Sulfide solid electrolyte and method for manufacturing same

Abstract

The present invention pertains to a sulfide solid electrolyte containing a sulfide-based glass phase, wherein: when differential scanning calorimetry is performed at a heating rate of 5°C / min, the sulfide-based glass phase has an exothermic peak at which a highly ionically conductive crystal is precipitated; the exothermic peak can be separated into two or more peaks; and the temperature difference between the peak top of the peak farthest on the high temperature side and the peak top of the peak farthest on the low temperature side among the two or more peaks separated by the peak separation is 0.5°C or more.

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 solid electrolytes 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 smaller cases.

[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] As a solid electrolyte having excellent water resistance and high ionic conductivity, Patent Document 1 discloses Li ２ S / P ２ S ５ Sulfide-based glass in which Li / LiI=63 / 21 / 16 or 52 / 17 / 31, ２ S / P ２ S ５ A sulfide-based glass-ceramic solid electrolyte is disclosed, which is a sulfide-based glass-ceramic obtained by partially crystallizing a sulfide-based glass having a composition of Li / LiBr=64 / 21 / 14. These are less likely to generate hydrogen sulfide even when exposed to a gas containing moisture.

[0006] Japanese Patent Application Publication No. 2018-049834

[0007] However, since the sulfide-based glass obtained by the manufacturing method disclosed in Patent Document 1 is in a powder form, it is difficult to produce a sulfide solid electrolyte sheet, that is, a bulk inorganic sheet.

[0008] Therefore, an object of the present invention is to provide a sulfide solid electrolyte containing a sulfide-based glass phase that has excellent water resistance and ionic conductivity and high strength when formed into a sheet, and a method for producing the same.

[0009] As a result of extensive research, the present inventors have found that, when obtaining a sulfide-based glass phase by heating, melting, and cooling raw materials, the heating and cooling are performed under normal pressure conditions, the stirring during the melting is adjusted, and the cooling is performed at a very fast cooling rate, a novel sulfide-based glass phase having different physical properties can be obtained even if the composition is conventionally known.The present inventors have also found that the above-mentioned problems can be solved by using a sulfide solid electrolyte containing such a sulfide-based glass phase, and have completed the present invention.

[0010] That is, the present invention relates to the following [1] to [8]. [1] A sulfide solid electrolyte including a sulfide-based glass phase, wherein the sulfide-based glass phase has an exothermic peak at which highly ion-conductive crystals precipitate when subjected to differential scanning calorimetry at a heating rate of 5°C / min, the exothermic peak can be separated into two or more peaks, and the temperature difference between the highest and lowest peaks among the two or more separated peaks is 0.5°C or more. [2] The sulfide solid electrolyte according to [1] above, wherein the temperature difference between the peak tops is 20°C or less. [3] The sulfide solid electrolyte according to [1] above, wherein the exothermic peak is present at 190 to 250°C. [4] The sulfide solid electrolyte according to any one of [1] to [3], wherein the sulfide solid electrolyte contains Li, P, and S as constituent elements, and the composition of the sulfide solid electrolyte satisfies Li: 30 to 50 at%, P: 5 to 15 at%, and S: 30 to 60 at%, and 2.5≦Li / P≦ 7. [5] The sulfide solid electrolyte according to [4], wherein the sulfide solid electrolyte further contains Ha as a constituent element, and the Ha is at least one selected from the group consisting of F, Cl, Br, and I, and the composition of the sulfide solid electrolyte further satisfies Ha: 1 to 12 at%. [6] The sulfide solid electrolyte according to any one of [1] to [5], wherein the content of the sulfide-based glass phase is 10 mass% or more. [7] The sulfide solid electrolyte according to any one of [1] to [6], which has a thickness of 5 μm or more and is in the form of a flake or sheet.

[0011] [8] A method for producing a sulfide solid electrolyte according to any one of [1] to [7] above, the method comprising the steps of mixing raw materials to obtain a raw material mixture, heating the raw material mixture to obtain a melt, and cooling and solidifying the melt to obtain the sulfide-based glass phase, in that order; wherein during the heating, the melt is not stirred or is stirred using a stirrer at a speed of less than 40 rpm; the heating and the cooling and solidifying are performed under normal pressure conditions; and the cooling rate during the cooling and solidifying is 100°C / second or more.

