Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
1999-06-07
2002-07-02
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
Reexamination Certificate
active
06413486
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nonaqueous secondary battery of high energy density and high repeating stability usable as power source for electronic appliance, constituent elements of battery, and electrochemical elements.
The invention further relates to a novel secondary battery of large size, small size, thin type, and light weight usable in the fields of electronic appliance, electric vehicles and others, and more particularly to a lithium ion secondary battery of high energy density of which current collector is composed of a flexible graphite sheet.
2. Description of the Prior Art
Along with the enhancement of performance of electronic appliances, the appliances are required to be smaller in size and portable. As a result, secondary batteries of small size and large capacity are demanded. On the other hand, for use as power source for electric vehicle, secondary batteries of large size, light weight, and large capacity are demanded.
Existing secondary batteries include the lead storage battery and nickel-cadmium battery, among others, but to replace them, lithium secondary batteries of higher energy density are attracting wide attention. In lithium secondary batteries, it was first attempted to use metal lithium as active material, but as charging and discharging were repeated, dendritic metals grow on the electrode surface, and if the growth is excessive, it is known to lead to overheating of the battery.
As one of the methods to prevent this, it has been proposed to use a carbonaceous material for absorbing lithium between layers, instead of metal lithium. When a carbonaceous material is used, lithium dendrite does not grow, and it is effective to prevent overheating of the battery.
However, when graphite is used as carbonaceous material, the upper limit of the capacity is 372 mAh/g. Instead of graphite, by using a material obtained by baking pitch at low temperature of 1000° C. or less, it is known that a capacity exceeding 372 mAh/g is obtained.
In this low temperature baking of pitch, however, the potential in charging and discharging fluctuates largely depending on the depth of charging and discharging, and it is hard to handle in control of power source.
This capacity is the electric charge density, and for increase of capacity from the viewpoint of energy density, it is disadvantageous when the flat zone of potential is small.
As other method of suppressing growth of lithium dendrite, it is proposed to solidify or gelate the electrolyte solution between positive and negative electrodes. In the conventional battery, liquid electrolyte was used, and dendrites grew, but by solidifying or gelating, it has been known that the dendrite growth is notably suppressed in the direction of the solid electrolyte.
Moreover, by solidifying or gelating, if metal lithium can be used as active material, the lithium performing oxidation and reduction reaction can be directly used as the electrode, and the upper limit of the capacity restricted when using carbon can be increased, and a large capacity is realized.
In the conventional battery, to prevent leak of electrolyte solution, a rigid container and a seal structure were used, which was hindrance to reduction of weight and thickness. Yet, to seal a container in a shape having notch or the like, an expensive and complicated device was needed. By solidifying or gelating the electrolyte, a simple container or seal structure can be used as compared with the case of using liquid, and the battery can be reduced in thickness and formed in a desired shape. More preferably, flash point and other heat resistant stability tend to be higher than in liquid, and it is expected to be beneficial in the assembling and manufacturing process.
Not only larger capacity, but also longer life of secondary battery is also demanded. In most secondary batteries containing lithium ions in the electrolyte, transition metal oxides such as LiCoO
2
, LiMn
2
O
4
, and V
2
O
5
are used in the positive electrode, but these transition metal oxides change in the volume significantly depending on lithium ions moving in and out. Accordingly, as the battery repeats charging and discharging, the performance as secondary battery deteriorates, and finally failing to charge and discharge sufficiently.
In the condenser, on the other hand, liquid electrolyte was used in the inexpensive electrolytic type, but evaporation of electrolyte was one of the factors of aging deterioration of characteristics. To prevent such aging deterioration, instead of liquid electrolyte, it has been attempted to use manganese dioxide or conductive high polymer. Alternatively, by gelating the electrolyte, evaporation may be suppressed. In the case of the gel, a stronger restoration action of the condenser is expected, as compared with solid electrolyte such as manganese dioxide.
In conventional lithium secondary batteries, as disclosed in Japanese Laid-open Patent No. 62-90863, Japanese Laid-open Patent No. 63-121260, and Japanese Laid-open Patent No. 3-49155, a transition metal compound oxide mainly composed of lithium and cobalt is used in the positive active material, and a carbon material in the negative active material. The positive active material is disposed on a metal current collector of aluminum, stainless steel or the like, and the negative active material on a metal current collector made of copper foil of 10 to 20 &mgr;m in thickness that is, an aqueous binder or nonaqueous binder is added to the active material, and is applied and held on one side or both sides of the current collector.
Thus, in the conventional lithium secondary battery, since a metal of large specific gravity is used in the current collector, the energy density per unit weight of the battery is not so high. Besides, such current collector is poor in contact with the active material, and the contact resistance increases, which causes impedance increase and cycle deterioration.
Recently, on the other hand, from the standpoint of environmental problems such as air pollution and global warming, large-sized secondary batteries of large capacity as power source for electric vehicles are being developed intensively. As the power source for electric vehicle, nickel-hydrogen absorbing alloy battery, lead storage battery, and nickel-cadmium battery are being put in practical use.
However, the total weight of the battery is very heavy, about 300 to 500 kg, and the energy density per unit weight is small, and the driving distance per one charge is limited, and development of secondary battery of high energy density per unit weight is urgently needed.
As the nonaqueous secondary battery used in the power source for electronic appliance, a larger capacity for longer time of continuous use, smaller size, and lighter weight is demanded. At the same time, high repeating stability for longer life is required. To satisfy these requirements, the nonaqueous secondary battery of high energy density and high repeating stability is demanded. However, to realize the nonaqueous secondary battery or electrochemical elements satisfying both high energy density and high repeating stability, there were problems as mentioned in the prior art.
The invention is to solve these problems, and it is hence an object thereof to realize constituent elements for manufacturing the nonaqueous secondary battery having high energy density and high repeating stability, and nonaqueous secondary battery and electrochemical elements using them.
It is also an object of the invention to present a lithium ion secondary battery of large capacity, excellent in cycle characteristics by improving the current collector to reduce the battery weight.
SUMMARY OF THE INVENTION
It is an object of the invention to realize constituent elements for manufacturing a nonaqueous secondary battery having high energy density and high repeating stability, and a nonaqueous secondary battery and electrochemical elements using them.
It is also an object of the invention to present a lithium ion secondary battery of large capac
Miyamoto Akihito
Nanai Norishige
Nichogi Katsuhiro
Tsuchiya Soji
Watanabe Kazuhiro
Hendrickson Stuart L.
Matsushita Electric - Industrial Co., Ltd.
Ratner & Prestia
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