JP2019160510A - Sulfide solid electrolyte - Google Patents

Abstract

To provide a sulfide solid electrolyte in which high ion conductivity is maintained and the generation of hydrogen sulfide is reduced.SOLUTION: There is provided a LiS-PS-LiI-LiBr-based sulfide solid electrolyte, in which a part of LiS is substituted by KS and the percentage of KS based on the total molar amount of LiS and KS is 3.0 mol% or less.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to sulfide solid electrolytes.

  With the rapid spread of information-related devices such as personal computers, video cameras, and mobile phones, and communication devices in recent years, the development of batteries that are used as power sources for them has been emphasized. In the automobile industry, the development of high-power and high-capacity batteries for electric vehicles or hybrid vehicles is underway. Among such various batteries, lithium ion batteries have attracted attention from the viewpoint of high energy density.

  Furthermore, among lithium ion batteries, all solid lithium ion batteries that use a solid electrolyte as the electrolyte do not use an organic solvent that is a liquid in the battery, so that the device can be simplified and output characteristics, etc. Is considered excellent.

A sulfide solid electrolyte is known as a solid electrolyte used for such a solid electrolyte layer. For example, in Patent Document 1, in a method for producing a sulfide solid electrolyte containing at least Li ₂ S, P ₂ S ₅ , LiI, and LiBr, Li ion conductivity is obtained by performing a non-crystallization step and a heating step. It is disclosed to obtain a high sulfide solid electrolyte.

  Patent Document 2 describes that an alkaline compound is mixed in a solid electrolyte, and the ratio of the molar amount of the alkali metal contained in the alkaline compound to the molar amount of Li contained in the solid electrolyte is 1/1000 or more and 1/25. It is disclosed that generation of hydrogen sulfide and a decrease in ionic conductivity are suppressed by the following. In this document, the alkaline compound preferably contains at least one of Li, Na, and K, and preferably further contains O.

Japanese Patent Laid-Open No. 2015-011898 JP 2017-120728 A

Since the sulfide solid electrolyte has high ion conductivity, it is useful for increasing the output of the battery. However, the sulfide solid electrolyte has a problem that hydrogen sulfide (H ₂ S) is generated by exposure to the atmosphere. That is, the sulfide solid electrolyte generates hydrogen sulfide when in contact with moisture.

  Inhibitors added to suppress the generation of hydrogen sulfide generally cause a decrease in the ionic conductivity of the sulfide solid electrolyte. Therefore, it has been difficult to provide a sulfide solid electrolyte that suppresses the generation of hydrogen sulfide and at the same time exhibits high ionic conductivity. In particular, when an alkaline compound containing oxygen element (O) is used as a hydrogen sulfide suppressant, the ionic conductivity may be significantly reduced.

  The present disclosure has been made in view of the above problems, and an object thereof is to provide a sulfide solid electrolyte in which generation of hydrogen sulfide is suppressed and high ionic conductivity is maintained.

  The present disclosure achieves the above object by the following means.

In _{Li 2 S-P 2 S 5} -LiI-LiBr -based sulfide solid electrolyte, a part of Li ₂ S has been substituted with a _{K 2} S, K to the molar amount of the sum of Li ₂ S and _{K 2} S _The sulfide solid electrolyte whose ratio of ₂ S is 3.0 mol% or less.

  According to the present disclosure, there is provided a sulfide solid electrolyte in which generation of hydrogen sulfide is suppressed and high ion conductivity is maintained.

FIG. 1 is a plot of the relationship between the substitution ratio with K ₂ S and the hydrogen sulfide generation rate of the sulfide solid electrolyte. FIG. 2 is a plot of the relationship between the substitution rate with K ₂ S and the ionic conductivity of the sulfide solid electrolyte.

The sulfide solid electrolyte of the present disclosure, a part of Li ₂ S has been substituted with a _{K 2} S, the ratio of _{K 2} S to the molar amount of the sum of Li ₂ S and _{K 2} S is less 3.0 mol% It is.

