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, 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 cycling life and self-discharge properties in the battery systems.

Summary of the Invention Accordingly, it is an object of the present invention to provide an electrolyte composition having improved overcharge-safety, cycling life and self-discharge 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, an organic solvent and a lithium salt.

Brief Description of the Drawings The above and other objects and features of the present invention will become apparent from the following description of the invention, when taken in conjunction with the accompanying drawings, which respectively show: FIG. 1: variations of capacity retention (%) of the lithium secondary batteries obtained in Examples 1 to 3 as a function of cycling number.

FIG. 2: variations of capacity retention (%) of the lithium secondary batteries obtained in Comparative Examples 1 to 3 as a function of cycling number.

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 biphenyl as an overcharge protection additive into the electrolytic solution comprising an organic solvent and a lithium salt.

The nitrogen-containing compound and biphenyl may be used in an amount ranging from 0.1 to 5% by weight and from 3 to 12% 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 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 biphenyl is less than 3% by weight, overcharge- safety cannot be ensured; and when more than 12% by weight, poor self- discharge property results.

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 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.

Besides the nitrogen-containing compound and biphenyl, 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, a, a'- 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 (CF3SO2) 2, LiBF4, LiCF3SO3, 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, 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, LiMn204 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 high safety and performance 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 LiCoO2 (Seimi), 40 g of acetylene black (Chevron), 50 g of

polyvinylidene 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 um thick cathode sheet.

1000 g of mesophase carbon micro bead (MCMB 25-28, Osaka gas), 15 g of acetylene black (Chevron), 100 g of polyvinylidene 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, um thick anode sheet.

A polypropylene separator sheet (25, am, 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), 3, 4-epoxycyclohexylmethyl-3', 4'-epoxycyclohexane carboxylate (ECMEC, Aldrich) and biphenyl (BP, Aldrich) were dissolved into a 1 : 1 : 1 weight mixture of ethylene carbonate, diethyl carbonate and dimethyl carbonate (EC-DEC-DMC, Ferro), and then 15.2 g LiPF6 was added thereto 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 biphenyl was used in the amount as shown in Table 1, to obtain three comparative lithium secondary batteries.

Battery Performance Characteristics The capacity (on 225 mAh discharge), capacity retention (%, at 2C discharge rate) and cycling property (at IC discharge rate) of each of the lithium secondary batteries obtained in Examples and Comparative Examples were measured with Maccor's testing system, and overcharge- safety (with 12 volts, at 1C & 2C discharge rate), with Power Supply (Hewlett Packard). The results are shown in Table 1, and FIGs. 1 (Examples 1 to 3) and 2 (Comparative Examples 1 to 3), respectively.

Table 1

N-containing ECMEC BP Capacity Capacity vercharge Compound retention (wt. %) (wt. %) (wt. %) (mAh) (%)-safety Comp. Ex. 1 0 0 0 945 94. 5 fire Comp. Ex. 2 0 0 2.5 942 94.0 fire Comp. Ex. 3 0 0 4 880 80.2 safe Ex. 1 PVPS (2) 0.5 4 939 94.1 safe Ex. 2 PVPS (2) 0.5 6 942 94.6 safe Ex. 3 PVPS (2) 0.5 8 945 93.8 safe Ex. 4 PVPS (2) 0.5 12 943 93.5 safe Pyrimidine Ex. 5 0 4 930 90.5 safe (2) Ex. 6 Triazine (2) 0 4 939 93.6 safe Ex. 7 Triethyl 0 4 920 89. 5 safe amine (2) Ex. 8 Polyvinyl 0 4 909 85.3 safe pyridine (2) As shown in Table 1, and FIGs. 1 and 2, the batteries obtained in Examples 1 to 8 exhibit much improved properties in terms of the capacity, capacity retention, cycling life and overcharge-safety, as compared with the batteries obtained in Comparative Examples 1 to 3. Further, when the amount of the overcharge protection additive, biphenyl, is less than 3 % 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 has satisfactory overcharge- safety but exhibited some observable loss in cycling capacity and capacity retention. The above results suggest that the inventive electrolyte composition comprising biphenyl as an overcharge protection additive together with a nitrogen-containing compound can be advantageously used in preparing an improved lithium secondary battery having 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.