JP7615815B2 - Manufacturing method of LGPS type solid electrolyte - Google Patents

Description

The present invention relates to a method for producing an LGPS-type solid electrolyte. The LGPS-type solid electrolyte refers to a solid electrolyte having a specific crystal structure containing Li, P, and S, and examples of such solid electrolytes include solid electrolytes containing Li, M (wherein M is one or more elements selected from the group consisting of Ge, Si, and Sn), P, and S.

In recent years, the demand for lithium-ion secondary batteries has been increasing for applications such as mobile information terminals, mobile electronic devices, electric vehicles, hybrid electric vehicles, and even stationary power storage systems. However, current lithium-ion secondary batteries use flammable organic solvents as electrolytes, and require a strong exterior to prevent the organic solvent from leaking. In addition, restrictions on the structure of devices such as portable personal computers are being imposed, such as the need to adopt a structure that addresses the risk of electrolyte leakage.

Furthermore, their use is expanding to mobile objects such as automobiles and airplanes, and large capacity is required for stationary lithium-ion secondary batteries. Under these circumstances, there is a tendency to place more importance on safety than ever before, and efforts are being made to develop all-solid-state lithium-ion secondary batteries that do not use harmful substances such as organic solvents.

For example, the use of oxides, phosphate compounds, organic polymers, sulfides, and the like as solid electrolytes in all-solid-state lithium ion secondary batteries has been considered.
Among these solid electrolytes, sulfides have the characteristics of being high in ionic conductivity, relatively soft, and easy to form solid-solid interfaces. They are also stable against active materials, and development is underway as a practical solid electrolyte.

Among sulfide solid electrolytes, there is an LGPS-type solid electrolyte that has a specific crystal structure (Non-Patent Document 1 and Patent Document 1). Among sulfide solid electrolytes, LGPS-type solid electrolytes have extremely high ionic conductivity and can operate stably from low temperatures of -30°C to high temperatures of 100°C, and there are high expectations for their practical application.

Since LGPS-type solid electrolytes react easily with moisture and oxygen to produce hydroxides and oxides, which causes a decrease in ionic conductivity, all manufacturing processes are carried out in an environment that does not contain moisture or oxygen. Non-Patent Document 2 shows that LGPS-type solid electrolytes with high ionic conductivity can be manufactured by sintering while flowing argon gas. However, when considering industrial manufacturing, a huge amount of gas is used, and the use of argon gas poses the problem of high production costs.

Patent No. 3125507

Nature Energy 1, Article number: 16030 (2016) Journal of Power Source 473 (2020) 228524

Under these circumstances, it is desirable to provide a method for producing LGPS-type solid electrolytes that have high ionic conductivity while keeping production costs low.

The inventors have conducted intensive research in light of the above problems and have discovered that an LGPS-type solid electrolyte with high ionic conductivity can be produced by synthesizing the LGPS-type solid electrolyte by firing the electrolyte in a nitrogen gas atmosphere containing more than 100 ppm and not more than 300 ppm of moisture.

That is, the present invention is as follows.
<1> A method for producing an LGPS type solid electrolyte, comprising the step of firing an LGPS type solid electrolyte precursor in a nitrogen gas atmosphere containing moisture in the range of more than 100 ppm and not more than 300 ppm.
<2> The method for producing an LGPS type solid electrolyte according to the above <1>, wherein the LGPS type solid electrolyte has peaks at least at positions of 2θ=20.18°±0.50°, 20.44°±0.50°, 26.96°±0.50°, and 29.58°±0.50° in X-ray diffraction (CuKα: λ=1.5405 Å).
<3> The method for producing an LGPS-type solid electrolyte according to <1> or <2> above, wherein the sintering temperature is 200° C. to 700° C.
<4> The method for producing an LGPS type solid electrolyte according to any one of <1> to <3> above, wherein the LGPS type solid electrolyte contains Ge and has an ionic conductivity of 3.2 mS/cm or more at 25° C.
<5> The method for producing an LGPS type solid electrolyte according to any one of <1> to <3> above, wherein the LGPS type solid electrolyte contains Sn and has an ionic conductivity of 2.8 mS/cm or more at 25° C.

