Lithium fluorophosphates and their use as conducting salts

Details Lithium fluorophosphates and their use as conducting salts Lithium fluorophosphates and their use as conducting salts

2000-03-08

2001-04-03

Diamond, Alan (Department: 1753)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S189000, C429S324000, C429S200000, C429S203000, C252S062200, C423S301000, C423S323000, C423S179500, C568S016000

Reexamination Certificate

active

06210830

ABSTRACT:

The invention relates to novel lithium fluorophosphates of the general formula
Li
+
[PF
a
(CH
b
F
c
(CF
3
)
d
)
e
]
−
,  (I)
wherein
a is 1, 2, 3, 4 or 5,
b is 0 or 1,
c is 0, 1, 2 or 3,
d is 0, 1, 2 or 3 and
e is 1, 2, 3 or 4,
with the condition that the sum of a+e is equal to 6, the sum of b+c+d is equal to 3 and b and c are not simultaneously 0, with the proviso that the ligands (CH
b
F
c
(CF
3
)
d
) may be different,
a method for producing said compounds,
their use in electrolytes, and also lithium batteries produced with said electrolytes.
The invention also relates to compounds of the general formula
[PF
a
(CH
b
F
c
(CF
3
)
d
)
e
]  (Ia),
wherein, as in formula (I),
b is 0 or 1,
c is 0, 1, 2 or 3,
d is 0, 1, 2 or 3 and
e is 1, 2, 3 or 4,
but
a is 1, 2, 3 or 4,
which are needed as intermediates for producing compounds of the formula (I).
Normally, lithium hexafluorophosphate is used as conducting salt in lithium secondary batteries. A disadvantage is that this salt has relatively little resistance to hydrolysis. Various experiments were therefore performed to find a replacement for said salt. For example, WO 88/03331 describes salts of cyclic perfluoroalkanebis(sulphonyl)imides, including also their lithium salts, which can be used as conducting salts in nonaqueous electrolytes for lithium secondary batteries. As experiments have revealed, such compounds can, however, be produced only at great expense and have to be freed from undesirable byproducts after the synthesis. The purification of the salts is indispensable since reproducible properties are indispensable for application as constituents of battery electrolytes.
The object of the invention is therefore to provide suitable conducting salts for electrolytes for use in lithium batteries. The object of the invention is also to provide a process by which the conducting salts according to the invention can be produced easily and inexpensively.
The object according to the invention is achieved by novel lithium fluorophosphates of the general formula
Li
+
[PF
a
(CH
b
F
c
(CF
3
)
d
)
e
]
−
,  (I),
wherein
a is 1, 2, 3, 4 or 5,
b is 0 or 1,
c is 0, 1, 2 or 3,
d is 0, 1, 2 or 3 and
e is 1, 2, 3 or 4,
with the condition that the sum a+e is equal to 6, the sum b+c+d is equal to 3 and b and c are not simultaneously 0 and with the condition that the ligands (CH
b
F
c
(CF
3
)
d
) may be different,
which lithium fluorophosphates can replace the lithium hexafluorophosphate conventionally used as conducting salt in lithium secondary batteries, and can also be used as a mixture with the latter.
The invention therefore relates to the novel salts according to the invention of the formula (I) and also to a process for producing them and compounds of the general formula (Ia),
[PF
a
(CH
b
F
c
(CF
3
)
d
)
e
]  (Ia),
wherein, as in formula (I)
b is 0 or 1,
c is 0, 1, 2 or 3,
d is 0, 1, 2 or 3 and
e is 1, 2, 3 or 4,
but
a is 1, 2, 3 or 4,
which are needed as intermediates for producing the salts in accordance with the general formula (I).
Finally, the invention also relates to electrolytes which comprise the salts according to the invention and electrochemical cells produced using said electrolytes. Such electrochemical cells can be either primary or secondary batteries which comprise the lithium compounds according to formula (I).
The invention relates, in particular, to the following lithium salts
their use as conducting salts in electrolytes, electrolytes comprising them, and also lithium batteries in which said compounds are comprised as conducting salts.
