Method for producing lithium sulfide with reduced / no carbon impurities, lithium sulfide with reduced / no carbon impurities and use thereof in production of solid electrolyte and solid battery

Abstract

The invention relates to a method for producing lithium sulfide with reduced or no carbon impurities, in which lithium sulfate and optionally lithium sulfite, lithium disulfate and / or other lithium sulfur oxides, as well as the carbon impurities containing lithium sulfide, are treated with hydrogen gas at a temperature in the range of 300 to 600 DEG C. The invention also relates to a lithium sulfide which can be produced in this way and which has a carbon impurity content of less than 2.0% by weight, based on the weight of the lithium sulfide. The lithium sulfide is used to produce battery components, preferably solid electrolytes, and solid state batteries.

Description

[0001] This invention relates to a method for preparing lithium sulfide with reduced or no carbon impurities, or for purifying lithium sulfide to effectively remove or avoid impurities (such as residual carbon or other carbon-containing impurities and lithium sulfate) from lithium sulfide / lithium sulfide for use in electronic and electrical materials.

[0002] Furthermore, the present invention relates to the purified lithium sulfide, a solid electrolyte for a rechargeable lithium battery, and a solid-state battery containing such a solid electrolyte.

[0003] Lithium sulfide has attracted considerable interest as a raw material for preparing solid electrolytes in solid-state batteries (Lee et al., Acc. Chem. Res, 54, 3390, 2021). Compared to existing technologies, solid-state batteries offer higher energy density and faster charging speeds. Furthermore, solid-state batteries are generally considered safer because they do not contain highly flammable organic solvents (Lee et al., Acc. Chem. Res, 54, 3390, 2021). Additionally, lithium sulfide can be used as a cathode material in lithium / sulfur batteries (EP 2 896 085 A1). Compared to conventional lithium-ion batteries, lithium / sulfur batteries also exhibit significantly higher energy density, thus possessing potential applications in the electric vehicle sector.

[0004] Low purity of raw materials (such as solid electrolytes used in rechargeable batteries) accelerates component aging. Therefore, the purity level of solid electrolytes or other raw materials must be very high (EP 1 681 263 A1). In particular, the use of graphitized carbon from lithium sulfide as a raw material for solid electrolytes must be avoided as much as possible, as it leads to poor electronic conductivity in solid electrolytes (Nikodimos et al., Energy Environ. Sci., 2022, 15, 991).

[0005] Methods for preparing lithium sulfide are well known, and lithium sulfide can be prepared in a simple manner by these methods (e.g., EP 0 802 575 A1).

[0006] A known method describes the production of lithium sulfide from lithium sulfate and carbon via carbothermic reduction at high temperatures (CN106229487 A). This is essentially an economical and simple method because the production steps can be carried out continuously. Furthermore, the raw materials lithium sulfate and carbon are readily available. However, carbothermic reduction typically results in lithium sulfide containing a significant amount of impurities. These are usually unreacted reactants, such as carbon or lithium sulfate. Additionally, lithium sulfite, lithium carbonate, and / or lithium oxide may also be formed.

[0007] Another method describes the high-temperature reduction of lithium sulfate to lithium sulfide in a melt at approximately 1150°C using hydrogen (US-A 2 840 455). This melt solidifies upon cooling and does not produce the desired powdered lithium sulfide. This makes the reduction to lithium sulfide using hydrogen economically unattractive.

[0008] If lithium sulfide is produced via the carbothermal method, the typical residual carbon contamination will cause lithium sulfide to have additional poor electronic conductivity as a raw material for solid electrolytes in rechargeable lithium batteries, thus failing to achieve the required battery performance and long-term stability.

[0009] One object of the present invention is to solve this problem by providing a method for producing lithium sulfide with reduced or no carbon impurities, wherein the content of carbon and / or carbonaceous impurities in the lithium sulfide constituting the solid electrolyte feedstock of rechargeable lithium batteries is minimized or completely avoided.

