Sulfide-based solid electrolyte and method for producing the same

Abstract

The present invention relates to a sulfide-based solid electrolyte for use in lithium-ion secondary batteries, wherein in an X-ray diffraction pattern, within the range of 2θ=30.3±0.5°, there are peaks A and B with half-peak widths of 0.07° or more, and the difference in diffraction angles (2θ) between the peaks A and B is 0.05° or more, and the electrolyte has a Li a PS b Ha c The crystal structure of the argyrodite type is represented by (5≤a≤7, 4≤b≤6 and 0<c≤2, Ha is a halogen element).

Description

Technical Field

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

[0002] Lithium-ion secondary batteries are widely used in portable electronic devices such as mobile phones and laptops.

[0003] Until now, lithium-ion secondary batteries have used liquid electrolytes, but this has raised concerns about leakage and fire, requiring larger housings for safety reasons. Furthermore, improvements are needed to address issues such as short battery life and a narrow operating temperature range.

[0004] In view of this, all-solid-state lithium ion secondary batteries using a solid electrolyte as the electrolyte of lithium ion secondary batteries are attracting attention from the perspectives of improved safety, high-speed charging and discharging, and miniaturization of the housing.

[0005] Solid electrolytes are broadly classified into sulfide-based solid electrolytes and oxide-based solid electrolytes. The sulfide ions that constitute the sulfide-based solid electrolytes have a higher polarizability than the oxide ions that constitute the oxide-based solid electrolytes, and exhibit higher ion conductivity. As sulfide-based solid electrolytes, Li 10 GeP2S 12 LGPS type crystals, Li6PS5Cl and other argyrodite type crystals, Li7P3S 11 Crystallized glass, LPS crystallized glass, etc.

[0006] As an example of disclosing an argyrodite-type sulfide-based solid electrolyte, Patent Document 1 can be cited. The sulfide-based solid electrolyte disclosed in Patent Document 1 contains the following compounds, and L * a * b * The brightness L value of the color system is 60.0 or more, and the compound has a crystal structure belonging to the cubic system and the space group F-43m, and is composed of the formula: Li 7-x PS 6-X Ha X (Ha is Cl or Br) (x = 0.2 to 1.8) The purpose is to increase lithium ion conductivity and reduce electron conductivity to improve charge and discharge efficiency and cycle characteristics.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: International Publication No. 2015 / 012042 Summary of the Invention

[0010] Sulfide-based solid electrolytes using argyrodite-type crystals can achieve high lithium-ion conductivity due to their inclusion of halogen elements. However, halides are highly corrosive, and excessive halogen content in sulfide-based solid electrolytes can corrode metals and other components of lithium-ion secondary batteries.

[0011] Therefore, an object of the present invention is to provide a sulfide-based solid electrolyte for use in lithium-ion secondary batteries, and a method for producing the same, wherein the sulfide-based solid electrolyte exhibits high lithium ion conductivity even when the element ratio of halogen elements in argyrodite-type crystals is set to a certain value or less.

[0012] The present inventors conducted intensive studies and, as a result, found that the above-mentioned problems can be solved by producing a sulfide-based solid electrolyte having an argyrodite-type crystal structure with specific peaks in its X-ray diffraction pattern, thereby completing the present invention.

[0013] That is, the present invention relates to the following [1] to [7].

[0014] [1] A sulfide-based solid electrolyte for use in a lithium-ion secondary battery, wherein in an X-ray diffraction pattern using Cu-Kα radiation, a peak A and a peak B each having a half-peak width of 0.07° or greater are present within the range of 2θ=30.3±0.5°, a difference in diffraction angle (2θ) between the peak A and the peak B is 0.05° or greater, and the electrolyte has a Li a PS b Ha c The crystal structure of the argyrodite type is represented by (5≤a≤7, 4≤b≤6 and 0<c≤2, Ha is a halogen element).

[0015] [2] The sulfide-based solid electrolyte according to [1], further comprising a peak C between the peak A and the peak B.

[0016] [3] The sulfide-based solid electrolyte according to [1] or [2], wherein the peak A and the peak B are derived from the lattice constant difference. The above two peaks are of argyrodite-type crystal structures.

[0017] [4] The sulfide-based solid electrolyte according to any one of [1] to [3] above, wherein in an X-ray diffraction pattern using Cu-Kα radiation after heat treatment at a temperature of 400° C. or higher and below the thermal decomposition temperature for 1 hour, at least one of a decrease in the peak intensity ratio of the peak A, an increase in the peak intensity ratio of the peak B, and the appearance of peak C or an increase in the peak intensity ratio of peak C is observed, and the peak C is present on the high-angle side of the peak A and on the low-angle side of the peak B.

[0018] [5] A sulfide-based solid electrolyte, which is a sulfide-based solid electrolyte used in a lithium-ion secondary battery, wherein in an X-ray diffraction pattern using Cu-Kα radiation, there are peaks D and E with half-peak widths of 0.07° or more in the range of 2θ=30.3±0.5°, and the difference in diffraction angle (2θ) between the peak D and the peak E is 0.02 to 0.4°, and there is Li a PS b Ha c The argyrodite-type crystal structure represented by (5≤a≤7, 4≤b≤6 and 0<c≤2, Ha is a halogen element) will not change the above X-ray diffraction pattern even if it is heat treated at a temperature above 400°C and below the thermal decomposition temperature for 1 hour.

[0019] [6] A method for producing a sulfide-based solid electrolyte for use in lithium-ion secondary batteries, comprising: mixing raw materials containing Li, P, S, and Ha, heating and melting them, and then crystallizing them by rapid cooling under normal pressure; wherein the Ha is a halogen element, and the sulfide-based solid electrolyte has Li a PS b Ha c Two or more different argyrodite-type crystal structures represented by (5≤a≤7, 4≤b≤6 and 0<c≤2, Ha is a halogen element).

[0020] [7] The method for producing a sulfide-based solid electrolyte according to [6] above, wherein after the crystallization, a heat treatment is performed at 200 to 600° C. for 0.1 to 10 hours.

[0021] The sulfide-based solid electrolyte of the present invention achieves high lithium-ion conductivity even when the halogen element ratio in argyrodite-type crystals is below a certain level. Therefore, it is very useful as a solid electrolyte for lithium-ion secondary batteries, and is expected to improve the battery characteristics of lithium-ion secondary batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Part of the XRD pattern of the solid electrolyte of Example 1.

[0023] Figure 2 Part of the XRD pattern of the solid electrolyte of Example 2. DETAILED DESCRIPTION

[0024] Hereinafter, the present invention will be described in detail, but the present invention is not limited to the following embodiments and can be arbitrarily changed without departing from the scope of the present invention. In addition, when using the " to " representing a numerical range, it refers to the numerical value of the preceding and following records as the lower limit and the upper limit.

[0025] <Sulfide-based solid electrolyte>

[0026] The sulfide-based solid electrolyte (hereinafter also referred to as "solid electrolyte") according to this embodiment is used for lithium-ion secondary batteries and has an argyrodite-type crystal structure. a PS b Ha c When denoted as , the ratio of each element satisfies the relationship of 5≤a≤7, 4≤b≤6, and 0<c≤2. In addition, Ha represents a halogen element.

