Lithium ion-conducting garnet-like compounds

Abstract

A lithium ion-conducting compound, having a garnet-like crystal structure, and having the general formula: Li.sub.n[A.sub.(3-a′-a″)A′.sub.(a′)A″.sub.(a″)][B.sub.(2-b′-b″)B′.sub.(b′)B″.sub.(b″)][C′.sub.(c′)C″.sub.(c″)]O.sub.12, where A, A′, A″ stand for a dodecahedral position of the crystal structure, where A stands for La, Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and/or Yb, A′ stands for Ca, Sr and/or Ba, A″ stands for Na and/or K, 0<a′<2 and 0<a″<1, where B, B′, B″ stand for an octahedral position of the crystal structure, where B stands for Zr, Hf and/or Sn, B′ stands for Ta, Nb, Sb and/or Bi, B″ stands for at least one element selected from the group including Te, W and Mo, 0<b′<2 and 0<b″<2, where C and C″ stand for a tetrahedral position of the crystal structure, where C stands for Al and Ga, C″ stands for Si and/or Ge, 0<c′<0.5 and 0<c″<0.4, and where n=7+a′+2.Math.a″−b′−2.Math.b″−3.Math.c′−4.Math.c″ and 5.5<n<6.875.

Claims

1. A lithium ion conductor, comprising: a compound that includes a garnet-like crystal structure of the general chemical formula Li.sub.n[A.sub.(3-a′-a″)A′.sub.(a′)A″.sub.(a″)][B.sub.(2-b′-b″)B′.sub.(b′)B″.sub.(b″)][C′.sub.(c′)C″.sub.(c″)]O.sub.12, wherein: A stands for a dodecahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of La, Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb; A′ stands for a dodecahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Ca, Sr and Ba; A″ stands for a dodecahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Na and K;
0≦a′<2;
0≦a″<1; B stands for an octahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Zr, Hf and Sn; B′ stands for an octahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Ta, Nb, Sb and Bi; B″ stands for an octahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Te, W and Mo;
0≦b′≦2;
0≦b″≦2; C′ stands for a tetrahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Al and Ga; C″ stands for a tetrahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Si and Ge;
0≦c′≦0.5;
0≦c″≦0.4;
n=7+a′+2.Math.a″−b′−2.Math.b″−3.Math.c′−4.Math.c″;
5.5≦n≦6.875; when b′=2, 6.0<n<6.875 or 5.5≦n≦6.875, and c′+c″>0; and when B′ is Nb, 6.0<n<6.4 or 5.5≦n≦6.875 and at least one of c′+c″>0 and a′+a″>0.

2. The lithium ion conductor of claim 1, wherein 5.9≦n≦6.6.

3. The lithium ion conductor of claim 1, wherein 6.0<n<6.5.

4. The lithium ion conductor of claim 1, wherein B′ stands for Ta.

5. The lithium ion conductor of claim 1, wherein:
0<b′≦2, and B′ stands for Ta and at least one element selected from the group consisting of Nb, Sb and Bi.

6. The lithium ion conductor of claim 1, wherein b′+b″>0.

7. The lithium ion conductor of claim 1, wherein at least one of C′ stands for Al, C″ stands for Si, and c′+c″>0.

8. The lithium ion conductor of claim 1, wherein a′+a″>0.

9. The lithium ion conductor of claim 1, wherein: the garnet-like crystal structure is of the general chemical formula Li.sub.n[La.sub.(3-a′)A′.sub.(a′)][Zr.sub.(2-b′) B′.sub.(b′)][Al.sub.(c′)Si.sub.(c″)]O.sub.12; La and A′ stand for a dodecahedral position of the garnet-like crystal structure; A′ stands for at least one element selected from the group including Ca, Sr and Ba;
0<a′<2; Zr stands for an octahedral position of the garnet-like crystal structure; B′ stands for an octahedral position of the garnet-like crystal structure and stands for at least one element selected from the group including Ta, Nb, Sb and Bi;
0<b′≦2; Al and Si stand for a tetrahedral position of the garnet-like crystal structure;
0≦c′≦0.5;
0<c″≦0.4;
c′+c″>0;
n=7+a′+2.Math.a″−b′−2.Math.b″−3.Math.c′−4.Math.c″; and
5.5≦n≦6.875.

10. The lithium ion conductor of claim 9, wherein 5.9≦n≦6.6.

11. The lithium ion conductor of claim 9, wherein 6.0≦n≦6.5.

12. The lithium ion conductor of claim 9, wherein 0<c′≦0.5.

13. The lithium ion conductor of claim 1, wherein C′ and C″ stand for tetrahedral position 24d of the garnet-like crystal structure.