[0012] According to the present invention, a sulfide solid electrolyte containing a sulfide-based glass phase can be obtained, which has excellent water resistance and ionic conductivity and high strength when formed into a sheet. Therefore, when the sulfide solid electrolyte is used in an all-solid-state lithium secondary battery, good battery characteristics can be achieved.

[0013] FIG. 1 is a flow diagram showing a method for producing a sulfide solid electrolyte according to this embodiment. FIG. 2 is a DSC curve showing the relationship between heat quantity and temperature when the sulfide solid electrolyte obtained in Example 1 was subjected to differential scanning calorimetry (DSC) measurement at a heating rate of 5°C / min. FIG. 3 is a DSC curve obtained by enlarging the exothermic peak at which highly ion-conductive crystals precipitate in the DSC curve obtained in FIG. 2 and performing curve fitting. FIG. 4 is a DSC curve showing the relationship between heat quantity and temperature when the sulfide solid electrolyte obtained in Example 2 was subjected to differential scanning calorimetry (DSC) measurement at a heating rate of 5°C / min. FIG. 5 is a DSC curve obtained by enlarging the exothermic peak at which highly ion-conductive crystals precipitate in the DSC curve obtained in FIG. 4 and performing curve fitting. FIG. 6 is a scanning electron microscope (SEM) image of the sulfide solid electrolyte obtained in Example 1 after pulverization for strength testing. FIG. 7 is an SEM image of the sulfide solid electrolyte obtained in Example 2 after pulverization for strength testing.

[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 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.

[0015] The sulfide solid electrolyte according to this embodiment includes a sulfide-based glass phase, which exhibits an exothermic peak at which highly ion-conductive crystals precipitate when measured by differential scanning calorimetry (DSC) at a heating rate of 5°C / min.

[0016] The exothermic peak at which the highly ion-conductive crystals precipitate can be separated into two or more peaks, and the temperature difference between the peak tops of the highest temperature peak and the peak tops of the lowest temperature peak among the two or more separated peaks is 0.5°C or more.

[0017] <Sulfide-Based Glass Phase> As described above, the sulfide-based glass phase in this embodiment has an exothermic peak at which highly ion-conductive crystals precipitate when measured by DSC at a temperature rise rate of 5°C / min, and the exothermic peak can be separated into two or more peaks. Of the two or more separated peaks, the temperature difference between the peak tops on the highest temperature side and the peak tops on the lowest temperature side is 0.5°C or more.

[0018] The method for determining whether an exothermic peak is due to the precipitation of highly ion-conductive crystals is as follows. First, a sulfide solid electrolyte is subjected to DSC measurement at a heating rate of 5°C / min, and then heat-treated for 10 minutes at the peak top temperature of the observed exothermic peak. Since the exothermic peak indicates crystallization, even if the measured sample is a sulfide solid electrolyte, the exothermic peak can be determined to be the exothermic peak of a sulfide-based glass phase contained in the sulfide solid electrolyte. If an increase in the ionic conductivity of the sulfide solid electrolyte is observed before and after the heat treatment, the exothermic peak can be determined to be due to the precipitation of highly ion-conductive crystals.

[0019] Here, the increase in ionic conductivity refers to the conductivity of carrier elements contained in the constituent elements of the sulfide-based glass phase. For example, when the sulfide-based glass phase contains lithium (Li), it is sufficient to check whether or not the lithium ion conductivity increases before and after the heat treatment. In addition to Li, examples of carrier elements include sodium (Na), potassium (K), magnesium (Mg), and fluorine (F). However, when the sulfide solid electrolyte is applied to an all-solid-state lithium secondary battery, Li is preferred.

[0020] The increase in ionic conductivity before and after the heat treatment can be 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 plots is taken as the ionic conductivity.

[0021] If it is determined from the above that the exothermic peak is due to the precipitation of highly ion-conductive crystals, the exothermic peak is then separated. Peak separation is performed by fitting using a Lorentz function. Specifically, after baseline correction is performed on the exothermic peak of the DSC curve, a predetermined number of Lorentz functions are added together to create a fitting function. The parameters of this fitting function are determined using the least squares method from the curve data of the DSC curve. Then, peak separation is performed to determine the peak top temperature of each peak.

