JP2021093344A - Garnet-type solid electrolyte separator and method of producing the same - Google Patents

Abstract

To provide a garnet-type solid electrolyte separator with high short circuit resistance, and a method of producing the same.SOLUTION: The garnet-type solid electrolyte separator includes: a garnet-type solid electrolyte; a carbon-enriched layer present on at least one surface of the garnet-type solid electrolyte separator; and a carbon-enriched part present in a range from the surface to a depth of 10 μm.SELECTED DRAWING: None

Description

The present application relates to a garnet type solid electrolyte separator and a method for producing the same.

In all-solid-state lithium-ion batteries, it has been conventionally studied to use a garnet-type solid electrolyte as a material for a separator.

Patent Document 1 discloses an oxide all-solid-state battery using a garnet-type oxide solid electrolyte sintered body as a separator.
Patent Document 2 discloses a technique for suppressing a short circuit by coating a thin lithium carbonate layer on the surface of a solid electrolyte substrate containing a garnet-type solid electrolyte. Specifically, the following items are described. The crystal particle size of the solid electrolyte is 10 μm or less. As a result, the unevenness of the surface of the solid electrolyte is reduced and the specific surface area is improved, so that short circuits due to non-uniform distance between terminals are suppressed, and interfacial resistance can be reduced. By reducing the interfacial resistance, short circuit can be suppressed by concentrating the current locally. Further, a lithium carbonate layer having a thickness of 100 nm or less is formed on the solid electrolyte substrate. As a result, holes and scratches on the solid electrolyte substrate can be filled, and short circuits can be further suppressed.

JP-A-2018-142432 JP-A-2017-199539

As described in Patent Document 2, by coating the surface of the solid electrolyte layer with a lithium carbonate layer, short circuits are certainly suppressed. However, this effect is limited, and a short circuit cannot be suppressed under the condition that a current is passed for several tens of minutes or more at a high current density of ^{4 mA / cm 2 or more and the dissolution and precipitation of metallic lithium are repeated.} .. Therefore, in the invention described in Patent Document 2, there is still room for improvement in short-circuit resistance.

Therefore, in view of the above circumstances, it is an object of the present application to provide a garnet-type solid electrolyte separator having high short-circuit resistance and a method for producing the same.

As a result of diligent studies on the above problems, the present inventors have further improved short-circuit resistance by providing a carbon-concentrated layer on the surface of the garnet-type solid electrolyte sintered body and further providing a carbon-concentrated portion inside the vicinity of the surface. It was found that it was improved. Based on this finding, the present application discloses the following means for solving the above problems.

That is, the present application contains a garnet-type solid electrolyte as one means for solving the above-mentioned problems, a carbon-concentrated layer is present on at least one surface, and carbon is provided in a range from the surface to a depth of 10 μm. Disclosed is a garnet-type solid electrolyte separator in which a concentrated portion is present.

In the above garnet type solid electrolyte separator, the garnet type solid electrolyte is _{selected from the group consisting of (Li 7-3Y-Z} , Al _Y ) (La ₃ ) (Zr _2-Z , M _Z ) O ₁₂ (M = Nb, Ta). At least one or more elements, Y and Z are preferably any number in the range of 0 ≦ Y <0.22, 0 ≦ Z ≦ 2). Further, it is preferable that a peak of _{Li 2} CO ₃ is present in the XPS spectrum of the surface of the garnet type solid electrolyte separator. The surface composition calculated by EDX preferably has a carbon concentration of 6.8% by mass or more. Alternatively, the surface composition calculated by EDX preferably has a carbon / oxygen ratio of 0.17 or more and a carbon / zirconium ratio of 0.23 or more on a mass basis. Further, in the composition of the fractured surface calculated by EDX when the garnet type solid electrolyte separator is divided in the thickness direction so that the carbon-enriched portion is exposed, the average carbon in the range from the surface to the position of 10 μm in depth is obtained. The concentration is preferably 10% by mass or more. More preferably, the average carbon concentration is 20% by mass or more.

Further, in the present application, as one means for solving the above-mentioned problems, a solvent contacting step of bringing a solvent containing an oxygen element into contact with the surface of a garnet-type solid electrolyte sintered body, and a carbon enrichment of the surface after the solvent contacting step. Disclosed is a method for producing a garnet-type solid electrolyte separator, which comprises a heating step of heating at a temperature equal to or higher than the layer formation temperature.

In the method for producing the garnet-type solid electrolyte separator, the solvent is preferably alcohol. Further, in the heating step, it is preferable to heat the surface at a temperature of 450 ° C. or higher and lower than 700 ° C.

According to the present disclosure, it is possible to provide a garnet-type solid electrolyte separator having a high critical current density and excellent short-circuit resistance as compared with the prior art. Further, it is possible to provide a method for producing the garnet-type solid electrolyte separator.

