Lithium sulfide powder with improved ionic conductivity retention and preparation method thereof

Abstract

The present invention relates to a lithium sulfide powder and a preparation method thereof, and provides a lithium sulfide powder comprising lithium sulfide (Li2S) and lithium oxide (Li2O) and having an oxygen content of 2.4 to 7.1 wt %.

Description

Lithium sulfide powder with improved ionic conductivity and method for producing the same

[0001] The present invention relates to an all-solid-state electrolyte, and more particularly, to a lithium sulfide powder having improved ionic conductivity and a method for producing the same.

[0002] Secondary batteries are widely used in everything from small electronic devices like mobile phones and laptops to larger devices like electric vehicles (EVs) and energy storage systems (ESS). As secondary batteries' applications expand across all aspects of daily life, the demand for them is increasing, not only for high energy density and long lifespan, but also for stability.

[0003] Conventionally, most electrolytes used in lithium secondary batteries have been liquid electrolytes utilizing organic solvents. However, due to the risk of leakage or fire hazards associated with these liquid electrolytes, strict packaging has become necessary, and this strict packaging has limited the ability to increase energy density beyond a certain level. Consequently, the need for all-solid-state batteries utilizing inorganic solid electrolytes rather than organic liquid electrolytes has emerged.

[0004] The above-mentioned all-solid-state battery can be safely manufactured by excluding organic solvents such as liquid electrolytes. Furthermore, because the inorganic solid electrolyte is stable and does not decompose over a wide voltage range, it has the advantage of enabling the use of high-voltage electrode materials.

[0005] The above solid electrolytes are classified into oxide-based and sulfide-based, and the sulfide-based solid electrolyte has the characteristic of higher ionic conductivity compared to the oxide-based solid electrolyte. The main raw material of the sulfide-based solid electrolyte is lithium sulfide (Li2S). The lithium sulfide synthesis methods utilized include a synthesis method using a high-energy ball mill, a synthesis method using a wet plasma process, and a wet / dry method using lithium metal.

[0006] In the case of the lithium sulfide described above, not only purity but also atmospheric stability is considered an important factor when applied to solid electrolytes. Specifically, there is growing interest in providing lithium sulfide that improves the atmospheric stability of solid electrolytes when applied to solid electrolytes.

[0007] The technical problem to be solved by the present invention is to provide a lithium sulfide powder that provides a solid electrolyte with improved atmospheric stability by not only improving purity but also reducing the decrease in ionic conductivity after exposure to the atmosphere when applied to a solid electrolyte.

[0008] Another technical problem to be solved by the present invention is to provide a manufacturing method for manufacturing lithium sulfide powder having the aforementioned advantages.

[0009] According to one embodiment of the present invention, the lithium sulfide powder relates to a lithium sulfide powder for an all-solid-state electrolyte, which includes lithium sulfide (Li2S) and lithium oxide (Li2O), and may have an oxygen content of 2.4 to 7.1 wt%. In one embodiment, the lithium sulfide powder may satisfy the following equation 1.

[0010] <Formula 1>

[0011] 0.70 < [Li2O] / Oxygen content ≤ 2.0

[0012] (In the above formula 1, [Li2O] means the XRD peak ratio (%) of lithium oxide in lithium sulfide powder, and the oxygen content means the oxygen content (wt%) in lithium sulfide powder)

[0013] In one embodiment, the above formula 1 may be 0.80 to 1.20. In one embodiment, the lithium oxide (Li2O) may satisfy an XRD peak ratio (%) of 10.0% or less. In one embodiment, the content ratio of the lithium oxide (Li2O) and lithium carbonate (Li2CO3) in the lithium sulfide powder may satisfy the following formula 2.

[0014] <Formula 2>

[0015] Lithium oxide (Li2O) > lithium carbonate (Li2CO3) ≥ 0

[0016] In one embodiment, the lithium sulfide powder may not include lithium carbonate (Li2CO3). In another embodiment of the present invention, a method for producing lithium sulfide powder may include the steps of mixing a carbon raw material and a lithium compound to produce a lithium-carbon compound, filtering a solution in which the lithium-carbon compound and a solvent are mixed, drying the filtrate at a temperature range of 30 to 170°C, and heat-treating the dried resultant at a temperature range of 650 to 950°C. In one embodiment, in the step of filtering the solution in which the lithium-carbon compound and the solvent are mixed, the dielectric constant value of the solvent may be 20 to 35.

