Solid electrolyte, all-solid-state battery, and method for manufacturing a solid electrolyte

Abstract

This disclosure is made to solve the problems of the prior art and aims to provide a solid electrolyte with improved ionic conductivity. [Solution] The solid electrolyte of this disclosure contains Al and has an argyrodite-type crystal structure. The solid electrolyte of this disclosure has the chemical formula Li 7-x-3y Al y PS 6-x Ha x It is represented as follows: In the above chemical formula, Ha is one or more elements selected from halogen elements, and 0

Description

[Technical Field]

[0001] This disclosure relates to a solid electrolyte, an all-solid-state battery, and a method for manufacturing a solid electrolyte. [Background technology]

[0002] To achieve higher energy density and improved stability, development is underway on all-solid-state batteries that replace the organic electrolyte in conventional lithium-ion batteries with a solid electrolyte. Solid electrolytes include oxides, sulfides, and chlorides, but sulfide solid electrolytes, such as argyrodite-based electrolytes, are being considered due to their superior moldability and electrochemical properties. 10 GeP2S 12 These materials are attracting attention as materials for all-solid-state batteries.

[0003] Furthermore, the development of lithium metal anodes with a theoretical capacity approximately 10 times higher than conventional graphite, and all-solid-state lithium batteries using them, is attracting attention. However, many sulfide solid electrolytes have low electrochemical stability with respect to lithium metal anodes. Because lithium metal has a low potential, it undergoes a reduction reaction upon contact with a solid electrolyte, causing the solid electrolyte to decompose. This decomposition reaction promotes the growth of lithium dendrites, which can lead to short circuits in the battery.

[0004] In recent years, research has been progressing on using Al as a material to improve the electrochemical stability of Li metal anodes. Patent document 1 discloses an all-solid-state battery in which Al metal particles are added to the solid electrolyte layer to suppress dendrite growth. Non-patent document 1 discloses a Li-Al alloy and an all-solid-state battery that prevent the reduction reaction of sulfide solid electrolytes. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Special Publication No. 2022-529975 [Non-patent literature]

[0006] [Non-Patent Document 1] Hui Pan et al., “Carbon-free and binder-free Li-Al alloy anode enabling an all-solid-state Li-S battery with high energy and stability”, Sci. Adv.8, eabn4372(2022).DOI:10.1126 / sciadv.abn4372 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, the Al metal particles used in Patent Document 1 have a maximum diameter of 1 μm, which inhibits Li ion conduction in the solid electrolyte. Furthermore, Non-Patent Document 1 requires pre-lithiation, such as electrodeposition, to obtain the Li-Al alloy, which increases mass production costs. Therefore, in order to improve the electrochemical stability of all-solid-state batteries and reduce mass production costs, it is necessary to develop a solid electrolyte with high ionic conductivity and excellent electrochemical stability with respect to the Li metal anode.

[0008] This disclosure was made to solve the problems of the prior art described above, and one of its objectives is to improve the electrochemical stability of the Li metal anode using only a solid electrolyte.

[0009] Another objective of this disclosure is to provide a solid electrolyte with improved ionic conductivity. [Means for solving the problem]

[0010] As a result of diligent research into the above-mentioned problems, the inventors of the present invention discovered that in the case of a solid electrolyte having a specific composition and a specific XRD peak area ratio, the electrochemical properties and ionic conductivity of the Li metal anode are improved, and thus completed the present invention.

[0011] To address the above issues, this disclosure includes the following configuration.

[0012] The solid electrolyte according to this embodiment is has an argyrodite-type crystal structure, wherein the sulfide-based solid electrolyte has the chemical formula Li 7-x-3y Al y PS 6-x Ha x and is represented by In the chemical formula, the Ha is one or more elements selected from halogen elements, satisfies 0 < x < 2.5 and 0 < y < 0.1, and the ratio I2 / I1 of the maximum peak intensity I2 at 30.15° ≤ 2θ ≤ 30.25° to the maximum peak intensity I1 at 29.9° ≤ 2θ ≤ 30.1° measured by X-ray diffraction method is 0.15 or less.

[0013] In one embodiment, Al may be present at the 48h site of the argyrodite-type crystal structure.

[0014] In one embodiment, in the chemical formula, Ha may include Cl and Br.

[0015] In one embodiment, when the D50 of the solid electrolyte is 5 μm or more and 50 μm or less, the solid electrolyte may have an ionic conductivity of 12 mS / cm or more.

[0016] In one embodiment, when the D50 of the solid electrolyte is 0.1 μm or more and less than 5 μm, the solid electrolyte may have an ionic conductivity of 7 mS / cm or more.

[0017] In one embodiment, the solid electrolyte may have a critical current density of 1.8 mA / cm 2 or more.

[0018] In one embodiment, the solid electrolyte may have a change rate of the resistance value at the interface between the solid electrolyte and Li metal of 25% or less.

[0019] The present disclosure is a method for manufacturing a sulfide-based solid electrolyte according to any one of the above embodiments, A step of mixing a lithium source, an aluminum source, a phosphorus source, a sulfur source, and a halogen source to obtain a mixture, The steps include firing the mixture at a temperature of 450°C or lower, This provides a method that includes [something].

[0020] This disclosure relates to an all-solid-state battery comprising a positive electrode, a negative electrode, and a solid electrolyte layer sandwiched between the positive electrode and the negative electrode, The present invention provides an all-solid-state battery in which the solid electrolyte layer includes the solid electrolyte described in any one of the above embodiments.

[0021] In one embodiment, the negative electrode may be a Li metal negative electrode.

[0022] In one embodiment, the all-solid-state battery may include an Al-containing compound between the solid electrolyte layer and the negative electrode.

[0023] In one embodiment, the Al-containing compound may further contain Li. [Effects of the Invention]

[0024] In one aspect, the present disclosure provides a solid electrolyte that has electrochemical stability with respect to a Li metal anode.

