JP2003229170A - Nonaqueous electrolyte solution and lithium battery using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte solution capable of giving birth to a secondary battery with high capacity and excellent self-discharge characteristics. <P>SOLUTION: The nonaqueous electrolyte solution has lithium salt dissolved in a nonaqueous organic solvent including a fluorine-containing diphenyl compound expressed in formula (1) below. In the formula, A denotes an alkylene group substituted with fluorine. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte solution and a lithium secondary battery using the same.

[0002]

2. Description of the Related Art Lithium secondary batteries have the advantages that they have a high energy density and that they do not easily self-discharge.
Therefore, in recent years, it has been widely used as a power source for consumer mobile devices such as mobile phones, notebook computers, and PDAs.
The electrolytic solution for a lithium secondary battery is composed of a lithium salt that is a supporting electrolyte and a non-aqueous organic solvent. The non-aqueous organic solvent is required to have a high dielectric constant for dissociating the lithium salt, exhibit high ionic conductivity in a wide temperature range, and be stable in the battery. Since it is difficult to achieve these requirements with one solvent,
Usually, a high boiling point solvent represented by propylene carbonate or ethylene carbonate and a low boiling point solvent such as dimethyl carbonate or diethyl carbonate are used in combination.

Further, many methods have been reported for adding various additives to an electrolytic solution in order to improve initial capacity, rate characteristics, cycle characteristics, high temperature storage characteristics, low temperature characteristics, trickle charge characteristics, self-discharge characteristics, and the like. It was For example, JP-A-11
-162512 discloses that the overcharge prevention property is improved by adding 2,2-diphenylpropane or the like, and in JP 2001-15154A, the high temperature storage property is added by adding diphenylmethane or the like. Is disclosed to be improved.

[0004]

[Problems to be Solved by the Invention] However, in recent years, the demand for higher performance has become higher and higher. In particular, the self-discharge characteristic is one of the advantages over the nickel-hydrogen battery and the like, and therefore further improvement in characteristics has been desired.

[0005]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have found that a non-aqueous electrolyte solution contains a specific compound to maintain high capacity while maintaining high capacity. The present invention has been completed by finding that the discharge characteristics are significantly improved. That is, the gist of the present invention is
A non-aqueous electrolytic solution in which a lithium salt is dissolved in a non-aqueous organic solvent, wherein the non-aqueous organic solvent contains a fluorine-containing diphenyl compound represented by the following general formula (1). In the liquid.

[0006]

[Chemical 2]

(In the formula, A represents a fluorine-substituted alkylene group) Another subject matter of the present invention resides in a lithium secondary battery characterized by using the above-mentioned non-aqueous electrolyte solution. When the electrolytic solution containing the above-mentioned fluorine-containing diphenyl compound is used, self-discharge is less likely to occur. The details of the factors that bring about the improvement of the self-discharge characteristics of the electrolytic solution containing the above-mentioned fluorine-containing diphenyl compound are unknown, but it is reduced at the initial stage of charging to form a stable SEI (interface protective film) on the negative electrode. It is thought that this is due to that. Regarding the manifestation of self-discharge, a mechanism is considered in which the reaction between lithium and a non-aqueous solvent such as carbonates proceeds in the negative electrode, and the reduced solvent species move to the positive electrode to reduce the active material. It is presumed that stable SEI is formed on the negative electrode and the reaction between lithium and the non-aqueous solvent is hindered, so that self-discharge is less likely to occur.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below. The non-aqueous electrolytic solution of the present invention is one in which a lithium salt is dissolved in a non-aqueous organic solvent and further a specific fluorine-containing diphenyl compound is contained. In the present invention, a fluorine-containing diphenyl compound represented by the following general formula (1) is used as an additive.

