JPH10199567A - Nonaqueous electrolyte secondary cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary cell with high stability and optimum battery characteristics. SOLUTION: This nonaqueous electrolyte secondary cell consists of a negative electrode 2 whose active material is a material capable of storing and releasing lithium electrochemically, a positive electrode whose active material is a material capable of storaging and releasing lithium electrochemically, and nonaqueous electrolyte whose lithium salt is dissolved in nonaqueous solvent as electrolyte. The nonaqueous electrolyte made out of mixture of one or more types of cyclic ester and one or more types of chain ester, a volume ratio of the cyclic ester against the whole solvent is 60 to 90%, at least one type of the cyclic ester is 4-trifluoromethyl or 3-dioxolan-2-one, and a volume ratio of the 4- trifluoromethy-1 or 3-dioxolan-2-one against the whole solvent is 60 to 90%, and a volume ratio of the chain ester against the whole solvent is 10 to 40%.

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, for example, JP-A-2-10666.
JP-A-4-162370 and USP No. 519262, in which a lithium salt is dissolved in a solvent in which a chain carbonate is mixed with propylene carbonate, as disclosed in JP-A No. 4-162370.
As disclosed in Japanese Patent Application Publication No. 9-203, there is known an electrolytic solution in which a lithium salt is dissolved in a solvent in which a chain carbonate is mixed with ethylene carbonate.

[0003]

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.

[0005]

In order to achieve the above object, as a solvent for an electrolytic solution used in a non-aqueous electrolyte secondary battery,
A mixture of a cyclic ester and a chain ester is used. As the cyclic ester, 4-trifluoromethyl-1,3-dioxolan-2-one having a structure represented by the general formula [1] is used as a main component. The above-mentioned problem was solved by using a mixed solvent obtained by mixing with a cyclic ester and further mixing an appropriate amount of a chain ester.

[0006]

Embedded image

The cyclic ester used in the present invention includes:
4-trifluoromethyl-1,3-dioxolan-2-
ON can be used alone or in combination with propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, vinylene carbonate, 2-methyl-γ-butyrolactone, acetyl-γ-butyrolactone, γ-valerolactone, or the like.

Examples of the chain ester include 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,
There are diisopropyl carbonate, propyl butyl carbonate, dibutyl carbonate, alkyl propionate, dialkyl malonate, alkyl acetate and the like, and these can be used alone or in combination, but dimethyl carbonate, diethyl carbonate, dipropyl carbonate, It is desirable to use one or more chain carbonates selected from the group consisting of diisopropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl propyl carbonate, and methyl isopropyl carbonate.

The amount of the cyclic ester in the total solvent is 60 to 90% by volume, and the volume of the chain ester is 1 to 90%.
0-40% and 4-trifluoromethyl-1,
The volume ratio of 3-dioxolan-2-one to the total solvent is desirably 60 to 90%. If the volume ratio of 4-trifluoromethyl-1,3-dioxolan-2-one to the total solvent is less than 60%, the flammability of the solvent is high and sufficient safety cannot be ensured. If the volume ratio of -1,3-dioxolan-2-one to the total solvent is more than 90%, the rate of insertion and extraction of lithium at the electrode is slow, and charging and discharging at a high current rate cannot be performed. The characteristics deteriorate.

Further, as the lithium salt used as the electrolyte, LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAs which dissociates in the above non-aqueous solvent and supplies lithium ions.
Inorganic lithium salts such as F ₆ , LiF, LiCl, LiBr, and LiB (C ₆ H ₅ ) ₄ , LiN (SO ₂ CF ₃ ) ₂ , Li
C (SO ₂ CF ₃ ) ₃ , LiOSO ₂ CF ₃ , LiOSO 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 absorbing and releasing lithium, and a tin oxide compound capable of absorbing and releasing lithium, and the like, and a material 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 inserting and extracting lithium can be classified into graphite, graphitizable carbon, and non-graphitizable carbon, and propylene such as 4-trifluoromethyl-1,3-dioxolan-2-one. When a compound having a carbonate structure is used as a solvent for the non-aqueous electrolyte, it is desirable to use non-graphitizable carbon or graphitizable carbon from the viewpoint of battery characteristics.

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 , MoO ₃ , V ₂
A chalcogen compound such as O ₅ , TiS ₂ , MoS ₂ or the like is used, and a high discharge voltage, high electrochemical stability, LiC
Lithium compounds having an α-NaCrO ₂ structure, such as oO ₂ , LiNiO ₂ , and LiMnO ₂ , and LiMn ₂ O ₄ are desirable.

