JPS63114075A - Electrolyte for lithium secondary battery - Google Patents

Abstract

PURPOSE:To increase the charge-discharge efficiency of lithium and to increase the conductivity of an electrolyte by using a mixture of acetonitrile and ether as the solvent of the electrolyte of a lithium battery. CONSTITUTION:1.5M (M: mole/l) liAsF6 is dissolved in a mixed solvent (volume ratio is 1:1) of acetonitrile (AN) and tetrahydrofuran (THF) to prepare an electrolyte. The charge-discharge performance of a lithium electrode is increased and the conductivity of the electrolyte for lithium battery is also increased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application 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.

[Prior art to 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 simultaneously satisfy two requirements: (1) to have high electrical conductivity, and (2) to have high lithium charging and discharging efficiency.

Acetonitrile has low viscosity and high ion mobility, so it has high conductivity when lithium salt is dissolved therein.

However, it has a practical drawback in that it is highly reactive with lithium and cannot dissolve lithium salts at low temperatures, and as a sole solvent, it is virtually impossible to use it as a solvent for electrolyte solutions for lithium batteries. In order to improve this drawback, attempts were made to mix propylene carbonate with acetonitrile (
J, Power 5sources+ Volume 10, 33
41, 1983], but propylene carbonate having a polar double bond also had high reactivity with lithium and its properties were insufficient.

[Problems to be solved by the invention]

The present invention has been made in view of the current situation, and its main purpose is to provide an electrolytic solution for lithium secondary batteries that has high lithium charging/discharging efficiency and high conductivity.

[Means to solve the problem]

Therefore, the electrolytic solution for lithium 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 mainly composed of a mixed solvent of acetonitrile and ethers. It is characterized by certain things.

According to the present invention, by using a mixed solvent of acetonitrile and ethers as an electrolyte for a lithium battery, it is possible to provide a non-aqueous electrolyte for a lithium battery that has excellent charge/discharge characteristics of a lithium electrode and has high conductivity. Can be done.

The present invention will be explained in more 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 battery is dissolved in a solvent, and the organic solvent used is a mixed solvent of acetonitrile and ethers as a main component.

Usually, the solute (electrolyte) used in the electrolyte solution for lithium batteries is a Lewis acid salt of lithium, such as LiClO4, L
iAsFe, LiBFa, LiPF6, LiC
F35Oa, LiCF3C0g, LiA]CIa, 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, in order to obtain high conductivity, it is considered effective to improve the mobility of ions by using a solvent with low viscosity.

Acetonitrile is a low viscosity solvent (0.375 centivoise at 15° C.) and exhibits high electrical conductivity when lithium salts are dissolved. However, it has a strong reactivity with lithium, causing a reaction as shown in formula (I), and has the practical disadvantage of not being able to dissolve the lithium salt when it becomes a lithium salt at low temperatures. Of course, it is virtually impossible to use it as an electrolyte solvent for lithium secondary batteries. In order to improve this drawback while taking advantage of the advantages of acetonitrile, it is necessary to use acetonitrile, which has low reactivity with lithium and
It is considered effective to mix a solvent with a low viscosity. From this point of view, it is considered most effective to mix ethers with acetonitrile.

2Li + 4CI3CN -→LiCN+CH4+CH3-C=CH-CN+Li
CH2CNHe The viscosity of the solvent to be mixed as described above is preferably 2
It is preferable that it is less than cP (centipoise).

This is because if the viscosity exceeds 2 cP, there is a possibility that the effective viscosity reduction of acetonitrile cannot be expected.

Specific examples of such ethers include, for example, tetrahydrofuran, 2-methyltetrahydrofuran, 2,
5-dimethyltetrahydrofuran, 1,2-dimethoxyethane, ■, 2-jethoxyethane, 1-methoxy-2
It is possible to use one or more selected from -ethoxyethane, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 2-methyl-1,3-dioxolane, tetrahydropyran, and the like.

Volume mixing ratio of acetonitrile and ethers is 5-95%
and preferably the volumetric mixing ratio of the ethers is 50 to
It is 80%. If the volume mixing ratio of either solvent is less than 5%, not only will the difference in electrical conductivity or lithium charge/discharge efficiency from that of a single solvent become small, but also the lithium salt may not be able to be dissolved.

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

For example, LiAsFe, LiCIO4, Li5bF
e, LiPFe, LiBFa, LiAlC1a
, LiCF3503 , LiCF3 COi+
−.

One or more lithium salts such as Li2B 10C110 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 deteriorate significantly, and 2
This is because if it exceeds 5M, it may become difficult to dissolve the solute.

As mentioned above, the organic solvent of the electrolytic solution used in the present invention is mainly composed of a mixed solvent of acetonitrile and ethers.

Small amounts of additives can be used in such mixed solvents for reasons such as improving the solubility of solutes. As such an additive, one or more compounds selected from, for example, hexamethylphosphoric acid triamide, N,N,N',N''-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 may be the negative electrode active material used in conventional lithium batteries, i.e., lithium or lithium that can discharge lithium ions. Alloys can be used.