It is a schematic diagram paying attention to the vicinity of the surface of the split cross section of a separator. It is a flowchart of manufacturing method 10. It is a figure explaining the estimation mechanism of carbon enrichment. It is XPS spectrum (O-1s) of Example 1 and Comparative Examples 1 and 2. It is XPS spectrum (C-1s) of Example 1 and Comparative Examples 1 and 2. 6 is an SEM image of a split cross section of the separator according to Comparative Example 1. 6 is an SEM image of a split cross section of the separator according to Comparative Example 2. 6 is an SEM image of a split cross section of the separator according to the first embodiment. This is the result of calculating the average carbon concentration of the fractured surface of the separator according to Comparative Example 1 by SEM-EDX. This is the result of calculating the average carbon concentration of the fractured surface of the separator according to Comparative Example 2 by SEM-EDX. This is the result of calculating the average carbon concentration of the fractured surface of the separator according to Comparative Example 3 by SEM-EDX. This is the result of calculating the average carbon concentration of the fractured surface of the separator according to Example 1 by SEM-EDX. This is the result of calculating the average carbon concentration of the fractured surface of the separator according to Example 2 by SEM-EDX.

In the present specification, the notation "A to B" for the numerical values A and B means "A or more and B or less". When a unit is attached only to the numerical value B in such a notation, the unit shall be applied to the numerical value A as well.

[Garnet type solid electrolyte separator]
The garnet-type solid electrolyte separator of the present disclosure contains a garnet-type solid electrolyte, has a carbon-enriched layer on at least one surface, and has a carbon-enriched portion in a range from the surface to a depth of 10 μm. It is characterized by being.

The garnet-type solid electrolyte separator of the present disclosure has the above-mentioned characteristics, has a higher current density than the conventional one, and exhibits excellent short-circuit resistance.

Each configuration will be described below.

(Garnet type solid electrolyte)
In the present disclosure, the garnet-type solid electrolyte is a solid electrolyte having Li ion conductivity, containing at least Li, and having a garnet-type crystal structure represented by the chemical composition of _{Li x} A ₃ B ₂ O _12. .. Here, X satisfies the relationship of X = 24-3a-2b, where a is the valence of A and b is the valence of B. Examples of such a garnet-type solid electrolyte include a group of (Li _7-3Y-Z , Al _Y ) (La ₃ ) (Zr _2-Z , M _Z ) O ₁₂ (M = Nb, Ta) called LLZ. At least one or more elements selected from the above. Y and Z are arbitrary numbers in the range of 0 ≦ Y <0.22 and 0 ≦ Z ≦ 2). As the LLZ, one having a known composition can be adopted. For example, Li ₇ La ₃ Zr ₂ O ₁₂ can be mentioned as a typical composition.

The content of the garnet-type solid electrolyte is preferably 50% by mass or more, more preferably 80% by mass or more, and 90% by mass or more, when the garnet-type solid electrolyte separator is 100% by mass. Is even more preferable. If the content of the garnet-type solid electrolyte is less than 50% by mass, the lithium ion conductivity may decrease. The upper limit of the content of the garnet-type solid electrolyte is not particularly limited, and may be 99% by mass or less.

(Carbon concentrated layer)
The garnet-type solid electrolyte separator of the present disclosure has a carbon-concentrated layer on at least one surface. The "carbon-enriched layer" refers to a portion in which a region having a higher carbon element concentration than the other portion exists in a layered manner. The carbon-enriched layer may be provided on both surfaces of the garnet-type solid electrolyte separator. Here, the “surface” means a surface that is in contact with or faces the positive electrode layer or the negative electrode layer when the garnet type solid electrolyte separator is used in a lithium ion battery.

The carbon-enriched layer is _{preferably mainly composed of Li 2} CO ₃ (lithium carbonate). _{“Mainly” means that the content of Li 2} CO ₃ in the carbon-enriched layer is 50% by mass or more. It is preferably 80% by mass or more, and more preferably 90% by mass or more. The upper limit of the content of Li ₂ CO ₃ is not particularly limited, and the carbon-enriched layer may be entirely composed of Li ₂ CO ₃ .

The garnet-type solid electrolyte separator can improve short-circuit resistance by providing a carbon-concentrated layer on the surface. This is because defects such as microcracks existing on the surface of the solid electrolyte sintered body are covered with the carbon-concentrated layer, and it is considered that the precipitation and progress of lithium originating from the defects can be suppressed. ..

The thickness of the carbon-enriched layer is not particularly limited, but it is preferable that the thickness is such that the above effects are sufficiently exhibited. This can be judged from the analysis results of XPS (X-ray photoelectron spectroscopy) and / or EDX (energy dispersive X-ray analysis) as follows.

When analyzing a surface having a carbon-enriched layer by XPS, it is preferable that a peak _{of Li 2} CO _{3 is present in the XPS spectrum. The fact that the presence of the Li 2} CO ₃ peak can be confirmed in the XPS spectrum indicates that the carbon-enriched layer has a thickness such that the peak can be confirmed. _{If the presence of the Li 2} CO ₃ peak can be confirmed in the XPS spectrum, it can be determined that the thickness of the carbon-enriched layer is sufficient to exert the above effect. The peak of Li ₂ CO ₃ is present, for example, near 531.5 ± 0.3 eV in the XPS O1s spectrum and around 289.6 ± 0.5 eV in the C1s spectrum.

