Sulfide-based solid electrolyte and method for producing the same

Abstract

The present invention relates to a sulfide solid electrolyte for use in a lithium ion secondary battery, which has an argyrodite type crystal structure represented by LiaPSbHac (5 ≤ a ≤ 7, 4 ≤ b ≤ 6, 0< c ≤ 2, Ha is a halogen element) which has peaks A and B each having a full width at half maximum of 0.07° or more in the range of 2θ = 30.3 ± 0.5° in an X-ray diffraction spectrum, the difference between the diffraction angles (2θ) thereof being 0.05° or more.

Description

Technical Field

[0001] The present invention relates to a sulfide-based solid electrolyte used in a lithium-ion secondary battery and a method for manufacturing the same.

Background Art

[0002] Lithium-ion secondary batteries are widely used in portable electronic devices such as mobile phones and notebook computers. Conventionally, liquid electrolytes have been used in lithium-ion secondary batteries, but there are concerns about liquid leakage and ignition, and it has been necessary to increase the size of the case for safety design. In addition, improvement has been desired for the short battery life and narrow operating temperature range.

[0003] On the other hand, all-solid-state lithium-ion secondary batteries using a solid electrolyte as the electrolyte of the lithium-ion secondary battery have attracted attention because they can be expected to improve safety, enable high-speed charge and discharge, and reduce the size of the case.

[0004] Solid electrolytes are roughly classified into sulfide-based solid electrolytes and oxide-based solid electrolytes. Sulfide ions constituting the sulfide-based solid electrolyte have a larger polarizability than oxide ions constituting the oxide-based solid electrolyte and exhibit high ionic conductivity. As the sulfide-based solid electrolyte, Li * , X , 7-x , , 12 ,

[0005] , * , * , 6-X , , 11 , 10 GeP2S 12 Crystals of the LGPS type such as etc., crystals of the argyrodite type such as Li6PS5Cl, and LPS crystallized glass such as Li7P3S 11 Crystallized glass are known. <00,00199>

[0005] As an example in which an argyrodite-type sulfide-based solid electrolyte is disclosed, Patent Document 1 can be cited. The sulfide-based solid electrolyte disclosed in Patent Document 1 has a crystal structure belonging to the cubic space group F-43m and a composition formula: Li 7-x PS 6-X Ha X (Ha is Cl or Br)(x = 0.2 to 1.8), and contains a compound represented by and L * a * b *The lightness L value of the color system is 60.0 or higher. This is intended to improve charge / discharge efficiency and cycle characteristics by increasing lithium-ion conductivity and decreasing electronic conductivity. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2015 / 012042 [Overview of the project] [Problems that the invention aims to solve]

[0007] Sulfide-based solid electrolytes using argyrodite crystals can achieve high lithium-ion conductivity because they contain halogen elements. On the other hand, halides are highly corrosive, and if the amount of halogen elements in sulfide-based solid electrolytes is too high, there are concerns about corrosion of the metals that make up lithium-ion secondary batteries.

[0008] Therefore, the present invention aims to provide a sulfide-based solid electrolyte for use in lithium-ion secondary batteries, and a method for producing the same, which exhibits high lithium-ion conductivity even when the elemental ratio of halogen elements in an argyrodite-type crystal is kept below a certain level. [Means for solving the problem]

[0009] As a result of diligent research, the inventors of the present invention have found that the above problems can be solved by using a sulfide-based solid electrolyte having an argyrodite-type crystal structure with a specific peak in the X-ray diffraction spectrum, and have completed the present invention.

[0010] In other words, the present invention relates to the following [1] to [7]. [1] A sulfide-based solid electrolyte used in a lithium-ion secondary battery, having, in an X-ray diffraction spectrum using Cu-Kα rays, peak A and peak B with a full width at half maximum of 0.07° or more within the range of 2θ = 30.3 ± 0.5°, and the difference in diffraction angle (2θ) between peak A and peak B is 0.05° or more, Li a PS b Ha c (where 5 ≤ a ≤ 7, 4 ≤ b ≤ 6 and 0 < c ≤ 2, and Ha is a halogen element), and having an alditol-type crystal structure, a sulfide-based solid electrolyte. [2] The sulfide-based solid electrolyte according to [1], further having peak C between peak A and peak B. [3] The sulfide-based solid electrolyte according to [1] or [2], wherein peak A and peak B are peaks derived from two alditol-type crystal structures having lattice constants different by 0.02 Å or more. [4] In an X-ray diffraction spectrum using Cu-Kα rays after heat treatment at a temperature of 400 °C or higher and lower than the thermal decomposition temperature for 1 hour, at least one of the following phenomena is observed: a decrease in the peak intensity ratio of peak A, an increase in the peak intensity ratio of peak B, and the appearance of peak C or an increase in the peak intensity ratio of peak C, and peak C is present on the higher angle side than peak A and on the lower angle side than peak B. The sulfide-based solid electrolyte according to any one of [1] to [3]. [5] A sulfide-based solid electrolyte used in a lithium-ion secondary battery, having, in an X-ray diffraction spectrum using Cu-Kα rays, peak D and peak E with a full width at half maximum of 0.07° or more within the range of 2θ = 30.3 ± 0.5°, and the difference in diffraction angle (2θ) between peak D and peak E is 0.02 - 0.4°, Li a PS b Ha c (where 5 ≤ a ≤ 7, 4 ≤ b ≤ 6 and 0 < c ≤ 2, and Ha is a halogen element), and having an alditol-type crystal structure, and the X-ray diffraction spectrum does not change even after heat treatment at a temperature of 400 °C or higher and lower than the thermal decomposition temperature for 1 hour, a sulfide-based solid electrolyte. [6] A method for producing a sulfide-based solid electrolyte used in a lithium-ion secondary battery, comprising mixing and heat-melting raw materials containing Li, P, S, and Ha, and then crystallizing by rapid cooling under normal pressure, wherein the Ha is a halogen element, and Li a PS b Ha c (5 ≤ a ≤ 7, 4 ≤ b ≤ 6, and 0 < c ≤ 2, and Ha is a halogen element.) A method for producing a sulfide-based solid electrolyte having two or more different alditol-type crystal structures represented by [7] After the crystallization, a heat treatment is performed at 200 to 600 °C for 0.1 to 10 hours. The method for producing a sulfide-based solid electrolyte according to [6] above.

Advantages of the Invention

[0011] According to the sulfide-based solid electrolyte of the present invention, even if the element ratio of the halogen element in the alditol-type crystal is below a certain level, it is possible to achieve high lithium ion conductivity. Therefore, it is very useful as a solid electrolyte for lithium-ion secondary batteries, and thereby, an improvement in the battery characteristics of lithium-ion secondary batteries can be expected.

Brief Description of the Drawings

[0012] [Figure 1] FIG. 1 is a part of the XRD spectrum of the solid electrolyte of Example 1. [Figure 2] FIG. 2 is a part of the XRD spectrum of the solid electrolyte of Example 2.

Modes for Carrying Out the Invention

[0013] Hereinafter, the present invention will be described in detail, but the present invention is not limited to the following embodiments, and can be arbitrarily modified and implemented without departing from the gist of the present invention. Also, "~" indicating a numerical range is used to mean including the numerical values described before and after it as the lower limit value and the upper limit value.

[0014] <Sulfide-based solid electrolyte> The sulfide-based solid electrolyte according to this embodiment (hereinafter sometimes simply referred to as "solid electrolyte") is used in a lithium-ion secondary battery and has an argyrodite-type crystal structure. The argyrodite-type crystal structure, when represented by Li a PS b Ha c satisfies the relationship of 5 ≦ a ≦ 7, 4 ≦ b ≦ 6 and 0 < c ≦ 2 for each element ratio. Also, Ha represents a halogen element.

