JP2024153497A - Positive electrode for non-aqueous electrolyte storage element and non-aqueous electrolyte storage element - Google Patents

Abstract

To provide a positive electrode for a nonaqueous electrolyte power storage element, capable of suppressing a decline in capacity retention rate after the nonaqueous electrolyte power storage element has been stored in a high temperature environment, and also to provide a nonaqueous electrolyte power storage element, capable of suppressing a decline in capacity retention rate after the nonaqueous electrolyte power storage element has been stored in a high temperature environment.SOLUTION: A positive electrode for a nonaqueous electrolyte power storage element according to one aspect of the present invention includes a positive electrode material layer containing a positive electrode active material and a binder. The positive electrode active material has an α-NaFeO2 type crystal structure, and contains a lithium transition metal composite oxide including a nickel element, a cobalt element, and at least one of an aluminum element and a manganese element. On a surface of the positive electrode active material, there exist a tungsten element, a boron element, a sulfur element, a phosphorus element, a silicon element, a titanium element, a nitrogen element, a germanium element, an aluminum element, a zirconium element, and a heterogeneous element of a combination thereof. The positive electrode active material has a tensile elongation of 1.0% or less.SELECTED DRAWING: None

Description

The present invention relates to a positive electrode for a non-aqueous electrolyte storage element and a non-aqueous electrolyte storage element.

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, due to their high energy density. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as non-aqueous electrolyte storage elements.

Various lithium transition metal composite oxides containing transition metal elements such as cobalt, nickel, and manganese have been developed and are widely used as active materials contained in the positive electrodes of nonaqueous electrolyte storage elements (see Patent Documents 1 and 2).

JP 2013-182783 A JP 2017-45725 A

The positive electrode active material contains a lithium transition metal composite oxide having an α- _NaFeO2 type crystal structure and containing nickel, cobalt, and at least one of aluminum and manganese, thereby improving the energy density of the nonaqueous electrolyte storage element. However, a nonaqueous electrolyte storage element using a positive electrode containing a positive electrode active material containing such a lithium transition metal composite oxide has a problem in that the capacity retention rate after storage in a high-temperature environment is insufficient.

The present invention aims to provide a positive electrode for a nonaqueous electrolyte storage element that can suppress a decrease in the capacity retention rate after storage in a high-temperature environment. It also aims to provide a nonaqueous electrolyte storage element that can suppress a decrease in the capacity retention rate after storage in a high-temperature environment.

A positive electrode for a non-aqueous electrolyte storage element according to one aspect of the present invention comprises a positive electrode active material layer containing a positive electrode active material and a binder, the positive electrode active material having an α- _NaFeO2 type crystal structure and containing a lithium transition metal composite oxide containing nickel, cobalt, and at least one of aluminum and manganese, a different element selected from tungsten, boron, sulfur, phosphorus, silicon, titanium, nitrogen, germanium, aluminum, zirconium, or a combination thereof is present on a surface of the positive electrode active material, and the positive electrode active material has a tensile elongation of 1.0% or less.

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

The positive electrode for a nonaqueous electrolyte storage element according to one aspect of the present invention can suppress the decrease in the capacity retention rate of the nonaqueous electrolyte storage element after storage in a high-temperature environment. Furthermore, the nonaqueous electrolyte storage element according to one aspect of the present invention can suppress the decrease in the capacity retention rate of the nonaqueous electrolyte storage element after storage in a high-temperature environment.

FIG. 1 is a perspective view showing one embodiment of a nonaqueous electrolyte electricity storage element. FIG. 2 is a schematic diagram showing one embodiment of an electricity storage device formed by assembling a plurality of nonaqueous electrolyte electricity storage elements.

First, we will provide an overview of the positive electrode for a nonaqueous electrolyte storage element and the nonaqueous electrolyte storage element disclosed in this specification.

(1) A positive electrode for a non-aqueous electrolyte storage element according to one embodiment of the present invention comprises a positive electrode active material layer containing a positive electrode active material and a binder, the positive electrode active material having an α-NaFeO2 _type crystal structure and containing a lithium transition metal composite oxide containing nickel, cobalt, and at least one of aluminum and manganese, a different element selected from tungsten, boron, sulfur, phosphorus, silicon, titanium, nitrogen, germanium, aluminum, zirconium, or a combination thereof is present on a surface of the positive electrode active material, and the positive electrode active material has a tensile elongation of 1.0% or less.

The positive electrode for a nonaqueous electrolyte storage element described in (1) above can suppress a decrease in the capacity retention rate of a nonaqueous electrolyte storage element after storage in a high-temperature environment. The reason for this is unclear, but the following reason is presumed. In the positive electrode for a nonaqueous electrolyte storage element, the positive electrode active material has an α- _NaFeO2 crystal structure and contains a lithium transition metal composite oxide containing nickel, cobalt, and at least one of aluminum and manganese, thereby improving the energy density of the nonaqueous electrolyte storage element.
On the other hand, a positive electrode active material having an α-NaFeO ₂ type crystal structure and containing the lithium transition metal composite oxide is likely to generate by-products such as fluorides due to a side reaction with the non-aqueous electrolyte, and it is believed that the reaction between the by-products and the binder is a factor in the expansion of the binder in a high-temperature environment. In the positive electrode for a non-aqueous electrolyte storage element, the surface of the positive electrode active material is appropriately covered by the presence of the different elements on the surface of the positive electrode active material, and the side reaction with the non-aqueous electrolyte is suppressed. As a result, it is presumed that the suppression of the generation of the by-products results in the effect of suppressing the expansion of the binder in a high-temperature environment.
Furthermore, by making the tensile elongation of the positive electrode small to 1.0% or less, even if the binder expands due to a reaction between the decomposition products of the nonaqueous electrolyte and the binder, the positive electrode active material layer is unlikely to expand, so that the decrease in the capacity retention rate can be suppressed. Therefore, it is presumed that the positive electrode for a nonaqueous electrolyte storage element described in (1) above can suppress the decrease in the capacity retention rate after storage of the nonaqueous electrolyte storage element in a high-temperature environment.

