Solid electrolyte and preparation method thereof

Abstract

The invention relates to a solid electrolyte and a preparation method thereof. The solid electrolyte includes sulfide-based solid electrolyte particles and a lithium-metal-oxide disposed on a surface of the particles, wherein a full width at half maximum of a main peak in X-ray diffraction analysis of the solid electrolyte is 0.160 at most.

Description

Technical Field

[0001] Disclosed are a solid electrolyte and a method for preparing the same. Background Art

[0002] Recently, there have been reports that batteries using liquid electrolytes have the risk of explosion, so the development of all-solid-state batteries has been actively progressing. However, compared with liquid electrolytes, solid electrolytes have low ion conductivity, resistance at the interface with solid particles of the positive electrode active material in the battery, and deterioration of ion conductivity due to the formation of a depletion layer through solid-solid bonding.

[0003] To solve these problems, the conventionally used technology is to dope various elements into the positive electrode active material particles used with the solid electrolyte, and to form a buffer layer including elements such as B, Nb, and Zr on the surface of the positive electrode active material particles. However, these methods may be difficult to mass produce, causing cost and environmental problems, and still have limitations in improving the performance of all-solid-state batteries.

[0004] Accordingly, there is a need to develop solid electrolytes with high ionic conductivity and appropriate particle size distribution. Summary of the invention

[0005] Provided are a solid electrolyte having uniform particle size distribution, high crystallinity and high ion conductivity and improved moisture stability and a method for preparing the same.

[0006] In an embodiment, the solid electrolyte includes sulfide solid electrolyte particles and lithium-metal-oxide on the surface of the sulfide solid electrolyte particles, wherein the full width at half maximum (FWHM) of the main peak in X-ray diffraction (XRD) analysis of the solid electrolyte is less than or equal to 0.160.

[0007] Another embodiment provides a method for preparing a solid electrolyte, including mixing sulfide-based solid electrolyte particles and lithium-metal-oxide and performing a heat treatment at 250°C to 350°C.

[0008] The solid electrolyte according to the embodiment has uniform particle size distribution, high crystallinity and ion conductivity, and improved moisture stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 1 and 2 are particle size distribution curves of the solid electrolytes of Example 2, Comparative Example 1 and Comparative Example 2.

[0010] Figure 2 1 is the particle size distribution curve of the solid electrolyte of Example 2, Example 3, Example 4 and Comparative Example 2.

[0011] Figure 3 The X-ray diffraction patterns of the solid electrolytes of Examples 1, 2 and 5 and Comparative Examples 1 and 2 and lithium-zirconium-oxide (LZO).

[0012] Figure 4 The X-ray diffraction patterns of the solid electrolytes of Examples 2 to 4 and Comparative Example 1, LZO, lithium-aluminum-oxide (LAO), and lithium-titanium-oxide (LTO).

[0013] Figure 5 It is a graph showing the half width at half maximum of the main peak (bar graph, left vertical axis) and the ion conductivity (dashed line graph, right vertical axis) in X-ray diffraction analysis of the solid electrolytes of Examples 1 to 5 and Comparative Examples 1 and 2.

[0014] Figure 6 : is a graph for evaluating the moisture stability of the solid electrolytes of Examples 1, 2, and 5 and Comparative Example 2, and is a graph showing the ion conductivity before and after standing for 3 days.

[0015] Figure 7 It is a graph for evaluating the moisture stability of the solid electrolytes of Examples 2 to 4 and Comparative Example 2, and is a graph showing the ion conductivity before and after being left for 3 days. DETAILED DESCRIPTION

[0016] Hereinafter, specific embodiments will be described in detail so that those skilled in the art can easily implement them. However, the present disclosure can be implemented in many different forms and is not to be construed as limited to the example embodiments set forth herein.

[0017] The terms used herein are for describing the embodiments only and are not intended to limit the present invention. A singular expression includes a plural expression unless the context clearly indicates otherwise.

[0018] As used herein, "combinations thereof" means mixtures, laminates, composites, copolymers, alloys, blends, reaction products, and the like, of the components.

[0019] In this document, it should be understood that terms such as "includes," "comprising," or "having" are intended to indicate the presence of the stated features, quantities, steps, elements, or combinations thereof, but do not exclude the possibility of the presence or addition of one or more other features, quantities, steps, elements, or combinations thereof.

[0020] In the drawings, the thickness of layers, films, panels, regions, etc., are exaggerated for clarity, and the same reference numerals denote the same elements throughout the specification. It will be understood that when an element (such as a layer, film, region, or substrate) is referred to as being "on" another element, it may be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being "directly on" another element, there are no intervening elements.

[0021] In addition, the "layer" herein includes not only a shape formed on the entire surface when viewed from a plan view but also a shape formed on a partial surface.

[0022] In addition, the average particle size can be measured by methods well known to those skilled in the art, for example, it can be measured by a particle size analyzer, or it can be measured by a transmission electron microscope photo or a scanning electron microscope photo. Alternatively, the size can be measured by using a dynamic light scattering method, performing data analysis, counting the particles of each particle size range, and calculating the average particle size value. The average particle size can be measured with a microscope image or a particle size analyzer, and can mean the diameter (D50) of the particles with a cumulative volume of 50% by volume in the particle size distribution.

