JP2003100343A - Lithium cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium cell which holds the cell performance on a sufficient level even at a high discharge rate, has a long life, an excellent stable preserving performance and an excellent safety in abuse or high temperatures, by holding the ionic conductivity of the nonaqueous electrolyte at a high level and achieving a smooth lithium ion migration therein. SOLUTION: The nonaqueous electrolyte includes g-butyrolactone of 1-25 vol.%, and at least LiPF6 and LiBF4 as lithium salts, wherein the concentration of all the lithium salts is 1-3 mol in one liter of the electrolyte, and the content of LiBF4 is 25 mol% or less in all the lithium salts and 0.4 mol or less in one liter of the electrolyte.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium battery, and more particularly to improvement of a non-aqueous electrolyte of a lithium battery.

[0002]

2. Description of the Related Art In recent years, portable devices such as mobile phones, PHSs, and small personal computers have been remarkably miniaturized and lightened with the progress of electronics technology, and even in batteries used as a power source for these devices. Miniaturization and weight reduction are required.

One of the batteries that can be expected for such applications is a lithium battery. In addition to a lithium primary battery that has already been put into practical use, a lithium secondary battery can be put into practical use, has a high capacity, and has a long life. It has been demanded. In particular, the conventional lithium-ion secondary batteries are mainly cylindrical or prismatic, whereas various researches have been carried out for practical use of thin-shaped lithium-ion secondary batteries and lithium polymer secondary batteries. Development is being done.

In the case of a cylindrical or prismatic lithium secondary battery, a process of inserting a pole group consisting of a positive electrode, a negative electrode and a separator into a cylindrical or prismatic metal battery case and then injecting a liquid non-aqueous electrolyte. It is produced through. On the other hand, in the thin-shaped lithium polymer secondary battery,
The positive electrode and the negative electrode are made to face each other with the gel electrolyte interposed therebetween, and then packed with a metal resin composite material, which is advantageous in manufacturing. However, such a lithium polymer secondary battery has a drawback that it has poor high-rate charge / discharge performance and low-temperature performance as compared with a cylindrical or prismatic lithium secondary battery.

The following factors can be cited as the cause of this. That is, in the case of a cylindrical or prismatic battery, since a liquid non-aqueous electrolyte is injected, the lithium ion conductivity in the electrode and the separator is generally 1 × 10 ⁻³ S / S which is said to be a level required for battery operation. It is easy to secure the cm order. On the other hand, in the case of a lithium polymer secondary battery, since the electrolyte is in a solid state, a liquid injection step is generally not required when preparing the electrolyte, but the lithium ion conductivity is inevitably lower than that of the liquid system. Generally, it was difficult to secure the conductivity of the order of 1 × 10 ⁻³ S / cm. Therefore, there is a drawback that the charge and discharge performance is inferior.

Therefore, a thin lithium ion secondary battery packed with a metal resin composite material using a liquid non-aqueous electrolyte, which has the advantages of a cylindrical or prismatic lithium secondary battery and the advantages of a lithium polymer secondary battery. Various researches and developments have been made on batteries, and in recent years, they have almost been put into practical use.

[0007]

However, such a thin-shaped lithium-ion secondary battery has almost the same non-aqueous electrolyte as the non-aqueous electrolyte used in a cylindrical or prismatic lithium secondary battery. I use the one. Organic solvents used in these non-aqueous electrolytes are generally volatile and highly flammable, and are classified as flammable substances. Therefore, a thin lithium-ion secondary battery that uses a metal-resin composite material, which is inferior in mechanical strength compared to a metal battery case, for the exterior body is safe when it is overused, overdischarged, or short-circuited. There was a problem.

The present invention has been made in view of the above problems, and maintains the ionic conductivity of a non-aqueous electrolyte at a high level without the need for a special manufacturing process, etc. The smooth movement of the battery keeps the battery performance at a sufficient level even during high-rate discharge, provides a long battery life and stable battery performance with excellent storage performance. It is an object of the present invention to provide a lithium battery that is excellent in safety in time and in a high temperature environment.

