Method for making lithium aluminide compound with high lithium content

Abstract

A method for making lithium aluminide compound with high lithium content is disclosed. The method includes applying an electrolyte composed of lithium chloride, potassium chloride, and calcium chloride to accomplish a diffusive electrolysis under 380˜550° C. temperature. The lithium atoms of the electrolyte are reduced and diffused into an aluminum cathode, and liquid lithium aluminide is formed and floats on the electrolyte. The liquid lithium aluminide is further scooped up from the upper electrolyte and solidified to a lithium aluminide compound with 40˜62 wt % lithium content.

Description

RELATED APPLICATIONS

The application claims priority to Taiwan Application Serial Number 96123839, filed Jun. 29, 2007, which is herein incorporated by reference.

BACKGROUND

1. Field of Invention

The present invention relates to a method for making lithium aluminide (AlLi) compound. More particularly, the present invention relates to a method for making lithium aluminide compound with high lithium content.

2. Description of Related Art

The application of the lithium aluminide compound is limited because the ingredients of the lithium aluminide (AlLi) compound are difficult to control. Conventionally, the intermetallic FeLi compound or intermetallic CuLi compound is prepared by a ball-grinding method in a chamber with a protective gaseous environment. The ball-grinding method consumes a lot of energy resources and time to force the metal powder blend into the lithium granule by mechanical metallurgy. In the case of manufacturing the lithium aluminide compound, both the aluminum powder and solid lithium are extremely unstable in air, so that the ball grinding process is not adequate for the preparation of the lithium aluminide compound.

The conventional lithium aluminide compound is prepared by putting solid lithium into an aluminum melt under a layer of flux or in an argon atmosphere. The above process gets even more complicated with increasing lithium content in the aluminum alloy melt. Moreover, solid lithium is extremely unstable in air because the air humidity may result in dangerous flash lithium gasification. Solid lithium is very dangerous during the transportation and storage period, and the cost cannot be lowered in regions without lithium ore that relies on importing the lithium from the place of origin.

Therefore, the conventional process of preparing a lithium containing alloy is not only difficult to operate but also expensive.

SUMMARY

A method for making lithium aluminide compound with high lithium content in accordance with the present invention satisfies the need to lower the danger of conventional processes and the cost of storing and transporting the solid lithium.

The method comprises providing an electrolytic bath that employs an aluminum material as cathode. The electrolytic bath fills an electrolyte that essentially consists of lithium chloride, potassium chloride, and calcium chloride. The lithium chloride of the electrolyte provides a lithium source for making lithium aluminide compound, and the potassium chloride is a flux to decrease the temperature and improve the fluidity of the electrolyte.

An electric current with a current density of 0.08 A˜0.1 A per unit area of cathode surface (cm²) is exerted to the electrolyte in atmospheric environment, and the temperature of the electrolyte is kept within a range of 380° C.˜550° C. to perform a diffusive electrolysis on the cathode. The lithium ions of the electrolyte are reduced into lithium atoms on an aluminum cathode at a high temperature, and the lithium atoms simultaneously diffuse into the aluminum cathode to form a lithium aluminide compound.

The lithium deposed on the surface and subsurface of the aluminum cathode is increased with progressing of diffusive electrolysis, when the lithium content in the lithium aluminide compound is approximately 50% by weight (assuming that the temperature is at 500° C.), the formed lithium aluminide compound is in a liquid state. Because the density of the liquid-aluminide is less than the density of the melted electrolyte, the liquid lithium aluminide floats on the upper electrolyte. The liquid lithium aluminide is further scooped up from the upper electrolyte and solidified to a lithium aluminide compound with high lithium content.

