Solid electrolyte, method for producing the same, and lithium secondary battery including the same

Abstract

The present invention relates to a solid electrolyte, a method for manufacturing the same, and a lithium secondary battery including the same. The sulfide-based solid electrolyte according to the present invention is represented by the following Chemical Formula 1. [Chemical formula 1] Li 6(1-y) A 2y P 1-y S 5-2y X 1-y In the above formula 1, A is a boron group element, X is a halogen element, and 0

Description

[Technical Field]

[0001] The present invention relates to a solid electrolyte, a method for producing the same, and a lithium secondary battery including the same, and more particularly to a sulfide-based solid electrolyte doped with a boron group element. [Background technology]

[0002] All-solid-state batteries are batteries that use a solid electrolyte. All-solid-state batteries replace flammable liquid electrolytes with solid electrolytes, which reduces the risk of explosion and provides excellent stability. Therefore, all-solid-state batteries containing solid electrolytes are attracting attention as next-generation batteries.

[0003] In particular, sulfide-based solid electrolytes exhibit superior ionic conductivity compared to other solid electrolytes such as polymers and oxides. Because the ionic conductivity of sulfide-based solid electrolytes is similar to that of liquid electrolytes, all-solid-state batteries containing sulfide-based solid electrolytes are considered to be the closest to practical application compared to other all-solid-state batteries.

[0004] In particular, Li6PS5X, which has an argyrodite structure, exhibits high ionic conductivity and high stability, and is used as a solid electrolyte in many organizations.

[0005] For commercialization, batteries must be constructed as full cells and evaluated in battery form. However, when argyrodite-structured solid electrolytes are constructed as full cells and evaluated in the form of all-solid-state batteries, they show inferior performance compared to conventional commercial lithium-ion batteries. This is because argyrodite-structured solid electrolytes have higher resistance and still low ionic conductivity compared to currently used liquid electrolytes. In other words, the high resistance and low ionic conductivity result in inferior cell characteristics.

[0006] Therefore, efforts are currently being made to improve the ionic conductivity of solid electrolytes with argyrodite structures, which should enhance the performance of solid-state batteries and expedite their commercialization. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention provides a solid electrolyte, a method for manufacturing the same, and a lithium secondary battery including the same, more specifically, a sulfide-based solid electrolyte doped with a boron group element. [Means for solving the problem]

[0008] The sulfide-based solid electrolyte according to the present invention is represented by the following Chemical Formula 1. [Chemical formula 1] Li 6(1-y) A 2y P 1-y S 5-2y X 1-y In chemical formula 1, A is a boron group element, X is a halogen element, and 0 <y≦1である。

[0009] A may be any one or more of boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Ta).

[0010] A may be boron (B).

[0011] y may be 0.02 to 0.16.

[0012] X may be any one or more of fluorine (F), chlorine (Cl), bromine (Br) and iodine (I).

[0013] X may be chlorine (Cl).

[0014] The sulfide-based solid electrolyte according to the present invention may include a crystalline phase having an argyrodite-based crystalline structure.

[0015] A method for producing a sulfide-based solid electrolyte according to the present invention includes the steps of preparing a mixture containing lithium sulfide (LiS), a sulfur compound, a halogen compound, and a compound containing a boron group element, and heat-treating the mixture.

[0016] In the step of producing a mixture containing lithium sulfide (Li2S), a sulfur compound, a halogen compound, and a compound containing a boron group element, the compound containing a boron group element may be B2S3.

[0017] In the step of producing a mixture containing lithium sulfide (Li2S), a sulfur compound, a halogen compound, and a compound containing a boron group element, the concentration of the compound containing a boron group element may be 0.02 to 0.16 mol %.

[0018] In the step of preparing a mixture containing lithium sulfide (Li2S), a sulfur compound, a halogen compound, and a compound containing a boron group element, the sulfur compound may be diphosphorus pentasulfide (P2S5).