[0017] In one embodiment, in the step of preparing a lithium-carbon compound by mixing a carbon raw material and a lithium compound, the weight ratio of the lithium compound to the carbon raw material may be 0.25 to 0.5. In one embodiment, the step of preparing a lithium-carbon compound by mixing a carbon raw material and a lithium compound may include a step of heat-treating a mixture of the carbon raw material and the lithium compound at a temperature range of 850 to 950° C.

[0018] In one embodiment, the step of heat-treating the mixture of the carbon raw material and the lithium compound at a temperature in the range of 850 to 950°C may be performed at a heating rate of 3 to 7°C / min. In one embodiment, the step of drying the filtrate may be performed at a vacuum pressure in the range of 10 to 500 mbar.

[0019] In one embodiment, the step of heat-treating the dried result may be performed at a rate of 5 to 20°C / min. In one embodiment, the step of heat-treating the dried result may be performed in an inert gas atmosphere. In one embodiment, the step of heat-treating the dried result may be performed at a flow rate of the inert gas of 2.0 to 3.0 L / min.

[0020] According to one embodiment of the present invention, a lithium sulfide powder has a high oxygen content and a small decrease in ionic conductivity after exposure to the atmosphere when a solid electrolyte is applied, thereby providing a lithium sulfide powder capable of improving atmospheric stability.

[0021] According to another embodiment of the present invention, a method for producing lithium sulfide powder provides a method for producing lithium sulfide powder having the aforementioned advantages by controlling a drying temperature and a heat treatment temperature.

[0022] Figure 1 shows the XRD peaks of the final heat-treated lithium sulfide crystal powder according to the drying temperature.

[0023] Figure 2 shows the XRD peaks of lithium sulfide crystal powder according to heat treatment conditions.

[0024] The terms first, second, and third, etc., are used to describe, but are not limited to, various parts, components, regions, layers, and / or sections. These terms are used solely to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Accordingly, 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.

[0025] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the present invention. As used herein, the singular forms "a," "an," and "the" include plural forms as well, unless the context clearly dictates otherwise. The word "comprising," as used herein, specifies particular features, regions, integers, steps, operations, elements, and / or components, but does not exclude the presence or addition of other features, regions, integers, steps, operations, elements, and / or components.

[0026] When a part is referred to as being "on" or "over" another part, it can be directly on or over the other part, or there may be other parts intervening. Conversely, when a part is referred to as being "directly on" another part, there are no other parts intervening.

[0027] Although not defined otherwise, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Terms defined in commonly used dictionaries are additionally interpreted as having meanings consistent with the relevant technical literature and the present disclosure, and are not to be construed in ideal or overly formal meanings unless otherwise defined. In addition, unless specifically stated, % means weight percent, and 1 ppm is 0.0001 weight percent.

[0028] Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement them. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

[0029] According to one embodiment of the present invention, the lithium sulfide powder relates to a lithium sulfide powder for an all-solid-state electrolyte, and may include lithium sulfide (Li2S) and lithium oxide (Li2O). Specifically, the lithium sulfide powder may include lithium oxide (Li2O) by undergoing a drying and heat treatment process.

[0030] In one embodiment, the oxygen content in the lithium sulfide powder may be from 2.4 wt% to 7.1 wt% based on 100 wt% of the lithium sulfide powder. Specifically, the oxygen content may be from 2.7 wt% to 4.9 wt%.

[0031] If the oxygen content is outside the aforementioned range, there is a problem that the ionic conductivity retention rate before and after exposure to the atmosphere is reduced when argyrodite is synthesized using Li2S. In addition, there is a problem that it is affected by the ratio of impurities such as Li2O, Li2CO3, Li2SO4, or LiOH. Specifically, even if the oxygen content is within the aforementioned range, there is a problem that the ionic conductivity retention rate due to exposure to the atmosphere is reduced when Li2CO3 is present above a certain level in addition to Li2O.

