Secondary battery

Abstract

A cylindrical battery (1) is provided with an electrode body (10). A positive electrode (11) constituting an electrode body (10) has a long positive electrode core (13) and a positive electrode mixture layer (14) disposed on the positive electrode core (13). A plurality of exposed parts (15), which expose the surface of the positive electrode core (13), are provided in the lengthwise direction of the positive electrode (11) at the widthwise end of the positive electrode (11). The positive electrode mixture layer (14) includes a first region (14a) and a second region (14b), and the compressive modulus of elasticity of the first region (14a) is greater than the compressive modulus of elasticity of the second region (14b).

Description

Technical Field

[0001] The present application relates to a secondary battery. Background Art

[0002] In recent years, secondary batteries such as lithium-ion batteries have been widely used in applications such as automotive applications and power storage applications that require high capacity, high durability, and rapid charging performance. Since the positive electrode, as the main component of the secondary battery, has a significant impact on these properties, a large amount of research has been conducted on the positive electrode. The positive electrode generally has a structure in which a composite layer containing a positive electrode active material is formed on a core composed of metal foil. For example, Patent Document 1 discloses a positive electrode having a plurality of leads welded at intervals along the length direction.

[0003] When multiple leads are joined at intervals along the length of the positive electrode, as in the positive electrode disclosed in Patent Document 1, it is necessary to form multiple exposed portions in the length direction, where the surface of the positive electrode core is exposed, as the portion where the leads are joined. In the positive electrode of Patent Document 1, a strip-shaped exposed portion is formed at the width end portion along the length direction. However, from the perspective of increasing the capacity of the battery, it is preferable to set the area of ​​the exposed portion to the minimum necessary limit. Specifically, it is conceivable to alternately arrange the exposed portion and the mixture layer along the length direction at the width end portion of the positive electrode.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2001-176489 Summary of the Invention

[0007] However, when using a positive electrode to produce an electrode body in which the exposed portion of the core and the mixture layer are alternately arranged along the length of the positive electrode's widthwise ends, it is believed that a large surface pressure difference will occur during charge and discharge between the widthwise ends of the positive electrode and the portion other than the widthwise ends, that is, the portion where the positive electrode mixture layer is continuously provided along the lengthwise direction. As a result, gaps may form between the electrode plates that constitute the electrode body in the portion corresponding to the widthwise ends of the positive electrode. If such gaps form, for example, there is a risk of the mixture layer peeling off during charge and discharge cycles.

[0008] The secondary battery of the present application is characterized in that it has an electrode body, which includes a positive electrode, a negative electrode and a separator and is formed by winding the positive electrode and the negative electrode with the separator interposed therebetween. The positive electrode has an elongated positive electrode core and a positive electrode mixture layer arranged on the positive electrode core. At the width end of the positive electrode, a plurality of exposed portions are provided along the length direction of the positive electrode, exposing the surface of the positive electrode core. When the area of ​​the positive electrode mixture layer that is parallel to the exposed portion along the length direction of the positive electrode is defined as the first area, and the area where the positive electrode mixture layer is continuous along the length direction of the positive electrode is defined as the second area, the compressive elastic modulus of the first area is greater than the compressive elastic modulus of the second area.

[0009] According to the secondary battery of the present application, it is possible to effectively suppress the generation of gaps between the electrode plates constituting the electrode assembly. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 It is a cross-sectional view of a secondary battery as an example of an embodiment.

[0011] Figure 2 It is a front view of a positive electrode as an example of an embodiment. DETAILED DESCRIPTION

[0012] Hereinafter, an example of an embodiment of the secondary battery of the present application will be described in detail with reference to the accompanying drawings. It should be noted that configurations formed by selectively combining the components of the various embodiments and modifications described below are within the scope of the present application.

[0013] In the embodiment described below, a cylindrical battery 10 is exemplified as a secondary battery in which a wound electrode body 14 is housed in a cylindrical outer can 16 with a bottom. However, the outer can of the battery is not limited to a cylindrical outer can, and the shape of the cylindrical battery is not limited to a cylindrical outer can. Figure 1 Other embodiments of the secondary battery of the present application include a square battery with a square outer can, a soft-pack battery with an outer package composed of a laminate sheet including a metal layer and a resin layer, etc. In addition, although the electrolyte may also be an aqueous electrolyte, a non-aqueous electrolyte is used in this embodiment.

[0014] Figure 1 1 is a cross-sectional view of a cylindrical battery 1 as an example of an embodiment. Figure 1 As shown, the cylindrical battery 1 includes an electrode body 10, a non-aqueous electrolyte, a bottomed cylindrical outer can 20 for storing the electrode body 10 and the non-aqueous electrolyte, and a sealing body 30 for sealing the opening of the outer can 20. The outer can 20 has a groove 23 formed in the side wall 21, and the sealing body 30 is supported by the groove 23 to seal the opening of the outer can 20. The electrode body 10 has a positive electrode 11 (see below). Figure 2 ), a negative electrode and a separator, with a structure in which the positive electrode 11 and the negative electrode are wound with the separator interposed therebetween. Hereinafter, for convenience of description, the sealing body 30 side of the cylindrical battery 1 is set as the top, and the bottom 22 side of the outer can 20 is set as the bottom.

[0015] The cylindrical battery 1 further includes a positive electrode lead 12 that connects the positive electrode to the current collector 32 of the sealing body 30, and an upper insulating plate 40 disposed between the electrode body 10 and the sealing body 30. In this embodiment, the positive electrode lead 12 electrically connects the positive electrode to the sealing body 30, and the negative electrode lead electrically connects the negative electrode to the outer can 20. Therefore, the sealing body 30 functions as the positive electrode external terminal, and the outer can 20 functions as the negative electrode external terminal. It should be noted that the cylindrical battery 1 may also include a lower insulating plate disposed between the electrode body 10 and the can bottom 22.

