Solid-state secondary batteries

Abstract

To provide a solid secondary battery including a solid electrolyte layer containing only an oxide solid electrolyte as the solid electrolyte, in which short-circuiting or the decrease in battery capacity due to separation of lithium can be suppressed sufficiently.SOLUTION: A solid secondary battery includes a positive electrode layer, a negative electrode layer, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer. The solid electrolyte layer contains only an oxide solid electrolyte as the solid electrolyte. The binding strength between the negative electrode layer and the solid electrolyte layer is 14 mN / mm or more and 100 mN / mm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Solid-state secondary batteries using lithium as the negative electrode active material include those that use lithium deposited on the negative electrode layer upon charging as the active material. In such solid-state secondary batteries, if the lithium deposited on the negative electrode layer grows into branches that weave through the gaps in the solid electrolyte layer, it not only causes a short circuit in the battery but also reduces the battery capacity.

[0003] Therefore, Patent Document 1 has been proposed as an all-solid-state secondary battery capable of suppressing the generation and growth of lithium dendrites in the solid electrolyte layer. In the all-solid-state secondary battery described in Patent Document 1, an alloying element that forms an alloy or compound with lithium is used as the negative electrode active material. This allows lithium to be absorbed into the negative electrode active material layer at the beginning of charging, and after the charge capacity of this negative electrode active material layer is exceeded, lithium can be deposited inside the negative electrode active material layer or on the back side (current collector side) of the negative electrode active material layer. As a result, the generation and growth of lithium dendrites in the solid electrolyte layer can be suppressed, thereby preventing short circuits and a decrease in battery capacity. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-096610 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when the anode layer described in Patent Document 1 was used in combination with a solid electrolyte layer containing only an oxide-based solid electrolyte as the solid electrolyte, the expected short-circuit suppression effect could not be obtained. Therefore, it is considered that there is room for improvement in suppressing lithium dendrites when used in combination with a solid electrolyte layer containing only an oxide-based solid electrolyte as the solid electrolyte. Therefore, an object of the present invention is to provide a solid secondary battery that can sufficiently suppress the generation and growth of lithium dendrites and suppress short circuits and reductions in battery capacity, even in a solid secondary battery that has a solid electrolyte layer that contains only an oxide-based solid electrolyte as the solid electrolyte. [Means for solving the problem]

[0006] The present invention was completed only after the inventors conducted extensive research to solve the above-mentioned problems and found that, by controlling the binding strength between the anode layer and the solid electrolyte layer, it is possible to achieve a sufficient short-circuit suppression effect even in a solid secondary battery having a solid electrolyte layer containing only an oxide-based solid electrolyte as the solid electrolyte.

[0007] That is, the solid secondary battery according to the present invention comprises a positive electrode layer, a negative electrode layer, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer, wherein the solid electrolyte layer contains only an oxide-based solid electrolyte as the solid electrolyte, and the bonding strength between the negative electrode layer and the solid electrolyte layer is 14 mN / mm or more and 100 mN / mm or less.

[0008] In a solid secondary battery configured in this manner, the adhesive strength between the anode layer and the solid electrolyte layer is set to 14 mN / mm or more and 100 mN / mm or less, so that even when a solid electrolyte layer made of an oxide-based solid electrolyte, which is harder than a solid electrolyte layer containing a sulfide-based solid electrolyte, is used, lithium precipitation between the anode layer and the solid electrolyte layer can be suppressed, and the generation and growth of lithium dendrites can be sufficiently suppressed, thereby suppressing short circuits and capacity reduction of the solid secondary battery due to the generation and growth of lithium dendrites.

[0009] The negative electrode layer preferably includes a plate-shaped or foil-shaped negative electrode current collector and a negative electrode active material layer laminated on the negative electrode current collector, and the film strength of the negative electrode active material layer is 16 MPa or more and 85 MPa or less, because this can prevent lithium from being deposited in the negative electrode active material layer and prevent cracks from occurring in the negative electrode active material layer.

[0010] In a specific embodiment of the present invention, the negative electrode active material layer contains a negative electrode active material that forms an alloy or compound with lithium. [Effects of the Invention]

[0011] According to the solid secondary battery of the present invention, in a solid secondary battery in which lithium is precipitated in the negative electrode layer by charging the negative electrode active material layer beyond the initial charge capacity, even when an oxide-based solid electrolyte is used as the solid electrolyte that forms the solid electrolyte layer, it is possible to suppress short circuits and reductions in battery capacity due to lithium precipitation. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a cross-sectional view showing a schematic configuration of a solid secondary battery according to one embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view schematically illustrating the deposition of lithium when the solid secondary battery according to the present embodiment is charged. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant explanations will be omitted.

[0014] <1. Basic configuration of the solid secondary battery according to this embodiment> The solid state secondary battery according to this embodiment is an all-solid state secondary battery 1 including a positive electrode layer 10, a negative electrode layer 20, and a solid electrolyte layer 30, as shown in FIG.

[0015] (1-1. Positive electrode layer) The positive electrode layer 10 includes a positive electrode current collector 11 and a positive electrode active material layer 12. Examples of the positive electrode current collector 11 include a plate-shaped or foil-shaped body made of indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), germanium (Ge), lithium (Li), or an alloy thereof. The positive electrode current collector 11 may be omitted.

[0016] The positive electrode active material layer 12 includes a positive electrode active material and a solid electrolyte. The solid electrolyte included in the positive electrode layer 12 may be an oxide-based solid electrolyte or a sulfide-based solid electrolyte, which will be described in the section on the solid electrolyte layer 30.

