JP2000323169A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a charge / discharge efficiency by suppressing a reaction between a carbon material and an electrolytic solution, and to use a solvent which cannot be applied to a lithium secondary battery using a carbon material as an electrode active material as an electrolytic solution. An easy-to-use method that can be applied at low cost, and a lithium secondary battery with excellent temperature characteristics and flame retardancy. SOLUTION: In a lithium secondary battery containing a carbon material as an active material and having a non-aqueous electrolytic solution in which a lithium salt is dissolved in an organic solvent, the non-aqueous electrolytic solution has a general formula shown in FIG. R _{1 to} R ₅ in the formula (I) are any of H, F, CF ₃ and at least one is F or CF ₃ , and R _H is C _n H _{2n +1} (1 ≦ n ≦ 3), which contains a fluorinated or trifluoromethylated benzoate compound.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery having excellent temperature characteristics.

[0002]

2. Description of the Related Art Among secondary batteries, lithium secondary batteries are characterized by being light in weight, having a high charge / discharge voltage, and having a large charge / discharge capacity, and can be used for various purposes. It is expected as the next battery. Conventional lithium secondary batteries mainly use lithium metal as a negative electrode. On the other hand, in recent years, a carbon material has attracted attention as a new active material. Graphite, coke, and the like are typical examples of the carbon material.

[0003]

[Problems to be solved] By the way, the above carbon materials are
It is easy to decompose the electrolyte composed of the organic solvent. In particular, propylene carbonate (propylene carbonate) with excellent temperature characteristics
When an electrolytic solution containing carbon is used, there is a problem that propylene carbonate reacts violently with the carbon material and the capacity is greatly reduced.

[0004] In order to solve this problem, an electrolytic solution containing ethylene carbonate (ethylene carbonate) as a main solvent is generally used as a solvent instead of propylene carbonate. An electrolyte containing ethylene carbonate as a main solvent has relatively low reactivity with a carbon material and is excellent in battery capacity and cycle characteristics. However, ethylene carbonate
Since it has the property of being solid at room temperature, ethylene carbonate precipitates or the electrolyte solidifies at low temperatures, causing a problem that the charge / discharge characteristics at low temperatures are degraded.

[0005] Further, even when an attempt is made to use a highly flame-retardant molecule such as a phosphorous-containing molecule represented by a phosphoric ester or a molecule containing a halogen atom in the molecule as an electrolyte solution solvent, Again, the intense reaction with the carbon material causes problems such as a decrease in discharge capacity and deterioration in cycle characteristics.

As means for suppressing the reaction between the carbon material and the electrolyte solvent, the following methods are known. For example, Japanese Patent Application Laid-Open Nos. 4-23738 and 5-12
No. 1066, Japanese Unexamined Patent Publication No. 9-237638, etc., amorphous carbon is attached to the surface of graphite to cover a highly reactive site on the graphite surface, thereby suppressing the reaction with the electrolyte solvent. There is a technique to do.

However, such a method requires a complicated process of attaching amorphous carbon to the surface of graphite as a carbon material, and has a problem in cost. Another problem is that it is difficult to completely cover the graphite surface with amorphous carbon.

[0008] Also, Japanese Patent Application Laid-Open No. 10-255836,
As disclosed in Japanese Patent Application Laid-Open No. 9-22722, benzoic acid ester is added to an electrolytic solution or applied as an electrolytic solution solvent, and the carbon material is stabilized by a film formed on the surface of the carbon material to improve cycle characteristics. There is a technique called. However, in this case, for example, when a highly reactive solvent such as propylene carbonate is used, it is not enough to suppress the reaction between the carbon material and propylene carbonate, and a problem such as a decrease in discharge capacity occurs. Occurs.

Further, as shown in Japanese Patent Application Laid-Open No. 9-180721, after a graphite electrode is subjected to oxidation-reduction treatment in an electrolytic solution using a fluorine-substituted compound represented by a fluorine-substituted carbonate as a solvent, propylene carbonate or phosphorus There is a method of changing to an electrolytic solution containing an acid ester. This method requires a complicated process of changing the electrolytic solution after performing the electrochemical treatment, and the application range of the fluorine-substituted compound is required to be about 10% to 90% by volume based on the whole solvent. It is necessary to use a large amount of a fluorine-substituted compound having a problem of cost.

