Solid secondary battery

Abstract

To provide a solid secondary battery that can be manufactured without applying external pressure.SOLUTION: A solid secondary battery includes a positive electrode layer containing lithium, a negative electrode layer containing lithium and silver, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer, and the negative electrode layer includes a coating layer, a lithium alloy layer, and a negative electrode current collector, and the bonding rate at an interface between the coating layer and the lithium alloy layer is 85% or more, and the total number of moles of lithium contained in the positive electrode layer and the negative electrode layer is 30 to 120 times the total number of moles of silver contained in the solid secondary battery.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a solid secondary battery. [Background technology]

[0002] The capacity density of lithium (capacity per unit mass) is approximately 10 times higher than that of graphite, which has traditionally been used as a negative electrode active material. Therefore, thin, high-output all-solid-state lithium secondary batteries that use lithium as the negative electrode active material have been proposed.

[0003] In such all-solid-state lithium secondary batteries, lithium deposited in the negative electrode layer during charging becomes lithium ions and dissolves during discharging, creating voids in the negative electrode layer. This can lead to damage to the negative electrode layer due to repeated charging and discharging, resulting in a significant decrease in battery performance.

[0004] Therefore, in the solid-state lithium-ion secondary battery described in Patent Document 1, a solid-state secondary battery has been developed in which the film strength of the negative electrode active material layer is set to 50 MPa or more and 250 MPa or less, thereby allowing lithium to be deposited between the negative electrode active material layer and the negative electrode current collector during battery operation. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-109033 Summary of the Invention [Problem to be solved by the invention]

[0006] According to the solid-state secondary battery as described above, end plates or the like for applying external pressure during battery operation are not required. Furthermore, if the pressurizing step in the battery manufacturing stage could also be eliminated, not only would the degree of freedom in battery size and shape be significantly improved by the pressurizing jig used in the pressurizing step, but omitting the pressurizing step would also be expected to result in a significant reduction in manufacturing costs.

[0007] The present invention has been made in view of the above-mentioned problems, and has an object to provide a solid secondary battery that can be manufactured without applying external pressure as much as possible. [Means for solving the problem]

[0008] That is, the solid secondary battery and the method for manufacturing the same according to the present invention are as follows. [1] A positive electrode layer containing lithium; a negative electrode layer containing lithium and silver; a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer, the negative electrode layer includes a coating layer, a lithium alloy layer, and a negative electrode current collector, the bonding rate of the interface between the coating layer and the lithium alloy layer is 85% or more; A solid-state secondary battery in which the total number of moles of lithium contained in the solid-state secondary battery is 30 times or more and 120 times or less the total number of moles of silver contained in the solid-state secondary battery. [2] The solid secondary battery according to [1], wherein the coating layer contains a binder, and the content of the binder relative to 100% by mass of the coating layer is 1% by mass or more and 5% by mass or less. [3] The solid secondary battery according to any one of [1] and [2], wherein the coating layer has a thickness of 4 μm or less. [4] The solid secondary battery according to [2], wherein the binder is a water-based binder. [5] The solid secondary battery according to [4], wherein the binder is a water-soluble acrylic acid-acrylonitrile copolymer. [6] The solid secondary battery according to any one of [1] to [5], wherein the solid electrolyte layer contains an oxide-based solid electrolyte. [7] The solid secondary battery according to any one of [1] to [6], wherein the positive electrode layer contains a liquid electrolyte. [8] A method for producing a solid electrolyte layer-negative electrode layer composite including a solid electrolyte layer and a negative electrode layer, a coating step of coating a negative electrode slurry on the solid electrolyte to form a coating layer; and a current-passing step of laminating a lithium source on a surface of the solid electrolyte layer opposite to the surface on which the coating layer is formed, and then passing current therethrough to deposit a lithium alloy layer in the negative electrode layer. [9] A method for producing a solid secondary battery by further laminating a positive electrode layer on the solid electrolyte layer-negative electrode layer composite produced by the method for producing a solid electrolyte layer-negative electrode layer composite according to [8], the lithium source is a member different from the positive electrode layer, the lithium source is removed after the energizing step, and the positive electrode layer is laminated on the solid electrolyte layer.

[10] A method for charging the solid secondary battery according to any one of [1] to [7], A method for charging a solid secondary battery, wherein the amount of change in thickness of the lithium alloy layer during charging is 10 μm or more and 60 μm or less. [Effects of the Invention]

[0009] According to the solid secondary battery of the present invention, the pressurizing step can be omitted during manufacturing, which not only significantly improves the degree of freedom in battery size and shape, but also significantly reduces manufacturing costs by omitting the pressurizing step. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view showing a schematic configuration of a solid secondary battery according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant explanations will be omitted.

[0012] <1. Configuration of the Solid State Secondary Battery According to the Present Embodiment> The solid secondary battery according to this embodiment is a solid secondary battery 1 including a positive electrode layer 10, a negative electrode layer 20, and a solid electrolyte layer 30, as shown in FIG.

[0013] (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 a plate-shaped or foil-shaped body 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 an alloy thereof. The positive electrode current collector 11 may be omitted.

[0014] The positive electrode active material layer 12 includes a positive electrode active material and a solid electrolyte. The solid electrolyte included in the positive electrode layer 12 may be an oxide-based solid electrolyte or a sulfide-based solid electrolyte, which will be described in the section on the solid electrolyte layer 30.

[0015] The positive electrode active material may be any positive electrode active material that can reversibly store and release lithium ions.

