METHOD FOR PRODUCING SULFIDE SOLID ELECTROLYTE INCLUDING Sn

Abstract

A method for producing a sulfide solid electrolyte includes: preparing a uniform solution that includes at least elemental lithium (Li), elemental tin (Sn), elemental phosphorus (P), and elemental sulfur (S) in an organic solvent; removing the organic solvent from the uniform solution to obtain a precursor; and heat-treating the precursor to obtain a sulfide solid electrolyte.

Claims

1. A method for producing a sulfide-based solid electrolyte which comprises: preparing a homogeneous solution that includes at least elemental lithium (Li), elemental tin (Sn), elemental phosphorus (P) and elemental sulfur (S) in an organic solvent; removing the organic solvent from the homogeneous solution to obtain a precursor; and heat-treating the precursor to obtain a sulfide-based solid electrolyte.

2. The method according to claim 1, wherein the preparing of the homogenous solution comprises: mixing Li.sub.2S and P.sub.2S.sub.5 with each other in the organic solvent to prepare an Li—P—S homogeneous solution; preparing an Li—Sn—S homogeneous solution that includes at least elemental lithium (Li), elemental tin (Sn) and elemental sulfur (S) in the organic solvent; and mixing the Li—P—S homogeneous solution and the Li—Sn—S homogeneous solution with each other to prepare the homogeneous solution.

3. The method according to claim 1, wherein the preparing of the homogenous solution comprises: a solution making step mixing Li.sub.2S and P.sub.2S.sub.5 with each other in the organic solvent to prepare an Li—P—S homogeneous solution; preparing an Li—Sn—S homogeneous solution that includes at least elemental lithium (Li), elemental tin (Sn) and elemental sulfur (S) in the organic solvent; mixing Li.sub.2S and S with each other in the organic solvent to prepare an Li—S homogeneous solution; and mixing the Li—P—S homogeneous solution, the Li—Sn—S homogeneous solution and the Li—S homogeneous solution with each other to prepare the homogeneous solution.

4. The method according to claim 1, wherein the preparing of the homogenous solution comprises: mixing Li.sub.2S and P.sub.2S.sub.5 with each other in the organic solvent to prepare an Li—P—S homogeneous solution; preparing an Li—Sn—S homogeneous solution that includes at least elemental lithium (Li), elemental tin (Sn) and elemental sulfur (S) in the organic solvent; mixing Li.sub.2S and S with each other in the organic solvent to prepare an Li—S homogeneous solution; preparing an Li—Si—S homogeneous solution that includes at least elemental lithium (Li), elemental silicon (Si) and elemental sulfur (S) in the organic solvent; and mixing the Li—P—S homogeneous solution, the Li—Sn—S homogeneous solution, the Li—S homogeneous solution and the Li—Si—S homogeneous solution with each other to prepare the homogeneous solution.

5. The method according to claim 2, wherein the solution making s preparing of the Li—Sn—S homogeneous solution comprises mixing Li.sub.2S, SnS and S with each other in the organic solvent to prepare the Li—Sn—S homogeneous solution.

6. The method according to claim 4, wherein the preparing of the Li—Si—S homogeneous solution comprises mixing Li.sub.2S, SiS.sub.2 and S with each other in the organic solvent to prepare the Li—Si—S homogeneous solution.

7. A method for producing a sulfide-based solid electrolyte which comprises: preparing an Li.sub.3PS.sub.4-containing slurry; a solution making step preparing an Li—Sn—S homogeneous solution that includes at least elemental lithium (Li), elemental tin (Sn) and elemental sulfur (S) in an organic solvent; mixing the Li.sub.3PS.sub.4-containing slurry and the Li—Sn—S homogeneous solution with each other to prepare a mixed slurry; removing the organic solvent from the mixed slurry to obtain a precursor; and heat-treating the precursor to obtain a sulfide-based solid electrolyte.

8. The method according to claim 7, wherein the preparing of the Li—Sn—S homogeneous solution comprises adding Li.sub.2S, SnS and S to the organic solvent to be mixed therewith to prepare the Li—Sn—S homogeneous solution.

9. The method according to claim 1, wherein the organic solvent is at least one selected from the group consisting of an ether-based solvent, a nitrile-based solvent and an ester-based solvent.

10. The method according to claim 1, wherein the organic solvent is at least one selected from the group consisting of tetrahydrofuran, acetonitrile, ethyl acetate and methyl acetate.

11. The method according to claim 1, wherein during the removing of the organic solvent the temperature is 60 to 280° C.

12. The method according to claim 1, wherein during the heat-treating of the precursor the temperature is 200° C. to 700° C.

13. The method according to claim 1, wherein the sulfide-based solid electrolyte contains an LGPS-based solid electrolyte and has peaks at at least 2θ=19.90°±0.50°, 20.20°±0.50°, 26.70°±0.50° and 29.20°±0.50° in X-ray diffraction (CuKα: λ=1.5405 Å).

14. The method according to claim 7, wherein the organic solvent is at least one selected from the group consisting of an ether-based solvent, a nitrile-based solvent and an ester-based solvent.

15. The method according to claim 7, wherein the organic solvent is at least one selected from the group consisting of tetrahydrofuran, acetonitrile, ethyl acetate and methyl acetate.

16. The method according to claim 7, wherein during the removing of the organic solvent the temperature is 60 to 280° C.

17. The method according to claim 7, wherein during the heat-treating of the precursor the temperature is 200° C. to 700° C.

18. The method according to claim 7, wherein the sulfide-based solid electrolyte contains an LGPS-based solid electrolyte and has peaks at at least 2θ=19.90°±0.50°, 20.20°±0.50°, 26.70°±0.50° and 29.20°±0.50° in X-ray diffraction (CuKα: λ=1.5405 Å).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1 is a schematic view showing a crystal structure of a sulfide-based solid electrolyte according to one embodiment of the present invention.

[0036] FIG. 2 is a schematic cross sectional view of an all-solid-state battery according to one embodiment of the present invention.

[0037] FIG. 3 is a graph showing the results of the X-ray diffraction measurement of the sulfide-based solid electrolytes obtained in Examples 1-4 and Comparative Examples 1-2.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

[0038] Hereinafter, the present invention will be described in detail. Note that materials, constitutions, etc. described below do not limit the present invention and can be modified variously within the range of the gist of the present invention.

