JPH0817468A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

(57) [Summary] [Structure] A positive electrode composed of Li _x MO ₂ (wherein M represents one or more transition metals, and 0.05 ≦ x ≦ 1.10), and doping / de-doping of lithium. In a non-aqueous electrolyte secondary battery comprising a negative electrode made of a possible material and a non-aqueous electrolyte solution, two types of electrolyte solutions, lithium borofluoride and lithium hexafluorophosphate, are dissolved in the non-aqueous electrolyte solution. ing. [Effects] When a mixed solute of lithium borofluoride and lithium hexafluorophosphate is used as an electrolyte, the charge and discharge characteristics and the cycle characteristics of the battery are improved as compared with the case where the conventionally used electrolyte is used alone. An excellent non-aqueous electrolyte secondary battery can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to improvement of a non-aqueous electrolyte solution.

[0002]

2. Description of the Related Art In recent years, with the spread of portable devices such as video cameras and radio cassettes, there is an increasing demand for rechargeable secondary batteries instead of disposable primary batteries.

Most of the secondary batteries currently in use are nickel-cadmium batteries using an alkaline electrolyte. However, this battery has a low voltage (about 1.2
V), it is difficult to improve the energy density.
There is also a drawback that the self-discharge rate is high (20% or more per month).

Therefore, a non-aqueous electrolyte secondary battery using a light metal such as lithium for the negative electrode has been studied. This non-aqueous electrolyte secondary battery has the advantages of high voltage (3 V or more), high energy density, little self-discharge, and light weight. However, in a non-aqueous electrolyte secondary battery using this lithium or the like as a negative electrode, when charge and discharge are repeated, metallic lithium or the like crystallizes in dendrite form from the negative electrode and contacts the positive electrode, resulting in a short circuit inside the battery Therefore, it is difficult to put it into practical use.

Therefore, a non-aqueous electrolyte secondary battery has been proposed in which lithium or the like is alloyed with another metal and this alloy is used for the negative electrode. However, this battery has a problem that the alloy forming the negative electrode is granulated when the charge and discharge are repeated, and it is difficult to put the alloy into practical use.

Therefore, as disclosed in, for example, Japanese Patent Application Laid-Open No. 62-90863, a non-aqueous electrolyte secondary battery using a carbonaceous material such as coke as a negative electrode active material has been proposed. This non-aqueous electrolyte secondary battery utilizes doping / dedoping of lithium ions into the carbon layer in the negative electrode reaction, and deposition of metallic lithium as in the case of using metallic lithium or a lithium alloy as the negative electrode active material. ， There is no atomization of the alloy. Therefore, good cycle characteristics can be obtained. Then, as a positive electrode active material, JP-A 1-3
As disclosed in Japanese Patent Publication No. 04664, for example, Li _x M
When the lithium transition metal composite oxide represented by O ₂ (M represents one kind or more than one kind of transition metal and 0.05 ≦ x ≦ 1.10) is used, the battery capacity is improved. A non-aqueous electrolyte secondary battery having high energy density can be obtained.

[0007]

In the non-aqueous electrolyte secondary battery as described above, a non-aqueous electrolytic solution prepared by dissolving an electrolyte in a non-aqueous solvent is used as the electrolytic solution. As this electrolyte, LiAsF ₆ , LiPF ₆ , LiB
F ₄ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiCl
It has been reported that one of O ₄ is used, and above all, LiAsF ₆ and LiPF ₆ can impart excellent charge / discharge performance to the battery. However, LiPF ₆ or L
Although iAsF ₆ is excellent in charge and discharge performance, it cannot be said that iAsF ₆ is necessarily optimum in terms of cycle characteristics.

Therefore, the present invention has been proposed in view of such conventional circumstances, and an object thereof is to provide a non-aqueous electrolyte secondary battery excellent in charge / discharge performance and charge / discharge cycle characteristics. .

[0009]

In order to achieve the above object, the present inventors searched for a suitable solute as an electrolyte of an electrolytic solution of a non-aqueous electrolyte secondary battery, and found that lithium borofluoride (LiBF ₄ ) And lithium hexafluorophosphate (LiPF
By using two kinds were mixed solute and _6), it has led to finding that improves charge-discharge cycle characteristics of the battery.

