Solid electrolyte, electrode mixture containing the same, solid electrolyte layer, and solid-state battery, and evaluation method of solid electrolyte

Abstract

The objective of this invention is to provide a solid electrolyte for solid-state batteries that allows for safe use even when operating in harsh environments. One solid electrolyte is provided in which the integrated intensity of the diffraction peak observed at a position of 2θ = 15.4° ± 1° is set as I in an X-ray diffraction pattern measured using a CuKα ray X-ray diffraction apparatus. A Let the integrated intensity of the diffraction peak observed at the position 2θ = 17.8° ± 1° be I. B Let the integrated intensity of the diffraction peak observed at 2θ = 25.3° ± 1° be I. C Let the integrated intensity of the diffraction peak observed at 2θ = 29.7° ± 1° be I. D At that time, I A / I B The value is 1.0 or higher, and I C / I D The value is less than 0.90.

Description

Technical Field

[0001] This invention relates to solid electrolytes. Furthermore, this invention relates to electrode mixtures comprising solid electrolytes, solid electrolyte layers, and solid-state batteries. Moreover, this invention relates to methods for evaluating solid electrolytes. Background Technology

[0002] In recent years, secondary batteries have garnered attention as a countermeasure to prevent global warming by reducing carbon dioxide emissions. Among these, solid-state batteries using sulfide solid electrolytes have attracted particular interest. Solid-state batteries using sulfide solid electrolytes offer advantages such as simplified safety devices and superior manufacturing costs and productivity because they do not use flammable organic solvents. Furthermore, since ions other than lithium ions do not move within the electrolyte, side reactions caused by anion movement do not occur, which is beneficial from the perspective of improving safety and durability.

[0003] With the aim of improving the performance of solid-state batteries, the applicant previously proposed a sulfide solid electrolyte having a cubic crystal system with a sulfide-germanium sulfide-type crystal structure, and composed of Li 7-x PS 6-x Cl y Br z (Refer to Patent Document 1). According to this sulfide solid electrolyte, it is possible to achieve low elasticity while maintaining high ionic conductivity. Therefore, by using this sulfide solid electrolyte as a material for solid-state batteries, it has the advantage of enabling the solid-state battery to achieve low resistance.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: US2020 / 127325A1 Summary of the Invention

[0007] The problem the invention aims to solve

[0008] The inventors conducted in-depth research to further improve the performance of solid-state batteries and discovered the following safety issues: During the use of solid-state batteries, unexpected changes in the environment, such as changes in ambient temperature, can lead to adverse conditions such as thermal runaway of the solid-state battery.

[0009] That is, the objective of this invention is to provide a solid electrolyte that can be used safely even when the operating environment of a solid-state battery changes.

[0010] Solution for solving the problem

[0011] This invention provides a solid electrolyte, which, in an X-ray diffraction pattern measured using an X-ray diffraction apparatus employing CuKα rays,

[0012] Let the integrated intensity of the diffraction peak observed at 2θ = 15.4° ± 1° be denoted as I. A ,

[0013] Let the integrated intensity of the diffraction peak observed at 2θ = 17.8° ± 1° be I. B ,

[0014] Let the integrated intensity of the diffraction peak observed at 2θ = 25.3° ± 1° be I. C ,

[0015] Let the integrated intensity of the diffraction peak observed at 2θ = 29.7° ± 1° be I. D hour,

[0016] I A / I B The value is 1.0 or higher, and I C / I D The value is less than 0.90.

[0017] Furthermore, this invention provides a solid electrolyte in which the maximum peak intensity of 2471 eV ± 1 eV in the K absorption limit spectrum of sulfur obtained by the all-electron yield method of X-ray absorption fine structure method is set as I. X The maximum peak intensity of 2475eV±2eV is set as I. Y At that time, I X / I Y The value is below 0.89.

[0018] Furthermore, the present invention provides a method for evaluating solid electrolytes, wherein a transition metal oxide is mixed with a solid electrolyte, and differential thermal analysis is performed to evaluate the antioxidant properties of the solid electrolyte based on the temperature at which an exothermic peak is generated. Attached Figure Description

[0019] Figure 1 The X-ray diffraction patterns (2θ = 10~40°) of the solid electrolytes obtained in the examples and comparative examples are shown.

[0020] Figure 2 The K absorption limit spectrum of sulfur element was obtained by measuring the solid electrolytes obtained in the examples and comparative examples using the all-electron yield method of X-ray absorption fine structure method. Detailed Implementation

[0021] The present invention will now be described based on its preferred embodiments.

[0022] This invention relates to solid electrolytes. The solid electrolyte of this invention preferably has lithium-ion conductivity. As long as it has lithium-ion conductivity, the type and composition of the solid electrolyte are not particularly limited, and a suitable solid electrolyte can be selected according to the application, desired physical properties, etc. A sulfide solid electrolyte is particularly preferred.

[0023] The sulfide solid electrolyte preferably contains lithium (Li), phosphorus (P), sulfur (S), and a halogen (X). Examples of halogen (X) include fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). The halogen (X) can be one of these elements, or a combination of two or more. A combination of chlorine (Cl) and bromine (Br) is particularly preferred.

[0024] Examples of solid electrolytes include Li₂S-P₂S₅-LiX (where "X" represents one or more halogen elements), Li a PS b X c (Solid electrolytes with a crystal phase having a sulfide-germanium ore-type crystal structure, where "X" represents one or more halogen elements) etc.

[0025] The solid electrolyte of the present invention is preferably a crystalline substance. Preferably, the solid electrolyte of the present invention exhibits diffraction peaks at specific angles in the diffraction pattern obtained by X-ray diffraction. According to the solid electrolyte of the present invention exhibiting diffraction peaks at specific angles, even if the operating environment of the solid-state battery containing the solid electrolyte unexpectedly becomes harsh, such as at high temperatures, it is less likely to react with the active material, thus exhibiting the advantage of safe operation.