[0010] The problem of the reaction between the carbon material and the electrolyte can occur not only when the carbon material is used as the negative electrode active material but also when it is used as the positive electrode active material.

The present invention has been made in view of such conventional problems, and in a lithium secondary battery using a carbon material as an electrode active material, the reaction between the carbon material and the electrolyte is suppressed to improve the charge / discharge efficiency. In addition, it aims to provide a lithium secondary battery with excellent temperature characteristics and flame retardancy by easily and inexpensively providing a method for applying a solvent that could not be applied conventionally to an electrolyte. .

[0012]

According to a first aspect of the present invention, there is provided a lithium secondary battery including a non-aqueous electrolyte containing a carbon material as an active material and a lithium salt dissolved in an organic solvent. In the formula (I), R _{1 to} R ₅ are any one of H, F and CF ₃ and at least one is F or CF ₃ . And R _H contains a fluorinated or trifluoromethylated benzoate compound of C _n H _{2n + 1} (1 ≦ n ≦ 3) in the lithium secondary battery.

What is most notable in the present invention is that the nonaqueous electrolyte contains a benzoate compound in which the aromatic ring is substituted by a fluorine group or a trifluoromethyl group.

The fluorinated or trifluoromethyl
The benzoic acid ester compound is formed on the surface of the carbon material.
Caused by electrochemical and local decomposition at
Is substituted with a fluorine or trifluoromethyl group
It is characterized by being stabilized by an aromatic ring. In addition,
To decompose locally on the carbon material surface, it is necessary to use an aromatic ring.
It is necessary that the functional group to be bonded is an ester group.
In this case, the estimated reaction of the compound represented by the formula (I) (FIG. 1)
The response mechanism is expressed by the following equation (II). Formula (II): R_F(Ar) COOR_H+ E⁻ → R_F(Ar) COO⁻+ R_H   Where R_F(Ar) represents an aromatic ring in the formula (I)
ing.

The carboxylate anion formed is stabilized by an aromatic ring whose electron-withdrawing property is enhanced by a fluorine group or a trifluoromethyl group. Therefore, equation (I)
It is necessary that at least one of R _{1 to} R ₅ is a fluorine group (—F) or a trifluoromethyl group (—CF ₃ ) having a strong electron-withdrawing property. In addition, the stability of the carboxylate anion increases as the number of the fluorine group or the trifluoromethyl group as the substituent increases. Further, a fluorine group and a trifluoromethyl group may be mixed and substituted.

Further, C _n H as R _H in the formula (I)
_When 1> n in _{2n + 1} , protonic acid is formed, and thus problems such as polymerization of the electrolyte solvent, dissolution of elements from the electrode active material, and decomposition of the binder occur. When n> 3, the viscosity of the molecules increases, so that even if a small amount is added, the viscosity of the electrolytic solution increases, the ionic conductivity decreases, and the gas generated during decomposition degrades the electrode or electrode activity. The detrimental effect of destroying the substance occurs.

The present invention provides a coating comprising a stable anion by electrochemically decomposing a fluorinated or trifluoromethylated benzoate compound on the surface of an active material during an initial reduction reaction of an electrode active material comprising a carbon material. Is selectively formed on the surface of the active material. It is difficult to selectively form a film on the surface of an active material by a method of adding a fluorinated or trifluoromethylated benzoate to an electrolytic solution in advance.

Next, the non-aqueous electrolyte is composed of a solvent and a supporting salt. According to the present invention, when the total weight of the solvent and the supporting salt is taken as 100%, 0.05%
It is preferred to contain ５5% by weight of a fluorinated or trifluoromethylated benzoate compound.
The fluorinated or trifluoromethylated benzoic acid ester compound may be used alone or in combination of two or more.

