Solid electrolyte and method for producing the same

Abstract

The present invention relates to a solid electrolyte comprising sulfide-based solid electrolyte particles and a lithium-metal-oxide located on the surface of the particles, wherein the half-width of the main peak in an X-ray diffraction analysis of the solid electrolyte is 0.160 or less, and a method for producing the same.

Description

[Technical Field]

[0001] The present invention relates to a solid electrolyte and a method for producing the same. [Background technology]

[0002] In recent years, reports of the risk of explosion in batteries using liquid electrolytes have led to active development of all-solid-state batteries. However, solid electrolytes have lower ionic conductivity than liquid electrolytes, and they have problems such as resistance occurring at the interface with solid particles such as the positive electrode active material in the battery, and the formation of a depletion layer at the solid-solid junction, which reduces ionic conductivity.

[0003] To solve this problem, conventional techniques have been used to dope various elements into the positive electrode active material particles used with the solid electrolyte, or to form a buffer layer containing elements such as B, Nb, and Zr on the surface of the positive electrode active material particles. However, these methods are difficult to mass-produce and pose cost and environmental issues, limiting the ability to improve the performance of all-solid-state batteries.

[0004] Therefore, there is a need to develop a solid electrolyte that has high ionic conductivity and an appropriate particle size distribution. Summary of the Invention [Problem to be solved by the invention]

[0005] Provided are a solid electrolyte having a uniform particle size distribution, high crystallinity and ionic conductivity, and improved water stability, and a method for producing the same. [Means for solving the problem]

[0006] In one embodiment, a solid electrolyte is provided, comprising sulfide-based solid electrolyte particles and a lithium-metal-oxide located on the surface of the sulfide-based solid electrolyte particles, wherein the full width at half maximum (FWHM) of a main peak in X-ray diffraction (XRD) analysis of the solid electrolyte is 0.160 or less.

[0007] In another embodiment, there is provided a method for producing a solid electrolyte, comprising mixing sulfide-based solid electrolyte particles and a lithium-metal-oxide, and heat-treating the mixture at 250°C to 350°C. [Effects of the Invention]

[0008] The solid electrolyte according to one embodiment has a uniform particle size distribution, high crystallinity and ionic conductivity, and excellent water stability. [Brief explanation of the drawings]

[0009] [Figure 1] 1 shows particle size distribution curves for the solid electrolytes of Example 2, Comparative Example 1, and Comparative Example 2. [Figure 2] 1 shows particle size distribution curves for the solid electrolytes of Examples 2, 3, and 4 and Comparative Example 2. [Figure 3] 1 is an X-ray diffraction graph for the solid electrolytes of Examples 1, 2, and 5, Comparative Examples 1 and 2, and lithium zirconium oxide (LZO). [Figure 4] 1 is an X-ray diffraction graph for the solid electrolytes of Examples 2 to 4 and Comparative Example 1, as well as LZO, lithium aluminum oxide (LAO), and lithium titanium oxide (LTO). [Figure 5] 1 is a graph showing the half-width of the main peak (bar graph, left vertical axis) and the ionic conductivity (dotted 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. [Figure 6]1 is a graph showing the evaluation of water stability for the solid electrolytes of Examples 1, 2, and 5 and Comparative Example 2, and showing ionic conductivities before and after being left standing for 3 days. [Figure 7] 1 is a graph showing the evaluation of water stability for the solid electrolytes of Examples 2 to 4 and Comparative Example 2, and showing ionic conductivities before and after being left standing for 3 days. DETAILED DESCRIPTION OF THE INVENTION

[0010] Although the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein, specific embodiments will be described in detail below so that those skilled in the art can easily implement the present invention.

[0011] The terms used herein are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless the context clearly indicates otherwise.

[0012] As used herein, "combinations thereof" refers to mixtures, laminates, composites, copolymers, alloys, blends, reaction products, and the like of compositions.

[0013] As used herein, terms such as "comprise," "comprise," or "have" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, and should be understood as not precluding the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0014] To clearly illustrate the various layers and regions in the drawings, thicknesses have been exaggerated, and similar parts have been given the same reference numerals throughout the specification. When a layer, film, region, plate, or other part is said to be "on" or "above" another part, this includes not only the case where it is "directly on" the other part, but also the case where there is another part between them. Conversely, when a part is said to be "directly on" another part, it means that there is no other part between them.

[0015] Here, the term "layer" includes not only shapes formed on the entire surface but also shapes formed on a portion of the surface when observed in a plan view.

