SINTERING AID MIXTURE, SOLID-STATE ION CONDUCTOR, AND METHOD FOR PRODUCING SOLID-STATE ION CONDUCTORS

Abstract

A sintering aid mixture for sintering solid-state ion conductors, electrode materials, or the like for solid-state batteries is provided. The mixture includes at least one sol-gel precursor and/or at least one sol-gel direct precursor produced from at least one sol-gel precursor.

Claims

1. A sintering aid mixture comprising a sintering precursor selected from a group consisting of a sol-gel precursor, a sol-gel direct precursor prepared from the sol-gel precursor, and any combinations thereof.

2. The sintering aid mixture of claim 1, wherein the sintering aid mixture is adapted for a use selected from a group consisting of a solid-state ion conductor, an electrode material, and a solid-state battery component.

3. The sintering aid mixture of claim 1, wherein the sol-gel direct precursor is a powder.

4. The sintering aid mixture of claim 3, wherein the sol-gel direct precursor is a stoichiometric mixture of at least two sol-gel precursors.

5. The sintering aid mixture of claim 1, wherein the sintering precursor comprises lithium.

6. The sintering aid mixture of claim 1, wherein the sol-gel precursor is free of inorganic anions and/or non-oxidic anions.

7. The sintering aid mixture of claim 6, wherein the sol-gel precursor comprises a compound selected from a group consisting of an organometallic compound, an alkoxide, an acetate, lithium acetate.

8. The sintering aid mixture of claim 7, wherein the sol-gel precursor further comprises a nitrate.

9. The sintering aid mixture of claim 1, wherein the sintering precursor has a sintering temperature selected from a group consisting of less than 1100° C., less than 1000° C., less than 900° C., less than 850° C., less than 840° C., less than 800° C., and less than 700° C.

10. A solid-state ion conductor made using the sintering aid mixture of claim 1.

11. The solid-state ion conductor of claim 10, further comprising lithium-lanthanum-zirconium oxide and/or lithium-aluminum-titanium phosphate.

12. The solid-state ion conductor of claim 10, wherein the solid-state ion conductor was sintered in the presence of the sintering aid mixture in a proportion between 0.01 wt % and at most 15 wt %.

13. A solid-state ion conductor, comprising lithium-lanthanum-zirconium oxide and/or lithium-aluminum-titanium phosphate sintered at a temperature of less than 1100° C. in the presence of a sintering precursor selected from a group consisting of a sol-gel precursor, a sol-gel direct precursor prepared from a sol-gel precursor, and any combinations thereof.

14. The solid-state ion conductor of claim 13, wherein the presence of the sintering precursor comprises a proportion between 0.01 wt % and at most 15 wt %.

15. The solid-state ion conductor of claim 13, wherein the sol-gel direct precursor has a characteristic selected from a group consisting of: present in powder form; a stoichiometric mixture of at least two sol-gel precursors; comprises lithium; free of inorganic anions; for of non-oxidic anions; is an organometallic compound; is an alkoxide, is an acetate, is lithium acetate; comprises a nitrate; and any combinations thereof.

16. The solid-state ion conductor of claim 13, wherein the temperature is less than 850° C.

17. A method for producing solid-state ion conductors, comprising: sintering in the presence of a sintering aid mixture, wherein the sintering aid mixture comprises a precursor is selected from a group consisting of a sol-gel precursor, a sol-gel direct precursor prepared from a sol-gel precursor, and any combinations thereof.

18. The method of claim 17, wherein the sintering step comprises sintering at a temperature selected from a group consisting of less than 1100° C., less than 900° C., less than 840° C., and less than 800° C.

19. The method of claim 17, wherein prior to the sintering step, the method further comprises introducing the sintering aid mixture in a proportion of between 0.01 wt % and at most 15 wt %.

20. The method of claim 17, wherein prior to the sintering step, the method further comprises introducing the sintering aid mixture to an oxide-based ceramic material comprising lithium-lanthanum-zirconium oxide and/or lithium-aluminum-titanium phosphate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] FIGS. 1a and 1b are schematic illustrations of the normalized area of powder compacts in accordance with embodiments of the invention as a function of the temperature; and

[0041] FIG. 2 is a table for the total conductivity and the relative density of compacts in accordance with embodiments of the invention.

