Solid electrolyte layer and solid secondary battery

Abstract

Provided is a solid secondary battery in which performance degradation due to charging and discharging is suppressed without the application of external pressure, or under low confining pressure. The solid electrolyte layer has an assumed shear strength of 15-70 MPa.

Description

Solid electrolyte layer and solid secondary battery

[0001] The present invention relates to a solid electrolyte layer and a solid secondary battery having the same.

[0002] When charging and discharging a precipitation-type all-solid-state secondary battery, it is necessary to ensure that lithium is uniformly precipitated regardless of the shape of the negative electrode layer and to suppress volume changes in the active material. Conventionally, all-solid-state lithium-ion secondary batteries were charged and discharged using a pressurizing jig while applying high external pressure in the thickness direction.

[0003] The use of a pressurizing jig that applies high external pressure to an all-solid-state secondary battery is not only disadvantageous in terms of price, but also reduces the energy density of the secondary battery module.

[0004] Accordingly, it is necessary to reduce the external pressure applied to the all-solid-state secondary battery as much as possible. For example, in an all-solid-state secondary battery having a 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, it is considered that the ratio of the binder contained in the solid electrolyte layer be 20% by volume or more and 30% by volume or less. Furthermore, in addition to lithium salt and lithium-containing sulfide solid electrolytes, the use of a solid electrolyte containing an organic electrolyte is also considered. However, a technology for sufficiently reducing the external pressure applied to the all-solid-state secondary battery has not yet been established, and further improvements are required.

[0005] The present invention is intended to solve the aforementioned problems, and aims to provide an all-solid-state secondary battery capable of suppressing performance degradation even when charging and discharging under low confining pressure without applying external pressure or using a simple external pressure applying mechanism. The low confining pressure is, for example, 1.0 MPa or less, more preferably 0.5 MPa or less.

[0006] The solid electrolyte layer and solid secondary battery according to the present invention are as follows.

[0007] According to one embodiment of the present invention, a solid electrolyte layer having a shear strength of 15 MPa or more and 70 MPa or less is provided.

[0008] The above solid electrolyte layer may include ionic viscous crystals.

[0009] The above ionic viscosity determination may include at least one cation selected from the group consisting of spiro-type ammonium cations, pyroridinium cations, piperidinium cations, and tetraalkylammonium cations.

[0010] The above ionic viscosity determination may include at least one anion selected from the group consisting of sulfonylimide anions and borate anions.

[0011] The above anion may include at least one anion selected from the group consisting of a bis(fluorosulfonyl)imido anion (FSI), a 1,1,2,2,3,3-hexafluoropropane-1,3-disulfone imide anion (CFSI), a tetrafluoroborate anion (BF4), and a trifluoro(trifluoromethyl)borate anion (CF3BF3).

[0012] The above solid electrolyte layer may include succinonitrile.

[0013] The above solid electrolyte layer may include a lithium-containing sulfide solid electrolyte.

[0014] The above lithium-containing sulfide solid electrolyte may contain a halogen element.

[0015] The above solid electrolyte layer includes a binder, and the content of the binder relative to the entire solid electrolyte layer may be 1 mass% or more and 10 mass% or less.

[0016] The above-mentioned shear strength may be 20 MPa or more and 60 MPa or less.

[0017] The content of the solid electrolyte in the entire solid electrolyte layer may be 80 mass% or more and 99 mass% or less.

[0018] According to one embodiment of the present invention, a solid secondary battery is provided, which includes a positive electrode layer, a negative electrode layer, and a solid electrolyte layer. The solid electrolyte layer may have an assumed shear strength of 15 MPa or more and 70 MPa or less.

[0019] The cathode layer may include at least one selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, and zinc, and amorphous carbon.

[0020] The cathode layer may include silver and amorphous carbon.

[0021] According to one embodiment of the present invention, a solid secondary battery is provided, which includes a positive electrode layer, a negative electrode layer, and a solid electrolyte layer. The solid electrolyte layer has an assumed shear strength of 15 MPa or more and 70 MPa or less, and a ratio of the initial charge capacity of the positive electrode layer to the initial charge capacity of the negative electrode layer can satisfy the following mathematical expression 1.

[0022] [Mathematical Formula 1]

[0023] 0.01

[0024] a: Initial charge capacity of the positive electrode layer (mAh)

[0025] b: Initial charge capacity of the cathode layer (mAh)

[0026] According to one embodiment of the present invention, a charging method for a solid secondary battery including a positive electrode layer, a negative electrode layer, and a solid electrolyte layer is provided. The solid electrolyte layer has an assumed shear strength of 15 MPa or more and 70 MPa or less, and can be charged in excess of the charging capacity of the negative electrode layer.

[0027] It can be charged in a range of 2 to 100 times the charging capacity of the above cathode layer.

[0028] ​According to the present invention, since the deemed shear strength of the solid electrolyte layer is 15 MPa or more and 70 MPa or more, it is possible to sufficiently suppress the deterioration of the performance of the solid secondary battery even when no external pressure is applied or when a low external pressure is applied.

[0029] Figure 1 is a graph showing the shear strength of a solid electrolyte layer and the performance of a solid secondary battery according to an embodiment and a comparative example of the present invention.

[0030] Hereinafter, optimal embodiments according to the present invention will be described in detail.

