JP2002313416A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-aqueous electrolyte secondary battery with less deterioration of capacity in charging and discharging cycles and a long service life. SOLUTION: At least one type of alkyl (trimethylsilyl) acetate shown by formula 4 is contained in a non-aqueous electrolyte. In the formula 4, R denotes an alkyl group.

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.

[0002]

2. Description of the Related Art In recent years, with the rapid reduction in size and weight and diversification of consumer mobile phones, portable devices, portable information terminals, and the like, the batteries used as power sources have been reduced in size, weight and energy density. Further, there is a strong demand for the development of a secondary battery capable of repeatedly charging and discharging for a long period of time. Above all,
Compared to lead batteries and nickel cadmium batteries that use aqueous electrolytes, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are the most promising secondary batteries that meet these needs, and active research is underway. It is being done.

[0003] The positive electrode active material of a non-aqueous electrolyte secondary battery includes general materials such as titanium disulfide, vanadium pentoxide and molybdenum trioxide, as well as lithium cobalt composite oxide, lithium nickel composite oxide and spinel type manganese oxide. Various compounds represented by the formula Li _x MO ₂ (where M is one or more transition metals) are being studied. Among them, lithium cobalt composite oxide, lithium nickel composite oxide, spinel type lithium manganese oxide, and the like are charged and discharged at a very noble potential of 4 V (vs Li / Li ⁺ ) or more, so that they are used as a positive electrode. A battery having a high discharge voltage can be realized.

As the negative electrode active material of the nonaqueous electrolyte secondary battery, various materials such as lithium metal, a lithium alloy, and a carbon material capable of inserting and extracting lithium have been studied. Then, there is an advantage that a battery having a long cycle life is obtained and safety is high.

[0005] The electrolyte of a non-aqueous electrolyte secondary battery generally includes a mixed solvent of a high dielectric constant solvent such as ethylene carbonate or propylene carbonate and a low viscosity solvent such as dimethyl carbonate or diethyl carbonate, such as LiPF ₆ or LiBF ₄ . An electrolytic solution in which a supporting salt is dissolved is used.

[0006]

However, in the non-aqueous electrolyte secondary battery, as the charge-discharge cycle progresses, the decomposition of the supporting salt and the solvent in the non-aqueous electrolyte progresses on the negative electrode, and the electrolyte decreases. Also, there is a problem that a film having poor conductivity is deposited on the surface of the negative electrode, hinders the movement of lithium ions, and reduces the discharge capacity.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a non-aqueous electrolyte secondary battery which has a small capacity deterioration during a charge / discharge cycle and a long life. It is in.

[0008]

According to a first aspect of the present invention, there is provided a non-aqueous electrolyte secondary battery, wherein the non-aqueous electrolyte contains at least one alkyl (trimethylsilyl) acetate represented by the following chemical formula (2). And However, [Formula 2]
In the above, R represents an alkyl group.

[0009]

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According to the first aspect of the present invention, a long-life non-aqueous electrolyte secondary battery that has a small capacity deterioration during a charge / discharge cycle can be obtained.

According to a second aspect of the present invention, in the nonaqueous electrolyte secondary battery, the alkyl (trimethylsilyl) acetate is ethyl (trimethylsilyl) acetate. According to a third aspect of the present invention, in the non-aqueous electrolyte secondary battery, the alkyl (trimethylsilyl) acetate is methyl (trimethylsilyl) acetate.

According to the second or third aspect of the present invention, a non-aqueous electrolyte secondary battery having good cycle characteristics can be obtained.

According to a fourth aspect of the present invention, in the non-aqueous electrolyte secondary battery, the non-aqueous electrolyte contains 2.0% by weight or less of alkyl (trimethylsilyl) acetate.

According to the fourth aspect of the present invention, a non-aqueous electrolyte secondary battery having a large initial discharge capacity and good cycle characteristics can be obtained.

[0015]

Embodiments of the present invention will be described below.

The present invention relates to a non-aqueous electrolyte secondary battery,
The non-aqueous electrolyte contains at least one kind of alkyl (trimethylsilyl) acetate represented by the following formula (3). However, in [Formula 3], R represents an alkyl group.