[0039] The first embodiment of the present invention is a method for producing a sulfide-based solid electrolyte, the method including:

[0040] a solution making step for preparing a homogeneous solution that includes at least elemental lithium (Li), elemental tin (Sn), elemental phosphorus (P) and elemental sulfur (S) in an organic solvent;

[0041] a drying step for removing the organic solvent from the homogeneous solution to obtain a precursor; and

[0042] a heat treatment step for heat-treating the precursor to obtain a sulfide-based solid electrolyte.

[0043] In the present invention, a homogeneous solution that includes at least elemental lithium (Li), elemental tin (Sn), elemental phosphorus (P) and elemental sulfur (S) in an organic solvent is defined as a solution which includes at least elemental lithium (Li), elemental tin (Sn), elemental phosphorus (P) and elemental sulfur (S) in an organic solvent, wherein no undissolved substance is precipitated.

[0044] In the first embodiment of the present invention, it is preferred that the solution making step includes: a solution making step 1 for mixing Li.sub.2S and P.sub.2S.sub.5 with each other in the organic solvent to prepare an Li—P—S homogeneous solution; and a solution making step 2 for preparing an Li—Sn—S homogeneous solution that includes at least elemental lithium (Li), elemental tin (Sn) and elemental sulfur (S) in the organic solvent, and that the method includes mixing the Li—P—S homogeneous solution and the Li—Sn—S homogeneous solution with each other to prepare a homogeneous solution.

[0045] Moreover, in the first embodiment of the present invention, it is also preferred that the solution making step includes, in addition to the solution making steps 1 and 2, a solution making step 3 for mixing Li.sub.2S and S with each other in the organic solvent to prepare an Li—S homogeneous solution, and that the method includes mixing the Li—P—S homogeneous solution, the Li—Sn—S homogeneous solution and the Li—S homogeneous solution with each other to prepare a homogeneous solution.

[0046] Furthermore, in the first embodiment of the present invention, it is also preferred that the solution making step includes, in addition to the solution making steps 1-3, a solution making step 4 for preparing an Li—Si—S homogeneous solution that includes at least elemental lithium (Li), elemental silicon (Si) and elemental sulfur (S) in the organic solvent, and that the method includes mixing the Li—P—S homogeneous solution, the Li—Sn—S homogeneous solution, the Li—S homogeneous solution and the Li—Si—S homogeneous solution with each other to prepare a homogeneous solution.

[0047] Hereinafter, the solution making steps 1-4, the drying step and the heat treatment step will be described in detail.

[0048] In the present invention, the “Li—P—S homogeneous solution” is defined as a solution which includes at least elemental lithium (Li), elemental phosphorus (P) and elemental sulfur (S) in an organic solvent, wherein no undissolved substance is precipitated. Similarly, the “Li—Sn—S homogeneous solution” is defined as a solution which includes at least elemental lithium (Li), elemental tin (Sn) and elemental sulfur (S) in an organic solvent, wherein no undissolved substance is precipitated. The “Li—S homogeneous solution” is defined as a solution which includes at least elemental lithium (Li) and elemental sulfur (S) in an organic solvent, wherein no undissolved substance is precipitated. Further, the “Li—Si—S homogeneous solution” is defined as a solution which includes at least elemental lithium (Li), elemental silicon (Si) and elemental sulfur (S) in an organic solvent, wherein no undissolved substance is precipitated.

[0049] The solution making step 1 is a step in which Li.sub.2S and P.sub.2S.sub.5 are mixed with each other in an organic solvent to prepare an Li—P—S homogeneous solution. Preferably, Li.sub.2S and P.sub.2S.sub.5 are mixed with each other in an organic solvent so that the Li.sub.2S/P.sub.2S.sub.5 molar ratio becomes from 0.7 to 1.5 to prepare an Li—P—S homogeneous solution.

[0050] At the time of mixing in the solution making step 1, a substrate is dispersed to provide a slurry state, but soon a reaction occurs. There is no need to carry out special stirring operation for crushing particles, and it is sufficient when stirring power that can enable suspension and dispersion of the slurry is provided.

[0051] Regarding the reaction temperature in the solution making step 1, the reaction slowly proceeds even at room temperature, but heating can be performed for increasing the reaction rate. When heating is performed, it is sufficient when it is performed at a temperature that is the boiling point of the organic solvent or lower. The temperature varies depending on the organic solvent to be used, but is usually lower than 120° C. Heating can also be performed under the pressurized state using an autoclave or the like. However, when mixing is carried out at a high temperature (120° C. or higher), there is concern that a side reaction may proceed.

[0052] The reaction time in the solution making step 1 varies depending on the type of the organic solvent and the particle diameter and concentration of raw materials, but for example, by performing the reaction for 0.1 to 24 hours, the reaction is completed and a solution can be made.

[0053] The Li—P—S homogeneous solution is preferably produced by mixing Li.sub.2S and P.sub.2S.sub.5 with each other in the organic solvent so that the Li.sub.2S/P.sub.2S.sub.5 molar ratio becomes from 0.7 to 1.5 to cause a reaction, and the Li.sub.2S/P.sub.2S.sub.5 molar ratio is more preferably from 0.75 to 1.4, and particularly preferably from 0.8 to 1.35. When the Li.sub.2S/P.sub.2S.sub.5 molar ratio is 0.7 to 1.5, a solution can be made from Li.sub.2S and P.sub.2S.sub.5 at room temperature. When the molar ratio is not within the above-described range, precipitation may occur.

[0054] This solution may contain unreacted Li.sub.2S and P.sub.2S.sub.5. Further, the solution may contain impurities derived from Li.sub.2S and P.sub.2S.sub.5. The impurities are scarcely dissolved in the solvent and most of them precipitate. For this reason, it is preferred to apply filtration or centrifugation to the obtained solution to remove the precipitate and separate the solution, thereby obtaining a high-purity Li—P—S homogeneous solution.

[0055] As Li.sub.2S, a synthesized product or commercially-available product can be used. The lower the content of moisture is, the better it is because mixing of moisture deteriorates the other raw materials and the precursor. The content is more preferably 300 ppm or less, and particularly preferably 50 ppm or less. The smaller the particle diameter of Li.sub.2S is, the better it is because a higher reaction rate is obtained. The particle diameter is preferably 10 nm to 100 μm, more preferably 100 nm to 30 μm, and particularly preferably 300 nm to 10 μm. The particle diameter can be measured by means of SEM, a particle size distribution measurement apparatus utilizing laser scattering, or the like. As Li.sub.2S to be used in the solution making steps 2-4 which will be described later, the same material as above can be preferably used.

