JP2548573B2 - Electrolyte for lithium battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electrolytic solution for a lithium battery, and more specifically to improvement of an electrolytic solution used for lithium primary and secondary batteries.

[Conventional technology]

Lithium has a high standard single electrode potential of −3.03V (standard hydrogen electrode standard), an extremely strong reducing power, and an atomic weight of 6.
Since it is as small as 941, the capacity density per weight is as large as 3.86 Ah / g.

Therefore, a battery using lithium as a negative electrode active material (hereinafter referred to as a lithium battery) has been studied as a battery having a small size and a high energy density, and a battery using manganese dioxide, fluorinated graphite or the like as a positive electrode active material has already been used. It is commercially available. However, these commercially available lithium batteries are primary batteries, and the present situation is that a rechargeable lithium secondary battery that can be used for ordinary practical use has not yet been realized.
If lithium batteries can be recharged while taking advantage of the discharge characteristics of high energy density, batteries with extremely superior characteristics will be realized compared to conventional battery systems, and it will be used in industries such as portable electronic devices. It has a great effect on the world.

In order to make a lithium battery secondary, there are many problems to be solved such as development of a positive electrode active material, selection of an electrolytic solution, and improvement of a battery construction method. In particular, the selection of the electrolytic solution is an important issue. It is desirable from the practical point of view to use a non-aqueous electrolytic solution for a lithium secondary battery that operates at room temperature, but the conductivity of the electrolytic solution is two orders of magnitude less than that of an aqueous solution system used in a conventional battery system.
There was a drawback that it was low. Therefore, it is essential to improve the conductivity of the electrolytic solution in order to improve the discharge utilization rate of the battery. At the same time, in order to apply it to the secondary battery, it is naturally required that the charge / discharge efficiency of lithium in the non-aqueous electrolyte is high. That is, the electrolytic solution used for the lithium secondary battery needs to simultaneously satisfy the two points of high conductivity and high lithium charge / discharge efficiency.

As an electrolyte with relatively high lithium charge / discharge efficiency, Li
AsF _{6-2-methyltetrahydrofuran-based} electrolyte it has been proposed (see U.S. Pat. No. 4,118,559). However, the conductivity of the electrolyte is low (4.3M with 1.5M LiAsF _6).
(10 ⁻³ S · cm ⁻¹ , 25 ° C.), and when used in a lithium battery, it had a drawback of low charge / discharge efficiency.

In addition, an electrolyte using a solvent having a polar double bond such as propylene carbonate or ethylene carbonate, or having a relatively high conductivity (for example, 1.5M LiAsF
₆ at 5.3 × 10 ^-3 S ・ cm ^-1 and 6.2 × 10 ^-3 S ・ c respectively
m ⁻¹ ), lithium has a drawback of low charge and discharge efficiency.

[Problems to be solved by the invention]

That is, up to now, an electrolyte solution for a lithium secondary battery, which has a high conductivity and a high lithium charge / discharge efficiency, has not been realized.

The present invention has been made in view of such a current situation, and an object thereof is an electrolyte for a lithium battery that has a high conductivity and is capable of producing a lithium primary and secondary battery having excellent charge / discharge characteristics of lithium. To provide.

[Means for solving problems]

The lithium battery electrolyte according to the present invention is a lithium battery electrolyte obtained by dissolving a lithium salt in an organic solvent,
The most main feature is that the dicarboxylic acid compound is contained in a concentration range of 10 ^-3 to 10 ^-1 M (mole /).

As a result, in contrast to a lithium battery electrolyte that has conventionally been difficult to achieve both high conductivity and excellent charge / discharge efficiency, a lithium battery electrolyte having high conductivity and excellent charge / discharge efficiency. Is realized.

 The present invention will be described in more detail.

In a lithium battery, the negative electrode active material is a lithium alloy capable of discharging lithium or lithium ions, the positive electrode active material is a material that electrochemically reversibly reacts with lithium ions, and the electrolyte solution uses a lithium salt as an organic solvent. It is composed of a dissolved solution.

According to the present invention, it is an object of the present invention to provide an electrolytic solution as described above having excellent properties. That is, a dicarboxylic acid compound is contained in a solution in which a lithium salt is dissolved in an organic solvent in a concentration range of 10 ^-3 to 10 ^-1 M, and this is used as an electrolyte for a lithium battery.

