Energy storage element

Abstract

A power storage element according to one aspect of the present invention comprises a wound-type electrode body having a negative electrode and a positive electrode, and a sealable flat container for accommodating the electrode body. The electrode body has a winding core in the center, the compressive strength of the material constituting the winding core is 55 MPa or more, and the inside of the container is in a state of negative pressure.

Description

[Technical Field]

[0001] The present disclosure relates to an energy storage element. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, typified by lithium-ion non-aqueous electrolyte secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, automobiles, etc., due to their high energy density. The non-aqueous electrolyte secondary batteries generally include an electrode assembly having a pair of electrodes electrically isolated by a separator, and a non-aqueous electrolyte interposed between the electrodes, and are configured to charge and discharge by transferring ions between the electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium-ion capacitors and electric double layer capacitors are also widely used as energy storage elements.

[0003] As such a storage element, for example, a configuration has been disclosed that includes an insulating plate disposed at the bottom of an electrode group formed by winding a positive electrode plate and a negative electrode plate with a separator interposed therebetween, and a core disposed in a space formed in the center of the electrode group formed by the winding configuration (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2005-243521 Summary of the Invention [Problem to be solved by the invention]

[0005] The inventors have been studying ways to reduce the gap between electrodes by creating a negative pressure inside the container of an energy storage element from the viewpoint of improving performance. However, in the case of a wound electrode assembly, there have been cases where the gap between the electrodes is not sufficiently reduced even when the inside of the container of the energy storage element is created under a negative pressure.

[0006] An object of the present invention is to provide an energy storage element that is excellent in the effect of reducing the gap between electrodes. [Means for solving the problem]

[0007] An energy storage element according to one aspect of the present invention comprises a wound electrode body having a negative electrode and a positive electrode, and a sealable flat container for accommodating the electrode body, wherein the electrode body has a winding core in the center, the material constituting the winding core has a compressive strength of 55 MPa or more, and the inside of the container is in a negative pressure state. [Effects of the Invention]

[0008] The energy storage element according to one aspect of the present invention is excellent in the effect of reducing the gap between electrodes. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view showing an embodiment of an energy storage element. [Figure 2] FIG. 2 is a schematic cross-sectional view of an energy storage element according to one embodiment of the present invention. [Figure 3] FIG. 3 is a schematic diagram showing an embodiment of an electricity storage device configured by assembling a plurality of electricity storage elements. DETAILED DESCRIPTION OF THE INVENTION

[0010] First, an overview of the energy storage element disclosed in this specification will be described.

[0011] An energy storage element according to one aspect of the present invention comprises a wound electrode body having a negative electrode and a positive electrode, and a sealable flat container for accommodating the electrode body, wherein the electrode body has a winding core in the center, the material constituting the winding core has a compressive strength of 55 MPa or more, and the inside of the container is in a negative pressure state.

[0012] The energy storage element, having the above-described configuration, exhibits an excellent effect of reducing the gap between electrodes. While the reason for this is unclear, the following reason is presumed, for example. By creating a negative pressure inside the container of the energy storage element, a force is generated that pulls the container inward, compressing the electrode assembly and thereby reducing the gap between the electrodes. However, the inventors have discovered that if a winding core with low compressive strength is used when winding the electrodes, the winding core may be compressed by an external load, or the winding core may distort, causing the entire electrode plate to distort, thereby reducing the effect of reducing the gap between the electrodes. By creating a negative pressure inside the container and using a winding core made of a material with a compressive strength of 55 MPa or more, the energy storage element can improve the effect of reducing the gap between the electrodes while suppressing deflection and deformation of the winding core. Furthermore, since the compression on the electrode assembly caused by creating a negative pressure in the container is due to atmospheric pressure, the load change on the electrode assembly is small, and gaps are less likely to occur between the electrodes. Therefore, the energy storage element is considered to have an excellent effect of reducing the gap between the electrodes. Here, "the inside of the container is in a negative pressure state" means that the pressure in the excess space inside the container is lower than the pressure outside the container. "Excess space inside the container" means the space inside the container minus the portions occupied by structures such as the electrode assembly, electrolyte, and current collector.

[0013] The method for creating a negative pressure inside the container is not particularly limited. Examples of methods for creating a negative pressure inside the container include a method of sealing the container in a state where the pressure inside the container is reduced using a vacuum pump or the like, a method of housing a gas-adsorbing member inside the container, and a method of housing a gas soluble in the electrolyte inside the container. The methods for creating a negative pressure inside the container can be used alone or in combination.

[0014] The winding core preferably contains polyacetal, polyimide, vinyl chloride, or a combination thereof as its main component. By using polyacetal, polyimide, vinyl chloride, or a combination thereof as its main component, the effect of suppressing deformation of the winding core and the effect of reducing the gap between the electrodes can be further enhanced. Here, "main component" refers to the component with the largest mass among the materials constituting the winding core. Although not particularly limited, the main component of the winding core may account for, for example, 50% by mass or more of all components of the materials constituting the winding core, preferably 60% by mass or more, more preferably 75% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more (for example, 98% by mass or more).

[0015] The energy storage element preferably contains an electrolyte solution and a gas soluble in the electrolyte solution inside the container. The energy storage element contains an electrolyte solution and a gas soluble in the electrolyte solution inside a sealed container, and the gas dissolves in the electrolyte solution. As a result, the pressure inside the container decreases, and the inside of the container can be more reliably placed in a negative pressure state. Furthermore, since the pressure on the electrode body caused by containing an electrolyte solution and a gas soluble in the electrolyte solution inside the container is due to atmospheric pressure, the load change on the electrode body is small and gaps between the electrodes are unlikely to occur. Note that the "gas soluble in the electrolyte solution" in the present invention refers to a gas soluble in the electrolyte solution that dissolves in the electrolyte solution when 1 cm of the electrolyte solution at 25°C under 1 atmosphere is applied. 3 The solubility in 3 The above gases.

[0016] The core preferably has a hollow structure, which allows a gas soluble in the electrolyte to be sealed in a hollow region formed in the center of the electrode assembly, thereby more effectively reducing the pressure inside the container.