[0026] In detail, in the X-ray diffraction pattern measured by an X-ray diffraction apparatus, the solid electrolyte of the present invention preferably exhibits diffraction peak A at a position of 2θ = 15.4° ± 1°, diffraction peak B at a position of 2θ = 17.8° ± 1°, diffraction peak C at a position of 2θ = 25.3° ± 1°, and diffraction peak D at a position of 2θ = 29.7° ± 1°. Furthermore, the range of each diffraction peak can be ±0.7°, ±0.5°, or ±0.3°.

[0027] When obtaining the aforementioned X-ray diffraction pattern, Cu-Kα was used as the X-ray source. Hereafter, in all cases where X-ray diffraction patterns are mentioned in this specification, Cu-Kα will be used as the X-ray source.

[0028] In addition to observing diffraction peaks at the aforementioned angles, the solid electrolyte of the present invention preferably exhibits a specific relationship between the intensity ratios of two particular diffraction peaks. Therefore, even if the operating environment of the solid-state battery containing the solid electrolyte of the present invention unexpectedly becomes harsh, such as at high temperatures, it is less likely to react with the active material, further enhancing its safe operation. In particular, it can suppress the reaction between the solid electrolyte of the present invention and the active material. The main reason for this is not yet clear, but the inventors have considered the following. It is known that argillium sulfide-germanium ore type solid electrolytes (space group: F-43m) have anion sites at the Wyckoff positions 4a and 4d. Here, the inventors' research results show that when the integrated intensity of diffraction peak A is set to I... A Let the integrated intensity of diffraction peak B be I. B Let the integral intensity of diffraction peak C be I. C Let the integral intensity of diffraction peak D be I. D At that time, I A / I B and I C / I D It is closely related to the electron density at the 4d site. Specifically, the amount of light elements present at the 4d site increases, thus leading to I A / I B Increase, I C / I D The tendency to decrease. It can be considered that: since the 4d site is surrounded by Li sites (24g, 48h) that act as cations, increasing the proportion of ions with low electron density at the 4d site can reduce the likelihood of Li... + The electrostatic attraction increases, stabilizing the structure. In this invention, the aforementioned problem is solved by increasing the proportion of light elements in the constituent elements and controlling appropriate manufacturing conditions to increase the proportion of light elements at the 4d sites.

[0029] Based on the above viewpoints, if the integral intensity of diffraction peak A is set as I... A Let the integrated intensity of diffraction peak B be I. B At that time, I A / I B The value is preferably 1.0 or higher, more preferably 1.2 or higher, and even more preferably 1.3 or higher.

[0030] Furthermore, from the perspective of maintaining ionic conductivity, I A / I B The value is preferably 3 or less, more preferably 2.5 or less, even more preferably 2.0 or less, and even more preferably 1.5 or less.

[0031] Furthermore, in the solid electrolyte of the present invention, the integrated intensity of diffraction peak C is set to I. C Let the integral intensity of diffraction peak D be I. D At that time, I C / I D The value is preferably less than 0.90, and more preferably less than 0.89.

[0032] Furthermore, from the perspective of maintaining ionic conductivity, I C / I D The value is preferably 0.6 or higher, more preferably 0.7 or higher, even more preferably 0.75 or higher, even more preferably 0.8 or higher, and particularly preferably 0.85 or higher.

[0033] As described above, the solid electrolyte of the present invention is preferably a crystalline substance, wherein, from the viewpoint of excellent lithium-ion conductivity, the solid electrolyte preferably has a sulforaphite-germanium type crystal structure.

[0034] In the case where the solid electrolyte of the present invention has a crystal phase with a sulforaphite-germanium-type crystal structure, from the viewpoint of improving lithium-ion conductivity, the solid electrolyte is preferably composed of the formula (I): Li a PS b X c (X represents at least one of the elements fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).)

[0035] In the case where the solid electrolyte of the present invention has a crystal phase with a steric sulfide-germanium-type crystal structure, the diffraction peak A corresponds to the 111 plane in that crystal structure. Diffraction peak B corresponds to the 020 plane in the steric sulfide-germanium-type crystal structure. Diffraction peak C corresponds to the 220 plane in the steric sulfide-germanium-type crystal structure. Diffraction peak D corresponds to the 311 plane in the steric sulfide-germanium-type crystal structure.

[0036] The inventors' research results show that, for the solid electrolyte of the present invention, in order to achieve the above-mentioned strength ratio I A / I B and I C / I D It is advantageous to adjust the molar ratio of element X to element P (hereinafter also referred to as "X / P") in the aforementioned composition formula (I) to a relatively low value. Specifically, if the molar ratio X / P is less than 1.2, the aforementioned strength ratio I can be easily achieved. A / I B and I C / I D Therefore, it is preferred. From this point of view, the molar ratio X / P is preferably less than 1.0, and more preferably less than 0.9.

[0037] In addition, the molar ratio X / P can be 0.1 or higher, or 0.3 or higher, or 0.6 or higher.

[0038] By keeping the molar ratio X / P within this range, X, as a monovalent anion, decreases, while S, as a divalent anion, decreases. 2- The proportion of the solid electrolyte in the overall composition increases. As a result, the electrostatic energy between the solid electrolyte and lithium ions increases, and the lattice energy of the solid electrolyte is improved. Therefore, it is preferred in terms of suppressing the reactivity between the solid electrolyte and the active material.

[0039] The inventors' research results show that, for the solid electrolyte of the present invention, in order to achieve the above-mentioned strength ratio I A / I B and I C / I D Adjusting the molar ratio of S to P in the aforementioned composition (I) (hereinafter also referred to as "S / P") is also advantageous. Specifically, if the molar ratio S / P is greater than 4.8, the aforementioned strength ratio I can be easily achieved. A / I B and I C / I D Therefore, it is preferred. From this point of view, the molar ratio S / P is preferably 5.0 or higher, and more preferably 5.1 or higher.

[0040] The molar ratio S / P can be below 6.8, or below 6.0, below 5.6, or below 5.3.

