Ion-conducting glass ceramic having garnet-like crystal structure

Abstract

A glass ceramic containing lithium-ions and having a garnet-like main crystal phase having an amorphous proportion of at least 5% is disclosed. The garnet-like main crystal phase preferably has the chemical formula Li.sub.7+xyM.sub.x.sup.IIM.sub.3x.sup.IIIM.sub.2y.sup.IVM.sub.y.sup.VO.sub.12, wherein M.sup.II is a bivalent cation, M.sup.III is a trivalent cation, M.sup.IV is a tetravalent cation, M.sup.V is a pentavalent cation. The glass ceramic is prepared by a melting technology preferably within a Skull crucible and has an ion conductivity of at least 5.Math.10.sup.5 S/cm, preferably of at least 1.Math.10.sup.4 S/cm.

Claims

1. A method of producing a glass ceramic comprising a garnet-like main crystal phase, wherein the garnet-like main crystal phase has a chemical formula Li.sub.7+xyM.sub.x.sup.IIM.sub.3x.sup.IIIM.sub.2y.sup.IVM.sub.y.sup.VO.sub.12, wherein M.sup.II is a bivalent cation, M.sup.III is a trivalent cation, M.sup.IV is a tetravalent cation, M.sup.V is a pentavalent cation, wherein 0x<3 and 0y<2, the method comprising the steps of: providing starting materials for the glass ceramic; melting and homogenizing the starting materials in an inductively heated Skull crucible to yield a glass melt; and cooling the glass melt and temperature treating to generate the glass ceramic comprising the garnet-like main crystal phase with an amorphous proportion of at least 5% wt.-%.

2. The method of claim 1, wherein the inductively heated Skull crucible has a plurality of metal tubes that are traversed by a coolant having slot-like intervals between the plurality of metal tubes, and further comprise having an induction coil surrounding the plurality of metal tubes.

3. The method of claim 2, wherein the plurality of metal tubes are shorted with each other.

4. The method of claim 1, wherein the glass ceramic comprises 10-25 wt.-% of Li.sub.2O.

5. The method of claim 1, wherein the glass ceramic comprises in total 40-60 wt.-% of an oxide of at least one lanthanoid.

6. The method of claim 1, wherein the glass ceramic comprises 10-25 wt.-% of Li.sub.2O and in total 40-60 wt.-% of an oxide of at least one lanthanoid.

7. The method of claim 1, wherein 0x2 and 0y1.

8. The method of claim 1, wherein the amorphous proportion has a maximum of 40 wt.-%.

9. The method of claim 1, wherein the glass ceramic has an ion-conductivity of at least 5.Math.10.sup.5 S/cm.

10. The method of claim 1, wherein the glass ceramic comprises in total 40-60 wt.-% of La.sub.2O.sub.3.

11. The method of claim 1, wherein the glass ceramic comprises 15-35 wt.-% of ZrO.sub.2.

12. The method of claim 1, wherein the glass ceramic comprises 1-5 wt.-% of one oxide being selected from a group consisting of Al.sub.2O.sub.3, Bi.sub.2O.sub.3, Ga.sub.2O.sub.3, Y.sub.2O.sub.3, Fe.sub.2O.sub.3, Cr.sub.2O.sub.3, In.sub.2O.sub.3, and mixtures thereof.

13. The method of claim 12, wherein the glass ceramic comprises 1-5 wt.-% of one oxide being selected from a group consisting of Al.sub.2O.sub.3, Bi.sub.2O.sub.3, Ga.sub.2O.sub.3, Y.sub.2O.sub.3, Fe.sub.2O.sub.3, Cr.sub.2O.sub.3, In.sub.2O.sub.3, and mixtures thereof.

14. The method of claim 1, wherein the glass ceramic comprises 1-20 wt.-% of an oxide being selected from a group consisting of Ta.sub.2O.sub.5, Nb.sub.2O.sub.5, V.sub.2O.sub.5, P.sub.2O.sub.5, and mixtures thereof.

15. The method of claim 1, wherein the glass ceramic comprises 1-5 wt.-% of an oxide being selected from a group consisting of TiO.sub.2, HfO.sub.2, SnO.sub.2, and mixtures thereof.

16. The method of claim 1, wherein the glass ceramic comprises 1-10 wt.-% of at least of an oxide being selected from a group consisting of RO, ZnO, and mixtures thereof, wherein R is an alkaline earth ion.