In order to improve the conductivity of the electrolyte used in the lithium battery and the charge / discharge efficiency of lithium, a solvent having a low electron transfer reactivity from lithium to the solvent or a lithium salt in the solvent is easily dissociated, and the lithium ion It is considered necessary to have high mobility. The dicarboxylic acid compound in the present invention is easily adsorbed on the lithium negative electrode without disturbing the dissociation of the lithium salt or the mobility of lithium ions, and suppresses the reaction between lithium and the solvent, which becomes a factor of reducing the charge / discharge efficiency of lithium. It is added for the purpose of facilitating the deposition of lithium ions on the negative electrode due to charging and discharging and the elution from the negative electrode into the electrolytic solution.

The dicarboxylic acid compound used in the present invention is a compound having two carboxyl groups -COOH, but one or two H are simultaneously substituted with an alkyl group, a phenyl group, a benzyl group and derivatives thereof. Monoester or diester compound (R'OOC-R-COOH, R'OOC
-R-COOR "), or those substituted with an alkali or alkaline earth metal or the like (-COOX) can also produce the same effect. The group bonded to the carboxyl group -COOH of the dicarboxylic acid compound is aromatic. It can be of any type, non-aromatic.

The reason why the dicarboxylic acid compound used in the present invention shows effective characteristics when added to the electrolytic solution is not necessarily clear, but one idea is that the additive physically or chemically adsorbs to the negative electrode lithium. By doing so, a layer that suppresses the loss of lithium due to the reaction between lithium as an active material to be used in the charge / discharge cycle and the electrolytic solution is formed at the negative electrode / electrolytic solution interface. Also, this layer is Li ⁺ during charging
The effect of improving the deposition state of ions on the negative electrode and suppressing the generation of precipitated lithium that becomes electrochemically inactive is expected. After charging with an electron microscope, the surface of the negative electrode after charging was observed in the battery system described in Example 1. The results show that in the system without the additive, the diameter effective for improving the lithium charge / discharge efficiency is shown. Along with about 10 μm of particulate lithium, acicular lithium with a diameter of 1 to 2 μm and a length of about 10 μm, which causes a decrease in charge / discharge efficiency of lithium, is present on the surface of the negative electrode, and the acicular lithium is deposited with the charge / discharge cycle. The percentage has increased. On the other hand, in the battery using the electrolytic solution of the present invention to which the additive was added, the presence of the acicular lithium was not observed, and the particulate lithium having a diameter of about 2 to 5 μm was uniformly deposited. The effectiveness of the electrolytic solution of the present invention due to the addition of the additive was confirmed.

The addition concentration of the dicarboxylic acid compound in the present invention is 10 ⁻³
~ 10 ^-1 M, but if the concentration is less than 10 ^-3 M, the characteristics are not improved by the addition as with the characteristics of the conventional electrolyte solution that is not added, while the concentration exceeds 10 ^-1 M And the excess of additives adversely affects the mobility of ions and the electrochemical reaction of the negative electrode, and the characteristics deteriorate as compared with before the addition.

The lithium salt and the organic solvent that compose the electrolytic solution in the present invention are not basically limited. For example,
In lithium _{salt, LiAsF 6, LiClO 4, LiSbF} 6, LiBF
One or more of ₄ , LiPF ₆ , LiAlCl ₄ , LiCF ₃ SO ₃ and LiCF ₃ CO ₂ can be effectively used. In addition, as the organic solvent, propylene carbonate, ethylene carbonate, 2-methyltetrahydrofuran, tetrahydrofuran, 2-methyldioxolane, 4-methyldioxolane, dioxolane, 1,2-dimethoxyethane, γ-butyrolactone, dimethylsulfoxide, acetonitrile,
One or more aprotic organic solvents such as formamide, dimethylformamide and nitromethane can be effectively used.

The negative electrode active material used in the lithium battery using the electrolytic solution according to the present invention is basically not limited, and the negative electrode active material used in the conventional lithium battery, that is, lithium or lithium that can discharge lithium ions. Alloys can be used.

Similarly, the positive electrode active material used in the present invention is basically not limited, and is a positive electrode active material used in conventional lithium secondary batteries, that is, a material that electrochemically reversibly reacts with lithium ions. be able to.

