JPH0684542A - Nonaqueous electrolytic solution secondary battery - Google Patents

Abstract

PURPOSE:To reduce the lowering of a battery capacity even at a low temperature not higher than -20 deg.C by using a mixed solvent of a predetermined amount of ethylene carbonate and cyclic carbonic ester and a cyclic carbonic ester for the solvent of a nonaqueous electrolytic solution. CONSTITUTION:A mixed solvent consisted of cyclic carbonic ester of 10 to 30% by weight and non-cyclic carbonic ester of 20 to 80% by weight, which is at least one kind selected from the group of ethylene carbonate of 10 to 50% by weight, propylene carbonate and the like is used for the solvent of a nonaqueous electrolytic solution. And for non-cyclic carbonic ester, dimethyl carbonate and/or diethyl carbonate are used. For carbon materials capable of absorbing and discharging lithium, coke, preferably an organic sintered substance made by sintering refined coke of not less than 99% in purity, cellulose and the like, graphite, glassy carbon and the like are used. Thereby, the lowering of a battery capacity at a low temperature not higher than -20 deg.C can be reduced.

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, and more particularly to improvement of a solvent in a non-aqueous electrolyte solution for the purpose of improving low temperature characteristics.

[0002]

2. Description of the Related Art In recent years,
As a negative electrode material for a non-aqueous electrolyte secondary battery, a carbon material such as graphite is a conventional metallic lithium (plate or It is being investigated as a negative electrode material to replace foil). In particular, graphite is a negative electrode material that has recently been studied for practical use because it has a large amount (capacity) capable of inserting and extracting lithium.

However, since graphite has a large number of active sites as compared with coke, if an easily decomposable cyclic carbonic acid ester such as propylene carbonate or butylene carbonate is used alone as an electrolytic solution solvent for a battery of this system, Since these are adsorbed to the active sites and generate decomposition gas, insertion of lithium in graphite during charging is hindered,
At discharge, the polarization due to gas overvoltage becomes large,
It turns out that there is a problem that the battery capacity decreases.

As a result of research conducted by the present inventors, it has been found that this problem can be solved by using hard-to-decompose ethylene carbonate instead of easily-decomposable propylene carbonate or the like. In particular, it has been found that a mixed solvent of ethylene carbonate and a non-cyclic carbonic acid ester having a low boiling point has substantially excellent physical properties as an electrolytic solution solvent.

However, the nonaqueous electrolytic solution using only one kind of ethylene carbonate as the cyclic carbonic acid ester has a relatively high freezing point (melting point) and thus has poor ionic conductivity at low temperatures. Therefore, the nonaqueous electrolytic solution of this system is used. It has been found that the battery has a problem that the battery capacity is extremely reduced at a low temperature of −20 ° C. or lower.

This is because even when ethylene carbonate having a freezing point of about 39 to 40 ° C. is used as a mixed solvent with an acyclic carbonate such as dimethyl carbonate,
It is presumed that the freezing point of the non-aqueous electrolyte solution is about -20 ° C at most, even if the freezing point depression due to mixing is taken into consideration.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a non-aqueous electrolyte secondary battery which has a small decrease in battery capacity even at a low temperature of -20 ° C or lower. To provide.

[0008]

A non-aqueous electrolyte secondary battery according to the present invention (hereinafter, referred to as "invention battery") for achieving the above object is a carbon material capable of inserting and extracting lithium. A non-aqueous electrolyte secondary battery as a negative electrode material,
As a solvent for the non-aqueous electrolyte solution, 10 to 50% by weight of ethylene carbonate (A), propylene carbonate,
10 to 30% by weight of at least one cyclic carbonate (B) selected from the group consisting of butylene carbonate and vinylene carbonate, and an acyclic carbonate (C)
It is characterized in that a mixed solvent consisting of 20 to 80% by weight is used.

