Sulfide-based solid electrolyte and method for producing sulfide-based solid electrolyte

Abstract

An object of the present invention is to provide a sulfide-based solid electrolyte with improved ionic conductivity and a method for producing the same. SOLUTION The present invention is a sulfide-based solid electrolyte containing a Group 2 element and having an argyrodite-type crystal structure, wherein the sulfide-based solid electrolyte has the chemical formula Li 7-x-2y M y PS 6-x Ha x wherein in the chemical formula, M is one or more elements selected from Group 2 elements, Ha is one or more elements selected from halogen elements, Ha contains Br, 0<x<2.5 and 0<y<0.45 are satisfied, and the lattice volume of the sulfide-based solid electrolyte is 950Å 3 or more and 980Å 3 or less, is provided as the sulfide-based solid electrolyte.

Description

Technical Field

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

[0002] Development of all-solid-state batteries, in which the electrolyte solution of a lithium ion battery is replaced with a solid electrolyte, is underway for achieving higher safety, longer service life, and higher energy density. Among various solid electrolytes, Li 10 GeP2S 12 and other sulfide-based solid electrolytes have the advantages of high ionic conductivity close to that of electrolyte solutions, as well as softness that facilitates adhesion with active materials, and the practical application of all-solid-state batteries using sulfide-based solid electrolytes is expected.

[0003] Lithium metal has attracted attention as a negative electrode material for all-solid-state batteries because it can increase the gravimetric energy density (Wh / kg) due to its low weight per unit volume and large theoretical capacity. However, Li 10 GeP2S 12 and other sulfide-based solid electrolytes have low stability against lithium metal, and thus have the problem that they are difficult to use together with a lithium metal negative electrode.

[0004] To solve this problem, Patent Documents 1 to 3 disclose a Li that is stable against lithium metal 7-x-2y PS 6-x-y Cl x have reported a sulfide-based solid electrolyte having an argyrodite-type crystal structure represented by the above formula. Patent Document 4 discloses a Li 10 GeP2S 12 has reported a sulfide-based solid electrolyte with improved stability against lithium metal achieved by precisely controlling the composition of the sulfide-based solid electrolyte having the above crystal structure.

[0005] However, the prior art has a problem that the sulfide-based solid electrolyte has low ionic conductivity. Prior Art Documents Patent Documents

[0006] [Patent Document 1] Japanese Patent No. 5873533 [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2018-45997 [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2018-203569 [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2016-27545 [Summary of the Invention] [Problem to be Solved by the Invention]

[0007] An object of the present invention is to provide a sulfide-based solid electrolyte with improved ionic conductivity and a method for producing the same. [Means for Solving the Problem]

[0008] To achieve the above object, the present invention provides: A sulfide-based solid electrolyte containing a Group 2 element and having an argyrodite-type crystal structure, wherein the sulfide-based solid electrolyte is represented by the chemical formula Li 7-x-2y M y PS 6-x Ha x , in the chemical formula, M is one or more elements selected from Group 2 elements, Ha is one or more elements selected from halogen elements, and Ha contains Br, 0 < x < 2.5 and 0 < y < 0.45 are satisfied, the lattice volume of the sulfide-based solid electrolyte is 950 Å 3 or more and 980 Å 3 or less.

[0009] In one embodiment, y may satisfy 0 < y < 0.1.

[0010] In one embodiment, M may be one or more selected from the group consisting of Mg, Ca, Sr, and Ba.

[0011] In one embodiment, M may be Ca.

[0012] In one embodiment, the sulfide-based solid electrolyte may have an ionic conductivity of 2 mS / cm or more.

[0013] In one embodiment, the sulfide-based solid electrolyte does not need to contain impurity phases other than the argyrodite phase.

[0014] In one embodiment, the lattice volume of the sulfide-based solid electrolyte is 958 Å. 3 The above 966 Å 3 The following is also acceptable.

[0015] The present invention relates to a method for producing a sulfide-based solid electrolyte according to any one of the above embodiments, The process involves mixing lithium sulfide (Li2S), phosphorus pentasulfide (P2S5), lithium halide (LiHa), and a sulfide containing a group 2 element to obtain a mixture. The process involves firing the mixture in an inert atmosphere at a temperature of 250°C to 600°C, This provides a method that includes [something]. [Effects of the Invention]

[0016] The present invention can provide a sulfide-based solid electrolyte with improved ionic conductivity and a method for producing the same. [Brief explanation of the drawing]

[0017] [Figure 1] The X-ray diffraction (XRD) patterns of Examples 8, 12, and 19, and Comparative Example 1 are shown. [Figure 2] The XRD patterns of Examples 2, 3, 10, and 11 are shown. [Figure 3] This graph shows the lithium ion conductivity as a function of sulfide-based solid electrolyte composition. [Modes for carrying out the invention]

[0018] The present invention will be described in more detail below.

