Solid electrolyte layer and solid secondary battery

Abstract

Provided is a solid secondary battery that can suppress deterioration of battery performance due to charging and discharging even when no external pressure is applied or even under low confining pressure. SOLUTION: A solid electrolyte layer having an assumed shear strength of 15 MPa or more and 70 MPa or less.

Description

[Technical Field]

[0001] The present invention relates to a solid electrolyte layer and a solid secondary battery including the solid electrolyte layer. [Background technology]

[0002] For example, as shown in Patent Document 1, when charging and discharging a precipitation-type all-solid-state secondary battery in which lithium metal is precipitated in the negative electrode layer, conventionally, the all-solid-state lithium-ion secondary battery is sandwiched between end plates or the like in the thickness direction and charged and discharged while a high external pressure is applied, so that the lithium metal in the negative electrode layer is uniformly precipitated along the shape of the negative electrode layer and the effect of volumetric changes in the active material during charging and discharging is suppressed.

[0003] The presence of a pressure jig for applying a high external pressure to an all-solid-state secondary battery is not only disadvantageous in terms of cost, but also causes a decrease in the energy density of the entire battery module including the all-solid-state secondary battery and the pressure jig.

[0004] Therefore, it is required to minimize the external pressure applied to all-solid-state secondary batteries, and for example, Patent Document 1 considers setting the binder ratio in the solid electrolyte layer to 20% by volume or more and 30% by volume or less for an all-solid-state secondary battery including an electrode laminate having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer. Also, Patent Document 2 considers using a solid electrolyte containing a certain organic electrolyte in addition to a lithium salt and a lithium-containing sulfide solid electrolyte. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-036160 [Patent Document 2] Japanese Patent Publication No. 2020-198270 Summary of the Invention [Problem to be solved by the invention]

[0006] However, a technology for sufficiently reducing the external pressure applied to a solid secondary battery has not yet been established, and further improvements are required.

[0007] The present invention has been made in view of the above-mentioned problems, and aims to provide a solid secondary battery that can suppress deterioration of battery performance even when charged and discharged without applying external pressure or under a low confining pressure that can be achieved with a simple jig. The low confining pressure means, for example, a confining pressure of 1.0 MPa or less, more preferably 0.5 MPa or less. [Means for solving the problem]

[0008] That is, the solid electrolyte layer and the solid secondary battery according to the present invention are as follows. [1] A solid electrolyte layer having an assumed shear strength of 15 MPa or more and 70 MPa or less. [2] The solid electrolyte layer according to [1], which contains ionic plastic crystals. [3] The solid electrolyte layer according to [2], wherein the ionic plastic crystal contains at least one cation selected from the group consisting of spiro-type ammonium cations, pyrrolidinium cations, piperidinium cations, and tetraalkylammonium cations. [4] The solid electrolyte layer according to [2] or [3], wherein the ionic plastic crystal contains, as an anion, at least one anion selected from the group consisting of a sulfonylimide anion and a borate anion. [5] The solid electrolyte layer according to [4], wherein the anion contains at least one anion selected from the group consisting of bis(fluorosulfonyl)imide anion (FSI), 1.1.2.2.3.3-hexafluoropropane-1.3-disulfonimide anion (CFSI), tetrafluoroborate anion (BF4), and trifluoro(trifluoromethyl)borate anion (CF3BF3). [6] The solid electrolyte layer according to [1], which contains succinonitrile. [7] The solid electrolyte layer according to any one of [1] to [6], which contains a lithium-containing sulfide solid electrolyte. [8] The solid electrolyte layer according to [7], wherein the lithium-containing sulfide solid electrolyte contains a halogen element. [9] The solid electrolyte layer according to any one of [1] to [8], further comprising a binder, wherein the content of the binder relative to the entire solid electrolyte layer is more than 1 mass % and 10 mass % or less.

[10] The solid electrolyte layer according to any one of [1] to [9], wherein the assumed shear strength is 20 MPa or more and 60 MPa or less.

[11] The content of the solid electrolyte in the entire solid electrolyte layer is 80% by mass or more but 99% by mass or less The solid electrolyte layer according to any one of [1] to

[10] below:

[12] A solid-state secondary battery comprising a positive electrode layer, a negative electrode layer, and a solid electrolyte layer, The solid secondary battery, wherein the solid electrolyte layer is any one of the solid electrolyte layers described in [1] to

[11] .

[13] The solid secondary battery according to

[12] , wherein the negative electrode layer contains one or more materials selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, and zinc, and amorphous carbon.

[14] The solid secondary battery according to

[13] , wherein the negative electrode layer contains silver and amorphous carbon.

[15] The solid secondary battery according to any one of

[12] to

[14] , wherein the ratio of the initial charge capacity of the positive electrode layer to the initial charge capacity of the negative electrode layer satisfies the following formula (1): 0.01 a: Initial charge capacity of the positive electrode layer (mAh) b: Initial charge capacity of the negative electrode layer (mAh)

[16] A method for charging a solid secondary battery, comprising charging the solid secondary battery according to any one of

[12] to

[15] above in excess of the charge capacity of the negative electrode layer.

