Amorphous lithium lanthanum titanate thin films manufacturing method

Abstract

An amorphous lithium lanthanum titanate (LLTO) thin film is produced by the sol-gel method wherein a polymer is mixed with a liquid alcohol to form a first solution. A second solution is then prepared by mixing a lanthanum alkoxide with an alcohol. The first solution is then mixed with the lanthanum based second solution. A lithium alkoxide and a titanium alkoxide are then also added to the lanthanum based second solution. This process produces a batch of LLTO precursor solution. The LLTO precursor solution is applied to a substrate to form a precursor layer which is then dried. The coating techniques that may be used include spin coating, spraying, casting, dripping, and the like, however, the spin coating technique is the preferred method recited herein.

Description

TECHNICAL FIELD

This invention relates generally to the manufacturing of amorphous lithium lanthanum titanate thin films, and especially for the manufacturing of amorphous lithium lanthanum titanate thin films that may be used as an electrolyte or composite cathode electrode.

BACKGROUND OF THE INVENTION

The need for a high performance and reliable energy source is well understood. Lithium batteries represent a very attractive solution to these energy needs due to their superior energy density and high performances. Lithium batteries having solid electrolytes offer significant advantages over other lithium battery families because of the elimination of fear of electrolyte instability and combustion, a wider operating temperature range, and relative ease of miniaturization. The solid electrolyte is generally applied in thin film form to minimize losses in the electrolyte. Currently, the most widespread solid lithium electrolyte is Li_3.3PO_3.9N_0.17 (LiPON). However, LiPON electrolytes are sensitive to moisture and oxygen in ambient air and as such limits their applicability.

Lithium lanthanum titanate (LLTO) has been identified as an attractive alternative to LiPON electrolytes. Currently, the La_0.5Li_0.5TiO₃ form of LLTO and its cation deficient modifications have been found to have high lithium ion conductivity. Despite all crystalline LLTO modifications having high conductivity, they turn out to be unstable for lithium solid film battery applications because they are unstable in contact with lithium metal anodes. This instability manifests itself in the crystalline LLTO electrolytes becoming an electronic conductor when in contact with lithium metal due to Li⁺ ion insertion into the LLTO electrolyte. This effect is facilitated by the presence of spatially extended electronic states in the crystalline LLTO electrolyte. Amorphous versions of LLTO electrolytes typically do not exhibit the electronic conduction instability of the crystalline LLTO electrolytes while maintaining the high lithium ion conductivity of its crystalline counterpart.

Amorphous LLTO electrolyte thin films have been prepared using pulsed laser deposition (PLD) techniques from crystalline targets. Measured lithium conductivities of these amorphous LLTO electrolyte samples are at least an order of magnitude higher than LiPON electrolytes conductivity and range for 1E-5 to 1E-3 S/cm⁻¹ Stability of the amorphous LLTO electrolyte was successfully demonstrated by fabricating a solid lithium ion battery that used amorphous LLTO as its electrolyte, lithium metal as its anode and LiCoO₂ as its cathode. This battery was successfully cycled between 4.3 and 3.3V at room temperature. While amorphous LLTO electrolytes show excellent promise as a lithium battery solid electrolyte candidate material, its pulse laser deposition technique method of preparation is not convenient for large scale manufacturing purposes.

It would be beneficial to provide an amorphous LLTO material which may be suitable for use in batteries, or other electrochemical devices or lithium ion conductive systems. Accordingly, it is to the provision of such that the present invention is primarily directed.

SUMMARY OF THE INVENTION

In a preferred form of the invention a method of producing an amorphous lithium lanthanum titanate layer comprises the steps of (a) providing a quantity of polymer; (b) providing a quantity of lanthanum alkoxide; (c) providing a quantity of lithium alkoxide; (d) providing a quantity of titanium alkoxide; (e) mixing the quantities of polymer, lanthanum alkoxide, lithium alkoxide and titanium alkoxide with an alcohol to form a precursor solution; (f) applying the precursor solution to a substrate to form a precursor layer, and (g) heating the precursor layer to form an amorphous lithium lanthanum titanate layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an air battery embodying principles of the invention in a preferred form.

