Sulfide solid electrolyte, method for producing sulfide solid electrolyte, and all-solid-state battery

Abstract

To provide a sulfide solid electrolyte having good ion conductivity.SOLUTION: There are provided a sulfide solid electrolyte for use in an all-solid-state battery, the sulfide solid electrolyte has a composition represented by (100-x)[yLi2S (1-y)P2S5] xLiBH4 (x is a number that satisfies 50<x<75, and y is a number that satisfies 0.72≤y≤0.78), and an ionic conductivity at 25°C of 5.0 mS / cm or more.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a sulfide solid electrolyte, a method for producing the sulfide solid electrolyte, and an all-solid-state battery. [Background technology]

[0002] All-solid-state batteries are batteries that have a solid electrolyte layer between a positive electrode active material layer and a negative electrode active material layer, and have the advantage of being easier to simplify safety devices compared to liquid-based batteries that have an electrolyte solution containing a flammable organic solvent. Sulfide solid electrolytes are known as solid electrolytes used in all-solid-state batteries.

[0003] For example, Patent Document 1 discloses a method for producing an ion conductor (sulfide solid electrolyte), which includes mixing LiBH4 and P2S5 in a molar ratio of LiBH4:P2S5=x:(1-x) [wherein x=more than 0.85 and 0.98 or less] to obtain a mixture, and heat-treating the mixture, as well as the ion conductor (sulfide solid electrolyte) produced by the method.

[0004] Furthermore, Non-Patent Document 1 discloses a sulfide solid electrolyte represented by (100-x)(0.75Li2S·0.25P2S5)·xLiBH4. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2016 / 103894 [Non-patent literature]

[0006] [Non-Patent Document 1] Mechanochemically Prepared Li2S P2S5 LiBH4 Solid Electrolytes with an Argyrodite Structure(ACS Omega 2018, 3, 5453 5458)

Summary of the Invention

Problems to be Solved by the Invention

[0007] From the perspective of improving battery performance, a solid electrolyte with good ionic conductivity is required. The present disclosure has been made in view of the above situation, and the main object is to provide a sulfide solid electrolyte with good ionic conductivity.

[0008] The present disclosure has been made in view of the above situation, and the main object is to provide a sulfide solid electrolyte with good ionic conductivity.

Means for Solving the Problems

[0009] [1] A sulfide solid electrolyte used in an all-solid-state battery, having a composition represented by (100 - x)[yLi2S·(1 - y)P2S5]·xLiBH4 (where x is a number satisfying 50 < x < 75, and y is a number satisfying 0.72 ≦ y ≦ 0.78), and having an ionic conductivity at 25°C of 5.0 mS / cm or more.

[0010] [2] The sulfide solid electrolyte according to [1], wherein the ionic conductivity is 8.0 mS / cm or more.

[0011] [3] The sulfide solid electrolyte according to [1] or [2], having one peak in the range of 2θ = 13° or more and 16° or less in X-ray diffraction measurement using CuKα radiation.

[0012] [4]<00[1] to [3]. A method for producing a sulfide solid electrolyte according to any one of [1] to [3], comprising: a first mechanical milling step of mechanically milling a mixture containing Li2S and P2S5 to obtain a first sulfide glass; and a second mechanical milling step of adding LiBH4 to the first sulfide glass and mechanically milling the resulting mixture to obtain a second sulfide glass, wherein the mechanical milling is performed in each of the first and second mechanical milling steps so that the gravity Gn1 calculated from the following mathematical formula (1) is 6 G or greater:

number

[0013] [5] The method for producing a sulfide solid electrolyte according to [4], wherein Gn1 is 15G or more.

[0014] [6] The method for producing a sulfide solid electrolyte according to [4] or [5], further comprising a heat treatment step of heat treating the sulfide glass, wherein the temperature in the heat treatment is 190°C or lower.