14. The lithium ion conductor of claim 13, wherein C′ stands for Al.

15. The lithium ion conductor of claim 13, wherein C″ stands for Si.

16. The lithium ion conductor of claim 1, wherein B, B′ and B″ stand for octahedral position 16a of the garnet-like crystal structure.

17. The lithium ion conductor of claim 16, wherein B stands for Zr.

18. The lithium ion conductor of claim 1, wherein A, A′ and A″ stand for the dodecahedral position 24c of the garnet-like crystal structure.

19. The lithium ion conductor of claim 1, wherein A′ stands for the dodecahedral position 24c of the garnet-like crystal structure.

20. The lithium ion conductor of claim 1, wherein the lithium ion conductor is part of a galvanic cell.

21. The lithium ion conductor of claim 20, wherein the galvanic cell is a lithium-sulfur cell, a lithium-oxygen cell, a lithium-ion cell, or a battery thereof.

22. The lithium ion conductor of claim 20, wherein the lithium ion conductor separates a cathode and an anode of the cell.

23. The lithium ion conductor of claim 1, wherein: the lithium ion conductor is synthesized by: (a) providing a powder mixture that includes: at least one lithium compound; at least one compound of an element selected from the group consisting of La, Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb; and at least one compound of an element selected from the group consisting of Zr, Hf, Sn, Ta, Nb, Sb, Bi, Te, W and Mo; (b) calcining the powder mixture at a temperature t1 in a temperature range of from 600° C.≦t1≦1000° C.; and (c) sintering the powder mixture or the molded body at a temperature t2 in a temperature range of 900° C.≦t2≦1250° C.; and the powder mixture contains the compounds of elements different from lithium in stoichiometric amounts which are selected in such a way that the lithium content of the compound is in a range of 5.5≦n≦6.875.

24. The lithium ion conductor of claim 23, wherein the synthesis further includes pressing the powder mixture to form a molded body.

25. The lithium ion conductor of claim 24, wherein the pressing is under a uniaxial and isostatic pressure.

26. The lithium ion conductor of claim 23, wherein the powder mixture further includes at least one compound of an element selected from the group consisting of Al, Ga, Si and Ge.

27. The lithium ion conductor of claim 23, wherein the powder mixture further includes at least one compound of an element selected from the group consisting of Ca, Sr, Ba, Na and K.

28. The lithium ion conductor of claim 23, wherein the calcining temperature t1 is in a temperature range of 850° C.≦t1≦950° C.

29. The lithium ion conductor of claim 23, wherein the lithium content of the compound is in a range of 5.9≦n≦6.6.

30. The lithium ion conductor of claim 23, wherein the lithium content of the compound is in a range of 6.0≦n≦6.5.

31. The lithium ion conductor of claim 23, wherein the sintering temperature t2 is in a temperature range of 1100° C.≦t2≦1200° C.

32. A lithium ion conductor, comprising: a compound that includes a garnet-like crystal structure of the general chemical formula Li.sub.n[A.sub.(3-a′-a″)A′.sub.(a′)A″.sub.(a″)][B.sub.(2-b′-b″)B′.sub.(b′)B″.sub.(b″)][C′.sub.(c′)C″.sub.(c″)]O.sub.12, wherein: A stands for a dodecahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of La, Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb; A′ stands for a dodecahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Ca, Sr and Ba; A″ stands for a dodecahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Na and K;
0≦a′<2;
0<a″<1; B stands for an octahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Zr, Hf and Sn; B′ stands for an octahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Ta, Nb, Sb and Bi; B″ stands for an octahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Te, W and Mo;
0≦b′≦2;
0≦b″≦2; C′ stands for a tetrahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Al and Ga; C″ stands for a tetrahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Si and Ge;
0≦c′≦0.5;
0<c″≦0.4;
n=7+a′+2.Math.a″−b′−2.Math.b″−3.Math.c′−4.Math.c″;
5.5≦n≦6.875; when b′=2, 6.0<n<6.875 or 5.5≦n≦6.875, and c′+c″>0; and when B′ is Nb, 6.0<n<6.4 or 5.5≦n≦6.875 and at least one of c′+c″>0 and a′+a″>0.