[0031] <1. Basic configuration of a solid secondary battery according to the present invention>

[0032] A solid secondary battery according to one embodiment of the present invention includes, for example, a positive electrode layer, a negative electrode layer, and a solid electrolyte layer.

[0033] (1-1. Bipolar layer)

[0034] The positive electrode layer may include, for example, a positive electrode current collector and a positive electrode composite material layer.

[0035] The positive electrode current collector may be composed of, for example, indium, copper, magnesium, stainless steel, titanium, iron, cobalt, nickel, zinc, aluminum, germanium, lithium, or an alloy thereof. The positive electrode current collector may have a plate shape or a foil shape. The positive electrode current collector may be omitted.

[0036] The positive electrode composite material layer may include a positive electrode active material and a solid electrolyte. Meanwhile, the solid electrolyte of the positive electrode composite material layer may be a sulfide solid electrolyte, which may be the same as or different from the solid electrolyte layer described below.

[0037] The cathode active material can reversibly absorb and desorb lithium ions. The cathode active material can be formed of a lithium salt including, for example, 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, and / or lithium iron phosphate. Furthermore, the cathode active material can be formed of nickel sulfide, copper sulfide, lithium sulfide, sulfur, iron oxide, or vanadium oxide. These cathode active materials may be used alone or may be formed by combining two or more thereof.

[0038] It is preferable that the cathode active material contains a lithium salt of a transition metal oxide having a layered rock salt structure. Here, the 'layered rock salt structure' means a cubic rock salt structure. <111> It is a structure in which oxygen and metal atomic layers are arranged alternately and regularly in a direction, and as a result, each atomic layer forms a two-dimensional plane. The 'cubic rock salt structure' refers to the sodium chloride structure, which is one of the crystal structures, and specifically, it refers to a structure in which the face-centered cubic lattices formed by each of the positive ions (cations) and negative ions (anions) are arranged offset from each other by half of the ridge of the unit cell.

[0039] Lithium salts of transition metal oxides having this layered rock salt structure are, for example, LiNix Co y Al z O2(NCA) or LiNi x Co y Mn z O2(NCM) (but, 0 <x <1, 0 <y <1, 0 <z <1, 그리고 x+y+z=1)과 같은 삼원계 전이 금속산화물의 리튬염을 포함할 수 있다.

[0040] When the positive electrode active material contains a lithium salt of a ternary transition metal oxide having the layered rock salt structure, the energy density and thermal stability of the all-solid-state secondary battery can be improved.

[0041] The positive electrode active material may be covered by a coating layer. The coating layer of the present embodiment may be any known coating layer for the positive electrode active material of an all-solid-state secondary battery. The coating layer may be, for example, Li2-ZrO2.

[0042] 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, the capacity density of the all-solid-state secondary battery increases, and metal dissolution into the positive electrode active material in a charged state can be reduced. Accordingly, the long-term reliability and cycle characteristics in a charged state of the all-solid-state secondary battery according to the present embodiment can be improved.

[0043] Here, the positive electrode active material may have a particle shape, such as a true spherical shape or an elliptical spherical shape. Furthermore, 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 of conventional all-solid-state secondary batteries. Meanwhile, the content of the positive electrode active material in the positive electrode layer is not particularly limited, and may be within a range applicable to positive electrode layers of conventional all-solid-state secondary batteries.

[0044] In addition, in addition to the above-described positive electrode active material and solid electrolyte, additives such as a conductive additive, a binder, a filler, a dispersant, or an ion conductive additive may be additionally and appropriately blended into the positive electrode composite material layer.

[0045] The conductive agent may be, for example, graphite, carbon black, acetylene black, ketjen black, carbon fiber, or metal powder. The binder may be, for example, styrene-butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, or polyethylene. The filler, dispersant, and ion conductive agent that can be mixed into the positive electrode composite material layer may be any known material that can be used in the electrode of an all-solid-state secondary battery.

[0046] (1-2. Cathode layer)

[0047] The negative electrode layer may include a negative electrode current collector and a negative electrode composite material layer laminated on the negative electrode current collector. The negative electrode current collector is preferably composed of a material that does not react with lithium, i.e., does not form both an alloy and a compound. The material constituting the negative electrode current collector may be, for example, copper, stainless steel, titanium, iron, cobalt, or nickel.

[0048] The negative electrode composite material layer may include a negative electrode active material, a carbon material, and a binder.

[0049] The negative active material may be, for example, an element that forms an alloy or compound with lithium through an electrochemical reaction during charging (i.e., an alloy-forming element). The alloy-forming element may include one or more elements selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, and zinc.

[0050] When the alloying element is one or more of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, and zinc, the negative active material is preferably in the form of particles, for example, and the particle size is preferably 4 μm or less, more preferably 300 nm or less. In this case, the characteristics of the all-solid-state secondary battery can be further improved. Here, the particle size of the negative active material can be, for example, a median diameter (i.e., D50) measured using a laser particle size distribution meter.

[0051] The carbon material may be, for example, carbon black and / or amorphous carbon such as graphene. The carbon black may be acetylene black, furnace black, or ketjen black.

[0052] In addition to the above, the negative electrode composite material layer may appropriately include additives used in conventional all-solid-state secondary batteries (e.g., fillers, dispersants, ion conductors, or solid electrolytes).

