Lithium fluoroalkylphosphates and their use as electrolyte...

Details Lithium fluoroalkylphosphates and their use as electrolyte... Lithium fluoroalkylphosphates and their use as electrolyte...

2000-05-18

2002-07-23

Chaney, Carol (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S199000, C252S062200

Reexamination Certificate

active

06423454

ABSTRACT:

The present invention relates to lithium fluoroalkyl-phosphates, a process for preparing them and their use as electrolyte salts in batteries, capacitors, supercapacitors and electrolytic cells.
The spread of portable electronic appliances such as laptop and palmtop computers, mobile telephones and video cameras, and thus also the need for light and powerful batteries has increased dramatically world-wide in recent years.
In view of this jump in the demand for batteries and the associated ecological problems, the development of rechargeable batteries having a long life is steadily increasing in importance.
Since the early 1990s, rechargeable lithium ion batteries have been commercially available. Most of these lithium ion batteries employ lithium hexafluorophosphate as electrolyte salt. However, this lithium salt is an extremely hydrolysis-sensitive compound having a low thermal stability, so that the corresponding lithium batteries can, owing to this property of the salt, only be produced by very complicated and thus also very costly methods.
The sensitivity of this lithium salt also reduces the life and the performance of these lithium batteries and also impairs their use under extreme conditions, e.g. at high temperatures.
There has therefore been no lack of attempts to provide lithium salts having improved properties. Thus, U.S. Pat. Nos. 4,505,997 and 9,202,966 describe the use of lithium bis(trifluoromethylsulfonyl)imide and lithium tris(trifluoromethylsulfonyl)methanide salts as electrolyte salts in batteries. Both salts display high anodic stability and form solutions having a high conductivity with organic carbonates. However, lithium bis(tri-fluoromethylsulfonyl)imide has the disadvantage that it does not sufficiently passivate the aluminium metal which functions as cathodic terminal lead in lithium batteries.
Lithium tris(trifluoromethylsulfonyl)methanide, on the other hand, is very expensive to produce and purify, so that the use of this salt as electrolyte salt in batteries greatly increases the production costs for such lithium batteries.
DE 196 411 38 teaches the use of lithium fluorophosphates preferably having perfluorinated or partially fluorinated ethyl and isopropyl groups as ligands. Although the thermal stability and the hydrolysis resistance of these lithium salts are significantly higher than those of lithium hexafluorophosphate, they hydrolyse within days in the presence of small traces of water, so that when using these electrolyte salts, too, the performance and life of the batteries based on these salts are reduced.
It is therefore an object of the invention to provide electrolyte salts which display no or only very slight signs of hydrolysis over a long period of time. A further object of the invention is to extend or improve the life and the performance of batteries, capacitors, supercapacitors and electrolytic cells.
This object is surprisingly achieved by the provision of lithium fluoroalkylphosphates of the general formula (I),
Li
+
[PF
x
(C
y
F
2y+l−z
H
z
)
6−x
]
−
  (I)
where
1≦x≦5
3≦y≦8 and
0≦z≦2y+1
and the ligands (C
y
F
2y+l−z
H
z
) can be identical or different, with the exception of compounds of the general formula (I′)
Li
+
[PF
a
(CH
b
F
c
(CF
3
)
d
)
e
]
−
  (I′)
in which a is an integer from 2 to 5, b=0 or 1, c=0 or 1, d=2 and e is an integer from 1 to 4, with the provisos that b and c are not simultaneously 0 and the sum of a+e is 6 and the ligands (CH
b
F
c
(CF
3
)
d
) can be identical or different.
Preference is given to lithium fluoroalkylphosphates of the general formula (I) in which 1≦x≦5, 3≦y≦8 and z=0.
Particular preference is given to the following lithium fluoroalkylphosphates of the general formulae (I):
