Sulfide-based solid electrolyte and method for producing same

Abstract

The present invention relates to a sulfide solid electrolyte which is used for lithium ion secondary batteries and contains a crystal phase and anions, wherein: the crystal phase contains an argyrodite crystal that contains Li, P, S and Ha (Ha is F, Cl, Br and / or I); the anions comprise oxide anions that have a P-O bond; at least some of the anions are polyanion structures that are different from the anions that constitute the argyrodite crystal; if the overall composition thereof is expressed by LiaMSbHacOx (wherein M represents at least one element including P, the at least one element being selected from the group consisting of group 2-15 metal elements of the periodic table and semimetal elements), 4 < a < 7, 3 < b < 6, 0 < c < 2, 0 < x and 3 < (b + c) < (6 – (x / 2)) are satisfied; and 60% or more of the total content of O is bonded to M.

Description

[Technical Field]

[0001] The present invention relates to a sulfide-based solid electrolyte for use in lithium-ion secondary batteries and a method for producing the same. [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. Furthermore, improvements to lithium-ion secondary batteries' short battery life and narrow operating temperature range have been desired.

[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. The sulfide ions that make up sulfide-based solid electrolytes have a higher polarizability than the oxide ions that make up oxide-based solid electrolytes, and therefore exhibit high lithium ion 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 containing argyrodite-type crystals 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] Sulfide-based solid electrolytes containing argyrodite-type crystals have a large intergranular resistance, known as grain boundary resistance. Therefore, reducing the grain boundary resistance through sintering is considered a way to achieve high lithium ion conductivity.

[0008] However, when heat-treated at high temperatures for sintering, the lithium ion conductivity actually decreases due to thermal decomposition. Specifically, for example, in the case of an argyrodite-type crystal represented by the composition Li6PS5Cl, when heat-treated at a high temperature of about 600°C, it thermally decomposes into Li2S, LiCl, and Li3PS4. As described above, there is a limit to the high-temperature heat treatment for reducing the grain boundary resistance of sulfide-based solid electrolytes containing argyrodite-type crystals.

[0009] Therefore, an object of the present invention is to provide a sulfide-based solid electrolyte having the lithium ion conductivity required for a solid electrolyte used in a lithium ion secondary battery and also having excellent heat resistance, and a method for producing the same. [Means for solving the problem]

[0010] As a result of intensive studies, the inventors of the present invention have found that, without reducing the P component constituting the argyrodite-type crystal, by providing a polyanion structure, which is an oxide anion having a P—O bond, separately from the anions constituting the argyrodite-type crystal, the above problems can be solved, and thus the present invention has been completed.

[0011] That is, the present invention relates to the following [1] to

[11] . [1] A sulfide-based solid electrolyte used in a lithium-ion secondary battery, comprising a crystal phase and anions, wherein the crystal phase contains an argyrodite-type crystal containing Li, P, S, and Ha, and Ha is at least one element selected from the group consisting of F, Cl, Br, and I; the anions contain an oxide anion having a P—O bond in which P and O are bonded; at least a part of the oxide anion having a P—O bond is a polyanion structure different from the anions constituting the argyrodite-type crystal; when the overall composition of the sulfide-based solid electrolyte is represented as Li a MS b Ha c O x aM bHa cO xS, the relationships 4 < a < 7, 3 < b < 6, 0 < c < 2, 0 < x, and 3 < (b + c) < {6-(x / 2)} are satisfied; M is at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 15 of the periodic table, including at least P; and more than 60% of the total content of O is bonded to M. [2] The polyanion structure contains at least PO4 3- and PO2S2 3- and satisfies the relationship 0.1 ≦ {[PO4 3- / ([PO4 3- +[POS3 3- +[PO2S2 3- +[PO3S 3- )} ≦ 0.5; and among PO4 3- 、POS3 3- 、PO2S2 3- and PO3S 3- PO2S2 3-The sulfide-based solid electrolyte according to [1], which has the highest content. Let the total content of [3] O be [O]. T When the total content of O constituting the crystal structure of the crystal phase is [O]. A When 0.5 < {([O]. T -[O]. A ) / [O]. T}< 1 is satisfied, the sulfide-based solid electrolyte according to [1] or [2]. [4] The total content of lithium halide, lithium phosphate, and lithium thiophosphate is 0.6% by mass or less, the sulfide-based solid electrolyte according to any one of [1] to [3]. [5] In the overall composition of Li. a MS. b Ha. c O. x When 0 < x < 1 and {3.5 - (x / ​​​​​​​​​[9] 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 with a compound serving as an oxygen source to obtain a raw material mixture; heating the raw material mixture to obtain a melt; and cooling the melt to obtain argyrodite-type crystals and a polyanion structure of oxide anions having a P-O bond different from the anions constituting the argyrodite-type crystals, wherein the Ha is at least one element selected from the group consisting of F, Cl, Br, and I, the compound serving as the oxygen source includes at least one element selected from the group consisting of Li2CO3, Li2SO4, and Li3PO4, and the molar ratio of the P content to the Ha content in the raw material mixture is 0.5 to 0.8.

[10] The method for producing a sulfide-based solid electrolyte according to [9] above, wherein the heating is carried out at a temperature of 650°C or higher.

[11] The method for producing a sulfide-based solid electrolyte according to [9] or

[10] above, wherein the cooling is carried out at a cooling rate of 0.01°C / second or more. [Effects of the Invention]

[0013] According to the present invention, a sulfide-based solid electrolyte containing argyrodite-type crystals with excellent heat resistance can be obtained. Therefore, when applied to lithium-ion secondary batteries, even if heat treatment is performed to reduce grain boundary resistance, high lithium ion conductivity can be achieved without thermal decomposition. This is expected to improve 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 sometimes simply referred to as "solid electrolyte") is used in a lithium-ion secondary battery and contains a crystal phase and anions. The crystal phase contains an argyrodite-type crystal containing Li, P, S, and Ha, and Ha is at least one element selected from the group consisting of F, Cl, Br, and I. The anion contains an oxide anion having a P-O bond in which P and O are bonded. At least a part of the oxide anion having this P-O bond is a polyanion structure different from the anion constituting the argyrodite-type crystal.

[0016] When the overall composition of the sulfide-based solid electrolyte is Li a MS b Ha c O x it satisfies the relationships of 4 < a < 7, 3 < b < 6, 0 < c < 2, 0 < x, and 3 < (b + c) < {6 - (x / 2)}. Here, M is at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 15 of the periodic table containing at least P. Among the total content of O contained in the sulfide-based solid electrolyte, 60% or more is bonded to the above M.

[0017] Separate from the anion constituting the argyrodite-type crystal, the existence of the polyanion structure of the oxide anion having a P-O bond improves the heat resistance of the argyrodite-type crystal due to the strength of the electrostatic attraction between the polyanion structure and the lithium cation. Therefore, even when heat treatment is performed at a high temperature, the argyrodite-type crystal can exist stably without thermal decomposition. As a result, when applying to a lithium-ion secondary battery, the sulfide-based solid electrolyte can be sintered to reduce the grain boundary resistance.

