JP7853764B2 - All-solid-state secondary battery - Google Patents

Description

This invention relates to an all-solid-state secondary battery.

While all-solid-state rechargeable batteries are generally safer than rechargeable batteries using organic electrolytes, exposing them to high temperatures above 200°C may cause oxygen to be generated during charging, depending on the composition of the positive electrode active material. If this oxygen reacts exothermically with the sulfide-based solid electrolyte, the temperature of the all-solid-state battery will rise further, potentially leading to ignition if flammable materials are nearby.

Japanese Patent Publication No. 1999-191417

This invention aims to provide an all-solid-state secondary battery that can suppress the aforementioned exothermic reaction even in high-temperature environments such as 200°C or higher, in order to further enhance the safety of all-solid-state secondary batteries.

As a technology to suppress heat generation in secondary batteries, for example, as shown in Patent Document 1, a liquid-based secondary battery containing a compound whose decomposition reaction is endothermic (hereinafter also referred to as an endothermic substance) has been considered.

However, when the inventors actually incorporated the aforementioned endothermic substance into an all-solid-state secondary battery, they discovered that the decomposition reaction of the endothermic substance, i.e., the endothermic reaction, is less likely to occur in the all-solid-state secondary battery compared to the liquid-based secondary battery described in Patent Document 1.
Therefore, after diligently studying the conditions necessary to sufficiently induce an endothermic reaction in an all-solid-state secondary battery, as well as the content of the endothermic substance, we have completed the present invention.

In other words, the all-solid-state secondary battery according to the present invention comprises a positive electrode layer, a negative electrode layer, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer. The positive electrode layer comprises a plate-shaped positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. The positive electrode layer contains an endothermic substance that absorbs heat through a decomposition reaction, and the content of the endothermic substance in the positive electrode layer is in the range of 1 part by mass or more and 30 parts by mass or less, based on a total mass of 100 parts by mass of the positive electrode active material layer.

In this type of all-solid-state secondary battery, since the positive electrode layer, which is in an oxidizing environment during charging, contains a heat-absorbing material, the decomposition reaction of the heat-absorbing material is more likely to occur during charging, allowing for sufficient heat absorption even at temperatures below 200°C. As a result, the all-solid-state secondary battery can prevent rapid heat generation at temperatures above 200°C, where oxygen may be generated during charging.

Furthermore, since the content of the heat-absorbing substance in the positive electrode layer is in the range of 1 part by mass to 30 parts by mass, based on a total mass of 100 parts by mass of the positive electrode active material layer, a sufficient heat-absorbing effect can be achieved while ensuring sufficient charging capacity of the positive electrode layer.

It is preferable that the heat-absorbing material is contained in the positive electrode active material layer or placed between the positive electrode active material layer and the positive electrode current collector, as this ensures the heat-absorbing material is reliably placed in an oxidizing environment during charging.

Specific embodiments of the present invention include those in which the heat-absorbing substance is a carbonate compound and/or a hydroxide compound. Examples of the carbonate compound include lithium carbonate. Examples of the hydroxide compound include aluminum hydroxide.

If the solid electrolyte layer contains a sulfide-based solid electrolyte, the exothermic reaction is more likely to occur when the battery is placed in a high-temperature environment, thus allowing the effects of the present invention to be exhibited more significantly.

From the viewpoint of suppressing battery deformation, it is preferable that the difference between the volume inside the casing at 80°C and the volume at 25°C be within 5% of the volume at 25°C.

The exterior further comprises an outer casing that houses the positive electrode layer, the negative electrode layer, and the solid electrolyte layer inside,
When the outer casing is in the form of a film, the volume change due to the decomposition reaction of the heat-absorbing substance is likely to affect the overall volume of the battery, thus allowing the effects of the present invention to be exhibited more significantly.

According to the present invention, in order to further enhance the safety of all-solid-state secondary batteries, the aforementioned heat-generating reactions can be suppressed even in high-temperature environments, thereby suppressing rapid heat generation during charging.

This is a cross-sectional view showing the schematic configuration of an all-solid-state secondary battery according to one embodiment of the present invention. This is a cross-sectional view showing a schematic configuration of an all-solid-state secondary battery according to another embodiment of the present invention. This is a cross-sectional view showing a schematic configuration of an all-solid-state secondary battery according to another embodiment of the present invention.

Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings. In this specification and the drawings, components having substantially the same functional configuration are denoted by the same reference numerals, thus omitting redundant explanations.

<1. Basic configuration of the all-solid-state secondary battery according to this embodiment>
As shown in Figure 1, the all-solid-state secondary battery 1 according to this embodiment comprises a positive electrode layer 10, a negative electrode layer 20, a solid electrolyte layer 30, and an outer casing 40 that houses these components inside.

(1-1. Positive electrode layer)
The positive electrode layer 10 includes a positive electrode current collector 11 and a positive electrode active material layer 12.
Examples of the positive electrode current collector 11 include plate-shaped or foil-shaped bodies made of indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), germanium (Ge), lithium (Li), or alloys thereof. The positive electrode current collector 11 may be omitted.

The positive electrode active material layer 12 contains a positive electrode active material and a solid electrolyte. The solid electrolyte contained in the positive electrode active material layer 12 may or may not be the same type as the solid electrolyte contained in the solid electrolyte layer 30. Details of the solid electrolyte will be explained in detail in the section on the solid electrolyte layer 30.

The positive electrode active material can be any positive electrode active material capable of reversibly intercalating and releasing lithium ions.