The present invention has been completed based on such knowledge, and Li _x MO ₂ (where M represents one or more transition metals, and 0.05 ≦ x ≦ 1.10. .)
In a non-aqueous electrolyte secondary battery comprising a positive electrode made of, a negative electrode made of a material capable of doping / dedoping lithium, and a non-aqueous electrolyte, the non-aqueous electrolyte contains lithium borofluoride and hexafluoride. It is characterized in that two types of electrolyte solutions of lithium phosphate are dissolved.

The present invention is applied to a non-aqueous electrolyte secondary battery comprising a positive electrode made of a lithium transition metal composite oxide, a negative electrode made of a material capable of doping / dedoping lithium, and a non-aqueous electrolyte solution. .

In the present invention, in order to further improve the cycle characteristics of such a battery, a mixed solute of lithium borofluoride (LiBF ₄ ) and lithium hexafluorophosphate (LiPF ₆ ) is used as the electrolyte of the electrolytic solution. use. When these mixed solutes of lithium borofluoride and lithium hexafluorophosphate are used as the electrolyte, both charge and discharge characteristics and cycle characteristics of the battery are improved as compared with the case where the conventionally used electrolyte is used alone. It

Here, the weight ratio of LiBF ₄ and LiPF ₆ is such that LiBF ₄ is 5 to 40 with respect to LiPF ₆ .
It is preferably in the weight%. If the weight ratio is out of this range, sufficient charge / discharge cycle characteristics cannot be obtained.

The total concentration of these electrolytes dissolved in the electrolytic solution is preferably 1 to 1.4 mol / l. When the total concentration of the electrolyte dissolved in the electrolytic solution is out of the above range, the electrical conductivity of the electrolytic solution may be low and the charge / discharge cycle characteristics may be insufficient.

As the solvent used in the non-aqueous electrolyte secondary battery in which the above two kinds of electrolytes are dissolved,
Any of the batteries normally used in this type of battery can be used.

For example, propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyl lactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl- 1,3-
Dioxolane, diglymes, triglymes, sulfolane, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, etc. may be used alone or in combination of two or more.

As the negative electrode active material and the positive electrode active material,
Any of the commonly used types of this type can be used.

First, as the negative electrode active material, a carbon material capable of being doped / dedoped with lithium is used. In addition to conductive polymers such as polyacetylene and polypyrrole, coke, polymer charcoal, carbon fibers, etc., per unit volume Pyrolytic carbons, cokes (petroleum coke, pitch coke, coal coke, etc.), carbon black (acetylene black, etc.), glassy carbon, organic polymer material fired body (organic polymer material) Is fired at a suitable temperature of 500 ° C. or higher in an inert gas stream or in a vacuum), carbon fiber and the like are preferable.

On the other hand, as the positive electrode active material, Li _x MO ₂
(However, M is at least one transition metal, preferably Co, N
represents at least one of i and Mn, and 0.05 ≧ x ≧ 1.
It is 10. ), A composite metal oxide composed of lithium and a transition metal, or an intercalation compound containing lithium is used.

The battery shape to which the present invention is applied is not particularly limited, and is a so-called coin type, a button type, a spiral type (so-called jelly roll type), a cylindrical type, a laminated type rectangular type, or a card type. It may be either.

[0021]

In the non-aqueous electrolyte secondary battery of the present invention, since the mixed solute of lithium borofluoride and lithium hexafluorophosphate is used as the electrolyte of the electrolytic solution, excellent charge / discharge performance and cycle characteristics can be obtained.

[0022]

EXAMPLES Preferred examples of the present invention will be described based on experimental results.

Investigation of Optimum Total Concentration of Electrolyte First, the negative electrode 1 was prepared as follows. Petroleum pitch is used as a starting material, and a functional group containing oxygen in 10
After introducing 20% (so-called oxidative crosslinking), it was fired at 1000 ° C. in an inert gas stream to obtain a soft graphitized carbon material.
Regarding the non-graphitizable carbon material obtained at this time, X
As a result of line diffraction measurement, the spacing between (002) planes was 3.
It was 76Å and the true specific gravity was 1.58.

