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

Abstract

To provide a solid electrolyte for a secondary battery, a method of manufacturing the same and a lithium secondary battery which improve an ionic conductivity maintenance rate (atmospheric stability) and critical current density (interfacial stability).SOLUTION: A solid electrolyte has an argyrodite crystal structure, and contains lithium, phosphorus, sulfur, element M, and a halogen element, where the element M is at least one selected from elements with an oxidation number of 1+ to 6+, and a substitution rate DS (%) of the element M represented by the formula in the figure is 0.1 to 1%. In the formula, [Li] and [M] are contents based on the number of atoms of lithium and element M, respectively, and f is the oxidation number of element M.SELECTED DRAWING: Figure 1

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. In contrast, all-solid-state batteries using solid electrolytes instead of electrolytes have been studied.

[0003] Unlike liquid electrolytes composed of organic substances, solid electrolytes can themselves 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 leakage of liquid due to external shocks. Therefore, since there is no electrolyte and separator membrane, there is an advantage that the ratio of the active material that increases the energy density can be increased, and a high-density battery can be realized.

[0004] Solid electrolytes are roughly 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 sulfide-based solid electrolytes have relatively lower chemical stability than oxide-based solid electrolytes, improvement in stability is required. Specifically, sulfide-based solid electrolytes are likely to react with moisture in the air or moisture that flows in during the process due to various factors such as residual Li2S and cross-linked sulfur such as P2S7 contained in the structure. As a result, there are 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

[0007] The present invention aims to synthesize a new solid electrolyte composition while maintaining the argyrodite crystal structure by incorporating element M into a conventional solid electrolyte of Li-PSX (X=Cl, Br, I) having an argyrodite crystal structure. Element M can substitute for a portion of the Li site.

[0008] Furthermore, an object of the present invention is 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) and critical current density (interfacial stability) by incorporating (doping) at least one element M selected from elements with an oxidation state of 1+ to 6+ into a conventional sulfide-based solid electrolyte.

[0009] Furthermore, the present invention aims to provide a sulfide-based solid electrolyte with improved atmospheric stability and interfacial stability by incorporating a dopant at a specific substitution ratio. [Means for solving the problem]

[0010] One embodiment of the present invention provides a solid electrolyte having an argyrodite crystal structure, containing lithium, phosphorus, sulfur, element M, and a halogen element, wherein element M is at least one selected from elements with an oxidation state of 1+ to 6+, and the substitution rate DS (%) of element M, represented by the following formula 1, is 0.1 to 1%.

number

[0011] In Formula 1, [Li] and [M] are the content ratios based on the number of atoms of lithium and element M, respectively, and f is the oxidation number of element M.

[0012] The element M may be strontium, cobalt, or a combination thereof.

[0013] Another embodiment of the present invention provides a solid electrolyte having an argyrodite crystal structure, containing lithium, phosphorus, sulfur, element M, and a halogen element, wherein the element M is at least one selected from elements having an oxidation number of 1+ to 6+, and is represented by the following Chemical Formula 1.

[0014]

Chemical Formula

[0015] In Chemical Formula 1, X is chlorine, bromine, or iodine, 0.005 ≤ a ≤ 0.05, 1 ≤ b ≤ 2, and f is the oxidation number of element M.

[0016] The element M may be strontium, cobalt, or a combination thereof.

[0017] The solid electrolyte may have an ion conductivity of 3.0 mS / cm or more at 25°C.

[0018] The solid electrolyte may have an ion conductivity retention rate (%) of 60% 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 Formula 2.

[0019]

Formula

[0020] The solid electrolyte may have a ratio (CCD2 / CCD1) of the critical current density (CCD2) measured with a solid electrolyte having DS(%) of 0.1 to 1% in Formula 1 or 0.005 ≤ a ≤ 0.05 in Chemical Formula 1 to the critical current density (CCD1) measured with a solid electrolyte having DS(%) of 0% in Formula 1 or a = 0 in Chemical Formula 1 of 1.2 or more.

[0021] Another embodiment of the present invention provides a method for manufacturing a solid electrolyte, including a step of mixing a precursor containing lithium, phosphorus, sulfur, element M, and a halogen element, and a step of firing the mixture.

[0022] The precursor containing lithium, phosphorus, sulfur, and element M may include at least one selected from cobalt sulfide, cobalt chloride, strontium sulfide, and strontium chloride.