[0019] In the step of producing a mixture containing lithium sulfide (LiS), a sulfur compound, a halogen compound, and a compound containing a boron group element, the halogen compound is LiX, where X may be one or more of fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

[0020] The lithium secondary battery according to the present invention includes a positive electrode, a negative electrode, and a solid electrolyte layer located between the positive electrode and the negative electrode, and at least one of the positive electrode, the negative electrode, and the solid electrolyte layer includes the sulfide-based solid electrolyte according to the present invention described above. [Effects of the Invention]

[0021] The sulfide-based solid electrolyte according to the present invention is doped with boron, which increases the mobility of lithium ions, resulting in high ionic conductivity and excellent cell capacity characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0022] Terms such as "first," "second," and "third" are used to describe various parts, components, regions, layers, and / or sections, but are not limited thereto. These terms are used only to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Therefore, a first part, component, region, layer, or section described below may be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention.

[0023] The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. As used herein, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. As used in the specification, the term "comprising" embodies certain features, regions, integers, steps, operations, elements, and / or components, and does not exclude the presence or addition of other features, regions, integers, steps, operations, elements, and / or components.

[0024] When a part is referred to as being "on" another part, it may be directly on top of the other part, or there may be other parts between them. In contrast, when a part is referred to as being "directly on top" of another part, there are no other parts between them.

[0025] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention pertains. Terms defined in commonly used dictionaries are additionally interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted in an ideal or very formal sense unless otherwise defined.

[0026] The sulfide-based solid electrolyte according to the present invention has been invented to improve the low ionic conductivity of conventional undoped sulfide-based solid electrolytes and to enhance the performance of cells containing such solid electrolytes.

[0027] More specifically, this is to improve the ionic conductivity and cell characteristics of the solid electrolyte expressed as Li6PS5X, where X represents a halogen element.

[0028] The sulfide-based solid electrolyte according to the present invention is represented by the following Chemical Formula 1. [Chemical formula 1] Li 6(1-y) A 2y P 1-y S 5-2y X 1-y In chemical formula 1, A is a boron group element, X is a halogen element, and 0 <y≦1である。

[0029] The sulfide-based solid electrolyte according to the present invention may be a solid electrolyte represented by Li6PS5X doped with a boron group element, more specifically, a solid electrolyte represented by Li6PS5Cl doped with boron (B).

[0030] More specifically, A may be a boron group element, i.e., an element in group 13. Even more specifically, A may be one or more of boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Ta), and even more specifically, A may be boron (B).

[0031] The boron group element may be a doping element. In this specification, "doping" can mean not only the substitution of a part of an element in a compound with a new element, but also the doping element becoming a component of the crystalline phase of the compound.

[0032] In the solid electrolyte crystal, lithium ions in the existing crystal may be deficient due to the doping of boron group elements. When lithium ions are deficient, vacancies are generated at that site. As a result, lithium ions can move more smoothly through a large number of generated vacancies. Therefore, the ionic conductivity of the solid electrolyte can be increased.

[0033] Since the solid electrolyte according to an embodiment of the present invention may have lithium ion deficiency, 6(1 - y) in Chemical Formula 1 may be the number of moles of lithium (Li). Here, 6(1 - y) may be 5 to 6. More specifically, it may be 5 to 5.6. Even more specifically, it may be 5 to 5.5. Even more specifically, it may be 5.2 to 5.45.

[0034] On the other hand, in Chemical Formula 1, 2y represents the doping amount of boron group elements in terms of the number of moles. At this time, y satisfies 0 < y ≤ 1.

[0035] More specifically, y may be 0.02 to 0.16. If y is excessively small, it means that the doping amount of boron group elements is excessively small, and if y is excessively large, it means that the doping amount of boron group elements is excessively large. However, if y is excessively small or large, the ionic conductivity of the solid electrolyte may be low and the cell characteristics may not be good. More specifically, if the doping amount of boron group elements is excessively small, it does not deviate much from the basic Li6PS5Cl argyrodite composition, the number of formed vacancies is small, and there is no doping effect. Also, if the doping amount of boron group elements is excessively large, the crystal structure of the argyrodite of the sulfide - based solid electrolyte having ionic conductivity may be greatly deformed, and the movement of lithium ions may not be smooth.