[0017] The positive electrode active material may be any positive electrode active material that can reversibly store and release lithium ions.

[0018] For example, the positive electrode active material can be formed using lithium salts such as lithium cobalt oxide (hereinafter referred to as LCO), lithium nickel oxide, lithium nickel cobalt oxide, lithium nickel cobalt 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.

[0019] 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-mentioned lithium salts. Here, the "layered rock salt structure" is a structure in which oxygen atom layers and metal atom layers are alternately and regularly arranged in the <111> direction of the cubic rock salt structure, and as a result, each atom layer forms a two-dimensional plane. The "cubic rock salt structure" represents a sodium chloride-type structure, which is a type of crystal structure. Specifically, it represents a structure in which the face-centered cubic lattices formed by each of the cations and anions are displaced from each other by 1 / 2 of the edge of the unit lattice.

[0020] 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.

[0021] 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 all-solid-state secondary battery 1 can be improved.

[0022] The positive electrode active material may be covered with a coating layer. Here, the coating layer of this embodiment may be any known coating layer for a positive electrode active material of an all-solid-state secondary battery. Examples of the coating layer include Li2O-ZrO2.

[0023] 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, it is possible to increase the capacity density of the all-solid-state secondary battery 1 and reduce metal elution from the positive electrode active material in a charged state. This allows the all-solid-state secondary battery 1 according to this embodiment to have improved long-term reliability and cycle characteristics in a charged state.

[0024] 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 all-solid-state secondary batteries. The content of the positive electrode active material in the positive electrode layer 10 is also not particularly limited, and may be within a range applicable to positive electrode layers in conventional all-solid-state secondary batteries.

[0025] In addition to the above-described positive electrode active material and solid electrolyte, the positive electrode layer 10 may contain additives such as a conductive aid, a binder, a filler, a dispersant, an ion conductive aid, or the like, as appropriate.

[0026] Examples of conductive additives that can be blended into the positive electrode layer 10 include graphite, carbon black, acetylene black, ketjen black, carbon fiber, and metal powder. Examples of binders that can be blended into the positive electrode layer 10 include styrene butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene. Furthermore, known materials generally used in electrodes of all-solid-state secondary batteries can be used as fillers, dispersants, ion conductive additives, and the like that can be blended into the positive electrode layer 10.

[0027] (1-2. Negative electrode layer) The negative electrode layer 20 includes a negative electrode current collector 21 and a negative electrode active material layer 22 laminated on the negative electrode current collector 21. The negative electrode current collector 21 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 to make the negative electrode current collector 21 include copper (Cu), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), and nickel (Ni). The negative electrode current collector 21 may be made of any one of these metals, or may be made of an alloy or clad material of two or more metals. The negative electrode current collector 21 is, for example, in the form of a plate or foil.

[0028] The negative electrode active material layer 22 contains a negative electrode active material. The negative electrode active material may include, for example, amorphous carbon and an alloying element that forms 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. Examples of amorphous carbon include carbon black and graphene. Examples of carbon black include acetylene black, furnace black, and ketjen black. To improve electronic conductivity, the silicon surface may be coated with a carbon layer having a thickness of approximately 1 nm to 10 nm.

[0029] 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 preferably, for example, granular, with a particle size of 4 μm or less, more preferably 300 nm or less. In this case, the characteristics of the all-solid-state secondary battery 1 are further improved. Here, the particle size of the negative electrode active material is determined by the median diameter (so-called D ) measured using, for example, a laser particle size distribution system. 50 ) can be used.

[0030] In addition to the above, the negative electrode active material layer 22 may also contain additives used in conventional all-solid-state secondary batteries, such as binders, fillers, dispersants, ion conductive agents, solid electrolytes, and the like, as appropriate.

[0031] (1-3.Solid electrolyte layer) The solid electrolyte layer 30 is formed between the positive electrode layer 10 and the negative electrode layer 20 and includes a solid electrolyte.

[0032] In this embodiment, an oxide-based solid electrolyte is used as the solid electrolyte. Examples of oxide-based solid electrolytes include Li 1.3 Al 0.3 Ti1.7 (PO4)3, Li 0.34 La 0.51 TiO 2.94 , Li 1.07 Al 0.69 Ti 1.46 (PO4)3, 50Li4SiO4-50Li2BO3, 90Li3BO3-10Li2SO 4、 Li 2.9 PO 3.3 N 0.46 , Li7La3Zr2O 12 The following can be mentioned:

[0033] The solid electrolyte layer 30 may further contain a binder. Examples of binders include styrene butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene. The binder in the solid electrolyte layer 30 may be the same as or different from the binder in the positive electrode active material layer 12 and the negative electrode active material layer 22.

[0034] In this embodiment, the solid electrolyte layer 30 contains only the above-mentioned oxide-based solid electrolyte as the solid electrolyte. More specifically, the solid electrolyte layer 30 of this embodiment is made of only the oxide-based solid electrolyte.