For example, the positive electrode active material can be formed using lithium salts such as lithium cobalt oxide (hereinafter referred to as LCO), lithium nickel oxide, lithium nickel cobalt oxide, lithium nickel cobalt aluminate (hereinafter referred to as NCA), lithium nickel cobalt manganese oxide (hereinafter referred to as NCM), lithium manganese oxide, lithium iron phosphate, nickel sulfide, copper sulfide, lithium sulfide, sulfur, iron oxide, or vanadium oxide. These positive electrode active materials may be used individually or in combination of two or more.

Furthermore, the positive electrode active material is preferably formed by including a lithium salt of a transition metal oxide having a layered rock salt structure among the lithium salts described above. Here, "layered rock salt structure" refers to a structure in which oxygen atomic layers and metal atomic layers are alternately and regularly arranged in the <111> direction of a cubic rock salt structure, resulting in each atomic layer forming a two-dimensional plane. "Cubic rock salt structure" refers to a type of crystalline structure, specifically a sodium chloride type structure, where the face-centered cubic lattices formed by the cations and anions are offset from each other by half the edge of the unit cell.

Examples of lithium salts of transition metal oxides having such a layered rock salt-type structure include lithium salts of ternary transition metal oxides such as LiNi _x Co _y Al _z O ₂ (NCA) or LiNi _x Co _y Mn _z O ₂ (NCM) (where 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1).

When the positive electrode active material contains a lithium salt of a ternary transition metal oxide having the layered rock salt-type structure described above, the energy density and thermal stability of the all-solid-state secondary battery 1 can be improved.

The positive electrode active material may be covered by a coating layer. Here, the coating layer in this embodiment can be any known coating layer for the positive electrode active material of an all-solid-state secondary battery. Examples of coating layers include _Li₂O - _ZrO₂ .

Furthermore, when the positive electrode active material is formed from a lithium salt of a ternary transition metal oxide such as NCA or NCM, and contains nickel (Ni), the capacity density of the all-solid-state secondary battery 1 can be increased, and metal leaching from the positive electrode active material in the charged state can be reduced. As a result, the all-solid-state secondary battery 1 according to this embodiment can improve long-term reliability and cycle characteristics in the charged state. To further exhibit these characteristics, a higher nickel (Ni) content is more preferable. The nickel content in the positive electrode active material is preferably at least 60 mol%, and more preferably 80 mol% or more. On the other hand, a content of 95 mol% or less is preferable because it can suppress the decrease in battery capacity during charge-discharge evaluation.

Here, the shape of the positive electrode active material can be, for example, a perfect sphere or an ellipsoid. Furthermore, the particle size of the positive electrode active material is not particularly limited and should be within a range applicable to the positive electrode active material of conventional all-solid-state secondary batteries. The content of the positive electrode active material in the positive electrode layer 10 is also not particularly limited and should be within a range applicable to the positive electrode layer 10 of conventional all-solid-state secondary batteries.

Furthermore, in addition to the positive electrode active material and solid electrolyte described above, the positive electrode active material layer 12 may appropriately contain additives such as conductive additives, binders, fillers, dispersants, and ionic conductive additives.

Examples of conductive additives that can be incorporated into the positive electrode active material layer 12 include graphite, carbon black, acetylene black, Ketjen black, carbon fiber, and metal powder. Examples of binders that can be incorporated into the positive electrode active material layer 12 include styrene-butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene. Furthermore, known materials generally used in electrodes for all-solid-state secondary batteries can be used as fillers, dispersants, and ion conductive additives that can be incorporated into the positive electrode active material layer 12.

(1-2. Negative electrode layer)
The negative electrode layer 20 includes a negative electrode current collector 21 and a negative electrode active material layer 22. The negative electrode current collector 21 is preferably composed of a material that does not react with lithium, i.e., does not form any alloys or compounds. Examples of materials constituting the negative electrode current collector 21 include copper (Cu), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), and nickel (Ni). The negative electrode current collector 21 may be composed of any one of these metals, or of an alloy or clad material of two or more metals. The negative electrode current collector 21 may be, for example, in the form of a plate or foil.

The negative electrode active material layer 22 contains the negative electrode active material.
The negative electrode active material may include, for example, amorphous carbon and an alloying element that forms an alloy with lithium. The alloying element may be one or more selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, and zinc. Examples of amorphous carbon include carbon black, graphene, and graphite. Examples of carbon black include acetylene black, furnace black, and ketjen black. Furthermore, to improve electronic conductivity, the silicon may be coated with a carbon layer about 1 to 10 nm thick.

Here, when one or more of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, and zinc are used as the alloy-forming elements, these negative electrode active materials are preferably in granular form, with a particle size of 4 μm or less, more preferably 300 nm or less. In this case, the characteristics of the all-solid-state secondary battery 1 are further improved. Here, the particle size of the negative electrode active material can be, for example, the median diameter (so-called _D50 ) measured using a laser particle size distribution system.

In addition to those mentioned above, the negative electrode active material layer 22 may also contain additives used in conventional all-solid-state secondary batteries, such as binders, fillers, dispersants, and ion conduction enhancers, as appropriate.

(1-3. Solid electrolyte layer)
The solid electrolyte layer 30 is formed between the positive electrode layer 10 and the negative electrode layer 20 and contains a solid electrolyte.

