Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-03-23
2002-06-11
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S218100, C429S231800
Reexamination Certificate
active
06403258
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a lithium secondary battery provided with a positive electrode, a negative electrode, and a non-aqueous electrolyte, and more particularly, to a lithium secondary battery whose charge/discharge cycle performance is improved upon improvement of a positive electrode active material used for its positive electrode or a negative electrode active material used for its negative electrode.
2. Description of the Related Art
In recent years, secondary batteries have begun to be used in various fields such as electronic equipment and the like. Lithium secondary batteries are attracting great attention as one of new-type batteries having high power and high energy density, and various efforts have been made to develop such lithium secondary batteries.
In order to improve charge/discharge cycle performance of such lithium secondary batteries, one so adapted as to employ as a negative electrode active material an oxide such as tungsten trioxide which is lithiumized has been proposed, as disclosed in Japanese Patent Laid-Open No. Hei8(1996)-241707.
Unfortunately, however, the above-mentioned tungsten trioxide has an unstable crystal structure, as described in the literature (D. J. Murphy, F. J. Di Salvo, J. N. Carides and J. V., Mat. Res. Bull., 13, 1395, 1978). When the tungsten trioxide is used as a negative electrode active material in a lithium secondary battery, the tungsten trioxide is degraded in capacity of occluding and discharging lithium due to the transformation of its crystal structure. Accordingly, there still exists a problem that the battery cannot attain an adequate improvement in charge/discharge cycle performance.
SUMMARY OF THE INVENTION
An object of the present invention is to improve charge/discharge cycle performance of a lithium secondary battery provided with a positive electrode, a negative electrode, and a non-aqueous electrolyte upon improvement of a positive electrode active material used for said positive electrode or a negative electrode active material used for said negative electrode.
A lithium secondary battery according to the present invention is a lithium secondary battery provided with a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein a tungsten oxide W
20
O
58
or W
18
O
49
, or the tungsten oxide to which lithium is added is used as a positive electrode active material for said positive electrode or a negative electrode active material for said negative electrode.
In the above-mentioned lithium secondary battery, tungsten oxide W
20
O
58
or W
18
O
49
used as the positive electrode active material or the negative electrode active material has a stable crystal structure. Accordingly, the composite oxide hardly suffers the transformation of its crystal structure and hence, is hardly degraded in capacity of occluding and discharging lithium. The lithium secondary battery with excellent charge/discharge cycle performance thus can be obtained.
Further, a lithium secondary battery according to the present invention is a lithium secondary battery provided with a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein a tungsten composite oxide comprising a tungsten oxide containing at least one type of metal element selected from the group consisting of Mn, Cu, V, Cr, Fe, Co, and Ni and having a W
20
O
58
-type or W
18
O
49
-type crystal structure or the tungsten composite oxide to which lithium is added is used as a positive electrode active material for said positive electrode or a negative electrode active material for said negative electrode.
The literature (Binary Alloy Phase Diagrams, 1986, American Society for Metals: M—O binary phase diagram) shows that the metal element M, which is selected from the group consisting of Mn, Cu, V, Cr, Fe, Co, and Ni, in the above-mentioned composite oxide forms a stable compound whose decomposition temperature is more than 1000° C. in combination with an oxygen atom O. Further, W
20
O
58
or W
18
O
49
is used as a tungsten oxide to contain at least one type of metal element selected from the group consisting of Cu, V, Cr, Mn, Fe, Co, and Ni.
When the metal element M selected form the group consisting of Cu, V, Cr, Mn, Fe, Co, and Ni is added to the tungsten oxide to give a tungsten composite oxide having a W
20
O
58
-type or W
18
O
49
-type crystal structure, the metal elements M are incorporated in some of crystal lattices of W
20
O
58
or W
18
O
49
to attain a relatively strong chemical bond with oxygen atoms O, thereby stabilizing the crystal structure of the tungsten composite oxide.
Accordingly, when the above-mentioned tungsten composite oxide having a W
20
O
58
-type or W
18
O
49
-type crystal structure or the tungsten composite oxide to which lithium is added is used as a positive electrode active material or a negative electrode active material in a lithium secondary battery, the crystal structure of the tungsten composite oxide is further prevented from being transformed and hence, degradation in capacity of occluding and discharging lithium is further prevented. The lithium secondary battery with more excellent charge/discharge cycle performance thus can be obtained.
Further, when a tungsten composite oxide represented by a chemical formula M
x
W
y
O
z
(wherein M denotes at least one type of metal element selected from the group consisting of Mn, Cu, V, Cr, Fe, Co, and Ni; and the conditions of 0.005<x/(x+y)<0.2 and z=58 or 49 are satisfied) and having a W
20
O
58
-type or W
18
O
49
-type crystal structure is used as a positive electrode active material or a negative electrode active material in a lithium secondary battery, the battery can attain still more excellent charge/discharge cycle performance because the crystal structure of the tungsten composite oxide is further stabilized.
When other element such as Cd, La, Ce, Sm, or Mo, which forms highly stable compound in combination with an oxygen atom O as described above, is used as the metal element M in the above-mentioned tungsten composite oxide, the resultant composite oxide is still expected to be effective in improving charge/discharge cycle performance of a lithium secondary battery.
When the above-mentioned tungsten oxide, the above-mentioned tungsten composite oxide, or the tungsten oxide or tungsten composite oxide to which lithium is added is used as a positive electrode active material in the lithium secondary battery of the present invention, various materials generally used in lithium secondary batteries may be used as a negative electrode active material. Examples of a usable material include carbon materials capable of electrochemically occluding and discharging Li, such as natural graphite, artificial graphite, coke, and calcined products of organic substances; Li alloys such as an Li—Al alloy, an Li—Mg alloy, an Li—In alloy, and an Li—Al—Mn alloy; and Li metals. However, when the Li alloys or Li metals is used as the negative electrode active material, branch-like dendrite crystals grow during the charging and discharging of the battery so that a short circuit may occur in the battery. Therefore, it is preferable to use the carbon materials as the negative electrode active material.
On the other hand, when the above-mentioned tungsten oxide, the above-mentioned tungsten composite oxide, or the tungsten oxide or tungsten composite oxide to which lithium is added is used as a negative electrode active material in the lithium secondary battery of the present invention, various materials generally used in lithium secondary batteries may be used as a positive electrode active material. For example, it is preferable to use a lithium-containing transition metal oxide such as LiCoO
2
, LiNiO
2
, LiMn
2
O
4
, LiMnO
2
, LiCo
0.5
Ni
0.5
O
2
, LiNi
0.7
Co
0.2
Mn
0.1
O
2
, LiCo
0.9
Ti
0.1
O
2
, or LiCo
0.5
Ni
0.4
Zr
0.1
O
2
.
Then, a case where the tungsten oxide, the tungsten composite oxide, or the tungsten oxide or tungsten composite oxide to which lithium is added is us
Fujimoto Masahisa
Fujitani Shin
Yoshimura Seiji
Armstrong Westerman & Hattori, LLP
Chaney Carol
Sanyo Electric Co,. Ltd.
Yuan Dah-Wei D.
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