[0032] In one embodiment, the lithium sulfide powder may satisfy the following equation 1.

[0033] <Formula 1>

[0034] 0.70 < [Li2O] / Oxygen content ≤ 2.0

[0035] (In the above formula 1, [Li2O] means the XRD peak ratio (%) of lithium oxide in lithium sulfide powder, and the oxygen content means the oxygen content (wt%) in lithium sulfide powder)

[0036] The above formula 1 refers to the ratio of lithium oxide to oxygen content in lithium sulfide powder. The above formula 1 may be 0.70 to 2.0 or less. Specifically, the above formula 1 may be 0.80 to 1.20, and more specifically, the above formula 1 may be 0.86 to 1.20.

[0037] If the above formula 1 is outside the upper limit of the above-mentioned range, there may be a problem of reduced reactivity, such as residual unreacted material during argyrodite synthesis, due to high crystallinity of Li2O. If the above formula 1 is outside the lower limit of the above-mentioned range, there may be a problem of reduced atmospheric stability of argyrodite applied with other lithium compounds containing oxygen other than Li2O, such as impurities such as Li2CO3 and Li2SO4, increasing.

[0038] In one embodiment, lithium oxide (Li2O) can satisfy an XRD peak ratio (%) of 10.0% or less. Specifically, lithium oxide (Li2O) can satisfy an XRD peak ratio (%) of 8.2% or less, and more specifically, 2.1 to 8.2%.

[0039] When the above lithium oxide (Li2O) has an XRD peak ratio (%) that satisfies the above-mentioned range, the decrease in ionic conductivity is small, and thus lithium sulfide that can be used as a raw material for a solid electrolyte with excellent atmospheric stability can be secured. When the above lithium oxide (Li2O) exceeds the upper limit of the above-mentioned range, the purity of the lithium sulfide powder may decrease, and when it exceeds the lower limit of the above-mentioned range, there is a problem that the atmospheric stability of the solid electrolyte decreases.

[0040] In one embodiment, the content ratio of lithium oxide (Li2O) and lithium carbonate (Li2CO3) in the lithium sulfide powder may satisfy the following equation 2.

[0041] <Formula 2>

[0042] Lithium oxide (Li2O) > lithium carbonate (Li2CO3) ≥ 0

[0043] The above equation 2 is an indicator that the content of lithium oxide is greater than the content of lithium carbonate, and the content of lithium carbonate can be greater than or equal to 0. Lithium carbonate has higher chemical stability than lithium oxide, but has a problem of lowering the yield due to low reactivity during solid electrolyte synthesis. This means that the higher the proportion of lithium carbonate in the lithium sulfide powder, the lower its value as a solid electrolyte raw material with excellent atmospheric stability. Therefore, there is an advantage of improving atmospheric stability when applying a solid electrolyte by having the lithium sulfide powder not contain the lithium carbonate or only have a very small amount.

[0044] In one embodiment, the lithium sulfide powder may not include lithium carbonate (Li2CO3). As described above, lithium carbonate has low reactivity during solid electrolyte synthesis, which can reduce the yield. Therefore, by not including lithium carbonate, atmospheric stability is improved when applying the solid electrolyte.

[0045] According to another embodiment of the present invention, a method for producing lithium sulfide comprises the steps of mixing a carbon raw material and a lithium compound to produce a lithium-carbon compound, filtering a solution containing the lithium-carbon compound and a solvent, drying the filtrate, and heat-treating the dried product. For a description of lithium sulfide, reference may be made to the description given above.

[0046] In one embodiment, the carbon source may include, but is not limited to, at least one of soft carbon, hard carbon, petroleum coke, coal-based needle coke, coal-based pitch coke, natural graphite, and artificial graphite. In one embodiment, the lithium-carbon compound may include at least one of lithium sulfate, lithium hydroxide, lithium oxide, and lithium carbonate.

[0047] In one embodiment, in the step of preparing a lithium-carbon compound by mixing a carbon raw material and a lithium compound, the weight ratio of the lithium compound to the carbon raw material may be 0.25 to 0.5. Specifically, the ratio may be 0.30 to 0.35.

