Secondary battery

Abstract

This secondary battery is provided with: a wound electrode group; a core member disposed in a hollow of the electrode group; and a nonaqueous electrolyte. The electrode group includes a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. The core member is a cylindrical body including at least one selected from the group consisting of a helical structure and a braided structure. The cylindrical body is formed from a linear or belt-like material. In the helical structure, the material is helically wound. In the braided structure, the material is braided into a net shape.

Description

secondary battery CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2024-124442, filed on July 31, 2024 in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to secondary batteries.

[0003] The secondary battery includes a wound electrode group and a non-aqueous electrolyte. The electrode group is formed by spirally winding a positive electrode, a negative electrode, and a separator therebetween.

[0004] Patent Document 1 proposes "a nonaqueous electrolyte secondary battery including an electrode group having a structure in which a negative electrode plate and a positive electrode plate are wound with a separator interposed therebetween, and including a cylindrical winding core having electrolyte resistance in the center of the electrode group, the nonaqueous electrolyte secondary battery being characterized in that an electrolyte permeation portion is provided on the side surface of the cylindrical winding core."

[0005] Japanese Patent Application Laid-Open No. 2000-340263

[0006] In recent years, there has been a demand for further improvement in the reliability of secondary batteries.

[0007] One aspect of the present disclosure relates to a secondary battery including a wound electrode group having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, a core member disposed in a hollow of the electrode group, and a non-aqueous electrolyte, wherein the core member is a tubular body including at least one selected from the group consisting of a spiral structure and a braided structure, and the tubular body is made of a linear or strip-shaped material, and in the spiral structure, the material is wound in a spiral shape, and in the braided structure, the material is braided in a mesh shape.

[0008] According to the present disclosure, the reliability of secondary batteries can be improved. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0009] FIG. 1 is a front view schematically showing an example of a core member; FIG. 2 is a front view schematically showing another example of a core member; FIG. 3 is a perspective view schematically showing yet another example of a core member; FIG. 4 is a longitudinal sectional view schematically showing an example of a secondary battery according to an embodiment of the present disclosure; FIG. 5 is a sectional view schematically showing a part of an electrode group; FIG. 6 is a top view showing an example of a spacer; FIG. 7 is a top view showing another example of a spacer.

[0010] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits are exemplified for numerical values ​​of specific physical properties or conditions, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more materials may be used in combination.

[0011] A secondary battery according to an embodiment of the present disclosure includes a wound electrode group having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, a core member disposed in the hollow of the electrode group, and a nonaqueous electrolyte. The core member is a cylindrical body including at least one selected from the group consisting of a spiral structure and a braided structure. The cylindrical body is composed of a linear or strip-shaped material (hereinafter also referred to as material A). In the spiral structure, material A is wound in a spiral shape, and in the braided structure, material A is braided in a mesh shape.

[0012] The reliability of the secondary battery is significantly improved by disposing a cylindrical body having a specific structure made of material A as a core member in the hollow portion of the electrode group. The core member disposed in the hollow portion of the electrode group presses the electrode group from the inner circumferential side, thereby stabilizing the shape of the electrode group and suppressing buckling of the electrodes near the hollow portion of the electrode group during charging and discharging.

[0013] The cylindrical body having the above-described specific structure and made of material A has excellent strength, and therefore can contribute to ensuring the shape stability of the electrode group as a core member and suppress buckling of the electrodes. Furthermore, the cylindrical body has moderate flexibility (deformability), and can play a role in absorbing impact forces when a strong external impact is applied to the secondary battery. Even if the battery is deformed by impact, the cylindrical body's deformation can alleviate local stress concentrations occurring in the electrode group, thereby suppressing the occurrence of internal short circuits due to stress concentration. Since material A is linear or strip-shaped, the cylindrical body can not only deform by being crushed (dented) to absorb impact, but also deform so as to stretch in the axial direction, thereby achieving a significant shock absorption effect. When a cylindrical body with a spiral structure deforms so as to stretch in the axial direction, the spacing between the spirally wound portions of material A increases when viewed from the side of the cylindrical body. When a cylindrical body with a braided structure deforms so as to stretch in the axial direction, the axial diameter of the mesh formed by the braided material A increases when viewed from the side of the cylindrical body.

[0014] From the viewpoint of easily obtaining a cylindrical body with excellent axial deformability, when the cylindrical body is viewed from the side (periphery), it is preferable that material A is arranged so that the length direction of material A intersects with the axial direction of the cylindrical body. It is preferable that material A is arranged at an angle with respect to the axial direction of the cylindrical body. The acute angle formed by the length direction of material A and the axial direction of the cylindrical body may be 10° or more and 80° or less, 25° or more and 75° or less, or 30° or more and 70° or less. When the above angle is 10° or more, the shape (structure) of the electrode group is easily maintained stably. When the above angle is 80° or less, it is easy to obtain a cylindrical body with excellent axial deformability. In the electrode group, the axis of the cylindrical body coincides with the winding axis of the electrode group.

[0015] From the viewpoint of excellent flexibility (especially axial deformability) and shape retention, the cylindrical body is preferably a spring body. A cylindrical body having the above-mentioned specific structure made of material A is likely to have good spring properties (stretchability). In the case of a spring body, the cylindrical body may deform so as to expand and contract in the axial direction in response to the expansion and contraction of the electrode group during charge and discharge, and this deformation may relieve stress caused by the expansion and contraction of the electrode group.

[0016] In a tubular body having a braided structure, the material A may be braided in a mesh shape so as to close an opening at at least one axial end of the tubular body. For example, the material A at at least one axial end of the tubular body may be converged to close the opening at that end. In other words, the braided structure may be a woven basket structure in which the opening at at least one axial end of the tubular body is closed.

[0017] The tubular body may have both a braided structure and a helical structure. The tubular body may include a region having a helical structure (first tubular body) and a region having a braided structure (second tubular body). The tubular body may be formed by stacking a first tubular body having a helical structure and a second tubular body having a braided structure in the axial direction. Either the first tubular body or the second tubular body may be disposed on the inner circumferential side and the other on the outer circumferential side to form a tubular body having a dual structure of a helical structure and a braided structure in the radial direction.

[0018] The material A constituting the cylindrical body may be a metal material or a resin material.

[0019] Examples of metallic materials include stainless steel (SUS), carbon steel, copper alloy, nickel alloy, titanium alloy, aluminum alloy, etc. Among them, stainless steel is preferred from the viewpoint of both shape retention and deformability and durability against non-aqueous electrolytes. Metallic materials are advantageous in terms of ensuring the strength of the cylindrical body.

[0020] Examples of resin materials include polyimide resins, polyamide resins, polyolefin resins (e.g., polyethylene, polypropylene, etc.), polyethylene naphthalate resins, polyacrylonitrile resins, polyphenylene sulfide resins, polycarbonate resins, polyether ether ketone resins, polyether sulfone resins, etc. Among these, polyimide resins are preferred from the viewpoints of strength (shape retention), thermal stability, and durability against non-aqueous electrolytes. Resin materials are advantageous in terms of suppressing internal short circuits.

[0021] The electrode group may have an outer shape of either a cylindrical or elliptical cylinder. The cylindrical body used as the core member may have an outer shape of either a cylindrical or elliptical cylinder. The diameter (maximum diameter) of the cylindrical body is, for example, 1.5 mm or more and 8 mm or less.

