JP2001210364A - Nonaqueous electrolytic solution and secondary battery using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolytic solution and secondary battery using it, which can prevent overcharge of battery. SOLUTION: This is the nonaqueous electrolytic solution which has a lithium salt as a nonaqueous organic solvent and solute, and in which biphenyl derivatives having one or more of electron donating group are further added in more than one kind. Furthermore, this is the nonaqueous secondary battery which has a positive electrode containing lithium metal complex oxide as an active substance, a negative electrode which contains a substance as an active substance which can occlude/discharge lithium, and the nonaqueous 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 and a secondary battery using the same. Specifically, the electrolyte solution is characterized in that the electrolyte solution contains an organic solvent and a lithium salt as a solute, and one or more specific biphenyl derivatives are added. ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide an electrolyte with improved safety, especially in the case of overcharging, and a non-aqueous secondary battery using the same.

[0002]

2. Description of the Related Art In recent years, as electronic devices have been reduced in size and weight, non-aqueous electrolyte secondary batteries using a carbon material capable of occluding and releasing lithium as a negative electrode and a lithium metal oxide as a positive electrode have become increasingly high voltage. -Due to its high energy density and excellent storage properties, it has been widely used as a power source for consumer electronic devices such as handy video cameras and portable personal computers. Non-aqueous electrolyte secondary batteries that exhibit battery functions by inserting and extracting lithium ions
It is called a lithium-ion secondary battery, and competition for its development and commercialization is increasing. Further, due to environmental problems, attention has been paid to a sealed, maintenance-free lithium-ion secondary battery having a large capacity, a high energy density, and a high capacity, such as for an electric vehicle and for load leveling of electric power.

As a positive electrode active material of a lithium ion secondary battery, a layered lithium cobalt oxide (LiCoO ₂ ), a lithium nickel oxide (LiNiO ₂ ), or the like is mainly used because of its large capacity per weight. However, these have major problems. This is because these lithium metal oxides become very unstable in the overcharged state (the state in which lithium ions are almost desorbed), causing a rapid exothermic reaction with the electrolytic solution or depositing lithium metal on the negative electrode. In the worst case, the battery may burst or ignite.

In order to solve such a problem, it has been attempted to ensure safety against overcharging of a battery by adding a small amount of an aromatic compound as an additive to an electrolyte of a lithium ion secondary battery. For example, JP-A-7-3
No. 02614, JP-A-9-50822, JP-A-9-106835, JP2939469, and the like.

[0005] In JP-A-7-302614 and JP-A-9-50822, as an additive in an electrolytic solution, a potential no higher than the positive electrode potential at the time of full charge of a secondary battery having a molecular weight of 500 or less is used. The use of organic low-molecular compounds such as anisole derivatives having a π-electron orbit with a reversible oxidation-reduction potential has been proposed. This additive is called a redox shuttle, and consumes an overcharge current at the time of overcharge, whereby a protection mechanism is established.

[0006] In Japanese Patent Application Laid-Open No. 9-106835, when the battery voltage in the electrolyte is higher than the maximum operating voltage of the battery,
It has been proposed that the additive polymerizes to increase the internal voltage of the battery and protect the battery in the event of overcharge abuse. Japanese Patent No. 2939469 proposes adding a terphenyl derivative having a specific structure to a solvent in an electrolytic solution. It is stated that this additive starts a polymerization reaction at a voltage in an overcharged state, becomes a polymer which acts as a resistor and is hardly redissolved, and effectively acts on overcharge.

[0007]

However, the anisoles proposed in JP-A-7-302614 and JP-A-9-50822 certainly function as a redox shuttle at the time of overcharging, but they do not use ordinary batteries. It was found that the reaction occurred in the voltage range, and that the discharge capacity was reduced and the cycle characteristics and storage were adversely affected.