[0041] By keeping the molar ratio S / P within this range, high ionic conductivity is achieved, and the electrostatic energy between the solid electrolyte and lithium ions is increased, thereby enhancing the lattice energy of the solid electrolyte. Therefore, it is preferred in terms of suppressing the reactivity between the solid electrolyte and the active material.

[0042] Furthermore, in the solid electrolyte of the present invention, the molar ratio of Li to P (hereinafter also referred to as "Li / P") is preferably 4.0 or more, more preferably 5.0 or more, and particularly preferably 6.0 or more. On the other hand, the molar ratio Li / P is preferably 7.0 or less, more preferably 6.8 or less, and particularly preferably 6.4 or less.

[0043] By setting the Li / P molar ratio to this range, the sulfogermanite-type crystal structure near room temperature (25°C), especially the cubic sulfogermanite-type crystal structure, becomes more stable, thereby enabling the full introduction of lithium-ion holes into the structure. As a result, lithium-ion conductivity can be effectively improved.

[0044] The sulfide-germanium type crystal phase has the composition shown in formula (II) below:

[0045] Li7-d PS 6-d X d …(II)

[0046] The composition shown in formula (II) is the stoichiometric composition of the silver-germanium sulfide crystal phase.

[0047] In the sulfide solid electrolyte of the present invention, from the viewpoint of having high ionic conductivity and taking into account the low reactivity of the solid electrolyte and the active material, d is preferably 0.1 or more and less than 1.2, more preferably 0.3 or more and less than 1.0, and even more preferably 0.6 or more and less than 0.9.

[0048] In formula (II), a portion of P can be replaced by one or more elements selected from silicon (Si), germanium (Ge), tin (Sn), lead (Pb), boron (B), aluminum (Al), gallium (Ga), arsenic (As), antimony (Sb), and bismuth (Bi).

[0049] The ratio of elements contained in a solid electrolyte can be determined, for example, using high-frequency inductively coupled plasma (ICP) emission spectroscopy, energy dispersive X-ray spectroscopy (EDS), or fluorescence X-ray analysis (XRF).

[0050] Solid electrolytes typically suffer from reduced crystallinity due to smaller particle size; however, the solid electrolyte of the present invention preferably has a small particle size and high crystallinity. High crystallinity means fewer reactive sites in the solid electrolyte. Therefore, even if the operating environment of the solid-state battery containing the solid electrolyte of the present invention unexpectedly becomes harsh, such as at high temperatures, it is less likely to react with the active material.

[0051] The crystallinity of a solid electrolyte can be evaluated using the half-width at half-maximum (WWHM) of an X-ray diffraction peak. A smaller WWHM indicates higher crystallinity of the solid electrolyte. In the solid electrolyte of this invention, the WWHM of diffraction peak D is preferably 0.16° or less, more preferably 0.15° or less, and even more preferably 0.14° or less. The reason for selecting diffraction peak D in the evaluation of the WWHM is that, in the solid electrolyte of this invention, diffraction peak D is the diffraction peak with the highest intensity, and its WWHM is easily calculated.

[0052] The crystallinity of the solid electrolyte of the present invention can also be evaluated by its crystallite size. Specifically, the crystallite size can be calculated based on the X-ray diffraction pattern measured for the solid electrolyte of the present invention. The crystallite size of the solid electrolyte of the present invention is preferably 450 Å or more, more preferably 550 Å or more, and even more preferably 600 Å or more.

[0053] There is no particular upper limit to the crystallite size; for example, it can be below 2000 Å, below 1000 Å, or below 700 Å.

[0054] The method for determining the crystallite size is described in the examples described later.

[0055] The crystallinity of the solid electrolyte of the present invention can also be evaluated by the ratio of the solid electrolyte particle size to the crystallite size. Specifically, the cumulative particle size at a cumulative volume of 50% obtained by laser diffraction scattering particle size distribution determination is defined as D. 50 When the crystallite size is set to C (Å), (D) 50 ×10 4 The value of ) / C is preferably 2 or more, more preferably 6 or more, and even more preferably 7 or more.

[0056] In addition, (D) 50 ×10 4 The value of ) / C is preferably 15 or less, more preferably 12 or less, even more preferably 10 or less, even more preferably 9 or less, and particularly preferably 8 or less.

[0057] (D) 50 ×10 4 The closer the value of ) / C is to 1, the closer the particles are to a single crystal state, reflecting the degree of crystallinity relative to the particle size.

[0058] (D) 50 ×10 4 The value of ) / C is as described above, but the particle size D 50 The particle size is preferably less than 5 μm, more preferably less than 2 μm, even more preferably less than 1 μm, and still more preferably less than 0.8 μm. By having such a particle size, the contact points and contact area between the solid electrolyte and the active material particles are increased, effectively improving the input-output characteristics of the battery.

[0059] For particle size D 50 There are no particular restrictions on the lower limit value; for example, it can be 0.1 μm or more, or 0.3 μm or more, or 0.5 μm or more. By having such a particle size, excessive increase in the surface area of ​​the solid electrolyte can be suppressed, thereby inhibiting the increase in resistance. Furthermore, it is easily mixed with active materials.

[0060] One of the characteristics of the solid electrolyte of the present invention is the K absorption limit spectrum of sulfur obtained by the all-electron yield method of X-ray absorption fine structure (hereinafter also referred to as "XAFS"). XAFS is a method for analyzing the absorption spectrum obtained by irradiating a substance with X-rays. In the spectrum obtained by irradiating a substance with X-rays, an increase in absorption intensity, known as the absorption limit, is observed at the incident X-ray energy inherent to the element contained in the substance. The structure appearing in the vicinity of this absorption limit by about ±50 eV is called XANES (X-ray Absorption Near Edge Structure), which can be used to evaluate the chemical state (valence, coordination structure) of the element of interest in the sample. However, in the region of incident X-ray energy below 4000 eV, which contains a large number of the main components contained in the solid electrolyte of the present invention, the X-ray transmittance of the sample is extremely low. Therefore, it is difficult to obtain the spectrum by the transmission method of measuring the X-ray intensity before and after the incident on the sample. In this case, the total electron yield (TEY) method, which utilizes the phenomenon of electrons being released from the sample during X-ray irradiation and determines the X-ray absorption intensity by measuring the sample current, is effective. By using the TEY method for XAFS determination, the solid electrolyte of this invention can be used as the analyte, and the chemical state of sulfur can be obtained elementally selectively.

