JPS63119170A - Electrolyte for use in lithium battery - Google Patents

Abstract

PURPOSE:To obtain an electrolyte having good electrical conductivity and charging and discharging efficiency for use in lithium battery by dissolving lithium salt in a specific organic solvent. CONSTITUTION:An electrolyte for use in lithium battery, prepared by dissolving lithium salt in organic solvent, wherein ethylene carbonate, propylene carbonate or mixture thereof and 2-methyl tetrahydrofurane and a mixed solvent with nonproton polar solvent with viscosity of under 2.5 cP at 20 deg.C are used as a main ingredient of organic solvent. 0.5-2.5 mole/L of lithium salt is preferable to add to the mixed solvent. In order to improve solubility of a solute, a small amount of additives can be used. As the additives, hexamethylphosphoric triamide, N, N, N', N'-tetramethyl ethylene diamine and the like are used.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an electrolyte for lithium batteries, and more particularly to an electrolyte for use in lithium secondary batteries.

[Technical background of the invention]

Batteries that use lithium as the negative electrode active material (hereinafter referred to as lithium batteries) are being researched as small, high-energy-density batteries, and batteries that use manganese dioxide, graphite fluoride, etc. as the positive electrode active material are already on the market. . However, these commercially available lithium batteries are primary batteries, and at present, a rechargeable and dischargeable lithium secondary battery for practical use has not been realized.

In order to convert a lithium battery into a secondary battery, there are many problems that need to be solved, such as the selection of a positive electrode active material and the battery construction method.

In particular, the selection of electrolyte is an important issue. From a practical standpoint, it is desirable to use a non-aqueous electrolyte in a lithium secondary battery that operates at room temperature, but the disadvantage is that the conductivity of the electrolyte is one to two orders of magnitude lower than that of the aqueous solution used in conventional battery systems. There is.

Therefore, in order to improve the discharge utilization rate of batteries, it is essential to improve the conductivity of the electrolyte. At the same time, in order to apply it to secondary batteries, high charging and discharging efficiency of lithium in the non-aqueous electrolyte is required. That is, the electrolytic solution used in a lithium secondary battery needs to satisfy two requirements at the same time: (1) having high conductivity and (2) having high lithium charging and discharging efficiency.

An ethylene carbonate/2-methyltetrahydrofuran mixed solvent electrolyte has been proposed as an electrolyte that achieves high conductivity and high lithium charge/discharge efficiency (1981).
(Patent Application No. 245718). However, under special usage conditions of lithium batteries where only the discharge characteristics at low temperatures below 0°C are a problem, or in lithium batteries using positive electrode active materials in which the diffusion of Lf" ions in the solid phase is extremely fast, electrolysis is difficult. When the diffusion rate of ions in the liquid greatly affects the discharge or charge current value, or under usage conditions where a high acquired current value is required rather than an extremely long charge/discharge life, the ethylene carbonate/ Although there is a need for further improvement in the conductivity characteristics of the 2-methyltetrahydrofuran mixed solvent electrolyte, this problem has not been solved.

[Summary of the invention]

The first object of the present invention is to improve an ethylene carbonate/2-methyltetrahydrofuran mixed solvent electrolyte, which has a lithium charge/discharge efficiency almost equivalent to that of the above-mentioned electrolyte and a higher conductivity. An object of the present invention is to provide an electrolytic solution for secondary batteries.

A second object of the present invention is to provide an electrolytic solution for use in lithium batteries that can be used satisfactorily under special conditions such as use at low temperatures.

A third object of the present invention is that it can be used in lithium batteries where the diffusion rate of Li"" ions greatly affects the discharge or charge current value, or where a high acquisition current is required rather than a long charge/discharge life. An object of the present invention is to provide an electrolyte for lithium secondary batteries.

Other objects according to the invention will become more apparent from the following description.

Therefore, the electrolytic solution for lithium secondary batteries according to the present invention is an electrolytic solution for lithium batteries in which a lithium salt is dissolved in an organic solvent, and the organic solvent of the electrolytic solution is: i) ethylene carbonate, propylene carbonate, or ethylene carbonate; and propylene carbonate, and 1i) 2-methyltetrahydrofuran, and 1ii) with a viscosity of 2 at 20°C.
．． It is characterized in that its main component is a mixed solvent with an aprotic polar solvent of less than 5 cP (centipoise).