When the surface having the carbon-concentrated layer is analyzed by EDX, the carbon concentration of the surface composition calculated by EDX is preferably 6.8% by mass or more, and more preferably 7.3% by mass or more. When the carbon concentration of the surface composition is 6.8% by mass or more, it can be determined that the thickness of the carbon-enriched layer is sufficient to exert the above effect. The upper limit of the carbon concentration of the surface composition is not particularly limited, but the carbon concentration is preferably less than 15.5% by mass, and more preferably 9.4 or less. If the carbon concentration of the surface composition is 15.5% by mass or more, the carbon-concentrated layer becomes too thick, and the interfacial resistance of the garnet-type solid electrolyte separator may increase.

Here, in EDX, the measurement conditions may affect the analysis result. Therefore, the standardized values may be used as follows. That is, in the surface composition calculated by EDX, the carbon / oxygen ratio is preferably 0.17 or more, and the carbon / zirconium ratio is preferably 0.23 or more in terms of mass ratio. The upper limit of the mass ratio is not particularly limited, but the carbon / oxygen ratio is preferably less than 0.24, more preferably 0.19 or less. The carbon / zirconium ratio is preferably less than 1.2, more preferably 0.37 or less.

As the EDX, SEM-EDX combined with SEM (scanning electron microscope) can be used.

(Carbon enrichment part)
The garnet-type solid electrolyte separator of the present disclosure has a carbon-enriched portion in a range from the surface (the surface on the side where the carbon-enriched layer is formed) to a position at a depth of 10 μm.

"From the surface to a position of 10 μm in depth" defines the range in the thickness direction of the garnet-type solid electrolyte separator, and defines the range in the stacking direction when the garnet-type solid electrolyte separator is used in a battery. is there. As described above, the range in the thickness direction is the range from the surface to the position of 10 μm.
The "carbon-enriched portion" is a region in which the carbon element concentration is higher than that of other portions, and it is preferable that the _{"carbon-enriched portion" is mainly composed of Li 2} CO _3. The presence or absence of the carbon-enriched portion can be specified by an SEM image or the like. The carbon-enriched portion is formed in a defect (gap between solid electrolytes, etc.) existing near the surface of the garnet-type solid electrolyte separator (within a depth of 10 μm from the surface). Since the solid electrolyte sintered body, which is a raw material, usually has many such defects, many carbon-enriched portions in the present disclosure are formed scattered near the surface.

In the garnet-type solid electrolyte separator of the present disclosure, the critical current density can be improved and the short-circuit resistance can be improved by having the carbon-enriched portion in a range from the surface to a depth of 10 μm. it can. This is because it is considered that the progress of lithium dendrite is suppressed by the formation of the carbon-enriched portion in the defect which is the starting point of the precipitation and growth of lithium.

Here, as described above, a large number of carbon-enriched portions are formed scattered near the surface of the garnet-type solid electrolyte separator. The fact that a large number of carbon-enriched portions are scattered in this way can be identified by an SEM image or the like, but it is difficult to quantify them. Therefore, in the present application, the degree of distribution of the carbon-enriched portion is quantified from the average carbon concentration calculated from EDX. As the EDX, for example, SEM-EDX can be used.

Specifically, it is performed as follows. First, the garnet-type solid electrolyte separator is cut in the thickness direction so that the carbon-enriched portion is exposed. In general, ceramics tend to break from a portion having a high impurity concentration. In the garnet type solid electrolyte separator, since the carbon-enriched portion corresponds to an impurity, the carbon-enriched portion can be cut so as to be exposed by a usual cutting method. As a cutting method, for example, a method of immersing the surface of the separator in ethanol or the like to remove Li existing on the surface, then making a linear mark on the surface with a diamond pen or the like, and cleaving along the surface. Can be done with.
When such a cutting method is used, the carbon-enriched portion is cut so as to be exposed, so that a difference may occur between the carbon concentration on the surface and the carbon concentration on the split section near the surface. Therefore, the carbon concentration of the fractured surface near the surface tends to be higher than that of the surface.

Next, surface analysis is performed on the fractured surface by EDX. FIG. 1 shows a schematic view focusing on the vicinity of the surface of the fractured surface. The vertical direction of the paper surface in FIG. 1 is the thickness direction, and the horizontal direction of the paper surface is the direction orthogonal to the thickness direction. A is a carbon-enriched layer, and B is a carbon-enriched portion. The range C shown by the broken line is the measurement range of the surface analysis. O is the center point of the measurement range, and the center point is the intersection of the thickness direction of the measurement range and the intersection of straight lines that vertically divide the length in the direction orthogonal to the thickness direction into two equal parts.

The surface analysis is performed so that the center point is included in the range from the surface to a depth of 10 μm. Further, the measurement is performed so that the ratio of the measurement range in the thickness direction to the range from the surface to the depth of 10 μm is 60% or more. This is because it is difficult to perform surface analysis only in the range from the surface to 10 μm. However, from the viewpoint of improving the accuracy, it is preferable to measure so that the ratio of the measurement region is 70% or more, and more preferably, the measurement is performed so that the ratio of the measurement region is 80% or more. Yes, and more preferably, the measurement is performed so that the ratio of the measurement region is 90% or more. Here, two or more surface analyzes may be performed for a range from the surface to a depth of 10 μm. In that case, the above ratio is calculated from the total of the measurement ranges in the thickness direction. The range in the thickness direction of the surface analysis is 5 to 10 μm. The range of the direction orthogonal to the thickness direction of the surface analysis is not particularly limited, but is, for example, 25 to 30 μm.

