Power storage element

Abstract

A power storage element according to one aspect of the present invention is provided with a positive electrode that includes a transition metal element, a negative electrode that comprises a negative electrode active material layer, and a separator. The positive electrode and the negative electrode are layered with the separator interposed therebetween. The separator comprises an inorganic layer that includes inorganic particles. The BET specific surface area of the negative electrode active material layer is at least 2 m2 / g. The mass per unit area of the inorganic layer is at least 0.2 mg / cm2.

Description

Energy storage element

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

[0002] Non-aqueous electrolyte secondary batteries, such as lithium-ion batteries, are widely used in electronic devices like personal computers and communication terminals, as well as automobiles, due to their high energy density. Generally, non-aqueous electrolyte secondary batteries consist of a pair of electrodes electrically isolated by a separator, and a non-aqueous electrolyte interposed between these electrodes. They are configured to charge and discharge by transferring charge-transporting ions, such as lithium ions, between the two electrodes. Other energy storage elements, such as lithium-ion capacitors and electric double-layer capacitors, are also widely used.

[0003] In some cases, a composite oxide containing a transition metal element is used as the positive electrode active material for an energy storage element (Patent Document 1).

[0004] Japanese Patent Application Publication No. 2016-173886

[0005] The inventors of this invention have been considering increasing the surface area of ​​the negative electrode in order to increase the initial output of the energy storage element. However, when the surface area of ​​the negative electrode is increased and a composite oxide containing a transition metal element is used as the positive electrode active material, phenomena such as a decrease in output after long-term storage at high temperatures have been observed.

[0006] The objective of the present invention is to provide an energy storage element that can suppress the decrease in output after long-term storage at high temperatures.

[0007] An energy storage element according to one aspect of the present invention comprises a positive electrode containing a transition metal element, a negative electrode having a negative electrode active material layer, and a separator, wherein the positive electrode and the negative electrode are laminated via the separator, the separator has an inorganic layer containing inorganic particles, and the BET specific surface area of ​​the negative electrode active material layer is 2 m². ２ The amount is 0.2 mg / cm² or more, and the mass per unit area of ​​the inorganic layer is 0.2 mg / cm². ２ That's all.

[0008] According to one aspect of the present invention, it is possible to provide an energy storage element that can suppress the decrease in output after long-term storage at high temperatures.

[0009] Figure 1 is a perspective view showing an energy storage element according to one embodiment of the present invention. Figure 2 is a schematic diagram showing an energy storage device equipped with multiple energy storage elements according to one embodiment of the present invention. Figure 3 is a graph showing the relationship between the BET specific surface area of ​​the negative electrode active material layer and the power maintenance rate of an energy storage element according to one embodiment of the present invention.

[0010] First, an overview of the energy storage elements disclosed herein will be provided.

[0011] [1] An energy storage element according to one aspect of the present invention comprises a positive electrode containing a transition metal element, a negative electrode having a negative electrode active material layer, and a separator, wherein the positive electrode and the negative electrode are laminated with the separator in between, the separator has an inorganic layer containing inorganic particles, and the BET specific surface area of ​​the negative electrode active material layer is 2 m² ２ The amount is 0.2 mg / cm² or more, and the mass per unit area of ​​the inorganic layer is 0.2 mg / cm². ２ That's all.

[0012] The energy storage element described in [1] above can suppress the decrease in output after long-term storage at high temperatures. The reason for this is not clear, but the following reason is speculated. In conventional energy storage elements equipped with a positive electrode containing a transition metal element, transition metal ions may dissolve from the positive electrode into the electrolyte during charging and discharging, pass through the pores of the separator, and precipitate on the surface of the negative electrode, forming a passivation layer. In such energy storage elements, if the surface area of ​​the negative electrode is increased in order to increase the initial output of the energy storage element, a passivation layer derived from transition metal ions is more likely to form on the surface of the negative electrode. In other words, the output is likely to decrease after long-term storage at high temperatures. In contrast, the energy storage element described in [1] above has an inorganic layer containing inorganic particles in the separator, so that transition metal ions passing through the pores of the separator can be captured by the inorganic particles. Furthermore, the mass per unit area of ​​the inorganic layer is 0.2 mg / cm². ２ Because it is adjusted as described above, the BET specific surface area of ​​the negative electrode active material layer is 2 m² ２ Even when set to a value of / g or higher, the inorganic particles can sufficiently capture transition metal ions to the extent that the formation of passivation originating from transition metal ions on the negative electrode surface is suppressed. For this reason, it is presumed that the energy storage element described in [1] above can suppress the decrease in output after long-term storage at high temperatures.

[0013] The BET specific surface area of ​​the negative electrode active material layer shall be the value obtained by the following procedure: 0.5 g of the negative electrode active material layer is placed in a sample tube for measurement and dried under reduced pressure at 80°C for 1.5 hours. Next, the adsorption isotherm is measured using the nitrogen gas adsorption method with liquid nitrogen, within the range of 0 to 1 for relative pressure P / P0 (P0 = approximately 770 mmHg). The measuring device used is the "FlowSorb III 2310" manufactured by micromeritics. The BET specific surface area is calculated from the amount of adsorption at one point on the obtained adsorption isotherm when P / P0 = 0.3.

[0014] The sample of the negative electrode active material layer used for measuring the BET specific surface area is taken from the negative electrode before the assembly of the energy storage element, if possible. If the negative electrode is prepared from an assembled energy storage element, it is prepared according to the following procedure. First, the energy storage element is discharged with a current of 0.5C to the discharge termination voltage under normal use, and then discharged at a constant voltage for 2 hours at the discharge termination voltage. Here, "under normal use" refers to using the energy storage element under the charge and discharge conditions recommended or specified for the element. After discharge, the element is disassembled, the negative electrode is removed, and a half-cell is assembled with the removed negative electrode as the working electrode and metallic lithium as the counter electrode. If the open-circuit voltage after assembly of this half-cell is less than 0.7V, the closed-circuit potential of the negative electrode is set to 2.0V (vs. Li / Li) by applying a current of 50mA per gram of negative electrode active material to the half-cell. ＋ A constant current discharge is performed until the negative electrode is fully discharged. Note that the discharge in the above half-cell refers to the oxidation reaction in which charge transport ions are released from the negative electrode active material. The device is disassembled again and the negative electrode is removed. The removed negative electrode is washed with dimethyl carbonate, then washed with water and dried, and a sample of the negative electrode active material layer is taken from the obtained negative electrode. The disassembly of the energy storage element and test battery is carried out in a dry air atmosphere with a dew point of -40°C or lower.

[0015] "The "mass per unit area of the inorganic layer" means the average mass per unit area of the inorganic layer. In this specification, the "average mass per unit area" means the average value of the masses per unit area measured at any three locations. Note that the inorganic layer is usually laminated on one or both surfaces of the base material layer of the separator. When the inorganic layer is laminated on both surfaces of the base material layer, the "mass per unit area of the inorganic layer" is calculated by summing the masses per unit area of the inorganic layers laminated on both surfaces of the base material layer. Specifically, when the separator is composed of a base material layer and an inorganic layer, the "mass per unit area of the inorganic layer" is obtained by measuring the masses of the separator and the base material layer of an arbitrary area respectively, and dividing the difference between the mass of the separator and the mass of the base material layer by the area of one surface of the separator."

[0016] 〔2〕In the power storage element described in the above 〔1〕, the BET specific surface area of the negative electrode active material layer may be 3.5 m ２ / g or more and 6.0 m ２ / g or less."

[0017] The power storage element described in the above 〔2〕 can increase the initial output. Also, it can more effectively suppress the decrease in output after long-term storage at high temperature."

[0018] 〔3〕In the power storage element described in the above 〔1〕 or 〔2〕, the mass per unit area of the inorganic layer may be less than 0.45 mg / cm ２ ."

[0019] The power storage element described in the above 〔3〕 can increase the initial output. Also, it can more effectively suppress the decrease in output after long-term storage at high temperature."

[0020] 〔4〕In the power storage element described in the above 〔1〕 to 〔3〕, the BET specific surface area of the negative electrode active material layer may be 5.0 m ２ / g or less."

