Lithium-ion-conductive solid electrolyte and...

Details Lithium-ion-conductive solid electrolyte and... Lithium-ion-conductive solid electrolyte and...

1998-12-08

2001-08-21

Ryan, Patrick (Department: 1725)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S344000, C429S345000

Reexamination Certificate

active

06277524

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lithium-ion-conductive substance which can be used as an electrolyte of solid electrochemical devices, such as solid-electrolyte batteries, capacitors and solid electro-chromic display units. Moreover, the present invention relates to a solid-electrolyte lithium battery as well.
2. Description of the Related Art
Solid-electrolyte lithium batteries of high safety have been developed as the new-generation lithium secondary battery. The materials for making the solid electrolytes are required to exhibit conductivity as high as possible, and to be stable chemically as well as electrochemically. Crystalline solid electrolytes are most desired materials in terms of stability. However, no crystalline substances of high conductivity have yet been discovered.
The studies on solid electrolytes are the history of investigative researches on how to stabilize the high ionic conduction, which is usually present at elevated temperatures, at low temperatures; and how to synthesize materials exhibiting the high ionic conduction at low temperatures. The researches started with crystalline substances, and have developed into glassy and polymeric substances.
In the development of the next-generation high-energy-density lithium battery, the following are reported: an inorganic glassy electrolyte of high ionic conductivity and of good chemical-and-electrochemical stability; an Li
2
S-SiS
2
-Li
3
PO
4
solid electrolyte disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 4-202,024; and an Li
2
S-GeS
2
-LiI glassy solid electrolyte disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 59-151,770.
The advantages of the glassy solid electrolyte are as follows: it is easy to synthesize the glassy solid electrolyte; it is relatively easy to carry out the material designing of the glassy solid electrolyte; the glassy solid electrolyte exhibits the isotropic ionic conduction; and the glassy solid electrolyte can be readily turned into an actual electrolyte. In particular, when the cooling speed of the glassy solid electrolyte is controlled, the glassy solid electrolyte exhibits an expanded vitrification range so that it is possible to attain compositions and structures which are appropriate for the ionic conduction of lithium ion.
It has been said that the crystalline solid electrolytes and the amorphous solid electrolytes depend on different basic ideas in terms of the material designing. The amorphous solid electrolytes exhibit the degree of freedom in the material designing far greater than the crystalline solid electrolytes. However, if the crystalline solid electrolytes can form structures which are adequate for the ionic conduction, there can exist substances which are far superior to the amorphous solid polymer electrolytes in terms of the ionic conductivity and the chemical-and-electrochemical stability.
Crystalline Rb
4
Cu
14
I
7
Cl
13
is a substance which has exhibited the maximum ion conductivity at room temperature so far. The ion conductivity of crystalline Rb
4
Cu
14
I
7
Cl
13
is better than those of the corresponding glassy silver solid electrolytes and copper solid electrolytes by a couple of digits, and additionally is larger than those of liquid electrolytes at room temperature. Considering the relationship between the crystalline solid electrolytes and amorphous solid electrolytes, it is possible to think of the presence of corresponding crystalline solid electrolyte, since there exist the glassy solid electrolytes whose structures are based on SiS
4
and PO
4
tetrahedrons. Moreover, on the analogy of the silver or copper ion conductors, it is natural that such crystalline solid electrolyte would exhibit a much higher ion conductivity than the glassy substances.
A substance which is famous as a ceramics ion conductor is an oxide, which is usually referred to as “LISICON”. In the substance, a framework structure is formed. The framework structure includes SiO
4
, PO
4
and ZnO
4
tetrahedrons. Lithium ions diffuse in the framework structure. These oxide substances are well known, since they exist as minerals on the earth. However, crystalline substances whose structures are based on SiS
4
and PS
4
tetrahedrons have not been known yet to exist as such oxide substances, since they are not present in nature. Hence, the inventor of the present invention has decided to study these novel substances.
Thus, the present inventor has started investigating the crystalline substances whose structures are based on SiS
4
, PO
4
, PS
4
and PN
4
tetrahedrons. Then, he aimed at constructing substance systems which are appropriate for the lithium ion diffusion, and at applying them to solid electrolytes for secondary batteries.
SUMMARY OF THE INVENTION
The present inventor investigated into Li
2
S-GeS
2
-Ga
2
S
3
, Li
2
S-GeS
2
-ZnS and Li
2
S-GeS
2
-P
2
S
5
ternary systems for novel substances, and revealed whether the electrochemical characteristics of the investigated novel substances are appropriate for solid electrolytes. As a result, he discovered that one of the Li
2
S-GeS
2
-Ga
2
S
3
ion conductors exhibited a remarkably high ion conductivity of 6.49×10
−5
Scm
−1
at room temperature. Thus, he completed the present invention.
A lithium-ion-conductive solid electrolyte according to the present invention comprises:
a lithium-ion-conductive substance expressed by a general formula li
2
S-GeS
2
-X wherein “X” is at least one member selected from the group consisting of Ga
2
S
3
and ZnS, or by a general formula Li
2
S-SiS
2
-P
2
S
5
.
The present lithium-ion-conductive solid electrolyte is a novel substance whose composition differs from those of the conventional lithium-ion solid electrolytes. Moreover, the present lithium-ion-conductive solid electrolyte is crystalline. In addition, the present lithium-ion-conductive solid electrolyte having a specific composition exhibits a remarkably high ion conductivity of 6.49×10
−5
Scm
−1
at room temperature.
A solid-electrolyte lithium battery according to the present invention employs the above-described lithium-ion-conductive substance as a solid electrolyte. Namely, the present solid-electrolyte lithium battery comprises, the solid electrolyte, a cathode, and an anode. The present solid-electrolyte lithium battery is superb in terms of stability and safety at elevated temperatures, since the electrolyte is a crystalline solid which can exhibit a high ion conductivity.
The present lithium-ion-conductive solid electrolyte can be crystalline, and accordingly can exhibit a high lithium-ion conductivity. The lithium-ion-conductive solid electrolyte can be used as the solid electrolyte for the present solid-electrolyte lithium battery, since it is stable and safe at elevated temperatures.


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Solid State Ionics; Yamashita et al; “Thin Film preparation of the Li2S-GeS2-Ga2S3 glass system by sputtering”; vol. 89, pp. 299-304, 1996.*
Materials Research Society Symposium Proceedings/Crystallisation and Related Phenomena in Amorphous Materials; Akai et al; “Ionic Motion and structure of ion conductive glasses”, vol. 321, pp. 191-196, 1994.*
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