JP7384088B2 - Garnet type solid electrolyte separator and its manufacturing method - Google Patents

Description

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

In all-solid-state lithium ion batteries, the use of garnet-type solid electrolytes as separator materials has been considered.

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 short circuits by coating the surface of a solid electrolyte substrate containing a garnet-type solid electrolyte with a thin lithium carbonate layer. Specifically, the following items are listed: The crystal grain size of the solid electrolyte is 10 μm or less. This reduces unevenness on the solid electrolyte surface and improves the specific surface area, thereby suppressing short circuits due to uneven distances between terminals and reducing interfacial resistance. By reducing the interfacial resistance, short circuits can be suppressed by localized current concentration. Further, a lithium carbonate layer with a thickness of 100 nm or less is formed on the solid electrolyte substrate. Thereby, holes and scratches in the solid electrolyte substrate can be filled and short circuits can be further suppressed.

Japanese Patent Application Publication No. 2018-142432 Japanese Patent Application Publication No. 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 short circuits cannot be suppressed under conditions where a current is passed for more than several tens of minutes at a high current density of 4 mA/ ^cm2 or more and the melting and precipitation of metallic lithium is 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 with high short circuit resistance and a method for manufacturing the same.

As a result of intensive studies on the above-mentioned problems, the present inventors have found that by providing a carbon-enriched layer on the surface of the garnet-type solid electrolyte sintered body and further providing a carbon-enriched portion inside near the surface, short circuit resistance can be further improved. I found that it was improved. Based on this knowledge, the present application discloses the following means for solving the above problems.

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

In the 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). (Y and Z are arbitrary numbers in the range of 0≦Y<0.22, 0≦Z≦2). Furthermore, it is preferable that a Li ₂ CO ₃ peak be present in the XPS spectrum of the surface of the garnet type solid electrolyte separator. The composition of the surface calculated by EDX preferably has a carbon concentration of 6.8% by mass or more. Alternatively, the composition of the surface 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. Furthermore, when the garnet-type solid electrolyte separator is cut in the thickness direction so that the carbon-enriched portion is exposed, the composition of the cut surface calculated by EDX shows the average carbon content in the range from the surface to a depth of 10 μm. It is preferable that the concentration is 10% by mass or more. More preferably, the average carbon concentration is 20% by mass or more.

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

In the above method for producing a garnet type solid electrolyte separator, the solvent is preferably alcohol. Further, in the heating step, the surface is preferably heated at a temperature of 450°C or more and less than 700°C.

According to the present disclosure, it is possible to provide a garnet-type solid electrolyte separator that has a higher limiting current density and excellent short circuit resistance than conventional techniques. Furthermore, a method for manufacturing the garnet type solid electrolyte separator can be provided.

FIG. 3 is a schematic diagram focusing on the vicinity of the surface of the cut surface of the separator. 2 is a flowchart of manufacturing method 10. It is a figure explaining the presumed mechanism by which carbon becomes concentrated. These are XPS spectra (O-1s) of Example 1 and Comparative Examples 1 and 2. These are XPS spectra (C-1s) of Example 1 and Comparative Examples 1 and 2. 3 is a SEM image of a cut surface of a separator according to Comparative Example 1. 3 is a SEM image of a cut surface of a separator according to Comparative Example 2. 1 is a SEM image of a cut surface of a separator according to Example 1. This is the result of calculating the average carbon concentration of the cut surface of the separator according to Comparative Example 1 using SEM-EDX. This is the result of calculating the average carbon concentration of the cut surface of the separator according to Comparative Example 2 using SEM-EDX. This is the result of calculating the average carbon concentration of the cut surface of the separator according to Comparative Example 3 using SEM-EDX. These are the results of calculating the average carbon concentration of the cut surface of the separator according to Example 1 using SEM-EDX. This is the result of calculating the average carbon concentration of the cut surface of the separator according to Example 2 using SEM-EDX.

In this specification, the notation "A to B" for numerical values A and B means "above A and below B". In such a notation, if a unit is attached only to the numerical value B, the unit shall also be applied to the numerical value A.

[Garnet type solid electrolyte separator]
The garnet-type solid electrolyte separator of the present disclosure includes 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

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

Each configuration will be explained below.