[0025] Furthermore, in another aspect, this disclosure makes it possible to provide a solid electrolyte with improved ionic conductivity. [Brief explanation of the drawing]

[0026] [Figure 1] This figure shows the DTA curves obtained by thermogravimetric analysis (TG-DTA) measurements for Example 2 and Comparative Examples 1 and 4. [Figure 2] This figure shows the X-ray diffraction (XRD) patterns of Example 2 and Comparative Example 1. [Figure 3]This figure shows the ratio I2 / I1 of the peak intensities of the maximum peak intensity I2 at 30.15°≦2θ≦30.25° to the maximum peak intensity I1 at 29.9°≦2θ≦30.1° as measured by XRD in Comparative Example 1. [Figure 4] This figure shows the XRD peak shift of the argyrodite phase in Comparative Example 1. [Figure 5A] This figure shows the XRD patterns of Examples 1-3 and Comparative Examples 2 and 4. [Figure 5B] This is a magnified view of the XRD patterns of Examples 1-3 and Comparative Examples 2 and 4, near the maximum peak at 29.9° ≤ 2θ ≤ 30.1°. [Figure 6] This is a schematic diagram of a Li metal symmetric cell containing a sulfide solid electrolyte layer doped with a group 3 element (Al). [Figure 7] This is a schematic diagram of a Li metal symmetric cell containing a sulfide solid electrolyte layer without the addition of a group 3 element (Al). [Figure 8] This graph shows the polarization voltage and its current density obtained from the CCD (Critical Current Density) test for Examples 4-6 and Comparative Example 3. [Figure 9] This graph shows the impedance measurement results for Examples 4-6 and Comparative Example 3 during critical current density measurement. [Modes for carrying out the invention]

[0027] The following provides a more detailed explanation of this disclosure.

[0028] Terms and words used in this specification and in the claims should not be interpreted in a limited sense, either in their ordinary or dictionary sense, but rather in a sense and concept consistent with the technical idea of ​​this disclosure, in accordance with the principle that inventors can appropriately define the concepts of terms in order to best describe their invention.

[0029] In order to clearly illustrate the present invention, the drawings omit parts that are not relevant to the description, and similar parts throughout the specification are denoted by similar reference numerals. Furthermore, the sizes and relative sizes of the components shown in the drawings are independent of the actual scale and may be reduced or exaggerated for clarity of explanation.

[0030] In this specification, "Dn" refers to the particle size distribution, and means the particle size at the n% point of the cumulative particle number distribution by particle size. That is, D50 is the particle size (central particle size, average particle size) at the 50% point of the cumulative particle number distribution by particle size, D90 is the particle size at the 90% point of the cumulative particle number distribution by particle size, and D10 is the particle size at the 10% point of the cumulative particle number distribution by particle size. The particle size distribution may also be measured using the laser diffraction method. Specifically, after dispersing the powder to be measured in a dispersion medium, it is introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500), and the difference in diffraction patterns due to particle size as the particles pass through the laser beam is measured to calculate the particle size distribution.

[0031] [Solid electrolyte for all-solid-state batteries] The solid electrolyte for all-solid-state batteries disclosed herein includes a sulfide solid electrolyte. In addition to sulfides, the electrolyte for all-solid-state batteries disclosed herein may also include oxide-based solid electrolytes and polymer-based solid electrolytes. The electrolyte for all-solid-state batteries disclosed herein is preferably 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 contain additives such as lithium salts, conductive materials, and binder resins.

[0032] <Sulfide solid electrolyte> Sulfide-based solid electrolytes may have a crystalline structure. Sulfide-based solid electrolytes having a crystalline structure can promote the conduction of lithium ions and have high lithium ion conductivity.

[0033] Sulfide-based solid electrolytes generally have argyrodite, nassycon, perovskite, garnet, or LGPS crystal structures. Preferably, the sulfide-based solid electrolyte according to this disclosure has 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 a negative electrode material.

[0034] The sulfide-based solid electrolyte may be in the form of amorphous, glass, or glass-ceramic material.

[0035] 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.

[0036] Sulfide-based solid electrolytes can include a crystalline phase and an amorphous phase. The sulfide-based solid electrolyte can include a crystalline phase (also referred to herein as the argyrodite phase) having an argyrodite-type crystal structure and other phases (also referred to herein as impurity phases). The argyrodite-type crystal structure is preferably cubic. The other phases may be a crystalline phase or an amorphous phase. The other phases, regardless of whether they are a crystalline phase or an amorphous phase, can include a Li2S phase, a P2S5 phase, a LiCl phase, a LiBr phase, a Li3PS4 phase, an Al2S3 phase, a Ga2S3 phase, an In2S3 phase, and the like. Preferably, the sulfide-based solid electrolyte does not contain or substantially does not contain impurity phases 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 impurity phases, lithium ion conduction is less likely to be inhibited, so the sulfide-based solid electrolyte can have a high lithium ion conductivity.

[0037] The sulfide-based solid electrolyte according to one embodiment of the present disclosure has the chemical formula Li 7-x-3y M y PS 6-x Ha x as represented. In the above chemical formula, M is one or more elements selected from Group 13 elements, and Ha is one or more elements selected from halogen elements, satisfying 0 < x < 2.5 and 0 < y < 0.1. Such a sulfide-based solid electrolyte can have a high lithium ion conductivity. As an example, the composition of the solid electrolyte according to the present disclosure, that is, the values of x and y in the above chemical formula, can be determined by ICP measurement. However, it is not limited to the above determination method, and a composition determination method generally used in the art can be utilized.

[0038] The sulfide-based solid electrolyte of the present disclosure is Li 7-x PS 6-x Ha xIn this mixture, some of the lithium may be replaced by a group 13 element M that can become a trivalent cation. The group 13 element M used to replace lithium may be one or more selected from the group consisting of aluminum (Al), gallium (Ga), and indium (In). Aluminum (Al), gallium (Ga), and indium (In) may be used individually or in combination. The ionic radius (4-coordinate) of lithium (Li) is 59 pm, while the ionic radii (4-coordinate) of aluminum (Al), gallium (Ga), and indium (In) are 39 pm, 47 pm, and 62 pm, respectively. Based on the valence of the elements, three lithium atoms may be replaced by one group 13 element M. Substitution of lithium sites by group 13 element M can create lithium site vacancies, potentially improving lithium ion conductivity. Furthermore, substitution of lithium sites by group 13 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.

[0039] Furthermore, sulfide-based solid electrolytes contain Li, a group 13 element M that can form a trivalent cation. 7-x PS 6-x Ha x It may also penetrate into the crystal lattice. The Group 13 element M that penetrates into the crystal lattice may be one or more selected from the group consisting of aluminum (Al), gallium (Ga), and indium (In). Aluminum (Al), gallium (Ga), and indium (In) may be used individually or in combination. Li 7-x PS 6-x Ha x The intrusion of group 13 element M into the crystal lattice can alter the lattice constant and volume of sulfide-based solid electrolytes, potentially resulting in a crystal structure suitable for lithium ion conduction.

