JPH11317232A - Flame retarder of electrolyte for lithium battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolyte having flame retardance while maintaining battery characteristics by including trialkyl phosphate. SOLUTION: Trialkyl phosphate giving flame retardance to an electrolyte is represented by formula I. In the formula, R1 -R3 are the same or different straight-chain or branched alkyl group having 1-4 carbon atoms. Specifically trimethyl phosphate and dimethyl phosphate may be mentioned. The content of the phosphoric ester in the electrolyte depends on the capacity requirement of a battery, but when the whole amount of the solvent is replaced by the phosphoric ester, the flame retardance of the battery is maximized. For increasing the flame retardance by adding to the electrolyte as an assistant solvent, 15 wt.% or more phosphoric ester is preferable, and addition of 30 wt.% or more phosphoric ester is more preferable. As the solvent for mixing, ethylene carbonate, for example, is mentioned.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an electrolytic solution used for a lithium battery. According to the present invention, a highly safe lithium battery can be obtained.

[0002]

2. Description of the Related Art Conventionally, lithium batteries have been used as electrolytes such as propylene carbonate, γ-butyrolactone,
A solution in which a solute such as lithium perchlorate, lithium borofluoride, lithium phosphofluoride, or lithium trifluoromethanesulfonate is dissolved in a single solvent such as 1,2-dimethoxyethane or a mixed solvent thereof is used.

On the other hand, as flame retardants, inorganic compounds such as antimony oxide and zinc borate, and organic compounds containing phosphorus or halogen in the molecule are known. However, when imparting flame retardancy to the electrolyte, electrical conductivity,
It is necessary that the basic performance as an electrolyte, such as the working potential range, the working temperature range, and the compatibility with the electrode material, be not hindered. For example, the above-mentioned inorganic compounds and halides are mostly solid substances, are insoluble in organic solvents, and lower the electric conductivity. Further, halogenated hydrocarbons such as methylene chloride, which are generally used as an organic solvent, have a low dielectric constant and lower electric conductivity, and therefore cannot be used as a solvent for an electrolytic solution.

[0004]

Since the above-mentioned lithium battery uses a very flammable solvent, when the battery is broken due to an internal short circuit or the like, a spark ignites the electrolyte and damages the equipment. And can lead to fire. In particular, in recent years, lithium batteries have been mounted on portable devices, and the safety of lithium batteries has become more and more important, and is becoming a social problem.

[0005]

Means for Solving the Problems The present inventor obtains an electrolyte exhibiting flame retardancy while maintaining the characteristics as a battery by using a lower phosphate as a solvent or a cosolvent for the electrolyte. The present invention was completed successfully.

That is, the present invention provides a compound represented by the general formula (I):

[0007]

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(Wherein, R _{1 to} R ₃ are linear or branched alkyl groups having 1 to 4 carbon atoms, which may be the same or different from each other). The present invention relates to a flame retardant for an electrolyte for a lithium battery; and to a method for flame retarding an electrolyte for a lithium battery, wherein the solvent for the electrolyte contains 15% by weight or more of the flame retardant.

A flame-retardant electrolytic solution can be obtained by using a solvent containing a phosphate ester in an electrolytic solution for a lithium battery in which a lithium salt is dissolved in an organic solvent. That is, the present invention provides a flame retardant and a flame retardation method for making an electrolyte for a lithium battery flame retardant.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The phosphate ester used is a trialkyl phosphate (I) represented by the following general formula, a monocyclic phosphate (II) and a bicyclic phosphate (III) in which alkyl groups are bonded to each other. It is.

[0011]

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(Wherein, R _{1 to} R ₄ are a linear or branched alkyl group having 1 to ₄ carbon atoms, and R _{1 to} R ₃ may be different from each other. Is a unit having one carbon atom in a linear or branched hydrocarbon group,
k, l, m, and n indicate the number of carbon atoms of the hydrocarbon group, and k =
2 to 8, 1, m, and n are integers of 0 to 12. )

Specific examples of the phosphate represented by formula (I) include trimethyl phosphate, dimethyl ethyl phosphate, methyl ethyl propyl phosphate, methyl diethyl phosphate, triethyl phosphate, tripropyl phosphate, and tributyl phosphate; ) As methylethylene phosphate and methyltrimethylene phosphate; and as represented by general formula (III):

[0014]

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And trimethylolethane phosphate. Among these, phosphate esters having a small molecular weight are preferable because they dissolve solutes well and have high electric conductivity. In particular, trimethyl phosphate is most preferable because it has the highest electric conductivity and the highest phosphorus content in the molecular structure, and therefore has high flame retardancy and does not catch fire.