Examples of the carbon material capable of inserting and extracting lithium in the present invention include coke, preferably purified coke having a purity of 99% or more, an organic fired body obtained by firing cellulose, graphite, glassy carbon (glassy carbon), etc. Is mentioned. These porous carbon materials may be used alone or in combination of two or more as needed. Among them, the lattice plane (0
02) The graphite having a d value of 3.40 Å or less, or the crystallite size in the c-axis direction in X-ray diffraction of 150
Graphite having a volume of Å or more is preferable because it has a large amount (capacity) capable of inserting and extracting lithium.

These carbon materials are polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVD).
It is kneaded with a binder such as F) and used as a mixture in the negative electrode.

In the battery of the present invention, a predetermined amount of the non-aqueous electrolyte solution containing a mixed solvent of ethylene carbonate (A) and acyclic carbonic acid ester (C) as an electrolyte solution solvent at a low temperature is prevented in order to prevent a decrease in capacity. Specific cyclic carbonic acid ester (B), that is, propylene carbonate, butylene carbonate or vinylene carbonate is blended.
These cyclic carbonic acid esters (B) may be used alone or in combination of two or more as required.

Examples of the acyclic carbonic acid ester (C) include dimethyl carbonate and diethyl carbonate,
These non-cyclic carbonic acid esters (C) may be used alone or in combination of two or more if necessary.

In the present invention, the ratio of ethylene carbonate (A), cyclic carbonic acid ester (B) and acyclic carbonic acid ester (C) is 10 to 50% by volume of (A) and 1 of (B).
It is regulated within the range of 0 to 30% by weight and (C) 20 to 80% by weight.

The reason why the ratio of each solvent component is regulated in this way is that the discharge capacity at a low temperature decreases when the ratio is out of the above range, as shown in Examples described later.

As described above, the present invention is intended to reduce the decrease in discharge capacity at low temperature of the non-aqueous electrolyte secondary battery initially conceived by the present inventors using ethylene carbonate alone as the cyclic carbonic acid ester. In addition, it is characterized in that a specific cyclic carbonate having a lower freezing point than ethylene carbonate is mixed with ethylene carbonate and an acyclic carbonate at a predetermined ratio. Therefore, for other elements constituting the battery, such as electrolyte solution solute and positive electrode active material, various materials conventionally used for non-aqueous electrolyte secondary batteries or proposed can be used. Is.

For example, the electrolyte solute may be LiP.
F ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiAsF ₆ ,
LiSbF ₆ , LiC (CF ₃ SO ₂ ) ₃ , LiN (C
Typical examples are fluorine-containing lithium salts such as F ₃ SO ₂ ) ₂ . These fluorine-containing lithium salts may be used alone or in combination of two or more if necessary.

As the positive electrode active material, any compound capable of inserting and extracting lithium can be used without particular limitation. For example, oxides having so-called tunnel-shaped holes such as TiO ₂ and V ₂ O ₅ can be used. Examples thereof include metal chalcogenides having a layered structure such as TiS ₂ and MoS ₂ , and particularly preferable positive electrode active materials include the composition formula Li _x MO ₂ or L.
i _y M ₂ O ₄ (where M is a transition element, 0 ≦ x ≦ 1, 0
The complex oxide represented by ≦ y ≦ 2) is exemplified. Specific examples of such a composite oxide include LiCoO ₂ and LiMn.
_{O 2, LiNiO 2, LiCrO 2} , LiMn 2 O 4 and the like.

[0018]

In the battery of the present invention, the mixed solvent in which ethylene carbonate (A), the specific cyclic carbonic acid ester (B) and the non-cyclic carbonic acid ester (C) having a lower freezing point than that of ethylene carbonate (A) are mixed at a predetermined ratio is used as the electrolytic solution. Since it is used as a solvent, the decrease in discharge capacity at a low temperature of -20 ° C or lower is small. In addition, since the compounding ratio of the specific cyclic carbonic acid ester (B) is regulated within a specific range (10 to 30% by weight), the easily decomposable cyclic carbonic acid ester (B) and the carbon material as the negative electrode material are There is little decrease in capacity at room temperature due to the generation of decomposition gas due to the reaction.