[0021] When increasing the mass per unit area of the inorganic layer of the separator, the initial output is likely to decrease. In contrast, the power storage element described in the above 〔4〕 can suppress the decrease in the initial output because the BET specific surface area of the negative electrode active material layer is within the above range."

[0022] [5] In the energy storage element described in [1] to [4] above, the inorganic particles may include alumina particles or boehmite particles.

[0023] The energy storage element described in [5] above can further suppress the decrease in output after long-term storage at high temperatures.

[0024] [6] In the energy storage element described in [1] to [5] above, the thickness of the inorganic layer may be 1.8 μm or more.

[0025] The energy storage element described in [6] above can further suppress the decrease in output after long-term storage at high temperatures. Here, "thickness of the inorganic layer" means the average thickness of the inorganic layer. In this specification, "average thickness" means the average value of the thickness measured at any 10 locations. The inorganic layer is usually laminated on one or both sides of the substrate layer of the separator. If the inorganic layer is laminated on both sides of the substrate layer, the "thickness of the inorganic layer" is calculated by summing the thicknesses of the inorganic layers laminated on both sides of the substrate layer. Specifically, if the separator is composed of a substrate layer and an inorganic layer, the "thickness of the inorganic layer" is determined by measuring the thickness of the separator and the thickness of the substrate layer using a micrometer (terminal diameter 10 mm, measuring pressure 55 kPa), and calculating the difference between them.

[0026] An embodiment of the present invention, including an energy storage element, a method for manufacturing the energy storage element, an energy storage device, and other embodiments, will be described in detail below.

[0027] <Energy Storage Element> An energy storage element according to one embodiment of the present invention comprises a positive electrode, a negative electrode, a separator, an electrolyte, and a container housing these. In this energy storage element, the positive electrode and the negative electrode are stacked with a separator in between. The separator is interposed between the positive electrode and the negative electrode to electrically insulate them. The positive electrode, the negative electrode, and the separator usually constitute an electrode body. At least a portion of the electrolyte is present in a state of permeating the electrode body. An energy storage element according to one embodiment of the present invention may further comprise other components. By having the configuration described later, this energy storage element has the effect of suppressing a decrease in output after long-term storage at high temperatures.

[0028] For example, the energy storage element 1 shown in Figure 1, according to one embodiment of the present invention, comprises an electrode body 2, an electrolyte (not shown), and a rectangular parallelepiped container 3 that houses them. The energy storage element 1 in Figure 1 further comprises a positive electrode lead 4, a positive electrode external terminal 5, a negative electrode lead 6, and a negative electrode external terminal 7. The positive electrode lead 4 and the negative electrode lead 6 are housed together with the electrode body 2, etc., inside the container 3. The positive electrode external terminal 5 and the negative electrode external terminal 7 are provided outside the container 3. The positive electrode constituting the electrode body 2 is electrically connected to the positive electrode external terminal 5 via the positive electrode lead 4. The negative electrode constituting the electrode body 2 is electrically connected to the negative electrode external terminal 7 via the negative electrode lead 6.

[0029] The energy storage element of the present invention may be a non-aqueous electrolyte secondary battery. Below, the main components constituting the energy storage element according to one embodiment of the present invention will be described in detail, focusing on the case where the energy storage element is a non-aqueous electrolyte secondary battery (particularly a lithium-ion secondary battery), but this is not intended to limit the scope of application of the present invention.

[0030] Furthermore, the lower and upper limits of each numerical range described in the embodiments of the present invention can be combined in any way (however, the upper limit must be greater than the lower limit). Also, regarding the lower and upper limits of a numerical range, it means that the numerical range includes both the lower and upper limits. That is, a lower limit of A means that it is greater than or equal to A. Similarly, an upper limit of B means that it is less than or equal to B.

[0031] (Positive electrode) The positive electrode comprises a positive electrode substrate and a positive electrode active material layer laminated directly to the positive electrode substrate or via an intermediate layer. Typically, the positive electrode has a portion where the positive electrode substrate is exposed. This exposed portion of the positive electrode substrate is usually connected to the positive electrode lead described above. The positive electrode may have a shape such as a sheet, plate, or strip.

[0032] The thickness of the positive electrode is set appropriately according to the application of the energy storage element. The average thickness of the positive electrode may be, for example, 30 μm or more and 1,000 μm or less. The lower limit of the average thickness of the positive electrode may be 50 μm, 100 μm, or 200 μm. The upper limit of the average thickness of the positive electrode may be 500 μm, 400 μm, 300 μm, 200 μm, or 100 μm. The average thickness of the positive electrode is the average thickness of the portion in which the positive electrode active material layer is laminated directly to the positive electrode substrate or via an intermediate layer. If there are portions in which the positive electrode active material layer is laminated on both sides of the positive electrode substrate and portions in which the positive electrode active material layer is laminated on only one side of the positive electrode substrate, then the average thickness of the portion in which the positive electrode active material layer is laminated on both sides of the positive electrode substrate shall be used.

[0033] The positive electrode substrate is conductive. In this specification, "having conductivity" means that the volume resistivity is 10 －２ This means that the volume resistivity is Ω·cm or less. The volume resistivity shall be the value measured in accordance with JIS-H-0505 (1975). On the other hand, in this specification, "not conductive" or "having (electrical) insulating properties" means that the above volume resistivity is 10 ７ This means it is greater than or equal to Ω·cm.

[0034] Examples of materials for the positive electrode substrate include metals such as aluminum, titanium, iron, and their alloys (stainless steel, etc.). Among these, aluminum or aluminum alloys are preferred from the viewpoint of high potential resistance, high electronic conductivity, and cost.

[0035] The positive electrode substrate has a shape such as a sheet, plate, or strip. Examples of positive electrode substrate forms include foil, vapor-deposited film, mesh, and porous material, with foil being preferred. The positive electrode substrate may also be, for example, aluminum foil or aluminum alloy foil.

[0036] The average thickness of the positive electrode substrate may be, for example, 3 μm or more and 50 μm or less. The lower limit of the average thickness of the positive electrode substrate may be 5 μm, 8 μm, 10 μm, or 15 μm. The upper limit of the average thickness of the positive electrode substrate may be 40 μm, 30 μm, 20 μm, or 15 μm.

[0037] The intermediate layer is a layer placed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer includes, for example, a conductive agent and a binder. When the intermediate layer contains a conductive agent, the contact resistance between the positive electrode substrate and the positive electrode active material layer can be reduced. Examples of conductive agents and binders used in the intermediate layer are the same as those used in the positive electrode active material layer, which will be described later.

[0038] The positive electrode active material layer contains a positive electrode active material. The positive electrode active material layer may optionally contain conductive agents, binders, thickeners, fillers, and other optional components. The positive electrode active material layer may be formed from a positive electrode mixture containing the positive electrode active material and other optional components. The positive electrode active material layer may be provided on only one side of a positive electrode substrate having a shape such as a sheet, or on both sides.

[0039] The positive electrode active material is typically a material capable of intercalating and releasing lithium ions. In one embodiment of the present invention, the positive electrode active material includes a transition metal element. Examples of positive electrode active materials include lithium transition metal composite oxides, polyanion compounds containing transition metal elements, and chalcogen compounds containing transition metal elements. Among these, lithium transition metal composite oxides and polyanion compounds containing transition metal elements are preferred from the viewpoint of significantly obtaining the effects of the present invention, and lithium transition metal composite oxides are more preferred. One or more positive electrode active materials can be used.

[0040] Examples of transition metal elements included in lithium transition metal composite oxides include nickel, cobalt, and manganese. Lithium transition metal composite oxides may also contain typical metal elements such as aluminum. An example of a lithium transition metal composite oxide is α-NaFeO ２ Examples include lithium transition metal composite oxides having a crystalline structure and lithium transition metal composite oxides having a spinel-type crystalline structure. Among these, α-NaFeO ２ Lithium transition metal composite oxides having a specific crystal structure are preferred.

