Method for producing sulfide-based solid electrolytes

Abstract

This invention provides a sulfide-based solid electrolyte for use in lithium-ion secondary batteries that exhibits high lithium-ion conductivity in a pressurized body at low pressure, even when the elemental ratio of halogen elements in the argyrodite-type crystalline phase is not as high as in conventional systems. [Solution] A crystalline phase of the argyrodite type containing Li, P, S, and Ha, with an [Ha] / [P] (atomic ratio) of 1.3 or higher, measured under specific conditions. 35 In the Cl-NMR spectrum, the area intensity S of the peak observed between 0 and 30 ppm. A And the area intensity S of the peak observed between -150 and 0 ppm. B S using B / S A A sulfide-based solid electrolyte used in lithium-ion secondary batteries, wherein the coefficient of variation is 3.5 or higher, or a peak is observed in the range of -150 to 0 ppm, and no peak is observed in the range of 0 to 30 ppm.

Description

[Technical Field]

[0001] This invention relates to a sulfide-based solid electrolyte used in lithium-ion secondary batteries and a method for producing the same. [Background technology]

[0002] Lithium-ion rechargeable batteries are widely used in portable electronic devices such as mobile phones and laptop computers. Traditionally, lithium-ion secondary batteries have used liquid electrolytes, but concerns about leakage and fire necessitated larger cases for safety design. Furthermore, improvements were desired in lithium-ion secondary batteries due to their short battery life and narrow operating temperature range.

[0003] In contrast, all-solid-state lithium-ion secondary batteries, which use a solid electrolyte as the electrolyte for lithium-ion secondary batteries, are attracting attention because they offer advantages such as improved safety, faster charging and discharging, and smaller case size.

[0004] Solid electrolytes are broadly classified into sulfide-based solid electrolytes and oxide-based solid electrolytes. Sulfide ions, which constitute sulfide-based solid electrolytes, have a higher polarizability and exhibit higher lithium ion conductivity compared to oxide ions, which constitute oxide-based solid electrolytes. As an example of a sulfide-based solid electrolyte, Li 10 GeP2S 12 LGPS-type crystals such as Li6PS5Cl, argyrodite-type crystals such as Li7P3S 11 Crystallized glass, such as LPS crystallized glass, is a well-known example.

[0005] Patent Document 1 is an example of a disclosure of an argyrodite-type sulfide-based solid electrolyte. The sulfide-based solid electrolyte disclosed in Patent Document 1 has a cubic crystal structure belonging to the space group F-43m, and its compositional formula is Li 7-x PS 6-X Ha X It contains a compound represented by (Ha is Cl or Br) (x=0.2~1.8), and L * a * b* The lightness L value of the color system is 60.0 or more. This is aimed at enhancing the charge-discharge efficiency and cycle characteristics by increasing the lithium ion conductivity and reducing the electronic conductivity.

[0006] In the argyrodite-type crystal structure, the Cl element occupying the 4d site is related to the energy barrier associated with the movement of lithium ions and affects the jump frequency of lithium ions to adjacent sites. That is, it is an important factor for realizing high lithium ion conductivity. Non-Patent Document 1 discloses a method of increasing the halogen element occupying the 4d site by increasing the ratio of the halogen element in the composition of the argyrodite-type crystal structure.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Non-Patent Documents

[0008]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] However, other than adopting a composition with a high ratio of halogen elements, a method of increasing the Cl element occupying the 4d site in the argyrodite-type crystal structure is not known.

[0010] Furthermore, conventional argyrodite-type sulfide-based solid electrolytes required pressurized molding at high pressures of around 300-600 MPa or hot-press molding by heating and pressurizing in order to achieve high lithium-ion conductivity. However, even if high lithium-ion conductivity can be achieved using the above methods, pressurized molding over a large area at high pressure is not practical when considering use in actual batteries.

[0011] In view of the above, the present invention aims to provide a sulfide-based solid electrolyte for use in lithium-ion secondary batteries and a method for producing the same, which exhibits high lithium-ion conductivity in a press-molded body at low pressure, even when the elemental ratio of halogen elements in the argyrodite-type crystalline phase is not as high as in conventional methods. [Means for solving the problem]

[0012] As a result of diligent research, the inventors of this invention have found that an argyrodite-type crystalline phase is 35 We discovered that the above problems can be solved by satisfying certain conditions in the Cl-NMR spectrum, and thus completed the present invention. Furthermore, we found a manufacturing method that involves a homogeneous intermediate as one way to obtain such an argyrodite-type crystalline phase. A homogeneous intermediate refers to a product resulting from the reaction of raw materials, where the structure becomes, for example, amorphous or melted into a molten state.

[0013] In other words, the present invention relates to the following [1] to

[13] . [1] A sulfide-based solid electrolyte used in lithium-ion secondary batteries, comprising an argyrodite-type crystalline phase containing Li, P, S, and Ha, wherein Ha is one or more halogen elements, including at least Cl, and the content ratio expressed as [Ha] / [P] (atomic ratio) using the content of P [P] and the total content of Ha [Ha] is 1.3 or more, and is measured under the following conditions. 35 In the Cl-NMR spectrum, the area intensity S of the peak observed between 0 and 30 ppm. A And the area intensity S of the peak observed between -150 and 0 ppm. BUsing S B / S A A sulfide-based solid electrolyte in which the ratio of area intensities represented by is 3.5 or greater, or in which a peak is observed between -150 and 0 ppm, and no peak is observed between 0 and 30 ppm. (Conditions) Measurement was performed using LiCl as an external standard, and the results were obtained. 35 The chemical shift position with the highest intensity in the Cl-NMR spectrum was set to 9.93 ppm, and the following conditions were used: probe: 3.2 mm solid-state, measurement conditions: single pulse method, pulse width: 2.45 μs, observation center: 20 ppm, observation width: 1000 ppm, relaxation delay: 10 sec, number of integrations: 1024, rotation speed: 15 kHz. 35 Cl-NMR measurement was performed. 35 Obtain the Cl-NMR spectrum. [2] The sulfide-based solid electrolyte according to [1], wherein Ha is two or more halogen elements including Cl. [3] The sulfide-based solid electrolyte according to [1] or [2], wherein Ha is two or more halogen elements including Cl and Br. [4] The sulfide-based solid electrolyte according to [3], wherein the ratio of the content, expressed as [Cl] / [Br] (atomic ratio) using the content of Cl [Cl] and the content of Br [Br], is 0.2 to 3.0. [5] The argyrodite-type crystalline phase is Li a PS b Ha c A sulfide-based solid electrolyte according to any one of [1] to [4] above, wherein, when expressed as, the elemental ratios represented by a, b, and c satisfy the relationships 5 ≤ ​​a ≤ 7, 4 ≤ b ≤ 6, and 1.3 ≤ c ≤ 2. [6] The sulfide-based solid electrolyte according to any one of [1] to [5], wherein the argyrodite-type crystalline phase contains an anion in its crystalline structure, the anion containing an oxide anion having an MO bond in which M and O are bonded, and M is at least one element selected from the group consisting of metallic and metalloid elements of groups 2 to 14 of the periodic table. [7] The sulfide-based solid electrolyte according to [6], wherein the total content of the elements Li, P, S, Ha, M, and O is 90% by mass or more.

[0014] [8] A method for producing a sulfide-based solid electrolyte used in lithium-ion secondary batteries, comprising: mixing raw materials containing Li, P, S and Ha to obtain a raw material mixture; heating the raw material mixture to obtain a molten product as a homogeneous intermediate compound; and cooling the molten product to precipitate argyrodite-type crystals, wherein the Ha is one or more halogen elements containing at least Cl, and the molten product contains an argyrodite-type crystalline phase. [9] The method for producing a sulfide-based solid electrolyte according to [8], wherein the heating is carried out under conditions of 600 to 800°C and for 5 minutes or more in an inert atmosphere.

[0015]

[10] A method for producing a sulfide-based solid electrolyte used in lithium-ion secondary batteries, comprising mixing raw materials containing Li, P, S and Ha to obtain an amorphous intermediate compound, and heating and calcining the intermediate compound to precipitate an argyrodite-type crystal, wherein Ha is one or more halogen elements including at least Cl, and the method for producing a sulfide-based solid electrolyte containing an argyrodite-type crystalline phase.

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

[10] , wherein the intermediate compound does not show any peaks originating from the raw materials in the Raman spectrum.

[12] A method for producing a sulfide-based solid electrolyte according to

[10] or

[11] , wherein the mixing is carried out using a mechanical milling method with a ball mill, in an environment with a dew point of -60°C or higher, at a rotational speed of 400 rpm or higher for 12 hours or more.

