JPH07176323A - Electrolytic solution and negative electrode for li secondary battery - Google Patents

Abstract

PURPOSE:To provide an electrolytic solution and a negative electrode capable of forming a Li secondary battery excellent in the charge/discharge efficiency, discharge capacity, and charge/discharge cycle life and rarely generating LiOH on the surface of the negative electrode. CONSTITUTION:Carbon dioxide is dissolved in an electrolytic solution dissolved with lithium salt in a nonaqueous solvent. The surface is coated with lithium carbonate 2. Even when Li ion-conductive lithium carbonate 2 is generated or exists on the surface of the negative electrode and moisture exists in the electrolyte, the inhibition of the battery reaction due to the generation or existence of LiOH can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolytic solution and a negative electrode capable of forming a Li secondary battery which is less likely to generate lithium hydroxide on the negative electrode surface and is excellent in charge / discharge efficiency, discharge capacity, charge / discharge cycle life and the like.

[0002]

2. Description of the Related Art Conventionally, as an electrolytic solution used for forming a Li secondary battery, a solution in which a lithium salt is dissolved in a non-aqueous solvent such as an organic solvent has been known. However, there have been problems that charge and discharge efficiency is lowered by repeated charge and discharge, the discharge capacity is lowered, and the charge and discharge cycle life is poor. In addition, when lithium or a lithium alloy is used for the negative electrode, there are problems that dendrites are easily generated on the surface and easily grow, resulting in a significant decrease in battery function and a short circuit between the positive and negative electrodes due to penetration of the separator (electrolyte layer) Even in the case of a carbon-based negative electrode, there is a problem that the negative electrode performance is significantly deteriorated.

The inventors of the present invention have made extensive studies to overcome the above-mentioned problems, and have found out that the problems such as the decrease in charge / discharge efficiency are due to the generation of LiOH and the like in the negative electrode. That is, LiOH or Li ₂ O is generated on the surface of a negative electrode such as lithium, a lithium alloy system, or a carbon system in which lithium is intercalated between layers, and Li ions are conducted more than Li ₂ O having Li ion conductivity. Not LiO
H is overwhelmingly generated and inhibits charge / discharge on the surface of the negative electrode, and in the case of lithium or lithium alloy type negative electrode, current is concentrated on the surface portion where charge / discharge is not inhibited by generation of LiOH and dendrite is generated and grown. In the case of a carbon-based negative electrode, it was found that Li ions, which cannot be intercalated, are electrodeposited on the electrode surface due to the decrease in the negative electrode capacity due to the formation of LiOH. Further, the production of LiOH or the like is carried out with a trace amount of water in which lithium or its ions are present in the electrolytic solution, usually 20 to
It was also clarified that it was due to a reaction with about 50 ppm of water as shown in the following formulas (a) and (b). _{(B) 2Li + 2H 2 O = 2LiOH} + H 2 ( _{b) 2Li + H 2 O = Li} 2 O + H 2

[0004]

Therefore, the present invention is excellent in charge / discharge efficiency, discharge capacity, charge / discharge cycle life and the like.
The purpose of the present invention is to obtain an electrolytic solution and a negative electrode that can form a secondary battery and in which LiOH is not easily generated on the negative electrode surface.

[0005]

The present invention is characterized in that carbon dioxide is dissolved in an electrolytic solution in which a lithium salt is dissolved in a non-aqueous solvent, and an electrolytic solution for a Li secondary battery, and a carbon dioxide surface The present invention provides a negative electrode for a Li secondary battery, which is characterized by being coated with lithium.

[0006]

[Function] By dissolving carbon dioxide in the electrolyte,
Based on the reaction represented by the following formulas (a) and (b), LiOH
And Li ₂ O change to Li ₂ CO ₃ , and due to this, the battery reaction inhibitor generated on the negative electrode surface is changed to Li ₂ CO ₃ which is Li ion conductive of LiOH, so that charging and discharging can also be performed at that part. Thus, a Li secondary battery having excellent charge / discharge efficiency, discharge capacity, charge / discharge cycle life and the like can be formed. (A) 2LiOH + CO ₂ = Li ₂ CO ₃ + H ₂ O (b) Li ₂ O + CO ₂ = Li ₂ CO ₃

In the above formula (a), water is produced,
By repeating the reaction of the above formula (a) and the above formula (a), the portion covered with lithium carbonate on the negative electrode surface gradually increases and reaches the entire negative electrode surface, whereby the original negative electrode portion is electrolyzed. Since it does not come into direct contact with the liquid, L in the above formula (a)
The iOH generation reaction etc. stops.

In the above, it is understood that the production of LiOH can be prevented by eliminating the water in the electrolytic solution. However, as described above, LiOH is produced in the presence of a trace amount of water of about 20 to 50 ppm, and the LiOH is formed in the electrolytic solution. It is practically difficult to reduce the water content to below such a level, and external contamination may occur. Therefore, the present invention has a practically advantageous advantage of allowing the presence of water in the electrolytic solution.