Further, biphenyl proposed in Japanese Patent Application Laid-Open No. 9-106835 occurs when an oxidative polymerization reaction occurs in an overcharge region of a battery, but a rapid polymerization reaction does not completely occur as the overcharge is stopped. It was found that the effect of preventing overcharge was insufficient. Furthermore, it has been found that 3-chlorothiophene, furan, and the like are easily oxidatively decomposed, and an oxidation reaction occurs under normal battery use conditions, which adversely affects battery characteristics. Further, it has been found that the terphenyl derivative disclosed in Japanese Patent No. 2939469 has polymerization reactivity, but has a high molecular weight and is not easily dissolved in an electrolytic solution, so that the battery performance is lowered.

The present invention has been made to solve the above problems, and has no adverse effect on battery characteristics under normal use conditions, and the resistance of the electrolyte rapidly increases during overcharge.
It is an object of the present invention to provide a non-aqueous electrolyte and a secondary battery that can prevent overcharging of the battery.

[0010]

According to the present invention, first, a non-aqueous organic solvent and a lithium salt as a solute are contained, and at least one biphenyl derivative having at least one electron donating group is added. A non-aqueous electrolyte characterized by the fact that, second, a positive electrode containing a lithium metal composite oxide as an active material,
An electrolyte for a non-aqueous secondary battery having a negative electrode containing a substance capable of occluding and releasing lithium as an active material, wherein the electrolyte contains an organic solvent and a lithium salt as a solute, and further has an electron donating group. A non-aqueous secondary battery electrolyte solution characterized by the addition of at least one biphenyl derivative having at least one, a third positive electrode containing a lithium metal composite oxide as an active material, and the storage and release of lithium A non-aqueous secondary battery includes a negative electrode containing a substance as an active material and any one of the non-aqueous electrolytes.

In the present invention, the addition of the biphenyl derivative has an oxidation onset potential higher than the upper limit potential of the positive electrode corresponding to the normally used maximum operating voltage of the battery, and the biphenyl alone is added. It is preferable to have the oxidation onset potential at a potential lower than the oxidation onset potential of the electrolytic solution in the case of. Further, the biphenyl derivative is contained in the electrolytic solution at a concentration of 0.1%.
Preferably, it is added in an amount of from 0.01 to 0.8 mmol / g. The biphenyl derivative preferably has a molecular weight of 210 or less. Preferred biphenyl derivatives in the present invention are those represented by the general formula (I)

[0012]

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(Wherein A ^{1 to} A ¹⁰ are each independently a hydrogen atom or an alkyl group, and at least one is an alkyl group) or represented by the general formula (II)

[0014]

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(Wherein, B ^{1 to} B ¹⁰ are each independently a hydrogen atom, an alkyl group or a methoxy group, and at least one is a methoxy group); III)

[0016]

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(Wherein C ¹ -C ⁶ are each independently a hydrogen atom or an alkyl group, and at least one is an alkyl group). In the present invention, the positive electrode preferably contains lithium and a metal composite oxide essentially containing cobalt or nickel as an active material.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The additive used in the non-aqueous electrolyte of the present invention, preferably the electrolyte for a non-aqueous secondary battery, is a biphenyl derivative having at least one electron donating group. In the present invention, the electron donating group means a substituent that promotes an electrophilic substitution reaction when substituted with an aromatic ring,
Specifically, it has 12 carbon atoms such as amino group, methylamino group, ethylamino group, dimethylamino group, methylethylamino group, diethylamino group and methylphenylamino group.
A dialkyl-substituted or alkylphenyl-substituted amino group having preferably 7 or less carbon atoms; a hydroxyl group; a carbon atom having 1 carbon atom such as a methyl group, an ethyl group, an n-propyl group, an iso-propyl group, an n-butyl group, a t-butyl group; To 5, preferably an alkyl group having 1 to 3 carbon atoms, a methoxy group, an ethoxy group or the like, having 1 to 5 carbon atoms, preferably an alkoxy group having 1 to 3 carbon atoms,
Can be exemplified.

Of these electron donating groups, preferred are an alkyl group and an alkoxy group, and particularly preferred are a methyl group and a methoxy group. The biphenyl derivative having one or more electron donating groups used in the present invention is preferably represented by the following general formula (I) or (II).

[0020]

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(In the formula, A ^{1 to} A ¹⁰ are each independently a hydrogen atom or an alkyl group, and at least one is an alkyl group.)