The solid electrolyte is composed of, for example, a sulfide-based solid electrolyte material. Examples of sulfide-based solid electrolyte materials include _Li₂S - _P₂S₅ , _Li₂S - _{P₂S₅ -} LiX (where X is a halogen element, e.g. _, I, Cl), _Li₂S - _P₂S₅ - _Li₂O , Li₂S- _P₂S₅ -Li₂O-LiI, Li₂S- _SiS₂ , _Li₂S - _{SiS₂ - LiI} , _Li₂S - _{SiS₂ -} LiBr, _Li₂S -SiS₂ _- LiCl, _{Li₂S - SiS₂ -B₂S₃-} LiI, _{Li₂S - SiS₂} - _P₂S₅ - _LiI , _{Li₂S - B₂S₃} , _{and Li₂ Examples} include S- _P2S5 - _ZmSn ( _where m and n are positive numbers, and Z is either Ge, Zn _, or Ga), _Li2S - _GeS2 , _Li2S - _SiS2 - _Li3PO4 , _Li2S - _{SiS2 - LipMOq} (where p and q are positive numbers, and M is either P, Si, Ge, B, Al, Ga, or In). Here, sulfide-based solid electrolyte materials are produced by processing starting materials (e.g., _Li2S , _P2S5 , _etc. ) by methods such as melt-quenching or mechanical milling. Further heat treatment may be performed after these processes. The solid electrolyte may be amorphous, crystalline, or a mixture of both.

Furthermore, it is preferable to use a sulfide solid electrolyte material from among the above-mentioned materials that contains at least sulfur (S), phosphorus (P), and lithium (Li) as constituent elements, and it is more preferable to use one that contains _{Li₂S - P₂S₅} .

Here, when using _a sulfide-based solid electrolyte material containing _Li₂S - _P₂S₅ to form the solid electrolyte, the molar _{ratio of Li₂S to P₂S₅ may be selected in the range of, for example, Li₂S:P₂S₅ = 50:50 to 90:10} . The solid electrolyte layer ₃₀ may also further contain a binder. Examples of binders included in the solid electrolyte layer 30 include styrene-butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene. The binder in the solid electrolyte layer 30 may be the same as or different from the binders in the positive electrode active material layer 12 and the negative electrode active material layer 22.

(1-4. Exterior)
As described above, the outer casing 40 houses the positive electrode layer 10, the negative electrode layer 20, and the solid electrolyte layer 30, and can be made of a flexible film, for example. Examples of such films include laminate films formed by sandwiching a thin metal film such as aluminum or stainless steel between resins such as polypropylene or polyethylene.
Examples of the thickness of the laminate film used as the outer casing 40 include those with a thickness of 30 μm or more and 150 μm or less.
Other materials may include rigid metals. For example, cans made of aluminum or stainless steel can be used. In this case, the shape of the outer casing may be rectangular (cuboidal) or cylindrical.

<2. Characteristic Configuration of the All-Solid-State Secondary Battery According to This Embodiment>
However, the positive electrode active material layer 12 contains an endothermic substance, which is a substance that absorbs heat through a decomposition reaction.

Examples of the heat-absorbing substances include carbonate compounds, hydroxide compounds, and compounds having crystal water.
Examples of the aforementioned carbonate compounds include carbonates and bicarbonates. Among these, lithium carbonate, rubidium carbonate, barium carbonate, cobalt carbonate, iron carbonate, nickel carbonate, zinc carbonate, sodium bicarbonate, potassium bicarbonate, rubidium bicarbonate, and cesium bicarbonate are particularly noteworthy.
Examples of the hydroxide compounds include zinc hydroxide, aluminum hydroxide, cadmium hydroxide, chromium hydroxide, cobalt hydroxide, nickel hydroxide, manganese hydroxide, calcium hydroxide, magnesium hydroxide, zirconium hydroxide, iron hydroxide oxide, and nickel hydroxide oxide.
Examples of compounds containing water of crystallization include aluminum oxide hydrate, barium nitrate hydrate, calcium sulfate hydrate, cobalt phosphate hydrate, antimony oxide hydrate, tin oxide hydrate, titanium oxide hydrate, bismuth oxide hydrate, and tungsten oxide hydrate.
As the heat-absorbing material, you may use only one of the aforementioned materials, or you may use multiple materials in combination.

The amount of the heat-absorbing substance is preferably in the range of 1 to 30 parts by mass when the total positive electrode active material layer 12 is considered to be 100 parts by mass. More preferably, the amount of the heat-absorbing substance is 5 to 25 parts by mass, and even more preferably 5 to 10 parts by mass.

<3. Method for manufacturing an all-solid-state secondary battery according to this embodiment>
Next, a method for manufacturing the all-solid-state secondary battery 1 according to this embodiment will be described. The all-solid-state secondary battery 1 according to this embodiment can be manufactured by first manufacturing a positive electrode layer 10, a negative electrode layer 20, and a solid electrolyte layer 30, then stacking the above layers and finally covering them with an outer casing 40.

(3-1. Cathode layer fabrication process)
First, a slurry (the slurry may be a paste; other slurries may also be produced) is prepared by adding the materials constituting the positive electrode active material layer 12 (positive electrode active material, heat-absorbing material, binder, etc.) to a non-polar solvent such as dehydrated xylene. Next, the obtained slurry is applied to the positive electrode current collector 11 and dried. Then, the positive electrode layer 10 is produced by pressurizing the resulting laminate (for example, by applying hydrostatic pressure). The pressurizing step may be omitted. The positive electrode layer 10 may also be produced by compacting the mixture of materials constituting the positive electrode active material layer 12 into a pellet or by stretching it into a sheet. When the positive electrode layer 10 is produced by these methods, the positive electrode current collector 11 may be pressed onto the produced pellet or sheet.

