomes the negative electrode terminal.

[0016] A gasket 28 is provided between the outer casing 16 and the sealing body 17 to ensure airtightness inside the battery. The outer casing 16 has a grooved portion 22 formed on its side surface, which protrudes inward to support the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17. The sealing body 17 is fixed to the upper part of the outer casing 16 by the grooved portion 22 and the open end of the outer casing 16 which is crimped to the sealing body 17.

[0017] The sealing body 17 has a structure in which 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 in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except 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, with the insulating member 25 interposed between their respective peripheries. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 towards the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the vent hole in the cap 27.

[0018] Next, the electrode body 14 will be described in detail with further reference to Figure 2. Figure 2 is a perspective view of the electrode body 14, showing a portion of the outer end unfolded.

[0019] As shown in Figures 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 along the longitudinal direction via a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are wound in a spiral shape and 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.

[0020] As will be described in more detail later, the positive electrode 11 has a positive electrode current collector 30 and a positive electrode mixture layer 32 disposed on the positive electrode current collector 30. The positive electrode current collector 30 can be made of a metal foil that is stable in the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film with the metal disposed on its surface. The positive electrode mixture layer 32 contains a positive electrode active material, a conductive agent, and a binder.

[0021] The negative electrode 12 comprises a negative electrode current collector 40 and a negative electrode mixture layer 42 disposed on the negative electrode current collector 40. The negative electrode current collector 40 can be made of a metal foil that is stable within the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with the metal disposed on its surface. The negative electrode mixture layer 42 contains a negative electrode active material and a binder. An exposed current collector portion is formed on the inner end side of the negative electrode 12 where the negative electrode current collector 40 is exposed, and a negative electrode lead 21 is connected to this exposed current collector portion.

[0022] The negative electrode mixture layer 42 generally contains a carbon material that reversibly intercepts and releases lithium ions as the negative electrode active material. A suitable example of the carbon material is graphite such as natural graphite like flake graphite, lump graphite, or clay graphite, or artificial graphite such as lump graphite (MAG) or graphitized mesophase carbon microbeads (MCMB). In addition, a material containing at least one of an element that alloys with Li, such as Si or Sn, and a material containing such an element may be used as the negative electrode active material. Among these, composite materials containing Si are preferred.

[0023] A preferred example of a composite material containing Si is SiO ２Examples include materials in which Si fine particles are dispersed in a phase or silicate phase such as lithium silicate, or materials in which Si fine particles are dispersed in an amorphous carbon phase. A conductive layer, such as a carbon film, is formed on the particle surface of the composite material. Using a carbon material and a Si-containing composite material in combination as a negative electrode active material is preferable from the viewpoint of achieving both high capacity and high durability of the battery.

[0024] The binder contained in the negative electrode mixture layer 42 may include fluororesins, PAN, polyimide, acrylic resins, polyolefins, etc., but styrene-butadiene rubber (SBR) is preferred. Furthermore, the negative electrode mixture layer 42 preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. In particular, a combination of SBR and CMC or a salt thereof, PAA or a salt thereof is preferred. The negative electrode mixture layer 42 may also contain a conductive agent such as CNT.

[0025] The separator 13 is made of a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator 13.

[0026] In this embodiment, the negative electrode compound layer 42 is not formed on the outermost periphery of the electrode body 14, and the negative electrode current collector 40 is exposed, with an exposed current collector portion that contacts the inner surface of the outer casing 16. By providing a negative electrode lead 21 and having the exposed current collector portion contact the inner surface of the outer casing 16, the internal resistance of the battery can be further reduced. A separator 13 may also be placed on the outermost periphery of the electrode body 14.

[0027] Next, with reference to Figure 3, the positive electrode 11 constituting the electrode body 14 will be described in detail. Figure 3 is a schematic diagram showing a cross-section of the positive electrode 11.

[0028] As shown in Figure 3, the positive electrode 11 has a positive electrode current collector 30 and a positive electrode mixture layer 32 disposed on the positive electrode current collector 30. Preferably, the positive electrode mixture layer 32 is formed on both sides of the positive electrode current collector 30. As described above, the positive electrode current collector 30 can be made of a metal foil that is stable in the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film with the metal disposed on its surface. The thickness of the positive electrode current collector 30 is, for example, 5 μm or more and 30 μm or less.