(Garnet type solid electrolyte)
In the present disclosure, a garnet-type solid electrolyte is a solid electrolyte that has Li ion conductivity, contains at least Li, and has a garnet-type crystal structure represented by the chemical composition of Li _x A ₃ B ₂ O ₁₂ . . Here, X satisfies the relationship X=24-3a-2b, where the valence of A is a and the valence of B is b. Examples of such garnet-type solid electrolytes include (Li _7-3Y-Z , Al _Y ) (La ₃ ) (Zr _2-Z , M _Z )O ₁₂ (M=Nb, Ta), which is called LLZ. (Y and Z are arbitrary numbers in the range of 0≦Y<0.22, 0≦Z≦2.). As LLZ, one having a known composition can be employed. For example, a typical composition is Li ₇ La ₃ Zr ₂ O ₁₂ .

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, 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 enriched layer)
The garnet-type solid electrolyte separator of the present disclosure includes a carbon-enriched layer on at least one surface. The term "carbon-enriched layer" refers to a layered region in which the carbon element concentration is higher than in other regions. The carbon enriched layer may be provided on both surfaces of the garnet type solid electrolyte separator. Here, the term "surface" means the surface that contacts 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.

It is preferable that the carbon-enriched layer mainly contains Li ₂ CO ₃ (lithium carbonate). "Mainly containing" means that the content of Li ₂ CO ₃ in the carbon-enriched layer is 50% by mass or more. Preferably it is 80% by mass or more, 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-enriched layer on the surface. This is because defects such as microcracks that exist on the surface of the solid electrolyte sintered body are covered with a carbon-enriched layer, which is thought to suppress the precipitation and progress of lithium starting from the defects. .

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

When a surface having a carbon-enriched layer is analyzed by XPS, it is preferable that a Li ₂ CO ₃ peak exists in the XPS spectrum. This is because 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 that allows the peak to be confirmed. If the presence of a 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 produce the above effects. The peak of Li ₂ CO ₃ exists, for example, in the vicinity of 531.5±0.3 eV in the O1s spectrum of XPS, and in the vicinity of 289.6±0.5 eV in the C1s spectrum.

When a surface having a carbon-enriched layer is analyzed by EDX, the carbon concentration of the surface composition calculated by EDX is preferably 6.8% by mass or more, more preferably 7.3% by mass or more. When the carbon concentration in 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 achieve the above effects. The upper limit of the carbon concentration in the surface composition is not particularly limited, but the carbon concentration is preferably less than 15.5% by mass, more preferably 9.4 or less. If the carbon concentration in the surface composition is 15.5% by mass or more, the carbon-enriched layer may become 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 results. Therefore, a value standardized as follows may be used. That is, in the surface composition calculated by EDX, the carbon/oxygen ratio in terms of mass ratio is preferably 0.17 or more, and the carbon/zirconium ratio is preferably 0.23 or more. Although the upper limit of the mass ratio is not particularly limited, the carbon/oxygen ratio is preferably less than 0.24, more preferably 0.19 or less. Further, the carbon/zirconium ratio is preferably less than 1.2, more preferably 0.37 or less.

As the EDX, SEM-EDX using a scanning electron microscope (SEM) can be used.

(Carbon enrichment department)
In the garnet-type solid electrolyte separator of the present disclosure, a carbon-enriched portion exists in a range from the surface (the surface on which the carbon-enriched layer is formed) to a depth of 10 μm.

"From the surface to a depth of 10 μm" 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. be. As mentioned above, the range in the thickness direction is the range from the surface to a position of 10 μm.
The “carbon-enriched portion” is a region where the carbon element concentration is higher than other portions, and is preferably mainly composed of Li ₂ CO ₃ . The presence or absence of a carbon-enriched portion can be identified by a SEM image or the like. The carbon-enriched portions are formed in defects (such as gaps between solid electrolytes) that exist near the surface of the garnet-type solid electrolyte separator (within a depth of 10 μm from the surface). Normally, the solid electrolyte sintered body that is the raw material has many such defects, so the carbon-enriched parts in the present disclosure are formed in a large number scattered near the surface.

