WO2004006379A1

WO2004006379A1 - Electrolyte composition for lithium secondary battery having low swelling level at high temperature

Info

Publication number: WO2004006379A1
Application number: PCT/KR2003/001360
Authority: WO
Inventors: Kwonsun Roh; Chi-Kyun Park; Jonha Lee; Jaemyoung Lee
Original assignee: Skc Limited
Priority date: 2002-07-09
Filing date: 2003-07-09
Publication date: 2004-01-15
Also published as: KR20040006058A; KR100560210B1; AU2003250550A1

Abstract

An electrolyte composition comprising a nitrogen-containing compound, biphenyl, propane sultone, an organic solvent and a lithium salt is advantageously used for the preparation of a lithium secondary battery having low swelling level at high temperature, and high overcharge-safety, cycling life and rate capability properties.

Description

ELECTROLYTE COMPOSITION FOR LITHIUM SECONDARY

BATTERY HAVING LOW SWELLING LEVEL

AT HIGH TEMPERATURE

Field of the Invention

The present invention relates to an electrolyte composition for lithium secondary batteries, which provides low high-temperature swelling property, high overcharge-safety and enhanced performance at the same time.

Background of the Invention

Lithium secondary batteries are classified into two types depending on the kind of electrolyte used: a lithium ion battery which employs a liquid electrolyte; and a lithium ion polymer battery, a gel polymer electrolyte.

Lithium secondary batteries are sensitive to certain types of abuse, particularly overcharge abuse of exceeding the normal operating voltage during recharge, and such overcharging causes heating of the battery, which can lead to fire. Accordingly, many efforts have been made to develop a method to improve the safety of lithium batteries by incorporating various overcharge protection additives into the electrolyte. For example, U.S. Patent Nos. 5,879,834 and 6,033,797 disclose a method of adding certain aromatic compounds such as biphenyl, 3-chlorothiophene, furan, etc., to the electrolyte. The aromatic compound employed in this method is electrochemically polymerized at voltages greater than the maximum operating voltage thereby increasing the internal resistance of the battery sufficiently for overcharge protection.

Unfortunately, however, this method had a problem in that biphenyl is polymerized by an acid catalyst such as HF and a Lewis acid existing in the electrolytic solution even at normal operating voltage, which results in adversely affecting the cycling life and self-discharge properties in the battery systems. In addition, when the temperature of the battery is increased to a high temperature of about 90 ^°C , the polymerization of overcharge protection additive (e.g., biphenyl, etc.) in the electrolyte may be accelerated on the interface between the anode and the electrolyte to produce a larger amount of H₂ gas which brings about increasing the degree of swelling of the battery. As for swelling problem of batteries at high temperature, on the other hand, there have been reported several methods to lower swelling level of lithium secondary batteries by incorporating certain additives into the electrolyte. For example, Japanese Laid-Open Publication Nos. 10-50342, 12-3724, 12-123868, 12-323171 and 13-52738 describe a method of adding propane sultone to an electrolyte. Propane sulton in the electrolyte forms a passivation film on the surface of the carbonaceous anode and may lower swelling level of lithium batteries by inhibiting the reaction between the anode and the electrolyte to decrease the amount of gases(e.g., ethane, ethylene, CO₂, CO, etc.) generated due to the electrochemical reduction of the organic solvent in the electrolyte.

Therefore, in order to lower swelling level at high temperature and simultaneously to improve the safety of lithium batteries, Japanese Laid- Open Publication No. 13-307773 suggests a method of adding lithium carbonate to an electrode active material with the addition of propane sultone to an electrolyte. However, this method has a problem in that the content of the electrode active material becomes low, resulting in a loss of energy density. Summary of the Invention

Accordingly, it is an object of the present invention to provide an electrolyte composition for lithium secondary battery having low swelling level at high temperature, and simultaneously high overcharge-safety and performance properties.

It is another object of the present invention to provide a lithium secondary battery comprising such an electrolyte. In accordance with one aspect of the present invention, there is provided an electrolyte composition comprising a nitrogen-containing compound, biphenyl, propane sultone, an organic solvent and a lithium salt.

Detailed Description of the Invention

The inventive electrolyte composition in accordance with the present invention is characterized by inco orating a nitrogen-containing compound as an acid-scavenger together with propane sultone as a swelling-inhibiting additive and biphenyl as an overcharge protection additive into the electrolytic solution comprising an organic solvent and a lithium salt.