According to the present invention, it is possible to provide an electrolyte for lithium secondary batteries that has almost the same lithium charging and discharging efficiency as the conventional ethylene carbonate/2-methyltetrahydrofuran mixed solvent electrolyte and has higher conductivity. There are advantages. For this reason, for lithium secondary batteries used under special conditions, where the diffusion rate of Li" ions greatly affects the discharging or charging current value, or for lithium secondary batteries that have a long charging/discharging life, the obtained current is Another advantage is that it can be effectively used in lithium batteries, which require high performance.

[Detailed description of the invention]

The present invention will be explained in further detail.

In a lithium secondary battery, the negative electrode active material is lithium or a lithium alloy that allows lithium ions to be discharged, the positive electrode active material is a material that electrochemically performs a reversible reaction with lithium ions, and the electrolyte is an organic According to the present invention, the organic solvent is i) ethylene carbonate, propylene carbonate, or a mixed solvent of ethylene carbonate and propylene carbonate, and 1i) 2-methyltetrahydrofuran, and 1ii). ) A mixed solvent with a non-protic polar solvent having a viscosity of 2.5 cP at 20° C. is used as the main component.

Ethylene carbonate, propylene carbonate, or a mixed solvent of ethylene carbonate and propylene carbonate, and a mixed solvent-based electrolyte of 2-methyltetrahydrofalane have particularly high lithium charging and discharging efficiency, and the conductivity is also higher than that of single-solvent electrolytes. Show value. However, under special battery usage conditions where an extremely long charge/discharge cycle life is not required and a particularly high acquired current value is required, ethylene carbonate/2-methylna-1-lahydrofuran mixed solvent There is a possibility that the electrical conductivity of the system electrolyte may not necessarily be sufficient. Usually, the solute (electrolyte) used in the electrolyte for lithium batteries is a Lewis acid salt of lithium, such as LiCl04, LiA
sF t3 , LiBF4 , LiPFB, LiCF
3SO3, L4CF3 CO2, LiAlCl4, etc. are known. In these lithium salts, the anion has a large radius and the electrostatic Coulomb attraction between the cation and anion is small, so the degree of ion dissociation is high. Therefore, it is considered effective to reduce the viscosity of the solvent and improve the mobility of ions in order to improve the conductivity. The viscosity at 20°C is approximately 2.5 cP for propylene carbonate, and approximately 2.4 cP for a mixed solvent of propylene carbonate and ethylene carbonate (volume mixing ratio, 1:4).
P (ethylene carbonate has a melting point of 36° C. and is solid at 20° C.), and it is considered that mixing a solvent with a viscosity of less than 2.5 cP is effective in improving the electrical conductivity. However, the disadvantage is that the mixing of a low viscosity solvent causes the lithium charging and discharging efficiency to be extremely lower than that of the ethylene carbonate/2-methyltetrahydrofuran mixed system. However, as shown in the Examples below, by using the mixed solvent of the present invention, compared to the ethylene carbonate/2-methyltetrahydrofuran mixed solvent electrolyte, lithium charging and discharging efficiency was not significantly deteriorated. , it is possible to provide an electrolyte solution for lithium secondary batteries with improved conductivity.

Examples of the above-mentioned low viscosity solvent include methyl formate, methyl acetate, tetrahydropyran, 1,3-dioxolane, 1
, 2-dimethoxyethane, 1,2-jethoxyethane,
2-methyl-1,3-dioxolane, 4-methyl-1,
It is possible to use one or more selected from 3-dioxolane, acetonitrile, tetrahydropyran, etc. The volumetric mixing amount of these low viscosity solvents is less than 80% of the total solvent amount of the electrolytic solution, particularly preferably 30 to 50%.
%. If the mixing amount is 80% or more, there is a possibility that the charging and discharging efficiency of lithium will be extremely low.