[0061] In the solid electrolyte of this invention, the maximum peak intensity observed at 2471 eV ± 1 eV in the K absorption limit spectrum of sulfur is set as I. X The maximum peak intensity observed at 2475eV±2eV is set as I. Y At that time, I X / I Y The XAFS intensity ratio (hereinafter also referred to as "XAFS intensity ratio") is preferably 0.89 or less. By giving the solid electrolyte such an XAFS intensity ratio, the beneficial effect of being less likely to cause reaction with the active material and being able to be used safely is further demonstrated when the operating environment of the solid-state battery containing the solid electrolyte unexpectedly becomes harsh. The reason for this is not yet clear, but the inventors assume the following. The peak observed at 2471 eV ± 1 eV originates from PS4 in the solid electrolyte. 3- The peak of the unit. It can be assumed that: if the S in the solid electrolyte is not bonded to P... 2- If the proportion of S in the total element increases, then I X / I Y Reduced. Therefore, with Li +The increased electrostatic attraction stabilizes the structure, making it less prone to reaction with active materials. In particular, the present invention achieves both reduced reactivity of the solid electrolyte with active materials and increased ionic conductivity of the solid electrolyte. From the viewpoint of making this advantage even more significant, the XAFS strength is higher than that of I... X / I Y Further preferred is below 0.88.

[0062] Furthermore, considering both high ionic conductivity and low reactivity with active substances, XAFS intensity is higher than I... X / I Y It can be 0.70 or higher, or 0.80 or higher, or 0.83 or higher.

[0063] The method for determining the K absorption limit spectrum of sulfur element based on the all-electron yield method using XAFS is described in the examples below.

[0064] The solid electrolyte of the present invention exhibits lithium-ion conductivity in a solid state. Preferably, the solid electrolyte has a lithium-ion conductivity of 0.5 mS / cm or higher at room temperature, i.e., 25°C, and more preferably, a lithium-ion conductivity of 1.0 mS / cm or higher. The lithium-ion conductivity can be measured using the methods described in the examples below.

[0065] Next, a preferred method for manufacturing the solid electrolyte of the present invention will be described. The solid electrolyte can be suitably manufactured by subjecting a raw material composition to a solid-phase reaction involving heating and sintering. The aforementioned raw material composition is a mixture of raw material powders comprising the aforementioned elements constituting the solid electrolyte. The raw material composition comprises one or more compounds containing at least one element selected from Li, P, S, and X.

[0066] The aforementioned raw material powder may be, for example, a compound containing Li, a compound containing S, a compound containing P, or a compound containing X.

[0067] In the aforementioned raw material powder, a compound may contain at least two or more elements selected from Li, P, S, and X. For example, as the aforementioned raw material powder, compounds containing Li and X, compounds containing P and S, compounds containing Li and S, compounds containing P and X, and compounds containing S and X may be used.

[0068] Lithium halides can be used as compounds containing both Li and X elements.

[0069] Phosphorus sulfides, such as phosphorus trisulfide (P2S3) and phosphorus pentasulfide (P2S5), can be used as compounds containing P and S elements.

[0070] Lithium sulfide (Li2S) can be used as a compound containing both Li and S elements.

[0071] Phosphorus halides such as PX3 and P2X5 can be used as compounds containing P and X elements.

[0072] As compounds containing both sulfur (S) and x (X) elements, for example, SX2, SX4, SX6, and S2X can be used. 10 Isohalated sulfur.

[0073] The aforementioned raw material powders are preferably fed into a grinding process and adjusted to a specified particle size before being mixed. Media-stirred mills, such as ball mills or bead mills, can be used in the grinding process.

[0074] When using a media stirring mill for pulverization, the slurry of the aforementioned raw material powder is fed into a container, and ceramic or metal balls or beads are added to the container. The container is rotated, causing the raw material powder to collide with the balls or beads inside the container, thus pulverizing the raw material powder. Pulverization tends to reduce the crystallinity of the raw material powder, but in this manufacturing method, pulverization is preferably performed in a manner that minimizes the reduction of the crystallinity of the raw material powder being pulverized. By performing such pulverization, a solid electrolyte with small particle size and high crystallinity can be obtained, resulting in a solid electrolyte that satisfies the aforementioned X-ray intensity ratio.

[0075] After the aforementioned raw material powders are pulverized, the raw material powders are mixed to obtain a raw material composition. From the viewpoint that a solid electrolyte satisfying the above-mentioned strength ratio can be easily obtained, the mixing of each raw material powder is preferably carried out in such a way that the molar ratio X / P in the target solid electrolyte is less than 1.2.

[0076] From the same point of view, the mixing of each raw material powder is preferably carried out in such a way that the molar ratio S / P in the target solid electrolyte is greater than 4.8.

[0077] A media stirring mill is preferably used in the mixing of the various raw material powders. In this case, it is preferable to mix in a manner that does not reduce the crystallinity of each raw material powder as much as possible. By performing such mixing, a solid electrolyte with small particle size and high crystallinity can be obtained, and as a result, a solid electrolyte that meets the above-mentioned strength ratio can be obtained.

[0078] Next, the raw material composition is fed into a firing process to undergo a solid-phase reaction, yielding a crystalline fired product. The firing atmosphere can be an inert gas atmosphere, such as argon or nitrogen, or a hydrogen sulfide atmosphere. From the viewpoint of adjusting the proportion of sulfur in the solid electrolyte, an inert gas atmosphere is preferred.

[0079] From the viewpoint of reliably generating a solid-state reaction of the raw material composition, the firing temperature is preferably, for example, 200°C or higher, more preferably 300°C or higher, even more preferably 350°C or higher, and still more preferably 400°C or higher. On the other hand, considering industrial production feasibility and economic efficiency, the firing temperature is preferably, for example, 700°C or lower, more preferably 600°C or lower, and even more preferably 550°C or lower.

