JPH0850923A - Nonaqueous electrolytic lithium secondary cell - Google Patents

Abstract

PURPOSE:To provide a cell excellent in discharging characteristics at low temperatures by employing carbonate in a specified composition as organic solvent in electrolytic solution where LiPF6, is dissolved in organic solution. CONSTITUTION:Let ethylene carbonate, dimethyl carbonate and ethylene methyl carbonate be included in organic solvent used for electrolytic solution. When each ratio of ethylene carbonate, dimethyl, carbonate and ethylene methyl carbonate to the sum of ethylene carbonate, dimethyl carbonate and ethylene methyl carnonate is x, y and z, they shall be 10<=x<=35, 10<=y<=-85, and 5<=z<=80. By this constitution, a cell excellent in discharging characteristics at low temperatures can thereby be obtained without degrading charging/discharging efficiency at room temperature, high amperage current discharging characteristics and cyclic characteristics.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a lithium secondary battery using a non-aqueous electrolyte.

[0002]

_2. Description of the Related Art A positive electrode containing, as an active material, a layered lithium composite oxide having an α-NaFeO ₂ type structure as a matrix or a transition metal oxide having a spinel type structure as a matrix, and a carbon material capable of being doped and dedoped with lithium. A lithium ion secondary battery using a negative electrode containing an active material has a high energy density per volume and weight, is easily miniaturized and lightweight, and is excellent in charge / discharge cycle property and safety, It has received a great deal of attention as a secondary battery for portable electric devices such as mobile phones and portable video cameras or a secondary battery for electric vehicles.

When the above-mentioned electrode active material is used, both the positive electrode active material and the negative electrode active material are extremely electrochemically active, so that it is necessary to use an electrolytic solution which is electrochemically stable to these materials. An aprotic non-aqueous solvent is generally suitable as a non-aqueous solvent for an electrolytic solution having such high oxidation resistance and reduction resistance.

As an aprotic solvent for a non-aqueous electrolytic solution of a lithium battery, an electrolytic solution in which a lithium salt is dissolved in a mixed solvent of a cyclic carbonate and an acyclic carbonate has been proposed (JP-A-2-172162). , JP-A-2-
172163, Japanese Patent Laid-Open No. 4-171674).

On the other hand, in order to further improve the capacity characteristics of the battery, the battery active material itself has been widely studied. Particularly, in the case of the carbon material of the negative electrode, the charging / discharging capacity per unit weight is large and the charging / discharging process is not performed. It has been pointed out that the graphite-based material is an excellent material in that it has a low average potential and a large energy density (JP-A-57-208079).
JP-A-5-13088). However, when a graphite-based material is used as the carbon material of the negative electrode, a conventional electrolytic solution using propylene carbonate, butylene carbonate, or the like (Japanese Patent Laid-Open Nos. 2-10666 and 4-184).
No. 872) decomposes at the negative electrode, and thus is a non-aqueous electrolyte solution that is excellent in both discharge characteristics after initial use and after storage and also excellent in cycle characteristics when a graphite-based carbon material is used as the negative electrode material. Further, an electrolytic solution containing a mixed solvent of three components of ethylene carbonate, dimethyl carbonate and diethyl carbonate has been proposed as being excellent in both a large current discharge characteristic at room temperature and a low temperature discharge characteristic (Japanese Patent Laid-Open No. 13088/1993). Gazette).

[0006]

However, as a result of studying an electrolytic solution having a composition proposed hitherto, a discharge capacity lower than that expected from the electric conductivity at -20 ° C. was obtained, and the low temperature discharge characteristics were obtained. Turned out not to be enough.
An object of the present invention is to provide a lithium secondary battery using a graphite-based material as a negative electrode carbon material, which is excellent in low temperature discharge characteristics without impairing charge / discharge efficiency at room temperature, large current discharge characteristics and cycle characteristics. To provide a secondary battery.

[0007]

[Means for Solving the Problems] In view of such circumstances,
As a result of intensive investigations by the present inventors, ethylene carbonate, dimethyl carbonate, which are organic solvents of the electrolytic solution,
It was found that the above problems can be solved when the three components of ethylmethyl carbonate are within a specific composition range,
The present invention has been completed.