[0015] The solid electrolyte according to this embodiment has peak A and peak B within the range of 2θ = 30.3 ± 0.5° in an X-ray diffraction (XRD) spectrum using Cu-Kα rays. For convenience, the peak on the low-angle side is defined as peak A, and the peak on the high-angle side is defined as peak B. The difference in diffraction angle (2θ) between peak A and peak B is 0.05° or more. Also, the full width at half maximum of the peaks of peak A and peak B is 0.07° or more, respectively.

[0016] Li a PS b Ha c (5 ≦ a ≦ 7, 4 ≦ b ≦ 6 and 0 < c ≦ 2) The argyrodite-type crystal structure represented by contains Li, P, S, and Ha. Ha is a halogen element and is at least one halogen element selected from the group consisting of F, Cl, Br, and I.

[0017] In the XRD spectrum using Cu-Kα rays, the solid electrolyte according to this embodiment has peak A and peak B in order from the low-angle side within the range of 2θ = 30.3 ± 0.5°. The difference in diffraction angle (2θ) between these peaks is 0.05° or more. This means that the solid electrolyte has two or more argyrodite-type crystal structures with different compositions.

[0018] A solid electrolyte having two or more argyrodite-type crystal structures is different from a mixture in which two or more powders having argyrodite-type crystal structures with different compositions are mixed. This can be distinguished by the full width at half maximum of peak A and peak B. Specifically, for the solid electrolyte according to this embodiment, in the XRD spectrum, the full width at half maximum (FWHM) of peak A and peak B is 0.07° or more. On the other hand, in a mixture obtained by mixing two or more kinds of powders having different compositions and an argyrodite-type crystal structure, the FWHM of the peaks in the XRD spectrum is less than 0.07°. That is, the solid electrolyte according to this embodiment is not a mixture coexisting on the order of mm or μm, but a solid electrolyte in which two or more kinds of argyrodite-type crystals coexist on the order of several hundred nm or several tens of nm. The FWHM of the peaks will be described later.

[0019] The coexistence of two or more kinds of argyrodite-type crystals on the order of several hundred nm or several tens of nm enables Li a PS b Ha c Even when the elemental ratio of the halogen element represented by c in the formula (5 ≦ a ≦ 7, 4 ≦ b ≦ 6 and 0 < c ≦ 2) is a low value of 2 or less, a high lithium ion conductivity can be realized. The reason for this is not clear, but when the solid electrolyte is manufactured through a melting process and then cooled to obtain an argyrodite-type crystal, the solid electrolyte according to this embodiment is realized. In the cooling process from the molten state, the first precipitated seed crystal is presumed to be a high-temperature stable phase. Therefore, the argyrodite-type crystal that grows with this seed crystal as a nucleus has different lithium ion, sulfur anion, and halogen anion existence sites from those of crystals produced by ordinary solid-state reactions, and it is considered that this affects the lithium ion conductivity. Generally, high-temperature stable phases tend to have high lithium ion conductivity, but in the argyrodite-type crystal in this embodiment, multiple-composition argyrodite-type crystals containing a high-temperature stable phase precipitate in the cooling process from the molten state. Therefore, it is considered that a high lithium ion conductivity can be realized.

[0020] The coexistence of two or more kinds of argyrodite-type crystals on the order of several hundred nm or several tens of nm instead of on the order of mm or μm can also be confirmed from the XRD spectra before and after heat treatment of the obtained solid electrolyte in addition to the value of the full width at half maximum of the peaks. Specifically, if, after heat treatment at a temperature of 400°C or higher but below the thermal decomposition temperature of the solid electrolyte for 1 hour, at least one of the following phenomena is observed: a decrease in the peak intensity ratio of peak A, an increase in the peak intensity ratio of peak B, and the appearance of peak C or an increase in the peak intensity ratio of peak C, then it can be said that two or more types of argyrodite crystals coexist on the order of several hundred nanometers or tens of nanometers. Here, peak C is a peak that is located at a higher angle than peak A and at a lower angle than peak B. The phenomenon of a decrease in the peak intensity ratio of peak A includes the phenomenon of peak A disappearing. Furthermore, regarding peak C, the phenomenon of an increase in the peak intensity ratio due to heat treatment presupposes that peak C was observed before the heat treatment. The phenomenon of peak C newly appearing due to heat treatment presupposes that peak C was not observed before the heat treatment. On the other hand, in the case of powders having argyrodite-type crystal structures with different compositions, i.e., mixtures on the order of mm or μm, the above-mentioned changes are not observed in the XRD pattern even when the same heat treatment is performed.

[0021] As an example, Figure 1 shows the XRD spectrum of Example 1 (described later) around 2θ = 30.3°, and Figure 2 shows the XRD spectrum of Example 2 (described later) around 2θ = 30.3°. Example 2 is a solid electrolyte obtained by heat-treating the solid electrolyte obtained in Example 1 at 500°C for 1 hour. Furthermore, the XRD spectra shown in Figures 1 and 2 represent the diffraction pattern of the Cu-Kα2 line, which is the source of the Cu-Kα line, after the Cu-Kα2 line diffraction pattern has been removed. Details of the removal of the Cu-Kα2 line diffraction pattern, along with the peak separation method, will be described later.

[0022] In Figure 1, the raw data (shown by the solid line) of the XRD spectrum before heat treatment clearly shows peaks at 2θ = 29.99°, corresponding to peak A, and at 2θ = 30.24°, corresponding to peak B. However, when the peaks are actually separated through analysis, in addition to peak A (shown by the dashed line) and peak B (shown by the dashed line), there is another peak at 30.11° near 2θ = {(peak A + peak B) / 2}, shown by the dotted line. This corresponds to peak C. Thus, through analysis, the spectrum can be separated into three peaks: peak A, peak B, and peak C. The peak separation method will be described later. Furthermore, a peak is also observed at 2θ = 30.66° in Example 1. While the precise assignment of this peak is difficult, it is thought to be a peak in a cubic argyrodite crystal with reduced symmetry, or in a cubic argyrodite crystal with a lower lattice constant due to a deficiency of lithium, etc. In addition to peaks A, B, and C, the solid electrolyte according to this embodiment may also have such a peak.

[0023] In contrast, in the XRD spectrum after heat treatment at 500°C for 1 hour shown in Figure 2, the raw data, indicated by the solid line, shows a sharp peak around 2θ = 30.11°. However, when peak separation is performed, it can be seen that in addition to peak C, indicated by the dotted line, a peak corresponding to peak B, indicated by the dashed line, is also observed at 2θ = 30.20°. Furthermore, the peak corresponding to peak A, indicated by the dashed line in Figure 1, either disappears or its peak intensity ratio decreases to the point where it cannot be detected.

[0024] As shown in Examples 1 and 2 in Figures 1 and 2, the solid electrolyte according to this embodiment undergoes a phenomenon in which the peak intensity ratio of peak C, which is located between peak A and peak B, increases after heat treatment. Furthermore, if peak C is not present before heat treatment, a phenomenon in which peak C newly appears may occur after heat treatment.

