JP2000030740A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery with high battery characteristics, especially high charge/discharge cycle characteristics, high fire resistance, and capable of producing at low cost. SOLUTION: This lithium secondary battery consists of a positive electrode capable of absorbing/releasing lithium, a negative electrode using a graphite base carbon material as a negative active material, and an organic electrolyte prepared by dissolving a lithium salt in an organic solvent, and the organic solvent contains 15-50 vol.% ethylene carbonate based on 100 vol.% of the total volume, and also contains 0.5-2.5 vol.% phosphazene compound.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a lithium secondary battery using a graphite-based carbon material as a negative electrode active material,
More specifically, the present invention relates to a device that can be used for a battery of an automobile.

[0002]

2. Description of the Related Art A lithium secondary battery using a graphite-based carbon material as a negative electrode active material has a high output voltage and a large discharge capacity, and thus is used in the field of electronic devices such as portable personal computers, mobile phones, and video cameras. And has been widely used. The electrolyte of such a lithium secondary battery has a high dielectric constant, a high chemical stability and electrochemical stability to the electrode, a low viscosity so as not to hinder the movement of lithium ions, and An organic electrolyte solution in which a lithium salt is dissolved in an organic solvent is generally required, for example, high safety is required.

[0003] The high dielectric constant is considered with priority because it relates to the basic performance of the battery. Conventionally, ethylene carbonate (E) has been used as an organic solvent in an organic electrolytic solution.
Those having a high dielectric constant solvent such as C) as a main component are widely used. In particular, such an organic electrolytic solution contains diethyl carbonate (DEC) in order to reduce the viscosity of the solvent.
For example, those containing a low-viscosity solvent as the second component are often used. Such an organic electrolyte can be used for any battery using a graphite-based carbon material or a non-graphite-based carbon material as a negative electrode active material. In particular, in batteries using highly reactive graphite-based carbon material as the negative electrode active material, EC has excellent chemical and electrochemical stability to the electrodes, and is extremely effective in obtaining excellent battery performance. It is known that

It is desirable to use a graphite-based carbon material for the negative electrode of a lithium secondary battery that can be mounted on an automobile because of its excellent voltage flatness and large current performance. It is necessary to provide a stable and inexpensive electrolyte solution for such a graphite-based carbon material. Furthermore, when using a lithium secondary battery for an automobile, a battery excellent in noncombustibility or flame retardancy is required from the viewpoint of safety. However, in a lithium secondary battery using an organic electrolyte containing EC as a main component, the organic electrolyte may be ignited. Among them, DEC
Since the flash point of such a low-viscosity solvent is near room temperature, there is a risk of ignition due to the organic electrolyte.

[0005] As a method of making such an organic electrolyte flame-retardant, a method of introducing a fluorine atom into the organic electrolyte solution (JP-A-10-50343 and JP-A-10-12272).
) And phosphazene compounds having a -PN- bond (Japanese Patent Laid-Open No. 6-13108 and Reference Journa).
l of Electrochemical soci
et al., Vol. 143, p. 209, 1996). However, in the former method, there is a problem that complicated synthesis needs to be performed in order to introduce a fluorine atom into an organic solvent, and the cost increases. In the latter method, when a phosphazene compound is used, JP-A-6-13108 discloses that
There is a statement that the invention can be applied to batteries using a graphite-based carbon material as a negative electrode active material, but as a result of additional tests by the present inventors according to the description of the embodiment, it was found that the graphite-based carbon material and the phosphazene compound were sufficient. It was found that the charge / discharge efficiency decreased as the charge / discharge cycle was repeated, for example, by reacting during the discharge.

As described above, none of the conventional lithium secondary batteries has all of excellent charge / discharge cycle characteristics, flame retardancy and low cost.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has excellent battery characteristics, in particular, charge-discharge cycle characteristics and flame retardancy, and can be manufactured at low cost. The task is to provide

[0008]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have studied the solvent components used for the electrolyte in a lithium secondary battery using a graphite-based carbon material as a negative electrode active material, and have particularly studied the EC and EC. With respect to the organic electrolyte containing a phosphazene compound, the optimum composition was studied diligently. as a result,
We found the following: The contents of EC and the phosphazene compound are values when the total amount of the organic electrolyte is 100% by volume.