[0035] (1-4. Relationship between charge capacity of positive electrode layer and negative electrode layer) The all-solid-state secondary battery 1 according to this embodiment is configured so that the ratio of the charge capacity of the positive electrode active material layer 12 to the charge capacity of the negative electrode active material layer 22, i.e., the capacity ratio, satisfies the requirement of the following mathematical formula (1). 0.01 a: Charging capacity (mAh) of the positive electrode active material layer 12 b: Charging capacity (mAh) of the negative electrode active material layer 22

[0036] ​Here, the charge capacity of the positive electrode active material layer 12 is obtained by multiplying the charge capacity density (mAh / g) of the positive electrode active material by the mass of the positive electrode active material in the positive electrode active material layer 12. When multiple types of positive electrode active materials are used, the value of charge capacity density x mass is calculated for each positive electrode active material, and the sum of these values ​​is taken as the charge capacity of the positive electrode active material layer 12. The charge capacity of the negative electrode active material layer 22 is also calculated in a similar manner. That is, the charge capacity of the negative electrode active material layer 22 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 active material layer 22. 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 taken as the capacity of the negative electrode active material layer 22. Here, the charge capacity densities of the positive and negative electrode active materials are capacities estimated using an all-solid-state half cell using lithium metal as the counter electrode. In practice, the charge capacities of the positive electrode active material layer 12 and the negative electrode active material layer 22 are directly measured by measurement using an all-solid-state half cell.

[0037] Specific methods for directly measuring the charge capacity include the following. First, the charge capacity of the positive electrode active material layer 12 is measured by preparing a test cell using the positive electrode active material layer 12 as the working electrode and Li as the counter electrode, and performing CC-CV charging from the open circuit voltage (OCV) to an upper charge voltage. The upper charge voltage is defined 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 a Different Upper Charge Voltage is Applied) of JIS C 8712:2015 for other positive electrodes. The charge capacity of the negative electrode active material layer 22 is measured by preparing a test cell using the negative electrode active material layer 22 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.

[0038] The test cell described above can be fabricated, for example, by the following method. The positive electrode active material layer 12 or negative electrode active material layer 22 for which the charge capacity is to be measured is punched out into a disk shape with a diameter of 13 mm. 200 mg of the same solid electrolyte powder used in the all-solid-state secondary battery 1 is compacted at 40 MPa to form a pellet with a diameter of 13 mm and a thickness of approximately 1 mm. This pellet is placed inside a cylinder with an inner diameter of 13 mm, and the punched-out disk-shaped positive electrode active material layer 12 or negative electrode active material layer 22 is placed on one side, and 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 under a constant pressure of 22 MPa to form a test cell. The charge capacity of the positive electrode active material layer 12 can be measured by CC charging the test cell prepared as described above at a current density of, for example, 0.1 mA, and then CV charging it down to 0.02 mA.

[0039] 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 active material layer 12 and the negative electrode active material layer 22 may be the initial charge capacities measured during the first charge cycle. This value was used in the examples described below.

[0040] As such, the charge capacity of the positive electrode active material layer 12 becomes excessively large relative to the charge capacity of the negative electrode active material layer 22. As will be described later, in this embodiment, the all-solid-state secondary battery 1 is charged beyond the charge capacity of the negative electrode active material layer 22. That is, the negative electrode active material layer 22 is overcharged. At the initial stage of charging, lithium is absorbed into the negative electrode active material layer 22. That is, the negative electrode active material forms an alloy with lithium ions that have migrated from the positive electrode layer 10. When further charging is performed beyond the capacity of the negative electrode active material layer 22, as shown in FIG. 2, lithium is precipitated on the back side of the negative electrode active material layer 22, i.e., between the negative electrode current collector 21 and the negative electrode active material layer 22, and a lithium precipitate layer 23 is formed by this lithium. The lithium precipitate layer 23 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 active material layer 22 and the lithium precipitate layer 23 ionizes and migrates to the positive electrode layer 10. Therefore, in the all-solid-state secondary battery 1 according to this embodiment, the precipitated lithium can be used as the negative electrode active material. Furthermore, the negative electrode active material layer 22 covers the lithium precipitate layer 23, and therefore functions as a protective layer for the lithium precipitate layer 23 and can suppress the precipitation and growth of dendrites. This suppresses short circuits and capacity reduction in the all-solid-state secondary battery 1, and ultimately improves the characteristics of the all-solid-state secondary battery 1.

[0041] Here, the capacity ratio is greater than 0.01. If the capacity ratio is 0.01 or less, the characteristics of the all-solid-state secondary battery 1 deteriorate. One reason for this is that the anode active material layer 22 no longer functions adequately as a protective layer. For example, if the thickness of the anode active material layer 22 is very thin, the capacity ratio may be 0.01 or less. In this case, repeated charge / discharge cycles may cause the anode active material layer 22 to collapse, leading to the precipitation and growth of dendrites. This results in a deterioration in the characteristics of the all-solid-state secondary battery 1. It is presumed that the characteristics of the all-solid-state secondary battery in Patent Document 1 were not sufficiently improved because the interface layer or carbon layer was too thin. Furthermore, the capacity ratio is preferably less than 0.5. This is because a capacity ratio of 0.5 or more may reduce the amount of lithium precipitation in the anode, resulting in a decrease in battery capacity. For the same reason, it is considered more preferable that the capacity ratio be less than 0.25. Furthermore, a capacity ratio of less than 0.25 can further improve the output characteristics of the battery.

[0042] The thickness of the anode active material layer 22 is not particularly limited as long as it satisfies the requirement 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 anode active material layer 22 is less than 1 μm, the characteristics of the all-solid-state secondary battery 1 may not be sufficiently improved. If the thickness of the anode active material layer 22 exceeds 20 μm, the resistance value of the anode active material layer 22 increases, and as a result, the characteristics of the all-solid-state secondary battery 1 may not be sufficiently improved. The thickness of the negative electrode active material layer 22 can be estimated, for example, by assembling an all-solid-state secondary battery, pressure-molding the battery, and then observing the average thickness of the cross section with a scanning electron microscope (SEM).

