JPH097635A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a non-aqueous electrolyte secondary battery having excellent charge-discharge cycle durability and high energy density. [Constitution] A carbonaceous material capable of absorbing and desorbing lithium ions, a negative electrode mainly composed of a carbonaceous material or lithium or a lithium alloy, and a compound capable of absorbing and desorbing lithium ions are allowed to absorb lithium ions. A positive electrode containing the above compound as a main component and a mixed solvent containing a fluorinated dioxolane are combined.

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 secondary battery having excellent charge / discharge cycle durability and high energy density.

[0002]

2. Description of the Related Art Batteries using an alkali metal as a material for a negative electrode have been attracting attention because they have a high energy density. Among these, batteries using lithium or a lithium alloy as a material for the negative electrode have a particularly high energy density. It has excellent reliability such as storability and is now widely used as a primary battery in power supplies for electronic devices. Recently, there is a great need for lithium secondary batteries as the next step.

The electrolytic solution of a lithium battery is prepared by adding one or more organic solvents such as propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, 1,2-dimethoxyethane and 2-methyltetrahydrofuran to the electrolyte. Lithium salts, for example
LiClO ₄ , LiCF ₃ SO ₃ , LiBF ₄ , LiP
_{F 6, LiAsF 6, LiSbF 6} , LiCF 3 CO
_A solution of ₂ , ₂ , Li ₂ B ₁₀ Cl ₁₀ , LiN (CF ₃ SO ₂ ) _{2 and the} like is used.

Further, in a primary battery in which a negative electrode made of lithium or a lithium alloy is combined with a positive electrode mainly composed of a compound such as manganese dioxide or fluorinated carbon having a performance of 3V, a solvent of an electrolytic solution has a high dielectric constant. In order to improve the conductivity of the electrolytic solution at low temperature by using propylene carbonate, γ-butyrolactone or the like, a mixed solvent obtained by adding 1,2-dimethoxyethane or the like having low viscosity to these is widely used.

Further, a negative electrode made of lithium or a lithium alloy, a positive electrode mainly made of a compound such as a metal oxide or a metal sulfide having a performance of 3V class, or a positive electrode mainly made of a conductive organic compound such as polyaniline. In the combined secondary battery, 1,2-dimethoxyethane of low viscosity is added to propylene carbonate, ethylene carbonate, etc. having a high dielectric constant as the solvent of the electrolytic solution in order to improve the conductivity at low temperature (Japanese Patent Publication No. 3-51068). , 2-methyltetrahydrofuran (Japanese Patent Publication No. 5-20874), ethyl propionate (JP-A-5-74487) and the like using a mixed solvent, and dimethyl carbonate (JP-A-5-82167) for high withstand voltage. Many combinations such as those containing a halogen atom-substituted carbonic acid ester (Japanese Patent Laid-Open No. 6-199992) have been detected. It is.

Furthermore, recently, carbon materials capable of occluding and desorbing lithium ions such as organic substances heat-treated under various conditions (for example, fired products such as resins having a large residual carbon ratio during heat treatment), artificial graphite, natural graphite, etc. The main negative electrode is LiCo
A secondary battery having a higher energy density has been proposed in which a positive electrode mainly composed of a compound capable of inserting and extracting lithium ions having a high operating potential of 4 V or more such as O ₂ and LiMn ₂ O ₄ is combined.

The electrolyte of this type of secondary battery is electrochemically stable with respect to a high positive electrode potential and contains artificial graphite,
An electrolyte solution capable of stably intercalating lithium ions or lithium is used for a negative electrode mainly composed of natural graphite, an organic substance heat-treated under various conditions, and the like. As a solvent for such an electrolytic solution, propylene carbonate having a high dielectric constant, ethylene carbonate, etc., a chain carbonate such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, etc. having a lower viscosity in order to improve the conductivity at low temperature. It has been proposed to use added mixed solvents.

With the recent popularization of portable equipment,
There is a strong need for a battery having a higher energy density, particularly a secondary battery having a higher energy density, and for that purpose, it is effective to use a positive electrode material having a higher operating potential.

