JP2002270222A - Non-aqueous electrolyte and secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-aqueous electrolyte that does not give an adverse effect to the characteristics under the normal usage condition and can improve safety at the time of over-charge of the battery, and further that does not have a problem of irritating smell and is safe, and a secondary battery. SOLUTION: In the non-aqueous electrolyte which contains a non-aqueous organic solvent and lithium salt, 0.001-0.18 mmol/g of orthoester is contained in the non-aqueous electrolyte.

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.

[0002]

2. Description of the Related Art In recent years, as electronic devices have been reduced in size and weight, non-aqueous secondary batteries using a carbon material capable of inserting and extracting lithium as a negative electrode and a lithium metal oxide as a positive electrode have been developed at high voltage and 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. Such a non-aqueous secondary battery that exhibits a battery function by inserting and extracting lithium ions is called a lithium ion secondary battery, and competition for its development and commercialization is increasing. Furthermore, due to environmental issues, for electric vehicles,
Attention has also been focused on a sealed, maintenance-free lithium-ion secondary battery that has a large capacity, a high energy density, and is used for power leveling.

As a positive electrode active material of a lithium ion secondary battery, a layered lithium cobalt composite oxide (LiCoO ₂ ), a lithium nickel composite 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 even burst or ignite.

In order to solve such a problem, an attempt is made to ensure safety against overcharging of a battery by adding an aromatic compound as an additive to an electrolyte of a lithium ion secondary battery. However, for example, Japanese Patent Laid-Open No. 7-3026
No. 14, JP-A-9-50822, JP-A-9-508
No. 106835, Japanese Patent No. 2939469, Japanese Patent No. 2983205, 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 to increase the internal voltage of the battery by polymerizing an additive (eg, biphenyl) to 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 initiates a polymerization reaction at a voltage in an overcharged state, acts as a resistor and becomes a polymer which is hardly redissolved, and effectively acts on overcharge.

[0007] In Japanese Patent No. 2983205,
It has been proposed to add an ether derivative having a specific structure (for example, diphenyl ether) to a solvent in an electrolytic solution. When the battery voltage reaches an overcharged voltage, the additive starts a decomposition reaction to generate gas, starts a polymerization reaction, acts as a resistor, and redissolves in the electrolyte. The polymer is said to be less likely to occur, and effectively acts against overcharging.

[0008]

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, it has been found that biphenyl proposed in Japanese Patent Application Laid-Open No. 9-106835 has an effect of preventing overcharge, but adversely affects the output characteristics of the battery. Furthermore, 3-
It has been found that chlorothiophene, furan, and the like are easily oxidatively decomposed, and an oxidation reaction occurs under normal battery use conditions, which adversely affects battery characteristics. Patent No. 2939
It has been found that the terphenyl derivative disclosed in Japanese Patent No. 469 has polymerization reactivity, but has a high molecular weight and is not easily dissolved in an electrolytic solution, resulting in a decrease in battery performance.

The ether derivative (eg, diphenyl ether) disclosed in Japanese Patent No. 2983205 has a problem that it has a strong pungent odor and is difficult to handle as an additive. That is, even if the above-mentioned aromatic compound is added to the electrolytic solution, various problems still remain, and development of a new compound is awaited.

The present invention has been made in view of the above circumstances, and an object of the present invention is to improve the safety of a battery during overcharge without adversely affecting battery characteristics under normal use conditions. Another object of the present invention is to provide a non-aqueous electrolyte and a secondary battery that do not have the problem of pungent odor.

[0012]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, by incorporating a compound having a structure different from that of a conventionally used aromatic compound into an electrolytic solution. It has been found that overcharging can be effectively prevented without deteriorating battery characteristics under normal use conditions, and the present invention has been accomplished. That is, the first gist of the present invention is a non-aqueous electrolyte containing a non-aqueous organic solvent and a lithium salt, wherein the non-aqueous electrolyte contains 0.001 to 0.18 mmol / g of orthoester with respect to the non-aqueous electrolyte. Exists in electrolyte. Further, the second gist of the present invention is to provide a positive electrode containing a lithium transition metal composite oxide,
A non-aqueous secondary battery includes a negative electrode containing a substance that can be released, and the non-aqueous electrolyte.

