Sulfide solid electrolyte and method for manufacturing same

Abstract

The present invention relates to a sulfide solid electrolyte containing 60 mass% or more of a sulfide-based glass phase, wherein: the sulfide solid electrolyte contains Li, P, and S as constituent elements; the composition of the sulfide solid electrolyte satisfies Li / P ≥ 2.5; and the lithium ion conductivity at 25°C when the sulfide solid electrolyte is formed into a green compact at 380 MPa is more than 1 mS / cm.

Description

Sulfide solid electrolyte and method for producing same

[0001] The present invention relates to a sulfide solid electrolyte and a method for producing the same.

[0002] Lithium-ion secondary batteries are widely used in portable electronic devices such as mobile phones and laptop computers, as well as in automobiles. Conventionally, liquid electrolytes have been used in lithium-ion secondary batteries, but concerns about leakage and fire have led to the need for larger casings for safety reasons. Furthermore, improvements have been needed to address the short battery life and narrow operating temperature range.

[0003] In response to this, all-solid-state lithium secondary batteries, which use a solid electrolyte as the electrolyte for 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 classified into sulfide solid electrolytes and oxide solid electrolytes. Sulfide ions constituting sulfide solid electrolytes have higher polarizability than oxide ions constituting oxide solid electrolytes, and exhibit high ionic conductivity. As sulfide solid electrolytes, Li １０ GeP ２ S １２ LGPS type crystals such as Li ６ P.S. ５ Argyrodite-type crystals such as Cl, Li ７ P ３ S １１ LPS crystallized glass and the like are known.

[0005] In all-solid-state batteries using solid electrolytes, the interfacial contact between the active material and the solid electrolyte is important. Therefore, the solid electrolyte must be made of a soft material that facilitates interface formation. Examples of soft materials include glass.

[0006] As an ionic conductor of sulfide-based glass, Non-Patent Document 1 describes (Li ４ P ２ S ７ ) １－ｘ -LiI ｘ (0≦x≦0.45)-based glasses are disclosed. Patent Document 1 also discloses LPSI-based and LPSBr-based glasses.

[0007] Japanese Patent Application Publication No. 2013-201110

[0008] R. Mercier et al. , SUPERIONIC CONDUCTION IN Li2S-P2S5-LiI-GLASSES, Solid State Ionics 5 (1981) 663-666

[0009] The sulfide-based glass described in Non-Patent Document 1 is produced by a melt-quenching method, and the sulfide-based glass described in Patent Document 1 is produced by a mechanical milling method.

[0010] However, these conventional glass materials have insufficient battery characteristics when used as solid electrolytes. Specifically, sulfide solid electrolytes containing sulfide-based glasses produced by melt quenching have relatively high ionic conductivity but low oxidation resistance. Furthermore, sulfide solid electrolytes containing sulfide-based glasses produced by mechanical milling have poor high-rate characteristics due to low ionic conductivity.

[0011] Therefore, an object of the present invention is to provide a sulfide solid electrolyte containing a sulfide-based glass phase that is excellent in oxidation resistance and high-rate characteristics, and a method for producing the same.

[0012] As a result of extensive research, the present inventors have found that the above-mentioned problems can be solved by performing the heating and cooling under normal pressure conditions and at a very fast cooling rate when obtaining a sulfide-based glass phase by heating and melting raw materials and then cooling them, and have thus completed the present invention.

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

[10] . [1] A sulfide solid electrolyte containing 60 mass% or more of a sulfide-based glass phase, wherein the sulfide solid electrolyte contains Li, P, and S as constituent elements, and the composition of the sulfide solid electrolyte satisfies Li / P≧2.5, and the lithium ion conductivity at 25°C when compacted under 380 MPa is greater than 1 mS / cm. [2] The sulfide solid electrolyte according to [1], wherein the sulfide solid electrolyte further contains Ha as a constituent element, wherein the Ha is at least one element selected from the group consisting of F, Cl, Br, and I, and the composition of the sulfide solid electrolyte further satisfies [(F + Cl + Br) / Ha]≧0.5. [3] The sulfide solid electrolyte according to [1] or [2], wherein the composition of the sulfide solid electrolyte further satisfies Li / P≦7.0. [4] H when kept in an atmosphere with a dew point of -20°C for 5 hours ２ The sulfide solid electrolyte according to any one of [1] to [3], wherein the amount of S generated is 1.0 mL / g or less. [5] The sulfide solid electrolyte according to any one of [1] to [4], wherein the composition of the sulfide solid electrolyte satisfies Li: 30 to 50 at %, P: 5 to 15 at %, S: 30 to 60 at %, and Ha: 1 to 12 at %.

[0014] [6] A method for producing a sulfide solid electrolyte containing 60 mass% or more of a glass phase, the method comprising the steps of: mixing raw materials to obtain a raw material mixture containing Li, P, and S; heating the raw material mixture to obtain a melt; and cooling and solidifying the melt to obtain the glass phase, wherein the heating and the cooling and solidifying are performed under atmospheric pressure conditions, and the cooling rate during the cooling and solidifying is 100°C / second or more. [7] A method for producing a sulfide solid electrolyte according to [6], wherein the melting temperature during the heating is 600 to 800°C. [8] A method for producing a sulfide solid electrolyte according to [6] or [7], wherein the composition of the sulfide solid electrolyte satisfies Li / P≧2.5. [9] A method for producing a sulfide solid electrolyte according to [8], wherein the composition of the sulfide solid electrolyte further satisfies Li / P≦7.

[10] The method for producing a sulfide solid electrolyte according to any one of [6] to [9], wherein the heating is performed under a pressure of gauge pressure ±10 kPa, and a composition ratio of the sulfide solid electrolyte expressed as [P / Li] relative to a composition ratio of the raw material mixture expressed as [P / Li] is 95 to 105%.

[0015] According to the present invention, a sulfide solid electrolyte containing a sulfide-based glass phase that has excellent oxidation resistance and high-rate characteristics can be obtained. Therefore, when the sulfide solid electrolyte is used in an all-solid-state lithium secondary battery, good battery characteristics can be achieved.

[0016] FIG. 1 is a flow diagram showing a method for producing a sulfide solid electrolyte according to this embodiment.

[0017] 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 term "to" indicating a range of values ​​is used to mean that the values ​​before and after it are included as the lower and upper limits.

[0018] Sulfide Solid Electrolyte The sulfide solid electrolyte according to this embodiment contains 60 mass % or more of a sulfide-based glass phase. The sulfide solid electrolyte contains Li, P, and S as constituent elements, and its composition satisfies Li / P≧2.5. When the sulfide solid electrolyte is compressed under 380 MPa, the lithium ion conductivity at 25° C. is greater than 1 mS / cm.

[0019] The sulfide solid electrolyte according to this embodiment contains Li, P, and S as constituent elements. In addition to these, it is preferable to contain Ha from the viewpoint of lithium ion conductivity. Here, Ha is at least one selected from the group consisting of F, Cl, Br, and I. From the viewpoint of obtaining good oxidation resistance and water resistance, it is more preferable to contain at least one of Cl and Br, and it is even more preferable to contain Br. It is also more preferable to contain both Cl and Br.

[0020] The composition of the sulfide solid electrolyte according to this embodiment has a composition ratio, expressed as Li / P, of 2.5 or more, more preferably 2.5 to 7.0, even more preferably 3.0 to 6.5, and even more preferably 3.2 to 6.2. From the viewpoint of obtaining high lithium ion conductivity, the composition ratio is 2.5 or more, preferably 3.0 or more, and more preferably 3.2 or more. Furthermore, from the viewpoint of widening the vitrification range, the composition ratio is preferably 7.0 or less, more preferably 6.5 or less, and even more preferably 6.2 or less. In this specification, the term "composition ratio" refers to the ratio of the content (at%) of each element in the sulfide solid electrolyte.