[0022]

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(In the formula, B ^{1 to} B ¹⁰ are each independently a hydrogen atom, an alkyl group or a methoxy group, and at least one is a methoxy group.) In the general formula (II), the number of methoxy groups is Is particularly preferable. In the present invention, the substitution of the electron donating group increases the reactivity of biphenyl, and the biphenyl derivative in the electrolytic solution is rapidly oxidized at the time of overcharging of the battery, thereby increasing the internal resistance of the battery. It is thought to stop the charging reaction.

In the biphenyl derivatives represented by the above general formulas (I) and (II), the alkyl group preferably has 1 to 5, preferably 1 to 3 carbon atoms, particularly preferably a methyl group. The number is preferably 6 or less, more preferably 4 or less, particularly preferably 2 or less. If the molecular weight is too large, it is difficult to dissolve in the electrolytic solution, which causes deterioration of battery characteristics. If the number of electron donating groups is too large, it is easily oxidized, which is not preferable. In the present invention, the molecular weight of the biphenyl derivative is preferably 210 or less. In addition, due to steric hindrance due to the position of the substituent, 2
The case where two aromatic rings are twisted, the interaction between the rings becomes small, and the ring is hardly oxidized is not preferable, and the biphenyl derivative represented by the general formula (III) is preferable.

[0025]

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(Wherein C ¹ -C ⁶ are each independently a hydrogen atom or an alkyl group, and at least one is an alkyl group.) In the general formula (III) as well, the preferred substituent size and For the numbers, the conditions described for general formulas (I) and (II) apply. In the present invention, the concentration of the biphenyl derivative in which one or more electron-donating groups are added to the electrolytic solution is preferably 0.01 mmol / g or more and 0.8 mmol / g or less per weight of the electrolytic solution. More preferably 0.05 mmol / g or more and 0.5 mmol
/ G or less, particularly preferably 0.1 mmol / g or more and 0.3 mmol / g or less. If the amount is less than the above range, the effect of preventing overcharge is reduced, and if the amount is too large, ordinary battery characteristics are deteriorated.

The electrolytic solution of the present invention to which one or more kinds of biphenyl derivatives substituted with one or more electron-donating groups are added can be used in a non-aqueous electrolyte secondary battery containing the electrolytic solution in a normal operating maximum operating voltage. It is desirable to have an oxidation onset potential at a potential nobler than the corresponding upper limit potential of the positive electrode, and preferably to have an oxidation onset potential at a potential lower than the oxidation onset potential of the electrolytic solution when biphenyl is added alone. As described above, the oxidation onset potential of this electrolytic solution is evaluated by cyclic voltammetry measurement (hereinafter referred to as CV measurement). The measurement is performed as follows.

The working electrode is made of platinum, the counter electrode and the reference electrode are made of lithium metal, and an H-type cell in which the working electrode side and the counter electrode side are separated by a glass filter is used. On the oxidation side at a running speed of 20 mV / sec. The potential at which a current density of 0.5 mA / cm ² flows is defined as the oxidation onset potential. If the oxidation start potential is lower than the upper limit potential of the positive electrode corresponding to the normal operating maximum operating voltage of the battery, the added biphenyl derivative reacts during normal use of the battery, which not only deteriorates the battery characteristics but also prevents it during overcharging thereafter. Does not function. On the other hand, if the oxidation starting potential is lower than the oxidation starting potential of the electrolytic solution to which biphenyl alone is added, the overcharging of the battery proceeds too much, resulting in a dangerous state.