[0016] 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 aluminum oxide (hereinafter referred to as NCA), lithium nickel cobalt manganese oxide (hereinafter referred to as NCM), lithium manganate, lithium iron phosphate, nickel sulfide, copper sulfide, lithium sulfide, sulfur, iron oxide, vanadium oxide, etc. These positive electrode active materials may be used alone or in combination of two or more.

[0017] Further, 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 above-described lithium salts. Here, the "layered rock salt structure" is a structure in which oxygen atom layers and metal atom layers are alternately and regularly arranged in the <111> direction of the cubic rock salt structure, and as a result, each atom layer forms a two-dimensional plane. The "cubic rock salt structure" represents a sodium chloride type structure, which is a type of crystal structure. Specifically, it represents a structure in which the face-centered cubic lattices formed by each of the cations and anions are displaced from each other by 1 / 2 of the edge of the unit lattice.

[0018] Examples of the lithium salt of the transition metal oxide having such a layered rock salt structure include, for example, LiNi x Co y Al z O2 (NCA), or LiNi x Co y Mn z O2 (NCM) (where 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1), and lithium salts of ternary transition metal oxides such as these can be mentioned.

[0019] When the positive electrode active material includes the lithium salt of the ternary transition metal oxide having the above-described layered rock salt structure, the energy density and thermal stability of the solid secondary battery 1 can be improved.

[0020] The positive electrode active material may be covered with a coating layer. Here, the coating layer of the present embodiment may be any known coating layer for the positive electrode active material of the solid secondary battery. Examples of the coating layer include, for example, Li2O-ZrO2 and the like.

[0021] [[ID=二十八]]Further, when the positive electrode active material is formed of a lithium salt of a ternary transition metal oxide such as NCA or NCM and includes nickel (Ni) as the positive electrode active material, the capacity density of the solid secondary battery 1 can be increased, and metal elution from the positive electrode active material in the charged state can be reduced. As a result, the solid secondary battery 1 according to the present embodiment can improve the long-term reliability and cycle characteristics in the charged state.

[0022] Here, examples of the shape of the positive electrode active material include particle shapes such as spherical and oval spheres. The particle size of the positive electrode active material is not particularly limited, provided that it is within a range applicable to positive electrode active materials in conventional solid-state secondary batteries. The content of the positive electrode active material in the positive electrode layer 10 is also not particularly limited, provided that it is within a range applicable to positive electrode layers in conventional solid-state secondary batteries.

[0023] In addition to the above-described positive electrode active material and solid electrolyte, the positive electrode layer 10 may contain additives such as a conductive aid, a binder, a filler, a dispersant, an ion conductive aid, or the like, as appropriate.

[0024] Examples of conductive additives that can be blended into the positive electrode layer 10 include graphite, carbon black, acetylene black, ketjen black, carbon fiber, and metal powder. Examples of binders that can be blended into the positive electrode layer 10 include styrene butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene. Furthermore, known materials generally used in electrodes for solid secondary batteries can be used as fillers, dispersants, ion conductive additives, and the like that can be blended into the positive electrode layer 10.

[0025] The positive electrode layer 10 may further contain a liquid electrolyte in addition to the above-mentioned components. A specific example of the positive electrode layer 10 containing a liquid electrolyte is one in which the positive electrode active material layer 12 is impregnated with the liquid electrolyte.

[0026] The liquid electrolyte may contain a lithium salt and one or more of an ionic liquid and a polymeric ionic liquid. The liquid electrolyte is preferably non-volatile.

[0027] An ionic liquid has a melting point below room temperature and refers to a salt that is composed only of ions and is in a liquid state at room temperature or a room-temperature molten salt. The ionic liquid comprises A) one or more cations selected from ammonium, pyrrolidium, pyridinium, pyrimidinium, imidazolium, piperidinium, pyrazolium, oxazolium, pyridazinium, phosphonium, sulfonium, triazolium, and mixtures thereof; and B) BF4 - , PF6 - , AsF6 - , SbF6 - , AlCl4 - , HSO4 - , ClO4 - , CH3SO3 - , CF3CO2 - , Cl - , Br - , I - , SO4 - , CF3SO3 - , (FSO2)2N - , (C2F5SO2)2N - , (C2F5SO2)(CF3SO2)N - , and (CF3SO2)2N - and one or more anions selected from the above.

[0028] Specific examples of ionic liquids include N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide, N-propyl-N-methylpyrrolidinium bis(3-trifluoromethylsulfonyl)imide, N-butyl-N-methylpyrrolidinium bis(3-trifluoromethylsulfonyl)imide, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide, and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide.

[0029] The polymeric ionic liquid comprises A) one or more cations selected from ammonium, pyrrolidium, pyridinium, pyrimidinium, imidazolium, piperidinium, pyrazolium, oxazolium, pyridazinium, phosphonium, sulfonium, triazolium, and mixtures thereof; and B) BF4 - , PF6 - , AsF6 - , SbF6 - , AlCl4 - , HSO4 - , ClO4 - , CH3SO3 - , CF3CO2 - , (CF3SO2)2N - , (FSO2)2N - , Cl - , Br - , I - , SO4 - , CF3SO3 - , (C2F5SO2)2N - , (C2F5SO2)(CF3SO2)N - , NO3 - , Al2Cl7 - , (CF3SO2)3C - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , SF5CF2SO3 - , SF5CHFCF2SO3 - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (O(CF3)2C2(CF3)2O)2PO - and one or more anions selected from the group consisting of:

[0030] Any lithium salt that is used in the art can be used. Examples of the lithium salt include LiPF, LiBF, LiSbF, LiAsF, LiClO, LiCF, SO, Li(CF, SO)N, Li(FSO)N, LiCF, SO, LiAlO, LiAlCl, and LiN(CxF 2x+1 SO2)(CyF 2y+1 SO2) (where x and y are natural numbers), LiCl, LiI, or a mixture thereof.