[0019] Terms and words used in this specification and in the claims should not be interpreted restrictively in their usual or dictionary sense, but rather in a sense and concept consistent with the technical idea of ​​the present invention, in accordance with the principle that inventors can appropriately define the concepts of terms in order to best describe their invention.

[0020] [Solid electrolyte for all solid-state batteries] The solid electrolyte for all-solid-state batteries of the present invention may comprise one or more of sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer-based solid electrolytes. Preferably, the solid electrolyte for all-solid-state batteries of the present invention is a sulfide-based solid electrolyte. The solid electrolyte for all-solid-state batteries may be mixed with a positive electrode mixture and used as a positive electrode material, mixed with a negative electrode mixture and used as a negative electrode material, or used as a separator. Depending on the application, the solid electrolyte for all-solid-state batteries may further comprise additives such as lithium salts, conductive materials, and binder resins.

[0021] <Sulfide solid electrolyte> The sulfide-based solid electrolyte is not particularly limited as long as it contains sulfur (S), and any known sulfide-based solid electrolyte can be used.

[0022] Sulfide-based solid electrolytes may have a crystalline structure. Sulfide-based solid electrolytes having a crystalline structure can promote lithium ion conduction and have high lithium ion conductivity.

[0023] Sulfide-based solid electrolytes may have argyrodite, nassycon, perovskite, garnet, or LGePS crystal structures. Preferably, sulfide-based solid electrolytes have an argyrodite crystal structure. Sulfide-based solid electrolytes having an argyrodite crystal structure have high stability with respect to lithium metal, making it possible to use lithium metal with a high mass energy density as the negative electrode material.

[0024] The sulfide-based solid electrolyte may be in the form of amorphous, glass, or glass-ceramic materials.

[0025] Sulfide-based solid electrolytes are those that have the ionic conductivity of metals belonging to Group 1 or Group 2 of the periodic table, and may include Li-PS-based glasses and Li-PS-based glass ceramics. Non-restrictive examples of such sulfide-based solid electrolytes include Li2S-P2S5, Li2S-LiI-P2S5, Li2S-LiI-Li2O-P2S5, Li2S-LiBr-P2S5, Li2S-Li2O-P2S5, Li2S-Li3PO4-P2S5, Li2S-P2S5-P2O5, Li2S-P2S5-SiS2, Li2S-P2S5-SnS, Li2S-P2S5-Al2S3, Li2S-GeS2, and Li2S-GeS2-ZnS, and may include one or more of these. However, they are not particularly limited to these.

[0026] The sulfide-based solid electrolyte may comprise a crystalline phase and an amorphous phase. The sulfide-based solid electrolyte may comprise a crystalline phase including an argyrodite-type crystal structure (also referred to as an argyrodite phase in the present specification) and another phase (also referred to as an impurity phase or an unknown phase in the present specification). The argyrodite-type crystal structure is preferably cubic. The other phase may be a crystalline phase or an amorphous phase. Regardless of whether the other phase is a crystalline phase or an amorphous phase, the other phase may include a Li₂S phase, a P₂S₅ phase, a LiCl phase, a LiBr phase, a Li₃PS₄ phase, a MgS phase, a CaS phase, a SrS phase, a BaS phase, and the like. Preferably, the sulfide-based solid electrolyte does not contain, or substantially does not contain, any impurity phase other than the argyrodite phase. That is, preferably, the sulfide-based solid electrolyte may consist only of the argyrodite phase. When the sulfide-based solid electrolyte does not contain or substantially does not contain an impurity phase, lithium ion conduction is less likely to be inhibited, and thus the sulfide-based solid electrolyte can have high lithium ion conductivity.

[0027] The proportion of crystalline phases contained in the sulfide-based solid electrolyte can be evaluated quantitatively or semi-quantitatively from an XRD pattern. As one method, the proportion of crystalline phases can be evaluated by comparing peak intensities (heights or areas) of the XRD pattern.

[0028] The sulfide-based solid electrolyte has the chemical formula Li 7-x-2y M y PS 6-x Ha x represented by . In the chemical formula, M is one or more elements selected from Group 2 elements, Ha is one or more elements selected from halogen elements, Ha contains Br, 0<x<2.5 and 0<y<0.45 are satisfied, and the unit cell volume of the sulfide-based solid electrolyte is 950 Å 3 or more and 980 Å 3 or less. Such a sulfide-based solid electrolyte can have high lithium ion conductivity.