[0048] If the above ratio exceeds the upper limit, unreacted lithium compound raw material, specifically lithium sulfate, remains within the lithium-carbon compound after heat treatment. If the above ratio exceeds the lower limit, the amount of carbon raw material used increases, which has a negative impact on process cost.

[0049] In one embodiment, the step of preparing a lithium-carbon compound by mixing a carbon raw material and a lithium compound may be performed by heat-treating the carbon raw material and the lithium compound. Specifically, the heat treatment may be performed at a temperature range of 850 to 950°C on the mixture of the carbon raw material and the lithium compound.

[0050] In one embodiment, the step of performing a mixture of the carbon raw material and the lithium compound at a temperature in the range of 850 to 950°C may be performed at a heating rate of 3 to 7°C / min. Specifically, the heating rate may be performed at a heating rate of 4 to 6°C / min.

[0051] The step of filtering the solution containing the above lithium-carbon compound and solvent may be a step for controlling the content of lithium oxide in lithium sulfide through solvent extraction. Specifically, solvent extraction may be performed to separate the resulting lithium sulfide from the remaining reducing agent after thermal reduction of lithium sulfate in the gas phase using a reducing agent such as carbon.

[0052] The solvent may be, for example, an alcohol solvent. A non-limiting example of the alcohol solvent may be ethanol. When ethanol is used as the alcohol solvent, lithium sulfide is easily dissolved, but carbon is not dissolved, so that only lithium sulfide can be extracted as a solution during the filtration process.

[0053] In one embodiment, in the step of extracting the filtrate, the dielectric constant of the solvent may be 20 to 35 at room temperature. By using a solvent having the above dielectric constant value range, lithium sulfide can be dissolved at room temperature, making it easy to separate from the remaining reducing agent.

[0054] Specifically, the extraction reactant can be partially dissociated in the solution phase or simultaneously achieve dynamic equilibrium with an ion pair by the following reaction scheme 1.

[0055] [Reaction Formula 1]

[0056] Li2S(s) + CH3CH2OH(l) → LiSH + Li(CH3CH2O) ↔ 2Li + + HS - + (CH3CH2O) -

[0057] The step of drying the above filtrate can be performed at a temperature range of 30 to 170°C. Specifically, the temperature range can be performed at a temperature range of 40 to 165°C, and more specifically, at a temperature range of 60 to 150°C.

[0058] The step of drying the above filtrate may be a step of slowly drying the solution containing lithium sulfide extracted through filtration by evaporation. Through the drying step, the solution containing lithium sulfide may be evaporated to obtain a white powder, which may be expressed according to the following reaction formula 2.

[0059] [Reaction Formula 2]

[0060] 2Li + + HS - + (CH3CH2O) - + nCH3CH2OH(l) → Li2S(s) + (n+1)CH3CH2OH(l)↑

[0061] The white powder obtained according to the above reaction formula 2 may be a crude dried product (crude) in the form of ethanol solvent adsorbed or absorbed around a mixture between lithium sulfide or extraction reactants.

[0062] In addition, the heat applied during the drying process of the solution causes hydrosulfide ions (HS) to be formed. - ) and ethanol molecules, a difference in acid-base change occurs, and some of the hydrosulfide ions can escape from the system in the form of hydrogen sulfide gas through a side reaction as shown in the following reaction formula 3.

[0063] [Reaction Formula 3]

[0064] 2Li + + HS - + (CH3CH2O)- + CH3CH2OH(l) → 2Li(CH3CH2O) + H2S(g)↑

[0065] In one embodiment, the step of drying the filtrate may be performed at a vacuum pressure ranging from 10 to 500 mbar. Specifically, the vacuum pressure may range from 10 to 400 mbar, more specifically from 10 to 250 mbar, and even more specifically from 15 to 200 mbar. By drying the filtrate under vacuum pressure conditions within the aforementioned range, the oxygen content can be reduced, thereby minimizing the generation of impurities.

[0066] The step of heat-treating the dried product may be performed at a temperature range of 650 to 950°C. Specifically, the temperature range may be performed at a temperature range of 700 to 900°C.

