Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1999-02-26
2001-02-27
Krynski, William (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S375000, C428S402000, C428S403000
Reexamination Certificate
active
06194067
ABSTRACT:
TECHNICAL FIELD
The present invention relates to novel carbonaceous particles and carbonaceous fibers and to lithium secondary batteries. More specifically, the invention relates to carbonaceous particles and carbonaceous fibers which are covered on their surfaces with boron nitride thin films and are suitable for applications as reinforcing materials, lubricating materials and electronic materials such as lithium secondary battery materials, as well as to lithium secondary batteries which employ them as negative electrode active materials.
BACKGROUND ART
Carbonaceous materials are commonly used as reinforcing materials for plastics, metals and carbon because of their mechanical high strength and high elastic modulus. In addition, because of their high crystallographic and mechanical anisotropy, they are also useful as lubricating materials for sliding parts. Their unique microstructures are also utilized for applications in capacitor batteries and lithium secondary batteries. However, carbonaceous materials have a drawback in that they undergo degradation at temperatures of a few hundred ° C. and above by oxidation in the presence of oxygen and other oxidative gases, or react with the metals or other matrixes to produce carbides, so that their expected performance cannot be exhibited; applications of carbonaceous materials as reinforcing materials and lubricating materials and so on have therefore been limited.
Boron nitride has also been applied as a sliding material, etc. Boron nitride is a chemically stable material with high oxidation resistance, but is difficult in control of its crystallinity and form than carbonaceous materials.
It has been attempted to utilize the similarities and differences between carbonaceous materials and boron nitride to produce materials which are combinations thereof. Japanese Unexamined Patent Publication (Kokai) No. 8-245206, for example, discloses a boron carbonitride material which is an intermediate compound of graphite and boron nitride wherein carbon, nitrogen and boron are in a molar composition of 6:2:1. Also, Japanese Unexamined Patent Publication (Kokai) No. 5-171444 discloses a material with excellent wear resistance comprising a bi-layer film composed of a hard carbon film and a boron nitride film which is formed on a coated substrate by vapor phase synthesis. Japanese Unexamined Patent Publication (Kokai) No. 7-187883 discloses a process for coating a silicon substrate with a thin film of boron nitride alloyed with carbon, which can be employed for wear resistant uses and for semiconductors across a wide range of temperatures. The processes for forming these boron nitride films involve vapor phase reaction under reduced pressure, but while such processes are effective for producing films on flat surfaces, they are difficult for materials with not flat surfaces, such as carbonaceous particles and carbonaceous fibers.
Carbonaceous particles and carbonaceous fibers containing boron atoms and nitrogen atoms are disclosed in Japanese Unexamined Patent Publication (Kokai) No.
3-245458
, No. 5-266880, No. 5-290843, No. 8-306359 and No. 8-31422. These carbonaceous particles and carbonaceous fibers, however, have the boron atoms or nitrogen atoms in solid solution in the carbon, or have carbon atoms replaced with boron atoms or nitrogen atoms, or are stoichiometrically simple compounds of carbon, nitrogen and boron.
Graphite is also used as a negative electrode active material in lithium secondary batteries, and Japanese Unexamined Patent Publication (Kokai) No. 9-63584 discloses the use of a carbon material which has been subjected to graphite treatment in the presence of a boron compound as a negative electrode active material in a lithium secondary battery, in order to achieve increased activity for the negative electrode active material. However, graphitization treatments in the presence of a boron compound it does not make possible to obtain a graphite material coated with boron nitride, and no improvement in chemical stability is achieved.
It is an object of the present invention to modify the chemically unstable surfaces of conventional carbonaceous particles and carbonaceous fibers by coating them with a boron nitride thin film, to thus provide carbonaceous particles and carbonaceous fibers with a widened range of applications.
It is another object of the invention to provide lithium secondary batteries with improved chemical stability of their negative electrode active materials by using the aforementioned boron nitride thin film-coated carbonaceous particles and carbonaceous fibers as the negative electrode active materials.
DISCLOSURE OF THE INVENTION
The present inventors have carried out diligent research in the light of reaction theory relating to different boron compounds and starting carbonaceous materials under various types of heat treatment conditions, such as that of the atmosphere gas, with the aim of modifying the surfaces of carbonaceous particles and carbonaceous fibers into a more chemically stable form. As a result, the present inventors have found that by addition of an appropriate amount of a boron compound as a graphitizing catalyst, sealing the mixture in a crucible, raising the temperature of the crucible to above a given temperature in an atmosphere of an inert gas containing an appropriate amount of nitrogen gas or a reducing gas containing an appropriate amount of nitrogen gas, and then lowering the temperature at a slow rate after holding it for a prescribed time or immediately without holding it, the resulting carbonaceous particles or carbonaceous fibers have a unique structure wherein the surfaces are covered with a chemically stable boron nitride thin film. It was also found that by using the carbonaceous particles and carbonaceous fibers covered with the chemically stable boron nitride thin film as negative electrode active materials in lithium secondary batteries, it is possible to provide excellent secondary battery characteristics as well as improved chemical stability for the lithium secondary batteries. The present invention has been completed on the basis of these findings.
In other words, the present invention provides the following.
(1) Carbonaceous particles characterized in that the surfaces of the carbonaceous particles are coated with a boron nitride thin film, wherein the atomic concentration of boron C(B), the atomic concentration of carbon C(C) and the atomic concentration of nitrogen C(N) satisfy the following inequalities:
0.05<C(B)/(C(B)+C(C)+C(N))<0.3
0.05<C(N)/(C(B)+C(C)+C(N))<0.3
in the surface region as measured by photoelectron spectroscopy;
(2) Carbonaceous fibers characterized in that the surface of the carbonaceous fibers are coated with a boron nitride thin film, wherein the atomic concentration of boron C(B), the atomic concentration of carbon C(C) and the atomic concentration of nitrogen C(N) satisfy the following inequalities:
0.05<C(B)/(C(B)+C(C)+C(N))<0.3
0.05<C(N)/(C(B)+C(C)+C(N))<0.3
in the surface region as measured by photoelectron spectroscopy;
(3) Carbonaceous particles or carbonaceous fibers according to (1) and (2) above, wherein the 002 plane spacing (d
oo2
) as determined by X-ray diffraction is 0.337 nm or less;
(4) A lithium secondary battery which employs a reversible charge/discharge material as the positive electrode active material, a lithium ion-containing nonaqueous electrolyte, a separator which is permeable to lithium ions, and a carbonaceous material as the negative electrode active material, characterized in that the lithium secondary battery uses the carbonaceous particles according to (1) above or carbonaceous fibers according to (2) above as the negative electrode active material;
(5) A lithium secondary battery which employs a reversible charge/discharge material as the positive electrode active material, a lithium ion-containing nonaqueous electrolyte, a separator which is permeable to lithium ions, and a carbonaceous material as the negative electrode active material, cha
Hamada Takeshi
Inada Koki
Kohno Taro
Sugiura Tsutomu
Suzuki Kimihito
Gray J. M.
Keynon & Kenyon
Krynski William
Nippon Steel Corporation
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