Here, the "heterogeneous element" may be present on at least a part of the surface of the positive electrode active material. The "heterogeneous element" refers to an element whose content in the positive electrode active material is 4.0 mol% or less with respect to the metal elements excluding the lithium element and the heterogeneous elements contained in the positive electrode active material. That is, when the positive electrode active material contains any of the elements tungsten, boron, sulfur, phosphorus, silicon, titanium, nitrogen, germanium, aluminum, and zirconium in an amount of more than 4.0 mol% with respect to the metal elements excluding the lithium element and the heterogeneous elements contained in the positive electrode active material, the element is not included in the heterogeneous elements. The "tensile elongation [%]" is measured by cutting the positive electrode into a measurement sample having a length of 170 mm and a width of 20 mm, using an autograph, in accordance with JIS-K-7161 (2014). The positive electrode is provided with a positive electrode active material layer on both sides of the positive electrode substrate, and the measurement environment is performed at room temperature of about 20 ° C. The measurement sample is prepared in the following procedure. If the positive electrode before assembling the nonaqueous electrolyte storage element can be prepared, it is used as is. When preparing from an assembled nonaqueous electrolyte storage element, the nonaqueous electrolyte storage element is first discharged at a constant current of 0.1 C to the discharge end voltage during normal use to make it in a discharged state. Note that "normal use" refers to the use of the nonaqueous electrolyte storage element under the recommended or specified charge and discharge conditions for the nonaqueous electrolyte storage element. The nonaqueous electrolyte storage element in this discharged state is disassembled, the positive electrode is removed, and the components (electrolyte, etc.) attached to the positive electrode are thoroughly washed with dimethyl carbonate, and then dried under reduced pressure at room temperature for 24 hours. The operations from disassembling the nonaqueous electrolyte storage element to preparing the positive electrode to be measured are carried out in a dry air atmosphere with a dew point of -40°C or less.

(2) In the positive electrode for a nonaqueous electrolyte storage element described in (1) above, the content of the binder in the positive electrode active material layer may be 0.5% by mass or more and 2.0% by mass or less.

According to the positive electrode for a nonaqueous electrolyte storage element described in (2) above, by setting the content of the binder to be equal to or greater than the lower limit and equal to or less than the upper limit, the tensile elongation of the positive electrode can be reduced, and the effect of suppressing the decrease in the capacity retention rate of the nonaqueous electrolyte storage element after storage in a high-temperature environment can be improved.

(3) In the positive electrode for a nonaqueous electrolyte storage element described in (1) or (2) above, the binder may be a fluororesin.

According to the positive electrode for a nonaqueous electrolyte storage element described in (3) above, the binder is a fluororesin, which enhances the effect of suppressing the decrease in the capacity retention rate of the nonaqueous electrolyte storage element after storage in a high-temperature environment.

(4) A nonaqueous electrolyte storage element according to one embodiment of the present invention comprises a positive electrode for a nonaqueous electrolyte storage element described in any one of (1) to (3) above.

The nonaqueous electrolyte storage element described in (4) above includes a positive electrode for a nonaqueous electrolyte storage element described in any one of (1) to (3) above, and therefore can suppress a decrease in the capacity retention rate after storage in a high-temperature environment.

The configuration of a positive electrode for a nonaqueous electrolyte storage element according to one embodiment of the present invention, the configuration of a nonaqueous electrolyte storage element, the configuration of a storage device, and a method for manufacturing a nonaqueous electrolyte storage element, 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.

<Positive electrode for non-aqueous electrolyte storage element>
The positive electrode for a nonaqueous electrolyte storage element (hereinafter also simply referred to as the 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.

[Positive electrode substrate]
The positive electrode substrate has electrical conductivity. Whether or not the substrate has "electrical conductivity" is determined by using a volume resistivity of 10 ^-2 Ω·cm measured in accordance with JIS-H-0505 (1975) as a threshold value. As the material of the positive electrode substrate, metals such as aluminum, titanium, tantalum, stainless steel, and alloys thereof are used. Among these, aluminum or aluminum alloys are preferred from the viewpoints of potential resistance, high electrical conductivity, and cost. As the positive electrode substrate, foil, vapor deposition film, mesh, porous material, etc. are mentioned, and foil is preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. As the aluminum or aluminum alloy, A1085, A3003, A1N30, etc., as specified in JIS-H-4000 (2014) or JIS-H-4160 (2006) can be exemplified.

The average thickness of the positive electrode substrate is preferably 3 μm or more and 50 μm or less, more preferably 5 μm or more and 40 μm or less, even more preferably 8 μm or more and 30 μm or less, and particularly preferably 10 μm or more and 25 μm or less. By setting the average thickness of the positive electrode substrate within the above range, it is possible to increase the strength of the positive electrode substrate while increasing the energy density per volume of the nonaqueous electrolyte storage element. "Average thickness" means the average value of thicknesses measured at any 10 points. The "average thickness" of other components, etc. is defined in the same manner.

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 include, for example, a binder and a conductive agent.

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

The positive electrode active material contains a lithium transition metal composite oxide having an α-NaFeO ₂ type crystal structure and containing nickel, cobalt, and at least one of aluminum and manganese. As the positive electrode active material, a lithium transition metal composite oxide having an α-NaFeO ₂ type crystal structure and containing nickel, cobalt, and manganese, or a lithium transition metal composite oxide containing nickel, cobalt, and aluminum, is preferable. By using such a lithium transition metal composite oxide, it is possible to increase the energy density, etc.

The lithium transition metal composite oxide is preferably a compound represented by the following formula 1.
Li _1+α Me _1-α O ₂ ...1
In formula 1, Me is a metal element (excluding Li) including Ni, Co, and at least one of Al and Mn, and 0≦α<1.

It is more preferable that Me in formula 1 is substantially composed of the three elements Ni, Co, and Mn, or the three elements Ni, Co, and Al. However, Me may contain other metal elements.

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.

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 or 0.3 is more preferable. On the other hand, the upper limit of this molar ratio (Ni/Me) is preferably 0.9, and in some cases 0.8, 0.7, 0.6, 0.5, or 0.4 is more preferable.

In formula 1, the lower limit of the molar ratio of Co to Me (Co/Me) is preferably 0.05, and in some cases 0.1, 0.2, or 0.3 is more preferable. On the other hand, the upper limit of this molar ratio (Co/Me) is preferably 0.7, and in some cases 0.5 or 0.4 is more preferable.

In formula 1, the lower limit of the molar ratio of Mn to Me (Mn/Me) is preferably 0.05, and in some cases 0.1, 0.2, or 0.3 is more preferable. On the other hand, the upper limit of this molar ratio (Mn/Me) is preferably 0.6, and in some cases 0.5 or 0.4 is more preferable.

In formula 1, the molar ratio of Al to Me (Al/Me) is preferably greater than 0.04, and in some cases may be more preferably 0.05 or greater. On the other hand, the upper limit of this molar ratio (Al/Me) is preferably 0.20, and in some cases may be more preferably 0.10 or 0.08.

