Positive electrode for non-aqueous electrolyte power storage element, non-aqueous electrolyte power storage element, and power storage device

Abstract

A positive electrode for a non-aqueous electrolyte power storage element according to one aspect of the present invention comprises a positive electrode active material layer containing carbon nanotubes and a positive electrode active material in which the ratio of the average particle diameter to the average primary particle diameter is 5 or less and the average particle diameter is 10 μm or less. The content of the carbon nanotubes in the positive electrode active material layer is less than 1.0 mass%. The number of aggregates of the carbon nanotubes having a diameter of 5 μm or more per 10,000 μm2 of a cross-section of the positive electrode active material layer is 9.0 or less.

Description

Positive electrode for non-aqueous electrolyte storage element, non-aqueous electrolyte storage element and storage device

[0001] The present invention relates to a positive electrode for a non-aqueous electrolyte electricity storage element, a non-aqueous electrolyte electricity storage element, and an electricity storage device.

[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, as well as automobiles. 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 charge-transporting ions between the electrodes. Other non-aqueous electrolyte energy storage elements besides non-aqueous electrolyte secondary batteries include capacitors such as lithium ion capacitors and electric double layer capacitors.

[0003] A positive electrode for a non-aqueous electrolyte storage element typically includes a substrate and a positive electrode active material layer formed on the substrate. The positive electrode active material layer generally includes a positive electrode active material, a conductive agent such as carbon black for improving electrical conductivity, and a binder for binding these components. In recent years, the use of carbon nanotubes as a conductive agent in the positive electrode active material layer, either in addition to or instead of carbon black, has been considered (see Patent Documents 1 and 2).

[0004] JP 2012-221672 A JP 2019-9001 A

[0005] The positive electrode of a nonaqueous electrolyte energy storage element is required to have a high energy density, etc. On the other hand, carbon nanotubes as a conductive agent can sufficiently increase the electronic conductivity of the positive electrode active material layer even in a relatively small amount, and are therefore expected to be a conductive agent for increasing the energy density. However, if a positive electrode is fabricated using a small amount of carbon nanotubes to increase the energy density, etc., the nonaqueous electrolyte energy storage element obtained using the positive electrode may be prone to an increase in DC resistance with charge / discharge cycles.

[0006] An object of the present invention is to provide a positive electrode for a nonaqueous electrolyte energy storage element in which the positive electrode active material layer has a component composition that can increase the energy density and has a low rate of increase in DC resistance after charge-discharge cycling, and a nonaqueous electrolyte energy storage element and an energy storage device that include such a positive electrode.

[0007] A positive electrode for a non-aqueous electrolyte storage element according to one aspect of the present invention comprises a positive electrode active material having a ratio of an average particle size to an average primary particle size of 5 or less and an average particle size of 10 μm or less, and a positive electrode active material layer containing carbon nanotubes, wherein the content of the carbon nanotubes in the positive electrode active material layer is less than 1.0 mass %, and the cross-sectional area of ​​the positive electrode active material layer is 10,000 μm. ２ The number of agglomerates of the carbon nanotubes having a diameter of 5 μm or more per particle is 9.0 or less.

[0008] A nonaqueous electrolyte storage element according to another aspect of the present invention includes the positive electrode for a nonaqueous electrolyte storage element according to the aspect of the present invention.

[0009] An electricity storage device according to another aspect of the present invention includes two or more electricity storage elements and one or more nonaqueous electrolyte electricity storage elements according to the one aspect of the present invention.

[0010] According to one aspect of the present invention, it is possible to provide a positive electrode for a nonaqueous electrolyte energy storage element in which the positive electrode active material layer has a component composition that can increase the energy density and has a low rate of increase in DC resistance after charge-discharge cycling, and a nonaqueous electrolyte energy storage element and an energy storage device that include such a positive electrode.

[0011] Fig. 1 is a perspective view showing one embodiment of a nonaqueous electrolyte electricity storage element, and Fig. 2 is a schematic view showing one embodiment of an electricity storage device formed by assembling a plurality of nonaqueous electrolyte electricity storage elements.

[0012] First, an outline of the positive electrode for a nonaqueous electrolyte storage element and the nonaqueous electrolyte storage element disclosed in this specification will be described.

[0013] [1] A positive electrode for a non-aqueous electrolyte storage element according to one aspect of the present invention comprises a positive electrode active material having a ratio of an average particle size to an average primary particle size of 5 or less and an average particle size of 10 μm or less, and a positive electrode active material layer containing carbon nanotubes, wherein the content of the carbon nanotubes in the positive electrode active material layer is less than 1.0 mass %, and the cross-sectional area of ​​the positive electrode active material layer is 10,000 μm ２ The number of agglomerates of the carbon nanotubes having a diameter of 5 μm or more per particle is 9.0 or less.

[0014] In the positive electrode for a nonaqueous electrolyte storage battery element described in [1] above, the positive electrode active material layer has a component composition capable of increasing energy density and a low rate of increase in DC resistance after charge-discharge cycling. While the reasons for this are unclear, the following reasons are presumed. First, in the positive electrode for a nonaqueous electrolyte storage battery element described in [1] above, the carbon nanotube (hereinafter also referred to as "CNT") content in the positive electrode active material layer is less than 1.0 mass %, allowing for a large positive electrode active material content, resulting in a positive electrode active material layer with a component composition capable of increasing energy density. Furthermore, a positive electrode active material having a ratio of average particle size to average primary particle size of 5 or less (hereinafter also referred to as a "single-particle positive electrode active material") is a single particle that exists in the form of a substantially unagglomerated primary particle, or a secondary particle formed by aggregating a relatively small number of primary particles. Such a single-particle positive electrode active material has fewer grain boundaries than a positive electrode active material that is a secondary particle formed by aggregating a large number of primary particles, and is therefore less susceptible to cracking and other problems associated with charge-discharge cycling. Cracks occurring in the positive electrode active material during charge-discharge cycling can reduce the electronic conductivity between particles of the positive electrode active material, resulting in increased resistance. Therefore, using a single-particle positive electrode active material that is less susceptible to cracking tends to reduce the rate of increase in DC resistance after charge-discharge cycling. However, such single-particle positive electrode active materials generally have a small average particle size. Therefore, when a single-particle positive electrode active material and a small amount of CNTs are used, it is difficult to sufficiently disperse the CNTs when kneading the components to prepare a positive electrode mixture paste for forming the positive electrode active material layer. In other words, when a positive electrode mixture paste using a single-particle positive electrode active material with a small average particle size is used, it is difficult to apply sufficient force to break down CNT agglomerates during kneading, which makes it easy for CNT agglomerates to form in the resulting positive electrode active material layer. If the CNT content in the positive electrode active material layer is low and the CNT dispersibility is poor, the positive electrode active material layer cannot maintain good electronic conductivity, and the rate of increase in DC resistance after charge-discharge cycling cannot be sufficiently reduced. In contrast, the cross section of the positive electrode active material layer is 10,000 μm ２When the number of CNT agglomerates having a diameter of 5 μm or more per unit area is 9.0 or less, the number of CNT agglomerates in the positive electrode active material layer is small and the CNTs are sufficiently dispersed, thereby maintaining good electronic conductivity in the positive electrode active material layer. For these reasons, it is presumed that the positive electrode for a nonaqueous electrolyte storage element described in [1] above has a component composition that can increase the energy density and has a low rate of increase in DC resistance after charge-discharge cycling.