For example, in a lithium ion secondary battery using a lithium cobalt oxide or a lithium cobalt oxide in which the cobalt site is replaced by another metal for the positive electrode, the upper limit potential of the positive electrode corresponding to the maximum operating voltage is Li / Li counter electrode.
Since it is usually 4.1 to 4.2 V on the + basis, it is preferable that the electrolyte has an oxidation onset potential higher than this. The oxidation onset potential of biphenyl is about 4.5 at the counter electrode Li / Li +.
Since it is around V, the electrolyte having the additive of the present invention has an oxidation onset potential of 4.2 V or more and 4.5 V with respect to Li / Li +.
The following is preferable, and 4.25V or more and 4.45V or less are more preferable, Furthermore, 4.3V or more and 4.4V or less are more preferable. Note that in a lithium ion secondary battery using lithium nickel oxide or a lithium nickel oxide in which the nickel site is replaced with another metal for the positive electrode, the maximum operating voltage drops by about 0.05 V to 0.1 V, and the lithium manganese oxide In a lithium ion secondary battery using a lithium manganese oxide in which a manganese site is replaced with another metal, the maximum operating voltage is 0.05 V to 0.1 V.
Since such a positive electrode is used, it is preferable that the lower limit of the oxidation onset potential of the additive is raised or lowered by about 0.05 to 0.1 V from the value in the case of the above lithium cobalt oxide positive electrode battery. .

Further, it is preferable that the electrolyte solution containing the additive is polymerized by oxidation to form a reaction-inactive film having high resistance in electron conduction or ion conduction on the positive electrode surface. This determination can also be made by the aforementioned CV measurement. When the CV measurement is started from the natural potential, and once scanned to the oxidation potential and even more noble potential, for example, about 5 V with the counter electrode Li / Li +, the operation is returned to the starting potential, and the operation of scanning to 5 V again is performed. It is preferable that the amount of oxidation current of the first layer flows only a smaller amount than that of the first time. At this time, it is preferable that a film is formed on the platinum surface of the working electrode. It is considered that the oxidation current in the second scan is smaller than that in the first scan because the film formed by the oxidation is inert.

The non-aqueous organic solvent used in the present invention is not particularly limited, and a known organic solvent can be used.
For example, carbonates, ethers, ketones, sulfolane compounds, lactones, nitriles, halogenated hydrocarbons, amines, esters, amides, phosphate compounds, and the like can be used. When these typical ones are listed, propylene carbonate, ethylene carbonate, chloroethylene carbonate, trifluoropropylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, diisopropyl carbonate, vinylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, 1, 4-dioxane, 4-methyl-2-pentanone, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, 1,3-dioxolane,
-Methyl-1,3-dioxolane, diethyl ether,
A single solvent or a mixture of two or more solvents such as sulfolane, methylsulfolane, acetonitrile, propionitrile, benzonitrile, butyronitrile, valeronitrile, 1,2-dichloroethane, dimethylformamide, dimethylsulfoxide, trimethyl phosphate, triethyl phosphate and the like can be used.

In the present invention, it is preferable that a high dielectric constant solvent is contained in order to dissociate the electrolyte. here,
The high dielectric constant solvent means a compound having a relative dielectric constant of 20 or more at 25 ° C. Among the high dielectric constant solvents, ethylene carbonate, propylene carbonate, and compounds in which hydrogen atoms thereof are substituted with another element such as halogen or an alkyl group are preferable. The proportion of the high dielectric constant compound in the electrolytic solution is preferably 20% by weight or more, more preferably 30% by weight or more, and most preferably 40% by weight or more.

Preferred solvents used in combination with the high dielectric constant solvent include low viscosity solvents such as chain carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, 1,2-dimethoxyethane, and 1,2 dimethoxyethane.
Chain ethers such as -diethoxyethane and dimethoxymethane, and these can be used alone or in combination of two or more.

As the solute used in the present invention, a known solute can be used.
Lithium salts. Specifically, LiClO_Four,
LiAsF₆, LiPF₆, LiBF_Four, LiB (C
₆H_Five)_Four, LiCl, LiBr, LiCH_ThreeSO_Three, Li
CF_ThreeSO_Three, LiN (SO_TwoCF_Three) _Two, LiN (SO_TwoC
_TwoF_Five)_Two, LiC (SO_TwoCF_Three)_Three, LiN (SO_ThreeC
F_Three)_TwoAnd the like.
May be used as a mixture of two or more.