[0031] The concentrations of the lithium salt, ionic liquid, and polymeric ionic liquid contained in the liquid electrolyte are preferably 0.1M to 5M.

[0032] The amount of liquid electrolyte impregnated into the positive electrode active material layer 12 may be, when the mass of the positive electrode active material layer 12 not including the liquid electrolyte is taken as 100 parts by mass, more than 0 parts by mass but not more than ...50 parts by mass, more than 0 parts by mass but not more than 30 parts by mass, more than 0 parts by mass but not more than 20 parts by mass, more than 0 parts by mass but not more than 10 parts by mass, or more than 0 parts by mass but not more than 5 parts by mass, etc.

[0033] (1-2.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.

[0034] The solid electrolyte contains, for example, a sulfide-based solid electrolyte material and / or an oxide-based solid electrolyte. Examples of sulfide-based solid electrolyte materials include Li2S-P2S5, Li2S-P2S5-LiX (X is a halogen element, such as I or Cl), Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, and Li2 S -SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-Z m S n(m and n are positive numbers, Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li p MO q (p and q are positive numbers, and M is any of P, Si, Ge, B, Al, Ga, and In).

[0035] Examples of oxide-based solid electrolytes include garnet-type composite oxides, perovskite-type oxides, LISICON-type composite oxides, NASICON-type composite oxides, Li-alumina-type composite oxides, LiPON, and oxide glass. Among these oxide-based solid electrolytes, it is preferable to select an oxide-based solid electrolyte that can be stably used with lithium metal.

[0036] As the oxide-based solid electrolyte as described above, for example, Li 1.3 Al 0.3 Ti 1.7 (PO4)3, Li 0.34 La 0.51 TiO 2.94 , Li 1.07 Al 0.69 Ti 1.46 (PO4)3, 50Li4SiO4-50Li2BO3, 90Li3BO3-10Li2SO 4、 Li 2.9 PO 3.3 N 0.46 , Li7La3Zr2O 12、 Li 3.6 Si 0.6 P 0.4 O4, Li 1.5 Al 10.5 Ge 1.5 (PO4)3 and the like.

[0037] The solid electrolyte layer 30 may further contain a binder, and examples of the binder include styrene butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, etc. The binder in the solid electrolyte layer 30 may be the same as or different from the binder in the positive electrode active material layer 12 and the negative electrode layer 20.

[0038] In this embodiment, the solid electrolyte layer 30 contains only the above-mentioned oxide-based solid electrolyte as the solid electrolyte. More specifically, the solid electrolyte layer 30 of this embodiment is made of only the oxide-based solid electrolyte.

[0039] (1-3. Negative electrode layer) The negative electrode layer 20 includes a negative electrode current collector 21 and a negative electrode active material layer laminated on the negative electrode current collector 21 .

[0040] The negative electrode current collector 21 is preferably made of a material that does not react with lithium, i.e., that does not form any alloy or compound. Examples of materials that make up 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 made of any one of these metals, or may be made of an alloy or clad material of two or more metals. The negative electrode current collector 21 is preferably in the form of, for example, a plate or foil.

[0041] In this embodiment, the negative electrode active material layer includes a coating layer 22 and a lithium alloy layer 23 .

[0042] The coating layer 22 contains a negative electrode active material, a carbon material, and a binder. Examples of the negative electrode active material include alloy-forming elements that form alloys or compounds with lithium through an electrochemical reaction during charging. The alloying element may be at least one selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, and zinc.

[0043] When one or more of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, and zinc are used as the alloying element, the negative electrode active material is, for example, granular, and the particle size is preferably 4 μm or less, more preferably 300 nm or less. In this case, the characteristics of the solid secondary battery 1 are further improved. Here, the particle size of the negative electrode active material is measured, for example, using a laser particle size distribution system, as the median diameter (so-called D 50 ) can be used.

[0044] Examples of the carbon material include carbon black, graphene, etc. Examples of carbon black include acetylene black, furnace black, ketjen black, etc.

[0045] As the binder, a wide variety of binders used in solid secondary batteries can be used, but it is preferable that the binder does not react with the negative electrode active material or the solid electrolyte, and it is more preferable to use an aqueous binder that can be dispersed in an aqueous solvent. A specific example of the water-based binder is a water-soluble acrylic acid-acrylonitrile copolymer. The water-soluble acrylic acid-acrylonitrile copolymer contains a water-soluble copolymer A containing units derived from a (meth)acrylic acid monomer and units derived from a (meth)acrylonitrile monomer, and a water-soluble copolymer B containing units derived from a monomer represented by the following (Chemical Formula 1).

[0046] [ka] (In the formula, R1 is a hydrogen atom or a methyl group. R2 is a C(=0)-O- group or a C(=0)-NH- group. R3 is an alkyl group having 1 to 4 carbon atoms or a functional group represented by the following (Chemical Formula 2).)

[0047] [ka] (wherein n is an integer of 1 or more).

[0048] The water-soluble acrylic acid-acrylonitrile copolymer preferably contains, for example, units derived from acrylic acid monomers in the range of 40% by mass to 70% by mass.