[0041] α-NaFeO ２ Li is a lithium transition metal composite oxide having a type crystal structure. １＋αMa １－α O ２ Examples include those represented as follows: (Ma is a metallic element other than lithium, containing one or more transition metal elements. 0 ≤ α < 1.) It is preferable that Ma contains one or more of Ni, Co, and Mn. The total content of Ni, Co, and Mn relative to Ma ((Ni + Co + Mn) / Ma) is preferably 90 mol% or more, and more preferably 98 mol% or more.

[0042] Li is a lithium transition metal composite oxide having a spinel-type crystal structure. β Mb ２ O ４ Examples include those expressed as follows: (Mb is a metallic element other than lithium, containing one or more transition metal elements. 0 < β ≤ 1.2.) Mb preferably contains Mn. The Mn content relative to Mb (Mn / Mb) is preferably 50 mol% or more, and more preferably 80 mol% or more.

[0043] A polyanionic compound containing a transition metal element is a compound composed of a polyanion (i.e., a polyvalent oxo acid anion) and a cation of a transition metal element. An example of a polyanionic compound containing a transition metal element is LiFePO. ４ LiMnPO ４ LiMn ｘ Fe １－ｘ PO ４ (0<x<1), LiNiPO ４ LiCoPO ４ Li ３ V ２ (PO ４ ) ３ Li ２ MnSiO ４ Li ２ CoPO ４ Examples include F. The surface of the polyanionic compound particles may be coated with other materials (e.g., carbon materials).

[0044] Examples of chalcogen compounds containing transition metal elements include titanium disulfide, molybdenum disulfide, and molybdenum dioxide.

[0045] The atoms or polyanions in these materials, which are the positive electrode active materials, may be partially substituted with atoms or anions of other elements. These materials may also be coated on the surface with other materials.

[0046] The positive electrode active material is usually in particulate form. 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 facilitates the manufacture and handling of the positive electrode active material. Setting the average particle size of the positive electrode active material below the upper limit improves the electronic conductivity of the positive electrode active material layer. When a composite material of the positive electrode active material and other materials is used, the average particle size of the composite material is considered the average particle size of the positive electrode active material. "Average particle size" refers to the value (D50) at which the volume-based integrated distribution, calculated according to JIS-Z-8819-2 (2001), is 50%, based on the particle size distribution measured by laser diffraction / scattering on a dilution of particles diluted with a solvent, in accordance with JIS-Z-8825 (2013). For obtaining particles of the positive electrode active material and the negative electrode active material described later with predetermined particle sizes, known methods using, for example, pulverizers and classifiers can be employed.

[0047] The content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass or more and 99% by mass or less, more preferably 70% by mass or more and 98% by mass or less, and may also be 80% by mass or more and 95% by mass or less. 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.

[0048] Conductive agents are typically components made of conductive materials. Even if the volume resistivity of a conductive agent cannot be directly measured, the volume resistivity is 10 －２Materials whose conductivity is known to be Ω·cm or less are classified as conductive agents. Examples of conductive agents include carbon materials, metals, and conductive ceramics. A carbon material is a material whose main constituent element is carbon. The main constituent element is the element with the highest mass content. For example, the carbon content in a carbon material may be 80% by mass or more, 90% by mass or more, 95% by mass, 99% by mass, or 99.9% by mass or more. It is preferable that the carbon material is a carbon material other than a non-carbonized polymer compound. Examples of carbon 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 (CNT), and fullerene. Conductive agents can take the form of powder or fibers. One or more conductive agents can be used. These materials may also be used as a composite conductive agent. For example, a composite material of carbon black and CNTs may be used.

[0049] The conductive agent content in the positive electrode active material layer is preferably 0.1% by mass or more and 10% by mass or less, more preferably 1% by mass or more and 9% by mass or less, and even more preferably 3% by mass or more and 8% by mass or less. The upper limit of the conductive agent content may be 5% by mass, 4% by mass, or 3% by mass. By setting the conductive agent content within the above range, it is possible to increase the energy density of the energy storage element.

[0050] Examples of binders include water-based binders and organic solvent-based binders.

[0051] A water-based binder is a binder that dissolves or disperses in water. A water-based binder may be one in mass or more that dissolves or disperses in 100 parts by mass of water at 20°C. When forming a positive electrode active material layer using a positive electrode mixture paste in which the dispersion medium is water or a mixed solvent mainly composed of water, a water-based binder (a water-soluble or water-dispersible polymer material) can be used. Examples of water-based binders include polyethylene oxide, polypropylene oxide, polyvinyl alcohol, polyacrylic acid, polymethacrylic acid, polytetrafluoroethylene, styrene-butadiene rubber, polyethylene, polypropylene, nitrile-butadiene rubber, and cellulose.

[0052] An organic solvent-based binder is a binder that dissolves or disperses in an organic solvent (e.g., N-methylpyrrolidone). An organic solvent-based binder may be a binder that dissolves or disperses at 1 part by mass or more per 100 parts by mass of an organic solvent (e.g., N-methylpyrrolidone) at 20°C. When forming a positive electrode active material layer using a positive electrode mixture paste in which the dispersion medium is an organic solvent or a mixed solvent mainly composed of an organic solvent, an organic solvent-based binder (a polymer material having solubility or dispersibility in an organic solvent) can be used. Examples of organic solvent-based binders include polyvinylidene fluoride, copolymers of vinylidene fluoride and hexafluoropropylene, copolymers of ethylene and vinyl alcohol, polyacrylonitrile, polyphosphazene, polysiloxane, polyvinyl acetate, polymethyl methacrylate, polystyrene, polycarbonate, polyamide, polyimide, polyamideimide, crosslinked polymers of cellulose and chitosan pyrrolidone carboxylate, and derivatives of chitosan.

[0053] The binder may be a fluororesin (polytetrafluoroethylene, polyvinylidene fluoride, etc.), a polyolefin (polyethylene, polypropylene, etc.), an elastomer (ethylene propylene diene rubber, styrene butadiene rubber, fluororubber, etc.), a polysaccharide polymer (cellulose, chitosan derivatives, etc.), etc. One or more types of binders may be used.

[0054] The binder content in the positive electrode active material layer is preferably 0.1% by mass or more and 10% by mass or less, more preferably 1% by mass or more and 9% by mass or less, and even more preferably 2% by mass or more and 8% by mass or less. The upper limit of the binder content may be 5% by mass, 4% by mass, or 3% by mass. By setting the binder content within the above range, the positive electrode active material can be stably maintained. The technology disclosed herein can also be implemented in a form in which the positive electrode active material layer does not contain a binder.

[0055] Examples of thickening agents include polysaccharide polymers such as carboxymethylcellulose and methylcellulose. If the thickening agent has a functional group that reacts with lithium, etc., this functional group may be deactivated beforehand by methylation or the like. The thickening agent may also function as a binder. One or more types of thickening agents can be used. When the positive electrode active material layer contains a thickening agent, the content of the thickening agent in the positive electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, more preferably 5% by mass or less, and even more preferably 2% by mass or less. The technology disclosed herein can also be carried out in a form in which the positive electrode active material layer does not contain a thickening agent.

[0056] The filler is not particularly limited. The filler may be a component other than the positive electrode active material, conductive agent, binder, and thickener, and may be a component that is intentionally included. The filler may be included as a component that fills gaps in the positive electrode active material layer, or it may be included for other purposes. The filler may be an organic substance such as a polyolefin, or an inorganic substance such as an inorganic oxide, hydroxide, or carbonate. One or more types of fillers may be used. When the positive electrode active material layer contains a filler, the filler content in the positive electrode active material layer can be 0.1% by mass or more and 8% by mass or less, usually preferably 5% by mass or less, and more preferably 2% by mass or less. The technology disclosed herein can also be carried out in a form in which the positive electrode active material layer does not contain a filler.