The garnet-type solid electrolyte separator of the present disclosure has a carbon-enriched portion in a range from the surface to a depth of 10 μm, thereby improving the critical current density and short-circuit resistance. can. This is because it is thought that the growth of lithium dendrites is suppressed by the formation of carbon-enriched areas in defects that serve as starting points for lithium precipitation and growth.

Here, as described above, a large number of carbon enriched parts are formed in the vicinity of the surface of the garnet type solid electrolyte separator. The fact that many carbon-enriched areas are scattered in this manner can be identified by SEM images, etc., but it is difficult to quantify. Therefore, in this application, the degree of distribution of the carbon-enriched portion is quantified from the average carbon concentration calculated from EDX. For example, SEM-EDX can be used as EDX.

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. Generally, ceramics tend to break in areas with high impurity concentrations. In the garnet type solid electrolyte separator, since the carbon-enriched portion corresponds to an impurity, it can be cut using a normal cutting method to expose the carbon-enriched portion. The cleaving method includes, for example, immersing the surface of the separator in ethanol to remove Li existing on the surface, and then applying a linear mark to the surface with a diamond pen or the like, and cleaving along this line. It can be done with
When such a cutting method is used, the carbon-enriched portion is cut to be exposed, so that a difference may occur between the carbon concentration on the surface and the carbon concentration on the cut surface near the surface. Therefore, the carbon concentration on the fractured surface near the surface tends to be higher than that on the surface.

Next, a surface analysis is performed on the cut surface using EDX. FIG. 1 shows a schematic diagram focusing on the vicinity of the surface of the fractured surface. The vertical direction of the paper in FIG. 1 is the thickness direction, and the horizontal direction of the paper is a direction perpendicular to the thickness direction. A is a carbon-enriched layer, and B is a carbon-enriched part. Range C indicated by a broken line is the measurement range of area analysis. O is the center point of the measurement range, and the center point is the intersection of straight lines that vertically bisect the length of the measurement range in the thickness direction and in the direction perpendicular to the thickness direction.

The surface analysis is performed such that the center point is included within a depth of 10 μm from the surface. Further, the measurement is performed so that the ratio of the measurement range in the thickness direction to the range from the surface to a depth of 10 μm is 60% or more. This is because it is difficult to perform surface analysis only on the range up to 10 μm from the surface. However, from the viewpoint of improving accuracy, it is preferable to measure so that the ratio of the above measurement area is 70% or more, and more preferably to measure so that the ratio of the above measurement area is 80% or more. More preferably, the ratio of the measurement area is 90% or more. Here, two or more surface analyzes may be performed in a range from the surface to a depth of 10 μm. In that case, the above ratio is calculated from the total measurement range in the thickness direction. The range of surface analysis in the thickness direction is 5 to 10 μm. The range of surface analysis in the direction perpendicular to the thickness direction is not particularly limited, but is set to, 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 cut surface and a range of 25 μm in the direction orthogonal to the thickness direction. In this case, it is set to analyze 63% of the range from the surface to a position of 10 μm in the thickness direction.

By performing surface analysis on the fractured surface using EDX so as to satisfy the above measurement conditions, it is possible to calculate the average carbon concentration in the range from the surface to a position at a depth of 10 μm. When two or more surface analyzes are performed in the range from the surface to a 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 a depth of 10 μm. Here, the "average carbon concentration" in this specification refers to the ratio of the mass of carbon to the total mass of carbon, zirconium, and oxygen calculated by EDX (mass ratio of carbon/(carbon + zirconium + oxygen)) means.

Further, when calculating the average carbon concentration, the following circumstances may be taken into consideration. As described later, in the method for manufacturing a garnet-type solid electrolyte separator of the present disclosure, a solvent that is a material for forming a carbon-enriched layer and the like is brought into contact with a garnet-type solid electrolyte sintered body. When a garnet type solid electrolyte separator is manufactured in this manner, the average carbon concentration tends to decrease from the surface toward the inside. Such a tendency can be identified by performing surface analysis on multiple 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 taking this into consideration. For example, the average carbon concentration value measured to include 60% or more of the range from the surface to a depth of 10 μm, and the value set appropriately in the range of 10 μm or more (the value to be appropriately set is closer to 10 μm) The average carbon concentration in the range from the surface to a position at a depth of 10 μm may be calculated, assuming that the average carbon concentration value measured in the above manner is in a proportional relationship.

