Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-10-28
2002-05-28
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
Reexamination Certificate
active
06395426
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a non-aqueous cell comprising, in a cell case, a positive electrode having a positive electrode active material composed of lithium cobalt oxide, a negative electrode, and an electrolyte.
(2) Description of the Prior Art
In recent years, lithium ion cells have attracted attention as high capacity batteries. In such lithium ion cells, lithium cobalt oxides and lithium manganese oxides are used as a material for the positive electrode, and alloys or carbon materials are used for a negative electrode active material thereof Such lithium ion cells, however, have such drawbacks that, as charge-discharge cycles are repeated, a charge and discharge capacity and a charge and discharge efficiency are degraded.
Japanese Patent Publication No. 63-59507, for example, discloses a non-aqueous electrolyte cell in which Li
x
M
y
O
2
(‘M’ consists of Ni or Co, x<0.8, and y≈1) is used as a positive electrode active material and a lithium metal is used for the negative electrode, and this cell exhibits a high electromotive force of 4 V or higher and a high energy density. However, this cell as well has such drawbacks that a charge and discharge capacity is susceptible to degrading, and so forth. The reasons for such drawbacks seem to be that an irreversible change occurs in part of the crystal structure of the positive electrode active material, causing deterioration in the capability of absorbing and releasing lithium ions, and that decomposition of the electrolytic solution or the like is caused by overcharge and overdischarge that exceed an appropriate electric potential range, inducing a shortage of the electrolytic solution, undesirable effects by the decomposed matter, and so forth. In view of this drawback, a technique to replace part of the crystal of the positive electrode active material with other types of metallic elements has been suggested as a means to prevent such undesirable phenomena from occurring.
For example, Japanese Unexamined Patent Publication Nos. 4-329267 and 5-13082 disclose a battery using a positive electrode active material in which a titanium compound is added to a lithium cobalt oxide in a state of solid solution.
In addition, Japanese Unexamined Patent Publication No. 4-319260 discloses a battery using a positive electrode active material in which a zirconium is added to a lithium cobalt oxide in a state of solid solution.
Also, Japanese Unexamined Patent Publication No. 4-253162 discloses a battery using a positive electrode active material in which a one element selected from lead, bismuth, and boron is added to a lithium cobalt oxide in a state of solid solution.
The techniques as listed above can improve the problems as described above, but on the other hand, they induce other drawbacks, such that an initial capacity of the cell is reduced, and that a temperature at which the positive electrode active material starts to generate heat is lowered and thereby safety of the cell is degraded.
As portable appliances such as portable computers and mobile phones have increasingly become popular in the market, a need for a cell having an excellent low-temperature cycle characteristic and increased safety is accordingly growing. In addition, in order to further reduce sizes and weight of batteries, a thin type sealed cell in which a power-generating component is enclosed in a cell case composed of a flexible and lightweight laminated material (such a cell case is hereinafter also referred to as a “laminated container”) has been developed. However, such a laminated container has a small strength against a cell internal pressure, and therefore easily expands when an internal gas is formed in the cell, causing problems such as deformation of the cell, leakage of the electrolyte, and rupture of the cell. For this reason, particularly in such a thin type cell, a positive electrode active material that can reduce an internal gas formation is desired.
SUMMARY OF THE INVENTION
In view of the foregoing problems and drawbacks in prior art, it is an object of the present invention to provide a non-aqueous electrolyte cell having a high working voltage, excellent low-temperature discharge characteristics, a reduced internal gas generation, and an increased safety.
This and other objects are accomplished in accordance with the present invention, by providing a non-aqueous electrolyte cell comprising in a cell case a positive electrode having a positive electrode active material composed of a lithium cobalt oxide, a negative electrode, and an electrolyte comprising a non-aqueous solvent,
the positive electrode active material comprising a Ti-attached LiCoO
2
in which a particle of a titanium and/or a titanium compound is attached on a surface of a particle of the lithium cobalt oxide.
In the Ti-attached LiCoO
2
having such a configuration as described above, titanium particles and/or titanium compound particles are attached on at least a surface of lithium cobalt oxide, and the titanium particles and/or titanium compound particles serve to decompose a film derived from the non-aqueous solvent (the film is formed such as to surround the positive electrode active material), or to facilitate the exfoliation of the formed film. Therefore, according to the configuration as described above, degradation of discharge performance caused by a poor ionic conductivity can be suppressed, and consequently a significant decrease in discharge capacity under low temperature can be avoided. Note here that in the case of prior art positive electrode active materials, a surface of the positive electrode active material is surrounded by a film derived from a non-aqueous solvent, inhibiting the contact between the active material particles and the electrolyte. Therefore, in the case of prior art positive electrode active materials, the discharge performance is significantly degraded under a low temperature environment of 0° C. or lower, where the ionic conductivity is low.
Moreover, according to the configuration as described above, not only do the titanium particles and/or titanium compound particles, which are present on the surface of the lithium cobalt oxide, serve to facilitate the charge-discharge reaction, but also serve to inhibit decomposition of the electrolytic solution, thereby suppressing formation of internal gas.
Furthermore, the Ti-attached LiCoO
2
has a higher temperature point at which heat is generated, when compared to titanium-lithium cobalt oxide in a form of solid solution. Therefore, by employing the Ti-attached LiCoO
2
, it is made possible to provide cell having an improved safety compared to the case where the titanium-lithium cobalt oxide in a form of solid solution is employed.
In the above-described configuration of the non-aqueous electrolyte cell, it is preferable that a mole ratio in the Ti-attached LiCoO
2
of the titanium and/or the titanium compound to the lithium cobalt oxide be in the range of from 0.00001 to 0.02, and more preferably be in the range of from 0.00001 to 0.01.
When the mole ratio x is restricted to be 0.00001≦x≦0.02, a working voltage and a discharge capacity at a low temperature of 0° C. or below can be remarkably increased without substantially decreasing a discharge capacity at room temperature. Moreover, when the mole ratio (x) is restricted to be 0.00001≦x≦0.01, the decrease of the discharge capacity at room temperature can be further lessened, and thus the low-temperature discharge characteristics are further improved. It is noted here that if the mole ratio x is less than 0.00001, the low-temperature discharge characteristics cannot be sufficiently improved. If the mole ratio x is more than 0.02, no further improvement in the low-temperature discharge characteristics is observed, and the discharge characteristics under room temperature are deteriorated. Hence, the mole ratio x is preferable to be 0.00001≦x≦0.02, and more preferably be 0.00001≦x≦0.01.
Further, in the above-described configuration of the non-aqueous electrolyte cell
Imachi Naoki
Kodama Yasunobu
Nakane Ikuro
Narukawa Satoshi
Yoshida Ichiro
Armstrong Westerman & Hattori, LLP
Brouillette Gabrielle
Sanyo Electric Co,. Ltd.
Tsang-Foster Susy
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