Positive electrode for power storage element, power storage element

Abstract

A positive electrode for a power storage element according to one aspect of the present invention has a positive electrode active material layer containing a positive electrode active material and a conductive auxiliary agent, the conductive auxiliary agent contains carbon nanotubes, the content of the conductive auxiliary agent in the positive electrode active material layer is 3.0 mass% or less, and the BET specific surface area of the positive electrode active material layer is 1.00 m2 / g to 3.00 m2 / g.

Description

TECHNICAL FIELD

[0001] The present application relates to a positive electrode for a power storage element, a power storage element, and a power storage device. BACKGROUND

[0002] A nonaqueous electrolyte secondary battery typified by a lithium ion secondary battery is used in electronic devices such as personal computers and communication terminals, and automobiles, because of its high energy density. The nonaqueous electrolyte secondary battery is generally configured to have a pair of electrodes electrically separated by a separator and a nonaqueous electrolyte interposed between the electrodes, and is charged and discharged by accepting and releasing charge transfer ions between the electrodes. Capacitors such as lithium ion capacitors and electric double layer capacitors, and power storage elements using electrolytes other than nonaqueous electrolytes, in addition to the nonaqueous electrolyte secondary battery, have also been widely popularized.

[0003] As a power storage element, it is desirable to have a large energy density and excellent output characteristics. In order to improve the performance of such a power storage element, a conductive agent (conductive aid) or the like is sometimes added to a positive electrode mixture layer (positive electrode active material layer) (see Patent Document 1).

[0004] PRIOR ART DOCUMENTS

[0005] PATENT DOCUMENTS

[0006] Patent Document 1: Japanese Patent Application Publication No. 2018-073687 SUMMARY

[0007] It can be considered that the energy density of a power storage element can be increased by relatively increasing the amount of positive electrode active material in the positive electrode active material layer. For example, by reducing the content of the conductive aid in the positive electrode active material layer, the energy density of the power storage element can be increased. On the other hand, if the content of the conductive aid is reduced, the conductive path between the positive electrode active materials cannot be sufficiently formed, and the resistance can increase.

[0008] The present application was completed in view of the above circumstances, and aims to provide a positive electrode for a power storage element that can both increase the energy density of the power storage element and reduce the resistance, and a power storage element and a power storage device that have a large energy density and a small resistance.

[0009] The positive electrode for a power storage element according to an aspect of the present application has a positive electrode active material layer containing a positive electrode active material and a conductive aid, the conductive aid includes carbon nanotubes, the content of the conductive aid in the positive electrode active material layer is 3.0% by mass or less, and the BET specific surface area of the positive electrode active material layer is 1.00 m 2 / g to 3.00 m 2 / g.

[0010] The power storage element according to another aspect of the present application has the positive electrode for a power storage element.

[0011] The power storage device according to still another aspect of the present application includes two or more power storage elements, and includes one or more power storage elements according to the above aspect.

[0012] The power storage element according to one aspect of the present application can increase the energy density and reduce the resistance of the power storage element.

[0013] The power storage element and the power storage device according to another aspect of the present application have a large energy density and a small resistance. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 is a perspective view showing one embodiment of a power storage element including the positive electrode for power storage element according to the present application.

[0015] Figure 2 is a schematic view of one embodiment of a power storage device including a plurality of power storage elements including the positive electrode for power storage element according to the present application. DETAILED DESCRIPTION

[0016] First, a summary of the positive electrode for power storage element, the power storage element, and the power storage device disclosed in the present specification will be described.

[0017] (1) The positive electrode for power storage element according to one aspect of the present application includes a positive electrode active material layer including a positive electrode active material and a conductive aid, the conductive aid includes carbon nanotubes, the content of the conductive aid in the positive electrode active material layer is 3.0 mass% or less, and the BET specific surface area of the positive electrode active material layer is 1.00 m 2 / g to 3.00 m 2 / g.

[0018] The positive electrode for power storage element according to the above (1) can increase the energy density and reduce the resistance of the power storage element. The reason is not yet determined, but it is presumed to be the following reason. In the positive electrode for power storage element, the content of the conductive aid is 3.0 mass% or less. In the conventional positive electrode for power storage element, the content of the positive electrode active material in the positive electrode active material layer is relatively large, and the energy density of the power storage element can be increased, but the conductive path between the positive electrode active materials cannot be sufficiently formed, and the resistance is easily increased. In contrast, in the positive electrode for power storage element according to the above (1), since the conductive aid includes carbon nanotubes, the conductive path connecting the positive electrode active materials can be easily formed with a small amount of the conductive aid. In addition, since the BET specific surface area of the positive electrode active material layer is controlled to an appropriate range, the conductive aid is easily and uniformly dispersed in the positive electrode active material layer. That is, the conductive path between the positive electrode active materials is uniformly and easily formed. Therefore, the positive electrode for power storage element according to the above (1) can increase the energy density and reduce the resistance.

[0019] (2) The positive electrode for a power storage element according to the above (1) can have a content of the positive electrode active material in the positive electrode active material layer of 95.0 mass% or more.

[0020] The positive electrode for a power storage element according to the above (2) can further increase the energy density of the power storage element.

[0021] (3) The power storage element according to another aspect of the present application includes the positive electrode for a power storage element according to the above (1) or (2).

[0022] The power storage element according to the above (3) has a large energy density and a small electric resistance because it includes the positive electrode for a power storage element according to the above (1) or (2).

[0023] In the present application, the "content of the electrically conductive aid (percentage of the content)" in the positive electrode active material layer is obtained based on thermogravimetric differential thermal analysis (TG-DTA) by the following steps.

[0024] First, the mass W0 [mg] of a sample of about 10 mg of the positive electrode active material layer is measured at room temperature with a precision of 0.01 mg. Next, using a thermogravimetric differential thermal analysis device, the sample is heated from 30°C to 900°C at a temperature increase rate of 2°C / min in an atmospheric atmosphere. At this time, the mass W1 [mg] at 460°C and the mass W2 [mg] at 700°C of the sample are measured with a precision of 0.01 mg, respectively. The content P of the electrically conductive aid in the positive electrode active material layer is calculated by the following formula. m [Mass%] by the following formula.