[0009]

In order to solve the above-mentioned problems, the invention according to claim 1 provides a lithium battery comprising a non-aqueous electrolyte composed of at least an organic solvent and a lithium salt, wherein the organic solvent is γ. -Butyrolactone, and the lithium salt is LiPF ₆ and LiBF ₄
At least, and the non-aqueous electrolyte is Li
The lithium battery is characterized in that the molar concentration of BF ₄ is 0.4 mol / liter or less.

According to the invention of claim 1, since the organic solvent contains at least γ-butyrolactone, γ-butyrolactone has features such as high dielectric constant, low viscosity, low vapor pressure, and high flash point. As a result, the mobility of lithium ions in the non-aqueous electrolyte can be sufficiently obtained, the battery performance can be maintained at a sufficient level even at a high rate discharge, and long life and stable battery performance can be obtained. Not only that, it becomes possible to improve the safety during use. Further, the lithium salt is at least LiPF ₆ and Li.
By containing two kinds of BF ₄ , LiBF ₄ is stabilized by capturing water contained in the battery to form a complex, so that decomposition of LiPF ₆ and generation of HF are suppressed, It becomes possible to obtain stable battery performance with longer life. Furthermore, with LiBF ₄ as compared to LiPF ₆ is lower dissociation degree, .gamma.-butyrolactone contained in the nonaqueous electrolyte is selectively coordinated to LiBF _4, to promote the dissociation of LiBF _4, .gamma. Degradation of butyrolactone and undissociated LiBF ₄ is suppressed, and it is possible to obtain stable battery performance with a longer life and excellent storage performance. At this time, since the LiBF ₄ content is 0.4 mol or less per liter of the non-aqueous electrolyte, it is possible to obtain particularly stable battery performance with a long life and excellent storage performance.

The LiBF ₄ content in the lithium salt is
The amount is preferably 0.4 mol or less, and more preferably 0.05 to 0.2 mol, per liter of the non-aqueous electrolyte. If the LiBF ₄ content exceeds 0.2 mol, γ-
Since there is LiBF ₄ that is not coordinated with butyrolactone, the dissociation of LiBF ₄ is not sufficient and the undissociated LiB 4
Due to the decomposition of F ₄ , sufficient life performance and storage performance cannot be obtained. In addition, when the LiBF ₄ content in the lithium salt is less than 0.05 mol, water that does not form a complex with LiBF ₄ exists, so that LiPF ₆ is decomposed and HF is generated, and sufficient life performance is obtained. I can't. further,
Since γ-butyrolactone that does not coordinate to LiBF ₄ exists, γ-butyrolactone is decomposed, and life performance and storage performance cannot be sufficiently obtained. In view of the above points, the above concentration range is preferable.

According to a second aspect of the present invention, in a lithium battery provided with a non-aqueous electrolyte composed of at least an organic solvent and a lithium salt, the organic solvent contains γ-butyrolactone, and the lithium salt is LiPF 4. A lithium battery containing at least ₆ and LiBF ₄ , and wherein the lithium salt has a molar ratio of LiBF ₄ of 25 mol% or less.

According to the invention of claim 2, since the organic solvent contains at least γ-butyrolactone, γ-butyrolactone has features such as high dielectric constant, low viscosity, low vapor pressure and high flash point. As a result, the mobility of lithium ions in the non-aqueous electrolyte can be sufficiently obtained, the battery performance can be maintained at a sufficient level even at a high rate discharge, and long life and stable battery performance can be obtained. Not only that, it becomes possible to improve the safety during use. Further, the lithium salt is at least LiPF ₆ and Li.
By containing two kinds of BF ₄ , LiBF ₄ is stabilized by capturing water contained in the battery to form a complex, so that decomposition of LiPF ₆ and generation of HF are suppressed, It becomes possible to obtain stable battery performance with longer life. Furthermore, with LiBF ₄ as compared to LiPF ₆ is lower dissociation degree, .gamma.-butyrolactone contained in the nonaqueous electrolyte is selectively coordinated to LiBF _4, to promote the dissociation of LiBF _4, .gamma. Degradation of butyrolactone and undissociated LiBF ₄ is suppressed, and it is possible to obtain stable battery performance with a longer life and excellent storage performance. At this time, the LiBF ₄ content in the lithium salt is 25
By having a mole percentage or less, especially long life,
In addition, stable battery performance with excellent storage performance can be obtained.