〔Example〕

Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited to these Examples.

Example 1 1.5M LiAsF ₆ -ethylene carbonate (hereinafter abbreviated as EC) / 2-methyltetrahydrofuran (hereinafter, 2Me TH)
(Abbreviated as F) (volume mixing ratio 1/1) to prepare an electrolyte solution in which the impurities in the mixture are controlled to 100 ppm or less, and various dicarboxylic acids that have been sufficiently dried by leaving them in a vacuum dryer at 80 ° C for 1 week in advance. The compound was dissolved at 10 ^{−4 to} 0.5 M and used as an electrolytic solution of a lithium secondary battery manufactured by the manufacturing method described below.

For the positive electrode, 95mole% V ₂ O ₅ −5mole% P ₂ O ₅ as an active material
70% by weight of an amorphous material having a composition of 25% by weight, acetylene black as a conductive agent by weight of 25%, and Teflon as a binder by weight of 5% by weight.
φ) and metallic lithium (17 mmφ, 15 mA) as the negative electrode
A coin-type lithium battery was produced by using h) and a microporous polypropylene sheet as a separator.

This lithium battery at room temperature, current value of 1mA, 2-3.5V
A charge / discharge test was performed in the voltage range of 1 to evaluate the charge / discharge characteristics of the electrolytic solution.

An example of the results is shown in FIG. That is, Fig. 1 shows 1.5M
In LiAsF ₆ −EC / 2Me THF (1/1), tridecanedioic acid [HOOC
- (CH _{2) 11} -COOH] of 0.01M the added electrolyte, 15 mAh Li
3 is a graph showing the relationship between the number of cycles (horizontal axis) and the discharge capacity (mAh, vertical axis) when a charge / discharge test was performed in a voltage range of 1 mA and 2 to 3.5 V for the battery of the present invention using. As is clear from FIG. 1, all the lithium that can participate in the discharge is consumed on the negative electrode side for each discharge, and the capacity thereof gradually decreases according to the charge / discharge efficiency E. That is, assuming that the charge / discharge efficiency of the negative electrode is E and the capacity of the nth discharge is Cn, Cn = E × C _n-1 = E ^n-1 × C ₁ holds, and lnCn = (n-1 ) LnE + lnC ₁ is obtained, and the charging / discharging efficiency E of the negative electrode lithium can be calculated from the slope of the straight line portion of the graph showing the discharge capacity on the vertical axis in a logarithmic scale as shown in FIG.

The charging / discharging efficiency E in the electrolytic solution containing various dicarboxylic acid compounds was calculated by the above formula, and the results are shown in Table 1. For comparison, Table 1 also shows the results of a similar test in an electrolyte solution containing no additive.

As is clear from the results in Table 1, it was found that the addition of 10 ^-3 to 10 ^-1 M of the dicarboxylic acid compound exhibited higher lithium charge / discharge efficiency than the case of no addition.

Example 2 An electrolytic solution in which impurities of 1.5M LiAsF ₆ -EC / 2Me THF (1/1) electrolytic solution were controlled was prepared, and 0.01 M of tridecanedioic acid vacuum-dried at 80 ° C. for 2 weeks was added to the electrolytic solution. This was used as an electrolytic solution for a lithium battery.

A positive electrode and a negative electrode were prepared in the same manner as in Example 1, and a coin-type lithium battery was prepared in the same manner as in Example 1. Furthermore, a charge / discharge test was performed under the same conditions as in Example 1 to evaluate the lithium charge / discharge efficiency.

The results are shown in Table 2. As is clear from Table 2, it was found that the additive does not decompose and deteriorate with time and maintains stable charge / discharge efficiency.

Example 3 Succinic acid [HOOC (CH ₂ ) ₂ vacuum dried at 80 ° C. for 2 weeks
COOH], tridecanedioic acid [HOOC (CH ₂ ) ₁₁ COOH] and 1,4
- cyclohexanedicarboxylic acid _{[C 6 H 10 - (COOH)} 2 ]
Each additive of 1M LiClO ₄ -propylene carbonate (hereinafter abbreviated as PC) / 1,2-dimethoxyethane (hereinafter abbreviated as DME) (1/1), 1.5M LiAsF ₆ -PC, 1.5M
LiAsF ₆ −2Me THF and 1.5M LiPF ₆ −EC / 2Me THF (1/1) were added to various controlled electrolytes at a concentration of 10 ^{−4 to} 0.5M and used as electrolytes for lithium batteries. .