[0057] The positive electrode active material layer may further contain other components besides the positive electrode active material, conductive agent, binder, thickener, and filler. These other components may include those unintentionally present in the positive electrode active material layer. Furthermore, the positive electrode active material layer may contain impurities unintentionally present as these other components, insofar as they achieve the effects of the present invention. The upper limit of the content of these other components in the positive electrode active material layer may be 10% by mass, or 5%, 2%, 1%, 0.1%, or 0.01% by mass. The upper limit of the content of unintentionally present components in the positive electrode active material layer may be 10% by mass, or 5%, 2%, 1%, 0.1%, or 0.01% by mass. The upper limit of the content of unintentionally present impurities in the positive electrode active material layer may be 10% by mass, or 5%, 2%, 1%, 0.1%, or 0.01% by mass.

[0058] The thickness of the positive electrode active material layer is set appropriately according to the type of positive electrode active material, the application of the energy storage element, etc. The average thickness of one positive electrode active material layer may be, for example, 5 μm or more and 1,000 μm or less. The lower limit of the average thickness of one positive electrode active material layer may be 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, or 100 μm. The upper limit of the average thickness of one positive electrode active material layer may be 800 μm, 500 μm, 200 μm, 100 μm, 80 μm, 60 μm, or 40 μm. The mass per unit area of ​​one positive electrode active material layer may be, for example, 4 mg / cm². ２ 100mg / cm or more ２ The following is also acceptable: The lower limit of the mass per unit area of ​​one positive electrode active material layer is 6 mg / cm². ２ , 8 mg / cm ２ or 10 mg / cm² ２ It may also be the case that the upper limit of the mass per unit area of ​​one positive electrode active material layer is 50 mg / cm². ２ , 20 mg / cm ２ , 15 mg / cm ２ , 12 mg / cm ２ or 10 mg / cm² ２ That's fine.

[0059] The porosity of the positive electrode active material layer may be, for example, 20% or more and 50% or less. The lower limit of the porosity of the positive electrode active material layer may be 25%, 30%, or 35%. The upper limit of the porosity of the positive electrode active material layer may be 45%, 40%, or 35%. The "porosity (%)" of the positive electrode active material layer and the negative electrode active material layer described later is calculated by taking the apparent volume (volume including voids) of the positive (negative) electrode active material layer as V １ Let V be the sum of the actual volumes of each material constituting the positive (negative) electrode active material layer. ２ In that case, (1-V ２ / V １ It is calculated using the formula ) × 100. V is the sum of the actual volumes of each material constituting the positive (negative) electrode active material layer. ２ This can be calculated from the content of each material in the positive (negative) electrode active material layer and the true density of each material.

[0060] (Method for Manufacturing a Positive Electrode) A positive electrode can be manufactured by known methods. A positive electrode can be manufactured, for example, by applying a paste-like positive electrode mixture (positive electrode mixture paste) directly to a positive electrode substrate or via an intermediate layer, and drying it to form a positive electrode active material layer. The positive electrode mixture paste usually contains a positive electrode active material, other optional components, and a dispersion medium. After drying, the positive electrode active material layer may be pressed or otherwise subjected to other processes.

[0061] (Negative electrode) The negative electrode comprises a negative electrode substrate and a negative electrode active material layer laminated directly to the negative electrode substrate or via an intermediate layer. Typically, the negative electrode has a portion where the negative electrode substrate is exposed. This exposed portion of the negative electrode substrate is usually connected to the negative electrode lead described above. The negative electrode may have a shape such as a sheet, plate, or strip.

[0062] The thickness of the negative electrode is set appropriately according to the application of the energy storage element. The average thickness of the negative electrode may be, for example, 30 μm or more and 1,000 μm or less. The lower limit of the average thickness of the negative electrode may be 50 μm, 100 μm, or 200 μm. The upper limit of the average thickness of the negative electrode may be 500 μm, 400 μm, 300 μm, 200 μm, or 100 μm. The average thickness of the negative electrode is the average thickness of the portion in which the negative electrode active material layer is laminated directly to the negative electrode substrate or via an intermediate layer. If there are portions in which the negative electrode active material layer is laminated on both sides of the negative electrode substrate and portions in which the negative electrode active material layer is laminated on only one side of the negative electrode substrate, then the average thickness of the portion in which the negative electrode active material layer is laminated on both sides of the negative electrode substrate shall be used.

[0063] The negative electrode substrate is electrically conductive. Examples of materials for the negative electrode substrate include metals such as copper, nickel, iron, and their alloys (such as stainless steel), as well as carbon materials. Among these, copper or copper alloys are preferred.

[0064] The negative electrode substrate has a shape such as a sheet, plate, or strip. Examples of negative electrode substrate forms include foil, vapor-deposited film, mesh, and porous material, with foil being preferred. The negative electrode substrate may also be, for example, copper foil or copper alloy foil.

[0065] The average thickness of the negative electrode substrate may be, for example, 2 μm or more and 35 μm or less. The lower limit of the average thickness of the negative electrode substrate may be 3 μm, 4 μm, 5 μm, or 10 μm. The upper limit of the average thickness of the negative electrode substrate may be 30 μm, 20 μm, 15 μm, or 10 μm.

[0066] The configuration of the negative electrode intermediate layer is not particularly limited; for example, it can be selected from the configurations exemplified for the positive electrode intermediate layer.

[0067] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer optionally contains optional components such as a conductive agent, binder, thickener, and filler. The optional components such as the conductive agent, binder, thickener, and filler can be selected from the materials exemplified above for the positive electrode. The negative electrode active material layer may be formed from a negative electrode mixture containing the negative electrode active material and other optional components. The negative electrode active material layer may be provided on only one side of a negative electrode substrate having a shape such as a sheet, or on both sides.

[0068] For the negative electrode active material, known negative electrode active materials can be used. For lithium-ion secondary batteries, materials that can intercept and release lithium ions are usually used as negative electrode active materials. Examples of negative electrode active materials include metallic lithium; metals or metalloids such as silicon and tin; metal oxides or metalloid oxides such as silicon oxide, titanium oxide, and tin oxide; Li ４ Ti ５ O １２ LiTio ２、 TiNb ２ O ７ Examples include titanium-containing oxides; polyphosphate compounds; silicon carbide; and carbon materials such as graphite and non-graphitic carbon. Among these materials, carbon materials are preferred, and graphite or non-graphitic carbon is more preferred. Graphite may have its surface coated with other materials such as non-graphitic carbon. One or more types of negative electrode active materials can be used.

[0069] "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 This refers to carbon materials with a n-scale of 0.33 nm or more and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite.

[0070] "Non-graphite carbon" refers to the average lattice plane spacing (d) of the (002) plane, which is determined by X-ray diffraction before charging / discharging or during the discharge state. ００２ ) refers to carbon materials with a nautical index of 0.34 nm or more and 0.42 nm or less. Non-graphitic carbons include poorly graphitizable carbons and easily graphitizable carbons. "Poorly graphitizable carbon" refers to the above d ００２ This refers to carbon materials with a wavelength of 0.36 nm or more and 0.42 nm or less. "Easily graphitizable carbon" refers to the above d ００２ This refers to carbon materials with a nautical radius of 0.34 nm or more and less than 0.36 nm.

[0071] 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 half-cell using a negative electrode containing a carbon material as the negative electrode active material as the working electrode and metallic lithium as the counter electrode, this is a state in which the open-circuit voltage is 0.7V or higher.

[0072] The negative electrode active material may be in particulate form. The average particle size of the negative electrode active material can be, for example, 1 nm to 100 μm. If the negative electrode active material is a carbon material, titanium-containing oxide, or polyphosphate compound, its average particle size may be 1 μm to 100 μm. If the negative electrode active material is Si, Sn, Si oxide, or Sn oxide, its average particle size may be 1 nm to 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 negative electrode active material layer.

[0073] The content of the negative electrode active material in the negative electrode active material layer is preferably, for example, 60% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 99% 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.

[0074] When the negative electrode active material layer contains a conductive agent, the content of the conductive agent in the negative electrode active material layer is preferably 0.5% by mass or more and 10% by mass or less, and more preferably 1% by mass or more and 9% by mass or less. The content of the conductive agent in the negative electrode active material layer may be 5% by mass or less, or 2% by mass or less. The technology disclosed herein can also be carried out in a form in which the negative electrode active material layer does not contain a conductive agent.

