Sulfide-based solid electrolyte for use in lithium-ion secondary batteries, its manufacturing method, and lithium-ion secondary batteries

Abstract

The present invention relates to a sulfide-based solid electrolyte used in a lithium ion secondary battery, containing a crystal phase and anions present in a crystal structure of the crystal phase, wherein: the crystal phase contains argyrodite-type crystals containing Li, P, S, and Ha; the Ha indicates at least one type of element selected from the group consisting of F, Cl, Br, and I; the anions include oxide anions in a Q0 structure having M-O bonds in which M and O are bonded to each other; and the M indicates at least one type of element selected from the group consisting of metal elements and metalloid elements of Groups 2-14 in the periodic table.

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, and a lithium ion secondary battery. [Background technology]

[0002] Lithium ion secondary batteries are widely used in portable electronic devices such as mobile phones and laptop computers. Conventionally, liquid electrolytes have been used in lithium-ion secondary batteries, but concerns about leakage and fire have led to the need for larger cases for safety reasons. Improvements were also desired regarding the short battery life and narrow operating temperature range.

[0003] In response to this, all-solid-state lithium ion secondary batteries that use solid electrolytes as the electrolyte of lithium ion secondary batteries are attracting attention because they are expected to offer improved safety, high-speed charging and discharging, and a smaller case.

[0004] Solid electrolytes are broadly divided into sulfide-based solid electrolytes and oxide-based solid electrolytes. Sulfide ions have a higher polarizability than oxide ions and exhibit high ionic conductivity. 10 GeP2S 12 LGPS-type crystals such as Li6PS5Cl, argyrodite-type crystals such as Li7P3S 11 LPS crystallized glass and the like are known.

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

[0006] [Patent Document 1] International Publication No. 2015 / 012042 Summary of the Invention [Problem to be solved by the invention]

[0007] However, when sulfide-based solid electrolytes using argyrodite-type crystals are sintered by heat treatment at high temperatures to reduce the resistance between particles, known as grain boundary resistance, the lithium ion conductivity actually decreases due to thermal decomposition. Specifically, for example, Li6PS5Cl decomposes into Li3PS4+Li2S+LiCl. Therefore, there are limitations to the high-temperature heat treatment that can be performed to reduce grain boundary resistance.

[0008] Therefore, an object of the present invention is to provide an argyrodite-type sulfide-based solid electrolyte that can remain stable without decomposition even when subjected to heat treatment at high temperatures, and a method for producing the same. [Means for solving the problem]

[0009] As a result of extensive research, the present inventors have discovered that the above-mentioned problems can be solved by having an oxide anion of a specific structure present in an argyrodite-type crystal structure, and have thus completed the present invention.

[0010] That is, the present invention relates to the following [1] to

[17] . [1] A sulfide-based solid electrolyte for use in a lithium-ion secondary battery, a crystalline phase and an anion present in the crystalline structure of the crystalline phase; The crystal phase includes an argyrodite-type crystal containing Li, P, S, and Ha, where the Ha is at least one element selected from the group consisting of F, Cl, Br, and I, the anion includes an oxide anion having a Q0 structure with an M-O bond in which M and O are bonded, the M is at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 14 of the periodic table, a sulfide-based solid electrolyte. [2] The sulfide-based solid electrolyte according to [1] above, wherein the content (at%) of each element satisfies the relationship of {(M + O) / (Li + P + S + Ha + M + O)} ≤ 0.2. [3] When the ratio of the content (at%) of the elements contained in the argyrodite-type crystal is Li a -P-S b -Ha c represented by, the sulfide-based solid electrolyte according to [1] or [2] above, which satisfies the relationships of 5 < a < 7, 4 < b < 6, and 0 < c < 2. [4] The sulfide-based solid electrolyte according to any one of [1] to [3] above, wherein the total content of the elements of Li, P, S, Ha, M, and O is 90% by mass or more. [5] The sulfide-based solid electrolyte according to any one of [1] to [4] above, wherein the M constituting the oxide anion includes at least one element selected from the group consisting of Si, Al, Zr, and B. [6] When α = (Li + P + S + Ha), β = (Si + Al + Zr + B + O), and γ = (Ha / P) using the content (at%) of each element, the sulfide-based solid electrolyte according to any one of [1] to [5] above, which satisfies the following relational expression. 0 < {β / (α + β)} < (D + 0.1) D = (0.1104 × γ - 0.1133) [7] The sulfide-based solid electrolyte according to any one of [1] to [6] above, wherein the Ha includes Cl and Br, and when the content of Cl in the argyrodite-type crystal is x (at%) and the content of Br is y (at%), (x / y) is 0.1 or more and 10 or less. [8] The Ha contains Cl and Br, and the ratio of the content (at%) of the elements contained in the argyrodite-type crystal is Li a -PS b -Cl c1 -Br c2 The sulfide-based solid electrolyte according to any one of [1] to [7] above, wherein, when expressed as: c1 is 0.1 or more and 1.5 or less, and c2 is 0.1 or more and 1.9 or less. [9] The argyrodite-type crystal contains at least one element selected from the group consisting of Na, K, Mg, and Ca, represented by R, and the ratio of the content (at%) of the element contained in the argyrodite-type crystal is Li a1 -R a2 -PS b -Ha c The sulfide-based solid electrolyte according to any one of [1] to [8] above, wherein a2 is 0.001 to 0.4 when expressed as

[10] The sulfide-based solid electrolyte according to any one of [1] to [9] above, wherein when an all-solid-state lithium ion secondary battery is produced by the following method and subjected to a charge-discharge test, the capacity retention rate, as expressed by the following formula, is 80% or more. (method) (Preparation of positive electrode mixture) A layered rock salt LiCoO powder (volume average particle size: 10 μm) coated with a 7 nm LiNbO coating was used as the positive electrode active material, and 35 parts of the sulfide-based solid electrolyte, 60 parts of the positive electrode active material, and 5 parts of a conductive additive (acetylene black, manufactured by Denka Co., Ltd., HS100) were mixed to prepare a positive electrode composite. The thickness of the LiNbO coating was determined by observation with a transmission electron microscope (TEM). (Fabrication of all-solid-state lithium-ion secondary batteries) 80 mg of the sulfide-based solid electrolyte was placed in a plastic cylinder with a diameter of 10 mm and pressure-molded to form a solid electrolyte layer. Next, 10 mg of the positive electrode composite was placed in the cylinder and pressure-molded again to form a positive electrode layer. Furthermore, indium foil and lithium foil were placed on the opposite side of the positive electrode composite to form a negative electrode layer. In this way, an all-solid-state lithium-ion secondary battery was fabricated, and a charge-discharge test was performed at a confining pressure of 10 kN. (Charge / discharge test) Using the all-solid-state lithium ion secondary battery, a constant current charge / discharge test is carried out for 100 cycles at 25° C. with a charge / discharge current density of 0.1 C and a charge / discharge potential range of 1.9 to 3.7 V. From the results of the charge / discharge test, the capacity retention rate (%) is calculated using the following formula. Capacity retention rate (%) = (discharge capacity at 100th cycle / discharge capacity at 1st cycle) x 100

[11] The sulfide-based solid electrolyte according to any one of [1] to

[10] , wherein an oxide layer is formed at the interface between the solid electrolyte layer and a positive electrode layer when a lithium ion secondary battery including a solid electrolyte layer containing the sulfide-based solid electrolyte is charged and discharged.

[0011]

[12] A lithium ion secondary battery comprising the sulfide-based solid electrolyte according to any one of [1] to

[11] above.

[0012]

[13] A method for producing a sulfide-based solid electrolyte for use in a lithium-ion secondary battery, comprising: Mixing raw materials containing Li, P, S, and Ha, and crystallizing them by heating to obtain argyrodite-type crystals; Obtaining a mixture in which the argyrodite-type crystal is mixed with an oxide having an MO bond in which M and O are bonded; and and heat-treating the mixture to obtain argyrodite-type crystals in which oxide anions of a Q0 structure having an MO bond are present in the crystal structure. the Ha is at least one element selected from the group consisting of F, Cl, Br, and I; M is at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 14 of the periodic table, A method for producing a sulfide-based solid electrolyte containing argyrodite-type crystals in which oxide anions with a Q0 structure and MO bonds exist within the crystal structure.

[14] A method for producing a sulfide-based solid electrolyte for use in a lithium-ion secondary battery, comprising: A raw material containing Li, P, S, and Ha is mixed with an oxide having an MO bond in which M and O are bonded to obtain a raw material composition; heat-treating the raw material composition to obtain argyrodite-type crystals in which oxide anions of a Q0 structure having an MO bond exist in the crystal structure, the Ha is at least one element selected from the group consisting of F, Cl, Br, and I; M is at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 14 of the periodic table, A method for producing a sulfide-based solid electrolyte containing argyrodite-type crystals in which oxide anions with a Q0 structure and MO bonds exist within the crystal structure.

[15] A method for producing a sulfide-based solid electrolyte for use in a lithium-ion secondary battery, comprising: reacting raw materials containing Li, P, S and Ha to obtain an intermediate product; Obtaining a mixture by mixing the intermediate product with an oxide having an MO bond in which M and O are bonded; and and heat-treating the mixture to obtain argyrodite-type crystals in which oxide anions of a Q0 structure having an MO bond are present in the crystal structure. the Ha is at least one element selected from the group consisting of F, Cl, Br, and I; M is at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 14 of the periodic table, A method for producing a sulfide-based solid electrolyte containing argyrodite-type crystals in which oxide anions with a Q0 structure and MO bonds exist within the crystal structure.