[0009]

[Chemical 3]

(In the formula, A represents a fluorine-substituted alkylene group) In the general formula (1), A represents a fluorine-substituted alkylene group. Examples of the fluorine-substituted alkylene group include a fluoromethylene group, a difluoromethylene group,
2,2-difluoroethylidene group, 2,2,2-trifluoroethylidene group, 1,1-difluoropropane-2
-Ylidene group, 1,1,1-trifluoropropane-2
-Substituted methylene group such as ylidene group and 1,1,1,3,3,3-hexafluoropropane-2-ylidene group, 1-
Fluoroethylene group, 1,2-difluoroethylene group,
Substituted ethylene groups such as 1,1,2,2-tetrafluoroethylene group may be mentioned, preferably difluoromethylene group, 2,2,2-trifluoroethylidene group, 1,1,
1,3,3,3-hexafluoropropane-2-ylidene group and 1,1,2,2-tetrafluoroethylene group.

Examples of the above-mentioned fluorine-containing diphenyl compound include fluorodiphenylmethane, difluorodiphenylmethane, 1,1-difluoro-2,2-diphenylpropane, 1,1,1-trifluoro-2,2-diphenylpropane and hexafluoro-. Substituted diphenylmethanes such as 2,2-diphenylpropane, 1-fluoro-
1,2-diphenylethane, 1,2-difluoro-1,
Substituted diphenylethanes such as 2-diphenylethane and tetrafluoro-1,2-diphenylethane may be mentioned. Among these, fluorodiphenylmethane and hexafluoro-2,2-diphenylpropane are preferably used because they have a great effect of preventing self-discharge. You may use these additives in mixture of 2 or more types.

The amount of the above-mentioned fluorine-containing diphenyl compound added is not particularly limited, but is usually 0.1% with respect to the non-aqueous electrolyte.
It is 1 to 10% by weight, preferably 0.5 to 5% by weight.
If the amount added is too large, the ionic conductivity tends to decrease, and battery characteristics such as rate characteristics tend to deteriorate. Further, if the addition amount is too small, a sufficient self-discharge preventing effect is not exhibited.

The lithium salt used as the supporting electrolyte in the present invention is not particularly limited.
_{F 6, LiAsF 6, LiBF 4} , LiSbF 6, LiAl
Cl ₄ , LiClO ₄ , CF ₃ SO ₃ Li, C ₄ F ₉ SO ₃ L
i, CF ₃ COOLi, (CF ₃ CO) ₂ NLi, (CF ₃
Examples include lithium salts such as SO ₂ ) ₂ NLi and (C ₂ F ₅ SO ₂ ) ₂ NLi. Particularly, a lithium salt selected from the group consisting of LiPF ₆ , LiBF ₄ , CF ₃ SO ₃ Li and (CF ₃ SO ₂ ) ₂ NLi which is easily soluble in a solvent and has a high dissociation degree is preferably used. The concentration of the lithium salt in the non-aqueous electrolyte is usually 0.5 to 2 m with respect to the non-aqueous electrolyte.
It is preferably used in the range of ol / L.

As the non-aqueous organic solvent used in the present invention,
It is not particularly limited as long as it can dissolve a lithium salt, but among them, as a solvent that expresses high ionic conductivity, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), methylpropyl are usually used. Carbonate, chain carbonates such as ethylpropyl carbonate, ethylene carbonate (EC), propylene carbonate (P
C), cyclic carbonates such as butylene carbonate (BC), unsaturated carbonates such as vinylene carbonate and vinylethylene carbonate, ethers such as 1,2-dimethoxyethane and tetrahydrofuran, γ-butyrolactone, γ-valerolactone and the like. Chain esters such as cyclic esters, methyl formate, methyl acetate and methyl propionate are preferably used.

These organic solvents are usually mixed and used so as to achieve appropriate physical properties. For example, it is generally preferable to use the chain carbonates and the cyclic carbonates together. Further, among the above-mentioned chain carbonates, it is particularly preferable to mix and use an asymmetric carbonate such as ethylmethyl carbonate, methylpropyl carbonate or ethylpropyl carbonate. Among them, ethylmethyl carbonate is preferably used because it has a low viscosity, so that it not only enhances the mobility of lithium, but also has a relatively high boiling point, it is difficult to volatilize, is easy to handle, and has little reaction with Li. When unsaturated carbonates such as vinylene carbonate and vinyl ethylene carbonate are mixed and used, these unsaturated carbonates are easily reduced at the time of initial charge and a stable surface protective film (SEI) is obtained.
Is preferred because it contributes to the formation of