[0016] Although the details will be described later, the cylindrical battery 1 has a plurality of positive electrode leads 12. Moreover, at the widthwise end of the positive electrode, as a portion to which the positive electrode lead 12 is joined, a plurality of exposed portions are provided along the longitudinal direction of the positive electrode, where the surface of the positive electrode core is exposed. In this embodiment, the upper insulating plate 40 includes a first insulating plate 41 and a second insulating plate 42 that is arranged on the sealing body 30 side relative to the first insulating plate 41. The first insulating plate 41 is sandwiched between the electrode body 10 and the positive electrode lead 12 to prevent contact between the negative electrode and the positive electrode lead 12. The second insulating plate 42 is sandwiched between the groove 23 of the outer can 20 and the positive electrode lead 12 to prevent contact between the outer can 20, which serves as the external terminal of the negative electrode, and the positive electrode lead 12.

[0017] The non-aqueous electrolyte has lithium ion conductivity and can be a liquid electrolyte (electrolyte) or a solid electrolyte.

[0018] The liquid electrolyte (electrolyte) contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more thereof. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents thereof. The non-aqueous solvent may contain a halogen-substituted product (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms in these solvents are replaced with halogen atoms such as fluorine. Examples of the electrolyte salt include lithium salts such as LiPF6.

[0019] As a solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc. can be used. As an inorganic solid electrolyte, materials known in 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. A polymer electrolyte, for example, comprises a lithium salt and a matrix polymer, or comprises a non-aqueous solvent, a lithium salt, and a matrix polymer. As the matrix polymer, for example, a polymer material that gels after absorbing a non-aqueous solvent can be used. Examples of polymer materials include fluororesins, acrylic resins, and polyether resins.

[0020] The positive electrode, negative electrode and separator constituting the electrode body 10 are all long strips, which are wound into a spiral and thus alternately stacked along the radial direction of the electrode body 10. In order to prevent the precipitation of lithium, the negative electrode is formed with a size that is one circle larger than the positive electrode. That is, the negative electrode is formed longer in the length direction and width direction than the positive electrode. The separator is formed with a size that is at least one circle larger than the positive electrode, for example, two pieces are arranged in a manner of clamping the positive electrode. The electrode body 10 has a positive electrode lead 12 connected to the positive electrode by welding or the like and a negative electrode lead connected to the negative electrode by welding or the like.

[0021] The diameter of the electrode body 10 is, for example, 40 mm or more. The larger the diameter of the electrode body 10, the more effective the positive electrode structure of the present application is. The upper limit of the diameter of the electrode body 10 is not particularly limited, and is 100 mm as an example. A suitable example of the diameter of the electrode body 10 is 40 mm or more and 80 mm or less, or 45 mm or more and 70 mm or less. It should be noted that the diameter of the electrode body 10 mainly depends on the length of the electrode plate.

[0022] The positive electrode comprises a positive electrode core and a positive electrode mixture layer disposed on the positive electrode core. The positive electrode core can be made of a metal foil stable within the potential range of the positive electrode, such as aluminum, aluminum alloy, stainless steel, or titanium, or a film having such a metal disposed on the surface. The positive electrode mixture layer comprises a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core, excluding the portion connected to the positive electrode lead 12. The positive electrode active material uses a lithium transition metal composite oxide containing transition metal elements such as Ni, Co, and Mn.

[0023] The negative electrode comprises a negative electrode core and a negative electrode mixture layer disposed on the negative electrode core. The negative electrode core can be made of a metal foil that is stable within the potential range of the negative electrode, such as copper, copper alloy, stainless steel, nickel, nickel alloy, or a film having the metal disposed on the surface. The negative electrode mixture layer comprises a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core excluding the portion connected to the negative electrode lead. The negative electrode active material generally uses a carbon material that reversibly absorbs and releases lithium ions. The negative electrode active material can also use elements such as Si and Sn that alloy with Li, or materials containing such elements.

[0024] The separator uses a porous sheet with ion permeability and insulating properties. Specific examples of porous sheets include microporous films, woven fabrics, and non-woven fabrics. Suitable materials for the separator include polyolefins such as polyethylene and polypropylene, and cellulose. The separator can have a single-layer or multi-layer structure. For example, the separator can have a multi-layer structure comprising a thermoplastic resin layer such as a polyolefin and a cellulose fiber layer, a two-layer structure of polyethylene (PE) / polypropylene (PP), or a three-layer structure of PE / PP / PE.

[0025] The outer packaging can 20 is a metal container with a bottom and a cylindrical shape that is open at one axial end (the upper end). It has a side wall 21 formed in a cylindrical shape and a bottom 22 that is circular when viewed from above. The outer packaging can 20 is usually made of a metal with iron as a main component, but it can also be made of a metal with aluminum or the like as a main component. In addition, the outer packaging can 20 has a groove portion 23 formed along the circumferential direction of the side wall 21. The groove portion 23 is formed near the opening of the outer packaging can 20 at a position that is a predetermined length away from the edge of the opening (the upper end of the outer packaging can 20). In this embodiment, a safety valve is formed on the bottom 22 of the outer packaging can 20 for releasing gas due to an increase in internal pressure when an abnormality occurs.

[0026] The groove portion 23 is a portion of the side wall 21 that protrudes toward the inside of the outer can 20, and is formed, for example, by spinning the side wall 21 from the outside. It should be noted that at the location where the groove portion 23 is formed, the diameter of the outer can 20 is reduced, forming a thin linear groove on the outer circumferential surface of the side wall 21. The groove portion 23 has a roughly U-shaped cross-section and is preferably formed in an annular shape along the entire circumferential length of the side wall 21. After the electrode body 10 is housed in the outer can 20, the side wall 21 is processed to form the groove portion 23.

