Energy storage element

Abstract

An electricity storage element according to an aspect of the present invention is provided with: an electrode assembly in which a positive electrode and a negative electrode are laminated via a separator; an electrolytic solution; and a container containing the electrode assembly and the electrolytic solution. The electrode assembly is in a loaded state, wherein a pressure P0(MPa) being applied to the electrode assembly in discharged state and a distortion A when the separator is compressed with a pressure of 2 MPa under the environment of 45℃ in the thickness direction satisfy equation 1.　Equation 1: P0(1-A)<1.2(MPa)

Description

[Technical Field]

[0001] This invention relates to an energy storage element. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, as well as in automobiles, due to their high energy density. In addition to non-aqueous electrolyte secondary batteries, other energy storage elements such as lithium-ion capacitors and electric double-layer capacitors, and energy storage elements using electrolytes other than non-aqueous electrolytes are also widely used.

[0003] In battery packs having multiple energy storage elements, the energy storage elements are sometimes arranged without gaps between them and used in a state where they are restrained by a restraining member or the like in order to increase energy density. Patent Document 1 describes a secondary battery comprising an electrode body formed by stacking a positive electrode and a negative electrode in a stacking direction with a separator in between, an aluminum battery case housing the electrode body, an insulating film provided between the electrode body and the inner wall of the battery case, and a restraining member disposed outside the battery case that restrains the electrode body by applying pressure in the stacking direction through the battery case and the insulating film. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2018-45797 [Overview of the project] [Problems that the invention aims to solve]

[0005] To increase energy density by constraining the energy storage element, it is preferable to restrain the energy storage element with sufficient pressure. However, when a load is applied to the electrode body of the energy storage element, the discharge capacity at high current densities may decrease significantly.

[0006] The present invention has been made based on the above circumstances, and an object thereof is to provide a power storage element in which a load is applied to an electrode body and a decrease in discharge capacity at a high current density is suppressed.

Means for Solving the Problems

[0007] The power storage element according to one aspect of the present invention includes an electrode body in which a positive electrode and a negative electrode are laminated via a separator, an electrolytic solution, and a container that houses the electrode body and the electrolytic solution. The electrode body is in a state where a load is applied, and the pressure P0 (MPa) applied to the electrode body in the discharge state and the strain A when the separator is compressed in the thickness direction at a pressure of 2 MPa in an environment of 45 °C satisfy the following formula (1). P0(1 - A) < 1.2 (MPa) ··· 1

[0008] The power storage element according to another aspect of the present invention includes an electrode body in which a positive electrode and a negative electrode are laminated via a separator, an electrolytic solution, and a container that houses the electrode body and the electrolytic solution. The electrode body is in a state where a load is applied, and the difference between the pressure P0 applied to the electrode body in the discharge state and the pressure P1 applied to the electrode body in the charge state is 0.90 MPa or less.

Advantages of the Invention

[0009] According to the present invention, it is possible to provide a power storage element in which a load is applied to an electrode body and a decrease in discharge capacity at a high current density is suppressed.

Brief Description of the Drawings

[0010] [Figure 1] FIG. 1 is a perspective view showing an embodiment of a power storage element. <9800010>2>FIG. 2 is a schematic view showing an embodiment of a power storage device configured by aggregating a plurality of power storage elements. [Figure 3]Figure 3 is a graph showing the relationship between the pressure difference (P1-P0) and the discharge capacity ratio in each energy storage element of the examples and comparative examples. [Figure 4] Figure 4 is a graph showing the relationship between P0(1-A) and the pressure difference (P1-P0) in each energy storage element of the examples and comparative examples. [Figure 5] Figure 5 is a graph showing the discharge capacity retention rate in charge-discharge cycle tests for each energy storage element in the reference example. [Modes for carrying out the invention]

[0011] First, an overview of the energy storage elements disclosed herein will be provided.

[0012] An energy storage element according to one aspect of the present invention comprises an electrode body in which a positive electrode and a negative electrode are stacked with a separator in between, an electrolyte, and a container for housing the electrode body and the electrolyte, wherein the electrode body is under load, and the pressure P0 (MPa) applied to the electrode body in the discharge state and the strain A when the separator is compressed in the thickness direction at a pressure of 2 MPa in an environment of 45°C satisfy the following formula 1. P0(1-A)<1.2(MPa) ···1

[0013] An energy storage element according to one aspect of the present invention is an energy storage element in which a load is applied to the electrode body, and the decrease in discharge capacity at high current density is suppressed. The reason for this effect is not clear, but the following reasons are speculated. The difference in pressure applied to the electrode body between the discharge state and the charge state becomes larger when the expansion of the negative electrode is suppressed, such as when the amount of strain when the separator is compressed in the thickness direction is small compared to the amount of expansion of the active material of the negative electrode in the thickness direction due to charging. In such a case, the expansion of the active material of the negative electrode increases the contact area between the active material and the binder, resulting in a decrease in the conductivity of the negative electrode. As discharge occurs, the negative electrode contracts and returns to its original state, but at high current density discharge, the recovery cannot keep up, and as a result of discharge occurring with low conductivity of the negative electrode, the discharge capacity is thought to decrease. Another factor is that the expansion of the active material of the negative electrode reduces the void of the negative electrode, and the electrolyte in the void of the negative electrode flows out of the electrode body. As discharge occurs, the void in the negative electrode returns to its original size and electrolyte flows into the void from outside the electrode. However, at high current densities, the electrolyte inflow cannot keep up, and the amount of electrolyte around the active material of the negative electrode decreases locally, increasing the reaction resistance and thus reducing the discharge capacity. On the other hand, in an energy storage element according to one aspect of the present invention, a separator with strain A close to 1, i.e., one that is easily compressible, is sufficiently compressed when the active material of the negative electrode expands during charging. This reduces the increase in pressure applied to the electrode during charging, i.e., the difference between the pressure P0 applied to the electrode in the discharge state and the pressure P1 applied to the electrode in the charging state (P1-P0). On the other hand, a separator with strain A close to 0, i.e., one that is not easily compressible, is not sufficiently compressed when the active material of the negative electrode expands during charging. However, by making the pressure P0 even smaller, the pressure difference (P1-P0) can be reduced. Therefore, when equation 1 is satisfied, the decrease in discharge capacity at high current densities is further suppressed.