[0067] In one embodiment, the alcohol solvent may be at least one of, but not limited to, ethanol, methanol, and propanol. By controlling the drying temperature in the step of drying the filtrate, the dielectric constant of the solvent may vary, thereby changing the degree of dissociation of ions. Specifically, as the drying temperature increases, the dielectric constant of the solvent decreases, making it difficult for ions to dissociate in the solvent. This weakens the basicity of hydrosulfide ions, thereby delaying or inhibiting the generation of hydrogen sulfide, thereby inducing the generation of lithium sulfide in terms of the overall reaction. This is expressed as in the following reaction formula 4, and means that the oxygen content in the final lithium sulfide powder can be controlled by controlling the drying temperature.

[0068] [Reaction Formula 4]

[0069] LiSH + Li(CH3CH2O) + CH3CH2OH(l) → Li2S(s) + 2CH3CH2OH(l)

[0070] The step of heat-treating the dried material is to apply heat to the dried material to obtain the final lithium sulfide (Li2S) powder. In one embodiment, the dried material may be heated at a temperature range of 650 to 950°C, specifically, at a temperature range of 700 to 900°C. In one embodiment, the temperature increase rate of the heat-treating step may be performed at 5 to 20°C per minute. Specifically, the temperature increase rate may be performed at 5 to 15°C / minute, more specifically, at 8 to 12°C / minute.

[0071] In one embodiment, the step of heat-treating the dried material may be performed for a maximum temperature maintenance time of 1 to 6 hours. In one embodiment, the step of heat-treating the dried material may include a step of naturally cooling after performing the heat treatment for the aforementioned time.

[0072] If the upper limit of the temperature, heating rate, and holding time mentioned above is exceeded, the lithium sulfide powder including lithium oxide is excessively sintered or hardened, which causes problems such as excessive load being applied in the grinding process for product packaging, thereby reducing the ease of handling. If the lower limit of the temperature, heating rate, and holding time mentioned above is exceeded, the purity of the lithium sulfide is lowered because impurities such as lithium hydroxide or lithium carbonate are not easily removed.

[0073] In one embodiment, the step of heat-treating the dried material may be performed in an inert gas atmosphere. The inert gas may include, for example, at least one of helium, neon, krypton, xenon, nitrogen, and argon.

[0074] In one embodiment, the step of heat treating the dried resultant may be performed at a flow rate of the inert gas of 2.0 to 3.0 L / min. Specifically, the flow rate may be performed at a flow rate of 2.3 to 3.8 L / min.

[0075] If the flow rate exceeds the upper limit of the aforementioned range, there is a problem of increased process costs due to the use of excessive inert gas in the manufacturing process. If the flow rate exceeds the lower limit of the aforementioned range, there is a problem of organic solvents or decomposition products of organic solvents evaporating from the dried product not being smoothly discharged but remaining in the form of carbides, thereby lowering the purity of lithium sulfide.

[0076]

[0077] Specific examples of the present invention are described below. However, the following examples are merely specific examples of the present invention, and the present invention is not limited to the following examples.

[0078]

[0079] <Experimental Example>

[0080] <Example 1-6> Effect of drying temperature

[0081] 720 g of graphite and 240 g of lithium sulfate monohydrate were evenly mixed together, and then heated to 900°C at a rate of 5°C per minute in an inert gas (Ar) atmosphere. Subsequently, heat treatment was performed at 900°C for 2 hours, and lithium sulfate was reduced to lithium sulfide, which was obtained in a mixed state with graphite.

[0082] Afterwards, the obtained mixture was naturally cooled in an inert gas atmosphere and stored in a glove box. Then, it was divided into small portions and used in all examples and comparative experiments.

[0083] In an inert gas atmosphere, 20 g of the thermal reduction product is added to 100 mL of ethanol and stirred thoroughly. After stopping the stirring, the mixed slurry is filtered. The graphite that is insoluble in ethanol remains wet in the filter funnel, and the remainder is obtained as a solution in the filter flask. At this time, the color of the solution is light yellow-green.

