JP7794587B2 - Manufacturing method of energy storage element - Google Patents

Description

The present invention relates to a dispersion, a method for producing a dispersion, a slurry, a method for producing an electric storage element, and an electric storage element.

Patent Document 1 describes a method for manufacturing an all-solid-state battery, which includes a step of forming an electrolyte-containing layer using a slurry containing a sulfide solid electrolyte containing Li, P, and S elements, a PVDF-based binder, and a solvent, the solvent containing 50% by volume or more of a ketone solvent represented by a specific molecular structure formula as a first solvent.

Japanese Patent Application Laid-Open No. 2019-091632

An object of the present invention is to provide an electric storage element in which particles of a sulfide solid electrolyte are well bound to each other by fluororesin particles and in which deterioration of the sulfide solid electrolyte during production is suppressed, and a method for producing the electric storage element.
Another object of the present invention is to provide a dispersion that can satisfactorily bond sulfide solid electrolyte particles together using fluororesin particles and that can suppress deterioration of the sulfide solid electrolyte in the production of an energy storage element, a solid electrolyte-containing slurry that includes the dispersion, and a method for producing the dispersion.

A dispersion according to one aspect of the present invention includes a low-polarity solvent and fluororesin particles dispersed in the low-polarity solvent,
the low-polarity solvent is at least one of a hydrocarbon compound and a compound having only a hydrocarbon group and an ether group in the molecule;
The particles have an average particle size of 200 nm or less.

A method for producing a dispersion liquid according to another aspect of the present invention includes preparing a stock solution containing a highly polar solvent containing water and fluororesin particles having an average particle size of 200 nm or less dispersed in the highly polar solvent;
and subjecting the raw liquid to a substitution treatment in which the water is substituted with a low-polarity solvent having a lower polarity than the high-polarity solvent, thereby obtaining a dispersion liquid of the fluororesin particles,
The low polarity solvent is at least one of a hydrocarbon compound and a compound having only a hydrocarbon group and an ether group in the molecule.

A slurry according to yet another aspect of the present invention comprises the above-described dispersion and
The battery includes at least one of particles of a sulfide solid electrolyte, particles of a lithium transition metal composite oxide, and particles of a carbon material.

A method for producing an electric storage element according to another aspect of the present invention includes preparing a solid electrolyte-containing slurry containing a low-polarity solvent, fluororesin particles, and sulfide solid electrolyte particles;
and preparing an electrolyte-containing layer containing fluororesin and sulfide solid electrolyte particles from the coated material of the solid electrolyte-containing slurry,
In preparing the solid electrolyte-containing slurry, the low-polarity solvent is at least one of a hydrocarbon compound and a compound having only a hydrocarbon group and an ether group in the molecule, and the particles have an average particle size of 200 nm or less;
In preparing the electrolyte-containing layer, the coating is subjected to a drying treatment for volatilizing the low-polarity solvent.

According to yet another aspect of the present invention, there is provided an electric storage element including an electrolyte-containing layer formed by drying an applied material of a solid electrolyte-containing slurry including a low-polarity solvent, fluororesin particles, and sulfide solid electrolyte particles;
the low-polarity solvent is at least one of a hydrocarbon compound and a compound having only a hydrocarbon group and an ether group in the molecule, and the average particle size of the fluororesin particles is 200 nm or less;
In the electrolyte-containing layer, the fluororesin is present between particles of the sulfide solid electrolyte.

In the energy storage element according to one aspect of the present invention, the particles of the sulfide solid electrolyte are well bound together by the fluororesin particles, and deterioration of the sulfide solid electrolyte during production is suppressed.
Furthermore, by the method for producing an electric storage element according to another aspect of the present invention, the particles of the sulfide solid electrolyte can be well bound together by the fluororesin particles, and deterioration of the sulfide solid electrolyte can be suppressed.
Furthermore, according to the dispersion liquid or the solid electrolyte-containing slurry containing the dispersion liquid according to another aspect of the present invention, in the production of an electric storage element, the particles of the sulfide solid electrolyte can be well bound together by the fluororesin particles, and deterioration of the sulfide solid electrolyte can be suppressed.
Furthermore, by a method for producing a dispersion according to yet another aspect of the present invention, it is possible to obtain a dispersion that can be used for producing an energy storage element, in which the particles of the sulfide solid electrolyte can be well bound together by the fluororesin particles and deterioration of the sulfide solid electrolyte during production can be suppressed.

FIG. 1 is a perspective view of an energy storage device according to this embodiment. FIG. 2 is a perspective view of a structural unit housed in the energy storage device of FIG. FIG. 3 is a schematic cross-sectional view of an electrode body that constitutes the energy storage element according to this embodiment. FIG. 4 is a schematic diagram showing an example of the process for producing a positive electrode active material layer. FIG. 5 is a schematic diagram showing an example of the process for producing a solid electrolyte layer. FIG. 6 is a schematic diagram of an energy storage device including a plurality of energy storage elements according to this embodiment.

This section provides an overview of the energy storage element 1 and the method for manufacturing the energy storage element disclosed in this specification. It also provides an overview of the dispersion liquid and the method for manufacturing the dispersion liquid disclosed in this specification. It also provides an overview of the slurry disclosed in this specification.

An energy storage device 1 according to one aspect of the present invention includes:
The electrochemical cell has an electrolyte-containing layer formed by drying an applied layer of a solid electrolyte-containing slurry containing a low-polarity solvent, fluororesin particles, and sulfide solid electrolyte particles,
the low-polarity solvent is at least one of a hydrocarbon compound and a compound having only a hydrocarbon group and an ether group in the molecule, and the average particle size of the fluororesin particles is 200 nm or less;
In the electrolyte-containing layer, the fluororesin is present between particles of the sulfide solid electrolyte.
In an energy storage element having such a configuration, the fluororesin particles are relatively small during production, and therefore the particles of the sulfide solid electrolyte are well bound together by the fluororesin particles, and deterioration of the sulfide solid electrolyte during production is suppressed.

A method for manufacturing an energy storage element according to another aspect of the present invention includes:
preparing a stock solution containing a highly polar solvent containing water and fluororesin particles having an average particle size of 200 nm or less dispersed in the highly polar solvent;
and subjecting the raw liquid to a substitution treatment in which the water is substituted with a low-polarity solvent having a lower polarity than the high-polarity solvent, thereby obtaining a dispersion liquid of the fluororesin particles,
The low polarity solvent is at least one of a hydrocarbon compound and a compound having only a hydrocarbon group and an ether group in the molecule.
In the manufacturing method having such a configuration, since the fluororesin particles are relatively small, the particles of the sulfide solid electrolyte can be well bound together by the fluororesin particles, and deterioration of the sulfide solid electrolyte can be suppressed.

A dispersion according to yet another aspect of the present invention comprises:
A low-polarity solvent and fluororesin particles dispersed in the low-polarity solvent,
the low-polarity solvent is at least one of a hydrocarbon compound and a compound having only a hydrocarbon group and an ether group in the molecule;
The particles have an average particle size of 200 nm or less.
By using a dispersion liquid having such a configuration, in the production of an electricity storage element, the fluororesin particles are relatively small, so that the particles of the sulfide solid electrolyte can be well bound together by the fluororesin particles, and deterioration of the sulfide solid electrolyte can be suppressed.

A method for producing a dispersion according to another aspect of the present invention includes the steps of:
preparing a stock solution containing a highly polar solvent containing water and fluororesin particles having an average particle size of 200 nm or less dispersed in the highly polar solvent;
and subjecting the raw liquid to a substitution treatment in which the water is substituted with a low-polarity solvent having a lower polarity than the high-polarity solvent, thereby obtaining a dispersion liquid of the fluororesin particles,
The low polarity solvent is at least one of a hydrocarbon compound and a compound having only a hydrocarbon group and an ether group in the molecule.
By using the method for producing a dispersion having such a configuration, it is possible to obtain a dispersion in which the fluororesin particles are relatively small, so that the particles of the sulfide solid electrolyte can be well bound together by the fluororesin particles, and further, deterioration of the sulfide solid electrolyte during production can be suppressed. The obtained dispersion can be used for producing an energy storage element.

A slurry according to yet another aspect of the present invention comprises:
The dispersion liquid,
The battery includes at least one of particles of a sulfide solid electrolyte, particles of a lithium transition metal composite oxide, and particles of a carbon material.
In the production of an electric storage element using a slurry having such a configuration, the fluororesin particles are relatively small, so that the particles of the sulfide solid electrolyte can be well bound together by the fluororesin particles, and deterioration of the sulfide solid electrolyte can be suppressed.