[0075] When the negative electrode active material layer contains a binder, the binder content in the negative electrode active material layer is preferably 0.1% by mass or more and 10% by mass or less, and more preferably 0.5% by mass or more and 8% by mass or less. The binder content in the negative electrode active material layer may be 5% by mass or less, or 2% by mass or less. The technology disclosed herein can also be carried out in a form in which the negative electrode active material layer does not contain a binder.

[0076] When the negative electrode active material layer contains a thickening agent, the content of the thickening agent in the negative electrode active material layer is preferably 0.1% by mass or more and 10% by mass or less, and more preferably 0.5% by mass or more and 8% by mass or less. The content of the thickening agent in the negative electrode active material layer may be 5% by mass or less, or 2% by mass or less. The technology disclosed herein can also be carried out in a form in which the negative electrode active material layer does not contain a thickening agent.

[0077] The filler in the negative electrode active material layer is a component other than the negative electrode active material, conductive agent, binder, and thickener, and may be a component that is intentionally included. The filler may be included as a component that fills gaps in the negative electrode active material layer, or it may be included for other purposes. When the negative electrode active material layer contains a filler, the filler content in the negative electrode active material layer can be 0.1% by mass or more and 8% by mass or less, usually preferably 5% by mass or less, and more preferably 2% by mass or less. The technology disclosed herein can also be carried out in a form in which the negative electrode active material layer does not contain a filler.

[0078] The negative electrode active material layer may further contain other components besides the negative electrode active material, conductive agent, binder, thickener, and filler. These other components may include those unintentionally present in the negative electrode active material layer. Furthermore, the negative electrode active material layer may contain impurities that have been unintentionally included as these other components, insofar as they achieve the effects of the present invention. The upper limit of the content of these other components in the negative electrode active material layer may be 10% by mass, 5% by mass, 2% by mass, 1% by mass, 0.1% by mass, or 0.01% by mass. The upper limit of the content of unintentionally present components in the negative electrode active material layer may be 10% by mass, 5% by mass, 2% by mass, 1% by mass, 0.1% by mass, or 0.01% by mass. The upper limit of the content of unintentionally present impurities in the negative electrode active material layer may be 10% by mass, 5% by mass, 2% by mass, 1% by mass, 0.1% by mass, or 0.01% by mass.

[0079] The thickness of the negative electrode active material layer, etc., is set appropriately according to the type of negative electrode active material, the application of the energy storage element, etc. The average thickness of one negative electrode active material layer may be, for example, 5 μm or more and 1,000 μm or less. The lower limit of the average thickness of one negative electrode active material layer may be 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, or 100 μm. The upper limit of the average thickness of one negative electrode active material layer may be 800 μm, 500 μm, 200 μm, 100 μm, 80 μm, 60 μm, or 40 μm. The average thickness of one negative electrode active material layer may be greater than or equal to any of the lower limits and less than or equal to any of the upper limits.

[0080] The porosity of the negative electrode active material layer may be, for example, 30% or more and 70% or less. The lower limit of the porosity of the negative electrode active material layer may be 35%, 40%, or 45%. The upper limit of the porosity of the negative electrode active material layer may be 60%, 50%, or 40%. The porosity of the negative electrode active material layer may be above any of the lower limits and below any of the upper limits.

[0081] The lower limit of the BET specific surface area of ​​the negative electrode active material layer is 2 m ２ / g, and 2.5m ２ It may also be / g, 3.0m ２ / g is preferred, and 3.5m ２ / g is more preferable, 4.0m ２ / g is even more preferable. On the other hand, the upper limit of the BET specific surface area of ​​the negative electrode active material layer is 6.5 m ２ / g is preferred, and 6.0m ２ / g is more preferable, 5.5m ２ A value of / g is even more preferable. Having the BET specific surface area of ​​the negative electrode active material layer within the above range allows for higher initial output and further suppression of output degradation after prolonged storage at high temperatures. From the viewpoint of suppressing the decrease in initial output, the upper limit of the BET specific surface area of ​​the negative electrode active material layer is 5.0 m². ２ / g is even more preferable, 4.5m ２ / g may be particularly preferred in some cases. The BET specific surface area of ​​the negative electrode active material layer may be greater than or equal to any of the lower limits and less than or equal to any of the upper limits. The BET specific surface area of ​​the negative electrode active material layer can be adjusted by adjusting the average particle size of the negative electrode active material, adjusting the press pressure during negative electrode manufacturing, etc.

[0082] The lower limit of the mass per unit area of ​​the negative electrode active material layer is 0.5 mg / cm². ２ Preferably, 1.5 mg / cm ２ More preferably, 2.5 mg / cm² ２ More preferably, 3 mg / cm ２ In some cases, this may be even more preferable. On the other hand, the upper limit for the mass per unit area of ​​the negative electrode active material layer is 10 mg / cm³. ２ Preferably, 9 mg / cm ２ More preferably, 8.0 mg / cm² ２ A more preferable concentration is 7.8 mg / cm³. ２ In some cases, this may be even more preferable. The mass per unit area of ​​the negative electrode active material layer may be greater than or equal to any of the lower limits and less than or equal to any of the upper limits. The effects of the present invention can be better realized when the mass per unit area of ​​the negative electrode active material layer is within this range. The negative electrode active material layer is usually laminated on one or both sides of the negative electrode substrate. The "mass per unit area of ​​the negative electrode active material layer" is calculated by adding up the masses per unit area of ​​the negative electrode active material layers laminated on both sides of the negative electrode substrate when the negative electrode active material layer is laminated on both sides of the negative electrode substrate. When the negative electrode active material layer is laminated on both sides of the negative electrode substrate, it is preferable that the mass per unit area of ​​each negative electrode active material layer (one side) is the same (i.e., half the above sum). Furthermore, if both portions of the negative electrode substrate have negative electrode active material layers laminated on both sides and portions have negative electrode active material layers laminated on only one side, the calculation is performed by summing the mass per unit area of ​​the negative electrode active material layer in the portions where it is laminated on both sides of the negative electrode substrate.

[0083] (Method for Manufacturing the Negative Electrode) The negative electrode can be manufactured by known methods. The negative electrode can be manufactured, for example, in the same way as the method for manufacturing the positive electrode described above, by applying a paste-like negative electrode mixture (negative electrode mixture paste) directly to the negative electrode substrate or via an intermediate layer, and drying it to form a negative electrode active material layer. The negative electrode mixture paste usually contains a negative electrode active material, other optional components, and a dispersion medium. After drying, the negative electrode active material layer may be pressed or otherwise subjected to the process.

[0084] (Separator) In one embodiment of the present invention, the separator comprises a base layer and an inorganic layer formed on one or both surfaces of the base layer. The separator may be selected from known separators.

[0085] Examples of the substrate 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. The material of the substrate layer of the separator is not particularly limited as long as it has insulating properties, but resins such as polyolefins (polyethylene, polypropylene, etc.), polyimide, and aramid are preferred.

[0086] The inorganic layer contains inorganic particles. The inorganic layer usually also contains a binder.

[0087] Examples of inorganic compounds constituting inorganic particles include oxides such as iron oxide, silicon oxide, aluminum oxide (alumina), aluminum oxide hydrate (boehmite), titanium oxide, zirconium oxide, calcium oxide, and magnesium oxide; 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; mineral resource-derived materials such as talc, zeolite, kaolin, bentonite, and mica, or their artificial counterparts. From the viewpoint of effectively capturing transition metal ions passing through the separator pores, inorganic particles containing the above oxides are preferred, those containing alumina particles or boehmite particles are more preferred, and those consisting of alumina particles or boehmite particles are even more preferred. One or more types of inorganic particles can be used. The average particle size of the inorganic particles is preferably, for example, 0.5 μm or more and 10 μm or less.

[0088] The content of the inorganic particles in the inorganic layer is not particularly limited, but is preferably 50% by mass or more and 99% by mass or less, more preferably 60% by mass or more and 99% by mass or less, and even more preferably 80% by mass or more and 99% by mass or less. By setting the content of the inorganic particles in the inorganic layer within the above range, transition metal ions passing through the pores of the separator can be appropriately captured by the inorganic particles.