According to the present invention, it is possible to provide a method for producing an LGPS-type solid electrolyte that has high ionic conductivity while suppressing production costs. Furthermore, according to the present invention, it is possible to provide a molded body obtained by heating and molding the LGPS-type solid electrolyte, and an all-solid-state battery that includes the LGPS-type solid electrolyte.

FIG. 1 is a schematic diagram showing the crystal structure of an LGPS type solid electrolyte according to one embodiment of the present invention. FIG. 2 is a schematic cross-sectional view of an all-solid-state battery according to one embodiment of the present invention. FIG. 3 is a graph showing the results of measuring the ionic conductivity of the Ge-type LGPS solid electrolytes obtained in Examples 1 to 4 and Comparative Example 1. FIG. 4 is a graph showing the results of measuring the ionic conductivity of the Sn-type LGPS solid electrolytes obtained in Examples 5 to 8 and Comparative Example 2. FIG. 5 is a graph showing the results of X-ray diffraction measurement of the Ge-type LGPS solid electrolytes obtained in Examples 1 to 4 and Comparative Example 1. FIG. 6 is a graph showing the results of X-ray diffraction measurement of the Sn-type LGPS solid electrolytes obtained in Examples 5 to 8 and Comparative Example 2. FIG. 7 is a schematic diagram of a heating vessel for carrying out a firing step in a method for producing an LGPS type solid electrolyte according to one embodiment of the present invention.

The present invention will be described in detail below. Note that the materials and configurations described below do not limit the present invention, and various modifications can be made within the scope of the present invention.

In this specification, the term LGPS-type solid electrolyte refers to a solid electrolyte that contains Li, P, and S and has a specific crystal structure, and examples of such solid electrolytes include solid electrolytes that contain Li, M (wherein M is one or more elements selected from the group consisting of Ge, Si, and Sn), P, and S.

The crystal structure of the LGPS-type solid electrolyte, as shown in FIG. 1, has an octahedron O composed of Li and S, a tetrahedron T1 composed of one or more elements selected from the group consisting of P, Ge, Si, and Sn and S, and a tetrahedron T2 composed of P and S, and the tetrahedron T1 and the octahedron O share an edge, and the tetrahedron T2 and the octahedron O share a vertex.

The LGPS-type solid electrolyte has peaks at least at 2θ=20.18°±0.50°, 20.44°±0.50°, 26.96°±0.50°, and 29.58°±0.50° in X-ray diffraction (CuKα: λ=1.5405 Å).

The LGPS-type solid electrolyte of the present invention is produced by mixing the raw materials in the desired molar ratio, reacting them while grinding them to produce an LGPS-type solid electrolyte precursor, and then calcining the produced LGPS-type solid electrolyte precursor.

<Production of LGPS-type solid electrolyte precursor>
The LGPS-type solid electrolyte precursor, which is the subject of the LGPS-type solid electrolyte manufacturing method of the present invention, can be manufactured by known methods such as the mechanical milling method described in Japanese Patent No. 5527673 and the liquid phase synthesis method described in International Publication No. WO2019/044517.

Mechanical Milling Method As an example of the mechanical milling method, a _LGPS -type solid electrolyte precursor can be obtained by mixing powders of _Li2S , _P2S5 , and _MS2 (M is one or more elements selected from the group consisting of Ge, Si, and Sn) and then grinding and mixing the mixture using a planetary ball mill or the like. The mechanical milling method is characterized in that it homogenizes the composition and makes the mixture amorphous by applying strong mechanical energy.

Liquid-phase synthesis method As an example of the liquid-phase synthesis method, Li ₂ S and P ₂ S ₅ are reacted in an organic solvent such as acetonitrile or tetrahydrofuran to dissolve the solution, and powders of Li ₂ S and MS ₂ (M is one or more elements selected from the group consisting of Ge, Si, and Sn) are added to the solution to obtain a suspension. Next, the organic solvent is distilled off to obtain a LGPS-type solid electrolyte precursor. The liquid-phase synthesis method is characterized in that the composition is homogenized and amorphous by reacting and dissolving a part or all of the raw material in an organic solvent.