[0014] Note that strain A is the average thickness T of the separator under no load conditions in a 45°C environment. a And, in an environment of 45°C, the average thickness T of the separator when compressed in the thickness direction at a pressure of 2 MPa. bTherefore, the value is obtained by the following equation 2. Furthermore, the average thickness of the separator is the average of the thickness measurements taken at any five locations. A=(T a -T b ) / (T a ) ···2

[0015] It is preferable that the difference between the above-mentioned pressure P0 and the pressure P1 applied to the electrode body in the charged state is 0.90 MPa or less. In such a case, since the difference between the pressure P0 applied to the electrode body in the discharged state and the pressure P1 applied to the electrode body in the charged state (P1-P0) is small (0.90 MPa or less), a decrease in the conductivity of the negative electrode and leakage of electrolyte from the void of the negative electrode are less likely to occur even in the charged state. As a result, the decrease in discharge capacity at high current densities when a load is applied to the electrode body is further suppressed.

[0016] The pressure applied to the electrode body is defined as the pressure applied in the thickness direction (Y direction in Figure 1) of the stacked positive electrode, negative electrode, and separator. Furthermore, the pressure applied to the electrode body is the value measured by the following method. First, the thickness of the energy storage element with a load applied by a pressurizing member, etc. (thickness in the Y direction in Figure 1) is measured. Next, the load from the pressurizing member, etc. is released, and the energy storage element is pressed in the direction of the above thickness using a compression tester so that it returns to the thickness with the above load applied. The force shown by the compression tester at this time is defined as the load applied to the electrode body. The value obtained by dividing this load applied to the electrode body by the area of ​​the contact surface between the probe of the compression tester and the energy storage element is defined as the pressure applied to the electrode body. This measurement is performed for each energy storage element in both the discharged and charged states.

[0017] Furthermore, the "charged state" and "discharged state" of the energy storage element refer to the state when the following operations are performed. The "charged state" is defined as the state in which the energy storage element is charged with a constant current of 1.0C until it reaches the charging termination voltage for normal use, and then charged at a constant voltage for 3 hours. The "discharged state" is defined as the state in which, after a 10-minute rest from the charging state, the energy storage element is discharged with a constant current of 1.0C until it reaches the lower limit voltage for normal use. Here, "normal use" refers to the case in which the energy storage element is used by adopting the charge and discharge conditions recommended or specified for the energy storage element, and if a charger for the energy storage element is provided, it refers to the case in which the energy storage element is used by applying that charger.

[0018] The strain A is preferably 0.05 or more and 0.3 or less. When the strain A is above the lower limit, sufficient compression occurs when a load is applied to the separator in the thickness direction, thus further suppressing the decrease in discharge capacity at high current densities when a sufficient load is applied to the electrode body. On the other hand, when the strain A is below the upper limit, excessive compression occurs when a load is applied to the separator in the thickness direction, which reduces ion permeability, and thus further suppresses the decrease in discharge capacity at high current densities when a sufficient load is applied to the electrode body.

[0019] It is preferable that the air permeability resistance of the above separator is 250 seconds / 100 mL or less. In such a case, sufficient ion permeability can be ensured even when the separator is compressed, so the decrease in discharge capacity at high current densities when a load is applied to the electrode body is further suppressed.

[0020] Furthermore, the "air permeability resistance" of the separator is a value measured in accordance with JIS-P8117 (2009), and is the average value measured at any five locations.

[0021] It is preferable that the above pressure P0 is between 0.40 MPa and 1.40 MPa. In such a case, the reduction in discharge capacity at high current densities is further suppressed because excessive pressure is less likely to be applied to the electrode body. Furthermore, by having the above pressure P0 above the lower limit, the gaps within the electrode body are reduced, and gas generated by charging and discharging is less likely to accumulate within the electrode body, thereby increasing energy density and the discharge capacity maintenance rate during the charge-discharge cycle.

[0022] It is preferable that the above pressure P1 is between 1.30 MPa and 2.00 MPa. Setting the above pressure P1 above the lower limit can increase energy density, discharge capacity maintenance rate during charge-discharge cycles, etc. On the other hand, setting the above pressure P1 below the upper limit suppresses excessive pressure on the electrode body during the charging state, and further suppresses the decrease in discharge capacity at high current density.

[0023] The average thickness of the separator is preferably 3 μm or more and 100 μm or less. Having an average thickness of the separator above the lower limit ensures reliable electrical isolation between the positive and negative electrodes, and the expansion of the negative electrode thickness due to charging can be sufficiently absorbed by the compression of the separator, thereby significantly reducing the difference between the pressure P1 and the a-pressure P0 (P1-P0). Furthermore, having an average thickness of the separator below the upper limit allows for increased energy density of the energy storage element.

[0024] Preferably, the separator has a base layer and a coating layer containing particles and a binder formed on one or both surfaces of the base layer. In this case, the strength of the separator can be increased.

[0025] Another embodiment of the present invention provides a power storage device comprising the power storage element described above. Preferably, the power storage device comprises the power storage element constrained to a fixed size. In this case, the effect of suppressing the decrease in discharge capacity at high current densities can be enjoyed to the fullest extent.

[0026] An automobile according to another aspect of the present invention is equipped with the above-mentioned energy storage element.

[0027] Another embodiment of the present invention provides a power supply for electronic equipment, comprising the above-mentioned energy storage element.

[0028] Another aspect of the present invention provides an energy storage element comprising an electrode body in which a positive electrode and a negative electrode are stacked with a separator in between, an electrolyte, and a container for housing the electrode body and the electrolyte, wherein the electrode body is under load, and the difference between the pressure P0 applied to the electrode body in the discharge state and the pressure P1 applied to the electrode body in the charge state is 0.90 MPa or less.