[0049] The water-soluble acrylic acid-acrylonitrile copolymer is preferably a copolymer of 40% by mass or more and 70% by mass or less of a (meth)acrylic acid monomer, 30% by mass or more and 60% by mass or less of (meth)acrylonitrile, and 0% by mass or more and 30% by mass or less of another monomer copolymerizable with the (meth)acrylic acid monomer and (meth)acrylonitrile.

[0050] If the content of structural units derived from (meth)acrylic acid monomers in the water-soluble acrylic acid-acrylonitrile copolymer is 40% by mass or more, the water-soluble copolymer A becomes more soluble in water, and the dispersibility of the negative electrode active material and the storage stability of the slurry can be further improved, which is preferable. Also, if the content of structural units derived from (meth)acrylic acid monomers in the water-soluble acrylic acid-acrylonitrile copolymer is 70% by mass or less, the occurrence of cracks in the coating layer 22 during the application and drying steps of the slurry can be further suppressed, which is preferable.

[0051] It is preferable that the content of structural units derived from (meth)acrylonitrile in the water-soluble acrylic acid-acrylonitrile copolymer is 30% by mass or more, since this further improves the adhesion of the coating layer 22 to the solid electrolyte layer 30. It is also preferable that the content of structural units derived from (meth)acrylonitrile in the water-soluble acrylic acid-acrylonitrile copolymer is 60% by mass or less, since this makes the water-soluble acrylic acid-acrylonitrile copolymer more soluble in water, which further improves the dispersibility of the negative electrode active material and the storage stability of the slurry.

[0052] It is preferable that the content of structural units derived from (meth)acrylic acid monomers and other monomers copolymerizable with (meth)acrylonitrile in the water-soluble acrylic acid-acrylonitrile copolymer is 30 mass% or less, since this can further suppress electrode swelling of the negative electrode layer 20.

[0053] The (meth)acrylic acid-based monomer preferably contains at least one selected from the group consisting of (meth)acrylic acid, metal salts of (meth)acrylic acid, ammonium salts of (meth)acrylic acid, and amine salts of (meth)acrylic acid.

[0054] As the metal salt of (meth)acrylic acid, it is preferable to use, for example, an alkali metal salt of (meth)acrylic acid. Specific examples of the metal salt of (meth)acrylic acid include sodium acrylate, lithium acrylate, potassium acrylate, calcium acrylate, magnesium acrylate, sodium methacrylate, lithium methacrylate, potassium methacrylate, calcium methacrylate, etc. Among these, it is more preferable to use sodium acrylate.

[0055] Examples of the ammonium salt of (meth)acrylic acid include ammonia-neutralized (meth)acrylic acid, monoethanolamine-neutralized (meth)acrylic acid, diethanolamine-neutralized (meth)acrylic acid, hydroxylamine-neutralized (meth)acrylic acid, etc. Among these, it is more preferable to use ammonia-neutralized (meth)acrylic acid.

[0056] The viscosity of an aqueous solution of the water-soluble acrylic acid-acrylonitrile copolymer with a solid content of 8% by mass at 25°C is preferably in the range of 500 mPa·s or more and 5000 mPa·s or less. It is preferable that the viscosity of the aqueous solution is 500 mPa·s or more, since this allows the coating layer 22 to exhibit good adhesion to the solid electrolyte layer 30. It is also preferable that the viscosity of the aqueous solution is 5000 mPa·s or less, since this allows the negative electrode active material to be well dispersed, making it easier to obtain a negative electrode layer 20 with excellent cycle characteristics.

[0057] When the coating layer 22 is taken as 100% by mass, the content of the negative electrode active material is preferably 50% by mass or more and 70% by mass or less, and more preferably 55% by mass or more and 65% by mass or less. When the coating layer 22 is taken as 100% by mass, the content of the carbon material is preferably 30% by mass or more and 50% by mass or less, and more preferably 35% by mass or more and 45% by mass or less. When the coating layer 22 is taken as 100% by mass, the content of the binder is preferably 1% by mass to 5% by mass, and more preferably 1% by mass to 4% by mass. A binder content of 1% by mass or more allows the film strength of the coating layer 22 to be within an appropriate range, which is preferable because it minimizes the generation of voids due to the deposition of metallic lithium in the coating layer 22 or between the solid electrolyte layer 30 and the coating layer 22.

[0058] In addition to the above, the coating layer 22 may contain additives used in conventional solid secondary batteries, such as fillers, dispersants, ion conductive agents, solid electrolytes, etc.

[0059] The thickness of the coating layer 22 is preferably more than 0.5 μm and not more than 4 μm, more preferably 1.0 μm or more and 3.5 μm or less, and even more preferably 1.5 μm or more and 2.5 μm or less. Setting the thickness of the coating layer 22 to be more than 0.5 μm and not more than 4 μm is preferable because it is possible to sufficiently protect the lithium alloy layer 23 with the coating layer 22 while minimizing the precipitation of metallic lithium in the coating layer 22. The thickness of the coating layer 22 can be measured by observing the average thickness of the coating layer 22 in a cross section of the all-solid-state secondary battery with a scanning electron microscope (SEM).

[0060] The lithium alloy layer 23 is a layer disposed between the coating layer 22 and the negative electrode current collector. This lithium alloy layer 23 contains an alloy of metallic lithium and silver, and further contains metallic lithium deposited by a current-carrying step described below. Note that this lithium alloy layer 23 is bonded to the coating layer 22 by the metallic lithium deposited by the current-carrying step.