If the content of EC is less than 15% by volume, not only the dielectric constant of the electrolytic solution is low, and sufficient battery characteristics cannot be obtained, but also the gap between the negative electrode active material of the graphite-based carbon material and the organic electrolytic solution. The decomposition reaction easily occurs. Therefore, the charge / discharge efficiency is reduced while the charge / discharge is repeated. Also,
When the content of EC exceeds 50% by volume, EC precipitates at a temperature lower than around -10 ° C, or the electrolytic solution itself freezes. For this reason, the charge / discharge efficiency in a low temperature range is reduced.

On the other hand, when the content of the phosphazene compound is 0.5
When the content is less than the volume%, an organic electrolyte having sufficient flame retardancy cannot be obtained. Further, the content of the phosphazene compound is 2.
If it exceeds 5% by volume, the decomposition reaction on the negative electrode surface during charge and discharge increases. For this reason, the charge / discharge efficiency is greatly reduced during repeated charge / discharge. Thus, the present inventors have solved the above problems by using an organic electrolyte containing EC in the range of 15 to 50% by volume and a phosphazene compound in the range of 0.5 to 2.5% by volume. I finally found something that could be solved. The present invention has been made based on such findings.

That is, the lithium secondary battery of the present invention is:
In a lithium secondary battery including a positive electrode capable of inserting and extracting lithium ions, a negative electrode in which a graphite-based carbon material is used as a negative electrode active material, and an organic electrolyte in which a lithium salt is dissolved in an organic solvent, The organic electrolyte,
Assuming that the total amount is 100% by volume, ethylene carbonate is contained in the range of 15 to 50% by volume, and the phosphazene compound is contained in the range of 0.5 to 2.5% by volume.

The lithium secondary battery of the present invention has battery characteristics,
In particular, it is excellent in charge / discharge cycle characteristics and flame retardancy, and can be manufactured at low cost. The reason is considered as follows. The content of EC contained in the organic electrolyte is 1
Since the content is limited to the range of 5 to 50% by volume, a sufficiently high dielectric constant is obtained in the organic electrolytic solution, and a decomposition reaction between the negative electrode active material and the organic electrolytic solution hardly occurs. Further, excellent charge / discharge efficiency can be obtained even in a low temperature range.

On the other hand, when the content of the phosphazene compound is 0.5
Since the content is limited to the range of about 2.5% by volume, sufficient flame retardancy is obtained in the organic electrolytic solution, and the decomposition reaction on the negative electrode surface during charge / discharge is not increased. Also,
It is presumed that the occurrence of solvation of the phosphazene compound by EC does not cause loss of the high flame retardancy of the phosphazene compound and suppresses its reactivity with the graphite-based carbon material.

A phosphazene compound is added in an amount of 0.5 to the organic electrolyte.
In order to make the content within the range of up to 2.5% by volume, a complicated synthesis method or the like is not required, and it is only necessary to simply mix with other components. Therefore, the organic electrolyte used in the present invention can be prepared at very low cost.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION A lithium secondary battery of the present invention comprises a positive electrode, a negative electrode, an electrolytic solution interposed between the electrodes,
It is basically composed of a battery case containing these. The electrode structure (laminated structure) is not particularly limited, and may be a known electrode structure.
For example, using a flat electrode plate for both the positive and negative electrodes,
Those obtained by laminating them with a plate-shaped separator interposed therebetween are also mentioned. Batteries having this electrode structure include coin-type batteries and prismatic batteries. In addition, there is also an example in which a strip-shaped electrode plate is used for each of the positive electrode and the negative electrode, and a separator is interposed therebetween and wound in a substantially columnar shape.