[0027] The solid electrolyte according to this embodiment has a Cu-Kα X-ray diffraction (XRD) pattern with peaks A and B within the range of 2θ = 30.3 ± 0.5°. For convenience, the peak on the lower angle side is defined as peak A, and the peak on the higher angle side is defined as peak B. The difference in diffraction angle (2θ) between peaks A and B is 0.05° or greater. Furthermore, the half-value widths of peaks A and B are each 0.07° or greater.

[0028] Li a PS b Ha c The argyrodite-type crystal structure represented by (5≤a≤7, 4≤b≤6, and 0<c≤2) contains Li, P, S, and Ha. Ha is a halogen element, which is at least one halogen element selected from F, Cl, Br, and I.

[0029] In an XRD pattern using Cu-Kα radiation, the solid electrolyte according to this embodiment has Peak A and Peak B, in the range of 2θ = 30.3 ± 0.5°, starting from the low-angle side. The difference in the diffraction angle (2θ) between these peaks is 0.05° or greater. This indicates that the solid electrolyte has two or more argyrodite-type crystal structures with different compositions.

[0030] A solid electrolyte having two or more argyrodite-type crystal structures is different from a mixture of two or more argyrodite-type crystal structure powders having different compositions. This can be distinguished by the half-value widths of Peak A and Peak B.

[0031] Specifically, the solid electrolyte according to this embodiment has a half-width (FWHM) of Peak A and Peak B of 0.07° or greater in its XRD pattern. On the other hand, in a mixture obtained by mixing two or more powders having an argyrodite-type crystal structure with different compositions, the FWHM of the peaks in the XRD pattern is less than 0.07°. In other words, the solid electrolyte according to this embodiment is not a mixture in which two or more argyrodite-type crystals coexist on the order of mm or μm, but rather a solid electrolyte in which two or more argyrodite-type crystals coexist on the order of hundreds of nanometers or tens of nanometers. The FWHM of the peaks will be discussed in detail below.

[0032] By making two or more argyrodite-type crystals coexist on the order of hundreds of nm or tens of nm, even if Li a PS b Ha c In the formula (5≤a≤7, 4≤b≤6, and 0<c≤2), the element ratio of the halogen element represented by c is a low value of 2 or less, and high lithium ion conductivity can still be achieved.

[0033] The reason for this is still uncertain, but the solid electrolyte involved in this embodiment can be achieved when, during the manufacture of a solid electrolyte, argyrodite-type crystals are obtained by cooling after undergoing a melting process. It can be speculated that the seed crystals that are initially precipitated during the cooling process from the molten state are high-temperature stable phases. Therefore, in the argyrodite-type crystals that grow from this seed crystal into a core, the existence sites of lithium ions, sulfur anions, and halogen anions are different from those of crystals produced by conventional solid-phase reactions, which is considered to affect the lithium ion conductivity. Generally speaking, high-temperature stable phases tend to have high lithium ion conductivity, but in the argyrodite-type crystals of this embodiment, argyrodite-type crystals of various compositions containing high-temperature stable phases are precipitated during the cooling process from the molten state. Therefore, it is considered that high lithium ion conductivity is achieved.

[0034] The coexistence of two or more argyrodite-type crystals on the order of hundreds of nm or tens of nm rather than mm or μm can be confirmed not only by the half-width value of the peak but also by the XRD patterns before and after heat treatment of the obtained solid electrolyte.

[0035] Specifically, if at least one of a decrease in the peak intensity ratio of Peak A, an increase in the peak intensity ratio of Peak B, and the appearance or increase in the peak intensity ratio of Peak C is observed after heat treatment for one hour at a temperature of 400°C or higher and below the thermal decomposition temperature of the solid electrolyte, it can be said that two or more argyrodite-type crystals coexist on the order of hundreds of nanometers or tens of nanometers. Here, Peak C refers to a peak that exists on the high-angle side of Peak A and the low-angle side of Peak B.

[0036] The phenomenon of a decrease in the peak intensity ratio of Peak A includes the phenomenon of disappearance of Peak A. Furthermore, the phenomenon of an increase in the peak intensity ratio of Peak C due to heat treatment is based on the premise that Peak C was observed before the heat treatment. The phenomenon of a new appearance of Peak C due to heat treatment is based on the premise that Peak C was not observed before the heat treatment.

[0037] On the other hand, in the case of powders having argyrodite-type crystal structures with different compositions, that is, a mixture of particles on the mm or μm order, no such changes are observed in the XRD pattern even after the same heat treatment.

[0038] As an example, Figure 1 The XRD pattern of Example 1 described later near 2θ=30.3° is shown in FIG. Figure 2 The XRD pattern of Example 2 described later is shown in the figure near 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. Figure 1 and Figure 2 The XRD pattern shown is a pattern obtained by eliminating the diffraction pattern caused by Cu-Kα2 radiation from the Cu-Kα radiation source. The details of eliminating the diffraction pattern caused by Cu-Kα2 radiation will be described later together with the peak separation method.

[0039] Figure 1 In the XRD pattern before heat treatment, according to the raw data (raw data) shown by the solid line, that is, the unprocessed spectrum, peaks can be clearly observed at 2θ = 29.99° corresponding to Peak A and 2θ = 30.24° corresponding to Peak B. However, if the peaks are actually separated by analysis, in addition to Peak A shown by the dotted line and Peak B shown by the dashed line, there is also a peak at 30.11° near 2θ = {(Peak A + Peak B) / 2} shown by the dotted line. This corresponds to Peak C. In this way, by performing analysis, it is possible to separate the peaks into three peaks: Peak A, Peak B, and Peak C. The method of peak separation is described in detail below.

[0040] In Example 1, a peak is also observed at 2θ = 30.66°. While the precise attribution of this peak is difficult, it is believed to be a peak resulting from a decrease in symmetry due to cubic argyrodite-type crystals, or a peak resulting from a decrease in the lattice constant due to the absence of lithium or the like. The solid electrolyte according to this embodiment may also have such peaks in addition to Peaks A, B, and C.

[0041] In contrast, in Figure 2 In the XRD pattern after heat treatment at 500°C for 1 hour, a sharp peak can be seen near 2θ=30.11° in the raw data shown by the solid line. However, if the peaks are separated, in addition to the peak C shown by the dotted line, a peak corresponding to peak B at 2θ=30.20° can be observed as shown by the dotted line. Figure 1 The peak corresponding to peak A shown by the dotted line disappears, or the peak intensity ratio decreases to such an extent that the peak cannot be detected.

[0042] Like the Figure 1 and Figure 2 As shown in Examples 1 and 2, the solid electrolyte according to this embodiment undergoes heat treatment, resulting in a phenomenon in which the peak intensity ratio of Peak C, which exists between Peak A and Peak B, increases. In addition, when Peak C does not exist before heat treatment, a phenomenon in which Peak C newly appears during heat treatment can occur.

[0043] Furthermore, simultaneously with or instead of the above phenomenon, a phenomenon in which the peak intensity ratio of Peak A decreases and the peak intensity ratio of Peak B increases may occur by performing heat treatment.