The fluorinated or trifluoromethylated benzoic acid ester compound is used in an amount of 0.05 to 5.0 w.
A small amount of about t% exhibits a sufficient effect, and a film can be formed on the carbon material surface without using a relatively expensive fluorine compound in a large amount. When the above addition amount is 0.
If the content is less than 05 wt%, a film cannot be sufficiently formed on the surface of the carbon material, and a sufficient effect may not be obtained. On the other hand, when the content exceeds 5.0 wt%, the battery characteristics may be affected.

It is also possible to electrochemically coat the surface of the carbon material in an electrolyte containing a fluorinated or trifluoromethylated benzoate compound, and then replace the carbon material with another electrolyte. . However, since this method has a problem in production cost and the like, it is desirable to first add a fluorinated or trifluoromethylated benzoate compound to an electrolyte having a desired function and use it as it is.

The specific fluorinated or trifluoromethylated benzoic acid ester compound includes, for example,
Methyl 2-fluorobenzoate, methyl 3-fluorobenzoate, methyl 4-fluorobenzoate, methyl 2,6-difluorobenzoate, methyl 2,3,4,5,6-pentafluorobenzoate, 2-trifluoro Methyl methyl benzoate, Methyl 3, trifluoromethylbenzoate, Methyl 4-trifluoromethylbenzoate, Methyl 2,6-ditrifluoromethylbenzoate, Ethyl 2,6-difluorobenzoate, 2,6-Difluorobenzoic acid Propyl and the like.

Further, the present invention is difficult to apply to a lithium secondary battery containing a carbon material as an electrode active material by adding the fluorinated or trifluoromethylated benzoate compound to an electrolytic solution. This makes it possible to use a suitable electrolyte solvent. Examples of solvents applicable to a lithium secondary battery containing a carbon material according to the present invention include the following.

Cyclic carbonates such as propylene carbonate, butylene carbonate, vinylene carbonate, cyclic esters such as gamma butyrolactone, gamma valerolactone, and chain carbonates such as diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate , Methyl acetate, ethyl acetate, chain esters represented by methyl propionate,
Examples thereof include ethers represented by 1,2-dimethoxyethane, tetrahydrofuran, and methyltetrahydrofuran.

Also, a molecule containing a sulfur atom in a molecule represented by sulfolane, 3-metersulfolane, butane sultone, dimethyl sulfite, a molecule containing nitrogen in a molecule represented by dimethylformamide, and a molecule represented by trimethyl phosphate Examples of such a molecule include a molecule containing phosphorus in the molecule to be formed, and a molecule in which a part of the above molecule is substituted with a halogen element such as fluorine, chlorine, and bromine.

These molecules have high reactivity with carbon materials, and it has been difficult to use them as electrolyte solvents even when used alone or when mixed with ethylene carbonate or the like. Some of the above-mentioned molecules are generally used as an electrolyte solvent when mixed with ethylene carbonate. However, according to the present invention, it can be used as an electrolyte solvent without being mixed with ethylene carbonate. become.

In addition, molecules that cannot be used as a solvent, such as propylene carbonate, sulfolane, and trimethyl phosphate, which can react with a carbon material even after being mixed with ethylene carbonate, can be used according to the present invention. These molecules can be used as a mixed solvent with ethylene carbonate depending on the purpose of use.

A solvent to which the fluorinated or trifluoromethylated benzoate compound is added;
By mixing with ethylene carbonate, the stability with the carbon material electrode is further increased, and the cycle characteristics, energy density, and the like are improved. When ethylene carbonate is mixed, if the content is too high, the low-temperature characteristics of the electrolytic solution are deteriorated. Therefore, the content is preferably 50% or less.

As the carbon material used for the negative electrode active material, for example, graphite, coke, carbon granules obtained by calcining raw coke can be used. In addition, as the above graphite, a carbon material obtained by heat-treating graphitizable carbon such as coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fiber, and pyrolytic vapor growth carbon fiber as raw materials to improve the graphitic property. , Graphite whiskers, natural graphite, and the like.