[0010] Another object of the present invention is to provide such lithium sulfide with reduced or no carbon impurities, a solid electrolyte using such lithium sulfide (particularly for rechargeable lithium-ion batteries), and a solid battery with very little or no carbon impurities.

[0011] These objectives are achieved by a method for producing lithium sulfide with reduced or no carbon impurities, characterized by treating lithium sulfide containing lithium sulfate and optionally lithium sulfite, lithium disulfide and / or other lithium sulfide oxides and carbon impurities with hydrogen at a temperature in the range of 300 to 600°C.

[0012] Hydrogen-treated lithium sulfide is preferably produced by the reaction of lithium sulfate with a carbon source (preferably carbon black), wherein lithium sulfate is added in stoichiometric excess.

[0013] Unreacted lithium sulfate, optionally generated lithium sulfite, lithium disulfide, or other lithium sulfide oxides are then reduced to lithium sulfide by hydrogen at a temperature in the range of 300 to 600 °C.

[0014] Therefore, the present invention provides lithium sulfide with low or no carbon impurities, which is produced by the method according to the present invention.

[0015] Furthermore, the present invention relates to the use of such lithium sulfide in the production of battery components (preferably in solid electrolytes).

[0016] Therefore, the present invention also relates to a method for purifying lithium sulfide, which can effectively remove impurities from lithium sulfide, such as lithium sulfate, lithium sulfite, lithium disulfide or other lithium sulfur oxides and carbonaceous impurities.

[0017] Furthermore, the present invention relates to such a solid electrolyte for rechargeable lithium-ion batteries and the corresponding solid battery.

[0018] Surprisingly, according to the present invention, 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 a temperature in the range of 300 to 600°C, without the disadvantages anticipated in the prior art. In particular, melt formation can be completely avoided, which is typically observed when lithium sulfate is directly reduced with hydrogen.

[0019] The residual carbon / residual carbon compound content of the hydrogen-treated lithium sulfide according to the invention is less than 2.0 wt%, preferably less than 1.0 wt%, more preferably less than 0.5 wt%, particularly less than 0.3 wt%, and even more preferably less than 0.25 wt%. Ideally, it is less than 0.05 wt%, or even 0 wt%.

[0020] The lithium sulfide according to the invention for producing lithium sulfide with reduced / free carbon impurities is preferably produced by first reducing a stoichiometric excess of lithium sulfate to lithium sulfide using a carbon source (preferably carbon black).

[0021] Carbon sources or carbon impurities can include crystalline and amorphous forms of carbon. Crystalline forms include graphite, graphitic 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), brine, or recycled lithium-ion batteries. Specifically, a specific surface area of ​​1 to 1000 m² is used. 2 / g, preferably 100 to 200 m 2 / g of carbon black was used as the carbon source.

[0023] To produce the most homogeneous lithium sulfate / carbon mixture possible, the two components are best mixed in a zirconia-lined surface ball mill. Zirconia balls can also be added to the grinding bowl for better mixing. Grinding time is typically 1 to 24 hours, preferably 1 to 3 hours.

[0024] Lithium sulfate / carbon mixtures are typically reacted under inert conditions at a temperature range of 650 to 900°C, preferably 750 to 850°C. For the purposes of this invention, inert conditions should be understood as operating under an inert gas atmosphere, with air and moisture excluded. For this purpose, the lithium sulfate / carbon mixture is weighed, mixed as uniformly as possible, placed in a high-temperature resistant crucible, such as alumina, boron nitride, or glassy carbon, and reacted according to the following reaction equation: (1+x) Li2SO4+ 2 C → Li2S + x Li2SO4+ 2 CO2 Where x represents excess lithium sulfate (x = 0.0005 to 1).