[0044] That is, it is preferred that at least one of 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 be observed by heat treatment from the perspective of improving lithium ion conductivity. Furthermore, it is more preferred that an increase in the peak intensity ratio of at least one of Peak B and Peak C be observed, and it is even more preferred that an increase in the peak intensity ratio of at least one of Peak B and Peak C be observed while the peak intensity ratio of Peak A is decreased. Furthermore, it is also preferred that a decrease in the peak intensity ratio of Peak C and an increase in the peak intensity ratio of Peak B be observed.

[0045] On the other hand, in the case of a mixture of powders having an argyrodite-type crystal structure with different compositions, even after heat treatment, no such changes are observed in the XRD pattern, and the difference in diffraction angle and the relative peak intensity ratio remain unchanged.

[0046] right Figure 1 and Figure 2 The composition of the argyrodite-type crystal structure represented by the peak at 2θ=29.99° corresponding to Peak A before heat treatment is assumed to be Li a1 PS b1 Ha c1 The composition of the argyrodite-type crystal structure represented by the peak at 2θ=30.24° corresponding to peak B is assumed to be Li a2 PS b2 Ha c2 The composition of the argyrodite-type crystal structure represented by the peak at 2θ=30.11° corresponding to peak C is assumed to be Li a3 PS b3 Ha c3 .

[0047] According to the relationship of the peak intensity ratio, the content ratio of each component in the solid electrolyte before the heat treatment of Example 1 is Li a2 PS b2 Ha c2 >Li a1 PS b1 Ha c1 >Li a3 PS b3 Ha c3 In contrast, in Example 2 after heat treatment, the above-mentioned content ratio is Li a3 PS b3 Ha c3 >Li a2 PS b2 Hac2 >>Li a1 PS b1 Ha c1 relationship.

[0048] That is, in Example 2, the presence ratio of the three argyrodite-type crystal structures in Example 1 before the heat treatment was changed by heat treatment, and the Li a1 PS b1 Ha c1 With Li a2 PS b2 Ha c2 The composition of Li a3 PS b3 Ha c3 The amount of the argyrodite-type crystal structure in the solid electrolyte is increased.

[0049] Thus, the appearance of Peak C or the increase in its peak intensity ratio by heat treatment means that the composition of at least one of Peak A and Peak B changes to the composition of Peak C. Of these, it is preferred that the composition of Peak A changes to the composition of Peak C from the perspective of improving lithium ion conductivity. In addition, from the perspective of improving lithium ion conductivity, it is also preferred that the composition of at least one of Peak A and Peak C changes to the composition of Peak B by heat treatment.

[0050] Peaks A and B having a diffraction angle (2θ) difference of 0.05° or more indicate two argyrodite-type crystal structures having different compositions. Focusing on the peak positions of 2θ = 30.3 ± 0.5° where Peaks A and B are observed, it is preferable that the lattice constants of the two crystals having a diffraction angle (2θ) difference of 0.05° or more differ from each other in terms of improving lithium ion conductivity, particularly for cubic argyrodite-type crystals. The difference in lattice constant is more preferably Above, more preferably In addition, from the perspective of crystal stability, the difference in lattice constant is preferably The following are more preferably The following is more preferably the following.

[0051] Peak B on the high-angle side indicates the composition of the argyrodite-type crystal Li a PS b Ha cIn (5≤a≤7, 4≤b≤6, and 0<c≤2), c, which represents the element ratio of Ha, is preferably 0.3 or greater, more preferably 1 or greater, and even more preferably 1.5 or greater, from the perspective of high lithium ion conductivity. Furthermore, from the perspective of suppressing corrosion of the metal current collector, c is 2 or less, preferably 1.85 or less, and more preferably 1.7 or less.

[0052] The composition of the argyrodite-type crystal represented by peak A on the low-angle side is Li a PS b Ha c In (5≤a≤7, 4≤b≤6, and 0<c≤2), c, which represents the element ratio of Ha, is greater than 0, preferably 0.5 or greater, and more preferably 1 or greater, from the perspective of high lithium ion conductivity. Furthermore, from the perspective of suppressing corrosion of the metal current collector, c is 1.7 or less, more preferably 1.6 or less, and even more preferably 1.5 or less.

[0053] Peak A represents the argyrodite-type crystal Li a1 PS b1 Ha c1 Peak B represents the argyrodite-type crystal Li a2 PS b2 Ha c2 The ratio (Li a1 PS b1 Ha c1 :Li a2 PS b2 Ha c2 ) From the perspective of high lithium ion conductivity, it is preferably 1:99 to 95:5, more preferably 1:99 to 50:50, and even more preferably 2:98 to 10:90.

[0054] The difference in the diffraction angle (2θ) between Peak A and Peak B is 0.05° or more. From the perspective of improving lithium ion conductivity, it is preferably 0.06° or more, and more preferably 0.07° or more. In addition, from the perspective of crystal stability, the difference in the diffraction angle (2θ) is 0.8° or less, more preferably 0.6° or less, and even more preferably 0.4° or less. It should be noted that in the peak analysis, the difference in the diffraction angle (2θ) between Peak A and Peak B may be less than 0.05°. In this case, these peaks are collectively considered as one peak.

[0055] The solid electrolyte according to this embodiment further has a peak C between the peaks A and B, which is preferable from the viewpoint of improving lithium ion conductivity. The composition of the argyrodite-type crystal represented by peak C is Li a PS b Ha cIn (5≤a≤7, 4≤b≤6, and 0<c≤2), c, which represents the element ratio of Ha, is preferably 0.5 or greater, more preferably 1 or greater, and even more preferably 1.5 or greater from the perspective of high lithium ion conductivity. Furthermore, from the perspective 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.

[0056] From the viewpoint of improving lithium ion conductivity, Li a2 PS b2 Ha c2 He Li a3 PS b3 Ha c3 The proportion is large, Li a1 PS b1 Ha c1 :(Li a2 PS b2 Ha c2 +Li a3 PS b3 Ha c3 ) is preferably 1:99 to 95:5, more preferably 1:99 to 50:50, and even more preferably 2:98 to 10:90.

[0057] The XRD pattern of the solid electrolyte involved in this embodiment may also have peaks at positions other than 2θ = 30.3 ± 0.5°. Peaks observed at other positions can also be separated into two or more peaks by performing peak separation, similar to the peak within the range of 2θ = 30.3 ± 0.5°.

[0058] Preferred crystal structures of argyrodite-type crystals are, for example, cubic systems such as F-43m, but rhombohedral, tetragonal, or orthorhombic systems with reduced symmetry may exist, as well as monoclinic systems with further reduced symmetry.

[0059] Given that the argyrodite-type crystal structure is cubic and has lattice constants of a=b=c, it is believed that structural changes due to heat treatment occur isotropically. Therefore, even for peaks observed outside the range of 2θ=30.3±0.5°, it is believed that heat treatment for one hour at a temperature of 400°C or higher and below the thermal decomposition temperature can lead to observations such as an increase or decrease in the intensity ratio of peaks representing each crystal structure and the appearance of new peaks.

[0060] Even when the argyrodite-type crystal structure is not a cubic system, some structural changes based on the crystal structure may occur by the above-mentioned heat treatment.

[0061] Furthermore, a solid electrolyte subjected to the following heat treatment is also included in one embodiment of the present invention.