According to the method of the present invention, the solvent used in the electrolytic solution is 4-trifluoromethyl-1,3-dioxolan-2-one, which is flame-retardant and has self-extinguishing properties, as a main component, and has a chain-like structure. By mixing an appropriate amount of the ester and the cyclic ester, a non-aqueous electrolyte secondary battery having high safety and excellent battery characteristics can be obtained.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION

(Examples 1 to 42) The following tests were conducted 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.

[0016]

[Table 1]

In Table 1, TFPC is 4-trifluoromethyl-1,3-dioxolan-2-one, EC is ethylene carbonate, PC is propylene carbonate, DMC is dimethyl carbonate, and DEC is diethyl carbonate. , MEC is methyl ethyl carbonate, MPC is methyl propyl carbonate, MiP
C represents methyl isopropyl carbonate, respectively.

In order to test the flammability of the non-aqueous electrolyte prepared in this manner, 2 cc of each of the electrolytes shown in Table 1 was impregnated into Manila linen cut to a width of 10 mm and a length of 100 mm. Was ignited with a lighter from one end, and the state of combustion was observed. Table 2 shows the results.

For comparison, electrolytic solutions mixed at the volume ratios shown in Comparative Examples 1 to 6 were prepared, and a flammability test was performed using Manila linen cloth in the same manner as described above. Table 2 shows the results.

[0020]

[Table 2]

As can be seen from the results in Table 2, when the volume ratio of 4-trifluoromethyl-1,3-dioxolan-2-one to the total solvent is less than 60%, all of the Manila linen impregnated with the electrolytic solution is used. Although it was burned, it was confirmed that when it was 60% or more, it did not ignite and the flammability of the electrolyte was significantly reduced.

Next, an AA type battery having the structure shown in FIG. 1 was fabricated, and a battery safety test was performed by an overcharge test.

LiCoO ₂ powder as a positive electrode 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 of a positive electrode 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 an anode active material, a polyvinylidene fluoride resin as a binder, and N-methyl-2-pyrrolidone as a solvent were mixed to obtain a slurry-like anode 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. Using the thus obtained positive electrode and negative electrode as separators, a polypropylene microporous membrane having a thickness of 25 μm was 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. Further, one end of an aluminum positive electrode lead 3 was attached to the positive electrode 1, and the other end was connected to a battery lid via a current interrupting thin plate 11 for interrupting an electric current according to the internal pressure of the battery. Then, various electrolytic solutions having the compositions shown in Table 1 were injected,
The battery is swaged via the insulating sealing gasket 8 and the battery cover 7
Was fixed to produce a non-aqueous electrolyte secondary battery.

The non-aqueous electrolyte secondary battery thus produced was first charged with a charging voltage of 4.2 V, a charging current of 280 mA,
Charging was performed under the condition of a charging time of 5 hours, followed by discharging under the conditions of a discharge current of 280 mA and a discharge end voltage of 2.8 V. After confirming the operation, an overcharge test was performed at a charging current of 2800 mA. The state of the battery change and the maximum temperature of the battery surface were measured. Table 3 shows the results.

[0026]

[Table 3]

As can be seen from Table 3, when the volume ratio of 4-trifluoromethyl-1,3-dioxolan-2-one to the total solvent was less than 60%, ignition was observed at the time of overcharging. , 60% or more, no ignition was observed during overcharging, and a safe battery could be obtained.

Also, in Comparative Examples 1 and 5, in which the temperature of the battery surface during overcharge did not include 4-trifluoromethyl-1,3-dioxolan-2-one, the current interrupting thin plate was operated.
Even after the current was cut off, the temperature rose sharply, and the temperature of the battery surface rose to 150 ° C. or higher, whereas the volume of 4-trifluoromethyl-1,3-dioxolan-2-one relative to the total solvent was increased. When the ratio is 60% or more, it can be seen that the battery is safe without the surface temperature exceeding 100 ° C. even during overcharge.

Next, in order to test the characteristics of the positive electrode or the single electrode of the negative electrode, the coin-type battery shown in FIG.
A test of battery characteristics was performed.

A slurry-like positive electrode active material mixture prepared in the same manner as that for preparing the AA type battery is applied as a positive electrode current collector to one side of a 20 μm thick aluminum foil by a doctor blade method, and dried. Then, an active material layer having a thickness of 50 μm was formed, compressed by a press, and heat-treated in a vacuum oven to remove moisture, thereby producing a positive electrode.