[0023] The firing step may be performed at a temperature of 400 to 600 °C for 1 to 16 hours under an inert atmosphere.

[0024] Another embodiment of the present invention provides a lithium secondary battery including the solid electrolyte.

Advantages of the Invention

[0025] The present invention can provide a sulfide-based solid electrolyte with improved ionic conductivity, improved air stability and interfacial stability. Specifically, it can ensure an ionic conductivity of 3.0 mS / cm or more at 25 °C, and maintain an ionic conductivity retention rate (%) of 60% or more after 2 days under dry conditions in an air atmosphere having a dew point of less than -60 °C, and develop a solid electrolyte in which the critical current density (CCD, Critical current density) increases 1.2 to 1.7 times compared to before doping.

Brief Description of the Drawings

[0026] [Figure 1]This photograph shows the cross-sectional state of the solid electrolyte pellet after CCD analysis and repeated charge-discharge cycling (40 cycles) of the symmetrical cells (Comparative Example 1, Examples 3 and 6) were disassembled. [Modes for carrying out the invention]

[0027] The advantages and features of the present invention, and methods for achieving them, will become apparent with reference to the embodiments described below in detail 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, provided that these embodiments are made possible to complete the disclosure of the present invention and to allow a person ordinary skill in the art to fully understand the scope of the invention, and the present invention is defined only by the scope of the claims.

[0028] Unless otherwise defined, all terms used herein (including technical and scientific terms) should be used in a way that can be commonly understood by a person of ordinary skill in the art to which the invention pertains. Throughout the specification, when a part “includes” a component, this means, unless otherwise stated, that it may include other components rather than excluding them. Also, the singular form includes the plural form unless otherwise specified in the context.

[0029] One embodiment of the present invention provides a solid electrolyte having an argyrodite crystal structure and containing lithium, phosphorus, sulfur, element M, and a halogen element, wherein element M is at least one element selected from elements with an oxidation state of 1+ to 6+.

[0030] The solid electrolyte of the present invention improves atmospheric stability by increasing the ionic conductivity retention rate and critical current density (CCD), and also improves the interfacial stability between lithium (electrode) and the solid electrolyte. The solid electrolyte is characterized by containing element M 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 oxidation states of 1+ to 6+, and specifically, it may be Sr with an oxidation state of 2+ and / or Co with an oxidation state of 4+.

[0031] In conventional solid electrolytes of Li-PSX (X=Cl, Br, I) having an argyrodite crystal structure, if element M is present, element M can substitute for a portion of the Li site, allowing for the synthesis of solid electrolytes of novel compositions while maintaining the argyrodite crystal structure. When element M is Sr with an oxidation state of 2+, one Sr atom is substituted for every two Li atoms (with an oxidation state of 1+). When element M is Co with an oxidation state of 4+, four Li atoms (with an oxidation state of 1+) are substituted.

[0032] When some of the lithium and phosphorus are replaced with element M, according to the Hard-Soft Acid-Base (HSAB) theory, the weak acid M strongly bonds with the weak base S (oxidation state 2-) to form an MS bond. This suppresses the H2O reaction in the atmosphere, improving atmospheric stability.

[0033] Furthermore, the crystal structure of the argyrodite 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.

[0034] The solid electrolyte of the present invention is characterized in that the substitution rate DS (%) of the element M represented by the following formula 1 is 0.1 to 1%.

[0035]

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[0036] In Equation 1, [Li] and [M] are the atomic-based content of lithium and element M, respectively, and f is the oxidation state of element M.

[0037] The aforementioned DS(%) is 0.1 to 1%, for example, 0.1 to 0.8%, preferably 0.1 to 0.6%. If the DS value is less than 0.1%, the doping effect will not be sufficiently expressed, and conversely, if it exceeds 1%, the ionic conductivity will rapidly decrease to less than 3.0 mS / cm, which may make application difficult.

[0038] On the other hand, DS(%) represents the percentage (%) of atoms substituted by element M at the Li site. When a portion of lithium is substituted by element M, the ionic conductivity maintenance rate and critical current density (CCD) are improved within the DS(%) design range, improving atmospheric stability and enhancing interfacial stability between lithium (electrode) and solid electrolyte.