[0015] The "average primary particle diameter" of a positive electrode active material is the average value of the primary particle diameters of any 50 primary particles constituting the positive electrode active material observed under a scanning electron microscope (SEM). Primary particles are particles for which no grain boundaries are observed externally under the SEM. The primary particle diameters of the primary particles are determined as follows: The shortest diameter passing through the center of the smallest circumscribing circle of the primary particle is defined as the minor diameter, and the diameter passing through the center and perpendicular to the minor diameter is defined as the major diameter. The average value of the major diameter and the minor diameter is defined as the primary particle diameter of the primary particles. When there are two or more shortest diameters, the diameter perpendicular to the minor diameter is defined as the minor diameter.

[0016] The "average particle size" of the positive electrode active material is the value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 50% (D50: median diameter) based on the particle size distribution measured by laser diffraction / scattering in a diluted solution of the positive electrode active material diluted with a solvent in accordance with JIS-Z-8815 (2013). It has been confirmed that the average particle size based on the above measurement is approximately equal to the average secondary particle size, which is the average value of the particle sizes of 50 particles extracted from an SEM image of the particles, avoiding extremely large and extremely small particles. The particle size of each secondary particle based on the measurement from this SEM image is determined as follows. The SEM image is obtained in the same manner as when determining the "average primary particle size" described above. The shortest diameter passing through the center of the smallest circumscribing circle of each secondary particle is defined as the minor diameter, and the diameter passing through the center and perpendicular to the minor diameter is defined as the major diameter. The average value of the major and minor diameters is taken as the particle diameter of each secondary particle. When there are two or more shortest diameters, the diameter that intersects them at right angles is taken as the longest diameter.

[0017] The positive electrode active material for measuring the average primary particle size and average particle size is the positive electrode active material before charge / discharge or when fully discharged by the following method. First, the nonaqueous electrolyte storage element is charged at a constant current of 0.05 C until the end-of-charge voltage in normal use is reached, and then fully charged. After a 30-minute rest, the element is discharged at a constant current of 0.05 C until the lower limit voltage in normal use is reached. The element is disassembled, the positive electrode is removed, and a half cell is assembled with 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 3.0 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 device is disassembled again, and the positive electrode is removed. The nonaqueous electrolyte adhering to the removed positive electrode is thoroughly washed with dimethyl carbonate, and the device is dried at room temperature for one day and one night, after which the positive electrode active material is collected. The collected positive electrode active material is subjected to measurement. The operations from disassembly of the nonaqueous electrolyte storage element to collection of the positive electrode active material are carried out in an argon atmosphere with a dew point of −60°C or lower. Here, "normal use" refers to the use of the nonaqueous electrolyte storage element under charge and discharge conditions recommended or specified for the nonaqueous electrolyte storage element. If a device such as a charger for the nonaqueous electrolyte storage element is available, the nonaqueous electrolyte storage element may be used with the charger or other device.

[0018] "Cross section of the positive electrode active material layer 10,000 μm ２ The "number of CNT aggregates having a diameter of 5 μm or more per 10,000 μm" is determined based on an SEM image of a cross section of the positive electrode active material layer. ２ to 20,000 μm ２ The number of CNT agglomerates with a diameter of 5 μm or more in the SEM image is counted. ２ Three SEM images of the cross section of the positive electrode active material layer were obtained from different fields of view, and each SEM image was analyzed in the same manner as above to obtain a cross section of 10,000 μm. ２ The number of CNT agglomerates per 10,000 μm cross section was calculated based on three SEM images. ２The maximum number of CNT aggregates per 10,000 μm of the cross section of the positive electrode active material layer is defined as ２ The diameter of a CNT agglomerate is defined as the major axis (the longest diameter passing through the center of the smallest circumscribing circle).

[0019] [2] In the positive electrode for a nonaqueous electrolyte storage element according to [1] above, the content of the positive electrode active material in the positive electrode active material layer may be 97.0% by mass or more and 99.0% by mass or less.

[0020] The positive electrode for a non-aqueous electrolyte storage element described in [2] above has a large content of single-particle positive electrode active material in the positive electrode active material layer, and can increase the energy density.

[0021] [3] In the positive electrode for a nonaqueous electrolyte storage element according to the above [1] or [2], the positive electrode active material layer may further contain a fluororesin, and the content of the fluororesin in the positive electrode active material layer may be 2.0 mass% or less.

[0022] The positive electrode for a nonaqueous electrolyte storage element described in [3] above is a preferred embodiment of the present invention, and particularly satisfactorily exhibits the effects that the positive electrode active material layer has a component composition that can increase the energy density and that the rate of increase in direct current resistance after charge-discharge cycling is low.

[0023] [4] A nonaqueous electrolyte storage element according to another aspect of the present invention includes the positive electrode for a nonaqueous electrolyte storage element according to any one of [1] to [3] above.

[0024] The nonaqueous electrolyte storage element described in [4] above includes the positive electrode for a nonaqueous electrolyte storage element described in any one of [1] to [3] above, and the positive electrode active material layer has a component composition that can increase the energy density and has a low rate of increase in direct current resistance after charge-discharge cycling.

[0025] [5] A power storage device according to yet another aspect of the present invention includes two or more power storage elements, and includes one or more of the nonaqueous electrolyte power storage elements described in [4] above.

[0026] The energy storage device described in [5] above includes one or more nonaqueous electrolyte energy storage elements described in [4] above, and the positive electrode active material layer has a component composition that can increase energy density and has a low rate of increase in direct current resistance after charge-discharge cycles.

[0027] A positive electrode for a nonaqueous electrolyte storage element, a manufacturing method for a positive electrode for a nonaqueous electrolyte storage element, a nonaqueous electrolyte storage element, an electricity storage device, a manufacturing method for a nonaqueous electrolyte storage element, and other embodiments according to one embodiment of the present invention will be described in detail below. Note that the names of the components (elements) used in each embodiment may differ from the names of the components (elements) used in the background art.

