JPH11283669A - Fire resistant electrolyte and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fire resistant electrolyte having excellent fire retardancy without adversely affecting electrical characteristics and a nonaqueous electrolyte secondary batter which uses the fire resistant electrolyte. SOLUTION: This fire resistant electrolyte is obtained by dissolving electrolyte salts in an organic solvent which contains at least on kind of phosphorus compound represented by the general formula (1). This nonaqueous electrolyte secondary battery uses the fire retardancy electrolyte. In the formula, R<1> is a C2-15 alkylene group or arylene group which can be substituted with F atoms and R<2> is a C1-10 alkyl group which can be substituted with F atoms.

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 having excellent flame retardancy, and more particularly, to a flame-retardant electrolyte containing a phosphorus-based flame retardant having a specific structure. The present invention relates to a non-aqueous electrolyte secondary battery using an electrolyte.

[0002]

2. Description of the Related Art In order to reduce the size and weight of portable devices such as notebook personal computers, video cameras, and mobile phones, there is an increasing need for secondary batteries having higher energy density. . Also, practical use of an electric vehicle that does not emit air pollutants is being studied. However, for practical use of electric vehicles, it is necessary to develop batteries having higher energy density than current lead batteries.

[0003] As a battery having such a high energy density, a lithium battery is known. In the lithium battery, as a non-aqueous electrolyte, a solvent such as propylene carbonate, γ-butyrolactone, and sulfolane, which are high dielectric constant solvents, and a low-viscosity solvent such as dimethoxyethane, tetrahydrofuran, and 1,3-dioxolane were mixed. LiBF ₄ , LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiAl
A solution in which an electrolyte such as Cl ₄ or LiSiF ₆ is dissolved is used.

However, since the solvent used in such a non-aqueous electrolyte is a flammable compound, it may cause a fire due to a short circuit or the like. In particular, in the case of a battery with a high energy density, if a short circuit occurs, the high energy of the battery is released at a time, so that measures to prevent a fire are indispensable.

[0005] Conventionally, overcharging,
Measures have been taken to avoid fires due to overdischarge and external shorts. However, if a short circuit occurs inside the battery,
There was a problem that the external safety mechanism did not work. Therefore, there is a need to develop a safe battery that does not depend on an external safety mechanism.

Hitherto, it has been known to make a solvent nonflammable as a safety method independent of an external safety device. As a flame-retarding method, it has been proposed to add a flame-retardant compound such as a phosphoric acid ester or a halogen compound to an electrolytic solution. For example, JP-A-4-184870, JP-A-6-283205 and JP-A-8-22839 propose to use a phosphate compound such as trimethyl phosphate and tricresyl phosphate. However, when these compounds are used, the charging / discharging efficiency is lowered, and lithium is deposited in a dendritic manner during charging, which is not satisfactory in practical use.

Japanese Patent Application Laid-Open No. 244565/1990 proposes to use a small amount of a phosphorus compound such as phosphate or phosphonate in order to improve the impregnation of the electrolyte with the separator. There is no description that a phosphate compound is effective for improving flammability.

Accordingly, an object of the present invention is to provide a non-aqueous electrolyte secondary battery using the non-aqueous electrolyte and the non-aqueous electrolyte using the non-aqueous electrolyte having excellent non-flammability without adversely affecting the electrical characteristics. To provide.

[0009]

As a result of various studies, the present inventors have found that the above object can be achieved by adding a specific phosphorus compound to a non-aqueous electrolyte.

The present invention has been made based on the above findings. In an electrolytic solution obtained by dissolving an electrolyte salt in an organic solvent, the organic solvent is represented by the following general formula (2) (same as the above (1)): An object of the present invention is to provide a flame-retardant electrolytic solution containing at least one phosphorus compound represented by the formula (I).