As the carbon material, a material in which the surface of a graphite crystal is coated with an amorphous carbon material can be used. Further, a graphite crystal surface coated with fine particles of silver or a tin compound can also be applied.
Such a coating material can further reduce the reactivity between the electrolyte solvent and the carbon material.

The carbon material may be used as a positive electrode active material or a negative electrode active material. A typical example of a battery using a carbon material as the positive electrode active material is a lithium secondary battery using graphite for the positive electrode and lithium metal for the negative electrode. On the other hand, a typical example of a battery using a carbon material as a negative electrode active material is a lithium secondary battery using graphite as a negative electrode and a lithium metal oxide represented by lithium manganate, lithium cobaltate, and lithium nickelate as a positive electrode. Batteries and the like.

As the above supporting salt, for example, Li
PF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiN
Lithium salts such as (CF ₃ SO ₂ ) ₂ and LiN (CF ₃ CF ₂ SO ₂ ) ₂ are exemplified, but not limited thereto. Also,
For example, even in a supporting salt such as LiBF ₄ which reacts with the negative electrode active material to form a coating and undergoes significant cycle deterioration, the present invention can suppress the reaction between the supporting salt and the carbon material. Therefore, in the present invention, it is possible to use a supporting salt which has been difficult to be applied because it has been reacted with the carbon material and the cycle deterioration is remarkable.

Next, the operation and effect of the present invention will be described.
In the present invention, the specific fluorinated or fluoroalkylated benzoate compound is contained in the nonaqueous electrolyte. Therefore, as described above, when the carbon material is used as the active material, the reaction between the carbon material and the electrolyte is suppressed to improve the charge / discharge efficiency, and an excellent solvent, which could not be applied conventionally, is used as the electrolyte. Can be applied. Therefore, according to the present invention, a lithium secondary battery having excellent temperature characteristics and flame retardancy can be provided.

That is, in a lithium secondary battery containing a carbon material as an electrode active material, when a fluorinated or trifluoromethylated benzoate compound represented by the above formula (I) is added to an electrolytic solution, At the time of the first reduction, when the potential of the carbon material is changed to 1.3-1.1 V (vs. Li / Li ⁺ ), the benzoate compound fluorinated or trifluoromethylated on the surface of the carbon material is decomposed. A film can be selectively formed on the surface of a carbon material.

The coating has high lithium ion permeability and suppresses the reaction between the electrolyte solvent and the carbon material.
For this reason, various solvents such as propylene carbonate, which have conventionally been considered difficult to apply to graphite-based carbon materials, can be used as the electrolyte solvent.

Next, it is preferable that the carbon material is graphite. As a carbon material,
As described above, graphite and coke are typical examples. Among them, graphite is a carbon material having high crystallinity, and has advantages such as flat potential and small irreversible capacity as compared with coke and the like having low crystallinity. Therefore, by using graphite as an active material, a useful lithium secondary battery having a high energy density can be obtained.

It is preferable that the carbon material is used as the negative electrode active material and the lithium metal composite oxide is used as the positive electrode active material. In this case, in particular, it is possible to provide a lithium secondary battery having excellent charge / discharge cycle characteristics at a high temperature, for example, at a temperature exceeding 60 ° C.

The reason can be considered as follows. First, from the analysis of the current-potential curve of the battery and the electrode material, the fluorinated or fluoroalkylated benzoate compound of the present invention has a negative electrode potential of 1.3-1.1 V ( Vs. Li / L
In the vicinity of (i ⁺ ), it was clarified that it was reacting with the carbon material of the negative electrode. The reaction was not observed after the second cycle. From the above results, it is expected that the negative electrode active material reacts with the fluorinated or fluoroalkylated benzoate compound at the time of charging in the first cycle to form a film on the negative electrode surface. This coating has very good properties, for example, the effect of suppressing the reaction between the electrolyte solvent and the negative electrode active material and the effect of preventing the metal ions eluted from the positive electrode from depositing on the negative electrode. It is believed that there is.

