Secondary battery

Abstract

A secondary battery according to an embodiment of the present disclosure comprises a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode and the negative electrode are wound with the separator therebetween. The positive electrode comprises a band-shaped positive electrode current collector and a positive electrode mixture layer disposed on at least one surface of the positive electrode current collector. The positive electrode has a first region including one end of the positive electrode in the transverse direction, and a second region excluding the first region. The first region has exposed portions of the positive electrode current collector which are partially provided at one or more positions along the longitudinal direction of the positive electrode current collector. The exposed portions do not include the positive electrode mixture layer from the one end of the positive electrode in the traverse direction to the second region. The positive electrode mixture layer includes a positive electrode active material and carbon black. The peel strength of the positive electrode mixture layer as measured by SAICAS method at two specific positions in the thickness direction satisfies a specific relationship.

Description

secondary battery

[0001] This invention relates to a secondary battery.

[0002] Rechargeable batteries, such as lithium-ion batteries, are used in a variety of applications due to their high capacity and other characteristics. A rechargeable battery comprises a positive electrode, a negative electrode, an electrolyte, and a separator. In such a rechargeable battery, the separator is positioned between the positive and negative electrodes. A laminate in which the positive electrode, separator, and negative electrode are stacked in this order can be wound and used as a wound electrode body.

[0003] When a positive electrode comprises a positive electrode current collector and a positive electrode mixture layer disposed on at least one surface of the positive electrode current collector, cracking may occur in the positive electrode mixture layer when a wound electrode body is formed using such a positive electrode. Furthermore, the positive electrode mixture layer may delaminate from the positive electrode current collector. Note that cracking in the positive electrode mixture layer is equivalent to a portion of the positive electrode mixture layer delaminating from the rest. Various proposals have been made to address the above problems.

[0004] Patent Document 1 discloses a method for manufacturing a lithium-ion secondary battery, comprising a slitting step of slitting a wide electrode substrate on which an electrode mixture is coated on the surface of a foil, and a winding step of winding the slit electrode slit material, characterized in that a judgment threshold is set to determine the quality of peeling in the slitting step or the winding step based on the peel strength in the film thickness direction measured by the SAICAS® method.

[0005] Further, Patent Document 1 discloses that the peeling of the electrode binder in the winding process is bending peeling caused by stress concentration due to bending, and the starting point thereof exists on the surface layer of the electrode binder, and the lower the surface layer strength, the higher the flexibility of the surface layer of the electrode binder, so it is considered that the generation of the starting point of bending peeling can be suppressed on the surface layer of the electrode binder. Further, Patent Document 1 discloses that by managing the surface layer strength of the electrode binder by the SAICA S (registered trademark) test and performing the winding process, peeling in the electrode binder can be suppressed. That is, Patent Document 1 proposes to suppress peeling in the electrode binder by determining the above determination threshold based on the surface layer strength of the electrode binder in the winding process. Note that the surface layer strength is the peeling strength at a measurement position 2 μm from the surface of the electrode binder.

[0006] Patent Document 2 discloses a method for manufacturing an electrode for a lithium-ion secondary battery, the method including: a step of preparing a current collector; a step of preparing granulated powder containing active material particles and a binder; a step of forming an active material layer made of the granulated powder on the current collector by supplying and pressing the granulated powder onto the surface of the current collector, wherein an inclined surface that inclines from the uppermost part of the active material layer as viewed from the current collector side toward the current collector is provided at an edge of the active material layer; a step of applying an adhesive to the inclined surface; and a step of curing the adhesive to form an adhesive layer.

[0007] Further, Patent Document 2 discloses that in the production of a wound electrode body, although a problem occurs in that a large stress (load) is applied to the electrode during winding, particularly, the edge of the active material layer is likely to peel off or the active material is likely to slip off, such a problem can be addressed by using the electrode for a lithium-ion secondary battery obtained by the above production method.

[0008] Japanese Unexamined Patent Application Publication No. 2013 - 109853 Japanese Unexamined Patent Application Publication No. 2016 - 71955

[0009] Incidentally, when forming a wound electrode body using a positive electrode having a positive electrode current collector and a positive electrode mixture layer disposed on at least one surface of the positive electrode current collector, in order to improve the permeability of the electrolyte into the positive electrode mixture layer, the positive electrode may be configured to have an exposed portion of the positive electrode current collector. For example, it has a first region including one end in the short side direction of the positive electrode and a second region other than the first region, and the first region has an exposed portion of the positive electrode current collector provided partially at one or more locations along the longitudinal direction of the strip-shaped positive electrode current collector, and the positive electrode may be configured such that the exposed portion does not have a positive electrode mixture layer from one end in the short side direction of the positive electrode to the second region.

[0010] In the positive electrode having the above configuration, the positive electrode mixture layer existing at the boundary portion with the exposed portion of the positive electrode current collector often becomes locally thick. Therefore, when forming a wound electrode body using the positive electrode having the above configuration, during winding, a stress load (for example, a load due to bending tensile stress) is likely to be applied to the locally thick portion (hereinafter also referred to as the thick layer portion) in the positive electrode mixture layer. As a result, cracks are likely to occur in the thick layer portion of the positive electrode mixture layer due to this stress load.

[0011] Also, when forming the wound electrode body, in order to sufficiently adhere the overlapping portions of the laminates of the positive electrode, the separator, and the negative electrode to each other, winding is usually performed such that an external force acts from the outside toward the winding axis. On the other hand, when forming the wound electrode body, a reaction force against the above external force is generated in the laminate. Therefore, in the positive electrode, an external force such that the positive electrode mixture layer presses the positive electrode current collector and a reaction force such that the positive electrode current collector pushes back the positive electrode mixture layer are generated. And when the adhesion force of the positive electrode mixture layer to the positive electrode current collector exceeds the above reaction force, the positive electrode mixture layer peels off from the positive electrode current collector. Also, after the positive electrode mixture layer is peeled off from the positive electrode current collector, the positive electrode mixture layer may be deformed so as to shrink.

[0012] Furthermore, when the positive electrode mixture layer contains a binder and a conductive material, the binder is often present in the positive electrode mixture layer attached to the conductive material. When carbon nanotubes are used as the conductive material, the carbon nanotubes are included in the positive electrode mixture layer while in linear contact with the positive electrode active material. Therefore, the binder can be present over a wide area of ​​the positive electrode mixture layer while attached to the carbon nanotubes. In this case, cracking in the positive electrode mixture layer can be sufficiently suppressed. On the other hand, when carbon black is used as the conductive material, the carbon black is included in the positive electrode mixture layer while in point-like contact with the positive electrode active material. Therefore, excessive unevenness in the distribution of carbon black occurs in the positive electrode mixture layer, and consequently, excessive unevenness in the distribution of the binder can occur. In this case, sufficient binding is difficult to obtain in areas of the positive electrode mixture layer where there is little binder, and cracking may occur in these areas. In particular, if excessive unevenness in the distribution of the binder occurs in the thicker parts of the positive electrode mixture layer, cracking will become more pronounced in these thicker parts.

[0013] Therefore, the object of this disclosure is to provide a secondary battery having a wound electrode body and in which the positive electrode mixture layer contains carbon black, while suppressing peeling of the positive electrode mixture layer from the positive electrode current collector and cracking of the positive electrode mixture layer.

[0014] One aspect of the present invention relates to a secondary battery. The secondary battery comprises a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode and the negative electrode are wound around the separator. The positive electrode comprises a strip-shaped positive electrode current collector and a positive electrode mixture layer disposed on at least one surface of the positive electrode current collector. The positive electrode has a first region including one end in the short direction of the positive electrode and a second region other than the first region. The first region has one or more exposed portions of the positive electrode current collector provided along the longitudinal direction of the positive electrode current collector. The exposed portions do not have the positive electrode mixture layer from one end in the short direction of the positive electrode to the second region. The positive electrode mixture layer comprises a positive electrode active material and carbon black. With respect to the positive electrode mixture layer, when the peel strength measured by the SAICAS® method at a position 2 μm away from the surface of the positive electrode current collector in the thickness direction is defined as PS1 (kN / m), and the peel strength measured by the SAICAS® method at a position 10 μm away from the outermost surface in the thickness direction is defined as PS2 (kN / m), then PS1 and PS2 satisfy either of the following conditions (a) or (b): (a) 0.073 kN / m ≤ PS1 ≤ 0.097 kN / m and PS2 / PS1 ≥ 2.8. (b) 0.098 kN / m ≤ PS1.

[0015] According to this disclosure, it is possible to provide a secondary battery having a wound electrode body and in which the positive electrode mixture layer contains carbon black, while suppressing peeling of the positive electrode mixture layer from the positive electrode current collector and cracking of the positive electrode mixture layer.

[0016] This is a schematic cross-sectional view showing an example of a secondary battery according to an embodiment of the present disclosure. This is a schematic plan view showing an example of a positive electrode of a secondary battery according to an embodiment of the present disclosure. This is a schematic plan view showing an example of a negative electrode of a secondary battery according to an embodiment of the present disclosure.

[0017] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values, materials, etc. may be applied as long as the effects of this disclosure are obtained. Notwithstanding, known components may be applied to components of parts that are characteristic of this disclosure. In this specification, when "the range of numerical values ​​A to numerical values ​​B" is used, that range includes numerical values ​​A and B.

[0018] In the following explanation, when examples are given for the lower and upper limits of numerical values ​​related to specific physical properties or conditions, any combination of either of the given lower limits and any of the given upper limits is permitted, as long as the lower limit does not exceed the upper limit. When multiple materials are given as examples, unless otherwise specified, one type may be selected and used alone, or two or more types may be used in combination.