[0015] [7] An all-solid-state battery having a positive electrode active material layer, a negative electrode active material layer, and a solid electrolyte layer disposed between the positive electrode active material layer and the negative electrode active material layer, wherein at least one of the positive electrode active material layer, the negative electrode active material layer, and the solid electrolyte layer contains the sulfide solid electrolyte according to any one of [1] to [3]. [Effects of the Invention]

[0016] The present disclosure has an effect of providing a sulfide solid electrolyte with good ion conductivity. [Brief explanation of the drawings]

[0017] [Figure 1] It is a flowchart illustrating a method for manufacturing a sulfide solid electrolyte in the present disclosure. [Figure 2] It is a schematic cross-sectional view illustrating a all-solid-state battery in the present disclosure. [Figure 3] It is a graph showing the measurement results of ionic conductivity in the examples. [Figure 4] It is a graph showing the measurement results of X-ray diffraction in the examples. [Figure 5] It is a graph showing the measurement results of discharge capacity retention rate in the examples. [Figure 6] It is a graph showing the measurement results of ionic conductivity in the examples.

Mode for Carrying Out the Invention

[0018] Hereinafter, the sulfide solid electrolyte, the method for manufacturing the sulfide solid electrolyte, and the all-solid-state battery in the present disclosure will be described in detail.

[0019] A. Sulfide Solid Electrolyte The sulfide solid electrolyte in the present disclosure is a sulfide solid electrolyte used in an all-solid-state battery, and has a composition represented by (100-x)[yLi2S·(1-y)P2S5]·xLiBH4 (where x is a number satisfying 50 < x < 75, and y is a number satisfying 0.72 ≦ y ≦ 0.78), and the ionic conductivity at 25°C is 5.0 mS / cm or more.

[0020] In the present disclosure, since it exhibits an ionic conductivity of 5.0 mS / cm or more within a predetermined composition range, it becomes a sulfide solid electrolyte with good ionic conductivity.

[0021] The sulfide solid electrolyte disclosed in Patent Document 1 does not use LiS, and has a maximum ionic conductivity of 2.0 mS / cm (measured at 27°C). The sulfide solid electrolyte disclosed in Non-Patent Document 1 has a maximum ionic conductivity of 1.8 mS / cm (measured at 25°C). In contrast, the sulfide solid electrolyte of the present disclosure exhibits an ionic conductivity of 5.0 mS / cm or more in a given composition, and has significantly better ionic conductivity than the sulfide solid electrolytes in the above-mentioned documents.

[0022] The sulfide solid electrolyte of the present disclosure has a composition represented by (100-x)[yLi2S·(1-y)P2S5]·xLiBH4. In the above composition, x is a number greater than 50, and may be a number greater than or equal to 52, a number greater than or equal to 55, or a number greater than or equal to 60. On the other hand, x is a number less than 75, and may be a number less than or equal to 70, a number less than or equal to 67, a number less than or equal to 65, or a number less than or equal to 63. In the above composition, y is a number greater than or equal to 0.72, and may be a number greater than or equal to 0.73. On the other hand, y is a number less than or equal to 0.78, and may be a number less than or equal to 0.75. In particular, y is preferably a number that allows the Li3PS4 structure to be primarily obtained in the first mechanical milling step described below. In particular, it is preferable that y is 0.75, i.e., the ratio of Li2S to P2S5 is 75:25.

[0023] The sulfide solid electrolyte according to the present disclosure has an ionic conductivity of 5.0 mS / cm or more at 25° C. The ionic conductivity may be 5.3 mS / cm or more, 5.5 mS / cm or more, 6.0 mS / cm or more, 8.0 mS / cm or more, or 10.0 mS / cm or more.

[0024] The sulfide solid electrolyte in the present disclosure may be sulfide glass, crystallized sulfide glass (glass ceramics), or a crystalline material obtained by subjecting a mixture to a solid-state reaction treatment.

[0025] The sulfide glass can be obtained by amorphizing a mixture containing Li2S, P2S5, and LiBH4. Examples of amorphizing processes include mechanical milling, which will be described later.

[0026] Crystallized sulfide glass can be obtained, for example, by heat treating sulfide glass at a temperature equal to or higher than the crystallization temperature.

[0027] Furthermore, the sulfide solid electrolyte of the present disclosure preferably has one peak in the range of 2θ=13° or more and 16° or less in X-ray diffraction measurement using CuKα radiation. "Having one peak in the range of 2θ=13° or more and 16° or less" means that one peak top is observed in the range of 2θ=13° or more and 16° or less. When a shoulder peak is observed, the shoulder peak itself is regarded as one peak.