  As described above, the inhibitor added to the sulfide solid electrolyte in order to suppress the generation of hydrogen sulfide generally causes a decrease in ionic conductivity. In particular, when an alkali compound that is an oxide is used as a suppressant, since the affinity between oxygen and phosphorus is high, a P—O bond is formed by a reaction between phosphorus and the oxide in the sulfide solid electrolyte. Is preferentially formed, leading to a decrease in ionic conductivity.

The inventors of the present disclosure have generated hydrogen sulfide in a sulfide solid electrolyte without significantly reducing the ionic conductivity of the sulfide solid electrolyte by substituting a specific proportion of Li ₂ S with K ₂ S. Has been found to be effectively suppressed.

In the sulfide solid electrolyte of the present disclosure, by replacing a part of Li ₂ S with strongly basic K ₂ S, the basicity of the sulfide solid electrolyte is improved. The occurrence is considered to be suppressed. That is, in the sulfide solid electrolyte partially substituted with K ₂ S, the basicity of the deliquescent liquid layer formed on the surface of the sulfide solid electrolyte is improved when it comes into contact with moisture. Since hydrogen sulfide is acidic in an aqueous solution, it is considered that the generated hydrogen sulfide is neutralized by this alkaline deliquescent solution, and as a result, out-of-system diffusion of hydrogen sulfide is suppressed.

Furthermore, the inventor of the present disclosure examined a substitution ratio for substituting Li ₂ S with K ₂ S. As a result, the ratio of _{K 2} S, by more than 3.0 mol% based on the molar amount of the sum of Li ₂ S and _{K 2} S, while maintaining the high ionic conductivity of the sulfide solid electrolyte, sulfide It has been found that generation of hydrogen can be effectively suppressed.

  As described above, according to the present disclosure, there is provided a sulfide solid electrolyte in which generation of hydrogen sulfide is suppressed and high ion conductivity is maintained.

  Hereinafter, embodiments of the sulfide solid electrolyte of the present disclosure will be described.

The sulfide solid electrolyte of the present disclosure, a part of Li 2 _S is replaced with K _{2 S.}

<Replacement ratio>
Ratio of Li ₂ S is replaced by _{K 2} S is no more than 3.0 mol% based on the molar amount of the sum of Li ₂ S and _{K 2} S, for example, 2.5 mol% or less, 2.0 mol% Or 1.0 mol% or less, and / or 0.1 mol% or more, 0.3 mol% or more, or 0.5 mol% or more.

In the sulfide solid electrolyte in which a part of Li ₂ S is substituted with K ₂ S according to the present disclosure, for example, K ₂ S is added to a starting material containing Li ₂ S in the production process of the sulfide solid electrolyte. Can be obtained.

<Sulfide solid electrolyte>
The sulfide solid electrolyte in the present disclosure is a Li ₂ S—P ₂ S ₅ —LiI—LiBr-based sulfide solid electrolyte. Regarding the sulfide solid electrolyte of the present disclosure and the production thereof, the description of Patent Document 1 can be referred to together with the following description.

The sulfide solid electrolyte according to the present disclosure in which a part of Li ₂ S is substituted with K ₂ S is manufactured, for example, by the following method. First, a raw material composition containing a lithium composition containing at least Li ₂ S, LiI, and LiBr, K ₂ S, and P ₂ S ₅ is prepared. Next, a sulfide solid electrolyte as sulfide glass is obtained by performing an amorphization process on these raw materials. In some cases, a sulfide solid electrolyte as a crystallized sulfide glass is obtained by heat-treating the produced sulfide glass.

  Examples of the amorphization treatment include mechanical milling and melt quenching, and mechanical milling is preferable among them. This is because processing at room temperature is possible, and the manufacturing process can be simplified. Mechanical milling is not particularly limited as long as the raw material composition is mixed while applying mechanical energy. Examples of the mechanical milling include a ball mill, a vibration mill, a turbo mill, a mechanofusion, and a disk mill. Among these, a ball mill is preferable, and a planetary ball mill is particularly preferable. This is because the desired sulfide glass can be obtained efficiently.

