JP7803085B2 - Positive electrode and non-aqueous electrolyte storage element - Google Patents

Description

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

Non-aqueous electrolyte secondary batteries, typified by 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. Non-aqueous electrolyte secondary batteries generally comprise an electrode assembly having a pair of electrodes electrically isolated by a separator, and a non-aqueous electrolyte sandwiched between the electrodes. They are configured to charge and discharge by transferring charge-carrying ions between the electrodes. In addition to secondary batteries, capacitors such as lithium-ion capacitors and electric double-layer capacitors are also widely used as non-aqueous electrolyte energy storage elements.

The positive electrode of such nonaqueous electrolyte storage elements is commonly made by laminating a positive electrode active material layer containing a positive electrode active material onto a conductive positive electrode substrate such as metallic aluminum. Known positive electrode active materials include lithium transition metal composite oxides, and positive electrode active materials containing nickel, such as lithium nickel composite oxides, are also used (see Patent Document 1).

JP 2017-107827 A

In order to increase the electrical capacity of non-aqueous electrolyte storage elements, the use of positive electrode active materials with a high nickel content has been considered. However, the inventors have discovered that using positive electrode active materials containing nickel can degrade the metallic aluminum of the positive electrode substrate, resulting in a deterioration in the performance of the non-aqueous electrolyte storage element, such as output performance.

The present invention was made based on the above circumstances, and its purpose is to provide a positive electrode that, when used in a nonaqueous electrolyte storage element, enables the nonaqueous electrolyte storage element to exhibit large electrical capacity and good output performance, and a nonaqueous electrolyte storage element equipped with such a positive electrode.

A positive electrode according to one embodiment of the present invention is a positive electrode for a non-aqueous electrolyte storage element, comprising: a positive electrode substrate containing metallic aluminum; an intermediate layer containing a conductive agent; and a positive electrode active material layer containing a lithium transition metal composite oxide, in this order; the lithium transition metal composite oxide contains nickel; the nickel content per unit area of the positive electrode active material layer is 1.1 × 10 ^-4 mol/cm ² or more; and the mass per unit area of the positive electrode active material layer is 0.017 g/cm ² or less.

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

When used in a nonaqueous electrolyte storage element, the positive electrode according to one embodiment of the present invention enables the nonaqueous electrolyte storage element to exhibit high electrical capacity and good output performance.

A nonaqueous electrolyte energy storage element according to another aspect of the present invention can exhibit large electrical capacity and good output performance.

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 configured by assembling a plurality of nonaqueous electrolyte electricity storage elements.

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

A positive electrode according to one embodiment of the present invention is a positive electrode for a non-aqueous electrolyte storage element, comprising: a positive electrode substrate containing metallic aluminum; an intermediate layer containing a conductive agent; and a positive electrode active material layer containing a lithium transition metal composite oxide, in this order; the lithium transition metal composite oxide contains nickel; the nickel content per unit area of the positive electrode active material layer is 1.1 × 10 ^-4 mol/cm ² or more; and the mass per unit area of the positive electrode active material layer is 0.017 g/cm ² or less.

When this positive electrode is used in a nonaqueous electrolyte storage element, the nonaqueous electrolyte storage element can exhibit a large electrical capacity and good output performance. While the reason for this is unclear, the following reason is presumed. In a conventional positive electrode for a nonaqueous electrolyte storage element, when a positive electrode active material with a high nickel content is used, the metallic aluminum of the positive electrode substrate deteriorates due to an increase in the alkaline component contained in the positive electrode active material. Specifically, it is presumed that the metallic aluminum on the surface of the positive electrode substrate reacts with the alkaline component to form aluminum hydroxide, reducing the conductivity and degrading the current collection performance of the positive electrode substrate. Therefore, in a conventional positive electrode, if the nickel content per unit area of the positive electrode active material layer is increased to 1.1 × ¹⁰ mol/cm ² or more, the initial electrical capacity increases, but the output performance decreases due to degradation of the positive electrode substrate. In contrast, in a positive electrode according to one embodiment of the present invention, an intermediate layer is provided between the positive electrode substrate and the positive electrode active material layer, thereby preventing the alkaline component from reaching the positive electrode substrate from the positive electrode active material layer, and the mass per unit area of the positive electrode active material layer is kept to 0.017 g/ ^cm2 or less, thereby preventing deterioration of the positive electrode substrate. Furthermore, in a positive electrode according to one embodiment of the present invention, the nickel element content per unit area of the positive electrode active material layer is 1.1 × ^10-4 mol/ ^cm2 or more, and therefore the electric capacity is also large. For these reasons, it is presumed that the use of a positive electrode according to one embodiment of the present invention enables a nonaqueous electrolyte storage element to exhibit large electric capacity and good output performance.

The "nickel element content per unit area of the positive electrode active material layer (mol/cm ² )" can be determined by the following method. A test battery is assembled using a positive electrode to be measured, a metallic lithium negative electrode, a separator, and a non-aqueous electrolyte, and fully discharged using the same method as in the measurement of the "discharge capacity per unit area of the positive electrode active material layer" described below. The battery is then disassembled again, and the positive electrode is removed. Components (such as the non-aqueous electrolyte) adhering to the removed positive electrode are thoroughly washed using dimethyl carbonate, and the battery is dried under reduced pressure at room temperature for 24 hours. A sample of the positive electrode active material layer with a predetermined area is then sampled, and the amount of nickel (mol) contained in this sample is measured using inductively coupled plasma atomic emission spectroscopy (ICP). The measured amount of nickel (mol) is divided by the area (cm ² ) of the positive electrode active material layer corresponding to the sample used for measurement, and the value obtained is the "nickel element content per unit area of the positive electrode active material layer (mol/cm ² )." When a positive electrode active material layer is provided on each side of the positive electrode substrate, the "nickel element content per unit area of the positive electrode active material layer (mol/cm ² )" refers to the nickel element content of the positive electrode active material layer on one side. When a positive electrode active material layer is provided on only one side of the positive electrode substrate, the "nickel element content per unit area of the positive electrode active material layer (mol/cm 2 )" refers to the nickel element content of that one positive electrode active material layer.