[0016] The average particle size can be measured by methods well known to those skilled in the art, such as by using a particle size analyzer or by using a transmission electron microscope or scanning electron microscope. Another method involves measuring using dynamic light scattering, analyzing the data, counting the number of particles in each particle size range, and then calculating the average particle size. The average particle size can be measured using a microscope image or a particle size analyzer, and can refer to the diameter of the particles that make up 50% of the cumulative volume in the particle size distribution (D50).

[0017] Here, "or" is not to be construed as exclusive; for example, "A or B" is to be construed as including A, B, A+B, etc.

[0018] Method for producing solid electrolyte In one embodiment, there is provided a method for producing a solid electrolyte, comprising mixing sulfide-based solid electrolyte particles and a lithium-metal-oxide, and heat-treating the mixture at 250°C to 350°C.

[0019] In general, solid electrolytes should have an appropriate particle size distribution and excellent particle flowability to achieve high ionic conductivity and high energy density in batteries, thereby enabling high-density electrodes and electrolyte membranes. At the same time, the solid electrolyte should maintain high crystallinity and exhibit improved ionic conductivity.

[0020] Among various solid electrolytes, sulfide-based solid electrolytes are capable of achieving high ionic conductivity, but immediately after synthesis at high temperatures, the particles are heavily agglomerated and have large particle sizes, so they need to be crushed. However, the crushing process reduces ionic conductivity, and if heat treatment is then performed to increase ionic conductivity, the particles will re-agglomerate and grow again.

[0021] In one embodiment, to solve these problems, crushed sulfide-based solid electrolyte particles are coated with a lithium-metal-oxide and then heat-treated at a temperature range of 250°C to 350°C. This method increases the crystallinity of the solid electrolyte, improving ionic conductivity, and simultaneously suppresses particle agglomeration and growth, resulting in high-density electrodes and electrolyte membranes with an appropriate particle size distribution. Such solid electrolytes also have high water stability and can improve battery capacity characteristics, initial charge / discharge efficiency, and life characteristics.

[0022] For example, if the heat treatment is performed at a temperature below 250°C, the crystallinity may not be sufficiently high and high ionic conductivity may not be achieved. Also, if the heat treatment is performed at a temperature above 350°C, the particles may aggregate and grow, making it impossible to have an appropriate particle size distribution, which may result in a decrease in crystallinity. The higher the heat treatment temperature, the more coating agent is required, which may result in a decrease in ionic conductivity.

[0023] The heat treatment can be carried out in an inert gas atmosphere such as He, Ar, or N2, or in a nitrogen atmosphere. The heat treatment is carried out for 0.5 to 10 hours, for example, 1 to 8 hours. When the heat treatment is carried out under these conditions, the produced solid electrolyte can achieve an appropriate particle size distribution while exhibiting excellent ionic conductivity.

[0024] The lithium metal oxide is mixed in an amount of 0.01 to 3 parts by weight relative to 100 parts by weight of the sulfide-based solid electrolyte particles, 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 within these amounts, the resulting solid electrolyte exhibits high ionic conductivity and has an appropriate particle size distribution without particle agglomerations. In particular, when 0.01 to 0.8 parts by weight of lithium metal oxide is mixed relative to 100 parts by weight of the sulfide-based solid electrolyte particles, the surface of the sulfide-based solid electrolyte is evenly coated with an appropriate amount of lithium metal oxide, further improving the ionic conductivity of the solid electrolyte.

[0025] A method for producing a solid electrolyte according to one embodiment may include, for example, mixing sulfur-containing raw materials, performing heat treatment to synthesize a sulfide-based solid electrolyte, pulverizing the synthesized sulfide-based solid electrolyte, mixing the pulverized sulfide-based solid electrolyte particles with a lithium metal oxide, and performing heat treatment at 250°C to 350°C to obtain a solid electrolyte in which the lithium metal oxide is located on the surface of the sulfide-based solid electrolyte particles.

[0026] In one embodiment, mixing sulfide-based solid electrolyte particles with a lithium metal oxide and heat-treating the mixture can be considered a type of dry coating method. That is, the method for manufacturing a solid electrolyte according to one embodiment can be considered a method for coating sulfide-based solid electrolyte particles, for example, dry-coating the surface of sulfide-based solid electrolyte particles with a lithium metal oxide. Unlike other oxide-based inorganic solid electrolytes or cathode active materials, sulfide-based solid electrolytes are difficult to wet-coat and susceptible to high-temperature heat treatment, making them materials that require careful design of coating conditions. Furthermore, wet-coating methods typically use alcohol-based solvents or alkoxide-based raw materials, which can leave carbon components locally remaining after coating, which can adversely affect electrical conductivity, etc. The method for manufacturing a solid electrolyte according to one embodiment differs from the conditions for coating other types of solid electrolyte particles and is therefore distinct from general wet-coating methods.