Of the above, LiBF ₄ and LiPF ₆ are preferably used. The electrolyte for a non-aqueous secondary battery of the present invention includes known additives, for example, on the surface of the negative electrode, substantially preventing the decomposition of the electrolyte and the efficiency of lithium ion, as long as the effects of the present invention are not impaired. Enable good charge and discharge,
Even when the temperature of the battery becomes high, an additive that forms a favorable film that is unlikely to be broken due to dissolution, decomposition, or the like may be added to the electrolyte solution.

As the material of the negative electrode constituting the battery of the present invention, any material may be used as long as it contains a substance capable of inserting and extracting lithium as an active material. Specific examples of the carbonaceous material include, for example, thermal decomposition products of organic substances under various thermal decomposition conditions, artificial graphite,
And natural graphite. Preferably artificial graphite and graphitized mesophase small spheres produced by high-temperature heat treatment of graphitic pitch obtained from various raw materials, other artificial graphite such as graphitized mesophase pitch-based carbon fiber and purified natural graphite, or a mixture thereof. A material obtained by performing various surface treatments including pitch on graphite is used.

These carbonaceous materials preferably have a lattice plane (002 plane) d-value (interlayer distance) of 0.335 to 0.34 nm, as determined by X-ray diffraction according to the Gakushin method.
Those having a thickness of 335 to 0.337 nm are more preferred. The ash content is preferably 1% by weight or less, more preferably 0.5% by weight or less, and particularly preferably 0.1% by weight or less. The crystallite size (Lc) determined by X-ray diffraction according to the Gakushin method is preferably 30 nm or more, more preferably 50 nm or more, and 100 nm or more.
The above is particularly preferred.

A negative electrode material capable of inserting and extracting lithium can be further mixed with these carbonaceous materials and used. Examples of the negative electrode material capable of occluding and releasing lithium other than the carbonaceous material include metal oxide materials such as tin oxide and silicon oxide, as well as lithium metal and various lithium alloys. These negative electrode materials may be used as a mixture of two or more.

The method for producing a negative electrode using these negative electrode materials is not particularly limited. For example, a negative electrode can be manufactured by adding a binder, a thickener, a conductive material, a solvent, and the like to a negative electrode material as needed to form a slurry, applying the slurry to a current collector substrate, and drying. Alternatively, the negative electrode material may be directly roll-formed into a sheet electrode,
A pellet electrode can be obtained by compression molding.

The binder used in the production of the electrode is not particularly limited as long as it is a material that is stable with respect to the solvent and the electrolyte used in the production of the electrode. Specific examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, styrene / butadiene rubber, isoprene rubber, and butadiene rubber. As a thickener,
Examples include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein and the like.

Examples of the conductive material include metal materials such as copper and nickel, and carbon materials such as graphite and carbon black. As the material of the current collector for the negative electrode, metals such as copper, nickel, and stainless steel are used, and among these, copper foil is preferable from the viewpoint of easy processing into a thin film and cost. The material of the positive electrode constituting the battery of the present invention may be any material containing a lithium metal composite oxide as an active material, and is preferably a lithium cobalt oxide, a lithium nickel oxide, a lithium manganese oxide or the like. It is a transition metal composite oxide. In particular, those containing lithium and a metal composite oxide essentially containing cobalt or nickel as an active material are preferable.

The method for manufacturing the positive electrode is not particularly limited, and the positive electrode can be manufactured according to the above-described method for manufacturing the negative electrode. As for the shape, a binder, a conductive material, a solvent, and the like are added to the positive electrode material as necessary and mixed, and then applied to a current collector substrate to form a sheet electrode, or a pellet electrode formed by press molding. It can be. As a material of the current collector for the positive electrode, a metal such as aluminum, titanium, and tantalum or an alloy thereof is used. Among these, aluminum or its alloy is desirable in terms of energy density because it is lightweight.

The material and shape of the separator used in the battery of the present invention are not particularly limited. However, it is preferable to select from materials that are stable with respect to the electrolytic solution and have excellent liquid retention. It is preferable to use a porous sheet or nonwoven fabric made of a polyolefin such as polyethylene or polypropylene as a raw material. The method for producing the battery of the present invention having at least the negative electrode, the positive electrode, and the nonaqueous electrolyte is not particularly limited, and can be appropriately selected from commonly employed methods.