[0029] Another aspect of the present invention relates to an energy storage element in which a load is applied to the electrode body, and the decrease in discharge capacity at high current densities is suppressed. The reason for this effect is not clear, but the following reason is presumed. In the energy storage element according to another aspect of the present invention, the difference (P1-P0) between the pressure P0 applied to the electrode body in the discharge state and the pressure P1 applied to the electrode body in the charge state is small, at 0.90 MPa or less, so even in the charge state, a decrease in the conductivity of the negative electrode and leakage of electrolyte from the void of the negative electrode are unlikely to occur. For this reason, it is presumed that, according to the energy storage element according to another aspect of the present invention, the decrease in discharge capacity at high current densities is suppressed despite the load being applied to the electrode body.

[0030] This document describes in detail an energy storage element, an energy storage device, a method for manufacturing an energy storage element, and other embodiments related to one embodiment of the present invention. Note that the names of the components (each element) used in each embodiment may differ from the names of the components (each element) used in the background art.

[0031] <Energy storage element> An energy storage element according to one embodiment of the present invention comprises an electrode body having a positive electrode, a negative electrode, and a separator, an electrolyte, and a container for housing the electrode body and the electrolyte. The positive electrode and the negative electrode are stacked with a separator in between. The electrode body is usually a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with a separator in between, or a wound type in which the positive electrode and the negative electrode are wound in a stacked state with a separator in between. The electrolyte exists in a state of impregnation of the positive electrode, the negative electrode, and the separator. As an example of an energy storage element, a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as a "secondary battery") will be described.

[0032] (positive electrode) The positive electrode comprises a positive electrode substrate and a positive electrode active material layer disposed directly on the positive electrode substrate or via an intermediate layer.

[0033] The positive electrode substrate is conductive. Whether or not it is conductive is determined by the volume resistivity measured in accordance with JIS-H-0505 (1975), which is 10 7 The determination is made using Ω·cm as the threshold. The positive electrode substrate material can be a metal such as aluminum, titanium, tantalum, or stainless steel, or an alloy thereof. Among these, aluminum or an aluminum alloy is preferred from the viewpoint of high potential resistance, high conductivity, and cost. Examples of positive electrode substrates include foil, vapor-deposited film, mesh, and porous material, 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).

[0034] 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, it is possible to increase the strength of the positive electrode substrate while increasing the energy density per unit volume of the secondary battery.

[0035] 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. For example, it contains a binder and a conductive agent.

[0036] The positive electrode active material layer contains a positive electrode active material. The positive electrode active material layer may contain optional components such as a conductive agent, a binder, a thickener, and a filler as required.

[0037] The positive electrode active material can be appropriately selected from known positive electrode active materials. As the positive electrode active material for a lithium-ion secondary battery, a material that can occlude and release lithium ions is usually used. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO2-type crystal structure, lithium transition metal composite oxides having a spinel-type crystal structure, polyanion compounds, chalcogen compounds, sulfur, etc. Examples of the lithium transition metal composite oxide having an α-NaFeO2-type 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), etc. Examples of the lithium transition metal composite oxide having a spinel-type crystal structure include Li x Mn2O4, Li x Ni γ Mn (2-γ)Examples include O4. Examples of polyanion compounds include LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, Li2CoPO4F, etc. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, molybdenum dioxide, etc. Some atoms or polyanions in these materials may be substituted with atoms or anions 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 mixture form.

[0038] 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. Setting the average particle size of the positive electrode active material above the lower limit makes it easier to manufacture or handle 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 material of the positive electrode active material and other materials is used, the average particle size of the composite material is used as the average particle size of the positive electrode active material. "Average particle size" refers to the value at which the volume-based integrated distribution calculated in accordance with JIS-Z-8819-2 (2001), based on the particle size distribution measured by laser diffraction / scattering method on a dilution of particles diluted with a solvent, in accordance with JIS-Z-8825 (2013), becomes 50%.

[0039] To obtain powder with a predetermined particle size, grinders and classifiers are used. Examples of grinding methods include using mortars, ball mills, sand mills, vibrating ball mills, planetary ball mills, jet mills, counter-jet mills, swirling airflow jet mills, or sieves. Wet grinding, which involves the coexistence of water or organic solvents such as hexane, can also be used during grinding. For classification, sieves and wind classifiers are used as needed, both dry and wet.

[0040] The content of the positive electrode active material in the positive electrode active material layer is preferably 50% to 99% by mass, more preferably 70% to 98% by mass, and even more preferably 80% to 95% by mass. By setting the content of the positive electrode active material within the above range, it is possible to achieve both high energy density and manufacturability in the positive electrode active material layer.

[0041] The conductive agent is not particularly limited as long as it is a conductive material. 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 can take the form of powder or fiber. One of these materials may be used alone as the conductive agent, or two or more may be used in mixture form. These materials may also be used in composite form. For example, a composite material of carbon black and CNTs may be used. Among these, carbon black is preferred from the viewpoint of electronic conductivity and coating properties, and acetylene black is particularly preferred.

[0042] 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 setting the content of the conductive agent within the above range, the energy density of the secondary battery can be increased.

[0043] 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.

[0044] 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.

[0045] Examples of thickening agents include polysaccharide polymers such as carboxymethylcellulose (CMC) and methylcellulose. If the thickening agent has a functional group that reacts with lithium or the like, this functional group may be deactivated beforehand by methylation or the like.

[0046] 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 resource-derived materials such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof.

[0047] The positive electrode active material layer may contain typical nonmetallic 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.

[0048] (Negative electrode) The negative electrode comprises a negative electrode substrate and a negative electrode active material layer disposed directly on the negative electrode substrate or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from, for example, the configurations exemplified in the positive electrode.

[0049] The negative electrode substrate is electrically conductive. Suitable materials for the negative electrode substrate include metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, or alloys thereof, as well as 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.

[0050] 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, it is possible to increase the strength of the negative electrode substrate while increasing the energy density per unit volume of the secondary battery.

[0051] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer optionally contains conductive agents, binders, thickeners, fillers, and other optional components. These optional components can be selected from the materials exemplified above for the positive electrode.

[0052] The negative electrode active material layer may contain typical nonmetallic 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 metallic 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.

