JP7746795B2 - All solid state battery - Google Patents

Description

The present invention relates to an all-solid-state battery that has excellent flexibility, dendrite resistance, and good heat resistance.

In recent years, in an effort to reduce the weight and increase the energy density of batteries, there has been active research into metal Li anode batteries, all-solid-state batteries, and air batteries, with progress being made in the design of positive electrodes, negative electrodes, separators, electrolytes, and other components with a view to practical application. All-solid-state batteries, in particular, do not require flammable electrolytes like conventional secondary batteries; instead, they use a solid electrolyte membrane capable of ion conduction, drawing attention as a highly safe secondary battery that can avoid the risk of fire or explosion.

Batteries typically use separators made of porous membranes or nonwoven fabrics with through-holes measuring tens of nanometers to several micrometers in diameter to enable ionic conduction between the positive and negative electrodes while preventing short circuits due to contact between the electrodes. However, using porous separators poses the issue of dendrite growth. Solid electrolytes can be used to solve this problem, and are broadly divided into inorganic and organic types. Organic electrolytes are further divided into polymer gel electrolytes and polymer solid electrolytes (true polymer electrolytes). Organic solid electrolytes are lighter and more flexible than inorganic solid electrolytes, and because they do not require complex processes such as vacuum sputtering or high-temperature firing, they can be easily processed over large areas and are expected to reduce manufacturing costs.

Inorganic solid electrolytes are composed of anionic lattice sites and metal ions, and many have been reported to have practical ionic conductivity (see, for example, Patent Document 1). Among organic electrolytes, the application of polymer gel electrolytes, in which the electrolyte solution is semi-solidified with a polymer, to batteries began with a report by Feuillade et al. in 1975 (Non-Patent Document 1). Since then, various reports have been made (see, for example, Patent Document 2), and these have been put to practical use in lithium polymer batteries.

Research into polymer solid electrolytes began with a paper by Wright published in 1975 (Non-Patent Document 2), and many successes have been reported to date, primarily focusing on polyether-based electrolytes (for example, Patent Document 3). Furthermore, recent reports have included those that improve ionic conductivity by applying a polymer electrolyte to the negative electrode (Patent Document 4), those that improve the shape retention and mechanical strength of polymers (Patent Document 5), and those that use ionic liquids to improve ionic conductivity at low temperatures (Patent Document 6).

JP 2014-13772 A Japanese Patent Application Laid-Open No. 2008-159496 Japanese Patent Application Laid-Open No. 2007-103145 JP 2016-167434 A Patent No. 6061096 Japanese Patent Application Laid-Open No. 2020-035587

G. Feuillade, Ph. Perche, J. Appl. Electrochem. , 5, 63 (1975). P. V. Wright, Br. Polym. J. , 7.319 (1975).

However, the polymer electrolyte described in Patent Document 4 is manufactured by applying a polyether-based polymer electrolyte to the negative electrode and allowing the polymer electrolyte to penetrate into the negative electrode, but the softening point of the polymer film is low, resulting in poor heat resistance. Furthermore, the polymer electrolyte described in Patent Document 5 also has poor thermal properties when the temperature inside the battery reaches high temperatures. The polymer electrolyte described in Patent Document 6 is a mixture of a polyether-based polymer, an ionic liquid, and an inorganic electrolyte, but has issues with the shape retention and flexibility of the polymer film. Therefore, in consideration of the above circumstances, the present invention aims to provide an all-solid-state battery that has excellent flexibility and dendrite resistance, as well as good heat resistance.

In order to solve the above problems, the present invention has the following features.
(1)
An all-solid-state battery having a current collector layer, an electrode mixture layer, and an electrolyte layer, wherein the electrode mixture layer and the electrolyte layer are layers containing an ion-conductive polymer, and the softening point of the ion-conductive polymer is 80°C or higher.
(2)
The all-solid-state battery according to (1), wherein the electrolyte layer and the electrode mixture layer contain the same ion-conductive polymer.
(3)
The all-solid-state battery according to (1) or (2), wherein the electrolyte layer is provided on at least one surface layer of the electrode mixture layer, and the surface gloss (60°) of the electrolyte layer is 10 or more and 300 or less.
(4)
The all-solid-state battery according to any one of (1) to (3), having a cell resistance at 25°C of 30 Ω cm or ^less .
(5)
The all-solid-state battery according to any one of (1) to (4), wherein the electrolyte layer contains a Li salt in an amount of 5 mass% or more relative to the total mass of the ion-conductive polymer in the electrolyte layer.
(6)
The all-solid-state battery according to any one of (1) to (5), wherein the electrolyte layer contains 50 μg or more of Li element per 1 g of the ion-conductive polymer in the electrolyte layer.
(7)
The all-solid-state battery according to any one of (1) to (6), wherein the ion-conductive polymer includes an aromatic polyamide.

The present invention provides an all-solid-state battery that is highly flexible, has excellent dendrite resistance, and also has good heat resistance.

The present invention is described in detail below.

The present invention relates to an all-solid-state battery having a current collector layer, an electrode mixture layer, and an electrolyte layer, wherein the electrode mixture layer and the electrolyte layer contain an ion-conductive polymer, and the softening point of the polymer is 80°C or higher. To achieve the effects of the present invention, all of the above characteristics must be satisfied simultaneously.

