Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-07-09
2002-01-01
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231800, C423S44500R
Reexamination Certificate
active
06335121
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an electrode material for non-aqueous secondary battery. More particularly, it relates to an improvement of a negative electrode material of a lithium ion battery.
BACKGROUND ART
Portable appliances have recently been miniaturized and lightened, remarkably. Consequently, miniaturization and lightening of batteries as a power source have remarkably required and various non-aqueous electrolyte solution batteries such as lithium battery have been suggested.
In case of a secondary lithium battery, reduction of the capacity as a result of an irreversible change of lithium (e.g. formation of dendritic lithium, etc.) and a problem on safety arise during repeating of charge/discharge. Therefore, a carbon material is generally used in place of a lithium metal as the negative electrode. As the carbon material, amorphous carbon such as coke (e.g. Japanese Patent Kokai Publication No. 62-122066 and 1-204361), glassy carbon (e.g. Japanese Patent Kokai Publication No. 2-66856) or the like; and graphite such as natural graphite (e.g. Japanese Patent Kokoku Publication No. 62-23433) and manufactured graphite (e.g. Japanese Patent Kokai Publication No. 4-190555) or the like have been suggested.
However, in a secondary lithium ion battery using an amorphous carbon, the discharge capacity of carbon is not sufficient and the potential of the battery changes largely during the charge/discharge. Furthermore, it is necessary to make the potential of carbon during the discharge closer considerably to that of a metal lithium and there is a risk of deposition of a dendritic lithium.
On the other hand, as the method of improving problems such as low charge/discharge capacity, large change in voltage of the battery during the charge/discharge and potential of carbon during the charge, for example, Japanese Patent Kokai Publication No. 3-245458 discloses that the capacity is improved by containing 0.1 to 2.0% by weight of boron in the carbonaceous material, but the problem of large change in voltage during the charge/discharge is still to be solved. Japanese Patent Kokai Publication No. 7-73898 discloses that an improvement in capacity and a change in potential of carbon can be performed by using a material of B
z
C
1−z
(0<z<0.17) made by the chemical vapor deposition method, but the problem of large change in voltage during the charge/discharge is still to be solved. Furthermore, in Japanese Patent Kokai Publication No. 5-290843, the capacity is improved by using a compound wherein a part of carbon atoms constituting a carbon network skeleton of pitch coke is substituted with a boron atom and a nitrogen atom (BC
3
, BC
3
N). However, since electrical performances of the compound are similar to those of a semiconductor, overvoltage during the charge/discharge becomes severe and high capacity can not obtained in the practical charge/discharge voltage region. Furthermore, in Japanese Patent Kokai Publication No. 8-31422, a carbon material having high graphitization degree is obtained by heat-treating a combination of pitch coke and a boron compound at the ultra-high temperature, thereby solving the problem on the capacity. However, since B
4
C is formed as a result of the heat treatment at the ultra-high temperature (not less than 2500° C.), high capacity can not be obtained.
DISCLOSURE OF THE INVENTION
In a conventional electrode material, a relation between a mechanism of occlusion of lithium, which is related to a charge/discharge capacity, and a crystalline structure of a carbon material has not been elucidated, sufficiently. Hence, an object of the present invention is to elucidate the above mechanism and to provide a carbon material wherein the discharge capacity is large and the change in potential during the charge/discharge is small.
The present inventors have intensively studied to solve the above object. As a result, the present inventors have found that, by using a carbonaceous material, wherein a value of R (degree of graphitization), which is defined as a ratio of a Raman spectrum intensity at 1580 cm
−1
to a Raman spectrum intensity at 1360 cm
−1
in the Raman spectrum analysis, is not more than 4.0 and a length of crystallite (Lc) oriented along a crystallographic c axis obtained by a wide angle X-ray diffraction method is from 25 to 35 nm as an electrode of a lithium ion battery, the discharge capacity of the resulting battery is large and the change in potential during the charge/discharge is small.
The fact that “the length of crystallite (Lc) oriented along a crystallographic c axis obtained by a wide angle X-ray diffraction method is from 25 to 35 nm” means that a graphite crystal has grown sufficiently and the safety of the battery is not adversely affected by the gas evolved by overvoltage during the charge/discharge, and that charge/discharge repeating properties are excellent. On the other hand, referring to the degree of graphitization in the Raman spectrum analysis, the small value of R means that a structure containing an amorphous portion, in addition to the graphite crystalline portion, is formed. It should be surprised that the length of crystallite (Lc) oriented along a crystallographic c axis is from 25 to 35 nm and the amorphous portion is contained. Therefore, it is assumed that this structure makes it possible to store at the portion other than the interlaminar portion of crystals, resulting in large discharge capacity.
Accordingly, the present invention provides a negative electrode material for non-aqueous secondary battery, comprising a carbonaceous material wherein a value of R (degree of graphitization), which is defined as a ratio of a Raman spectrum intensity at 1580 cm
−1
to a Raman spectrum intensity at 1360 cm
−1
in the Raman spectrum analysis, is not more than 4.0 and a length of crystallite (Lc) oriented along a crystallographic c axis obtained by a wide angle X-ray diffraction method is from 25 to 35 nm.
It has also been found that, since this carbon material has a specific structure comprising a crystalline portion and an amorphous portion, a peak derived from d
002
in the X-ray diffraction appears between 0.354 nm (second stage) and 0.370 nm (first stage) in the process of charging Li using the carbon material as the negative electrode. It is assumed by this fact that this carbon material extends gradually from the second stage to the first stage without rapidly extending the interlaminar space in the charging process, which means that carbon material is easily charged and the structure is hardly broken by repeating of the charge/discharge.
This carbon material has a dielectric loss (obtained by ESR spectrum analysis) of not more than 0.4. When the dielectric loss is small, the electrical conductivity is high and the material is easily charged and discharged. Therefore, it is specific to the electrode material having an initial charge/discharge capacity of not less than 250 mAh/g.
The carbonaceous material of the present invention can be obtained by calcining an organic material or a carbon material under a non-oxidizing atmosphere at 800 to 3000° C., and preferably from 2000 to 2500° C., in the presence of (a) boron or its compound and (b) silicon or its compound or germanium or its compound. By limiting the kind or amount of the boron material to be added and calcination temperature, formation of B
4
C is particularly controlled. When the amount of the boron material is large, by-products, which do not take part in the charge/discharge, are formed, resulting in reduction of the capacity. Accordingly, the present invention provides a negative electrode material for non-aqueous secondary battery, comprising a carbonaceous material obtained by heat-treating under a non-oxidizing atmosphere in the presence of (a) boron or its compound and (b) silicon or its compound or germanium or its compound, wherein a peak derived from B
4
C does not appear in the X-ray diffraction pattern, that is, the content of B
4
C is about 5% by weight or less.
BEST MODE FOR CARRYING OUT THE I
Isshiki Nobuyuki
Kajiura Yoshio
Nakanishi Kuniyuki
Suzuki Atsushi
Chaney Carol
Kao Corporation
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