[0025] P m = {(W1 - W2) / W0} x 100

[0026] In the present application, the "BET specific surface area" is set to a value obtained from an adsorption isotherm obtained by a nitrogen adsorption method. Specifically, an adsorption isotherm is measured by a nitrogen adsorption method using liquid nitrogen in a range of relative pressure P / P0 (P0 = about 770 mmHg) from 0 to 1. Five points are extracted from a region of P / P0 = 0.05 to 0.3 of the obtained adsorption isotherm, a BET curve is plotted, and the BET specific surface area is calculated from the y-intercept and the slope of the straight line.

[0027] It should be noted that the sample of the positive electrode active material layer for the measurement of the above-mentioned thermal gravimetric differential thermal analysis and the above-mentioned nitrogen adsorption method is prepared in the following procedure in the case where the positive electrode active material layer is incorporated in a positive electrode of an electricity storage device. The electricity storage device is subjected to constant current discharge at a current of 0.1 C to a discharge termination voltage in ordinary use, and is brought to a discharged state. Here, "in ordinary use" means a case where the electricity storage device is used under the charge / discharge conditions recommended or specified for the electricity storage device. Next, the electricity storage device in the discharged state is disassembled, and the positive electrode is taken out, and the components (electrolyte, etc.) adhered to the positive electrode are sufficiently washed with dimethyl carbonate. Then, the positive electrode is dried, and the positive electrode active material layer is collected from the dried positive electrode. The operation from the disassembly of the electricity storage device to the collection of the positive electrode active material layer is performed in an argon atmosphere at a dew point of -60°C or lower.

[0028] (4) The electricity storage device according to still another aspect of the present application includes two or more electricity storage devices, and includes one or more electricity storage devices according to the above-mentioned (3).

[0029] The electricity storage device according to the above-mentioned (4) includes one or more electricity storage devices according to the above-mentioned (3), and thus has a large energy density and a small electric resistance.

[0030] Hereinafter, the positive electrode for an electricity storage device, the electricity storage device, the electricity storage device manufacturing method, and other embodiments according to an embodiment of the present application will be described in detail. It should be noted that the names of the components (elements) used in each embodiment are sometimes different from the names of the components (elements) used in the background art.

[0031] [Positive electrode for an electricity storage device]

[0032] The positive electrode for an electricity storage device according to an embodiment of the present application includes a positive electrode substrate and a positive electrode active material layer disposed directly on the positive electrode substrate or via an intermediate layer.

[0033] <Positive electrode substrate>

[0034] The positive electrode substrate has conductivity. Whether or not a material has "conductivity" is determined by the volume resistivity measured in accordance with JIS-H-0505 (1975) as 10 -2Ω·cm. As the material of the positive electrode base material, a metal such as aluminum, titanium, tantalum, stainless steel, or an alloy thereof can be used. Among them, from the viewpoint of potential resistance, high conductivity, and cost, aluminum or an aluminum alloy is preferable. As the positive electrode base material, a foil, an evaporation film, a mesh, a porous material, or the like can be given, and from the viewpoint of cost, a foil is preferable. Therefore, an aluminum foil or an aluminum alloy foil is preferable as the positive electrode base material. As the aluminum or the aluminum alloy, A1085, A3003, A1N30, or the like specified in JIS-H-4000 (2014) or JIS-H-4160 (2006) can be given.

[0035] The average thickness of the positive electrode base material is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, further preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By making the average thickness of the positive electrode base material within the above range, both the strength of the positive electrode base material and the energy density per unit volume of the power storage element can be improved.

[0036] The intermediate layer is a layer disposed between the positive electrode base material and the positive electrode active material layer. The intermediate layer reduces the contact resistance between the positive electrode base material and the positive electrode active material layer by containing a conductive aid such as carbon particles. The structure of the intermediate layer is not particularly limited, and for example, contains a binder and a conductive aid. The conductive aid can also be the same as the conductive aid in the positive electrode active material layer described later.

[0037] <Positive electrode active material layer>

[0038] In one embodiment of the present application, the positive electrode active material layer contains a positive electrode active material and a conductive aid. The positive electrode active material layer can contain any component, such as a binder, a thickening agent, a filler, or the like, as needed.

[0039] As the positive electrode active material, a known positive electrode active material can be appropriately selected. As the positive electrode active material for a lithium ion secondary battery, a material capable of occluding and releasing lithium ions is generally used. As the positive electrode active material, for example, a lithium transition metal complex oxide having an α-NaFe02-type crystal structure, a lithium transition metal complex oxide having a spinel crystal structure, a polyanion compound, a chalcogen compound, sulfur, or the like can be given. As the lithium transition metal complex oxide having an α-NaFe02-type crystal structure, for example, Li[Li x Ni (1 - x) ]02(0≤x<0.5), Li[Li x Ni γ Co (1 - x - γ) ]02(0≤x<0.5, 0<γ<1, 0<1-x-γ), 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, 0<1-x-γ), Li[Li x Ni γ MnβCo (1 - x - γ - β ) ]O2(0≤x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1-x-γ-β), Li[Li x Ni γ CoβAl (1 - x - γ - β ) ]O2(0≤x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1-x-γ-β), etc. As the lithium transition metal composite oxide having a spinel crystal structure, Li x Mn2O4, Li x Ni γ Mn (2 - γ) O4, etc. As the polyanion compound, LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, Li2CoPO4F, etc. As the chalcogenide compound, titanium disulfide, molybdenum disulfide, molybdenum dioxide, etc. The atoms or polyanion in these materials can be partially substituted with atoms or anionic substances composed of other elements. The surface of these materials can also be covered with other materials. In the positive electrode active material layer, one of these materials can be used alone, or two or more kinds can be used in mixture.

[0040] As the positive electrode active material, from the viewpoint of increasing the energy density, a lithium transition metal composite oxide is preferable, a lithium transition metal composite oxide having an α-NaFeO2type crystal structure is more preferable, and a lithium transition metal composite oxide represented by the chemical formula Li x Ni γ MnβCo (1 - x - γ - β ) ]O2(0≤x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1-x-γ-β) is further preferable, and a lithium transition metal composite oxide in which 0.55≤γ in the above chemical formula is still further preferable.