(3-2. Negative electrode layer fabrication process)
First, a slurry is prepared by adding the negative electrode active material (negative electrode active material, alloy non-forming elements, binder, etc.) constituting the negative electrode active material layer 22 to a polar or non-polar solvent. Next, the obtained slurry is applied onto the negative electrode current collector 21 and dried. Then, the negative electrode layer 20 is prepared by pressurizing the resulting laminate (for example, by applying hydrostatic pressure). The pressurizing step may be omitted.

(3-3. Solid Electrolyte Layer Fabrication Process)
The solid electrolyte layer 30 can be made from a solid electrolyte formed from a sulfide-based solid electrolyte material.

First, the starting material is processed using methods such as melt-and-quench method or mechanical milling.

For example, when using the melt-and-cool method, a sulfide-based solid electrolyte material can be produced by mixing a predetermined amount of starting materials (e.g., _Li₂S , _P₂S₅ , _etc. ), forming them into pellets, reacting them in a vacuum at a predetermined reaction temperature, and then rapidly cooling them. The reaction temperature of the _{Li₂S and P₂S₅ mixture} is preferably 400°C to 1000°C, more preferably 800°C to 900°C. The reaction time is preferably 0.1 hours to 12 hours, more preferably 1 hour to 12 hours. Furthermore, the rapid cooling temperature of the reactants is usually 10°C or lower, preferably 0°C or lower, and the rapid cooling rate is usually about 1°C/sec to 10000°C/sec, more preferably about 1°C/sec to 1000°C/sec.

Furthermore, when using the mechanical milling method, sulfide-based solid electrolyte materials can be produced by stirring and reacting the starting materials (e.g., _Li₂S , _P₂S₅ , _etc. ) using a ball mill or the like. While the stirring speed and time in the mechanical milling method are not particularly limited, a faster stirring speed can increase the rate of sulfide-based solid electrolyte material production, and a longer stirring time can increase the conversion rate of the raw materials to sulfide-based solid electrolyte material.

Subsequently, the mixed raw materials obtained by the melt-quenching method or mechanical milling method can be heat-treated at a predetermined temperature and then pulverized to produce particulate solid electrolytes. If the solid electrolyte has a glass transition temperature, it may change from amorphous to crystalline during heat treatment.

Next, the solid electrolyte obtained by the above method can be used to create a solid electrolyte layer 30 by forming a film using a known film formation method such as aerosol deposition, cold spray, or sputtering. Alternatively, the solid electrolyte layer 30 may be prepared by pressurizing individual solid electrolyte particles. Furthermore, the solid electrolyte layer 30 may be prepared by mixing the solid electrolyte with a solvent and a binder, coating and drying the mixture, and then pressurizing it.

(3-4. Assembly process for all-solid-state secondary batteries)
The positive electrode layer 10, negative electrode layer 20, and solid electrolyte layer 30 prepared by the above method are laminated so that the solid electrolyte layer 30 is sandwiched between the positive electrode layer 10 and the negative electrode layer 20, and the entire assembly is covered from both sides with a laminate film that forms an outer casing 40, and then pressurized (for example, by applying hydrostatic pressure) to produce the all-solid-state secondary battery 1 according to this embodiment.

<4. Effects of this embodiment>
In this all-solid-state secondary battery configuration, the positive electrode active material layer, which is in an oxidizing environment during charging, contains an endothermic substance. As a result, the decomposition reaction of the endothermic substance occurs appropriately within an appropriate temperature range, allowing for a sufficient endothermic effect.

Since the heat-absorbing material content is between 1 and 30 parts by mass, the heat-absorbing effect can be fully exerted while ensuring sufficient charging capacity for the positive electrode active material layer 12.

<5. Other Embodiments of the Invention>
<5-1. Configuration of an all-solid-state secondary battery according to the second embodiment of the present invention>
As shown in Figure 2, the positive electrode layer 10 may further include a conductive layer 13 formed between the positive electrode current collector 11 and the positive electrode active material layer 12.
The conductive layer 13 is a layer that protects the positive electrode current collector, and for example, contains a conductive material and a binder.

The conductive material contained in the conductive layer 13 can be graphite, carbon black, acetylene black, Ketjen black, carbon fiber, metal powder, etc.
Furthermore, examples of binders contained in the conductive layer 13 include styrene-butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene.
The conductive layer 13 configured in this manner may also contain the aforementioned heat-absorbing substance. Specific examples of the conductive layer 13 containing a heat-absorbing substance include a conductive layer 13 containing 6% to 54% by mass of conductive material, 24% to 81% by mass of heat-absorbing substance, and a binder containing 10% to 40% by mass.

The amount of heat-absorbing material contained in the conductive layer 13 is such that the total amount of heat-absorbing material contained in the positive electrode active material layer 12 and the amount of heat-absorbing material contained in the conductive layer 13 is between 1 and 30 parts by mass, based on a total mass of 100 parts by mass of the positive electrode active material layer 12.
In this embodiment, the heat-absorbing material may be contained in both the positive electrode active material layer 12 and the conductive layer 13, or it may be contained only in the conductive layer 13.
In this embodiment as well, similar to the previously described embodiment, a sufficient heat absorption effect can be obtained by setting the content of the heat-absorbing material to 1 part by mass or more when the total mass of the positive electrode active material layer 12 is 100 parts by mass. Furthermore, by setting the content of the heat-absorbing material to 30 parts by mass or less, the thickness of the conductive layer 13 can be reduced while maintaining the conductivity of the conductive layer 13, thereby preventing the volume of the all-solid-state secondary battery 1 from increasing. The thickness of this conductive layer is preferably in the range of 0.5 μm to 10 μm, and more preferably 1 μm to 5 μm.
In this embodiment, the conductive layer 13 containing the heat-absorbing material is a layer that exhibits a heat-absorbing effect, and can therefore also be called a heat-absorbing layer.