[0056] As P.sub.2S.sub.5, a synthesized product or commercially-available product can be used. The higher the purity of P.sub.2S.sub.5 is, the better it is because the amount of impurities mixed into the solid electrolyte becomes smaller. The smaller the particle diameter of P.sub.2S.sub.5 is, the better it is because a higher reaction rate is obtained. The particle diameter is preferably 10 nm to 100 μm, more preferably 100 nm to 30 μm, and particularly preferably 300 nm to 10 μm. The lower the content of moisture is, the better it is because mixing of moisture deteriorates the other raw materials and the precursor. The content is more preferably 300 ppm or less, and particularly preferably 50 ppm or less.

[0057] The organic solvent is not particularly limited as long as it does not react with Li.sub.2S or P.sub.2S.sub.5. Examples thereof include an ether-based solvent, an ester-based solvent, a hydrocarbon-based solvent and a nitrile-based solvent. Specific examples thereof include tetrahydrofuran, cyclopentyl methyl ether, diisopropyl ether, diethyl ether, dimethyl ether, dioxane, methyl acetate, ethyl acetate, butyl acetate and acetonitrile. Among them, at least one selected from the group consisting of tetrahydrofuran, acetonitrile, ethyl acetate and methyl acetate is preferred, and acetonitrile is particularly preferred. Since acetonitrile does not contain any oxygen atom in the structure thereof, oxygen is less likely to be introduced into the raw material composition, and change in quality is suppressed. Further, for preventing deterioration of the raw material composition, it is preferred to remove oxygen and water in the organic solvent in advance. In particular, regarding the moisture content, it is preferably 100 ppm or less, and more preferably 50 ppm or less. As the organic solvents to be used in the solution making steps 2-4 which will be described later, the same materials as above can be preferably used.

[0058] The solution making step 2 is a step of preparing an Li—Sn—S homogeneous solution that includes at least elemental lithium (Li), elemental tin (Sn) and elemental sulfur (S) in the organic solvent. Preferably, in the solution making step 2, Li.sub.2S, SnS and S (elemental sulfur) are mixed with each other in the organic solvent to prepare an Li—Sn—S homogeneous solution.

[0059] For conventional sulfide-based solid electrolytes containing Sn, SnS.sub.2 that is a sulfide of Sn is often used as a raw material. However, SnS.sub.2 has poor solubility in solvents. For this reason, it is difficult to homogeneously disperse Sn in a solid electrolyte at the time of synthesis in a solvent, and there is a problem that it is difficult to obtain a solid electrolyte providing stable performance. The present inventors found that, by using the combination of Li.sub.2S, SnS and S, an Li—Sn—S homogeneous solution in which these materials are dissolved in an organic solvent can be prepared.

[0060] In the solution making step 2, more preferably, Li.sub.2S, SnS and S are mixed with each other in an organic solvent so that the molar ratio of Li.sub.2S:SnS:S becomes 2:1:12 to 6:1:36, and particularly preferably 3:1:18 to 5:1:30 to prepare an Li—Sn—S homogeneous solution.

[0061] At the time of mixing in the solution making step 2, a substrate is dispersed to provide a slurry state, but soon a reaction occurs. There is no need to carry out special stirring operation for crushing particles, and it is sufficient when stirring power that can enable suspension and dispersion of the slurry is provided.

[0062] Regarding the reaction temperature in the solution making step 2, it is sufficient when it is performed at a temperature that is the boiling point of the organic solvent or lower. The temperature varies depending on the organic solvent to be used, but is usually lower than 120° C. The reaction temperature is preferably 50 to 100° C., and more preferably 60 to 90° C. Heating can also be performed under the pressurized state using an autoclave or the like. However, when mixing is carried out at a high temperature (120° C. or higher), there is concern that a side reaction may proceed.

[0063] The reaction time in the solution making step 2 varies depending on the type of the organic solvent and the particle diameter and concentration of raw materials, but for example, by performing the reaction for 0.1 to 24 hours, the reaction is completed and a solution can be made.

[0064] This solution may contain unreacted Li.sub.2S, SnS and S. Further, the solution may contain impurities derived from Li.sub.2S, SnS and S. The impurities are scarcely dissolved in the solvent and most of them precipitate. For this reason, it is preferred to apply filtration or centrifugation to the obtained solution to remove the precipitate and separate the solution, thereby obtaining a high-purity Li—Sn—S homogeneous solution.

[0065] The concentration of each element in the obtained homogeneous solution is analyzed by means of ICP. The molar ratio of Li:Sn:S is preferably 4:1:15 to 12:1:43, more preferably 5:1:18 to 11:1:40, and particularly preferably 6:1:22 to 10:1:36.

[0066] As SnS, a synthesized product or commercially-available product can be used. The higher the purity of SnS is, the better it is because the amount of impurities mixed into the solid electrolyte becomes smaller. The smaller the particle diameter of SnS is, the better it is because a higher reaction rate is obtained. The particle diameter is preferably 10 nm to 100 μm, more preferably 100 nm to 30 μm, and particularly preferably 300 nm to 10 μm. The particle diameter can be measured by means of SEM, a particle size distribution measurement apparatus utilizing laser scattering, or the like. Note that there is no problem even when a part of the above-described raw materials to be used are amorphous. The lower the content of moisture is, the better it is because mixing of moisture deteriorates the other raw materials and the precursor. The content is more preferably 300 ppm or less, and particularly preferably 50 ppm or less.

[0067] As elemental sulfur, a synthesized product or commercially-available product can be used, but in general, cyclic S8 sulfur is used. The lower the content of moisture is, the better it is because mixing of moisture deteriorates the other raw materials and the precursor. The content is more preferably 300 ppm or less, and particularly preferably 50 ppm or less. The smaller the particle diameter of elemental sulfur is, the better it is because a higher reaction rate is obtained. The particle diameter is preferably 10 nm to 100 μm, more preferably 100 nm to 30 μm, and particularly preferably 300 nm to 10 μm. As S (elemental sulfur) to be used in the solution making steps 3 and 4 which will be described later, the same material as above can be preferably used.

