JPH0226345B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a non-aqueous electrolyte used in lithium secondary batteries.

BACKGROUND TECHNOLOGY Batteries using lithium as a negative electrode active material are being researched as small-sized batteries with high energy density, but secondaryization has become a major problem.

As a positive electrode active material that can be secondaryized, V ₂ O ₅ ,
Metal oxides such as TiO ₂ and layered compounds such as TiS ₂ and WS ₂ are known as compounds that undergo topochemical reactions with Li. Batteries using niobium selenide, selenide, telluride (see US Pat. No. 4,089,052), and batteries using niobium selenide (J.
Electrochem.Soc., vol.124, No.7, pages 968 and
325 (1977)) etc. are disclosed.

Problems to be Solved by the Invention However, compared to such research on positive electrode active materials for secondary batteries, research on the charging and discharging characteristics of Li electrodes is not sufficient. ,
The search for electrolytes with good charge/discharge characteristics such as charge/discharge efficiency and cycle life has become a serious issue.

In an attempt to improve the charging and discharging efficiency of Li electrodes,
An attempt was made to add additives such as nitromethane and SO ₂ to LiClO ₄ /propylene carbonate (hereinafter abbreviated as PC) [Electorchmica.Acta., vol. 122, pp. 75-83 (1977)] and use of LiClO ₄ /methyl acetate. Although attempts have been made, they are not necessarily satisfactory, and there is a need for an electrolytic solution for lithium secondary batteries with even better characteristics.

Means for Solving the Problems The present invention was made in view of the current situation, and its purpose is to provide a non-aqueous electrolyte for lithium secondary batteries that has excellent charging and discharging characteristics for Li electrodes. be.

Therefore, the non-aqueous electrolyte for lithium secondary batteries according to the present invention is a non-aqueous electrolyte in which an inorganic lithium salt is dissolved in an organic solvent, and N, N, N', N' are added as additives to the non-aqueous electrolyte. -It is characterized by using tetramethylethylenediamine.

According to the present invention, the additives include N, N,
By using N',N'-tetramethylethylenediamine, a lithium secondary battery with good Li electrode charge/discharge characteristics can be realized.

The present invention will be explained in more detail.

The organic solvent used in the non-aqueous electrolyte for a lithium secondary battery according to the present invention may be any organic solvent as long as it is conventionally used in this type of electrolyte. For example, one or more organic solvents selected from propylene carbonate, tetrahydrofuran, dimethyl sulfoxide, γ-butyrolactone, dioxirane, 1,2-dimethoxyethane, and 2-methyltetrahaladofuran can be used.

Furthermore, the inorganic lithium salt that is the solute is not limited like the above-mentioned organic solvent. For example _LiClO4 ,
Inorganic lithium salts that are generally used as solutes in nonaqueous electrolytes, such as one or more selected from LiBF ₄ , LiAsF ₆ , LiPE ₆ , and LiAlCl ₄ , can be effectively used.

The amount of solute dissolved in the organic solvent is preferably
It is 0.5-2.5N. This is because if it is less than 0.5N, the charge-discharge characteristics will be significantly deteriorated, and if it exceeds 2.5N, it will become difficult to dissolve, the viscosity will increase, and the charge-discharge characteristics will deteriorate easily. Particularly preferably, for example in the case of LiClO ₄ , it is around 1N.

In the present invention, the additive added to the organic solvent is N,N,N',N'-tetramethylethylenediamine, but the reason why its addition improves the charge-release characteristics is not necessarily clear.

However, the following things can be considered.

The main reason for the decrease in Li charging and discharging efficiency is that the precipitated activated Li chemically reduces the solvent, and Li changes into an electrochemically inert compound (cannot discharge Li ⁺ ions), and eventually Li It is recognized that the energy is consumed. In order to suppress this chemical reaction,
Possible methods include 1) using a solvent with low reactivity with Li, or 2) using an additive that forms a protective film on the surface of Li, but method 1) There is a possibility that other characteristics may deteriorate.

The additive of the present invention aims at the effect 2) above, and N,N,N',N'-tetramethylethylenediamine is one of the solvents with high solvation power for Li ⁺ ions. , is known to react with Li metal.

For this reason, N, N,
When N′,N′-tetramethylethylenediamine is added, electrodeposition reaction occurs in any solvent.
The Li ⁺ ions are selectively solvated by the additive to form stable one-to-one complexes. For this reason, it is thought that N,N,N',N'-tetramethylethylenediamine reacts with Li to form a protective film on the Li surface.

