Positive electrode and storage element

Abstract

To provide a positive electrode exhibiting good performance at low temperatures even after charge-discharge cycles.SOLUTION: A positive electrode according to an embodiment of the present invention includes a positive electrode active material layer, and the positive electrode active material layer includes a positive electrode active material, a dielectric, and carbon nanotubes, and the dielectric has a dielectric constant of 30 or more, and the carbon nanotubes have an average tube diameter of 20 nm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a positive electrode and an energy storage element. [Background technology]

[0002] Due to their high energy density, non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, automobiles, etc. Non-aqueous electrolyte secondary batteries generally have a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte interposed between the electrodes, and are configured to charge and discharge by transferring ions between the electrodes. Furthermore, as energy storage elements other than non-aqueous electrolyte secondary batteries, capacitors such as lithium ion capacitors and electric double layer capacitors, as well as energy storage elements using electrolytes other than non-aqueous electrolytes, are also widely used.

[0003] As the active material contained in the positive electrode of such an electricity storage element, various lithium transition metal composite oxides containing transition metals such as cobalt, nickel, and manganese have been developed and are widely used (see Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-182783 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-45725 Summary of the Invention [Problem to be solved by the invention]

[0005] Energy storage elements are required to have various performance characteristics depending on the environment in which they are used, their intended use, and the like. For example, when considering use in low-temperature environments, it is desirable for the elements to exhibit good performance at low temperatures even after charge-discharge cycles.

[0006] The present invention has been made in light of the above circumstances, and an object of the present invention is to provide a positive electrode that can suppress an increase in DC resistance at low temperatures that occurs with charge / discharge cycles of an electricity storage element. [Means for solving the problem]

[0007] One aspect of the present invention is a positive electrode for a storage element, comprising a positive electrode active material layer, the positive electrode active material layer containing a positive electrode active material, a dielectric, and carbon nanotubes, the dielectric having a relative dielectric constant of 30 or more, and the carbon nanotubes having an average tube diameter of 20 nm or less. [Effects of the Invention]

[0008] The positive electrode according to one aspect of the present invention can suppress an increase in DC resistance at low temperatures that occurs with charge / discharge cycles of the energy storage element. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view showing one embodiment of a nonaqueous electrolyte electricity storage element. [Figure 2] FIG. 2 is a schematic diagram showing one embodiment of an electricity storage device configured by assembling a plurality of nonaqueous electrolyte electricity storage elements. DETAILED DESCRIPTION OF THE INVENTION

[0010] First, an overview of the positive electrode and the energy storage element disclosed in this specification will be described.

[0011] One aspect of the present invention is a positive electrode for a storage element, comprising a positive electrode active material layer, the positive electrode active material layer containing a positive electrode active material, a dielectric, and carbon nanotubes, the dielectric having a relative dielectric constant of 30 or more, and the carbon nanotubes having an average tube diameter of 20 nm or less.

[0012] In this positive electrode, the positive electrode active material layer contains a positive electrode active material, a dielectric, and carbon nanotubes, and the dielectric has a relative permittivity of 30 or more, and the carbon nanotubes have an average tube diameter of 20 nm or less. This suppresses an increase in DC resistance at low temperatures associated with charge-discharge cycling of the energy storage device. While the reason for this is unclear, the following is presumed. If the positive electrode does not contain carbon nanotubes or a dielectric, the positive electrode active material layer expands with charge-discharge cycling, which can cut the conductive paths between the positive electrode active material particles and electronically isolate the active material particles. By containing only carbon nanotubes in the positive electrode active material layer, even if the positive electrode active material layer expands with charge-discharge cycling, the fibrous carbon nanotubes maintain the conductive paths between the positive electrode active material particles, slightly suppressing an increase in DC resistance at low temperatures. However, the suppression of the increase in DC resistance is insufficient due to factors such as an increase in interfacial resistance at the surface of the positive electrode active material. When only a dielectric is contained in the positive electrode active material layer, the increase in interfacial resistance at the positive electrode active material surface can be suppressed, but the conductive paths between the positive electrode active material particles cannot be maintained, resulting in insufficient suppression of DC resistance. On the other hand, when both carbon nanotubes with an average tube diameter of 20 nm or less and a dielectric are contained in the positive electrode active material layer, the carbon nanotubes and the dielectric strongly interact, changing the charge state around the dielectric and increasing polarization. This promotes desolvation of ions such as lithium ions on the positive electrode active material surface, thereby suppressing DC resistance at low temperatures. By narrowing the carbon nanotubes to a sufficiently small average tube diameter of 20 nm or less, the carbon nanotubes and the dielectric strongly interact with each other, preventing the positive electrode active material particles from becoming isolated, even if the positive electrode active material layer expands with charge / discharge cycling. This maintains the effect of suppressing DC resistance at low temperatures with charge / discharge cycling. As a result, it is believed that this positive electrode can suppress DC resistance at low temperatures with charge / discharge cycling of an energy storage device. Here, the "average tube diameter" is the average value of the tube diameters of any 10 carbon nanotubes observed under a scanning electron microscope or a transmission electron microscope.