33. A lithium ion conductor, comprising: a compound that includes a garnet-like crystal structure of the general chemical formula Li.sub.n[A.sub.(3-a′-a″)A′.sub.(a′)A″.sub.(a″)][B.sub.(2-b′-b″)B′.sub.(b′)B″.sub.(b″)][C′.sub.(c′)C″.sub.(c″)]O.sub.12, wherein: A stands for a dodecahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of La, Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb; A′ stands for a dodecahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Ca, Sr and Ba; A″ stands for a dodecahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Na and K;
0≦a′<2;
0≦a″<1; B stands for an octahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Zr, Hf and Sn; B′ stands for an octahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Ta, Nb, Sb and Bi; B″ stands for an octahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Te, W and Mo;
0≦b′≦2;
0<b″≦2; C′ stands for a tetrahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Al and Ga; C″ stands for a tetrahedral position of the garnet-like crystal structure and stands for at least one element selected from the group consisting of Si and Ge;
0≦c′≦0.5;
0<c″≦0.4;
n=7+a′+2.Math.a″−b′−2.Math.b″−3.Math.c′−4.Math.c″;
5.5≦n≦6.875; when b′=2, 6.0<n<6.875 or 5.5≦n≦6.875, and c′+c″>0; and when B′ is Nb, 6.0<n<6.4 or 5.5≦n≦6.875 and at least one of c′+c″>0 and a′+a″>0.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a graph to illustrate calculated results with respect to the dependence of the number of mobile charge carriers on lithium content n.

(2) FIG. 2 shows a graph to illustrate experimental results with respect to the dependence of the ion conductivity on lithium content n.

(3) FIG. 3 shows an impedance spectrum of Li.sub.6.25La.sub.3Ta.sub.0.75Zr.sub.1.25O.sub.12.

(4) FIG. 4 shows an impedance spectrum of Al.sub.0.17Li.sub.6.49La.sub.3Zr.sub.2O.sub.12.

(5) FIG. 5 shows a REM-EDS/WDS spectrum of a randomly selected crystal grain in a sample of Al.sub.0.2Li.sub.6.4La.sub.3Zr.sub.2O.sub.12.

DETAILED DESCRIPTION

(6) FIG. 1 shows the results of calculations of the mobile charge carriers for compounds of varying composition having a garnet-like crystal structure starting from the original compound Li.sub.7La.sub.3Zr.sub.2O.sub.12 as a function of the lithium content per garnet formula unit. Occupation numbers were used to calculate the charge carriers. The activation energy for activation of mobile charge carriers was not taken into account in the calculations.

(7) The curve labeled with reference numeral 2 illustrates the results for occupation of the octahedral position, otherwise occupied by Zr.sup.4+, with a pentavalent element, for example, Ta.sup.5+, where the curve labeled with reference numeral 1 illustrates the results when the tetrahedral position otherwise occupied by Li.sup.+ is occupied by a trivalent element, for example, Al.sup.3+, while the zirconium content remains the same.

(8) FIG. 1 shows that, according to the carried out calculations, a high ion conductivity is to be expected for occupation of the octahedral position in a range from 5.9 to 6.5 and for occupation of the tetrahedral position in a range from 5.3 to 6.0, the calculated maximum ascertained for the mobile charge carriers is at approximately 6.3 lithium atoms per garnet formula unit for occupation of the octahedral position, and 5.8 lithium atoms per garnet formula unit for occupation of the tetrahedral position. FIG. 1 suggests that the maximum ion conductivity is to be expected in a range between 5.5 and 6.5 when the octahedral and tetrahedral positions are doped at the same time.

(9) Compounds having a garnet-like structure and containing Ta, Nb, Al, Si and Sr were prepared by a solid-state reaction at elevated temperatures, starting in particular from the original structure Li.sub.7La.sub.3Zr.sub.2O.sub.12. Stoichiometric amounts of the starting materials of a high purity were used for the synthesis. Lithium in the form of Li.sub.2CO.sub.3, in particular having a purity of >99.0%, for example, was used in a 10% excess in particular. The excess was used to compensate for the loss of lithium during the sintering process. Lanthanum was used in the form of La.sub.2O.sub.3 in particular and in a purity of >99.99%, for example, having been dried for 12 hours at 900° C. in particular. Zirconium and strontium, in particular in the form of ZrO.sub.2 and SrCO.sub.3 in a purity of >99%, for example, were used in particular. Aluminum was used in the form of γ-Al.sub.2O.sub.3, for example, in a purity of 99.60%, for example. Silicon, tantalum and niobium, in particular in the form of SiO.sub.2 and Ta.sub.2O.sub.5 and Nb.sub.2O.sub.5 in a purity of >99.85% in particular were used. The weighed powders were mixed with water for 1 hour at approximately 100° C. in a rotary evaporator. The powder mixtures were calcined at 900° C. The products were pressed under a uniaxial and isostatic pressure to form tablets which were sintered for 5 hours at 1150° C. The heating rate was 3 K/min in all cases.