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

[10] to

[12] , wherein the particle size of the intermediate compound is 0.1 to 2 μm. [Effects of the Invention]

[0016] According to the sulfide-based solid electrolyte of the present invention, even when the elemental ratio of halogen elements in the argyrodite-type crystalline phase is lower than in conventional methods, high lithium-ion conductivity can be achieved in a pressurized body at low pressure. Therefore, it is extremely useful as a solid electrolyte for lithium-ion secondary batteries, and this is expected to improve the battery characteristics of lithium-ion secondary batteries. [Brief explanation of the drawing]

[0017] [Figure 1] Figure 1 shows the 35Cl-NMR spectrum of the sulfide-based solid electrolyte of Example 9, and is a diagram used to explain how to determine the ratio of area intensities expressed as SB / SA. [Figure 2] Figure 2 shows the 35Cl-NMR spectra of sulfide-based solid electrolytes for Examples 1, 2, 5, and 9. [Modes for carrying out the invention]

[0018] The present invention will be described in detail below, but the present invention is not limited to the following embodiments and can be modified and implemented as appropriate without departing from the spirit of the invention. Furthermore, the "~" indicating a numerical range is used to mean that the numbers written before and after it are included as the lower limit and upper limit.

[0019] <Sulfide solid electrolyte> The sulfide-based solid electrolyte (hereinafter sometimes simply referred to as "solid electrolyte") according to this embodiment is used in lithium-ion secondary batteries and contains an argyrodite-type crystalline phase containing Li, P, S, and Ha. Ha is one or more halogen elements, including at least Cl.

[0020] (Argyrodite-type crystalline phase) In an argyrodite-type crystalline phase, the content ratio expressed as [Ha] / [P] (atomic ratio), where [P] is the content of P and [Ha] is the total content of Ha, is 1.3 or greater. That is, in an argyrodite-type crystalline phase, Li a PS b Ha cWhen expressed as such, the elemental ratio of the total halogen elements represented by c is 1.3 or greater.

[0021] Solid electrolytes are measured under the following conditions: 35 In the Cl-NMR spectrum, peaks are observed at 0-30 ppm and -150-0 ppm, or a peak is observed at -150-0 ppm but not at 0-30 ppm. If peaks are observed between 0 and 30 ppm and between -150 and 0 ppm, the area intensity S of the peak observed between 0 and 30 ppm is... A And the area intensity S of the peak observed between -150 and 0 ppm. B Using S B / S A The ratio of area intensity represented by is 3.5 or greater. Note that a sharp peak may be detected in the range of -50 to 0 ppm, which is due to S B Treat it as part of something.

[0022] (conditions) The measurement was performed using LiCl as an external standard, and the results were obtained. 35 The chemical shift position with the highest intensity in the Cl-NMR spectrum was set to 9.93 ppm, and the following conditions were used: probe: 3.2 mm solid-state, measurement conditions: single pulse method, pulse width: 2.45 μs, observation center: 20 ppm, observation width: 1000 ppm, relaxation delay: 10 sec, number of integrations: 1024, rotation speed: 15 kHz. 35 Cl-NMR measurement was performed. 35 Obtain the Cl-NMR spectrum.

[0023] Measured under the above conditions 35 The Cl-NMR spectrum was analyzed using the software "ALICE2 FOR WINDOWS® Ver.6," and the area intensity S of the peak observed between 0 and 30 ppm was determined. A And the area intensity S of the peak observed between -150 and 0 ppm. B We seek.

[0024] Area strength SA and area intensity S B As a method for calculating this, see the example 9 shown in Figure 1. 35 This will be explained using Cl-NMR spectroscopy. In Figure 1, looking at the range of 0 to 30 ppm, peak A1 is observed with its peak top at around 9.5 ppm. As spinning sidebands of peak A1, peak A2 with its peak top at around 228 ppm and peak A3 with its peak top at around -208 ppm are observed. Here, with the area intensity of peak A1 set to 1, the total area intensity of peaks A1, A2, and A3 is defined as area intensity S. A The following calculation was performed to determine the area intensity: the integration range for peak A1 was set to 0-20 ppm, for peak A2 to 220-240 ppm, and for peak A3 to -220-200 ppm. Although spinning sidebands originating from peak A1 were observed around ±450 ppm, they were too weak to be included in the area intensity calculation.

[0025] Next, looking at the range of -150 to 0 ppm in Figure 1, a peak B is observed with its peak top around -60 ppm. Therefore, the area intensity of peak B is calculated using the integration range of -160 to 20 ppm, and the area intensity of peak A, whose integration range overlaps with this peak, is subtracted from this value to obtain the area intensity S of peak B. B Let's assume that.

[0026] Area strength S obtained above A and area intensity S B And then, S B / S A The ratio of area intensities represented by this formula can be determined.

[0027] In addition to the above, the inventors have determined the following from first-principles calculations regarding argyrodite-type crystals, as shown below. 35 Cl-NMR spectra were obtained. Density functional theory was used for first-principles calculations. As a unit cell, Li 22 P4S 18For Cl3Br3, an initial structure was set with one Cl and two Br at the 4a site, and two Cl and one Br at the 4d site. In addition, Li was used as a unit cell. 22 P4S 18 For Cl6, an initial structure was set with two Cl atoms at the 4a site and four Cl atoms at the 4d site. Then, structural optimization was performed using VASP, followed by NMR calculations. Thus, from first-principles calculations 35 Cl-NMR spectroscopy revealed that in the argyrodite crystal structure, the peak at the 4a site is located at a lower magnetic field, i.e., at a higher ppm value, compared to the peak at the 4d site.

[0028] From these results, S B / S A A larger ratio of the area intensity, as expressed by , is thought to mean that the amount of Cl located at the 4a site is relatively small. This means that the proportion of chlorine (Cl) at the 4d site, surrounded by lithium (Li), is higher.

[0029] 4d site is S 2- Ka Ha - It is composed of anions represented by S 2- Ha - It has a higher valence than Li + It is easy to restrict the lithium ion. As a result, lithium-ion conductivity decreases. In contrast, in the sulfide-based solid electrolyte according to this embodiment, Ha containing Cl with a lower valency occupies a relatively large number of 4d sites, Li + The interaction with Li decreases + This makes it easier for the lithium ion to move. As a result, we believe that the lithium ion conductivity will increase.

[0030] In addition to the above, first-principles calculations revealed that a relatively smaller amount of Cl at the 4a site resulted in a lower estimated elastic modulus. This means that even when press-molded at a lower pressure, the interfacial contact is good and the interfacial resistance is small. In other words, the sulfide-based solid electrolyte according to this embodiment can achieve high lithium-ion conductivity even as a press-molded body at a low pressure of about 50 to 100 MPa.

[0031] When a sulfide-based solid electrolyte contains halogen elements other than Cl, the smaller the amount of those halogen elements located at the 4a site, the lower the elastic modulus, and the higher the lithium ion conductivity, even when molded under low pressure. For example, if a sulfide-based solid electrolyte contains Br as a halogen element other than Cl, 79 When Br-NMR is measured, 35 We believe that this will allow for similar analysis as with Cl-NMR spectra.

[0032] Furthermore, we will compare the argyrodite-type crystal phase in this embodiment with conventional methods that increase the proportion of halogen elements in the composition of an argyrodite-type crystal structure, thereby increasing the amount of halogen elements occupying the 4d sites. Referring to Non-Patent Document 1, the general formula for argyrodite crystals is Li 6-x PS 5-x Cl 1+x This can be expressed as follows: Conventionally, lithium ion conductivity was increased by increasing the proportion of halogen elements, thereby increasing the proportion of halogen elements occupying the 4d site relative to sulfur elements. Increasing the proportion of halogen elements means increasing the content ratio expressed as [Ha] / [P] (atomic ratio) of halogens, which means increasing the proportion of halogen elements relative to sulfur elements. According to Non-Patent Literature 1, the larger the content ratio expressed as [Ha] / [P] (atomic ratio) of halogens, the higher the S B / S A The ratio of area intensities, expressed as , tends to decrease.

[0033] In contrast, the sulfide-based solid electrolyte according to this embodiment, even with a composition in which the content ratio expressed as [Ha] / [P] (atomic ratio) is increased, contains S B / S A The ratio of area intensity, expressed as [Ha] / [P](atomic ratio), can be increased, specifically, the ratio of area intensity must be 3.5 or higher. Furthermore, if the above ratio of area intensity is 3.5 or higher, high lithium ion conductivity can be achieved even in pressurized bodies at low pressure, even if the content ratio expressed as [Ha] / [P](atomic ratio) is not as large as in conventional methods.

[0034] Thus, S B / S A The ratio of area intensity represented by is 3.5 or higher, and from the viewpoint of obtaining a higher lithium-ion conductivity, it is preferably 4 or higher, more preferably 5 or higher, even more preferably 10 or higher, and particularly preferably 30 or higher. Also, S B / S A There is no particular upper limit; the higher the better.

[0035] Furthermore, the solid electrolyte according to this embodiment is 35 In the Cl-NMR spectrum, a peak may be observed between -150 and 0 ppm, and no peak may be observed between 0 and 30 ppm. This means that the Cl at the 4a site is 0, i.e., absent. B / S A The ratio of area intensities represented by this formula becomes infinite.