In formula 1, the upper limit of the molar ratio of Li to Me (Li/Me), i.e., (1+α)/(1-α), is preferably 1.6, and in some cases 1.4 or 1.2 is more preferable.

The composition ratio of the lithium transition metal composite oxide refers to the composition ratio when the lithium transition metal composite oxide is in a fully discharged state by the following method. First, the non-aqueous electrolyte storage element is discharged at a constant current of 0.05C to the lower limit voltage during normal use. Dismantled, the positive electrode is removed, and a test battery is assembled with metal Li as the counter electrode. A constant current discharge is performed at a discharge current of 10 mA per 1 g of positive electrode active material until the positive electrode potential becomes 3.0 V vs. Li/Li ⁺ , and the positive electrode is adjusted to a fully discharged state. Dismantled again, the positive electrode is removed. Dimethyl carbonate is used to thoroughly wash the components (electrolyte, etc.) attached to the removed positive electrode, and the lithium transition metal composite oxide of the positive electrode active material is collected after drying under reduced pressure at room temperature for 24 hours. The collected lithium transition metal composite oxide is subjected to measurement. The operation from dismantling the non-aqueous electrolyte storage element to collecting the lithium transition metal composite oxide for measurement is performed in an argon atmosphere with a dew point of -60°C or less.

Suitable examples of lithium transition metal composite oxides include LiNi1 _{/3Co1 / 3Mn1} /3O2, LiNi3 _{/ 5Co1 / 5Mn1 / 5O2 ,} LiNi1/ _{2Co1 / 5Mn3 /} 10O2, LiNi1/ _2Co3 /10Mn1 _{/ 5O2 ,} LiNi8 _{/ 10Co1} / _10Mn1 / _10O2 , _{LiNi0.80Co0.15Al0.05O2} , and the _like .

The positive electrode active material particles may contain other positive electrode active material particles other than the lithium transition metal composite oxide containing at least one of the above-mentioned nickel element, cobalt element, aluminum element, and manganese element. The other positive electrode active material particles can be appropriately selected from known positive electrode active materials. As the positive electrode active material for lithium ion secondary batteries, a material capable of absorbing and releasing lithium ions is usually used. As the other positive electrode active material, for example, lithium transition metal composite oxides having a spinel crystal structure, polyanion compounds, chalcogen compounds, sulfur, etc. can be mentioned. As the lithium transition metal composite oxides having a spinel crystal structure, Li _x Mn ₂ O ₄ , Li _x Ni _γ Mn _(2-γ) O ₄ , etc. can be mentioned. 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. Surfaces of these materials may be coated with other materials.

The material of the positive electrode active material particles may be used alone or in combination of two or more. In particular, the positive electrode active material particles preferably contain lithium transition metal composite oxide containing the above nickel element, cobalt element, and at least one of aluminum element and manganese element in a proportion of 50 mass% or more (preferably 70 mass% to 100 mass%, more preferably 80 mass% to 100 mass%) of the total positive electrode active material particles used, and it is more preferable to use a positive electrode active material consisting essentially of lithium transition metal composite oxide containing the above nickel element, cobalt element, and at least one of aluminum element and manganese element.

The positive electrode active material is usually a particle (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 can be easily manufactured or handled. 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. 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. "Average particle size" means a 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 a laser diffraction/scattering method for a diluted solution obtained by diluting particles with a solvent in accordance with JIS-Z-8825 (2013).

In order to obtain powder with a specified particle size, a pulverizer or a classifier is used. Examples of pulverization methods include those using a mortar, ball mill, sand mill, vibrating ball mill, planetary ball mill, jet mill, counter jet mill, swirling airflow type jet mill, or a sieve. Wet pulverization in the presence of water or a non-aqueous solvent such as hexane can also be used during pulverization. As for classification methods, sieves and air classifiers are used as necessary for both dry and wet methods.

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

In the positive electrode for a non-aqueous electrolyte storage element, a different element, which is a tungsten element, a boron element, a sulfur element, a phosphorus element, a silicon element, a titanium element, a nitrogen element, a germanium element, an aluminum element, a zirconium element, or a combination thereof, is present on the surface of the positive electrode active material. In the positive electrode for a non-aqueous electrolyte storage element, the presence of the different element on the surface of the positive electrode active material reduces side reactions between the positive electrode active material and the non-aqueous electrolyte in a high-temperature environment. Among the different elements, tungsten element, boron element, silicon element, titanium element, nitrogen element, germanium element, aluminum element, zirconium element, or a combination thereof is preferred in that side reactions between the positive electrode active material and the non-aqueous electrolyte in a high-temperature environment are further reduced, and tungsten element, boron element, or a combination thereof is more preferred. The different element may be present on at least a part of the surface of the positive electrode active material. The different element may be scattered on the surface of the positive electrode active material, or may form a coating layer. The heterogeneous elements may also be present inside the positive electrode active material. When the positive electrode active material is a secondary particle, the heterogeneous elements may be present at the grain boundaries of the primary particles of the positive electrode active material. The heterogeneous elements may be present on the surface of the positive electrode active material as a compound different from the positive electrode active material. The content of each heterogeneous element contained in the positive electrode active material is 4.0 mol% or less with respect to the metal elements excluding the lithium element and the heterogeneous elements contained in the positive electrode active material. That is, when the positive electrode active material contains any of the elements tungsten, boron, sulfur, phosphorus, silicon, titanium, nitrogen, germanium, aluminum, and zirconium in an amount of more than 4.0 mol% with respect to the metal elements excluding the lithium element and the heterogeneous elements contained in the positive electrode active material, the element is not included in the heterogeneous elements.

The lower limit of the content of the heterogeneous elements contained in the positive electrode active material is preferably 0.2 mol% to 1.5 mol%, more preferably 0.5 mol% to 1.2 mol%, based on the metal elements excluding the lithium element and heterogeneous elements contained in the positive electrode active material. When the content of the heterogeneous elements contained in the positive electrode active material is within the above range based on the metal elements excluding the lithium element and heterogeneous elements contained in the positive electrode active material, the decrease in the capacity retention rate after storage in a high-temperature environment of a nonaqueous electrolyte storage element having the positive electrode for the nonaqueous electrolyte storage element can be further improved. When multiple heterogeneous elements are present, the content of the heterogeneous elements refers to the content of each heterogeneous element.