[0019] This disclosure includes any combination of two or more claims that can be arbitrarily selected from the claims set forth in the attached claims. In other words, any combination of two or more claims that can be arbitrarily selected from the claims set forth in the attached claims is possible, as long as it does not result in a technical inconsistency.

[0020] [Secondary Battery] A secondary battery according to an embodiment of the present disclosure comprises a positive electrode, a negative electrode, an electrolyte, and a separator. In the secondary battery according to an embodiment of the present disclosure, the positive electrode and the negative electrode are wound around a separator. In the secondary battery according to an embodiment of the present disclosure, the positive electrode comprises a strip-shaped positive electrode current collector and a positive electrode mixture layer disposed on at least one surface of the positive electrode current collector, and the positive electrode has a first region including one end in the short direction of the positive electrode and a second region other than the first region. In the secondary battery according to an embodiment of the present disclosure, the first region has one or more exposed portions of the positive electrode current collector provided along the longitudinal direction of the positive electrode current collector, and the exposed portions do not have a positive electrode mixture layer from one end in the short direction of the positive electrode to the second region.

[0021] In the secondary battery according to the embodiment of this disclosure, the positive electrode mixture layer includes a positive electrode active material and carbon black. In the secondary battery according to the embodiment of this disclosure, when the peel strength of the positive electrode mixture layer measured by the SAICAS® method at a position 2 μm away from the surface of the positive electrode current collector in the thickness direction is defined as PS1 (kN / m), and the peel strength measured by the SAICAS® method at a position 10 μm away from the outermost surface in the thickness direction is defined as PS2 (kN / m), then PS1 and PS2 satisfy either of the following conditions (a) or (b): (a) 0.073 kN / m ≤ PS1 ≤ 0.097 kN / m and PS2 / PS1 ≥ 2.8. (b) 0.098 kN / m ≤ PS1.

[0022] In the secondary battery according to the embodiment of this disclosure, it is important that (i) the positive electrode mixture layer contains carbon black, and (ii) the peel strength PS1 of the positive electrode mixture layer measured at a predetermined position near the positive electrode current collector and the peel strength PS2 of the positive electrode mixture layer measured at a predetermined position near the outermost surface of the positive electrode mixture layer satisfy the following conditions: (a) 0.073 kN / m ≤ PS1 ≤ 0.097 kN / m and PS2 / PS1 ≥ 2.8, or (b) 0.098 kN / m ≤ PS1. The reasons for this will be explained below.

[0023] When forming a wound electrode body using a laminate of a strip-shaped positive electrode, a strip-shaped separator, and a strip-shaped negative electrode, winding is usually carried out so that an external force acts from the outside toward the winding axis in order to ensure sufficient contact between the overlapping portions of the laminates. On the other hand, when forming the wound electrode body, a reaction force is generated in the laminate that opposes the above external force. Specifically, a reaction force is generated in the laminate that acts outward from the winding axis. Therefore, in the positive electrode, an external force is generated in which the positive electrode mixture layer presses against the positive electrode current collector, and a reaction force (hereinafter referred to as the first reaction force) is generated in which the positive electrode current collector pushes back against the positive electrode mixture layer.

[0024] Furthermore, if the strip-shaped positive electrode has a first region including one end in the short direction and a second region other than the first region, and the first region has one or more exposed portions of the positive electrode current collector provided along the longitudinal direction of the positive electrode current collector, and the exposed portions of the positive electrode current collector do not have a positive electrode mixture layer from one end in the short direction of the positive electrode current collector to the second region, then tape may be applied to cover the exposed portions of the positive electrode current collector. In this case, when forming the wound electrode body, an external force is generated in which the tape presses against the positive electrode mixture layer, and a reaction force (hereinafter referred to as the second reaction force) is generated in which the positive electrode mixture layer pushes back against the tape.

[0025] Furthermore, in the case of the positive electrode, if tape is not applied to cover the exposed portion of the positive electrode current collector, that is, if the exposed portion of the positive electrode current collector remains exposed, the positive electrode mixture layer will peel off from the positive electrode current collector if the adhesion force of the positive electrode mixture layer to the positive electrode current collector exceeds the first reaction force. Also, in the case of the positive electrode, if tape is applied to cover the exposed portion of the positive electrode current collector, the positive electrode mixture layer will peel off from the positive electrode current collector if the adhesion force of the positive electrode mixture layer to the positive electrode current collector exceeds the sum of the first and second reaction forces. In addition, after the positive electrode mixture layer has peeled off from the positive electrode current collector, the positive electrode mixture layer may deform to shrink.

[0026] Furthermore, as described above, when the positive electrode has an exposed portion of the positive electrode current collector, the positive electrode mixture layer at the boundary with the exposed portion of the positive electrode current collector often becomes locally thicker. Therefore, when a wound electrode is formed using such a positive electrode, stress loads (for example, loads due to bending tensile stress) are easily applied to the locally thickened portion of the positive electrode mixture layer (hereinafter also referred to as the thickened portion) during winding. As a result, cracks are more likely to occur in the thickened portion of the positive electrode mixture layer due to these stress loads.

[0027] Furthermore, when the positive electrode mixture layer contains both a binder and a conductive material, the binder is often present in the positive electrode mixture layer attached to the conductive material. When carbon black is used as the conductive material, the carbon black is included in the positive electrode mixture layer while in point-like contact with the positive electrode active material. As a result, excessive unevenness in the distribution of carbon black can occur in the positive electrode mixture layer, and consequently, excessive unevenness in the distribution of the binder can occur. In this case, cracking may occur in areas of the positive electrode mixture layer where there is less binder. In particular, if excessive unevenness in the binder distribution occurs in the thicker portions of the positive electrode mixture layer as described above, cracking will become more pronounced in these thicker portions.

[0028] However, in the secondary battery according to the embodiment of this disclosure, the peel strength PS1 of the positive electrode mixture layer measured at a predetermined position near the positive electrode current collector, and the peel strength PS2 of the positive electrode mixture layer measured at a predetermined position near the outermost surface of the positive electrode mixture layer, satisfy either (a) 0.073 kN / m ≤ PS1 ≤ 0.097 kN / m and PS2 / PS1 ≥ 2.8, or (b) 0.098 kN / m ≤ PS1. Therefore, it is considered that a sufficient peel strength PS1 of the positive electrode mixture layer near the positive electrode current collector can be secured in either case (a) or (b). This is considered to suppress the peeling of the positive electrode mixture layer from the positive electrode current collector during the formation of the wound electrode body. Furthermore, under condition (a), since PS2 / PS1 ≥ 2.8, it is considered that the conductive material, acetylene black, and the binder are sufficiently distributed to the outermost surface of the positive electrode mixture layer. In other words, it is considered that excessive bias in the distribution of conductive material and binder in the positive electrode mixture layer can be suppressed. This is considered to suppress cracking in the positive electrode mixture layer when the wound electrode body is formed. In the case of condition (b), it is considered that the amount of conductive material and binder distributed near the positive electrode current collector is greater than in the case of condition (a), so the amount of conductive material and binder distributed near the outermost surface of the positive electrode mixture layer is considered to be relatively smaller. Therefore, it is considered that bias in the distribution of conductive material and binder in the positive electrode mixture layer can be suppressed more sufficiently than in the case of condition (a). This is considered to suppress cracking in the positive electrode mixture layer when the wound electrode body is formed, even in the case of condition (b).

[0029] The peel strengths PS1 and PS2 of the positive electrode mixture layer can be measured using a surface-interface property analysis device. Specifically, the peel strengths PS1 and PS2 of the positive electrode mixture layer can be measured using a "SAICAS® EN-WA type" manufactured by Typela Wintes, Inc., in constant speed mode (horizontal speed: 100 μm / sec). A diamond cutting blade (width 1 mm) can be used as the cutting blade. In the above device, the cutting blade is attached to the shaft. Specifically, when the shaft is positioned along the vertical direction, the cutting blade is attached to the shaft so as to be inclined downward at a predetermined angle with respect to the vertical direction.

[0030] The peel strength using the above apparatus can be measured by inserting the cutting blade to a predetermined depth in the thickness direction of the positive electrode mixture layer, and then obtaining the cutting speed when the cutting blade is moved at a horizontal speed of 100 μm / sec. The peel strength PS1 can be obtained as the cutting speed at a position 2 μm away from the surface of the positive electrode current collector in the thickness direction. Specifically, the peel strength PS1 can be obtained as the cutting speed when the cutting blade is inserted to a position 2 μm away from the surface of the positive electrode current collector in the thickness direction, and then moved at a horizontal speed of 100 μm / sec. The peel strength PS2 can be obtained as the cutting speed at a position 10 μm away from the outermost surface in the thickness direction. Specifically, the peel strength PS2 can be obtained as the cutting speed when the cutting blade is inserted to a position 10 μm away from the outermost surface in the thickness direction, and then moved at a horizontal speed of 100 μm / sec. The peel strengths PS1 and PS2 are the arithmetic mean values ​​of the cutting speeds obtained at any five locations in the positive electrode mixture layer, respectively.

[0031] Furthermore, in the case of condition (a), the ratio of the peel strength PS2 to the peel strength PS1 (PS2 / PS1) may be 4.0 or less, or 3.5 or less. Also, in the case of condition (b), the peel strength PS1 may be 0.15 kN / m or less, or 0.12 kN / m or less. Furthermore, in the case of condition (b), PS2 / PS1 may be 1.5 or more, or 1.8 or more. Also, in the case of condition (b), PS2 / PS1 may be 3.5 or less, or 3.0 or less, or 2.8 or less.