[0040] Preferably, the Group 13 element M is aluminum (Al). When the Group 13 element M is aluminum (Al), the sulfide-based solid electrolyte can have a high degree of crystallinity, and thus, the sulfide-based solid electrolyte can have a high ionic conductivity. This is considered to be because the ionic radius of lithium (Li) is 59 pm and the ionic radius of aluminum (Al) is 39 pm, and the argyrodite-type crystal structure is likely to be maintained even after substitution by the Group 13 element M.

[0041] In the chemical formula Li 7-x-3y M y PS 6-x Ha x The addition amount y of the Group 13 element M in satisfies 0 < y < 0.2. When the Group 13 element M is aluminum (Al), preferably, y satisfies 0 < y < 0.1, more preferably, 0.01 ≦ y ≦ 0.08, even more preferably, 0.02 ≦ y ≦ 0.07, and most preferably, 0.03 ≦ y ≦ 0.06. The lower limit of the range of y may be any of more than 0, 0.01 or more, 0.02 or more, 0.03 or more, or 0.04 or more, and the upper limit of the range of y may be any of less than 0.1, 0.08 or less, 0.07 or less, or 0.06 or less. When y satisfies the above range, the sulfide-based solid electrolyte can have a high ionic conductivity. When y is 0, a change in the crystal structure due to substitution of the Group 13 element M cannot be obtained, and the ionic conductivity may be low. When y is 0.2 or more, the argyrodite-type crystal structure of the sulfide-based solid electrolyte may not be maintained, and the impurity phase that inhibits lithium ion conduction in the sulfide-based solid electrolyte may increase, resulting in a decrease in ionic conductivity.

[0042] When the Group 13 element M is aluminum (Al), the argyrodite crystal structure of the sulfide-based solid electrolyte can be maintained even with a large amount of added element y. When the Group 13 element M is gallium (Ga) or indium (In), the argyrodite crystal structure of the sulfide-based solid electrolyte can be maintained with a relatively small amount of added element y. Although not bound by theory, this is thought to be due to the differences in properties between aluminum (Al) and gallium (Ga) and indium (In). For example, when used as a component of an alloy, gallium (Ga) has the property of easily diffusing at the grain boundaries of the alloy, thus promoting alloying with other metallic elements. Such properties can be observed in indium (In), which, like gallium (Ga), has electrons in its d orbitals. Due to these specifics, it is thought that gallium (Ga) or indium (In) can maintain the argyrodite crystal structure of the sulfide-based solid electrolyte with a relatively small amount of added element y compared to aluminum (Al).

[0043] Preferably, the group 13 element M (preferably Al) is located at the 48h site of the argyrodite crystal structure. More preferably, the group 13 element M is located only at the 48h site of the argyrodite crystal structure. A site is represented by a combination of a number and an English letter, where the number represents the number of positions in the crystal structure where the element can be located, and the English letter is a, b, c, d… in order of crystallinity, based on the crystallographic symmetry of that site. The argyrodite crystal structure can be cubic, hexagonal, tetragonal, orthorhombic, monoclinic, triclinic, etc. When the argyrodite crystal structure is cubic (space group F-43m), the group 13 element M can be located at the 48h site in the crystal structure. The presence of the group 13 element M at the 48h site in the crystal structure can be determined by neutron crystal structure analysis. While not strictly bound by theory, if Al is present at the 48h site, which is the conduction pathway for Li ions, it is thought that Al can also conduct through the Li ion conduction pathway.

[0044] The aforementioned chemical formula Li 7-x-3y M yPS 6-x Ha x In PS, the halogen (Ha) is one or more elements selected from halogen elements. The halogen (Ha) preferably contains bromine (Br). More preferably, the halogen (Ha) contains chlorine (Cl) and bromine (Br). Without being bound by theory, sulfur (S) is a divalent anion and exists in the PS4 tetrahedron and the isolated sites of the 4a sites in the argyrodite-type crystal structure. By including chlorine (Cl) and bromine (Br), which are monovalent anions, the occupancy of sulfur (S) at the isolated sites decreases and the halogen occupancy increases. Considering the charge balance of the entire structure, when a part of the divalent sulfur (S) is replaced by a monovalent halogen element, the occupancy of lithium decreases accordingly. Therefore, voids occur in the lithium sites, and the lithium ion mobility can be enhanced. As a result, it is considered that the lithium ion conductivity can be improved. Also, without being bound by theory, bromine (Br) can combine with 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 can lower the lithium ion conductivity and improve the lithium ion conductivity of the sulfide-based solid electrolyte.

[0045] The chemical formula Li 7-x-3y M y PS 6-x Ha x The ratio x of the halogen (Ha) in Li

[0046] The ionic conductivity of sulfide-based solid electrolytes can be influenced by their crystallinity. Crystallinity can be evaluated from the XRD pattern. When other phases besides the argyrodite crystalline phase (such as crystalline or amorphous phases like Li2S, P2S5, LiCl, LiBr, Li3PS4, Al2S3, Ga2S3, and In2S3) are not observed or are hardly observed in the XRD pattern, sulfide-based solid electrolytes can have high ionic conductivity. Furthermore, even among the argyrodite crystalline phases, the design composition (e.g., Li 7-x-3y M y PS 6-x Ha x Sulfide-based solid electrolytes may have high ionic conductivity if no or very few deviations from the standard composition (e.g., halogen-rich systems) are observed.

[0047] The proportion of the crystalline phase in sulfide-based solid electrolytes can be quantitatively or semi-quantitatively evaluated from XRD patterns. One method involves comparing the peak intensities (height or area) of the XRD patterns to assess the proportion of the crystalline phase.

[0048] Furthermore, the proportion of impurity phases other than the argyrodite phase in the solid electrolyte according to this disclosure can be evaluated by the ratio I2 / I1 of the peak intensities of the maximum peak intensity I2 at 30.15°≦2θ≦30.25° to the maximum peak intensity I1 at 29.9°≦2θ≦30.1° measured by XRD. The maximum peak at 29.9°≦2θ≦30.1° is the peak of the argyrodite-type crystalline phase having the design composition according to this disclosure, and the maximum peak at 30.15°≦2θ≦30.25° is the peak of the argyrodite-type crystalline phase having a composition deviating from the design composition. When the ratio of the peak intensities I2 / I1 between the maximum peak intensity I1 at 29.9°≦2θ≦30.1° and the maximum peak intensity I2 at 30.15°≦2θ≦30.25° is small, it means that in the solid electrolyte, the proportion of argyrodite-type crystalline phases having compositions deviating from the design composition is small compared to argyrodite-type crystalline phases having the design composition. I2 / I1 may be 0.15 or less, preferably 0.12 or less, 0.10 or less, more preferably 0.08 or less, 0.05 or less, 0.03 or less, and most preferably 0.01 or less. Also, I2 / I1 may be 0 or greater. I2 / I1=0 indicates that there are no argyrodite-type crystalline phases having compositions deviating from the design composition. When I2 / I1 has a value of 0.15 or less, the proportion of argyrodite-type crystalline phases having the design composition is high, and the ionic conductivity of the solid electrolyte may improve.