<Firing>
The heating temperature varies depending on the type, and for those containing Ge, Si or Sn, the range is usually preferably 200 to 700° C., more preferably 350 to 650° C., and particularly preferably 400 to 600° C. If the temperature is lower than the above range, the desired crystals are unlikely to form, while if the temperature is higher than the above range, crystals other than the intended one may be formed.

The heating time varies slightly depending on the heating temperature, but usually crystallization is sufficient within the range of 0.1 to 24 hours. Heating at a high temperature for a long period of time beyond the above range is not recommended, as this may cause deterioration of the LGPS-type solid electrolyte.

In the present invention, the firing is performed under a nitrogen gas atmosphere from the viewpoint of suppressing production costs. Considering industrial production, a huge amount of gas is used, so using an inert gas such as helium gas or argon gas instead of nitrogen gas is not preferable from the viewpoint of production costs, and it is preferable to use inexpensive nitrogen gas.
Generally, it is considered preferable that the moisture content in the inert gas used for calcination is low, and high-purity gases with a moisture content of less than 5 ppm have been used. Even when a standard grade inert gas is used, the moisture content is often less than 10 ppm.

According to the inventors' research, it is presumed that when the nitrogen gas used contains a small amount of moisture, the moisture reacts with the surface of the LGPS solid electrolyte, and oxygen is taken into the structure. Then, it is presumed that the size of the tunnel through which the Li ions present in the crystal pass changes to a size that is more conductive, and the ion conductivity is improved.
However, if the amount of water contained in the nitrogen gas is too much, it exceeds the solid solubility range of oxygen, and an impurity layer is formed, which is thought to reduce ionic conductivity. Therefore, the amount of water contained in the nitrogen gas should not be too much or too little.
When the LGPS type solid electrolyte obtained by the present invention contains Ge, the ionic conductivity at 25° C. is preferably 3.2 mS/cm or more, and more preferably 3.5 mS/cm or more. When the LGPS type solid electrolyte obtained by the present invention contains Sn, the ionic conductivity at 25° C. is preferably 2.8 mS/cm or more, and more preferably 2.9 mS/cm or more. In the present invention, the method described in the examples below can be used as the method for measuring the ionic conductivity.

In the present invention, the amount of water contained in the nitrogen gas needs to be more than 100 ppm and not more than 300 ppm. It is preferably between 120 ppm and 295 ppm. It is particularly preferably between 140 ppm and 290 ppm. By making the amount of water contained in the nitrogen gas more than 100 ppm and not more than 300 ppm, it is possible to obtain an LGPS solid electrolyte having high ionic conductivity.
A method for adjusting the amount of moisture contained in nitrogen gas to more than 100 ppm and not more than 300 ppm can be, for example, by preparing nitrogen gas with a moisture content of 100 ppm or less and nitrogen gas with a moisture content of 300 ppm or more and mixing them in any ratio.
A dew point meter can be used to measure the amount of moisture contained in nitrogen gas.

Figure 7 is a schematic diagram of an example of a heating vessel in which the firing process is performed. The heating vessel 11 has a glass inner cylinder 12 and a glass outer cylinder 13. The LGPS-type solid electrolyte precursor 14 is placed in the glass inner cylinder 12. The heating vessel 11 is sealed with a stopper (not shown), and nitrogen gas is introduced and discharged through a branch pipe. A thermocouple 15 is also provided to measure the temperature inside the glass inner cylinder 12. Here, the heating vessel 11 is heated using a tubular furnace 16, and the LGPS-type solid electrolyte precursor 14 is heat-treated.

The LGPS-type solid electrolyte of the present invention obtained as described above can be formed into a desired molded body by various means and used in various applications including the all-solid-state battery described below. The molding method is not particularly limited. For example, the same molding method as the molding method for each layer constituting the all-solid-state battery described in <All-solid-state battery> below can be used.

<All-solid-state battery>
The LGPS-type solid electrolyte of the present invention can be used, for example, as a solid electrolyte for an all-solid-state battery. According to a further embodiment of the present invention, there is provided an all-solid-state battery including the above-mentioned solid electrolyte for an all-solid-state battery.

