Sulfide-based solid electrolyte used in lithium-ion secondary batteries, method for producing the same, solid electrolyte layer, and lithium-ion secondary battery

Abstract

The present invention relates to a sulfide-based solid electrolyte used for a lithium ion secondary battery. The sulfide-based solid electrolyte comprises an argyrodite-type crystals represented by the composition formula Lia–M–Zb–Hac, in which M, Z, and Ha are at least one element selected from predetermined elements; Z includes S; and a, b, and c of the composition formula represent the content ratio (unit: at%) of each element, and satisfy 5 < a < 7, 4 < b < 6, and 0 < c < 2. The maximum distance between the Li / Li-ions in the crystals is 2.54 Å or less.

Description

Technical Field

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

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 in terms of 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 a lithium-ion secondary battery have attracted attention because they can be expected to improve safety, high-speed charge and discharge, cycle characteristics, and miniaturization of the case.

[0004] Solid electrolytes are roughly classified into sulfide-based solid electrolytes and oxide-based solid electrolytes. Sulfide ions have a larger polarization rate than oxide ions and exhibit high ionic conductivity. As sulfide-based solid electrolytes, LGPS-type crystals such as Li 10 GeP2S 12 etc., argyrodite-type crystals such as Li6PS5Cl, and LPS crystallized glass such as Li7P3S 11 are known.

[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 has 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 L * a * b *The lightness L value of the color system is 60.0 or more. This is aimed at improving the charge-discharge efficiency and cycle characteristics by increasing the lithium ion conductivity and decreasing the electronic conductivity.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] For use in a lithium ion secondary battery, a sulfide-based solid electrolyte is required to have a higher lithium ion conductivity. In the case of an argyrodite-type crystal as described in Patent Document 1, it is known that the higher the content ratio of Ha (halogen element), the higher the lithium ion conductivity. However, there is a concern that Ha, especially chlorine, corrodes aluminum or the like used as a current collector foil in a solid battery. Therefore, there is a limit to the improvement of the lithium ion conductivity by increasing the content of Ha in an argyrodite-type crystal.

[0008] An object of the present invention is to provide a sulfide-based solid electrolyte having excellent lithium ion conductivity, a method for producing the same, a solid electrolyte layer, and a lithium ion secondary battery.

Means for Solving the Problems

[0009] As a result of intensive studies, the present inventors have found that in an argyrodite-type crystal, by focusing on the arrangement of lithium ions and setting the maximum distance between Li-Li ions within a specific range, the lithium ion conductivity can be further improved, and thus the present invention has been completed.

[0010] That is, the present invention includes the following aspects. 1. A sulfide-based solid electrolyte used in a lithium-ion secondary battery, comprising an aldite-type crystal, said crystal having a composition formula: Li a -M-Z b -Ha c represented by, said M being at least one element selected from Na, K, and elements present as cations having a valence of 2 to 5 in said crystal, said Z being at least one element selected from elements present as divalent anions in said crystal, said Z containing S, said Ha being at least one element selected from the group consisting of F, Cl, Br, and I, a, b, and c in said composition formula representing the ratios of the contents (unit: at%) of the respective elements, satisfying 5 < a < 7, 4 < b < 6, and 0 < c < 2, and a sulfide-based solid electrolyte in which the maximum distance between Li-Li ions in said crystal is 2.54 Å or less. 2. The sulfide-based solid electrolyte according to 1, wherein said Z further contains O. 3. Said crystal contains M n+ and said M n+ is a cation having a valence of 1 to 3, M in said M n+ is at least one element (M1) selected from Al, Ca, Mg, Na, and K, and the sulfide-based solid electrolyte according to 2, wherein said M n+ exists at the site of Li. 4. The sulfide-based solid electrolyte according to 3, wherein the distance between said M n+ and said O is 2.3 Å or less. 5. Said M mainly contains P, said M further contains at least one element (M2) of Si and B, and the sulfide-based solid electrolyte according to any one of 2 to 4, wherein said element (M2) exists at the site of said P. 6. A sulfide-based solid electrolyte according to any one of 2 to 5, comprising an oxide anion having a Q0 structure in which M and O are bonded, wherein the M constituting the MO bond comprises at least one element (M3) selected from Si, Al, Zr, and B.

[0011] 7. A method for producing a sulfide-based solid electrolyte used in lithium-ion secondary batteries, The process involves mixing raw materials containing Li, M, Z, and Ha, heat-treating them, and obtaining argyrodite-type crystals during the cooling process. The cooling process includes keeping the temperature between 500°C and 400°C for at least one minute. The aforementioned M is at least one selected from Na, K, and elements present as 2- to 5-valent cations in the crystal. The aforementioned Z is at least one element selected from the elements that exist as divalent anions in the crystal, The aforementioned Ha is at least one selected from the group consisting of F, Cl, Br, and I. A method for producing sulfide-based solid electrolytes. 8. A method for producing a sulfide-based solid electrolyte used in lithium-ion secondary batteries, The process includes mixing raw materials containing Li, M, Z, and Ha, heat-treating them, and obtaining argyrodite-type crystals during the cooling process, and reheating the obtained argyrodite-type crystals and keeping them at 500°C to 400°C for at least one minute. The aforementioned M is at least one selected from Na, K, and elements present as 2- to 5-valent cations in the crystal. The aforementioned Z is at least one element selected from the elements that exist as divalent anions in the crystal, The aforementioned Ha is at least one selected from the group consisting of F, Cl, Br, and I. A method for producing sulfide-based solid electrolytes. 9. A method for producing a sulfide-based solid electrolyte used in lithium-ion secondary batteries, The process involves mixing raw materials containing Li, M, Z, and Ha, heat-treating them, and obtaining argyrodite-type crystals during the cooling process. In the aforementioned cooling process, the cooling rate from 500°C to 400°C is 500°C / second or more. The aforementioned M is at least one selected from Na, K, and elements present as 2- to 5-valent cations in the crystal. The aforementioned Z is at least one element selected from the elements that exist as divalent anions in the crystal, The aforementioned Ha is at least one selected from the group consisting of F, Cl, Br, and I. A method for producing sulfide-based solid electrolytes. 10. A method for producing a sulfide-based solid electrolyte according to any one of 7 to 9, comprising performing the heat treatment in an atmosphere containing sulfur elements. 11. A method for producing a sulfide-based solid electrolyte according to any one of 7 to 10, wherein the temperature of the heat treatment is 500°C or higher.