[0021] In the composition of the sulfide solid electrolyte according to this embodiment, the composition ratio represented by S / P is preferably 3.0 to 5.0, more preferably 3.5 to 4.5, and even more preferably 3.8 to 4.2. From the viewpoint of lithium ion conductivity, the composition ratio is preferably 3.0 or more, more preferably 3.5 or more, and even more preferably 3.8 or more. From the viewpoint of water resistance, the composition ratio is preferably 5.0 or less, more preferably 4.5 or less, and even more preferably 4.2 or less.

[0022] When the sulfide solid electrolyte further contains Ha, the composition ratio represented by [(F + Cl + Br) / Ha] is preferably 0.5 or more, more preferably 0.5 to 1.0, even more preferably 0.6 to 1.0, and particularly preferably 0.8 to 1.0. Here, from the viewpoint of oxidation resistance, the composition ratio is preferably 0.5 or more, more preferably 0.6 or more, and even more preferably 0.8 or more. The upper limit of the composition ratio is not particularly limited and may be 1.0. The composition ratio of 1.0 means that I is not contained as Ha. When the sulfide solid electrolyte contains I as Ha, from the viewpoint of obtaining the effect of I, the composition ratio may be 0.8 or less, or may be 0.6 or less.

[0023] When the sulfide solid electrolyte further contains Ha, in the composition of the sulfide solid electrolyte according to this embodiment, the composition ratio represented by Ha / P is preferably 0.01 to 1.5, more preferably 0.1 to 1.2, and even more preferably 0.2 to 1.0. From the viewpoint of water resistance, the composition ratio is preferably 0.01 or more, more preferably 0.1 or more, and even more preferably 0.2 or more. Furthermore, from the viewpoint of lithium halide crystal precipitation, the composition ratio is preferably 1.5 or less, more preferably 1.2 or less, and even more preferably 1.0 or less. Here, Ha refers to the sum of F, Cl, Br, and I.

[0024] More specifically, the composition of the sulfide solid electrolyte according to this embodiment preferably contains 30 to 50 at% Li, 5 to 15 at% P, and 30 to 60 at% S, and more preferably contains 30 to 50 at% Li, 5 to 15 at% P, 30 to 60 at% S, and 1 to 12 at% Ha. In this specification, at% means atomic %.

[0025] In the sulfide solid electrolyte according to this embodiment, Li is an element responsible for ion conduction as a solid electrolyte. In the composition of the sulfide solid electrolyte, the Li content is preferably 30 to 50 at%. 35 to 45 at% is preferred, and 37 to 43 at% is more preferred. From the viewpoint of increasing lithium ion conductivity, the Li content is preferably 30 at% or more, more preferably 35 at% or more, and even more preferably 37 at% or more. Furthermore, from the viewpoint of widening the vitrification range, the Li content is preferably 50 at% or less, more preferably 45 at% or less, and even more preferably 43 at% or less.

[0026] In the sulfide solid electrolyte according to this embodiment, P is an element that forms a glass network, and the P-S bond is highly resistant to both oxidation and reduction among sulfides. Therefore, as a solid electrolyte, it has a wide potential window and excellent electrochemical stability. In the composition of the sulfide solid electrolyte, the P content is preferably 5 to 15 at%, more preferably 6 to 13 at%, and more preferably 7 to 12 at%. Here, from the viewpoint of widening the vitrification range, the P content is preferably 5 at% or more, more preferably 6 at% or more, and even more preferably 7 at% or more. Furthermore, from the viewpoint of increasing lithium ion conductivity, the P content is preferably 15 at% or less, more preferably 13 at% or less, and even more preferably 12 at% or less.

[0027] In the sulfide solid electrolyte according to this embodiment, S is an element that forms a P-S bond together with P and is an essential element for forming a glass phase. In the composition of the sulfide solid electrolyte, the S content is preferably 30 to 60 at%, more preferably 33 to 50 at%, and even more preferably 35 to 47 at%. Here, from the viewpoint of widening the vitrification range, the S content is preferably 30 at% or more, more preferably 33 at% or more, and even more preferably 35 at% or more. Furthermore, from the viewpoint of increasing lithium ion conductivity, the S content is preferably 60 at% or less, more preferably 50 at% or less, and even more preferably 47 at% or less.

[0028] When the sulfide solid electrolyte according to this embodiment contains Ha, Ha is an element that contributes to high lithium ion conductivity. In the composition of the sulfide solid electrolyte, the Ha content is preferably 1 to 12 at%, more preferably 3 to 10 at%, and even more preferably 3.5 to 8 at%. Here, from the viewpoint of obtaining high lithium ion conductivity, the Ha content is preferably 1 at% or more, more preferably 3 at% or more, and even more preferably 3.5 at% or more. Furthermore, from the viewpoint of preventing precipitation of lithium halide crystals, the Ha content is preferably 12 at% or less, more preferably 10 at% or less, and even more preferably 8 at% or less. Here, the Ha content refers to the total content of F, Cl, Br, and I.

[0029] Ha is at least one element selected from the group consisting of F, Cl, Br, and I. As described above, from the viewpoint of obtaining good oxidation resistance and water resistance, it is more preferable that at least one of Cl and Br is contained, and it is even more preferable that Br is contained. It is also more preferable that both Cl and Br are contained. For example, of the Ha content, the total content of Cl and Br preferably accounts for 50% or more, more preferably 70% or more, and may be 100%, i.e., it may consist only of Cl and Br. Furthermore, the content ratio expressed as Cl:Br is preferably 100:0 to 0:100, more preferably 80:20 to 0:100, and even more preferably 60:40 to 0:100.

[0030] The sulfide solid electrolyte according to this embodiment may contain other elements as constituent elements in addition to the above-mentioned Li, P, S, and Ha. Examples of other elements that may be contained include Si, B, Ge, Al, O, Na, K, Mg, Ca, Sr, Ba, Y, Zr, Cr, Zn, Ga, Sn, and Sb. However, if the other elements are SiO ２ Or B ２ O ３ , P ２ O ５ , Al ２ O ３In the case of forming oxides such as those mentioned above, it is preferable that such oxides are not contained because they become crystal nuclei. The total content of oxides is preferably 1 mol% or less, more preferably 0.1 mol% or less, and even more preferably 0.05 mol% or less, expressed in mol% based on the oxides, and may be zero, i.e., not contained.

[0031] The sulfide solid electrolyte according to this embodiment may contain, among the other elements, for example, Si, B, Ge, Al, Sn, and Sb, which are elements that can occupy the position of P when the sulfide-based glass phase is crystallized.

[0032] When the sulfide solid electrolyte according to this embodiment contains Si as a constituent element, Si has the effect of increasing the viscosity of the melt and promoting vitrification. The Si content is preferably 0 to 15 at%, more preferably 0.1 to 10 at%, and even more preferably 1 to 5 at%. Here, from the viewpoint of optimally obtaining the effect of Si, the Si content is preferably 0.1 at% or more, and even more preferably 1 at% or more. Furthermore, from the viewpoint of electrochemical stability, the Si content is preferably 15 at% or less, more preferably 10 at% or less, and even more preferably 5 at% or less. However, if Si has a Si-O bond or SiO ２ When Si is contained as an oxide, the Si content is appropriately adjusted so that the total content of oxides is 1 mol % or less, or so that the total content of oxides that become network formers of glass is 3 to 20 mol %, as described above.