A coin-type battery was produced in the same manner as in Example 1, and a charge / discharge test was performed under the same conditions as in Example 1. Table 3 shows the lithium charge / discharge efficiency obtained from the test.

As is clear from Table 3, it was revealed that by adding the additive in any of the electrolytic solutions, higher charging / discharging efficiency was exhibited as compared with the case where no additive was added.

Example 4 1.5M LiAsF ₆ -EC / 2Me THF with controlled impurities
(1/1) 80 ° C. in the electrolyte solution, 2 weeks vacuum dried tridecanedioic acid [HOOC- (CH _{2) 11} -COOH] to a 0.01M added and the electrolytic solution for lithium battery.

As the positive electrode, a positive electrode material mixture pellet prepared in the same manner as in Example 1 using an amorphous material having a composition of 95 mole% V ₂ O ₅ −5 mole% P ₂ O ₅ as an active material was used, and 60 mAh was used as the negative electrode.
A coin-type lithium battery was produced in the same manner as in Example 1 by using the metallic lithium (17 mmφ).

The lithium battery was subjected to a charge / discharge test at room temperature under a discharge current of 3 mA / cm ² , a charge current of 1 mA / cm ² , and a voltage regulation of ² to 3.5 V to evaluate the charge / discharge characteristics. For comparison, a coin-type battery was manufactured using an electrolyte solution containing no additive, and the same test was performed.

Results are shown in FIG. That is, Fig. 2 shows 1.5M LiAsF
Discharge current 3mA / cm ² , charging current 1mA / cm ² , 2 to 6 for each battery using _6- EC / 2Me THF (1/1) electrolyte and 0.01M tridecanedioic acid It is the graph which showed the change of the discharge capacity (mAh, a vertical axis) with the number of cycles (horizontal axis) at the time of performing a charging / discharging test by 3.5V voltage regulation. In FIG. 2, a curve A shows a case of a battery using an electrolytic solution containing 0.01 M of tridecanedioic acid as an additive, and a curve B shows a case of a battery using an electrolytic solution containing no additive.

As is clear from FIG. 2, the battery using the electrolytic solution containing tridecanedioic acid as an additive exhibited excellent charge-discharge cycle stability as compared with the battery using the electrolytic solution containing no additive. .

Example 5 1.5M LiAsF ₆ -EC / 2Me THF with controlled impurities
(1/1) in the electrolytic solution, 80 ° C., 2 weeks vacuum dried tridecanedioic acid [HOOC- (CH _{2) 11} COOH] was 0.01M added and the lithium battery electrolyte.

Using this electrolytic solution, a coin type lithium battery was produced in the same manner as in Example 4. This battery has a discharge current of 1 mA / cm ² , ² mA / cm ² , 4 mA / cm ² , and a charging current of 1 mA / cm ² ,
A charge / discharge test was conducted under the voltage regulation of 2 to 3.5V. For comparison, a coin-type lithium battery was prepared in the same manner using an electrolytic solution containing no additive, and a charge / discharge test was conducted under the same conditions.

Results are shown in FIG. That is, FIG. 3 shows the discharge capacity (mA) according to the number of charge / discharge cycles (horizontal axis) of the lithium battery.
It is a graph showing the change of (h, vertical axis). In FIG. 3, curve C shows the case of a battery using an electrolyte solution added with tridecanedioic acid in a charge / discharge test under the conditions of a discharge current of 1 mA / cm ² and a charge current of 1 mA / cm ² , and curve D shows the same conditions. In the case of a battery using an electrolyte solution containing no additive in the charge and discharge test, the curve E shows a discharge current of ² mA / cm ² and a charge current of 1 mA / cm ²
This is the case of the battery using the electrolyte solution added with tridecanedioic acid in the charge / discharge test under the conditions of, and the curve F is the case of the battery using the electrolyte solution without the additive in the charge / discharge test under the same conditions. Further, in FIG. 3, a curve G is the case of a battery using an electrolyte solution containing tridecanedioic acid as an additive in a charge / discharge test under the conditions of a discharge current of 4 mA / cm ² and a charge current of 1 mA / cm ² . , Curve H is the case of a battery using an electrolyte solution containing no additive in a charge / discharge test under the same conditions.