[0009]

Problems to be Solved by the Invention LiCoO ₂ , LiM
The potential of the positive electrode mainly composed of a compound capable of inserting and extracting lithium ions having a high operating potential of 4 V or more such as n ₂ O ₄ can be further increased by extracting the lithium ions. In order to maximize the performance of this type of positive electrode, it is necessary to raise the operating potential to 4.5 V or more with respect to the lithium negative electrode, but at present, there is an upper limit to the oxidative decomposition potential of the electrolytic solution, and the lithium negative electrode 4.2V to 4.V.
It is used at an operating potential of 3V. Therefore, so far, the secondary battery is used only with about 2/3 the capacity of this type of positive electrode.

The present invention overcomes this limitation to realize an electrolytic solution which is stable at a high operating potential and which can be used in a wide temperature range. By combining this electrolytic solution with the above-mentioned positive electrode, the capacity of the positive electrode is maximized. It is an object of the present invention to provide a secondary battery having such a high energy density.

[0011]

The non-aqueous electrolyte secondary battery of the present invention is mainly composed of a carbonaceous material obtained by storing lithium ions in a carbon material capable of storing and releasing lithium ions, or lithium or a lithium alloy. A negative electrode, and a positive electrode mainly composed of a compound capable of occluding and desorbing lithium ions (also referred to as a positive electrode active material) which occludes lithium ions
A non-aqueous electrolyte solution, and the solvent of the non-aqueous electrolyte solution is a mixed solvent containing fluorinated dioxolane.

In the non-aqueous electrolyte secondary battery of the present invention, the oxidative decomposition potential of the electrolyte is increased by combining the solvent of the electrolyte with the mixed solvent containing fluorinated dioxolane, and the capacity of the positive electrode is maximized. You can withdraw. Thus, according to the present invention, it is possible to obtain a secondary battery having a large energy density that can be used at a higher operating potential than before.

As the fluorinated dioxolane, 2,2-bis (trifluoromethyl) -1,3-dioxolane and 2,2-bis (trifluoromethyl) -1,3-dioxolane can be used because the oxidative decomposition potential of the electrolytic solution can be increased and the freezing point is low and the viscosity is low. , 2-bis (trifluoromethyl) -4,5-difluoro-1,3-dioxolane, 2,2-bis (trifluoromethyl) -4,
4,5,5-tetrafluoro-1,3-dioxolane,
2,2-dimethyl-4,4,5,5-tetrafluoro-
1,3-dioxolane or 2,2-dimethyl-4,5-
Preference is given to using difluoro-1,3-dioxolane.

In addition to fluorinated dioxolane, preferably one or more solvents selected from cyclic carbonate, sulfolane and sulfolane derivative are added to the solvent of the electrolytic solution to prepare a mixed solvent. When this kind of solvent having a high dielectric constant is mixed, the lithium salt of the electrolyte is easily dissolved in the solvent, the concentration of the electrolyte in the electrolytic solution can be increased, and a secondary battery having a low internal resistance can be obtained.

That is, the solvent of the electrolytic solution is a mixed solvent containing a fluorinated dioxolane having a high oxidative decomposition potential and a low viscosity, and at least one solvent having a high dielectric constant selected from a cyclic carbonic acid ester, sulfolane or a sulfolane derivative. Then, an electrochemically stable electrolytic solution can be obtained, and the initial charge / discharge efficiency can be improved. In addition, since the fluorinated dioxolane has a sufficiently low freezing point, the electrolyte solution in which the electrolyte is dissolved does not become a solid state even at a temperature of -30 ° C or lower, and is remarkably excellent in low-temperature characteristics as well, and has excellent charge-discharge cycle durability. And a high energy density non-aqueous electrolyte secondary battery can be obtained.

The negative electrode of the secondary battery of the present invention is mainly composed of 1) a carbonaceous material prepared by absorbing and desorbing lithium ions into a carbon material capable of absorbing and desorbing lithium ions, or 2) lithium or 3) lithium alloy. However, examples of carbon materials that can be used for this purpose include artificial graphite, natural graphite, soil graphite, expansive graphite, flake graphite, and calcined organic materials that have been heat-treated under various conditions.