[0033] When the sulfide solid electrolyte according to this embodiment contains B as a constituent element, B has the effect of increasing the viscosity of the melt and promoting vitrification. The B content is preferably 0 to 15 at %, more preferably 0.1 to 10 at %, and even more preferably 1 to 5 at %. From the viewpoint of optimally obtaining the effects of B, the B content is preferably 0.1 at % or more, and more preferably 1 at % or more. Furthermore, from the viewpoint of lithium ion conductivity, the B content is preferably 15 at % or less, more preferably 10 at % or less, and even more preferably 5 at % or less. However, if B has a B-O bond or B ２ O ３When B is contained as an oxide, the B content is appropriately adjusted so that the total content of oxides is 1 mol % or less, or so that the total content of oxides that become network formers of the glass is 3 to 20 mol %, as described above.

[0034] When the sulfide solid electrolyte according to this embodiment contains Ge as a constituent element, Ge has the effect of promoting an improvement in lithium ion conductivity. The Ge content is preferably 0 to 15 at %, more preferably 0.1 to 10 at %, and even more preferably 1 to 5 at %. Here, from the viewpoint of lithium ion conductivity, the Ge content is preferably 0.1 at % or more, and more preferably 1 at % or more. Furthermore, from the viewpoint of ease of glass formation, the Ge content is preferably 15 at % or less, more preferably 10 at % or less, and even more preferably 5 at % or less. However, if Ge has a Ge—O bond or is a GeO ２ When Ge is contained as an oxide, the Ge content is appropriately adjusted so that the total content of the oxides is 1 mol % or less, as described above.

[0035] When the sulfide solid electrolyte according to this embodiment contains Al as a constituent element, Al has the effect of increasing the viscosity of the melt and promoting vitrification. The Al content is preferably 0 to 15 at%, more preferably 0.1 to 10 at%, and even more preferably 1 to 5 at%. Here, from the viewpoint of optimally obtaining the effect of Al, the Al content is preferably 0.1 at% or more, and more preferably 1 at% or more. Furthermore, from the viewpoint of lithium ion conductivity, the Al content is preferably 15 at% or less, more preferably 10 at% or less, and even more preferably 5 at% or less. However, if Al has an Al—O bond or Al ２ O ３ When Al is contained as an oxide, the Al content is appropriately adjusted so that the total content of oxides is 1 mol % or less, or so that the total content of oxides that become network formers of glass is 3 to 20 mol %, as described above.

[0036] When the sulfide solid electrolyte according to this embodiment contains Sn as a constituent element, Sn has the effect of increasing the viscosity of the melt and promoting vitrification. The Sn content is preferably 0 to 15 at%, more preferably 0.1 to 10 at%, and even more preferably 1 to 5 at%. Here, from the viewpoint of optimally obtaining the effects of Sn, the Sn content is preferably 0.1 at% or more, and more preferably 1 at% or more. Furthermore, from the viewpoint of achieving high lithium ion conductivity, the Sn content is preferably 15 at% or less, more preferably 10 at% or less, and even more preferably 5 at% or less. However, if Sn has an Sn—O bond or SnO ２ When Sn is contained as an oxide, the Sn content is appropriately adjusted so that the total content of the oxides is 1 mol % or less, as described above.

[0037] When the sulfide solid electrolyte according to this embodiment contains Sb as a constituent element, Sb has the effect of increasing the viscosity of the melt and promoting vitrification. The Sb content is preferably 0 to 15 at %, more preferably 0.1 to 10 at %, and even more preferably 1 to 5 at %. Here, from the viewpoint of optimally obtaining the effects of Sb, the Sb content is preferably 0.1 at % or more, and more preferably 1 at % or more. Furthermore, from the viewpoint of lithium ion conductivity, the Sb content is preferably 15 at % or less, more preferably 10 at % or less, and even more preferably 5 at % or less. However, if Sb has an Sb—O bond or Sb ２ O ３ When Sb is contained as an oxide, the Sb content is appropriately adjusted so that the total content of oxides is 1 mol % or less, or so that the total content of oxides that become network formers of glass is 3 to 20 mol %, as described above.

[0038] The constituent elements of the sulfide solid electrolyte and the method for determining their respective contents (composition ratios) vary depending on the element. For example, P and S can be determined by ICP emission spectroscopy, Li by atomic absorption spectroscopy, and Ha by ion chromatography. Details of each analysis will be described in the Examples section below.

[0039] The sulfide solid electrolyte according to this embodiment may be heat-treated to crystallize the sulfide-based glass phase that constitutes the sulfide solid electrolyte. In this case, the crystal phase resulting from the crystallization of the sulfide-based glass phase may have a thio-lisicon or LGPS crystal structure. The thio-lisicon or LGPS crystal phase has high lithium ion conductivity and can also be used as a solid electrolyte.

[0040] The proportion of the sulfide-based glass phase in the sulfide solid electrolyte according to this embodiment is 60% by mass or more, preferably 60 to 100% by mass, more preferably 65 to 95% by mass, and even more preferably 70 to 90% by mass. From the viewpoint of lithium ion conductivity, the proportion of the sulfide-based glass phase is 60% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 70% by mass or more. Alternatively, the proportion of the sulfide-based glass phase may be 100% by mass, i.e., the sulfide solid electrolyte may consist solely of a sulfide-based glass phase. However, when obtaining the sulfide-based glass phase, a crystalline phase may also be obtained, and attempting to obtain only the sulfide-based glass phase is time-consuming and costly. Therefore, from the viewpoint of the manufacturability of the sulfide solid electrolyte, the proportion of the sulfide-based glass phase is preferably 95% by mass or less, more preferably 90% by mass or less. The proportion of the sulfide-based glass phase can be determined by powdering the sulfide solid electrolyte, performing powder X-ray diffraction (XRD) measurement together with a crystalline powder serving as an internal standard, determining the proportion of crystals, and subtracting this from 100% by mass to perform Rietveld analysis.

[0041] The constituent elements and composition ratios of the sulfide-based glass phase in this embodiment can be considered to be substantially the same as the constituent elements and composition ratios of the sulfide solid electrolyte according to this embodiment. The difference between the constituent elements and composition ratios of the two becomes smaller as the proportion of the sulfide-based glass phase in the sulfide solid electrolyte increases. Therefore, the preferred aspects of the constituent elements and composition ratios of the sulfide solid electrolyte can be interpreted as the preferred aspects of the constituent elements and composition ratios of the sulfide-based glass phase.

[0042] <Other Phases> The sulfide solid electrolyte according to this embodiment may also contain phases other than the sulfide-based glass phase. Examples of other phases include, for example, the crystalline phases obtained during the production of the sulfide-based glass phase, as described above. The total proportion of such other phases in the sulfide solid electrolyte is 40% by mass or less, preferably 1 to 40% by mass, more preferably 2 to 35% by mass, and even more preferably 5 to 30% by mass. From the viewpoint of fully utilizing the properties of the sulfide-based glass phase, the total proportion of the other phases is 40% by mass or less, preferably 35% by mass or less, and even more preferably 30% by mass or less. Furthermore, while the other phases may not be present, from the viewpoint of the manufacturability of the sulfide solid electrolyte, the total proportion of the other phases is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 5% by mass or more.