The nitrogen-containing compound, biphenyl and propane sultone may be used in an amount ranging from 0.1 to 5% by weight, from 3 to 10% by weight and from 0.05 to 1.5% by weight, respectively, based on the total weight of the electrolytic solution. Preferably, biphenyl and propane sultone may be used in an amount ranging from 5 to 8% by weight and from 0.05 to 1% by weight, respectively, based on the total weight of the electrolytic solution. Biphenyl used in the inventive composition is electrochemically polymerized at voltages above the maximum operating charging-voltage of the battery resulting in the formation of the insulating polymer on the cathode surfaces, and thus substantially raise the internal resistance of a battery to enhance overcharge-safety.

When the amount of biphenyl is less than 3% by weight, overcharge- safety cannot be ensured; and when more than 10% by weight, poor self- discharge property results.

The nitrogen-containing compound used in the present invention removes HF or a Lewis acid typically existing in the electrolytic solution to inhibit acid-catalyzed polymerization of biphenyl at normal operating voltage, thereby making biphenyl accomplish the intended role as an overcharge protection additive during overcharge.

The nitrogen-containing compound which may be used in the present invention includes a tertiary amine, an aromatic nitrogen-containing heterocyclic compound and a polymeric form thereof, among which an aromatic or non-aromatic tertiary amine, a 6-membered aromatic heterocyclic compound and a 5-membered fused aromatic heterocyclic compound are preferred. Representative examples of the 6-membered aromatic heterocyclic compound may include pyridine, pyridazine, pyrimidine, pyrazine and triasine; and the 5-membered fused aromatic heterocyclic compound, triazole, thiazole and thiadiazole. In addition, preferred as the aromatic or non-aromatic tertiary amines are those which contain 1 or more nitrogen atoms and 5 or more carbon atoms. When the amount of the nitrogen-containing compound is less than

0.1% by weight, the acid such as HF in the electrolytic solution may not be removed effectively; and when more than 5% by weight, poor self-discharge property results.

Further, propane sultone used in the inventive composition forms a passivation film at high temperature on the surface of the anode, and thus the electrochemical polymerization of biphenyl on the interface between the electrolyte and the anode at high temperature is suppressed without adversely affecting the normal reactions of the battery to inhibit the production of H₂ gas, which results in decreasing swelling level of lithium batteries.

When the amount of propane sultone is less than 0.05% by weight, the reaction between the electrolyte and the anode cannot be suppressed effectively; and when more than 1.5% by weight, the capacity becomes poor.

Besides the nitrogen-containing compound, biphenyl and propane sultone, a halogen- or epoxy-containing compound may be further added to the said electrolyte composition in an amount ranging from 0.02 to 1.5% by weight based on the total weight of the electrolytic solution, if desired. The halogen- or epoxy-containing compound may react with a nitrogen- containing compound in the inventive electrolyte composition to undergo gelling at high temperature. Thus, the inventive electrolyte composition can be changed to a gel polymer electrolyte by adding a halogen- or epoxy- containing compound to the electrolytic solution and then heating it.

The halogen-containing compound which may be used in the present invention includes unsubstituted or substituted alkylene halide and aromatic halide, and polymers, copolymers and olygomers thereof. Among these halogen-containing compounds, preferred are the aromatic halide such as halomethylbenzene, halomethylnaphthalene, halomethylbiphenyl, bis(halomethyl)benzene, bis(halomethyl)naphthalene, bis(halomethyl)biphenyl, tris(halomethyl)benzene, tris(halomethyl)naphthalene, tris(halomethyl) biphenyl, tetrakis(halomethyl)benzene, tetrakis(halomethyl)naphthalene, tetrakis(halomethyl)biphenyl and halomethylstyrene; and the alkylene halide such as diiodoalkane, triiodoalkane and tetraiodoalkane, wherein halomethyl group means chloromethyl, bromomethyl or iodomethyl, and alkylene halide means haloalkane compound containing 2 or more carbon atoms. The most preferable examples of unsubstituted or substituted alkylene halide and aromatic halide may include bis(bromomethyl)benzene, α ,α '- dibromoxylene and diiodoalkane.

