US20120295167A1

US20120295167A1 - Phase-pure lithium aluminium titanium phosphate and method for its production and its use

Info

Publication number: US20120295167A1
Application number: US13/502,285
Authority: US
Inventors: Michael Holzapfel; Max Eisgruber; Gerhard Nuspl
Original assignee: Sued Chemie AG
Current assignee: Sued Chemie IP GmbH and Co KG
Priority date: 2009-10-16
Filing date: 2010-10-14
Publication date: 2012-11-22
Also published as: CA2777784A1; EP2488451A1; KR20120093958A; CN102648154A; JP2013507318A; JP5749727B2; EP2488451B1; US9748557B2; KR101426350B1; CA2777784C; TW201125813A; CN102648154B; JP2015145335A; DE102009049693A1; TWI461355B; WO2011045067A1; JP6063503B2; US20150236339A1

Abstract

The present invention relates to a method for producing lithium aluminum titanium phosphates of the general formula Li_1+xTi_2−xAl_x(PO₄)_3,wherein x is ≦0.4, a method for their production as well as their use as solid-state electrolytes in lithium ion accumulators.

Description

The present invention relates to phase-pure lithium aluminum titanium phosphate, a method for its production, its use, as well as a secondary lithium ion battery containing the phase-pure lithium aluminum titanium phosphate.
Recently, battery-powered motor vehicles have increasingly become the focal point of research and development because of the increasing lack of fossil raw materials in the near future.
In particular lithium ion accumulators (also called secondary lithium ion batteries) proved to be the most promising battery models for such applications.
These so-called “lithium ion batteries” are also widely used in fields such as power tools, computers, mobile telephones etc. In particular the cathodes and electrolytes, but also the anodes, consist of lithium-containing materials.
LiMn₂O₄and LiCoO₂for example have been used for some time as cathode materials. Recently, in particular since the work of Goodenough et al. (U.S. Pat. No. 5,910,382), also doped or non-doped mixed lithium transition metal phosphates, in particular LiFePO₄.
Normally, for example graphite or also, as already mentioned above, lithium compounds such as lithium titanates are used as anode materials in particular for large-capacity batteries.
By lithium titanates are meant here the doped or non-doped lithium titanium spinels of the Li_1+xTi_2−xO₄type with 0≦x≦⅓ of the space group Fd3m and all mixed titanium oxides of the generic formula Li_xTi_yO(0≦x, y≦1).
Normally, lithium salts or their solutions are used for the electrolyte in such lithium ion accumulators.
Ceramic separators such as Separion® commercially available in the meantime for example from Evonik Degussa (DE 196 53 484 A1) have also been proposed. However, Separion contains, not a solid-state electrolyte, but ceramic fillers such as nanoscale Al₂O₃and SiO₂.
Lithium titanium phosphates have for some time been mentioned as solid electrolytes (JP A 1990 2-225310). Lithium titanium phosphates have, depending on the structure and doping, an increased lithium ion conductivity and a low electrical conductivity, which, also in addition to their hardness, makes them very suitable as solid electrolytes in secondary lithium ion batteries.
Aono et al. have described the ionic (lithium) conductivity of doped and non-doped lithium titanium phosphates (J. Electrochem. Soc., Vol. 137, No. 4, 1990, pp. 1023-1027, J. Electrochem. Soc., Vol. 136, No. 2, 1989, pp. 590-591).
Systems doped with aluminum, scandium, yttrium and lanthanum in particular were examined. It was found that in particular doping with aluminum delivers good results because, depending on the degree of doping, aluminum brings about the highest lithium ion conductivity compared with other doping metals and, because of its cation radius (smaller than Ti⁴⁺) in the crystal, it can well take the spaces occupied by the titanium.
Kosova et al. in Chemistry for Sustainable Development 13 (2005) 253-260 propose suitable doped lithium titanium phosphates as cathodes, anodes and electrolytes for rechargeable lithium ion batteries.
Li_1.3Al_0.3Ti_1.7(PO₄) was proposed in EP 1 570 113 B1 as ceramic filler in an “active” separator film which has additional lithium ion conductivity for electrochemical components.
Likewise, further doped lithium titanium phosphates, in particular doped with iron, aluminum and rare earths, were described in U.S. Pat. No. 4,985,317.
However, very expensive production by means of solid-state synthesis starting from solid phosphates, in which the obtained lithium titanium phosphate is normally contaminated by further foreign phases such as for example AlPO₄or TiP₂O₇, is common to all of the above-named lithium titanium phosphates. Phase-pure lithium titanium phosphate or phase-pure doped lithium titanium phosphate has been unknown thus far.