<5-2. Method for manufacturing an all-solid-state secondary battery according to the second embodiment of the present invention>
Next, a method for manufacturing the all-solid-state secondary battery 1 according to the second embodiment will be described.
In the positive electrode layer 10 manufacturing step of the manufacturing method for the all-solid-state secondary battery 1 according to the first embodiment, a slurry is formed by adding materials constituting the conductive layer 13 (conductive substance, heat-absorbing substance, binder, etc.) to a non-polar solvent, and this slurry is applied to the positive electrode current collector 11 and dried to form the conductive layer 13. A slurry for forming the positive electrode active material layer 12 is applied to this conductive layer 13 and dried to form the positive electrode active material layer 12, and the positive electrode layer 10 is manufactured by applying pressure. The all-solid-state secondary battery 1 can be manufactured in the same manner as in the first embodiment for the other steps.

<5-3. Configuration of an all-solid-state secondary battery according to the third embodiment of the present invention>
In the embodiments described above, an all-solid-state secondary battery 1 comprising one positive electrode layer 10, one negative electrode layer 20, and one solid electrolyte layer 30 was described. However, for example, as shown in Figure 3, an all-solid-state secondary battery 1 may also be configured such that the solid electrolyte layer 30 is arranged to sandwich the positive electrode layer 10 from both sides, and the negative electrode layer 20 sandwiches these solid electrolyte layers 30 from the outside.
Even in the case of an all-solid-state secondary battery 1 with this configuration, a conductive layer 13 can be provided between the positive electrode current collector 11 and the positive electrode active material layer 12. This conductive layer 13 does not necessarily need to be formed on both sides of the positive electrode current collector 11; as shown in Figure 3, the conductive layer 13 may be formed on only one side of the positive electrode current collector 11.
Furthermore, the positive electrode active material layer 12 and/or conductive layer 13 of this all-solid-state secondary battery 1 may contain an endothermic substance.

Although preferred embodiments of the present invention have been described in detail above with reference to the attached drawings, the present invention is not limited to these examples. It is clear to any person with ordinary skill in the art to which the present invention pertains that various modifications or alterations can be conceived within the scope of the technical idea described in the claims, and these, too, are naturally understood to fall within the technical scope of the present invention.

The all-solid-state secondary battery according to the present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

(Example 1)
[Generation of the positive electrode layer]
LiNi _0.8 Co _0.15 Al _0.05 O ₂ (NCA) ternary powder as the positive electrode active material, Li ₂ S-P ₂ S ₅ (80:20 mol%) amorphous powder as the sulfide-based solid electrolyte, and vapor-grown carbon fiber powder as the positive electrode layer conductive material (conductive additive) were weighed in a mass percentage ratio of 60:35:5 and mixed using a rotation-orbit mixer. Furthermore, 5 parts by mass of lithium carbonate were weighed out per 100 parts by mass of this mixed powder and added to the mixed powder, and mixed using a rotation-orbit mixer.
Next, a dehydrated xylene solution in which SBR as a binder was dissolved was added so that the amount of SBR was 5.0% by mass relative to the total mass of the mixed powder containing the endothermic substance, thereby producing a primary mixture. Since the solid component remaining after removing the solvent such as the dehydrated xylene solution from this primary mixture should be equal in mass to that of the positive electrode active material layer, in this embodiment, the endothermic substance (lithium carbonate) is contained at 4.5% by mass relative to the entire positive electrode active material layer.
Furthermore, a secondary mixture was produced by adding an appropriate amount of dehydrated xylene to this primary mixture to adjust its viscosity. To further improve the dispersibility of the mixed powder, 5 mm diameter zirconia balls were added to the secondary mixture so that the space, mixed powder, and zirconia balls each occupied 1/3 of the total volume of the mixing container. The resulting tertiary mixture was then added to a rotary-orbit mixer and stirred at 3000 rpm for 3 minutes to produce a coating solution for the positive electrode active material layer.
Next, a 20 μm thick aluminum foil current collector was prepared as the positive electrode current collector. The positive electrode current collector was placed on a desktop screen printing machine, and a positive electrode active material layer coating solution was applied to the sheet using a metal mask with a hole diameter of 2.0 cm x 2.0 cm and a thickness of 150 μm. After that, the sheet coated with the positive electrode active material layer coating solution was dried on a hot plate at 60°C for 30 minutes, and then vacuum dried at 80°C for 12 hours. This formed a positive electrode active material layer on the positive electrode current collector. The total thickness of the positive electrode current collector and positive electrode active material layer after drying was approximately 165 μm.

[Generation of the negative electrode layer]
Graphite powder (vacuum-dried at 80°C for 24 hours) as the negative electrode active material and PVDF as a binder were weighed in a mass percentage ratio of 95.0:5.0. These materials, along with an appropriate amount of N-methyl-2-pyrrolidone (hereinafter referred to as NMP), were placed in a rotary-orbit mixer and stirred at 3000 rpm for 3 minutes, followed by a 1 minute degassing treatment to produce a negative electrode active material layer coating solution.
Next, a copper foil current collector with a thickness of 16 μm was prepared as the negative electrode current collector, and a negative electrode active material layer coating solution was applied to the copper foil current collector using a blade. The thickness (gap) of the negative electrode active material layer coating solution on the copper foil current collector was approximately 150 μm.
A sheet coated with the negative electrode active material coating solution was placed in a dryer heated to 80°C and dried for 15 minutes. Furthermore, the dried sheet was vacuum-dried at 80°C for 24 hours. This created the negative electrode layer. The thickness of the negative electrode layer was approximately 140 μm.