[0029] The positive electrode mixture layer 32 includes a positive electrode active material, a conductive agent, and a binder. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing the positive electrode active material, a conductive agent, and a binder onto a positive electrode current collector 30, drying the coating film, and then compressing it to form the positive electrode mixture layer 32 on both sides of the positive electrode current collector 30.

[0030] As shown in Figure 3, the positive electrode mixture layer 32 of this embodiment includes a first positive electrode mixture layer 34 and a second positive electrode mixture layer 36 whose volume density per unit mass (hereinafter simply referred to as "volume density") is greater than that of the first positive electrode mixture layer 34.

[0031] The positive electrode mixture layer 32 has a first region 32A near the inner end 32X of the positive electrode mixture layer 32, where only the first positive electrode mixture layer 34 is arranged on the positive electrode current collector 30. The positive electrode mixture layer 32 also has a second region 32B near the outer end 32Y of the positive electrode mixture layer 32, where only the second positive electrode mixture layer 36 is arranged on the positive electrode current collector 30. The positive electrode mixture layer 32 also has a third region 32C between the first region 32A and the second region 32B in the longitudinal direction of the positive electrode 11, where the second positive electrode mixture layer 36 is arranged on the first positive electrode mixture layer 34.

[0032] Here, in the third region 32C, the ratio of the thickness of the second positive electrode mixture layer 36 to the thickness of the first positive electrode mixture layer 34 continuously increases from the inner end 32X side to the outer end 32Y side of the positive electrode mixture layer 32, while the total thickness of the positive electrode mixture layer 32 continuously decreases. As described above, the capacity density of the second positive electrode mixture layer 36 is greater than that of the first positive electrode mixture layer 34. Therefore, in the third region 32C, the capacity density of the positive electrode mixture layer 32 continuously increases from the inner end 32X side to the outer end 32Y side of the positive electrode mixture layer 32, while the total thickness of the positive electrode mixture layer 32 continuously decreases.

[0033] In a wound electrode body 14, a step is formed at the outer end 32Y of the positive electrode 11. When the electrode body 14 undergoes volume changes (expansion and contraction) due to charging and discharging, and is pressed radially inward from the outer casing 16, the pressure applied to the electrode body 14 may change in plane due to the step. This can cause unevenness in the charge-discharge reaction inside the electrode body 14, potentially degrading the cycle characteristics. As in this embodiment, by continuously reducing the total thickness of the positive electrode mixture layer 32 from the inner end 32X to the outer end 32Y of the positive electrode mixture layer 32, the step formed at the outer end 32Y of the positive electrode 11 can be reduced. As a result, the in-plane pressure difference is reduced, unevenness in the reaction inside the electrode body 14 is suppressed, and the cycle characteristics are improved.

[0034] On the other hand, if the total thickness of the positive electrode mixture layer 32 is simply continuously reduced from the inner end 32X to the outer end 32Y, the capacity of the positive electrode mixture layer 32 at the outer end 32Y will decrease, resulting in a decrease in the overall capacity of the battery. Therefore, as in this embodiment, by continuously increasing the capacity density of the positive electrode mixture layer 32 from the inner end 32X to the outer end 32Y, the capacity of the positive electrode mixture layer 32 at the outer end 32Y is improved, ensuring the overall capacity of the battery. In other words, by configuring the positive electrode mixture layer 32 according to this embodiment, it is possible to improve cycle characteristics while ensuring battery capacity.

[0035] In this embodiment, the capacitance density in the first region 32A and the second region 32B is substantially uniform respectively. The capacitance density of the positive electrode active material layer 32 (the second positive electrode active material layer 36) at the outer winding end 32Y of the positive electrode active material layer 32 is, for example, 1.1 times or more, preferably 1.2 times or more, the capacitance density of the positive electrode active material layer 32 (the first positive electrode active material layer 34) at the inner winding end 32X of the positive electrode active material layer 32.

[0036] The capacitance density of the positive electrode active material layer 32 (the first positive electrode active material layer 34) in the first region 32A is, for example, 90 mAh / g or more and 250 mAh / g or less. Further, the capacitance density of the positive electrode active material layer 32 (the second positive electrode active material layer 36) in the second region 32B may be larger than the capacitance density of the first positive electrode active material layer 34, for example, 50 mAh / g or more and 200 mAh / g or less. Although details will be described later, the capacitance density of the positive electrode active material layer 32 can be adjusted by the capacitance density of the positive electrode active material contained in the positive electrode active material layer 32 or the content rate of the conductive agent or the binder.