DETAILED DESCRIPTION

[0042] Preparation of reference samples without addition of a sintering aid mixture in accordance with any embodiment of the present invention.

[0043] In the following, the synthesis of the materials GK-LLZO and SG-LLZO as materials will be described first. They will then be pressed to form compacts and the compacts will be sintered.

[0044] Synthesis of glass-ceramic LLZO (GK-LLZO). GK-LLZO was synthesized by means of a melting method, such as described, for example, in DE 10 2014 100 684 B4, which is incorporated herein by reference. In this case, the oxides of the metal cations were fused together and homogenized in a skull crucible. In this case, the skull crucible consisted of vertical metal tubes, which were cooled with water. The educts were combined in the crucible and the batch was preheated using a burner in order to achieve a sufficient conductivity. Further heating was carried out by irradiation with high-frequency energy via an induction coil. Formed on the cooled metal tubes was a layer of solidified melt, which separated the crucible wall from the liquid melt. It was possible in this way to prevent any possible reaction between the crucible material and the melt. The mix was homogenized by means of a water-cooled stirrer. After the reaction had ended, the heating was switched off and the melt solidified in the crucible in the form of a solid block.

[0045] The solid block was first broken apart using a hammer and chisel to form smaller fragments. These were then fed in one or more throughputs to a jaw crusher until fragments with a size of at most 10 mm in the longest extension were formed. They, in turn, were ground on a disk mill to a size of d.sub.99<1 mm.

[0046] 1 kg of coarsely ground GK-LLZO powder with a grain size of <1 mm is fed to a counterjet mill. A connecting sieve provides a powder fraction that—after a further separation of fine fractions in a cyclone—has a particle size distribution of d.sub.50=1.4 μm, d.sub.99=2.9 μm, and d.sub.99=4.1 μm. The measurement of particle sizes is made with the use of the statistical light scattering method in accordance with the standard ISO 13320-1 on a CILAS Model 1064 particle size measuring instrument. The measurement is carried out in water as medium and analyzed by the Fraunhofer method.

[0047] Synthesis of ceramic LLZO, produced via the sol-gel route (SG-LLZO). The synthesis of SG-LLZO as a reference material relative to the powders that were prepared via a glass-ceramic route was carried out via a sol-gel variant. For this purpose, a zirconium oxide direct precursor powder (Zr-VP) was produced first.

[0048] For the production of the Zr-VP, zirconium propoxide solution (70% in 1-propanol) (234.04 g, 0.5 mol, 1.0 eq.) was placed in a round-bottom flask. Via a dropping funnel, acetyl acetone (50.08 g, 0.5 mol, 1.0 eq.) was added under strong stirring and, after the addition had ended, the resulting yellowish solution was stirred for one hour at room temperature. The solution was treated dropwise with water (23.78 g, 1.32 mol, 2.6 eq.) under constant stirring, and, after the addition had ended, stirring was continued for a further 30 minutes. The addition of water caused the solution to turn deep orange and to become more viscous.

[0049] The solution was freed of solvent rapidly on a rotary evaporator, whereby an orange-yellow powder was obtained. The powder was kept in a crystallization dish for 5 hours at 125° C. in the oven in order to remove all residues of solvent.

[0050] For the production of SG-LLZO, lanthanum (III) acetate sesquihydrate (30.75 g, 0.09 mol, 1 eq.) in approximately 130 mL of ethanol was placed in a round-bottom flask and treated dropwise with 2-(2-methoxyethoxy) acetate, so that a milky, cloudy solution formed. Zr-VP (12.68 g, 0.06 mol, 0.67 equivalent concentration eq.) was dissolved in 60 mL of ethanol and added to the lanthanum (III) acetate solution while stirring. Lithium acetate dihydrate (23.58 g, 0.23 mol, 2.57 eq.) and aluminum chloride hexahydrate (1.74 g, 0.007 mol, 0.08 eq.) were added to the solution, and the orange cloudy solution was stirred overnight at room temperature. It was freed of the solvent by means of a rotary evaporator and the resulting yellowish orange powder was calcined in a ZrO.sub.2 crucible at a heating rate of 10 K/min for 7 hours at 1000° C. The resulting colorless powder was ground using a mortar and pestle under nitrogen atmosphere and then ground using a planetary mill.

