Positive electrode active material, positive electrode...

Details Positive electrode active material, positive electrode... Positive electrode active material, positive electrode...

2000-05-12

2003-07-08

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S212000, C429S218100, C429S231300, C429S231800, C429S231950, C429S338000

Reexamination Certificate

active

06589694

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to a non-aqueous electrolytic secondary battery. More particularly, the present invention relates to an improvement in a lithium ion secondary battery, specifically, a positive electrode active material capable of improving charge and discharge cycle characteristic and safety of lithium ion secondary battery, a positive electrode active material composition, and to a lithium ion secondary battery having improved charge and discharge cycle characteristic, low temperature characteristic and safety.
BACKGROUND OF THE INVENTION
Lithium ion secondary batteries have superior electromotive force and battery capacity, and are more advantageous than nickel-cadmium battery etc. in that they show high energy density, high voltage etc. While they have been drawing much attention in recent years, they have been more often employed as driving force of portable devices, such as cellular phones and note type personal computers. Motivated by the situation as described, various studies have been undertaken in the pertinent field to provide a higher performance product. To be specific, such studies focus on the properties and preferable combinations of the constituent materials of the Lithium ion secondary batteries: positive electrode active materials, negative electrode active materials, electrolytes and the like.
As a positive electrode active material for lithium ion secondary batteries, there have been proposed a number of Li-transition metal composite oxides, such as Li—Mn type composite oxide, Li—Ni type composite oxide, Li—Co type composite oxide and the like. Of these, Li—Co type composite oxides have been predominantly put into practice, because they are chemically stable, can be handled easily and are capable of producing secondary batteries having high capacity. There are many suggestions and reports to further improve Li—Co type composite oxides to ultimately improve the properties of the secondary battery that uses a Li—Co type composite oxide as a positive electrode active material. For example, JP-B-7-118318 discloses that LiCoO
2
can improve the discharge capacity of a secondary battery, which LiCoO
2
is obtained by adding a rich amount of a lithium compound and a cobalt compound, heating the compounds and removing, by washing with water, unreacted lithium compound and lithium carbonate byproduct in the reaction product.
Mostly, the positive electrode active material for a lithium ion secondary battery is a layer made from a composition (hereinafter to be also referred to as a positive electrode active material composition) consisting of a conductive material and a binder made from an organic polymer. As the conductive material, various graphites and carbon black are used. The positive electrode active material is generally used in the form of particles, each particle dispersed in a non-conductive binder in the composition. Absence of a conductive material leads to an electrically insulated state of each particle in the positive electrode active material due to the action of the binder, which in turn makes the layer of the positive electrode active material composition (hereinafter to be also referred to as a positive electrode active material layer) substantially electrically insulating. The conductive material is used to make this layer conductive by its presence between the particles of the positive electrode active material to electrically connect the particles. Consequently, the positive electrode active material layer as a whole becomes conductive. The positive electrode active material is used in the form of particles as mentioned earlier. When the particle size is too small, the reactivity during charge and discharge of a secondary battery sometimes becomes too great to the extent that abnormal cell reaction is induced to a dangerous level. The present inventors have found that, from the aspect of the safety of the secondary battery, the preferable average particle size of a positive electrode active material is not less than 10 &mgr;m. However, an average particle size of not less than 10 &mgr;m lowers the conductivity of the positive electrode active material layer, frequently causing degraded charge and discharge cycle characteristic.
With regard to the negative electrode active material and electrolyte, for example, JP-A-6-36802 discloses that charge and discharge cycle characteristic of a lithium ion secondary battery can be improved by using a positive electrode active material made from a Li-transition metal composite oxide, a negative electrode active material made from a specific pitch type carbon fiber, and a mixed solvent of one or more members from a group of ethylene carbonate, propylene carbonate, butylene carbonate, &ggr;-butyrolactone, sulfolane, 3-methylsulfolane, tetrahydrofuran, 2-methyltetrahydrofuran, acetonitrile, dimethoxyethane, diethoxyethane, dimethylsulfoxide, dioxolane, 4-methyldioxolane and diethyl carbonate, as an electrolyte. U.S. Pat. No. 5,561,005 discloses that charge and discharge cycle characteristic of a lithium ion secondary battery can be improved by using a positive electrode active material made from a Li-transition metal composite oxide, a graphite type carbon material as a negative electrode active material, and a mixed solvent of ethylmethyl carbonate and dimethyl carbonate as an electrolyte, and further a mixture of this mixed solvent and ethylene carbonate or propylene carbonate as an additional component. When such component is to be added, it should be noted that addition of propylene carbonate to a graphite type carbon material as the negative electrode active material results in the decomposition of the solvent, and in this case, therefore, ethylene carbonate is preferably used. When the negative electrode active material is a carbon material other than graphite, propylene carbonate is preferably used (see paragraph 59).
As mentioned above, lithium ion secondary battery has many superior characteristics as compared to nickel-cadmium battery etc., in high energy density, high voltage and the like. On the other hand, it has unpreferable low temperature characteristic in that discharge at a low temperature results in lower discharge capacity and lower discharge voltage than in the case of discharge at room temperature. Particularly, at an extremely low temperature of not more than −20° C. (and not more than −35° C.), the discharge voltage shows a sharp drop at the initial stage of discharge. In the discharge curve [axis of abscissas: discharge capacity rate (%), axis of ordinate: discharge voltage (V) ] of lithium ion secondary battery at a low temperature, the minimum value and the maximum value sequentially appear in the direction of increase of the discharge capacity rate. However, at an extremely low temperature of not more than −20° C., the difference between the minimum value and the maximum value becomes about 0.3 (V)-0.5 (V) and the difference between the minimum value and the discharge voltage, when the discharge capacity rate is 0%, becomes about 0.5 (V)-1.0 (V). This causes a drastic drop of voltage at the initial stage of discharge. In effect, the discharge voltage could fall under the stop voltage set for each equipment, thereby making the equipment practically unoperatable. This problem of low temperature characteristic prevents application of the battery to observation monitors, communication devices, electric automobiles, electric power reservoir and the like, that may be used in frigid places. While some measure for preventing the battery from falling to a temperature below a certain level by, for example, placing the battery in a warmer place or applying a heater, will enable use of the battery for the above-mentioned equipment, this results in an increased cost of the equipment. Therefore, the major problem of the lithium ion secondary battery is to overcome the low temperature characteristic. Lithium ion secondary battery typically has a structure wherein a positive electrode sheet and a negative electrode sheet

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