[0023] In this document, “or” is not interpreted as being exclusive. For example, “A or B” is interpreted as including A, B, A+B, etc.

[0024] Preparation method of solid electrolyte

[0025] An embodiment provides a method for preparing a solid electrolyte, the method including mixing sulfide-based solid electrolyte particles and lithium-metal-oxide and performing a heat treatment at 250°C to 350°C.

[0026] In general, solid electrolytes should have an appropriate particle size distribution to show excellent ion conductivity in batteries and achieve high energy density, and should achieve excellent particle fluidity, that is, high-density electrode plates and electrolyte membranes. At the same time, solid electrolytes should be able to show improved ion conductivity by maintaining high crystallinity.

[0027] Sulfide-based solid electrolytes are materials that can achieve high ion conductivity among various solid electrolytes. Since the particles are severely agglomerated or the particle size is large just after synthesis at high temperature, they need to be crushed. However, the ion conductivity decreases due to the crushing operation, and if heat treatment is performed to increase the ion conductivity, the particles re-aggregate and grow.

[0028] In an embodiment, in order to solve these problems, heat treatment is performed at a temperature range of 250°C to 350°C while coating lithium-metal-oxide on the crushed sulfide solid electrolyte particles. According to this method, the crystallinity of the solid electrolyte is increased, the ion conductivity is improved, and particle aggregation and growth are suppressed, thereby achieving a high-density electrode plate and electrolyte membrane with an appropriate particle size distribution. This solid electrolyte also has high moisture stability and can improve the capacity characteristics, initial charge / discharge efficiency, and cycle life characteristics of the battery.

[0029] In the heat treatment, for example, if the heat treatment is performed at a temperature of less than 250°C, the crystallinity may not be sufficiently increased, and thus high ion conductivity cannot be achieved. In addition, when the heat treatment is performed at a temperature of more than 350°C, aggregation and growth of particles occur, so that an appropriate particle size distribution is not obtained, and thus the crystallinity decreases. The higher the heat treatment temperature at a higher temperature, the more coating agent is required, which may lead to a problem in which the ion conductivity is deteriorated instead.

[0030] The heat treatment may be carried out in a nitrogen atmosphere or an inert gas (such as He, Ar or N 2 ). In addition, the heat treatment can be performed for 0.5 to 10 hours, for example, 1 to 8 hours. When the heat treatment is performed under these conditions, the prepared solid electrolyte can achieve an appropriate particle size distribution while showing excellent ionic conductivity.

[0031] Based on 100 parts by weight of sulfide solid electrolyte particles, lithium-metal-oxide can be mixed in an amount of 0.01 to 3 parts by weight (e.g., 0.01 to 2 parts by weight, 0.01 to 1 part by weight, 0.01 to 0.8 parts by weight, or 0.1 to 0.75 parts by weight). When mixed in this content range, the prepared solid electrolyte can have an appropriate particle size distribution without particle size aggregation while showing high ionic conductivity. In particular, if 0.01 to 0.8 parts by weight of lithium-metal-oxide is mixed based on 100 parts by weight of sulfide solid electrolyte particles, the surface of the sulfide solid electrolyte is uniformly coated with an appropriate amount of lithium-metal-oxide to further improve the ionic conductivity of the solid electrolyte.

[0032] A method for preparing a solid electrolyte according to an embodiment includes: for example, mixing a sulfur-containing raw material and heat-treating it to synthesize a sulfide solid electrolyte; crushing the synthesized sulfide solid electrolyte; and mixing the crushed sulfide solid electrolyte particles and lithium-metal-oxide and heat-treating them at 250°C to 350°C to obtain a solid electrolyte in which the lithium-metal-oxide is disposed on the surface of the sulfide solid electrolyte particles.

[0033] In an embodiment, mixing sulfide solid electrolyte particles and lithium-metal-oxide and heat treating them can be referred to as a dry coating method. That is, the method for preparing a solid electrolyte according to an embodiment is a method for coating sulfide solid electrolyte particles, and can be, for example, a method for dry coating lithium-metal-oxide on the surface of sulfide solid electrolyte particles. Unlike other oxide inorganic solid electrolytes or positive electrode active materials, sulfide solid electrolytes are difficult to wet-coat and are susceptible to high-temperature heat treatment, so sulfide solid electrolytes need to be designed for harsh coating conditions. In addition, generally, the wet coating method uses an alcohol solvent or an alkoxide raw material, and accordingly, the carbon component will remain locally after coating, which may have an adverse effect on conductivity, etc. The method for preparing a solid electrolyte according to an embodiment has different conditions from coating other types of solid electrolyte particles, and is also different from general wet coating.