[0053] The negative electrode active material can be appropriately selected from known negative electrode active materials. For lithium-ion secondary batteries, materials capable of intercalating and releasing lithium ions are typically used as negative electrode active materials. Examples of negative electrode active materials include: metallic Li; metals or metalloids such as Si and Sn; metal oxides or metalloid oxides such as Si oxide, Ti oxide, and Sn oxide; and Li4Ti5O 12Examples of materials include titanium-containing oxides such as LiTiO2 and TiNb2O7; polyphosphate compounds; silicon carbide; and carbon materials such as graphite and non-graphitizable carbon (easily graphitizable carbon or poorly graphitizable carbon). Among these materials, graphite and non-graphitizable carbon are preferred. In the negative electrode active material layer, one of these materials may be used alone, or two or more may be used in mixture form.

[0054] "Graphite" refers to the average lattice plane spacing (d) of the (002) plane, determined by X-ray diffraction before charging or discharging, or during the discharge state. 002 ) refers to carbon materials with a n-scale between 0.33 nm and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the standpoint of obtaining materials with stable physical properties.

[0055] "Non-graphite carbon" refers to the average lattice plane spacing (d) of the (002) plane, which is determined by X-ray diffraction before charging or during the discharge state. 002 This refers to carbon materials with a nautical radius of 0.34 nm or more and 0.42 nm or less. Non-graphitized carbons include poorly graphitizable carbons and easily graphitizable carbons. Examples of non-graphitized carbons include resin-derived materials, petroleum pitch or materials derived from petroleum pitch, petroleum coke or materials derived from petroleum coke, plant-derived materials, and alcohol-derived materials.

[0056] Here, the "discharge state" of the carbon material refers to a state in which sufficient lithium ions that can be absorbed and released during charging and discharging are released from the carbon material, which is the negative electrode active material. For example, in a monoelectrode battery using a negative electrode containing a carbon material as the negative electrode active material as the working electrode and metallic Li as the counter electrode, this is the state in which the open-circuit voltage is 0.7V or higher.

[0057] "Non-graphitizable carbon" refers to the above d 002 This refers to carbon materials with a wavelength between 0.36 nm and 0.42 nm.

[0058] "Easily graphitizable carbon" refers to the above d 002 This refers to carbon materials with a wavelength of 0.34 nm or more and less than 0.36 nm.

[0059] 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, between 1 nm and 100 μm. If the negative electrode active material is a carbon material, titanium-containing oxide, or polyphosphate compound, its average particle size may be between 1 μm and 100 μm. If the negative electrode active material is Si, Sn, Si oxide, or Sn oxide, its average particle size may be between 1 nm and 1 μm. Setting the average particle size of the negative electrode active material above the lower limit makes it easier to manufacture or handle. Setting the average particle size of the negative electrode active material below the upper limit improves the electronic conductivity of the active material layer. To obtain powder with a predetermined particle size, a pulverizer or classifier is used. The pulverizing method and classification method can be selected from, for example, the methods exemplified above for the positive electrode. If the negative electrode active material is a metal such as metallic Li, the negative electrode active material may be in the form of foil.

[0060] 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 setting the content of the negative electrode active material within the above range, it is possible to achieve both high energy density and manufacturability in the negative electrode active material layer.

[0061] (Separator) The separator can be appropriately selected from known separators. Examples of separators include a separator consisting only of a base layer, or a separator in which a coating layer containing particles and a binder is formed on one or both sides of the base layer. Examples of the base layer form of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these forms, porous resin film is preferred from the viewpoint of strength, and nonwoven fabric is preferred from the viewpoint of liquid retention of non-aqueous electrolytes. From the viewpoint of shutdown function, polyolefins such as polyethylene and polypropylene are preferred as the material for the base layer of the separator, and from the viewpoint of oxidative degradation resistance, polyimide and aramid are preferred. A composite material of these resins may also be used as the base layer of the separator. The base layer may consist of two or more layers.

[0062] From the viewpoint of heat resistance, the particles contained in the coating layer are preferably those whose mass loss when heated from room temperature to 500°C in an air atmosphere of 1 atmosphere is 5% or less, and more preferably those whose mass loss when heated from room temperature to 800°C is 5% or less. Inorganic compounds are examples of materials whose mass loss is below a predetermined level. The coating layer may be an inorganic particle layer in which particles of an inorganic compound are used as the particles. 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; covalent crystals such as silicon and diamond; mineral resource-derived materials or artificial products thereof such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica. These inorganic compounds may be used individually or in combination, or two or more may be used in mixtures. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicates are preferred from the viewpoint of safety of the energy storage element.

[0063] The porosity of the separator is preferably 80 volume% or less from the viewpoint of strength, and preferably 20 volume% or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value and means the measurement value obtained using a mercury porosimeter.

[0064] The air permeability resistance of the separator is preferably 50 seconds / 100 mL to 250 seconds / 100 mL, more preferably 80 seconds / 100 mL to 240 seconds / 100 mL, and even more preferably 120 seconds / 100 mL to 230 seconds / 100 mL. When the air permeability resistance of the separator is within the above range, the decrease in discharge capacity at high current densities when a sufficient load is applied to the electrode body is further suppressed. In particular, when the air permeability resistance of the separator is below the above upper limit, sufficient ion permeability can be ensured even when the separator is compressed, so the decrease in discharge capacity at high current densities is further suppressed.

[0065] When the separator is compressed in the thickness direction at a pressure of 2 MPa in an environment of 45°C, the strain A is preferably 0.05 to 0.3, more preferably 0.10 to 0.25, and even more preferably 0.20 or less. When the strain A is above the lower limit, sufficient compression occurs when a load is applied to the separator in the thickness direction, thus further suppressing the decrease in discharge capacity at high current densities when a sufficient load is applied to the electrode body. On the other hand, when the strain A is below the upper limit, excessive compression occurs when a load is applied to the separator in the thickness direction, which reduces ion permeability, and thus further suppresses the decrease in discharge capacity at high current densities when a sufficient load is applied to the electrode body. The strain A can be adjusted by the material, structure, porosity, etc., of the separator.