[16] A method for producing a sulfide-based solid electrolyte for use in a lithium-ion secondary battery, comprising: Mixing raw materials containing Li, P, S, and Ha, and crystallizing them by heating to obtain argyrodite-type crystals; Obtaining a composition containing an oxide anion of a Q0 structure having an MO bond in which M and O are bonded; Obtaining a mixture by mixing the argyrodite-type crystals with the composition; and and heat-treating the mixture to obtain argyrodite-type crystals in which oxide anions of a Q0 structure having an MO bond are present in the crystal structure. the Ha is at least one element selected from the group consisting of F, Cl, Br, and I; M is at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 14 of the periodic table, A method for producing a sulfide-based solid electrolyte containing argyrodite-type crystals in which oxide anions with a Q0 structure and MO bonds exist within the crystal structure.

[17] The method for producing a sulfide-based solid electrolyte according to any one of

[13] to

[16] above, wherein the heat treatment is carried out at a temperature of 500° C. or higher. [Effects of the Invention]

[0013] The sulfide-based solid electrolyte according to the present invention can remain stable without decomposition even when subjected to high-temperature heat treatment, thereby reducing grain boundary resistance due to sintering. As a result, improved lithium ion conductivity can be expected, making it extremely useful as a solid electrolyte for lithium ion secondary batteries. This improves the battery characteristics of lithium ion secondary batteries. DETAILED DESCRIPTION OF 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 as desired without departing from the spirit of the present invention. Furthermore, the symbol "to" indicating a numerical range is used to mean that the numerical values ​​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, may be simply referred to as "solid electrolyte") is used in a lithium-ion secondary battery, and includes a crystalline phase and an anion present in the crystalline structure of the crystalline phase. The crystalline phase contains argyrodite-type crystals containing Li, P, S, and Ha, where Ha is at least one element selected from the group consisting of F, Cl, Br, and I. The anion includes an oxide anion of a Q0 structure having an MO bond in which M and O are bonded (hereinafter, this may be simply referred to as an "oxide anion" or an "oxide anion of a Q0 structure"), where M is at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 14 of the periodic table.

[0016] (oxide anion) The presence of Q0 oxide anions in the crystalline phase of the solid electrolyte improves the heat resistance of the argyrodite-type crystals, allowing them to remain stable without decomposition even when heat-treated at high temperatures. This allows for heat treatment that reduces grain boundary resistance while maintaining lithium ion conductivity as an electrolyte.

[0017] The Q0 structure is a structure in which all oxygen atoms bonded to the central cation M are non-bridging oxygen atoms. For example, when M is Si, the oxide SiO2 is formed by bonding to silicate ions, i.e., SiO4 4- This means that it exists as an oxide anion.

[0018] The presence of Q0 oxide anions in a crystal structure can also be described as a state in which they are fixed in the crystal structure. Specifically, this refers to the case in which an argyrodite-type crystal phase is formed with Q0 oxide anions as crystal nuclei, the case in which Q0 oxide anions are incorporated into the argyrodite-type crystal structure, or both. This fixation of Q0 oxide anions is a phenomenon unique to argyrodite-type crystals containing at least the three elements Li, S, and Ha. On the other hand, Li-PS ternary crystals containing the three elements Li, P, and S but not Ha, such as Li7P3S 11 and LGPS-type crystals, e.g., Li 10 GeP2S 12 In such cases, the oxide anion of the Q0 structure is not properly fixed in the crystal structure.

[0019] It is unclear why the presence of Q0 structure oxide anions allows argyrodite-type crystals to exist stably without decomposition even at high temperatures. However, it is thought that the Q0 structure oxide anions act as crystal nuclei to form an argyrodite-type crystalline phase, thereby stabilizing crystallization, or that the Q0 structure oxide anions are incorporated into the argyrodite-type crystal structure, resulting in stabilization. Furthermore, when both Q0 structure oxide anions that act as crystal nuclei and Q0 structure oxide anions that are incorporated into the crystal structure are present, it is speculated that the combined effects of these effects allow argyrodite-type crystals to exist more stably.

[0020] For example, if the argyrodite-type crystal is Li6PS5Cl, the anion that constitutes Li6PS5Cl is PS4 3- At this anion site, SiO4 4- It is thought that the inclusion of oxide anions with a Q0 structure such as SiO4 strengthens the electrostatic attraction and improves the heat resistance of the crystal. In addition, lithium ion conductive argyrodite-type crystals are often in a high-temperature stable phase, and 4- It is speculated that the inclusion of oxide anions with a Q0 structure such as these provides a thermodynamic stabilizing effect. Furthermore, it is speculated that the formation or re-formation of argyrodite-type crystals using oxide anions with a Q0 structure as crystal nuclei allows the formation of small, dense crystallites.

[0021] The elements constituting the oxide anion may include M and O, where M is at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 14 of the periodic table. The oxide anion may be of one type only, or may contain multiple types. Metal elements of Groups 2 to 14 are elements of Groups 2 to 12, elements of Group 13 other than B, and elements of Group 14 other than C, Si, and Ge in the periodic table. The metalloid elements of Groups 2 to 14 are B, Si, and Ge among the elements of Groups 13 and 14 of the periodic table.

[0022] Even if an element M having a weak MO bonding strength is selected, the above-mentioned effect of an oxide anion having a Q0 structure can be obtained, but there is a possibility that oxygen may be lost or volatilized during the heating process, or that oxygen atoms may be incorporated into unexpected anion sites in the crystal, etc. Therefore, it is preferable to include an element M having an MO bonding strength of 400 kJ / mol or more, more preferably an element M having an MO bonding strength of 450 kJ / mol or more, and even more preferably an element M having an MO bonding strength of 500 kJ / mol or more. There is no particular upper limit to the MO bond strength, but it is generally 1000 kJ / mol or less. For the above reasons, M preferably contains at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 14 of the periodic table, and more preferably contains at least one element selected from the group consisting of Al, B, Ge, La, Ta, Nb, Ti, Si, Sn, V, Y, and Zr. In this specification, the MO bond strength refers to bond dissociation energy, and each bond has its own unique bond dissociation energy. The calculation method is defined as the standard enthalpy change (ΔHf298) in the dissociation reaction of MO → M + O, as ΔHf298 = ΔHf298(M) + ΔHf298(O) - ΔHf298(MO).

[0023] In addition, it is preferable that the oxide anion has a wide potential window, because, in consideration of its use as a solid electrolyte in a lithium ion secondary battery, it is preferable that it is not easily oxidized or reduced during charging and discharging of the lithium ion secondary battery. For the above reasons, the stable M of M is n+ Regarding ions, M n+ It is more preferable that M contains at least one element selected from the group of elements having a reduction potential of −0.5 V (vs. SHE) or less. Furthermore, from the viewpoint of the oxidation-reduction resistance of the oxide of M, it is even more preferable that M contains at least one element selected from the group consisting of Si, Al, Zr, and B.

[0024] The presence of MO bonds can be confirmed by Raman spectroscopy or nuclear magnetic resonance (NMR) measurements. In Raman spectroscopy, the presence of MO bonds is confirmed in the range of 750–1500 cm -1 For example, the Si-O bond can be confirmed as a Raman scattering spectrum of 800 to 1300 cm -1 , Al-O bond is 775~1275cm -1 , Zr-O bond is 850~1350cm -1 , BO bond is 925~1425cm -1 have peaks at In NMR measurement, the nuclides are limited, for example, 29 In Si-NMR, the peak of the Si-O bond is observed in the range of -125 to -50 ppm (standard substance is tetramethylsilane). 27 In Al-NMR, the peak of the Al-O bond is observed in the range of 0 to 80 ppm (standard substance is aluminum chloride). 11 In B-NMR, the peak of the BO bond is seen in the range of -10 to 20 ppm (the reference substance is boron trifluoride diethyl ether complex). 91 Although Zr-NMR is available as a nuclide, it is not a commonly used analytical method at present.

[0025] The Q0 structure of the oxide anion can be confirmed by Raman spectroscopy or nuclear magnetic resonance (NMR) measurements. In Raman spectroscopy, for example, the Si-O bond in the Q0 structure has a wavelength of 800-900 cm -1 , the Al-O bond in the Q0 structure is 775-875 cm -1 , the Zr-O bond in the Q0 structure is 850-950 cm -1 , the B-O bond of the Q0 structure is 925-1025 cm -1 have peaks at In NMR measurements, for example, 29 In Si-NMR, the peak of the Si-O bond of the Q0 structure is observed in the range of -70 to -60 ppm (standard substance is tetramethylsilane). 27 In Al-NMR, the peak of the Al-O bond of the Q0 structure is observed in the range of 50 to 80 ppm (standard substance is aluminum chloride). 11In B-NMR, the peak of the BO bond of the Q0 structure is observed in the range of -5 to 15 ppm (the reference substance is boron trifluoride diethyl ether complex).

[0026] The presence of oxide anions 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.

[0027] The oxide anions in the solid electrolyte do not all need to have the Q0 structure, and they do not all need to exist in the argyrodite-type crystal structure. The effects of the present invention can be obtained even if the ratio of the Q0 structure oxide anions to the total amount of oxide anions is small.

[0028] (Argyrodite crystal) The argyrodite-type crystal contains Li, P, S, and Ha, where Ha is at least one halogen element selected from the group consisting of F, Cl, Br, and I. The argyrodite-type crystal may contain only one type of crystal with the same composition, or two or more types with different compositions.