[0034] Sulfide solid electrolyte particles

[0035] The sulfide solid electrolyte particles may be, for example, Li 2 SP 2 S 5 , Li 2 SP 2 S 5 -LiX (X is a halogen element, such as I or Cl), Li 2 SP 2 S 5 -Li 2 O. Li 2 SP 2 S 5 -Li 2 O-LiI、Li 2 S-SiS 2 , Li 2 S-SiS 2 -LiI, Li 2 S-SiS 2 -LiBr, Li 2 S-SiS 2 -LiCl, Li 2 S-SiS 2 -B 2 S 3 -LiI, Li 2 S-SiS 2 -P 2 S 5 -LiI, Li 2 SB 2 S 3 , Li 2 SP 2 S5 -Z m S n (m and n are each an integer and Z is Ge, Zn or Ga), Li 2 S-GeS 2 , Li 2 S-SiS 2 -Li 3 PO 4 , Li 2 S-SiS 2 -Li p MO q (p and q are integers and M is P, Si, Ge, B, Al, Ga or In) or a combination thereof.

[0036] Such a sulfide-based solid electrolyte can be prepared by mixing Li 2 S and P 2 S 5 and optionally heat-treated. Within the above mixing ratio range, a sulfide-based solid electrolyte having excellent ion conductivity can be prepared. By further including SiS 2 ,GeS 2 , B 2 S 3 As other components, the ion conductivity can be further improved.

[0037] The method of mixing sulfur-containing raw materials to prepare sulfide-based solid electrolytes can be mechanical grinding or solution method. Mechanical grinding is a method of forming particles and mixing the starting materials by placing the starting materials into a reactor and vigorously stirring with a ball mill. In the case of using a solution method, the starting materials can be mixed in a solvent to obtain a solid electrolyte in the form of a precipitate. In addition, in the case of heat treating the mixture, the crystals of the solid electrolyte can be more solid and the ion conductivity can be improved. For example, a sulfide-based solid electrolyte can be prepared by mixing sulfur-containing raw materials and heat treating them twice or more. In this case, a sulfide-based solid electrolyte with high ion conductivity and robustness can be prepared.

[0038] The sulfide-based solid electrolyte particles according to the embodiment can be prepared, for example, by: a first heat treatment, mixing a sulfur-containing raw material and firing at 120°C to 350°C; and a second heat treatment, mixing the result of the first heat treatment and firing at 350°C to 800°C. The first heat treatment and the second heat treatment may be performed under an inert gas or nitrogen atmosphere, respectively. The first heat treatment may be performed for 1 hour to 10 hours, and the second heat treatment may be performed for 5 hours to 20 hours. The first heat treatment may be ground into small particles, and the final solid electrolyte may be synthesized by grinding through the second heat treatment. By this two or more heat treatments, a sulfide-based solid electrolyte with high ionic conductivity and high performance may be obtained, and this solid electrolyte may be suitable for mass production. The temperature of the first heat treatment may be, for example, 150°C to 330°C or 200°C to 300°C, and the temperature of the second heat treatment may be, for example, 380°C to 700°C or 400°C to 600°C.

[0039] For example, the sulfide-based solid electrolyte particles may include argyrodite-type sulfides. Argentodite-type sulfides may be, for example, of the chemical formula Li a M b P c S d A e (wherein a, b, c, d and e are all equal to or greater than 0 and equal to or less than 12, M is Ge, Sn, Si or a combination thereof, and A is F, Cl, Br or I), and as a specific example, it can be a chemical formula Li 7-x PS 6- x A x (wherein x is equal to or greater than 0.2 and equal to or less than 1.8, and A is F, Cl, Br or I). Specifically, the argyrodite-type sulfide may be Li 3 PS 4 , Li 7 P 3 S 11 , Li 7 PS 6 , Li 6 PS 5 Cl, Li 6 PS 5 Br, Li 5.8 PS 4.8 Cl 1.2 , Li 6.2 PS 5.2 Br 0.8 wait.

[0040] The sulfide-based solid electrolyte particles including this argyrodite-type sulfide may have a thickness of approximately 10 -4 ~10 -2S / cm range, which is the ionic conductivity of a general liquid electrolyte at room temperature, and forms a close bond between the positive electrode active material and the solid electrolyte without causing a decrease in ionic conductivity, and further forms a close interface between the electrode layer and the solid electrolyte layer. An all-solid-state battery including it can have improved battery performance (such as rate performance, coulombic efficiency, and cycle life characteristics).

[0041] For example, by mixing lithium sulfide and phosphorus sulfide and optionally lithium halide, an argyrodite-type sulfide-based solid electrolyte can be prepared. After mixing them, heat treatment can be performed. The heat treatment can include, for example, two or more heat treatment steps.

[0042] The method for preparing a solid electrolyte according to an embodiment includes: preparing argyrodite-type sulfide solid electrolyte particles by mixing lithium sulfide, phosphorus sulfide and optionally lithium halide, mixing the prepared argyrodite-type sulfide solid electrolyte particles and lithium-metal-oxide, and heat treating at 250°C to 350°C.

