Energy storage element

Abstract

An electricity storage element according to one aspect of the present invention comprises an electrode body in which a positive electrode and a negative electrode are laminated with a separator interposed therebetween, an electrolyte, and a sealable container for accommodating the electrode body and the electrolyte, the compressive elastic modulus of the separator being 15 MPa or greater, and the interior of the container being in a negative pressure state.

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] With the aim of improving the characteristics of such electricity storage elements, such as the capacity retention rate, many studies have been conducted on additives to electrolyte solutions, for example (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2007-165125 Summary of the Invention [Problem to be solved by the invention]

[0005] However, depending on the additives in the electrolyte, the additives may decompose during the initial charge / discharge cycle, generating gases such as carbon monoxide. Even if the electrolyte does not contain additives, gas may be generated due to oxidation-reduction decomposition of the electrolyte. When gas is generated due to the decomposition of the additives or the electrolyte, this gas tends to accumulate in the gap between the electrodes. Furthermore, gas present inside the container may flow into and accumulate in the gap between the electrodes due to the expansion and contraction of the electrodes caused by charge / discharge. When gas accumulates between the electrodes, the active material of the electrode facing the gas accumulation area may be unable to charge or discharge, which may result in a decrease in the performance of the energy storage device.

[0006] An object of the present invention is to provide an energy storage element that can reduce the occurrence of gas accumulation between electrodes. [Means for solving the problem]

[0007] An energy storage element according to one aspect of the present invention comprises an electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, an electrolyte, and a sealable container for accommodating the electrode assembly and the electrolyte, wherein the separator has a compressive modulus of elasticity of 15 MPa or more, and the inside of the container is in a negative pressure state. [Effects of the Invention]

[0008] An energy storage device according to one aspect of the present invention can reduce the occurrence of gas accumulation between electrodes. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic exploded perspective view showing an energy storage element according to one embodiment of the present invention. [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 an electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, an electrolyte, and a sealable container for accommodating the electrode assembly and the electrolyte, wherein the separator has a compressive modulus of elasticity of 15 MPa or more, and the inside of the container is in a negative pressure state.

[0012] The energy storage element, having the above configuration, can reduce the occurrence of gas accumulation between the electrodes. While the reason for this is unclear, the following reason is presumed, for example. If the energy storage element is physically compressed from the outside to reduce the gap between the electrodes, the load change on the electrode assembly due to the expansion and contraction of the electrodes during charging and discharging is large, which may actually create a gap between the electrodes and make gas accumulation between the electrodes more likely. On the other hand, if the inside of the container of the energy storage element is placed under negative pressure, a force pulling the container inward is generated, compressing the electrode assembly, thereby discharging gas present between the electrodes to the outside of the electrode assembly, thereby reducing the occurrence of gas accumulation between the electrodes. However, the inventors have found that if the compressive modulus of the separator is small, the separator is compressed when the electrode assembly is compressed, thereby reducing the effect of reducing the gap between the electrodes. In the energy storage element, by placing the inside of the container under negative pressure and setting the compressive modulus of the separator to 15 MPa or more, gas present between the electrodes can be effectively discharged to the outside of the electrode assembly, thereby reducing the occurrence of gas accumulation between the electrodes. Furthermore, since the pressure 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 between the electrodes are less likely to occur. Therefore, it is believed that this energy storage element can reduce the occurrence of gas accumulation 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 energy storage element preferably contains a gas soluble in the electrolyte solution in the container. The energy storage element contains a gas soluble in the electrolyte solution in 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 the gas soluble in the electrolyte solution contained in 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 is dissolved under 1 atmosphere at 25°C. 3 The solubility in 3 The above gases.

[0015] 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.

[0016] <Configuration of energy storage element> An energy storage element according to one embodiment of the present invention includes an electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, an electrolyte, and a sealable container for containing the electrode assembly and the electrolyte. The interior of the container is under negative pressure. In this embodiment, a gas soluble in the electrolyte is contained within the container. The electrolyte is present in a state contained in the positive electrode, the negative electrode, and the separator. Hereinafter, a nonaqueous 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.