The LiBF ₄ content in the lithium salt is
It is preferably 25 mol% or less, and more preferably 5 to 20 mol%. When the LiBF ₄ content in the lithium salt exceeds 25 mol%, LiBF ₄ that is not coordinated with γ-butyrolactone is present, so that LiBF ₄ is not sufficiently dissociated and undissociated LiBF ₄ is decomposed. Lifetime performance and storage performance cannot be fully obtained. In addition, LiBF in lithium salt
_{When the} content of ₄ is less than 5 mol%, water that does not form a complex with LiBF ₄ exists, so that decomposition of LiPF ₆ and generation of HF occur, and a sufficient life performance cannot be obtained. Furthermore, since there is γ-butyrolactone that is not coordinated with LiBF ₄ , decomposition of γ-butyrolactone occurs, and sufficient life performance and storage performance cannot be obtained. In view of the above points, the above concentration range is preferable.

A third aspect of the present invention is the lithium battery, wherein the non-aqueous electrolyte has the lithium salt concentration of 1 mol / liter or more and 3 mol / liter or less.

According to the third aspect of the invention, since the total concentration of the lithium salt is 1 to 3 mol with respect to 1 liter of the non-aqueous electrolyte, good diffusion of lithium ions can be realized and sufficient. Since the number of carriers of various electric charges can be secured, sufficient battery performance can be obtained.

The concentration of the lithium salt contained in the non-aqueous electrolyte is preferably 1 to 3 mol, and more preferably 1 to 2 mol per 1 liter of the electrolytic solution. If the lithium salt concentration is less than 1 mol, the lithium ion concentration is too low, and the number of charge carriers becomes insufficient, so that sufficient battery performance cannot be obtained. On the other hand, when the lithium salt concentration exceeds 3 mols, the lithium salt concentration is too high, the viscosity of the non-aqueous electrolyte increases, and not only the diffusion of lithium ions slows but also the dissociation degree decreases, resulting in a sufficient Battery performance is not obtained. In addition, precipitation of lithium salt easily occurs. In view of the above points, the above concentration range is preferable.

According to a fourth aspect of the present invention, the organic solvent is
γ-butyrolactone content is 1 volume percent or more 25
It is a lithium battery characterized by being below a volume percent. According to the invention of claim 4, since the content of γ-butyrolactone in the organic solvent is 1 to 25 volume%, γ-butyrolactone contained in the non-aqueous electrolyte is selectively coordinated with LiBF _4. Therefore, LiBF ₄
And the decomposition of γ-butyrolactone and undissociated LiBF ₄ are suppressed, and the stable battery performance having a longer life and excellent storage performance can be obtained. At this time, since the content of γ-butyrolactone in the organic solvent is 1 to 25 volume%, it is possible to obtain stable battery performance with a particularly long life and excellent storage performance.

The γ-butyrolactone content in the organic solvent is preferably 1 to 25 volume percent,
More preferably, it is 5 to 20 volume percent. When the content of γ-butyrolactone in the organic solvent exceeds 25% by volume, γ-butyrolactone which is not coordinated with LiBF ₄ is present, so that γ-butyrolactone is decomposed and the life performance and storage performance cannot be sufficiently obtained. . Also,
If the LiBF ₄ content in the lithium salt is less than 1 volume percent, LiBF ₄ not coordinated to γ-butyrolactone
For There exist, not sufficient dissociation of LiBF _4, since the decomposition of LiBF ₄ undissociated occurs, life performance and storage performance can not be obtained sufficiently. In view of the above points, the above content range is preferable.