[0043] Specifically, the preparation method of the solid electrolyte includes: preparing argyrodite-type sulfide solid electrolyte particles by mixing lithium sulfide, phosphorus sulfide and optional lithium halide and heat treating them; crushing the prepared argyrodite-type sulfide solid electrolyte particles; and mixing the crushed argyrodite-type sulfide solid electrolyte particles and lithium-metal-oxide and heat treating them at 250°C to 350°C.

[0044] Herein, in the step of preparing an argyrodite-type sulfide solid electrolyte, the heat treatment may include: for example, a first heat treatment in which the raw materials are mixed and fired at 120°C to 350°C; and a second heat treatment in which the result of the first heat treatment is mixed again and fired at 350°C to 800°C.

[0045] The average particle size (D50) of the sulfide solid electrolyte particles according to the embodiment may be less than or equal to 5.0 μm, for example, 0.1 μm to 5.0 μm, 0.1 μm to 4.0 μm, 0.1 μm to 3.0 μm, 0.5 μm to 2.0 μm or 0.1 μm to 1.5 μm. Sulfide solid electrolyte particles having this particle size range can effectively pass between the positive electrode active material, and have excellent contact with the positive electrode active material and excellent connectivity between the solid electrolyte particles. The average particle size of the sulfide solid electrolyte particles can be measured using a microscope image, and for example, the particle size distribution can be obtained by measuring the size of about 20 particles in a scanning electron microscope image, and D50 can be calculated therefrom.

[0046] Lithium-metal-oxide

[0047] Lithium-metal-oxide according to an embodiment may refer to an oxide including lithium and other metals other than lithium. In this article, metal is a concept including conventional metals, transition metals and semimetals. In lithium-metal-oxide, the metal may be one or more elements selected from, for example, Al, B, Ca, Ce, Cr, Fe, Mg, Mo, Nb, Si, Sn, Sr, Ta, V, W and Zr.

[0048] The lithium-metal-oxide may be amorphous. According to an embodiment, when amorphous lithium-metal-oxide is coated on sulfide-based solid electrolyte particles, the prepared solid electrolyte can achieve higher ion conductivity and interface resistance with other solid particles in the battery (such as positive electrode active materials), prevent aggregation of solid electrolyte particles, improve ion conductivity, and improve capacity characteristics, cycle life characteristics, etc.

[0049] The lithium-metal-oxide mixed in the preparation method of the solid electrolyte may be in the form of particles, and its average particle size (D50) may be, for example, 0.01 μm to 1.0 μm, 0.01 μm to 0.9 μm, 0.01 μm to 0.8 μm, or 0.01 μm to 0.5 μm. The average particle size of the lithium-metal-oxide may be smaller than the average particle size of the sulfide solid electrolyte particles. When using a lithium-metal-oxide having such a particle size range, it can be uniformly coated on the surface of the sulfide solid electrolyte particles, fully increasing the ionic conductivity of the solid electrolyte and improving moisture stability.

[0050] Solid Electrolyte

[0051] In an embodiment, the solid electrolyte includes sulfide solid electrolyte particles and lithium-metal-oxide on surfaces of the sulfide solid electrolyte particles, wherein the full width at half maximum (FWHM) of a main peak in an X-ray diffraction (XRD) analysis of the solid electrolyte is less than or equal to 0.160.

[0052] Lithium-metal-oxide may exist in the form of a film or island on the surface of the sulfide-based solid electrolyte particles. For example, the solid electrolyte according to the embodiment may include sulfide-based solid electrolyte particles and a coating on the surface of the particles, and the coating may include lithium-metal-oxide.

[0053] The solid electrolyte according to the embodiment is in a form in which lithium-metal-oxide is coated on the surface of sulfide solid electrolyte particles, and the crystallinity of the solid electrolyte is high enough to achieve excellent ionic conductivity, while obtaining a suitable particle size distribution without particle aggregation. As the crystallinity of the solid electrolyte increases or the size of the crystal increases, the full width at half maximum (FWHM) of the main peak in the X-ray diffraction analysis can be reduced, and the solid electrolyte according to the embodiment has a full width at half maximum of the main peak less than or equal to 0.160. In this article, the main peak refers to the peak with the highest diffraction intensity in the X-ray diffraction analysis. The full width at half maximum of the main peak in the X-ray diffraction analysis of the solid electrolyte according to the embodiment may be, for example, less than or equal to 0.159, or less than or equal to 0.155. It is known that if the full width at half maximum decreases, that is, if the crystallinity increases, the ionic conductivity will improve, and for example, it should be understood that as the size of the crystal increases, the grain boundaries will decrease and the ionic conductivity will be improved.

[0054] As described above, the particles of the sulfide-based solid electrolyte may aggregate or have a large particle size just after synthesis, and when a process such as pulverization is performed to adjust the particle size to a suitable battery, its crystallinity may decrease and the ionic conductivity may decrease. In the solid electrolyte according to the embodiment, by heating the sulfide-based solid electrolyte particles within a specific temperature range while coating the lithium-metal-oxide, the crystallinity can be increased and the half-maximum full width of the main peak can be adjusted to less than or equal to 0.160, while having a uniform particle size distribution without particle aggregation or growth, to improve the ionic conductivity.