[0022] The core member (cylindrical body) is inserted into the hollow of the electrode group after the electrode group is constructed, for example. At the time of inserting the core member into the hollow of the electrode group, a gap may exist between the inner peripheral surface of the electrode group and the outer peripheral surface of the core member. The size of the gap may be designed so that the inner peripheral surface of the electrode group and the outer peripheral surface of the core member come into contact with each other in a discharged state through subsequent charging and discharging.

[0023] Here, Fig. 1 is a front view showing an example of a core member as viewed from the side (peripheral surface). The X direction in Fig. 1 is the axial direction of the core member (cylindrical body).

[0024] The core member 29 shown in Fig. 1 is a cylindrical body 100 having a spiral structure in which a linear material 110 is wound in a spiral shape. The cylindrical body 100 in Fig. 1 is a spring body formed by spirally winding the linear material 110 while keeping it in close contact with the material. The diameter of the linear material 110 is, for example, 0.05 mm or more and 3 mm or less. The linear material 110 is, for example, a metal material such as stainless steel.

[0025] In Fig. 1, when the cylindrical body 100 is viewed from the side (periphery), the linear material 110 is arranged so that the length direction of the linear material 110 intersects with the axial direction (X direction) of the cylindrical body 100. In the case of Fig. 1, the acute angle formed by the length direction of the linear material 110 and the axial direction (X direction) of the cylindrical body 100 is, for example, not less than 70° and not more than 85°.

[0026] In the cylindrical body 100 of FIG. 1 , the linear material 110 is spirally wound while being tightly packed, thereby forming a gap-free side of the cylindrical body 100. However, as shown in FIG. 2 , the linear material 110 may be spirally wound without being tightly packed, thereby forming a gap 120 on the side of the cylindrical body 100. From the viewpoint of shape stability of the electrode group, the cylindrical body 100 of FIG. 1 may be used. From the viewpoint of the fluidity of the non-aqueous electrolyte inside the battery and the deformability of the cylindrical body, the cylindrical body 100 of FIG. 2 may be used. In the case of FIG. 2 , the acute angle formed by the length direction of the linear material 110 and the axial direction (X direction) of the cylindrical body 100 is, for example, 65° or more and 85° or less. In FIG. 2 , when the cylindrical body 100 is viewed from the side (periphery), the distance between adjacent linear material 110 is, for example, 0.01 mm or more and 10 mm or less.

[0027] 3 is a perspective view showing another example of the core member, in which the X direction is the axial direction of the core member (cylindrical body).

[0028] The core member 29 shown in FIG. 3 is a tubular body 200 having a braided structure in which linear materials 210 are braided in a net-like shape. The tubular body 200 in FIG. 3 is formed by braiding the linear materials 210 and can serve as a spring. Furthermore, at both axial end portions 220 of the tubular body 200 (X direction), converging portions 230 are formed by converging the linear materials 210. The converging portions 230 are fixed by welding or the like. In other words, the tubular body 200 in FIG. 3 has a woven basket structure in which the openings at both axial end portions 220 (X direction) are closed. With a woven basket structure, the shape stability of the tubular body is easily ensured even if the tubular body is deformed by stretching in the axial direction. The converging portion 230 may be formed by converging the linear material 210 at both end portions 220 in the axial direction (X direction) of the cylindrical body 200, housing the ends of the converged linear material 210 in a cap-shaped member, and integrating the ends with the cap-shaped member by welding or the like. The linear material 210 is, for example, a metal material such as stainless steel. The diameter of the linear material 210 is, for example, 0.05 mm or more and 3 mm or less.

[0029] Known braiding methods can be used. The braiding method is not particularly limited, but examples include stockinette knitting, circular knitting, plain weave, and twill weave. From the perspective of the deformability and elasticity of the tubular body, the linear material 210 may be woven in a spiral or loop pattern. For example, multiple linear materials (half of the first linear materials and half of the second linear materials) may be prepared, and the first linear materials may be spirally wound around a core rod at regular intervals in the same direction, and the second linear materials may be spirally wound around a core rod at regular intervals in a direction opposite to that of the first linear materials. In this case, a mesh is formed by the first and second linear materials intersecting with each other. The shape of the mesh in FIG. 3 is not particularly limited, and examples include a rectangle. From the perspective of the deformability of the tubular body, the mesh shape may be a figure surrounded by curves, including arcs and sinusoidal shapes. From the viewpoint of the shape stability of the cylindrical body, the maximum diameter of the mesh formed by the braided structure is, for example, not less than 0.1 mm and not more than 10 mm.

[0030] 3, when the cylindrical body 200 is viewed from the side (periphery), the linear material 210 is arranged so that the length direction of the linear material 210 intersects with the axial direction (X direction) of the cylindrical body 200. The acute angle formed by the length direction of the linear material 210 and the axial direction (X direction) of the cylindrical body 200 may be, for example, 30° or more and 70° or less.

[0031] 4 is a perspective view showing a still further example of the core member, in which the X direction is the axial direction of the core member (cylindrical body).

[0032] The core member 29 shown in FIG. 4 is a cylindrical body 300 with a spiral structure, in which a strip-shaped material 310 is spirally wound. The cylindrical body 300 in FIG. 4 is obtained by spirally winding the strip-shaped material 310 while overlapping its widthwise ends, and then bonding the overlapping portions 320 of the wound strip-shaped material 310 by ultrasonic welding or the like. The overlapping portions are in close contact when the cylindrical body 300 is inserted into the hollow of the electrode group and during normal use. When a strong impact is applied to the battery, the ends of the overlapping portions (seams) separate, forming slits on the sides of the cylindrical body 300. The formation of the slits allows the cylindrical body 300 to deform and stretch in the axial direction. The overlapping portions 320 may be bonded continuously along the overlapping portions 320 (the widthwise ends of the strip-shaped material 310) or discontinuously (at predetermined intervals).

[0033] The width of the overlapping portion 320 (the end portion in the width direction of the strip-shaped material 310) is, for example, 0.03 to 0.3 times the width of the strip-shaped material 310. The strip-shaped material 310 has, for example, a thickness of 0.01 to 3 mm and a width of 2 to 40 mm. The strip-shaped material 310 is, for example, a resin material such as polyimide resin.

[0034] 4, when the cylindrical body 300 is viewed from the side (periphery), the strip material 310 is arranged so that the length direction of the strip material 310 intersects with the axial direction (X direction) of the cylindrical body 300. The acute angle formed by the length direction of the strip material 310 and the axial direction (X direction) of the cylindrical body 300 may be, for example, 30° or more and 70° or less.

[0035] Secondary batteries include lithium secondary batteries (lithium metal secondary batteries), lithium ion batteries, and the like. For example, the negative electrode of a lithium secondary battery is an electrode in which lithium metal precipitates during charging and dissolves in the non-aqueous electrolyte during discharging. The negative electrode of a lithium ion battery is an electrode in which lithium ions are absorbed into the negative electrode active material during charging and released from the negative electrode active material during discharging. Among secondary batteries, lithium secondary batteries have a large expansion rate of the negative electrode due to Li precipitation during charging, resulting in a large volume change in the electrode group. Therefore, the effects of the above-mentioned cylindrical body are significantly obtained.