[0028] The sulfide solid electrolyte of the present disclosure is used in an all-solid-state battery, which will be described later.

[0029] B. Method for producing sulfide solid electrolyte Fig. 1 is a flow diagram illustrating a method for producing a sulfide solid electrolyte according to the present disclosure. As shown in Fig. 1, the method for producing a sulfide solid electrolyte according to the present disclosure is the same as the method for producing a sulfide solid electrolyte described above, and includes at least a first mechanical milling step and a second mechanical milling step.

[0030] 1. First mechanical milling process The first mechanical milling step in the present disclosure is a step of mechanically milling a mixture containing Li2S and P2S5 to obtain a first sulfide glass.

[0031] The ratio of Li2S and P2S5 in the mixture is not particularly limited as long as the above-mentioned sulfide solid electrolyte is obtained. That is, the ratio (molar ratio) of Li2S to P2S5 is y:1-y (0.72≦y≦0.78) in the above-mentioned composition formula.

[0032] Furthermore, in the first mechanical milling step, mechanical milling is performed so that the gravity Gn1 calculated from the following formula (1) is 6 G or more. The following formula (1) converts the force applied in mechanical milling into gravity. Note that Gn1 can also be regarded as gravitational acceleration. By performing mechanical milling so that the gravity-equivalent force is 6 G or more, a first sulfide glass in which a Li3PS4 skeleton is well formed can be obtained. By using such a first sulfide glass, a sulfide solid electrolyte with good ionic conductivity can be obtained.

[0033]

number

[0034] Gn1 calculated from formula (1) may be 8 G or more, 10 G or more, 15 G or more, or 18 G or more. On the other hand, Gn1 may be, for example, 25 G or less, 22 G or less, or 20 G or less.

[0035] The time for mechanical milling in the first mechanical milling step is not particularly limited, and is, for example, 5 hours or more and 24 hours or less.

[0036] The type of mechanical milling may be wet mechanical milling or dry mechanical milling, and examples of mechanical milling include ball mills such as planetary ball mills.

[0037] 2. Second mechanical milling process The second mechanical milling step in the present disclosure is a step of adding LiBH4 to the first sulfide glass and performing mechanical milling to obtain a second sulfide glass.

[0038] The ratio of the first sulfide glass to LiBH4 is not particularly limited as long as the above-described sulfide solid electrolyte can be obtained. In other words, the ratio (molar ratio) of the first sulfide glass to LiBH4 is 100-x:x (50 <x<75)である。

[0039] Furthermore, in the second mechanical milling step, mechanical milling is performed so that the gravity Gn1 calculated from the above formula (1) is 6 G or more. Formula (1) and Gn1 are the same as above. In the first mechanical milling step and the second mechanical milling step, Gn1 may be the same or different.

[0040] The time and type of mechanical milling in the second mechanical milling step are the same as those in the first mechanical milling step described above.

[0041] 3.Heat treatment process The method for producing a sulfide solid electrolyte according to the present disclosure may include a heat treatment step of heat treating the sulfide glass, wherein the heat treatment temperature is 190° C. or lower.

[0042] It is believed that if the heat treatment temperature is too high, a different phase may be formed in the resulting sulfide solid electrolyte. On the other hand, it is believed that if the heat treatment temperature is set to 190°C or less, the formation of a different phase is suppressed, and a sulfide solid electrolyte with better ionic conductivity (a sulfide solid electrolyte with good crystallinity) can be obtained. This is believed to be because the hydride (LiBH4) used in the sulfide solid electrolyte of the present disclosure undergoes a phase transition at around 400K (127°C) and exhibits good ionic conductivity in the high-temperature phase of a hexagonal crystal structure.

[0043] The heat treatment temperature is, for example, 50° C. or higher, and may be 75° C. or higher, or 100° C. or higher. On the other hand, the heat treatment temperature is 190° C. or lower, and may be 160° C. or lower, or 150° C. or lower.

[0044] The heat treatment time is not particularly limited and can be adjusted appropriately depending on the heat treatment temperature, and is, for example, 30 minutes or more and 4 hours or less.