[0025] Furthermore, simultaneously with or in lieu of the above phenomenon, heat treatment may cause phenomena such as a decrease in the peak intensity ratio of peak A or an increase in the peak intensity ratio of peak B. In other words, it is preferable from the viewpoint of improving lithium-ion conductivity that the heat treatment results in at least one of the following phenomena: a decrease in the peak intensity ratio of peak A, an increase in the peak intensity ratio of peak B, and the appearance of peak C or an increase in the peak intensity ratio of peak C. It is even more preferable that the peak intensity ratio of at least one of peak B and peak C increases, and it is even more preferable that the peak intensity ratio of at least one of peak B and peak C increases along with a decrease in the peak intensity ratio of peak A. It is also preferable that the peak intensity ratio of peak C decreases and the peak intensity ratio of peak B increases.

[0026] On the other hand, in the case of a mixture of powders having argyrodite-type crystal structures with different compositions, no such changes are observed in the XRD pattern even after heat treatment, and the differences in diffraction angles and relative peak intensity ratios remain unchanged.

[0027] Figures 1 and 2 will be explained in more detail. Before heat treatment, the composition of the argyrodite-type crystal structure shown by the peak at 2θ = 29.99° corresponding to peak A is Li a1 PS b1 Ha c1 The composition of the argyrodite-type crystal structure indicated by the peak at 2θ = 30.24°, corresponding to peak B, is Li a2 PS b2 Ha c2 The composition of the argyrodite-type crystal structure indicated by the peak at 2θ = 30.11°, corresponding to peak C, is Li a3 PS b3 Ha c3 Let's assume that...

[0028] From the relationship of the peak intensity ratios described above, the content ratio of each component in the solid electrolyte before heat treatment, which is Example 1, is Li a2 PS b2 Ha c2 >Li a1 PS b1Ha c1 >Li a3 PS b3 Ha c3 The relationship is as follows. In contrast, in Example 2 after heat treatment, the above content ratio is Li a3 PS b3 Ha c3 >Li a2 PS b2 Ha c2 >>Li a1 PS b1 Ha c1 This is the relationship.

[0029] In other words, in Example 2, by performing heat treatment, the relative abundance of the three types of argyrodite crystal structures in Example 1 before heat treatment changes, Li a1 PS b1 Ha c1 and Li a2 PS b2 Ha c2 The composition between, a3 PS b3 Ha c3 The amount of argyrodite-type crystal structure increased, resulting in a solid electrolyte.

[0030] Thus, the appearance of peak C or an increase in its peak intensity ratio due to heat treatment means that the composition of at least one of peaks A and B has changed to the composition of peak C. In particular, a change in the composition of peak A to become the composition of peak C is preferable from the viewpoint of improving lithium ion conductivity. Furthermore, from the viewpoint of improving lithium ion conductivity, it is also preferable that the composition of at least one of peaks A and C changes to become the composition of peak B due to heat treatment.

[0031] Peaks A and B with a difference in diffraction angle (2θ) of 0.05° or more mean that they have two different compositions of the argyrodite-type crystal structure. Focusing on the peak positions at 2θ = 30.3 ± 0.5° where peaks A and B are observed, for argyrodite-type crystals, especially cubic argyrodite-type crystals, it is preferable from the viewpoint of improving lithium ion conductivity that the lattice constants of two crystals with a difference in diffraction angle (2θ) of 0.05° or more differ by 0.02 Å or more. The difference in lattice constants is more preferably 0.03 Å or more, and even more preferably 0.05 Å or more. Also, from the viewpoint of crystal stability, the difference in lattice constants is preferably 0.2 Å or less, more preferably 0.15 Å or less, and even more preferably 0.1 Å or less.

[0032] The argyrodite-type crystal indicated by the peak B on the high-angle side has the composition Li a PS b Ha c (5 ≤ a ≤ 7, 4 ≤ b ≤ 6 and 0 < c ≤ 2), c indicating the element ratio of Ha is preferably 0.3 or more, more preferably 1 or more, and even more preferably 1.5 or more from the viewpoint of high lithium ion conductivity. Also, from the viewpoint of suppressing metal current collector corrosion, c is 2 or less, preferably 1.85 or less, and more preferably 1.7 or less.

[0033] The argyrodite-type crystal indicated by the peak A on the low-angle side has the composition Li a PS b Ha c (5 ≤ a ≤ 7, 4 ≤ b ≤ 6 and 0 < c ≤ 2), c indicating the element ratio of Ha is more than 0, preferably 0.5 or more, and more preferably 1 or more from the viewpoint of high lithium ion conductivity. Also, from the viewpoint of suppressing metal current collector corrosion, c is preferably 1.7 or less, more preferably 1.6 or less, and even more preferably 1.5 or less.

[0034] The argyrodite-type crystal Li a1 PS b1 Ha c1 and the argyrodite-type crystal Li a2 PS b2 Ha c2 The ratio of (Li a1 PSb1 Ha c1 :Li a2 PS b2 Ha c2 ) is preferably in the range of 1:99 to 95:5, more preferably 1:99 to 50:50, and even more preferably 2:98 to 10:90 from the viewpoint of high lithium ion conductivity.

[0035] The difference in diffraction angle (2θ) between peak A and peak B may be 0.05° or more, preferably 0.06° or more, and more preferably 0.07° or more from the viewpoint of improving lithium ion conductivity. Also, from the viewpoint of crystal stability, the difference in diffraction angle (2θ) is preferably 0.8° or less, more preferably 0.6° or less, and even more preferably 0.4° or less. In addition, in peak analysis, there may be a case where the result shows that the difference in diffraction angle (2θ) between peak A and peak B is less than 0.05°. In this case, these peaks shall be regarded as one peak.

[0036] From the viewpoint of improving lithium ion conductivity, it is preferable that the solid electrolyte according to this embodiment further has peak C between peak A and peak B. The aldite-type crystal shown by peak C has a composition of Li a PS b Ha c (5 ≤ a ≤ 7, 4 ≤ b ≤ 6 and 0 < c ≤ 2), and c indicating the element ratio of Ha is preferably 0.5 or more, more preferably 1 or more, and even more preferably 1.5 or more from the viewpoint of high lithium ion conductivity. Also, from the viewpoint of suppressing corrosion of the metal current collector, c is preferably 1.9 or less, more preferably 1.8 or less, and even more preferably 1.7 or less.

[0037] From the viewpoint of improving lithium ion conductivity, it is preferable that the proportions of Li a2 PS b2 Ha c2 and Li a3 PS b3 Ha c3 are large, and Li a1 PS b1 Ha c1 :(Li a2 PS b2 Ha c2 +Lia3 PS b3 Ha c3 The ratio represented by ) is preferably 1:99 to 95:5, more preferably 1:99 to 50:50, and even more preferably 2:98 to 10:90.

[0038] The XRD spectrum of the solid electrolyte according to this embodiment may have peaks at positions other than 2θ = 30.3 ± 0.5°. The peaks observed at other positions may also be separable into two peaks or more than two peaks when peak separation is performed, similar to the peaks within the range of 2θ = 30.3 ± 0.5°.

[0039] The preferred crystal structure of the al-djrite type crystal is, for example, a cubic crystal such as F-43m, but there may also be rhombohedral crystals, tetragonal crystals, orthorhombic crystals, etc. with reduced symmetry, and even monoclinic crystals with further reduced symmetry. When the crystal structure of the al-djrite type is cubic and the lattice constants are a = b = c, it is considered that the structural change due to heat treatment occurs isotropically. Therefore, even at other peaks observed in the range other than 2θ = 30.3 ± 0.5°, by performing heat treatment at a temperature of 400 °C or higher and lower than the thermal decomposition temperature for 1 hour, phenomena such as an increase or decrease in the intensity ratio of the peaks representing each crystal structure or the appearance of new peaks may be observed. Also, even when the crystal structure of the al-djrite type is not cubic, some structural change corresponding to the crystal structure may occur due to the above heat treatment.