The ion-conducting polymer used in embodiments of the present invention (hereinafter sometimes referred to as the "ion-conducting polymer of the present invention") is a polymer that enables ion conduction between the positive and negative electrodes when used in an all-solid-state battery. In conventional technology, the electrode mixture layer that forms the positive or negative electrode of an all-solid-state battery, and the electrolyte layer placed between the positive and negative electrodes, are primarily composed of inorganic materials. In embodiments of the present invention, the electrode mixture layer and electrolyte layer contain this ion-conducting polymer, improving the flexibility of each layer and enhancing the handling and processability during the molding of the all-solid-state battery. Furthermore, cracking and chipping of each layer due to impact during battery use can be prevented, thereby suppressing short circuits and fires caused by the formation of dendrites. Furthermore, the smooth interface between the electrode mixture layer and the electrolyte layer improves adhesion, reducing interfacial resistance and improving battery characteristics.

The softening point of the ion-conductive polymer according to the embodiment of the present invention must be 80° C. or higher. Normally, the electrolyte layer of an all-solid-state battery is a solid membrane, and the risk of ignition at high temperatures is lower than in secondary batteries that use non-aqueous electrolyte solutions. However, in all-solid-state batteries intended for long life and high energy capacity, the softening point of the polymer is set to 80° C. or higher to further increase heat resistance and enhance safety.
The softening point of a polymer is the temperature at which the polymer begins to deform when heated, regardless of whether the polymer is a single component or a composite component.

The polymers that can be used in the embodiments of the present invention are not particularly limited, but examples include polymers having an aromatic ring on the main chain and fluorine-containing polymers such as polyvinylidene fluoride. In particular, polymers having an aromatic ring on the main chain are preferably used, such as aromatic polyamide (aramid), aromatic polyimide, aromatic polyamideimide, aromatic polyetherketone, aromatic polyetheretherketone, aromatic polyarylate, aromatic polysulfone, aromatic polyethersulfone, aromatic polyetherimide, and aromatic polycarbonate. Blends of multiple polymers may also be used. Among these, aromatic polyamides (including aromatic polyamic acids, which are aromatic polyimide precursors), aromatic polyimides, and aromatic polyamideimides are more preferred, as they tend to maintain high strength when thinned, and aromatic polyamides are particularly preferred.
That is, the ion-conductive polymer according to the embodiment of the present invention preferably contains an aromatic polyamide.

The content of the ion-conductive polymer in the electrode mixture layer and the electrolyte layer is not particularly specified, but is preferably 0.05% by mass or more of the entire layer. In particular, from the viewpoints of flexibility and adhesion to the electrode mixture layer, the electrolyte layer preferably contains 5% by mass or more, and even more preferably 10% by mass or more. Furthermore, from the viewpoint of improving ion conductivity by incorporating a lithium salt as described below, the content is preferably 99% by mass or less, and more preferably 95% by mass or less.

Ion-conductive polymers that can be suitably used in the present invention preferably include polymers having a structure represented by any of the following chemical formulas (1) to (3). Examples of aromatic polyamides include those having a repeating unit represented by the following chemical formula (1), aromatic polyimides include those having a repeating unit represented by the following chemical formula (2), and aromatic polyamideimides include those having a repeating unit represented by the following chemical formula (3).

Chemical formula (1):

Chemical formula (2):

Chemical formula (3):

Here, Ar ₁ and Ar ₂ in chemical formulas (1) to (3) are groups containing aromatic groups, and each may be a single group or a multi-component copolymer containing multiple groups. Furthermore, the bonds constituting the main chain on the aromatic ring may be either meta-oriented or para-oriented. Furthermore, some of the hydrogen atoms on the aromatic ring may be substituted with any group.

In embodiments of the present invention, when an aromatic polyamide, aromatic polyimide, or aromatic polyamideimide is used as the ion-conductive polymer, it is preferable that the main chain or side chain has an ether bond or a thioether bond (in the main chain or on the side chain). The unshared electron pair of the ether bond or thioether bond speeds up the movement of ions required for battery operation, making it possible to improve ion conductivity.

The confirmation of each component and its content in the membrane containing the ion-conducting polymer according to the embodiment of the present invention (hereinafter, sometimes simply referred to as the polymer membrane) is not limited to a specific method, but proton nuclear magnetic resonance spectroscopy ( ¹ H-NMR) or Fourier transform infrared spectroscopy (FT-IR) can be used. Furthermore, multiple methods can be used in combination for confirmation as necessary.

The electrolyte layer and electrode mixture layer in the all-solid-state battery according to an embodiment of the present invention preferably contain the same ion-conductive polymer. By containing the same ion-conductive polymer in the electrolyte layer and electrode mixture layer, it is possible to reduce the interfacial resistance between the layers, and to dramatically improve ion conductivity and cycle characteristics, thereby improving the performance of the all-solid-state battery. There are no particular restrictions on the method for making the electrolyte layer and electrode mixture layer contain the same ion-conductive polymer. The electrode mixture layer and electrolyte layer may be molded separately, or they may be processed by integral molding in which the electrode mixture material and polymer are mixed and applied to the current collector layer, and then the layers are separated. Alternatively, a liquid of the ion-conductive polymer dissolved in an organic solvent may be applied to the electrode mixture layer and allowed to penetrate.