[0018] From the above viewpoint, the more O that is bonded to P out of the total amount of oxygen (O) contained in the sulfide-based solid electrolyte, i.e., the total content of O, the better, i.e., the more O that bonds with M containing P to form M-O bonds. Therefore, in the sulfide-based solid electrolyte according to this embodiment, 60% or more of the total content of O is bonded with M to form M-O bonds.

[0019] When an oxygen source compound is added to form the above-mentioned MO bond, while the MO bond is formed, some of the P that would have originally formed the argyrodite-type crystal also forms a PO bond, resulting in a shortage of P components. When the P component is insufficient, the ratio of P to Ha required to form the argyrodite-type crystal becomes inappropriate. As a result, in addition to the argyrodite-type crystal, impurity crystals such as lithium halide crystals, lithium phosphate crystals, and lithium thiophosphate crystals precipitate. The presence of impurity crystals leads to a decrease in lithium ion conductivity.

[0020] Therefore, in the sulfide-based solid electrolyte according to this embodiment, the proportion of M containing P in the overall composition is increased. Specifically, the overall composition of the sulfide-based solid electrolyte is increased by increasing the proportion of Li a MS b Ha c O x When expressed as , the relationship 3<(b+c)<{6-(x / 2)} is satisfied. Among these, {6-(x / 2)} is a polyanion structure, PO4 3- In addition to POS3 3- , PO2S2 3- , PO3S 3- Among them, PO2S2 3- This is derived from the fact that Li tends to be the most commonly observed. 7―y PS 6-y Ha y Therefore, the value expressed by (b+c) is usually 6 for P. Next, PO2S2 3-When the polyanion structure is formed, half (1 / 2) of the amount of oxygen (O) contributes to the formation of the polyanion structure. Therefore, it is assumed that half (x / 2), of the amount of oxygen (O) in the overall composition, x, is used to oxidize M containing P and form an MO bond. That is, the above relational expression corresponds to the amount of M that is required in excess due to the presence of oxide anions when forming an argyrodite-type crystal. By satisfying the above relationship, the P component that constitutes the argyrodite-type crystal is not insufficient, and a polyanion structure of oxide anions having P-O bonds can be formed in addition to the anions that constitute the argyrodite-type crystal.

[0021] (Polyanion structure) The anions contained in the sulfide-based solid electrolyte according to this embodiment include oxide anions having a P-O bond in which P and O are bonded together. At least a portion of the oxide anions having a P-O bond are polyanion structures that are different from the anions that constitute argyrodite-type crystals.

[0022] The sulfide-based solid electrolyte contains a polyanion structure that is different from the anions that constitute the argyrodite-type crystals. As described above, this creates a strong electrostatic attraction between the sulfide-based solid electrolyte and the lithium cations, improving the heat resistance of the argyrodite-type crystals.

[0023] The polyanion structure is not particularly limited as long as it is an oxide anion having a PO bond. For example, PO4 3- ,POS3 3- , PO2S2 3- , PO3S 3- Among them, at least PO4 3- and PO2S2 3- tend to contain PO2S2 3- tends to be the most common. Among them, the content expressed in mole percent is used to determine {[PO4 3- ] / ([PO4 3- ]+[POS3 3- ]+[PO2S23- ]+[PO3S 3- The ratio of the content expressed by the formula {} is preferably 0.1 or more, more preferably 0.2 or more, from the viewpoint of increasing the proportion of P-O bonds. Moreover, from the viewpoint of preventing deterioration of battery performance due to excessive hardness when applied to a lithium ion secondary battery, the ratio of the content is preferably 0.5 or less, more preferably 0.4 or less.

[0024] The presence of polyanion structures is 31 The amount of crystalline state can be confirmed by P-NMR (nuclear magnetic resonance) analysis or Raman spectroscopy, and the amount of crystalline state can be confirmed by powder X-ray diffraction (XRD) measurement. Specifically, the presence of P-O bonds can be detected by Raman spectroscopy at 960 cm -1 This can be confirmed by the appearance of a peak near the center. The fact that these are not the anions that make up the argyrodite-type crystals can be confirmed by the absence of any peaks in XRD measurements. Even if a peak is observed in XRD measurements, the weight percentage of oxygen (O) can be roughly calculated from the pattern, and if this weight percentage is small compared to the total content of O contained in the sulfide-based solid electrolyte as a whole, it can be confirmed that they are included as a polyanion structure different from the anions that make up the crystals.

[0025] Also, 31 In P-NMR analysis, PO4 3- ,POS3 3- , PO2S2 3- , PO3S 3- Since the peak positions are different for each compound, the content ratio of each polyanion structure can be determined from the area intensity ratio. in particular, 31 In P-NMR analysis, PO4 3- Peaks were detected in the range of 0 to 20 ppm for POS3. 3- A peak was detected in the range of 75 to 80 ppm, and PO2S2 3- A peak was detected in the range of 60 to 75 ppm for PO3S. 3- A peak is detected in the range of 30 to 50 ppm.

[0026] (Overall composition) Regarding a sulfide-based solid electrolyte, the overall composition composed of a crystal phase consisting of an al-djerroudite-type crystal and anions is Li a MS b Ha c O x When it is set as such, the relationships of 4 < a < 7, 3 < b < 6, 0 < c < 2, 0 < x, and 3 < (b + c) < {6 - (x / 2)} are satisfied.

[0027] The above overall composition is a composition considering an oxide anion having a M-O bond including a polyanion structure of PO4 3- which is different from the crystal phase consisting of an al-djerroudite-type crystal and the anions constituting the crystal. Also, when there are other crystal phases other than the al-djerroudite-type crystal and other anions, they are also considered. The content of each element and their total are determined by composition analysis using ICP emission spectroscopy, atomic absorption spectrometry, ion chromatography, etc.

[0028] When the sulfide-based solid electrolyte contains other crystal phases other than the al-djerroudite-type crystal, there is concern about a decrease in performance such as lithium ion conductivity and water resistance. Therefore, in all components constituting the sulfide-based solid electrolyte, it is preferable that the crystal phase consisting of the al-djerroudite-type crystal is the main component. Here, the main component means the component having the highest content in terms of mass ratio among all components constituting the sulfide-based solid electrolyte, and such content is preferably more than 50% by mass, and more preferably 51% by mass or more.

[0029] Also, from the viewpoint of realizing high lithium ion conductivity, the content of the crystal phase consisting of the al-djerroudite-type crystal in the sulfide-based solid electrolyte is more preferably 65% by mass or more, even more preferably 80% by mass or more, still more preferably 85% by mass or more, and particularly preferably 90% by mass or more. Also, from the viewpoint of obtaining the effect of containing an oxide anion having a M-O bond, the content of the crystal phase consisting of the al-djerroudite-type crystal is preferably 99% by mass or less, more preferably 97% by mass or less, even more preferably 95% by mass or less, and even more preferably 90% by mass or less. The content of each crystalline phase in the sulfide-based solid electrolyte 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.