Therefore, since the above-mentioned actions are synergistically obtained, the present invention is excellent in reliability and high in charge / discharge performance.
It is possible to easily provide a lithium battery having excellent low-temperature performance, life performance, storage performance, safety and the like.

Examples of the lithium salt that can be contained in the non-aqueous electrolyte include lithium salts having an inorganic or organic anion, in addition to LiPF ₆ and LiBF ₄ . Examples of inorganic anions include Cl
O ₄ ⁻ , AsF ₆ ⁻ , SCN ^{− and the} like can be mentioned. Examples of organic anions, for example _{C n F 2n + 1 SO 3} - (n = 0~
5), C (SO ₂ C _n F _{2n + 1} ) (SO ₂ C _m F _{2m + 1} ) (SO ₂ C _p
F _{2P + 1} ) ^- (n, m, p = 0 to 5), N (SO ₂ C _n F
_{2n + 1} ) (SO ₂ C _m F _{2m + 1} ) ^- (n, m = 0 to 5), RCOO
^{-And the} like, but are not limited to these. These lithium salts may be used alone or in combination of two or more, in addition to LiPF ₆ and LiBF ₄ , if necessary.

Examples of the organic solvent contained in the non-aqueous electrolyte include lactones {propiolactone, γ-butyrolactone, γ-valerolactone, etc.}, chain esters {methyl acetate, methyl propionate, ethyl propionate. Etc.}, carbonic acid esters {ethylene carbonate,
Propylene carbonate, diethyl carbonate, methyl ethyl carbonate, dimethyl carbonate, diphenyl carbonate, etc.}, cyclic ethers {tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, etc.}, chain ethers {dimethoxyethane, diethoxyethane, methoxy Ethoxyethane, ethylene glycol methyl ethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, polyethylene glycol di (C ₁ -C ₄ ) alkyl ether having a degree of polymerization of 3 or more, propylene glycol dimethyl ether, polypropylene glycol di (C ₁ C _4), such as alkyl ether}, N-methyl oxazolidone Gino Unless they already exist, sulfolane compounds {sulfolane, 2-methyl Ruforane, etc., Nitriles, {acetonitrile, etc.}, Sulfoxides, {Dimethylsulfoxide, etc.}, Amides, {N, N-Dimethylformamide, etc.}, Pyrrolidones, {N-Methylpyrrolidone, etc.}, Phosphates, Phosphoric acid Trimethyl, triethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate, tripropyl phosphate, tributyl phosphate, tri (trifluoromethyl) phosphate, tri (trifluoroethyl) phosphate, tri (triperfluoro phosphate) Ethyl) and the like}, and the like, but are not limited thereto. These organic solvents may be used alone or in combination of two or more, in addition to γ-butyrolactone, if necessary.

Above all, it is preferable to contain ethylene carbonate and / or propylene carbonate in addition to the γ-butyrolactone. In this case, it is more preferable that the ethylene carbonate content be higher than the propylene carbonate content. Further, in this case, the content of ethylene carbonate is 5% of the total solvent volume.
Most preferably, it is 0% by volume or less.

The non-aqueous electrolyte in the present invention may be solidified in a gel state by combining a lithium salt and an organic solvent, as well as a polymer. Here, examples of the polymer include polyethylene oxide, polypropylene oxide, polyacrylonitrile, polymethylmethacrylate, polyvinylidene fluoride, various acrylic monomers, methacrylic monomers, acrylamide monomers, allyl monomers, and styrene monomers. Examples include, but are not limited to, polymers. These may be used alone or in combination of two or more.

