JP7056530B2 - Method for producing modified sulfide solid electrolyte - Google Patents

Description

The present disclosure relates to a method for producing a modified sulfide solid electrolyte, particularly a method for producing a modified sulfide solid electrolyte by removing a solvent from a water / ethanol solution of the sulfide solid electrolyte.

Lithium-ion batteries have a high energy density and are used not only in applications such as mobile phones and portable personal computers, but also in stationary applications such as in-vehicle devices and social infrastructure. In addition, from the viewpoint of preventing global warming due to the increase in carbon dioxide, interest in electric vehicles is increasing, and the use of lithium secondary batteries as a power source is being considered.

Further, in recent years, an all-solid-state lithium-ion battery using a solid electrolyte having lithium ion conductivity has attracted attention. The all-solid-state lithium-ion battery is considered to be excellent in terms of simplification of the apparatus and improvement of output characteristics.

Regarding such lithium-ion batteries, technological development is underway for the purpose of improving cycle characteristics, high durability, heat resistance, etc., and in particular, a technique for coating the surface of an active material with various substances is disclosed. ing.

Patent Document 1 discloses a method for producing a positive electrode active material for a lithium ion secondary battery. In this document, a positive electrode active material, a lithium alkoxide, and an alkoxide of an element M forming a covalent bond with phosphorus are mixed in an alcohol solution to obtain a mixed solution, and a solvent is used by a rotary evaporator while irradiating ultrasonic waves. It is described that the coating precursor is fired in dry air at a temperature of 750 ° C. or lower.

Patent Document 2 discloses a method for producing a coated positive electrode active material for a lithium secondary battery. In this document, the raw material of the solid electrolyte is dissolved in 1-propanol and water, LiCoO ₂ as an active substance is added to the mixed solution of these, the solvent is blown off by an evaporator, and vacuum drying is performed at 60 ° C. for 24 hours. It describes what to do and how to bake at 700 ° C. for 1 hour.

Here, among the solid electrolytes, the sulfide solid electrolyte has high lithium ion conductivity, and it is proposed to use it as a solid electrolyte of an all-solid lithium ion battery. Further, a technique for coating an active material or the like with a solution in which a sulfide solid electrolyte is dissolved in various solvents is being developed.

Non-Patent Document 1 discloses that LiCoO ₂ is impregnated with an ethanol solution of Li ₆ PS ₅ Cl as a sulfide solid electrolyte. The document describes that the electrodes of a lithium-ion battery were immersed in an ethanol solution of Li ₆ PS ₅ Cl.

Non-Patent Document 2 describes the acquisition of coated LiCoO ₂ with a methanol solution of Li ₄ SnS ₄ and Li I as a sulfide solid electrolyte. The document discloses the synthesis of LiI-Li ₄ SnS ₄ using an aqueous solution, and describes that the raw material is dissolved in anhydrous methanol and then heat-treated in vacuum.

Non-Patent Document 3 discloses a method using an aqueous solution for coating the active material LiCoO ₂ with a sulfide solid electrolyte. In this document, an aqueous solution of Li ₄ SnS ₄ dissolved in water is prepared, powdered LiCoO ₂ is added to the aqueous solution, and the solution is dried under vacuum to provide the active material coated with Li ₄ SnS ₄ . It is described to prepare.

Japanese Unexamined Patent Publication No. 2016-110714 Japanese Unexamined Patent Publication No. 2016-018610

Dong Hyeon Kim et al. , Nano Lett. , 2017, 17, 3013-3020. Kern Ho Park et al. , Adv. Mater. , 2016, 28, 1874-1883. Young En Choi et al. , ChemSusChem, 2017, 10, 2605-2611.

In an all-solid lithium ion secondary battery using a solid electrolyte such as a sulfide solid electrolyte, the active material layer is coated with a solution in which the sulfide solid electrolyte is dissolved in an organic solvent or the like for the purpose of ensuring ionic conductivity. Technology has been developed. In such a technique, a drying treatment for removing the solvent by heating or the like is required, but it is difficult to perform drying while keeping the ionic conductivity of the sulfide solid electrolyte relatively high. That is, when the treatment is performed at a high temperature, the crystal structure of the sulfide solid electrolyte cannot be maintained, which leads to a decrease in ionic conductivity. On the other hand, when the drying treatment is performed at a low temperature, water remains in the sulfide solid electrolyte, which may lead to a decrease in ionic conductivity.

