Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
Lithium secondary battery
Tcchnical Field
This invention relates to a lithium secondary battery,
especially to its positive active material.
Background Art
In resent years, studies have been carries actively on
a lithium secondary battery and those having a working voltage
of about 3V are now partially put in practical use. However,
it can not be said that these lithium secondary batteries are
provided with satisfactory characteristics with respect to
energy density and service life.
Recently, batteries which utilize a positive active
material having a working voltage of about 4V in order to
increase the energy density and which utilize an organic burnt
material as a negative electrode in order to prolong the
service life, are reported and attract public attention.
The inventors begun investigations into 4V-group positive
active materials having a working voltage of about 4V on the
basis of consideration that, even when the service life was
prolonged by utilizing the organic burnt material as the
negative electrode, a battery having a high energy density
could not be obtained if the working voltage of positive
e~lectrode was not high. As the result, we came into
conclusion that LiCoO2 shown by Mizushima et al., "Material
Research Bulletin" vol. 15, pp. 783, 1980, was the favorable
4V-group positive active material in the present stage.
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However, the following problems have become clear from
energetic studies on the LiCoO2. Although the LiCoO2 had a
h:igh working voltage of about 4V and a high energy density,
the reversibility of LiCoO2 was poor when it was subjected to
deep charging and discharging. In other words, it was -
d:ifficult to put the LiCoO2 into practical use as the positive
active material of lithium secondary battery if the problem
oE poor reversibility in the LiCoO2 was not improved.
An object of this invention is to improve the poor
reversibility of LiCoO2 and to provide a lithium secondary
battery having a large discharge capacity.
Disclosure of the Invention
In a lithium secondary battery utilizing LiCoO2 as its
positive active material, this invention is characterized by
that a peak strength of X-ray diffraction in (101) plane of
LiCoO2 lies within a range of 5 to 15 when a peak strength of
X-ray diffraction in (003) plane is 100.
When the LiCoO2 is subjected to deep charging, a part of
crystal structure changes to a discontinuous one. It can be
considered that this change in crystal structure is a cause
of the poor reversibility. It became clear from studies on
the crystal structure that LiCoO2 prepared by an ordinary
method, i.e. LiCoO2 obtained by mixing, burning and slowly
cooling a raw material, had a high orientati~n to c-axis and
its crystal grew remarkably in a direction of c-axis.
The inventors considered that the poor reversibility
could be prevented when the crystal structure of high
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orientation in c-axis was changed, so that we examined to
optimize a superposition in c-axis direction and a spreading
in a-axis direction. We found that LiCoO2 excellent in the
reversibility and able to increase the discharge capacity
could be obtained when the peak strength of X-ray diffraction
in (101) plane able to represent the spreading in a-axis
direction too, as mentioned above, lay within the range of 5
to 15, if the peak strength of X-ray diffraction in (003)
plane representing the superposition in c-axis direction was
l~DO.
The reason why the LiCoO2 excellent in the reversibility
and able to increase the discharge capacity could be obtained
is that an absolute value of change in crystal structure at
time of charging can be restrained to a small by restraining
the orientation in c-axis direction. In this case, the
capacity will be decreased when the peak strength of X-ray
diffraction in (101) plane in relation to (003) plane exceeds
by excessively restraining the orientation in c-axis
direction, and the reversibility will be decreased when the
peak strength is smaller than 5.
Brief Description of the Drawinqs
Fig. 1 is a characteristic diagram showing a discharge
capacity of first cycle for a peak strength of X-ray
diffraction in (101) plane in relation to (003) plane. Fig.
2 is a characteristic diagram showing a capacity retention -~
factor for a peak strenqth of X-ray diffraction in (101) plane
in relation to (003) plane.