[0029] The crystal structure can be analyzed from the X-ray powder diffraction (XRD) pattern. If the XRD pattern has peaks at 2θ=15.7±0.5° and 30.2±0.5°, the crystal is said to be of the argyrodite type. In addition to the above, the XRD pattern preferably also has peaks at 2θ=18.0±0.5°, and more preferably has peaks at 2θ=25.7±0.5°.

[0030] The ratio of the content (at%) of elements that make up the argyrodite-type crystal (the ratio of the content (at%) of elements contained in the argyrodite-type crystal) is Li a -PS b -Ha cWhen expressed by [the formula], it is preferable that the relationships of 5 < a < 7, 4 < b < 6 and 0 < c < 2 are satisfied because the crystal is likely to be of the alvite type. It is more preferable that such an elemental ratio satisfies the relationships of 5.1 < a < 6.3, 4 < b < 5.3 and 0.7 < c < 1.9, and it is even more preferable that the relationships of 5.2 < a < 6.2, 4.1 < b < 5.2 and 0.8 < c < 1.8 are satisfied. That is, for a, 5 < a < 7 is preferable, 5.1 < a < 6.3 is more preferable, and 5.2 < a < 6.2 is even more preferable. For b, 4 < b < 6 is preferable, 4 < b < 5.3 is more preferable, and 4.1 < b < 5.2 is even more preferable. For c, 0 < c < 2 is preferable, 0.7 < c < 1.9 is preferable, and 0.8 < c < 1.8 is even more preferable. In this specification, the "elemental ratio" means the ratio of the content (at%) of the elements. The preferred crystal structure is cubic (e.g., F-43m), but there may be present hexagonal, tetragonal, orthorhombic, monoclinic, etc. with reduced symmetry, and even triclinic, etc. with further reduced symmetry.

[0031] The halogen element represented by Ha is at least one selected from the group consisting of F, Cl, Br, and I. However, since the crystal is likely to be of the alvite type, it is preferable to contain at least one of Cl and Br, more preferably to contain Cl, and even more preferably to be Cl alone or a mixture of Cl and Br.

[0032] When Ha contains Cl and Br, where x (at%) is the Cl content in the argyrodite-type crystal and y (at%) is the Br content, (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 is likely to be improved. 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. Furthermore, when (x / y) satisfies the above range, the cycle characteristics of lithium ion secondary batteries are likely to be improved.

[0033] In addition, when Ha contains Cl and Br, the ratio of the content (at%) of the elements constituting the argyrodite-type crystal is Li a -PS b -Cl c1 -Br c2 When expressed as above, c1 is preferably 0.1 or more, more preferably 0.3 or more, and even more preferably 0.5 or more. c1 is preferably 1.5 or less, more preferably 1.4 or less, and even more preferably 1.3 or less. c2 is preferably 0.1 or more, more preferably 0.3 or more, and even more preferably 0.5 or more. c2 is preferably 1.9 or less, more preferably 1.6 or less, and even more preferably 1.4 or less. When c1 and c2 each satisfy the above ranges, the proportion of halide ions in the crystal is optimized, and a stable argyrodite-type crystal is obtained while reducing the interaction between anions and lithium ions in the crystal. This tends to improve the lithium ion conductivity of the solid electrolyte after heat treatment. Furthermore, when c1 and c2 satisfy the above ranges, the cycle characteristics of the lithium ion secondary battery are likely to be improved. Here, it is preferable that a, b, and (c1+c2) satisfy the same relationship as that of a, b, and c described above.

[0034] The crystallite size of the argyrodite-type crystals is preferably small in order to obtain good lithium ion conductivity when the solid electrolyte is finely pulverized and made into a battery. Specifically, the crystallite size is preferably 1000 nm or less, more preferably 500 nm or less, and even more preferably 250 nm or less. The lower limit of the crystallite size is not particularly limited, but is usually 5 nm or more. The crystallite size can be calculated using the half-width of the peak in the XRD pattern and Scherrer's equation.

[0035] The raw materials for argyrodite-type crystals will be described in detail later, but a mixture containing lithium sulfide (LiS) is preferably used as such a raw material. Here, it is widely known that lithium sulfide is produced from lithium hydroxide (LiOH), which 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. When argyrodite-type crystals are produced using raw materials containing R, the argyrodite-type crystals may also contain R. Here, in order to reduce the R content in the argyrodite-type crystals, it is necessary to use high-purity raw materials, which may increase production costs. The present inventors have investigated the influence of R contained in argyrodite-type crystals on the physical properties of a solid electrolyte and have found that the lithium ion conductivity and heat resistance of the solid electrolyte are not affected as long as the content ratio of R is within a predetermined range.

[0036] That is, the argyrodite-type crystal may contain at least one element selected from the group consisting of Na, K, Mg, and Ca, represented by R. When R is contained, the ratio of the content (at%) of the elements constituting the argyrodite-type crystal is expressed as Li a1 -R a2 -PS b -Ha cIn view of suppressing the manufacturing cost, a2 is preferably 0.001 or more, more preferably 0.01 or more, and even more preferably 0.02 or more. In view of suppressing the decrease in lithium ion conductivity, a2 is preferably 0.4 or less, more preferably 0.3 or less, and even more preferably 0.2 or less. The argyrodite-type crystal does not need to contain R. Here, it is preferable that a1, b, and c satisfy the same relationship as the above-mentioned a, b, and c, respectively. However, the value of a1 may be smaller than the above-mentioned preferable range of a depending on the value of a2. In that case, it is preferable that the value of (a1 + a2) satisfies 5<(a1 + a2)<7.

[0037] Among Na, K, Mg, and Ca, Mg and Ca can also be contained in the solid electrolyte as M constituting the oxide anions described above. When the solid electrolyte contains at least one of Mg and Ca, whether these elements are contained as M or R can be determined by analyzing the crystal structure through Rietveld analysis of the XRD pattern. When the above elements are contained as R, Mg and Ca are present in the crystal structure and O is present nearby. 2- The result is that there is no O 2- The absence of O within the second nearest neighboring elements of Mg and Ca 2- It means that does not exist.

[0038] (solid electrolyte) The total content of elements constituting the crystalline phase including argyrodite-type crystals and oxide anions relative to all components constituting the solid electrolyte is preferably 90% by mass or more, more preferably 92% by mass or more, and even more preferably 94% by mass or more, from the viewpoint of achieving high ionic conductivity. The upper limit of the total content is not particularly limited and may be 100% by mass. The crystalline phase may contain amorphous phase in addition to argyrodite-type crystals. The solid electrolyte may also contain another argyrodite-type crystalline phase consisting of at least one of Li, P, S, and Ha. The total content is, for example, the total content of the elements Li, P, S, Ha, M, and O, and when the argyrodite-type crystal contains R, the total also includes the content of R. In this specification, the content of Ha is the total content of F, Cl, Br, and I. The content of each element and their total content can be determined by composition analysis using ICP emission spectrometry, atomic absorption spectrometry, ion chromatography, or the like.

[0039] In order to achieve high lithium ion conductivity, the proportion of argyrodite-type crystals in the solid electrolyte is preferably 50% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 80% by mass or more. The upper limit is not particularly limited and may be 100% by mass, but is generally 99% by mass or less. The proportion of argyrodite-type crystals can be calculated by adding an internal standard substance, measuring by XRD or neutron scattering, and then comparing the peak intensity with that of the internal standard substance.

[0040] In addition to the crystalline phase containing argyrodite-type crystals and the oxide anions, the solid electrolyte may contain Li3PS4, Li4P2S6, Li2S, LiHa (Ha is at least one halogen element selected from F, Cl, Br, and I), etc.

[0041] The ratio of the total (at%) of elements constituting the oxide anion to the total (at%) of elements constituting the argyrodite-type crystal and the oxide anion, i.e., the element ratio, can be expressed, for example, as {(M+O) / (Li+P+S+Ha+M+O)}. The ratio {(M+O) / (Li+P+S+Ha+M+O)} is preferably 0.2 or less, more preferably 0.18 or less, and even more preferably 0.15 or less, because a large amount of oxide anions reduces ionic conductivity and increases susceptibility to oxidation during electrolyte preparation. Even a small amount of oxide anions can have a stabilizing effect on high-temperature heat treatment, and to effectively obtain this effect, the ratio is preferably 0.0005 or more. To obtain higher conductivity, the ratio is preferably 0.05 or more, and even more preferably 0.01 or more. For example, when the argyrodite-type crystal contains R, the element ratio is expressed as {(M+O) / (Li+P+S+Ha+R+M+O)}.

[0042] Furthermore, the ratio of the total (at %) of elements constituting the oxide anion to the total (at %) of elements constituting the argyrodite-type crystal and oxide anion depends on the composition, particularly the Ha content. The ratio of the element content (at%) of argyrodite-type crystals is Li 7-γ PS 6-γ Ha γ When the value of γ is small, there are many S (sulfur) atoms with weak bonding strength in the isolated anion sites in the argyrodite-type crystal structure. As a result, it becomes more susceptible to oxidative decomposition, making it difficult to add many oxide anions. On the other hand, when the value of γ is large, crystals of lithium halide such as LiCl are more likely to precipitate. Therefore, it is preferable to add more oxide anions to stabilize the material.

[0043] When producing a sulfide-based solid electrolyte, the oxide added to form the oxide anion may be in the form of a nano-sized fine powder, a micro-sized powder, or coarse particles. It may also be intentionally mixed in from the container used during production. The oxide does not need to have a Q0 structure when added during production of the sulfide-based solid electrolyte.

