JP7800534B2 - Energy storage element - Google Patents

Description

This disclosure relates to an energy storage element.

Due to their high energy density, non-aqueous electrolyte secondary batteries, such as lithium-ion non-aqueous electrolyte secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, as well as automobiles. Non-aqueous electrolyte secondary batteries generally comprise an electrode assembly having a pair of electrodes electrically isolated by a separator, and a non-aqueous electrolyte interposed between the electrodes. They 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.

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

Japanese Patent Application Publication No. 2005-243521

With a view to improving performance, the inventors have been studying the possibility of reducing the gap between the electrodes by creating a negative pressure inside the container of the energy storage element. 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 under negative pressure.

The object of the present invention is to provide an energy storage element that is excellent in reducing the gap between electrodes.

An energy storage element according to one aspect of the present invention comprises a wound electrode assembly having a negative electrode and a positive electrode, and a sealable, flat container for housing the electrode assembly, the electrode assembly having a winding core in the center, the material constituting the winding core having a compressive strength of 55 MPa or more, and the interior of the container being under negative pressure.

The energy storage element according to one aspect of the present invention is excellent in reducing the gap between electrodes.

FIG. 1 is a perspective view showing an embodiment of an energy storage element. FIG. 2 is a schematic cross-sectional view of an energy storage element according to one embodiment of the present invention. FIG. 3 is a schematic diagram showing an embodiment of an electricity storage device configured by assembling a plurality of electricity storage elements.

First, we will provide an overview of the energy storage element disclosed in this specification.

An energy storage element according to one aspect of the present invention comprises a wound electrode assembly having a negative electrode and a positive electrode, and a sealable, flat container for housing the electrode assembly, the electrode assembly having a winding core in the center, the material constituting the winding core having a compressive strength of 55 MPa or more, and the interior of the container being under negative pressure.

The energy storage element, with the above-described configuration, exhibits excellent inter-electrode gap reduction effects. While the reason for this is unclear, the following is speculated: By creating a negative pressure inside the energy storage element's container, a force is generated that pulls the container inward, compressing the electrode assembly and thereby reducing the inter-electrode gap. 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, and the winding core may distort, causing the entire electrode plate to distort, thereby reducing the inter-electrode gap reduction effect. 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 suppress deflection and deformation of the winding core while improving the inter-electrode gap reduction effect. Furthermore, because 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, making it less likely for gaps to form between the electrodes. Therefore, the energy storage element is considered to have excellent inter-electrode gap reduction effects. 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.

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 using a vacuum pump or the like to reduce the pressure inside the container and then sealing it, placing a gas-adsorbing material inside the container, or placing 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.

It is preferable that the winding core is primarily composed of polyacetal, polyimide, vinyl chloride, or a combination thereof. By using polyacetal, polyimide, vinyl chloride, or a combination thereof as the primary component of the winding core, the effect of suppressing deformation of the winding core and the effect of reducing the gap between electrodes can be further enhanced. Here, "primary component" refers to the component with the largest mass among the materials constituting the winding core. While not particularly limited, the primary component of the winding core may, for example, account for 50% or more by mass of all components of the materials constituting the winding core, preferably 60% or more by mass, more preferably 75% or more by mass, even more preferably 90% or more by mass, and particularly preferably 95% or more by mass (e.g., 98% or more by mass).

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, making it possible to more reliably create a negative pressure state inside the container. Furthermore, since the pressure on the electrode body caused by containing the electrolyte solution and the 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 less likely to occur. In the present invention, the term "gas soluble in the electrolyte solution" refers to a gas whose solubility in 1 ^cm3 of electrolyte solution at 25°C and 1 atmosphere is 1 ^cm3 or more.

It is preferable that the winding core has a hollow structure. By having the winding core have a hollow structure, a hollow region is formed in the center of the electrode body and a gas soluble in the electrolyte is sealed in, thereby more effectively reducing the pressure inside the container.

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, are described in detail below. Note that the names of the components used in each embodiment may differ from the names of the components used in the background art.

<Configuration of Energy Storage Element>
An energy storage element according to one embodiment of the present invention includes 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.

