Manufacturing method for lithium hexafluoro phosphate

Details Manufacturing method for lithium hexafluoro phosphate Manufacturing method for lithium hexafluoro phosphate

1999-12-29

2002-05-14

Langel, Wayne (Department: 1754)

Chemistry of inorganic compounds

Phosphorus or compound thereof

Halogen containing

C423S483000

Reexamination Certificate

active

06387340

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of preparing lithium hexafluoro phosphate (LiPF
6
) at a high yield and purity, which is one of the essential elements for a lithium ion secondary battery cell and a lithium polymer battery cell.
2. Description of the Prior Art
A lithium ion battery cell or a lithium polymer battery cell is fundamentally composed of an anode of lithium oxide, such as LiCoO
2
, LiNiO
2
, LiMn
2
O
4
or the like, a cathode of a carbon material, such as graphite, and an electrolytic solution consisting of an electrolyte dissolved in an organic solvent. Examples of the organic solvent include nonaqueous solvents, such as a mixed solvent of ethylene carbonate (hereinafter, called “EC”) and dimethyl carbonate (hereinafter, called “DMC”), a mixed solvent of EC and diethyl carbonate (hereinafter, called “DEC”), and the like. Moreover, lithium hexafluoro phosphate is typically used as the electrolyte.
When the battery cell is electrically charged, Li ion is inserted into, and attached to a void in the cathode, whereas, when it is electrically discharged, the Li ion attached to the cathode is returned to the anode. In this case, the Li ion moves through the electrolytic solution. As a result, in order to maintain performance of the battery throughout its life cycle, lithium hexafluoro phosphate, which is an electrolyte used in the electrolytic solution, is very strictly limited in specifications, such as purity, etc.
As a rule, lithium hexafluoro phosphate is prepared by reacting, in a concentrated hydrofluoric acid solution, lithium fluoride (LiF) with phosphorus pentafluoride (PF
5
) which is obtained by a reaction of phosphorus pentachloride (PCl
5
) and hydrofluoric acid (HF). In this reaction, if the reaction system for preparing lithium hexafluoro phosphate contains moisture, lithium oxyfluoro phosphate (LiPO
X
F
Y
) is produced as a by-product, and lithium oxyfluoro phosphate is also partially resolved into LiF which then remains as an impurity in lithium hexafluoro phosphate. Consequently, it is preferred that the moisture content in the reaction system for preparing lithium hexafluoro phosphate is as low as possible.
A metal component contained in a raw material used in a preparing process of lithium hexafluoro phosphate, or a metal component introduced by a corrosion of an equipment, changes lithium metal ionization potential. Thus, where the metal component is contained in the electrolyte, the life of the battery cell may be shortened.
Furthermore, if moisture is contained in the final product, then lithium hexafluoro phosphate is resolved into LiF, HF, and PF
5
, that are then converted into a gaseous state, thereby forming an internal pressure with in the battery cell. Also, as HF is reacted with the organic solvent, it has an effect on the corrosion of a case enclosing the battery cell. It adversely affects a stability of the battery cell. For these reasons, lithium hexafluoro phosphate (LiPF
6
) used as the electrolyte is strictly limited in specifications, such as purity, moisture content, metal content, free hydrofluoric acid content, and the like.
SUMMARY OF THE INVENTION
The present invention is a method wherein HF dried completely by means of F
2
is used as one of raw materials for the preparation of lithium hexafluoro phosphate to fundamentally prevent lithium oxyfluoro phosphate (LiPO
X
F
Y
) from being produced as a by-product and also to prevent lithium hexafluoro phosphate (LiPF
6
) from resolving by moisture, thereby enabling lithium hexafluoro phosphate (LiPF
6
) to be prepared at a high purity without reducing yield.
Also, the present invention is a method wherein F
2
gas of a high purity is introduced at a desired amount during a reaction for preparing lithium hexafluoro phosphate (LiPF
6
) to fundamentally prevent lithium oxyfluoro phosphate (LiPO
X
F
Y
) from being produced as a by-product because of moisture contained in a solid type raw material such as PCl
5
, LiX (X=F, Br, Cl, or I), or the like and other moisture which can be introduced during the reaction, thereby enabling lithium hexafluoro phosphate (LiPF
6
) to be prepared at a high purity.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In a general method for preparing hydrogen fluoride, the preparation of hydrogen fluoride is carried out after basic raw materials are completely dried to control moisture. In a last step, hydrogen fluoride is passed through anhydrous sulfuric acid to remove moisture contained therein. After undergoing this drying process, hydrogen fluoride typically contains moisture of about 100 ppm, and also sulfuric acid of about 50 ppm. It is a high boiling point material, due to the passage through anhydrous sulfuric acid.
As described above, in the battery cell-grade electrolyte, the content of the metal component is very critical. Because all equipment used for the preparation of HF is made of steel, however, hydrogen fluoride itself will contain a large number of metal components.
Anhydrous hydrogen fluoride resulting from the process is unsuitable for use as a raw material for the preparation of a battery cell-grade electrolyte, because it is high in content of moisture, sulfuric acid, metal components, and the like. Therefore, this needs to be dried and purified, again.
In the present invention, the reaction formula, according to which the remaining moisture in anhydrous hydrogen fluoride is dried, is as follows:
H
2
O(in HF)+2F
2
→2HF+OF
2
(↑)
This drying is carried out under the following conditions:
Bubbling time: 1 to 3 hours
Temperature: −10 to 30° C.
Flow rate: 1 to 1,000 g/hr
As shown in the reaction formula, as moisture in hydrogen fluoride is reacted with F
2
gas to be converted into the hydrogen fluoride, oxyfluoride (OF
2
) is discharged which is very low in boiling point, such as −145° C., so that it is easily volatilized out after the reaction.
The present invention also relates to a purification of PCl
5
and LiCl that are solid raw materials. Commercially available compounds generally have respective specifications inherent in purity, moisture content, metal component, and the like. Even though it is well known that the use of a raw material of a high purity results in an increase in quality of a final product, this causes the manufacturing-cost to be increased, thereby decreasing competitiveness. Thus, the present invention adopts PCl
5
and LiCl that are relatively cheap raw materials containing some impurities, such as moisture, etc.
As described above, if the raw material contains moisture, lithium oxyfluoro phosphate (LiPO
X
F
Y
) may be produced which is very difficult to be isolated by a recrystallization.
The present invention is characterized in that F
2
gas is introduced into a reaction system to completely remove moisture contained in the solid raw materials, thereby fundamentally preventing lithium oxyfluoro phosphate (LiPO
X
F
Y
) from being produced.
Moreover, even when lithium oxyfluoro phosphate (LiPO
X
F
Y
) is produced during the reaction, this is converted into lithium hexafluoro phosphate (LiPF
6
) and oxyfluoride (OF
2
) by F
2
gas in accordance with the present invention, thereby increasing yield.
In addition, another characteristic of the present invention is that, for controlling the metal components, equipment coated with a fluorine resin, such as polytetrafluoroethylene (hereinafter, called “PTFE”), is used for all processes, and that pipe lines made of a PFA resin, such as tetrafluoroethylene perfluoroalkylvinylether copolymer are also used. Futhermore, two reactors are used.
The reasons why all the equipment is coated with the fluorine resin and the pipe lines are made of the PFA resin, are as follows.
In the prior art, there was used a reactor made of a material capable of withstanding anhydrous hydrofluoric acid, such as a monel metal, a Ni base alloy, a Cr base alloy, or the like. However, such a material is very expensive, and therefore disadvantageous in view of a commercial factors. Moreover, if th

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