In preparing the non-aqueous electrolyte of the present invention,
It is preferable to dehydrate each raw material of the non-aqueous electrolyte solution in advance. Water content is usually 50 ppm or less, preferably 30 pp
It is preferably m or less. When a large amount of water is present, electrolysis of water, reaction with lithium metal, hydrolysis of lithium salt, and the like may occur, which may be inappropriate as an electrolyte for a lithium secondary battery. The dehydration means is not particularly limited, but in the case of a liquid such as a solvent, a molecular sieve or the like may be used. Further, in the case of a solid such as a lithium salt, it may be dried below the temperature at which decomposition occurs.

The non-aqueous electrolytic solution in the present invention is useful as an electrolytic solution for a lithium secondary battery. Hereinafter, the lithium secondary battery of the present invention will be described. The basic configuration of a lithium secondary battery to which the non-aqueous electrolyte solution of the present invention can be applied is the same as that of a conventionally known lithium secondary battery, in which the positive electrode and the negative electrode have a porous film and the non-aqueous electrolyte solution of the present invention. It is stored in a case and configured. The positive electrode and the negative electrode used in the secondary battery of the present invention may be appropriately selected according to the type of battery, but include at least active materials corresponding to the positive electrode and the negative electrode. Moreover, you may contain the binder for fixing an active material.

Examples of the positive electrode active material that can be used in the lithium secondary battery of the present invention include oxides having transition metals such as Fe, Co, Ni and Mn, composite oxides with lithium,
Inorganic compounds such as sulfides may be mentioned. Specifically, Mn
O, V ₂ O ₅ , V ₆ O ₁₃ , transition metal oxides such as TiO ₂ ,
Composite oxides of lithium and transition metals such as lithium nickelate, lithium cobaltate and lithium manganate, T
Examples thereof include transition metal sulfides such as iS ₂ and FeS. Further, as the positive electrode active material, for example, an organic compound such as a conductive polymer such as polyaniline can be used. You may mix and use two or more types of the said active material. When the active material is granular, the particle size is usually 1 to 30 μm, preferably 1 to 10 from the viewpoint of excellent battery characteristics such as rate characteristics and cycle characteristics.
It is about μm.

As the negative electrode active material that can be used in the lithium secondary battery of the present invention, lithium metal or lithium alloy can be used, but since the effect of forming a film by using the additive is great, the lithium ion is absorbed and released. It is particularly preferable to use carbonaceous substances such as coke, acetylene black, mesophase microbeads and graphite as possible compounds. The particle size of the granular negative electrode active material is usually 1 to 50 μm, preferably about 15 to 30 μm from the viewpoint of excellent battery characteristics such as initial efficiency, rate characteristics, and cycle characteristics.

Further, the above carbonaceous substance is mixed with an organic substance or the like.
A calcined material, or a material in which amorphous carbon is formed on at least a part of the surface by using a CVD method or the like as compared with the above carbonaceous material can also be preferably used as the carbonaceous material. Examples of the organic matter include coal tar pitch from soft pitch to hard pitch; coal-based heavy oil such as dry-distilled liquefied oil; straight-run heavy oil such as atmospheric residual oil and vacuum residual oil; heat of crude oil, naphtha, etc. Examples include petroleum heavy oils such as cracked heavy oils (for example, ethylene heavy ends) that are by-produced during decomposition. Moreover, what grind | pulverized the solid residue obtained by distilling these heavy oils at 200-400 degreeC to 1-100 micrometers can also be used. Further, a vinyl chloride resin or a resin precursor thereof which becomes a phenol resin or an imide resin by firing can be used.