[0049] The average particle size of solid electrolytes can be controlled depending on the application.

[0050] The average particle size (D50) of a solid electrolyte can be controlled by changing conditions such as whether or not grinding is performed after firing, and the rotation speed and time of the ball mill during grinding. This makes it possible to produce coarse solid electrolyte powder with a relatively large average particle size and fine solid electrolyte powder with a relatively small average particle size. As an example, D50 can also be measured using the laser diffraction method. Specifically, after dispersing the powder to be measured in a dispersion medium, it is introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500), and the difference in diffraction patterns due to particle size as the particles pass through the laser beam is measured to calculate the particle size distribution and measure D50. In addition, when the solid electrolyte is in an all-solid-state battery, the D50 of the solid electrolyte can be determined by confirming the particle size distribution, for example, by SEM observation.

[0051] The average particle size of the solid electrolyte coarse powder may be larger than the average particle size of the solid electrolyte fine powder. The average particle size of the solid electrolyte coarse powder is 5 μm or more and 50 μm or less, preferably 8 μm or more and 30 μm or less, and more preferably 10 μm or more and 20 μm or less. The average particle size of the solid electrolyte fine powder is 0.1 μm or more and less than 5 μm, preferably 0.5 μm or more and less than 5 μm, and more preferably 1 μm or more and 3 μm or less.

[0052] The average particle size of the sulfide-based solid electrolyte contained in the solid electrolyte layer may be larger than the average particle size of the sulfide-based solid electrolyte contained in the positive electrode active material layer. Coarse solid electrolyte powder has a large average particle size and few grain boundaries per unit volume, so it can exhibit high ionic conductivity when used in the solid electrolyte layer. Therefore, by making the average particle size of the sulfide-based solid electrolyte contained in the solid electrolyte layer larger than the average particle size of the sulfide-based solid electrolyte contained in the positive electrode active material layer, the discharge capacity and cycle characteristics of the all-solid-state battery can be improved. On the other hand, fine solid electrolyte powder has a small average particle size and many grain boundaries per unit volume, so it tends to have lower ionic conductivity compared to coarse solid electrolyte powder. However, when used in the positive electrode active material layer, it has the advantage of being able to enter the gaps between positive electrode active material particles and provide lithium ion conduction pathways to the positive electrode active material. Furthermore, when fine solid electrolyte powder is used together with a lithium metal electrode, it has the advantage of suppressing dendrite growth of lithium metal and preventing short circuits compared to coarse solid electrolyte powder because of its small average particle size and fewer gaps between solid electrolyte layers.

[0053] 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 a sulfide-based solid electrolyte is used in an all-solid-state battery, it is practically desirable for the ionic conductivity to be 4 mS / cm or higher. In one embodiment of this disclosure, the ionic conductivity of a sulfide-based solid electrolyte may be 10 mS / cm or higher in the case of coarse solid electrolyte powder, preferably 12 mS / cm or higher, and more preferably 13 mS / cm or higher. In one embodiment of this disclosure, the ionic conductivity of a sulfide-based solid electrolyte may be 4 mS / cm or higher in the case of fine solid electrolyte powder, preferably 6 mS / cm or higher, more preferably 7 mS / cm or higher, and most preferably 7.5 mS / cm or higher.

[0054] The electrochemical stability of sulfide-based solid electrolytes can be evaluated, for example, by fabricating a Li-metal symmetric cell using a sulfide-based solid electrolyte and performing a CCD (Critical Current Density) test. A current density is applied to the Li-metal symmetric cell, causing Li to precipitate and dissolve on both sides of the solid electrolyte. Gradually increasing the current density leads to decomposition of the solid electrolyte due to Li precipitation and dissolution, as well as a short circuit. The maximum current density at which this decomposition and short circuit occur is called the CCD or critical current density. A larger CCD indicates higher electrochemical stability with respect to Li metal. In the case of the sulfide-based solid electrolyte according to this disclosure, the critical current density is 1.5 mA / cm². 2 The above is possible, preferably 1.8 mA / cm². 2 More preferably, 2.0 mA / cm² 2 In summary, the most preferred value is 2.2 mA / cm². 2 It could be any of the above.

[0055] Furthermore, AC impedance measurements can be performed simultaneously with the CCD test to measure the interfacial resistance at the interface between the sulfide-based solid electrolyte and the Li metal. In the case of the sulfide-based solid electrolyte according to this disclosure, the rate of increase in resistance at each current density calculated by Equation 1 below is 1.5 mA / cm². 2 The current density may be 25% or less, preferably 20% or less, more preferably 10% or less, even more preferably 5% or less, and most preferably 1% or less. 2 If the rate of increase in resistance is 25% or less, the solid electrolyte according to this disclosure can have improved electrochemical stability relative to the Li metal electrode. [Formula 1] (Percentage increase in resistance (%)) = 100 × {(Interface resistance (Ω) at each current density) - (0 mA / cm²) 2 Interfacial resistance (Ω) at (0mA / cm) 2 (Interfacial resistance (Ω))

[0056] A sulfide-based solid electrolyte according to one embodiment of the present disclosure can be obtained by a manufacturing method comprising the steps of: mixing a lithium source, a group 13 element source, a phosphorus source, a sulfur source, and a halogen source to obtain a mixture; and calcining the mixture at a temperature of 450°C or less under an inert atmosphere such as argon gas and nitrogen gas.

[0057] The lithium source, the group 13 element source, the phosphorus source, the sulfur source, and the halogen source may be compounds such as sulfides, oxides, or 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, a sulfide can be used as the group 13 element source. Alternatively, sulfur can be supplied from other element sources. That is, one or more of the lithium source, group 13 element source, phosphorus source, and halogen source may also serve as the sulfur source.