[0043] <Physical Properties of Sulfide Solid Electrolyte> The sulfide solid electrolyte according to this embodiment has a lithium ion conductivity of more than 1 mS / cm at 25°C when compacted under 380 MPa. This is due to the high lithium ion conductivity of the sulfide-based glass phase. However, since it is difficult to extract only the sulfide-based glass phase from the sulfide solid electrolyte, the lithium ion conductivity of the sulfide-based glass phase is evaluated based on the lithium ion conductivity of a sulfide solid electrolyte containing 60 mass% or more of the sulfide-based glass phase.

[0044] Although it is unclear why the lithium ion conductivity of the sulfide-based glass phase in this embodiment is higher than that of conventional sulfide-based glasses, it is believed that this is because the glass phase (amorphous phase) does not have any regular atomic arrangement and is not a completely amorphous structure. In other words, the sulfide-based glass phase in this embodiment is a glass phase with a certain degree of order, i.e., a periodic structure, which is believed to contribute to the realization of high lithium ion conductivity.

[0045] The lithium ion conductivity of the sulfide solid electrolyte according to this embodiment may be greater than 1 mS / cm, and this value varies depending on the composition and the proportion of the sulfide-based glass phase. For example, when the Br content of the Ha composition is 90 at% or more, the lithium ion conductivity is preferably 1.3 mS / cm or more. Furthermore, when the Cl content of the Ha composition is 90 at% or more, the lithium ion conductivity is preferably 1.1 mS / cm or more. The lithium ion conductivity in this specification is determined by AC impedance measurement using a measurement sample obtained by compacting a sulfide solid electrolyte powder at 380 MPa. Specifically, the AC impedance of the measurement sample is measured at a measurement frequency of 100 Hz to 1 MHz, a measurement voltage of 100 mV, and a measurement temperature of 25°C, and the value obtained from the resulting Nyquist plot is taken as the lithium ion conductivity.

[0046] The sulfide solid electrolyte according to this embodiment was kept in an atmosphere with a dew point of −20° C. for 5 hours. ２ The amount of sulfur generated is preferably 10 mL / g or less, more preferably 1 mL / g or less, and even more preferably 0.1 mL / g or less, and the smaller the better. ２ More specific conditions for measuring the amount of sulfur generated are as follows: First, the sulfide solid electrolyte is passed through a 100 μm mesh to obtain powder with an average particle size of 10 to 20 μm. Then, 10 mg of this powder is mixed with nitrogen (N ２ ) gas at a flow rate of 0.5 L / min for 5 hours. ２ S) Monitor the amount of generation and calculate the total amount as H ２ The amount of S generated is the above H ２ The amount of S generated is a value that serves as an index of the hydrolysis resistance, i.e., water resistance, of the sulfide solid electrolyte, and can be achieved by having a sulfide-based glass phase content of 60 mass% or more and by including Ha as a constituent element. In particular, it is preferable that at least one of Cl and Br is included as Ha, and it is more preferable that Br is included. It is also more preferable that both Cl and Br are included.

[0047] The average particle size (D50) of the sulfide solid electrolyte according to this embodiment is preferably 5 to 300 μm, more preferably 10 to 200 μm, and even more preferably 10 to 150 μm. From the viewpoint of reducing the load of the pulverization process when using the sulfide solid electrolyte in a secondary battery, the average particle size is preferably 300 μm or less, more preferably 200 μm or less, and even more preferably 150 μm or less. Furthermore, from the viewpoint of powder handling, the average particle size is preferably 5 μm or more, more preferably 10 μm. The average particle size (D50) can be adjusted by subjecting the obtained sulfide solid electrolyte to coarse pulverization. Conventional known methods can be used for coarse pulverization, including, for example, a cutter mill, a planetary ball mill, and a jet mill, and dry pulverization is preferred.

[0048] The sulfide solid electrolyte according to this embodiment is suitable as an electrolyte for lithium ion secondary batteries. When used in lithium ion secondary batteries, the sulfide solid electrolyte forms a solid electrolyte layer together with other components such as a binder, as necessary. Conventionally known binders and other components can be used. The content of the sulfide solid electrolyte according to this embodiment is preferably 80 mass % or more, and more preferably 90 mass % or more, of the entire solid electrolyte layer.

[0049] The sulfide solid electrolyte according to this embodiment 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, and the like used in the positive electrode layer or the negative electrode layer may be conventionally known materials.

[0050] The lithium ion secondary battery includes a positive electrode layer, a solid electrolyte layer, and a positive electrode layer. It is preferable that at least one layer selected from the group consisting of the positive electrode layer, the solid electrolyte layer, and the positive electrode layer contains the sulfide solid electrolyte according to the present embodiment.

[0051] The lithium ion secondary battery may have any of a variety of shapes, including a coin shape, a sheet shape (film shape), a folded shape, a wound bottomed cylindrical shape, and a button shape, and may be appropriately selected depending on the intended use.

[0052] <<Method for Producing Sulfide Solid Electrolyte>> The method for producing a sulfide solid electrolyte according to this embodiment includes the following steps S1 to S3 in order, as shown in FIG. 1 : Step S1: Mixing raw materials to obtain a raw material mixture containing Li, P, and S; Step S2: Heating the raw material mixture obtained in step S1 to obtain a melt; Step S3: Cooling and solidifying the melt obtained in step S2 to obtain a glass phase; Step S4: Cooling and solidifying the melt obtained in step S3 to obtain a glass phase;

[0053] The heating in step 2 and the cooling and solidification in step 3 are carried out under atmospheric pressure, and the cooling rate during the cooling and solidification is 100°C / sec or more. This results in a sulfide solid electrolyte containing 60 mass% or more of a glass phase. In this specification, "under atmospheric pressure" means a pressure range of approximately (gauge pressure ±15 kPa).

[0054] In addition, in the above step 1, by mixing the raw materials so that the composition of the sulfide solid electrolyte to be obtained satisfies Li / P≧2.5, the sulfide solid electrolyte described in the above <<Sulfide Solid Electrolyte>> can be obtained.

[0055] Each step will be explained below.

[0056] <Step 1> In this embodiment, step S1 is step 1 in which raw materials are mixed to obtain a raw material mixture containing Li, P, and S. Specifically, a raw material containing Li, a raw material containing P, and a raw material containing S are mixed to obtain a raw material mixture. Furthermore, when it is desired to obtain a glass phase further containing Ha as a constituent element, a raw material containing Ha is further mixed in addition to the raw material containing Li, the raw material containing P, and the raw material containing S to obtain a raw material mixture containing Li, P, S, and Ha. Here, the Ha element refers to at least one element selected from the group consisting of F, Cl, Br, and I.

[0057] Examples of raw materials containing Li include lithium sulfide (Li ２ S), lithium carbonate (Li ２ CO ３ ), lithium sulfate (Li ２ SO ４ ), lithium oxide (Li ２Examples of the substance containing Li include lithium compounds such as lithium ion (LiO), lithium hydroxide (LiOH), and metallic lithium. The substance containing Li may be used alone or in combination of two or more.

[0058] From the viewpoint of obtaining a sulfide-based glass phase, it is preferable to use lithium sulfide as the raw material containing Li. Furthermore, when the obtained sulfide solid electrolyte contains a halogen element, lithium halide (LiHa, where Ha is a halogen element) is also preferable as the raw material containing Li. Lithium halide will be described later.

[0059] Examples of raw materials containing the P element include diphosphorus pentasulfide (P ２ S ５ ), diphosphorus trisulfide (P ２ S ３ ) and other phosphorus sulfides, sodium phosphate (Na ３ P.O. ４ The phosphorus-containing substance may be used singly or in combination of two or more.