[0040] Also, the solid electrolyte after heat treatment as described below is also included in one embodiment of the present invention. Such an embodiment is Li a PS b Ha c(It has an argyrodite-type crystal structure represented by (5 ≦ a ≦ 7, 4 ≦ b ≦ 6 and 0 < c ≦ 2), and is a sulfide-based solid electrolyte used in a lithium-ion secondary battery. This solid electrolyte has peaks D and E within the range of 2θ = 30.3 ± 0.5° in an X-ray diffraction spectrum using Cu-Kα rays. The full width at half maximum of these peaks is both 0.07° or more, and the difference in diffraction angle (2θ) between these two peaks is 0.02 to 0.4°. Also, even when heat treatment is performed at a temperature of 400°C or higher and lower than the thermal decomposition temperature for 1 hour, no change is seen in the X-ray diffraction spectrum.)

[0041] Peaks D and E with a difference in diffraction angle (2θ) of 0.02 to 0.4° respectively correspond to peak A or peak B before heat treatment and peak C, and are two different types of argyrodite-type crystals.) When peak D is on the low-angle side and peak E is on the high-angle side, peak D and peak E respectively correspond to peak A and peak C before heat treatment, or peak C and peak B before treatment.)

[0042] When peaks D and E respectively correspond to peaks A and C, it may further have a high-angle peak F corresponding to peak B. Also, when peaks D and E respectively correspond to peaks C and B, it may further have a low-angle peak G corresponding to peak A. At this time, the full width at half maximum of peaks F and G is preferably 0.07° or more. Also, the difference in diffraction angle (2θ) between peak F and peak E is preferably 0.05 to 0.4°, and the difference in diffraction angle (2θ) between peak G and peak D is preferably 0.05 to 0.4°.)

[0043] The difference in diffraction angle (2θ) between peaks D and E only needs to be 0.02 to 0.4°, but from the viewpoint of improving lithium-ion conductivity, 0.05° or more is preferable, 0.07° or more is more preferable, and 0.09° or more is even more preferable. Also, from the viewpoint of crystal stability, the difference in diffraction angle (2θ) is preferably 0.3° or less, and 0.2° or less is more preferable. Also, the preferable ranges of the difference in diffraction angle (2θ) between peak F and peak E and the difference in diffraction angle (2θ) between peak G and peak D are the same as above.)

[0044] The solid electrolyte according to this embodiment, which includes the crystal structures of each al-dilodite type indicated by peak D and peak E, can adjust the crystal ratio, the difference in diffraction angle, etc. according to the heat treatment conditions. Whether the solid electrolyte has been heat-treated or not can be determined not only by the fact that there is no change in the XRD spectrum when additional heat treatment is performed, but also by the degree of change in lithium ion conductivity, etc. The change in the XRD spectrum means that the values of the diffraction angles (2θ) of peak D and peak E do not change, or even if they change, the difference in diffraction angle before and after heat treatment is 0.05° or less. Also, it is preferable that the change in the peak intensity ratio before and after heat treatment is also 0.1 or less. The degree of change in lithium ion conductivity before and after heat treatment is preferably 0.5 mS / cm or less. This additional heat treatment is performed for 1 hour in the temperature range of 400°C or higher and the thermal decomposition temperature or lower.

[0045] Li indicating the composition of the al-dilodite type crystal a PS b Ha c (5 ≤ a ≤ 7, 4 ≤ b ≤ 6 and 0 < c ≤ 2), the halogen element represented by Ha is at least one selected from the group consisting of F, Cl, Br, and I. Since the crystal is likely to be of the al-dilodite type, it is preferable to contain at least one of Cl and Br, more preferably to contain Cl, and even more preferably to be Cl alone or a mixture of Cl and Br.

[0046] The crystallite size of the al-dilodite type crystal is preferably small from the viewpoint of obtaining good lithium ion conductivity when the solid electrolyte is finely pulverized to manufacture a lithium ion battery. Specifically, it is preferably 1000 nm or less, more preferably 500 nm or less, and even more preferably 250 nm or less. The lower limit of the crystallite size is not particularly limited, but is usually 5 nm or more. The crystallite size after heat treatment becomes larger than before heat treatment, but even after heat treatment, it is preferable that the crystallite size is within the above range. Crystallite size can be calculated from the full width at half maximum of the peaks in the XRD pattern.

[0047] The secondary particle size of the argyrodite-type crystal is preferably small from the viewpoint of obtaining good lithium-ion conductivity when manufacturing lithium-ion batteries by finely grinding the solid electrolyte. Specifically, it is preferably 10 μm or less, more preferably 3 μm or less, and even more preferably 1 μm or less. There is no particular lower limit to the secondary particle size, but it is usually 0.1 μm or more. Secondary particle size can be measured using a microtrac device.

[0048] The full width at half maximum (FMAX) of each peak should be 0.07° or greater, preferably 0.075° or greater, more preferably 0.08° or greater, and even more preferably 0.09° or greater, as smaller crystallite sizes are preferable. There is no specific upper limit for the FMAX, but it is usually obtained within 0.5°. If the FMAX is greater than this, it is better to increase the number of peaks and re-analyze. The analysis method for determining the FMAX will be described later.

[0049] The argyrodite-type crystal structure in this embodiment may contain oxide anions. For example, from the viewpoint of improving the heat resistance of the crystal and ensuring that it remains stable without decomposition even after heat treatment at high temperatures, it is preferable to include oxide anions with a Q0 structure in which a metal atom (M) and an oxygen atom (O) are bonded. Here, the Q0 structure refers to a structure in which all oxygen atoms bonded to the central cation M are non-bridged oxygen atoms. For example, if M is Si, the oxide SiO2 becomes a silicate ion, i.e., SiO4. 4- This means that it exists as an oxide anion.

[0050] The elements constituting the oxide anion may include M and O, where M is at least one element selected from the group consisting of metallic and metalloid elements in groups 2-14 of the periodic table. Furthermore, the oxide anion may contain only one element or multiple elements. Metallic elements of groups 2-14 of the periodic table are the elements of groups 2-12, the elements of group 13 other than B, and the elements of group 14 other than C, Si, and Ge. The metalloid elements of groups 2-14 are B, Si, and Ge, which are elements in groups 13 and 14 of the periodic table. The presence of MO bonds and the Q0 structure in oxide anions can be confirmed by Raman spectroscopy and nuclear magnetic resonance (NMR) measurements. Furthermore, the presence of Q0 structured oxide anions in the crystal structure, i.e., at the anion sites of the crystal, can be confirmed by X-ray powder diffraction (XRD) and neutron scattering measurements.

[0051] With respect to all components constituting the solid electrolyte according to this embodiment, the total content of two or more elements constituting an argyrodite-type crystal structure is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 90% by mass or more, even more preferably 92% by mass or more, and particularly preferably 94% by mass or more, from the viewpoint of achieving high lithium ion conductivity. Furthermore, there is no particular upper limit to the total content, and it may be 100% by mass. In addition, the solid electrolyte according to this embodiment may also include other crystals or amorphous materials other than argyrodite-type crystals. The total content mentioned above refers to, for example, the sum of Li, P, S, and Ha. If oxide anions are present in the crystal, it refers to the sum of the content of the elements Li, P, S, Ha, M, and O. In this specification, the Ha content refers to the sum of the content of F, Cl, Br, and I. The content of each element and their total amounts can be determined by compositional analysis using methods such as ICP emission spectrometry, atomic absorption spectrometry, and ion chromatography.