[0036] The above 35 In addition to the characteristics of the Cl-NMR spectrum, in the argyrodite-type crystalline phase, the content ratio expressed as [Ha] / [P] (atomic ratio), where the content of P [P] and the total content of Ha [Ha] are used, is 1.3 or greater. That is, the argyrodite-type crystalline phase is Li a PS b Ha c When expressed as such, the elemental ratio of the total halogen elements represented by c is 1.3 or higher. This enables high lithium-ion conductivity.

[0037] The ratio of the content represented by [Ha] / [P] (atomic ratio) is preferably 1.3 to 2, more preferably 1.4 to 1.9, and even more preferably 1.5 to 1.8. Here, from the viewpoint of obtaining a higher lithium ion conductivity, the [Ha] / [P] (atomic ratio) is preferably 1.4 or more, and more preferably 1.5 or more. On the other hand, from the viewpoint of suppressing the corrosion of the metal current collector, the [Ha] / [P] (atomic ratio) is preferably 2 or less, more preferably 1.9 or less, and even more preferably 1.8 or less.

[0038] The argyrodite-type crystal phase may be only one kind of the same composition or may include two or more kinds of different compositions. When two or more kinds of different compositions are included, at least one kind of composition is the above 35 It is only necessary to have the characteristics of the Cl-NMR spectrum and the characteristics of [Ha] / [P] (atomic ratio), but it is preferable that other compositions also have such characteristics, and it is more preferable that all the compositions included have such characteristics.

[0039] The crystal structure constituting the argyrodite-type crystal phase can be analyzed from the X-ray powder diffraction (XRD) pattern. In the XRD pattern, it can be said that the crystal is of the argyrodite type by having peaks at positions of 2θ = 15.7 ± 0.5° and 30.2 ± 0.5°. In addition to the above, the XRD pattern preferably further has a peak at a position of 2θ = 18.0 ± 0.5°, and more preferably further has a peak at a position of 2θ = 25.7 ± 0.5°.

[0040] Regarding the value of c when the argyrodite-type crystal phase is represented by Li a PS b Ha c is as described above, and further, it is preferable that the relationships of 5 ≤ a ≤ 7, 4 ≤ b ≤ 6 and 1.3 ≤ c ≤ 2 are satisfied because the crystal is likely to be of the argyrodite type. Such elemental ratios more preferably satisfy the relationships of 5.1 < a < 6.3, 4 < b < 5.3 and 1.4 ≤ c ≤ 1.9, and even more preferably satisfy the relationships of 5.2 < a < 6.2, 4.1 < b < 5.2 and 1.5 ≤ c ≤ 1.8. Specifically, for a, 5 or more is preferred, greater than 5.1 is more preferred, greater than 5.2 is even more preferred, and 7 or less is preferred, less than 6.3 is more preferred, and less than 6.2 is even more preferred. For b, 4 or more is preferred, greater than 4 is more preferred, greater than 4.1 is even more preferred, and 6 or less is preferred, less than 5.3 is more preferred, and less than 5.2 is even more preferred. For c, 1.3 or more is preferred, 1.4 or more is more preferred, 1.5 or more is even more preferred, and 2 or less is preferred, 1.9 or less is more preferred, and 1.8 or less is even more preferred.

[0041] A preferred crystal structure for argyrodites is, for example, cubic, such as F-43m. However, hexagonal, tetragonal, orthorhombic, monoclinic, and even triclinic crystals with reduced symmetry may also exist.

[0042] The halogen element represented by Ha is one or more elements, including at least Cl. The presence of Cl makes it easier for the crystal to form an argyrodite type. As for halogen elements other than Cl, at least one selected from the group consisting of F, Br, and I is preferred.

[0043] The halogen element may be just one type, Cl, but it may also be two or more types including Cl, in which case two or more types including Cl and Br are preferred. Including Br can improve the lithium ion conductivity of the solid electrolyte itself and also contribute to a decrease in elastic modulus, thus improving the lithium ion conductivity when the body is molded under pressure at lower pressures.

[0044] If the halogen element contains Br, 35 S in Cl-NMR spectrum B / S A Similar to the ratio of area intensity expressed by, 79 The relative deficiency of Br located at the 4a site can be expressed using the ratio of the area intensities of specific peaks in the Br-NMR spectrum. As mentioned earlier, from the first-principles calculations 35 Using a method similar to the one used to determine the Cl-NMR spectrum, first-principles calculations can be performed from...79 The Br-NMR spectrum was obtained. As a result, in the alditol-type crystal structure, it was found that the peak at the 4a site is located on the lower magnetic field side, that is, on the side with a higher ppm value, compared to the peak at the 4d site.

[0045] (Condition) Measured using KBr as an external standard, and the 79 position of the chemical shift with the highest intensity in the Br-NMR spectrum was set as 54.5 ppm, and under the conditions of probe: for 3.2 mm solid, measurement condition: Hahn Echo method, pulse width: 1.4 μs, observation center: 5.0 ppm, observation width: 1 MHz, Relaxation delay: 0.3 sec, number of integrations: 300000, rotation speed: 15 kHz 79 Br-NMR measurement was performed, 79 and a Br-NMR spectrum was obtained.

[0046] In the Br-NMR spectrum measured under the above conditions, two overlapping peaks are observed at -400 to 4@0 ppm. The ratio of the area intensity S 79 of the peak on the lower magnetic field side of the observed peaks A2 to the area intensity S B2 of the peak on the higher magnetic field side of the observed peaks, using S B2 / S A2 If the ratio of the area intensity represented by is 1 or more, it means that the proportion of bromine (Br) at the 4d site surrounded by lithium (Li) is high.

[0047] When the halogen elements include Cl and Br, the ratio of the Cl content [Cl] and Br content [Br] in the argyrodite-type crystalline phase, expressed as [Cl] / [Br] (atomic ratio), is preferably 0.1 to 10, more preferably 0.2 to 3.0, even more preferably 0.3 to 1.6, and particularly preferably 0.5 to 0.8. Here, the lower limit of the [Cl] / [Br] (atomic ratio) is preferably 0.1 or higher, more preferably 0.2 or higher, even more preferably 0.3 or higher, and particularly preferably 0.5 or higher. Furthermore, the [Cl] / [Br] (atomic ratio) is preferably 10 or less, more preferably 3.0 or less, even more preferably 1.6 or less, and particularly preferably 0.8 or less. When the [Cl] / [Br] (atomic ratio) satisfies the above range, the interaction between lithium ions and halide ions weakens, and the lithium ion conductivity of the solid electrolyte tends to improve. This is thought to be due to the mixed anion effect, which weakens the interaction between cations and anions by mixing bromide ions, which have a larger ionic radius than chloride ions. In addition, when the [Cl] / [Br] (atomic ratio) satisfies the above range, the cycle characteristics of lithium-ion secondary batteries tend to improve.

[0048] When halogen elements are present, the argyrodite type crystal phase is Li a PS b Cl c1 Br c2 When expressed as [C1+C2], the ratio of the content expressed as [Ha] / [P] (atomic ratio) is the same as the value expressed as (c1+c2). (c1+c2) should be 1.3 or more, preferably 1.3 to 2, more preferably 1.4 to 1.9, and even more preferably 1.5 to 1.8. Here, similar to the value of c above, from the viewpoint of obtaining high lithium ion conductivity, (c1+c2) is preferably 1.4 or more, and more preferably 1.5 or more. On the other hand, from the viewpoint of suppressing corrosion of metal current collectors, (c1+c2) is preferably 2 or less, more preferably 1.9 or less, and even more preferably 1.8 or less.

[0049] Li a PS b Cl c1 Br c2The value represented by c1 in this formula is the same as the ratio of the content expressed as [Cl] / [P] (atomic ratio) using the content of P [P] and the content of Cl [Cl]. c1 is preferably 0.1 to 1.9, more preferably 0.5 to 1.5, and even more preferably 0.7 to 1.0. Here, the lower limit of c1 is preferably 0.1 or higher, more preferably 0.5 or higher, and even more preferably 0.7 or higher. Also, c1 is preferably 1.9 or lower, more preferably 1.5 or lower, and even more preferably 1.0 or lower.

[0050] Li a PS b Cl c1 Br c2 The value represented by c2 is the same as the content ratio expressed as [Br] / [P] (atomic ratio) using the content of P [P] and the content of Br [Br]. c2 is preferably 0.1 to 1.9, more preferably 0.5 to 1.5, and even more preferably 0.7 to 1.0. Here, the lower limit of c2 is preferably 0.1 or higher, more preferably 0.5 or higher, and even more preferably 0.7 or higher. Also, c2 is preferably 1.9 or lower, more preferably 1.5 or lower, and even more preferably 1.0 or lower.

[0051] When c1 and c2 satisfy the above ranges, the proportion of halide ions in the crystalline phase is optimized, and a more stable argyrodite-type crystalline phase is obtained while reducing the interaction between anions and lithium ions. As a result, lithium-ion conductivity tends to be good. In addition, when c1 and c2 satisfy the above ranges, the cycle characteristics of lithium-ion secondary batteries tend to be improved. Furthermore, it is preferable that a, b, and (c1+c2) satisfy the same relationships as a, b, and c described above.