[0012] 12. A solid electrolyte layer containing a sulfide-based solid electrolyte as described in any one of items 1 to 6 above. 13. A lithium-ion secondary battery containing a sulfide-based solid electrolyte as described in any one of items 1 to 6 above. [Effects of the Invention]

[0013] According to the present invention, by adjusting the maximum distance between Li-Li ions in an argyrodite-type crystal to a specific range, it is possible to provide a sulfide-based solid electrolyte with superior lithium-ion conductivity, a method for producing the same, a solid electrolyte layer, and a lithium-ion secondary battery. [Modes for carrying out the invention]

[0014] The present invention will be described in detail below, but the present invention is not limited to the following embodiments and can be modified and implemented as appropriate without departing from the spirit of the invention. Furthermore, the "~" indicating a numerical range is used to mean that the numbers written before and after it are included as the lower limit and upper limit.

[0015] <Sulfide solid electrolyte> The sulfide-based solid electrolyte according to this embodiment (hereinafter sometimes simply referred to as "this solid electrolyte") is a sulfide-based solid electrolyte used in a lithium-ion secondary battery and contains an argyrodite-type crystal. In this solid electrolyte, the crystal has a composition formula: Li a -M-Z b -Ha c represented by, where M is at least one element selected from Na, K, and elements present as cations with a valence of 2 to 5 in the crystal, Z is at least one element selected from elements present as divalent anions in the crystal, and Ha is at least one element selected from the group consisting of F, Cl, Br, and I. a, b, and c in the composition formula represent the ratios of the contents (unit: at%) of the respective elements and satisfy 5 < a < 7, 4 < b < 6, and 0 < c < 2. In this solid electrolyte, the maximum distance between Li-Li ions in the crystal is 2.54 Å or less.

[0016] In this specification, the maximum distance between Li-Li ions means the distance between Li-Li ions that is the maximum value among the distances between adjacent lithium ions in an argyrodite-type crystal. The method for measuring the maximum distance between Li-Li ions is specifically as follows. That is, synchrotron X-ray diffraction measurement is performed on the sulfide-based solid electrolyte, and structure refinement analysis is performed by the Rietveld method. Then, among the following ion distances (J. Am. Chem. Soc. 2017, 139, 10909-10918 Figure 1) considered as the migration paths of lithium ions in the argyrodite-type crystal structure obtained from the structure refinement analysis, the maximum distance can be determined as the maximum distance between Li-Li ions. (1) Doublet (48h-48h) jump (2) 48h-24g jump (3) Intra-cage jump (distance within the cage) (4) Inter-cage jump (distance between cages)

[0017] By reducing the maximum distance between Li-Li ions to 2.54 Å or less, the lithium ion conductivity of this solid electrolyte is improved. The inventors have found that by appropriately adjusting the manufacturing conditions of the argyrodite-type crystal, the maximum distance between Li-Li ions can be made smaller than in conventional methods. In this solid electrolyte, it is believed that the relatively small maximum distance between Li-Li ions allows Li ions to move more easily within the crystal structure, thereby improving lithium ion conductivity. The maximum distance between Li-Li ions is 2.54 Å or less, preferably 2.5 Å or less, more preferably 2.4 Å or less, and even more preferably 2.3 Å or less. While a smaller maximum distance between Li-Li ions is preferable, from the viewpoint of maintaining the crystal structure and the ionic radii of the various elements constituting the crystal, a distance of 1.8 Å or more is practical.

[0018] (Argyrodite-type crystals) This solid electrolyte has the composition formula: Li a -MZ b -Ha c It contains argyrodite-type crystals represented by [formula]. The argyrodite-type crystals may consist of only one type with the same composition, or two or more types with different compositions may be included.

[0019] From the viewpoint of measurement precision, the crystal structure is preferably analyzed from the synchrotron X-ray powder diffraction (XRD) pattern. The crystal structure may also be analyzed from the XRD pattern of a general-purpose device. In the XRD pattern when the radiation source is Cu-Kα rays, the presence of peaks at 2θ = 15.7 ± 0.5° and 30.2 ± 0.5° indicates that the crystal is of the argyrodite type. In addition to the above, it is preferable that the XRD pattern also has a peak at 2θ = 18.0 ± 0.5°, and even more preferable that it has a peak at 2θ = 25.7 ± 0.5°. Furthermore, the arrangement of each element in the crystal structure can be determined by refining the crystal structure using the Rietveld method on the XRD pattern measured with synchrotron X-rays. Furthermore, the content of each element and their total can be determined by compositional analysis using methods such as ICP emission spectrometry, atomic absorption spectrometry, and ion chromatography. By refining the crystal structure using the Rietveld method based on these values, the crystal composition can be determined with even greater accuracy.

[0020] M is at least one element selected from Na, K, and elements that exist as divalent to pentavalent cations in argyrodite crystals. Specific examples of elements that exist as divalent to pentavalent cations in argyrodite crystals include B, Mg, Al, Si, P, Ca, Ti, V, Fe, Zn, Ga, Sr, Y, Zr, Nb, Mo, Sn, Sb, Ba, Ta, W, and Bi.

[0021] From the viewpoint of the oxidation-reduction potential of the elements, M preferably mainly contains P. Specifically, M mainly containing P means that the ratio of the P content (at%) to the M content (at%) in the argyrodite-type crystal is 0.6 or more, preferably 0.7 or more, and more preferably 0.8 or more. There is no particular upper limit to the ratio of the P content (at%) to the M content (at%), and it may be as low as 1, preferably 0.97 or less, and more preferably 0.95 or less.

[0022] M may contain at least one element selected from Na, K, Mg, and Ca. A mixture containing lithium sulfide (Li2S) is sometimes suitably used as a raw material for argyrodite-type crystals. Here, lithium sulfide is widely known to be produced from lithium hydroxide (LiOH), but lithium hydroxide may contain at least one element selected from the group consisting of Na, K, Mg, and Ca (hereinafter also referred to as "R") as an impurity. That is, M may contain R derived from such impurities in the raw material. Reducing the content of R derived from impurities may require high-purity raw materials, which raises concerns about increased manufacturing costs. In this case, the ratio of the content of R (at%) to the content of M (at%) is preferably 0.001 or higher, more preferably 0.01 or higher, and even more preferably 0.02 or higher from the viewpoint of suppressing manufacturing costs. The ratio of the content of R (at%) to the content of M (at%) is preferably 0.4 or lower, and more preferably 0.3 or lower, from the viewpoint of suppressing a decrease in lithium ion conductivity. Furthermore, there is no prejudice to intentionally include R in argyrodite crystals, or to include it in a higher proportion than described above. For example, R can also be included in argyrodite crystals as element (M1), as described later.

[0023] Z is at least one element selected from elements that exist as divalent anions in argyrodite crystals. Examples of such elements include S, O, Se, and Te.