The object of the present invention was therefore to provide phase-pure lithium aluminum titanium phosphate, because this combines the characteristics of a high lithium ion conductivity with a low electrical conductivity. In particular, phase-pure lithium aluminum titanium phosphate should have an even better ionic conductivity compared with lithium aluminum titanium phosphate of the state of the art because of the absence of foreign phases.
This object is achieved by the provision of phase-pure lithium aluminum titanium phosphate of the formula Li_1+x,Ti_2−xAl_x(PO₄)₃, wherein x is ≦0.4 and the level of magnetic metals and metal compounds of the elements Fe, Cr and Ni therein is 1 ppm.
Here, by the term “phase-pure” is meant that reflexes of foreign phases cannot be recognized in the X-ray powder diffractogram (XRD). The absence of foreign-phase reflexes in the lithium aluminum titanium phosphate according to the invention, as is shown by way of example in FIG. 2 below, corresponds to a maximum proportion of foreign phases, such as e.g. AlPO₄and TiP₂O₇, of 1%.
Foreign phases reduce the intrinsic ion conductivity, with the result that, compared with those of the state of the art, all of which contain foreign phases, the phase-pure lithium aluminum titanium phosphates according to the invention have a higher intrinsic conductivity than the lithium aluminum titanium phosphates of the state of the art.
Surprisingly, it was also found that the total level of magnetic metals and metal compounds of Fe, Cr and Ni (ΣFe+Cr+Ni) in the lithium aluminum titanium phosphate according to the invention is ≦1 ppm. In the case of lithium aluminum phosphates of the state of the art (obtained according to JP A 1990-2-225310) this value is normally between 2 and 3 ppm. When account is also taken of any disruptive zinc, the total content Σ Fe+Cr+Ni+Zn=1.1 ppm in the lithium aluminum titanium phosphate according to the invention, compared with 2.3-3.3 ppm of a lithium aluminum titanium phosphate according to the above-named state of the art.
In particular, the lithium aluminum titanium phosphate according to the invention displays only an extremely small contamination by metallic or magnetic iron and magnetic iron compounds (such as e.g. Fe₃O₄) of <0.5 ppm. The determination of the concentrations of magnetic metals or metal compounds is described in detail below in the experimental section. Customary values for magnetic iron or magnetic iron compounds in the lithium aluminum titanium phosphates previously known from the state of the art are approx. 1-1000 ppm. The result of contamination by metallic iron or magnetic iron compounds is that in addition to the formation of dendrites associated with a drop in current the danger of short circuits within an electrochemical cell in which lithium aluminum titanium phosphate is used as solid electrolyte increases significantly and thus represents a risk for the production of such cells on an industrial scale. This disadvantage can be avoided with the phase-pure lithium aluminum titanium phosphate here.
Surprisingly, the phase-pure lithium aluminum titanium phosphate according to the invention also has a relatively high BET surface area of <4.5 m²/g. Typical values are for example 2.0 to 3.5 m²/g. Lithium aluminum titanium phosphates known from the literature on the other hand have BET surface areas of less than 1.5 m²/g.
The lithium aluminum titanium phosphate according to the invention preferably has a particle-size distribution of d₉₀<6 μm, d₅₀<2.1 μm and d₁₀<1 μm, which results in the majority of the particles being particularly small and thus a particularly high ion conductivity being achieved. This confirms similar findings from the above-mentioned Japanese unexamined patent application, where it was also attempted to obtain smaller particle sizes by means of various grinding processes. Because of the extreme hardness of the lithium aluminum titanium phosphate (Mohs' hardness>7, i.e. close to diamond), this is difficult to obtain with customary grinding processes, however.
In further preferred embodiments of the present invention, the lithium aluminum titanium phosphate has the following empirical formulae: Li_1.2Ti_1.8Al_0.2(PO₄)_3,which has a very good total ion conductivity of approx. 5×10⁻⁴S/cm at 293 K and—in the particularly phase-pure form—Li_1.3Ti_1.7Al_0.3(PO₄)_3,which has a particularly high total ion conductivity of 7×10⁻⁴S/cm at 293 K.
The object of the present invention was furthermore to provide a method for producing the phase-pure lithium aluminum titanium phosphate according to the invention. This object is achieved by a method which comprises the following steps:
a) providing a concentrated phosphoric acid,
b) adding a mixture of a lithium compound, titanium dioxide and an oxygen-containing aluminum compound,
c) heating the mixture in order to obtain a solid intermediate product,
d) calcining the solid intermediate product.
Surprisingly it was found that, unlike all previously known syntheses of the state of the art, a liquid phosphoric acid can also be used instead of solid phosphoric acid salts. The method according to the invention thus proceeds as a defined precipitation of an aqueous precursor suspension. The use of a phosphoric acid makes possible a simpler execution of the method and thus also the option of removing impurities already in solution or suspension and thus also obtaining products with greater phase purity.