[0052] Other materials that may be included in solid electrolytes include Li3PS4, Li4P2S6, Li2S, and LiHa (where Ha is at least one halogen element selected from F, Cl, Br, and I).

[0053] As an index of heat resistance, the thermal decomposition test temperature of the solid electrolyte is preferably 400 °C or higher, more preferably 450 °C or higher, further preferably 500 °C or higher, still more preferably 600 °C or higher, and particularly preferably 650 °C or higher. The upper limit is not particularly limited, but is usually 900 °C or lower. However, in the case of an alluaudite-type crystal with a high proportion of halogen elements, the thermal decomposition temperature tends to be low. Therefore, considering the balance with the properties desired for the solid electrolyte, the composition of the alluaudite-type crystal contained in the solid electrolyte according to the present embodiment is selected. The thermal decomposition test of the solid electrolyte can be evaluated by putting the solid electrolyte into a sealed container that does not react with the solid electrolyte, performing heat treatment at a predetermined temperature for 10 to 60 minutes, and examining the change in lithium ion conductivity before and after the heat treatment. The smaller the change in lithium ion conductivity before and after the heat treatment, the more preferable it is. If the lithium ion conductivity after the heat treatment is less than half of the lithium ion conductivity before the heat treatment, it can be said that the thermal decomposition resistance, that is, the heat resistance, is low. In this specification, the lithium ion conductivity means the lithium ion conductivity at 25 °C, and is obtained from a Nyquist plot obtained by alternating current impedance measurement.

[0054] <Method for producing sulfide-based solid electrolyte> The method for producing a sulfide-based solid electrolyte according to the present embodiment is Li a PS b Ha c (5 ≦ a ≦ 7, 4 ≦ b ≦ 6 and 0 < c ≦ 2), and is not particularly limited as long as two or more different alluaudite-type crystal structures coexisting on the order of several hundred nanometers or several tens of nanometers can be obtained. As one embodiment, a production method including a step of mixing and heat-melting raw materials containing Li, P, S, and Ha, and then a step of crystallizing by rapid cooling is preferable. By rapid cooling under normal pressure, a solid electrolyte in which two or more alluaudite-type crystals coexist on the order of several hundred nanometers or several tens of nanometers can be obtained.

[0055] The raw materials containing Li, P, S, and Ha are those conventionally known as materials for obtaining argyrodite-type crystals containing Li, P, S, and Ha. Specifically, Li element or Li-containing compounds, P element or P-containing compounds, S element or S-containing compounds, and Ha-containing compounds can be used in appropriate combinations. These compounds may also contain two or more of Li, P, S, and Ha. For example, phosphorus pentasulfide (P2S5) is an example of a compound that contains both S and P. Lithium halides are an example of a compound that contains both Li and Ha.

[0056] Examples of lithium-containing compounds include lithium sulfide (Li2S), lithium carbonate (Li2CO3), lithium sulfate (Li2SO4), lithium oxide (Li2O), and lithium hydroxide (LiOH). From the viewpoint of ease of handling, lithium sulfide is preferred. On the other hand, since lithium sulfide is expensive, it is preferable to use lithium compounds other than lithium sulfide, or metallic lithium, from the viewpoint of reducing manufacturing costs. Specifically, it is preferable to use one or more selected from the group consisting of metallic lithium, lithium carbonate (Li2CO3), lithium sulfate (Li2SO4), lithium oxide (Li2O), and lithium hydroxide (LiOH). These may be used individually or in combination of two or more.

[0057] Examples of sulfur-containing compounds include phosphorus sulfides such as phosphorus trisulfide (P2S3) and phosphorus pentasulfide (P2S5), other sulfur compounds containing phosphorus, elemental sulfur, and sulfur-containing compounds. Examples of sulfur-containing compounds include H2S, CS2, and iron sulfide (FeS, Fe2S3, FeS2, Fe 1-x S, etc.), bismuth sulfide (Bi2S3), copper sulfide (CuS, Cu2S, Cu 1-xExamples include sulfur (S). Among these, phosphorus sulfide is preferred, and diphosphorus pentasulfide (P2S5) is more preferred, from the viewpoint of preventing the inclusion of elements other than those constituting the target sulfide-based solid electrolyte. These may be used individually or in combination of two or more. Note that phosphorus sulfide is a compound that contains both sulfur and phosphorus.

[0058] Examples of phosphorus-containing compounds include phosphorus sulfides such as phosphorus trisulfide (P2S3) and phosphorus pentasulfide (P2S5), and phosphorus compounds such as sodium phosphate (Na3PO4). Among these, phosphorus sulfide is preferred, and phosphorus pentasulfide (P2S5) is more preferred, from the viewpoint of preventing the inclusion of elements other than those constituting the target sulfide-based solid electrolyte. These may be used individually or in combination of two or more.

[0059] Examples of compounds containing halide include lithium halides such as lithium fluoride (LiF), lithium chloride (LiCl), lithium bromide (LiBr), and lithium iodide (LiI), as well as phosphorus halides, phosphoryl halides, sulfur halides, sodium halides, and boron halides. Among these, lithium halides are preferred, and LiCl, LiBr, and LiI are more preferred, from the viewpoint of preventing the inclusion of elements other than those constituting the target sulfide-based solid electrolyte. These compounds may be used individually or in combination of two or more.

[0060] The raw materials can be mixed by methods such as mortar and pestle, media-based mixing such as a planetary ball mill, or media-less mixing such as a pin mill, powder agitator, or airflow mixing. The raw materials may be amorphous by mixing before heating.

[0061] The specific method for heating and melting the mixture of raw materials is not particularly limited, but one example is to place the raw materials in a heat-resistant container and heat it in a heating furnace. The heat-resistant container is not particularly limited, but examples include heat-resistant containers made of carbon, heat-resistant containers containing oxides such as quartz, quartz glass, borosilicate glass, aluminosilicate glass, alumina, zirconia, and mullite, heat-resistant containers containing nitrides such as silicon nitride and boron nitride, and heat-resistant containers containing carbides such as silicon carbide. Furthermore, these heat-resistant containers may be formed as a bulk from the above materials, or they may be containers in which layers of carbon, oxides, nitrides, carbides, etc., are formed.

[0062] When heating and melting the mixture of raw materials, the heating temperature is preferably 600°C or higher, more preferably 630°C or higher, and even more preferably 650°C or higher, from the viewpoint of increasing the fluidity of the melt. Furthermore, from the viewpoint of suppressing deterioration and decomposition of components in the melt due to heating, the heating temperature is preferably 900°C or lower, more preferably 850°C or lower, and even more preferably 800°C or lower.

[0063] From the viewpoint of allowing the reaction to proceed, the heating and melting time is preferably 0.1 hours or more, more preferably 0.5 hours or more, even more preferably 0.7 hours or more, and even more preferably 1 hour or more. Furthermore, from the viewpoint of suppressing deterioration and decomposition of components in the molten liquid due to heating, the heating and melting time is preferably 10 hours or less, more preferably 9.5 hours or less, and even more preferably 9 hours or less.

[0064] Furthermore, while the pressure during heating and melting is not particularly limited, it is preferably atmospheric pressure to slightly pressurized, and more preferably atmospheric pressure.

[0065] To prevent side reactions with water vapor, oxygen, etc., during heating and melting, the dew point is preferably -20°C or lower. While there is no particular lower limit, it is usually around -80°C. Furthermore, the oxygen concentration during heating and melting is preferably 1000 ppm or lower.