The lithium metal composite oxide used as the positive electrode active material is, for example, LiMn ₂ O ₄ , L
Spinel structure such as iV ₂ O ₄ , LiMnO ₂ , Li
Examples include layered compounds such as CoO ₂ and LiNiO ₂ , and compounds obtained by replacing these compounds with different elements.

[0039]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 In this embodiment, a reaction between a carbon material and a non-aqueous electrolyte containing propylene carbonate (PC) was clarified in order to clarify the mechanism of action in the present invention. That is, cyclic voltammogram (CV) measurement using a three-electrode cell revealed the reaction between the positive electrode containing graphite as an active material and the non-aqueous electrolyte containing PC to which a fluorinated benzoate compound was added. did. And it was shown that the fluorinated benzoic acid ester compound suppressed the reaction between PC and graphite, and that graphite was charged and discharged smoothly even in an electrolytic solution using PC as a solvent. Hereinafter, Example E1 and three comparative examples C1 to C
3 will be described.

Example E1 First, a method for manufacturing a graphite electrode will be described. Spherical artificial graphite (manufactured by Osaka Gas, MCMB25
-28) as an electrode active material, and 5 parts by weight of polyvinylidene fluoride as a binder.
The mixture was kneaded with methyl-2-pyrrolidone to obtain an electrode material paste. This is coated on one side of a strip-shaped copper foil, and N-methyl-2
After vaporizing and removing pyrrolidone, compression molding was performed to obtain a strip-shaped electrode sheet. The electrode sheet was punched out into a disk shape of 18 mmφ to form an electrode.

Next, the cell for CV measurement will be described. The above-mentioned electrode using graphite as an active material is used as a working electrode and is 19 mm in diameter.
Using a disc-shaped lithium foil of φ as a counter electrode, sandwiched via a polyethylene separator, and housed in a coin-type tripolar cell equipped with a lithium chip as a reference electrode.

Thereafter, ethylene carbonate (EC; manufactured by Toyama Pharmaceutical Co., Ltd.), PC (manufactured by Toyama Pharmaceutical Co., Ltd.) and diethyl carbonate (DEC; manufactured by Toyama Pharmaceutical Co., Ltd.) were used in a volume ratio of 3: 3.
LiPF ₆ (manufactured by Toyama Pharmaceutical Co., Ltd.) was dissolved at a concentration of 1 mol / liter in a solvent mixed at a ratio of 4: 7). In this electrolytic solution, methyl 2,6-difluorobenzoate (manufactured by ThinkQuest) was dissolved at a concentration of 0.5% by weight to prepare an electrolytic solution. After this electrolyte was injected into the coin-type three-electrode cell, it was sealed.

The CV measurement was performed as follows. Using a Hokuto Denko HA3001 potentiostat, sweep from the natural electrode potential of the graphite electrode (about 3 V / vs. Li / Li ⁺ ) to 0 V (vs. Li / Li ⁺ ) at a sweep rate of 0.1 mV / sec. Immediately after the voltage reached 0 V, the potential sweep direction was reversed, and the voltage was swept to about 3 V to perform CV measurement.

(Comparative Example C1) Instead of methyl 2,6-difluorobenzoate, 0.5% of methyl benzoate was added to the electrolytic solution.
A coin-type three-electrode cell was prepared and subjected to CV measurement in the same manner as in Example E1, except that the addition was performed in wt%.

(Comparative Example C2) A coin-type three-electrode cell was prepared and subjected to CV measurement in the same manner as in Example E1, except that methyl 2,6-difluorobenzoate was not added.

(Comparative Example C3) EC and DEC were used as electrolyte solvents.
Using a solvent in which is mixed in a volume ratio (3: 7),
A coin-type tripolar cell was prepared and CV measurement was performed in the same manner as in Example E1, except that methyl 2,6-difluorobenzoate was not added. Table 1 summarizes the composition of the electrolytic solution used for the CV measurement.

[0047]

[Table 1]

FIG. 2 shows Example E1 and Comparative Example C1 in FIG.
Shows CV measurement results of Comparative Examples C2 and C3.
2 and 3, the horizontal axis indicates the potential (V (vs. Li / Li ⁺ )).
And the vertical axis represents the current (mA).

