Process for producing reduced / free carbon impurity lithium sulfide, the reduced / free carbon impurity lithium sulfide, and its use for producing solid electrolytes and solid-state batteries

Abstract

The present invention relates to a process for producing lithium sulfide with reduced or no carbon impurities, in which lithium sulfide containing carbon impurities is treated with hydrogen gas at a temperature in the range of 450-1000°C. The present invention further relates to lithium sulfide thus produced, the carbon impurity content of which is less than 0.3 wt. % based on the weight of the lithium sulfide. This lithium sulfide is used in the production 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 having reduced or no carbon impurities, or a method for purifying lithium sulfide to efficiently remove impurities such as residual carbon or other carbon-containing impurities from the lithium sulfide, respectively, for use in electronic and electrical materials. Furthermore, the present invention relates to the purified lithium sulfide, a solid electrolyte for rechargeable lithium batteries, and a solid-state battery comprising such a solid electrolyte.

[0002] Lithium sulfide is currently attracting much interest as a raw material for the preparation of solid-state electrolytes for all-solid-state batteries (Lee et al., Acc. Chem. Res., 54, 3390, 2021). All-solid-state batteries offer higher energy densities and faster charging capabilities compared to the state of the art. Furthermore, all-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). Furthermore, lithium sulfide has applications as a cathode material in lithium / sulfur batteries (EP 2896085 A1). Lithium / sulfur batteries are also attracting interest for their potential applications in the field of electromobility due to their significantly higher energy density compared to conventional lithium-ion batteries.

[0003] Low purity levels of raw materials, such as solid electrolytes, used in rechargeable batteries can accelerate the aging of components. Therefore, the purity levels of the solid electrolyte or other raw materials must be high (EP1681263A1). In particular, graphitized carbon in lithium sulfide as a raw material for solid electrolytes must be completely avoided as much as possible because it can cause undesirable electronic conductivity in the solid electrolyte (Nikodimos et al., Energy Environ. Sci., 2022, 15, 991).

[0004] Lithium sulfide can be prepared by simple means, and processes for the preparation of lithium sulfide are well known (for example EP 0 802 575 A1).

[0005] One known process describes the production of lithium sulfide from lithium sulfate and carbon by carbothermal reduction at high temperatures (CN106229487A). It is essentially an economical and simple process, since the production steps can be performed continuously. Furthermore, the raw materials lithium sulfate and carbon are readily available. However, carbothermal reduction often results in significant impurities in the lithium sulfide. These are usually unreacted reactants such as carbon or lithium sulfate. Furthermore, lithium sulfite, lithium carbonate, and / or lithium oxide may be formed.

[0006] Another process describes the reduction of lithium sulfate to lithium sulfide with hydrogen at high temperatures (US Pat. No. 2,840,455). The drawback here is that the reaction rate to lithium sulfide is very slow. The reaction rate can be significantly increased by increasing the temperature, but this leads to the formation of a melt that solidifies after cooling and does not yield the desired powdered lithium sulfide. In these circumstances, reduction to lithium sulfide with hydrogen becomes economically unattractive.

[0007] When lithium sulfide is produced by the carbothermal method, due to typical contamination with residual carbon, lithium sulfide generates additional undesirable electronic conduction as a raw material of solid electrolyte for rechargeable lithium batteries, and therefore cannot achieve the desired battery performance and long-term stability.

[0008] 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 carbon and / or carbonaceous impurities contained in the lithium sulfide constituting the raw material for the solid electrolyte of rechargeable lithium batteries is minimized or avoided entirely.

[0009] Another object of the present invention is to provide such lithium sulfides with reduced or no carbon impurities, and in particular solid electrolytes for rechargeable lithium-ion batteries using such lithium sulfides, and solid-state batteries in which carbon impurities are minimized or absent.

[0010] These objects are achieved by a process for producing lithium sulfide with reduced or no carbon impurities, characterized in that the lithium sulfide containing carbon impurities is treated with hydrogen gas at a temperature in the range of 450 to 1000°C.

[0011] Thus, the present invention provides low or no carbon impurities lithium sulfide produced by the process according to the present invention.