[17] The method for charging a solid secondary battery according to

[16] , wherein the battery is charged to a capacity in the range of 2 to 100 times the charge capacity of the negative electrode layer. ​[Effects of the Invention]

[0009] According to the present invention, the deemed shear strength of the solid electrolyte layer is set to 15 MPa or more and 70 MPa or less, so that the deterioration of the battery performance of the solid secondary battery can be sufficiently suppressed even when no external pressure or a low external pressure is applied. [Brief explanation of the drawings]

[0010] [Figure 1] 3 is a graph showing the deemed shear strength of the solid electrolyte layer and the battery performance of the solid secondary battery according to the examples of the present invention and the comparative examples. DETAILED DESCRIPTION OF THE INVENTION

[0011] Preferred embodiments of the present invention will be described in detail below.

[0012] <1. Basic configuration of the solid secondary battery according to this embodiment> The solid secondary battery according to this embodiment is, for example, a lithium secondary battery including a positive electrode, a negative electrode, and a solid electrolyte layer.

[0013] (1-1. Positive electrode) The positive electrode includes, for example, a positive electrode current collector and a positive electrode composite layer. Examples of the positive electrode current collector include a plate or foil 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 may be omitted.

[0014] The positive electrode composite layer includes, for example, a positive electrode active material and a solid electrolyte. The solid electrolyte included in the positive electrode is preferably a sulfide solid electrolyte of the same type or a different type from that described in the section on the solid electrolyte layer.

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

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

[0017] In addition, the positive electrode active material preferably contains 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 atomic 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), etc., lithium salts of ternary transition metal oxides.

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

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

[0021] Furthermore, when the positive electrode active material is formed of a lithium salt of a ternary transition metal oxide such as NCA or NCM and contains nickel (Ni) as the positive electrode active material, the capacity density of the solid secondary battery can be increased and metal elution from the positive electrode active material in a charged state can be reduced, thereby improving the long-term reliability and cycle characteristics of the solid secondary battery in a 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, and may be within a range applicable to positive electrode active materials in conventional solid secondary batteries. The content of the positive electrode active material in the positive electrode is also not particularly limited, and may be within a range applicable to positive electrodes in conventional solid secondary batteries.

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

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

[0025] (1-2. Negative electrode) The negative electrode includes, for example, a negative electrode current collector and a negative electrode composite layer laminated on the negative electrode current collector. The negative electrode current collector is preferably made of a material that does not react with lithium, i.e., does not form any alloy or compound with lithium. Examples of materials that can be used for the negative electrode current collector include copper (Cu), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), and nickel (Ni).

[0026] The negative electrode mixture layer contains, for example, a negative electrode active material, a carbon material, and a binder. Examples of the negative electrode active material include alloy-forming elements that form an alloy or compound 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.

[0027] 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 properties of the solid secondary battery 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.

[0028] Examples of the carbon material include amorphous carbon such as carbon black and graphene. Examples of carbon black include acetylene black, furnace black, and ketjen black.

[0029] In addition to the above, the negative electrode mixture layer may also contain additives used in conventional solid secondary batteries, such as fillers, dispersants, ion conductive agents, solid electrolytes, etc., as appropriate.

[0030] (1-3.Solid electrolyte layer) The solid electrolyte layer is formed between the positive electrode and the negative electrode and includes a solid electrolyte. The configuration of the solid electrolyte layer is a characteristic feature of the solid secondary battery according to this embodiment, and will be described in detail later.

[0031] (1-4. Relationship between charge capacity of positive electrode and negative electrode) The solid secondary battery according to this embodiment is preferably configured so that the ratio of the charge capacity of the positive electrode mixture layer to the charge capacity of the negative electrode mixture layer, i.e., the capacity ratio, satisfies the requirement of the following mathematical formula (1). 0.01 a: Charging capacity of the positive electrode composite layer (mAh) b: Charging capacity of the negative electrode composite layer (mAh)

[0032] ​Here, the charge capacity of the positive electrode mixture layer 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 mixture layer. 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 ​​is used as the charge capacity of the positive electrode mixture layer. The charge capacity of the negative electrode mixture layer is calculated in a similar manner. That is, the charge capacity of the negative electrode mixture layer is obtained by multiplying the charge capacity density (mAh / g) of the negative electrode active material by the mass of the negative electrode active material in the negative electrode mixture layer. When multiple types of negative electrode active materials are used, the value of charge capacity density x mass is calculated for each negative electrode active material, and the sum of these values ​​is used as the capacity of the negative electrode mixture layer. Here, the charge capacity densities of the positive and negative electrode active materials are capacities estimated using half cells of a solid secondary battery using lithium metal as the counter electrode. In practice, the charge capacities of the positive electrode mixture layer and the negative electrode mixture layer are directly measured by measurement using a half cell of a solid secondary battery.