[0028] <Positive electrode for non-aqueous electrolyte storage element> A positive electrode for a non-aqueous electrolyte storage element according to one embodiment of the present invention (hereinafter also simply referred to as "positive electrode") has a positive electrode substrate and a positive electrode active material layer disposed on the positive electrode substrate directly or via an intermediate layer. The positive electrode is a positive electrode for a non-aqueous electrolyte storage element, and is preferably a positive electrode for a non-aqueous electrolyte secondary battery, and more preferably a positive electrode for a non-aqueous electrolyte secondary battery.

[0029] The positive electrode substrate has electrical conductivity. Whether or not it has electrical conductivity is determined by whether or not the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 －２ The resistance is determined using Ω cm as a threshold value. Metals such as aluminum, titanium, tantalum, and stainless steel, or alloys thereof, are used as the material for the positive electrode substrate. Among these, aluminum or aluminum alloys are preferred from the viewpoints of potential resistance, high conductivity, and cost. Examples of the positive electrode substrate include foil, vapor-deposited film, mesh, and porous material, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30, as specified in JIS-H-4000 (2014) or JIS-H-4160 (2006).

[0030] 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 nonaqueous electrolyte storage element.

[0031] 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.

[0032] The positive electrode active material layer contains a single-particle positive electrode active material (a positive electrode active material having a ratio of average particle size to average primary particle size of 5 or less) and CNTs. The positive electrode active material layer contains optional components such as a positive electrode active material other than the single-particle positive electrode active material, a conductive agent other than CNTs, a binder, a thickener, a filler, etc., as needed.

[0033] The upper limit of the ratio of the average particle size to the average primary particle size of the single-particle positive electrode active material is 5, preferably 4, more preferably 3, and even more preferably 2.5. When the ratio of the average particle size to the average primary particle size is equal to or less than the above upper limit, the advantages of the single-particle positive electrode active material, such as resistance to cracking due to charge / discharge cycles, can be fully exhibited. The lower limit of the ratio of the average particle size to the average primary particle size of the single-particle positive electrode active material may be 1. Due to differences in the methods for measuring the average primary particle size and the average particle size, the lower limit of the ratio of the average particle size to the average primary particle size may be less than 1, for example, 0.9. The ratio of the average particle size to the average primary particle size of the single-particle positive electrode active material may be a range that combines any of the above lower limits with any of the above upper limits.

[0034] The single-particle positive electrode active material may be a particle (single particle) made of primary particles, a secondary particle formed by aggregation of a plurality of primary particles, or a mixed particle of a single particle and a secondary particle, as long as the ratio of the average particle size to the average primary particle size is 5 or less.

[0035] The upper limit of the average particle size of a single-particle positive electrode active material (a positive electrode active material having a ratio of the average particle size to the average primary particle size of 5 or less) is 10 μm, preferably 8 μm, and more preferably 6 μm. By setting the average particle size of the single-particle positive electrode active material to the above upper limit or less, the electronic conductivity of the positive electrode active material layer is improved. In addition, when the average particle size of a single-particle positive electrode active material is generally set to the above upper limit or less, the dispersibility of CNTs decreases during kneading when preparing the positive electrode mixture paste, and the increase rate of DC resistance after charge-discharge cycling tends to increase. Therefore, by applying one embodiment of the present invention to a positive electrode using such a single-particle positive electrode active material, the advantages of improving the dispersibility of CNTs and reducing the increase rate of DC resistance after charge-discharge cycling can be fully obtained. The lower limit of the average particle size of the single-particle positive electrode active material is preferably 0.5 μm, more preferably 1 μm, even more preferably 2 μm, and even more preferably 3 μm. By setting the average particle size of the single-particle positive electrode active material to be equal to or greater than the above-mentioned lower limit, the single-particle positive electrode active material can be easily produced or handled. The average particle size of the single-particle positive electrode active material may be within a range that combines any of the above-mentioned lower limits with any of the above-mentioned upper limits.

[0036] The average primary particle diameter of the primary particles constituting the single-particle positive electrode active material may be, for example, 0.5 μm or more and 10 μm or less, 1 μm or more and 6 μm or less, or 1.5 μm or more and 4 μm or less.

[0037] The single-particle positive electrode active material can be produced by a known method, or a commercially available product can be used. In the production process of the positive electrode active material, the single-particle positive electrode active material can be obtained by increasing the calcination temperature or the calcination time to grow the crystals of multiple primary particles and increase the primary particle diameter. Alternatively, the single-particle positive electrode active material can be obtained by crushing the secondary particles.

[0038] In order to obtain a single particle positive electrode active material with a predetermined particle size (average particle size), a pulverizer, a classifier, or the like is used. Examples of pulverization methods include methods 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 swirling airflow type jet mill, or a sieve. During pulverization, 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 is used as needed for both dry and wet methods.

[0039] The positive electrode active material constituting the single particle positive electrode active material can be appropriately selected from known positive electrode active materials. As the positive electrode active material for a lithium ion secondary battery, a material capable of absorbing and releasing lithium ions is usually used. As the positive electrode active material, for example, α-NaFeO ２ Examples of suitable lithium transition metal composite oxides include those having a α-type crystal structure, those having a spinel type crystal structure, polyanion compounds, chalcogen compounds, and sulfur. ２ As the lithium transition metal composite oxide having a crystalline structure, for example, Li[Li ｘ Ni （１－ｘ） ]O ２ (0≦x<0.5), Li[Li ｘ Ni γ Co （１－ｘ－γ） ]O ２ (0≦x<0.5, 0<γ<1, 0<1-x-γ), Li[Li ｘ Co （１－ｘ） ]O ２ (0≦x<0.5), Li[Li ｘ Ni γ Mn （１－ｘ－γ） ]O ２ (0≦x<0.5, 0<γ<1, 0<1-x-γ), Li[Li ｘ Ni γ Mn β Co （１－ｘ－γ－β） ]O ２ (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1−x−γ−β), Li[Li ｘ Ni γ Co β Al （１－ｘ－γ－β） ]O ２(0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1−x−γ−β), etc. Examples of lithium transition metal composite oxides having a spinel crystal structure include Li ｘ Mn ２ O ４ , Li ｘ Ni γ Mn （２－γ） O ４ Examples of polyanion compounds include LiFePO ４ , LiMnPO ４ , LiNiPO ４ , LiCoPO ４ , Li ３ V ２ (P.O. ４ ) ３ , Li ２ MnSiO ４ , Li ２ CoPO ４ Examples of the chalcogen compound 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 positive electrode active material constituting the single particle positive electrode active material may be one of these materials alone or a mixture of two or more of them.