That is, the present invention is the invention described below. (1) A positive electrode containing a lithium composite oxide containing at least one transition metal as an active material, a negative electrode containing a carbon material mainly containing graphite as an active material, and an electrolyte solution in which at least LiPF ₆ is dissolved in an organic solvent as an electrolyte. , A non-aqueous electrolyte lithium secondary battery comprising a separator,
The organic solvent contains ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, the composition ratio of ethylene carbonate, dimethyl carbonate,
When the ratios of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate to the total amount of ethyl methyl carbonate are expressed as x, y, and z (unit volume%), 10 ≦ x ≦ 35, 10 ≦ y ≦ 85, and 5 ≦ z
A non-aqueous electrolyte lithium secondary battery characterized in that ≦ 80.

(2) The non-aqueous electrolyte lithium secondary battery according to (1), wherein the concentration range of LiPF _{6 in} the electrolyte is 0.5 to 1.5 mol / liter.

Next, the present invention will be described in detail. The non-aqueous electrolyte lithium secondary battery of the present invention includes a positive electrode containing a lithium composite oxide containing at least one transition metal as an active material, a negative electrode containing a carbon material mainly containing graphite as an active material, and at least LiPF 6 as an electrolyte. An electrolytic solution in which ₆ is dissolved in an organic solvent and a separator are provided. In the non-aqueous electrolytic solution lithium secondary battery of the present invention, the organic solvent used for the electrolytic solution contains ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, the composition ratio is ethylene carbonate, dimethyl carbonate,
When the ratios of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate to the total amount of ethyl methyl carbonate are expressed as x, y, and z (unit volume%), 10 ≦ x ≦ 35, 10 ≦ y ≦ 85, and 5 ≦ z
It is characterized in that ≦ 80. The ratio is preferably 1
5 ≦ x ≦ 35, 15 ≦ y ≦ 70, 10 ≦ z ≦ 70, and more preferably 20 ≦ x ≦ 35, 20 ≦ y ≦ 6.
0 and 10 ≦ z ≦ 60.

When the volume fraction of ethylene carbonate is less than 10% by volume as the organic solvent used for the electrolytic solution, the discharge capacity deteriorates greatly in each cycle, which is not preferable. Although the cause of the large deterioration of the discharge capacity in each cycle has not been clarified yet, it is considered that ethylene carbonate has an action of suppressing an irreversible electrochemical reaction on the electrode surface. In addition, ethylene carbonate is 35% by volume
When it exceeds, the discharge capacity at a low temperature (-20 ° C) is significantly deteriorated regardless of the mixing ratio of dimethyl carbonate and ethyl methyl carbonate, which is not preferable. The reason for this is not yet clear, but it is considered that when a direct current is applied as in a battery, non-uniform ion concentration is likely to occur near the electrodes, and the influence of concentration polarization is relatively large.

When the volume fraction of dimethyl carbonate is less than 10% by volume as the organic solvent used in the electrolytic solution, cycle characteristics are deteriorated, which is not preferable. On the other hand, when the content of dimethyl carbonate exceeds 85% by volume, the electrolytic solution is solidified at a low temperature and discharge becomes impossible, which is not preferable.

As the organic solvent used in the electrolytic solution, when the volume fraction of ethyl methyl carbonate is less than 5% by volume,
It is not preferable because the discharge capacity at -20 ° C is small. It is considered that this is because both ethylene carbonate and dimethyl carbonate have high freezing points of 34 ° C. and 0 ° C., and therefore the electrolytic solution is solidified even if the solute is dissolved at about 1 mol / liter. Further, when the content of ethylmethyl carbonate exceeds 80% by volume, cycle characteristics are deteriorated, which is not preferable.

When the ratio of dimethyl carbonate to the total of dimethyl carbonate and ethyl methyl carbonate in the organic solvent is represented by w, 0.15 <w <4 in terms of low temperature discharge capacity and large current discharge capacity. Is preferred.

The positive electrode in the non-aqueous electrolyte lithium secondary battery of the present invention uses a lithium composite oxide containing at least one transition metal as an active material. Specifically, as the positive electrode, an active material powder of the lithium composite oxide, an auxiliary conductive agent powder, a binder for fixing these powders, and the like are uniformly mixed and then pressure-molded, or a solvent or the like is used. An example is a composition in which it is made into a paste and applied on a current collector, dried, and pressed, and then fixed to a current collector sheet.

