JPH08339825A - Battery electrolyte and lithium secondary battery - Google Patents

Abstract

PURPOSE: To provide a battery electrolyte which can keep high conductivity even at a low temperature and to provide a lithium secondary battery which can keep high charge and discharge capacity even at a low temperature by using the excellent battery electrolyte. CONSTITUTION: A lithium salt is properly dissolved in a nonaqueous solution in which ethylene carbonate, propylene carbonate, dimethyl carbonate and tert- butyl formate are mixed to make a battery electrolyte. A lithium secondary battery is constituted by using the electrolyte.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved electrolytic solution for a battery and a lithium secondary battery using the improved electrolytic solution.

[0002]

2. Description of the Related Art In a lithium secondary battery, a lithium metal composite oxide is used as a positive electrode active material, a carbon material or metallic lithium is used as a negative electrode active material, and an electrolyte solution in which a lithium salt is appropriately dissolved in a non-aqueous solvent is used. are doing. As the non-aqueous solvent, ethylene carbonate (EC), propylene carbonate (PC), dimethoxyethane (DME), dimethyl carbonate (DMC), diethyl carbonate (D
One or two or more kinds of EC) are appropriately mixed and used.

LiPF6 is a solute lithium salt.
One or more selected from ₆ , LiClO ₄ , LiBF _4, etc. are used. A coin type or a cylindrical type is generally used as the form of the battery.

[0004]

In the conventional lithium secondary battery, when the charge and discharge are performed in a low temperature environment of about -20 ° C, the capacity is significantly reduced as compared with the charge and discharge at room temperature. this is,
This is because the conductivity of the electrolytic solution is remarkably lowered at a low temperature and the internal resistance is remarkably increased. Also, the charge / discharge cycle characteristics were not sufficiently high.

The present invention has been made in view of the above-mentioned conventional problems, and an object thereof is to provide an electrolytic solution for a battery which can maintain a high conductivity even at a low temperature. Another object of the present invention is to provide a lithium secondary battery which can maintain a high charge / discharge capacity even at a low temperature by using the electrolytic solution and has excellent charge / discharge cycle characteristics.

[0006]

The battery electrolyte of the present invention comprises a lithium salt appropriately dissolved in a non-aqueous solvent prepared by mixing ethylene carbonate, propylene carbonate, dimethyl carbonate and tert-butyl formate. Then, a lithium secondary battery is constructed by using the electrolytic solution of the present invention.

[0007]

[Function] Ethylene carbonate, propylene carbonate and dimethyl carbonate are components that have been conventionally used in battery electrolytes, and battery electrolytes in which a lithium salt is dissolved in a solvent in which these three components are mixed are known. . In the present invention, tert-butyl formate is a novel component of an electrolytic solution for a battery which has not been known so far. It is a feature of the present invention that tert-butyl formate which is a new component is mixed with the above-mentioned three known components.

The electrolytic solution according to the present invention maintains high conductivity even at low temperatures. Therefore, if a lithium secondary battery is constructed using this electrolytic solution, the charge / discharge capacity can be kept high even at low temperatures, and excellent charge / discharge cycle characteristics can be realized.

[0009]

【Example】

[Electrolytic Solution C of Conventional Example] A known mixture of three-component ethylene carbonate (EC), propylene carbonate (PC) and dimethyl carbonate (DMC) in a volume ratio of 1: 1: 2 is a lithium salt in a solvent. An electrolyte solution C is a solution of LiPF ₆ . This is also referred to below as the base electrolyte C.

[Electrolytic Solution B of Comparative Example] An electrolytic solution B is prepared by appropriately mixing the above base electrolytic solution C with methyl formate as a fourth component.

[Electrolytic Solution A of the Present Invention] An electrolytic solution A is prepared by appropriately mixing the above-mentioned base electrolytic solution C with tert-butyl formate as a fourth component.

<< Comparison of Conductivities of Electrolytes >> Samples with various amounts of mixed methyl formate in the electrolytic solution B and samples with various amounts of mixed tert-butyl formate in the electrolytic solution A were compared. Then, the electrical conductivity of these and the base electrolyte C at 20 ° C. and −20 ° C. were measured. The result is shown in FIG.

As is clear from the graph of FIG. 1, an appropriate amount (10 to 50% by volume) of methyl formate is mixed with the base electrolytic solution C or an appropriate amount (10 to 10% by volume) of the base electrolytic solution C.
In the electrolytic solution A in which tert-butyl formate (about 50% by volume) is mixed, the conductivity is obviously improved as compared with the base electrolytic solution C. In particular, the electrolytic solution A of the present invention has better conductivity than the electrolytic solution B of the comparative example.