Further, the epoxy-containing compound which may be used in the present invention includes 3,4-epoxycyclohexylmethyl-3',4'- epoxycyclohexane carboxylate, glycidyl dodecafluoroheptylether, polypropyleneglycol diglycidylether, butadiene diepoxide, butanediol diglycidylether, cyclohexene oxide, cyclopentene oxide, diepoxy cyclooctane, ethyleneglycol diglycidylether and 1,2-epoxyhexane. Exemplary lithium salts that may be used in the present invention are

LiPF₆, LiAsF₆, LiC10₄, LiN(CF₃S0₂)₂, LiBF₄, LiCF₃S0₃, LiSbF₆ and a mixture thereof. The lithium salt may be present at a concentration ranging from 0.5 to 2 M in an organic solvent. When the concentration of the salt is less than 0.5 M, the capacity becomes poor; and when more than 2 M, poor cycling life property results.

Representative examples of the organic solvent used in the present invention include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, dipropyl carbonate, dimethoxyethane, diethoxyethane, vinylene carbonate, gamma-butyrolactone, ethylene sulfite and propylene sulfite.

The inventive electrolytic solution may be prepared by simply mixing the nitrogen-containing compound, biphenyl, propane sultone, the lithium salt and the organic solvent. Also, as mentioned above, in order to form a gel polymer electrolyte, a halogen- or epoxy-containing compound may be further added to said electrolyte composition.

In accordance with another aspect of the present invention, there is provided a lithium secondary battery comprising a cathode, an anode, a separator interposed between the cathode and the anode, and said electrolyte composition. The present invention may be applied to any type of lithium batteries.

Typically, a cathode composition, i.e., a mixture of a cathode active material, a conducting agent, a binder and a solvent, may be coated directly on an aluminum current collector, or laminated in the form of a film on an aluminum current collector to form a cathode sheet.

The cathode active material may be lithium-containing metal oxides such as LiCo0₂, LiMn₂θ₄ and LiNi0₂. The conducting agent may be carbon black; the binder may be vinylidene fluoride/hexafluoropropylene copolymers, polyvinylidene fluoride (PVDF), polyacrilonitrile, polymethylmetacrilate or polytetrafluoroethylene; and the solvent may be N- methylpyrrolidone (NMP) or acetone. The conducting agent, the binder and the solvent may be used in an amount ranging from 1 to 10 parts by weight, from 2 to 10 parts by weight and from 30 to 100 parts by weight based on 100 parts by weight of the cathode active material, respectively.

Also, an anode composition, i.e., a mixture of an anode active material, a conducting agent, a binder and a solvent, may be coated directly on a copper current collector, or laminated in the form of a film on a copper current collector to form an anode sheet.

Representative examples of the anode active material may include carbon-based materials and graphite. The conducting agent, the binder and the solvent, which may be the same as those used in the cathode composition, may be used in an amount of below 10 parts by weight, ranging from 2 to 10 parts by weight and from 30 to 100 parts by weight based on 100 parts by weight of the anode active material, respectively. If necessary, a plasticizer may be further added to said cathode and anode compositions to form porous electrode sheets.

Further, a separator which is interposed between the cathode and the anode sheets may be of a microporous sheet made from, for example, a polymeric material such as polyethylene and polypropylene. An appropriate separator sheet is located between the cathode and the anode sheets to form an electrode stack. The electrode stack may be wound or stacked, placed into a cylindrical or angular battery case and then sealed, followed by injecting the inventive electrolyte composition thereinto to prepare a lithium secondary battery. In addition, in case of preparing a lithium ion polymer battery, the process for making a battery further comprises the step of gelating the electrolytic solution comprising a halogen- or epoxy-containing compound by heating at 30 to 130 °C .

The inventive battery prepared in accordance with the present invention is characterized by having low high-temperature swelling, and high overcharge-safety and performance properties at the same time.

The following Example and Comparative Examples are given for the purpose of illustration only, and are not intended to limit the scope of the invention.