[0040] The positive electrode active material constituting the single particle positive electrode active material is α-NaFeO ２ In other words, the single particle positive electrode active material is preferably a lithium transition metal composite oxide having an α-NaFeO ２Preferably, the lithium transition metal composite oxide particles have a crystalline structure. Such lithium transition metal composite oxides may contain nickel, nickel and cobalt, or nickel, cobalt, and at least one of manganese and aluminum. Lithium transition metal composite oxides containing such elements are typically used in the form of secondary particles formed by agglomeration of a large number of primary particles, with a ratio of the average particle size to the average primary particle size exceeding 5, from the viewpoint of electronic conductivity, etc., and are prone to cracking during charge-discharge cycling. Therefore, when an embodiment of the present invention is applied to a positive electrode using a lithium transition metal composite oxide containing such elements, the advantage of reducing the rate of increase in DC resistance after charge-discharge cycling is particularly sufficiently obtained.

[0041] The content of nickel element relative to metal elements other than lithium element in the lithium transition metal composite oxide is preferably 30 mol % or more and 100 mol % or less, more preferably 40 mol % or more and 90 mol % or less, and even more preferably 50 mol % or more and 85 mol % or less.

[0042] The content of cobalt element relative to the metal elements other than lithium element in the lithium transition metal composite oxide is preferably 3 mol % or more and 70 mol % or less, more preferably 5 mol % or more and 50 mol % or less, and even more preferably 10 mol % or more and 40 mol % or less.

[0043] The content of manganese element relative to metal elements other than lithium element in the lithium transition metal composite oxide is preferably 2 mol % or more and 70 mol % or less, more preferably 3 mol % or more and 50 mol % or less, and even more preferably 4 mol % or more and 40 mol % or less.

[0044] The content of aluminum element relative to metal elements other than lithium element in the lithium transition metal composite oxide is preferably 0 mol % or more and 10 mol % or less, and may be 0.1 mol % or more and 5 mol % or less. The content of aluminum element relative to metal elements other than lithium element in the lithium transition metal composite oxide may be 1 mol % or less, or may be 0 mol %.

[0045] The lithium transition metal composite oxide may further contain metal elements other than lithium, nickel, cobalt, manganese, and aluminum, provided that the total content of nickel, cobalt, manganese, and aluminum relative to the metal elements other than lithium in the lithium transition metal composite oxide is preferably 80 mol % or more, 90 mol % or more, 95 mol % or more, 99 mol % or more, or 100 mol %.

[0046] As the lithium transition metal composite oxide, a compound represented by the following formula (1) can be suitably used.

[0047] Li １＋α Me １－α O ２ In formula (1), Me is a metal element other than Li, and 0≦α<1.

[0048] In formula (1), α may be 0 or more and 0.5 or less, 0 or more and 0.3 or less, 0 or more and 0.1 or less, or even 0. Me may be composed of one or more metal elements. The types and contents (composition ratios) of specific metal elements constituting Me (metal elements other than Li) can be determined from the values ​​of each metal element contained in the lithium transition metal composite oxide and their preferred contents.

[0049] The composition ratio of the lithium transition metal composite oxide refers to the composition ratio before charge and discharge, or the composition ratio when the positive electrode active material is fully discharged by the above-described method.

[0050] The lower limit of the content of the single particle positive electrode active material in the positive electrode active material layer may be, for example, 50 mass%, 70 mass%, 90 mass%, or 95 mass%, but is preferably 97.0 mass%, more preferably 97.5 mass%, and even more preferably 98.0 mass%. By setting the content of the single particle positive electrode active material to the above lower limit or more, it is possible to increase the energy density, etc. The upper limit of the content of the single particle positive electrode active material in the positive electrode active material layer is preferably 99.0 mass%, more preferably 98.5 mass%. The content of the single particle positive electrode active material may be a range that combines any of the above lower limits and any of the above upper limits.

[0051] The positive electrode active material layer may contain a positive electrode active material other than the single-particle positive electrode active material. The single-particle positive electrode active material and the other positive electrode active material may be positive electrode active materials composed of different materials. However, the content of the single-particle positive electrode active material in all the positive electrode active materials contained in the positive electrode active material layer is preferably 80% by mass or more and 100% by mass or less, more preferably 95% by mass or more and 100% by mass or less, and even more preferably 100% by mass. The upper limit of the ratio of the average particle size to the average primary particle size based on all the positive electrode active material particles contained in the positive electrode active material layer is preferably 5, more preferably 4, 3, or 2.5. The lower limit of the ratio of the average particle size to the average primary particle size based on all the positive electrode active material particles contained in the positive electrode active material layer may be 0.9 or 1. The ratio of the average particle size to the average primary particle size based on all the positive electrode active material particles contained in the positive electrode active material layer may be a range that combines any of the above-mentioned lower limits and any of the above-mentioned upper limits.

[0052] The lower limit of the content of all positive electrode active materials in the positive electrode active material layer may be, for example, 50 mass%, 70 mass%, 90 mass%, or 95 mass%, but is preferably 97.0 mass%, more preferably 97.5 mass%, and even more preferably 98.0 mass%. By setting the content of all positive electrode active materials to the above lower limit or more, it is possible to increase the energy density, etc. The upper limit of the content of all positive electrode active materials in the positive electrode active material layer is preferably 99.0 mass%, more preferably 98.5 mass%. The content of all positive electrode active materials may be in a range that combines any of the above lower limits and any of the above upper limits.

[0053] Carbon nanotubes (CNTs) are components that function as conductive agents. Examples of CNTs include single-walled carbon nanotubes (SWCNTs) formed from a single layer of graphene and multi-walled carbon nanotubes (MWCNTs) formed from two or more layers (e.g., 2 to 20 layers) of graphene. The structure of the CNTs is not particularly limited and may be any type, such as a chiral (spiral) type, a zigzag type, or an armchair type. The CNTs may also contain catalyst metals (e.g., Fe, Co, and platinum group elements (Ru, Rh, Pd, Os, Ir, Pt)) used in the synthesis of the CNTs.

[0054] The average diameter of the CNTs may be, for example, 0.3 nm or more and 100 nm or less, 0.5 nm or more and 50 nm or less, or 1 nm or more and 20 nm or less.

[0055] The average aspect ratio of the CNTs (average length to average diameter) is not particularly limited, but is, for example, 10 or more. The lower limit of the aspect ratio of the CNTs may be 20, 50, 100, 500, or 1,000. The upper limit of the aspect ratio of the CNTs may be, for example, 100,000, 50,000, 20,000, or 10,000. The use of CNTs with a relatively high aspect ratio tends to facilitate the formation of good electron conduction paths.