A negative electrode active material mixture having a thickness of 20 μm was prepared by a doctor blade method using a slurry-like negative electrode active material mixture prepared by the same method as that for preparing the AA type battery. Coated on one side of copper foil, dried to form a 50 μm thick active material layer, compressed with a press,
A negative electrode was prepared by heat treatment in a vacuum oven to remove moisture.

The positive electrode or negative electrode thus obtained was punched into a circle, the positive electrode or the negative electrode was inserted into the bottom of a stainless steel can 16 in which a packing 17 was previously mounted, and a circular punch was formed on the top. 25
μm polypropylene microporous membrane separator 14 stacked,
Further, metal lithium punched out in a circular shape as a counter electrode was superimposed on the upper portion, each of the electrolytes shown in Table 1 was injected, a stainless steel lid 15 was attached, and then the battery can was caulked and sealed. Such a coin-type non-aqueous electrolyte secondary battery for unipolar evaluation was produced.

The prepared coin-type secondary battery has a voltage range of 3 to 4.2 V for the positive electrode and a voltage range of 0.01 to 1 V for the negative electrode, and has a charge / discharge current of 0.1 / cm ² of the electrode. The battery was charged and discharged at a constant current in the range of 5 to 1.5 mA, and the battery capacity was measured. The results are shown in FIGS. The relative discharge capacity in the figure indicates the discharge capacity at the current density when the current density at the time of charge / discharge is 0.5 mA / cm ² and the discharge capacity is 1.

As can be seen from FIGS. 3 and 4, when the volume ratio of 4-trifluoromethyl-1,3-dioxolan-2-one to the total solvent is more than 90%, the charge / discharge current is high when the charge / discharge current is high. A remarkable decrease in the capacity was observed as compared with the case where the charge / discharge rate was low. However, when the charge / discharge current was 90% or less, a decrease in the capacity was small and a battery having good characteristics could be obtained.

From the above results, the volume ratio of 4-trifluoromethyl-1,3-dioxolan-2-one to the total solvent is required to make the electrolyte solution flame-retardant and obtain good battery characteristics. , 60-90%.

[0036]

As described above, according to the present invention, 4-trifluoromethyl-1,3-dioxolan-2-one is used as a main component of an electrolytic solution, and an appropriate amount of a chain carbonate is mixed, whereby safety is ensured. It is possible to obtain a non-aqueous electrolyte secondary battery having excellent battery characteristics and good battery characteristics.

[Brief description of the drawings]

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

FIG. 2 is a cross-sectional view of a coin-type non-aqueous electrolyte secondary battery used in carrying out the present invention.

FIG. 3 is a diagram showing the relationship between charge / discharge current and discharge capacity of a coin-type nonaqueous electrolyte secondary battery having a positive electrode and metallic lithium as counter electrodes.

FIG. 4 is a diagram showing the relationship between charge / discharge current and discharge capacity of a coin-type nonaqueous electrolyte secondary battery having a negative electrode and metallic lithium as counter electrodes.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Positive electrode, 2 ... Negative electrode, 3 ... Positive electrode lead, 4 ... Negative electrode lead, 5, 14 ... Separator, 6, 16 ... Battery can, 7, 1
5 ... battery lid, 8 ... sealing gasket, 9, 10 ... insulating plate,
11 ... thin plate for interrupting current, 12 ... positive electrode or negative electrode, 13 ...
Metal lithium, 17 ... packing.

Claims (1)

[Claims] 1. A negative electrode using a material capable of electrochemically storing and releasing lithium as an active material, a positive electrode using a material capable of electrochemically storing and releasing lithium as an active material, and lithium as an electrolyte in a non-aqueous solvent. A non-aqueous electrolyte secondary battery comprising a salt-dissolved non-aqueous electrolyte, wherein the non-aqueous electrolyte comprises a mixture of at least one cyclic ester and at least one chain ester, Volume ratio to
90% and at least one of the cyclic esters is 4%
-Trifluoromethyl-1,3-dioxolan-2-one, wherein the volume ratio of 4-trifluoromethyl-1,3-dioxolan-2-one to all solvents is 60 to 90%.
And the volume ratio of the chain ester to the total solvent is 10 to 10.
Non-aqueous electrolyte secondary battery characterized by being 40%.