[0052] From the viewpoint of obtaining good lithium-ion conductivity when the solid electrolyte is finely pulverized and made into a battery, a smaller crystallite size is preferable for the argyrodite-type crystalline phase. Specifically, a crystallite size of 1000 nm or less is preferable, 500 nm or less is more preferable, and 250 nm or less is even more preferable. There is no particular lower limit to the crystallite size, but it is usually 5 nm or more. Crystallite size can be calculated using the full width at half maximum (FMAX) of the peaks in the XRD pattern and Scherrer's equation.

[0053] The raw materials for the argyrodite-type crystalline phase will be described in detail later, but a mixture containing lithium sulfide (Li2S) is preferably used as such raw material. Here, it is widely known that lithium sulfide is produced from lithium hydroxide (LiOH), but lithium hydroxide may contain at least one element selected from the group consisting of Na, K, Mg, and Ca (hereinafter sometimes referred to as "R") as an impurity. When an argyrodite-type crystalline phase is produced using raw materials containing R, the argyrodite-type crystalline phase may also contain R. Here, reducing the R content in the argyrodite-type crystalline phase requires the use of high-purity raw materials, which raises concerns about increased manufacturing costs. However, if the R content ratio in the argyrodite-type crystalline phase is within a predetermined range, it will have no effect on the lithium ion conductivity or heat resistance of the solid electrolyte, or if it does, the effect will be negligibly small.

[0054] Therefore, the argyrodite-type crystalline phase may contain at least one element selected from the group consisting of Na, K, Mg, and Ca, represented by R. A argyrodite-type crystalline phase containing R is Li a1 R a2 PS b Ha cWhen expressed as such, the ratio of the content expressed as [R] / [P] (atomic ratio), using the content of R [R] and the content of P [P], is the same as the value expressed as a2. a2 is preferably 0.001 to 0.4, more preferably 0.01 to 0.3, and even more preferably 0.02 to 0.2. Here, from the viewpoint of suppressing manufacturing costs, a2 is preferably 0.001 or more, more preferably 0.01 or more, and even more preferably 0.02 or more. Also, from the viewpoint of suppressing a decrease in lithium ion conductivity, a2 is preferably 0.4 or less, more preferably 0.3 or less, and even more preferably 0.2 or less. The argyrodite-type crystalline phase does not have to contain R. Furthermore, it is preferable that a1, b, and c each satisfy the same relationship as a, b, and c described above. However, the value of a1 may be smaller than the preferred numerical range of a described above, depending on the value of a2. In that case, it is preferable that the value of (a1+a2) satisfies the relationship 5≦(a1+a2)<7.

[0055] Furthermore, Mg and Ca among the R elements can also be present in the solid electrolyte as M elements that constitute oxide anions, as described later. When the solid electrolyte contains at least one of the elements Mg and Ca, whether these elements are present as M or R can be determined by analyzing the crystal structure using Rietveld analysis of the XRD pattern. If the above elements are present as R, then Mg and Ca are present in the crystal structure, and O is nearby. 2- This result indicates that there is no such thing. Here, in the neighborhood O 2- The absence of O means that O is within the second nearest neighbor element of Mg and Ca. 2- This means that it does not exist.

[0056] (Oxide anion) The argyrodite-type crystal phase contains anions in its crystal structure, and it is preferable that such anions include oxide anions having MO bonds between M and O. This allows for heat treatment that reduces grain boundary resistance while maintaining high lithium ion conductivity as a solid electrolyte. The crystal structure may contain only one type of oxide anion or multiple types.

[0057] M is at least one element selected from the group consisting of metallic and metalloid elements in groups 2-14 of the periodic table. Metal elements in groups 2 through 14 of the periodic table are the elements in groups 2 through 12, the elements in group 13 other than B, and the elements in group 14 other than C, Si, and Ge. The metalloid elements of groups 2-14 are B, Si, and Ge, which are elements in groups 13 and 14 of the periodic table. In particular, it is more preferable to include at least one element selected from the group consisting of Al, B, Ge, La, Ta, Nb, Ti, Si, Sn, V, Y, and Zr.

[0058] The presence of MO bonds in oxide anions can be confirmed by Raman spectroscopy or nuclear magnetic resonance (NMR) measurements.

[0059] When oxide anions are included in the crystal structure of the argyrodite-type crystal phase, the total content of the elements constituting the argyrodite-type crystal phase and oxide anions relative to the total components constituting the solid electrolyte is preferably 90% by mass or more, more preferably 92% by mass or more, and even more preferably 94% by mass or more, from the viewpoint of achieving high lithium ion conductivity. Furthermore, there is no particular upper limit to the total content, and it may be 100% by mass. In addition to argyrodite-type crystals, amorphous crystals may also be included in the crystal phase. In addition, another argyrodite-type crystal phase consisting of at least one of Li, P, S, and Ha may also be included.

[0060] The total content mentioned above refers to the sum of the content of elements such as Li, P, S, Ha, M, and O, and if the argyrodite crystal contains R, the sum also includes the content of R. In this specification, the Ha content refers to the sum of the content of F, Cl, Br, and I. The content of each element and their total amounts can be determined by compositional analysis using methods such as ICP emission spectrometry, atomic absorption spectrometry, and ion chromatography.

[0061] In the solid electrolyte, the proportion of the argyrodite-type crystalline phase is preferably 50% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 80% by mass or more, from the viewpoint of achieving high lithium-ion conductivity. Furthermore, there is no particular upper limit to the proportion of the argyrodite-type crystalline phase, and it may be 100% by mass, but generally it is 99% by mass or less. The proportion of the argyrodite-type crystalline phase can be calculated by including an internal standard material, measuring it by XRD or neutron scattering, and then comparing the peak intensity with that of the internal standard material.

[0062] In addition to argyrodite-type crystalline phases and oxide anions, other elements that may be included in the solid electrolyte include Li3PS4, Li4P2S6, Li2S, and LiHa (where Ha is at least one halogen element selected from F, Cl, Br, and I).

[0063] The sulfide-based solid electrolyte according to this embodiment achieves high lithium ion conductivity even when molded under low pressure, where low pressure is, for example, in the range of 50 to 100 MPa. As an indicator of lithium ion conductivity in a molded body under low pressure, for example, when molded under 80 MPa, the lithium ion conductivity at 25°C is preferably 2 mS / cm or higher, more preferably 3 mS / cm or higher, even more preferably 4 mS / cm or higher, and higher is preferable.

[0064] In order to achieve lithium-ion conductivity within the above range, and from the viewpoint of reducing interfacial resistance between particles forming the solid electrolyte, i.e., grain boundary resistance, the elastic modulus of the sulfide-based solid electrolyte is preferably 40 GPa or less, more preferably 30 GPa or less, and even more preferably 20 GPa or less. The elastic modulus of the sulfide-based solid electrolyte can be determined by measuring the compacted powder with a nanoindenter or by fixing the powder and measuring it with an atomic force microscope (AFM).

[0065] <Method for producing sulfide-based solid electrolytes> In the method for producing a sulfide-based solid electrolyte used in the lithium-ion secondary battery according to this embodiment, it is important that the intermediate compound obtained before obtaining the argyrodite-type crystalline phase is a homogeneous compound. A homogeneous compound is not simply a mixture of multiple raw materials, but rather a compound formed by the reaction of the raw materials themselves, resulting in a structure that is, for example, amorphous or melted into a molten state.

[0066] To obtain an amorphous product, one method involves mixing the raw materials in a media-less grinder such as a mixer mill or pin mill, and then mechanically mixing them in a media grinder such as a planetary ball mill, bead mill, or Atriator (registered trademark) to induce a mechanochemical reaction. To further promote the conventionally known mechanochemical reaction, it is preferable to carry out the reaction in a humidified atmosphere. This yields an amorphous, homogeneous intermediate. The specific environment for the humidified atmosphere in the above mixing process to obtain the amorphous material is preferably a dew point of -60 to -30°C, and more preferably -50 to -30°C. Here, the lower limit of the dew point is preferably -60°C or higher, and more preferably -50°C or higher. Furthermore, from the viewpoint of preventing the generation of large amounts of hydrogen sulfide due to the reaction of sulfides and water during synthesis, an environment with a dew point of -30°C or lower is preferred. Note that the dew point referred to here means the dew point of the entire facility. To obtain a melt, the raw materials are mixed using a mixer mill or pin mill before dissolving. This allows the melt to be converted into a homogeneous intermediate compound. Alternatively, the raw materials may be dissolved first in an organic solvent.

[0067] The method for producing the sulfide-based solid electrolyte according to this embodiment is not particularly limited as long as an argyrodite-type crystalline phase can be obtained via such a homogeneous intermediate compound, but two methods, i and ii described below, can be cited as examples.

[0068] (Manufacturing method i) Step i-1: A step of mixing raw materials containing Li, P, S and Ha to obtain a raw material mixture. Step i-2: A step of heating the above raw material mixture to obtain a molten product as a homogeneous intermediate compound, and Step i-3: A step of cooling the above molten material to precipitate argyrodite-type crystals. The above Ha is one or more halogen elements, including at least Cl.

