Sulfide solid electrolyte for lithium secondary batteries

Abstract

Disclosed is a sulfide solid electrolyte for lithium secondary batteries. The present invention provides a sulfide solid electrolyte represented by the composition formula LixPS(5.7-0.9y)Hav (where Ha is at least one type of element selected from the group consisting of Cl, Br, and I, x=6.4-0.8y, and 0.5≤y≤1.7).

Description

Sulfide-based solid electrolyte for lithium secondary batteries

[0001] The present invention relates to a solid electrolyte for a lithium secondary battery, and more particularly, to a sulfide-based solid electrolyte.

[0002] An all-solid-state battery (SSL) replaces the liquid electrolyte of conventional batteries with a solid one. This design eliminates the electrolyte and separator required in conventional lithium-ion batteries and instead utilizes a solid electrolyte. Therefore, all-solid-state batteries are attracting attention as next-generation secondary batteries due to their reduced risk of explosion or fire, simplified manufacturing process, and potential for high energy density compared to conventional batteries using liquid electrolytes.

[0003] Among inorganic materials, sulfide-based solid electrolytes (SSEs) are attracting attention as solid-state electrolytes (SSEs) for all-solid-state batteries. However, most sulfide-based solid electrolytes are prepared by mixing raw materials such as Li2S, P2S5, and LiCl, subjecting them to extensive high-energy milling. These are then calcined to enhance crystallinity and ionic conductivity. This calcination process increases the particle size of the solid electrolyte. Therefore, the synthesized assembly material must be appropriately micronized for the cell application. However, sulfide-based solid electrolytes are difficult to micronize due to their soft nature. Furthermore, mechanical grinding of the solid electrolyte can lead to adhesion to the grinding medium or reagglomeration, reducing the recovery rate of the solid electrolyte. Furthermore, wet grinding can also lead to residual impurities, such as the solvent used as a dispersion medium, reducing the ionic conductivity of the solid electrolyte.

[0004] To address these issues with solid-state synthesis, efforts are being made to manufacture sulfide-based solid electrolytes using liquid-phase synthesis. This method, compared to solid-state synthesis, allows for the synthesis of sulfide-based solid electrolytes in a relatively short period of time and with less energy, making it suitable for mass production. However, it suffers from low ionic conductivity due to impurities and high electronic conductivity due to sulfur vacancies or carbonization of organic solvents. It also has the disadvantages of dark and uneven color.

[0005] For example, in the conventional liquid-phase synthesis of LPSCl, a sulfide-based solid electrolyte, a two-step synthesis method is applied, in which Li3PS4 is synthesized followed by additional addition of Li2S and LiCl. The reaction formula of the two-step synthesis method is as follows.

[0006] [Reaction Formula 1]

[0007] 3Li2S + P2S5→2Li3PS4

[0008] [Reaction Formula 2]

[0009] Li3PS4+ Li2S + LiCl → Li6PS5Cl

[0010] In this regard, the inventors of the present invention have found that the injected Cl binds with P and the existing PS4 3- It was found that S in the bond exists in an unstable bond state and is lost during heat treatment, and as a result, the manufactured LPSCl has increased electronic conductivity and takes on a dark color.

[0011] To solve these problems, the inventors of the present invention aim to provide a sulfide-based solid electrolyte composition suitable for achieving high ionic conductivity despite the reaction mechanism of LPSCl, and a method for producing the same.

[0012] In order to achieve the above technical task, the present invention comprises a composition Li x PS (5.7-0.9y) Ha v(Here, Ha is at least one element selected from the group consisting of Cl, Br, and I, and x=6.4-0.8y, 0.5≤y≤1.7) provides a sulfide-based solid electrolyte.

[0013] In the present invention, the sulfide-based solid electrolyte may include an argyrodite-type crystal structure.

[0014] At this time, in the above composition formula, 0.8≤y≤1.5 may be satisfied.

[0015] In the present invention, Ha may be Cl.