[0033] Specific methods for directly measuring the charge capacity include the following. First, the charge capacity of the positive electrode composite layer is measured by preparing a test cell using the positive electrode composite layer as the working electrode and Li as the counter electrode, and performing CC-CV charging from the open circuit voltage (OCV) to the upper charge voltage limit. The upper charge voltage limit is specified in JIS C 8712:2015, and refers to 4.25 V for lithium cobalt oxide-based positive electrodes, and the voltage determined by applying the provisions of A.3.2.3 (Safety requirements when applying different upper charge voltages) of JIS C 8712:2015 for other positive electrodes. The charge capacity of the negative electrode composite layer is measured by preparing a test cell using the negative electrode composite layer as the working electrode and Li as the counter electrode, and performing CC-CV charging from the open circuit voltage (OCV) to 0.01 V.

[0034] The test cell described above can be prepared, for example, by the following method. The positive electrode composite layer or negative electrode composite layer for which the charge capacity is to be measured is punched into a disk shape with a diameter of 13 mm. 200 mg of the solid electrolyte powder described below 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 disk-shaped positive electrode composite layer or negative electrode composite layer is placed on one side, and 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 compressed axially 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 to form a test cell. The charge capacity of the positive electrode composite layer can be measured by, for example, CC charging the test cell prepared as described above at a current density of 0.1 mA, followed by CV charging to 0.02 mA.

[0035] The charge capacity density is calculated by dividing this charge capacity by the mass of each active material. The initial charge capacities of the positive electrode mixture and the negative electrode mixture layer may be the initial charge capacities measured during the first cycle of charging.

[0036] In this way, it is preferable to make the charge capacity of the positive electrode mixture layer excessively large relative to the charge capacity of the negative electrode mixture layer. As will be described later, in this embodiment, the solid secondary battery is charged beyond the charge capacity of the negative electrode mixture layer. That is, the negative electrode mixture layer is overcharged. At the beginning of charging, lithium is absorbed into the negative electrode mixture layer. That is, the negative electrode active material forms an alloy with lithium ions that have migrated from the positive electrode. When further charging is performed beyond the capacity of the negative electrode mixture layer, lithium is precipitated on the back side of the negative electrode mixture layer, i.e., between the negative electrode current collector and the negative electrode mixture layer, and a lithium precipitate layer is formed by this lithium. The lithium precipitate layer is mainly composed of lithium (mainly metallic lithium), although it also contains trace amounts of elements other than lithium. This phenomenon occurs when the negative electrode active material contains a specific substance, i.e., an alloying element that forms an alloy or compound with lithium. During discharge, lithium in the negative electrode mixture layer and the lithium deposit layer ionizes and migrates to the positive electrode. Therefore, in the solid secondary battery according to this embodiment, the precipitated lithium can be used as the negative electrode active material. Furthermore, the negative electrode composite layer covers the lithium precipitate layer, and therefore functions as a protective layer for the lithium precipitate layer and can suppress the precipitation and growth of dendrites. This suppresses short circuits and capacity reduction in the solid secondary battery, and ultimately improves the characteristics of the solid secondary battery.

[0037] Here, the capacity ratio is preferably greater than 0.01. By increasing the capacity ratio to greater than 0.01, degradation of the solid secondary battery's characteristics can be suppressed. This is thought to be because the negative electrode composite layer functions as a protective layer for the lithium deposition layer. For example, by increasing the thickness of the negative electrode composite layer to a certain extent, the capacity ratio can be increased to greater than 0.01. In this case, the negative electrode composite layer can be prevented from collapsing due to repeated charge / discharge cycles, and dendrite deposition and growth can be suppressed. As a result, degradation of the solid secondary battery's characteristics can be suppressed. Furthermore, the capacity ratio is preferably less than 0.5. This is because when the capacity ratio is less than 0.5, the amount of lithium deposition in the negative electrode can be maintained, suppressing a decrease in battery capacity. For the same reason, it is thought that the capacity ratio is more preferably less than 0.25. Furthermore, when the capacity ratio is less than 0.25, the battery's output characteristics can be further improved.

[0038] The thickness of the negative electrode composite layer is not particularly limited as long as it satisfies the requirements of the above mathematical formula (1), but is preferably 1 μm or more and 20 μm or less, more preferably 1 μm or more and 10 μm or less. If the thickness of the negative electrode composite layer is 1 μm or more, the characteristics of the solid secondary battery can be further improved, which is preferable. If the thickness of the negative electrode composite layer is 20 μm or less, the resistance value of the negative electrode composite layer can be prevented from increasing, and as a result, the characteristics of the solid secondary battery can be improved. The thickness of the negative electrode mixture layer can be estimated, for example, by assembling a solid secondary battery, pressure-molding the battery, and then observing the average thickness of the cross section with a scanning electron microscope (SEM).

[0039] 2. Characteristic Configuration of the Solid State Secondary Battery According to the Present Embodiment The solid electrolyte layer in this embodiment contains, for example, a solid electrolyte, plastic crystals, a binder (binder for solid electrolyte layer), and a dispersant, and has an assumed shear strength measured by a Surface and Interface Analysis System (SAICAS) of 15 MPa or more and 70 MPa or less, and more preferably 20 MPa or more and 60 MPa or less.