[0060] The raw material containing the P element is preferably phosphorus sulfide, and more preferably diphosphorus pentasulfide (P), from the viewpoint of preventing the inclusion of elements other than those constituting the target sulfide solid electrolyte. ２ S ５ ) is more preferred.

[0061] Examples of raw materials containing the S element include lithium sulfide (Li ２ S), diphosphorus trisulfide (P ２ S ３ ), diphosphorus pentasulfide (P ２ S ５ ), other sulfur compounds containing phosphorus, and compounds containing sulfur. ２ S, CS ２ , iron sulfide (FeS, Fe ２ S ３ , FeS ２ , Fe １－ｘ S, etc.), bismuth sulfide (Bi ２ S ３ ), copper sulfide (CuS, Cu ２ S, Cu １－ｘThe substance containing the S element may be used alone or in combination of two or more.

[0062] The raw material containing the S element is preferably lithium sulfide or phosphorus sulfide from the viewpoint of preventing the inclusion of elements other than those constituting the target sulfide solid electrolyte. As the phosphorus sulfide, diphosphorus pentasulfide (P ２ S ５ Lithium sulfide is a compound that serves as both a raw material containing Li and a raw material containing S, and phosphorus sulfide is a compound that serves as both a raw material containing S and a raw material containing P.

[0063] Examples of raw materials containing Ha element include lithium halides such as lithium fluoride (LiF), lithium chloride (LiCl), lithium bromide (LiBr), and lithium iodide (LiI), phosphorus halides, phosphoryl halides, sulfur halides, sodium halides, and boron halides. One or more substances containing Ha element may be used, or two or more may be used in combination.

[0064] From the viewpoint of reactivity, the raw material containing the Ha element is preferably a lithium halide, more preferably LiCl, LiBr, or LiI, and even more preferably LiCl or LiBr.

[0065] A raw material mixture may be obtained by further mixing raw materials containing other elements according to the desired composition of the sulfide solid electrolyte. As described above, examples of the other elements include Si, B, Ge, Al, O, Na, K, Mg, Ca, Sr, Ba, Y, Zr, Cr, Zn, Ga, Sn, and Sb. Among the other elements, for example, Si, B, Ge, Al, Sn, and Sb may be included. These Si, B, Ge, Al, Sn, and Sb are elements that can occupy the P position when the sulfide-based glass phase is crystallized.

[0066] As raw materials containing these other elements, conventionally known raw materials can be used. For example, as a raw material containing Si element, SiO ２ , SiS ２ Among them, from the viewpoint of lithium ion conductivity, SiO ２These compounds may be used alone or in combination of two or more. Examples of raw materials containing B element include B ２ O ３ , B ２ S ３ Among them, from the viewpoint of water resistance of glass, B ２ O ３ These compounds may be used alone or in combination of two or more. The raw material containing Ge element is GeO ２ , GeS, GeS ２ , GeCl ２ Among them, from the viewpoint of lithium ion conductivity, GeS ２ , GeCl ２ is preferred, and GeS ２ These compounds may be used alone or in combination of two or more. Examples of raw materials containing Al element include Al ２ S ３ , Al ２ O ３ , AlCl ３ Among them, Al is the most suitable from the viewpoint of lithium ion conductivity and water resistance. ２ S ３ , AlCl ３ is preferred, and Al ２ S ３ These compounds may be used alone or in combination of two or more. Examples of raw materials containing Sn include SnS, SnS ２ , SnO, SnO ２ , SnCl ２ Among them, from the viewpoint of lithium ion conductivity, SnS ２ , SnCl ２ is preferred, and SnS ２ These compounds may be used alone or in combination of two or more. Examples of raw materials containing Sb include Sb ２ S ３ , Sb ２ O ３ , Sb ２ O ５ , SbCl３ , SbCl ５ Among them, from the viewpoint of lithium ion conductivity and water resistance, Sb ２ S ３ , SbCl ３ is preferred, and Sb ２ S ３ These compounds may be used alone or in combination of two or more.

[0067] These raw materials are mixed appropriately according to the composition of the sulfide solid electrolyte to be obtained. Specifically, to obtain the sulfide solid electrolyte described in the above "Sulfide Solid Electrolyte", the raw materials are mixed so that the composition of the sulfide solid electrolyte satisfies Li / P≧2.5. Furthermore, it is preferable that the composition of the sulfide solid electrolyte further satisfies Li / P≦7. In addition to the above, preferred aspects of the obtained sulfide solid electrolyte are the same as the preferred aspects of the above "Sulfide Solid Electrolyte".

[0068] It is preferable that the particle size of each raw material be small from the viewpoint of shortening the holding time during heating and melting in the subsequent step 2. Furthermore, if the particle size of the raw material is too large, it may affect the homogeneity of the sulfide solid electrolyte, and from this viewpoint as well, it is preferable that the particle size of the raw material be small to a certain extent. However, the production method according to this embodiment has excellent composition controllability. Therefore, even if raw materials with particle sizes that may reduce homogeneity in conventional production methods are used, a more homogeneous sulfide solid electrolyte can be produced by the production method according to this embodiment.

[0069] From the above viewpoint, specifically, the particle size of each raw material is preferably 1 mm or less, more preferably 500 μm or less, even more preferably 250 μm or less, even more preferably 100 μm or less, and particularly preferably 50 μm or less. Smaller particle sizes are preferable, but a practical lower limit is about 0.1 μm, and the particle size may be 1 μm or more, or may be 5 μm or more. Furthermore, as described above, according to the production method of this embodiment, a homogeneous sulfide solid electrolyte is easily obtained even when raw materials with relatively large particle sizes are used. In consideration of this, for example, from the viewpoint of reducing production costs, the particle size of each raw material may be 10 μm or more, 100 μm or more, or 250 μm or more.

[0070] From these, the particle size of each raw material is preferably 0.1 μm to 1 mm, more preferably 1 to 500 μm, even more preferably 5 to 250 μm, even more preferably 5 to 100 μm, and particularly preferably 5 to 50 μm. Furthermore, from the viewpoint of production costs, the particle size of each raw material is preferably 10 μm to 1 mm, more preferably 100 μm to 1 mm, and even more preferably 250 to 500 μm. In this specification, the particle size of each raw material refers to the average particle size (D50) expressed as the median diameter obtained from a volume-based particle size distribution chart obtained by measuring the particle size distribution using a laser diffraction particle size distribution analyzer MT3300EXII manufactured by Microtrac.

[0071] The raw materials can be mixed, for example, by mixing in a mortar, 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.

[0072] <Step 2> Step S2 in this embodiment is step 2 in which the raw material mixture obtained in step 1 is heated to obtain a melt.

[0073] The specific method for heating and melting the raw material mixture is not particularly limited, as long as the heating is performed under atmospheric pressure conditions. In the production method according to this embodiment, the heating for melting the raw material mixture is performed under atmospheric pressure conditions, thereby reducing the compositional difference between the composition of the raw material mixture and the composition of the resulting sulfide solid electrolyte. Furthermore, by performing the heating under a gas atmosphere containing the element S, the compositional difference can be further suppressed. Note that in the production method according to this embodiment, the heating and melting are performed in an atmosphere-controlled environment under atmospheric pressure conditions, and do not need to be performed in a sealed tube. However, this does not in any way exclude the use of a sealed tube.

[0074] Here, the difference between the sealed tube environment and the controlled atmosphere environment under normal pressure conditions is that in the sealed tube environment, the pressure inside the container is a vacuum, i.e., less than (gauge pressure -15 kPa), whereas in the controlled atmosphere environment under normal pressure conditions, the pressure inside the container during heating and melting is within the range of (gauge pressure ±15 kPa).

