Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-09-25
2002-07-02
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231800
Reexamination Certificate
active
06413672
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lithium secondary battery comprising an anode of a sintered material that contains silicon as an active material, and a method of producing the same.
2. Description of the Related Art
As portable electronic apparatuses such as video cameras and cellular phones become popular, high-energy density lithium secondary batteries have attracted special interest. The lithium secondary battery comprises a cathode and an anode that contain active materials capable of incorporating and releasing lithium ions, and a lithium ion conductive electrolytic solution.
Such attempts have been made as silicon or a compound thereof capable of providing greater capacity per unit volume than carbonaceous materials is used as the anode active material of the lithium secondary battery thereby forming the anode. For example, Japanese Laid-Open Patent Publication No. 7-29602 discloses a method of producing a anode by using Li
x
Si (0≦x≦5) as the anode active material and molding the anode active material with graphite used as an electrically conducting filler and a binder added thereto into pellets.
Silicon is capable of incorporating a greater amount of lithium than the conventionally used carbonaceous materials. However, electrical resistance of a anode that contains silicon increases since volume thereof changes significantly as charging and discharging operations are repeated, and the capacity of the battery decreases with the number of charge and discharge cycles that have been experienced. Since silicon has lower electrical conductivity than the carbonaceous material, it is necessary to add a large amount of carbonaceous material as electrically conducting filler in order to ensure the electrical conductivity of the anode. As a result, there has been such a problem that the capacity of the battery per unit volume decreases.
There has also been such a problem that a battery having silicon-based anode falls in the state of over-discharge when having been left without charging after discharge, and this causes the dissolution of silicon from the anode or dissolution of copper from a current collector thus resulting in decreased battery capacity.
Fluorine-containing lithium salt having high electrical conductivity has been used in the prior art for the electrolyte contained in a non-aqueous electrolytic solution. However, the electrolytic solution contains hydrogen fluoride. Hydrogen fluoride is produced through reaction of the fluorine-containing lithium salt and moisture that inevitably contaminates the electrolytic solution. Hydrogen fluoride is also likely to contaminate as an impurity during the production process. There has also been such a problem that hydrogen fluoride contained in the electrolytic solution reacts with silicon contained in the anode thus decreasing the amount of active material and decreasing the conductivity of the electrolytic solution, thus degrading the charge-discharge cycle property.
To incorporate lithium into the silicon-based anode, for example, such a method is employed as lithium is supplied from the cathode to the anode by charging. Since silicon particles lose crystallinity and changes to an amorphous structure on the first charging, reversible incorporation and release of lithium ions is made possible. However, there has been such a problem that, although the silicon-based anode can incorporate a large amount of lithium, there is a significant difference between the amount of lithium incorporated and the amount of lithium released (irreversible capacity) at the first charging, and satisfactory cycle property cannot be obtained.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a lithium secondary battery having high capacity and good cycle property by improving the electrical conductivity of the anode.
It is another object of the present invention to provide a lithium secondary battery having good storage stability, high capacity and good cycle property by suppressing the dissolution of silicon from the anode and copper from the current collector at a time of over-discharge.
It is still another object of the present invention to provide a lithium secondary battery having high capacity and good cycle property by restricting the amount of hydrogen fluoride contained in the electrolytic solution within a predetermined level.
It is still another object of the present invention to provide a lithium secondary battery having high capacity and good cycle property by decreasing the irreversible capacity.
It is still another object of the present invention to provide a method of producing the lithium secondary battery having high capacity and good cycle property.
According to an aspect of the present invention, there is provided a lithium secondary battery with a cathode and an anode, each electrode containing an active material capable of incorporating and releasing lithium ions, said anode comprises a sintered material which contains, at least, 50 to 99% by weight of silicon and 1 to 50% by weight of a carbonaceous material, and the sintered material has an electrical conductivity not less than 1 S/cm.
According to the present invention, since the anode is made from a sintered material that contains an anode active material consisting of 50 to 99% by weight of silicon and 1 to 50% by weight of the carbonaceous material, the packing density of the anode active material can be increased and the battery capacity per unit volume can be increased. The electrical conductivity of the anode having 1 S/cm or higher makes it possible to provide a battery of high capacity and good cycle property.
It is preferable that silicon contained in the sintered material exists in the form of particles and the particles are covered with the carbonaceous material. The carbonaceous material that covers the silicon particles prevents the silicon particles from making direct contact with the electrolytic solution. This makes it possible to suppress the irreversible reaction of the silicon particles and the electrolytic solution even in the state of over-discharge, suppress the decrease of the battery capacity even during storage and maintain high capacity over a longer period of storage time.
According to another aspect of the present invention, there is provided a lithium secondary battery, the anode is made from the sintered material that contains 50 to 99% by weight of silicon and 1 to 50% by weight of carbonaceous material. The sintered material containing high proportion of silicon provides a high capacity.
According to another aspect of the present invention, there is provided a lithium secondary battery with a cathode and an anode, each electrode containing an active material capable of incorporating and releasing lithium ions, said anode is made from the sintered material that contains 50 to 99% by weight of silicon and 1 to 50% by weight of a carbonaceous material, and lithium is incorporated to a degree equivalent to a capacity of 200 to 800 mAh per 1 g of silicon when fully charged. In a battery capable of incorporating lithium in the anode active material equivalent to 200 to 800 mAh when fully charged, irreversible capacity can be decreased and the cycle property can be improved. In the range described above, it is assumed that decay of the crystal structure of silicon and formation of material that do not participate in the charging and discharging reactions, which are considered to cause the irreversible capacity, are suppressed.
According to another aspect of the present invention, there is provided a lithium secondary battery with a cathode, an anode and a lithium conductive electrolytic solution, each electrode containing an active material capable of incorporating and releasing lithium ions, said anode is made from the sintered material that contains 50 to 99% by weight of silicon and 1 to 50% by weight of a carbonaceous material, and said electrolytic solution contains fluorine-containing lithium salt and hydrogen fluoride with the concentration of not ore t
Isshiki Nobuyuki
Kusakabe Hatsuo
Kusakabe Hideko
Kusakabe Tetsuya
Kuskabe Miharu
Brouillette Gabrielle
Kao Corporation
Kusakabe Hatsuo
Kusakabe Hideko
Kuskabe Miharu
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