Liquid purification or separation – Processes – Chemical treatment
Reexamination Certificate
2000-05-18
2001-03-06
Anthony, Joseph D. (Department: 1714)
Liquid purification or separation
Processes
Chemical treatment
C210S753000, C252S182320, C252S182350, C429S200000, C429S188000, C423S301000, C423S596000
Reexamination Certificate
active
06197205
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to lithium cell's electrolytic solution containing lithium hexafluorophosphate, and a method for producing the electrolytic solution, and a method for purifying the electrolytic solution.
Hitherto, there have ben proposed various methods for producing lithium hexafluorophophate. For example, Japanese Patent Unexamined Publication JP-A-Sho-A-64-72901 discloses a method in which lithium fluoride which is in the form of solid is reacted with phosphorus pentafluoride which is in the form of gas, without using any solvent. In this method, after the reaction has been started, particles of lithium fluoride are coated with the reaction product (i.e., hexafluorophosphate). With this, the reaction does not proceed further, and thus the unreacted lithium fluoride is left in the reaction system. As another example, J. Chem. Soc. Part 4, 4408 (1963) discloses a method in which lithium fluoride dissolved in anhydrous hydrogen fluoride is reacted with phosphorus pentafluoride which is in the form of gas. It is, however, difficult to handle this reaction, beacause anhydrous hydrogen fluoride, which is high in vapor pressure, is used as a solvent. Furthermore, when the reaction product, lithium hexafluorophosphate, is taken in the form of crystals, an impurity, hydrogen fluoride, remains therein. If this impurity still remains in the electrolytic solution of a sithium cell, it interferes with the operation thereof. As mentioned hereinabove, the conventional methods for producing lithium hexafluorophosphate are not satisfactory in yield of the reaction, easiness to handle the reaction, and/or purity of the reaction product.
Lithium hexafluorophosphate is very stable, when it is ionically dissociated in a nonaqueous solvent. It is, however, very unstable and thus susceptible to hydrolysis and thelike during storage or transportation under a solid condition.
The above-mentioned lithium hexafluorophosphate and other lithium salts of fluorine-containing compounds are commonly used as electrolytes (solutes) dissolved in electrolytica solutions for lithium cells. These electrolytic solutions generally contain various acid impurities such as hydrogen fluoride. Of the electrolytes, lithium hexafluorophosphate and the like are easily decomposed by water contained in the electrolytic solution, thereby to produce acid impurities such as hydrogen fluiride, phosphoric acid, and oxygluorophosphoric acid. If an electrolytic solution containing acid impurities is used in a lithium cell, the acid impurities are reacted with the athode, the anode, and the solvent of teh electrolytic solution, thereby to lower the discharge capacity of the cell, to increase the internal resistance, to shorten lifetime of the cell, and to cause other problems. Hitherto, there have been various proposals for purging acid impurities from the electrolytic solution. For example, there have been proposals for purging water from the electrolytic solution in order to suppress the formation of acid impurities. However, it has been difficult to completely purge water therefrom. Thus, the electrolytic solutions purified by conventional purging methods were unsatisfactory in purity.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for producing an electrolytic solution containing a solute of lithium hexafluorophosphate, which is free of the drawbacks of the above-mentioned conventional methods.
It is a specific object of the present invention to provide such method in which yield of the reaction is high, the reaction can easily be managed, and purity of the reaction product is sufficiently high.
It is another object of the present invention to provide a method for producing an electrolytic solution containing a solute of lithium hexafluorophosphate and a solvent of cyclic carbonic acid ester.
It is still another object of the present invention to provide a method for uprifying an electrolytic solution used for a lithium cell, which method enables the electrolytic solutino to have a substantially low concentration of acid impurities.
According to a first aspect of the present invention, ther is provided a first method for producing an electrolytic solution containing a solute of lithium hexafluorophosphate. The first method comprises a step of (a) reacting lithium fluoride with phosphorus pentafluoride, in a nonaqueous organic solvent that is used for producing a lithium cell's electrolytic solution, thereby to form said lithium hexafluorophosphate dissolved in said solvent. According to the first method, yield of the reaction is high, the reaction can easily be managed, and the reaction product is sufficiently high in purity. By this reaction, lithium hexafluorophosphate is produced, and at the same time it is ionically dissociated. Therefore, the obtained electrolytic solution is very stable and thus is not subjected to hydrolysis and the like during storage or transportation. The electrolytic solution itself obtained by the first method may be used as a lithium cell's electrolytic solution. Alternatively, the lithium hexafluorophosphate may be separated from the solvent, thereby to obtain the same in the form of crystals.
When there is used, as the nonaqueous organic solvent of the first method, a monofunctional chain compound which is stable and thus does not polymerize in the step (a) by the phosphorus pentafluoride, the obtained electrolytic solution itself may be used as a lithium cell's electrolytic solution, as mentioned hereinabove. Even if the monofunctional chain compound is partially decomposed in the step (a) of the first method, the decomposition products, which have smaller molecular weights as compared with that of the monofunctional chain compound, are not polymerized, but are puged from the reaction system in the form of gas. Therefore, this partial decomposition does not have an adverse effect on the electrolytic solution.
As an alternative to the above-mentioned direct use of the electrolytic solution obtained by the first method, another nonaqueous organic solvent, that is, a cyclic carbonic acid ester (e.g., ethylene carbonate and propylene carbonate) and/or the like, which is polymerizable or decomposable in the step (a) by phosphorus pentafluoride acting as a Lewis acid, may be added to the electrolytic solution prepared by the first method. The thus obtained electrolytic solution may also be used as a lithium cell's electrolytic solution. In contrast, in case that only the another nonaqueous organic solvent must be contained in a lithium cell's electrolytic solution, it is not preferable to use the another solvent in the step (a) of the first method, because it may polymerize by lithium pentafluoride, and the thus formed polymer remains in the solution, causing an adverse effect (e.g., the viscosity increase) on the electrolytic solution. In this case, it is preferable that the nonaqueous organic solvent contained in the electrolytic solution prepared by the first method is replaced with the another solvent, after the step (a). This replacement is conducted by steps of(b) adding the another solvent to the electrolytic solution prepared by the first method; and (c) purging only the nonaqueous organic solvent used in the step (a) from the electrolytic solution.
According to the invention, there is provided another method for producing a lithium cell's electrolytic solution containing a solute of lithium hexafluorophosphate, which another method has steps (a) and (b) that are respectively analogous to the above-mentioned steps (b) and (c). In fact, the another method comprises steps of (a) adding a second nonaqueous organic solvent used for a lithium cell, which second solvent is preferably a cyclic carbonic acid ester, to an electrolytic solution containing a solute of lithium hexafluorophosphate dissolved in a first nonaqueous solvent that is preferably a monofunctional chain compoung; and (b) purging only the first nonaqueous solvent from the electrolytic solution.
According to a second aspect o
Itou Hisakazu
Kawashima Tadayuki
Sasaki Hiromi
Sato Keiji
Takahata Mituo
Anthony Joseph D.
Central Glass Company Limited
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
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