[0068] The solution making step 3 is a step in which Li.sub.2S and S (elemental sulfur) are mixed with each other in an organic solvent to prepare an Li—S homogeneous solution.

[0069] In the solution making step 3, Li.sub.2S and S are mixed with each other in an organic solvent so that the molar ratio of Li.sub.2S:S becomes preferably 1:4 to 1:10, and more preferably 1:5 to 1:8 to prepare an Li—S homogeneous solution.

[0070] At the time of mixing in the solution making step 3, a substrate is dispersed to provide a slurry state, but soon a reaction occurs. There is no need to carry out special stirring operation for crushing particles, and it is sufficient when stirring power that can enable suspension and dispersion of the slurry is provided.

[0071] Regarding the reaction temperature in the solution making step 3, it is sufficient when it is performed at a temperature that is the boiling point of the organic solvent or lower. The temperature varies depending on the organic solvent to be used, but is usually lower than 120° C. The reaction temperature is preferably 50 to 100° C., and more preferably 60 to 90° C. Heating can also be performed under the pressurized state using an autoclave or the like. However, when mixing is carried out at a high temperature (120° C. or higher), there is concern that a side reaction may proceed.

[0072] The reaction time in the solution making step 3 varies depending on the type of the organic solvent and the particle diameter and concentration of raw materials, but for example, by performing the reaction for 0.1 to 24 hours, the reaction is completed and a solution can be made.

[0073] This solution may contain unreacted Li.sub.2S and S. Further, the solution may contain impurities derived from Li.sub.2S and S. The impurities are scarcely dissolved in the solvent and most of them precipitate. For this reason, it is preferred to apply filtration or centrifugation to the obtained solution to remove the precipitate and separate the solution, thereby obtaining a high-purity Li—S homogeneous solution.

[0074] The solution making step 4 is a step in which an Li—Si—S homogeneous solution, which contains at least elemental lithium (Li), elemental silicon (Si) and elemental sulfur (S) in an organic solvent, is prepared. By using the Li—Si—S homogeneous solution, a sulfide-based solid electrolyte with little impurities, which has high ion conductivity, can be stably obtained. When using SiS.sub.2 as a starting material, it is difficult to homogeneously disperse Si in a solid electrolyte at the time of synthesis. Further, SiS.sub.2 has high reactivity with the atmosphere and often contains an oxygen-containing compound or Si that is an unreacted raw material. For such reasons, it is difficult to prepare SiS.sub.2 not containing impurities. Further, it is difficult to remove these impurities from SiS.sub.2.

[0075] Meanwhile, in the case of the Li—Si—S homogeneous solution, Si is easily dispersed homogeneously in the solid electrolyte at the time of synthesis and a side reaction is not easily caused because impurities are reduced by the subsequent operation of removing the precipitate. For this reason, it is considered that a sulfide-based solid electrolyte having high ion conductivity can be stably produced by using it.

[0076] It is preferred that the Li—Si—S homogeneous solution, in which Li, Si and S are dissolved, is provided by mixing Li.sub.2S, SiS.sub.2 and S (elemental sulfur) with each other in an organic solvent to cause a reaction. This solution may contain unreacted Li.sub.2S, SiS.sub.2 and S. Further, the solution may contain impurities derived from Li.sub.2S, SiS.sub.2 and S.

[0077] More preferably, the solution obtained is subjected to filtration or centrifugation to remove the precipitate and separate the solution, thereby obtaining the Li—Si—S homogeneous solution. The concentration of each element in the obtained homogeneous solution is analyzed by means of ICP. The Li/Si molar ratio is preferably from 0.6 to 2.0. In this regard, the above-described molar ratio is more preferably Li/Si=0.7 to 1.6, and particularly preferably Li/Si=0.8 to 1.4.

[0078] The precipitate can be removed by means of filtration or centrifugation. When performing filtration using a filter, the pore size of the filter is desirably 10 μm or less. The pore size is more preferably 5 μm or less, and particularly preferably 2 μm or less.

[0079] The precipitate obtained includes unreacted raw materials such as Li.sub.2S and SiS.sub.2 and impurities derived from SiS.sub.2. Examples of the impurities include oxygen-containing compounds of Si or SiS.sub.2 and SiO.sub.2.

[0080] As SiS.sub.2, a synthesized product or commercially-available product can be used. The higher the purity of SiS.sub.2 is, the better it is because the amount of impurities mixed into the solid electrolyte becomes smaller. The smaller the particle diameter of SiS.sub.2 is, the better it is because a higher reaction rate is obtained. The particle diameter is preferably 10 nm to 100 μm, more preferably 100 nm to 30 μm, and particularly preferably 300 nm to 10 μm. The particle diameter can be measured by means of SEM, a particle size distribution measurement apparatus utilizing laser scattering, or the like. Note that there is no problem even when a part of the above-described raw materials to be used are amorphous. The lower the content of moisture is, the better it is because mixing of moisture deteriorates the other raw materials and the precursor. The content is more preferably 300 ppm or less, and particularly preferably 50 ppm or less.

[0081] The total concentration of Li, Si and S in the organic solvent is preferably 0.5 to 20% by mass, more preferably 1 to 15% by mass, and particularly preferably 2 to 10% by mass. When the total concentration of Li, Si and S in the organic solvent is higher than 20% by mass, it is difficult to obtain ahomogeneous solution because of precipitation of solids. Meanwhile, when the total concentration of Li, Si and S in the organic solvent is lower than 0.5% by mass, a large amount of the organic solvent must be used and a load of solvent recovery is increased. In addition, it causes excessive increase in the size of a reactor.

[0082] In the first embodiment of the present invention, it is preferred to prepare a homogeneous mixed solution by (i) mixing the Li—P—S homogeneous solution obtained in the solution making step 1 and the Li—Sn—S homogeneous solution obtained in the solution making step 2 with each other, or (ii) mixing the Li—P—S homogeneous solution obtained in the solution making step 1, the Li—Sn—S homogeneous solution obtained in the solution making step 2 and the Li—S homogeneous solution obtained in the solution making step 3 with each other, or (iii) mixing the Li—P—S homogeneous solution obtained in the solution making step 1, the Li—Sn—S homogeneous solution obtained in the solution making step 2, the Li—S homogeneous solution obtained in the solution making step 3 and the Li—Si—S homogeneous solution obtained in the solution making step 4 with each other.