[0024] Z contains S. From the viewpoint of lithium ion conductivity, Z preferably mainly contains S. Specifically, Z mainly containing S means that the ratio of the S content (at%) to the Z content (at%) in the argyrodite-type crystal is 0.6 or more, preferably 0.7 or more, and more preferably 0.8 or more. There is no particular upper limit to the ratio of the S content (at%) to the Z content (at%), and it may be as low as 1, preferably 0.95 or less, and more preferably 0.9 or less.

[0025] The halogen element represented by Ha is at least one element selected from the group consisting of F, Cl, Br, and I. Since the crystal tends to be of the argyrodite type, Ha preferably contains at least one of Cl and Br, more preferably contains Cl, and even more preferably Cl alone or a mixture of Cl and Br. Furthermore, from the viewpoint of further improving lithium ion conductivity, a mixture of Cl and Br is preferred for Ha.

[0026] Here, when Ha contains Cl and Br, let x (at%) be the Cl content and y (at%) be the Br content in the argyrodite-type crystal. Then (x / y) is preferably 0.1 or greater, more preferably 0.3 or greater, and even more preferably 0.5 or greater. Furthermore, (x / y) is preferably 10 or less, more preferably 3 or less, and even more preferably 1.6 or less. When (x / y) satisfies the above range, the interaction between lithium ions and halide ions is weakened, and the lithium ion conductivity of the solid electrolyte after heat treatment tends to be good. This is thought to be due to the mixed anion effect, which weakens the interaction between cations and anions by mixing bromide ions, which have a larger ionic radius than chloride ions. In addition, when (x / y) satisfies the above range, the cycle characteristics of lithium-ion secondary batteries tend to be improved.

[0027] Furthermore, if Ha contains Cl and Br, the ratio of the element content (at%) that constitutes the argyrodite type crystal is Li a -MZ b -Cl c1 -Br c2When expressed by [parameters], c1 is preferably 0.1 or more, more preferably 0.3 or more, and still more preferably 0.5 or more. c1 is preferably 1.5 or less, more preferably 1.4 or less, and still more preferably 1.3 or less. c2 is preferably 0.1 or more, more preferably 0.3 or more, and still more preferably 0.5 or more. c2 is preferably 1.9 or less, more preferably 1.6 or less, and still more preferably 1.4 or less. By c1 and c2 each satisfying the above ranges, the proportion of halide ions in the crystal is optimized, and a stable argyrodite-type crystal can be obtained while reducing the interaction between anions and lithium ions in the crystal. Thereby, the lithium ion conductivity of the solid electrolyte after heat treatment tends to be good. Further, by c1 and c2 satisfying the above ranges, the cycle characteristics of the lithium ion secondary battery tend to be improved. Here, a, b, and (c1 + c2) preferably satisfy the same relationships as a, b, and c described below.

[0028] Composition formula: Li a -M-Z b -Ha c In [formula], a, b, and c each represent the ratio of the content (unit: at%) of each element. Here, a, b, and c satisfy 5 < a < 7, 4 < b < 6, and 0 < c < 2. By satisfying such a relationship, the crystal is likely to be of the argyrodite type. a more preferably satisfies the relationship of 5.1 < a < 6.3, and still more preferably satisfies the relationship of 5.2 < a < 6.2. b more preferably satisfies the relationship of 4 < b < 5.3, and still more preferably satisfies the relationship of 4.1 < b < 5.2. c more preferably satisfies the relationship of 0.7 < c < 1.9, and still more preferably satisfies the relationship of 0.8 < c < 1.8.

[0029] The preferred crystal structure of the argyrodite-type crystal is a cubic crystal (for example, F-43m), but there may be hexagonal, tetragonal, orthorhombic, monoclinic, etc. with reduced symmetry, and even triclinic with further reduced symmetry.

[0030] Here, from the viewpoint of keeping the maximum distance between Li-Li ions at 2.54 Å or less, it is preferable that the argyrodite-type crystal has, for example, the following characteristics.

[0031] It is preferable that Z contains O. The presence of O in Z means that the argyrodite-type crystal contains O. 2- This means that it contains argyrodite crystals. 2- The inclusion of O makes it easier to reduce the lattice constant and to keep the maximum distance between Li-Li ions below 2.54 Å. In this case, in the argyrodite type crystal, O 2- The sites where O exists are not particularly limited. For example, in argyrodite crystals 2- One way to make it present is to add oxides such as SiO2, Al2O3, B2O3, P2O5, Li2O, and Li4SiO4 to the raw materials, but if SiO2 is added, O 2- It is more likely to be present at the 16e site, and when Al2O3 is added, O 2- It is likely to be present at the 4a site. Also, when Li2O is added, O 2- It is more likely to be found on 4a sites.

[0032] When Z contains O, the ratio of O content (at%) to Z content (at%) in the argyrodite-type crystal is preferably 0.03 or higher, more preferably 0.05 or higher, and even more preferably 0.1 or higher. From the viewpoint of ionic conductivity, the ratio of O content (at%) to Z content (at%) is preferably 0.4 or lower, and more preferably 0.2 or lower.

[0033] If Z contains O, then M contains at least one element (M1) selected from Al, Ca, Mg, Na and K, and element (M1) is M n+ It is preferable that it exists at the site of Li. Here, M n+ represents a 1- to 3-valent cation. In other words, argyrodite crystals are M n+ It is preferable that it contains M n+ In this, M is the element (M1), and M n+It is present at the Li site. The element (M1) more preferably contains Al. If Z contains O, then M is added to the argyrodite type crystal. n+ By including this, the lattice constant tends to decrease, and it is more preferable from the viewpoint of making it easier to keep the maximum distance between Li-Li ions below 2.54 Å. n+ The method of incorporating the element is not particularly limited, but examples include adding a compound containing the element (M1) to the raw materials, or adding the element (M1) in its elemental form or a mixture containing the element. Preferred compounds containing the element (M1) are oxides, sulfides, halides, etc., of the element (M1).

[0034] Argyrodite type crystals M n+ If it contains, the M content (at%) in the argyrodite type crystal is n+ The ratio of the total content (at%) of is preferably 0.001 or higher, more preferably 0.01 or higher, and even more preferably 0.1 or higher. n+ The ratio of the total content (at%) is preferably 0.4 or less, and more preferably 0.3 or less, from the viewpoint of maintaining the crystal structure.

[0035] Argyrodite type crystals M n+ If it contains M n+ M and O may exist, for example, adjacent to each other. In this case, it is preferable that the distance between M and O is 2.3 Å or less. n+ The distance between and O is more preferably 2.2 Å or less, and even more preferably 2.1 Å or less. n+ The distance between M and O can be calculated in the same way as the distance between M and O, which will be described later.