[0039] Another embodiment of the present invention provides a solid electrolyte having an argyrodite crystal structure, containing lithium, phosphorus, sulfur, element M, and a halogen element, wherein element M is at least one selected from elements with an oxidation state of 1+ to 6+, and is represented by the following chemical formula 1.

[0040] [ka]

[0041] In chemical equation 1, X may be chlorine, bromine, or iodine, and may be 0.005 ≤ a ≤ 0.05 and 1 ≤ b ≤ 2.

[0042] Furthermore, in Equation 1, 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+), and if element M is an element with an oxidation state of 4+ (M4) or Co, one element M4 (Co) is substituted for four Li (oxidation state 1+).

[0043] Furthermore, X may be chlorine, bromine, or iodine as a halogen element, with chlorine being preferred. The halogen element can substitute for a portion of the S site, thereby securing a new pathway for lithium ions within the crystal lattice and improving ionic conductivity.

[0044] The solid electrolyte may have values ​​of 0.005≦a≦0.05 and 1≦b≦2, specifically 0.009≦a≦0.05, 0.01≦a≦0.05, 0.01≦a≦0.03, 0.015≦a≦0.025, 1.2≦b≦1.8, and 1.4≦b≦1.6.

[0045] The solid electrolyte of the present invention may have an ionic conductivity of 3.0 mS / cm or higher at 25°C. For example, the ionic conductivity after initial synthesis of the solid electrolyte may be 3.3 mS / cm or higher, 3.5 mS / cm or higher, 3.7 mS / cm or higher, 4.0 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.

[0046] Furthermore, the solid electrolyte may have an ionic conductivity retention rate (%) expressed in the following formula 2 after 2 days in a dry air atmosphere with a dew point of less than -60°C, which may be 60% or more, for example, 65% or more, 68% or more, 70% or more, or 75% or more. The upper limit is not particularly limited, but it may be 95% or less or 90% or less. This has the effect of reducing reactivity to moisture and oxygen and improving atmospheric stability.

[0047]

number

[0048] Furthermore, the solid electrolyte may have a ratio (CCD2 / CCD1) of 1.2 or greater to the critical current density (CCD1) measured with a solid electrolyte where DS(%) is 0% in Equation 1 or a is 0 in Formula 1, where DS(%) is 0.1 to 1% in Equation 1 or a is 0% in Formula 1. For example, CCD2 / CCD1 may be 1.2 or greater, 1.4 or greater, 1.5 or greater, or 1.7 or greater. This has the effect of improving the interfacial stability between the solid electrolyte and lithium (electrode) and suppressing dendrite growth.

[0049] 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 lithium, phosphorus, sulfur, a precursor containing element M and a halogen element, and calcining the mixture.

[0050] The mixing step is not particularly limited as long as it is carried out by a mixing method known in the art. For example, mechanical milling, melt-quenching, or liquid-phase milling may be applied. As an example, in the case of mechanical milling, a precursor is obtained by mechanically grinding compounds containing Li, P, S, halogens, and Sr or Co as raw materials in a predetermined ratio corresponding to the composition of the target solid electrolyte.

[0051] The precursor containing lithium, phosphorus, sulfur, and element M may be CoS2, SrS, or SrCl2. The mixing step may further involve mixing a precursor containing a halogen element (X=Cl, Br, I), for example, LiCl.

[0052] The firing process may be carried out at a temperature of 400-600°C for 1-16 hours under an inert atmosphere, or for example, at a temperature of 450-600°C, 500-600°C, or 530-600°C under an N2 or Ar gas atmosphere for 2-16 hours, 2-14 hours, or 2-12 hours. The conditions for the firing process are not particularly limited, as long as the firing process is carried out sufficiently to the extent that the crystalline structure of the solid electrolyte argyrodite is formed.

[0053] Another embodiment of the present invention provides a lithium secondary battery containing the solid electrolyte.

[0054] 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.

[0055] The positive and negative electrodes can be any known technology used in the field of lithium secondary batteries without any particular limitations.

[0056] 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.

[0057] In this case, 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.

[0058] The present invention will be described in detail below through 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.

[0059] Examples (Mixing process) Lithium precursor Li2S, phosphorus precursor P2S5, chlorine precursor LiCl, and M precursors (CoS2, WS2, WCl6, BaS, BaCl2, SrS, SrCl2, TaCl5) were weighed in stoichiometric ratios using a high-energy milling method in a dry room (DP-60℃) to obtain the compositions shown in Table 1 below.