Conventionally, electrodes of nonaqueous electrolyte batteries (energy storage elements), such as lithium-ion secondary batteries, often contain a fluororesin, such as polyvinylidene fluoride (PVDF), as a binder. In the process of fabricating such electrodes, the fluororesin is generally used in the form of a solution dissolved in a polar solvent, such as N-methylpyrrolidone (NMP), a slurry containing the solution, or a powder. However, polar solvents containing polar groups, such as carbonyl groups, such as NMP, may deteriorate the sulfide solid electrolyte in the solid electrolyte during the electrode fabrication process. Meanwhile, when fluororesin powder is dry-mixed with an electrode material in a dry state, the fluororesin particles aggregate, and even if a solvent is added, the fluororesin particles may be unevenly distributed in the electrode, potentially resulting in insufficient binding performance of the fluororesin particles. Increasing the blending ratio of the fluororesin particles in order to fully exert the binding performance not only affects the charge/discharge reaction by inhibiting electronic conduction between the active material particles, ionic conduction between the active material particles and the solid electrolyte particles, and ionic conduction between the solid electrolyte particles, but also reduces the energy density of the electrode.
Therefore, when producing an electrode using a sulfide solid electrolyte as the solid electrolyte, it has been desired to fully utilize the binding properties of the fluororesin without using the above-mentioned polar solvents that deteriorate the sulfide solid electrolyte.

In this regard, the inventors believed that the sulfide solid electrolyte deteriorated because the polar solvent contained in the slurry, as described above, has the property of dissolving fluororesin. Therefore, instead of preparing a slurry from a fluororesin solution prepared in a solvent that dissolves fluororesin, the inventors focused on preparing a slurry from a fluororesin dispersion prepared in a solvent that does not easily dissolve fluororesin. They then discovered an energy storage element containing a sulfide solid electrolyte prepared using such a slurry, as well as a method for producing an energy storage element using such a slurry.

As described above, in one aspect of the present invention, the average particle diameter of the fluororesin particles in the slurry or dispersion is sufficiently small, at 200 nm or less. Therefore, in any electrolyte-containing layer prepared using the slurry, the number of fluororesin particles is sufficiently large, assuming the same volumetric content of fluororesin. Therefore, with the fluororesin sufficiently dispersed in the electrolyte-containing layer, uneven distribution of the fluororesin particles between sulfide solid electrolyte particles can be suppressed. This allows the fluororesin particles to effectively bond the sulfide solid electrolyte particles together. Furthermore, since the average particle diameter of the fluororesin particles is small, the amount of fluororesin required to achieve sufficient bonding is reduced. This allows the total area of the contact interfaces between sulfide solid electrolyte particles to be increased. This reduces the adverse effect of ionic conduction between sulfide solid electrolyte particles being inhibited by the fluororesin particles. Furthermore, because the average particle diameter of the fluororesin particles is sufficiently small, settling of the fluororesin when the slurry is applied can be suppressed. This allows for the preparation of a more uniform electrolyte-containing layer.

The configuration of the energy storage element 1 according to one embodiment of the present invention, the configuration of the energy storage device 100, and a method for manufacturing the energy storage element 1, 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.

<Configuration of Energy Storage Element>
As shown in FIGS. 1 to 3 , an energy storage element 1 according to one embodiment of the present invention includes an electrode assembly 2 having a positive electrode 40, a negative electrode 50, and a solid electrolyte layer 60, and a container 3 that houses the electrode assembly 2. Because the energy storage element 1 according to this embodiment substantially does not contain an electrolyte solution, it may be referred to as an “all-solid-state energy storage element” hereinafter. In this embodiment, the electrode assembly 2 is a stacked electrode assembly in which the positive electrode 40, the negative electrode 50, and the solid electrolyte layer 60 are stacked. The container 3 houses the electrode assembly 2 such that the electrode assembly 2 is sandwiched between two sheets. In other words, the container 3 is a so-called flat pouch container. The positive electrode 40 has a positive electrode active material layer 42 containing a solid electrolyte and a positive electrode active material, and the negative electrode 50 has a negative electrode active material layer 52 containing a solid electrolyte and a negative electrode active material. The solid electrolyte layer 60 is disposed between the positive electrode 40 and the negative electrode 50 and mainly contains a solid electrolyte. As an example of the all-solid-state energy storage element, an all-solid-state lithium ion secondary battery (hereinafter simply referred to as "secondary battery") will be described.

1 to 3 , the energy storage element 1 of this embodiment includes a stacked electrode assembly 2 in which a positive electrode 40 and a negative electrode 50 are stacked with a solid electrolyte layer 60 interposed therebetween, the container 3 that houses the electrode assembly 2, and two external terminals (a positive electrode terminal 4 and a negative electrode terminal 5). The positive electrode terminal 4 is formed by a tab portion that is formed by a portion of a positive electrode substrate 41 (described later) of the positive electrode 40 extending outward. Similarly, the negative electrode terminal 5 is formed by a tab portion that is formed by a portion of a negative electrode substrate 51 (described later) of the negative electrode 50 extending outward.
The energy storage element 1 is configured so that electricity flows between the electrode body 2 and an arbitrary external device via a positive electrode terminal 4 and a negative electrode terminal 5 during charging and discharging. In the energy storage element 1 of this embodiment, one electrode body 2 is housed in a container 3, and the electrode body 2 is configured to undergo a charging and discharging reaction.
Furthermore, the container 3 is preferably in a state in which a compressive force is applied from the outside to the inside in the thickness direction, which can further reduce the interfacial resistance between the positive electrode 40 and the negative electrode 50 and the solid electrolyte layer 60. The container 3 may contain one or more electrode bodies 2.

The electrode assembly 2 is formed by stacking a rectangular sheet-shaped positive electrode 40 and a rectangular sheet-shaped negative electrode 50 with a solid electrolyte layer 60 interposed between them. As shown in FIG. 3 , the solid electrolyte layer 60 is arranged to electrically insulate the positive electrode 40 and the negative electrode 50. The solid electrolyte layer 60 does not contain either a positive electrode active material or a negative electrode active material. In this embodiment, the electrode assembly 2 and the container 3 both have a flat shape. The electrode assembly 2 is arranged in the container 3 so that the stacking direction of the electrodes in the electrode assembly 2 and the thickness direction of the container 3 are the same.

In the electrode assembly 2 of this embodiment, at least one of the positive electrode active material layer 42 of the positive electrode 40, the negative electrode active material layer 52 of the negative electrode 50, and the solid electrolyte layer 60 contains a solid electrolyte. Specifically, in this embodiment, at least one of the positive electrode active material layer 42, the negative electrode active material layer 52, and the solid electrolyte layer 60 contains sulfide solid electrolyte particles, which will be described in detail later, as the solid electrolyte. In one example of this embodiment, the positive electrode active material layer 42, the negative electrode active material layer 52, and the solid electrolyte layer 60 all contain a sulfide solid electrolyte. Therefore, hereinafter, as shown in FIG. 3, the positive electrode active material layer 42, the negative electrode active material layer 52, and the solid electrolyte layer 60 are referred to as an electrolyte-containing layer A. In one aspect of the present invention, a fluororesin (described below) may be present between the sulfide solid electrolyte particles in at least one of the electrolyte-containing layers A (at least one of the positive electrode active material layer 42, the negative electrode active material layer 52, and the solid electrolyte layer 60). The following description will focus on an embodiment in which fluororesin (described below) is present between particles of sulfide solid electrolyte at least in the positive electrode active material layer 42.

(positive electrode)
The positive electrode 40 has a positive electrode substrate 41 and a positive electrode active material layer 42 that overlaps the positive electrode substrate 41 directly or via a predetermined layer. In this embodiment, the positive electrode substrate 41 and the positive electrode active material layer 42 overlap directly in the positive electrode 40. In this embodiment, the positive electrode active material layer 42 is overlapped on one surface (one side) of the positive electrode substrate 41. The positive electrode active material layer 42 and the negative electrode active material layer 52 undergo a charge/discharge reaction between each other.

The positive electrode substrate 41 is conductive. Whether or not a material has "conductivity" is determined by using a volume resistivity of 10 ⁷ Ω·cm measured in accordance with JIS-H-0505 (1975) as a threshold value. The material for the positive electrode substrate 41 may be a metal such as aluminum, titanium, tantalum, or stainless steel, or an alloy thereof. Among these, aluminum or an aluminum alloy is preferred from the viewpoints of potential resistance, high conductivity, and cost. Examples of the positive electrode substrate 41 include foils and vapor-deposited films, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred for the positive electrode substrate 41. Examples of aluminum or aluminum alloys include A1085 and A3003 specified in JIS-H-4000 (2014).