[0041] The positive electrode active material is generally in the form of particles (powder). The lower limit of the average particle diameter of the positive electrode active material is preferably 0.1 μm, more preferably 1 μm, and further preferably 3 μm. On the other hand, the upper limit of the average particle diameter of the positive electrode active material is preferably 20 μm, more preferably 16 μm, and further preferably 12 μm. By setting the average particle diameter of the positive electrode active material to be equal to or greater than the lower limit described above, the production or handling of the positive electrode active material is facilitated. By setting the average particle diameter of the positive electrode active material to be equal to or less than the upper limit described above, the electronic conductivity of the positive electrode active material layer is improved. Note that the average particle diameter of the positive electrode active material can be equal to or greater than any of the lower limits described above and equal to or less than any of the upper limits described above. Note that in the case where a composite of the positive electrode active material and another material is used, the average particle diameter of the composite is regarded as the average particle diameter of the positive electrode active material. The "average particle diameter" refers to a value at which the volume-based standard cumulative distribution is 50% according to the particle size distribution measured by a laser diffraction / scattering method on a dilute solution obtained by diluting particles in a solvent, in accordance with JIS-Z-8825 (2013), and calculated in accordance with JIS-Z-8819-2 (2001). It is confirmed that the average particle diameter determined based on the measurement described above is substantially the same as the average particle diameter determined by extracting 100 positive electrode active material particles from a SEM image obtained using a scanning electron microscope (SEM) of the positive electrode active material layer, while avoiding extremely large positive electrode active material particles and extremely small positive electrode active material particles. Note that the particle diameter of each positive electrode active material particle in the measurement based on the SEM image is the Feret diameter, and the volume of each positive electrode active material particle is calculated as a sphere having the Feret diameter as the diameter.

[0042] In order to obtain a powder having a predetermined particle diameter, a pulverizer, a classifier, or the like can be used. As a pulverization method, for example, a method using a mortar, a ball mill, a sand mill, a vibration ball mill, a planetary ball mill, a jet mill, a Counter Jet Mill, a rotary airflow type jet mill, or a sieve, or the like can be given. Wet pulverization in the presence of water or an organic solvent such as hexane can also be used. As a classification method, a sieve, an air classifier, or the like can be used, and either dry or wet classification can be used as needed.

[0043] The content of the positive electrode active material in the positive electrode active material layer is preferably 95.0% by mass to 99.0% by mass, more preferably 96.0% by mass to 99.0% by mass, and further preferably 97.0% by mass to 99.0% by mass. By setting the content of the positive electrode active material to be within the range described above, both the high energy density of the positive electrode active material layer and the productivity can be achieved.

[0044] In one embodiment of the present application, the electrically conductive assistant contains carbon nanotubes (CNT). As the CNT, single-walled carbon nanotubes (SWCNT) formed of 1 layer of graphene, multi-walled carbon nanotubes (MWCNT) formed of 2 or more layers (for example, 2 to 20 layers, typically 2 to 60 layers) of graphene, and the like can be given. The CNT can also be one containing both SWCNT and MWCNT in any ratio (mass ratio of SWCNT:MWCNT is, for example, 100:0 to 0:100, preferably 100:0 to 80:20). It is particularly preferable that the CNT be composed substantially only of SWCNT. The structure of the CNT is not particularly limited, and can be any of a chiral (helical) type, a zigzag type, and a armchair type. In addition, a catalyst metal (for example, Fe, Co, and platinum group elements (Ru, Rh, Pd, Os, Ir, Pt)) and the like used in the synthesis of the CNT can also be contained.

[0045] From the viewpoint of appropriately controlling the BET specific surface area of the positive electrode active material layer described later, the electrically conductive assistant can also contain a material other than CNT. As such a material, for example, a carbonaceous material other than CNT, a metal, an electrically conductive ceramic, and the like can be given. As the carbonaceous material other than CNT, graphite, non-graphitic carbon, graphene-based carbon, and the like can be given. As the non-graphitic carbon, carbon nanofibers, pitch-based carbon fibers, carbon black, and the like can be given. As the carbon black, furnace black, acetylene black, Ketjen black, and the like can be given. As the graphene-based carbon, graphene and fullerene, and the like can be given. As the shape of these materials, powder, fibrous, and the like can be given. The material other than CNT can be contained singly as one kind, or two or more kinds can be contained in mixture. In addition, the CNT can also be used in combination with these materials. For example, a material in which CNT is combined with carbon black can be used. Among the materials other than CNT, from the viewpoints of electronic conductivity and coatability, carbon black is preferable, and among them, acetylene black is preferable.

[0046] As the lower limit of the content of the electrically conductive assistant in the positive electrode active material layer, 0.1% by mass is preferable, 0.3% by mass is more preferable, and 0.5% by mass is further preferable. On the other hand, the upper limit of the content of the electrically conductive assistant in the positive electrode active material layer is 3.0% by mass, 2.5% by mass is preferable, 2.0% by mass is more preferable, and 1.5% by mass is further preferable. By making the content of the electrically conductive assistant be the above lower limit or more, the electronic conductivity of the positive electrode active material layer is improved. In addition, if the content of the electrically conductive assistant is made to be the above upper limit or less, the energy density of the power storage element can be increased. As the content of the electrically conductive assistant, it can be the above lower limit or more and the above upper limit or less.

[0047] As a lower limit of the content of the carbon nanotube (CNT) in the positive electrode active material layer, 0.1% by mass is preferable, 0.2% by mass is more preferable, and 0.3% by mass is further preferable. On the other hand, as an upper limit of the content of the CNT in the positive electrode active material layer, 2.0% by mass is preferable, 1.5% by mass is more preferable, and 1.0% by mass is further preferable. If the content of the CNT is equal to or higher than the above lower limit, the electronic conductivity of the positive electrode active material layer is improved. In addition, if the content of the CNT is equal to or lower than the above upper limit, the BET specific surface area of the positive electrode active material layer described later is easily controlled appropriately. The content of the CNT can be equal to or higher than any of the above lower limits and equal to or lower than any of the above upper limits.

[0048] As the binder, for example, a thermoplastic resin such as a fluororesin (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), or the like), polyethylene, polypropylene, polyacrylic acid, polyimide, or the like; an elastomer such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, or the like; a polysaccharide polymer, or the like can be given.

[0049] The content of the binder in the positive electrode active material layer is preferably 1% by mass to 10% by mass, and more preferably 1.5% by mass to 9% by mass. By setting the content of the binder within the above range, the positive electrode active material can be stably maintained.