[0027] The sealing body 30 includes a lid 31, a current collector plate 32, and a gasket 33, and is formed into a disc shape as a whole. The sealing body 30 is placed on the groove 23 of the outer can 20 and fixed to the upper end of the outer can 20. The upper end of the outer can 20 is bent inward and caulked to the sealing body 30. In other words, the sealing body 30 is fixed to the upper end of the outer can 20 using the groove 23 and the caulked portion of the outer can 20, thereby sealing the opening of the outer can 20. The caulked portion is formed in an annular shape along the circumference of the outer can 20 and, together with the groove 23, holds the sealing body 30.

[0028] The cover 31 is a disc-shaped metal member that is exposed to the outside of the outer can 20 and forms the top surface of the cylindrical battery 1. The cover 31 has a shape with the radial center portion protruding toward the outside of the cylindrical battery 1. When the cylindrical battery 1 is modularized to form a battery pack, wiring materials are connected to the cover 31. Therefore, the cover 31 functions as an external terminal of the cylindrical battery 1 and is also called an external terminal or top cover. In this embodiment, the positive electrode lead 12 is connected to the current collector plate 32, and the cover 31 functions as the positive electrode external terminal.

[0029] The collector plate 32 is a metal member having a diameter approximately equal to that of the lid 31 and is positioned closer to the electrode body 10 than the lid 31. The collector plate 32 is annular and has an opening 32a at its radial center. The lid 31 and the collector plate 32 are welded together, for example, at a position closer to the outer periphery than the radial center of the lid 31. A protrusion 32b is formed on the collector plate 32, which serves as the weld to the lid 31.

[0030] The gasket 33 is provided around the outer periphery of the stack of the lid 31 and the current collector plate 32. The gasket 33 is a resin member that seals the interior of the battery while preventing contact between the lid 31 and the current collector plate 32 and the outer can 20, thereby ensuring insulation between the outer can 20 and the sealing body 30. The gasket 33 is an annular resin member with an opening 33a formed in its radial center that vertically overlaps the opening 32a of the current collector plate 32.

[0031] Below, in reference Figure 2 At the same time, the positive electrode 11 constituting the electrode body 10 will be described in detail. Figure 2 It is a front view of the positive electrode 11 to which the positive electrode lead 12 is connected, and shows the positive electrode 11 in a developed state.

[0032] like Figure 2 As shown, the positive electrode 11 is a long, strip-shaped object, with a plurality of positive electrode leads 12 connected at predetermined intervals along the length of the positive electrode 11. As described above, the positive electrode 11 includes a long, strip-shaped positive electrode core 13 and positive electrode mixture layers 14 provided on both surfaces of the positive electrode core 13. Furthermore, at the widthwise ends of the positive electrode 11, as portions for connecting the positive electrode leads 12, a plurality of exposed portions 15 are provided along the lengthwise direction of the positive electrode 11, where the surface of the positive electrode core 13 is exposed. The plurality of exposed portions 15 are arranged in a row along the lengthwise direction of the positive electrode 11 at one end (one widthwise end) of the positive electrode 11 located on the upper end side of the electrode body 10.

[0033] The length direction of the positive electrode 11 is along Figure 2 The first direction of the left and right direction, the width direction of the positive electrode 11 is along Figure 2The length of the positive electrode 11 (length in the first direction) is, for example, 30 to 70 times the width of the positive electrode 11 (length in the second direction), and as an example, 3000 to 4000 mm. The width of the positive electrode 11 is, for example, 60 to 80 mm, and is constant over the entire length.

[0034] The positive electrode lead 12 is a strip-shaped conductive member, for example, made of a metal with aluminum as the main component. The multiple positive electrode leads 12 extend in the same direction and extend upward from one end in the width direction of the positive electrode 11. In this embodiment, one end side of each positive electrode lead 12 in the longitudinal direction is joined to the positive electrode 11, and the other end side in the longitudinal direction is joined to the collector plate 32 of the sealing body 30. The number of positive electrode leads 12 is not particularly limited, and as an example, it is 4 or more and 15 or less. In this embodiment, 8 positive electrode leads 12 are connected to the positive electrode 11.

[0035] The same number of exposed portions 15 as positive electrode leads 12 is provided. Preferably, one positive electrode lead 12 is joined to each exposed portion 15 by welding or the like. The positive electrode lead 12 is joined only to one side of the positive electrode core 13, but the exposed portions 15 are provided on both sides of the positive electrode 11. The exposed portions 15 are formed to have substantially the same size so as to overlap along the thickness direction of the positive electrode 11. The joining position of the positive electrode lead 12 in each exposed portion 15 is not particularly limited, and the positive electrode lead 12 can be arranged within the range of the exposed portion 15 so as not to overlap with the positive electrode mixture layer 14.

[0036] Figure 2 In the example shown, the exposed portions 15 are formed with the same size and at equal intervals, but the sizes of the exposed portions 15 may be different from each other, and the exposed portions 15 may be formed at different intervals. The exposed portion 15 preferably has a rectangular shape when viewed from the front. The arrangement of the positive lead 12 in the electrode body 10, the connection position of the positive lead 12 relative to the collector plate 32, etc. can be appropriately set according to the battery performance such as the capacity and output characteristics of the cylindrical battery 1. Therefore, the size and interval of each exposed portion 15 can be determined based on the arrangement, etc. In addition, in order to reliably arrange the positive lead 12 within the range of the exposed portion 15, the length of the exposed portion 15 can also be made different on the inner and outer sides of the electrode body 10.

[0037] The length of the exposed portion 15 along the longitudinal direction of the positive electrode 11 must be greater than the width of the positive electrode lead 12, and is, for example, 5 mm to 50 mm, preferably 10 mm to 40 mm. The length (width) of the exposed portion 15 along the width direction of the positive electrode 11 is preferably 25% or less of the width of the positive electrode 11, and more preferably 10% to 20% of the width of the positive electrode 11. The width of each exposed portion 15 is preferably the same, and for example, is 7 mm to 15 mm. Each exposed portion 15 is formed with a constant width from one end of the width direction of the positive electrode 11.