[0040] The solid electrolyte is in powder form, and is, for example, a sulfide solid electrolyte made of a sulfide solid electrolyte material. The sulfide solid electrolyte material is, for example, a lithium-containing sulfide solid electrolyte containing lithium. Examples of the lithium-containing sulfide solid electrolyte include Li2S-P2S5, Li2S-P2S5-LiX (X is a halogen element, for example, I, Br, or Cl), Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, and Li2S-Si These solid electrolytes contain one or more solid electrolyte materials selected from the group consisting of S2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-ZmSn (m and n are positive numbers, Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, and Li2S-SiS2-LipMOq (p and q are positive numbers, M is P, Si, Ge, B, Al, Ga, or In). Such solid electrolytes are prepared by processing starting materials (e.g., Li2S, P2S5, etc.) using methods such as melt quenching and mechanical milling. These processes may be followed by further heat treatment. The solid electrolyte may be amorphous, crystalline, or a mixture of both.

[0041] Plastic crystals are an intermediate phase between solid crystals and liquids, and refer to a state in which the three-dimensional positions of the constituent particles are regular, but the orientation of the particles is irregular. In this embodiment, by incorporating these plastic crystals into the solid electrolyte layer, the solid electrolyte layer becomes relatively soft, and the deemed shear strength of the solid electrolyte layer is adjusted to fall within the aforementioned range. The plastic crystals in this embodiment are ionic and contain cations and anions.

[0042] The cation is, for example, one or more cations selected from ammonium-based cations, pyrrolidium-based cations, pyridinium-based cations, pyrimidinium-based cations, imidazolium-based cations, piperidinium-based cations, pyrazolium-based cations, oxazolium-based cations, pyridazinium-based cations, phosphonium-based cations, sulfonium-based cations, triazolium-based cations, and mixtures thereof. Among these, spiro-type ammonium cations, tetraalkylammonium cations, pyrrolidium cations, and pyridinium cations are preferred. Specific examples of such cations include one or more selected from the group consisting of 5-Azoniaspiro[4,4]nonane (SPB), N-ethyl-N-methylpyrrolidinium (P12), tetramethylammonium (TMA), N,N-dimethyl-piperidinium (PP11), triethylmethylammonium (TEMA), tetrapropylammonium (TPA), ethyltrimethylammonium (ETMA), and the like.

[0043] The anion is preferably, for example, a sulfonylimide anion and / or a borate anion, and the anion may be, for example, one or more selected from the group consisting of bis(fluorosulfonyl)imide anion (FSI), 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide anion (CFSI), tetrafluoroborate anion (BF), and trifluoro(trifluoromethyl)borate anion (CFBF).

[0044] As described above, an ionic plastic crystal containing a cation and an anion can be expressed as 5-Azoniaspiro[4,4]nonane1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide anion (SPBCFSI), for example, when using 5-Azoniaspiro[4,4]nonane as the cation and 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide anion as the anion. As described above, for the ionic plastic crystal according to this embodiment, there are multiple candidate substances for the cation and anion, so various types of ionic plastic crystals can be used by replacing the cation or anion with other types and changing the combination of cation and anion.

[0045] The content of the ionic plastic crystals in the entire solid electrolyte layer (i.e., when the mass of the entire solid electrolyte layer is 100% by mass) is preferably more than 1% by mass and not more than 20% by mass, and more preferably 1.5% by mass or more and 15% by mass or less.

[0046] The ionic plastic crystal may further contain a lithium salt. 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. The concentration of the lithium salt contained in the ionic plastic crystal is preferably 0.1M or more and 5M or less.

[0047] The solid electrolyte layer may further contain a binder (a binder for solid electrolyte). The binder for the solid electrolyte layer is, for example, one or more selected from the group consisting of styrene butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, etc. The binder in the solid electrolyte layer may be the same as or different from the binder contained in the positive electrode mixture layer and the negative electrode mixture layer.

[0048] When the solid electrolyte layer contains a binder for a solid electrolyte, the content of the binder for a solid electrolyte relative to the entire solid electrolyte layer is preferably 0.1 mass % or more and 5 mass % or less, and more preferably 0.5 mass % or more and 3 mass % or less.

[0049] The solid electrolyte layer may further contain a dispersant. The dispersant makes it difficult for the solid electrolyte to aggregate and facilitates uniform mixing when forming the solid electrolyte layer, and any known material generally used for electrodes of solid secondary batteries can be used.

[0050] 3. Method for manufacturing a solid secondary battery according to this embodiment Next, a method for manufacturing the solid secondary battery according to this embodiment will be described. The solid secondary battery according to this embodiment can be manufactured by first manufacturing a positive electrode, a negative electrode, and a solid electrolyte layer, and then laminating the above layers.