As can be seen from Comparative Example C3 in FIG.
In a general electrolytic solution using a mixed solvent of DEC and DEC, 0.1.
At 9 V (vs. Li / Li ⁺ ), the reduction current (minus side) starts to flow, and strongly at a potential region of 0.5 V (vs. Li / Li ⁺ ) or less. The corresponding oxidation current (positive side) is 0 to 0.88 (relative to Li / L).
The current flowing through i ⁺ ) is observed, but these currents correspond to the insertion and desorption reactions of lithium ions between graphite layers.

On the other hand, as seen in Comparative Example C2, the electrolyte using PC as a solvent
From 0.9 V (vs. Li / Li ⁺ ), a reduction current larger than C3 flows. This reduction current is based on the electrochemical volume 62, 1
023 (1994) and Electrochemistry 61, 421 (1
According to 993) and the like, it is a current generated due to a decomposition reaction of PC on the surface of graphite. The graphite layer is destroyed by this PC decomposition, and the oxidation current value corresponding to the desorption of lithium ions decreases.

On the other hand, as can be seen from Example E1 in FIG. 2, in the electrolytic solution to which methyl 2,6-difluorobenzoate was added, 1.3 to 1.1 V (relative to Li / L) at the first sweep.
At i ⁺ ), a small reduction current peak is observed. This is the current resulting from the reductive decomposition of 2,6-difluoromethyl benzoate. The decomposition current of PC generated from 0.9 V (vs. Li / Li ⁺ ) observed in Comparative Example C2 was not observed in E1. In addition, oxidation / reduction currents corresponding to lithium ion desorption / insertion reactions were observed as in Comparative Example C3.

However, in the electrolyte of Comparative Example C1 to which methyl benzoate in which the aromatic ring was not substituted with a fluorine group was added, a reduction current began to flow at 1.1 V (vs. Li / Li ⁺ ), and 0.8- A strong reduction current with a peak at 0.9 V (vs. Li / Li ⁺ ) was observed. Further, the oxidation current value corresponding to the desorption of lithium ions is smaller than that of Example E1. From a comparison between Example E1 and Comparative Example C1.
It is clear that higher charge / discharge efficiency can be obtained by adding methyl 2,6-difluorobenzoate in which the aromatic ring of methyl benzoate is fluorinated than methyl benzoate.

From the above results, the following reaction mechanism is estimated. When methyl 2,6-difluorobenzoate is added, 1.3-1.1 V (relative to Li / Li)
^At a potential of ⁺ ), methyl 2,6-difluorobenzoate undergoes reductive decomposition. The reduced decomposition product becomes a coating covering the graphite surface, and suppresses the reaction between PC and graphite. The coating has high permeability of lithium ions, and the desorption and insertion of lithium ions between graphite layers proceed smoothly.

On the other hand, when methyl benzoate was added, the reductive decomposition reaction of methyl benzoate was 1.1 V (vs. Li).
/ Li ⁺ ), but a film that suppresses the reaction between PC and graphite is not formed, and the decomposition of PC and the decomposition reaction of methyl benzoate occur simultaneously to destroy the graphite layer. As a result, the capacity decreases and the oxidation current value corresponding to the desorption of lithium ions decreases.

Thus, the addition of methyl benzoate in which the aromatic ring is fluorinated suppresses the reaction between graphite and PC,
It became clear that high charge / discharge efficiency was obtained. still,
In Example E1 of this example, methyl 2,6-difluorobenzoate was mentioned as an additive, but the same applies to the case where benzoic acid esters in which the aromatic ring of methyl benzoate is trifluoromethylated are used. The effect of is obtained. In addition, although PC is mentioned as a representative example of the solvent, the same effect as that of Example E1 of the present example can be obtained even in a solvent having high reactivity with graphite such as phosphate ester and gamma-butyrolactone.

