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

Abstract

The invention relates to a method for producing lithium sulfide with reduced or no carbon impurities, wherein the lithium sulfide containing carbon impurities is treated with hydrogen gas at a temperature in the range of 450 to 1000 DEG C. The invention also relates to lithium sulfide which can be produced in this way and which has a carbon impurity content of less than 0.3% by weight, based on the weight of the lithium sulfide. Such lithium sulfide is used to produce battery components, preferably solid electrolytes, and solid state batteries.

Description

[0001] The present invention relates to a method for preparing lithium sulfide with reduced or no carbon impurities, or a method for purifying lithium sulfide to effectively remove impurities such as residual carbon or other carbon-containing impurities, for use in electronic and electrical materials. The present invention also relates to the purified lithium sulfide, a solid electrolyte for use in rechargeable lithium batteries, and a solid battery containing such a solid electrolyte.

[0002] Lithium sulfide is currently a raw material for preparing solid electrolytes for solid-state batteries and has attracted great interest (Lee et al., Acc. Chem. Res, 54, 3390, 2021). Compared with the prior art, solid-state batteries have higher energy density and faster charging speed. In addition, 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). In addition, lithium sulfide can also be used as a cathode material for lithium / sulfur batteries (EP 2 896085 A1). Compared with conventional lithium-ion batteries, the energy density of lithium / sulfur batteries is also significantly improved, and therefore has potential application value in the field of electric vehicles.

[0003] If the purity of raw materials (such as solid electrolytes used in rechargeable batteries) is low, it will accelerate 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 because it leads to poor electronic conductivity in the solid electrolyte (Nikodimos et al., Energy Environ. Sci., 2022, 15, 991).

[0004] Processes for preparing lithium sulfide are known and can be used to prepare lithium sulfide in a simple manner (eg EP 0 802 575 A1).

[0005] A known method describes the production of lithium sulfide from lithium sulfate and carbon by carbothermal reduction at high temperatures (CN106229487 A). This is essentially an economical and simple method, as the production steps can also be carried out continuously. In addition, the raw materials lithium sulfate and carbon are also readily available. However, carbothermal reduction generally results in the lithium sulfide containing a large amount of impurities. These are usually unreacted reactants, such as carbon or lithium sulfate. In addition, lithium sulfite, lithium carbonate and / or lithium oxide may also be formed.

[0006] Another method describes the reduction of lithium sulfate to lithium sulfide with hydrogen at high temperatures (US-A 2840455). A disadvantage here is that the reaction rate of the lithium sulfide is very slow. Although the reaction rate can be significantly increased by increasing the temperature, this leads to the formation of a melt that solidifies after cooling and does not produce the desired powdered lithium sulfide. These circumstances make the reduction of lithium sulfide with hydrogen economically unattractive.

[0007] If lithium sulfide is produced via a carbothermal process, the typical residual carbon contamination results in additional poor electronic conductivity for lithium sulfide as a raw material for solid electrolytes in rechargeable lithium batteries, making it impossible to achieve the desired battery performance and long-term stability.

[0008] An 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 contained in the lithium sulfide constituting the raw material of the solid electrolyte for rechargeable lithium batteries is minimized or completely avoided.

[0009] Another object of the present invention is to provide such lithium sulfide with reduced or no carbon impurities, solid electrolytes using such lithium sulfide (especially solid electrolytes for rechargeable lithium ion batteries), and solid batteries with very little or no carbon impurities.

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

[0011] Thus, the present invention provides lithium sulfide having low or no carbon impurities, which is produced by this method according to the present invention.

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

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

[0014] Furthermore, the present invention relates to a solid electrolyte for a rechargeable lithium-ion battery and a corresponding solid battery.

[0015] Surprisingly, it has been demonstrated 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 temperatures without the disadvantages expected in the prior art.

[0016] According to the present invention, the residual carbon / residual carbon compound content of the lithium sulfide treated with hydrogen is less than 0.3% by weight, preferably less than 0.2% by weight, in particular less than 0.1% by weight, and ideally is 0% by weight.

[0017] According to the present invention, the lithium sulfide used to produce lithium sulfide with reduced / no carbon impurities is preferably produced by first reducing lithium sulfate to lithium sulfide with a carbon source, preferably carbon black.