[0051] (3-1. Positive electrode manufacturing process) First, a mixture of materials (positive electrode active material, solid electrolyte, binder, etc.) that constitute the positive electrode composite layer is processed into a sheet. The positive electrode composite sheet thus obtained is cut into an appropriate shape and stacked on a positive electrode current collector as a positive electrode composite layer material, and the resulting stack is pressurized (for example, pressurized using hydrostatic pressure) to produce a positive electrode comprising a positive electrode current collector and a positive electrode composite layer.

[0052] (3-2. Negative electrode manufacturing process) First, a slurry is prepared by adding the materials for the negative electrode composite layer (negative electrode active material, non-alloying element, binder, etc.) constituting the negative electrode composite layer to a polar or non-polar solvent. The obtained slurry is then applied to a negative electrode current collector and dried. The obtained laminate is then pressed (for example, by applying hydrostatic pressure) to prepare a negative electrode. The pressing step may be omitted. Alternatively, the negative electrode may be prepared by a method in which a negative electrode composite layer is separately formed, and then laminated on a negative electrode current collector and pressed.

[0053] (3-3. Solid electrolyte sheet manufacturing process) The solid electrolyte layer can be produced, for example, by the following procedure or steps. First, the starting material is treated by melt quenching or mechanical milling.

[0054] For example, when using the melt quenching method, a sulfide-based solid electrolyte material can be produced by mixing predetermined amounts of starting materials (e.g., Li2S, P2S5, etc.), forming them into pellets, reacting them in a vacuum at a predetermined reaction temperature, and then quenching them. The reaction temperature for the Li2S and P2S5 mixture is preferably 400°C to 1000°C, more preferably 800°C to 900°C. The reaction time is preferably 0.1 hours to 12 hours, more preferably 1 hour to 12 hours. The quenching temperature for the reaction product is usually 10°C or lower, preferably 0°C or lower, and the quenching rate is usually about 1°C / sec to 10,000°C / sec, preferably about 1°C / sec to 1000°C / sec.

[0055] Furthermore, when mechanical milling is used, a sulfide-based solid electrolyte material can be produced by stirring and reacting starting materials (e.g., Li2S, P2S5, etc.) using a ball mill or the like. The stirring speed and stirring time in mechanical milling are not particularly limited, but the faster the stirring speed, the faster the rate at which the sulfide-based solid electrolyte material is produced, and the longer the stirring time, the higher the conversion rate of the raw materials to the sulfide-based solid electrolyte material.

[0056] The mixed raw materials obtained by melt quenching or mechanical milling are then heat-treated at a predetermined temperature and pulverized to produce a powdered (particulate) solid electrolyte. If the solid electrolyte has a glass transition point, it may change from amorphous to crystalline by heat treatment.

[0057] Subsequently, the solid electrolyte obtained by the above method may be mixed with ionic plastic crystals, and if necessary, a solvent and a binder may be added and mixed, and then the mixture may be coated, dried, and pressed to produce a solid electrolyte sheet.

[0058] (3-4. Assembly process of solid secondary batteries) The positive electrode, negative electrode, and solid electrolyte sheet prepared by the above method are laminated so that the solid electrolyte sheet is sandwiched between the positive electrode and the negative electrode to form a solid electrolyte layer, and then pressurized (for example, by applying pressure using hydrostatic pressure), thereby producing the solid secondary battery according to this embodiment.

[0059] 4. Charging Method of Solid-State Secondary Battery According to the Present Embodiment Next, a method for charging a solid secondary battery will be described. In this embodiment, as described above, the solid secondary battery is charged beyond the charge capacity of the negative electrode mixture layer. That is, the negative electrode mixture layer is overcharged. At the beginning of charging, lithium is absorbed in the negative electrode mixture layer. When charging is performed beyond the charge capacity of the negative electrode mixture layer, for example, lithium is precipitated on the back side of the negative electrode mixture layer, i.e., between the negative electrode current collector and the negative electrode mixture layer, and this lithium forms a lithium precipitate layer that was not present at the time of manufacturing. During discharge, the lithium in the negative electrode mixture layer and the lithium precipitate layer is ionized and moves to the positive electrode side. The charge amount is preferably set to a value between 2 and 100 times the charge capacity of the negative electrode mixture layer, and more preferably in the range of 4 to 100 times. The thickness of the lithium deposition layer deposited in the negative electrode during charging is preferably 10 μm or more, more preferably 30 μm or more, and is preferably in the range of 60 μm or less, which is the upper limit feasible for a solid secondary battery. The thickness of this lithium deposition layer can be estimated by observing the average thickness of a cross section of the solid secondary battery after charging with a scanning electron microscope (SEM).

[0060] <5. Effects of this embodiment> According to the solid electrolyte layer and solid secondary battery of this embodiment, the deemed shear strength of the solid electrolyte layer is 15 MPa or more and 70 MPa or less. Therefore, even if the volume change of the battery is large, such as when the negative electrode capacity is exceeded and overcharged, causing lithium to precipitate, as described above, deterioration of battery performance can be sufficiently suppressed. Therefore, the solid secondary battery can be charged and discharged without any problem by applying no external pressure or by applying a sufficiently low external pressure.