[0058] The firing temperature for sulfide-based solid electrolytes having an argyrodite crystal structure is generally around 400°C to 550°C. However, a decomposition reaction from the argyrodite phase of the design composition may occur above a certain temperature, and it is more preferable to fire at a temperature below the decomposition reaction temperature. In the case of the solid electrolyte according to this disclosure, the firing temperature is preferably 450°C or lower, more preferably 400°C to 450°C, and even more preferably 410°C to 440°C. When the firing temperature is within the above range, the formation of the argyrodite crystal structure is promoted, the decomposition reaction does not occur, and the sulfide-based solid electrolyte can have a high degree of crystallinity and a low impurity phase ratio. As a result, a sulfide-based solid electrolyte with high ionic conductivity can be obtained.

[0059] [All-solid battery] The electrolyte for all-solid-state batteries disclosed herein can be used in all-solid-state batteries including a positive electrode, a negative electrode, and a solid electrolyte layer. The solid electrolyte for all-solid-state batteries can be used together with an 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.

[0060] <Solid electrolyte layer> The solid electrolyte layer is located between the positive and negative electrodes. In this disclosure, 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 100 kgf / cm². 2 The above is approximately 2,000 kgf / cm². 2 The solid electrolyte layer may have the following tensile strengths. Furthermore, the solid electrolyte layer may have a porosity of 15 vol% or less, or about 10 vol% or less. Thus, the solid electrolyte layer according to this disclosure can have high mechanical strength despite being a thin film.

[0061] <Positive and negative electrodes> In this disclosure, 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 conductive materials and binder resins 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.

[0062] In the present disclosure, as the negative electrode active material, any material can be used as long as it can be used as the negative electrode active material of a lithium-ion secondary battery. For example, the negative electrode active material includes carbon such as graphitizable carbon and graphite 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, Group 3 elements of the periodic table, halogen; 0 < x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8), etc. metal composite oxides; lithium metal; lithium alloy; silicon metal; silicon-based alloy; indium metal; indium alloy; tin-based alloy; 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; lithium titanium oxides, etc. One or more selected therefrom can be used. In a specific embodiment, the negative electrode active material can contain a carbon-based material and / or Si.

[0063] In the present disclosure, a Li metal negative electrode can preferably be used as the negative electrode. Without being bound by theory, Al in the solid electrolyte conducts via the Li ion conduction path, and a compound (precipitate) containing Al is formed at the interface with the Li metal negative electrode in contact with the solid electrolyte, thereby improving the electrochemical stability of the Li metal negative electrode, and therefore, an improvement in the cycle characteristics of the battery is expected. The compound (precipitate) containing Al preferably also contains Li.

[0064] In the case of the positive electrode, the electrode active material can be used without limitation as long as it can be used as the positive electrode active material of a lithium-ion secondary battery. For example, the positive electrode active material includes 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-xLithium manganese oxides such as O4 (where x is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5, 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 x Lithium manganese composite oxide represented by O2 (A = Co, Ni, Fe, Cr, Zn or Ta, x = 0.01 to 0.1) or Li2Mn3AO8 (A = Fe, Co, Ni, Cu or Zn); LiNi x Mn 2-x Spinel-structured lithium manganese composite oxide represented by O4; Li(Ni a Co b Mn c )O2 (a, b, c are atomic fractions of independent elements, 0 < a < 1, 0 < b < 1, 0 < c < 1, and a + b + c = 1).) NCM-based composite oxide; LiMn2O4 in which part of the Li in the chemical formula is substituted with alkaline earth metal ions; disulfide compounds; It can include, but is not limited to, Fe2(MoO4)3, etc.

[0065] In the present disclosure, the current collector can be an electrically conductive current collector such as a metal plate that is known in the field of secondary batteries and can be appropriately used according to the polarity of the electrode.

[0066] In this disclosure, the conductive material is usually added in an amount of 1% to 30% by weight based on the total weight of the mixture containing the electrode active material. Such conductive materials are not particularly limited as long as they do not induce chemical changes in the battery and are conductive, and may include, for example, one or more mixtures of conductive materials selected from graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; 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 conductive materials such as polyphenylene derivatives.

[0067] In this disclosure, 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, and 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 included 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.

[0068] In this disclosure, the electrode active material layer may contain one or more additives as needed, such as oxidation stabilizers, reduction stabilizers, flame retardants, heat stabilizers, and anti-fogging agents.

[0069] This disclosure provides a secondary battery having the structure described above. This disclosure 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 electric motors; 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. [Examples]

[0070] The present disclosure will be described in more detail below using examples and comparative examples, but the scope of the present disclosure is not limited to the examples.

[0071] <1. Manufacturing of Solid Electrolytes> (Example 1) The raw materials used were lithium sulfide (Li2S, Mitsuwa Chemical Co.), diphosphorus pentasulfide (P2S5, Aldrich), aluminum sulfide (Al2S3, High Purity Chemical Co.), lithium chloride (LiCl, Aldrich), and lithium bromide (LiBr, Aldrich). These raw materials were used to create a Li 5.4-3y Al y PS 4.4 Cl 1.0 Br 0.6A mixed powder was obtained by weighing and mixing in a mortar and pestle in an Ar gas flow glove box so that (y=0.025). This mixed powder was placed in a zirconia pot along with zirconia (ZrO2) balls and sealed to obtain a sealed pot. This sealed pot was placed in a planetary ball mill and ball milling was performed at 380 rpm for 20 hours. After that, the pot was opened in an Ar gas flow glove box and the precursor powder was recovered. This precursor powder was placed in a carbon crucible, sealed, and then calcined at 430°C for 8 hours while flowing Ar gas. The calcined powder was ground in a mortar and pestle for 5 minutes to obtain 5 g of crude solid electrolyte powder.

[0072] (Example 2) Composition is Li 5.4-3y Al y PS 4.4 Cl 1.0 Br 0.6 The manufacturing process was the same as in Example 1, except that the raw materials were weighed so that (y=0.05).

[0073] (Example 3) Composition is Li 5.4-3y Al y PS 4.4 Cl 1.0 Br 0.6 The manufacturing process was the same as in Example 1, except that the raw materials were weighed so that (y=0.075).

[0074] (Examples 4-6: Micronized solid electrolytes) The solid electrolyte was manufactured in the same manner as in Examples 1 to 3, except that the crude solid electrolyte powder obtained in Examples 1 to 3 was placed in a zirconia pot, and wet grinding was performed at 250 rpm for 1 hour using zirconia balls and anisole solvent to obtain fine solid electrolyte powder for use as a cathode composite material.

[0075] (Comparative Example 1) A crude solid electrolyte powder was obtained in the same manner as in Example 2, except that the firing temperature was set to 460°C.