[0011]

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Further, the present invention provides a non-aqueous electrolyte secondary battery using the above-mentioned flame-retardant electrolyte as an electrolyte.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the flame-retardant electrolytic solution of the present invention will be described in detail. In the phosphorus compound represented by the general formula (I) used in the present invention, examples of the alkylene group having 2 to 15 carbon atoms represented by R ¹ include ethylene, propylene, trimethylene, tetramethylene,
Linear or branched alkylene groups such as pentamethylene, 2,2-dimethylpropylene, 2-ethyl-2-butylpropylene, 2-ethyl-2-octylpropylene, hexamethylene, heptamethylene, octamethylene, and decamethylene; And alkylene groups substituted by fluorine atoms include 2,2-bis (trifluoromethyl) propylene, 1,1,2,2,3,3-hexafluoropropylene, 1-fluoromethylethylene, 1-difluoromethyl Ethylene, 1-trifluoromethylethylene,
1,2-difluoro-1,2-dimethylethylene, 1,
1-difluoro-2-methylethylene, 1,1-difluoro-2-ethylethylene, 1,1,2-trifluoro-2-ethylethylene, 1,1-difluoro-2-
(2,2-difluoroethyl) ethylene, 1,2-difluoro-1,2-bis (trifluoromethyl) ethylene, 1-fluoro-2-ethylethylene, 1-fluoro-1,2-dimethylethylene, 2-bis (trifluoromethyl) ethylene, 1,1,2-trifluoro-2
-Trifluoromethylethylene and the like.

In the above general formula (I), examples of the arylene group represented by R ¹ include 1,2-phenylene and diphenylmethane-2,2'-diyl.

In the general formula (I), examples of the alkyl group having 1 to 10 carbon atoms represented by R ² include methyl, ethyl, propyl, butyl, hexyl, heptyl, octyl, isooctyl, Straight-chain or branched alkyl groups such as -ethylhexyl, isononyl, decyl, and isodecyl; and the alkyl group substituted with a fluorine atom includes trifluoromethyl, 1,1,1-
Trifluoroethyl, 1,1,1,2,2-pentafluoropropyl, 1,1,1,3,3,3-hexafluoroisopropyl and the like.

Accordingly, the phosphorus compound represented by the above general formula (I) used in the present invention includes, for example, the following compound Nos. 3 to 16: 1 to 14 and the like. However, this does not limit the present invention at all.

[0017]

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[0018]

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[0019]

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[0020]

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[0021]

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[0022]

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[0023]

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[0024]

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[0025]

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[0026]

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[0027]

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[0028]

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[0029]

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[0030]

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Among these phosphorus compounds, those in which R ¹ is an alkylene group, especially an alkylene group having 6 or less carbon atoms, or a compound in which R ² is an alkyl group having 5 or less carbon atoms have a low viscosity and have a low viscosity. It is preferable because it has excellent solubility and excellent flame retardancy. Further, a compound in which R ¹ and / or R ² is a group substituted with a fluorine atom is particularly preferable because of its excellent initial battery capacity. Preferred specific examples of the phosphorus compound include 5,5-dimethyl-1,
3,2-dioxaphosphine-2-oxa-2- (2,
2,2-trifluoroethyloxy) [compound No.
1], 5,5-ditrifluoromethyl-1,3,2-dioxaphosphine-2-oxa-2- (2,2,2-trifluoroethyloxy) [compound No. 2], 5, 5
-Dimethyl-1,3,2-dioxaphosphine-2-oxa-2-ethyloxy [Compound No. 3], 1, 3,
2-dioxaphosphor-2-oxa- (2,2,2-trifluoroethyloxy) [compound No. 4] and the like, and particularly, 5,5-dimethyl-1,3,2-dioxaphosphine-2-oxa-2- (2,2,2-trifluoroethyloxy) [compound No. 1] is preferred.

The phosphorus compound according to the present invention can be used as an organic solvent in an electrolytic solution by itself. However, since the compound has excellent compatibility with other organic solvents, it can reduce the viscosity,
It may be mixed with another organic solvent to increase the electric conductivity.