The term "all-solid-state battery" as used herein refers to an all-solid-state lithium-ion secondary battery. FIG. 2 is a schematic cross-sectional view of an all-solid-state battery according to one embodiment of the present invention. The all-solid-state battery 10 has a structure in which a solid electrolyte layer 2 is disposed between a positive electrode layer 1 and a negative electrode layer 3. The all-solid-state battery 10 can be used in various devices including mobile phones, personal computers, automobiles, etc.

The LGPS type solid electrolyte of the present invention may be contained as a solid electrolyte in at least one of the positive electrode layer 1, the negative electrode layer 3, and the solid electrolyte layer 2. When the LGPS type solid electrolyte of the present invention is contained in the positive electrode layer 1 or the negative electrode layer 3, the LGPS type solid electrolyte of the present invention is used in combination with a known positive electrode active material or negative electrode active material for lithium ion secondary batteries. The amount ratio of the LGPS type solid electrolyte of the present invention contained in the positive electrode layer 1 or the negative electrode layer 3 is not particularly limited.
The LGPS type solid electrolyte of the present invention may be composed of a single material, or may be used in appropriate combination with an oxide solid electrolyte (e.g., Li ₇ La ₃ Zr ₂ O ₁₂ ), a sulfide solid electrolyte (e.g., Li ₂ S—P ₂ S ₅ ), or other complex hydride solid electrolytes (e.g., LiBH ₄ , 3LiBH ₄ —LiI), if necessary.

The all-solid-state battery is produced by molding and stacking each of the layers described above, but there are no particular limitations on the method of molding and stacking each layer.

For example, there are methods in which a solid electrolyte and/or an electrode active material is dispersed in a solvent to form a slurry, which is then applied using a doctor blade or spin coater, and then rolled to form a film; gas-phase methods in which films are formed and laminated using vacuum deposition, ion plating, sputtering, laser ablation, etc.; and pressure molding methods in which powder is molded using hot pressing or cold pressing without applying heat, and then laminated.

Since the LGPS-type solid electrolyte of the present invention is relatively soft, it is particularly preferable to produce an all-solid-state battery by forming and stacking each layer by pressure molding. Pressure molding methods include hot pressing, which is performed with heating, and cold pressing, which is not performed with heating, but cold pressing can also be used to sufficiently form the electrolyte.

The present invention also includes a molded body obtained by heat-molding the LGPS-type solid electrolyte of the present invention. The molded body is suitable for use as an all-solid-state battery. The present invention also includes a method for producing an all-solid-state battery, which includes a step of heat-molding the LGPS-type solid electrolyte of the present invention.

The present embodiment will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples.
The amount of moisture contained in the nitrogen gas used in the following Examples and Comparative Examples was measured using a dew point meter (TK-100 online dew point meter manufactured by Techne Measurement Co., Ltd.).

Example 1
<Preparation of Ge-type LGPS precursor>
In a glove box under an argon atmosphere, Li ₂ S (Sigma-Aldrich, purity 99.8%), P ₂ S ₅ (Sigma-Aldrich, purity 99%), and GeS ₂ (Sigma-Aldrich, purity 99%) were weighed out and premixed in an agate mortar so that the molar ratio of Li ₂ S:P ₂ S _{5 :} GeS ₂ = 5:1:1.
Next, the premixed raw materials were put into a 45 mL zirconia pot, and zirconia balls (Nikkato Corporation "YTZ", φ10 mm, 15 pieces) were further put in to completely seal the pot. This pot was attached to a planetary ball mill (Fritsch "P-7") and mechanical milling was performed at a rotation speed of 500 rpm for 10 hours to obtain a Ge-type LGPS precursor.
<Baking of Ge-type LGPS precursor>
The obtained precursor was sintered using a heating vessel 11 shown in Fig. 7. 0.5 g of the precursor was placed in a glass inner cylinder 12 in a glove box under an argon atmosphere, and the glass inner cylinder 12 was placed in a tubular furnace 16 so that the precursor was not exposed to the atmosphere. The gas in the heating vessel 11 was replaced from argon gas to nitrogen gas (water content: 145 ppm), and then the temperature was raised to 550°C over 3 hours while blowing in nitrogen gas (water content: 145 ppm), and then the precursor was sintered at 550°C for 8 hours to obtain a Ge-type LGPS solid electrolyte.

