Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
2001-04-17
2003-12-02
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S332000, C252S364000
Reexamination Certificate
active
06656642
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a non-aqueous electrolytic solution and a lithium secondary battery employing the non-aqueous electrolytic solution. In particular, the invention relates to a lithium secondary battery having improved electric capacity and cycle characteristics, and a non-aqueous electrolytic solution and non-aqueous solvent which are advantageously employable for preparing the lithium secondary battery.
BACKGROUND OF THE INVENTION
At present, potable small electronic devices such as personal computers, cellular phones, and video recorders equipped with camera are widely used, and a small sized secondary battery having light weight and high electric capacity is desired to provide an electric source for driving such small electronic devices. From the viewpoints of small size, light weight, and high electric capacity, a lithium secondary battery is paid attention.
The lithium secondary battery employs a positive active electrode material comprising a complex oxide such as lithium cobaltate, lithium nickelate, or lithium manganate, a negative active electrode material comprising a carbonaceous material into which lithium ions are able to intercalate and from which lithium ions are able to release, and a non-aqueous electrolytic solution of a lithium salt in a non-aqueous solvent comprising a cyclic carbonate and a linear carbonate. The lithium secondary battery is now studied for improving its characteristics.
Among the carbonaceous materials into which lithium ions are able to intercalate and from which lithium ions are able to release, graphite is considered to be the most preferred negative active electrode material of a lithium secondary battery because of its large electric capacity and advantageous flat electric potential curve, and therefore is employed widely in the art.
There is a problem, however, in that the graphite electrode shows exfoliation on its surface when it is employed in a lithium secondary battery in combination with a non-aqueous solvent for the electrolytic solution which comprises a cyclic carbonate such as ethylene carbonate (EC), propylene carbonate (PC), or butylene carbonate (BC). Simultaneously, the cyclic carbonate is decomposed on the surface of the graphite electrode. The exfoliation of the graphite electrode and decomposition of the cyclic carbonate of the non-aqueous solvent cause decrease of battery characteristics such as electric capacity, cycle characteristics, and storage stability. Particularly, the decrease is apparently observed when the graphite electrode is employed in an electrolytic solution containing propylene carbonate. It is sometimes noted that propylene carbonate decomposes on the surface of the graphite negative electrode when it is subjected to initial charging procedure and that further discharging-charging procedures cannot be done.
For obviating decomposition of an electrolytic solution on the surface of the graphite negative electrode material and exfoliation of the graphite, it has been proposed addition of additive material classified into various compounds. For instance, J. Electrochem. Soc., Vol. 140, No. 6, L 101 (1993) describes that addition of a crown-ether compound (12-crown-4) to an electrolytic solution comprising propylene carbonate and ethylene carbonate obviates decomposition of the electrolytic solution. In this case, however, it is required to use a relatively large amount of an expensive crown-ether compound for effectively obviating the decomposition. Further, the addition of crown-ether still cannot impart to the battery well satisfactory electric characteristics.
U.S. Pat. No. 5,626,981 describes an electrolytic solution comprising a lithium salt and a mixture of at least two aprotic organic solvents of which the first solvent has a high dielectric constant and the second solvent has low viscosity and further contains a soluble compound of the same type as at least one of the solvents and contains at least one unsaturated bond and which can be reduced at the anode at a potential of more than 1 volt with respect to lithium to form a passivation layer. This patent describes that the additive compound is reduced on the anode when the battery is charged, to form a passivation layer on the graphite surface and obviate reduction of other solvent components.
According to the study of the inventors, however, the methods described above cannot give satisfactorily high Coulomb efficiency (i.e., charge-discharge efficiency) at the initial stage. Further, the electric capacity gradually decreases after the charge-discharge cycle is repeated. Thus, the known improvement methods fail to impart satisfactory cycle characteristics and storage stability to the lithium secondary battery.
Further, 1997 Joint International Meeting of The Electrochemical Society, Inc. and International Society of Electrochemistry, Abstracts, P. 153 (1997) describes that a voltamograph obtained in a battery cell comprising a graphite electrode (working electrode)/Li (counter electrode)/Li (reference electrode) and an electrolytic solution of 1M LiPF
6
in a solvent of PC/EC/DMC (DMC: dimethyl carbonate) of 1/1/3 by a volume ratio shows a reduction peak at 1 volt, and that the passivation film is formed on the negative electrode at that voltage so as to keep other solvent components from reducing.
Furthermore, J. Electrochem. Soc., Vol. 140, No. 9, L 161 (1995) describes that addition of chloroethylene carbonate to an electrolytic solution is effective to keep propylene carbonate (PC) from decomposing on the graphite electrode surface. It is assumed that a decomposed product of chloroethylene carbonate forms a passivation film on the graphite surface. However, the inhibition of decomposition of the electrolytic solution is not satisfactorily high.
According to the above-described improvement methods, it has become possible to use a cyclic carbonate (which is an excellent non-aqueous solvent) and a carbonaceous electrode having high crystallinity such as a graphite electrode in combination. Nevertheless, the use of the above-mentioned solvent component is still not able to provide a lithium secondary battery showing well satisfactory battery characteristics.
SUMMARY OF THE INVENTION
The present inventors have focused their studies on the use of a non-aqueous solvent mixture of a cyclic carbonate (which shows excellent characteristics as a non-aqueous solvent for an electrolytic solution) and particularly on the effect of vinylene carbonate (VC) for keeping the electrolytic solution from decomposing on the graphite electrode surface.
As a result, it has been discovered that a vinylene carbonate product prepared by a conventional synthetic process does not provide satisfactory battery characteristics, and further the resulting battery does not have reliable battery characteristics. It is further discovered that the vinylene carbonate product prepared by conventional synthetic processes contains a not small amount of chlorine atom-containing organic compounds which are produced in the process for the preparation of vinylene carbonate as by-products. The by-produced chlorine atom-containing organic compounds are incorporated into a non-aqueous solvent of an electrolytic solution when the vinylene carbonate product is mixed with other non-aqueous solvent components. The chlorine atom-containing organic compounds in the non-aqueous solvent of an electrolytic solution bring about increase of reduction potential of the non-aqueous electrolytic solution and cause lowering of the battery characteristics and reliability of the battery.
The present invention resides in a non-aqueous electrolytic solution which comprises a non-aqueous solvent comprising a cyclic carbonate, a linear carbonate and vinylene carbonate, and an electrolyte dissolved in the non-aqueous solvent, and which shows a reduction potential of less than 1 volt (or a reduction potential higher by less than 1 volt), with reference to lithium.
The invention further resides in a non-aqueous electrolytic solution which comprises a non-aqueous solvent co
Abe Koji
Hamamoto Toshikazu
Takai Tsutomu
Ueki Akira
Kalafut Stephen
Reed Smith LLP
Ube Industries Ltd.
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