[0050] As the thickening agent, for example, a polysaccharide polymer such as carboxymethyl cellulose (CMC), methyl cellulose, or the like can be given. In the case where the thickening agent has a functional group that reacts with lithium or the like, the functional group can be inactivated in advance by methylation or the like. In the case where the thickening agent is used, the content of the thickening agent in the positive electrode active material layer can be set to 0.1% by mass to 8% by mass, and is generally preferably 5% by mass or less, and more preferably 2% by mass or less. The technology disclosed herein is preferably implemented in such a manner that the positive electrode active material layer does not contain the thickening agent.

[0051] The filler is not particularly limited. As the filler, a polyolefin such as polypropylene, polyethylene, an inorganic oxide such as silicon dioxide, aluminum oxide, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate, a hydroxide such as magnesium hydroxide, calcium hydroxide, aluminum hydroxide, a carbonate such as calcium carbonate, a sparingly soluble ionic crystal such as calcium fluoride, barium fluoride, barium sulfate, a nitride such as aluminum nitride, silicon nitride, a substance from a mineral resource such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, mica, or a man-made substance thereof, or the like can be given. In the case where the filler is used, the content of the filler in the positive electrode active material layer can be set to 0.1% by mass to 8% by mass, and is generally preferably 5% by mass or less, and more preferably 2% by mass or less. The technology disclosed herein is preferably implemented in such a manner that the positive electrode active material layer does not contain the filler.

[0052] The positive electrode active material layer can also contain, as components other than the positive electrode active material, a conductive aid, a binder, a thickening agent, a filler, and the like, and can contain typical non-metal elements such as B, N, P, F, Cl, Br, and I, and typical metal 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.

[0053] The lower limit of the BET specific surface area of the positive electrode active material layer is 1.00 m2 / g, preferably 1.02 m2 / g, and more preferably 1.05 m2 / g. On the other hand, the upper limit of the BET specific surface area of the positive electrode active material layer is 3.00 m2 / g, preferably 2.50 m2 / g, and more preferably 2.00 m2 / g. 2 2 2 2 2 2 If the BET specific surface area of the positive electrode active material layer is equal to or greater than the lower limit described above, the conductive aid is easily uniformly dispersed in the positive electrode active material layer. That is, a conductive path between the positive electrode active materials is uniformly formed, and the resistance can be reduced. If the BET specific surface area of the positive electrode active material layer is equal to or less than the upper limit described above, a conductive path between the positive electrode active materials is easily formed even with a small amount of the conductive aid. In addition, if the BET specific surface area of the positive electrode active material layer is equal to or less than the upper limit described above, decomposition of the electrolyte and the like in contact with the positive electrode of the power storage element can be suppressed, and the capacity reduction during durability (during charge and discharge cycles, and the like) can be suppressed. The BET specific surface area of the positive electrode active material layer can be equal to or greater than any of the lower limits described above and equal to or less than any of the upper limits described above. Note that the BET specific surface area of the positive electrode active material layer can be controlled by adjusting the content ratio of the conductive aid, the kneading method when the positive electrode mixture is prepared, the rotation speed of the kneader, the press pressure when the positive electrode active material layer is formed on the positive electrode substrate, and the like.

[0054] [Power storage element]

[0055] The power storage element according to one embodiment of the present application includes a positive electrode, a negative electrode, and a separator. The positive electrode includes a positive electrode active material layer, a positive electrode current collector, and a positive electrode binder. The positive electrode active material layer contains a positive electrode active material, a conductive aid, and a binder. The positive electrode current collector is provided with the positive electrode active material layer. The positive electrode binder is provided with the positive electrode active material layer. The negative electrode includes a negative electrode active material layer, a negative electrode current collector, and a negative electrode binder. The negative electrode active material layer contains a negative electrode active material, a conductive aid, and a binder. The negative electrode current collector is provided with the negative electrode active material layer. The negative electrode binder is provided with the negative electrode active material layer. The separator is provided with the positive electrode active material layer and the negative electrode active material layer. The power storage element according to one embodiment of the present application is a nonaqueous electrolyte secondary battery.

[0056] [Negative electrode]​​​​​

[0057] The negative electrode has a negative electrode substrate and a negative electrode active material layer disposed directly on the negative electrode substrate or via an intermediate layer. The constitution of the intermediate layer is not particularly limited and can be selected from the constitutions exemplified above for the positive electrode.

[0058] The negative electrode substrate has electrical conductivity. As the material of the negative electrode substrate, a metal such as copper, nickel, stainless steel, nickel-plated steel, aluminum, or an alloy thereof, a carbon material, or the like can be used. Among them, copper or a copper alloy is preferred. As the negative electrode substrate, a foil, an evaporation film, a mesh, a porous material, or the like can be given, and from the viewpoint of cost, a foil is preferred. Therefore, as the negative electrode substrate, a copper foil or a copper alloy foil is preferred. As examples of the copper foil, a rolled copper foil, an electrolytic copper foil, or the like can be given.

[0059] The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, further preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By making the average thickness of the negative electrode substrate within the above range, both the strength of the negative electrode substrate and the energy density per unit volume of the power storage element can be improved.

[0060] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer can contain, as necessary, any of a conductive aid, a binder, a thickener, a filler, or the like. Any of the conductive aid, the binder, the thickener, the filler, or the like can be selected from the materials exemplified above for the positive electrode.

[0061] The negative electrode active material layer can also contain, as a component other than the negative electrode active material, a conductive aid, a binder, a thickener, a filler, or the like, a typical nonmetallic element such as B, N, P, F, Cl, Br, or I, a typical metallic element such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, or Ba, or a transition metal element such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, or W.

[0062] The negative electrode active material can be appropriately selected from known negative electrode active materials. The negative electrode active material for a lithium-ion secondary battery can generally use a material that can occlude and release lithium ions. As the negative electrode active material, for example, metal Li; a metal or semimetal such as Si or Sn; a metal oxide or semimetal oxide such as a Si oxide, a Ti oxide, or a Sn oxide; a titanium-containing oxide such as Li4Ti5O12, LiTiO2, or TiNb2O7; a polyphosphoric acid compound; silicon carbide; a carbon material such as graphite, non-graphitizable carbon, or easily graphitizable carbon; or the like can be given. Among them, graphite and non-graphitizable carbon are preferred. In the negative electrode active material layer, one of these materials can be used alone, or two or more kinds thereof can be used in mixture. 12 ​

[0063] "Graphite" means a carbon material in which the average interlattice spacing (d 002 ) of the (002) plane determined by X-ray diffraction method is 0.33 nm or more and less than 0.34 nm in the charged and discharged state or the discharged state.