[0036] More specifically, y may be 0.07 to 0.16. Even more specifically, y may be 0.08 to 0.16. Even more specifically, y may be 0.09 to 0.13. Even more specifically, y may be 0.1 to 0.12. Even more specifically, y may be 0.11 to 0.13.

[0037] In the above formula 1, X may be a halogen element, that is, one or more of fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

[0038] More specifically, X may be chlorine (Cl).

[0039] The sulfide-based solid electrolyte according to the present invention may include a crystalline phase having an argyrodite-based crystalline structure, which has the advantage of high ionic conductivity.

[0040] A method for producing a sulfide-based solid electrolyte according to the present invention includes the steps of preparing a mixture containing lithium sulfide (LiS), a sulfur compound, a halogen compound, and a compound containing a boron group element, and heat-treating the mixture.

[0041] Hereinafter, the method for preparing a sulfide-based solid electrolyte according to the present invention will be described step by step.

[0042] First, a mixture containing lithium sulfide (LiS), a sulfur compound, a halogen compound, and a compound containing a boron group element is prepared. That is, lithium sulfide, a sulfur compound, a halogen compound, and a compound containing a boron group element are used as raw materials for the sulfide-based solid electrolyte according to the present invention.

[0043] In this case, the sulfur compound may be a mixture of sulfur (S) and an element selected from the group consisting of phosphorus (P), silicon (Si), germanium (Ge), aluminum (Al), boron (B), and mixtures thereof, and more specifically, may be diphosphorus pentasulfide (P2S5).

[0044] In this case, the halogen compound may be LiX. More specifically, X may be a halogen element, and X may be one or more of fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). Therefore, X may be one or more of LiF, LiCl, LiBr, and LiI.

[0045] On the other hand, a compound containing a boron group element may be A2S3. More specifically, A may be a boron group element, i.e., a group 13 element. Even more specifically, A may be one or more of boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Ta).

[0046] The boron group element may more specifically be boron (B), that is, the compound containing the boron group element may be B2S3.

[0047] In this stage, the concentration of the compound containing a boron group element may be 0.02 to 0.16 mol %.

[0048] If the doping amount of the boron group element is too low, the basic Li6PS5Cl argyrodite composition will not deviate significantly, resulting in a small number of vacancies and no doping effect. Furthermore, if the doping amount of the boron group element is too high, the crystal structure of the argyrodite, an ion-conductive sulfide-based solid electrolyte, will be significantly distorted, potentially hindering the smooth movement of lithium ions.

[0049] More specifically, the concentration y of the compound containing a boron group element may be 0.07 to 0.16 mol%. Even more specifically, it may be 0.08 to 0.16 mol%. Even more specifically, it may be 0.09 to 0.13 mol%. Even more specifically, it may be 0.1 to 0.12 mol%. Even more specifically, it may be 0.11 to 0.13 mol%.

[0050] For example, if B2S3 is added to Li6PS5X to prepare a B-doped sulfide-based solid electrolyte, the solid electrolyte can be prepared according to the following reaction formula 1. [Reaction Scheme 1] (1-y)Li6PS5X+xB2S3→ Li 6(1-y) B 2y P 1-y S 5-2y X 1-y

[0051] The mixing at this stage may be carried out by a dry method or a wet method, more specifically, the mixing at this stage may be carried out by dry milling.

[0052] More specifically, the dry milling may be a ball mill, a vibration mill, a turbo mill, a mechanofusion, a disc mill, a bead mill, a planetary mill, or even more specifically, a planetary mill.