Examples of the lithium composite oxide containing at least one transition metal in the positive electrode include lithium composite oxide containing at least one transition metal such as vanadium, manganese, iron, cobalt and nickel. Of these, cobalt is preferable in that the average discharge potential is high.
Examples thereof include a layered lithium composite oxide having an α-NaFeO ₂ type structure such as nickel as a base material, or a lithium composite oxide having a spinel type structure such as manganese as a base material. Among them, the layered lithium composite oxide mainly composed of the lithium-nickel composite oxide is preferable in terms of excellent cycle characteristics.

As the auxiliary conductive material powder used for the positive electrode,
Has a conductive effect, resistance to the non-aqueous electrolyte used,
Any material having resistance to the electrochemical reaction at the positive electrode may be used, and examples thereof include graphite powder, carbon black, coke powder, and conductive polymers. The amount of the auxiliary conductive material is 1 to 2 with respect to 100 parts by weight of the active material powder used.
It is preferably about 0 parts by weight.

The negative electrode in the non-aqueous electrolyte lithium secondary battery of the present invention uses a carbon material mainly containing graphite as an active material. The proportion of graphite in the carbon material is preferably 70% by weight or more, more preferably 90% by weight or more. Examples of carbon materials other than graphite include carbon black and coke. As the negative electrode in the present invention, specifically, a carbon material powder mainly containing graphite, and a binder or the like for further binding the powder to each other are uniformly mixed, and then pressure-molded, or a solvent or the like is used. An example is a structure in which it is fixed to a current collector sheet by making it a paste, coating it on a current collector, drying it, and pressing it.

Examples of the graphite include natural graphite and artificial graphite.
Be done. As the graphite, specifically, a case in X-ray diffraction
Facet spacing (d₀₀₂) Is less than 3.37Å and the true specific gravity is
2.23 or more is preferable. More preferably X-ray
Lattice plane spacing in diffraction (d ₀₀₂) Is less than 3.36Å
Yes, the true specific gravity is 2.24 or more. Grid here
Surface spacing (d₀₀₂) Means that CuKα rays are used as X-rays,
X-ray diffraction method using high-purity silicon as standard material [Otani cedar
Ruro, Carbon Fiber, pp. 733-742 (1986), modern edit
Company].

The ash content of the graphite used in the present invention is preferably 0.5% by weight or less, more preferably 0.1% by weight or less. In the case of natural graphite, it depends on the production area,
Since the contained ash content is as large as several wt% or more, it is preferably treated at a high temperature of 2500 ° C. or higher, more preferably 2800 ° C. or higher, and the ash content is preferably 0.5 wt% or lower,
More preferably, it is 0.1 wt% or less. Here, ash means a value according to JIS M8812.

The artificial graphite used in the present invention is, for example, flake graphite (trade name SGP5, SGP1 manufactured by SEC).
5, SGO5, SGX5; manufactured by LONZA, trade name SF
G6, SFG15, KS6, KS15), spherical graphite (Osaka Gas Co., Ltd., trade name MCMB6-28, MCMB20-
28), fibrous graphite (trade name SG24, manufactured by Osaka Gas Co., Ltd.)
1, F500) and the like. The particle size of the graphite-based carbon material used in the present invention is not particularly limited, but an average particle size of about 1 to 50 μm is preferable. More preferably, it is 2 to 20 μm.

Examples of carbon materials other than graphite in the present invention include carbon black and coke, and so-called pseudo-graphitic carbon black obtained by heat-treating carbon black at a temperature of about 1500 to 3000 ° C.

As the binder used for the positive electrode and the negative electrode, any binder may be used as long as it has a binding effect and has resistance to the non-aqueous electrolyte used and electrochemical reaction at the positive electrode and the negative electrode. Examples thereof include fluororesins such as tetrafluoroethylene (hereinafter sometimes referred to as PTFE), polyvinylidene fluoride (hereinafter sometimes referred to as PVdF), polyethylene, polypropylene and the like. The amount of the binder is the active material powder 10 used.
It is preferably about 1 to 20 parts by weight with respect to 0 parts by weight.