[0018] Carbon sources or carbon impurities can include crystalline carbon and amorphous 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 (such as carbides) and organic carbon compounds.

[0019] To produce the most homogeneous lithium sulfate / carbon mixture possible, the two components are preferably mixed in a zirconium dioxide-lined flat ball mill. For better mixing, zirconium dioxide balls can also be added to the milling bowl. The milling time is typically 1 to 24 hours, preferably 1 to 3 hours.

[0020] The lithium sulfate / carbon mixture is typically reacted under inert conditions at a temperature range of 650 to 900° C., preferably at a temperature range of 750 to 850° C. For the purposes of the present invention, inert conditions are understood to mean working under an inert gas atmosphere with the exclusion of air and moisture. To this end, the lithium sulfate / carbon mixture is weighed out, mixed as homogeneously as possible, placed in a high-temperature crucible, such as aluminum oxide, boron nitride, or glassy carbon, and reacted according to the following reaction equation:

[0021] Li2SO4+(2+x)C→Li2S+xC+2CO2(x=0bis2)

[0022] where x represents the excess carbon.

[0023] Thus, the lithium sulfate / carbon molar ratio is in the range of 1:2 to 1:2+x, wherein x=0 to 2, preferably, there is a stoichiometric excess of carbon in the range of 1-10 wt. %, even more preferably in the range of 1-5 wt. %, calculated on the lithium sulfate.

[0024] This reaction produces lithium sulfide contaminated with carbon. Due to the inhomogeneity of the starting mixture, undesirable residual carbon cannot usually be completely avoided.

[0025] The problem is solved by the following steps of purifying lithium sulfide according to the present invention:

[0026] According to the present invention, the contaminated lithium sulfide is treated with a hydrogen-containing gas mixture, wherein the hydrogen content may be 1% to 100% by volume, preferably 5% to 10% by volume, and the remainder of the hydrogen is nitrogen and / or argon.

[0027] According to the invention, the treatment with hydrogen is carried out in a temperature range of 450 to 1000°C, preferably 650 to 1000°C, preferably 750 to 1000°C, more preferably 800 to 1000°C, still more preferably 800 to 950°C, in particular 800 to 900°C.

[0028] According to the present invention, the treatment time with hydrogen is specifically 1 to 10 hours, preferably 1 to 8 hours, more preferably 1 to 5 hours.For this purpose, for example, commercially available "synthetic gas", i.e. a mixture of hydrogen and nitrogen and / or argon, can be used.

[0029] According to the present invention, carbon-contaminated lithium sulfide can be treated with a synthesis gas containing 5% by volume of hydrogen at 800 to 1000° C. for 1 to 10 hours, for example, according to the following equation. The amount of H2 required is at least twice the stoichiometric amount of residual carbon:

[0030] Li2S+x C+2x H2→Li2S+CH4.

[0031] According to the reaction equation, in this reaction, the remaining carbon is removed from the lithium sulfide by forming gaseous methane. This leaves purified white crystalline lithium sulfide. The exemplary isolated material shows only the desired Li2S (content >99 wt%) line in its X-ray diffraction pattern, with a carbon content of <0.3 wt%.

[0032] According to the present invention, it is preferred to allow a hydrogen stream to overflow the lithium sulfide during the treatment.

[0033] 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. 2 / g, preferably 100 to 200m 2 / g of carbon black as the carbon source.

[0034] Measurement method

[0035] The phase purity of the samples was checked using a Bruker D2-Phaser X-ray powder diffractometer in Bragg-Brentano geometry. An X-ray tube with Cu-Kα radiation (λ=0.15418 nm) was used as the radiation source.

[0036] Quantitative analysis of lithium, sulfur, and carbon in lithium sulfide was performed using the elemental analysis unit of a Keyence VHX-7000 digital microscope. A small amount of sample (<1 mg) was evaporated and atomized using a UV laser (λ = 250 nm; P = 0.01 mW). Characteristic atomic emission lines were detected and used for quantification.

[0037] The body color of the resulting samples was determined using RAL color charts from RAL GmbH. Using these standardized color charts, the corresponding CIELAB color coordinates can be determined.

