Stock material or miscellaneous articles – Self-sustaining carbon mass or layer with impregnant or...
Reexamination Certificate
1999-04-16
2004-07-20
Hendrickson, Stuart (Department: 1754)
Stock material or miscellaneous articles
Self-sustaining carbon mass or layer with impregnant or...
C423S448000
Reexamination Certificate
active
06764767
ABSTRACT:
RELATED APPLICATION DATA
The present application claims priority to Japanese Application No. P10-111001 filed Apr. 21, 1998 which application is incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to graphite powders having a novel structure suitable as a carbonaceous material for a negative terminal of a lithium ion secondary battery. More particularly, it relates to graphite powders that are able to fabricate a negative electrode of a lithium ion secondary battery having a high discharge capacity and superior charging/discharging efficiency, a method for producing these graphite powders, a material for a negative electrode of the lithium ion secondary battery formed of these graphite powders, and a lithium ion secondary battery having the negative electrode which is fabricated from this negative terminal material.
2. Description of Related Art
A lithium secondary battery is among non-aqueous secondary batteries employing lithium as an active material for a negative electrode, an oxide of a transition metal or chalcogenides, such as sulfides or selenides, as an active material for the positive electrode, and a solution of an inorganic or organic lithium salt in a non-protonic organic solvent, as an electrolytic solution.
Since lithium is a metal having an extremely base potential, it is possible with the battery employing this as a negative electrode to take out a large voltage easily. Consequently, a lithium secondary battery is recently stirring up notice as a secondary battery of high electromotive force and a high energy density, such that expectations are made of applications thereof as a distribution or portable type battery in a wide range of applications, such as electronic equipments, electric cars or power storage. It is already being put to use as a small-sized battery.
In an early version of the lithium secondary battery, use is made of a foil-shaped metal lithium as a negative electrode material. In this case, a charging/discharging reaction proceeds by dissolution (ionization) and precipitation of lithium. However, since metal lithium tends to be precipitated as a needle on the negative electrode in the reaction of Li
+
→Li, repeated charging/discharging leads to precipitation of a dendritic lithium (lithium dendrite) on the surface of the negative electrode. If growth of this lithium dendrite is allowed to proceed, shorting with the negative electrode tends to occur through a separator (partition), thus leading to a fatal defect of an extremely short repetitive charging/discharging cyclic life.
As means for solving the problem of the lithium secondary battery, it is proposed in, for example, Japanese Laying-Open Patent S-57-208079 to use a carbon material capable of storing and yielding lithium ions, such as natural graphite, artificial graphite, petroleum coke, sintered resin, carbon fibers, pyrocarbon, carbon black etc, as a negative electrode material. In this case, the negative electrode material may substantially be formed only of the carbon material, and an electrode operating as a negative electrode usually can be fabricated by allowing powders of the carbon material to be deposited on a metal current collector along with a suitable resin binder.
Although the electrode reaction of a lithium secondary battery, the negative terminal of which is prepared from this carbonaceous material, is not known precisely, it may be presumed that, during charging, electrons are forwarded to the carbon material of the negative electrode and charged to the negative polarity such that lithium ions in the electrolytic solution are accumulated by electrochemical intercalation in the carbon material of the negative electrode charged to the negative polarity. Conversely, during the discharging, lithium ions are desorbed (de-intercalated) from the carbon material of the negative electrode and emitted into the electrolytic solution. That is, charging/discharging occurs due to accumulation and emission of lithium ions in or from the negative electrode material. Therefore, this sort of the battery is generally termed a lithium ion secondary battery. In the lithium ion secondary battery, in which metal lithium is not precipitated during the electrode reaction, there is raised no problem of deterioration of the negative electrode due to dendritic precipitation. The lithium secondary battery now in use is mainly of this type, that is, a lithium ion secondary battery the negative electrode of which is formed of a carbon material.
The theoretical capacity of the lithium ion secondary battery, the negative electrode of which is formed only of metal lithium, is as high as approximately 3800 mAH. Conversely, the theoretical capacity of the lithium ion secondary battery, the negative electrode of which is formed of a lithium/graphite interlayer compound (C
6
Li), is 372 mAH/g, this capacity being retained to be a limit or threshold capacity. It is noted that the lithium/graphite interlayer compound (C
6
Li) is an inter-layer compound in which lithium ions are packed densely in a regular pattern between layers of graphite which is the most crystalline carbonaceous material.
However, since surface activated sites which inhibit intrusion of lithium ions into the carbon material of the negative electrode and a dead zone against packing of lithium ions exist in actuality in the carbon material of the negative electrode, it has been extremely difficult to achieve the threshold capacity of 372 mAH/g even with the use of the high crystalline graphite as the carbon material for the negative electrode of the lithium ion secondary battery.
Meanwhile, the carbon material may be classified into hard carbon (low-crystalline amorphous carbon) and soft carbon (high-crystalline graphite carbon). The above-mentioned threshold capacity, which holds for the soft carbon, fails to hold for the hard carbon, there being a material manifesting a higher capacity per weight. However, the capacity per volume is lowered because of the lower density of the hard carbon.
If the graphite, as the high-crystalline carbon material, is used as the negative electrode material, there is deposited an inactivated skin film in the course of charging with the above-mentioned decomposition of the electrolytic solution. Since the electrical quantity used at this time represents the loss, the charging/discharging efficiency [discharging capacity/charging capacity×100 (%)], as one of battery indices, is lowered. This is a considerable demerit for a usage such as a small-sized battery having a pre-set shape standard because the quantity of the negative electrode material needs to be estimated to a larger value at the time of battery designing.
For approaching the discharging capacity of the lithium ion secondary battery to the above-mentioned threshold capacity as much as possible, various proposals have so far been made as to the manufacturing method for the carbonaceous material for the negative electrode.
For example, it is proposed in Japanese Laying-Open Patent H-4-115458, Japanese Laying-Open Patent H-5-234584 and Japanese Laying-Open Patent H-5-307958 to use carbides of mesophase globules generated in the pitch carbonization process. The mesophase globules are spherically-shaped particles exhibiting optical isomerism (properties of liquid crystal) and which are generated on heat treatment of pitches for several hours at approximately 400 to 550° C. On continued heat treatment, the globules grow in size and coalesce to become a bulk mesophase which exhibits optical isomerism in their entirety. This bulk mesophase can also be used as the carbon material. However, the discharging capacity of the lithium ion secondary battery employing this negative electrode material is as yet rather low.
In the Japanese Laying-Open Patent H-7-282812, attempts are made to raise the regularity of the layered disposition of the graphite layers in association with graphized carbon fibers to raise the capacity of the lithium ion secondary
Abe Masaru
Kamei Kazuhito
Kaminaka Hideya
Moriguchi Koji
Nagamine Masayuki
Bell Boyd & Lloyd LLC
Hendrickson Stuart
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