[0069] (Manufacturing method ii) Step ii-1: A step of mixing raw materials containing Li, P, S and Ha to obtain an amorphous intermediate compound, and Step ii-2: The process includes heating and calcining the above intermediate compound to precipitate an argyrodite-type crystal, wherein the Ha is one or more halogen elements, including at least Cl, and the intermediate compound obtained in step ii-1 is an amorphous intermediate compound in which no peaks originating from the above raw materials are observed in powder X-ray diffraction measurements.

[0070] First, let's explain manufacturing method i. Step i-1 is a step in which raw materials containing Li, P, S, and Ha are mixed to obtain a raw material mixture. The raw materials containing Li, P, S, and Ha can be those conventionally known as materials for obtaining argyrodite-type crystals containing Li, P, S, and Ha. For example, a mixture of a compound containing Li (lithium), a compound containing P (phosphorus), a compound containing S (sulfur), and a compound containing Ha (halogen) can be used.

[0071] Examples of lithium-containing compounds include lithium compounds such as lithium sulfide (Li2S), lithium oxide (Li2O), lithium carbonate (Li2CO3), lithium hydroxide (LiOH), and lithium sulfate (Li2SO4), as well as elemental lithium metal.

[0072] Examples of compounds containing phosphorus (P) include phosphorus sulfides such as diphosphorus trisulfide (P2S3) and diphosphorus pentasulfide (P2S5), phosphorus compounds such as lithium phosphate (LiPO3, Li4P2O7, Li3PO4) and sodium phosphate (NaPO3, Na4P2O7, Na3PO4), and elemental phosphorus. Examples of sulfur-containing compounds include lithium sulfide (Li2S), phosphorus sulfide (P2S3, P2S5), and hydrogen sulfide (H2S), and elemental sulfur can also be used. Among compounds containing ha, those containing chlorine (Cl) include, for example, lithium chloride (LiCl), phosphorus trichloride (PCl3), phosphorus pentachloride (PCl5), phosphorus tetrachloride (P2Cl4), phosphoryl chloride (POCl3), sulfur dichloride (SCl2), disulfur dichloride (S2Cl2), sodium chloride (NaCl), and boron trichloride (BCl3). Among compounds containing ha (Ha), examples of compounds containing br (bromine) include lithium bromide (LiBr), phosphorus tribromide (PBr3), phosphoryl chloride (POBr3), disulfur dibromide (S2Br2), sodium bromide (NaBr), and boron tribromide (BBr3). In particular, a combination of lithium sulfide, phosphorus sulfide, and at least one of lithium chloride and lithium bromide is preferred.

[0073] These raw materials are highly unstable in the atmosphere and can decompose upon contact with water, potentially generating hydrogen sulfide gas and undergoing oxidation. Therefore, it is preferable to mix them in an inert atmosphere.

[0074] The raw materials can be mixed using, for example, a mixer mill, pin mill, powder agitator, or media-less mixing such as airflow mixing. Some of the raw materials may become amorphous by mixing before step i-2.

[0075] Step i-2 is a step in which the obtained raw material mixture is heated to obtain a molten product as an intermediate compound. Confirmation of a molten state means that no peaks originating from the raw materials are observed in high-temperature X-ray diffraction measurements. In the case of a Li-PS-Ha composition, the molten material mixes well, which means that it is a homogeneous molten compound distinct from the raw materials. Another simple way to confirm whether it is molten is to observe the state of the raw materials inside the furnace. If no unmelted material is observed, it is completely dissolved and can be considered a homogeneous intermediate compound. By passing through a homogeneous intermediate compound, 35 In the Cl-NMR spectrum, S B / S A A sulfide-based solid electrolyte is obtained in which the ratio of area intensities represented by is 3.5 or greater, or in which a peak is observed between -150 and 0 ppm, and no peak is observed between 0 and 30 ppm.

[0076] High-temperature X-ray diffraction measurements are performed after setting the measurement temperature, holding time, atmosphere, etc., to match the heating conditions used to obtain the molten material. Furthermore, by performing high-temperature X-ray diffraction measurements while varying the measurement temperature, it becomes possible to track changes in the crystalline state, such as phase transitions, and estimate the heating temperature at which an intermediate compound can be obtained.

[0077] The heating conditions required to obtain a molten product vary depending on the composition of the raw materials or raw material mixture used. The heating temperature should be above the temperature at which a homogeneous amorphous intermediate compound as described above is obtained. For example, 550 to 950°C is preferred, 600 to 900°C is more preferred, 600 to 850°C is even more preferred, and 650 to 800°C is particularly preferred. Here, the lower limit of the heating temperature is preferably 550°C or higher, more preferably 600°C or higher, and even more preferably 650°C or higher. Furthermore, from the viewpoint of suppressing compositional deviations due to volatilization of components, the heating temperature is preferably 950°C or lower, more preferably 900°C or lower, even more preferably 850°C or lower, and particularly preferably 800°C or lower. The temperature may also be changed in steps within the above temperature range. In addition, the higher the halogen content expressed as [Ha] / [P] (atomic ratio) of the composition, the lower the heating temperature can be.

[0078] The heating time should be at least the time required to obtain the homogeneous amorphous intermediate compound described above. This varies depending on the scale, but for example, 2 to 360 minutes is preferred, 5 to 180 minutes is more preferred, and 10 to 120 minutes is even more preferred. Here, the lower limit of the heating time is preferably 2 minutes or more, more preferably 5 minutes or more, and even more preferably 10 minutes or more. Furthermore, from the viewpoint of productivity, the heating time is preferably 360 minutes or less, more preferably 180 minutes or less, and even more preferably 120 minutes or less.

[0079] By stirring during heating and melting, the molten material, which will become an amorphous intermediate compound, can be made more homogeneous. Furthermore, the more homogeneous the mixing in step i-1, the shorter the heating time in the subsequent step ii-2 can be.

[0080] As mentioned above, it is preferable to mix the raw materials in an inert atmosphere, and it is also preferable to obtain the molten product by heating it in the same inert atmosphere. Examples of an inert atmosphere include an Ar atmosphere and a nitrogen atmosphere, but from the viewpoint of production costs, a nitrogen atmosphere is more preferable. Alternatively, heating may be performed in a vacuum-sealed state.

[0081] When Ha in an argyrodite-type crystalline phase contains Br in addition to Cl, Br has a larger ionic radius than Cl and therefore does not easily enter the crystalline structure. For this reason, adjusting the heating conditions to obtain the molten material by raising the heating temperature above the above range, increasing the heating time, or doing both, makes it easier to obtain a homogeneous intermediate compound. However, in manufacturing method i, where the molten material is an intermediate compound, the intermediate compound has high fluidity and therefore tends to be very homogeneous. As a result, Br, which has a large ionic radius, can easily enter the crystal structure, and furthermore, it can easily enter the 4d site, so the above-mentioned adjustments are not necessarily required.

[0082] For amorphous intermediate compounds, it is acceptable if no peaks originating from the raw materials are observed in high-temperature X-ray diffraction measurements. However, from the viewpoint of homogeneity, it is preferable that the melt is homogeneous and not phase-separated. In the case of Li-PS-Ha composition, the melt mixes well without phase separation, but the presence or absence of phase separation can be confirmed visually or optically by checking whether light is transmitted without scattering.

[0083] Step i-3 is a process of cooling the molten intermediate compound, which is a homogeneous intermediate compound, to precipitate argyrodite-type crystals. As mentioned above, the argyrodite-type crystals may contain impurities such as R originating from the raw materials, as long as they do not affect the lithium ion conductivity or elastic modulus of the solid electrolyte.

[0084] The cooling conditions required for crystal precipitation vary depending on the composition and the desired crystallization rate. The cooling rate is not particularly limited as long as argyrodite-type crystals precipitate, but is preferably 5 to 2000°C / min, more preferably 10 to 1000°C / min, and even more preferably 30 to 300°C / min. From the viewpoint of productivity, the cooling rate is preferably 5°C / min or more, more preferably 10°C / min or more, and even more preferably 30°C / min or more. Furthermore, from the viewpoint of increasing the crystallization rate, the cooling rate is preferably 2000°C / min or less, more preferably 1000°C / min or less, and even more preferably 300°C / min or less.

[0085] During cooling, extending the residence time between 200 and 450°C to allow for crystal growth or crystal structure reconstruction is more preferable from the viewpoint of improving lithium-ion conductivity.

[0086] The cooling atmosphere should preferably be an inert atmosphere, similar to the atmosphere used during mixing the raw materials and heating to obtain the molten product. Furthermore, if the heating to obtain the molten product is performed in a vacuum-sealed container, the cooling may also be performed in the same vacuum-sealed container.

[0087] Thus, by adopting the manufacturing method i according to this embodiment, 35 In the Cl-NMR spectrum, S B / SA By making the area intensity ratio represented by 3.5 or higher, or by obtaining a solid electrolyte in which a peak is observed between -150 and 0 ppm and no peak is observed between 0 and 30 ppm, high lithium ion conductivity can be achieved in pressurized molded bodies at low pressure without increasing the elemental ratio of halogen elements as in conventional methods. The preferred embodiment of the obtained solid electrolyte is the same as the preferred embodiment described above for <sulfide-based solid electrolyte>.