[0036] Furthermore, when Z contains O and M mainly contains P, it is also preferable that M further contains at least one element (M2) of Si and B, and that element (M2) is present at the P site. It is more preferable that element (M2) contains Si. When M satisfies the above requirements, the element (M2) can more easily be incorporated into the crystal structure in the form of an M2-O bond with oxygen. This is more preferable from the viewpoint of making it easier to reduce the lattice constant and to keep the maximum distance between Li-Li ions below 2.54 Å. The method for incorporating the element (M2) into the argyrodite-type crystal and placing it at the P site is not particularly limited, but examples include adding a compound containing the element (M2) to the raw material, or adding the element (M2) itself or a mixture containing the element while blowing in oxygen. Preferred compounds containing the element (M2) include oxides, sulfides, nitrides, and carbides of the element (M2).

[0037] When M contains elements (M2) present at the P sites, the ratio of the total content (at%) of elements (M2) present at the P sites to the content (at%) of M in the argyrodite-type crystal is preferably 0.01 or higher, more preferably 0.03 or higher, and even more preferably 0.05 or higher. From the viewpoint of maintaining the crystal structure, the ratio of the total content (at%) of elements (M2) present at the P sites to the content (at%) of M is preferably 0.3 or lower, and more preferably 0.2 or lower.

[0038] In argyrodite crystals having at least one of the embodiments described above, the maximum distance between Li-Li ions can be made relatively small, which is preferable. However, these embodiments are not essential for making the maximum distance between Li-Li ions 2.54 Å or less. For example, it is thought that appropriately adjusting the manufacturing conditions also contributes to reducing the maximum distance between Li-Li ions, and this point will be described in detail later.

[0039] When M contains, for example, an element present at the P site (M2), and Z contains O, the argyrodite crystal has adjacent regions of M and O, and possesses an MO bond. The bond is not particularly limited, but could be, for example, a covalent bond. In this MO bond, the distance between M and O is preferably 2.04 Å or less. The distance between M and O is more preferably 2.03 Å or less, and even more preferably 2.02 Å or less. In this case, it means that the argyrodite crystal contains an oxide anion with a Q0 structure in its crystal structure, which has an MO bond between M and O. The presence of an oxide anion with a Q0 structure in the crystal structure, i.e., at the anion site of the crystal, can be confirmed by X-ray powder diffraction (XRD) measurement or neutron scattering measurement. A Q0 structure is a structure in which all oxygen atoms bonded to M, which forms the central cation, 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. The presence of MO bonds can be confirmed by Raman spectroscopy and nuclear magnetic resonance (NMR) measurements. In Raman spectroscopy, the presence of MO bonds is observed at 750–1500 cm⁻¹. -1 This can be confirmed as a Raman scattering spectrum. For example, the Si-O bond is 800-1300 cm⁻¹. -1 The Al-O bond is 775-1275 cm². -1 The Zr-O bond is 850-1350 cm². -1 The BO bond is 925-1425 cm -1 Each has a peak. In NMR measurements, the radionuclides are limited, but for example, 29 In Si-NMR, the Si-O bond peak is observed in the range of -125 to -50 ppm (reference material is tetramethylsilane). 27 In Al-NMR, the Al-O bond peak is observed in the range of 0-80 ppm (reference material is aluminum chloride). 11 In 1B-NMR, the BO bond peak is observed in the range of -10 to 20 ppm (reference material is boron trifluoride diethyl ether complex). Zr is 91Although Zr-NMR is a known radionuclide, it is not a very common analytical method at present.

[0040] Furthermore, the distance between M and O can be determined from the crystal structure obtained by refining the crystal structure using the Rietveld method. However, the atomic positions obtained by refining the crystal structure using the Rietveld method are average atomic positions. That is, if there are two or more atoms (for example, two types of atoms, O and S) that bond with M (or adjacent atoms), the distance obtained is not the distance between pure M and O, but an average distance corresponding to the abundance of each element. In this specification, "distance between M and O" means the average distance obtained by refining the crystal structure using the Rietveld method.

[0041] The presence of Q0 structure oxide anions in the argyrodite crystal structure improves the heat resistance of the argyrodite crystal, allowing it to remain stable without decomposition even under high-temperature heat treatment. Therefore, heat treatment can be performed while maintaining lithium-ion conductivity as an electrolyte. One method for introducing Q0 structure oxide anions into the crystal structure is to add and mix at least one of a composition containing an oxide having MO bonds and an oxide anion with a Q0 structure having MO bonds to the raw materials for obtaining the argyrodite crystal.

[0042] It is preferable that the Q0 structure oxide anion has a wide potential window. This is because, considering its use as a solid electrolyte in lithium-ion secondary batteries, it is preferable that it is less susceptible to oxidation and reduction during charging and discharging of lithium-ion secondary batteries. In the Q0 structure oxide anion, the M that forms the MO bond preferably contains at least one element (M3) selected from Si, Al, Zr, and B, from the viewpoint of excellent oxidation-reduction resistance of the oxide of M.

[0043] In argyrodite crystals where Z contains O, M and O do not necessarily have to be bonded. M and O may exist, for example, adjacent to each other. In that case, the distance between M and O is preferably 2.3 Å or less. The distance between M and O is more preferably 2.2 Å or less, and even more preferably 2.1 Å or less.

[0044] The crystallite size of argyrodite-type crystals is preferably small from the viewpoint of obtaining good lithium-ion conductivity when the solid electrolyte is finely crushed and made into a battery. Specifically, it is preferably 1000 nm or less, more preferably 500 nm or less, and even more preferably 250 nm or less. There is no particular lower limit to the crystallite size, but it is usually 5 nm or more. Crystallite size can be calculated using the full width at half maximum (FMAX) of the peaks in the XRD pattern and Scherrer's formula.

[0045] (Sulfide solid electrolyte) In sulfide-based solid electrolytes, the proportion of argyrodite-type crystals is preferably 50% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 80% by mass or more, from the viewpoint of achieving high lithium-ion conductivity. Furthermore, there is no particular upper limit, and it may be 100% by mass, but generally it is 99% by mass or less. This can be calculated by including an internal standard material, measuring by XRD or neutron scattering, and then comparing the peak intensity with that of the internal standard material.

[0046] In addition to argyrodite-type crystals, other materials that may be included in this solid electrolyte include Li3PS4, Li4P2S6, Li2S, LiHa (where Ha is at least one halogen element selected from F, Cl, Br, and I), and oxide crystals containing M.