The average thickness of the positive electrode substrate 41 is preferably 1 μm or more and 50 μm or less, more preferably 30 μm or less, and even more preferably 15 μm or less. By keeping the average thickness of the positive electrode substrate 41 within the above range, the strength of the positive electrode substrate 41 can be increased while also increasing the energy density per volume of the secondary battery.

The positive electrode active material layer 42 contains a positive electrode active material, particles of a sulfide solid electrolyte, and a fluororesin. The fluororesin is present at least between the particles of the sulfide solid electrolyte. The positive electrode active material layer 42 may contain optional components such as a conductive agent, a binder (binding agent) other than the fluororesin, and a thickener, as needed.

The positive electrode active material can be appropriately selected from known positive electrode active materials. A material capable of absorbing and releasing lithium ions is typically used as the positive electrode active material for lithium ion secondary batteries. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO ₂ -type crystal structure, lithium transition metal composite oxides having a spinel-type crystal structure, polyanion compounds, chalcogen compounds, and sulfur. Examples of lithium transition metal composite oxides having α-NaFeO ₂ type crystal structure include Li[Li _x Ni _(1-x) ]O ₂ (0≦x<0.5), Li[Li _x Ni _γ Co _(1-x-γ) ]O ₂ (0≦x<0.5, 0<γ<1), Li[Li _x Co _(1-x) ]O ₂ (0≦x<0.5), Li[LiNiMn _(1-x-γ) ]O ₂ (0≦x<0.5, 0<γ<1), Li[Li _x Ni _γ Mn _β Co _(1-x-γ-β) ]O ₂ (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), Li[Li _x Ni _γ Co _β Al _(1-x-γ-β) ]O ₂ (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), etc. Examples of lithium transition metal composite oxides having a spinel crystal structure include Li _x Mn ₂ O ₄ and Li _x Ni _γ Mn _(2-γ) O ₄ , etc. Examples of polyanion compounds include LiFePO ₄ , LiMnPO ₄ , LiNiPO ₄ , LiCoPO ₄ , Li ₃ V ₂ (PO ₄ ) ₃ , Li ₂ MnSiO ₄ , and Li ₂ CoPO ₄ F, etc. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, iron sulfide, cobalt sulfide, copper sulfide, nickel sulfide, and copper Chevrel. Sulfur, bismuth oxide, bismuth plumbate, copper oxide, vanadium oxide, etc. can also be used as the positive electrode active material.
The atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements. The surfaces of these materials may be coated with other materials. In the positive electrode active material layer 42, one of these materials may be used alone, or two or more of them may be used in combination.
Among these materials, lithium transition metal composite oxides are preferred as the positive electrode active material.
It is preferable to select a positive electrode active material that is coated with a coating layer containing a material such as a lithium ion conductive oxide. Examples of lithium ion conductive oxides include _LiNbO3 , _Li2WO4 , _{Li4Ti5O12 , and Li3PO4} . Among these, it is preferable to select _LiNbO3 . The coating layer may cover the entire surface of _the positive electrode active material, or may cover only _a portion of the surface.

The positive electrode active material is typically particulate. The average particle diameter of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. Having an average particle diameter of the positive electrode active material equal to or greater than the above-mentioned lower limit facilitates the production and handling of the positive electrode active material. Having an average particle diameter of the positive electrode active material equal to or less than the above-mentioned upper limit improves the electronic conductivity of the positive electrode active material layer 42. When a composite of the positive electrode active material and another material is used (e.g., when the surface of the positive electrode active material is coated with a coating layer), the average particle diameter of the composite is taken as the average particle diameter of the positive electrode active material. "Average particle diameter" 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).

To obtain powder of the positive electrode active material with the specified particle size, a grinder or classifier is used. Grinding methods include, for example, 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 grinding in the presence of water or an organic solvent such as hexane can also be used. Sieves or air classifiers can be 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 42 is preferably 10% by mass or more and 95% by mass or less, more preferably 20% by mass or more and 90% by mass or less, and even more preferably 30% by mass or more and 80% by mass or less. By having the content of the positive electrode active material in this range, it is possible to achieve both a high energy density of the positive electrode active material layer 42 and relatively simple manufacturing of the positive electrode active material layer 42.

The solid electrolyte of the positive electrode active material layer 42 contains at least a sulfide solid electrolyte. The solid electrolyte of the positive electrode active material layer 42 may further contain a solid electrolyte other than the sulfide solid electrolyte, which will be described in detail later, but preferably contains only a sulfide solid electrolyte. The solid electrolyte contained in the positive electrode active material layer 42 may be the same type as the solid electrolyte contained in the solid electrolyte layer 60, or may be a different type. The solid electrolyte content in the positive electrode active material layer 42 is preferably 5% by mass or more and 90% by mass or less, more preferably 10% by mass or more and 80% by mass or less, and even more preferably 15% by mass or more and 70% by mass or less. When the solid electrolyte content in the positive electrode active material layer 42 is within the above range, the energy storage device can have a relatively large electrical capacity.

The conductive agent is not particularly limited as long as it is a material that is conductive. Examples of such conductive agents include carbon materials, metals, and conductive ceramics. Carbon materials include graphitized carbon, non-graphitized carbon, and graphene-based carbon. Non-graphitized carbon includes carbon fiber, 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 granular or fibrous. One type of conductive agent may be used alone, or two or more types may be mixed. The above materials may also be used in combination. For example, a composite of carbon black and CNT may be used. Of the above conductive agents, carbon fiber is preferred from the perspective of electronic conductivity.

When the positive electrode active material layer 42 contains a conductive agent, the content of the conductive agent in the positive electrode active material layer 42 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. Having the conductive agent content in this range can increase the energy density of the secondary battery.

Examples of fluororesins include polytetrafluoroethylene (PTFE) and vinylidene fluoride polymers. Examples of vinylidene fluoride polymers include polyvinylidene fluoride (PVDF) and copolymers of vinylidene fluoride and compounds other than vinylidene fluoride. Monomers copolymerizable with vinylidene fluoride include vinyl fluoride, trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, and perfluoroalkyl vinyl ether. As a fluororesin, vinylidene fluoride polymers are preferred, and polyvinylidene fluoride (PVDF) is more preferred, due to their wider potential window. The shape of the fluororesin present between particles of the sulfide solid electrolyte is not particularly limited. For example, the shape may be particulate or an undefined shape.

Examples of binders other than fluororesin that may be further included in the positive electrode active material layer 42 include thermoplastic resins such as polyethylene, polypropylene, polyacrylic, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

The binder content (total amount of binder including fluororesin) in the positive electrode active material layer 42 is preferably 0.01% by mass or more and 10% by mass or less, and more preferably 0.1% by mass or more and 7.0% by mass or less. Having the binder content within this range allows the active material to be stably maintained.

The positive electrode active material layer 42 may contain only a fluororesin as a binder. The positive electrode active material layer 42 may contain only polyvinylidene fluoride (PVDF) as a binder. The proportion of the fluororesin (particularly PVDF) in the binder may be 95 mass% or more.

When the positive electrode active material layer 42 contains a thickener, examples of the thickener 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.

In this embodiment, at least the positive electrode active material layer 42 of the electrolyte-containing layer A is formed by drying an applied solid electrolyte-containing slurry containing a low-polarity solvent, fluororesin particles with an average particle size of 200 nm or less, and sulfide solid electrolyte particles, as described later in the manufacturing method of the energy storage device. Because the fluororesin particles are relatively small during manufacturing, the sulfide solid electrolyte particles are well bonded together by the fluororesin particles, and deterioration of the sulfide solid electrolyte during manufacturing is suppressed.

As described above, the solid electrolyte-containing slurry contains fluororesin particles with a sufficiently small average particle diameter of 200 nm or less. Therefore, in a positive electrode active material layer 42 fabricated using the above slurry, the number of fluororesin particles is sufficiently large, assuming the same volumetric content of fluororesin. Therefore, while the fluororesin is sufficiently dispersed within the positive electrode active material layer, uneven distribution of the fluororesin particles between sulfide solid electrolyte particles can be suppressed. This allows the fluororesin particles to effectively bond the sulfide solid electrolyte particles together. Furthermore, since the average particle diameter of the fluororesin particles is small, less fluororesin is required to achieve sufficient bonding. This allows the total area of the contact interfaces between sulfide solid electrolyte particles to be increased. This reduces the adverse effect of fluororesin particles impeding ionic conduction between sulfide solid electrolyte particles. Furthermore, because the average particle diameter of the fluororesin particles is sufficiently small, settling of the fluororesin can be suppressed when the slurry is applied. This allows for the fabrication of a more uniform positive electrode active material layer 42.