[0009]

EXAMPLES The electrolytic solution of the present invention was prepared by dissolving carbon dioxide in an electrolytic solution prepared by dissolving a lithium salt in a non-aqueous solvent, and the negative electrode of the present invention had a surface coated with lithium carbonate. , These are for forming a Li secondary battery. Examples of the negative electrode are shown in FIGS. Reference numeral 1 is a negative electrode, and 2 is a lithium carbonate coating layer. As is clear from the illustrated example, the lithium carbonate coating layer may be provided at least in a portion of the negative electrode which is in contact with the electrolytic solution.

As the non-aqueous solvent in the electrolytic solution, a suitable non-water solvent that dissociates the lithium salt can be used. Examples thereof include ethylene carbonate, propylene carbonate, sulfolane, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, methyl formate, methyl acetate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, and the like, or two or more thereof. Examples include those used.

As the lithium salt, an appropriate lithium compound capable of dissociating Li (complex) ions can be used. Examples include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiCF ₃ S.
O ₃ , LiAsF ₆ , LiAlCl ₄ , LiI and the like can be mentioned. The amount of the lithium salt used is generally 0.1 to 3 mol per liter of the non-aqueous solvent, but is not limited to this and may be appropriately determined depending on the desired battery performance and the like.

Dissolution of carbon dioxide can be carried out by an appropriate method such as a method of bubbling carbon dioxide gas to the electrolytic solution or a method of bringing carbon dioxide gas into contact with the electrolytic solution to apply pressure. The amount to be dissolved is at least an amount capable of converting LiOH or the like produced by the reaction with water in the electrolytic solution into Li ₂ CO ₃ , or Li ₂ C required to cover the required area of the negative electrode.
It is an amount that can generate O ₃ . Generally, even if carbon dioxide remains in the electrolytic solution, there is no problem, and therefore an excessive amount of carbon dioxide is dissolved.

The electrolyte is formed by, for example, a method of dissolving carbon dioxide in a non-aqueous solvent in which a predetermined amount of lithium salt is dissolved, a method of dissolving the lithium salt in a non-aqueous solvent in which carbon dioxide is dissolved, or the like. be able to. In that case,
2-methylfuran, thiophene, pyrrole, if necessary
Dendritic growth inhibitors such as crown ether,
4-dicyanobutane, 1,4-dicyano-2-butene,
Additives such as an electrolytic solution decomposition inhibitor (improvement of charging voltage) composed of a cyano group-containing compound such as succinonitrile can also be blended. When forming the electrolytic solution, it is preferable to reduce the content of water as much as possible, for example, by using the one purified by distillation.

The electrolytic solution of the present invention is for forming a Li secondary battery, but the Li secondary battery to be formed is not particularly limited except that such an electrolytic solution is used. Therefore, there is no particular limitation on the forming material such as the positive electrode and the negative electrode,
Further, there is no particular limitation on the battery form or the like. The electrolyte is
A method that is used as it is according to the form of the battery for which it is formed,
It can be used in an appropriate method such as a method of holding it on a support.

Incidentally, as the positive electrode, for example, MnO ₂ ,
LiCoO ₂ , Li _w Co _1-xy M _x P _y O _{2 + z} (where M is one or more transition metals, w is 0 <w ≦ 2, and x is 0).
≦ x <1, y is 0 <y <1, and z is −1 ≦ z ≦ 4. ), Or Li or Li · Co phosphate and /
Alternatively, the active material may be, for example, an oxide containing Co or Li · Co and containing 0.1 mol or more of Co and 0.2 mol or more of P per 1 mol of Li as an active material.

Examples of the negative electrode include an electrode made of lithium or a lithium alloy and a carbon-based electrode in which lithium is intercalated between layers. Examples of lithium alloys include Li and Al, Mg, Pb, Sn,
Appropriate binary or ternary or more alloys with metals such as In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, and Sr, and C can be used. If necessary, Si, Cd, Zn, La or the like may be added, or an alloy of Li and Pb may be added with La or the like to improve mechanical properties.

Further, when forming a coin-type or button-type battery, or a battery in which positive and negative electrodes are wound with a tape-like material, the separator interposed between the positive and negative electrodes is also made of polypropylene or the like. It can be carried out by an appropriate method according to a conventional method such as a method of impregnating or filling a porous polymer film, a glass filter, or a gel-like substance with an electrolytic solution.

The tape-shaped positive and negative electrodes may be made of a conductive material such as acetylene black or Ketjen black, and polytetrafluoroethylene, polyethylene, polyolefin oxide, polyvinyl alcohol, or a solid electrolyte as necessary. It can be formed by an appropriate method such as a casting method, a compression molding method, or a roll molding method together with the binder.
The positive and negative electrodes may be bonded to a current collector tape by an appropriate method such as soldering, brazing, ultrasonic welding, spot welding, coating with a binder resin, or the like.

