Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-09-17
2002-07-16
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231400, C423S44500R
Reexamination Certificate
active
06420070
ABSTRACT:
2. BACKGROUND OF THE INVENTION
2.1 Field of the Invention
The present invention relates to an anode made of a graphite material capable of intercalating and de-intercalating lithium ions, and a nonaqueous electrolyte secondary battery using the same.
2.2 Description of the Prior Art
In the recent trend of rapid progress in portable and cordless structure of consumer electronic appliances, the lithium secondary battery is noticed as their driving power source.
Hitherto, as the materials for anode of lithium secondary battery, lithium metal, lithium alloy, and carbon capable of intercalating and de-intercalating lithium ions have been discussed, but the former two materials produce dendritic lithium or fine lithium alloy respectively along with process of charging and discharging of battery, possibly leading to internal short-circuit of battery. Recently, therefore, the lithium ion battery using carbon is in the mainstream of merchandise.
When carbon is used as the anode, since lithium is intercalated between carbon layers at the time of charging, lithium is not present on the anode surface in metal state, and it is hence said that the safety of the battery may be enhanced.
Among carbon, graphite is particularly small in the initial irreversible capacity, and is likely to raise the electrode density, and it is studied in various aspects.
Such graphite materials include natural graphite, and artificial graphite obtained by calcining pitch, coke or other organic material. Generally, graphite particles are composed as polycrystals of graphite crystallites with crystallite size ranging from several nm to hundreds of nm in the in-plane direction ((110) or (100) direction) or C-axis direction ((004) or (002) direction). In such graphite particles, the C-axis of crystallites tends to face nearly same direction, and the same tendency is noted in the particles after grinding and sieving. Accordingly, the in-plane direction and C-axis direction are respectively uniform as if entire graphite particles were one crystallite.
When grinding the graphite in order to reduce the particle size, the graphite is likely to be cleaved by the shearing force between layers, that is, in the in-plane direction of crystal. Usually, therefore, the graphite particles ground to particle size of scores of microns are shaped like scales, the particle size is small in the C-axis direction of crystallite, and the aspect ratio of particle size in the in-plane direction of crystallite and particle size of C-axis direction tends to be larger.
Using such graphite material as the anode material, when paste is prepared together with binder and others and applied and rolled on the current collector, the filling density of graphite material in the electrode is raised, and owing to the large aspect ratio of particles in the in-plane direction and C-axis direction, the C-axis direction of particles tends to coincide with the vertical direction of the current collector. That is, the basement surface of crystallite in the graphite particles (C-axis (004) or (002) direction) tends to orient in the same direction as the surface of the current collector.
The orientation of graphite material in the electrode can be known from the peak intensity ratio R of the diffraction line (110) in the in-plane direction obtained from the wide-angle X-ray diffraction and the diffraction line (004) in the C-axis direction.
R
=
(
110
)
peak
integral
intensity
I
(
110
)
(
004
)
peak
integral
intensity
I
(
004
)
The intensity ratio R of graphite material measured in the powder state before application is measured in a state in which each particle does not have orientation in the measuring surface of wide angle X-ray diffraction, and therefore the obtained value corresponds to the size ratio of crystal size in the in-plane direction of graphite material and crystal size in C-axis direction. By contrast, in the electrode prepared by applying and rolling paste compound of graphite material on the current collector, the basement surface of graphite particles tends to orient in the same direction as the current collector surface. Therefore, crystallites composing graphite particles also orient according to the orientation of particles, and when the electrode surface is measured by X-ray, as compared with the powder state before application, the peak intensity I (110) of the in-plane direction of crystallites is weak, and the peak intensity I (004) in the C-axis direction is strong, so that the peak intensity ratio R varies. Thus, from the change in the peak intensity ratio R of wide angle X-ray diffraction, the degree of orientation of particles in the electrode may be known.
When the conventional electrode was measured in the above method, the peak intensity ratio R was about 0.01 to 0.05, and the ratio P(=R/R
o
) of R to the peak intensity ratio R
o
obtained from the powder before preparation of electrode was about 0.05.
In such electrode, on the electrode surface at the interface to the electrolyte, the ratio of existence of the basement surface of graphite crystal is large, while the ratio of existence of edge of graphite crystal inducing intercalation of lithium ions is small. Hence, in charging and discharging reaction, lithium ions cannot move smoothly at the interface of electrolyte and electrode, and polarization is likely to occur, and therefore favorable high rate charging and discharging characteristic or charging and discharging cycle characteristic cannot be obtained.
To solve such problems, as disclosed in Japanese Laid-open Patent No. 4-190556, Japanese Laid-open Patent No. 4-190557, and Japanese Laid-open Patent No. 6-318459, for example, it has been proposed to reduce the crystal size ratio (aspect ratio) in the in-plane direction and C-axis direction of graphite crystallites. In spite of these proposals, however, the problems are not solved completely, and in particular nothing is considered about restriction of orientation of graphite particles on the electrode.
In Japanese Laid-open Patent No. 8-83609 or Japanese Laid-open Patent No. 8-180873, graphites having various particle shapes are proposed, but nothing is still considered about restriction of orientation of graphite particles on the electrode.
The invention is devised to solve these problems, and it is hence an object thereof to present a nonaqueous electrolyte secondary battery using an anode particularly excellent in high rate discharging characteristic and charging and discharging cycle characteristic.
3. SUMMARY OF THE INVENTION
The object of the invention is to present a nonaqueous electrolyte secondary battery using an anode particularly excellent in high rate discharging characteristic and charging and discharging cycle characteristic. To achieve such object, as the anode for nonaqueous electrolyte secondary battery, the invention uses an anode mainly composed of a graphite material, with its peak intensity ratio R (=I(110)/I(004)) ranging from 0.05 to 0.5. As a result, it prevents extreme parallel orientation of crystal layer of graphite material on the current collector to the current collector plane, and enhances the high rate discharging characteristic.
Also in the anode, the ratio P (=R/R
o
) of peak intensity ratio R obtained from the electrode prepared by applying and rolling a graphite material on a current collector and peak intensity ratio R
o
obtained from the powder before preparation of the electrode is in a range of 0.1 to 0.7.
It hence controls extreme orientation of graphite particles in the electrode in the electrode preparation process, and enhances the high rate discharging characteristic.
By using such anode, a nonaqueous electrolyte secondary battery excellent in high rate discharging characteristic is obtained.
The invention as set forth in claim 1 uses an electrode of which peak intensity ratio R (=I(110)/I(004)) of lattice planes (110) and (004)
Kasamatsu Shinji
Muraoka Hiroki
Nitta Yoshiaki
Watanabe Shoichiro
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