[0037] Here, the capacitance density of the positive electrode active material layer 32 can use the value obtained by the following method. First, the positive electrode 11 having the positive electrode active material layer 32 is cut out into a predetermined size (for example, 10 mm square), and a coin cell is fabricated using metallic Li as the counter electrode. Then, using the fabricated coin cell, it is charged at a constant current of 0.3C until the battery voltage reaches 4.2V in a temperature environment of 25°C, and then charged at a constant voltage of 4.2V until the current value reaches 0.02C. Thereafter, discharge is performed at a constant current of 0.5C until the battery voltage reaches 2.5V, and the discharge capacity (mAh) at this time is measured. Then, the capacitance density (mAh / g) of the positive electrode active material layer 32 is calculated by dividing the measured discharge capacity by the mass (g) of the cutout positive electrode active material layer 32.

[0038] In this embodiment, the thickness (T32A) of the positive electrode active material layer 32 (the first positive electrode active material layer 34) in the first region 32A is substantially uniform. Similarly, the thickness (T32B) of the positive electrode active material layer 32 (the second positive electrode active material layer 36) in the second region 32B is also substantially uniform.

[0039] Here, T32A and T32B are preferably set in consideration of the capacity of the positive electrode active material layer 32. Specifically, T32A and T32B are preferably set such that the capacity of the positive electrode active material layer 32 at the inner end 32X of the winding of the positive electrode active material layer 32 is 0.9 times or more and 1.1 times or less the capacity of the positive electrode active material layer 32 at the outer end 32Y of the winding of the positive electrode active material layer 32. In this case, the current density distribution in the plane is made uniform, and the reaction unevenness of the charge-discharge reaction in the electrode body 14 becomes less likely to occur. As a result, for example, the cycle characteristics can be improved.

[0040] The ratio (T32B / T32A) of the thickness (T32B) of the positive electrode active material layer 32 (second positive electrode active material layer 36) in the second region 32B to the thickness (T32A) of the positive electrode active material layer 32 (first positive electrode active material layer 34) in the first region 32A is, for example, 0.1 or more and 0.8 or less, and may be 0.2 or more and 0.6 or less.

[0041] The thickness of the positive electrode active material layer 32 (first positive electrode active material layer 34) in the first region 32A is, for example, 50 μm or more and 200 μm or less on one side of the positive electrode current collector 30. Also, the thickness of the positive electrode active material layer 32 (second positive electrode active material layer 36) in the second region 32B is, for example, 20 μm or more and 150 μm or less on one side of the positive electrode current collector 30.

[0042] The length of the first region 32A in the longitudinal direction of the positive electrode 11 is, for example, 1% or more and 50% or less of the length of the positive electrode 11, and may be 3% or more and 40% or less. Also, the length of the second region 32B in the longitudinal direction of the positive electrode 11 is, for example, the same as the length of the first region 32A, 1% or more and 50% or less of the length of the positive electrode 11, and may be 3% or more and 40% or less. Note that the length of the first region 32A and the length of the second region 32B in the longitudinal direction of the positive electrode 11 may be the same or different from each other. Also, the length of the third region 32C in the longitudinal direction of the positive electrode 11 is, for example, 20% or more of the length of the positive electrode 11, and may be 30% or more, or 50% or more.

[0043] The positive electrode mixture layer 32 contains a positive electrode active material. As the positive electrode active material, for example, particulate lithium transition metal composite oxide can be used. The lithium transition metal composite oxide is a composite oxide containing metal elements such as Ni, Co, Mn, and Al in addition to Li. The metal elements constituting the lithium transition metal composite oxide are, for example, at least one selected from Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, and Bi. Among these, it is preferable to contain at least one selected from Co, Ni, Al, and Mn. Examples of suitable composite oxides include lithium transition metal composite oxides containing Ni, Co, and Mn, and lithium transition metal composite oxides containing Ni, Co, and Al. Note that the positive electrode active material contained in the positive electrode mixture layer 32 is not limited to the above materials.