[0017]

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It is particularly effective when the alkyl (trimethylsilyl) acetate is ethyl (trimethylsilyl) acetate or methyl (trimethylsilyl) acetate.

By including alkyl (trimethylsilyl) acetate in the non-aqueous electrolyte, a good SEI is formed on the surface of the negative electrode active material, so that the subsequent decomposition of the non-aqueous electrolyte on the surface of the negative electrode active material is suppressed. As a result, a non-aqueous electrolyte secondary battery with a small capacity deterioration during a charge / discharge cycle and a long life can be obtained. The alkyl (trimethylsilyl) acetate contained in the non-aqueous electrolyte is 1
Seeds may be used, or two or more kinds may be mixed.

Here, SEI (Solid Electr)
“Olyte Interphase” refers to a passivation film formed on the surface of the lithium metal or carbon material when the lithium metal or carbon material is initially charged in a non-aqueous electrolyte by reducing the solvent in the electrolyte. Edited by Masayuki Oo and Akiya Ozawa, "Lithium ion secondary batteries-materials and applications", Nikkan Kogyo Shimbun (1996). Then, the SEI formed on the surface of the lithium metal or carbon material functions as a lithium ion conductive protective film, and the subsequent reaction between the lithium metal or the carbon material and the solvent is suppressed.

Further, the present invention is characterized in that the non-aqueous electrolyte contains 2.0% by weight or less of alkyl (trimethylsilyl) acetate. If the non-aqueous electrolyte contains a small amount of alkyl (trimethylsilyl) acetate, good SEI is formed on the surface of the negative electrode active material. %, The initial discharge capacity becomes smaller.

As the positive electrode active material of the non-aqueous electrolyte secondary battery according to the present invention, any compound can be used as long as it can absorb and release lithium ions.
i _x MO ₂ (however, M represents one or more transition metals) is preferable to use singly or in two or more compounds mainly and _Li x _M 2 _{O 4,}
Further, from the height of the discharge voltage, as the transition metal M, C
It is more preferable to use at least one transition metal selected from the group consisting of o, Ni and Mn.

The negative electrode may be a carbon material such as coke, vitreous carbon, graphite, non-graphitizable carbon, pyrolytic carbon, carbon fiber, or metal lithium, lithium alloy, polyacene or the like. Although the above can be used in combination, it is particularly preferable to use a carbonaceous material from the viewpoint of high safety.

As the solvent of the non-aqueous electrolyte, ethylene carbonate, propylene carbonate, butylene carbonate, trifluoropropylene carbonate, γ-butyrolactone, sulfolane, 1,2-dimethoxyethane,
1,2-diethoxyethane, tetrahydrofuran, 2-
Non-aqueous solvents such as methyltetrahydrofuran, 3-methyl-1,3-dioxolane, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, methyl propyl carbonate, etc. , Alone or a mixed solvent thereof can be used. The non-aqueous electrolyte is used by dissolving a supporting salt in these non-aqueous solvents. As the supporting salt, LiClO ₄ ,
LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ C
O ₂ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , L
iCF ₃ CF ₂ CF ₂ SO ₃ , LiN (SO ₂ CF ₃ )
₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ , LiN (COC
F ₃ ) ₂ , LiN (COCF ₂ CF ₃ ) ₂ and LiP
Salts such as F ₃ (CF ₂ CF ₃ ) ₃ or mixtures thereof can be used.

Also, a solid ion-conductive polymer electrolyte can be used instead of a liquid electrolyte. When the polymer electrolyte membrane is made of polyethylene oxide, polyacrylonitrile, polyethylene glycol, or a modified product thereof, it is lightweight and flexible, and is advantageous when used for a wound electrode plate. Furthermore, an ion conductive polymer electrolyte membrane and an organic electrolyte can be used in combination. In addition, as the electrolyte, in addition to the polymer electrolyte, a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte,
Alternatively, any known material such as an inorganic solid powder bound by an organic binder can be used.

The non-aqueous electrolyte secondary battery according to the present invention is generally composed of a combination of a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte, and the separator is a porous polyvinyl chloride membrane. Such a porous polymer membrane or a lithium ion or ionic conductive polymer electrolyte membrane can be used alone or in combination.