[0080] The firing time is not a critical time; it is simply the time required to obtain a fired product with the desired composition. Specifically, a firing time that allows sufficient solid-state reaction of the raw material composition to occur is preferred. The firing time can be, for example, 30 minutes or more, 2 hours or more, or 3 hours or more. On the other hand, the firing time can be, for example, 10 hours or less, or 5 hours or less.

[0081] The solid electrolyte obtained in this way can be used alone or in combination with other solid electrolytes. For example, the solid electrolyte of the present invention can be used as a material constituting a lithium battery, such as a material constituting a solid electrolyte layer, a positive electrode layer, or a negative electrode layer.

[0082] Specifically, the solid electrolyte of the present invention can be used in batteries having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer located between the positive and negative electrode layers. That is, the solid electrolyte can be used in so-called solid-state batteries. More specifically, it can be used in lithium solid-state batteries.

[0083] Lithium solid-state batteries can be primary or secondary batteries. There are no particular restrictions on the shape of the battery; for example, they can be laminated, cylindrical, or prismatic. The term "solid-state battery" includes not only those batteries that completely do not contain liquid or gel-like substances as electrolytes, but also those containing, for example, less than 50% by mass, less than 30% by mass, or less than 10% by mass of liquid or gel-like substances as electrolytes.

[0084] When the solid electrolyte layer contains the solid electrolyte of the present invention, the solid electrolyte layer can be manufactured, for example, by methods such as: dripping a slurry containing a solid electrolyte, a binder, and a solvent onto a substrate and then grinding it with a scraper or the like; cutting it with an air knife after the substrate has come into contact with the slurry; or forming a coating film by screen printing or the like, and then removing the solvent by heating and drying. Alternatively, it can be manufactured by pressing the powdered solid electrolyte into a pressed powder and then processing it appropriately.

[0085] To balance the prevention of short circuits and volumetric capacity density, the thickness of the solid electrolyte layer is typically 5 μm or more and 300 μm or less, and more preferably 10 μm or more and 100 μm or less.

[0086] The solid electrolyte of the present invention can also be used with active materials to form an electrode mixture. Typically, the proportion of solid electrolyte in the electrode mixture is 10% by mass or more and 50% by mass or less. The electrode mixture may also contain other materials such as conductive materials as needed.

[0087] Electrode mixtures, binders, and solvents are mixed to form a paste, which is then applied to current collectors such as aluminum foil and dried to produce positive and negative electrodes.

[0088] As the cathode material constituting the cathode layer, cathode materials used as cathode active materials in lithium-ion batteries can be appropriately used. Examples include lithium-containing cathode active materials, specifically spinel-type lithium transition metal oxides and lithium metal oxides with layered structures. By using high-voltage cathode materials, energy density can be increased. In addition to the cathode active material, the cathode material may also contain conductive materials, or other materials.

[0089] As the negative electrode material constituting the negative electrode layer, a negative electrode material used as the negative electrode active material in lithium-ion batteries can be appropriately used. The solid electrolyte of this invention is electrochemically stable, therefore lithium metal can be used, or at a low potential comparable to lithium metal (approximately 0.1V vs. Li). + Materials that are charged and discharged under Li (Li) conditions, namely carbon-based materials such as graphite, artificial graphite, natural graphite, and non-graphitizable carbon (hard carbon), can be used as anode materials. This significantly improves the energy density of solid-state batteries. Alternatively, silicon or tin, which are expected to be high-capacity materials, can be used as active materials. Regarding anode materials, in addition to the active material, conductive materials or other materials may also be included.

[0090] According to the present invention, a method for evaluating solid electrolytes is also provided. This method involves evaluating the antioxidant properties of solid electrolytes. In the use of solid-state batteries, unexpected environmental changes, such as increases in ambient temperature, can sometimes cause the solid electrolyte to react with oxygen contained in the active material, leading to adverse conditions in the solid-state battery. Therefore, it is desirable to evaluate the antioxidant properties of the solid electrolyte before assembling it into a solid-state battery. The evaluation method of the present invention is extremely useful from this perspective.

[0091] In this evaluation method, transition metal oxides are mixed with a solid electrolyte, and differential thermal analysis is performed. The oxidation resistance of the solid electrolyte is evaluated based on the temperature at which an exothermic peak is generated. The transition metal oxides simulate the charged state of a cathode where lithium ions are intercalated or deintercalated from spinel-type lithium manganese oxide or rock salt layered cathode active materials. From this perspective, the transition metal in the transition metal oxide is preferably an element used as a cathode active material in lithium solid-state batteries. Examples of such transition metals include manganese (Mn) and nickel (Ni).

[0092] Examples of oxides containing the aforementioned transition metals include MnO2, Mn3O4, Mn2O3, MnO, and Mn2O7, which are Mn oxides. Additionally, NiO2 and NiO, which are Ni oxides, are examples. These transition metal oxides can be used individually or in combination of two or more.

[0093] Differential thermal analysis (DTA) is performed on a mixture of solid electrolyte and transition metal oxide. Regarding the mass ratio of the solid electrolyte to the transition metal oxide in this mixture, from the viewpoint of improving evaluation accuracy, the mass of the solid electrolyte relative to the total mass of the solid electrolyte and transition metal oxide is preferably set to, for example, 1% by mass or more and 90% by mass or less, more preferably 5% by mass or more and 50% by mass or less. Furthermore, the mass of the transition metal oxide relative to the total mass of the solid electrolyte and transition metal oxide is preferably set to 10% by mass or more and 99% by mass or less, more preferably 50% by mass or more and 95% by mass or less.

[0094] It should be noted that the mixture of solid electrolyte and transition metal oxide may contain only these two substances, or it may contain substances commonly blended in lithium solid-state batteries as a third component. However, from the viewpoint of further improving evaluation accuracy, the aforementioned mixture preferably contains only solid electrolyte and transition metal oxide.