The shape of the battery is not particularly limited, and is a cylinder type in which a sheet electrode and a separator are formed in a spiral shape, a cylinder type having an inside-out structure in which a pellet electrode and a separator are combined, and a coin type in which a pellet electrode and a separator are laminated. Etc. can be used.

[0045]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, which should not be construed as limiting the scope of the invention. (CV measurement) An electrolyte to be measured is placed in an H-type cell whose center is partitioned by a glass filter, a platinum electrode as a working electrode and a lithium metal wire as a reference electrode are placed on one side, and lithium as a counter electrode is placed on the other side. The measurement was performed using a metal foil. In addition, the platinum electrode of the working electrode has a cylindrical shape, the periphery of which is covered with a resin, and only the exposed cross section (circle having a diameter of 1.6 mm) is in contact with the electrolyte.

In the first measurement, the potential of the working electrode was swept from the spontaneous potential to the oxidation side, swept to 5 V on the basis of Li / Li +, and subsequently swept to the reduction side to 2.5 V. Subsequently, the second measurement was again run to 5 V on the oxidizing side and continued to 2.5 V on the reducing side.
I ran to V. The running speed was a constant speed of 20 mV / sec. The oxidation onset potential is set to 0.1 at the first run to the oxidation side.
It was defined as the potential at which a current of 5 mA / cm ² began to flow. In addition, the presence or absence of the formation of a film on the Pt electrode and the increase and decrease of the oxidation current in the first run and the second run were also observed.

(Battery Measurement) (Preparation of Positive Electrode) The positive electrode was composed of 90% by weight of lithium cobalt oxide (LiCoO ₂ ) as a positive electrode active material, 5% by weight of acetylene black as a conductive agent, and polyfluoride as a binder. After 5% by weight of vinylidene (PVdF) is mixed with N-methylpyrrolidone in a solvent to form a slurry,
It was applied to one side of a 20 μm aluminum foil, dried, and rolled by a press machine, and punched out with a punch having a diameter of 12 mm.

(Preparation of Negative Electrode) A negative electrode was prepared by mixing 95% by weight of graphite (surface spacing: 0.336 nm) as a negative electrode active material and 5% by weight of polyvinylidene fluoride (PVdF) as a binder in an N-methylpyrrolidone solvent. And after slurrying, 2
One side of a copper foil having a thickness of 0 μm was applied, dried, and rolled by a press machine to be punched out with a diameter of 12 mm.

When constructing the battery, the weight W of the positive electrode active material
The ratio between (c) and the active material weight W (a) of the negative electrode is preferably in a range where lithium ions released from the positive electrode do not cause precipitation of lithium metal on the opposite negative electrode at the normal upper limit voltage of the battery. That is, the electric capacity per weight of the positive electrode active material under the condition corresponding to the initial charging condition of the battery is Q (c) mAh / g, and the negative electrode active material capable of maximally storing lithium without depositing lithium metal. Assuming that the electric capacity per weight of the substance is Q (a) mAh / g, the capacity ratio Rq = Q (a) × W (a) / {Q (c) × W (c)}.
Must be 1.0 or more. In this embodiment and the comparative example, W is set so that 1.1 ≦ Rq ≦ 1.2.
(C) / W (a) was set. Q (c) or Q
(A) can be measured by using a positive electrode or a negative electrode as a working electrode and lithium metal as a counter electrode, and assembling a test cell via a separator in the same electrolytic solution as used for forming a battery. That is, the capacity at which the positive electrode can be charged (release of lithium ions from the positive electrode) at the lowest possible current density up to the upper limit potential of the positive electrode or the lower limit potential of the negative electrode corresponding to the initial charging conditions of the target battery system, and the negative electrode discharges (Occlusion of lithium ions in the negative electrode) It can be obtained as a capacity that can be obtained.