[0013] It is preferable that the molar ratio of the metal element contained in the dielectric relative to the carbon nanotubes is 10% or more and 20% or less. When the molar ratio of the metal element contained in the dielectric relative to the carbon nanotubes is within the above range, the mixing ratio of the carbon nanotubes and the dielectric is optimized and the interaction between the carbon nanotubes and the dielectric is improved, thereby improving the effect of suppressing an increase in DC resistance at low temperatures due to the charge / discharge cycles of the energy storage element. Note that when calculating the molar ratio of the metal element contained in the dielectric relative to the carbon nanotubes, the formula weight of the carbon nanotubes is set to 12 and the number of moles of carbon nanotubes is calculated.

[0014] An energy storage device according to one aspect of the present invention includes the positive electrode. Because the energy storage device includes the positive electrode, an increase in DC resistance at low temperatures that occurs with charge / discharge cycles of the energy storage device can be suppressed.

[0015] The configuration of a positive electrode, the configuration of an energy storage element, the configuration of an energy storage device, a method for manufacturing a positive electrode, and a method for manufacturing an energy storage element according to one embodiment of the present invention, as well as other embodiments, will be described in detail. Note that the names of the components (elementary components) used in each embodiment may differ from the names of the components (elementary components) used in the background art.

[0016] <Positive electrode> A positive electrode according to one embodiment of the present invention has a positive electrode substrate and a positive electrode active material layer disposed on the positive electrode substrate directly or via an intermediate layer.

[0017] [Positive electrode substrate] The positive electrode substrate is electrically conductive. Whether or not it has "electrical conductivity" is determined by whether the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 7The threshold value is Ω·cm. The material of the positive electrode substrate is a metal such as aluminum, titanium, tantalum, or stainless steel, or an alloy of these. Among these, aluminum or an aluminum alloy is preferred from the viewpoints of potential resistance, high conductivity, and cost. Positive electrode substrates include foils, vapor-deposited films, meshes, and porous materials, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30, as specified in JIS-H-4000 (2014) or JIS-H4160 (2006).

[0018] The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, even more preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be increased while increasing the energy density per volume of the energy storage device. The "average thickness" of the positive electrode substrate and the negative electrode substrate described below refers to the value obtained by dividing the punched mass when a substrate of a predetermined area is punched out by the true density and punched area of ​​the substrate.

[0019] The intermediate layer is a layer disposed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer contains a conductive agent such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The configuration of the intermediate layer is not particularly limited, and may contain, for example, a binder and a conductive agent.

[0020] [Cathode active material layer] The positive electrode active material layer contains a positive electrode active material, a dielectric, and carbon nanotubes, and may contain other optional components such as a conductive agent, a binder, a thickener, and a filler, as needed.

[0021] (Cathode active material) The positive electrode active material can be appropriately selected from known positive electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the positive electrode active material for a lithium ion secondary battery. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO2 crystal structure, lithium transition metal composite oxides having a spinel crystal structure, polyanion compounds, chalcogen compounds, and sulfur. Examples of lithium transition metal composite oxides having an α-NaFeO2 crystal structure include Li[Li x Ni (1-x) ]O2(0≦x<0.5), Li[Li x Ni γ Co (1-x-γ) ]O2(0≦x<0.5, 0<γ<1), Li[Li x Co (1-x) ]O2(0≦x<0.5), Li[Li x Ni γ Mn (1-x-γ) ]O2(0≦x<0.5, 0<γ<1), Li[Li x Ni γ Mn β Co (1-x-γ-β) ]O2(0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), Li[Li x Ni γ Co β Al (1-x-γ-β) ]O2 (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1). Lithium transition metal composite oxides with spinel crystal structure include Li x Mn2O4, Li x Ni γ Mn (2-γ)Examples of polyanion compounds include LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, and Li2CoPO4F. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. Atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements. The surfaces of these materials may be coated with other materials. In the positive electrode active material layer, one of these materials may be used alone, or two or more may be used in combination.

[0022] Among the above-mentioned positive electrode active materials, the positive electrode active material preferably contains a lithium transition metal composite oxide having an α-NaFeO2-type crystal structure, and the lithium transition metal composite oxide preferably contains nickel, cobalt, and at least one of aluminum and manganese. The use of such a lithium transition metal composite oxide can increase the energy density.

[0023] Specifically, the lithium transition metal composite oxide having an α-NaFeO2 type crystal structure is preferably a compound represented by the following formula (1). Li 1+α Me 1-α O2···(1) In formula (1), Me is a metal including Ni, Co, and Mn (excluding Li), and α satisfies 0≦α<1.

[0024] In formula (1), Me is preferably substantially composed of the three elements Ni, Co, and Mn, and more preferably composed of the three elements Ni, Co, and Mn, although Me may contain other metals.

[0025] From the viewpoint of achieving a larger electric capacity, the preferred contents (composition ratios) of the constituent elements in the compound represented by formula (1) are as follows: Note that the molar ratio is equal to the atomic ratio.

[0026] In formula (1), the lower limit of the molar ratio of Ni to Me (Ni / Me) is preferably 0.1, and in some cases, 0.2, 0.3, or 0.4 is more preferable, while the upper limit of this molar ratio (Ni / Me) is preferably 0.9, and in some cases, 0.8, 0.7, or 0.6 is more preferable.

[0027] In formula (1), the lower limit of the molar ratio of Co to Me (Co / Me) is preferably 0.01, and in some cases, 0.1 or 0.2 is more preferable, while the upper limit of this molar ratio (Co / Me) is preferably 0.7, and in some cases, 0.5, 0.4, or 0.3 is more preferable.