<< Prototype Example of Lithium Secondary Battery >> A lithium secondary battery having a structure shown in FIG. In FIG. 2, a positive electrode mixture 1 in which carbon powder as a conductive agent and Teflon powder as a binder are mixed with LiCoO ₂ is formed into a sheet shape, which is pressure-bonded to a current collector 2 made of titanium net to form a positive electrode. Further, a negative electrode mixture 3 obtained by mixing carbonaceous powder with EPDM (ethylene propylene diene monomer) as a binder is formed into a sheet,
This is pressure-bonded to a nickel net current collector 4 to form a negative electrode. The sheet shape of the positive electrode mixture 1 and the negative electrode mixture 3 is 1
It is a square of 0 mm × 10 mm. An electrolyte solution described below was put in the beaker 5, and the positive electrode and the negative electrode were placed in the beaker 5 so as to face each other as shown in the drawing.

A prototype battery of the above structure using a conventional base electrolyte C as an electrolyte is referred to as a conventional example (c). A prototype battery using an electrolyte solution B prepared by adding 50% methyl formate to a base electrolyte solution C as an electrolyte solution is referred to as Comparative Example (b). Example (a) is a prototype battery in which 50% tert-butyl formate is added to the base electrolyte solution C as an electrolyte solution.

<< Charge / Discharge Test of Prototype Battery >> A charge / discharge cycle characteristic test was performed at 20 ° C. for the above-mentioned three types of prototype batteries (a), (b) and (c). Charge current and discharge current are 0.
75mA, end-of-charge voltage 4.2V, end-of-discharge voltage 2.8V
And The test results are shown in FIG. The characteristic of the embodiment (a) of the present invention is the best, and then the conventional example (c).
In Comparative Example (b), the capacity was significantly deteriorated as the cycle progressed. Even with the same formate ester, the performance of the electrolytic solution A containing tert-butyl formate according to the present invention was good, and the performance of the comparative electrolytic solution B containing ethyl formate was extremely poor.

It is considered that this is because hydrogen in the aldehyde group of methyl formate is easily released and hydrogen ions are easily generated. Hydrogen ions may accelerate the decomposition of other solvents, causing a significant capacity deterioration during charge / discharge cycles. On the other hand, in tert-butyl formate, the presence of three methyl groups stabilizes the hydrogen of the aldehyde group, and hydrogen ions are less likely to be generated. Therefore, it is presumed that stable charge / discharge cycle characteristics are exhibited without decomposing the solvent.

In the charge / discharge test, the 100th cycle of discharge was performed at a low temperature of -20 ° C. Part 1
The discharge capacity at the 00th cycle is 100 times the discharge capacity at the first time.
When it was set, it was 75% in Example (a) of the present invention, 2% in Comparative Example (b), and 53% in Conventional Example (c).
In the example (a) of the present invention, the capacity decrease at a low temperature is extremely small, and the characteristics are clearly improved as compared with the conventional example (c). The low temperature characteristics of Comparative Example (b) are so poor that they cannot be used at all.

The electrical conductivity characteristics of the electrolytic solution at low temperature shown in FIG. 1 were better than those of the electrolytic solution B, but the charge / discharge cycle characteristics were remarkably deteriorated for the reasons described above. In other words, as the fourth component added to the base electrolyte C, any formate ester is not necessary, but tert-butyl formate is preferable.

In this example, L was used as the lithium salt.
iPF ₆ , LiClO ₄ , LiBF _4, LiCF ₃ S
One or more kinds are appropriately selected from O ₃ , Li (CF ₃ SO ₂ ) ₂ N and the like.

[0021]

INDUSTRIAL APPLICABILITY In the present invention, tert-butyl formate is a novel component of an electrolytic solution for a battery which has hitherto been unknown. By adding an appropriate amount of this tert-butyl formate to the known three components (ethylene carbonate, propylene carbonate, and dimethyl carbonate), this electrolytic solution maintains high conductivity even at low temperatures. Therefore, if a lithium secondary battery is constructed using this electrolyte, the charge / discharge capacity can be kept high even at low temperatures. Moreover, the charge / discharge cycle characteristics are also improved as compared with the conventional one.

[Brief description of drawings]

FIG. 1 is a graph showing conductivity characteristics of an electrolytic solution according to a test example of the present invention.

FIG. 2 is a structural diagram of a beaker cell of a lithium secondary battery used in a test of the present invention.

FIG. 3 is a graph showing charge / discharge cycle characteristics of a prototype battery according to the test of the present invention.

[Explanation of symbols]

 1 Positive electrode mixture 2 Current collector 3 Negative electrode mixture 4 Current collector 5 Beaker

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Nozomu Narita 5-36-11 Shimbashi, Minato-ku, Tokyo Inside Fuji Electric Chemical Co., Ltd.

Claims (2)

[Claims] 1. An electrolytic solution for a battery, which is obtained by appropriately dissolving a lithium salt in a non-aqueous solvent in which ethylene carbonate, propylene carbonate, dimethyl carbonate and tert-butyl formate are mixed. 2. A lithium secondary battery using the battery electrolyte according to claim 1, wherein the lithium metal composite oxide is used as a positive electrode active material, and a carbon material or metallic lithium is used as a negative electrode active material.