JPH10189046A - Nonaqueus electrolyte and nonaqueous electrolytic secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the safety of a nonaqueous electrolyte formed out of an electrolyte and a nonaqueous solvent containing the specified amount of a compound having a low melting point, and prevent the occurrence of a flashing danger such as liquid leakage or the like during the manufacture of a battery or the storage thereof by specifying the flashing point and burning point of the nonaqueous electrolyte. SOLUTION: In a nonaqueous electrolyte consisting of an electrolyte and a nonaqueous solvent containing a compound having a melting point equal to or lower than 10 deg.C, the content of the compound having the melting point equal to or lower than 10 deg.C is taken at a value between 2wt% and 40wt%, and the flashing point of the nonaqueous electrolyte is taken at a value equal to or above 40 deg.C. Furthermore, the burning point of the electrolyte is taken at a value equal to or above 60 deg.C. The nonaqueous solvent containing the compound having the melting point equal to or lower than 40 deg.C is a mixed solution of both ethylene carbonate and the chain carbonate or phosphate expressed by the formula, where R<1> and R<2> are an alkyl group or a haloaklyl group having carbons between 1 and 3. Also, the total amount of the ethylene carbonate and the electrolyte such as LiPF6 is preferably 60% to 98% of the nonaqueous electrolyte. A secondary battery having high safety and a high battery function can be provided by using the nonaqueous electrolyte so prepared.

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 and a non-aqueous electrolyte secondary battery, and more particularly, to a non-aqueous electrolyte requiring a high degree of safety and to use such a non-aqueous electrolyte. It relates to a non-aqueous electrolyte.

[0002]

BACKGROUND OF THE INVENTION Lithium metal has an oxidation-reduction potential of -3.03 V and is the lowest metal on the earth. Since the voltage of the battery is determined by the potential difference between the positive electrode and the negative electrode, the highest electromotive force can be obtained by using lithium as the negative electrode active material. The atomic weight is 6.94 and the density is 0.53.
4 g / cm ³ , both of which are small, so that the weight per unit amount of electricity is small and the energy density is high. Therefore, a small and lightweight battery can be obtained.

However, lithium metal has a very high reactivity, for example, reacts violently when it comes into contact with water to generate hydrogen gas, so that an aqueous solution cannot be used as an electrolytic solution as in a conventional battery.

For this reason, in a secondary battery using lithium metal as a negative electrode active material, propylene carbonate which is a high dielectric constant solvent,
γ-butyrolactone, sulfolane, etc., and low viscosity solvents such as dimethoxyethane and tetrahydrofuran, 1,3-
LiBF ₄ , LiP
Non-aqueous electrolytes in which electrolytes such as F ₆ , LiClO ₄ , LiAsF ₆ , and LiCF ₃ SO ₃ are dissolved have been proposed.

[0005] However, such low-viscosity solvents are:
Since the flash point is below room temperature, there is a danger of ignition due to liquid leakage during battery manufacture or battery storage. Although safety is ensured even with a conventional electrolytic solution, a higher level of safety may be required when the size of the battery is increased.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems associated with the prior art as described above, and an object of the present invention is to provide an electrolyte which is more excellent in safety. Another object of the present invention is to provide a non-aqueous electrolyte secondary battery using such a non-aqueous electrolyte with excellent safety.

[0007]

SUMMARY OF THE INVENTION A non-aqueous electrolyte according to the present invention comprises: an electrolyte;
A non-aqueous electrolyte comprising a non-aqueous solvent containing a compound having a melting point of 10 ° C. or lower, wherein the non-aqueous electrolyte contains a compound having a melting point of 10 ° C. or lower in an amount of 2 to 40% by weight. 4 points
It is characterized by a temperature of 0 ° C. or higher.

The non-aqueous solvent containing the compound having a melting point of 10 ° C. or less is a mixed solution of ethylene carbonate and a compound selected from a chain carbonate ester and a phosphate ester. The total amount of the electrolyte is preferably 60 to 98% by weight. Further, the burning point of the non-aqueous electrolyte is preferably 60 ° C. or higher.