[0019]

The present invention will be described in more detail based on the following examples, but the invention is not intended to be limited by the following examples, and various modifications can be made without departing from the scope of the invention. Is possible.

Example 1 [Preparation of Positive Electrode] LiNiO ₂ , carbon black as a conductive agent, and polyvinylidene fluoride as a binder were mixed at a weight ratio of 90: 5: 5 to prepare a positive electrode mixture. Got This positive electrode mixture was applied and rolled on an aluminum foil as a current collector, and heat-treated under vacuum at 250 ° C. for 2 hours to produce a positive electrode.

[Preparation of Negative Electrode] Natural graphite (d ₀₀₂ : 3.35Å, Lc: 2) as a negative electrode material of 400 mesh pass
000Å) and polyvinylidene fluoride as a binder were mixed in a weight ratio of 95: 5 to obtain a negative electrode mixture.
This negative electrode mixture is applied and rolled on a copper foil as a current collector, and 25
A negative electrode was prepared by heat treatment under vacuum at 0 ° C. for 2 hours.

[Preparation of Non-Aqueous Electrolyte Solution] Li as a solute in a mixed solvent of ethylene carbonate, propylene carbonate and dimethyl carbonate in a volume ratio of 1: 1: 2.
PF ₆ was dissolved at 1 mol / liter to prepare a non-aqueous electrolyte solution.

[Preparation of Battery of the Present Invention] A cylindrical battery BA1 of the present invention was prepared using the positive and negative electrodes and the non-aqueous electrolyte solution described above (battery size: diameter 14.2 mm; length 50.0 m).
m). As the separator, a microporous thin film made of polypropylene (trade name "Celguard 3401" manufactured by Polyplastics Co., Ltd.) was used.

FIG. 1 is a cross-sectional view of the manufactured battery BA1 of the present invention. The illustrated battery BA1 of the present invention includes a positive electrode 1 and a negative electrode 2, a separator 3 for separating these electrodes, a positive electrode lead 4, a negative electrode lead 5, and a positive electrode. The external terminal 6 and the negative electrode can 7 are included. The positive electrode 1 and the negative electrode 2 are housed in the negative electrode can 7 in a spirally wound state via the separator 3 into which the electrolytic solution is injected, and the positive electrode 1 is connected to the positive electrode external terminal 6 via the positive electrode lead 4. Further, the negative electrode 2 is connected to the negative electrode can 7 through the negative electrode lead 5 so that the chemical energy generated inside the battery BA1 can be taken out as electric energy to the outside.

(Comparative Example 1) Example 1 except that a solution obtained by dissolving 1 mol / liter of LiPF ₆ as a solute in an equal volume mixed solvent of ethylene carbonate and dimethyl carbonate was used as the non-aqueous electrolyte. A comparative battery BC1 was produced in the same manner as in 1.

(Comparative Example 2) Example 2 except that a solution obtained by dissolving 1 mol / liter of LiPF ₆ as a solute in an equal volume mixed solvent of propylene carbonate and dimethyl carbonate was used as the non-aqueous electrolyte. A comparative battery BC2 was produced in the same manner as in 1.

(Cycle Test) The battery BA1 of the present invention and the comparative batteries BC1 and BC2 were charged at a charging current of 200 mA at a charge current of 200 mA to a charge end voltage of 4.1 V, and then at a discharge current of 200 mA. Then, a cycle test was conducted in which the process of discharging to a discharge end voltage of 2.75 V was set as one cycle.

FIG. 2 is at room temperature, and FIG. 3 is −20.
3 is a graph showing the discharge characteristics at the initial stage of each battery at ° C, in which the vertical axis represents the battery voltage (V) and the horizontal axis represents the capacity (mAh).