[0036] The secondary battery will be described in detail below.

[0037] (Lithium secondary battery) A lithium secondary battery includes a positive electrode, a negative electrode on which lithium metal precipitates during charging and dissolves in a non-aqueous electrolyte during discharge, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The negative electrode includes at least a negative electrode current collector, and lithium metal precipitates on the negative electrode current collector during charging. The non-aqueous electrolyte has lithium ion conductivity.

[0038] In a lithium secondary battery, for example, 70% or more of the rated capacity is achieved by the deposition and dissolution of lithium metal. The movement of electrons at the negative electrode during charging and discharging is mainly due to the deposition and dissolution of lithium metal at the negative electrode. Specifically, 70 to 100% (e.g., 80 to 100% or 90 to 100%) of the movement of electrons (or current from another perspective) at the negative electrode during charging and discharging is due to the deposition and dissolution of lithium metal. In other words, the negative electrode of a lithium secondary battery differs from a negative electrode in which the movement of electrons at the negative electrode during charging and discharging is mainly due to the absorption and release of lithium ions by the negative electrode active material (e.g., graphite).

[0039] (Separator) The separator includes at least a sheet-like substrate. The separator may include a spacer. That is, the separator may include a substrate and a spacer disposed on a main surface of the substrate.

[0040] (Substrate) A porous sheet having ion permeability and insulating properties is used as the substrate. Examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. The material of the porous sheet is not particularly limited, but may be a polymer material. Examples of the polymer material include an olefin resin, a polyamide resin, and cellulose. Examples of the olefin resin include polyethylene, polypropylene, and a copolymer of ethylene and propylene. The substrate may contain an additive, if necessary. Examples of the additive include an inorganic filler.

[0041] The thickness of the substrate is not particularly limited, but is, for example, 5 μm or more and 20 μm or less, and more preferably 10 μm or more and 20 μm or less.

[0042] The substrate may include a porous sheet and a composite material layer (heat-resistant layer). The composite material layer may be formed on one or both main surfaces of the porous sheet. The composite material layer is a layer that allows lithium ions to permeate. The composite material layer contains inorganic particles. The composite material layer may contain a resin material as needed. The thickness of the composite material layer may be 5% to 50% of the total thickness of the substrate.

[0043] The composite material layer may be disposed on the side of the porous sheet facing the positive electrode, or on the side of the porous sheet facing the negative electrode. When the composite material layer is disposed on the positive electrode side, deterioration of the porous sheet due to oxidation reactions can be suppressed. When the composite material layer is disposed on the negative electrode side, deterioration of the porous sheet due to reduction reactions can be suppressed. A spacer may be disposed on the composite material layer. In this case, the effect of suppressing thermal shrinkage of the substrate is particularly enhanced.

[0044] The inorganic particles are preferably particles of an inorganic compound that is thermally stable and insulating, and thus unlikely to melt or decompose during abnormal heat generation due to a short circuit in the battery, etc. Examples of inorganic particle materials include oxides, hydroxides, nitrides, carbides, sulfides, etc. Examples of oxides include aluminum oxide, boehmite, magnesium oxide, titanium oxide, zirconium oxide, silicon oxide, yttrium oxide, zinc oxide, etc. Examples of nitrides include silicon nitride, aluminum nitride, boron nitride, titanium nitride, etc. Examples of carbides include silicon carbide and boron carbide, etc. Examples of sulfides include barium sulfate, etc. Examples of hydroxides include aluminum hydroxide, etc. The median diameter in the volume-based particle size distribution of the inorganic particles may be 0.2 to 2.0 μm.

[0045] The median diameter in the volume-based particle size distribution of inorganic particles can be measured, for example, using a laser diffraction / scattering particle size distribution measuring device (for example, Microtrac manufactured by Nikkiso Co., Ltd.) Alternatively, the cross section of the substrate may be observed with a transmission electron microscope (TEM), a TEM image may be taken, the area surrounded by the outlines of any 100 inorganic particles may be calculated, the diameter of an equivalent circle (perfect circle) having the same area as the calculated area may be determined, and the average diameter of the 100 equivalent circles may be calculated.

[0046] Examples of resin materials contained in the composite material layer (heat-resistant layer) include fluorine-containing resins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene, fluorine-containing rubbers such as vinylidene fluoride-tetrafluoroethylene copolymer and ethylene-tetrafluoroethylene copolymer, styrene-butadiene copolymer or hydrogenated product thereof, acrylonitrile-butadiene copolymer or hydrogenated product thereof, methacrylic acid ester-acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, ethylene propylene rubber, polyvinyl alcohol, and rubbers such as polyvinyl acetate. Examples of the resin include cellulose derivatives such as ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, and carboxymethyl cellulose; acrylic resins such as acrylic acid-methacrylic acid copolymers; polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyimide, polyamides such as wholly aromatic polyamide (aramid); polyamideimide, polyacrylonitrile, polyvinyl alcohol, polyether, polyacrylic acid, polymethacrylic acid, polyester, polyolefin, silicone resin, urethane resin, melamine resin, urea resin, and epoxy resin.

[0047] The resin material contained in the composite material layer (heat-resistant layer) is preferably a polymeric material having higher heat resistance than the material of the porous sheet. Such a polymeric material preferably includes at least one selected from the group consisting of aromatic polyamides, aromatic polyimides, and aromatic polyamideimides. These are known to have high heat resistance. From the viewpoint of heat resistance, aramids, i.e., meta-aramids (meta-type wholly aromatic polyamides) and para-aramids (para-type wholly aromatic polyamides), are preferred.

[0048] The content of the inorganic particles in the composite material layer may be in the range of 50% by weight to 99% by weight (for example, in the range of 85% by weight to 99% by weight).

[0049] The composite material layer is formed, for example, by applying a coating liquid containing inorganic particles, a resin material, and a liquid component (dispersion medium) to a porous sheet and then drying the coating film. Examples of the liquid component include N-methyl-2-pyrrolidone.

[0050] (Spacer) The separator may further include a spacer disposed on the main surface of the substrate. By using the above-mentioned cylindrical body as a core member, buckling of the electrodes near the hollow of the electrode group is suppressed even without disposing a spacer, but a spacer may be disposed between the positive and negative electrodes to suppress volumetric changes of the electrode group. Disposing a spacer further improves the shape stability of the electrode group during charge and discharge, and further suppresses buckling of the electrodes.

[0051] From the viewpoint of ease of fabrication of the electrode group, the substrate and the spacer may be integrated by forming a spacer on the main surface of the substrate. The electrode and the spacer may be integrated by forming a spacer on the main surface of the electrode. It is sufficient that the spacer is disposed on the main surface of the substrate when constructing the electrode group.