[0066] Crystallization is carried out by rapidly cooling a mixture of heated and molten raw materials under atmospheric pressure. This yields a solid electrolyte in which two or more argyrodite-type crystals, each on the order of several hundred nanometers or tens of nanometers, coexist.

[0067] Rapid cooling means that the cooling rate is 1°C / second or more, preferably 10°C / second or more, and more preferably 100°C / second or more. Furthermore, there is no particular upper limit to the cooling rate, but considering the cooling rate of twin rollers, which are generally said to have the fastest rapid cooling rate, the upper limit is 1,000,000°C / second or less.

[0068] When referring to rapid cooling under atmospheric pressure, it means that the pressure is not controlled during the cooling process. Specifically, this is around 0.8 to 1.2 atm.

[0069] Stabilization treatment may be performed by rapidly cooling under atmospheric pressure followed by further heat treatment. Heat treatment increases crystallinity. Depending on the heat treatment conditions, new argyrodite crystals with compositions between those of the multiple argyrodite crystals before heat treatment can also be obtained.

[0070] The duration of the heat treatment, which is a stabilization process, is preferably 0.1 hours or more, and more preferably 0.2 hours or more, from the viewpoint of ensuring more reliable crystal precipitation. Furthermore, from the viewpoint of obtaining new argyrodite-type crystals, the heat treatment duration is preferably 0.5 hours or more, and more preferably 1 hour or more. On the other hand, from the viewpoint of suppressing thermal degradation due to heating, the heat treatment time is preferably 10 hours or less, and more preferably 5 hours or less. Furthermore, from the viewpoint of preventing excessive crystallization and a decrease in lithium ion conductivity, the heat treatment time is preferably 3 hours or less, and more preferably 2 hours or less.

[0071] The temperature of the heat treatment, which is a stabilization process, is preferably above the glass transition temperature of the solid electrolyte, specifically 200°C or higher, and more preferably 250°C or higher. Furthermore, from the viewpoint of obtaining new argyrodite-type crystals, the heat treatment temperature is preferably 350°C or higher, and more preferably 400°C or higher. On the other hand, from the viewpoint of preventing thermal degradation and thermal decomposition, the heat treatment temperature is preferably below the thermal decomposition temperature, for example, preferably 600°C or lower, and more preferably 575°C or lower. Furthermore, from the viewpoint of preventing excessive crystallization and a decrease in lithium ion conductivity, the heat treatment temperature is preferably 550°C or lower, and more preferably 530°C or lower.

[0072] When the solid electrolyte according to this embodiment contains an oxide anion with a Q0 structure having an MO bond, the timing of adding the oxide having the MO bond is not particularly limited. For example, the oxide may be mixed with the raw materials and then heated and melted. Alternatively, the oxide may be mixed with crystals obtained by rapid cooling under normal pressure and then subjected to heat treatment.

[0073] The obtained solid electrolyte, when used in lithium-ion secondary batteries, contains other components such as binders as needed to form a solid electrolyte layer. Conventionally known binders and other components are used. The content of the solid electrolyte according to this embodiment relative to the entire solid electrolyte layer is preferably 80% by mass or more, and more preferably 90% by mass or more. There is no particular upper limit to the content of the solid electrolyte, and it may be 100% by mass. Furthermore, from the viewpoint of suppressing deformation, inorganic fillers and organic fillers may be mixed, in which case the content of the solid electrolyte is preferably 99% by mass or less.

[0074] Conventional methods can also be used to form the solid electrolyte layer. For example, the components constituting the solid electrolyte layer can be dispersed or dissolved in a solvent to form a slurry, which can then be coated in layers (sheets), dried, and optionally pressed to form the solid electrolyte layer. If necessary, heat may be applied to remove the binder. The thickness of the solid electrolyte layer can be easily adjusted by adjusting the amount of slurry applied. Alternatively, instead of wet molding, the solid electrolyte layer may be formed by dry press molding of the solid electrolyte powder, etc., according to this embodiment, on the surface of the positive or negative electrode, etc. Alternatively, the solid electrolyte layer may be formed on another substrate and then transferred to the surface of the positive or negative electrode, etc.

[0075] The solid electrolyte according to this embodiment may be mixed with a positive electrode active material or a negative electrode active material and used as a positive electrode layer or a negative electrode layer. Conventionally known materials can be used for the positive electrode active material or negative electrode active material, current collector, binder, conductive additive, etc. used in the positive electrode layer or negative electrode layer.

[0076] The lithium-ion secondary battery using the solid electrolyte according to this embodiment includes the solid electrolyte layer, a positive electrode layer, and a negative electrode layer. The casing material for the lithium-ion secondary battery can also be one of conventionally known materials. The shape of the lithium-ion secondary battery can also be one of conventionally known shapes, such as coin-shaped, sheet-shaped (film-shaped), foldable, wound-type bottomed cylindrical, button-shaped, etc., which can be appropriately selected depending on the application. [Examples]

[0077] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. Examples 1, 2, and 4-9 are examples, while Example 3 is a comparative example.

[0078] [evaluation] (Lithium-ion conductivity) The lithium-ion conductivity was measured using an AC impedance measuring device (Bio-Logic Sciences Instruments, potentiostat / galvanostat VSP) with the obtained sulfide-based solid electrolyte powder compacted into a powder sample at a pressure of 380 kN. The measurement conditions were: measurement frequency: 100Hz to 1MHz, measurement voltage: 100mV, and measurement temperature: 25℃.

[0079] (Powder X-ray diffraction) The XRD spectra of sulfide-based solid electrolytes were measured using an X-ray diffractometer (Rigaku Corporation, SmartLab). Since sulfide-based solid electrolytes used as measurement samples deteriorate when exposed to air, the samples were prepared and measured in an atmosphere that was not exposed to air. Sample preparation: The sulfide-based solid electrolyte powder, ground in a mortar, is passed through a sieve with a mesh size of 100 μm to obtain the 50% particle size D.50 A sulfide-based solid electrolyte powder with a particle size distribution of 5-10 μm was prepared. The particle size distribution was measured using a Microtrac MT3300EXII laser diffraction particle size distribution analyzer, and the 50% particle size D was determined from the obtained volume-based particle size distribution chart. 50 We measured it. The measurement conditions for powder X-ray diffraction are as follows: Source: CuKα radiation (λ=1.5418Å), Tube voltage: 45kV, Tube current: 200mA, Scanning angle: 10~100°, Scanning speed: 5° / min, Step count: 0.01° / step.

[0080] The analysis method for peak separation and the method for determining the full width at half maximum from the raw XRD spectrum data obtained are as follows. Baseline removal and Cu-Kα2 radiation removal were performed using the integrated powder X-ray analysis software PDXL2, which is included with the X-ray diffractometer (SmartLab, manufactured by Rigaku Corporation). Next, focusing on the peak that appears at 2θ = 30.3 ± 0.5°, we defined a pseudo-Voigt function for peak fitting as shown in equation (1).