(10) Table 1 shows the composition and ion conductivity of the compounds synthesized:

(11) TABLE-US-00001 TABLE 1 Composition and ion conductivity Ion conductivity Composition (S/cm) Li.sub.7La.sub.3Zr.sub.2O.sub.12 .sup. 1 .Math. 10.sup.−7 Li.sub.6.875La.sub.3Ta.sub.0.125Zr.sub.1.875O.sub.12 4.0 .Math. 10.sup.−5 Li.sub.6.75La.sub.3Ta.sub.0.25Zr.sub.1.75O.sub.12 1.3 .Math. 10.sup.−4 Li.sub.6.5La.sub.3Ta.sub.0.5Zr.sub.1.5O.sub.12 1.5 .Math. 10.sup.−4 Li.sub.6.25La.sub.3Ta.sub.0.75Zr.sub.1.25O.sub.12 2.7 .Math. 10.sup.−4 Li.sub.6La.sub.3TaZrO.sub.12 2.0 .Math. 10.sup.−4 Li.sub.5.5La.sub.3Ta.sub.1.5Zr.sub.0.5O.sub.12 4.0 .Math. 10.sup.−5 Li.sub.5La.sub.3Ta.sub.2O.sub.12 2.8 .Math. 10.sup.−5 Al.sub.0.1Li.sub.6.7La.sub.3Zr.sub.2O.sub.12 1.3 .Math. 10.sup.−5 Al.sub.0.17Li.sub.6.49La.sub.3Zr.sub.2O.sub.12 3.2 .Math. 10.sup.−4 Al.sub.0.23Li.sub.6.31La.sub.3Zr.sub.2O.sub.12 5.2 .Math. 10.sup.−4 Al.sub.0.29Li.sub.6.13La.sub.3Zr.sub.2O.sub.12 4.4 .Math. 10.sup.−4 Al.sub.0.35Li.sub.5.95La.sub.3Zr.sub.2O.sub.12 9.4 .Math. 10.sup.−5 Al.sub.0.3Li.sub.5.85Sr.sub.0.25La.sub.2.75Nb.sub.0.5Zr.sub.1.5O.sub.12 1.5 .Math. 10.sup.−4 Si.sub.0.05Li.sub.5.3La.sub.3Zr.sub.2O.sub.12 1.5 .Math. 10.sup.−6 Si.sub.0.1Li.sub.6.6La.sub.3Zr.sub.2O.sub.12 2.8 .Math. 10.sup.−6 Si.sub.0.2Li.sub.6.2La.sub.3Zr.sub.2O.sub.12 4.4 .Math. 10.sup.−5 Si.sub.0.3Li.sub.5.8La.sub.3Zr.sub.2O.sub.12 1.5 .Math. 10.sup.−5 Si.sub.0.4Li.sub.5.4La.sub.3Zr.sub.2O.sub.12 7.3 .Math. 10.sup.−6

(12) The ion conductivity of the sintered tablets was measured in air at room temperature with the aid of an impedance spectrometer (Solatron; 0.05 Hz-10 MHz) using lithium-blocking gold electrodes.

(13) FIG. 3 shows the impedance spectrum of Li.sub.6.25La.sub.3Ta.sub.0.75Zr.sub.1.25O.sub.12 and FIG. 4 shows the impedance spectrum of Al.sub.0.17Li.sub.6.49La.sub.3ZrO.sub.12 as examples. The rising branch in the low-frequency range of the spectra is an indication of the lithium-ion conductivity of the material.

(14) The ion conductivity values ascertained at room temperature are shown in Table 1 and FIG. 2. FIG. 2 shows a graph in which the ion conductivity of the compounds synthesized is plotted as a function of the lithium content per garnet formula unit. FIG. 2 illustrates the fact that the ion conductivity surprisingly has a maximum at a lithium content of approximately 6.3 for occupation of the octahedral position with Ta and Nb and optionally in the dodecahedral position with Sr as well as for occupation of the tetrahedral position with Al and Si. The values ascertained experimentally follow a curve which is very similar to the curve ascertained by computer. It is noteworthy that the maximums of the tetrahedrally doped compounds (1a: Al; 1b: Si) do not occur at a lithium content of 5.8, as predicted by the theoretical calculation (1), but instead also occur at 6.3, like those of the octahedrally doped compounds (2: Ta). This may be explained by the fact that in the theoretical calculation the influence of the activation energy for the movement of lithium ions was not taken into account, although in practice, this evidently results in a shift in the curve ascertained by computer for tetrahedrally doped compounds (1) in the direction of a higher lithium content n, in particular having a maximum in the range around 6.3.

