Solvate of lithium hexafluorophosphate and pyridine, its prepara

Details Solvate of lithium hexafluorophosphate and pyridine, its prepara Solvate of lithium hexafluorophosphate and pyridine, its prepara

1998-01-23

1999-11-30

Langel, Wayne

Chemistry of inorganic compounds

Phosphorus or compound thereof

Halogen containing

546 22, 546347, C01B 2510, C07D21320, C07F 928

Patent

active

059937670

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

1. Technical Field
The invention relates to a novel compound constituted by a solvate of lithium hexafluorophosphate and pyridine usable for the preparation of lithium hexafluorophosphate.
It more particularly applies to the preparation of lithium hexafluorophosphate used as an electrolyte in lithium-carbon batteries.
At present lithium batteries are being extensively developed for various applications, particularly in electric vehicles, portable equipments such as portable telephones and camescopes, as well as in space. These batteries use an electrolyte constituted by one or more organic solvents containing in solution a lithium salt. Among the usable lithium salts, lithium hexafluorophosphate (LiPF.sub.6) is at present the most widely used, due to its high solubility in organic solvents, its conductivity and its safety.
2. Prior Art
The conventional method for the preparation of LiPF.sub.6 consists of reacting PF.sub.5 with LiF in anhydrous hydrofluoric acid, but the purity of LiPF.sub.6 prepared in this way is only 90 to 95%, whereas purities of at least 99% are required for use in a lithium battery in order to satisfy the LiPF.sub.6 storage stability and solubility requirements.
In order to overcome this difficulty, it is still possible to obtain LiPF.sub.6 of higher purity by the process described in U.S. Pat. No. 3,654,330. According to this process, lithium fluoride, anhydrous hydrofluoric acid and phosphorus pentafluoride PF.sub.5 are reacted to obtain impure LiPF.sub.6, which is then purified by reaction with acetonitrile. Thus, tetraacetonitrilolithium hexafluorophosphate Li(CH.sub.3 CN).sub.4 PF.sub.6 is produced, which then regenerates lithium hexafluorophosphate by heating in a partial vacuum. It is also possible to directly produce Li(CH.sub.3 CN).sub.4 PF.sub.6 by reacting PF.sub.5 in a suspension of LiF in acetonitrile.
This process makes it possible to obtain high purity lithium hexafluorophosphate, but it suffers from the disadvantage of requiring the use of PF which is a very difficult product.
Another way for obtaining alkali metal hexafluorophosphates has been investigated by Syed Mohamed et al in J. Fluorine Chem., 23, 1983, pp 509-514. According to this document, the starting product is pyridinium hexafluorophosphate C.sub.5 H.sub.5 NHPF.sub.6, which is then treated with an ammonium or alkali metal hydroxide in an aqueous medium, but this process has not made it possible to isolate solid LiPF.sub.6 from said medium. Moreover, the preparation of the starting product C.sub.5 H.sub.5 NHPF.sub.6 involves a low temperature reaction (-80.degree. C.) between the pyridine and the anhydrous hydrofluoric acid, followed by the reaction of the product obtained with phosphoryl chloride. This involves difficult performance using expensive reagents (POCL.sub.3), which is difficult to carry out on an industrial scale.
Lange et al in BER. 63B, 1058-70, 1930 describe the preparation of C.sub.5 H.sub.5 NHPF.sub.6 by reacting pyridine in acetic acid with ammonium hexafluorophosphate. However, they did not envisage using this pyridinium hexafluorophosphate for preparing lithium hexafluorophosphate.
Thus, none of the presently known processes makes it possible to prepare high purity lithium hexafluorophosphate LiPF.sub.6 under easily implementable conditions and without using onerous reagents.


DESCRIPTION OF THE INVENTION

The present invention relates to the preparation of high purity LiPF.sub.6 by a simple, uncomplicated process using inexpensive, commercial products, with as the intermediate a solvate of lithium hexafluorophosphate and pyridine. It also relates to the solvate and its preparation process.
The solvate complies with the formula: because it is stable in air at ambient temperature, whereas LiPF.sub.6 decomposes into PF.sub.5 and LiF and must be manipulated in a glove box. It also makes it possible to generate LiPF.sub.6 with a high purity level of 99.8% using simple processes.
According to the invention, this solvate can be prepared by a process comprising the following stag

REFERENCES:
patent: 3654330 (1972-04-01), Wiesboeck
patent: 4996320 (1991-02-01), Omemoto et al.
patent: 5378445 (1995-01-01), Salmon et al.
patent: 5616636 (1997-04-01), Avar et al.
Chemical Abstracts, vol. 100, No. 21, 21 Mai 1984, Columbus, Ohio, US; abstract no 174608c, Mohamed, K. er at.: "pyridinium poly (hydrogen fluoride)--a reagent for the preparation of hexafluorophosphates" XP002024671.

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