Hereinafter, embodiments of the lithium secondary battery of the present invention will be described for each component. The positive electrode is not particularly limited in its form as long as it can occlude and release lithium ions.A known form can be used, but it is preferable to use the following form . The type of the positive electrode active material is not particularly limited, and a known positive electrode active material can be used.
It is preferable to use a lithium transition metal composite oxide such as iCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ . By using such a lithium transition metal composite oxide as the positive electrode active material, extremely excellent battery characteristics can be obtained.

It is preferable to use an electrode body in which a positive electrode active material layer mainly composed of a positive electrode active material is uniformly formed on the surface of a suitable current collector. About the formation method,
There is no particular limitation, and it can be formed by a known forming method. On the other hand, the negative electrode is not particularly limited in its form as long as the graphite-based carbon material is used as the negative electrode active material and can absorb and release lithium ions released from the positive electrode. It is preferable to use the form.

As for the type of the negative electrode active material, natural graphite,
It is limited to those containing a graphite-based carbon material such as artificial graphite. As such a negative electrode active material, a material in which a non-graphite-based carbon material such as hard carbon or heat-treated coke is mixed with a graphite-based carbon material may be used. Alternatively, a graphite-based carbon material whose surface is coated with an amorphous carbon material may be used. As a raw material of such a graphite-based carbon material, a spherical, flaky, fibrous, etc. graphite-based carbon material can be used.

As for the negative electrode, it is preferable to use an electrode body in which a negative electrode active material layer mainly composed of a negative electrode active material is uniformly formed on the surface of an appropriate current collector. The formation method is not particularly limited, and it can be formed by a known formation method. The organic electrolyte solution most characteristic of the present invention is a solution in which a lithium salt is dissolved in an organic solvent, and when the total amount is 100% by volume,
EC is contained in the range of 15 to 50% by volume, and the phosphazene compound is contained in the range of 0.5 to 2.5% by volume. Preferably, the organic electrolyte has the following form.

The lithium salt is not particularly limited, and may be LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAs
Known lithium salts such as F ₆ , LiCF ₃ SO ₃ , and lithium trifluoromethanesulfonimide can be used. An organic electrolyte containing a plurality of these lithium salts may be used. On the other hand, the phosphazene compound includes a cyclic compound and a chain compound, and any of the compounds may be used in the lithium secondary battery of the present invention.

When the former phosphazene compound is used, it is preferable to use a cyclic trimer or cyclic tetramer (n = 3, 4) phosphazene compound represented by the chemical formula 1.

[0022]

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The side chain X of the P atom has an alkoxy group. However, an alkoxy group having 5 or more carbon atoms is preferably an alkoxy group having 1 to 4 carbon atoms because it is easily decomposed during charge and discharge. Specific examples of such an alkoxy group having 1 to 4 carbon atoms include a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group, a butoxy group, a SEC-butoxy group and a tert-
Butoxy group and the like. Further, an alkoxy group in which a part of the alkoxy group is fluorinated can also be used. on the other hand,
Chlorine or bromine can be considered as the side chain of the P atom, but it is not preferable because the reduction potential is low and it is easily decomposed.

When the latter phosphazene compound is used, it is preferable to use a chain phosphazene compound represented by the chemical formula 2.

[0025]

Embedded image

The side chain Y of the P atom may be the same as X in Formula 1. However, when the number m of repeating units in Chemical Formula 2 is 5 or more, it becomes difficult to dissolve this phosphazene compound in an electrolytic solution. The remaining solvent components of the EC and the phosphazene compound in the organic electrolyte are not particularly limited in their types, but DEC which is a low-viscosity solvent, dimethyl carbonate, ethyl methyl carbonate, dimethoxymethane, 2-methylhydrofuran It is preferable to include the like. An organic electrolyte containing a plurality of these organic solvents may be used.

Further, a high dielectric constant solvent such as sulfolane, gamma-butyrolactone, propylene carbonate and the like can be contained. A solvent containing a plurality of these organic solvents may be used. The battery case is not particularly limited in its form, and a known battery case can be used.