[0032] The peel strengths PS1 and PS2 of the positive electrode mixture layer can be adjusted by adjusting the amount of binder included in the positive electrode mixture slurry used to form the positive electrode mixture layer. Specifically, the more binder is included in the positive electrode mixture slurry, the higher the values ​​of the peel strengths PS1 and PS2 can be. In addition, the peel strengths PS1 and PS2 can also be adjusted by adjusting the application speed when applying the positive electrode mixture slurry to the positive electrode current collector. Specifically, by slowing down the application speed of the positive electrode mixture slurry, it is possible to suppress excessive unevenness in the distribution of the binder in the positive electrode mixture layer, so that the binder can be sufficiently distributed not only near the positive electrode current collector but also near the outermost surface of the positive electrode mixture layer. This makes it possible to sufficiently increase the peel strengths PS1 and PS2.

[0033] The secondary batteries according to the embodiments of this disclosure include non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries, lithium metal secondary batteries, and solid-state batteries containing a gel electrolyte or a solid electrolyte. That is, the secondary batteries according to the embodiments of this disclosure may be liquid-type secondary batteries containing an electrolyte solution, or all-solid-state secondary batteries containing a solid electrolyte. Furthermore, the electrolyte solution may be a non-aqueous electrolyte solution.

[0034] As described above, in the secondary battery according to the embodiment of this disclosure, the positive electrode and the negative electrode are wound around a separator. That is, the secondary battery according to the embodiment of this disclosure has a wound electrode group. The wound electrode group is also referred to as a wound electrode body. In the wound electrode body, the shape of the cross section perpendicular to the winding axis is, for example, circular or elliptical. The outer shape of the wound electrode body is, for example, cylindrical or elliptical. In the wound electrode body, the shape of the cross section perpendicular to the winding axis may be other than circular or elliptical, and the outer shape of the wound electrode body may be other than cylindrical or elliptical.

[0035] (Positive electrode) As described above, the positive electrode comprises a strip-shaped positive electrode current collector and a positive electrode mixture layer disposed on at least one surface of the positive electrode current collector. The positive electrode mixture layer may be formed in the form of a film. The positive electrode has a positive electrode current collector and a positive electrode mixture layer formed (or supported) on a portion of the surface of at least one of the positive electrode current collectors. Specifically, the positive electrode has a first region including one end in the short direction of the positive electrode and a second region other than the first region. That is, the length of the first region in the short direction is smaller than the length of the positive electrode current collector in the short direction. The first region may be called the positive electrode edge because it includes one end in the short direction of the positive electrode, and the second region may be called the positive electrode main part because it includes the central part which is the main part of the positive electrode. The first region has one or more exposed portions of the positive electrode current collector provided along the longitudinal direction of the positive electrode current collector. The exposed portion of the positive electrode current collector does not have a positive electrode mixture layer from one end in the short direction of the positive electrode to the second region. The exposed portion of the positive electrode current collector may be provided intermittently at multiple locations along the longitudinal direction of the positive electrode current collector. The exposed portion of the positive electrode current collector may be a partially exposed portion that is roughly rectangular and has predetermined dimensions along both the longitudinal and short directions.

[0036] The positive electrode mixture layer is composed of a positive electrode mixture. In the secondary battery according to the embodiment of this disclosure, the positive electrode mixture includes a positive electrode active material and carbon black as essential components. The positive electrode mixture may also include binders and thickeners as optional components. As described above, the positive electrode mixture contains a positive electrode active material as an essential component, so the positive electrode mixture layer may also be called the positive electrode active material layer. As described above, the positive electrode mixture layer is arranged on at least one surface of the positive electrode current collector. Therefore, the positive electrode mixture layer may be supported on only one surface of the positive electrode current collector, or on both surfaces of the positive electrode current collector.

[0037] The thickness of the positive electrode mixture layer is preferably 75 μm or more per side of the positive electrode current collector. Having such a thickness in the positive electrode mixture layer allows for a sufficiently high capacity in the secondary battery. However, as the thickness of the positive electrode mixture layer increases, that is, as the volume of the positive electrode mixture layer increases, the distribution of the conductive material, carbon black, tends to become excessively uneven. Since the binder often adheres to the conductive material within the positive electrode mixture layer, excessive unevenness in the distribution of carbon black leads to excessive unevenness in the distribution of the binder as well. In this case, cracks are more likely to occur in the positive electrode mixture layer during the formation of the wound electrode body. Furthermore, as the thickness of the positive electrode mixture layer increases, the stress load on the thicker portion of the positive electrode mixture layer during the formation of the wound electrode body increases, making the positive electrode mixture layer more prone to cracking. However, in the secondary battery according to the embodiment of this disclosure, the peel strengths PS1 and PS2 of the positive electrode mixture layer satisfy either condition (a) or (b) above. Therefore, even if the thickness of the positive electrode mixture layer is as described above, it is possible to suppress the occurrence of cracks in the positive electrode mixture layer when forming the wound electrode body.

[0038] The positive electrode mixture layer can be formed by, for example, dispersing a positive electrode mixture containing particles of a positive electrode active material, which is an essential component, and carbon black, and an optional component (e.g., a binder) in a dispersion medium, applying the resulting positive electrode mixture slurry to at least one surface of a positive electrode current collector to obtain a coating film, and then drying this coating film. The dried coating film may be rolled if necessary. The positive electrode mixture layer may be formed on only one surface of the positive electrode current collector or on both surfaces of the positive electrode current collector. As the dispersion medium used for preparing the positive electrode mixture slurry, for example, N-methyl-2-pyrrolidone (NMP), cyclohexanone, alcohols, ethers, etc. can be used.

[0039] The positive electrode active material is, for example, a material that can reversibly occlude and release lithium ions. The positive electrode active material may be, for example, a lithium-containing transition metal oxide. Examples of the lithium-containing transition metal oxide include lithium cobalt oxide and lithium nickel oxide, which have a layered crystal structure and are of the rock salt type.

[0040] As the positive electrode active material, for example, a composite oxide containing lithium and transition metals such as Ni, Co, and Mn can be used. Examples of such composite oxides include Li ａ CoO ２ , Li ａ NiO ２ , Li ａ MnO ２ , Li ａ Co ｂ Ni １－ｂ O ２ , Li ａ Co ｂ M １－ｂ O ｃ , Li ａ Ni １－ｂ M ｂ O ｃ , Li ａ Mn ２ O ４ , Li ａ Mn ２－ｂ M ｂ O ４ , LiMPO ４ , Li ２ MPO ４Examples include F. However, M is at least one selected from the group consisting of Na, Mg, K, Ca, Rb, Sr, Sc, Y, Ti, Zr, V, Nb, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Also, 0 < a ≤ 1.2, 0 < b ≤ 0.9, and 2.0 ≤ c ≤ 2.3. Note that the value of a, which represents the molar ratio of lithium, increases or decreases with charging and discharging.

[0041] Among the various composite oxides mentioned above, Li ａ Ni １－ｂ M ｂ O ２ It is preferable to use a lithium nickel composite oxide represented by . However, M is at least one selected from the group consisting of Mn, Co, and Al, and 0 < a ≤ 1.2 and 0 < b < 0.7. From the viewpoint of increasing capacity, it is preferable that 0 < b < 0.2 is satisfied. Also, from the viewpoint of crystal structure stability, as M, Li containing Co and Al ａ Ni １－ｂ Co ｄ Al ｅ O ｃ , or as M, Li including Co and Mn ａ Ni １－ｂ Co ｄ Mn ｅ O ｃ This is even more preferable, where 0 < a ≤ 1.2, 0 < b < 0.2, 0 < d < 0.15, 0 < e ≤ 0.1, and b = d + e.

[0042] The volume-based median diameter D50 of the positive electrode active material is, for example, between 1 μm and 30 μm. The volume-based median diameter D50 of the positive electrode active material may be measured by separating the positive electrode active material from the positive electrode mixture layer, or it may be determined by analyzing an SEM image of the cross-section of the positive electrode mixture layer. Regardless of which method is used to determine the median diameter D50, approximately the same (without significant difference) median diameter D50 can be obtained.

[0043] To separate the positive electrode active material from the positive electrode mixture layer, first, the positive electrode mixture layer is peeled off the positive electrode current collector. Next, the peeled positive electrode mixture layer is immersed in a suitable solvent, and while other components (e.g., dispersants) other than the positive electrode active material are dissolved or swollen in this solvent, the positive electrode active material particles are dispersed to obtain a dispersion of positive electrode active material particles. Next, the positive electrode active material particles may be separated by centrifugation once or more times in this dispersion. The median diameter D50 of the separated positive electrode active material particles can be measured using a laser diffraction scattering particle size distribution analyzer.

[0044] When analyzing a SEM image of the cross-section of the positive electrode mixture layer, first, the positive electrode mixture layer and the positive electrode current collector are cut together along the width direction of the strip-shaped positive electrode to obtain a cross-section in the thickness direction of the positive electrode. At this time, the cross-section may be processed using a cross-section polisher (CP). Next, the cross-section of the positive electrode mixture layer is observed using a scanning electron microscope (SEM).