[0043] 2. Characteristic Configuration of the Solid State Secondary Battery According to the Present Embodiment The all-solid-state secondary battery 1 according to this embodiment is characterized in that the binding strength between the negative electrode active material layer 22 and the solid electrolyte layer 30 is 14 mN / mm or more and 100 mN / mm or less. This binding strength is more preferably 14 mN / mm or more and 98 mN / mm or less, and particularly preferably 20 mN / mm or more and 61 mN / mm or less. The adhesive strength between the negative electrode active material layer 22 and the solid electrolyte layer 30 can be measured by measuring the peel strength using an AGS-X manufactured by Shimadzu Corporation.

[0044] Various methods can be considered for adjusting the adhesive strength between the anode active material layer 22 and the solid electrolyte layer 30 to the aforementioned range. A specific example of such an approach is to adjust the surface roughness of the solid electrolyte layer 30, for example, to a range of 0.05 μm to 0.6 μm, more preferably 0.1 μm to 0.5 μm, and particularly preferably 0.2 μm to 0.4 μm, in terms of Sa (arithmetic mean height). Adjusting the surface roughness of the contact surface of the solid electrolyte layer 30 with the anode active material layer 22 in this manner increases the contact area between the solid electrolyte layer 30 and the anode active material layer 22, thereby improving the adhesive strength therebetween. The surface roughness of the solid electrolyte layer 30 can be controlled by the polishing conditions of the solid electrolyte layer 30 and the strength of the acid treatment, which will be described later. The strength of the acid treatment can be controlled, for example, by the concentration of the acid used, the treatment time, the treatment temperature, etc.

[0045] Furthermore, the negative electrode active material layer 22 of the all solid state secondary battery 1 according to this embodiment preferably has a film strength of 16 MPa or more and 85 MPa or less, and more preferably 18 MPa or more and 82 MPa or less. The film strength of the negative electrode active material layer 22 can be measured by measuring the shear strength using SAICAS manufactured by Daipla Wintes Co., Ltd. The film strength of the negative electrode active material layer 22 can be controlled, for example, by adjusting the combination of the type of amorphous carbon contained as the negative electrode active material, the type of binder, the binder content, etc. For example, when amorphous carbon with a relatively large specific surface area is used, the number of binders bonded to one amorphous carbon tends to increase, so the binder content needs to be increased to maintain film strength. Furthermore, when a string-like binder with a large molecular weight is used, the binders tend to adhere to each other or to the amorphous carbon, even when the binder content is kept low, which tends to increase film strength.

[0046] In this embodiment, examples of binders that can be used in the negative electrode active material layer 22 include styrene butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene difluoride, polyethylene, and polymethyl methacrylate (PMMA). As the binder, one of these may be used alone, or two or more may be used in combination.

[0047] The binder content to achieve ideal film strength is preferably 0.5 mass % to 30 mass % relative to the total mass of the negative electrode active material layer 22, and more preferably 3 mass % to 20 mass %. It is preferable that the content of the alloying element relative to the entire negative electrode active material layer 22 is 5% by mass or more and 25% by mass or less, and the content of amorphous carbon is 50% by mass or more and 90% by mass or less, and it is more preferable that the content of the alloying element is 8% by mass or more and 23% by mass or less, and the content of amorphous carbon is 60% by mass or more and 86% by mass or less.

[0048] 3. Method for manufacturing a solid secondary battery according to this embodiment Next, a description will be given of a method for manufacturing the all solid state secondary battery 1 according to this embodiment. The all solid state secondary battery 1 according to this embodiment can be manufactured by manufacturing the positive electrode layer 10, the negative electrode layer 20, and the solid electrolyte layer 30, and then laminating the above layers.

[0049] (3-1. Positive electrode layer manufacturing process) First, the materials (such as the positive electrode active material and binder) constituting the positive electrode active material layer 12 are mixed and then laminated on the positive electrode current collector 11. The resulting laminate is then pressed (for example, by hydrostatic pressure) to produce the positive electrode layer 10. The pressing step may be omitted. The positive electrode layer 10 may also be produced by compacting the mixture of materials constituting the positive electrode active material layer 12 into a pellet or by stretching it into a sheet. When producing the positive electrode layer 10 by these methods, the positive electrode current collector 11 may be pressed onto the produced pellet or sheet. The positive electrode active material layer 12 may also be formed by adding the materials constituting the positive electrode active material layer to a nonpolar solvent to produce a slurry (the slurry may be a paste, and the same applies to other slurries), applying the slurry to the positive electrode current collector 11, and drying the slurry.

[0050] (3-2. Negative electrode layer manufacturing process) First, a slurry is prepared by adding anode active material layer materials (anode active material, non-alloying element, binder, etc.) constituting the anode active material layer 22 to a polar solvent or a non-polar solvent. The obtained slurry is then applied to the anode current collector 21 and dried. The obtained laminate is then pressed (for example, by applying hydrostatic pressure) to prepare the anode layer 20. The pressing step may be omitted. Alternatively, the anode layer 20 may be prepared by a method in which the anode active material layer 22 is separately formed, and then laminated on the anode current collector 21 and pressed.

[0051] (3-3. Solid electrolyte layer manufacturing process) The solid electrolyte layer 30 can be produced, for example, by the following procedure or steps.