Figure 1 shows a storage element 1 as an example of a prismatic battery. Figure 2 is a schematic cross-sectional view of the storage element in Figure 1. As shown in Figure 1, the storage element 1 comprises a wound electrode body 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 body 2, respectively, and a sealable container 3 for containing these.

The container 3 has a flat, bottomed, rectangular tubular container body 3a and a slender rectangular plate-like lid 3b that can close the slender 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 made of a known metal container, resin container, or the like commonly used for containers of nonaqueous electrolyte secondary batteries. Examples of such metals include aluminum, stainless steel, and nickel-plated steel. As shown in Figure 2, a container 3 made of aluminum or an aluminum alloy is preferred because it is prone to distortion due to negative pressure (and thus creates a negative pressure inside the container, which more effectively compresses the electrode assembly). The container thickness 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).

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.

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 has a cylindrically shaped tubular portion and a hollow portion (cavity) formed inside the tubular portion. Although not particularly limited, the thickness of the tubular portion of the winding core 8 (the thickness of the material of the winding core forming the tubular 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. Although 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 is 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 material, separator, etc.) and does not include voids present between the active materials or within the separator. 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.

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, deflection 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 body 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 using a measurement method in accordance with ASTM D-395.

The material constituting the winding core 8 can be appropriately selected from known winding core materials and can be used based on its appropriate compressive strength. The material for the winding core 8 is preferably highly acid-resistant. It is also preferable to use a material that is highly resistant to organic solvents. Furthermore, it is preferable to use a material that is easy to manufacture and process. 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.

In the energy storage device 1, the interior 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 at 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 surfaces that make up the electrode assembly 2, and are each positioned opposite the wide side surfaces of the inner wall side surfaces that make up the container body. In this embodiment, the two flat portions are positioned 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 surfaces that make up the electrode assembly 2, and are positioned facing the bottom surface and lid of the container body, respectively. The two wound R portions are positioned so as not to contact the wide side surfaces of the container body. According to 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.

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 may be 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. The pressure inside the container 3 may be 0.03 MPa or more, or may be 0.04 MPa or more (e.g., 0.045 MPa or more), from the viewpoint of the container's resistance to negative pressure, etc. 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).

The energy storage element 1 preferably contains an electrolyte solution and a gas soluble in the electrolyte solution inside the container 3. By containing an electrolyte solution and a gas soluble in the electrolyte solution inside the sealed container 3, the gas dissolves in the electrolyte solution, effectively reducing the pressure inside the container 3 and more reliably creating a negative pressure state inside the container.

When the electrolyte is a non-aqueous electrolyte, examples of gases soluble in the non-aqueous electrolyte include carbon dioxide gas (solubility of 5 ^cm3 per 1 ^cm3 of non-aqueous electrolyte at 1 atmosphere and 25°C) and nitrous oxide gas. Carbon dioxide is preferred as the gas, as it is easy to handle and obtain. Carbon dioxide is easily soluble in the non-aqueous electrolyte, and therefore, even if gas (gas containing a high concentration 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 during charging and discharging, the gas can be quickly dissolved in the non-aqueous electrolyte, making it easier to eliminate the gas accumulation.

When carbon dioxide is used as the gas soluble in the nonaqueous electrolyte, the carbon dioxide content (concentration) in the excess 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 vol.% or more, more preferably 2.5 vol.% or more, and even more preferably 3 vol.% or more. In some embodiments, the carbon dioxide content may be 4 vol.% or more, or even 5 vol.% or more (e.g., 6 vol.% or more, typically 7 vol.% or more). The upper limit of the carbon dioxide content is not particularly limited, but may be approximately 80 vol.%. The carbon dioxide content may be, for example, 50 vol.% or less, or 30 vol.% or less (e.g., 20 vol.% or less, typically 15 vol.% or less). By storing carbon dioxide inside the container so that the above-mentioned atmosphere is achieved after the carbon dioxide dissolves in the nonaqueous electrolyte, the gap between the electrodes can be further reduced.

When carbon dioxide is used as the gas soluble in the non-aqueous electrolyte, the carbon dioxide content (concentration) in the non-aqueous electrolyte inside the container is not particularly limited, but from the viewpoint of maintaining a suitable negative pressure state 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.