[0029] The sulfide-based solid electrolyte is Li 7-x PS 6-x Ha xIn this material, some of the lithium is replaced by a group 2 element M that can become a divalent cation. The group 2 element M that replaces lithium may be one or more selected from the group consisting of magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). The ionic radius (6-coordinate) of lithium (Li) is 90 pm, while the ionic radii (6-coordinate) of magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba) are 86 pm, 114 pm, 132 pm, and 149 pm, respectively. Based on the valence of the elements, two lithium atoms may be replaced by one group 2 element M. Substitution by group 2 element M can create lithium site vacancies, potentially improving lithium ion conductivity. Furthermore, substitution by group 2 element M can alter the lattice constant and lattice volume of the sulfide-based solid electrolyte, resulting in a crystal structure suitable for lithium ion conduction.

[0030] Preferably, the group 2 element M is magnesium (Mg) and / or calcium (Ca), and particularly preferably calcium (Ca). When the group 2 element M is magnesium (Mg) and / or calcium (Ca), the sulfide-based solid electrolyte can have a high degree of crystallinity and, therefore, a high ionic conductivity. This is thought to be because the ionic radius of lithium (Li) is 90 pm, and the ionic radii of magnesium (Mg) and calcium (Ca) are 86 pm and 114 pm, respectively, so the argyrodite-type crystal structure is easily maintained even after substitution with the group 2 element M.

[0031] The aforementioned chemical formula Li 7-x-2y M y PS 6-x Ha xThe substitution amount y of the Group 2 element M in the above formula satisfies 0<y<0.45, preferably satisfies 0<y<0.1, more preferably satisfies 0.005≦y≦0.04, and still more preferably satisfies 0.01≦y≦0.03. When y satisfies the above range, the sulfide-based solid electrolyte can have high ionic conductivity. When y is 0, a change in crystal structure due to substitution of the Group 2 element M cannot be obtained, and the ionic conductivity may be low. When y is 0.45 or more, the argyrodite-type crystal structure of the sulfide-based solid electrolyte may not be maintained, and the ionic conductivity may decrease. In addition, an impurity phase that inhibits lithium ion conduction increases in the sulfide-based solid electrolyte, which may cause a decrease in ionic conductivity.

[0032] the chemical formula Li 7-x-2y M y PS 6-x Ha x The halogen (Ha) in the above formula is one or more elements selected from halogen elements, and contains at least bromine (Br). Preferably, the halogen (Ha) includes chlorine (Cl) and bromine (Br). When sulfur (S) is a divalent anion, it has a stronger force of attracting lithium ions than monovalent halogens, and can greatly inhibit the movement of lithium ions. By containing bromine (Br), the occupancy of sulfur (S) at a specific site in the argyrodite-type crystal structure decreases, the content of halogen increases, and the mobility of lithium ions around the bromine (Br) site can be activated. As a result, lithium ion conductivity can be improved. In addition, bromine (Br) can bind to Li in the sulfide-based solid electrolyte to form lithium bromide (LiBr), which is a water-absorbing substance. Lithium bromide (LiBr) can adsorb moisture that would otherwise reduce lithium ion conductivity, thereby improving the lithium ion conductivity of the sulfide-based solid electrolyte.

[0033] the chemical formula Li 7-x-2y M y PS 6-x Ha xThe proportion x of halogen (Ha) satisfies 0<x<2.5, preferably 1.0<x<2.0, and more preferably 1.3<x<1.8. When x satisfies the above range, the argyrodite-type crystal structure is stabilized, and the sulfide-based solid electrolyte can have high ionic conductivity.

[0034] The ionic conductivity of a sulfide-based solid electrolyte can be affected by the crystallinity of the sulfide-based solid electrolyte. Crystallinity can be evaluated from an XRD pattern. In the XRD pattern, when no or almost no phases other than the argyrodite crystal phase (crystalline phases or amorphous phases such as Li₂S phase, P₂S₅ phase, LiCl phase, LiBr phase, Li₃PS₄ phase, MgS phase, CaS phase, SrS phase and BaS phase) are observed, the sulfide-based solid electrolyte can have high ionic conductivity.

[0035] The lattice volume of the sulfide-based solid electrolyte can be reduced by substitution of lithium sites with the Group 2 element M. Without being bound by theory, it is considered that since the Group 2 element M exhibits the property of a divalent cation, the interaction with other anions present in the sulfide-based solid electrolyte is strengthened, resulting in a decrease in lattice volume. A decrease in lattice volume leads to a crystal structure suitable for lithium ion conduction, so that the sulfide-based solid electrolyte can have high ionic conductivity.