[0053] (1-3. Solid electrolyte layer)

[0054] A solid electrolyte layer may be laminated between the positive electrode layer and the negative electrode layer and may include a solid electrolyte. The composition of the solid electrolyte layer is a characteristic of the all-solid-state secondary battery related to the present embodiment, and will be described in detail later.

[0055] (1-4. Relationship between the positive and negative charge capacities)

[0056] It is preferable that the all-solid-state secondary battery according to the present embodiment be configured so that the ratio of the charging capacity of the positive electrode composite material layer to the charging capacity of the negative electrode composite material layer (i.e., capacity ratio) satisfies the requirement of the following mathematical expression 1.

[0057] [Mathematical Formula 1]

[0058] 0.01

[0059] a: Charging capacity (mAh) of the positive electrode composite layer

[0060] b: Charge capacity (mAh) of the negative composite layer

[0061] Here, the charge capacity of the positive electrode composite material layer can be obtained by multiplying the charge capacity density (mAh / g) of the positive electrode active material by the mass of the positive electrode active material. When multiple types of positive electrode active materials are used, the value of charge capacity density × mass is calculated for each positive electrode active material, and the sum of these can be referred to as the charge capacity of the positive electrode composite material layer. The charge capacity of the negative electrode composite material layer is also calculated in the same way. In other words, the charge capacity of the negative electrode composite material layer can be obtained by multiplying the charge capacity density (mAh / g) of the negative electrode active material by the mass of the negative electrode active material. When multiple types of negative electrode active materials are used, the value of charge capacity density × mass is calculated for each negative electrode active material, and the sum of these can be referred to as the capacity of the negative electrode composite material layer. Here, the charge capacity densities of the positive electrode active material layer and the negative electrode active material can be calculated using an all-solid-state half cell using lithium metal as the counter electrode. In practice, the charge capacities of the positive electrode composite material layer and the negative electrode composite material layer are directly measured using an all-solid-state half cell.

[0062] ​A specific method for directly measuring the charge capacity may be as follows. A test cell is manufactured using a positive electrode composite material layer as the working electrode and Li as the counter electrode, and CC-CV charging is performed from the OCV (open circuit voltage) to the upper limit charge voltage, and the charge capacity of the positive electrode composite material layer is measured. The upper limit charge voltage can be determined by the standard of JIS C 8712:2015. The upper limit charge voltage for a positive electrode composite material layer of lithium cobalt acid type is 4.25 V, and the upper limit charge voltage for other positive electrode composite material layers can be obtained by applying the provisions of A.3.2.3 (Safety requirements when applying a different upper limit charge voltage) of JIS C 8712:2015. A test cell is manufactured using a negative electrode composite material layer as the working electrode and Li as the counter electrode, and CC-CV charging is performed from the OCV (open circuit voltage) to 0.01 V, and the charge capacity of the negative electrode composite material layer is measured.

[0063] The above-described test cell can be manufactured, for example, by the following method. The positive electrode composite layer or negative electrode composite layer, the charge capacity of which is to be measured, is formed into a 13 mm diameter disc shape. 200 mg of solid electrolyte powder, similar to that used in all-solid-state secondary batteries, 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. A disc-shaped positive electrode composite layer or negative electrode composite layer is placed on one side of the cylinder, and a lithium foil with a diameter of 13 mm and a thickness of 0.03 mm is placed on the opposite side. One stainless steel disc is placed on each side of the cylinder, and the entire body is pressurized at 300 MPa for 1 minute in the axial direction of the cylinder to integrate the contents. The integrated contents are removed from the cylinder and sealed in a case so that a constant pressure of 22 MPa is applied, thereby creating a test cell. The test cell manufactured in the above manner can be CC-charged at a current density of, for example, 0.1 mA and then CV-charged to 0.02 mA, so that the charge capacity of the positive electrode composite material layer can be measured.

[0064] 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 composite material layer and the negative electrode composite material layer may be the initial charge capacity measured during the first cycle of charging.

[0065] As described below, according to the present embodiment, the all-solid-state secondary battery is charged beyond the charging capacity of the negative electrode composite layer. In other words, the negative electrode composite layer is overcharged. At the initial stage of charging, lithium is absorbed into the negative electrode composite layer. In other words, the negative electrode active material forms an alloy with lithium ions that have migrated from the positive electrode layer. When the charging is further performed beyond the capacity of the negative electrode composite layer, lithium is deposited on the back side of the negative electrode composite layer (i.e., between the negative electrode current collector and the negative electrode composite layer), and a lithium deposition layer is formed by this lithium. The lithium deposition layer contains trace amounts of elements other than lithium, but is mainly composed of lithium (mainly metallic lithium).

[0066] This phenomenon occurs because the negative active material contains a specific substance (i.e., an alloy-forming element that forms an alloy or compound with lithium). During discharge, lithium within the negative electrode composite layer and lithium precipitation layer ionizes and migrates toward the positive electrode layer.

[0067] Therefore, the all-solid-state secondary battery according to the present embodiment can use precipitated lithium as an anode active material. Furthermore, since the anode composite material layer covers the lithium precipitate layer, it functions as a protective layer for the lithium precipitate layer while simultaneously suppressing the precipitation and growth of dendrites. As a result, short-circuiting and capacity reduction of the all-solid-state secondary battery can be suppressed, and furthermore, the characteristics of the all-solid-state secondary battery can be improved.