FIG. 2 is a Nyquist plot of an amorphous lithium lanthanum titanate thin film layer produced of FIG. 1.

FIG. 3 is an SEM image of an amorphous lithium lanthanum titanate thin film layer produced of FIG. 1.

DETAILED DESCRIPTION

With reference next to the drawings, there is shown in a battery or electrochemical cell 10. The cell 10 includes a a cathode current collector 11, cathode 12, an electrolyte 13, an anode 14, and an anode current collector 16.

To produce the cell 10 an amorphous lithium lanthanum titanate (LLTO) thin film electrolyte is produced by the sol-gel method. The LLTO electrolyte may be manufactured in the following manner, however, it should be understood that the quantities involved are for exemplary purposes only. The manufacturing commences by introducing 0.05 grams of a polymer, preferably polyvinyl pyrrolidone (PVP), which may be in powder form, into 5 grams of liquid alcohol such as 2-methoxyethanol (2-ME). The solution is allowed to sit for approximately one hour so that the PVP powder is dissolved fully. This produces a first solution.

A second solution is then prepared by mixing approximately 0.868 grams of a lanthanum alkoxide, such as lanthanum methoxyethoxide, previously dissolved in an alcohol, such as 2-methoxyethanol. The lanthanum alkoxide constitutes approximately 10% by weight of the resulting solution while the alcohol constitutes 90% by weight of resulting solution.

Approximately 1 gram of the PVP solution (first solution) is then added to the lanthanum based second solution.

It should be understood that the just described steps of producing first and second solutions has been described because of the ease in obtaining a pre-produced or already dissolved solution containing lanthanum alkoxide. However, it should be understood that the solution resulting from the combination of the first and second solutions could be formed by simply combining the PVP and the lanthanum alkoxide in alcohol to form the resulting solution. Furthermore, it should be understood that the sequence of the mixing of polymer, lanthanum alkoxide, lithium alkoxide and titanium alkoxide is not important to the invention, as these components may be added to the alcohol in any sequence or simultaneously.

Approximately 0.11 grams of a lithium alkoxide, such as lithium butoxide, and approximately 1.21 grams of a titanium alkoxide, such as titanium propoxide, are then also added to the lanthanum based second solution. This process produces an approximately 10 to 11 grams batch of LLTO precursor solution. The amount of PVP is nominally 1% of PVP in the solid LLTO material free of the liquids, either solvents or products of the LLTO synthesis process.

The LLTO precursor solution is applied or otherwise coated on a substrate, such as a gold foil or cathode for a lithium battery, to form a precursor layer, coating or film. The coating techniques that may be used include spin coating, spraying, casting, dripping, and the like, however, the spin coating technique is the preferred method recited herein.

The spin coating process steps commence with spin coating the LLTO precursor solution at 5,000 rpms for 10 seconds. The freshly deposited LLTO coating and substrate are then placed in a closed container saturated by 2-ME vapors for a time period of 15 minutes. The 2-ME saturation is achieved by keeping an open dish containing 2-ME within the closed container. The LLTO coating and substrate and then moved to another closed container, without 2-ME therein, and left for approximately one hour. The LLTO coating and substrate are then heated to approximately 80 degrees Celsius for approximately 15 to 30 minutes in ambient air. The LLTO coating and substrate are then heated to approximately 300 degrees Celsius for approximately 15 to 30 minutes in ambient air and subsequently heated to approximately 350 to 600 degrees Celsius for approximately 15 to 30 minutes in ambient air. It should be understood that the drying times may be varied in accordance with different temperatures.

The just described spin coating process results in a material layer having a thickness in the range of approximately 100 to 150 nm. However, it should be understood that thicker films or layers may be formed by repeating the basic spin coating processing steps multiple times in order to achieve a multilayer film having the desired thickness.

A sample layer was produced in the just described manner which showed high lithium ion conductivity of approximately 1E-4. FIG. 2 illustrates a Nyquist plot of the amorphous LLTO sample. The graph illustrates the desired impedance with the layer acting as a good ionic conductor. FIG. 3 is an SEM image of the layer produced upon a gold substrate. The SEM image shows that the produced sample was essentially void of cracks, a problem associated with the prior art thin films.