[0044] As an index representing the above relationship, when the contents (at%) of each element are used, α=(Li+P+S+Ha), β=(Si+Al+Zr+B+O), and γ=(Ha / P), it is preferable that the following relationship be satisfied: When the argyrodite-type crystal contains R, α=(Li+P+S+Ha+R). 0<{β / (α+β)}<(D+0.1) D = (0.1104 × γ - 0.1133)

[0045] The value expressed by {β / (α+β)} is the same as the ratio expressed by {(M+O) / (Li+P+S+Ha+M+O)}, but the above relational expression means that the amount of oxide anions optimal for achieving the effects of the present invention increases as the amount of Ha contained in the solid electrolyte increases. The function expressing D indicates the optimal value of {β / (α+β)} for each element ratio of Ha to P, and is a function derived from the results of the Examples and Comparative Examples described below.

[0046] It is more preferable that the value represented by {β / (α+β)} satisfies the following relational expression. 0<{β / (α+β)} <E E=(0.2018×γ-0.1608)

[0047] As an index of heat resistance, the thermal decomposition resistance test temperature of the solid electrolyte is preferably 500° C. or higher, more preferably 600° C. or higher, and even more preferably 650° C. or higher. There is no particular upper limit, but it is usually 900° C. or lower. The thermal decomposition resistance test for a solid electrolyte can be performed by placing the solid electrolyte in a container that does not react with the solid electrolyte, heat-treating it at a specified temperature for 10 to 60 minutes, and examining the change in ionic conductivity before and after the heat treatment to evaluate its heat resistance. It is preferable that the ionic conductivity does not change before and after the heat treatment; if the ionic conductivity after the heat treatment is less than half of what it was before the heat treatment, it can be said to have low thermal decomposition resistance, i.e., low heat resistance.

[0048] As an index of the heat resistance, the lithium ion conductivity when heat treatment is carried out at a high temperature will be described in detail below. For example, if the lithium ion conductivity of an argyrodite-type crystal of a specific composition is σ1, and the lithium ion conductivity σ2 after heat treatment at a specific temperature is lower than σ1 (σ1 > σ2), this indicates that at least a portion of the crystal has undergone thermal decomposition. In contrast, if the lithium ion conductivity σ3 of a solid electrolyte according to this embodiment containing argyrodite-type crystals of the same composition after heat treatment under the same conditions is higher than σ2 (σ3 > σ2), it can be determined that the thermal decomposition has been suppressed, and the solid electrolyte has excellent heat resistance. Furthermore, if σ3 is higher than σ1 (σ3 > σ1), it can be determined that the solid electrolyte has been reconstructed into an argyrodite-type crystal with even higher lithium ion conductivity by heat treatment. Furthermore, using argyrodite-type crystals of such a composition is believed to achieve both improved bulk lithium ion conductivity and reduced grain boundary resistance during the sintering process. In this specification, the lithium ion conductivity means the lithium ion conductivity at 25° C., and is determined from a Nyquist plot obtained by AC impedance measurement.

[0049] The solid electrolyte according to this embodiment satisfies the relationship σ3>σ2. The solid electrolyte according to this embodiment preferably satisfies the relationship σ3≧1.5×σ2, more preferably satisfies the relationship σ3≧3×σ2, and the higher σ3, the better. In addition, the solid electrolyte according to this embodiment preferably satisfies the above relationship σ3>σ1.

[0050] Although the solid electrolyte according to this embodiment exhibits high heat resistance, it is preferable to control the cooling rate after heat treatment. From the viewpoint of further reducing the crystallite size, it is preferable to cool at a cooling rate of 1°C / sec or more. Furthermore, from the viewpoint of crystal growth during cooling, it is preferable to cool at a cooling rate of 1°C / sec or less. In this way, it is preferable to change the cooling rate depending on the desired physical properties.

[0051] When used in a lithium ion secondary battery, the solid electrolyte forms a solid electrolyte layer together with other components such as a binder, if necessary. Conventionally known binders and other components are used. The content of the solid electrolyte in the entire solid electrolyte layer is preferably 80 mass % or more, and more preferably 90 mass % or more.

[0052] The solid electrolyte layer can be formed by 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 optionally pressed to form the solid electrolyte layer. If necessary, the binder may be removed by applying heat. The thickness of the solid electrolyte layer can be easily adjusted by adjusting the amount of the slurry applied, etc. Furthermore, instead of wet molding, the solid electrolyte layer may be formed by dry press molding of a solid electrolyte powder or the like on the surface of a positive electrode or a negative electrode, etc. Alternatively, the solid electrolyte layer may be formed on another substrate and then transferred onto the surface of a positive electrode or a negative electrode, etc.

[0053] The solid electrolyte may be mixed with a positive electrode active material or a negative electrode active material to form a positive electrode layer or a negative electrode layer. The positive electrode active material or a negative electrode active material, current collector, binder, conductive additive, etc. used in the positive electrode layer or the negative electrode layer may be any known material.

[0054] A lithium ion secondary battery using a solid electrolyte includes the solid electrolyte layer, a positive electrode layer, and a negative electrode layer. The lithium ion secondary battery may be made of a conventionally known material for its exterior. The shape of the lithium ion secondary battery may be made of a conventionally known material, such as a coin, sheet (film), folded, wound, cylindrical with a bottom, or button, and may be selected appropriately depending on the application.

[0055] The lithium ion secondary battery containing the sulfide-based solid electrolyte according to this embodiment has excellent cycle characteristics. For example, when an all-solid-state lithium ion secondary battery is fabricated by the following method and subjected to a charge-discharge test, the capacity retention rate represented by the following formula is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. The higher the capacity retention rate, the better, and it may even be 100%.

[0056] (method) (Preparation of positive electrode mixture) A layered rock salt LiCoO powder (volume average particle size: 10 μm) coated with a 7 nm LiNbO coating was used as the positive electrode active material, and 35 parts of the sulfide-based solid electrolyte according to this embodiment, 60 parts of the positive electrode active material, and 5 parts of a conductive additive (acetylene black, manufactured by Denka Co., Ltd., HS100) were mixed to prepare a positive electrode composite. The thickness of the LiNbO coating was determined by observation with a transmission electron microscope (TEM).

[0057] (Fabrication of all-solid-state lithium-ion secondary batteries) 80 mg of the sulfide-based solid electrolyte according to this embodiment is placed in a plastic cylinder with a diameter of 10 mm and pressure-molded to form a solid electrolyte layer. Next, 10 mg of the positive electrode composite prepared above is placed in the same cylinder and pressure-molded again to form a positive electrode layer. Furthermore, indium foil and lithium foil are placed on the opposite side of the positive electrode composite to form a negative electrode layer. In this way, an all-solid-state lithium-ion secondary battery is prepared, and a charge-discharge test is performed at a confining pressure of 10 kN. The sulfide-based solid electrolyte used in the positive electrode mixture and the solid electrolyte layer is preferably one that has been pulverized into a powder form by pulverization or the like.

[0058] (Charge / discharge test) Using the fabricated all-solid-state lithium-ion secondary battery, a constant current charge / discharge test is carried out for 100 cycles at 25°C with a charge / discharge current density of 0.1 C and a charge / discharge potential range of 1.9-3.7 V. From the results of the charge / discharge test, the capacity retention rate (%) is calculated using the following formula. Capacity retention rate (%) = (discharge capacity at 100th cycle / discharge capacity at 1st cycle) x 100

[0059] The higher the capacity retention rate, the more the deterioration of battery characteristics during repeated charge and discharge is suppressed, and the better the cycle characteristics can be determined. The reason why a lithium-ion secondary battery containing a sulfide-based solid electrolyte according to this embodiment has excellent cycle characteristics is thought to be that, for example, in a charge and discharge test under the above conditions, a thin oxide layer derived from oxygen in the solid electrolyte is formed at the interface between the solid electrolyte layer and the positive electrode layer. This makes it difficult for sulfide ions in the solid electrolyte to decompose, which is thought to improve the cycle characteristics. Even when the sulfide-based solid electrolyte according to this embodiment is used in the solid electrolyte layer of a lithium-ion secondary battery having a different configuration from the all-solid-state lithium-ion secondary battery used in the charge and discharge test, a similar oxide layer can be formed by charging and discharging the battery. In particular, when Ha in the argyrodite-type crystal contains Cl and Br and the element ratios thereof are in the preferred ranges described above, the cycle characteristics are more likely to be improved, which is preferable.

[0060] <Method for producing sulfide-based solid electrolyte> The method for producing the sulfide-based solid electrolyte used in the lithium-ion secondary battery according to this embodiment is not particularly limited as long as it can produce an argyrodite-type crystal containing Li, P, S, and Ha, in which an oxide anion with a Q structure having an MO bond in which M and O are bonded in the crystal structure is present. Specifically, for example, by subjecting a mixture of at least one of a plurality of compounds that serve as materials for argyrodite-type crystals and argyrodite-type crystals, and at least one of a composition containing an oxide having an MO bond and an oxide anion having a Q0 structure and having an MO bond, an argyrodite-type crystal having an oxide anion having a Q0 structure and having an MO bond in its crystal structure is obtained.

[0061] The timing of the heat treatment is not particularly limited, and the sulfide-based solid electrolyte according to this embodiment can be obtained by using, for example, any of the following production methods (i) to (iv).