[0022] In the sulfide-based glass phase of this embodiment, the exothermic peak can be separated into two or more peaks by peak separation. Of the two or more separated peaks, the peak top temperature of the lowest temperature peak is designated as T1, and the peak top temperature of the highest temperature peak is designated as T2. The temperature difference between the peak tops of the highest temperature peak and the lowest temperature peak, i.e., the temperature represented by (T2 - T1), is 0.5°C or more. This allows the sulfide solid electrolyte to have high strength when formed into a sheet.

[0023] The reason why the strength of the sulfide solid electrolyte increases when the temperature difference (T2-T1) is 0.5°C or more is not clear, but it is believed to be as follows. The sulfide-based glass phase in the sulfide solid electrolyte according to this embodiment, as will be described in detail later, is obtained by heating and melting a raw material mixture and then rapidly cooling it. If the melt is not stirred or is stirred weakly during heating and melting, the melt structure becomes somewhat heterogeneous. Then, if the melt is rapidly cooled, the somewhat heterogeneous structure is maintained, and a glass having a heterogeneous structure is obtained. As a result, the surface area newly generated when a crack propagates increases, and the surface energy increases, which is believed to increase the fracture toughness value and result in a high-strength sulfide solid electrolyte.

[0024] The temperature difference represented by (T2-T1) above may be 0.5°C or more, and is preferably 0.5 to 20°C, more preferably 1 to 10°C, even more preferably 1.5 to 8°C, and even more preferably 2 to 5°C. From the viewpoint of achieving high strength, the temperature difference is 0.5°C or more, preferably 1.0°C or more, more preferably 1.5°C or more, and even more preferably 2.0°C or more. Furthermore, from the viewpoint that ionic conductivity is impaired when the non-uniformity of the glass material becomes too high, the temperature difference is preferably 20°C or less, more preferably 10°C or less, even more preferably 8°C or less, and even more preferably 5°C or less.

[0025] The exothermic peak is preferably present at 190 to 250°C, more preferably at 200 to 245°C, and even more preferably at 205 to 240°C. Here, from the viewpoint of increasing the heat treatment and accelerating the crystallization rate, the temperature at which the exothermic peak is present is preferably 190°C or higher, more preferably 200°C or higher, and even more preferably 205°C or higher. Furthermore, from the viewpoint of suppressing deformation of the electrolyte during heat treatment, the temperature at which the exothermic peak is present is preferably 250°C or lower, more preferably 245°C or lower, and even more preferably 240°C or lower. Here, the temperature at which the exothermic peak is present means the temperature at the peak top. When the exothermic peak has two or more maxima, it is preferable that two or more of the maxima are within the above temperature range, and it is more preferable that the highest and lowest maxima among the maxima are within the above temperature range.

[0026] After the exothermic peak is separated, the temperature T1 of the lowest peak top is preferably 190 to 245° C., more preferably 200 to 240° C., and even more preferably 205 to 235° C. From the viewpoint of increasing the heat treatment and accelerating the crystallization rate, the temperature T1 is preferably 190° C. or higher, more preferably 200° C. or higher, and even more preferably 205° C. or higher. From the viewpoint of suppressing deformation of the electrolyte during the heat treatment, the temperature T1 is preferably 245° C. or lower, more preferably 240° C. or lower, and even more preferably 235° C. or lower.

[0027] After the exothermic peak is separated, the temperature T2 of the highest peak top is preferably 195 to 250° C., more preferably 205 to 245° C., and even more preferably 210 to 240° C. From the viewpoint of thermal stability, the temperature T2 is preferably 195° C. or higher, more preferably 205° C. or higher, and even more preferably 210° C. or higher. From the viewpoint of suppressing deformation of the electrolyte during heat treatment, the temperature T2 is preferably 250° C. or lower, more preferably 245° C. or lower, and even more preferably 240° C. or lower.

[0028] <Constituent Elements and Composition> The sulfide solid electrolyte according to this embodiment preferably contains Li, P, and S as constituent elements. In addition to these, it is more preferable that it contains 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 that it contains at least one of Cl and Br, and even more preferable that it contains Br. It is also more preferable that it contains both Cl and Br.

[0029] In the composition of the sulfide solid electrolyte according to this embodiment, the composition ratio represented by Li / P is preferably 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 preferably 2.5 or more, more preferably 3.0 or more, and even more preferably 3.2 or more. 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 composition ratio refers to the ratio of the content (at %) of each element in the sulfide solid electrolyte.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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 %.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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 at% or more, more preferably 70 at% or more, and may be 100 at%, 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.