As the chain carbonate, a compound represented by the general formula [I]:
Is preferably represented by

[0010]

Embedded image

(In the formula, R ¹ and R ² may be the same or different and each represents an alkyl group or a haloalkyl group having 1 to 3 carbon atoms.) As such a chain carbonate, methyl propyl carbonate is used. , Dipropyl carbonate, methyl isopropyl carbonate, methyl isopropyl carbonate, diisopropyl carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl 2,2,2-trifluoroethyl carbonate, methyl 2,2,3,3-tetrafluoropropyl At least one chain carbonate selected from carbonate and 2,2,3,3,3-pentafluoropropyl carbonate is preferred.

As the phosphate, at least one phosphate selected from trimethyl phosphate, triethyl phosphate, and tris (2,2,2-trifluoroethyl) phosphate is preferable. As the electrolyte, LiPF ₆ , LiB
F ₄ , LiOSO ₂ CF ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN
At least one electrolyte selected from (SO ₂ CF ₂ CF ₃ ) ₂ and LiC (SO ₂ CF ₃ ) ₃ is preferred.

The non-aqueous electrolyte secondary battery according to the present invention comprises a negative electrode using a carbon material capable of doping and undoping lithium ions as a negative electrode active material, and a composite oxide of lithium and a transition metal as a positive electrode active material. , A separator, and the non-aqueous electrolyte.

[0014]

DETAILED DESCRIPTION OF THE INVENTION The non-aqueous electrolyte according to the present invention and a secondary battery using such a non-aqueous electrolyte will be specifically described below.

Nonaqueous Electrolyte The nonaqueous electrolyte according to the present invention is a nonaqueous electrolyte comprising an electrolyte and a nonaqueous solvent containing a compound having a melting point of 10 ° C. or lower. 2 to 40
%, Preferably 20 to 40% by weight, and the non-aqueous electrolyte has a flash point of 40 ° C. or higher (for example, 40 to 200%).
° C), particularly preferably 60 ° C or higher. Compounds having a melting point of 10 ° C. or lower include hexane, heptane, octane,
Examples thereof include hydrocarbons such as toluene and xylene, ethers such as diethyl ether, dimethoxyethane and tetrahydrofuran, esters such as ethyl acetate, methyl propionate and γ-butyrolactone, chain carbonates and phosphates.

As the chain carbonate, a carbonate represented by the following general formula [I] is preferable.

[0017]

Embedded image

(Wherein R ¹ and R ² may be the same or different and represent an alkyl group having 1 to 3 carbon atoms or a haloalkyl group) Examples of such a chain carbonate include dimethyl carbonate, methyl Ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl 2,2,2-trifluoroethyl carbonate, methyl 2,2,3,3-tetrafluoropropyl carbonate, 2,2,3,3,3- Examples include pentafluoropropyl carbonate, dipropyl carbonate and the like.

As the phosphoric acid ester, a phosphoric acid ester such as trimethyl phosphate, triethyl phosphate, and tris (2,2,2-trifluoroethyl) phosphate is preferable. These compounds having a melting point of 10 ° C. or lower can be used alone or in combination of two or more.

In the present specification, the flash point is JIS K 2265
It is measured as follows by the tag closed flash point test method specified in -1980. (1) Put 50 ml of the sample into the sample cup, close the lid,
Heat at a rate of minutes. (2) Each time the temperature of the sample rises by 0.5 ° C, the test flame is removed. (3) The operation of (2) is repeated until the ignition occurs, and the temperature at the time of ignition becomes the flash point.

The combustion point of such a non-aqueous electrolyte is 6
It is preferably 0 ° C or more (for example, 60 to 250 ° C), particularly preferably 80 ° C or more. The combustion point is AS
It is measured as follows using a tag release type flash point measuring device specified in TM D1310-1986. (1) The liquid level of the test sample is 3.2 mm from the top of the sample cup
The sample is placed so as to be below, and the sample is heated at a rate of 1 ° C./min. (2) Each time the temperature of the sample increases by 1 ° C., the test flame is passed through the upper part of the cup. (3) The temperature at which combustion lasts for 5 minutes is defined as the combustion point.

[0022] As the electrolyte used for the non-aqueous electrolyte according to the electrolyte present invention,
LiPF ₆ , LiBF ₄ , LiOSO ₂ CF ₃ , LiN (S
A lithium compound selected from O ₂ CF ₃ ) ₂ and LiC (SO ₂ CF ₃ ) ₃ is preferred. Such an electrolyte can be used alone or in combination of two or more.

Each of these electrolytes dissociates in a non-aqueous electrolyte to generate Li ions. Such an electrolyte is usually 0.5 to 2 mol / L in the non-aqueous electrolyte, although it depends on the type of the electrolyte used, the type of the solvent, and the like.
Preferably, it may be contained in the range of 0.7 to 1.5 mol / liter.