The "mass per unit area of the positive electrode active material layer (g/cm ² )" is the value obtained by dividing the mass (g) of the positive electrode active material layer by the area (cm ² ) over which the positive electrode active material layer is laminated. When the positive electrode active material layer is formed by coating, the "mass per unit area of the positive electrode active material layer (g/cm ² )" is the coating mass converted to solid content per unit area. When positive electrode active material layers are provided on both sides of the positive electrode substrate, the "mass per unit area of the positive electrode active material layer (g/cm ^{2 )" refers to the mass of the positive electrode active material layer on one side. When positive electrode active material layers are provided on only one side of the positive electrode substrate, the "mass per unit area of the positive electrode active material layer (g/cm 2} )" refers to the mass of that one positive electrode active material layer.

In the positive electrode according to one embodiment of the present invention, the discharge capacity per unit area of the positive electrode active material layer is preferably 3.00 mAh/ ^cm2 or ^more when the positive electrode potential in a fully charged state is 4.23 V vs. Li/Li ⁺ and the positive electrode potential in a fully discharged state is 3.00 V vs. Li/Li+.

This positive electrode can increase the electrical capacity of nonaqueous electrolyte storage elements. The discharge capacity measurement is performed using the following method. A test battery is assembled using a test positive electrode, a metallic lithium negative electrode, a separator, and a nonaqueous electrolyte. The nonaqueous electrolyte is prepared by dissolving LiPF6 at a concentration of 1.2 mol/dm3 in a nonaqueous solvent containing EC (ethylene carbonate), EMC (ethyl methyl carbonate), and DMC (dimethyl carbonate ₎ in a volume ratio of 30:35:35. The assembled test battery is charged at a constant current of 1.00 mA/ ^{cm2 and} a cut-off voltage of 4.23 V at 25°C for 5 hours to a fully charged state. After a 20-minute rest period, the battery is discharged at a constant current of 1.00 mA/ ^cm2 and a cut-off voltage of 3.00 V to a fully discharged state. The discharged electrical quantity is the discharge capacity. In addition, since the test battery uses a metallic lithium negative electrode as the negative electrode, the positive electrode potential relative to the oxidation-reduction potential of lithium at the end-of-charge voltage and the end-of-discharge voltage is substantially equal to the end-of-charge voltage and the end-of-discharge voltage of the test battery.

The mass per unit area of the intermediate layer is preferably 0.05 g/m ² or more and 1 g/m ² or less.

Such a positive electrode can exhibit a sufficient blocking function against alkaline components while preventing the positive electrode from becoming thick, thereby further improving output performance. Note that when a positive electrode active material layer and an intermediate layer are provided on both sides of the positive electrode substrate, the "mass per unit area of the intermediate layer (g/cm ^{2 )" refers to the mass of the intermediate layer on one side. When a positive electrode active material layer and an intermediate layer are provided on only one side of the positive electrode substrate, the "mass per unit area of the intermediate layer (g/cm 2} )" refers to the mass of that one intermediate layer.

It is preferable that the nickel content of the transition metal elements in the lithium transition metal composite oxide be 50 mol % or more.

This type of positive electrode uses a lithium transition metal composite oxide with a high nickel content, which allows for a higher electrical capacity.

It is preferable that the average particle size of the conductive agent contained in the intermediate layer be 40 nm or less.

This type of positive electrode improves adhesion between conductive agents in the intermediate layer and reduces voids, thereby improving the conductivity of the intermediate layer and its blocking function against alkaline components.

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

This non-aqueous electrolyte storage element can exhibit large electrical capacity and good output performance.

A positive electrode and a nonaqueous electrolyte storage element according to one embodiment of the present invention will now be described in detail. 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.

<Positive electrode>
A positive electrode according to one embodiment of the present invention includes a positive electrode substrate, an intermediate layer, and a positive electrode active material layer, in this order. The intermediate layer and the positive electrode active material layer may be laminated on both sides of the positive electrode substrate, respectively, or may be laminated on only one side of the positive electrode substrate. The positive electrode may typically be a multilayer sheet having these layers. The positive electrode is an electrode for a non-aqueous electrolyte storage element such as a non-aqueous electrolyte secondary battery.

(Positive electrode substrate)
The positive electrode substrate is electrically conductive. "Electrical conductivity" means that the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 ⁷ Ω·cm or less, and "non-conductive" means that the volume resistivity is greater than 10 ⁷ Ω·cm.

The positive electrode substrate contains metallic aluminum. The material of the positive electrode substrate is usually metallic aluminum alone or an aluminum alloy. The metallic aluminum content in the positive electrode substrate is preferably 90% by mass or more, more preferably 99% by mass or more, and may be substantially 100% by mass. The positive electrode substrate may contain components other than metallic aluminum.

The positive electrode substrate is in the form of a film or plate. Examples of positive electrode substrates include foil and vapor-deposited film, with foil being preferred from a cost perspective. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum metal or aluminum alloys include A1085, A3003, and A1N30 as specified in JIS-H-4000 (2014) or JIS-H4160 (2006).

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, the strength of the positive electrode substrate can be increased while also increasing the energy density per volume of the nonaqueous electrolyte storage element. The "average thickness" of the positive electrode substrate refers to the value obtained by dividing the punched mass when a positive electrode substrate of a specified area is punched out by the true density and punched area of the positive electrode substrate. The average thickness of the "negative electrode substrate" described below is defined in the same way.

(middle class)
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. The conductive agent is a component that has electrical conductivity, and is typically in the form of conductive particles. In this positive electrode, the presence of the intermediate layer prevents alkaline components from reaching the surface of the positive electrode substrate from the positive electrode active material layer, thereby suppressing deterioration of the positive electrode substrate, allowing the nonaqueous electrolyte storage element to exhibit good output performance. Furthermore, the presence of the conductive agent in the intermediate layer reduces contact resistance between the positive electrode substrate and the positive electrode active material layer.