[0064] "Non-graphitizable carbon" means a carbon material in which the average interlattice spacing (d 002 ) of the (002) plane determined by X-ray diffraction method is 0.34 nm to 0.42 nm in the charged and discharged state or the discharged state. As the non-graphitizable carbon, for example, there can be mentioned hard graphitizable carbon and easy graphitizable carbon. As the non-graphitizable carbon, for example, there can be mentioned a material from a resin, petroleum pitch or a material from petroleum pitch, petroleum coke or a material from petroleum coke, a material from a plant, a material from an alcohol, and the like.

[0065] Here, the "discharged state" of the carbonaceous material means a state in which lithium ions that can be occluded and released are sufficiently released from the carbonaceous material as the negative electrode active material with charging and discharging. For example, in a half cell in which a negative electrode containing a carbon material as the negative electrode active material is used as the working electrode and metallic Li is used as the counter electrode, the open circuit voltage is 0.7 V or more.

[0066] "Hard graphitized carbon" means a carbon material in which the above d 002 is 0.36 nm to 0.42 nm.

[0067] "Easy graphitized carbon" means a carbon material in which the above d 002 is 0.34 nm or more and less than 0.36 nm.

[0068] The negative electrode active material is generally in the form of a particle (powder). The average particle diameter of the negative electrode active material can be, for example, 1 nm to 100 μm. In the case where the negative electrode active material is a carbon material, a titanium-containing oxide, or a polyphosphoric acid compound, the average particle diameter thereof can also be 1 μm to 100 μm. In the case where the negative electrode active material is Si, Sn, a Si oxide, a Sn oxide, or the like, the average particle diameter thereof can also be 1 nm to 1 μm. By setting the average particle diameter of the negative electrode active material to be the above lower limit or more, the manufacture or handling of the negative electrode active material is facilitated. By setting the average particle diameter of the negative electrode active material to be the above upper limit or less, the electronic conductivity of the negative electrode active material layer is improved. In order to obtain a powder of a prescribed particle diameter, a pulverizer, a classifier, or the like can be used. The pulverizing method and the classifying method can be selected, for example, from the methods exemplified above for the positive electrode. When the negative electrode active material is a metal such as metallic Li, the negative electrode active material layer can also be in the form of a foil.

[0069] The content of the negative electrode active material in the negative electrode active material layer is preferably 60 to 99 mass%, more preferably 90 to 98 mass%. By setting the content of the negative electrode active material within the above range, both high energy density of the negative electrode active material layer and workability can be achieved. When the negative electrode active material is metallic Li, the content of the negative electrode active material in the negative electrode active material layer can be 99 mass% or more, or 100 mass%.

[0070] <Separator>

[0071] The separator can be appropriately selected from known separators. As the separator, for example, a separator composed only of a base material layer, a separator in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the base material layer, or the like can be used. As the shape of the base material layer of the separator, woven fabric, nonwoven fabric, and porous resin film, or the like can be given. Among these shapes, from the viewpoint of strength, a porous resin film is preferred, and from the viewpoint of liquid retention of the nonaqueous electrolyte, a nonwoven fabric is preferred. As the material of the base material layer of the separator, from the viewpoint of the cutting function, for example, polyolefin such as polyethylene or polypropylene is preferred, and from the viewpoint of oxidation decomposition resistance, polyimide, aramid, or the like is preferred. As the base material layer of the separator, a material in which these resins are compounded can also be used.

[0072] The heat-resistant particles contained in the heat-resistant layer preferably have a mass reduction of 5% or less when warmed from room temperature to 500°C in an air atmosphere at 1 atmosphere, and further preferably have a mass reduction of 5% or less when warmed from room temperature to 800°C. As a material having a mass reduction of 5% or less, an inorganic compound can be given. As the inorganic compound, for example, oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalent crystals such as silicon and diamond; and substances derived from mineral resources such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, mica, and artificial substances thereof can be given. As the inorganic compound, a single substance or a composite of these substances can be used alone, or two or more kinds thereof can be used in mixture. Among these inorganic compounds, from the viewpoint of safety of the power storage element, silicon oxide, aluminum oxide, or aluminosilicate is preferred.

[0073] From the viewpoint of strength, the porosity of the separator is preferably 80% or less by volume, and from the viewpoint of discharge performance, is preferably 20% or more by volume. Here, the "porosity" refers to a value on a volume basis, and is measured using a mercury porosimeter.

[0074] As the separator, a polymer gel composed of a polymer and an electrolyte can be used. As the polymer, for example, polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, polyvinylidene fluoride, or the like can be mentioned. By using the polymer gel, an effect of suppressing liquid leakage can be obtained. As the separator, a porous resin film or a nonwoven fabric or the like as described above can be used in combination with the polymer gel.

[0075] <Non-aqueous electrolyte>

[0076] As the non-aqueous electrolyte, a non-aqueous electrolyte solution can also be used. The non-aqueous electrolyte solution contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent.

[0077] As the non-aqueous solvent, one can be appropriately selected from publicly known non-aqueous solvents. The non-aqueous solvent includes cyclic carbonates, chain carbonates, carboxylic acid esters, phosphoric acid esters, sulfonic acid esters, ethers, amides, nitriles, and the like. As the non-aqueous solvent, one in which a part of hydrogen atoms contained in these compounds is substituted with a halogen atom can be used.

[0078] As the cyclic carbonate, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), chloro vinylene carbonate, fluorinated ethylene carbonate (FEC), difluorinated ethylene carbonate (DFEC), styrene carbonate, 1-phenyl vinylidene carbonate, and 1,2-diphenyl vinylidene carbonate, or the like can be mentioned. Among them, EC is preferred.

[0079] As the chain carbonate, diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, di(trifluoroethyl) carbonate, or the like can be mentioned. Among them, EMC is preferred.

[0080] As the non-aqueous solvent, it is preferred to use a cyclic carbonate or a chain carbonate, and more preferably, a cyclic carbonate and a chain carbonate are used in combination. By using a cyclic carbonate, dissociation of the electrolyte salt can be promoted, and the ionic conductivity of the non-aqueous electrolyte solution can be increased. By using a chain carbonate, the viscosity of the non-aqueous electrolyte solution can be controlled to be low. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate : chain carbonate) is, for example, preferably in the range of 5 : 95 to 50 : 50.