[0047] When used in lithium-ion secondary batteries, this solid electrolyte forms a solid electrolyte layer together with other components such as binders, as needed. Conventionally known binders and other components are used. The content of this solid electrolyte relative to the entire solid electrolyte layer is preferably 80% by mass or more, and more preferably 90% by mass or more.

[0048] 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, a solid electrolyte layer may be formed by dry press molding of solid electrolyte powder or the like on the surface of the positive or negative electrode. In addition, a solid electrolyte layer may be formed on another substrate and then transferred onto the surface of the positive or negative electrode.

[0049] This solid electrolyte 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 such as the positive electrode active material or negative electrode active material, current collector, binder, and conductive additive used in the positive electrode layer or negative electrode layer can be used.

[0050] A lithium-ion secondary battery using this solid electrolyte can be any battery that contains this solid electrolyte, for example, one that 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.

[0051] <Method for producing sulfide-based solid electrolytes> The method for producing this solid electrolyte is not particularly limited as long as it is a method that can produce a solid electrolyte that satisfies the above requirements, but for example, either of the following production methods (I) or (II) is preferred.

[0052] Manufacturing method (I): A method for producing a sulfide-based solid electrolyte used in lithium-ion secondary batteries, A method for producing a sulfide-based solid electrolyte, comprising: mixing raw materials containing Li, M, Z, and Ha and heat-treating them (heat treatment step); obtaining argyrodite-type crystals during the cooling process (cooling step); and at least one of the following (i) and (ii). (i) During the cooling process, the temperature must be maintained at 400°C for at least one minute, rather than from 500°C. (ii) Reheat the obtained argyrodite-type crystals and keep them at 500°C to 400°C for at least 1 minute (reheating step).

[0053] Manufacturing method (II): A method for producing a sulfide-based solid electrolyte used in lithium-ion secondary batteries, A method for producing a sulfide-based solid electrolyte, comprising mixing raw materials containing Li, M, Z, and Ha and heat-treating them (heat treatment step), and obtaining argyrodite-type crystals in a cooling process (cooling step), wherein in the cooling process, the cooling rate from 500°C to 400°C is 500°C / second or more.

[0054] Here, in manufacturing methods (I) and (II), Li, M, Z, and Ha are the compositional formula of the solid electrolyte: Li a -MZ b -Ha c These are the same as Li, M, Z, and Ha in the formula. That is, M is at least one element selected from Na, K, and elements that exist as divalent to pentavalent cations in argyrodite crystals, and Z is at least one element selected from elements that exist as divalent anions in argyrodite crystals. If M is an element that exists as a divalent to pentavalent cation in argyrodite crystals, then that element may also exist as a divalent to pentavalent cation in the raw materials, or it may not exist as a divalent to pentavalent cation in the raw materials. Similarly, Z may or may not exist as a divalent anion in the raw materials.

[0055] (Heat treatment process) In the heat treatment process, raw materials containing Li, M, Z, and Ha are mixed and then heat-treated.

[0056] As raw materials containing Li, M, Z, and Ha, for example, compounds or individual elements containing Li, M, Z, and Ha can be used in appropriate combinations. Specifically, for example, if M mainly contains P and Z mainly contains S, a combination of a Li-containing compound, a P-containing compound, a S-containing compound, and a Ha-containing compound, with other components added as needed, can be used as raw materials.

[0057] Examples of lithium-containing compounds include lithium sulfide (Li2S), lithium oxide (Li2O), lithium carbonate (Li2CO3), lithium hydroxide (LiOH), and lithium sulfate (Li2SO4), and elemental lithium metal can also be used.

[0058] Examples of phosphorus-containing compounds include phosphorus sulfides such as phosphorus trisulfide (P2S3) and phosphorus pentasulfide (P2S5), phosphorus compounds such as lithium phosphate (LiPO3, Li4P2O7, Li3PO4) and sodium phosphate (NaPO3, Na4P2O7, Na3PO4), and elemental phosphorus can also be used.

[0059] Examples of sulfur-containing compounds include lithium sulfide (Li2S), phosphorus sulfide (P2S3, P2S5), and hydrogen sulfide (H2S), and elemental sulfur can also be used.

[0060] Among compounds containing ha, those containing chlorine (Cl) include, for example, lithium chloride (LiCl), phosphorus trichloride (PCl3), phosphorus pentachloride (PCl5), phosphorus tetrachloride (P2Cl4), phosphoryl chloride (POCl3), sulfur dichloride (SCl2), disulfur dichloride (S2Cl2), sodium chloride (NaCl), and boron trichloride (BCl3). Among compounds containing ha (Ha), examples of compounds containing br (bromine) include lithium bromide (LiBr), phosphorus tribromide (PBr3), phosphoryl chloride (POBr3), disulfur dibromide (S2Br2), sodium bromide (NaBr), and boron tribromide (BBr3). When combining these compounds, for example, a combination of lithium sulfide, phosphorus sulfide, and at least one of lithium chloride and lithium bromide is preferred.

[0061] Other components that may be added as needed include compounds containing M other than P as exemplified above, and compounds containing Z other than S as exemplified above. As other components, compounds or elements containing the above-mentioned element (M1) or element (M2) are preferred because they make it easier to obtain argyrodite-type crystals with a relatively small maximum distance between Li-Li ions. Oxides of element (M1) or elements of element (M2) are more preferred, and oxides of element (M1) or elements of element (M2) are more preferred, with Al2O3, SiO2, and B2O3 being even more preferred. When using element (M1) or element (M2) in their elemental form, it is preferable to oxidize them in the heat treatment step described later. Methods of oxidation include adding the element while blowing in oxygen.

[0062] These raw materials are highly unstable in the atmosphere and can decompose upon contact with water, potentially generating hydrogen sulfide gas and undergoing oxidation. Therefore, it is preferable to mix them in an inert atmosphere.

[0063] The raw materials can be mixed using media such as a planetary ball mill, or media-free mixing such as a pin mill, powder agitator, or airflow mixing. The raw materials may be amorphous by mixing before heating.

[0064] After mixing the raw materials, a heat treatment is performed. The heat treatment process includes at least a second heat treatment as described below. The heat treatment process may further include a first heat treatment as described below, if necessary.

[0065] The first heat treatment involves mixing some or all of the raw materials and subjecting the mixture to heating or other treatments to obtain an intermediate or a substance containing the intermediate. If the first heat treatment is performed on only a portion of the raw materials, the remaining raw materials may be added to and mixed with the obtained intermediate or substance containing the intermediate before performing the second heat treatment. Obtaining the intermediate or substance containing the intermediate in advance during the first heat treatment is preferable because it improves the reactivity in the second heat treatment. Furthermore, the intermediate or substance containing the intermediate may have improved atmospheric stability compared to the raw materials, which have poor atmospheric stability and handling properties.