[0028] In formula (1), the lower limit of the molar ratio of Mn to Me (Mn / Me) may be 0, and in some cases is preferably 0.05, and more preferably 0.1 or 0.2. On the other hand, the upper limit of this molar ratio (Mn / Me) is preferably 0.6, and in some cases is more preferably 0.4 or 0.3.

[0029] In formula (1), the molar ratio of Li to Me (Li / Me), i.e., (1+α) / (1−α), may be 1, and in some cases, it may be preferably greater than 1.0 (α>0) or 1.1 or greater. On the other hand, the upper limit of this molar ratio (Li / Me) is preferably 1.6, and in some cases, 1.4 or 1.2 is more preferable.

[0030] The composition ratio of the lithium transition metal composite oxide refers to the composition ratio when the battery is fully discharged using the following method. First, the storage element is charged at a constant current of 0.05 C until it reaches the end-of-charge voltage for normal use, and then fully charged. After a 30-minute rest, it is discharged at a constant current of 0.05 C until it reaches the lower limit voltage for normal use. The battery is then disassembled, the positive electrode is removed, and a test battery is assembled using a metallic lithium electrode as the counter electrode. The positive electrode potential is measured at a current of 10 mA per 1 g of positive electrode active material until the positive electrode potential reaches 2.0 V vs. Li / Li. +The positive electrode is adjusted to a fully discharged state by constant current discharge until the positive electrode reaches a fully discharged state. The test battery is disassembled and the positive electrode is removed. The nonaqueous electrolyte adhering to the removed positive electrode is thoroughly washed using dimethyl carbonate, and after drying at room temperature for one day, the lithium transition metal composite oxide, the positive electrode active material, is extracted. The extracted lithium transition metal composite oxide is subjected to measurement. The operations from disassembling the energy storage element to extracting the lithium transition metal composite oxide are carried out in an argon atmosphere with a dew point of -60°C or below. Here, "normal use" refers to the case where the energy storage element is used under the recommended or specified charge / discharge conditions for the energy storage element, and if a charger for the energy storage element is provided, the energy storage element is used with the charger.

[0031] Suitable lithium transition metal composite oxides include, for example, LiNi 3 / 5 Co 1 / 5 Mn 1 / 5 O2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 1 / 2 Co 1 / 5 Mn 3 / 10 O2, LiNi 1 / 2 Co 3 / 10 Mn 1 / 5 O2, LiNi 8 / 10 Co 1 / 10 Mn 1 / 10 Examples include O2.

[0032] The positive electrode active material may be used singly or in combination of two or more. In particular, the positive electrode active material preferably contains the lithium transition metal composite oxide in an amount of 50 mass % or more (preferably 70 to 100 mass %, more preferably 80 to 100 mass %) of the total positive electrode active material used, and it is more preferable to use a positive electrode active material consisting essentially of the lithium metal composite oxide.

[0033] The positive electrode active material is usually in the form of particles (powder). The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. By setting the average particle size of the positive electrode active material to the above lower limit or more, the positive electrode active material becomes easier to manufacture and handle. By setting the average particle size of the positive electrode active material to the above upper limit or less, the electronic conductivity of the positive electrode active material layer is improved. Note that when a composite of the positive electrode active material and another material is used, the average particle size of the composite is taken as the average particle size of the positive electrode active material. The "average particle size" refers to the value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 50% based on the particle size distribution measured by laser diffraction / scattering in a diluted solution obtained by diluting particles with a solvent in accordance with JIS-Z-8825 (2013).

[0034] To obtain powders with a predetermined particle size, grinders, classifiers, etc. are used. Grinding methods include, for example, methods using a mortar, ball mill, sand mill, vibration ball mill, planetary ball mill, jet mill, counter jet mill, swirling airflow jet mill, or sieves. Wet grinding in the presence of water or an organic solvent such as hexane can also be used during grinding. As classification methods, sieves, air classifiers, etc. are used as needed for both dry and wet methods.

[0035] The content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass to 99% by mass, more preferably 70% by mass to 98% by mass, and even more preferably 80% by mass to 95% by mass. By setting the content of the positive electrode active material within this range, both high energy density and manufacturability of the positive electrode active material layer can be achieved.

[0036] (carbon nanotubes) Carbon nanotubes (hereinafter also referred to as CNTs), which are graphene-based carbon, are a component that functions as a conductive agent in the positive electrode active material layer.

[0037] The upper limit of the average tube diameter of the carbon nanotubes is 20 nm, preferably 10 nm. On the other hand, the lower limit of the average tube diameter of the carbon nanotubes is preferably 1 nm, more preferably 3 nm, and even more preferably 5 nm. When the average tube diameter of the carbon nanotubes is within the above range, the interaction with the dielectric becomes stronger, thereby further enhancing the effect of suppressing the increase in DC resistance at low temperatures associated with the charge / discharge cycle of the energy storage element. Note that the "average tube diameter" of the carbon nanotubes is the average value of the tube diameters of any 10 carbon nanotubes observed with a scanning electron microscope or a transmission electron microscope.