[0017] 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 shown in FIG. 1. The energy storage element 1 includes a wound electrode assembly 2 formed by winding a positive electrode and a negative 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, a container 3 for accommodating these, and a positive electrode terminal 4 and a negative electrode terminal 5 provided on the container 3. The container 3 may be a known metal container, a 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 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 container 3 according to this embodiment is made of aluminum or an aluminum-based alloy. The thickness of the container is not particularly limited, but may be approximately 0.2 mm to 2 mm (for example, 0.3 mm to 1.5 mm, typically 0.35 mm to 1 mm).

[0018] The container 3 has a container body 3a in the shape of a flattened, bottomed, rectangular cylinder, and a lid 3b in the shape of an elongated rectangular plate that can close an 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. In this embodiment, the container body 3a has a pair of opposing wide side surfaces, a pair of opposing narrow side surfaces, and a bottom surface that faces the lid 3b, with the electrode assembly 2 sandwiched therebetween, and the electrode assembly 2 is in direct or indirect contact with the inner surfaces of the pair of opposing wide side surfaces and the bottom surface of the container body 3a.

[0019] 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.

[0020] In the energy storage element 1, the inside of the container 3 is in a negative pressure state. When the inside of the container 3 is in a negative pressure state, that is, when a force pulling the container 3 inward is generated, gas present between the electrodes is discharged to the outside of the electrode body. Therefore, the energy storage element 1 can reduce the occurrence of gas accumulation between the electrodes.

[0021] 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 = 1 atmosphere = 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 reducing the occurrence of gas accumulation between the electrodes. In some embodiments, the pressure may be 0.07 MPa or less or 0.065 MPa or less (e.g., 0.055 MPa). The lower limit of the pressure inside the container 3 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).

[0022] In this embodiment, a gas soluble in the electrolyte is contained inside the container 3. By containing a gas soluble in the electrolyte inside the sealed container 3, the gas dissolves in the electrolyte, which can effectively reduce the pressure inside the container 3 and more reliably create a negative pressure state inside the container.

[0023] 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.

[0024] When carbon dioxide is used as the gas soluble in the nonaqueous electrolyte, the carbon dioxide content (concentration) in the residual 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 100% by volume (e.g., 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 above-mentioned composition atmosphere is achieved after the carbon dioxide dissolves in the electrolyte, the occurrence of gas accumulation between the electrodes can be better reduced. When carbon dioxide is used as the gas soluble in the nonaqueous electrolyte, the carbon dioxide content (concentration) in the electrolyte inside the container is not particularly limited, but is preferably 0.001% by volume or more, and more preferably 0.003% by volume or more, from the viewpoint of maintaining a suitable negative pressure inside the container, etc. In some embodiments, the carbon dioxide content (concentration) in the electrolyte inside the container may be 0.0035% by volume or more, or may be 0.005% by volume or more.

[0025] On the other hand, when the electrolyte is a nonaqueous electrolyte, examples of the gas that is 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 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 nonaqueous electrolyte is dissolved in the electrolyte, the gas soluble in the nonaqueous 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 occurrence of gas accumulation 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% by volume, the proportion of carbon dioxide can 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 occurrence of gas accumulation 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.

[0026] In the energy storage element, it is preferable that the container further contains a member capable of adsorbing gas soluble in the electrolyte solution. By containing a member capable of adsorbing gas soluble in the electrolyte solution in the container, the content (concentration) of gas soluble in the electrolyte solution in the excess space inside the container is more likely to be reduced, and the inside of the container is placed in a more suitable negative pressure state, making it easier for gas between the electrodes to be discharged to the outside of the electrode body. Therefore, the effect of reducing the occurrence of gas accumulation between the electrodes can be further improved. Furthermore, since the member capable of adsorbing gas soluble in the electrolyte solution also adsorbs the gas soluble in the electrolyte solution, the time until the inside of the container reaches a negative pressure state after the container is sealed can be shortened.

