Chemistry of inorganic compounds – Phosphorus or compound thereof – Halogen containing
Reexamination Certificate
2001-03-23
2002-12-31
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Phosphorus or compound thereof
Halogen containing
Reexamination Certificate
active
06500399
ABSTRACT:
The present invention relates to a process for manufacturing lithium hexafluorophosphate (LiPF
6
).
Lithium hexafluorophosphate is used mainly as an electrolyte in rechargeable batteries.
In this application, the current specifications regarding the weight contents of the main impurities are as follows:
hydrogen fluoride (HF)<200 ppm
lithium fluoride (LiF)<1000 ppm.
The contents of phosphorus oxyfluorides (O=PF
3
, O=PF
2
OH) are not explicitly indicated, but it is considered that their sum should be less than 1000 ppm.
In the rechargeable batteries industry, it has been noted that traces of acid impurities (labile protons H
+
) are harmful to the electrochemical behaviour of the battery, in particular to the formation of the passivation layer at the interface of the lithium electrode/electrolyte or Li
x
C
6
/electrolyte, which has the consequence of adversely affecting the capacitance of the battery and its lifetime (number of charging and discharging cycles).
The problem arises of industrially obtaining LiPF
6
with a very low content of acid impurities in order to obtain a high-quality electrolyte.
Virtually all of the known processes for synthesizing LiPF
6
involve a step in which the crystalline LiPF
6
is either dissolved or placed in contact with a large excess of liquid HF. The strategy then consists in obtaining, by crystallization, large (1 to 3 mm) LiPF
6
crystals tending towards being as perfect as possible (quasi-monocrystals) in order to minimize the amounts of HF which are dissolved and/or occluded therein.
Thus, in Japanese patent application 60-251 109, PCl
5
is added to a solution of LiF in liquid anhydrous HF in order to obtain LiPF
6
in solution, and the HF is then evaporated off slowly. The product obtained is in the form of crystals 1 to 3 mm in size, accompanied by a white powder containing LiF. Example 1 indicates the production of 40 g of LiPF
6
in a yield of 65.1% and a purity of at least 99%. However, the methods for the analytical determination of this purity are not indicated.
Japanese patent application 4-175 216 teaches the production of gaseous PF
5
by reacting PCl
5
with anhydrous HF in large excess at −30° C. to give firstly white crystals of HPF
6
in HF, and the concomitant evaporation of the hydrogen chloride (HCl) produced in the reaction. The temperature is then raised to −10° C. to give gaseous PF
5
according to the reaction:
HPF
6
→PF
5
+HF
The gaseous PF
5
thus obtained is introduced continuously into a solution of LiF.HF in HF in order to obtain LiPF
6
crystals with a uniform diameter of 2 to 3 mm. These crystals are separated from the filtrate by an ordinary technique which is not specified. The yield is 70% and the purity indicated is 99.98%. The analytical methods indicated for the lithium and phosphorus are, respectively, atomic absorption spectrometry and absorptiometry. On the other hand, the HF assay method is not indicated.
Japanese patent application 5-279 003 discloses a process for synthesizing LiPF
6
which can be used as an electrolyte in rechargeable batteries and which contains no phosphorus oxyfluoride (O=PF
3
). PCl
5
is reacted with HF to give a gaseous mixture of PF
5
and HCl. This mixture is cooled to between −40° C. and −84° C. in order to remove the O=PF
3
by separation, and is then introduced into a solution of LiF in HF to form LiPF
6
. The HF is distilled off under nitrogen in order to concentrate the solution, and this solution is then cooled to between 0° C. and −20° C. to give LiPF
6
crystals. These crystals are separated out by filtration and then flushed with nitrogen at between 40° C. and 80° C. to remove the residual HF. Examples 1, 2 and 3 indicate respective yields of 65.6%, 62.3% and 64.9% crystalline LiPF
6
having respective residual contents of acid compounds, assayed by titrimetry in aqueous medium, of 11, 12 and 12 ppm of H
+
, which corresponds for a person skilled in the art to 220, 240 and 240 ppm of HF.