[0084] Afterwards, the prepared filtrate was transferred to a 1 L round-bottom flask, which was then mounted on a rotary evaporator. The solution was evaporated and concentrated using a vacuum pump to proceed with the drying process. At this time, the experiment was conducted under vacuum for a total of 30 minutes at different drying temperatures, and the experimental conditions for each example are summarized in Table 1 below.

[0085] Specifically, the vacuum pressure was controlled within the range of 15-200 mbar, and an external thermostatic bath was used for the drying temperature. When drying at a temperature below 60°C, a heated water bath was used, and when a temperature above 80°C was required, a thermostatic bath containing silicone oil was used. The powder thus dried was heat-treated at 800°C for 2 hours to obtain lithium sulfide powder. The heat treatment temperature increase rate was 10°C / min, and the Ar gas flow rate was 2.5 L / min.

[0086]

[0087] <Comparative Example 1> Effect of drying temperature

[0088] The same method as described in Examples 1 to 6 was performed, except that the drying temperature and pressure were 180°C and 600 mbar, respectively.

[0089]

[0090] <Examples 7 and 8> Effect of heat treatment temperature

[0091] The procedure for extracting lithium sulfide solution from thermal reduction of lithium sulfate was performed in the same manner as described in Examples 1 to 6 above. The extracted solution was dried under reduced pressure at 100°C to obtain a white dried product, and the dried powders were heat-treated at 700°C and 900°C for 2 hours, respectively. The heat treatment temperature increase rate was 10°C / min, and the flow rate of Ar gas was 2.5 L / min.

[0092]

[0093] <Comparative Examples 2 and 3> Effect of heat treatment temperature

[0094] The procedure for extracting lithium sulfide solution from thermal reduction of lithium sulfate was performed in the same manner as described in Examples 1 to 6 above. The extracted solution was dried under reduced pressure at 100°C to obtain a white dried product, and the dried powders were heat-treated at 500°C and 600°C for 2 hours, respectively. The heat treatment temperature increase rate was 10°C / min, and the flow rate of Ar gas was 2.5 L / min.

[0095]

[0096] Table 1 below shows the heat treatment conversion rate, Li2S ratio, impurity content ratio, and oxygen content according to the drying temperature and heat treatment temperature.

[0097] The heat treatment conversion rate is determined by comparing the amount of dried material with the amount of Li2S heat treatment product containing Li2O obtained by heat treating the dried material, thereby determining the degree of drying of the dried material according to the drying temperature. Specifically, the heat treatment conversion rate was measured as follows.

[0098] (Heat treatment conversion rate, %) = (Weight after heat treatment) / (Weight before heat treatment) × 100

[0099] The crystal phase of lithium sulfide powder was analyzed using an X-ray diffractometer with a D / MAX 2500 equipment from Rigaku.

[0100] The oxygen content in the powder was measured using an elemental analyzer with the ON836-MC equipment from Leco.

[0101] Drying temperature, Drying pressure, Heat treatment temperature, Heat treatment conversion rate, Li2S ratio (XRD), Impurity content ratio (XRD) 1)Oxygen content Li2O(%) / Oxygen content XRD peak ratio (Li2S / Li2O)crude→Li2SLi2OLi2CO3Li2SO4(℃)(mbar)(℃)(%)(%)(%)(wt%)(% / wt%)Example 14015800321008.2--7.11.1512.2Example 26030800431006.1--5.11.2016.4Example 38030800641004.1--3.61.1424.4Example 4100100800681002.7--2.61.0437.0Example 5120100800 711002.4--2.41.0041.7Example 6150200800741002.1--2.50.8447.6Comparative Example 1180600800841002.51.18.710.50.1140.0Comparative Example 2100100500711002.10.7-4.00.5347.6Comparative Example 3100100600711002.30.3-3.30.7043.5Example 7100100700701002.4--2.80.8641.7Example 8100100900691002.6--2.51.0438.51) Relative ratio of the intensity of the main diffraction peak line of each impurity crystal phase to the intensity of the main diffraction peak line of Li2S