The current collector used for the positive electrode and the negative electrode may be one having resistance to the non-aqueous electrolyte used and resistance to the electrochemical reaction at the positive electrode and the negative electrode, and examples thereof include nickel and titanium. Examples include stainless steel, copper and aluminum. The thickness of the current collector is preferably as thin as possible so long as the strength is maintained, from the viewpoint of increasing the volume energy density of the battery, and is preferably about 5 to 100 μm. As the current collector for the positive electrode, aluminum foil is preferable because it can be easily processed into a thin film and is inexpensive.
As a current collector for the negative electrode, it is difficult to form an alloy with lithium,
A copper foil is preferable because it can be easily processed into a thin film.

In the non-aqueous electrolyte lithium secondary battery of the present invention, the separator has a function of preventing contact between both electrodes and having an insulating property, holding the non-aqueous electrolyte and allowing lithium ions to permeate. Any material may be used as long as it has resistance to the non-aqueous electrolyte used and electrochemical resistance at the positive electrode and the negative electrode, and examples thereof include fluorinated resins, polyethylene, olefin resins such as polypropylene, nonwoven fabrics such as nylon, and woven cloth. it can. The thickness of the separator is preferably as thin as possible so that the volume energy density as a battery increases and the internal resistance decreases, so long as the mechanical strength is maintained, and preferably about 10 to 200 μm.

[0026]

EXAMPLES The present invention will now be described in more detail by way of examples, which should not be construed as limiting the invention thereto. (I) Specifications of Lithium Secondary Battery Subjected to Test As a positive electrode, lithium nickel oxide powder obtained by mixing lithium nitrate and nickel carbonate and firing at 700 ° C. for 15 hours in an oxygen stream, and artificial graphite powder, Polyvinylidene fluoride (manufactured by Kureha Chemical Co., Ltd.) in a weight ratio of 87: 10: 3
The mixed powder prepared as above was dispersed in a 2-methylpyrrolidone solution to form a slurry, which was applied onto an aluminum foil, vacuum-dried, and pressed to obtain a sheet-shaped electrode having a size of 1.5 cm × 2.0 c.
A piece cut out to a size of m was used. When X-ray diffraction measurement of the lithium nickelate powder was performed, α-Na
It was confirmed to have a FeO ₂ type structure.

As the negative electrode, scaly natural graphite powder from Madagascar treated at 3000 ° C., carbon black treated at 3800 ° C. (manufactured by Tokai Carbon Co.) and polyvinylidene fluoride (manufactured by Kureha Chemical Co., Ltd.) were used in a weight ratio of 86. : 4: 1
The mixed powder of 0 was dispersed in a 2-methylpyrrolidone solution to form a slurry, which was applied on a copper foil, dried, and pressed to prepare a sheet-shaped electrode cut into a size of 1.5 cm × 2 cm. I was there. A polypropylene porous film (manufactured by Daicel Chemical Co., Ltd., trade name Celgard # 2240) was used as the separator.

(II) Charge / Discharge Test (1) Charging: Charging is always performed at 20 ° C. Maximum charging voltage Vmax =
4.1V, 6mA, constant current constant voltage charging for 3 hours was carried out. (2) Discharge (i) Room temperature discharge test: A discharge test was performed at 20 ° C. with a cutoff voltage of 2.75 V and a constant current of 1.2 mA. This corresponds to a discharge condition of 0.2C. (Ii) Low temperature discharge test: A discharge test was carried out at −20 ° C. with a cutoff voltage of 2.75 V and a constant current of 1.2 mA. This corresponds to a discharge condition of 0.2C. (Iii) Large current discharge test: A discharge test was carried out at 20 ° C. with a cutoff voltage of 2.75 V and a constant current of 6 mA. This corresponds to the 1C discharge condition. (3) Cycle test The test was conducted at room temperature (25 ° C). Charging was performed at 20 ° C. with a maximum charging voltage Vmax of 4.1 V, 6 mA, constant current and constant voltage charging for 3 hours, and after 0.5 hour rest, discharge was performed at a cutoff voltage of 2.75 V and 1.2 mA. A discharge test was performed at a constant current, and the test was rested for 0.5 hour. This corresponds to 1C charge and 0.2C discharge. This charge / discharge cycle is 2
Repeated 0 times. The ratio of the 20th capacity to the 1st discharge capacity is shown as the capacity retention rate.