[0053] Dry milling in this stage may be carried out for 5 to 12 hours. More specifically, it may be carried out for 6 to 10 hours. If it is carried out for an excessively short time, there is a problem that mixing is not performed sufficiently. Furthermore, since all mixing is performed within a certain time or more, and the mixed state is the same even if it is carried out for a long time, it is preferable to carry out the dry milling within an appropriate time from the viewpoint of productivity.

[0054] In this step, the rotation speed of the planetary mill may be 150 rpm to 450 rpm. More specifically, it may be 200 rpm to 400 rpm. If the rotation speed is too slow, the balls entering the planetary mill may not penetrate deep into the powder particles, resulting in insufficient mixing of the powder particles overall, or the energy may be too low to achieve sufficient atomization of the powder particles. Furthermore, if the rotation speed is too fast, the powder particles may be concentrated in one area, resulting in insufficient uniform mixing.

[0055] The method can then further include forming the mixture into pellets.

[0056] The pressure in the pelletizing step may be 150 MPa to 450 MPa. More specifically, it may be 200 MPa to 400 MPa. If the pressure is too low, there is a disadvantage that the bonding (adhesion) between the powder particles is insufficient, which may result in high interfacial resistance. Furthermore, if the pressure in the pelletizing step exceeds a certain level, the powder particles will bond together, and even if more pressure is applied, the bonding state will not change. Therefore, from the perspective of productivity, it is preferable to produce pellets at an appropriate pressure.

[0057] The mixture is then heat treated to produce a solid electrolyte.

[0058] The heat treatment may be carried out in the range of 200 to 700° C. More specifically, it may be carried out in the range of 300 to 600° C. If the heat treatment temperature is too low, the effect of the heat treatment is small, whereas if the heat treatment temperature is too high, the elements constituting the solid electrolyte are vaporized, resulting in a loss of the solid electrolyte.

[0059] The heat treatment may be performed in an inert gas atmosphere, more specifically, in an argon (Ar) atmosphere.

[0060] Additionally, the synthesized solid electrolyte can be crushed and made into pellets, and then used as a working electrode to prepare a cell.

[0061] The lithium secondary battery according to the present invention includes a positive electrode, a negative electrode, and a solid electrolyte layer located between the positive electrode and the negative electrode, and at least one of the positive electrode, the negative electrode, and the solid electrolyte layer includes the sulfide-based solid electrolyte according to the present invention described above.

[0062] The positive electrode may include one or more of a positive electrode active material, a conductive material, a binder, and the solid electrolyte described above.

[0063] The negative electrode may be a metal negative electrode or a composite negative electrode, which may include a negative electrode active material, a conductive material, a binder, and one or more of the solid electrolytes described above.

[0064] Since the details of the sulfide-based solid electrolyte have been described above, the specific details will be omitted below. [Example]

[0065] The following detailed description of the present invention is provided by way of example only and does not limit the scope of the present invention, which is defined solely by the scope of the claims that follow.

[0066] <Comparative Example 1 - No B2S5 added> (1) Synthesis of solid electrolyte The solid electrolyte was synthesized by dry milling. Lithium sulfide (Li2S), diphosphorus pentasulfide (P2S5), and lithium chloride (LiCl) were mixed using a planetary mill at 300 rpm for approximately 8 hours. Then, pellets were produced at 300 MPa. Then, the mixture was heat-treated at 500°C under an argon (Ar) atmosphere to synthesize Li6PS5Cl.

[0067] (2) Measurement of the ionic conductivity of the cell The synthesized solid electrolyte was crushed and then pelletized at 300 MPa, and a cell was then fabricated using a sus as the working electrode. The cell was used to measure ionic conductivity via impedance. The measurement results are shown in Table 1 below.

[0068] <Example 1 - 0.02 mol% B2S5 added> The synthesis was carried out under the same conditions as in Comparative Example 1, except that 0.02 mol % of B2S5 was further added in the mixing step of Comparative Example 1. The synthesized B-doped solid electrolyte was used to prepare a cell in the same manner as in Comparative Example 1, and then the ionic conductivity was measured in the same manner. The measurement results are shown in Table 1 below.