To produce the salts according to the invention, in a first step, suitable monochloro- or fluoro-, dichloro- or difluoro-, chlorofluoroalkylphosphanes, chloromono-, chlorodi-, chlorotri-, or chlorotetraalkylphosphoranes, fluoromono-, fluorodi-, fluorotri- or fluorotetraalkylphosphoranes or trifluoromonohydroalkylphosphoranes are taken up in a solvent and electrochemically fluorinated in a known manner at a temperature of −15 to 20° C. under normal pressure. Hydrogen fluoride is suitable as a solvent for this reaction. The fluorination reaction is terminated when 90 to 150%, in particular 110 to 130%, of the theoretical quantity of electricity has been consumed. This is determined by coulometric measurement.
Depending on the duration of the reaction, product mixtures are obtained which comprise both compounds completely saturated with fluorine and those which are partially fluorinated. For example, the following products are obtained in the fluorination of chlorodiisopropylphosphane:
The actual salts according to the invention of the formula (I) are obtained by the fluorinated alkylphosphoranes (II) obtained in the first stage being taken up as a product mixture, preferably after distillative separation, under anhydrous conditions in a suitable aprotic, polar solvent, such as, for example, dimethyl ether, dimethoxyethanes or their mixtures and reacted with lithium fluoride to form compounds of the formula (I), depending on reactivity, at a temperature of −35 to 60° C., preferably adhering to room temperature.
In the electrolytes according to the invention, both the pure compounds of the formula (I) and the mixtures obtained by the fluorination reaction can be used. Preferably, the pure compounds are used to produce the electrolyte solutions because of the reproducibility of the electrolyte properties.
Surprisingly, it was found by experiments that compounds of the formula (I) are resistant to hydrolysis in aprotic polar solvents at room temperature and, to be specific, those compounds, in particular, whose alkyl radicals are completely saturated with fluorine. In this connection, the resistance to hydrolysis increases with the number of fluorine atoms in the molecule.
Aprotic polar solvents are to be understood as meaning solvents such as
nitriles
acetonitrile or benzonitrile,
ethers
diethyl ether, dimethoxyethane, tetra-
hydrofuran, dioxane or dimethyltetra-
hydrofuran,
esters
methyl or ethyl esters of formic acid,
acetic acid, propionic acid, and also
cyclic esters, such as butyrolactone, and
organic carbonates, such as, for example,
dimethyl carbonate, diethyl carbonate,
ethyl methyl carbonate, ethylene
carbonate or propylene carbonate,
amides
dimethylformamide, diethylformamide, N-
methylpyrolidine
or
sulphones
dimethyl sulphone, tetramethylene
sulphone or other sulpholanes.
The salts according to the invention have, in addition, an excellent solubility in said solvents and, in particular, the compounds completely saturated with fluorine have little hygroscopy.
Experiments have shown that these compounds are extremely stable. With dry storage in the solid state, perfluorinated compounds exhibit no decomposition at temperatures below 100° C. They prove to be thermally table even on increasing the temperature further. It is only at temperatures above 130° C. that they exhibit slight discolorations.
Even on storage in solution, for example in dimethoxyethane, no colour changes can be observed or decomposition products detected even after weeks.
For this reason, the compounds according to the invention, in particular the compounds of the formulae III, IV, V, VI and VII, are already excellently suited as conducting salts in nonaqueous electrolytes for lithium batteries.
Furthermore, electrolyte solutions which contain these compounds have remarkable chemical and electrochemical stability. In particular, oxidation of the corresponding anion cannot be detected prior to the lithium deposition.
Such electrolytes comprise, in addition to organolithium salts, such as the compounds according to the invention of the formula (I) one or more nonaqueous organic solvents and, optionally, further additives. If desired, in addition to the compounds according to the invention, already known lithium salts can also be added to the electrolyte as conducting salts. Further details on such electrolytes and the structure or mode of operation of lithium batteries are

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