[0030] The proportion of the crystalline phase consisting of argyrodite-type crystals in the crystalline phase constituting the sulfide-based solid electrolyte is more preferably 70 mass% or more, further preferably 85 mass% or more, and may be 100 mass%, i.e., the crystalline phase may consist solely of the crystalline phase consisting of argyrodite-type crystals. The total content of the crystalline phase consisting of argyrodite-type crystals and oxide anions having MO bonds in the sulfide-based solid electrolyte is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, from the viewpoint of further enjoying the effects of the present invention. The upper limit of the total content is not particularly limited, and may be 100% by mass, i.e., the sulfide-based solid electrolyte may consist only of a crystalline phase consisting of argyrodite-type crystals and oxide anions. However, since the sulfide-based solid electrolyte may contain other components such as impurity crystals or an amorphous phase during production, the total content may be, for example, 99.5% by mass or less, 99% by mass or less, 97% by mass or less, or 95% by mass or less. In this specification, the crystalline phase consisting of argyrodite-type crystals may contain two or more types of argyrodite-type crystals. The content of each crystalline phase relative to the total content of the crystalline phases constituting the sulfide-based solid electrolyte 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.

[0031] Among oxide anions having P-O bonds in the sulfide-based solid electrolyte, the content of polyanion structures, which are oxide anions having P-O bonds different from the anions constituting the crystals that form the crystalline phase, is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more from the viewpoint of achieving heat resistance. Furthermore, from the viewpoint of preventing impurities from being precipitated, the content of polyanion structures is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less. The content of the polyanion structure is the total amount of MO bonds. 31 P-NMR and 29 It is determined by quantifying the MO bonds that make up the crystal using NMR analysis such as Si-NMR, and then quantifying them separately using XRD measurement or neutron scattering measurement, and then calculating the difference between these.

[0032] The overall composition of the sulfide-based solid electrolyte is Li a MS b Ha c O x When this is done, the total amount of M, which is the basis for the ratios of Li, S, Ha, and O, is mainly derived from crystalline phases such as argyrodite-type crystals and oxide anions with M-O bonds that are different from the anions that make up the crystals.

[0033] In the overall composition of the sulfide-based solid electrolyte, the ratio of Li to the total amount of M is represented by a, which is mainly derived from argyrodite-type crystals. The value of a is greater than 4, and from the viewpoint of preventing excessive deviation from the argyrodite composition, it is preferably 4.3 or greater, more preferably 4.7 or greater, and even more preferably 5.0 or greater. Furthermore, from the viewpoint of suppressing impurity deposition, the value of a is less than 7, and is preferably 6.5 or less, more preferably 6.0 or less, and even more preferably 5.5 or less. The overall composition of the sulfide-based solid electrolyte can be determined quantitatively using ICP emission spectroscopy and oxygen, nitrogen, and hydrogen analysis.

[0034] In the overall composition of the sulfide-based solid electrolyte, the ratio of S to the total amount of M is represented by b, which is mainly derived from argyrodite-type crystals. The value of b is greater than 3, and from the viewpoint of preventing excessive deviation from the argyrodite composition, it is preferably 3.3 or more, more preferably 3.7 or more, and even more preferably 4.0 or more. Furthermore, from the viewpoint of suppressing impurity deposition, the value of b is less than 6, and is preferably 5.0 or less, more preferably 4.7 or less, and even more preferably 4.3 or less.

[0035] In the overall composition of the sulfide-based solid electrolyte, the ratio of the total amount of Ha to the total amount of M is represented by c, which mainly originates from the argyrodite-type crystal. The value of c only needs to be greater than 0, and from the perspective of not deviating too much from the argyrodite composition, it is preferably 0.8 or more, more preferably 1.0 or more, and even more preferably 1.2 or more. Also, from the perspective of suppressing impurity precipitation, the value of c is less than 2, preferably 1.8 or less, more preferably 1.6 or less, and even more preferably 1.4 or less.

[0036] In the overall composition of the sulfide-based solid electrolyte, the amount ratio of O to the total amount of M is represented by x, which mainly originates from the oxide anions having M-O bonds containing P-O bonds, and when a part of the anions constituting the crystal is substituted by oxide anions, it also originates from the substituted oxide anions. The value of x only needs to be greater than 0, and from the perspective of increasing the amount of P-O bonds, it is preferably 0.03 or more, more preferably 0.2 or more, and even more preferably 0.4 or more. Also, the value of x is less than 6, and from the perspective that impurities are likely to precipitate when the amount of the compound serving as the oxygen source to be added is large, it is preferably 3.0 or less, more preferably less than 1.

[0037] Also, in the overall composition of the sulfide-based solid electrolyte, the ratio (b + c) of the total amount of S and Ha to the total amount of M and the value represented by {6 - (x / 2)} using the amount ratio of O to the total amount of M satisfy the relationship (b + c) < {6 - (x / 2)}. This {6 - (x / 2)} is, as described above, among the polyanion structures having P-O bonds, the content of PO2S2 3- tends to be the highest.

[0038] The value represented by (b + c) is greater than 3, but in the range of 0 < x < 1, it is preferable to satisfy {3.5 - (x / 2)} < (b + c). This is a relational expression defined from the perspective of preventing the composition of the argyrodite-type crystal itself from shifting due to an excessive content of M in the overall composition. The value represented by (b + c) is more preferably 3.5 or more, and even more preferably 4.5 or more.

[0039] The value represented by (b+c) is less than {6-(x / 2)}, but from the viewpoint of reducing the content of impurity crystals and realizing high lithium ion conductivity, the difference between {6-(x / 2)} and (b+c) is preferably 0.1 or more, more preferably 0.3 or more, and even more preferably 0.5 or more. The upper limit of the difference is less than 3.

[0040] In the overall composition of the sulfide-based solid electrolyte, M is at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 15 of the periodic table, including at least P. P is mainly derived from argyrodite-type crystals and oxide anions having P-O bonds. Furthermore, if the sulfide-based solid electrolyte contains another crystalline phase that contains P, P is also derived from the other crystalline phase. When M contains at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 15 of the periodic table other than P, it may be derived from other crystalline phases or other oxide anions. For example, when the other crystalline phase contains a crystalline phase containing LGPS-type crystals, Ge is contained in M. Furthermore, as the other oxide anion, for example, SiO4 4- and AlO4 4- , BO3 4- When M contains Si, Al, B, etc., M also includes Si, Al, B, etc.

[0041] M may contain only P, but when it contains elements other than P, it preferably contains an oxide anion having an MO bond other than a PO bond from the viewpoint of heat resistance. More preferably, M contains at least one element selected from the group consisting of Si, Al, Zr, and B.

[0042] From the viewpoint of heat resistance, the oxide anion having an MO bond other than a PO bond is preferably an oxide anion having a Q0 structure. 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., SiO44- This means that it exists as an oxide anion.

[0043] The oxide anion of the Q0 structure may exist as an anion constituting the crystal, or may exist as a polyanion structure different from the anion constituting the crystal. The anions that constitute the crystal can also be said to be fixed in the crystal structure. Specifically, this refers to the case where an argyrodite-type crystal is formed with an oxide anion of the Q0 structure as a crystal nucleus, the case where an oxide anion of the Q0 structure is incorporated into the argyrodite-type crystal structure, or both cases. This fixation of the oxide anion of the Q0 structure 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.