[0062] This embodiment has Li a PS b Ha c A sulfide-based solid electrolyte for lithium-ion secondary batteries having an argyrodite-type crystal structure represented by (5≤a≤7, 4≤b≤6, and 0<c≤2). The X-ray diffraction pattern of this solid electrolyte using Cu-Kα radiation shows peaks D and E within the range of 2θ=30.3±0.5°. The half-value widths of these peaks are both greater than 0.07°, and the difference in diffraction angles (2θ) between the two peaks is 0.02-0.4°. Furthermore, even after heat treatment for 1 hour at a temperature of 400°C or higher and below the thermal decomposition temperature, no change in the X-ray diffraction pattern is observed.

[0063] Peak D and peak E, whose diffraction angle (2θ) difference is 0.02 to 0.4°, correspond to peak A or peak B and peak C before heat treatment, respectively, and are two different argyrodite-type crystals.

[0064] 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.

[0065] When Peak D and Peak E correspond to Peak A and Peak C, respectively, Peak F may be further present on the high-angle side corresponding to Peak B. Furthermore, when Peak D and Peak E correspond to Peak C and Peak B, respectively, Peak G may be further present on the low-angle side corresponding to Peak A. In this case, the half-value widths of Peaks F and G are preferably 0.07° or greater. Furthermore, 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 more preferably 0.05 to 0.4°.

[0066] The difference in diffraction angle (2θ) between Peak D and Peak E may be 0.02 to 0.4°, but from the perspective of improving lithium ion conductivity, it is preferably 0.05° or more, more preferably 0.07° or more, and even more preferably 0.09° or more. Furthermore, from the perspective of crystal stability, the difference in diffraction angle (2θ) is preferably 0.3° or less, more preferably 0.2° or less. The preferred ranges for 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 those described above.

[0067] The solid electrolyte according to the present embodiment, which includes the argyrodite-type crystal structures represented by Peak D and Peak E, can adjust the crystal ratio, diffraction angle difference, and the like by adjusting the heat treatment conditions.

[0068] It should be noted that whether the solid electrolyte has been heat treated can be determined not only by whether the XRD pattern changes when the additional heat treatment is performed, but also by the degree of change in lithium ion conductivity. The change in the XRD pattern means that the value of the diffraction angle (2θ) of peak D and peak E does not change, or even if the difference in the diffraction angle before and after the heat treatment is changed, it is less than 0.05°. In addition, the change in the peak intensity ratio before and after the heat treatment is preferably less than 0.1. The degree of change in lithium ion conductivity before and after the heat treatment is preferably less than 0.5mS / cm.

[0069] This additional heat treatment can be performed at a temperature range of 400° C. or higher and below the thermal decomposition temperature for 1 hour.

[0070] Li represents the composition of argyrodite-type crystals 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 of F, Cl, Br, and I. Since the crystal easily forms an argyrodite type, it preferably contains at least one of Cl and Br, more preferably contains Cl, and even more preferably is a single substance of Cl or a mixture of Cl and Br.

[0071] From the perspective of achieving good lithium ion conductivity when manufacturing lithium-ion batteries by finely pulverizing the solid electrolyte, the crystallite size of the argyrodite-type crystals is preferably small. 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 generally 5 nm or greater.

[0072] Although the crystallite size after the heat treatment becomes larger than that before the heat treatment, it is preferable that the crystallite size remains within the above range even after the heat treatment.

[0073] The crystallite size can be calculated from the half-value width of the peak in the XRD pattern.

[0074] From the perspective of achieving good lithium ion conductivity when manufacturing lithium-ion batteries by finely pulverizing the solid electrolyte, the secondary particle size of the argyrodite-type crystal is preferably small. Specifically, it is preferably 10 μm or less, more preferably 3 μm or less, and even more preferably 1 μm or less. The lower limit of the secondary particle size is not particularly limited, but is generally 0.1 μm or greater.

[0075] The secondary particle size can be measured using a Microtrac device.

[0076] From the perspective of the smaller the crystallite size, the half-width of each peak is 0.07° or more, preferably 0.075° or more, more preferably 0.08° or more, and even more preferably 0.09° or more. In addition, the upper limit of the half-width is not particularly limited, but is generally required to be within 0.5°. If the half-width is larger than this, it is better to further increase the number of peaks and re-analyze. The analytical method for obtaining the half-width will be described in detail below.

[0077] The argyrodite-type crystal structure of this embodiment may contain oxide anions. For example, from the perspective of improving the heat resistance of the crystal and maintaining stability without decomposition even after high-temperature heat treatment, oxide anions containing a Q0 structure formed by bonding a metal atom (M) and an oxygen atom (O) are preferred.

[0078] Here, the Q0 structure refers to a structure in which all oxygen atoms bonded to M as the central cation are non-crosslinked oxygen atoms. For example, when M is Si, it refers to the oxide of SiO2 as a silicate ion, i.e., SiO4. 4－ This oxide anion exists.

[0079] The elements constituting the oxide anion may include M and O, where M is at least one element selected from metal elements and semimetal elements of Groups 2 to 14 of the periodic table. The oxide anion may be of one or more types.

[0080] The metal elements of Groups 2 to 14 refer to elements of Groups 2 to 12 of the periodic table, elements of Group 13 except B, and elements of Group 14 except C, Si, and Ge.

[0081] The semimetallic elements of Groups 2 to 14 refer to elements of Groups 13 and 14 of the periodic table, B, Si, and Ge.

[0082] The presence of M—O bonds and the Q0 structure in oxide anions can be confirmed by Raman spectroscopy and nuclear magnetic resonance (NMR) measurements. Furthermore, the presence of oxide anions with a Q0 structure in the crystal structure, i.e., at the anion site in the crystal, can be confirmed by X-ray powder diffraction (XRD) and neutron beam scattering measurements.

[0083] From the perspective of achieving high lithium ion conductivity, the total content of elements comprising two or more argyrodite-type crystal structures relative to the total components of the solid electrolyte according to this embodiment is 80% by mass or greater, more preferably 85% by mass or greater, even more preferably 90% by mass or greater, even more preferably 92% by mass or greater, and particularly preferably 94% by mass or greater. The upper limit of the total content is not particularly limited and may be 100% by mass. Furthermore, the solid electrolyte according to this embodiment may further include other crystals besides argyrodite-type crystals, or an amorphous structure.

[0084] The total of the above contents refers to, for example, the total of Li, P, S, and Ha. When the crystal contains oxide anions, it refers to the total content of the elements Li, P, S, Ha, M, and O. In this specification, the content of Ha refers to the total content of F, Cl, Br, and I.

[0085] The content of each element and their total can be determined by composition analysis using ICP emission spectrometry, atomic absorption spectrometry, ion chromatography, or the like.

[0086] Other substances that can be contained in the solid electrolyte include Li3PS4, Li4P2S6, Li2S, LiHa (Ha is at least one halogen element selected from F, Cl, Br, and I), etc.

[0087] As an indicator of heat resistance, the solid electrolyte's thermal decomposition test temperature is preferably 400°C or higher, more preferably 450°C or higher, even more preferably 500°C or higher, even more preferably 600°C or higher, and particularly preferably 650°C or higher. The upper limit is not particularly limited, but is typically 900°C or lower. However, in the case of argyrodite-type crystals with a high halogen content, the thermal decomposition temperature tends to be lower. Therefore, the composition of the argyrodite-type crystals contained in the solid electrolyte according to this embodiment is selected in consideration of the balance with the desired properties of the solid electrolyte.