[0038] The positive electrode 11 is characterized in that, with the region of the positive electrode mixture layer 14 aligned with the exposed portion 15 along the length of the positive electrode 11 defined as the first region 14a, and the region where the positive electrode mixture layer 14 continues along the length of the positive electrode 11 defined as the second region 14b, the compressive elastic modulus (Eα) of the first region 14a is greater than the compressive elastic modulus (Eβ) of the second region 14b. This effectively suppresses the formation of gaps between the electrode plates of the electrode assembly 10. It is believed that because the amount of positive electrode active material in the first region 14a is smaller than that in the second region 14b by the area of ​​the exposed portion 15, a large surface pressure difference occurs between the regions during battery charge and discharge, resulting in a tendency for gaps to form in the first region 14a. When the condition of Eα > Eβ is satisfied, it is believed that the surface pressure in the first region 14a can be increased, reducing this surface pressure difference and effectively suppressing the formation of gaps.

[0039] In this embodiment, each exposed portion 15 has the same width, and the width of the first region 14a is the same as that of each exposed portion 15. However, if the width of each exposed portion 15 is not constant, the range of length from one end of the width direction of the positive electrode 11 corresponding to the average width of each exposed portion 15 is defined as the first region 14a. Furthermore, in this embodiment, the compressive modulus (Eα) of the entire first region 14a (the same applies to the second region 14b) exhibits substantially the same value. When observing the first region 14a as a whole, as long as the condition Eα > Eβ is satisfied, the compressive modulus (Eα) of a portion of the first region 14a may be less than or equal to the compressive modulus (Eβ) of the second region 14b. If the compressive modulus of each region is not constant, it is sufficient that the average value of the compressive modulus of each region satisfies the condition Eα > Eβ.

[0040] The above-mentioned effects are achieved when the positive electrode 11 satisfies the condition Eα>Eβ. However, the compressive modulus (Eα) of the first region 14a is preferably 18 GPa or higher, more preferably 20 GPa or higher, and particularly preferably 23 GPa or higher. In this case, the above-mentioned effects are even more pronounced. There is no particular upper limit for the compressive modulus (Eα), but it is conceivable that if it is too high, the balance with the compressive modulus (Eβ) of the second region 14b will be lost, and gaps between the electrode plates may be more likely to form. The upper limit of the compressive modulus (Eα) is, for example, 40 GPa. An example of a suitable range for the compressive modulus (Eα) is 18 GPa or higher and 35 GPa or lower.

[0041] The compressive modulus of the positive electrode mixture layer 14 was measured using a dynamic ultra-microhardness tester (DUH-211S) manufactured by Shimadzu Corporation. Measurement conditions were based on JIS Z2255. While the compressive modulus of the positive electrode mixture layer 14 may vary depending on the type and amount of the conductive agent and binder, it is significantly dependent on the hardness of the positive electrode active material. The hardness of the positive electrode active material can be evaluated using particle fracture strength. It should be noted that the compressive modulus is calculated based on the displacement of the electrode plate from the start of the measurement until it reaches a compressive pressure of 0.4 GPa.

[0042] A positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder is applied to both surfaces of the positive electrode core 13, and the coating is dried and compressed to form the positive electrode mixture layer 14. For example, by using two slurries with different positive electrode active materials to form the first region 14a and the second region 14b, Eα>Eβ can be achieved. Specifically, a first positive electrode active material with high particle rupture strength is added to the slurry forming the first region 14a, and a second positive electrode active material with low particle rupture strength is added to the slurry forming the second region 14b. It should be noted that the first region 14a can be formed by intermittently applying the positive electrode mixture slurry along the length direction of the core 13.

[0043] The average particle breaking strength of the positive electrode active material contained in the first region 14a is preferably higher than the average particle breaking strength of the positive electrode active material contained in the second region 14b. The average particle breaking strength of the positive electrode active material contained in the first region 14a is preferably 100 MPa or greater, more preferably 150 MPa or greater, particularly preferably 250 MPa or greater, and may be 500 MPa or greater. The upper limit of the average particle breaking strength is not particularly limited, and for example, is 1000 MPa or 700 MPa.

[0044] The particle breaking strength of the positive electrode active material was measured using a micro compression tester (MCT-W201) manufactured by Shimadzu Corporation. The measurement procedure is as follows.

[0045] (1) Spread the positive electrode active material on the lower pressure plate (SKS plate) of the measuring device.

[0046] (2) Using an optical microscope, select particles with a size close to the volume-based median diameter (D50).

[0047] (3) A diamond flat indenter with a diameter of 50 μm was used as an upper presser so that only one particle existed between the upper presser and the lower presser plate.

[0048] (4) The upper presser is slowly lowered, and a load is applied at a constant acceleration from the moment it contacts the graphite particles (the lowering speed changes).

[0049] (5) The relationship between load and particle deformation was measured. The point where the particle deformation changed dramatically (the inflection point of the load-deformation curve) was defined as the point of failure. The load at that point and the particle diameter were used to calculate the breaking strength using the following formula. The average value of the particle breaking strength was calculated by averaging the measured values ​​of five graphite particles.

[0050] St=2.8P / πd 2

[0051] St: Fracture strength [MPa], P: Load [N], d: Particle diameter [mm]

[0052] The compressive elastic modulus (Eβ) of the second region 14b is smaller than the compressive elastic modulus (Eα) of the first region 14a, and is preferably 20 GPa or less, more preferably 19 GPa or less, and particularly preferably 18 GPa or less. In this case, the above-mentioned effect becomes more significant. The lower limit of the compressive elastic modulus (Eβ) is not particularly limited, but as an example, it is 15 GPa. An example of a suitable range of the compressive elastic modulus (Eβ) is 15 GPa or more and 20 GPa or 16 GPa or more and 18 GPa or less. The average value of the particle breaking strength of the positive electrode active material contained in the second region 14b is preferably 250 MPa or less, more preferably 150 MPa or less, and may be 100 MPa or less. The lower limit of the average value of the particle breaking strength is not particularly limited, but, for example, it is 50 MPa.

