Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-11-23
2003-10-14
Gulakowski, Randy (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C252S502000
Reexamination Certificate
active
06632569
ABSTRACT:
TECHNICAL FIELD
This invention relates to a carbonaceous material for electrode and a non-aqueous solvent secondary battery using this material. More specifically, this invention relates to a carbonaceous material for electrode capable of constituting a non-aqueous solvent secondary battery having adequately quick charging and discharging property, more preferably, to a carbonaceous material for anode.
RELATED ART
According to recent trends rendering electronics apparatuses further compact, secondary batteries are required to have a large capacity. Lithium secondary battery specially has been received attentions as having a higher energy density in comparison. with nickel-cadmium batteries and nickel-hydride batteries. As a material for making an anode, use of a lithium metal was tried at an initial stage, but it was turned out that the lithium was deposited in a resin shape (dendrite shape) during repeating of charging and discharging and might reach the cathode in penetrating a separator, thereby raising a risk that the anode and the cathode are short-circuited. Therefore, carbonaceous materials that can prevent dendrite from occurring have been receiving attentions instead of metal electrodes.
As a non-aqueous electrolyte secondary battery using a carbonaceous material a battery in which a non-graphitizable carbonaceous material having a low crystallinity is used as an anode material first has been made commercially available. Subsequently, a battery using a graphite group having a high crystallinity has been made commercially available, and this situation is going on. The electrical capacity of graphite is 372 mAh/g, maximum theoretically, and a battery having a large charging and discharging capacity can be obtained by proper selection of electrolytes.
Some carbonaceous material having a multilayer structure, as shown in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 4-171,677 also has been studied. This is based on a theory that, in combination of an advantage of the graphite having a high crystallinity (large capacity and small irreversible capacity) and a disadvantage (decomposition of a propylene carbonate type) and an advantage of the carbonaceous material having a low crystallinity (good stability in an electrolyte) and a disadvantage (large irreversible capacity), the advantages are mutually utilized while the disadvantages are compensated.
The graphite groups (graphites and multilayered carbonaceous materials containing graphite) have a high crystallinity in comparison with the non-graphitizable carbonaceous material and a high true density. Where the anode is structured of carbonaceous materials of those graphite groups, high electrode filling property is obtainable, and the battery can have a higher volume energy density. It is general, in a case where graphite powders constitute an anode, that a slurry is produced upon addition of a dispersion medium where powers and binders are mixed and is coated on a metal foil as a current collector and thereafter the dispersion medium is dried. During this process, it is also general to provide a step of compression molding to press the powders to the current collector, to unify the thickness of the electrode plate, and to improve the capacity of the electrode plate. With this compression molding, the plate density of the anode is improved, and the energy density per volume of the battery is further improved.
The general graphite materials, having a high crystallinity, industrially available, however, have particle forms of a flaky shape, crystalline (or scale-like) shape, or plate shape. The reason that the particle forms are in a flaky shape, crystalline shape, or plate shape is thought that carbon crystallization mesh surface grows accumulatively in a single direction to form graphite crystallization graphite. Where those graphite materials are used for an anode of the non-aqueous solvent secondary battery, the materials indicate small irreversible capacity and large discharging capacity due to a high crystallinity, but the materials show a lower capacity in rapid charging and discharging in a high current density because crystal edge surfaces, at which lithium ions can enter and exit, exist in a small amount where the particle forms are in a flaky shape, crystalline shape, or plate shape while basal surfaces not involving entry and exit of the lithium ions exist in a large amount. Where the graphite particles are made into electrode plates through the step of manufacturing the plates, the plate density may increase, but on the other hand, because particle spacing is not adequately ensured, the lithium ions are disturbed from moving, so that the rapid charging and discharging ability as a battery may be lowered.
Where graphite powders in a plate shape are molded in electrodes, the plate surface of the powders is arranged in parallel to the electrode plate surface with a high possibility from the influences of the slurry coating step and the plate compression step. Therefore, the edge surfaces of the graphite crystallite constituting respective powder grains are molded in a vertical positional relation to the electrode surface with a relatively high possibility. When charging and discharging are performed under such a plate situation, the lithium ions entered in and separated from the graphite upon traveling between the anode and the cathode are required to go around the powder surface once, and suffer from considerable disadvantages in terms of moving efficiency of ions in the electrolyte. Moreover, spaces left over in the electrodes after the molding are disadvantageously sealed with respect to the exterior of the electrodes because the particles are formed in a plate shape. That is, because free communication of the electrolyte to the exterior of the electrodes may be disturbed, the material raises a problem that the lithium ions are disturbed from moving.
On the other hand, graphitized materials of mesocarbon micro-beads are proposed as an anode material having a spherical shape guaranteeing spaces necessary for movements of lithium ions in the plates and are already made commercially available. Where the ratio of the edge surface is high the areas that the lithium ions can enter in the particles increase, and where the shape is spherical, selective arrangement does not occur in respective powder particles and the isotropy of the edge surfaces is maintained even after the plate compression process described above, so that good moving velocity of the ions in the electrode plates are kept. Spaces remaining in the electrodes are in a state connecting to the exterior of the electrodes on the ground of the particle shapes, and therefore, the lithium ions can move relatively freely, so that the electrode structure is applicable to quick charging and discharging. The mesocarbon micro-beads however, since having a low crystal structure level as a graphite, has a low limitation of electric capacity of 300 mAh/g, and such inferiority has been known well in comparison with graphite in a flaky shape, crystalline shape, or plate shape.
Some inventions have been conceived, in consideration of those problems, in which shapes of the graphite used for the non-aqueous solvent secondary battery are restricted. For example, in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 8-180,873, an invention is disclosed in which a ratio of particles in a flaky shape to particles in a relatively non-flaky shape and the like is restricted. On the other hand, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 8-83,610, in opposition to the above, describes that particles in flaky shapes are preferable.
Practical batteries require electrodes having a large electric capacity and excellently quick charging and discharging ability. However, such an electrode satisfying those demands is not yet provided. Therefore, it is strongly desired to improve the quick charging and discharging ability of the graphitized materials in a flaky shape, crystalline shape, or plate shape.
It is an object of the invention to solve the problems
Ishihara Tadashi
Kameda Takashi
Armstrong Westerman & Hattori, LLP.
Gulakowski Randy
Mitsubishi Chemical Corporation
Wills M.
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