[0039] 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 ３ In 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.

[0040] 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.

[0041] 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.

[0042] 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 ３ When 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] The proportion of the sulfide-based glass phase in the sulfide solid electrolyte according to this embodiment is preferably 10% by mass or more, more preferably 10 to 100% by mass, even more preferably 20 to 99% by mass, even more preferably 40 to 98% by mass, even more preferably 40 to 95% by mass, and particularly preferably 60 to 90% by mass. From the viewpoints of lithium ion conductivity and formability, the proportion of the sulfide-based glass phase is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 40% by mass or more, and even more preferably 60% 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 the 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. From the viewpoint of the manufacturability of the sulfide solid electrolyte, the proportion of the sulfide-based glass phase is preferably 99% by mass or less, more preferably 98% by mass or less, even more preferably 95% by mass or less, and particularly 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 as an internal standard, determining the proportion of crystals, and subtracting the obtained value from 100% by mass to perform Rietveld analysis.

[0050] 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.

[0051] <Other Phases> The sulfide solid electrolyte according to this embodiment may 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. Furthermore, a highly ion-conductive crystalline phase may be contained. The proportion of such other phases in the sulfide solid electrolyte is preferably 90% by mass or less, more preferably 1 to 80% by mass, even more preferably 2 to 60% by mass, even more preferably 5 to 40% by mass, and particularly preferably 10 to 40% by mass. Here, from the viewpoint of fully utilizing the properties of the sulfide-based glass phase, the proportion of the other phases is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 60% by mass or less, and even more preferably 40% by mass or less. Furthermore, the other phases may not be contained, but from the viewpoint of the manufacturability of the sulfide solid electrolyte, the proportion of the other phases is preferably 1% by mass or more, more preferably 2% by mass or more, even more preferably 5% by mass or more, and even more preferably 10% by mass or more.

[0052] <Characteristics of Sulfide Solid Electrolyte> The sulfide solid electrolyte according to this embodiment is preferably in the form of a flake or sheet having a thickness of 5 μm or more. The thickness of the sulfide solid electrolyte can be determined using a thickness gauge or the like. Furthermore, if the long side (longest diameter) of each surface of the sulfide solid electrolyte observed by surface observation using an SEM or optical microscope is 5 or more times the thickness, the sulfide solid electrolyte can be determined to be in the form of a flake or sheet.

[0053] The sulfide solid electrolyte is obtained in a powder form when produced by, for example, a mechanical milling method. However, the sulfide solid electrolyte according to this embodiment can be made into the above-described flake- or sheet-shaped sulfide solid electrolyte by employing a melting method described below and a very fast cooling rate. Therefore, the sulfide solid electrolyte according to this embodiment can also achieve high strength.

[0054] The thickness of the sulfide solid electrolyte is preferably 5 μm or more, more preferably 10 to 5000 μm, and even more preferably 20 to 1000 μm. From the viewpoint of handling, the thickness is preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 20 μm or more. From the viewpoint of producing a thin battery, the thickness is preferably 5000 μm or less, and more preferably 1000 μm or less.

[0055] When the sulfide solid electrolyte according to this embodiment is compressed at 380 MPa, the lithium ion conductivity at 25°C is preferably greater than 1 mS / cm. This is achievable due to the high lithium ion conductivity of the sulfide-based glass phase. However, because 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 the sulfide solid electrolyte containing the sulfide-based glass phase.

[0056] 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.

[0057] The lithium ion conductivity of the sulfide solid electrolyte according to this embodiment is preferably greater than 1 mS / cm, but this value varies depending on the composition and the proportion of the sulfide-based glass phase. For example, when the Br content of Ha is 90 at% or more, the lithium ion conductivity is more preferably 1.3 mS / cm or more, and when the Cl content of Ha is 90 at% or more, the lithium ion conductivity is more preferably 1.1 mS / cm or more.

[0058] 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 plots is taken as the lithium ion conductivity.

[0059] 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 increasing the content of the sulfide-based glass phase or 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.

[0060] The sulfide solid electrolyte according to this embodiment can be formed into a bulk inorganic sheet and is suitable as an electrolyte for lithium-ion secondary batteries. The inorganic sheet can be used, for example, as a separator layer. In this case, the separator layer can be composed solely of an inorganic sheet made of the sulfide solid electrolyte. However, this does not preclude the use of other components, such as a binder, in the separator layer.

