Process for producing lithium sulfide with reduced carbon impurities / free of carbon impurities, said lithium sulfide with reduced carbon impurities / free of carbon impurities, and its use for producing solid electrolytes and all-solid-state batteries.

Abstract

The present invention relates to a process for producing lithium sulfide with reduced or no carbon impurities, comprising treating lithium sulfate and optionally lithium sulfite, lithium disulfate and / or other lithium sulfur oxides, as well as lithium sulfide containing carbon impurities, with hydrogen gas at a temperature in the range of 300 to 600°C. The present invention further relates to lithium sulfide thus produced, wherein the carbon impurity content is less than 2.0% by weight, based on the weight of the lithium sulfide. This lithium sulfide is used in the manufacture of battery components, preferably solid electrolytes, and all-solid-state batteries.

Description

【Technical Field】 【0001】 The present invention relates to a method for preparing lithium sulfide with reduced carbon impurities or free of carbon impurities, or respectively, a method for purifying lithium sulfide to efficiently remove or avoid impurities such as residual carbon or other carbon-containing impurities and lithium sulfate used in electronic materials and electrical materials from lithium sulfide. 【0002】 Furthermore, the present invention relates to the above purified lithium sulfide, a solid electrolyte for a rechargeable lithium battery, and a solid battery including such a solid electrolyte. 【Background Art】 【0003】 Lithium sulfide is currently attracting much attention as a raw material for preparing solid electrolytes for all-solid-state batteries (Lee et.al, Acc. Chem.Res, 54, 3390, 2021). All-solid-state batteries offer higher energy density and faster charging capabilities compared to the state of the art. Furthermore, all-solid-state batteries are generally considered to be safer because they do not contain highly flammable organic solvents (Lee et.al, Acc. Chem.Res, 54, 3390, 2021). Furthermore, lithium sulfide has applications as a cathode material for lithium / sulfur batteries (EP2896085A1). Lithium / sulfur batteries are also of interest for potential applications in the field of electromobility because they have a significantly higher energy density compared to conventional lithium-ion batteries. 【0004】 If the purity level of raw materials such as solid electrolytes used in rechargeable batteries is low, the aging deterioration of components may be accelerated. Therefore, the purity level of solid electrolytes or other raw materials must be high (EP1681263A1). In particular, graphitic carbon in lithium sulfide as a raw material for solid electrolytes can lead to undesirable electronic conductivity in the solid electrolyte and must be avoided as completely as possible (Nikodimos et.al, Energy Environ. Sci., 2022, 15, 991). 【0005】 Lithium sulfide can be prepared by simple means, and the process for preparing lithium sulfide is well known (e.g., EP0802575A1). 【0006】 One known process describes the production of lithium sulfide by high-temperature carbonthermal reduction from lithium sulfate and carbon (CN106229487A). Since the production steps can be carried out in sequence, it is essentially an economical and simple process. Furthermore, the raw materials, lithium sulfate and carbon, are readily available. However, carbonthermal reduction often results in significant impurities in the lithium sulfide. These are typically unreacted reactants such as carbon or lithium sulfate. Additionally, lithium sulfite, lithium carbonate, and / or lithium oxide may be formed. 【0007】 Another process describes the reduction of lithium sulfate to lithium sulfide by hydrogen at high temperatures, in a molten state at approximately 1150°C (US-A2840455). Such molten materials solidify after cooling and do not yield the desired powdered lithium sulfide. In these circumstances, the reduction to lithium sulfide by hydrogen becomes economically unattractive. 【0008】 When lithium sulfide is produced by the carbon thermal process, typical contamination by residual carbon means that lithium sulfide is an additional, undesirable raw material for solid electrolytes in rechargeable lithium batteries. This does not result in electronic conduction, and therefore the desired battery performance and long-term stability cannot be achieved. 