Examples of the binder that can be used for the positive electrode or the negative electrode include various materials from the viewpoint of weather resistance, chemical resistance, heat resistance, flame retardancy, and the like. Specifically, inorganic compounds such as silicate and glass, alkane polymers such as polyethylene, polypropylene and poly-1,1-dimethylethylene; unsaturated polymers such as polybutadiene and polyisoprene; polystyrene, polymethylstyrene,
Polymers having a ring such as polyvinyl pyridine and poly-N-vinyl pyrrolidone; polymethyl methacrylate, polyethyl methacrylate, polybutyl methacrylate, polymethyl acrylate, polyethyl acrylate, polyacrylic acid, polymethacrylic acid, poly Acryl derivative polymers such as acrylamide; Fluorine resins such as polyvinyl fluoride, polyvinylidene fluoride, and polytetrafluoroethylene; CN group-containing polymers such as polyacrylonitrile and polyvinylidene cyanide; polyvinyl alcohols such as polyvinyl acetate and polyvinyl alcohol. Polymer
A halogen-containing polymer such as polyvinyl chloride or polyvinylidene chloride; a conductive polymer such as polyaniline can be used. Also, a mixture of the above polymers, a modified product, a derivative, a random copolymer, an alternating copolymer, a graft copolymer, a block copolymer and the like can be used. The weight average molecular weight of these resins is usually 10,000 to 30.
It is about 100,000, preferably about 100,000 to 1,000,000. If the molecular weight is too low, the strength of the electrode tends to decrease. on the other hand,
If the molecular weight is too high, the viscosity becomes high and it may be difficult to form an electrode. Preferred binder resins are fluororesins and CN group-containing polymers.

The amount of binder used is usually 0.1 part by weight or more, preferably 1 part by weight or more, and usually 30 parts by weight or less, preferably 20 parts by weight, relative to 100 parts by weight of the active material.
It is less than or equal to parts by weight. If the amount of the binder is too small, the strength of the electrode tends to decrease, and if the amount of the binder is too large, the ionic conductivity tends to decrease. In the electrode,
In order to improve the conductivity and mechanical strength of the electrode, a conductive material, an additive such as a reinforcing material that exhibits various functions, a powder, and a filler may be contained. The conductive material is not particularly limited as long as it can impart conductivity by mixing an appropriate amount of the active material, but is usually acetylene black, carbon black, carbon powder such as graphite, fibers of various metals, Examples include foil. As the reinforcing material, various inorganic or organic spherical or fibrous fillers can be used.

The electrode can be formed by applying and drying a coating material containing a component such as an active material and a binder and a solvent. The thickness of the electrode is usually 1 μm or more, preferably 10 μm or more, more preferably 20 μm or more, most preferably 40 μm or more, and usually 20 μm or more.
It is 0 μm or less, preferably 150 μm or less, and more preferably 100 μm or less. If it is too thin, not only will it be difficult to apply the coating, it will be difficult to ensure uniformity, but the capacity of the battery may become too small. On the other hand, if it is too thick, the rate characteristics may deteriorate too much.

At least one of the positive electrode and the negative electrode is
Usually, it is formed on the current collector. Various kinds of current collectors can be used, but metals or alloys are usually used. Specifically, examples of the current collector of the positive electrode include aluminum, nickel and SUS, and examples of the current collector of the negative electrode include copper, nickel and SUS. Preferably, aluminum is used as the current collector of the positive electrode and copper is used as the current collector of the negative electrode. In order to improve the binding effect with the positive and negative electrode layers, it is preferable to preliminarily roughen the surfaces of these current collectors. As a method of surface roughening,
Methods such as blasting or rolling with a rough surface roll,
The mechanical polishing method, the electrolytic polishing method, the chemical polishing method, and the like, in which the current collector surface is polished by a polishing cloth paper to which abrasive particles are fixed, a grindstone, an emery buff, a wire-brush equipped with a steel wire, and the like can be mentioned.

Further, in order to reduce the weight of the battery, that is, to improve the weight energy density, a perforated type current collector such as expanded metal or punching metal can be used. In this case, the weight can be freely changed by changing the aperture ratio. In addition, when an active material is present on both sides of such a hole-type current collector, the rivet effect of the coating film through the holes tends to make it more difficult to peel the coating film, but the aperture ratio is too high. When it becomes higher, the contact area between the coating film and the current collector becomes smaller, so that the adhesive strength may be rather lowered.