17. A method of producing a glass ceramic comprising a garnet-like main crystal phase, wherein the garnet-like main crystal phase has the chemical formula: Li.sub.7+xyM.sub.x.sup.IIM.sub.3x.sup.IIIM.sub.2y.sup.IVM.sub.y.sup.VO.sub.12, wherein M.sup.II is a bivalent cation, M.sup.III is a trivalent cation, M.sup.IV is a tetravalent cation, M.sup.V is a pentavalent cation, the method comprising the steps of: providing starting materials for the glass ceramic; melting and homogenizing the starting materials to yield a glass melt; cooling the glass melt and temperature treating to generate the glass ceramic comprising the garnet-like main crystal phase with an amorphous proportion of at least 5% wt.-%.

18. The method of claim 1, wherein 0x2 and 0y1.

19. The method of claim 1, wherein the amorphous proportion has a maximum of 40 wt.-%.

20. A method of producing a glass ceramic comprising a garnet-like main crystal phase, wherein the garnet-like main crystal phase has the chemical formula Li.sub.7+xyM.sub.x.sup.IIM.sub.3x.sup.IIIM.sub.2y.sup.IVM.sub.y.sup.VO.sub.12, wherein M.sup.II is a bivalent cation, M.sup.III is a trivalent cation, M.sup.IV is a tetravalent cation, M.sup.V is a pentavalent cation, the method comprising the steps of: providing starting materials for the glass ceramic; melting and homogenizing the starting materials to yield a glass melt; cooling the glass melt and temperature treating to generate the glass ceramic comprising the garnet-like main crystal phase with an amorphous proportion of at least 5% wt.-% and an ion-conductivity of at least 5.Math.10.sup.5 S/cm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the invention can be taken from the subsequent description of preferred embodiments with reference to the drawings. In the drawings show:

(2) FIG. 1 is an X-ray diffraction diagram of a glass ceramic prepared within a platinum crucible by melting, comprising tetragonal LLZO (Li.sub.7La.sub.3Zr.sub.2O.sub.12) as a main phase and small amounts of elementary Pt as a side phase;

(3) FIG. 2 a SEM picture of the LLZO prepared by melting, and

(4) FIG. 3 a picture taken by polarization microscopy of a LLZO glass ceramic according to the invention which was molten within a Skull crucible, wherein the dark regions show the amorphous phase.

DETAILED DESCRIPTION

Examples

(5) A glass ceramic according to the invention preferably has the garnet-like main crystal phase according to the chemical formula:
Li.sub.7+xyM.sub.x.sup.IIM.sub.3x.sup.IIIM.sub.2y.sup.IVM.sub.y.sup.VO.sub.12,
wherein M.sup.II is a bivalent cation, M.sup.III a trivalent cation, M.sup.IV a tetravalent cation, M.sup.V a pentavalent cation, wherein preferably 0x<3, more preferred 0x2, 0y<2, and particularly preferred 0y1.

(6) TABLE-US-00001 TABLE 1 shows different compositions (in wt. % on oxide basis) which were used for preparing a glass ceramic according to the invention; the specifics with respect to the proportions of cubic, tetragonal, and amorphous phase are given in vol.%. Example 1 2 3 4 5 6 7 8 9 Li.sub.2O 14.02 14-16 14.31 13.87 14.32 12.86 13.23 13.94 14.65 La.sub.2O.sub.3 55.24 55.15 55.06 55.34 55.12 56.07 55.83 53.64 56.37 ZrO.sub.2 27.86 27.81 27.76 27.90 24.32 24.74 24.63 20.29 21.32 Al.sub.2O.sub.3 2.88 2.88 2.87 2.89 Ta.sub.2O.sub.5 6.23 6.34 6.31 12.13 Nb.sub.2O.sub.5 7.66 Cubic phase 62% 55% 40% 38% 27% 49% 75% 100% 100% Tetragonal Phase 38% 45% 60% 62% 73% 51% 25% Foreign Phases none none none none none none none none none Amorphous Proportion ca. 15% ca. 10% 16% 21% n.d. n.d. n.d. n.d. n.d. Conductivity (S/cm) 1 .Math. 10.sup.4 n.d. 1 .Math. 10.sup.4 n.d. 3 .Math. 10.sup.4 2.9 .Math. 10.sup.4 3 .Math. 10.sup.4 5 .Math. 10.sup.4 3 .Math. 10.sup.4 n.d.not determined

(7) The melting is performed within a so-called Skull crucible, such as described in DE 199 39 782 C1.