[0052] The spacer may be disposed on the main surface of the substrate facing the positive electrode (the main surface of the substrate on the positive electrode side), or on the main surface of the substrate facing the negative electrode (the main surface of the substrate on the negative electrode side), or on both main surfaces. When the spacer is disposed on the main surface of the substrate facing the positive electrode, Li precipitates between the spacers so as to stretch the substrate in the direction of the positive electrode, compared to when the spacer is disposed on the main surface of the substrate facing the negative electrode, so that compressive stress is generated in the precipitated Li, making it easier for Li to precipitate densely. From the viewpoint of improving discharge efficiency and cycle characteristics, it is preferable to dispose the spacer on the main surface of the substrate facing the positive electrode. On the other hand, when the spacer is disposed on the main surface of the substrate facing the negative electrode, a space is formed in advance between the substrate and the negative electrode, so the tensile load on the substrate that occurs due to the precipitation of Li is reduced. In other words, it is advantageous in that it is easier to maintain the insulation properties of the substrate or the short-circuit resistance of the substrate.

[0053] In lithium secondary batteries, the main role of the spacer is to form a space for lithium metal to deposit, which prevents the negative electrode from expanding during charging.

[0054] The spacer may include a resin material (e.g., an insulating resin) or may include a resin material and particles. The proportion of the resin material in the spacer may be 10% by volume or more, 30% by volume or more, or 50% by volume or more, or may be 100% by volume or less, or 80% by volume or less.

[0055] Examples of resin materials contained in the spacer include fluorine-containing resins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene, fluorine-containing rubbers such as vinylidene fluoride-tetrafluoroethylene copolymer and ethylene-tetrafluoroethylene copolymer, styrene-butadiene copolymer or its hydrogenated product, acrylonitrile-butadiene copolymer or its hydrogenated product, methacrylic acid ester-acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, rubbers such as ethylene propylene rubber, polyvinyl alcohol, and polyvinyl acetate, Examples of the resin include cellulose derivatives such as ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, and carboxymethyl cellulose; acrylic resins such as acrylic acid-methacrylic acid copolymers; polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyimide, polyamides such as wholly aromatic polyamide (aramid); polyamideimide, polyacrylonitrile, polyvinyl alcohol, polyether, polyacrylic acid, polymethacrylic acid, polyester, polyolefin, silicone resin, urethane resin, melamine resin, urea resin, and epoxy resin.

[0056] Among the above resin materials, preferred materials that are impermeable to lithium ions include polyimide, polyvinylidene fluoride, and acrylonitrile-acrylic acid ester copolymers, and polyimide may also be used. A non-porous spacer of a certain height or greater made of these resin materials is a layer that is impermeable to lithium ions and has a non-porous structure that does not allow lithium ions to pass through. The placement of such a spacer is preferred from the viewpoint of suppressing an increase in the gas generation reaction rate during an internal short circuit.

[0057] The particles may be inorganic or organic. Examples of inorganic particles include insulating metal oxides, metal hydroxides, metal nitrides, metal carbides, and metal sulfides. Examples of metal oxides include aluminum oxide (alumina and boehmite), magnesium oxide, titanium oxide (titania), zirconium oxide, and silicon oxide (silica). Examples of metal hydroxides include aluminum hydroxide. Examples of metal nitrides include silicon nitride, aluminum nitride, boron nitride, and titanium nitride. Examples of metal carbides include silicon carbide and boron carbide. Examples of metal sulfides include barium sulfate. Minerals such as aluminosilicates, layered silicates, barium titanate, and strontium titanate may also be used. Among these, alumina, silica, and titania are preferred.

[0058] The average particle size of the particles is not particularly limited, but may be 0.1 μm or more, 0.5 μm or more, or 10 μm or less, 5 μm or less, or 2 μm or less. The average particle size can be measured by the following method. First, a cross-section of the spacer in the thickness direction of the separator is photographed using an electron microscope to obtain an image of the cross-section. Next, the image is subjected to image processing such as binarization to identify the particle portion. Next, the diameter of a circle having the same area as the cross-section area of ​​each particle (equivalent circle diameter) is determined, and the arithmetic mean of the determined equivalent circle diameters can be used as the average particle size. The arithmetic mean can be determined, for example, from 100 or more particles. The average particle size of other particles contained in the electrode plate and separator can also be determined by a similar method.

[0059] When the spacer contains a resin material and particles, the particle content in the spacer is preferably 50% by volume or less, which makes it easier to ensure sufficient strength of the spacer.

[0060] The spacer includes a protrusion. The spacer may include a linear protrusion and / or a dot-shaped protrusion. From one viewpoint, the linear protrusion is a ridge-shaped protrusion. The linear protrusion may be arranged continuously or intermittently. The linear protrusion may be straight or curved.

[0061] The width of the linear convex portion may be 100 μm or more, or 200 μm or more, and may be 2000 μm or less, or 1000 μm or less.

[0062] The spacer preferably includes protrusions having a predetermined repeating pattern. The linear protrusions may be arranged in a stripe pattern or a mesh pattern. The mesh pattern may be a collection of polygons. An example of a mesh pattern includes a shape in which polygons are combined so as to share a side. Polygons include triangles, quadrilaterals, hexagons, etc. Different types of polygons may be combined. The mesh pattern may be a honeycomb pattern. Furthermore, dot-shaped protrusions may be arranged in a predetermined repeating pattern.

[0063] The thickness TC of the spacer (height of the convex portion) may be greater than the thickness T of the substrate. The ratio of the height TC to the thickness T: TC / T may be greater than 1, and 1.5 or greater, or 2 or greater. TC / T may be 5 or less, 4 or less, or 3 or less. From the viewpoint of suppressing expansion of the electrode group, TC / T may be, for example, greater than 1 (or 1.5 or greater) and 3 or less.

[0064] The spacers are formed, for example, by applying a coating liquid containing spacer components and a liquid component to a predetermined location on the substrate (or electrode) and drying the coating. Examples of the liquid component include N-methyl-2-pyrrolidone. The coating may be performed using a dispenser or by known printing methods such as gravure printing, inkjet printing, and screen printing. The drying may be performed by known methods such as heating or natural drying. The thickness of the spacers can be adjusted by changing the amount applied or the viscosity of the coating liquid.

[0065] (Negative electrode) The negative electrode includes a negative electrode current collector. In a lithium secondary battery, lithium metal is deposited on the surface of the negative electrode upon charging. More specifically, lithium ions contained in the non-aqueous electrolyte receive electrons on the negative electrode upon charging, becoming lithium metal, which is then deposited on the surface of the negative electrode. The lithium metal deposited on the surface of the negative electrode dissolves as lithium ions in the non-aqueous electrolyte upon discharging. The negative electrode may be composed of only the negative electrode current collector, or may include a thin lithium metal foil previously pressed onto the negative electrode current collector.

[0066] The negative electrode may also include a lithium ion occlusion layer (a layer that develops capacity by occlusion and release of lithium ions by the negative electrode active material) supported on the negative electrode current collector. In this case, the open circuit potential of the negative electrode at full charge may be 70 mV or less relative to lithium metal (dissolution and deposition potential of lithium). If the open circuit potential of the negative electrode at full charge is 70 mV or less relative to lithium metal, lithium metal is present on the surface of the lithium ion occlusion layer at full charge. In other words, the negative electrode develops capacity by deposition and dissolution of lithium metal.

[0067] The lithium ion occlusion layer is a layer of a negative electrode mixture containing a negative electrode active material. The negative electrode mixture may contain a binder, a thickener, a conductive agent, etc. in addition to the negative electrode active material.

