JP2002033118A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode for a secondary battery suppressing sub-reaction of the electrolyte on the surface of the electrode under a high temperature environment, in particular, during charging and discharging, having high adhesion property with a collector for improving a cycle service life of the lithium secondary battery and making an active material difficult to peel from the collector and to provide the secondary battery with little electrode deterioration and a long cycle service life. SOLUTION: In the lithium secondary battery comprising electrolyte containing lithium ion between a positive electrode made of mainly lithium composite oxide and a negative electrode made of mainly a carbon material, the electrolyte comprising organic solvent with dissolved lithium metallic salt and surface active agent of 1×10-4 to 10 wt.% is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery in which a side reaction between an electrode and an electrolyte is suppressed.
This is a lithium secondary battery with improved cycle life.

[0002]

2. Description of the Related Art In recent years, lithium secondary batteries have attracted attention as batteries having high energy density and high voltage.
Such lithium secondary batteries include lithium manganate,
An organic solvent containing lithium ions was filled between a positive electrode using a lithium-containing composite oxide such as lithium cobalt oxide and lithium nickel oxide as an active material and a negative electrode using a carbon material such as graphite or coke as an active material. Things. In general, as an electrolyte for such a lithium secondary battery, an aprotic organic solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), or propylene carbonate, or a mixed organic solvent of two or more of these solvents is used. , LiBF ₄ , LiClO ₄ , LiPF _{6 and the} like, in which lithium salts are dissolved.

[0003]

However, in such a lithium secondary battery, the environment in which the electrolyte is placed during charging and discharging is an environment in which the reducing action is very strong near the surface of the negative electrode, and an environment in which the reducing action is very strong near the surface of the positive electrode. Since the environment has a very strong oxidizing effect, the reduction reaction and / or oxidation reaction of the electrolyte on these electrode surfaces cannot be avoided, and the electrolyte causes a side reaction with the active material constituting the electrode to decompose and degrade. Therefore, there is a problem that the battery capacity is reduced. This phenomenon was particularly remarkable at high temperatures.

Conventionally, in order to prevent such a side reaction between the electrode and the electrolyte, a side reaction is actively advanced between the electrode and the electrolyte beforehand, and a solid electrolyte comprising a by-product on the surface of the electrode active material. A method called aging for forming an interface film (SEI film) has been proposed. However, in such an aging method, it is difficult to obtain a uniform SEI film without lowering the electrode capacity, and the SEI film is grown on the SEI film by new by-products by repeating charge and discharge. The effect was not enough. Other methods for preventing a side reaction between the electrode and the electrolyte include a method using an active material coated with a surfactant, and a method of forming a surfactant film on the electrode surface in advance. . However, in the former method, when the active material is slurried, the coating may be peeled off from the surface.In the latter method, a step of immersing the electrode in a solution containing a surfactant is required, Therefore, there has been a problem that the manufacturing process of the battery becomes complicated.

The present invention has been made in view of the above circumstances, and an object thereof is to suppress a side reaction of an electrolytic solution on an electrode surface during charge and discharge, particularly in a high-temperature environment, thereby improving the cycle life of a lithium secondary battery. To make it happen.

[0006]

In order to solve the above problems, in a lithium secondary battery of the present invention, a surfactant film is formed on an electrode surface by adding a surfactant to an electrolytic solution. By preventing side reactions of the electrolytic solution on the electrode surface, the cycle life of the lithium secondary battery is improved. In particular, it suppresses a side reaction of the electrolytic solution at high temperatures. That is, the lithium secondary battery of the present invention is a lithium secondary battery comprising an electrolyte containing lithium ions between a positive electrode mainly containing a lithium composite oxide and a negative electrode mainly containing a carbon material. And
The electrolytic solution contains an organic solvent in which a lithium metal salt is dissolved and 1 × 10 ⁻⁴ to 10% by weight of a surfactant. The surfactant is preferably selected from an anionic surfactant, a cationic surfactant, a neutral surfactant, and an amphoteric surfactant. Also, a perfluoroalkyl sulfonate, a perfluoroalkyl phosphate, a perfluoroalkyl carboxylate, a trimethyl ammonium perfluoroalkyl salt,
It is desirable that the surfactant is one or more surfactants selected from the group of ethylene oxide addition type ammonium salts.