[0102] FIG. 1 is an XRD graph showing changes in impurities in the final heat-treated lithium sulfide according to the drying temperature of the present invention. Looking at Table 1 and FIG. 1, it was confirmed that the main impurity in the lithium sulfide heat-treated at a temperature of 800 ℃ was lithium oxide (Li2O), and it was confirmed that as the drying temperature increased, the impurity content decreased and the total oxygen content decreased. Specifically, when the drying temperature was excessively low, it took an excessively long time to obtain a dried product in powder form, and even if the dried product was obtained, it was judged that it would be easy to deform when exposed to the outside air and inconvenient to handle because it was excessively moist. In addition, when the drying temperature was excessively high, as can be confirmed in FIG. 1 and Comparative Example 1, not only lithium oxide but also lithium carbonate (Li2CO3) and lithium sulfate (Li2SO4) were simultaneously generated as impurities in the heat-treated lithium sulfide, and in particular, it was confirmed that the amount of lithium sulfate greatly increased, so that the total oxygen content increased to 10 wt% or more. FIG. 2 is an XRD graph showing changes in impurities in the lithium sulfide according to the heat-treatment temperature of the present invention.

[0103] Referring to Figure 2, it can be confirmed that when the temperature is low, Li2O and Li2CO3 are present in lithium sulfide, and when the temperature is above 700 ℃, only Li2O is observed.

[0104]

[0105] <Evaluation example>

[0106] The solid electrolyte (Li6PS5Cl) was synthesized using a dry milling method. Li2S, P2S5, and LiCl were mixed at 300 rpm for about 8 hours using a planetary mill, and then pellets were made at 300 MPa and synthesized through heat treatment at 550°C in an Ar atmosphere. At this time, the oxygen content of the lithium sulfide used was as described in Table 2 below. In the case of lithium sulfide containing only lithium oxide, the samples indicated in Examples 2 and 4 were used. In addition, for comparison, reagent-grade lithium sulfide with a purity of 99.9% was used in Comparative Example 4. In Example 9, in the case of lithium sulfide containing both lithium oxide and lithium carbonate, a sample was used in which the extracted lithium sulfide solution was dried under reduced pressure at 60°C and then heat-treated at 600°C for 2 hours.

[0107] For comparison with Example 9, the sample shown in Comparative Example 3 was used. After producing a solid electrolyte pellet, the positive electrode was bonded to the upper portion and the counter electrode, In, to the lower portion, and then the density was increased to 500 MPa. After assembling the all-solid-state battery cell, formation cycles were performed at 0.1 C in a 30°C chamber, and the ionic conductivity values ​​were measured.

[0108] To evaluate the atmospheric stability of the solid electrolyte, the solid electrolyte synthesized under the conditions described above was exposed to a dry room atmosphere (dew point -55 to -45°C) for 8 hours, and then a battery cell was manufactured using the same method as described above, and the ionic conductivity value was measured under the same conditions.

[0109] Lithium sulfide solid electrolyte purity and impurity content ratio 1) Oxygen content [Li2O] / oxygen content Ion conductivity Li2OLi2CO3 Before exposure After exposure Retention rate (%) (%) (%) (wt%) (mS cm -1 )(mS cm -1)(%)Example 26.1-5.11.204.03.075Example 42.7-2.61.043.02.377Example 94.80.44.11.172.92.379Comparative Example 32.30.33.30.703.12.374Comparative Example 499.9--0.4-3.52.6741) Relative ratio value of the intensity of the main diffraction peak line of each impurity crystal phase to the intensity of the main diffraction peak line of Li2S

[0110] Looking at Table 2 above, it was confirmed that by controlling the oxygen content within the range of the present invention, an ionic conductivity value superior to that of the solid electrolyte using reagent-grade lithium sulfide of Comparative Example 4 was exhibited. This is believed to be because, in the crystal structure of the argyrodite material, which is a solid electrolyte, some of the sulfur atom sites are replaced by oxygen atoms of lithium oxide, which are more stable to the atmosphere, thereby improving the maintenance of ionic conductivity due to exposure to the atmosphere. As in Example 9, when the oxygen content was controlled to 4.1 wt% and the value of [Li2O] / oxygen content in Formula 1, which contains both lithium oxide and lithium carbonate but has a relatively high proportion of lithium oxide, is 1 or more, it was confirmed that an ionic conductivity value superior to that of the solid electrolyte using reagent-grade lithium sulfide was exhibited even when lithium sulfide was used. However, as can be seen in Comparative Example 3, even though the oxygen content was adjusted to 3.3 wt%, when the ratio of lithium carbonate in lithium sulfide was higher than that of lithium oxide, and the value of [Li2O] / oxygen content in Equation 1 was 0.7, the ionic conductivity value of the solid electrolyte applied with this was shown to decrease.