The thickness of the current collector is usually 1 μm or more, preferably 5 μm or more, and usually 100 μm or less, preferably 50 μm or less. If it is too thick, the capacity of the whole battery will be too low, and if it is too thin, handling may be difficult. The non-aqueous electrolytic solution of the present invention may be gelled with a polymer to be a semi-solid state. The amount of the non-aqueous electrolyte solution used in the semi-solid electrolyte is usually 30% by weight based on the total amount of the semi-solid electrolyte.
Or more, preferably 50% by weight or more, more preferably 7
5% by weight or more, and usually 99.95% by weight or less,
It is preferably 99% by weight or less, more preferably 98% by weight or less. If the amount used is too large, it becomes difficult to retain the electrolytic solution and liquid leakage easily occurs. On the contrary, if the amount is too small, charge and discharge efficiency and capacity may be insufficient.

A porous spacer is preferably provided between the positive electrode and the negative electrode in order to prevent a short circuit.
That is, in this case, the electrolytic solution is used by impregnating the porous spacer. As the material of the spacer, polyolefin such as polyethylene or polypropylene, polytetrafluoroethylene, polyether sulfone or the like can be used, but polyolefin is preferable. The spacer has a thickness of usually 1 μm or more, preferably 5 μm or more, more preferably 10 μm or more, and usually 5 μm or more.
It is 0 μm or less, preferably 40 μm or less, and more preferably 30 μm or less. If the porous film is too thin, the insulation properties and mechanical strength may deteriorate, and if it is too thick, not only the battery performance such as rate characteristics may deteriorate, but also the energy density of the battery as a whole may decrease. The porosity of the spacer is usually 20% or more, preferably 35% or more, more preferably 45% or more, and usually 90% or more.
% Or less, preferably 85% or less, more preferably 75%
% Or less. If the porosity is too small, the film resistance will increase and the rate characteristics will tend to deteriorate. On the other hand, if it is too large, the mechanical strength of the film is lowered and the insulating property tends to be lowered.
The average pore diameter of the spacer is usually 0.5 μm or less, preferably 0.2 μm or less, and usually 0.05 μm or more. If it is too large, a short circuit is likely to occur, and if it is too small, the film resistance may increase and the rate characteristics may deteriorate.

[0028]

[Examples] Specific examples of the present invention will be described below with reference to examples, but the present invention is not limited to these examples as long as the gist thereof is not exceeded. Example 1 [Production of positive electrode] Lithium cobalt oxide (LiCoO ₂ ) 9
0% by weight and polyvinylidene fluoride (PVdF) 5% by weight
And 5% by weight of acetylene black were mixed and N-methylpyrrolidone was added to form a slurry, which was applied to one side of a current collector made of aluminum and dried to obtain a positive electrode.

[Production of Negative Electrode] Graphite powder 87.4
% By weight, 9.7% by weight of PVdF and acetylene black
2.9 wt% was mixed and N-methylpyrrolidone was added to form a slurry, which was applied to both surfaces of a current collector made of copper and dried to obtain a negative electrode. [Preparation of Electrolyte Solution] 100 parts by weight of a mixed solvent of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate (mixing volume ratio 2: 3: 3) containing LiPF ₆ at a ratio of 1.25 mol / L, and 2 parts by weight of vinylene carbonate. Was added as a base electrolytic solution, and 4 parts by weight of hexafluoro-2,2-diphenylpropane was added to this as an electrolytic solution.

[Production of lithium secondary battery] The above positive electrode, negative electrode, and film thickness 16 μm, porosity 45%, average pore diameter 0.0
A 5 μm polyethylene biaxially-stretched porous membrane film was coated and impregnated with the electrolytic solution, and then laminated in order of a negative electrode, a separator, a positive electrode, a separator, and a negative electrode. The battery element thus obtained was first sandwiched between PET films, and then the positive and negative electrode terminals were projected on a laminate film in which both sides of an aluminum layer were covered with a resin layer, and vacuum sealing was performed to form a sheet-like lithium secondary material. A battery was made. Further, in order to enhance the adhesion between the electrodes, the sheet-shaped battery was sandwiched between silicon rubber and a glass plate, and pressure was applied at 0.35 kg / cm ² . FIG. 1 shows a schematic sectional view of a secondary battery.