Such surface analysis is performed, for example, as follows. That is, the surface analysis is performed by setting a range of 8.7 ± 5 μm in the thickness direction of the fractured surface and a range of 25 μm in the direction orthogonal to the thickness direction. In this case, 63% of the range from the surface to the position of 10 μm in the thickness direction is set to be analyzed.

By performing surface analysis on the fractured surface using EDX so as to satisfy the above measurement conditions, the average carbon concentration in the range from the surface to the position at a depth of 10 μm can be calculated. When two or more surface analyzes are performed in the range from the surface to the depth of 10 μm, the average value of the obtained results is taken as the average carbon concentration in the range from the surface to the depth of 10 μm. Here, the "average carbon concentration" in the present specification is the ratio of the mass of carbon to the total mass of carbon, zirconium, and oxygen calculated by EDX (mass ratio: carbon / (carbon + zirconium + oxygen)). Means.

Further, when calculating the average carbon concentration, the following circumstances may be taken into consideration when calculating the average carbon concentration. As will be described later, in the method for producing a garnet-type solid electrolyte separator of the present disclosure, a solvent that is a material for producing a carbon-enriched layer or the like is brought into contact with the garnet-type solid electrolyte sintered body. When the garnet type solid electrolyte separator is produced in this way, the average carbon concentration tends to decrease from the surface to the inside. Such a tendency can be identified by performing surface analysis at a plurality of locations on the fractured surface of the garnet-type solid electrolyte separator. Therefore, if a specific tendency is observed in the average carbon concentration of the fractured surface, the average carbon concentration may be calculated in consideration of this tendency. For example, the average carbon concentration value measured so as to include 60% or more of the range from the surface to the position of 10 μm in depth and the appropriate setting in the range of 10 μm or more in depth (the value to be appropriately set is closer to 10 μm). You may calculate the average carbon concentration in the range from the surface to the position of 10 μm in depth, assuming that the average carbon concentration value measured in (may) is in a proportional relationship.

In this way, the average carbon concentration in the range from the surface to the position of 10 μm in depth can be calculated. The preferred average carbon concentration is as follows. That is, in the composition of the fractured surface calculated by EDX, the average carbon concentration in the range from the surface to the position at a depth of 10 μm is preferably 10% by mass or more. When the average carbon concentration is 10% by mass or more, the limit current density of the separator can be improved and the short circuit resistance can be improved. More preferably, the average carbon concentration is 20% by mass or more. The upper limit of the average carbon concentration is not particularly limited, and the average carbon concentration may be 40% by mass or less. When the average carbon concentration exceeds 40% by mass, it indicates that a large amount of carbon-enriched portions are formed inside the separator near the surface. If a large amount of carbon-enriched portion is formed in this way, the strength of the garnet-type solid electrolyte separator is lowered, which causes a problem of causing cracks and the like.

Further, it is preferable that the average carbon concentration in the range from the surface to the position of 10 μm in depth is at least twice the average carbon concentration in the range exceeding 10 μm in depth. As a result, a large number of carbon-enriched portions are scattered near the surface as compared with the inside, so that the short-circuit resistance is improved. Here, the average carbon concentration in the range exceeding 10 μm is the average carbon concentration when the surface analysis is performed so that the center point is included in the range exceeding 10 μm, and two or more surface analyzes are performed. In the case, it is the average value of the obtained average carbon concentration. When calculating the average carbon concentration in the range exceeding 10 μm in depth, surface analysis is performed so that the center point of the measurement range is included in the range of more than 10 μm and 50 μm or less.

(Garnet type solid electrolyte separator)
The garnet-type solid electrolyte separator of the present disclosure has been described above. As described above, the garnet-type solid electrolyte separator of the present disclosure has a carbon-enriched layer on the surface, and further has a carbon-enriched portion inside near the surface. As a result, the critical current density can be synergistically improved, so that excellent short-circuit resistance can be exhibited.

The garnet-type solid electrolyte separator of the present disclosure can be used as a separator for an all-solid-state battery. For example, it can be suitably used as a separator for a lithium ion all-solid-state battery.

[Manufacturing method of garnet type solid electrolyte separator]
Next, the method for producing the above-mentioned garnet-type solid electrolyte separator will be described. The method for producing the above-mentioned garnet-type solid electrolyte separator is not particularly limited, but the following production method is disclosed in the present application.

That is, a solvent contact step of contacting the surface of the garnet type solid electrolyte sintered body with a solvent containing an oxygen element, and a heating step of heating the surface at a temperature equal to or higher than the formation temperature of the carbon-enriched layer after the solvent contact step. Disclose a method for producing a garnet-type solid electrolyte separator having the above.

Hereinafter, the method for producing the garnet-type solid electrolyte separator of the present disclosure will be described using the method 10 for producing the garnet-type solid electrolyte separator according to the embodiment (hereinafter, may be referred to as “production method 10”). FIG. 2 is a flowchart of the manufacturing method 10.

As shown in FIG. 2, the manufacturing method 10 includes a solvent contacting step S1 and a heating step S2.