[0038] Compared with the prior art, the advantages of the method according to the present invention are as follows:

[0039] Direct purification of lithium sulfide obtained by carbothermal reduction and the resulting low-carbon / carbon-free lithium sulfide for use in the production of solid-state electrolytes;

[0040] Use commercially available starting materials;

[0041] Avoid contact with solids that are sensitive to air and moisture, such as lithium metal, lithium hydride, alkyl lithium, aryl lithium, or lithium amide;

[0042] Avoid contact with toxic sulfur sources, such as hydrogen sulfide or carbon disulfide;

[0043] Direct use of lithium sulfate recovered from lithium-ion batteries without the need for energy-intensive conversion to, for example, lithium hydroxide;

[0044] • Avoidance of further purification of the lithium sulfide by further process steps using organic solvents such as THF.

[0045] All operations were preferably performed in an argon-filled glove box.

[0046] The above-described measurement methods were used in the following examples to determine product characteristics. Example

[0047] Example 1:

[0048] Weigh 4.4 g (40 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) and 0.96 g (80 mmol) of a quartz crystal with a specific surface area of ​​about 176 m 2 / g of carbon black (Cabot Vulcan P Fluffy) and then ground thoroughly in an agate mortar. The lithium sulfate / carbon mixture is then transferred to a Fritsch zirconium dioxide lined grinding bowl. 12 grinding balls with a diameter of 10nm are added. The grinding bowl is then sealed under inert gas and placed in a Fritsch Pulverisette 7 planetary ball mill. The mixture is ground at 600rpm for 2 hours. After the grinding process is completed, the zirconium dioxide balls are sieved out. The homogeneous lithium sulfate / carbon mixture is then transferred to a corundum annealing box. The mixture is converted into lithium sulfide at 850°C under a nitrogen flow for 3.3 hours. The lithium sulfide, which is still contaminated with carbon, is then post-processed by switching to synthesis gas containing 5% volume hydrogen at 900°C for 6 hours. After cooling, it is purged with nitrogen. The purified lithium sulfide loses its gray discoloration due to residual carbon and becomes pure white. In addition, the phase purity is checked by X-ray diffraction. The obtained lithium sulfide is a microcrystalline powder showing no sintering or other agglomerates.

[0049]

[0050] Example 2:

[0051] Weigh 4.4 g (40 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) and 0.96 g (80 mmol) of a quartz crystal with a specific surface area of ​​about 176 m 2 / g of carbon black (Cabot Vulcan P Fluffy) and then thoroughly ground in an agate mortar. The lithium sulfate / carbon mixture is then transferred to a Fritsch zirconium dioxide lined grinding bowl. Twelve grinding balls with a diameter of 10nm are added. The grinding bowl is then sealed under inert gas and placed in a Fritsch Pulverisette 7 planetary ball mill. The mixture is ground at 600rpm for 20 hours. After the grinding process is completed, the zirconium dioxide balls are sieved out. The homogeneous lithium sulfate / carbon mixture is then transferred to a corundum annealing box. The mixture is converted into lithium sulfide at 800°C under a nitrogen flow for 8 hours. The lithium sulfide, which is still contaminated with carbon, is then post-processed by switching to synthesis gas containing 5% volume hydrogen at 850°C for 8 hours. After cooling, it is purged with nitrogen. The purified lithium sulfide loses its gray discoloration due to residual carbon and becomes pure white. In addition, the phase purity is checked by X-ray diffraction. The obtained lithium sulfide is a microcrystalline powder showing no sintering or other agglomerates.

[0052]

[0053] Comparative Example 1: Conversion without H2 workup.

[0054] Weigh 4.4 g (40 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) and 0.96 g (80 mmol) of a quartz crystal with a specific surface area of ​​about 176 m 2 / g of carbon black (Cabot Vulcan P Fluffy) and then thoroughly ground in an agate mortar. The lithium sulfate / carbon mixture is then transferred to a Fritsch zirconium dioxide lined grinding bowl. Twelve grinding balls with a diameter of 10nm are added. The grinding bowl is then sealed under inert gas and placed in a Fritsch Pulverisette 7 planetary ball mill. The mixture is ground at 600rpm for 2 hours. After the grinding process is completed, the zirconium dioxide balls are sieved out. The homogeneous lithium sulfate / carbon mixture is then transferred to a corundum annealing box. The mixture is converted into lithium sulfide at 850°C under a nitrogen stream for 3.3 hours.