The conductive agent is not particularly limited as long as it is a material that is conductive. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Carbonaceous materials include graphite, non-graphitized carbon, and graphene-based carbon. Non-graphitized carbon includes carbon nanofiber, pitch-based carbon fiber, and carbon black. Carbon black includes furnace black, acetylene black, and ketjen black. Graphene-based carbon includes graphene, carbon nanotubes (CNT), and fullerene. The conductive agent may be in the form of powder or fiber. As the conductive agent, one of these materials may be used alone, or two or more may be mixed. These materials may also be used in combination. For example, a composite of carbon black and CNT may be used. Among these, from the standpoints of conductivity and coatability, carbonaceous materials are preferred, carbon black is more preferred, and acetylene black is even more preferred.

The surface of the conductive agent, such as a carbonaceous material, is preferably not coated with other components, such as inorganic oxides (e.g., metal oxides), and is more preferably composed essentially of carbonaceous material alone. Carbonaceous materials are generally hydrophobic, while metal oxides and the like are generally hydrophilic. Using a hydrophobic conductive agent can more effectively prevent alkaline components (e.g., moisture containing alkaline components) from penetrating the intermediate layer from the positive electrode active material layer and reaching the positive electrode substrate surface. This more reliably provides a sufficient blocking function against alkaline components.

The average particle size of the conductive agent is preferably, for example, 1 nm or more and 200 nm or less, more preferably 10 nm or more and 100 nm or less, with an upper limit of 40 nm being even more preferable. By setting the average particle size of the conductive agent within this range, it is possible to further improve conductivity and blocking properties against alkaline components. "Average particle size" refers to the value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 50%, based on the particle size distribution measured by laser diffraction/scattering in a diluted solution obtained by diluting particles with a solvent in accordance with JIS-Z-8825 (2013). Below, the average particle size of other materials is defined in the same way.

The content of the conductive agent in the intermediate layer is preferably 10% by mass or more and 80% by mass or less, more preferably 20% by mass or more and 60% by mass or less, and even more preferably 25% by mass or more and 40% by mass or less. By keeping the content of the conductive agent in the intermediate layer within this range, it is possible to achieve a good balance between the adhesion between the conductive agents themselves or with other layers, as well as the conductivity, thereby improving the output performance, etc. of the nonaqueous electrolyte storage element. In addition, it is possible to prevent alkaline components (e.g., moisture containing alkaline components) from penetrating into the intermediate layer from the positive electrode active material layer and reaching the surface of the positive electrode substrate, thereby providing a sufficient blocking function against alkaline components.

In addition to the conductive agent, the intermediate layer preferably further contains a binder. 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. Polysaccharide polymers are preferred as binders in the intermediate layer. Examples of polysaccharide polymers include chitosan, chitin, cellulose, and derivatives thereof. Of these, chitosan, chitin, and their derivatives are preferred, with chitosan and its derivatives being more preferred. Chitosan derivatives include hydroxyalkyl chitosans such as hydroxyethyl chitosan and hydroxypropyl chitosan. Furthermore, from the perspective of durability, derivatives of chitosan and the like that have a crosslinked structure obtained by reacting them with organic acids such as salicylic acid, pyromellitic acid, citric acid, and trimellitic acid can also be used.

The binder content in the intermediate layer is preferably 20% by mass or more and 80% by mass or less, and more preferably 50% by mass or more and 75% by mass or less. By keeping the binder content within this range, the conductive agent can be stably retained, and good conductivity can be maintained and exhibited.

The intermediate layer may further contain components other than the conductive agent and binder. Examples of such components include non-conductive fillers (e.g., metal oxides). However, the total content of the conductive agent and binder in the intermediate layer is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and even more preferably 99% by mass or more. The intermediate layer may be composed essentially of only the conductive agent and binder. The technology disclosed herein is preferably implemented in an embodiment in which the intermediate layer contains no components other than the conductive agent and binder. This configuration of the intermediate layer allows the intermediate layer to exhibit good conductivity while also becoming hydrophobic, sufficiently reducing the reach of alkaline components from the positive electrode active material layer to the positive electrode substrate. Therefore, this configuration can increase the electrical capacity of the non-aqueous electrolyte storage element and further improve its output performance.

The mass per unit area of the intermediate layer is preferably 0.05 g/m ² or more and 1 g/m ² or less, and more preferably 0.1 g/m ² or more and 0.5 g/m ² or less. By setting the mass per unit area of the intermediate layer within the above range, it is possible to suppress an increase in thickness of the positive electrode and to exhibit a sufficient blocking function against alkaline components.

(Positive electrode active material layer)
The positive electrode active material layer contains a positive electrode active material and, if necessary, optional components such as a conductive agent, a binder, a thickener, and a filler.

The positive electrode active material includes a lithium transition metal composite oxide containing nickel. The lower limit of the content ratio of nickel (Ni) to the transition metal element (Me) in the lithium transition metal composite oxide containing nickel (Ni/Me) may be, for example, 30 mol%, but is preferably 50 mol%, more preferably 70 mol%, and even more preferably 80 mol%. Using such a lithium transition metal composite oxide with a high content ratio of nickel can increase the electrical capacity. On the other hand, the upper limit of this nickel content ratio (Ni/Me) may be, for example, 100 mol% or 90 mol%. The nickel content ratio (Ni/Me) may be equal to or greater than any of the above lower limits and equal to or less than any of the above upper limits.

The lithium transition metal composite oxide containing nickel preferably further contains at least one of manganese and cobalt, and more preferably both manganese and cobalt. The manganese (Mn) content (Mn/Me) in the transition metal element (Me) in the lithium transition metal composite oxide containing nickel is preferably 1 mol% or more and 30 mol% or less, and more preferably 5 mol% or more and 15 mol% or less. The cobalt (Co) content (Co/Me) in the transition metal element (Me) in the lithium transition metal composite oxide containing nickel is preferably 1 mol% or more and 30 mol% or less, and more preferably 5 mol% or more and 15 mol% or less.