[0061] The total number of moles of lithium contained in the positive electrode layer and the negative electrode layer, including the lithium contained in this lithium alloy layer 23, is set to be 30 times or more and 120 times or less the total number of moles of silver contained in the positive electrode layer and the negative electrode layer.

[0062] In this embodiment, the bonding rate of the interface between the coating layer 22 and the lithium alloy layer 23 is 85% or more and 100% or less. This bonding rate of the interface can be achieved by forming a good bonding interface between the coating layer 22 and the lithium alloy layer 23 through a current application process described below. The bonding refers to a state in which the interface between the coating layer 22 and the lithium alloy layer 23 is in physical contact with each other. This bonding rate can be calculated using the following method. First, an SEM image is obtained at a magnification of 1000 times, depicting an arbitrary portion of the cross section of the solid secondary battery 100, with the bonding interface between the coating layer 22 and the lithium alloy layer 23 at its center. Next, the ratio of the bonding portion to the total length of the bonding interface included in this image (bonding rate) is calculated. This operation is performed at five different locations on the same solid secondary battery 100, and the average of the bonding rates calculated for each location is calculated. This average value is used as the bonding rate between the coating layer 22 and the lithium alloy layer 23 in the solid secondary battery 100. The bonding rate is also calculated using the method described here in the examples and comparative examples described below. The bonding rate can be made nearly 100% by the current application process described below, and the bonding rate is preferably 90% or more and 100% or less, more preferably 95% or more and 100% or less, and even more preferably 97% or more and 100% or less.

[0063] 2. Method for manufacturing a solid secondary battery according to this embodiment Next, a description will be given of a method for manufacturing the solid secondary battery 1 according to this embodiment. The solid secondary battery 1 according to this embodiment can be manufactured by forming the anode layer 20 on the solid electrolyte layer 30, and laminating a separately manufactured cathode layer 10 thereon so that the solid electrolyte layer 30 is located between the cathode layer 10 and the anode layer 20.

[0064] (2-1. Solid electrolyte layer manufacturing process) The solid electrolyte layer 30 can be produced, for example, by the following procedure or steps.

[0065] The solid electrolyte layer 30 includes a blending step of blending raw materials to obtain a blended material and a firing step of firing the obtained blended material. Here, as an example, a method for producing the solid electrolyte layer 30 made of a sintered body of a garnet-type oxide will be described.

[0066] In the blending step, raw materials containing at least a Li component, a La component, and a Zr component are blended to obtain a blended material. The components contained in the blended material are in a ratio that allows a lithium ion conductive ceramic material having a garnet-type crystal structure or a crystal structure similar to the garnet-type crystal structure to be obtained.

[0067] A sintering aid such as boron oxide is added to the blended materials, and the materials are mixed in a ball mill or jet mill to obtain a precursor powder. The precursor powder thus obtained is placed in a mold or the like and pressure-molded to obtain precursor pellets.

[0068] Next, in the firing step, the precursor pellets formed as described above are heated at a temperature of about 900°C to 1250°C for about 1 hour to 36 hours to obtain the solid electrolyte layer 30. The sintering temperature and sintering time can be changed as appropriate depending on the blending of materials, etc.

[0069] The heating method is not particularly limited, and can be resistance heating, microwave heating, etc. The sintering process can be divided into two stages: preliminary sintering and main sintering, or the sintering process can include the molding process described above and employ electric current sintering, spark plasma sintering, etc.

[0070] Furthermore, the surface roughness of the fired solid electrolyte layer 30 may be adjusted by polishing, acid treatment, etc. The surface roughness can be controlled by the polishing conditions and the strength of the acid treatment, and the strength of the acid treatment can be controlled by, for example, the concentration of the acid used, the treatment time, the treatment temperature, etc.

[0071] (2-2. Negative electrode layer manufacturing process) First, the materials (negative electrode active material, carbon material, binder, etc.) that constitute the coating layer 22 are added to a polar solvent or a non-polar solvent to prepare a slurry. Next, the obtained slurry is applied to the surface of the solid electrolyte layer 30 prepared as described above and dried to form the coating layer 22. The coating layer 22 is formed by applying the slurry to the surface of the solid electrolyte layer 30 as described herein, and is not pressure-molded.

[0072] Next, an Ag-Li / Cu laminate including an Ag-Li alloy layer and a Cu layer is laminated on the coating layer 22. The Ag-Li alloy layer is laminated so as to be in contact with the coating layer 22. Finally, a lithium source is laminated on the surface of the solid electrolyte layer 30 opposite to the surface in contact with the coating layer 22, and current is applied. This current application process deposits Ag-Li between the Ag-Li alloy layer and the coating layer 22 to form a bonding interface, and forms a lithium alloy layer 23 between the coating layer 22 and the negative electrode current collector 21, thereby producing a solid electrolyte total-negative electrode layer composite in which the negative electrode layer 20 is laminated on the solid electrolyte layer 30.

[0073] The lithium source used in the current application step may be any source capable of supplying lithium, and may be the positive electrode layer 10. However, in this embodiment, a lithium source (for example, plate-shaped metallic lithium) separate from the positive electrode layer 10 is used.

[0074] The above-mentioned current application step is preferably carried out over a long period of time at the lowest possible current density, since this allows for the formation of a dense lithium alloy layer 23. The current application conditions in the current application step are, for example, 10 μA / cm 2 More than 200μA / cm 2 The capacity of the lithium alloy layer 23 formed on the negative electrode layer can be controlled by adjusting the current flow time. 2 More than 180μA / cm 2 More preferably, it is 30 μA / cm or less. 2 More than 160μA / cm 2The current application time is preferably 1 hour or more and 150 hours or less, more preferably 3 hours or more and 120 hours or less, and even more preferably 4 hours or more and 112 hours or less. The thickness of the metallic lithium deposited on the layer in this current application step is preferably 0.5 μm or more and 20 μm or less, and more preferably 1.0 μm or more and 13 μm or less. The change in thickness of the lithium alloy layer in this current application step can be measured by observing the average thickness of the lithium alloy layer in a cross section of the all-solid-state secondary battery with a scanning electron microscope (SEM).