[0044] 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.

[0045] 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 will be lost or volatilized during the heating process, or that oxygen atoms will 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 15 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).

[0046] Such oxide anions having a Q structure may be actively added when producing a sulfide-based solid electrolyte. For example, when M is Si, an example of an active addition method is to add SiO2 as a raw material. The presence of the oxide anion of the Q0 structure can be confirmed by NMR analysis. For example, when a compound has an Si-O bond, the formation of an Si-OP bond can be confirmed by NMR analysis. 29 This can be confirmed by Si-NMR analysis.

[0047] Of the total content of O contained in the sulfide-based solid electrolyte, 60% or more is bonded to M. This allows for excellent heat resistance. The proportion of O bonded to M of the total content of O is preferably 70% or more, more preferably 80% or more, and even more preferably 90% or more, with the higher the percentage the better. Therefore, the upper limit may be 100%. The percentage of O bonded to M can be determined from the results of oxygen, nitrogen, and hydrogen analysis and NMR analysis. Specifically, the total O content in the sulfide-based solid electrolyte is quantified by oxygen, nitrogen, and hydrogen analysis. For oxygen, nitrogen, and hydrogen analysis, a measurement device can be selected depending on the O content. For example, if the oxygen content is 0.05 ppm to 5.0% (weight percentage), LECO's ONH836 is good, and if it is 5.0% or more, energy dispersive X-ray analysis is good. On the other hand, the MO bond is quantified by NMR analysis using M as the measurement nucleus. For example, when M is only P, the NMR analysis 31 P-NMR analysis is performed. When M contains Si in addition to P, 31 In addition to P-NMR analysis, 29 Si-NMR analysis is also performed. The proportion of MO bonds is then calculated from the results of the oxygen, nitrogen, and hydrogen analysis and NMR analysis described above.

[0048] The total content of O contained in the sulfide-based solid electrolyte is [O] T Similarly, the total content of O that constitutes the crystal structure of the crystalline phase contained in the sulfide-based solid electrolyte is [O] A When the above-mentioned crystal structure is not composed of oxygen, the ratio of the amount of oxygen that does not compose the crystal structure is {([O] T -[O] A ) / [O] T The oxygen that does not constitute the crystal structure is, for example, a polyanion structure that is different from the anions that constitute the argyrodite-type crystal, and does not include the O contained in the anions that constitute the argyrodite-type crystal or the O that constitutes impurity crystals such as Li3PO4.

[0049] {([O] T -[O] A ) / [O] T From the viewpoint of heat resistance, the value represented by {} is preferably more than 0.5, more preferably 0.6 or more, and even more preferably 0.7 or more. Although the reason for this is unclear, it is believed that the thermal decomposition of the argyrodite-type crystal structure is due to the elimination of sulfur from the argyrodite-type crystal structure. In contrast, if non-crystalline oxygen, i.e., polyanion structures such as oxide anions with P-O bonds, are present at the grain boundaries of the argyrodite-type crystals, this sulfur elimination can be suppressed, resulting in improved heat resistance. On the other hand, if the amount of non-crystalline oxygen is low, impurity crystals such as Li3PO4 will be present in relatively large amounts in addition to the O contained in the anions that make up the argyrodite-type crystals. It is believed that if impurity crystals are present near the argyrodite-type crystals and an interface is formed between the crystals, the above-mentioned inhibition of sulfur elimination will not occur. In addition to the heat resistance as described above, the presence of oxygen that does not constitute crystals at the grain boundaries makes the interfaces between the crystals harder than when oxygen is not present, and therefore, when crushed, cracks start from these interfaces, making it easier to crush.

[0050] {([O] T -[O] A ) / [O] T The upper limit of the value represented by {} is not particularly limited, but is usually less than 1, and may be 0.95 or less, or 0.90 or less.

[0051] The impurity crystals contained in the sulfide-based solid electrolyte may lead to a decrease in lithium ion conductivity and, as mentioned above, may also lead to a decrease in heat resistance. Therefore, the total content of lithium halide (LiHa), lithium phosphate (Li3PO4), and lithium thiophosphate (Li3PS4) contained in the sulfide-based solid electrolyte is preferably 3.0 mass% or less, more preferably 1.5 mass% or less, and even more preferably 0.6 mass% or less.

[0052] (crystalline phase) The crystalline phase in the sulfide-based solid electrolyte contains argyrodite-type crystals containing Li, P, S, and Ha. Ha is at least one element selected from the group consisting of F, Cl, Br, and I. The argyrodite-type crystals may be of the same composition or may contain two or more types of crystals with different compositions. Furthermore, the crystalline phase may contain crystals having a crystal structure other than the argyrodite-type.

[0053] The crystal structure can be analyzed from the pattern obtained by XRD measurement. If the XRD pattern has peaks at 2θ=15.7±0.5° and 30.2±0.5°, it can be said to contain argyrodite-type crystals. In addition to the above, the XRD pattern preferably also has a peak at 2θ=18.0±0.5°, and more preferably has a peak at 2θ=25.7±0.5°.

[0054] The ratio of the content (at%) of elements that make up the argyrodite-type crystal, that is, the ratio of the content of elements contained in the argyrodite-type crystal, is calculated by dividing the composition by Li α PS β Ha γ When expressed as above, it is preferable that the relationships 5<α<7, 4<β<6, and 0<γ<2 are satisfied, as this makes it easier for the crystal to become an argyrodite type. The element ratios more preferably satisfy the relationships 5.1<α<6.3, 4<β<5.3, and 0.7<γ<1.9, and even more preferably satisfy the relationships 5.2<α<6.2, 4.1<β<5.2, and 0.8<γ<1.8. That is, α is preferably greater than 5, more preferably greater than 5.1, and even more preferably greater than 5.2, and is preferably less than 7, more preferably less than 6.3, and even more preferably less than 6.2. β is preferably greater than 4, more preferably greater than 4.1, and is preferably less than 6, more preferably less than 5.3, and even more preferably less than 5.2. Regarding γ, it is preferably greater than 0, more preferably greater than 0.7, and even more preferably greater than 0.8, and is preferably less than 2, more preferably less than 1.9, and even more preferably less than 1.8.

[0055] The preferred crystal structure of argyrodite-type crystals is a cubic crystal such as F-43m, but crystals with reduced symmetry such as hexagonal, tetragonal, orthorhombic, monoclinic, and even less symmetrical crystals such as triclinic may also exist.

[0056] The halogen element represented by Ha is at least one selected from the group consisting of F, Cl, Br, and I. Since the crystal is likely to be of the argyrodite type, it preferably contains at least one of Cl and Br, more preferably contains Cl, and even more preferably contains Cl alone or a mixture of Cl and Br.