Note that when ethylenediamine is used as an additive, hydrogen in the amino group reacts with lithium metal to generate hydrogen gas. This hydrogen gas causes ignition and poses a safety problem for batteries.

On the other hand, N,N,N',N'-tetramethylethylenediamine is produced by replacing the hydrogen in the amino group of ethylenediamine with a methyl group, and does not generate hydrogen gas even when used as an additive, making it highly safe. ing.

The amount of N,N,N',N'-tetramethylethylenediamine added is preferably 2 or less, most preferably 1.5 or less in molar ratio to the Li ⁺ ion concentration. This is because if it exceeds 2, the charge/discharge characteristics will deteriorate.

Example Examples of the present invention will be described below.

Example 1 A cell was assembled using a Pt electrode as a working electrode, Li as a counter electrode, and Li as a reference electrode, and Li was deposited on the Pt electrode to measure the charge/discharge characteristics of the Li electrode. The electrolyte was made by adding N,N,N',N'-tetramethylethylenediamine (hereinafter abbreviated as TMEDA) to 1NLiClO ₄ /propylene carbonate (hereinafter abbreviated as PC) at a volume mixing ratio of 3%. Using. The structural formula of the above additive is shown in the following formula (). The equivalent conductivity of the above electrolyte is 6.5
Ω ⁻¹ mol ⁻¹ cm ² , which was higher than the equivalent conductivity of 1NLiClO ₄ /PC alone, which was 6.0 Ω ⁻¹ mol ⁻¹ cm ² .

(CH ₃ ) ₂ NCH ₂ CH ₂ N (CH ₃ ⁾ ₂ () First, the Pt
After depositing Li on the Pt pole and charging it, a cycle test was conducted in which the Li deposited on the Pt pole was discharged as Li ⁺ ions at a constant current of 5 mA/cm ² . Charge/discharge efficiency is Pt
Obtain the potential change of the electrode, and change the Li deposited on the Pt electrode to Li ⁺
Amount of electricity required to discharge as ions and Pt
It was calculated from the ratio to the amount of electricity required to deposit Li on the top.

Figure 1 is a diagram showing the relationship between the charge/discharge efficiency of Li electrodes and the number of cycles. In the figure, a shows the case where the above electrolyte is used, and b shows the case where the electrolyte of 1NLiClO ₄ /PC alone is used. The charge/discharge characteristics in the case where the battery is used are shown as a reference example. As can be seen from Figure 1, compared to independent system b,
In system a to which TMEDA was added, the charge-discharge cycle characteristics were clearly improved.

Example 2 The charging and discharging characteristics of the Li electrode were measured in the same manner as in Example 1, except that 1NLiClO ₄ /PC with TMEDA added at a volume mixing ratio of 0.5% was used as the electrolyte. The equivalent conductivity of the above electrolyte is 6.2Ω ^-1 mol ^-1 cm ²
This was higher than the equivalent conductivity of 1NLiClO ₄ /PC alone, which was 6.0Ω ^- mol ^-1 cm ² .

Figure 2 is a diagram showing the relationship between charge/discharge efficiency and number of cycles. In the figure, a shows the case when the above electrolyte is used, and b shows the case when the electrolyte of 1NLiCl ₄ /PC alone is used. The charge/discharge characteristics are shown as a reference example.

As can be seen from Figure 2, compared to independent system b,
In system a to which TMEDA was added, the charge-discharge characteristics were clearly improved.

Effects of the Invention As is clear from the above explanation, according to the present invention, by adding N,N,N',N'-tetramethylethylenediamine to an electrolytic solution in which an inorganic lithium salt is dissolved in an organic solvent, It is possible to realize a non-aqueous electrolyte for lithium secondary batteries with good Li electrode charge/discharge characteristics.

[Brief explanation of drawings]

FIGS. 1 and 2 are diagrams showing the relationship between the charge/discharge efficiency of a lithium electrode and the number of cycles in an example of the present invention.

Claims (1)

[Claims] 1. A non-aqueous electrolyte in which an inorganic lithium salt is dissolved in an organic solvent, characterized in that N,N,N',N'-tetramethylethylenediamine is used as an additive in the non-aqueous electrolyte. Non-aqueous electrolyte for secondary batteries.