[0053] The ratio (Eα / Eβ) of the compressive elastic modulus (Eα) of the first region 14a to the compressive elastic modulus (Eβ) of the second region 14b is preferably 1.15 or greater, more preferably 1.30 or greater, and particularly preferably 1.50 or greater. In this case, the aforementioned effect becomes more pronounced. The upper limit of the ratio (Eα / Eβ) is not particularly limited, but is 2.50 as an example.

[0054] The first region 14a and the second region 14b contain different types of positive electrode active materials. The positive electrode active materials contained in each region may have different compositions, or they may have substantially the same composition but different particle shapes. Alternatively, each region may contain two or more types of positive electrode active materials, and their mass ratios may differ. The combination of positive electrode active materials in each region is determined so as to satisfy the condition Eα>Eβ. Specific examples of positive electrode active materials are described below.

[0055] The first region 14a may contain a first positive electrode active material in a non-agglomerated single particle state. In this specification, a single particle refers to a single primary particle, that is, a single crystal particle without a grain boundary inside the particle, or a secondary particle formed by a collection of 15 or less primary particles. Compared with secondary particles formed by the aggregation of multiple primary particles, single particles have higher particle destruction strength, thereby increasing the compressive elastic modulus of the positive electrode mixture layer 14. The particle destruction strength can be controlled, for example, by the sintering conditions of the positive electrode active material. Under conditions of high sintering temperature and long sintering time, the destruction strength tends to increase. In addition, the crystallite size also tends to become larger.

[0056] The non-agglomerated state of the positive electrode active material is observed using particle cross-section SEM images obtained using a scanning electron microscope (SEM). For example, the positive electrode 11 is embedded in a resin, and a cross-section of the positive electrode is produced using a process such as cross-section polishing (CP). The cross-section of the positive electrode mixture layer in this cross-section is then photographed using an SEM. Alternatively, the powder of the positive electrode active material is embedded in a resin, and a particle cross-section is produced using a process such as CP. This cross-section is then photographed using an SEM. To quantify the aggregation state of primary particles, first, particles whose particle diameter, as determined in the cross-section SEM image, is within 10% of the volume average particle diameter and the primary particle size is determined. The primary particles and aggregated particles are each spherical, and the volume of the primary particles is determined based on the ratio of the volume of the primary particles to the volume assumed for the volume average particle.

[0057] The first region 14a and the second region 14b may contain a first positive electrode active material as a single particle and a second positive electrode active material as secondary particles formed by agglomeration of multiple primary particles having an average particle size of 50 nm to 5 μm. In this case, the proportion of the first positive electrode active material in the first region 14a is preferably higher than the proportion of the first positive electrode active material in the second region 14b. The average particle size of the primary particles is calculated by measuring the diameters of 100 circumscribed circles of the extracted primary particles using scanning electron microscope (SEM) images of secondary particle cross-sections and averaging the measured values.

[0058] The proportion of the first positive electrode active material in the positive electrode active material contained in the first region 14a is, for example, 20% by mass or more or 30% by mass or more, and may be substantially 100%. The compressive elastic modulus of the first region 14a containing a large number of single particles tends to be higher. The proportion of the first positive electrode active material in the positive electrode active material contained in the second region 14b is, for example, 30% by mass or less or 25% by mass or less, and may be 15% by mass or more and 30% by mass or less. The first region 14a may substantially only contain the first positive electrode active material, and the second region 14b may substantially only contain the second positive electrode active material.

[0059] The volume-based median diameter (D50) of the first positive electrode active material (single particle) is preferably 2 μm or more and 12 μm or less. Furthermore, the D50 of the second positive electrode active material (secondary particle) is greater than the D50 of the first positive electrode active material and is preferably 10 μm or more and 20 μm or less. It should be noted that in this specification, D50 refers to the particle size at which the cumulative frequency of the smaller particle size in the volume-based particle size distribution reaches 50%. The particle size distribution of the positive electrode active material can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II manufactured by Microtrac Bel Co., Ltd.) using water as the dispersion medium.

[0060] The positive electrode active material is particles of a lithium metal composite oxide containing transition metal elements such as Ni, Co, and Mn. Examples of the metal elements contained in the lithium metal composite oxide include Li, Ni, Co, Mn, Al, Be, B, Na, Mg, Si, K, Ca, Sc, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, Ta, W, Pb, and Bi. The crystal structure of the lithium metal composite oxide is, for example, a layered rock salt structure belonging to space group R-3m.

[0061] Lithium metal composite oxides can be synthesized, for example, by mixing a composite oxide raw material containing Ni, Co, Mn, etc. and a Li raw material such as lithium hydroxide and calcining the mixture. The calcined product can be pulverized, classified, or washed with water. Composite oxide raw materials containing Ni, Co, Mn, etc. can be obtained, for example, by precipitating (coprecipitating) a composite hydroxide containing Ni, Co, Mn, etc. and heat-treating the composite hydroxide.

[0062] The composite hydroxide can be synthesized, for example, by adding an alkaline solution such as sodium hydroxide while stirring a solution containing a metal salt such as Ni, Co, or Mn, and adjusting the pH to the alkaline side (e.g., 8.5 or higher and 12.5 or lower). The particle size of the composite hydroxide tends to decrease as the pH during synthesis increases, and can also be controlled by adjusting the amount of the metal salt solution added. Lithium metal composite oxides having different particle sizes can be produced separately by controlling the particle size of the composite hydroxide.