Example 2
<Preparation of Ge-type LGPS precursor>
The same operation as in Example 1 was carried out to obtain a Ge-type LGPS precursor.
<Baking of Ge-type LGPS precursor>
A Ge-type LGPS solid electrolyte was obtained in the same manner as in Example 1, except that the water content of the nitrogen gas was set to 203 ppm.

Example 3
<Preparation of Ge-type LGPS precursor>
The same operation as in Example 1 was carried out to obtain a Ge-type LGPS precursor.
<Baking of Ge-type LGPS precursor>
A Ge-type LGPS solid electrolyte was obtained in the same manner as in Example 1, except that the water content of the nitrogen gas was set to 289 ppm.

Example 4
<Preparation of Ge-type LGPS precursor>
The same operation as in Example 1 was carried out to obtain a Ge-type LGPS precursor.
<Baking of Ge-type LGPS precursor>
A Ge-type LGPS solid electrolyte was obtained in the same manner as in Example 1, except that the water content of the nitrogen gas was set to 123 ppm.

(Comparative Example 1)
<Preparation of Ge-type LGPS precursor>
The same operation as in Example 1 was carried out to obtain a Ge-type LGPS precursor.
<Baking of Ge-type LGPS precursor>
A Ge-type LGPS solid electrolyte was obtained in the same manner as in Example 1, except that the moisture content of the nitrogen gas was set to 23 ppm.

Example 5
<Preparation of Sn-type LGPS precursor>
In a glove box under an argon atmosphere, Li ₂ S (Sigma-Aldrich, purity 99.8%), P ₂ S ₅ (Sigma-Aldrich, purity 99%), and SnS ₂ (Kojundo Chemical Co., Ltd.) were weighed out and premixed in an agate mortar so that the molar ratio of Li ₂ S:P ₂ S ₅ :SnS ₂ = 5:1:1.
Next, the premixed raw materials were put into a 45 mL zirconia pot, and zirconia balls (Nikkato Corporation "YTZ", φ10 mm, 15 pieces) were further put in to completely seal the pot. This pot was attached to a planetary ball mill (Fritsch "P-7") and mechanical milling was performed at a rotation speed of 800 rpm for 10 hours to obtain a Sn-type LGPS precursor.
<Firing of Sn-type LGPS precursor>
The obtained precursor was sintered using a heating vessel 11 shown in Fig. 7. 0.5 g of the precursor was placed in a glass inner cylinder 12 in a glove box under an argon atmosphere, and the glass inner cylinder 12 was placed in a tubular furnace 16 so that the precursor was not exposed to the atmosphere. The gas in the heating vessel 11 was replaced from argon gas to nitrogen gas (water content: 145 ppm), and then the temperature was raised to 550°C over 3 hours while blowing in nitrogen gas (water content: 145 ppm), and then the precursor was sintered at 550°C for 8 hours to obtain a Sn-type LGPS solid electrolyte.

Example 6
<Preparation of Sn-type LGPS precursor>
The same procedure as in Example 5 was carried out to obtain a Sn-type LGPS precursor.
<Firing of Sn-type LGPS precursor>
The same operation as in Example 5 was carried out except that the moisture content of the nitrogen gas was set to 203 ppm, to obtain a Sn-type LGPS solid electrolyte.

(Example 7)
<Preparation of Sn-type LGPS precursor>
The same procedure as in Example 5 was carried out to obtain a Sn-type LGPS precursor.
<Firing of Sn-type LGPS precursor>
The same operation as in Example 5 was carried out except that the water content of the nitrogen gas was set to 289 ppm, to obtain a Sn-type LGPS solid electrolyte.

(Example 8)
<Preparation of Sn-type LGPS precursor>
The same procedure as in Example 5 was carried out to obtain a Sn-type LGPS precursor.
<Firing of Sn-type LGPS precursor>
The same operation as in Example 5 was carried out except that the water content of the nitrogen gas was set to 123 ppm, to obtain a Sn-type LGPS solid electrolyte.