[Evaluation of initial characteristics of battery] The discharge amount of lithium cobalt oxide per hour was set to 138 mAh / g, and the discharge rate 1C was calculated from this and the amount of the active material of the positive electrode of the lithium secondary battery for evaluation to set the rate. Then, after charging to 4.2V at 0.2C, discharge to 3V at 0.2C,
We had an early formation. Then, charge to 4.2V at 0.5C, then discharge to 3V at 0.2C,
The 0.2 C discharge capacity was determined. The results are shown in Table-1. The cut current during charging was set to 0.05C.

[Evaluation of Self-Discharge Characteristics] The battery whose initial characteristics had been evaluated was charged to 0.5 V at 4.2 V and then stored in a constant temperature bath at 60 ° C. for 7 days in order to carry out an accelerated test of self-discharge characteristics. Then, the battery was taken out and discharged at 0.2 C, and the remaining 0.2 C discharge capacity after storage was determined. The self-discharge maintenance rate was calculated from the 0.2 C discharge capacity before and after storage by the following formula. The results are shown in Table-1.

[0033]

[Equation 1] Self-discharge maintenance rate (%) = 0.2 C remaining after storage
Discharge capacity (mAh / g) /0.2C discharge capacity (mAh /
g) Comparative Example 1 A lithium secondary battery was prepared in the same manner as in Example 1 except that an electrolyte solution containing no hexafluoro-2,2-diphenylpropane was used, and a battery characteristic test similar to that in Example 1 was performed. Carried out. The results are shown in Table-1. It was found that the self-discharge maintenance rate was as low as about 70%.

Comparative Example 2 A lithium secondary battery was prepared in the same manner as in Example 1 except that an electrolyte solution containing diphenylmethane described in JP 2001-15154 A was used in place of hexafluoro-2,2-diphenylpropane. Was prepared and the same battery characteristic test as in Example 1 was carried out. The results are shown in Table-1. It was found that the self-discharge characteristics were rather deteriorated with diphenylmethane containing no fluorine.

[0035]

[Table 1]

As is clear from Table 1 above, when the non-aqueous electrolyte solution of the present invention is used, there is little decrease in capacity when fully stored and self-discharge characteristics are greatly improved.

[0037]

According to the present invention, a secondary battery having high capacity and excellent self-discharge characteristics can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the structure of a lithium secondary battery embodying the present invention.

[Explanation of symbols]

1 positive electrode 2 Negative electrode 3 separator 4 PET film 5 silicone rubber 6 glass plates 7 Laminated film 8 Lead with sealing material

Continued front page    (72) Inventor Hiroshi Susada             1000 Kamoshida-cho, Aoba-ku, Yokohama-shi, Kanagawa             Within Mitsubishi Chemical Corporation F-term (reference) 5H029 AJ04 AK02 AK03 AK05 AK16                       AK18 AL06 AL12 AM02 AM03                       AM05 AM07 BJ04 BJ12 HJ02

Claims (5)

[Claims] 1. A non-aqueous electrolytic solution in which a lithium salt is dissolved in a non-aqueous organic solvent, wherein the non-aqueous organic solvent contains a fluorine-containing diphenyl compound represented by the following general formula (1). Non-aqueous electrolyte solution. [Chemical 1] (In the formula, A represents a fluorine-substituted alkylene group) 2. The content of the fluorine-containing diphenyl compound is 0.1 to 10% by weight with respect to the non-aqueous electrolyte solution.
The non-aqueous electrolyte solution according to claim 1. 3. The non-aqueous electrolytic solution according to claim 1, wherein the non-aqueous organic solvent contains unsaturated carbonate. 4. The non-aqueous electrolytic solution according to claim 1, wherein the non-aqueous organic solvent contains an asymmetric carbonate. 5. A lithium secondary battery using the non-aqueous electrolyte solution according to claim 1.