[0055] The detailed description of the sulfide-based solid electrolyte particles and the lithium-metal-oxide is omitted here because they have been described above.

[0056] In the solid electrolyte according to the embodiment, based on 100wt% of the solid electrolyte, lithium-metal-oxide may be included in an amount of 0.01wt% to 3wt% (e.g., 0.01wt% to 2wt%, 0.01wt% to 1wt%, 0.01wt% to 0.8wt% or 0.1wt% to 1.0wt%). When the lithium-metal-oxide is the above content, the solid electrolyte may show an appropriate particle size distribution without particle aggregation while achieving high ionic conductivity. In particular, if the content of lithium-metal-oxide satisfies the range of 0.01wt% to 0.8wt% based on 100wt% of the solid electrolyte, the surface of the sulfide solid electrolyte particles may be uniformly coated with lithium-metal-oxide. Accordingly, the ionic conductivity and moisture stability of the solid electrolyte may be further improved, and the efficiency and cycle life characteristics of the battery may be further improved.

[0057] In the solid electrolyte, the lithium-metal-oxide located or coated on the surface of the sulfide-based solid electrolyte particles may be in an amorphous form. When coated with amorphous lithium-metal-oxide, the solid electrolyte exhibits excellent ion conductivity and reduces interface resistance, thereby improving battery performance.

[0058] The average particle size (D50) of the solid electrolyte may be 0.1 μm to 5.0 μm, for example, 0.1 μm to 4.0 μm, 0.1 μm to 3.0 μm, 0.5 μm to 2.0 μm or 0.1 μm to 1.5 μm. Such a solid electrolyte can effectively pass between the positive electrode active materials, and has excellent contact with the positive electrode active materials and excellent connectivity between the solid electrolyte particles. The average particle size of the solid electrolyte can be measured using a microscope image, and, for example, the particle size distribution can be obtained by measuring the size of about 20 particles in a scanning electron microscope image, and D50 can be calculated therefrom.

[0059] The solid electrolyte according to the embodiment is characterized in that it has a uniform particle size distribution without particle aggregation. For example, the (D90-D10) / D50 value in the particle size distribution of the solid electrolyte may be greater than 1 and less than or equal to 5, for example, 1.1-4.0, 1.1-3.0 or 1.1-2.0. The (D90-D10) / D50 value may indicate the width of the peak in the particle size distribution diagram of the solid electrolyte, specifically, the horizontal axis represents the particle size (μm) and the vertical axis represents the cumulative volume of the particles. %. The smaller the corresponding number, the narrower the peak width of the graph, which can be interpreted as having a uniform particle size. In this article, D10 means the diameter of the particles with a cumulative volume of 10% by volume in the particle size distribution, D50 means the diameter of the particles with a cumulative volume of 50% by volume in the particle size distribution, and D90 means the diameter of the particles with a cumulative volume of 90% by volume in the particle size distribution.

[0060] The D10 of the solid electrolyte may be, for example, 0.05 μm to 0.7 μm, 0.05 μm to 0.6 μm, 0.1 μm to 0.5 μm, or 0.2 μm to 0.4 μm. In addition, the D90 of the solid electrolyte may be, for example, 0.9 μm to 5.0 μm, 1.0 μm to 4.0 μm, 1.0 μm to 3.0 μm, or 1.2 μm to 2.0 μm. When the solid electrolyte has such a particle size distribution, the battery performance can be improved by achieving high energy density while achieving excellent ionic conductivity.

[0061] The solid electrolyte may have an ionic conductivity greater than or equal to 2.9 mS / cm, for example, 2.9 mS / cm to 5.0 mS / cm, 3.0 mS / cm to 4.5 mS / cm, or 3.0 mS / cm to 4.0 mS at 25° C. The ionic conductivity may be measured by electrochemical impedance spectroscopy (EIS).

[0062] Hereinafter, embodiments of the present invention and comparative examples are described. However, it should be understood that the examples are for illustrative purposes and should not be construed as limiting the present invention.

[0063] Example 1

[0064] 1. Preparation of sulfide solid electrolyte particles

[0065] Argentum-type sulfide solid electrolytes were synthesized by the method described later. All processes before and after mixing the raw materials and heat treatment were carried out in a glove box under an argon atmosphere. Specifically, lithium sulfide (Li 2 S), phosphorus pentasulfide (P 2 S 5 ) and lithium chloride (LiCl) were mixed in a molar ratio of 2.5:0.5:1 to prepare a mixed powder. The mixed powder was uniformly mixed with a Henschel mixer and then preliminarily fired at 250° C. for 5 hours in a tube furnace with argon flowing at a constant speed of 8 SLM.

[0066] The preliminarily fired powder was uniformly mixed again with a Henschel mixer, sieved, and then secondary fired at 500°C for 10 hours in a tube furnace with argon flowing at a constant speed of 8 SLM. The secondary fired powder was crushed and sieved to obtain Li 6 PS 5 Cl sulfide-based solid electrolyte particles The obtained sulfide-based solid electrolyte particles had a size (D50) of 0.85 μm.

