Phase-pure lithium aluminium titanium phosphate and method for its production and its use

Abstract

The present invention relates to a method for producing lithium aluminum titanium phosphates of the general formula Li.sub.1+xTi.sub.2−xAl.sub.x(PO.sub.4).sub.3, wherein x is ≦0.4, a method for their production as well as their use as solid-state electrolytes in lithium ion accumulators.

Claims

1. A method for producing a phase-pure lithium aluminum titanium phosphate of the formula Li.sub.1+xTi.sub.2−xAl.sub.x(PO.sub.4).sub.3 comprising the steps of: a) providing an aqueous liquid phosphoric acid, b) adding a mixture of a lithium compound, titanium dioxide and an oxygen-containing aluminum compound to the aqueous phosphoric acid of step a), c) heating the mixture of step b) in order to obtain a solid intermediate product, and d) calcining the solid intermediate product of step c), wherein x is ≦0.4 and >0, and the concentration of magnetic metals and magnetic metal compounds of the elements Fe, Cr and Ni in the phase-pure lithium aluminum titanium phosphate is ≦1 ppm.

2. The method according to claim 1, wherein the aqueous liquid phosphoric acid is aqueous liquid orthophosphoric acid.

3. The method according to claim 1, wherein lithium carbonate is used as lithium compound.

4. The method according to claim 1, wherein Al(OH).sub.3 is used as oxygen-containing aluminum compound.

5. The method according to claim 1, wherein the step of heating is carried out at a temperature of from 200 to 300° C.

6. The method according to claim 5, wherein the calcining takes place at a temperature in the range from 850° C. to 1000° C.

7. The method according to claim 6, wherein the calcining is carried out over a period of from 5 to 24 hours.

8. The method according to claim 1, wherein a stoichiometric excess of lithium compound is used in step b).

9. A method for producing a phase-pure lithium aluminum titanium phosphate of the formula Li.sub.1+xTi.sub.2−xAl.sub.x(PO.sub.4).sub.3 comprising the steps of: a) providing 85% orthophosphoric acid, b) adding a mixture of a lithium compound, titanium dioxide and an oxygen-containing aluminum compound to the 85% orthophosphoric acid of step a), c) heating the mixture of step b) in order to obtain a solid intermediate product, and d) calcining the solid intermediate product of step c), wherein x is ≦0.4 and >0, and the concentration of magnetic metals and magnetic metal compounds of the elements Fe, Cr and Ni in the phase-pure lithium aluminum titanium phosphate is ≦1 ppm.

10. The method according to claim 9, wherein lithium carbonate is used as lithium compound.

11. The method according to claim 9, wherein Al(OH).sub.3 is used as oxygen-containing aluminum compound.

12. The method according to claim 9, wherein the step of heating is carried out at a temperature of from 200 to 300° C.

13. The method according to claim 12, wherein the calcining takes place at a temperature in the range from 850° C. to 1000° C.

14. The method according to claim 13, wherein the calcining is carried out over a period of from 5 to 24 hours.

15. The method according to claim 9, wherein a stoichiometric excess of lithium compound is used in step b).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in more detail below with the help of drawings and examples which are not to be understood as limiting the scope of the present invention. There are shown in:

(2) FIG. 1 the structure of the phase-pure lithium aluminium titanium phosphate according to the invention,

(3) FIG. 2 an X-ray powder diffractogram (XRD) of a lithium aluminium titanium phosphate according to the invention,

(4) FIG. 3 an X-ray powder diffractogram (XRD) of a conventionally produced lithium aluminium titanium phosphate,

(5) FIG. 4 the particle-size distribution of the lithium aluminium titanium phosphate according to the invention.

1. MEASUREMENT METHODS

(6) The BET surface area was determined according to DIN 66131 (DIN-ISO 9277),

(7) The particle-size distribution was determined according to DIN 66133 by means of laser granulometry with a Malvern Mastersizer 2000.

(8) The X-ray powder diffractogram (XRD) was measured with an X'Pert PRO diffractometer, PANalytical: Goniometer Theta/Theta, Cu anode PW 3376 (max. output 2.2 kW), detector X'Celerator, X'Pert Software.

(9) The level of magnetic constituents in the lithium aluminium titanium phosphate according to the invention is determined by separation by means of magnets followed by decomposition by acid and subsequent ICP analysis of the formed solution.

(10) The lithium aluminium titanium phosphate powder to be examined is suspended in ethanol with a magnet of a specific size (diameter 1.7 cm, length 5.5 cm<6000 Gauss). The ethanolic suspension is exposed to the magnet in an ultrasound bath with a frequency of 135 kHz for 30 minutes. The magnet attracts the magnetic particles from the suspension or the powder. The magnet with the magnetic particles is then removed from the suspension. The magnetic impurities are dissolved with the help of decomposition by acid and this is examined by means of ICP (ion chromatography) analysis, in order to determine the precise quantity as well as the composition of the magnetic impurities. The apparatus for ICP analysis was an ICP-EDS, Varian Vista Pro 720-ES.