[Formation of a solid electrolyte layer]
A primary mixture was prepared by adding a dehydrated xylene solution containing dissolved SBR to _an amorphous _Li₂S - _P₂S₅ (80:20, mol%) sulfide-based solid electrolyte powder, such that the SBR content was 2.0% by mass relative to the total mass of the mixed powder. A secondary mixture was then prepared by adding an appropriate amount of dehydrated xylene to this primary mixture to adjust the viscosity. To further improve the dispersibility of the mixed powder, 5 mm diameter zirconia balls were added to the tertiary mixture so that the space, mixed powder, and zirconia balls each occupied 1/3 of the total volume of the mixing container. The resulting tertiary mixture was then added to a rotary-orbit mixer and stirred at 3000 rpm for 3 minutes to produce an electrolyte layer coating solution.
The negative electrode layer was placed on a desktop screen printing machine, and the electrolyte layer coating solution was applied to the negative electrode active material layer using a 500 μm metal mask. The sheet coated with the electrolyte layer coating solution was then dried on a 40°C hot plate for 10 minutes, followed by vacuum drying at 40°C for 12 hours. This formed a solid electrolyte layer on the negative electrode layer. The total thickness of the solid electrolyte layer after drying was approximately 300 μm.

[Generation of all-solid-state batteries]
A sheet consisting of a negative electrode layer and a solid electrolyte layer was die-cut to 3.5 cm x 3.5 cm, and the positive electrode layer to 3.0 cm x 3.0 cm using a die-cutting machine. These were then bonded together using a dry lamination method with a roll press machine having a roll gap of 150 μm to produce a single cell of an all-solid-state battery. The layer thickness of the single cell was approximately 400 μm.

[Encapsulation of all-solid-state batteries]
The generated single cells were placed in an aluminum laminate film with terminals attached, evacuated to 100 Pa using a vacuum machine, and then heat-sealed to form a pack. The layer thickness of the all-solid-state battery was approximately 600 μm.

[Battery characteristics evaluation]
The capacity (mAh) of a single cell was measured using a Toyo Systems TOSCAT-3100 charge/discharge evaluation device. Charging and discharging were performed in an environment of 60°C. The cell was charged to 4.20V with a current of 0.1mA and discharged to 2.50V with a current of 0.1mA to determine the capacity of the single cell.

[Heating test]
The solid-state battery was charged to 4.20V and then stored in a constant temperature bath at 80°C for 24 hours. The change in battery thickness was measured before and after storage. In the case of the type of battery sealed in a laminate bag as described in this embodiment, the percentage change in thickness directly represents the percentage change in the total volume of the battery.

[DSC Test]
The battery was charged to 4.20V, and DSC samples were prepared in an Ar-atmosphere glove box using the following procedure. A single cell was removed from the laminated outer bag and punched out to a diameter of φ2.5mm using a die. It was placed in a stainless steel sample pan, the lid was set, and the pan was crimped and sealed with a press. The DSC samples prepared in this way were measured using a Hitachi High-Tech Science DSC7000X DSC measuring instrument. Measurements were taken from room temperature to 500°C, and the cumulative heat capacity was estimated. The cumulative heat capacity of Comparative Example 1 was set to 100%, and the percentage change in cumulative heat capacity in each evaluation example was estimated as the reduction rate.

(Example 2)
The process for forming the positive electrode layer is the same as in Example 1, except that the amount of lithium carbonate added to the positive electrode active material layer is 10 parts by mass per 100 parts by mass of the mixed powder forming the positive electrode active material layer.

(Example 3)
In the formation of the positive electrode layer, the amount of lithium carbonate added to the positive electrode active material layer was the same as in Example 1, except that it was 1 part by mass per 100 parts by mass of the mixed powder forming the positive electrode active material layer.

(Example 4)
In the formation of the positive electrode layer, the amount of lithium carbonate added to the positive electrode layer was 25 parts by mass per 100 parts by mass of the mixed powder forming the positive electrode active material layer, except that it was the same as in Example 1.

(Example 5)
In the production of the positive electrode layer, lithium carbonate was not added to the positive electrode active material layer, but rather to the conductive layer formed between the positive electrode current collector and the positive electrode active material layer. The specific manufacturing method is described below.
Acetylene black, lithium carbonate, and acid-modified PVDF as a binder were weighed in a mass ratio of 30:40:30 as the conductive material for the conductive layer. These materials, along with an appropriate amount of NMP, were then added to a rotary-orbit mixer and stirred at 3000 rpm for 5 minutes to produce a conductive layer coating solution.
Next, a 20 μm thick aluminum foil was placed on a desktop screen printing machine, and a conductive layer coating solution was applied to the aluminum foil using a 400-mesh screen. Afterward, the aluminum foil coated with the conductive layer coating solution was vacuum-dried at 80°C for 12 hours. This formed a conductive layer on the positive electrode current collector. The thickness of the conductive layer after drying was 15 μm. The thickness of this conductive layer was adjusted so that the lithium carbonate content in the conductive layer was 5 parts by mass per 100 parts by mass of the mixed powder forming the positive electrode active material layer.
A positive electrode layer was formed by coating the conductive layer thus created with the same positive electrode active material layer coating solution as in Example 1, except that it does not contain lithium carbonate, and then drying it. The procedure from the formation of the negative electrode layer onward was the same as in Example 1.