Japanese Patent Application Laid-Open No. 9-199171 discloses that
A technique has been disclosed in which an orthoester is contained in a non-aqueous electrolyte to improve cycle characteristics under high voltage and heavy load discharge conditions. According to this known document, it is assumed that the cause of the deterioration of the cycle characteristics is water in the electrolytic solution, and the orthoester and the water are positively reacted with each other to produce alcohol and the like as a by-product, thereby deteriorating the cycle characteristics. Preventing. For this reason, it is necessary to contain orthoester in an amount comparable to the amount of water contained in the non-aqueous electrolyte. Specifically, it is described that the content of the orthoester in the non-aqueous solvent is at least 5% by volume of the non-aqueous solvent because the cycle characteristics cannot be improved if the content is too small. 0015]).

The present invention has found a completely new effect that the safety at the time of overcharge can be improved by including the orthoester in the non-aqueous electrolyte. And
The orthoester is used in an amount of 0.001 to
It is characterized by containing as little as 0.18 mmol / g.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The nonaqueous electrolytic solution of the present invention contains an orthoester in addition to a nonaqueous organic solvent and a lithium salt. The orthoester may be an ester of orthoacid generated by hydration of carboxylic acid or carbonic acid. The properties of the orthoester are not particularly limited, but are preferably liquid at 20 ° C. and 1 atm from the viewpoint of compatibility with other non-aqueous organic solvents. Examples of the orthoester include orthocarbonate and orthocarboxylate.

As the orthocarbonate, those represented by the following general formula (i) are preferably used.

[0017]

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(I) In the formula, R ₁ , R ₂ , R ₃ and R ₄ are
Each independently represents an alkyl group or an aryl group. Preferred are an aryl group and an alkyl group having 1 to 10 carbon atoms, and particularly preferred are a methyl group, an ethyl group and a phenyl group. Specific examples of the orthocarbonate include, for example, tetraethoxymethane (tetraethyl orthocarbonate), tetramethyl orthocarbonate and the like. Among them, industrially preferred is tetraethoxymethane.

As the orthocarboxylic acid ester, those represented by the following general formula (ii) are preferably used.

[0020]

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(Ii) In the formula, R ₅ represents a hydrogen atom, an alkyl group or an aryl group. It is preferably a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, or an aryl group, and particularly preferably a hydrogen atom, a methyl group, an ethyl group, or a phenyl group. In the formula (ii), R ₆ , R ₇ , and R ₈
Represents an alkyl group or an aryl group each independently. Preferably, it is an aryl group or an alkyl group having 1 to 10 carbon atoms, and particularly preferably, a methyl group, an ethyl group, or a phenyl group.

Specific examples of the orthocarboxylate include, for example, orthoformate such as trimethyl orthoformate and triethyl orthoformate, and orthoacetate such as trimethyl orthoacetate and triethyl orthoacetate. Among these compounds, industrially preferred are triethyl orthoformate and triethyl orthoacetate. Of course, a plurality of types of orthoesters may be contained in the electrolytic solution. In addition, an orthoester may be used in combination with an additive which is conventionally known as improving the safety against overcharging.

The content of the orthoester in the non-aqueous electrolyte is usually 0.001 mmol / g or more, preferably 0.01 mmol / g or more, particularly preferably 0.03 m / g or more.
mol / g or more, usually 0.18 mmol / g or less, preferably 0.17 mmol / g or less, particularly preferably 0.1 mol / g or less.
It is 16 mmol / g or less. If the content is too large, the stability during storage of the battery and the normal cycle stability may be adversely affected. If the content is too small, the effect of improving safety during overcharging may not be sufficiently exhibited.