[0044] As described above, the capacity density of the positive electrode active material greatly affects the capacity density of the positive electrode mixture layer 32. Therefore, it is preferable that the capacity density of the positive electrode active material contained in the second positive electrode mixture layer 36 is greater than the capacity density of the positive electrode active material contained in the first positive electrode mixture layer 34.

[0045] The second positive electrode mixture layer 36 contains a positive electrode active material with the general formula Li ｘ Ni ａ Co ｂ Al ｃ M ｄ O ２－ｙ The lithium transition metal composite oxide may include the following: (wherein the formula, 0.9 < x < 1.2, 0.70 ≤ a ≤ 0.95, 0 ≤ b ≤ 0.25, 0 ≤ c ≤ 0.25, 0 ≤ d ≤ 0.10, 0 ≤ y < 0.05, a + b + c + d = 1, and M is at least one element selected from the group consisting of Mn, Sr, Nb, Ba, Mg, Ca, Ti, V, Cr, Zr, Mo, and W). Generally, the higher the Ni content in the lithium transition metal composite oxide, the higher the capacity density of the positive electrode active material.

[0046] The positive electrode mixture layer 32 contains a positive electrode active material, a conductive agent, and a binder. Examples of conductive agents included in the positive electrode mixture layer 32 include carbon black such as acetylene black and Ketjen black, graphite, carbon nanotubes (CNTs), carbon nanofibers, and graphene. Examples of binders included in the positive electrode mixture layer 32 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resins, and polyolefins. In addition, these resins may be used in combination with carboxymethylcellulose (CMC) or its salts, polyethylene oxide (PEO), etc.

[0047] The binder content in the second positive electrode mixture layer 36 relative to the mass of the second positive electrode mixture layer 36 may be smaller than the binder content in the first positive electrode mixture layer 34 relative to the mass of the first positive electrode mixture layer 34. In this case, it becomes easier to make the volume density of the second positive electrode mixture layer 36 larger than the volume density of the first positive electrode mixture layer 34. The binder content in the first positive electrode mixture layer 34 relative to the mass of the first positive electrode mixture layer 34 is, for example, 0.5% by mass or more and 15% by mass or less. Also, the binder content in the second positive electrode mixture layer 36 relative to the mass of the second positive electrode mixture layer 36 is, for example, 0.1% by mass or more and 10% by mass or less.

[0048] Furthermore, the content of the conductive agent in the second positive electrode mixture layer 36 relative to the mass of the second positive electrode mixture layer 36 may be smaller than the content of the conductive agent in the first positive electrode mixture layer 34 relative to the mass of the first positive electrode mixture layer 34. In this case, it becomes easier to make the capacity density of the second positive electrode mixture layer 36 greater than that of the first positive electrode mixture layer 34. The content of the conductive agent in the first positive electrode mixture layer 34 relative to the mass of the first positive electrode mixture layer 34 is, for example, 0.5% by mass or more and 15% by mass or less. Also, the content of the conductive agent in the second positive electrode mixture layer 36 relative to the mass of the second positive electrode mixture layer 36 is, for example, 0.1% by mass or more and 10% by mass or less.

[0049] Next, an example of a method for manufacturing the positive electrode 11 having the positive electrode mixture layer 32 of this embodiment will be described. Note that the method for manufacturing the positive electrode 11 is not limited to the method described below.

[0050] The method for manufacturing the positive electrode 11 comprises a preparation step of preparing a first positive electrode mixture slurry and a second positive electrode mixture slurry having a higher capacity density than the first positive electrode mixture slurry, and a coating step of applying the first positive electrode mixture slurry and the second positive electrode mixture slurry to the surface of the positive electrode current collector 30. As described above, the capacity density of the positive electrode mixture slurry can be adjusted by the type and mixing ratio of the positive electrode active material and the content of the binder or conductive agent.