[0027] Sulfide solid electrolyte particles Examples of the sulfide-based solid electrolyte particles include Li2S-P2S5, Li2S-P2S5-LiX (X is a halogen element, for example, I or Cl), Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, and Li2S-P2S5-Z m S n (m and n are integers, and Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li p MO q (p, q are integers, and M is P, Si, Ge, B, Al, Ga, or In), or a combination thereof.

[0028] For example, such a sulfide-based solid electrolyte can be obtained by mixing Li2S and P2S5 in a molar ratio of 50:50 to 90:10 or 50:50 to 80:20 and selectively heat-treating the mixture. This mixing ratio range allows for the production of a sulfide-based solid electrolyte with excellent ionic conductivity. The addition of other components, such as SiS2, GeS2, or B2S3, can further improve ionic conductivity.

[0029] Mechanical milling and solution methods can be used to mix sulfur-containing raw materials to produce sulfide-based solid electrolytes. Mechanical milling involves placing the starting materials in a reactor and vigorously stirring them with a ball mill or similar to finely grind the starting materials and mix them. When using the solution method, the starting materials are mixed in a solvent to obtain a solid electrolyte as a precipitate. Furthermore, when heat-treated after mixing, the crystals of the solid electrolyte become even stronger, improving ionic conductivity. For example, sulfide-based solid electrolytes can be produced by mixing sulfur-containing raw materials and heat-treating them two or more times. In this case, a strong sulfide-based solid electrolyte with high ionic conductivity can be produced.

[0030] For example, sulfide-based solid electrolyte particles according to one embodiment can be produced by a first heat treatment in which sulfur-containing 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 and fired at 350°C to 800°C. The first and second heat treatments can be carried out in an inert gas or nitrogen atmosphere, respectively. The first heat treatment can be carried out for 1 hour to 10 hours, and the second heat treatment can be carried out for 5 hours to 20 hours. The first heat treatment can achieve the effect of milling small raw materials, and the second heat treatment can synthesize the final solid electrolyte. Such two or more heat treatments can produce a high-performance sulfide-based solid electrolyte with high ionic conductivity and robustness, which is suitable for mass production. The temperature of the first heat treatment can be, for example, 150°C to 330°C or 200°C to 300°C, and the temperature of the second heat treatment can be, for example, 380°C to 700°C or 400°C to 600°C.

[0031] For example, the sulfide-based solid electrolyte particles may include argyrodite-type sulfide. The argyrodite-type sulfide may be, for example, Li a M b P c S d A e(wherein a, b, c, d, and e are all 0 to 12, M is Ge, Sn, Si, or a combination thereof, and A is F, Cl, Br, or I), and a specific example is Li 7-x PS 6-x A x (where x is 0.2 or more and 1.8 or less, and A is F, Cl, Br, or I). The argyrodite-type sulfide is specifically represented by the chemical formula Li3PS4, Li7P3S 11 , Li7PS6, Li6PS5Cl, Li6PS5Br, Li 5.8 PS 4.8 Cl 1.2 , Li 6.2 PS 5.2 Br 0.8 etc. may also be used.

[0032] The sulfide-based solid electrolyte particles containing such argyrodite-type sulfides have an ionic conductivity of 10 -4 ~10 -2 It has high ionic conductivity close to the S / cm range, and can form a tight bond between the positive electrode active material and the solid electrolyte without causing a decrease in ionic conductivity, and can also form a tight interface between the electrode layer and the solid electrolyte layer. All-solid-state batteries containing this material can improve battery performance such as rate characteristics, coulombic efficiency, and life characteristics.

[0033] The argyrodite-type sulfide-based solid electrolyte can be prepared by mixing, for example, lithium sulfide and phosphorus sulfide, and optionally, lithium halide. After mixing, the mixture can be subjected to a heat treatment. The heat treatment can include, for example, two or more heat treatment steps.

[0034] A method for producing a solid electrolyte according to one embodiment may include mixing lithium sulfide, phosphorus sulfide, and optionally lithium halide to produce argyrodite-type sulfide-based solid electrolyte particles, mixing the produced argyrodite-type sulfide-based solid electrolyte particles with a lithium-metal-oxide, and heat-treating the mixture at 250°C to 350°C.