The electrolyte layer according to the embodiment of the present invention is provided on at least one surface layer of the electrode mixture layer, and the surface gloss (60°) of the electrolyte layer is preferably 10 or more and 300 or less. It is more preferably 15 or more and 280 or less, and even more preferably 20 or more and 250 or less. If the surface gloss (60°) is 10 or more, inorganic solid electrolytes that may be contained in the electrolyte layer and precipitates generated during molding processing do not protrude to the surface, and gaps are less likely to occur at the contact surface with the other electrode mixture layer when used as an all-solid-state battery, making it easier to obtain sufficient ionic conductivity. Furthermore, the low content of ion-conductive polymer in the electrolyte layer prevents the occurrence of continuous voids in the electrolyte layer, thereby suppressing short circuits due to dendrite growth when used as an all-solid-state battery. Furthermore, by setting the gloss (60°) to 300 or less, the inorganic solid electrolyte does not become too dense on the surface of the electrolyte layer, resulting in good flexibility.
The surface glossiness (60°) of the electrolyte layer is the surface glossiness of the electrolyte layer measured in accordance with JIS Z8741 (1997) at an incident angle and a receiving angle of 60°.

Examples of how the surface gloss (60°) can be adjusted to fall within the above range include ensuring that the ion-conductive polymer content in the electrolyte layer falls within the above range, or isolating and redissolving the ion-conductive polymer using the method described below and removing any precipitates beforehand. By achieving a surface gloss (60°) within the above range, an all-solid-state battery with excellent ionic conductivity, dendrite resistance, and flexibility can be obtained.

The electrolyte layer according to the embodiment of the present invention preferably contains 5 mass% or more of a Li salt relative to the total mass of the ion-conductive polymer in the electrolyte layer. The lithium salt is not particularly specified, but from the viewpoints of ionic conductivity, the ion migration rate at the interface between each layer, thermal and electrochemical stability, discharge capacity, cycle characteristics, etc., LiPF ₆ , LiAsF ₆ , LiClO ₄ , LiBF ₄ , LiBr, lithium bis(oxalate)borate, lithium difluoro(oxalate)borate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethanesulfonyl)imide, etc. are preferred. These lithium salts may be used alone or in combination of two or more.

The content of the lithium salt is preferably 5% by mass or more, more preferably 7% by mass or more, and even more preferably 10% by mass or more, relative to the total mass of the ion-conductive polymer in the electrolyte layer. By keeping the lithium salt content within this range, an all-solid-state battery with excellent ionic conductivity, discharge capacity, and cycle characteristics can be obtained. There is no particular upper limit, but from the perspective of the handleability and moldability of the ion-conductive polymer, it is preferable that the upper limit be 95% by mass or less. When two or more types of lithium salts are used, it is preferable to adjust the total amount so that it is within the above range.

Furthermore, the electrolyte layer according to an embodiment of the present invention preferably contains 50 μg or more of Li element per gram of ion-conductive polymer in the electrolyte layer, more preferably 100 μg or more, and even more preferably 200 μg or more. By setting the lithium element content within the above range, an all-solid-state battery with excellent ionic conductivity, discharge capacity, and cycle characteristics can be obtained. Furthermore, while there is no particular upper limit, it is preferably 100,000 μg or less from the perspective of the handleability and moldability of the ion-conductive polymer. To achieve the lithium element content within the above range, it is preferable that the lithium salt content relative to the ion-conductive polymer be within the above range and that the molding conditions be within the range of the manufacturing method described below. The lithium element content can be evaluated using atomic absorption spectroscopy, which will be described later.

In addition, the electrolyte layer according to an embodiment of the present invention may contain materials such as inorganic solid electrolytes and ionic liquids in order to increase ionic conductivity and improve discharge capacity and cycle characteristics.

The all-solid-state battery according to the embodiment of the present invention preferably has a cell resistance at 25°C of 30 Ω· ^cm2 or less. More preferably, it is 25 Ω· ^cm2 or less, and even more preferably, it is 20 Ω· ^cm2 or less. By setting the cell resistance within the above range, when used as an all-solid-state battery, high ionic conductivity, excellent output characteristics and cycle characteristics can be obtained, and capacity loss during repeated use can be suppressed. In order to set the cell resistance within the above range, it is preferable to set the structure of the ion-conductive polymer, the amount of lithium salt added, molding conditions, and the like within the ranges of the manufacturing conditions described below.

Next, a method for manufacturing an all-solid-state battery according to an embodiment of the present invention will be described below.
In the all-solid-state battery according to the embodiment of the present invention, a positive electrode-side current collector layer, an electrode mixture layer containing a positive electrode active material (sometimes referred to as a positive electrode layer), an electrolyte layer, an electrode mixture layer containing a negative electrode active material (sometimes referred to as a negative electrode layer), and a negative electrode-side current collector layer are laminated in this order or in the reverse order. The ion-conductive polymer of the present invention is contained in the electrolyte layer and the electrode mixture layer, but in the electrode mixture layer, it may be contained in either the positive electrode layer or the negative electrode layer, or in both layers. The all-solid-state battery can be assembled using known battery components.

The current collector layer in the all-solid-state battery according to the embodiment of the present invention is not particularly limited, but metal foils made of, for example, gold, silver, aluminum, copper, stainless steel, nickel, titanium, alloys of these, carbon-based materials, etc. can be used.