The ratio of lithium (Li) to transition metal (Me) in the lithium transition metal composite oxide containing nickel (Li/Me) is preferably 1 or more and less than 1.5, and may be 1.

Examples of lithium transition metal composite oxides containing nickel include Li[Li _x Ni _(1-x) ]O ₂ (0≦x≦0.5), Li[Li _x Ni _α Co _(1-x-α) ] _{O 2 (} 0≦x≦0.5, 0<α<1, 0<x+α<1), and Li[Li _x Ni _α Mn _β Co _(1-x-α-β) ]O ₂ (0≦x<0.5, 0<α<1, 0<β<1, 0<x+α+β<1), which have a layered α-NaFeO 2 type crystal structure, and Li _x Ni _γ Mn _(2-γ) O ₄ , which has a spinel type crystal structure. The surfaces of these materials may be coated with other materials. These materials may be used alone or in combination of two or more.

Among these, compounds having a layered α-NaFeO ₂ type crystal structure are preferred, and Li[Li _x Ni _α Mn _β Co _(1-x-α-β) ]O ₂ (0≦x<0.5, 0<α<1, 0<β<1, 0<x+α+β<1) is more preferred. In this case, it is more preferred that 0.7<α<0.9, 0.05<β<0.2, and 0.05<1-x-α-β<0.2.

Specific examples of nickel-containing lithium transition metal composite oxides include LiNi3 _{/5Co1 /5Mn1 / 5O2} , LiNi1 _/2Co1 / _{5Mn3 / 10O2} , LiNi1 _/2Co3 / _{10Mn1 /5O2 ,} and LiNi8 _/10Co1 / _{10Mn1 / 10O2} . The chemical formulas representing the lithium transition metal composite oxides above represent the compositions in a state before the first charging process (i.e., the first charging process performed after assembling battery components such as the positive electrode, negative electrode, and electrolyte) or the compositions in a state after being fully discharged by the method described above.

The content of the nickel-containing lithium transition metal composite oxide in the positive electrode active material layer is preferably 70% by mass or more and 99% by mass or less, more preferably 80% by mass or more and 99% by mass or less, and even more preferably 90% by mass or more and 98.5% by mass or less. By setting the content of the nickel-containing lithium transition metal composite oxide within the above range, both high energy density and manufacturability of the positive electrode active material layer can be achieved. Furthermore, when the content of the nickel-containing lithium transition metal composite oxide in the positive electrode active material layer is high, the alkaline component increases and the binder content relatively decreases, which tends to cause peeling of the positive electrode active material layer and reduce output performance in conventional positive electrodes. Therefore, the advantages of the present invention are more effectively achieved when the content of the nickel-containing lithium transition metal composite oxide in the positive electrode active material layer is, for example, 92% by mass or more, or even 94% by mass or more. In some embodiments, the content of the lithium transition metal composite oxide may be 95% by mass or more, or even 98% by mass or more.

The positive electrode active material layer may further contain a positive electrode active material other than the lithium transition metal composite oxide containing nickel. Examples of such other positive electrode active materials include conventionally known positive electrode active materials such as lithium transition metal composite oxides that do not contain nickel, polyanion compounds, chalcogen compounds, and sulfur. The surfaces of these materials may be coated with other materials. Furthermore, these materials may be used alone or in combination of two or more.

The lower limit of the content of the lithium transition metal composite oxide containing nickel in the positive electrode active material is preferably 80% by mass, more preferably 90% by mass, even more preferably 95% by mass, and even more preferably 99% by mass. The positive electrode active material may essentially consist of only lithium transition metal composite oxide containing nickel. In this way, by increasing the content of the lithium transition metal composite oxide containing nickel in the positive electrode active material, the electrical capacity of the nonaqueous electrolyte storage element can be further increased.

The positive electrode active material is usually in particulate form. The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. Setting the average particle size of the positive electrode active material to be above the above lower limit makes the positive electrode active material easier to manufacture and handle. Setting the average particle size of the positive electrode active material to be below the above upper limit improves the conductivity of the positive electrode active material layer. Note that when a composite of the positive electrode active material and another material is used, the average particle size of the composite is taken as the average particle size of the positive electrode active material.

To obtain particles of a specified particle size, mills and classifiers are used. Examples of milling methods include those using a mortar, ball mill, sand mill, vibrating ball mill, planetary ball mill, jet mill, counter jet mill, swirling air jet mill, or sieve. Wet milling, in which water or an organic solvent such as hexane is present, can also be used. Sieves and air classifiers are used as classification methods, both dry and wet, as needed.

The content of the positive electrode active material in the positive electrode active material layer is preferably 70% by mass or more and 99% by mass or less, more preferably 80% by mass or more and 99% by mass or less, and even more preferably 90% by mass or more and 98% by mass or less. By keeping the content of the positive electrode active material within this range, it is possible to achieve both high energy density and manufacturability of the positive electrode active material layer.

Conductive agents in the positive electrode active material layer can be the same as those exemplified as conductive agents in the intermediate layer. The content of the conductive agent in the positive electrode active material layer is preferably 0.2% by mass or more and 10% by mass or less, more preferably 0.5% by mass or more and 8% by mass or less, and even more preferably 1% by mass or more and 5% by mass or less. By keeping the content of the conductive agent in the positive electrode active material layer within the above range, the energy density of the nonaqueous electrolyte storage element can be increased.