(15) Table 1 and FIG. 2 show that, of the compounds synthesized, Al.sub.0.23Li.sub.6.31La.sub.3Zr.sub.2O.sub.12 at 5.2.Math.10.sup.−4 S/cm has the highest ion conductivity. FIG. 2 shows the ion conductivity of Al.sub.0.3Li.sub.5.85Sr.sub.0.25La.sub.2.75Nb.sub.0.5Zr.sub.1.5O.sub.12 labeled with reference numeral 3. Table 1 shows that Al.sub.0.3Li.sub.5.85Sr.sub.0.25La.sub.2.75Nb.sub.0.5Zr.sub.1.5O.sub.12 at 1.5.Math.10.sup.−4 S/cm has an ion conductivity similar to that of Li.sub.6.5La.sub.3Ta.sub.0.5Zr.sub.1.5O.sub.12 at 1.5.Math.10.sup.−4 S/cm. FIG. 2 illustrates that lithium content n of Al.sub.0.3Li.sub.5.85Sr.sub.0.25La.sub.2.75Nb.sub.0.5Zr.sub.1.5O.sub.12 (3) at n=5.85 is too low to achieve an optimal ion conductivity (n≈6.3). Lithium content n could be achieved here by reducing the aluminum content and/or increasing the strontium content, for example, among other things. It is therefore to be expected that a definitely higher lithium-ion conductivity than that of Li.sub.6.5La.sub.3Ta.sub.0.5Zr.sub.1.5O.sub.12 could be achieved by modifying Al.sub.0.3Li.sub.5.85Sr.sub.0.25La.sub.2.75Nb.sub.0.5Zr.sub.1.5O.sub.12 to form a similar compound having a lithium content n of approximately 6.3. The curve ascertained experimentally and corrected by computer, in particular with respect to the activation energy, indicates that an ion conductivity greater than 1.5.Math.10.sup.−4 S/cm and in particular even greater than 4.4.Math.10.sup.−4 S/cm could be achieved at an aluminum content of 0.15 or a strontium content of 0.7, for example, or an aluminum content of 0.15 to 0.3 and a strontium content of 0.25 to 0.7.

(16) On the whole, the experimental findings illustrated in FIG. 2 confirm that ion conductivity may be increased by forming voids, increasing lithium ion mobility, and by occupation of tetrahedral positions, which act as lithium traps, by other elements, and a maximum ion conductivity in the range of a lithium content n of 6.3 may surprisingly be achieved.

(17) It has been demonstrated by an Ab Initio computer simulation that a crystal structure in which Al.sup.3+ occupies the tetrahedral positions has the lowest total energy. This permits the conclusion to be drawn that Al.sup.3+ favors the tetrahedral positions of a garnet-like crystal structure.

(18) The result of the Ab Initio computer simulation is consistent with the result ascertained with the aid of x-ray diffractometry and Rietveld refinement for the occupation of aluminum in the garnet-like compound Al.sub.0.2Li.sub.6.4La.sub.3Zr.sub.2O.sub.12. For better resolution, pure κ-α1 radiation was used in the measurement. The TOPAS program was used for adaptation with the aid of the Rietveld method. The results of the Rietveld refinement were of a good quality. Table 2 summarizes the results of the Rietveld refinement:

(19) TABLE-US-00002 TABLE 2 Results of the Rietveld refinement with respect to the occupation of the tetrahedral and octahedral positions in Al.sub.0.2Li.sub.6La.sub.3Zr.sub.2O.sub.12 Position Relative Al occupation Tetrahedron 24d 0.2 Octahedron 48g 0 Octahedron 96h <0.03

(20) Table 2 shows that tetrahedral position 24d in Al.sub.0.2Li.sub.6.4La.sub.3Zr.sub.2O.sub.12 is occupied by an x-ray-sensitive element, in particular aluminum, but octahedral positions 48g and 96h are not occupied by an x-ray-sensitive element. The positions of the lithium atoms could not be determined from the x-ray diffraction measurements.

(21) FIG. 5 shows the result of one of many REM-EDS/WDS analyses of different randomly selected crystal grains in a cross-section polish through a tablet of Al.sub.0.2Li.sub.6.4La.sub.3Zr.sub.2O.sub.12. FIG. 5 shows that aluminum is present in the expected amounts in both crystals. This permits the conclusion to be drawn that the aluminum atoms are incorporated into the garnet-like crystal structure and are not present as a secondary phase in the material. This also supports the result of the Rietveld refinement, namely that aluminum occupies tetrahedral positions 24d.