[0035] Specifically, the method for producing the solid electrolyte may include mixing lithium sulfide, phosphorus sulfide, and optionally lithium halide, followed by heat treatment to produce argyrodite-type sulfide-based solid electrolyte particles, pulverizing the produced argyrodite-type sulfide-based solid electrolyte particles, mixing the pulverized argyrodite-type sulfide-based solid electrolyte particles with a lithium-metal-oxide, and heat treatment at 250°C to 350°C.

[0036] Here, in the step of preparing the argyrodite-type sulfide-based solid electrolyte, the heat treatment may include, for example, a first heat treatment in which raw materials are mixed and fired at 120°C to 350°C, and a second heat treatment in which a result of the first heat treatment is mixed again and fired at 350°C to 800°C.

[0037] According to an embodiment, the sulfide-based solid electrolyte particles may have an average particle size (D50) of 5.0 μm or less, 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-based solid electrolyte particles within this particle size range can effectively penetrate into the positive electrode active material, providing excellent contact with the positive electrode active material and excellent connectivity between the solid electrolyte particles. The average particle size of the sulfide-based solid electrolyte particles may be measured using a microscope image. For example, the size of approximately 20 particles may be measured using a scanning electron microscope image to obtain a particle size distribution, from which the D50 may be calculated.

[0038] lithium-metal-oxide According to an embodiment, the lithium-metal-oxide may refer to an oxide containing lithium and a metal other than lithium. Here, the metal includes general metals, transition metals, and metalloids. In the lithium-metal-oxide, the metal may be one or more elements selected from the group consisting of Al, B, Ca, Ce, Cr, Fe, Mg, Mo, Nb, Si, Sn, Sr, Ta, V, W, and Zr.

[0039] The lithium metal oxide may be amorphous. According to one embodiment, when the amorphous lithium metal oxide is coated on the sulfide-based solid electrolyte particles, the resulting solid electrolyte may have higher ionic conductivity, thereby reducing interfacial resistance with other solid particles, such as a positive electrode active material, in a battery and preventing the solid electrolyte particles from clumping together, thereby improving ionic conductivity and capacity characteristics and life characteristics.

[0040] The lithium metal oxide mixed in the solid electrolyte manufacturing method 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 particle size of the sulfide-based solid electrolyte particles. When a lithium metal oxide having such a particle size range is used, it is evenly coated on the surface of the sulfide-based solid electrolyte particles, thereby sufficiently increasing the ionic conductivity of the solid electrolyte and improving its water stability.

[0041] solid electrolyte In one embodiment, a solid electrolyte is provided, comprising sulfide-based solid electrolyte particles and a lithium-metal-oxide located on the surface of the sulfide-based solid electrolyte particles, wherein an X-ray diffraction analysis of the solid electrolyte shows a half-width of a main peak of 0.160 or less.

[0042] The lithium metal oxide may be present on the surface of sulfide-based solid electrolyte particles in the form of a film or islands. For example, a solid electrolyte according to one embodiment may include sulfide-based solid electrolyte particles and a coating layer disposed on the surface of the particles, and the coating layer may include lithium metal oxide.

[0043] A solid electrolyte according to one embodiment is formed by coating the surface of sulfide-based solid electrolyte particles with a lithium metal oxide. The solid electrolyte has sufficiently high crystallinity, achieving excellent ionic conductivity, while also exhibiting an appropriate particle size distribution without particle agglomeration. The higher the crystallinity or the larger the crystal size of the solid electrolyte, the smaller the full width at half maximum of the main peak in X-ray diffraction analysis. The solid electrolyte according to one embodiment is characterized by a full width at half maximum of the main peak of 0.160 or less. Here, the term "main peak" refers to the peak with the highest diffraction intensity in X-ray diffraction analysis. The full width at half maximum of the main peak of the solid electrolyte according to one embodiment may be, for example, 0.159 or less, or 0.155 or less. It is known that a smaller full width at half maximum, i.e., higher crystallinity, improves ionic conductivity. For example, it is understood that larger crystal size reduces grain boundaries and improves ionic conductivity.

[0044] As described above, sulfide-based solid electrolytes tend to have large particle sizes or aggregated particles immediately after synthesis, and when they undergo processes such as pulverization to adjust the particle size to a size suitable for use in batteries, the crystallinity decreases and the ionic conductivity decreases. According to one embodiment, a solid electrolyte is provided by coating sulfide-based solid electrolyte particles with a lithium-metal-oxide and then heat-treating them within a specific temperature range, thereby increasing the crystallinity and adjusting the full width at half maximum of the main peak to 0.160 or less. At the same time, the solid electrolyte has a uniform particle size distribution without particle agglomeration or growth, resulting in improved ionic conductivity.