A concentrated phosphoric acid, i.e. for example 85% orthophosphoric acid, is preferably used as phosphoric acid, although in less preferred further embodiments of the present invention other concentrated phosphoric acids can also be used, such as for example metaphosphoric acid etc. All condensation products of orthophosphoric acid can also be used according to the invention such as: catenary polyphosphoric acids (diphosphoric acid, triphosphoric acid, oligophosphoric acids, etc.) annular metaphosphoric acids (tri-, tetrametaphosphoric acid) up to the anhydride of phosphoric acid P₂O₅(in water).
According to the invention any suitable lithium compound can be used as lithium compound, such as Li₂CO₃, LiOH, Li₂O, LiNO₃, wherein lithium carbonate is particularly preferred because it is the most cost-favourable source of raw material.
Practically any oxide or hydroxide or mixed oxide/hydroxide of aluminum can be used as oxygen-containing aluminum compound. Aluminum oxide Al₂O₃is preferably used in the state of the art because of its ready availability. In the present case it was found, however, that the best results are achieved with Al(OH)₃. Al(OH)₃is even more cost-favourable compared with Al₂O₃and also more reactive than Al₂O₃, in particular in the calcining step. Of course, Al₂O₃can also be used in the method according to the invention, albeit less preferably; however, the calcining in particular then lasts longer compared with using Al(OH)₃.
The step of heating the mixture is carried out at a temperature of from 200 to 300° C., preferably 200 to 260° C. and quite particularly preferably of from 200 to 240° C. A gentle reaction which moreover can still be controlled is thereby guaranteed.
The calcining takes place preferably at temperatures of from 830-1000° C., quite particularly preferably at 880-900° C., as below 830° C. the danger of the occurrence of foreign phases is particularly great.
Typically, the vapour pressure of the lithium in the compound Li_1+xTi_2−xAl_x(PO₄)₃increases at temperatures >950° C., i.e. at temperatures >950° C. the formed compounds Li_1+xTi_2−xAl_x(PO₄)₃lose more and more lithium which settles as Li₂O and Li₂CO₃on the oven walls in an air atmosphere. This can be compensated for e.g. by the lithium excess described below, but the precise setting of the stoichiometry becomes more difficult. Therefore, lower temperatures are preferred and surprisingly also possible by the previous execution of the method compared with the state of the art. This result can be attributed to the use of aqueous concentrated phosphoric acid compared with solid phosphates of the state of the art.
Moreover, temperatures >1000° C. make greater demands of the oven and crucible material.
The calcining is carried out over a period of from 5 to 10 hours. In further even more preferred embodiments of the present invention, a second calcining step is carried out at the same temperature and preferably for the same period, whereby a particularly phase-pure product is obtained.
In other preferred developments of the present invention, a stoichiometric excess of the lithium compound is used in step b). Lithium compounds are, as already said above, often volatile at the reaction temperatures used, with the result that, depending on the lithium compound, work must here often be carried out with an excess. Here, preferably a stoichiometric excess of approx. 8% is then used which represents a reduction in quantity of expensive lithium compound of approx. 50% compared with the solid-state methods of the state of the art. Moreover, because the method is carried out via an aqueous precipitation process, monitoring of the stoichiometry is made particularly easy compared with a solid-state method.
The subject of the present invention is also a phase-pure lithium aluminum titanium phosphate of the formula Li_1+x,Ti_2−xAl_x(PO₄)₃wherein x is 0.4, which can be obtained by the method according to the invention and can be obtained particularly phase-pure within the meaning of the above definition by the execution of the method, and contains small quantities of ≦1 ppm of magnetic impurities, as already described above. Also, all previously known products obtainable by solid-state synthesis methods—as already said above—had further foreign phases in addition to increased quantities of disruptive magnetic compounds, something which can be avoided here by executing the method according to the invention in particular by using an (aqueous) concentrated phosphoric acid instead of solid phosphates.
The subject of the invention is also the use of the phase-pure lithium aluminum titanium phosphate according to the invention as solid electrolyte in a secondary lithium ion battery.