In the positive electrode active material layer 42 produced using the solid electrolyte-containing slurry as described above, the fluororesin particles are arranged between the sulfide solid electrolyte particles. The fluororesin particles in this state do not necessarily maintain a particle shape, such as a spherical shape, making it difficult to measure their average particle diameter. Therefore, it is nearly impossible and impractical to specify the average particle diameter of the fluororesin particles contained in the positive electrode active material layer 42.

(Negative electrode)
The negative electrode 50 has a negative electrode substrate 51 and a negative electrode active material layer 52 that overlaps the negative electrode substrate 51 directly or via a predetermined layer or the like. In this embodiment, the negative electrode substrate 51 and the negative electrode active material layer 52 overlap directly in the negative electrode 50. In this embodiment, the negative electrode active material layer 52 is overlapped on one surface (one side) of the negative electrode substrate 51.

The negative electrode substrate 51 is electrically conductive. Materials used for the negative electrode substrate 51 include metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, as well as alloys of these. Among these, stainless steel, nickel or nickel alloys, and copper or copper alloys are preferred. Examples of the negative electrode substrate 51 include metal foils and vapor-deposited films, with metal foils being preferred from a cost perspective. Examples of the negative electrode substrate 51 include stainless steel foils, nickel foils or nickel alloy foils, and copper foils or copper alloy foils. Examples of copper foils include rolled copper foils and electrolytic copper foils.

The average thickness of the negative electrode substrate 51 is preferably 1 μm or more and 50 μm or less, more preferably 30 μm or less, and even more preferably 15 μm or less. By setting the average thickness of the negative electrode substrate 51 within the above range, the strength of the negative electrode substrate 51 can be increased while also increasing the energy density per volume of the secondary battery.

The negative electrode active material layer 52 contains a negative electrode active material and particles of a solid electrolyte. The negative electrode active material layer 52 may contain optional components such as a conductive agent, binder, thickener, and filler as needed. The optional components such as the conductive agent, binder, thickener, and filler can be selected from the materials exemplified for the positive electrode 40 above. Like the positive electrode active material layer 42, the negative electrode active material layer 52 may contain only a fluororesin as the binder, or only polyvinylidene fluoride (PVDF). The proportion of the fluororesin (particularly PVDF) in the binder may be 95% by mass or more.

The negative electrode active material layer 52 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, solid electrolyte, 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, "discharged state" refers to a state in which the negative electrode active material, a carbonaceous material, has been discharged so that lithium ions capable of being absorbed and released during charging and discharging are sufficiently released. For example, in a single-electrode battery using a negative electrode containing a carbonaceous material as the negative electrode active material as the working electrode and metallic Li 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.

"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 typically in the form of particles (powder). The average particle diameter of the negative electrode active material can be, for example, 1 nm to 100 μm. When the negative electrode active material is a carbon material, a titanium-containing oxide, or a polyphosphate compound, the average particle diameter may be 1 μm to 100 μm. When the negative electrode active material is Si, Sn, Si oxide, Sn oxide, or the like, the average particle diameter may be 1 nm to 1 μm. Setting the average particle diameter of the negative electrode active material to be equal to or greater than the above lower limit facilitates the production and handling of the negative electrode active material. Setting the average particle diameter of the negative electrode active material to be equal to or less than the above upper limit improves the electronic conductivity of the active material layer. A pulverizer, classifier, or the like is used to obtain powder with a specified particle size. The pulverization and classification methods can be selected from, for example, the methods exemplified for the positive electrode above. 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 52 is preferably 10% by mass or more and 95% by mass or less, more preferably 20% by mass or more and 90% by mass or less, and even more preferably 30% by mass or more and 80% by mass or less. Having the content of the negative electrode active material in this range enables both a high energy density of the negative electrode active material layer 52 and relatively simple manufacturing of the negative electrode active material layer 52.

The solid electrolyte of the anode active material layer 52 can be appropriately selected from the solid electrolytes exemplified in the detailed description of the solid electrolyte layer 60 below. The solid electrolyte contained in the anode active material layer 52 may be the same type as the solid electrolyte contained in the solid electrolyte layer 60, or may be a different type. In one example of this embodiment, the solid electrolyte contained in the anode active material layer 52 is a sulfide solid electrolyte. The content of the solid electrolyte in the anode active material layer 52 is preferably 5% by mass or more and 90% by mass or less, more preferably 10% by mass or more and 80% by mass or less, and even more preferably 15% by mass or more and 70% by mass or less. When the content of the solid electrolyte in the anode active material layer 52 is within the above range, the energy storage element can have a relatively large electrical capacity. The anode active material layer 52 may contain multiple different types of solid electrolytes.

Like the above-described positive electrode active material layer 42, the negative electrode active material layer 52 may be formed by drying an applied solid electrolyte-containing slurry containing a low-polarity solvent, fluororesin particles having an average particle size of 200 nm or less, and sulfide solid electrolyte particles.

In the electrode body 2, the positive electrode active material layer 42 and the negative electrode active material layer 52 face each other with the solid electrolyte layer 60 interposed therebetween. When the electrode body 2 is viewed from one side in the stacking direction (thickness direction), the area of the negative electrode active material layer 52 may be larger than the area of the positive electrode active material layer 42. In other words, at least a portion of the peripheral edge of the negative electrode active material layer 52 does not have to face the positive electrode active material layer 42 in the thickness direction.

(solid electrolyte layer)
In the electrode body 2 of this embodiment, a solid electrolyte layer 60 containing a solid electrolyte but no active material is disposed between the positive electrode 40 and the negative electrode 50. The thickness of the solid electrolyte layer 60 may be 5 μm or more and 200 μm or less, or may be 10 μm or more and 100 μm or less.

The solid electrolyte layer 60 may further contain, as a binder, the fluororesin mentioned in the description of the positive electrode active material layer 42 above. In other words, the solid electrolyte layer 60 may contain a binder similar to the binder that may be contained in the positive electrode active material layer 42, and specifically, may contain the above-mentioned fluororesin as a binder.

The solid electrolyte is an ion-conductive material that conducts ions such as lithium, sodium, potassium, magnesium, and calcium, and is a compound that remains solid even in a nitrogen atmosphere at 1 atmosphere pressure and 25°C. Examples of the solid electrolyte include sulfide solid electrolytes (described in detail below) and solid electrolytes other than sulfide solid electrolytes (described in detail below). In one example of this embodiment, the solid electrolyte layer 60 contains at least a sulfide solid electrolyte. It is preferable that the solid electrolyte layer 60 contains only a sulfide solid electrolyte as the solid electrolyte.

Using a sulfide solid electrolyte as the solid electrolyte has the advantage of increasing the conductivity of ions such as lithium in the solid electrolyte layer. Furthermore, because sulfide solid electrolyte particles are more easily deformed, the contact area between solid electrolyte particles and the contact area between the solid electrolyte and the active material can be increased. This has the advantage of reducing the interfacial resistance between solid electrolyte particles and between the solid electrolyte and the active material.

A sulfide solid electrolyte is a compound that contains sulfur element as an essential component and remains solid even in a nitrogen atmosphere at 1 atmosphere pressure and 25° C. When used in a lithium ion secondary battery, examples of sulfide solid electrolytes include Li ₂ S—P ₂ S ₅ , Li ₂ S—GeS ₂ , LiI—Li ₂ S—P ₂ S ₅ , Li ₁₀ Ge—P ₂ S ₁₂ , and Li ₆ PS ₅ X (X = Cl, Br, I). Here, Li ₁₀ Ge—P ₂ S ₁₂ is called an LGPS type, and Li ₆ PS ₅ X is called an argyrodite type.

Other solid electrolytes that may be included in the solid electrolyte layer 60 include oxide solid electrolytes, oxynitride solid electrolytes, and polymer solid electrolytes. In the solid electrolyte layer 60, the proportion of sulfide solid electrolytes in the solid electrolyte may be 95 mass% or more.