【0009】 The object of the present invention is to solve this problem by providing a process for producing lithium sulfide with reduced or no carbon impurities, in which the content of carbonaceous impurities in the lithium sulfide constituting the raw material for the solid electrolyte of a rechargeable lithium battery is minimized or completely avoided. 【0010】 Another object of the present invention is to provide such lithium sulfide with reduced or no carbon impurities, in particular a solid electrolyte for rechargeable lithium-ion batteries using such lithium sulfide, and a solid battery in which carbon impurities are minimized or absent. 【0011】 These objectives are addressed by a process for producing lithium sulfide with reduced or no carbon impurities, characterized by treating lithium sulfate and optionally lithium sulfite, lithium disulfate and / or other lithium sulfur oxides, as well as lithium sulfide containing carbon impurities, with hydrogen gas at a temperature in the range of 300 to 600°C. 【0012】 The hydrogen gas-treated lithium sulfide is preferably produced by the reaction of lithium sulfate with a carbon source, preferably carbon black, and the lithium sulfate is added in stoichiometric excess. 【0013】 Unreacted lithium sulfate, optionally produced lithium sulfite, lithium disulfate, or other lithium sulfur oxides are then reduced to lithium sulfide by hydrogen gas at temperatures ranging from 300 to 600°C. 【0014】 Therefore, the present invention provides lithium sulfide with low or no carbon impurities, which is produced by this process according to the present invention. 【0015】 Furthermore, the present invention relates to the use of such lithium sulfide, preferably in a solid electrolyte, for the manufacture of battery components. 【0016】 Therefore, the present invention also relates to a lithium sulfide purification process that can efficiently remove impurities such as lithium sulfate, lithium sulfite, lithium disulfide, or other lithium sulfur oxides and carbonaceous impurities from lithium sulfide. 【0017】 Furthermore, the present invention relates to such solid electrolytes for rechargeable lithium-ion batteries, and to corresponding solid batteries. 【0018】 Surprisingly, the present invention has shown that carbon impurities in lithium sulfide, such as carbon or carbon-containing inorganic or organic compounds, can be minimized or even completely avoided by using a stoichiometric excess of lithium sulfate and by post-treatment with hydrogen at temperatures in the range of 300-600°C, without the drawbacks expected in the prior art. In particular, it is possible to completely avoid the formation of molten material that is usually observed when lithium sulfate is directly reduced with hydrogen. 【0019】 The residual carbon / residual carbon compound content of lithium sulfide treated with hydrogen gas according to the present invention is less than 2.0% by weight, preferably less than 1.0%, and more preferably less than 0.5%. In particular, it should be less than 0.3%, and more preferably less than 0.25%. Ideally, it should be less than 0.05% by weight, or even 0% by weight. 【0020】 The lithium sulfide used in the production of lithium sulfide with reduced or no carbon impurities according to the present invention is preferably produced by a carbothermal method, in which stoichiometric excess lithium sulfate is first reduced to lithium sulfide using a carbon source, preferably carbon black. 【0021】 Carbon sources or carbon impurities may include carbon in crystalline and amorphous forms. Crystalline forms include graphite, graphite-like carbon (including carbon black or activated carbon), graphene, fullerene, or carbon nanotubes. Carbonaceous impurities include both inorganic carbon compounds (e.g., carbides) and organic carbon compounds. 【0022】 Preferably, the lithium sulfate used is high-purity anhydrous lithium sulfate obtained from lithium-containing minerals such as spodumene and brine, or from recycled lithium-ion batteries. In particular, a specific surface area of ​​1 to 1000 m² is preferred. 2 / g, preferably 100 to 200 m 2 Carbon black of 2 / g is used as a carbon source. 【0023】 To produce a lithium sulfate / carbon mixture that is as homogeneous as possible, it is preferable to mix the two components in a planetary ball mill lined with zirconium dioxide. To mix better, zirconium dioxide balls can also be added to the grinding bowl. The grinding time is generally 1 to 24 hours, preferably 1 to 3 hours. 