[0063] The calcination process of the mixture of the composite oxide raw material and the Li raw material may be a multi-stage calcination process including a first calcination process and a second calcination process at a higher temperature than the first calcination process. The calcination of the mixture is carried out in an oxygen atmosphere, in which case the oxygen concentration is set to 85% or more, for example. Although the appropriate first calcination temperature may vary slightly depending on the composition of the mixture, it is, for example, 500°C or more and 750°C or less. The appropriate second calcination temperature is, for example, 800°C or more and 1150°C or less. The temperature difference between the calcination processes is preferably 50°C or more.

[0064] By adjusting the calcination temperature of the raw material mixture, the compressive strength of the final composite oxide particles can also be controlled, and the particles in a non-agglomerated state can also be adjusted. When the high-Ni-containing oxide particles are used as particles in a non-agglomerated state and the compressive strength is set to be greater than 250 MPa, the calcination temperature of the raw material mixture is preferably in the range of 750°C to 1100°C, for example. The calcination temperature is preferably 20 hours to 150 hours, more preferably 20 hours to 100 hours. It should be noted that when the calcination time of the high-Ni-containing composite oxide particles is greater than 150 hours, there is a possibility of deterioration in material properties and electrochemical characteristics compared to the case of less than 150 hours.

[0065] Examples of the conductive agent contained in the positive electrode mixture layer 14 include carbon black such as acetylene black and Ketjen black, graphite, carbon nanotubes (CNTs), carbon nanofibers, graphene, metal fibers, metal powder, conductive whiskers, etc. The conductive agent may be used alone or in combination of two or more.

[0066] Examples of the binder contained in the positive electrode mixture layer 14 include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF); olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymers, and ethylene-propylene-butadiene copolymers; and acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide, and ethylene-acrylic acid copolymers. Furthermore, these resins may be used in combination with carboxymethylcellulose (CMC) or its salts, polyethylene oxide (PEO), and the like. The binder may be used alone or in combination of multiple types.

[0067] The mass ratios of the positive electrode active material, the conductive agent, and the binder in the first region 14a and the second region 14b may be the same or different. The content of the positive electrode active material in the positive electrode mixture layer 14 is, for example, 95% by mass or more and 99% by mass or less. The content of the conductive agent and the binder in the positive electrode mixture layer 14 is, for example, 0.5% by mass or more and 2.0% by mass or less, respectively.

[0068] Example

[0069] Hereinafter, the present application will be further described using examples, but the present application is not limited to these examples.

[0070] [Synthesis of Lithium Metal Composite Oxide X1]

[0071] A composite hydroxide containing Ni, Co, and Al in a molar ratio of 85:10:5 was synthesized using a coprecipitation method and then heat-treated at 600°C to produce a composite oxide. During the synthesis of the composite hydroxide, the pH and amount of the metal salt solution were adjusted to achieve a D50 of approximately 15 μm for the resulting lithium-metal composite oxide. The resulting composite oxide was mixed with lithium hydroxide such that the molar ratio of the metal element (Me) in the composite oxide to the Li in the lithium hydroxide (Li / Me ratio) was 1:1.020. This mixture was placed in a calcining furnace and calcined in two stages.

[0072] During the firing process, under an oxygen flow with an oxygen concentration of 95% (per 10 cm 3 The temperature was raised from room temperature to 650°C (first calcination temperature) at a rate of 3°C / min (first calcination rate) at a flow rate of 2 mL / min per kg of the mixture and 5 L / min per kg of the mixture. Subsequently, the temperature was raised from 650°C to 750°C (second calcination temperature) at a rate of 1°C / min (second calcination rate) and maintained at 750°C for 3 hours. The calcined product was pulverized and washed with water to obtain a lithium metal composite oxide X1.

[0073] The volume-based D50 of the lithium metal composite oxide X1, measured using a Microtrac Bel MT3000II with water as the dispersion medium, was 12 μm. Furthermore, the particle fracture strength, measured using the aforementioned method using a Shimadzu Corporation microcompression tester (MCT-W201), was 198 MPa. Furthermore, SEM images of particle cross-sections confirmed that the composite oxide consisted of secondary particles formed by the aggregation of primary particles with an average particle size of 500 nm.

[0074] [Synthesis of Lithium Metal Composite Oxide X2]

[0075] Secondary particle-type lithium metal composite oxide X2 was obtained in the same manner as in the synthesis of the lithium metal composite oxide X1 except that the second calcination temperature and calcination time were adjusted so as to achieve D50 of 12 μm and particle fracture strength of 117 MPa.

[0076] [Synthesis of Lithium Metal Composite Oxide X3]

[0077] Secondary particle-type lithium metal composite oxide X3 was obtained in the same manner as in the synthesis of the lithium metal composite oxide X1 except that the second calcination temperature and calcination time were adjusted so as to achieve D50 of 12 μm and particle fracture strength of 90 MPa.

[0078] [Synthesis of Lithium Metal Composite Oxide Y1]

[0079] Single-particle lithium metal composite oxide Y1 was obtained in the same manner as in the synthesis of lithium metal composite oxide X1 except that the calcination temperature was set to 1000° C. and the calcination time was adjusted so as to achieve D50 of 6 μm and particle fracture strength of 650 MPa.

[0080] [Synthesis of Lithium Metal Composite Oxide Y2]

[0081] A single-particle lithium metal composite oxide Y2 was obtained in the same manner as in the synthesis of the lithium metal composite oxide Y1 except that the calcination time was adjusted so that D50 became 6 μm and the particle fracture strength became 570 MPa.

[0082] <Example 1>

[0083] [Preparation of the First Positive Electrode Mixture Slurry]

[0084] Lithium metal composite oxide Y1 was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride (PVdF) were mixed at a solid content mass ratio of 98:1:1, and N-methyl-2-pyrrolidone (NMP) was used as the dispersion medium to prepare a first positive electrode mixture slurry.

[0085] [Preparation of the Second Positive Electrode Mixture Slurry]

[0086] A second positive electrode mixture slurry was prepared by the same method as the first positive electrode mixture slurry except that the lithium metal composite oxide X2 was used as the positive electrode active material.