This non-graphitizable carbon material was crushed to obtain a carbon material powder having an average particle diameter (average volume diameter) of 10 μm. Then, 90 parts by weight of this carbon material powder is mixed with 10 parts by weight of polyvinylidene fluoride as a binder to prepare a negative electrode mixture, and this negative electrode mixture is dispersed in a solvent N-methyl-2-pyrrolidone to form a slurry. To prepare a negative electrode slurry.

Then, the negative electrode slurry thus obtained is uniformly applied to both sides of a strip-shaped copper foil having a thickness of 10 μm which serves as a negative electrode collector, dried and then compression-molded by a roll press machine to form a strip-shaped strip. A negative electrode 1 was produced.

Next, the positive electrode 2 was produced as follows. Lithium carbonate and cobalt carbonate were mixed in a ratio of 0.5 mol: 1 mol, and the mixture was baked in air at 900 ° C. for 5 hours to obtain LiCoO ₂ .

LiCoO ₂ 91 thus obtained
6 parts by weight of graphite as a conductive material and 3 parts by weight of polyvinylidene fluoride as a binder are mixed to prepare a positive electrode mixture, and the positive electrode mixture is dispersed in a solvent N-methyl-2-pyrrolidone. The mixture was made into a slurry to prepare a positive electrode slurry.

The positive electrode slurry thus obtained was evenly applied to both sides of a 20 μm-thick strip-shaped aluminum foil serving as a positive electrode current collector, dried, and then compression-molded by a roll press machine to form a strip-shaped strip. A positive electrode 2 was produced.

Then, as shown in FIG. 1, the strip negative electrode 1,
The strip-shaped positive electrode 2 and the separator 3 made of a microporous polypropylene film are preliminarily length-sized so as to be properly accommodated in a battery can 5 having a diameter of 18.0 mm and a height of 65.0 mm when used as a spiral electrode element. The width and the width of the spiral electrode were adjusted in advance to produce a spiral electrode.

The spiral electrode thus produced was housed in a nickel-plated iron battery can 5, and insulating plates 4 were arranged on both upper and lower surfaces of the housed spiral electrode. Then, from the positive electrode current collector 11 to the aluminum positive electrode lead 13
The nickel negative electrode lead 12 was led out from the negative electrode current collector 10 and welded to the battery can 5.

Then, lithium hexafluorophosphate (LiPF ₆ ) was dissolved at a concentration shown in Table 1 in a mixed solvent of propylene carbonate 50% by volume and diethyl carbonate 50% by volume to prepare an electrolytic solution. The liquid was poured into the battery can 5. Then, the battery lid 5 was fixed by caulking the battery can 5 via the insulating gasket 6 coated with asphalt, and a cylindrical nonaqueous electrolyte battery having a diameter of 18.0 mm and a height of 65.0 mm was produced.

[0032]

[Table 1]

Regarding the battery 1 produced as described above,
The charge / discharge cycle was repeated under the condition of a temperature of 23 ° C., and the capacity retention ratio for the second cycle was examined at each cycle. The result is shown in FIG. During the charging / discharging cycle, charging was performed at a charging voltage of 4.2 V at a maximum and a constant current of 1 A for 2.0 hours, and discharging was performed at a constant current of 700 mA to a final voltage of 2.75V.

From FIG. 2, the concentration of the electrolytic solution is 1.6 mol / l.
It can be seen that if it is too high, the cycle characteristics deteriorate. Therefore, from this, it is understood that the total concentration of the electrolyte dissolved in the electrolytic solution is appropriately 1.0 mol / l to 1.4 mol / l. In the following Examples 1 to 5 and Comparative Example 1, the electrolyte concentration was 1.2 mol based on this result.
/ L was set to make a battery, and the characteristics were examined.

Examples 1 to 5 When preparing the electrolytic solution, the procedure was carried out except that LiPF ₆ and LiBF ₄ were dissolved in a mixed solvent of 50% by volume of propylene carbonate and 50% by volume of diethyl carbonate at the concentrations shown in Table 2. A cylindrical non-aqueous electrolyte battery was produced in the same manner as in Example 1.