In the present invention, the content of the above-mentioned heterogeneous elements other than the nitrogen element and the metal elements excluding the lithium element and the heterogeneous elements contained in the positive electrode active material can be determined by high-frequency inductively coupled plasma optical emission spectroscopy (ICP). The content of the above-mentioned heterogeneous elements other than the nitrogen element and the metal elements contained in the positive electrode active material is measured by the following procedure. First, the positive electrode active material is collected from the positive electrode that has been fully discharged by the above-mentioned method, and the positive electrode active material is completely dissolved in an acid capable of dissolving the positive electrode active material and the heterogeneous elements by a microwave decomposition method. Next, this solution is diluted to a certain amount with pure water to obtain a measurement solution. Then, using a multi-type ICP optical emission spectrometer ICPE-9820 (manufactured by Shimadzu Corporation), the concentration of the heterogeneous elements and the metal elements contained in the positive electrode active material of the measurement solution is measured by ICP optical emission spectroscopy. From the obtained concentrations of the heterogeneous elements and the metal elements contained in the positive electrode active material, the content of the heterogeneous elements in the positive electrode active material and the metal elements contained in the positive electrode active material is quantified. In addition, in calculating the concentration of the heterogeneous elements in the measurement solution and the metal elements contained in the positive electrode active material, a calibration curve method can be used in which a calibration curve is created from a solution of heterogeneous elements and metal elements contained in the positive electrode active material of known concentrations, and the concentration of the heterogeneous elements in the measurement solution and the metal elements contained in the positive electrode active material is calculated. In addition, the content of the nitrogen element can be calculated using an oxygen/nitrogen analyzer in the following procedure. A positive electrode active material is collected from a positive electrode that has been fully discharged by the above method, and the nitrogen element in the positive electrode active material is extracted as nitrogen gas using an oxygen/nitrogen analyzer and detected by a thermal conductivity detector, and the content of the nitrogen element is quantified. In addition, the presence of heterogeneous elements on the surface of the positive electrode active material can be confirmed by observing the distribution of the heterogeneous elements on the surface of the positive electrode active material and the metal elements contained in the positive electrode active material using a scanning electron microscope-energy dispersive X-ray analyzer (SEM-EDX), an electron probe microanalyzer (EPMA), or the like.

The conductive agent is not particularly limited as long as it is a material having electrical conductivity. Examples of such conductive agents include carbonaceous materials, metals, conductive ceramics, etc. Examples of carbonaceous materials include graphite, non-graphitic carbon, graphene-based carbon, etc. Examples of non-graphitic carbon include carbon nanofibers, pitch-based carbon fibers, carbon black, etc. Examples of carbon black include furnace black, acetylene black, ketjen black, etc. Examples of graphene-based carbon include graphene, carbon nanotubes (CNT), fullerene, etc. Examples of the conductive agent include powder and fiber. As the conductive agent, one of these materials may be used alone, or two or more types may be mixed and used. These materials may also be used in combination. For example, a material in which carbon black and CNT are combined may be used. Among these, carbon black is preferred from the viewpoint of electronic conductivity and coatability, and acetylene black is preferred among them.

The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By setting the content of the conductive agent in the above range, the energy density of the nonaqueous electrolyte storage element can be increased.

Examples of binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic, polyimide, etc.; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, etc.; polysaccharide polymers, etc. Among these, fluororesins are preferred, and polyvinylidene fluoride is more preferred, from the viewpoint of being more effective in suppressing the decrease in the capacity retention rate of the non-aqueous electrolyte storage element after storage in a high-temperature environment.

The lower limit of the weight average molecular weight of the binder contained in the positive electrode active material layer is preferably 700,000, and more preferably 800,000. On the other hand, the upper limit of the weight average molecular weight of the binder is not particularly limited, but may be, for example, 1.5 million. By having the weight average molecular weight of the binder be equal to or greater than the lower limit and equal to or less than the upper limit, the application properties of the positive electrode mixture paste can be maintained well, and the effect of suppressing the decrease in the capacity retention rate after storage of the non-aqueous electrolyte storage element in a high-temperature environment can be further improved. The "weight average molecular weight" refers to the weight average molecular weight measured using gel permeation chromatography (GPC) in accordance with JIS-K-7252-1 (2008) "Plastics - Method for determining the average molecular weight and molecular weight distribution of polymers by size exclusion chromatography - Part 1: General rules".

The lower limit of the binder content in the positive electrode active material layer is preferably 0.1 mass%, more preferably 0.5 mass%, and even more preferably 1.0 mass%. The upper limit of the binder content is preferably 4.0 mass%, more preferably 2.5 mass%, and even more preferably 2.0 mass%. By setting the binder content to be equal to or greater than the lower limit and equal to or less than the upper limit, the tensile elongation of the positive electrode can be reduced, and the effect of suppressing the decrease in the capacity retention rate after storage of the nonaqueous electrolyte storage element in a high-temperature environment can be improved.

Examples of thickeners include polysaccharide polymers such as carboxymethylcellulose (CMC) and methylcellulose. If 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.

The filler is not particularly limited. Examples of the filler include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, aluminum oxide, 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 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, or man-made products thereof.

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

The average thickness of the positive electrode active material layer is preferably 30 μm or more and 100 μm or less, and more preferably 40 μm or more and 90 μm or less. By setting the average thickness of the positive electrode active material layer within the above range, it is possible to achieve both the capacity and output of the nonaqueous electrolyte storage element within an appropriate range. Note that, when the positive electrode active material layer is provided on both sides of the positive electrode substrate, the average thickness of the positive electrode active material layer is the sum of the average thicknesses of the two layers provided on each side.

The ratio of the average thickness of the positive electrode active material layer to the average thickness of the positive electrode substrate is preferably 10 to 20, and more preferably 12 to 15. By setting the ratio of the average thickness of the positive electrode active material layer to the average thickness of the positive electrode substrate within the above range, the tensile elongation of the positive electrode can be further reduced.

The upper limit of the porosity of the positive electrode active material layer is preferably 40%, more preferably 35%. On the other hand, the lower limit of this porosity is preferably 25%, more preferably 30%. By setting the porosity of the positive electrode active material layer to 40% or less, the contact between the positive electrode active material particles can be improved. By setting the porosity of the positive electrode active material layer to 25% or more, deformation of the positive electrode substrate can be suppressed. In addition, by setting the porosity within the above range, the tensile elongation of the positive electrode can be reduced, and the effect of suppressing the decrease in the capacity retention rate of the nonaqueous electrolyte storage element after storage in a high-temperature environment can be improved.