[0067] 2. Coating of sulfide solid electrolyte particles

[0068] 100 parts by weight of the prepared sulfide solid electrolyte particles and 0.25 parts by weight of lithium-zirconium-oxide (LZO) as a coating agent were mixed with a Henschel mixer, the lithium-zirconium-oxide having a D50 of 0.13 μm and amorphous as a result of X-ray diffraction analysis. The mixed powder was heat treated at 250° C. for 5 hours in a tube furnace where argon gas flowed at a constant speed of 8 SLM. Thus, a solid electrolyte coated with lithium-zirconium-oxide on the surface of the sulfide solid electrolyte particles was prepared.

[0069] Example 2

[0070] A solid electrolyte was prepared in the same manner as in Example 1, except that the content of the coating agent was changed to 0.5 parts by weight.

[0071] Example 3

[0072] A solid electrolyte was prepared in the same manner as in Example 2, except that amorphous lithium-aluminum-oxide (LAO; D50=0.06 μm) was used as a coating agent.

[0073] Example 4

[0074] A solid electrolyte was prepared in the same manner as in Example 2, except that amorphous lithium-titanium-oxide (LTO; D50=0.06 μm) was used as a coating agent.

[0075] Example 5

[0076] A solid electrolyte was prepared in the same manner as in Example 1, except that the content of the coating agent was changed to 1.0 part by weight.

[0077] Comparative Example 1

[0078] A solid electrolyte was prepared in the same manner as in Example 1, except that the coating process was performed by heat treatment at 250° C. for 5 hours without using a coating agent.

[0079] Comparative Example 2

[0080] A solid electrolyte was prepared in the same manner as in Example 1, except that the coating process was not performed. In other words, the sulfide-based solid electrolyte particles themselves prepared in Step 1 of Example 1 were used as the solid electrolyte.

[0081] The various solid electrolytes of Examples 1 to 5 and Comparative Examples 1 and 2 were designed as shown in Table 1.

[0082] (Table 1)

[0083]

[0084] Evaluation Example 1: Evaluation of Particle Size Distribution

[0085] The particle size distribution of the solid electrolytes of Examples 1 to 5 and Comparative Examples 1 and 2 was measured respectively. Xylene from which water was removed was used as a solvent, and the particle size distribution was measured using a particle size analyzer using laser diffraction.

[0086] Figure 1The particle size distribution curves of the solid electrolytes of Example 2 (LZO 0.5), Comparative Example 1 (250°C-None) and Comparative Example 2 (None-None) are shown, and Figure 2 The particle size distribution curves of the solid electrolytes of Example 2 (LZO 0.5), Example 3 (LAO 0.5), Example 4 (LTO 0.5) and Comparative Example 2 (None-None) are shown. Figure 1 and Figure 2 In the particle size distribution curve of , the horizontal axis is the particle size (μm), and the vertical axis is the cumulative volume of the particles (volume %).

[0087] In addition, in each particle size distribution of the solid electrolytes of Examples 1 to 5 and Comparative Examples 1 and 2, the particle size D10 of 10% cumulative volume, the particle size D50 of 50% cumulative volume, and the particle size D90 of 90% cumulative volume are shown in Table 2. In addition, (D90-D10) / D50 was calculated to compare the width of each particle size distribution, and then provided as a span in Table 2.

[0088] (Table 2)

[0089] Unit: μm D10 D50 D90 span Example 1 0.286 0.942 1.564 1.4 Example 2 0.387 0.938 1.556 1.2 Example 3 0.333 0.919 1.454 1.2 Example 4 0.286 0.884 1.492 1.4 Example 5 0.355 1.112 2.327 1.8 Comparative Example 1 0.369 1.144 12.890 10.9 Comparative Example 2 0.260 0.848 1.644 1.6

[0090] refer to Figure 1 , compared with Comparative Example 2 before heat treatment, Comparative Example 1 in which only heat treatment was performed without using a coating agent showed several peaks, which confirmed the agglomeration of solid electrolyte particles. In addition, Table 1 shows that D90 and span of Comparative Example 1 increased significantly compared with Comparative Example 2 due to particle agglomeration. In other words, when heat treatment is additionally performed in order to increase the ionic conductivity of the pulverized sulfide-based solid electrolyte particles, etc., there is a problem of agglomeration between particles.

[0091] On the contrary, reference Figure 1 and Figure 2 As shown in Table 2, Examples 1 to 5 show very uniform particle size distribution without agglomeration or growth of particles after coating.

[0092] In addition, the particle size distribution analysis shows that in the final solid electrolyte, the coating agent particles do not exist independently from the sulfide-based solid electrolyte particles, but are uniformly coated on the surfaces of the sulfide-based solid electrolyte particles.