In the present invention, when the battery is used in a normal state, it is preferable to suppress the decomposition of the orthoester in the non-aqueous electrolyte. When the orthoester is decomposed, the effect of improving the safety at the time of overcharging is hardly exhibited. Therefore, the reaction between the orthoester and water should be avoided as much as possible. In other words, in the present invention, it is preferable to suppress the content of alcohol generated by the above reaction to a certain level or less. Specifically, the amount of alcohol in the non-aqueous electrolyte is usually 100 ppm or less, preferably 50 ppm or less, particularly preferably 30 ppm or less. By controlling the amount within the above range, substantial overcharge prevention can be effectively performed. Incidentally, the amount of alcohol in the non-aqueous electrolyte may be determined by, for example, a known GC-MS
(Gas chromatography / mass spectrometry).

[0025] As described above, in the use of normal state of the battery, since it is preferable to suppress the decomposition of the ortho ester in the nonaqueous electrolytic solution, a non-water may contribute to orthoester decomposition H ₂ O It is preferable that the content in the electrolytic solution is as small as possible. When the content of H ₂ O is increased, the orthoester is decomposed, so that the effect of improving safety during overcharge is hardly exhibited. Specifically, the amount of H ₂ O in the non-aqueous electrolytic solution is set to 10
It is preferable that the content be 0 ppm or less. Particularly preferably 5
0 ppm or less, most preferably 30 ppm or less. It is preferable that both the amount of H ₂ O and the amount of the alcohol in the non-aqueous electrolyte are controlled to be equal to or less than the predetermined amount, since the effect of improving safety during overcharge is further exhibited. The amount of H ₂ O in the non-aqueous electrolyte can be determined, for example, by using a known Karl Fischer method.

As described above, when the battery is used in a normal state, it is preferable to suppress the decomposition of the orthoester in the nonaqueous electrolytic solution. It is preferable that the content in the medium is as small as possible. Specifically, the amount of the acid component in the non-aqueous electrolyte is preferably set to 100 ppm or less. It is particularly preferably at most 50 ppm, most preferably at most 30 ppm. Here, the acid content refers to a protic acid represented by HF or the like. It is preferable to further improve the safety at the time of overcharging, by controlling both the amount of the acid and the amount of the alcohol in the non-aqueous electrolyte solution to be equal to or less than the predetermined amount, and more preferably, the amount of the acid , The H ₂ O amount and the alcohol amount are both controlled to be equal to or less than the predetermined amount. The acid content in the non-aqueous electrolyte can be determined, for example, by using a known acid-base titration method.

The non-aqueous organic solvent contained in the non-aqueous electrolyte 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-dioxolan, 4-methyl-1,3-dioxolan, diethyl ether, Sulfolane, methylsulfolane, acetonitrile, propionitrile, benzonitrile, butyronitrile, valeronitrile, 1,2-dichloroethane, dimethylformamide, Methyl sulfoxide, trimethyl phosphate, triethyl phosphate, and the like. In the present invention, the above-mentioned organic solvents may be used alone or as a mixture of two or more.

In the present invention, it is preferable to use a high dielectric constant solvent as the non-aqueous organic solvent in order to sufficiently dissociate the electrolyte. Here, the high dielectric constant solvent is 25
It means a compound having a relative dielectric constant at 20 ° C. of 20 or more. Preferred solvents used as the high dielectric constant solvent include ethylene carbonate, propylene carbonate and cyclic carbonates in which hydrogen atoms thereof are substituted with another element such as halogen or an alkyl group, or dimethyl carbonate which is a low-viscosity solvent. Chain carbonates such as diethyl carbonate and ethyl methyl carbonate;
Examples thereof include chain ethers such as dimethoxyethane, 1,2-diethoxyethane, and dimethoxymethane, and these can be used alone or in combination of two or more.
The ratio of the high dielectric constant solvent to the electrolyte is preferably 20%.
% By weight, more preferably 30% by weight or more, most preferably 40% by weight or more.