The phosphate ester used in the present invention is a trialkyl phosphate represented by the general formula (I).

The proportion of the above phosphate ester in the electrolytic solution varies depending on the required performance of the lithium battery, but when the total amount of the solvent is phosphate ester, a lithium battery having the highest flame retardancy can be obtained. In order to improve the flame retardancy by adding it to a conventional electrolytic solution as a cosolvent, good flame retardancy can be obtained by using 15% by weight or more, preferably 30% by weight or more.

Examples of the solvent for mixing the above phosphate ester include carbonate solvents such as ethylene carbonate, propylene carbonate and butylene carbonate;
Lactone solvents such as butyrolactone; and ether solvents such as 1,2-dimethoxyethane, 1,3-dioxolan, and tetrahydrofuran.

As the solute, LiClO ₄ , Li
BF ₄ , LiPF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ , L
iAlCl _{4 and the} like can be exemplified.

[0020]

EXAMPLES The present invention will be described more specifically with reference to reference examples and comparative examples.

As a method for evaluating the flame retardancy of the electrolytic solution, the burning rate of paper impregnated with the electrolytic solution was employed. The flash point was measured with a Pensky-Martens closed tester.

Example 1 An electrolytic solution prepared by dissolving LiBF ₄ at a concentration of 1 mol in trimethyl phosphate (electrical conductivity at 25 ° C. 5.6 mS / cm)
In addition, width 15mm, length 320mm, thickness 40μm, density 0.
6 g / cm ³ of manila paper was soaked for 1 minute and suspended vertically for 3 minutes to remove excess electrolyte. The manila paper impregnated with the electrolyte in this manner was horizontally fixed to a sample holder having support needles at intervals of 25 mm, and one end of the paper was ignited with a match.

COMPARATIVE EXAMPLE 1 An electrolytic solution in which 1 mol of LiBF ₄ was dissolved in γ-butyrolactone (electric conductivity at 25 ° C .: 7.8 mS / cm)
Manila paper was immersed under the same conditions as in Example 1, and the burning rate was determined from the burning time of 300 mm by the same ignition test. The burning rate was 10 mm / s.

Examples 2 and 3 In Example 1, the solvent was changed to triethyl phosphate (Example 2) and a mixed solvent of γ-butyrolactone and trimethyl phosphate at a weight ratio of 1: 1 (Example 3).
A similar ignition test was performed. All exhibited high flame retardancy.

Comparative Example 2 The same test was performed as in Comparative Example 1 except that the solvent was changed to propylene carbonate. As a result, the composition was flammable.

Reference Examples 1 to 7 As Reference Example 1, Manila paper impregnated with nothing, Reference Examples 2 to 7
A similar ignition test was performed on Manila paper impregnated with only a solvent.

The results of these ignition tests; and the flash point of each Example, Comparative Example and Reference Example (excluding Reference Example 1) samples;
Table 1 summarizes the electrical conductivities of the samples of Examples and Comparative Examples.
Shown in

The following abbreviations are used in Table 1. GBL: γ-butyrolactone PC: propylene carbonate TMP: trimethyl phosphate TEP: triethyl phosphate TBP: tributyl phosphate

[0029]

[Table 1]

[0030]

According to the present invention, it has become possible to obtain an electrolyte having excellent solubility of a lithium salt solute, suitable for an electrolyte of a lithium battery, and exhibiting excellent flame retardancy.

The electrolyte for a lithium battery of the present invention is used as an electrolyte for a highly safe lithium battery, particularly a portable lithium battery.

Claims (3)

[Claims] 1. A compound of the general formula (I): (Wherein, R _{1 to} R ₃ are linear or branched alkyl groups having 1 to 4 carbon atoms, which may be the same or different from each other). Flame retardant for battery electrolyte. 2. The flame retardant according to claim 1, wherein R _{1 to} R ₃ are methyl groups. 3. A method for flame retarding an electrolyte for a lithium battery, comprising the flame retardant according to claim 1 or 15% by weight or more in a solvent of the electrolyte.