[0083] Regarding the concentration of elements constituting the homogeneous mixed solution obtained by (i) above, the molar ratio of Li:Sn:P is preferably 8:1:1 to 15:1:4, and more preferably 10:1:2 to 13:1:3.

[0084] Regarding the concentration of elements constituting the homogeneous mixed solution obtained by (ii) above, the molar ratio of Li:Sn:P is preferably 8:1:1 to 15:1:4, and more preferably 10:1:2 to 13:1:3.

[0085] Further, regarding the concentration of elements constituting the homogeneous mixed solution obtained by (iii) above, the molar ratio of Li:Sn:Si:P is preferably 32:1:2:4 to 44:1:6:8, and more preferably 35:1:3:5 to 41:1:5:7.

[0086] The type and concentration of elements can be confirmed, for example, by using an ICP emission spectrometer. Since performances of the sulfide-based solid electrolyte significantly change due to slight deviation of the composition, it is preferred that the elemental composition is accurately controlled by subjecting the homogeneous solution to ICP emission spectrometry.

[0087] Note that a halogen compound can be added thereto. In this case, it is preferred that the halogen compound is also dissolved in the organic solvent. Preferred specific examples of the halogen compound include LiCl, LiBr, LiI, PCIs, PCl.sub.3, PBr.sub.5 and PBr.sub.3, and more preferred are LiCl, LiBr and LiI. These materials may be used solely, or two or more of them may be used in combination.

[0088] The drying step is a step in which the obtained homogeneous solution is dried to remove the organic solvent, thereby obtaining a precursor. Drying is preferably carried out by means of heated-air drying or vacuum drying under inert gas atmosphere.

[0089] The drying temperature is preferably 60 to 280° C., and more preferably 100 to 250° C. The optimum temperature range slightly varies depending on the type of the organic solvent, but the temperature range is important. When the drying temperature is set to be too high in a state where the organic solvent exists, the quality of the precursor changes in almost all cases. Further, when the drying temperature is too low, the amount of the residual solvent is larger, and when the next heat treatment step is directly carried out, the organic solvent is carbonized and a sulfide-based solid electrolyte obtained has high electron conductivity. It may be preferred that the solid electrolyte has electron conductivity depending on the method for using it, but the solid electrolyte to be used for the portion 2 in FIG. 2 is required to have sufficiently low electron conductivity. In the case of using the solid electrolyte for such applications, the amount of the residual solvent must be decreased as much as possible.

[0090] The drying time slightly varies depending on the type of the organic solvent and the drying temperature, but the organic solvent can be sufficiently removed by drying for 1 to 24 hours. Note that by removing the organic solvent under reduced pressure as in the case of vacuum drying, and by flowing an inert gas such as nitrogen and argon in which the moisture content is sufficiently low, the temperature at the time of removing the organic solvent can be lowered and the required time can be shortened.

[0091] Note that the heat treatment step described below and the drying step can be carried out simultaneously.

[0092] The heat treatment step is a step in which the precursor obtained in the drying step is heated to obtain a sulfide-based solid electrolyte.

[0093] Usually, the heating temperature is preferably 200 to 700° C., more preferably 350 to 650° C., and particularly preferably 400 to 600° C. When the temperature is lower than the above-described range, desired crystals are not easily generated, and when the temperature is higher than the above-described range, crystals other than those desired may be generated.

[0094] The heating time slightly varies depending on the heating temperature, but usually, crystallization can be sufficiently performed when the heating time is 0.1 to 24 hours. It is not preferred that heating is carried out at a high temperature for a long period of time which exceeds the above-described range because there is concern for change in quality of the sulfide-based solid electrolyte.

[0095] Heating can be performed under vacuum or inert gas atmosphere, but preferably performed under inert gas atmosphere. As the inert gas, nitrogen, helium, argon or the like can be used, and among them, argon is preferred. The contents of oxygen and moisture are preferably low, and conditions thereof are the same as those at the time of mixing in the slurry forming step.

[0096] The second embodiment of the present invention is a method for producing a sulfide-based solid electrolyte, the method including:

[0097] a slurry forming step 1 for preparing an Li.sub.3PS.sub.4-containing slurry;

[0098] a solution making step for preparing an Li—Sn—S homogeneous solution that includes at least elemental lithium (Li), elemental tin (Sn) and elemental sulfur (S) in an organic solvent;

[0099] a slurry forming step 2 for mixing the Li.sub.3PS.sub.4-containing slurry and the Li—Sn—S homogeneous solution with each other to prepare a mixed slurry;

[0100] a drying step for removing the organic solvent from the mixed slurry to obtain a precursor; and

[0101] a heat treatment step for heat-treating the precursor to obtain a sulfide-based solid electrolyte.

[0102] The slurry forming step 1 in the second embodiment is not particularly limited as long as the Li.sub.3PS.sub.4-containing slurry can be prepared thereby, but it is preferred to mix Li.sub.2S and P.sub.2S.sub.5 with each other in an organic solvent so that the Li.sub.2S/P.sub.2S.sub.5 molar ratio becomes from 2 to 4, thereby preparing the Li.sub.3PS.sub.4-containing slurry.

[0103] The other steps in the second embodiment can be carried out according to the steps explained with respect to the first embodiment.

[0104] The LGPS type crystal structure has an octahedron O composed of elemental Li and elemental S, a tetrahedron T1 composed of at least one element selected from the group consisting of P, Ge, Si and Sn and elemental S, and a tetrahedron T2 composed of elemental P and elemental S (PS.sub.4.sup.3− anion), wherein the tetrahedron T1 and the octahedron O share an edge and the tetrahedron T2 and the octahedron O share an apex. A solid electrolyte having the LGPS type crystal structure is more preferred because ion conductivity thereof is particularly high. When an LGPS-based solid electrolyte containing Si is brought into contact with water, hydrogen sulfide tends to be generated. Meanwhile, when Sn is brought into contact with water, hydrogen sulfide is not generated, and it has the advantage that high safety is provided and the production can be easily carried out.

[0105] The solid electrolyte having the LGPS type crystal structure that is a preferred embodiment of the present invention preferably has peaks at at least 2θ=19.90°±0.50°, 20.20°±0.50°, 26.70°±0.50° and 29.20°±0.50° in X-ray diffraction (CuKα: λ=1.5405 Å).