In this way, the average carbon concentration in the range from the surface to a depth of 10 μm 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 a depth of 10 μm is preferably 10% by mass or more. When the average carbon concentration is 10% by mass or more, the limiting 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. If the average carbon concentration exceeds 40% by mass, this indicates that a large amount of carbon-enriched portions are formed inside the separator near the surface. If a large amount of carbon-enriched portions are formed in this manner, the strength of the garnet type solid electrolyte separator decreases, causing problems such as cracking.

Further, it is preferable that the average carbon concentration in the range from the surface to a depth of 10 μm is at least twice the average carbon concentration in the range exceeding the depth of 10 μm. As a result, a larger number of carbon-enriched portions are scattered near the surface than in the interior, which improves short-circuit resistance. Here, the average carbon concentration in a range exceeding a depth of 10 μm is the average carbon concentration when a surface analysis is performed so that the center point is included in a range exceeding a depth of 10 μm, and two or more surface analyzes are performed. is the average value of the obtained average carbon concentration. When calculating the average carbon concentration in a range exceeding a depth of 10 μm, a surface analysis is performed so that the center point of the measurement range is included in a 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 explained 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 limiting 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 all-solid-state batteries. For example, it can be suitably used as a separator for lithium ion all-solid-state batteries.

[Method for manufacturing garnet type solid electrolyte separator]
Next, a method for manufacturing the above-mentioned garnet type solid electrolyte separator will be explained. Although the method for manufacturing the above-described garnet-type solid electrolyte separator is not particularly limited, the following manufacturing method is disclosed in this application.

That is, a solvent contacting step of bringing a solvent containing an oxygen element into contact with the surface of the garnet-type solid electrolyte sintered body, 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 contacting step. Disclosed is a method for manufacturing a garnet-type solid electrolyte separator having the following.

Hereinafter, a method for manufacturing a garnet-type solid electrolyte separator of the present disclosure will be described using a method 10 for manufacturing a garnet-type solid electrolyte separator (hereinafter sometimes referred to as "manufacturing method 10"), which is one embodiment. 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 in which a solvent containing an oxygen element is brought 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 react with each other in the heating step S2, which will be described later, and Li ₂ CO ₃ can be precipitated. When Li ₂ CO ₃ precipitates on the surface, it becomes a carbon-enriched layer, and when it precipitates 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 garnet type solid electrolyte in an amount of 50% by mass or more, 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 material containing a garnet-type solid electrolyte at 400° C. to 500° C. in an argon atmosphere.

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

The method of bringing the solvent into contact with the surface of the garnet-type solid electrolyte sintered body is not particularly limited, but includes, for example, 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. Although the solvent may be brought into contact with at least a portion of the surface, it is preferable that the solvent is brought into contact with the entire surface.

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

Further, in the solvent contacting 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). Thereby, the surface can be made smooth and the thickness of the carbon-enriched layer can also be made uniform. Sandpaper or the like can be used for polishing.

(Heating process S2)
The heating step S2 is performed after the solvent contacting 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-enriched layer. In the solvent contacting step S1, the above-mentioned solvent is immersed in the surface of the garnet-type solid electrolyte sintered body and defects near the surface (sintering defects, microvoids, microcracks), so the surface is heated in the heating step S2. By doing so, 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 near the surface.

Here, the present inventors estimate 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 a solvent. As described above, defects (such as communicating holes) exist on the surface of the garnet-type solid electrolyte sintered body and inside the vicinity of the surface. Therefore, (a) ethanol comes into contact with the defect in the solvent contact step S1, and (b) penetrates. Also, (c) CO ₂ in the atmosphere is absorbed by the moisture remaining in the ethanol. Then, by heating the surface in the (d) heating step S2, the reaction between the garnet-type solid electrolyte sintered body, water, and CO ₂ is promoted to generate Li ₂ CO ₃ , thereby concentrating carbon. It is thought that a carbon-enriched layer and a carbon-enriched portion are formed by this mechanism. Note that, as described above, since carbon-enriched portions are easily cracked, the cutting performed during (e) EDX measurement tends to cause cracking starting from such portions.