[0027] [Electrode body] The electrode assembly 2 may be a wound type in which a positive electrode and a negative electrode are wound in a stacked state with a separator interposed therebetween, or a stacked type in which multiple positive electrodes and multiple negative electrodes are stacked with separators interposed therebetween. In this embodiment, the electrode assembly 2 is a flat-shaped wound electrode assembly. Compared to a stacked electrode assembly, a wound electrode assembly is less likely to allow gas generated between the electrodes to be discharged to the outside of the electrode assembly, so the effect of applying this aspect can be more effectively exerted. In this embodiment, the electrode assembly 2 has two wound R portions and two flat portions. The two flat portions correspond to flat portions of the outer wall side surface that constitutes the electrode assembly 2 and are each arranged opposite the wide side surface of the inner wall side surface that constitutes the container body. In this embodiment 1, the two flat portions are arranged so as to contact the wide side surface of the container body. The two wound R portions correspond to curvature portions (curved portions) of the outer wall side surface that constitutes the electrode assembly 2 and are each arranged facing the bottom surface and lid of the container body. The two wound R portions are arranged so as not to contact the wide side surface 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 3 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, 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, it is possible to more effectively reduce the occurrence of gas accumulation between the electrodes.

[0028] When the electrode assembly 2 is a wound type, the electrode assembly 2 may further include a winding core in the center and be wound around the winding core. The winding core may have either a hollow structure or a solid structure, but a hollow core is preferred. When the electrode assembly 2 includes a hollow core, a gas soluble in the electrolyte is sealed in the hollow region formed in the center of the electrode assembly 2, so that the pressure inside the container 3 can be reduced more effectively.

[0029] (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.

[0030] 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).

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] In a preferred embodiment, the positive electrode active material is composed of a lithium transition metal composite oxide having an α-NaFeO2-type crystal structure, from the viewpoints of charge / discharge performance, energy density, etc. Examples of the lithium transition metal composite oxide include a nickel-containing lithium transition metal composite oxide containing at least nickel (Ni) as a constituent element in addition to Li, a cobalt-containing lithium transition metal composite oxide containing at least cobalt (Co) as a constituent element, and a manganese-containing lithium transition metal composite oxide containing at least manganese (Mn) as a constituent element. Of these, a nickel-containing lithium transition metal composite oxide is preferred, and a lithium transition metal composite oxide containing nickel, manganese, and cobalt is more preferred.

[0036] 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).

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] The mass per unit area of ​​the positive electrode active material layer (one side) is not particularly limited, but is, for example, 2.0 mg / cm in terms of solid content. 2 More than 30.0mg / cm 2 The mass per unit area of ​​the positive electrode active material layer may be 3.0 mg / cm or less. 2 More than 20.0mg / cm 2 Less than 4.0 mg / cm is preferred 2 More than 15.0mg / cm 2 Less than 5.0 mg / cm is more preferable. 2 More than 10.0mg / cm 2 The following is even more preferred:

[0047] (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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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-graphitizable carbon (easily graphitizable carbon or non-graphitizable carbon). Among these materials, graphite and non-graphitizable carbon are preferred, and non-graphitizable carbon (especially non-graphitizable carbon) is more 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.

[0053] "Graphite" refers to graphite that has an average lattice spacing (d002 ) 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.

[0054] "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.

[0055] 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.

[0056] "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.

[0057] "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.

[0058] 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.

[0059] 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.

[0060] The mass per unit area of ​​the negative electrode active material layer (one side) is not particularly limited, but is, for example, 0.5 mg / cm in terms of solid content. 2 More than 20.0mg / cm 2 The mass per unit area of ​​the negative electrode active material layer may be 0.8 mg / cm or less. 2 More than 15.0mg / cm 2 Less than 1.0 mg / cm is preferred 2 More than 10.0mg / cm 2 Less than 2.0 mg / cm is more preferred. 2 More than 7.0mg / cm 2 The following is even more preferred:

[0061] (separator) The lower limit of the compressive elastic modulus of the separator of the energy storage element of this embodiment is 15 MPa, preferably 17 MPa, and more preferably 19 MPa (e.g., 20 MPa). When the compressive elastic modulus of the separator is equal to or greater than the lower limit, compression of the separator when compressed is suppressed, thereby improving the effect of reducing the occurrence of gas accumulation between the electrodes. On the other hand, the upper limit of the compressive elastic modulus of the separator is preferably 50 MPa, more preferably 40 MPa, and even more preferably 30 MPa. When the compressive elastic modulus of the separator is equal to or less than the upper limit, the compressive force applied to the electrode assembly is less likely to be uneven, thereby improving the effect of reducing the occurrence of gas accumulation between the electrodes. A separator having such a compressive elastic modulus is also suitable from the viewpoint of reducing the resistance of the energy storage element. The compressive elastic modulus of the separator can be adjusted by changing the porosity, material, stretching method, or, in the case of a polymer material, the molecular weight, etc.