[0036]

[Table 2]

Comparative Example 1 Example 1 was repeated except that LiPF ₆ alone was dissolved at a concentration of 1.2 mol / l in a mixed solvent of propylene carbonate 50% by volume and diethyl carbonate 50% by volume when preparing an electrolytic solution.
A cylindrical non-aqueous electrolyte battery was produced in the same manner as in.

With respect to the battery manufactured as described above, the charge / discharge cycle was repeated in the same manner as described above, and the capacity retention rate at each cycle was examined. The result is shown in FIG.

As can be seen from FIG. 3, in the batteries of Examples 1 to 5 in which lithium hexafluorophosphate (LiPF ₆ ) and lithium borofluoride (LiBF ₄ ) were mixed and used as the electrolyte of the electrolytic solution, the capacity was maintained in each cycle. The rate is larger than that of the battery of Comparative Example 1 in which only LiPF ₆ is used as the electrolytic solution, and the cycle characteristics are good. From these results, the use of the mixed solute of lithium hexafluorophosphate (LiPF ₆ ) and lithium borofluoride (LiBF ₄ ) as the solute of the electrolytic solution of the non-aqueous electrolyte secondary battery improves the cycle characteristics of the battery. It turns out that is possible.

In particular, LiPF ₆ : LiBF ₄ is 2:
Excellent cycle characteristics were obtained with the batteries of Examples 3 to 5 in which the ratio was 1 to 11: 1 (about 5% to 31% in terms of molar ratio and weight ratio). From this, it is understood that it is appropriate that the weight ratio of LiBF ₄ to LiPF _{6 be} 5 to 40% by weight.

In this experimental example, the carbon material was used as the negative electrode, but it goes without saying that other materials may be used. The positive electrode active material may be Li _x M as described above.
O ₂ (where M is at least one transition metal, preferably C
represents at least one of o, Ni, Mn, and 0.05 ≧ x
≧ 1.10. In the above description, a composite metal oxide composed of lithium and a transition metal, an intercalation compound containing lithium, and the like are described, but other materials may be used as a matter of course.

[0042]

As is apparent from the above description, in the present invention, the mixed solute of lithium borofluoride and lithium hexafluorophosphate is used as the electrolyte of the electrolytic solution in the non-aqueous electrolyte secondary battery. It is possible to obtain a non-aqueous electrolyte secondary battery that is excellent in terms of charge / discharge performance and cycle characteristics. Therefore, it will greatly contribute to the popularization of portable devices using secondary batteries.

[Brief description of drawings]

FIG. 1 is a schematic vertical cross-sectional view showing the structure of a non-aqueous electrolyte secondary battery to which the present invention is applied.

FIG. 2 is a characteristic diagram showing charge / discharge cycles of a battery using a mixed solute of LiPF ₆ and LiBF ₄ as an electrolyte and a battery using LiPF ₆ alone.

FIG. 3 is a characteristic diagram showing a charge / discharge cycle when the electrolyte concentration of an electrolytic solution is changed.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Negative electrode 2 Positive electrode 3 Separator 4 Insulating plate 5 Battery can 6 Insulation sealing gasket 7 Battery lid 10 Negative electrode current collector 11 Positive electrode current collector 12 Negative electrode lead 13 Positive electrode lead

Claims (3)

[Claims] 1. A positive electrode composed of Li _x MO ₂ (wherein M represents one or more transition metals and 0.05 ≦ x ≦ 1.10) and a material capable of being doped / dedoped with lithium. In a non-aqueous electrolyte secondary battery comprising a negative electrode and a non-aqueous electrolyte solution, two kinds of electrolyte solutions, lithium borofluoride and lithium hexafluorophosphate, are dissolved in the non-aqueous electrolyte solution. Characteristic non-aqueous electrolyte secondary battery. 2. The non-aqueous electrolyte secondary according to claim 1, wherein the content of lithium borofluoride in the non-aqueous electrolyte is 5 to 40% by weight of the content of lithium hexafluorophosphate. battery. 3. The total concentration of the electrolyte in the non-aqueous electrolyte is 1 to
It is 1.4 mol / l, The non-aqueous electrolyte secondary battery of Claim 1 or Claim 2 characterized by the above-mentioned.