[0038] The average aspect ratio of the carbon nanotubes (ratio of average length to average tube diameter) is preferably 10 or more and 10,000 or less, more preferably 100 or more, and even more preferably 1,000 or more. Using carbon nanotubes with an average aspect ratio in the above range can provide better conductivity of the positive electrode active material layer. The "average length" of carbon nanotubes refers to the average length of 10 arbitrary carbon nanotubes observed with a scanning electron microscope or a transmission electron microscope. When calculating the average aspect ratio, it is preferable to use the same particles for the 10 carbon nanotubes used to determine the average tube diameter and the 10 carbon nanotubes used to determine the average length. However, the average tube diameter and average length may also be determined for 10 different particles, and the ratio between these may be used as the average aspect ratio. In other words, the average aspect ratio may also be the ratio of the average length of 10 arbitrary different carbon nanotubes to the average tube diameter of 10 arbitrary carbon nanotubes observed with a scanning electron microscope or a transmission electron microscope.

[0039] The average lattice spacing (d 002 ) of the carbon nanotube is preferably less than 0.340 nm. 002 When the average lattice spacing (d 002The lower limit of the (002) angle can be, for example, 0.330 nm. The half-width (002) of the peak corresponding to the (002) plane of the carbon nanotube measured by X-ray diffraction is, for example, 0.5° or more. The half-width (002) of the carbon nanotube is preferably less than 0.7°.

[0040] The upper limit of the carbon nanotube content in the positive electrode active material layer is preferably 5.0% by mass, more preferably 3.0% by mass, and may be 2.5% by mass. The lower limit of the carbon nanotube content in the positive electrode active material layer is preferably 0.05% by mass, more preferably 0.2% by mass, even more preferably 0.5% by mass, and in some cases even more preferably 1.0% by mass. That is, the carbon nanotube content in the positive electrode active material layer is preferably 0.05% by mass or more and 5.0% by mass or less, preferably 0.2% by mass or more and 3.0% by mass or less, and more preferably 0.5% by mass or more and 2.5% by mass or less. By setting the carbon nanotube content at or above the lower limit, the interaction between the carbon nanotubes and the dielectric can be further enhanced, and the increase in DC resistance at low temperatures due to the charge / discharge cycles of the energy storage device can be further reduced. On the other hand, by setting the carbon nanotube content to the above upper limit or less, it is possible to sufficiently reduce the increase in DC resistance at low temperatures that occurs during the charge / discharge cycle of the storage element, while reducing production costs and suppressing the occurrence of lumps during the manufacturing process of the positive electrode mixture paste.

[0041] (dielectric) The positive electrode active material layer of the positive electrode contains a dielectric.

[0042] The dielectric constant of the dielectric is 30 or more, preferably 100 or more, and when the dielectric constant is 30 or more, the effect of suppressing an increase in DC resistance at low temperatures due to charge / discharge cycles of the energy storage element can be improved. The "dielectric constant" is a value at 20°C, and can be measured in accordance with JIS-C2138 (2007).

[0043] Examples of dielectrics with a relative dielectric constant of 30 or more include barium titanate (BaTiO3), titanium dioxide (TiO2), lead titanate (PbTiO3), lead zirconate titanate (Pb(Zr,Ti)O3), potassium niobate (KNbO3), lithium niobate (LiNbO3), lithium tantalate (LiTaO3), calcium titanate (CaTiO3), bismuth ferrite (BiFeO3), zirconia (ZrO2), etc. Among these, barium titanate is preferred as the dielectric from the viewpoint of improving the effect of suppressing an increase in DC resistance at low temperatures that accompanies charge-discharge cycles of the energy storage element.

[0044] The dielectric may be contained in the positive electrode active material layer by coating at least a portion of the surface of the positive electrode active material or by being present between the positive electrode active materials. The content of the dielectric in the positive electrode active material layer is, for example, preferably 0.5% by mass to 4.0% by mass, more preferably 1.0% by mass to 3.5% by mass, and even more preferably 1.5% by mass to 3.0% by mass. By having the content of the dielectric in the positive electrode active material layer within the above range, the conductivity of the positive electrode active material layer can be improved and the effect of suppressing an increase in DC resistance at low temperatures due to charge / discharge cycles of the energy storage device can be improved.

[0045] The upper limit of the molar ratio of the metal element contained in the dielectric to the carbon nanotubes in the positive electrode active material layer is preferably 30%, more preferably 25%, and even more preferably 20%. The lower limit of this molar ratio is preferably 6%, more preferably 7%, and even more preferably 10%. That is, the molar ratio of the metal element contained in the dielectric to the carbon nanotubes in the positive electrode active material layer is preferably 6% to 30%, and more preferably 10% to 20%. When the molar ratio of the metal element contained in the dielectric to the carbon nanotubes is within the above range, the mixing ratio of the carbon nanotubes and the dielectric is optimized and the interaction between the carbon nanotubes and the dielectric is improved, thereby improving the effect of suppressing the increase in DC resistance at low temperatures associated with the charge / discharge cycles of the energy storage element.

[0046] The mass ratio of the dielectric to the carbon nanotubes is preferably 30% to 300%, more preferably 50% to 250%, and even more preferably 100% to 200%. When the mass ratio of the dielectric to the carbon nanotubes is within the above range, the effect of suppressing an increase in DC resistance at low temperatures due to charge / discharge cycles of the energy storage element can be improved.

[0047] (optional ingredient) The positive electrode active material layer may contain a conductive agent other than carbon nanotubes. Examples of the conductive agent include carbonaceous materials other than carbon nanotubes, metals, conductive ceramics, etc. Examples of the carbonaceous material include graphite, non-graphitic carbon, graphene-based carbon, etc. Examples of the non-graphitic carbon include carbon black, etc. Examples of the carbon black include furnace black, acetylene black, ketjen black, etc. Examples of the graphene-based carbon include graphene, fullerene, etc. Among these, carbonaceous materials other than carbon nanotubes are preferred, and carbon black is more preferred.