The active material contained in the electrode mixture layer according to the embodiment of the present invention is not particularly specified, but known positive electrode active materials such as lithium metal oxides (such as lithium cobalt oxide and lithium manganese oxide) containing lithium and at least one transition metal selected from manganese, cobalt, nickel, and titanium can be used as the positive electrode active material. Furthermore, any material capable of absorbing and releasing metal ions can be used as the negative electrode active material. For example, known negative electrode active materials such as Li, Sn, Si, In, lithium alloy particles (lithium alloy particles of lithium and titanium, magnesium, aluminum, etc.), and carbon-based materials (carbon, hard carbon, soft carbon, graphite, etc.) can be used as the negative electrode active material.

Furthermore, the electrode mixture layer and the electrolyte layer may contain an inorganic solid electrolyte, a conductive aid, or the like, for the purpose of improving ionic conductivity. While neither is particularly specified, for example, known inorganic solid electrolytes can be appropriately used, such as sulfide-based solid electrolytes such as Li ₂ S—P ₂ S ₅ and Li ₇ P ₃ S ₁₁ , and oxide-based solid electrolytes such as LiI, Li ₂ O— _{B 2 O 3 —P 2 O 5 , and Li 7 La 3 Zr 2 O 12 (} LLZ). Furthermore, known conductive aids, such as carbon materials such as carbon black, acetylene black (AB), ketjen black (KB), carbon nanotubes (CNT), carbon nanofibers (CNF), and vapor-grown carbon fibers, and metal materials, can be appropriately used as the conductive aid.

Methods for obtaining polymers that can be used as ion-conductive polymers according to embodiments of the present invention will be described using aromatic polyamides, aromatic polyimides, or their precursors, polyamic acids, as examples, but the polymers that can be used and the polymerization methods thereof are not limited to these.

Various methods are available for producing aromatic polyamides. For example, when using low-temperature solution polymerization with acid dichlorides and diamines as raw materials, they are synthesized in aprotic organic polar solvents such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, dimethylformamide, and dimethyl sulfoxide. In the case of solution polymerization, to obtain high-molecular-weight polymers, the water content of the solvent used for polymerization is preferably 500 ppm or less (by mass, hereinafter), and more preferably 200 ppm or less. Furthermore, a metal salt may be added to promote polymer dissolution. Examples of such metal salts include alkali metal or alkaline earth metal halides that dissolve in aprotic organic polar solvents, such as lithium chloride, lithium bromide, sodium chloride, sodium bromide, potassium chloride, and potassium bromide. Since using equal amounts of acid dichlorides and diamines can result in the production of ultrahigh-molecular-weight polymers, it is preferable to adjust the molar ratio of one to the other so that one is 95.0 to 99.95 mol%. Furthermore, the polymerization reaction of aromatic polyamides is exothermic, and if the temperature of the polymerization system rises, side reactions may occur and the degree of polymerization may not increase sufficiently; therefore, it is preferable to cool the temperature of the solution during polymerization to 40°C or below. Furthermore, when acid dichlorides and diamines are used as raw materials, hydrogen chloride is produced as a by-product during the polymerization reaction. To neutralize this, it is recommended to use inorganic neutralizing agents such as lithium carbonate, calcium carbonate, and calcium hydroxide, or organic neutralizing agents such as ethylene oxide, propylene oxide, ammonia, triethylamine, triethanolamine, and diethanolamine.

On the other hand, when the aromatic polyimide or its precursor, polyamic acid, usable in the present invention is polymerized using, for example, a tetracarboxylic acid anhydride and an aromatic diamine as raw materials, it can be synthesized by solution polymerization in an aprotic organic polar solvent such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, dimethylformamide, or dimethyl sulfoxide. Using equal amounts of the tetracarboxylic acid anhydride and aromatic diamine raw materials can result in the production of an ultra-high molecular weight polymer, so it is preferable to adjust the molar ratio so that one is 90.0 to 99.5 mol% of the other. Furthermore, while the polymerization reaction is exothermic, if the temperature of the polymerization system rises, precipitation due to the imidization reaction may occur. Therefore, it is preferable to keep the solution temperature below 70°C during polymerization. Methods for imidizing the aromatic polyamic acid synthesized in this way to obtain an aromatic polyimide include heat treatment, chemical treatment, and a combination of these. Heat treatment generally involves heating the polyamic acid at approximately 100 to 500°C to achieve imidization. On the other hand, chemical treatments include methods that use a tertiary amine such as triethylamine as a catalyst and a dehydrating agent such as an aliphatic acid anhydride or aromatic acid anhydride, or methods that use an imidizing agent such as pyridine.

The viscosity η _inh of aromatic polyamides and aromatic polyimides, or their precursor polyamic acids, is preferably 0.5 to 7.0 dl/g. By setting the viscosity within this range, a polymer with excellent toughness and strength and good ionic conductivity can be obtained. The viscosity η can be measured, for example, by the method described below.

Next, a membrane-forming stock solution (hereinafter referred to as membrane-forming stock solution) used when producing the electrode mixture layer and the electrolyte layer according to the embodiment of the present invention will be described.
The polymer solution after polymerization may be used as is as the membrane-forming solution, but if the solution contains a large amount of unnecessary substances such as neutralization salts, it is preferable to isolate the polymer and then redissolve it in an organic solvent such as the above-mentioned aprotic organic polar solvent or sulfuric acid before use. The method for isolating the polymer is not particularly limited, but examples include a method in which the polymer solution after polymerization is poured into a large amount of water to extract the solvent and neutralization salts into the water, and then the precipitated polymer is separated and dried.