[0016] In the present invention, the solid electrolyte may have an ionic conductivity of 1.6 mS / cm or more, or 2.0 mS / cm or more.

[0017] In the present invention, the solid electrolyte may have an electronic conductivity of 10 nS / cm or less.

[0018] In the present invention, the solid electrolyte may have a brightness (L*) of 60 or more in the CIE L*a*b* color space.

[0019] In order to achieve the above other technical tasks, the present invention comprises a composition Li x PS (5.7+0.665x-1.565y) Ha y (wherein Ha is at least one element selected from the group consisting of Cl, Br and I, and x=6.4-0.8y, 0.5≤y≤1.7) a step of mixing a lithium source, a phosphorus source, a sulfur source and a halogen source; and reacting the sources to form Li x PS (5.7-0.9y) Ha y A method for producing a sulfide-based solid electrolyte is provided, which comprises a step of synthesizing a compound.

[0020] In the present invention, the mixing step may include a step of dissolving each source in an organic solvent and stirring.

[0021] In the present invention, the mixing step may include a step of mixing a lithium source, a phosphorus source, and a sulfur source to produce an LPS compound; and a step of mixing a halogen source into the LPS compound.

[0022] At this time, the LPS compound may include Li3PS4.

[0023] In the present invention, the synthesis step may include a step of calcining the reaction product at a temperature of 300 to 600°C.

[0024] In the present invention, the synthesized compound may have an azirodite-type crystal structure.

[0025] In the composition formula of the present invention, 0.8≤y≤1.5 may be satisfied.

[0026] In the present invention, Ha may be Cl.

[0027] According to the present invention, it is possible to provide a sulfide-based solid electrolyte powder having high ionic conductivity and low electronic conductivity.

[0028] In addition, according to the present invention, it is possible to manufacture a sulfide-based solid electrolyte powder having high ionic conductivity and low electronic conductivity based on a liquid-phase synthesis method.

[0029] Figure 1 is a flow chart illustrating a method for manufacturing a sulfide-based solid electrolyte according to one embodiment of the present invention.

[0030] Figure 2 is a schematic diagram illustrating a two-step synthesis process according to one embodiment of the present invention.

[0031] Figures 3 and 4 are graphs showing the results of X-ray diffraction analysis of a powder sample manufactured according to one embodiment of the present invention.

[0032] The present invention will be described in detail by explaining a preferred embodiment of the present invention with reference to the drawings below.

[0033] In the specification of the present invention, the LPS compound or LPS refers to a compound obtainable from a ternary starting composition of lithium (Li)-phosphorus (P)-sulfur (S), and the LPSHa compound or LPSHa refers to a compound obtainable from a quaternary starting composition of Li-PS-Ha (wherein Ha is at least one selected from the group consisting of Cl, Br, and I). In the present invention, the LPS or LPSHa compound may each be a mixture of two or more compounds.

[0034] Meanwhile, LPS may include not only ternary compounds but also binary compounds such as Li2S or P2S5 as unreacted products or byproducts obtained from Li, P, and S as raw materials. The same applies to LPSHa compounds.

[0035] FIG. 1 is a schematic diagram illustrating a method for manufacturing an LPSCl sulfide-based solid electrolyte according to one embodiment of the present invention.

[0036] Referring to Fig. 1, a lithium source, a phosphorus source, a sulfur source, and a halogen source are mixed to satisfy the following composition formula (S110).

[0037] [Composition 1]

[0038] Li x PS (5.7+0.665x-1.565y) Ha y (x=6.4-0.8y, 0.5≤y≤1.7)

[0039] An LPSHa compound having the following composition formula is synthesized by mixing and calcining the compounded composition (S120).

[0040] [Composition 2]

[0041] Li x PS (5.7-0.9y) Ha y (x=6.4-0.8y, 0.5≤y≤1.7)

[0042] In the specification of the present invention, the numbers expressed as subscripts after the symbols of each element, such as x and y in the composition formulae 1 and 2, represent the number of moles of individual elements constituting the entire synthesized compound, and are expressed based on the case where the number of moles of P is 1.

