JPH1140195A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolytic solution with flame resistance and self-extinguishing property by using a mixed solvent of one or more cyclic esters and one or more chain esters, which are mutually compatible as a nonaqueous solvent and using a halogenated chain carbonate with a specified composition for the chain ester. SOLUTION: This battery comprises a negative electrode and a positive electrode, containing a material which can electrochemically occlude and desorb lithium and a non-aqueous electrolytic substance produced by dissolving a lithium salt as an electrolytic substance in a nonaqueous solvent. The nonaqueous solvent is a mixed solvent containing one or more cyclic esters and one or more chain esters, which are mutually compatible as a nonaqueous solvent, and at least one chain ester is halogenated chain carbonate with a specified composition having a defined formula (wherein n and n' are each 1-3; X is F, C, Br, or I; m+m'=1 to 2(n+n'+1) for the chain ester. The safety of the buttery is high by using a mixture which has such a self-extinguishing property for the solvent in the electrolytic solution.

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 a non-aqueous electrolyte secondary battery having high safety against overcharge or short circuit.

[0002]

2. Description of the Related Art A material capable of electrochemically occluding and releasing lithium, such as metallic lithium, a lithium alloy, or a carbon material, is used as a negative electrode active material, and electrochemically occluding and releasing lithium.
Lithium secondary batteries using materials that can be released as positive electrode active materials have been researched and developed, and some of them have been put to practical use. This lithium secondary battery has a feature that the battery voltage is high and the energy density per weight and volume is higher than other secondary batteries. For this reason, it is used as a power source for portable electronic devices such as mobile phones, notebook computers, and camera-integrated VTRs.

In such a lithium secondary battery,
Aqueous electrolytes such as those used in other secondary batteries such as lead-acid batteries, nickel-cadmium batteries, and nickel-metal hydride batteries cannot be used due to inconveniences such as reaction with lithium. A non-aqueous electrolyte in which a lithium salt is dissolved in a solvent is used.

Examples of such a non-aqueous electrolyte include a solution obtained by dissolving a lithium salt in a solvent obtained by mixing a chain carbonate with propylene carbonate, as described in, for example, JP-A-2-10666. As shown in JP-A-4-162370 and USP No. 5192629,
An electrolytic solution in which a lithium salt is dissolved in a solvent in which a chain carbonate is mixed with ethylene carbonate is known.

[0005]

However, an electrolytic solution using such an organic solvent is highly flammable, and may be ignited during overcharging or when an external short circuit or an internal short circuit occurs for some reason. There is a demand for improved safety.

SUMMARY OF THE INVENTION In order to solve the above-mentioned conventional problems, the present invention provides a non-aqueous electrolyte with flame retardancy and self-extinguishing properties. An object is to provide a non-aqueous electrolyte secondary battery having excellent characteristics.

[0007]

In order to achieve the above object, as a solvent for an electrolytic solution used in a non-aqueous electrolyte secondary battery,
It is a mixed solvent containing at least one kind of cyclic ester and at least one kind of chain ester mutually compatible, and at least one kind of chain ester is a halogenated chain carbonate represented by the general formula (1). The above problem was solved by using a solvent.

[0008]

Embedded image

The chain ester used in the present invention includes:
In addition to the compounds represented by the general formula (1), dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate, ethyl propyl carbonate, ethyl isopropyl carbonate,
Ethyl butyl carbonate, dipropyl carbonate,
Chain carbonates such as diisopropyl carbonate, propyl butyl carbonate and dibutyl carbonate can be mixed. Further, a carboxylic acid ester such as an alkyl propionate, a dialkyl malonate and an alkyl acetate may be mixed.

As the cyclic ester, propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, vinylene carbonate, 2-ene
Hydrocarbon-based cyclic esters such as methyl-γ-butyrolactone, acetyl-γ-butyrolactone, and γ-valerolactone, or chloroethylene carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one,
Halogenated cyclic esters such as bromo-γ-butyrolactone can be used alone or as a mixture.

Further, it is necessary that the mixed solvent containing the chain ester and the cyclic ester be compatible with each other.
When the number of halogens in the chain ester represented by the general formula (1) is large, the compatibility with a hydrocarbon-based cyclic ester or chain ester is deteriorated, and phase separation may occur. Can not be used.