Embodiment 2 In this embodiment, a lithium manganese oxide was used as a positive electrode active material, graphite was used as a negative electrode active material, and an electrolyte containing a PC to which a fluorobenzoate compound was added was used as an electrolyte. The 60 ° C cycle characteristics of the secondary battery were examined, and the lithium secondary battery using graphite as the negative electrode active material
It has been shown that a C solvent-based electrolyte can be applied.
Hereinafter, description will be made using Example E2 and three comparative examples C4, C5, and C6.

Example E2 A cylindrical lithium secondary battery was manufactured as follows. First, a method for manufacturing a positive electrode will be described below. Li _1.10 Mn _1.90 O ₄ (manufactured by Honjo Chemical) having a composition formula of lithium manganese oxide of 85 parts by weight as a positive electrode active material, 10 parts by weight of graphite as a conductive agent, and 5 parts by weight of polyvinylidene fluoride as a binder And N-methyl-2-pyrrolidone was added and kneaded to obtain a positive electrode material paste. This was coated on both sides of a strip-shaped aluminum foil, and N-methyl-2-pyrrolidone was vaporized and removed, followed by compression molding to obtain a strip-shaped positive electrode.

Next, a method for manufacturing the negative electrode will be described below. Spherical artificial graphite (manufactured by Osaka Gas, MCMB25-28) was mixed with 95 parts by weight as a negative electrode active material and 5 parts by weight of polyvinylidene fluoride as a binder, and further kneaded by adding N-methyl-2-pyrrolidone. A negative electrode material paste was obtained. This was coated on both sides of a strip-shaped copper foil, and N-methyl-2-pyrrolidone was vaporized and removed, followed by compression molding to obtain a strip-shaped negative electrode.

To obtain current collection of the positive electrode and the negative electrode, a positive electrode lead made of aluminum was welded to the strip-shaped positive electrode, and a negative electrode lead made of nickel was welded to the strip-shaped negative electrode. Then, using a separator made of a microporous polyethylene film having a thickness of 25 μm, a positive electrode, a separator and a negative electrode are laminated in this order,
Nickel-plated iron battery cans (18 mm outside diameter, 6 height
5 mm) to form a spiral electrode.

After placing the electrodes in the battery can, the negative electrode lead wire was welded to the battery can, and the positive electrode lead wire was welded to the terminal of a polyethylene battery lid provided with aluminum terminals. After that, EC (Toyama Pharmaceutical) and PC (Toyama Pharmaceutical)
And DEC (manufactured by Toyama Pharmaceutical Co., Ltd.) in a ratio of 3: 4: 7 by volume, and LiPF ₆ (manufactured by Toyama Pharmaceutical Co., Ltd.)
Was dissolved to a concentration of 1 mol / l.

In this electrolyte, methyl 2,6-difluorobenzoate (manufactured by ThinkQuest) was dissolved at a concentration of 0.5 wt%, and the electrolyte was poured into a battery can. The battery lid was fixed by caulking the battery can and the battery lid, and a cylindrical non-aqueous electrolyte secondary battery (lithium secondary battery) having a diameter of 18 mm and a height of 65 mm was produced.

Thereafter, the following high-temperature cycle test was performed using this lithium secondary battery. That is, the cylindrical non-aqueous electrolyte secondary battery produced as described above was charged to 4.20 V at a constant current of 1 mA / cm ² at a temperature of 25 ° C., and then charged at 4.20 V. The battery was charged at the voltage for 2.5 hours. This total amount of current is defined as the initial charging capacity. Thereafter, discharging was performed to 3.00 V at a constant current of 0.33 mA / cm ² . This total discharge flow rate is defined as the first group capacitance.

Next, the battery was charged at a constant current of 1 mA / cm ² to 4.20 V at a temperature of 60 ° C., and then charged at a constant current of 1 mA / cm ² to 3.0.
Discharge was performed to 0V. This charge / discharge cycle at 60 ° C. was performed up to 100 cycles. Hereinafter, the cycle test at 60 ° C. is referred to as a high-temperature cycle test.

Comparative Example C4 A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example E2 except that methyl benzoate (manufactured by Wako) was used instead of methyl 2,6-difluorobenzoate. , High temperature cycle test.

