WO2004040687A1

WO2004040687A1 - Electrolyte composition for lithium secondary battery having high overcharge-safety

Info

Publication number: WO2004040687A1
Application number: PCT/KR2003/002274
Authority: WO
Inventors: Kwonsun Roh; Jonghyuk Choi; Jaemyoung Lee; Jonha Lee
Original assignee: Skc Limited
Priority date: 2002-10-29
Filing date: 2003-10-27
Publication date: 2004-05-13
Also published as: KR20040037534A; AU2003272137A1; KR100560211B1

Abstract

An electrolyte composition comprising a nitrogen-containing compound, cyclohexyl benzene, an organic solvent and a lithium salt is advantageously used for the preparation of a lithium secondary battery having high overcharge-safety, cycling life and high-temperature swelling properties at the same time.

Description

ELECTROLYTE COMPOSITION FOR LITHIUM SECONDARY BATTERY HAVING HIGH OVERCHARGE-SAFETY

Field of the Invention

The present invention relates to an electrolyte composition for lithium secondary batteries, which provides 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 secondaiy 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, 6,033,797, 6,074,776 and 6,074,777, and Japanese Laid-Open Publication Nos. 9-17447, 9-50822, 8-293323, 8-306387, 9-22722, 5-36439, 2000-215909, 2001-15155 and 2002-203594 disclose a method of adding certain aromatic compounds such as biphenyl, 3-chlorothiophene, furan, cyclohexyl benzene, 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, the overcharge protection additive may be 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 self-discharge property in the battery systems. Further, polymerization of the overcharge protection additive may produce gaseous by-products, i.e., H₂ gas, thereby increasing the swelling level of lithium batteries. Accordingly, when the amount of an overcharge protection additive incorporated into the electrolyte is increased, cycling life and high-temperature swelling properties are decreased.

As a result, it is very difficult to identify electrolyte compositions for lithium secondary battery having high overcharge-safety, cycling life and high-temperature swelling properties at the same time. Particularly, it is more difficult to meet all these requirements in case of a lithium ion polymer battery, since more amount of an overcharge protection additive is required to be incorporated into the electrolyte for ensuring the safety of batteries due to the low mobility of the additive in a gel polymer electrolyte.

Summary of the Invention

Accordingly, it is an object of the present invention to provide an electrolyte composition for lithium secondary battery having high overcharge-safety, and enhanced cycling life and high-temperature swelling properties at the same time. 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, cyclohexyl benzene, 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 incorporating a nitrogen-containing compound as an acid-scavenger together with cyclohexyl benzene as an overcharge protection additive into the electrolytic solution comprising an organic solvent and a lithium salt.

The nitrogen-containing compound and cyclohexyl benzene may be used in an amount ranging from 0.1 to 5% by weight and from 4 to 10% by weight, respectively, based on the total weight of the electrolytic solution.

Cyclohexyl benzene used in the inventive composition is electrochemically polymerized at voltages above the maximum operating charging-voltage of the battery resulting in the formation of an insulating polymer on the cathode surfaces, and thus substantially raise the internal resistance of a battery to enhance overcharge-safety.

When the amount of cyclohexyl benzene is less than 4% by weight, overcharge-safety cannot be ensured; and when more than 10% by weight, poor self-discharge property results. Particularly, in case of a lithium ion polymer battery, the amount of cyclohexyl benzene is preferably more than 5% by weight based on the total weight of the electrolytic solution, due to the low mobility of cyclohexyl benzene in a gel polymer electrolyte. Further, 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 cyclohexyl benzene at normal operating voltage, thereby making cyclohexyl benzene accomplish the intended role as an overcharge protection additive during overcharge without lowering cycling life and high-temperature swelling properties.

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.

Besides the nitrogen-containing compound and cyclohexyl benzene, an 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 epoxy-containing compound may react with a nitrogen-containing compound in the inventive electrolyte composition to undergo gelling at a temperature ranging from 25 to 130 °C . Thus, the inventive electrolyte composition can be changed into a gel polymer electrolyte by adding an epoxy-containing compound to the electrolytic solution. 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.