In the manufacturing process for the electrode mixture layer and electrolyte layer according to embodiments of the present invention, it is preferable to add an active material, inorganic solid electrolyte, conductive aid, lithium salt, etc. to the ion-conductive polymer. There are no limitations on the timing of adding these materials, and they may be added during the polymer polymerization process, the membrane-forming solution preparation process, or the membrane-forming process. However, it is preferable to add them during the membrane-forming solution preparation process, as this allows them to be uniformly dispersed with the polymer. They may also be added in multiple processes, or in multiple batches within the same process.

The concentration of the ion-conductive polymer in the film-forming solution is preferably 3 to 30% by mass, more preferably 4 to 20% by mass. Inorganic or organic particles may be added to the film-forming solution to improve the strength, heat resistance, and ion permeability of the resulting polymer film, reduce the static friction coefficient, or otherwise, as long as the effects of the present invention are not impaired. Examples of inorganic particles include wet and dry silica, colloidal silica, aluminum silicate, titanium oxide, calcium carbonate, calcium phosphate, barium sulfate, alumina, aluminum hydroxide, magnesium hydroxide, magnesium carbonate, zinc carbonate, titanium oxide, zinc oxide (zinc oxide), antimony oxide, cerium oxide, zirconium oxide, tin oxide, lanthanum oxide, magnesium oxide, barium carbonate, zinc carbonate, basic lead carbonate (white lead), barium sulfate, calcium sulfate, lead sulfate, zinc sulfide, mica, titanium mica, talc, clay, kaolin, lithium fluoride, and calcium fluoride. Examples of organic particles include particles obtained by crosslinking a polymer compound with a crosslinking agent. Examples of such crosslinked particles include crosslinked particles of polymethoxysilane compounds, crosslinked particles of polystyrene compounds, crosslinked particles of acrylic compounds, crosslinked particles of polyurethane compounds, crosslinked particles of polyester compounds, crosslinked particles of fluorine compounds, and mixtures thereof.

The film-forming solution obtained by isolating and redissolving the polymer from the polymerized polymer solution as described above can be used to form a film using the so-called solution film-forming method. Solution film-forming methods include dry-wet, dry, and wet methods. However, the dry method is preferred from the perspective of protecting the functionality of the active material in the electrode mixture layer and the lithium salt in the electrolyte layer. Furthermore, any of the following methods can be suitably used to manufacture the electrode mixture layer and electrolyte layer: a method in which the active material of the electrode mixture and the film-forming solution are mixed and applied to a current collector and dried, followed by application of the film-forming solution that will become the electrolyte layer, drying, and molding; a method in which the active material of the electrode mixture is mixed with the film-forming solution and applied to a current collector, followed by layer separation and drying, followed by processing by integral molding; or a method in which an ion-conductive polymer solution dissolved in an organic solvent is applied to the electrode mixture layer and allowed to penetrate.

Here, we will explain a method in which the active material for the electrode mixture layer and a film-forming source solution are mixed, applied to a current collector, and dried, and then the film-forming source solution for the electrolyte layer is applied on top of it, dried, and molded.
A film-forming solution containing an ion-conductive polymer, a positive or negative electrode active material, an inorganic solid electrolyte, a conductive additive, and the like is extruded onto a metal foil serving as a current collector using a die or the like to form a film, which is then dried to obtain a positive or negative electrode mixture layer. Drying conditions can be, for example, 50 to 220°C and 120 minutes or less. However, if polyamic acid is used as the ion-conductive polymer and a film made of polyamic acid is to be obtained without imidization, the drying temperature is preferably 50 to 150°C. A temperature of 50 to 130°C under reduced pressure is more preferably used. Next, a film-forming solution containing an ion-conductive polymer, a lithium salt, an inorganic solid electrolyte, and the like is applied over the formed electrode mixture layer and further dried to form an electrolyte layer, thereby obtaining a laminate. The drying temperature during electrolyte layer formation is within the above-mentioned range, as with the electrode mixture layer. If necessary, the laminate may also be heat-treated at a temperature of 80°C to 500°C, preferably 100°C to 300°C, for several seconds to several tens of minutes. However, when polyamic acid is used as the ion-conductive polymer and a membrane made of polyamic acid is to be obtained without imidization, the heat treatment temperature is preferably 80 to 150°C, more preferably 80 to 130°C under reduced pressure.

The thickness of the electrolyte layer according to an embodiment of the present invention is not particularly specified, but is preferably 0.05 to 30 μm, more preferably 0.10 to 20 μm, and even more preferably 0.20 to 15 μm. By keeping the thickness within the above range, the polymer film has sufficient strength, excellent flexibility and dendrite resistance, and does not increase in resistance due to the film thickness, making it suitable for use. The thickness of the electrolyte layer can be controlled by various conditions, such as the concentration of the membrane-forming solution, the viscosity of the membrane-forming solution, the type and concentration of additives in the membrane-forming solution, the casting thickness of the polymer film, the heat treatment temperature, and the stretching conditions.