(Solvent contact step S1)
The solvent contact step S1 is a step of bringing a solvent containing an oxygen element into contact with the surface of the garnet type solid electrolyte sintered body. By bringing a solvent containing an oxygen element into contact with the surface of the garnet-type solid electrolyte sintered body, the garnet-type solid electrolyte and the solvent can react with each other in the heating step S2 described later to precipitate _{Li 2} CO _3. When Li ₂ CO ₃ is deposited on the surface, it becomes a carbon-enriched layer, and when it is deposited inside near the surface, it becomes a carbon-enriched part.

The garnet-type solid electrolyte sintered body is a sintered body mainly composed of a garnet-type solid electrolyte. Specifically, it contains 50% by mass or more of the garnet-type solid electrolyte, preferably 80% by mass or more, and more preferably 90% by mass or more. The upper limit of the content of the garnet-type solid electrolyte is not particularly limited, and the garnet-type solid electrolyte sintered body may consist only of the garnet-type solid electrolyte. Such a garnet-type solid electrolyte sintered body can be produced by a known method. For example, it can be produced by sintering a sintered body material containing a garnet-type solid electrolyte under the conditions of 400 ° C. to 500 ° C. in an argon atmosphere.

The solvent containing an oxygen element is not particularly limited as long as it is a solvent _{containing an oxygen element and capable of precipitating Li 2} CO ₃ in the heating step S2 described later. Examples thereof include polar solvents such as water and solvents containing oxygen elements in which materials containing carbon elements such as alcohol and wax are dissolved (ketones (acetone, methyl ethyl ketone, etc.), ethers (diethyl ether, dibutyl ether, etc.), etc.). be able to. From the viewpoint of water immersion in the garnet-type solid electrolyte sintered body, water or alcohol is preferable. More preferably, it is alcohol. This is because the _{Li 2} CO ₃ formation reaction proceeds more gently with alcohol than with water. When water is used, lithium ions in the garnet-type solid electrolyte sintered body are eluted in water and replaced with hydrogen ions, which may reduce the lithium ion conductivity. Examples of the alcohol include methanol, ethanol, propanol, butanol and the like. Ethanol is preferred.

The method of bringing the solvent into contact with the surface of the garnet-type solid electrolyte sintered body is not particularly limited, and examples thereof include a method of dropping the solvent using a pipette or the like to impregnate the surface. Further, the surface of the garnet-type solid electrolyte sintered body may be immersed in the above solvent. The contact of the solvent with the surface may be at least a part of the surface, but it is preferable to bring the solvent into contact with the entire surface.

The amount of the solvent to be brought into contact with the surface of the garnet-type solid electrolyte sintered body is appropriately set so that the carbon-enriched layer and the carbon-enriched portion are appropriately formed in the heating step S2. This is because the required amount may differ depending on the type of solvent.

Further, in the solvent contact step S1, the surface of the garnet-type solid electrolyte sintered body may be brought into contact with the solvent and the surface may be polished (wet polishing). As a result, the surface can be smoothed and the thickness of the carbon-enriched layer can be made uniform. Sandpaper or the like can be used for polishing.

(Heating step S2)
The heating step S2 is performed after the solvent contact step S1 and is a step of heating the surface of the garnet-type solid electrolyte sintered body at a temperature equal to or higher than the formation temperature of the carbon-concentrated layer. Since the solvent is immersed in the surface of the garnet-type solid electrolyte sintered body and defects (sintering defects, microvoids, microcracks) near the surface in the solvent contact step S1, the surface is heated by the heating step S2. As a result, a carbon-enriched layer can be formed on the surface of the garnet-type solid electrolyte sintered body, and a carbon-enriched portion can be formed inside the vicinity of the surface.

Here, the present inventors presume that the carbon-enriched layer and the carbon-enriched portion are formed by the following carbon enrichment mechanism. FIG. 3 shows a schematic diagram when ethanol is used as the solvent. As described above, there are defects (communication holes, etc.) on the surface of the garnet-type solid electrolyte sintered body and inside near the surface. Therefore, ethanol comes into contact with the defect in (a) solvent contact step S1 and (b) permeates. Further, (c) CO ₂ in the atmosphere is absorbed by the moisture remaining in ethanol. Then, by heating the surface in (d) heating step S2, _{the reaction of the garnet-type solid electrolyte sintered body, water and CO 2} is promoted to _{generate Li 2} CO ₃ , which concentrates carbon. It is considered that a carbon-enriched layer and a carbon-enriched portion are formed by such a mechanism. As described above, since the carbon-enriched portion is easily cracked, the splitting performed at the time of (e) EDX measurement tends to be cracked starting from such a portion.

The heating temperature in the heating step S2 is not particularly limited as long as it is a temperature equal to or higher than the formation temperature of the carbon-enriched layer, but is preferably 450 ° C. or higher from the viewpoint of promoting the reaction. However, if the reaction rate is too fast, the carbon-enriched layer formed becomes too thick, and it becomes difficult to form an interface between the separator and the negative electrode when manufacturing the battery. In addition, a large number of irregularities are generated in the carbon-enriched layer, which is not preferable. Therefore, the heating temperature is preferably less than 700 ° C, more preferably 550 ° C or lower.

The heating time in the heating step S2 is not particularly limited, and is appropriately set according to the heating temperature. For example, it is preferable to set it between 15 minutes and 1 hour.
The heating method in the heating step S2 is not particularly limited, but it can be performed using, for example, a hot plate or the like. The heating atmosphere may be an atmospheric atmosphere.