[0017] The configuration of an energy storage element, the configuration of an energy storage device, and a method for manufacturing an energy storage element according to one embodiment of the present invention, as well as other embodiments, will be described in detail. Note that the names of the components (elementary components) used in each embodiment may differ from the names of the components (elementary components) used in the background art.

[0018] <Configuration of energy storage element> An energy storage element according to one embodiment of the present invention comprises an electrode assembly having a positive electrode, a negative electrode, and a separator, a non-aqueous electrolyte, and a sealable container for accommodating the electrode assembly and the non-aqueous electrolyte. The interior of the container is under negative pressure. The electrode assembly is a wound type in which the positive electrode and the negative electrode are wound in a stacked state with the separator interposed therebetween. The non-aqueous electrolyte is present in a state contained in the positive electrode, the negative electrode, and the separator. Hereinafter, a non-aqueous electrolyte secondary battery (hereinafter simply referred to as a "secondary battery") will be described as an example of an energy storage element with reference to the drawings. Note that the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships.

[0019] Fig. 1 shows an energy storage element 1 as an example of a prismatic battery. Fig. 2 is a schematic cross-sectional view of the energy storage element in Fig. 1. As shown in Fig. 1, the energy storage element 1 includes a wound electrode assembly 2 having a negative electrode and a positive electrode stacked with a separator interposed therebetween, a positive electrode current collector 14 and a negative electrode current collector 15 connected to both ends of the electrode assembly 2, respectively, and a sealable container 3 for housing these.

[0020] The container 3 has a container body 3a in the shape of a flat, bottomed, rectangular cylinder and a lid body 3b in the form of an elongated rectangular plate that can close the elongated rectangular opening of the container body 3a. The electrode assembly 2 is in direct or indirect contact with the inner surface of the container body 3a. The container 3 can be a known metal container, resin container, or the like commonly used for containers of nonaqueous electrolyte secondary batteries. Examples of the metal include aluminum, stainless steel, and nickel-plated steel. As shown in FIG. 2 , it is preferable to use a container 3 made of aluminum or an aluminum alloy because aluminum or an aluminum alloy is easily distorted by negative pressure (which in turn creates a negative pressure inside the container and more effectively compresses the electrode assembly). The thickness of the container is not particularly limited, but can be approximately 0.2 mm to 2 mm (e.g., 0.3 mm to 1.5 mm, typically 0.35 mm to 1 mm).

[0021] The lid 3b is provided with a positive electrode terminal 4 and a negative electrode terminal 5 that are electrically connected to the outside. The positive electrode is electrically connected to the positive electrode terminal 4 via a positive electrode current collector 14 that is connected to the positive electrode substrate, and the negative electrode is electrically connected to the negative electrode terminal 5 via a negative electrode current collector 15 that is connected to the negative electrode substrate.

[0022] As shown in FIG. 2, the electrode assembly 2 has a winding core 8 in its center. The positive electrode, negative electrode, and separator are wound flat around the winding core 8, forming a wound electrode assembly. The winding core 8 may have either a hollow or solid structure, but a hollow core is preferred. In this embodiment, the winding core 8 has a hollow structure. In this embodiment, the winding core 8 includes a cylindrical portion and a hollow portion (cavity) formed inside the cylindrical portion. Although not particularly limited, the thickness of the cylindrical portion of the winding core 8 (the thickness of the material of the winding core forming the cylindrical portion) can be, for example, 0.01 mm to 1 mm, typically 0.05 mm to 0.5 mm. The width of the hollow portion (the length along the long side of the rectangle in the cross-sectional view of the lid body 3b in FIG. 2) can be, for example, 0.01 mm to 5 mm, preferably 0.1 mm to 3 mm. The hollow structure of the winding core 8 allows a gas soluble in the electrolyte to be sealed in a hollow region formed at the center of the electrode assembly 2, thereby more effectively reducing the pressure inside the container 3. While not particularly limited, the ratio of the volume of the hollow portion of the winding core 8 to the volume of the excess space inside the container is preferably 30% by volume or more. The volume ratio of the hollow portion may be preferably 50% by volume or more, more preferably 70% by volume or more. When the volume ratio of the hollow portion is equal to or greater than the lower limit, a suitable negative pressure state can be created inside the container. Here, the "volume of the excess space inside the container" refers to the volume obtained by subtracting the volumes occupied by the electrode assembly, electrolyte, current collector, and other structural components from the internal volume of the container. The volume of the electrode assembly refers to the actual volume of the components of the electrode assembly (active materials, separators, etc.) and does not include voids between the active materials or within the separators. In other words, the volume of the excess space inside the container means the volume of gas contained inside the container when the pressure inside the container is 1 atmosphere (0.1013 MPa) at 25°C.

[0023] The lower limit of the compressive strength of the material constituting the winding core 8 is 55 MPa, preferably 60 MPa, and more preferably 70 MPa. When the compressive strength of the material constituting the winding core 8 is equal to or greater than the lower limit, bending and deformation of the winding core 8 are suppressed when the interior of the container is in a negative pressure state, thereby improving the effect of reducing the gap between the electrodes. On the other hand, the upper limit of the compressive strength of the material constituting the winding core 8 is not particularly limited, but may be, for example, 300 MPa. From the viewpoint of ease of manufacturing the electrode assembly 2, the upper limit of the compressive strength of the material constituting the winding core 8 is preferably 150 MPa. In some embodiments, the compressive strength of the material constituting the winding core 8 may be, for example, 110 MPa or less, or 105 MPa or less. The compressive strength (MPa) of the material constituting the winding core 8 is a value measured by a measurement method in accordance with ASTM D-395.