[0048] The content of all conductive agents (carbon nanotubes and other conductive agents) in the positive electrode active material layer is, for example, preferably 0.3% by mass to 10% by mass, more preferably 0.5% by mass to 8% by mass. The content of other conductive agents other than carbon nanotubes in the positive electrode active material layer may be, for example, 0.1% by mass to 10% by mass, but is preferably 0.5% by mass to 6% by mass, more preferably 0.5% by mass to 5% by mass. By using carbon nanotubes and other conductive agents in such a content, it is possible to reduce production costs and sufficiently suppress an increase in DC resistance at low temperatures due to charge / discharge cycles.

[0049] Examples of binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

[0050] The binder content in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the binder content within this range, the active material can be stably held.

[0051] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC), methyl cellulose, etc. When the thickener has a functional group that reacts with lithium or the like, this functional group may be deactivated in advance by methylation or the like.

[0052] The filler is not particularly limited, and examples of the filler include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, carbonates such as calcium carbonate, sparingly soluble ion crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, substances derived from mineral resources such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, and artificial products thereof.

[0053] The positive electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, and W as components other than the positive electrode active material, dielectric, carbon nanotubes, other conductive agents, binders, thickeners, and fillers.

[0054] The positive electrode can be prepared, for example, by applying a positive electrode mixture paste to a positive electrode substrate directly or via an intermediate layer, and then drying the paste. The positive electrode mixture paste contains the components constituting the positive electrode active material layer, such as a positive electrode active material, a dielectric, carbon nanotubes, and optional components such as other conductive agents and binders. The positive electrode mixture paste usually further contains a dispersion medium. An organic solvent is preferably used as the dispersion medium. Examples of organic solvents used as dispersion mediums in preparing the positive electrode mixture paste include N-methylpyrrolidone (NMP) and toluene.

[0055] <Energy storage element> An energy storage element according to one embodiment of the present invention comprises an electrode assembly having a positive electrode, a negative electrode, and a separator, an electrolyte, and a container for accommodating the electrode assembly and the electrolyte. The electrode assembly is typically a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with separators interposed therebetween, or a wound type in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween and wound. The electrolyte is present in a state of being impregnated in the positive electrode, the negative electrode, and the separator. As an example of an energy storage element, a nonaqueous electrolyte secondary battery (hereinafter also simply referred to as a "secondary battery") will be described.

[0056] [Positive electrode] The positive electrode of the energy storage element is as described above. Since the energy storage element includes the positive electrode, an increase in DC resistance at low temperatures due to charge / discharge cycles of the energy storage element can be suppressed. Therefore, the energy storage element can be suitably used in high-power applications.

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

[0058] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, alloys thereof, and carbonaceous materials are used as the material for the negative electrode substrate. Among these, copper or copper alloys are preferred. Examples of the negative electrode substrate include foils, vapor-deposited films, meshes, and porous materials, with foils being preferred from the viewpoint of cost. Therefore, copper foil or copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0059] The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, even more preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By setting the average thickness of the negative electrode substrate within the above range, the strength of the negative electrode substrate can be increased while increasing the energy density per volume of the secondary battery.

[0060] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer contains optional components such as a conductive agent, a binder, a thickener, and a filler, as needed. The optional components such as the conductive agent, the binder, the thickener, and the filler can be selected from the materials exemplified for the positive electrode above.

[0061] The negative electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

[0062] The negative electrode active material can be appropriately selected from known negative electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the negative electrode active material for lithium ion secondary batteries. Examples of the negative electrode active material include metallic Li; metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; and Li4Ti5O 12 , LiTiO 2、 Examples of the material include titanium-containing oxides such as TiNbO; polyphosphate compounds; silicon carbide; and carbon materials such as graphite and non-graphitic carbon (easily graphitizable carbon or non-graphitizable carbon). Among these materials, graphite and non-graphitic carbon are preferred. In the negative electrode active material layer, one of these materials may be used alone, or two or more may be used in combination.

[0063] "Graphite" refers to a graphite material that has an average lattice spacing (d 002 ) is 0.33 nm or more and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the viewpoint of being able to obtain a material with stable physical properties.

[0064] "Non-graphitic carbon" refers to carbon that has an average lattice spacing (d 002 ) is 0.34 nm or more and 0.42 nm or less. Non-graphitic carbon includes non-graphitizable carbon and graphitizable carbon. Examples of non-graphitic carbon include resin-derived materials, petroleum pitch or petroleum pitch-derived materials, petroleum coke or petroleum coke-derived materials, plant-derived materials, and alcohol-derived materials.

[0065] Here, the "discharged state" refers to a state in which the negative electrode active material, a carbonaceous material, is discharged so that lithium ions capable of being absorbed and desorbed during charging and discharging are sufficiently released. For example, in a single-electrode battery using a negative electrode containing a carbonaceous material as a negative electrode active material as a working electrode and metallic Li as a counter electrode, this refers to a state in which the open circuit voltage is 0.7 V or higher.

[0066] "Non-graphitizable carbon" means the above d 002 This refers to carbon materials with a particle size of 0.36 nm or more and 0.42 nm or less.