[0062] (Manufacturing method (i)) (Step i-1) a step of mixing raw materials containing Li, P, S, and Ha, and crystallizing the mixture by heating to obtain argyrodite-type crystals; (Step i-2) A step of obtaining a mixture by mixing the obtained argyrodite-type crystals with an oxide having an MO bond in which M and O are bonded; and (Step i-3) The resulting mixture is heat-treated to obtain argyrodite-type crystals in which oxide anions of a Q0 structure having MO bonds exist in the crystal structure. Ha is at least one element selected from the group consisting of F, Cl, Br, and I. M is at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 14 of the periodic table.

[0063] (Manufacturing method (ii)) (Step ii-1) A step of mixing a raw material containing Li, P, S, and Ha with an oxide having an MO bond in which M and O are bonded to obtain a raw material composition; and (Step ii-2) The raw material composition thus obtained is heat-treated to obtain argyrodite-type crystals in which oxide anions of a Q0 structure having MO bonds exist in the crystal structure. Ha is at least one element selected from the group consisting of F, Cl, Br, and I. M is at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 14 of the periodic table.

[0064] (Manufacturing method (iii)) (Step iii-1) A step of reacting raw materials containing Li, P, S, and Ha to obtain an intermediate product; (Step iii-2) A step of mixing the obtained intermediate product with an oxide having an MO bond in which M and O are bonded to obtain a mixture; and (Step iii-3) A step of heat-treating the obtained mixture to obtain argyrodite-type crystals in which oxide anions of a Q0 structure having MO bonds exist in the crystal structure. Ha is at least one element selected from the group consisting of F, Cl, Br, and I. M is at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 14 of the periodic table.

[0065] (Manufacturing method (iv)) (Step iv-1) A step of mixing raw materials containing Li, P, S, and Ha, and crystallizing the mixture by heating to obtain argyrodite-type crystals; (Step iv-2) A step of obtaining a composition containing an oxide anion having a QO structure and an MO bond in which M and O are bonded; (Step iv-3) A step of obtaining a mixture by mixing the obtained argyrodite-type crystals with the obtained composition; and (Step iv-4) The resulting mixture is heat-treated to obtain argyrodite-type crystals in which oxide anions of a Q0 structure having MO bonds exist in the crystal structure. (Step iv-1) and (Step iv-2) may be carried out in either order, or may be carried out simultaneously. Ha is at least one element selected from the group consisting of F, Cl, Br, and I. M is at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 14 of the periodic table.

[0066] The raw material containing Li, P, S, and Ha in (step i-1) of the above production method (i) may be any known material for obtaining argyrodite-type crystals containing Li, P, S, and Ha. For example, a mixture of a compound containing Li (lithium), a compound containing P (phosphorus), a compound containing S (sulfur), and a compound containing Ha (halogen) can be mentioned.

[0067] Examples of compounds containing Li include lithium compounds such as lithium sulfide (Li2S), lithium oxide (Li2O), lithium carbonate (Li2CO3), lithium hydroxide (LiOH), and lithium sulfate (Li2SO4), as well as elemental lithium metal. Examples of compounds containing P include phosphorus sulfides such as diphosphorus trisulfide (P2S3) and diphosphorus pentasulfide (P2S5), phosphorus compounds such as lithium phosphate (LiPO3, Li4P2O7, Li3PO4) and sodium phosphate (NaPO3, Na4P2O7, Na3PO4), and elemental phosphorus. Examples of compounds containing S include the above-mentioned lithium sulfide (Li2S), the above-mentioned phosphorus sulfides (P2S3, P2S5), and hydrogen sulfide (H2S), and elemental sulfur can also be used. Among the compounds containing Ha, examples of compounds containing Cl (chlorine) include lithium chloride (LiCl), phosphorus trichloride (PCl3), phosphorus pentachloride (PCl5), diphosphorus tetrachloride (P2Cl4), phosphoryl chloride (POCl3), sulfur dichloride (SCl2), disulfur dichloride (S2Cl2), sodium chloride (NaCl), and boron trichloride (BCl3). Among compounds containing Ha, compounds containing Br (bromine) include, for example, lithium bromide (LiBr), phosphorus tribromide (PBr3), phosphoryl chloride (POBr3), disulfur dibromide (S2Br2), sodium bromide (NaBr), and boron tribromide (BBr3). Among these, a combination of lithium sulfide, phosphorus sulfide, and at least one of lithium chloride and lithium bromide is preferred.

[0068] These raw materials are very unstable in the atmosphere and may react with water to decompose, generating hydrogen sulfide gas and oxidizing, so it is preferable to mix them in an inert atmosphere.

[0069] The raw materials can be mixed by, for example, mixing using media such as a planetary ball mill, or medialess mixing such as a pin mill, a powder mixer, or airflow mixing. The raw materials may be made amorphous by mixing before heating.

[0070] Argyrodite-type crystals are obtained by heating raw materials containing Li, P, S, and Ha or amorphous sulfide solid electrolyte materials to crystallize them. As mentioned above, impurities such as R derived from the raw materials may be contained in the argyrodite-type crystals as long as they do not affect the lithium ion conductivity or heat resistance of the solid electrolyte. The heating is preferably carried out, for example, in an inert gas atmosphere, a hydrogen sulfide gas atmosphere, a sulfur gas atmosphere, or in a vacuum sealed tube. The heating 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 a solid-phase reaction, i.e., crystallization. Furthermore, from the viewpoint of suppressing thermal decomposition, the heating temperature is preferably lower than 600° C., and more preferably 575° C. or lower.

[0071] From the same viewpoint, the heating time is preferably 1 hour or more, more preferably 2 hours or more, and even more preferably 4 hours or more, and is preferably 100 hours or less, more preferably 50 hours or less, and even more preferably 24 hours or less.

[0072] In (Step i-2), a mixture is obtained by mixing the argyrodite-type crystal obtained in (Step i-1) with an oxide having an MO bond, in which M and O are bonded. The oxide having an MO bond may be manufactured, or a commercially available product may be used as is.

[0073] The mixing method is not particularly limited, and for example, the materials may be pulverized and mixed using a ball mill, a bead mill, a homogenizer, a pin mill, a powder agitator, or an airflow mixer.

[0074] In (Step i-3), the mixture obtained in (Step i-2) is heat-treated, thereby obtaining argyrodite crystals in which oxide anions with a Q0 structure and MO bonds exist in the crystal structure. Although the mechanism is unclear, the inventors believe that the oxide becomes a Q0-structure oxide anion upon heat treatment, and that this Q0-structure oxide anion acts as a crystal nucleus to reconstruct an argyrodite-type crystal, or that the Q0-structure oxide anion enters the site of the argyrodite-type crystal structure, thereby achieving high heat resistance.

[0075] The heat treatment conditions vary depending on the composition, but from the viewpoint of favorably forming oxide anions with a Q0 structure, 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. From the viewpoint of reducing grain boundary resistance, the heat treatment temperature is preferably 500°C or higher, more preferably 600°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, more preferably 800°C or lower. The heat treatment atmosphere is preferably, for example, in an inert gas atmosphere, a hydrogen sulfide gas atmosphere, a sulfur gas atmosphere, or a vacuum sealed tube.

[0076] For the same reasons, the heat treatment time is preferably 0.1 hours or more, more preferably 0.25 hours or more, and even more preferably 0.5 hours or more, and is preferably 4 hours or less, more preferably 2 hours or less, and even more preferably 1 hour or less.

[0077] The raw materials containing Li, P, S, and Ha and the oxide having an MO bond formed by bonding M and O in (step ii-1) of production method (ii) are similar to the raw materials and the oxide having an MO bond formed by bonding M and O in (step i-1) and (step i-2) of production method (i), respectively.

[0078] In (step ii-1), a raw material composition is obtained by mixing a raw material containing Li, P, S, and Ha with an oxide having an M-O bond before precipitating the raw material into argyrodite-type crystals, which is different from the production method (i). The raw material and the oxide having an MO bond are mixed, for example, in an inert atmosphere using a ball mill, a bead mill, a homogenizer, a pin mill, a powder mixer, or an air mixer.

[0079] In (step ii-2), the raw material composition obtained in (step ii-1) is heat-treated to obtain argyrodite-type crystals in which oxide anions with a Q structure and MO bonds exist in the crystal structure. The heat treatment in (step ii-2) is the same as the heat treatment in (step i-3) of production method (i).

[0080] The method for producing the intermediate product from raw materials containing Li, P, S, and Ha in (step iii-1) of production method (iii) is not particularly limited. A mechanochemical reaction may be carried out using a mixer, or sulfide crystals may be produced by heat treatment, or both may be used.

[0081] In (step iii-1), raw materials are reacted to obtain an intermediate product. The intermediate product may be an amorphous sulfide solid electrolyte material or a crystalline sulfide solid electrolyte material, and has improved atmospheric stability compared to the raw material, which has poor atmospheric stability and poor handling properties. The intermediate product may be prepared by appropriately changing the manufacturing conditions for the raw materials according to the desired solid electrolyte.

[0082] In (step iii-2), a mixture is obtained by mixing the intermediate product obtained in (step iii-1) with an oxide having an MO bond in which M and O are bonded. The oxide having an MO bond in which M and O are bonded here is the same as the oxide having an MO bond in which M and O are bonded in (step i-2) of production method (i). The intermediate product and the oxide having an MO bond may be mixed in a dry inert atmosphere, but from the viewpoint of reducing production costs, they may also be mixed in a dry air atmosphere using, for example, a ball mill, a bead mill, a homogenizer, a pin mill, a powder agitator, or airflow mixing.

[0083] In (step iii-3), the mixture obtained in (step iii-2) is heat-treated to obtain argyrodite-type crystals in which oxide anions with a Q0 structure and MO bonds exist in the crystal structure. The heat treatment in (step iii-3) is the same as the heat treatment in (step i-3) of production method (i).