In conclusion, the embodiment of the present invention provides a method to prepare lithium aluminide compound with high lithium content. The method includes applying an aluminum cathode and an electrolyte with lithium chloride, a stable chemical in atmospheric environment, to perform a diffusive electrolysis. The lithium is increasingly reduced and deposited on the aluminum cathode in diffusive electrolysis to form the liquid lithium aluminide, and solidify to a lithiumaluminide compound with high lithium content. The embodiment of the present invention provides a safer, lower cost and more convenient way to transport and store the lithium material by employing the lithium chloride as the raw material of the lithium aluminide compound.

It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:

FIG. 1 is a flowchart of steps in accordance with an embodiment of the present invention;

FIG. 2 is an operational cross-section view of a electrolytic bath in accordance with an embodiment of the present invention; and

FIG. 3 is a phase diagram of the lithium aluminide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

Refer to FIG. 1. FIG. 1 is a flowchart of steps in accordance with an embodiment of the present invention. The steps of the embodiment include providing an electrolytic bath with an electrolyte and the temperature of the electrolyte is kept within a range of 380° C.˜550° C.; and exerting an electric current with current density of 0.08 A˜0.1 A per unit area of cathode surface (cm²) to the electrolyte to perform a diffusive electrolysis. The lithium ions of the electrolyte are reduced into lithium atoms and diffused into the aluminum cathode. The lithium content of the aluminum cathode is increased with diffusive electrolysis and forms liquid lithium aluminide floated on the upper of the electrolyte. Finally, scooping liquid lithium aluminide and solidifying to a lithiumaluminide compound with high lithium content.

In accordance with an embodiment of the present invention, the electrolytic bath comprises a cathode and an anode wherein the cathode is formed by aluminum material. The aluminum cathode receives the lithium atoms to form the lithium aluminide.

Refer to FIG. 2. FIG. 2 is an operational cross-section view of an electrolytic bath in accordance with an embodiment of the present invention. The electrolytic bath 200 comprises an external tank 210 and an internal tank 220 wherein the internal 220 is formed by a heat insulation material such as aluminum oxide. A lid 211 covers the top of the external tank 210 wherein the lid 211 is movable. A plank 221 is set in the bottom of the internal tank 220, and a container 230 is mounted on the plank 221 wherein the container 230 is a steelmade container to load an electrolyte 241.

A heater 250 surrounds the container 230, and a thermocouple 260, an anode 242 and a cathode 243 are placed in the container 230. The heater 250 heats the electrolyte 241 to keep the temperature. The anode 242 and the cathode 243 are supported by a support 244 and immersed partially into the electrolyte 241. In accordance with an embodiment of the invention, an aluminum material is used as the cathode 243 and the material of the anode 242 comprises graphite, polymer or alloy material. High temperature and electric current triggers the lithium ion of the electrolyte 241 reduced to lithium atoms and diffused into the cathode 243. The thermocouple 260 is immersed partially into the electrolyte 241 to detect the temperature variation of the electrolytic system.

In accordance with an embodiment of the invention, the electrolyte 241 essentially consists of 20˜40% by weight (wt. %) of lithium chloride (LiCl), 30˜50 wt. % of potassium chloride (KCI), and less than 10 wt. % of calcium chloride (CaCl₂). The electrolyte 241 provides the lithium source from lithium chloride to form the lithium aluminide compound, and other components of the electrolyte, potassium chloride and calcium chloride, are employed as a flux to reduce the temperature and improve the flow rate of the electrolyte.

In accordance with the present invention, to perform a diffusive electrolysis, an electric current with current density of 0.08 A per unit area of the cathode surface (cm²) is exerted on the electrolysis system at 500° C. in atmospheric environment. The lithium atoms of the electrolyte are reduced on the surface of aluminum cathode 243 and diffused into the aluminum to form a lithium aluminide compound. The lithium content of the lithium aluminide compound is increased with progressing of the diffusive electrolytic, and the lithium aluminide compound is transformed into liquid state when approximately 55% lithium content of the lithium aluminide compound is reached.