On the other hand, when the electrolyte is a nonaqueous electrolyte, examples of gases that are poorly soluble or insoluble in the nonaqueous electrolyte include oxygen gas, nitrogen gas, and methane gas. In a preferred embodiment, the nitrogen content (concentration) in the excess space inside the container may be 50% by volume or less (e.g., 45% by volume or less). In some embodiments, the nitrogen content (concentration) may be 40% by volume or less, or 30% by volume or less. The lower limit of the nitrogen content is not particularly limited, but may be approximately 10% by volume. The nitrogen content may be, for example, 15% by volume or more, or 20% by volume or more. After the gas soluble in the nonaqueous electrolyte dissolves in the nonaqueous electrolyte, the container may contain the gas soluble in the nonaqueous electrolyte so that the nitrogen concentration is such that a negative pressure state suitable for reducing the gap between the electrodes can be achieved. 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% by volume, the proportion of carbon dioxide may be 4% by volume or more (e.g., 4% by volume or more and 20% by volume or less). The proportion of carbon dioxide is preferably 6% by volume or more (e.g., 6% by volume or more and 18% by volume or less), and more preferably 8% by volume or more (e.g., 8% by volume or more and 16% by volume or less). In some embodiments, the proportion of carbon dioxide may be 10% by volume or more, or may be 12% by 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 achieved. The content (concentration) of each gas in the excess space inside the container can be measured by gas chromatography.

[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.

(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.

The positive electrode substrate is conductive. Whether or not a material is "conductive" is determined by a volume resistivity of 10 ⁷ Ω·cm measured in accordance with JIS-H-0505 (1975). Metals such as aluminum, titanium, tantalum, and stainless steel, or alloys thereof, are used as the material for the positive electrode substrate. Among these, aluminum or aluminum alloys are preferred from the viewpoints of potential resistance, high conductivity, and cost. Examples of 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).

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 keeping the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be increased while also increasing the energy density per volume of the secondary battery.

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 composition of the intermediate layer is not particularly limited, and may include, for example, a binder and a conductive agent.

The positive electrode active material layer contains a positive electrode active material. If necessary, the positive electrode active material layer may contain optional components such as a conductive agent, a binder, a thickener, and a filler.

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 typically used as the positive electrode active material for lithium ion secondary batteries. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO ₂ -type crystal structure, lithium transition metal composite oxides having a spinel-type crystal structure, polyanion compounds, chalcogen compounds, and sulfur. Examples of lithium transition metal composite oxides having α-NaFeO ₂ type crystal structure include Li[Li _x Ni _(1-x) ]O ₂ (0≦x<0.5), Li[Li _x Ni _γ Co _(1-x-γ) ]O ₂ (0≦x<0.5, 0<γ<1), Li[Li _x Co _(1-x) ]O ₂ (0≦x<0.5), Li[Li _x Ni _γ Mn _(1-x-γ) ]O ₂ (0≦x<0.5, 0<γ<1), Li[Li _x Ni _γ Mn _β Co _(1-x-γ-β) ]O ₂ (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), Li[Li _x Ni Examples of _lithium transition metal composite oxides having a spinel crystal structure include Li _{x Mn 2} O ₄ and Li _x Ni _γ Mn _{(2-γ) O 4. Examples} of polyanion compounds include LiFePO ₄ , LiMnPO 4 , LiNiPO 4 , LiCoPO ₄ , Li ₃ V ₂ (PO ₄ ) ₃ , Li ₂ MnSiO ₄ , _and Li ₂ CoPO ₄ F. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, _and molybdenum dioxide. The 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 of them may be used in combination.

The positive electrode active material is typically in the form of particles (powder). The average particle size of the positive electrode active material is preferably, for example, between 0.1 μm and 20 μm. Setting the average particle size of the positive electrode active material above the lower limit facilitates manufacturing and handling of the positive electrode active material. Setting the average particle size of the positive electrode active material below the upper limit improves the electronic conductivity of the positive electrode active material layer. 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. "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 of particles diluted with a solvent in accordance with JIS-Z-8825 (2013).

