Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1997-06-12
2001-01-16
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231800
Reexamination Certificate
active
06174625
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a nonaqueous electrolyte secondary cell, more particularly to an active anode substance of the nonaqueous electrolyte secondary cell and a production method thereof, and a nonaqueous electrolyte secondary cell incorporating the anode substance.
2. Description of the Prior Art
Recently with spread of portable appliances such as video cameras, demand for reusable secondary cells in place of conventional disposable cells is increasing. The majority of secondary cells now available in the market are nickel-cadmium cells involving alkali solutions. It has been difficult, however, to improve further the energy density of a conventional cell because its voltage is about 1.2V. It presents with another problem: its self discharge rate rises as high as 20% when left at room temperature for one month.
To meet these problems studies have been directed to a nonaqueous electrolyte secondary cell which incorporates a nonaqueous solvent as an electrolyte solution and a light metal like lithium as a material of the anode, thereby to raise the voltage up to 3V, to increase the energy density, and to reduce the self-discharge rate. With such a secondary cell, however, the metal lithium used as a material of the anode grows into dendritic processes under the influence of repeated charges/discharges, and the processes may come into contact with the cathode to form a shunt within the cell, which may cause the life of the cell to shorten. This problem hinders the cell from being put into practice.
To meet this problem, a nonaqueous electrolyte secondary cell which incorporates an alloy of lithium or the like with other metals as a material of the anode has been proposed. The same problem still holds for this cell: repeated charges/discharges turn the alloy into fine particles, which may lead to the shortened life of the cell.
To meet above problems, for example, as disclosed in Japanese Unexamined Patent Publication No. 62-90863, a nonaqueous electrolyte secondary cell has been proposed in which a carbon material like cokes is employed as an active anode substance. This secondary cell, being free from above problems, is excellent in endurance to cyclic activation. Further, as disclosed by the present authors in Japanese Unexamined Patent Publication No. 63-135099, when LixMO
2
(where M represents one or more transition metals and x is a number not less than 0.05 and not more than 1.10) is used as an active cathode substance, the resulting nonaqueous electrolyte secondary cell will have a long life and a high energy density.
The nonaqueous electrolyte secondary cell which incorporates a carbon material as an active anode substance is more excellent in endurance to cyclic activation and safety as compared with the secondary cell which incorporates a metal lithium as an active anode substance, but is inferior in energy density to the latter. To improve this defect measures have been taken to improve the filling density, but it still remains a technically unsolved problem.
One of the reasons why this type of cell is inferior in energy density may lie in the fact that it requires the addition of a binder for adherence of a powder acting as an active substance. Namely, when a carbon material is used, a material like pitch is sintered and ground, or the material, after being ground, is sintered anew to produce a powder to be ready for use. Then, to this powder are added a binder such as rubber and a dispersant to produce a slurry. The slurry is applied on an electricity collecting body or is molded into pellets, to produce an anode. Thus, the electrode is composed of three elements; a carbon material, a binder and an electricity collecting body. The binder typically is added by 3-20%. The content of the active substance (filling density) in the cell is restricted by that amount of the binder, which will impose a limit on the capacity of the cell.
SUMMARY OF THE INVENTION
This invention has been proposed as a remedy for these problems, and intends to provide an active anode substance allowing the resulting secondary cell to have a high capacity, requiring nevertheless only carbon materials conventionally used, and a production method thereof, and a nonaqueous electrolyte secondary cell having a high energy density and a long life.
This invention has been proposed to meet above problems, and depends on a method by which to produce an active anode substance which consists of preparing two carbon materials different in calcination temperature from mesophase carbon, combining the two carbon materials at a specified ratio, turning the mixture into particles, converting the particles into a mold, and sintering the mold in an atmosphere of an inert gas or in a vacuum. This invention solves above problems by employing the active anode substance as a material of the anode and making a nonaqueous electrolyte secondary cell which incorporates the anode.
Another nonaqueous electrolyte secondary cell this invention provides comprises an electrode-layered body having a laminated structure with three or more layers when layers around the cathode and anode being summed, and whose discharge capacity per unit reaction area is adjusted to 6-100 mAh/cm
2
. The discharge here concerned occurs under the condition where the charged electricity is maintained at 0.1C or less.
Mesophase carbon is used as a starting material, two carbon materials different in calcination temperature are prepared therefrom, and the two carbon materials are submitted to a specified process to produce a sintered body which, when used as an active anode substance, enables the resulting secondary cell to have a high capacity.
Further, electrode layers around the cathode and anode to form an electrode-layered body are allowed to be three or more in number, and the thickness of electrodes and the number of layers are so adjusted as to allow the electrode-layered body to have a discharge capacity per unit reaction area of 6-100 mAh/cm
2
. This adjustment will enable the resulting cell to have a high capacity.
If the discharge capacity per unit reaction area exceeds 100 mAh/cm
2
, the number of layers will be too small, and the resulting cell will not show a satisfactory performance when used as a secondary cell shaped like a square or cylinder like conventional cells, although they can be used with profit for backup purposes.
On the contrary, if the discharge capacity per unit reaction area is under 6 mAh/cm
2
, the number of layers will become too large, and the resulting cell will have a discharge capacity similar to that of conventional cells.
REFERENCES:
patent: 5028500 (1991-07-01), Fong et al.
patent: 5344724 (1994-09-01), Ozaki et al.
patent: 5451477 (1995-09-01), Omaru et al.
patent: 5571638 (1996-11-01), Satoh et al.
patent: 5622793 (1997-04-01), Iijima et al.
patent: 5672446 (1997-09-01), Barker et al.
patent: 0 627 777 A2 (1994-05-01), None
patent: PCT/US95/05776 (1995-10-01), None
ISO R349 Dec. 1963.
Patent Abstracts of Japan Application No. 06114568 dated Apr. 28, 1994.
Mamada Norio
Takeuchi Yoshiaki
Yamahira Takayuki
Chaney Carol
Sonnenschein Nath & Rosenthal
Sony Corporation
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