[0068] Examples of the negative electrode active material include a carbonaceous material, a Si-containing material, and a Sn-containing material. The negative electrode may contain one type of negative electrode active material, or a combination of two or more types. When the negative electrode contains a Si-containing material as the negative electrode active material, the expansion rate of the negative electrode during charging is large, and the volume change of the electrode group is large, so the effect of the cylindrical body described above is significantly obtained. Examples of the carbonaceous material include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon).

[0069] The conductive material is, for example, a carbon material, such as carbon black, acetylene black, ketjen black, carbon nanotubes, and graphite.

[0070] Examples of the binder include fluororesin, polyacrylonitrile, polyimide resin, acrylic resin, polyolefin resin, rubber polymer, etc. Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, etc.

[0071] The negative electrode current collector may be a conductive sheet, such as a foil or film.

[0072] The material of the negative electrode current collector (conductive sheet) may be any conductive material other than lithium metal and lithium alloys. The conductive material may be a metallic material such as a metal or alloy. A conductive material that does not react with lithium is preferred. More specifically, a material that does not form an alloy or intermetallic compound with lithium is preferred. Examples of such conductive materials include copper (Cu), nickel (Ni), iron (Fe), alloys containing these metal elements, and graphite with a preferentially exposed basal surface. Examples of alloys include copper alloys and stainless steel (SUS). Among these, copper and / or copper alloys, which have high conductivity, are preferred. Alternatively, a laminate sheet in which a metal or alloy such as stainless steel, nickel, nickel alloy, copper, or copper alloy is laminated on the surface of a resin film may be used as the negative electrode current collector. The resin material of the resin film is not particularly limited, but examples include polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), polyethylene, polypropylene, polyamide, and polyimide.

[0073] The thickness of the negative electrode current collector is not particularly limited and is, for example, 5 μm or more and 300 μm or less.

[0074] (Positive Electrode) The positive electrode includes, for example, a positive electrode current collector and a positive electrode composite layer supported on the positive electrode current collector. The positive electrode composite layer includes, for example, a positive electrode active material, a conductive material, and a binder. The positive electrode composite layer may be formed on only one side of the positive electrode current collector, or may be formed on both sides. The positive electrode is obtained, for example, by applying a positive electrode composite slurry including the positive electrode active material, the conductive material, and the binder to both sides of the positive electrode current collector, drying the coating, and then rolling.

[0075] The positive electrode active material is a material that absorbs and releases lithium ions. Examples of the positive electrode active material include lithium-containing transition metal oxides, transition metal fluorides, polyanions, fluorinated polyanions, and transition metal sulfides. Among these, lithium-containing transition metal oxides are preferred because of their low production cost and high average discharge voltage.

[0076] The lithium contained in the lithium-containing transition metal oxide is released from the positive electrode as lithium ions during charging and precipitates as lithium metal on the negative electrode or negative electrode current collector. During discharging, the lithium metal dissolves from the negative electrode, releasing lithium ions, which are then absorbed into the composite oxide of the positive electrode. In other words, the lithium ions involved in charging and discharging are generally derived from the solute in the nonaqueous electrolyte and the positive electrode active material.

[0077] Examples of transition metal elements contained in the lithium-containing transition metal oxide include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. The lithium-containing transition metal oxide may contain one or more transition metal elements. The transition metal element may be Ni, Co, and / or Mn. The lithium-containing transition metal oxide may contain one or more typical elements as needed. Examples of typical elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. The typical element may be Al, etc.

[0078] Among lithium-containing transition metal oxides, composite oxides containing Ni, Co, and / or Mn as transition metal elements, and which may contain Al as an optional component, and which have a layered rock-salt crystal structure are preferred in terms of obtaining high capacity. In this case, in the lithium secondary battery, the molar ratio mLi / mM of the total amount of lithium contained in the positive electrode and negative electrode to the amount mM of metal M other than lithium contained in the positive electrode is set to, for example, 1.1 or less.

[0079] As the binder, conductive material, etc., for example, those exemplified for the negative electrode can be used. The shape and thickness of the positive electrode current collector can be selected from the shape and range of the positive electrode current collector.

[0080] Examples of materials for the positive electrode current collector (conductive sheet) include metal materials containing Al, Ti, Fe, etc. The metal material may be Al, Al alloy, Ti, Ti alloy, Fe alloy, etc. The Fe alloy may be stainless steel (SUS). Alternatively, a laminate sheet in which a metal or alloy such as stainless steel, aluminum, aluminum alloy, or titanium is laminated on the surface of a resin film may be used as the positive electrode current collector. The resin material for the resin film is not particularly limited, and examples include polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), polyethylene, polypropylene, polyamide, and polyimide.

[0081] The thickness of the positive electrode current collector is not particularly limited and is, for example, 5 μm or more and 300 μm or less.

[0082] (Non-aqueous electrolyte) The non-aqueous electrolyte has ion conductivity (for example, lithium ion conductivity). The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution), a gel electrolyte, or a solid electrolyte. The liquid electrolyte is, for example, an electrolytic solution containing a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the electrolytic solution is, for example, 0.5 mol / L or more and 2 mol / L or less. The electrolytic solution may contain known additives.

[0083] The gel electrolyte contains a lithium salt and a matrix polymer, or a lithium salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer is, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, polyether resin, and polyethylene oxide.

[0084] As the solid electrolyte, for example, a material known in all-solid-state lithium ion secondary batteries (for example, oxide-based solid electrolyte, sulfide-based solid electrolyte, halide-based solid electrolyte, etc.) can be used.

[0085] In a liquid (gel) non-aqueous electrolyte, lithium salt dissolves in a non-aqueous solvent to generate lithium ions and anions.

[0086] The anion is BF４ － , ClO ４ － , P.F. ６ － , C.F. ３ SO ３ － , C.F. ３ CO ２ － , anions of imides, anions of oxalate complexes, etc. Examples of the anions of imides include N(SO ２ CF ３ ) ２ － , N(C ｍ F ２ｍ＋１ SO ２ ) ｘ (C ｎ F ２ｎ＋１ SO ２ ) ｙ － (m and n are each independently an integer of 0 or 1 or more, and x and y are each independently 0, 1, or 2, satisfying the relationship x+y=2). The anion of the oxalate complex may contain boron and / or phosphorus. Examples of the anion of the oxalate complex include bisoxalate borate anion and difluorooxalate borate anion (BF ２ (C ２ O ４ ) － ), P.F. ４ (C ２ O ４ ) － , P.F. ２ (C ２ O ４ ) ２ － The non-aqueous electrolyte may contain one of these anions alone or two or more of them.

[0087] From the viewpoint of suppressing the deposition of lithium metal in a dendritic form, the nonaqueous electrolyte preferably contains an anion of an oxalate complex, and more preferably contains an oxalate complex anion having fluorine. The interaction between the oxalate complex anion having fluorine and lithium facilitates the uniform deposition of lithium metal in the form of fine particles. This makes it easier to suppress local deposition of lithium metal. The oxalate complex anion having fluorine may be combined with another anion. The other anion may be PF ６ － and / or an anion of an imide.

[0088] Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and halogen-substituted products thereof. The non-aqueous electrolyte may contain one or more of these non-aqueous solvents. Examples of halogen-substituted products include fluorides. From the viewpoint of suppressing decomposition of the non-aqueous electrolyte due to contact with lithium metal, the non-aqueous electrolyte preferably contains an ether-based solvent, which has excellent resistance to reduction.