[0012] Furthermore, the present invention relates to the use of such lithium sulfide for the manufacture of battery components, preferably in solid electrolytes.

[0013] Therefore, the present invention also relates to a process for purifying lithium sulfide, which can efficiently remove impurities such as residual carbon or other carbonaceous impurities from lithium sulfide.

[0014] Furthermore, the present invention relates to such solid electrolytes for rechargeable lithium-ion batteries and to corresponding solid-state batteries.

[0015] Surprisingly, it has been shown according to the present invention that carbon impurities in lithium sulfide, such as excess carbon or carbon-containing inorganic or organic compounds, can be removed by a specific post-treatment with hydrogen at high temperature without the drawbacks expected in the prior art.

[0016] The content of residual carbon / carbon compounds in the lithium sulfide treated with hydrogen gas according to the present invention is less than 0.3 wt%, preferably less than 0.2 wt%, and in particular less than 0.1 wt%. Ideally, the content is 0 wt%.

[0017] The lithium sulfide used to produce reduced / free carbon impurity lithium sulfide according to the present invention is preferably produced carbothermally by first reducing lithium sulfate with a carbon source, preferably carbon black, to lithium sulfide.

[0018] The carbon source or carbon impurities can include crystalline and amorphous forms of carbon. Crystalline forms include graphite, graphite-like carbon (including carbon black or activated carbon), graphene, fullerenes, or carbon nanotubes. Carbonaceous impurities include both inorganic carbon compounds (e.g., carbides) and organic carbon compounds.

[0019] To produce a lithium sulfate / carbon mixture as homogeneous as possible, it is preferable to mix the two components in a plain ball mill lined with zirconium dioxide. For better mixing, zirconium dioxide balls may also be added to the milling bowl. The milling time is generally 1 to 24 hours, preferably 1 to 3 hours.

[0020] The lithium sulfate / carbon mixture is usually reacted under inert conditions in the temperature range of 650-900°C, preferably in the temperature range of 750-850°C. For the purposes of the present invention, inert conditions are understood to mean working under inert gas, excluding air and moisture. For this purpose, the lithium sulfate / carbon mixture is weighed, mixed as homogeneously as possible, filled into a heat-resistant crucible (for example, aluminum oxide, boron nitride, or glassy carbon), and reacted according to the following reaction scheme: Li2SO4+(2+X)C→Li2S+xC+2CO2(x=0bis2) In the formula, x represents an excess of carbon.

[0021] Thus, the lithium sulfate / carbon molar ratio is in the range of 1:2 to 1:2+x, where x=0 to 2, and preferably there is a stoichiometric excess of carbon in the range of 1 to 10 wt.%, and even more preferably in the range of 1 to 5 wt.%, based on lithium sulfate.

[0022] This reaction results in lithium sulfide contaminated with carbon. Due to the heterogeneity of the starting mixture, undesirable carbon residues usually cannot be completely avoided.

[0023] The problem is solved by the step of purifying lithium sulfide according to the invention as follows:

[0024] According to the present invention, contaminated lithium sulfide is treated with a hydrogen-containing gas mixture, the hydrogen content of which may be 1-100% by volume, preferably 5-10% by volume, the remainder of the hydrogen gas being nitrogen and / or argon.

[0025] The treatment with hydrogen gas according to the present invention is carried out in the temperature range of 450 to 1000°C, preferably 650 to 1000°C, preferably 750 to 1000°C, more preferably 800 to 1000°C, even more preferably 800 to 950°C, and particularly 800 to 900°C.

[0026] The treatment time with hydrogen gas according to the invention is in particular 1 to 10 hours, preferably 1 to 8 hours, preferably 1 to 5 hours. For this purpose, for example, commercially available "forming gas", i.e. a mixture of hydrogen with nitrogen and / or argon, can be used.

[0027] According to the present invention, carbon-contaminated lithium sulfide can be treated with forming gas containing 5% by volume of hydrogen at 800-1000°C for 1-10 hours, for example, according to the following formula: The amount of H2 required is at least twice the stoichiometric amount of residual carbon: Li2S+xC+2xH2→Li2S+CH4.