[0084] The step iv-1 of the production method (iv) is the same as the step i-1 of the production method (i).

[0085] In (step iv-2), a composition is obtained that contains an oxide anion of a Q0 structure having an MO bond in which M and O are bonded. Specifically, an oxide similar to the oxide having an MO bond in which M and O are bonded in (step i-2) of production method (i) is used to obtain a composition that exists as an oxide anion of a Q0 structure.

[0086] For example, argyrodite-type crystals obtained by methods such as production methods (i) to (iii), in which oxide anions with a Q0 structure having an MO bond are present in the crystal structure, may be used as the composition. Furthermore, the composition is not limited to argyrodite-type crystals, and may be crystals with other crystal structures as long as the composition contains an oxide present as an oxide anion with a Q0 structure. Furthermore, the composition is not limited to crystals, and may be amorphous.

[0087] In (step iv-3), the argyrodite-type crystals obtained in (step iv-1) and the composition obtained in (step iv-2) are mixed to obtain a mixture. The argyrodite-type crystals and the composition are mixed, for example, in a dry inert atmosphere using a ball mill, a bead mill, a homogenizer, a pin mill, a powder mixer, an air mixer, or the like.

[0088] In (step iv-4), the mixture obtained in (step iv-3) is heat-treated to obtain argyrodite-type crystals in which oxide anions with a Q0 structure and MO bonds exist in the crystal structure. The heat treatment in (step iv-4) is the same as the heat treatment in (step i-3) of production method (i). It is presumed that by carrying out the heat treatment in (step iv-4), the oxide anions with the Q0 structure in the composition obtained in (step iv-2) are reconstructed into the argyrodite-type crystal structure obtained in (step iv-1). [Example]

[0089] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. Examples 2 to 5, 7 to 9, 11 to 16, 18 to 28, 30 to 34, 36, 38, 39, 41, 42, 44, 46, 48, 50, 52 to 54, 1-1 to 1-3, 2-1 to 2-3, and 3-1 to 3-10 are working examples, while Examples 1, 6, 10, 17, 29, 35, 37, 40, 43, 45, 47, 49, and 51 are comparative examples.

[0090] [evaluation] The obtained powder was compressed at a pressure of 380 kN to form a measurement sample, and the lithium ion conductivity was measured using an AC impedance measuring device (potentiostat / galvanostat VSP, manufactured by Bio-Logic Sciences Instruments). The measurement conditions were as follows: measurement frequency: 100 Hz to 1 MHz, measurement voltage: 100 mV, and measurement temperature: 25°C.

[0091] [Example 1] Lithium sulfide powder (Sigma, purity 99.98%), diphosphorus pentasulfide powder (Sigma, purity 99%), and lithium chloride powder (Sigma, purity 99.99%) were weighed in a dry nitrogen atmosphere to achieve the element ratios listed in Table 1, and 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 argyrodite-type crystals of Li6PS5Cl. The obtained crystals were pelletized, vacuum sealed again in a carbon-coated quartz tube, and heat-treated at 650°C for 30 minutes to obtain a sulfide-based solid electrolyte. The lithium ion conductivity of the argyrodite-type crystal of Li6PS5Cl before heat treatment was 2.7 mS / cm, and the lithium ion conductivity of the obtained sulfide-based solid electrolyte was 1.1 mS / cm. The table also shows the values ​​of {(M+O) / (Li+P+S+Ha+M+O)}, {β / (α+β)}, and (D+0.1) using the element ratios of the elements constituting the crystal and the elements constituting the oxide anion. In the table, blanks in the element ratio column mean that the element is not contained, and are equivalent to "0.000."

[0092] [Example 2] SiO2 powder (pulverized using SJT series quartz test tubes manufactured by AS ONE Corporation) was added to argyrodite-type crystals of Li6PS5Cl obtained in the same manner as in Example 1, and mixed in a mortar to obtain a mixture. The amount of SiO2 powder added was adjusted so that the element ratios of Si and O were as shown in Table 1. The resulting mixture was pelletized in the same manner as in Example 1, and again vacuum sealed in a carbon-coated quartz tube. Heat treatment was then performed at 650°C for 30 minutes to obtain a sulfide-based solid electrolyte containing argyrodite-type crystals in which oxide anions with a Q0 structure having Si-O bonds were present. The lithium ion conductivity of the obtained sulfide-based solid electrolyte is shown in Table 1.

[0093] The composition of the argyrodite-type crystal, Li6PS5Cl, and the composition of the oxide, SiO2, are both raw material compositions and therefore do not strictly correspond to the actual compositions. However, based on the results of ICP atomic emission spectroscopy of the elements P and S, atomic absorption spectroscopy of the element Li, and ion chromatography of the element Cl, such differences are within ±10% at most and do not affect the results or discussion. The same is true for other examples.

[0094] [Examples 3 to 5] A sulfide-based solid electrolyte containing argyrodite-type crystals containing oxide anions with a Q0 structure having Si-O bonds was obtained in the same manner as in Example 2, except that the amount of SiO2 powder added was changed to the amount shown in Table 1. The lithium ion conductivity of the obtained sulfide-based solid electrolyte is shown in Table 1.

[0095] [Examples 6 to 34] Sulfide-based solid electrolytes containing argyrodite-type crystals containing oxide anions of a Q0 structure having an MO bond were obtained in the same manner as in Example 2, except that the element ratios of the argyrodite-type crystals and the types and amounts of oxides added were changed to those shown in Table 1 or Table 2. Note that in Examples 6, 10, 17, and 29, sulfide-based solid electrolytes were obtained by performing heat treatment without adding any oxide, as in Example 1. Table 1 or Table 2 shows the lithium ion conductivity of the argyrodite-type crystal before heat treatment and the resulting sulfide-based solid electrolyte. When the crystal contained Br, lithium bromide powder (Sigma, purity 99.995%) was used as the Br source. When Al was contained as an oxide anion, Al2O3 powder (Sigma, Type CG-20) was used, and when Zr was contained, ZrO2 powder (Sigma, purity 99%) was used.

[0096] [Table 1]

[0097] [Table 2]

[0098] [Examples 35 to 44] Lithium sulfide powder (Sigma, purity 99.98%), diphosphorus pentasulfide powder (Sigma, purity 99%), lithium chloride powder (Sigma, purity 99.99%), lithium bromide powder (Sigma, purity 99.995%), and SiO powder (AS ONE Corporation, SJT series quartz test tube used after pulverization) were weighed out in a dry nitrogen atmosphere so as to obtain the element ratios shown in Table 3, and mixed in a mortar to obtain a raw material composition. The obtained raw material composition was vacuum-sealed in a carbon-coated quartz tube and heat-treated at 750°C for 1 hour to obtain a sulfide-based solid electrolyte or a sulfide-based solid electrolyte containing argyrodite-type crystals containing Q0-structure oxide anions with MO bonds. Table 3 shows the lithium ion conductivity of sulfide-based solid electrolytes.

[0099] [Table 3]

[0100] [Examples 45 to 52] The sulfide-based solid electrolytes obtained in Examples 1, 4, 17, 26, 35, 36, 40, and 41 were each subjected to a heat treatment again at 650°C for 30 minutes. Table 4 shows the change in lithium ion conductivity of the sulfide-based solid electrolytes before and after the re-heat treatment. The lithium ion conductivity after reheat treatment was estimated based on the fact that in Examples 1 and 17, the conductivity was reduced to half or more after 30 minutes of heat treatment at 650°C, and that in Examples 4 and 27, the decrease in conductivity was small or the conductivity improved. Furthermore, in Examples 49 to 52, due to their manufacturing methods, no crystals were formed before mixing with the oxide. Therefore, the "crystals" in Table 4 are left blank to indicate that there is no lithium ion conductivity.

[0101] [Table 4]

[0102] [Example 53] A mixture is obtained by mixing equimolar amounts of the sulfide-based solid electrolyte of Example 35 and the sulfide-based solid electrolyte containing oxide anions with a Q0 structure and M-O bonds of Example 36. The mixture is heat-treated at 650°C for 30 minutes to obtain a sulfide-based solid electrolyte containing argyrodite-type crystals containing oxide anions with a Q0 structure and M-O bonds. The change in lithium ion conductivity before and after the heat treatment is shown in Table 5. The element ratio of the crystal and oxide anion in the mixture is the average of the element ratios in Examples 35 and 36. The lithium ion conductivity before the heat treatment is a value that is within the range considered with reference to Examples 1 to 5 in Table 1. The lithium ion conductivity after the heat treatment is a value estimated in the same way as in Examples 45 to 52.

[0103] [Example 54] A sulfide-based solid electrolyte containing argyrodite-type crystals containing oxide anions having a Q0 structure and having an M-O bond is obtained in the same manner as in Example 53, except that the sulfide-based solid electrolyte of Example 40 is used instead of the sulfide-based solid electrolyte of Example 35, and the sulfide-based solid electrolyte containing oxide anions having a Q0 structure and having an M-O bond of Example 41 is used instead of the sulfide-based solid electrolyte of Example 36. The change in lithium ion conductivity before and after the heat treatment is calculated in the same manner as in Example 53. The results are shown in Table 5.