The present disclosure has been made in view of the above problems, and an object of the present invention is to provide a method for producing a modified sulfide solid electrolyte from a solution while maintaining a relatively high ionic conductivity of the sulfide solid electrolyte. Is.

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

To provide a sulfide solid electrolyte solution in which the raw sulfide solid electrolyte containing Li ₄ SnS ₄ is dissolved in a mixed solvent containing water and ethanol.
The solution is dried at 300 ° C. or lower under vacuum conditions to obtain a dried sulfide solid electrolyte, and
The drying-treated sulfide solid electrolyte is further dried at 300 ° C. or lower in an argon atmosphere.
A method for producing a modified sulfide solid electrolyte.

According to the method of the present disclosure, a modified sulfide solid electrolyte can be produced from a solution while maintaining a relatively high ionic conductivity of the sulfide solid electrolyte.

FIG. 1 shows the ionic conductivity of a modified sulfide solid electrolyte when the solution is dried by various modes when the weight ratio of water to ethanol in the solvent is 2: 1. FIG. 2 shows the ionic conductivity of a modified sulfide solid electrolyte when the solution is dried by various modes when the weight ratio of water to ethanol in the solvent is 1: 1. FIG. 3 shows the ionic conductivity of the modified sulfide solid electrolyte when dried at various drying times.

Hereinafter, embodiments of the present disclosure will be described in detail. The specific configuration is not limited to this embodiment, and even if there are design changes and the like within the scope of the gist of the present disclosure, they are included in the present disclosure.

The methods of the present disclosure for producing modified sulfide solid electrolytes are:
A providing step of providing a sulfide solid electrolyte solution in which the raw sulfide solid electrolyte containing Li ₄ SnS ₄ is dissolved in a mixed solvent containing water and ethanol.
A vacuum drying step, wherein the solution is dried at 300 ° C. or lower under vacuum conditions to obtain a dried sulfide solid electrolyte.
Argon atmosphere drying step, in which the dried sulfide solid electrolyte is further dried at 300 ° C. or lower under an argon atmosphere.
Includes.

As described above, for the purpose of improving the battery characteristics of the lithium ion battery, a technique of coating the active material with a solid electrolyte solution such as a sulfide solid electrolyte is useful. Further, as a method for producing LiI-Li ₄ SnS ₄ , there is a method using a solution containing LiI and Li ₄ SnS ₄ . In such a technique, it is important to sufficiently dry the liquefied solid electrolyte in order to ensure the relatively high ionic conductivity of the produced solid electrolyte. However, in the drying method involving heating at a high temperature, it becomes difficult to maintain the crystal structure of the sulfide solid electrolyte due to the high temperature, and as a result, the ionic conductivity may decrease.

The inventors of the present disclosure have found a method for producing a modified sulfide solid electrolyte from a solution by heating at a relatively low temperature while maintaining a relatively high ionic conductivity of the sulfide solid electrolyte.

In the method of the present disclosure, the drying temperature is set to 300 ° C. or lower. It is considered that this suppresses the change in the crystal structure of the sulfide solid electrolyte due to high temperature heating, and as a result, a modified sulfide solid electrolyte having a relatively high ionic conductivity can be obtained.

Further, in the method of the present disclosure, in addition to drying in a vacuum, drying in an argon atmosphere is performed. It is considered that this makes it possible to sufficiently remove the water even in the drying treatment at a relatively low temperature, and to achieve sufficient drying. As a result, it is considered that a modified sulfide solid electrolyte having a relatively high ionic conductivity can be obtained.

As described above, by following the method of the present disclosure, a modified sulfide solid electrolyte can be produced from the solution while maintaining a relatively high ionic conductivity of the sulfide solid electrolyte.

By applying the method of the present disclosure to a known technique for coating an active material or the like with a sulfide solid electrolyte solution, the active material or the like is coated with a modified sulfide solid electrolyte having a relatively high ionic conductivity. It is possible to do.

Hereinafter, the method of the present disclosure will be described in detail.

≪Providing process≫
The providing step in the method of the present disclosure provides a sulfide solid electrolyte solution in which the raw sulfide solid electrolyte containing Li ₄ SnS ₄ is dissolved in a mixed solvent containing water and ethanol.

The solid sulfide electrolyte solution in the step may be provided, for example, by dissolving the raw sulfide solid electrolyte containing Li ₄ SnS ₄ in a solvent containing water and ethanol.

<solvent>
The solvent according to the present disclosure includes water and ethanol.