In the lithium secondary battery of the present invention, if the distance between the positive electrode and the negative electrode can be maintained, it is not always necessary to interpose these separators. By interposing, the lamination of those electrode plates can be facilitated. If a microporous separator made of polyethylene or polypropylene is used for the separator, a current shut-down function can be obtained, and the safety of the battery can be improved.

[0029]

The present invention will be described below in detail with reference to examples. (Example 1) In this example, cyclic hexaethoxytricyclophosphazene (H
An organic electrolyte solution containing (ETCPN) was prepared as follows. Hexachlorotricyclophosphazene (Fluka)
And sodium ethylate (manufactured by Wako Pure Chemical Industries, Ltd.) were reacted in tetrahydrofuran (THF) while refluxing THF to synthesize HETCPN.

HETCPN, EC,
DEC and LiPF ₆ (manufactured by Toyama Pharmaceutical Co., Ltd.) were mixed at a predetermined ratio, and EC and HET were mixed as shown in Table 1.
Nine types of organic electrolytes having different CPN contents were prepared. Note that all of these nine types of organic electrolytes were LiPF ₆
Is 1 mol / l and the content of DEC is 50% by volume. These organic electrolytes were used as samples 11 to 1
It was set to 9. [Flame retardancy test] length 120mm, thickness 25μm, width 8m
and 9 polyethylene separators (product name: Cetera, manufactured by Tonen Chemical Co., Ltd.). After these separators were vacuum-dried at 50 ° C. to sufficiently remove water, each separator was immersed in the above-mentioned 9 types of organic electrolytes (samples 1 to 9), and the organic electrolytes were respectively applied to the separators. Impregnated. Subsequently, each of these separators was taken out to the atmosphere, and one end was fixed and hung, and then fired by a match. After the fire was extinguished, the remaining separator was sufficiently washed with dimethoxyethane, and the weight after drying was measured. The measurement results are also shown in Table 1. In this flame retardancy test, it can be said that those having a residual separator amount of 8.5 mg or more have excellent flame retardancy. [Charge / Discharge Cycle Characteristics] Using the above nine kinds of organic electrolytes (samples 11 to 19), lithium secondary batteries were produced as follows. In the following description, “part” means a weight ratio.

18.5 parts of lithium manganese composite oxide (manufactured by Honjo Chemical), 1.5 parts of carbon black (manufactured by Tokai Carbon), polyvinylidene fluoride (PVDF)
8 parts of powder (manufactured by Kureha Chemical Co., Ltd.) and 72 parts of N-methylpyrrolidone (manufactured by Wako Pure Chemical Industries, Ltd.) were prepared, and these were sufficiently mixed to obtain a slurry for a positive electrode. This positive electrode slurry was applied on a 20 μm-thick aluminum foil to obtain a positive electrode material.

On the other hand, 95 parts of artificial graphite (MCMB) (manufactured by Osaka Gas Chemical) and 5 parts of PVDF were prepared, and these were dissolved in 100 parts of N-methylpyrrolidone to obtain a slurry for a negative electrode. This negative electrode slurry was applied on a copper foil to obtain a negative electrode material. Next, nine disk-shaped positive and negative electrode plates each having a diameter of 17 mm were punched from the above-described positive electrode material and negative electrode material. A polyethylene separator was sandwiched between these disc-shaped electrode plates. The above nine kinds of organic electrolytes (samples 11 to 1)
9) were respectively impregnated to produce 9 types of lithium secondary batteries.

For each lithium secondary battery, 1 mA / c
After charging to 4.2 V at a constant current of m ² , charging was further continued at a constant voltage of 4.2 V. Each of the charged batteries was discharged at a constant current of 0.5 mA / cm ² , and the initial discharge capacity and the discharge capacity after 10 cycles were measured. Table 1 also shows the discharge capacity of each battery after 10 cycles. The initial discharge capacity of each battery is 39 to
It was 72 mAh / g. In this charge / discharge test, it can be said that those having a discharge capacity after 45 cycles of 45 mAh / g or more have excellent charge / discharge cycle characteristics.