[0024] The material constituting the winding core 8 can be appropriately selected from known materials constituting winding cores and can be used if it has an appropriate compressive strength. The material for the winding core 8 is preferably a material with high acid resistance. It is also preferable to use a material that is highly resistant to organic solvents. Furthermore, it is preferable to use a material with excellent manufacturing processability. Specific examples of materials constituting such winding cores include polyacetal (PMC, 103 MPa), polyimide (PI, 111 MPa), polyvinyl chloride (PVC, 83 MPa), polymethyl methacrylate (PMMA, 120 MPa), polyethylene terephthalate (PET, 97 MPa), polycarbonate (PC, 77 MPa), polyether ether ketone (PEEK, 119 MPa), polyphenylene sulfide (PPS, 148 MPa), polyetherimide (PEI, 118 MPa), polyamideimide (PAI, 118 MPa), and polybenzimidazole (PBI, 294 MPa). These materials may also be used in combination. Among these, from the viewpoints of acid resistance, organic solvent resistance, and manufacturing processability, it is preferable that the material constituting the winding core 8 be primarily composed of polyacetal, polyimide, vinyl chloride, or a combination thereof, and it is particularly preferable that the material be primarily composed of polyacetal, vinyl chloride, or a combination thereof.

[0025] In the energy storage device 1, the inside of the container 3 is under negative pressure. By creating a negative pressure inside the container 3, a force is generated that pulls the container 3 inward, compressing the electrode assembly 2. Therefore, the energy storage device 1 is excellent in reducing the gap between the electrodes. In this embodiment, the electrode assembly 2 is a flat, wound electrode assembly. The electrode assembly 2 has two wound R portions and two flat portions. The two flat portions correspond to the flat portions of the outer wall side surface that constitutes the electrode assembly 2, and are each arranged opposite the wide side surfaces of the inner wall side surface that constitutes the container body. In this embodiment, the two flat portions are arranged so as to contact the wide side surfaces of the container body. The two wound R portions correspond to the curvature portions (curved portions) of the outer wall side surface that constitutes the electrode assembly 2, and are arranged facing the bottom surface and lid of the container body, respectively. The two wound R portions are arranged so as not to contact the wide side surfaces of the container body. With this configuration, by creating a negative pressure inside the container 3, a force is generated that pulls the container 3 inward, causing at least one wide side surface of the container body to bend, and the flat portion (flat surface) of the flat electrode body 2 is compressed in the thickness direction (the short side direction of the rectangular plate-shaped lid body 3b, which is the stacking direction of the positive electrode, negative electrode, and separator). In this way, by compressing the flat portion (flat surface) of the electrode body 2 in the thickness direction, the gap between the electrodes can be further reduced.

[0026] The pressure (absolute pressure) inside the container 3 is not particularly limited as long as it is lower than the pressure outside the container 3 (typically atmospheric pressure = 0.1013 MPa). The pressure inside the container 3 is preferably 0.09 MPa or less, more preferably 0.085 MPa or less, and even more preferably 0.075 MPa or less, from the viewpoint of, for example, better achieving the effect of reducing the gap between the electrodes. In some embodiments, the pressure inside the container 3 may be 0.07 MPa or less or 0.065 MPa or less (e.g., 0.055 MPa). The lower limit of the pressure is not particularly limited, but may be, for example, 0.02 MPa. From the viewpoint of the container's resistance to negative pressure, the pressure inside the container 3 may be 0.03 MPa or more or 0.04 MPa or more (e.g., 0.045 MPa or more). The technology disclosed herein can be preferably implemented in an embodiment in which the pressure inside the container 3 is 0.02 MPa or more and 0.09 MPa or less (preferably 0.03 MPa or more and 0.07 MPa or less).

[0027] In the energy storage element 1, an electrolytic solution and a gas soluble in the electrolytic solution are preferably contained inside the container 3. By containing the electrolytic solution and a gas soluble in the electrolytic solution inside the sealed container 3, the gas dissolves in the electrolytic solution, which can effectively reduce the pressure inside the container 3 and more reliably create a negative pressure state inside the container.

[0028] When the electrolyte is a non-aqueous electrolyte, examples of the gas soluble in the non-aqueous electrolyte include carbon dioxide gas (1 cm of non-aqueous electrolyte at 25°C under 1 atmosphere). 3 Solubility in 5cm 3 ), nitrous oxide gas, etc. Carbon dioxide is preferred as the gas, as it is easy to handle and obtain. Carbon dioxide is easily dissolved in the non-aqueous electrolyte, and therefore, even if gas (gas containing high concentrations of carbon dioxide) present inside the container flows into and accumulates in the gap between the electrodes due to expansion and contraction of the electrodes caused by charging and discharging, the gas can be quickly dissolved in the non-aqueous electrolyte, which makes it easier to eliminate the gas accumulation.

[0029] When carbon dioxide is used as the gas soluble in the non-aqueous electrolyte, the content (concentration) of carbon dioxide in the surplus space inside the container is not particularly limited. However, from the viewpoint of maintaining a suitable negative pressure inside the container, it is preferably 2% by volume or more, more preferably 2.5% by volume or more, and even more preferably 3% by volume or more. In some embodiments, the carbon dioxide content may be 4% by volume or more, or 5% by volume or more (e.g., 6% by volume or more, typically 7% by volume or more). The upper limit of the carbon dioxide content is not particularly limited, but may be approximately 80% by volume. The carbon dioxide content may be, for example, 50% by volume or less, or 30% by volume or less (e.g., 20% by volume or less, typically 15% by volume or less). By storing carbon dioxide inside the container so that the atmosphere has such a composition after the carbon dioxide dissolves in the non-aqueous electrolyte, the gap between the electrodes can be further reduced.

[0030] When carbon dioxide is used as the gas soluble in the nonaqueous electrolyte, the carbon dioxide content (concentration) in the nonaqueous electrolyte inside the container is not particularly limited, but from the viewpoint of maintaining a suitable negative pressure inside the container, it is preferably 0.001% by volume or more, more preferably 0.003% by volume or more, and even more preferably 0.005% by volume or more.