[0076] (Comparative Example 2) A solid electrolyte crude powder free of Al was obtained in the same manner as in Example 2, except that aluminum sulfide was not used among the raw materials. The composition of the solid electrolyte obtained in Comparative Example 2 was Li 5.4 PS 4.4 Cl 1.0 Br 0.6 That was the case.

[0077] (Comparative Example 3) The Al-free solid electrolyte crude powder obtained in Comparative Example 2 was placed in a zirconia pot and finely ground using zirconia balls and anisole solvent at 250 rpm for 1 hour, except that it was prepared in the same manner as in Comparative Example 2.

[0078] (Comparative Example 4) Composition is Li 5.4-3y Al y PS 4.4 Cl 1.0 Br 0.6 The manufacturing process was the same as in Example 1, except that the raw materials were weighed so that (y=0.1).

[0079] <2. Evaluation Methods for Solid Electrolytes> (1) TG-DTA measurement To optimize the synthesis temperature, TG-DTA measurements were performed. Approximately 10 mg of the precursor powder obtained in Example 2 and Comparative Examples 1 and 4 was placed in an aluminum oxide (Al2O3) pan, and the phase transition temperature was measured using a TG-DTA apparatus installed in an Ar gas flow glove box. The measurement temperature range was from room temperature (approximately 25°C) to 600°C, and the Ar flow rate was 450 mL / min.

[0080] (2)XRD measurement The solid electrolyte powders of Examples 1-3 and Comparative Examples 1, 2, and 4, in predetermined amounts (approximately 30 mg), were placed in sealed holders within an Ar gas flow glove box, and XRD measurements were performed. Measurements were performed using a SmartLab XRD instrument with a CuKα1 tube in the measurement range 2θ = 10° to 60°.

[0081] (3) Particle size distribution measurement The solid electrolyte powders of Examples 1 to 6 and Comparative Examples 1 to 4 were put into a heptane solvent, and a solid electrolyte dispersion solution was prepared using Span80 as a dispersant. Using this dispersion solution, the particle size distribution was measured with a particle size distribution measuring device Mastersizer3000. The refractive index of the solid electrolyte used for data analysis was set to 2.16.

[0082] (4) Ion conductivity measurement After weighing a predetermined amount (about 50 mg) of the solid electrolyte powders of Examples 1 to 7 and Comparative Examples 1 to 4, a pellet molding jig (lower press pin) and a Macor tube with a diameter of 5 mm were assembled, and the weighed solid electrolyte powder was put into the Macor tube. Then, in combination with the pellet molding jig (upper press pin), it was press-molded at about 370 MPa with a uniaxial press. After placing a predetermined amount (about 20 mg) of gold powder on both sides of the obtained pellet, it was press-molded at about 554 MPa with a uniaxial press. The produced Macor tube cell was installed in an electrochemical measurement jig cell, and pressure was applied up to 80 MPa using a torque wrench to obtain an ion conductivity measurement cell. This measurement cell was connected to an impedance measurement device, the resistance value of the solid electrolyte pellet was measured, and the ion conductivity was derived.

[0083] (5) Crystal structure analysis Using the obtained solid electrolyte crude powders of Example 2 (Li 5.25 Al 0.05 PS 4.4 Cl 1.0 Br 0.6 ) and Comparative Example 2 (Li 5.4 PS 4.4 Cl 1.0 Br 0.6 ), neutron diffraction measurement was performed. Using the obtained data, crystal structure analysis was performed with the Z-Rietveld program.

[0084] (6) CCD test (6-1) Fabrication of Li metal symmetric cell To evaluate the electrochemical stability against Li metal, a Li metal symmetric cell was fabricated. After weighing 150 mg of the solid electrolyte fine powder obtained in Examples 4 to 6 and Comparative Example 3, it was placed in a molding jig and pressure-molded at 550 MPa for 1 minute to obtain a solid electrolyte layer pellet. Next, Li foils were placed on both sides of the solid electrolyte pellet, and then a SUS plate was placed on the Li foils. Thereafter, it was pressurized at 18 MPa for 2 seconds. After placing this in a jig cell, a constraint pressure of about 8 MPa was applied to obtain a Li metal symmetric cell.

[0085] Typical schematic diagrams of the Li metal symmetric cells of the examples and comparative examples are shown in FIGS. 6 and 7. As shown in FIGS. 6 and 7, a sulfide solid electrolyte layer (3) to which a Group 3 element (Al) is added or a sulfide solid electrolyte layer (4) to which no Group 3 element (Al) is added is in direct contact with the Li foil (2). By depositing and dissolving Li on both sides of the solid electrolyte layer of this Li metal symmetric battery, the electrochemical stability against Li metal can be evaluated.

[0086] (6-2) CCD test of Li metal symmetric cell Using the Li metal symmetric cell obtained in (6-1), a CCD (Critical Current Density) test was conducted at 25°C. The current density was from 0 mA / cm 2 to 3 mA / cm 2 or less. The deposition / dissolution time was 1 hour, and the current density increment was 0.1 mA / cm2.

[0087] (7) Impedance measurement Using the Li metal symmetric cell obtained in (6-1), an alternating current impedance measurement was conducted simultaneously with the CCD test to observe the change in the interfacial resistance between the solid electrolyte and the Li metal. The impedance measurement was carried out in the range from 7 MHz to 100 mHz. Immediately after the fabrication of the cell, it was measured every time the current density increment was 0.5 mA / cm 2 . Also, using the measured interfacial resistance values, the increase rate of resistance at each current density was calculated based on the following formula 1. [Formula 1] (Increase rate of resistance (%)) = 100 × {(Interfacial resistance (Ω) at each current density) - (0 mA / cm2 Interfacial resistance (Ω) at (0mA / cm) 2 (Interfacial resistance (Ω))

[0088] <3. Evaluation results of solid electrolytes> (1) TG-DTA measurement Table 1 shows the evaluation results of the phase transition temperature by TG-DTA measurement. Figure 1 shows the obtained DTA curves. In all measured compositions, an endothermic peak appeared around 460°C, indicating that some kind of reaction occurred. Furthermore, the endothermic peak temperature tended to decrease to around 456°C with increasing Al content.

[0089] [Table 1]

[0090] (2)XRD measurement (2-1) Optimization of firing temperature To identify the products around the endothermic peak at approximately 456°C to 460°C, which were confirmed to exist by TG-DTA measurement, XRD measurements were performed on the samples from Example 2 and Comparative Example 1. The obtained XRD patterns are shown in Figure 2. As shown in Figure 2, an impurity phase appeared in Comparative Example 1, which was calcined at 460°C, whereas no impurity phase was present in Example 1.