The thickness of the electrode mixture layer according to the embodiment of the present invention is not particularly specified, but is preferably 1 to 1500 μm, more preferably 5 to 1000 μm, and even more preferably 10 to 500 μm. By maintaining the thickness within the above range, sufficient energy density can be maintained. The thickness of the electrode mixture layer can be controlled by various conditions, such as the concentration of the membrane-forming solution, the viscosity of the membrane-forming solution, the type and concentration of additives in the membrane-forming solution, the casting thickness of the polymer film, the heat treatment temperature, and the stretching conditions.

An all-solid-state battery can be manufactured by stacking, for example, a separately molded laminate consisting of another electrode mixture layer/another current collector layer on top of the thus obtained laminate consisting of a current collector layer/electrode mixture layer/electrolyte layer.

When the ion-conducting polymer according to the embodiment of the present invention is used in the electrode mixture layer or electrolyte layer, excellent properties can be obtained, such as suppressing short circuits caused by dendrites. It is also expected to prevent degradation of the active material contained in the electrode mixture layer, and to reduce the size and increase the capacity of the battery by reducing the thickness.

The all-solid-state battery according to an embodiment of the present invention can be suitably used as a power source for small electronic devices, transportation such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), and large industrial equipment such as industrial cranes. It can also be suitably used as a power storage device for leveling power in solar cells, wind power generation equipment, and the like, and for smart grids. It can also be suitably used as a battery for use in special environments such as space.

The present invention will be explained in more detail below with reference to the following examples. The physical properties of the examples were measured using the following methods.

(1) Logarithmic viscosity η _inh
A polymer is dissolved in N-methylpyrrolidone (NMP) containing 2.5% by mass of lithium bromide (LiBr) at a concentration of 0.5 g/dl, and the flow time is measured using an Ubbelohde viscometer at 30° C. The flow time of a blank LiBr 2.5% by mass/NMP solution in which no polymer is dissolved is also measured in the same manner, and the logarithmic viscosity η _inh (dl/g) can be calculated using the following formula.

Logarithmic viscosity η _inh (dl/g) = [ln (t/t ₀ )]/0.5
t ₀ : Blank flow time (seconds)
t: sample flow time (seconds)

(2) Softening point of ion-conductive polymer (°C)
The softening point of the ion-conductive polymer was determined from the inflection point of the storage modulus (E') by dynamic mechanical analysis (DMA) in accordance with ASTM E1640-13. DMA was performed using the following apparatus and conditions.
The softening points of the ion-conductive polymers used in the electrolyte layers were measured by peeling off the positive electrode layer and the positive electrode current collector layer from the laminates each consisting of a positive electrode current collector layer/positive electrode layer/electrolyte layer obtained in each Example and Comparative Example. The softening points of the ion-conductive polymers used in the positive electrode layers were measured by peeling off the positive electrode current collector layer and the electrolyte layer from the laminates.
Apparatus: Viscoelasticity measuring device DMS6100 (manufactured by Seiko Instruments Inc.)
Measurement mode: Tensile mode Measurement frequency: 1 Hz
Temperature rise rate: 5°C/min Temperature range: 25°C to 400°C
Holding time: 2 minutes

(3) Surface gloss (60°)
The gloss (60°) of the electrolyte layer surface was determined in accordance with JIS Z8741 (1997).
As a sample, a laminate consisting of a positive electrode current collector layer/positive electrode layer/electrolyte layer obtained in each of the examples and comparative examples was used, and the surface on the electrolyte layer side was measured.
Equipment: Suga Testing Instruments Digital Variable Goniophotometer UVG-5D
Measurement angle: 60° for both incident and receiving light

(4) Cell resistance (Ω·cm ² )
An all-solid-state battery cell was prepared, and AC impedance was measured under conditions of a 25°C atmosphere, a voltage amplitude of 10 mV, and a frequency of 10 Hz to 5,000 kHz. The cell resistance (Ω) was calculated from the Cole-Cole plot and normalized by the measurement area. The cell was prepared in an argon-filled glove box by laminating the negative electrode current collector (copper foil), the negative electrode layer (metallic lithium), and the laminate (electrolyte layer/positive electrode layer/positive electrode current collector layer) obtained in each example and comparative example in this order, and then using a manual press to obtain an all-solid-state battery cell.

(5) Li element content (μg/g)
The amount of Li element contained per 1 g of polymer was determined using an atomic absorption analyzer. 0.1 g of sample was weighed out, sulfuric acid was added, and the mixture was heated to carbonize, followed by heating to incineration. The incinerated product was thermally decomposed with sulfuric acid and hydrofluoric acid, and dissolved in dilute nitric acid by heating to a constant volume. The Li element content of this solution was measured using atomic absorption spectrometry to determine the content in the sample. As a sample, only the electrolyte layer was peeled off from the laminate consisting of a positive electrode current collector layer/positive electrode layer/electrolyte layer obtained in each example and comparative example, and then measured.
Apparatus: Atomic absorption spectrophotometer Z-2300 manufactured by Hitachi High-Technologies

(6) Dendrite Resistance Dendrite resistance was evaluated when metallic Li was used for the negative electrode. In metallic Li negative electrodes, Li ions precipitate on the metal surface during charging and dissolve during discharging. Li dendrites are formed when the reaction on the metallic Li surface is uneven, resulting in rapid Li deposition in areas with low resistance. When dendrites penetrate the electrolyte layer, fractures are observed in the electrolyte membrane. Therefore, dendrite resistance was evaluated using the following method.