[0052] The solid electrolyte layer 30 includes a blending step of blending raw materials to obtain a blended material and a firing step of firing the obtained blended material. Here, as an example, a method for producing the solid electrolyte layer 30 made of a sintered body of a garnet-type oxide will be described.

[0053] In the blending step, raw materials containing at least a Li component, a La component, and a Zr component are blended to obtain a blended material. The components contained in the blended material are in a ratio that allows a lithium ion conductive ceramic material having a garnet-type crystal structure or a crystal structure similar to the garnet-type crystal structure to be obtained.

[0054] A sintering aid such as boron oxide is added to the blended materials, and the materials are mixed in a ball mill or jet mill to obtain a precursor powder. The precursor powder thus obtained is placed in a mold or the like and pressure-molded to obtain precursor pellets.

[0055] Next, in the firing process, the precursor pellets formed as described above are heated for about 1 to 36 hours at a temperature of about 900°C to 1250°C. The sintering temperature and time can be changed as appropriate depending on the material composition, etc.

[0056] The heating method is not particularly limited, and can be resistance heating, microwave heating, etc. The sintering process can be divided into two stages: preliminary sintering and main sintering, or the sintering process can include the molding process described above and employ electric current sintering, spark plasma sintering, etc.

[0057] In this embodiment, the surface of the solid electrolyte layer 30 thus fabricated is subjected to an acid treatment. This acid treatment step can be carried out using, for example, phosphoric acid. The concentration of phosphoric acid used in the phosphate treatment is preferably 1 mol / L to 10 mol / L, more preferably 5 mol / L to 6 mol / L. The temperature during the acid treatment is preferably 20°C to 60°C, more preferably 30°C to 55°C. The acid treatment time is preferably 5 seconds to 10 minutes, more preferably 30 seconds to 5 minutes, and particularly preferably 1 minute to 2 minutes. The acid concentration, treatment temperature, treatment time, etc. used in the acid treatment can be changed as appropriate depending on the material of the solid electrolyte layer used, etc.

[0058] Before the acid treatment, the surface roughness may be adjusted by polishing the surface of the solid electrolyte layer 30. Polishing is preferable because it makes the solid electrolyte layer 30 less likely to become brittle than when the surface roughness of the solid electrolyte layer 30 is adjusted only by the acid treatment. When polishing the solid electrolyte layer 30, it is preferable to polish it with abrasive paper of #280 or more and #5000 or less, and then perform finish polishing with a lapping film of #300 or more and #10000 or less. It is more preferable to use abrasive paper of #300 or more and #2000 or less, and particularly preferable to use a lapping film of #600 or more and #1500 or less. It is more preferable to use a lapping film of #400 or more and #4000 or less, and particularly preferable to use a lapping film of #600 or more and #2000 or less, for finish polishing.

[0059] (3-4. Assembly process of all-solid-state secondary batteries) The positive electrode layer 10, the negative electrode layer 20, and the solid electrolyte layer 30 produced by the above-described method are stacked so that the solid electrolyte layer 30 is sandwiched between the positive electrode layer 10 and the negative electrode layer 20, and pressurized (for example, pressurized using hydrostatic pressure), thereby producing the all-solid-state secondary battery 1 according to this embodiment.

[0060] When the all-solid-state secondary battery 1 produced by the above method is operated, it is preferable to do so while applying pressure to the all-solid-state battery.

[0061] The pressure may be 0.5 MPa or more and 10 MPa or less. The pressure may be applied by sandwiching the all-solid-state battery between two hard plates such as stainless steel, brass, aluminum, or glass, and fastening the two plates with screws.

[0062] 4. Charging Method of Solid-State Secondary Battery According to the Present Embodiment Next, a method for charging the all-solid-state secondary battery 1 will be described. In this embodiment, as described above, the all-solid-state secondary battery 1 is charged beyond the charge capacity of the anode active material layer 22. That is, the anode active material layer 22 is overcharged. In the initial stage of charging, lithium is absorbed in the anode active material layer 22. When charging is performed beyond the charge capacity of the anode active material layer 22, for example, as shown in FIG. 2, lithium is precipitated on the back side of the anode active material layer 22, i.e., between the anode current collector 21 and the anode active material layer 22, and this lithium forms a lithium precipitate layer 23 that was not present at the time of manufacture. During discharge, the lithium in the anode active material layer 22 and the lithium precipitate layer 23 is ionized and moves toward the cathode layer 10. The charge amount is preferably set to a value between 2 and 100 times the charge capacity of the negative electrode active material layer 22, more preferably in the range of 4 to 100 times. The thickness of the lithium deposition layer 23 deposited in the negative electrode layer during charging is preferably 10 μm or more, more preferably 20 μm or more, and is preferably in the range of 60 μm or less, which is the upper limit value feasible for a solid secondary battery. The thickness of the lithium deposition layer 23 can be estimated by observing the average thickness of a cross section of the all-solid-state secondary battery 1 after charging with a scanning electron microscope (SEM).

[0063] <5. Effects of this embodiment> In the all-solid-state secondary battery 1 configured as described above, the binding strength between the anode active material layer 22 and the solid electrolyte layer 30 is 20 mN / mm or more and 100 mN / mm or less, and therefore, lithium that precipitates when the anode active material layer 22 is overcharged can be prevented from precipitating between the anode active material layer 22 and the solid electrolyte layer 30.