[0104] [Table 5]

[0105] The results in Tables 1 and 2 indicate that when sulfide-based solid electrolytes do not contain Q0-structure oxide anions in their crystal structure, their lithium ion conductivity decreases by half or even 20% after heat treatment at 650°C for 30 minutes. This indicates that the heat treatment causes thermal decomposition of the crystal. On the other hand, sulfide-based solid electrolytes containing Q0-structure oxide anions in their crystal structure exhibited a reduced rate of decrease in lithium ion conductivity, confirming improved stability at high temperatures. Furthermore, depending on the crystal structure and the type and amount of oxide anions, they exhibited higher lithium ion conductivity than before heat treatment. This is thought to be due to the reconstitution of the argyrodite-type crystals, which have excellent stability at high temperatures and further increase lithium ion conductivity through heat treatment. Furthermore, using argyrodite-type crystals with such compositions is thought to achieve both improved bulk ionic conductivity and reduced grain boundary resistance during the sintering process. The inventors believe that the results estimated in Table 4 are reasonable given the high-temperature stability.

[0106] The results in Table 3 confirm the above effects, demonstrating extremely high thermal stability, even when the heat treatment temperature was increased to 750°C. Furthermore, a sulfide-based solid electrolyte containing Q0-structure oxide anions in its crystal structure was obtained by heat-treating a raw material composition containing a mixture of raw materials containing Li, P, S, and Ha and oxides without forming argyrodite-type crystals. In other words, it was confirmed that a sulfide-based solid electrolyte with the desired structure can be obtained whether the heat treatment is performed after forming argyrodite-type crystals in advance, or whether the heat treatment is performed in conjunction with crystallization. Furthermore, from the results in Table 5, it is inferred that when a composition containing oxide anions with a Q0 structure is mixed with argyrodite-type crystals that do not contain oxides and then heat-treated, the oxide anions with a Q0 structure in the composition enter the argyrodite-type crystal structure and are reconstructed, resulting in a sulfide-based solid electrolyte that is stable at high temperatures.

[0107] For the sulfide-based solid electrolyte of Example 4, to which oxide SiO2 powder was added, synchrotron X-ray diffraction measurements were performed, and structural refinement analysis was performed using the Rietveld method. Three inter-ion distances that are considered to be the migration paths of lithium ions in the argyrodite crystal structure were calculated: (1) the lithium site 48h-24g-48h distance (doublet jump distance), (2) the intra-cage distance (intra-cage jump distance), and (3) the inter-cage distance (inter-cage jump distance) (J.Am.Chem.Soc. 2017, 139, 10909-10918 Figure 1). The results showed that the distance (1) in Example 4 was 2.26396 Å, the distance (2) was 2.28013 Å, and the distance (3) was 2.42153 Å. The lattice constant was also calculated; the lattice constant for Example 4 was 9.85087 Å. In the argyrodite-type crystal (Li6PS5Cl) of the basic structure shown in Figure 1 of J.Am.Chem.Soc. 2017, 139, 10909-10918, the distance of (1) is 1.95772 Å, the distance of (2) is 2.25333 Å, and the distance of (3) is 2.76089 Å. Also, according to the same document, the lattice constant of the argyrodite-type crystal of the basic structure is 9.85980 Å.

[0108] [Examples 1-1 to 1-3] Lithium sulfide powder (Sigma, purity 99.98%), diphosphorus pentasulfide powder (Sigma, purity 99%), and lithium chloride powder (Sigma, purity 99.99%) were weighed in a dry nitrogen atmosphere to achieve the element ratios listed in Table 6, and mixed for 4 hours at 400 rpm 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. When Br was contained in the crystals, lithium bromide powder (Sigma, purity 99.995%) was used as the Br source. SiO2 powder (pulverized using SJT series quartz test tubes manufactured by AS ONE Corporation) was added to the obtained argyrodite-type crystals and mixed in a mortar to obtain a mixture. The amount of SiO2 powder added was adjusted so that the element ratios of Si and O were as shown in Table 6. The resulting 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 containing oxide anions with a Q0 structure and Si-O bonds. The argyrodite-type crystals in Examples 1-2 and 1-3 both contain Cl and Br as Ha. The Cl content in the argyrodite-type crystals is x (at%), and the Br content is y (at%), with (x / y) = 7 in Example 1-2 and (x / y) = 1 in Example 1-3. Table 6 shows the lithium ion conductivity of the argyrodite-type crystal before heat treatment and the resulting sulfide-based solid electrolyte.

[0109] [Table 6]

[0110] [Examples 2-1 to 2-3] The cycle characteristics of the all-solid-state lithium ion secondary batteries using the sulfide-based solid electrolytes of Examples 1-1 to 1-3 were evaluated. (Preparation of solid electrolyte powder) First, the sulfide-based solid electrolytes of Examples 1-1, 1-2, and 1-3 were placed in carbon containers, and heat-treated by heating at 450°C for 60 minutes in an N atmosphere with a dew point of -60°C or lower, to obtain the sulfide-based solid electrolytes of Examples 2-1, 2-2, and 2-3, respectively. The composition formulas and lithium ion conductivities of the sulfide-based solid electrolytes of Examples 2-1, 2-2, and 2-3 are shown in Table 7. The composition formulas here are values ​​obtained by composition analysis using the following method, with the exception that the amount of O was twice that of Si. (Composition analysis method) The solid electrolyte powder was weighed in a glove box and dissolved in an alkaline aqueous solution. Compositional analysis was performed by ICP atomic emission spectroscopy (instrument: Hitachi High-Tech Science Corporation, model PS3520UVDDII) for P, S, and Si, atomic absorption spectroscopy (instrument: Hitachi High-Tech Corporation, model ZA3300; when measuring Li, CsCl was added to make the solution concentration 0.1%), and ion chromatography (instrument: Thermo Fisher Scientific Corporation, model ICS-2100 (column: AS11HC); a small amount of H2O2 was added and the solution was diluted with ultrapure water for measurement) for elemental Cl and Br.

[0111] The sulfide-based solid electrolytes obtained in Examples 2-1 to 2-3 were dry-pulverized in a planetary ball mill (manufactured by Ito Seisakusho Co., Ltd., model number LP-4) using alumina balls with a particle size of 2 mm, and passed through a sieve with 43 μm openings to obtain sulfide-based solid electrolyte powders with a particle size distribution of D50 of 3 μm. The lithium ion conductivity in Table 7 is the value measured before the pulverization. All operations were carried out in a dry nitrogen atmosphere. The particle size distribution was measured using a Microtrac laser diffraction particle size distribution analyzer MT3300EXII, and the average particle size (D50) was determined from the obtained volume-based particle size distribution chart.

[0112] (Preparation of positive electrode mixture) A positive electrode composite was prepared by mixing 35 parts of the sulfide-based solid electrolyte powder prepared above, 60 parts of the positive electrode active material, and 5 parts of a conductive additive (acetylene black, manufactured by Denka Co., Ltd., HS100), with LiNbO3-coated layered rock salt LiCoO2 powder (volume average particle diameter: 10 μm) as the positive electrode active material. TEM observation revealed that the thickness of the LiNbO3 coating was 7 nm.

[0113] (Fabrication of all-solid-state lithium-ion secondary batteries) 80 mg of the sulfide-based solid electrolyte powder prepared above was placed in a 10 mm diameter plastic cylinder and pressure-molded to form a solid electrolyte layer. Next, 10 mg of the cathode composite prepared above was placed in the same cylinder and pressure-molded again to form a cathode layer. Furthermore, indium foil and lithium foil were placed on the opposite side of the cathode composite to form a cathode layer. An all-solid-state lithium-ion secondary battery was thus prepared, and a charge-discharge test was performed at a confining pressure of 10 kN.

[0114] (Charge / discharge test) Using the fabricated all-solid-state lithium-ion secondary batteries, a constant-current charge / discharge test was performed for 100 cycles at 25°C, with a charge / discharge current density of 0.1 C and a charge / discharge potential range of 1.9-3.7 V. From the results of the charge / discharge test, the capacity retention rate (%) expressed by the following formula was calculated, and the cycle characteristics of the all-solid-state lithium-ion secondary batteries were evaluated. The capacity retention rate (%) for each example is shown in Table 7. Capacity retention rate (%) = (discharge capacity at 100th cycle / discharge capacity at 1st cycle) x 100

[0115] [Table 7]

[0116] [Example 3-1 to Example 3-10] Assuming that the raw materials for the argyrodite-type crystals contained impurities, sulfide-based solid electrolytes were produced under the same conditions as in Example 26, except that part of the lithium sulfide was replaced with NaS, KS, MgS, or CaS. Specifically, the sulfide-based solid electrolytes of Examples 3-1 to 3-10 were obtained in the same manner as in Example 26, except that the mixing ratio of the raw materials was adjusted so that the argyrodite-type crystals had the composition ratios shown in Table 8. Here, when the argyrodite-type crystals contained Na, NaS powder (manufactured by Kojundo Chemical Laboratory Co., Ltd., 2N) was used; when they contained K, KS powder (manufactured by Kojundo Chemical Laboratory Co., Ltd., 2Nup) was used; when they contained Mg, MgS powder (manufactured by Kojundo Chemical Laboratory Co., Ltd., 3N) was used; and when they contained Ca, CaS powder (manufactured by Kojundo Chemical Laboratory Co., Ltd., 2Nup) was used. In Table 8, R represents at least one element selected from the group consisting of Na, K, Mg, and Ca.

[0117] [Table 8]

[0118] The results in Table 6 show that the sulfide-based solid electrolytes of Examples 1-1 to 1-3 all contain Q0-structure oxide anions in their crystal structures, resulting in higher lithium ion conductivities after the heat treatment than before the heat treatment for 30 minutes at 750° C. Among these, Examples 1-2 and 1-3, in which the argyrodite-type crystals contain Cl and Br as Ha and the ratios thereof are adjusted within a preferred range, showed particularly high lithium ion conductivities after the heat treatment.