By providing the above steps, the garnet-type solid electrolyte separator of the present disclosure can be produced.

Hereinafter, the garnet-type solid electrolyte separator of the present disclosure will be further described with reference to Examples.

[Preparation of garnet type solid electrolyte separator]
An LLZ sintered body was used as the sintered body that became the basis of the garnet type solid electrolyte separator. The LLZ sintered body is manufactured by Toyoshima Seisakusho and has a disk shape of φ11.2 mm × t3 mm. The composition is Li _6.6 La ₃ Zr _1.6 Ta _0.4 O ₁₂ , and the relative density is 97% or more. The following surface treatment was performed on this LLZ sintered body to prepare garnet-type solid electrolyte separators according to Examples 1 to 3 and Comparative Examples 1 to 4.

The separator according to Comparative Example 1 was produced by polishing the entire surface of one of the above LLZ sintered bodies with # 2000 sandpaper in an argon atmosphere.
For the separator according to Comparative Example 2, the entire surface of one of the above LLZ sintered bodies was sanded with # 2000 sandpaper in the air, and then the surface was heated in the air on a hot plate at 450 ° C. for 15 minutes. After that, it was produced by allowing it to cool.
The separator according to Comparative Example 3 was polished with # 2000 sandpaper while dropping ethanol in the air on the entire surface of one surface of the above LLZ sintered body, and then on a hot plate at 400 ° C. in the air. The surface was heated for 10 minutes and then allowed to cool.
The separator according to Example 1 was polished with # 2000 sandpaper while dropping ethanol in the air on the entire surface of one surface of the above LLZ sintered body, and then on a hot plate at 450 ° C. in the air. The surface was heated for 15 minutes and then allowed to cool.
The separator according to Example 2 was polished with # 2000 sandpaper while dropping ethanol in the air on the entire surface of one surface of the LLZ sintered body, and then on a hot plate at 550 ° C. in the air. The surface was heated for 1 hour and then allowed to cool.
The separator according to Example 3 was polished with # 2000 sandpaper while dropping water in the air on the entire surface of one surface of the above LLZ sintered body, and then on a hot plate at 550 ° C. in the air. The surface was heated for 1 hour and then allowed to cool.
The separator according to Comparative Example 4 was polished with # 2000 sandpaper while dropping ethanol in the air on the entire surface of one surface of the above LLZ sintered body, and then on a hot plate at 700 ° C. in the air. The surface was heated for 1 hour and then allowed to cool.

[Evaluation]
(Measurement of critical current density)
For each of the separators according to Examples 1 and 2 and Comparative Examples 1 to 4, a metallic lithium foil (thickness 50 μm) was adhered to the surface subjected to the above treatment, and a copper foil (thickness 10 μm) was adhered to the other surface. Then, the evaluation batteries (half cells) according to Examples 1 and 2 and Comparative Examples 1 to 4 were produced.

Next, each of the evaluation batteries manufactured above was restrained by applying a load of 200 to 300 kgf using two iron pins of φ11.28 mm from the upper and lower surfaces in the stacking direction, and the electrochemical evaluation device (Hokuto). It was heated to 60 ° C. while connected to an electrochemical charge / discharge evaluation system).
Then, for the evaluation battery, ± 0.1 mA / cm ² , ± 0.25 mA / cm ² , ± 0.4 mA / cm ² , ± 0.5 mA / cm ² , ± 1 mA / cm ² , in the low current mode, ^{While gradually increasing ± 2} mA / cm 2, ± 4 mA / cm ² , ± 8 mA / cm ² , and ± 16 mA / cm ² , the current was applied for 1 hour for each current mode, and the voltage at that time was measured. It was determined that a short circuit occurred when the voltage value suddenly decreased to near 0 V, and the critical current density (CCD: Critical Current Density) at that time was set to a current density one step lower than that at the time of the short circuit. The results are shown in Table 1. Here, Example 1 and Comparative Examples 1 and 3 were performed in 4 cases each, and Example 2 and Comparative Example 2 were performed in 2 cases each. Further, Comparative Example 4 could not be measured because the carbon-concentrated layer on the surface of the LLZ sintered body was thick and it was difficult to form an interface with the metal Li.

As shown in the results of Table 1, the limit current densities of Comparative Examples 1 to 3 ^{were 2} mA / cm 2 or less, but the limit current density of Example 1 was 8 mA / cm ^{2 at the} maximum, and the limit current density of Example 2 was. The maximum was 4 mA / cm ² . From this result, it can be said that Examples 1 and 2 have a higher critical current density than Comparative Examples 1 to 3 and exhibit excellent short-circuit resistance.

(Surface analysis by XPS)
For the separators according to Example 1 and Comparative Examples 1 and 2, the surface on the treated side was measured by XPS. As the XPS apparatus, PHI-5000 VersaProve II manufactured by ULVAC-PHI was used. The obtained spectra (O-1s spectrum and C-1s spectrum are shown in FIGS. 4 and 5).

_{From FIGS. 4 and 5, clear Li 2} CO ₃ peaks were observed in the XPS spectra of Example 1 and Comparative Example 2. _{Therefore, it is considered that a layer of Li 2} CO ₃ (carbon-enriched layer) exists on the surface of the separator according to Example 1 and Comparative Example 2, and the carbon-enriched layer is more than observable by XPS. It is considered to have a thickness. On the other hand, _{the peak of Li 2} CO ₃ could not be observed in Comparative Example 1.