Examples of binders in the positive electrode active material layer include those exemplified as binders in the intermediate layer. The binder content in the positive electrode active material layer is preferably 0.1% by mass or more and 10% by mass or less, and more preferably 0.3% by mass or more and 5% by mass or less. By maintaining the binder content in the positive electrode active material layer within the above range, the positive electrode active material can be stably maintained. In particular, if the content of nickel-containing lithium transition metal composite oxide in the positive electrode active material layer is high and the binder content is relatively low, conventional positive electrodes tend to be prone to peeling of the positive electrode active material layer and a decrease in output performance. Therefore, the advantages of the present invention are more effectively achieved when the binder content in the positive electrode active material layer is, for example, 2% by mass or less, or even 1% by mass or less.

Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. 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.

Examples of fillers 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; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica; or synthetic versions of these.

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

The positive electrode active material layer preferably contains substantially no neutralizing agent. In this case, the positive electrode active material layer in the positive electrode is typically alkaline. To prevent alkaline components from reaching the surface of the positive electrode substrate from the positive electrode active material layer, it is possible to incorporate a neutralizing agent into the positive electrode active material layer and reduce the alkaline component content in the positive electrode active material layer. However, incorporating a neutralizing agent into the positive electrode active material can result in impurities that affect the performance of the nonaqueous electrolyte storage element. In other words, by substantially eliminating the neutralizing agent from the positive electrode active material layer, the output performance of the nonaqueous electrolyte storage element can be further improved. The content of the neutralizing agent in the positive electrode active material layer is preferably 1% by mass or less, more preferably 0.1% by mass or less, and even more preferably 0.01% by mass or less (e.g., 0.001% by mass or less). Examples of neutralizing agents include organic acids such as citric acid and maleic acid.

The lower limit of the nickel element content per unit area of the positive electrode active material layer is 1.1×10 ⁻⁴ mol/cm ² , preferably 1.2×10 ⁻⁴ mol/cm ² , and in some cases, more preferably 1.25×10 ⁻⁴ mol/cm ^2. By setting the nickel element content per unit area of the positive electrode active material layer to be equal to or greater than the above lower limit, the electrical capacity of the nonaqueous electrolyte storage element can be increased. The upper limit of the nickel element content per unit area of the positive electrode active material layer is, for example, 2×10 ⁻⁴ mol/cm ² , or may be 1.6×10 ⁻⁴ mol/cm ² , 1.4×10 ⁻⁴ mol/cm ² , or 1.3×10 ⁻⁴ mol/cm ^2. The nickel element content per unit area of the positive electrode active material layer may be equal to or greater than any of the above lower limits and equal to or less than any of the above upper limits.

The upper limit of the mass per unit area of the positive electrode active material layer is 0.017 g/cm ² , preferably 0.0165 g/cm ^2. By setting the mass per unit area of the positive electrode active material layer to the above upper limit or less, the amount of alkaline component contained in the positive electrode active material layer that reaches the positive electrode substrate can be reduced, thereby improving the output performance of the non-aqueous electrolyte storage element. In some embodiments, the mass per unit area of the positive electrode active material layer may be 0.0161 g/cm ² or less, or may be 0.0158 g/cm ² or less. The lower limit of the mass per unit area of the positive electrode active material layer is preferably 0.014 g/cm ² , more preferably 0.015 g/cm ² , and even more preferably 0.0155 g/cm ^2. By setting the mass per unit area of the positive electrode active material layer to the above lower limit or more, the electrical capacity of the non-aqueous electrolyte storage element can be further increased. The mass per unit area of the positive electrode active material layer can be set to be equal to or greater than any of the above lower limits and equal to or less than any of the above upper limits.

When the positive electrode potential in the fully charged state of the positive electrode is 4.23 V vs. Li/Li ⁺ and the positive electrode potential in the fully discharged state is 3.00 V vs. Li/Li ⁺ , the discharge capacity per unit area of the positive electrode active material layer is not particularly limited and may be, for example, 2.5 mAh/ ^cm2 or more, preferably 3.00 mAh/ ^cm2 or more, and more preferably 3.05 mAh/ ^cm2 or more. In such a case, the electrical capacity of the nonaqueous electrolyte storage element can be further increased. On the other hand, the discharge capacity per unit area of this positive electrode active material layer may be, for example, 3.5 mAh/ ^cm2 or less, 3.3 mAh/ ^cm2 or less, or 3.2 mAh/ ^cm2 or less. The discharge capacity per unit area of this positive electrode active material layer can be equal to or greater than any of the above lower limits and equal to or less than any of the above upper limits.

<Method of manufacturing positive electrode>
A positive electrode according to one embodiment of the present invention can be obtained, for example, by a manufacturing method including forming an intermediate layer on the surface of a positive electrode substrate and forming a positive electrode active material layer on the surface of the intermediate layer.

The intermediate layer can be formed by applying and drying an intermediate layer-forming paste containing the components that make up the intermediate layer and a dispersion medium. The positive electrode active material layer can be formed by applying and drying a positive electrode active material layer-forming paste (positive electrode mixture paste) containing the components that make up the positive electrode active material layer and a dispersion medium.

The method for applying the intermediate layer forming paste and the positive electrode active material layer forming paste is not particularly limited, and can be performed by known methods such as roller coating, screen coating, and spin coating. After application and drying, the intermediate layer and positive electrode active material layer may be pressed in the thickness direction by known methods.

<Non-aqueous 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 comprises a positive electrode, a negative electrode, and a nonaqueous electrolyte. The positive electrode and the negative electrode are usually stacked or wound with a separator interposed therebetween to form an electrode assembly. This electrode assembly is housed in a container, and the container is filled with a nonaqueous electrolyte. The nonaqueous electrolyte is interposed between the positive electrode and the negative electrode. 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 is the positive electrode according to one embodiment of the present invention described above.

(Negative electrode)
The negative electrode has a negative electrode substrate and a negative electrode active material layer disposed on the negative electrode substrate directly or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from the configurations exemplified for the positive electrode above. The negative electrode may usually be a multilayer sheet having these layers.