[0106] The sulfide-based solid electrolyte of the present invention obtained in the above-described manner can be formed into a desired formed body by various means and can be used for various applications including an all-solid-state battery described below. The forming method is not particularly limited. For example, a method similar to the method for forming respective layers constituting the all-solid-state battery described in <All-solid-state battery> below can be used.

<All-Solid-State Battery>

[0107] The sulfide-based solid electrolyte of the present invention can be used, for example, as a solid electrolyte for all-solid-state batteries. Further, according to another embodiment of the present invention, an all-solid-state battery comprising the above-described solid electrolyte for all-solid-state batteries is provided.

[0108] In this regard, the “all-solid-state battery” is an all-solid-state lithium ion secondary battery. FIG. 2 is a schematic cross sectional view of an all-solid-state battery according to one embodiment of the present invention. An all-solid-state battery 10 has a structure in which a solid electrolyte layer 2 is arranged between a positive electrode layer 1 and a negative electrode layer 3. The all-solid-state battery 10 can be used in various devices including mobile phones, personal computers and automobiles.

[0109] The sulfide-based solid electrolyte of the present invention may be contained as a solid electrolyte in at least one of the positive electrode layer 1, the negative electrode layer 3 and the solid electrolyte layer 2. In the case where the sulfide-based solid electrolyte of the present invention is contained in the positive electrode layer 1 or negative electrode layer 3, the sulfide-based solid electrolyte of the present invention is used in combination with a publicly-known positive electrode active material or negative electrode active material for lithium ion secondary batteries. The quantitative ratio of the sulfide-based solid electrolyte of the present invention to be contained in the positive electrode layer 1 or negative electrode layer 3 is not particularly limited.

[0110] The sulfide-based solid electrolyte of the present invention may be used solely, or according to need, may be suitably used in combination with an oxide solid electrolyte (e.g., Li.sub.7La.sub.3Zr.sub.2O.sub.12), a sulfide-based solid electrolyte (e.g., Li.sub.2S—P.sub.2S.sub.5), other complex hydride solid electrolytes (e.g., LiBH.sub.4 and 3LiBH.sub.4—LiI), etc.

[0111] The all-solid-state battery is prepared by forming and layering the above-described layers, and the forming method and layering method for the respective layers are not particularly limited.

[0112] Examples thereof include: a method in which a solid electrolyte and/or an electrode active material are dispersed in a solvent to provide a slurry-like mixture, which is applied by a doctor blade, spin coating or the like and subjected to rolling to form a film; a gas phase method in which film forming and layering are performed by using a vacuum deposition method, ion plating method, sputtering method, laser ablation method or the like; and a pressing method in which powder is formed by hot pressing or cold pressing (not heating) and layering is performed.

[0113] Since the sulfide-based solid electrolyte of the present invention is relatively soft, it is particularly preferred to prepare the all-solid-state battery by forming the respective layers by means of the pressing method and layering the layers. As the pressing method, there are hot pressing in which heating is performed and cold pressing in which heating is not performed, but forming the layers can be sufficiently carried out even by means of cold pressing.

[0114] Note that the present invention includes a formed body obtained by heating and forming the sulfide-based solid electrolyte of the present invention. The formed body is suitably used as the all-solid-state battery. Further, the present invention includes a method for producing an all-solid-state battery, which includes a step of heating and forming the sulfide-based solid electrolyte of the present invention.

EXAMPLES

[0115] Hereinafter, the embodiments of the present invention will be more specifically described by way of examples, but the embodiments are not limited to the examples.

Example 1

[0116] In a glovebox under argon atmosphere, 101 mg of Li.sub.2S (manufactured by Sigma-Aldrich, purity: 99.8%) and 487 mg of P.sub.2S.sub.5 (manufactured by Sigma-Aldrich, purity: 99%) were weighed so that the molar ratio of Li.sub.2S:P.sub.2S.sub.5 became 1:1. Next, to 6.0 g of acetonitrile (manufactured by Wako Pure Chemical Industries, Ltd., super dehydrated grade), Li.sub.2S and P.sub.2S.sub.5 were added in this order so that the concentration of (Li.sub.2S+P.sub.2S.sub.5) became about 10% by mass, and mixing was carried out at room temperature for 3 hours. The mixture was gradually dissolved, and an Li—P—S homogeneous solution was obtained.

[0117] In a glovebox under argon atmosphere, 1.0 g of Li.sub.2S (manufactured by Sigma-Aldrich, purity: 99.8%), 1.0 g of SnS (manufactured by Kojundo Chemical Laboratory Co., Ltd.) and 3.7 g of S (manufactured by Kojundo Chemical Laboratory Co., Ltd.) were weighed so that the molar ratio of Li.sub.2S:SnS:S became 3:1:18. Next, 100 g of acetonitrile (manufactured by Wako Pure Chemical Industries, Ltd., super dehydrated grade) was added thereto so that the concentration of (Li.sub.2S+SnS+S) became about 6% by mass, and mixing was carried out at 80° C. for 24 hours. The mixture was gradually dissolved, but at this stage, insoluble matters remained.

[0118] The obtained solution was filtered with a membrane filter (PTFE, pore size: 1.0 μm), thereby obtaining 300 mg of a filter residue and 100 g of a filtrate (Li—Sn—S homogeneous solution). When the Li—Sn—S homogeneous solution was subjected to ICP analysis, Li:Sn:S (molar ratio) was 6:1:26. Further, the concentration of Sn was 0.76% by mass.

[0119] In a glovebox under argon atmosphere, 1.0 g of Li.sub.2S (manufactured by Sigma-Aldrich, purity: 99.8%) and 3.7 g of S (manufactured by Kojundo Chemical Laboratory Co., Ltd.) were weighed so that the molar ratio of Li.sub.2S:S became 1:6. Next, 75 g of acetonitrile (manufactured by Wako Pure Chemical Industries, Ltd., super dehydrated grade) was added thereto so that the concentration of (Li.sub.2S+S) became about 6% by mass, and mixing was carried out at 80° C. for 24 hours. The mixture was gradually dissolved, but at this stage, insoluble matters remained.

[0120] The obtained solution was filtered with a membrane filter (PTFE, pore size: 1.0 μm), thereby obtaining 200 mg of a filter residue and 75 g of a filtrate (Li—S homogeneous solution). When the Li—S homogeneous solution was subjected to ICP analysis, Li:S (molar ratio) was 1:3. Further, the concentration of Li was 0.5% by mass.