The positive electrode active material used in the present invention is not particularly limited, but in order to obtain the effect of the present invention remarkably, it is preferable to use a lithium-containing transition metal oxide. As the lithium-containing transition metal oxide, a general formula Li
_y MX ₂ , Li _y MN _x X ₂ (M and N represent metals of groups I to VIII, and X represents a chalcogen compound such as oxygen or sulfur.)
And, for example, LiyCo _1-x M _x O ₂ , Li _y Mn _2-x M _x
O ₄ (M is a metal of group I to VIII (for example, Li, C
a, Cr, Ni, Fe, one or more elements of Co) and the like. The x value indicating the substitution amount of the different element of the lithium-containing transition metal oxide is effective up to the maximum substitution amount, but preferably 0 ≦ x ≦ 1 from the viewpoint of discharge capacity. As for the y value indicating the amount of lithium, the maximum amount that can reversibly use lithium is effective, and preferably 0 ≦ y ≦ 2 in terms of discharge capacity. ),
It is not limited to these.

The negative electrode material used in the present invention is not particularly limited, but in order to obtain the effect of the present invention remarkably,
As the negative electrode material, lithium metal, lithium alloy (lithium-aluminum, lithium-lead, lithium-tin,
Lithium-aluminum-tin, lithium-gallium,
And lithium metal-containing alloys such as wood alloys), alloys capable of inserting and extracting lithium, carbon materials (for example, graphite, hard carbon, low temperature calcined carbon, amorphous carbon, etc.) and the like. Among these, graphite is preferable as a negative electrode material because it has an operating potential extremely close to that of metallic lithium, and therefore self-discharge can be reduced and irreversible capacity during charge / discharge can be reduced when a lithium salt is used as an electrolyte salt. For example, artificial graphite,
Natural graphite is preferred. In particular, graphite in which the surface of the negative electrode active material particles is modified with amorphous carbon or the like is desirable because it generates less gas during charging.

The analysis results of X-ray diffraction of graphite which can be preferably used are shown below: Lattice plane spacing (d ₀₀₂ ) 0.333 to 0.350 nm Crystallite size La in the a-axis direction La 20 nm or more c Axial crystallite size Lc 20 nm or more True density 2.00 to 2.25 g / cm ³

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail below, but the present invention is not limited by these descriptions.

(Example 1) FIG. 1 is a sectional view of a lithium battery according to the present invention.

The lithium battery according to the present invention comprises the positive electrode 1,
The electrode group 4 is composed of the negative electrode 2 and the separator 3, a non-aqueous electrolyte, and a metal resin composite film 5. The positive electrode 1 is formed by coating the positive electrode mixture 11 on the positive electrode current collector 12. The negative electrode 2 is formed by coating the negative electrode mixture 21 on the negative electrode current collector 22. The non-aqueous electrolyte is impregnated in the pole group 4. The metal resin composite film 5 covers the pole group 4 and is sealed on all four sides by heat welding.

Next, a method of manufacturing the battery having the above structure will be described.

The positive electrode 1 was obtained as follows. First, LiCoO ₂ that is a positive electrode active material and acetylene black that is a conductive agent are mixed, and then an N-methyl-2-pyrrolidone solution of polyvinylidene fluoride is mixed as a binder.
This mixture is applied to one surface of the positive electrode current collector 12 made of aluminum foil and then dried, so that the thickness of the positive electrode mixture 11 is 0.1 mm.
Pressed so that The positive electrode 1 was obtained through the above steps.