[0045] The analysis of SEM images can be performed as follows. First, using the contour image of one positive electrode active material particle in the SEM image, the area of ​​the region enclosed by the contour is determined. Next, the diameter of a circle (equivalent circle) having the same area as the region enclosed by the contour is determined, and this is taken as the particle size of one positive electrode active material particle. Then, the volume of a sphere having the same diameter as the equivalent circle is considered to be the volume of one positive electrode active material particle. This is performed for any 100 or more (preferably 1000 or more) positive electrode active material particles to determine the diameter and volume of the equivalent circle for any 100 or more (preferably 1000 or more) positive electrode active material particles. Using these values ​​of the diameter and volume of the equivalent circle, a volume-based particle size distribution is obtained. Then, the median diameter D50 of the positive electrode active material particle is determined from this volume-based particle size distribution.

[0046] The positive electrode mixture layer contains carbon black as an essential component. The carbon black functions as a conductive material. Examples of carbon black include furnace black and acetylene black. The positive electrode mixture layer may also contain other conductive materials besides carbon black. Examples of other conductive materials include graphite, carbon fiber, and graphene. The carbon fiber may be carbon nanotubes (CNTs) or other types of carbon fibers. The other conductive materials may be used individually or in combination of two or more. However, from a cost standpoint, it is desirable that the conductive material be substantially composed of carbon black only, and preferably the mass ratio of carbon black to the total conductive material is 50% by mass or more. Note that substantially composed of carbon black means that the mass ratio of carbon black to the total conductive material is 99% by mass or more. Furthermore, as described above, a high mass ratio of carbon black to the total conductive material makes it easier for the distribution of the conductive material to become uneven in the positive electrode mixture layer, and consequently, for the distribution of the binder to become uneven as well. Therefore, the effects of the present invention are more easily obtained in such cases.

[0047] Carbon black typically forms a structural structure consisting of multiple interconnected carbon nanoparticles with a diameter of approximately 20 nm. In other words, although carbon black has a linear structure, the length of the linear portion is shorter than that of carbon nanotubes. Therefore, in the positive electrode mixture layer, carbon black often exists in a point-like contact with the positive electrode active material, rather than in a linear contact state like carbon nanotubes. Consequently, when carbon black is used as a conductive material in the positive electrode mixture layer, excessive unevenness may occur in the distribution of carbon black in the positive electrode mixture layer, as well as in the distribution of binders adhering to the carbon black. In this case, cracking may occur in areas of the positive electrode mixture layer where there is less binder.

[0048] However, in the secondary battery according to the embodiment of this disclosure, the peel strengths PS1 and PS2 in the positive electrode mixture layer satisfy either condition (a) or (b) above. Therefore, in the secondary battery according to the embodiment of this disclosure, even when carbon black is used as the conductive material to be included in the positive electrode mixture layer, it is considered that excessive bias in the distribution of carbon black in the positive electrode mixture layer can be suppressed, and consequently, excessive bias in the distribution of the binder can also be suppressed. As a result, it is considered that cracking in the positive electrode mixture layer can be suppressed when the wound electrode body is formed.

[0049] The positive electrode mixture layer may contain 0.5 parts by mass or more, 0.6 parts by mass or more, or 0.7 parts by mass or more of conductive material per 100 parts by mass of positive electrode active material. Alternatively, the positive electrode mixture layer may contain 2.0 parts by mass or less, 1.5 parts by mass or less, or 1.1 parts by mass or less of conductive material per 100 parts by mass of positive electrode active material.

[0050] The positive electrode mixture layer may contain a binder as an optional component. The binder may include, for example, a fluorine-based polymer. Fluorine-based polymers can exhibit high binding strength. A fluorine-based polymer is a general term for polymers that have fluorine atoms bonded to carbon atoms that constitute the main chain. Because fluorine atoms have a small atomic radius and polarizability, the carbon-fluorine bond contained in fluorine-based polymers exhibits high stability. Therefore, fluorine-based polymers have excellent crystallinity. As a result, fluorine-based polymers also have excellent heat resistance, weather resistance, and chemical resistance.

[0051] Examples of fluorinated polymers include polyvinylidene fluoride polymers and polytetrafluoroethylene polymers, but the fluorinated polymer is not limited to these. It is preferable that the fluorinated polymer includes a polyvinylidene fluoride polymer. The polyvinylidene fluoride polymer may account for 50% or more by mass of the fluorinated polymer, or 80% or more by mass. Furthermore, all of the fluorinated polymer may be a polyvinylidene fluoride polymer; that is, 100% of the fluorinated polymer may be a polyvinylidene fluoride polymer.

[0052] Polyvinylidene fluoride polymers are fluorinated polymers containing vinylidene fluoride units. Polyvinylidene fluoride polymers can exhibit high binding strength. Because polyvinylidene fluoride polymers maintain polarity through the vinylidene fluoride units, they have a higher affinity for polar solvents such as N-methyl-2-pyrrolidone (NMP) than other fluorinated polymers.

[0053] The polyvinylidene fluoride polymer may be polyvinylidene fluoride (PVDF) or a copolymer of vinylidene fluoride and other monomers. Examples of other monomers include ethylene, propylene, tetrafluoroethylene (TFE), and hexafluoropropylene (HFP). The molar ratio of vinylidene fluoride units to total monomer units is preferably in the range of 50 to 100 mol%, and more preferably in the range of 75 to 100 mol%.

[0054] The polyvinylidene fluoride polymer may be polyvinylidene fluoride (PVDF), its modified form, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, or vinylidene fluoride-pentafluoropropylene copolymer. The polyvinylidene fluoride polymer may be used alone or in combination of two or more types.

[0055] Polytetrafluoroethylene polymers are fluorine-based polymers containing tetrafluoroethylene units. Polytetrafluoroethylene polymers may be polytetrafluoroethylene (PTFE) or copolymers of tetrafluoroethylene with other monomers. Examples of other monomers include ethylene, propylene, and hexafluoropropylene (HFP). The molar ratio of tetrafluoroethylene units to total monomer units is preferably in the range of 50 to 100 mol%, and more preferably in the range of 75 to 100 mol%.

[0056] The weight-average molecular weight Mw of the polyvinylidene fluoride polymer and the polytetrafluoroethylene polymer is, for example, 300,000 to 2,000,000, and may also be 500,000 to 1,500,000, or 1,000,000 to 1,500,000. This increases the strength of the positive electrode mixture layer, thereby suppressing the peeling of the positive electrode mixture layer from the positive electrode current collector. The weight-average molecular weight Mw is a polystyrene equivalent value determined by gel permeation chromatography (GPC).

[0057] The positive electrode mixture layer may contain binders other than fluorine-based polymers. Examples of other binders include styrene-butadiene copolymers and their hydrides, acrylonitrile-butadiene copolymers and their hydrides, acrylonitrile-butadiene-styrene copolymers and their hydrides, and N-vinylacetamide. The mass ratio of fluorine-based polymers in the binder is preferably 80% by mass or more, and more preferably 90% by mass or more.

[0058] The positive electrode mixture layer may contain 0.5 parts by mass or more, 0.6 parts by mass or more, or 0.7 parts by mass or more of binder per 100 parts by mass of positive electrode active material. Alternatively, the positive electrode mixture layer may contain 2.0 parts by mass or less, 1.5 parts by mass or less, or 1.1 parts by mass or less of binder per 100 parts by mass of positive electrode active material.

[0059] The positive electrode mixture layer may contain a thickening agent as an optional component. Examples of thickening agents include carboxymethylcellulose (CMC) and its modified forms (including salts such as Na salts), cellulose derivatives such as methylcellulose (e.g., cellulose ethers), and saponified polymers having vinyl acetate units such as polyvinyl alcohol. The thickening agent may be used alone or in combination of two or more types.

[0060] The positive electrode mixture layer may contain a dispersant as an optional component. That is, the positive electrode mixture slurry for forming the positive electrode mixture layer may contain a dispersant. By including a dispersant in the positive electrode mixture slurry, the dispersibility of carbon black, which is a conductive material, can be improved within the slurry. This prevents excessive uneven distribution of carbon black in the positive electrode mixture layer. Examples of dispersants include nitrile group-containing rubber, polyvinylpyrrolidone resin, and cellulose resin. Cellulose resin is also referred to as a cellulose-based polymer. The dispersant may be used alone or in combination of two or more types.

[0061] Examples of nitrile group-containing rubbers include copolymers of monomers containing acrylonitrile and a diene (e.g., butadiene). Specific examples of nitrile group-containing rubbers include acrylonitrile-butadiene rubber (NBR) and acrylonitrile-butadiene rubber (H-NBR), among other acrylonitrile-based rubbers.

[0062] The weight-average molecular weight of the nitrile group-containing rubber may be in the range of 5,000 to 500,000, or in the range of 10,000 to 300,000.

[0063] Examples of polyvinylpyrrolidone resins include polyvinylpyrrolidones and derivatives of polyvinylpyrrolidones. Derivatives of polyvinylpyrrolidones include polymers in which the hydrogen atoms of polyvinylpyrrolidone are substituted with other substituents. An example of such a polymer is alkylated polyvinylpyrrolidone. As polyvinylpyrrolidones, only polyvinylpyrrolidone may be used, or a copolymer of vinylpyrrolidone and another monomolecule may be used. Examples of other monomolecules include styrene monomolecules and vinyl acetate monomolecules.

[0064] The weight-average molecular weight of the polyvinylpyrrolidone resin may be in the range of 1,000 to 2,000,000, or in the range of 5,000 to 1,000,000.