[0061] <6. Other embodiments of the present invention> Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to these examples. For example, in the above-described embodiment, the deemed shear strength of the solid electrolyte layer is adjusted to a desired range by incorporating ionic plastic crystals into the solid electrolyte layer, but this is not limiting and the solid electrolyte layer may also contain, for example, succinonitrile. The content of succinonitrile in the entire solid electrolyte layer is preferably in the range of 0.1 mass % to 20 mass %.

[0062] The method for adjusting the deemed shear strength of the solid electrolyte layer within the desired range is not limited to the method using the plastic crystal as described above, and examples thereof include a method of adjusting the type of solid electrolyte contained in the solid electrolyte layer. In this case, it is preferable to use a solid electrolyte containing a halogen element. By setting the content of the halogen element in the solid electrolyte to 1% by mass or more and 20% by mass or less, more preferably 5% by mass or more and 15% by mass or less, the solid electrolyte can be made relatively soft, and the deemed shear strength of the solid electrolyte layer can be adjusted to fall within the aforementioned preferred range. The halogen element is preferably, for example, chlorine and / or bromine.

[0063] Another method for adjusting the deemed shear strength of the solid electrolyte layer to fall within the desired range is to adjust the content of the binder for the solid electrolyte layer. In this case, the content of the above-mentioned solid electrolyte binder in the entire solid electrolyte layer is preferably 1% by mass or more and 10% by mass or less, and more preferably more than 1% by mass or more and 5% by mass or less.

[0064] In addition, a person having ordinary knowledge in the technical field to which the present invention pertains will understand that various modifications or alterations that can be conceived within the scope of the technical ideas described in the claims also fall within the technical scope of the present invention. [Example]

[0065] 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.

[0066] Example 1 (Fabrication of solid electrolyte sheets) In this example, an ionic plastic crystal was added to soften the solid electrolyte layer. 5-Azoniaspiro[4.4]nonane 1.1.2.2.3.3-Hexafluoropropane-1.3-disulfonimide (SBPCFSI) was selected as the ionic plastic crystal. The synthesis of ionic plastic crystals was carried out as follows. 10 g of 5-azoniaspiro[4.4]nonane chloride (SBPCl) was dissolved in 100 mL of ultrapure water. Next, 18.5 g of Lithium 1.1.2.2.3.3-Hexafluoropropane-1.3-disulfonimide (LiCFSI) was dissolved in 100 mL of ultrapure water, and SBPCl was added dropwise to this solution. The resulting white solid was collected by filtration, washed with distilled water, and then vacuum-dried at 120 °C for 12 hours to obtain the desired ionic plastic crystals. A solid electrolyte sheet was then fabricated using the following method. Li6PS5Cl solid electrolyte, PVDF binder, and SBPCFSI containing 10 mol% LiTFSI were mixed in a mass ratio of 96.5:1:2.5 solid electrolyte:binder:ionic plastic crystal. Xylene and isobutyl isobutyrate were added and stirred to form a slurry. This slurry was then applied to a PET sheet using a blade coater, dried in air at 40°C, and then vacuum-dried at 40°C for 12 hours to obtain a solid electrolyte sheet. The resulting solid electrolyte sheet was subjected to isostatic pressure treatment at 490 MPa. The assumed shear strength of the solid electrolyte sheet was measured using a Surface and Interface Properties Analysis System (SAICAS) and found to be 49.0 MPa. The assumed shear strength of the solid electrolyte sheet was measured using a surface / interface property analyzer (SAICAS DN-GS model, manufactured by Daipla Wintes) under the following conditions: Cutting speed: horizontal 2μm / s, vertical 0.2μm / s Cutting depth: 20μm Blade width: 1mm Angle: Rake 10°, Relief 20°

[0067] (Preparation of negative electrode) Porous carbon black, silver particles, and a binder (PVDF) were mixed in a mass ratio of 70:23:7, and NMP was gradually added while stirring. The slurry was applied to a 10-micron-thick stainless steel foil using a blade coater, dried in air at 80°C for approximately 20 minutes, and then vacuum-dried at 100°C for approximately 12 hours to form a negative electrode.

[0068] (Preparation of positive electrode) Next, LiNi 0.8 Co 0.15 Mn 0.05 Using O2 (NCM), Li6PS5Cl solid electrolyte, carbon nanotubes (CNT) as a conductive agent, and PTFE as a binder were mixed in a mass ratio of positive electrode active material: solid electrolyte: CNT: PTFE = 85:14.45:0.25:0.3, and the mixture was stretched into a sheet to form the positive electrode composite. Next, the ionic plastic crystal SBPCFSI was dissolved in dichloromethane to a concentration of 2% by mass. The positive electrode sheet was immersed in this solution and evacuated to -0.05 MPa, thereby impregnating the positive electrode sheet with the ionic plastic crystal. The positive electrode sheet was removed from the solution and dried to obtain a positive electrode sheet impregnated with the ionic plastic crystal. This was then pressed onto an 18 μm thick aluminum foil positive electrode current collector to produce a positive electrode. The surface of the NCM used in this example was coated with Li2O-ZrO2.