The negative electrode 2 was obtained as follows. First, graphite, which is a negative electrode active material, and an N-methyl-2-pyrrolidone solution of polyvinylidene fluoride, which is a binder, are mixed, and this mixture is applied to one surface of a negative electrode current collector 22 made of a copper foil. It is dried and the negative electrode mixture 21 has a thickness of 0.1.
It was pressed to have a size of mm. Through the above steps, the negative electrode 2
Got

On the other hand, the separator 3 was obtained as follows. First, an ethanol solution in which 3% by weight of a bifunctional acrylate monomer having the structure shown in [Chemical Formula 1] was dissolved was prepared, and a polyethylene microporous membrane (average pore size 0.1 micron, porosity 50 %, Thickness 23 microns, weight 12.52 g / m ² , air permeability 89 seconds / 10
(0 ml), the monomer was crosslinked by electron beam irradiation to form an organic polymer layer, and dried at a temperature of 60 ° C. for 5 minutes. The separator 3 was obtained through the above steps.
The obtained separator 3 had a thickness of 24 microns, a weight of 13.04 g / m ² , and an air permeability of 103 seconds / 100 ml, and the weight of the organic polymer layer was about 4 weight relative to the weight of the porous material. %, The thickness of the crosslinked layer was about 1 micron, and the pores of the porous substrate were maintained almost as they were.

[0035]

[Chemical 1]

The electrode group 4 was constructed by placing the positive electrode mixture 11 and the negative electrode mixture 21 facing each other, disposing the separator 3 therebetween, and laminating the positive electrode 1, the separator 3 and the negative electrode 2 in this order. The non-aqueous electrolyte contains γ-butyrolactone, ethylene carbonate and propylene carbonate in a volume ratio of 25: 5.
It was obtained by dissolving 1 mol of LiPF ₆ and 0.3 mol of LiBF ₄ in 1 liter of a mixed solvent mixed at a ratio of 0:25. Next, the electrode group 4 was immersed in the non-aqueous electrolyte to impregnate the electrode group 4 with the non-aqueous electrolyte.
Further, the electrode group 4 was covered with the metal-resin composite film 5, and its four sides were sealed by heat welding.

The battery obtained by the above manufacturing method is referred to as Battery A of the present invention. The designed capacity of the battery A of the present invention is 10 m.
It is Ah.

Example 2 As a non-aqueous electrolyte, 1 mol of LiPF ₆ and 0.2 was added to 1 liter of a mixed solvent in which γ-butyrolactone, ethylene carbonate and propylene carbonate were mixed at a volume ratio of 20:50:30. A polymer electrolyte battery having a capacity of 10 mAh was produced by the same raw material and production method as in Example 1 except that one in which mol LiBF ₄ was dissolved was used, and was designated as Battery B of the present invention.

Example 3 As a non-aqueous electrolyte, γ-butyrolactone, ethylene carbonate, propylene carbonate and diethylene carbonate were used in a volume ratio of 15: 5.
By using the same raw material and the same production method as in Example 1, except that 1 mol of LiPF ₆ and 0.2 mol of LiBF ₄ were dissolved in 1 liter of a mixed solvent mixed at a ratio of 0:25:10. A polymer electrolyte battery having a capacity of 10 mAh was produced, and was used as a battery C of the invention.

Comparative Example 1 As a non-aqueous electrolyte, 1 mol of LiPF ₆ and 0.5 was added to 1 liter of a mixed solvent in which γ-butyrolactone, ethylene carbonate and propylene carbonate were mixed at a volume ratio of 25:50:25. A polymer electrolyte battery having a capacity of 10 mAh was produced by the same raw material and production method as in Example 1 except that a solution obtained by dissolving a molar amount of LiBF ₄ was used.

Comparative Example 2 0.4 mol of LiPF ₆ and 0 was added to 1 liter of a mixed solvent in which γ-butyrolactone, ethylene carbonate and propylene carbonate were mixed with a non-aqueous electrolyte in a volume ratio of 25:50:25. A polymer electrolyte battery having a capacity of 10 mAh was prepared as Comparative Battery E by the same raw material and manufacturing method as in Example 1 except that a solution in which 4 mol of LiBF ₄ was dissolved was used.