Example 1 As an electrolytic solution, ethylene carbonate (EC) and dimethyl carbonate (DM) having the compositions shown in Tables 1 and 2 were used.
Both C) and ethyl methyl carbonate (EMC) mixed solvent were prepared by using LiPF ₆ as an electrolyte so as to be 1 mol / liter, and the positive electrode and the negative electrode obtained as described above were opposed to each other through a separator, The batteries A1 to A11 were manufactured by accommodating them in a stainless steel container. The discharge capacity of the obtained battery was about 6 mAh. Table 1 shows the composition ratio of the electrolyte solution of the prepared battery and the discharge capacity under various discharge conditions.
And shown in Table 2.

[0030]

[Table 1]

[0031]

[Table 2]

Comparative Example 1 As an electrolytic solution, ethylene carbonate (EC) and dimethyl carbonate (D) having the composition ratios shown in Tables 3 and 4 were used.
MC), ethylmethyl carbonate (EMC) mixed solvent was prepared, and batteries R1 to R12 were prepared in the same manner as in Example 1 except that LiPF ₆ was used as an electrolyte so that the amount thereof was 1 mol / liter. It was made. The charge / discharge test was performed in the same manner as in Example 1, and the electrolytic solution composition ratios of the manufactured batteries and the discharge capacities are shown in Tables 3 and 4.

[0033]

[Table 3]

[0034]

[Table 4]

Comparative Example 2 A mixed solvent of ethylene carbonate (EC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC) = 30: 67: 3 (volume%) was prepared as an electrolytic solution, and 1 LiPF ₆ was used as an electrolyte. It was prepared to be mol / liter. Since this electrolytic solution solidified at -20 ° C, it was unsuitable as an electrolytic solution for a secondary battery.

The batteries A1 to A11, which are the products of the present invention, are batteries R having a composition ratio x of ethylene carbonate of 40% by volume or more.
It can be seen that the discharge capacity is remarkably excellent especially at low temperatures as compared with 1 to R6. Batteries A1 to A11 that are products of the present invention
Are batteries R9 to R1 not containing ethylene carbonate
It can be seen that the cycle characteristics are superior to those of No. 2. It is understood that the batteries A1 to A11, which are the products of the present invention, have excellent cycle characteristics as compared with the batteries R7 and R8 in which the content of dimethyl carbonate is less than 5% by volume.

Example 2 A mixed solvent of ethylene carbonate (EC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC) = 30: 35: 35 (volume%) was prepared as an electrolytic solution, and LiPF ₆ was used as an electrolyte. A battery B1 was produced in the same manner as in Example 1 except that the one prepared to be 1 mol / liter was used. The charging test was performed in the same manner as in Example 1 to measure the discharge characteristics in the operating temperature range of -20 ° C to 80 ° C. The results are shown in Table 5.

[0038]

[Table 5] It was shown to operate in a wide temperature range of -20 ° C to 80 ° C.

[0039]

INDUSTRIAL APPLICABILITY Although the non-aqueous electrolyte lithium secondary battery of the present invention is a lithium secondary battery using a graphite-based material as the negative electrode carbon material, the charge / discharge efficiency at room temperature, large energy density, and cycle A large discharge capacity is maintained even at a low temperature of −20 ° C. without deteriorating preferable characteristics such as characteristics, and an extreme reduction in discharge capacity is not brought about even when discharging a large current of 1 C or more. Furthermore, since it operates even at a high temperature of 80 ° C., it has great industrial value as a portable electronic device used outdoors or as a transportation device such as an electric vehicle.

Claims (2)

[Claims] 1. A positive electrode containing a lithium composite oxide containing at least one transition metal as an active material, a negative electrode containing a carbon material mainly containing graphite as an active material, and an electrolyte in which at least LiPF ₆ is dissolved as an electrolyte in an organic solvent. Liquid, in a non-aqueous electrolyte lithium secondary battery comprising a separator, the organic solvent contains ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, the composition ratio of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate When the ratios of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate to the total are expressed as x, y, and z (unit volume%), 10 ≦ x ≦ 35, 10 ≦ y ≦ 85, and 5 ≦ z ≦ 80. A non-aqueous electrolyte lithium secondary battery characterized by: 2. The concentration range of LiPF _{6 in} the electrolytic solution is 0.5.
The non-aqueous electrolyte lithium secondary battery according to claim 1, wherein the lithium secondary battery is about 1.5 mol / liter.