[0034]

[Table 1] Table 1 shows that the lithium secondary batteries using the organic electrolytes of Samples 13 to 15 are excellent in both charge / discharge cycle characteristics and flame retardancy. In all of the organic electrolytes of these lithium secondary batteries, the total amount of the organic electrolyte was 100% by volume.
Then, EC is contained in the range of 15 to 50% by volume, and the phosphazene compound (HETCPN) is contained in the range of 0.5 to 2.
It is contained within the range of 5% by volume. Comparative Example 1 HETCP synthesized in the same manner as in Example 1.
N and DEC were mixed at a volume ratio of 1: 1 to obtain an organic solvent. Using this organic solvent, in the same manner as in Example 1,
The concentration of LiPF ₆ is 1 mol / l and the DE
An organic electrolyte solution having a C content of 50% by volume was prepared.

With respect to the organic electrolyte thus prepared,
A flame retardancy test was performed in the same manner as in Example 1. Table 2 shows the results. Further, a lithium secondary battery was prepared using this organic electrolyte in the same manner as in Example 1, and a charge / discharge test was performed. Table 2 also shows the discharge capacity at the tenth cycle.
The initial discharge capacity was 32 mAh / g.

Table 2 shows that the lithium secondary battery of this comparative example exhibits high flame retardancy, but is inferior in charge / discharge cycle characteristics.

[0037]

[Table 2] (Example 2) In this example, cyclic hexa (trifluoroethoxy) tricyclophosphazene (HFETCP) was used as the phosphazene compound instead of HETCPN.
An organic electrolyte solution identical to the organic electrolyte solution of Example 1 except that N) was used was prepared as follows.

By reacting trifluoroethanol (Wako Pure Chemical) with sodium hydride (Wako Pure Chemical), sodium trifluoroethoxide was obtained. This sodium trifluoroethoxide and hexachlorotricyclophosphazene (manufactured by Fluka) were reacted in THF while refluxing THF to synthesize HFETCPN. HFET CPN, EC, DEC and LiPF thus synthesized
₆ (manufactured by Toyama Pharmaceutical Co., Ltd.) were mixed at predetermined ratios to prepare eight kinds of organic electrolytes having different contents of EC and HFETCPN as shown in Table 3. In each of these eight types of organic electrolytes, the concentration of LiPF ₆ was 1 mol / mol.
Liter and the content of DEC is 50% by volume. These organic electrolytes were used as Samples 21 to 28.

Each of the thus prepared organic electrolytes (sample 2
1 to 28), a flame retardancy test was conducted in the same manner as in Example 1. Table 3 shows the results. Further, a lithium secondary battery was produced using each of these organic electrolytes in the same manner as in Example 1, and a charge / discharge test was similarly performed on each battery. Table 3 also shows the discharge capacity at the 10th cycle of each battery. The initial discharge capacity of each battery was 33 to
It was 72 mAh / g.

[0040]

[Table 3] Table 3 shows that all of the lithium secondary batteries using each of the organic electrolytes of Samples 22 to 25 have excellent charge / discharge cycle characteristics and flame retardancy. All of the organic electrolytes of these lithium secondary batteries contain EC in the range of 15 to 50% by volume, and the phosphazene compound (HFETCP) when the total amount of the organic electrolyte is 100% by volume.
N) in the range of 0.5 to 2.5% by volume. (Comparative Example 2) HFETC synthesized in the same manner as in Example 2
PN and DEC were mixed at a volume ratio of 1: 1 to obtain an organic solvent. Using this organic solvent, an organic electrolytic solution having a LiPF ₆ concentration of 1 mol / liter and a DEC content of 50% by volume was prepared in the same manner as in Example 1.

With respect to the organic electrolyte thus prepared,
A flame retardancy test was performed in the same manner as in Example 1. Table 2 shows the results. Further, a lithium secondary battery was prepared using this organic electrolyte in the same manner as in Example 1, and a charge / discharge test was performed. Table 2 also shows the discharge capacity at the tenth cycle.
The initial discharge capacity was 25 mAh / g.