Comparative Example C5 A non-aqueous electrolyte secondary battery was prepared and subjected to a high-temperature cycle test in the same manner as in Example E2 except that methyl 2,6-difluorobenzoate was not added.

(Comparative Example C6) Example E was repeated except that a solvent in which EC and DEC were mixed at a volume mixing ratio of 3: 7 was used, and methyl 2,6-difluorobenzoate was not added.
In the same manner as in Example 2, a non-aqueous electrolyte secondary battery was prepared and subjected to a high-temperature cycle test. Table 2 shows the configurations of the electrolytic solutions used in Example E2 and Comparative Examples C4 to C6.

[0067]

[Table 2]

Table 3 shows the initial charge capacity and the initial discharge capacity of the battery using each electrolytic solution. FIG. 4 shows the results of the high-temperature cycle test at 60 ° C. In the figure, the horizontal axis represents the number of charge / discharge cycles, and the vertical axis represents the discharge capacity per positive electrode active material (m
Ah / g).

As is clear from Table 3, Example E2 to which methyl 2,6-difluorobenzoate was added exhibited a discharge capacity equivalent to that of Comparative Example C6 using a commonly used electrolytic solution. . On the other hand, when the electrolytic solution of Comparative Example C5 was used, the graphite negative electrode was broken by the decomposition of PC, and the discharge capacity was small. Further, as in Comparative Example C4, when methyl benzoate is added, the discharge capacity is increased more than in the case of C5 without addition, but is lower than that in Example E2.

As can be seen from the high-temperature cycle characteristics at 60 ° C. in FIG. 4, the electrolyte solution of Example E2 is superior to the electrolyte solution of Comparative Example C6 which is generally used. It was revealed.

From the above results, it has been clarified that the electrolyte using PC as a solvent can be applied to a lithium secondary battery using graphite as an active material by adding methyl 2,6-difluorobenzoate. . The embodiment E of this example
In Example 2, methyl 2,6-difluorobenzoate was mentioned as an additive, but the same effect as in Example E2 can be obtained when benzoic acid esters in which the aromatic ring of methyl benzoate is trifluoromethylated are used. Is obtained. In addition, although PC is mentioned as a typical example of the solvent, the same effect as that of Example E2 can be obtained in a solvent having high reactivity with graphite, such as phosphate ester and gamma butyrolactone.

[0072]

[Table 3]

[0073]

As described above, according to the present invention, the reaction between the carbon material and the electrolytic solution is suppressed to improve the charge / discharge efficiency, and cannot be applied to a lithium secondary battery using a carbon material as an electrode active material. It is possible to easily and inexpensively provide a technique that can apply the used solvent to the electrolytic solution. Further, a lithium secondary battery having excellent temperature characteristics and flame retardancy can be provided.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a general formula of a fluorinated or fluoroalkylated benzoate compound.

FIG. 2 shows Example E1 and Comparative Example C in Embodiment 1;
FIG. 4 is an explanatory view showing a CV measurement result of FIG.

FIG. 3 shows C of Comparative Examples C2 and C3 in Embodiment 1;
Explanatory drawing which shows a V measurement result.

FIG. 4 is an explanatory diagram showing a high-temperature cycle test result in the second embodiment.

Claims (3)

[Claims] 1. A lithium secondary battery having a non-aqueous electrolyte containing a carbon material as an active material and a lithium salt dissolved in an organic solvent, wherein the non-aqueous electrolyte has a general formula shown in FIG. R _{1 to} R ₅ in the formula (I) are any of H, F and CF ₃ , at least one of which is F or CF ₃ , and R _H
Is a lithium secondary battery containing a fluorinated or trifluoromethylated benzoate compound of C _n H _{2n + 1} (1 ≦ n ≦ 3). 2. The lithium secondary battery according to claim 1, wherein the carbon material is graphite. 3. The lithium secondary battery according to claim 1, wherein the carbon material is used as a negative electrode active material, and a lithium metal composite oxide is used as a positive electrode active material.