Here, the "porosity" of the positive electrode active material layer refers to a value calculated from the true density of the positive electrode active material layer calculated from the true density of each component constituting the positive electrode active material layer and the packing density by the following formula. The packing density refers to a value obtained by dividing the mass of the positive electrode active material layer by the apparent volume of the positive electrode active material layer. The apparent volume refers to a volume including voids, and in the case of the positive electrode active material layer, it can be calculated as the product of the thickness and the area.
Porosity (%) = 100 - (filling density/true density) x 100

The tensile elongation of the positive electrode is 1.0% or less, preferably 0.8% or less, more preferably 0.6% or less, and even more preferably 0.4% or less. By setting the tensile elongation of the positive electrode to 1.0% or less, the effect of suppressing the decrease in the capacity retention rate after storage in a high-temperature environment in the nonaqueous electrolyte storage element is good. The lower limit of the tensile elongation of the positive electrode may be, for example, 0%, and 0.1% may be preferable. The tensile elongation of the positive electrode can be adjusted by the microcompressive strength of the positive electrode active material, the type and content of the binder, the material and average thickness of the positive electrode substrate, the average thickness and porosity of the positive electrode active material layer, etc.

<Configuration of Nonaqueous Electrolyte Energy Storage Element>
A nonaqueous electrolyte storage element (hereinafter, also simply referred to as "storage element") according to one embodiment of the present invention includes an electrode assembly having a positive electrode, a negative electrode, and a separator, a nonaqueous electrolyte, and a container that contains the electrode assembly and the nonaqueous electrolyte. The electrode assembly is usually a laminated type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with a separator 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 nonaqueous electrolyte exists in a state in which it is impregnated in the positive electrode, the negative electrode, and the separator. As an example of a nonaqueous electrolyte storage element, a nonaqueous electrolyte secondary battery (hereinafter, also simply referred to as "secondary battery") will be described.

(Positive electrode)
The positive electrode of the nonaqueous electrolyte storage element is as described above. Since the nonaqueous electrolyte storage element includes the positive electrode, it is possible to suppress a decrease in the capacity retention rate after storage in a high-temperature environment.

(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.

The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, or alloys thereof, carbonaceous materials, etc. 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 deposition film, mesh, and porous materials, and foil is 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.

The average thickness of the negative electrode substrate is preferably 2 μm or more and 35 μm or less, more preferably 3 μm or more and 30 μm or less, even more preferably 4 μm or more and 25 μm or less, and particularly preferably 5 μm or more and 20 μm or less. By setting the average thickness of the negative electrode substrate within the above range, it is possible to increase the strength of the negative electrode substrate while increasing the energy density per volume of the nonaqueous electrolyte storage element.

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 necessary.

The negative electrode active material layer may contain typical nonmetallic elements such as B, N, P, F, Cl, Br, and I, typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba, and transition 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.

The negative electrode active material can be appropriately selected from known negative electrode active materials. As the negative electrode active material for lithium ion secondary batteries, a material capable of absorbing and releasing lithium ions is usually used. 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; titanium-containing oxides such as Li ₄ Ti ₅ O ₁₂ , LiTiO _{2, and} TiNb ₂ O ₇ ; polyphosphate compounds; silicon carbide; carbon materials such as graphite and non-graphitic carbon (easily graphitized 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 mixed and used.

"Graphite" refers to a carbon material having an average lattice spacing (d ₀₀₂ ) of the (002) plane determined by X-ray diffraction before charging and discharging or after discharging of 0.33 nm or more and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. Natural graphite is preferred from the viewpoint of excellent input/output characteristics.

"Non-graphitic carbon" refers to a carbon material having an average lattice spacing (d002) of 0.34 nm or more and 0.42 nm or less of ( ₀₀₂ ) plane determined by X-ray diffraction before charging and discharging or in a discharged state. Examples of non-graphitic carbon include carbon that is difficult to graphitize and carbon that is easy to graphitize. 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.

Here, the "discharged state" of a carbon material such as graphite means a state in which the carbon material, which is the negative electrode active material, is discharged so that charge transport ions such as lithium ions that can be absorbed and released during charging and discharging are sufficiently released. For example, in a half cell using a negative electrode containing a carbon material as the negative electrode active material as the working electrode and a lithium metal (Li) electrode as the counter electrode, the open circuit voltage is 0.7 V or more.

The term "non-graphitizable carbon" refers to a carbon material having the above _d002 of 0.36 nm or more and 0.42 nm or less.

The term "graphitizable carbon" refers to a carbon material having the above _d002 of 0.34 nm or more and less than 0.36 nm.

The negative electrode active material is usually a particle (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 the above lower limit or more, the negative electrode active material can be easily manufactured or handled. By setting the average particle size of the negative electrode active material to the above upper limit or less, the electronic conductivity of the negative electrode active material layer is improved. In order to obtain powder with a predetermined particle size, a pulverizer, a classifier, or the like is used. The pulverizing method and the classifying 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.

The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 98% by mass or less. By setting the content of the negative electrode active material in the above range, it is possible to achieve both high energy density and manufacturability of the negative electrode active material layer.

Optional components such as conductive agents, thickeners, and fillers can be selected from the materials exemplified for the positive electrode above.

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; polysaccharide polymers, etc.

The binder content 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. By setting the binder content within the above range, the negative electrode active material can be stably maintained.

(Separator)
The separator can be appropriately selected from known separators. For example, a separator consisting of only a substrate layer, a separator in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the substrate layer, etc. can be used as the separator. Examples of the shape of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, a porous resin film is preferred from the viewpoint of strength, and a nonwoven fabric is preferred from the viewpoint of non-aqueous electrolyte retention. As the material of the substrate layer of the separator, polyolefins such as polyethylene and polypropylene are preferred from the viewpoint of shutdown function, and polyimide and aramid are preferred from the viewpoint of oxidation decomposition resistance. A material obtained by combining these resins may be used as the substrate layer of the separator.

The heat-resistant particles contained in the heat-resistant layer preferably have 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 have a mass loss of 5% or less when heated from room temperature to 800°C. Examples of materials with a mass loss of a predetermined amount 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 ion crystals such as calcium fluoride, barium fluoride, and barium titanate; covalent crystals such as silicon and diamond; mineral resource-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or 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 mixed and used. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicates are preferred from the viewpoint of the safety of the nonaqueous electrolyte storage element.

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

As the separator, a polymer gel composed of a polymer and a non-aqueous electrolyte may be used. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, polyvinylidene fluoride, etc. Using a polymer gel has the effect of suppressing leakage. As the separator, a polymer gel may be used in combination with the porous resin film or nonwoven fabric as described above.