(Surface analysis by EDX)
For the separators according to Examples 1 to 3 and Comparative Examples 1 to 4, the surface on the treated side was measured by SEM-EDX, and the surface composition (mass%) was calculated. As the SEM, SU-8000 manufactured by Hitachi High-Technologies Corporation was used. The EDX was measured using an X-max 80 mm ² manufactured by HORIBA, Ltd. with an acceleration voltage of 5 kV. The measurement was carried out in any three regions, and the contents of carbon, oxygen and zirconium were calculated. The results are shown in Tables 2 and 3.

Table 2 describes in detail the results of Example 1 and Comparative Examples 1 and 2. Element comparison is calculated based on the average value.
Table 3 shows Examples 1 to 3 and Comparative Examples 1 to 4 so that the surface treatment conditions and the results of the surface composition can be compared. The carbon concentration, C / Zr ratio, and C / O ratio in Table 3 are calculated based on the average values as in Table 2.

From Table 2, it was found that the carbon concentration of Comparative Example 2 was higher than the carbon concentration of Comparative Example 1, and the carbon concentration of Example 1 was higher than the carbon concentration of Comparative Example 2. Since the carbon concentration in the surface composition _{is considered to be derived from Li 2} CO _3, it is considered that the higher the carbon concentration, the thicker the carbon-enriched layer. A similar tendency can be read from the standardized carbon / zirconium ratio and carbon / oxygen ratio.
From this, it is considered that a thicker carbon-enriched layer can be formed by contacting the surface of the LLZ sintered body with a solvent and then performing a heat treatment.

Next, the surface treatment conditions were examined from the results in Table 3. From Examples 1 to 3, it was confirmed that either water or alcohol can be used as the solvent. Further, it was confirmed that no particular problem occurred in the heating temperature in the range of 450 ° C. to 550 ° C. On the other hand, Comparative Example 3 was heat-treated at 400 ° C., and the carbon concentration was lower than that of Example. Therefore, it is probable that the thickness of the carbon-enriched layer was not sufficient and the critical current density was low as shown in Table 1. Further, in Comparative Example 4, since the heat treatment was performed at 700 ° C., the carbon concentration was higher than that in Example, but a carbon-concentrated layer having a large number of irregularities was formed on the surface, so that it was not suitable for use in a battery. ..

(Analysis of fractured surface by SEM-EDX)
The separators according to Example 1 and Comparative Examples 1 and 2 were cut, and the split cross sections thereof were analyzed by SEM-EDX. The cutting is performed by making a linear mark on the surface of the separator using a diamond pen, holding both ends of the separator with pliers across the linear mark, and applying bending stress to cleave along the linear mark. It was. 6 to 8 show SEM images near the surface (metal Li foil / LLZ (separator) interface) of the separator according to Example 1 and Comparative Examples 1 and 2. FIG. 6 is an SEM image of Comparative Example 1, FIG. 7 is an SEM image of Comparative Example 2, and FIG. 8 is an SEM image of Example 1.

From FIGS. 6 to 8, it was confirmed that the SEM images of Comparative Examples 1 and 2 had few black regions showing light elements, but the SEM images of Example 1 had black regions concentrated near the surface. From this, it is considered that some element is concentrated near the surface of the separator of Example 1. Since it can be inferred from the method for producing the separator that Li ₂ CO ₃ is also generated inside the separator, it is expected that the black region is a region where carbon is concentrated. Therefore, the carbon concentration was calculated using EDX for the fractured faces of the separators according to Examples 1 and 2 and Comparative Examples 1 to 3.

In the analysis by EDX, the average carbon concentration (mass%) of the measurement region was calculated by performing surface analysis for each of the two visual fields and dividing the cross section into several regions. The surface analysis of Comparative Example 1 was performed in the range of 10 μm in the thickness direction of the separator and 25 μm in the direction orthogonal to the thickness direction. The surface analysis of Comparative Examples 2 and 3 and Examples 1 and 2 was performed in the range of 5 μm in the thickness direction of the separator and 25 μm in the direction orthogonal to the thickness direction. The position of the center point of the analysis range in the thickness direction was described in the result. The average carbon concentration was calculated separately for the vicinity of the surface (the range from the surface to the position of 10 μm in depth) and the inside (the range of more than 10 μm in depth from the surface), and 2 or more were measured in each region. In the case, the average value thereof was taken as the average carbon concentration in each region.

In Comparative Example 1, surface analysis was performed at distances of 8.3 μm, 18 μm, 29 μm, 41 μm, 53 μm, and 66 μm from the surface to the center point. In Comparative Example 2, surface analysis was performed at distances of 3.75 μm, 9.67 μm, 16.1 μm, 22.1 μm, 29.1 μm, 36.1 μm, 42.9 μm, and 49.7 μm from the surface to the center point. It was. In Comparative Example 3, surface analysis was performed at distances of 2.46 μm, 6.93 μm, 11.7 μm, 16.48 μm, 21.10 μm, 25.48 μm, and 30.12 μm from the surface to the center point. In Example 1, surface analysis was performed at distances of 2.46 μm, 6.93 μm, 11.7 μm, 16.5 μm, 21.2 μm, 25.5 μm, and 30.1 μm from the surface to the center point. In Example 2, surface analysis was performed at the distances from the surface to the center point at 2.41 μm, 7.04 μm, 11.9 μm, 16.9 μm, 22.2 μm, 28.0 μm, and 34.4 μm.
The results are shown in FIGS. 9 to 13. 9 is the result of Comparative Example 1, FIG. 10 is the result of Comparative Example 2, FIG. 11 is the result of Comparative Example 3, FIG. 12 is the result of Example 1, and FIG. 13 is the result of Example 2. Is the result of.