Comparative Example 3 As a non-aqueous electrolyte, 1 mol of LiPF ₆ and 0.2 was added to 1 liter of a mixed solvent in which γ-butyrolactone, ethylene carbonate and propylene carbonate were mixed at a volume ratio of 30:50:20. A polymer electrolyte battery having a capacity of 10 mAh was produced by the same raw material and production method as in Example 1 except that a solution obtained by dissolving a molar amount of LiBF ₄ was used.

Comparative Example 4 As a non-aqueous electrolyte, 1 mol of LiPF ₆ and 0.2 was added to 1 liter of a mixed solvent in which ethylene carbonate, propylene carbonate and diethyl carbonate were mixed at a volume ratio of 50:25:25.
A polymer electrolyte battery having a capacity of 10 mAh was prepared as Comparative Battery G by the same raw material and manufacturing method as in Example 1 except that one in which mol LiBF ₄ was dissolved was used.

(High Rate Discharge Characteristic Test) The high rate discharge characteristics of these batteries A, B and C of the present invention and comparative batteries D, E, F and G were obtained. The test temperature was 20 ° C. The charging was a constant-current constant-voltage charge with a current of 2 mA and a final voltage of 4.2 V, and the discharging was a constant-current discharge with various current values, and the final voltage was 2.7 V. The ratio with the battery design capacity was defined as the discharge capacity (%). The results are shown in Figure 2.

In FIG. 2, comparing the discharge capacities at a discharge current of 30 mA, the comparative battery D has a design capacity of 35.
%, About 20% of the designed capacity of the comparative battery E, about 32% of the designed capacity of the comparative battery F, and about 28% of the designed capacity of the comparative battery G. It was found that the discharge capacity of about 52% of the designed capacity, the discharge capacity of about 45% of the designed capacity of the battery B of the present invention, and the discharge capacity of 47% of the battery C of the invention were obtained.

(Charge / Discharge Cycle Characteristic Test) Next, charge / discharge cycle characteristics of the batteries A, B and C of the present invention and the comparative batteries D, E, F and G were obtained. Test temperature is 2
It was set to 0 ° C. Charging is a constant current constant voltage charging with a current of 2 mA and an end voltage of 4.2 V, discharging is 2 mA and an end voltage of 2.7 V.
Constant current discharge. The ratio of the discharge capacity after 200 cycles to the initial discharge capacity was defined as the cycle characteristic (%).
The results are shown in Table 1.

[0047]

[Table 1]

In Table 1, comparing the after-cycle capacities after 200 cycles of charging and discharging, the comparative battery D has a capacity of about 68%, and the comparative battery E has a capacity of about 65%.
In comparison battery F, about 70% of the initial capacity, and in comparison battery G, only about 75% of the initial capacity can be obtained, whereas in the invention battery A, about 80% of the initial capacity, and in the invention battery B, the initial capacity is about 80%. About 82% of the capacity, 85 with the battery C of the present invention
It was found that a discharge capacity of% could be obtained.

(High-temperature storage test) Further, with respect to these batteries A, B and C of the present invention and comparative batteries D, E, F and G,
A high temperature storage test was conducted. First, after charging / discharging for several cycles under the same conditions as the charge / discharge cycle test at a temperature of 20 ° C., each battery in a charged state was stored in a constant temperature bath at 60 ° C. for 20 days. Next, each battery was taken out from the constant temperature bath and the residual discharge capacity was measured. The ratio of the remaining discharge capacity to the discharge capacity before storage was defined as the remaining discharge capacity rate (%). Further, the thickness of each battery was measured before and after storage, and the battery thickness after storage relative to the battery thickness before storage was defined as the battery thickness change rate (%). The results are shown in Table 2.