To obtain powders of a specified particle size, mills and classifiers are used. Grinding methods include, for example, using a mortar, ball mill, sand mill, vibrating ball mill, planetary ball mill, jet mill, counter jet mill, swirling air jet mill, or sieve. Wet grinding, in which water or an organic solvent such as hexane is present, can also be used. Sieves and air classifiers are used as classification methods, both dry and wet, as needed.

The content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass or more and 99% by mass or less, more preferably 70% by mass or more and 98% by mass or less, and even more preferably 80% by mass or more and 95% by mass or less. By keeping the content of the positive electrode active material within the above range, it is possible to achieve both high energy density and manufacturability of the positive electrode active material layer.

The conductive agent is not particularly limited as long as it is a material that is conductive. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Carbonaceous materials include graphite, non-graphitic carbon, and graphene-based carbon. Non-graphitic carbon includes carbon nanofiber, pitch-based carbon fiber, and carbon black. Carbon black includes furnace black, acetylene black, and ketjen black. Graphene-based carbon includes graphene, carbon nanotubes (CNT), and fullerene. 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 CNT may be used. Among these, carbon black is preferred from the standpoints of electronic conductivity and coatability, and acetylene black is particularly preferred.

The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By keeping the content of the conductive agent within this range, the energy density of the secondary battery can be increased.

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; polysaccharide polymers, etc.

The binder content in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By keeping the binder content within the above range, the active material can be stably maintained.

Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. If 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.

The filler is not particularly limited. Examples of fillers 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; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica; and synthetic versions of these.

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.

(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.

The negative electrode substrate is electrically conductive. Materials used for the negative electrode substrate include metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, or alloys of these metals, and carbonaceous materials. Among these, copper or copper alloys are preferred. Examples of negative electrode substrates include foil, vapor-deposited film, mesh, and porous materials, with foil being preferred from a cost perspective. 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.

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

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, binder, thickener, and filler as needed. The optional components such as a conductive agent, binder, thickener, and filler can be selected from the materials exemplified for the positive electrode above.

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.

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 typically used as the negative electrode active material for lithium ion secondary batteries. Examples of negative electrode active materials include metal Li; metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; titanium-containing oxides such as Li ₄ Ti ₅ O ₁₂ , LiTiO _{2 , and} TiNb ₂ O ₇ ; polyphosphate compounds; silicon carbide; 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 mixed and used.

"Graphite" refers to a carbon material in which the average lattice spacing (d ₀₀₂ ) of the (002) plane, as determined by X-ray diffraction before charge/discharge or in a discharged state, 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.

"Non-graphitic carbon" refers to a carbon material in which the average lattice spacing (d ₀₀₂ ) of the (002) plane, as determined by X-ray diffraction before charge/discharge or in a discharged state, is 0.34 nm or more and 0.42 nm or less. Examples of non-graphitic carbon include 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.

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

The term "non-graphitizable carbon" refers to a carbon material having the above d ₀₀₂ of 0.36 nm or more and 0.42 nm or less.

The term "easily graphitizable carbon" refers to a carbon material having the above d ₀₀₂ of 0.34 nm or more and less than 0.36 nm.

The negative electrode active material is typically in the form of particles (powder). The average particle size of the negative electrode active material can be, for example, 1 nm to 100 μm. 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 to 100 μm. When the negative electrode active material is Si, Sn, Si oxide, Sn oxide, or the like, the average particle size may be 1 nm to 1 μm. Setting the average particle size of the negative electrode active material to be above the lower limit facilitates the production and handling of the negative electrode active material. Setting the average particle size of the negative electrode active material to be below the upper limit improves the electronic conductivity of the active material layer. To obtain powder with a specified particle size, a pulverizer or classifier is used. The pulverization and classification methods can be selected from, for example, the methods exemplified for the positive electrode above. 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.

The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 98% by mass or less. By keeping the content of the negative electrode active material within this range, it is possible to achieve both high energy density and manufacturability of the negative electrode active material layer.

(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.

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 less than the specified value 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 aluminosilicates; 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.