[0089] Examples of the binder used in the inorganic layer include carboxymethyl cellulose (CMC), methyl cellulose, etc., in addition to the binders exemplified in the positive electrode active material layer. Among these, polysaccharide polymers are preferred, and carboxymethyl cellulose is more preferred. As the content of the binder in the inorganic layer, for example, 20% by mass or less is preferred, 15% by mass or less is more preferred, and 10% by mass or 5% by mass or less is even more preferred. The inorganic layer may have other components other than the inorganic particles and the binder. However, the total content of the inorganic particles and the binder in the inorganic layer is preferably 90% by mass or more, and may be more preferably 99% by mass or more.

[0090] The lower limit of the mass per unit area of the inorganic layer is 0.2 mg / cm ２ and 0.25 mg / cm ２ is preferred, 0.30 mg / cm ２ is more preferred, 0.32 mg / cm ２ is even more preferred. When the mass per unit area of the inorganic layer is at least the above lower limit, transition metal ions passing through the pores of the separator can be sufficiently captured. That is, the output decrease after long-term storage of the power storage element at high temperature can be more effectively suppressed. On the other hand, the upper limit of the mass per unit area of the inorganic layer is preferably 0.60 mg / cm ２ and 0.55 mg / cm ２ is more preferred, 0.50 mg / cm ２ is even more preferred, 0.45 mg / cm ２ less than is even more preferred, 0.40 mg / cm ２This is particularly preferable. The mass per unit area of ​​the inorganic layer may be greater than or equal to any of the lower limits and less than or equal to any of the upper limits. By having the mass per unit area of ​​the inorganic layer be less than or equal to the upper limit, the initial output of the energy storage element can be increased.

[0091] The lower limit of the average thickness of the inorganic layer is preferably 1.8 μm, more preferably 2.0 μm, even more preferably 2.2 μm, and even more preferably 2.5 μm. An average thickness of the inorganic layer greater than or equal to the above lower limit allows for sufficient capture of transition metal ions passing through the separator vacancies. On the other hand, the upper limit of the average thickness of the inorganic layer is preferably 5.0 μm, more preferably 4.0 μm, and even more preferably 3.0 μm, from the viewpoint of increasing the initial output of the energy storage element. The average thickness of the inorganic layer may be greater than or equal to any of the lower limits and less than or equal to any of the upper limits.

[0092] The porosity of the separator may be, for example, 20% or more and 80% or less. The lower limit of the separator's porosity may be 30%, 40%, or 50% from the viewpoint of discharge performance, etc. The upper limit of the separator's porosity may be 70%, 60%, or 50% from the viewpoint of strength, etc. The above-mentioned separator's porosity may be above any of the lower limits and below any of the upper limits. In this specification, "porosity" is a volume-based value and is calculated from the mass per unit area of ​​the separator, its thickness, and the true density of the constituent material.

[0093] The average thickness of the separator may be, for example, 5 μm or more and 40 μm or less, 10 μm or more and 30 μm or less, or 15 μm or more and 20 μm or less. The average thickness of the separator may be greater than or equal to any of the lower limits and less than or equal to any of the upper limits.

[0094] The lower limit of the BET specific surface area of ​​the separator (total BET specific surface area of ​​the substrate layer and inorganic layer) is 20 m². ２ / g is preferred, 25m ２ / g is more preferable. On the other hand, the upper limit of the BET specific surface area of ​​the separator is 50 m ２ / g is preferred, 40m ２ / g is more preferable, 30m ２ / g is even more preferable. The BET specific surface area of ​​the separator may be greater than or equal to any of the lower limits and less than or equal to any of the upper limits. The effects of the present invention can be easily obtained if the BET specific surface area of ​​the separator is within the above range. The "BET specific surface area" of the separator shall be calculated by placing 0.3g of the separator in a sample tube for measurement, drying under reduced pressure at 90°C for 6 hours, and then using the nitrogen gas adsorption method under the same conditions as the "BET specific surface area" of the negative electrode active material layer described above.

[0095] (Electrode Body) As the electrode body, known structures such as wound electrode bodies and laminated electrode bodies can be used.

[0096] A wound electrode has a structure in which the positive electrode and negative electrode are wound together while being insulated. The wound electrode may be cylindrical or flattened. The electrode 2 of the energy storage element 1 in Figure 1 is a flattened wound electrode. A wound electrode can be manufactured, for example, by the following procedure. First, a laminate is obtained by stacking a positive electrode, a separator, and a negative electrode, each formed in a strip shape, in that order. A wound electrode is obtained by winding this laminate.

[0097] A laminated electrode body has a structure in which one or more positive electrodes and one or more negative electrodes are stacked in an insulated state. For example, a laminated electrode body can be obtained by stacking rectangular positive electrodes, separators, and negative electrodes in this order.

[0098] Other electrode structures can also be used, such as those in which at least one of the positive and negative electrodes is folded in a bellows-like manner and stacked.

[0099] (Non-aqueous electrolyte) Known non-aqueous electrolytes can be used as the non-aqueous electrolyte. A non-aqueous electrolyte is a medium that is responsible for transporting charge transport ions (e.g., lithium ions) between the positive electrode and the negative electrode, and which is substantially free of water. The water content in the non-aqueous electrolyte may be, for example, 10,000 ppm or less, 5,000 ppm or less, 1,000 ppm or less, 500 ppm or less, 100 ppm or less, preferably 50 ppm or less, and more preferably 20 ppm or less. Examples of non-aqueous electrolytes include non-aqueous electrolyte solutions and solid electrolytes. Non-aqueous electrolyte solutions and solid electrolytes may be used in combination. It is preferable to use a non-aqueous electrolyte solution, and more preferable to use only a non-aqueous electrolyte solution.

[0100] (Non-aqueous electrolyte) A non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in this non-aqueous solvent.

[0101] Any known non-aqueous solvent can be used. Examples of non-aqueous solvents include carbonates, esters, ethers, amides, and nitriles. Examples of carbonates include cyclic carbonates and linear carbonates. Examples of esters include carboxylic acid esters, phosphate esters, and sulfonic acid esters. As non-aqueous solvents, compounds in which some of the hydrogen atoms contained in these compounds are substituted with halogen atoms may also be used. One or more non-aqueous solvents can be used.

[0102] A cyclic carbonate is a carbonate having a ring structure containing a carbonate group (-O-C(=O)-O-). Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, vinylethylene carbonate, chloroethylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, styrene carbonate, 1-phenylvinylene carbonate, and 1,2-diphenylvinylene carbonate. A cyclic carbonate may be a saturated cyclic carbonate such as ethylene carbonate, or an unsaturated cyclic carbonate such as vinylene carbonate. "Saturated" means not having carbon-carbon unsaturated bonds (carbon-carbon double bonds and carbon-carbon triple bonds). "Unsaturated" means having carbon-carbon unsaturated bonds. As the cyclic carbonate, saturated cyclic carbonates are preferred, and ethylene carbonates are more preferred.

[0103] A chain-like carbonate refers to a carbonate that does not have a ring structure containing a carbonate group. Examples of chain-like carbonates include diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, diphenyl carbonate, trifluoroethyl methyl carbonate, and bis(trifluoroethyl) carbonate. The chain-like carbonate may be a saturated chain-like carbonate such as dimethyl carbonate, or an unsaturated chain-like carbonate such as diphenyl carbonate. A saturated chain-like carbonate is preferred as the chain-like carbonate, and ethyl methyl carbonate is more preferred.

[0104] The non-aqueous solvent preferably contains a carbonate, more preferably contains a cyclic carbonate and a chain carbonate. The content of the carbonate in the non-aqueous solvent is preferably 80% by volume or more and 100% by volume or less, may be 99% by volume or more and 100% by volume, or may be 100% by volume. By using the cyclic carbonate, the dissociation of the electrolyte salt can be promoted and the ionic conductivity of the non-aqueous electrolyte can be increased. By using the chain carbonate, the viscosity of the non-aqueous electrolyte can be kept low. When the cyclic carbonate and the chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate: chain carbonate) is preferably, for example, in the range of 5:95 to 50:50.

