Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1999-09-27
2002-11-26
Tung, T. (Department: 1743)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C429S006000, C429S319000, C429S320000, C429S322000, C501S002000, C501S004000, C501S007000, C501S008000, C501S010000, C501S046000, C501S048000, C501S073000, C501S103000, C501S152000
Reexamination Certificate
active
06485622
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a lithium ion conductive glass-ceramics suitable for use as wholly solid electric cells, gas sensors and electrochemical devices of various types, and electric cells and gas sensors using such glass-ceramics.
Recent development in electronics has brought about high-performance electronic devices of a compact and light-weight design and, as a power source of such electronic devices, development of an electric cell of a high energy density and a long life is strongly desired for.
Lithium has the highest oxidation-reduction potential of Li/Li
+
of all metal elements and has the smallest mass per 1 mol and, therefore, a lithium cell can provide a higher energy density than other types of cells. Moreover, if a lithium ion conductive solid electrolyte is used, this electrolyte can be made very thin and, therefore, a cell of a thin film can be formed and increase in energy density per unit volume can thereby be realized.
A lithium ion cell which has been realized to date uses an organic electrolyte solution as its electrolyte and this makes it difficult to achieve a cell of a compact design such as a thin film design. This lithium ion cell has additional disadvantages that it has likelihood of leakage of electrolyte solution and likelihood of spontaneous combustion. If this lithium ion cell is replaced by a cell employing an inorganic solid electrolyte, a wholly solid cell of a high reliability will be realized.
Carbon dioxide gas produced by combustion of fossil fuel is a main cause of a hothouse effect which has recently become a serious problem and it has become necessary to incessantly watch the concentration of carbon dioxide gas. Therefore, establishment of a system for detecting carbon dioxide gas is a matter of increasing importance for the maintenance of a comfortable life in the future human society.
Carbon dioxide gas detection systems which are currently in use are generally of a type utilizing absorption of infrared ray. These systems, however, are large and costly and besides are susceptible to contamination. For these reasons, studies have recently been actively made to develop a compact carbon dioxide gas sensor using a solid electrolyte. Particularly, many reports have been made about studies using a lithium ion solid electrolyte.
For realizing such gas sensor using solid electrolyte, development of a solid electrolyte which is highly conductive, chemically stable and sufficiently heat proof is indispensable.
Among known electrolytes, Li
3
N single crystal (Applied Physics Letters, 30(1977) P621-22), LiI—Li
2
S—P
2
S
5
(Solid State Ionics, 5(1981) P663), LiI—Li
2
S—SiS
4
(J. Solid State Chem. 69(1987) P252) and LiI—Li
2
S—B
2
S
3
(Mat. Res. Bull., 18(1983) 189) glasses have high conductivity of 10
−3
S/cm or over at room temperature. These materials, however, have the disadvantage that preparation of these materials is difficult and these materials are not chemically stable and not sufficiently heat proof. Particularly, these materials have the fatal disadvantage that decomposition voltage of these materials is so low that, when they are used for an electrolyte of a solid cell, a sufficiently high terminal voltage cannot be obtained.
An oxide lithium solid electrolyte does not have the above described disadvantages and has a decomposition voltage which is higher than 3V and, therefore, it has possibility of usage as a wholly solid lithium cell if it exhibits a high conductivity at room temperature. It is known in the art that conductivity in an oxide glass can be increased by increasing lithium ion concentration. However, there is limitation in increasing the lithium ion concentration even if rapid quenching is employed for glass formation and conductivity of this glass at room temperature is below 10
−6
S/cm at the highest.
Japanese Patent Application Laid-open Publication No. Hei 8-239218 discloses a gas sensor using a thin film of a lithium ion conductive glass. The conductivity of this lithium ion conductive glass thin film is within a range from 1.7×10
−7
S/cm to 6.1×10
−7
S/cm. This is not a sufficiently high value and a solid electrolyte having a higher conductivity is desired for.
There are many reports about oxide ceramics (sintered products) having a high conductivity. For example, Li
4
GeO
4
—Li
3
VO
4
exhibits conductivity of 4×10
−5
S/cm at room temperature (Mat. Res. Bull. 15(1980) P1661), and Li
1+X
Al
X
Ge
2−X
(PO
4
)
3
exhibits conductivity of 1.3×10
−4
S/cm at room temperature (Proceedings of 8th International Meeting on Lithium Batteries, Jun. 6-21, 1996, Nagoya, Japan P316-317). Oxide ceramics are superior in conductivity to oxide glasses but have the disadvantages that they require a complicated and troublesome process for manufacturing and that they are difficult to form, particularly to a thin film.
In short, the prior art lithium ion solid electrolytes have the problems that they are either low in conductivity, hard to handle or hard to form to a compact design such as a thin film.
It is, therefore, an object of the invention to provide glass-ceramics which have solved these problems and exhibit a high lithium ion conductivity at room temperature.
It is another object of the invention to provide a lithium electric cell and a gas sensor of a high performance by utilizing such glass-ceramics.
SUMMARY OF THE INVENTION
As described above, ceramics exhibit conductivity of 10
−4
S/cm or over at room temperature. These ceramics, however, have pores and a large grain boundary which cannot be eliminated completely and existence of these pores and grain boundary results in decrease in conductivity. If, therefore, glass-ceramics including the above crystal are provided, there will be no pores and the grain boundary will be improved and, as a result, a solid electrolyte having a higher conductivity is expected to be produced. Besides, glass-ceramics which share a feature of glass can be easily formed into various shapes including a thin film by utilizing this feature of glass. For these reasons, glass-ceramics are considered to have practical advantages over ceramics made by sintering.
As a result of studies and experiments made by the inventor of the present invention on the basis of the above described basic concept, the inventor has succeeded in obtaining glass-ceramics having a high lithium ion conductivity at room temperature by producing glasses including ingredients of P
2
O
5
, SiO
2
, GeO
2
, TiO
2
, ZrO
2
, M
2
O
3
(where M is en element or elements selected from the group consisting of In, Fe, Cr, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu), Al
2
O
3
, Ga
2
O
3
and Li
2
O and causing a crystal phase of conductive crystal Li
1+X
(M, Al, Ga)
X
(Ge
1−X
Ti
Y
)
2−X
(PO
4
)
3
(where 0<X≦0.8 and 0≦Y≦1.0) to precipitate from the glasses by heat treating these glasses. The inventor has also found that a lithium electric cell and a gas sensor using the glass-ceramics exhibit excellent characteristics.
For achieving the above described objects of the invention, there are provided lithium ion conductive glass-ceramics comprising, in mol %:
P
2
O
5
30-45%
SiO
2
0-15%
GeO
2
+ TiO
2
25-50%
in which
GeO
2
0-50%
TiO
2
0-50%
ZrO
2
0-8%
M
2
O
3
0<-10%
where M is an element or elements selected from the group consisting of In, Fe, Cr, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu
Al
2
O
3
0-12%
Ga
2
O
3
0-12%
Li
2
O
10-25%
and containing Li
1+X
(M, Al, Ga)
X
(Ge
1−Y
Ti
Y
)
2−X
(PO
4
)
3
(where 0<X≦0.8 and 0≦Y≦1.0) as a predominant crystal phase.
In one aspect of the invention, there is provided a solid electrolyte for a lithium electric cell using these lithium ion conductive glass-ceramics.
In another aspect of the invention, there is provided a solid electrolyte for a gas sensor using these lithium ion conductive glass-ceramics.
In another aspect of the invention, there is provided a lithium electric cell co
Hedman & Costigan ,P.C.
Kabushiki Kaisha Ohara
Tung T.
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