[0091] To compare the amount of impurity phase more specifically, the ratio I2 / I1 of the peak intensities of the maximum peak intensity I2 at 30.15°≦2θ≦30.25° to the maximum peak intensity I1 at 29.9°≦2θ≦30.1°, as measured by XRD, was calculated. The results for Comparative Example 1 are shown in Figure 3. When the maximum peak I1 that appeared around 30° is normalized to 1, the maximum peak I2, which originated from the chlorine-based Li6PS5Cl of the impurity phase that appeared at 30.2°, becomes 0.16, and therefore the value of I2 / I1 was calculated to be 0.16. On the other hand, when a similar analysis was performed, the value of I2 / I1 in Example 2 was 0.

[0092] Figure 2 also shows the results of identifying the impurity phase around 29°~32°, which was confirmed by the XRD pattern of Comparative Example 1. The identification of the impurity phase revealed that all impurities corresponded to argyrodite-type solid electrolytes: bromine-based Li6PS5Br, chlorine-based Li6PS5Cl, and chlorine-excess Li. 5.5 PS 4.5 Cl 1.5 The following was detected. Comparing the ionic radii of the anions in the phase, Br (1.96 Å) > S (1.84 Å) ≈ Cl (1.81 Å). As the ionic radius increases, the lattice constant also increases, so using 30° as a reference, the bromine system appears at 29.8° on the lower angle side, and the chlorine system appears at 30.2° on the higher angle side. Furthermore, a chlorine-rich system with a decrease in lithium and an increase in chlorine appears at 30.4°. Considering the TG-DTA measurement results described above, the endothermic peak around 456°C to 460°C is thought to be a reaction in which the phase with the design composition containing Al decomposes into the respective argyrodite phases corresponding to the impurity phases mentioned above.

[0093] Furthermore, Figure 4 shows the peak shift of the argyrodite phase in Comparative Example 1, which was calcined at 460°C, compared to Example 2, which was calcined at 430°C. No peak splitting was observed in either sample, and a single peak was observed. In Comparative Example 1, compared to Example 2, both the peaks around 30° and 31.4° were slightly (about 0.01°) shifted to a higher angle. This shift to a higher angle means that the lattice constant has decreased, i.e., the lattice has shrunk. When the lattice shrunk, Li ions become less likely to conduct. On the other hand, no broadening of the full width at half maximum was observed visually in the XRD pattern. Therefore, it is thought that there is only one main phase, rather than multiple main phases, and that the presence of the aforementioned impurity phase slightly shifted the stoichiometric composition, causing the lattice to shrunk.

[0094] Based on these results, it was determined that a firing temperature of 450°C or lower is desirable when producing solid electrolytes having an argyrodite crystal structure. Therefore, in the examples, the firing temperature was set to 430°C.

[0095] (2-2) Confirmation of the crystalline phase Table 2 shows the evaluation results of the crystalline phases identified from the XRD patterns obtained by XRD measurement. The XRD patterns are also shown in Figures 5A and 5B. As shown in Table 2 and Figure 5A, in the solid electrolyte powders of Examples 1 to 3 (y=0.025 to 0.075) and Comparative Example 2 (y=0), almost only the argyrodite phase peak appeared, and no impurity phase peak appeared. However, in Comparative Example 4 (y=0.1), an impurity phase peak appeared around 2θ=29°. Also, as shown in Figure 5B, in Examples 1 to 3 and Comparative Examples 2 and 4, a peak at 29.9°≦2θ≦30.1° (a peak of the argyrodite-type crystalline phase with the designed composition) appeared, but no peak at 30.15°≦2θ≦30.25° (a peak of the argyrodite-type crystalline phase with a composition deviating from the designed composition) appeared. Therefore, for Examples 1-3 and Comparative Examples 2 and 4, the ratio of peak intensities I2 / I1 between the maximum peak intensity I1 at 29.9°≦2θ≦30.1° and the maximum peak intensity I2 at 30.15°≦2θ≦30.25°, as measured by XRD, was calculated to be 0.

[0096] [Table 2]

[0097] (3) Particle size distribution measurement Table 3 shows the particle size distribution measurement results. The D50 values ​​for the coarse solid electrolyte powders of Examples 1 to 3 and the fine solid electrolyte powders of Examples 4 to 6 were 11.1 μm to 15.0 μm and 2.04 μm to 2.29 μm, respectively. The D50 values ​​for the coarse solid electrolyte powder of Comparative Example 2 (y=0) and the fine solid electrolyte powder of Comparative Example 3 were 15.7 μm and 2.20 μm, respectively. These results confirm that solid electrolytes can be micronized by the solid electrolyte micronization process.

[0098] [Table 3]

[0099] (4) Measurement of ionic conductivity The measurement results of ionic conductivity are shown in Table 2. As shown in Table 2, the ionic conductivity was 13.1 mS / cm or higher for the coarse solid electrolyte powder of Examples 1 to 3, and 6.46 mS / cm or higher for the fine solid electrolyte powder of Examples 4 to 6. In addition, it was 11.5 mS / cm for the coarse solid electrolyte powder of Comparative Example 2, and 6.98 mS / cm for the fine solid electrolyte powder of Comparative Example 3. In the examples and comparative examples, it was confirmed that ionic conductivity decreased with micronization.

[0100] Furthermore, Comparative Example 1, which was calcined at 460°C, showed an ionic conductivity of 5.9 mS / cm, which is significantly lower than that of Example 2, which had the same composition and was calcined at 430°C. This is because the bromine-based Li6PS5Br, chlorine-based Li6PS5Cl, and chlorine-rich Li were confirmed in the XRD pattern of Comparative Example 1. 5.5 PS 4.5 Cl 1.5 This is thought to be because the ionic conductivity of the impurity phases is low, at approximately 5 mS / cm, 3 mS / cm, and 8 mS / cm, respectively. Furthermore, as mentioned above, in Comparative Example 1, the peak of the argyrodite phase shifted to the higher angle side and it was confirmed that the lattice contracted compared to Example 2. From these results, it is possible to consider two possible causes for the decrease in ionic conductivity in Comparative Example 1, which was calcined at 460°C: (a) the formation of impurity phases, and (b) the contraction of the ion conduction pathway due to lattice contraction.