In such a lithium secondary battery, a film of the above-mentioned surfactant can be formed on both electrode surfaces of the negative electrode and the positive electrode only by adding a surfactant to the electrolyte, and the active material and the active material can be formed on the electrode surface. Since direct contact with the electrolyte can be prevented, side reactions of the electrolyte can be suppressed. Therefore, a lithium secondary battery with little battery deterioration due to a side reaction of the electrolyte and a long cycle life can be obtained.

[0008]

Embodiments of the present invention will be described below. The electrolyte used for the lithium secondary battery of the present invention is obtained by dissolving a lithium salt in an organic solvent and adding a surfactant.

As the above-mentioned organic solvent, those which are usually used can be used. For example, ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate, ethylene carbonate-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran , Dioxolan, 4-methyldioxolan, sulfolane, 1,2-dimethoxyethane, diethyl carbonate, dimethyl sulfoxide, acetonitrile, N, N-
An aprotic solvent such as dimethylformamide, diethylene glycol, and dimethyl ether, or a mixed solvent of two or more of these solvents is used. Lithium salts added to the organic solvent include LiBF ₄ , L
iClO ₄ , LiAsF ₆ , LiSbF ₆ , LiAlO ₄ ,
LiAlCl ₄ , LiPF ₆ , LiN (CxF ( _{2X + 1} ) S
O ₂ ) (CyF ( _{2Y + 1} ) SO ₂ ) ( _X and _Y are natural numbers), Li
Cl, LiI, etc. can be used. These may be used alone or as a mixture of two or more.

Since the surfactant is contained in the electrolytic solution and incorporated in the lithium secondary battery, it has high dispersibility in the electrolytic solution and does not decompose by the oxidation-reduction reaction of the battery. It has low reactivity, high heat resistance with stable properties without denaturation or decomposition even at high temperatures, and high ion conductivity that does not hinder the ionic conductivity of the electrolyte. As long as it has certain properties, various surfactants such as anionic surfactants, cationic surfactants, neutral surfactants, amphoteric surfactants, etc. may be used alone or in combination of two or more. Can be used.

The above-mentioned anionic surfactants include N-acyl amino acid salts, alkyl ether carboxylates, acylated peptides, alkyl sulfonates, alkyl benzene sulfonates, alkyl naphthalene sulfonates, sulfon succinates, α -Olefin sulfonate, N-acyl sulfonate, alkyl sulfate, alkyl ether sulfate, alkyl allyl ether sulfate, alkyl amide sulfate, alkyl phosphate, alkyl ether phosphate, alkyl allyl ether phosphate A perfluoroalkyl sulfonate represented by the following chemical formula (1):
A perfluoroalkyl phosphate represented by the following chemical formula (2), a perfluoroalkyl carboxylate represented by the following chemical formula (3), and the like can be given. Among them, a fluorine-containing anionic surfactant is preferred because of its low reactivity and excellent heat resistance.

[0012]

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[0013]

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[0014]

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In the above chemical formulas (1) to (3), R1 is (C
H_Two)_nH, (CF_Three)_nF, (CH_Two)_n-(CF _Three)_mAny of F
And n and m represent an integer of 5 to 20. X is L
i, Na, ammonium compounds, etc.

Examples of the cationic surfactant include an alkylamine salt, a monoalkyltrimethylammonium salt, an alkylpyridinium salt, an imidazolinium salt, a trimethylammonium perfluoroalkyl salt represented by the following chemical formula (4), and a following chemical formula (5) ), And the like. Among them, a fluorine-containing cationic surfactant is preferred because of its low reactivity and excellent heat resistance.

[0017]

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[0018]

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In the above chemical formulas (4) and (5), R1 is (C
_{H 2) n H, (CF} 3) n F, (CH 2) n - (CF 3) indicates one of groups of _m F, R2 represents a CH ₃ or CF _3, R3 is (CH _{2) n} It indicates H, R4 represents a _{(CH 2 CH 2 O) n} H, n, m is an integer of 5-20. X is F, C
Halogen such as l, Br, I or the following chemical formula (6)
Organic anions and the like shown in (8) can be mentioned. )

[0020]

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[0021]

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[0022]

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In the above chemical formulas (6) to (8), R1 is (C
H ₂ ) _n H, (CF ₃ ) _n F, or any group of (CH ₂ ) _n- (CF ₃ ) _m F, wherein n is an integer of 5 to 20, and m is an integer of 5 to 20 Show.

