JPH07142090A - Lithium secondary battery electrolyte containing additive - Google Patents

Abstract

PURPOSE:To provide an electrolyte for a lithium secondary battery containing an additive for enhancing charge/discharge reversibility of a metallic lithium negative electrode. CONSTITUTION:A compound comprising a cation of metal capable of alloying with lithium and an iodide anion is contained as an additive in a range of 10ppm to 2000ppm in an electrolyte prepared by dissolving a lithium salt in a nonaqueous solvent. Metallic lithium is used in a negative electrode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in an electrolytic solution for a lithium secondary battery having a metallic lithium negative electrode.

[0002]

2. Description of the Related Art A lithium secondary battery using metallic lithium as a negative electrode has an excellent advantage that it has a very high energy density. This kind of battery is used for various household electric appliances,
It is being considered for use as a power source for computer memory backup, and also as a power source for motive power of automobiles and the like, and is expected to be used in a wide range of fields.

However, in the lithium secondary battery using metallic lithium as the negative electrode, the reversibility of dissolution and precipitation of metallic lithium during charge / discharge cycles is a major obstacle, and the battery has sufficient performance. Has not been obtained. That is, it has been pointed out that lithium is deposited in a dendrite form on the negative electrode during charging, and as a result, a part of the lithium electrode is detached or the battery causes an internal short circuit, resulting in a decrease in the repeated life of the battery. .

The electrolyte of this type of lithium secondary battery includes
It is essential to use an organic solvent, but the main conditions required are high ionic conductivity, chemical,
They are electrochemically stable and do not impair the reversibility of the electrode reaction. Generally, as the electrolytic solution, a solution prepared by dissolving a lithium salt in an organic solvent system mainly containing propylene carbonate, ethylene carbonate and the like which are non-pront organic solvents is often used. However, even if these electrolytes are used as they are, they cannot be said to be sufficient to improve the charge-discharge cycle characteristics of the above-mentioned lithium metal, and are characterized by not impairing the reversibility of the charge-discharge reaction of the metal lithium negative electrode. Developing an electrolyte system is currently considered to be an important issue for improving battery characteristics.

On the other hand, in order to break through such a difficult situation, metallic lithium is not used as a negative electrode as it is, but a carbonaceous material doped with lithium or alloyed with another metal is used as a negative electrode. Currently, lithium secondary batteries are being actively developed. These batteries are
The cycle life of the negative electrode has been improved considerably, and practical secondary batteries have been developed.

[0006]

However, the fate that these batteries do not use metallic lithium itself in the negative electrode is that the theoretical capacity per weight of the negative electrode is considerably lower than that of the negative electrode composed of metallic lithium itself. is there. Under such circumstances, the development of secondary batteries using metallic lithium for the negative electrode is still in progress, and the development of an electrolytic solution capable of enhancing the charge-discharge reversibility of the lithium load is desired.

Therefore, the problem to be solved by the present invention is to provide an electrolytic solution containing an additive for improving the charge-discharge reversibility of a metal lithium negative electrode from such a background.

[0008]

In the present invention, the charge and discharge of a metal lithium negative electrode is made possible by adding a compound consisting of a cation of a metal alloying with lithium and a compound of an iodide anion to an electrolyte solution solvent as an additive. It has been found that the cycle characteristics are improved. The electrolytic solution for a lithium secondary battery is characterized in that, in an electrolytic solution in which a lithium salt is dissolved in a non-aqueous solvent, these metal iodides as additives are contained in the range of 10 ppm to 2000 pmm with respect to the electrolytic solution. To do.

The organic solvent used in the above-mentioned electrolytic solution is not particularly limited, but for example, a high dielectric constant solvent such as propylene carbonate is used alone, or a high dielectric constant solvent such as propylene carbonate or ethylene carbonate. In addition, a low-viscosity solvent such as 1,2-dimethoxyethane, diethyl carbonate, or dimethyl carbonate mixed in a volume percentage of 10% to 90% can be used.

On the other hand, examples of the lithium salt include lithium perchlorate, lithium hexafluorophosphate, lithium trifluoromethanesulfonate, lithium tetrafluoroborate, and the like, and 0.5 mol of any of these is used. It can be used by dissolving it in an electrolytic solution in the range of concentration to 1.5 molar. The lithium salt used here is most preferably 1.0 molar.

Further, as an additive, a compound comprising a cation of a metal alloying with lithium and an iodide anion is dissolved in an electrolyte solution within a range of 10 ppm to 2000 ppm, and the resulting electrolyte for a lithium secondary battery. Use as liquid. The additive used is preferably aluminum iodide, tin iodide, etc.
Most preferably, it is pm.

[0012]

The electrochemical deposition and dissolution test of the lithium metal negative electrode, that is, the charge-discharge test, was carried out using the organic electrolyte solution containing the additive according to the present invention. The reversibility of charge and discharge was greatly improved as compared with the test results of. It is considered that this is because the metal cation of the additive is reduced at the time of charging, and this is alloyed with metal lithium to improve the precipitation morphology as compared with the case without the additive, that is, dendrite is hardly formed. Furthermore, iodide ion is used as an anion of the additive, which becomes lithium iodide even when it reacts with lithium on the electrode in the oxidized state, forming a protective film with lithium ion conductivity, and forming a lithium metal and solvent. It is considered that this is because the effect of suppressing side reactions with the above appears.