The other organic solvent is not particularly limited as long as it is an organic solvent usually used for an electrolyte of a non-aqueous electrolyte secondary battery, and examples thereof include a carbonate compound, a lactone compound, an ether compound, a sulfolane compound, and a dioxolane. Compounds, ketone compounds, nitrile compounds, halogenated hydrocarbon compounds and the like, specifically, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, ethylene glycol dimethyl carbonate, propylene glycol dimethyl carbonate, ethylene Carbonates such as glycol diethyl carbonate and vinylene carbonate; lactones such as γ-butyrolactone; tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-di Ethers such as xane, anisole and monoglyme; sulfolane such as sulfolane and 3-methylsulfolane; dioxolanes such as 1,3-dioxolane; ketones such as 4-methyl-2-pentanone; acetonitrile, propionitrile and butyronitrile And nitriles such as valeronitrile and benzonitrile; halogenated hydrocarbons such as 1,2-dichloroethane; and others such as methylformate, dimethylformamide, dimethylthioformamide, and dimethylsulfoxide, and a plurality of mixtures thereof. It may be. Of these organic solvents, one or more nonaqueous solvents selected from the group consisting of carbonates, lactones, ethers, sulfolanes and dioxolanes are preferred because of their excellent solubility, dielectric constant and viscosity of the electrolyte.

The amount of the phosphorus compound to be used is preferably 5 to 100% by weight in the organic solvent constituting the electrolytic solution.
100% by weight is particularly preferred. If it is less than 5% by weight, a sufficient flame retarding effect may not be obtained.

The method for synthesizing the phosphorus compound is not particularly limited, and may be any of the synthesis methods usually employed for organic phosphorus compounds.

The electrolyte salt used in the present invention includes LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , CF ₃ SO ₃ Li, N (CF ₃ S
O ₂ ) ₂ Li, C (CF ₃ SO ₂ ) ₃ Li, LiI, LiAlCl ₄ , NaClO ₄ , NaBF
₄ , NaI, etc., among which LiPF ₆ , LiBF ₄ , LiCl
Inorganic salts such as O ₄ and LiAsF ₆ , and CF ₃ SO ₃ Li, N (CF ₃ SO ₂ ) ₂
One or a combination of two or more salts selected from the group consisting of organic salts such as Li and C (CF ₃ SO ₂ ) ₃ Li is preferable because of its excellent electrical properties.

The above-mentioned electrolyte salt has a concentration in the electrolytic solution of:
It is preferable to dissolve in the above-mentioned organic solvent so as to be 0.1 to 3.0 mol / l, particularly 0.5 to 2.0 mol / l. If the concentration of the electrolyte is less than 0.1 mol / liter, a sufficient current density may not be obtained,
If it is more than 3.0 mol / liter, the stability of the electrolyte may be impaired.

The flame-retardant electrolytic solution of the present invention can be prepared by dissolving the electrolyte salt in an organic solvent comprising the phosphorus compound and, if necessary, the other organic solvent according to a generally known method. .

The flame-retardant electrolyte of the present invention is suitably used as a non-aqueous electrolyte constituting a primary or secondary battery, particularly a non-aqueous electrolyte secondary battery described later.

Next, the non-aqueous electrolyte secondary battery of the present invention will be described in detail. The non-aqueous electrolyte secondary battery of the present invention uses the above-described flame-retardant electrolytic solution of the present invention as an electrolytic solution. The constituent materials other than the organic solvent and the electrolyte salt constituting the nonaqueous electrolyte secondary battery of the present invention, that is, the positive electrode, the negative electrode, the separator and the like are not particularly limited, and are conventionally used in nonaqueous electrolyte secondary batteries. Can be used as they are.

Here, as the positive electrode active material constituting the positive electrode, for example, TiS ₂ , TiS ₃ , MoS ₃ , FeS ₂ , Li _(1-x) Mn
O ₂ , Li _(1-x) Mn ₂ O ₄ , Li _(1-x) CoO ₂ , Li _(1-x) NiO ₂ , V ₂
O ₅ , V ₆ O _{13 and the} like. X in the examples of the positive electrode active material indicates a number of 0 to 1.

Examples of the negative electrode active material constituting the negative electrode include lithium, lithium alloys, inorganic compounds such as tin compounds, carbonaceous materials, and conductive polymers.