[0065] The cellulose resin may be a cellulose derivative. Examples of cellulose derivatives include alkylcellulose, hydroxyalkylcellulose, and their alkali metal salts. Examples of alkylcellulose include methylcellulose and ethylcellulose. Examples of alkali metals that form alkali metal salts include potassium and sodium. It is preferable to use at least one selected from the group consisting of methylcellulose, ethylcellulose, and hydroxypropylmethylcellulose as the cellulose resin.

[0066] The weight-average molecular weight of the cellulose resin may be in the range of 1,000 to 1,000,000, 10,000 to 1,000,000, or 10,000 to 200,000.

[0067] The cathode mixture slurry for forming the cathode mixture layer preferably contains 3 parts by mass or more, and more preferably 7 parts by mass or more, of a dispersant per 100 parts by mass of carbon black. The upper limit of the dispersant content is, for example, 20 parts by mass. By including the dispersant within the above range, the carbon black can be suitably dispersed in the cathode mixture slurry.

[0068] As the positive electrode current collector, a non-porous conductive substrate (e.g., metal foil) or a porous conductive substrate (e.g., mesh, net, and perforated sheet) can be used. Examples of materials constituting the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium. The thickness of the positive electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0069] (Negative electrode) The negative electrode comprises a strip-shaped negative electrode current collector. The negative electrode may comprise a negative electrode current collector and a negative electrode layer disposed on at least one surface of the negative electrode current collector. The negative electrode layer may be formed in the form of a film. In other words, the negative electrode may have a negative electrode current collector and a negative electrode layer formed (or supported) on all or part of at least one surface of the negative electrode current collector. The negative electrode layer may be supported on only one surface of the negative electrode current collector, or on both surfaces of the negative electrode current collector.

[0070] The negative electrode layer may be composed of a negative electrode mixture. In this case, the negative electrode layer is referred to as the negative electrode mixture layer. Since the negative electrode mixture contains a negative electrode active material as an essential component, the negative electrode mixture layer may also be referred to as the negative electrode active material layer. The negative electrode active material may be a material that reversibly intercepts and releases lithium ions, a lithium metal, or a lithium alloy. The negative electrode layer composed of materials other than the negative electrode mixture is composed of at least one selected from the group consisting of lithium metal and lithium alloy. The negative electrode layer composed of materials other than the negative electrode mixture may be a metal layer composed of at least one selected from the group consisting of lithium metal and lithium alloy.

[0071] The negative electrode mixture contains a negative electrode active material as an essential component, and optional components such as a binder, a conductive material, and a thickener. The negative electrode mixture layer can be formed, for example, by dispersing a negative electrode mixture containing particles of the essential negative electrode active material and optional components in a dispersion medium, applying the resulting negative electrode mixture slurry to at least one surface of a negative electrode current collector to form a coating, and then drying this coating. The dried coating may be rolled if necessary.

[0072] If the negative electrode layer is a negative electrode mixture layer, the negative electrode mixture layer may contain an alloying material as the negative electrode active material. The alloying material contains a phase that reversibly forms an alloy with lithium. The phase that reversibly forms an alloy with lithium may be, for example, silicon (silicon phase). Silicon phases and the like expand and contract very greatly due to charging and discharging. In the negative electrode mixture layer, the negative electrode edge side may contain more alloying material than the negative electrode main part side. This makes it possible to increase the expansion rate of the negative electrode mixture layer on the negative electrode edge side than on the negative electrode main part side. The negative electrode edge is the region that includes one end in the short direction of the negative electrode, and the negative electrode main part is the region that includes the central part which is the main part of the negative electrode.

[0073] Examples of alloying materials include Si-containing materials, Sn-containing materials, Si-Sn-Si alloys, and Sn alloys. Among these, Si-containing materials are preferred from the viewpoint of stably obtaining high capacity. Si-containing materials contain a silicon phase. Silicon can reversibly form alloys with lithium. Therefore, Si-containing materials are materials that can reversibly intercept and release lithium ions.

[0074] The silicon-containing material may be a composite particle comprising a silicon phase and a matrix phase in which the silicon phase is dispersed. The matrix phase may be composed of a material having lithium-ion conductivity. For example, the matrix phase may include at least one selected from the group consisting of a silicon oxide phase and a carbon phase.

[0075] The silicon oxide phase contains Si and O. The silicon oxide phase may also contain other elements besides Si and O. The silicon oxide phase is SiO ２ It may be composed of, or it may be composed of lithium silicate, or SiO ２ It may consist of both and lithium silicate.

[0076] The silicon-containing composite particles, specifically those comprising a silicon phase and a matrix phase in which the silicon phase is dispersed, may take any of the following forms (a) to (c).

[0077] (a) A silicon phase and silicon dioxide (SiO₂) in which the silicon phase is dispersed. ２ (b) A first composite particle containing a silicon phase and a lithium silicate phase in which the silicon phase is dispersed (c) A third composite particle containing a silicon phase and a carbon phase in which the silicon phase is dispersed

[0078] The negative electrode mixture layer may contain negative electrode active materials other than Si-containing materials. Examples of other negative electrode active materials include carbon materials, spinel-type lithium titanium oxide, and spinel-type lithium manganese oxide. Among these, it is preferable to use a carbon material as the other negative electrode active material. The carbon material may be graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). Among these, it is preferable to use graphite as the carbon material because it provides stable charge-discharge characteristics and has low irreversible capacity.

[0079] Graphite refers to a carbon material in which the interplanar spacing d002 of the (002) planes, as measured by X-ray diffraction, is, for example, 0.340 nm or less. The crystallite size Lc(002) of graphite, as measured by X-ray diffraction, may be, for example, 5 nm or more, 5 nm to 300 nm, or 10 nm to 200 nm. The average grain size of graphite is, for example, 1 μm to 30 μm.

[0080] When graphite and silicon-containing material are used together as the negative electrode active material, the mass ratio of the silicon-containing material to the total negative electrode active material (graphite and silicon-containing material) is, for example, 1% by mass or more and 20% by mass or less. The above mass ratio may also be 3% by mass or more and 15% by mass or 3% by mass or more and 10% by mass or less. By keeping the above mass ratio within the numerical range described above, it becomes easier to achieve a good balance between improved cycle characteristics and increased capacity.

[0081] Resin materials can be used as binders. Examples of resin materials include fluororesins, polyolefin resins, polyamide resins, polyimide resins, acrylic resins, vinyl resins, and rubber-like materials. Examples of fluororesins include polytetrafluoroethylene and polyvinylidene fluoride (PVDF), examples of polyolefin resins include polyethylene and polypropylene, examples of polyamide resins include aramid resin, examples of acrylic resins include polyacrylic acid, methyl polyacrylate, and ethylene-acrylic acid copolymers, examples of vinyl resins include polyacrylonitrile and polyvinyl acetate, and examples of rubber-like materials include styrene-butadiene copolymer rubber (SBR). Polyvinylpyrrolidone and polyethersulfone may also be used as resin materials. The binder may be one of the above resin materials used alone, or two or more may be used in combination.

[0082] Examples of conductive materials that can be used include carbon compounds such as acetylene black, carbon fibers (e.g., carbon nanotubes (CNTs) and carbon fibers other than CNTs), graphene, metal fibers, and metal powders such as aluminum. Conductive materials may be used individually or in combination of two or more types.

[0083] Examples of thickening agents that can be used include carboxymethylcellulose (CMC) and its modified forms (including salts such as Na salts), cellulose derivatives such as methylcellulose (e.g., cellulose ethers), and saponified polymers having vinyl acetate units such as polyvinyl alcohol. The thickening agent may be used alone or in combination of two or more types.

[0084] As the negative electrode current collector, a non-porous conductive substrate (e.g., metal foil) or a porous conductive substrate (e.g., mesh, net, and perforated sheet) can be used. Examples of materials constituting the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0085] (Electrolyte) The electrolyte may be a liquid electrolyte (electrolyte solution), a gel electrolyte, or a solid electrolyte. A liquid electrolyte is, for example, an electrolyte solution containing a non-aqueous solvent and a salt dissolved in the non-aqueous solvent. Such a liquid electrolyte is also called a non-aqueous electrolyte (non-aqueous electrolyte solution). The concentration of the salt in the electrolyte solution is, for example, 0.5 mol / L or more and 2.0 mol / L or less. The electrolyte solution may contain various known additives.

[0086] The gel-like electrolyte comprises a salt and a matrix polymer, or a salt, a non-aqueous solvent, 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, and polyethylene oxide.

[0087] As the solid electrolyte, for example, known materials used in all-solid-state lithium-ion secondary batteries can be used. Examples of such known materials include oxide-based solid electrolytes, sulfide-based solid electrolytes, and halide-based solid electrolytes.

[0088] Liquid non-aqueous electrolytes can be prepared, for example, by dissolving a salt in a non-aqueous solvent. The salt is an electrolyte salt that undergoes ion dissociation in the electrolyte, and may include, for example, lithium salts. Various additives may be included in the electrolyte. Electrolytes are usually used in liquid form, but their fluidity may be restricted by gelling agents or other means.

[0089] Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC) and ethylene carbonate (EC). As environmental carbonate esters, cyclic carbonate esters having unsaturated bonds, such as vinylene carbonate (VC), or cyclic carbonate esters having fluorine atoms, such as fluoroethylene carbonate (FEC), may be used. Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of linear carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. Non-aqueous solvents may be used individually or in combination of two or more.