As is clear from FIG. 3, in any charging / discharging test, when the electrolytic solution of the present invention to which tridecanedioic acid was added was used, the capacity was increased as compared with the case of using the additive-free electrolytic solution. No significant decrease was observed, and long-term charge / discharge cycle stability was shown.

Example 6 1.5M LiPF ₆ -EC / 2Me THF with controlled impurities
(1/1) Various dicarboxylic acid esters obtained by dehydrating the electrolytic solution in advance, that is, di-n-butyl adipate, dibenzyl adipate, and di-n-amyl phthalate were used.
0.01 M was added to obtain a lithium battery electrolyte.

A positive electrode and a negative electrode were produced in the same manner as in Example 1, and a coin-type lithium battery was produced in the same manner as in Example 1. The produced battery was subjected to a charge / discharge test under the same conditions as in Example 1 to evaluate lithium charge / discharge efficiency. For comparison, a coin-type lithium battery was similarly prepared using an electrolytic solution containing no additive, and a charge / discharge test under the same conditions was also performed.

The results are shown in Table 4. As is clear from Table 4,
It was found that the lithium charge / discharge efficiency is improved by adding the additive to the electrolytic solution as compared with the case where the additive is not added.

The dehydration treatment was carried out at room temperature for di-n-butyl adipate and di-n-amyl phthalate, at 50 ° C for dibenzyl adipate in a thermostatic sieve.
3A was put in each and left for one day and night.

(Effects of the Invention) As described above, when the dicarboxylic acid compound according to the present invention is added in the concentration range of 10 ^-3 to 10 ^-1 M and the lithium salt-dissolved organic solvent-based electrolytic solution is used in a lithium battery, There is an advantage that a high energy density battery having a large discharge capacity and an excellent cycle life can be realized and can be widely used in various fields.

[Brief description of drawings]

FIG. 1 shows a battery of the present invention using an electrolyte solution in which 0.01M tridecanedioic acid is added to 1.5M LiAsF ₆ -EC / 2Me THF (1/1), which is 1mA of a battery using 15mAh lithium as a negative electrode. 2-3.5V
Fig. 2 is a graph showing the relationship between the number of cycles and the discharge capacity when conducting the charge / discharge test in the voltage range of 1.5 LiAsF ₆ −.
Discharge current 3mA / of a battery using an electrolyte solution containing 0.01M tridecanedioic acid in EC / 2Me THF (1/1) and an electrolyte solution not containing it
A graph showing the change in discharge capacity with the number of cycles when conducting a charge / discharge test under voltage regulation of cm ² , charging current 1mA / cm ² , and 2V to 3.5V, Fig. 3 shows the discharge of the same system battery. Current is 1mA / cm ² , 2mA / cm ² , 4mA / cm ² , charging current is 1mA / cm ² , 2V
It is the graph which showed the change of the discharge capacity with the cycle at the time of carrying out a charging / discharging test by the voltage regulation of -3.5V.

Continuation of the front page (72) Inventor ▲ Yoshi ▼ Matsu Isamu 162 Shirahane, Shirahoji, Tokai-mura, Naka-gun, Ibaraki Prefecture, Nippon Steel Telephone Corporation Ibaraki Dentsu Tsushin Laboratory (72) Inventor Shinichi Tobajima Naka-gun, Ibaraki Prefecture Tokaimura Shirahata 162 Shirane, Nippon Telegraph and Telephone Corporation Ibaraki Dentsu Communication Research Laboratories (72) Inventor Masayasu Arakawa Tokaimura Nakamura, Ibaraki 162 Shirahone Shirane Shiken, Nippon Telegraph and Telephone Corporation Ibaraki Dentsu Communication Research In-house (56) Reference JP 63-969 (JP, A) JP 62-288815 (JP, A) JP 60-41774 (JP, A) JP 54-75533 (JP, A)

Claims (1)

(57) [Claims] 1. A lithium battery electrolyte in which a lithium salt is dissolved in an organic solvent, which contains a dicarboxylic acid compound as an additive in a concentration range of 10 ^-3 to 10 ^-1 M (mole /). A featured electrolyte for lithium batteries.