According to FIG. 9, the average carbon concentration near the surface of Comparative Example 1 was 8.92%, and the average carbon concentration inside was 6.15%. According to FIG. 10, the average carbon concentration near the surface of Comparative Example 2 was 3.62%, and the average carbon concentration inside was 2.55%. As described above, the average carbon concentrations of Comparative Examples 1 and 2 were not significantly different between the vicinity of the surface and the inside. On the other hand, according to FIG. 11, the average carbon concentration near the surface of Comparative Example 3 was 9.67%, and the average carbon concentration inside was 3.08%. As described above, in Comparative Example 3, since the average carbon concentration near the surface was higher than the average carbon concentration inside, it was confirmed that the carbon element was concentrated near the surface. However, the value was still smaller than that of Examples 1 and 2 described later.

On the other hand, according to FIGS. 12 and 13, it was found that in Examples 1 and 2, there was a large difference in the average carbon concentration between the vicinity of the surface and the inside, and the average carbon concentration was higher as it was closer to the surface. Specifically, the average carbon concentration near the surface in Example 1 was 49.3%, the average carbon concentration inside was 10.3%, and the average carbon concentration near the surface in Example 2 was 22.4%, inside. The average carbon concentration of was 8.46%. From these results, the black region observed in the SEM image is considered to be a carbon-enriched region, and considering the previous results, this carbon is considered to be derived from _{Li 2} CO _3. Further, it is considered that excellent short-circuit resistance is exhibited when the average carbon concentration near the surface is 10% or more, preferably 20% or more.

(Discussion)
From the above results, it was found that the vicinity of the surface of the garnet-type solid electrolyte separator is a carbon-rich structure derived from _{Li 2} CO _3. It is known that brittle materials such as ceramics are cut by developing cracks starting from minute recesses or cracks on the surface. Therefore, it is considered that defects such as voids and cracks generated during sintering are exposed near the surface of the fractured surface. In Example 1, _{since a large amount of Li 2} CO ₃ is distributed in such defects (Fig. 8), ethanol permeates the defects during surface treatment, and ethanol reacts with the garnet-type solid electrolyte by heat treatment. By doing so, it is considered that _{Li 2} CO _{3 was generated.}
Since Li ₂ CO ₃ has extremely low wettability with metallic lithium, it is considered that the lithium carbonate covering the defective portion hinders the progress of lithium dendrite, thereby improving the short-circuit resistance (Table 1).

Claims (10)

A garnet-type solid electrolyte separator containing a garnet-type solid electrolyte, having a carbon-concentrated layer on at least one surface and a carbon-concentrated portion in a range from the surface to a depth of 10 μm. A garnet-type solid electrolyte is at least one element selected from the group consisting of _{(Li 7-3Y-Z} , Al _Y ) (La ₃ ) (Zr _2-Z , M _Z ) O _{12 (M = Nb, Ta).} The garnet-type solid electrolyte separator according to claim 1, wherein Y and Z are arbitrary numbers in the range of 0 ≦ Y <0.22 and 0 ≦ Z ≦ 2. The garnet-type solid electrolyte separator according to claim 1 or 2, wherein a peak of _{Li 2} CO ₃ is present in the XPS spectrum of the surface. The garnet-type solid electrolyte separator according to any one of claims 1 to 3, wherein the carbon concentration is 6.8% by mass or more in the surface composition calculated by EDX. The method according to any one of claims 1 to 4, wherein the surface composition calculated by EDX has a carbon / oxygen ratio of 0.17 or more and a carbon / zirconium ratio of 0.23 or more on a mass basis. Garnet type solid electrolyte separator. When the garnet-type solid electrolyte separator is divided in the thickness direction so that the carbon-enriched portion is exposed, the average carbon in the range from the surface to a depth of 10 μm in the composition of the fractured surface calculated by EDX. The garnet-type solid electrolyte separator according to any one of claims 1 to 5, wherein the concentration is 10% by mass or more. The garnet-type solid electrolyte separator according to claim 6, wherein the average carbon concentration is 20% by mass or more. A solvent contact step in which a solvent containing an oxygen element is brought into contact with the surface of a garnet-type solid electrolyte sintered body,
After the solvent contact step, a heating step of heating the surface at a temperature equal to or higher than the formation temperature of the carbon-enriched layer, and a heating step.
A method for producing a garnet-type solid electrolyte separator. The method for producing a garnet-type solid electrolyte separator according to claim 8, wherein the solvent is alcohol. The method for producing a garnet-type solid electrolyte separator according to claim 8 or 9, wherein in the heating step, the surface is heated at a temperature of 450 ° C. or higher and lower than 700 ° C.

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