[0036] The lattice volume of the sulfide-based solid electrolyte is 950Å 3 or more and 980Å 3 or less, preferably 958Å 3 or more and 966Å 3 or less, more preferably 960Å 3 or more and 964Å 3 or less, even more preferably 961Å 3 or more and 963Å 3 or less. The lattice constant and lattice volume can be evaluated from an XRD pattern. When the lattice volume satisfies the above range, lithium ion conduction in the sulfide-based solid electrolyte is promoted, and the sulfide-based solid electrolyte can have high ionic conductivity.

[0037] The ionic conductivity of a sulfide-based solid electrolyte (also referred to herein as "lithium ion conductivity") refers to the ionic conductivity at room temperature (25°C, 298K) and atmospheric pressure (1 atm), unless otherwise specified. When used in all-solid-state batteries, it is practically desirable for the ionic conductivity to be 4 mS / cm or higher. The ionic conductivity of a sulfide-based solid electrolyte according to one embodiment of the present invention is 2 mS / cm or higher, preferably 4 mS / cm or higher, more preferably 10.8 mS / cm or higher, even more preferably 12 mS / cm or higher, and most preferably 13 mS / cm or higher.

[0038] A sulfide-based solid electrolyte according to one embodiment of the present invention can be obtained by a manufacturing method comprising the steps of: mixing a lithium source, a group 2 element source, a phosphorus source, a sulfur source, and a halogen source to obtain a mixture; and calcining the mixture at a temperature of 250°C to 600°C in an inert atmosphere such as argon gas and nitrogen gas.

[0039] The lithium source, group 2 element source, phosphorus source, sulfur source, and halogen source may be compounds such as sulfides, oxides, and nitrides. Lithium sulfide (Li2S) can be used as the lithium source, diphosphorus pentasulfide (P2S5) as the phosphorus source, and lithium halides (LiHa) such as lithium chloride (LiCl) and lithium bromide (LiBr) as the halogen source. For example, the group 2 element source may be a sulfide. Sulfur can be supplied from other element sources. That is, one or more of the lithium source, group 2 element source, phosphorus source, and halogen source may also serve as the sulfur source.

[0040] The firing temperature for sulfide-based solid electrolytes having an argyrodite-type crystal structure is preferably 400°C to 550°C, more preferably 420°C to 530°C, and even more preferably 450°C to 500°C. When the firing temperature is within the above range, the sulfide-based solid electrolyte can have a high degree of crystallinity, and therefore, a sulfide-based solid electrolyte with high ionic conductivity can be obtained.

[0041] [All-solid battery] The electrolyte for all-solid-state batteries of the present invention can be used in an all-solid-state battery comprising a positive electrode, a negative electrode, and a solid electrolyte layer. The solid electrolyte for all-solid-state batteries can be used together with the active material in the electrode active material layer of the positive electrode and the negative electrode. The solid electrolyte for all-solid-state batteries can be used as a material for the solid electrolyte layer. The average particle size of the electrolyte for all-solid-state batteries can be controlled depending on the application. By controlling the average particle size of the electrolyte for all-solid-state batteries, the ionic conductivity can be improved.

[0042] <Solid electrolyte layer> In the present invention, the solid electrolyte layer may have a thickness of about 50 μm or less, preferably about 15 μm to 50 μm. The thickness can be appropriate within the above range, taking into consideration the ionic conductivity, physical strength, and energy density of the battery to which it is applied. For example, in terms of ionic conductivity and energy density, the thickness may be 10 μm or more, 20 μm or more, or 30 μm or more. On the other hand, in terms of physical strength, the thickness may be 50 μm or less, 45 μm or less, or 40 μm or less. Furthermore, the solid electrolyte layer has a thickness range and a load capacity of about 100 kgf / cm². 2 ~Approx. 2,000kgf / cm 2 It can have a tensile strength of [value missing]. Furthermore, the solid electrolyte layer can have a porosity of 15 vol% or less, or about 10 vol% or less. Thus, the solid electrolyte layer according to the present invention can have high mechanical strength despite being a thin film.

[0043] <Positive and negative electrodes> In the present invention, the positive electrode and the negative electrode include a current collector and an electrode active material layer formed on at least one surface of the current collector, the electrode active material layer including a plurality of electrode active material particles and a solid electrolyte. The electrodes may further include one or more of a conductive material and a binder resin as needed. The electrodes may also further include a variety of additives for the purpose of complementing or improving the physicochemical properties of the electrodes.