[0067] "Graphitizable carbon" means the above d 002 This refers to carbon materials with a particle size of 0.34 nm or more and less than 0.36 nm.

[0068] The negative electrode active material is usually in the form of particles (powder). The average particle size of the negative electrode active material can be, for example, 1 nm or more and 100 μm or less. When the negative electrode active material is a carbon material, a titanium-containing oxide, or a polyphosphate compound, the average particle size may be 1 μm or more and 100 μm or less. When the negative electrode active material is Si, Sn, Si oxide, Sn oxide, or the like, the average particle size may be 1 nm or more and 1 μm or less. By setting the average particle size of the negative electrode active material to be equal to or greater than the above lower limit, the negative electrode active material can be easily produced or handled. By setting the average particle size of the negative electrode active material to be equal to or less than the above upper limit, the electronic conductivity of the active material layer is improved. To obtain powder with a predetermined particle size, a pulverizer, a classifier, or the like is used. The pulverization method and the powder classification method can be selected from, for example, the methods exemplified for the positive electrode. When the negative electrode active material is a metal such as metallic Li, the negative electrode active material may be in the form of a foil.

[0069] The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass to 99% by mass, more preferably 90% by mass to 98% by mass. By setting the content of the negative electrode active material within this range, both high energy density and manufacturability of the negative electrode active material layer can be achieved.

[0070] [Separator] The separator can be appropriately selected from known separators. Examples of separators that can be used include separators consisting of only a substrate layer and separators in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the substrate layer. Examples of the shape of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, porous resin films are preferred from the viewpoint of strength, and nonwoven fabrics are preferred from the viewpoint of non-aqueous electrolyte retention. Materials for the substrate layer of the separator are preferably polyolefins such as polyethylene and polypropylene from the viewpoint of shutdown function, and polyimide and aramid from the viewpoint of oxidative decomposition resistance. A composite material of these resins may also be used for the substrate layer of the separator.

[0071] The heat-resistant particles contained in the heat-resistant layer preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C in an air atmosphere at 1 atmosphere pressure, and more preferably exhibit a mass loss of 5% or less when heated from room temperature to 800°C. Examples of materials exhibiting a mass loss of a predetermined value or less include inorganic compounds. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalently bonded crystals such as silicon and diamond; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, as well as artificial products thereof. As the inorganic compound, these substances may be used alone or in the form of a complex, or two or more of them may be used in combination. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferred from the viewpoint of the safety of the energy storage device.

[0072] The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and is preferably 20% by volume or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0073] The separator may be a polymer gel composed of a polymer and a non-aqueous electrolyte. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. The use of a polymer gel has the effect of suppressing leakage. The separator may be a combination of the porous resin film or nonwoven fabric described above and a polymer gel.

[0074] [Non-aqueous electrolyte] The nonaqueous electrolyte can be appropriately selected from known nonaqueous electrolytes. The nonaqueous electrolyte may be a nonaqueous electrolytic solution. The nonaqueous electrolytic solution contains a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent.

[0075] The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, carboxylic acid esters, phosphate esters, sulfonic acid esters, ethers, amides, and nitriles. Non-aqueous solvents in which some of the hydrogen atoms contained in these compounds have been substituted with halogens may also be used.

[0076] Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate, etc. Among these, EC is preferred.

[0077] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, bis(trifluoroethyl) carbonate, etc. Among these, EMC is preferred.

[0078] It is preferable to use a cyclic carbonate or a chain carbonate as the non-aqueous solvent, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. The use of a cyclic carbonate can promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate can reduce the viscosity of the non-aqueous electrolyte. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is preferably in the range of, for example, 5:95 to 50:50.

[0079] The electrolyte salt can be appropriately selected from known electrolyte salts, such as lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Of these, lithium salts are preferred.

[0080] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2, lithium oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP), and lithium salts having a halogenated hydrocarbon group such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0081] The content of electrolyte salt in the non-aqueous electrolyte is 0.1 mol / dm at 20°C and 1 atmosphere. 3More than 2.5mol / dm 3 It is preferable that the value is 0.3 mol / dm or less. 3 More than 2.0mol / dm 3 It is more preferable that it is 0.5 mol / dm or less. 3 More than 1.7mol / dm 3 It is more preferable that it is 0.7 mol / dm or less. 3 More than 1.5mol / dm 3 By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0082] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of the additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); oxalates such as lithium bis(oxalato)borate (LiBOB), lithium difluorooxalatoborate (LiFOB), and lithium bis(oxalato)difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, and cyclohexyl. Aromatic compounds such as benzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, succinic anhydride, Glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, propane sultone, propene sultone, butane sultone, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2- dioxo-1,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propene sultone, 1,3-propane sultone, 1,4-butane sultone, 1,4-butene sultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, lithium monofluorophosphate, lithium difluorophosphate, and the like.These additives may be used alone or in combination of two or more.

[0083] The content of the additive contained in the non-aqueous electrolyte is preferably 0.01% by mass to 10% by mass, more preferably 0.1% by mass to 7% by mass, even more preferably 0.2% by mass to 5% by mass, and particularly preferably 0.3% by mass to 3% by mass. By setting the content of the additive within the above range, it is possible to improve the capacity retention performance or cycle performance after high-temperature storage, and further improve safety.

[0084] The non-aqueous electrolyte may be a solid electrolyte, or a non-aqueous electrolytic solution and a solid electrolyte may be used in combination.