[0028] According to the reaction equation, the remaining carbon is removed from the lithium sulfide in this reaction by the formation of gaseous methane. What remains is purified white crystalline lithium sulfide. Exemplary isolated material exhibits lines in the X-ray diffraction pattern only for the desired LiS (content >99 wt%) and has a carbon content <0.3 wt%.

[0029] According to the invention, the lithium sulfide is preferably overflowed with a stream of hydrogen gas during treatment.

[0030] Preferably, the lithium sulfate used is high-purity anhydrous lithium sulfate obtained from lithium-containing minerals such as spodumene, brine, or from recycled lithium-ion batteries. 2 / g, preferably 100 to 200m 2 / g of carbon black is used as the carbon source.

[0031] Measurement method The phase purity of the samples was confirmed using a Bruker D2-Phaser X-ray powder diffractometer of the Bragg-Brentano type. An X-ray tube with Cu-Kα radiation (λ = 0.15418 nm) was used as the radiation source.

[0032] Quantification of lithium, sulfur, and carbon in lithium sulfide was performed using the elemental analysis unit of a Keyence VHX-7000 digital microscope. A UV laser (λ = 250 nm, P = 0.01 mW) was used to vaporize and atomize small amounts of sample (<1 mg). Characteristic atomic radiation was detected and used for quantification.

[0033] The body color of the obtained samples was determined using RAL comparison cards from RAL GmbH. Using these standardized color charts, the respective CIELAB color coordinates can be determined.

[0034] The advantages of the process according to the invention, compared to the state of the art, are therefore: Direct purification of lithium sulfide obtained by carbothermal reduction and the availability of the resulting low-carbon / carbon-free lithium sulfide for the production of solid electrolytes; Use of commercially available starting materials, Avoidance of working with air- and moisture-sensitive solids such as Li metal, Li hydride, Li alkyl, Li aryl or Li amide; Avoid working with toxic sulfur sources such as hydrogen sulfide or carbon disulfide; Direct use of lithium sulfate, e.g. from recycling lithium-ion batteries, without energy-intensive conversion to lithium hydroxide, Avoidance of organic solvents (e.g. THF) for further purification of lithium sulfide by further process steps.

[0035] All manipulations are preferably carried out in an Ar-filled glove box.

[0036] The above measurement methods were used in the following examples to determine product properties. [Example]

[0037] Example 1: 4.4 g (40 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) was added to approximately 176 ml 20.96 g (80 mmol) of carbon black (Cabot Vulcan P Fluffy) with a specific surface area of ​​0.96 g / g was weighed out and then intimately 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 Pulveriset 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 carbon, was then post-treated at 900 °C for 6 hours in forming gas containing 5% hydrogen by volume. After cooling, it was purged with nitrogen. The purified lithium sulfide was pure white, free of any gray coloring due to residual carbon. Furthermore, the phase purity was confirmed by X-ray diffraction. The resulting lithium sulfide was a microcrystalline powder that showed no sintering or other agglomerations. Li2S content:>99 Residual carbon content: <0.3% Li2S color: Pure white Color coordinates CIELAB: L=94.57, a=-0.47, b=4.14

[0038] Example 2: 4.4 g (40 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) was added to approximately 176 ml 20.96 g (80 mmol) of carbon black (Cabot Vulcan P Fluffy) with a specific surface area of ​​0.96 g / g was weighed out and then intimately 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 Pulveriset 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 lithium sulfide, still contaminated with carbon, was then post-treated at 850 °C for 8 hours in forming gas containing 5% hydrogen by volume. After cooling, it was purged with nitrogen. The purified lithium sulfide was pure white, free of any gray coloring due to residual carbon. Furthermore, the phase purity was confirmed by X-ray diffraction. The resulting lithium sulfide was a microcrystalline powder that showed no sintering or other agglomerations. Li2S content:>99 Residual carbon content: <0.3% Li2S color: Pure white Color coordinates CIELAB: L=94.57, a=-0.47, b=4.14