Li
+
[F
6−x
P(CF
2
—CF
2
—CF
3
)
x
]
−
and
Li
+
[F
6−x
P(CF
2
—CF
2
—CF
2
—CF
3
)
x
]
−
in which 1 ≦x≦3 in each case.
The lithium fluoroalkylphosphates of the general formula (I) can be used, either in pure form or in the form of their mixtures, as electrolyte salts in primary and secondary batteries, capacitors, supercapacitors and electrolytic cells. It is also possible to use the lithium fluoroalkylphosphates of the invention together with further lithium salts known to those skilled in the art as electrolyte salt. The lithium fluoroalkylphosphates of the invention are preferably used in pure form as electrolyte salt, since particularly good reproducibility of the electrochemical properties can be achieved in this way.
The invention likewise provides a process for preparing the novel lithium fluoroalkylphosphates of the general formula (I), which is characterised in that at least one compound of the general formula
H
m
P(C
n
H
2n+1
)
3−m
  (III),
OP(C
n
H
2n+1
)
3
  (IV),
Cl
m
P(C
n
H
2n+1
)
3−m
  (V),
F
m
P(C
n
H
2n+1
)
3−m
  (VI),
 Cl
o
P(C
n
H
2n+1
)
5−o
  (VII)
or
F
o
P(C
n
H
2n+1
)
5−o
  (VIII),
where in each case
0≦m≦2
3≦n≦8 and
0≦o≦4,
is fluorinated by electrolysis in hydrogen fluoride, the resulting mixture of fluorination products is fractionated by extraction, phase separation and/or distillation, preferably by fractional distillation, and the resulting fluorinated alkylphosphorane is reacted with lithium fluoride in an aprotic solvent or solvent mixture in the absence of moisture, and the resulting novel lithium fluoroalkylphosphate of the general formula (I) is purified and isolated by customary methods.
The electrolysis is preferably carried out at a temperature of from −20 to +40° C., particularly preferably from −10 to +10° C. and very particularly preferably at from −5 to +5° C.; the pressure is preferably from 0.5 to 3 bar, particularly preferably from 0.5 to 1.5 bar and very particularly preferably atmospheric pressure.
The applied voltage during the electrolysis is preferably from 4 to 6 V, particularly preferably from 4.5 to 5.5 V, and the current density is preferably from 0.2 to 5 A/dm
2
, particularly preferably from 0.2 to 2 A/dm
2
and very particularly preferably from 0.5 to 1.5 A/dm
2
.
The compounds of the general formulae (V) and (VII) can also be reacted with inorganic fluorinating agents, preferably with SbF
3
, SbF
5
, MoF
5
, VF
5
or mixtures thereof so as to replace the chlorine atoms by fluorine atoms prior to the electrolysis in hydrogen fluoride. The reaction conditions under which these fluorinations are carried out are known to those skilled in the art.
The reaction of the fluorinated alkylphosphorane with lithium fluoride is preferably carried out at a temperature of from −35 to 60° C., particularly preferably from −20 to +50° C. and very particularly preferably from −10 to 25° C.
Solvents used for the reaction of the fluorinated alkylphosphoranes with lithium fluoride are preferably carbonates, nitrites, ethers, esters, amides, sulfones or mixtures thereof.
Particular preference is given to using solvents or solvent mixtures which are suitable for direct use in a primary or secondary battery, a capacitor, a supercapacitor or an electrolytic cell, for example dimethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, ethyl methyl carbonate, methyl propyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, methyl acetate, y-butyrolactone, ethyl acetate, methyl propionate, ethyl propionate, methyl butyrate, ethyl butyrate, dimethylsulfoxide, dioxolane, sulfolane, acetonitrile, acrylonitrile, tetrahydrofuran, 2-methyltetrahydrofuran or mixtures thereof. The electrolytes obtained in this way are suitable for use in primary batteries, secondary batteries, capacitors, supercapacitors and electrolytic cells and are likewise provided by the present invention.
The concentration of the lithium fluoroalkylphosphate(s) of the invention in these elect

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