[0081] As the electrolyte salt, one can be appropriately selected from publicly known electrolyte salts. As the electrolyte salt, lithium salts, sodium salts, potassium salts, magnesium salts, calcium salts, and the like can be mentioned. Among them, lithium salts are preferred.

[0082] ​As the lithium salt, inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, LiN(SO2F)2, lithium oxalate salts such as lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiFOB), lithium difluoro bis(oxalato)phosphate (LiFOP), lithium salts having a halogenated hydrocarbon group such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, LiC(SO2C2F5)3, and the like can be mentioned. Among these, inorganic lithium salts are preferred, and LiPF6is more preferred.

[0083] The content of the electrolyte salt in the nonaqueous electrolyte solution is preferably 0.1 mol / dm3at 20°C under 1 atm. 3 ~ 2.5 mol / dm3 3 More preferably, it is 0.3 mol / dm3 3 ~ 2.0 mol / dm3 3 Further preferably, it is 0.5 mol / dm3 3 ~ 1.7 mol / dm3 3 Particularly preferably, it is 0.7 mol / dm3 3 ~ 1.5 mol / dm3 3 By making the content of the electrolyte salt within the above range, the ionic conductivity of the nonaqueous electrolyte solution can be improved.

[0084] In the nonaqueous electrolyte, in addition to the nonaqueous solvent and the electrolyte salt, an additive can be contained. As the additive, halogenated carbonates such as fluorine ethylene carbonate (FEC), difluoro ethylene carbonate (DFEC); lithium oxalate salts such as lithium bis(oxalato)borate (LiBOB), lithium difluoro oxalate borate (LiFOB), lithium difluoro bis(oxalato)phosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, tert-butylbenzene, tert-amylbenzene, diphenyl ether, dibenzofuran; partially halogenated compounds of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, 3,5-difluoroanisole; vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, pentenedioic anhydride, itaconic anhydride, cyclohexane dicarboxylic anhydride; vinyl sulfite, propylene sulfite, dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2-dioxo-l,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo-l,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridyl disulfide, 1,3-propene sultone, 1,3-propane sultone, 1,4-butane sultone, 1,4-butene sultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetratrimethylsilyl titanate, lithium monofluorophosphate, lithium difluorophosphate, and the like can be exemplified. One of these additives can be used alone, or two or more kinds thereof can be used in combination.

[0085] The content of the additive in the nonaqueous electrolyte is preferably 0.01 to 10% by mass, more preferably 0.1 to 7% by mass, further preferably 0.2 to 5% by mass, and particularly preferably 0.3 to 3% by mass, relative to the total mass of the nonaqueous electrolyte. By setting the content of the additive within the above range, the capacity retention after high-temperature storage or the charge-discharge cycle characteristics can be improved, and the safety can be further improved.

[0086] A solid electrolyte can be used in the nonaqueous electrolyte, or the nonaqueous electrolyte and the solid electrolyte can be used in combination.

[0087] The solid electrolyte can be selected from any material that has ion conductivity and is solid at room temperature (e.g., 15° C. to 25° C.), such as lithium, sodium, and calcium. Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and polymer solid electrolytes.

[0088] As sulfide solid electrolytes, in the case of lithium ion secondary batteries, for example, Li2S-P2S5, LiI-Li2S-P2S5, Li 10 Ge-P2S 12 wait.

[0089] The shape of the electric storage element of the present embodiment is not particularly limited, and examples thereof include cylindrical batteries, rectangular batteries, flat batteries, coin batteries, and button batteries.

[0090] Figure 1 This figure shows an energy storage element 1, which is an example of a rectangular battery. Note that this figure shows the interior of the container. An electrode assembly 2, comprising a positive electrode and a negative electrode wound with a separator interposed therebetween, is housed in a rectangular container 3. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode lead 41. The negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode lead 51.

[0091] [Electricity Storage Device]

[0092] The energy storage element of this embodiment can be installed as an energy storage unit (battery module) composed of a plurality of energy storage elements, such as an electric vehicle (EV), a hybrid electric vehicle (HEV), or a plug-in hybrid electric vehicle (PHEV), as a power supply for vehicles such as personal computers and communication terminals, or as a power supply for power storage. In this case, the technology of the present invention can be applied to at least one energy storage element included in the energy storage unit.

[0093] An energy storage device according to one embodiment of the present invention includes two or more energy storage elements, including one or more energy storage elements according to the above-described embodiment of the present invention (hereinafter referred to as the "second embodiment"). The at least one energy storage element included in the energy storage device of the second embodiment may be a single energy storage element according to the above-described embodiment of the present invention, one or more energy storage elements other than the above-described embodiment of the present invention, or two or more energy storage elements according to the above-described embodiment of the present invention, as long as the technology of the above-described embodiment of the present invention is applied.

[0094] Figure 2FIG. 2 shows an example of a secondary battery 30 of the second embodiment in which two or more of the power storage units 20 in which two or more of the power storage elements 1 are electrically connected are further collected. The secondary battery 30 can also include bus bars (not shown) for electrically connecting two or more of the power storage elements 1, bus bars (not shown) for electrically connecting two or more of the power storage units 20, and the like. The power storage unit 20 or the secondary battery 30 can also include a state monitoring device (not shown) for monitoring the state of one or more of the power storage elements.

[0095] [Method for manufacturing power storage element]

[0096] The method for manufacturing the power storage element of the present embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing an electrode body, preparing a nonaqueous electrolyte, and housing the electrode body and the nonaqueous electrolyte in a container. The preparation of the electrode body includes, for example, preparing a positive electrode and a negative electrode, and forming an electrode body by stacking or winding the positive electrode and the negative electrode via a separator.

[0097] The method for housing the nonaqueous electrolyte in the container can be appropriately selected from known methods. For example, in the case of using a nonaqueous electrolytic solution as the nonaqueous electrolyte, the nonaqueous electrolytic solution can be injected from an injection port formed in the container, and then the injection port can be sealed.

[0098] [Other embodiments]

[0099] Note that the positive electrode for power storage element and the power storage element of the present application are not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present application. For example, the configuration of one embodiment can be supplemented with the configuration of another embodiment, and a part of the configuration of one embodiment can be replaced with the configuration of another embodiment or a known technique. Furthermore, a part of the configuration of one embodiment can be deleted. In addition, a known technique can be added to the configuration of one embodiment.