Examples 1 to 3 polyvinylidene fluoride (PVDF, Solvay) and 200 g of N-methylpyrrolidone (NMP, Aldrich) were mixed to form a cathode composition. The cathode composition was coated on an aluminum foil, dried and pressed to prepare a 122 μ thick cathode sheet. 93.76 g of mesophase carbon micro bead (MCMB 25-28, Osaka gas),

6.24 g of polyvinylidene fluoride (PVDF, Solvay) and 180 g of N- methylpyrrolidone (NMP, Aldrich) were mixed to form an anode composition. The anode composition was coated on a copper foil, dried and pressed to prepare a 120 μm thick anode sheet. A polypropylene separator sheet (25 μm, 2300 microporous film;

Cellgard) was disposed between the cathode and anode sheets to form an electrode stack. The electrode stack was wound in a jellyroll manner, placed into an aluminum can and then sealed with a bar sealer.

Various amounts of propane sultone, 2 g of poly(vinylpyridine-co- styrene) (PVPS, Aldrich), 0.5 g of butanediol diglycidylether (BDDGE, Aldrich) and 6 g of biphenyl (Aldrich) were dissolved into 1M LiPF₆ in a 1 : 1 volume mixture of ethylene carbonate and diethyl carbonate (EC-DEC, Merck) to form 100 g each of various electrolytic solutions as shown in Table 1. Each of the electrolytic solutions was injected into the sealed can through an inlet, and then allowed to gel by heating at 65 ^°C for 24 hours, to obtain three lithium secondary batteries (Examples 1 to 3, respectively).

Comparative Example 1

The procedure of Example 1 was repeated except that propane sultone was not employed, to obtain a comparative lithium secondary battery. Battery Performance Characteristics

The capacity (on 225 mAh discharge), rate capability (%, at 2C discharge rate) and cycling life (%, at 150 cycles) of each of the lithium secondary batteries obtained in Examples and Comparative Example were measured with Maccor's testing system, and overcharge-safety (with 12 volts, at 1C & 2C discharge rate), with Power Supply (Hewlett Packard), and the swelling level (%) was measured after being placed at 90 ^°C for four hours. The results are shown in Table 1.

Table 1

As shown in Table 1, the batteries obtained in Examples 1 to 3 exhibit much improved high-temperature swelling property, as compared with the battery obtained in Comparative Example 1. The above results suggest that the inventive electrolyte composition comprising propane sultone as a swelling-preventing agent and biphenyl as an overcharge protection additive together with a nitrogen-containing compound can be advantageously used in preparing an improved lithium secondary battery having low swelling level at high temperature as well as high safety and performance characteristics.

While the embodiments of the subject invention have been described and illustrated, it is obvious that various changes and modifications can be made therein without departing from the spirit of the present invention which should be limited only by the scope of the appended claims.

Claims

What is claimed is:

1. An electrolyte composition comprising a nitrogen-containing compound, biphenyl, propane sultone, an organic solvent and a lithium salt.

2. The composition of claim 1, wherein the nitrogen-containing compound, biphenyl and propane sultone are used in an amount ranging from 0.1 to 5% by weight, from 3 to 10% by weight and from 0.05 to 1.5% by weight, respectively, based on the total weight of the composition.

3. The composition of claim 1, wherein the nitrogen-containing compound is selected from the group consisting of a tertiary amine, an aromatic nitrogen-containing heterocyclic compound and a polymeric form thereof.

4. The composition of claim 3, wherein the tertiary amine is an aromatic or non-aromatic tertiary amine and the aromatic nitrogen-containing heterocyclic compound is selected from the group consisting of a 6- membered aromatic heterocyclic compound and a 5-membered fused aromatic heterocyclic compound.

5. The composition of claim 4, wherein the tertiary amine is an aromatic or non-aromatic tertiary amine containing 1 or more nitrogen atoms and 5 or more carbon atoms and the aromatic nitrogen-containing heterocyclic compound is selected from the group consisting of pyridine, pyridazine, pyrimidine, pyrazine, triasine, triazole, thiazole and thiadiazole.

6. The composition of claim 1, which further comprises a halogen- or epoxy-containing compound.

7. The composition of claim 6, wherein the halogen- or epoxy-containing compound is used in an amount ranging from 0.02 to 1.5% by weight based on the total weight of the composition.

8. The composition of claim 1, wherein the lithium salt is selected from the group consisting of LiPF₆, LiAsF₆, LiC10₄, LiN(CF₃S0₂)₂, LiBF₄, LiCF₃S0₃ and LiSbF₆.

9. The composition of claim 8, wherein the concentration of the lithium salt in the organic solvent is in the range from 0.5 to 2 M.

10. A lithium secondary battery comprising the electrolyte composition of any one of claims 1 to 9.