Further, as a lithium salt used as an electrolyte, LiClO ₄ , LiBF ₄ , LiPF ₆ and LiA which dissociate in the above-mentioned non-aqueous solvent and supply lithium ions are used.
Inorganic lithium salts such as sF ₆ , LiF, LiCl, LiBr and the like, and LiB (C ₆ H ₅ ) ₄ , LiN (SO ₂ CF ₃ ) ₂ , Li
C (SO ₂ CF ₃ ) ₃ , LiOSO ₂ CF ₃ , LiOS
O ₂ C ₂ F ₅ , LiOSO ₂ C ₃ F ₇ , LiOSO ₂ C
₄ F ₉ , LiOSO ₂ C ₅ F ₁₁ , LiOSO ₂ C ₆ F ₁₃ , L
and organic lithium salts such as iOSO ₂ C ₇ F _15, but lithium salts of inorganic is high and desirable flame retardancy of the electrolyte. Among them, LiPF ₆ is particularly preferable for use as a lithium salt because of its excellent battery characteristics and flame retardancy.

The negative electrode active material includes lithium metal, lithium alloy, a carbon material capable of storing and releasing lithium, and a tin oxide compound capable of storing and releasing lithium, and other materials capable of electrochemically storing and releasing lithium. However, among them, it is desirable to use a carbon material capable of inserting and extracting lithium from the viewpoint of safety. More specifically, carbon materials capable of occluding and releasing lithium can be classified into graphite, graphitizable carbon, and non-graphitizable carbon, but when a compound having a propylene carbonate structure is used as a solvent for a non-aqueous electrolyte, From the viewpoint of battery characteristics, it is desirable to use non-graphitizable carbon or graphitizable carbon, or to mix and use a solvent having an ethylene carbonate structure.

The positive electrode active material is, for example, LiCo.
Lithium-containing composite oxides such as O ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , TiO ₂ , MnO ₂ , Mo
Α-NaCrO ₂ structure such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , which uses a chalcogen compound such as O ₃ , V ₂ O ₅ , TiS ₂ , MoS ₂ , and has a high discharge voltage and high electrochemical stability Lithium compounds and LiM
n ₂ O _{4 and the} like are desirable.

That is, according to the method of the present invention, the use of a mixture of a flame-retardant, self-extinguishing halogenated chain carbonate, a cyclic ester, and a chain ester as a solvent in the electrolytic solution allows the safety to be improved. And a nonaqueous electrolyte secondary battery excellent in battery characteristics can be obtained.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION

(Examples 1 to 29) The following tests were performed to confirm the flammability of the electrolyte used in the battery of the present invention, the safety of the battery of the present invention, and the battery characteristics.

LiPF ₆ was used as a lithium salt, and was dissolved in various mixed solvents shown in Table 1 to prepare an electrolyte so that the lithium salt concentration was 1 mol / l. The mixing ratio indicates the volume ratio of the solvent.

[0018]

[Table 1]

In Table 1, EC is ethylene carbonate, PC is propylene carbonate, DMC is dimethyl carbonate, TFMEC is 2,2,2-trifluoroethylmethyl carbonate, and TBMEC is 2,
2,2-Tribromoethylmethyl carbonate was added to TI
MEC is 2,2,2-triiodoethyl methyl carbonate, and HFDEC is 2,2,2,2 ', 2', 2 '
-Hexafluorodiethyl carbonate, HCDEC
Is 2,2,2,2 ', 2', 2'-hexachlorodiethyl carbonate, and DFDEC is 2,2,2 ', 2',
3,3,3,3 ', 3', 3'-decafluorodipropyl carbonate, DCDEC is 2,2'-dichlorodiethyl carbonate, DBDEC is 2,2'-dibromodiethyl carbonate, 1-CDEC is 1-chloroethyl, ethyl carbonate and 2-CDEC are 2-chloroethyl ethyl carbonate and DCDMC are 1,1 '
-Dichlorodimethyl carbonate is DBDMC 1,
1'-dibromodimethyl carbonate, CDMC is chloromethyl methyl carbonate, BDMC is bromomethyl methyl carbonate, TFPC is 4-trifluoromethyl-1,3-dioxolan-2-one, C1-EC
Represents chloroethylene carbonate, respectively.

Next, a cylindrical battery having a diameter of 18 mm and a length of 650 mm having the structure shown in FIG. 1 was manufactured, and a capacity measurement by charging / discharging and a safety test of the battery by an overcharge test were performed.