[0088] The thermal decomposition resistance test for solid electrolytes involves placing the solid electrolyte in a sealed container that is non-reactive with the solid electrolyte and heat-treating it at a specified temperature for 10 to 60 minutes. The change in lithium ion conductivity before and after the heat treatment is then measured to evaluate heat resistance. The smaller the change in lithium ion conductivity before and after the heat treatment, the better. If the lithium ion conductivity after heat treatment is less than half of the pre-heat treatment value, the thermal decomposition resistance, or heat resistance, is considered low.

[0089] In this specification, the lithium ion conductivity refers to the lithium ion conductivity at 25° C., which can be obtained from the Nyquist plot obtained by AC impedance measurement.

[0090] <Method for producing sulfide-based solid electrolyte>

[0091] Regarding the method for producing the sulfide-based solid electrolyte according to this embodiment, as long as Li a PS b Ha c The structure is not particularly limited to two or more different argyrodite-type crystal structures represented by (5≤a≤7, 4≤b≤6, and 0<c≤2) and coexisting on the order of hundreds of nm or tens of nm.

[0092] As one embodiment, a preferred production method includes the following steps: mixing raw materials containing Li, P, S, and Ha, heating and melting them, and subsequently rapidly cooling them for crystallization. Rapid cooling at normal pressure can produce a solid electrolyte in which two or more argyrodite-type crystals coexist on the order of hundreds or tens of nanometers.

[0093] The raw material containing Li, P, S, and Ha means that conventionally known substances can be used as materials for obtaining argyrodite-type crystals containing Li, P, S, and Ha.

[0094] Specifically, Li simple substance, a compound containing Li, P simple substance, a compound containing P, S simple substance, a compound containing S, and a compound containing Ha can be used in appropriate combination. These compounds can be compounds containing two or more of Li, P, S and Ha at the same time. For example, as a compound that serves as both a compound containing S and a compound containing P, phosphorus pentasulfide (P2S5) and the like can be cited. In addition, as a compound that serves as both a compound containing Li and a compound containing Ha, lithium halide can be cited.

[0095] Examples of the Li-containing compound include lithium compounds such as lithium sulfide (Li2S), lithium carbonate (Li2CO3), lithium sulfate (Li2SO4), lithium oxide (Li2O), and lithium hydroxide (LiOH). From the perspective of ease of handling, lithium sulfide is preferably used.

[0096] On the other hand, lithium sulfide is expensive. Therefore, from the perspective of reducing production costs, it is preferable to use lithium compounds other than lithium sulfide, metallic lithium, etc. Specifically, it is preferable to use one or more selected from metallic lithium, lithium carbonate (Li2CO3), lithium sulfate (Li2SO4), lithium oxide (Li2O), and lithium hydroxide (LiOH). These can be used alone or in combination of two or more.

[0097] Examples of compounds containing sulfur include phosphorus sulfide such as phosphorus trisulfide (P2S3) and phosphorus pentasulfide (P2S5), other sulfur compounds containing phosphorus, elemental sulfur, and compounds containing sulfur. Examples of compounds containing sulfur include H2S, CS2, iron sulfide (FeS, Fe2S3, FeS2, Fe 1-x S, etc.), bismuth sulfide (Bi2S3), copper sulfide (CuS, Cu2S, Cu 1-x S, etc.). Among them, from the viewpoint of preventing the inclusion of elements other than the elements constituting the target sulfide-based solid electrolyte, phosphorus sulfide is preferred, and phosphorus pentasulfide (P2S5) is more preferred. These may be used alone or in combination of two or more. It should be noted that phosphorus sulfide is a compound that serves as both a compound containing S and a compound containing P.

[0098] Examples of compounds containing P 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 perspective of preventing the inclusion of elements other than those constituting the target sulfide-based solid electrolyte. These may be used alone or in combination of two or more.

[0099] As the compound containing Ha, for example, lithium fluoride (LiF), lithium chloride (LiCl), lithium bromide (LiBr), lithium iodide (LiI) and other lithium halides, phosphorus halides, phosphorus oxyhalides, sulfur halides, sodium halides, boron halides, etc. can be mentioned. Among them, from the viewpoint of preventing the inclusion of elements other than the elements constituting the target sulfide-based solid electrolyte, lithium halides are preferred, and LiCl, LiBr, and LiI are more preferred. These compounds can be used alone or in combination of two or more.

[0100] The raw materials can be mixed by, for example, mixing in a mortar, mixing using media such as a planetary ball mill, or mixing without media such as a pin mill, a powder blender, or air flow mixing. The raw materials can also be amorphized by mixing before heating.

[0101] The specific method for heating and melting the raw material mixture is not particularly limited. For example, the raw materials may be placed in a heat-resistant container and heated in a furnace. Examples of heat-resistant containers are not particularly limited, and include heat-resistant containers made of carbon, quartz, quartz glass, borosilicate glass, aluminosilicate glass, heat-resistant containers containing oxides such as aluminum oxide, zirconium oxide, 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 block from the aforementioned materials, or may be containers formed with layers of carbon, oxides, nitrides, carbides, and the like.

[0102] From the perspective of improving the fluidity of the melt, the heating temperature when heating and melting the raw material mixture is preferably 600°C or higher, more preferably 630°C or higher, and even more preferably 650°C or higher. Furthermore, from the perspective of suppressing degradation 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.

[0103] From the perspective of promoting the reaction, the heating and melting time is preferably 0.1 hours or longer, more preferably 0.5 hours or longer, even more preferably 0.7 hours or longer, and even more preferably 1 hour or longer. Furthermore, from the perspective of suppressing degradation and decomposition of components in the melt due to heating, the heating and melting time is preferably 10 hours or shorter, more preferably 9.5 hours or shorter, and even more preferably 9 hours or shorter.

[0104] The pressure during heating and melting is not particularly limited, but is preferably normal pressure to slightly increased pressure, and more preferably normal pressure.

[0105] From the viewpoint of preventing side reactions with water vapor, oxygen, etc. during heating and melting, the dew point is preferably -20°C or lower. The lower limit is not particularly limited, but is usually around -80°C. The oxygen concentration during heating and melting is preferably 1000 ppm or lower.

[0106] The heated and molten raw material mixture is rapidly cooled at normal pressure to crystallize, thereby obtaining a solid electrolyte containing two or more argyrodite-type crystals of the order of hundreds or tens of nanometers.

[0107] For rapid cooling, the cooling rate may be 1°C / second or higher, preferably 10°C / second or higher, and more preferably 100°C / second or higher. While there is no particular upper limit on the cooling rate, generally speaking, when using a twin-roll cooling method, which is known to have the fastest rapid cooling rate, the upper limit is 1,000,000°C / second or lower.

[0108] Normal pressure during rapid cooling means that the pressure is not controlled during cooling, specifically, about 0.8 to 1.2 atm.

[0109] After rapid cooling at normal pressure, further heat treatment can be performed to stabilize the crystal. Heat treatment can improve crystallinity. Depending on the heat treatment conditions, new argyrodite-type crystals with a composition intermediate between the various types of argyrodite-type crystals before heat treatment can also be obtained.