Non-aqueous solvent containing a compound having a melting point of 10 ° C. or lower The non-aqueous solvent containing a compound having a melting point of 10 ° C. or lower used in the non-aqueous electrolytic solution of the present invention includes ethylene carbonate,
It is preferably a mixed solution with a compound having a melting point of 10 ° C. or less, which is a compound selected from a chain carbonate ester and a phosphate ester. Ethylene carbonate is contained in the non-aqueous electrolyte in a total amount of 60 to 98% by weight with the electrolyte,
Preferably, it is contained in an amount of 60 to 80% by weight.

Non-Aqueous Electrolyte Secondary Battery A non-aqueous electrolyte secondary battery according to the present invention comprises a negative electrode using a carbon material capable of doping / dedoping lithium ions as a negative electrode active material, and a composite of lithium and a transition metal. It comprises a positive electrode using an oxide as a positive electrode active material, a separator, and the non-aqueous electrolyte.

Such a non-aqueous electrolyte secondary battery can be applied to, for example, a cylindrical non-aqueous electrolyte secondary battery. As shown in FIG. 1, a cylindrical nonaqueous electrolyte secondary battery has a negative electrode 1 formed by applying a negative electrode active material to a negative electrode current collector 9 and a positive electrode formed by applying a positive electrode active material to a positive electrode current collector 10. 2 is wound through a separator 3 into which a non-aqueous electrolyte is injected, and is housed in a battery can 5 in a state where insulating plates 4 are placed above and below the wound body. A battery lid 7 is attached to the battery can 5 by caulking via a sealing gasket 6, and is electrically connected to the negative electrode 1 or the positive electrode 2 via a negative electrode lead 11 and a positive electrode lead 12, respectively. It is configured to function as a positive electrode. The separator is a porous film.

In this battery, the positive electrode lead 12 may be electrically connected to the battery lid 7 via the current interrupting thin plate 8. In such a battery, when the pressure inside the battery rises, the current interrupting thin plate 8 is pushed up and deformed, and the positive electrode lead 12 is cut off leaving the portion welded to the thin plate 8 to cut off the current.

As the negative electrode active material constituting such a negative electrode 1, a carbon material capable of doping and undoping lithium ions is used. Such a carbon material may be graphite or amorphous carbon, and any carbon material such as activated carbon, carbon fiber, carbon black, and mesocarbon microbeads can be used.

The positive electrode active material constituting the positive electrode 2 is L
iCoO_Two, LiMnO_Two, LiMn _TwoO_Four, LiNiO_Two
Use a composite oxide consisting of lithium and transition metal
be able to.

The non-aqueous electrolyte secondary battery according to the present invention comprises:
The electrolyte contains the non-aqueous electrolyte described above, and the shape and form of the battery are not limited to those in FIG.
The coin shape shown in FIG. 2 or a square shape may be used.

[0031]

The non-aqueous electrolyte according to the present invention has a melting point of 10
The compound containing the compound having a flash point of 40 ° C.
As described above, it is possible to obtain a secondary battery excellent in safety and battery performance such that it is difficult to catch fire and does not react at a voltage of 4.2 V.

The secondary battery according to the present invention is excellent in safety and battery performance.

[0033]

EXAMPLES Hereinafter, the present invention will be described more specifically based on examples, but the present invention is not limited to these examples.

[0034]

EXAMPLE 1 3.8 g (25 mmol) of lithium hexafluorophosphate was dissolved as an electrolyte in a solvent in which dipropyl carbonate and ethylene carbonate were mixed at a volume ratio of 1: 1 to obtain 25 ml of a non-aqueous electrolyte. . The weight composition of the non-aqueous electrolyte was 36% by weight of dipropyl carbonate, and 6% in total of ethylene carbonate and lithium hexafluorophosphate.
It was 4% by weight. The compound having a melting point of 10 ° C. or less in the non-aqueous electrolyte is dipropyl carbonate, and its amount is 3
It was 6% by weight. The flash point of this electrolytic solution was measured by a closed tag type and the combustion point was measured by an open tag type.
° C and the burning point was 85 ° C.

As a result of measuring the electric conductivity of the obtained non-aqueous electrolyte at 25 ° C. and 10 kHz using an impedance meter, the electric conductivity was 5.1 mS / cm.