(Assembly of Battery) A battery was prepared using a 2032 type coin cell having the structure shown in FIG. 1 in a dry box in an argon atmosphere. That is, the positive electrode 2 is placed on the positive electrode can 1, a 25 μm porous polyethylene film is placed thereon as the separator 3, pressed with a polypropylene gasket 4, the negative electrode 5 is placed, and the spacer 6 for thickness adjustment is placed. After that, an electrolyte was added and the battery was sufficiently impregnated into the battery, and then the battery was sealed with the negative electrode can 7 placed thereon. In this example and the comparative example, the capacity of the battery was set to a charging upper limit of 4.1 V,
The design is such that the discharge lower limit is about 3.0 mAh at the lower limit of 3.0 V.

(Evaluation Method) The battery was evaluated in the following order: initial charge / discharge (capacity check) → discharge rate measurement test → full charge operation → overcharge test.・ In the initial charge (confirmation of capacity), charge is 1C (3.7mA),
The battery was charged by a constant current and constant voltage method with an upper limit of 4.1 V. The charge was cut when the current value reached 0.05 mA. Discharging was performed at a constant current up to 3.0 V at 0.2 C. The discharge rate measurement test was performed for 2 cycles, and all charging was performed at a constant current and constant voltage of 1 C and 4.1 V (0.05 mA).
Cut) and the discharge rate is 0.2C and 1C
And The discharge was cut at 3V.

It should be noted that 1C / 0.2C discharge rate = (1C discharge capacity / 0.2C discharge capacity) × 100 (%) was used as an index for determining the superiority of the discharge rate characteristics. The larger the value, the better the rate characteristics. -Full charge operation is a constant current and constant voltage method with an upper limit of 4.1 V (0.0
(5 mA cut).・ The overcharge test is 4.9V cut to 3hr cut at 1C (cut off whichever arrives first), and discharge is 0.
The test was performed at a constant current up to 3.0 V at 2C.

As an index for determining the degree of overcharge prevention effect, the overcharge rate = 100 + (overcharge capacity / full charge capacity) from the overcharge capacity at the time of the overcharge test and the full charge capacity of the full charge operation.
× 100 (%) was used. The smaller this value is, the higher the overcharge prevention effect is. Using the electrolyte solutions shown in the following Examples and Comparative Examples, the CV measurement and the battery measurement described above were performed and compared. The results are collectively shown in Tables 1 and 2.

Example 1 As an electrolytic solution, lithium hexafluorophosphate (LiPF ₆ ) was used at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3: 7. 0.015mmo in dissolved electrolyte
One to which 4-methylbiphenyl was added at a concentration of 1 / g was used.

Example 2 The same electrolytic solution as in Example 1 was used except that 3,3'-methylbiphenyl was added instead of 4-methylbiphenyl. Example 3 The same electrolytic solution as in Example 1 was used except that 4,4'-methylbiphenyl was added instead of 4-methylbiphenyl.

COMPARATIVE EXAMPLE 1 In the electrolyte solution of Example 1, to which 4-methylbiphenyl was not added, ie, a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) having a volume ratio of 3: 7, a concentration of 1 mol / liter. And an electrolyte solution in which lithium hexafluorophosphate (LiPF ₆ ) was dissolved.

Comparative Example 2 The same electrolytic solution as in Example 1 was used except that biphenyl was added instead of 4-methylbiphenyl. Comparative Example 3 1,4-Dimethoxy-in place of 4-methylbiphenyl
The same electrolytic solution as in Example 1 was used except that 4-bromobenzene was added. Comparative Example 4 The same electrolytic solution as in Example 1 was used except that meta-terphenyl was added instead of 4-methylbiphenyl.

[0058]

[Table 1]

[0059]

[Table 2]

Example 4-methylbiphenyl, 3,3 '
-Methylbiphenyl and 4,4'-methylbiphenyl increased in resistance immediately after the start of overcharge, and the current was cut off at the set upper limit voltage. These overcharge rates are smaller than the blank of Comparative Example 1 in overcharge capacity, and safety is improved. In addition, these examples show that the initial capacity and 1C / 0.2
It is considered that the C discharge rate did not significantly decrease compared to the level of the blank of Comparative Example 1, and the decrease in battery performance during normal use was small.