[0069] <Example 2 - 0.04 mol% B2S5 added> The synthesis was carried out under the same conditions as in Comparative Example 1, except that 0.04 mol % of B2S5 was further added in the mixing step of Comparative Example 1. The synthesized B-doped solid electrolyte was used to prepare a cell in the same manner as in Comparative Example 1, and the ionic conductivity was measured in the same manner. The measurement results are shown in Table 1 below.

[0070] <Example 3 - 0.06 mol% B2S5 added> The synthesis was carried out under the same conditions as in Comparative Example 1, except that 0.06 mol % of B2S5 was further added in the mixing step of Comparative Example 1. The synthesized B-doped solid electrolyte was used to prepare a cell in the same manner as in Comparative Example 1, and then the ionic conductivity was measured in the same manner. The measurement results are shown in Table 1 below.

[0071] <Example 4 - 0.08 mol% B2S5 added> The synthesis was carried out under the same conditions as in Comparative Example 1, except that 0.08 mol % of B2S5 was further added in the mixing step of Comparative Example 1. The synthesized B-doped solid electrolyte was used to prepare a cell in the same manner as in Comparative Example 1, and then the ionic conductivity was measured in the same manner. The measurement results are shown in Table 1 below.

[0072] Example 5 - 0.1 mol% B2S5 added The synthesis was carried out under the same conditions as in Comparative Example 1, except that 0.1 mol % of B2S5 was further added in the mixing step of Comparative Example 1. The synthesized B-doped solid electrolyte was used to prepare a cell in the same manner as in Comparative Example 1, and then the ionic conductivity was measured in the same manner. The measurement results are shown in Table 1 below.

[0073] <Example 6 - 0.12 mol% B2S5 added> The synthesis was carried out under the same conditions as in Comparative Example 1, except that 0.12 mol % of B2S5 was further added in the mixing step of Comparative Example 1. The synthesized B-doped solid electrolyte was used to prepare a cell in the same manner as in Comparative Example 1, and then the ionic conductivity was measured in the same manner. The measurement results are shown in Table 1 below.

[0074] <Example 7 - 0.14 mol% B2S5 added> The synthesis was carried out under the same conditions as in Comparative Example 1, except that 0.14 mol % of B2S5 was further added in the mixing step of Comparative Example 1. The synthesized B-doped solid electrolyte was used to prepare a cell in the same manner as in Comparative Example 1, and then the ionic conductivity was measured in the same manner. The measurement results are shown in Table 1 below.

[0075] <Example 8 - 0.16 mol% B2S5 added> The synthesis was carried out under the same conditions as in Comparative Example 1, except that 0.16 mol % of B2S5 was further added in the mixing step of Comparative Example 1. The synthesized B-doped solid electrolyte was used to prepare a cell in the same manner as in Comparative Example 1, and then the ionic conductivity was measured in the same manner. The measurement results are shown in Table 1 below.

[0076] <Example 9 - 0.18 mol% B2S5 added> The synthesis was carried out under the same conditions as in Comparative Example 1, except that 0.18 mol % of B2S5 was further added in the mixing step of Comparative Example 1. The synthesized B-doped solid electrolyte was used to prepare a cell in the same manner as in Comparative Example 1, and then the ionic conductivity was measured in the same manner. The measurement results are shown in Table 1 below.

[0077] Example 10 - 0.2 mol% B2S5 added The synthesis was carried out under the same conditions as in Comparative Example 1, except that 0.2 mol % of B2S5 was further added in the mixing step of Comparative Example 1. The synthesized B-doped solid electrolyte was used to prepare a cell in the same manner as in Comparative Example 1, and then the ionic conductivity was measured in the same manner. The measurement results are shown in Table 1 below.

[0078] [Table 1]

[0079] The ionic conductivities of the cells containing the sulfide-based solid electrolytes produced in Comparative Example 1 and Examples 1 to 10 are compared.