[0051] In the coating process, for example, using a coating device having two discharge ports, the first positive electrode mixture slurry and the second positive electrode mixture slurry are simultaneously applied to the surface of the positive electrode current collector 30, in the order of first positive electrode mixture slurry and second positive electrode mixture slurry from the surface side of the positive electrode current collector 30. Specifically, at the inner end of the winding of the electrode plate, only the first positive electrode mixture slurry is applied to the surface of the positive electrode current collector 30. Then, as you move from the inner side of the winding of the electrode plate to the outer side, the discharge amount of the first positive electrode mixture slurry is reduced, and the discharge amount of the second positive electrode mixture slurry is continuously increased. By changing the discharge amounts of the first positive electrode mixture slurry and the second positive electrode mixture slurry, the ratio of the thicknesses of the first positive electrode mixture layer 34 and the second positive electrode mixture layer 36 can be changed. Furthermore, at this time, the total discharge amount of the first positive electrode mixture slurry and the second positive electrode mixture slurry is continuously reduced as you move from the inside to the outside of the electrode plate winding. This allows the thickness of the positive electrode mixture layer 32 to be continuously reduced as you move from the inside to the outside of the electrode plate winding. Then, at the outer end of the electrode plate winding, only the second positive electrode mixture slurry is applied to the surface of the positive electrode current collector 30. Finally, the coating film produced in the coating process is dried and rolled to produce the positive electrode 11 of this embodiment.

[0052] Next, a modified example of the positive electrode 11 will be described with reference to Figures 4 to 6. Figures 4 to 6 schematically show cross-sections of the modified positive electrode 11.

[0053] The positive electrode 11 shown in Figure 4 differs from the positive electrode 11 shown in Figure 3 in that the positive electrode mixture layer 32 does not have a first region 32A composed only of the first positive electrode mixture layer 34, and a second region 32B composed only of the second positive electrode mixture layer 36. In other words, in the positive electrode 11 shown in Figure 4, the capacity density of the positive electrode mixture layer 32 increases continuously from the inner end 32X to the outer end 32Y of the winding, while the total thickness of the positive electrode mixture layer 32 continuously decreases.

[0054] The positive electrode mixture layer 32 in the positive electrode 11 shown in Figure 5 differs from the positive electrode 11 shown in Figure 4 in that the first positive electrode mixture layer 34 is arranged on top of the second positive electrode mixture layer 36. Even in this case, the step formed at the outer end 32Y of the winding of the positive electrode 11 can be reduced. As a result, reaction unevenness inside the electrode body 14 is suppressed, and the cycle characteristics are improved.

[0055] The positive electrode mixture layer 32 in the positive electrode 11 shown in Figure 6 differs from the positive electrode 11 shown in Figures 3 to 5 in that both the first positive electrode mixture layer 34 and the second positive electrode mixture layer 36 are arranged at the inner end 32X and the outer end 32Y of the positive electrode mixture layer 32. In the positive electrode 11 shown in Figure 6, the capacity density of the positive electrode mixture layer 32 increases continuously from the inner end 32X to the outer end 32Y, while the total thickness of the positive electrode mixture layer 32 continuously decreases. Even in this case, the step formed at the outer end 32Y of the positive electrode 11 can be reduced. As a result, reaction unevenness inside the electrode body 14 is suppressed, and the cycle characteristics are improved.

[0056] In addition, the positive electrode mixture layer 32 in the positive electrode 11 shown in Figures 3 to 6 all have a first positive electrode mixture layer 34 and a second positive electrode mixture layer 36, but the positive electrode mixture layer 32 may also have a single-layer structure. A positive electrode 11 having a single-layer structure can be manufactured, for example, by applying a mixture of the first positive electrode mixture slurry and the second positive electrode mixture slurry, mixed while changing the mixing ratio, to the surface of the positive electrode current collector 30.

[0057] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited to these examples.

[0058] <Example 1> [Fabrication of the positive electrode] LiCoO with a capacity density of 140 mAh / g as the positive electrode active material２ (the first positive electrode active material) and LiMnO with a capacity density of 100 mAh / g ２ (the second positive electrode active material) were used. Then, the first positive electrode active material, the second positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed at a mass ratio of 5:85:5:5 to prepare a first positive electrode binder slurry. Also, the first positive electrode active material, the second positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed at a mass ratio of 85:5:5:5 to prepare a second positive electrode binder slurry.