[0045] 4.Sulfide solid electrolyte The sulfide solid electrolyte produced by the above-described method is similar to the content described in "A. Sulfide solid electrolyte," and therefore will not be described here.

[0046] C. All-solid-state battery The sulfide solid electrolyte of the present disclosure is used in an all-solid-state battery. That is, the present disclosure can also provide an all-solid-state battery having a positive electrode active material layer, a negative electrode active material layer, and a solid electrolyte layer disposed between the positive electrode active material layer and the negative electrode active material layer, wherein at least one of the positive electrode active material layer, the negative electrode active material layer, and the solid electrolyte layer contains the sulfide solid electrolyte.

[0047] In the all-solid-state battery according to the present disclosure, at least one of the positive electrode active material layer, the negative electrode active material layer, and the solid electrolyte layer contains the sulfide solid electrolyte described above, and therefore the all-solid-state battery has a good discharge capacity retention rate.

[0048] Fig. 2 is a schematic cross-sectional view illustrating an example of an all-solid-state battery according to the present disclosure. The all-solid-state battery 10 shown in Fig. 2 includes a positive electrode active material layer 1, a negative electrode active material layer 2, and a solid electrolyte layer 3 disposed between the positive electrode active material layer 1 and the negative electrode active material layer 2. The all-solid-state battery 10 also includes a positive electrode current collector 4 that collects electrons from the positive electrode active material layer 1, and a negative electrode current collector 5 that collects electrons from the negative electrode active material layer 2. At least one of the positive electrode active material layer 1, the negative electrode active material layer 2, and the solid electrolyte layer 3 contains the sulfide solid electrolyte described above.

[0049] 1.Cathode active material layer The positive electrode active material layer contains at least a positive electrode active material. The positive electrode active material layer preferably contains at least one of a solid electrolyte, a conductive material, and a binder, as necessary. Examples of the positive electrode active material, the conductive material, and the binder include conventionally known materials. Furthermore, the positive electrode active material layer preferably contains the above-mentioned sulfide solid electrolyte as the solid electrolyte.

[0050] 2.Negative electrode active material layer The negative electrode active material layer contains at least a negative electrode active material. The negative electrode active material layer preferably contains at least one of a solid electrolyte, a conductive material, and a binder, as necessary. Examples of the negative electrode active material, the conductive material, and the binder include conventionally known materials. Furthermore, the negative electrode active material layer preferably contains the above-mentioned sulfide solid electrolyte as the solid electrolyte.

[0051] 3.Solid active material layer The solid electrolyte layer contains at least a solid electrolyte and may contain a binder as needed. The binder is as described above. The solid electrolyte is preferably the sulfide solid electrolyte described above.

[0052] 4. Positive and negative electrode current collectors The material of the positive electrode current collector and the negative electrode current collector can be a conventionally known metal material such as Al, SUS, Cu, and Ni.

[0053] 5.All-solid-state battery The all-solid-state battery in the present disclosure is typically a lithium-ion secondary battery. Applications of the all-solid-state battery include, for example, power sources for vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), electric vehicles (BEVs), gasoline-powered vehicles, and diesel-powered vehicles. It is particularly preferred that the all-solid-state battery be used as a driving power source for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), or electric vehicles (BEVs). The all-solid-state battery may also be used as a power source for mobile objects other than vehicles (for example, railways, ships, and aircraft), and may also be used as a power source for electrical appliances such as information processing devices.

[0054] The present disclosure is not limited to the above-described embodiments. The above-described embodiments are merely examples, and any configuration that is substantially identical to the technical idea described in the claims of the present disclosure and that provides similar effects is included within the technical scope of the present disclosure. [Example]

[0055] [Example 1-1] Li2S (Mitsuwa Chemical) and P2S5 (Merck) were prepared as raw materials in a molar ratio of 75:25. These were dry-milled (ball diameter 5 mm, container 80 mL, rotation speed 510 rpm, operating time 10 hours) to obtain the first sulfide glass (0.75Li2S 0.25P2S5) (first mechanical milling step). LiBH4 (Aldrich) was added to the first sulfide glass so that the molar ratio of the first sulfide glass to LiBH4 was 48:52, and this was dry-milled (ball diameter 5 mm, container 80 mL, rotation speed 510 rpm, operating time 15 hours) (second mechanical milling step). This resulted in a sulfide solid electrolyte (second sulfide glass) with a composition of 48(0.75Li2S 0.25P2S5) 52LiBH4. The above composition corresponds to the formula (100-x)[yLi2S·(1-y)P2S5]·xLiBH4 where x = 52 and y = 0.75. The mechanical milling equipment used was a Fritsch Premium Line PL-7 planetary ball mill.