[0066] Examples of intermediates include amorphous sulfide solid electrolyte materials, crystalline sulfide solid electrolyte materials, and argyrodite-type crystals as intermediates. Argyrodite-type crystals as intermediates refer to materials that have an argyrodite-type crystal structure but do not correspond to the argyrodite-type crystals in the solid electrolyte described above. Alternatively, they may contain some of the target argyrodite-type crystals.

[0067] The conditions for the first heat treatment are not particularly limited and can be adjusted as appropriate depending on the composition of the target intermediate. For example, in the first heat treatment, the heat treatment temperature is preferably 250°C or higher, and preferably 650°C or lower. The heat treatment time is preferably 0.1 hours or higher, and preferably 20 hours or lower.

[0068] As an example of the first heat treatment, specifically, if M mainly contains P and Z mainly contains S, the following process can be considered. That is, as the first heat treatment, raw materials containing Li, P, S, and Ha are mixed and heated to obtain a substance containing argyrodite-type crystals as an intermediate. Then, raw materials containing desired M other than P and desired Z other than S are appropriately mixed and heated, and then the second heat treatment can be performed.

[0069] If the first heat treatment is to obtain an argyrodite-type crystal as an intermediate as illustrated, the heat treatment temperature is preferably 400°C or higher, more preferably 450°C or higher, and even more preferably 500°C or higher, from the viewpoint of promoting solid-phase reaction, i.e., crystallization. Furthermore, the heat treatment temperature is preferably less than 600°C, and more preferably 575°C or lower, from the viewpoint of suppressing thermal decomposition.

[0070] Similarly, when obtaining argyrodite-type crystals as intermediates, the heat treatment time is preferably 1 hour or more, more preferably 2 hours or more, and even more preferably 4 hours or more. Furthermore, the heat treatment time is preferably 100 hours or less, more preferably 50 hours or less, and even more preferably 24 hours or less. Such heat treatment is preferably carried out, for example, under an inert gas atmosphere, or under an atmosphere containing sulfur elements such as a hydrogen sulfide gas atmosphere or a sulfur gas atmosphere, or under a vacuum-sealed tube.

[0071] The first heat treatment may be performed multiple times as needed. Alternatively, instead of the first heat treatment, a process may be carried out to obtain an intermediate by mechanochemically reacting a mixture of raw materials using a mixer, or both this process and the first heat treatment may be carried out.

[0072] If the first heat treatment is performed, the second heat treatment is performed after the first heat treatment. Alternatively, if the first heat treatment is not performed, the raw materials are mixed, and then the mixture is subjected to the second heat treatment. The second heat treatment requires obtaining argyrodite-type crystals during the cooling process after the second heat treatment. The second heat treatment is essential in the method for producing this solid electrolyte.

[0073] The conditions for the second heat treatment vary depending on the composition, but from the viewpoint of homogenizing the structure of the resulting electrolyte and obtaining argyrodite-type crystals during the cooling process, the heat treatment temperature is preferably 500°C or higher, more preferably 600°C or higher, even more preferably 650°C or higher, and particularly preferably 700°C or higher. On the other hand, from the viewpoint of preventing thermal decomposition due to unnecessary high temperatures, the heat treatment temperature is preferably 900°C or lower, and more preferably 800°C or lower. The atmosphere for the second heat treatment is preferably an inert gas atmosphere, or an atmosphere containing sulfur elements such as a hydrogen sulfide gas atmosphere or a sulfur gas atmosphere, or it is preferably carried out in a vacuum-sealed tube.

[0074] For similar reasons, the heat treatment time in the second heat treatment is preferably 0.1 hours or more, more preferably 0.25 hours or more, and even more preferably 0.5 hours or more. Furthermore, the heat treatment time is preferably 4 hours or less, more preferably 2 hours or less, and even more preferably 1 hour or less.

[0075] (cooling process) Following the second heat treatment described above, in the cooling process, argyrodite-type crystals are obtained during the cooling of the mixture after the second heat treatment. Here, "obtaining argyrodite-type crystals during the cooling process" means that, unlike cases where argyrodite-type crystals are obtained during the heat treatment (by the heat treatment itself) through solid-phase reactions, etc., the mixture does not have an argyrodite-type crystalline structure during or immediately after the heat treatment, but argyrodite-type crystals are formed during the cooling process as the temperature decreases. Typically, if the mixture of raw materials is melted in the second heat treatment and the melt is cooled to obtain argyrodite-type crystals, then argyrodite-type crystals will be obtained during the cooling process. In other words, the conditions for the second heat treatment described above are typically preferably conditions that allow the mixture of raw materials to be melted.

[0076] In the manufacturing method according to this embodiment, in addition to obtaining argyrodite-type crystals during the cooling process, the maximum distance between Li-Li ions can be made relatively small by adjusting the conditions during the cooling process to a specific range.

[0077] In other words, in manufacturing method (I), while obtaining argyrodite-type crystals during the cooling process of the mixture after the second heat treatment, the maximum distance between Li-Li ions can be made relatively small by satisfying at least one of the following conditions (i) and (ii). (i) During the cooling process, the temperature is kept between 500°C and 400°C for at least one minute. (ii) Reheat the obtained argyrodite-type crystal and keep it at 500°C to 400°C for at least 1 minute (reheating step).

[0078] The reason for this is thought to be that, in (i) or (ii) above, by keeping the lithium ions at a temperature of 400°C for a predetermined period of time or longer, the lithium ions are positioned in a thermodynamically stable location.

[0079] In (i), it is preferable to keep the temperature between 500°C and 400°C for at least one minute, more preferably for at least two minutes, and even more preferably for at least five minutes. Furthermore, from the viewpoint of productivity, the upper limit of the time to be kept is preferably 120 minutes or less, and more preferably 100 minutes or less.

[0080] In (ii), it is preferable to keep the temperature between 500°C and 400°C for at least one minute, more preferably for at least two minutes, and even more preferably for at least five minutes. Furthermore, from the viewpoint of productivity, the upper limit of the time to be kept is preferably 120 minutes or less, and more preferably 100 minutes or less.

[0081] Furthermore, in manufacturing method (II), while obtaining argyrodite-type crystals during the cooling process, the maximum distance between Li-Li ions can be made relatively small by maintaining a cooling rate of 500°C / second or more from 500°C to 400°C.

[0082] The reason for this is thought to be that the cooling rate from 500°C to 400°C is above a predetermined value, which minimizes the movement of lithium during the cooling process. In other words, at high temperatures such as 500°C, a structure is formed that allows lithium ions to move easily, and it is thought that this structure can be reflected by cooling very quickly.