[0043] In the present invention, the LPSHa compound has high ionic conductivity. The ionic conductivity of the LPSHa compound of the present invention may be 1.6 mS / cm or more, 2.0 mS / cm or more, 2.5 mS / cm or more, 3 mS / cm or more, 3.5 mS / cm or more, or 4 mS / cm or more.

[0044] Additionally, the LPSHa compound of the present invention has high brightness. The brightness (L*) of the LPSHa compound of the present invention may be 65 or more, 66 or more, 67 or more, 68 or more, 69 or more, or 70 or more.

[0045] The sulfide-based solid electrolyte composition described above in the present invention can be manufactured through a two-step process based on reaction formulas 1 and 2. This will be described below.

[0046] First, a precursor for manufacturing LPS including a lithium source, a phosphorus (P) source, and a sulfur (S) source is provided (S210).

[0047] For example, the precursor may include lithium sulfide and phosphorus sulfide. In addition, the lithium sulfide in the present invention may include at least one compound selected from the group consisting of Li2S2, Li2S3, Li2S4, Li2S6, and Li2S8. The phosphorus sulfide in the present invention may include P2S5. In the present invention, the lithium sulfide and phosphorus sulfide as the precursor may be combined in consideration of the mixing ratio of the above-described reaction scheme 1.

[0048] In the present invention, excess sulfur (S) according to the above composition 1 can be used in the first-step synthesis for LPS production. In the present invention, S8 can be used as the excess sulfur source.

[0049] Next, the aforementioned precursor is dissolved in an organic solvent such as THF, DMC, ACN, or DME and stirred to synthesize an LPS compound (S220). At this time, the stirring can be performed at a temperature of 50 to 80°C and a speed of 200 to 500 rpm for 1 to 10 hours. Preferably, the stirring step is performed in an inert gas atmosphere such as Ar.

[0050] Optionally, a step of drying the LPS compound produced by stirring may be added. For example, the LPS compound may be dried at a temperature of 80 to 140°C for 1 to 24 hours.

[0051] Next, a lithium halide is added to the synthesized LPS compound to synthesize an LPSHa compound. For example, LPSHa can be synthesized by mixing the LPS compound and LiHa in a suitable organic solvent such as THF and stirring at 200-500 rpm for 1-10 hours at a temperature of 50-80°C or milling in a ball mill at 100-500 rpm for 1-10 hours. Highly crystalline LPSHa can be manufactured by calcining the synthesized LPSHa at a temperature of 300-600°C in an inert atmosphere.

[0052] The present invention can provide an all-solid-state secondary battery including the aforementioned solid electrolyte. The all-solid-state secondary battery of the present invention can include a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a solid electrolyte layer disposed between the positive electrode and the negative electrode.

[0053] In the present invention, the cathode active material may be any cathode active material commonly used in secondary batteries without limitation. For example, it may be a lithium transition metal oxide, a transition metal sulfide, etc. For example, at least one compound oxide of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof may be used.

[0054] In addition, in the present invention, the negative electrode active material may include at least one selected from the group consisting of lithium metal, lithium metal alloy, or a combination thereof, a metal alloyable with lithium, a transition metal oxide, a non-transition metal oxide, and a carbon-based material.

[0055] Meanwhile, the solid electrolyte in the present invention may include the aforementioned sulfide-based solid electrolyte. Furthermore, the aforementioned sulfide-based solid electrolyte can of course be used as the cathode active material.

[0056] According to the present invention, it is possible to provide an all-solid-state secondary battery with reduced internal resistance and improved cycle characteristics.

[0057] Hereinafter, a preferred embodiment of the present invention will be described.