[0089] Examples of esters include carbonate esters and carboxylic acid esters. Examples of cyclic carbonate esters include ethylene carbonate, propylene carbonate, and fluoroethylene carbonate (FEC). Examples of chain carbonate esters include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate. Examples of cyclic carboxylic acid esters include γ-butyrolactone and γ-valerolactone. Examples of chain carboxylic acid esters include ethyl acetate, methyl propionate, and methyl fluoropropionate.

[0090] Examples of the ether include cyclic ethers and chain ethers. Examples of the cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of the chain ethers include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methyl phenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, and diethylene glycol dimethyl ether.

[0091] The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 3.5 mol / L or less. The concentration of the anion in the non-aqueous electrolyte may be 0.5 mol / L or more and 3.5 mol / L or less. Furthermore, the concentration of the anion of the oxalate complex in the non-aqueous electrolyte may be 0.05 mol / L or more and 1 mol / L or less.

[0092] The non-aqueous electrolyte may contain an additive. The additive may form a coating on the negative electrode. The formation of a coating derived from the additive on the negative electrode makes it easier to suppress the formation of dendrites. Examples of such additives include vinylene carbonate, FEC, and vinyl ethyl carbonate (VEC).

[0093] (Lithium-ion battery) A lithium-ion battery includes a positive electrode, a negative electrode containing a negative electrode active material that absorbs and releases lithium ions, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The positive electrode and non-aqueous electrolyte may be the same as those exemplified for lithium secondary batteries. The positive electrode composite and positive electrode current collector contained in the positive electrode may be appropriately selected from those exemplified above. The non-aqueous solvent and lithium salt (anion) contained in the non-aqueous electrolyte may be appropriately selected from those exemplified above.

[0094] The negative electrode includes, for example, a negative electrode current collector and a negative electrode composite layer (the above-mentioned lithium ion occlusion layer) supported on a main surface of the negative electrode current collector. The negative electrode composite layer may be supported on one main surface of the negative electrode current collector, or may be supported on both main surfaces of the negative electrode current collector. The negative electrode composite and negative electrode current collector included in the negative electrode can be appropriately selected from those exemplified above.

[0095] The substrate and spacer included in the separator can be the same as those used in lithium secondary batteries. The substrate may include a porous sheet containing a polymer material. The substrate may further include a composite material layer containing a resin material and inorganic particles. The spacer may include a resin material.

[0096] In the case of a lithium-ion battery, for example, the spacer may be disposed at a location where stress is likely to increase when the negative electrode expands, such as the innermost periphery of a wound electrode assembly, or at a location of a flat electrode assembly with a small radius of curvature.

[0097] An example of a secondary battery according to the present embodiment will be specifically described below with reference to the drawings. The components described above can be applied to the components of the example secondary battery described below. The components of the example described below can be modified based on the above description. The matters described below may also be applied to the above embodiment. In the secondary battery described below, components that are not essential for the secondary battery according to the present disclosure may be omitted. Note that the scale of the components has been changed in the following drawings to facilitate understanding.

[0098] (Embodiment 1) FIG. 5 is a longitudinal cross-sectional view schematically illustrating a lithium secondary battery as an example of a secondary battery according to Embodiment 1. Note that FIG. 5 does not illustrate spacers and the spaces formed by the spacers. The cylindrical lithium secondary battery 10 shown in FIG. 5 includes a cylindrical battery case and a wound electrode group 14 and a nonaqueous electrolyte (not shown) housed within the battery case. A core member 29 is disposed within the hollow of the electrode group 14. The core member 29 is the aforementioned cylindrical body. The battery case includes a case body 15, which is a cylindrical metal container with a bottom, and a sealing member 16 that seals the opening of the case body 15. A gasket 27 is disposed between the case body 15 and the sealing member 16. The gasket 27 ensures the hermeticity of the battery case. Within the case body 15, insulating plates 17 and 18 are disposed at both ends of the electrode group 14 in the direction of the winding axis.

[0099] Case body 15 has a step 21 formed, for example, by pressing a portion of the side wall of case body 15 from the outside. Step 21 may be formed in an annular shape along the circumferential direction of case body 15 on the side wall of case body 15. In this case, sealing body 16 is supported by the surface of step 21 on the opening side.

[0100] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. These components are stacked in this order in the sealing body 16. The sealing body 16 is attached to the opening of the case body 15 so that the cap 26 is located outside the case body 15 and the filter 22 is located inside the case body 15. The above-mentioned components constituting the sealing body 16 are, for example, disk-shaped or ring-shaped. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, and an insulating member 24 is interposed between their respective peripheral edges. The filter 22 and the lower valve body 23 are connected to each other at their respective centers. The upper valve body 25 and the cap 26 are connected to each other at their respective centers. In other words, all components except the insulating member 24 are electrically connected to each other.

[0101] A vent hole (not shown) is formed in the lower valve body 23. Therefore, if the internal pressure of the battery case increases due to abnormal heat generation or the like, the upper valve body 25 bulges toward the cap 26 and separates from the lower valve body 23. This cuts off the electrical connection between the lower valve body 23 and the upper valve body 25. If the internal pressure increases further, the upper valve body 25 breaks, and gas is discharged from an opening (not shown) formed in the cap 26.

[0102] 6 is an enlarged view of a portion of the electrode group 14. The electrode group 14 includes a positive electrode 11, a negative electrode 12, and a separator (a substrate 50 and a spacer 53). The positive electrode 11, the negative electrode 12, and the separator substrate 50 are all strip-shaped. The electrode group 14 is formed by winding the positive electrode 11, the negative electrode 12, and the separator (the substrate 50 with the spacer 53 disposed thereon) so that the separator is disposed between the positive electrode 11 and the negative electrode 12.

[0103] The positive electrode 11 includes a positive electrode current collector 11a and a positive electrode composite layer 11b. The positive electrode current collector 11a is electrically connected to a cap 26, which functions as a positive electrode terminal, via a positive electrode lead 19. In FIG. 6, the negative electrode 12 is shown as a negative electrode (negative electrode current collector) on which no lithium metal is deposited. The negative electrode 12 is electrically connected to a case body 15, which functions as a negative electrode terminal, via a negative electrode lead 20.

[0104] The substrate 50 has a main surface 50a facing the positive electrode 11 and a main surface 50b facing the negative electrode 12. The substrate 50 of embodiment 1 includes a porous sheet 51 and a composite material layer 52 (heat-resistant layer). The composite material layer 52 is formed on one of the two main surfaces of the porous sheet 51, the main surface facing the negative electrode 12. In embodiment 1, a spacer 53 is formed on the main surface 50a facing the positive electrode 11. The spacer 53 is formed on the composite material layer 52 and is in contact with the positive electrode 11. The spacer 53 forms a space 14s between the positive electrode 11 and the negative electrode 12 (between the negative electrode 12 and the substrate 50). Figure 6 shows the height h of the spacer 53.

[0105] 6, the spacer 53 is disposed on the main surface 50a of the substrate 50 on the positive electrode 11 side, but may also be disposed on the main surface 50b of the substrate 50 on the negative electrode 12 side. The spacer 53 is formed on the composite material layer 52, but may also be formed on the porous sheet 51. The composite material layer 52 of the substrate 50 is disposed on the positive electrode 11 side, but may also be disposed on the negative electrode 12 side.