[0093] Evaluation Example 2: X-ray diffraction analysis

[0094] After performing X-ray diffraction analysis on the solid electrolytes and LZO of Examples 1, 2 and 5 and Comparative Examples 1 and 2, the results are Figure 3In addition, after performing another X-ray diffraction analysis of the solid electrolytes of Examples 2 to 4 and Comparative Example 1 and LZO, LAO and LTO, the results are Figure 4 In addition, in the X-ray diffraction analysis of the solid electrolytes of Examples 1 to 5 and Comparative Examples 1 and 2, the full width at half maximum of the peak (main peak) with the highest diffraction intensity at about 30° was calculated, and then Figure 5 Shown in a bar graph.

[0095] refer to Figure 3 Since no separate peak of LZO itself was shown in Examples 1, 2 and 5 in which LZO coating was performed, and even Example 5 in which a high content of coating agent was used did not show an LZO peak, it was confirmed that the lithium-zirconium-oxide on the surface of the sulfide solid electrolyte particles had a very low degree of crystallinity, that is, existed in an amorphous state. Figure 4 The results of using different types of coating agents are shown in , and since Example 3 does not show a LAO peak, and Example 4 does not show a LTO peak, it is confirmed that the lithium-metal-oxide on the surface of the sulfide-based solid electrolyte particles is amorphous. Summarizing Evaluation Examples 1 and 2, the lithium-metal-oxide is uniformly coated on the surface of the sulfide-based solid electrolyte particles in an amorphous manner.

[0096] In addition, Figure 3 In the embodiment 1, 2 and 5 and the comparative example 1 show higher diffraction peaks than the comparative example 2 which is not subjected to the coating process, wherein the embodiment 1, 2 and 5 show higher diffraction peaks than the comparative example 1 which is heat-treated without adding the coating agent. Accordingly, when the amorphous lithium-metal-oxide is coated and then heat-treated within a specific temperature range, the coating agent inhibits the agglomeration of the solid electrolyte particles and promotes the growth of crystals.

[0097] Refer to the full width at half maximum of the main peak shown in the form of a bar graph Figure 5 , Comparative Example 2 before heat treatment (i.e., the crushed sulfide-based solid electrolyte particles before heat treatment) shows a high half-peak full width of 0.175, but Examples 1 to 5 show a half-peak full width of less than or equal to 0.155, and accordingly, the half-peak full width is significantly reduced, so that the crystal size is larger and the crystallinity is higher. Compared with Comparative Example 2, Comparative Example 1, which is heat treated at 250°C without adding a coating agent, shows a slight decrease in the half-peak full width, which indicates crystal growth, but the increase in crystal size is less than that of the embodiment. As shown in Evaluation Example 1, Comparative Example 1 shows agglomeration of particles, which results in a loss of thermal energy and a less increase in crystal size.

[0098] Accordingly, when sulfide-based solid electrolyte particles are appropriately coated with amorphous lithium-metal-oxide on the surface and heat-treated within a specific temperature range, aggregation of the solid electrolyte particles is suppressed while the crystal size increases.

[0099] Evaluation Example 3: Evaluation of ion conductivity

[0100] 0.15 g of each of the solid electrolytes of Examples 1 to 5 and Comparative Examples 1 and 2 were charged and heated at 40 kgf / cm 2 The torque battery cell was manufactured by pressing under a pressure of . The ionic conductivity of the manufactured battery cell was calculated by electrochemical impedance spectroscopy (EIS), and the results were Figure 5 The dotted line diagram shows the amplitude of 10mV and the frequency of 0.1Hz~10 6 EIS was performed at a frequency of Hz in an air atmosphere at 25° C. The resistance value was obtained from the arc of the Nyquist plot by ESI, and the ion conductivity was calculated in consideration of the thickness and area of ​​each battery cell.

[0101] refer to Figure 5 , Comparative Example 1 in which an additional heat treatment was performed showed a low full width at half maximum (FWHM) and improved crystallinity compared to Comparative Example 2 in which the coating process was not performed, but particle agglomeration occurred, and thus ion conductivity decreased.

[0102] Compared with the comparative example, the embodiment shows improved ionic conductivity. Examples 2 to 4, in which the same amount of coating agent is used but different types, show that the crystal growth and ionic conductivity of the solid electrolyte are different depending on the type of coating agent. The reason is that the coating agent D50 of Example 2 is 0.13μm, but the coating agent D50 of the coating agent of Examples 3 and 4 is 0.06μm, so coatings with different thicknesses or shapes are formed on the surface of the sulfide solid electrolyte particles. Example 5, in which the content of the coating agent is increased, shows a slight decrease in ionic conductivity due to the agglomeration of the sulfide solid electrolyte particles or the resistance on their surface.

[0103] Evaluation Example 4: Evaluation of Moisture Stability

[0104] The solid electrolytes of Examples 1 to 5 and Comparative Examples 1 and 2 were left in a dry room at a dew point of -45°C for 3 days, and then the ion conductivity was measured in the same manner as in Evaluation Example 3. Figure 6 The results of Examples 1, 2 and 5 and Comparative Example 2 are shown, and Figure 7 The results of Examples 2 to 4 and Comparative Example 2 are shown. Figure 6 and 7 The ionic conductivity before placement is shown as a black bar graph and the ionic conductivity after placement is shown as an orange bar graph.