The object of the invention is further achieved by providing an improved secondary lithium ion battery which contains the phase-pure lithium aluminum titanium phosphate according to the invention, in particular as solid electrolyte. Because of its high lithium ion conductivity, the solid electrolyte is particularly suitable and particularly stable and also resistant to short circuits because of its phase purity and low iron content.
In preferred developments of the present invention, the cathode of the secondary lithium ion battery according to the invention contains a doped or non-doped lithium transition metal phosphate as cathode, wherein the transition metal of the lithium transition metal phosphate is selected from Fe, Co, Ni, Mn, Cr and Cu. Doped or non-doped lithium iron phosphate LiFePO₄is particularly preferred.
In yet further preferred developments of the present invention, the cathode material additionally contains a doped or non-doped mixed lithium transition metal oxo compound different from the lithium transition metal phosphate used. Lithium transition metal oxo compounds suitable according to the invention are e.g. LiMn₂O₄, LiCoO₂, NCA (LiNi_1−x−yCo_xAl_yO₂, e.g. LiNi_0.8Co_0.15Al_0.05O₂) or NCM (LiNi_1/3Co_1/3Mn_1/3O₂) . The proportion of lithium transition metal phosphate in such a combination lies in the range of from 1 to 60 wt. %. Preferred proportions are e.g. 6-25 wt. %, preferably 8-12 wt. % in an LiCoO₂/LiFePO₄mixture and 25-60 wt. % in an LiNiO₂/LiFePO₄mixture.
In yet further preferred developments of the present invention, the anode material of the secondary lithium ion battery according to the invention contains a doped or non-doped lithium titanate. In less preferred developments the anode material contains exclusively carbon, for example graphite etc. The lithium titanate in the preferred development mentioned above is typically doped or non-doped Li₄Ti₅O₁₂, with the result that for example a potential of 2 volts vis-a-vis the preferred cathode of doped or non-doped lithium transition metal phosphate can be achieved.
As already stated above, both the lithium transition metal phosphates of the cathode material as well as the lithium titanates of the anode material of the preferred development are either doped or non-doped. Doping takes place with at least one further metal or also with several, which leads in particular to an increased stability and cycle stability of the doped materials when used as cathode or anode. Metal ions such as Al, B, Mg, Ga, Fe, Co, Sc, Y, Mn, Ni, Cr, V, Sb, Bi, Nb or several of these ions, which can be incorporated in the lattice structure of the cathode or anode material, are preferred as doping material. Mg, Nb and Al are quite particularly preferred. The lithium titanates are normally preferably rutile-free and thus equally phase-pure.
The doping metal cations are present in the above-named lithium transition metal phosphates or lithium titanates in a quantity of from 0.05 to 3 wt. %, preferably 1 to 3 wt. % relative to the total mixed lithium transition metal phosphate or lithium titanate. Relative to the transition metal (values in at %) or in the case of lithium titanates, relative to lithium and/or titanium, the quantity of doping metal cation(s) is 20 at %, preferably 5-10 at %.
The doping metal cations occupy either the lattice positions of the metal or of the lithium. Exceptions to this are mixed Fe, Co, Mn, Ni, Cr, Cu, lithium transition metal phosphates which contain at least two of the above-named elements, in which larger quantities of doping metal cations may also be present, in the extreme case up to 50 wt. %.
Typical further constituents of an electrode of the secondary lithium ion battery according to the invention are, in addition to the active material, i.e. the lithium transition metal phosphate or the lithium titanate, carbon blacks as well as a binder.
Binders known per se to a person skilled in the art may be used here as binder, such as for example polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF), polyvinylidene difluoride hexafluoropropylene copolymers (PVDF-HFP), ethylene-propylene-diene terpolymers (EPDM), tetrafluoroethylene hexafluoropropylene copolymers, polyethylene oxides (PEO), polyacrylonitriles (PAN), polyacryl methacrylates (PMMA), carboxymethylcelluloses (CMC), and derivatives and mixtures thereof.
Within the framework of the present invention, typical proportions of the individual constituents of the electrode material are preferably 80 to 98 parts by weight active material electrode material, 10 to 1 parts by weight conductive carbon and 10 to 1 parts by weight binder.
Within the framework of the present invention, preferred cathode/solid electrolyte/anode combinations are for example LiFePO₄/Li_1.3Ti_1.7Al_0.3(PO₄)₃/Li_xTi_yO with a single-cell voltage of approx. 2 volts which is well suited as substitute for lead-acid cells or LiCo_zMn_yFe_xPO₄/Li_1.3Ti_1.7Al_0.3(PO₄)₃/Li_xTi_yO, wherein x, y and z are as defined further above, with increased cell voltage and improved energy density.