[0045] Detailed explanations of the sulfide-based solid electrolyte particles and the lithium-metal-oxide have been given above and will be omitted here.

[0046] In one embodiment of the solid electrolyte, the lithium metal oxide may be present in an amount of 0.01 wt % to 3 wt %, for example, 0.01 wt % to 2 wt %, 0.01 wt % to 1 wt %, 0.01 wt % to 0.8 wt %, or 0.1 wt % to 1.0 wt %, based on 100 wt % of the solid electrolyte. When the lithium metal oxide content is within this range, the solid electrolyte can exhibit high ionic conductivity while exhibiting an appropriate particle size distribution without particle agglomeration. In particular, when the lithium metal oxide content is within the range of 0.01 wt % to 0.8 wt % based on 100 wt % of the solid electrolyte, the lithium metal oxide is uniformly coated on the surface of the sulfide-based solid electrolyte particles, thereby further improving the ionic conductivity and water stability of the solid electrolyte and further improving the efficiency and life characteristics of the battery.

[0047] In the solid electrolyte, the lithium metal oxide located on or coating the surface of the sulfide-based solid electrolyte particles may be amorphous. When the amorphous lithium metal oxide is coated, the solid electrolyte exhibits superior ionic conductivity and lowers interfacial resistance, thereby improving battery performance.

[0048] The solid electrolyte may have an average particle size (D50) of 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 penetrate into the positive electrode active material, providing excellent contact with the positive electrode active material and excellent connectivity between solid electrolyte particles. The average particle size of the solid electrolyte may be measured using a microscope image. For example, the size of approximately 20 particles may be measured using a scanning electron microscope image to obtain a particle size distribution, from which the D50 may be calculated.

[0049] A solid electrolyte according to one embodiment is characterized by a uniform particle size distribution without particle agglomerations. For example, in the particle size distribution of the solid electrolyte, the (D90-D10) / D50 value may be greater than 1 and less than or equal to 5, such as 1.1 to 4.0, 1.1 to 3.0, or 1.1 to 2.0. The (D90-D10) / D50 value indicates the degree of peak width in a particle size distribution graph for the solid electrolyte, specifically, a graph showing particle size (μm) on the horizontal axis and cumulative particle volume percentage on the vertical axis. A smaller value indicates a narrower peak width and more uniform particle size. Here, D10 refers to the diameter of particles at 10% by volume of the cumulative volume in the particle size distribution, D50 refers to the diameter of particles at 50% by volume of the cumulative volume in the particle size distribution, and D90 refers to the diameter of particles at 90% by volume of the cumulative volume in the particle size distribution.

[0050] 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. 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, it is possible to achieve high energy density while realizing excellent ionic conductivity, thereby improving battery performance.

[0051] The ionic conductivity of the solid electrolyte at 25° C. may be 2.9 mS / cm or more, 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 / cm. The ionic conductivity may be measured by electrochemical impedance spectroscopy (EIS).

[0052] Examples of the present invention and comparative examples are described below. The following examples are merely examples of the present invention, and the present invention is not limited to the following examples.

[0053] Example 1 1. Production of sulfide-based solid electrolyte particles An argyrodite-type sulfide-based solid electrolyte was synthesized using the method described below. All mixing of raw materials, pre-treatment and post-treatment for heat treatment were carried out in a glove box under an argon atmosphere. A mixed powder was prepared by mixing lithium sulfide (Li2S), phosphorus pentasulfide (P2S5), and lithium chloride (LiCl) as raw materials in a molar ratio of 2.5:0.5:1. The mixed powder was mixed uniformly using a Henschel mixer and then subjected to primary firing at 250°C for 5 hours in a tubular furnace with argon gas flowing at a constant rate of 8 SLM.

[0054] The primary fired powder was mixed uniformly again in a Henschel mixer, sieved, and then secondary fired at 500°C for 10 hours in a tubular furnace where argon gas was flowing at a constant rate of 8 SLM.

[0055] The secondary fired powder was crushed and sieved to obtain sulfide-based solid electrolyte particles of Li6PS5Cl. The size (D50) of the obtained sulfide-based solid electrolyte particles was 0.85 μm.