[0044] In the present invention, any negative electrode active material can be used as long as it can be used as a negative electrode active material for lithium ion secondary batteries. For example, the negative electrode active material may be carbons such as non-graphitizable carbon and graphitic carbon; Li x Fe2O3 (0≦x≦1), Li x WO2 (0≦x≦1), Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, Group 1, Group 2 and Group 3 elements of the periodic table, halogen; 0<x≦1; 1≦y≦3; 1≦z≦8); lithium metal; lithium alloys; silicon metal; silicon-based alloys; indium metal; indium alloys; tin-based alloys; metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4 and Bi2O5; conductive polymers such as polyacetylene; Li-Co-Ni based materials; titanium oxides; and lithium titanium oxides. In one specific embodiment, the negative electrode active material may comprise a carbon-based material and / or Si.

[0045] In the case of a positive electrode, any electrode active material can be used without limitation as long as it can be used as a positive electrode active material for lithium ion secondary batteries. For example, the positive electrode active material may be layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals; chemical formula Li 1+x Mn 2-x O4 (x is 0 to 0.33), lithium manganese oxides such as LiMnO3, LiMn2O3 and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5 and Cu2V2O7; chemical formula LiNi 1-x A x Ni-site type lithium nickel oxide represented by O2 (A=Co, Mn, Al, Cu, Fe, Mg, B or Ga, x=0.01 to 0.3); chemical formula LiMn 2-x A xLithium manganese composite oxide represented by O₂ (A = Co, Ni, Fe, Cr, Zn or Ta, x = 0.01~0.1) or Li₂Mn₃AO₈ (A = Fe, Co, Ni, Cu or Zn); LiNi x Mn 2-x Lithium manganese composite oxide with a spinel structure represented by O₄; Li(Ni a Co b Mn c )O₂ (a, b and c each independently represent an atomic fraction of an element, satisfying 0<a<1, 0<b<1, 0<c<1, and a+b+c=1) NCM-based composite oxide represented by the above formula; LiMn₂O₄ in which a part of Li in the chemical formula is substituted with alkaline earth metal ions; disulfide compounds; Fe₂(MoO₄)₃, etc., but is not limited thereto.

[0046] In the present invention, as the current collector, any current collector known in the secondary battery field that has electrical conductivity, such as a metal plate, can be appropriately used according to the polarity of the electrode.

[0047] In the present invention, the conductive material is usually added in an amount of 1% by weight to 30% by weight based on the total weight of the mixture containing the electrode active material. Such a conductive material is not particularly limited as long as it has electrical conductivity without inducing chemical changes in the battery, and examples thereof include: graphites such as natural graphite and artificial graphite; carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and one or a mixture of two or more selected from conductive materials such as polyphenylene derivatives.

[0048] In the present invention, the binder resin is not particularly limited as long as it is a component that assists in the bonding of the active material to the conductive material and to the current collector. Examples include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers. The binder resin can usually be contained in an amount of 1 to 30% by weight, or 1 to 10% by weight, based on 100% by weight of the electrode active material layer.

[0049] In the present invention, the electrode active material layer may contain one or more additives as needed, such as oxidation stabilizing additives, reduction stabilizing additives, flame retardants, heat stabilizers, and anti-fogging agents.

[0050] The present invention provides a secondary battery having the structure described above. The present invention also provides a battery module including a secondary battery as a unit battery, a battery pack including a battery module, and a device including a battery pack as a power source. Specific examples of the device include, but are not limited to, power tools driven by an electric motor; electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric two-wheeled vehicles including electric bicycles (E-bikes) and electric scooters (E-scooters); electric golf carts; and power systems.

[0051] The present invention will be described in more detail below with reference to examples, but the following examples are for illustrative purposes only and the scope of the present invention is not limited to them.

[0052] Example 1 As raw materials, lithium sulfide (Li₂S, Mitsuwa Chemical), phosphorus pentasulfide (P₂S₅, Aldrich), magnesium sulfide (MgS, Kojundo Chemical), lithium chloride (LiCl, Aldrich), and lithium bromide (LiBr, Aldrich) were used to obtain a composition of Li 5.4-2y M y PS 4.4 Cl 1.0 Br 0.6 (substitution amount y = 0.025), weighing and mortar mixing were performed in an Ar gas-flow glove box to obtain a mixed powder. The mixed powder was placed together with ZrO₂ balls in a ZrO₂ pot to obtain a sealed pot. The sealed pot was set in a planetary ball mill, and after performing ball milling at 380 rpm for 20 hours, the pot was opened in the glove box, and the powder was recovered. The powder was placed in a carbon crucible, sealed, and then fired at 460°C for 8 hours while flowing Ar gas. The fired powder was pulverized in a mortar for 10 minutes to obtain a solid electrolyte.