As the above separator, for example, a separator made of a thermoplastic resin is used. As the thermoplastic resin used for producing the separator made of a thermoplastic resin, for example, high density polyethylene, low density polyethylene, linear Low density polyethylene, polypropylene,
Polybutene-1, poly-3-methylpentene, polyolefins such as ethylene-propylene copolymers, fluorine-based resins such as polytetrafluoroethylene, polystyrene, polymethyl methacrylate, polydimethylsiloxane and the like, and mixtures thereof, In particular, polyolefin is preferred from the viewpoint of moldability, chemical resistance, mechanical strength, and the like.

The above-mentioned thermoplastic resin separator is formed by laminating a low-melting thermoplastic resin film and a high-melting thermoplastic resin film or a non-woven fabric in order to prevent ejection or explosion of the battery contents due to heat generation due to short circuit. It may have a multi-layered structure such as a laminated structure.

The non-aqueous battery of the present invention having the above-described structure is not particularly limited in its shape, and can be used as batteries of various shapes such as a flat type (button type), a cylindrical type, and a square type. FIG. 1 shows an example of a cylindrical type non-aqueous electrolyte secondary battery of the present invention.

[0046]

The present invention will be described below in detail with reference to examples. However, the present invention is not limited by the following examples. FIG. 2 is a schematic diagram showing a basic configuration of a lithium secondary battery, which is a nonaqueous electrolyte secondary battery of the present invention, using the flame-retardant electrolytic solution of the present invention.

The lithium secondary battery, which is the nonaqueous electrolyte secondary battery 10 shown in FIG. 2, has a negative electrode 1, a negative electrode current collector 2, a positive electrode 3, and a positive electrode current collector composed of at least lithium or a lithium alloy as an active material. And a separator 6, a positive electrode terminal 7 and a negative electrode terminal 8. In addition, the lithium secondary battery can use, if necessary, constituent materials other than those described above that are usually used for nonaqueous batteries.

In the lithium secondary battery, since the flame-retardant electrolytic solution of the present invention containing the phosphorus compound represented by the general formula (I) is used as the electrolytic solution 5, ignition of the electrolytic solution is suppressed. This is effective for improving the safety of the battery.

In the above-mentioned lithium secondary battery, in the discharge reaction, at the same time as lithium ions dissolve into the electrolyte from the negative electrode active material forming the negative electrode, lithium in the electrolyte flows into the positive electrode active material forming the positive electrode. It is captured. On the other hand, in the charging reaction, lithium ions in the electrolyte are taken into the negative electrode active material as lithium. At this time, the lithium ions are precipitated as lithium metal, and at the same time, the lithium ions in the positive electrode active material are dissolved in the electrolytic solution. In general, at the time of such a charging in which the charging reaction occurs, lithium deposited as lithium metal on the negative electrode does not uniformly deposit on the lithium surface but locally deposits, and lithium grows in a dendritic manner using the lithium as a growth nucleus ( Dendrites), grow in the electrolyte, and finally contact the positive electrode,
Cause a short. At this time, a large current may flow in a short time, which may cause a fire.

Even if the negative electrode active material is not lithium or a lithium alloy, but is a material that absorbs and releases lithium ions, for example, a carbon material, at the time of overcharge, lithium metal precipitates on the carbon surface, and this is the growth nucleus. As a result, dendrite may grow and cause a short circuit.

When there is a risk of such a fire,
When a flame-retardant base material comprising a halogen compound or a phosphorus compound other than the one according to the present invention is contained in the electrolytic solution, the ignition of the electrolytic solution is suppressed to some extent but is not sufficient, and there are various disadvantages and it is not practically satisfactory. .

On the other hand, when the flame-retardant electrolytic solution of the present invention containing the phosphorus compound represented by the general formula (I) is used, even when a substance other than lithium or a lithium alloy is used for the negative electrode, The ignition of the liquid is suppressed, which is effective for improving the safety of the battery.

The non-aqueous electrolyte secondary battery of the present invention is not limited to the above-mentioned lithium battery, but may be a battery whose configuration is appropriately changed as long as the effects of the present invention are not impaired. For example, the nonaqueous electrolyte secondary battery of the present invention in which the active material in the lithium battery is changed is also effective because the electrolyte containing the phosphorus compound represented by the general formula (I) is used. .