First, a cell was prepared in the same manner as in (4), and charged for a total of 48 hours using a charge/discharge device (manufactured by Hokuto Denko Corporation) by constant current charging at 0.05 C up to 4.15 V and constant voltage charging at 4.15 V. After 48 hours of charging, the electrolyte layer was removed from the evaluation cell and its condition was observed, and the dendrite resistance was evaluated according to the following criteria.
No electrolyte layer rupture: 〇 Electrolyte layer rupture: ×

(6) Flexibility Using a mandrel conforming to JIS-K5600-5-1 (1999), the laminates each composed of a positive electrode current collector layer/positive electrode layer/electrolyte layer obtained in each Example and Comparative Example were bent and left to stand in an atmosphere at 25° C. At this time, the laminates were bent so that the electrolyte layer was on the outside.
Sample dimensions: short side 50 mm x long side 100 mm
Sample placement: Placed so that the folding line (cylinder contact part) is at 50 mm in the long side direction. Cylinder dimensions: Radius 2 mm
Test time: 72 hours Evaluation criteria: After the test, the surface of the electrolyte layer was observed and evaluated according to the following criteria.
○: No waviness, cracks, or breakage was observed, and the product was good. △: Waviness was observed, but within the practical range. ×: Cracks or breakage was observed, and the product was outside the practical range.

(Reference Example 1) Polymer solution P-1
4,4'-Diaminodiphenyl ether (manufactured by Tokyo Chemical Industry Co., Ltd.) as a diamine was dissolved in dehydrated NMP (N-methyl-2-pyrrolidone, manufactured by Mitsubishi Chemical Corporation) under a nitrogen stream and cooled to below 30°C. To this solution, 2-chloroterephthaloyl chloride (manufactured by Nippon Light Metal Co., Ltd.) equivalent to 99 mol% of the total amount of diamine was added over 30 min under a nitrogen stream while maintaining the system at below 30°C. After the entire amount was added, the mixture was stirred for approximately 2 hours to polymerize aromatic polyamide (P-1). The resulting polymerization solution was neutralized with 97 mol% of lithium carbonate (manufactured by Honjo Chemical Co., Ltd.) and 6 mol% of diethanolamine (manufactured by Tokyo Chemical Industry Co., Ltd.) based on the total amount of acid chloride, to obtain a polymer solution. The viscosity η _inh of the resulting polymer was 2.5 dl/g. Next, this polymer solution was poured into purified water in a mass ratio of 10 times or more, the solvent and neutralization salt were extracted into water, and only the precipitated polymer was separated and then dried at 80° C. for 10 hours to obtain a polymer powder. Thereafter, the polymer was redissolved in dehydrated NMP (manufactured by Mitsubishi Chemical Corporation) so that the polymer concentration was 8 mass %, to obtain polymer solution P-1.

(Reference Example 2) Polymer solution P-2
A polymer solution P-2 was obtained in the same manner as in Reference Example 1, except that 4,4'-diaminodiphenyl ether was replaced with 1,4-bis(4-aminophenoxy)benzene (manufactured by Tokyo Chemical Industry Co., Ltd.) as the diamine. The viscosity η _inh of the obtained polymer was 2.5 dl/g.

(Reference Example 3) Polymer solution P-3
A polymer solution P-3 was obtained in the same manner as in Reference Example 1, except that 4,4'-thiodianiline (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the diamine instead of 4,4'-diaminodiphenyl ether. The viscosity η _inh of the obtained polymer was 1.6 dl/g.

(Reference Example 4) Polymer solution P-4
A polymer solution P-4 was obtained in the same manner as in Reference Example 1, except that 2-chloroterephthaloyl chloride was changed to 2-phenoxyterephthaloyl chloride (manufactured by Ihara Nikkei Chemical Industry Co., Ltd.) The viscosity η _inh of the obtained polymer was 1.5 dl/g.

Example 1
The polymer solution P-1 obtained in Reference Example 1 was used as a polymer solution for the electrolyte layer, and was applied to the surface of the positive electrode layer side of a positive electrode sheet (manufactured by Hosen Co., Ltd.) using lithium cobalt oxide (LiCoO ₂ ) as the active material and polyvinylidene fluoride (PVDF) as the ion-conductive polymer, 40 μm thick, so that the thickness after drying would be 5 μm. The sheet was then vacuum dried at 120°C for 60 minutes to obtain a laminate consisting of a positive electrode current collector layer/positive electrode layer/electrolyte layer.
The evaluation results of the obtained laminate are shown in Table 1. The dendrite resistance and flexibility were good.

Example 2
The polymer solution P-1 obtained in Reference Example 1 was mixed with LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) at a mass ratio of 95:5, and the mixture was stirred and degassed using a mixer (manufactured by THINKY Corporation, model number: AR-250) to obtain a polymer solution for an electrolyte layer. The obtained polymer solution was applied to the surface of the positive electrode layer side of a 40 μm-thick positive electrode sheet (manufactured by Hosen Co., Ltd.) using lithium cobalt oxide (LiCoO ₂ ) as the active material and PVDF as the ion-conductive polymer, so that the thickness after drying would be 5 μm. The resulting solution was then vacuum dried at 120° C. for 60 minutes to obtain a laminate of a positive electrode current collector layer/positive electrode layer/electrolyte layer. The evaluation results of the obtained laminate, which was evaluated as described above, are shown in Table 1. The cell resistance, flexibility, and dendrite resistance were good.