[0100] In the above-described embodiments, the positive electrode for power storage element is described as being used in a nonaqueous electrolyte secondary battery (e.g., a lithium ion secondary battery) that can be charged and discharged, but the type, shape, size, capacity, and the like of the power storage element in which the positive electrode for power storage element is used are arbitrary. The present application is also applicable to various secondary batteries, electric double layer capacitors or lithium ion capacitors, and power storage elements using an electrolyte other than a nonaqueous electrolyte.

[0101] Examples

[0102] Hereinafter, the positive electrode production step, the power storage element production step, and the evaluation step of the above-described power storage element of Examples 1 and 2 and Comparative Examples 1 to 6 as examples will be described. The present application is not limited to the following examples.

[0103] [Example 1]

[0104] As the positive electrode active material, LiNi 0.6 Mn 0.2 Co 0.2 O2(also referred to as "active material A" hereinafter). The average particle diameter of this positive electrode active material was 12 μm. This average particle diameter was a value measured by the above-described method. Next, a mixture containing the above-described positive electrode active material, a conductive aid, and polyvinylidene fluoride (PVDF) as a binder in a mass ratio of 97.0:1.5:1.5, converted into solid content, was prepared using N-methylpyrrolidone (NMP) as a dispersion medium. As the conductive aid, acetylene black (AB) and carbon nanotube (CNT) were used, and the content in the above-described mixture was set to 1.0 mass% and 0.5 mass%, respectively. Note that the above-described carbon nanotube used was a multi-walled carbon nanotube (MWCNT).

[0105] In addition, as the kneader, a high-speed mixer (PRIMIX Co., Ltd., "Filmix" (registered trademark)) was used, and the mixture was kneaded at a rotation speed of 7600 rpm for 20 seconds to produce a positive electrode mixture slurry. In addition, the above-described positive electrode mixture slurry was directly applied to both surfaces of an aluminum foil as a positive electrode base material, and after drying, pressing was performed. Thus, a positive electrode of Example 1 in which a positive electrode active material layer was laminated on both surfaces of the above-described positive electrode base material was obtained.

[0106] [Comparative Example 1]

[0107] The mass ratio of the positive electrode active material, acetylene black (AB) as a conductive aid, and a binder in the above-described mixture was set to 94.5:4.0:1.5, converted into solid content, and using a mixed type kneader (Thinky Co., Ltd., "Awa Tori Rantaro" (registered trademark)) as a kneader, the above-described mixture was kneaded at a rotation speed of 2000 rpm for 5 minutes to produce a positive electrode mixture slurry, and otherwise, by the same steps as Example 1, a positive electrode of Comparative Example 1 was obtained.

[0108] [Comparative Example 2]

[0109] The mass ratio of the positive electrode active material, carbon nanotube (CNT) as a conductive aid, and a binder in the above-described mixture was set to 97.9:0.6:1.5, converted into solid content, and otherwise, by the same steps as Example 1, a positive electrode of Comparative Example 2 was obtained.

[0110] [Example 2]

[0111] As the positive electrode active material, LiNi 0.83 Mn 0.05 Co 0.12O2 (hereinafter, also referred to as "active material B"), and the same procedure as in Example 1, except that the above mixture was mixed using the above mixing-type mixer, to thereby obtain a positive electrode of Comparative Example 3. In addition, the above mixture was mixed using the above mixing-type mixer, and the same procedure as in Example 2, except that the above mixture was mixed using the above mixing-type mixer, to thereby obtain a positive electrode of Comparative Example 6.

[0112] [Comparative Examples 3, 6]

[0113] The mass ratio of the positive electrode active material, acetylene black (AB) as the conductive aid, and the binder in the above mixture was set to 94.5:4.0:1.5 in terms of solid components, and the above mixing-type mixer was used in the same manner as in Comparative Example 1, and the same procedure as in Example 2, except that the above mixture was mixed using the above mixing-type mixer, to thereby obtain a positive electrode of Comparative Example 3. In addition, the above mixture was mixed using the above mixing-type mixer, and the same procedure as in Example 2, except that the above mixture was mixed using the above mixing-type mixer, to thereby obtain a positive electrode of Comparative Example 6.

[0114] [Comparative Examples 4, 5]

[0115] The mass ratio of the positive electrode active material, carbon nanotube (CNT) as the conductive aid, and the binder in the above mixture was set to 97.9:0.6:1.5 in terms of solid components, and the same procedure as in Example 2, except that the above mixture was mixed using the above mixing-type mixer, to thereby obtain a positive electrode of Comparative Example 4. In addition, the mass ratio of the positive electrode active material, carbon nanotube (CNT) as the conductive aid, and the binder in the above mixture was set to 97.5:1.0:1.5 in terms of solid components, and the same procedure as in Example 2, except that the above mixture was mixed using the above mixing-type mixer, to thereby obtain a positive electrode of Comparative Example 5.

[0116] The content of the conductive aid and the BET specific surface area of the positive electrode active material layer in each of the obtained positive electrodes are shown in Table 1. The BET specific surface area of the positive electrode active material layer is a value determined by the above method.

[0117] [Production of Power Storage Element]

[0118] A power storage element was produced from each of the obtained positive electrodes as follows.

[0119] (Production of Negative Electrode)

[0120] A negative electrode binder slurry was prepared by mixing graphite (Gr) as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickening agent, and water as a dispersion medium. Note that the mass ratio of Gr, SBR, and CMC was set to 96:2:2 (in terms of solid components). The above negative electrode binder slurry was applied to both sides of a copper foil as a negative electrode base material, and dried. Then, roll-pressing was performed, to thereby obtain a negative electrode.

[0121] (Preparation of Non-aqueous Electrolyte)

[0122] A nonaqueous solvent was prepared by mixing EC, DMC and EMC at a volume ratio of 30:35:35, and LiPF6 as an electrolyte salt was dissolved in the nonaqueous solvent at a concentration of 1.2 mol / dm 3 The above solution was prepared as a nonaqueous electrolyte.

[0123] (Production of power storage element)

[0124] The above positive electrode and the above negative electrode were stacked via a separator of a microporous film made of a polyolefin, whereby an electrode body was produced. The electrode body was housed in a container made of a metal resin composite film, and after the inside was injected with the above nonaqueous electrolyte, a heat seal was performed to obtain a power storage element.