[0105] As the electrolyte salt, known electrolyte salts can be used. Examples of the electrolyte salt include lithium salts, sodium salts, potassium salts, magnesium salts, onium salts, etc. Among these, lithium salts are preferred. One or more kinds of electrolyte salts can be used.

[0106] Examples of the lithium salt include inorganic lithium salts such as LiPF ６ , LiPO ２ F ２ , LiBF ４ , LiClO ４ , etc., imide salts such as LiN(SO ２ F) ２ , LiN(SO ２ CF ３ ) ２ , LiN(SO ２ C ２ F ５ ) ２ , LiN(SO ２ [[ID=3�]]CF ３ )(SO ２ C ４ F ９ ), etc., and LiB(C ２ O ４ ) ２ , LiBF ２ (C ２ O ４ ), LiPF ２ (C ２ O ４ ) ２Examples include lithium oxalate salts such as LiN(SO4). ２ F) ２ This also applies to inorganic lithium salts. Among these, inorganic lithium salts are preferred, and LiPF ６ This is more preferable. In some cases, an imide salt may also be preferable.

[0107] The electrolyte salt content in the non-aqueous electrolyte is 0.1 mol / dm³ at 20°C and 1 atm. ３ 2.5mol / dm or more ３ The following is preferred: 0.3 mol / dm ３ 2.0mol / dm or more ３ The following is more preferable: 0.5 mol / dm ３ More than 1.7mol / dm ３ The following is even more preferable: 0.7 mol / dm ３ 1.5mol / dm or more 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.

[0108] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and electrolyte salt. One or more types of additives may be used. When additives are used in the non-aqueous electrolyte, the additive content in the non-aqueous electrolyte is preferably 0.01% by mass or more and 10% by mass or less, more preferably 0.1% by mass or more and 7% by mass or less, even more preferably 0.2% by mass or more and 5% by mass or less, and particularly preferably 0.3% by mass or more and 3% by mass or less.

[0109] (Solid Electrolyte) The solid electrolyte can be selected from any material that has ionic conductivity, such as lithium, sodium, and calcium, and is solid at room temperature (e.g., 20°C). Examples of solid electrolytes include sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, halide solid electrolytes, and polymer solid electrolytes. One or more types of solid electrolytes can be used.

[0110] (Container) The container houses the electrode body and non-aqueous electrolyte in its internal space. The container material can be a metal material such as aluminum or stainless steel, or a resin material, with metal materials being preferred from the viewpoint of strength, etc. A composite material of metal and resin materials can also be used.

[0111] The shape of the container is not particularly limited, but it can be cylindrical, rectangular (square), disc-shaped, etc. The container may also be in the form of a sheet or other shape formed from a metal-resin composite film.

[0112] (Shape and application of the energy storage element) The shape of the energy storage element according to one embodiment of the present invention is not particularly limited. The energy storage element may be, for example, a cylindrical battery, a prismatic battery, a flat battery, a coin cell battery, a button cell battery, etc.

[0113] The applications of the energy storage element according to one embodiment of the present invention are not particularly limited. The energy storage element can be used, for example, as a power source for automobiles such as electric vehicles, hybrid vehicles, and plug-in hybrid vehicles, as a power source for electronic devices such as personal computers and communication terminals, and as a power storage power source.

[0114] The energy storage element of the present invention can be used individually or in combination. When the required output and voltage are small, the energy storage element may be used individually. On the other hand, when at least one of the required output and voltage is large, the energy storage element may be used in a combined energy storage device with other energy storage elements. In an energy storage device composed of multiple energy storage elements, at least one of the energy storage elements included in the device may be an energy storage element according to one embodiment of the present invention. The energy storage device will be described in detail later.

[0115] In one embodiment of the present invention, the energy storage element may be constrained to maintain a certain thickness, or it may not be constrained in this way. Alternatively, it may be constrained to have a certain load applied to it. When the container is constrained, expansion of the container due to charge-discharge cycles, etc., may be suppressed, and a decrease in charge-discharge performance may be suppressed. When the container is constrained, a load may or may not be applied to the electrode body inside the container. For example, the energy storage element or energy storage device may be provided with a restraining member that performs such restraint.

[0116] <Method for Manufacturing an Energy Storage Element> An energy storage element according to one embodiment of the present invention can be manufactured by a known method. The method for manufacturing the energy storage element includes, for example, preparing a positive electrode, preparing a negative electrode, preparing an electrolyte, and housing the positive electrode, negative electrode, and electrolyte in a container. The manufacturing method may also include preparing a separator, and manufacturing an electrode body using the positive electrode, negative electrode, and separator. Housing the positive electrode, negative electrode, and electrolyte in a container may be equivalent to housing the electrode body and electrolyte in a container.

[0117] Preparing the positive electrode may also mean manufacturing the positive electrode. The positive electrode can be manufactured by the method described above. Preparing the negative electrode may also mean manufacturing the negative electrode. The negative electrode can be manufactured by the method described above. Preparing the electrolyte may also mean preparing the electrolyte. If the electrolyte is a non-aqueous electrolyte, the preparation of the non-aqueous electrolyte can be done, for example, by dissolving the electrolyte salt in a non-aqueous solvent. The positive electrode, negative electrode, separator, electrolyte, etc., may be prepared by purchase or other means.

[0118] The electrode body (or positive and negative electrodes) and electrolyte can be housed in a container by known methods. If the electrolyte is a non-aqueous electrolyte, for example, the electrode body (or positive and negative electrodes) can be housed in the container first, and then the non-aqueous electrolyte can be injected through an inlet provided in the container. The inlet is sealed after the non-aqueous electrolyte is injected. The method for manufacturing the energy storage element may further include initial charging and discharging of the assembled uncharged and discharged energy storage element.

[0119] The energy storage element according to one embodiment of the present invention may be manufactured by other methods. For example, if the energy storage element according to one embodiment of the present invention is an all-solid-state battery, it may be manufactured by pressing the forming materials of the positive electrode, separator, and negative electrode individually or collectively.

[0120] <Energy Storage Device> The energy storage device 30 in Figure 2 comprises a plurality of energy storage units 20. Each energy storage unit 20 comprises a plurality of electrically connected energy storage elements 1. The energy storage device 30 may also include busbars (not shown) that electrically connect the plurality of energy storage elements 1, busbars (not shown) that electrically connect the plurality of energy storage units 20, etc. The energy storage unit 20 or the energy storage device 30 may also include a condition monitoring device (not shown) that monitors the state of one or more energy storage elements 1.

[0121] <Other Embodiments> 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. Furthermore, a part of the configuration of one embodiment may be deleted. Also, well-known technology may be added to the configuration of one embodiment.

[0122] In the above embodiment, the case in which the energy storage element is used as a non-aqueous electrolyte secondary battery that can be charged and discharged 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, lithium-ion capacitors, and other capacitors.

[0123] 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.

[0124] [Example 1] (Preparation of positive electrode) As the positive electrode active material, α-NaFeO ２ It has a type crystal structure, LiNi ０．３３ Co ０．３３ Mn ０．３３ O ２A lithium transition metal composite oxide represented by [formula] was used. A positive electrode mixture paste was prepared by mixing the above positive electrode active material with 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 above positive electrode active material, AB, and PVDF was 92.0:5.0:3.0 in terms of solid content. This positive electrode mixture paste was coated on both sides of an aluminum foil, which was the positive electrode substrate, dried, and roll-pressed to form a positive electrode active material layer, thereby obtaining the positive electrode.