[0066] The average thickness of the separator is preferably 3 μm to 100 μm, more preferably 5 μm to 50 μm, and even more preferably 10 μm to 30 μm. Having an average separator thickness above the lower limit ensures reliable electrical isolation between the positive and negative electrodes, and the expansion of the negative electrode thickness during charging is sufficiently absorbed by the compression of the separator, thereby significantly reducing the pressure difference (P1-P0). Furthermore, having an average separator thickness below the upper limit allows for increased energy density of the secondary battery.

[0067] (Load on the electrode) The electrode body is subjected to a load in all states, from the discharge state to the charge state. This load on the electrode body is in the thickness direction of each of the stacked positive electrode, negative electrode, and separator, i.e., in the stacking direction. Note that there may be parts of the electrode body, such as the edges, that are not subjected to a load. In this secondary battery, the energy density can be increased by applying a load to the electrode body in this way, and the discharge capacity retention rate in the charge-discharge cycle can be increased. The reason why the discharge capacity retention rate is increased by applying a load to the electrode body is not entirely clear, but it is speculated that it is because the gaps within the electrode body are reduced, making it less likely for gases generated during charging and discharging to accumulate inside the electrode body. The load on the electrode body can be applied by a pressurizing member, etc., as described later.

[0068] In one embodiment of the present invention, the difference (P1-P0) between the pressure P0 applied to the electrode body in the discharge state and the pressure P1 applied to the electrode body in the charge state is 0.90 MPa or less. The above pressure difference (P1-P0) is preferably 0.85 MPa or less, and more preferably 0.80 MPa or less. By keeping the pressure difference (P1-P0) below the above upper limit, the decrease in discharge capacity at high current density is suppressed. The lower limit of the above pressure difference (P1-P0) may be 0 MPa, or it may be 0.20 MPa, 0.40 MPa, or 0.60 MPa.

[0069] The pressure P0 applied to the electrode body in the discharge state may be, for example, 0.10 MPa or more and 2.00 MPa or less, but is preferably 0.40 MPa or more and 1.40 MPa or less, more preferably 0.60 MPa or more and 1.10 MPa or less, and even more preferably 0.80 MPa or more and 1.00 MPa or less. By setting the pressure P0 above the lower limit, it is possible to increase the energy density, the discharge capacity maintenance rate in the charge-discharge cycle, etc. On the other hand, by setting the pressure P0 below the upper limit, excessive pressure is suppressed on the electrode body in all states from the discharge state to the charge state, and the decrease in discharge capacity at high current density is further suppressed.

[0070] The pressure P1 applied to the electrode body in the charged state may be, for example, 0.50 PMa or more and 2.90 MPa or less, but is preferably 0.80 MPa or more and 2.30 MPa or less, more preferably 1.30 MPa or more and 2.00 MPa or less, and even more preferably 1.40 MPa or more and 1.60 MPa or less. Setting the pressure P1 above the lower limit can increase the energy density, the discharge capacity maintenance rate in the charge-discharge cycle, etc. On the other hand, setting the pressure P1 below the upper limit can suppress excessive pressure on the electrode body in the charged state, and further suppress the decrease in discharge capacity at high current density.

[0071] In one embodiment of the present invention, the pressure P0 (MPa) applied to the electrode body in the discharge state and the strain A when the separator is compressed in the thickness direction at a pressure of 2 MPa in an environment of 45°C satisfy the following equation 1. P0(1-A)<1.2(MPa) ···1

[0072] The upper limit of P0(1-A) is preferably 1.1, and may be more preferably 1.0, 0.9, 0.8, or 0.7. When P0(1-A) is less than or equal to the above upper limit, the increase in pressure applied to the electrode body due to charging, i.e., the pressure difference (P1-P0), becomes small, thus suppressing the decrease in discharge capacity at high current density. The lower limit of P0(1-A) is greater than 0, and may be 0.1, 0.3, 0.5, or 0.7.

[0073] (Non-aqueous electrolyte) A non-aqueous electrolyte secondary battery comprises a non-aqueous electrolyte. The non-aqueous electrolyte can be appropriately selected from known non-aqueous electrolytes. Typically, a non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in this non-aqueous solvent.

[0074] As the non-aqueous solvent, it 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. As the non-aqueous solvent, compounds in which some of the hydrogen atoms contained in these compounds are substituted with halogens may also be used.

[0075] 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, and 1,2-diphenylvinylene carbonate. Among these, EC is preferred.

[0076] Examples of linear carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, and bis(trifluoroethyl) carbonate. Among these, EMC is preferred.

[0077] It is preferable to use a cyclic carbonate or a linear carbonate as the non-aqueous solvent, and it is more preferable to use a cyclic carbonate and a linear carbonate in combination. Using a cyclic carbonate can promote the dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. Using a linear carbonate can keep the viscosity of the non-aqueous electrolyte low. When using a cyclic carbonate and a linear carbonate in combination, the volume ratio of the cyclic carbonate to the linear carbonate (cyclic carbonate:linear carbonate) is preferably in the range of 5:95 to 50:50.

[0078] 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. Among these, lithium salts are preferred.

[0079] Examples of lithium salts 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 difluorooxalate borate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); and lithium salts having halogenated hydrocarbon groups 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.

[0080] The electrolyte salt content in the non-aqueous electrolyte is 0.1 mol / dm³ at 20°C and 1 atm. 3 More than 2.5mol / dm 3 Preferably, it is 0.3 mol / dm³ 3 More than 2.0mol / dm 3 It is more preferable that it be less than or equal to 0.5 mol / dm 3 More than 1.7mol / dm 3 It is even more preferable that the following is the case: 0.7 mol / dm 3 More than 1.5mol / dm 3 The following is particularly preferable. By setting the electrolyte salt content within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0081] Non-aqueous electrolytes may contain additives in addition to the non-aqueous solvent and electrolyte salt. Examples of additives include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partially halides of the aforementioned 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, succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic acid anhydride; ethylene sulfite, propylene sulfite, sulfurous acid Examples include dimethyl acid, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethylsulfone, dimethyl sulfoxide, diethylsulfoxide, 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-propensultone, 1,3-propanesultone, 1,4-butanesultone, 1,4-butensultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakithtrimethylsilyl titanate, lithium monofluorophosphate, and lithium difluorophosphate. These additives may be used individually or in combination of two or more.

