Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-08-24
2003-06-10
Henderickson, Stuart L. (Department: 1754)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C423S448000
Reexamination Certificate
active
06576369
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a graphite powder having a novel microstructure, which is suitable for use as a carbon material for negative electrodes of lithium ion secondary batteries. More specifically, the present invention pertains to a graphite powder capable of producing negative electrodes for lithium ion secondary batteries having a high discharge capacity and an improved charge/discharge coulombic efficiency and to a process for the preparation of such a graphite powder. It is also concerned with a material for negative electrodes of lithium ion secondary batteries and a lithium ion secondary battery having a negative electrode formed from such a material.
BACKGROUND ART
Lithium secondary or storage batteries are a class of nonaqueous secondary batteries using lithium as an active material for a negative electrode, an oxide or chalcogenide (e.g., sulfide or selenide) of a transition metal as an active material for a positive electrode, and a solution of an inorganic or organic lithium salt dissolved in an aprotic organic solvent as an electrolyte.
Since lithium is a base metal having a very low electric potential, use of lithium as a negative electrode in a battery provides the battery with the ability to readily obtain a high potential therefrom. For this reason, lithium secondary batteries have recently attracted increasing attention as promising secondary batteries having a high electromotive force and a high energy density, and they are expected to find applications as distributed batteries or portable batteries in a wide variety of fields including electronic equipment, electric equipment, electric vehicles, and electric power storage. Lithium secondary batteries have already been put into practical use as compact batteries.
Initially, lithium metal in the form of foil was used to form a negative electrode by itself in lithium secondary batteries. In such cases, the discharging and charging reactions proceed with dissolution (ionization) and deposition of lithium. In the reaction of Li
+
→Li during charging cycles, however, the metallic lithium tends to deposit as acicular crystals on the negative electrode, and repeated discharging and charging cycles cause the formation of lithium dendrite (tree-like branching crystals) on that electrode. As the lithium dendrite grows, it may break through the separator of the battery, leading to an internal short circuit by direct contact of the dendrite with the positive electrode. Therefore, these batteries have a fatal drawback of a very short cycle life in a repeated discharging and charging cycle test.
In order to eliminate this problem in lithium secondary batteries having a negative electrode of lithium metal, it was proposed to use carbon materials (e.g., naturally-occurring graphite, artificial graphite, petroleum coke, carbonized resins, carbon fibers, pyrolyzed carbon, carbon black, and the like), which are capable of reversibly receiving and releasing lithium ions, to form a negative electrode of these batteries [see, e.g., Published Unexamined Japanese Patent Application No. 57-208079(1982)]. In such batteries, the material for the negative electrode may be comprised substantially entirely of the carbon material. Such a negative electrode can be manufactured by attaching the carbon material in powder form to a metal base serving as a current collector, normally with the aid of a suitable binder.
The electrode reactions of a lithium secondary battery having a negative electrode made of a carbon material have not been elucidated completely but may be considered to be as follows. While the battery is charged, electrons are delivered to the carbon material of the negative electrode, thereby causing the carbon material to be negatively charged. The electrolyte contains lithium ions, which are attracted toward the negatively charged carbon material of the negative electrode and are received therein by an electrochemical intercalation reaction. Conversely, during a discharging cycle, the lithium ions contained in the carbon material are removed (deintercalated) from the negative electrode to release them into the electrolyte solution. Thus, charge and discharge occur by receipt of lithium ions into the negative electrode material and release of them from the material. In view of this mechanism, this type of battery is generally called a lithium “ion” secondary battery. Lithium ion secondary batteries do not involve the deposition of metallic lithium on the negative electrode during electrode reactions, thereby avoiding the problem of deposition of lithium dendrite, which deteriorates the negative electrode significantly. Lithium secondary batteries which are commercially used at present are mostly of this type, i.e., of the type having a negative electrode of a carbon material.
The theoretical capacity of a lithium secondary battery having a negative electrode of lithium metal is very high, i.e., on the order of 3800 mAh/g. In the case of a lithium ion secondary battery having a negative electrode of a carbon material which receives lithium ions therein, its theoretical capacity is limited to 372 mAh/g even when the negative electrode is comprised of a lithium-graphite intercalation compound (C
6
Li), which is a graphite (a highly crystalline carbon material) having lithium ions densely and regularly incorporated in the interstices between the layer crystal lattices of graphite.
In practice, however, the carbon material used as a negative electrode has surface active sites which interfere with entry of lithium ions and dead regions incapable of receiving lithium ions. Therefore, even if a highly crystalline graphite is used to form a negative electrode of a lithium ion secondary battery, it is extremely difficult to achieve a capacity of 372 mAh/g, the theoretical capacity of C
6
Li.
In addition, when a highly crystalline carbon material or graphite is used to form a negative electrode, the surface of the electrode has a higher reactivity than the inside thereof since the crystal structure is interrupted on the surface, and a passivated film tends to deposit on the more reactive surface as a component of the electrolyte is slightly decomposed by the action of a high charge voltage. The quantity of electricity consumed for the decomposition is lost wastefully, thereby decreasing the charge/discharge coulombic efficiency (ampere-hour efficiency) of the electrode, i.e., the ratio of discharged to charged quantity of electricity, which is an indication of the performance of a secondary battery, calculated by the equation [(discharge capacity)/(charge capacity)×100 (%)]. The use of such a material requires that a battery be designed using an extra amount of material for the positive electrode to allow for the decrease in charge/discharge coulombic efficiency. This is disadvantageous for applications such as compact batteries which have a given shape defined by specifications.
In order to increase the discharge capacity of a carbon negative electrode for lithium ion secondary batteries to as close to the above-described theoretical capacity as possible, various methods have been proposed for the production of a carbon material for the negative electrode.
For example, Published Unexamined Japanese Patent Applications Nos. 4-115458 (1992), 5-234584 (1993), and 5-307958 (1993) disclose the use of a carbonized product of mesophase microbeads which are formed in the course of carbonization of pitch. The mesophase microbeads are liquid crystalline spherical particles exhibiting optical anisotropy and are formed by subjecting pitch to heat treatment for a few hours or more at about 400-550° C. When the heat treatment is further continued, the microbeads are grown and finally united with each other to form a mass, called a bulk mesophase, which exhibits optical anisotropy as a whole. The bulk mesophase may be used as a material for carbonization. However, the carbonized products of these materials, when used as a negative electrode, do not have a sufficiently high discharge capac
Abe Masaru
Kamei Kazuhito
Kaminaka Hideya
Moriguchi Koji
Negi Noriyuki
Burns Doane , Swecker, Mathis LLP
Henderickson Stuart L.
Sumitomo Metal Industries Ltd.
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