(Non-aqueous electrolyte)
The nonaqueous electrolyte may be appropriately selected from known nonaqueous electrolytes. The nonaqueous electrolyte may be a nonaqueous electrolyte solution. The nonaqueous electrolyte solution includes a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent.

The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, carboxylate esters, phosphate esters, sulfonate esters, ethers, amides, and nitriles. Non-aqueous solvents in which some of the hydrogen atoms contained in these compounds are substituted with halogens may also be used.

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.

Examples of chain carbonates 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.

As the non-aqueous solvent, it is preferable to use a cyclic carbonate or a chain carbonate, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. By using a cyclic carbonate, it is possible to promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. By using a chain carbonate, it is possible to keep the viscosity of the non-aqueous electrolyte low. When a cyclic carbonate and a chain carbonate are used in combination, it is preferable that the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is, for example, in the range of 5:95 to 50:50.

The electrolyte salt can be appropriately selected from known electrolyte salts. Examples of the electrolyte salt include lithium salts, sodium salts, potassium salts, magnesium salts, onium salts, etc. Among these, lithium salts are preferred.

Examples of the lithium salt include inorganic lithium salts such as _LiPF6 , _{LiPO2F2 , LiBF4} , _LiClO4 , _and LiN( _SO2F ) ₂ ; lithium oxalate salts such as lithium bis(oxalate)borate (LiBOB), lithium 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.

The content of the electrolyte salt in the nonaqueous electrolyte solution is preferably 0.1 mol/dm ³ or more and 2.5 mol/dm ³ or less, more preferably 0.3 mol/dm ³ or more and 2.0 mol/dm ³ or less, even more preferably 0.5 mol/dm ³ or more and 1.7 mol/dm ³ or less, and particularly preferably 0.7 mol/dm ³ or more and 1.5 mol/dm ³ or less, at 20° C. and 1 atmospheric pressure. By setting the content of the electrolyte salt in the above range, the ionic conductivity of the nonaqueous electrolyte solution can be increased.

The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and electrolyte salt. Examples of additives include oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, 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, methylvinylene carbonate, ethylvinylene carbonate, succinic anhydride, glutaric anhydride, and maleic anhydride. , citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2-dioxo-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, etc. These additives may be used alone or in combination of two or more.

The content of the additives 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 additives within the above range, it is possible to improve the capacity retention performance or cycle performance after high-temperature storage, and to further improve safety.

A solid electrolyte may be used as the non-aqueous electrolyte, or a non-aqueous electrolyte solution and a solid electrolyte may be used in combination.

The solid electrolyte can be selected from any material that has ionic conductivity such as 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, nitride solid electrolytes, polymer solid electrolytes, and gel polymer electrolytes.

In the case of a lithium ion secondary battery, examples of the sulfide solid electrolyte include Li ₂ S—P ₂ S ₅ , LiI—Li ₂ S—P ₂ S ₅ , and Li ₁₀ Ge—P ₂ S ₁₂ .

The shape of the nonaqueous electrolyte storage element of this embodiment is not particularly limited, and examples include cylindrical batteries, square batteries, flat batteries, coin batteries, button batteries, etc.

Figure 1 shows a nonaqueous electrolyte storage element 1 as an example of a square battery. Note that this figure is a see-through view of the inside of the container. An electrode body 2 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in a square 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.

<Configuration of Power Storage Device>
The nonaqueous electrolyte storage element of the present embodiment can be mounted as an electricity storage device including an electricity storage unit (battery module) constituted by assembling a plurality of nonaqueous electrolyte 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 and a communication terminal, or a power storage power source, etc. In this case, it is sufficient that the technology of the present invention is applied to at least one of the nonaqueous electrolyte storage elements included in the electricity storage device.
2 shows an example of an electricity storage device 30 in which electricity storage units 20, each of which is an assembly of two or more electrically connected nonaqueous electrolyte electricity storage elements 1, are further assembled. The electricity storage device 30 may include a bus bar (not shown) that electrically connects the two or more nonaqueous electrolyte electricity storage elements 1, a bus bar (not shown) that electrically connects the two or more electricity storage units 20, and the like. The electricity storage unit 20 or the electricity storage device 30 may include a status monitoring device (not shown) that monitors the status of the one or more nonaqueous electrolyte electricity storage elements 1.

<Method of Manufacturing Nonaqueous Electrolyte Storage Element>
The method for producing the nonaqueous electrolyte storage element of the present embodiment can be appropriately selected from known methods. The method for producing the nonaqueous electrolyte storage element of the present embodiment includes, for example, preparing an electrode body, preparing a nonaqueous electrolyte, and housing the electrode body and the nonaqueous electrolyte in a container. The preparation of the electrode body includes preparing a positive electrode and a negative electrode, and stacking or winding the positive electrode and the negative electrode with a separator interposed therebetween to form the electrode body.

The positive electrode can be produced, for example, by applying a positive electrode mixture paste directly or through an intermediate layer to the positive electrode substrate and drying it. After drying, pressing or the like may be performed as necessary. The positive electrode mixture paste contains each component constituting the positive electrode active material layer, such as a positive electrode active material, a binder, and an optional conductive agent. The positive electrode mixture paste usually further contains a dispersion medium. Examples of methods for making a different element such as tungsten element, boron element, sulfur element, phosphorus element, silicon element, titanium element, nitrogen element, germanium element, aluminum element, zirconium element, or a combination thereof present on the surface of the positive electrode active material include a method of immersing positive electrode active material particles in a solution containing ions of the different elements, a method of spraying a solution containing ions of the different elements onto positive electrode active material particles, and a method of mixing positive electrode active material particles with a compound containing the different elements. Heat treatment may be performed after the method for making the different elements present. In addition, the method of making the above-mentioned different elements present can be carried out before preparing the positive electrode mixture paste.

The method of storing the nonaqueous electrolyte in the container can be appropriately selected from known methods. For example, when a nonaqueous electrolyte solution is used as the nonaqueous electrolyte, the nonaqueous electrolyte solution can be injected through an injection port formed in the container, and then the injection port can be sealed. Details of each component constituting the nonaqueous electrolyte storage element are as described above.

The nonaqueous electrolyte storage element of this embodiment can suppress the decrease in capacity retention rate after storage in a high-temperature environment.