[0062] The compressive elastic modulus of the separator is the compressive elastic modulus in the thickness direction, and is calculated using the following formula from the thickness change (μm) when a predetermined load is applied to the separator in the thickness direction and the compressive stress reaches 1 MPa, and the thickness (μm) of one separator before compression. Compressive elastic modulus = 1 / {change in thickness per separator (μm) / thickness of one separator before compression (μm)}

[0063] In a preferred embodiment of the energy storage element disclosed herein, the relationship between the compressive modulus X (MPa) of the separator and the pressure P (absolute pressure: MPa) inside the container satisfies 180≦(X / P)≦600. By appropriately setting the relationship between the compressive modulus X of the separator and the pressure P inside the container, the effect of reducing the occurrence of gas accumulation between the electrodes can be further improved. The technology disclosed herein can be preferably implemented, for example, in an embodiment in which the relationship between X and P satisfies 200≦(X / P)≦580, more preferably 220≦(X / P)≦550, even more preferably 300≦(X / P)≦500, and particularly preferably 350≦(X / P)≦450.

[0064] The separator can be appropriately selected from known separators having an appropriate compressive modulus. 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 substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these, porous resin film is preferred from the viewpoint of strength. Materials for the substrate layer of the separator include, in order to ensure an appropriate compressive modulus of the separator, polyolefins such as polyethylene (PE) and polypropylene (PP), polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacrylonitrile, polyphenylene sulfide, polyimide, and fluororesin. Among these, polyolefins are preferred. Copolymers of the monomers constituting these resins may also be used. In a preferred embodiment, the substrate layer of the separator is made of a polyethylene-based resin. Ethylene homopolymers and ethylene copolymers are preferably used as the polyethylene-based resin. The ethylene copolymer is a resin containing 50% by mass or more of repeating units derived from ethylene, and can be a copolymer obtained by polymerizing an olefin copolymerizable with ethylene or a copolymer obtained by polymerizing at least one monomer copolymerizable with ethylene. Examples of olefins copolymerizable with ethylene include propylene. Examples of other monomers include conjugated dienes (e.g., butadiene) and acrylic acid. Furthermore, a uniaxially or biaxially stretched porous resin film can be suitably used as the substrate layer of the separator. Among these, a porous resin film uniaxially stretched in the machine direction (MD) can be suitably used. Here, "uniaxial stretching" refers to stretching in only one direction (e.g., the machine direction) in a process in which a resin film is stretched at or above its glass transition temperature to orient the molecules, and "biaxial stretching" refers to stretching in two orthogonal directions (e.g., the machine direction and the longitudinal direction). The width direction refers to a direction parallel to the conveying plane of the resin film and perpendicular to the machine direction.The method for making the separator substrate layer porous in the manufacturing process is not particularly limited. For example, a dry substrate layer can be formed by dry stretching (e.g., uniaxial stretching) after drying, or a wet substrate layer can be formed by wet stretching (e.g., biaxial stretching) in a wet state (e.g., a state in which the raw material resin and a solvent are mixed). Among these, a dry substrate layer is preferred. Dry stretching of a microporous membrane can be performed by roll stretching while heating. A dry-stretched substrate layer is particularly preferred because it is easy to adjust the compressive modulus of the separator to the preferred values ​​disclosed herein, and it has appropriate strength and little thermal shrinkage in the width direction. A composite material of these resins can also be used for the separator substrate layer. For example, the substrate layer can have a single-layer structure, a mixed structure (e.g., a mixed structure of PP and PE), or a multilayer structure (e.g., a three-layer structure of PP / PE / PP or a two-layer structure of PP / PE). Of these, a three-layer structure of PP / PE / PP is preferred.

[0065] The porosity of the substrate layer of the separator is not particularly limited, but the lower limit is preferably 20% by volume, and more preferably 30% by volume. In some embodiments, the porosity of the separator may be, for example, 35% by volume or more, and typically 40% by volume or more. On the other hand, the upper limit of the porosity is preferably 80% by volume, and more preferably 70% by volume. In some embodiments, the porosity of the separator may be, for example, 65% by volume or less, and typically 60% by volume or less (e.g., 55% by volume or less). The porosity of the separator may be, for example, 50% by volume or less, and may be 45% by volume or less. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0066] 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 under 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, boehmite, or aluminosilicate is preferred from the viewpoint of the safety of the energy storage device.

