Solid electrolyte for secondary batteries and method for manufacturing the same, lithium secondary battery

Abstract

To provide a sulfide-based solid electrolyte that may suppress the decrease in ionic conductivity and improve ionic conductivity maintenance rate (atmospheric stability).SOLUTION: One embodiment of the present invention provides a solid electrolyte which has an argyrodite crystal structure and contains lithium, phosphorus, sulfur, element M, oxygen, and halogen elements, where the element M is at least one selected from elements (M2) with an oxidation number of 2+ and elements (M6) with an oxidation number of 6+, a substitution rate DS1 (%) of the element M represented by Relational Formula 1 is 0.1 to 1%, and a substitution rate DS2 (%) of the oxygen represented by Relational Formula 2 is 0.15% to 2%.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a solid electrolyte for a secondary battery, a method for manufacturing the same, and a lithium secondary battery.

Background Art

[0002] Currently, commercially available lithium-ion secondary batteries use an electrolyte containing a flammable organic solvent. Therefore, in the event of a short circuit, there is a risk of overheating and fire. As a result, all-solid-state batteries using a solid electrolyte instead of the electrolyte are being studied.

[0003] Unlike a liquid electrolyte composed of an organic substance form, a solid electrolyte can itself serve as a separator membrane, so the conventionally applied separator membrane can be removed. In addition, there is no risk of side reactions due to temperature changes or tears due to external impacts. As a result, since there is no electrolyte and separator membrane, there is an advantage that the proportion of the active material that increases the energy density can be increased, and a high-density battery can be realized.

[0004] Solid electrolytes can be broadly classified into three types: sulfide-based, oxide-based, and polymer-based. Among these, sulfide-based electrolytes can widely form an interface between the electrode and the electrolyte and have the characteristic of high lithium ion conductivity.

[0005] However, since the chemical stability of sulfide-based solid electrolytes is relatively lower than that of oxide-based solid electrolytes, improvement in stability is required. Specifically, sulfide-based solid electrolytes are likely to react with moisture in the atmosphere or moisture flowing in during the process due to various factors such as residual Li2S and cross-linked sulfur such as P2S7 contained in the structure. Therefore, it has problems in the manufacturing process such as a decrease in ion conductivity due to the generation of hydrogen sulfide gas by the reaction with moisture.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] China Patent Publication CN 108493479 A [Patent Document 2] Korean Patent Registration KR 10-2269019 B1 [Patent Document 3] Korean Patent Publication KR 10-2021-0053181 A [Overview of the project] [Problems that the invention aims to solve]

[0007] The present invention aims to synthesize a solid electrolyte with a novel composition while maintaining the argyrodite crystal structure by incorporating elements M and oxygen into a conventional solid electrolyte of Li-PSX (X=Cl, Br, or I) having an argyrodite crystal structure. Element M can substitute for part of the Li site, part of the P site, or both, and oxygen can substitute for part of the S site.

[0008] Furthermore, the present invention aims to provide a sulfide-based solid electrolyte that can suppress the decrease in ionic conductivity due to doping and improve the ionic conductivity maintenance rate (atmospheric stability) by simultaneously incorporating (doping) at least one element selected from elements with an oxidation state of 2+ (M2) and elements with an oxidation state of 6+ (M6) and oxygen into a conventional sulfide-based solid electrolyte. Furthermore, the present invention aims to provide a sulfide-based solid electrolyte in which ionic conductivity is improved and atmospheric stability is enhanced by incorporating a dopant at a specific substitution rate. [Means for solving the problem]

[0009] One embodiment of the present invention provides a solid electrolyte having an argyrodite crystal structure and containing lithium, phosphorus, sulfur, element M, oxygen, and a halogen element, wherein element M is at least one selected from elements with an oxidation state of 2+ (M2) and elements with an oxidation state of 6+ (M6), the substitution rate DS1 (%) of element M, as shown in the following relational formula 1, is 0.1 to 1%, and the substitution rate DS2 (%) of oxygen, as shown in the following relational formula 2, is 0.15 to 2%.