[0095] From the perspective of improving measurement accuracy, differential thermal analysis is preferably performed under an inert gas atmosphere. Examples of inert gases include nitrogen, as well as rare gases such as argon and helium.

[0096] If the temperature range for differential thermal analysis is above 25°C and below 500°C, it is sufficient to evaluate the antioxidant properties of solid electrolytes. However, it is acceptable to set the upper limit of the measurement temperature above or below 500°C.

[0097] From the perspective of improving measurement accuracy, the heating rate of differential thermal analysis is preferably set to more than 1℃ / min and less than 50℃ / min.

[0098] Differential thermal analysis was performed on a mixture containing a solid electrolyte and transition metal oxides, and an exothermic peak was observed at a specified temperature. This exothermic peak was observed due to the oxidation of the solid electrolyte. The higher the temperature at which the exothermic peak was observed, the higher the antioxidant capacity of the solid electrolyte can be evaluated.

[0099] In this evaluation method, mass spectrometry analysis can be performed simultaneously with differential thermal analysis. Mass spectrometry analysis allows for the evaluation of the types and quantities of gases produced by the solid electrolyte during the heating process.

[0100] Examples of gases produced by solid electrolytes include sulfur oxides, sulfur, and hydrogen sulfide. In this evaluation method, it is useful to evaluate the types of gases produced, considering their harmfulness to humans and ignition temperature, as well as the reaction between the solid electrolyte and the active material. On the other hand, it is useful to evaluate the quantity of gases produced, based on the explosive limits of the gases produced during the reaction and the rate of increase in internal pressure within the battery cell.

[0101] To perform differential thermal analysis and mass spectrometry analysis simultaneously, a device such as a TG-DTA-MS can be used.

[0102] Regarding the above embodiments, the present invention further discloses the following evaluation methods for solid electrolytes, electrode mixtures, electrode layers, solid-state batteries, and solid electrolytes.

[0103] [1] A solid electrolyte, wherein, in an X-ray diffraction pattern measured using an X-ray diffraction apparatus employing CuKα rays,

[0104] Let the integrated intensity of the diffraction peak observed at 2θ = 15.4° ± 1° be denoted as I. A ,

[0105] Let the integrated intensity of the diffraction peak observed at 2θ = 17.8° ± 1° be I. B ,

[0106] Let the integrated intensity of the diffraction peak observed at 2θ = 25.3° ± 1° be I. C ,

[0107] Let the integrated intensity of the diffraction peak observed at 2θ = 29.7° ± 1° be I. D hour,

[0108] I A / I B The value is 1.0 or higher, and I C / I DThe value is less than 0.90.

[0109] [2] A solid electrolyte, wherein, in the K absorption limit spectrum of sulfur obtained by the total electron yield method of X-ray absorption fine structure method, the maximum peak intensity of 2471 eV ± 1 eV is set as I. X The maximum peak intensity of 2475eV±2eV is set as I. Y At that time, I X / I Y The value is below 0.89.

[0110] [3] The solid electrolyte according to [1] or [2] contains lithium (Li), phosphorus (P), sulfur (S) and halogen (X) elements.

[0111] The molar ratio of halogen (X) to phosphorus (P) is less than 1.2.

[0112] [4] According to the solid electrolyte described in [3], the molar ratio of sulfur (S) to phosphorus (P) is greater than 4.8.

[0113] [5] The solid electrolyte according to any one of [1] to [4], wherein the cumulative particle size D when the cumulative volume obtained by laser diffraction scattering particle size distribution determination method is 50% by volume. 50 Less than 5μm

[0114] In the aforementioned X-ray diffraction pattern, the half-width of the diffraction peak observed at the position 2θ = 29.7° ± 1° is less than 0.16°.

[0115] [6] According to the solid electrolyte described in [5], wherein the aforementioned particle size D 50 (μm) of 10 4 The ratio to the crystallite size (Å) determined based on the aforementioned X-ray diffraction pattern is greater than 1 and less than 15.

[0116] [7] The solid electrolyte according to any one of [1] to [6] comprises a crystal phase having a sulfogermanium-type crystal structure.

[0117] [8] An electrode mixture comprising any one of [1] to [7] a solid electrolyte and an active substance.

[0118] [9] A solid electrolyte layer containing any one of [1] to [7].

[0119]

[10] A solid-state battery having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer located between the aforementioned positive electrode layer and the aforementioned negative electrode layer, wherein the solid-state battery contains any one of [1] to [7].

[0120]

[11] A method for evaluating solid electrolytes, wherein a transition metal oxide is mixed with a solid electrolyte and differential thermal analysis is performed to evaluate the antioxidant properties of the solid electrolyte based on the temperature at which an exothermic peak is generated.

[0121]

[12] According to the evaluation method described in

[11] , the aforementioned transition metal oxide is MnO2, Mn3O4, Mn2O3, MnO, Mn2O7, NiO2 or NiO.

[0122]

[13] According to the evaluation method described in

[11] or

[12] , mass spectrometry is performed simultaneously with the aforementioned differential thermal analysis to evaluate the type and amount of gas generated by the aforementioned solid electrolyte.

[0123] Example

[0124] The present invention will now be described in more detail through embodiments. However, the scope of the present invention is not limited to these embodiments.

[0125] [Example 1]

[0126] Lithium sulfide (Li₂S) powder, phosphorus pentasulfide (P₂S₅) powder, lithium chloride (LiCl) powder, and lithium bromide (LiBr) powder were used as raw material powders. Toluene was added to each raw material powder to prepare a slurry. Each slurry was placed in a polyamide container and placed in a Fritsch planetary ball mill P-5. Zirconia balls with a diameter of 5 mm were used as the grinding medium. The ball mill was operated at 100 rpm for 10 hours for wet grinding. The ground slurry was then vacuum dried at 150°C to remove toluene. This yielded the ground raw material powders. Each raw material powder was weighed and mixed according to the composition shown in Table 1 below, and toluene was added to prepare a slurry. Each slurry was placed in a polyamide container and placed in a planetary ball mill. Zirconia balls with a diameter of 5 mm were used as the grinding medium. The ball mill was operated at 100 rpm for 10 hours for wet mixing. The mixed slurry was vacuum dried at 80°C to remove toluene. This yielded the raw material composition.