[0050]

[Table 2]

In Table 2, comparing the residual discharge capacities after storage at 60 ° C. for 20 days, the comparative battery C has a storage capacity of about 82%, the comparative battery D has a storage capacity of about 85%, and the comparative battery E has a storage capacity of about 85%. The discharge capacity of about 87% of the pre-storage capacity and about 78% of the pre-storage capacity of the comparative battery F can be obtained, whereas the discharge capacity of the present invention battery A is about 95% of the pre-storage capacity, and the storage capacity of the present invention battery B is It was found that a discharge capacity of about 92% of the capacity and a capacity of about 90% of the pre-storage capacity of the battery C of the present invention were obtained. Further, comparing the rate of change in battery thickness after storage at 60 ° C. for 20 days, it was found that the swelling of the battery in the battery of the present invention was suppressed to be equal to or less than that of the battery in the comparative battery.

From the above three test results, the battery A of the present invention,
It was found that B and C have good battery performance with respect to the comparative batteries D, E, F and G.

[0053]

According to the invention of claim 1, an organic solvent
Contains at least γ-butyrolactone
Keeps battery performance at a sufficient level even during high-rate discharge
It is possible to obtain stable battery performance with a long life.
It Further, the lithium salt is at least LiPF₆And Li
BF_FourLonger life due to containing 2 kinds of
And stable battery performance with excellent storage performance
be able to. In addition, LiBF _FourNon-aqueous electrolyte content
Since it is 0.4 mol or less per liter,
The effect can be effectively obtained.

According to the invention described in claim 2, since the organic solvent contains at least γ-butyrolactone, the battery performance is maintained at a sufficient level even at a high rate discharge,
It becomes possible to obtain stable battery performance with a long life. Further, since the lithium salt contains at least two kinds of LiPF ₆ and LiBF ₄ , it has a longer life and
It is possible to obtain stable battery performance with excellent storage performance. Furthermore, when the LiBF ₄ content in the lithium salt is 25 mol% or less, the above effect can be effectively obtained.

According to the third aspect of the invention, since the total concentration of the lithium salt is 1 to 3 mol with respect to 1 liter of the non-aqueous electrolyte, the above effect can be obtained more effectively.

According to the fourth aspect of the present invention, the above effect can be more effectively obtained because the γ-butyrolactone content in the organic solvent is 1 to 25% by volume.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a lithium battery of the present invention.

FIG. 2 shows the present invention batteries A, B and C and comparative batteries D and E,
It is a figure which shows the discharge rate characteristic of F and G.

[Explanation of symbols]

1 positive electrode 11 Positive electrode mixture 12 Positive electrode current collector 2 Negative electrode mixture 21 Negative electrode mixture 22 Negative electrode current collector 3 separator 4 pole group 5 Metal resin composite film

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Takaaki Iguchi             2-32 Kosobe-cho, Takatsuki City, Osaka Prefecture Stock             Ceremony company Yuasa Corporation (72) Inventor Hiroyuki Yoshida             2-32 Kosobe-cho, Takatsuki City, Osaka Prefecture Stock             Ceremony company Yuasa Corporation F-term (reference) 5H029 AJ04 AJ06 AJ12 AJ14 AK03                       AL06 AM00 AM01 AM04 AM05                       AM07 BJ03 BJ12 HJ02 HJ10

Claims (4)

[Claims] 1. A lithium battery comprising a non-aqueous electrolyte composed of at least an organic solvent and a lithium salt,
The organic solvent contains γ-butyrolactone, the lithium salt contains at least LiPF ₆ and LiBF ₄ , and the non-aqueous electrolyte has a LiBF ₄ molar concentration of 0.4 mol / liter or less. A lithium battery characterized in that 2. A lithium battery comprising a non-aqueous electrolyte composed of at least an organic solvent and a lithium salt,
The organic solvent contains γ-butyrolactone, the lithium salt contains at least LiPF ₆ and LiBF ₄ , and the lithium salt has a molar ratio of LiBF ₄ of 25 mol% or less. Lithium battery. 3. The lithium battery according to claim 1, wherein the non-aqueous electrolyte has the lithium salt concentration of 1 mol / liter or more and 3 mol / liter or less. 4. The lithium battery according to claim 1, wherein the organic solvent has a γ-butyrolactone content of 1 volume percent or more and 25 volume percent or less.