[0053] Examples 2 to 20 As shown in Table 1, a solid electrolyte was obtained in the same manner as in Example 1, except that the Group 2 element M was changed, the substitution amount y of the Group 2 element M was changed within the range of 0 < y < 0.45, and the firing temperature was changed. As a raw material, instead of magnesium sulfide (MgS, Kojundo Chemical), when the Group 2 element M is Ca, calcium sulfide (CaS, Kojundo Chemical) was used; when the Group 2 element M is Sr, strontium sulfide (SrS, Kojundo Chemical) was used; when the Group 2 element M is Ba, barium sulfide (BaS, Kojundo Chemical) was used. In Example 20, magnesium sulfide (MgS, Kojundo Chemical) and calcium sulfide (CaS, Kojundo Chemical) were used as raw materials.

[0054] Comparative Example 1 As shown in Table 1, a solid electrolyte was obtained in the same manner as in Example 1, except that the Group 2 element M was changed, the substitution amount y of the Group 2 element M was set to 0.45, and the firing temperature was set to 430°C.

[0055] Comparative Example 2 As shown in Table 1, a solid electrolyte was obtained in the same manner as in Example 1, except that the group 2 element M was not added.

[0056] [Table 1]

[0057] [evaluation] The following evaluations were performed using the obtained solid electrolyte.

[0058] (XRD measurement) A predetermined amount of solid electrolyte was placed in a sealed holder within an Ar gas-flow glove box, and XRD measurements were performed. The lattice constant, lattice volume, and full width at half maximum (FWHM) were calculated from the obtained XRD (X-ray diffraction) patterns. The FWHM was calculated from the (311) plane crystal peak of the argyrodite-type crystal structure observed around 2θ = 30° in Figure 1.

[0059] (Ionic conductivity measurement) A predetermined amount of solid electrolyte was placed inside a Machor tube, and the Machor tube and pellet molding jig (upper and lower press pins) were combined and press-formed at 5 MPa using a single-screw press. After that, a predetermined amount of gold powder was placed on both sides of the pellet, and then press-formed at 7.5 MPa using a single-screw press to obtain a Machor tube cell. The obtained Machor tube cell was placed in an electrochemical measurement jig cell, and pressurized to 5.0 N·m using a torque wrench to obtain an ion conductivity measurement cell. The obtained ion conductivity measurement cell was connected to an impedance measuring device, and the resistance value of the solid electrolyte pellet was measured at room temperature (298 K) and atmospheric pressure (1 atm) to derive the ion conductivity [mS / cm] of the solid electrolyte.

[0060] (Initial charge / discharge capacity measurement) NCM-based positive electrode active material and solid electrolyte were weighed in a mass ratio of 70:30. Carbon black was added at 1.5 wt% as a conductive additive and mixed to obtain a positive electrode mixture. 80 mg of the solid electrolyte obtained above was weighed, placed in a molding jig, and pressure-molded at 6 MPa for 1 minute to obtain a solid electrolyte pellet. 10 mg of the positive electrode mixture obtained above was placed on one side of the obtained solid electrolyte pellet, and then pressed down with a SUS press pin of the molding jig to flatten it and form a positive electrode layer. An Al plate was placed on the obtained positive electrode layer and pressure-molded at 30 MPa for 1 minute. Then, a Li-Cu foil was placed on the other side of the solid electrolyte pellet and pressure-molded at 3 MPa for 30 seconds. This was combined with a SUS press pin to fabricate a Macol tube cell. The obtained Macol tube cell was placed in a battery cell and a torque of 4 N·m was applied to obtain an all-solid-state battery cell.

[0061] Using the obtained all-solid-state batteries, charge-discharge tests were performed with a voltage range of 4.25V-3.0V, charging conditions of CC(0.05C)-CV(0.01C cutoff), and discharging conditions of CC(0.05C). The initial charge capacity and initial discharge capacity were determined from the obtained charge-discharge curves.

[0062] [Evaluation Results] (crystalline phase) Table 1 shows the evaluation results of the crystalline phase (crystal structure) identified from the XRD patterns obtained by XRD measurement. Representative XRD patterns are also shown in Figures 1 and 2.

[0063] As shown in Table 1 and Figure 1, in Examples 1, 2, 3, 6-11, 20, and Comparative Example 2, the impurity phase (also called the unknown phase) was hardly observed, and almost only the argyrodite phase peak was present. In particular, in Example 8, where the group 2 element M is Ca and the substitution amount y = 0.025, a clean argyrodite phase peak without any observed impurity phase was observed. On the other hand, in Examples 4, 5, and 12-19, the argyrodite phase peak and trace amounts of impurity phases other than the argyrodite phase, such as Li2S, CaS, and BaS, were present. In Comparative Example 1, the argyrodite phase peak and large amounts of impurity phases other than the argyrodite phase, such as Li2S, CaS, and LiBr, were present.