[0027]

EXAMPLES The present invention will be described below with reference to preferred examples, but it goes without saying that the present invention is not limited to the following without departing from the gist of the present invention.

Example 1 A prismatic nonaqueous electrolyte secondary battery using lithium cobalt oxide as a positive electrode active material and a carbon material as a negative electrode active material was manufactured. FIG. 1 is a diagram showing a cross-sectional structure of a prismatic nonaqueous electrolyte secondary battery. In FIG. 1, 1 is a prismatic nonaqueous electrolyte secondary battery, 2 is a wound electrode group, 3 is a positive electrode,
Is a negative electrode, 5 is a separator, 6 is a battery case, 7 is a battery cover, 8 is a safety valve, 9 is a positive electrode terminal, and 10 is a positive electrode lead. The wound electrode group 2 is obtained by winding a positive electrode 3 and a negative electrode 4 via a separator 5. The wound electrode group 2 is housed in a battery case 6, a safety valve 8 is provided in the battery case 6, and the battery lid 7 and the battery case 6 are sealed by laser welding. Positive electrode terminal 9 is connected to positive electrode lead 10, and negative electrode 4 is connected to the inner wall of battery case 6 by contact.

[0029] The positive electrode mixture, LiCoO ₂ 9 as the active material
0 parts by weight, 5 parts by weight of acetylene black as a conductive agent,
A paste was manufactured by mixing 5 parts by weight of polyvinylidene fluoride (PVdF) as a binder to prepare a positive electrode mixture and dispersing the mixture in N-methyl-2-pyrrolidone (NMP). This paste was uniformly applied to an aluminum current collector having a thickness of 20 μm, dried, and then compression molded by a roll press to produce a positive electrode.

The negative electrode mixture was prepared by mixing 90 parts by weight of a carbon material capable of inserting and extracting lithium ions and 10 parts by weight of PVdF as a binder, appropriately adding NMP and dispersing the mixture to prepare a slurry. The slurry was uniformly applied to a 15 μm-thick copper current collector, dried, dried at 100 ° C. for 5 hours, and then compression-molded by a roll press to produce a negative electrode.

A microporous polyethylene film having a thickness of about 20 μm was used as a separator. These positive and
The negative electrode and the separator were wound to form a wound electrode group. In the electrolyte, 1.0 M of LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 1: 1. Ethyl (trimethylsilyl) acetate was contained in the electrolyte at 0.01 wt%. A prismatic nonaqueous electrolyte secondary battery was prepared using the nonaqueous electrolyte.

Example 2 Ethyl (trimethylsilyl)
A prismatic nonaqueous electrolyte secondary battery of Example 2 was prepared in the same manner as in Example 1, except that the content of acetate was 0.1 wt%.

Example 3 Ethyl (trimethylsilyl)
A prismatic nonaqueous electrolyte secondary battery of Example 3 was produced in the same manner as in Example 1 except that the content of acetate was 0.5 wt%.

Example 4 Ethyl (trimethylsilyl)
A prismatic nonaqueous electrolyte secondary battery of Example 4 was produced in the same manner as in Example 1 except that the content of acetate was changed to 1.0% by weight.

Example 5 Ethyl (trimethylsilyl)
A prismatic nonaqueous electrolyte secondary battery of Example 5 was produced in the same manner as in Example 1 except that the content of acetate was 2.0 wt%.

Example 6 Ethyl (trimethylsilyl)
A prismatic nonaqueous electrolyte secondary battery of Example 6 was produced in the same manner as in Example 1, except that the content of acetate was 5.0 wt%.

Example 7 Ethyl (trimethylsilyl)
A prismatic nonaqueous electrolyte secondary battery of Example 7 was prepared in the same manner as in Example 1 except that methyl (trimethylsilyl) acetate was used instead of acetate, and the content of methyl (trimethylsilyl) acetate was changed to 0.01 wt%. did.

Example 8 Methyl (trimethylsilyl)
A prismatic nonaqueous electrolyte secondary battery of Example 8 was produced in the same manner as in Example 7, except that the content of acetate was 0.1 wt%.

Example 9 Methyl (trimethylsilyl)
A prismatic nonaqueous electrolyte secondary battery of Example 9 was produced in the same manner as in Example 7, except that the content of acetate was changed to 0.5% by weight.