[0101] (5) Crystal structure analysis Tables 4 and 5 show the neutron crystal structure analysis results for the crude solid electrolyte powders obtained in Example 2 (y=0.05) and Comparative Example 2 (y=0), respectively. In the tables, g represents the occupancy rate of each site. x, y, and z are parameters determined by the site, representing the position of an element as a ratio of lattice constants in the x, y, and z directions. Biso is the atomic displacement parameter, representing the distribution of displacement due to the thermal vibration of atoms. Neutron crystal structure analysis allows for the distinction between Li (-1.90) and Al (3.449) based on the difference in atomic scattering factors, and confirms whether Al acts as a carrier ion or a framework-forming element. Compared with Comparative Example 2, it was found that in the crude solid electrolyte powder obtained in Example 2, Al was present at the 48h site, which corresponds to the Li ion conduction pathway in the crystal structure. The ionic radii of Al and Li are 0.39 Å and 0.59 Å, respectively, with the ionic radius of Al being smaller than that of Li. Therefore, Al and Li can coexist at the 48h site. Therefore, it is thought that even in trace amounts, Al also conducts as a carrier ion via the Li ion conduction pathway. The lattice constants of Example 2 and Comparative Example 2, determined from crystal structure analysis, are 9.869164(7) Å and 9.868944(6) Å, respectively, with Example 2, which has Al added, being slightly larger. This increase in the lattice constant is thought to broaden the Li ion conduction pathway, contributing to improved ionic conductivity.

[0102] Neutron crystal structure analysis results of the solid electrolyte crude powder of Example 2 [Table 4]

[0103] Neutron crystal structure analysis results of the solid electrolyte crude powder of Comparative Example 2 [Table 5]

[0104] (6) CCD test Figure 8 shows the polarization voltage and current density obtained by the CCD (Critical Current Density) test. As the current density increases, the polarization voltage also increases symmetrically. However, if one side of the solid electrolyte interface decomposes, the shape of the polarization voltage becomes asymmetric, and this can lead to a short circuit. The maximum current density at which this asymmetrical state occurs is the critical current density (CCD). The electrochemical stability of the Li metal anode can be evaluated using this CCD value. As shown in Figure 8, for Example 4 (y=0.025), Example 5 (y=0.05), and Example 6 (y=0.075), the CCD was 2.4 mA / cm². 2 , 2.1mA / cm 2 , 1.5mA / cm 2 In Comparative Example 3, the CCD was 1.5 mA / cm². 2 The values ​​were lower than those of Examples 4 and 5. In other words, it was confirmed that the addition of Al improved the CCD. Although not bound by theory, it is thought that the Al in the solid electrolyte was conducted via the Li ion conduction pathway, and that precipitates containing Al were formed at the interface with the Li metal in contact with the solid electrolyte, thereby improving the electrochemical stability.

[0105] (7) Impedance measurement Figure 9 shows the impedance measurement results for Examples 4 to 6 and Comparative Example 3 during critical current density measurement. The resistance of the solid electrolyte-Li metal interface corresponding to 1 MHz to 1 kHz, and the charge transfer resistance of the Li metal corresponding to 1 kHz or less, are observed. Tables 6A and 6B show the resistance values ​​(Table 6A) and their increase rates (Table 6B) of the solid electrolyte-Li metal interface for Examples 4 to 6 and Comparative Example 3. As the current density increased, the resistance value increased significantly in Comparative Example 3, whereas it did not increase as much in Examples 4 to 6. Specifically, 1.5 mA / cm 2The increase in resistance at the solid electrolyte-Li metal interface with respect to current density was a maximum of 21.3% in Examples 4 to 6, compared to 53.8% in Comparative Example 3. In particular, Example 4 showed a small increase of less than 1%. These results confirm that the addition of Al suppresses the increase in interfacial resistance at the interface between the solid electrolyte and Li metal. Thus, adding Al to the solid electrolyte suppresses reductive decomposition due to contact between the solid electrolyte and the Li metal anode, improving the increase in interfacial resistance and interfacial non-uniformity caused by decomposition products. Therefore, it is considered that Li dendrite growth caused by interfacial non-uniformity can be suppressed. As described above, the addition of Al suggests improved electrochemical stability for the Li metal anode, and its application to all-solid-state batteries is expected.

[0106] [Table 6A]

[0107] [Table 6B] [Explanation of Symbols]

[0108] 1: Current collector 2:Li foil 3: Solid electrolyte layer (with added aluminum) 4: Solid electrolyte layer (without added aluminum)

Claims

1. It has an argyrodite-type crystal structure, Chemical formula Li ７－ｘ－３ｙ Al ｙ PS ６－ｘ Ha ｘ It is represented as, In the above chemical formula, The Ha is one or more elements selected from halogen elements, Satisfying 0 < x < 2.5 and 0 < y < 0.1, The maximum peak intensity I measured by X-ray diffraction at 29.9° ≤ 2θ ≤ 30.1° １ Maximum peak intensity I for 30.15° ≤ 2θ ≤ 30.25° ２ Ratio of peak intensity I ２ / I １ A solid electrolyte in which the ratio is 0.15 or less.

2. The solid electrolyte according to claim 1, wherein Al is present at the 48h site of the argyrodite crystal structure.

3. The solid electrolyte according to claim 1, wherein in the chemical formula, Ha comprises Cl and Br.

4. The solid electrolyte according to claim 1, wherein when the D50 of the solid electrolyte is 5 μm or more and 50 μm or less, the solid electrolyte has an ionic conductivity of 12 mS / cm or more.

5. The solid electrolyte according to claim 1, wherein when the D50 of the solid electrolyte is 0.1 μm or more and less than 5 μm, the solid electrolyte has an ionic conductivity of 7 mS / cm or more.

6. The solid electrolyte has a critical current density of 1.8 mA / cm ２ or more, and is the solid electrolyte according to claim 1.

7. The solid electrolyte according to claim 1, wherein the rate of change in the resistance value at the interface between the solid electrolyte and the Li metal is 25% or less.

8. A method for producing a solid electrolyte according to any one of claims 1 to 7, A step of mixing a lithium source, an aluminum source, a phosphorus source, a sulfur source, and a halogen source to obtain a mixture, The steps include firing the mixture at a temperature of 450°C or lower, A method for producing a solid electrolyte, including the following:

9. It has a positive electrode, a negative electrode, and a solid electrolyte layer sandwiched between the positive electrode and the negative electrode. A solid-state battery wherein the solid electrolyte layer comprises the solid electrolyte described in any one of claims 1 to 7.

10. The all-solid-state battery according to claim 9, wherein the negative electrode is a Li metal negative electrode.

11. The all-solid-state battery according to claim 9, wherein a compound containing Al is included between the solid electrolyte layer and the negative electrode.

12. The all-solid-state battery according to claim 11, wherein the compound further comprises Li.

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