It should be understood that an amorphous LLTO layer is believed to be an improvement over the prior art crystalline LLTO layer, since the amorphous layer appears to provide greater characteristics for electrochemical conversion systems, such as batteries, such as better lithium ion conductivity.

The LLTO precursor solution could also be used as an alternative to prior art liquid electrolytes. As such, the LLTO precursor solution would allowed to permeate the cathode and allowed to dry. The LLTO precursor solution would thus enhance the ability of ions to move through the cathode. Alternatively, the LLTO precursor solution could be dried to obtain a powder and the powder would then be mixed with the solid cathode components and a liquid solution used to fill the leftover pores. As another alternative, the cathode components could be mixed with the LLTO precursor solution to form a cathode. Lastly, the precursor solution may be thickened (increased viscosity) by solvent evaporation in an inert atmosphere and heated. The resulting solution is used to permeate the initial cathode material.

It should be understood that as an alternative to the PVP other polymers may be utilized, such as polyvinyl butyral (PVB). Similarly, alternatives to the 2-ME include butanol, propanol, methoxypropanol, propoxyethanol, glycol ethers, and others. Alternatives to the lanthanum methoxyethoxide include lanthanum propoxide and lanthanum ethoxide. Alternatives to the lithium butoxide include lithium methoxide and lithium propoxide. Lastly, alternatives to the titanium propoxide include titanium butoxide and titanium ethoxide.

It thus is seen that a simple method of manufacturing an amorphous lithium lanthanum titanate thin film by sol-gel method is now provided. It should of course be understood that many modifications may be made to the specific preferred embodiment described herein, in addition to those specifically recited herein, without departure from the spirit and scope of the invention as set forth in the following claims.

Claims

1. A method of producing an amorphous lithium lanthanum titanate layer comprising the steps of: (a) providing a quantity of polymer;(b) providing a quantity of lanthanum alkoxide;(c) providing a quantity of lithium alkoxide;(d) providing a quantity of titanium alkoxide;(e) mixing the quantities of polymer, lanthanum alkoxide, lithium alkoxide and titanium alkoxide with an alcohol to form a precursor solution;(f) applying the precursor solution to a substrate to form a precursor layer, and(g) heating the precursor layer to form an amorphous lithium lanthanum titanate layer.

2. The method of claim 1 wherein step (a) the polymer is selected from the group consisting of polyvinyl pyrrolidone and polyvinyl butyral.

3. The method of claim 2 wherein step (e) the alcohol is selected from the group consisting of s 2-methoxyethanol, butanol, propanol, methoxypropanol, propoxyethanol, and glycol ethers.

4. The method of claim 1 wherein step (e) the alcohol is selected from the group consisting of s 2-methoxyethanol, butanol, propanol, methoxypropanol, propoxyethanol, and glycol ethers.

5. The method of claim 1 wherein step (b) the lanthanum alkoxide is selected from the group consisting of lanthanum methoxyethoxide, lanthanum propoxide, and lanthanum ethoxide.

6. The method of claim 2 wherein step (b) the lanthanum alkoxide is selected from the group consisting of lanthanum methoxyethoxide, lanthanum propoxide, and lanthanum ethoxide.

7. The method of claim 1 wherein step (c) the lithium alkoxide is selected from the group consisting of lithium butoxide, lithium methoxide, and lithium propoxide.

8. The method of claim 2 wherein step (c) the lithium alkoxide is selected from the group consisting of lithium butoxide, lithium methoxide, and lithium propoxide.

9. The method of claim 1 wherein step (d) the titanium alkoxide is selected from the group consisting of titanium propoxide, titanium butoxide and titanium ethoxide.

10. The method of claim 2 wherein step (d) the titanium alkoxide is selected from the group consisting of titanium propoxide, titanium butoxide and titanium ethoxide.

11. The method of claim 7 wherein step (d) the titanium alkoxide is selected from the group consisting of titanium propoxide, titanium butoxide and titanium ethoxide.

12. The method of claim 1 further comprising the step (h) electrically coupling the amorphous lithium lanthanum titanate layer between a cathode and an anode.