[0056] Here, in the mechanical milling device, the revolution radius rs was 0.07 m, the container radius rp1 was 0.0240 m, and the ratio iw of rotation to revolution was -2.0. The gravity Gn1 calculated by substituting these values ​​and the rotation speed into the formula (1) was 18 G for both the first mechanical milling process and the second mechanical milling process.

[0057] [Examples 1-2 to 1-5 and Comparative Examples 1-1 to 1-7] A sulfide solid electrolyte was prepared in the same manner as in Example 1-1, except that the ratios of Li2S, P2S5, and LiBH4 were changed so that the composition of the sulfide solid electrolyte would be the values shown in Table 1 below.

[0058] [Evaluation 1] (Measurement of ionic conductivity) Each of the sulfide solid electrolytes of Examples 1-1 to 1-5 and Comparative Examples 1-1 to 1-7 was weighed in the range of 0.08 g to 0.1 g, pressed at a pressure of 6 t, and pelletized. Then, impedance measurement was performed at 25°C. Using the obtained resistance value R (Ω), the thickness L (cm) of the pellet, and the bottom area A (cm , ,

[0062] ), the ionic conductivity (σ) was calculated from the following formula. The results are shown in Figure 3 and Table 1. ​​​​​​​​​​​​​​​​​​​​​​As shown in Figure 4(a), when x was too low, no peak was observed in the range of 2θ = 13° or more and 16° or less. Also, as shown in Figure 4(b), in Example 1-3, one peak was observed in the range of 2θ = 13° or more and 16° or less, but in Comparative Example 1-7, two peaks, including a shoulder, were observed. Although not shown, two peaks were also observed in other Comparative Examples, similar to Comparative Example 1-7.

[0063] (Measurement of discharge capacity retention rate) Using the sulfide solid electrolytes of Examples 1-3 and Comparative Examples 1-4, evaluation batteries were fabricated as follows. First, the sulfide solid electrolyte was micronized by mechanical milling in a mixed solvent of heptane and dibutyl ether. Next, it was dried at 100°C for 2 hours. Then, it was mixed with spherical graphite in a volume ratio of 35:65 to prepare a negative electrode composite. The negative electrode composite, separator layer, and counter electrode (Li-In foil) were laminated in this order. In this way, evaluation batteries (negative electrode half cells) were fabricated. Note that the sulfide solid electrolyte (second sulfide glass) after the second mechanical milling process was used for the separator layer.

[0064] Each of the obtained evaluation batteries was subjected to three cycles of CCCV charging and discharging at 25°C and 0.1C. In the CCCV charging and discharging, the cutoff voltage was 0.05V (Li + / Li) and the cutoff current was set to 0.01C. Then, the charge rate was changed from 0.1 to 0.5C, and the discharge rate was fixed at 0.1C, and CC charge / discharge was performed. The cutoff voltage in CC charge / discharge was 0.05V (Li + The discharge capacity retention rate (%) was calculated by dividing the CC charge / discharge capacity by the initial capacity. The results are shown in Figure 5.

[0065] 5, as the charge rate increased, the charge / discharge capacity in Example 1-3 became larger than that in Comparative Example 1-4. In particular, when the charge rate was 0.5 C, Example 1-3 had a capacity retention rate 1.2 times higher than that of Comparative Example 1-4.

[0066] [Example 2-1] A sulfide solid electrolyte was prepared in the same manner as in Example 1-1, except that the proportions of Li2S, P2S5, and LiBH4 were changed so that the composition of the sulfide solid electrolyte was 46(0.75Li2S 0.25P2S5) 54LiBH4. Note that this composition corresponds to the composition where x = 54 and y = 0.75 in (100-x)[yLi2S (1-y)P2S5] xLiBH4.