<Other embodiments>
The nonaqueous electrolyte storage element of the present invention is not limited to the above-described embodiment, and various modifications may be made without departing from the scope of the present invention. For example, the configuration of one embodiment may be added to the configuration of another embodiment, and part of the configuration of one embodiment may be replaced with the configuration of another embodiment or a well-known technique. Furthermore, part of the configuration of one embodiment may be deleted. Also, a well-known technique may be added to the configuration of one embodiment.

In the above embodiment, the nonaqueous electrolyte storage element is used as a chargeable and dischargeable nonaqueous electrolyte secondary battery (e.g., a lithium ion secondary battery), but the type, shape, size, capacity, etc. of the nonaqueous electrolyte storage element are arbitrary. The present invention can also be applied to various secondary batteries, electric double layer capacitors, lithium ion capacitors, and other capacitors.

In the above embodiment, the electrode body in which the positive electrode and the negative electrode are stacked with a separator interposed therebetween has been described, but the electrode body 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.

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

[Example 1]
(Preparation of Positive Electrode)
A positive electrode mixture paste was prepared using LiNi _0.6 Co _0.2 Mn _0.2 O ₂ (NCM622) as a positive electrode active material, acetylene black (AB) as a conductive agent, high molecular weight polyvinylidene fluoride (weight average molecular weight 800,000 or more) as a binder, and N-methylpyrrolidone (NMP) as a dispersion medium. The mass ratio of the positive electrode active material, the conductive agent, and the binder was 95:4:1 (solid content conversion). In addition, the positive electrode active material was used in which boron element was present as a different element on the surface in advance, and at least a part of the surface of the positive electrode active material was coated with the boron compound. The content of the boron element as a different element was 1.0 mol% with respect to the metal elements excluding the lithium element and the different elements contained in the positive electrode active material. The positive electrode mixture paste was applied to both sides of an aluminum foil (A1085) with an average thickness of 15 μm as a positive electrode substrate, and dried. Then, roll pressing was performed to obtain a positive electrode. The coating mass of the positive electrode active material layer was 2.0 g/100 ^cm2 , and the average thickness was 120 μm. The coating mass and average thickness of the positive electrode active material layer are the total values of the two layers provided on both sides of the positive electrode substrate. The tensile elongation [%] of the positive electrode measured by the above measurement method was 0.2%.

(Preparation of negative electrode)
A negative electrode mixture paste was prepared by mixing graphite as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion medium. The mass ratio of the negative electrode active material, binder, and thickener was 97:2:1 (solid content conversion). The negative electrode mixture paste was applied to both sides of copper foil as a negative electrode substrate, and dried. Then, roll pressing was performed to obtain a negative electrode. The coating mass of the negative electrode active material layer was 1.0 g/100 cm ^2. The coating mass of the negative electrode active material layer is the total value of the two layers provided on both sides of the negative electrode substrate.

(Non-aqueous electrolyte)
_LiPF6 was dissolved at a concentration of 1.0 mol/ ^dm3 in a solvent in which ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate were mixed in a volume ratio of 30:35:35 to obtain a non-aqueous electrolyte.

(Separator)
As the separator, a polyolefin microporous film having a thickness of 20 μm was used.

(Assembly of non-aqueous electrolyte storage element)
The positive electrode, the negative electrode, and the separator were used to obtain a wound electrode body, which was then housed in a rectangular container, into which a nonaqueous electrolyte was poured and the container was sealed to obtain a nonaqueous electrolyte storage element of Example 1.

[Examples 2 to 4 and Comparative Example 1]
The nonaqueous electrolyte storage elements of Examples 2 to 4 and Comparative Example 1 were obtained in the same manner as in Example 1, except that the content of the high molecular weight polyvinylidene fluoride binder and the tensile elongation of the positive electrode were changed as shown in Table 1. The tensile elongation of the positive electrode was adjusted by changing the content of the binder. The content of the binder in the positive electrode mixture paste was as shown in Table 1, the content of the conductive agent was 4 mass%, and the content of the positive electrode active material was the remainder.

[Example 5]
A nonaqueous electrolyte storage element of Example ₅ was obtained in the same manner as in Example _{1 ,} except that the positive electrode active material was changed from _{LiNi0.6Co0.2Mn0.2O2} ( _{NCM622 )} to _{LiNi0.5Co0.2Mn0.3O2} ( _NCM523 ).

[Example 6]
A nonaqueous electrolyte storage element of Example ₆ was obtained _in the same manner as in Example 1 _, except that the positive electrode active material was changed from _{LiNi0.6Co0.2Mn0.2O2} ( _{NCM622 )} to _{LiNi0.8Co0.1Mn0.1O2} ( _NCM811 ).

[Examples 7 to 10]
Non-aqueous electrolyte storage elements of Examples 7 to 10 were obtained in the same manner as in Example 1, except that an aluminum alloy foil (A3003) was used instead of an aluminum foil (A1085) as the positive electrode substrate, and the binder content and the tensile elongation of the positive electrode were changed as shown in Table 1. The tensile elongation of the positive electrode was adjusted by changing the binder content. The binder content in the positive electrode mixture paste was as shown in Table 1, the conductive agent content was 4 mass%, and the positive electrode active material content was the remainder.

[Comparative Example 2]
A nonaqueous electrolyte storage element of Comparative Example 2 was obtained in the same manner as in Example 2, except that no different element was present on the surface of the positive electrode active material.

[Comparative Example 3]
A nonaqueous electrolyte storage element of Comparative Example 3 was obtained in the same manner as Comparative Example 1, except that a low molecular weight polyvinylidene fluoride (weight average molecular weight of 500,000 or more and less than 800,000) was used as the binder and the tensile elongation was changed as shown in Table 1. The tensile elongation of the positive electrode was adjusted by changing the type of binder.

[Comparative Example 4]
A nonaqueous electrolyte storage element of Comparative Example 4 was obtained in the same manner as in Comparative Example 3, except that no different element was present on the surface of the positive electrode active material.

[Comparative Example 5]
A positive electrode mixture paste was prepared in the same manner as in Example 1, except that a low molecular weight polyvinylidene fluoride (weight average molecular weight of 500,000 or more and less than 800,000) was used as the binder, and the mass ratio of the positive electrode active material, the conductive agent and the binder was 93.5:4:2.5 (solid content equivalent). The positive electrode mixture paste was applied to both sides of an aluminum foil (A1085) having an average thickness of 15 μm as a positive electrode substrate, and dried. However, when the roll press was performed, the positive electrode active material layer peeled off from the positive electrode substrate, and the adhesion with the positive electrode substrate could not be ensured, so the positive electrode and nonaqueous electrolyte storage element of Comparative Example 5 could not be produced.