From both figures, at room temperature, the discharge capacities of the battery BA1 of the present invention and the comparative battery BC1 are both 500 m.
Although there is no difference with Ah, at -20 ° C, the capacity of the battery BA1 of the present invention is reduced only to about 400 mAh, whereas the capacity of the comparative battery BC1 is reduced to about 160 mAh, which is low temperature. It can be seen that the lower capacity is large. The comparative battery BC2 had a very small discharge capacity at both room temperature (25 ° C.) and −20 ° C., and the mixed solvent of propylene carbonate and dimethyl carbonate (acyclic carbonic acid ester) was used. It can be seen that it is difficult to obtain a non-aqueous electrolyte secondary battery having a practicable capacity.

Example 2 [Preparation of Positive Electrode] LiCoO ₂ , carbon black as a conductive agent, and polyvinylidene fluoride as a binder were mixed at a weight ratio of 90: 5: 5 to prepare a positive electrode mixture. Got This positive electrode mixture was applied and rolled on an aluminum foil as a current collector, and heat-treated under vacuum at 250 ° C. for 2 hours to produce a positive electrode.

[Preparation of Negative Electrode] Natural graphite (d ₀₀₂ : 3.37Å, Lc: 3) as a negative electrode material of 400 mesh pass
00 Å), polyvinylidene fluoride as a binder,
The mixture was mixed at a weight ratio of 95: 5 to obtain a negative electrode mixture. This negative electrode mixture is applied and rolled on a copper foil as a current collector, and the temperature is set to 250 °.
A negative electrode was produced by heating at C for 2 hours under vacuum.

[Preparation of Non-Aqueous Electrolyte Solution] LiP as a solute was added to a mixed solvent of ethylene carbonate, butylene carbonate and diethyl carbonate at a volume ratio of 1: 1: 2.
F ₆ was dissolved at 1 mol / liter to prepare a non-aqueous electrolyte solution.

[Preparation of Battery of the Present Invention] A battery BA2 of the present invention was prepared in the same manner as in Example 1 except that the positive and negative electrodes and the non-aqueous electrolyte solution described above were used.

(Comparative Example 3) Example 3 except that a solution obtained by dissolving 1 mol / liter of LiPF ₆ as a solute in an equal volume mixed solvent of ethylene carbonate and butylene carbonate was used as the non-aqueous electrolyte. A comparative battery BC3 was produced in the same manner as in 2.

(Comparative Example 4) Example 4 except that a solution obtained by dissolving 1 mol / liter of LiPF ₆ as a solute in an equal volume mixed solvent of butylene carbonate and diethyl carbonate was used as the non-aqueous electrolyte solution. A comparative battery BC4 was produced in the same manner as in 2.

(Cycle Test) The battery BA2 of the present invention and the comparative batteries BC3 and BC4 were subjected to a cycle test under the same conditions as in the previous cycle test at room temperature and −20 ° C.

FIG. 4 is at room temperature, and FIG. 5 is -20.
6 is a graph showing the discharge characteristics at the initial stage of each battery at ° C, with the vertical axis representing battery voltage (V) and the horizontal axis representing capacity (mAh).

From both figures, at room temperature, the discharge capacities of the battery BA2 of the present invention and the comparative battery BC3 are both 500 m.
At about -20 ° C, the capacity of the battery BA2 of the present invention is reduced to about 430 mAh, but the capacity of the comparative battery BC3 is reduced to about 200 mAh at -20 ° C. It can be seen that the lower capacity is large. The comparative battery BC4 had a very small discharge capacity at both room temperature and −20 ° C., and when a mixed solvent of butylene carbonate and diethyl carbonate (acyclic carbonic acid ester) was used, a practically usable capacity was obtained. It can be seen that it is difficult to obtain the non-aqueous electrolyte secondary battery of the present invention.