13. A method of producing an amorphous lithium lanthanum titanate battery electrolyte layer comprising the steps of: (a) mixing a quantity of polymer with a quantity of alcohol;(b) mixing a quantity of lanthanum alkoxide with the quantity of alcohol;(c) mixing a quantity of lithium alkoxide with the quantity of alcohol;(d) mixing a quantity of titanium alkoxide with the alcohol;(e) mixing the polymer, lanthanum alkoxide, lithium alkoxide, titanium alkoxide, and alcohol to form a precursor solution;(f) applying the precursor solution to a substrate to form a precursor layer;(g) drying the precursor layer to a dry state to form an amorphous lithium lanthanum titanate electrolyte layer, and(h) electrically coupling the amorphous lithium lanthanum titanate layer between a battery cathode and a battery anode.

14. The method of claim 13 wherein step (a) the polymer is selected from the group consisting of polyvinyl pyrrolidone and polyvinyl butyral.

15. The method of claim 14 wherein step (a) the alcohol is selected from the group consisting of s 2-methoxyethanol, butanol, propanol, methoxypropanol, propoxyethanol, and glycol ethers.

16. The method of claim 13 wherein step (a) the alcohol is selected from the group consisting of s 2-methoxyethanol, butanol, propanol, methoxypropanol, propoxyethanol, and glycol ethers.

17. The method of claim 13 wherein step (b) the lanthanum alkoxide is selected from the group consisting of lanthanum methoxyethoxide, lanthanum propoxide, and lanthanum ethoxide.

18. The method of claim 14 wherein step (b) the lanthanum alkoxide is selected from the group consisting of lanthanum methoxyethoxide, lanthanum propoxide, and lanthanum ethoxide.

19. The method of claim 13 wherein step (c) the lithium alkoxide is selected from the group consisting of lithium butoxide, lithium methoxide, and lithium propoxide.

20. The method of claim 14 wherein step (c) the lithium alkoxide is selected from the group consisting of lithium butoxide, lithium methoxide, and lithium propoxide.

21. The method of claim 13 wherein step (d) the titanium alkoxide is selected from the group consisting of titanium propoxide, titanium butoxide and titanium ethoxide.

22. The method of claim 14 wherein step (d) the titanium alkoxide is selected from the group consisting of titanium propoxide, titanium butoxide and titanium ethoxide.

23. The method of claim 19 wherein step (d) the titanium alkoxide is selected from the group consisting of titanium propoxide, titanium butoxide and titanium ethoxide.

24. The method of claim 13 wherein step (g) the precursor layer is dried by applying heat to the precursor layer.

25. A method of producing an amorphous lithium lanthanum titanate layer comprising the steps of: (a) mixing a quantity of polymer with a quantity of alcohol to form a first solution;(b) mixing a quantity of lanthanum alkoxide with a quantity of alcohol to form a second solution;(c) mixing a quantity of the first solution with the second solution;(d) mixing a quantity of lithium alkoxide with the second solution;(e) mixing a quantity of titanium alkoxide with the second solution, the combination of the second solution with the amount of first solution, lithium alkoxide and titanium alkoxide forming a precursor solution;(f) applying the precursor solution to a substrate to form a precursor layer, and(g) drying the precursor layer to form an amorphous lithium lanthanum titanate layer.

26. The method of claim 25 wherein step (a) the polymer is selected from the group consisting of polyvinyl pyrrolidone and polyvinyl butyral.

27. The method of claim 25 wherein step (b) the lanthanum alkoxide is selected from the group consisting of lanthanum methoxyethoxide, lanthanum propoxide, and lanthanum ethoxide.

28. The method of claim 25 wherein step (d) the lithium alkoxide is selected from the group consisting of lithium butoxide, lithium methoxide, and lithium propoxide.

29. The method of claim 25 wherein step (e) the titanium alkoxide is selected from the group consisting of titanium propoxide, titanium butoxide and titanium ethoxide.

30. The method of claim 25 further comprising the step (h) electrically coupling the amorphous lithium lanthanum titanate layer between a cathode and an anode.