[0036]

Embodiment 2 An outer diameter of 20 mm and a height of 3.2 m as shown in FIG.
m coin type non-aqueous electrolyte secondary battery was produced. Negative electrode 13
As the positive electrode 14, LiCo ₂ O 8 was used.
A mixture obtained by adding 12 parts by weight of graphite as a conductive material and 3 parts by weight of a fluororesin as a binder to 5 parts by weight and pressing the mixture into a coin shape was used. Such a negative electrode 13 and a positive electrode 14 are connected to a separator 15 made of polyethylene.
To the sealing plate 16 and the case 17, respectively. As the electrolytic solution, the non-aqueous electrolytic solution prepared in Example 1 was used and sealed from the sealing gasket 18.

With respect to the battery thus manufactured,
The battery was charged for 10 hours at a current of 1.0 mA with an upper limit voltage of 4.2 V, and then discharged at a current of 1.0 mA until the voltage reached 2.5 V.
With this as one cycle, a predetermined number of charge / discharge cycles were repeated, and the charge capacity at each cycle number was measured. The results are shown in FIG.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing one embodiment of a secondary battery according to the present invention.

FIG. 2 is a schematic cross-sectional view of a coin battery manufactured in Example 2 of the present invention.

FIG. 3 shows a change in discharge capacity with respect to the number of charge / discharge cycles of the coin-shaped battery manufactured in Example 2.

[Explanation of symbols]

 1, 13 ··· Negative electrode 2, 14 ··· Positive electrode 3, 15 ··· Separator 4 ··· Insulating plate 5 ··· Battery can 6 ··· Sealing gasket 7 ··· Battery lid 8: Current interrupting thin plate 9: Negative electrode current collector 10: Positive electrode current collector 11: Negative electrode lead 12: Positive electrode lead 16: Sealing plate 17 ··· Case 18 ··· Gasket

Claims (8)

[Claims] 1. A non-aqueous electrolyte comprising an electrolyte and a non-aqueous solvent containing a compound having a melting point of 10 ° C. or less, wherein the non-aqueous electrolyte contains 2 to 40% by weight of a compound having a melting point of 10 ° C. or less.
Wherein the flash point of the non-aqueous electrolyte is 40 ° C. or higher. 2. The non-aqueous solvent containing a compound having a melting point of 10 ° C. or lower is a mixed solution of ethylene carbonate and a compound selected from a chain carbonate ester and a phosphate ester. The non-aqueous electrolyte according to claim 1, wherein the total amount of the electrolyte and the electrolyte is 60 to 98% by weight. 3. The non-aqueous electrolyte according to claim 1, wherein the combustion point of the non-aqueous electrolyte is 60 ° C. or higher. 4. The non-aqueous electrolyte according to claim 2, wherein the chain carbonate is represented by the general formula [I]. Embedded image (In the formula, R ¹ and R ² may be the same or different from each other and represent an alkyl group having 1 to 3 carbon atoms or a haloalkyl group.) 5. A method according to claim 1, wherein said chain carbonate is methyl propyl carbonate, dipropyl carbonate, methyl isopropyl carbonate, methyl isopropyl carbonate, diisopropyl carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl 2,2,2-trifluoro. Claims are at least one chain carbonate selected from ethyl carbonate, methyl 2,2,3,3-tetrafluoropropyl carbonate and 2,2,3,3,3-pentafluoropropyl carbonate. Item 4. The non-aqueous electrolyte according to item 2 or 3. 6. The method according to claim 1, wherein the phosphate is at least one phosphate selected from the group consisting of trimethyl phosphate, triethyl phosphate, and tris (2,2,2-trifluoroethyl) phosphate. Item 4. The non-aqueous electrolyte according to item 2 or 3. 7. The method according to claim 1, wherein the electrolyte is LiPF ₆ , LiBF ₄ , LiO
SO ₂ CF ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂
CF _{3) 2} and LiC (claim 1, wherein the SO ₂ CF ₃₎ is at least one electrolyte selected from ₃
The non-aqueous electrolyte according to any one of the above. 8. A negative electrode using a carbon material capable of doping / dedoping lithium ions as a negative electrode active material, a positive electrode using a composite oxide of lithium and a transition metal as a positive electrode active material, a separator, 7. A non-aqueous electrolyte secondary battery comprising the non-aqueous electrolyte according to any one of 7.