Because the density of the liquid lithium aluminide is less than the density of the melted electrolyte, the liquid-aluminide floats on the upper surface of the electrolyte. The liquid lithium aluminide is further scooped up from the upper surface of the electrolyte and solidified to a lithium aluminide compound with 40˜62% wt. of lithium. In an embodiment, the liquid lithium aluminide is solidified using a mold to shape the lithium aluminide with high lithium content into a specific shape. Furthermore, the high lithium content is out compass of the present measuring instrument, so that the lithium content of the lithium aluminide compound of the embodiment of present invention cannot yet be measured. In this embodiment, the lithium content of the lithiumaluminide compound is determined in accordance with a phase diagram shown in FIG. 3.

Refer to FIG. 3. FIG. 3 is a phase diagram of the lithium aluminide compound. The vertical axis indicates the different temperature, the lower horizontal axis indicates the atomic lithium percent of the lithium aluminide compound, and the upper horizontal axis indicates the lithium weight percent of the lithium aluminide compound. Refer to FIG. 3, assuming a diffusive electrolysis is performed at 550° C., for example, when the accumulated lithium on the cathode surface reaches 41 wt. %, the lithium aluminide compound is presented in a Li—Al liquid phase zone. The determined density of the Al41% Li is about 1.84 g/cm³ as calculated by referring the density of aluminum (2.74 g/cm³) and the lithium (0.54 g/cm³). The density of the melting electrolyte is about 2 g/cm³. Because the density of the liquid Al—Li is less than the density of the melted electrolyte, the liquid Al—Li floats on the upper surface of the electrolyte continuously under an applicable current density and temperature condition, therefore the liquid lithium aluminide can be scooped up from the upper electrolyte and solidified to a lithium aluminide compound with high lithium content.

The lithium aluminide compound with high lithium content made by the method of the present invention possesses stable metallicity, and it is therefore inactive at atmospheric environment to avoid the danger of storing solid lithium. In addition, putting the lithium aluminide compound into water can cause acute reaction that proves the lithium aluminide compound contains a high content of lithium.

Although the electrolyte includes potassium chloride as a flux, the lithium aluminide compound merely contains less than 50 ppm of potassium as a result of the poor affinity between the potassium and the aluminum cathode. Sodium is a very common impurity in chloride salt and the lithiumaluminide compound contains less than 30 ppm of sodium.

Although the present invention has been described in considerable detail with certain embodiments referenced herein, other embodiments are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiments contained herein.

It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.

Claims

1. A method for making lithiumaluminide compound with high lithium content, and the method comprising: providing an electrolytic bath, where the electrolytic bath includes an anode and a cathode, and the cathode is formed by an aluminum material;filling an electrolyte in the electrolytic bath, the electrolyte essentially consisting of lithium chloride, potassium chloride, and calcium chloride, and having a temperature kept within a range of 380° C.˜550° C.;exerting an electric current with current density of 0.08 A˜0.1 A per unit area of cathode surface (cm2) to the electrolyte to reduce and diffuse a plurality of lithium atoms of the lithium chloride into a cathode surface of the cathode, and form a liquid lithium aluminum floating on the electrolyte; andremoving the liquid lithium aluminum from the electrolyte and solidifying the liquid lithium aluminum to a lithium aluminide compound.

2. The method of claim 1, wherein the electrolyte essentially consists of 20˜40 wt. % lithium chloride, 30˜50 wt. % potassium chloride, and less than 10 wt. % calcium chloride.

3. The method of claim 1, wherein the temperature of the electrolyte is kept at 500° C.

4. The method of claim 1, wherein the current density of the electric current is kept at 0.08 A per unit area of cathode surface (cm2).

5. The method of claim 1, wherein the anode is a graphite anode.

6. The method of claim 1, wherein the composition of the lithiumaluminide compound is within the range of Al-40 wt. % Li˜Al-62 wt. % Li.

7. The method of claim 1, further comprises using a mold to solidify the liquid lithium aluminum.