On the other hand, the blank of Comparative Example 1 has a large overcharge capacity and a large discharge capacity after overcharge. Further, the biphenyl of Comparative Example 2 did not show a substantial improvement in the overcharge rate as compared with the blank of Comparative Example 1. 1,4- of Comparative Example 3
Dimethoxy-4-bromobenzene greatly deteriorated under normal use conditions before overcharging, and did not lead to an overcharge test.
It is considered that the reaction started under normal conditions due to the low oxidation onset potential, causing the deterioration of the battery.

Although the overcharge rate of meta-terphenyl of Comparative Example 4 was smaller than that of Biphenyl of Comparative Example 2, it was 1 C / 0.2 C
The discharge rate is poor, and the battery characteristics during normal use are greatly reduced. From the results of the CV measurement and the coin cell measurement, the biphenyl derivative used in the present invention has an oxidation onset potential of 4.2 to 4.2.
It is preferable that the voltage is in the range of 4.5 V, and that a film is formed on the electrode, and the oxidation current from the first time to the second time tends to decrease.

[0063]

According to the present invention, a non-aqueous electrolyte having a function of suppressing thermal runaway during overcharge and a secondary battery using the same can be obtained, which has great industrial significance.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a structural example of a coin cell battery.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode can 2 Positive electrode 3 Separator 4 Gasket 5 Negative electrode 6 Spacer 7 Negative electrode can

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Akiko Torikai 3-1-1 Chuo, Ami-cho, Inashiki-gun, Ibaraki Pref. Mitsubishi Chemical Corporation Tsukuba Research Laboratory F-term (reference) EJ11 HJ00 HJ02

Claims (10)

[Claims] 1. A non-aqueous electrolyte solution comprising a non-aqueous organic solvent, a lithium salt as a solute, and one or more biphenyl derivatives having at least one electron-donating group. 2. An electrolyte for a non-aqueous secondary battery having a positive electrode containing a lithium metal composite oxide as an active material and a negative electrode containing a substance capable of occluding and releasing lithium as an active material, wherein the electrolyte is An electrolyte for a non-aqueous secondary battery, comprising an organic solvent and a lithium salt as a solute, and further added at least one biphenyl derivative having at least one electron donating group. 3. When the biphenyl derivative is added, the oxidation start potential is higher than the upper limit potential of the positive electrode corresponding to the normal operating maximum operating voltage of the battery, and the biphenyl derivative is added alone. The electrolyte for a non-aqueous secondary battery according to claim 2, wherein the electrolyte has an oxidation onset potential lower than the oxidation onset potential of the electrolyte. 4. The method according to claim 1, wherein the biphenyl derivative is contained in the electrolyte at a concentration of 0.01
The electrolyte solution according to any one of claims 1 to 3, wherein the electrolyte solution is added in an amount of from 0.8 to 0.8 mmol / g. 5. The electrolytic solution according to claim 1, wherein the molecular weight of the biphenyl derivative is 210 or less. 6. The electrolytic solution according to claim 1, wherein the biphenyl derivative is represented by the general formula (I). Embedded image (In the formula, A ^{1 to} A ¹⁰ are each independently a hydrogen atom or an alkyl group, and at least one is an alkyl group.) 7. The electrolytic solution according to claim 1, wherein the biphenyl derivative is represented by the general formula (II). Embedded image (In the formula, B ^{1 to} B ¹⁰ are each independently a hydrogen atom, an alkyl group or a methoxy group, and at least one is a methoxy group.) 8. The electrolytic solution according to claim 1, wherein the biphenyl derivative is represented by the general formula (III). Embedded image (In the formula, C ^{1 to} C ⁶ are each independently a hydrogen atom or an alkyl group, and at least one is an alkyl group.) 9. The electrolytic solution according to claim 1, wherein the positive electrode contains lithium and a metal composite oxide essentially containing cobalt or nickel as an active material. 10. A positive electrode comprising a lithium metal composite oxide as an active material, a negative electrode comprising a substance capable of occluding and releasing lithium as an active material, and an electrolytic solution according to any one of claims 1 to 9. Non-aqueous secondary battery.