The oxide solid electrolyte is a compound that contains oxygen as an essential component and remains solid even in a nitrogen atmosphere at 1 atmosphere pressure and 25° C. Examples of the oxide solid electrolyte include perovskite-type oxides, NASICON-type oxides, LISICON-type oxides, and garnet-type oxides.
Examples of perovskite oxides include oxides expressed by Li _x La _1-x TiO ₃ (Li-La-Ti-O based perovskite oxides), etc. Examples of such oxides include Li _0.29 La _0.57 TiO ₃ and Li _0.35 La _0.55 TiO ₃ .
An example of the NASICON type oxide is Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ .
Examples of LISICON type oxides include Li ₄ SiO ₄ —Li ₃ PO ₄ and Li ₃ BO ₃ —Li ₃ PO ₄ .
Examples of garnet-type oxides include Li—La—Zr—O-based oxides such as Li ₇ La ₃ Zr ₂ O ₁₂ .

An example of the oxynitride solid electrolyte is Li ₃ N.

Examples of polymer solid electrolytes include ion-conductive polymers. Examples of such ion-conductive polymers include chemically modified and crosslinked polyether, polyester, polyamine, and polysulfide polymers.

In addition, like the above-described positive electrode active material layer 42, the solid electrolyte layer 60 may be formed by drying an applied solid electrolyte-containing slurry (not including active material) containing a low-polarity solvent, fluororesin particles with an average particle size of 200 nm or less, and sulfide solid electrolyte particles.

<Method of manufacturing all-solid-state energy storage element>
The method for manufacturing the energy storage element of this embodiment includes, for example, the steps of producing an electrode body and housing the electrode body in a container 3 to assemble the energy storage element.
By fabricating the electrode body, the positive electrode 40, the negative electrode 50, and the solid electrolyte layer 60 are fabricated.

The step of preparing the electrode body includes:
preparing a solid electrolyte-containing slurry containing a low-polarity solvent, fluororesin particles, and sulfide solid electrolyte particles;
and preparing an electrolyte-containing layer A containing fluororesin and sulfide solid electrolyte particles from the applied solid electrolyte-containing slurry.
In the step of preparing the solid electrolyte-containing slurry, the low-polarity solvent is at least one of a hydrocarbon compound and a compound having only a hydrocarbon group and an ether group in the molecule, and the particles have an average particle size of 200 nm or less. In the step of producing the electrolyte-containing layer A, the coating is subjected to a drying treatment to volatilize the low-polarity solvent.

In the electrode assembly fabrication step, a dispersion containing a low-polarity solvent and fluororesin particles dispersed in the low-polarity solvent is prepared to fabricate at least one of the electrolyte-containing layers A. The prepared dispersion is then mixed with at least one of sulfide solid electrolyte particles, lithium transition metal composite oxide particles, and carbon material particles to prepare a slurry. The applied slurry is then dried to volatilize the low-polarity solvent from the slurry, thereby fabricating at least one of the positive electrode active material layer 42, the negative electrode active material layer 52, and the solid electrolyte layer.

The following describes each of the above steps in detail. The following describes in detail an example in which the layers constituting the electrolyte-containing layer A (positive electrode active material layer 42, negative electrode active material layer 52, and solid electrolyte layer 60) are produced using the above-mentioned slurry and dispersion liquid.

(Step of producing electrode body)
In the process of producing an electrode assembly, first, the above-described dispersion liquid is prepared. The dispersion liquid includes a low-polarity solvent and fluororesin particles dispersed in the low-polarity solvent. The low-polarity solvent is at least one of a hydrocarbon compound and a compound having only a hydrocarbon group and an ether group in the molecule, and the particles have an average particle size of 200 nm or less. In this embodiment, the above-described dispersion liquid is a dispersion liquid for producing an electrode assembly of an energy storage element, as described below.

The dispersion can be prepared as follows: A method for preparing the dispersion includes preparing a stock solution containing a highly polar solvent containing water and fluororesin particles having an average particle size of 200 nm or less dispersed in the highly polar solvent;
and subjecting the raw liquid to a substitution treatment in which the water is substituted with the low-polarity solvent having a lower polarity than the high-polarity solvent, thereby obtaining a dispersion liquid of the fluororesin particles.

[Preparation of stock solution for preparing dispersion]
The highly polar solvent is a solvent containing at least water. The highly polar solvent may further contain a solvent other than water (organic solvent). The content of the organic solvent in the highly polar solvent is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 1% by mass or less.

High-polarity solvents have a higher polarity than the low-polarity solvents mentioned above. In other words, the polarity of the solvent contained in the stock solution is higher than the polarity of the solvent contained in the dispersion liquid prepared by the substitution process. The level of polarity can be determined by the Hildebrand solubility parameter (δ SP value), even if the high-polarity solvent or low-polarity solvent is a mixed solvent. The higher the SP value, the higher the polarity.

The highly polar solvent may be a solvent that contains, in addition to water, an organic solvent that dissolves in water. Examples of such organic solvents include acetonitrile, isopropanol, and N-methylpyrrolidone.

The stock solution can be prepared, for example, as follows. A commercially available fluororesin (e.g., polyvinylidene fluoride powder) is mixed with acetonitrile while heating to a fluororesin concentration of, for example, 1% by mass, to obtain a solution in which the fluororesin is dissolved. To this solution, water is added in an amount, for example, 10 times the mass of the fluororesin (solid content equivalent). The fluororesin solution, heated to approximately 70 to 80°C, is continuously added to water in an amount 100 times the mass of the fluororesin (solid content equivalent), to obtain a dispersion in which fluororesin particles are dispersed in water and acetonitrile. Most of the acetonitrile and water contained in this dispersion are evaporated by vacuum concentration. An appropriate amount of water is then added to prepare a stock solution with a concentration of, for example, 5% to 10% by mass. This stock solution contains the fluororesin in particulate form.

[Preparation of dispersion liquid (substitution treatment)]
The stock solution prepared as described above is subjected to a substitution treatment to substitute at least the water contained in the high-polarity solvent with the low-polarity solvent. The substitution treatment results in a dispersion in which fluororesin particles are dispersed. The substitution treatment can be carried out by a general substitution treatment method, including a dispersion operation using ultrasonic irradiation, a volatilization operation, a centrifugal separation operation, and a solvent removal operation by decantation.

Low-polarity solvents are at least one of hydrocarbon compounds and compounds containing only hydrocarbon groups and ether groups in the molecule.

Examples of hydrocarbon compounds include aliphatic hydrocarbons such as octane, nonane, and decane, and aromatic hydrocarbons such as benzene, toluene, xylene, mesitylene, and tetralin. On the other hand, examples of compounds containing only hydrocarbon groups and ether groups in the molecule include aliphatic ethers such as dihexyl ether, diheptyl ether, and dibutyl ether, and aromatic ethers such as anisole (methoxybenzene), methoxytoluene, methoxyxylene, methoxymesitylene, and ethoxybenzene.

The low-polarity solvent is preferably at least one selected from the group consisting of octane, nonane, decane, benzene, toluene, xylene, mesitylene, tetralin, dihexyl ether, diheptyl ether, dibutyl ether, anisole (methoxybenzene), methoxytoluene, methoxyxylene, methoxymesitylene, and ethoxybenzene.

A variety of commonly known procedures can be used in the replacement process. For example, a dispersion operation using ultrasonic irradiation can be performed in the replacement process to thoroughly disperse the fluororesin particles. Furthermore, after the dispersion operation, a volatilization operation can be performed using an evaporator or the like to volatilize the water in the stock solution. The volatilization operation can be performed under reduced pressure while heating the stock solution to 100°C or below. Furthermore, after the volatilization operation, a centrifugation operation can be performed in the replacement process to precipitate the fluororesin particles, and a solvent removal operation can be performed in which the supernatant liquid after centrifugation is removed by decantation. This series of operations can be repeated multiple times.

If necessary, an intermediate solvent (e.g., isopropanol) that is less polar than water but more polar than the low-polarity solvent may be used in the replacement process. Specifically, after performing the above series of operations to replace the water in the stock solution with the intermediate solvent, a series of operations to replace the intermediate solvent with a low-polarity solvent may be performed. In this case, the operation to volatilize the water may volatilize not only the water but also the intermediate solvent.

The intermediate solvent is preferably an organic solvent that dissolves in both water and low-polarity solvents. In other words, the intermediate solvent is preferably a protic organic solvent that has good solubility in both water and low-polarity solvents. Examples of intermediate solvents include monohydric alcohols with 3 or fewer carbon atoms, such as methyl alcohol, ethyl alcohol, and isopropanol.

The dispersion obtained through the substitution process contains the above-mentioned low-polarity solvent as a solvent. The solvent of the dispersion may also contain solvents other than the above-mentioned low-polarity solvent. The proportion of the low-polarity solvent in the solvent of the dispersion is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more. Furthermore, the proportion of solvents other than the above-mentioned low-polarity solvent in the solvent of the dispersion is preferably 1% by mass or less. For example, the proportion of carbonyl group-containing solvents (compounds having a carbonyl group in the molecule) in the solvent of the dispersion is 1% by mass or less.