The heating temperature in the heating step S2 is not particularly limited as long as it is higher than the carbon-enriched layer formation temperature, 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 produced becomes too thick, making it difficult to form an interface between the separator and the negative electrode during battery production. Moreover, many unevennesses occur in the carbon-enriched layer, which is not preferable. Therefore, the heating temperature is preferably less than 700°C, more preferably 550°C or less.

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 the time between 15 minutes and 1 hour.
Although the heating method in the heating step S2 is not particularly limited, it can be performed using, for example, a hot plate. The heating atmosphere may be air atmosphere.

By including the above steps, the garnet type solid electrolyte separator of the present disclosure can be manufactured.

The garnet type solid electrolyte separator of the present disclosure will be further described below using Examples.

[Production of garnet type solid electrolyte separator]
An LLZ sintered body was used as the basis of the garnet-type solid electrolyte separator. The LLZ sintered body is manufactured by Toyoshima Seisakusho and has a disc 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. This LLZ sintered body was subjected to the following surface treatment to produce garnet type solid electrolyte separators according to Examples 1 to 3 and Comparative Examples 1 to 4.

A separator according to Comparative Example 1 was produced by polishing the entire surface of one side of the LLZ sintered body with #2000 sandpaper in an argon atmosphere.
The separator according to Comparative Example 2 was prepared by polishing one entire surface of the LLZ sintered body in the atmosphere with #2000 sandpaper, and then heating the surface on a hot plate at 450°C for 15 minutes in the atmosphere. , and then left to cool.
The separator according to Comparative Example 3 was prepared by polishing one entire surface of the LLZ sintered body with #2000 sandpaper while dropping ethanol in the air, and then polishing it with 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 prepared by polishing one entire surface of the LLZ sintered body with #2000 sandpaper while dropping ethanol in the atmosphere, and then polishing it with a hot plate at 450°C in the atmosphere. The surface was heated for 15 minutes and then allowed to cool.
The separator according to Example 2 was prepared by polishing one entire surface of the LLZ sintered body with #2000 sandpaper while dropping ethanol in the atmosphere, and then polishing it with a hot plate at 550°C in the atmosphere. The surface was heated for 1 hour and then allowed to cool.
The separator according to Example 3 was prepared by polishing one entire surface of the LLZ sintered body with #2000 sandpaper while dropping water in the atmosphere, and then polishing it with a hot plate at 550°C in the atmosphere. The surface was heated for 1 hour and then allowed to cool.
The separator according to Comparative Example 4 was prepared by polishing one entire surface of the LLZ sintered body with #2000 sandpaper while dropping ethanol in the atmosphere, and then polishing it with a hot plate at 700°C in the atmosphere. The surface was heated for 1 hour and then allowed to cool.

[evaluation]
(Measurement of limiting current density)
For each of the separators according to Examples 1 and 2 and Comparative Examples 1 to 4, metal lithium foil (thickness: 50 μm) was adhered to the above-treated side, and copper foil (thickness: 10 μm) was adhered to the other side. Then, evaluation batteries (half cells) according to Examples 1 and 2 and Comparative Examples 1 to 4 were produced.

Next, each of the evaluation batteries produced above was restrained using two iron pins of 11.28 mm in diameter from the upper and lower surfaces in the stacking direction, applying a load of 200 to 300 kgf, and using an electrochemical evaluation device (Hokuto). The battery was heated to 60°C while connected to a charge/discharge evaluation system (manufactured by Denko).
Then, for the evaluation battery, in the low current mode, ±0.1 mA/cm ² , ±0.25 mA/cm ² , ±0.4 mA/cm ² , ±0.5 mA/cm ² , ±1 mA/cm ² , While increasing the voltage stepwise in the order of ±2 mA/cm ² , ±4 mA/cm ² , ±8 mA/cm ² , and ±16 mA/cm ² , current was applied for 1 h in 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) at that point was set to one step lower than the current density at the time of the short circuit. The results are shown in Table 1. Here, four examples each of Example 1 and Comparative Examples 1 and 3 were conducted, and two examples each of Example 2 and Comparative Example 2 were conducted. Furthermore, in Comparative Example 4, the carbon-enriched layer on the surface of the LLZ sintered body was thick and it was difficult to form an interface with metal Li, so it could not be measured.