[0069] (Fabrication of solid secondary batteries) The solid electrolyte sheet and negative electrode prepared as described above were stacked with the PET sheet and stainless steel foil facing outward, and after isostatic pressing at 50 MPa, the PET sheet was peeled off to transfer the solid electrolyte sheet onto the negative electrode. The positive electrode was placed on the surface of the solid electrolyte sheet exposed by peeling off the PET sheet, with the aluminum foil facing outward, and sealed in a laminate film in a vacuum to produce a solid secondary battery with a solid electrolyte layer stacked between the negative and positive electrodes. A portion of each of the aluminum foil of the positive electrode and the stainless steel foil of the negative electrode was exposed from the laminate film so as not to break the vacuum in the battery, and these exposed portions served as the positive and negative electrode terminals, respectively. This solid secondary battery was subjected to an isostatic pressure treatment at 490 MPa to prepare a solid secondary battery.

[0070] (Evaluation of solid secondary batteries) The charge-discharge characteristics of the solid secondary battery thus fabricated under a confining pressure of 0.3 MPa were evaluated under the following conditions. The measurement was carried out by placing the solid secondary battery in a thermostatic chamber at 25°C. The current was 1.5 mA / cm until the battery voltage reached 4.25 V. 2 The battery was charged at a constant current of 0.5mA / cm 2 The battery was charged at a constant voltage of 4.25 V until the discharge rate reached 1.5 mA / cm. 2 The test was continued at a constant current of 100 V until the battery voltage reached 2.5 V. This charge and discharge cycle was repeated 100 times. The capacity retention rate, which is the percentage of the discharge capacity at the 100th cycle, assuming that the discharge capacity at the first cycle of this charge and discharge test is 100%, was 93.2%. In addition, the average coulomb efficiency, which is the percentage of the discharge capacity at each cycle of the charge and discharge test relative to 100% charge capacity at each cycle, was 99.9%.

[0071] Examples 2 to 7 In Examples 2 to 7, solid electrolyte sheets and solid secondary batteries were produced in the same manner as in Example 1, except that the content of ionic plastic crystals contained in the solid electrolyte sheets was different. The content of ionic plastic crystals in the solid electrolyte sheet was adjusted to 1.5 mass% in Example 2, 2 mass% in Example 3, 5 mass% in Example 4, 10 mass% in Example 5, 15 mass% in Example 6, and 20 mass% in Example 7. During this adjustment, the content of the binder in the solid electrolyte sheet was not changed, and the amount of solid electrolyte was increased or decreased. The deemed shear strength and capacity retention rate of the solid secondary batteries measured by SAICAS for the solid electrolyte sheets produced in Examples 2 to 7 are shown in the graph of Figure 1. The deemed shear strength, capacity retention rate, and average coulombic efficiency measured for each Example are shown in Table 1.

[0072] (Examples 8 to 10) In Examples 8 to 10, solid electrolyte sheets and solid secondary batteries were produced in the same manner as in Example 1, except that the type of ionic plastic crystals contained in the solid electrolyte sheets was changed. The types of ionic plastic crystals used in each example are as follows: spiro-(1,1')-bipyrrolidinium bis(fluorosulfonyl) (SBPFSI) in Example 8, N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl) imide (PCFSI) in Example 9, and N-ethyl-N-methylpyrrolidinium trifluoro[trifluoromethyl]borate (PCFBF) in Example 10. The deemed shear strength and capacity retention rate of the solid secondary batteries measured by SAICAS for the solid electrolyte sheets produced in Examples 8 to 10 are shown in the graph of Figure 1. The deemed shear strength, capacity retention rate, and average coulombic efficiency measured for each Example are shown in Table 1.

[0073] Example 11 In Example 11, the amount of binder was increased as a method for softening the solid electrolyte layer. (Preparation of solid electrolyte layer) In Example 11, a solid electrolyte sheet was fabricated using the following method. 4% by mass of PVDF binder was added to 96% by mass of Li6PS5Cl solid electrolyte. Xylene and isobutyl isobutyrate were added to the mixture while stirring to create a slurry. This slurry was applied to a PET sheet using a blade coater, dried in air at 40°C, and then vacuum-dried at 40°C for 12 hours. The resulting solid electrolyte sheet was subjected to isostatic pressure treatment at 490 MPa, and its assumed shear strength was measured using SAICAS, yielding a value of 52.9 MPa. (Fabrication of solid secondary batteries) A solid secondary battery was fabricated using the produced solid electrolyte sheet, and a charge-discharge test was carried out in the same manner as in Example 1. As shown in Table 1, the capacity retention rate was 73.4%, and the average coulomb efficiency was 99.7%.