[0083] The cooling rate from 500°C to 400°C is preferably 500°C / second or more, more preferably 1000°C / second or more, and even more preferably 3000°C / second or more. Furthermore, there is no particular upper limit to the cooling rate from 500°C to 400°C, but 10 6 Temperatures below °C / second are practical. [Examples]

[0084] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. Examples 1, 3, 5, 6, and 8 are examples, while Examples 2, 4, and 7 are comparative examples.

[0085] [evaluation] (Lithium-ion conductivity) The lithium-ion conductivity was measured using an AC impedance measuring device (Bio-Logic Sciences Instruments, potentiostat / galvanostat VSP) after the obtained powder was compressed into a compact at a pressure of 380 kN to form a measurement sample. The measurement conditions were: measurement frequency: 100Hz to 1MHz, measurement voltage: 100mV, and measurement temperature: 25℃.

[0086] (Refinement of crystal structure using synchrotron X-ray diffraction measurement and Rietveld method) Synchrotron X-ray diffraction measurements were performed on each example of sulfide-based solid electrolyte, and the crystal structure was refined using the Rietveld method with RIETAN-FP software. This allowed us to identify the lattice constants of the argyrodite-type crystals in the sulfide-based solid electrolyte and the following interionic distances (1) to (4) that are considered to be the migration pathways of lithium ions in the argyrodite-type crystal structure (J.Am.Chem.Soc. 2017, 139, 10909-10918 Figure 1). Of the interionic distances (1) to (4), the longest distance was defined as the maximum distance between Li-Li ions. (1) Doublet (48h-48h) jump (2) 48h-24g jump (3) Intra-cage jump (distance within the cage) (4) Inter-cage jump (distance between cages) Rietveld analysis was performed, and the structure with the lowest Rwp value was identified as the crystal structure for each example. The Rwp value is a reliability factor (R-weighted pattern) that is generally used as a guideline for the entire analysis range in structural refinement fitting using Rietveld analysis. A lower Rwp value is better, and in this analysis, the lowest Rwp value was less than 10% in all cases. Note that depending on the crystal structure, some of the interion distances (1) to (4) shown in Table 1 may not be definable. Interion distances of types that cannot be defined in such crystal structures are indicated as blanks in Table 1.

[0087] The conditions for synchrotron X-ray diffraction measurements are as follows: Measurement method: Powder X-ray diffraction Light energy used for measurement: 17.71 keV Sample shape: 0.3 mm diameter capillary Measurement angle range: 2θ = 0.1 to 95° Step size (Δ2θ) = 0.010° Detectors: Debye-Scherrer camera and two-dimensional semiconductor detector

[0088] (MO distance) For each example of sulfide-based solid electrolyte, the distance between M and O in argyrodite-type crystals was confirmed by measuring the interatomic distance from the crystal structure obtained through refinement of the crystal structure using the Rietveld method.

[0089] [Example 1] 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 to the composition shown in Table 1, and mixed at 400 rpm for 4 hours using a planetary ball mill in the same atmosphere. The mixture was then vacuum-sealed in a carbon-coated quartz tube and heated at 550°C for 5 hours to obtain argyrodite-type crystals. The obtained argyrodite-type crystals were mixed with SiO2 powder (manufactured by Sigma, purity over 99%) in a mortar to obtain a mixture. The amount of SiO2 powder added was adjusted so that the Si and O content ratios met the compositions shown in Table 1. The obtained mixture was pelletized, vacuum-sealed in a carbon-coated quartz tube, and heat-treated at 650°C for 30 minutes to obtain a sulfide-based solid electrolyte containing argyrodite-type crystals during the cooling process. The cooling rate during the cooling process was 20°C / second, and the mixture was cooled to room temperature. Subsequently, it was reheated at 500°C for 1 hour to obtain a sulfide-based solid electrolyte containing argyrodite-type crystals having the composition described in Table 1.

[0090] [Example 2] 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 to achieve the compositions shown in Table 1. The mixture was then mixed in the same atmosphere at 400 rpm for 4 hours using a planetary ball mill. The mixture was then vacuum-sealed in a carbon-coated quartz tube and heated at 550°C for 5 hours to obtain a sulfide-based solid electrolyte containing argyrodite-type crystals having the compositions shown in Table 1.

[0091] [Example 3] Except for adjusting the mixing ratio of the raw materials to achieve the composition shown in Table 1 and performing reheating at 450°C for 1 hour after cooling, a sulfide-based solid electrolyte containing argyrodite-type crystals having the composition shown in Table 1 was obtained in the same manner as in Example 1.

[0092] [Example 4] A sulfide-based solid electrolyte containing argyrodite-type crystals having the composition shown in Table 1 was obtained in the same manner as in Example 3, except that the cooling rate during the cooling process was set to 5°C / second after heat treatment at 650°C for 30 minutes, and no reheating was performed after the cooling process.

[0093] [Example 5] Except for adjusting the mixing ratio of the raw materials to achieve the composition shown in Table 1, a sulfide-based solid electrolyte containing argyrodite-type crystals having the composition shown in Table 1 was obtained in the same manner as in Example 3. In Example 5, lithium bromide powder (manufactured by Sigma, 99.995% purity) was used as the Br source.

[0094] [Example 6] Under a dry nitrogen atmosphere, 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) were weighed to the composition shown in Table 1, and mixed at 400 rpm for 4 hours using a planetary ball mill in the same atmosphere. The mixture was then vacuum-sealed in a carbon-coated quartz tube and heated at 450°C for 5 hours to obtain argyrodite-type crystals. The obtained argyrodite-type crystals were mixed with SiO2 powder (Sigma, purity over 99%) in a mortar to obtain a mixture. The amount of SiO2 powder added was adjusted so that the Si and O content ratios met the compositions shown in Table 1. The obtained mixture was pelletized, vacuum-sealed in a quartz tube, and heat-treated at 750°C for 30 minutes to obtain a sulfide-based solid electrolyte containing argyrodite-type crystals during the cooling process. The cooling rate during the cooling process was 10°C / second, and the mixture was cooled to room temperature. Subsequently, the sulfide-based solid electrolyte was placed in a carbon container and reheated at 450°C for 1 hour in an N2 atmosphere with a dew point of -60°C or lower to obtain a sulfide-based solid electrolyte containing argyrodite-type crystals having the composition described in Table 1.