The negative electrode substrate is electrically conductive. Metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, or alloys of these, are used as materials for the negative electrode substrate. Of these, copper or copper alloys are preferred. Examples of negative electrode substrates include foils and vapor-deposited films, with foils being preferred from a cost perspective. 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 keeping the average thickness of the negative electrode substrate within the above range, the strength of the negative electrode substrate can be increased while also increasing the energy density per volume of the secondary battery.

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

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.

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 typically used as the negative electrode active material for lithium ion secondary batteries. Examples of negative electrode active materials 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 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 mixed and used.

"Graphite" refers to a carbon material in which the average lattice spacing (d ₀₀₂ ) of the (002) plane, as determined by X-ray diffraction before charge/discharge or in a discharged state, 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.

"Non-graphitic carbon" refers to a carbon material in which the average lattice spacing (d ₀₀₂ ) of the (002) plane, as determined by X-ray diffraction before charge/discharge or in a discharged state, is 0.34 nm or more and 0.42 nm or less. Examples of non-graphitic carbon include non-graphitizable carbon and graphitizable carbon. Examples of non-graphitic carbon include resin-derived materials, petroleum pitch or petroleum pitch-derived materials, petroleum coke or petroleum coke-derived materials, plant-derived materials, and alcohol-derived materials.

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

The term "non-graphitizable carbon" refers to a carbon material having the above d ₀₀₂ of 0.36 nm or more and 0.42 nm or less.

The term "easily graphitizable carbon" refers to a carbon material having the above d ₀₀₂ of 0.34 nm or more and less than 0.36 nm.

The negative electrode active material is typically particulate. The average particle size of the negative electrode active material can be, for example, 1 μm 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. Setting the average particle size of the negative electrode active material to be above the above lower limit facilitates the production and handling of the negative electrode active material. Setting the average particle size of the negative electrode active material to be below the above upper limit improves the conductivity of the negative electrode active material layer. A pulverizer, classifier, or the like is used to obtain particles with the specified particle size. The pulverization method and powder classification method can be selected, for example, from the methods exemplified above for the positive electrode.

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 keeping the content of the negative electrode active material within this range, it is possible to achieve both high energy density and manufacturability of the negative electrode active material layer.

(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.

The heat-resistant particles contained in the heat-resistant layer preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C in an air atmosphere at 1 atmosphere pressure, and more preferably exhibit a mass loss of 5% or less when heated from room temperature to 800°C. Examples of materials exhibiting a mass loss of less than the specified value 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 aluminosilicates; hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide; 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 may be mixed and used. Of these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferred from the standpoint of safety of the nonaqueous electrolyte storage element.

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

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

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

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 in these compounds have been substituted with halogens may also be used.

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, and 1,2-diphenylvinylene carbonate. Of these, EC is preferred.

Examples of chain carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, and bis(trifluoroethyl) carbonate. Of these, EMC is preferred.

It is preferable to use a cyclic carbonate or a chain carbonate as the non-aqueous solvent, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. The use of a cyclic carbonate promotes dissociation of the electrolyte salt, improving the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate reduces 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.

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

Examples of lithium salts include inorganic _lithium salts such as _LiPF6 , _LiPO2F2 , _LiBF4 , _LiClO4 , _and LiN( _SO2F ) _{2 , 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 non-aqueous electrolyte solution at 20°C and 1 atmosphere is preferably 0.1 mol/ ^dm3 or more and 2.5 mol/dm3 ^{or less} , more preferably 0.3 mol/dm3 or ^more and 2.0 mol/dm3 ^or less, even more preferably 0.5 mol/dm3 or more and 1.7 mol/dm3 or ^less , and particularly preferably ^0.7 mol/dm3 or ^more and 1.5 mol/dm3 or less. By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte solution can be increased.

The non-aqueous electrolyte may contain additives. Examples of 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 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, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, and cyclohexane. Dicarboxylic acid anhydrides; ethylene sulfite, propylene sulfite, dimethyl sulfite, 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, etc. These additives may be used alone or in combination of two or more.

The content of additives contained in the non-aqueous electrolyte is preferably 0.01% by mass or more and 10% by mass or less, more preferably 0.1% by mass or more and 7% by mass or less, even more preferably 0.2% by mass or more and 5% by mass or less, and particularly preferably 0.3% by mass or more and 3% by mass or less, relative to the total mass of the non-aqueous electrolyte. By keeping the additive content within the above range, it is possible to improve capacity retention 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 is conductive to ions such as lithium, sodium, and calcium, and is solid at room temperature (e.g., 15°C to 25°C). Examples of solid electrolytes include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, and polymer solid electrolytes.

Examples of sulfide solid electrolytes include Li ₂ SP ₂ S ₅ , LiI-Li ₂ SP 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, prismatic batteries, flat batteries, coin batteries, and button batteries.

Figure 1 shows a nonaqueous electrolyte storage element 1 as an example of a prismatic battery. Note that this 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.

<Electricity storage device>
The nonaqueous electrolyte energy storage element of this embodiment can be mounted as an energy storage unit (battery module) comprising a plurality of nonaqueous electrolyte energy storage elements 1 in automotive power sources such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), power sources for electronic devices such as personal computers and communication terminals, or power storage power sources, 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.

Figure 2 shows an example of an energy storage device 30 that further assembles energy storage units 20, each of which is an assembly of two or more electrically connected nonaqueous electrolyte energy storage elements 1. The energy storage device 30 may include a bus bar (not shown) that electrically connects two or more nonaqueous electrolyte energy storage elements 1, and a bus bar (not shown) that electrically connects two or more energy storage units 20. The energy storage unit 20 or the energy storage device 30 may also include a status monitoring device (not shown) that monitors the status of one or more nonaqueous electrolyte energy storage elements.

<Method of manufacturing nonaqueous electrolyte energy 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. Housing 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 may be injected through an injection port formed in the container, and the injection port may then be sealed.

<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 may be added to the configuration of another embodiment, or part of the configuration of one embodiment may be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment may be deleted. Also, well-known technology may be added to the configuration of one embodiment.