[0125] (Preparation of the negative electrode) A negative electrode mixture paste was prepared by mixing graphite as the negative electrode active material, styrene-butadiene rubber (SBR) as a binder, carboxymethylcellulose (CMC) as a thickener, and water as a dispersion medium. The mass ratio of the negative electrode active material, SBR, and CMC was 98.4:1.0:0.6 in terms of solid content. This negative electrode mixture paste was coated onto both sides of a copper foil, which was the negative electrode substrate, dried, and roll-pressed to form a negative electrode active material layer, thereby obtaining the negative electrode. The BET specific surface area of ​​the negative electrode active material layer was 5.07 m². ２ The mass was / g. Furthermore, the mass per unit area of ​​the negative electrode active material layer was 7.72 mg / cm². ２ That was the case.

[0126] (Non-aqueous electrolyte) A solvent prepared by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 30:35:35, with 1.2 mol / dm³ added. ３ LiPF at this concentration ６ A non-aqueous electrolyte was obtained by dissolving it.

[0127] (Separator) As a separator, a porous resin film with a polypropylene / polyethylene / polypropylene three-layer structure was used, with an inorganic layer containing alumina particles as inorganic particles and CMC as a binder laminated on one side of the base layer. The thickness of the inorganic layer was 2.5 μm, and the mass per unit area was 0.36 mg / cm². ２ The BET specific surface area of ​​the separator was 28.3 m². ２ It was / g.

[0128] (Assembly of the energy storage element) A positive electrode, a negative electrode, and a separator were stacked to create an electrode body. The obtained electrode body was placed in a container, and then a non-aqueous electrolyte was poured into the container and sealed to obtain the energy storage element of Example 1.

[0129] [Examples 2, 3 and Comparative Examples 1 to 3] Except for the BET specific surface area of ​​the negative electrode active material layer being as shown in Table 1 and using a different separator than in Example 1, the energy storage elements of Examples 2, 3 and Comparative Examples 1 to 3 were obtained using the same procedure as in Example 1. The separators of Examples 2, 3 and Comparative Examples 1 to 3 all consisted of a base layer which was a porous polyethylene resin film, with an inorganic layer containing alumina particles as inorganic particles and CMC as a binder laminated on one side. The thickness and mass per unit area of ​​the inorganic layer were as shown in Table 1. The mass per unit area of ​​the negative electrode active material layer was 7.57 mg / cm² for Examples 2 and Comparative Examples 1, 2, and 3. ２ Example 3 had a concentration of 7.63 mg / cm³. ２ That was the case.

[0130] [Evaluation] (Initial Charge / Discharge) Initial charge and discharge were performed on each obtained energy storage element according to the following procedure. Constant current and constant voltage charging was performed at 25°C with a charging current of 1.0C and a charging termination voltage of 3.9V. The charging termination condition was when the total charging time reached 2 hours. After that, the elements were stored at 45°C for 15 hours. After that, they were stored at 25°C for 3 hours, and then constant current discharge was performed with a discharge current of 1.0C and a discharge termination voltage of 3.0V.

[0131] (Initial Capacity Verification Test) Next, an initial capacity verification test was performed on each energy storage element at 25°C in the following manner. Constant current and constant voltage charging was performed with a charging current of 1.0C and a charging termination voltage of 4.1V. The charging termination condition was set to a total charging time of 2 hours. After that, a 10-minute rest period was provided. After that, constant current discharge was performed with a discharge current of 1.0C and a discharge termination voltage of 3.0V. The discharge capacity at this time was defined as the "initial discharge capacity". The fully charged state based on the initial discharge capacity was defined as SOC (State Of Charge) 100%.

[0132] (Initial Output Verification Test) After the initial capacity verification test described above, each energy storage element was charged at a constant current and voltage of 1.0C in a constant temperature bath at 25°C until the SOC reached 50%. The charging was terminated when the total charging time reached 1 hour. Next, the elements were discharged for 12 seconds at discharge currents of 15C, 22.5C, 30C, 37.5C, 57.5C, 70C, and 75C. After each discharge, a 1-minute rest period was observed, followed by supplemental charging at a charging current of 7.5C, 15C, 22.5C, 30C, 37.5C, and 50C until the SOC reached 50%. The voltage at 1 second after the start of each discharge was plotted on the vertical axis and the discharge current on the horizontal axis to obtain a current-voltage characteristic graph. The output [W] at 1 second was determined by the IV method with a lower voltage limit of 2.5V and this was defined as the "initial output". The results are shown in Table 1.

[0133] (Output confirmation test after long-term storage at high temperatures) After the initial output confirmation test, the output of each energy storage element after long-term storage at high temperatures was measured in the following manner. Constant current charging was performed with a charging current of 1.0C in a constant temperature bath at 25°C, and the SOC was adjusted to 65%. In this state of SOC 65%, it was stored for 2500 hours in a constant temperature bath at 55°C. After the long-term storage at high temperatures described above, the output of each energy storage element after long-term storage at high temperatures was measured in the same manner as the initial output confirmation test described above. Furthermore, the percentage of this output after long-term storage at high temperatures relative to the initial output was calculated as the "output retention rate after storage". The results are shown in Table 1 and Figure 3. The dashed line in Figure 3 is an approximate curve for the example and comparative example, in which the mass per unit area of ​​the inorganic layer is equivalent.

[0134]

[0135] As shown in Table 1 and Figure 3, the mass per unit area of ​​the inorganic layer of the separator is 0.2 mg / cm³. ２ In Comparative Examples 1 to 3, the energy storage elements, which were less than 0.2 mg / cm³, showed a tendency for the power retention rate after long-term storage at high temperatures to decrease as the BET specific surface area of ​​the negative electrode active material layer increased. In contrast, the inorganic layer of the separator had a mass per unit area of ​​0.2 mg / cm³. ２The energy storage elements of Examples 1 and 2 maintained an output retention rate of over 93% after long-term storage at high temperatures, even when the BET specific surface area of ​​the negative electrode active material layer was increased. Furthermore, from a comparison of the approximate straight lines for Examples 1 and 2 and the approximate straight lines for Comparative Examples 1 to 3, it was found that when the BET specific surface area of ​​the negative electrode active material layer was increased by 2 m² ２ The amount is 0.2 mg / cm² or more, and the mass per unit area of ​​the inorganic layer of the separator is 0.2 mg / cm². ２ In the above cases, it was confirmed that good output retention rates could be obtained after long-term storage at high temperatures. Furthermore, from a comparison between Examples 1 and 3 and Comparative Example 1, and between Example 2 and Comparative Example 3, it was found that even when the mass per unit area of ​​the inorganic layer of the separator was increased, a similar initial output was maintained. In particular, when the BET specific surface area of ​​the negative electrode active material layer was 5.00 m² ２ When the amount was less than or equal to / g (Example 2 and Comparative Example 3), the decrease in initial output when the mass per unit area of ​​the inorganic layer of the separator was increased was suppressed.

[0136] 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.

[0137] 1. Energy storage element 2. Electrode body 3. Container 4. Positive electrode lead 5. Positive electrode external terminal 6. Negative electrode lead 7. Negative electrode external terminal 20. Energy storage unit 30. Energy storage device

Claims

1. The device comprises a positive electrode containing a transition metal element, a negative electrode having a negative electrode active material layer, and a separator, wherein the positive electrode and the negative electrode are laminated via the separator, the separator has an inorganic layer containing inorganic particles, and the BET specific surface area of ​​the negative electrode active material layer is 2 m². ２ The amount is 0.2 mg / cm² or more, and the mass per unit area of ​​the inorganic layer is 0.2 mg / cm². ２ The above describes the energy storage element.

2. The BET specific surface area of ​​the above negative electrode active material layer is 3.5 m². ２ / g or more 6.0m ２ The energy storage element according to claim 1, wherein the value is less than or equal to / g.

3. The mass per unit area of ​​the above inorganic layer is 0.45 mg / cm³. ２ A storage element according to claim 1 or claim 2, wherein the value is less than [value missing].

4. The BET specific surface area of ​​the above-mentioned negative electrode active material layer is 5.0 m². ２ The energy storage element according to claim 1 or claim 2, wherein the energy is less than or equal to / g.

5. The energy storage element according to claim 1 or claim 2, wherein the inorganic particles include alumina particles or boehmite particles.

6. The energy storage element according to claim 1 or claim 2, wherein the thickness of the inorganic layer is 1.8 μm or more.

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