The lithium salt dissolved in the above-mentioned mixed solvent is not fundamentally limited in the present invention.

For example, LiAsFe, LiCl0a, Li5bFe
,, LiPF6, LiBF4, LiAlCl4, L
iCF3SO3, LiCF3CO2,1, iz R10
One or more lithium salts such as C110 can be effectively used.

Such a lithium salt is added to the mixed solvent in an amount of 0.5 to 2.
It is preferable to add 5 mol/β (M). If it is less than 0.5M, the charging and discharging characteristics of lithium will be significantly deteriorated, and if it exceeds 2.5M, it may become difficult to dissolve the solute.

The organic solvent of the electrolyte used in the present invention is i) ethylene carbonate or propylene carbonate or a mixed solvent of ethylene carbonate and propylene carbonate, 1i) 2-methyltetrahydrofuran, and 1ii) a viscosity of 2. The main component is a mixed solvent with a 5cP non-grooved aprotic polar solvent.

A small amount of additive can be used in such a mixed solvent for reasons such as improving the solubility of the solute. Such additives include, for example, hexamethylphosphoric triamide, N, N, N', N
One or more compounds selected from '-tetramethylethylenediamine, diglyme, triglyme, tetraglyme, etc. can be used.

The negative electrode active material used in the lithium battery using the electrolyte according to the present invention is basically not limited, and any negative electrode active material used in conventional lithium batteries, that is, lithium or lithium ions, can be discharged. Lithium alloys can be used.

Similarly, the positive electrode active material used in the present invention is not fundamentally limited, and must be a material that electrochemically undergoes a reversible reaction with the positive electrode active material used in conventional lithium secondary batteries, that is, lithium ions. I can do it.

Examples will be described below.

Example 1 Ethylene carbonate (hereinafter abbreviated as EC) and 2-methyltetrahydrofuran (hereinafter 2MeT) were used as electrolytes.
(abbreviated as IIF) and tetrahydrofuran (hereinafter referred to as TIIF)
) and a mixed solvent (volume mixing ratio, 1:1:1) of 1.5
A product was prepared by melting LiAsF 6 of M (M: mol/β). The electrical conductivity of the electrolytic solution at -10 to 30°C is shown in FIG. In FIG. 1, as a reference example for showing the effects of the present invention, 1.5MLiAsF e E
The conductivity of C/2MeTHF (volume mixing ratio, l/I) is also shown. From FIG. 1, it can be seen that EC/2Me according to the present invention
The TllF/TIIF mixed electrolyte is EC/2MeT
It can be seen that the conductivity is higher than that of the IIF type.

Example 2 As electrolyte, 1.5MLiAsF e -F, C/2
MeTIIF/TIIF (volume mixing ratio, 1/1/'1
) was prepared, and the lithium charge/discharge efficiency was determined by the method described below.

The charge/discharge efficiency (Ea) was measured as follows by preparing a battery using platinum as a working electrode and lithium as a counter electrode and a reference electrode. The measurement was first carried out using f & (4
,8C/ctJ), a part of this precipitated lithium (
A cycle test was repeated in which the battery was discharged at a capacity of 1.2 C/cJ) as Li1 ions, and then further discharged at a capacity of 1.2 C/cnl. The charge/discharge efficiency ([a) is determined by a change in the potential of the platinum electrode, and if n is the number of cycles showing an apparent efficiency of 100%, the Ha can be determined from the following formula (T).

Ea= (1,2-(4,8-1,,2)/n)
/1.2 X100 (%) --(T) The results are shown in Table 1. Table 1 shows 1.5MLi as a comparative example.
AsF e -EC/2MeTIIF (volume mixing ratio.

1/1) is also shown. EC/2MeTIIF/TIIF mixed system [1st
Table (A)] shows the EC/2MeTIIF mixed system [Table 1 (B)
)].

Example 3 As electrolyte solution, 1.5ML+As'F e -EC/2
MeTHF/methyl formate (hereinafter referred to as MP) (volume mixing ratio, 415/6) was prepared, and the electrical conductivity was measured. Figure 2 shows the results. As can be seen from FIG. 2, by using the EC/2MeTHP/MP mixed system according to the present invention, higher conductivity can be obtained than the EC/2MeTHF mixed system.