Example 1

(11) Production of Li.sub.1.3Al.sub.0.3Ti.sub.1.7(PO.sub.4).sub.3

(12) 1037.7 g orthophosphoric acid (85%) was introduced into a reaction vessel, A mixture of 144.3 g Li.sub.2CO.sub.3, 431.5 g TiO.sub.2 (in anatase form) and 46.8 g Al(OH.sub.3) (Gibbsite) was added slowly via a fluid channel accompanied by vigorous stirring with a Teflon-coated anchor stirrer. As the Li.sub.2CO.sub.3 with the phosphoric acid reacted off accompanied by strong foaming of the suspension because of the formation of CO.sub.2, the admixture was added very slowly over a period of from 1 to 1.5 hours. Towards the end of the addition, the white suspension became more viscous but remained capable of forming drops.

(13) The mixture was then heated to 225° C. in an oven and left at this temperature for two hours. A hard, friable crude product, only partly removable from the reaction vessel with difficulty, forms. The complete solidification of the suspension from liquid state via a rubbery consistency took place relatively quickly. However, e.g. a sand or oil bath can also be used instead of an oven.

(14) The crude product was then finely ground over a period of 6 hours in order to obtain a particle size of <50 μm.

(15) The finely ground premixture was heated from 200 to 900° C. within six hours at a heat-up rate of 2° C. per minute, as otherwise crystalline foreign phases were detectable in the X-ray powder diffractogram (XRD). The product was then sintered at 900° C. for 24 hours and then finely ground in a ball mill with porcelain spheres. The total quantity of magnetic Fe, Cr and Ni or their magnetic compounds was 0.75 ppm. The total quantity of Fe and its magnetic compounds was 0.25 ppm.

Example 2

(16) Li.sub.1.3Al.sub.0.3Ti.sub.1.7(PO.sub.4).sub.3 was synthesized as in Example 1, but after the end of the addition of the mixture of lithium carbonate, TiO.sub.2 and Al(OH).sub.3, the white suspension was transferred into a vessel with anti-adhesion coating, for example into a vessel with Teflon walls. The removal of the cured intermediate product was thereby greatly simplified compared with Example 1. The analysis data corresponded to those of Example 1.

Example 3

(17) Li.sub.1.3Al.sub.0.3Ti.sub.1.7(PO.sub.4).sub.3 was synthesized as in Example 2, except that the ground intermediate product was also pressed into pellets before the sintering. The analysis data corresponded to those of Example 1.

Example 4

(18) Li.sub.1.3Al.sub.0.3Ti.sub.1.7(PO.sub.4).sub.3 was synthesized as in Example 2 or 3, except that both with the pellets and with the finely ground intermediate product, a first calcining was carried out over 12 hours after cooling to room temperature followed by a second calcining over a further 12 hours at 900° C. In the case of the latter, no signs of foreign phases were found in the product. The total quantity of magnetic Fe, Cr and Ni or their magnetic compounds was 0.76 ppm. The quantity of Fe and its magnetic compound was 0.24 ppm. A comparison example produced according to JP A 1990 2-225310 showed, on the other hand, a quantity Σ of Fe, Cr, Ni of 2.79 ppm and of magnetic iron or iron compounds of 1.52 ppm.

(19) The structure of the product Li.sub.1.3Al.sub.0.3Ti.sub.1.7(PO.sub.4).sub.3 obtained according to the invention is shown in FIG. 1 and is similar to a so-called NASiCON (Na.sup.+ superionic conductor) structure (see Nuspl et al, J. Appl. Phys. Vol. 06, No, 10, p. 5484 et seq. (1999)).

(20) The three-dimensional Li.sup.+ channels of the crystal structure and a simultaneously very low activation energy of 0.30 eV for the Li migration in these channels bring about a high intrinsic Li ion conductivity. The Al doping scarcely influences this intrinsic Li.sup.+ conductivity, but reduces the Li ion conductivity at the particle boundaries.

(21) In addition to Li.sub.3xLa.sub.2/3−xTiO.sub.3 compounds, Li.sub.1.3Al.sub.0.3Ti.sub.1.7(PO.sub.4).sub.3 is the solid-state electrolyte with the highest Li.sup.+ ion conductivity known in literature.

(22) As can be seen from the X-ray powder diffractogram (XRD) of the product from Example 4 in FIG. 2, particularly phase-pure products result from the reaction process according to the invention.

(23) FIG. 3 shows, in comparison to this, an X-ray powder diffractogram of a lithium aluminium titanium phosphate of the state of the art produced according to JP A 1990 2-225310 with foreign phases such as TiP.sub.2O.sub.7 and AlPO.sub.4. The same foreign phases are also found in the material described by Kosova et al. (see above).

(24) The particle-size distribution of the product from Example 4 is shown in FIG. 4 which has a purely monomodal particle-size distribution with values for d.sub.90 of <6 μm, d.sub.50 of <2.1 μm and d.sub.10<1 μm.