[0082] The additive content in the non-aqueous electrolyte is preferably 0.01% to 10% by mass relative to the total mass of the non-aqueous electrolyte, more preferably 0.1% to 7% by mass, even more preferably 0.2% to 5% by mass, and particularly preferably 0.3% to 3% by mass. By setting the additive content within the above range, it is possible to improve the capacity retention performance or cycle performance after high-temperature storage, and to further improve safety.

[0083] (container) The container is a sealed container that houses the electrode body and the non-aqueous electrolyte. The material of the container can be any material that has the sealing properties to seal the electrode body and the non-aqueous electrolyte, and the strength to protect the electrode body, for example, it may be resin or metal. Alternatively, the container may be a flexible bag or other container formed from, for example, a composite film in which heat-weldable resin and metal foil are laminated.

[0084] The shape of the energy storage element in this embodiment is not particularly limited, and examples include cylindrical batteries, prismatic batteries, flat batteries, coin-type batteries, button-type batteries, and the like.

[0085] Figure 1 shows an example of a rectangular battery, specifically a storage element 1. The figure is a transparent view of the inside of the container. An electrode body 2, having a positive electrode and a negative electrode wound around a separator, is housed in a rectangular container 3. The container 3 further contains an electrolyte (not shown), such as a non-aqueous electrolyte. The positive electrode is electrically connected to the positive electrode terminal 4 via a positive electrode lead 41. The negative electrode is electrically connected to the negative electrode terminal 5 via a negative electrode lead 51.

[0086] The load on the electrode body 2 in the energy storage element 1 shown in Figure 1 can be applied, for example, by a pressurizing member (not shown) that pressurizes the container 3 from the outside. The pressurizing member may be a restraining member that constrains the shape of the container 3. The pressurizing member (restraining member) is provided to pressurize the electrode body 2 by sandwiching it from both sides (the front side and the back side in Figure 1) in the thickness direction (stacking direction: Y direction in Figure 1) via the container 3. Both of the above-mentioned sides of the electrode body 2 are in contact with the inner surface of the container 3, either directly or via other members (not shown). Therefore, when the container 3 is pressurized, a load is applied to the electrode body 2. It is preferable that both of the above-mentioned sides of the electrode body 2 are in contact with the inner surface of the container 3, either directly or via other members (not shown), when the container 3 is not pressurized. Examples of pressurizing members (restraining members) include restraining bands and metal frames. Alternatively, as shown in Figure 2, multiple energy storage elements 1 may be arranged in the stacking direction of the electrode body 2 (Y direction in Figure 1, left-right direction in Figure 2), and the multiple energy storage elements 1 may be fixed using a restraining member 21 such as a frame while being pressurized from both ends of this stacking direction.

[0087] As a restraining member for restricting the shape of an energy storage element, a member that restrains it to a fixed size (a certain dimension) is known. Restraining to a fixed size means restraining the energy storage element so that its size and shape do not change substantially. When a restraining member that restrains to a fixed size is used, the restraining member and the energy storage element do not deform substantially after being attached to the energy storage element, which has advantages such as ease of design. However, when a restraining member that restrains to a fixed size is used, the load applied to the electrode body tends to increase due to the expansion of the energy storage element during charging, so the above pressure difference (P1-P0) generally tends to become larger. For this reason, when one embodiment of the present invention is applied to an energy storage element using a restraining member that restrains to a fixed size, the effect of suppressing the decrease in discharge capacity at high current density can be enjoyed to the fullest extent.

[0088] <Energy storage device> The energy storage element of this embodiment can be mounted as an energy storage unit (battery module) composed of multiple energy storage elements in power supplies for vehicles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), power supplies for electronic devices such as personal computers and communication terminals, or power storage devices. In this case, it is sufficient that the technology of the present invention is applied to at least one of the energy storage elements included in the energy storage device.

[0089] Figure 2 shows an example of a power storage device 30 which is formed by further assembling power storage units 20, each of which is an assembly of two or more electrically connected power storage elements 1. In the power storage unit 20, multiple power storage elements 1 are arranged without gaps in the thickness direction and are constrained by a fixed-size restraining member 21 so as to maintain a constant thickness and to apply pressure from the thickness direction (left-right direction in Figure 2). The power storage device 30 may include busbars (not shown) that electrically connect two or more power storage elements 1, busbars (not shown) that electrically connect two or more power storage units 20, etc. The power storage unit 20 or the power storage device 30 may include a condition monitoring device (not shown) that monitors the state of one or more power storage elements.

[0090] <Manufacturing method for energy storage elements> The method for manufacturing the energy storage element of this embodiment can be appropriately selected from known methods. This manufacturing method includes, for example, preparing an electrode body, preparing an electrolyte such as a non-aqueous electrolyte, and housing the electrode body and electrolyte in a container. Preparing the electrode body includes preparing a positive electrode and a negative electrode, and forming the electrode body by stacking or winding the positive electrode and negative electrode via a separator.

[0091] The method for housing the electrolyte in a container can be appropriately selected from known methods. For example, the electrolyte can be injected through an inlet formed in the container, and then the inlet can be sealed. The manufacturing method may further include attaching a pressurizing member such as a restraining member. Furthermore, a manufacturing method for an energy storage device comprising multiple energy storage elements may include attaching pressurizing members such as restraining members to the multiple energy storage elements.

[0092] <Other Embodiments> Furthermore, the energy storage element of the present invention is not limited to the embodiments described above, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment may be added to the configuration of another embodiment, and a part of the configuration of one embodiment may be replaced with the configuration of another embodiment or with well-known technology. In addition, a part of the configuration of one embodiment may be deleted. Also, well-known technology may be added to the configuration of one embodiment.