[0087] [Production of positive electrode]

[0088] The first and second positive electrode mixture slurries are applied to both sides of a positive electrode core body made of aluminum foil to form a coating film, and the coating film is dried. The first positive electrode mixture slurry is intermittently applied to a portion corresponding to one end portion in the width direction of the positive electrode core body, leaving an exposed portion. Thus, a first region of the positive electrode mixture layer is formed that is aligned with the exposed portion along the longitudinal direction of the positive electrode (becoming a coating film of the first region). The second positive electrode mixture slurry forming the second region is continuously applied along the longitudinal direction of the positive electrode core body. Then, the coating film is rolled using a roller, and the positive electrode core body is cut into a specified electrode size to obtain a positive electrode having a positive electrode mixture layer formed on both sides of the positive electrode core body.

[0089] In this example, the positive electrode has a length of 3650 mm and a width of 68 mm. The ratio of the width of the first region to the width of the second region is 1:6. Furthermore, eight exposed portions of equal size (length: 30 mm, width: 9.7 mm) are provided at equal intervals along the length of the positive electrode. The compressive elastic modulus (Eα) of the first region and the compressive elastic modulus (Eβ) of the second region were measured using a dynamic microhardness tester (DUH-211S) manufactured by Shimadzu Corporation. The results showed that Eα was 34 GPa and Eβ was 18 GPa.

[0090] [Production of negative electrode]

[0091] The negative electrode active material is a mixture of natural graphite and a silicon-containing material (a composite material composed of a finely dispersed Si phase within a silicon oxide phase) at a mass ratio of 98:2. A negative electrode mixture slurry is prepared by mixing the negative electrode active material, sodium carboxymethylcellulose (CMC-Na), and a dispersion of styrene-butadiene rubber (SBR) at a solids mass ratio of 100:1:1, using water as the dispersion medium. This negative electrode mixture slurry is applied to both sides of a negative electrode core made of copper foil. After drying, the coating is rolled using a roller and cut to the specified electrode size, resulting in a negative electrode with a negative electrode mixture layer formed on both sides of the negative electrode core. It should be noted that a portion of the negative electrode has an exposed portion that exposes the surface of the negative electrode core.

[0092] [Preparation of non-aqueous electrolyte]

[0093] A non-aqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1.2 mol / L in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (MEC), and dimethyl carbonate (DMC) at a volume ratio of 3:3:4 (25° C.).

[0094] [Preparation of test batteries]

[0095] An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to the exposed portion of the negative electrode. The positive and negative electrodes were then spirally wound with a polyolefin separator interposed therebetween to produce a wound electrode assembly. This electrode assembly was housed in a bottomed cylindrical outer can, and after the non-aqueous electrolyte was injected, the opening of the outer can was sealed with a sealant to produce a cylindrical test cell.

[0096] <Example 2>

[0097] A test cell was produced in the same manner as in Example 1, except that the lithium metal composite oxide Y2 was used as the positive electrode active material in the preparation of the first positive electrode mixture slurry.

[0098] <Example 3>

[0099] A test cell was produced in the same manner as in Example 1, except that the lithium metal composite oxide X1 was used as the positive electrode active material in the preparation of the first positive electrode mixture slurry.

[0100] <Example 4>

[0101] A test cell was prepared in the same manner as in Example 1, except that the lithium metal composite oxide X2 was used as the positive electrode active material in the preparation of the first positive electrode mixture slurry, and the lithium metal composite oxide X3 was used as the positive electrode active material in the preparation of the second positive electrode mixture slurry. Note that Eα was 18 GPa and Eβ was 17 GPa.

[0102] <Example 5>

[0103] A test cell was produced in the same manner as in Example 1, except that a mixture of lithium metal composite oxide Y2 and X1 at a mass ratio of 35:65 was used as the positive electrode active material in the preparation of the first positive electrode mixture slurry, and a mixture of lithium metal composite oxide Y2 and X1 at a mass ratio of 15:85 was used as the positive electrode active material in the preparation of the second positive electrode mixture slurry. It should be noted that Eα was 23 GPa and Eβ was 19 GPa.

[0104] <Example 6>

[0105] A test cell was produced in the same manner as in Example 5, except that a mixture of lithium metal composite oxides Y2 and X1 at a mass ratio of 25:75 was used as the positive electrode active material in the preparation of the second positive electrode mixture slurry.

[0106] <Example 7>

[0107] A test cell was produced in the same manner as in Example 1, except that a mixture of lithium metal composite oxides Y2 and X1 at a mass ratio of 25:75 was used as the positive electrode active material in the preparation of the first positive electrode mixture slurry, and a mixture of lithium metal composite oxides Y2 and X1 at a mass ratio of 20:80 in the exposed portion was used as the positive electrode active material in the preparation of the second positive electrode mixture slurry. It should be noted that Eα was 20 GPa and Eβ was 18 GPa.

[0108] Comparative Example 1

[0109] A test cell was produced in the same manner as in Example 2, except that the first positive electrode mixture slurry was used to form the second region and the second positive electrode mixture slurry was used to form the first region.

[0110] Comparative Example 2

[0111] A test cell was produced in the same manner as in Example 2, except that the first positive electrode mixture slurry was used to form the entire positive electrode mixture layer. Note that both Eα and Eβ were 31 GPa.

[0112] Comparative Example 2

[0113] A test cell was produced in the same manner as in Example 2, except that the second positive electrode mixture slurry was used to form the entire positive electrode mixture layer. Note that both Eα and Eβ were 18 GPa.