[0067] The thickness of the separator (when a heat-resistant layer is included, the total thickness of the base layer and the heat-resistant layer) is not particularly limited, but the lower limit is preferably 3 μm, more preferably 5 μm. In some embodiments, the thickness of the separator may be, for example, 8 μm or more, typically 10 μm or more. On the other hand, the upper limit of the thickness is preferably 30 μm, more preferably 25 μm. In some embodiments, the thickness of the separator may be, for example, 20 μm or less, typically 15 μm or less (e.g., 12 μm or less).

[0068] (electrolyte) When the energy storage element is a nonaqueous electrolyte secondary battery, a nonaqueous electrolyte is used as the electrolyte. The nonaqueous electrolyte can be appropriately selected from known nonaqueous electrolytes. The nonaqueous electrolyte contains a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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. 3 More 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.

[0076] 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.

[0077] 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.

[0078] <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, making it easier for gas between the electrodes to be discharged outside the electrode body, thereby improving the effect of reducing the occurrence of gas accumulation between the electrodes.

[0079] <Method of manufacturing an energy storage element> The method for manufacturing an energy storage device 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 solution in the container (hereinafter also referred to as an electrolyte solution 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 negative pressure forming step includes housing a gas soluble in the electrolyte solution in the container (hereinafter also referred to as a gas housing step) after the electrolyte solution housing step and before the sealing step. Furthermore, the method for manufacturing an energy storage device 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).

[0080] (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.

[0081] 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 (NMP) and toluene. The positive electrode active material layer may be laminated directly on the positive electrode substrate or via an intermediate layer.

[0082] (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.

[0083] (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 flat wound electrode body having 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 or wound with the separator interposed therebetween to form an alternately stacked electrode body.

[0084] (Electrode body accommodation process) In the electrode assembly housing step, the electrode assembly in which the negative electrode and the positive electrode are stacked is housed in a container.

[0085] (Electrolyte containing process) In the electrolyte solution containing step, the electrolyte solution is contained in the container. The containing of the electrolyte solution can be performed by a known method. When the energy storage element is a non-aqueous electrolyte secondary battery, the non-aqueous electrolyte solution is contained in the container by, for example, pouring the non-aqueous electrolyte solution through an inlet provided in the container.

[0086] (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.

[0087] 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.

[0088] The amount of the 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, from the viewpoint of further reducing the pressure inside the container. The amount of the 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 the 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. Here, the "volume of the excess space inside the container" refers to the volume obtained by subtracting the volumes of the structures such as the electrode assembly, electrolyte, and current collector from the internal volume of the container. Furthermore, the volume of the electrode assembly refers to the actual volume of the components of the electrode (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 the gas contained inside the container when the pressure inside the container is 1 atmosphere (0.1013 MPa) at 25°C.

[0089] 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.

[0090] (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.

[0091] 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 gas that dissolves in the electrolyte or is released to the outside of the container through the injection port by diffusion before the sealing step. 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] <Other embodiments> The energy storage element of the present invention is not limited to the above-described embodiments and may be modified in various ways without departing from the spirit and scope of the present invention. For example, the configuration of one embodiment may be added to the configuration of another embodiment, or a portion of the configuration of one embodiment may be replaced with the configuration of another embodiment or known technology. Furthermore, a portion of the configuration of one embodiment may be deleted. Known technology may be added to the configuration of one embodiment. For example, in the above-described embodiment, the flat-shaped wound electrode body has two rounded portions and two flat portions. The two flat portions are arranged facing the wide side surfaces of the container body so as to directly or indirectly contact the wide side surfaces. The two rounded portions are arranged facing the bottom and lid of the container body so as not to contact the wide side surfaces. However, this is not limiting. The two rounded portions may also be arranged facing the narrow side surfaces of the container body so as not to contact the wide side surfaces. In such a wound electrode body, gas is less likely to escape than in a laminated electrode body, and therefore the effect of applying this embodiment can be more effectively exhibited.