[0110] From the perspective of more reliable crystal precipitation, the stabilization treatment, i.e., the heat treatment time, is preferably 0.1 hours or longer, more preferably 0.2 hours or longer. Furthermore, from the perspective of obtaining new argyrodite-type crystals, the heat treatment time is preferably 0.5 hours or longer, more preferably 1 hour or longer.

[0111] On the other hand, from the perspective of suppressing thermal degradation caused by heating, the heat treatment time is preferably 10 hours or less, more preferably 5 hours or less. In addition, from the perspective of preventing excessive crystallization from causing excessive reduction in lithium ion conductivity, the heat treatment time is preferably 3 hours or less, more preferably 2 hours or less.

[0112] The temperature of the stabilization treatment, i.e., the heat treatment, is preferably at least the glass transition temperature of the solid electrolyte, specifically preferably at least 200° C., more preferably at least 250° C. Furthermore, from the perspective of obtaining new argyrodite-type crystals, the heat treatment temperature is preferably at least 350° C., more preferably at least 400° C.

[0113] On the other hand, from the perspective of preventing thermal degradation and thermal decomposition, the heat treatment temperature is preferably below the thermal decomposition temperature, for example, preferably 600°C or below, more preferably 575°C or below. Furthermore, from the perspective of preventing excessive crystallization and an excessive decrease in lithium ion conductivity, the heat treatment temperature is preferably 550°C or below, more preferably 530°C or below.

[0114] When the solid electrolyte according to this embodiment contains an oxide anion having a Q0 structure and an M—O bond, the timing of adding the oxide having an M—O bond is not particularly limited. For example, the raw materials and the oxide may be mixed together and heated to melt. Alternatively, the crystalline mixed oxide obtained by rapid cooling at normal pressure may be subjected to heat treatment.

[0115] When the obtained solid electrolyte is used in a lithium ion secondary battery, other components such as a binder are optionally included to form a solid electrolyte layer. Conventionally known substances can be used as the binder and other components.

[0116] The solid electrolyte involved in this embodiment is preferably 80% by mass or more, more preferably 90% by mass or more relative to the entire solid electrolyte layer. The upper limit of the content of the solid electrolyte is not particularly limited and can be 100% by mass. In addition, from the perspective of suppressing deformation, inorganic fillers and organic fillers can be mixed. In this case, the content of the solid electrolyte is preferably 99% by mass or less.

[0117] The solid electrolyte layer can also be formed using a conventionally known method. For example, the components constituting the solid electrolyte layer are dispersed or dissolved in a solvent to form a slurry, which is then applied in a layer (sheet), dried, and pressed arbitrarily to form the solid electrolyte layer. It can also be heated as needed to perform a debindering treatment. By adjusting the amount of the slurry applied, the thickness of the solid electrolyte layer can be easily adjusted.

[0118] Alternatively, instead of wet molding, the solid electrolyte powder of this embodiment may be dry-pressed onto the surface of a positive electrode or negative electrode to form a solid electrolyte layer. Alternatively, the solid electrolyte layer may be formed on another substrate and transferred onto the surface of a positive electrode or negative electrode.

[0119] The solid electrolyte involved in this embodiment can be mixed with a positive electrode active material or a negative electrode active material to be used as a positive electrode layer or a negative electrode layer. The positive electrode active material or negative electrode active material, current collector, binder, conductive additive, etc. used in the positive electrode layer or the negative electrode layer can use conventionally known materials.

[0120] The lithium ion secondary battery using the solid electrolyte according to this embodiment includes the above-described solid electrolyte layer, a positive electrode layer, and a negative electrode layer.

[0121] The material of the outer packaging of the lithium ion secondary battery can also use existing known substances. The shape of the lithium ion secondary battery can also use existing known shapes, for example, coin-shaped, sheet-shaped (membrane-shaped), folded, wound-type bottomed cylindrical, button-shaped, etc. can be enumerated, and can be appropriately selected according to the purpose.

[0122] Example

[0123] Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited thereto.

[0124] Examples 1, 2, and 4 to 9 are embodiments, and Example 3 is a comparative example.

[0125] [evaluate]

[0126] (Lithium ion conductivity)

[0127] The lithium ion conductivity was measured using an AC impedance analyzer (VSP, a potentiostat / galvanostat, manufactured by Bio-Logic Sciences Instruments) by pressing the obtained sulfide-based solid electrolyte powder into a powder compact as a measurement sample at a pressure of 380 kN.

[0128] The measurement conditions were as follows: measurement frequency: 100 Hz to 1 MHz, measurement voltage: 100 mV, and measurement temperature: 25°C.

[0129] (Powder X-ray Diffraction)

[0130] The XRD pattern of the sulfide-based solid electrolyte was measured using an X-ray diffractometer (SmartLab, manufactured by Rigaku Co., Ltd.) Since the sulfide-based solid electrolyte used as the measurement sample deteriorates if exposed to the atmosphere, the sample was prepared in an environment not exposed to the atmosphere and subjected to measurement.

[0131] Sample preparation: The sulfide-based solid electrolyte powder was ground in a mortar and passed through a 100 μm mesh to prepare a 50% particle size D 50 The sulfide-based solid electrolyte powder has a particle size distribution of 5 to 10 μm. The particle size distribution was measured using a laser diffraction particle size distribution measuring instrument MT3300EXII manufactured by Microtrac, and the 50% particle size D was determined from the obtained volume-based particle size distribution chart. 50 .

[0132] The measurement conditions of powder X-ray diffraction are as follows.

[0133] Radiation source: CuKα radiation Tube voltage: 45 kV, tube current: 200 mA, scanning angle: 10-100°, scanning speed: 5° / min, step number: 0.01° / step.

[0134] The analysis method for peak separation and the method for determining the half-peak width from the raw data of the obtained XRD pattern are as follows.

[0135] The baseline and Cu-Kα2 line were removed using the integrated powder X-ray analysis software PDXL2 that comes with the X-ray diffractometer (SmartLab, manufactured by Rigaku Corporation).

[0136] Next, focusing on the peak appearing at 2θ=30.3±0.5°, the pseudo-Voigt function used for peak fitting is defined as in formula (1).

[0137] f(2θ)=A[a*L(2θ)+(1-a)*G(2θ)]+B Formula (1)

[0138]

[0139]

[0140] In formula (1), A represents the peak coefficient, B represents the height correction factor, ω represents the half-peak width, and a represents the mixing ratio. Formula (2) represents the Lorentzian function, and formula (3) represents the Gaussian function. x0 represents the peak center value, expressed as a 2θ value. Generally speaking, if the XRD spectrum is expressed using the 2θ value on the horizontal axis, it will become a left-right asymmetric spectrum. Therefore, the mixing ratio in formula (1) is changed before and after the peak (=x0) to express it, and thus analysis is performed.

[0141] This asymmetry is primarily due to the instrumentation and the X-ray penetration depth into the sample. Therefore, this analysis used the mixing ratio determined from the analysis of Example 3, prepared using the existing solid-phase method. Specifically, a ratio of 1.00 was used for x0 ≥ x, and 0.36 for x0 < x. Using this mixing ratio, the peaks analyzed for silicon single crystals showed remarkably consistent results.