[0125] [Evaluation]

[0126] (Initial charge and discharge)

[0127] Each power storage element produced from each positive electrode was subjected to initial charge and discharge under the following conditions.

[0128] In a thermostat set to 25°C, constant current charging was performed at a charge current of 1.0 C and a charge termination voltage of 4.20 V, and then constant voltage charging was performed at 4.20 V. The end condition of charging was set to a charge current of 0.01 C. Then, a rest period of 10 minutes was provided. Then, constant current discharge was performed at a discharge termination voltage of 2.50 V and a current of 0.2 C. Two cycles of this charge and discharge were performed. From the second discharge, the initial discharge capacity [Ah] and the average discharge voltage [V] of the power storage element were measured.

[0129] For each power storage element, the product of the above initial discharge capacity and the above average discharge voltage was divided by the volume of the power storage element, and the energy density per unit volume [Wh / L] of the power storage element was calculated. The energy density of each power storage element is shown in Table 1. In Table 1, the energy density is a relative value with respect to the energy density of the power storage element obtained in Comparative Example 1 for Example 1, Comparative Examples 1 and 2, and with respect to the energy density of the power storage element obtained in Comparative Example 3 for Example 2 and Comparative Examples 3 to 6.

[0130] (Direct current resistance test)

[0131] Each power storage element after the above initial charge and discharge was evaluated for direct current resistance (DCR) by the following procedure.

[0132] The 50% of the initial discharge capacity was charged at 0.1 C in a constant temperature chamber at 25°C. The SOC (State of Charge) of each power storage element was set to 50% under this condition. Next, each power storage element was discharged at 0.2 C, 0.5 C, and 1.0 C for 30 seconds after being left in a constant temperature chamber at 25°C for 4 hours. After each discharge, the SOC was adjusted to 50% by constant current charging at 0.1 C. The voltage 10 seconds after the start of each discharge was plotted against the discharge current, and the value corresponding to the gradient, i.e., the DC resistance [mΩ], was obtained from the current-voltage characteristic diagram as the 25°C DCR. The DC resistance (DCR) of each power storage element is shown in Table 1. Note that the DC resistance (DCR) in Table 1 is a relative value with respect to the DCR of the power storage element obtained in Comparative Example 1 for Example 1, Comparative Examples 1 and 2, and a relative value with respect to the DCR of the power storage element obtained in Comparative Example 3 for Example 2 and Comparative Examples 3 to 6.

[0133]

[0134] As shown in Table 1, the total content of the conductive aids in the positive electrode active material layer of Comparative Example 1 was 4.0 mass%, whereas that of Comparative Example 2 was 0.6 mass%. Therefore, the content of the positive electrode active material (active material A) was relatively large in Comparative Example 2, and the energy density was larger than that of Comparative Example 1. On the other hand, since the BET specific surface area of the positive electrode active material layer of Comparative Example 2 was less than 1.00 m 2 / g, the conductive paths between the positive electrode active materials were not sufficiently formed, and the DCR was large (more than 101).

[0135] The total content of the conductive aids in the positive electrode active material layer of Example 1 was 1.5 mass%, which was smaller than that of Comparative Example 2. Also in Example 1, the BET specific surface area of the positive electrode active material layer was 1.00 m 2 / g or more. Therefore, in Example 1, the conductive paths between the positive electrode active materials were uniformly formed, and the DCR was small (less than 101).

[0136] As in the case of the power storage element having the active material B, the total content of the conductive aids in the positive electrode active material layer of Comparative Example 3 was 4.0 mass%, whereas that of Comparative Example 4 was 0.6 mass%. Therefore, the content of the positive electrode active material (active material B) was relatively large in Comparative Example 4, and the energy density was larger than that of Comparative Example 3. On the other hand, since the BET specific surface area of the positive electrode active material layer of Comparative Example 4 was less than 1.00 m 2 / g, the conductive path between the positive electrode active materials is not sufficiently formed, and the DCR is large (more than 101). For Comparative Examples 5 and 6, the energy density is large compared to Comparative Example 3, but since the BET specific surface area is less than 1.00 m 2 / g, the DCR is large (more than 101). From Comparative Examples 4 to 6, it can be inferred that by simply increasing the total content of the conductive aid, and controlling the ratio of the components of the conductive aid, it is difficult to sufficiently reduce the DCR.

[0137] The total content of the conductive aid of the positive electrode active material layer of Example 2 is 1.5 mass%, which is small, and as with Comparative Examples 4 to 6, the energy density is larger than that of Comparative Example 3. In addition, the positive electrode active material layer of Example 2 contains CNT as the conductive aid, and the BET specific surface area is 1.00 m 2 / g or more. Therefore, in Example 2, the conductive path between the positive electrode active materials is uniformly formed, and the DCR is small (101 or less).

[0138] From the above, it can be considered that by setting the total content of the conductive aid containing CNT to a certain value or less, while controlling the BET specific surface area of the positive electrode active material layer to a certain value, both the increase in energy density and the reduction in resistance can be achieved.

[0139] Industrial Applicability

[0140] The present application can be applied to power storage elements and the like used as power sources for electronic devices such as personal computers and communication terminals, and vehicles and the like.

[0141] Explanation of Symbols

[0142] 1 Power storage element

[0143] 2 Electrode body

[0144] 3 Container

[0145] 4 Positive electrode terminal

[0146] 41 Positive electrode lead wire

[0147] 5 Negative electrode terminal

[0148] 51 Negative electrode lead wire

[0149] 20 Power storage unit

[0150] 30 Power storage device

Claims

1. A positive electrode for an electric storage device, comprising a positive electrode active material layer containing a positive electrode active material and a conductive auxiliary agent, The conductive additive comprises carbon nanotubes, The content of the conductive additive in the positive electrode active material layer is 3.0% by mass or less. The BET specific surface area of ​​the positive electrode active material layer is 1.00 m 2 / g~3.00m 2 / g.

2. The positive electrode for an electric storage element according to claim 1, wherein The content of the positive electrode active material in the positive electrode active material layer is 95.0 mass % or more. 3 . An electric storage device comprising the positive electrode for an electric storage device according to claim 1 . 4 . A power storage device comprising two or more power storage elements, and comprising one or more power storage elements according to claim 3 .

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