Table 2 shows that the lithium secondary battery of this comparative example exhibited high flame retardancy, but was inferior in charge / discharge cycle characteristics. Comparative Example 3 In this comparative example, an organic solvent was obtained by mixing EC and DEC at a volume ratio of 1: 1. LiBF4 was dissolved in this organic solvent at 1 mol / liter to prepare an organic electrolyte.

With respect to the organic electrolyte thus prepared,
A flame retardancy test was performed in the same manner as in Example 1. Table 2 shows the results. Further, a lithium secondary battery was prepared using this organic electrolyte in the same manner as in Example 1, and a charge / discharge test was performed. Table 2 also shows the discharge capacity at the tenth cycle.
The initial discharge capacity was 73 mAh / g.

Table 2 shows that the lithium secondary battery of this comparative example had excellent charge / discharge cycle characteristics, but was inferior in flame retardancy. (Example 3) In this example, the same organic electrolyte as the organic electrolyte of Example 1 except that a chain-like dibutoxyphosphonylphosphoric tributoxide (BPPTB) was used as the phosphazene compound instead of HETCPN. Was prepared as follows.

Ammonium sulfate (manufactured by Wako Pure Chemical) and phosphorus pentachloride (manufactured by Wako Pure Chemical) are reacted in tetrachloroethane (Wako Pure Chemical) heated to 134 ° C. to produce chlorophosphonylphosphoric trichloride (CPPTC). I got
This CPPTC and sodium butoxide (Hanai Chemical)
Is reacted in THF while refluxing THF to obtain BP.
PTB was synthesized.

The BPPTB, EC, D thus synthesized
EC and LiBF4 (manufactured by Toyama Pharmaceutical Co., Ltd.) were mixed at predetermined ratios, and as shown in Table 4, EC and BPPT were mixed.
Six types of organic electrolytes having different B contents were prepared. In addition,
All of these six types of organic electrolytes have a LiPF ₆ concentration of 1 mol / liter and a DEC content of 70 mol / l.
% By volume. These organic electrolytes were designated as Samples 31 to 36.

Each of the thus prepared organic electrolytes (sample 3
1 to 36), a flame retardancy test was performed in the same manner as in Example 1. Table 4 shows the results. Further, a lithium secondary battery was produced using each of these organic electrolytes in the same manner as in Example 1, and a charge / discharge test was similarly performed on each battery. Table 4 also shows the discharge capacity at the 10th cycle of each battery. The initial discharge capacity of each battery is 45 to 45.
It was 74 mAh / g.

[0048]

[Table 4] Table 4 shows that the lithium secondary batteries using each of the organic electrolytes of Samples 33 to 35 are excellent in both cycle characteristics and flame retardancy. Assuming that the total amount of the organic electrolyte of these lithium secondary batteries is 100% by volume, E
C is contained in the range of 15 to 50% by volume, and the phosphazene compound (BPPTB) is contained in the range of 0.5 to 2.5% by volume. (Comparative Example 4) The CPPTC (substituted with chlorine) obtained in Example 3 was mixed with EC, DEC and LiBF4 (manufactured by Toyama Pharmaceutical Co., Ltd.) at a predetermined ratio, respectively, to give an EC content of 25% by volume. And an organic electrolyte solution having a CPPTC content of 5% by volume was prepared. In this organic electrolyte, the concentration of LiBF ₄ was 1 mol / liter, and the content of DEC was 70% by volume.

With respect to the organic electrolyte thus prepared,
A flame retardancy test was performed in the same manner as in Example 1. Table 2 shows the results. Further, a lithium secondary battery was fabricated using the organic electrolyte in the same manner as in Example 1, and a similar charge / discharge test was performed. Table 2 also shows the discharge capacity at the tenth cycle. The initial discharge capacity was 49 mAh / g.

From Table 2, it can be seen that the lithium secondary battery of this comparative example shows high flame retardancy, but is considerably inferior in cycle characteristics. (Comparative Example 5) An organic electrolyte solution similar to that of Comparative Example 4 was prepared except that the content of CPPTC was 0.1%.