Example 3
Lithium cobalt oxide and carbon black were blended with the polymer solution P-1 obtained in Reference Example 1 so that the ratio of polymer, lithium cobalt oxide, and carbon black was 5:92:3 by mass, and the mixture was stirred and degassed using a mixer (THINKY Corporation, model number: AR-250) to obtain a polymer solution for the positive electrode layer. The obtained polymer solution for the positive electrode layer was applied to a 10 μm thick Al foil so that the thickness after drying was 30 μm, and the resulting solution was vacuum dried at 120 ° C for 60 minutes to obtain a positive electrode sheet. A laminate of a current collector layer for the electrode/positive electrode layer/electrolyte layer was obtained in the same manner as in Example 2, except that the obtained positive electrode sheet was used. The evaluation results of the above evaluations performed using the obtained laminate are shown in Table 1. Cell resistance was improved.

(Examples 4 to 6)
A laminate of a positive electrode current collector layer/positive electrode layer/electrolyte layer was obtained in the same manner as in Example 3, except that the ratio of LiTFSI in the electrolyte layer was changed as shown in Table 1. The above evaluations were performed using the obtained laminate, and the results are shown in Table 1. Even when the ratio of polymer to LiTFSI in the electrolyte layer was changed, the cell resistance, flexibility, and dendrite resistance were all good.

Examples 7 to 9
A laminate was obtained in the same manner as in Example 4, except that the type of polymer solution was changed as shown in Table 1.

Example 10
A laminate was obtained in the same manner as in Example 3, except that the _oxide -based inorganic solid electrolyte _Li7La3Zr2O12 ( _LLZ ) was added to the polymer solution for _the electrolyte layer as shown in Table 1. A laminate consisting of a positive electrode current collector layer/positive electrode layer/electrolyte layer was obtained. The above evaluations were performed using the obtained laminate, and the results are shown in Table 1. Even when an oxide-based inorganic solid electrolyte was added, the cell resistance, flexibility, and dendrite resistance were all good.

Example 11
A laminate consisting of a positive electrode current collector layer/positive electrode layer/electrolyte layer was obtained in the same manner as in Example 10, except that the compounding ratio of the polymer, lithium salt, and inorganic solid electrolyte in the polymer solution for the electrolyte layer was changed as shown in Table 1. The above evaluations were carried out using the obtained laminate, and the results are shown in Table 1. Although the flexibility was slightly inferior compared to the results of Example 10 due to the large amount of oxide-based inorganic solid electrolyte added, both the cell resistance and dendrite resistance were good.

(Comparative Example 1)
Polyethylene oxide E-45 (PEO) (manufactured by Meisei Chemical Industry Co., Ltd.) was dissolved in acetonitrile (manufactured by Tokyo Chemical Industry Co., Ltd.) at 60°C to obtain a 10% by mass solution, which was used to obtain a polymer for the electrolyte layer. This was applied to the surface of the positive electrode layer side of a 40 μm-thick positive electrode sheet (manufactured by Hosen Co., Ltd.) that used lithium cobalt oxide (LiCoO ₂ ) as the active material and PVDF as the ion-conductive polymer, so that the thickness after drying would be 5 μm, and the resulting solution was vacuum dried at 80°C for 30 minutes to obtain a laminate of a positive electrode current collector layer/positive electrode layer/electrolyte layer. The evaluation results of the obtained laminate are shown in Table 1. Although the dendrite resistance and flexibility were good, the softening point of the ion-conductive polymer contained in the electrolyte layer was low, resulting in poor heat resistance, and the laminate was therefore unsuccessful.

(Comparative Example 2)
An electrolyte sheet was compression-molded to a thickness of 10 μm using only the oxide-based inorganic solid electrolyte LLZ as the electrolyte layer, and the resulting sheet was placed on the surface of the positive electrode layer of a 40 μm-thick positive electrode sheet (manufactured by Hosen Co., Ltd.) using lithium cobalt oxide (LiCoO ₂ ) as the active material and PVDF as the ion-conductive polymer, and press-molded to obtain a positive electrode current collector layer/positive electrode layer/electrolyte layer laminate. The evaluation results for the obtained laminate, which were evaluated as described above, are shown in Table 1. It was rejected due to poor dendrite resistance and flexibility.

Claims (6)

An all-solid-state battery having a current collector layer, an electrode mixture layer, and an electrolyte layer, wherein the electrolyte layer is provided on at least one surface layer of the electrode mixture layer, the surface gloss (60°) of the electrolyte layer is 10 or more and 300 or less, the electrode mixture layer and the electrolyte layer are layers containing an ion-conductive polymer, and the softening point of the ion-conductive polymer is 80°C or higher. The all-solid-state battery according to claim 1, wherein the electrolyte layer and the electrode mixture layer contain the same ion-conducting polymer. 3. The all-solid-state battery according to claim 1, wherein the cell resistance at 25°C is 30 Ω·cm ² or less. 4. The all-solid-state battery according to claim 1 , wherein the electrolyte layer contains 5 mass % or more of a Li salt relative to the total mass of the ion-conductive polymer in the electrolyte layer. 5. The all-solid-state battery according to claim 1 , wherein the electrolyte layer contains 50 μg or more of Li element per 1 g of the ion-conducting polymer in the electrolyte layer. The all-solid-state battery according to any one of claims 1 to 5 , wherein the ion-conducting polymer comprises an aromatic polyamide.