[0090] Examples of lithium salts include LiClO ４ LiBF ４ LiPF ６ LiAlCl ４ LiSbF ６ , LiSCN, LiCF ３ SO ３ LiCF ３ CO ２ LiAsF ６ LiB １０ Cl １０Examples of these include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, and imide salts. Examples of borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bisfluorosulfonylimide (LIN(FSO ２ ) ２ ), bisfluoromethanesulfonate lithium (LIN(CF ２ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), and bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ Examples include the following. The lithium salt may be used alone or in combination of two or more types. The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2.0 mol / L or less.

[0091] (Separator) As the separator, a porous sheet having ion permeability and insulating properties can be used. As the porous sheet, for example, a thin film, woven fabric, and nonwoven fabric having microporous properties can be used. The material constituting the separator is not particularly limited, and for example, polymer materials can be used. Examples of polymer materials include polyolefin resins, polyamide resins, and cellulose. Examples of polyolefin resins include polyethylene resins, polypropylene resins, and copolymers of ethylene and propylene. The separator may contain additives (such as inorganic fillers) as needed. The thickness of the separator is not particularly limited and may be 10 μm or more, or 15 μm or more. The thickness of the separator may be 30 μm or less, or 20 μm or less.

[0092] (Outer casing) Various known outer casings (battery cases) can be used. The outer casing may include an outer can and a sealing body that seals the opening of the outer can. In this case, the outer can functions as the negative terminal, and the sealing body functions as the positive terminal. The sealing body may include a sealing plate and a gasket.

[0093] The outer casing (battery case) houses the electrode group and the non-aqueous electrolyte. The electrode group consists of a positive electrode, a negative electrode, and a separator interposed between the positive and negative electrodes. The configuration of the electrode group is not particularly limited. The electrode group may be wound or laminated. A wound electrode group is formed by winding a laminate of a positive electrode, a negative electrode, and a separator interposed between the positive and negative electrodes. The form of the secondary battery is not particularly limited. The form of the secondary battery may be cylindrical, rectangular, coin-shaped, button-shaped, or laminated.

[0094] Hereinafter, an example of a secondary battery according to an embodiment of the present disclosure will be described with reference to Figure 1. Figure 1 is a schematic cross-sectional view showing an example of a secondary battery according to an embodiment of the present disclosure. Furthermore, hereinafter, examples of the positive electrode and negative electrode of the secondary battery according to an embodiment of the present disclosure will be described with reference to Figures 2 and 3, respectively. Figure 2 is a schematic plan view showing an example of the positive electrode of the secondary battery according to an embodiment of the present disclosure, and Figure 3 is a schematic plan view showing an example of the negative electrode of the secondary battery according to an embodiment of the present disclosure.

[0095] The secondary battery 10 may be, for example, a lithium-ion secondary battery or a lithium secondary battery (lithium metal secondary battery). As shown in Figure 1, the secondary battery 10 comprises a non-polar case 11, a wound electrode group 14, a plurality of positive electrode leads 112 made of conductors, a positive electrode terminal 16 made of conductors, an end face current collector plate 19 made of conductors, a negative electrode current collector plate 22 made of conductors, and a sealing plate 23.

[0096] The case 11 is formed in a bottomed cylindrical shape with an opening at one end (the lower end in Figure 1). The case 11 is made of metal. A through hole 12 is formed in the center of the bottom of the case 11 (the upper end in Figure 1), through which the positive electrode terminal 16 is inserted. The case 11 houses the electrolyte (not shown) together with the electrode group 14. Near the opening in the case 11, a recess 13 is formed that is indented radially inward of the case 11.

[0097] The electrode group 14 has a positive electrode 110 and a negative electrode 120. The electrode group 14 is a wound-type electrode group formed by winding the positive electrode 110 and the negative electrode 120 with a separator (not shown) in between. The electrode group 14 as a whole is generally cylindrical.

[0098] Each of the multiple positive electrode leads 112 is connected to the exposed portion 113b of the positive electrode current collector in the first region (positive electrode edge) 113 of the positive electrode 110. The other ends of the multiple positive electrode leads 112 are provided so as to be planted from one end face of the electrode group 14.

[0099] Multiple positive electrode leads 112 are stacked on top of each other and connected to the positive electrode terminal 16 by welding. In one example of a secondary battery according to the embodiment of this disclosure, the number of positive electrode leads 112 is eight, but is not limited to this. Also, in Figure 1, only four of the eight positive electrode leads 112 are shown.

[0100] The materials that make up each positive electrode lead 112 include, for example, stainless steel, aluminum, aluminum alloy, nickel, and nickel alloy.

[0101] An insulating member 24 is placed between the electrode group 14 and the bottom of the case 11 to electrically insulate them. The insulating member 24 is made of, for example, an insulating resin. The insulating member 24 may be attached to the bottom of the case 11.

[0102] The positive electrode terminal 16 is located on the opposite side of the electrode group 14, sandwiching the multiple positive electrode leads 112. The positive electrode terminal 16 is inserted through a through hole 12 at the bottom of the case 11 and penetrates the bottom of the case 11. The positive electrode terminal 16 is made of metal. For example, a rivet-type terminal can be used as the positive electrode terminal 16. An insulating plate 25 is placed between the positive electrode terminal 16 and the electrode group 14 to electrically insulate them from each other.

[0103] The positive electrode terminal 16 has a first terminal member 17 that extends both inside and outside the case 11, and a disc-shaped second terminal member 18 that is joined to the first terminal member 17 and exposed to the outside of the case 11. The first terminal member 17 comprises a disc-shaped first portion 17a, a hollow cylindrical second portion 17b that is formed continuously with the first portion 17a and inserted through the through hole 12, and a third portion 17c that extends radially outward from the end of the second portion 17b and to which the second terminal member 18 is joined. The first terminal member 17 is welded to a plurality of positive electrode leads 112 at the first portion 17a by a laser irradiated in the direction from the first terminal member 17 toward the electrode group 14. Therefore, the positive electrode terminal 16 is electrically connected to the positive electrode 110 via the plurality of positive electrode leads 112 and functions as an external positive electrode terminal of the secondary battery 10. The first terminal member 17 is an example of a terminal member.

[0104] Of the multiple positive electrode leads 112, at least the positive electrode lead 112 located closest to the electrode group 14 (the lowermost positive electrode lead 112 in Figure 1) is formed by folding a portion of this positive electrode lead 112 (specifically, a portion of the tip side), and has a folded portion 112a on which a portion of the laser trace LM formed by the laser is formed. The folded portion 112a is positioned on the opposite side of the electrode group 14, with the insulating plate 25 in between.

[0105] The end face current collector plate 19 is made of metal. The shape of the end face current collector plate 19 is not particularly limited; for example, it may be roughly cross-shaped overall. The end face current collector plate 19 is electrically connected to the negative electrode 120 of the electrode group 14.

[0106] The negative electrode current collector plate 22 is electrically connected to the end face current collector plate 19 via a metal connecting plate 21. The connecting plate 21 may be formed, for example, in the shape of a ring. Thus, the negative electrode current collector plate 22 is electrically connected to the negative electrode 120. The negative electrode current collector plate 22 and the connecting plate 21 may be welded to each other (for example, by laser welding). Alternatively, the negative electrode current collector plate 22 may be directly connected to the end face current collector plate 19. In this case, the connecting plate 21 is not necessary. The negative electrode current collector plate 22 has one or more injection holes 22a for injecting electrolyte into the case 11. The negative electrode current collector plate 22 is welded (for example, by laser welding) to a recess 13 in the case 11 at its outer edge. Thus, the case 11 is electrically connected to the negative electrode 120 via the negative electrode current collector plate 22, etc.

[0107] The sealing plate 23 seals the opening of the case 11. The sealing plate 23 is made of metal. The sealing plate 23 is generally disc-shaped. The sealing plate 23 is insulated from the case 11 by the negative electrode gasket 27. In this embodiment, the sealing plate 23 is not electrically connected to either the positive electrode 110 or the negative electrode 120 of the electrode group 14, but is not limited to this. The sealing plate 23 has an explosion-proof mechanism (not shown) that activates when the internal pressure of the case 11 exceeds a predetermined value.

[0108] The positive electrode 110 shown in Figure 2 represents its state before being wound as part of the electrode group 14. In Figure 2, the direction of arrow Y1 is the longitudinal direction of the positive electrode 110, and also the winding direction of the positive electrode 110 when manufacturing the electrode group 14. Also in Figure 2, the direction of arrow Y2, which is perpendicular to arrow Y1, is the short direction of the positive electrode 110, and also the winding axis direction of the positive electrode 110 after winding. In other words, the direction of arrow Y2 is also the winding axis direction of the electrode group 14.

[0109] As shown in Figure 2, the positive electrode 110 has a first region (positive electrode edge) 113 that includes one end 110a in the short direction of the positive electrode 110, and a second region (positive electrode main part) 114 other than the first region 113. The first region 113 includes the positive electrode central end 113a on the opposite side of the one end 110a in the short direction of the positive electrode 110. The second region 114 is the region from the positive electrode central end 113a of the first region 113 to the other end 110b in the short direction of the positive electrode 110. The ratio of the width (length in the short direction) WP1 of the first region 113 to the width (length in the short direction) WP2 of the second region 114 may be, for example, in the range of WP1:WP2 = 1:15 to 3:4, or in the range of WP1:WP2 = 1:12 to 1:6.

[0110] The first region 113 of the positive electrode 110 has an exposed portion 113b of the positive electrode current collector where the positive electrode mixture layer is not placed on the positive electrode current collector, and a first positive electrode mixture portion 113c where the positive electrode mixture layer is placed on the positive electrode current collector. The second region 114 of the positive electrode 110 has a second positive electrode mixture portion 114c where the positive electrode mixture layer is placed on the positive electrode current collector.