[0068] Here, the capacity ratio is preferably 0.01 or greater. When the capacity ratio is 0.01 or greater, the deterioration of the characteristics of the all-solid-state secondary battery can be suppressed. This is because the negative electrode composite material layer functions as a protective layer for the lithium precipitation layer. For example, the capacity ratio can be set to 0.01 or greater by increasing the thickness of the negative electrode composite material layer as much as possible. In this case, the collapse of the negative electrode composite material layer due to repeated charge and discharge can be prevented, and the precipitation and growth of dendrites can be suppressed. As a result, the deterioration of the characteristics of the all-solid-state secondary battery can be suppressed. In addition, the capacity ratio is preferably 0.5 or less. When the capacity ratio is 0.5 or less, the amount of lithium precipitation in the negative electrode layer can be secured, and the decrease in battery capacity can be suppressed. For the same reason, the capacity ratio is more preferably less than 0.25. When the capacity ratio is less than 0.25, the output characteristics of the secondary battery can be further improved.

[0069] The thickness of the negative electrode composite material layer is preferably 1 µm or more and 20 µm or less, which satisfies the requirements of mathematical formula 1, and more preferably 1 µm or more and 10 µm or less. When the thickness of the negative electrode composite material layer is 1 µm or more, the characteristics of the all-solid-state secondary battery can be sufficiently improved. When the thickness of the negative electrode composite material layer is 20 µm or less, the resistance value of the negative electrode composite material layer is small, so the characteristics of the all-solid-state secondary battery can be further improved.

[0070] The thickness of the aforementioned negative composite material layer can be estimated, for example, by observing the average thickness of a cross-section after assembling and pressurizing an all-solid-state secondary battery using a scanning electron microscope (SEM).

[0071] <2. Characteristics of the all-solid-state secondary battery according to the present invention>

[0072] In one embodiment of the present invention, the solid electrolyte layer may include, for example, a solid electrolyte, a plastic crystal, a binder (a binder for the solid electrolyte layer), and a dispersant. The shear strength measured by a surface interface property analysis device (SAICAS) is preferably 15 MPa or more and 70 MPa or less, and more preferably 20 MPa or more and 60 MPa or less.

[0073] The solid electrolyte is in powder form and may be, for example, a sulfide solid electrolyte. The sulfide solid electrolyte may contain, for example, lithium.

[0074] Lithium-containing sulfide solid electrolytes include, for example, Li2S-P2S5, Li2S-P2S5-LiX (X is a halogen element, for example, I, Br, Cl), Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-Z m S n (m, n are defined numbers, Z is any one of Ge, Zn or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, and Li2S-SiS2-Li p MO q (p, q are integers, M is any one of P, Si, Ge, B, Al, Ga or In) may be at least one selected from the group consisting of. Such a solid electrolyte may be formed by treating a starting material (e.g., Li2S, P2S5, etc.) by a melt quenching method or mechanical milling. In addition, an additional heat treatment may be provided after such treatment. The solid electrolyte may be amorphous, crystalline, or a mixed state thereof.

[0075] Viscous crystals are an intermediate state 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 the present embodiment, the solid electrolyte layer contains viscous crystals, making the solid electrolyte layer relatively soft, and thus allowing the shear strength of the solid electrolyte layer to be adjusted within the aforementioned range. The viscous crystals in the present embodiment are ionic and may contain cations and anions.

[0076] The cation may be, for example, at least one selected from among ammonium cations, pyrrolidium cations, pyridinium cations, pyrimidinium cations, imidazolium cations, piperidinium cations, pyrazolium cations, oxazolium cations, pyridazine cations, phosphonium cations, sulfonium cations, triazolium cations, and mixtures thereof.

[0077] Among them, spiro-type ammonium cations, tetraalkyl ammonium cations, pyrrolidium cations, and pyridinium cations are preferable.

[0078] Specific examples of such cations may be one or more selected from the group consisting of 5-Azoniaspiro[4,4]nonane (SBP), N-ethyl-N-methylpyrrolidinium (P12), tetramethylammonium (TMA), NN-dimethyl-piperidinium (PP11), triethylmethylammonium (TEMA), tetrapropylammonium (TPA), and ethyltrimethylammonium (ETMA).

[0079] The anion is preferably, for example, sulfonylimide and / or borate. The anion may be, for example, one or more selected from the group consisting of bis(fluorosulfonyl)imido (FSI), 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide (CFSI), tetrafluoroborate (BF4), and trifluoro(trifluoromethyl)borate (CF3BF3).

[0080] For example, when 5-Azoniaspiro [4,4] nonane is used as the cation and 1,1,2,2,3,3-hexafluoropropane-1,3-disulfone imide is used as the anion, the ionic viscous crystal may be 5-Azoniaspiro [4,4] nonane 1,1,2,2,3,3-hexafluoropropane-1,3-disulfone imide (SBPCFSI). The ionic viscous crystal according to the present embodiment may include various combinations of cations and anions.

[0081] For the entire solid electrolyte layer (i.e., when the mass of the entire solid electrolyte layer is 100 mass%), the content of ionic viscous crystals is preferably 1 mass% or more and 20 mass% or less, and more preferably 1.5 mass% or more and 15 mass% or less.