[0060] A precursor in a stoichiometric ratio was placed in a 330 ml ZrO2 jar, and then 10 times the amount of ZrO2 3 mm balls was added to complete the process.

[0061] I used a planetary mill and milled for a total of 18 hours at 330 rpm.

[0062] (Firing process) The mixture produced in the mixing process was placed in an Al2O3 crucible, and then placed in a Tube Furnace for firing.

[0063] To suppress the reaction with the gas present inside the Tube Furnace, firing was performed while flowing inert N2Gas. The firing conditions were 510°C, heating rate of 3°C / min, maintenance time of 12 hours, and natural cooling.

[0064] Experimental example (Evaluation method) Measurement of ionic conductivity Electrochemical impedance spectroscopy (EIS) analysis was used to evaluate the ionic conductivity of solid electrolytes. A pressure cell was prepared by cold-pressing 200 mg of powder into a 10 mm diameter mold at 4 tons for 2 minutes, and measurements were taken at frequencies from 1 MHz to 1 Hz.

[0065] The ionic conductivity was calculated using the formula σ = t / RA. Here, R represents the resistance of the solid electrolyte, t represents the thickness of the solid electrolyte, and A represents the area of ​​the solid electrolyte.

[0066] Ionic conductivity maintenance rate The synthetic products of the examples and comparative examples were exposed to a dry room (DP-60℃) for 48 hours, and their ionic conductivity was analyzed. The ionic conductivity retention rate was then calculated using the following formula.

[0067] [Calculation formula] Ionic conductivity retention rate (%) = (Ionic conductivity after 48 hours of exposure) / (Ionic conductivity after synthesis) × 100 (%)

[0068] Critical current density (CCD) 1) Manufacturing of Li symmetric cells (Li / LPSCl / Li) 200 mg of the solid electrolyte powder prepared in the examples and comparative examples was cold-pressed using a 10 mm diameter mold at 4 tons for 2 minutes.

[0069] A pressure cell was fabricated by placing Li metal at both ends (Li / SE / Li) of a solid electrolyte, using Li metal with a thickness of 0.02T.

[0070] 2) CCD evaluation A constant current was applied to the manufactured lithium symmetric cell, and repeated charging and discharging cycles were performed. As the cycling progressed, the applied current density was increased to identify the point where a short circuit occurred.

[0071] 3) Confirmation of the pellet cross-sectional state of the solid electrolyte after CCD analysis and cycling (40 cycles). After CCD analysis and repeated charge-discharge cycling (40 cycles), the symmetrical cells (Comparative Example 1, Examples 3 and 6) were disassembled, and photographs of the cross-sectional state of the solid electrolyte pellets were visually inspected. The results are shown in Figure 1.

[0072] [Experimental Example 1]: Analysis of ionic conductivity, ionic conductivity maintenance rate, and critical current density

[0073] [Table 1]

[0074] Referring to Table 1, it was confirmed that when Sr or Co was doped in Examples 1 to 6, the ionic conductivity was maintained at 3 mS / cm or higher, and the ionic conductivity retention rate improved to 60% or higher, preferably 70% or higher.

[0075] Furthermore, in the evaluation of critical current density, it was confirmed that it increased by 1.2 to 1.7 times (Examples 4 and 3) or more compared to Comparative Example 1. Therefore, it is analyzed that this exhibits suppression of Li dendrite growth and stable Li plating / stripping behavior, improving the interface stability between Li metal and solid electrolyte.

[0076] When the Co or Sr substitution rate exceeds 1%, the ionic conductivity is 3 mS / cm or less, which is significantly lower than in Ref. (Comparative Example 1).

[0077] [Experimental Example 2]: Confirmation of the cross-sectional state of the solid electrolyte pellet after CCD analysis and cycling (40 cycles). Referring to Figure 1, in Comparative Example 1, where Dopant was not added, it was confirmed that Li grew on the cross-section of the internal solid electrolyte. On the other hand, in Examples 3 and 6, in which Dopant of the present invention was added, it was confirmed that the cross-section of the internal solid electrolyte remained in its initial state without any abnormalities. As a result, it is predicted that the solid electrolyte of the present invention will improve Li dendrite growth suppression and symmetric cell lifetime through improved Li / SE interface stability.