[0064] Figure 2 shows the XRD patterns for Examples 2, 3, 10, and 11. In Examples 2 and 10, where the substitution amount y = 0.05, almost only the argyrodite phase peak was present, but peaks for impurity phases such as Li2S and CaS, as well as an unknown phase, were detected. On the other hand, in Examples 3 and 11, where the substitution amount y = 0.075, almost only the argyrodite phase peak was present.

[0065] When the group 2 element M is Mg, an impurity phase was observed at y = 0.1 or higher, and when the group 2 element M is Ca, an impurity phase was observed at y = 0.125 or higher. This indicates that as the substitution amount y of the group 2 element M increases, the formation of an impurity phase becomes more likely. When the group 2 element M is Sr and Ba, an impurity phase was observed even when the substitution amount y was small.

[0066] When the substitution amount y = 0.075 and the firing temperature was 460°C, no impurity phase was observed for Mg (Example 3) and Ca (Example 11), which have relatively small ionic radii, but an impurity phase was observed for Sr (Example 14) and Ba (Example 15), which have relatively large ionic radii. This indicates that an argyrodite-type crystal structure without an impurity phase is easily formed in the case of Mg and Ca, which have relatively small ionic radii. In particular, in the case of Ca, no impurity phase was observed, and a solid electrolyte with a clean argyrodite-type crystal structure was obtained. Sulfide-based solid electrolytes with a high degree of crystallinity are thought to promote lithium ion hopping conduction and contribute to an increase in ionic conductivity.

[0067] (Lattice volume) The lattice constants derived from the XRD pattern in this example range from 9.8624 Å to 9.8826 Å, and the lattice volume is 959.3 Å. 3 From 965.2 Å 3 The range was as follows. On the other hand, in Comparative Example 1, where the amount of Ca substitution y = 0.45, the lattice constant was 9.8171 Å and the lattice volume was 946.1 Å. 3 Furthermore, in Comparative Example 2, where the lithium sites of the sulfide-based solid electrolyte were not substituted with the group 2 element M, the lattice constant was 9.9471 Å and the lattice volume was 984.2 Å. 3 It was shown that substituting the lithium sites of the argyrodite-type crystal structure with group 2 element M reduces the crystal volume by approximately 1.9-2.5%. Although not bound by theory, it is thought that the crystal volume of the sulfide-based solid electrolyte decreases because one of the two lithium sites is substituted with group 2 element M, and the other becomes a lithium vacancy. The lithium vacancy is thought to serve as a pathway for lithium ion hopping conduction, contributing to an increase in ionic conductivity. In addition, the group 2 element M substituted at the lithium site can be divalent, and can attract anions around the group 2 element M site more strongly compared to monovalent lithium ions. This is thought to be the reason why the crystal volume of the sulfide-based solid electrolyte decreases.

[0068] In the examples, the half width of the (311) plane crystal peak of the argyrodite crystal structure was in the range of 0.05° to 0.07°. On the other hand, it was 0.11° in Comparative Example 1 and 0.08° in Comparative Example 2. In the examples where the substitution amount satisfies 0<y<0.45, the half width is small. A small half width corresponds to a large crystallite size and is considered to contribute to an increase in ionic conductivity.

[0069] (Ionic Conductivity) The measurement results of ionic conductivity are shown in Table 1. Figure 3 shows the composition of the sulfide-based solid electrolyte Li 5.4-2y M y PS 4.4 Cl 1.0 Br 0.6 is a graph with the substitution amount y of the Group 2 element M as the horizontal axis, and the ionic conductivity measured at 25°C and atmospheric pressure as the vertical axis. Each point in Figure 3 corresponds to Examples 1 to 8, 10 to 15, and 20 fired at 430°C and 460°C, Comparative Examples 1 and 2 (corresponding to "no addition" in the figure), and the example where y of Example 1 is 0.0125. The ionic conductivity of the example where y of Example 1 is 0.0125 was 10.1 mS / cm.

[0070] As shown in Table 1, in the examples, the ionic conductivity was in the range of 2.14 mS / cm to 13.20 mS / cm. On the other hand, in Comparative Example 1 where the Ca substitution amount y=0.45, the ionic conductivity was 0.09 mS / cm. In this way, the ionic conductivity of each example could be made higher than that of Comparative Example 1. In Comparative Example 1, the substitution amount y of Group 2 element M was too large, so it is considered that the crystallinity of the argyrodite-type crystal structure decreased, leading to a decrease in ionic conductivity. In addition, in Comparative Example 2 where no Group 2 element M was added, the ionic conductivity was 10.70 mS / cm.