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:

FIG. 1 the structure of the phase-pure lithium aluminum titanium phosphate according to the invention,

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

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

FIG. 4 the particle-size distribution of the lithium aluminum titanium phosphate according to the invention.

1. Measurement Methods
The BET surface area was determined according to DIN 66131 (DIN-ISO 9277).
The particle-size distribution was determined according to DIN 66133 by means of laser granulometry with a Malvern Mastersizer 2000.
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.
The level of magnetic constituents in the lithium aluminum 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.
The lithium aluminum 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-EOS, Varian Vista Pro 720-ES.

Example 1

Production of Li_1.3Al_0.3Ti_1.7(PO₄)₃
1037.7 g orthophosphoric acid (85%) was introduced into a reaction vessel. A mixture of 144.3 g Li₂CO₃, 431.5 g TiO₂(in anatase form) and 46.8 g Al(OH₃) (Gibbsite) was added slowly via a fluid channel accompanied by vigorous stirring with a Teflon-coated anchor stirrer. As the Li₂CO₃with the phosphoric acid reacted off accompanied by strong foaming of the suspension because of the formation of CO₂, 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.
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.
The crude product was then finely ground over a period of 6 hours in order to obtain a particle size of <50 μm.
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

Li_1.3Al_0.3Ti_1.7(PO₄)₃was synthesized as in Example 1, but after the end of the addition of the mixture of lithium carbonate, TiO₂and Al(OH)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

Li_1.3Al_0.3Ti_1.7(PO₄)₃was 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