[0081]

number

[0082] In equation (1), A is the peak coefficient, B is the height correction coefficient, ω is the full width at half maximum, and a is the hybridization ratio. Equation (2) represents a Lorentz function, and equation (3) represents a Gaussian function. x0 is the peak center value and is expressed as a value of 2θ. Generally, when an XRD spectrum is represented on the horizontal axis as a value of 2θ, the spectrum becomes asymmetrical. Therefore, the hybridization ratio in equation (1) is changed before and after the peak value (=x0) to represent and analyze the spectrum. The cause of this asymmetry is mainly due to the device and the penetration depth of X-rays into the sample. Therefore, in this analysis, the mixing ratio obtained from the analysis of Example 3 prepared by the conventional solid-phase method was adopted. Specifically, 1.00 was adopted when x0≧x, and 0.36 was adopted when x0<x. When the peak of single-crystal silicon was analyzed using this mixing ratio, it was in good agreement.

[0083] To evaluate the consistency of the analysis of the fitting function, an index called Rwp was used. This is an index used for the Rietveld analysis of the XRD spectrum. For the peak appearing within the range of 2θ = 30.3 ± 0.5°, the lower the value of Rwp obtained when applying one or more fitting functions defined above is more preferable. Specifically, the value of Rwp is preferably 15% or less, more preferably 12.5% or less, and even more preferably 10% or less. Note that the value of Rwp is a positive value.

[0084] [Example 1] Under a dry nitrogen atmosphere, lithium sulfide powder (manufactured by Sigma, purity 99.98%), diphosphorus pentasulfide powder (manufactured by Sigma, purity 99%), and lithium chloride powder (manufactured by Sigma, purity 99.99%) were weighed so as to have a composition ratio of Li6PS5Cl and mixed in a mortar. The obtained mixture was put into a heat-resistant container and heated and melted at 730°C for 0.5 hours in an atmosphere with a dew point of -60°C. Then, it was cooled to room temperature at a cooling rate of 10°C / second to obtain a solid that became a sulfide-based solid electrolyte. Among the XRD spectra of the obtained solid electrolyte, the spectrum around 2θ = 30.3° is shown in FIG. 1. When peak separation was performed on the untreated spectrum shown by the solid line, three peaks were observed within the range of 2θ = 30.3 ± 0.5°. The details of this peak are shown in Table 1.

[0085] As a result of the Rietveld analysis of the XRD spectrum, the diffraction peak at 2θ = 29.99° corresponding to peak A of the obtained solid electrolyte is Li 6.9 PS 5.9 Cl 0.1The diffraction peak at 2θ = 30.11°, corresponding to peak C, is derived from the argyrodite crystal of the composition, and is Li 5.9 PS 4.9 Cl 1.1 The diffraction peak at 2θ = 30.24°, corresponding to peak B, is derived from the argyrodite crystal of the composition, and is Li 5.5 PS 4.5 Cl 1.5 It was determined that the peak originated from the argyrodite crystal of the composition. The molar ratio of these three types of argyrodite crystals is approximately Li 6.9 PS 5.9 Cl 0.1 :Li 5.9 PS 4.9 Cl 1.1 :Li 5.5 PS 4.5 Cl 1.5 The ratio was 2:1:3, and the lattice constants of these three argyrodite crystals were 9.904 Å, 9.851 Å, and 9.804 Å, respectively.

[0086] In Table 1, "basic composition" refers to the target composition when mixing the raw materials. In Example 1, this is Li6PS5Cl, but the actual composition of the argyrodite-type crystal obtained is as determined from peaks A, B, and C above, Li 6.9 PS 5.9 Cl 0.1 Li 5.5 PS 4.5 Cl 1.5 , and Li 5.9 PS 4.9 Cl 1.1 These elements coexist and are different from the basic composition. In Table 1, a "-" next to Peak A or Peak B indicates that the peak disappeared or that the peak intensity ratio was too small to detect. The peak intensity ratio is a relative ratio expressed with the sum of the peak intensities of Peaks A, B, and C set to 1.

[0087] [Example 2] A sulfide-based solid electrolyte was obtained by further heat treatment of the solid obtained in Example 1 at 500°C for 1 hour under a nitrogen atmosphere. The XRD spectrum of the obtained solid electrolyte around 2θ = 30.3° is shown in Figure 2. Peak separation was performed on the untreated spectrum, shown by the solid line, and two peaks were observed within the range of 2θ = 30.3 ± 0.5°. Details of the peaks are shown in Table 1. The Rietveld analysis of the XRD spectrum revealed that the obtained solid electrolyte was Li 5.9 PS 4.9 Cl 1.1 :Li 5.5 PS 4.5 Cl 1.5 It was found to possess two types of argyrodite crystals with a molar ratio of 5:1.

[0088] [Example 3] The same mixture as in Example 1 was mixed using a planetary ball mill at 400 rpm for 4 hours. The resulting mixture was vacuum-sealed in a carbon-coated quartz tube and heated at 550°C for 5 hours to obtain a solid sulfide-based solid electrolyte. In the XRD spectrum of the obtained solid electrolyte, one peak was observed within the range of 2θ = 30.3 ± 0.5°. Details of the peak are shown in Table 1. Rietveld analysis of the XRD spectrum revealed that the obtained solid electrolyte contains a type of argyrodite crystal of Li6PS5Cl.

[0089] [Example 4] Li 5.4 PS 4.4 Cl 1.6A solid sulfide-based solid electrolyte was obtained in the same manner as in Example 1, except that a mixture of lithium sulfide powder (Sigma, 99.98% purity), phosphorus pentasulfide powder (Sigma, 99% purity), and lithium chloride powder (Sigma, 99.99% purity) was weighed to achieve the composition ratio specified. In the XRD spectrum of the obtained solid electrolyte, three peaks were observed within the range of 2θ = 30.3 ± 0.5°, with 2θ = 30.15°, 30.29°, and 30.33°, respectively. Of these, the two peaks at 2θ = 30.29° and 2θ = 30.33°, which have a diffraction angle difference of less than 0.05°, have nearly similar compositions, and Li 5.4 PS 4.4 Cl 1.6 It is presumed to have two types of argyrodite crystals with a composition close to that of Li. The argyrodite crystal with a peak at 2θ = 30.15° is Li 5.7 PS 4.7 Cl 1.7 It is presumed to have a composition similar to that of [another substance].

[0090] [Example 5] A sulfide-based solid electrolyte was obtained by further heat treatment of the solid obtained in Example 4 at 450°C for 1 hour under a nitrogen atmosphere. In the XRD spectrum of the obtained solid electrolyte, three peaks were observed within the range of 2θ = 30.3 ± 0.5°. Compared to the peaks in Example 4, the peak intensity ratio on the high-angle side corresponding to peak B was higher. Compositional analysis showed no change in composition before and after heat treatment. Therefore, the obtained solid electrolyte is Li 5.4 PS 4.4 Cl 1.6 It can be said that it has the composition described above and possesses an argyrodite-type crystal with a lower lattice constant than before heat treatment.

[0091] [Example 6] Li 5.6 PS 4.4 Cl 1.5 Br 0.2A solid sulfide-based solid electrolyte was obtained in the same manner as in Example 1, except that a mixture of lithium sulfide powder (Sigma, 99.98% purity), phosphorus pentasulfide powder (Sigma, 99% purity), lithium chloride powder (Sigma, 99.99% purity), and lithium bromide powder (Sigma, 99.995% purity) was weighed to achieve the composition ratio specified. In the XRD spectrum of the obtained solid electrolyte, three peaks were observed within the range of 2θ = 30.3 ± 0.5°, with 2θ = 30.16°, 30.34°, and 30.41°, respectively. Of these, the peak corresponding to peak C at 2θ = 30.34° was Li 5.6 PS 4.4 Cl 1.5 Br 0.2 It is presumed that the crystals originate from an argyrodite type with a composition similar to that of the original crystal, and the two peaks corresponding to peak A and peak B are presumed to be from argyrodite type crystals with a slightly different composition or lattice constant.