Examples of the neutral surfactant include polyoxyethylene glycerin fatty acid ester, polyethylene glycol fatty acid ester, sorbitamin fatty acid ester, polyoxyethylene fatty acid amide, polyoxyethylene alkylamine, perfluoroalkyl EO adduct, and perfluoroalkyl Amine oxide and the like.

Examples of the amphoteric surfactant include alkyl dimethyl carboxy betaine, alkyl amino carboxylate,
Perfluoroalkyl betaine and the like.

The surfactant is added in an amount of 1 × 10 ^-4 to 10% by weight, preferably 1 × 1 ^-4 to 10% by weight based on the weight of the electrolyte.
It is preferably from 0 ^{-4 to} 5% by weight. If it is less than 1 × 10 ⁻⁴ wt%, a uniform coating cannot be formed on the electrode surface, and if it exceeds 10 wt%, the ionic conductivity of the electrolytic solution will be impaired.

The above-mentioned surfactant is dissolved in the electrolytic solution, and when incorporated into a lithium secondary battery, the surfactant self-assembles collectively adheres to the electrode surface due to electrostatic interaction with the electrode. To form a coating. By this surfactant coating, the active material of the electrode does not come into direct contact with the electrolytic solution, and a side reaction of the electrolytic solution is suppressed. Due to the nature of the ions, it is considered that an anionic surfactant easily adheres to the positive electrode, and a cationic surfactant easily adheres to the negative electrode. Therefore, it is preferable to use an anionic surfactant and a cationic surfactant in combination in the electrolytic solution. In this way, a uniform coating is formed on both the positive electrode surface and the negative electrode surface. In addition, since the surfactant is dissolved in the electrolyte, the film is formed even during charging and discharging, so that the film does not peel off from the electrode as in the conventional case, and the side reaction of the electrolyte does not occur for a long time. Can be suppressed.

The electrolyte containing the surfactant is impregnated into the separator, sandwiched between the negative electrode and the positive electrode, and incorporated into a lithium secondary battery. The above separator,
A commonly used polyolefin-based porous membrane such as a non-woven fabric such as microporous polypropylene can be used. As the negative electrode, a material obtained by integrating a carbon material such as graphite and coke with a binder such as polyvinylidene fluoride (PVDF) as an active material usually used for a lithium secondary battery can be used.

As the positive electrode, those which are usually used for a lithium secondary battery may be used. For example, a positive electrode obtained by integrating the following active materials with a binder such as PVDF can be used. (1) TiS ₂ , MoS ₃ , NbSe, FeS, VS ₂ ,
Metal chalcogenides having a layered structure such as VSe ₂ . (2) CoO ₂ , Cr ₃ O ₅ , TiO ₂ , CuO, V ₃ O ₆ , M
oO, V ₂ O ₅ , Mn ₂ O metal oxides. (3) Conductive conjugated polymer compounds such as polyacetylene, polyaniline, polyparaphenylene, polythiophene, and polypyrrole. (4) LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , and a lithium-containing composite oxide in which Co, Ni, or Mn in each is partially replaced with another element, for example, Co, Mn, Fe, Ni, or the like. In particular, a positive electrode using the lithium-containing composite oxide shown in (4) as an active material is suitably used as the positive electrode. In addition, a conductive agent such as graphite, acetylene black, and carbon black can be added to these active materials as needed.

In such a lithium secondary battery,
Since a surfactant film is formed on the surface of the positive electrode and the surface of the negative electrode, the active material of the electrode does not come into direct contact with the electrolytic solution and side reactions are suppressed, so that the cycle life is extended.