[0071] In particular, in Examples 6 to 8 and 11 where the Group 2 element M is Ca and the substitution amount y<0.1, the ionic conductivity was higher than that of Comparative Example 2.

[0072] On the other hand, as can be seen from Figure 3, when the substitution amount of Mg or Ca is y=0.05 (Examples 2 and 10), the ionic conductivity is lower compared to the adjacent cases of y=0.025 and y=0.075. Referring to the XRD pattern in Figure 2, trace amounts of impurity phase (unknown phase) are observed in Examples 2 and 10. Therefore, this decrease in ionic conductivity is thought to be due to trace amounts of impurities present in the sulfide-based solid electrolyte. Thus, in order to increase the ionic conductivity of sulfide-based solid electrolytes, it is preferable that they do not contain impurity phases other than the argyrodite phase.

[0073] When examining the ionic conductivity for each type of Group 2 element M, the ionic conductivity ranged from 2.19 mS / cm to 10.40 mS / cm when M=Mg. When M=Ca, the ionic conductivity ranged from 2.14 mS / cm to 13.20 mS / cm. When M=Sr, the ionic conductivity ranged from 4.10 mS / cm to 4.90 mS / cm. When M=Ba, the ionic conductivity ranged from 3.76 mS / cm to 5.24 mS / cm. For both Mg and Ca, the ionic conductivity was 5.51 mS / cm. Thus, in these examples, the ionic conductivity could be significantly increased when the Group 2 element M was Mg or Ca.

[0074] As shown in Figure 3 and Table 1, when the amount of Ca substitution y was 0.2 or less, the ionic conductivity was greater than 2 mS / cm. When the amount of Ca substitution y was less than 0.10 (except for the cases y=0.035 and y=0.05), the ionic conductivity was greater than 10 mS / cm. When the amount of Ca substitution y was less than 0.035, the ionic conductivity was greater than 11 mS / cm.

[0075] (Battery characteristics) When the solid electrolyte of Example 8 was used in an all-solid-state battery, stable charging and discharging was possible with respect to the lithium metal as the negative electrode material. The relative ratio of the all-solid-state battery capacity when using the solid electrolyte of Example 8 to the all-solid-state battery capacity when using the solid electrolyte of Comparative Example 2 was 107% of the initial discharge capacity. In other words, by using the solid electrolyte of Example 8, which has high ionic conductivity, in an all-solid-state battery, the initial discharge capacity of the all-solid-state battery could be improved.

[0076] Although the present invention has been described above with reference to limited embodiments and drawings, the present invention is not limited thereto and can be modified and transformed in various ways within the equivalent scope of the technical idea and the appended claims by persons with ordinary skill in the art to which the present invention pertains.

Claims

1. A sulfide-based solid electrolyte containing group 2 elements and having an argyrodite-type crystal structure, The aforementioned sulfide-based solid electrolyte has the chemical formula Li ７－ｘ－２ｙ M ｙ PS ６－ｘ Ha ｘ It is represented as, In the above chemical formula, The aforementioned M is one or more elements selected from Group 2 elements, The Ha is one or more elements selected from halogen elements, and the Ha includes Br. Satisfying 0 < x < 2.5 and 0 < y < 0.45, The lattice volume of the sulfide-based solid electrolyte is 950 Å. ３ The above is 980 Å. ３ The following are sulfide-based solid electrolytes.

2. The sulfide-based solid electrolyte according to claim 1, wherein y satisfies 0 < y < 0.

1.

3. The sulfide-based solid electrolyte according to claim 1 or 2, wherein M is one or more selected from the group consisting of Mg, Ca, Sr, and Ba.

4. The sulfide-based solid electrolyte according to any one of claims 1 to 3, wherein M is Ca.

5. The sulfide-based solid electrolyte according to any one of claims 1 to 4, wherein the sulfide-based solid electrolyte has an ionic conductivity of 2 mS / cm or more.

6. The sulfide-based solid electrolyte according to any one of claims 1 to 5, wherein the sulfide-based solid electrolyte does not contain any impurity phases other than the argyrodite phase.

7. The lattice volume of the sulfide-based solid electrolyte is 958 Å. ３ The above is 966 Å. ３ The following is a sulfide-based solid electrolyte according to any one of claims 1 to 6.

8. A method for producing a sulfide-based solid electrolyte according to any one of claims 1 to 7, A step of mixing a lithium source, a group 2 element source, a phosphorus source, a sulfur source, and a halogen source to obtain a mixture, The process involves firing the mixture in an inert atmosphere at a temperature of 250°C to 600°C, Methods that include...

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