[0075] That is, in the production method according to the present embodiment, the heating and melting in step 2 is performed under normal pressure conditions (gauge pressure ±15 kPa). At this time, the composition ratio of the resulting sulfide solid electrolyte expressed as [P / Li] relative to the composition ratio of the raw material mixture expressed as [P / Li] is preferably 95 to 105%.

[0076] The pressure during the heat melting may be normal pressure conditions of (gauge pressure ±15 kPa), preferably (gauge pressure ±10 kPa), more preferably (gauge pressure ±5 kPa), and even more preferably (gauge pressure ±2 kPa). Furthermore, the composition ratio of the resulting sulfide solid electrolyte expressed as [P / Li] relative to the composition ratio expressed as [P / Li] of the raw material mixture at this time is more preferably 95 to 105%, even more preferably 97 to 103%, even more preferably 99 to 101%, and the closer to 100%, the more preferable. Here, the composition ratio is preferably 95% or more, more preferably 97% or more, even more preferably 99% or more, and preferably 105% or less, more preferably 103% or less, and even more preferably 101% or less.

[0077] With respect to the composition ratio of the raw material mixture expressed as [P / Li] and the composition ratio of the resulting sulfide solid electrolyte expressed as [P / Li], Li is an element that is less likely to volatilize, and P is an element that is more likely to volatilize. Therefore, comparing the composition ratios expressed as [P / Li] between the raw material mixture and the sulfide solid electrolyte is appropriate as an index of the compositional difference between the composition of the raw material mixture and the composition of the resulting sulfide solid electrolyte.

[0078] Examples of heat-resistant containers for containing the raw material mixture include heat-resistant containers made of carbon, heat-resistant containers containing oxides such as quartz, quartz glass, borosilicate glass, aluminosilicate glass, alumina, zirconia, and mullite, heat-resistant containers containing nitrides such as silicon nitride and boron nitride, and heat-resistant containers containing carbides such as silicon carbide. These heat-resistant containers may be formed in bulk from the above-mentioned materials, or may be containers on which a layer of carbon, oxide, nitride, carbide, or the like is formed, such as a carbon-coated quartz tube.

[0079] The melting temperature when the raw material mixture is heated varies depending on the raw materials used and the composition of the raw material mixture, but is, for example, preferably 600 to 950°C, more preferably 630 to 850°C, and even more preferably 650 to 750°C. Here, from the viewpoint of reaction rate, the melting temperature is preferably 600°C or higher, more preferably 630°C or higher, and even more preferably 650°C or higher. Furthermore, from the viewpoint of suppressing composition deviation due to volatilization of components, the melting temperature is preferably 950°C or lower, more preferably 850°C or lower, and even more preferably 750°C or lower.

[0080] The heat-melting time varies depending on the scale, but is preferably 10 minutes to 10 hours, more preferably 30 minutes to 9.5 hours, even more preferably 45 minutes to 9 hours, and particularly preferably 1 to 9 hours. From the viewpoint of smoothly progressing the reaction, the heat-melting time is preferably 10 minutes or more, more preferably 30 minutes or more, even more preferably 45 minutes or more, and particularly preferably 1 hour or more. From the viewpoint of productivity, 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.

[0081] The atmosphere in the container during heat melting is not particularly limited. For example, it may be a nitrogen atmosphere, an argon atmosphere, or a gas atmosphere containing an S element. Among these, a gas atmosphere containing an S element is preferred from the viewpoint of more suitably suppressing composition deviation.

[0082] When the gas atmosphere contains S element, examples of the S element source include sulfur gas, hydrogen sulfide gas, sulfur dioxide gas, etc., and sulfur gas is preferred from the viewpoint of reactivity with the raw materials. Alternatively, the gas containing S element may be introduced by adding elemental sulfur powder and vaporizing the elemental sulfur powder by heating during melting. The elemental sulfur powder may be added together with the raw materials when mixing the raw materials to obtain a raw material mixture, or may be added separately after obtaining the raw material mixture. However, when the elemental sulfur powder is added together with the raw materials when obtaining the raw material mixture, the mass of the obtained raw material mixture does not include the mass of the elemental sulfur powder. The gas containing S element may be introduced by adding the above-mentioned sulfur gas, hydrogen sulfide gas, sulfur dioxide gas, etc., and the gas containing S element resulting from the addition of elemental sulfur powder.

[0083] The gas containing the S element is preferably used as a mixed gas with, for example, an inert gas, such as the S element source gas. The mixing ratio with the inert gas is arbitrary and is not particularly limited as long as the cumulative amount of S element introduced relative to the mass of the raw material mixture can be set to a desired value.

[0084] Examples of the inert gas include nitrogen gas, argon gas, and helium gas, and these may be used alone or in combination of two or more.

[0085] The dew point during heat melting is preferably −20° C. or lower, and although there is no particular lower limit, it is usually about −80° C. The oxygen concentration is preferably 1000 ppm by volume or lower.

[0086] In step 2, whether the molten mixture of raw materials is dissolved can be confirmed by the absence of peaks derived from crystals in high-temperature X-ray diffraction measurement. It can also be confirmed by heating the molten mixture at a predetermined temperature and tilting the molten mixture to check whether it has fluidity.

[0087] <Step 3> Step S3 in this embodiment is step 3 in which the melt obtained in step 2 is cooled and solidified to obtain a glass phase.

[0088] Cooling and solidification are carried out under atmospheric pressure conditions. As mentioned above, atmospheric pressure conditions refer to an atmosphere under a pressure of approximately (gauge pressure ±15 kPa). The pressure during cooling and solidification may be atmospheric pressure conditions of (gauge pressure ±15 kPa), preferably (gauge pressure ±10 kPa), more preferably (gauge pressure ±5 kPa), and even more preferably (gauge pressure ±2 kPa).

[0089] The cooling rate during cooling and solidification is 100°C / sec or higher, preferably 100 to 100,000°C / sec, more preferably 500 to 50,000°C / sec, and even more preferably 1,000 to 10,000°C / sec. Here, it has been discovered that by using ultra-rapid cooling at a rate of 100°C / sec or higher, which is faster than conventional methods, a sulfide solid electrolyte containing 60 mass% or more of a sulfide-based glass phase containing Li, P, and S as constituent elements can be obtained. This sulfide-based glass phase can achieve higher lithium ion conductivity than conventional sulfide-based glasses, thereby increasing the lithium ion conductivity of the sulfide solid electrolyte. Furthermore, by performing the cooling and solidification under atmospheric pressure conditions, the compositional difference between the composition of the raw material mixture and the composition of the resulting sulfide solid electrolyte can be reduced.

[0090] The cooling rate is 100°C / sec or more, but from the viewpoint of ease of glass formation, it is preferably 500°C / sec or more, more preferably 1000°C / sec or more. Furthermore, the upper limit of the cooling rate is not particularly limited, but from the viewpoint of equipment capacity, it is preferably 100,000°C / sec or less, more preferably 50,000°C / sec or less, and even more preferably 10,000°C / sec or less. The above-mentioned ultra-rapid cooling can be achieved, for example, by using a rapid-cooling twin roll.