[0085] The solid electrolyte can be selected from any material that has ionic conductivity of lithium, sodium, calcium, etc. and is solid at room temperature (e.g., 15° C. to 25° C.) Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, and polymer solid electrolytes.

[0086] Examples of sulfide solid electrolytes include Li2S-P2S5, LiI-Li2S-P2S5, and Li 10 Ge-P2S 12 etc.

[0087] [Configuration of the power storage device] The energy storage element of this embodiment can be mounted as an energy storage device configured by assembling a plurality of energy storage elements in an automobile power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), or a plug-in hybrid electric vehicle (PHEV), a power source for electronic devices such as a personal computer or a communication terminal, or a power storage power source, etc. In this case, the technology of the present invention may be applied to at least one energy storage element included in the energy storage device. 2 shows an example of an energy storage device 30 in which energy storage units 20, each of which is an assembly of two or more electrically connected energy storage elements 1, are further assembled. The energy storage device 30 may include a bus bar (not shown) that electrically connects two or more energy storage elements 1, a bus bar (not shown) that electrically connects two or more energy storage units 20, etc. The energy storage units 20 or the energy storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more energy storage elements.

[0088] [Method of manufacturing an energy storage element] The method for manufacturing the energy storage element of this embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing an electrode assembly, preparing an electrolyte, and housing the electrode assembly and the electrolyte in a container. Preparing the electrode assembly includes preparing the above-mentioned positive electrode and negative electrode, and forming the electrode assembly by stacking or winding the positive electrode and negative electrode with a separator interposed therebetween.

[0089] The method for placing the electrolyte in the container can be appropriately selected from known methods. For example, when a non-aqueous electrolyte solution is used as the electrolyte, the non-aqueous electrolyte solution may be poured into the container through an inlet formed in the container, and the inlet may then be sealed.

[0090] According to the energy storage element, by including the positive electrode, an increase in direct current resistance at low temperatures that occurs with charge / discharge cycles of the energy storage element can be suppressed.

[0091] <Other embodiments> The energy storage device of the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, and part of the configuration of one embodiment can be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment can be deleted. Also, well-known technology can be added to the configuration of one embodiment.

[0092] In the above embodiment, the case where the energy storage element is used as a chargeable and dischargeable non-aqueous electrolyte secondary battery (for example, a lithium ion secondary battery) has been described, but the energy storage element may be of any type, shape, size, capacity, etc. The present invention can also be applied to various secondary batteries, electric double layer capacitors, lithium ion capacitors, and other capacitors.

[0093] In the above embodiment, the electrode assembly in which the positive electrode and the negative electrode are stacked with a separator interposed therebetween has been described, but the electrode assembly may not include a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other in a state in which a non-conductive layer is formed on the active material layer of the positive electrode or the negative electrode. [Example]

[0094] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0095] [Examples 1 to 3] (Preparation of positive electrode) As the positive electrode active material, NCM622 (LiNi 0.6 Co 0.2 Mn 0.2 Particles of ZnO (O2) were used, and barium titanate (BaTiO3) was used as the dielectric. A positive electrode mixture paste containing a positive electrode active material, a dielectric, carbon nanotubes with an average tube diameter of 20 nm, carbon black, and a PVDF binder, and N-methyl-pyrrolidone (NMP) as a dispersion medium, was prepared. The solid content of the positive electrode active material, dielectric, carbon nanotubes, and carbon black was as shown in Table 1, and the binder content was the remainder of 100 mass%. This positive electrode mixture paste was applied to the surface of aluminum foil, which was the positive electrode substrate, and dried to prepare a positive electrode active material layer. Then, roll pressing was performed to obtain a positive electrode.

[0096] (Preparation of negative electrode) A negative electrode mixture paste containing graphite as the negative electrode active material, styrene butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the thickener in a mass ratio of 98:1:1 (solid content equivalent), with water as the dispersion medium, was prepared. This negative electrode mixture paste was applied to the surface of copper foil as the negative electrode substrate and dried to form a negative electrode active material layer. The negative electrode was then obtained by roll pressing.

[0097] (Fabrication of energy storage element) An electricity storage element was assembled using the positive electrode and the negative electrode. The non-aqueous electrolyte was a non-aqueous solvent made by mixing EC (ethylene carbonate), EMC (ethyl methyl carbonate), and dimethyl carbonate (DMC) in a volume ratio of 6:7:7, and lithium hexafluorophosphate (LiPF) was added as an electrolyte salt at a concentration of 1.0 mol / dm 3 A solution in which the compound was dissolved at a concentration of 100% was used, and a polyolefin microporous film was used as the separator. In this manner, the energy storage elements of Examples 1 to 3 were produced.

[0098] [Example 4] An electricity storage element of Example 4 was produced in the same manner as in Example 1, except that titanium dioxide (TiO2) was used as the dielectric in producing the positive electrode.

[0099] [Comparative Example 1] The energy storage element of Comparative Example 1 was produced in the same manner as in Example 1, except that no dielectric was used in the preparation of the positive electrode and the contents of the positive electrode active material, carbon nanotubes, and carbon black were set to the values ​​shown in Table 1.

[0100] Comparative Example 2 The energy storage element of Comparative Example 2 was manufactured in the same manner as in Example 1, except that no dielectric or carbon nanotubes were used in the preparation of the positive electrode, and the contents of the positive electrode active material and carbon black were set to the values ​​shown in Table 1.