  Various conditions of mechanical milling are set so that a desired sulfide glass can be obtained. For example, when a sulfide solid electrolyte is produced by a planetary ball mill, the raw material composition and grinding balls are added to the pot, and the treatment is performed at a predetermined rotation speed and time. In general, the higher the number of rotations, the faster the production rate of the sulfide solid electrolyte, and the longer the treatment time, the higher the conversion rate from the raw material composition to sulfide glass.

The number of rotations when performing the planetary ball mill is preferably in the range of 200 rpm to 500 rpm, for example. Moreover, it is preferable that the processing time at the time of performing a planetary ball mill is, for example, within a range of 1 hour to 100 hours, and particularly within a range of 1 hour to 50 hours. In addition, examples of the material of the container and ball for pulverization used in the ball mill include ZrO ₂ and Al ₂ O ₃ . Moreover, the diameter of the ball for grinding | pulverization exists in the range of 1 mm-20 mm, for example.

  The heat treatment temperature of the heat treatment performed as necessary is not particularly limited as long as the desired crystallized sulfide glass can be obtained. For example, 160 ° C. or higher, 180 ° C. or higher, 195 ° C. or higher, or 200 ° C. And / or 320 ° C. or lower, 300 ° C. or lower, 250 ° C. or lower, or 220 ° C. or lower. The heat treatment time is preferably in the range of 1 minute to 24 hours, for example. The heat treatment is preferably performed in an inert gas atmosphere (for example, Ar gas atmosphere) or a reduced pressure atmosphere (particularly vacuum). Although the method of heat processing is not specifically limited, For example, the method using a baking furnace can be mentioned.

The ratio of each compound in the raw material composition is not particularly limited. For _example, in the case of producing a sulfide solid electrolyte using a Li _{2 S,} a raw material composition containing a _P 2 S 5, LiI and LiBr, the ratio of Li ₂ S to the total of Li ₂ S and _P 2 _{S 5} However, it is preferable that it exists in the range of 70 mol%-80 mol%. This is because a sulfide solid electrolyte having high chemical stability can be obtained.

  The total ratio of LiI and LiBr in the raw material composition is not particularly limited as long as the desired sulfide solid electrolyte can be obtained. For example, the ratio is in the range of 10 mol% to 35 mol%. Is preferably in the range of 10 mol% to 30 mol%, and more preferably in the range of 15 mol% to 25 mol%.

  The sulfide solid electrolyte of the present disclosure may be a sulfide glass obtained by vitrifying a raw material composition, or may be a crystallized sulfide glass obtained by heat-treating the sulfide glass. good. Since sulfide glass is softer than crystallized sulfide glass, it can be considered that the expansion and contraction of the active material can be absorbed when an all-solid battery is produced, and the cycle characteristics are excellent. On the other hand, crystallized sulfide glass may have higher Li ion conductivity than sulfide glass.

  The sulfide solid electrolyte of the present disclosure preferably includes, for example, an ion conductor having Li, P, and S, and LiI and LiBr. It is preferable that at least a part of LiI and LiBr exist in a state of being incorporated into the structure of the ionic conductor as a LiI component and a LiBr component, respectively. Further, the sulfide solid electrolyte of the present disclosure may or may not have a LiI peak in X-ray diffraction measurement, but the latter is preferable. This is because the Li ion conductivity is high. The same applies to LiBr.

The ionic conductor includes, for example, Li, P, and S. The ionic conductor preferably has an ortho composition. This is because the chemical stability of the sulfide solid electrolyte is increased. Here, ortho generally refers to one having the highest degree of hydration among oxo acids obtained by hydrating the same oxide. In the present disclosure, the crystal composition to which Li ₂ S is most added in the sulfide is referred to as an ortho composition. For example, in the Li ₂ S—P ₂ S ₅ system, Li ₃ PS ₄ corresponds to the ortho composition. In the case of a Li ₂ S—P ₂ S ₅ -based sulfide solid electrolyte, the ratio of Li ₂ S and P ₂ S ₅ for obtaining the ortho composition is Li ₂ S: P ₂ S ₅ = 75: 25.