[0082] The ionic viscous crystals may contain lithium salts. Any lithium salt used in the relevant technical field may be used. Lithium salts include, for example, LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiCF3SO3, Li(CF3SO2)2N, Li(FSO2)2N, LiC4F9SO3, LiAlO2, LiAlCl4, LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2) (wherein x and y are natural numbers), LiCl, and LiI, or a mixture thereof. The concentration of the lithium salt is preferably 0.1 M or more and 5 M or less.

[0083] The solid electrolyte layer may further include a binder (a binder for the solid electrolyte). The binder for the solid electrolyte layer may be, for example, at least one selected from the group consisting of styrene-butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene. The binder in the solid electrolyte layer may be the same as the binder contained in the positive electrode composite material layer and the negative electrode composite material layer, but is not limited thereto.

[0084] The content of the solid electrolyte binder for 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.

[0085] The solid electrolyte layer may further include a dispersant. The dispersant is used to prevent the solid electrolyte from agglomerating when forming the solid electrolyte layer, and may be a well-known material that can be generally used in the electrodes of solid secondary batteries.

[0086] <3. Method for manufacturing a solid secondary battery according to the present invention>

[0087] A method for manufacturing a solid secondary battery according to one embodiment of the present invention is described. A solid secondary battery can be manufactured by manufacturing a positive electrode layer, a negative electrode layer, and a solid electrolyte layer, respectively, and then laminating the respective layers.

[0088] (3-1. Bipolar manufacturing process)

[0089] First, the materials that make up the positive electrode composite layer (positive electrode active material, solid electrolyte, binder, etc.) are mixed and processed into a sheet shape. These sheets are cut into an appropriate shape, and a positive electrode composite layer is laminated onto a positive electrode current collector to obtain a laminate. This laminate is then pressurized (e.g., using hydrostatic pressure) to produce a positive electrode layer comprising a positive electrode current collector and a positive electrode composite layer.

[0090] (3-2. Cathode manufacturing process)

[0091] First, materials constituting the negative electrode composite layer (negative active material, non-alloying elements, binder, etc.) are added to a polar or non-polar solvent to produce a slurry. Next, the slurry is applied onto the negative electrode current collector and dried. The resulting laminate is then pressurized (e.g., using hydrostatic pressure) to produce the negative electrode. The pressurization process may be omitted.

[0092] Meanwhile, the negative electrode can be manufactured by forming a negative composite material layer separately, then laminating and pressing it on the negative electrode current collector.

[0093] (3-3. Solid electrolyte sheet manufacturing process)

[0094] The solid electrolyte layer can be manufactured, for example, by the following sequence or process.

[0095] First, the starting raw material is processed using a melt quenching method or mechanical milling method.

[0096] For example, in the case of the melt quenching method, starting materials (e.g., Li2S, P2S5, etc.) are mixed in a predetermined amount, made into a pellet shape, reacted at a predetermined reaction temperature in a vacuum, and then quenched to produce a sulfide-based solid electrolyte material. Meanwhile, the reaction temperature of the mixture of Li2S and P2S5 is preferably 400°C to 1000°C, and more preferably 800°C to 900°C. The reaction time is preferably 0.1 hour to 12 hours, and more preferably 1 hour to 12 hours. The quenching temperature of the reactants is usually 10°C or lower, and preferably 0°C or lower. The quenching speed is usually about 1°C / sec to 10,000°C / sec, and preferably about 1°C / sec to 1000°C / sec.

[0097] In the case of the mechanical milling method, starting raw materials (e.g., Li2S, P2S5, etc.) can be stirred and reacted using a ball mill or the like to produce a sulfide-based solid electrolyte material. Meanwhile, the stirring speed and stirring time in the mechanical milling method are not particularly limited, but a fast stirring speed can accelerate the formation speed of the sulfide-based solid electrolyte material, and a long stirring time can increase the conversion rate of the raw materials into the sulfide-based solid electrolyte material.

[0098] Thereafter, the mixed raw material obtained by the melting quenching method or mechanical milling method can be heat-treated at a predetermined temperature and then pulverized to produce a solid electrolyte in particle form. If the solid electrolyte has a glass transition point, it can be transformed from an amorphous state to a crystalline state through heat treatment.

[0099] Subsequently, the solid electrolyte obtained by the above method can be mixed with an ionic viscous crystal (a solvent and binder are added if necessary), applied, dried, and then pressed to produce a solid electrolyte sheet.

[0100] (3-4. Assembly process of solid secondary batteries)

[0101] A solid secondary battery according to the present embodiment can be manufactured by stacking and pressing (for example, pressing using hydrostatic pressure) a positive electrode layer, a negative electrode layer, and a solid electrolyte sheet so that the solid electrolyte sheet is inserted between the positive electrode layer and the negative electrode layer.

[0102] <4. Charging method of a solid secondary battery according to the present invention>

[0103] Next, a charging method of a solid-state secondary battery will be described. According to one embodiment of the present invention, as described above, the solid-state secondary battery is charged beyond the charging capacity of the negative electrode composite material layer. In other words, the negative electrode composite material layer is overcharged. At the initial stage of charging, lithium is absorbed into the negative electrode composite material layer. When the negative electrode composite material layer is charged beyond the charging capacity, lithium is deposited, for example, on the back side of the negative electrode composite material layer (i.e., between the negative electrode current collector and the negative electrode composite material layer), thereby forming a lithium deposition layer that did not exist during manufacture. During discharge, lithium in the negative electrode composite material layer and the lithium deposition layer is ionized and moves toward the positive electrode.