[0064] Furthermore, since the film strength of the negative electrode active material layer 22 is 16 MPa or more and 85 MPa or less, deposition of lithium inside the negative electrode active material layer 22 can be suppressed.

[0065] As described above, according to the all-solid-state secondary battery 1 according to this embodiment, lithium can be selectively deposited only between the negative electrode active material layer 22 and the negative electrode current collector 21. As a result, the negative electrode active material layer 22 functions as a protective layer for the lithium deposit layer 23 deposited between the negative electrode current collector 21 and the negative electrode active material layer 22, making it possible to more effectively suppress the deposition and growth of lithium dendrites.

[0066] For the reasons explained above, in the all solid state secondary battery 1 according to this embodiment, short circuits and capacity reductions are suppressed, and the characteristics of the all solid state secondary battery are improved.

[0067] <6. Other embodiments of the present invention> The solid electrolyte layer is not limited to one containing only the oxide-based solid electrolyte as described above, but may also contain an oxide-based solid electrolyte and a binder or the like.

[0068] In the above embodiment, the case of an all-solid-state secondary battery in which the positive electrode layer, the negative electrode layer, and the solid electrolyte layer are all solid has been described. However, the present invention is applicable to any solid-state secondary battery that includes a solid negative electrode layer and a solid solid electrolyte layer, and is also applicable to, for example, a solid-state secondary battery in which part or all of the positive electrode layer is not solid, a solid-state secondary battery that contains an electrolytic solution in addition to a solid electrolyte, and the like.

[0069] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention. [Example]

[0070] 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.

[0071] <Fabrication of solid secondary batteries> (Production of negative electrode layer) For each example and comparative example, a silver-carbon mixed particle thin film or a silicon-carbon mixed particle thin film was prepared as the negative electrode active material by the following method. Silver or silicon particles with a particle diameter of approximately 60 nm were used. Carbon black (two types: CB-1 and CB-2) manufactured by Asahi Carbon Co., Ltd. and acetylene black (AB) manufactured by Denka Corporation were used. 6 g of carbon and 2 g of silver particles (or silicon particles) were placed in a container, and an NMP solution containing a binder was added. The mixture was stirred while gradually adding NMP to prepare a slurry. This slurry was applied to a 10-micron-thick SUS negative electrode current collector 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 layer. The binder used was either PVdF, PMMA, or SBR. The composition of the negative electrode active material layer for each example and comparative example is shown in Table 1 below. The ratios listed in the column for the composition of the negative electrode active material layer in Table 1 represent the mass ratios of each component. Note that the carbon CB-1 described above has a specific surface area of ​​approximately 50 m 2 / g, and CB-2 has a specific surface area of ​​approximately 300m 2 / g, AB has a specific surface area of ​​approximately 50m 2 / g.

[0072] (Fabrication of solid electrolyte layer) The solid electrolyte layer was made of oxide-based solid electrolyte (LLZO) pellets (Toshima Manufacturing Co., Ltd., Li7La3Zr2-xTaxO12 (LLZ-Ta)) that had been treated with 5 mol / L phosphoric acid for 1 minute and then vacuum-dried. In Examples 15 and 16, a polishing treatment was performed before the acid treatment. More specifically, after polishing with #1000 abrasive paper, a finish polishing was performed with #600 lapping film. The grit sizes of the abrasive paper and lapping film are not limited to those mentioned above, and similar effects can be obtained even if grit sizes within a range of about ±20% of the mentioned grit sizes are used.

[0073] (Preparation of positive electrode layer) LiNi as the positive electrode active material 0.8 Co 0.15 Al 0.05 The cathode active material layer was fabricated by mixing O2 (NCA), Li6PS5Cl solid electrolyte, carbon nanofiber (CNF) conductive material, and PTFE binder (Daikin) in a ratio of 88:12:2:1 (by mass) of cathode active material, solid electrolyte, CNF, and PTFE binder, and stretching the mixture into a sheet. A 10 μm thick aluminum foil was used as the cathode current collector, and the cathode active material layer was laminated on this aluminum foil to fabricate the cathode layer.

[0074] (Fabrication of solid secondary batteries) The cathode, solid electrolyte, and anode layers were stacked in this order, sealed in a laminate film in a vacuum, and subjected to hydrostatic pressure treatment at 490 MPa for 30 minutes to produce an all-solid-state secondary battery. A portion of each of the cathode and anode layers was exposed to the laminate film so as not to break the vacuum in the battery, and these exposed portions served as the cathode and anode terminals, respectively.

[0075] <Measurement of the adhesive strength between the negative electrode active material layer and the solid electrolyte layer> Peel strength was measured using an AGS-X tester manufactured by Shimadzu Corporation. Specifically, a negative electrode layer was placed on one side of a solid electrolyte layer using the same procedure as in the secondary battery fabrication process described above, and then subjected to hydrostatic pressure treatment. The negative electrode current collector was pulled at a negative acceleration of 100 mm / min to measure the 90° peel strength. The peel strength was measured by calculating the average value of the measurements from the start to the end of peeling. For samples in which the negative electrode active material layer could not be peeled from the surface of the solid electrolyte layer even after peeling the negative electrode current collector, adhesive tape was applied to the surface of the negative electrode active material layer after peeling the negative electrode current collector, and the 90° peel strength was measured when peeling the adhesive tape.