[0075] (2-3. Positive electrode layer manufacturing process) The materials constituting the positive electrode active material layer 12 (e.g., positive electrode active material, binder, etc.) are mixed and then laminated on the positive electrode current collector 11. The resulting laminate is then pressed (e.g., by hydrostatic pressure) to produce the positive electrode layer 10. This pressing step may be omitted. The positive electrode layer 10 may 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 producing the positive electrode layer 10 by these methods, the positive electrode current collector 11 may be pressed onto the produced pellet or sheet. The positive electrode active material layer 12 may be formed by adding the materials constituting the positive electrode active material layer to a nonpolar solvent to produce a slurry (the slurry may be a paste, and the same applies to other slurries), applying the slurry to the positive electrode current collector 11, and drying the slurry. In this embodiment, the positive electrode active material layer 12 thus formed is impregnated with an ionic liquid electrolyte.

[0076] (2-4. Assembly process of solid secondary batteries) The solid secondary battery 1 according to this embodiment can be produced by laminating the cathode layer 10 on the composite of the solid electrolyte layer 30 and the anode layer 20 produced by the above method so that the solid electrolyte layer 30 is sandwiched between the cathode layer 10 and the anode layer 20. At this time, pressure may be applied as needed, but this pressurizing step may be omitted. For example, in this embodiment, the cathode active material layer 12 is impregnated with an ionic liquid electrolyte, and therefore its surface is adhesive. Therefore, the cathode layer 10 can be laminated on the surface of the solid electrolyte layer 30 simply by placing it on the surface of the solid electrolyte layer 30 and pressing it lightly.

[0077] <3. Charging method for solid-state secondary batteries> When the solid secondary battery 1 produced by the above method is operated, the solid secondary battery may be operated with or without pressure being applied thereto.

[0078] When pressure is applied, the pressure may be, for example, 0.01 MPa or more and 10 MPa or less, preferably 0.01 MPa or more and 1 MPa or less, and more preferably 0.05 MPa or more and 0.3 MPa or less. Pressure may also be applied by sandwiching the solid state secondary battery 1 between two hard plates such as stainless steel, brass, aluminum, or glass, and fastening the two plates together with a screw.

[0079] Furthermore, the temperature at which the solid secondary battery 1 according to this embodiment is operated is not particularly limited, but is preferably 15°C or higher and 70°C or lower, more preferably 15°C or higher and 50°C or lower, and is particularly preferably operated at room temperature, such as 20°C or higher and 30°C or lower.

[0080] In this embodiment, since the lithium alloy layer 23 is present in the negative electrode layer 20, lithium ions that have migrated from the positive electrode layer 10 are precipitated in the lithium alloy layer 23 of the negative electrode layer 20. A solid secondary battery in which lithium is precipitated in the negative electrode to form the lithium alloy layer 23 during charging in this manner is called a precipitation-type solid secondary battery.

[0081] Because lithium deposited in the anode layer 20 during charge migrates to the cathode layer 10 during discharge, the thickness of the lithium alloy layer 23 in the anode layer 20 changes with charge and discharge. The amount of change in the thickness of the lithium alloy layer 23 at this time, i.e., the difference between the thickness of the lithium alloy layer 23 after charge and the thickness of the lithium alloy layer 23 before charge, is preferably set to 10 μm or more and 60 μm or less. The thickness of this lithium alloy layer can be measured by observing the average thickness of the lithium alloy layer in a cross section of the all-solid-state secondary battery before and after charge using a scanning electron microscope (SEM).

[0082] As described above, the thickness of the lithium alloy layer 23 changes due to charge and discharge, and therefore, in this embodiment, the charge capacity of the positive electrode active material layer 12 directly affects the thickness change of the lithium alloy layer 23. In this embodiment, the charge capacity of the positive electrode active material layer 12 is 2 mAh / cm 2 More than 12mAh / cm 2 It is preferable that: Here, the charge capacity of the positive electrode active material layer 12 is obtained by multiplying the charge capacity density (mAh / g) of the positive electrode active material by the mass of the positive electrode active material in the positive electrode active material layer 12. When multiple types of positive electrode active materials are used, the value of charge capacity density x mass is calculated for each positive electrode active material, and the sum of these values ​​can be taken as the charge capacity of the positive electrode active material layer 12.

[0083] Specific methods for directly measuring the charge capacity include the following. First, the charge capacity of the positive electrode active material layer 12 is measured by preparing a test cell using the positive electrode active material layer 12 as the working electrode and Li as the counter electrode, and performing CC-CV charging from the OCV (open circuit voltage) to the upper limit charge voltage. The upper limit charge voltage is specified in JIS C 8712:2015, and refers to 4.25 V for lithium cobalt oxide-based positive electrodes, and the voltage obtained by applying the provisions of A.3.2.3 (Safety requirements when a different upper limit charge voltage is applied) of JIS C 8712:2015 for other positive electrodes.