[0031] On the other hand, when the electrolyte is a non-aqueous electrolyte, examples of the gas that is poorly soluble or insoluble in the non-aqueous electrolyte include oxygen gas, nitrogen gas, and methane gas. In a preferred embodiment, the nitrogen content (concentration) in the surplus space inside the container can be 50% by volume or less (e.g., 45% by volume or less). In some embodiments, the nitrogen content (concentration) can be 40% by volume or less, or 30% by volume or less. The lower limit of the nitrogen content is not particularly limited, but can be approximately 10% by volume. The nitrogen content can be, for example, 15% by volume or more, or 20% by volume or more. After the gas soluble in the non-aqueous electrolyte is dissolved in the non-aqueous electrolyte, the gas soluble in the non-aqueous electrolyte can be placed inside the container so that the nitrogen concentration is as described above, thereby creating a negative pressure state suitable for reducing the gap between the electrodes. In a preferred embodiment, when the total volume of gases other than nitrogen gas and oxygen gas (e.g., carbon dioxide gas, methane gas, carbon monoxide gas, hydrogen gas, etc.) in the excess space inside the container is taken as 100 volume %, the proportion of carbon dioxide can be 4 volume % or more (e.g., 4 volume % or more and 20 volume % or less). The proportion of carbon dioxide is preferably 6 volume % or more (e.g., 6 volume % or more and 18 volume % or less), and more preferably 8 volume % or more (e.g., 8 volume % or more and 16 volume % or less). In some embodiments, the proportion of carbon dioxide may be 10 volume % or more, or may be 12 volume % or more. By storing carbon dioxide inside the container so that the proportion of carbon dioxide is such, a negative pressure state suitable for reducing the gap between the electrodes can be created. The content (concentration) of each gas in the excess space inside the container can be measured by gas chromatography.

[0032] [Electrode body] The electrode assembly 2 is a wound type in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween and wound.

[0033] (positive electrode) The positive electrode has a positive electrode substrate and a positive electrode active material layer disposed on the positive electrode substrate directly or via an intermediate layer.

[0034] The positive electrode substrate is electrically conductive. Whether or not it has "electrical conductivity" is determined by whether the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 7 The threshold value is Ω·cm. The material of the positive electrode substrate is a metal such as aluminum, titanium, tantalum, or stainless steel, or an alloy of these. Among these, aluminum or an aluminum alloy is preferred from the viewpoints of potential resistance, high conductivity, and cost. Positive electrode substrates include foils, vapor-deposited films, meshes, and porous materials, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30, as specified in JIS-H-4000 (2014) or JIS-H4160 (2006).

[0035] The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, even more preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be increased while increasing the energy density per volume of the secondary battery.

[0036] The intermediate layer is a layer disposed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer contains a conductive agent such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The configuration of the intermediate layer is not particularly limited, and may contain, for example, a binder and a conductive agent.

[0037] The positive electrode active material layer contains a positive electrode active material and, if necessary, optional components such as a conductive agent, a binder, a thickener, and a filler.

[0038] The positive electrode active material can be appropriately selected from known positive electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the positive electrode active material for a lithium ion secondary battery. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO2 crystal structure, lithium transition metal composite oxides having a spinel crystal structure, polyanion compounds, chalcogen compounds, and sulfur. Examples of lithium transition metal composite oxides having an α-NaFeO2 crystal structure include Li[Li x Ni (1-x) ]O2(0≦x<0.5), Li[Li x Ni γ Co (1-x-γ) ]O2(0≦x<0.5, 0<γ<1), Li[Li x Co (1-x) ]O2(0≦x<0.5), Li[Li x Ni γ Mn (1-x-γ) ]O2(0≦x<0.5, 0<γ<1), Li[Li x Ni γ Mn β Co (1-x-γ-β) ]O2(0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), Li[Li x Ni γ Co β Al (1-x-γ-β) ]O2 (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1). Lithium transition metal composite oxides with spinel crystal structure include Li x Mn2O4, Li x Ni γ Mn (2-γ)Examples of polyanion compounds include LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, and Li2CoPO4F. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. Atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements. The surfaces of these materials may be coated with other materials. In the positive electrode active material layer, one of these materials may be used alone, or two or more may be used in combination.

[0039] The positive electrode active material is usually in the form of particles (powder). The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. By setting the average particle size of the positive electrode active material to the above lower limit or more, the positive electrode active material becomes easier to manufacture and handle. By setting the average particle size of the positive electrode active material to the above upper limit or less, the electronic conductivity of the positive electrode active material layer is improved. Note that when a composite of the positive electrode active material and another material is used, the average particle size of the composite is taken as the average particle size of the positive electrode active material. The "average particle size" refers to the value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 50% based on the particle size distribution measured by laser diffraction / scattering in a diluted solution obtained by diluting particles with a solvent in accordance with JIS-Z-8825 (2013).

[0040] To obtain powders with a predetermined particle size, grinders, classifiers, etc. are used. Grinding methods include, for example, methods using a mortar, ball mill, sand mill, vibration ball mill, planetary ball mill, jet mill, counter jet mill, swirling airflow jet mill, or sieves. Wet grinding in the presence of water or an organic solvent such as hexane can also be used during grinding. As classification methods, sieves, air classifiers, etc. are used as needed for both dry and wet methods.

[0041] The content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass to 99% by mass, more preferably 70% by mass to 98% by mass, and even more preferably 80% by mass to 95% by mass. By setting the content of the positive electrode active material within this range, both high energy density and manufacturability of the positive electrode active material layer can be achieved.

[0042] The conductive agent is not particularly limited as long as it is a material having electrical conductivity. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphite, non-graphitic carbon, and graphene-based carbon. Examples of non-graphitic carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNTs), and fullerenes. The conductive agent may be in the form of powder or fiber. As the conductive agent, one of these materials may be used alone, or two or more may be mixed. These materials may also be used in combination. For example, a composite of carbon black and CNTs may be used. Among these, carbon black is preferred from the viewpoints of electronic conductivity and coatability, and acetylene black is particularly preferred.

[0043] The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the content of the conductive agent within this range, the energy density of the secondary battery can be increased.

[0044] Examples of binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

[0045] The binder content in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the binder content within this range, the active material can be stably held.

[0046] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC), methyl cellulose, etc. When the thickener has a functional group that reacts with lithium or the like, this functional group may be deactivated in advance by methylation or the like.

[0047] The filler is not particularly limited, and examples of the filler include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, carbonates such as calcium carbonate, sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, substances derived from mineral resources such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, and artificial products thereof.