[0080] It was found that the solid electrolyte of Example 1, which was doped with B by adding 0.02 mol% of B2S5, had higher ionic conductivity than the solid electrolyte of Comparative Example 1, which was not doped with B. Since the ionic conductivity was higher when only a small amount of B was doped than when no B was doped, it was found that doping with B basically increases the ionic conductivity of the cell.

[0081] Furthermore, up to Example 5, in which B doping was performed by adding 0.1 mol % of B2S5, the ionic conductivity increased as the B doping concentration increased.

[0082] In particular, the ionic conductivities of Example 4, in which 0.08 mol% B2S5 was added for B doping, Example 5, in which 0.1 mol% B2S5 was added for B doping, Example 6, in which 0.12 mol% B2S5 was added for B doping, Example 7, in which 0.14 mol% B2S5 was added for B doping, and Example 8, in which 0.16 mol% B2S5 was added for B doping, exceeded 4 mS / cm.

[0083] In addition, in the case of Example 5 in which B-doping was performed by adding 0.1 mol % of B2S5, the ionic conductivity was 4.75 mS / cm, which was the highest.

[0084] The present invention is not limited to the above-described embodiments, and can be manufactured in various different forms, and those skilled in the art will understand that the present invention can be embodied in other specific forms without changing the technical spirit or essential characteristics of the present invention. Therefore, it should be understood that the above-described embodiments are illustrative in all respects and not limiting.

Claims

1. A sulfide-based solid electrolyte represented by the following chemical formula 1: [Chemical formula 1] Li ６（１－ｙ） A ２ｙ P １－ｙ S ５－２ｙ X １－ｙ In the formula 1, A is a boron group element, X is a halogen element, and 0<y≦1.

2. 2. The sulfide-based solid electrolyte according to claim 1, wherein A is one or more of boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Ta).

3. The sulfide-based solid electrolyte according to claim 2, wherein A is boron (B).

4. The sulfide-based solid electrolyte according to claim 1, wherein y is 0.02 to 0.

16.

5. The sulfide-based solid electrolyte according to claim 1, wherein X is one or more of fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

6. The sulfide-based solid electrolyte according to claim 5, wherein X is chlorine (Cl).

7. The sulfide-based solid electrolyte according to claim 1 , wherein the sulfide-based solid electrolyte includes a crystalline phase having an argyrodite-based crystalline structure.

8. Lithium sulfide (Li ２ S), preparing a mixture containing a sulfur compound, a halogen compound, and a compound containing a boron group element; heat treating the mixture; Including, A method for producing a sulfide-based solid electrolyte.

9. The lithium sulfide (Li ２ S), a sulfur compound, a halogen compound, and a compound containing a boron group element, The compound containing a boron group element is B ２ S ３ That is, The method for producing the sulfide-based solid electrolyte according to claim 8.

10. The lithium sulfide (Li ２ S), a sulfur compound, a halogen compound, and a compound containing a boron group element, the concentration of the compound containing a boron group element is 0.02 to 0.16 mol %; The method for producing the sulfide-based solid electrolyte according to claim 8.

11. The lithium sulfide (Li ２ S), a sulfur compound, a halogen compound, and a compound containing a boron group element, The sulfur compound is diphosphorus pentasulfide (P ２ S ５ ) The method for producing the sulfide-based solid electrolyte according to claim 8.

12. The lithium sulfide (Li ２ S), a sulfur compound, a halogen compound, and a compound containing a boron group element, the halogen compound is LiX, X is one or more of fluorine (F), chlorine (Cl), bromine (Br), and iodine (I); The method for producing the sulfide-based solid electrolyte according to claim 8.

13. a positive electrode, a negative electrode, and a solid electrolyte layer located between the positive electrode and the negative electrode; 2. A lithium secondary battery, wherein at least one of the positive electrode, the negative electrode, and the solid electrolyte layer contains the solid electrolyte according to claim 1.

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