[0055]

[0056] Comparative Example 2: Conversion without H2 workup.

[0057] Weigh 4.4 g (40 mmol) of anhydrous lithium sulfate (99.0%, Albemarle Germany GmbH) and 0.96 g (80 mmol) of a quartz crystal with a specific surface area of ​​about 176 m 2 / g of carbon black (Cabot Vulcan P Fluffy) and then thoroughly ground in an agate mortar. The lithium sulfate / carbon mixture is then transferred to a Fritsch zirconium dioxide lined grinding bowl. Twelve grinding balls with a diameter of 10nm are added. The grinding bowl is then sealed under inert gas and placed in a Fritsch Pulverisette 7 planetary ball mill. The mixture is ground at 600rpm for 20 hours. After the grinding process is completed, the zirconium dioxide balls are sieved out. The homogeneous lithium sulfate / carbon mixture is then transferred to a corundum annealing box. The mixture is converted into lithium sulfide at 800°C under a nitrogen stream for 8 hours.

[0058]

[0059] Example 3:

[0060] Preparation of solid electrolyte Li6PS5Cl

[0061] 2.140 g (46.57 mmol) of lithium sulfide prepared in Example 1 of the embodiment, 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) were weighed and then thoroughly ground. The mixture was then transferred to a Fritsch zirconium dioxide lined grinding bowl. 12 grinding balls with a diameter of 10 nm were added. The grinding bowl was then sealed under inert gas and placed in a Pulverisette 7 planetary ball mill from Fritsch. The mixture was ground at 600 rpm for 20 hours. After the grinding process was completed, the zirconium dioxide balls were removed. The homogeneous mixture was then transferred to a metal cylinder and sealed with a nut. After 48 hours at 370° C. in a box furnace, it was completely converted into the solid electrolyte Li6PS5Cl. The phase purity of the solid electrolyte was checked by X-ray powder diffraction.

Claims

1. A method for preparing lithium sulfide with reduced or no carbon impurities, characterized in that: Lithium sulfide containing carbon impurities is treated with hydrogen at temperatures ranging from 450 to 1000°C.

2. The method according to claim 1, characterized in that The carbon impurities are carbon or one or more inorganic or organic compounds containing carbon.

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

4. The method according to any one of the preceding claims, characterized in that The carbon impurity content of the lithium sulfide treated with the hydrogen after the treatment is less than 0.3% by weight, preferably less than 0.2% by weight, in particular less than 0.1% by weight, and particularly preferably 0% by weight, based on the weight of the treated lithium sulfide.

5. The method according to any one of the preceding claims, 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 method according to any one of the preceding claims, characterized in that The treatment with hydrogen is carried out at 650 to 1000°C, preferably 750 to 1000°C, preferably 800 to 1000°C, more preferably 800 to 950°C, still more preferably 800 to 900°C.

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

8. The method according to any one of the preceding claims, characterized in that The lithium sulfide to be treated with the hydrogen is prepared by reacting lithium sulfate with a carbon source, preferably carbon black, with a lithium sulfate / carbon molar ratio in the range of 1:2 to 1:2+x, wherein x=0 to 2, preferably using a stoichiometric carbon excess in the range of 1-10 wt.-%, even more preferably in the range of 1-5 wt.-%, calculated on the lithium sulfate.

9. A lithium sulfide, characterized in that: The carbon impurity content of the carbothermally produced lithium sulfide is less than 0.3 wt%, preferably less than 0.2 wt%, especially less than 0.1 wt%, and ideally 0 wt%, based on the weight of the lithium sulfide.

10. Lithium sulfide, which can be produced by the method according to any one of claims 1 to 8.

11. Use of the carbon-reduced / carbon-free lithium sulfide according to claim 9 or 10 for producing battery components, preferably in a solid electrolyte.

12. A solid electrolyte, in particular a solid electrolyte for a rechargeable lithium-ion battery, comprising the lithium sulfide according to claim 9 or 10. 13 . A solid battery comprising the solid electrolyte according to claim 12 .

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