Ethanol contained in the solvent is 5.0% by weight or more, 10.0% by weight or more, 25.0% by weight or more, 33.0% by weight or more, or 40.0% by weight, based on the total of water and ethanol. The above may be sufficient, and / or may be 90.0% by weight or less, 75.0% by weight or less, 60.0% by weight or less, 50.0% by weight or less, or 45.0% by weight or less.

<Dissolution>
The raw material sulfide solid electrolyte containing Li ₄ SnS ₄ may be dissolved in a solvent by a known method. For example, in one embodiment of the present disclosure, first, a solvent in which water and ethanol are mixed is prepared, and then a raw sulfide solid electrolyte containing Li ₄ SnS ₄ is added to the solvent, and the mixture is optionally stirred. , Dissolve.

Alternatively, in another embodiment of the present disclosure, first, a raw sulfide solid electrolyte containing Li ₄ SnS ₄ is added to water or ethanol and optionally stirred to prepare the solution. After that, ethanol or water is added to this solution, and the mixture is optionally stirred to obtain a sulfide solid electrolyte solution in which the Li ₄ SnS ₄ -containing sulfide solid electrolyte is dissolved in water and an ethanol-containing solvent.

<Sulfide solid electrolyte containing Li ₄ SnS ₄ >
In the method according to the present disclosure, a raw material sulfide solid electrolyte containing Li ₄ SnS ₄ is used. The raw sulfide solid electrolyte according to the present disclosure may contain a substance other than Li ₄ SnS ₄ . The method for synthesizing such a solid electrolyte is exemplified below.

<Method for synthesizing Li ₄ SnS ₄ >
(Starting raw material)
The starting material for synthesizing Li ₄ SnS ₄ is not particularly limited, but commercially available materials can be used. In particular, it is preferable to use a high-purity one. Examples of the starting material include lithium sulfide (Li 2S), tin sulfide ( _{SnS 2 )} , tin (Sn), sulfur (S), and the like, and any combination thereof can be used.

(Synthesis method)
The method for synthesizing Li ₄ SnS ₄ is not particularly limited and a known method may be used, and examples thereof include a method involving heat treatment and a mechanical milling treatment.

In methods involving heat treatment, for example, a stoichiometric mixture of lithium sulfide, tin, and sulfur as starting materials is heat treated in a vacuum-sealed quartz ampol. After that, the obtained powder is dissolved in ion-exchanged water, and undissolved impurities are removed by a filter. Then, the filtered solution is vacuum-treated and dried at a predetermined temperature under reduced pressure to obtain a powder of Li ₄ SnS ₄ .

The mechanical milling process is a method of grinding and mixing raw materials while applying mechanical energy. In the mechanical milling process, for example, a mechanical pulverizer such as a ball mill, a bead mill, a rod mill, a vibration mill, a disc mill, a hammer mill, or a jet mill can be used to perform mixed pulverization. By mechanical milling treatment, a sulfide solid electrolyte containing Li ₄ SnS ₄ can be obtained, for example, as a fine powder.

<Sulfide solid electrolyte containing LiI-Li ₄ SnS ₄ >
In one embodiment of the method according to the present disclosure, the sulfide solid electrolyte solution provided in the providing step further comprises LiI in addition to Li ₄ SnS ₄ . Preferably, the sulfide solid electrolyte solution provided in the providing step is adjusted so that the composition ratio x: 1-x of LiI to Li ₄ SnS ₄ satisfies 0.2 ≦ x ≦ 0.4. For example, in the case of 0.2LiI-0.8Li ₄ SnS ₄ , the concentration is preferably in the range of 1% by weight to 20% by weight with respect to the solvent.

The inclusion of LiI in addition to Li ₄ SnS ₄ further improves the ionic conductivity of the modified sulfide solid electrolyte purified from the sulfide solid electrolyte solution. The reason for the increase is not intended to be limited by logic, but due to the large ionic radius of the iodine ion, a relatively open crystal structure is obtained and / or the highly polarized iodine ion. It is conceivable that the energy barrier required for Li hopping is relatively low due to the property of Li hopping.

≪Vacuum drying process≫
In the vacuum drying step in the method of the present disclosure, the sulfide solid electrolyte solution is dried at 300 ° C. or lower under vacuum conditions to obtain a dried sulfide solid electrolyte.

Drying under vacuum conditions may be performed by a known method, and is not particularly limited as long as the solvent can be removed while heating.

The drying time in the vacuum drying step is not particularly limited, but may be 1 hour or more, 2 hours or more, or 3 hours or more, and / or 6 hours or less, 5 hours or less, or 4 hours or less. ..