As the lithium salt contained in the non-aqueous electrolyte, there may be mentioned known lithium salts. Specifically, L
iClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , Li
B (C ₆ H ₅ ) ₄ , LiCl, LiBr, LiCH ₃ S
O ₃ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN
(SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiN
Lithium salts such as (SO ₃ CF ₃ ) ₂ may be used, and these may be used alone or in combination of two or more. Among the above, LiBF ₄ and LiPF ₆ are preferably used. The content of these lithium salts is
Usually 0.5 to 2.5 mol / l to non-aqueous electrolyte
It is.

The non-aqueous electrolytic solution of the present invention contains a known additive, for example, substantially preventing decomposition of the electrolytic solution on the surface of the negative electrode, enabling efficient charge and discharge of lithium ions,
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. Non-aqueous electrolyte used in the present invention,
It can be used as a nonaqueous secondary battery (in this specification, it may be simply called a battery) sandwiched between a positive electrode containing a lithium transition metal composite oxide and a negative electrode containing a substance capable of inserting and extracting lithium. .

As the material of the negative electrode comprising the substance capable of occluding and releasing lithium, which constitutes the battery of the present invention, any material containing a substance capable of occluding and releasing lithium as an active material may be used. Those containing are preferred. Specific examples of the carbon material include thermal decomposition products of organic substances under various thermal decomposition conditions, artificial graphite, natural graphite, and the like. Preferably artificial graphite and graphitized mesophase spherules, graphitized mesophase pitch-based carbon fiber, other artificial graphite and purified natural graphite, or the like, produced by high-temperature heat treatment of graphitic pitch obtained from various raw materials. A material obtained by performing various surface treatments including pitch on graphite is used.

As for these carbon materials, the d value (interlayer distance) of the lattice plane (002 plane) determined by X-ray diffraction by the Gakushin method is preferably from 0.335 to 0.34 nm.
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 carbon materials and used. Examples of the negative electrode material capable of inserting and extracting lithium other than the carbon 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 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. The solvent is not particularly limited as long as it dissolves or disperses the binder, but usually an organic solvent is used. For example, N-methylpyrrolidone,
Examples thereof include dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, NN-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran. In addition, the active material can be slurried with a latex such as SBR by adding a dispersant, a thickener and the like to water.

As the material of the current collector for the negative electrode, metals such as copper, nickel, and stainless steel are used. Among them, copper foil is preferable in terms of easy processing into a thin film and cost. The material of the positive electrode containing the lithium transition metal composite oxide constituting the battery of the present invention may be any material containing a lithium transition metal composite oxide as an active material. Examples of the lithium transition metal composite oxide include lithium. Examples thereof include a cobalt composite oxide, a lithium nickel composite oxide, and a lithium manganese composite oxide. These lithium transition metal composite oxides may be used alone or in combination. Preferred as the lithium transition metal composite oxide is one containing a lithium cobalt composite oxide or a lithium nickel composite oxide. Since the lithium-cobalt composite oxide and the lithium-nickel composite oxide become extremely unstable in the overcharged state, the overcharge prevention effect of the present invention is remarkably exhibited.

In the lithium transition metal composite oxide, some of the sites occupied by the transition metal may be replaced with an element other than the transition metal. As a result, the stability of the crystal structure can be improved. At this time, other elements (hereinafter referred to as “substituting elements”) that partially replace the transition metal sites include Al,
Ti, V, Cr, Mn, Fe, Co, Li, Ni, C
u, Zn, Mg, Ga, Zr, etc., preferably Al, Cr, Fe, Co, Li, Ni, Mg, Ga, and more preferably Al. Note that the transition metal site may be substituted with two or more kinds of other elements.

When part of the cobalt site of the lithium-cobalt composite oxide is replaced with another element, the other element is
1, Ti, V, Cr, Fe, Mn, Li, Ni, Cu,
It is preferably at least one element selected from the group consisting of Zn, Mg, Ga and Zr, and Al is particularly preferred. When a part of the nickel site of the lithium nickel composite oxide is replaced with another element, the other element is Al, Ti,
V, Cr, Fe, Mn, Li, Co, Cu, Zn, M
It is preferably at least one element selected from the group consisting of g, Ga and Zr, and Al and Co are particularly preferred.