[0059] Then, using a coating device having two discharge ports, the first positive electrode binder slurry and the second positive electrode binder slurry were simultaneously coated on both sides of the positive electrode current collector so that the first positive electrode binder slurry and the second positive electrode binder slurry were coated in this order from the surface side of the aluminum foil as the positive electrode current collector, and the coating film was dried. At this time, as shown in FIG. 3, a first region where only the first positive electrode binder slurry was coated and a second region where only the second positive electrode binder slurry was coated were provided on both end sides in the longitudinal direction of the positive electrode, respectively. Then, after rolling the coating film using a roller, it was cut to a predetermined electrode size to produce a positive electrode in which a positive electrode binder layer was formed on both sides of the positive electrode current collector.

[0060] The thicknesses of the positive electrode binder layers at the inner end (the first region) and the outer end (the second region) of the winding of the positive electrode binder layer were made 126 μm and 93 μm, respectively, on one side of the positive electrode current collector. Also, the lengths of the first region and the second region in the longitudinal direction of the positive electrode were each made about 6% of the total length of the positive electrode. Further, as shown in FIG. 3, in the third region sandwiched between the first region and the second region, the positive electrode binder layer was produced such that the capacity density continuously increased and the thickness of the positive electrode binder layer continuously decreased from the inner end side of the winding to the outer end side of the winding. Also, the capacity density of the first positive electrode binder layer was 124 mAh / g, and the capacity density of the second positive electrode binder layer was 92 mAh / g.

[0061] [Fabrication of the negative electrode] Graphite, a carbon material, was used as the negative electrode active material. The negative electrode active material, styrene-butadiene copolymer, and carboxymethylcellulose were mixed in a mass ratio of 100:1:1.5, and an appropriate amount of water was added to prepare a negative electrode slurry to form the negative electrode slurry layer. Next, the negative electrode slurry was applied to both sides of a copper foil, which was to be used as the negative electrode current collector, and the coating was dried. Then, the coating was rolled using a roller, cut to a predetermined electrode size, and a negative electrode was fabricated with a negative electrode slurry layer formed on both sides of the negative electrode current collector.

[0062] [Preparation of non-aqueous electrolyte (electrolyte)] Add LiPF to a non-aqueous solvent. ６ The electrolyte was prepared by adding (lithium salt). LiPF in the electrolyte ６ The concentration was set to 1.0 mol / L. As the non-aqueous solvent, a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of EC:EMC = 3:7 was used.

[0063] [Fabrication of Test Cell (Non-Aqueous Electrolyte Secondary Battery)] An aluminum lead was attached to a portion of the positive electrode and a nickel lead to a portion of the negative electrode. The positive and negative electrodes were then wound in a spiral shape via a polyolefin separator to create a wound electrode body. Insulating plates were placed above and below the electrode body, and the electrode body was housed in a bottomed cylindrical outer casing. The negative electrode lead was welded to the bottom of the outer casing, and the positive electrode lead was welded to a sealing body. Then, electrolyte was injected into the outer casing, and the opening of the outer casing was sealed with a sealing body via a gasket to create a non-aqueous electrolyte secondary battery as a test cell.

[0064] [Evaluation of Cycle Characteristics] The fabricated test cell was charged at a constant current of 0.3C in a temperature environment of 25°C until the battery voltage reached 4.2V. Then, it was charged again at a constant voltage of 4.2V until the current value was 0.02C. After that, it was discharged at a constant current of 0.5C until the battery voltage reached 2.5V, and the discharge capacity at this time was defined as the initial discharge capacity. This charge-discharge cycle was considered one cycle, and 100 cycles were performed. The discharge capacity at the 100th cycle was determined, and the capacity retention rate was calculated using the following formula: Capacity retention rate (%) = Discharge capacity at the 100th cycle / Initial discharge capacity × 100

[0065] <Example 2> In the preparation of the positive electrode, as shown in Figure 4, a positive electrode mixture layer was prepared in the same manner as in Example 1, except that the second positive electrode mixture layer was placed on the surface of the first positive electrode mixture layer without having a first and second region. The cycle characteristics were then evaluated.

[0066] <Example 3> In the preparation of the positive electrode, as shown in Figure 5, a positive electrode mixture layer was prepared in the same manner as in Example 1, except that the first positive electrode mixture layer was placed on the surface of the second positive electrode mixture layer without having the first and second regions. The cycle characteristics were then evaluated.