[0048] The positive electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, and W as components other than the positive electrode active material, conductive agent, binder, thickener, and filler.

[0049] (Negative electrode) The negative electrode has a negative electrode substrate and a negative electrode active material layer disposed on the negative electrode substrate directly or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from the configurations exemplified for the positive electrode above, for example.

[0050] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, alloys thereof, and carbonaceous materials are used as the material for the negative electrode substrate. Among these, copper or copper alloys are preferred. Examples of the negative electrode substrate include foils, vapor-deposited films, meshes, and porous materials, with foils being preferred from the viewpoint of cost. Therefore, copper foil or copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0051] The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, even more preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By setting the average thickness of the negative electrode substrate within the above range, the strength of the negative electrode substrate can be increased while increasing the energy density per volume of the secondary battery.

[0052] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer contains optional components such as a conductive agent, a binder, a thickener, and a filler, as needed. The optional components such as the conductive agent, the binder, the thickener, and the filler can be selected from the materials exemplified for the positive electrode above.

[0053] The negative electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

[0054] The negative electrode active material can be appropriately selected from known negative electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the negative electrode active material for lithium ion secondary batteries. Examples of the negative electrode active material include metallic Li; metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; and Li4Ti5O 12 , LiTiO 2、Examples of the material include titanium-containing oxides such as TiNbO; polyphosphate compounds; silicon carbide; and carbon materials such as graphite and non-graphitic carbon (easily graphitizable carbon or non-graphitizable carbon). Among these materials, graphite and non-graphitic carbon are preferred. In the negative electrode active material layer, one of these materials may be used alone, or two or more may be used in combination.

[0055] "Graphite" refers to graphite that has an average lattice spacing (d 002 ) is 0.33 nm or more and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the viewpoint of being able to obtain a material with stable physical properties.

[0056] "Non-graphitic carbon" refers to the carbon that has an average lattice spacing (d 002 ) is 0.34 nm or more and 0.42 nm or less. Non-graphitic carbon includes non-graphitizable carbon and graphitizable carbon. Examples of non-graphitic carbon include resin-derived materials, petroleum pitch or petroleum pitch-derived materials, petroleum coke or petroleum coke-derived materials, plant-derived materials, and alcohol-derived materials.

[0057] Here, the "discharged state" refers to a state in which the negative electrode active material, a carbonaceous material, is discharged so that lithium ions capable of being absorbed and desorbed during charging and discharging are sufficiently released. For example, in a single-electrode battery using a negative electrode containing a carbonaceous material as a negative electrode active material as a working electrode and metallic Li as a counter electrode, this refers to a state in which the open circuit voltage is 0.7 V or higher.

[0058] "Non-graphitizable carbon" means the above d 002 This refers to carbon materials with a particle size of 0.36 nm or more and 0.42 nm or less.

[0059] "Graphitizable carbon" means the above d 002 This refers to carbon materials with a particle size of 0.34 nm or more and less than 0.36 nm.

[0060] The negative electrode active material is usually in the form of particles (powder). The average particle size of the negative electrode active material can be, for example, 1 nm or more and 100 μm or less. When the negative electrode active material is a carbon material, a titanium-containing oxide, or a polyphosphate compound, the average particle size may be 1 μm or more and 100 μm or less. When the negative electrode active material is Si, Sn, Si oxide, Sn oxide, or the like, the average particle size may be 1 nm or more and 1 μm or less. By setting the average particle size of the negative electrode active material to be equal to or greater than the above lower limit, the negative electrode active material can be easily produced or handled. By setting the average particle size of the negative electrode active material to be equal to or less than the above upper limit, the electronic conductivity of the active material layer is improved. To obtain powder with a predetermined particle size, a pulverizer, a classifier, or the like is used. The pulverization method and the powder classification method can be selected from, for example, the methods exemplified for the positive electrode. When the negative electrode active material is a metal such as metallic Li, the negative electrode active material may be in the form of a foil.

[0061] The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass to 99% by mass, more preferably 90% by mass to 98% by mass. By setting the content of the negative electrode active material within this range, both high energy density and manufacturability of the negative electrode active material layer can be achieved.

[0062] (separator) The separator can be appropriately selected from known separators. Examples of separators that can be used include separators consisting of only a substrate layer and separators in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the substrate layer. Examples of the form of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these forms, porous resin films are preferred from the viewpoint of strength, and nonwoven fabrics are preferred from the viewpoint of non-aqueous electrolyte retention. Materials for the substrate layer of the separator are preferably polyolefins such as polyethylene and polypropylene from the viewpoint of shutdown function, and polyimide and aramid from the viewpoint of oxidative decomposition resistance. A composite material of these resins may also be used for the substrate layer of the separator.

[0063] The heat-resistant particles contained in the heat-resistant layer preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C in an air atmosphere at 1 atmosphere pressure, and more preferably exhibit a mass loss of 5% or less when heated from room temperature to 800°C. Examples of materials exhibiting a mass loss of a predetermined value or less include inorganic compounds. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalently bonded crystals such as silicon and diamond; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, as well as artificial products thereof. As the inorganic compound, these substances may be used alone or in the form of a complex, or two or more of them may be used in combination. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferred from the viewpoint of the safety of the energy storage device.

[0064] The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and is preferably 20% by volume or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0065] The separator may be a polymer gel composed of a polymer and a non-aqueous electrolyte. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. The use of a polymer gel has the effect of suppressing leakage. The separator may be a combination of the porous resin film or nonwoven fabric described above and a polymer gel.

[0066] (non-aqueous electrolyte) When the energy storage element is a nonaqueous electrolyte secondary battery, a nonaqueous electrolyte solution is used as the nonaqueous electrolyte. The nonaqueous electrolyte solution can be appropriately selected from known nonaqueous electrolyte solutions. The nonaqueous electrolyte solution includes a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent.

[0067] The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, carboxylic acid esters, phosphate esters, sulfonic acid esters, ethers, amides, and nitriles. Non-aqueous solvents in which some of the hydrogen atoms contained in these compounds have been substituted with halogens may also be used.