[0091] The sulfide solid electrolyte containing 60 mass% or more of the glass phase obtained above may be used as is for at least one selected from the group consisting of a positive electrode layer, a solid electrolyte layer, and a negative electrode layer, or may be used after being crushed, dried, or the like depending on the application. Furthermore, the sulfide solid electrolyte according to this embodiment may be heat-treated at a temperature equal to or higher than the crystallization temperature of the glass phase to crystallize at least a portion of the glass phase and used as described above. In this case, the crystalline phase derived from the glass phase may have a thiolisicon-type or LGPS-type crystalline structure, which has high lithium ion conductivity.

[0092] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these. Examples 1 to 5 are working examples, and Examples 6 to 8 are comparative examples.

[0093] <Test Examples> <Example 1> Li under a dry nitrogen gas atmosphere ３．４ P １．０ S ４．０ Br ０．３ Lithium sulfide powder (Sigma, purity 99.98%), diphosphorus pentasulfide powder (Sigma, purity 99%), and lithium bromide powder (Sigma, purity 99.995%) were weighed and mixed in a mortar to obtain a raw material mixture (step 1). The obtained raw material mixture was placed in a carbon container under a nitrogen atmosphere containing sulfur powder (Sigma, purity 99.998%) as an S element source and a dew point of -50 ° C or less, and placed in an electric furnace inside a glove box. The pressure was increased to +1 kPa and the temperature was increased to 750 ° C for 1 hour to obtain a melt (step 2). The tip of the container was then heated to melt the mixture, and the melt was poured onto a twin roll and cooled to room temperature at a cooling rate of 10,000 ° C / s to obtain a sulfide solid electrolyte (step 3). The pressure at this time was (gauge pressure + 1 kPa), and the cooling rate was adjusted by the outflow rate of the melt, the roll gap between the twin rolls, and the rotation speed.

[0094] <Examples 2, 3, and 6> Sulfide solid electrolytes were obtained in the same manner as in Example 1, except that the cooling rate of the melt obtained by heating was changed to 5000°C / sec (Example 2), 1000°C / sec (Example 3), or 50°C / sec (Example 6).

[0095] <Example 4, Example 5, Example 7> Li ３．７ P １．０ S ４．０ Br ０．３ (Example 4), Li ３．４ P １．０ S ４．０ Cl ０．１ Br ０．２ (Example 5), or Li ３．３ P １．０ S ３．５ I １．３A sulfide solid electrolyte was obtained in the same manner as in Example 1, except that a raw material mixture was obtained by weighing and mixing the raw materials so as to have a composition ratio of (Example 7), and the cooling rate was changed to 5000°C / sec (Example 4), 10000°C / sec (Example 5), or 50°C / sec (Example 7). Note that in Example 5, lithium chloride powder (manufactured by Sigma, purity 99.995%) was further used as a raw material, and in Example 7, lithium iodide powder (manufactured by Sigma, purity 99.99%) was further used as a raw material.

[0096] Example 8: Li under a dry nitrogen gas atmosphere ３．４ P １．０ S ４．０ Br ０．３ Lithium sulfide powder (manufactured by Sigma, purity 99.98%), diphosphorus pentasulfide powder (manufactured by Sigma, purity 99%), and lithium bromide powder (manufactured by Sigma, purity 99.995%) were weighed and mixed in a mortar to obtain a raw material mixture. The obtained raw material mixture was further mixed using a planetary ball mill (manufactured by Fritsch, P-7) to obtain a sulfide solid electrolyte. Mixing using a planetary ball mill was performed using zirconia balls with a particle size of 4 mm and a zirconia pot, and mechanical milling was performed at 400 rpm for 20 hours in a nitrogen atmosphere. The dew point during operation was kept below -70 ° C.

[0097] <Evaluation> <Composition of sulfide-based glass phase> The obtained sulfide solid electrolyte was weighed in a glove box and dissolved in an alkaline aqueous solution, and composition analysis was performed for each element. Specifically, P and S were analyzed by ICP atomic emission spectroscopy (apparatus: Hitachi High-Tech Science Corporation, model PS3520UVDDII). Li was analyzed by atomic absorption spectroscopy (apparatus: Hitachi High-Tech Corporation, model ZA3300; when measuring Li, CsCl was added to the solution to make the concentration 0.1%). Cl, Br, and I were analyzed by ion chromatography (apparatus: Thermo Fisher Scientific, model ICS-2100 (column: AS11HC), H ２ O ２(The sulfide solid electrolyte was diluted with ultrapure water and measured.) The results are shown in the "Total" section of the "Composition" section of Table 1. The total composition here is a blended composition normalized to P of 1.0. In all examples, the composition of the obtained sulfide solid electrolyte did not deviate significantly from the composition of the raw material mixture. However, the composition ratio of the obtained sulfide solid electrolyte expressed as [P / Li] relative to the composition ratio of the raw material mixture expressed as [P / Li] is shown in "Composition Deviation (%)" in Table 1.

[0098] <Proportion of Sulfide-Based Glass Phase> The obtained sulfide solid electrolyte was crushed in a mortar and then passed through a 100 μm sieve to obtain a powder with an average particle size (D50) of approximately 20 μm. This was used as a sample. An X-ray diffraction (XRD) device (Rigaku Corporation, SmartLab) was used under the following conditions: radiation source: CuKα radiation (λ = 1.5418 Å), tube voltage: 45 kV, tube current: 200 mA, scan angle: 10-60°, scan rate: 5° / min, step number: 0.01° / step, and measurement was performed in an environment not exposed to air. From the obtained XRD pattern, the proportion of the crystalline phase to the sulfide-based glass phase was determined using a calibration curve based on the weight ratio of the crystalline phase contained in the sulfide solid electrolyte to the internal standard substance, Si. The proportion of the sulfide-based glass phase was shown in Table 1.

[0099] <Lithium ion conductivity> The obtained sulfide solid electrolyte was pulverized in a mortar and then passed through a 100 μm sieve to obtain a powder with an average particle size (D50) of approximately 20 μm, which was used as a sample. The above sample was compressed under a pressure of 380 MPa to obtain a measurement sample, and was measured using an AC impedance measurement device (potentiostat / galvanostat VSP, manufactured by Bio-Logic Sciences Instruments). The measurement conditions were a measurement frequency of 100 Hz to 1 MHz, a measurement voltage of 100 mV, and a measurement temperature of 25°C, and the lithium ion conductivity was calculated from the obtained Nyquist plot. The results are shown in Table 1 as "σ Ｌｉ＋ (mS / cm)".

[0100] <Battery Characteristics> The sulfide solid electrolyte was dry-pulverized in a planetary ball mill (manufactured by Ito Seisakusho Co., Ltd., model number LP-M2) using alumina balls with a particle size of 2 mm under a dry nitrogen atmosphere. The powder was then passed through a sieve with a mesh size of 43 μm to obtain a sulfide solid electrolyte powder having a particle size distribution with an average particle size (D50) of 3 μm. The average particle size (D50) was measured using a Microtrac laser diffraction particle size distribution analyzer MT3300EXII, and then determined from the volume-based particle size distribution chart obtained. As the positive electrode active material, layered rock salt type NMC811 powder (purchased from MTI Corporation, volume average particle diameter: 11.75 μm) coated with lithium niobate was used. 34 parts by mass of the sulfide solid electrolyte powder prepared above, 60 parts by mass of the positive electrode active material, and 6 parts by mass of a conductive additive (acetylene black, manufactured by Denka Co., Ltd., HS100) were mixed to prepare a positive electrode composite. 80 mg of the sulfide solid electrolyte powder prepared above was placed in a plastic cylinder with a diameter of 10 mm and pressure-molded to form a solid electrolyte layer. Next, 6 mg of the positive electrode composite prepared above was placed in the same 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 prepared, and a charge / discharge test was performed at a confining pressure of 10 kN.

