Electrolyte for lithium cells and method of producing the same

Details Electrolyte for lithium cells and method of producing the same Electrolyte for lithium cells and method of producing the same

2000-08-04

2002-05-07

Brouillette, Gabrielle (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S231950, C429S188000, C429S324000, C429S328000, C429S329000, C429S339000, C429S231100

Reexamination Certificate

active

06383688

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an electrolytic solution for lithium batteries. More specifically, the present invention relates to a method of removing water and free acids from an electrolytic solution for lithium batteries and to an electrolytic solution having a low water content and a low free acids content.
Prior Art
For lithium batteries, a non-aqueous electrolytic solution comprising a lithium electrolyte such as lithium hexafluorophosphate, LiPF
6
, in a non-aqueous organic solvent is used as an electrolytic solution. It is difficult to completely remove water contained in the solvent or in the electrolyte as an impurity, and water may further be introduced during storage of the electrolytic solution or a filling process of the electrolytic solution in batteries.
Further, a trace amount of free acids may be contained as impurities. Especially when an electrolyte which can be hydrolyzed or thermally decomposed with ease, such as LiPF
6
or the like, is used, hydrofluoric acid is produced by hydrolysis with a trace amount of water or thermal decomposition due to dissolution heat. This hydrofluoric acid not only decreases a capacity of a battery or degrades charge-discharge cycle characteristics, but also corrodes the inside of the battery.
As methods of removing water in an electrolytic solution, the use of metal oxide such as molecular sieves, phosphorus pentaoxide, activated alumina, and calcium oxide is described in Japanese Patent Application Laid-Open S59-9874; the use of molecular sieves of a lithium ion type is described in Japanese Patent Application Laid-Open S59-81869; and the use of activated alumina is described in Japanese Patent Publication H3-49180.
Meanwhile, to remove free acids, there are known the following methods: a method where acids are removed by adsorption with an adsorbent, such as aluminum oxide, contained in a battery, as described in Japanese Patent Application Laid-Open H4-28437 and Japanese Patent Application Laid-Open H5-315006; a method where acids are removed by distillation; a method where acids are removed by ammonium salt dissolved in an electrolytic solution, as described in Japanese Patent Application Laid-Open H3-119667; a method where acids are removed by neutralization with an alkaline agent such as lithium hydroxide and lithium hydride, as described in Japanese Patent Application Laid-Open H4-282563; and use is made of a metal fluoride, as described in Japanese Patent Application Laid-Open H8-321326.
However, the methods where water and free acids are removed by a solid powdery adsorbent contained in a battery are less preferred, because a design of the battery should be modified. The adsorption method with molecular sieves or the like has only a little effect of removing water or the like, when conducted alone, and an additional process for separating and recovering the used adsorbent is required.
Japanese Patent Application Laid-Open H1-286262 discloses a method of removing free acids by adding an organic lithium compound such as pentafluorophenyllithium to an electrolytic solution. The present inventors have found that additional generation of free acids is suppressed only in a short period of time.
Thus, the purpose of the present invention is to provide a method of removing water and free acids simultaneously from an electrolytic solution without a need of modifying designs of batteries and without separating and recovering an adsorbent used. Another purpose of the present invention is to provide a method where the effect of suppressing additional generation of free acids is maintained for a prolonged period of time. Still another purpose of the present invention is to provide an electrolytic solution where a water content and a free acids content are both low and to provide a lithium battery comprising the electrolytic solution.
DISCLOSURE OF THE INVENTION
The present invention is a method of preparing an electrolytic solution for a lithium battery, comprising dissolving a lithium electrolyte in a solvent comprising at least one organic solvent, characterized in that the method comprises steps of
(a) leading an inert gas through the solvent having a water content of 100 ppm or lower under heating of the solvent to vaporize water together with the solvent to thereby reduce the water content of the solvent, and
(b) dissolving the lithium electrolyte in the solvent while maintaining a temperature of the solvent at 20° C. or lower.
Preferably, the following step (c) follows after step (b),
(c) incorporating at least one lithium compound in the electrolytic solution, said lithium compound being selected from the group consisting of lithium amide compounds represented by the formula, LiNR
1
R
2
, lithium imide compounds represented by the formula, Li
2
NR
3
, lithium borohydride and derivatives thereof represented by the formula, LiBR
4
R
5
R
6
R
7
, organic lithium compounds represented by R
8
Li, lithium alkoxides represented by R
9
OLi, and lithium aluminum hydride and derivatives thereof represented by LiAlR
10
R
11
R
12
R
13
, wherein each of said R
1
-R
3
independently represents hydrogen or a hydrocarbon residue.
More preferably, each of said R
1
-R
13
is at least one independently selected from the group consisting of hydrogen, alkyl, aryl, and allyl.
The present invention also relates to an electrolytic solution for a lithium battery prepared by incorporating a lithium electrolyte in a solvent comprising at least one organic solvent, characterized in that the electrolytic solution has a water content of 3 ppm or lower and a free acids content, converted as hydrofluoric acid, of less than 1 ppm.
Further, the present invention relates to a lithium battery comprising the electrolytic solution obtainable by the method according to the present invention.
PREFERRED EMBODIMENTS OF THE INVENTION
The present method comprises (a) a step of leading an inert gas through an organic solvent having a water content of at most 100 ppm at a room temperature under heating of the organic solvent. If the initial water content of the organic solvent is higher than 100 ppm, a larger amount of the inert gas is required to flow under heating, which is not preferred in terms of time and costs. To make the water content 100 ppm or less, any method may be used such as adsorption with an adsorbent such as molecular sieves, usual distillation under atmospheric pressure or distillation under a sub-atmospheric pressure, or purging with an inert gas. The water content can be determined by, for example, a Karl-Fisher method which will be described later in the specification.
Examples of the inert gas used in the present invention include nitrogen gas, helium gas, and argon gas, among which nitrogen gas is preferred in costs. Preferably, the inert gas contains substantially no water so as to show a dew point of −40° C. or lower, preferably −60° C. or lower.
The inert gas is led through a tube resistant to the organic solvent, such as a glass tube and a stainless tube. A flow rate may be set, depending on the amount of the solvent to be treated and a size of a container, but typically may be in the range of from 3 to 5 liters/minute for treating about 4 liters of the organic solvent.
By leading an inert gas through the organic solvent, the water content is made preferably 60 ppm or lower. Practically, the water content of from 40 to 60 ppm can be achieved. To make the water content less than 40 ppm, a larger volume of the inert gas and a longer treatment time are required, which may be disadvantageous in costs.
Examples of the organic solvent used in the present invention include dimethyl carbonate, diethyl carbonate, 1,2-dimethoxyethane, ethylene carbonate, methylethyl carbonate, propylene carbonate, &ggr;-butyrolactone, sulfolane, tetrahydrofuran, 2-metylhydrofuran, dimethyl sulfoxide, dioxolan, dimethylformamide, acetonitrile or a mixture of two or more of them. Dimethyl carbonate and/or propylene carbonate are preferably used for their dielectric constants and viscosities.
When a mixture of two kinds of the orga

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