[0051] Production of the compacts. In order to determine the conductivity of GK-LLZO and SG-LLZO with and without a sintering aid and in order to carry out contacting tests with lithium, compacts of the various powders were produced. For this purpose, a green body was first prepared in air. Approximately 0.3-0.5 g of the powder was transferred to a cylindrical steel press die and the steel stamp was pressed in place in a handtight manner. Subsequently, the powder was pressed uniaxially with a defined force (30 kN) for two minutes and afterwards sintered at 1200° C.

[0052] For further tests and measurements, the surface of the compacts was polished in an argon glovebox (MBraun, H.sub.2O<1 ppm, O.sub.2<1 ppm) using silicon carbide sandpaper. The thickness of the compacts after polishing was approximately 1 mm.

[0053] Contacting of the compacts with a gold layer. For the sample preparation, the compacts were produced as described above, and, after being polished, were sputtered with a thin gold layer (˜130 nm) for four minutes and at a current intensity of 60 mA by using an instrument of the Leica company. The installation of the compacts in a suitable measuring cell was carried out in an argon glovebox.

[0054] The impedance of the compact was measured on a broadband dielectric spectrometer using a Novocontrol Alpha-A High Performance Frequency Analyzer with cryostats. The frequency range of the measurement was between 4 MHz and 10 MHz, with an AC voltage amplitude of 20 mV being applied.

[0055] The measurement data was analyzed using the software ZView 2.9 (Scribner Associates, Inc. USA).

[0056] Exemplary embodiments for the production of samples with addition of a sintering aid mixture in accordance with one embodiment of the present invention. The following now presents, by way of the example of a sol-gel direct precursor of lithium aluminate (LiAlO.sub.2) as a sintering aid, how the sintering temperature required for the sintering of GK-LLZO and SG-LLZO is reduced, and also shows the dependency of the conductivity on the quantity of the added sintering aid.

[0057] Synthesis of a lithium aluminate (LiAlO.sub.2) sol-gel direct precursor. Aluminum isopropoxide (21.20 g, 0.10 mol, 1.0 eq.)—used here as a sol-gel precursor for Al.sub.2O.sub.3—was dispersed in 67 mL of ethyl acetate and treated while stirring with 40 mL of acetic acid (41 g, 0.68 mol, 6.8 eq.). A milky white suspension was thereby obtained. Lithium acetate dihydrate (23.58 g, 0.23 mol, 2.57 eq.)—used here as a sol-gel precursor for Li.sub.2O—was dissolved in 57 mL of ethanol and added to the suspension while stirring. Stirring was continued for one hour at room temperature. The suspension was evaporated and the gel was dried for approximately 20 hours at 100° C. The obtained colorless powder was finely ground using a mortar and pestle. A colorless powder was obtained as lithium aluminate (LiAlO.sub.2) sol-gel direct precursor.

[0058] Lithium aluminate (LiAlO.sub.2) sol-gel direct precursor as a sintering aid mixture. In order to investigate the effect of a sintering additive or a sintering aid mixture, a sol-gel direct precursor of lithium aluminate (LiAlO.sub.2), LA, was produced via a sol-gel route and was combined, in each case in a proportion of 15 wt %, with GK-LLZO and SG-LLZO. Subsequently, the powder mixture was pressed to form a pellet analogous to the reference samples, sintered, and investigated electrochemically.

[0059] Characterization of samples in accordance with an embodiment of the present invention (that is, samples produced by addition of a sintering aid mixture and reference samples produced without addition of sintering additives in comparison). For a first phenomenological characterization of the sintering behavior, the LiAlO.sub.2 sol-gel direct precursor and the mixtures of GK-LLZO and the LiAlO.sub.2 sol-gel direct precursor and SG-LLZO and the LiAlO.sub.2 sol-gel direct precursor were each investigated in the hot-stage microscope (see FIG. 1). In detail, FIG. 1a shows the plot of the normalized area of the powder compact of the LiAlO.sub.2-sol-gel direct precursor (reference number 102), GK-LLZO (reference number 101), and GK-LLZO+15 wt % LiAlO.sub.2 sol-gel direct precursor (reference number 103) versus temperature and FIG. 1b shows the plot of the normalized area of the powder compact of the LiAlO.sub.2 sol-gel direct precursor (reference number 202), SG-LLZO (reference number 201), and SG-LLZO+15 wt % LiAlO.sub.2 sol-gel direct precursor (reference number 203) versus temperature. The thermocouple present in the measuring apparatus measures in a reliable manner only starting at a temperature >350° C., for which reason the illustration in FIGS. 1a and 1b begins only starting from this temperature.