[0096] 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 accommodated inside the container. However, this is not limiting. For example, a method of creating a negative pressure inside the container may be employed 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 creating process may include a decompression process of reducing the pressure inside the container using a vacuum pump or the like, instead of the gas accommodation process. Such a decompression process can be performed after the electrolyte accommodation process and before the sealing process. However, as in the above-described embodiment, a method of accommodating a gas soluble in the electrolyte solution inside the container is preferred because it makes it easier to eliminate gas accumulation caused by gas accumulating inside the container in the gap between the electrodes. That is, because the concentration of the gas soluble in the electrolyte solution increases in the excess space inside the container, even if gaps are generated 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 gap between the electrodes, it can quickly dissolve in the electrolyte solution, which is preferable because it makes it easier to eliminate gas accumulation.

[0097] 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.

[0098] 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. [Example]

[0099] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0100] [Examples 1 to 3 and Comparative Examples 1 to 3] (1) Positive electrode formation process LiNi as a positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3 A positive electrode mixture paste containing O2, PVDF as a binder, and acetylene black as a conductive agent, and using NMP as a dispersion medium, was prepared. The ratio of the positive electrode active material, binder, and conductive agent was 90:5:5 in terms of mass ratio of solid content. The positive electrode mixture paste was applied to both sides of a 12 μm thick aluminum foil serving as a positive electrode substrate, and dried to form a positive electrode active material layer, thereby obtaining positive electrodes of Examples 1 to 3 and Comparative Examples 1 to 3. After drying, the applied amount of the positive electrode mixture (the positive electrode mixture paste from which the dispersion medium has been evaporated) per unit area on one side was 8.5 mg / cm. 2 It was made to be like this.

[0101] (2) Negative electrode formation process A negative electrode mixture paste was prepared containing non-graphitizable carbon as the negative electrode active material, PVDF as the binder, and NMP as the dispersion medium. The ratio of the negative electrode active material to the binder was 95:5 by mass ratio converted to solid content. The negative electrode mixture paste was applied to both sides of an 8 μm-thick copper foil as the negative electrode substrate, and dried to form a negative electrode active material layer, thereby obtaining negative electrodes of Examples 1 to 3 and Comparative Examples 1 to 3. After drying, the coating amount of the negative electrode mixture (the negative electrode mixture paste from which the dispersion medium had been evaporated) per unit area on one side was 4 mg / cm. 2 It was made to be like this.

[0102] (3) Electrode body formation process The negative electrode and positive electrode were stacked with a 20 μm thick separator having the compressive modulus and porosity shown in Table 1, and the stack was wound around a hollow core to produce wound electrode bodies of Examples 1 to 3 and Comparative Examples 1 to 3. As the separator material, Examples 1 to 3 and Comparative Example 2 used a three-layer dry porous resin film separator made of PP / PE / PP, while Comparative Examples 1 and 3 used a single-layer wet porous resin film separator made of PE.

[0103] (4) Electrode assembly housing process The electrode assembly was housed in a flat, bottomed, rectangular cylindrical container body made of aluminum alloy.

[0104] (5) Electrolyte containing process A non-aqueous solvent containing PC, DMC, and EMC mixed in a volume ratio of 1:1:1 was added with LiPF6 at 1.2 mol / dm 3 In the energy storage elements of Examples 1 to 3 and Comparative Examples 1 to 3, the prepared electrolyte solution was added to the container in an amount of 30 cm 3 The volume of the extra space inside the container was 12 cm 3 It was.

[0105] (6) Gas storage process After the electrolyte solution was placed inside the container, preliminary charging was performed. Thereafter, in the energy storage elements of Examples 1 to 3 and Comparative Example 3, the pressure inside the container was reduced to 10,000 Pa using a vacuum pump, and then carbon dioxide gas and air, which are gases soluble in the electrolyte solution, were placed inside the container in the amounts shown in Table 1. In the energy storage elements of Comparative Example 1 and Comparative Example 2, air was placed inside the container instead of carbon dioxide gas.

[0106] (7) Sealing process Five minutes after the carbon dioxide gas or air was poured into the container, the container was sealed by sealing the injection port, and the test cells, that is, the energy storage elements of Examples 1 to 3 and Comparative Examples 1 to 3, were obtained.

[0107] [evaluation] (pressure inside the container) The pressure inside the container was measured by attaching an internal pressure measuring device to the container. The internal pressure (gauge pressure) of the container 48 hours after sealing is shown in Table 1. The absolute pressure inside the container was 0.0613 MPa for Examples 1 and 2 and Comparative Example 3, 0.0513 MPa for Example 3, and 0.1413 MPa for Comparative Examples 1 and 2.