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[0010] The element M may be strontium (M2), tungsten (M6), or a combination thereof.

[0011] The solid electrolyte may have a substitution rate of element M2 DS1-1 (%) represented by the following relational equation 1-1, which is 0.1 to 1%.

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[0012] The solid electrolyte may have a substitution rate of element M6 DS1-2(%) expressed by the following relational equation 1-2, which is 0.1 to 1%.

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[0013] Another embodiment of the present invention provides a solid electrolyte having an argyrodite crystal structure, containing lithium, phosphorus, sulfur, element M, oxygen, and a halogen element, wherein the element M is at least one selected from an element (M²) having an oxidation number of 2+ and an element (M6) having an oxidation number of 6+, and is represented by the following chemical formula 1, 2, or 3.

Chem.

Chem.

Chem.

[0014] The element M may be strontium (M²), tungsten (M6), or a combination thereof.

[0015] The solid electrolyte may have an ion conductivity of 4.0 mS / cm or more at 25°C. <​

[0016] The solid electrolyte may have an ion conductivity retention rate (%) of 70% or more after 2 days under dry conditions in an air atmosphere having a dew point of less than -60°C, as represented by the following relational expression 3.

Math.

[0017] Another embodiment of the present invention provides a method for producing a solid electrolyte according to claim 1 or 5, comprising the steps of mixing a precursor containing lithium, phosphorus, sulfur, and elements M and O, and calcining the mixture.

[0018] The precursor containing lithium, phosphorus, sulfur, and elements M and O may include at least one selected from WO3, BaO, and SrO. The firing process may be carried out at a temperature of 400 to 600°C for 1 to 16 hours under an activated atmosphere.

[0019] Another embodiment of the present invention provides a lithium secondary battery containing the solid electrolyte. [Effects of the Invention]

[0020] The present invention provides a sulfide-based solid electrolyte with improved ionic conductivity and enhanced atmospheric stability. Specifically, it is possible to develop a solid electrolyte that ensures an ionic conductivity of 4.0 mS / cm or higher at 25°C, while simultaneously improving the ionic conductivity retention rate (%) to 70% or higher after 2 days under dry conditions in an air atmosphere with a dew point of less than -60°C. [Modes for carrying out the invention]

[0021] The advantages and features of the present invention, and the methods for achieving them, will become clearer with reference to the embodiments described below in detail, along with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but can be embodied in a variety of different forms, and these embodiments are provided only to complete the disclosure of the present invention and to fully inform those who are ordinary skill in the art to which the present invention pertains, and the present invention is defined only by the scope of the claims.

[0022] Unless otherwise defined, all terms used herein (including technical and scientific terms) may be used in a way that is commonly understood by a person of ordinary skill in the art to which the invention pertains. Wherever any part of the specification “includes” a certain component, this means, unless otherwise stated, that it does not exclude other components, but rather that it may include other components. Furthermore, unless otherwise specified in the statement, singular forms also include plural forms.

[0023] One embodiment of the present invention provides a solid electrolyte having an argyrodite crystal structure and containing lithium, phosphorus, sulfur, element M, oxygen, and a halogen element, wherein element M is at least one selected from elements with an oxidation state of 2+ (M2) and elements with an oxidation state of 6+ (M6).

[0024] The solid electrolyte of the present invention can improve ionic conductivity and ionic conductivity retention. The solid electrolyte is characterized by containing element M and oxygen at a specific substitution rate in a conventional solid electrolyte containing lithium, phosphorus, sulfur, and halogen elements. Element M is at least one selected from elements with an oxidation state of 2+ (M2) and elements with an oxidation state of 6+ (M6). Specifically, the element with an oxidation state of 2+ (M2) may be Sr, and the element with an oxidation state of 6+ (M6) may be W.