The drying temperature in the vacuum drying step is 300 ° C. or lower. The drying temperature in the vacuum drying step may be 75 ° C. or higher, 100 ° C. or higher, 120 ° C. or higher, or 140 ° C. or higher, and / or 300 ° C. or lower, 250 ° C. or lower, 200 ° C. or lower, or 150 ° C. or lower. It's okay.

≪Argon atmosphere drying process≫
In the argon atmosphere drying step in the method of the present disclosure, the sulfide solid electrolyte that has been dried in the vacuum drying step is further dried at 300 ° C. or lower in an argon atmosphere.

Drying in an argon atmosphere may be performed by a known method, and is not particularly limited as long as the solvent can be removed while heating.

The drying time in the argon atmosphere drying step is not particularly limited, but may be 1 hour or more, 2 hours or more, or 3 hours or more, and / or 6 hours or less, 5 hours or less, or 4 hours or less. good.

The drying temperature in the argon atmosphere drying step is 300 ° C. or lower. The drying temperature in the argon atmosphere drying step may be 75 ° C. or higher, 100 ° C. or higher, 120 ° C. or higher, or 140 ° C. or higher, and / or at 300 ° C. or lower, 250 ° C. or lower, 200 ° C. or lower, or 150 ° C. or lower. It may be there.

Hereinafter, the method according to the present disclosure will be described in more detail with reference to examples.

≪Evaluation of drying temperature 1≫
As described in detail below, the relationship between the drying temperature and the ionic conductivity of the modified sulfide solid electrolyte obtained after drying was investigated.

<Synthesis of Li ₄ SnS ₄ >
Li 2S (Nippon Chemical Industrial, 99.9%) and _{SnS 2 (} high-purity chemistry, 99.9%) were used as raw materials. These were mixed to a stoichiometric ratio of Li ₄ SnS ₄ and baked at 700 ° C. under vacuum for 12 hours to synthesize Li ₄ SnS ₄ .

<Preparation of solution>
Li ₄ SnS ₄ and LiI (Aldrich, 99.9%) synthesized as described above are mixed in a solvent consisting of water and ethanol with a weight ratio of 2: 1 and Li ₄ SnS ₄ and LiI in total are 10% by weight. The sulfide solid electrolyte solution was prepared by dissolving the mixture so as to be. The composition ratio of LiI and Li ₄ SnS ₄ was 1: 4.

<Drying process>
The above sulfide solid electrolyte solution was dried by various modes. The mode of drying in each Example and each Comparative Example is shown in Table 1 below.

Comparative Example 1, Comparative Example 2, and Comparative Example 4 are the same as those of Example 1, Example 2, and Comparative Example 3, respectively, except that they are not dried in an argon atmosphere.

<Evaluation of ionic conductivity>
100 mg of the modified sulfide solid electrolyte obtained as a result of the above drying treatment was weighed and pressed at 6 ton / cm ² for 1 minute. Then, it was constrained with 6N, and the ionic conductivity was measured by the AC impedance.

The results are shown in FIG.

FIG. 1 shows the ionic conductivity of a modified sulfide solid electrolyte when the solution is dried by various drying modes when the weight ratio of water to ethanol in the solvent is 2: 1. In Example 1 in which drying at 200 ° C. was performed under vacuum conditions and an argon atmosphere, the ionic conductivity of the modified sulfide solid electrolyte was higher than that in Comparative Example 1 in which drying at 200 ° C. was performed only in vacuum. It turned out that it was high. Even in Example 2 in which drying at 300 ° C. was performed under vacuum conditions and an argon atmosphere, the ionic conductivity of the modified sulfide solid electrolyte was compared with Comparative Example 2 in which drying at 300 ° C. was performed only in vacuum. It turned out that was high. On the other hand, in Comparative Example 3 in which drying at 350 ° C. was performed under vacuum conditions and an argon atmosphere, the modified sulfide solid electrolyte was compared with Comparative Example 4 in which drying at 350 ° C. was performed only in vacuum. The ionic conductivity of was not high.

These results show that a modified sulfide solid electrolyte with relatively high ionic conductivity can be obtained by drying at 200 ° C. or 300 ° C. under vacuum conditions and an argon atmosphere. There is.

Further, from FIG. 1, when drying was performed at 200 ° C. under vacuum conditions and an argon atmosphere (Example 1), when drying was performed at 300 ° C. under vacuum conditions and an argon atmosphere (Example 2). By comparison, it can be seen that a modified sulfide solid electrolyte having a relatively high ionic conductivity could be obtained.