Further, 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 sulfϊte and propylene sulfite.

The inventive electrolytic solution may be prepared by simply mixing the nitrogen-containing compound, cyclohexyl benzene, the lithium salt and the organic solvent. Also, as mentioned above, in order to form a gel polymer electrolyte, an 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 an epoxy- containing compound by maintaining at a temperature of 25 to 130 ^°C . The inventive battery prepared in accordance with the present invention is characterized by having high safety, performance and high- temperature swelling characteristics 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 4

1000 g of LiCo0₂ (Seimi), 40 g of acetylene black (Chevron), 50 g of polyvinyhdene fluoride (PVDF, Solvay) and 1100 g of N-methylpyrrolidone

(NMP, Aldrich) were mixed with a plenary mixer to form a cathode composition. The cathode composition was coated on an aluminum foil, dried and pressed to prepare a 180 β thick cathode sheet.

1000 g of mesophase carbon micro bead (MCMB 25-28, Osaka gas), 15 g of acetylene black (Chevron), 100 g of polyvinyhdene fluoride (Solvay) and 800 g of N-methylpyrrolidone (NMP, Aldrich) were mixed with a plenary mixer to form an anode composition. The anode composition was coated on a copper foil, dried and pressed to prepare a 200 μ thick anode sheet.

A polypropylene separator sheet (25 βm, 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 poly(vinylpyridine-co-styrene) (PVPS, Aldrich), 1,4-butanediol diglycidylether (BDDGE, Aldrich) and cyclohexyl benzene (CHB, Aldrich) were dissolved into 1M LiPFβ in a 1:1:1 weight mixture of ethylene carbonate, diethyl carbonate and dimethyl carbonate (EC-DEC- DMC, Ferro) 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 under an argon gas atmosphere, and allowed to gel by heating at 65 °C for 36 hours, to obtain four lithium secondary batteries (Examples 1 to 4, respectively).

Examples 5 to 8

The procedure of Example 1 was repeated except that pyrimidine, triazine, triethylamine and polyvinyl pyridine, respectively, were used instead of PVPS as a nitrogen-containing compound as shown in Table 1, and the gelation process was omitted, to obtain four lithium secondary batteries (Examples 5 to 8, respectively).

Comparative Examples 1 to 3

The procedure of Example 5 was repeated except that the nitrogen- containing compound was not employed, and cyclohexyl benzene was used in the amount as shown in Table 1, to obtain three comparative lithium secondary batteries (Comparative Examples 1 to 3, respectively).

Battery Performance Characteristics

The cycling property (%, at 200 cycles) of each of the lithium secondary batteries obtained in Examples and Comparative Examples were measured with Maccor's testing system; the overcharge-safety (with 6 volts, at IC & 2C discharge rate), with Power Supply (Hewlett Packard); and the high-temperature swelling property (%), with hot-chamber (SKC) 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 8 exhibit much improved properties in terms of the cycling life, high- temperature swelling and overcharge-safety, as compared with the batteries obtained in Comparative Examples 1 to 3. Further, when the amount of the overcharge protection additive, cyclohexyl benzene, is less than 4 % by weight based on the total weight of the electrolytic solution (Comparative Examples 1 and 2), fire was observed during overcharge. Additionally, the battery obtained in Comparative Example 3 showed a satisfactory overcharge-safety property but exhibited poor cycling life and high- temperature swelling properties. The above results suggest that the inventive electrolyte composition comprising cyclohexyl benzene as an overcharge protection additive together with a nitrogen-containing compound can be advantageously used in preparing a lithium secondary battery having improved safety, cycling life and high-temperature swelling characteristics at the same time.

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, cyclohexyl benzene, an organic solvent and a lithium salt.

2. The composition of claim 1, wherein the nitrogen-containing compound and cyclohexyl benzene are used in an amount ranging from 0.1 to 5% by weight and 4 to 10% 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 an epoxy- containing compound.

7. The composition of claim 6, wherein the 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.