[0025] In conventional solid electrolytes of Li-PSX having an argyrodite crystal structure, when elements M and oxygen are present, element M can substitute for part of the Li sites, part of the P sites, or both, and oxygen can substitute for part of the S sites, allowing for the synthesis of solid electrolytes of novel compositions while maintaining the argyrodite crystal structure. When element M is an element with oxidation state 2+ (M2) or Sr, one element M2 (Sr) is substituted for two Li (oxidation state 1+). When element M is an element with oxidation state 6+ (M6) or W, one element M6 (W) is substituted for one Li (oxidation state 1+) and one P (oxidation state 5+). In the case of oxygen, one oxygen (oxidation state 2-) is substituted for one S (oxidation state 2-).

[0026] By substituting some of the lithium and phosphorus with element M, the weak acid M strongly bonds with the weak base S (oxidation state 2-) according to the Hard-Soft Acid-Base (HSAB) principle, forming an MS bond. This suppresses the reaction of H2O in the atmosphere, thereby improving atmospheric stability.

[0027] Furthermore, while doping with oxygen alone can significantly improve the retention rate of ionic conductivity, the ionic conductivity may deteriorate rapidly. Similarly, doping with only element M may not produce the desired effect, making product application difficult.

[0028] Furthermore, the argyrodite crystal structure is face-centered cubic (FCC) and belongs to space group F-43m. Such a crystal structure can be confirmed by X-ray diffraction analysis, and there are no particular limitations when applying known X-ray diffraction analysis.

[0029] The present invention provides a solid electrolyte in which the substitution rate DS1(%) of the element M, as shown in the following relational equation 1, is 0.1 to 1%, and the substitution rate DS2(%) of the oxygen, as shown in the following relational equation 2, is 0.15 to 2%.

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[0030] The DS1 value is 0.1 to 1%, and may be, for example, 0.1 to 0.5%, preferably 0.2 to 0.4%. If the DS1 value is less than 0.1%, the improvement in ionic conductivity is minimal, and conversely, if it exceeds 1%, the ionic conductivity is low, and the ionic conductivity retention rate may deteriorate.

[0031] The DS2 value is 0.15 to 2%, and may be, for example, 0.15 to 1%, 0.15 to 0.7%, 0.5 to 2%, 1.3 to 2%, preferably 0.2 to 0.7% or 0.6 to 1.5%. If the DS2 value is less than 0.15%, the effect of improving the ionic conductivity retention rate is minimal, and conversely, if it exceeds 2%, the ionic conductivity is low and the ionic conductivity retention rate may deteriorate.

[0032] Note that DS1 represents the percentage (based on the number of atoms) of Li and P sites that are substituted with element M, and DS2 represents the percentage (based on the number of atoms) of S sites that are substituted with element O. When some lithium and phosphorus are substituted with element M, and some sulfur is substituted with oxygen, both ionic conductivity and ionic conductivity retention can be improved simultaneously.

[0033] In this invention, an MO doping precursor can be applied in the manufacturing process of a solid electrolyte to simultaneously substitute M and oxygen. The relationship and numerical range of DS1 and DS2 values ​​can be determined by the elemental ratio of M and O in the precursor raw material. For example, the MO doping precursor may be WO3, BaO, SrO, Ta2O5, etc.

[0034] The solid electrolyte of the present invention may have a substitution rate DS1-1 (%) of element M2, represented by the following relational formula 1-1, of 0.1 to 1%, for example, 0.1 to 0.6%, preferably 0.25 to 0.45%. Furthermore, the substitution rate DS1-2 (%) of element M6, represented by the following relational formula 1-2, may also be 0.1 to 1%, for example, 0.1 to 0.5%, preferably 0.2 to 0.4%.

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[0035] If the DS1-1 value is less than 0.1%, the improvement in ionic conductivity is minimal. Conversely, if it exceeds 1%, the ionic conductivity is low, and the ionic conductivity retention rate may deteriorate. Similarly, if the DS1-2 value is less than 0.1%, the improvement in ionic conductivity is minimal. Conversely, if it exceeds 1%, the ionic conductivity is low, and the ionic conductivity retention rate may deteriorate.