(Example 6)
The same procedure as in Example 5, except that the thickness of the conductive layer was adjusted so that the lithium carbonate content in the conductive layer was 25 parts by mass per 100 parts by mass of the mixed powder forming the positive electrode active material layer.

(Example 7)
The process for forming the positive electrode layer is the same as in Example 5, except that the positive electrode active material layer coating solution is applied to the surface of the positive electrode current collector opposite to the side where the conductive layer is formed, and then dried.

(Example 8)
The same procedure as in Example 7, except that the thickness of the conductive layer was adjusted so that the lithium carbonate content in the conductive layer was 25 parts by mass per 100 parts by mass of the mixed powder forming the positive electrode active material layer.

(Example 9)
The process for forming the positive electrode layer is the same as in Example 1, except that aluminum hydroxide is included in the positive electrode active material layer instead of lithium carbonate.

(Example 10)
The process for forming the positive electrode layer is the same as in Example 9, except that the aluminum hydroxide content in the positive electrode active material layer is 10 parts by mass per 100 parts by mass of the mixed powder forming the positive electrode active material layer.

(Example 11)
The process for forming the positive electrode layer is the same as in Example 9, except that the aluminum hydroxide content in the positive electrode active material layer is 1 part by mass per 100 parts by mass of the mixed powder forming the positive electrode active material layer.

(Comparative Example 1)
The process for forming the positive electrode layer is the same as in Example 1, except that lithium carbonate was not added to the positive electrode active material layer.

(Comparative Example 2)
This is the same as Example 1, except that lithium carbonate is not included in the positive electrode active material layer, and in the formation of the negative electrode layer, 5 parts by mass (i.e., the same amount as used in Example 1) of lithium carbonate is included in the negative electrode active material layer per 100 parts by mass of the mixed powder used to form the positive electrode active material layer.

(Comparative Example 3)
This is the same as Example 1, except that lithium carbonate is not included in the positive electrode active material layer, and in the formation of the solid electrolyte layer, 5 parts by mass (i.e., the same amount as used in Example 1) of lithium carbonate is included in the solid electrolyte layer per 100 parts by mass of the mixed powder used to form the positive electrode active material layer.

(Comparative Example 4)
This is the same as Comparative Example 2, except that aluminum hydroxide is included in the negative electrode active material layer instead of lithium carbonate.

(Comparative Example 5)
This is the same as Comparative Example 3, except that aluminum hydroxide is included in the solid electrolyte layer instead of lithium carbonate.

(Comparative Example 6)
The process for forming the positive electrode layer is the same as in Example 1, except that the lithium carbonate content in the positive electrode active material layer is 0.3 parts by mass per 100 parts by mass of the mixed powder forming the positive electrode active material layer.

(Reference example 1)
This example involves incorporating an endothermic substance into a liquid-based secondary battery. The specific experimental method is described below.

[Generation of the positive electrode layer]
NCA ternary powder as the positive electrode active material and acetylene black as a conductive additive were weighed in a mass percentage ratio of 97:3 and mixed. Furthermore, 1 part by mass of lithium carbonate was weighed out for every 100 parts by mass of this mixed powder and added to the mixed powder and mixed.
Next, a primary mixture was prepared by adding an NMP solution containing PVdF as a binder to the mixed powder so that the amount of PVdF was 3.0% by mass relative to the total mass of the mixed powder. Furthermore, a secondary mixture was prepared by adding an appropriate amount of NMP to this primary mixture to adjust the viscosity. This secondary mixture was then introduced into a rotary-orbit mixer and stirred at 2000 rpm for 3 minutes to produce a coating solution for the positive electrode active material layer.
Next, a 20 μm thick aluminum foil current collector was prepared as the positive electrode current collector. The positive electrode current collector was placed on a desktop screen printing machine, and a positive electrode active material layer coating solution was applied to the sheet using a metal mask with a hole diameter of 2.0 cm × 2.0 cm and a thickness of 150 μm. After that, the sheet coated with the positive electrode active material layer coating solution was dried on a hot plate at 100°C for 30 minutes, and then vacuum dried at 180°C for 12 hours. This formed a positive electrode active material layer on the positive electrode current collector. The total thickness of the positive electrode current collector and positive electrode active material layer after drying was approximately 120 μm. The laminate formed in this way was then pressure-molded using a roll press to create the positive electrode layer. This positive electrode layer was punched out with a 3.0 cm × 3.0 cm die.

[Generation of the negative electrode layer]
Graphite powder (vacuum-dried at 80°C for 24 hours) as the negative electrode active material and PVdF as a binder were weighed in a mass percentage ratio of 95.0:5.0. These materials, along with an appropriate amount of N-methyl-2-pyrrolidone (hereinafter referred to as NMP), were placed in a rotary-orbit mixer and stirred at 3000 rpm for 3 minutes, followed by a 1 minute degassing treatment to produce a negative electrode active material layer coating solution.
Next, a copper foil current collector with a thickness of 16 μm was prepared as the negative electrode current collector, and a negative electrode active material layer coating solution was applied to the copper foil current collector using a blade. The thickness (gap) of the negative electrode active material layer coating solution on the copper foil current collector was approximately 150 μm.
A sheet coated with the negative electrode active material layer coating solution was placed in a dryer heated to 80°C and dried for 15 minutes. Furthermore, the dried sheet was vacuum-dried at 80°C for 24 hours. This created the negative electrode layer. The thickness of the negative electrode layer was approximately 140 μm. The negative electrode layer was then pressure-molded using a roll press. The negative electrode layer was punched out with a 3.5 cm x 3.5 cm die.