[0056] The average diameter and average aspect ratio of CNTs are the average values ​​measured from any 10 CNTs observed by SEM.

[0057] CNTs can be obtained by, for example, forming a polymer into a fiber form by a spinning method or the like and then heat-treating the fiber in an inert atmosphere, or by a vapor phase growth method in which an organic compound is reacted at high temperature in the presence of a catalyst, etc. Commercially available CNTs can be used.

[0058] The CNT content in the positive electrode active material layer is less than 1.0% by mass, preferably 0.9% by mass or less, more preferably 0.8% by mass or less, and even more preferably 0.7% by mass or less. By having the CNT content less than the above upper limit, the positive electrode active material layer has a component composition that can increase the energy density and also reduce material costs. The lower limit of the CNT content in the positive electrode active material layer is preferably 0.3% by mass, more preferably 0.5% by mass, and even more preferably 0.6% by mass. By having the CNT content equal to or greater than the above lower limit, sufficient electronic conductivity can be ensured in the positive electrode active material layer, and the rate of increase in DC resistance after charge / discharge cycling can be further reduced. The CNT content in the positive electrode active material layer may be a range that combines any of the above lower limits and any of the above upper limits.

[0059] In the positive electrode active material layer, the CNTs are preferably present in a dispersed state, but the CNTs may also exist as aggregates. ２ The upper limit of the number of CNT agglomerates having a diameter of 5 μm or more per 10,000 μm cross section of the positive electrode active material layer is 9.0, preferably 8.0, more preferably 7.0, and even more preferably 6.5. ２ When the number of CNT aggregates having a diameter of 5 μm or more per 10,000 μm cross section of the positive electrode active material layer is equal to or less than the upper limit, the rate of increase in DC resistance after charge-discharge cycles can be reduced. ２ The lower limit of the number of CNT agglomerates having a diameter of 5 μm or more per 10,000 μm cross section of the positive electrode active material layer may be 0, 0.5, 1.0, or 2.0. ２ The number of CNT aggregates having a diameter of 5 μm or more per 10,000 μm cross section of the positive electrode active material layer may be within a range combining any of the above lower limits and any of the above upper limits. ２ The number of CNT aggregates each having a diameter of 5 μm or more can be adjusted by the kneading conditions when preparing the positive electrode mixture paste.

[0060] The positive electrode active material layer may further contain another conductive agent other than CNT. The other conductive agent is not particularly limited as long as it is a material other than CNT and has conductivity. Examples of such conductive agents include non-fibrous carbon materials, metals, conductive ceramics, etc. Examples of carbon materials include graphite, non-graphitic carbon, graphene-based carbon, etc. Examples of non-graphitic carbon include carbon black, etc. Examples of carbon black include furnace black, acetylene black, ketjen black, etc. Examples of graphene-based carbon include fullerene, etc.

[0061] The upper limit of the content of the conductive agent other than CNT in the positive electrode active material layer is preferably 5% by mass, more preferably 1% by mass, even more preferably 0.5% by mass, and even more preferably 0.1% by mass. The positive electrode active material layer may not contain any conductive agent other than CNT. By using substantially only CNT as the conductive agent in the positive electrode active material layer, it is possible to increase the energy density, etc.

[0062] 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. Among these, fluororesins are preferred, and PVDF is more preferred. By using such binders, even a small amount can sufficiently hold the single-particle positive electrode active material, etc., and the rate of increase in DC resistance after charge-discharge cycles can be further reduced.

[0063] The upper limit of the binder content in the positive electrode active material layer may be, for example, 5.0% by mass or 3.0% by mass, but is preferably 2.0% by mass, more preferably 1.5% by mass, and even more preferably 1.3% by mass. By setting the binder content in the positive electrode active material layer to the above upper limit or less, it is possible to increase the energy density, etc. The lower limit of the binder content in the positive electrode active material layer is preferably 0.3% by mass, more preferably 0.5% by mass, and even more preferably 0.8% by mass. By setting the binder content to the above lower limit or more, it is possible to stably maintain the single-particle positive electrode active material, etc.

[0064] The lower limit of the total content of the single-particle positive electrode active material, CNTs, and binder in the positive electrode active material layer is preferably 95.0 mass%, more preferably 99.0 mass%, and even more preferably 99.9 mass%. A positive electrode having a positive electrode active material layer substantially composed of the single-particle positive electrode active material, CNTs, and binder is a preferred embodiment of the present invention.

[0065] 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.

[0066] When the positive electrode active material layer contains a thickener, the content of the thickener in the positive electrode active material layer is preferably 0.1% by mass or more and 10% by mass or less, and more preferably 0.5% by mass or more and 8% by mass or less. The content of the thickener in the positive electrode active material layer may be 5% by mass or less, or may be 2% by mass or less. The technology disclosed herein may also be implemented in an embodiment in which the positive electrode active material layer does not contain a thickener.

[0067] The filler is not particularly limited, and examples of the filler include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, carbonates such as calcium carbonate, sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, substances derived from mineral resources such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, and artificial products thereof.

[0068] When the positive electrode active material layer contains a filler, the content of the filler in the positive electrode active material layer can be 0.1 mass % or more and 8 mass % or less, and usually 5 mass % or less is preferable, and 2 mass % or less is more preferable. The technology disclosed herein can also be implemented in an embodiment in which the positive electrode active material layer does not contain a filler.

[0069] 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 single particle positive electrode active material and other positive electrode active materials, CNT and other conductive agents, binders, thickeners, and fillers.

[0070] <Method for manufacturing a positive electrode for a non-aqueous electrolyte storage element> A positive electrode according to one embodiment of the present invention can be manufactured, for example, by preparing a positive electrode mixture paste containing a single-particle positive electrode active material and CNTs, applying the positive electrode mixture paste to a positive electrode substrate directly or via an intermediate layer, and drying the paste. A positive electrode active material layer is formed by applying and drying the positive electrode mixture paste. After drying, the positive electrode active material layer may be pressed or the like.

[0071] The prepared positive electrode mixture paste typically contains, in addition to the single-particle positive electrode active material and CNT, other optional components such as a binder, and a dispersion medium. An organic solvent such as N-methylpyrrolidone can be suitably used as the dispersion medium. The specific forms of each component contained in the positive electrode mixture paste, other than the single-particle positive electrode active material and the dispersion medium such as CNT, are the same as the specific forms of each component contained in the positive electrode described above.

[0072] When preparing the positive electrode mixture paste, the dispersibility of the CNTs can be improved by mixing and kneading the components. For example, by using a Disper, which is a stirring device with a stirring blade that rotates at high speed, or a Filmix (registered trademark), which is a thin film rotation type stirring device, the dispersibility of the CNTs can be improved even when a single particle type positive electrode active material is used and the CNT content is low. Furthermore, when a stirring device is used, the dispersion state (a cross-section of 10,000 μm in the obtained positive electrode active material layer) can be improved by adjusting the stirring speed and stirring time, etc. ２ For example, when using Filmix, the number of agglomerates tends to decrease by slowing down the stirring speed.