[0106] During charging of the lithium secondary battery 10, lithium metal is deposited on the negative electrode 12. Because a space 14s exists between the positive electrode 11 and the negative electrode 12, the volume change of the electrode group 14 that accompanies the deposition of lithium metal is reduced, improving the cycle characteristics.

[0107] An example of the planar shape of the spacer 53 is shown in Fig. 7. Fig. 7 shows the spacer 53 arranged on the main surface of the substrate 50 on the composite material layer 52 side. The spacer 53 in Fig. 7 is composed of linear protrusions 53a arranged in a honeycomb pattern. The linear protrusions 53a are arranged continuously. Areas where the spacers 53 are not formed form spaces 14s.

[0108] Another example of the planar shape of the spacer 53 is shown in Fig. 8. The spacer 53 in Fig. 8 has a plurality of linear protrusions 53a arranged in a stripe pattern. Although six linear protrusions 53a are arranged in Fig. 5, the number of linear protrusions 53a is not limited to this. The linear protrusions 53a in Fig. 5 are arranged along the length direction of the strip-shaped substrate 50.

[0109] In the first embodiment, a cylindrical lithium secondary battery having a wound electrode group has been described. However, the secondary battery of this embodiment is not limited to the form of the first embodiment, and can be applied to other forms. The shape of the secondary battery can be appropriately selected from various shapes such as cylindrical, rectangular, and flat depending on the application.

[0110] <<Supplementary Notes>> The above embodiments disclose the following technologies. (Technology 1) A secondary battery comprising: a wound electrode group having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; a core member disposed in a hollow space of the electrode group; and a non-aqueous electrolyte, wherein the core member is a cylindrical body containing at least one selected from the group consisting of a spiral structure and a braided structure, and the cylindrical body is made of a linear or strip-shaped material, and in the spiral structure, the material is wound spirally, and in the braided structure, the material is braided in a mesh-like shape. (Technology 2) The secondary battery according to Technology 1, wherein, when viewed from the side of the cylindrical body, the material is arranged so that the length direction of the material intersects with the axial direction of the cylindrical body. (Technology 3) The secondary battery according to Technology 1 or 2, wherein the cylindrical body is a spring body. (Technology 4) The secondary battery according to any one of Technologies 1 to 3, wherein the negative electrode is an electrode in which lithium ions are absorbed into a negative electrode active material during charging and the lithium ions are released from the negative electrode active material during discharging. (Technology 5) The secondary battery according to any one of Technologies 1 to 3, wherein the negative electrode is an electrode in which lithium metal precipitates during charging and the lithium metal dissolves in the non-aqueous electrolyte during discharging. (Technology 6) The secondary battery according to any one of Technologies 1 to 5, wherein the separator includes a spacer, and the spacer includes protrusions having a predetermined repeating pattern. (Technology 7) The secondary battery according to any one of Technologies 1 to 6, wherein the non-aqueous electrolyte includes an ether-based solvent. (Technology 8) The secondary battery according to any one of Technologies 1 to 7, wherein the non-aqueous electrolyte includes an anion of an oxalate complex. (Technology 9) The secondary battery according to Technology 8, wherein the anion of the oxalate complex includes a difluorooxalatoborate anion.

[0111] [Examples] The secondary battery according to the present disclosure will be described in more detail below based on examples and comparative examples, although the present disclosure is not limited to the following examples.

[0112] Batteries A1 to A3, B1 (Preparation of Positive Electrode) 2.5 parts by mass of acetylene black and 2.5 parts by mass of polyvinylidene fluoride were added to 95 parts by mass of the positive electrode active material, and an appropriate amount of N-methyl-2-pyrrolidone was added and stirred to prepare a positive electrode mixture slurry. The positive electrode active material used was a rock salt type lithium-containing transition metal oxide having a layered structure containing Li, Ni, Co, and Al (the molar ratio of Li to the total of Ni, Co, and Al was 1.0).

[0113] The positive electrode composite slurry was applied to both sides of a strip of aluminum foil (positive electrode support), dried, and the coating was rolled to obtain a laminate in which a positive electrode composite layer was formed on both sides of the positive electrode support. The laminate was cut to a predetermined electrode size to obtain a strip of positive electrode. An aluminum positive electrode lead was attached to a predetermined position of the positive electrode.

[0114] (Preparation of Negative Electrode) In a dry atmosphere with a dew point of -30°C or less, lithium metal foil (10 μm thick) was pressure-bonded to both sides of a strip-shaped stainless steel foil (10 μm thick) serving as a negative electrode support, and a lithium metal layer (underlayer) was disposed. Thus, a negative electrode was prepared. A Ni negative electrode lead was attached to a predetermined position on the negative electrode. The lithium metal layer had a thickness of 30 μm during charging and 10 μm during discharging.

[0115] (Preparation of Separator Substrate) A 10 μm-thick polyethylene microporous thin film was prepared. One main surface of the microporous thin film was coated with a coating liquid containing paraphenylene terephthalamide, an aromatic polyamide, as the resin material and alumina as inorganic particles. The coating liquid was an N-methyl-2-pyrrolidone solution containing 5.8% by mass of dissolved calcium chloride, and the concentration was adjusted to 2% by mass of aromatic polyamide and 4% by mass of alumina. The substrate with the coating film formed was left for 1 hour in an atmosphere at 25°C and 70% relative humidity to precipitate the aromatic polyamide. Next, NMP and calcium chloride in the coating film were removed by water washing. The coating film was dried at 60°C for 5 minutes to form a 3 μm-thick composite material layer (heat-resistant layer). In this manner, a substrate comprising a microporous thin film and a composite material layer was obtained.

[0116] (Formation of spacers on the main surface of the substrate) A coating liquid containing polyvinylidene fluoride and alumina particles (inorganic filler) was applied to the surface of the microporous thin film of the substrate, and the coating was dried to form honeycomb-shaped spacers (linear convex portions). In this way, a separator having the substrate and the spacers was obtained.

[0117] (Preparation of non-aqueous electrolyte) 1,2-dimethoxyethane and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (CHF ２ CF ２ OCH ２ CF ３ ) in a mixed solvent (ether-based solvent) ６ and LiBF ２ (C ２ O ４ ) were dissolved in the non-aqueous electrolyte to prepare a non-aqueous electrolyte. ６ The concentration of LiBF in the non-aqueous electrolyte was 1 mol / L. ２ (C ２ O ４ The concentration of ) was 0.1 mol / L.

[0118] (Preparation of Core Member) For Battery A1, the core member was a cylindrical body (outer diameter 3 mm, spiral structure) as shown in FIG. 1 . A thin SUS wire (diameter 0.16 mm) was used as the linear material. The cylindrical body shown in FIG. 1 was produced by spirally winding the thin SUS wire around a core rod. After the cylindrical body was produced, the core rod was removed from the cylindrical body.