[0121] 6.6 g of the prepared Li—P—S homogeneous solution, 25.08 g of the prepared Li—Sn—S homogeneous solution and 7.97 g of the prepared Li—S homogeneous solution were mixed with each other so that the molar ratio of Li:Sn:P became 12:1:3, and stirring was carried out for 3 hours to prepare a homogeneous mixed solution.

[0122] The obtained homogeneous mixed solution was dried under vacuum at 180° C. for 4 hours to remove the solvent. Removal of the solvent was carried out while stirring the solution. After that, it was cooled to room temperature to obtain a precursor.

[0123] In the glovebox, the obtained precursor was put into a glass reaction tube, which was placed in an electric tube furnace in a manner such that the precursor was not exposed to atmosphere. Argon (G3 grade) was injected into the reaction tube, the temperature was increased to 550° C. over 3 hours, and then burning was carried out at 550° C. for 8 hours, thereby synthesizing Li.sub.9.81Sn.sub.0.81P.sub.2.19S.sub.12 crystal.

Example 2

[0124] An Li—P—S homogeneous solution was obtained in a manner similar to that in Example 1.

[0125] In a glovebox under argon atmosphere, 1.25 g of Li.sub.2S (manufactured by Sigma-Aldrich, purity: 99.8%), 1 g of SnS (manufactured by Kojundo Chemical Laboratory Co., Ltd.) and 5 g of S (manufactured by Kojundo Chemical Laboratory Co., Ltd.) were weighed so that the molar ratio of Li.sub.2S:SnS:S became 4.5:1:24. Next, 100 g of acetonitrile (manufactured by Wako Pure Chemical Industries, Ltd., super dehydrated grade) was added thereto so that the concentration of (Li.sub.2S+SnS+S) became about 7% by mass, and mixing was carried out at 80° C. for 24 hours. The mixture was gradually dissolved, but at this stage, insoluble matters remained.

[0126] The obtained solution was filtered with a membrane filter (PTFE, pore size: 1.0 μm), thereby obtaining 300 mg of a filter residue and 100 g of a filtrate (Li—Sn—S homogeneous solution). When the Li—Sn—S homogeneous solution was subjected to ICP analysis, Li:Sn:S (molar ratio) was 9:1:30. Further, the concentration of Sn was 0.89% by mass.

[0127] 6.6 g of the prepared Li—P—S homogeneous solution and 33.28 g of the prepared Li—Sn—S homogeneous solution were mixed with each other so that the molar ratio of Li:Sn:P became 12:1:3, and stirring was carried out for 3 hours to prepare a homogeneous mixed solution.

[0128] A precursor was obtained in a manner similar to that in Example 1.

[0129] Li.sub.9.81Sn.sub.0.81P.sub.2.19S.sub.12 crystal was synthesized in a manner similar to that in Example 1.

Comparative Example 1

[0130] An Li—P—S homogeneous solution was obtained in a manner similar to that in Example 1.

[0131] 6.6 g of the prepared Li—P—S homogeneous solution, 296 mg of SnS.sub.2 powder (manufactured by Kojundo Chemical Laboratory Co., Ltd.) and 350 mg of Li.sub.2S were mixed with each other so that the molar ratio of Li:Sn:P became 12:1:3, and stirring was carried out for 3 hours to prepare a slurry. In this regard, Sn was not in a state where it was completely dissolved in the organic solvent.

[0132] The obtained slurry was dried under vacuum at 180° C. for 4 hours to remove the solvent. Removal of the solvent was carried out while stirring the slurry. After that, it was cooled to room temperature to obtain a precursor.

[0133] Li.sub.9.81Sn.sub.0.81P.sub.2.19S.sub.12 crystal was synthesized in a manner similar to that in Example 1.

Example 3

[0134] An Li—P—S homogeneous solution was obtained in a manner similar to that in Example 1.

[0135] An Li—Sn—S homogeneous solution was obtained in a manner similar to that in Example 1.

[0136] An Li—S homogeneous solution was obtained in a manner similar to that in Example 1.

[0137] In a glovebox under argon atmosphere, 4.0 g of Li.sub.2S (manufactured by Sigma-Aldrich, purity: 99.8%), 16.0 g of SiS.sub.2 (manufactured by HANGZHOU) and 2.4 g of S (manufactured by Kojundo Chemical Laboratory Co., Ltd.) were weighed so that the molar ratio of Li.sub.2S:SiS.sub.2:S became 0.5:1:0.4. Next, 610 g of acetonitrile (manufactured by Wako Pure Chemical Industries, Ltd., super dehydrated grade) was added thereto so that the concentration of (Li.sub.2S+SiS.sub.2+S) became about 3.5% by mass, and mixing was carried out at room temperature for 24 hours. The mixture was gradually dissolved, but at this stage, impurities in the raw materials remained.

[0138] The obtained solution was filtered with a membrane filter (PTFE, pore size: 1.0 μm), thereby obtaining 2.0 g of a filter residue and 578 g of a filtrate (Li—Si—S homogeneous solution). When the Li—Si—S homogeneous solution was subjected to ICP analysis, Li:Si:S (molar ratio) was 1:1:3. Further, the concentration of (Li.sub.2S+SiS.sub.2+S) was 3.43% by mass.

[0139] 6.6 g of the prepared Li—P—S homogeneous solution, 11.11 g of the prepared Li—Sn—S homogeneous solution, 13.48 g of the prepared Li—Si—S homogeneous solution and 22.93 g of the prepared Li—S homogeneous solution were mixed with each other so that the molar ratio of Li:Sn:Si:P became 38:1:4:6, and stirring was carried out for 3 hours to prepare a homogeneous mixed solution.

[0140] The obtained homogeneous mixed solution was dried under vacuum at 180° C. for 4 hours to remove the solvent. Removal of the solvent was carried out while stirring the solution. After that, it was cooled to room temperature to obtain a precursor.

[0141] In the glovebox, the obtained precursor was put into a glass reaction tube, which was placed in an electric tube furnace in a manner such that the precursor was not exposed to atmosphere. Argon (G3 grade) was injected into the reaction tube, the temperature was increased to 550° C. over 3 hours, and then burning was carried out at 550° C. for 8 hours, thereby synthesizing Li.sub.10.35Sn.sub.0.27Si.sub.1.08P.sub.1.65S.sub.12 crystal.