Example 10 A prismatic nonaqueous electrolyte secondary battery of Example 10 was prepared in the same manner as in Example 7, except that the content of methyl (trimethylsilyl) acetate was changed to 1.0 wt%.

Example 11 A prismatic nonaqueous electrolyte secondary battery of Example 11 was prepared in the same manner as in Example 7, except that the content of methyl (trimethylsilyl) acetate was changed to 2.0% by weight.

Example 12 A prismatic nonaqueous electrolyte secondary battery of Example 12 was produced in the same manner as in Example 7 except that the content of methyl (trimethylsilyl) acetate was changed to 5.0 wt%.

Comparative Example 1 Ethyl (trimethylsilyl)
A prismatic non-aqueous electrolyte secondary battery of Comparative Example 1 was prepared in the same manner as in Example 1 except that no acetate was contained.

Each of the batteries of Examples 1 to 12 and Comparative Example 1 was manufactured by 10 cells. These batteries were charged at a constant current and constant voltage of 4.2 V with a current of 1 CA for 3 hours, and then discharged to 3 V with a current value of 1 CA.
The initial discharge capacity was confirmed. Thereafter, the same charge / discharge cycle was repeated 500 times, and the capacity retention (%) after 500 cycles was measured. Table 1 shows the results. Here, the “capacity retention rate” indicates the ratio (%) of the discharge capacity after 500 cycles to the initial discharge capacity.

[0045]

[Table 1]

As shown in Table 1, Examples 1 to 5 and Examples 7 to
In the battery of No. 11, the initial discharge capacity was 600 mAh or more,
Although the capacity retention was 85% or more, in the batteries of Example 6 and Example 12, the initial discharge capacity was smaller than 600 mAh, and the capacity retention was 81 to 82%.

Thus, ethyl (trimethylsilyl)
It was found that the cycle life characteristics of the battery were improved by adding 2.0 wt% or less of acetate and methyl (trimethylsilyl) acetate to the non-aqueous electrolyte. Although the cause is not clear, alkyl (trimethylsilyl) acetate is contained in the electrolyte.
It is considered that by containing 0 wt% or less, a good SEI coating was formed on the surface of the negative electrode active material, and the subsequent decomposition of the nonaqueous electrolyte on the negative electrode was suppressed.

In the examples and comparative examples, examples were given in which ethyl (trimethylsilyl) acetate and methyl (trimethylsilyl) acetate were used as the alkyl (trimethylsilyl) acetate, but other alkyl (trimethylsilyl) acetates were used. In this case, the same effect can be obtained.

In the examples and comparative examples, the electrolyte solvent is EC: DEC system. However, when the ratio of cyclic carbonate to chain carbonate is changed, or when DMC or EMC system is used as chain carbonate, the solvent is used. When γ-BL was used in place of the linear carbonate, the same tendency was observed.

[0050]

According to the present invention, since the non-aqueous electrolyte contains alkyl (trimethylsilyl) acetate, a good SEI is formed on the surface of the negative electrode active material. The decomposition of the non-aqueous electrolyte was suppressed, and as a result, it was possible to obtain a long-life non-aqueous electrolyte secondary battery with a small capacity deterioration during a charge / discharge cycle.

[Brief description of the drawings]

FIG. 1 is a diagram showing a cross-sectional structure of a prismatic battery according to an example of the present invention and a comparative example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Rectangular nonaqueous electrolyte secondary battery 2 Wound electrode group 3 Positive electrode 4 Negative electrode 5 Separator 6 Battery case 7 Battery cover 8 Safety valve 9 Positive electrode terminal 10 Positive electrode lead

Claims (4)

[Claims] 1. A non-aqueous electrolyte secondary battery characterized in that the non-aqueous electrolyte contains at least one kind of alkyl (trimethylsilyl) acetate represented by the following chemical formula (1). Embedded image (However, in the chemical formula 1, R represents an alkyl group.) 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the alkyl (trimethylsilyl) acetate is ethyl (trimethylsilyl) acetate. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the alkyl (trimethylsilyl) acetate is methyl (trimethylsilyl) acetate. 4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte contains 2.0% by weight or less of alkyl (trimethylsilyl) acetate.