≪Evaluation of drying temperature 2≫
The relationship between the drying temperature and the ionic conductivity of the modified sulfide solid electrolyte obtained after drying was investigated.

A sulfide solid electrolyte solution was prepared in the same manner as described above for Example 1 and the like, except that the weight ratio of water and ethanol in the solvent was 1: 1. Then, the solution was dried in various manners shown in Table 2 below to produce a modified sulfide solid electrolyte, and the ionic conductivity of the modified sulfide solid electrolyte was measured.

The results are shown in FIG.

FIG. 2 shows the ionic conductivity of a modified sulfide solid electrolyte when dried by various modes when the weight ratio of water to ethanol in the solvent is 1: 1. In Example 3 in which drying at 200 ° C. was performed under vacuum conditions and an argon atmosphere, the ionic conductivity of the sulfide solid electrolyte was higher than that in Comparative Example 5 in which drying at 200 ° C. was performed only in vacuum. It turned out that it was. Even in Example 4 in which drying at 300 ° C. was performed under vacuum conditions and an argon atmosphere, the ionic conductivity of the modified sulfide solid electrolyte was compared with Comparative Example 6 in which drying at 300 ° C. was performed only in vacuum. It turned out that was high. On the other hand, in Comparative Example 7 in which drying at 350 ° C. was performed under vacuum conditions and an argon atmosphere, the modified sulfide solid electrolyte was compared with Comparative Example 8 in which drying at 350 ° C. was performed only in vacuum. No increase in ionic conductivity was observed.

These results show that a modified sulfide solid electrolyte with relatively high ionic conductivity can be obtained by drying at 200 ° C. or 300 ° C. under vacuum conditions and an argon atmosphere. There is.

Further, from FIG. 2, when drying was performed at 200 ° C. under vacuum conditions and an argon atmosphere (Example 3), when drying was performed at 300 ° C. under vacuum conditions and an argon atmosphere (Example 4). By comparison, it can be seen that a modified sulfide solid electrolyte having a relatively high ionic conductivity could be obtained.

≪Evaluation of drying time≫
The ionic conductivity of the modified sulfide solid electrolyte was evaluated under various values of the drying time under vacuum conditions and an argon atmosphere.

A sulfide solid electrolyte solution was prepared by the same method as described above for Example 1 and the like. Then, drying was carried out for each Example and each Comparative Example in the manner shown in Table 3 below. The drying temperature was 200 ° C. The ionic conductivity of the obtained modified sulfide solid electrolyte was measured by the method described above.

The results are shown in Table 3 and FIG. 3 below.

In FIG. 3, the case where the ionic conductivity of less than 5.0 × 10 ^-5 S / cm is shown is indicated by ×, which is 5.0 × 10 ^-5 S / cm or more and 8.0 × 10. The case where the ionic conductivity of less than ^-5 S / cm is shown is indicated by Δ, and the case of showing the ionic conductivity of 8.0 × 10 ^-5 S / cm or more is indicated by ◯.

Looking at FIG. 3, in Comparative Example 9 and Comparative Example 10 in which only drying under vacuum conditions was performed, the ionic conductivity of the modified sulfide solid electrolyte was less than 5.0 × ^10-5 S / cm. have understood. In Example 8, Example 9, and Example 10 in which the drying time under vacuum conditions was 1 hour and / or the drying time under an argon atmosphere was 2 hours, the ion conduction of the modified sulfide solid electrolyte was performed. It was found that the degree was 5.0 × 10 ^-5 S / cm or more and less than 8.0 × 10 ^-5 S / cm. In Example 1, Example 5, Example 6, and Example 7 in which the drying time under vacuum conditions was 2 hours or more and the drying time under an argon atmosphere was 3 hours or more, the modified sulfide solid electrolyte was used. It was found that the ionic conductivity of the above was 8.0 × ^10-5 S / cm or more.

Claims (1)

To provide a sulfide solid electrolyte solution in which the raw sulfide solid electrolyte containing Li ₄ SnS ₄ is dissolved in a mixed solvent containing water and ethanol.
The solution is dried at 200 ° C. or higher and 300 ° C. or lower under vacuum conditions to obtain a dry-treated sulfide solid electrolyte, and the dry-treated sulfide solid electrolyte is dried at 200 ° C. or higher and 300 ° C. or lower. A method for producing a modified sulfide solid electrolyte, which comprises further drying in an argon atmosphere.

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