[0061] The content of the sulfide solid electrolyte according to this embodiment is preferably 80 mass % or more, and more preferably 90 mass % or more, relative to the entire solid electrolyte layer.

[0062] The sulfide solid electrolyte according to this embodiment may be pulverized from a flake or sheet state into a powder. Even in the powder state, the sulfide solid electrolyte can exhibit the high strength effect.

[0063] The powdered sulfide solid electrolyte may be mixed with an electrode active material such as a positive electrode active material or a negative electrode active material and used as an electrode layer such as a positive electrode layer or a negative electrode layer. It is believed that by including the sulfide solid electrolyte powder according to this embodiment in the electrode layer, cracking and deformation of the electrode layer can be suppressed even if volume changes occur due to expansion and contraction of the electrode active material during charge and discharge.

[0064] The positive electrode active material or negative electrode active material, current collector, binder, conductive aid, etc. used in the positive electrode layer or negative electrode layer may be any known material.

[0065] The average particle size (D50) of the sulfide solid electrolyte according to this embodiment is preferably 0.1 to 300 μm, more preferably 0.3 to 50 μm, and even more preferably 0.5 to 5 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 50 μm or less, and even more preferably 5 μm or less. Furthermore, from the viewpoint of powder handling, the average particle size is preferably 0.1 μm or more, more preferably 0.3 μm or more, and even more preferably 0.5 μm or more. The average particle size (D50) can be adjusted by subjecting the obtained sulfide solid electrolyte to coarse pulverization. Conventional known methods for coarse pulverization can be used, including, for example, a cutter mill, a planetary ball mill, and a jet mill, and dry pulverization is preferred. The resulting solid electrolyte is then further pulverized to a finer particle size using a wet or dry pulverization method such as a bead mill or a jet mill, and used as a battery material, such as a solid electrolyte for a positive electrode, a negative electrode, or a separator layer.

[0066] 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.

[0067] 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.

[0068] <<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;

[0069] The heating in step 2 and the cooling and solidification in step 3 are carried out under atmospheric pressure conditions, and the cooling rate during the cooling and solidification is 100°C / sec or higher. During the heating in step 2, the melt is not stirred, or is stirred at less than 40 rpm using a stirrer or the like. This provides the sulfide solid electrolyte described in the above section "Sulfide Solid Electrolyte." Specifically, a sulfide solid electrolyte is obtained that includes a sulfide-based glass phase having an exothermic peak at which highly ion-conductive crystals precipitate, which exothermic peak can be separated into two or more peaks with a peak-top temperature difference of 0.5 or more. Note that, in this specification, "under atmospheric pressure conditions" refers to a pressure range of approximately (gauge pressure ±15 kPa).

[0070] In the above step 1, the raw materials are preferably mixed so that the composition of the resulting sulfide solid electrolyte satisfies Li: 30 to 50 at %, P: 5 to 15 at %, and S: 30 to 60 at %.

[0071] Each step will be explained below.

[0072] <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.

[0073] 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 ２ Examples 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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－ｘ The substance containing the S element may be used alone or in combination of two or more.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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 ２ These 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 Al２ 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 , SbCl ３ , 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.

[0083] 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," it is preferable to mix the raw materials so that the composition of the resulting glass phase satisfies Li: 30 to 50 at%, P: 5 to 15 at%, and S: 30 to 60 at%. Furthermore, it is more preferable that the sulfide solid electrolyte further contains Ha as a constituent element, and in addition to the above composition, further satisfies Ha: 1 to 12 at%. In addition to the above, preferred aspects of the resulting sulfide solid electrolyte are the same as the preferred aspects of the above "Sulfide Solid Electrolyte."

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] <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.

[0089] The specific method for heating and melting the raw material mixture is not particularly limited, as long as the heating is carried out under normal pressure conditions and the melt is not stirred during heating, or if stirred, is stirred under gentle conditions.

[0090] In the production method according to this embodiment, the heating for melting the raw material mixture is performed under atmospheric pressure conditions, thereby making it possible to reduce 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 in 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 heat melting is performed in an atmosphere-controlled environment under atmospheric pressure conditions, and it is not necessary to perform the heat melting in a sealed tube. However, this does not in any way exclude the use of a sealed tube.

[0091] 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).

[0092] 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%.