[0073] When preparing the positive electrode mixture paste, a pre-mixing step may be performed in which only the CNTs are dispersed in a dispersion medium, and other components are added to the dispersion and further mixed to obtain the positive electrode mixture paste. The solid content concentration during the pre-mixing step is preferably, for example, 70% by mass or more and 85% by mass or less.

[0074] <Non-aqueous electrolyte storage element> A non-aqueous electrolyte storage element (hereinafter also simply referred to as "storage element") according to one embodiment of the present invention comprises an electrode assembly having a positive electrode, a negative electrode, and a separator, a non-aqueous electrolyte, and a container that accommodates the electrode assembly and the non-aqueous 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 non-aqueous electrolyte exists in a state in which it is impregnated into the positive electrode, the negative electrode, and the separator. As an example of a non-aqueous electrolyte storage element, a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as "secondary battery") will be described.

[0075] (Positive Electrode) The positive electrode used is the same as that for the nonaqueous electrolyte storage element.

[0076] 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.

[0077] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, and nickel-plated steel, alloys thereof, and carbon materials are used as the material of the negative electrode substrate. Among these, copper or copper alloys are preferred. Examples of the negative electrode substrate include foil, vapor-deposited film, mesh, and porous material, with foil 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.

[0078] 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 nonaqueous electrolyte storage element.

[0079] 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. The conductive agent used in the negative electrode active material layer may be either CNT or another conductive agent.

[0080] 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.

[0081] 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 a lithium ion secondary battery. Examples of the negative electrode active material include metal Li; metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; Li ４ Ti ５ O １２ , LiTiO ２、 TiNb ２ O ７ Examples of the material include titanium-containing oxides such as titanium dioxide, 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.

[0082] "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.

[0083] The term "non-graphitic carbon" refers to a carbon material having an average lattice spacing (d ００２ ) is 0.34 nm or more and 0.42 nm or less. Examples of non-graphitizable carbon include non-graphitizable carbon and graphitizable carbon. Examples of non-graphitizable 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.

[0084] Here, the "discharged state" of the carbon material refers to a state in which the carbon material, which is the negative electrode active material, is discharged so that lithium ions that can be absorbed and released during charging and discharging are sufficiently released from the carbon material. For example, this refers to a state in which the open circuit voltage is 0.7 V or higher in a half cell using a negative electrode containing a carbon material as the negative electrode active material as the working electrode and metallic Li as the counter electrode.

[0085] "Non-graphitizable carbon" refers to the above-mentioned d ００２ The term "carbon material" refers to a carbon material having a particle size of 0.36 nm or more and 0.42 nm or less.

[0086] "Graphitizable carbon" refers to the above-mentioned d ００２ The term "carbon material" refers to a carbon material having a particle size of 0.34 nm or more and less than 0.36 nm.

[0087] 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 production or handling of the negative electrode active material becomes easier. 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 negative electrode 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 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 layer may be in the form of a foil.

[0088] 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.

[0089] When the negative electrode active material layer contains a conductive agent, the content of the conductive agent in the negative electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. The content of the conductive agent in the negative electrode active material layer may be 5% by mass or less, or may be 2% by mass or less. The technology disclosed herein may also be implemented in an embodiment in which the negative electrode active material layer does not contain a conductive agent.

[0090] When the negative electrode active material layer contains a binder, the content of the binder in the negative electrode active material layer is preferably 0.1% by mass to 10% by mass, more preferably 0.5% by mass to 8% by mass, and may be 6% by mass or less, or may be 4% by mass or less.

[0091] When the negative electrode active material layer contains a thickener, the content of the thickener in the negative electrode active material layer is preferably 0.1% by mass or more and 10% by mass or less, and more preferably 0.5% by mass or more and 8% by mass or less. The content of the thickener in the negative electrode active material layer may be 5% by mass or less, or may be 2% by mass or less. The technology disclosed herein may also be implemented in an embodiment in which the negative electrode active material layer does not contain a thickener.

[0092] When the negative electrode active material layer contains a filler, the content of the filler in the negative electrode active material layer can be 0.1 mass % or more and 8 mass % or less, and usually 5 mass % or less is preferable, and 2 mass % or less is more preferable. The technology disclosed herein can also be implemented in an embodiment in which the negative electrode active material layer does not contain a filler.

[0093] The negative electrode can be manufactured by a known method. The negative electrode can be manufactured, for example, in the same manner as the above-mentioned positive electrode manufacturing method, by applying a negative electrode mixture paste to a negative electrode substrate directly or via an intermediate layer, and drying the paste to form a negative electrode active material layer. After drying, the negative electrode active material layer may be pressed or the like.

[0094] (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.

[0095] The heat-resistant particles contained in the heat-resistant layer preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C under an air atmosphere at 1 atmosphere, 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 safety of the nonaqueous electrolyte storage element.

[0096] 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.

[0097] 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.

[0098] (Non-aqueous electrolyte) The non-aqueous electrolyte can be appropriately selected from known non-aqueous electrolytes. The non-aqueous electrolyte may be a non-aqueous electrolyte solution. The non-aqueous electrolyte solution contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent.

[0099] 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.

[0100] 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. Of these, EC is preferred.

[0101] 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.

[0102] 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 cyclic carbonate and a chain carbonate in combination. 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.

[0103] 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.

[0104] The lithium salt is LiPF ６ , LiPO ２ F ２ , LiBF ４ , LiClO ４ , LiN(SO ２ F) ２ inorganic lithium salts such as lithium oxalate salts, lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); ３ CF ３ , LiN(SO ２ CF ３ ) ２ , LiN(SO ２ C ２ F ５ ) ２ , LiN(SO ２ CF ３ ) (SO２ C ４ F ９ ), LiC(SO ２ CF ３ ) ３ , LiC(SO ２ C ２ F ５ ) ３ Among these, inorganic lithium salts are preferred, and LiPF ６ is more preferred.

[0105] The content of the electrolyte salt in the non-aqueous electrolyte solution is 0.1 mol / dm at 20°C and 1 atmosphere. ３ 2.5mol / dm or more ３ It is preferably 0.3 mol / dm or less. ３ 2.0mol / dm or more 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 1.5mol / dm or more ３ By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0106] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of the additives include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; vinylene carbonate, methylvinylene carbonate, ethylvinylene carbonate, succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, and cyclohexanedicarboxylic anhydride; and ethylene sulfite. Examples of the additives include propylene sulfite, dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propene sultone, 1,3-propane sultone, 1,4-butane sultone, 1,4-butene sultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, and tetrakistrimethylsilyl titanate. These additives may be used alone or in combination of two or more.