LiCoO ₂ powder as a cathode active material, graphite powder as a conductive agent, polyvinylidene fluoride resin as a binder, and N-methyl-2-pyrrolidone as a solvent were mixed to obtain a slurry-like cathode active material mixture. . This slurry was applied to both sides of a 20 μm thick aluminum foil as a positive electrode current collector by a doctor blade method, dried to form an active material layer having a thickness of 50 μm, compressed by a press, and heat-treated in a vacuum oven. Water was removed to produce a positive electrode.

Also, a non-graphitizable carbon powder as a negative electrode active material, polyvinylidene fluoride resin as a binder, and N-methyl-2-pyrrolidone as a solvent were mixed to obtain a slurry of a negative electrode active material mixture. This slurry was applied to both sides of a copper foil having a thickness of 20 μm as a negative electrode current collector by a doctor blade method, and dried to form an active material layer having a thickness of 50 μm.
It was compressed by a press and heat-treated in a vacuum oven to remove water, thereby producing a negative electrode. The thus obtained positive electrode, negative electrode, and a polypropylene microporous membrane having a thickness of 25 μm as a separator were laminated and wound to produce a spiral electrode body. The electrode body was housed in a battery can 6, one end of a nickel negative electrode lead 4 was pressed against the negative electrode 2, and the other end was welded to the battery can. Also, one end of a positive electrode lead 3 made of aluminum was attached to the positive electrode 1, and the other end was connected to the battery lid via a current interrupting thin plate 11 for interrupting current according to the internal pressure of the battery. Then, various electrolytic solutions having the compositions shown in Table 1 were injected, and the battery was caulked through an insulating sealing gasket 8.
The battery cover 7 was fixed to produce a non-aqueous electrolyte secondary battery.

The capacity of the thus prepared non-aqueous electrolyte secondary battery was measured. A charging current of 280 mA, a charging current of 4.2 V, a constant current constant voltage charging and a discharging current of 280 mA,
Charge / discharge was performed three times with constant current discharge at a discharge end voltage of 2.5 V, capacity measurement was performed, and the third discharge capacity was defined as the capacity of the battery. Table 2 shows the results.

Next, an overcharge test was performed at a charging current of 2800 mA to measure the state of change of the battery and the maximum temperature of the battery surface. Table 2 shows the results.

[0025]

[Table 2]

As can be seen from Table 2, the battery using the electrolyte containing the halogenated chain carbonate did not show ignition at the time of overcharging, and a safe battery was obtained.

Also, in Comparative Example 1 in which the temperature of the battery surface during overcharge did not include the halogenated chain carbonate, the current interrupting thin plate was operated, and the temperature rapidly increased even after the current was interrupted. While the surface temperature of the battery was increased to 150 ° C. or higher, in Examples 1 to 29, it was found that the battery was safe without the surface temperature exceeding 80 ° C. even during overcharge.

[0028]

As described above, according to the present invention, a mixture of at least one kind of cyclic ester and at least one kind of chain ester compatible with each other in the electrolytic solution, wherein at least one kind of the chain ester is represented by the general formula: The use of an electrolyte in which a lithium salt is dissolved in a solvent which is a halogenated chain carbonate as shown in (1) to obtain a non-aqueous electrolyte secondary battery having excellent safety and good battery characteristics. Can be.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a cylindrical non-aqueous electrolyte secondary battery used for carrying out the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Positive electrode, 2 ... Negative electrode, 3 ... Positive electrode lead, 4 ... Negative electrode lead, 5 ... Separator, 6 ... Battery can, 7 ... Battery lid, 8 ... Sealing gasket, 9,10 ... Insulating plate, 11 ... Current interrupting thin plate .

Claims (1)

[Claims] 1. A negative electrode using a material capable of electrochemically storing and releasing lithium, a positive electrode using a material capable of electrochemically storing and releasing lithium, and a lithium salt dissolved as an electrolyte in a non-aqueous solvent. In a non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte, the non-aqueous solvent comprises a mixed solvent containing at least one cyclic ester and at least one chain ester,
A non-aqueous electrolyte secondary battery characterized in that the mixed solvents are mutually compatible and at least one of the chain esters is a halogenated chain carbonate represented by the general formula (1). Embedded image