In the above embodiment, the nonaqueous electrolyte storage element is described as being used as a chargeable and dischargeable nonaqueous electrolyte secondary battery (e.g., a lithium-ion secondary battery), but the type, shape, dimensions, 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.

The present invention will be explained in more detail below using examples. The present invention is not limited to the following examples.

(Preparation of Positive Electrode)
[Example 1]
An intermediate layer was formed on the surface of aluminum foil (average thickness 15 μm) as a positive electrode substrate in the following manner. Acetylene black, a conductive agent, and chitosan, a binder, were weighed in a mass ratio (solid content equivalent) of 30:70. These were mixed with water as a dispersion medium to prepare an intermediate layer forming paste. This intermediate layer forming paste was applied to one side of the aluminum foil. The resulting mixture was then dried to obtain an intermediate layer.

The positive electrode active material LiNi _8/10 Co _1/10 Mn _1/10 O ₂ , the conductive agent acetylene black, and the binder PVDF were weighed out in a mass ratio (solid content equivalent) of 96:3:1. These were mixed with NMP (N-methylpyrrolidone) as a dispersion medium to prepare a paste for forming a positive electrode active material layer. This paste for forming a positive electrode active material layer was applied to the surface of the intermediate layer and dried to remove the dispersion medium. The paste was then pressure-molded using a roller press to obtain the positive electrode of Example 1.
The mass per unit area of the intermediate layer in the obtained positive electrode was 0.5 g/m ^2. The mass per unit area of the positive electrode active material layer in the obtained positive electrode was 0.01617 g/cm ² , and the nickel element content per unit area of the positive electrode active material layer was 1.274 × 10 ^-4 mol/cm ² .

[Example 2]
A positive electrode of Example 2 was obtained in the same manner as in Example 1, except that the mass ratio (in terms of solid content) of the positive electrode active material, the conductive agent, and the binder was set to 94:4:2, the application amount of the paste for forming the positive electrode active material layer was changed, and the mass per unit area of the positive electrode active material layer was set as shown in Table 1.

[Example 3]
A positive electrode of Example 3 was obtained in the same manner as in Example 1, except that the mass ratio (in terms of solid content) of the positive electrode active material, the conductive agent, and the binder was set to 98:1:1, the amount of the positive electrode active material layer-forming paste applied was changed, and the mass per unit area of the positive electrode active material layer was set as shown in Table 1.

[Comparative Example 1]
A positive electrode of Comparative Example 1 was obtained in the same manner as in Example 1, except that the positive electrode active material layer was formed directly on the aluminum foil serving as the positive electrode substrate without providing an intermediate layer.

[Comparative Example 2]
A positive electrode of Comparative Example 2 was obtained in the same manner as in Comparative Example 1, except that citric acid as a neutralizing agent was further added when preparing the paste for forming a positive electrode active material layer.

[Comparative Example 3]
A positive electrode of Comparative Example 3 was obtained in the same manner as in Comparative Example 1, except that the amount of the paste for forming the positive electrode active material layer was changed to give a mass per unit area of the positive electrode active material layer as shown in Table 1.

[Comparative Example 4]
A positive electrode of Comparative Example 4 was obtained in the same manner as in Example 1, except that the amount of the paste for forming the positive electrode active material layer was changed to give a mass per unit area of the positive electrode active material layer as shown in Table 1.

[Comparative Example 5]
A positive electrode of Comparative Example 5 was obtained in the same manner as in Example 1, except that LiNi _3/5 Co _1/5 Mn _1/5 O ₂ was used as the positive electrode active material.

[Comparative Example 6]
A positive electrode of Comparative Example 6 was obtained in the same manner as in Comparative Example 5, except that the amount of the paste for forming the positive electrode active material layer was changed to give a mass per unit area of the positive electrode active material layer as shown in Table 1.

[Comparative Example 7]
A positive electrode of Comparative Example 7 was obtained in the same manner as in Comparative Example 5, except that the positive electrode active material layer was formed directly on the aluminum foil serving as the positive electrode substrate without providing an intermediate layer.

The nickel element content per unit area of the positive electrode active material layer of each positive electrode in Examples 2 to 3 and Comparative Examples 1 to 7 was as shown in Table 1.

(Discharge capacity per unit area of positive electrode active material layer)
For each of the positive electrodes obtained in the Examples and Comparative Examples, the discharge capacity per unit area of the positive electrode active material layer was measured by the method described above when the potential in a fully charged state was 4.23 V vs. Li/Li ⁺ and the positive electrode potential in a fully discharged state was 3.00 V vs. Li/ ^Li+ . The measurement results are shown in Table 1.

(Fabrication of non-aqueous electrolyte storage element)
[Fabrication of Positive Electrode for Non-Aqueous Electrolyte Storage Element]
In the same manner as the positive electrodes obtained in the above-described Examples and Comparative Examples, positive electrodes for nonaqueous electrolyte storage elements in the Examples and Comparative Examples were obtained in which an intermediate layer and a positive electrode active material layer were formed on both sides of an aluminum foil serving as a positive electrode substrate.
[Fabrication of Negative Electrode for Non-Aqueous Electrolyte Storage Element]
Graphite as a negative electrode active material, SBR as a binder, and CMC as a thickener were weighed in a mass ratio (solid content equivalent) of 98:1:1. These were mixed with water as a dispersion medium to prepare a paste for forming a negative electrode active material layer. This paste for forming a negative electrode active material layer was applied to both sides of copper foil as a negative electrode substrate and dried to remove the dispersion medium. The paste was then pressure-molded using a roller press to obtain a negative electrode for a nonaqueous electrolyte storage element.