[0058] <Example 1>

[0059] Jo Seong-sik Li x PS (5.7+0.665x-1.565y) Cl y The raw materials were mixed by varying the y value according to the following. At this time, 0.5≤y≤2.2. Li2S, P2S5, S8 and LiCl were used as Li, P, S and Cl sources. LPS compounds were prepared by mixing Li2S, P2S5 and S8. ACN was used as a solvent for stirring, and the mixture was stirred at 300 rpm for 5 hours at a temperature of 55°C in an Ar atmosphere. Subsequently, the mixed solution was dried in a vacuum atmosphere at a temperature of 80 to 140°C for 1 to 24 hours. Next, LiCl was added to synthesize the LPSCl compound, and the obtained LPSCl compound was calcined at a temperature of 350 to 600°C in an inert atmosphere.

[0060] A. Composition analysis of LPSCl compounds

[0061] The composition of the manufactured LPSCl was analyzed. The composition analysis was performed using ion chromatography (IC), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and a C / S-dedicated analyzer.

[0062] Specifically, the quantitative values ​​of Li and P were measured through ICP-AES analysis using iCAP 7400 from Thermo Fisher Scientific.

[0063] The quantitative value of Cl was measured through IC analysis using DIONEX's ICS 6000. The sample was introduced into the analytical column (dionex AS11-HC) at a rate of 0.25 ml / min via a pump, and the conductivity was measured using an electrical conductivity detector for ion chromatography.

[0064] In addition, the quantitative value of S was measured using the CS844 of LECO using the following method.

[0065] 1) Place an empty ceramic crucible on a scale, tare it, put about 1g of sample into the crucible, enter the weight, and then mount the crucible on a high-frequency induction furnace to automatically proceed with the analysis.

[0066] 2) CO2 contained in O2gas used as a mobile gas is removed with NaOH (see reaction formula 1 below), and H2O is removed with MgClO4 as a reagent (see reaction formula 2 below).

[0067] [Reaction Formula 3]

[0068] NaOH + CO2→ NaCO3

[0069] [Reaction Formula 4]

[0070] MgClO4+ H2O → MgCl(OH)4

[0071] 3) The C gas of the sample generated in the high-frequency induction heater combines with the combustion gas O2 to produce CO and CO2, and the S gas also combines with O2 to produce SO2.

[0072] 4) Remove moisture and dust that may affect the analysis from MgClO4 and Quartz Wool.

[0073] 5) The sulfur content is measured in the SO2 state gas using a SO2 infrared absorption detector.

[0074] 6) Meanwhile, since carbon is measured by the infrared detector of CO2, all gas in the form of CO2 is converted to CO2 via CuO (see reaction formula 3 below).

[0075] [Reaction Formula 5]

[0076] CO + CuO → CO2+ Cu

[0077] 7) The carbon content is measured in the CO2 state gas using a CO2 infrared absorption detector.

[0078] B. Measurement of ionic conductivity (IC) and electronic conductivity (EC) of LPSCl compounds

[0079] The ionic conductivity of the manufactured LPSCl compound was measured. 100 mg of the manufactured LPSCl compound powder was measured at 3 tons / cm 2A pellet specimen with a thickness of approximately 800 ㎛ and a diameter of approximately 10 mm was prepared by pressing at a pressure of 1000 ℃ for 1 minute. The impedance of the pellet was measured by the 2-probe method using an impedance analyzer (Biologic VSP400) for the specimen with indium electrodes placed on both sides. The frequency range was 0.1 Hz to 1 MHz, and the amplitude voltage was 100 mV. The measurement was performed at 25℃ in a dry air atmosphere. The resistance value was obtained from the arc of the Nyquist plot for the impedance measurement results, and the ionic conductivity was calculated considering the area and thickness of the specimen. The measurement results are shown in Table 1 below.

[0080] C. Brightness measurement of LPSCl compound

[0081] The luminosity (L*) was measured based on the CIE L*a*b* color space. The manufactured LPSCl powder was evenly filled in a 30 mm sample holder and measured using a spectrophotometer (Konica Minolta, CM-2600d). A D65 light source was used as the light source.