[0104] Meanwhile, the charging amount is preferably 2 to 100 times, more preferably 4 to 100 times, the charging capacity of the negative electrode composite material layer. The thickness of the lithium precipitation layer is preferably 10 μm or more, and more preferably 30 μm or more. Furthermore, the thickness of the lithium precipitation layer is preferably in the range of 60 μm or less, which is the upper limit feasible as a solid secondary battery. Meanwhile, the thickness of the lithium precipitation layer can be obtained by observing the average thickness of a cross-section after charging the solid secondary battery using a scanning electron microscope (SEM).

[0105] <5. Effects of the present invention>

[0106] According to one embodiment of the present invention, the shear strength of the solid electrolyte layer is 15 MPa or more and 70 MPa or less. Therefore, even when the volume change of the secondary battery is large due to lithium precipitation caused by overcharging exceeding the charging capacity of the negative electrode composite material layer, the deterioration of the secondary battery performance can be sufficiently suppressed. Accordingly, the solid secondary battery can be charged and discharged without problems even when no external pressure is applied or when a sufficiently low external pressure is applied.

[0107] <6. About other embodiments of the present invention>

[0108] Although the best embodiment of the present invention has been described in detail above, the present invention is not limited thereto. For example, according to the above-described embodiment, the shear strength of the solid electrolyte layer is adjusted within a desired range by the solid electrolyte layer including ionic viscous crystals. However, the present invention is not limited thereto, and for example, the solid electrolyte layer may include succinonitrile. The content of succinonitrile relative to the entire solid electrolyte layer is preferably 0.1 mass% or more and 20 mass% or less.

[0109] The method for adjusting the shear strength of the solid electrolyte layer to a desired range is not limited to using the viscosity determination described above, and can be achieved, for example, by changing the type of the solid electrolyte. In this case, the solid electrolyte preferably contains a halogen element. The content of the halogen element in the solid electrolyte may be 1 mass% or more and 20 mass% or less, more preferably 5 mass% or more and 15 mass% or less. Accordingly, the solid electrolyte can be made relatively soft, and the shear strength of the solid electrolyte layer can be adjusted to the optimal range described above. The halogen element is preferably, for example, chlorine or bromine.

[0110] Another method for adjusting the shear strength of the solid electrolyte to a desired range may be to adjust the content of the binder for the solid electrolyte layer. In this case, the content of the binder for the solid electrolyte relative to the entire solid electrolyte layer is preferably 1 mass% or more and 10 mass% or less, and more preferably 1 mass% or more and 5 mass% or less.

[0111] In addition, a person having ordinary knowledge in the technical field to which the present invention pertains will understand that various conceivable changes or modifications within the scope of the technical idea described in the patent claims also fall within the technical scope of the present invention.

[0112] In the following examples, the solid secondary battery according to the present invention is described in more detail, but the present invention is not limited to these examples.

[0113] <Example 1>

[0114] (Manufacturing of solid electrolyte sheets)

[0115] According to the present embodiment, an ionic viscous crystal was added as a method of softening the solid electrolyte layer. The ionic viscous crystal may be 5-Azoniaspiro[4,4]nonane 1.1.2.2.3.3-Hexafluoropropane-1.3-disulfonimide (SBPCFSI). Such ionic viscous crystal was formed by the following method. 10 g of 5-Azoniaspiro[4.4]nonane chloride (SBPCl) was dissolved in 100 mL of ultrapure water. Subsequently, 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 the solution. The white solid thus obtained was collected by filtration, washed with distilled water, and then vacuum dried at 120°C for 12 hours to obtain ionic viscous crystals (SBPCFSI).

[0116] After that, a solid electrolyte sheet was manufactured by the following method. Li6PS5Cl solid electrolyte, PVDF binder, and SBPCFSI containing LiTFSI at a mass ratio of 96.5: 1: 2.5 were mixed, and xylene and isobutyl isobutyrate were added thereto while stirring to manufacture a slurry. This was coated on 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. After the obtained solid electrolyte sheet was isotropically pressurized at 490 MPa, the shear strength of the solid electrolyte sheet measured using a surface interface material analysis device (SAICAS) was 49.0 MPa.

[0117] Meanwhile, the measurement of the shear strength of the solid electrolyte sheet using a surface interface property analysis device (SAICAS DN-GS type, DAIPLA WINTES Co. LTD) was performed under the following conditions.

[0118] Cutting speed: horizontal 2㎛ / s, vertical 0.2㎛ / s

[0119] Cutting depth: 20㎛

[0120] Blade width: 1mm

[0121] Angle: rake angle 10°, clearance angle 20°

[0122] (Fabrication of the cathode layer)

[0123] Porous carbon black, silver particles, and a binder (PVDF) were mixed in a mass ratio of 70:23:7, and stirred while adding NMP little by little to form a slurry. The slurry was applied onto a 10 μm thick stainless steel foil using a blade coater, dried in air at 80°C for about 20 minutes, and then vacuum-dried at 100°C for about 12 hours to produce a cathode layer.