As these carbon materials capable of occluding and desorbing lithium ions, it is preferable to use materials having a large capacity capable of reversibly occluding and desorbing lithium ions so as to obtain a higher energy density and a low operating potential. . As such a carbon material, it is preferable to use natural graphite having a low reversibility for occluding and releasing lithium ions and having a low operating potential or artificial graphite having a high degree of graphitization. It is also preferable to use an amorphous resin fired product having a large capacity for inserting and extracting lithium ions. Although there is a theory that carbonaceous materials and compounds are occluded not only in the form of lithium ions but also in the form of lithium, as long as they have the same practical function as the electrode of the secondary battery of the present invention, There is no problem in either case.

The positive electrode active material (compound capable of inserting and extracting lithium ions) of the secondary battery of the present invention is, for example, 4, 5, 6, 7, 8, 9, 10, 1 in the periodic table.
Chalcogenides or oxyhalides such as oxides, complex oxides and sulfides of metals belonging to Groups 1, 12, 13 and 14 can be used. Further, a conductive polymer material composed of a polyaniline derivative, a polypyrrole derivative, a polythiophene derivative, a polyacene derivative, a polyparaphenylene derivative or a copolymer of these derivatives can also be used.

As the positive electrode active material, it is preferable to use a material having a large capacity for reversibly occluding and desorbing lithium ions and having a high working potential because a high energy density can be obtained. As such a positive electrode active material, α-NaFe
LiCoO ₂ , LiNiO ₂ , LiM having O ₂ structure
nO ₂ and LiMn ₂ O ₄ having a spinel structure are preferable.

The solvent of the non-aqueous electrolyte used in the secondary battery of the present invention is preferably a mixture of at least one solvent selected from cyclic carbonate, sulfolane or sulfolane derivative and fluorinated dioxolane. Although it is a solvent, a third solvent may be mixed for the purpose of improving the ionic conductivity of the electrolytic solution, charge / discharge cycle characteristics and the like.

As the third solvent, γ-butyrolactone, 1,3-dioxolane, dimethylsulfoxide, formamide, dimethylformamide, dioxolane, phosphoric acid triester, 1,3-propanesultone, 4,5
-Dihydropyran derivative, nitrobenzene, 1,3-dioxane, 1,4-dioxane, 3-methyl-2-oxazolidinone, 1,2-dimethoxyethane, tetrahydrofuran, tetrahydrofuran derivative, sydnone compound, 2-methyltetrahydrofuran, dimethyl carbonate, One or more solvents selected from diethyl carbonate, ethylmethyl carbonate, acetonitrile, nitromethane, alkoxyethane, dimethylacetoacetamide and toluene can be mentioned.

Examples of the electrolyte of the electrolyte used in the secondary battery of the present invention, ClO ₄ by large electrolyte conductive of more soluble in the solvent is obtained ^{_{-, CF 3 SO 3 -,}} B
^{_{F 4 -, PF 6 -,}} AsF 6 -, SbF 6 -, CF 3 CO 2 -, B
One or more lithium salts having ₁₀ Cl ₁₀ ²⁻ or (CF ₃ SO ₂ ) ₂ N ⁻ as an anion can be preferably used. This type of electrolyte is preferably added to the mixed solvent in the range of 0.2 to 2.0 mol / liter. If it deviates from this range, the ionic conductivity of the electrolytic solution will decrease.

In the present invention, when a mixed solvent containing at least one solvent selected from cyclic carbonate, sulfolane or sulfolane derivative and fluorinated dioxolane is used as the solvent of the electrolytic solution, the electrolytic solution is used as a lithium negative electrode. On the other hand, it is stable at a high operating potential of about 4.5 V, and since the freezing point of fluorinated dioxolane is sufficiently low, the electrolyte solution in which the electrolyte is dissolved does not become a solid state even at a temperature of -30 ° C or lower, and the temperature is low. Since it also has excellent characteristics, a high energy density non-aqueous electrolyte secondary battery can be constructed.