[0107] 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, based on the total mass of the non-aqueous electrolyte. 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.

[0108] The non-aqueous electrolyte may be a solid electrolyte, or a non-aqueous electrolytic solution and a solid electrolyte may be used in combination.

[0109] 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., 20° C.). Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and polymer solid electrolytes.

[0110] As the sulfide solid electrolyte, in the case of a lithium ion secondary battery, for example, Li ２ S-P ２ S ５ , LiI-Li ２ S-P ２ S ５ , Li １０ Ge-P ２ S １２ etc.

[0111] The shape of the nonaqueous electrolyte storage element of this embodiment is not particularly limited, and examples thereof include cylindrical batteries, prismatic batteries, flat batteries, coin batteries, and button batteries.

[0112] Figure 1 shows a nonaqueous electrolyte storage element 1 as an example of a prismatic battery. The figure is a see-through view of the inside of the container. An electrode assembly 2 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in a prismatic 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.

[0113] <Electricity Storage Device> The nonaqueous electrolyte energy storage element of the present embodiment can be mounted as an energy storage unit (battery module) comprising a plurality of nonaqueous electrolyte energy storage elements in an automobile power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), etc., a power source for electronic devices such as a personal computer or a communication terminal, or a power source for power storage, etc. In this case, the technology of the present invention may be applied to at least one nonaqueous electrolyte energy storage element included in the energy storage unit.

[0114] An electricity storage device according to one embodiment of the present invention includes two or more electricity storage elements, and includes one or more nonaqueous electrolyte electricity storage elements according to one embodiment of the present invention described above (hereinafter referred to as the "second embodiment"). It is sufficient that the technology according to one embodiment of the present invention is applied to at least one nonaqueous electrolyte electricity storage element included in the electricity storage device according to the second embodiment, and the electricity storage device may include one nonaqueous electrolyte electricity storage element according to one embodiment of the present invention described above and one or more electricity storage elements not according to one embodiment of the present invention described above, or may include two or more nonaqueous electrolyte electricity storage elements according to one embodiment of the present invention described above.

[0115] 2 shows an example of an energy storage device 30 according to a second embodiment in which energy storage units 20, each of which is an assembly of two or more electrically connected nonaqueous electrolyte 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 nonaqueous electrolyte energy storage elements 1, a bus bar (not shown) that electrically connects two or more energy storage units 20, etc. The energy storage unit 20 or the energy storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more nonaqueous electrolyte energy storage elements.

[0116] <Method for manufacturing nonaqueous electrolyte storage element> The method for manufacturing the nonaqueous electrolyte storage element of this embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing an electrode assembly, preparing a nonaqueous electrolyte, and housing the electrode assembly and the nonaqueous electrolyte in a container. Preparing the electrode assembly includes preparing a positive electrode and a negative electrode, and forming the electrode assembly by stacking or winding the positive electrode and the negative electrode with a separator interposed therebetween.

[0117] The method for placing the non-aqueous electrolyte in the container can be appropriately selected from known methods. For example, when a non-aqueous electrolyte solution is used as the non-aqueous electrolyte, the non-aqueous electrolyte solution may be poured into the container through an inlet formed in the container, and then the inlet may be sealed.

[0118] <Other Embodiments> The nonaqueous electrolyte storage element 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, or 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.

[0119] In the above embodiment, the nonaqueous electrolyte storage element is used as a chargeable and dischargeable nonaqueous electrolyte secondary battery (lithium ion secondary battery), but the nonaqueous electrolyte 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.

[0120] 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.

[0121] 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.

[0122] [Example 1] (Fabrication of Positive Electrode) As a positive electrode active material, Lithium ion batteries having an average primary particle size of 2.0 μm, an average particle size of 4.0 μm, and a ratio of the average particle size to the average primary particle size of 2.0 were used. １．０ Ni ０．６ Co ０．２ Mn ０．２ O ２A single particle positive electrode active material (single particle type positive electrode active material) was prepared. A dispersion of CNT (solid content concentration 74% by mass) using NMP as a dispersion medium was prepared by pre-kneading. The above-mentioned positive electrode active material and the binder polyvinylidene fluoride (PVDF) were mixed with this dispersion, and an appropriate amount of NMP as a dispersion medium was added and kneaded to prepare a positive electrode mixture paste. The positive electrode active material, CNT, and PVDF were mixed in a mass ratio of 98.2:0.7:1.1 (solid content equivalent). Note that multi-walled carbon nanotubes (MWCNT) were used as the CNT. Furthermore, a Filmix was used for kneading at a stirring speed of 10 m / s. The obtained 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 the positive electrode of Example 1.

[0123] (Preparation of Negative Electrode) A negative electrode mixture paste containing graphite as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener in a mass ratio (solid content equivalent) of 96:3:1, and water as a dispersion medium, was prepared. The obtained negative electrode mixture paste was applied to the surface of copper foil as a negative electrode substrate and dried to prepare a negative electrode active material layer. Then, a roll press was performed to obtain a negative electrode.

[0124] (Preparation of non-aqueous electrolyte) A non-aqueous solvent was prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 30:35:35. ３ At a concentration of LiPF ６ was dissolved to prepare a non-aqueous electrolyte.

[0125] (Assembly of non-aqueous electrolyte storage element) The positive electrode and negative electrode were stacked with a polyethylene separator interposed therebetween to prepare an electrode assembly. The electrode assembly was placed in a container, and the non-aqueous electrolyte was poured into the container to obtain the non-aqueous electrolyte storage element of Example 1.

[0126] [Examples 2 and 3, Comparative Examples 1 to 6] The positive electrodes and nonaqueous electrolyte storage elements of Examples 2 and 3 and Comparative Examples 1 to 6 were obtained in the same manner as in Example 1, except that the type of positive electrode active material, the contents of the positive electrode active material, CNT, and PVDF in the solid content of the positive electrode mixture paste (i.e., the contents of the positive electrode active material, CNT, and PVDF in the obtained positive electrode active material layer), the solid content concentration of the dispersion liquid pre-kneaded during preparation of the positive electrode mixture paste, and the kneading method were as shown in Table 1. Note that, with regard to the type of positive electrode active material in Table 1, "single-particle particles" refers to the single-particle positive electrode active material used in Example 1. Furthermore, "secondary particles" refers to LiFePO4 particles having an average primary particle diameter of 0.4 μm, an average particle diameter of 12 μm, and a ratio of the average particle diameter to the average primary particle diameter of 30. １．０ Ni ０．６ Co ０．２ Mn ０．２ O ２ The values ​​in parentheses for the kneading method in Table 1 indicate the stirring speed (m / s).