[0101] Oxidation Resistance Test A charge / discharge test was performed under the following conditions: measurement temperature: 25°C, charge current density: 0.1 C, discharge current density: 0.1 C, charge / discharge potential range: 1.9 to 3.9 V; a constant current charge / discharge test was performed for 10 cycles. From the results of the charge / discharge test, the capacity retention rate (%) expressed by the following formula was calculated, and the battery characteristics of the all-solid-state lithium-ion secondary batteries were evaluated. Capacity retention rate (%) = (discharge capacity at 10th cycle / discharge capacity at 1st cycle) x 100 The results of the battery characteristics are shown in the "Oxidation Resistance Test" section of Table 1. The evaluation criteria were as follows: ○: capacity retention rate over 90% △: capacity retention rate over 80% but not more than 90% ×: capacity retention rate 80% or less

[0102] C-Rate Test In the charge-discharge test, first, one cycle of charge and discharge was performed under the following conditions: measurement temperature: 25°C, charge current density: 0.1 C, discharge current density: 0.1 C, and charge-discharge potential range: 1.9 to 3.9 V, and the discharge capacity was determined. Then, one cycle of charge and discharge was performed under the following conditions: measurement temperature: 25°C, charge current density: 3.0 C, discharge current density: 3.0 C, and charge-discharge potential range: 1.9 to 3.9 V, and the discharge capacity was determined. From the results of the charge-discharge test, the capacity retention rate (%) expressed by the following formula was calculated, and the battery characteristics of the all-solid-state lithium-ion secondary battery were evaluated. Capacity retention rate (%) = (discharge capacity at 3.0 C / discharge capacity at 0.1 C) × 100. The results of the battery characteristics are shown in the "C-Rate Test" section of Table 1. The evaluation criteria were as follows: ○: capacity retention rate over 50% △: capacity retention rate over 10% but not more than 50% ×: capacity retention rate 10% or less

[0103] <Water Resistance> The obtained sulfide solid electrolyte was crushed in a mortar and then passed through a 100 μm sieve to obtain a powder with an average particle size (D50) of about 20 μm, which was used as a sample. 10 mg of the sample was placed in a humidified N ２ The gas was allowed to flow and exposed for 5 hours, and the hydrogen sulfide concentration in the gas was monitored to determine the total amount of hydrogen sulfide generated. ２ The gas was passed through at 0.5 L / min. In this way, the total amount of hydrogen sulfide generated was detected to evaluate the hydrolysis resistance as an evaluation of water resistance. A hydrogen sulfide concentration meter (manufactured by Techne Measurement Co., Ltd., Model 3000RS) was used as the detector tube. The results are shown in "H" in Table 1. ２ The amount of sulfur generated (mL / g) is shown in the table below.

[0104]

[0105] From the above results, it was found that the sulfide solid electrolyte according to this embodiment has a high lithium ion conductivity of more than 1 mS / cm despite having a glass phase content of 60 mass % or more, and the results of the oxidation resistance test and the C rate test show that it has excellent cycle characteristics and rate characteristics. ３．４ P １．０ S ４．０ Br ０．３In addition, it was also obtained in the case where the amount of Li was relatively large as in Example 4, and in the case where a halogen element other than Br was contained as Ha as in Example 5.

[0106] A comparison of Examples 1 to 3 shows that a higher lithium ion conductivity can be achieved by increasing the content of the sulfide-based glass phase and decreasing the content of the devitrified crystalline phase. The content of the glass phase is adjusted by the cooling rate.

[0107] On the other hand, as shown in Example 6, when the cooling rate by the quenching twin roll was slow, the proportion of the resulting sulfide-based glass phase was as low as 30 mass%, and as a result, the lithium ion conductivity was also low. Furthermore, as shown in Example 7, even when iodine, which is expected to have high lithium ion conductivity, was used as the halogen element, when the cooling rate by the quenching twin roll was slow, the lithium ion conductivity was only 1.0 mS / cm. The proportion of the sulfide-based glass phase in Example 7 was extremely high, exceeding 95 mass%, and was significantly different from the proportion of the sulfide-based glass phase in Example 6, which had the same cooling rate. This is thought to be due to the fact that heavier elements are less likely to cause atomic arrangement, i.e., crystallization, during cooling. Furthermore, although Example 7 had a high proportion of the sulfide-based glass phase, the cooling rate was slow, so it is thought that the glass phase did not have a certain degree of order like Examples 1 to 5.

[0108] As shown in Example 8, the sulfide solid electrolyte obtained by mechanical milling had a very high sulfide-based glass phase ratio of over 95 mass%, but had low lithium ion conductivity. This suggests that although the sulfide-based glass phase in this embodiment and the sulfide-based glass phase obtained by mechanical milling are both classified as amorphous glass phases, their detailed states are different. Specifically, the sulfide-based glass phase in this embodiment is a glass phase having a certain degree of order, i.e., a periodic structure, which may contribute to the realization of high lithium ion conductivity.

[0109] 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. 2023-168392) filed on September 28, 2023, the contents of which are incorporated herein by reference.

Claims

1. A sulfide solid electrolyte containing 60 mass% or more of a sulfide-based glass phase, the sulfide solid electrolyte containing Li, P, and S as constituent elements, the composition of the sulfide solid electrolyte satisfying Li / P≧2.5, and the sulfide solid electrolyte having a lithium ion conductivity of more than 1 mS / cm at 25°C when compressed under 380 MPa.

2. The sulfide solid electrolyte according to claim 1, wherein the sulfide solid electrolyte further contains Ha as a constituent element, the Ha being at least one element selected from the group consisting of F, Cl, Br, and I, and the composition of the sulfide solid electrolyte further satisfies [(F+Cl+Br) / Ha]≧0.

5.

3. The sulfide solid electrolyte according to claim 1 or 2, wherein the composition of the sulfide solid electrolyte further satisfies Li / P≦7.

0.

4. H when held in an atmosphere with a dew point of -20°C for 5 hours ２ The sulfide solid electrolyte according to claim 1 or 2, wherein the amount of S generated is 1.0 mL / g or less.

5. The sulfide solid electrolyte according to claim 1 or 2, wherein the composition of the sulfide solid electrolyte satisfies Li: 30 to 50 at%, P: 5 to 15 at%, S: 30 to 60 at%, and Ha: 1 to 12 at%.

6. A method for producing a sulfide solid electrolyte containing 60 mass% or more of a glass phase, the method comprising the steps of: mixing raw materials to obtain a raw material mixture containing Li, P, and S; heating the raw material mixture to obtain a melt; and cooling and solidifying the melt to obtain the glass phase, wherein the heating and the cooling and solidifying are performed under normal pressure conditions, and the cooling rate in the cooling and solidifying is 100°C / sec or more.

7. The method for producing a sulfide solid electrolyte according to claim 6, wherein the melting temperature in the heating is 600 to 800°C.

8. The method for producing a sulfide solid electrolyte according to claim 6 or 7, wherein the composition of the sulfide solid electrolyte satisfies Li / P≧2.

5.

9. The method for producing a sulfide solid electrolyte according to claim 8, wherein the composition of the sulfide solid electrolyte further satisfies Li / P≦7.

10. The method for producing a sulfide solid electrolyte according to claim 6 or 7, wherein the heating is performed under a pressure of gauge pressure ±10 kPa, and a composition ratio of the sulfide solid electrolyte expressed as [P / Li] relative to a composition ratio of the raw material mixture expressed as [P / Li] is 95 to 105%.

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