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

A polymer gel composed of a polymer and a non-aqueous electrolyte may be used as the separator. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. Using a polymer gel has the effect of suppressing leakage. A polymer gel may also be used in combination with a porous resin film or nonwoven fabric, such as those described above, as the separator.

(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.

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

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. Of these, EC and PC are preferred.

Examples of chain carbonates 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.

It is preferable to use a cyclic carbonate or a chain carbonate as the non-aqueous solvent, and it is even more preferable to use a combination of a cyclic carbonate and a chain carbonate. The use of a cyclic carbonate promotes dissociation of the electrolyte salt, improving the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate reduces 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.

The electrolyte salt can be appropriately selected from known electrolyte salts. Examples of electrolyte salts include lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Of these, lithium salts are preferred.

Examples of the lithium salt include inorganic lithium salts such _{as LiPF6} , _LiPO2F2 , _LiBF4 , _LiClO4 , and _LiN ( _SO2F ) ₂ ; lithium oxalate salts 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.

The content of the electrolyte salt in the non-aqueous electrolyte solution at 20°C and 1 atmosphere is preferably 0.1 mol/dm3 or ^more and ^2.5 mol/dm3 or less, more preferably ^0.3 mol/dm3 or ^more and 2.0 mol/dm3 or less, even more preferably 0.5 mol/dm3 or ^more and 1.7 mol/dm3 or ^less , and particularly preferably 0.7 mol/dm3 or ^more and 1.5 mol/ ^dm3 or less. By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte solution can be increased.

In addition to the nonaqueous solvent and electrolyte salt, the nonaqueous electrolyte may contain additives. 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); biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, and silyl. Aromatic compounds such as 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.

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

A solid electrolyte may be used as the non-aqueous electrolyte, or a non-aqueous electrolyte solution and a solid electrolyte may be used in combination.

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

In the case of a lithium ion secondary battery, examples of sulfide solid electrolytes include Li ₂ SP ₂ S ₅ , LiI-Li ₂ SP ₂ S ₅ , and Li ₁₀ Ge-P ₂ S ₁₂ .

<Configuration of 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.

<Method of manufacturing an energy storage element>
The manufacturing method of the energy storage element of 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 manufacturing method of the energy storage element can 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).

(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.

The positive electrode mixture may be a positive electrode mixture paste containing a dispersion medium in addition to the optional components described above. Examples of the dispersion medium include aqueous solvents such as water and mixed solvents primarily containing water; and organic solvents such as N-methylpyrrolidone and toluene. The positive electrode active material layer may be laminated directly onto the positive electrode substrate or via an intermediate layer.

(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.

(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 and 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.

(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.

(Electrolyte containing step)
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.

(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.

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, this is undesirable because much of the gas soluble in the electrolyte solution dissolves in the electrolyte solution when the electrolyte solution is filled, resulting in excessive dissolution of the gas soluble in the electrolyte solution (e.g., saturation dissolution) in the electrolyte solution before the sealing process. 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 effective reduction of the pressure inside the container. Preferably, after the electrolytic 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 electrolytic 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 electrolytic 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 electrolytic 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 so that the pressure inside the container immediately after injecting the gas soluble in the electrolyte is close to atmospheric pressure, the inconvenience of the gas soluble in the electrolyte being dissolved too much in the electrolyte before the sealing step can be eliminated or alleviated, and the pressure inside the container can be effectively reduced after the sealing step.

From the viewpoint of further reducing the pressure inside the container, the amount of gas soluble in the electrolyte 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 gas soluble in the electrolyte 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 gas soluble in the electrolyte 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.

From the viewpoint of reducing the pressure inside the container, the content of the gas soluble in the electrolyte solution contained inside the container is preferably 80% by volume or more, preferably 98% by volume or more, and more preferably 100% by volume, of the total amount of gas contained inside the container. From the viewpoint of ease of handling the gas, the content of the gas soluble in the electrolyte solution may be 80% by volume or less of the total amount of gas contained inside the container.

(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.