[0067] <Comparative Example 1> In the preparation of the positive electrode, a test cell was prepared in the same manner as in Example 1, except that, as shown in Figure 7, a positive electrode mixture layer was prepared in which the second positive electrode mixture layer was arranged on the first positive electrode mixture layer over the entire longitudinal direction of the positive electrode, and the cycle characteristics were evaluated. The positive electrode mixture layer of Comparative Example 1 had a substantially uniform thickness over the entire longitudinal direction of the positive electrode, and the thickness of the positive electrode mixture layer was set to 110 μm. In addition, the ratio of the thickness of the first positive electrode mixture layer to the thickness of the second positive electrode mixture layer was set to 1:1.

[0068] Table 1 shows the results of the volume retention rate of the test cells for Examples 1-3 and Comparative Example 1. Table 1

[0069]

[0070] As shown in Table 1, the test cells of Examples 1 to 3 showed improved capacity retention and improved cycle characteristics compared to the test cell of Comparative Example 1. This is presumed to be because the test cells of Examples 1 to 3 were able to reduce the step formed at the outer end of the positive electrode winding, thereby suppressing reaction unevenness within the electrode body.

[0071] This disclosure is further illustrated by the following embodiments. Configuration 1: A non-aqueous electrolyte secondary battery comprising an electrode body in which a positive electrode and a negative electrode are wound longitudinally via a separator, wherein the positive electrode comprises a positive electrode current collector and a positive electrode mixture layer disposed on the surface of the positive electrode current collector, and the positive electrode mixture layer has a region in which the capacity density of the positive electrode mixture layer per unit mass continuously increases and the thickness of the positive electrode mixture layer continuously decreases from the inner end to the outer end. Configuration 2: The non-aqueous electrolyte secondary battery according to Configuration 1, wherein the capacity density of the positive electrode mixture layer per unit mass at the outer end of the positive electrode mixture layer is 1.1 times or more the capacity density of the positive electrode mixture layer per unit mass at the inner end of the positive electrode mixture layer. Configuration 3: The non-aqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the positive electrode mixture layer comprises a first positive electrode mixture layer and a second positive electrode mixture layer disposed on the surface of the positive electrode current collector or on the surface of the first positive electrode mixture layer, the second positive electrode mixture layer having a capacity density per unit mass greater than that per unit mass of the first positive electrode mixture layer, and having a region from the inner end to the outer end of the winding in which the ratio of the thickness of the second positive electrode mixture layer to the thickness of the first positive electrode mixture layer continuously increases and the thickness of the positive electrode mixture layer continuously decreases. Configuration 4: The non-aqueous electrolyte secondary battery according to Configuration 3, wherein the first positive electrode mixture layer is disposed on the surface of the second positive electrode mixture layer. Configuration 5: The non-aqueous electrolyte secondary battery according to Configuration 3, wherein the first positive electrode mixture layer is disposed between the positive electrode current collector and the second positive electrode mixture layer. Configuration 6: A non-aqueous electrolyte secondary battery according to any one of Configurations 3 to 5, wherein only the first positive electrode mixture layer is arranged at the inner end of the positive electrode mixture layer, and only the second positive electrode mixture layer is arranged at the outer end of the positive electrode mixture layer. Configuration 7: A non-aqueous electrolyte secondary battery according to any one of Configurations 3 to 5, wherein the first positive electrode mixture layer and the second positive electrode mixture layer are arranged at the inner and outer ends of the positive electrode mixture layer, respectively.Configuration 8: The non-aqueous electrolyte secondary battery according to any one of Configurations 3 to 7, wherein the first positive electrode mixture layer and the second positive electrode mixture layer each contain a binder, and the content of the binder in the second positive electrode mixture layer relative to the mass of the second positive electrode mixture layer is smaller than the content of the binder in the first positive electrode mixture layer relative to the mass of the first positive electrode mixture layer. Configuration 9: The second positive electrode mixture layer contains a positive electrode active material of general formula Li. ｘ Ni ａ Co ｂ Al ｃ M ｄ O ２－ｙ A non-aqueous electrolyte secondary battery according to any one of configurations 3 to 8, comprising a lithium transition metal composite oxide represented by the formula (wherein 0.9 < x < 1.2, 0.70 ≤ a ≤ 0.95, 0 ≤ b ≤ 0.25, 0 ≤ c ≤ 0.25, 0 ≤ d ≤ 0.10, 0 ≤ y < 0.05, a + b + c + d = 1, and M is at least one element selected from the group consisting of Mn, Sr, Nb, Ba, Mg, Ca, Ti, V, Cr, Zr, Mo, and W). Configuration 10: A non-aqueous electrolyte secondary battery according to any one of configurations 1 to 9, wherein the capacity of the positive electrode composite layer at the inner end of the winding of the positive electrode composite layer is 0.9 times or more and 1.1 times or less the capacity of the positive electrode composite layer at the outer end of the winding of the positive electrode composite layer.