The average particle size of the fluororesin particles contained in the dispersion is 200 nm or less. This average particle size may be 180 nm or less, 120 nm or less, or 100 nm or less. The average particle size may also be 10 nm or more. Because the average particle size is relatively small, 200 nm or less, when this dispersion is used to manufacture an energy storage element (including a solid electrolyte), the fluororesin particles tend to penetrate between the solid electrolyte particles. Therefore, the fluororesin particles can effectively bond the sulfide solid electrolyte particles together.

The average particle size of the fluororesin particles contained in the dispersion is directly reflected by the average particle size of the fluororesin particles contained in the stock solution used to prepare the dispersion. Therefore, when preparing the stock solution used to prepare the dispersion, for example, adding the entire amount of the fluororesin solution to water in a shorter time to prepare an aqueous dispersion of fluororesin particles can promote fusion between the fluororesin particles, thereby increasing the average particle size of the fluororesin particles in the dispersion. On the other hand, when preparing the stock solution used to prepare the dispersion, for example, slowing down the addition rate of the fluororesin solution (for example, 20 mass%/min or less) can suppress fusion between the fluororesin particles, thereby reducing the average particle size of the fluororesin particles in the dispersion.

The average particle size of fluororesin particles is measured as follows. A laser diffraction/scattering particle size distribution analyzer is used as the measuring instrument. A 0.5% by mass aqueous solution of polyoxyethylene-p-cumylphenyl ether (trade name "Nonal 912A", manufactured by Toho Chemical Industry Co., Ltd.) is used as the dispersion medium. A cumulative curve is calculated by analyzing the scattered laser light, with the total volume of the particles being 100%, and the particle size at the point where the cumulative curve reaches 50% (median diameter: d50) is taken as the average particle size of the fluororesin particles.

The next step in the process of making the electrode body is to prepare a slurry.

[Preparation of slurry]
The slurry contains the dispersion liquid prepared as described above and at least one of sulfide solid electrolyte particles, lithium transition metal composite oxide particles, and carbon material particles. A solid electrolyte-containing slurry containing the dispersion liquid, sulfide solid electrolyte particles, and lithium transition metal composite oxide particles (positive electrode active material particles) can be used to produce, for example, a positive electrode active material layer 42. A solid electrolyte-containing slurry containing the dispersion liquid, sulfide solid electrolyte particles, and carbon material particles (negative electrode active material particles) can be used to produce, for example, a negative electrode active material layer 52. A solid electrolyte-containing slurry containing the dispersion liquid and sulfide solid electrolyte particles but not containing either a positive electrode active material or a negative electrode active material can be used to produce, for example, a solid electrolyte layer 60.

For example, the solid electrolyte-containing slurry can be prepared by mixing the above-mentioned dispersion with at least one of sulfide solid electrolyte particles, lithium transition metal composite oxide particles, and carbon material particles. The lithium transition metal composite oxide can be any of the lithium transition metal composite oxides listed above in the description of the positive electrode active material. The carbon material particles can be any of the carbon material particles listed above in the description of the negative electrode active material.

A pre-prepared (commercially available) slurry may also be used. For example, such a slurry may contain the above-described dispersion liquid and at least one of lithium transition metal composite oxide particles and carbon material particles, but may not contain sulfide solid electrolyte particles. A solid electrolyte-containing slurry can be prepared by adding sulfide solid electrolyte particles to such a slurry.

For example, in the slurry (solid electrolyte-containing slurry), the total content of solvents other than the above-mentioned low-polarity solvents is 1 mass% or less. It is preferable that the slurry (solid electrolyte-containing slurry) is substantially free of solvents other than the above-mentioned low-polarity solvents (e.g., carbonyl group-containing solvents, protic organic solvents, etc.). For example, the slurry (solid electrolyte-containing slurry) is substantially free of ester group-containing solvents, ketone group-containing solvents, protic organic solvents, etc. This more sufficiently suppresses deterioration of the sulfide solid electrolyte.

The average particle size of the fluororesin particles in the slurry is 200 nm or less, the same as the average particle size in the dispersion liquid described above. Therefore, as described above, when such a slurry is used to manufacture an energy storage element (including a solid electrolyte), the fluororesin particles can effectively bond the sulfide solid electrolyte particles together. Furthermore, because the solvent contained in the slurry (solid electrolyte-containing slurry) is the specific solvent described above, deterioration of the sulfide solid electrolyte can be suppressed when the solid electrolyte-containing slurry is stored and when an energy storage element is manufactured as described below.

In the slurry, the average particle size of the fluororesin particles is preferably smaller than the average particle size of the sulfide solid electrolyte particles. That is, the ratio of the average particle size of the fluororesin particles to the average particle size of the sulfide solid electrolyte particles is preferably less than 1.0. This ratio may be 0.9 or less, or may be 0.5 or less. Furthermore, this ratio may be 0.1 or greater. A ratio of less than 1.0 allows the fluororesin particles to more easily penetrate between the sulfide solid electrolyte particles. Therefore, in the electrolyte-containing layer A, such as the positive electrode active material layer 42, the sulfide solid electrolyte particles can be more fully bound together. Furthermore, since this is more fully bound, the amount of fluororesin particles required to achieve the same binding properties can be reduced.

The average particle size of solid electrolyte particles is measured as follows. A laser diffraction/scattering particle size distribution analyzer is used as the measuring instrument. Tetralin is used as the dispersion medium. A cumulative curve is calculated, with the total volume of particles obtained by analyzing the scattered laser light set to 100%, and the particle size at the point where the cumulative curve reaches 50% (median diameter: d50) is taken as the average particle size of the solid electrolyte particles.

[Preparation of Electrolyte-Containing Layers (Positive Electrolyte Layer, Solid Electrolyte Layer, Negative Electrolyte Layer)]
The electrolyte-containing layers A, such as the solid electrolyte-containing positive electrode active material layer 42, the solid electrolyte layer 60, and the solid electrolyte-containing negative electrode active material layer 52, can each be prepared using a solid electrolyte-containing slurry. Specifically, the positive electrode active material layer 42 can be prepared using a solid electrolyte-containing slurry (slurry for the positive electrode active material layer) containing at least a low-polarity solvent, fluororesin particles, sulfide solid electrolyte particles, and lithium transition metal composite oxide particles (positive electrode active material particles). The solid electrolyte layer 60 can be prepared using a solid electrolyte-containing slurry (slurry for the solid electrolyte layer) containing a low-polarity solvent, fluororesin particles, and sulfide solid electrolyte particles, but not containing either positive electrode active material particles or negative electrode active material particles. The negative electrode active material layer 52 can be prepared using a solid electrolyte-containing slurry (slurry for the negative electrode active material layer) containing at least a low-polarity solvent, fluororesin particles, sulfide solid electrolyte particles, and carbon material particles (negative electrode active material particles).

Preparation of Positive Electrode Active Material Layer For example, the above-mentioned metal foil or alloy foil is prepared as the positive electrode substrate 41. As shown in Fig. 4, the positive electrode active material layer 42 can be prepared by carrying out the following operation.
(I) A slurry for a positive electrode active material layer (denoted as SL) is applied to the metal foil or alloy foil of the positive electrode substrate 41. In this way, the applied material is superimposed on the positive electrode substrate 41. A conventional general method can be used as the application method (coating method).
(II) The coated material is subjected to a drying treatment to volatilize the low-polarity solvent contained in the coated slurry. For example, the solvent is volatilized and removed from the coated material by a heat treatment. The set temperature during the heat treatment may be, for example, 40° C. or higher and 150° C. or lower. The heat treatment may be performed under reduced pressure.
(III) After the low-polarity solvent has evaporated, the coated product (positive electrode active material layer 42) is subjected to a pressing process (indicated by the dashed arrow). If necessary, the pressing process is performed while heating.
(IV) A pressed positive electrode active material layer 42 is formed.

Fabrication of Solid Electrolyte Layer As shown in FIG. 5, the solid electrolyte layer 60 can be fabricated by carrying out the following operations.
(i) A slurry for a solid electrolyte layer (denoted as SL′) is applied to the positive electrode active material layer 42. In this way, the applied material is superimposed on the positive electrode active material layer 42. A conventional general method can be used as the application method (coating method).
(ii) The coated material is subjected to a drying treatment to volatilize the low-polarity solvent contained in the coated slurry. For example, the solvent is volatilized and removed from the coated material by a heat treatment. The set temperature during the heat treatment may be within the temperature range employed when forming the positive electrode active material layer 42, for example.
(iii) After the low-polarity solvent has evaporated, the coated product (solid electrolyte layer 60) is subjected to a pressing process (shown by the dashed arrow). If necessary, the pressing process is performed while heating.
(iv) A pressed solid electrolyte layer 60 is formed.