The sealing process must be performed promptly after the electrolyte-soluble gas is sealed in. If the container is left for a long time after sealing in the electrolyte-soluble gas, the electrolyte-soluble gas may dissolve too much in the electrolyte before the sealing process, making it difficult for the electrolyte-soluble gas to further dissolve in the electrolyte after the sealing process, which could result in an insufficient negative pressure inside the container. The elapsed time from the time the electrolyte-soluble gas is sealed in until the injection port is sealed is preferably 1 hour or less, from the viewpoint of minimizing the amount of electrolyte-soluble gas that dissolves in the electrolyte or diffuses out of the container through the injection port before the sealing process. This 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) in the electrolyte before the sealing step, and it is possible to effectively reduce the pressure inside the container after the sealing step.

After the gas soluble in the electrolyte solution is placed in the container, a step of temporarily sealing the injection port may be included 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 stopper 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 diffusing to the outside of the container through the injection port. In this case, the step of sealing the injection port may involve removing the stopper member or the like and then blocking the injection port with a sealing member, which may then be fixed by laser welding or the like. Alternatively, the step of sealing the injection port may involve placing a sealing member that covers the stopper member or the like that blocks the injection port, and then fixing the sealing member by laser welding or the like.

After the sealing step, when the dissolution of the gas soluble in the electrolyte solution into the electrolyte solution reaches equilibrium, the pressure inside the container 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 into the electrolyte solution after the sealing step, the pressure inside the container can be reduced.

Details regarding the electrode body, electrolyte, gas soluble in the electrolyte, container, etc. in the manufacturing method of the above-mentioned storage element are as described above.

<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.

For example, in the above-described embodiment, a method of creating a negative pressure inside the container was described in which a gas soluble in the electrolyte solution was placed inside the container. However, this is not limiting. For example, a method of creating a negative pressure inside the container may be used in which the pressure inside the container is reduced using a vacuum pump or the like and then sealed. In this case, the negative pressure creation process may include a decompression process of reducing the pressure inside the container using a vacuum pump or the like, instead of the gas containing process. This decompression process can be performed after the electrolyte containing process and before the sealing process. However, as in the above-described embodiment, a method of placing a gas soluble in the electrolyte solution inside the container is preferable because it more reliably creates 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 form 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, thereby eliminating the gas accumulation and making it easier to reduce the gaps between the electrodes.

In the above embodiment, the storage element is described as being used as a chargeable and dischargeable non-aqueous electrolyte secondary battery (e.g., a lithium-ion secondary battery), but the type, shape, dimensions, capacity, etc. of the storage element are arbitrary. The present invention can also be applied to various secondary batteries, electric double layer capacitors, lithium-ion capacitors, and other capacitors.

In the above embodiment, an electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween was described, but the electrode assembly does not need to include a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other, with a non-conductive layer formed on the active material layer of the positive electrode or negative electrode.

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 an energy storage device 30 in which energy storage units 20, each of which is an assembly of two or more electrically connected energy storage elements 1, are further assembled. The energy storage device 30 may include a bus bar (not shown) that electrically connects two or more energy storage elements 1, a bus bar (not shown) that electrically connects two or more energy storage units 20, etc. The energy storage unit 20 or the energy storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more energy storage elements 1.

REFERENCE SIGNS LIST 1 Energy storage element 2 Electrode body 3 Container 3a Container body 3b Lid 4 Positive electrode terminal 5 Negative electrode terminal 8 Winding core 14 Positive electrode current collector 15 Negative electrode current collector 20 Energy storage unit 30 Energy storage device

Claims (8)

a wound electrode body having a negative electrode and a positive electrode;
a sealable flat container for accommodating the electrode assembly,
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 . The energy storage element according to claim 1, wherein the winding core is primarily composed of polyacetal, polyimide, vinyl chloride, or a combination thereof. The energy storage element described in claim 1 or claim 2, wherein the container contains an electrolyte and a gas soluble in the electrolyte. The energy storage element according to claim 1, claim 2, or claim 3, wherein the winding core has a hollow structure. The energy storage element according to claim 3, wherein the gas soluble in the electrolyte solution includes carbon dioxide. The energy storage element described in claim 5, wherein the carbon dioxide content in the excess space inside the container is 5% by volume or more. The energy storage element described in any one of claims 1 to 6, wherein the pressure inside the container is 0.07 MPa or less. 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.