[0024]

EXAMPLES The present invention will be specifically described below with reference to Examples (Examples 1 to 5) and Comparative Examples (Examples 6 to 8), but the present invention is not limited thereto.

Example 1 47 parts by weight of artificial graphite powder having an average particle size of 20 μm, 3 parts by weight of polyvinylidene fluoride (binder) and N-methyl-2-on a nickel foil having a thickness of 50 μm.
A slurry obtained by mixing 50 parts by weight of pyrrolidone was applied, heated to 180 ° C. and dried, punched out into a circle having a diameter of 19 mm, and pressed to obtain a negative electrode having a thickness of 0.3 mm. In order to remove water in the negative electrode, the negative electrode was heated at 180 ° C. for 4 hours under reduced pressure of 0.1 torr and dried.

A slurry obtained by mixing 42 parts by weight of LiMn ₂ O ₄ , 5 parts by weight of artificial graphite powder, 3 parts by weight of polyvinylidene fluoride and 50 parts by weight of N-methyl-2-pyrrolidone was formed into an aluminum foil having a thickness of 50 μm. After coating, heating at 180 ° C. and drying, it was punched into a disk having a diameter of 15 mm and pressed by a roll press to obtain a positive electrode having a thickness of 0.4 mm. In order to remove water in the positive electrode, the positive electrode was heated at 180 ° C. for 4 hours under reduced pressure of 0.1 torr.

Next, a separator comprising a polypropylene nonwoven fabric and a polypropylene microporous film is sandwiched between the positive electrode and the negative electrode, and the positive electrode, the negative electrode and the separator are separated by 2,2-
Bis (trifluoromethyl) -1,3-dioxolane 5
A mixed solvent consisting of 0% by volume and 50% by volume of ethylene carbonate was impregnated with an electrolytic solution in which 1 mol / liter of LiClO ₄ was dissolved and sealed in a metal container to obtain a coin-type secondary battery. FIG. 1 is a partial sectional view of the obtained coin-type secondary battery, in which 1 is a case of a metal container, 2 is a gasket, 3 is a lid of the metal container, 4 is a negative electrode, and 5 is a negative electrode. Is a separator and 6 is a positive electrode.

Example 2 2,2-bis (trifluoromethyl) -4,5-difluoro-1,3-dioxolane 50
A coin-type battery was obtained in the same manner as in Example 1 except that 1 mol / liter of LiPF ₆ was dissolved in a mixed solvent containing 50% by volume of ethylene carbonate and 50% by volume of ethylene carbonate.

Example 3 1 mol / min in a mixed solvent consisting of 30% by volume of 2,2-bis (trifluoromethyl) -4,4,5,5-tetrafluoro-1,3-dioxolane and 70% by volume of sulfolane. A coin-type battery was obtained in the same manner as in Example 1 except that 1 liter of LiPF ₆ was dissolved in the solution.

[Example 4] 2,2-dimethyl-as an electrolytic solution
The electrolyte solution was prepared by dissolving 1 mol / liter of LiPF ₆ in a mixed solvent of 30% by volume of 4,4,5,5-tetrafluoro-1,3-dioxolane and 70% by volume of ethylene carbonate. A coin type battery was obtained in the same manner as in.

[Example 5] 2,2-dimethyl-electrolyte
The electrolyte solution was prepared by dissolving 1 mol / liter of LiPF ₆ in a mixed solvent consisting of 30% by volume of 4,5-trifluoro-1,3-dioxolane and 70% by volume of ethylene carbonate. A coin type battery was obtained.

[Example 6] 1 mol / mol of ethylene carbonate
A coin-type battery was obtained in the same manner as in Example 1 except that 1 liter of LiPF ₆ was dissolved in the solution.

[Example 7] 50% by volume of ethylene carbonate
A coin-type battery was obtained in the same manner as in Example 1 except that 1 mol / liter of LiPF ₆ was dissolved in a mixed solvent of 50% by volume of diethyl carbonate and used as an electrolytic solution.