[0105] refer to Figure 6 , when the surface of the solid electrolyte is well protected by the coating agent, the stability to moisture is enhanced. As shown in Example 1, when the content of the coating agent is small, since the solid electrolyte surface is coated with a thinner coating or even exposed in large quantities, it leads to insufficient protection, and the ion conductivity is initially high but significantly reduced after being placed. In Example 5, in which an excessive amount of the coating agent is used, the coating agent itself agglomerates after the heat treatment and the exposure amount of the solid electrolyte surface increases, thereby reducing the ion conductivity after being placed.

[0106] refer to Figure 7 , when the solid electrolyte is appropriately coated with lithium-metal-oxide on the surface to protect the surface, the moisture stability is improved compared with Comparative Example 2 where no coating is performed.

[0107] While the invention has been described in conjunction with what are presently considered to be practical example embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but on the contrary is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the claims.

Claims

1. A solid electrolyte comprising sulfide-based solid electrolyte particles and lithium-metal-oxide on the surfaces of the sulfide-based solid electrolyte particles, in, The full width at half maximum (FWHM) of a main peak in an X-ray diffraction (XRD) analysis of the solid electrolyte is less than or equal to 0.

160.

2. The solid electrolyte according to claim 1, wherein In the lithium-metal-oxide, the metal is one or more selected from Al, B, Ca, Ce, Cr, Fe, Mg, Mo, Nb, Si, Sn, Sr, Ta, V, W and Zr.

3. The solid electrolyte according to claim 1, wherein The content of the lithium-metal-oxide is 0.01 wt % to 3 wt % based on 100 wt % of the solid electrolyte.

4. The solid electrolyte according to claim 1, wherein The content of the lithium-metal-oxide is 0.01 wt % to 0.8 wt % based on 100 wt % of the solid electrolyte.

5. The solid electrolyte according to claim 1, wherein The lithium-metal-oxide is amorphous.

6. The solid electrolyte according to claim 1, wherein The sulfide-based solid electrolyte particles include argyrodite-type sulfides.

7. The solid electrolyte according to claim 1, wherein The average particle size (D50) of the solid electrolyte is 0.1 μm to 5.0 μm.

8. The solid electrolyte according to claim 1, wherein The value of (D90-D10) / D50 in the particle size distribution of the solid electrolyte is greater than 1 and less than or equal to 5.

9. A method for preparing a solid electrolyte, comprising: The sulfide-based solid electrolyte particles and the lithium-metal-oxide are mixed and heat-treated at 250°C to 350°C.

10. The method according to claim 9, wherein The heat treatment is performed in an inert gas or nitrogen atmosphere for 0.5 to 10 hours.

11. The method according to claim 9, wherein The lithium-metal-oxide is mixed in an amount of 0.01 parts by weight to 3 parts by weight based on 100 parts by weight of the sulfide-based solid electrolyte particles.

12. The method according to claim 9, wherein The lithium-metal-oxide is mixed in an amount of 0.01 parts by weight to 0.8 parts by weight based on 100 parts by weight of the sulfide-based solid electrolyte particles.

13. The method according to claim 9, wherein The sulfide-based solid electrolyte particles include argyrodite-type sulfides.

14. The method according to claim 9, wherein The average particle size (D50) of the sulfide-based solid electrolyte particles is 0.1 μm to 5.0 μm.

15. The method according to claim 9, wherein In the lithium-metal-oxide, the metal is one or more selected from Al, B, Ca, Ce, Cr, Fe, Mg, Mo, Nb, Si, Sn, Sr, Ta, V, W and Zr.

16. The method according to claim 9, wherein The lithium-metal-oxide is amorphous.

17. The method according to claim 9, wherein The lithium-metal-oxide is in the form of particles, and an average particle size (D50) thereof is 0.01 μm to 1.0 μm.

18. The method according to claim 9, wherein The full width at half maximum (FWHM) of a main peak in the X-ray diffraction analysis of the prepared solid electrolyte is less than or equal to 0.

160.

19. The method according to claim 9, wherein The preparation method of the solid electrolyte comprises: The sulfur-containing raw materials are mixed and heat-treated to prepare a sulfide-based solid electrolyte. The prepared sulfide-based solid electrolyte is pulverized to obtain sulfide-based solid electrolyte particles having an average particle size (D50) of 0.1 μm to 0.5 μm, and The sulfide-based solid electrolyte particles and the lithium-metal-oxide are mixed and heat-treated at 250°C to 350°C.

20. The method according to claim 19, wherein The step of mixing the sulfur-containing raw materials and heat treating to prepare the sulfide-based solid electrolyte comprises: A first heat treatment, mixing the sulfur-containing raw materials and firing at 120° C. to 350° C.; And a second heat treatment, mixing the first heat treatment products and firing at 350°C to 800°C.

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