Production process of material for lithium-ion secondary...

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Reexamination Certificate

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C423S448000

Reexamination Certificate

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06485864

ABSTRACT:

BACKGROUND OF THE INVENTION
a) Field of the Invention
This invention relates to processes for the production of carbonaceous materials having high performance as negative-electrode materials, and especially to processes for the production of boron-substituted graphite, silicon-containing boron-substituted graphite and silicon-containing carbonaceous materials for lithium-ion secondary (rechargeable) batteries. This invention is also concerned with the boron-substituted graphite, silicon-containing boron-substituted graphite and silicon-containing carbonaceous materials obtained by the processes. This invention also pertains to lithium-ion secondary batteries making use of the boron-substituted graphite, silicon-containing boron-substituted graphite and silicon-containing carbonaceous materials as materials of negative electrodes.
b) Description of the Related Art
Recent years have seen rapid advancements in the size and weight reductions of electronic equipments and communication equipments, resulting in a strong demand for reductions in size and weight of secondary batteries used as drive power sources for them. Keeping in step with this, lithium-ion secondary batteries with high energy density and high voltage have been proposed. A lithium-ion secondary battery uses, for example, lithium cobaltate as a positive electrode and a carbonaceous material, such as graphite, as a negative electrode. Upon charging, lithium ions are caused to be occluded in the negative electrode, and upon discharging, these lithium ions are emitted from the negative electrode.
Employed as such negative-electrode materials are those available from carbonization or graphitization of carbonaceous raw materials or resins, such as MCMB (meso carbon microbeads) and fine particles of mesophase pitch, derived from petroleum pitch or coal tar pitch. However, dis-charge capacities available from these negative-electrode materials are not sufficiently high as batteries and moreover, initial efficiencies are not very high, either.
With a view to overcoming these problems, a variety of investigations have been made about boron-added carbonaceous materials. In 1992, J. R. Dahn et al. studied, as a host material for lithium intercalate, boron-substituted graphite which was obtained by adding, to artificial graphite, boron oxide in a proportion of about 8 wt. % in terms of boron and then conducting graphitization in a nitrogen gas atmosphere [Phys. Rev. B. 45(7), 3773 (1992)]. In this publication, however, properties or characteristics as an actual battery, such as dis-charge capacity and initial efficiency, are not disclosed.
The teaching of JP 5-266,880 A is only to conduct graphitization by simply adding boron to an organic substance as a raw material for graphite instead of the addition of boron oxide to artificial graphite as disclosed in the technical bulletin referred to in the above. The teaching of this patent publication is therefore not different at all from that of the technical bulletin.
In JP 8-31422 A and JP 9-63584 A, for example, the raw material for graphite to be produced is limited to pitch, or the graphitization temperature is limited to prevent an increase in the particle size of graphite. With these conditions alone, it is impossible to obtain a carbonaceous material optimal as a negative-electrode material for lithium-ion batteries. Further, JP 8-31422 A indicates that, when the thus-obtained carbonaceous material is used as a negative material in a battery, boron carbide remaining in the carbonaceous material leads to a reduction in the charge capacity of the battery. As a measure for dis-problem, this patent publication discloses merely to control the amount of a boron compound to be added. The carbonaceous material is likewise not considered to be optimal as a negative-electrode material for lithium-ion batteries.
Concerning industrial production of such boron-substituted graphite, JP 10-162829 A discloses that, after graphitization of a carbonaceous material is conducted in a nitrogen gas atmosphere, boron nitride formed in a surface layer is decreased by treating the graphitized carbonaceous material at 2,000° C. or higher under reduced pressure or subjecting it to halogen treatment.
From the standpoint of industrial production of a negative-electrode material, however, the inclusion of such treatment requires more complex production facilities and higher cost. To cope with this problem, JP 10-255799 A discloses to disperse a boron compound in softened or molten pitch so that the boron compound can be prevented from coming into contact with the nitrogen gas atmosphere. It is however very difficult to completely encapsulate the boron compound with the pitch. Effects of this proposal are therefore suspicious. Except for the above patent publications, there is no technical bulletin or patent publication that makes mention about an atmosphere gas to be employed upon graphitization of a carbonaceous material. Even in most of the above patent publications referred to in the above, nothing is disclosed beyond graphitization of a carbonaceous material “in an inert atmosphere”.
Carbonaceous materials, which can be obtained by either substituting boron for or adding boron to these boron-substituted graphites, are disclosed in JP 3-245458 A. As this patent publication discloses only treatments up to carbonization, the carbonaceous materials disclosed there are totally different from the previously-mentioned boron-substituted graphites in which portions of graphite carbons were replaced by boron.
Further, JP 7-73898 A discloses as a carbonaceous material graphite or amorphous graphite in which boron has substituted for a portion of carbon atoms. This carbonaceous material is however considerably different from the previously-mentioned boron-substituted graphite in which boron has substituted for a portion of carbon in the graphite, because its graphite layer spacing (d
002
) as determined by X-ray diffraction is 0.337 nm or greater and in addition, its use as a negative-electrode material in a battery provides a charge-discharge curve different from that available from a battery making use of a carbonaceous material.
As is disclosed inter alia in JP 7-73898 A and JP 6-333601, these boron-substituted or boron-added carbonaceous materials are synthesized by CVD processes each of which makes use of a mixed gas of a boron source gas and a carbon source gas. When industrial production is intended, these processes have no practical utility in cost.
In addition to boron-added carbonaceous materials, a variety of investigations have also been made about silicon-added carbonaceous materials. Silicon is known to be alloyed with lithium atoms, and as a specific capacity, approximately 4,017 mAh/g is achieved. However, these silicon-lithium alloys have large irreversible capacity, and due to expansion in volume resulting from the alloying, they involve a problem in stability.
To resolve this problem, JP 7-315822 A is proposing a material with atoms of an element capable of forming an alloy, such as silicon, incorporated in graphitized carbon as a host material without modification to the structure of an intact region in the host material. To product such a material, however, a costly synthetic process such as CVD is needed. Further, due to the insertion of silicon in carbon on the order of atoms, no improvements have been made yet as to the problem of large irreversible capacity in batteries.
Carbonaceous materials with boron and silicon mixed therein are disclosed, for example, in PCT International Publication WO 98/24134 and JP 11-40158 A. These carbonaceous materials are obtained by adding boron and silicon to the carbonaceous materials and then graphitizing the resulting mixture, and are not improved substantially in dis-charge capacity.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to industrially provide boron-substituted graphite and a silicon-containing carbonaceous material which, when used in lithium-ion secondary batteries, can produce constant discharge potentia

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