[0072] 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 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 section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode current collector, 32 Positive electrode mixture layer, 32A First region, 32B Second region, 32C Third region, 32X Inner end of winding, 32Y Outer end of winding, 34 First positive electrode mixture layer, 36 Second positive electrode mixture layer, 40 Negative electrode current collector, 42 Negative electrode mixture layer.

Claims

1. A non-aqueous electrolyte secondary battery comprising an electrode body in which a positive electrode and a negative electrode are wound longitudinally with a separator between them, wherein the positive electrode comprises a positive electrode current collector and a positive electrode mixture layer disposed on the surface of the positive electrode current collector, and the positive electrode mixture layer has a region in which the capacity density of the positive electrode mixture layer per unit mass continuously increases and the thickness of the positive electrode mixture layer continuously decreases from the inner end to the outer end of the winding.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the capacity density of the positive electrode mixture layer per unit mass at the outer end of the winding of the positive electrode mixture layer is 1.1 times or more the capacity density of the positive electrode mixture layer per unit mass at the inner end of the winding of the positive electrode mixture layer.

3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode mixture layer comprises: a first positive electrode mixture layer; and a second positive electrode mixture layer disposed on the surface of the positive electrode current collector or on the surface of the first positive electrode mixture layer, the second positive electrode mixture layer having a capacity density per unit mass greater than the capacity density per unit mass of the first positive electrode mixture layer, and having a region from the inner end to the outer end of the winding in which the ratio of the thickness of the second positive electrode mixture layer to the thickness of the first positive electrode mixture layer continuously increases and the thickness of the positive electrode mixture layer continuously decreases.

4. The non-aqueous electrolyte secondary battery according to claim 3, wherein the first positive electrode mixture layer is disposed on the surface of the second positive electrode mixture layer.

5. The non-aqueous electrolyte secondary battery according to claim 3, wherein the first positive electrode mixture layer is disposed between the positive electrode current collector and the second positive electrode mixture layer.

6. The non-aqueous electrolyte secondary battery according to claim 3, wherein only the first positive electrode mixture layer is arranged at the inner end of the winding of the positive electrode mixture layer, and only the second positive electrode mixture layer is arranged at the outer end of the winding of the positive electrode mixture layer.

7. The non-aqueous electrolyte secondary battery according to claim 3, wherein the first positive electrode mixture layer and the second positive electrode mixture layer are arranged at the inner and outer ends of the positive electrode mixture layer, respectively.

8. The non-aqueous electrolyte secondary battery according to claim 3, wherein the first positive electrode mixture layer and the second positive electrode mixture layer each contain a binder, and the content of the binder in the second positive electrode mixture layer relative to the mass of the second positive electrode mixture layer is smaller than the content of the binder in the first positive electrode mixture layer relative to the mass of the first positive electrode mixture layer.

9. The second positive electrode mixture layer contains a positive electrode active material of the general formula Li ｘ Ni ａ Co ｂ Al ｃ M ｄ O ２－ｙ A non-aqueous electrolyte secondary battery according to claim 3, comprising a lithium transition metal composite oxide represented by the formula (wherein 0.9 < x < 1.2, 0.70 ≤ a ≤ 0.95, 0 ≤ b ≤ 0.25, 0 ≤ c ≤ 0.25, 0 ≤ d ≤ 0.10, 0 ≤ y < 0.05, a + b + c + d = 1, and M is at least one element selected from the group consisting of Mn, Sr, Nb, Ba, Mg, Ca, Ti, V, Cr, Zr, Mo, and W).

10. The non-aqueous electrolyte secondary battery according to claim 1, wherein the volume of the positive electrode mixture layer at the inner end of the winding is 0.9 times or more and 1.1 times or less than the volume of the positive electrode mixture layer at the outer end of the winding.

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