[Production of liquid-based lithium-ion secondary batteries]
A porous polyethylene film (12 μm thick) was used as the separator. The electrode structure was fabricated by sandwiching the separator between a positive electrode layer and a negative electrode layer. This electrode structure was then placed in an aluminum laminate film to which terminals were attached.
An electrolyte was prepared by dissolving lithium hexafluorophosphate ( _LiPF6 ) at a concentration of 1.3 mol/L in a non-aqueous solvent prepared by mixing ethylene carbonate and dimethyl carbonate in a volume ratio of 3:7. The prepared electrolyte was injected into an aluminum laminate film to impregnate the separator. After evacuating to 100 Pa using a vacuum machine, the film was heat-sealed and packaged. This resulted in the production of a liquid-type lithium-ion secondary battery.

The evaluation results for Examples 1-11, Comparative Examples 1-6, and Reference Examples described above are shown in Table 1 below.

The heat-absorbing substance content listed in Table 1 indicates the amount of heat-absorbing substance added per 100 parts by mass of the mixed powder for the positive electrode active material layer before the addition of the binder.

From the results in Table 1, it was found that in Examples 1 to 11, in which an endothermic substance such as lithium carbonate or aluminum hydroxide was contained in the positive electrode layer at a rate of 1 part by mass or more per 100 parts by mass of the positive electrode active material layer, a significantly higher endothermic effect was observed compared to Comparative Examples 1 to 6.
Furthermore, it was found that the heat-absorbing material can exert a sufficient heat-absorbing effect even when it is contained in the conductive layer within the positive electrode layer. Based on these results, it is considered that all-solid-state secondary batteries capable of sufficiently suppressing rapid heat generation in the battery were fabricated in Examples 1 to 11.

It was found that the charging capacity of the positive electrode layer can be sufficiently maintained if the amount of heat-absorbing material contained in the positive electrode layer is 30 parts by mass or less per 100 parts by mass of the positive electrode active material layer. Although not described in the above examples, when the heat-absorbing material was contained at approximately 50 parts by mass per 100 parts by mass of the positive electrode active material layer, the battery did not operate. This is thought to be because the proportion of heat-absorbing material in the positive electrode active material layer that does not contribute to lithium-ion conductivity or electronic conductivity was too high.

In the all-solid-state batteries of the examples and comparative examples described here, the change in thickness represents the change in volume. As can be seen from the results in Table 1, the thickness when placed in an environment of 80°C is less than 1% compared to when at room temperature (25°C), indicating that there is almost no change.
On the other hand, in the example of a liquid-type lithium-ion secondary battery, even when the lithium carbonate content is 1 part by mass per 100 parts by mass of the positive electrode active material layer, the volume change at 80°C is a very large 32%.
These results show that in liquid-based secondary batteries, the decomposition reaction of the endothermic substance occurs vigorously even at low temperatures of around 80°C, whereas in the all-solid-state secondary battery according to the present invention, while the endothermic effect is fully exerted at temperatures above 80°C, the decomposition of the endothermic substance is relatively gradual up to temperatures of around 80°C, and deformation of the battery due to volume expansion can be suppressed.

1, 1a All-solid secondary battery 10 Positive electrode layer 11 Positive electrode current collector 12 Positive electrode active material layer 13 Conductive layer 20 Negative electrode layer 21 Negative electrode current collector 22 Negative electrode active material layer 30 Solid electrolyte layer

Claims (6)

The device comprises a positive electrode layer, a negative electrode layer, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer, wherein the positive electrode layer comprises a plate-shaped positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector,
The positive electrode layer further comprises a conductive layer containing an endothermic substance that absorbs heat through a decomposition reaction, a conductive substance, and a binder.
The conductive layer is disposed between the positive electrode current collector and the positive electrode active material layer, or on the surface of the positive electrode current collector opposite to the side on which the positive electrode active material layer is disposed.
The content of the conductive material in the conductive layer is in the range of 6% by mass or more and 54% by mass or less.
The content of the binder in the conductive layer is 10% by mass or more and 40% by mass or less .
An all-solid-state secondary battery characterized in that the heat-absorbing substance is lithium carbonate . The heat-absorbing substance is further contained in the positive electrode active material layer,
The all-solid-state secondary battery according to claim 1, wherein the total amount of the heat-absorbing substance contained in the conductive layer is such that the sum of the amount of the heat-absorbing substance contained in the positive electrode active material layer and the amount of the heat-absorbing substance contained in the conductive layer is 1 part by mass or more and 30 parts by mass or less, when the total mass of the positive electrode active material layer is 100 parts by mass. The all-solid-state secondary battery according to claim 1 or 2 , wherein the solid electrolyte layer contains a sulfide-based solid electrolyte. The exterior further comprises an outer casing that houses the positive electrode layer, the negative electrode layer, and the solid electrolyte layer inside,
The all-solid-state secondary battery according to any one of claims 1 to 3 , wherein the outer casing is in the form of a film. The all-solid-state secondary battery according to claim 4 , wherein the difference between the volume inside the outer casing at 80°C and the volume at 25°C is within 5% of the volume at 25°C. The all-solid-state secondary battery according to claim 1, wherein the thickness of the conductive layer is 0.5 μm or more and 10 μm or less.