(Examples 3 to 17 and Comparative Examples 5 to 38) The following Tables 1 and 2 differ in the mixing ratio of ethylene carbonate, propylene carbonate and dimethyl carbonate.
Inventive batteries BA3 to BA17 and comparative batteries BC5 to BC38 were produced in the same manner as in Example 1 except that the mixed solvent having the composition shown in was used. In both tables, EC, P
C and DMC represent ethylene carbonate, propylene carbonate and dimethyl carbonate, respectively.

[0040]

[Table 1]

[0041]

[Table 2]

(Cycle test) Batteries BA3 to BA of the present invention
For 17 and comparative batteries BC5 to BC38, a cycle test was performed at -20 ° C under the same conditions as in the previous cycle test.

6 to 13 show the discharge characteristics of each battery at -20.degree. C. at the beginning of the cycle, and the vertical axis shows the discharge capacity (mA).
3 is a graph showing h) with the horizontal axis representing the weight mixing ratio (%) of the mixed solvent.

From these figures, ethylene carbonate,
Battery BA of the present invention in which the mixing ratio of propylene carbonate and dimethyl carbonate is within the range regulated by the present invention
3 to BA17 have a larger discharge capacity at low temperature than the comparative batteries BC5 to BC38 in which the mixing ratio of these is out of the range regulated by the present invention.

In the above embodiments, the present invention has been described by taking the cylindrical battery as an example, but the shape of the battery is not particularly limited,
INDUSTRIAL APPLICABILITY The present invention can be applied to non-aqueous electrolyte secondary batteries of various shapes such as flat type and square type.

[0046]

INDUSTRIAL APPLICABILITY The battery of the present invention exhibits excellent peculiar effects such as a small decrease in capacity at low temperatures.

[Brief description of drawings]

FIG. 1 is a sectional view of a cylindrical battery BA1 of the present invention.

FIG. 2 is a discharge characteristic diagram at room temperature.

FIG. 3 is a discharge characteristic diagram at −20 ° C.

FIG. 4 is a discharge characteristic diagram at room temperature.

FIG. 5 is a discharge characteristic diagram at −20 ° C.

FIG. 6 is a discharge characteristic diagram at −20 ° C.

FIG. 7 is a discharge characteristic diagram at −20 ° C.

FIG. 8 is a discharge characteristic diagram at −20 ° C.

FIG. 9 is a discharge characteristic diagram at −20 ° C.

FIG. 10 is a discharge characteristic diagram at −20 ° C.

FIG. 11 is a discharge characteristic diagram at −20 ° C.

FIG. 12 is a discharge characteristic diagram at −20 ° C.

FIG. 13 is a discharge characteristic diagram at −20 ° C.

[Explanation of symbols]

 BA1 Cylindrical battery 1 of the present invention 1 positive electrode 2 negative electrode 3 separator

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Toshihiko Saito 2-18 Keihan Hondori, Moriguchi City, Osaka Sanyo Electric Co., Ltd. (72) Inventor Koji Nishio 2-18 Keihan Hondori, Moriguchi City, Osaka Sanyo Electric Within the corporation

Claims (3)

[Claims] 1. A non-aqueous electrolyte secondary battery using a carbon material capable of inserting and extracting lithium as a negative electrode material, wherein ethylene carbonate (A) 10-5 is used as a solvent for the non-aqueous electrolyte.
0 to 30% by weight, and at least one cyclic carbonate (B) 10 to 30 selected from the group consisting of propylene carbonate, butylene carbonate and vinylene carbonate.
% By weight, and 20-80% by weight of acyclic carbonate (C)
A non-aqueous electrolyte secondary battery, wherein a mixed solvent consisting of and is used. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-cyclic carbonic acid ester (C) is dimethyl carbonate and / or diethyl carbonate. 3. The carbon material has a d-value of a lattice plane (002) plane of 3.40Å or less in X-ray diffraction, or a crystallite size Lc in the c-axis direction in X-ray diffraction.
Is 150 Å or more, The non-aqueous electrolyte secondary battery according to claim 1.