In the present disclosure, the expression “having an ortho composition” means not only a strict ortho composition but also a composition in the vicinity thereof. Specifically, it is preferable that the ionic conductor mainly contains an anion structure (PS ₄ ^3- structure) having an ortho composition. The ratio of the anion structure of the ortho composition is preferably 60 mol% or more, more preferably 70 mol% or more, still more preferably 80 mol% or more, based on the total anion structure in the ion conductor, 90 mol % Or more is particularly preferable. The ratio of the anion structure of the ortho composition can be determined by Raman spectroscopy, NMR, XPS, or the like.

It is preferable that the sulfide solid electrolyte does not substantially contain Li ₂ S and bridging sulfur. This is because a sulfide solid electrolyte with a low hydrogen sulfide generation amount can be obtained. “Substantially free of Li ₂ S” can be confirmed by X-ray diffraction. Specifically, when it does not have a Li ₂ S peak (2θ = 27.0 °, 31.2 °, 44.8 °, 53.1 °), it is determined that it does not substantially contain Li ₂ S. can do. Cross-linked sulfur refers to cross-linked sulfur in a compound produced by the reaction of Li ₂ S and P ₂ S ₅ . For example, cross-linking sulfur _{S 3} PS-PS 3 structure Li ₂ S and _P 2 _{S 5} is formed by reaction, it corresponds to this. Such bridging sulfur easily reacts with water and easily generates hydrogen sulfide. “Substantially free of bridging sulfur” can be confirmed by measurement of a Raman spectrum. For example, in the case of a Li ₂ S—P ₂ S ₅ -based sulfide solid electrolyte, the peak of the S ₃ P—S—PS ₃ structure usually appears at 402 cm ⁻¹ . Therefore, it is preferable that this peak is not detected. Moreover, the peak of PS ₄ ³⁻ structure usually appears at 417 cm ⁻¹ . In the disclosed invention, the intensity _{I 402} at 402 cm ^-1 is preferably smaller than the intensity _{I 417} at 417 cm ^-1. More specifically, the strength I ₄₀₂ is preferably 70% or less, more preferably 50% or less, and even more preferably 35% or less with respect to the strength I ₄₁₇ .

Examples of the shape of the sulfide solid electrolyte include a particulate shape. The average particle diameter (D ₅₀ ) of the sulfide solid electrolyte is preferably in the range of 0.1 μm to ₅₀ μm, for example. Further, the sulfide solid electrolyte preferably has a high Li ion conductivity, and the Li ion conductivity at room temperature is preferably 1 × 10 ⁻⁴ S / cm or more, for example, 1 × 10 ⁻³ S / cm. More preferably.

  The invention of the present disclosure is not limited to the above embodiment. The above-described embodiment is an example, and has substantially the same configuration as the technical idea described in the claims of the present disclosure. Are also included in the technical scope of the invention of the present disclosure.

  The invention of the present disclosure will be described more specifically with reference to the following examples.

<Comparative Example 1: No replacement>
(Preparation of sulfide glass)
Li ₂ S (Fluuchi Chemical) 0.5503 g, P ₂ S ₅ (Aldrich) 0.8874 g, LiI (High Purity Chemical) 0.2850 g, and LiBr (High Purity Chemical) 0.2773 g as a starting material composition, The mixture was put into a zirconia pot (45 ml) containing zirconia balls having a diameter of 5 mm, 4 g of dehydrated heptane (Kanto Chemical Industries) was added, and the lid was capped. This was attached to a planetary ball mill apparatus (Fritch P-7), and mechanical milling was performed for 20 hours. Thereafter, heptane was removed to obtain a sulfide glass.

(Preparation of sulfide glass with small particle size)
Next, 2 g of this sulfide glass is again put into a zirconia pot containing 0.3 mm diameter zirconia balls, 2 g of dibutyl ether (Kishida Chemical) and 6 g of heptane are added, and the mixture is stirred for 20 hours. Diameter sulfide glass was obtained.

(Preparation of crystallized sulfide glass)
Furthermore, the obtained small-sized sulfide glass was baked by heating at a temperature equal to or higher than the crystallization temperature for 3 hours in an inert atmosphere. As a result, a sulfide solid electrolyte that was crystallized sulfide glass was obtained.