【0024】 The lithium sulfate / carbon mixture usually reacts in a temperature range of 650 to 900 °C, preferably in a temperature range of 750 to 850 °C, under inert conditions. For the purposes of the present invention, it is understood that inert conditions mean working under an inert gas, excluding air and humidity. For this purpose, the lithium sulfate / carbon mixture is weighed, mixed as uniformly as possible, filled into a heat-resistant crucible (e.g., aluminum oxide, boron nitride, or vitreous carbon), and reacted according to the following reaction formula. (1 + x) Li2SO4+ 2 C → Li2S + x Li2SO4+ 2 CO2 Here, x represents the excess lithium sulfate (x = 0.0005 to 1). 【0025】 The molar ratio of lithium sulfate to carbon is thus in the range of 1.0005:2 to 2:2 (corresponding to x = 0.0005 to 1), preferably with a stoichiometric excess of lithium sulfate in the range of 0.05 to 10 wt%, more preferably in the range of 0.1 to 5 mol%, based on lithium sulfate. 【0026】 This reaction results in lithium sulfide with reduced or no carbon. The excess lithium sulfate needs to be removed in a post-treatment step. 【0027】 That problem is solved by the step of purifying lithium sulfide according to the present invention as follows. 【0028】 According to the present invention, contaminated lithium sulfide is treated with a hydrogen-containing gas mixture, and the hydrogen content can be 1 to 100% by volume, preferably 5 to 10% by volume, and the remainder of the hydrogen gas is nitrogen and / or argon. 【0029】 The hydrogen gas treatment according to the present invention is carried out in a temperature range of 300 to 600 °C, preferably 400 to 575 °C, more preferably 450 to 550 °C, still more preferably 475 to 525 °C, and most preferably 49 0 to 510 °C, particularly 495 to 505 °C. 【0030】 The treatment time with hydrogen gas according to the present invention is preferably 1 to 10 hours, particularly 1 to 8 hours, and preferably 1 to 5 hours. For this purpose, for example, commercially available "forming gas", that is, a mixture of hydrogen and nitrogen and / or argon, can be used. 【0031】 According to the present invention, lithium sulfide contaminated with lithium sulfate can be treated at 300 to 600 °C for 1 to 10 hours in forming gas containing 5% hydrogen by volume, for example, according to the following formula. The amount of H2 required is at least 4 times the stoichiometric amount of the residual lithium sulfate portion: Li2S + y Li2SO4 + 4 y H2 → 1 + y Li2S + 4 y H2O According to the reaction formula, the residual lithium sulfate in this reaction is removed from lithium sulfide by generating gaseous water. What remains is purified white crystalline lithium sulfide. 【0032】 Exemplary isolated materials (see Examples 1 and 2) show lines only at the desired Li2S content (>99% by weight) in the X-ray diffraction pattern, and the carbon content is <0.06% by weight. 【0033】 According to the present invention, lithium sulfide is preferably overflowed with a flow of hydrogen gas during the treatment. 【0034】 Measurement method The phase purity of the sample was confirmed using a Bragg-Brentano type Bruker D2-Phaser X-ray powder diffractometer. An X-ray tube emitting Cu-Kα radiation (λ=0.15418 nm) was used as the radiation source. Lithium sulfate was quantified using the Rietveld method. 【0035】 The elemental analysis unit of the Keyence VHX-7000 digital microscope was used to quantify lithium, sulfur, and carbon in lithium sulfide. A UV laser (λ=250nm, P=0.01mW) was used to evaporate and atomize a small sample (<1mg). Characteristic atomic radiation was detected and used for quantification. 【0036】 Therefore, the advantages of the process according to the present invention, compared to the state of the art, are as follows: • Direct purification of lithium sulfide obtained by carbonothermal reduction, and the availability of low-carbon / carbon-free lithium sulfide for the production of the resulting solid electrolyte. • Use of commercially available and readily available starting materials, Avoid handling solids that are sensitive to air and moisture, such as Li metal, lithium hydride, lithium alkyl, lithium aryl, or lithium amide. Avoid handling toxic sulfur sources such as hydrogen sulfide or carbon disulfide. • From recycling lithium-ion batteries without energy-intensive conversion, to, for example, the direct use of lithium sulfate in lithium hydroxide. • Avoidance of organic solvents (e.g., THF) for further purification of lithium sulfide through additional process steps. 【0037】 It is preferable that all operations be performed inside a glove box filled with Ar. [Brief explanation of the drawing] 【0038】 [Figure 1]X-ray diffraction patterns of the Li2S sample (red) from Example 1 with H2 post-treatment and the Li2S sample (blue) without H2 post-treatment with visible Li2SO4 impurities. [Examples] 【0039】 The above measurement method was used in the following example to determine the properties of the product. 