Lithium secondary cell and nonaqueous electrolyte

Abstract

The sudden generation of heat being frequently caused in the case of the overcharge of a lithium secondary cell which have a positive electrode comprising a composite metal oxide of lithium and cobalt or a composite metal oxide of lithium and nickel, a negative electrode comprising metallic lithium, a lithium alloy or a material capable of occluding and releasing lithium, and a nonaqueous electrolyte solution comprising a nonaqueous solvent and an electrolyte dissolved therein can be efficiently prevented by the addition, to the nonaqueous electrolyte solution, of an organic compound which, when the lithium secondary cell is overcharged, decomposes into a decomposition product capable of dissolving out the cobalt or nickel contained in the positive electrode and depositing it ion the negative electrode (for example, a tert-alkylbenzene derivative).

Description

[0001] The present invention relates to a lithium secondary battery and a non-aqueous electrolytic solution which is favorably employable for the lithium secondary battery. The invention specifically relates to a lithium secondary battery improved in battery characteristics such as cycle performance, battery capacity and storage performance and further in its safety such as prevention of sudden heat generation which is caused in the case of overcharging, and a non-aqueous electrolytic solution which is favorably employable for the lithium secondary battery.BACKGROUND OF INVENTION[0002] At present, a lithium secondary battery is generally employed as an electric source for driving small electronic devices. The lithium secondary battery is expected not only for the use as a potable electronic / communication tool such as small size video camera, potable phone, and note-size personal computer but also an electric source of automobile. The lithium secondary battery essentially comprises a...

AI Technical Summary

Benefits of technology

[0014] The present invention resides in a method of preventing sudden heat generation when a lithium secondary battery comprising a positive electrode comprising a composite metal oxide of lithium and cobalt or a composite metal oxide of lithium and nickel, a negative electrode comprising a lithium metal, a lithium alloy or a material capable occluding and releasing lithium, and a non-aqueous electrolyte solution comprising an electrolyte in a non-aqueous solvent is overcharged, which comprises dissolving an organic compound in the non-aqueous electrolyte solution, decomposing the organic compound when the overcharging takes place, to give a decomposed product, the decomposed product functioning to dissolve cobalt or nickel out of the positive electrode, and deposit the cobalt or nickel an the negative electrode.

[0020] When the battery is overcharged, the above-mentioned compound contained in the non-aqueous electrolyte solution oxidatively decomposes at a potential in the range of +4.6 V to +5.0 V, as compared with the lithium, and cobalt or nickel in the positive electrode dissolves out and deposits on the negative electrode. It is assumed that the cobalt or nickel deposited on the negative electrode prevent reaction between the lithium metal deposited on the negative electrode and the carbonate contained in the non-aqueous electrolyte solution.

[0021] Further, the deposition of cobalt or nickel on the negative electrode of the battery may sometimes cause formation of short circuit so that the prevention of overcharging can be made. Thus, the safety of battery is sufficiently ensured.

[0023] Accordingly, a lithium secondary battery having not only good safety for preventing overcharging but also good battery characteristics such as cycle characteristics, battery capacity and storage stability is provided.

[0027] As the organic compound, cyclohexylbenzene (4.7 V) is also employed. Particularly, if a portion of the organic compound having a high oxidation potential such as within 4.8-5.0 V such as the above-mentioned tert-butylbenzene is replaced with cyclohexylbenzene having a low oxidation potential such as 4.7 V, the prevention of overcharging is enhanced. In the case that a portion of the tert-butylbenzene is replaced with cyclohexylbenzene, the tert-butylbenzene is preferably employed in an amount of 4 or less parts, more preferably 0.3 to 3 parts, most preferably 0.5 to 2.5 parts, per one part of cyclohexylbenzene. As is described above, the use of combination of two or more organic compounds having different oxidation potentials is effective to enhance the prevention of overcharging. However, the compound of the present invention is not restricted by these compounds, so long as the organic compounds decompose at a potential in the range of +4.6 V to +5.0 V and the cobalt or nickel in the positive electrode dissolves in the solution at the time of overcharging.

Problems solved by technology

When the lithium secondary battery is overcharged to a level higher than the ordinary working voltage, an excessive amount of lithium is released from the positive electrode, and simultaneously excessive lithium deposits on the negative electrode, and dendrite is produced.

Therefore, both of the positive electrode and the negative electrode are rendered chemically unstable.

If both of the positive and negative electrodes become chemically unstable, they soon react with carbonate in the non-aqueous electrolyte solution to decompose the carbonate, and sudden exothermic reaction occurs.

Accordingly, the battery as such generates abnormal heat, and trouble of lowering of battery safety occurs.

The trouble will be more serious in the case that the density of a lithium secondary battery increases.

Japanese Patent Provisional Publication 11-162512 points out a problem in the use of an additive such as biphenyl or the like in a battery in that the battery characteristics such as cycle characteristic are apt to lower when the cyclic procedure is repeated up to a voltage exceeding 4.1 V or the battery is discharged at a high temperature exceeding 40.degree. C. for a long period of time, and that this problem is more prominently observed when the addition amount of additive increases.

Although the anisole derivative disclosed in Japanese Patent Provisional Publication 7-302614 favorably functions by-redox shuttle in the case of overcharging, it has problems in that adverse effects are observed On cycle characteristics and storage stability.

Thus, the amount of an anisole derivative gradually decreases in the course of ordinary charge-discharge procedures, and hence the safety may not be ensured after the charge-discharge procedures of 300 cycles.

However, as is pointed out in the aforementioned Japanese Patent Provisional Publication 11-162512, they impart adverse effect to the cycle characteristics and storage stability.

Further, the adverse effect increases when the amount of biphenyl and the like is increased.

Thus, the amount of a biphenyl or the like gradually decreases in the course of ordinary charge-discharge procedures, and hence the safety may not be ensured after the charge-discharge procedures of 300 cycles.

Further, although the battery containing 2,2-diphenylpropane shows high cycle characteristics as compared with the cycle characteristics shown in the battery containing biphenyl, it still does not show such high cycle characteristics, as compared with a battery containing no additive.

In consequence, it does not satisfy either battery characteristics or safety such as prevention of overcharging.