Japanese patent application 6-56413 discloses a process comprising the following steps:
A PF
5
+5HCl gaseous mixture is generated by reacting solid PCl
5
with HF gas at between 60 and 165° C. The gaseous mixture thus obtained is introduced into a reactor-crystallizing vessel containing LiF dissolved in liquid HF. The crystals obtained are dissolved by raising the temperature to ambient temperature and the solution obtained is then crystallized by lowering the temperature to −20° C. to give crystals with a particle size of 1 to 3 mm. The crystals are collected and then dried under reduced pressure. Example 1 indicates a yield of 35% for a purity of at least 99%. The mother liquors from Example 1 which still contain 113 g of LiPF
6
are recycled in Example 2 with a fresh amount of LiF to give LiPF
6
in a yield of 95% calculated relative to the fresh amount of LiF and an indicated purity of at least 99%. The analysis methods for establishing the purity are not mentioned.
Japanese patent application 6-298 506 discloses a process for recrystallizing LiPF
6
in HF, comprising a final step of drying under vacuum or under a stream of dry nitrogen at between 60° C. and 130° C. The residual content of acid impurities indicated in Examples 1, 2 and 3 is, respectively, 150, 100 and 60 ppm, which is interpreted by a person skilled in the art as being an HF content. The method for determining these contents is not indicated.
Japanese patent application 9-268 005 discloses a process for crystallizing LiPF
6
in HF, comprising a final step. of drying. Examples 1 and 3 indicate respective contents of free acid of 100 and 99 ppm for yields which are not indicated. To a person skilled in the art, the free acid is HF. The contents of insoluble residues in these same examples are, respectively, 0.08% and 0.07% by weight. In addition, the method for determining these contents is not dscribed.
In summary, since the above prior art does not describe the methods for analysing the residual contents of HF and of phosphorus oxyfluorides, it does not allow a person skilled in the art to check by experimentation the values indicated.
Moreover, patent application WO 98/06666 discloses a process in which LiF is reacted with PCl
5
or POCl
3
at a reaction temperature of from −20° C. to 300° C. for 0.1 to 10 h to give LiPF
6
according to:
a) PCl
5
+6LiF→5LiCl+LiPF
6
b) 4POCl
3
+18LiF→12LiCl+Li
3
PO
4
+3LiPF
6
and LiPF
6
is isolated in solution form from the mixture: ethers, nitriles, esters, sulphones, carbonates, halogenated hydrocarbons and/or tertiary amines are used as solvent.
The purity obtained (Example 1) is only 99.8%, since 0.2% chloride remains in the LiPF
6
. Furthermore, the analysis methods are not described.
The aim of the present invention is to provide a process for manufacturing LiPF
6
with a residual content of HF of less than 20 ppm (lower limit of detection for the analytical method) by reacting LiF and PF
5
, in a yield, relative to the LiF reacted, of greater than 99%.
The process proposed must also be industrial to allow the manufacture of large amounts of LiPF
6
with an HF content of less than 20 ppm.
These objectives are achieved by the process for manufacturing LiPF
6
according to the invention, by reacting LiF with PF
5
in a liquid, characterized in that this liquid is sulphur dioxide (SO
2
).
Liquid SO
2
replaces liquid HF as reaction medium and thus makes it possible to avoid all the technical problems associated with the removal of HF from the solid LiPF
6
.
The liquid containing the LiF and PF
5
is advantageously at a temperature below 40° C.
This temperature preferably ranges from −10° C. to +10° C.
The PF
5
/LiF molar ratio is advantageously greater than or equal to 1.05. This excess of PF
5
avoids the reverse reaction of decomposition of the LiPF
6
into PF
5
and LiF.
Advantageously, the PF
5
which reacts with LiF in the. liquid SO
2
is derived from a gaseous mixture containing hydrogen chloride (HCl). Specifically,
Atofina
Langel Wayne A.
Millen White Zelano & Branigan P.C.
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