[0093] In the above embodiment, the case in which the energy storage element is used as a rechargeable non-aqueous electrolyte secondary battery (e.g., a lithium-ion secondary battery) has been described, but the type, shape, dimensions, capacity, etc. of the energy storage element are arbitrary. The present invention can also be applied to various secondary batteries, electric double-layer capacitors, or capacitors such as lithium-ion capacitors. [Examples]

[0094] 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.

[0095] [Example 1] (Fabrication of the positive electrode) LiNi 0.6 Co 0.2 Mn 0.2 A positive electrode mixture paste was prepared using O2, acetylene black (AB) as a conductive agent, polyvinylidene fluoride (PVDF) as a binder, and N-methylpyrrolidone (NMP) as a dispersion medium. The mass ratio of the positive electrode active material, AB, and PVDF was 90:5:5 (on a solid content basis). The positive electrode mixture paste was applied to both sides of an aluminum foil substrate and dried. After that, a roll press was performed to obtain the positive electrode.

[0096] (Fabrication of the negative electrode) A negative electrode mixture paste was prepared by mixing graphite (negative electrode active material), styrene-butadiene rubber (SBR) (binder), carboxymethylcellulose (CMC) (thickener), and water (dispersion medium). The mass ratio of the negative electrode active material, SBR, and CMC was 96:2:2 (based on solid content). The negative electrode mixture paste was applied to both sides of a copper foil, which served as the negative electrode substrate, and dried. Subsequently, a roll press was performed to obtain the negative electrode.

[0097] (Non-aqueous electrolyte) A solvent containing ethylene carbonate, propylene carbonate, and ethyl methyl carbonate in a volume ratio of 20:10:70 was mixed, and 1.2 mol / dm³ was added. 3 LiPF6 was dissolved at the specified concentration to obtain a non-aqueous electrolyte.

[0098] (Separator) For the separator, we used separator A, which consists of a base layer made of a porous polyethylene resin film with a coating layer formed of inorganic particles and a binder laminated on one side. The average thickness of separator A was 21 μm, the air permeability resistance was 90 seconds / 100 mL, and the strain A (strain when compressed in the thickness direction at a pressure of 2 MPa in a 45°C environment) was 0.18.

[0099] (Assembly of energy storage elements) An electrode body was obtained by laminating the positive electrode, negative electrode, and separator A described above. The electrode body was sealed in a container made of a composite film of heat-weldable resin and aluminum foil, and a non-aqueous electrolyte was injected and the container was sealed. A restraining member that constrains the electrode body to a fixed size was attached, and a load was applied so that the pressure P0 applied to the electrode body in the discharge state was 1.31 MPa, thereby obtaining the energy storage element of Example 1.

[0100] [Examples 2 to 7, Comparative Examples 1 to 4] Except for changing the type of separator used and the pressure P0 applied to the electrode body during the discharge state as shown in Table 1, the energy storage elements of Examples 2 to 7 and Comparative Examples 1 to 4 were obtained in the same manner as in Example 1.

[0101] Table 1 shows the material of the base layer, presence or absence of a coating layer, average thickness, air permeability resistance, strain A (strain when compressed in the thickness direction at a pressure of 2 MPa in a 45°C environment), and the value of P0(1-A) for each separator. Strain A was not measured for separators B and C. In Table 1, PE represents polyethylene and PP represents polypropylene. PP / PE / PP indicates a three-layer base layer structure in which PP, PE, and PP are laminated in that order. The coating layer in all cases is a layer formed from inorganic particles and a binder.

[0102] [evaluation] (Measurement of pressure P1 applied to the electrode body in the charged state) For each energy storage element in the examples and comparative examples, an initial charge-discharge process was performed at 25°C under predetermined conditions. Then, at 45°C, constant current charging was performed with a current of 1.0C up to 4.25V, followed by constant voltage charging at 4.25V to reach the charged state. The total charging time for constant current charging and constant voltage charging was 3 hours. The pressure P1 applied to the electrode body was then measured in the charged state. The measured pressure P1 and the pressure difference (P1-P0) are shown in Table 1.

[0103] (Measurement of discharge capacity at high current density) For each energy storage element in Examples 1 to 4 and Comparative Examples 1 to 4, which were charged by the measurement of the pressure P1 described above, a constant current discharge was performed at 45°C with a current of 3.0C down to 2.75V, and the discharge capacity X was determined. Furthermore, for each energy storage element in Examples 1 to 4 and Comparative Examples 1 to 4, with virtually no load applied to the electrode body, the discharge capacity Y at a current of 3.0C was determined in the same manner as above. The discharge capacity ratio (%) was calculated as the percentage of discharge capacity X to discharge capacity Y for each energy storage element. The results are shown in Table 1.

[0104] Furthermore, Figure 3 shows the relationship between the pressure difference (P1-P0) and the discharge capacity ratio in each energy storage element of Examples 1 to 4 and Comparative Examples 1 to 4. Figure 4 shows the relationship between P0(1-A) and the pressure difference (P1-P0) in each energy storage element of Examples 1, 4 to 7 and Comparative Examples 1 and 4.

[0105] [Table 1]

[0106] As shown in Table 1 and Figure 3, the discharge capacity ratio shows a high correlation with the pressure difference (P1-P0) between the charged and discharged states. When the pressure difference (P1-P0) is 0.90 MPa or less, the discharge capacity ratio is 86% or more. In other words, when the pressure difference (P1-P0) is 0.90 MPa or less, the decrease in discharge capacity at high current densities in energy storage elements with applied loads to the electrodes can be sufficiently suppressed.

[0107] Furthermore, as shown in Table 1 and Figure 4, the pressure difference between the charging state and the discharging state (P1-P0) shows a high correlation with P0(1-A), and when P0(1-A) is less than 1.2, the pressure difference (P1-P0) is 0.90 MPa or less. In other words, when P0(1-A) is less than 1.2, the pressure difference (P1-P0) is 0.90 MPa or less, which indicates that the decrease in discharge capacity at high current density in energy storage elements with a load applied to the electrode body can be sufficiently suppressed.

[0108] [Reference example 1] To confirm the difference in discharge capacity maintenance rate depending on whether or not a load is applied to the electrode body, the following reference example was performed.