[0057] When Ha contains Cl and Br, where c1 (at%) is the Cl content in the argyrodite-type crystal and c2 (at%) is the Br content, the content ratio (c1 / c2) is preferably 0.1 or greater, more preferably 0.3 or greater, and even more preferably 0.5 or greater. Furthermore, (c1 / c2) is preferably 10 or less, more preferably 3 or less, and even more preferably 1.6 or less. When (c1 / c2) satisfies the above range, the interaction between lithium ions and halide ions is weakened, and the lithium ion conductivity of the sulfide-based solid electrolyte tends to be improved. This is thought to be due to the mixed anion effect of mixing bromide ions, which have a larger ionic radius than chloride ions, and weakening the interaction between cations and anions. Furthermore, when (c1 / c2) satisfies the above range, the cycle characteristics of lithium ion secondary batteries tend to be improved.

[0058] In addition, when Ha contains Cl and Br, the ratio of the content (at%) of the elements constituting the argyrodite-type crystal is Li α PS β Cl γ1 Br γ2When expressed as above, γ1 is preferably 0.1 or more, more preferably 0.3 or more, even more preferably 0.5 or more, and preferably 1.5 or less, more preferably 1.4 or less, and even more preferably 1.3 or less. γ2 is preferably 0.1 or more, more preferably 0.3 or more, even more preferably 0.5 or more, and preferably 1.9 or less, more preferably 1.6 or less, and even more preferably 1.4 or less. When γ1 and γ2 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 sulfide-based solid electrolyte. Furthermore, when γ1 and γ2 satisfy the above ranges, the cycle characteristics of the lithium ion secondary battery are likely to be improved. Here, it is preferable that α, β, and (γ1+γ2) satisfy the same relationship as that of the above-mentioned α, β, and γ.

[0059] The anions that make up the argyrodite-type crystals are the anions that are fixed in the crystal structure. Specifically, although it varies depending on the composition of the argyrodite-type crystals, for example, PS4 3- , P2S6 4- , P2S7 4- etc. In addition, some of these anion sites may be substituted with oxide anions. In this case, the argyrodite-type crystal contains O in addition to Li, P, S, and Ha. The substituted oxide anions are, for example, SiO4 4- , PO4 3- , AlO4 3- Depending on the composition of the oxide anion, argyrodite-type crystals may also contain elements represented by M, such as Si, Al, Zr, and B.

[0060] The crystallite size of the crystals constituting the crystalline phase is preferably small in order to obtain good lithium ion conductivity when the sulfide-based solid electrolyte is used to form a sulfide-based solid electrolyte layer 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 the Scherrer equation.

[0061] (lithium-ion secondary battery) The sulfide-based solid electrolyte has high lithium ion conductivity and excellent heat resistance when used in lithium ion secondary batteries. The lithium ion conductivity before heat treatment is preferably 6.0 mS / cm or more, more preferably 7.0 mS / cm or more, and even more preferably 8.0 mS / cm or more, at 25°C when the material is pressure-molded under a pressure of 380 MPa. The higher the conductivity, the better. The lithium ion conductivity can be determined from a Nyquist plot obtained by AC impedance measurement.

[0062] The heat resistance is evaluated by the degree of decrease in lithium ion conductivity after heat treatment at high temperature. If the ratio of the lithium ion conductivity after heat treatment at 475°C for 30 minutes to the lithium ion conductivity before heat treatment is 50% or more, the heat resistance is said to be good, and this ratio is preferably 65% ​​or more, more preferably 70% or more, even more preferably 80% or more, even more preferably 85% or more, and particularly preferably 90% or more, and the higher the ratio, the better. The heat resistance of the sulfide-based solid electrolyte may be measured by its thermal decomposition temperature. The thermal decomposition temperature of the sulfide-based 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 temperature of a sulfide-based solid electrolyte can be confirmed by the XRD pattern of the sample after heat treatment. Specifically, heat treatment is performed at each temperature, and the presence or absence of impurity crystals is confirmed and quantified by analyzing the XRD pattern after heat treatment. The temperature at which the crystals contain 10 mass% or more of impurity crystals other than argyrodite-type crystals, such as Li3PS4, LiHa, and Li2S, is defined as the thermal decomposition temperature of the sulfide-based solid electrolyte.

[0063] When used in a lithium ion secondary battery, the sulfide-based 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 sulfide-based solid electrolyte in the entire solid electrolyte layer is preferably 80 mass % or more, and more preferably 90 mass % or more.

[0064] The solid electrolyte layer can also be formed by a conventionally known method. As an example of wet molding, the components constituting the solid electrolyte layer are dispersed or dissolved in a solvent to form a slurry, which is then coated in a layer, i.e., sheet form, 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 coating amount of the slurry, etc.

[0065] Furthermore, instead of wet molding, the solid electrolyte layer may be formed by dry press molding of a powder of a sulfide-based solid electrolyte or the like onto 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.

[0066] The sulfide-based 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.

[0067] A lithium ion secondary battery using a sulfide-based 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.

[0068] <Method for producing sulfide-based solid electrolyte> The method for producing the sulfide-based solid electrolyte according to this embodiment is not particularly limited as long as it can produce the solid electrolyte described in the above <Sulfide-based solid electrolyte>. That is, the solid electrolyte contains a crystal phase containing argyrodite-type crystals and oxide anions having a PO bond containing a polyanion structure different from the anions constituting the argyrodite-type crystals, and the overall composition of the sulfide-based solid electrolyte is Li a MS b Ha c O x When the above formula is satisfied, it is sufficient to obtain a sulfide-based solid electrolyte that satisfies the relationship 3<(b+c)<{6-(x / 2)} and in which 60% or more of the total O content is bonded to M.

[0069] In order to introduce oxide anions with P-O bonds, which contain polyanion structures different from the anions that make up argyrodite-type crystals, an oxygen source compound is added to the raw materials. Conventionally, Li2O and LiOH have been used as oxygen source compounds, but these have weak oxidizing power and make it difficult to form MO bonds containing P-O bonds.

[0070] In contrast, the inventors have discovered that, from the perspective of heat resistance, in order to efficiently form PO bonds, specific lithium-containing compounds, such as Li2CO3, Li2SO4, and Li3PO4, are preferred as oxygen sources. While the detailed mechanism behind this is unclear, Li2O and LiOH have strong oxidizing power and therefore tend to form crystals such as Li3PO4, whereas O contained in carbonates, sulfates, and phosphates has weak oxidizing power. Therefore, it is believed that, although the long-range order of a crystalline structure cannot be achieved, an amorphous polyanion structure with a certain degree of order is likely to form.

[0071] In the production method according to this embodiment, at least one compound selected from the group consisting of Li2CO3, Li2SO4, and Li3PO4 is used as the oxygen source compound, which provides a moderate oxidizing power and facilitates the formation of MO bonds including PO bonds. As a result, oxide anions having PO bonds, including a polyanion structure, are introduced, and 60% or more of the total O content is bonded to M to form MO bonds.

[0072] In addition to the above, when an oxygen source compound is introduced, the P to Ha ratio required to obtain argyrodite-type crystals shifts due to the formation of P-O bonds, which causes the precipitation of impurity crystals such as lithium halide, lithium phosphate, and lithium thiophosphate in addition to the argyrodite-type crystals, which is one of the causes of the decrease in lithium ion conductivity. In contrast, in the manufacturing method according to the present embodiment, the P content in the raw materials is made to be in excess of that in the conventional method, thereby preventing deviation of the composition required to obtain argyrodite-type crystals and suppressing the precipitation of impurity crystals.

