A Lithium Secondary Battery

LITHIUM SECONDARY BATTERY

Field of Invention

The present invention relates to a lithium secondary battery having excellent battery characteristics in cycle performance, electric capacity and storage property.

Background of Invention

Recently, a lithium secondary battery is generally employed as an electric source for driving small elec¬tronic devices. The lithium secondary battery essential¬ly comprises a positive electrode, a nonaqueous electro¬lytic solution, and a negative electrode. A lithium secondary battery utilizing a positive electrode of lith¬ium compound oxide such as LiCoO2 and a negative electrode of carbonaceous material or lithium metal is favorably used. As the electrolytic solution for the lithium sec¬ondary battery, a carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) is favorably used. Nevertheless, it is desired to provide a secondary battery showing improved characteristics in the cycle performance and electric capacity. A lithium secondary battery utilizing a positive electrode of LiCoO2, LiMn2O4 or LiNiO2 sometimes shows decrease of electric performances because a portion of the nonaqueous solvent in the nonaqueous electrolytic solution oxidatively decomposes in the course of charging and hence the produced deconposition product disturbs the desired electrochemical reaction. The decomposition is considered to be caused by electrochemical oxidation of the solvent on the interface between the positive elec¬trode and the nonaqueous electrolytic solution. On the other hand, a lithium secondary battery uti¬lizing a negative electrode of carbonaceous material of

high crystallization such as natural graphite or artifi¬cial graphite also shows decrease of electric performanc¬es because a solvent of the electrolytic solution reduc-tively decomposes on the surface of the negative elec¬trode in the course of charging. The reductive decompo-sition also occurs in the repeated charging-discharging procedures when EC (which is generally employed as the nonaqueous solvent of the electrolytic solution) is uti¬lized as the nonaqueous solvent. Japanese Patent Provisional Publication 10-74537 describes that the cycle performance and electric capaci¬ty are improved when a small amount of an aromatic com¬pound such as benzene having a hydrocarbon substituent (e.g./ cyclohexylbenzene) . Japanese Patent Provisional Publication 10-112335 describes that the cycle performance is improved when a small amount of a fluorine atom-containing aromatic com-pound such as f luorobenzene is added to a nonaqueous electrolytic solution of a lithium secondary battery.

Disclosure of Invention

The present invention has an object to provide a lithium secondary battery showing improved battery cycle performance, improved electric capacity, and improved storability in the charged condition. The present invention resides in a lithium secondary battery comprising a positive electrode, a negative elec¬trode of artificial graphite or natural graphite and a nonaqueous electrolytic solution having an electrolyte dissolved in a nonaqueous solvent, wherein 0.1 to 20 wt.% of a cyclohexylbenzene having a halogen atom bonded to a benzene ring thereof is contained in the nonaqueous elec¬trolytic solution. The cyclohexylbenzene having a halogen atom bonded to a benzene ring thereof employed in the invention pre¬ferably is a conpound having the following formula (I):

wherein X is a halogen atom, and the halogen atom is attached to an optional position. Preferred is l-halogeno-4-cyclohexylbenzene.

Detailed Explanation of the Invention

In the cyclohexylbenzene having a halogen atom bond¬ed to a benzene ring thereof (hereinafter referred to as "cyclohexyl-halogenobenzene") contained in the nonaqueous electrolytic solution containing an electrolyte dissolved in a nonaqueous solvent, the halogen atom preferably is a fluorine atom or a chlorine atom. Exanples of the cyclohexyl-halogenobenzenes include 1 - f luoro- 2 - cyclohexylbenzene, 1 - f luoro- 3 -cyclohexyl -benzene, 1 - f luoro - 4 - eye lohexylbenzene, 1 - chloro - 4 -cyclohexylbenzene, l-bromo-4-cyclohexylbenzene, and 1-iodo - 4 - eye lohexylbenzene. If the content of the cyclohexyl-halogenobenzene in the nonaqueous electrolytic solution is extremely large, the battery performances may lower. If the content of the cyclohexyl-halogenobenzene is extremely small, an expected improvement of the battery performances cannot be attained. Accordingly, the content preferably is in the range of 0.1-20 wt.%, more preferably 0.2-10 wt.%, most preferably 0.5-5 wt.%, based on the amount of the nonaqueous electrolytic solution, so that the cycle per¬formance can well improved. Examples of the non-aqueous solvents employed in the electrolytic solution of the invention are cyclic carbon¬ates such as ethylene carbonate (EC) , propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC) , lactones such as y-butyrolactone, linear carbonates such as dimethyl carbonate (DMC) , methyl ethyl carbonate

(MEC), and diethyl carbonate (DEC), ethers such as tetra-hydrofuran, 2 -methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2- diethoxyethane, and 1,2- dibutoxy-ethane, nitriles such as acetonitrile and adiponitrile, esters such as methyl propionate, methyl pivalate, butyl pivalate, octyl pivalate and dimethyl oxalate, amides such as dimethylformamide, and compounds containing S=0 group such as 1,3-propanesultone, glycol sulfite and divinyl sulfone. The non-aqueous solvents can be employed singly or in combination of two or more. There are no specific limitations with respect to the combination of the non¬aqueous solvents. Examples of the combinations include a combination of a cyclic carbonate and a linear carbonate, a combination of a cyclic carbonate and a lactone, and a combination of plural cyclic carbonates and linear car¬bonates . Exanples of the electrolytes employed in the inven¬tion include LiPFg, LiBF4, LiC104, LiN(SO2CF3) 2, LiN(SO2C2F5)2, LiC(SO2CF3)3, LiPF4(CF3)2, LiPF3 (C2F5) 3, LiPF3(CF3)3, LiPF3(iso-C3F7)3, and LiPF5(iso-C3F7) . These electrolytes can be employed singly or in combination of two or more. The electrolyte can be incorporated into the nonaqueous solvent generally in such an amount as to give an electrolytic solution of 0.1 M to 3 M, preferably 0.5 M to 1.5 M. The electrolytic solution of the invention can be prepared, for instance, by mixing the above-mentioned non-aqueous solvents; dissolving the above-mentioned electrolyte in the mixture; and further dissolving at least one of the above-mentioned cyclohexyl-halogeno-benzenes in the resulting mixture. For instance, the active material of positive elec¬trode is a compound metal oxide comprising lithium and cobalt or nickel. The active material of positive elec¬trode can be used singly or in combination. Examples of

the compound metal oxides include LiCoO2, LiNiO2, and LiCO1-xNixO2 (0.01