Li_1.3Al_0.3Ti_1.7(PO₄)₃was 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.
The structure of the product Li_1.3Al_0.3Ti_1.7(PO₄)₃obtained according to the invention is shown in FIG. 1 and is similar to a so-called NASiCON (Na⁺ superionic conductor) structure (see Nuspl et al. J. Appl. Phys. Vol. 06, No. 10, p. 5484 et seq. (1999)).
The three-dimensional Li⁺ 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⁺ conductivity, but reduces the Li ion conductivity at the particle boundaries.
In addition to Li_3xLa_2/3−xTiO₃compounds, Li_1.3Al_0.3Ti_1.7(PO₄)₃is the solid-state electrolyte with the highest Li⁺ ion conductivity known in literature.
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.
FIG. 3 shows, in comparison to this, an X-ray powder diffractogram of a lithium aluminum titanium phosphate of the state of the art produced according to JP A 1990 2-225310 with foreign phases such as TiP₂O₇and AlPO₄. The same foreign phases are also found in the material described by Kosova et al. (see above).
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 d₉₀of <6 μm, d₅₀of <2.1 μm and d₁₀<1 μm.

Claims

1. Phase-pure lithium aluminum titanium phosphate of the formula Li_1+xTi_2−xAl_x(PO₄)_3,wherein x is ≦0.4 and the level of magnetic metals and magnetic metal compounds of the elements Fe, Cr and Ni therein is ≦1 ppm.

2. Lithium aluminum titanium phosphate according to claim 1, the particle-size distribution d₉₀of which is <6 μm.

3. Lithium aluminum titanium phosphate according to claim 1 or 2, the metal iron and magnetic iron compounds content of which is <0.5 ppm.

4. Lithium aluminum titanium phosphate according to claim 3, wherein the value for x is 0.2 or 0.3.

5. Method for producing Li_1+xTi_2−xAl_x(PO₄)₃, wherein x is ≦0.4, according to one of the previous claims, comprising the steps of

a) providing a concentrated phosphoric acid

b) adding a mixture of a lithium compound, titanium dioxide and an oxygen-containing aluminum compound,

c) heating the mixture in order to obtain a solid intermediate product,

d) calcining the solid intermediate product.

6. Method according to claim 5, wherein liquid concentrated phosphoric acid or aqueous concentrated phosphoric acid is used as phosphoric acid; and/or wherein concentrated orthophosphoric acid or 85% orthophosphoric acid is used as phosphoric acid.

7. Method according to claim 5 or 6, wherein lithium carbonate is used as lithium compound.

8. Method according to one of claims 5 to 7, wherein Al(OH)₃is used as oxygen-containing aluminum compound.

9. Method according to one of claims 5 to 8, wherein the step of heating is carried out at a temperature of from 200 to 300° C.

10. Method according to claim 9, wherein the calcining takes place at 850 to 1000° C.

11. Method according to claim 10, wherein the calcining is carried out over a period of from 5 to 24 hours.

12. Method according to one of the previous claims 5 to 11, wherein a stoichiometric excess of lithium compound is used in step b).

13. Phase-pure lithium aluminum titanium phosphate of the formula Li_1+xTi_2−xAl_x(PO₄)₃, wherein x is ≦0.4, obtainable by the method according to one of the previous claims 5 to 12.

14. Use of phase-pure lithium aluminum titanium phosphate according to claim 1 to 4 or 13 as solid electrolyte in a secondary lithium ion battery.

15. Secondary lithium ion battery containing phase-pure lithium aluminum titanium phosphate according to one of claim 1 to 4 or 13.

16. Secondary lithium ion battery according to claim 15, further containing, as cathode material, a doped or non-doped lithium transition metal phosphate.

17. Secondary lithium ion battery according to claim 16, wherein the transition metal of the lithium transition metal phosphate is selected from Fe, Co, Ni, Mn, Cu, Cr.

18. Secondary lithium ion battery according to claim 17, wherein the transition metal is Fe.

19. Secondary lithium ion battery according to claim 18, wherein the cathode material contains a further doped or non-doped lithium transition metal oxo compound.

20. Secondary lithium ion battery according to one of claims 15 to 19, wherein the anode material contains a doped or non-doped lithium titanate.