[0111] The exposed portions 113b of the positive electrode current collector are provided intermittently at multiple locations along the longitudinal direction of the positive electrode current collector. In Figure 2, an example is shown in which the exposed portions 113b of the positive electrode current collector are provided intermittently at three locations along the longitudinal direction of the positive electrode current collector, but the number of exposed portions 113b is not limited to this. For example, the exposed portions 113b of the positive electrode current collector may be provided intermittently at eight locations along the longitudinal direction of the positive electrode current collector. The exposed portions 113b do not have a positive electrode mixture layer from one end 110a in the short direction of the positive electrode 110 to the second region 114.

[0112] The longitudinal length LE1 of each exposed portion 113b of the positive electrode current collector may be within the range of 0.5% to 10% of the total longitudinal length LT of the positive electrode current collector. Furthermore, the sum of the longitudinal lengths of all exposed portions 113b of the positive electrode current collector, LET, may be within the range of 1% to 20%, 5% to 20%, or 8% to 20% of the total longitudinal length LT of the positive electrode current collector.

[0113] It is desirable that the spacing I between adjacent exposed portions 113b of the positive electrode current collectors be as uniform as possible. For example, when the number of exposed portions 113b of the positive electrode current collectors is n, the spacing I may be between 0.8 × LT / n and 1.2 × LT / n. LT is the total length of the positive electrode current collector in the longitudinal direction, as described above.

[0114] Each of the exposed portions 113b of the positive electrode current collector is connected to a tab-shaped positive electrode lead 112. In other words, multiple tab-shaped positive electrode leads 112 are connected to the positive electrode 110. The multiple positive electrode leads 112 are bundled together and connected to the first portion 17a of the first terminal member 17 (see Figure 1).

[0115] The mass W1 per unit area of ​​the positive electrode mixture layer in the first positive electrode mixture section 113c may be the same as or different from the mass W2 per unit area of ​​the positive electrode mixture layer in the second positive electrode mixture section 114c. If W1 and W2 are different, the ratio of the difference between W1 and W2 (ΔW) to W1 (ΔW / W1 × 100) may be, for example, 4% or less, 3% or less, 2% or less, or 1% or less.

[0116] As shown in Figure 3, the negative electrode 120 has a negative electrode edge portion 123 that faces at least a part of the first region 113 of the positive electrode 110, and a negative electrode main portion 124 other than the negative electrode edge portion 123. The negative electrode main portion 124 faces at least a part of the second region 114 of the positive electrode 110. Preferably, the negative electrode edge portion 123 faces 70% or more of the first region 113. Preferably, the negative electrode main portion 124 faces 70% or more of the second region 114. In other words, the negative electrode 120 has a negative electrode edge portion 123 that includes one end 120a in the short direction of the negative electrode 120, and a negative electrode main portion 124 other than the negative electrode edge portion 123. In the short direction of the negative electrode 120, the negative electrode edge portion 123 includes a negative electrode central end portion 123a on the opposite side of the one end 120a. The negative electrode main portion 124 is the region between the negative electrode central end 123a of the negative electrode edge portion 123 and the other end 120b in the short direction of the negative electrode 120. The ratio of the width (length in the short direction) WN1 of the negative electrode edge portion 123 to the width (length in the short direction) WN2 of the negative electrode main portion 124 may be in the range of WN1:WN2 = 1:15 to 3:4, or in the range of WN1:WN2 = 1:12 to 1:6, similar to the positive electrode 110.

[0117] The negative electrode edge portion 123 of the negative electrode 120 has a first negative electrode mixture portion 123c in which the negative electrode mixture layer is arranged on the negative electrode current collector. The negative electrode main portion 124 of the negative electrode 120 has an exposed portion 123b of the negative electrode current collector in which the negative electrode mixture layer is not arranged on the negative electrode current collector, and a second negative electrode mixture portion 124c in which the negative electrode mixture layer is arranged on the negative electrode current collector. The exposed portion 123b of the negative electrode current collector is provided on the other end 120b side in the short direction of the negative electrode 120. The exposed portion 123b of the negative electrode current collector is provided along the longitudinal direction of the negative electrode 120. Therefore, the exposed portion 123b of the negative electrode current collector is exposed on the other end face of the electrode group 14. The exposed portion 123b of the negative electrode current collector is connected to the end face current collector plate 19 by, for example, laser welding (see Figure 1).

[0118] (Note) The following technologies are disclosed by the above description. (Technology 1) A positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode and the negative electrode are wound around the separator, the positive electrode comprises a strip-shaped positive electrode current collector and a positive electrode mixture layer disposed on at least one surface of the positive electrode current collector, the positive electrode has a first region including one end in the short direction of the positive electrode and a second region other than the first region, the first region has one or more exposed portions of the positive electrode current collector provided along the longitudinal direction of the positive electrode current collector, the exposed portions do not have the positive electrode mixture layer from one end in the short direction of the positive electrode to the second region, the positive electrode mixture layer comprises a positive electrode active material and carbon black, With respect to the positive electrode mixture layer, when the peel strength measured by the SAICAS® method at a position 2 μm away from the surface of the positive electrode current collector in the thickness direction is defined as PS1 (kN / m), and the peel strength measured by the SAICAS® method at a position 10 μm away from the outermost surface in the thickness direction is defined as PS2 (kN / m), the secondary battery satisfies either of the following conditions (a) or (b): (a) 0.073 kN / m ≤ PS1 ≤ 0.097 kN / m, and PS2 / PS1 ≥ 2.8. (b) 0.098 kN / m ≤ PS1. (Technical 2) The secondary battery according to Technical 1, wherein the thickness of the positive electrode mixture layer per side of the positive electrode current collector is 75 μm or more. (Technical 3) The secondary battery according to Technical 1 or 2, wherein the length in the short direction of the first region is smaller than the length in the short direction of the positive electrode current collector.

[0119] The present disclosure will be described below in detail based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0120] (Example 1) <Preparation of the positive electrode> (Positive electrode slurry) An appropriate amount of NMP was added to the positive electrode slurry to obtain a positive electrode slurry. The positive electrode slurry used was a mixture of lithium-containing composite oxide, which is the positive electrode active material, acetylene black, which is the conductive material, and polyvinylidene fluoride (PVDF), which is the binder. In Example 1, only acetylene black was used as the conductive material. LiNi０．８ Co ０．１ Mn ０．１ O ２ The following was used. Furthermore, as shown in Table 1 below, the positive electrode mixture contained 0.80 parts by mass of conductive material and 0.82 parts by mass of binder per 100 parts by mass of positive electrode active material.

[0121] ≪Formation of the Positive Electrode Mixture Layer≫ First, a coating film was obtained by applying a positive electrode mixture slurry to a predetermined thickness on both sides of an aluminum foil, which serves as the positive electrode current collector. Next, these coating films were dried and then rolled to form a positive electrode mixture layer with a thickness of 82 μm per side. The coating films were dried while applying hot air to them. This resulted in a positive electrode as shown in Figure 2. Specifically, a first coating film was obtained by intermittently applying a positive electrode mixture slurry to one end of the aluminum foil in the short direction, along the longitudinal direction of the aluminum foil, to a predetermined thickness. Then, a second coating film was obtained by applying the same thickness of positive electrode mixture slurry to the remaining part of the aluminum foil. Next, the first and second coating films were dried and then rolled to obtain a positive electrode having a first positive electrode mixture portion and a second positive electrode mixture portion. The first coating film corresponds to the first positive electrode mixture portion, and the second coating film corresponds to the second positive electrode mixture portion. Furthermore, the first positive electrode mixture portion was formed in a first region including one end in the short direction of the aluminum foil, and the second positive electrode mixture portion was formed in a second region other than the first region. Such first and second positive electrode mixture portions were formed on both sides of the aluminum foil. As described above, the thickness of each side of the first and second positive electrode mixture portions was 82 μm. In addition, eight exposed portions of the positive electrode current collector were provided in the first region.

[0122] The width (length in the shorter direction) of the first region was set to 12 mm, and the width (length in the shorter direction) of the second region was set to 62 mm. That is, the ratio of the length in the shorter direction of the first region to the length in the shorter direction of the second region was 1:5.2.

[0123] Furthermore, the sum of the longitudinal lengths of the eight exposed portions of the positive electrode current collector was 10% of the total length of the positive electrode current collector.

[0124] Furthermore, when the total length of the positive electrode current collector was set to 720 mm and the number of exposed parts of the positive electrode current collector was n = 8, the distance between adjacent exposed parts was 720 / 8 = 90.

[0125] <Fabrication of the negative electrode> SiO2, the active material of the negative electrode ｘ (x = 1.0) A suitable amount of water was added to a negative electrode mixture containing graphite as the negative electrode active material, styrene-butadiene copolymer rubber (SBR) as a binder, and carboxymethylcellulose (CMC) as a thickener to obtain a negative electrode mixture slurry. In the negative electrode mixture, the mass ratio of each component is SiO ｘ The ratio of graphite, SBR, and CMC was set to 5:93:1:1.

[0126] First, a negative electrode mixture slurry was applied to both sides of the copper foil, which served as the negative electrode current collector, to a predetermined thickness to obtain a coating. Next, these coatings were dried and then rolled to form a negative electrode mixture layer. This resulted in the negative electrode shown in Figure 3. However, in the negative electrode, the negative electrode mixture layer was formed in such a way that a portion of one end of the negative electrode current collector was left exposed. The thickness of the negative electrode mixture layer was appropriately varied according to the thickness of the positive electrode mixture layer.