[0073] In this way, the sulfide-based solid electrolyte according to this embodiment can be obtained by using at least one compound selected from the group consisting of Li2CO3, Li2SO4, and Li3PO4 as the oxygen source compound and by making the P content in the raw materials higher than that in the past. In addition, it is preferable to produce the sulfide-based solid electrolyte by a melt quenching method, rather than the solid-state reaction that has been widely used in the past.

[0074] Specifically, the production method A preferably includes the following steps A1 to A3. (Step A1) A step of mixing raw materials containing Li, P, S, and Ha with a compound serving as an oxygen source to obtain a raw material mixture; (Step A2) a step of heating the obtained raw material mixture to obtain a melt; and (Step A3) A step of cooling the resulting melt to obtain argyrodite-type crystals and a polyanion structure of oxide anions having P-O bonds different from the anions that constitute the argyrodite-type crystals. Here, Ha is at least one element selected from the group consisting of F, Cl, Br, and I. The compound serving as the oxygen source in step A1 contains at least one selected from the group consisting of Li2CO3, Li2SO4, and Li3PO4. In the raw material mixture obtained in step A1, the content of P relative to the content of Ha is 0.5 to 0.8 in terms of molar ratio.

[0075] Each step of the manufacturing method A will be described in detail below.

[0076] In step A1, a raw material mixture containing Li, P, S, and Ha is mixed with a compound serving as an oxygen source. The raw materials may be conventionally known materials. 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), hydrogen sulfide (H2S), Li2SO4, and the like, and elemental sulfur can also be used.

[0077] 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).

[0078] Among the above, when it is desired to obtain argyrodite-type crystals, a combination of lithium sulfide, phosphorus sulfide, and at least one of lithium chloride and lithium bromide is preferred.

[0079] The compound serving as the oxygen source is at least one selected from the group consisting of Li2CO3, Li2SO4, and Li3PO4. Li2CO3, Li2SO4, and Li3PO4 are compounds containing Li among the raw materials, but other Li-containing compounds such as those exemplified above may also be used in combination as the Li-containing compound. Similarly, when Li3PO4 is used as the oxygen source compound, it is also a P-containing compound among the raw materials, but other P-containing compounds such as those exemplified above may also be used in combination as the P-containing compound. When Li2SO4 is used as the oxygen source compound, it is also a S-containing compound among the raw materials, but other S-containing compounds such as those exemplified above may also be used in combination as the S-containing compound.

[0080] PO4, which is different from the anion that constitutes the argyrodite-type crystals. 3- From the viewpoint of forming an oxide anion having a P-O bond containing the polyanion structure, the oxygen source compounds are added so that the mass ratio of oxygen in the entire composition is preferably 1.0% or more, more preferably 2.0% or more, and even more preferably 3.0% or more. Furthermore, from the viewpoint of suppressing the precipitation of impurity crystals, the total amount of oxygen source compounds added is preferably 10% or less, more preferably 8.0% or less, and even more preferably 6.0% or less.

[0081] The P content in the raw material mixture is set to be excessive in order to compensate for the P used in forming oxide anions having P-O bonds. Furthermore, the P content in the raw material mixture is set to be excessive in order to suppress a decrease in lithium ion conductivity due to impurity crystals of LiHa (lithium halide). Specifically, the P content relative to the Ha content is set to a molar ratio of 0.5 or more, preferably 0.6 or more, and more preferably 0.65 or more. Furthermore, in order to suppress a decrease in lithium ion conductivity due to a shortage of Ha, the P content relative to the Ha content is set to a molar ratio of 0.8 or less, preferably 0.75 or less.

[0082] These raw materials are very unstable in the atmosphere and may react with water to decompose, generating hydrogen sulfide gas and oxidizing. Therefore, it is preferable to mix the raw materials with the oxygen source compound in an inert atmosphere.

[0083] The raw materials can be mixed using, for example, a media-based mixer such as a planetary ball mill, a pin mill, a powder mixer, a media-less mixer such as airflow mixer, etc. The raw materials may be made amorphous by mixing before heating.

[0084] In step A2, the resulting raw material mixture is heated to obtain a melt. The heat melting conditions may be conventionally known conditions. For example, in order to increase the fluidity of the melt, the heating temperature is preferably 600°C or higher, more preferably 630°C or higher, and even more preferably 650°C or higher. In addition, the heat-melting temperature is preferably 900°C or lower, more preferably 850°C or lower, and even more preferably 800°C or lower, from the viewpoint of suppressing deterioration or decomposition of components in the melt due to heating.

[0085] The heat-melting time is preferably 0.1 hours or more, more preferably 0.5 hours or more, even more preferably 0.7 hours or more, and even more preferably 1 hour or more in order to sufficiently homogenize the melt. The heat-melting time is preferably 10 hours or less, more preferably 9.5 hours or less, and even more preferably 9 hours or less in order to prevent deterioration or decomposition of components in the melt due to heating.

[0086] In step A3, the resulting melt is cooled to obtain argyrodite-type crystals and a polyanion structure of oxide anions having a P-O bond different from the anions that constitute the argyrodite-type crystals. The crystalline state varies depending on the cooling conditions. Specifically, the crystalline state can be adjusted by changing the cooling conditions, such as the cooling rate and pressure.

[0087] From the viewpoint of maintaining the composition obtained in the heat-melting step A2, the cooling rate is preferably 0.01°C / sec or more under normal pressure, more preferably 0.05°C / sec or more, and even more preferably 0.1°C / sec or more. The upper limit of the cooling rate is not particularly limited, but may be 1,000,000°C / sec or less. Furthermore, a stabilization treatment may be performed by further performing a heat treatment after cooling.

[0088] The method for producing a sulfide-based solid electrolyte powder according to this embodiment may further include other steps in addition to the above steps A1 to A3, as long as the effects of the present invention are not impaired. [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 1 to 8 are working examples, and Examples 9 to 12 are comparative examples.

[0090] (Example 1) Lithium sulfide powder (Sigma, purity 99.98%), diphosphorus pentasulfide powder (Sigma, purity 99%), lithium chloride powder (Sigma, purity 99.99%), and lithium carbonate powder (Sigma, purity 99.99%) were weighed out in a dry nitrogen atmosphere so that the overall composition of the sulfide-based solid electrolyte and the composition ratio of the argyrodite-type crystals were the ratios listed in the "Overall Composition (at Ratio)" and "Crystal Phase (Argyrodite) Composition Ratio" sections of Table 1, respectively, and mixed in the same atmosphere using a planetary ball mill at 400 rpm for 4 hours to obtain a raw material mixture. In the raw material mixture, the molar ratio of P to Ha was 0.67 (Step A1). The obtained raw material mixture was vacuum sealed in a carbon-coated quartz tube and heated at 750°C for 60 minutes to obtain a melt (step A2).Then, the obtained melt was cooled to room temperature (25°C) at a rate of 1°C / sec to obtain a sulfide-based solid electrolyte (step A3).