[0068] Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate, etc. Among these, EC and PC are preferred.

[0069] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, bis(trifluoroethyl) carbonate, etc. Among these, DMC and EMC are preferred.

[0070] It is preferable to use a cyclic carbonate or a chain carbonate as the non-aqueous solvent, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. The use of a cyclic carbonate can promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate can reduce the viscosity of the non-aqueous electrolyte. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is preferably in the range of, for example, 5:95 to 50:50.

[0071] The electrolyte salt can be appropriately selected from known electrolyte salts, such as lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Of these, lithium salts are preferred.

[0072] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2, lithium oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP), and lithium salts having a halogenated hydrocarbon group such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0073] The content of electrolyte salt in the non-aqueous electrolyte is 0.1 mol / dm at 20°C and 1 atmosphere. 3 More than 2.5mol / dm 3 It is preferable that the value is 0.3 mol / dm or less. 3 More than 2.0mol / dm 3 It is more preferable that it is 0.5 mol / dm or less. 3 More than 1.7mol / dm 3 It is more preferable that it is 0.7 mol / dm or less. 3More than 1.5mol / dm 3 By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0074] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; vinylene carbonate, methylvinylene carbonate, ethylvinylene carbonate, and the like. Carbonate, succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2-dioxo-1 ,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propene sultone, 1,3-propane sultone, 1,4-butane sultone, 1,4-butene sultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, lithium monofluorophosphate, lithium difluorophosphate, etc. These additives may be used alone or in combination of two or more.

[0075] The content of the additive contained in the non-aqueous electrolyte is preferably 0.01% by mass to 10% by mass, more preferably 0.1% by mass to 7% by mass, even more preferably 0.2% by mass to 5% by mass, and particularly preferably 0.3% by mass to 3% by mass. By setting the content of the additive within the above range, it is possible to improve the capacity retention performance or cycle performance after high-temperature storage, and further improve safety.

[0076] The non-aqueous electrolyte may be a solid electrolyte, or a non-aqueous electrolytic solution and a solid electrolyte may be used in combination.

[0077] The solid electrolyte can be selected from any material that has ionic conductivity of lithium, sodium, calcium, etc. and is solid at room temperature (e.g., 15° C. to 25° C.) Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, polymer solid electrolytes, and gel polymer electrolytes.

[0078] Examples of sulfide solid electrolytes for lithium ion secondary batteries include Li2S-P2S5, LiI-Li2S-P2S5, and Li 10 Ge-P2S 12 etc.

[0079] <Configuration of the power storage device> The shape of the energy storage element of this embodiment is not particularly limited, but is preferably, for example, a flat prismatic battery. When the shape of the energy storage element is a flat prismatic battery, if the interior of the container is in a negative pressure state, a force pulling the container inward is generated, making at least one side surface of the container body more likely to become depressed. The side surface of the electrode body facing the depressed side surface of the container body due to the negative pressure is pressurized in the thickness direction, thereby improving the effect of reducing the gap between the electrodes.

[0080] <Method of manufacturing an energy storage element> The method for manufacturing an energy storage device according to this embodiment includes, for example, housing an electrode assembly in which a negative electrode and a positive electrode are stacked in a container (hereinafter also referred to as an electrode assembly housing step), housing an electrolyte in the container (hereinafter also referred to as an electrolyte housing step), creating a negative pressure inside the container (hereinafter also referred to as a negative pressure forming step), and sealing the container (hereinafter also referred to as a sealing step). In this embodiment, the electrolyte housing step includes housing an electrolyte solution in the container (hereinafter also referred to as an electrolyte solution housing step), and the negative pressure forming step includes housing a gas soluble in the electrolyte solution in the container after the electrolyte solution housing step and before the sealing step (hereinafter also referred to as a gas housing step). Furthermore, the method for manufacturing an energy storage device may include other steps, such as forming a positive electrode (hereinafter also referred to as a positive electrode forming step), forming a negative electrode (hereinafter also referred to as a negative electrode forming step), and forming an electrode assembly (hereinafter also referred to as an electrode assembly forming step).

[0081] (Positive electrode formation process) In the positive electrode formation step, a positive electrode having a positive electrode substrate and a positive electrode active material layer is formed. In the positive electrode formation step, a positive electrode mixture containing a positive electrode active material is applied to the positive electrode substrate, so that the positive electrode mixture can be disposed along at least one surface of the positive electrode substrate. Specifically, for example, the positive electrode mixture is applied to the positive electrode substrate and dried to dispose the positive electrode active material layer.

[0082] The positive electrode mixture may be a positive electrode mixture paste containing a dispersion medium in addition to the optional components. Examples of the dispersion medium include water, aqueous solvents such as water-based mixed solvents, and organic solvents such as N-methylpyrrolidone and toluene. The positive electrode active material layer may be laminated directly on the positive electrode substrate or via an intermediate layer.

[0083] (Negative electrode formation process) In the negative electrode formation step, a negative electrode having a negative electrode substrate and a negative electrode active material layer is formed. In the negative electrode formation step, a negative electrode mixture containing a negative electrode active material can be applied to the negative electrode substrate, so that the negative electrode mixture is disposed along at least one surface of the negative electrode substrate. Specifically, for example, the negative electrode mixture is applied to the negative electrode substrate and dried to form the negative electrode active material layer. Furthermore, the negative electrode mixture may be a negative electrode mixture paste that further contains a dispersion medium in addition to the optional components described above. The dispersion medium can be selected from any of the dispersion mediums exemplified in the positive electrode formation step. The negative electrode active material layer may be laminated directly on the negative electrode substrate or via an intermediate layer.

[0084] (Electrode body formation process) In the electrode body forming step, an electrode body is formed using the positive electrode and the negative electrode. The electrode body is preferably a wound electrode body having a flat shape with a pair of opposing wound R portions and a flat portion located between the pair of wound R portions. In the electrode body forming step of the energy storage element, the positive electrode and the negative electrode are stacked and wound with the separator interposed therebetween to form an electrode body in which the positive electrode and the negative electrode are alternately stacked.