[0119] In Battery A2, the core member was a cylindrical body (outer diameter 3 mm, braided structure) as shown in Figure 3 . Stainless steel fine wires (diameter 0.13 mm) were used as the linear material. The cylindrical body shown in Figure 3 was fabricated by spirally winding 16 SUS fine wires (eight first fine wires and eight second fine wires) around a core rod. Specifically, the eight first fine wires were spirally wound around the core rod at regular intervals. The remaining eight second fine wires were spirally wound around the core rod at regular intervals in the opposite direction to the first fine wires. A mesh was formed by the intersection of the two oriented fine wires. At both axial ends of the cylindrical body, the linear material was converged, and the converged portions were housed in cap-shaped members and secured by welding to form convergent portions. The maximum diameter of the rectangular mesh formed by the braided structure was within the range of 0.5 mm to 3 mm. After fabrication of the cylindrical body, the core rod was removed from the cylindrical body.

[0120] In Battery A3, the core member was a cylindrical body (outer diameter 3 mm, spiral structure) as shown in Figure 4. A strip-shaped polyimide resin (PI) sheet (width 12 mm, thickness 0.075 mm) was used as the strip-shaped material. The cylindrical body in Figure 4 was fabricated by spirally winding the strip-shaped PI sheet around a core rod. The overlapping portions of the widthwise ends of the strip-shaped material were joined by ultrasonic welding. After fabrication of the cylindrical body, the core rod was removed from the cylindrical body.

[0121] 1, 3, and 4, when the cylindrical body is viewed from the side (periphery), material A is arranged so that the length direction of material A intersects with the axial direction of the cylindrical body. The acute angle formed by the length direction of material A and the axial direction of the cylindrical body is 88° for the cylindrical body of Fig. 1, 45° for the cylindrical body of Fig. 3, and 40° for the cylindrical body of Fig. 4.

[0122] In the battery B1, a single SUS tube (outer diameter 3 mm, thickness 0.2 mm) was used as the core member.

[0123] (Preparation of Electrode Assembly) A positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes were spirally wound in an inert gas atmosphere to prepare an electrode assembly. The electrode assembly had a configuration of (positive electrode / spacer / microporous membrane / composite material layer / negative electrode). The electrode assembly had a hollow with a diameter of 3.2 mm. A core member was inserted into this hollow. At this point, a gap existed between the inner surface of the electrode assembly and the outer surface of the core member. The size of the gap was designed so that the inner surface of the electrode assembly and the outer surface of the core member would contact each other in the discharged state after subsequent charging and discharging.

[0124] (Battery Assembly) The electrode group was inserted into a cylindrical battery can (case body) with a bottom, a non-aqueous electrolyte was poured into it, and the opening of the battery can was sealed with a sealing body. At this time, the positive electrode lead was connected to the sealing body, and the negative electrode lead was connected to the battery can. A gasket was placed between the sealing body and the battery can. In this way, a cylindrical lithium secondary battery was completed.

[0125] Battery B2 Battery B2 was obtained in the same manner as Battery B1, except that no core member was disposed in the hollow of the electrode group.

[0126] [Evaluation] The following evaluations were carried out for each of the obtained batteries.

[0127] (Crushing test) Each battery was subjected to a crushing test at 10 mA / cm under an environment of 25°C until the voltage reached 3.8 V. ２ After that, the current value was increased to 1 mA / cm ２ The battery was charged at a constant voltage of 3.8 V until the battery voltage V1 before the crush test was measured.

[0128] Thereafter, a crush test was conducted using the charged battery. Specifically, a semicircular jig was used to press the center of the side of the cylindrical battery by 1 / 3 of the diameter of the battery, deforming the battery. The battery voltage V2 after the crush test was measured. V1 - V2 was calculated to determine the voltage drop upon crushing.

[0129] (Charge-Discharge Cycle Test) A charge-discharge cycle test was performed on each battery in an environment of 25° C. Charging and discharging were performed under the following conditions: A 20-minute break was left between charging and discharging.

[0130] (Charging) 10mA / cm until the voltage reaches 4.1V ２ After that, the current value was increased to 1 mA / cm ２ The battery was charged at a constant voltage of 4.1 V until the battery reached a voltage of 0.1 V.

[0131] (Discharge) 10 mA / cm until the voltage reaches 3.0 V ２ A constant current discharge was carried out at a current of .

[0132] The charge-discharge test was performed up to 200 cycles, and the presence or absence of deformation of the electrode group (electrode buckling near the hollow of the electrode group) was examined using X-ray CT images of the cross section of the battery after 200 cycles. When the electrode near the hollow of the electrode group was bent toward the hollow of the electrode group and the bending angle was 150° or less, it was determined that buckling of the electrode had occurred.

[0133] The evaluation results are shown in Table 1. In Table 1, A1 to A3 are batteries of the example, and B1 and B2 are batteries of the comparative example.

[0134]

[0135] In Batteries A1 to A3, the battery voltage hardly changed when crushed, and buckling of the electrodes near the hollow of the electrode group during charge-discharge cycling was suppressed, improving reliability. In Battery B1, the battery voltage dropped significantly when crushed, and an internal short circuit occurred. In Battery B2, the electrodes near the hollow of the electrode group buckled during charge-discharge cycling.

[0136] The secondary battery of the present disclosure can be used in electronic devices such as mobile phones, smartphones, and tablet terminals, electric vehicles, hybrid vehicles, plug-in hybrid vehicles, and home storage batteries.

[0137] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0138] 10: Lithium secondary battery, 11: Positive electrode, 12: Negative electrode, 14: Electrode group, 15: Case body, 16: Sealing body, 17, 18: Insulating plate, 19: Positive electrode lead, 20: Negative electrode lead, 21: Step portion, 27: Gasket, 29: Core member, 50: Substrate, 53: Spacer, 53a: Linear convex portion, 100, 200, 300: Cylindrical body, 110, 210: Linear material, 310: Strip-shaped material

Claims

1. A secondary battery comprising: a wound electrode group having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; a core member disposed in a hollow space within the electrode group; and a non-aqueous electrolyte, wherein the core member is a cylindrical body containing at least one structure selected from the group consisting of a spiral structure and a braided structure, and the cylindrical body is made of a linear or strip-shaped material, and in the spiral structure, the material is wound in a spiral shape, and in the braided structure, the material is braided in a mesh shape.

2. A secondary battery according to claim 1, wherein the material is arranged so that the length direction of the material intersects with the axial direction of the cylindrical body when viewed from the side.

3. The secondary battery according to claim 1, wherein the cylindrical body is a spring body.

4. The secondary battery according to any one of claims 1 to 3, wherein the negative electrode is an electrode in which lithium ions are absorbed into the negative electrode active material during charging and from which lithium ions are released during discharging.

5. The secondary battery according to any one of claims 1 to 3, wherein the negative electrode is an electrode on which lithium metal precipitates during charging and from which the lithium metal dissolves in the non-aqueous electrolyte during discharging.

6. The secondary battery according to any one of claims 1 to 3, wherein the separator includes a spacer, and the spacer includes protrusions having a predetermined repeating pattern.

7. The secondary battery according to any one of claims 1 to 3, wherein the non-aqueous electrolyte contains an ether-based solvent.

8. The secondary battery according to any one of claims 1 to 3, wherein the non-aqueous electrolyte contains an anion of an oxalate complex.

9. The secondary battery according to claim 8, wherein the anion of the oxalate complex includes a difluorooxalate borate anion.

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