[0124] (Making of anode layer)

[0125] Next, LiNi as the positive electrode active material 0.8 Co 0.15 Mn 0.05 O2(NCM), Li6PS5Cl as a solid electrolyte, carbon nanotubes (CNT) as a conductor, and PTFE as a binder were mixed in a mass ratio of 85: 14.45: 0.25: 0.3, and stretched into a sheet shape to form a cathode sheet.

[0126] Subsequently, the ionic viscous crystal SBPCFSI was dissolved in dichloromethane to a concentration of 2 wt%. The positive electrode sheet was immersed in this solution and evacuated to 0.05 MPa to impregnate the positive electrode sheet with the ionic viscous crystal. The positive electrode sheet was taken out of the solution and dried, thereby obtaining a positive electrode sheet impregnated with the ionic viscous crystal. This was pressed onto a positive electrode current collector made of an 18 μm thick aluminum foil to produce a positive electrode layer. Meanwhile, the surface of the NCM used in this example was coated with Li2O-ZrO2.

[0127] (Manufacturing of solid secondary batteries)

[0128] Between a PET sheet and a stainless steel foil, a solid electrolyte sheet and a cathode layer, manufactured as described above, were laminated, and an isotropic pressure treatment was performed at 50 MPa. The PET sheet was peeled off to transfer the solid electrolyte sheet onto the cathode layer. A cathode layer was provided on the surface of the solid electrolyte sheet exposed by peeling off the PET sheet, and the solid electrolyte layer was sealed with a laminate film in a vacuum, thereby manufacturing a solid secondary battery in which a solid electrolyte layer was laminated between the cathode layer and the cathode layer.

[0129] The portions of the aluminum foil of the positive electrode layer and the stainless steel foil of the negative electrode layer that protrude outside the laminate film so as not to break the vacuum of the secondary battery become the positive terminal and the negative terminal, respectively.

[0130] These solid secondary batteries were isotropically pressurized at 490 MPa.

[0131] (Evaluation of solid secondary batteries)

[0132] The charge-discharge characteristics of the solid secondary battery manufactured in this way were evaluated under the following conditions under a confining pressure of 0.3 MPa. The measurements were performed in a constant temperature chamber at 25°C. The current was 1.5 mA / cm until the battery voltage reached 4.25 V. 2 Charge with a constant current of 0.5 mA / cm 2 It was charged at a constant voltage of 4.25 V until it reached 1.5 mA / cm. 2 The battery was discharged at a constant current until the battery voltage reached 2.5 V. This charging and discharging process was repeated 100 times. When the discharge capacity of the first charge / discharge test is 100%, the capacity retention rate, which is expressed as a percentage of the discharge capacity of the 100th time, was 93.2%. In addition, the average Coulomb efficiency, which is the average value of 100 times for the percentage of the discharge capacity to 100% of the charge capacity in each cycle of the charge / discharge test mentioned above, was 99.9%.

[0133] <Examples 2 to 7>

[0134] In Examples 2 to 7, a solid electrolyte sheet and a solid secondary battery were manufactured in the same order as Example 1, except that the content of the ionic viscous crystals included in the solid electrolyte sheet was different.

[0135] The content of the ionic viscous crystals included 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. At this time, the content of the binder included in the solid electrolyte sheet did not change, and the amount of the solid electrolyte was increased or decreased.

[0136] The shear strength and capacity retention rate of the solid secondary battery measured by SAICAS of the solid electrolyte sheets manufactured in Examples 2 to 7 are shown in the graph of Fig. 1.

[0137] The measured shear strength, capacity retention, and average Coulombic efficiency for each example are shown in Table 1.

[0138] <Examples 8 to 10>

[0139] In Examples 8 to 10, a solid electrolyte sheet and a solid secondary battery were manufactured in the same order as Example 1, except that the type of ionic viscous crystal included in the solid electrolyte sheet was changed.

[0140] The types of ionic viscosity determinations used in each example are as follows.

[0141] In Example 8, it was spiro-(1,1′bis(fluorosulfonyl)(SBPFSI), in Example 9, it was N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl) imide(P12CFSI), and in Example 10, it was N-ethyl-N-methylpyrrolidinium trifluoro [trifluoromethyl]borate(P12CF3BF3).

[0142] The shear strength and capacity retention ratio of the solid secondary battery measured by SAICAS of the solid electrolyte sheets manufactured in Examples 8 to 10 are shown in the graph of Fig. 1.

[0143] The measured shear strength, capacity retention, and average Coulombic efficiency in each example are shown in Table 1.

[0144] (Example 11)

[0145] Example 11 used a change in binder weight as a method of softening the solid electrolyte layer.

[0146] (Fabrication of solid electrolyte layer)

[0147] In Example 11, a solid electrolyte sheet was manufactured by the following method.

[0148] A slurry was prepared by adding 4 mass% of PVD binder to 96 mass% of Li6PS5Cl solid electrolyte, and stirring while adding xylene and isobutyl isobutyrate. The slurry was applied onto a PET sheet using a blade coater, dried in air at 40°C, and then vacuum-dried at 40°C for 12 hours. The obtained solid electrolyte sheet was isotropically pressurized to 490 MPa, and the shear strength measured by SAICAS was 52.9 MPa.

[0149] (Manufacturing of solid secondary batteries)

[0150] A solid secondary battery was fabricated using the fabricated solid electrolyte sheet, and a charge / discharge test was performed as in Example 1. As shown in Table 1, the capacity retention rate was 73.4%, and the average coulombic efficiency was 99.7%.