[0056] 2. Coating of sulfide-based solid electrolyte particles 100 parts by weight of the prepared sulfide-based solid electrolyte particles were mixed with 0.25 parts by weight of a coating agent, lithium zirconium oxide (LZO), which had a D50 of 0.13 μm and was found to be amorphous according to X-ray diffraction analysis, in a Henschel mixer. The mixed powder was heat-treated at 250°C for 5 hours in a tubular furnace with argon gas flowing at a constant rate of 8 SLM. This produced a solid electrolyte in which the surfaces of sulfide-based solid electrolyte particles were coated with lithium zirconium oxide.

[0057] Example 2 A solid electrolyte was prepared in the same manner as in Example 1, except that 0.5 parts by weight of the coating agent was mixed.

[0058] Example 3 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 the coating agent.

[0059] Example 4 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 the coating agent.

[0060] Example 5 A solid electrolyte was prepared in the same manner as in Example 1, except that 1.0 part by weight of the coating agent was mixed.

[0061] Comparative Example 1 A solid electrolyte was prepared in the same manner as in Example 1, except that a coating step of heat treatment at 250° C. for 5 hours was performed without adding a coating agent.

[0062] Comparative Example 2 A solid electrolyte was prepared in the same manner as in Example 1, except that the coating process was not performed. That is, the sulfide-based solid electrolyte particles prepared in the first step of Example 1 were used as the solid electrolyte.

[0063] The design details of the solid electrolytes in Examples 1 to 5 and Comparative Examples 1 and 2 are shown in Table 1 below.

[0064] [Table 1]

[0065] Evaluation example 1: Particle size distribution evaluation The particle size distribution of the solid electrolytes prepared in Examples 1 to 5 and Comparative Examples 1 and 2 was measured using a particle size analyzer that utilizes laser diffraction using xylene from which water had been removed as a solvent.

[0066] Figure 1 shows particle size distribution curves for the solid electrolytes produced in Example 2 (LZO 0.5), Comparative Example (1250°C-not), and Comparative Example 2 (not-not), and Figure 2 shows particle size distribution curves for the solid electrolytes produced in Example 2 (LZO 0.5), Example 3 (LAO 0.5), Example 4 (LTO 0.5), and Comparative Example 2 (not-not). In the particle size distribution curves in Figures 1 and 2, the horizontal axis represents particle size (μm), and the vertical axis represents the cumulative volume of the particles (volume %).

[0067] In addition, in the particle size distributions of the solid electrolytes of Examples 1 to 5 and Comparative Examples 1 and 2, the cumulative size at 10% by volume is represented as D10, the cumulative size at 50% by volume as D50, and the cumulative size at 90% by volume as D90, and these are shown in Table 2 below. In addition, to compare the breadth of the particle size distribution, the span was calculated as (D90-D10) / D50 and is also shown in Table 2.

[0068] [Table 2]

[0069] 1, in Comparative Example 1, in which only heat treatment was performed without a coating agent, peaks were observed at multiple locations, confirming the occurrence of clumping of solid electrolyte particles compared to Comparative Example 2, which was not subjected to heat treatment. Furthermore, Table 1 shows that the D90 value and Span value of Comparative Example 1 were significantly higher than those of Comparative Example 2 due to particle clumping. This indicates that additional heat treatment for the purpose of increasing the ionic conductivity of pulverized sulfide-based solid electrolyte particles can lead to the problem of particle clumping.

[0070] On the other hand, in the case of Examples 1 to 5, referring to FIGS. 1, 2 and Table 2, it can be seen that after coating, the particles have a very uniform particle size distribution without agglomeration or growth.

[0071] Furthermore, from the particle size distribution analysis, it can be seen that in the final solid electrolyte, the coating agent particles are not present separately from the sulfide-based solid electrolyte particles, but are evenly coated on the surfaces of the sulfide-based solid electrolyte particles.

[0072] Evaluation example 2: X-ray diffraction analysis X-ray diffraction analysis was performed on the solid electrolytes of Examples 1, 2, and 5 and Comparative Examples 1 and 2, and LZO, and the results are shown in Figure 3. X-ray diffraction analysis was performed on the solid electrolytes of Examples 2 to 4 and Comparative Example 1, and LZO, LAO, and LTO, and the results are shown in Figure 4. Furthermore, from the X-ray diffraction analysis of the solid electrolytes of Examples 1 to 5 and Comparative Examples 1 and 2, the half-widths of the peaks (main peaks) around 30° where the diffraction intensity is highest were calculated and are shown in the bar graph of Figure 5.