(8) In the Skull technology a water-cooled crucible is utilized, within which during the melting a cooler protective layer forms from the molten material. Thus, during the melting procedure no crucible material is dissolved. The energy introduction into the melt is realized by coupling using high frequency coupling via the surrounding induction coil into the liquid material. A condition in this regard is a sufficient conductivity of the melt which in the case of lithium-garnet melts is ensured by the high lithium content. During the melting procedure there is a lithium evaporation which can easily be corrected by a lithium excess. To this end usually there is worked with a 1.1- to a 2-fold lithium excess.

(9) The raw materials according to the composition given in TABLE 1 are mixed and then given into the Skull crucible that is open on the top side. The mixture had initially to be preheated to reach a certain minimum conductivity. To this end a burner heating was utilized. After reaching the coupling temperature the further heating and homogenizing of the melt was ensured by high frequency coupling via the induction coil.

(10) To improve the homogenization of the melt, it was stirred using a water-cooled stirrer.

(11) After full homogenization direct samples were taken from the melt (fast cooling), while the reminder of the melt was cooled slowly by switching off the high frequency.

(12) The material prepared in this way may basically be either directly solidified from the melt or by quenching, followed by a temperature treatment (ceramizing) transferred into a glass ceramic material with garnet-like main crystal phase.

(13) The samples taken from the melt independently from the cooling showed a spontaneous crystallization, so that a subsequent ceramization treatment could be dispensed with.

(14) The measurement of the ion-conductivity showed the positive influence of the amorphous phase contained within the glass ceramic which can be explained by a reduction of the conductivity decrease by the grain boundaries.

(15) From the glass ceramics obtained in this way samples for the impedance-spectroscopy were prepared for determining the conductivity, as well as for X-ray diffraction investigations (XRD). To avoid a degradation of the samples upon contact with water the sample preparation was performed water-free.

(16) The X-ray diffraction investigations (XRD) in all samples showed a mixture of tetragonal and cubic crystal phases.

(17) FIG. 1 shows a comparison of a tetragonal LLZO prepared within a platinum crucible by melting technology. Herein apart from the desired main phase (tetragonal LLZO (Li.sub.7La.sub.3Zr.sub.2O.sub.12)) as a side phase also reflexes of elementary platinum (main peak at 39.8) can be seen. This is due to an attack onto the melting crucible by the highly lithium-containing melt. Due to this reason the later melting processes with the compositions given in TABLE 1 were performed within a Skull crucible as described above.

(18) FIG. 2 shows the LLZO prepared by melting within a Pt-crucible under the scanning electron microscope with structures of a size of up to 300 m.

(19) For a precise determining of the phase content a Rietveld-analysis was performed. According to TABLE 1 the samples that are doped with aluminum (no. 1-4) contain a fraction of cubic phase between 38 and 62%, while the samples that are doped with tantalum (nos. 5-8) contain a cubic fraction of 27 to 100% (each without taking into account the amorphous fraction). Also using a doping with niob makes possible a fully stabilization of the cubic phase (confer example 9).

(20) Even in the samples with high tetragonal proportion there are considerably high conductivities. Thus example 3 that comprises 60% of tetragonal phase, the conductivity of which in literature is given with 1.6.Math.10.sup.6 S/cm, shows a conductivity of about 1.Math.10.sup.4 S/cm. This almost corresponds to the expected conductivity for aluminum-dopings of the pure cubic phase (2.Math.10.sup.4 S/cm).

(21) A determination of the amorphous proportion using Rietveld-improvements, or using an external standard, respectively, with respect to the samples did not lead to correct results, due to the strong reflex superimpositions of the cubic and tetragonal LLZO phases. The determination of the amorphous proportion, or the crystallinity, respectively, therefore was made by evaluating the underground. Also measurements on fully crystalline samples show a noise which is due to crystal lattice flaws, the influence of optical devices, fluorescence effects and dispersive effects. If the sample is not fully crystalline, then to this noise the contribution of the amorphous phase is added. The constant noise therefore was determined using standard samples with defined amorphous proportions. In this way the determination of the amorphous proportion within the samples was made possible which is between 10 and 21% according to TABLE 1.

(22) If the different conductivities of the cubic and tetragonal phases are taken into account, as well as the fact that in part considerable amounts of tetragonal phases are contained within the samples, then it can be derived that the amorphous phase contained within the glass ceramic increases the total conductivity which is a surprising result.

(23) Since the determination of the amorphous proportion is done only indirectly, a sample (composition according to example 5) was examined using a polarization microscope. Herein dark regions are found within the sample which is evidence of an amorphous proportion. FIG. 3 shows a picture of a sample which, for increasing the visibility of the amorphous regions was transferred in water for 2-3 days. About 10 m large amorphous structures can be clearly seen.