[0025] Therefore, the lithium sulfate / carbon molar ratio is in the range of 1.0005:2 to 2:2 (corresponding to x = 0.0005 to 1), preferably, based on lithium sulfate, there is a stoichiometric excess of lithium sulfate in the range of 0.05 to 10 mol%, and even more preferably, there is a stoichiometric excess of lithium sulfate in the range of 0.1 to 5 mol%.

[0026] The reaction produces lithium sulfide, which is carbon-reduced or carbon-free. Excess lithium sulfate must be removed in a post-processing step.

[0027] The problem is solved by the following steps for purifying lithium sulfide according to the present invention: According to the present invention, contaminated lithium sulfide is treated with a hydrogen-containing gas mixture, wherein the hydrogen content can be from 1% to 100% by volume, preferably from 5% to 10% by volume, and the remainder of the hydrogen is nitrogen and / or argon.

[0028] The hydrogen treatment according to the invention is carried out in a temperature range of 300 to 600°C, preferably 400 to 575°C, preferably 450 to 550°C, more preferably 475 to 525°C, still more preferably 490 to 510°C, and especially 495 to 505°C.

[0029] The hydrogen treatment time according to the invention is preferably 1 to 10 hours, particularly 1 to 8 hours, and more preferably 1 to 5 hours. For this purpose, commercially available "synthesis gases," i.e., mixtures of hydrogen and nitrogen and / or argon, can be used, for example.

[0030] According to the present invention, lithium sulfide contaminated with lithium sulfate can be treated for 1 to 10 hours at 300 to 600°C with a synthesis gas containing 5% by volume hydrogen, for example, according to the following equation. The amount of H2 required is at least four times the stoichiometric amount of the residual lithium sulfate fraction: Li2S + y Li2SO4+ 4 y H2→ 1+ y Li2S + 4 y H2O According to the reaction equation, in this reaction, the remaining lithium sulfate is removed from the lithium sulfide by the formation of gaseous water. What remains is purified white crystalline lithium sulfide.

[0031] The exemplary separated materials (see Examples 1 and 2) show only lines of Li2S (content > 99 wt%) and carbon content < 0.06 wt% in the X-ray diffraction pattern.

[0032] According to the present invention, it is preferable to allow the hydrogen gas to escape from the lithium sulfide during the processing.

[0033] Measurement methods Phase purity of the samples was examined using a Bruker D2-Phaser X-ray powder diffractometer with Bragg-Brentano geometry. An X-ray tube with Cu-Kα radiation (λ = 0.15418 nm) was used as the radiation source. Lithium sulfate was quantified using the Rietveld method.

[0034] The elemental composition of lithium, sulfur, and carbon in lithium sulfide was quantified using the elemental analysis unit of a Keyence VHX-7000 digital microscope. Small sample amounts (<1 mg) were evaporated and nebulized using a UV laser (λ = 250 nm; P = 0.01 mW). Characteristic atomic emission lines were detected and used for quantification.

[0035] Compared with the prior art, the method according to the present invention has the following advantages: • Direct purification of lithium sulfide obtained by carbothermal reduction and the resulting low-carbon / carbon-free lithium sulfide can be used to produce solid electrolytes. • Use commercially available starting materials; • Avoid contact with solids that are sensitive to air and moisture, such as lithium metal, lithium hydride, alkyl lithium, aryl lithium, or amino lithium; • Avoid contact with toxic sulfur sources, such as hydrogen sulfide or carbon disulfide; • Directly utilize lithium sulfate recovered from lithium-ion batteries without the need for energy-intensive conversion to, for example, lithium hydroxide; • Avoid using organic solvents (such as THF) for further purification of lithium sulfide through additional process steps.

[0036] All operations are best performed in an argon-filled glove box.

[0037] Example The above measurement method is used to determine product characteristics in the following embodiments.