[0088] Next, we will explain manufacturing method ii. Step ii-1 is a step in which raw materials containing Li, P, S, and Ha are mixed to obtain an amorphous intermediate compound.

[0089] Raw materials containing Li, P, S, and Ha can be used under the same conditions as those described in step i-1 of manufacturing method i.

[0090] By employing much stricter conditions than conventional methods in the mixing of these raw materials, amorphous intermediate compounds can be obtained. An amorphous (non-crystalline) intermediate compound means that, in X-ray diffraction measurements, no peaks originating from the raw materials are observed, which means that it is a homogeneous compound different from the raw materials. By going through an amorphous intermediate compound, 35 In the Cl-NMR spectrum, S B / S A A sulfide-based solid electrolyte is obtained in which the ratio of area intensities represented by is 3.5 or greater, or in which a peak is observed between -150 and 0 ppm, and no peak is observed between 0 and 30 ppm.

[0091] When using a mechanical milling method with a ball mill for mixing raw materials, examples include rotary ball mills that impart rotational motion to the container, vibrating ball mills that impart vibrational motion, planetary ball mills that impart both revolutionary and rotational motion, bead mills, and attritors (registered trademark). All of these can be applied if conditions are adopted that yield amorphous intermediate compounds. Among these, planetary ball mills, attritors (registered trademark), and bead mills, which have higher mixing and grinding power, are preferred.

[0092] In the mechanical milling method using a ball mill, the higher the rotation speed, the longer the mixing time, and the smaller the ball particle size, the higher the mixing and grinding power. The rotational speed varies depending on the type of ball mill used and other conditions, but is preferably 200 to 1000 rpm, more preferably 300 to 800 rpm, and even more preferably 400 to 800 rpm. Here, the lower limit of the rotational speed is preferably 200 rpm or higher, more preferably 300 rpm or higher, and even more preferably 400 rpm or higher. There is no particular upper limit to the rotational speed, but from the viewpoint of mechanical strength, it is preferably 1000 rpm or lower, and more preferably 800 rpm or lower. When the rotational speed is defined by peripheral speed, as in the case of a bead mill, the peripheral speed is preferably 8 m / sec or higher, and even more preferably 10 m / sec or higher. There is no particular upper limit to the peripheral speed, but from a practical standpoint, it is 20 m / sec or lower. Alternatively, you can combine multiple rotation speeds, such as mixing at a low speed for a certain period of time first, and then increasing the rotation speed to a high speed for another period of time.

[0093] The mixing time varies depending on the type of ball mill used and other conditions, but in the case of a planetary ball mill, it is preferably 5 to 300 hours, more preferably 8 to 300 hours, and even more preferably 12 to 150 hours. Here, the lower limit of the mixing time is preferably 5 hours or more, more preferably 8 hours or more, and even more preferably 12 hours or more. There is no particular upper limit to the mixing time, but from the viewpoint of productivity, it is preferably 300 hours or less, and more preferably 150 hours or less. In the case of a circulating processing system like a bead mill, the number of passes is preferably 20 to 500, more preferably 50 to 500, and even more preferably 100 to 300. Here, the lower limit of the number of passes is preferably 20 or more, more preferably 50 or more, and even more preferably 100 or more. Also, from the viewpoint of productivity, the number of passes is preferably 500 or less, and even more preferably 300 or less.

[0094] The ball diameter varies depending on the type of ball mill used and other conditions, but is preferably 0.1 to 5 mm, more preferably 0.1 to 4 mm, and even more preferably 0.3 to 1 mm. Here, the upper limit of the ball diameter is preferably 5 mm or less, more preferably 4 mm or less, and even more preferably 1 mm or less. The lower limit of the ball diameter is preferably 0.1 mm or more, and more preferably 0.3 mm or more, from the viewpoint of handling. In addition, two or more types of balls with different diameters may be used in combination.

[0095] The amount of balls used varies depending on the type of ball mill used and other conditions, but from the viewpoint of mixing force and grinding force, it is preferable that the amount of balls be 100% or more of the total weight of the raw materials, more preferably 200% or more, even more preferably 500% or more, and preferably 1000% or less.

[0096] Ball milling can be used for dry mixing or wet mixing using a dispersion medium, but dry mixing is preferred from the viewpoint of efficiently transferring energy.

[0097] As mentioned above, from the viewpoint of further promoting the mechanochemical reaction, the dew point of the entire facility during mixing is preferably -60°C or higher, and more preferably -50°C or higher. Furthermore, from the viewpoint of preventing the generation of large amounts of hydrogen sulfide due to the reaction of sulfides and water during synthesis, the dew point is preferably -30°C or lower. In manufacturing method ii-1, the dew point when handling the raw materials before mixing and after obtaining the amorphous intermediate compound may be -50°C or lower, or below -60°C. However, from the viewpoint of productivity, it is preferable that the dew point be the same as that of the atmosphere used for mixing. Furthermore, an inert atmosphere such as nitrogen or argon is preferred, but oxygen may be present as long as it is 5% or less.

[0098] Through the mixing process described above, not only are the raw materials mixed, but the mixed powder becomes amorphous, resulting in a homogeneous amorphous intermediate compound. As mentioned earlier, the resulting amorphous intermediate compound means that no XRD peaks originating from the raw materials are observed. However, if, in the Raman spectrum, the peak at the position originating from the raw materials is completely absent and not observed, and a peak appears at a different position, it can be determined that a more homogeneous amorphous intermediate compound has been obtained.

[0099] The particle size of the intermediate compound is preferably 30 nm to 5 μm, more preferably 0.1 to 3 μm, and even more preferably 0.1 to 2 μm. Powder X-ray diffraction measurement used to verify the obtained amorphous intermediate compound may not show a peak even if it is crystalline if the particle size of the sample powder is very small. Therefore, from the viewpoint of distinguishing between a mixture of raw materials with very small particle sizes and the amorphous intermediate compound, the particle size of the intermediate compound is preferably 30 nm or larger, and more preferably 0.1 μm or larger. On the other hand, from the viewpoint of increasing the surface area of ​​the powder and efficiently advancing the subsequent reaction, the particle size of the intermediate compound is preferably 5 μm or less, more preferably 3 μm or less, and even more preferably 2 μm or less. In this specification, the particle size of the intermediate compound refers to the primary particle size determined from the SEM image obtained by scanning electron microscopy (SEM) observation. Specifically, SEM measurements are performed in an air-free environment, observed at a magnification of 2000x and an acceleration voltage of 2kV, and the particle sizes of 20 particles within a suitable field of view are measured. The average value of these measurements is then defined as the particle size.

[0100] Step ii-2 is a process in which the amorphous intermediate compound obtained in step ii-1 is heated and calcined to precipitate argyrodite-type crystals. As mentioned above, the argyrodite-type crystals may contain impurities such as R originating from the raw materials, as long as they do not affect the lithium ion conductivity or elastic modulus of the solid electrolyte.

[0101] Heating is preferably carried out, for example, under an inert gas atmosphere, a hydrogen sulfide gas atmosphere, a sulfur gas atmosphere, or under a vacuum-sealed tube. The heating temperature is preferably 350°C or higher and less than 600°C, more preferably 400°C or higher and less than 600°C, and even more preferably 450°C to 575°C. Here, from the viewpoint of promoting solid-phase reaction, i.e., crystallization, the heating temperature is preferably 350°C or higher, more preferably 400°C or higher, and even more preferably 450°C or higher. Also, from the viewpoint of suppressing thermal decomposition, the heating temperature is preferably less than 600°C, and even more preferably 575°C or lower.

[0102] From a similar viewpoint, the heating time is preferably 1 to 100 hours, more preferably 2 to 50 hours, and even more preferably 4 to 24 hours. Here, the lower limit of the heating time is preferably 1 hour or more, more preferably 2 hours or more, and even more preferably 4 hours or more. Also, the heating time is preferably 100 hours or less, more preferably 50 hours or less, and even more preferably 24 hours or less.

[0103] Thus, by adopting the manufacturing method ii according to this embodiment, 35 In the Cl-NMR spectrum, S B / S A By making the area intensity ratio represented by 3.5 or higher, or by obtaining a solid electrolyte in which a peak is observed between -150 and 0 ppm and no peak is observed between 0 and 30 ppm, high lithium ion conductivity can be achieved in pressurized molded bodies at low pressure without increasing the elemental ratio of halogen elements as in conventional methods. The preferred embodiment of the obtained solid electrolyte is the same as the preferred embodiment described above for <sulfide-based solid electrolyte>.

[0104] <Lithium-ion rechargeable battery> The sulfide-based solid electrolyte according to this embodiment is used in lithium-ion secondary batteries. When used in lithium-ion secondary batteries, sulfide-based solid electrolytes form a solid electrolyte layer together with other components such as binders, as needed. Conventionally known binders and other components are used. The solid electrolyte content relative to the entire solid electrolyte layer is preferably 80% by mass or more, and more preferably 90% by mass or more.