[Comparative Example 6]
A nonaqueous electrolyte storage element of Comparative Example 6 was obtained in the same manner as in Comparative Example 1, except that an aluminum alloy foil (A3003) was used instead of the aluminum foil (A1085) as the positive electrode substrate.

[Comparative Example 7]
A nonaqueous electrolyte storage element of Comparative Example 7 was obtained in the same manner as in Comparative Example 2, except that an aluminum alloy foil (A3003) was used instead of the aluminum foil (A1085) as the positive electrode substrate.

[Comparative Example 8]
A nonaqueous electrolyte storage element of Comparative Example 8 was obtained in the same manner as in Comparative Example 3, except that an aluminum alloy foil (A3003) was used instead of the aluminum foil (A1085) as the positive electrode substrate.

[Comparative Example 9]
A nonaqueous electrolyte storage element of Comparative Example 9 was obtained in the same manner as in Comparative Example 4, except that an aluminum alloy foil (A3003) was used instead of the aluminum foil (A1085) as the positive electrode substrate.

[Comparative Example 10]
Except for using an aluminum alloy foil (A3003) instead of the aluminum foil (A1085) as the positive electrode substrate, the positive electrode mixture paste was applied to both sides of an aluminum alloy foil (A3003) having an average thickness of 15 μm as the positive electrode substrate, and then dried in the same manner as in Comparative Example 5. However, when roll pressing was performed, the positive electrode active material layer peeled off from the positive electrode substrate, and adhesion to the positive electrode substrate could not be ensured, so that the positive electrode and nonaqueous electrolyte storage element of Comparative Example 10 could not be produced.

[evaluation]
(Measurement of initial discharge capacity)
For each of the obtained nonaqueous electrolyte storage elements of Example 1 to Example 10, Comparative Example 1 to Comparative Example 4, and Comparative Example 6 to Comparative Example 9, constant current charging was performed at a charging current of 0.1C to 4.2V in a temperature environment of 25°C, and then constant voltage charging was performed at 4.2V. The charging termination condition was set to a total charging time of 15 hours. After a 10-minute rest period, constant current discharging was performed at a discharge current of 0.1C to 2.75V, and a 10-minute rest period was set. Subsequently, constant current charging was performed at a charging current of 1.0C to 4.2V, and then constant voltage charging was performed at 4.2V. The charging termination condition was set to a total charging time of 3 hours. After a 10-minute rest period, constant current discharging was performed at a discharge current of 1.0C to 2.75V. The discharge capacity at this discharge current of 1.0C was set as the "initial discharge capacity".

(Capacity retention rate after storage in high temperature environment)
After the measurement of the initial discharge capacity, each of the nonaqueous electrolyte storage elements of Examples 1 to 10, Comparative Examples 1 to 4, and Comparative Examples 6 to 9 was charged at a constant current of 1.0 C to 4.2 V in a temperature environment of 25 ° C., and then charged at a constant voltage of 4.2 V. The charging termination condition was that the charging current was until the charging current reached 0.2 C. After charging in this manner, the element was left standing in a thermostatic chamber at 60 ° C. for 14 days. After 14 days had passed, each of the nonaqueous electrolyte storage elements was left standing in a 25 ° C. environment for 3 hours, and then discharged at a constant current of 1.0 C to 2.75 V. The discharge capacity at this time was defined as "discharge capacity after storage in a high temperature environment". The percentage of the discharge capacity after storage in a high temperature environment relative to the initial discharge capacity was defined as "capacity retention rate [%] after storage in a high temperature environment". The capacity retention rate after storage in a high temperature environment is shown in Table 1.

As shown in Table 1, in Examples 1 to 10 in which the positive electrode active material has an α-NaFeO ₂ type crystal structure, contains a lithium transition metal composite oxide containing nickel, cobalt, and at least one of aluminum and manganese, has a different element present on the surface of the positive electrode active material, and is provided with a positive electrode having a tensile elongation of 1.0% or less, it is understood that, regardless of the type of positive electrode substrate, the decrease in capacity retention rate after storage in a high temperature environment is suppressed compared to Comparative Examples 1 to 4 and Comparative Examples 6 to 9.

In Comparative Example 1, Comparative Example 3, Comparative Example 6, and Comparative Example 8, which have a large binder content, the capacity retention rate after storage in a high-temperature environment decreased significantly, regardless of whether the binder contained a high-molecular-weight binder or a low-molecular-weight binder, and regardless of the type of positive electrode substrate. This is thought to be because the binder expanded significantly during storage in a high-temperature environment due to the large binder content. In Comparative Example 5 and Comparative Example 10, which have a reduced low-molecular-weight binder content, the positive electrode itself was difficult to manufacture due to peeling of the positive electrode active material layer during roll pressing. In Comparative Example 2 and Comparative Example 7, in which no foreign elements were present on the surface of the positive electrode active material, the capacity retention rate after storage in a high-temperature environment decreased significantly, regardless of the type of positive electrode substrate. In particular, in Comparative Example 4 and Comparative Example 9, in which the low-molecular-weight binder content was large and no foreign elements were present on the surface of the positive electrode active material, the capacity retention rate after storage in a high-temperature environment decreased significantly, regardless of the type of positive electrode substrate.

The above results show that the positive electrode can suppress the decrease in capacity retention rate of a non-aqueous electrolyte storage element after storage in a high-temperature environment.

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.

Reference Signs List 1 Non-aqueous electrolyte storage element 2 Electrode body 3 Container 4 Positive electrode terminal 41 Positive electrode lead 5 Negative electrode terminal 51 Negative electrode lead 20 Storage unit 30 Storage device

Claims (4)

A positive electrode active material layer containing a positive electrode active material and a binder,
the positive electrode active material contains a lithium transition metal composite oxide having an α- _NaFeO2 type crystal structure and containing nickel, cobalt, and at least one of aluminum and manganese;
a different element selected from a group consisting of tungsten, boron, sulfur, phosphorus, silicon, titanium, nitrogen, germanium, aluminum, zirconium, and combinations thereof, is present on a surface of the positive electrode active material;
A positive electrode for a non-aqueous electrolyte storage element, having a tensile elongation of 1.0% or less. The positive electrode for a nonaqueous electrolyte storage element according to claim 1, wherein the content of the binder in the positive electrode active material layer is 0.5% by mass or more and 2.0% by mass or less. The positive electrode for a non-aqueous electrolyte storage element according to claim 1 or 2, wherein the binder is a fluororesin. A non-aqueous electrolyte storage element comprising the positive electrode according to claim 1 or 2.

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