[Assembly of non-aqueous electrolyte energy storage element]
The positive electrode for the nonaqueous electrolyte storage element of each of the above Examples and Comparative Examples and the negative electrode for the nonaqueous electrolyte storage element were wound with a polyethylene separator interposed therebetween to obtain an electrode assembly in which the positive electrode and the negative electrode were wound. A nonaqueous electrolyte was also prepared by dissolving the electrolyte salt _LiPF6 at a concentration of 1.4 mol/ ^dm3 in a nonaqueous solvent in which EC and EMC were mixed in a volume ratio of 30:70. Each of the nonaqueous electrolyte storage elements of the Examples and Comparative Examples was fabricated using the electrode assembly and the nonaqueous electrolyte.

[Initial capacity]
The initial capacity of each nonaqueous electrolyte storage element of the Examples and Comparative Examples was measured by the following method. The following test was carried out in a thermostatic chamber at 25°C. After constant current charging at a charging current of 1 mA/ ^cm2 at an upper limit voltage of 4.15 V, constant voltage charging was carried out at 4.15 V. The charging was terminated after 5 hours from the start of charging. After a 10-minute rest period, constant current discharge was carried out at a discharging current of ¹ mA/cm2 at a lower limit voltage of 2.75 V, and the amount of electricity during this discharge was taken as the initial capacity. The initial capacity measurement results shown in Table 1 are relative values, with Example 1 being taken as 100%.

(Whether or not the surface of the positive electrode substrate is whitened)
For each of the positive electrodes of the Examples and Comparative Examples, which were separately prepared using the same procedure, the positive electrode active material layer and intermediate layer were peeled off, and the presence or absence of whitening of the positive electrode substrate surface was visually confirmed. Note that the whitening of the positive electrode substrate surface is thought to occur due to the reaction of metallic aluminum with an alkaline component to produce aluminum hydroxide. The results of the confirmation of the presence or absence of whitening are shown in Table 1.

(Output performance)
The output performance of each nonaqueous electrolyte storage element of the Examples and Comparative Examples was measured using the following method. 1 C was determined using the initial capacity test described above, and the discharged state was subjected to constant current charging at a temperature of 25°C, a charging current of 0.5 C, and a charging time of 1 hour, adjusting the SOC (State of Charge) to 50%. Then, at a temperature of 25°C, constant current discharges were performed for 10 seconds at discharge currents of 0.2 C, 0.5 C, and 1.0 C. After each discharge, constant current charging was performed at a current of 0.5 C, and supplementary charging was performed to compensate for the discharged amount of electricity, ensuring that the SOC did not deviate from 50%. The voltage at 1 second at each obtained current was plotted to obtain a straight line. The slope of the line was taken as the DC resistance DCR, and the intercept was taken as the pre-energization voltage Vo. The output was then calculated using the formula {(Vo - 2.5)/DCR} × 2.5. The output measurement results are shown in Table 1. The output measurement results shown in Table 1 are relative values with the output of Example 1 taken as 100%.

As shown in Table 1, Comparative Example 1, which uses a positive electrode without an intermediate layer and with a high nickel content per unit area of the positive electrode active material layer, exhibits whitening of the aluminum-containing positive electrode substrate and low output. In contrast to Comparative Example 1, Comparative Example 2, which uses a positive electrode with a neutralizing agent added to the positive electrode active material layer, suppresses whitening of the positive electrode substrate but has low output. Comparative Example 3, which uses a positive electrode without an intermediate layer and with a high mass per unit area of the positive electrode active material layer, and Comparative Examples 4 and 6, which use positive electrodes with an intermediate layer but a high mass per unit area of the positive electrode active material layer, all have high initial capacity but low output. Comparative Examples 5 and 7, which use positive electrodes with a low nickel content per unit area of the positive electrode active material layer, have low initial capacity.

In contrast to the above comparative examples, Examples 1 to 3, which used positive electrodes having an intermediate layer, a nickel element content per unit area of the positive electrode active material layer of 1.1 × ¹⁰ mol/cm ² or more, and a mass per unit area of the positive electrode active material layer of 0.017 g/cm ² or less, all had large initial capacities of 97% or more, suppressed whitening of the positive electrode substrate, and achieved outputs of 100% or more. These results demonstrate that when such positive electrodes are used in nonaqueous electrolyte storage elements, the nonaqueous electrolyte storage elements can exhibit large electrical capacity and good output performance. Furthermore, for example, comparisons between Example 1 and Comparative Examples 5 and 6 reveal that the presence or absence of whitening of the positive electrode substrate and the level of output are influenced by both the nickel element content per unit area of the positive electrode active material layer and the mass per unit area of the positive electrode active material layer.

REFERENCE SIGNS LIST 1 nonaqueous 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 (6)

a positive electrode substrate containing metallic aluminum, an intermediate layer containing a conductive agent, and a positive electrode active material layer containing a lithium transition metal composite oxide, in this order;
the lithium transition metal composite oxide contains nickel;
the positive electrode active material layer has a nickel element content per unit area of 1.1×10 ⁻⁴ mol/cm ² or more and 2×10 ⁻⁴ mol/cm ² or less;
A positive electrode for a non-aqueous electrolyte storage element, wherein the mass per unit area of the positive electrode active material layer is 0.014 g/cm ² or more and 0.017 g/cm ² or less. 2. The positive electrode of claim 1, wherein the positive electrode active material layer has a discharge capacity per unit area of 3.00 mAh/ ^cm2 or more when the positive electrode potential in a fully charged state is 4.23 V vs. Li/Li ⁺ and the positive electrode potential in a fully discharged state is 3.00 V vs. Li/Li ⁺ . 3. The positive electrode according to claim 1, wherein the mass per unit area of the intermediate layer is 0.05 g/m ^<2> or more and 1 g/m ^<2> or less. The positive electrode of any one of claims 1 to 3, wherein the content of nickel element in the transition metal elements in the lithium transition metal composite oxide is 50 mol % or more. The positive electrode of any one of claims 1 to 4, wherein the conductive agent contained in the intermediate layer has an average particle size of 1 nm or more and 40 nm or less. A nonaqueous electrolyte electricity storage element comprising the positive electrode of claim 1 .