(Comparative Example 2)
<Preparation of Sn-type LGPS precursor>
The same procedure as in Example 5 was carried out to obtain a Sn-type LGPS precursor.
<Firing of Sn-type LGPS precursor>
The same operation as in Example 5 was carried out except that the moisture content of the nitrogen gas was set to 23 ppm, to obtain a Sn-type LGPS solid electrolyte.

<Measurement of lithium ion conductivity>
The LGPS-type solid electrolytes obtained in Examples 1 to 8 and Comparative Examples 1 and 2 were subjected to uniaxial molding (420 MPa) to obtain disks with a thickness of about 1 mm and a diameter of 10 mm. Using an all-solid-state battery evaluation cell (manufactured by Hosen Co., Ltd.), AC impedance measurements were performed by the four-terminal method (Solartron's "SI1260 IMPEDANCE/GAIN-PHASE ANALYZER") at room temperature (25°C) to calculate the lithium ion conductivity.
Specifically, the sample was placed in a thermostatic chamber set at 25° C. and held for 30 minutes, and then the lithium ion conductivity was measured. The measurement frequency range was 0.1 Hz to 1 MHz, and the amplitude was 50 mV. The measurement results of the lithium ion conductivity are shown in FIG. 3 (Ge-type LGPS solid electrolyte) and FIG. 4 (Sn-type LGPS solid electrolyte).
3 (Ge-type LGPS solid electrolyte), Examples 1 to 4, in which the amount of water in the nitrogen gas is large, show high ionic conductivity compared to Comparative Example 1, in which the amount of water in the nitrogen gas is small. This is believed to be because the water in the nitrogen gas reacts with the surface of the solid electrolyte, and oxygen is taken into the structure, changing the size of the tunnel through which the Li ions present in the crystal pass to a size that is more conductive.
FIG. 4 (Sn-type LGPS solid electrolyte) also shows the same behavior of ion conductivity as FIG. 3 (Ge-type LGPS solid electrolyte).

<X-ray diffraction measurement>
For the LGPS-type solid electrolytes obtained in Examples 1 to 8 and Comparative Examples 1 and 2, X-ray diffraction measurements (PANalytical's "X'Pert3 Powder", CuKα: λ=1.5405 Å) were carried out in an Ar atmosphere at room temperature (25° C.).
As shown in the results of X-ray diffraction measurements in Figures 5 and 6, in Examples 1 to 8 and Comparative Examples 1 and 2, diffraction peaks were observed at least at 2θ = 20.18° ± 0.50°, 20.44° ± 0.50°, 26.96° ± 0.50°, and 29.58° ± 0.50°, and this pattern matched Li ₁₀ GeP ₂ S ₁₂ in the ICSD database, confirming that the material had an LGPS-type crystal structure.

Reference Signs List 1: Positive electrode layer 2: Solid electrolyte layer 3: Negative electrode layer 10: All-solid-state battery 11: Heating vessel 12: Glass inner cylinder 13: Glass outer cylinder 14: LGPS-type solid electrolyte precursor 15: Thermocouple 16: Tubular furnace

Claims (5)

A method for producing an LGPS-type solid electrolyte, comprising the step of sintering an LGPS-type solid electrolyte precursor in a nitrogen gas atmosphere containing moisture in the range of more than 100 ppm and not more than 300 ppm. The method for producing an LGPS-type solid electrolyte according to claim 1, wherein the LGPS-type solid electrolyte has peaks at least at 2θ=20.18°±0.50°, 20.44°±0.50°, 26.96°±0.50°, and 29.58°±0.50° in X-ray diffraction (CuKα: λ=1.5405 Å). The method for producing an LGPS-type solid electrolyte according to claim 1 or 2, wherein the temperature during the sintering is 200°C to 700°C. The method for producing an LGPS-type solid electrolyte according to any one of claims 1 to 3, wherein the LGPS-type solid electrolyte contains Ge and has an ionic conductivity of 3.2 mS/cm or more at 25°C. 4. The method for producing an LGPS type solid electrolyte according to claim 1, wherein the LGPS type solid electrolyte contains Sn and has an ionic conductivity of 2.8 mS/cm or more at 25° C.

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