[0039] Comparative Example 1: Conversion of H2 without post-treatment. 4.4 g (40 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) was added to approximately 176 ml 20.96 g (80 mmol) of carbon black (Cabot Vulcan P Fluffy) with a specific surface area of ​​0.96 g / g was weighed out and then intimately 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 Pulveriset 7 planetary ball mill. The mixture was milled at 600 rpm for 2 hours. After the milling process, the zirconium dioxide balls were sieved out. 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. Li2S content: 95% Residual carbon content: 5% Li2S color: pearl dark gray Color coordinates CIELAB: L=57.32, a=-0.31, b=-0.98

[0040] Comparative Example 2: Conversion of H2 without post-treatment. 4.4 g (40 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) was added to approximately 176 ml 2 0.96 g (80 mmol) of carbon black (Cabot Vulcan P Fluffy) with a specific surface area of ​​0.96 g / g was weighed out and then intimately 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 Pulveriset 7 planetary ball mill. The mixture was milled at 600 rpm for 20 hours. After the milling process, the zirconium dioxide balls were sieved out. 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. Li2S content: 97% Residual carbon content: 3% Li2S color: pearl light gray Color coordinates CIELAB: L=65.38, a=-0.43, b=-0.34

[0041] Example 3: Preparation of solid electrolyte Li6PS5Cl 2.140 g (46.57 mmol) of lithium sulfide prepared in Example 1, 2.070 g (9.312 mmol) of diphosphorus pentasulfide (99%, Sigma Aldrich), and 0.790 g (18.6 mmol) of lithium chloride (Battery grade, Albemarle Germany GmbH) were weighed and then intimately ground. The 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 Pulveriset 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 complete. The phase purity of the solid electrolyte was confirmed by X-ray powder diffraction.

Claims

1. A process for preparing lithium sulfide with reduced or no carbon impurities, characterized in that lithium sulfide containing carbon impurities is treated with hydrogen gas at a temperature range of 450 to 1000°C.

2. The process according to claim 1, characterized in that the carbon impurity is carbon or one or more carbon-containing inorganic compounds or organic compounds.

3. The process according to claim 1 or 2, characterized in that the carbon impurity content of the lithium sulfide to be processed is 0.5 to 10% by weight, preferably 0.5 to 5% by weight, based on the weight of the lithium sulfide.

4. The process according to claim 1 or 2, characterized in that the carbon impurity content of the lithium sulfide treated with the hydrogen gas after the above treatment is less than 0.3% by weight, preferably less than 0.2% by weight, particularly less than 0.1% by weight, and particularly preferably 0% by weight, based on the weight of the treated lithium sulfide.

5. The process according to claim 1 or 2, characterized in that 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 is nitrogen and / or argon.

6. The process according to claim 1 or 2, characterized in that the treatment with hydrogen gas is carried out at 650 to 1000°C, preferably 750 to 1000°C, preferably 800 to 1000°C, more preferably 800 to 950°C, and even more preferably 800 to 900°C.

7. The process according to claim 1 or 2, 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.

8. The process according to claim 1 or 2, 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, wherein the molar ratio of lithium sulfate to carbon is in the range of 1:2 to 1:2+x, where x = 0 to 2, and preferably, based on lithium sulfate, a stoichiometrically excess amount of carbon in the range of 1 to 10 wt.% is used, and more preferably, a stoichiometrically excess amount of carbon in the range of 1 to 5 wt.% is used.

9. Lithium sulfide characterized in that the carbon impurity content of the carbon-thermal produced lithium sulfide is less than 0.3% by weight, preferably less than 0.2% by weight, particularly less than 0.1% by weight, and ideally 0% by weight, based on the weight of lithium sulfide.

10. Lithium sulfide, which can be produced by a process defined in claim 1 or 2.

11. Preferably in a solid electrolyte, use of carbon-reduced / carbon-free lithium sulfide as defined in claim 9 for the manufacture of battery components.

12. Preferably in a solid electrolyte, the use of carbon-reduced / carbon-free lithium sulfide as defined in claim 10 for the manufacture of a battery component.

13. A solid electrolyte, particularly for a rechargeable lithium-ion battery, comprising lithium sulfide as defined in claim 9.

14. A solid electrolyte, particularly for a rechargeable lithium-ion battery, comprising lithium sulfide as defined in claim 10.

15. A solid battery comprising the solid electrolyte described in claim 13.

16. A solid battery comprising the solid electrolyte described in Claim 14.