[0127] The raw material composition is calcined to obtain a calcined product. Calcination is carried out using a tubular electric furnace. During calcination, 100% pure nitrogen gas is circulated within the furnace. The temperature is increased to 500°C at a rate of 200°C / h, maintained at 500°C for 4 hours, and then allowed to cool naturally to room temperature. This yields the calcined product.

[0128] The calcined material was pulverized in a mortar and sieved through a 250 μm sieve to obtain a powder. This powder was then further pulverized using a planetary ball mill. Zirconia balls with a diameter of 5 mm were used as the pulverizing medium. The container was made of polyamide. Toluene was used as the solvent. The ball mill was operated at 100 rpm for 3 hours. The resulting slurry was then vacuum-dried at 80°C to remove the solvent. This yielded the pulverized powder.

[0129] The obtained powder was further pulverized using a planetary ball mill. Zirconia balls with a diameter of 0.8 mm were used as the pulverizing medium. The container was made of polyamide. Toluene with added dispersant was used as the solvent. The ball mill was operated at 100 rpm until the target particle size was achieved. The resulting slurry was vacuum dried at 80°C to remove the solvent. This yielded the target solid electrolyte.

[0130] [Examples 2 and 3]

[0131] Li₂S powder, P₂S₅ powder, LiCl powder, and LiBr powder were weighed separately to obtain the raw material composition, which was prepared in the manner shown in Table 1 below. Otherwise, solid electrolyte powder was obtained in the same manner as in Example 1.

[0132] [Compare Examples 1 and 2]

[0133] Li₂S powder, P₂S₅ powder, LiCl powder, and LiBr powder were weighed separately to obtain the raw material composition, which was prepared in the manner shown in Table 1 below. Otherwise, solid electrolyte powder was obtained in the same manner as in Example 1.

[0134] [evaluate]

[0135] For the solid electrolytes obtained in the examples and comparative examples, X-ray diffraction measurements were performed using the following method to determine the intensity ratio I. A / I B and I C / I D The value of is also determined. Additionally, the half-width of the diffraction peak D is calculated. The crystallite size is further determined. X-ray diffraction patterns of the solid electrolytes for the examples and comparative examples are shown in [the figure]. Figure 1 .

[0136] In addition, the K absorption limit spectrum of sulfur was determined using the XAFS all-electron yield method as follows. The spectrum (excluding Example 3) is shown below. Figure 2 .

[0137] Furthermore, for the solid electrolytes obtained in the examples and comparative examples, the particle size D was measured. 50The conductivity and the reaction initiation temperature with manganese dioxide were determined. The results are shown in Table 1 below.

[0138] X-ray diffraction measurement

[0139] Measurements were performed using the Malvern Panalytical Aeris benchtop X-ray diffractometer, under non-atmospheric exposure conditions. The measurement conditions are described below.

[0140] • Radiation source: CuKα

[0141] • Tube voltage: 40kV

[0142] Tube current: 15mA

[0143] • Measurement method: Concentration method (reflectance method)

[0144] • Detector: One-dimensional semiconductor detector

[0145] • Incident Soler slit: The Soler slit is 0.02 rad.

[0146] • Length-limiting slit: 20mm

[0147] • Light receiving Soler slit: 0.02 rad

[0148] • Entrance slit: 1 / 2°

[0149] • Light receiving slit: Open

[0150] • Measurement range: 2θ = 10~105°

[0151] • Step width: 0.01°

[0152] • Scanning speed: 1.67° / minute

[0153] In addition, the background intensity obtained by subtracting the background intensity from the Kapton film of the non-exposed retainer was used to analyze the measurement results.

[0154] [Integral intensity, half-width, and crystallite size]

[0155] The X-ray diffraction pattern obtained by X-ray diffraction measurement is read into Smart Lab Studio II, and peak processing is performed to calculate the result.

[0156] The peak analysis settings are as follows: Peak shape: segmented pseudo-Voight function, Background type: B-spline, Fitting conditions: Automatic.

[0157] The integrated intensity (Count°) and half-width (FWHM) (°) of the corresponding peaks are read from the obtained peak list. The crystallite size (Å) is recorded as the value calculated based on the peak with 2θ = 29.7° ± 1°.

[0158] [K absorption limit spectrum of sulfur]

[0159] The K absorption limit spectrum of sulfur was determined by the all-electron yield method of XAFS according to the following method.

[0160] • Experimental facilities: Ritsumeikan University SR Center

[0161] • Experimental beamline: BL-10

[0162] • Spectrometer: Ge(111) dicrystalline spectrometer

[0163] • Incident X-ray dimensions: 2mm (length) × 5mm (width)

[0164] • Measurement method: Total Electronic Yield (TEY) method

[0165] • Measurement energy range: 2400~2850 eV

[0166] In TEY-based measurements, a carbon ribbon is attached to a SUS-made sample holder, and the test sample is coated onto it. The test sample is transported using a transfer container and introduced into the beamline under non-atmospheric exposure. At various incident X-ray energies (E, x-axis), the incident X-ray intensity (I0) and sample current (I0) are measured. e The absorption intensity of X-rays (y-axis) is calculated according to the following formula, and plotted on the x-axis-y-axis to obtain the XAFS spectrum.

[0167] X-ray absorption intensity μt=I e / I0

[0168] In the background processing and standardization of the data obtained above, "Athena" (Demeterver. 0.9.26) was used as the analysis software. After reading the XAFS spectrum using this software, the pre-edge region (approximately -70 eV to -30 eV based on the absorption limit) and the post-edge region (approximately -150 eV to -310 eV based on the absorption limit) were fitted and the background absorption was subtracted. The XAFS spectrum was standardized by ensuring that the absorbance before and after the absorption limit was equal to 1. The XAFS intensity ratio I was then calculated from the standardized XAFS spectrum. X / I Y .