[0119] Furthermore, the results in Table 7 show that the sulfide-based solid electrolytes of Examples 2-1, 2-2, and 2-3, which were subjected to heat treatment at 450°C for 60 minutes, exhibited further improved lithium ion conductivity. In other words, it is presumed that the heat treatment resulted in both improved bulk ion conductivity and reduced grain boundary resistance. Furthermore, the results of charge-discharge tests using all-solid-state lithium ion secondary batteries containing the sulfide-based solid electrolytes of Examples 2-1, 2-2, and 2-3 confirmed that Examples 2-1, 2-2, and 2-3 all had high capacity retention rates and excellent cycle characteristics. In particular, the sulfide-based solid electrolytes of Examples 2-2 and 2-3, in which the argyrodite-type crystals contained Cl and Br as Ha and the ratios were adjusted within a preferred range, exhibited particularly excellent cycle characteristics.

[0120] Furthermore, from the results in Table 8, even when the sulfide-based solid electrolyte contains R, the lithium ion conductivity after the heat treatment was greater than that before the heat treatment at 650°C for 30 minutes, and it was confirmed that the stability at high temperatures was improved, similar to when R was not contained.

[0121] 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 present invention. This application is based on Japanese patent applications filed on July 7, 2020 (Patent Application No. 2020-117194), October 13, 2020 (Patent Application No. 2020-172693), and May 21, 2021 (Patent Application No. 2021-086406), the contents of which are incorporated herein by reference.

Claims

1. A sulfide-based solid electrolyte for use in a lithium-ion secondary battery, a crystalline phase and an anion present in the crystalline structure of the crystalline phase; the crystalline phase comprises argyrodite-type crystals containing Li, P, S, and Ha; The Ha is at least one element selected from the group consisting of F, Cl, Br, and I, The argyrodite-type crystal contains Cl and Br as the Ha, the Cl content in the argyrodite-type crystal is x (at %), the Br content is y (at %), and (x / y) is 0.1 or more and 10 or less; The anion includes an oxide anion having a QO structure having an MO bond in which M and O are bonded, The sulfide-based solid electrolyte, wherein M is at least one element selected from the group consisting of Si, Al, Zr, and B.

2. A sulfide-based solid electrolyte for use in a lithium-ion secondary battery, a crystalline phase and an anion present in the crystalline structure of the crystalline phase; the crystalline phase comprises argyrodite-type crystals containing Li, P, S, and Ha; The Ha is at least one element selected from the group consisting of F, Cl, Br, and I, The argyrodite-type crystal contains Cl and Br as the Ha, The ratio of the content (at%) of elements contained in the argyrodite-type crystal is Li ａ -P-S ｂ -Cl ｃ１ -Br ｃ２ When expressed as above, c1 is 0.1 or more and 1.5 or less, and c2 is 0.1 or more and 1.9 or less, The anion includes an oxide anion having a QO structure having an MO bond in which M and O are bonded, The sulfide-based solid electrolyte, wherein M is at least one element selected from the group consisting of Si, Al, Zr, and B.

3. The argyrodite-type crystal contains at least one element selected from the group consisting of Na, K, Mg, and Ca, represented by R, and the ratio of the contents (at %) of the elements contained in the argyrodite-type crystal is Li ａ１ -R ａ２ -P-S ｂ -Ha ｃ The sulfide-based solid electrolyte according to claim 1 or 2, wherein a2 is 0.001 to 0.4 when expressed as

4. The sulfide-based solid electrolyte according to any one of claims 1 to 3, wherein when an all-solid-state lithium ion secondary battery is produced by the following method and subjected to a charge-discharge test, the capacity retention rate represented by the following formula is 80% or more. (method) (Preparation of positive electrode composite) As the positive electrode active material, a 7 nm thick LiNbO ３ Coated layered rock salt LiCoO ２ The LiNbO powder (volume average particle diameter: 10 μm) was mixed with 35 parts of the sulfide-based solid electrolyte, 60 parts of the positive electrode active material, and 5 parts of a conductive additive (acetylene black, manufactured by Denka Co., Ltd., HS100) to prepare a positive electrode composite. ３ The thickness of the coating is determined by observation with a transmission electron microscope (TEM). (Fabrication of all-solid-state lithium-ion secondary batteries) 80 mg of the sulfide-based solid electrolyte was placed in a plastic cylinder with a diameter of 10 mm and pressure-molded to form a solid electrolyte layer. Next, 10 mg of the positive electrode composite was placed in the cylinder and pressure-molded again to form a positive electrode layer. Furthermore, an indium foil and a lithium foil are placed on the opposite side of the positive electrode mixture to form a negative electrode layer. In this way, an all-solid-state lithium ion secondary battery is fabricated, and a charge-discharge test is carried out at a confining pressure of 10 kN. (Charge / discharge test) Using the all-solid-state lithium ion secondary battery, a constant current charge / discharge test is carried out for 100 cycles at 25° C. with a charge / discharge current density of 0.1 C and a charge / discharge potential range of 1.9 to 3.7 V. From the results of the charge / discharge test, the capacity retention rate (%) is calculated using the following formula. Capacity retention rate (%)=(discharge capacity at 100th cycle / discharge capacity at 1st cycle)×100

5. 5. The sulfide-based solid electrolyte according to claim 1, wherein an oxide layer is formed at an interface between the solid electrolyte layer and a positive electrode layer when a lithium ion secondary battery including a solid electrolyte layer containing the sulfide-based solid electrolyte is charged and discharged.

6. A lithium ion secondary battery comprising the sulfide-based solid electrolyte according to any one of claims 1 to 5.

7. A method for producing the sulfide-based solid electrolyte according to any one of claims 1 to 5, comprising: mixing raw materials containing Li, P, S, and Ha and crystallizing them by heating to obtain argyrodite-type crystals; Obtaining a mixture in which the argyrodite-type crystal is mixed with an oxide having an M-O bond in which M and O are bonded; and heat-treating the mixture to obtain argyrodite-type crystals in which oxide anions of a QO structure having an MO bond are present in the crystal structure; The Ha is at least one element selected from the group consisting of F, Cl, Br, and I, The argyrodite-type crystal contains at least one of Cl and Br as the Ha, The ratio of the content (at%) of elements contained in the argyrodite-type crystal is Li ａ -P-S ｂ -Ha ｃ When expressed as M is at least one element selected from the group consisting of Si, Al, Zr, and B, A method for producing a sulfide-based solid electrolyte comprising argyrodite-type crystals in which oxide anions of a Q0 structure having an MO bond exist in the crystal structure.

8. A method for producing the sulfide-based solid electrolyte according to claim 1, comprising: A raw material composition is obtained by mixing a raw material containing Li, P, S, and Ha with an oxide having an M-O bond in which M and O are bonded; and heat-treating the raw material composition to obtain argyrodite-type crystals in which oxide anions having a QO structure and an M-O bond exist in the crystal structure; The Ha is at least one element selected from the group consisting of F, Cl, Br, and I, The argyrodite-type crystal contains at least one of Cl and Br as the Ha, The ratio of the content (at%) of elements contained in the argyrodite-type crystal is Li ａ -P-S ｂ -Ha ｃ When expressed as M is at least one element selected from the group consisting of Si, Al, Zr, and B, A method for producing a sulfide-based solid electrolyte comprising argyrodite-type crystals in which oxide anions of a Q0 structure having an MO bond exist in the crystal structure.

9. A method for producing the sulfide-based solid electrolyte according to claim 1, comprising: reacting raw materials containing Li, P, S and Ha to obtain an intermediate product; Obtaining a mixture by mixing the intermediate product with an oxide having an M-O bond in which M and O are bonded; and heat-treating the mixture to obtain argyrodite-type crystals in which oxide anions of a QO structure having an MO bond are present in the crystal structure; The Ha is at least one element selected from the group consisting of F, Cl, Br, and I, The argyrodite-type crystal contains at least one of Cl and Br as the Ha, The ratio of the content (at%) of elements contained in the argyrodite-type crystal is Li ａ -P-S ｂ -Ha ｃ When expressed as M is at least one element selected from the group consisting of Si, Al, Zr, and B, A method for producing a sulfide-based solid electrolyte comprising argyrodite-type crystals in which oxide anions of a Q0 structure having an MO bond exist in the crystal structure.

10. A method for producing the sulfide-based solid electrolyte according to claim 1, comprising: mixing raw materials containing Li, P, S, and Ha and crystallizing them by heating to obtain argyrodite-type crystals; Obtaining a composition containing an oxide anion of a QO structure having an MO bond in which M and O are bonded; Obtaining a mixture by mixing the argyrodite-type crystals with the composition; and heat-treating the mixture to obtain argyrodite-type crystals in which oxide anions of a QO structure having an MO bond are present in the crystal structure; The Ha is at least one element selected from the group consisting of F, Cl, Br, and I, The argyrodite-type crystal contains at least one of Cl and Br as the Ha, The ratio of the content (at%) of elements contained in the argyrodite-type crystal is Li ａ -P-S ｂ -Ha ｃ When expressed as M is at least one element selected from the group consisting of Si, Al, Zr, and B, A method for producing a sulfide-based solid electrolyte comprising argyrodite-type crystals in which oxide anions of a Q0 structure having an MO bond exist in the crystal structure.

11. The method for producing a sulfide-based solid electrolyte according to any one of claims 7 to 10, wherein the heat treatment is carried out at a temperature of 500°C or higher.

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