Next, another example of the non-aqueous electrolyte secondary battery of the present invention (cylindrical non-aqueous electrolyte secondary battery using a carbonaceous material for the negative electrode) will be described. And electrical characteristics (charge / discharge efficiency)]. The effects of the battery were evaluated and measured according to the following methods, and the results are shown in [Table 1] below.

[Method of Evaluating Flame Retardancy of Electrolyte Solution] Ethylene carbonate (EC): diethyl carbonate (DEC):
Phosphorus compound (described in [Table 1] below) = 1: 1: X (described in [Table 1] below) [weight ratio] and an organic solvent composed of LiPF ₆
Is dissolved in an electrolyte solution having a concentration of 1 mol / liter,
A 0.04 mm-thick manila sheet for separator cut to a length of 320 mm and a length of 320 mm is immersed, and then suspended vertically for 3 minutes to remove excess electrolyte. In this manner, the manila paper impregnated with the electrolytic solution is pierced at 25 mm intervals into the support needles of the sample table having the support needles and fixed horizontally. This sample table was placed in a 250 mm × 250 mm × 500 mm metal box, and one end of the sample table was ignited with a lighter, and the burning rate (m
m / sec), the flame retardancy of the electrolyte was evaluated.

[Method of Manufacturing Test Battery and Measurement of Charge / Discharge Efficiency] A graphite carbon material having a true specific gravity of 2.20 g / cm ³ was pulverized to an average particle diameter of 10 μm, and 90 parts by weight of the carbon material powder was added with polyvinylidene fluoride 10 parts by weight. Parts by weight were mixed to obtain a negative electrode material. This negative electrode material was dispersed in N-methyl-2-pyrrolidone to form a slurry. This slurry has a thickness of 10
It was applied to both sides of a copper foil having a thickness of μm, dried, and then compression-molded with a roll press machine to produce a negative electrode.

Further, LiCoO ₂ is pulverized to an average particle size of 3 μm,
91 parts by weight of the pulverized LiCoO ₂ , 6 parts by weight of graphite and 3 parts by weight of polyvinylidene fluoride were mixed to prepare a positive electrode material. This positive electrode material was dispersed in N-methyl-2-pyrrolidone to form a slurry. Thick this slurry 2
It was applied to both sides of a 0 μm aluminum foil, dried, and then compression-molded with a roll press to produce a positive electrode.

A 25-μm-thick microporous polypropylene film was used as a separator, and the above-mentioned positive electrode and negative electrode were sequentially laminated, and then spirally wound many times to obtain a roll. Next, an insulator was inserted into the bottom of the nickel-plated iron battery can, and the wound body was housed. Then, a nickel negative electrode terminal was crimped to the negative electrode, and the other end was welded to the battery can. Also, a positive electrode terminal made of aluminum was attached to the positive electrode, and the other end was electrically connected to the battery lid via a current blocking thin plate.

An electrolytic solution obtained by dissolving LiPF ₆ at a concentration of 1 mol / liter in the same organic solvent containing a phosphorus compound as shown in the following [Table 1] and used in the above-mentioned test for evaluating flame retardancy was used as described above. Injected into the battery can produced in the 18mm diameter,
A cylindrical non-aqueous electrolyte secondary battery having a height of 65 mm was produced.

Using the above non-aqueous electrolyte secondary battery, 4.2
V, 1A, 2.5 hours constant current constant voltage charging,
And a discharge with a constant current of 1 A and a discharge voltage of 2.75 V was performed. This charge and discharge is performed for 100 cycles,
The discharge capacity at the 100th cycle when the discharge capacity at the second cycle was 100 was measured as the capacity retention ratio.