[0093] 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.

[0094] 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.

[0095] As described above, during heating, the raw material mixture and the resulting melt are not stirred, or if stirred, are stirred gently, which allows the melt structure to be somewhat heterogeneous, and the subsequent rapid cooling in step 3 allows a sulfide-based glass phase that maintains this structure.

[0096] When the melt is not stirred, the raw material mixture may be heated while the container is left standing without a stirrer in the container.

[0097] When stirring the melt, gentle conditions include stirring using a stirrer, i.e., stirring blades, at a speed of less than 40 rpm. However, the above conditions are not limited to these. For example, stirring methods include shaking, in addition to stirring using a stirrer. The degree of stirring may also vary depending on the type and size of the stirring vessel used, such as a general-purpose vessel, a sealed vessel, a pressurized vessel, etc. In the production method according to this embodiment, it is sufficient to use a sealed vessel and achieve a stirring degree that is similar to or gentler than the stirring degree achieved when a stirrer is rotated at less than 40 rpm.

[0098] When the melt is stirred using a stirrer, the rotation speed is set to less than 40 rpm. The rotation speed is preferably 30 rpm or less, more preferably 20 rpm or less. In addition to stirring with a stirrer in the container as described above, stirring may also be performed by shaking, thermal convection, or the like. In such cases, the stirring force should be set to a level at which the melt in the container is uniformly mixed within 10 minutes of starting stirring.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] The gas containing S element is preferably used as a mixed gas with, for example, an inert gas, which is 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.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] <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 sulfide-based glass phase.

[0109] 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).

[0110] The cooling rate during 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. It has been discovered that by using a cooling rate of 100°C / sec or higher, which is faster than conventional methods, a sulfide solid electrolyte containing a sulfide-based glass phase according to the present embodiment can be obtained. This sulfide-based glass phase can achieve higher lithium ion conductivity than conventional sulfide-based glasses. Furthermore, by performing this solidification under atmospheric pressure in addition to the heating described above, the compositional difference between the composition of the raw material mixture and the composition of the resulting sulfide solid electrolyte can be reduced.

[0111] 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.

[0112] The sulfide solid electrolyte containing the sulfide-based 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 pulverized, dried, or the like depending on the application. In particular, the sulfide solid electrolyte according to this embodiment can be made into a bulk inorganic sheet as described above, and is therefore suitable as a separator layer. Furthermore, it is also suitable to pulverize the sulfide solid electrolyte according to this embodiment into powder and incorporate it into an electrode layer such as a positive electrode layer or a negative electrode layer, from the viewpoint of utilizing its high strength to suppress cracking and deformation of the electrode layer.

[0113] 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 sulfide-based glass phase to crystallize at least a portion of the sulfide-based glass phase, and then used for the above purpose. In this case, the crystalline phase derived from the sulfide-based glass phase may have a thiolisicon-type or LGPS-type crystalline structure, which has high lithium ion conductivity.

[0114] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these. Example 1 is a reference example, and Examples 2 to 4 are working examples.

[0115] <Test Examples> <Example 1> Li under a dry nitrogen gas atmosphere ３．４ P １．０ S ４．０ Br ０．３Lithium 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 (Step 1). The obtained raw material mixture was placed in a carbon container containing sulfur powder (manufactured by Sigma, purity 99.998%) as an S element source under a nitrogen atmosphere with a dew point of −50° C. or less, together with a stirrer, and then placed in an electric furnace inside a glove box. Then, the mixture was heated for 1 hour under the conditions of pressure: gauge pressure +1 kPa, temperature: 750° C., and rotation speed: 40 rpm to obtain a melt (Step 2). Next, the tip of the container was heated to melt the melt, 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 during this process 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.

[0116] Examples 2 to 4 A sulfide solid electrolyte was obtained in the same manner as in Example 1, except that the rotation speed during heating of the raw material mixture in step 2 was changed to 0 rpm (no rotation, Example 2), 5 rpm (Example 3), or 10 rpm (Example 4).

[0117] <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%). Br was analyzed by ion chromatography (apparatus: Thermo Fisher Scientific, model ICS-2100 (column: AS11HC), H ２ O ２The results are shown in Table 1. The compositions here are blended compositions normalized to P of 1.0. In all examples, the compositions of the obtained sulfide solid electrolytes did not deviate significantly from the compositions of the raw material mixtures.