[0127] <<Fabrication of wound electrode group>> A wound electrode group was fabricated by winding the positive electrode and negative electrode with a separator in an inert gas atmosphere. In fabricating the wound electrode group, the positive electrode and negative electrode were stacked so that the edge of the positive electrode was positioned on one end face of the wound electrode group, and the exposed portion of the negative electrode current collector was positioned on the other end face of the wound electrode group.

[0128] (Example 2) A wound electrode group according to Example 2 was fabricated in the same manner as in Example 1, except that the thickness per side of the positive electrode mixture layer was changed to 75 μm.

[0129] (Example 3) A wound electrode group according to Example 3 was prepared in the same manner as in Example 2, except that an acetylene black dispersant, which is a conductive material, was added in the formation of the positive electrode mixture layer. A cellulose-based polymer was used as the acetylene black dispersant. The amount of acetylene black dispersant was 0.1 parts by mass per 100 parts by mass of positive electrode active material.

[0130] (Example 4) A wound electrode group according to Example 4 was prepared in the same manner as in Example 1, except that a dispersant for acetylene black, which is a conductive material, was added in the formation of the positive electrode mixture layer. A cellulose-based polymer was used as the dispersant for acetylene black. The amount of acetylene black dispersant was 0.1 parts by mass per 100 parts by mass of positive electrode active material.

[0131] (Example 5) In forming the positive electrode mixture layer, as shown in Table 1 below, the positive electrode mixture was prepared in the same manner as in Example 1, except that it contained 1.00 part by mass of conductive material and 1.02 parts by mass of binder per 100 parts by mass of positive electrode active material.

[0132] (Comparative Example 1) In forming the positive electrode mixture layer, as shown in Table 1 below, the positive electrode mixture was prepared in the same manner as in Example 1, except that 0.75 parts by mass of conductive material and 0.60 parts by mass of binder were added to 100 parts by mass of positive electrode active material.

[0133] (Comparative Example 2) In forming the positive electrode mixture layer, the positive electrode mixture slurry was not intermittently applied to one end of the aluminum foil in the short direction along the longitudinal direction of the aluminum foil at a predetermined thickness. In other words, the positive electrode according to Comparative Example 2 did not have an exposed portion of the positive electrode current collector.

[0134] (Comparative Example 3) A wound-type electrode group according to Comparative Example 3 was fabricated in the same manner as in Example 1, except that the coating speed of the positive electrode mixture slurry was increased and the drying temperature of the coating film formed by the positive electrode mixture slurry was increased (the temperature of the hot air was increased) in the formation of the positive electrode mixture layer. In Comparative Example 3, the drying time was shorter than in Example 1 because the drying temperature of the coating film was increased as described above.

[0135] (Comparative Example 4) A wound-type electrode group according to Comparative Example 4 was fabricated in the same manner as in Example 1, except that the winding velocity of the hot air applied to the coating film formed by the positive electrode mixture slurry was increased and an infrared heater was used in combination as a heat source during the formation of the positive electrode mixture layer.

[0136] (Reference Example 1) In forming the positive electrode mixture layer, carbon nanotubes (CNTs) were used instead of acetylene black as the conductive material, and as shown in Table 1 below, the positive electrode mixture contained 0.40 parts by mass of conductive material and 0.65 parts by mass of binder per 100 parts by mass of positive electrode active material. Otherwise, a wound electrode group according to Reference Example 1 was fabricated in the same manner as in Example 2.

[0137] Table 1 is shown below.

[0138]

[0139] [Evaluation] <Peel Strength> For each example (Examples 1-5, Comparative Examples 1-4, and Reference Example 1), the peel strength was measured at two specific locations in the thickness direction of the positive electrode mixture layer using the SAICAS® method. The first location in the thickness direction of the positive electrode mixture layer was 2 μm away from the surface of the positive electrode current collector in the thickness direction (hereinafter referred to as the first location), and the second location in the thickness direction of the positive electrode mixture layer was 10 μm away from the outermost surface of the positive electrode mixture layer in the thickness direction (hereinafter referred to as the second location). The SAICAS method measurement was performed according to the method described above. Table 2 below shows the value of the peel strength PS1 at the first location and the ratio of the value of the peel strength P2 at the second location to the value of the peel strength P1 at the first location (PS2 / PS1) for each example.

[0140] <Cracks and Delamination> After disassembling the wound electrode group for each example and removing the positive electrode, the positive electrode for each example was visually evaluated according to the following evaluation criteria to determine whether cracks were observed in the positive electrode mixture layer and whether delamination of the positive electrode mixture layer from the positive electrode current collector was observed. Table 2 below shows the evaluation results for cracks and delamination for each example. Evaluation Criteria ○: No cracks or delamination observed. △: At least one of cracks or delamination is observed, although at a level that does not pose a practical problem. ×: At least one of cracks or delamination is observed, at a level that poses a practical problem.

[0141]

[0142] As described above, the positive electrodes in each example (each embodiment and each comparative example) all contained acetylene black as a conductive material in the positive electrode mixture layer. However, even after forming a wound electrode group, no cracking or peeling was observed in the positive electrodes in each embodiment. This is thought to be because the positive electrode mixture layers in the positive electrodes of Examples 1 to 3 satisfied the conditions that (a) 0.073 kN / m ≤ PS1 ≤ 0.097 kN / m and PS2 / PS1 ≥ 2.8 for the peel strength PS1 at the first position and PS2 at the second position, and the positive electrode mixture layers in the positive electrodes of Examples 4 and 5 satisfied the condition that (b) 0.098 kN / m ≤ PS1 for the first peel strength PS1.

[0143] On the other hand, after forming a wound electrode group, the positive electrodes of Comparative Examples 1, 3, and 4 all exhibited at least one of cracking and delamination at a level that poses a practical problem, and the positive electrode of Comparative Example 2 exhibited at least one of cracking and delamination, although at a level that does not pose a practical problem. This is thought to be because the peel strength PS1 at the first position and the peel strength PS2 at the second position of the positive electrode mixture layer in each comparative example do not satisfy either condition (a) or (b) above.

[0144] Furthermore, in the positive electrode of Reference Example 1, carbon nanotubes (CNTs) are included in the positive electrode mixture layer instead of acetylene black as the conductive material. Therefore, it is believed that uneven distribution of the conductive material within the positive electrode mixture layer is suppressed. Consequently, even after forming a wound electrode group, neither cracking nor delamination was observed in the positive electrode of Reference Example 1.

[0145] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0146] The secondary battery according to this disclosure has a wound electrode body and the positive electrode mixture layer contains carbon black, and can be used in applications where it is required to suppress peeling of the positive electrode mixture layer from the positive electrode current collector and cracking of the positive electrode mixture layer.

[0147] 10: Secondary battery, 11: Case, 12: Through hole, 13: Recess, 14: Electrode group, 16: Positive terminal, 17: First terminal member, 17a: First part, 17b: Second part, 17c: Third part, 18: Second terminal member, 19: End face current collector plate, 21: Connecting plate, 22: Negative current collector plate, 22a: Injection hole, 23: Sealing plate, 24: Insulating member, 25: Insulating plate, 26: Positive gasket, 27: Negative gasket, 110: Positive electrode, 110a: One end, 110b: Other end, 112: Positive lead D, 112a: Folded portion, 113: First region (positive electrode edge), 113a: Central end of positive electrode, 113b: Exposed portion of positive electrode current collector, 113c: First positive electrode mixture portion, 114: Second region (positive electrode main part), 114c: Second positive electrode mixture portion, 120: Negative electrode, 120a: One end, 120b: Other end, 123: Negative electrode edge, 123a: Central end of negative electrode, 123b: Exposed portion of negative electrode current collector, 123c: First negative electrode mixture portion, 124: Main part of negative electrode, 124c: Second negative electrode mixture portion, LM: Laser mark

Claims

1. A positive electrode, a negative electrode, an electrolyte, and a separator are provided, wherein the positive electrode and the negative electrode are wound around the separator, the positive electrode comprises a strip-shaped positive electrode current collector and a positive electrode mixture layer disposed on at least one surface of the positive electrode current collector, the positive electrode has a first region including one end in the short direction of the positive electrode and a second region other than the first region, the first region has one or more exposed portions of the positive electrode current collector provided along the longitudinal direction of the positive electrode current collector, the exposed portions do not have the positive electrode mixture layer from one end in the short direction of the positive electrode to the second region, the positive electrode mixture layer comprises a positive electrode active material and carbon black, With respect to the positive electrode mixture layer, when the peel strength measured by the SAICAS® method at a position 2 μm away in the thickness direction from the surface of the positive electrode current collector is defined as PS1 (kN / m), and the peel strength measured by the SAICAS® method at a position 10 μm away in the thickness direction from the outermost surface is defined as PS2 (kN / m), the PS1 and PS2 satisfy either of the following conditions (a) or (b) in a secondary battery: (a) 0.073 kN / m ≤ PS1 ≤ 0.097 kN / m and PS2 / PS1 ≥ 2.

8. (b) 0.098 kN / m ≤ PS1.

2. The secondary battery according to claim 1, wherein the thickness of the positive electrode mixture layer on one side of the positive electrode current collector is 75 μm or more.

3. The secondary battery according to claim 1, wherein the length in the short direction of the first region is smaller than the length in the short direction of the positive electrode current collector.

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