Similarly, the positive electrode active material used in the present invention is not fundamentally limited, and may be a positive electrode active material used in conventional lithium secondary batteries, that is, a material that electrochemically undergoes a reversible reaction with lithium ions. be able to.

Examples will be described below.

Example 1 As an electrolytic solution, 1.5 M (M: mole/7
A solution of LiAsFe was used. -10
The electrical conductivity of the electrolyte at ~25°C is shown in Figure 1.

FIG. 1 shows, as a reference example to show the effects of the present invention,
1.5MLiAsFe-AN and 1.5MLiA
The graph shows the electrical conductivity of a sFe-THF single solvent electrolyte. In the case of AN alone system, the conductivity at 25°C is 5
Although the conductivity is high as xlo-2Scm-', the conductivity decreases dramatically as the temperature decreases. At temperatures below 18℃, AN/THFN
/ system exhibits higher conductivity than AN alone system, and also exhibits stable conductive properties even at low temperatures. Also, AN/
It can also be seen that the THFN/system exhibits higher conductivity than the T-liver alone system.

Example 2 1.5 MLiAsFs-8N/THF as electrolyte
(volume mixing ratio, l/1) and determine the charge/discharge efficiency of lithium using the method described below. was assembled and measured as follows. The measurement was performed by depositing lithium on a platinum electrode for 80 minutes at a constant current of 0.5 mA and 7 cm2 (2,4 C/c).
m2), a part of this precipitated lithium (0,6C/
cm2) is discharged as Li'' ions, and then further discharged as 0.cm2).
A cycle test of discharging at a capacity of 6C/cm" was repeated.The charge/discharge efficiency (Ea) was determined from the change in potential of the platinum electrode, and the apparent number of cycles at which the efficiency was 100% was n, as shown below. The above Ea can be determined from the equation (n).

Ea=(0,6(2,40,6)/n) 10.
6 xloo (%) - Song - (11”) Results as first
Shown in the table. Table 1 also shows the charging and discharging efficiency of lithium when 1.5 M LiAsFe AN is used as a comparative example to show the effects of this presentation. AN/TH
F Mixed system [Table 1 (8)] to 8 alone [Table 1 (B
)] shows higher lithium charge/discharge efficiency.

Example 3 As an electrolyte, 1M was added to a mixed solvent of AN and 1,2-dimethoxyethane (hereinafter referred to as DME) (volume mixing ratio, 3/2).
molten LiAsFe was used.

The electrical conductivity of the electrolytic solution at -10 to 25°C is shown in Figure 2. It can be seen that while the temperature characteristics of the conductivity of the AN single system are unstable, the AN/DME mixed system exhibits a stable and high conductivity even at low temperatures. In addition, with DME alone, 1.5M
LiAsFe could not be dissolved and an electrolytic solution could not be produced.

Example 4 IMLiAsFe-AN/DME as electrolyte
The charging and discharging efficiency of lithium was measured in the same manner as in Example 2 except that (volume mixing ratio, 3/2) was used.

The results are shown in Table 1. AN/DME mixed system [Table 1 (C
)] shows a higher lithium charge/discharge efficiency than the static one (Table 1 (B)).

Example 5 As an electrolyte, 1.5 M LiAsF s was mixed with AN and 2-methyltetrahydrofuran (hereinafter referred to as 2MeTHF) (
The mixture was dissolved at a volumetric mixing ratio of 1/1). -1
The conductivity of the electrolytic solution at 0 to 25° C. is shown in FIG. 3.

While the temperature characteristics of the conductivity of AN alone system are unstable,
It can be seen that the /2MeTHF mixed system exhibits stable and high conductivity even at low temperatures. It is also seen that the AN/2MeTIIF mixed system exhibits higher conductivity than the 2MeTHF single system.

Example 6 1.5MLiAsFe e-8N/2Me as electrolyte
The charging and discharging efficiency of lithium was measured in the same manner as in Example 2 except that THF (volume mixing ratio, 1/1) was used.

The results are shown in Table 1. It can be seen that the AN/2MeTllF mixed system [Table 1 (D)] exhibits higher lithium charging and discharging efficiency than AN alone [Table 1 (B)].

First Table Ratio [Effect of the Invention] As explained above, according to the electrolyte for lithium batteries according to the present invention, by using the electrolyte whose main component is a mixed solvent of acetonitrile and ethers, the electrical conductivity is high (
It is possible to provide an electrolytic solution for a lithium secondary battery that also has high lithium charging and discharging efficiency.

[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 Tadashi Amemiya Figure 1 ('C) Figure 2

Claims (1)

[Claims] (1) An electrolytic solution for a lithium secondary battery in which a lithium salt is dissolved in an organic solvent, wherein the organic solvent is a mixed solvent of one or more types of acetonitrile and ethers as a main component. Electrolyte for use.