[0084] The test cell described above can be fabricated, for example, by the following method. The cathode active material layer 12 for which the charge capacity is to be measured is punched into a disk shape with a diameter of 13 mm. 200 g of the same solid electrolyte powder used in the solid secondary battery 1 is compacted at 40 MPa to form a pellet with a diameter of 13 mm and a thickness of approximately 1 mm. This pellet is placed inside a cylinder with an inner diameter of 13 mm, and the punched-out disk-shaped cathode active material layer 12 is placed on one side. A lithium foil with a diameter of 13 mm and a thickness of 0.03 mm is placed on the other side. Two stainless steel disks are then placed on each side, and the entire contents are axially pressurized at 300 MPa for one minute to integrate the contents. The integrated contents are then removed from the cylinder and sealed in a case so that a constant pressure of 22 MPa is applied, forming a test cell. The charge capacity of the cathode active material layer 12 can be measured by, for example, CC charging the test cell fabricated as described above at a current density of 0.1 mA, followed by CV charging to 0.02 mA.

[0085] The charge capacity density is calculated by dividing this charge capacity by the mass of each active material. The initial charge capacity of the positive electrode active material layer 12 may be the initial charge capacity measured during the first charge cycle. This value was used in the examples described below.

[0086] <4. Effects of this embodiment> In the solid secondary battery 1 configured as described above, the negative electrode layer includes the coating layer 22, the lithium alloy layer 23, and the negative electrode current collector, the bonding rate of the interface between the coating layer 22 and the lithium alloy layer 23 is 85% or more, and the total number of moles of lithium contained in the solid secondary battery is 30 to 120 times the total number of moles of silver contained in the solid secondary battery, so that the pressurizing step can be omitted when manufacturing the solid secondary battery. As a result, not only is the degree of freedom in battery size and shape significantly improved, but the elimination of the pressurizing step can also significantly reduce manufacturing costs.

[0087] <5. Other embodiments of the present invention> The solid electrolyte layer is not limited to one containing only the oxide-based solid electrolyte as described above, but may also contain an oxide-based solid electrolyte and a binder or the like. Furthermore, the solid electrolyte layer is not limited to one containing an oxide-based solid electrolyte, and may contain a sulfide-based solid electrolyte.

[0088] In the above embodiment, the case of a solid secondary battery in which the positive electrode layer contains an ionic liquid electrolyte has been described. However, the present invention is applicable to any solid secondary battery that includes a solid negative electrode layer and a solid solid electrolyte layer, and is also applicable to, for example, a solid secondary battery that contains an electrolytic solution in addition to a solid electrolyte, an all-solid secondary battery in which the positive electrode layer is also solid, and the like.

[0089] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention. [Example]

[0090] The solid 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.

[0091] <Fabrication of solid secondary batteries> The solid secondary batteries shown in Table 1 were fabricated by the following procedure. (Fabrication of solid electrolyte / negative electrode laminate) First, a coating layer slurry was prepared using mixed particles of silver and carbon as the negative electrode active material using the following method. Silver particles with a particle size of approximately 60 nm were used, and CB carbon manufactured by Asahi Carbon Co., Ltd. was used. 4 g of carbon and 6 g of silver particles were placed in a container, and an aqueous solution containing a water-soluble acrylic acid-acrylonitrile copolymer as a binder was added. The mixture was stirred while gradually adding water to prepare a slurry. The coating layer composition is shown in Table 1. In Example 8, polyacrylic acid was used as the binder instead of the water-soluble acrylic acid-acrylonitrile copolymer. The molar ratios in Table 1 indicate the ratio (Li / Ag) of the total number of moles of lithium (Li) contained in the positive electrode layer and the negative electrode layer to the total number of moles of silver (Ag) contained in the solid-state secondary battery.

[0092] The solid electrolyte layer was made of phosphoric acid-treated oxide-based solid electrolyte (LLZO) pellets that were vacuum-dried. The above-mentioned slurry for the coating layer was applied to one side of the LLZO using a spin coater and vacuum dried at 30°C for about 12 hours to form a coating layer 22 on the LLZO. An Ag-Li alloy / Cu laminate was placed on this coating layer 22 so that the Ag-Li alloy surface and the coating layer 22 were in contact with each other. Furthermore, lithium metal, which is a lithium source, was attached to the side on which the LLZO coating layer 22 was not formed, and electricity was applied to form a solid electrolyte layer-negative electrode layer composite. 2 The current was passed through the capacitor at a current flow rate of 100 kJ / s, and the time was controlled so as to achieve the capacitance shown in Table 1.

[0093] (Preparation of positive electrode layer) LiNi as the positive electrode active material 0.8 Co 0.15 Al 0.05O2 (NCA), carbon nanotubes (CNTs), carbon black (CB), and PVDF were mixed in a ratio of 98:0.5:0.5:1 (by mass) to form a cathode active material: CNTs, CB, and PVDF. The mixture was stirred with NMP, as in the anode, to form a slurry. The slurry was applied to an aluminum current collector foil using a blade coater, dried in air at 80°C for approximately 20 minutes, and then vacuum-dried at 100°C for 12 hours. After drying, the cathode was impregnated with 2M LiFSI / N-propyl-N-methylpyrrolidinium bis(fluoromethanesulfonyl)imide (Pyr13FSI), an ionic liquid electrolyte, to form the cathode layer.

[0094] (Fabrication of solid secondary batteries) The prepared solid electrolyte layer-negative electrode layer composite and the positive electrode layer were laminated in the order of positive electrode layer / solid electrolyte layer / negative electrode layer. This laminate was sealed in a laminate film in a vacuum to prepare a solid secondary battery without a pressurizing step. A portion of each of the positive electrode current collector and the negative electrode current collector was designed to protrude from the laminate film so as not to break the vacuum in the battery, and this protruding portion served as the terminal of the positive electrode layer or the negative electrode layer, respectively.