[0092] [Example 7] A sulfide-based solid electrolyte was obtained by further heat treatment of the solid obtained in Example 6 at 450°C for 1 hour under a nitrogen atmosphere. In the XRD spectrum of the obtained solid electrolyte, two peaks were observed within the range of 2θ = 30.3 ± 0.5°. Compared to the peak in Example 6, the peak intensity ratio on the high-angle side corresponding to peak B was higher. Compositional analysis showed no change in composition before and after heat treatment. Therefore, the obtained solid electrolyte is Li 5.6 PS 4.4 Cl 1.5 Br 0.2 It can be said that it has the composition described above and possesses an argyrodite-type crystal with a lower lattice constant than before heat treatment.

[0093] [Example 8] Li 5.6 PS 4.4 Cl 0.8 Br 0.8A solid sulfide-based solid electrolyte was obtained in the same manner as in Example 1, except that a mixture of lithium sulfide powder (Sigma, 99.98% purity), phosphorus pentasulfide powder (Sigma, 99% purity), lithium chloride powder (Sigma, 99.99% purity), and lithium bromide powder (Sigma, 99.995% purity) was weighed to achieve the composition ratio specified. In the XRD spectrum of the obtained solid electrolyte, three peaks were observed within the range of 2θ = 30.3 ± 0.5°, with 2θ = 30.05°, 30.14°, and 30.33°, respectively. Of these, the peak at 2θ = 30.14°, corresponding to peak C, was Li 5.6 PS 4.4 Cl 0.8 Br 0.8 It is presumed that the crystals originate from an argyrodite type with a composition similar to that of the original crystal, and the two peaks corresponding to peaks A and B are presumed to be argyrodite crystals with a slightly different composition or lattice constant.

[0094] [Example 9] The solid obtained in Example 8 was further heat-treated at 450°C for 1 hour under a nitrogen atmosphere to obtain a sulfide-based solid electrolyte. In the XRD spectrum of the obtained solid electrolyte, two peaks were observed within the range of 2θ = 30.3 ± 0.5°. The peak at 2θ = 30.14°, which corresponds to peak C and can be considered the main peak due to its large peak intensity ratio, became sharper and its intensity ratio increased after the heat treatment. Therefore, Li, which has increased crystallinity, was obtained. 5.6 PS 4.4 Cl 0.8 Br 0.8 It is presumed to have argyrodite-type crystals with the following composition.

[0095] [Table 1]

[0096] In Examples 1, 2, and 4-9, the hybrid ratio during the fitting analysis, represented by parameter a in equation (1), was the same value as in Example 3. The analysis results all showed good values, with Rwp being 15% or less.

[0097] From the above results, the sulfide-based solid electrolyte of Example 3, obtained by the conventional solid-phase method, contained one type of argyrodite crystal and had a lithium ion conductivity of 1.2 mS / cm. On the other hand, the solid electrolytes of Examples 1 and 2, despite using the same types and amounts of raw materials as Example 3, contained two or three types of argyrodite crystals. Their lithium ion conductivity was 3.1 mS / cm and 2.7 mS / cm, respectively, which were significantly higher than that of Example 3.

[0098] Comparing Example 1 with Example 2, Example 4 with Example 5, Example 6 with Example 7, and Example 8 with Example 9, it was found that heat treatment sharpens the peaks in the XRD spectrum, causing the lattice constant to converge to the crystal showing the central peak, or to tend to lower the lattice constant. This is thought to increase the Coulomb interaction within the crystal, improving heat resistance, chemical stability, and electrochemical stability. Furthermore, although the lithium-ion conductivity decreases slightly after heat treatment compared to before treatment, it still shows a higher value than the solid electrolyte in Example 3, which contains only one type of argyrodite crystal.

[0099] The reason why the presence of two or more argyrodite crystals increases lithium ion conductivity is not entirely clear, but it is achieved in the case of argyrodite crystals obtained by rapidly cooling after a melting process during the production of solid electrolytes. Therefore, it is thought that the seed crystal that precipitates first during the cooling process from the molten state is a high-temperature stable phase, and the argyrodite crystals that grow around this seed crystal have different lithium ion, sulfur anion, and halogen anion sites than those produced by normal solid-phase reactions. Generally, high-temperature stable phases tend to have high ionic conductivity, and in the argyrodite crystals of this embodiment, it is believed that high lithium ion conductivity can be achieved because multiple compositions of argyrodite crystals containing high-temperature stable phases precipitate during cooling from the melt.

[0100] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. This application is based on Japanese Patent Application No. 2020-172693, filed on 13 October 2020, the contents of which are incorporated herein by reference.

Claims

1. A sulfide-based solid electrolyte used in lithium-ion secondary batteries, In the X-ray diffraction spectrum using Cu-Kα rays, peaks A and B are located within the range of 2θ = 30.3 ± 0.5°, with each peak having a full width at half maximum of 0.07° or more. The difference in diffraction angles (2θ) between peak A and peak B is 0.05° or more. The aforementioned peaks A and B are peaks originating from two argyrodite-type crystal structures with lattice constants differing by 0.02 Å or more. Li ａ PS ｂ Ha ｃ A sulfide-based solid electrolyte having two or more argyrodite-type crystal structures with different compositions, represented by (5 ≤ a ≤ 7, 4 ≤ b ≤ 6 and 0 < c ≤ 2, where Ha is a halogen element).

2. The sulfide-based solid electrolyte according to claim 1, further comprising a peak C between peak A and peak B.

3. The sulfide-based solid electrolyte according to claim 1 or 2, wherein, in an X-ray diffraction spectrum using Cu-Kα rays after heat treatment at a temperature of 400°C or higher but below the thermal decomposition temperature for 1 hour, at least one of the following phenomena is observed: a decrease in the peak intensity ratio of peak A, an increase in the peak intensity ratio of peak B, and the appearance of peak C or an increase in the peak intensity ratio of peak C, and peak C is located at a higher angle than peak A and at a lower angle than peak B.

4. A sulfide-based solid electrolyte used in lithium-ion secondary batteries, In the X-ray diffraction spectrum using Cu-Kα rays, peaks D and E are present within the range of 2θ = 30.3 ± 0.5°, with a full width at half maximum of 0.07° or more. The difference in diffraction angles (2θ) between peak D and peak E is 0.02 to 0.4°. Li ａ PS ｂ Ha ｃ It has two different argyrodite-type crystal structures corresponding to peak D and peak E, respectively, represented by (5 ≤ a ≤ 7, 4 ≤ b ≤ 6 and 0 < c ≤ 2, where Ha is a halogen element). A sulfide-based solid electrolyte in which the X-ray diffraction spectrum does not change even after heat treatment at a temperature above 400°C but below the thermal decomposition temperature for one hour.

5. A method for producing a sulfide-based solid electrolyte according to claim 1, 2, or 4, Mixing raw materials containing Li, P, S, and Ha and heating them to melt them. Next, crystallization occurs by rapid cooling under normal pressure. The aforementioned Ha is a halogen element, Li ａ PS ｂ Ha ｃ (where 5 ≤ a ≤ 7, 4 ≤ b ≤ 6 and 0 < c ≤ 2, and Ha is a halogen element). A method for producing a sulfide-based solid electrolyte having two or more different alditol-type crystal structures represented by

6. A method for producing a sulfide-based solid electrolyte according to claim 5, wherein, after the crystallization, a heat treatment is performed at 200 to 600°C for 0.1 to 10 hours.

Images