As shown in Table 1, the limiting current density of Comparative Examples 1 to 3 was 2 mA/cm ² or less, but the limiting current density of Example 1 was 8 mA/cm ² at maximum, and the limiting current density of Example 2 was The maximum was 4 mA/ ^cm2 . From this result, it can be said that Examples 1 and 2 have higher limiting current densities than Comparative Examples 1 to 3 and exhibit excellent short circuit resistance.

(Surface analysis by XPS)
Regarding the separators according to Example 1 and Comparative Examples 1 and 2, the treated surface was measured by XPS. As the XPS device, 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).

4 and 5, a clear Li ₂ CO ₃ peak was 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 furthermore, the carbon-enriched layer has a layer that is more than the level that can be observed by XPS. It is thought that it is thick. On the other hand, in Comparative Example 1, no peak of Li ₂ CO ₃ could be observed.

(Surface analysis by EDX)
Regarding 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 (% by mass) was calculated. The SEM used was SU-8000 manufactured by Hitachi High Technologies. EDX was measured using Horiba's X-max 80 mm ² at an accelerating voltage of 5 kV. Measurements were performed in three arbitrary regions, and the contents of carbon, oxygen, and zirconium were calculated. The results are shown in Tables 2 and 3.

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

From Table 2, it was found that the carbon concentration of Comparative Example 2 was higher than that of Comparative Example 1, and the carbon concentration of Example 1 was higher than that 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. Similar trends can be seen from the normalized 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 heat treatment.

Next, the conditions for surface treatment were examined based on the results shown in Table 3. From Examples 1 to 3, it was confirmed that both water and alcohol can be used as the solvent. Furthermore, regarding the heating temperature, it was confirmed that no particular problem occurred within 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 the Example. Therefore, the thickness of the carbon-enriched layer was not sufficient, and as shown in Table 1, it is thought that the limiting current density was low. In addition, in Comparative Example 4, the heat treatment was performed at 700°C, so the carbon concentration was higher than in the example, but a carbon-enriched layer with many irregularities was formed on the surface, making it unsuitable for battery use. .

(Analysis of cut surface using SEM-EDX)
The separators according to Example 1 and Comparative Examples 1 and 2 were cut, and the cut surfaces were analyzed by SEM-EDX. Cutting is performed by creating linear marks on the surface of the separator using a diamond pen, holding both ends of the separator with pliers across the linear marks, and applying bending stress to cleave along the linear marks. Ta. 6 to 8 show SEM images of the vicinity of the surface (metallic Li foil/LLZ (separator) interface) of the separators according to Example 1 and Comparative Examples 1 and 2. 6 is a SEM image of Comparative Example 1, FIG. 7 is a SEM image of Comparative Example 2, and FIG. 8 is a 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 areas indicating light elements, but the SEM image of Example 1 had black areas concentrated near the surface. From this, it is considered that some element is concentrated near the surface of the separator of Example 1. From the method of manufacturing the separator, it can be inferred that Li ₂ CO ₃ is also generated inside the separator, so the black areas are expected to be areas where carbon is concentrated. Therefore, the carbon concentration was calculated using EDX on the cut surfaces of the separators according to Examples 1 and 2 and Comparative Examples 1 to 3.

In the EDX analysis, the cross-section was divided into several regions in two fields at a time, and area analysis was performed on each region, and the average carbon concentration (mass %) of the measurement region was calculated. The surface analysis of Comparative Example 1 was performed in a range of 10 μm in the thickness direction of the separator and 25 μm in the direction perpendicular to the thickness direction. The surface analysis of Comparative Examples 2 and 3 and Examples 1 and 2 was carried out in a range of 5 μm in the thickness direction of the separator and 25 μm in the direction perpendicular to the thickness direction. The results indicate the position of the center point of the analysis range in the thickness direction. In addition, the average carbon concentration was calculated separately for the near surface (range from the surface to a depth of 10 μm) and the interior (range from the surface to a depth of more than 10 μm), and two or more measurements were taken in each region. In this case, the average value was taken as the average carbon concentration in each region.