[0060] For the LiAlO.sub.2 sol-gel direct precursor 102, 202, the shrinkage begins at approximately 810° C. slowly and becomes increasingly stronger until, starting at a temperature of 900° C., a linear decrease in the normalized area down to 85% of the original area takes place. Starting at 1020° C., the shrinkage reduces to approximately 82% up to the end of the measurement (1200° C.).

[0061] From the comparison of the hot-stage microscope (EHM) curves of GK-LLZO 101 and GK-LLZO +15 wt % LiAlO.sub.2 sol-gel direct precursor 103, the positive effect of the sintering aid mixture LiAlO.sub.2 sol-gel direct precursor 102 in regard to the decrease in the normalized sample area can readily be seen. Starting at approximately 550° C., a first minimal shrinkage appears, which, in the case of GK-LLZO 101, is evident only at 700° C. In the further course of 101, starting at 950° C., a stronger reduction in the area occurs, and, starting at a temperature of 1050° C., the curve transitions into a very steep course, so that the sample area shrinks by nearly 30% (1150° C.). The densification increases further with increasing temperature, but less strongly, to a normalized sample area of approximately 65%. For the mixture of SG-LLZO and the LiAlO.sub.2 sol-gel direct precursor (15 wt %) 103, there also results in the EHM a marked difference in comparison to the pure LLZO powder 101. The shrinkage begins here slightly even at 880° C., and the curve 103 transitions, starting at approximately 1050° C., to a steeper course until the minimal normalized area in the course of the curve (approximately 82%) is reached.

[0062] On the basis of the EHM data, consequently, the effect of the LiAlO.sub.2 sol-gel direct precursor 102 as a sintering aid mixture is evident: For both powders GK-LLZO and SG-LLZO, a sintering occurs first at lower temperatures and the densification of the samples is markedly greater.

[0063] In addition to the phenomenological EHM investigations, LiAlO.sub.2 was also combined with GK-LLZO and SG-LLZO (two mixtures, each with 5 wt % LiAlO.sub.2 sol-gel direct precursor) and sintered. In a purely visual manner, the effect of LiAlO.sub.2 sol-gel direct precursor as a sintering additive could be confirmed. The compacts produced appear overall more homogeneous and more stable. Presented in FIG. 2, in addition to the relative density of the compacts, are also the conductivities of the pure LLZO variants, that is, GK-LLZO and SG-LLZO, and the conductivities of the LLZO-LiAlO.sub.2 mixtures, each with 5 wt % LiAlO.sub.2 sol-gel direct precursor.

[0064] Even for a small addition of 5 wt % LiAlO.sub.2 sol-gel direct precursor as a sintering aid mixture, better conductivities were obtained in both cases than in the case of the pure LLZO variants (see FIG. 2).

[0065] Through the electrochemical characterization and the subsequent phase analysis by means of powder diffractometry of the compacts with subsequently added sintering additive (LiAlO.sub.2 sol-gel direct precursor), it was possible to ascertain that, for addition of a sintering aid mixture according to the invention as a sintering additive, the conductivity can be or is positively influenced.

[0066] In summary, at least one of the embodiments of the invention can provide at least one of the following advantages and/or features: Simply produced sintering aid; Reduction of the sintering temperature by addition of the sintering aid; Cost-effective sintering aid; Prevention of a reduction of the conductivity, in particular in the case of lithium-based materials; and Simpler production of solid-state batteries.

[0067] Although the present invention was described on the basis of preferred exemplary embodiments, it is not limited to these, but can be modified in a variety of ways.

LIST OF REFERENCE NUMBERS

[0068] 102 plot of LA sol-gel direct precursor [0069] 101 plot of GK-LLZO [0070] 103 plot of GK-LLZO+LA sol-gel direct precursor [0071] 202 plot of LA sol-gel direct precursor [0072] 201 plot of SG-LLZO [0073] 203 plot of SG-LLZO+LA sol-gel direct precursor