[0061]

[Table 1]

As can be seen from the results shown in Table 1, the non-aqueous battery using the electrolytic solution containing no phosphorus compound (Comparative Example 1-1) has a capacity retention ratio which is a performance as a battery. There is no problem, but there is no fire resistance and the risk of ignition is quite large. Further, the nonaqueous electrolyte secondary battery using the electrolyte solution (Comparative Examples 1-2 and 1-3) to which the phosphorus compound other than the present invention is added, although the flame retardancy is improved to some extent, but the ignition still occurs. There is a danger, and the capacity retention rate is also decreasing. On the other hand, a non-aqueous electrolyte secondary battery using the flame-retardant electrolyte of the present invention (Examples 1-1 to 1-8) to which the phosphorus compound represented by the general formula (I) is added is described. It is apparent that the battery has a high capacity retention rate, which is the performance of a battery, has excellent flame retardancy, and has extremely low risk of ignition.

[0063]

The flame-retardant electrolytic solution of the present invention has a high degree of flame retardancy without deteriorating the performance as a battery. Further, the non-aqueous electrolyte secondary battery of the present invention has excellent performance as a battery and has high flame retardancy.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a cross section of an internal structure of a lithium secondary battery (cylindrical type) as a nonaqueous electrolyte secondary battery of the present invention.

FIG. 2 is a schematic diagram showing a basic configuration of a lithium secondary battery as a non-aqueous electrolyte secondary battery of the present invention.

[Explanation of symbols]

 Reference Signs List 1 negative electrode 2 negative electrode current collector 1 'negative electrode plate 1 "negative electrode lead 3 positive electrode 4 positive electrode current collector 3' positive electrode plate 3" positive electrode lead 5 electrolyte 6 separator 7 positive electrode terminal 8 negative electrode terminal 10 nonaqueous electrolyte secondary battery 11 Case 12 Insulator 13 Gasket 14 Safety valve 15 PTC element

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Shinya Mashimo 7-35 Higashiogu, Arakawa-ku, Tokyo

Claims (9)

[Claims] 1. A flame-retardant solution comprising an electrolyte solution obtained by dissolving an electrolyte salt in an organic solvent, wherein the organic solvent contains at least one phosphorus compound represented by the following general formula (I). Electrolyte. Embedded image 2. The flame-retardant electrolytic solution according to claim 1, wherein R ¹ is an alkylene group. 3. The flame-retardant electrolytic solution according to claim 1, wherein said R ¹ and / or R ² are substituted with a fluorine atom. 4. The method according to claim 1, wherein the organic solvent contains at least one nonaqueous solvent selected from the group consisting of carbonates, lactones, ethers, sulfolane, and dioxolane. Flame retardant electrolyte. 5. The method according to claim 1, wherein the electrolyte salt is lithium ion and P
F₆, BF_Four, ClO_FourAnd AsF₆Anion selected from
Inorganic salt composed of and lithium ion and SO _ThreeC
F_Three, N (CF_ThreeSO_Two)_Two, C (CF_ThreeSO_Two)_ThreeAnd in these derivatives
Consisting of an organic salt composed of an anion selected from
One or a combination of two or more salts selected from the group
The flame-retardant electrolytic solution according to any one of claims 1 to 4. 6. The organic compound according to claim 1, wherein the phosphorus compound represented by the general formula (I) is present in an amount of 5% by weight or more.
6. The flame-retardant electrolytic solution according to any one of claims 1 to 5. 7. The method according to claim 1, wherein the phosphorus compound represented by the general formula (I) is 5,5-dimethyl-1,3,2-dioxaphosphine-2-oxa-2- (2,2,2-trifluoro Ethyloxy), 5,5-ditrifluoromethyl-1,3,
2-dioxaphosphine-2-oxa-2- (2,2,2
2-trifluoroethyloxy), 5,5-dimethyl-
1,3,2-dioxaphosphine-2-oxa-2-ethyloxy or 1,3,2-dioxaphosphor-2
-Oxa- (2,2,2-trifluoroethyloxy)
The flame-retardant electrolytic solution according to claim 1, comprising at least one of the following. 8. The method according to claim 1, wherein the phosphorus compound represented by the general formula (I) is 5,5-dimethyl-1,3,2-dioxaphosphine-2-oxa-2- (2,2,2-trifluoro The flame-retardant electrolytic solution according to claim 7, which is ethyloxy). 9. A non-aqueous electrolyte secondary battery using the flame-retardant electrolyte according to claim 1 as an electrolyte.