In Comparative Example 1, surface analysis was performed at positions 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 positions 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. Ta. In Comparative Example 3, surface analysis was performed at positions 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 positions 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 positions at distances of 2.41 μm, 7.04 μm, 11.9 μm, 16.9 μm, 22.2 μm, 28.0 μm, and 34.4 μm from the surface to the center point.
The results are shown in FIGS. 9 to 13. 9 shows the results of Comparative Example 1, FIG. 10 shows the results of Comparative Example 2, FIG. 11 shows the results of Comparative Example 3, FIG. 12 shows the results of Example 1, and FIG. 13 shows the results of Example 2. This is the result.

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, there was no significant difference in the average carbon concentration of Comparative Examples 1 and 2 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, the average carbon concentration near the surface was higher than the average carbon concentration inside, so 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, which will be 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 average carbon concentration between near the surface and inside, and the average carbon concentration was higher closer to the surface. Specifically, the average carbon concentration near the surface in Example 1 was 49.3% and the average carbon concentration inside was 10.3%, and the average carbon concentration near the surface in Example 2 was 22.4% and inside. The average carbon concentration was 8.46%. From these results, it is considered that the black region observed in the SEM image is a region where carbon is concentrated, and considering the previous results, this carbon is considered to be derived from Li ₂ CO ₃ . Moreover, 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.

(Consideration)
From the above results, it was found that the vicinity of the surface of the garnet type solid electrolyte separator has a structure rich in carbon derived from Li ₂ CO ₃ . It is known that brittle materials such as ceramics can be fractured by cracks that start from minute depressions 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 (Figure 8), ethanol penetrates into the defects during surface treatment, and the heat treatment causes a reaction between ethanol and the garnet-type solid electrolyte. It is thought that Li ₂ CO ₃ was generated by doing so.
Since Li ₂ CO ₃ has extremely low wettability with metallic lithium, it is thought that the lithium carbonate covering the defective parts hinders the growth of lithium dendrites, thereby improving the short circuit resistance (Table 1).

Claims (8)

a garnet-type solid electrolyte sintered body containing a garnet-type solid electrolyte;
a carbon-enriched layer present on at least one surface of the garnet-type solid electrolyte sintered body;
a carbon-enriched portion existing within the garnet-type solid electrolyte sintered body, the carbon-enriched portion extending from the surface of the carbon-enriched layer to a depth of 10 μm ;
In the composition of the surface of the carbon-enriched layer calculated by EDX, the carbon concentration is 6.8% by mass or more,
Garnet type solid electrolyte separator. The 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, 0≦Z≦2. The garnet-type solid electrolyte separator according to claim 1 or 2, wherein a Li ₂ CO ₃ peak is present in the XPS spectrum of the surface of the carbon-enriched layer. Any one of claims 1 to 3 , wherein in the composition of the surface of the carbon-enriched layer calculated by EDX, the carbon/oxygen ratio is 0.17 or more and the carbon/zirconium ratio is 0.23 or more on a mass basis. The garnet type solid electrolyte separator according to item 1. When the garnet-type solid electrolyte separator is cut in the thickness direction so that the carbon-enriched portion is exposed, the composition of the cut surface calculated by EDX is at a position at a depth of 10 μm from the surface of the carbon-enriched layer. The garnet type solid electrolyte separator according to any one of claims 1 to 4 , wherein the average carbon concentration inside the garnet type solid electrolyte sintered body is 10% by mass or more. The garnet type solid electrolyte separator according to claim 5 , wherein the average carbon concentration is 20% by mass or more. a solvent contacting step of bringing a solvent containing an oxygen element into contact with the surface of the garnet-type solid electrolyte sintered body;
After the solvent contacting step, a heating step of heating the surface at a temperature of 450° C. or more and less than 700° C.;
A method for producing a garnet-type solid electrolyte separator, comprising: The method for producing a garnet-type solid electrolyte separator according to claim 7 , wherein the solvent is alcohol.