Fabrication of Negative Electrode Active Material Layer Although not shown, the negative electrode active material layer 52 can be fabricated by the same method as the fabrication of the positive electrode active material layer or the fabrication of the solid electrolyte layer described above.

In the process of producing the electrode body, after producing a laminate of the positive electrode active material layer 42, the solid electrolyte layer 60, and the negative electrode active material layer 52 as described above, the negative electrode substrate 51 is further stacked on top of the laminate and pressed from both sides in the thickness direction, thereby producing the electrode body 2.
In the process of producing the electrode body, the positive electrode 40 may be produced by superposing the positive electrode active material layer 42 on the positive electrode substrate 41, and the negative electrode 50 may be produced by superposing the negative electrode active material layer 52 on the negative electrode substrate 51, and a laminate of the negative electrode 50 and the solid electrolyte layer 60 may be produced. Thereafter, the laminate and the positive electrode 40 may be superposed, and further, a pressing process may be performed from both sides in the thickness direction to produce the electrode body 2.

(Step of assembling the energy storage element)
In this step, the produced electrode body 2 is housed in a container 3, and the energy storage element 1 is assembled so that the positive electrode 40 and the negative electrode 50 of the electrode body 2 are electrically connected to the external terminals of the energy storage element 1. A general method can be used to assemble the energy storage element.

In this manner, the energy storage element 1 of this embodiment can be manufactured.

<Configuration of Power Storage Device>
The energy storage element 1 of this embodiment can be mounted as an energy storage device 100 (battery module) configured by assembling a plurality of energy storage elements 1 in an automobile power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), or a plug-in hybrid electric vehicle (PHEV), a power source for electronic devices such as a personal computer or a communication terminal, or a power source for power storage, etc. In this case, it is sufficient that the technology of the present invention is applied to at least one energy storage element 1 included in the energy storage device 100.
6 shows an example of an energy storage device 100 in which energy storage units 10, each of which is an assembly of two or more electrically connected energy storage elements 1, are further assembled. The energy storage device 100 may include a bus bar (not shown) that electrically connects two or more energy storage elements 1, a bus bar (not shown) that electrically connects two or more energy storage units 10, etc. The energy storage unit 10 or the energy storage device 100 may include a status monitoring device (not shown) that monitors the status of one or more energy storage elements.

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

In the above embodiment, the energy storage element 1 is described as being used as a chargeable and dischargeable all-solid-state secondary battery (e.g., an all-solid-state lithium-ion secondary battery), but the type, shape, dimensions, capacity, etc. of the energy storage element are arbitrary.

In the above embodiment, a positive electrode 40 in which a positive electrode active material layer 42 is superimposed on one side of a positive electrode substrate 41 and a negative electrode 50 in which a negative electrode active material layer 52 is superimposed on one side of a negative electrode substrate 51 are described. However, in the positive electrode, a positive electrode active material layer 42 may be superimposed on both sides of the positive electrode substrate 41, and in the negative electrode, a negative electrode active material layer 52 may be superimposed on both sides of the negative electrode substrate.

According to the manufacturing method of the energy storage element of the above embodiment, the fluororesin particles are relatively small, so that the particles of the sulfide solid electrolyte can be well bound together by the fluororesin particles, and deterioration of the sulfide solid electrolyte can be suppressed.
According to the dispersion liquid of the above embodiment or the solid electrolyte-containing slurry containing the dispersion liquid, in the production of an energy storage element, the fluororesin particles are relatively small, so that the particles of the sulfide solid electrolyte can be well bound together by the fluororesin particles, and deterioration of the sulfide solid electrolyte can be suppressed.
According to the method for producing a dispersion liquid of the above embodiment, since the fluororesin particles are relatively small, the particles of the sulfide solid electrolyte can be well bound together by the fluororesin particles, and a dispersion liquid can be obtained that can suppress deterioration of the sulfide solid electrolyte during production. This dispersion liquid can be used for producing an energy storage element.

Conventionally, when preparing an electrolyte-containing layer (such as a positive electrode active material layer, a negative electrode active material layer, and a solid electrolyte layer in an electrode assembly) in which sulfide solid electrolyte particles are bound with a binder, for example, fluororesin particles, which have good oxidation resistance, are used as the binder. However, fluororesin particles tend to aggregate, and therefore do not necessarily exhibit sufficient binding performance as a binder. Therefore, in order to sufficiently bind the sulfide solid electrolyte particles with the fluororesin, it is preferable to prepare an electrolyte-containing layer by drying a solid electrolyte-containing slurry prepared from a fluororesin solution in which the fluororesin is first dissolved in a solvent. However, fluororesins (such as PVDF) are soluble only in certain polar solvents (such as carbonyl group-containing solvents) that can deteriorate the sulfide solid electrolyte. Therefore, when attempting to sufficiently bind the sulfide solid electrolyte particles with the fluororesin binder, there is a problem that the sulfide solid electrolyte may deteriorate during production.
In contrast, in the above embodiment, while using fluororesin particles (such as PVDF particles) as a binder, it is possible to suppress deterioration of the sulfide solid electrolyte particles during production, and also to satisfactorily bind the sulfide solid electrolyte particles together with the fluororesin.

In the above embodiment, it is preferable that the above-mentioned specific low-polarity solvent is used, the fluororesin particles contained in the slurry are polyvinylidene fluoride particles, and the average particle diameter of such particles is 200 nm or less. By using the above-mentioned specific low-polarity solvent as the solvent contained in the slurry, deterioration of the sulfide solid electrolyte particles can be sufficiently suppressed during the production of the energy storage device. Furthermore, by having a sufficiently small average particle diameter of polyvinylidene fluoride contained in the slurry, the sulfide solid electrolyte particles can be more effectively bonded together.

The energy storage element 1 of the above embodiment is preferably an all-solid-state energy storage element. That is, the energy storage element 1 of the above embodiment preferably does not contain an electrolyte solution in the positive electrode active material layer 42, the negative electrode active material layer 52, and the solid electrolyte layer 60.

In the above embodiment, the all-solid-state energy storage element among the energy storage elements has been described in detail, but the energy storage element of the present invention is not limited to the all-solid-state energy storage element. The positive electrode active material layer, the negative electrode active material layer, and the solid electrolyte layer constituting the energy storage element of the present invention may contain an electrolyte solution. The electrolyte solution can be appropriately selected from known electrolyte solutions. When the energy storage element of the present invention is a lithium ion secondary battery, for example, the electrolyte solution can be one in which _LiPF6 is dissolved as an electrolyte salt in a solvent containing a mixture of a cyclic carbonate and a chain carbonate.

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

<Preparation of Dispersion>
Example 1
594 parts by mass of acetonitrile and 6 parts by mass of polyvinylidene fluoride powder (PVDF, model #9100, manufactured by Kureha) were mixed and heated to 80°C to dissolve, obtaining a fluororesin acetonitrile solution. 66 parts by mass of water was added to this solution to obtain a fluororesin solution. The fluororesin solution maintained at 80°C was added dropwise to 600 parts by mass of stirred water at 20°C at a rate of 0.08 dm ³ /min for 10 minutes, obtaining a dispersion in which fluororesin particles were dispersed in water and acetonitrile. Most of the acetonitrile and water contained in this dispersion were evaporated by vacuum concentration. Water was then added to the mixture so that the PVDF concentration was 5% by mass, preparing the stock solution of Example 1 in which PVDF was dispersed. The particle size distribution of the fluororesin particles contained in the stock solution was measured using the method described above, and the average particle diameter was found to be 70 nm. In this way, a stock solution according to Example 1 was prepared, which contained PVDF particles having an average particle size of 70 nm and water, with the PVDF particles dispersed in water at a concentration of 5% by mass.

Next, a sufficient amount of isopropanol (IPA) was added to the above-mentioned stock solution, and the mixture was dispersed using an ultrasonic vibrator. The solids (PVDF) were then precipitated using a centrifuge, and most of the supernatant was removed by decantation. This series of steps, including the addition of IPA, dispersion using an ultrasonic vibrator, settling of the solids using a centrifuge, and removal of the supernatant by decantation, was repeated three times.
Next, a sufficient amount of anisole was added as a low-polarity solvent, and dispersion treatment was carried out using an ultrasonic vibrator. After that, the solid (PVDF) was precipitated using a centrifuge, and most of the supernatant was removed by decantation. The series of operations consisting of the addition of the low-polarity solvent, dispersion treatment using an ultrasonic vibrator, settling of the solid using a centrifuge, and removal of the supernatant by decantation was carried out a total of three times.
Finally, a predetermined amount of anisole was added to produce a dispersion of Example 1 in which PVDF having an average particle size of 70 nm was dispersed in anisole at a concentration of 4.5% by mass.