[0073] Referring to Figure 3, no LZO peaks are apparent in Examples 1, 2, and 5, which were coated with LZO, and even in Example 5, which had a high coating agent content, no LZO peaks are apparent, confirming that the lithium zirconium oxide present on the surface of the sulfide-based solid electrolyte particles has very low crystallinity, i.e., exists in an amorphous state. Figure 4, which shows analyses using different coating agents, also shows that no LAO peaks appear in Example 3 and no LTO peaks appear in Example 4, indicating that all of the lithium metal oxide present on the surface of the sulfide-based solid electrolyte particles exists in an amorphous form. Combining Evaluation Examples 1 and 2, it can be seen that the lithium metal oxide was uniformly coated amorphously on the surface of the sulfide-based solid electrolyte particles.

[0074] 3 also shows that the diffraction peak heights of Examples 1, 2, 5, and Comparative Example 1 are higher than those of Comparative Example 2, which does not include a coating process, and that the diffraction peak heights of Examples 1, 2, and 5, which include a coating agent, are higher than those of Comparative Example 1, which is heat-treated without a coating agent. This shows that when coated with an amorphous lithium-metal-oxide and heat-treated within a specific temperature range, the coating agent suppresses the agglomeration of solid electrolyte particles and promotes crystal growth.

[0075] Referring to Figure 5, which shows the half-width of the main peak as a bar graph, Comparative Example 2, before heat treatment, i.e., in the state of pulverized sulfide-based solid electrolyte particles, exhibited a high half-width of 0.175, while Examples 1 to 5 exhibited a significant decrease in half-width, exhibiting levels of 0.155 or less, indicating an increase in crystal size and higher crystallinity. In Comparative Example 1, which was heat-treated at 250°C without a coating agent, the half-width decreased slightly compared to Comparative Example 2, indicating that crystal growth occurred, but the increase in crystal size was smaller compared to the Examples. In Comparative Example 1, as in Evaluation Example 1, agglomeration between particles occurred, and this is understood to be due to the loss of thermal energy, resulting in a smaller increase in crystal size.

[0076] This indicates that if the surface of sulfide-based solid electrolyte particles is appropriately coated with amorphous lithium-metal-oxide and then heat-treated within a specific temperature range, it is possible to increase the crystal size while suppressing the formation of agglomerations between solid electrolyte particles.

[0077] Evaluation example 3: Ion conductivity evaluation 0.15 g of each of the solid electrolytes produced in Examples 1 to 5 and Comparative Examples 1 and 2 was filled, and the pressure was 40 kgf / cm 2After pressurizing the cell at a pressure of 100 kJ / cm, a torque cell was fabricated. Electrochemical Impedance Spectroscopy (EIS) was performed on the fabricated cell to calculate the ionic conductivity, and the results are shown in the dotted line graph in Figure 5. EIS was performed at an amplitude of about 10 mV and a frequency of 0.1 Hz to 10 Hz. 6 The test was carried out at 100 Hz, in an air atmosphere, and at 25°C. The resistance value was determined from the arc of the Nyquist plot obtained by EIS, and the ionic conductivity was calculated taking into account the thickness and area of the cell.

[0078] Referring to FIG. 5, in the case of Comparative Example 1, in which an additional heat treatment was performed, the half-width was lower and the crystallinity was improved compared to Comparative Example 2, in which no coating process was performed. However, particle agglomeration occurred, and the ionic conductivity decreased.

[0079] The ionic conductivity of all the examples was improved compared to the comparative example. Comparing Examples 2 to 4, which have the same coating agent content but different types, it can be seen that the degree of crystal growth of the solid electrolyte and the ionic conductivity differ depending on the type of coating agent. This is understood to be due to differences in the thickness and shape of the coating formed on the surface of the sulfide-based solid electrolyte particles, with the coating agent of Example 2 having a D50 of 0.13 μm and the coating agents of Examples 3 and 4 having a D50 of 0.06 μm. In the case of Example 5, the increased coating agent content is understood to have caused aggregation on the surface of the sulfide-based solid electrolyte particles or acted as resistance, resulting in a slight decrease in ionic conductivity.

[0080] Evaluation example 4: Moisture stability evaluation The solid electrolytes produced in Examples 1 to 5 and Comparative Examples 1 and 2 were left in a dry room with a dew point temperature of -45°C for three days, and then the ionic conductivity was measured in the same manner as in Evaluation Example 3. Figure 6 shows the results of Examples 1, 2, and 5 and Comparative Example 2, and Figure 7 shows the results of Examples 2 to 4 and Comparative Example 2. In Figures 6 and 7, the ionic conductivity before leaving the electrolytes is shown by black bars, and the ionic conductivity after leaving the electrolytes is shown by orange bars.