[0082] Table 1 below summarizes the composition, ionic conductivity, electronic conductivity, and brightness measurement results of the synthesized LPSCl compound.

[0083] Sample composition S8 Additive IC (mS / cm) EC (nS / cm) Brightness (L* value) Li (x) PS Cl (y) #16.0 1.0 5.2 0.5 O 0.5 2 1.5 7 2.9 #25.8 1.0 5.0 0.8 O 1.7 9 2.9 6 4.7 #35.6 1.0 4.8 O 2.9 0 5.5 7 1.0 #45.3 1.0 4.5 1.3 4 O 5.4 0 1.5 7 5.2 #55.2 1.0 4.4 O 1.5 O 4.3 0 2.8 6 7 #65.1 1.0 4.2 1.7 O 0.6 7 2.5 5 0.3 #74.8 1.0 3.9 O - - 51.4 #84.7 1.0 3.7 O - - 49.4

[0084] From Table 1, it can be seen that the ionic conductivity values ​​of samples #2 to #5 are excellent, with values ​​exceeding 1 mS / cm. It can be seen that when the y value is less than 0.8 or the y value is greater than 1.5, the ionic conductivity value decreases to less than 1. In addition, it can be seen that as the y value increases, the brightness of the sample powder tends to decrease overall.

[0085] D. X-ray diffraction analysis of LPSCl compound

[0086] The manufactured LPSCl compound powder was subjected to X-ray diffraction analysis. Figure 3 is a graph showing the results of the X-ray diffraction analysis.

[0087] Referring to Figure 3, high LiCl peaks can be observed in samples #1, #7, and #8. On the other hand, it can be seen that samples #1 to #6, and especially #2 to #5, yield highly crystalline LPSCl compounds with the content of impurities such as Li2S and LiCl minimized.

[0088] <Example 2>

[0089] An LPS compound composition having the same composition as samples #3 and #4 of Example 1 was prepared by the high-temperature method.

[0090] According to the composition formula 1, Li2S, P2S5 and LiCl were weighed, excess S8 was added thereto, and then placed in a ball mill together with zirconia balls, followed by high-energy milling at 300 rpm for 10 hours in a ball milling device to synthesize LPSCl.

[0091] For comparison, according to the composition formula 2, Li2S, P2S5 and LiCl were weighed, but the raw materials without excess S8 were subjected to high-energy milling to synthesize LPSCl.

[0092] The composition of the powder of the synthesized LPSCl was analyzed, and the ionic conductivity, electronic conductivity, and brightness were measured, and X-ray diffraction analysis was performed.

[0093] Table 2 below is a table summarizing the analysis results, and Figure 4 is a graph showing the X-ray diffraction analysis results.

[0094] Sample composition S8 Additive IC (mS / cm) EC (nS / cm) Brightness (L* value) LiPSCl #95.6 1.04.81 O2.2 21.67 0.1 #105.3 1.04.5 1.34 O3.9 5 1.17 0.3 #115.6 1.04.81 X--71.8 #125.3 1.04.5 1.34 X--70.6

[0095] As can be seen in Table 2, when the solid-state synthesis method was applied, samples #9 and #10, which fall within the range of the composition formula 1 of the present invention, show high ionic conductivity values. However, samples #9 and #10 show lower ionic conductivity values ​​compared to samples #3 and #4 of Example 1, which were prepared using the liquid-state synthesis method with the same composition.

[0096] For reference, Psalms #11 and #12 fall within the range of composition formula 2.

[0097] <Comparative Example 1>

[0098] Compounds were prepared using the liquid method with the composition shown in Table 3 below.

[0099] The composition of the manufactured compound was analyzed, and IC, EC, and brightness were measured. The analysis results for each sample are shown in Table 3 below, and the X-ray diffraction analysis results are shown in Figure 4.