Example 2
A stock solution according to Example 2 was produced in the same manner as in Example 1, except that the dropping rate was 0.4 dm ³ /min and the dropping time was 2 min. The particle size distribution of the fluororesin particles contained in the stock solution was measured by the method described above, and the average particle diameter was found to be 172 nm. In this way, a stock solution according to Example 2 was prepared in which PVDF having an average particle diameter of 172 nm was dispersed in water at a concentration of 5% by mass. A dispersion according to Example 2 in which PVDF having an average particle diameter of 172 nm was dispersed in anisole at a concentration of 10% by mass was produced in the same manner as in Example 1, except that the stock solution was used.

(Comparative Example 1)
The particle size distribution of polyvinylidene fluoride powder (PVDF #9100 manufactured by Kureha) was measured using the method described above, and the average particle size was found to be 140 μm. Water was added to the PVDF to prepare a stock solution of Comparative Example 1 in which 140 μm PVDF was dispersed in water at a concentration of 5% by mass. A dispersion of Comparative Example 1 in which 10% by mass PVDF with an average particle size of 140 μm was dispersed in anisole was produced in the same manner as in Example 1, except for using the stock solution.

(Comparative Example 2)
Polyvinylidene fluoride powder (PVDF #9100 manufactured by Kureha) was dispersed in a mixed solvent (90 parts by mass of methyl isobutyl ketone/10 parts by mass of decane) to a concentration of 1% by mass, and then stirred at 80°C for 24 hours to produce a dispersion liquid according to Comparative Example 2.

<Visual evaluation of dispersibility of dispersion liquid>
The dispersions of Examples 1 and 2 and Comparative Examples 1 and 2 were each placed in a 20 ml reagent bottle with a lid and allowed to stand. As a result, it was visually observed that the dispersions of Examples 1 and 2 maintained good dispersibility even after one month. On the other hand, it was visually observed that the dispersions of Comparative Examples 1 and 2 lost uniformity after a few seconds, and after about 90 seconds lost dispersibility and completely separated.

Example 3
An energy storage element (all-solid-state lithium ion secondary battery) of Example 3 was manufactured as follows. The manufactured energy storage element is also referred to as an all-solid-state secondary battery. In the all-solid-state secondary battery described below, only the positive electrode active material layer was manufactured using the dispersion liquid of Example 1 described above.

<Preparation of solid electrolyte-containing slurry A>
A sulfide solid electrolyte having an argyrodite structure and containing lithium, phosphorus, sulfur, and chlorine was prepared as a solid electrolyte. The dispersion liquid of Example 1 was mixed with the solid electrolyte powder, and the mixture was stirred using a planetary centrifugal mixer to prepare a solid electrolyte-containing slurry A.

<Preparation of Solid Electrolyte-Containing Slurry B (Positive Electrode Mixture Paste)>
A lithium transition metal composite oxide (particulate active material) having _an α-NaFeO2 _- type crystal structure and represented by _{LiNi0.5Co0.2Mn0.3O2 and} carbon fiber as a conductive agent were added to the above solid electrolyte-containing slurry _A , and the mixture was stirred with a rotation/revolution mixer to prepare solid electrolyte-containing slurry B (positive electrode mixture paste).

<Preparation of positive electrode>
The solid electrolyte-containing slurry B, which is a cathode mixture paste, was applied to the carbon coating side of a 20 μm-thick aluminum foil (cathode substrate) on one side of which a carbon coating layer had been previously formed. A drying treatment was then performed to volatilize and remove the solvent from the slurry B. In this manner, a cathode was fabricated in which a 60 μm-thick cathode active material layer was formed on the cathode substrate.

<Fabrication of all-solid-state secondary battery>
The positive electrode prepared as described above was punched out in the thickness direction to a predetermined size.
A solid electrolyte was poured into the Macol tube from one side. Next, a SUS jig was used to press the tube at 100 MPa to form a solid electrolyte layer with a thickness of 800 μm. The positive electrode punched out as described above was then inserted into the Macol tube and pressed at 580 MPa. This resulted in the solid electrolyte layer of the positive electrode and the solid electrolyte layer being bonded inside the Macol tube.
Subsequently, a lithium-indium counter electrode was inserted from the other side of the Macol tube. This caused the inserted lithium-indium counter electrode to face the positive electrode inside the Macol tube with the solid electrolyte layer interposed between them. Then, inside the Macol tube, the positive electrode, the solid electrolyte layer, and the counter electrode were sandwiched between SUS jigs. Thereafter, the SUS jigs on both sides were fastened with bolts, and leads were connected to the SUS jigs on both sides as external terminals, thereby producing a half-cell (all-solid-state secondary battery) of Example 3.

<Performance evaluation>
(Float charge test)
The all-solid-state secondary battery of Example 3, which included a positive electrode active material layer prepared using the dispersion of Example 1, was subjected to constant current charging at a charge current of 0.1 C and a cut-off voltage of 3.93 V at a temperature of 25°C, and constant current discharging at a discharge current of 0.1 C and a cut-off voltage of 2.38 V. A 30-minute rest period was provided after each charge and discharge. Next, as a float charge test, constant current charging at a charge current of 0.1 C and a cut-off voltage of 3.93 V was performed at a temperature of 60°C for 120 hours. Subsequently, constant current discharging at a discharge current of 0.1 C and a cut-off voltage of 2.38 V was performed at 25°C. A 30-minute rest period was provided after each charge and discharge. As a result, the discharge capacity retention rate, which is the percentage of the discharged quantity of electricity after the float charge test relative to the discharged quantity of electricity before the float charge test, was 75%, confirming good float life performance.

The reason why the all-solid-state secondary battery of Example 3 exhibited good float life performance is that the dispersion liquid of Example 1 did not contain a solvent that would deteriorate the sulfide solid electrolyte, thereby suppressing deterioration of the sulfide solid electrolyte during production. Furthermore, because a fluororesin (PVDF), which has higher oxidation resistance than rubber-based binders, was used as the binder, the presence of the fluororesin not only between the sulfide solid electrolytes but also between the positive electrode active material or conductive agent and the sulfide solid electrolyte suppressed oxidative decomposition of the binder and the solid electrolyte.

(Comparative Example 3)
Except for using the dispersion liquid of Comparative Example 2, a solid electrolyte-containing slurry C (positive electrode mixture paste) was prepared in the same procedure as for the solid electrolyte-containing slurry A prepared in the production of the energy storage element of Example 3. The presence of many coarse lumps was observed in this solid electrolyte-containing slurry C.

A 20 μm thick aluminum foil (cathode substrate) was prepared with a carbon coating layer formed on one side. The solid electrolyte-containing slurry C, which is a cathode mixture paste, was applied to the carbon coating layer side. After application, streaks that significantly impaired the appearance were observed on the coated surface. A drying process was then performed to volatilize and remove the solvent from the slurry C. As a result, the cathode active material layer peeled off from the cathode substrate, and a cathode and the all-solid-state secondary battery of Comparative Example 3 using this cathode could not be manufactured.

1: Energy storage element (all-solid-state lithium-ion secondary battery),
2: electrode body,
3: Container,
4: positive electrode terminal, 5: negative electrode terminal,
40: positive electrode,
41: positive electrode base material, 42: positive electrode active material layer,
50: negative electrode,
51: negative electrode base material, 52: negative electrode active material layer,
60: solid electrolyte layer,
10: Power storage unit, 100: Power storage device.

Claims (1)

preparing a solid electrolyte-containing slurry containing a low-polarity solvent, fluororesin particles, and sulfide solid electrolyte particles;
and preparing an electrolyte-containing layer containing particles of the fluororesin and particles of the sulfide solid electrolyte from the applied material of the solid electrolyte-containing slurry,
In preparing the solid electrolyte-containing slurry, the low-polarity solvent is at least one selected from the group consisting of methoxybenzene, methoxytoluene, methoxyxylene, methoxymesitylene, and ethoxybenzene, and the average particle size of the fluororesin particles is 200 nm or less;
The method for producing an electric storage element includes, in forming the electrolyte-containing layer, subjecting the applied material to a drying treatment for volatilizing the low-polarity solvent.