[0095] [Example 7] 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 to achieve the compositions shown in Table 1. The mixture was then mixed in the same atmosphere at 400 rpm for 10 hours using a planetary ball mill. The mixture was then vacuum-sealed in a carbon-coated quartz tube and heated at 450°C for 10 hours to obtain a sulfide-based solid electrolyte containing argyrodite-type crystals having the compositions shown in Table 1.

[0096] [Example 8] Under a dry nitrogen atmosphere, lithium sulfide powder (Sigma, 99.98% purity), phosphorus pentasulfide powder (Sigma, 99% purity), lithium chloride powder (Sigma, 99.99% purity), lithium bromide powder (Sigma, 99.995% purity), and Al2O3 powder (Sigma Type CG-20) were weighed to the composition shown in Table 1, and mixed at 400 rpm for 4 hours using a planetary ball mill in the same atmosphere. The mixture was then vacuum-sealed in a carbon-coated quartz tube and heat-treated at 700°C for 30 minutes to obtain a sulfide-based solid electrolyte containing argyrodite-type crystals during the cooling process. The cooling rate during the cooling process was 20°C / second, and the mixture was cooled to room temperature. Subsequently, it was transferred to a carbon container and reheated at 450°C for 1 hour under a nitrogen atmosphere with a dew point of -60°C to obtain a sulfide-based solid electrolyte containing argyrodite-type crystals having the composition of Example 8.

[0097] The argyrodite crystal compositions for each example shown in Table 1 are based on atomic fraction values ​​obtained from ICP emission spectroscopy of elements P and S, atomic absorption spectroscopy of element Li, and ion chromatography of elements Cl and Br. These values ​​were used as initial values ​​for Rietveld analysis, and the converged values ​​are listed.

[0098] Table 1 shows the measured lithium ion conductivity, the lattice constant of the argyrodite crystal, and the following interion distances (1) to (4) that are considered to be the lithium ion migration pathways in the argyrodite crystal structure for each example of sulfide-based solid electrolyte. Of the interion distances (1) to (4), those in bold and italics represent the maximum distance between Li-Li ions. In Table 1, "Ha / M" represents the ratio of the Ha content (at%) to the M content (at%) in the argyrodite crystal. (1) Doublet (48h-48h) jump (2) 48h-24g jump (3) Intra-cage jump (distance within the cage) (4) Inter-cage jump (distance between cages)

[0099] Furthermore, in the argyrodite-type crystals of the sulfide-based solid electrolytes in each example, Examples 1, 3-6, and 8 contained O as Z. That is, the argyrodite-type crystals contained O 2- It contained [the following]. In addition, in Examples 1 and 3-6 which contained Si, Si was located at the P site. In Example 8 which contained Al, Al was a trivalent cation and was located at the Li site. Furthermore, in Example 1, there was a Si-O bond, and the distance between Si and O was 2.04 Å or less. And in Example 8, the distance between Al and O was 2.30 Å or less. As mentioned above, the "distance between Si and O" and the "distance between Al and O" here refer to the average distance obtained from the refinement of the crystal structure by the Rietveld method.

[0100] [Table 1]

[0101] Table 1 shows that the sulfide-based solid electrolytes in Examples 1, 3, 5, 6, and 8 exhibited superior lithium ion conductivity due to their smaller maximum distance between Li-Li ions. Furthermore, comparing Examples 1 and 2, despite having equivalent Ha / M values, Example 1, with its smaller maximum distance between Li-Li ions, demonstrated superior lithium ion conductivity. The same trend was observed between Examples 3 and 4, with Example 3 exhibiting superior lithium ion conductivity due to its smaller maximum distance between Li-Li ions.

[0102] 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. 2020-117194 filed on 7 July 2020, Japanese Patent Application No. 2020-130799 filed on 31 July 2020, Japanese Patent Application No. 2020-172693 filed on 13 October 2020, and Japanese Patent Application No. 2021-086406 filed on 21 May 2021, the contents of which are incorporated herein by reference.

Claims

1. A sulfide-based solid electrolyte used in lithium-ion secondary batteries, It contains argyrodite-type crystals, The aforementioned crystal has the chemical formula: Li ａ -M-Z ｂ -Ha ｃ It is represented as, The aforementioned M is at least one element selected from Na, K, and elements present as divalent to pentavalent cations in the crystal. The aforementioned Z is at least one element selected from the elements that exist as divalent anions in the crystal, The aforementioned Z includes S, The aforementioned Z further includes O, The aforementioned Ha is at least one element selected from the group consisting of F, Cl, Br, and I. In the above compositional formula, a, b, and c each represent the ratio of the content (unit: at%) of each element. Satisfying 5 < a < 7, 4 < b < 6 and 1.06 ≤ c ≤ 1.60, The maximum distance between Li-Li ions in the aforementioned crystal is 2.54 Å or less. The crystal comprises an oxide anion having a Q0 structure in which M and O are bonded, and the M constituting the M-O bond comprises at least one element (M3) selected from Si, Al, Zr, and B, wherein the crystal is a sulfide-based solid electrolyte.

2. The aforementioned crystal is M ｎ＋ Includes, Said M ｎ＋ These are 1- to 3-valent cations, Said M ｎ＋ In this, M is at least one element (M1) selected from Al, Ca, Mg, Na, and K. Said M ｎ＋ A sulfide-based solid electrolyte according to claim 1, wherein the sulfide is present at the site of Li.

3. The crystal is the M ｎ＋ The sulfide-based solid electrolyte according to claim 2, wherein the distance between the oxygen and the oxygen is 2.3 Å or less.

4. The aforementioned M mainly includes P, The aforementioned M further comprises at least one element (M2) of Si and B. The sulfide-based solid electrolyte according to any one of claims 1 to 3, wherein the element (M2) is present at the site of P.

5. A method for producing a sulfide-based solid electrolyte according to any one of claims 1 to 4, The process includes mixing raw materials containing Li, the aforementioned M, the aforementioned Z, and the aforementioned Ha, heat-treating them, and obtaining argyrodite-type crystals during the cooling process, and reheating the obtained argyrodite-type crystals and keeping them at a temperature of 500°C to 400°C for at least one minute. The aforementioned M is at least one selected from Na, K, and elements present as divalent to pentavalent cations in the crystal. A method for producing sulfide-based solid electrolytes.

6. A method for producing a sulfide-based solid electrolyte according to claim 5, comprising performing the heat treatment in an atmosphere containing sulfur elements.

7. A method for producing a sulfide-based solid electrolyte according to claim 5 or 6, wherein the temperature of the heat treatment is 500°C or higher.

8. A solid electrolyte layer comprising a sulfide-based solid electrolyte according to any one of claims 1 to 4.

9. A lithium-ion secondary battery comprising a sulfide-based solid electrolyte according to any one of claims 1 to 4.

Images