[0036] Note that DS1-1 represents the percentage (based on the number of atoms) of Li sites that are replaced by M2 elements, and DS1-2 represents the percentage (based on the number of atoms) of Li and P sites that are replaced by M6 elements. In metals with an oxidation state of 2+, such as Sr (M2+), some of the lithium is replaced by M2 elements, and in metals with an oxidation state of 6+, such as W (M6+), some of the lithium and phosphorus are simultaneously replaced by M6 elements.

[0037] The solid electrolyte of the present invention has an argyrodite crystal structure and contains lithium, phosphorus, sulfur, element M, oxygen, and a halogen element, wherein element M is at least one selected from elements with an oxidation state of 2+ (M2) and elements with an oxidation state of 6+ (M6), and may be represented by the following chemical formulas 1, 2, or 3. [ka] [ka] [ka] In chemical formulas 1, 2, and 3, X is chlorine, bromine, or iodine, and 0.005 ≤ a ≤ 0.05, 0.005 ≤ a1 + a2 ≤ 0.05, 0.005 ≤ b ≤ 0.1, and 1 ≤ c ≤ 2.

[0038] In chemical formulas 1-3, M2 is an element with an oxidation state of 2+, and M6 is an element with an oxidation state of 6+. For example, element M2 may be Sr, and M6 may be W.

[0039] Furthermore, in chemical formulas 1-3, if element M is an element with an oxidation state of 2+ (M2) or Sr, one element M2 (Sr) is substituted for two Li (oxidation state 1+). If element M is an element with an oxidation state of 6+ (M6) or W, one element M6 (W) is substituted for one Li (oxidation state 1+) and one P (oxidation state 5+). In the case of oxygen, one oxygen (oxidation state 2-) is substituted for one S (oxidation state 2-).

[0040] Furthermore, X may be chlorine, bromine, or iodine as the halogen element, and chlorine is preferred. The halogen element can substitute a portion of the S site, thereby securing a new pathway for lithium ions within the crystal lattice and improving ionic conductivity.

[0041] The solid electrolyte may also be 0.005≦a≦0.05, 0.005≦a1+a2≦0.05, 0.005≦b≦0.1, 1≦c≦2, specifically 0.009≦a≦0.05, 0.01≦a≦0.05, 0.01≦a≦0.03, 0.02≦a≦0.04, 0.009≦a1+a2≦0.05, 0.01≦a1+a2≦0.05, 0.01 The values ​​may also be ≤ a1+a2≦0.03, 0.02≦a1+a2≦0.04, 0.009≦b≦0.1, 0.01≦b≦0.1, 0.01≦b≦0.09, 0.01≦b≦0.05, 0.01≦b≦0.03, 0.02≦b≦0.04, 1.2≦c≦1.8, 1.4≦c≦1.6, and for example, it may have the composition listed in Table 1 below.

[0042] [Table 1]

[0043] The solid electrolyte of the present invention may have an ionic conductivity of 4.0 mS / cm or higher at 25°C. For example, after the initial synthesis of the solid electrolyte, the ionic conductivity may be 4.1 mS / cm or higher, or 4.2 mS / cm or higher. The upper limit is not particularly limited, but may be, for example, 10 mS / cm or less, 7 mS / cm or less, 5 mS / cm or less, or 4.5 mS / cm or less.

[0044] Furthermore, the solid electrolyte may have an ionic conductivity retention rate (%) expressed by the following relational equation 3 after 2 days in a dry air atmosphere with a dew point of -60°C or less, which may be 70% or more, for example, 75% or more, 78% or more, or 80% or more. The upper limit is not particularly limited, but may be 95% or less or 90% or less.

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[0045] Another embodiment of the present invention provides a method for producing a solid electrolyte. The production method is not particularly limited, but may include, for example, the steps of mixing a precursor containing lithium, phosphorus, sulfur, and elements M and O, and calcining the mixture.

[0046] The mixing step is not particularly limited if it is carried out by a mixing method known in the art. For example, mechanical milling, melt-quenching, liquid methods, etc., can be applied. As an example, in the case of mechanical milling, a precursor can be obtained by mechanically grinding compounds containing Li, P, S, halogens, and Sr or W and O in a predetermined proportion corresponding to the composition of the target solid electrolyte.