[0085] (Electrode body accommodation process) In the electrode assembly housing step, the electrode assembly in which the negative electrode and the positive electrode are stacked and wound is housed in a container.

[0086] (Electrolyte containing process) In the electrolyte containing step, the electrolyte is contained in the container. The containing of the electrolyte can be performed by a known method. When the energy storage element is a non-aqueous electrolyte secondary battery, the electrolyte containing step involves, for example, containing the non-aqueous electrolyte in the container by injecting the non-aqueous electrolyte through an inlet provided in the container.

[0087] (Gas storage process) In the gas containing step, after the electrolyte solution containing step, a gas soluble in the electrolyte solution is contained in the container. Specifically, after the electrolyte solution is contained in the container, the gas soluble in the electrolyte solution is injected into the container through the inlet, thereby containing the gas soluble in the electrolyte solution in the container. The injection of the gas soluble in the electrolyte solution may be carried out at atmospheric pressure, or may be carried out with the pressure inside the container reduced using a vacuum pump or the like. Here, "reduced pressure" refers to the pressure in the excess space inside the container being less than atmospheric pressure. Note that if the gas containing step is carried out under pressure, the gas soluble in the electrolyte solution will dissolve too much in the electrolyte solution before sealing, making it difficult for the gas soluble in the electrolyte solution to further dissolve in the electrolyte solution after the sealing step, and the inside of the container may not be sufficiently under negative pressure. In addition, the inlet may be provided separately from the inlet for injecting the electrolyte solution.

[0088] As described above, the energy storage element has a negative pressure state inside the container. One suitable method for achieving this negative pressure state is to first fill the container with the electrolyte solution and then fill the container with a gas soluble in the electrolyte solution. In this way, by injecting a gas soluble in the electrolyte solution into a container containing the electrolyte solution, the gas dissolves in the electrolyte solution after the sealing process, effectively reducing the pressure inside the container and creating a suitable negative pressure state inside the container. On the other hand, in an embodiment in which a gas soluble in the electrolyte solution is filled into the container and then the electrolyte solution is filled into the container, much of the gas soluble in the electrolyte solution dissolves in the electrolyte solution when the electrolyte solution is filled, and the gas dissolves too much in the electrolyte solution (e.g., reaches saturation) before the sealing process, which is not preferred. In other words, if too much gas soluble in the electrolyte solution dissolves in the electrolyte solution before the sealing process, it becomes difficult for the gas soluble in the electrolyte solution to further dissolve in the electrolyte solution after the sealing process, which may prevent the pressure inside the container from being effectively reduced. Preferably, after the electrolyte solution is placed in the container, pre-charging is performed, the pressure inside the container is reduced using a vacuum pump or the like, and then a gas soluble in the electrolyte solution is injected so that the pressure inside the container is close to atmospheric pressure. The pressure inside the container immediately after the injection of the gas soluble in the electrolyte solution is an important factor from the viewpoint of maintaining a suitable negative pressure inside the container after the sealing step. The pressure inside the container immediately after the injection of the gas soluble in the electrolyte solution is preferably 0.1 MPa or more and 0.2 MPa or less, more preferably 0.1 MPa or more and 0.15 MPa or less, even more preferably 0.1 MPa or more and 0.12 MPa or less, and particularly preferably 0.1 MPa or more and 0.11 MPa or less. In this way, by injecting the gas soluble in the electrolyte solution so that the pressure inside the container immediately after the injection of the gas soluble in the electrolyte solution is close to atmospheric pressure, the inconvenience of excessive dissolution of the gas soluble in the electrolyte solution in the electrolyte solution before the sealing step can be eliminated or alleviated, and the pressure inside the container can be effectively reduced after the sealing step.

[0089] From the viewpoint of further reducing the pressure inside the container, the amount of the gas soluble in the electrolytic solution is preferably 40% by volume or more, more preferably 70% by volume or more, and may be, for example, 95% by volume or more, relative to the volume of the excess space inside the container. The amount of the gas soluble in the electrolytic solution may be 100% by volume, relative to the volume of the excess space inside the container. The technology disclosed herein can be preferably implemented in an embodiment in which the amount of the gas soluble in the electrolytic solution is preferably 70% by volume or more and 100% by volume or less, more preferably 80% by volume or more and 95% by volume or less, relative to the volume of the excess space inside the container.

[0090] The content of the gas soluble in the electrolytic solution contained in the container is preferably 80% by volume or more, more preferably 98% by volume or more, and even more preferably 100% by volume, based on the total volume of gas contained in the container, from the viewpoint of reducing the pressure inside the container. From the viewpoint of ease of handling the gas, the content of the gas soluble in the electrolytic solution may be 80% by volume or less, based on the total volume of gas contained in the container.

[0091] (Sealing process) In the sealing step, the container is sealed while the gas soluble in the electrolyte is contained in the container. Specifically, after the gas is contained in the container, the injection port is sealed to obtain an energy storage device. The injection port is sealed, for example, by closing the injection port with a sealing member and fixing the sealing member by laser welding or the like.

[0092] The sealing step must be performed promptly after the gas soluble in the electrolyte is sealed in. If the container is left for a long time after the gas soluble in the electrolyte is sealed in, the gas will dissolve too much in the electrolyte before the sealing step, making it difficult for the gas soluble in the electrolyte to further dissolve in the electrolyte after the sealing step, and the inside of the container may not be sufficiently negatively pressurized. The time elapsed from the time the gas soluble in the electrolyte is contained until the injection port is sealed is preferably 1 hour or less, from the viewpoint of reducing the amount of the gas soluble in the electrolyte that dissolves in the electrolyte before the sealing step or is released to the outside of the container through the injection port by diffusion. The elapsed time is preferably 30 minutes or less (e.g., 1 minute to 30 minutes), more preferably 20 minutes or less, even more preferably 15 minutes or less, and particularly preferably 10 minutes or less (e.g., 5 minutes or less). By shortening the time that elapses from the time the gas soluble in the electrolyte is placed in the container until the injection port is sealed, it is possible to eliminate or mitigate the inconvenience of the gas soluble in the electrolyte being dissolved too much (typically to saturation) before the sealing step, and it is possible to effectively reduce the pressure inside the container after the sealing step.