【0040】 Example 1: 4.62g (42 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) and approximately 176m 2 0.96 g (80 mmol) of carbon black (Cabot's Vulcan P Fluffy) with a specific surface area of ​​1 / g was weighed and then finely ground in an agate mortar. The lithium sulfate / carbon mixture was then transferred to a Fritsch zirconium dioxide-lined grinding bowl. Twelve grinding balls with a diameter of 10 mm were added. The grinding bowl was then sealed under inert gas and placed in a Fritsch Palveriset 7 planetary ball mill. The mixture was ground at 600 rpm for 2 hours. After the grinding process, the zirconium dioxide balls were sieved. The homogenized lithium sulfate / carbon mixture was then transferred to a corundum annealing box. This mixture was converted to lithium sulfide at 850°C for 3.3 hours under a nitrogen stream. The lithium sulfide, still contaminated with lithium sulfate, was then treated at 900°C for 6 hours with a foaming gas containing 5 vol% hydrogen. After cooling, it was purged with nitrogen. The purity of the purified lithium sulfide phase was checked by X-ray diffraction. The obtained lithium sulfide was a microcrystalline powder, and it did not show signs of sintering or other aggregates. 【0041】 Li2S content:>99% Li2SO4 content:<1.0% Residual carbon content: <0.06% Li2S color: Pure white 【0042】 Example 2: 4.84g (44 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) and approximately 176m 2 0.96 g (80 mmol) of carbon black (Cabot's Vulcan P Fluffy) with a specific surface area of ​​1 / g was weighed and then finely ground in an agate mortar. The lithium sulfate / carbon mixture was then transferred to a Fritsch zirconium dioxide-lined grinding bowl. Twelve grinding balls with a diameter of 10 mm were added. The grinding bowl was then sealed under an inert gas and placed in a Fritsch Palveriset 7 planetary ball mill. The mixture was ground at 600 rpm for 20 hours. After the grinding process, the zirconium dioxide balls were sieved. The homogenized lithium sulfate / carbon mixture was then transferred to a corundum annealing box. This mixture was converted to lithium sulfide at 800°C for 8 hours under a nitrogen stream. The still carbon-contaminated lithium sulfide was then treated at 525°C for 8 hours with a foaming gas containing 5 vol% hydrogen. After cooling, it was purged with nitrogen. Phase purity was confirmed by X-ray diffraction. The obtained lithium sulfide was a microcrystalline powder, but it did not exhibit sintering or other aggregate formation. 【0043】 Li2S content:>99% Li2SO4 content:<1.0% Residual carbon content: <0.06% Li2S color: Pure white 【0044】 Comparative Example 1: Reaction without using excess lithium sulfate 4.4g (40 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) and approximately 176m 2 0.96g (80m²) has a specific surface area of ​​ / g. 100ml of carbon black (Cabot's Vulcan P Fluffy) was weighed and then finely ground in an agate mortar. The lithium sulfate / carbon mixture was then transferred to a Fritsch zirconium dioxide-lined grinding bowl. Twelve grinding balls, each 10 mm in diameter, were added. The grinding bowl was then sealed under inert gas and placed in a Fritsch Palveriset 7 planetary ball mill. The mixture was ground at 600 rpm for 2 hours. After the grinding process, the zirconium dioxide balls were sieved. The homogenized lithium sulfate / carbon mixture was then transferred to a corundum annealing box. This mixture was converted to lithium sulfide at 850°C for 3.3 hours under a nitrogen stream. The still contaminated lithium sulfide was then treated at 525°C for 8 hours with a foaming gas containing 5 vol% hydrogen. After cooling, it was purged with nitrogen. Phase purity was confirmed by X-ray diffraction. The obtained lithium sulfide was a microcrystalline powder, but it did not exhibit sintering or other aggregate formation. 【0045】 Li2S content:>95% Li2SO4 content: Undetectable Residual carbon content: Approximately 5% Li2S color: Pearl dark gray 【0046】 Comparative Example 2: Reaction using an excess amount of lithium sulfate, without H2 workup. 