[0091] (Examples 2 to 8, 10) A sulfide-based solid electrolyte was obtained in the same manner as in Example 1, except that the amounts of lithium sulfide powder, diphosphorus pentasulfide powder, lithium chloride powder, and lithium carbonate powder added were changed so that the overall composition of the sulfide-based solid electrolyte and the composition ratio of the argyrodite-type crystals would be the ratios shown in the items "Overall composition (at ratio)" and "Crystal phase (argyrodite-type) composition (at ratio)" in Table 1. The P content relative to the Ha content in the raw material mixture is as shown in the "P / Ha (molar ratio)" in Table 1. In Examples 4 to 8 and 11 to 12, lithium bromide powder (Sigma, purity 99.995%) was further added as a raw material serving as a Br source. In Example 8, SiO2 was also added as an M source for forming M-O bonds.

[0092] (Example 9, Example 11, Example 12) A sulfide-based solid electrolyte was obtained in the same manner as in Example 1, except that the amounts of lithium sulfide powder, diphosphorus pentasulfide powder, and lithium chloride powder added were changed so that the overall composition of the sulfide-based solid electrolyte and the composition ratio of the argyrodite-type crystals were the ratios shown in the "Overall composition (at ratio)" and "Crystal phase (argyrodite-type) composition (at ratio)" sections of Table 1, respectively, and lithium oxide powder was used instead of lithium carbonate powder. That is, lithium carbonate powder was not used as the oxygen source compound, and lithium oxide was a compound containing O. The P content relative to the Ha content is as shown in the "P / Ha (molar ratio)" section of Table 1.

[0093] (Overall composition) The sulfide-based solid electrolyte was weighed in a glove box and dissolved in an alkaline aqueous solution, and the composition was analyzed for each element. Specifically, P, S, and Si were analyzed by ICP optical emission spectroscopy (apparatus: Hitachi High-Tech Science Corporation, model number PS3520UVDDII). Li was analyzed by atomic absorption spectrometry (apparatus: Hitachi High-Technologies Corporation, Model No. ZA3300). When measuring Li, CsCl was added to the solution to make the concentration 0.1%. Cl and Br were analyzed by ion chromatography (apparatus: Thermo Fisher Scientific, model number ICS-2100 (column: AS11HC)). A small amount of H2O2 was added and diluted with ultrapure water before measurement. The O content was determined by the inert gas fusion-infrared absorption method described below using an oxygen, nitrogen, and hydrogen analyzer (manufactured by LECO, ONH836). The overall composition of the sulfide-based solid electrolyte obtained by analysis was normalized to 1.0, taking into account the excess P, and the resulting composition was shown in Table 1 as "Overall composition (Li a MS b Ha c O x The results of the composition analysis above were confirmed to be roughly consistent with the compounded composition within a range of ±5%. The overall composition of the sulfide-based solid electrolyte was also confirmed to be a MS b Ha c O x Table 1 also shows the values ​​expressed by (b+c) and {6-(x / 2)} when

[0094] (crystalline phase) An X-ray diffraction pattern was obtained for the sulfide-based solid electrolyte using an X-ray diffraction device (Rigaku Corporation, SmartLab) under the following measurement conditions. Radiation source: CuKα radiation (λ=1.5418 Å), tube voltage: 45 kV, tube current: 200 mA, scan angle: 10-100°, scan speed: 5° / min, step number: 0.01° / step. Measurements were performed in an environment not exposed to the atmosphere. The obtained XRD patterns confirmed that the sulfide-based solid electrolyte contained argyrodite-type crystals. Furthermore, when crystals other than argyrodite-type crystals were also contained, Rietveld analysis was performed on the XRD patterns to identify the other crystals as impurity crystals and determine their content. The results are shown in the item "Impurity crystals (mass %)" in Table 2. The impurity crystals in Examples 1 to 8 are the total contents of lithium halide, lithium phosphate, and lithium thiophosphate. The "crystalline phase (argyrodite) composition ratio" listed in Table 1 is the composition of only the argyrodite-type crystals, obtained by removing the composition before the P content becomes excessive, i.e., the composition that forms the MO bond, which is a polyanion structure different from the anions that constitute the argyrodite-type crystals, from the overall composition described above.

[0095] (Polyanion structure) Compared to sulfide-based solid electrolytes, 31 Analysis was carried out using P-NMR (JEOL, ECZ700). As a result, the peaks observed in the range of 0 to 20 ppm were 3- The peak observed in the range of 30 to 50 ppm is PO3S 3- The peak observed in the range of 60 to 75 ppm is PO2S2 3- The peak observed at 75-80 ppm is POS3 3- The molar ratio of each polyanion structure was calculated from the ratio of the area intensities of the peaks. As a result, in Example 5, [PO4 3- ]=0.2, [POS3 3- ]=0.1, [PO2S2 3- ]=0.4, [PO3S 3- ]=0.3, and {[PO4 3- ] / ([PO4 3- ]+[POS3 3- ]+[PO2S2 3- ]+[PO3S 3- ])} was 0.2. Also, in Example 12, [PO4 3- ]=0.8,[POS3 3- ]≦0.1, [PO2S2 3-]≦0.1,[PO3S 3- ]≦0.1, and {[PO4 3- ] / ([PO4 3- ]+[POS3 3- ]+[PO2S2 3- ]+[PO3S 3- The value expressed by ])} was 0.8.

[0096] (MO bond) The total O content in the sulfide-based solid electrolyte was determined by inert gas fusion-infrared absorption spectroscopy using an oxygen, nitrogen, and hydrogen analyzer (ONH836, manufactured by LECO Corporation). The specific measurement conditions are as follows: Conditions: A sulfide-based solid electrolyte was placed in a copper capsule in an environment not exposed to the atmosphere, sealed, and placed in a graphite crucible heated in a helium atmosphere using a 4500W electrode furnace. The oxygen in the sample reacted with the oxygen in the graphite crucible to form carbon monoxide and carbon dioxide, and the total oxygen content was calculated from the peak intensities of carbon monoxide and carbon dioxide observed in the infrared absorption spectrum and the charged weight of the sulfide-based solid electrolyte sample. Next, P, which is the element M whose presence was confirmed in the above (overall composition) analysis, was used as the measurement nucleus. 31 P-NMR analysis was performed to determine the amount of MO-bonded O. Specifically, the amount of MO-bonded O was determined by adding an internal standard substance and comparing the peak area. Note that in Example 8, the content of Si was very small, 29 Considering the measurement sensitivity of P-NMR analysis, it was determined that appropriate results would not be obtained, and calculations were therefore carried out assuming that all of the Si determined by ICP atomic emission spectroscopy formed Si-O bonds. The proportion of O bound to M in the total O content was calculated from the total O content obtained by the oxygen, nitrogen, and hydrogen analysis and the amount of O bound to M-O obtained by the NMR analysis. The results are shown in Table 2 under the heading "MO bond (%)."