[0013]

EXAMPLES The present invention will be specifically described below with reference to examples.

Comparative Example Commercially available battery grade propylene carbonate (PC) was used as the electrolyte catalyst. As the electrolyte salt, lithium perchlorate (LiClO ₄ ) was used at a molar concentration of 1. A three-electrode beaker cell was used for electrochemical measurement. That is, as the test electrode substrate, a nickel (Ni) plate having a diameter of 13 mm was attached to a holder made of fluororesin (for example, Teflon (trade name of DuPont)) and used. A lithium mesh (Li) foil bonded to a Ni mesh was used as the counter electrode, and a lithium piece was wound around the Ni wire as the reference electrode, and the electrolytic solution volume was 50 cm ³ .

The Coulombic efficiency of Li on the Ni substrate of the test electrode was measured at a constant current with a current density of ² mA / cm ² .
The Coulombic efficiency here is defined as the percentage of the amount of electricity required to dissolve Li with respect to the amount of electricity required to deposit Li on the Ni substrate in each charge / discharge cycle. Therefore, if this value is closer to 100%, it can be considered that the reversibility of charge and discharge is excellent. Furthermore, as a condition in the measurement of Coulomb here, charging, that is, Li
The amount of electricity required for the precipitation of was fixed at 0.2 coulomb, and the discharge, that is, the dissolution of Li, was performed until the test electrode reached 1.5 V with respect to the reference electrode Li. These measurements were performed at room temperature (18 to 25 ° C.) in dry Ar.

In this system, that is, when no additive was used, the Coulombic efficiency due to repeated charging / discharging was 65% in the first charging / discharging cycle and then gradually increased to 85% in the 5th cycle. . However, after that, the efficiency gradually decreased, and at the 10th cycle, 80%, 2
It decreased to 50% at the 0th cycle.

Example 1 The same battery grade propylene carbonate (PC) as described in Comparative Example was used as the electrolyte solvent. As the electrolyte salt, LiClO ₄ was used at a molar concentration of 1 as in the comparative example. Further, aluminum iodide was dissolved in 100 ppm of the electrolytic solution as an additive of the electrolytic solution. A three-electrode beaker cell was used for electrochemical measurement. As the test electrode substrate, a Ni plate having a diameter of 13 mm was attached to a holder made of fluororesin and used. Ni for the counter electrode
The mesh was obtained by crimping a Li foil, and the reference electrode was prepared by winding a Li piece around the Ni wire, and the electrolytic solution volume was set to 50 cm ³ .

The Coulombic efficiency of Li on the Ni substrate was measured at a constant current density of ² mA / cm ² . The charging / discharging conditions in this test are the same as in the comparative example. as a result,
The Coulomb efficiency in charge and discharge was 70% in the first charge and discharge cycle, but was 90% in the second time, and remained constant at 90% up to 20 cycles thereafter, showing excellent charge and discharge reversibility.

Example 2 The same battery grade propylene carbonate (PC) as that described in the comparative example was used as the electrolytic solution solvent. As the electrolyte salt, LiClO ₄ was used at a molar concentration of 1 as in the comparative example. Further, tin iodide as an additive of the electrolytic solution was dissolved in 100 ppm of the electrolytic solution. A three-electrode beaker cell was used for electrochemical measurement. As the test electrode substrate, a Ni plate having a diameter of 13 mm was attached to a holder made of fluororesin and used. The counter electrode was a Ni mesh crimped with a Li foil, and the reference electrode was a Ni wire wrapped with a Li piece.
It was set to 0 cm ³ .

The Coulombic efficiency of Li on the Ni substrate was measured at a constant current with a current density of ² mA / cm ² . The charging / discharging conditions in this test are the same as in the comparative example. as a result,
The coulombic efficiency in charge and discharge was 65% in the first charge and discharge cycle, then increased as the number of cycles increased, and reached 90% in the fifth cycle. After this 20
The efficiency gradually decreased until the cycle and reached 80% at the 20th cycle.

[0021]

As described above, for the negative electrode of lithium metal, the cation of the metal capable of alloying with lithium and iodide are added to the electrolytic solution in which the lithium salt is dissolved in an aprotic solvent such as propylene carbonate. It was found that good charge-discharge reversibility can be maintained by using a compound consisting of a substance anion as an additive. That is, the reversibility of dissolution and precipitation of metallic lithium was significantly improved as compared with the electrolyte solution without addition.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasushi Goto 2-47 Shiobara, Minami-ku, Fukuoka-shi, Fukuoka Inside Kyushu Electric Power Co., Inc. Research Institute (72) Inventor Kazuyuki Adachi Minami-ku, Fukuoka-ku, Fukuoka 2-Chome 1-47 Kyushu Electric Power Co., Inc. Research Institute

Claims (1)

[Claims] 1. A cation of a metal which forms an alloy with lithium in an electrolytic solution in which a lithium salt is dissolved in a non-aqueous solvent,
An electrolytic solution for a lithium secondary battery having a metallic lithium negative electrode, characterized in that a compound comprising an iodide anion is used as an additive and is contained in the range of 10 ppm to 2000 ppm with respect to the electrolytic solution.