[0038] Example 1: Weigh out 4.62 g (42 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) and 0.96 g (80 mmol) of a substance with a specific surface area of ​​approximately 176 m². 2 / g of carbon black (Cabot Vulcan P Fluffy) was then thoroughly ground in an agate mortar. The lithium sulfate / carbon mixture was then transferred to a Fritsch zirconia-lined grinding bowl. Twelve grinding balls with a diameter of 10 mm were added. The grinding bowl was then sealed under an inert gas atmosphere and placed in a Fritsch Pulverisette 7 planetary ball mill. The mixture was ground at 600 rpm for 2 hours. After grinding, the zirconia balls were sieved out. The homogenized lithium sulfate / carbon mixture was then transferred to a corundum annealing chamber. The mixture was converted to lithium sulfide at 850°C under a nitrogen atmosphere for 3.3 hours. The lithium sulfide, still contaminated with lithium sulfate, was then post-treated at 500°C for 6 hours by switching to a synthesis gas containing 5% by volume hydrogen. After cooling, it was purged with nitrogen. The phase purity of the purified lithium sulfide was examined by X-ray diffraction. The resulting lithium sulfide was a microcrystalline powder, showing no sintering or other agglomerates.

[0039] Example 2: Weigh out 4.84 g (44 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) and 0.96 g (80 mmol) of a substance with a specific surface area of ​​approximately 176 m². 2 / g of carbon black (Cabot Vulcan P Fluffy) was then thoroughly ground in an agate mortar. The lithium sulfate / carbon mixture was then transferred to a Fritsch zirconia-lined grinding bowl. Twelve grinding balls with a diameter of 10 mm were added. The grinding bowl was then sealed under an inert gas atmosphere and placed in a Fritsch Pulverisette 7 planetary ball mill. The mixture was ground at 600 rpm for 20 hours. After grinding, the zirconia balls were sieved out. The homogenized lithium sulfate / carbon mixture was then transferred to a corundum annealing chamber. The mixture was converted to lithium sulfide at 800°C under a nitrogen atmosphere for 8 hours. The lithium sulfide, still contaminated with lithium sulfate, was then post-treated at 525°C for 8 hours by switching to a synthesis gas containing 5% by volume hydrogen. After cooling, it was purged with nitrogen. Phase purity was examined by X-ray diffraction. The resulting lithium sulfide was a microcrystalline powder, showing no sintering or other agglomerates.

[0040] Comparative Example 1: Reaction without excess lithium sulfate Weigh out 4.4 g (40 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) and 0.96 g (80 mmol) of a substance with a specific surface area of ​​approximately 176 m². 2 / g of carbon black (Cabot Vulcan P Fluffy) was then thoroughly ground in an agate mortar. The lithium sulfate / carbon mixture was then transferred to a Fritsch zirconia-lined grinding bowl. Twelve grinding balls with a diameter of 10 mm were added. The grinding bowl was then sealed under an inert gas atmosphere and placed in a Fritsch Pulverisette 7 planetary ball mill. The mixture was ground at 600 rpm for 2 hours. After grinding, the zirconia balls were sieved out. The homogenized lithium sulfate / carbon mixture was then transferred to a corundum annealing chamber. The mixture was converted to lithium sulfide at 850°C under a nitrogen atmosphere for 3.3 hours. The still contaminated lithium sulfide was then post-treated at 525°C for 8 hours by switching to a synthesis gas containing 5% by volume hydrogen. After cooling, it was purged with nitrogen. Phase purity was examined by X-ray diffraction. The resulting lithium sulfide was a microcrystalline powder, showing no sintering or other agglomerates.

[0041] Comparative Example 2: The reaction was carried out with excess lithium sulfate and without H2 post-treatment. Weigh out 4.84 g (44 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) and 0.96 g (80 mmol) of a substance with a specific surface area of ​​approximately 176 m². 2 / g of carbon black (Cabot Vulcan P Fluffy) was then thoroughly ground in an agate mortar. The lithium sulfate / carbon mixture was then transferred to a Fritsch zirconia-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 Pulverisette 7 planetary ball mill. The mixture was ground at 600 rpm for 2 hours. After grinding, the zirconia balls were sieved out. The homogenized lithium sulfate / carbon mixture was then transferred to a corundum annealing chamber. The mixture was converted to lithium sulfide at 850°C under a nitrogen atmosphere for 8 hours. Figure 1 The X-ray diffraction patterns in the image show a comparison between Example 1 and Comparative Example 2.