[0097] (([O] T -[O] A ) / [O] T ) The total content of O in the sulfide-based solid electrolyte [O] was determined by oxygen, nitrogen, and hydrogen analysis in the same manner as described above (MO bond). T asked for. Next, from the results of Rietveld analysis of the XRD pattern obtained above (overall composition), the total content of O constituting the crystal structure of the crystalline phase [O] A asked for. Required [O] T and [O] A Using the value of , the proportion of O that does not form the crystal structure is calculated as {([O] T -[O] A ) / [O] T} was calculated from the formula. The results are shown in Table 2. T -[O] A ) / [O] T " section.

[0098] (Lithium ion conductivity) The sulfide-based solid electrolyte was compressed under a pressure of 380 MPa to prepare a measurement sample, and the lithium ion conductivity was measured using an AC impedance measurement device (potentiostat / galvanostat VSP, manufactured by Bio-Logic Sciences Instruments) under the following measurement conditions: measurement frequency: 100 Hz to 1 MHz, measurement voltage: 100 mV, and measurement temperature: 25°C. The results are shown in "σ25 (mS / cm)" in Table 2. Note that a lithium ion conductivity value of 4 mS / cm or higher can be said to be a favorable value for an electrolyte used in a lithium ion secondary battery.

[0099] (Evaluation: Heat resistance) The sulfide-based solid electrolyte was subjected to heat treatment at 475°C for 30 minutes, and then its lithium ion conductivity was measured using the same method as described above (Evaluation: Lithium ion conductivity). The percentage of lithium ion conductivity after heat treatment was calculated, assuming that the lithium ion conductivity before heat treatment was 100%. The results are shown in "Heat resistance (%)" in Table 2. It should be noted that if the percentage of lithium ion conductivity after heat treatment is 50% or more of the lithium ion conductivity before heat treatment, it can be said that the heat resistance is good.

[0100] [Table 1]

[0101] [Table 2]

[0102] From the above results, the sulfide-based solid electrolytes, except for Example 9, contain a crystalline phase containing argyrodite-type crystals and a polyanion structure with P-O bonds that are different from the anions that constitute these crystals. The results of Example 9 show that without the polyanion structure, the lithium ion conductivity is low and the heat resistance is also poor. Furthermore, when MO bonds including P-O bonds are present, as in Example 10, the heat resistance is improved, but a large amount of impurity crystals precipitates because the relationship (b + c) < {6 - (x / 2)} is not satisfied. As a result, high lithium ion conductivity was not obtained.

[0103] The sulfide-based solid electrolytes of Examples 1 to 8 have a high proportion of MO bonds, and the overall composition is a MS b Ha c O x When the relationship (b + c) < {6 - (x / 2)} is satisfied, high lithium ion conductivity and excellent heat resistance are exhibited. Furthermore, the larger the difference between (b + c) and {6 - (x / 2)}, the less impurity crystals there tend to be. In addition, Example 10 did not satisfy the condition (b + c) < {6 - (x / 2)}, resulting in an increase in impurity crystals and a decrease in lithium ion conductivity. In Examples 11 and 12, lithium oxide was used as the oxygen source compound, and even though the condition (b + c) < {6 - (x / 2)} was satisfied, the low proportion of MO bonds resulted in poor heat resistance.

[0104] The lithium ion conductivity increased when Br was included in addition to Cl as Ha, and there was a tendency for the lithium ion conductivity to increase as the amount of impurity crystals decreased.

[0105] 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 a Japanese patent application (Patent Application No. 2021-162228) filed on September 30, 2021, the contents of which are incorporated herein by reference.

Claims

1. A sulfide-based solid electrolyte for use in a lithium-ion secondary battery, comprising a crystalline phase and an anion, 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 anion includes an oxide anion having a P—O bond in which P and O are bonded together, at least a part of the oxide anions having a P—O bond is a polyanion structure different from the anions constituting the argyrodite-type crystal, The overall composition of the sulfide-based solid electrolyte is Li ａ MS ｂ Ha ｃ O ｘ When this is set, the relationships 4<a<7, 3<b<6, 0<c<2, 0<x, and 3<(b+c)<{6-(x / 2)} are satisfied, M is at least one element selected from the group consisting of metal elements and metalloid elements of Groups 2 to 15 of the periodic table, including at least P; A sulfide-based solid electrolyte in which 60% or more of the total content of O is bonded to the M.

2. The polyanion structure comprises at least PO ４ ３－ and P.O. ２ S ２ ３－ Including, The content expressed in mol% is 0.1≦{[PO ４ ３－ ] / ([PO ４ ３－ ]+[POS ３ ３－ ]+[PO ２ S ２ ３－ ]+[PO ３ S ３－ ])))≦0.5, and P.O. ４ ３－ , POS ３ ３－ , P.O. ２ S ２ ３－ and P.O. ３ S ３－ Among them, PO ２ S ２ ３－ The sulfide-based solid electrolyte according to claim 1, wherein the content of

3. The total content of O is [O] Ｔ The total content of O constituting the crystal structure of the crystalline phase is [O] Ａ When this is the case, 0.5 < {([O] Ｔ -[O] Ａ ) / [O] Ｔ 3. The sulfide-based solid electrolyte according to claim 1, wherein the sulfide-based solid electrolyte satisfies the relationship:}<1.

4. 3. The sulfide-based solid electrolyte according to claim 1, wherein the total content of lithium halide, lithium phosphate, and lithium thiophosphate is 0.6 mass% or less.

5. The overall composition is Li ａ MS ｂ Ha ｃ O ｘ 3. The sulfide-based solid electrolyte according to claim 1 or 2, further satisfying the relationships 0<x<1 and {3.5-(x / 2)}<(b+c)<{6-(x / 2)}.

6. The sulfide-based solid electrolyte according to claim 1 or 2, wherein the argyrodite-type crystal contains Cl as the Ha.

7. The sulfide-based solid electrolyte according to claim 1 or 2, wherein the argyrodite-type crystal contains Cl and Br as the Ha.

8. 8. The sulfide-based solid electrolyte according to claim 7, wherein the relationship of 0.1≦(c1 / c2)≦10 is satisfied, where c1 (at %) is the content of Cl in the argyrodite-type crystal, and c2 (at %) is the content of Br.

9. A method for producing a sulfide-based solid electrolyte used in a lithium-ion secondary battery, comprising: Mixing a raw material containing Li, P, S, and Ha with a compound serving as an oxygen source to obtain a raw material mixture; heating the raw material mixture to obtain a melt; and cooling the melt to obtain argyrodite-type crystals and a polyanion structure of oxide anions having a P—O bond different from that of the anions constituting the argyrodite-type crystals; The Ha is at least one element selected from the group consisting of F, Cl, Br, and I, The oxygen source compound is Li ２ CO ３ , Li ２ SO ４ and Li ３ P.O. ４ At least one selected from the group consisting of In the raw material mixture, the content of P relative to the content of Ha is 0.5 to 0.8 in terms of molar ratio.

10. The method for producing a sulfide-based solid electrolyte according to claim 9 , wherein the heating is carried out at a temperature of 650° C. or higher.

11. The method for producing a sulfide-based solid electrolyte according to claim 9 or 10, wherein the cooling is carried out at a cooling rate of 0.01°C / second or more.

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