[0109] (Fabrication of the positive electrode) LiNi 0.6 Co 0.2 Mn 0.2 A positive electrode mixture paste was prepared using O2, acetylene black (AB) as a conductive agent, polyvinylidene fluoride (PVDF) as a binder, and N-methylpyrrolidone (NMP) as a dispersion medium. The mass ratio of the positive electrode active material, AB, and PVDF was 90:5:5 (on a solid content basis). The positive electrode mixture paste was applied to both sides of an aluminum foil substrate and dried. After that, a roll press was performed to obtain the positive electrode.

[0110] (Fabrication of the negative electrode) A negative electrode mixture paste was prepared by mixing graphite (negative electrode active material), styrene-butadiene rubber (SBR) (binder), carboxymethylcellulose (CMC) (thickener), and water (dispersion medium). The mass ratio of the negative electrode active material, SBR, and CMC was 96:2:2 (based on solid content). The negative electrode mixture paste was applied to both sides of a copper foil, which served as the negative electrode substrate, and dried. Subsequently, a roll press was performed to obtain the negative electrode.

[0111] (Non-aqueous electrolyte) A solvent prepared by mixing ethylene carbonate, propylene carbonate, and ethyl methyl carbonate in a volume ratio of 25:5:70, with 1.0 mol / dm³ added. 3 LiPF6 was dissolved at the specified concentration to obtain a non-aqueous electrolyte.

[0112] (Separator) For the separator, a separator (overall average thickness 16 μm) was used, in which a base layer (average thickness 12 μm, porosity 40 vol%) made of a porous polyolefin resin film was laminated on one side, with a coating layer (thickness 4 μm, porosity 70 vol%) formed of inorganic particles and a binder.

[0113] (Assembly of energy storage elements) An electrode body was obtained by stacking the positive electrode, negative electrode, and separator described above. The electrode body was sealed in a container, and a non-aqueous electrolyte was injected and the container was sealed. A restraining member that constrains the electrode body at a fixed size was attached, and a load was applied so that the pressure P0 applied to the electrode body in the discharge state was 0.1 MPa, thereby obtaining the energy storage element of Reference Example 1.

[0114] [Reference example 2] The energy storage element of Reference Example 2 was obtained in the same manner as in Reference Example 1, except that a restraining member was attached to constrain the electrode body to a fixed length so that no load was applied to it.

[0115] (Charge-discharge cycle test) For each energy storage element in Reference Examples 1 and 2, an initial charge-discharge process was performed at 25°C under specified conditions, followed by the following charge-discharge cycle test. At 60°C, constant current charging was performed with a current of 1.0C up to 4.25V, and then constant voltage charging was performed at 4.25V. The charging termination condition was set to a total charging time of 3 hours. After that, a 10-minute rest period was provided. Constant current discharge was performed with a current of 1.0C up to 2.75V, followed by a 10-minute rest period. These charging and discharging processes constituted one cycle, and 900 cycles were performed. Figure 5 shows a graph illustrating the discharge capacity retention rate based on the discharge capacity of the first cycle in the above charge-discharge cycle test.

[0116] As shown in Figure 5, the energy storage element in Reference Example 1, which is fitted with a restraining member that constrains the electrode body to a fixed size so that a load is applied to it, exhibits a higher discharge capacity maintenance rate during the charge-discharge cycle compared to the energy storage element in Reference Example 2, which is fitted with a restraining member that constrains the electrode body to a fixed size so that no load is applied to it. [Industrial applicability]

[0117] This invention can be applied to electronic devices such as personal computers and communication terminals, as well as energy storage elements used as power sources for automobiles and the like. [Explanation of Symbols]

[0118] 1. Energy storage element 2 Electrode body 3 containers 4 Positive terminal 41 Positive lead 5 Negative terminal 51 Negative lead 20 Energy storage units 21 Restraining member 30 Energy storage devices

Claims

1. An electrode body in which a positive electrode and a negative electrode are stacked with a separator in between, Electrolyte, and A container for housing the electrode body and the electrolyte. Equipped with, The electrode body described above is in a state where a load is applied. Pressure P applied to the electrode body in the discharge state ０ A storage element in which (MPa) and the strain A obtained when the above separator is compressed in the thickness direction at a pressure of 2 MPa in an environment of 45°C satisfy the following equation 1. P ０ (1-A) <1.2 (MPa) ・・・1

2. The above pressure P ０ The pressure P applied to the electrode body in the charged state is... １ The energy storage element according to claim 1, wherein the difference is 0.90 MPa or less.

3. The energy storage element according to claim 1 or claim 2, wherein the above-mentioned strain A is 0.05 or more and 0.3 or less.

4. The energy storage element according to claim 1, claim 2, or claim 3, wherein the air permeability resistance of the separator is 250 seconds / 100 mL or less.

5. The above pressure P ０ A storage element according to any one of claims 1 to 4, wherein the current is 0.40 MPa or more and 1.40 MPa or less.

6. The above pressure P １ The energy storage element according to claim 2, wherein the current is 1.30 MPa or more and 2.00 MPa or less.

7. The energy storage element according to any one of claims 1 to 6, wherein the average thickness of the separator is 3 μm or more and 100 μm or less.

8. The energy storage element according to any one of claims 1 to 7, wherein the separator comprises a base layer and a coating layer formed on one or both surfaces of the base layer and containing particles and a binder.

9. A power storage device comprising a power storage element according to any one of claims 1 to 8, which is constrained to a fixed size.

10. An automobile equipped with an energy storage element according to any one of claims 1 to 8.

11. A power supply for electronic equipment comprising an energy storage element according to any one of claims 1 to 8.

12. An electrode body in which a positive electrode and a negative electrode are stacked with a separator in between, Electrolyte, and A container for housing the electrode body and the electrolyte. Equipped with, The electrode body described above is in a state where a load is applied. Pressure P applied to the electrode body in the discharge state ０ The pressure P applied to the electrode body in the charged state is... １ A storage element whose difference from the given value is 0.90 MPa or less.

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