[0076] <Measurement of film strength of negative electrode active material layer> The shear strength (deemed shear strength) was measured using SAICAS manufactured by Daipla Wintes Co., Ltd. Specifically, the film strength of the negative electrode active material layer was measured by cutting the layer with a diamond blade and measuring the shear strength at that time. The specific experimental conditions were as follows: Cutting blade specifications: clearance angle 10°, cutting angle 60°, rake angle 20°, cutting width 1mm Cutting blade movement conditions: horizontal 2μm / sec, vertical: 0.2μm / sec

[0077] <Evaluation of charging current density characteristics> The charging current density characteristics of the all-solid-state secondary battery thus fabricated were evaluated under the following conditions. The measurements were carried out by placing the all-solid-state secondary battery in a thermostatic chamber at 25°C. Charging to 4.25 V and discharging to 2.5 V were repeated, with the discharge current after charging always being 0.3 mA / cm. 2 The charging was performed at a constant current of 0.3 mA / cm. The current density was increased for each cycle, and in the first cycle, the current density was 0.3 mA / cm. 2 , then 0.1mA / cm 2 The maximum current at which charging was possible without causing a short circuit is shown in the CCD (Critical Current Density) column in Table 1. The CCD was 2.6 mA / cm 2 That is, 2.6mA / cm without short circuit. 2 In Table 1, the adhesive strength indicates the adhesive strength between the negative electrode active material layer and the solid electrolyte layer, and the film strength indicates the film strength of the negative electrode active material layer.

[0078] [Table 1]

[0079] The results in Table 1 show that in Examples 1 to 16, in which the adhesive strength between the negative electrode active material layer and the solid electrolyte layer was in the range of 14 mN / mm or more and 100 mN / mm or less, the CCD value was 1.0 or more, and a charging current density sufficient for practical use was achieved. On the other hand, in Comparative Examples 1 and 2, in which the binding strength was outside the range of 14 mN / mm or more and 100 mN / mm or less, it was found that a short circuit occurred when the charging current density was less than 1.0. From this, it is considered that, as long as the binding strength between the negative electrode active material layer and the solid electrolyte layer is within the range of 14 mN / mm or more and 100 mN / mm or less, short-circuiting of the solid secondary battery due to lithium precipitation can be sufficiently suppressed even when an oxide-based solid electrolyte is used. A higher binding strength is considered preferable because it can suppress lithium precipitation between the solid electrolyte layer and the negative electrode active material layer. However, the higher the binding strength, the more brittle the solid electrolyte layer becomes. Therefore, to easily balance these, the binding strength is preferably in the range of 14 mN / mm or more and 61 mN / mm or less. With current technology, achieving a binding strength of more than 100 mN / mm results in the problem of the solid electrolyte becoming brittle. However, if a technology is developed in the future that can increase the binding strength without embrittlement of the solid electrolyte as much as possible, it may be possible to realize a solid secondary battery with a binding strength exceeding this upper limit.

[0080] The results of Examples 1, 3 and 6 show that the binding strength tends to increase by increasing the temperature of the acid treatment of the solid electrolyte layer. Comparison of Examples 1, 4 and 5 revealed that similar binding strength could be obtained even when the type of carbon contained in the negative electrode active material layer was changed. The results of Examples 5, 9, and 10, which were under almost the same conditions except for the type of binder used, showed that similar binding strength could be obtained even if the type of binder contained in the negative electrode active material layer was changed. Furthermore, comparisons between Example 2 and Example 15, and Example 6 and Example 16 revealed that polishing the surface of the solid electrolyte layer before the acid treatment can minimize the brittleness of the solid electrolyte and further improve the binding strength. These results show that the adhesive strength between the negative electrode active material layer and the solid electrolyte layer can be controlled by changing the conditions of the acid treatment of the solid electrolyte layer, such as the temperature and polishing.

[0081] The surface roughness (Sa) of the surface of the solid electrolyte layer used in Example 1 that was in contact with the negative electrode active material layer was measured and found to be 0.21 μm. Since the Sa of the solid electrolyte layer in Example 6 was 0.28 μm and the Sa of the solid electrolyte layer in Example 16 was 0.35 μm, it is believed that the adhesive strength between the negative electrode active material layer and the solid electrolyte layer can be changed by adjusting the surface roughness of the solid electrolyte layer.

[0082] Furthermore, among Examples 1 to 16, in Examples 1 to 7 and 12 to 16 in which the film strength of the negative electrode active material layer was 16 MPa or more and 85 MPa or less, the CCD was 1.5 or more, and it was found that charging was possible without short-circuiting even at higher charging current densities.

[0083] From the results of Examples 4 and 5, it is believed that the film strength can be controlled by the type of amorphous carbon used in the negative electrode active material layer. Furthermore, a comparison between Examples 6, 7 and 11 reveals that increasing the binder content in the negative electrode active material layer tends to increase the film strength of the negative electrode active material layer. Furthermore, the results of Examples 5, 9 and 10 show that the film strength of the negative electrode active material layer also varies depending on the type of binder contained in the negative electrode active material layer. From this result, it is considered that the film strength of the negative electrode active material layer can be controlled by changing the type of amorphous carbon, the type of binder, and the binder content used in the negative electrode active material layer. [Explanation of symbols]

[0084] 1 Solid state secondary battery 10 Positive electrode layer 11 Positive electrode current collector 12 Cathode active material layer 20 negative electrode layer 21 Negative electrode current collector 22 Negative electrode active material layer 23 Lithium deposit layer 30 Solid electrolyte layer

Claims

1. A solid-state secondary battery comprising a positive electrode layer, a negative electrode layer, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer, the solid electrolyte layer contains only an oxide-based solid electrolyte as the solid electrolyte, the negative electrode layer includes a negative electrode active material layer, an arithmetic mean height Sa of a surface of the solid electrolyte layer in contact with the negative electrode active material layer is in the range of 0.05 μm or more and 0.6 μm or less; the adhesive strength between the negative electrode layer and the solid electrolyte layer is 14 mN / mm or more and 100 mN / mm or less, a negative electrode layer including a plate-shaped or foil-shaped negative electrode current collector and a negative electrode active material layer laminated on the negative electrode current collector, and the negative electrode active material layer has a film strength of 16 MPa or more and 85 MPa or less.