[0095] <Evaluation of solid secondary batteries> The charging current density characteristics of the all-solid-state lithium secondary batteries fabricated in this manner were evaluated under the following conditions. Measurements were performed by placing the all-solid-state batteries in a thermostatic chamber at 25°C. Charging up to 4.4 V and discharging down to 3 V were repeated, with the current density increasing with each cycle. The results are shown in Table 1. The numbers in the critical current density (CCD) column in Table 1 indicate the charging current density characteristics of each battery; for example, 4 mA / cm 2 If it says 4 mA / cm without short circuit, 2 This indicates that measurements were possible up to a charge / discharge current of 1000 kJ / s.

[0096] [Table 1]

[0097] <Consideration> From the results in Table 1, it was found that the solid secondary batteries of Examples 1 to 8, in which the total number of moles of lithium contained in the positive electrode layer and the negative electrode layer was 30 to 120 times the total number of moles of silver contained in the solid secondary battery and the connection rate of the interface between the coating layer and the lithium alloy layer was 85%, exhibited a high short-circuit suppression effect even when fabricated without applying external pressure (external pressure). Note that in Example 8, polyacrylic acid was used as the binder instead of the water-soluble acrylic acid-acrylonitrile copolymer, but results similar to those of Examples 1 to 7 were obtained. On the other hand, in Comparative Example 1, it was found that the number of moles of lithium contained in the entire solid secondary battery was too large relative to the number of moles of silver, and therefore a problem occurred in which voids were generated in the negative electrode layer when charging and discharging was repeated (the second cycle in Comparative Example 1). In Comparative Example 2, the number of moles of silver was too large relative to the number of moles of lithium, resulting in the deposition of silver and current concentration. In addition, in Comparative Example 3, which was manufactured without undergoing the current-passing process, or Comparative Example 4, in which the current-passing process was insufficient and the bonding rate at the interface between the coating layer and the lithium alloy layer was less than 85%, lithium was unevenly precipitated during charging, resulting in a short circuit. From the above results, it is believed that the purpose of applying external pressure in the manufacturing process of a solid secondary battery is mainly to form a uniform interface and densify the negative electrode layer. According to Examples 1 to 8 of the present invention, in particular, with regard to the formation of two interfaces (between the solid electrolyte layer and the negative electrode layer, and the coating layer and the lithium alloy layer) that are directly related to the prevention of short circuits, by applying a slurry directly to the solid electrolyte layer and by going through a current application step in the process of forming the negative electrode layer, these interfaces can be formed uniformly even without applying pressure, and as a result, a short circuit prevention effect can be obtained. [Explanation of symbols]

[0098] 1 Solid state secondary battery 10 Positive electrode layer 11 Positive electrode current collector 12 Cathode active material layer 20 negative electrode layer 21 Negative electrode current collector 22 Coating layer 23 Lithium alloy layer 30 Solid electrolyte layer

Claims

1. a positive electrode layer containing lithium; a negative electrode layer containing lithium and silver; a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer, the negative electrode layer includes a coating layer, a lithium alloy layer, and a negative electrode current collector, a bonding rate at the interface between the coating layer and the lithium alloy layer is 85% or more and 100% or less; A solid-state secondary battery in which the total number of moles of lithium contained in the positive electrode layer and the negative electrode layer is 30 times or more and 120 times or less the total number of moles of silver contained in the solid-state secondary battery.

2. 2. The solid secondary battery according to claim 1, wherein the coating layer contains a binder, and the content of the binder relative to 100% by mass of the coating layer is 1% by mass or more and 5% by mass or less.

3. The solid secondary battery according to claim 1 , wherein the coating layer has an average thickness of 4 μm or less.

4. The solid secondary battery according to claim 2 , wherein the binder is a water-based binder.

5. 5. The solid secondary battery according to claim 4, wherein the binder is a water-soluble acrylic acid-acrylonitrile copolymer.

6. The solid secondary battery according to claim 1 , wherein the solid electrolyte layer contains an oxide-based solid electrolyte.

7. The solid secondary battery according to claim 1 , wherein the positive electrode layer contains a liquid electrolyte.

8. a solid electrolyte layer and a negative electrode layer, the negative electrode layer includes a coating layer, a lithium alloy layer, and a negative electrode current collector, The bonding rate of the interface between the coating layer and the lithium alloy layer is 85% or more and 100% or less. A method for producing a solid electrolyte layer-negative electrode layer composite, comprising: a coating step of coating a negative electrode slurry on the solid electrolyte layer to form a coating layer; and a current-passing step of laminating a lithium source on a surface of the solid electrolyte layer opposite to the surface on which the coating layer is formed, and then passing current therethrough to deposit a lithium alloy layer in the negative electrode layer.

9. A method for producing a solid secondary battery by further laminating a positive electrode layer on the solid electrolyte layer-negative electrode layer composite produced by the method for producing a solid electrolyte layer-negative electrode layer composite according to claim 8, the lithium source is a member different from the positive electrode layer, the lithium source is removed after the energizing step, and the positive electrode layer is laminated on the solid electrolyte layer.

10. A method for charging the solid secondary battery according to any one of claims 1 to 7, comprising: A method for charging a solid secondary battery, wherein the change in average thickness of the lithium alloy layer during charging is 10 μm or more and 60 μm or less.

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