Example 8 50% by volume of ethylene carbonate
A coin-type battery was obtained in the same manner as in Example 1 except that 1 mol / liter of LiPF ₆ was dissolved in a mixed solvent containing 50% by volume of 1,3-dioxolane and was used as an electrolytic solution.

[Evaluation Method] The following tests were carried out on the obtained coin type batteries of Examples 1 to 8 to evaluate the characteristics. That is, in a room at 20 ° C, charging is performed at a constant current of 1 mA until the end-of-charge voltage reaches 4.4 V (potential regulation), and then at a constant current of 1 mA until the end-of-discharge voltage reaches 3.0 V (potential regulation). A charging / discharging cycle of discharging was performed. Example 1
The charge capacity and discharge capacity in the first cycle of ~ 8 were determined. This charge / discharge cycle was repeated 50 times, and 50 times the discharge capacity of the first cycle of each battery at this time
The ratio of the discharge capacity at the cycle was calculated.

Next, the batteries of Examples 1 to 8 were charged in a room at 20 ° C. with a constant current of 1 mA until the charge cutoff voltage was 4.4 V, and at a constant current of 1 mA at -30 ° C., the discharge cutoff voltage. Was discharged to 2.5 V. The discharge capacity at this time was determined. The results are shown in Table 1. Of these, the battery of Example 6 was unable to discharge because the electrolytic solution solidified.

From the above test results, in the secondary batteries of Examples 1 to 5, in which the solvent of the electrolytic solution was mainly the mixed solvent containing the fluorinated dioxolane, the solvent of the electrolytic solution was mainly ethylene carbonate. It can be seen that the discharge capacity is larger than those of the secondary batteries of Examples 6 to 8. This is because the discharge characteristics at low temperatures are improved.

[0038]

[Table 1]

[0039]

As can be seen from these results, the non-aqueous solvent secondary battery according to the present invention has a high withstand voltage because the mixed solvent containing the fluorinated dioxolane is combined with the solvent of the electrolytic solution. A secondary battery having a high energy density can be realized, and the secondary battery is remarkably excellent in low-temperature characteristics and charge / discharge cycle durability as compared with conventional secondary batteries. The secondary battery of the present invention having a high withstand voltage and a large energy density is suitable for many uses such as a power source for electronic devices.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional view of a secondary battery prototyped in Example 1 of the present invention.

[Explanation of symbols]

 1: Case of metal container 2: Gasket 3: Lid of metal container 4: Negative electrode 5: Separator 6: Positive electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Go Morimoto 1150 Hazawa-machi, Kanagawa-ku, Yokohama, Kanagawa Prefecture Asahi Glass Co., Ltd. Central Research Laboratory

Claims (4)

[Claims] 1. A carbonaceous material in which lithium ions are occluded and desorbed in a lithium ion-containing carbonaceous material, or a negative electrode mainly composed of lithium or a lithium alloy, and a lithium ion in a compound capable of occluding and deintercalating lithium ions. A non-aqueous electrolyte secondary battery comprising a positive electrode mainly containing a stored compound and a non-aqueous electrolyte, wherein the solvent of the non-aqueous electrolyte is a mixed solvent containing fluorinated dioxolane. . 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the mixed solvent contains at least one solvent selected from cyclic ester carbonate, sulfolane and sulfolane derivatives. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the mixed solvent contains 10 to 60% by volume of fluorinated dioxolane. 4. The fluorinated dioxolane is 2,2-bis (trifluoromethyl) -1,3-dioxolane,
2,2-bis (trifluoromethyl) -4,5-difluoro-1,3-dioxolane, 2,2-bis (trifluoromethyl) -4,4,5,5-tetrafluoro-1,
3-dioxolane, 2,2-dimethyl-4,4,5,5
-Tetrafluoro-1,3-dioxolane or 2,2-
The non-aqueous electrolyte secondary battery according to claim 1, which is dimethyl-4,5-difluoro-1,3-dioxolane.