[0074] Example 12 In this example, succinonitrile (SN) was added to soften the solid electrolyte layer. (Fabrication of solid electrolyte sheets) The solid electrolyte sheet was fabricated using the following method. 1 wt% binder and 2.5 wt% SN containing 5 mol% LiTFSI were added to the Li6PS5Cl solid electrolyte, and the mixture was stirred while adding xylene and isobutyl isobutyrate to create a slurry. This was applied to PET using a blade coater, dried in air at 40°C, and then vacuum dried at 40°C for 12 hours. The resulting solid electrolyte sheet was subjected to isostatic pressure treatment at 490 MPa, and the assumed shear strength was measured using SAICAS, resulting in a value of 49.7 MPa, which met the requirements. (Fabrication of solid secondary batteries) A solid secondary battery was fabricated using the produced solid electrolyte sheet, and a charge-discharge test was carried out in the same manner as in Example 1. As shown in Table 1, the capacity retention rate was 93.1%, and the average coulomb efficiency was 99.9%.

[0075] (Comparative Example 1) A solid secondary battery was fabricated in the same manner as in Example 1, except that no ionic plastic crystals were added and a solid electrolyte sheet consisting of only 99% by mass of Li6PS5Cl solid electrolyte and 1% by mass of PVDF binder was used. The assumed shear strength of the solid electrolyte sheet fabricated in Comparative Example 1, measured with SAICAS after isostatic pressing at 490 MPa, was 88.8 MPa. The solid secondary battery produced in Comparative Example 1 was subjected to a charge-discharge test similar to that in Example 1. As a result, as shown in Table 1, the capacity retention rate was 10.1% and the average coulomb efficiency was 96.8%.

[0076] [Table 1]

[0077] (Discussion of Examples and Comparative Examples) 1, it was found that by setting the deemed shear strength of the solid electrolyte layer to 15 MPa or more and 70 MPa or less, the cycle capacity retention rate of the solid secondary battery at low confining pressure can be maintained higher than before. One of the reasons for this result is thought to be that the softer solid electrolyte layer improves the adhesion between the solid electrolyte layer and the positive and negative electrodes, and the internal voids of the solid electrolyte layer itself become smaller, resulting in a more uniform reaction distribution.

Claims

1. A solid electrolyte layer having an assumed shear strength of 15 MPa or more and 70 MPa or less.

2. The solid electrolyte layer according to claim 1 , comprising ionic plastic crystals.

3. 3. The solid electrolyte layer according to claim 2, wherein the ionic plastic crystal contains at least one cation selected from the group consisting of spiro-type ammonium cations, pyrrolidinium cations, piperidinium cations, and tetraalkylammonium cations.

4. 3. The solid electrolyte layer according to claim 2, wherein the ionic plastic crystal contains at least one anion selected from the group consisting of a sulfonylimide anion and a borate anion.

5. 5. The solid electrolyte layer of claim 4, wherein the anion comprises at least one anion selected from the group consisting of bis(fluorosulfonyl)imide anion (FSI), 1.1.2.2.3.3-hexafluoropropane-1.3-disulfonimide anion (CFSI), tetrafluoroborate anion (BF), and trifluoro(trifluoromethyl)borate anion (CFBF).

6. The solid electrolyte layer according to claim 1 , comprising succinonitrile.

7. The solid electrolyte layer according to claim 1 , comprising a lithium-containing sulfide solid electrolyte.

8. The solid electrolyte layer according to claim 7 , wherein the lithium-containing sulfide solid electrolyte contains a halogen element.

9. The solid electrolyte layer according to claim 1 , further comprising a binder, wherein the content of the binder relative to the entire solid electrolyte layer is more than 1 mass % and not more than 10 mass %.

10. The solid electrolyte layer according to claim 1 , wherein the apparent shear strength is 20 MPa or more and 60 MPa or less.

11. 2. The solid electrolyte layer according to claim 1, wherein the content of the solid electrolyte in the entire solid electrolyte layer is 80% by mass or more and 99% by mass or less.

12. A solid-state secondary battery including a positive electrode layer, a negative electrode layer, and a solid electrolyte layer, The solid secondary battery according to claim 1, wherein the solid electrolyte layer is the one according to claim 1.

13. 13. The solid secondary battery according to claim 12, wherein the negative electrode layer contains one or more materials selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, and zinc, and amorphous carbon.

14. The solid state secondary battery according to claim 13 , wherein the negative electrode layer contains silver and amorphous carbon.

15. The solid secondary battery according to claim 12 , wherein a ratio of an initial charge capacity of the positive electrode layer to an initial charge capacity of the negative electrode layer satisfies the following formula (1): 0.01<b / a<0.5 (1) a: initial charge capacity of the positive electrode layer (mAh) b: Initial charge capacity of the negative electrode layer (mAh)

16. A method for charging a solid secondary battery, comprising charging the solid secondary battery according to claim 12 beyond the charge capacity of the negative electrode layer.

17. The method for charging a solid secondary battery according to claim 16, wherein charging is performed within a range of 2 times or more and 100 times or less the charge capacity of the negative electrode layer.

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