With respect to the organic electrolyte thus prepared,
A flame retardancy test was performed in the same manner as in Example 1. Table 2 shows the results. Further, a lithium secondary battery was fabricated using the organic electrolyte in the same manner as in Example 1, and a similar charge / discharge test was performed. Table 2 also shows the discharge capacity at the tenth cycle. The initial discharge capacity was 63 mAh / g.

Table 2 shows that the lithium secondary battery of this comparative example exhibited high flame retardancy, but was considerably inferior in cycle characteristics. (Example 4) The same organic electrolytic solution as in Example 3 was used except that di-tert-butoxyphosphonylphosphoric di (tert-butoxide) was used instead of BPPTB as the phosphazene compound. Was prepared. In this example, the content of tert-butoxide was 0.5% by volume.

With respect to the organic electrolyte thus prepared,
A flame retardancy test was performed in the same manner as in Example 1. Table 5 shows the results. Further, a lithium secondary battery was fabricated using the organic electrolyte in the same manner as in Example 1, and a similar charge / discharge test was performed. Table 5 also shows the discharge capacity at the tenth cycle. The initial discharge capacity was 60 mAh / g.

[0054]

[Table 5] Table 5 shows that the lithium secondary battery using this organic electrolyte is excellent in both charge / discharge cycle characteristics and flame retardancy. (Example 5) As a phosphazene compound, HETCPN
Is replaced by a linear polyphosphonic ethoxide (P
An organic electrolyte solution identical to the organic electrolyte solution of Example 1 except that PPET was used was prepared as follows.

Hexachlorotricyclophosphazene (Fluka) was thermally polymerized using ammonium persulfate as a catalyst. After dissolving the obtained polymer in benzene and filtering the insoluble reaction product, the filtrate (benzene solution)
And add sodium ethylate (Wako Pure Chemicals)
PPPET was synthesized. PPPE synthesized in this way
T, EC, DEC and LiBF ₄ (Toyama Pharmaceutical)
Are mixed at predetermined ratios, and E is mixed as shown in Table 4.
Five kinds of organic electrolytes having different contents of C and BPPTB were prepared. Note that these five types of organic electrolytes are all L
The concentration of iPF ₆ is 1 mol / l and DEC
Is 70% by volume. These organic electrolytes were used as samples 51 to 55.

Each of the thus prepared organic electrolytes (sample 5
1 to 55), a flame retardancy test was performed in the same manner as in Example 1. Table 6 shows the results. Further, a lithium secondary battery was produced using each of these organic electrolytes in the same manner as in Example 1, and a charge / discharge test was similarly performed on each battery. Table 6 also shows the discharge capacity at the tenth cycle.
The initial discharge capacity of each battery was 35 to 75 mA.
h / g.

[0057]

[Table 6] Table 6 shows that the lithium secondary batteries using each of the organic electrolytes of Samples 51 to 55 are excellent in both cycle characteristics and flame retardancy. Assuming that the total amount of the organic electrolyte of the lithium secondary battery is 100% by volume, the organic electrolyte contains EC in a range of 15 to 50% by volume and a phosphazene compound (PPPET) of 0.5 to 2.5% by volume. It is included within the range.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor: Noritake Takeichi 41-cho, Chuchu-ji, Yokomichi, Nagakute-cho, Aichi-gun, Aichi F-term in Toyota Central Research Laboratory, Inc. (reference) 4H028 AA38 BA05 5H029 AJ02 AJ05 AJ12 AJ14 AK03 AL07 AM02 AM03 AM04 AM05 AM07 BJ02 BJ03 BJ14 DJ08 EJ11 HJ07

Claims (1)

[Claims] 1. A positive electrode capable of occluding and releasing lithium ions, a negative electrode in which a graphite-based carbon material is used as a negative electrode active material, and an organic electrolyte in which a lithium salt is dissolved in an organic solvent. Assuming that the total amount is 100% by volume, the organic electrolytic solution contains ethylene carbonate in a range of 15 to 50% by volume and a phosphazene compound in a range of 0.5 to 50% by volume.
A lithium secondary battery, which is contained in a range of 2.5% by volume.