[0067] [Comparative Example 2-1] A sulfide solid electrolyte was produced in the same manner as in Example 2-1, except that in the first mechanical milling step, the ball diameter was set to 4 mm, the rotation speed was set to 280 rpm, and the gravity Gn1 calculated from Equation (1) was changed to 5.4 G, and the operation time was changed to 45 hours.

[0068] [Rating 2] (Measurement of ionic conductivity) The ionic conductivity of the sulfide solid electrolytes of Example 2-1 and Comparative Example 2-1 was measured in the same manner as in Evaluation 1. The results are shown in Table 2.

[0069] [Table 2]

[0070] As shown in Table 2, it was confirmed that the ionic conductivity was improved by performing mechanical milling in the first and second mechanical milling steps so that Gn1 was 6 G or more. This is presumably due to the better formation of the Li3PS4 framework.

[0071] [Example 3-1] A sulfide solid electrolyte (sulfide glass) having a composition of 42(0.75Li2S 0.25P2S5) 58LiBH4 was obtained in the same manner as in Examples 1-3. The obtained sulfide glass was heated at a temperature of 60°C for 2 hours. The sulfide solid electrolyte after heating was used as an evaluation sample.

[0072] [Examples 3-2 to 3-4] Except for changing the heat treatment temperature as shown in Table 3, sulfide solid electrolytes (evaluation samples) were obtained in the same manner as in Example 3-1.

[0073] [Rating 3] (Measurement of ionic conductivity) The ionic conductivity of the sulfide solid electrolytes of Examples 3-1 to 3-4 was measured in the same manner as in Evaluation 1. The results are shown in Table 3 and Fig. 6 together with the results of Example 1-3. Note that although it can be considered that no heat treatment was performed in Example 1-3, for convenience, the heat treatment temperature is described as room temperature (25°C).

[0074] [Table 3]

[0075] As shown in FIG. 6 and Table 3, it was confirmed that good ionic conductivity was obtained by heat treatment at 190°C or less, and that ionic conductivity was further improved by heat treatment at 160°C or less. Furthermore, the highest ionic conductivity was obtained when heat treatment was performed at 100°C. The hydride (LiBH4) used in the sulfide solid electrolyte of the present disclosure undergoes a phase transition near 400 K (127°C). It is known that LiBH4 improves its ionic conductivity by changing from a low-temperature phase, orthorhombic structure, to a high-temperature phase, hexagonal structure. Therefore, it is presumed that the sulfide solid electrolyte using LiBH4 also achieved the highest ionic conductivity when heat-treated at 100°C, which is near the phase transition temperature. It is also presumed that the sulfide solid electrolyte using LiBH4 was successfully converted into a glass-ceramic by heat treatment at around 100°C, resulting in good ionic conductivity. [Explanation of symbols]

[0076] 1...Cathode active material layer 2...Negative electrode active material layer 3...Solid electrolyte layer 4...Positive electrode current collector 5...Negative electrode current collector 10...All-solid-state battery

Claims

1. A sulfide solid electrolyte for use in an all-solid-state battery, (100-x)[yLi ２ S·(1−y)P ２ S ５ ]・xLiBH ４ (wherein x is a number satisfying 50<x<75, and y is a number satisfying 0.72≦y≦0.78), The ionic conductivity at 25°C is 5.0 mS / cm or more, In X-ray diffraction measurement using CuKα radiation, A sulfide solid electrolyte having one peak in the range of 2θ = 13° or more and 16° or less.

2. The sulfide solid electrolyte according to claim 1, wherein the ionic conductivity is 8.0 mS / cm or more.

3. An all-solid-state battery having a positive electrode active material layer, a negative electrode active material layer, and a solid electrolyte layer disposed between the positive electrode active material layer and the negative electrode active material layer, 3. An all-solid-state battery, wherein at least one of the positive electrode active material layer, the negative electrode active material layer, and the solid electrolyte layer contains the sulfide solid electrolyte according to claim 1 or 2.

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