[0093] After the gas soluble in the electrolyte solution is placed in the container, the method may include a step of temporarily sealing the injection port before the step of sealing the injection port. The step of temporarily sealing the injection port is, for example, a step of temporarily blocking the injection port using a rubber plug member or the like. By including the step of temporarily sealing the injection port, it is possible to prevent the gas soluble in the electrolyte solution placed in the container from being released to the outside of the container through the injection port due to diffusion. In this case, the step of sealing the injection port may include removing the plug member or the like and then blocking the injection port with a sealing member, and fixing the sealing member by laser welding or the like. Furthermore, the step of sealing the injection port may include disposing a sealing member that covers the plug member or the like that blocks the injection port, and fixing the sealing member by laser welding or the like.

[0094] After the sealing step, the pressure inside the container when the dissolution of the gas soluble in the electrolyte solution into the electrolyte solution reaches equilibrium is preferably 0.02 MPa or more and 0.09 MPa or less at 25°C, from the viewpoint of effectively suppressing an increase in the distance between the electrodes. The pressure inside the container immediately after sealing is preferably 0.1 MPa or more and 0.2 MPa or less. In other words, immediately after the sealing step, most of the gas soluble in the electrolyte solution is not dissolved in the electrolyte solution, and by dissolving the gas soluble in the electrolyte solution in the electrolyte solution after the sealing step, the pressure inside the container can be reduced.

[0095] The details of the electrode body, the electrolyte, the gas soluble in the electrolyte, the container, etc. in the method for producing the energy storage element are as described above.

[0096] <Other embodiments> The energy storage device of the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, and part of the configuration of one embodiment can be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment can be deleted. Also, well-known technology can be added to the configuration of one embodiment.

[0097] For example, in the above-described embodiment, a method of placing a gas soluble in the electrolyte solution inside the container has been described as an example of creating a negative pressure inside the container. However, this is not limiting. For example, a method of placing a negative pressure inside the container using a vacuum pump or the like to reduce the pressure inside the container and then sealing the container may be used. In this case, the negative pressure creating step may include a depressurization step of reducing the pressure inside the container using a vacuum pump or the like instead of the gas containing step. This depressurization step can be performed after the electrolyte containing step and before the sealing step. However, as in the above-described embodiment, the method of placing a gas soluble in the electrolyte solution inside the container is preferable because it can more reliably create a negative pressure inside the container. That is, because the concentration of the gas soluble in the electrolyte solution increases in the excess space inside the container, even if gaps occur between the electrodes due to expansion and contraction of the electrodes during charging and discharging, and gas present inside the container flows into and accumulates in the gaps between the electrodes, it can quickly dissolve in the electrolyte solution, which is preferable because it eliminates gas accumulation and makes it easier to reduce the gaps between the electrodes.

[0098] In the above embodiment, the case where the energy storage element is used as a chargeable and dischargeable non-aqueous electrolyte secondary battery (for example, a lithium ion secondary battery) has been described, but the energy storage element may be of any type, shape, size, capacity, etc. The present invention can also be applied to various secondary batteries, electric double layer capacitors, lithium ion capacitors, and other capacitors.

[0099] In the above embodiment, the electrode assembly in which the positive electrode and the negative electrode are stacked with a separator interposed therebetween has been described, but the electrode assembly may not include a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other in a state in which a non-conductive layer is formed on the active material layer of the positive electrode or the negative electrode.

[0100] The energy storage element of this embodiment can be mounted as an energy storage unit (battery module) configured by assembling a plurality of energy storage elements 1 in an automobile power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), or a plug-in hybrid electric vehicle (PHEV), a power source for electronic devices such as a personal computer or a communication terminal, or a power storage power source, etc. In this case, the technology of the present invention may be applied to at least one energy storage element included in the energy storage unit. 3 shows an example of a power storage device 30 in which power storage units 20, each of which is an assembly of two or more electrically connected power storage elements 1, are further assembled. The power storage device 30 may include a bus bar (not shown) that electrically connects two or more power storage elements 1, a bus bar (not shown) that electrically connects two or more power storage units 20, etc. The power storage unit 20 or the power storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more power storage elements 1. [Explanation of symbols]

[0101] 1. Energy storage element 2 Electrode body 3 containers 3a Container body 3b Lid body 4 Positive terminal 5 Negative terminal 8 core 14 Positive electrode current collector 15 Negative electrode current collector 20 Energy storage unit 30 Electricity storage device

Claims

1. a wound electrode body having a negative electrode and a positive electrode; a sealable flat container for accommodating the electrode assembly; Equipped with The electrode body has a winding core in the center, The compressive strength of the material constituting the core is 55 MPa or more, The winding core has a cylindrical portion formed in a cylindrical shape and a hollow portion formed inside the cylindrical portion, The volume of the hollow portion accounts for 30% or more of the volume of the excess space inside the container, The inside of the container is in a negative pressure state, and the electrode body is pressed by the container.

2. 2. The energy storage element according to claim 1, wherein the core is made primarily of polyacetal, polyimide, vinyl chloride, or a combination thereof.

3. 3. The electric storage element according to claim 1, wherein the container contains an electrolytic solution and a gas soluble in the electrolytic solution.

4. 4. The energy storage element according to claim 1, wherein the core has a hollow structure.

5. The electric storage element according to claim 3 , wherein the gas soluble in the electrolyte solution contains carbon dioxide.

6. The energy storage element according to claim 5 , wherein the carbon dioxide content in the surplus space inside the container is 5% by volume or more.

7. The energy storage element according to claim 1 , wherein the pressure inside the container is 0.07 MPa or less.

8. the electrode body is a flat wound electrode body obtained by winding the positive electrode and the negative electrode stacked with a separator interposed therebetween, The container has a container body in the shape of a flattened rectangular cylinder with a bottom, and a lid body that closes an opening of the container body, The energy storage device according to claim 1 , wherein at least one side surface of the container body is recessed toward the inside of the container.

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