[0111]

[0112] The present invention is not limited to the above-described embodiments and / or examples, but can be manufactured in various different forms. Those skilled in the art will understand that the present invention can be implemented in other specific forms without altering the technical spirit or essential characteristics of the present invention. Therefore, the above-described embodiments and / or examples should be understood as illustrative in all respects and not restrictive.

Claims

1. Regarding lithium sulfide powder for all-solid-state electrolyte, Lithium sulfide (Li2S); and Contains lithium oxide (Li2O), Lithium sulfide powder having an oxygen content of 2.4 to 7.1 wt%.

2. In paragraph 1, Lithium sulfide powder satisfying the following equation 1. <Formula 1> 0.70 < [Li2O] / Oxygen content ≤ 2.0 (In the above formula 1, [Li2O] means the XRD peak ratio (%) of lithium oxide in lithium sulfide powder, and the oxygen content means the oxygen content (wt%) in lithium sulfide powder) 3. In paragraph 1, The above formula 1 is a lithium sulfide powder having a pH of 0.80 to 1.

20.

4. In paragraph 1, The above lithium oxide (Li2O) is a lithium sulfide powder satisfying an XRD peak ratio (%) of 10.0% or less.

5. In paragraph 1, A lithium sulfide powder having a content ratio of lithium oxide (Li2O) and lithium carbonate (Li2CO3) in the lithium sulfide powder, wherein the content ratio satisfies the following formula 2. <Formula 2> Lithium oxide (Li2O) > lithium carbonate (Li2CO3) ≥ 0 6. In paragraph 1, Lithium sulfide powder not containing lithium carbonate (Li2CO3).

7. A step of producing a lithium-carbon compound by mixing a carbon raw material and a lithium compound; A step of filtering a solution containing the lithium-carbon compound and the solvent; A step of drying the above filtrate at a temperature range of 30 to 170°C; and A method for producing lithium sulfide powder, comprising the step of heat-treating the dried resultant at a temperature range of 650 to 950° C.

8. In paragraph 7, A method for producing lithium sulfide powder having a dielectric constant of 20 to 35 of the solvent in a step of filtering a solution mixed with the lithium-carbon compound and the solvent.

9. In paragraph 7, In the step of manufacturing a lithium-carbon compound by mixing carbon raw materials and lithium compounds, A method for producing lithium sulfide powder, wherein the weight ratio of the lithium compound to the carbon raw material is 0.25 to 0.

5.

10. In paragraph 7, A method for producing lithium sulfide powder, wherein the step of producing a lithium-carbon compound by mixing a carbon raw material and a lithium compound includes the step of heat-treating a mixture of the carbon raw material and the lithium compound at a temperature range of 850 to 950° C.

11. In clause 10, A method for producing lithium sulfide powder, wherein the step of heat-treating a mixture of the carbon raw material and the lithium compound at a temperature range of 850 to 950°C is performed at a heating rate of 3 to 7°C / min.

12. In paragraph 7, A method for producing lithium sulfide powder, wherein the step of drying the above filtrate is performed at a vacuum pressure in the range of 10 to 500 mbar.

13. In paragraph 7, A method for producing lithium sulfide powder, wherein the step of heat treating the dried resultant is performed at 5 to 20°C / min.

14. In paragraph 7, A method for producing lithium sulfide powder, wherein the step of heat treating the dried resultant is performed in an inert gas atmosphere.

15. In paragraph 14, A method for producing lithium sulfide powder, wherein the step of heat-treating the dried resultant is performed at a flow rate of the inert gas of 2.0 to 3.0 L / min.

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