4.84g (44 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) and approximately 176m 20.96 g (80 mmol) of carbon black (Cabot's Vulcan P Fluffy) with a specific surface area of ​​1 / g was weighed and then finely ground in an agate mortar. The lithium sulfate / carbon mixture was then transferred to a Fritsch zirconium dioxide-lined grinding bowl. Twelve grinding balls with a diameter of 10 mm were added. The grinding bowl was then sealed under inert gas and placed in a Fritsch Palveriset 7 planetary ball mill. The mixture was ground at 600 rpm for 2 hours. After the grinding process, the zirconium dioxide balls were sieved. The homogenized lithium sulfate / carbon mixture was then transferred to a corundum annealing box. This mixture was converted to lithium sulfide at 850°C for 8 hours under a nitrogen stream. The X-ray diffraction patterns in Figure 1 show Comparative Example 1 and Comparative Example 2. 【0047】 Li2S content: 97% Li2SO4 content: 3% Residual carbon content: <0.06% Li2S: pure white 【0048】 Example 3: Preparation of the solid electrolyte Li6PS5Cl 2.140 g (46.57 mmol) of lithium sulfide, 2.070 g (9.312 mmol) of phosphorus pentasulfide (99%, Sigma Aldrich), and 0.790 g (18.6 mmol) of lithium chloride (battery grade, Albemarle Germany GmbH), prepared in Example 1, were weighed and then finely ground. The mixture was then transferred to a grinding bowl lined with zirconium dioxide made by Fritsch. Twelve grinding balls with a diameter of 10 mm were added. The grinding bowl was then sealed under inert gas and placed in a Fritsch Pulverizet 7 planetary ball mill. The mixture was ground at 600 rpm for 20 hours. After the grinding process, the zirconium dioxide balls were removed. The homogenized mixture was then transferred to a metal cylinder and sealed with a screw cap. After 48 hours at 370°C in a chamber furnace, the conversion to the solid electrolyte Li6PS5Cl was completed. The phase purity of the solid electrolyte was confirmed by X-ray powder diffraction.

Claims

[Claim 1] A process for preparing lithium sulfide with reduced or no carbon impurities, characterized by treating lithium sulfide containing lithium sulfate and optionally lithium sulfite, lithium disulfate and / or other lithium sulfur oxides with hydrogen gas at a temperature range of 300 to 600°C. [Claim 2] The process according to claim 1, characterized in that the lithium sulfide treated with hydrogen gas is prepared by the reaction of lithium sulfate with a carbon source, preferably carbon black, and the lithium sulfate is added in a stoichiometric excess. [Claim 3] The process according to claim 2, characterized in that the lithium sulfate / carbon molar ratio is in the range of 1.0005 to 2:2, and preferably in a range where lithium sulfate is stoichiometrically in excess, and is used in an amount of 0.05 to 10 mol%, more preferably 0.1 to 5 mol%) relative to lithium sulfate. [Claim 4] The process according to any one of claims 1 to 3, characterized in that the hydrogen gas is a foaming gas, the hydrogen content of the hydrogen gas is 1 to 100% by volume, preferably 5 to 10% by volume, and the remainder of the hydrogen gas may be nitrogen and / or argon. [Claim 5] The process according to any one of claims 1 to 4, characterized in that the treatment with hydrogen is carried out at 450 to 550°C, particularly preferably at 500°C. [Claim 6] The process according to any one of claims 1 to 5, characterized in that the carbon impurity content of the treated lithium sulfide, based on the weight of the lithium sulfide, is less than 2.0% by weight, preferably less than 1.0% by weight, more preferably less than 0.5% by weight, particularly preferably less than 0.3% by weight, even more preferably less than 0.25% by weight, particularly preferably less than 0.05% by weight, or 0% by weight. [Claim 7] The process according to any one of claims 1 to 6, characterized in that the treatment with hydrogen gas is carried out at 400 to 575°C, preferably 450 to 550°C, more preferably 475 to 525°C, even more preferably 490 to 510°C, and most preferably 495 to 505°C. [Claim 8] The process according to any one of the prior claims, characterized in that the treatment time with hydrogen gas is 1 to 10 hours, preferably 1 to 8 hours, and particularly 1 to 5 hours. [Claim 9] Lithium sulfide that can be produced by a process defined in any one of claims 1 to 8. [Claim 10] Preferably, in a solid electrolyte, use of carbon-reduced / carbon-free lithium sulfide as defined in claim 9 for the manufacture of battery components. [Claim 11] A solid electrolyte, particularly for a rechargeable lithium-ion battery, comprising lithium sulfide as defined in claim 9. [Claim 12] A solid battery comprising a solid electrolyte as defined in claim 11.

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