[0151] (Example 12)

[0152] Example 12 added succinonitrile (SN) as a method of softening the solid electrolyte layer.

[0153] (Manufacturing of solid electrolyte sheets)

[0154] A solid electrolyte sheet was manufactured using the following method.

[0155] For the Li6PS5Cl solid electrolyte, SN containing 1 wt% of binder and 2.5 wt% of LiTFSI in a 5 mol% ratio was added, and a slurry was prepared by stirring while adding xylene and isobutyl isobutyrate. The slurry was applied onto PET using a blade coater, dried in air at 40°C, and then vacuum-dried at 40°C for 12 hours. The obtained solid electrolyte sheet was isotropically pressurized to 490 MPa, and the shear strength measured by SAICAS was 49.7 MPa, which met the requirements.

[0156] (Manufacturing of solid secondary batteries)

[0157] A solid secondary battery was manufactured using the manufactured solid electrolyte sheet, and a charge / discharge test was performed as in Example 1. As shown in Table 1, the capacity retention rate was 93.1%, and the average coulombic efficiency was 99.9%.

[0158] (Comparative Example 1)

[0159] A solid secondary battery was manufactured in the same manner as in Example 1, except that a solid electrolyte sheet composed of 99 mass% of Li6PS5Cl solid electrolyte and 1 mass% of PVDF binder was used without adding ionic viscous crystals. The shear strength measured by SAICAS after isotropically pressurizing the solid electrolyte sheet manufactured in Comparative Example 1 at 490 MPa was 88.8 MPa.

[0160] As a result of conducting a charge / discharge test similar to Example 1 on the solid secondary battery manufactured in Comparative Example 1, the capacity retention rate was 10.1% and the average coulombic efficiency was 96.8%, as shown in Table 1.

[0161] [Table 1]

[0162]

[0163] (Consideration of practical examples and comparative examples)

[0164] From the results of Examples 1 to 12 and Comparative Example 1 shown in Table 1 and Fig. 1, it was possible to maintain a higher cycle capacity retention rate of the solid secondary battery under low confining pressure than before by having the shear strength of the solid electrolyte layer be 15 MPa or more and 70 MPa or less. One of the reasons for this result may be the softening of the solid electrolyte layer, the improved adhesion between the solid electrolyte layer and the positive and negative electrode layers, and the uniformity of the reaction distribution due to the reduction in the internal gap of the solid electrolyte layer itself.

Claims

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

2. In claim 1, A solid electrolyte layer comprising ionic viscous crystals.

3. In claim 2, The above ionic viscosity determination is a solid electrolyte layer comprising at least one cation selected from the group consisting of spiro-type ammonium cations, pyrolithinium cations, piperidinium cations, and tetraalkylammonium cations.

4. In claim 2, The above ionic viscosity determination is a solid electrolyte layer comprising at least one anion selected from the group consisting of sulfonylimide anions and borate anions.

5. In claim 4, A solid electrolyte layer comprising at least one anion selected from the group consisting of a bis(fluorosulfonyl)imido anion (FSI), a 1,1,2,2,3,3-hexafluoropropane-1,3-disulfone imide anion (CFSI), a tetrafluoroborate anion (BF4), and a trifluoro(trifluoromethyl)borate anion (CF3BF3).

6. In claim 1, A solid electrolyte layer containing succinonitrile.

7. In claim 1, A solid electrolyte layer comprising a lithium-containing sulfide solid electrolyte.

8. In claim 7, The above lithium-containing sulfide solid electrolyte is a solid electrolyte layer containing a halogen element.

9. In claim 1, A solid electrolyte layer comprising a binder, wherein the content of the binder relative to the entire solid electrolyte layer is 1 mass% or more and 10 mass% or less.

10. In claim 1, The above-mentioned solid electrolyte layer having a shear strength of 20 MPa or more and 60 MPa or less.

11. In claim 1, A solid electrolyte layer having a solid electrolyte content of 80 mass% or more and 99 mass% or less for the entire solid electrolyte layer.

12. In a solid secondary battery including a positive electrode layer, a negative electrode layer, and a solid electrolyte layer, A solid secondary battery wherein the shear strength of the above solid electrolyte layer is 15 MPa or more and 70 MPa or less.

13. In claim 12, A solid secondary battery wherein the cathode layer comprises at least one selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, and zinc, and amorphous carbon.

14. In claim 13, The above cathode layer is a solid secondary battery comprising silver and amorphous carbon.

15. In a solid secondary battery including a positive electrode layer, a negative electrode layer, and a solid electrolyte layer, The shear strength of the above solid electrolyte layer is 15 MPa or more and 70 MPa or less, A solid secondary battery in which 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 mathematical expression 1: [Mathematical Formula 1] 0.01 a: Initial charge capacity of the positive electrode layer (mAh) b: Initial charge capacity of the cathode layer (mAh).

16. A charging method for a solid secondary battery including a positive electrode layer, a negative electrode layer, and a solid electrolyte layer, Including charging exceeding the charging capacity of the above cathode layer, The shear strength of the above solid electrolyte layer is 15 MPa or more and 70 MPa or less. A method for charging a solid secondary battery.

17. In claim 16, A method for charging a solid secondary battery, wherein the charging is performed in a range of 2 to 100 times the charging capacity of the cathode layer. ​

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