[0101] Comparative Example 3 An electricity storage element of Comparative Example 3 was produced in the same manner as in Example 1, except that aluminum oxide (Al2O3) was used instead of barium titanate in the production of the positive electrode.

[0102] Comparative Example 4 An electricity storage element of Comparative Example 4 was produced in the same manner as in Example 2, except that carbon nanotubes with an average tube diameter of 100 nm were used in the production of the positive electrode.

[0103] Comparative Example 5 An electricity storage element of Comparative Example 5 was produced in the same manner as in Comparative Example 1, except that carbon nanotubes with an average tube diameter of 100 nm were used in the production of the positive electrode.

[0104] [evaluation] (Initial DC resistance at low temperatures) Each of the resulting energy storage elements was subjected to an initial charge-discharge process under specified conditions at 25°C, followed by constant-current charging at 1.0C to bring the SOC to 50%. After storing the element in a thermostatic chamber at -10°C for at least 4 hours, it was discharged at -10°C at currents of 0.1C, 0.2C, and 0.3C in sequence for 30 seconds each. After each discharge, it was charged at a constant current of 0.05C to bring the SOC to 50%. The relationship between the current and the voltage 10 seconds after the start of discharge was plotted, and the DC resistance was calculated from the slope of the straight line obtained from the three plots, which was used as the initial DC resistance at low temperatures.

[0105] (Charge-discharge cycle test) Next, the following charge-discharge cycle test was performed on each energy storage element. After storing each energy storage element in a thermostatic chamber at 60°C for 4 hours, it was charged at a constant current of 1.0 C to 4.20 V, and then charged at a constant voltage of 4.20 V. The charging was terminated when the total charging time reached 3 hours. A 10-minute rest period was then provided. A constant current discharge was performed at a current of 1.0 C to 2.50 V, followed by a 10-minute rest period. This charge and discharge process constitutes one cycle, and 300 cycles were performed.

[0106] (DC resistance at low temperature after charge / discharge cycle test) For each energy storage element after the charge-discharge cycle test, the DC resistance at low temperature after the charge-discharge cycle test was determined in the same manner as for the DC resistance at initial low temperature. Then, the increase rate of the DC resistance at low temperature after the charge-discharge cycle test relative to the DC resistance at initial low temperature (-10°C DCR increase rate [%]) was calculated using the following formula. -10℃ DCR ​​increase rate [%] = (DC resistance at low temperature after charge / discharge cycle test) / (initial DC resistance at low temperature) × 100-100 Next, in order to demonstrate the improvement effect of adding a dielectric, the -10°C DCR increase rate [%] relative to Comparative Example 1, to which no dielectric was added, was used as the reference, and the -10°C DCR increase rate was calculated for each of Examples 1 to 5 and Comparative Examples 1 to 5.

[0107] [Table 1]

[0108] As shown in Table 1, Examples 1 to 4, in which the positive electrode active material layer contains a dielectric having a relative permittivity of 30 or more and carbon nanotubes having an average tube diameter of 20 nm or less, are able to suppress an increase in DC resistance at low temperatures associated with charge / discharge cycling of the energy storage element. In particular, Examples 2 and 4, in which the molar ratio of the metal elements contained in the dielectric to the carbon nanotubes is 10% or more and 20% or less, were excellent in reducing the increase in DC resistance at low temperatures associated with charge / discharge cycling.

[0109] On the other hand, Comparative Examples 1 to 5, in which the positive electrode active material layer does not contain either or both of a dielectric having a relative dielectric constant of 30 or more and carbon nanotubes having an average tube diameter of 20 nm or less, exhibited a poor effect of suppressing an increase in DC resistance at low temperatures due to charge-discharge cycling. In particular, Comparative Example 2, in which the positive electrode active material layer does not contain either a dielectric having a relative dielectric constant of 30 or more and carbon nanotubes having an average tube diameter of 20 nm or less, exhibited a poor effect of suppressing an increase in DC resistance at low temperatures due to charge-discharge cycling.

[0110] The above results demonstrate that the positive electrode can suppress an increase in DC resistance at low temperatures that occurs with charge-discharge cycles of an energy storage element. [Industrial Applicability]

[0111] The positive electrode can be applied to an electric storage element used as a power source for electronic devices such as personal computers and communication terminals, and automobiles. [Explanation of symbols]

[0112] 1. Energy storage element 2 Electrode body 3 Cases 4 Positive terminal 41 Positive lead 5 Negative terminal 51 Negative lead 20 Energy storage unit 30 Energy storage device

Claims

1. a positive electrode active material layer; the positive electrode active material layer contains a positive electrode active material, a dielectric material, and a conductive agent, the conductive agent contains carbon nanotubes and a carbon material other than carbon nanotubes, The dielectric has a relative dielectric constant of 100 or more, The carbon nanotubes have an average tube diameter of less than 10 nm, the content of the carbon nanotubes in the positive electrode active material layer is 0.5% by mass or more, the content of the carbon material in the positive electrode active material layer is 0.5% by mass or more and 5% by mass or less, A positive electrode for an electric storage element, wherein the molar ratio of the metal element contained in the dielectric to the carbon nanotubes is 10% or more and 20% or less, when the formula weight of the carbon nanotubes is 12.

2. An electric storage element comprising the positive electrode of claim 1.

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