[0078] 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 and halogen elements, The element M is at least one selected from elements with an oxidation state of 1+ to 6+. A solid electrolyte in which the substitution rate DS (%) of the element M represented by the following formula 1 is 0.1 to 1%, The solid electrolyte is A solid electrolyte in which the ratio (CCD2 / CCD1) of the critical current density (CCD2) measured with a solid electrolyte having a DS (%) of 0.1 to 1% in the following formula 1 to the critical current density (CCD1) measured with a solid electrolyte having a DS (%) of 0% in the following formula 1 is 1.2 or greater. [Math 1] ...Formula 1 In Equation 1, [Li] and [M] represent the atomic-based content of lithium and element M, respectively, and f is the oxidation state of element M.

2. The solid electrolyte according to claim 1, wherein the element M is strontium, cobalt, or a combination thereof.

3. It has an argyrodite crystal structure, It contains lithium, phosphorus, sulfur, element M and halogen elements, The element M is at least one selected from elements with an oxidation state of 1+ to 6+. The solid electrolyte is represented by the following formula 1, The solid electrolyte is A solid electrolyte in which the ratio (CCD2 / CCD1) of the critical current density (CCD1) measured with a solid electrolyte where a is 0 in the formula 1 below to the critical current density (CCD2) measured with a solid electrolyte where a is 0 in the formula 1 below is 1.2 or greater. 【Chemistry 1】 In chemical equation 1, X is chlorine, bromine, or iodine, with 0.005 ≤ a ≤ 0.05, 1 ≤ b ≤ 2, and f is the oxidation number of element M.

4. The solid electrolyte according to claim 3, wherein the element M is strontium, cobalt, or a combination thereof.

5. The solid electrolyte according to claim 1, wherein the solid electrolyte has an ionic conductivity of 3.0 mS / cm or more at 25°C.

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

7. It has an argyrodite crystal structure, It contains lithium, phosphorus, sulfur, element M and halogen elements, The element M is at least one selected from elements with an oxidation state of 1+ to 6+. A method for producing a solid electrolyte, wherein the substitution rate DS (%) of the element M represented by the following formula 3 is 0.1 to 1%, The solid electrolyte is In the formula 3 below, the ratio (CCD2 / CCD1) of the critical current density (CCD2) measured with a solid electrolyte having a DS (%) of 0.1 to 1% to the critical current density (CCD1) measured with a solid electrolyte having a DS (%) of 0% is 1.2 or more. A step of mixing a precursor containing lithium, phosphorus, sulfur, element M, and halogen elements, A method for producing a solid electrolyte, comprising the step of calcining the mixture. [Math 3] ...Formula 3 In Equation 3, [Li] and [M] represent the atomic-based content of lithium and element M, respectively, and f is the oxidation state of element M.

8. It has an argyrodite crystal structure, It contains lithium, phosphorus, sulfur, element M and halogen elements, The element M is at least one selected from elements with an oxidation state of 1+ to 6+. A method for producing a solid electrolyte, represented by the following chemical formula 2, The solid electrolyte is In the formula 2 below, the ratio (CCD2 / CCD1) of the critical current density (CCD1) measured with a solid electrolyte where a is 0 to the critical current density (CCD2) measured with a solid electrolyte where a is 0.005 in the formula 2 below is 1.2 or greater. A step of mixing a precursor containing lithium, phosphorus, sulfur, element M, and halogen elements, The process includes a step of calcining the mixture, A method for producing a solid electrolyte, wherein the precursor containing lithium, phosphorus, sulfur, and element M comprises at least one selected from Co sulfide, Co chloride, Sr sulfide, and Sr chloride. 【Chemistry 2】 In chemical equation 2, X is chlorine, bromine, or iodine, 0.005 ≤ a ≤ 0.05, 1 ≤ b ≤ 2, and f is the oxidation number of element M.

9. The method for producing a solid electrolyte according to claim 7, wherein the precursor containing lithium, phosphorus, sulfur, and element M includes at least one selected from Co sulfide, Co chloride, Sr sulfide, and Sr chloride.

10. The method for producing a solid electrolyte according to claim 7 or 8, wherein the firing step is carried out at a temperature of 400 to 600°C for 1 to 16 hours under an inert atmosphere.

11. A lithium secondary battery comprising the solid electrolyte described in claim 1.

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