Example 4 1.5M LiAsF s -EC/2M as electrolyte
eTtlF/MP (volume mixing ratio, 415/6) was produced, and the lithium charge/discharge efficiency was measured in the same manner as in Example 2. The results are shown in Table 1. EC/2MeTHF
/ MP mixed system [Table 1 (C)] is EC/2Me↑I
It can be seen that the lithium charging/discharging efficiency is almost the same as that of the IP mixed system [Table 1 (B)].

Example 5 As electrolyte, 1.5M LiAsF s EC/2M
eTllF/acetonitrile (hereinafter referred to as AN) (volume mixing ratio, 415/6") was prepared and its conductivity was measured. The results are shown in Figure 3. As can be seen from Figure 3, EC/2MeT according to the present invention ) It can be seen that higher conductivity can be obtained by using the IP/AN mixed system than the EC/2MeTtlP mixed system.

Example 6 As electrolyte, 1.5M LiAsF e -EC/2M
The charging and discharging efficiency of lithium was measured in the same manner as in Example 2, except that eTHF/8N (volume mixing ratio, 15/6) was used. The results are shown in Table 1. EC/2MeTII
The F/AN mixed system [Table 1 (D)] is EC/2MeT
It can be seen that the lithium charging/discharging efficiency is almost the same as that of the llF mixed system [Table 1 (B)].

Example 7 As the electrolyte, ■ IMLiAsF s -EC/propylene carbonate (hereinafter referred to as PC) /2MeTHF/T
IIF (volume mixing ratio, 1/1/2/2) or ■IMLiAsPe-EC/2MeTHF/L
, 2-dimethoxyethane (hereinafter referred to as D liver) (volume mixing ratio,
1/1 /1), or ■IMLiAsP
e-PC/2MeTHF/1.3-dioxolane (hereinafter referred to as DOL) (volume mixing ratio, 3/3/2) or ■
1.5MLiAsFs-EC/2MeTllF/methyl acetate (hereinafter referred to as methyl acetate) (volume mixing ratio, 1/1/1)
Or ■1.5MLi8sF e to EC/2MeT
Example 2 except that HF/1.2-jethoxyethane (hereinafter referred to as D) (volume mixing ratio, 1/1/1) was used.
Measure the charging and discharging efficiency of lithium in the same manner as above, and
The electrical conductivity was also measured. These results are shown in Table 2. From Table 2, the electrolytes of the above ■ to ■ according to the present invention are I
It can be seen that it exhibits higher conductivity than the IC/2MeTIIP mixed system, and also exhibits lithium charging and discharging efficiency almost equivalent to the EC/2MeTIIF system.

Table 1 (blank below) Table 2 □ [Effects of the invention] As explained above, according to the electrolyte for lithium batteries according to the present invention, i) ethylene carbonate, propylene carbonate, or a mixture of ethylene carbonate and propylene carbonate; 1 combined solvent, and ii) 2
- By using an electrolytic solution mainly composed of a mixed solvent of methyltetrahydrofuran and 1ii) an aprotic polar solvent with a viscosity of less than 2.5 cP at 20°C, the electrical conductivity is high and the lithium charging/discharging efficiency is good. It is possible to provide an electrolyte solution for lithium batteries.

[Brief explanation of the drawing]

FIGS. 1 to 3 are diagrams showing the relationship between the electrical conductivity and temperature of the electrolytic solution according to the present invention. Applicant's agent Masaki Amemiya (I8) Figure 1 Figure 2

Claims (1)

[Claims] (1) In an electrolytic solution for lithium batteries in which a lithium salt is dissolved in an organic solvent, the organic solvent is i) ethylene carbonate, propylene carbonate, or a mixed solvent of ethylene carbonate and propylene carbonate, and ii) 2-methyltetrahydrofuran. and iii)
An electrolytic solution for lithium batteries, characterized in that the main component is a mixed solvent with an aprotic polar solvent having a viscosity of less than 2.5 cP at 20°C.