[0042] Example 3: Preparation of solid electrolyte Li6PS5Cl Weigh 2.140 g (46.57 mmol) of lithium sulfide prepared in Example 1, 2.070 g (9.312 mmol) of phosphorus pentasulfide (V) (99%, Sigma Aldrich), and 0.790 g (18.6 mmol) of lithium chloride (battery grade, Albemarle Germany GmbH), and grind thoroughly. The mixture was then transferred to a Fritsch zirconia-lined grinding bowl. Twelve grinding balls with a diameter of 10 mm were added. The grinding bowl was then sealed under an inert gas atmosphere and placed in a Fritsch Pulverisette 7 planetary ball mill. The mixture was ground at 600 rpm for 20 hours. After grinding, the zirconia balls were removed. The homogenized mixture was then transferred to a metal cylinder and sealed with a nut. After 48 hours in a box furnace at 370°C, it was completely converted to the solid electrolyte Li6PS5Cl. The phase purity of the solid electrolyte was examined by X-ray powder diffraction.

Claims

1. A method for producing lithium sulfide with reduced or no carbon impurities, characterized by lithium sulfide containing lithium sulfate and optionally lithium sulfite, lithium disulfate and / or other lithium sulfur oxides is treated with hydrogen gas in a temperature range of 300 to 600 °C.

2. The method of claim 1, wherein The lithium sulfide treated with the hydrogen gas is prepared by reaction of lithium sulfate with a carbon source, preferably carbon black, wherein the lithium sulfate is added in stoichiometric excess.

3. The method of claim 2, wherein The lithium sulfate / carbon molar ratio is in the range of 1,0005 to 2:2, wherein preferably based on lithium sulfate, a stoichiometric excess of lithium sulfate in the range of 0.05 to 10 mol%, even more preferably in the range of 0.1 to 5 mol% is used.

4. The method according to any one of claims 1 to 3, characterized in that The hydrogen gas is a synthesis gas, wherein the hydrogen gas content is in the range of 1 to 100 vol%, preferably 5 to 10 vol%, wherein the remainder of the hydrogen gas can be nitrogen and / or argon.

5. The method according to any one of claims 1 to 4, characterized in that The treatment with the hydrogen gas is carried out at 450 to 550 °C, particularly preferred at 500 °C.

6. The method according to any one of claims 1 to 5, characterized in that The carbon impurity content of the lithium sulfide is less than 2.0 wt%, preferably less than 1.0 wt%, more preferably less than 0.5 wt%, particularly less than 0.3 wt%, still more preferably less than 0.25 wt%, particularly less than 0.05 wt%, or even 0 wt%, based on the weight of the treated lithium sulfide.

7. The method according to any one of claims 1 to 6, characterized in that The treatment with the hydrogen gas is carried out at 400 to 575 °C, preferably 450 to 550 °C, preferably 475 to 525 °C, more preferably 490 to 510 °C, still more preferably 495 to 505 °C.

8. The method according to any of the preceding claims, characterized in that The treatment time with the hydrogen gas is in the range of 1 to 10 hours, preferably 1 to 8 hours, particularly 1 to 5 hours.

9. A lithium sulfide producible by the method according to any one of claims 1 to 8.

10. Use of the carbon reduced / carbon free lithium sulfide according to claim 9 for the production of battery components, preferably in solid electrolytes.

11. A solid electrolyte, particularly for rechargeable lithium ion batteries, comprising the lithium sulfide according to claim 9.

12. A solid state battery comprising the solid electrolyte according to claim 11.

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