[0118] <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. X-ray diffraction (XRD) measurements were performed in an atmosphere-unexposed environment using an X-ray diffraction (XRD) device (Rigaku Corporation, SmartLab) 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. The obtained XRD pattern showed no diffraction peaks other than those of silicon added as an internal standard, confirming that nearly 100% by mass was glass phase.

[0119] <Exothermic Peak of Highly Ion-Conductive Crystals> 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. This powder was used as a sample. DSC measurements were performed on 20 mg of the sample using a differential scanning calorimeter (BRUKER AXS, DSC3300) at a heating rate of 5°C / min from 25 to 350°C, and a DSC curve was obtained. As examples, the DSC curve for Example 1 is shown in Figure 2, and the DSC curve for Example 2 is shown in Figure 3. Examples 3 and 4, like Example 2 shown in Figure 3, exhibited an exothermic peak with a split tip near 220°C.

[0120] Therefore, the sulfide solid electrolytes of Examples 1 to 4 were heated for 10 minutes at the peak top temperature of the exothermic peak (Example 1: 207°C, Example 2: 222°C, Example 3: 226°C, Example 4: 220°C), and the change in lithium ion conductivity before and after heating was measured. As a result, in all examples, the lithium ion conductivity increased by more than two times, and it was confirmed that the exothermic peak was the exothermic peak at which highly ion-conductive crystals precipitated. The measurement of lithium ion conductivity will be described in detail below.

[0121] After confirming that the exothermic peak was the peak at which highly ion-conductive crystals precipitated, the DSC curve in the range of 180 to 280°C was analyzed to separate the exothermic peak. The peak separation analysis was performed by fitting using a Lorentz function. Specifically, after baseline correction was performed on the exothermic peak of the DSC curve, a predetermined number of Lorentz functions were added together to create a fitting function. The parameters of this fitting function were determined using the least squares method from the curve data of the DSC curve. Then, peak separation was performed to determine the peak top temperature of each peak.

[0122] FIG. 4 shows the DSC curve obtained by enlarging the exothermic peak where high ion conductive crystals precipitate in Example 1 and performing curve fitting. FIG. 5 shows the DSC curve obtained by enlarging the exothermic peak where high ion conductive crystals precipitate in Example 2 and performing curve fitting. In FIGS. 4 and 5, the solid lines are the DSC curves actually obtained (measured values), and the dotted lines are curves (fitted values) curved by curve fitting through analysis using a Lorentz function. In FIG. 5, the peak was separated into two by the analysis, and the sum of the separated peaks (fitting total) is shown with a dashed dotted line. The peak top temperatures T1 and T2 of the analyzed peaks, as well as the temperature difference between them, are shown in Table 1. Note that for Example 1, the exothermic peak where high ion conductive crystals precipitate could not be separated into two or more peaks. Therefore, in Table 1, the peak top temperature is listed as "T1 (°C)" and "T2 (°C)" is marked with "-".

[0123] <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)". The lithium ion conductivity after heating was also determined in the same manner as above for the sulfide solid electrolyte after heating for 10 minutes at the peak top temperature of the exothermic peak described above in "Exothermic Peak of Highly Ion Conductive Crystal".

[0124] <Shape> The obtained sulfide solid electrolytes were observed using an optical microscope in an environment not exposed to the atmosphere, and the length of the long side (longest diameter) of the surface of each sulfide solid electrolyte was determined. The thickness of each sulfide solid electrolyte was also determined using a thickness gauge. The ratio of the long side to the thickness was 1,000.