Examples of the current collector tape include copper,
A conductive tape made of aluminum, silver, nickel or the like is used. Such a current collector tape is, for example, a conductor that is difficult to react with lithium such as nickel or iron or / and a liquid such as silver, copper, zinc, magnesium, aluminum, calcium, barium, bismuth, indium, lead, platinum, palladium, tin. It may be covered with a conductor having an affinity with lithium.

In the above, when a negative electrode having the surface of the present invention coated with lithium carbonate is formed in advance and used as the negative electrode of a Li secondary battery, an electrolytic solution in which carbon dioxide is not dissolved may be used. The negative electrode can be formed by, for example, a method of plating the negative electrode with a system according to the Li secondary battery using the carbon dioxide-dissolved electrolyte described above, and the coating thickness thereof is usually about 10 to 1000Å. However, it is not limited to this.

The Li secondary battery thus formed can be charged by an appropriate method such as a method of continuously supplying a constant current or a method of supplying a pulse current using a pulse power source. The charging method using a pulse current has the advantage that dendrites are more difficult to grow because the concentration change is suppressed because energization / stopping is repeated.

Example 1 LiBF ₄ vacuum-dried at 80 ° C. for 24 hours in a 50/50 (volume ratio) mixed solvent of distilled and purified ethylene carbonate and dimethyl carbonate was dissolved at a concentration of 1 mol / l, and its Karl Fischer was dissolved. 20 by measurement with a moisture meter
High-purity carbon dioxide gas having a water content of 0.1 ppm or less was bubbled into a 100 ppm or less solution at a flow rate of 100 ml / min for 1 hour (room temperature) to dissolve the carbon dioxide gas to obtain an electrolytic solution.

A porous polypropylene film having a thickness and a diameter of 25 μm is impregnated with the electrolytic solution to form a separator, and a positive electrode and a negative electrode are arranged above and below the separator to form a coin-type Li secondary battery cell. did.

In the above-mentioned positive electrode, lithium carbonate, basic cobalt carbonate and an aqueous phosphoric acid solution having a phosphoric acid content of 85% are mixed at an atomic ratio of Li: Co: P = 1: 0.5: 0.5, It is put in an alumina crucible and heat-treated at 900 ° C. for 24 hours to form a mixture of lithium phosphate, lithium cobalt phosphate and cobalt oxide (active material), which is crushed by a ball mill. Powder having a particle size of 20 μm or less, and mixed with 8 parts by weight of the powder, 1 part by weight of acetylene black and 1 part by weight of polyvinylidene fluoride
Press molding of 00mg onto nickel mesh, diameter 2
It is a disk-shaped positive electrode with a thickness of 0 mm, and the negative electrode has a thickness of 0.2.
A metal lithium sheet having a diameter of 20 mm is punched out to have a diameter of 20 mm, and a nickel mesh is press-bonded to one surface of the sheet to form a disk-shaped negative electrode.

Example 2 In accordance with Example 1 using an electrolyte solution in which carbon dioxide gas was not dissolved, L
An i secondary battery cell was formed, and the same carbon dioxide gas as in Example 1 was filled in the cell and a pressure of 3 atm was applied to obtain a Li secondary battery cell composed of an electrolytic solution in which carbon dioxide gas was dissolved.

Comparative Example A Li secondary battery cell was formed in the same manner as in Example 1 except that an electrolytic solution in which carbon dioxide gas was not dissolved was used.

Evaluation test: The Li secondary battery cells obtained in Examples and Comparative Examples were repeatedly charged and discharged under the conditions of a charge / discharge current of 0.5 mA, a charge voltage of 4.2 V and a discharge voltage of 3.0 V to examine the cycle characteristics. The results are shown in Table 1. In Table 1, the first cycle of Example 1 was set to 100, and other cases were evaluated based on this.

[0028]

[Table 1]

[0029]

According to the present invention, even if Li ion conductive lithium carbonate is produced or present on the surface of the negative electrode and water is present in the electrolytic solution, the inhibition of the battery reaction due to the production or persistence of LiOH can be prevented. It is possible to form a Li secondary battery having excellent charge / discharge efficiency, discharge capacity, charge / discharge cycle life and the like.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an example of a negative electrode.

FIG. 2 is a sectional view showing an example of another negative electrode.

[Explanation of symbols]

 1: Negative electrode 2: Lithium carbonate coating layer

Claims (2)

[Claims] 1. An electrolytic solution for a Li secondary battery, wherein carbon dioxide is dissolved in an electrolytic solution in which a lithium salt is dissolved in a non-aqueous solvent. 2. A negative electrode for a Li secondary battery, the surface of which is coated with lithium carbonate.