[0105] Conventional methods can be used to form the solid electrolyte layer. For example, the components constituting the solid electrolyte layer can be dispersed in a dispersion medium or dissolved in a solvent to form a slurry, which can then be coated in layers (sheets), dried, and optionally pressed to form the solid electrolyte layer. If necessary, heat may be applied to remove the binder. The thickness of the solid electrolyte layer can be easily adjusted by adjusting the amount of slurry applied.

[0106] Alternatively, instead of wet molding, a solid electrolyte layer may be formed by dry press molding of solid electrolyte powder or the like on the surface of the positive or negative electrode. In addition, a solid electrolyte layer may be formed on another substrate and then transferred onto the surface of the positive or negative electrode. The solid electrolyte according to this embodiment has a low elastic modulus and can achieve high lithium ion conductivity even when molded under low pressure. Therefore, when press molding as described above, a lower pressure than conventional methods, such as 50 to 100 MPa, can be used. An even lower pressure than 50 MPa may also be used.

[0107] The solid electrolyte may be mixed with a positive electrode active material or a negative electrode active material and used as the positive electrode layer or negative electrode layer. Conventionally known materials are used for the positive electrode active material or negative electrode active material, current collector, binder, conductive additive, etc. used in the positive electrode layer or negative electrode layer.

[0108] A lithium-ion secondary battery using a solid electrolyte includes the solid electrolyte layer, a positive electrode layer, and a negative electrode layer. The casing material for the lithium-ion secondary battery can also be one of conventionally known materials. The shape of the lithium-ion secondary battery can also be one of conventionally known shapes, such as coin-shaped, sheet-shaped (film-shaped), foldable, wound-type bottomed cylindrical, button-shaped, etc., which can be appropriately selected depending on the application. [Examples]

[0109] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. Examples 1-4, 7, and 8 are examples, while Examples 5, 6, 9, and 10 are comparative examples.

[0110] [Example 1] Under a dry nitrogen atmosphere, lithium sulfide powder (Sigma, 99.98% purity), phosphorus pentasulfide powder (Sigma, 99% purity), and lithium chloride powder (Sigma, 99.99% purity) were weighed to the composition ratios shown in Table 1, and the mixture was mixed in the same atmosphere using a mixer (WARING, X-TREME (MX1100XTM)) on High mode for 1 minute to obtain a raw material mixture (Step i-1). The obtained raw material mixture was vacuum-sealed into a quartz tube and heated at 700°C for 25 minutes to obtain a molten material (Step i-2). The molten material was cooled to room temperature at a rate of 300°C / min while still sealed, and argyrodite-type crystals were precipitated to obtain a sulfide-based solid electrolyte containing an argyrodite-type crystalline phase (Step i-3). Furthermore, when the raw materials inside the furnace were observed under the same conditions as the molten material, namely holding the raw material mixture at 700°C for 25 minutes, no unmelted material was observed. From this, it was confirmed that the molten material is a homogeneous intermediate compound.

[0111] [Examples 2 and 5] Except for changing the composition ratio and heating time in step i-2 to those listed in Table 1, sulfide-based solid electrolytes containing the argyrodite-type crystalline phase of Example 2 and Example 5 were obtained in the same manner as in Example 1. Furthermore, similar to Example 1, when the raw materials in the furnace were observed under the same conditions as the molten material, namely the raw material mixture held at 700°C for 60 minutes or 1 minute, no unmelted material was observed in Example 2, while some unmelted material-like substances were observed in Example 5. From this, it was confirmed that the molten material in Example 2 was a homogeneous intermediate compound. It is also considered that the conditions in Example 5 were such that the molten material did not yet become a completely homogeneous intermediate compound.

[0112] [Examples 3 and 4] Except for changing the composition ratio and heating time in step i-2 to those listed in Table 1, sulfide-based solid electrolytes containing the argyrodite-type crystalline phase of Example 3 and Example 4 were obtained in the same manner as in Example 1. In addition to the lithium sulfide powder, phosphorus pentasulfide powder, and lithium chloride powder used in Example 1, lithium bromide powder (manufactured by Sigma, purity 99.995%) was used as raw materials. Furthermore, similar to Example 1, when the raw material mixture was held at 700°C for 60 minutes under the same conditions as the molten material, the state of the raw materials inside the furnace was observed, and no unmelted material was observed. From this, it was confirmed that the molten materials in both Example 3 and Example 4 were homogeneous intermediate compounds.

[0113] [Example 6] Under a dry nitrogen atmosphere, lithium sulfide powder (Sigma, 99.98% purity), phosphorus pentasulfide powder (Sigma, 99% purity), and lithium chloride powder (Sigma, 99.99% purity) were weighed to the composition ratios shown in Table 1, and the raw material mixture was obtained by mixing them in a mortar in the same atmosphere (Step i-1). The obtained raw material mixture was vacuum-sealed into a quartz tube and heated at 700°C for 1 minute to obtain a molten material (Step i-2). The molten material was cooled to room temperature at a rate of 300°C / min while still sealed, and argyrodite-type crystals were precipitated to obtain a sulfide-based solid electrolyte containing an argyrodite-type crystalline phase (Step i-3). Furthermore, similar to Example 1, when the raw material mixture was held at 700°C for 1 minute under the same conditions as the molten material, the state of the raw materials inside the furnace was observed, and some unmelted material was seen. From this, it can be concluded that the conditions in Example 6 are such that the molten material does not become a completely homogeneous intermediate compound.

[0114] [Example 7] Under a dry nitrogen atmosphere with a dew point of -60°C, lithium sulfide powder (Sigma, 99.98% purity), phosphorus pentasulfide powder (Sigma, 99% purity), and lithium chloride powder (Sigma, 99.99% purity) were weighed and mixed in a mixer for 1 minute in the composition ratios shown in Table 1. The mixture was then further mixed using a planetary ball mill (Ito Seisakusho, LP-M2) to obtain an intermediate compound (Step ii-1). The mixing with the planetary ball mill was performed using balls with a particle size of 4 mm at 400 rpm for 50 hours, as shown in Table 1. The obtained intermediate compound was vacuum-sealed in a quartz tube and heated and calcined at 450°C for 5 hours to precipitate argyrodite-type crystals, obtaining a sulfide-based solid electrolyte containing an argyrodite-type crystalline phase (Step ii-2). The primary particle size of the intermediate compound before heating and calcination was confirmed to be 0.9 μm from SEM images. Furthermore, powder X-ray diffraction (Rigaku SmartLab) showed no peaks for the raw materials, lithium sulfide, diphosphorus pentasulfide, or lithium chloride, and an amorphous halo pattern was obtained. From this, it was confirmed that the intermediate compound is a homogeneous amorphous intermediate compound. The measurement conditions for X-ray diffraction are as follows: Source: CuKα radiation (λ=1.5418Å), Tube voltage: 45kV, Tube current: 200mA, Scanning angle: 10~100°, Scanning speed: 5° / min, Step count: 0.01° / step.

[0115] [Example 8] Except for changing the composition ratio to that shown in Table 1, setting the dew point of the entire facility to -50°C, and performing the preparation of raw materials, mixing by planetary ball milling, and vacuum sealing into quartz tubes under these conditions, a sulfide-based solid electrolyte containing an argyrodite-type crystalline phase was obtained in the same manner as in Example 7. In addition to the lithium sulfide powder, phosphorus pentasulfide powder, and lithium chloride powder used as raw materials in Example 7, lithium bromide powder (manufactured by Sigma, purity 99.995%) was used. The primary particle size of the intermediate compound before heating and calcination was 0.9 μm. Similar to Example 7, powder X-ray diffraction measurements showed no peaks for the raw materials—lithium sulfide, diphosphorus pentasulfide, lithium chloride, and lithium bromide—and yielded a halo pattern originating from the amorphous material. This confirmed that the intermediate compound was a homogeneous amorphous intermediate compound.

[0116] [Examples 9 and 10] In Example 9, the mixing conditions using a planetary ball mill in step ii-1 were changed, and in Example 10, the composition ratio and the mixing conditions using a planetary ball mill in step ii-1 were changed, but otherwise a sulfide-based solid electrolyte containing an argyrodite-type crystalline phase was obtained in the same manner as in Example 7. The mixing conditions for Example 9 and Example 10 were as shown in Table 1, using balls with a particle size of 10 mm and performed at 400 rpm for 4 hours. The primary particle size of the intermediate compound before heating and calcination was 7.3 μm. Powder X-ray diffraction measurements revealed peaks for the raw materials: lithium sulfide, phosphorus pentasulfide, and lithium chloride. This confirmed that the intermediate compound was not a homogeneous amorphous intermediate compound.