[0031]

Next, an embodiment of the present invention will be described in more detail. (Example 1) 1. Preparation of electrolytic solution EC and DMC were mixed so that the mixing ratio EC: DMC was 1: 2, and 1 M of LiPF ₆ was dissolved in this mixed solvent. To this solution, 0.1% by weight of lithium perfluoroalkylcarboxylate was added as a surfactant to prepare an electrolytic solution. 2. Preparation of negative electrode 95% by weight of graphite as negative electrode active material and PVD as binder
F5% by weight was mixed with NMP as a solvent so as to have a solid content of 50% to form a slurry. Next, this slurry was applied on one surface of a copper foil serving as a current collector.
For 1 hour. Next, the above-mentioned coating film was similarly formed on the other surface of the copper foil. Next, the coating film on the current collector was formed by a roller press so that the film density became 1.4 g / cm ³ . Then, the coating film on the current collector was
Heating under reduced pressure and drying at 0.01 to 1 atm at 12 ° C. for 12 hours produced the negative electrode in Example 1.

2. Preparation of Positive Electrode 90% by weight of lithium manganate as a positive electrode active material, 5% by weight of graphite as a conductive agent, and 5% by weight of PVDF as a binder were mixed with NMP as a solvent so as to have a solid content of 50%. The resulting slurry was applied on one surface of an aluminum foil serving as a current collector, and this coating film was dried at 80 ° C. for 1.5 hours.
Next, a coating film was similarly formed on the other surface of the aluminum foil. Next, the coating film on the current collector was formed by a roller press so that the film density became 2.4 g / cm ³ .
Next, the coating film on the current collector was dried by heating at 110 ° C. for 12 hours to produce a positive electrode.

4. Preparation of Lithium Secondary Battery A microporous polypropylene separator was sandwiched between the negative electrode and the positive electrode, housed in a stainless steel can, and then filled with the electrolytic solution in the stainless steel can. Was manufactured.

(Examples 2 and 3) The lithium secondary batteries of Examples 2 and 3 were obtained in the same manner as in Example 1 except that the amount of the surfactant was changed to the value shown in Table 1. . (Examples 4 and 5) Surfactants shown in Table 1 (Example 4 is alkyldimethylammonium hexafluorophosphate,
In Example 5, the lithium secondary batteries of Examples 4 and 5 were changed to polyoxyethylene alkylmethylammonium hexafluorophosphate, and the amount added was changed to 1% by weight. I got Comparative Example 1 A lithium secondary battery of Comparative Example 1 was obtained in the same manner as in Example 1, except that no surfactant was added to the electrolyte solution. Comparative Example 2 A lithium secondary battery of Comparative Example 2 was obtained in the same manner as in Example 1 except that the amount of the surfactant was changed to 20% by weight.

[Performance Test] Examples 1 to 5 and Comparative Example 1,
In the lithium secondary battery of No. 2, the initial capacity a and 10
The capacity b after 0 cycles was measured, and the capacity maintenance rate (b / a)
× 100 (%) was calculated. Table 1 shows the results.

[0036]

[Table 1]

As described above, the results of the examples show that the cycle life of the lithium secondary battery to which a specific amount of a surfactant is added is longer than that of the lithium secondary battery to which no surfactant is added.

[0038]

As described above, in the present invention, a surfactant film can be formed on the electrode surface only by adding a surfactant to the electrolytic solution, which requires a complicated process. It is possible to suppress the side reaction of the electrode solution at first, and since the side reaction is suppressed at a high temperature, the capacity reduction of the lithium secondary battery due to the deterioration to the electrolyte is small, and the cycle life is long. It becomes a lithium secondary battery.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Tsutomu Hashimoto 5-717-1, Fukabori-cho, Nagasaki-city, Nagasaki Pref. AM07 DJ08 DJ09 EJ11 HJ01

Claims (3)

[Claims] 1. A lithium secondary battery comprising an electrolyte solution containing lithium ions between a positive electrode containing lithium composite oxide as a main component and a negative electrode containing carbon material as a main component. A lithium secondary battery, wherein the liquid contains an organic solvent in which a lithium metal salt is dissolved and 1 × 10 ⁻⁴ to 10% by weight of a surfactant. 2. The lithium according to claim 1, wherein the surfactant is selected from an anionic surfactant, a cationic surfactant, a neutral surfactant, and an amphoteric surfactant. Rechargeable battery. 3. The surfactant is one selected from the group consisting of a perfluoroalkyl sulfonate, a perfluoroalkyl phosphate, a perfluoroalkyl carboxylate, a trimethylammonium perfluoroalkyl salt, and an ethylene oxide addition type ammonium salt. Alternatively, the lithium secondary battery according to claim 1 or 2, wherein the lithium secondary battery is two or more surfactants.