The specific surface area of the lithium transition metal composite oxide used in the present invention is usually at least 0.01 m ² / g, preferably at least 0.3 m ² / g, more preferably at least 0.5 m ² / g, In addition, it is usually 10 m ² / g or less, preferably 5.
0 m ² / g or less, more preferably 3.0 m ² / g or less, particularly preferably 2.0 m ² / g or less. If the specific surface area is too small, the rate characteristics and capacity will be reduced. If the specific surface area is too large, undesired reactions with the electrolytic solution and the like will be caused, and the cycle characteristics may be reduced. The measurement of the specific surface area follows the BET method.

The average particle diameter of the lithium transition metal composite oxide used in the present invention is usually 0.1 μm or more, preferably 0.1 μm or more.
2 μm or more, more preferably 0.3 μm or more, most preferably 0.5 μm or more, and usually 300 μm or less,
Preferably 100 μm or less, more preferably 50 μm
Or less, most preferably 20 μm or less. If the average particle size is too small, the cycle deterioration of the battery may increase or a problem may occur in safety.

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 same binder, conductive material, and solvent as those used for the negative electrode can be used for the positive electrode.

In a non-aqueous secondary battery, a separator is usually inserted between the positive electrode and the negative electrode. The material and shape of the separator used 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 non-aqueous secondary battery is not particularly limited, and can be appropriately selected from commonly employed methods. The shape of the battery is not particularly limited, and a cylinder type in which a sheet electrode and a separator are formed into 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 stacked are used. It is possible.

The use of the non-aqueous secondary battery of the present invention is not particularly limited, and may be, for example, a car use or a load leveling use of electric power. When installed in electronic devices, notebook computers, pen input computers, mobile computers, e-book players, mobile phones, cordless handsets, pagers, handy terminals,
Portable fax, portable copy, portable printer, headphone stereo, video movie, LCD TV, handy cleaner, portable CD, mini disk, electric shaver, transceiver, electronic notebook, calculator, memory card, portable tape recorder, radio, backup power supply, memory Cards, motors, lighting equipment, toys,
Game equipment, road conditioners, watches, strobes, (digital) cameras, medical equipment (pacemakers,
Hearing aids, shoulder massagers, etc.). Further, the non-aqueous secondary battery of the present invention may be used in combination with a solar cell.

[0046]

EXAMPLES Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to the following examples unless it exceeds the gist. (Battery Measurement) (Preparation of Positive Electrode) The positive electrode was composed of 75% by weight of lithium cobalt oxide (LiCoO ₂ ) as a positive electrode active material, 20% by weight of acetylene black as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder. ) Was mixed well in an agate mortar with 5% by weight to form a sheet.
Punched with a punch of, thickness about 250μm16m
It was prepared by crimping on an expanded metal (meshed foil) made of mφ aluminum. (Preparation of Negative Electrode) A negative electrode was prepared by mixing 95% by weight of graphite (surface interval: 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. After the slurry was formed, it was applied to one side of a copper foil having a thickness of 20 μm, dried, and then rolled by a press machine to be punched out with a diameter of 12 mmφ. (Ratio of the weight of the positive electrode active material to the weight of the negative electrode active material) When configuring the battery, the weight of the positive electrode active material W (c) and the weight of the negative electrode active material W
The ratio (a) is preferably within a range where lithium ions released from the positive electrode do not cause precipitation of lithium metal on the opposing negative electrode at the normal use 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. Q (a) is the electric capacity per weight of the substance.
mAh / g, the capacity ratio Rq = 比 Q (a) × W
(A) In general, {/ {Q (c) × W (c)} is set to 1.0 or more. In this example and the comparative example,
W (c) / W (a) was set so that 1 ≦ Rq ≦ 1.2.

Q (c) or Q (a) is obtained by assembling a test cell with a positive electrode or a negative electrode as a working electrode, lithium metal as a counter electrode, and a separator in the same electrolytic solution as used for forming a battery with a separator interposed therebetween. It was measured. 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) The capacity was determined as a possible capacity.