[0100] Sample composition S8 Additive IC (mS / cm) EC (nS / cm) Brightness (L* value) LiPSCl #136.0 1.05.01 X 1.30 12941.1 #145.5 1.04.5 1.5 X 1.50 200 46.4 #155.6 1.04.8 1 X--45.5 #165.3 1.04.5 1.34 X--46.5

[0101] Referring to Table 3, it can be seen that compounds manufactured by the liquid-phase method exhibit low ionic conductivity and high electronic conductivity. For compounds #15 and #16, the ionic conductivity values ​​were so low that the measurement was meaningless due to the presence of large amounts of impurities.

[0102] Although the present invention has been described above with reference to exemplary embodiments and drawings, these have been provided only to help a more general understanding of the present invention, and the present invention is not limited to the above embodiments, and those skilled in the art to which the present invention pertains will appreciate that various modifications and variations are possible without departing from the essential characteristics of the present invention. Therefore, the spirit of the present invention should not be limited to the described embodiments, and all technical ideas that are equivalent or equivalent to the claims as well as the claims should be interpreted as being included in the scope of the rights of the present invention.

[0103] The present invention can be applied to a lithium secondary battery and a solid electrolyte used therein.

Claims

1. Jo Seong-sik Li x PS (5.7-0.9y) Ha v (where Ha is at least one element selected from the group consisting of Cl, Br and I, and x=6.4-0.8y, 0.5≤y≤1.7) Sulfide-based solid electrolyte.

2. In paragraph 1, A sulfide-based solid electrolyte containing an argyrodite-type crystal structure.

3. In paragraph 1, Sulfide-based solid electrolyte with 0.8≤y≤1.

5.

4. In paragraph 1, Ha is a sulfide-based solid electrolyte containing Cl.

5. In paragraph 1, The above solid electrolyte is a sulfide-based solid electrolyte having an ionic conductivity of 1.6 mS / cm or more.

6. In paragraph 1, The above solid electrolyte is a sulfide-based solid electrolyte having an ionic conductivity of 2.0 mS / cm or more.

7. In paragraph 1, The above solid electrolyte is a sulfide-based solid electrolyte having an electronic conductivity of 10 nS / cm or less.

8. In paragraph 1, The above solid electrolyte is a sulfide-based solid electrolyte having a brightness (L*) of 60 or more in the CIE L*a*b* color space.

9. Jo Seong-sik Li x PS (5.7+0.665x-1.565y) Ha y (wherein, Ha is at least one element selected from the group consisting of Cl, Br and I, and x=6.4-0.8y, 0.5≤y≤1.7) a step of mixing a lithium source, a phosphorus source, a sulfur source and a halogen source; and Li reacts to the above sources x PS (5.7-0.9y) Ha y Comprising a step of synthesizing a compound, Method for producing a sulfide-based solid electrolyte.

10. In paragraph 9, The above mixing step is, Comprising the step of dissolving each source in an organic solvent and stirring it, Method for producing a sulfide-based solid electrolyte.

11. In paragraph 9, The above mixing step is, A step of producing an LPS compound by combining a lithium source, a phosphorus source, and a sulfur source; and Comprising a step of mixing a halogen source into the above LPS compound, Method for producing a sulfide-based solid electrolyte.

12. In paragraph 9, A method for producing a sulfide-based solid electrolyte, wherein the above LPS compound comprises Li3PS4.

13. In paragraph 9, The above synthesis step is, Comprising a step of calcining the above reaction product at a temperature of 300 to 600°C, Method for producing a sulfide-based solid electrolyte.

14. In paragraph 9, A method for producing a sulfide-based solid electrolyte, wherein the above-mentioned synthesized compound has an argyrodite-type crystal structure.

15. In paragraph 9, A method for manufacturing a sulfide-based solid electrolyte, wherein 0.8≤y≤1.

5.

16. In paragraph 9, A method for producing a sulfide-based solid electrolyte, wherein Ha is Cl.

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