[0081] Figure 6 shows that when the surface of the solid electrolyte is well protected by the coating agent, its stability against moisture is increased. When the amount of coating agent is small, as in Example 1, the coating layer on the solid electrolyte surface is thin or there is a large amount of exposed surface. This results in high initial ionic conductivity, but insufficient surface protection, resulting in a large decrease in ionic conductivity after storage. In Example 5, the amount of coating agent was excessive, and the coating agent solidified after heat treatment, increasing the exposure of the solid electrolyte surface, which is understood to have resulted in a large decrease in ionic conductivity after storage.

[0082] Referring to FIG. 7, it can be seen that when the surface of the solid electrolyte is properly coated with lithium-metal-oxide to protect the surface, the moisture stability is improved in all cases compared to Comparative Example 2, which does not have a coating.

[0083] Although the preferred embodiments have been described in detail above, the scope of the present invention is not limited to these, and various modifications and improvements made by those skilled in the art using the basic concepts defined in the following claims also fall within the scope of the present invention.

Claims

1. A solid electrolyte comprising sulfide-based solid electrolyte particles and a lithium-metal-oxide located on the surface of the sulfide-based solid electrolyte particles, The solid electrolyte has a main peak half width of 0.160 or less in an X-ray diffraction analysis.

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

3. 2. The solid electrolyte according to claim 1, wherein the content of the lithium-metal-oxide is 0.01% by weight to 3% by weight relative to 100% by weight of the solid electrolyte.

4. 2. The solid electrolyte according to claim 1, wherein the content of the lithium-metal-oxide is 0.01% by weight to 0.8% by weight relative to 100% by weight of the solid electrolyte.

5. 2. 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 an argyrodite-type sulfide.

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

8. 2. The solid electrolyte according to claim 1, wherein the particle size distribution of the solid electrolyte has a (D90-D10) / D50 value of more than 1 and not more than 5.

9. A method for producing a solid electrolyte, comprising mixing sulfide-based solid electrolyte particles and a lithium-metal-oxide, and heat-treating the mixture at 250°C to 350°C.

10. 10. The method for producing a solid electrolyte according to claim 9, wherein the heat treatment is carried out in an inert gas or nitrogen atmosphere for 0.5 to 10 hours.

11. 10. The method for producing a solid electrolyte according to claim 9, wherein 0.01 to 3 parts by weight of the lithium-metal-oxide is mixed with 100 parts by weight of the sulfide-based solid electrolyte particles.

12. 10. The method for producing a solid electrolyte according to claim 9, wherein 0.01 to 0.8 parts by weight of the lithium-metal-oxide is mixed with 100 parts by weight of the sulfide-based solid electrolyte particles.

13. The method for producing a solid electrolyte according to claim 9 , wherein the sulfide-based solid electrolyte particles contain an argyrodite-type sulfide.

14. 10. The method for producing a solid electrolyte according to claim 9, wherein the sulfide-based solid electrolyte particles have an average particle size (D50) of 0.1 μm to 5.0 μm.

15. 10. The method for producing a solid electrolyte according to claim 9, wherein the metal in the lithium-metal-oxide is one or more elements selected from the group consisting of Al, B, Ca, Ce, Cr, Fe, Mg, Mo, Nb, Si, Sn, Sr, Ta, V, W, and Zr.

16. The method for producing a solid electrolyte according to claim 9, wherein the lithium-metal-oxide is amorphous.

17. 10. The method for producing a solid electrolyte according to claim 9, wherein the lithium-metal-oxide is in the form of particles, and the average particle size (D50) is 0.01 μm to 1.0 μm.

18. 10. The method for producing a solid electrolyte according to claim 9, wherein the half width of the main peak in X-ray diffraction analysis of the produced solid electrolyte is 0.160 or less.

19. The method for producing the solid electrolyte includes: mixing sulfur-containing raw materials and heat treating them to produce a sulfide-based solid electrolyte; The produced sulfide-based solid electrolyte is pulverized to obtain sulfide-based solid electrolyte particles having an average particle size (D50) of 0.1 μm to 5.0 μm; The method for producing a solid electrolyte according to claim 9, comprising mixing the obtained sulfide-based solid electrolyte particles and a lithium-metal-oxide, and heat-treating the mixture at 250°C to 350°C.

20. 20. The method for producing a sulfide-based solid electrolyte according to claim 19, wherein the step of mixing the sulfur-containing raw materials and heat-treating the mixture comprises a first heat treatment of mixing the sulfur-containing raw materials and firing the mixture at 120°C to 350°C, and a second heat treatment of mixing a result of the first heat treatment and firing the mixture at 350°C to 800°C.

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