In the embodiment, Q (c) of the positive electrode is 160 mAh / g at the upper limit Li voltage of 4.3 V, and Q (c) of the negative electrode is
(a) is 380 mAh / g at the lower limit voltage of the counter electrode Li of 3 mV.
The amount of the positive electrode active material was about 11.5 mg / 12 mmφ, and the amount of the negative electrode active material was about 5.8 mg / 12 mmφ. (Battery assembly) In a dry box in an argon atmosphere,
A battery was prepared using a 2032 type coin cell having a configuration as shown in FIG. 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, a predetermined electrolytic solution was added and the battery was sufficiently impregnated into the battery. Then, the negative electrode can 7 was placed thereon and the battery was sealed. In the present embodiment and the comparative example, the capacity of the battery was set to a charging upper limit of 4.2 V
It was designed to have a discharge lower limit of 3.0 V to be about 1.6 mAh. (Method of Battery Evaluation) Battery evaluation was performed in the order of (1) initial charge / discharge (confirmation of capacity), (2) discharge rate measurement test, (3) full charge operation, and (4) overcharge test.

The initial charge (confirmation of the capacity) is 1 C (1.6 m).
A) The battery was charged by a constant current and constant voltage method with an upper limit of 4.2 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. Then, the ratio (%) of the initial discharge capacity to the initial charge capacity was calculated as the initial efficiency. The initial efficiency indicates the ratio of lithium ions returning to the positive electrode side during discharging to lithium ions moving to the negative electrode side during charging, and the higher the value, the higher the efficiency of lithium ion use. preferable.

The charge in the discharge rate measurement test was 1 C, 4.2.
The constant current and constant voltage method (cut off at 0.05 mA) of the upper limit of V was constant, and the discharge rates were 0.2 C and 2 C. The discharge was cut at 3V. It should be noted that 2C / 0.2C discharge rate = (2C discharge capacity / 0.2C discharge capacity) × 100 (%) was used as an index for checking the superiority of the discharge rate characteristics. The larger the value, the better the rate characteristics.

In the full charge operation, the battery was charged by a constant current and constant voltage method with an upper limit of 4.2 V (cut off at 0.05 mA). The overcharge test was performed at 1C at 4.99 V cut to 3 hr cut (whichever was reached first). As an index for determining the degree of overcharge prevention effect, the coin cell after overcharge was disassembled, and Li remaining in the positive electrode was quantified by elemental analysis. When the positive electrode composition after the overcharge test is expressed as Li _x CoO ₂ , as x (positive amount of Li remaining in the positive electrode) increases, overcharging does not proceed, and the effect of preventing overcharge increases.

Here, x (positive amount of Li remaining in the positive electrode) was determined from the molar ratio of Co and net Li in the positive electrode determined by elemental analysis (ICP emission analysis). Note also performs determination of phosphorus (P) in the positive electrode in a similar analysis, it was assumed by LiPF _6, by subtracting the Li molar number corresponding to LiPF ₆ from total Li moles in the positive electrode, the net Li The number of moles was determined.

Using the non-aqueous electrolytes shown in the following Examples and Comparative Examples, differences between the non-aqueous electrolytes were compared using the above-described battery evaluation method. The results are shown in Table 1. Example 1 As an electrolytic solution, lithium hexafluorophosphate (LiPF ₆ ) was dissolved at a concentration of 1 mol / liter in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3: 7. 0.15 mmol in electrolyte
/ G to which tetraethoxymethane was added.

Incidentally, tetraethoxymethane had little pungent odor. Example 2 As an electrolytic solution, lithium hexafluorophosphate (LiPF ₆ ) was dissolved at a concentration of 1 mol / liter in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3: 7. 0.15 mmol in electrolyte
/ G to which triethyl orthoformate was added was used.

Incidentally, triethyl orthoformate had little pungent odor. Comparative Example 1 Phosphorus hexafluoride was added to the electrolyte solution of Example 1 to which no additive was added, that is, a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3: 7 at a concentration of 1 mol / liter. Lithium oxide (LiPF ₆ )
Was used.