[0142] The Rwp index is used to evaluate the analytical integrity of the fitting function. This is an index used in the full spectrum fitting (Rietveld) analysis of the XRD pattern.

[0143] For peaks appearing within the range of 2θ = 30.3 ± 0.5°, the lower the Rwp value obtained when applying one or more of the above-defined fitting functions, the better. Specifically, the Rwp value is preferably 15% or less, more preferably 12.5% ​​or less, and even more preferably 10% or less. Note that Rwp values ​​are positive.

[0144] [Example 1]

[0145] Under a dry nitrogen atmosphere, lithium sulfide powder (Sigma, 99.98% purity), phosphorus pentasulfide powder (Sigma, 99% purity), and lithium chloride powder (Sigma, 99.99% purity) were weighed in a mortar to form a composition ratio of Li6PS5Cl. The mixture was placed in a heat-resistant container and heated to melt at 730°C for 0.5 hours in an environment with a dew point of -60°C. It was then cooled to room temperature at a cooling rate of 10°C / second to obtain a solid that became a sulfide-based solid electrolyte.

[0146] The XRD pattern of the obtained solid electrolyte near 2θ=30.3° is shown in FIG. Figure 1 Peak separation of the unprocessed spectrum shown by the solid line revealed three peaks within the range of 2θ = 30.3 ± 0.5°. The details of these peaks are shown in Table 1.

[0147] The results of the full spectrum fitting analysis of the XRD pattern showed that the diffraction peak at 2θ=29.99° corresponding to peak A of the obtained solid electrolyte was derived from Li 6.9 PS 5.9 Cl0.1 The peak of the argyrodite crystal composed of the above mentioned peaks is the diffraction peak at 2θ=30.11° corresponding to the peak C, which is derived from Li 5.9 PS 4.9 Cl 1.1 The peak of the argyrodite crystal composed of the above is the diffraction peak at 2θ=30.24° corresponding to peak B, which is derived from Li 5.5 PS 4.5 Cl 1.5 The molar ratio of these three argyrodite-type 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 =2:1:3, the lattice constants of these three argyrodite-type crystals are

[0148] The “basic composition” in Table 1 refers to the target composition when mixing raw materials. For example, in Example 1, it is Li6PS5Cl. However, the composition of the argyrodite-type crystals actually obtained is Li as determined from the above peaks A, B, and C. 6.9 PS 5.9 Cl 0.1 、Li 5.5 PS 4.5 Cl 1.5 He Li 5.9 PS 4.9 Cl 1.1 The coexisting composition is different from the basic composition.

[0149] "-" in Peak A or Peak B in Table 1 means that the peak disappeared or the peak intensity ratio was so small that the peak could not be detected. The peak intensity ratio is a relative intensity ratio expressed with the sum of the peak intensities of Peak A, Peak B, and Peak C being 1.

[0150] [Example 2]

[0151] The solid obtained in Example 1 was further heat treated at 500°C for 1 hour in a nitrogen atmosphere to obtain a sulfide-based solid electrolyte. The XRD pattern of the obtained solid electrolyte near 2θ=30.3° is shown in FIG. Figure 2 Peak separation of the unprocessed spectrum shown by the solid line revealed two peaks within the range of 2θ = 30.3 ± 0.5°. The peak details are shown in Table 1.

[0152] The results of the full spectrum fitting analysis of the XRD pattern show that the obtained solid electrolyte has Li 5.9PS 4.9 Cl 1.1 :Li 5.5 PS 4.5 Cl 1.5 =5:1 (molar ratio) of two argyrodite-type crystals.

[0153] [Example 3]

[0154] 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. The XRD pattern of the resulting solid electrolyte revealed a single peak within the range of 2θ = 30.3 ± 0.5°. The peak details are shown in Table 1.

[0155] As a result of full spectrum fitting analysis of the XRD pattern, it was found that the obtained solid electrolyte had one type of argyrodite-type crystal of Li6PS5Cl.

[0156] [Example 4]

[0157] To become Li 5.4 PS 4.4 Cl 1.6 In the manner of weighing lithium sulfide powder (manufactured by Sigma, purity 99.98%), phosphorus pentasulfide powder (manufactured by Sigma, purity 99%) and lithium chloride powder (manufactured by Sigma, purity 99.99%) in a composition ratio of 1.5-2.5% pyridine was used to prepare a mixture. Otherwise, the same procedure as in Example 1 was followed to obtain a solid which became a sulfide-based solid electrolyte. In the XRD spectrum of the obtained solid electrolyte, three peaks were confirmed within the range of 2θ=30.3±0.5°, namely 2θ=30.15°, 30.29° and 30.33°. Among them, the two peaks of 2θ=30.29° and 2θ=30.33°, whose diffraction angle difference was less than 0.05°, had almost similar compositions, and it can be inferred that they had the same XRD pattern as Li-ion. 5.4 PS 4.4 Cl 1.6 The composition of the two argyrodite-type crystals is similar. It can be inferred that the argyrodite-type crystal with a peak at 2θ = 30.15° is similar to Li 5.7 PS 4.7 Cl 1.7 The composition is close.

[0158] [Example 5]

[0159] The solid obtained in Example 4 was further heat treated at 450°C for 1 hour in a nitrogen environment to obtain a sulfide-based solid electrolyte. In the XRD spectrum of the obtained solid electrolyte, three peaks were observed in the range of 2θ=30.3±0.5°. The peak intensity ratio of the high-angle side corresponding to peak B increased relative to the peak of Example 4. According to the results of the composition analysis, no composition change was observed before and after the heat treatment. Therefore, it can be said that the obtained solid electrolyte is Li 5.4 PS 4.4 Cl 1.6 and having an argyrodite-type crystal with a lattice constant lower than that before heat treatment.

[0160] [Example 6]

[0161] To become Li 5.6 PS 4.4 Cl 1.5 Br 0.2 In the manner of weighing lithium sulfide powder (manufactured by Sigma, purity 99.98%), phosphorus pentasulfide powder (manufactured by Sigma, purity 99%), lithium chloride powder (manufactured by Sigma, purity 99.99%) and lithium bromide powder (manufactured by Sigma, purity 99.995%) in a composition ratio of 1, 2, 3 and 4, 5, 6, 7, 8 and 9, respectively, were prepared to form a mixture. The same procedure as in Example 1 was followed except that the above procedures were repeated to obtain a solid electrolyte which became a sulfide-based solid electrolyte. In the XRD spectrum of the obtained solid electrolyte, three peaks were confirmed within the range of 2θ=30.3±0.5°, namely 2θ=30.16°, 30.34° and 30.41°. Among them, it can be inferred that the peak at 2θ=30.34° corresponding to peak C comes from the peak C with Li 5.6 PS 4.4 Cl 1.5 Br 0.2 It can be inferred that the two peaks corresponding to peak A and peak B have argyrodite-type crystals with a similar composition, and have compositions or lattice constants slightly different from those of the peak corresponding to peak C.