[0114] [Evaluation of the plate gap]

[0115] For each test battery of the embodiment and the comparative example, a constant current charge of 0.3 It was performed at a temperature environment of 25°C until the battery voltage reached 4.2V, and then a constant voltage charge was performed at 4.2V until the current value reached 0.02It. Thereafter, a constant current discharge was performed at 0.5 It until the battery voltage reached 2.5V. This charge and discharge was set as 1 cycle, and 200 cycles were performed. Thereafter, a CT image of the upper cross-section of the test battery was obtained. The presence or absence of a gap between the electrode plates of the electrode body was confirmed based on the CT image, and the size of the gap was confirmed if a gap existed. Table 1 shows the physical properties of the positive electrode mixture layer and also shows its evaluation results.

[0116] In the area where a gap exists within the electrode assembly, the gap is calculated by subtracting half the thickness of each electrode plate and the thickness of the spacer from the distance between the centerline of the electrode plate on the inner and outer sides of the electrode assembly in the thickness direction. The gap is then categorized as none, small, medium, or large based on the following evaluation criteria.

[0117] None: gap less than 10μm

[0118] Small: The gap is 10 μm or more and less than 50 μm

[0119] Medium: Gap is 50 μm or more and less than 100 μm

[0120] Large: Gap of 100 μm or more

[0121] [Table 1]

[0122]

[0123] As shown in Table 1, the test batteries of the embodiments are more difficult to generate gaps between the electrode plates than the test batteries of Comparative Example 1. When the compressive elastic modulus Eα of the first region of the positive electrode mixture layer is smaller than the compressive elastic modulus Eβ of the second region, or when Eα=Eβ, as in the test batteries of the comparative example, it is confirmed that a large gap is generated between the electrode plates in the upper part of the electrode body. If such a large gap exists, there is a possibility of peeling of the mixture layer during the charge and discharge cycle. Based on this result, it can be understood that when Eα>Eβ, the generation of gaps between the electrode plates can be effectively suppressed. According to the test batteries of the embodiments, it is believed that the surface pressure of the first region (the axial end of the electrode body) becomes higher, which can alleviate the surface pressure difference with the second region (the axial center of the electrode body).

[0124] The present application is further described through the following embodiments.

[0125] Configuration 1: A secondary battery comprising an electrode body, wherein the electrode body includes a positive electrode, a negative electrode and a separator and is formed by winding the positive electrode and the negative electrode with the separator interposed therebetween, the positive electrode having an elongated positive electrode core and a positive electrode mixture layer arranged on the positive electrode core, and a plurality of exposed portions exposing the surface of the positive electrode core are provided along the longitudinal direction of the positive electrode at the width end of the positive electrode, and when the region of the positive electrode mixture layer that is aligned with the exposed portion along the longitudinal direction of the positive electrode is defined as a first region and the region where the positive electrode mixture layer is continuous along the longitudinal direction of the positive electrode is defined as a second region, the compressive elastic modulus of the first region is greater than the compressive elastic modulus of the second region.

[0126] Configuration 2: The secondary battery according to Configuration 1, wherein the compressive elastic modulus of the first region is 18 GPa or more and 35 GPa or less.

[0127] Configuration 3: The secondary battery according to Configuration 1 or 2, wherein the average value of the particle fracture strength of the positive electrode active material contained in the first region is 250 MPa or more.

[0128] Configuration 4: The secondary battery according to any one of Configurations 1 to 3, wherein the first region contains a first positive electrode active material in the form of a single particle.

[0129] Configuration 5: A secondary battery according to any one of Configurations 1 to 4, wherein the first region and the second region contain a first positive electrode active material as a single particle, and a second positive electrode active material as a secondary particle formed by agglomeration of primary particles having an average particle diameter of greater than 50 nm and less than 5 μm, the proportion of the first positive electrode active material in the positive electrode active material contained in the first region is greater than 30 mass%, and the proportion of the first positive electrode active material in the positive electrode active material contained in the second region is greater than 15 mass% and less than 30 mass%.

[0130] Description of Reference Numerals

[0131] 1 Cylindrical battery, 10 Electrode body, 11 Positive electrode, 12 Positive electrode lead, 13 Positive electrode core, 14 Positive electrode mixture layer, 14a First region, 14b Second region, 15 Exposed portion, 20 Outer can, 21 Side wall, 22 Can bottom, 23 Grooved portion, 30 Sealing member, 31 Lid, 32 Current collector, 32a, 33a Openings, 32b Protrusion, 33 Gasket, 40 Upper insulating plate, 41 First insulating plate, 42 Second insulating plate.

Claims

1. A secondary battery comprising an electrode body comprising a positive electrode, a negative electrode, and a separator, wherein the positive electrode and the negative electrode are wound with the separator interposed therebetween. The positive electrode comprises a long positive electrode core and a positive electrode mixture layer disposed on the positive electrode core. At the widthwise end of the positive electrode, a plurality of exposed portions exposing the surface of the positive electrode core are provided along the longitudinal direction of the positive electrode. When the area of ​​the positive electrode mixture layer that is parallel to the exposed portion along the longitudinal direction of the positive electrode is defined as the first area, and the area of ​​the positive electrode mixture layer that is continuous along the longitudinal direction of the positive electrode is defined as the second area, the compressive elastic modulus of the first area is greater than the compressive elastic modulus of the second area.

2. The secondary battery according to claim 1, wherein The compressive elastic modulus of the first region is 18 GPa or more and 35 GPa or less.

3. The secondary battery according to claim 1 or 2, wherein The average value of the particle fracture strength of the positive electrode active material contained in the first region is 250 MPa or more.

4. The secondary battery according to claim 1 or 2, wherein The first region contains a first positive electrode active material as a single particle.

5. The secondary battery according to claim 1 or 2, wherein The first region and the second region contain a first positive electrode active material as a single particle and a second positive electrode active material as a secondary particle formed by aggregation of primary particles. The first positive electrode active material accounts for 30% by mass or more of the positive electrode active material contained in the first region. The ratio of the first positive electrode active material to the positive electrode active material contained in the second region is 15 mass % or more and 30 mass % or less.

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