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Best Mode for Carrvinq Out the Invention
LiCoO2 forming the positive active material was prepared
first in the following way. Lithium carbonate and cobalt
carbonate were weighed so as to attain a molar ratio 1 of Li
t:o Co, and completely mixed while being ground in a ball mill.
q'he obtained mixture was put in an alumina crucible and
t:emporarily burnt at 650C for 5 hours in air, and then burnt
at 900C for 20 hours. Thereafter, it was cooled and ground
t:o be used for active materials.
In the above preparation, seven kinds of active materials
were obtained by controlling a cooling speed after burning.
All of these seven kinds of active materials were measured by
Y-ray diffraction, and it was ensured that these materials
were all LiCoO2. Seven kinds of LiCoO2 were denoted as A, B,
C, D, E, F and G respectively, and peak strengths of X-ray
diffraction of in (101) plane in relation to (003) plane
(expressed as I1ol/Ioo3 hereunder) were measured for each.
]Results are shown in Table 1.
[Table 1]
Active material A B C D E F ¦ G
I,ol/Ioo3 3.1 5.2 6.9 10.1 13.4 18.6 ¦35.5
Coin type lithium batteries A through G were made as
trial in the following way by using LiCoO2 A through G as the
positive active material respectively.
In the first place, the positive electrode was obtained
as follows. LiCoO2 powder, acetylene-black powder and
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polytetrafluoroethylene powder were mixed with a weight ratio
of 85 to 10 to 5, and thoroughly kneaded after adding
isopropyl alcohol to them. The obtained mixture was formed
by a roll press into a sheet having a thickness of 0.8 mm and
then punched to circular articles having a diameter of 16 mm,
and it was heat treated at 200C for 15 hours. The positive
electrode was used by being pressed against a positive
electrode can attached with a current collector.
A negative electrode was obtained by punching a lithium
foil having a thickness of 0.3 mm into circular article having
a diameter of 15 mm. The negative electrode was used by being
pressed against a negative electrode can through a current
collector.
A material prepared by dissolving LiBF~ of 1 mol/l to
~-butylolactone was used for an electrolyte, and a fine porous
membrane made of polypropylene was used for a separator.
A coin type lithium battery having a diameter of 20 mm
and a thickness of 1.6 mm was made up by using the
above-mentioned positive electrode, negative electrode,
electrolyte and separator.
In the second place, charge/discharge cycle tests were
p~erformed on the obtained batteries A through G, with test
conditions: charge current of 3mA, charge end voltage!of 4.5V, ~;
discharge current of 3mA and discharge end voltage of 3.0V.
Fig. 1 and Fig. 2 are characteristic diagrams showing
results of the above tests. Fig. 1 shows a discharge capacity
at first cycle in relation to Ilol/Ioo3~ and Fig. 2 shows a
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capacity retention factor in relation to Ilol/Ioo3. The
capacity retention factor was calculated by the following
equation.
Discharge capacity
at 50th cycle
Capacity retention factor(~) = x 100
Discharge capacity
at first cycle
As seen from Fig. 1, the discharge capacity decreased
when I10l/IO03 exceeded 15- As seen from Fig. 2, the
reversibility lowered when Ilol/Ioo3 became lower than 5, and
the discharge capacity decreased with the cycle.
Consequently, an optimum Ilol/Ioo3 for LiCoO2 excellent in the
reversibility and able to increase the discharqe capacity was
5 to 15.
Since the LiCoO2 having an Ilol/Ioo3 ranging from 5 to 15
is used for the positive active material as described above,
this invention can provide a lithium secondary battery which
is excellent in its reversibility and has a large discharge
capacity even when it is subjected to the deep charging and
d.ischarging.
In this invention, the start material and manufacturing
method of positive active material, the positive electrode,
t.he negative electrode, the electrolyte, the separator and the
battery shape etc. are not limited to those described in the
a.bove best mode. For example, a material synthesized from
hydroxides or oxides may be used for the positive active
material, an organic burnt material may be used for the
negative electrode, and a solid electrolyte may be used in
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p:Lace of the electrolyte and the separator.
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