[0056]

[Table 1]

From the results shown in Table 1, the batteries of Examples 1 and 2 (with orthoester added to the electrolytic solution) had an initial efficiency of 2 C / 0.2 C in comparison with the battery of Comparative Example 1 (without addition). While the ordinary battery characteristic of the discharge rate is hardly inferior, the amount of Li remaining in the positive electrode at the time of overcharging is much larger than that of the comparative example. , And 234%, respectively), indicating that the safety during overcharge is improved.

[0058]

According to the present invention, there is provided a non-aqueous electrolyte solution and a secondary battery capable of improving safety during overcharge by using a compound completely different from a conventionally used compound. be able to. Further, according to the present invention, it is possible to provide a non-aqueous electrolyte solution and a secondary battery capable of improving safety at the time of overcharging the battery without adversely affecting battery characteristics.

Further, according to the present invention, it is possible to provide a safe non-aqueous electrolyte and a secondary battery which do not have the problem of irritating odor.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing the structure 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

Continued on the front page (72) Inventor Katsuya Isada 1000 Kamoshita-cho, Aoba-ku, Yokohama-shi, Kanagawa Prefecture Inside Mitsubishi Chemical Co., Ltd. (72) Inventor Takao Kobayashi 1000 Kamoshida-cho, Aoba-ku, Yokohama-shi, Kanagawa Mitsubishi Stock Inside the Yokohama Research Institute, Inc. (72) Hiroshi Suzuda 1000, Kamoshidacho, Aoba-ku, Yokohama-shi, Kanagawa Prefecture Mitsubishi Chemical Corporation (72) Inventor Shinichiro Nakamura 1000, Kamoshida-cho, Aoba-ku, Yokohama-shi, Kanagawa Mitsubishi Chemical F-term in Yokohama Research Institute, Inc. (reference) 5H029 AJ12 AK03 AK18 AL02 AL06 AL07 AL12 AL18 AM01 AM02 AM03 AM04 AM05 AM07 BJ03 CJ08 DJ09 DJ16 DJ17 HJ02 HJ10

Claims (10)

[Claims] 1. A non-aqueous electrolyte containing a non-aqueous organic solvent and a lithium salt, wherein the orthoester is contained in an amount of 0.001 to 0.18 mmol / g with respect to the non-aqueous electrolyte. Electrolyte. 2. The non-aqueous electrolyte according to claim 1, wherein the orthoester is an orthocarbonate. 3. The orthocarbonate of the following general formula (i):
The non-aqueous electrolytic solution according to claim 1, wherein the non-aqueous electrolytic solution is represented by: Embedded image (In the formula, R ₁ , R ₂ , R ₃ , and R ₄ each independently represent an alkyl group or an aryl group.) 4. The non-aqueous electrolyte according to claim 1, wherein the orthocarbonate is tetraethoxymethane. 5. The non-aqueous electrolyte according to claim 1, wherein the orthoester is an orthocarboxylic acid ester. 6. The non-aqueous electrolyte according to claim 1, wherein the orthocarboxylic acid ester is represented by the following general formula (ii). Embedded image (In the formula, R ₅ represents a hydrogen atom, an alkyl group, or an aryl group, and R ₆ , R ₇ , and R ₈ each independently represent an alkyl group or an aryl group.) 7. The non-aqueous electrolyte according to claim 1, wherein the orthocarboxylate is triethyl orthoformate. 8. A non-aqueous system comprising: a positive electrode containing a lithium-transition metal composite oxide; a negative electrode containing a substance capable of inserting and extracting lithium; and the electrolytic solution according to claim 1. Rechargeable battery. 9. The non-aqueous secondary battery according to claim 8, wherein the lithium transition metal composite oxide contains a lithium cobalt composite oxide or a lithium nickel composite oxide. 10. The non-aqueous secondary battery according to claim 8, wherein the substance capable of inserting and extracting lithium is a carbon material.