[0047] The precursor containing lithium, phosphorus, sulfur, elements M and O, and halogens may also include precursors containing Li2S, P2S5, WO3, BaO, SrO, and halogen elements (such as LiCl).

[0048] The firing process may be carried out in an inert atmosphere at a temperature of 400 to 600°C for 1 to 16 hours, or for example, in an N2 or Ar gas atmosphere at a temperature of 450 to 600°C, 500 to 600°C, or 530 to 600°C for 2 to 16 hours, 2 to 14 hours, or 2 to 12 hours. The conditions for the firing process are not particularly limited, as long as the firing process is carried out sufficiently to form an argyrodite crystal structure of the solid electrolyte.

[0049] Another embodiment of the present invention provides a lithium secondary battery containing the solid electrolyte. The lithium secondary battery may include a positive electrode, a negative electrode positioned opposite the positive electrode, and the solid electrolyte interposed between the positive electrode and the negative electrode.

[0050] The positive and negative electrodes can be any known technology used in the field of lithium secondary battery technology, without any particular limitations.

[0051] Furthermore, the lithium secondary battery may also include a battery container (case) that houses an electrode assembly including a positive electrode, a negative electrode, and a solid electrolyte, and a sealing member that seals the battery container.

[0052] In this context, lithium secondary batteries can be classified into two types based on the shape of the battery container (case): can-type lithium secondary batteries in which the electrode assembly is housed in a metal can, and pouch-type lithium secondary batteries in which the electrode assembly is housed in a pouch made of a sheet such as aluminum laminate.

[0053] The present invention will be described in detail below based on examples, but these are for the purpose of explaining the present invention in more detail, and the scope of the rights of the present invention is not limited by the following examples.

[0054] Examples (Mixing process) In a drying chamber (DP: -60°C), lithium precursor Li2S, phosphorus precursor P2S5, chlorine precursor LiCl, and MO precursors (WO3, BaO, SrO, Ta2O5) were weighed in stoichiometric proportions using a high-energy milling method to obtain the compositions shown in Table 2 below. A 330 ml ZrO2 jar was filled with precursors in stoichiometric proportions, and then sealed with 10 times the amount of 3 mm ZrO2 balls. Milling was performed using a planetary mill at 330 rpm for a total of 18 hours.

[0055] (Firing process) The mixture produced in the mixing process was placed into an Al2O3 crucible, then charged into a tube furnace and fired. To suppress reactions with gases present inside the tube furnace, firing was carried out while flowing inert N2 gas. The firing conditions were 490°C, a heating rate of 3°C / min, and a holding time of 12 hours, after which it was allowed to cool naturally.

[0056] Experimental example (Evaluation method) Ionic conductivity 200 mg of the solid electrolyte powders from the examples and comparative examples were cold-pressed at 4 tons for 2 minutes using a 10 mm diameter pressure cell mold to create pellet-type materials for ionic conductivity measurement, after which AC impedance measurements were performed. (Applied frequency: 1 MHz ~ 0.01 Hz)

[0057] Ionic conductivity maintenance rate The solid electrolyte powders of the examples and comparative examples were exposed to a drying chamber (DP: -60°C) for 48 hours, and their ionic conductivity was analyzed. The ionic conductivity retention rate was then calculated using the following formula. [Calculation formula] Ionic conductivity retention rate (%) = (Ionic conductivity after 48 hours of exposure) / (Ionic conductivity after synthesis) × 100 (%)

[0058] [Table 2]

[0059] Referring to Table 2, it was confirmed that when Sr or W and oxygen were simultaneously doped in Examples 1-3 and 4-6, the ionic conductivity was maintained at 4 mS / cm or higher, and the ionic conductivity retention rate improved.

[0060] However, in Examples 7-9, when Ba and oxygen were doped simultaneously, the ionic conductivity retention rate was good at 80% compared to Sr, which has the same oxidation state (2+). However, the ionic conductivity was low at less than 4 mS / cm, so the improvement effect was not sufficient.