<Examples 1-3 and Comparative Example 2>
A sulfide solid electrolyte, which is crystallized sulfide glass, was prepared by the same method as in Comparative Example 1 except that the starting material composition was changed as shown in Table 1 below. In Examples 1 to 3 and Comparative Example 2, K ₂ S (high purity chemistry) was used.

<Hydrogen sulfide generation test>
Each sample obtained as described above was subjected to a hydrogen sulfide generation test. Specifically, first, a 20 cc crucible containing 10 cc of water was placed in a 1.7 L exposure desiccator with a fan inside, and left until the saturated water vapor amount was reached. 100 mg of a sample was placed in a 10 cc glass petri dish, and the glass petri dish was covered with a paraffin film. The glass petri dish was placed in a desiccator and the hydrogen sulfide sensor was activated. Then, the paraffin film was removed and exposure measurement was started. The slope was calculated from the relationship between exposure time and hydrogen sulfide concentration, and the hydrogen sulfide generation rate was calculated. The experimental results regarding Comparative Examples 1 and 2 and Examples 1 to 3 are shown in FIG.

<Ion conductivity measurement>
The ionic conductivity of each sample obtained as described above was measured. Specifically, a 6t press using a pellet molding machine was performed on 100 mg of a sample to produce a conductivity cell. And ion conductivity was computed from the resistance obtained by alternating current impedance measurement, and the thickness of a pellet. The experimental results regarding Comparative Examples 1 and 2 and Examples 1 to 3 are shown in FIG.

<Experimental result>
The experimental results regarding Comparative Examples 1 and 2 and Examples 1 to 3 are shown in FIGS.

1, the ratio of K _{2 S} to the molar amount of the sum of Li 2 _S and K _{2 S} and (K 2 _S substitution ratio by), plots the relationship between the hydrogen sulfide generation rate of the sulfide solid electrolyte . As can be seen in FIG. 1, in Examples 1, 2 and 3 where the substitution ratio with K ₂ S is 0.7 mol%, 1.3 mol%, and 2.7 mol%, the comparative example in which no substitution was performed Compared with 1, it was confirmed that the hydrogen sulfide generation rate was reduced. Of these examples, Example 3 showed the best effect, and the hydrogen sulfide generation rate was reduced by about 62% compared to the hydrogen sulfide generation rate in Comparative Example 1. .

On the other hand, according to FIG. 1, in Comparative Example 2 in which the substitution ratio with K ₂ S was 4.0 mol%, the hydrogen sulfide generation rate was increased as compared with Comparative Example 1 in which substitution was not performed. confirmed.

FIG. 2 is a plot of the relationship between the substitution rate with K ₂ S and the ionic conductivity of the sulfide solid electrolyte. As can be seen in FIG. 2, in Examples 1, 2 and 3 where the substitution ratio by K ₂ S is 0.7 mol%, 1.3 mol%, and 2.7 mol%, the comparative example without substitution Compared with 1, it was confirmed that the decrease in ionic conductivity was slight. On the other hand, in Comparative Example 2 in which the substitution ratio with K ₂ S was 4.0 mol%, it was confirmed that the ionic conductivity was significantly reduced as compared with Comparative Example 1 in which substitution was not performed.

Therefore, the sulfide solid electrolyte whose substitution ratio by K ₂ S is 0.7 mol%, 1.3 mol%, and 2.7 mol% exhibits a high hydrogen sulfide generation suppressing effect and maintains high ionic conductivity. It was confirmed that

  The embodiment of the present disclosure has been described in detail above with reference to the drawings. However, the specific configuration is not limited to the embodiment, and there are design changes and the like within a scope not departing from the gist of the present disclosure. They are also included in this disclosure.

Claims (1)

In _{Li 2 S-P 2 S 5} -LiI-LiBr -based sulfide solid electrolyte, a part of Li ₂ S has been substituted with a _{K 2} S, K to the molar amount of the sum of Li ₂ S and _{K 2} S _The sulfide solid electrolyte whose ratio of ₂ S is 3.0 mol% or less.

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