Comparative Example 2

[0142] An Li—P—S homogeneous solution was obtained in a manner similar to that in Example 1.

[0143] An Li—Si—S homogeneous solution was obtained in a manner similar to that in Example 3.

[0144] 6.6 g of the prepared Li—P—S homogeneous solution, 131 mg of SnS.sub.2 powder, 13.48 g of the Li—Si—S homogeneous solution and 461 mg of Li.sub.2S were mixed with each other so that the molar ratio of Li:Sn:Si:P became 38:1:4:6, and stirring was carried out for 3 hours to prepare a slurry. In this regard, Sn was not in a state where it was completely dissolved in the organic solvent.

[0145] The obtained slurry was dried under vacuum at 180° C. for 4 hours to remove the solvent. Removal of the solvent was carried out while stirring the slurry. After that, it was cooled to room temperature to obtain a precursor.

[0146] Li.sub.10.35Sn.sub.0.27Si.sub.1.08P.sub.1.65S.sub.12 crystal was synthesized in a manner similar to that in Example 3.

Example 4

[0147] In a glovebox under argon atmosphere, 236 mg of Li.sub.2S (manufactured by Sigma-Aldrich, purity: 99.8%) and 487 mg of P.sub.2S.sub.5 (manufactured by Sigma-Aldrich, purity: 99%) were weighed so that the molar ratio of Li.sub.2S:P.sub.2S.sub.5 became 2.4:1. Next, to 7.0 g of acetonitrile (manufactured by Wako Pure Chemical Industries, Ltd., super dehydrated grade), Li.sub.2S and P.sub.2S.sub.5 were added in this order so that the concentration of (Li.sub.2S+P.sub.2S.sub.5) became about 10% by mass, and mixing was carried out at room temperature for 12 hours. Precipitation of Li.sub.3PS.sub.4 was caused to obtain an Li.sub.3PS.sub.4-containing slurry.

[0148] An Li—Sn—S homogeneous solution was obtained in a manner similar to that in Example 1.

[0149] 7.72 g of the prepared Li.sub.3PS.sub.4-containing slurry and 20.71 g of the prepared Li—Sn—S homogeneous solution were mixed with each other so that the molar ratio of Li:Sn:P became 12:1:3, and stirring was carried out for 3 hours to prepare a slurry mixed solution.

[0150] The obtained slurry mixed solution was dried under vacuum at 180° C. for 4 hours to remove the solvent. Removal of the solvent was carried out while stirring the solution. After that, it was cooled to room temperature to obtain a precursor.

[0151] Li.sub.9.81Sn.sub.0.81P.sub.2.19S.sub.12 crystal was synthesized in a manner similar to that in Example 1.

<X-Ray Diffraction Measurement>

[0152] Powders of the sulfide-based solid electrolytes obtained in Examples 1-4 and Comparative Examples 1-2 were subjected to the X-ray diffraction measurement under Ar atmosphere at room temperature (25° C.) (“X' Pert3 Powder” manufactured by PANalytical, CuKα: λ=1.5405 Å).

[0153] The results of the X-ray diffraction measurement of the sulfide-based solid electrolytes obtained in Examples 1-4 and Comparative Examples 1-2 are shown in FIG. 3.

[0154] As shown in FIG. 3, in Examples 1-4 and Comparative Examples 1-2, the diffraction peaks were observed at at least 20=19.90°±0.50°, 20.20°±0.50°, 26.70°±40.50° and 29.20°±0.50°. The pattern corresponded to that of Li.sub.10GeP.sub.2S.sub.12 of ICSD database, and it was recognized that the LGPS type crystal structure is possessed.

[0155] Further, in Comparative Example 1, many peaks of impurities were observed. It is considered that these peaks of impurities were observed since Sn was not homogeneously dispersed in the solid electrolyte at the time of synthesis because SnS.sub.2 as the raw material has poor solubility in solvents.

[0156] Each of the sulfide-based solid electrolytes obtained in Examples 1-4 and Comparative Examples 1-2 was subjected to uniaxial molding (420 MPa) to obtain a disk having a thickness of about 1 mm and a diameter of 10 mm. Using a cell for evaluating all-solid-state batteries (manufactured by Hohsen Corporation), the AC impedance was measured by the four-terminal method at room temperature (25° C.) (“SI1260 IMPEDANCE/GAIN-PHASE ANALYZER” manufactured by Solartron), and the lithium ion conductivity was calculated.

[0157] Specifically, the sample was put into a thermostatic bath with its temperature being set at 25° C. and it was kept for 30 minutes, and after that, the lithium ion conductivity was measured. The measurement frequency range was 0.1 Hz to 1 MHz, and the amplitude was 50 mV. The measurement results of the lithium ion conductivity are shown in Table 1 below.

TABLE-US-00001 TABLE 1 Composition P component Sn component Si component Others Conductivity Example 1 Li.sub.9.81Sn.sub.0.81P.sub.2.19S.sub.12 Li—P—S Li—Sn—S Absent Li—S 2.6 mS/cm homogeneous homogeneous homogeneous solution solution solution Example 2 Li—P—S Li—Sn—S Absent Absent 2.8 mS/cm homogeneous homogeneous solution solution Comparative Li—P—S SnS.sub.2 powder Absent Li.sub.2S powder 1.4 mS/cm Example 1 homogeneous solution Example 3 Li.sub.10.35Sn.sub.0.27- Li—P—S Li—Sn—S Li—Sn—S Li—S 4.6 mS/cm Si.sub.1.08P.sub.1.65S.sub.12 homogeneous homogeneous homogeneous homogeneous solution solution solution solution Comparative Li—P—S SnS.sub.2 powder Li—Si—S Li.sub.2S powder 3.9 mS/cm Example 2 homogeneous homogeneous solution solution Example 4 Li.sub.9.81Sn.sub.0.81P.sub.2.19S.sub.12 Li—P—S Li—Sn—S Absent Absent 2.0 mS/em containing homogeneous slurry solution

EXPLANATIONS OF LETTERS OR NUMERALS

[0158] 1 positive electrode layer [0159] 2 solid electrolyte layer [0160] 3 negative electrode layer [0161] 10 all-solid-state battery

METHOD FOR PRODUCING SULFIDE SOLID ELECTROLYTE INCLUDING Sn

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Abstract

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