[0061] When the Sr or Ba substitution rate exceeds 1%, it was confirmed that the ionic conductivity is lower and the ionic conductivity retention rate deteriorates compared to Ref. (Comparative Example 1). On the other hand, in Comparative Examples 2-4, when only oxygen was doped, the ionic conductivity retention rate improved, but the ionic conductivity remained at a relatively low level.

[0062] Furthermore, in Comparative Examples 7-9, it was confirmed that both ionic conductivity and ionic conductivity retention were lower when Ta (oxidation state 5+) and oxygen were doped simultaneously. Furthermore, in Comparative Examples 5 and 6, even when Sr or Ba elements and oxygen were doped simultaneously, it was confirmed that when the Sr or Ba substitution rate exceeded 1% (over-doping), the ionic conductivity was lower and the ionic conductivity retention rate was inferior compared to Ref. (Comparative Example 1).

[0063] As described above, the present invention has been illustrated and explained in relation to specific embodiments, but it will be obvious to those ordinary in the art that the present invention can be improved and modified in various ways without departing from the technical spirit of the invention provided by the following claims.

Claims

1. It has an argyrodite crystal structure, It contains lithium, phosphorus, sulfur, element M, oxygen, and halogen elements. The element M is strontium (M2), tungsten (M6), or a combination thereof. A solid electrolyte in which the substitution rate DS1 (%) of element M, as shown in the following relational equation 1, is 0.1 to 1%, and the substitution rate DS2 (%) of oxygen, as shown in the following relational equation 2, is 0.15 to 2%: [Math 1] ...Relationship 1 [Math 2] ...Relationship 2 In relational equations 1 and 2, [Li], [P], [M], [O], and [S] are the atomic percentages of lithium, phosphorus, element M, and oxygen, respectively.

2. The solid electrolyte according to claim 1, wherein the substitution rate DS1-1 (%) of the element M2, as shown in the following relational formula 1-1, is 0.1 to 1%: [Math 3-1] ...Relationship 1-1 In relational equation 1-1, [Li] and [M2] are the atomic percentages of lithium and element M2, respectively.

3. The solid electrolyte according to claim 1, wherein the substitution rate DS1-2 (%) of the element M6, as shown in the following relational formula 1-2, is 0.1 to 1%: [Math 3-2] ...Relationship 1-2 In relational equation 1-2, [Li], [P], and [M6] are the atomic percentages of lithium, phosphorus, and element M6, respectively.

4. It has an argyrodite crystal structure, It contains lithium, phosphorus, sulfur, element M, oxygen, and halogen elements. The element M is strontium (M2), tungsten (M6), or a combination thereof. Solid electrolytes represented by the following chemical formulas 1, 2, or 3: 【Chemistry 1】 【Chemistry 2】 【Transformation 3】 In chemical formulas 1, 2, and 3, X is chlorine, brome, or iodine, and 0.005 ≤ a ≤ 0.05, 0.005 ≤ a1 + a2 ≤ 0.05, 0.005 ≤ b ≤ 0.1, and 1 ≤ c ≤ 2.

5. The solid electrolyte according to claim 1, wherein the solid electrolyte has an ionic conductivity of 4.0 mS / cm or more at 25°C.

6. The solid electrolyte according to claim 1, wherein the ionic conductivity retention rate (%) expressed by the following relational formula 3 after 2 days in a dry air atmosphere having a dew point of less than -60°C is 70% or more: [Math 4] ...Relationship 3

7. A step of mixing a precursor containing lithium, phosphorus, sulfur, and elements M and O, A method for producing a solid electrolyte according to claim 1 or 4, comprising the step of calcining the mixture.

8. The precursor containing lithium, phosphorus, sulfur, and elements M and O is WO ３ A method for producing a solid electrolyte according to claim 7, comprising at least one selected from BaO and SrO.

9. The method for producing a solid electrolyte according to claim 7, wherein the firing step is carried out in an inert atmosphere at a temperature of 400 to 600°C for 1 to 16 hours.

10. A lithium secondary battery comprising a solid electrolyte as described in claim 1.

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