[0117] [evaluation] ( 35 Cl-NMR) The obtained sulfide-based solid electrolyte was packed into a 3.2 mm diameter ZrO2 sample tube in a simple glove box and subjected to nuclear magnetic resonance (JEOL ECZ700) 35 Cl-NMR spectra were obtained. The measurement conditions are as follows: radionuclides: 35 Cl Probe: 3.2mm for solids Measurement conditions: Single pulse method Pulse width: 2.45 μs Observation center: 20 ppm Observation width: 1000 ppm Relaxation delay: 10 sec Total count: 1024 Rotation speed: 15kHz External standard: LiCl 35 The chemical shift position with the highest intensity in the Cl-NMR spectrum is defined as 9.93 ppm.

[0118] Measured 35 In the Cl-NMR spectrum, the area intensity S of the peak observed between 0 and 30 ppm. A And the area intensity S of the peak observed between -150 and 0 ppm. B Using S B / S A The ratio of area intensity represented by was determined. Specifically, for peak A1, which has its peak top in the range of 0 to 30 ppm, the area was calculated using an integration range of 0 to 20 ppm. Furthermore, as spinning sidebands of peak A, the areas of peak A2, which has its peak top around 228 ppm, and peak A3, which has its peak top around -208 ppm, were also calculated using integration ranges of 220 to 240 ppm and -220 to -200 ppm, respectively. The sum of the areas of peaks A1, A2, and A3 was used to determine the area intensity S of peak A. A This was the result. Additionally, spinning sidebands originating from peak A1 were observed around ±450 ppm, but they were too weak to be included in the area intensity calculation. Next, for peak B, which has its peak top in -75 to -25 ppm, the area is calculated using the integration range of -160 to 20 ppm. Subtracting the area of ​​peak A, whose integration range overlaps with that of peak B, gives the area intensity S of peak B. B That's what I decided. The results are shown in Table 1, "S B / S A ( 35 (Cl-NMR) is shown. Also, Examples 1, 2, 5 and 9 35 The Cl-NMR spectrum is shown in Figure 2.

[0119] (Lithium-ion conductivity) The obtained sulfide-based solid electrolyte was ground in a mortar, and after removing coarse particles using a 100 μm mesh pass, 100 mg was measured out, and the lithium ion conductivity of the sample was measured using an AC impedance measuring device (Bio-Logic Sciences Instruments, potentiostat / galvanostat VSP) while pressurizing and molding an area of ​​10 mm in diameter at 80 MPa. The measurement conditions were: measurement frequency: 100Hz to 1MHz, measurement voltage: 100mV, and measurement temperature: 25℃. The results are shown in Table 1 under "σ25 (mS / cm) (80MPa)".

[0120] [Table 1]

[0121] From the above results, 35 S in Cl-NMR spectrum B / S A It was found that there is a very good correlation between the ratio of area intensities represented by and the lithium ion conductivity of the press-molded body at low pressure. For example, in Example 1, even when the content ratio represented by [Cl] / [P] (atomic ratio) is 1.58, which is a relatively large amount of halogen, S B / S A With a value of 3.5 or higher, the lithium ion conductivity of the pressurized body at low pressure was a good value of 5.1 mS / cm. In addition, Examples 3 and 4 contain bromine in addition to chlorine as halogen elements. Because bromine has a large ionic radius, it does not easily enter the argyrodite-type crystal structure, and in particular, the inclusion of bromine can make it even more difficult for chlorine to enter the 4d site. However, by going through a homogeneous intermediate compound, a sulfide-based solid electrolyte with high lithium ion conductivity in the pressurized body at low pressure was obtained.

[0122] Furthermore, compared to the sulfide-based solid electrolyte of Example 9 obtained by the conventional solid-phase method, the sulfide-based solid electrolyte of Example 7, which has the same composition but uses a homogeneous amorphous intermediate compound with a smaller primary particle size, is S B / S AThe values ​​were large, and the lithium-ion conductivity of the pressurized molded body at low pressure was also high. Furthermore, in Examples 10 and 8, which included bromine in addition to chlorine as halogen elements, the results showed a similar trend to Examples 9 and 7 above. In addition, in Example 8, setting the dew point to -50°C and the atmosphere to humidified conditions further promoted the mechanochemical reaction.

[0123] Thus, the sulfide-based solid electrolyte according to this embodiment can achieve high lithium ion conductivity even as a press-molded body at low pressure. This is because the 4d site surrounded by lithium is S 2- We believe this is because, compared to other materials, the proportion of low-valence Cl or Cl-containing halogen elements is higher, reducing the interaction with lithium ions and making them more mobile. Furthermore, considering this in light of the results of first-principles calculations, as mentioned earlier, which show that a lower elastic modulus is associated with a relatively smaller amount of Cl at the 4a site, a higher proportion of Cl or Cl-containing halogen elements at the 4d site results in a lower elastic modulus. As a result, even when molded at a lower pressure, the interfacial contact is good and the grain boundary resistance is small, which is thought to enable high lithium ion conductivity even in pressurized bodies at low pressure.

[0124] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. This application is based on Japanese Patent Application No. 2021-162226, filed on 30 September 2021, the contents of which are incorporated herein by reference.

Claims

1. A sulfide-based solid electrolyte used in lithium-ion secondary batteries, It contains an argyrodite-type crystalline phase containing Li, P, S, and Ha, The Ha is one or more halogen elements, including at least Cl. The ratio of the content expressed as [Ha] / [P] (atomic ratio) using the content of P [P] and the total content of Ha [Ha] is 1.3 or more. Measured under the following conditions ３５ In the Cl-NMR spectrum, Area intensity S of the peak observed between 0 and 30 ppm Ａ And the area intensity S of the peak observed between -150 and 0 ppm. Ｂ Using S Ｂ / S Ａ The ratio of area intensity represented by is 3.5 or greater, or A sulfide-based solid electrolyte in which a peak is observed between -150 and 0 ppm, and no peak is observed between 0 and 30 ppm. (conditions) The measurement was performed using LiCl as an external standard, and the results were obtained. ３５ The chemical shift position with the highest intensity in the Cl-NMR spectrum was set to 9.93 ppm. The following conditions were used: probe: 3.2 mm solid-state, measurement conditions: Single pulse method, pulse width: 2.45 μs, observation center: 20 ppm, observation width: 1000 ppm, relaxation delay: 10 sec, number of integrations: 1024, rotation speed: 15 kHz. ３５ Cl-NMR measurement was performed. ３５ Obtain a Cl-NMR spectrum.

2. The sulfide-based solid electrolyte according to claim 1, wherein the Ha is two or more halogen elements including Cl.

3. The sulfide-based solid electrolyte according to claim 1 or 2, wherein the Ha is two or more halogen elements including Cl and Br.

4. The sulfide-based solid electrolyte according to claim 3, wherein the ratio of the content expressed as [Cl] / [Br] (atomic ratio) using the content of Cl [Cl] and the content of Br [Br] is 0.2 to 3.

0.

5. The aldite-type crystal phase is Li ａ PS ｂ Ha ｃ When represented by, the elemental ratios represented by a, b, and c satisfy the relationships of 5 ≤ a ≤ 7, 4 ≤ b ≤ 6, and 1.3 ≤ c ≤ 2. The sulfide-based solid electrolyte according to any one of claims 1 to 4.

6. The argyrodite-type crystal phase contains anions in its crystal structure, The anion includes an oxide anion having an M-O bond formed by the bonding of M and O. The sulfide-based solid electrolyte according to any one of claims 1 to 5, wherein M is at least one element selected from the group consisting of metallic and metalloid elements of groups 2 to 14 of the periodic table.

7. The sulfide-based solid electrolyte according to claim 6, wherein the total content of the elements Li, P, S, Ha, M, and O is 90% by mass or more.

8. A method for producing a sulfide-based solid electrolyte used in lithium-ion secondary batteries, A raw material mixture is obtained by mixing raw materials containing Li, P, S, and Ha. The raw material mixture is heated to obtain a molten product as a homogeneous intermediate compound, and The process includes cooling the molten material to precipitate argyrodite-type crystals, The Ha is one or more halogen elements, including at least Cl. A method for producing a sulfide-based solid electrolyte containing an argyrodite-type crystalline phase.

9. The method for producing a sulfide-based solid electrolyte according to claim 8, wherein the heating is performed under conditions of 600 to 800°C and for 5 minutes or more in an inert atmosphere.

10. A method for producing a sulfide-based solid electrolyte used in lithium-ion secondary batteries, The process involves mixing raw materials containing Li, P, S, and Ha to obtain an amorphous intermediate compound, and This includes heating and calcining the intermediate compound to precipitate argyrodite-type crystals. The Ha is one or more halogen elements, including at least Cl. A method for producing a sulfide-based solid electrolyte containing an argyrodite-type crystalline phase.

11. The method for producing a sulfide-based solid electrolyte according to claim 10, wherein the intermediate compound does not show any peaks originating from the raw materials in the Raman spectrum.

12. A method for producing a sulfide-based solid electrolyte according to claim 10 or 11, wherein the mixing is carried out using a mechanical milling method with a ball mill, in an environment with a dew point of -60°C or higher, at a rotation speed of 400 rpm or higher for 12 hours or more.

13. A method for producing a sulfide-based solid electrolyte according to any one of claims 10 to 12, wherein the particle size of the intermediate compound is 0.1 to 2 μm.

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