[0169] Particle size D 50 ]

[0170] The particle size distribution was determined using laser diffraction scattering particle size distribution measurement. Using an automatic sample feeder (Nikkiso Corporation's "Microtrac SDC"), a solid electrolyte was added to toluene, and the mixture was irradiated with 40W ultrasound at a flow rate of 40% for 360 seconds. The particle size distribution was then measured using a Nikkiso Corporation laser diffraction particle size distribution measuring machine, the "MT3000II". The particle size D was determined from the obtained volume-based particle size distribution map. 50 .

[0171] [Conductivity]

[0172] In a glove box purged with thoroughly dried Ar gas (dew point below -60°C), an application of approximately 6 t / cm was applied to the solid electrolyte. 2 The lithium-ion conductivity of the samples was determined by uniaxial compression molding under a load, producing granules with a diameter of 10 mm and a thickness of approximately 0.5 mm to 8 mm. The lithium-ion conductivity of the samples was measured using a Solartron 1255B impedance measuring device from TOYO Corporation. Measurements were performed by AC impedance method at a temperature of 25°C and a frequency of 0.1 Hz to 1 MHz.

[0173] [Reaction initiation temperature when the reaction with manganese dioxide begins]

[0174] Using spinel-type lithium manganese oxide (LiMn2O4), known as a positive electrode active material for lithium batteries, and manganese dioxide (MnO2), a model compound used as a rock salt layered positive electrode active material, the reactivity of manganese dioxide with the solid electrolytes obtained in the examples and comparative examples was evaluated using the following method. Manganese dioxide simulates the positive electrode in a charged state where lithium ions are intercalated or deintercalated from the spinel-type lithium manganese oxide or rock salt layered positive electrode active material.

[0175] Manganese dioxide (manufactured by High Purity Chemical Co., Ltd.) was mixed with a solid electrolyte at a mass ratio of 70:30 and sieved using a 53 μm sieve. 2 mg of the sieved mixture was packed into an aluminum dish for differential thermal analysis (DTA). The temperature range for DTA was set from 25 °C to 500 °C. DTA was performed using a STA 2500 Regulus (manufactured by NETZSH). The heating rate was set to 10 °C / min. The measurement atmosphere was a nitrogen flow.

[0176] The peak temperature of the exothermic peak observed by differential thermal analysis was determined as the reaction initiation temperature with manganese dioxide. A higher reaction initiation temperature means that the solid electrolyte is less likely to react with manganese dioxide, i.e., less likely to react with the active substance.

[0177] [Table 1]

[0178]

[0179] As clearly shown in Table 1, the solid electrolytes obtained in each embodiment have a higher reaction initiation temperature when reacting with manganese dioxide compared to the solid electrolytes obtained in the comparative examples. Therefore, it can be concluded that the solid-state batteries with the solid electrolytes obtained in each embodiment are less prone to reaction with the active materials even when accidentally exposed to high temperatures while charging, indicating high safety.

[0180] Industrial availability

[0181] As described above, according to the present invention, a solid electrolyte is provided that enables the safe use of solid-state batteries even when the operating environment changes from normal to harsh operating environments.

Claims

1. A solid electrolyte, wherein, In the X-ray diffraction pattern measured using an X-ray diffraction apparatus for CuKα rays Let the integrated intensity of the diffraction peak observed at 2θ = 15.4° ± 1° be denoted as I. A , Let the integrated intensity of the diffraction peak observed at 2θ = 17.8° ± 1° be I. B , Let the integrated intensity of the diffraction peak observed at 2θ = 25.3° ± 1° be I. C , Let the integrated intensity of the diffraction peak observed at 2θ = 29.7° ± 1° be I. D hour, I A / I B The value is 1.0 or higher, and I C / I D The value is less than 0.

90.

2. A solid electrolyte, wherein, In the K absorption limit spectrum of sulfur obtained by the all-electron yield method of X-ray absorption fine structure method, the maximum peak intensity of 2471 eV ± 1 eV is set as I. X The maximum peak intensity of 2475eV±2eV is set as I. Y At that time, I X / I Y The value is below 0.

89.

3. The solid electrolyte according to claim 1 or 2, comprising lithium (Li), phosphorus (P), sulfur (S), and halogen (X) elements. The molar ratio of halogen (X) to phosphorus (P) is less than 1.

2.

4. The solid electrolyte according to claim 3, wherein, The molar ratio of sulfur (S) to phosphorus (P) is greater than 4.

8.

5. The solid electrolyte according to claim 1, wherein, The cumulative particle size D at a cumulative volume of 50% was obtained by laser diffraction scattering particle size distribution determination. 50 Less than 5μm In the X-ray diffraction pattern, the half-width of the diffraction peak observed at the position 2θ = 29.7° ± 1° is less than 0.16°.

6. The solid electrolyte according to claim 5, wherein, The particle size D 50 (μm) of 10 4 The ratio to the crystallite size (Å) determined based on the X-ray diffraction pattern is greater than 1 and less than 15.

7. The solid electrolyte according to claim 1 or 2, comprising a crystal phase having a sulforaphite-germanium type crystal structure.

8. An electrode mixture comprising the solid electrolyte and active substance as described in any one of claims 1 to 7.

9. A solid electrolyte layer comprising the solid electrolyte according to any one of claims 1 to 7.

10. A solid-state battery having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer located between the positive electrode layer and the negative electrode layer, the solid-state battery containing the solid electrolyte according to claim 1 or 2.

11. A method for evaluating solid electrolytes, wherein, Transition metal oxides were mixed with solid electrolytes and subjected to differential thermal analysis. The antioxidant properties of the solid electrolytes were evaluated based on the temperature at which the exothermic peak was generated.

12. The evaluation method according to claim 11, wherein, The transition metal oxide is MnO2, Mn3O4, Mn2O3, MnO, Mn2O7, NiO2, or NiO.

13. The evaluation method according to claim 11 or 12, wherein, Mass spectrometry analysis was performed simultaneously with the differential thermal analysis to evaluate the type and amount of gas generated by the solid electrolyte.

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