2. The solid secondary battery according to claim 1 , wherein the negative electrode active material layer contains a negative electrode active material that forms an alloy or a compound with lithium.

3. An acid treatment step of subjecting a solid electrolyte layer containing only an oxide-based solid electrolyte as a solid electrolyte to an acid treatment; and an assembly step of laminating the solid electrolyte layer that has been subjected to the acid treatment step between a positive electrode layer and a negative electrode layer so that the binding strength between the solid electrolyte layer and the negative electrode layer is 14 mN / mm or more and 100 mN / mm or less.

4. The method for producing a solid secondary battery according to claim 3 , further comprising a polishing step of polishing the surface of the solid electrolyte layer.

5. A method for charging the solid secondary battery according to claim 1 or 2, A method for charging a solid secondary battery, comprising charging the negative electrode active material layer beyond its charge capacity.

6. The method for charging a solid secondary battery according to claim 5, wherein charging is performed so that the thickness of the lithium deposit layer deposited on the negative electrode layer is in the range of 20 μm to 60 μm.

7. 3. The solid secondary battery according to claim 2, wherein the negative electrode active material that forms an alloy or compound with lithium contains an alloy-forming element selected from gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, zinc, or a combination thereof.

8. 3. The solid secondary battery according to claim 2, wherein the negative electrode active material that forms an alloy or compound with lithium is in a particulate form, and the particle size of the particles is 4 μm or less.

9. The solid secondary battery according to claim 8 , further comprising a carbon layer disposed on the particle surface, wherein the carbon layer has a thickness of 1 nm or more and 10 nm or less.

10. The solid secondary battery according to claim 2 , wherein the negative electrode active material that forms an alloy or compound with lithium contains an alloy-forming element, and the negative electrode active material further contains amorphous carbon.

11. 11. The solid secondary battery according to claim 10, wherein the content of the alloying element is 5 parts by weight or more and 25 parts by weight or less, and the content of the amorphous carbon is 50 parts by weight or more and 90 parts by weight or less, relative to 100 parts by weight of the negative electrode active material layer.

12. 3. The solid secondary battery according to claim 2, wherein the negative electrode active material layer further contains a binder, and the content of the binder is 0.5 parts by weight or more and 30 parts by weight or less per 100 parts by weight of the negative electrode active material layer.

13. the positive electrode layer includes a positive electrode active material layer, and the negative electrode layer includes a negative electrode active material layer, 2. The solid secondary battery according to claim 1, wherein a ratio of a charge capacity of the positive electrode active material layer to a charge capacity of the negative electrode active material layer satisfies the following formula (1): 0.01<b / a<0.5 (1) a: Filling capacity of the positive electrode active material layer b: Filling capacity of the negative electrode active material layer

14. 2. The solid secondary battery according to claim 1, wherein the negative electrode active material layer has a thickness of 1 μm or more and 20 μm or less.

15. The oxide-based solid electrolyte is Li １.３ Al ０.３ Ti １.７ (PO ４ ) ３ , Li ０.３４ La ０.５１ TiO ２.９４ , Li １.０７ Al ０.６９ Ti １.４６ (PO ４ ) ３ , 50Li ４ SiO ４ -50Li ２ BO ３ , 90Li ３ BO ３ -10Li ２ SO ４ , Li ２.９ P.O. ３.３ N ０.４６ , Li ７ La ３ Zr ２ O １２ or a combination thereof.

16. the negative electrode layer further includes a plate-shaped or foil-shaped negative electrode current collector, the solid state secondary battery further includes a lithium deposition layer disposed between the negative electrode current collector and the negative electrode active material layer, the lithium deposit layer comprises lithium metal or a lithium alloy; 2. The solid secondary battery according to claim 1, wherein the lithium precipitate layer has a thickness of 10 μm or more and 60 μm or less.

17. the positive electrode layer includes a positive electrode active material layer, and the positive electrode active material layer includes a positive electrode active material, 2. The solid state secondary battery according to claim 1, wherein the positive electrode active material comprises a lithium salt, nickel sulfide, copper sulfide, lithium sulfide, sulfur, iron oxide, or vanadium oxide, or a combination thereof.

18. The solid secondary battery according to claim 17 , wherein the positive electrode active material comprises a lithium salt of a transition metal oxide having a layered structure represented by the following chemical formula 1 or 2: <Chemical formula 1> L)) ｘ Co ｙ Mn ｚ O ２ <Chemical formula 2> L)) ｘ Co ｙ Al ｚ O ２ (In the above Chemical Formula 1 or 2, 0<x<1, 0<y<1, 0<z<1, and x+y+z=1.)

19. the positive electrode layer comprises a positive electrode active material layer, and the positive electrode active material layer further contains a solid electrolyte; The solid secondary battery according to claim 1 , wherein the solid electrolyte comprises an oxide-based solid electrolyte or a sulfide-based solid electrolyte.

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