[0108] (Compressive elastic modulus of separator) The compressive elastic modulus (MPa) of the separator was calculated using the above formula by pressing a cylindrical indenter with a diameter of 50 mm against a sample of 200 stacked separators at 30°C using a load cell creep tester (manufactured by Mize Testing Instruments Co., Ltd.) until the compressive stress reached 1 MPa, maintaining that stress state, and measuring the change in thickness (μm) of the separator after 1 hour.

[0109] (Number of gas pockets between electrodes) The number of gas pockets that occurred between the electrodes in the examples and comparative examples was determined by disassembling the electricity storage element, unfolding the electrode assembly, and visually checking.

[0110] [Table 1]

[0111] As shown in Table 1, Examples 1 to 3, in which the separator had a compressive modulus of 15 MPa or more and the inside of the container was in a negative pressure state, were excellent in reducing the occurrence of gas accumulation between the electrodes. Furthermore, the lower the separator's compressive modulus and the pressure inside the container, the better the effect of reducing the occurrence of gas accumulation between the electrodes. On the other hand, Comparative Examples 1 and 3, in which the separator had a compressive modulus of less than 15 MPa, were unable to achieve a reduction in the occurrence of gas accumulation between the electrodes, regardless of the pressure inside the container. Furthermore, Comparative Example 2, in which the inside of the container was not in a negative pressure state, was unable to achieve a reduction in the occurrence of gas accumulation between the electrodes, even though the separator had a compressive modulus of 15 MPa or more.

[0112] The above results show that the energy storage element has a compressive modulus of elasticity of the separator of 15 MPa or more and a negative pressure state inside the container, thereby reducing the occurrence of gas accumulation between the electrodes. [Explanation of symbols]

[0113] 1. Energy storage element 2 Electrode body 3 containers 4 Positive terminal 14 Positive electrode current collector 5 Negative terminal 15 Negative electrode current collector 3a Container body 3b Lid body 20 Energy storage unit 30 Energy storage device

Claims

1. an electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween; An electrolyte; a sealable container for containing the electrode assembly and the electrolyte; Equipped with The separator has a compressive elastic modulus of 15 MPa or more, The inside of the container is in a negative pressure state, The pressure inside the container is 0.07 MPa or less.

2. 2. The energy storage device according to claim 1, wherein the container contains a gas soluble in the electrolyte.

3. The energy storage element according to claim 2 , wherein the gas soluble in the electrolyte solution contains carbon dioxide.

4. The energy storage element according to claim 3 , wherein the carbon dioxide content in the surplus space inside the container is 5% by volume or more.

5. The energy storage element according to claim 1 , wherein the separator has a compressive modulus of elasticity of 40 MPa or less.

6. The energy storage element according to claim 1 , wherein the negative electrode contains a carbon material as a negative electrode active material.

7. The energy storage element according to claim 1 , wherein the electrode body is a wound electrode body in which the positive electrode and the negative electrode are stacked with the separator interposed therebetween and wound.

8. the electrode body is a flat wound electrode body obtained by winding the positive electrode and the negative electrode stacked with the 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.

9. The container body sandwiches the electrode assembly therebetween. a pair of wide side surfaces facing each other, a pair of narrow side surfaces facing each other, and a bottom surface facing the lid; The flat wound electrode body has two wound R portions and two flat portions, the two flat portions are disposed opposite the wide side surfaces constituting the container body so as to be in direct or indirect contact with the wide side surfaces, The energy storage device according to claim 8 , wherein the two wound R portions are arranged facing the bottom surface of the container body and the lid so as not to come into contact with the wide side surfaces.

10. The container body sandwiches the electrode assembly therebetween. a pair of wide side surfaces facing each other, a pair of narrow side surfaces facing each other, and a bottom surface facing the lid; The flat wound electrode body has two wound R portions and two flat portions, the two flat portions are disposed opposite the wide side surfaces constituting the container body so as to be in direct or indirect contact with the wide side surfaces, The energy storage device according to claim 8 , wherein the two wound R portions are arranged facing the narrow side surfaces constituting the container body so as not to come into contact with the wide side surfaces.

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