[0125] <Strength> 2 g of the obtained sulfide solid electrolyte was placed in a 45 mL zirconia pot in flake form without being crushed. The flake sulfide solid electrolyte had a thickness of 50 to 150 μm and an area of ​​approximately 2 cm. ２The following sizes were selected. Zirconia balls with a diameter of 10 mm were also placed in the same zirconia pot, and dry pulverization was performed at 100 rpm for 30 minutes using a planetary ball mill (LP-M2, manufactured by Ito Seisakusho). Next, the zirconia balls were recovered, and 1 mm diameter zirconia balls were placed in the pot. 8 g of dibutyl ether was added, and wet pulverization was performed at 200 rpm for 60 minutes. The pot was then dried at 140°C for 6 hours to obtain a sulfide solid electrolyte powder. The obtained powder was observed using a scanning electron microscope (SU8230, manufactured by Hitachi High-Tech Corporation) at a magnification such that the powder contained approximately 50 particles, and the average particle size was determined. SEM images of the powders obtained by pulverizing the sulfide solid electrolytes of Examples 1 and 2 are shown in Figures 6 and 7, respectively. It can be said that the larger the average particle size after pulverization, the higher the strength of the sulfide solid electrolyte. Note that the sulfide solid electrolyte was subjected to all procedures, from the selection of the measurement sample to the observation after pulverization, in an environment not exposed to the atmosphere. The results are shown in Table 1, and the evaluation criteria were as follows: ⊚: The average particle size was 5 μm or more, and the strength was very high and extremely good; ○: The average particle size was 3 μm or more and less than 5 μm, and the strength was high and good; ×: The average particle size was less than 3 μm, and the strength was low and poor.

[0126] <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 (Model 3000RS, manufactured by Techne Measurement Co., Ltd.) was used as the detector tube. As a result, the total amount of hydrogen sulfide generated was 0 mL / g in all examples, confirming good water resistance.

[0127]

[0128] From the above results, it was found that the sulfide solid electrolytes of Examples 2 to 4 maintained high lithium ion conductivity and water resistance equivalent to those of Reference Example 1, while also exhibiting excellent strength. It was found that high lithium ion conductivity and water resistance could be achieved by performing the heating and melting of the raw material mixture and cooling and solidifying it under atmospheric pressure conditions and by increasing the cooling rate during solidification. Furthermore, by employing such a manufacturing method, the proportion of the sulfide-based glass phase in the resulting sulfide solid electrolyte could be increased. Furthermore, it was found that by employing gentle stirring conditions, such as not stirring the melt during heating and melting of the raw material mixture (Example 2) or using a rotation speed of 5 rpm (Example 3) or 10 rpm (Example 4), the fracture toughness value could be increased and high strength could also be achieved. Note that, while sulfide solid electrolytes containing a sulfide-based glass phase obtained using a conventional mechanical milling method (MM method) are in powder form, the sulfide solid electrolyte of Example 1 was obtained in sheet form. Therefore, although the sulfide solid electrolyte of Example 1 is inferior in strength to the sulfide solid electrolyte according to the present embodiment (evaluated as "x" in the table), it can be said to have good strength compared to the sulfide solid electrolyte produced by the conventional MM method, at the point when the above-described strength test can be carried out.

[0129] 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. 2023-168395) filed on September 28, 2023, the contents of which are incorporated herein by reference.

Claims

1. A sulfide solid electrolyte comprising a sulfide-based glass phase, wherein the sulfide-based glass phase has an exothermic peak at which highly ion-conductive crystals precipitate when subjected to differential scanning calorimetry at a heating rate of 5°C / min, the exothermic peak can be separated into two or more peaks, and the temperature difference between the peak tops of the highest peak and the peak tops of the lowest peak among the two or more separated peaks is 0.5°C or more.

2. The sulfide solid electrolyte according to claim 1, wherein the peak top temperature difference is 20° C. or less.

3. The sulfide solid electrolyte according to claim 1, wherein the exothermic peak is present at 190 to 250°C.

4. The sulfide solid electrolyte according to claim 1, wherein the sulfide solid electrolyte contains Li, P, and S as constituent elements, and the composition of the sulfide solid electrolyte satisfies Li: 30 to 50 at%, P: 5 to 15 at%, and S: 30 to 60 at%, and 2.5≦Li / P≦7.

5. The sulfide solid electrolyte according to claim 4, 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 Ha: 1 to 12 at%.

6. The sulfide solid electrolyte according to claim 1, wherein the content of the sulfide-based glass phase is 10 mass % or more.

7. The sulfide solid electrolyte according to claim 1, which is in the form of a flake or sheet having a thickness of 5 μm or more.

8. A method for producing a sulfide solid electrolyte according to any one of claims 1 to 7, comprising the steps of mixing raw materials to obtain a raw material mixture, heating the raw material mixture to obtain a melt, and cooling and solidifying the melt to obtain the sulfide-based glass phase, wherein during the heating, the melt is not stirred or is stirred at less than 40 rpm using a stirrer, the heating and the cooling and solidifying are performed under normal pressure conditions, and the cooling rate during the cooling and solidifying is 100°C / sec or more.

Images