Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-05-18
2001-03-27
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S223000, C429S231200, C423S594120, C423S600000
Reexamination Certificate
active
06207325
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a lithium-containing complex metal oxide suited to a cathode electroactive material for lithium secondary cells, a preparation method thereof, a cathode electroactive material using the same, and a lithium secondary cell. More specifically, it relates to a lithium nickelate-based complex metal oxide which has excellent thermal stability and can be used as a cathode electroactive material in a lithium secondary cell (battery) wherein metal lithium or a lithium-carbon (lithium-graphite) intercalation compound is used as an anode electroactive material, thus making the cell has a high capacity and good cycle characteristics.
2. Related Background Art
The non-aqueous secondary cell disposed lithium or lithium compound as an anode has been expected to the cell having a high voltage and high energy density, and therefore, many studies have been proceeded. Widely known cathode electroactive materials for the secondary batteries with non-aqueous electrolyte solutions include the complex metal oxide comprising of lithium and other metal or metals such as lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide; metal oxides such as manganese dioxide, titanium disulfide, molybdenum disulfide, vanadium pentaoxide, and niobium pentaoxide; and chalcogens. These oxides and compounds have layer or tunnel crystal structures and are capable of repeating the reversible release and intercalation of lithium ions on the charge/discharge, respectively. Especially, active studies are being made on lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide for their use in the cathode electroactive material for lithium secondary cells with non-aqueous electrolyte solutions as four-volt (V) type cell. Lithium cobalt oxide, which is relatively easy to prepare, has already been put into practical use.
However, cobalt is a very expensive metal and also a strategic material, and the places of its origin are localized within certain regions; thus, there are problems such as the troubled supply due to changes in political situations or rising in the price. On the other hand, nickel and manganese are relatively inexpensive metals and their stable supply is possible. Lithium manganese oxide has a smaller capacity compared with lithium cobalt oxide and lithium nickel oxide, and has problems in cycle characteristics. Lithium nickel oxide has also some problems in its cycle characteristics. LiNiO
2
results a change in its crystal structure from hexagonal to monoclinic, as it releases Li on the charge. This is believed to cause deterioration of the cycle characteristics. It has been found as a countermeasure to the deterioration that, if a part of the Ni in LiNiO
2
is substituted with Co, there will be no change from hexagonal to monoclinic and thus the cycle characteristics will be improved. See, T. Ohzuka et al., J. Electrochem. Soc., 140, 1862 (1993) and S. Arai, S. Okada, H. Ohtsuka, and J. Yamamoto, Battery Technology (Denchi Gijyutsu), 7, 98 (1995).
When LiNiO
2
is released Li through charging, NiO
2
is formed. NiO
2
is a very unstable compound, which generates heat while releasing oxygen. Accordingly, it is strongly desired that the thermal stability of LiNiO
2
is improved.
It has been found that if a part of the Ni in LiNiO
2
is substituted with Al, there will be a great improvement in its thermal stability. In this case, the charge capacity, however, lowers greatly. See, T. Ohzuka et al., J. Electrochem. Soc., 142, 4033 (1995).
Japanese Unexamined Patent Publication Sho 63-121,258 (1988) suggests a method to improve overpotential characteristics by substituting LiCoO
2
with a variety of other metals. Also, in Japanese Unexamined Patent Publication Hei 5-242,891 (1993), it is suggested that if LiNi
X
Co
Y
O
2
is further substituted with a variety of other metals, its discharge capacity will increase and that when Fe or Cu exists their thermal stability will be improved.
O. Zhong et al. have studied the synthesis and electrochemistry of LiAl
Y
Ni
1−Y
O
2
. See, O. Zhong and Ulchi von Sacken, J. Power Sources, 54, 221 (1995). Synthesis of LiAl
Y
Ni
1−Y
O
2
was first attempted with a mixture of LiOH, NiO, and Al
2
O
3
(or Al (OH)
3
), but the synthesis of a single-phase LiAl
Y
Ni
1−Y
O
2
was unsuccessful, as the product was contaminated with Al
2
O
3
as an impurity. Thus, they changed the Al source to a metal Al powder (300 mesh) and, for the first time, succeeded in synthesizing the single-phase product. Nevertheless, its discharge capacity was as small as 104-148 mAh/g.
SUMMARY OF THE INVENTION
This invention enables the obtaining of a lithium secondary cell that has a large discharge capacity and good cycle characteristics in addition to improved Coulomb efficiency at the first cycle of the charge/discharge. Moreover, an object of the invention is to provide a lithium-containing complex metal oxide having excellent thermal stability and suited to a cathode electroactive material (positive active material) for a lithium secondary cell.
Also, another object of the invention is to provide a method for preparing the above-mentioned lithium-containing complex metal oxide with good efficiency and relibility.
Further, another object of the invention is to provide a lithium secondary cell that is excellent in the cycle characteristics, the discharge capacity, and thermal stability and is improvided with respect to the Coulomb efficiency at the first cycle of the charge/discharge, as well as to provide a cathode electroactive material thereof.
As a result of intensive efforts to solve the above-mentioned problems, the present inventors discorvered that by substituting a part of the Ni LiNiO
2
with Co and Al, a lithium secondary cell with a large discharge capacity and good cycle characteristics can be obtained and further that a lithium-containing complex metal oxide, which serves as the cathode electroactive material with excellent thermal stability, can also be obtained. Furthermore, the present inventors discovered that in the preparation of this lithium-containing complex metal oxide, a method to blend (or mix) the raw material compounds containing Li, Ni, Co, and Al had an influence on characteristics of the products, particularly the thermal stability and the Coulomb efficiency at the first cycle of the charge/discharge, thus completing this invention.
The lithium-containing complex metal oxide of this invention is a lithium-containing complex metal oxide having a crystal structure of the &agr;-NaFeO
2
type and represented by the general formula:
Li
a
Ni
X
Co
Y
Al
Z
O
2
wherein 0.96≦a≦1.06, 0.70≦X<0.85, 0.05≦Y≦0.20, 0.10<Z≦0.25, and 0.98≦(X+Y+Z)≦1.02, and further wherein a separation between a peak position of (018) face and a peak position of (110) face in the powder X-ray diffraction pattern of said metal oxide using CuK&agr;-ray is in the range of from 0.520 to 0.700° as expressed in terms of &Dgr;2&thgr;((110)−(018)).
The lithium-containing complex metal oxide of the invention as described above, preferably, has a peak height ratio of 0.30 or less. The peak height ratio is calculated based on the following equation:
peak height ratio=peak height (&mgr;V)/((peak temperature)−(beginning temperature of the peak)),
in the exothermic peak curve by DTA measurement.
Also, the cathode electroactive material for a lithium secondary battery according to this invention contains a lithium-containing complex metal oxide of the invention as described above.
Further, the lithium secondary battery of the invention is a lithium secondary cell comprising:
a positive electrode plate;
a positive electrode electrically connected to the positive electrode plate;
a negative electrode plate;
a negative electrode electrically connected to the negative electrode plate; and
a separator positioned between the positive electrode and the negative electrode, said separator being impregnated with an electrolyte solution, wherein the posit
Matsui Yasushi
Shirao Masatoshi
Chaney Carol
Showa Denko K.K.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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