[0162] [Example 7]

[0163] The solid obtained in Example 6 was further heat treated at 450°C for 1 hour in a nitrogen environment to obtain a sulfide-based solid electrolyte. In the XRD spectrum of the obtained solid electrolyte, two peaks were observed in the range of 2θ=30.3±0.5°. The peak intensity ratio of the high-angle side corresponding to peak B increased relative to the peak of Example 6. According to the results of the composition analysis, no composition change was observed before and after the heat treatment. Therefore, it can be said that the obtained solid electrolyte is Li 5.6 PS 4.4 Cl 1.5 Br 0.2and having an argyrodite-type crystal with a lattice constant lower than that before heat treatment.

[0164] [Example 8]

[0165] To become Li 5.6 PS 4.4 Cl 0.8 Br 0.8 In the manner of weighing lithium sulfide powder (manufactured by Sigma, purity 99.98%), phosphorus pentasulfide powder (manufactured by Sigma, purity 99%), lithium chloride powder (manufactured by Sigma, purity 99.99%) and lithium bromide powder (manufactured by Sigma, purity 99.995%) in a composition ratio of 1, 2, 3 and 4, 5, 6 and 7, respectively, were prepared to form a mixture. The same procedure as in Example 1 was followed except that the mixture was prepared to obtain a solid sulfide-based solid electrolyte. In the XRD spectrum of the obtained solid electrolyte, three peaks were confirmed within the range of 2θ=30.3±0.5°, namely 2θ=30.05°, 30.14° and 30.33°. Among them, it can be inferred that the peak at 2θ=30.14° corresponding to peak C comes from the peak C with Li 5.6 PS 4.4 Cl 0.8 Br 0.8 It can be inferred that the two peaks corresponding to peak A and peak B have argyrodite crystals with compositions similar to those of the peak corresponding to peak C, which have compositions or lattice constants that are slightly different from those of the peak corresponding to peak C.

[0166] [Example 9]

[0167] The solid obtained in Example 8 was further heat treated at 450°C for 1 hour in a nitrogen environment to obtain a sulfide-based solid electrolyte. In the XRD pattern of the obtained solid electrolyte, two peaks were observed in the range of 2θ=30.3±0.5°. The peak intensity ratio of the peak at 2θ=30.14°, which corresponds to peak C, is large and can be called the main peak. The peak becomes sharper by heat treatment and the intensity ratio increases. Therefore, it can be inferred that it has Li-ion with further increased crystallinity. 5.6 PS 4.4 Cl 0.8 Br 0.8 The composition is argyrodite-type crystal.

[0168]

Table 1

[0169]

[0170] In Examples 1, 2, and 4 to 9, the mixing ratio represented by parameter a in equation (1) during the fitting analysis was the same value as that in Example 3. The analysis results showed that Rwp was 15% or less, which was a good value.

[0171] Based on the above results, the sulfide-based solid electrolyte of Example 3, obtained by the conventional solid-phase method, contains a single argyrodite-type crystal and exhibits a lithium ion conductivity of 1.2 mS / cm. Meanwhile, while the types and amounts of raw materials used in the solid electrolytes of Examples 1 and 2 are the same as those of Example 3, they contain two or three types of argyrodite-type crystals. Furthermore, their lithium ion conductivities are 3.1 mS / cm and 2.7 mS / cm, respectively, significantly higher than those of Example 3.

[0172] Comparisons of Examples 1 and 2, 4 and 5, 6 and 7, and 8 and 9 reveal that heat treatment sharpens the peaks in the XRD patterns, tending to concentrate the lattice constant in the crystals exhibiting the central peak, or to lower the lattice constant. This enhances the Coulomb interaction within the crystals, and is thought to improve heat resistance, chemical stability, and electrochemical stability. Furthermore, while the lithium ion conductivity decreases slightly compared to pre-heat treatment, it remains higher than that of the solid electrolyte of Example 3, which contains only one type of argyrodite-type crystal.

[0173] The reason why the inclusion of two or more argyrodite-type crystals improves lithium-ion conductivity is unclear, but this effect can be achieved when argyrodite-type crystals are obtained by rapid cooling after a melting step during solid electrolyte production. This is believed to be because the seed crystals that initially precipitate during cooling from the molten state are high-temperature stable phases. The argyrodite-type crystals that form the nucleus of these seed crystals exhibit different locations for lithium ions, sulfur anions, and halogen anions than those produced by conventional solid-phase reactions. Generally, high-temperature stable phases tend to have high ion conductivity. In the argyrodite-type crystals of this embodiment, argyrodite-type crystals containing multiple compositions of high-temperature stable phases precipitate during cooling from the melt, resulting in high lithium-ion conductivity.

[0174] Although the present invention has been described with reference to details or specific embodiments, it is clear to those skilled in the art that various changes and modifications can be implemented without departing from the spirit and scope of the present invention. This application is based on Japanese patent application (Japanese Patent Application No. 2020-172693) filed on October 13, 2020, the contents of which are incorporated herein by reference.

Claims

1. A sulfide-based solid electrolyte, which is a sulfide-based solid electrolyte used in a lithium-ion secondary battery, With Li a PS b Ha c The crystal structure of the argyrodite type is represented by 5≤a≤7, 4≤b≤6 and 0<c≤2, Ha is a halogen element, In an X-ray diffraction pattern using Cu-Kα radiation, the argyrodite-type crystal structure has a peak A and a peak B with half-peak widths of 0.07° or more, respectively, within a range of 2θ=30.3±0.5°. The difference in diffraction angle 2θ between the peak A and the peak B is 0.05° or more, The peak A and the peak B are peaks derived from two argyrodite-type crystal structures having lattice constants that differ by 0.02 Å or more.

2. The sulfide-based solid electrolyte according to claim 1, wherein Peak C is located between Peak A and Peak B.

3. The sulfide-based solid electrolyte according to claim 1 or 2, wherein After heat treatment at a temperature of 400° C. or higher and below the thermal decomposition temperature for 1 hour, at least one phenomenon selected from the following (i) to (iii) is observed in an X-ray diffraction pattern using Cu-Kα radiation. (i) a decrease in the peak intensity ratio of Peak A, (ii) an increase in the peak intensity ratio of Peak B, (iii) the appearance of a peak C or an increase in the peak intensity ratio of the peak C, the peak C existing on the high-angle side of the peak A and the low-angle side of the peak B.

4. The sulfide-based solid electrolyte according to claim 3, wherein After heat treatment at a temperature of 400° C. or higher and below the thermal decomposition temperature for 1 hour, the difference in diffraction angle 2θ between the peak A and the peak C is 0.02 to 0.4°, or the difference in diffraction angle 2θ between the peak C and the peak B is 0.02 to 0.4°, Even if heat treatment is further performed at a temperature of 400° C. or higher and below the thermal decomposition temperature for 1 hour, the X-ray diffraction pattern does not change.

5. A method for producing the sulfide-based solid electrolyte according to claim 1, which is a method for producing a sulfide-based solid electrolyte for use in a lithium-ion secondary battery, comprising: Raw materials containing Li, P, S and Ha are mixed and heated to melt. Then it is crystallized by rapid cooling at normal pressure; Said Ha is a halogen element, The sulfide-based solid electrolyte has Li a PS b Ha c Two or more different argyrodite-type crystal structures are represented, wherein 5≤a≤7, 4≤b≤6, and 0<c≤2, Ha is a halogen element, The rapid cooling is performed at a cooling rate of 1°C / second or more.

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

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