[0127] (Number of Aggregates) Separately from those incorporated into the nonaqueous electrolyte storage element, positive electrodes of the Examples and Comparative Examples were prepared, and SEM images of the cross sections of the positive electrode active material layer were obtained. Based on the SEM images, the number of aggregates was calculated by the above-mentioned method. ２ The number of CNT agglomerates with a diameter of 5 μm or more per particle was measured (number of agglomerates). The results are shown in Table 1.

[0128] [Evaluation] (Initial Charge / Discharge) Each of the obtained nonaqueous electrolyte storage elements was initially charged / discharged under the following conditions. In a thermostatic chamber at 25°C, constant-current charging was performed with a charging current of 1.0 C and a cut-off voltage of 4.20 V, followed by constant-voltage charging at 4.20 V. The charging was terminated until the total charging time reached 3 hours. A 10-minute rest period was then provided. Constant-current discharging was performed with a discharging current of 1.0 C and a cut-off voltage of 2.50 V, and the initial discharge capacity and average voltage during initial discharge were measured.

[0129] (Energy Density) For each nonaqueous electrolyte storage element, the energy density was calculated from the results of the initial charge and discharge using the following formula. The results are shown in Table 1 as a relative value with the energy density of the nonaqueous electrolyte storage element of Example 1 as the reference (100.0%). Energy density = (initial discharge capacity x average voltage during initial discharge) / volume of nonaqueous electrolyte storage element

[0130] (Initial DC Resistance) After the initial charge and discharge, each nonaqueous electrolyte storage element was charged at a constant current of 1.0 C in a thermostatic chamber at 25 ° C., with a quantity of electricity of 50% of the initial discharge capacity, to bring the SOC to 50%. Then, in a thermostatic chamber at -10 ° C., the element was discharged for 30 seconds at discharge currents of 0.1 C, 0.2 C, and 0.3 C, respectively. After each discharge, the element was charged at a constant current of 0.05 C to bring the SOC to 50%. The relationship between the discharge current and the voltage 10 seconds after the start of discharge at each discharge current was plotted, and the DC resistance was calculated from the slope of the straight line obtained from the plot of the three points, which was used as the initial DC resistance.

[0131] (Charge-Discharge Cycle Test) Next, the following charge-discharge cycle test was conducted. In a thermostatic chamber at 45°C, constant current / constant voltage charging was performed with a charging current of 1.0 C and a charge cut-off voltage of 4.20 V. The charge was terminated until the total charge time reached 3 hours. After that, a rest period of 10 minutes was provided. A constant current discharge was performed with a discharge current of 1.0 C and a discharge cut-off voltage of 2.50 V, followed by a rest period of 10 minutes. The above charge-discharge cycle was repeated 500 times.

[0132] (Increase in DC Resistance) After the charge-discharge cycle test, the DC resistance of each nonaqueous electrolyte storage element was determined in the same manner as in the above-mentioned "initial DC resistance," and this was used as the DC resistance after the charge-discharge cycle test. The increase in DC resistance after the charge-discharge cycle test was determined from the initial DC resistance and the DC resistance after the charge-discharge cycle test. The increase in DC resistance (DCR increase) is shown in Table 1.

[0133]

[0134] As shown in Table 1, the nonaqueous electrolyte storage elements of Comparative Examples 5 and 6, which used a secondary particle positive electrode active material, had an extremely high DCR increase rate. The nonaqueous electrolyte storage elements of Comparative Examples 3 and 4, which used a single particle positive electrode active material and had a CNT content of 1.0 mass% in the positive electrode active material layer, had a low DCR increase rate regardless of the number of CNT aggregates, but also had low energy density. The nonaqueous electrolyte storage elements of Comparative Examples 3 and 4, which used a single particle positive electrode active material and had a CNT content of 0.7 mass% in the positive electrode active material layer and had a CNT aggregate count of 9.0 / 10,000 μm, had a low DCR increase rate regardless of the number of CNT aggregates, but also had low energy density. ２ In contrast to these, the nonaqueous electrolyte storage elements of Comparative Examples 1 and 2, which used a single particle positive electrode active material, had a CNT content of 0.7 mass% in the positive electrode active material layer, and had a CNT aggregate number of 9.0 / 10,000 μm or less. ２ The nonaqueous electrolyte storage elements of Examples 1 to 3 below had high energy density and low DCR increase rates. Furthermore, the CNT content in the positive electrode active material layer of each of the nonaqueous electrolyte storage elements of Examples 1 to 3 was less than 1.0 mass%, and material costs were also reduced. When a positive electrode active material that was a secondary particle was used, as in the nonaqueous electrolyte storage elements of Comparative Examples 5 and 6, the number of CNT aggregates was small regardless of the CNT content. It was confirmed that the phenomenon in which CNT aggregates are more likely to form and affect the DCR increase rate when the CNT content in the positive electrode active material layer is less than 1.0 mass% is unique to the use of a single-particle positive electrode active material.

[0135] The present invention can be applied to nonaqueous electrolyte storage elements used as power sources for electronic devices such as personal computers and communication terminals, and automobiles.

[0136] REFERENCE SIGNS LIST 1 nonaqueous electrolyte energy storage element 2 electrode body 3 container 4 positive electrode terminal 41 positive electrode lead 5 negative electrode terminal 51 negative electrode lead 20 energy storage unit 30 energy storage device

Claims

1. A cathode active material layer containing carbon nanotubes, the ratio of the average particle size to the average primary particle size being 5 or less and the average particle size being 10 μm or less, wherein the content of the carbon nanotubes in the cathode active material layer is less than 1.0 mass %, and the cross section of the cathode active material layer is 10,000 μm. ２ the number of agglomerates of the carbon nanotubes having a diameter of 5 μm or more per positive electrode is 9.0 or less.

2. The positive electrode for a non-aqueous electrolyte storage element according to claim 1, wherein the content of the positive electrode active material in the positive electrode active material layer is 97.0% by mass or more and 99.0% by mass or less.

3. The positive electrode for a non-aqueous electrolyte storage element according to claim 1 or 2, wherein the positive electrode active material layer further contains a fluororesin, and the content of the fluororesin in the positive electrode active material layer is 2.0 mass % or less.

4. A non-aqueous electrolyte storage element comprising the positive electrode for a non-aqueous electrolyte storage element according to claim 1 or 2.

5. An electricity storage device comprising two or more electricity storage elements, and comprising one or more nonaqueous electrolyte electricity storage elements according to claim 4.

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